A Lejeune

M Rabiller-Baudry

I F J Vankelecom

Dr Thierry Renouard
email: thierry.renouard@univ-rennes1.fr

On the relative influence of the hydrodynamics of lab-scale set-ups and the membrane materials on the rejection of homogeneous metal catalysts in solvent resistant nanofiltration

Keywords: Solvent resistant nanofiltration, hydrodynamics, cross-flow, deadend, homogeneous metal catalyst rejection

On the relative influence of the hydrodynamics of lab-scale set-ups and the membrane materials on the rejection of homogeneous metal catalysts in solvent resistant nanofiltration

Solvent resistant nanofiltration is a promising process for homogeneous metal catalyst recovery. However, catalyst rejections are determined using various setups with different hydrodynamics, which questions the comparison of the results.

The analysis of the membrane performance is another issue even when comparing membranes made of the same material. This study compares the hydrodynamics of three experimental set-ups. Rejections from 57.8 to 80.8% between cross-flow and dead-end SRNF are observed with a Rh catalyst (MW = 258 g•mol -1 ) in toluene. A high throughput screening system is used to analyze the performance of a set of membranes in terms of permeate fluxes and catalyst rejections.

Introduction

Production of fine chemicals suffers from difficult separation of multicomponent postreaction mixtures. In fact, the usual purification steps have many drawbacks because they can be energy-consuming (distillation), need phase change (distillation, crystallization) or consume large amounts of solvents (liquid-liquid extraction). Solvent resistant nanofiltration (SRNF) is a promising separation process (1-4) since it can be performed at low temperature, in liquid phase only and without any solvent addition. This makes SRNF an eco-friendly process, favorable to maintain the integrity of fragile molecules. In that sense, SRNF has a huge potential in the field of homogeneous metal catalyst recycling because thermal separation processes can be responsible for their degradation (5). Many papers focus on the recycling of homogeneous and pseudohomogeneous catalysts by SRNF (6-14). Different strategies are used to improve their This paper deals with the comparison of the performances of 3 lab-scale set-ups.

Two of them are cross-flow systems and the third one allows dead-end nanofiltration.

Their hydrodynamics are first compared using Reynolds numbers and the cross-flow velocity. Then, the toluene fluxes and the rejections of homogeneous metal catalysts obtained with these 3 set-ups are compared to determine the influence of the hydrodynamic conditions. Finally, a dead-end set-up allowing fast membrane screening is used to discuss the effects of solvent and membrane on the catalyst rejection.

Materials and methods

Solvents and metal catalysts

Toluene of analytical grade (>99.3% purity) was provided by Sigma-Aldrich and dimethyl carbonate (DMC, >99% purity) was purchased from Alfa Aesar. The Grubbs-Hoveyda II (Fig. 1-a) and Rh(acac)(CO)2 (Fig. 1-b) catalysts were provided by Sigma-Aldrich. The properties of the solvents and the metal catalysts are summarized in Table 1.

[Figure 1]

[Table 1]
UV-Vis spectra in toluene and in DMC were recorded in the 280-800 nm range with a Jasco V-630 spectrophotometer equipped with a quartz tube of 1 cm path length. The background was recorded with pure solvent. The concentration of pure Grubbs-Hoveyda II was estimated from its maximum absorbance at λ max = 377 nm and the one of Rh(acac)(CO)2 at λ max = 334 nm using the Beer-Lambert relationship. The accuracy A c c e p t e d M a n u s c r i p t of the analyzes was 5%.

Membranes

Various membranes based on polydimethylsiloxane (PDMS) and polyimide (PI) were tested. Their main characteristics are summarized in Table 2. Membranes were cut as flat sheet coupons prior to SRNF. For all membranes, one coupon was used for toluene experiments and another one for DMC experiments. The membranes were first compacted in the respective solvent at room temperature under 40 bar pressure until a constant flux was reached. The first milliliters that passed through each membrane sample were discarded because of the possible presence of preservatives. The membrane conditioning can have an impact on its performance (32). For better comparison, the same conditioning was applied to all membranes, even though the Solsep NF070706 was not supposed to be used up to 40 bar (see Table 2). It has to be noticed that no damage was observed on the Solsep NF070706 surface after use and several papers already reported on its use under 35-40 bar pressure (33,34).

[Table 2]

Filtration conditions and experimental set-ups

For all SRNF experiments, the Grubbs-Hoveyda II initial concentration was 0.5 mM and that of Rh(acac)(CO)2 was 0.05 mM, which corresponds to what can be encountered in catalytic reactions. All nanofiltration experiments were operated at room temperature and at TMP=30 bar, unless otherwise specified, with a volume reduction ratio (VRR) up A c c e p t e d M a n u s c r i p t to 2. For all experimental set-ups, sealing was obtained with Viton® O-rings. Wetted rings were FEP-coated to prevent any damage due to the organic solvents.

Cross-flow SRNF with the MET set-up

Flat sheet membranes (17 cm²) were inserted in circular cells on a commercial stainless steel filtration unit (MET-Evonik, UK) (Fig. 2). Two cells were connected in series to make the retentate of the first cell to feed the second one. The cross-flow mode was obtained thanks to a recirculation pump (part of the MET set-up) allowing a flowrate of 1.24 L•min -1 . Since this feed flowrate was high compared to the permeate flux, an equivalent feed was considered for both membranes. The height of the feed channel was 5 mm. The pressure was obtained thanks to nitrogen pressure applied on the feed tank.

[Figure 2]

Cross-flow SRNF with the SEPA cell

A SEPA cell for flat sheet membranes (139 cm²) was used (Fig. 3). The pressure was imposed thanks to a HPLC pump (AZURA P 4.1S, Knauer, Germany) allowing a feed flowrate up to 50 mL•min -1 . The cross-flow mode was obtained thanks to a recirculation pump allowing a flowrate of 1.24 L•min -1 . A spacer (Fig. 4) was inserted in the retentate compartment in order to promote turbulence (65 mil -thickness of 1.65 mm, Sterlitech, US) as in a spiral module. Without spacer, the height of the feed channel was 2 mm. modules (HTML, Belgium) (37,38) called HT-system, allowing for simultaneous filtrations with 6 or 16 membrane samples under the same operating conditions (Fig. 5).

The 16 positions set-up was used for all experiments in toluene and for some measurements in DMC, while the 6 positions set-up was used for all other DMC experiments (solvent flux and SRNF of solutes). Membrane samples were supported by a porous stainless steel disc. The active area of each membrane sample was 4.52 cm 2 for the 6 positions set-up and 1.54 cm² for the 16 positions set-up. The pressure was obtained thanks to nitrogen pressure applied on the feed tank. Feeds were stirred magnetically at 320 rpm (length of the stirrer: 8 cm). Inventors of the system have already checked that all positions were equivalent dealing with fluxes and rejections of rose Bengal in 2-propanol (37), but the system was not characterized yet with respect to hydrodynamics.

[Figure 5]

Hydrodynamics

In order to compare the performances of the different set-ups, they were first characterized by their hydrodynamics. The Reynolds number Re is an easy and commonly used way to compare hydrodynamic conditions. It is also often used in correlation to determine the mass transfer coefficient (39). For sake of simplicity, a uniform flow distribution has been assumed above the permeating area of the membranes, even if this assumption does not always correspond with reality (40,41).

Cross-flow SRNF

For a cross-flow nanofiltration, the Reynolds number was calculated according to equation [START_REF] Marchetti | Molecular Separation with Organic Solvent Nanofiltration: A Critical Review[END_REF]. It has to be underlined that for a flow channel (MET set-up, SEPA cell), turbulent conditions start at Re CF > 2,000.

A c c e p t e d M a n u s c r i p t

Re CF = ρ⨯d H ⨯u η [START_REF] Marchetti | Molecular Separation with Organic Solvent Nanofiltration: A Critical Review[END_REF] with ρ the solution density (kg•m -3 ), d H the hydraulic diameter of the liquid flow (m), u the free liquid velocity (m•s -1 ) and η the solution viscosity (Pa•s). Since the solutions were diluted, toluene density and viscosity data were used. d H and u had to be determined according to some assumptions.

The hydraulic diameter was calculated with equation ( 2) (39,42).

d H = 2⨯L⨯H L+H (2)
with L the length of the channel (m) and H the height of the channel (m).

The feed flowrate Q (m 3 •s -1 ) was imposed by a pump and the liquid velocity u (m•s -1 ) in the membrane cell was calculated as follows:

u = Q S (3)
Rectangular cell. Fig. 6 is a layout of the rectangular SEPA cell. The Reynolds number for the SEPA cell was calculated considering a free liquid channel without spacer. It was assumed that the feed passed through the whole length L at the membrane cell entrance. For such a configuration, the flow section S was calculated with equation ( 4).

S = L ⨯ H

[Figure 6] Circular cell. For the circular cells of the MET set-up, Reynolds calculations were done considering that the liquid moved as a whirlwind, as illustrated in Fig. 7.

[Figure 7]

A c c e p t e d M a n u s c r i p t

The length of the flow section was assumed to be the diameter of the pipe where the liquid came from (d=2 mm). The flow section S was calculated with equation 5.

S = d ⨯ H

(5)

Dead-end SRNF

For a dead-end circular cell, the Reynolds number was calculated thanks to equation 6 (43). For a stirred cell (such as the HT-system), a turbulent flow starts at Re DE > 2•10 5

(44).

Re DE = ρ⨯D stirrer 2 ⨯w η [START_REF] Peeva | Continuous Consecutive Reactions with Inter-Reaction Solvent Exchange by Membrane Separation[END_REF] with D stirrer the diameter of the stirrer (m) and w the angular speed (rad•s -1 ).

SRNF experiments

The same procedure was applied for each experiment whatever the experimental set-up.

All nanofiltrations were performed at room temperature (27 ± 4 °C). The first step was to measure the pure solvent flux J 0 (L•m -2 •h -1 ) at the desired transmembrane pressure (TMP) using equation 7.

J 0 = V p A⨯t (7)
with V p the volume of permeate (L) recovered during t time (h) and A the membrane filtering area (m²).

The membrane permeance

L P 0 (L•m -2 •h -1 •bar -1
) was also determined thanks to Darcy's law:

J 0 = L P 0 ⨯ TMP (8)
A c c e p t e d M a n u s c r i p t

Then the solution to filtrate was prepared at the initial concentration (0.05 mM for Rh(acac)(CO)2 and 0.5 mM for Grubbs-Hoveyda II) and the set-up was filled (200 mL of solution for the MET set-up and the HT-system and 260 mL for the SEPA cell). The filtration was performed up to a volume reduction ratio (VRR) equal to 2 (i.e. half of the initial feed volume passed through the membrane, equation 9).

VRR =

V initial feed V final feed [START_REF] Siew | Efficient and productive asymmetric Michael addition: development of a highly enantioselective quinidine-based organocatalyst for homogeneous recycling via nanofiltration[END_REF] with V initial feed the initial volume of the mixture in the feed tank (L) and V final feed the final volume of the mixture in the feed tank (L).

Permeate samples were taken during the filtration in order to measure the permeate flux (J SRNF ) and the solute concentration. The final retentate was also analyzed in order to calculate the rejection at VRR=2 using equation [START_REF] Cano-Odena | Solvent-resistant nanofiltration for product purification and catalyst recovery in click chemistry reactions[END_REF].

Rejection VRR=2 (%) = (1 - C P C R
) ⨯ 100 [START_REF] Cano-Odena | Solvent-resistant nanofiltration for product purification and catalyst recovery in click chemistry reactions[END_REF] with C P the concentration of the last permeate sample (mol•L -1 ) and C R the final retentate concentration (mol•L -1 ).

The remaining solution was then taken off and the set-up was thoroughly rinsed with solvent. Lastly, the pure solvent flux after rinsing (J R ) was measured in order to be compared with the initial solvent flux. For each experiment, the flux measurements (J 0 , J SRNF , J R ) were repeated twice and the accuracy was 2%.

Results and discussion

Hydrodynamics of the 3 lab-scale set-ups

The fluid velocities and the Reynolds numbers were calculated for all set-ups (Table 3).

For the MET set-up, according to Fig. 7, the flow was supposed to behave as a The HT-system was developed to work in turbulent conditions, which is highlighted by a high Reynolds number (> 300,000), but it is commonly accepted that dead-end filtrations can increase concentration polarization. Moreover, the calculated Reynolds number is an average value and the local variations along the stirred cell were not taken into account. Actually, the mass transfer decreased continuously with the distance from the sidewall, to reach zero at the center of the set-up (21,25).

[Table 3]

A c c e p t e d M a n u s c r i p t

Comparison of the performances in toluene for the 3 set-ups

The previous flow characterization has outlined that different hydrodynamics were encountered with 3 different lab set-ups, thus the influence of hydrodynamics on SRNF results has to be determined. The reported results were calculated using data obtained at final VRR=2, but it has been checked that both flux and rejection were constant throughout the experiments (Appendix 1).

Pure toluene fluxes

First, the toluene fluxes of the Sulzer Pervap 4060 (PDMS) and the Starmem 122 (polyimide) membranes were measured with all set-ups, each time using a new membrane coupon (Fig. 8). For the SEPA cell, the experiments were performed only with Sulzer Pervap 4060.

For the 3 set-ups, similar toluene fluxes were obtained with Sulzer Pervap 4060 in the 10-30 bar range. For the MET set-up and the SEPA cell, fluxes were so close that they needed to be differently colored to be distinguished in Fig. 8. The same trend was also observed with Starmem 122 in the MET set-up and the HT-system. One can guess that the same flux would have been found with the SEPA cell. These results were expected since the solvent flux is known to be independent of the hydrodynamics when the permeate flux is small compared to the feed flow. It can be noticed that the PDMS membrane fluxes were systematically five-fold those measured with the polyimide membrane.

[Figure 8]

Fluxes of model solutions

Fluxes during SRNF at TMP=30 bar of the Grubbs-Hoveyda II and Rh(acac)(CO)2 catalysts in toluene (J SRNF ) were compared to the pure solvent flux (J 0 ) at the same A c c e p t e d M a n u s c r i p t pressure, for both membranes. Table 4 gives the J SRNF /J 0 ratios.

For SRNF of Grubbs-Hoveyda II in the HT-system with Starmem 122, the flux with the catalyst was 10% lower than the pure solvent flux (the membrane recovered its initial flux after rinsing) which revealed that concentration polarization occurred in spite of turbulent conditions. Rabiller-Baudry et al. (18) obtained a more pronounced effect at the same concentration during dead-end SRNF experiments using an apparatus operating under laminar conditions. Actually, they observed that only 35% of the initial flux was maintained, this flux reduction being fully reversible. By changing either the set-up (MET set-up with Starmem 122) or the membrane (HT-system with Sulzer Pervap 4060), the J SRNF /J 0 ratio was close to 1. An additional experiment was realized with the MET cell and the SEPA cell mounted in series (analogous to the replacement of membrane 2 by the SEPA cell in Fig. 2) so that both cells were studied exactly under the same operational conditions. Similar J SRNF /J 0 ratios were obtained, thus the influence of the hydrodynamics was limited with these diluted solutions.

For SRNF of Rh(acac)(CO)2, the J SRNF /J 0 ratio was close to 1 for all experiments, meaning that there was no flux decrease. So no concentration polarization can be evidenced through the flux data. This result was expected because the concentration of this catalyst was very small and its effect on the flux was negligible.

Nevertheless, an approach combining the film theory and the solution-diffusion model developed by Murthy and Gupta (51) can be used to evaluate the solute concentration at the membrane. If the membrane concentration over the bulk concentration ratio (C m /C bulk ) is higher than 1 then concentration polarization occurs. Experimental data obtained with the MET set-up at a TMP of 10 and 30 bar during SRNF of Rh(acac)(CO)2 were used for the calculations (Table 5). No concentration polarization was detected with Sulzer Pervap 4060, although concentration polarization was A c c e p t e d M a n u s c r i p t evidenced with Starmem 122, even if this was not observed with flux measurements. In the tested 10-30 bar pressure range, the higher the transmembrane pressure, the more significant was the concentration polarization in agreement with the results of Imbrogno and Schäfer (21) in water systems. The results obtained with both catalysts emphasize that not only hydrodynamics but also subtle interactions between the solute and the membrane play a major role in SRNF performances.

[Table 4]

[Table 5]

Rejections of solutes

The rejections of the Grubbs-Hoveyda II and the Rh(acac)(CO)2 catalysts at TMP=30 bar were then compared with the 3 set-ups, using both membranes (Table 4). As a general trend, the rejections obtained with Starmem 122 were higher than those obtained with Sulzer Pervap 4060. This can be put in parallel to the much lower fluxes of the former membrane.

The rejection of the Grubbs-Hoveyda II catalyst with Starmem 122 was the same with the MET set-up and the HT-system (99.6%). Both cross-flow set-ups also revealed similar catalyst rejections with Sulzer Pervap 4060 (ca. 95% at TMP=10 bar).

For that experiment, the slightly lower rejection can be partly attributed to the lower pressure applied during SRNF. For a strongly retained solute, it seemed that the crossflow or dead-end hydrodynamics had no impact on the rejection.

The rejection of Rh(acac)(CO)2 varied from 35% to 81%, depending on the setup or the membrane used. Similar to what was observed with the Grubbs-Hoveyda II catalyst, both cross-flow set-ups exhibited comparable rejections. However, the rejection was at least 20 percent higher with the HT-system (dead-end mode) whatever the membrane. This can be correlated to the optimized hydrodynamics developed for the HT-system as pointed out by its high Reynolds number.

These preliminary results highlight that the membrane material mainly drives the SRNF performances and that hydrodynamics has a small impact on solvent flux and rejection for very highly retained solutes (for concentrations in the millimolar range) whereas hydrodynamics plays a major role for solutes with intermediate rejections.

Hence, in the following, a membrane screening was performed with the HT-system, allowing to study the membranes performances exactly under the same operational conditions and hydrodynamics.

Membrane screening in toluene and DMC with the HT-system

A set of PDMS and polyimide membranes were tested. Their characteristics are summarized in Table 2. The filtrated solutions were prepared with the catalyst as a single solute. It has to be noticed that real reaction mixtures would be more complex, since mainly composed of some remaining substrate, the target product and several byproducts, in addition to the catalyst residue. The substrate/product concentrations are usually high (up to 1 mol•L -1 (52)) which may induce a rejection variation due to either interactions between the catalyst and the organic products or modification of the solution viscosity.

Solvent fluxes

Pure solvent fluxes at TMP=30 bar were first measured (Fig. 9). Toluene fluxes were measured with the 16 positions HT-system. Fluxes in DMC were measured with the 6 and 16 positions HT-system and proved to be equivalent with both set-ups. The DMC viscosity and density were close to those of toluene ( [Figure 9]

Solvent fluxes were also measured during SRNF experiments. In toluene, the J SRNF /J 0 ratio was close to 1 for all membranes during the filtration of all catalysts (Data not shown). This was also true in DMC, except for SRNF of Grubbs-Hoveyda II with the Starmem 240 membrane (J SRNF /J 0 =0.56). Keraani et al. (53) were the first ones to perform SRNF of Grubbs-Hoveyda II in DMC. They used a Starmem 228 membrane in a dead-end set-up and observed a J SRNF /J 0 ratio of 0.82 at a TMP of 25 bar.

Catalyst rejections

The Grubbs-Hoveyda II catalyst rejections were first determined (Fig. 10). For toluene [Figure 10]

In toluene, Rh(acac)(CO)2 always had a lower rejection than the Grubbs-Hoveyda II catalyst (Fig. 10). This result was expected because of their molecular weight difference. Its rejection was higher than 80% only with Starmem 122.

In the case of DMC solutions, the Rh(acac)(CO) except for the Grubbs-Hoveyda II rejection in DMC. This final point outlines that even though a clear tendency can be observed when comparing the membrane performance, the extrapolation of results is still delicate.

Conclusion

The hydrodynamics of 3 lab-scale set-ups were characterized thanks to simple but realistic assumptions and different flow conditions were revealed. SRNF of two organometallic catalysts was performed with the 3 set-ups, to study to what extent hydrodynamics can affect the filtration results. Rejections varied from 57.8 to 80.8% between cross-flow and dead-end apparatuses for Rh(acac)(CO)2 (rejection), while rejections always remained high for Grubbs-Hoveyda II, whatever the set-up. These results highlight that for solutes of intermediate rejections, the hydrodynamics may play a more important role on the rejection than for highly retained solutes.

The HT-system working in dead-end mode and allowing simultaneous SRNF with several membranes was used for fast membrane screening. Promising combinations (solvent, catalyst and membrane) were found (e.g. Starmem 122 gave almost full rejection of Grubbs-Hoveyda II in toluene), but no systematic trend could be withdrawn highlighting that "trial and error" experiments might still be necessary to select an optimal combination. In addition to the time consuming "trial and error" approach, the design of experiments (DoE) analysis could reveal further trends.

A c c e p t e d M a n u s c r i p t

A c c e( 28 , 29 )

 2829 p t e d M a n u s c r i p t separation from the final mixture. Variations of membranes (10,11), solvents (15,16) and operating conditions (mainly transmembrane pressure (16,17) and sometimes temperature (10)) are studied as the most important process design parameters. All these experimental results have been obtained with different lab equipment exhibiting different hydrodynamics (10,16,18,19) and the question of their comparison and their transposition to larger flat membranes or industrial spiral-wound modules is open, since fluxes and rejections can vary from one set-up to another (20). Some studies report on the effect of hydrodynamics on membrane performances in aqueous systems (21-25) highlighting (i) the positive effect of cross-flow systems to decrease concentration polarization compared to dead-end filtration (21) and (ii) the influence of the module scale on the mass transfer for cross-flow (22-24) and dead-end filtration (25). However, few studies focus on the influence of hydrodynamic conditions and the scalability of the results in SRNF. Tsibranska and Tylkowsky (26) compared fluxes and rejections of phenolics and flavoids with Duramem 300 and 500 membranes in ethanol with a cross-flow set-up (ca. 100% rejection) and a stirred dead-end set-up (70-100% rejection). Patterson et al. (27) also compared cross-flow and dead-end set-ups with the Starmem 122 membrane in toluene. The filtrated trialkylamine bases had lower rejections in cross-flow mode, which was explained by differences in concentration polarization and fouling between the 2 operating modes, even though for one solute the rejection was 19% in dead-end mode and more than 80% in cross-flow SRNF. Shi et al. studied the hydrodynamics of industrial SRNF modules with the PuraMem S600 membrane. Using data from a 1.8"x12" spiral-wound module tested with solutions of sucrose octaacetate in ethyl acetate, correlations were determined to characterize the fluid dynamics as well as the mass transfer coefficient. The solution-diffusion model and the film theory were also used to successfully predict the performances of the

A c c e

  p t e d M a n u s c r i p t module of larger size. In addition to the influence of the hydrodynamics, it has to be mentioned that the variance in data is also important since it can affect the ultimate separation process (30).

[Figure 3 ][Figure 4 ]

 34 Dead-end SRNF with the HT-systemDead-end filtrations were carried out in 2 different high-throughput (HT) filtration

A c c e p t e d M a n u s c r i p t whirlwind and so

  a turbulent regime was operated. Cortés-Juan et al. (45) used Computational Fluid Dynamics to highlight that the flow was partly tangential to the cell wall for a circular cell even if the feed arrived straight inside the cell. Dzhonova-Atanasova et al. (46) also considered a tangential orientation of the feed pipe resulting in a swirling flow in the cell for a lab-scale set-up (from MET society) with the same geometry as the MET set-up but with an active membrane area of 54 cm². They performed Computational Fluid Dynamics simulations considering a pure water solution and found that the speed of the solution was in the range 0.3-0.9 m•s -1 for a feed flowrate of 1.2 L•min -1 . In these conditions, the Reynolds number was estimated to 8,500. All these data are in accordance with our results.The SEPA cell seemed to work in laminar conditions (Reynolds < 2,000) but the calculation was performed considering a free liquid channel. It must be underlined that this apparent laminar regime was probably not encountered in the cell because a retentate spacer was added to promote turbulence (40,47). The fluid velocity in the SEPA cell was estimated to 0.13 m•s -1 . It's noteworthy that in industrial membrane spiral-wound modules, the fluid velocity in a free liquid channel can be estimated by simple assumptions (48) and usually ranges from 0.1 to 0.6 m•s -1 (49) and sometimes reaches 2 m•s -1 (50).

A c c e p t e d M a n u s c r i p t

A c c e p t e d M a n u s c r i p t

  As a general trend, the DMC flux was higher than the toluene flux (which was not measured for the Solsep NF030306F and both Duramem membranes, since they had an extremely low permeance), except for Sulzer Pervap 4060, Solsep NF070706 and Starmem 240 exhibiting a much higher toluene flux. The membrane material was not the only factor influencing the permeance since the 2 PDMS membranes Solsep NF070706 and NF030306 had respectively the highest and the lowest toluene fluxes and the 2 PI membranes Starmem 122 and 240 had respectively the highest and the lowest DMC fluxes. The MWCO is commonly considered as being the main difference between the last 2 PI membranes, but other manufacturing differences (active layer thickness, additives,…) are necessary to tackle their reversed behavior from toluene to DMC.

A c c e p t e d M a n u s c r i p t

A c c e p t e d M a n u s c r i p t

A c c e p t e d M a n u s c r i p tFigure 1 . 2 Figure 2 .Figure 3 .

 1223 Figure 1. Structure of the catalysts: a) Grubbs-Hoveyda II ; b) Rh(acac)(CO)2

Figure 4 .Figure 5 .Figure 6 .

 456 Figure 4. Picture of the spacer used in the SEPA cell (left) and its geometric characteristics (right)

Figure 7 .Figure 8 .Figure 9 .Figure 10 .Figure 1 . 2 Figure 2 .Figure 3 .Figure 4 .Figure 5 .Figure 6 .Figure 7 .Figure 8 .Figure 9 .Figure 10 .

 78910122345678910 Figure 7. Picture of a membrane coupon after its use with the MET set-up

Table 1

 1

) meaning that the flow remained turbulent in DMC (Re DE =395,283 > 200,000).

31,54,55).

  relationship between toluene flux and catalyst rejection cannot be drawn, as could be expected from membrane permeations where affinity of the solvent or the solute for the membrane also plays a role (

	Three membranes were tested for the Grubbs-Hoveyda II rejection in DMC. The
	M maximum rejection was 94.8% with Starmem 240, comparable to the maximum rejection observed with toluene. This should be of importance for productivity reasons since the rejection of the reaction product is an issue quite frequently omitted: the high MWCO of Starmem 240 compared to Starmem 122 should favor the reaction product permeance. For the 2 PDMS Solsep membranes, the rejections were quite different: 53.6% with NF030306 and almost 2.3% with NF070706, thus the highest rejection was a n u s c r i p t obtained with the membrane exhibiting the highest MWCO.
	solutions, the rejection reached 99.6% with Starmem 122 and 90.0% with Sulzer Pervap A c c e p t e d
	4060 but was lower than 65% with all other membranes. The highest rejection was
	obtained with Starmem 122 which also had the lowest flux, whereas Sulzer Pervap 4060
	exhibited one of the highest rejection as well as the highest flux. Thus, a clear

Table 1 .

 1 Solvents and solutes properties

	Component	Molecular weight	Density Viscosity	r	Solubility
		(g•mol -1 )		at 25°C		parameter
				(mPa•s)		(J•cm -3 ) 0.5
	Toluene	92	0.867	0.55	2.38	18.2 (31)
	Dimethyl carbonate	90	1.069	0.58	3.08	20.2 (31)
	Grubbs-Hoveyda II	627	-	-	-	-
	Rh(acac)(CO)2	258	-	-	-	-

Table 2 .

 2 SRNF membranes characteristics(a)

	Provider	Reference	Active layer material	MWCO (g•mol -1 )	T max (°C)	TMP max (bar)	Solubility parameter
							(J•cm -3 ) 0.5
	Sulzer	Pervap 4060	PDMS	-	-	-	15.5 (35)
	Solsep	NF030306	PDMS	500	150	40	15.5 (35)
	Solsep	NF030306F	PDMS	500	150	40	15.5 (35)
	Solsep	NF070706	PDMS	250	100	20	15.5 (35)
	MET-Evonik Duramem 300 PI	300	50	60	23.1 (36)
	MET-Evonik Duramem 200 PI	200	50	60	23.1 (36)
	MET-Evonik Starmem 122	PI	220	60	60	27.0 (36)
	MET-Evonik Starmem 240	PI	400	60	60	27.0 (36)

(a)

Manufacturer data -PDMS : polydimethylsiloxane ; PI : polyimide

Table 3 .

 3 Hydrodynamic characteristics of the 3 lab-scale set-ups

		MET set-up SEPA cell	HT-system
	Mode of SRNF	Cross-flow Cross-flow Dead-end
	d H (mm) / D stirrer (cm) u (m•s -1 ) / w (rpm)	3 2.07	4 0.13	8 320
	Reynolds number (-)	9308	803	338,076

Table 4 .

 4 SRNF performances in toluene at TMP=30 bar and VRR=2

	Membrane Set-up	Duration	Grubbs-Hoveyda II	Rh(acac)(CO)2
			(min)	Initial concentration=0.5	Initial concentration=0.05
				mM		mM
				J SRNF /J 0	Rejection	J SRNF /J 0	Rejection
					(%)		(%)
	Starmem	MET set-up 100	1.05	99.6±7.0	1.01	57.8±4.1
	122	HT-system	120	0.90	99.6±7.0	1.12	80.8±5.7
	Sulzer	MET set-up 30	0.89 (a)	95.4±6.7 (a)	1.05	40.0±2.8
	Pervap	SEPA cell	30	0.93 (a)	94.9±6.7 (a)	1.02	35.1±2.5
	4060	HT-system	50	1.00	90.0±6.4	1.00	60.9±4.3

(a)

MET and SEPA cells in series, TMP=10 bar

Table 5 .

 5 Membrane over bulk concentration ratio (𝐶 𝑚 /𝐶 𝑏𝑢𝑙𝑘 ) of Rh(acac)(CO)2 during SRNF in toluene with the MET set-up (initial concentration=0.05 mM)

		TMP=10 bar	TMP=30 bar
	Starmem 122	1.49	3.42
	Sulzer Pervap 4060	1.01	1.03

Acknowledgements

The authors thank the french ANR (Agence Nationale de la Recherche) for the financial support of the project MemChem n° 14-CE06-0022 and the french clusters IAR and Axelera for labelling this project. A c c e p t e d M a n u s c r i p t


Keywords: Decentralization, energy transition, policy networks, multi-level governance, energy regulation, transformation of State, local authorities

Driven by many actors, the issue of energy decentralization has reappeared on the French political agenda. Celebrated in the past for its capacity to modernize and electrify territories, the centralized model now appears in crisis and is disputed for its inability to meet the new challenges of energy transition. On the contrary, local authorities are put forward, given the strengths they may have, in terms of knowledge of territories, proximity to citizens or synergy. Symbolizing this growing interest, the reference to "territories", to "local authorities", to "decentralization" has now become unavoidable in legal texts, giving rise to an abundant academic literature.

There is a significant gap, in France, between this new rhetoric and the institutional reality, and the centralized model remains firmly established. We propose to explain this permanence by using a multilevel analysis approach mobilizing the concept of "policy networks". We first draw a mapping of the different forces involved in the issue of energy decentralization, highlighting ideas and interests they defend, as well as the resources they can mobilize. In a second part, we show how these different networks have shaped an institutional framework in which the role of local authorities remains highly controlled. In the energy industry sector, it is essentially reduced to the distribution segment. In other areas where energy is involved (housing, urban and regional planning, etc.), this role is more important, since local authorities are more perceived as allies by centralizing forces, able to territorialize the energy policy objectives they defend.

Introduction

Driven by many political forces, large environmental NGOs1 and several transnational networks of local authorities (such as Energy Cities, Climate Alliance, Cities for climate protection, etc.), the issue of decentralization in energy has emerged in Europe on the national political agendas. Whereas previously it had been celebrated for its capacity to modernize and quickly electrify territories, the centralized model now appears in crisis and is disputed for its inability to meet the new challenges of energy transition. On the contrary, local authorities are put forward, given the strengths they may have. The arguments are of various kinds. Some highlight efficiency issues of decentralized models: in-depth knowledge of territories by local actors (better exploitation of resource potential), proximity to citizens (mobilization and awareness of energy issues), ability to manage problems in a way more horizontal (synergies between jurisdictions in transport, urban planning or housing) [START_REF] Poupeau | Central-Local Relations in French Energy Policy-Making: Towards a New Pattern of Territorial Governance[END_REF]. Others insist more on the political dimensions of democracy [START_REF] Becker | Energy democracy: Mapping the debate on energy alternatives[END_REF]. Symbolizing this growing interest, the reference to "territories", to "local authorities", to "decentralization", to "territorialization" has now become unavoidable in legal texts, giving rise to an abundant academic literature on the subject (for some recent examples, see Urban Studies, 2014;Energy Policy, 2015;[START_REF] Environment | Sub-national government and pathways to sustainable energy[END_REF]).

Yet, in France, there is a significant gap between this new rhetoric of the "local" and the institutional reality. Despite many evolutions and the emergence of political alternatives, the centralized model remains firmly established, sending observers back to the now famous Lampedusa's formula, according to which "everything must change in order to stay as it is". We propose in this article to explain this permanence by using a multilevel analysis approach mobilizing the concept of "policy networks" [START_REF] Rhodes | Beyond Westminster and Whitehall: The Sub-Central Governments of Britain[END_REF][START_REF] Marsh | Policy Networks in British Government[END_REF]. We will develop this approach in section 2. In order to analyse ongoing changes in the institutional frame, we will rely on a corpus of materials that we have been building for some twenty years on energy policies in France. It is made up of numerous surveys and observations, which have enabled us to better identify the major coalitions of actors who are positioning themselves on the issues of decentralization in energy. Given the editorial constraints, we will develop only the case of electricity. In France, this energy plays a decisive role in the energy transition trajectory, both physically and institutionally. Although it represents only 22% of final consumption energy, its weight is very important in the political decisions related to decentralization, because of the nuclear issue and the still very important place occupied by EDF, the former state monopoly. This weight of electricity is also likely to strengthen in the future, in France but also in the world. IEA International Report 2018 says electricity should be the main driver of energy transition2 .

After exposing our theoretical framework (section 2), we will present our results in two steps. We will first draw up a mapping of the main forces involved, highlighting ideas and interests they defend, as well as the resources they can mobilize (section 3). Presented in the first part, this inventory shows that decentralization issues are carried by three main networks of actors, which we will call "historical Jacobins" (dominant in public decision), "alternative decentralizers" (still emerging) and "moderate decentralizers" (discrete but influential). In a second part, we will see how these different networks have shaped an institutional framework in which the role of local authorities remains highly controlled (section 4). In terms of the industrial organization of the electricity sector, it is essentially reduced to the distribution segment, because of the historical role played by local authorities. Local government action, however, remains very limited to a weak player role. In other areas where energy is involved (housing, urban and regional planning, etc.), this role is more important, since local authorities are more perceived as allies by centralizing forces, able to territorialise the energy policy objectives they defend. But, again, it encounters a strong national framing, including the financial capacity of local authorities.

We will then discuss this results and conclude the article (section 5).

Theory, methods and material

In this first section, we specify the theoretical frame as well as the methodology and the corpus of materials we have used in this article.

Theory

Energy decentralization issues can be analyzed by using theories on intergovernmental relations or multi-level governance, which focus on understanding how different levels of government, differentiated on a political point of view, interact to manage public problems and, more generally, to ensure the regulation of our contemporary societies [START_REF] Pickvance | Introduction: the significance of local power in theory and practice[END_REF][START_REF] Stoker | Intergovernmental Relations[END_REF][START_REF] Thoenig | Territorial Politics[END_REF]Poupeau, 2017a). Among these, it seems to us that the theory of policy networks is particularly heuristic [START_REF] Rhodes | Beyond Westminster and Whitehall: The Sub-Central Governments of Britain[END_REF][START_REF] Marsh | Policy Networks in British Government[END_REF]. This approach, which has spread in the field of multilevel analysis from the 1990s onwards, is very useful to analyse the ongoing recomposition of the governance system, insofar as it makes it possible to better understand the coalitions involved in the decentralization issues, some of them trying to control the rise of local authorities.

The network approach is based on the following assumptions (Poupeau, 2017a).

1. the analysis of multi-level relationships must emancipate from a reading grid based on formal categories inherited, in particular, from law or administrative science, which ranks actors according to their nature, duties and sphere of belonging (public vs. private, economic vs. non-economic, political vs. administrative, etc.). In particular, it invites us to use very carefully the notion of "level" (European Union vs. states, state vs. local authorities, regions vs. cities, etc.) which does not allow us to understand the forms of complex relationships that can link these actors and do not necessarily follow the logics of opposition (rivalries, competition, conflicts, etc.).

2. another way of analysing these relationships is to start from the ideas and interests that actors defend in relation to a given problem, and to identify the networks, informal or formal, these actors build on these issues, as well as the meta-networks that encompass them. There is a policy network when stable exchanges intervene over time, which allow to share representations and consolidate agreements about an issue (energy decentralization in this article). Meta-networks are networks of networks. They are made up of actors who do not necessarily have direct relationships with each other, although their ideas and interests may converge on the vision of a specific problem. These meta-networks are based on looser structures of exchange between networks and appear, in the form of a broad coalition, when an important event occurs (vote of a law, reform of a sector, decision-making that may have financial implications, etc.). On this occasion, actors at the interface between several policy networks play the role of intermediaries, which allow them to build a large coalition defending a common cause.

Methods

This type of theory requires a two-step methodology.

First, it is necessary to identify the different types of networks that exist around the phenomenon under scrutiny (here the setting of energy decentralization on the political agenda). This task requires a detailed knowledge of the field studied, made possible thanks to two types of materials.

1. the study of official documents in order to "capture" the positions of the actors on the issue, directly (when they are put on the agenda) or indirectly (when they appear at the turn of other issues affecting the sector as a whole). In particular, the aim is to identify for each actor his logic of action and the stakes that energy decentralization issues represent for him. These stakes are of different nature: political, economic, institutional, cultural, etc. They are both material (the materiality of production units or infrastructures in our case) and immaterial (symbolic aspects, etc.).

2. interviews conducted with these actors on the subject, which allow the information to be completed in two directions. First, interviews enrich the understanding of the positions of the various stakeholders, their logics and the interests they defend. Second, they are an opportunity to identify the resources that each of the players can mobilize, alone or within a network, to express its point of view and try to influence the institutional framework (in our case for regulating the electricity sector).

The following table presents the analytical tool used in this first phase. This task makes it possible to map the actors presented in section 3 (figure 1) and to draw the table 2.

The identification of these networks and their resources allows us to understand the distribution of legal duties between State and local authorities in the electricity sector, considering the industrial organization (production, transmission, distribution, supply) and some major areas in which local actors are involved (housing, mobility, climate change, planning, fuel poverty, etc.). The analysis is developed in section 4 (result 2), which draws the general frame of intergovernmental relations in energy in France.

Materials

To carry out such a work, it is necessary to gather a sufficiently large material to be able to "catch" the system of actors in all its complexity and in its long-term dynamics. Our corpus is based on an observation of more than 20 years of the field of energy and issues of decentralization. Here is the list of the materials used (surveys, seminars, participation in consultation processes, supervision of student work) to map the networks of actors and identify the resources they mobilize to shape the institutional framework of electricity governance.

1. Personal research on the role of French local authorities in the regulation of energy: historical aspects; decentralization and liberalization of the energy market (1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000); EDF strategy towards local authorities; role of regions in the energy planning process; local energy climate policies; the rise of metropolises, etc.

2. Supervision of three PhD theses: action of operators held by local authorities (Pauline Gabillet, thesis defended in 2015, see references); role of European associations of local authorities on energy legislation (Corinne Belvèze, thesis to be defended in 2020); strategies of metropolises in terms of energy transition (Mathilde Marchand, thesis in progress).

3.

Participation, as an expert, in the national debate on energy transition (2013), preparing the vote on the Loi sur la transition énergétique et la croissance verte (energy transition and green growth act, voted in August 2015): member of the working group on "governance" (reading the actors' booklets, writing a synthesis note on local energy governance, co-writing the final note of the group).

4.

Consultation of the documents written by 200 actors during the consultation launched by the French State for the multiannual energy planning (2018): reading the actors' booklets, identifying arguments, mapping positions, etc.

5.

Animation of a seminar organized with the Ministry of ecology on the role of local authorities in energy (2015).

6.

Co-animation of a seminar practitioners-researchers on the financing of the energy transition by local authorities (2017-2019).

Result 1. Actors and networks involved in the decentralization issues

Since the 1990s, the question of decentralization, which had never completely disappeared from the concerns of energy stakeholders (Poupeau, 2017b), has been put back on the French political agenda, thanks to several joint dynamics. First, liberalization has opened up new windows of opportunity for local authorities, calling into question the existence of former national monopolies and, indirectly, the importance of the role played by the State. Once criticised, local authorities, and especially municipalities, were thus able to re-appropriate historical competences in the organization of the market, particularly in the area of public distribution [START_REF] Poupeau | Le service public à la française face aux pouvoirs locaux[END_REF]. Parallel to this process, which has not yet been completed in France, the decentralization laws have strengthened local authorities in several areas concerning energy management: housing, urban planning, mobility, social policies, etc. Finally, the last major change has been the fight against climate change. In a complementary way to the major international negotiations (the different rounds of the COP3 ), local authorities took full account of the subject, in order to legitimize their role in the sector, like many of their counterparts abroad [START_REF] Bulkeley | Cities and Climate Change[END_REF]Urban Studies, 2014;Energy Policy, 2015).

These different processes have brought in new players, which complicates the governance of the electricity sector. The diagram in figure 1 maps the three main policy networks that are now positioned on the issue of decentralization, directly (by their claims) or indirectly (by the discrete influence they may have on major public policy choices). It distinguishes 5 main spheres of activity: politics (the political class and this issue); state administrations; operators of the energy sector; trade unions, NGOs and professional associations and, finally, associations and networks of local elected officials. As policy networks postulate, these spheres are not compartmentalized from one another. Between them circulate ideas and interests around the issue of decentralization, that give rise to large and mixed alliances.

Historical Jacobins

The network of "historical Jacobins" brings together actors who have been dominant for a long time in the French electricity system. It is made up of four main circles of actors, who defend interests and logics of action that contribute to the maintenance of a centralized model, based on nuclear energy and led by the State.

Strengthening the domestic market to conquer the international: the lobby of the nuclear industry

This network was built largely around the industrial complex that was formed in France from the nuclear sector, in the 1960s, in its first military and then civil dimensions (Simonnot, 1978;Frost 1991;[START_REF] Hecht | The Radiance of France: Nuclear Power and National Identity after World War II[END_REF]. Its main industrial players are Orano (former Areva) and EDF, which, in 2017, bought a part of the first 5 . Orano is the company responsible for the operation, transport and management of nuclear fuels. EDF builds and operates, in France but also abroad, the plants that use this fuel. Around them are other public or parapublic institutions, such as the CEA, the SFEN and the UFE 6 , which bring together nuclear actors, as well as the powerful Corps des Mines. As one of the grands corps de l'État, its members are very present in the sector, occupying the main management positions in companies and administrations (for example in EDF and Orano).

It is from this first nucleus that the French electronuclear industry develops. Now, it accounts for about 70% of electricity production and 16% of primary energy consumption in metropolitan France 7 . Its economic weight is very important, in terms of capital assets. The stakes are also commercial, since the French nuclear industry has been positioning itself internationally, facing US and Chinese competitors. Even though the IEA statistics show that the sector is clearly losing momentum in the world, especially with regard to the development of renewable energies 8 , in France, nuclear proponents want to remain present on the market, by developing new units, in particular the European Pressurised Reactors (EPRs). Partnerships and equity investments have already been initiated with England (Hinkley Point), Finland, but also China and India. For those involved in this sector, removing France from nuclear power would entail heavy economic and industrial consequences. Such a political decision would contribute to disequilibrate a little more the French trade balance, with electricity sales worth around 2 billion euros each year 9 . It would lead to job losses in regions affected by site closures, that are sometimes subject to unemployment. It would result in higher tariffs for all French consumers. Finally, it would compromise the international strategy of Orano and EDF, giving a very negative signal to EDF's potential partners.

Supporting "national champions" and keeping low prices: the Ministry of Finance

The nuclear industry finds a strong ally in the Ministry of Finance. Whereas it had been reluctant to develop nuclear plants in the 1970s, considering the cost they represented (Simonnot, 1978), it is now one of the strongest supports of this industry. Four actors are involved within this ministry, or are closed to, which weighs very heavily on French political and administrative decisions: the DGE, the APE 10 , the Treasury and the Budget. 5 APE, Rapport d'activité 2016-2017, page 40. 6 CEA: Commissariat à l'énergie atomique (research center); SFEN: Société française d'énergie nucléaire (scientific association); UFE: Union française de l'électricité (professional association which brings together the main energy operators: production, transport, distribution, supply, services managing different functionalities, etc.). 7 Sources: RTE (Statistiques production, consommation échanges, 2018) and MTES (CGDD, Chiffres clés de l'énergie, édition 2018, page 18). 8 According to the IEA, nuclear power now accounts for only about 10% of global electricity generation (World Energy Statistics or Report, 2018). The growth rate of this industry is much lower than that of renewable energies. 9 Based on data from the UFE (which does not take into account uranium imports to establish this balance). 10 DGE: Direction générale des entreprises; APE : Agence des participations de l'État.

We will not dwell on the DGE, which, in charge of the French industry, supports the strategy of consolidation and expansion of the electronuclear sector. It is supported by the MEDEF11 , which represents the interests of large French energy-consuming companies, and which is very favorable to nuclear energy. Both the DGE and the MEDEF consider that the nuclear industry provides low prices, among the best in Europe, both for industrial and domestic consumers 12 . For them, it is then essential to continue to operate existing nuclear plants and not to dismantle them too early. It is also important to launch new nuclear programs (such as EPRs of new generation), in order to favor the development of the French nuclear industry. In comparison, supporting renewable energy is considered as less important and as a public expenditure, more than a strategic long term investment for the sector. This argument is shared by the Budget Department.

The APE represents the interests of the State-shareholder. As such, it is heavily involved in the management of large energy companies, as it has significant shares in the sector. EDF alone accounts for 22% of the total assets held by the APE, an amount of 23 billion euros in 2017 13 . Destabilizing this company by calling into question its pro-nuclear strategy would be likely to drop the share price (already historically low compared to its rating) and to reduce the high level of dividends that are paid each year to the State (1.7 billion euros in 2017, or 50% of the total amount collected by the State on its participations14 ). Thus, in a certain way, the APE has little choice today to speak in opposition to EDF (and Orano) on their nuclear strategy. The high level of dividends paid each year by EDF also explains the support of the Budget and Treasury Departments, who are seeking resources to increase the State's revenue and contribute to its deleveraging, in a context of reduced public spending. Finally, the Budget Department aims to maintain a high level of production from nuclear power plants as a means to avoid reducing consumption, particularly by subsidizing the thermal renovation of buildings (tax credits, subsidies, etc.). This short-term strategy undeniably serves the interests of the electronuclear industry, delaying the consideration of the real costs of dismantling power stations in the longer term.

The fear of dismantling: the major network operators

Network operators in charge of transport and distribution lines are also involved in debates on institutional organization. Although they present themselves as neutral actors, their interests converge to maintain a rather centralized model, which legitimizes their historical existence, built around the construction of interconnected infrastructures serving the entire territory (universalization of networks). Indeed, against the idea of an energy system based on autonomy and post-networks territories [START_REF] Coutard | The rise of post-networked cities in Europe? Recombining infrastructural, ecological and urban transformations in low carbon transitions[END_REF], they plead for preserving and strengthening national and local infrastructures, to foster the development of renewable energies (fight against intermittence by pooling production sources), electric vehicles (France has very ambitious targets in this area) and smart networks (to better drive demand). These new challenges require, according to them, significant investments to adapt and modernize networks15 . They also require strong coordination, under the aegis of integrated operators, and not dispersed. Beyond these technical and economic arguments, grid operators know that they too are playing their survival, as integrated organizations, at a time when discussions on energy decentralization are increasing. RTE16 , in charge of the transport network, is still relatively little impacted by the issue because the company owns its infrastructure and is not really challenged in its role of national monopoly. Enedis, which operates the distribution lines17 , is much more threatened by ongoing developments. The company does not own its networks. These are the property of local authorities, since the law of June 15, 1906. The management of these infrastructures which, for the time being, has escaped the liberalization process, could be put in competition in the future, some local elected officials claiming to be able to call on other operators or to create their own structure. Enedis therefore seeks to defend its unity, by highlighting the advantages of centralized network management. This interest is also that of EDF, which owns 100% of the capital of Enedis, and which, as such, receives each year a very large amount of dividends (659 million euros in 2017, 513 in 201818 ).

A strong ideological attachment to centralization: the support of the French political class

The majority of the French political class also remains very faithful to the historical model, based on the role of the State and recourse to the electronuclear industry. This was illustrated by the last presidential campaign in 2017 and the victory of Emmanuel Macron, when several arguments have been put forward in favour of centralization. The values of solidarity between territories and the fight against urban-rural inequalities continue to be very present in debates on governance issues. Discussions about the territorial differentiation of electricity rates, which was overwhelmingly rejected by elected officials, attest to this. These actors continue to want to place the State as the guarantor of national unity, in the face of the risks of excessive decentralization.

The nuclear issue is associated with these Jacobin values, insofar as it embodies, in the eyes of many elected officials, the vision of a general interest served by the grands corps de l'État, a symbol of power and radiance of the country [START_REF] Hecht | The Radiance of France: Nuclear Power and National Identity after World War II[END_REF]. From this point of view, the 2011 Fukushima nuclear accident did not, surprisingly, give rise to a real questioning of this political support. On the contrary, as some researchers have shown (Brouard and alii, 2013), the major pro-nuclear political parties (notably located on the right of the political spectrum) developed, shortly after this accident, an intense activity of reframing rhetoric to relegitimize the use of nuclear power, in the face of a destabilized public opinion. The arguments put forward were mostly economic (independence of France, low cost of the current energy mix).

Initially not very present in the media sphere, they then progressively imposed themselves in the debates, competing those put forward by the opponents of the nuclear industry, around the issues of industrial risk and impact on health and environment. One could add another complementary rhetoric, developed since the 2000s. It concerns climate change issues.

Nuclear energy is indeed often presented as the "champion " of decarbonation [START_REF] Topçu | Urban Studies. Urban energy transitions: Places, processes and politics of socio-technical change[END_REF], compared to other sources such as renewables, presumed to be less effective because of their intermittency. The association Sauvons le Climat, of which the SFEN is a member, or the consultant firm Carbone 4 defend this link between recourse to nuclear energy and preservation of the environment.

Alternative decentralizers

Faced with this broad and strong alliance, the forces embodying an alternative decentralized model have emerged more recently in France, around three main types of actors.

Challenging the electronuclear industry: NGOs and ecologist parties

The opposition to nuclear energy played a key role in challenging the centralized model [START_REF] Topçu | Urban Studies. Urban energy transitions: Places, processes and politics of socio-technical change[END_REF]. It was formed from the 1970s, around a network of activists from which will be born associations such as Greenpeace or Sortir du nucléaire, very present in the debates on energy. Their criticism relates both to the risks that nuclear poses to society (environment, health, etc.) and to the political regime that it underlies. Opponents denounce the existence of a kind of "State technocracy", bringing together a small number of actors who impose their choices on citizens, without any real consultation. Nuclear power is thus associated with a denial of democracy, which serves the interests of a ruling class defending a highly contested productivist model. This associative mobilization has been expanding gradually to other types of actors. On the side of the trade unions, if the CGT, well established at EDF, remains very favorable to the centralized and nuclear model, the CFDT, another great federation, is very early against the nuclear and promotes a stronger decentralization 19 . But it is above all the ecologist parties that are making these demands on a broader, national scale. Appeared on the political scene in the 1970s, they enshrine the decline (as a first step) and the exit (longer term) of electronuclear production in all their political programs. Other forces have joined them more recently, located to the left of the political chessboard, such as the party France Insoumise. For them, beyond the question of risks, it is a decentralized model of society that is put forward, energy being considered as an important area in democratic life. Against the nuclear, these parties and historical NGOs plead for the massive use of renewable energies, which, according to them, favor the reappropriation of energy by citizens. As these develop, alternative operators to major companies join this activist network, often more discreetly and less politically: citizen cooperatives (Enercoop), small companies specialized in renewable energy, energy communities, etc.

Energy demand management: the emergence of an alternative within the State

At a time when opposition to nuclear power is growing, France, like most of the European countries, is plunging into a major energy crisis, following the oil shock of 1973. Whereas one of the answers to this event is the launch of the electronuclear program, other ways are explored. They call for a rethinking of the energy consumption model supported by the centralizers, based on nuclear energy. Against the opinion of EDF and the Ministry of Finance, a few State actors argue in favor of an ambitious energy saving policy, to reduce the French global energy bill. First isolated within the administrative sphere, they manage to convince the government to create a specific agency dedicated to these issues. The AEME 20 was born in 1973, with the mission of proposing measures to save energy, especially in buildings [START_REF] Pautard | Vers la sobriété électrique[END_REF]Pour mémoire, 2015). In particular, it targets electric heating, which is widespread in France and which the proponents of the electronuclear programme seek then to develop. In doing so, competing visions appear within the State, which somewhat undermine the unanimity that existed previously.

Heir of the AEME and its successor agencies, the ADEME 21 , created in 1992, today embodies this public policy of energy savings. It argues for a strategy of a sharp decrease in consumption which, as in the past, goes against the interests of nuclear proponents, who wish to extend the life of their plants and build new EPRs. Over the years, the ADEME also defends the massive use of renewable energies, bringing it closer to the actors described above. This support is reflected in the publication of numerous reports, one of which, published in 2016, proposes a 100% renewable energy production mix, in total opposition to the scenario advocated by the nuclear industry 22 . Around the ADEME are many organizations that share its vision of the production mix and the decrease in consumption. This includes Negawatt, CLER and RAC 23 . These environmental actors, who have a strong expertise in energy, challenge EDF's argument that nuclear power would be the most relevant solution in the fight against climate change (decarbonated energy, problem of intermittent renewable energies). For these associations, implementing ambitious public policies to decrease energy consumption and to produce renewable energy can make it possible to reduce greenhouse gas emissions (GGE) just as effectively.

Implementing energy-climate jurisdiction: the commitment of big cities

Long removed from regulation (Poupeau, 2017b), big cities have reappeared on the decisionmaking scene since the 1990s, particularly in light of the issues they represent in the energy sector 24 . Liberalization has opened up new opportunities, allowing them to switch suppliers and to produce renewable energy by themselves or by using new operators. The fight against climate change has also legitimized their role. The City of Paris led the launch of the C40, an informal organization of major global cities that intend to pursue ambitious policies to reduce GGE. In many other cities, local elected officials and citizens have also made this issue a high political priority, believing that local level has its role to play, facing, according to them, the inertia of states and major international organizations. This rise has translated into a political will of greater decentralization. Cities believe that it is essential to give them more power and autonomy in energy management, as they face many economic, social and environmental challenges. Much of the consumption is now on an urban 20 Agence pour l'économie et la maîtrise de l'énergie. 21 ADEME: Agence de l'environnement et de la maîtrise de l'énergie. 22 ADEME, Artelys, Armines-Persee, Énergies demain, A 100% renewable electricity mix? Analyses and optimisations. Testing the boundaries of renewable energy-based electricity development in metropolitan France by 2050, January 2016. 23 CLER: Comité de liaison sur les énergies renouvelables (now named CLER-Réseau pour la transition énergétique); RAC: Réseau action climat. 24 In 2008, an IEA-OECD report estimated that urban areas account for about two-thirds of overall energy consumption (IEA, World energy outlook, 2008). scale. Air pollution issues have become urgent to address. The thermal renovation of buildings remains a major stake. Fuel poverty requires a fine intervention with the populations concerned. Lastly, the international competitiveness of French metropolises also depends on their ability to set up efficient energy systems. For all these reasons, cities are questioning the unified nature of the French model and are calling for new leeways. They join to act, through different networks of actors, including France Urbaine (Urban France). France Urbaine is a large national association created in 2015 to defend the interests of big cities. It asks (discreetly but regularly) for completing the process of liberalization by allowing cities and all local authorities to put Enedis in competition. Thus, cities could call on other operators or even create their own structure, giving them more leverage to implement their public policies. Although its interests diverge sometimes with those of France Urbaine, the ADCF, which federates all the intercommunal structures, is quite close to this position 25 .

Moderate decentralizers

We can identify a third and last network of actors, which is positioned more midway. We propose to call it "moderate decentralizers". This policy network advocates a form of relaxation of the centralised model, without switching to a new model, in which local authorities would have very extensive powers.

Territorializing public policies: the DGEC

The vision and strategy of the State are not unified concerning (de)centralization issues. Between supporters of maintaining or strengthening the historical model (Finance) and those (ADEME) who advocate for its profound transformation, the DGEC 26 takes an intermediate attitude. This Directorate-General of the Ministry of ecology is in charge of implementing France's strategic orientations in energy policy: liberalization of the French market, fight against climate change, development of renewable energies, energy efficiency, etc. To carry out these missions, in a context of tension on public expenditure, the DGEC tries to rely on relay actors to multiply its action as close as possible to the territories. Among them, local authorities play a particularly important role, in relation to the assets they may have (knowledge of resources, proximity to the inhabitants, synergies between sectors, etc.).

The challenge for the DGEC is therefore to mobilize the local level to support it in its multiple missions, if necessary by strengthening certain regional or metropolitan areas jurisdictions in targeted fields (territorial planning, housing and urban planning). This is evidenced by the references to "territory" in many pieces of legislation or administrative discourse, or the multiplication of the term "territorialization" for some fifteen years. Different from "decentralization", it refers, for them, to the mobilization of local authorities around the achievement of the objectives of the State, which are often quite ambitious with regard to the weak means involved. This strategy includes the implementation of regional energy planning schemes, co-produced by the State and the regions [START_REF] Poupeau | Quand l'État territorialise la politique énergétique. L'expérience des schémas régionaux du climat, de l'air et de l'énergie[END_REF]. Other tools are mobilized too, such as "labels" or call for projects. Even if they are based on limited financial resources, these instruments are a way for the DGEC to build new relationships with the actors of the territories, in particular with local authorities, in order to achieve its national goals.

25 ADCF: Association des communautés de France. 26 DGEC: Direction générale énergie climat.

Preserving rural interests vis-à-vis cities: the FNCCR

For rural stakeholders, the decentralization of energy represents a real threat. Indeed, the historical model that was built in France from the interwar period was accompanied by numerous mechanisms of economic redistribution in favor of rural territories [START_REF] Poupeau | La fabrique d'une solidarité territoriale. État et élus ruraux dans l'adoption d'une péréquation des tarifs de l'électricité en France[END_REF](Poupeau, , 2017b)): standardization of tariffs regardless of the place of consumption (tariff equalization), very important investments in rural areas (State subsidies and EDF funds), etc. For many elected officials and citizens living in the countryside, the rise in power of cities could challenge these measures, while the context is often difficult in their territories (rising unemployment, deindustrialization, closure of public services, etc.).

These claims are now relayed by a powerful national association: the FNCCR27 . Founded in 1933, it has always defended the interests of rural areas, against private operators and, after nationalization, against EDF. Over the years, it has built a network of influence that has heavily influenced the choices of the State and EDF (Poupeau, 2017b). It was at the origin of the national equalization rates, which was adopted in France in the 1960s. Since liberalization, the action of FNCCR has been maintained, if not reinforced, through alliances concluded with EDF and the State, to preserve the centralized model [START_REF] Poupeau | Le service public à la française face aux pouvoirs locaux[END_REF]. The association today advocates moderate decentralization, which may strengthen the powers of local authorities while maintaining a strong national framework managed by the State, and guaranteeing powerful mechanisms of territorial redistribution between rural and urban areas.

Small local public operators

Ninety-five percent of public energy distribution is provided by EDF's subsidiary Enedis. In the remaining 5% of the territory, local public or semi-public operators, created before the nationalization, provide this essential public service. While some large urban structures exist (Strasbourg, Metz, Chartres, Bordeaux, Colmar, etc.), in most cases, local public operators are small and located in rural areas or small towns. Their survival has hitherto depended heavily on the redistribution mechanisms put in place by the State to guarantee homogeneous and universal access to energy networks: aids to balance budgets, subsidies, advantageous energy purchase rates to EDF, etc [START_REF] Gabillet | Les entreprises locales de distribution à Grenoble et Metz : des outils de gouvernement énergétique urbain partiellement appropriés[END_REF]. This relatively strong dependence on the State explains the attachment of these small operators to a centralized system. Very few of them are in favor of greater decentralization, of which they fear losing out. A total liberalization of the French distribution sector, now preserved, could jeopardize their position on the market, which today enjoys multiple forms of protection, thanks to the centralized model (no competition in their territory, except for supply). Only the biggest operators could draw their pin from the game, which explains why some of them, however rare, are tempted to join the ranks of the supporters of a broad decentralization.

Summary of the arguments and resources of the three main stakeholder networks

The table 2 sums up the positions of the main actors on the (de)centralization issue, as well as the resources they have. It shows a great asymmetry in favor of the "historical Jacobins". They continue to accumulate numerous and diverse resources, which gives them an even prominent place in the evolution of institutional games.

Table 2. Arguments, stakes and resources used by the three policy networks

Policy networks Arguments and stakes Resources

Historical Jabobins Economic Low cost nuclear energy (for households, for businesses).

Nuclear contribution to the French trade balance (exports).

The nuclear industry as a strong job creator.

The necessity to support EDF and Orano in their international expansion strategy.

Environmental and health

Decarbonisation of the energy mix. Nuclear power, an indispensable complement for developing renewable energies (problem of intermittence, security of supply).

Social

Difficult redevelopment of nuclear sites. Low cost of nuclear energy for low income consumers.

Political and symbolic

Radiance of France. Unity and equality (through equalization of tariffs between regions).

Very important

Control of the political agenda.

Modereate decentralizers

Mix between the arguments of "historical Jacobins" (preservation of the national framework, support for EDF) and "alternative decentralizers" (local development, territorial balances between poor and rich regions in energy, and between rural and urban areas, protection of the most precarious populations).

Medium

Discrete access to major actors (EDF, Ministry of ecology). Discrete weight on Parliament. Strong expertise.

Result 2. A hybrid model of energy regulation

These three main networks of actors draw a new model of energy governance in France, which can be described as hybrid (see diagram in figure 2). It is no longer completely centralized because the action of decentralizing coalitions has had effects on the redistribution of powers between the State and local authorities. It is not however decentralized, many features characteristic of the old model remain still very present, under the influence of the alliance of Jacobin forces. To understand emerging forms of regulation, two main dimensions must be distinguished: the industrial organization of the sector on the one hand, and public policies with a strong energy component on the other.

Industrial organization

The industrial organization of the energy sector shows a great continuity with the historical model. As illustrated in figure 2 and in appendix (different jurisdictions), local authorities generally play a weak role.

A weak role in the production, transmission and supply of electricity

In production, major operators remain dominant, whether they come from nuclear power (70% of electricity production) or renewable energies (20%). In this latter segment of the market, the smaller alternative operators that appeared in the 1970s and 1980s have given way to larger companies, including EDF and Engie, which have profoundly transformed this sector (Évrard, 2013). They now have subsidiaries producing renewable energies, managed in a very centralized way. In this context, local authorities play a marginal role. They can invest in local projects, in the form of shareholdings in public or semi-public operators. But their intervention should not be overstated, given their real economic weight and the influence of this intervention on the entire electricity system. Indeed, in many cases, projects are viable only to the extent that they benefit from purchase prices subsidized by all consumers, which do not call into question the regulation of the sector.

In transport, local authorities action is even more limited, with no jurisdiction to significantly influence RTE. RTE has its own infrastructure and is mainly owned by EDF. It is rather during the process of concerted action around major infrastructure projects that local authorities can intervene, to try to modify the passage of power lines on the public domain.

The supply segment also offers little direct control, with tariffs set by economic operators (for market offers) or by the State (regulated tariffs, with a light consultation of local authorities).

Local authorities can mainly act as energy consumers (to reduce their bills, buy green energy, etc.), by using their bargaining power, alone or through purchasing entities. They do not have the power to group all or part of the energy consumers in their territory, to negotiate on their behalf offers with suppliers. Such an option, rejected until now by the State, would constitute a significant change, which would amount to re-establishing the concession regime that prevailed until the creation of EDF (Poupeau, 2017b).

A more important but thwarted role in the distribution

It is in the distribution that the action of the local authorities is today the most significant. It is explained by the maintenance, despite the nationalization (1946) and the liberalization, of a historical jurisdiction of the communes and their unions: their function of "energy distribution organizing authorities" (AODE, autorités organisatrices de la distribution d'énergie). This competence, which somehow also exists in Germany, forces an energy distributor to ask permission from the local authority to serve a territory. In return, the operator is committed to providing energy to all consumers. It also pays concession fees to the local authority and assigns ownership of the networks to it at the end of the contract (20 years in general). The role of local authorities is therefore, in theory, important, considering the weakness of their margins of maneuver elsewhere.

Distribution activity, however, is still very strongly regulated by the centralizing coalition.

The stakes are primarily financial, especially for Enedis and EDF, its main shareholder. As we saw in section 3, Enedis is a very profitable enterprise at the moment, which allows its parent company EDF, highly indebted, to better balance its accounts28 . Giving more power to local authorities by allowing them to change their distributor Enedis could have negative effects on these financial transfers. The strategy of EDF and Enedis, endorsed by the State, therefore consists in getting the distribution sector out of the liberalization process. In doing so, the risk of dismantling Enedis is eliminated, as local authorities are still forced to sign contracts with this company. This inability to use an alternative operator considerably limits the local bargaining power, in particular the amount of royalties paid to AODEs. This amount reached 316 million euros in 201429 , plus a part of local electricity taxes (400 million euros in 201130 ). These sums, very important, could increase if Enedis would be put in competition, especially in cities, the most attractive territories for distributors.

Public policies integrating energy

Local authorities have a little more room for maneuver with regard to public policies integrating energy (housing, planning, mobility, etc.), insofar as their action does not directly affect the industrial organization of the energy sector and the economic and financial strategies of EDF. In these areas, their action is even sought by the State, in particular by the ADEME and the DGEC, which seek to find local relays to implement their national objectives (reduction of GGE, efficiency energy, fight against fuel poverty, etc.). However, local intervention is not free of constraints, as other sectoral interests may oppose too radical measures in favor of energy. To take only the example of housing, the most emblematic, the issue of thermal renovation of buildings stumbles on the financing of measures, insufficiently provided by the State. Public and private landlords seek not to bear the burden of ambitious public policy alone, which could have a significant impact on real estate markets. As a result, they maintain a constant pressure on local authorities, so that they do not impose excessive regulations.

In these areas, it is rather indirectly, via planning tools, that local authorities are entering the game of energy regulation. If they can impose certain rules in housing (thermal consumption of new buildings, installation of photovoltaic panels) or in the development of new areas (production of renewable energies, buildings with high environmental quality), their action largerly involves the drafting of energy-climate planning documents at the regional (SRADDET) and local (PCAET) levels31 (see appendix 1). These documents define the main strategic orientations of regions and cities, in terms of planning, mobility, development of renewable energies or housing policy. The major challenge, and the main obstacle to greater local intervention, lies in the financial capacities of local authorities. While these can create new economic tools (local operators, third-party finance companies, public-private partnerships), their room for maneuver remains limited. Over the past few years, their demands have focused on the proceeds of the "climate energy contribution", introduced in 2014 by the State to tax CO 2 emissions. Aimed at fuelling policies in favor of energy transition, it represents several billion euros that currently abound the general budget of the State. Shortly after it was set up, the main associations of local authorities demanded to receive a part of it, in view of their jurisdictions in the field of energy transition. But this request today stumbles on the veto of the Ministry of Finance, which seeks to keep the proceeds of the tax. This blocking shows that, despite the existence of levers of action higher than in the energy industry, local and regional authorities must cope with a system of dominant actors seeking to limit the effects of any decentralization.

Discussion and conclusion

Since the 1990s, issues of decentralization in energy have (re)appeared on the political agenda in France. They are carried by more and more numerous and diversified actors, coming from the world of NGOs, experts and consultants, trade unions, associations of local authorities, economic operators but also State administrations. These actors, who have long been disregarded, mobilize economic, technical, political and social arguments that legitimize the implementation of alternative models of regulation of the energy sector, more anchored in the territories. In France, as in other countries, the academic world has echoed these initiatives by analyzing local projects, modes of territorial governance, the rise of decentralized energy, thus moving the center of gravity research questions from the national to the local level.

By replacing the action of local authorities in multi-level actors' games, we have shown, like other researchers [START_REF] Emelianoff | Local Energy Transition and Multilevel Climate Governance: The Contrasted Experiences of Two Pioneer Cities (Hanover, Germany, and Växjö, Sweden)[END_REF][START_REF] Goldthau | Rethinking the governance of energy infrastructure: Scale, decentralization and polycentrism[END_REF][START_REF] Jaglin | Urban Energy Policies and the Governance of Multilevel Issues in Cape Town[END_REF][START_REF] Rohracher | The Interplay of Urban Energy Policy and Socio-technical Transitions: The Eco-cities of Graz and Freiburg in Retrospect[END_REF], that it is necessary to move out of a purely "localist" focus of local intervention forms, to articulate it with other scales of regulation. Indeed, at a time when the legitimacy of a territorial approach is put forward by many actors, others develop ideas and strategies that are instead based on a stronger centralization. It is therefore important to reconcile academic fields that are often disjointed, between, on the one hand, those who analyze national public policies and their integration into larger regional groups, such as the European Union (with authors not very sensitive to the territorial question; see the manuals on European energy policies, etc.), and, on the other hand, those studying the emergence of local solutions to the energy transition, in the assumption that it foreshadows profound transformations.

This article demonstrates that these two simultaneous dynamics come together to draw, in France, a new model of energy regulation, of hybrid nature. No more centralized as in the past, but not too much decentralized, it gives rise to a selective integration, by the dominant historical coalition, of local authorities. For that, this coalition can mobilize many resources, especially political (a certain conception of society and solidarity between citizens and territories) and economic (the weight of past choices in terms of production and industrial organization of the sector). The industry organization is still largerly locked, including distribution where local jurisdictions are most important. Public policies impacted by energy issues are more open to local authorities, but these actors do not really have all the levers, particularly financial ones, to act fully. Another distinction, quite close, can be done between generation and use of electricity. Although some actors would like to change the regulatory model, recent reforms tend to confine local authorities to a role of supporting new modes of energy consumption, in buildings and transport namely. In doing so, local authorities are weakened in their potential role of questioning the energy system, and instead are turning into relays of the state and large operators in the territories.

It is in this perspective that we must understand the reference to the title of this article, which insists on the elements of inertia due to the permanence of institutional games very anchored in history, and which can not be ignored in the analyzes on energy transition. Admittedly, technological innovations, such as the decentralization of the means of production, have effects on regulatory regimes. But these cannot be studied in a univocal way, technical and institutional systems having links of reciprocal influence [START_REF] Hughes | Networks of Power[END_REF]. Similarly, the emergence of digital, in which some see the end of vertical and hierarchical systems [START_REF] Rifkin | The Third Industrial Revolution: How Lateral Power Is Transforming Energy, the Economy, and the World[END_REF], gives way to forms of differentiated appropriation, depending on the strategies of actors and the institutional frameworks. The example of France, one of the most centralized countries, offers a stimulating case study that can be tested in other countries with a more federal or decentralized tradition, in which the actions of territories and local authorities are also questionned.

Fig. 1 .

 1 Fig. 1. Mapping of networks of actors involved in decentralization issues in France 4

Fig. 2 .

 2 Fig. 2. Articulation between European Union, State and local authorities in the energy sector in France

Table 1 . Analytic tools used in the article Type of actor What are the arguments for him concerning energy decentralization? What is at stake? What are the resources (economic, political, technical, etc.) he can mobilize to put forward his point of view? Network 1 Actor 1 Actor 2 …/… Actor n Network 2 …/… Network n

 1

To avoid burdening the text, we have not developped all the acronyms used in this article. See the complete list, translated into English, in the abbreviations list.

International energy agency, World Energy Outlook 2018.

Conférence des parties (conference of the parties).

Acronyms are developed in the article.

MEDEF: Mouvement des entreprises de France.

The idea that the nuclear industry offers the lowest prices is the subject of major controversies, namely around the cost of dismantling nuclear power plants. Despite this, the DGE and the MEDEF remain faithful, now, to their positions.

APE, Rapport d'activité 2016-2017.

Ibidem.

According to RTE (Réseau de transport d'électricité), the energy transition is expected to cost around 33 billion euros in France over 15 years (source : website of Le Monde, 17 septembre 2019).

Réseau de transport d'électricité.

With a few exceptions, since only 5% of the French territory is served by local public companies, inherited from history[START_REF] Gabillet | Les entreprises locales de distribution à Grenoble et Metz : des outils de gouvernement énergétique urbain partiellement appropriés[END_REF] Author, 2017b).

EDF, Document de référence 2018 incluant le rapport financier annuel, page 457.

CGT: Confédération générale du travail; CFDT: Confédération française démocratique du travail.

FNCCR: Fédération nationale des collectivités concédantes et régies.

This does not mean that EDF can drain its subsidiary as it sees fit. Rules exist that govern Enedis' profits and prevent it and its parent company from abusing the monopoly position of Enedis. These rules are set by the CRE, which regulates the energy sector.

Source: CRE (Commission de régulation de l'énergie).

Source: Cour des comptes.

SRADDET: Schéma régional d'aménagement, de développement durable et d'égalité des territoires; PCAET: Plan climat air énergie territorial.

union]. CFDT : Confédération française démocratique du travail [French democratic confederation of labour, a trade union]. CLER: Comité de liaison sur les énergies renouvelables [Renewable energy liaison committee, a NGO now named CLER-Réseau pour la transition énergétique, CLER-Network for energy transition]. COP : Conférence des parties [Conference of the parties]. CRE : Commission de régulation de l'énergie [Commission of the energy regulation]. DGE : Direction générale des entreprises [Directorate-General for enterprise]. DGEC: Direction générale énergie climat [Directorate-General for energy and climate]. EDF: Électricité de France. EPR : European pressurised reactor. FNCCR: Fédération nationale des collectivités concédantes et régies [National federation of local authorities responsible for public services]. GGE : Greenhouse gas emissions. IEA: International energy agency. MEDEF : Mouvement des entreprises de France [French union of enterprises, the largest employers' organization]. NGO: Non governmental organization. OECD: Organization for economic cooperation and development. PCAET: Plan climat air énergie territorial [Territorial climate air energy scheme]. PDU(I): Plan de déplacement urbain (intercommunal) [(Intercommunal] urban transport plan]. PLH: Programme local de l'habitat [Local housing program]. PLU(I): Plan local d'urbanisme (intercommunal) [(Intercommunal) local development plan]. RAC: Réseau action climat [Climate action network]. RTE : Réseau de transport d'électricité [National power grid]. SCoT: Schéma de cohérence territoriale [Territorial coherence scheme]. SFEN: Société française d'énergie nucléaire [French society of nuclear energy]. SRADDET: Schéma régional d'aménagement, de développement durable et d'égalité des territoires [Regional development scheme]. UFE: Union française de l'électricité [French union of electricity].


Marco Di Lorenzo

Pierre Brequigny

Fabrice Foucher

Christine Mounaïm-Rousselle

Experimental Investigation of Turbulent Premixed Flames under Spark-Ignition Engine-Like Conditions

Conclusions

No-grid configuration:

• Large volume setup allows to reproduce laminar-like flow conditions during the temperature and pressure plateau.

• In the small volume the high compression ratio promotes the early insurgence of the turbulence (not interesting due to the possibly flame shifting).

Large Volume (GRID)

Methane -φ=0.9 -EGR0% -5bar -423K u'=1.5 m/s

Small Volume (GRID)

Methane -φ=0.9 -EGR0% -21bar -650K u'=2.5 m/s

Next future steps

• Validate the NOSE measurements by testing experimental points in common with the sphere in turbulent and laminar case.

• Test the dilution limit of the TRF-E with increasing temperature and pressure.

• Obtaining turbulent burning velocity measurements from NOSE using Double-View Schlieren and Tomography.

Introduction•

  Nowadays, one of the main challenge in the automotive sector concerns the increase of internal combustion engine (ICEs) efficiency in order to improve the fuel economy and cut the pollutant missions • The main reason is due to the great impact of transport sector on Greenhouse Gas (GHG) emissions. Particularly due to the light vehicles • Downsizing is one of the most exploited solution: reducing the displacement by maintaining the performance through a turbocharger. Downsizing means reduced friction, thermal losses and mass moved, boosting fuel economy and cutting CO2 emissions • A turbo provides the engine with the mass of air needed to ensure highly efficient and clean combustion • The higher intake conditions lead to an increase of the abnormal combustions • High dilution rates may be used in order to mitigate such phenomena Increasing dilution, turbulence and pressure may cause the shifting from the actual flamelet regime to the thin reaction zone NECESSITY OF A THOROUGH MODEL

  Great spatial and temporal resolution, allowing the flow characterization during compression and plateau phases. Possibility of performing tomography to detect flame contour. PIV performed ZOOMED and UNZOOMED.

-SPHERE -EGR20% -473K -5baru'=1.77 m/s 1 -SPHERE -EGR0% -473K -1baru'=0.77 m/s 2 -SPHERE -EGR0% -473K -1baru'=1.77 m/s

5 -SPHERE -EGR20% -473K -5baru'=2.77 m/s

Optical techniques: Double-View Schlieren

Direct optical access from two views allows flame speed propagation and a tridimensional vision of the flame shape.

Grid configuration:

Large volume:

• Turbulence may be considered homogeneous and isotropic.

• The turbulence intensity strongly decreases during the plateau, but it does not show significant oscillations.

Small volume:

• Turbulence may be considered homogeneous and isotropic as well.

• During the plateau the turbulence intensity mean value is constant or slightly increases, even though it presents a lot of oscillations.

• High turbulence intensity values, combined with small integral lengths, allows to reach high Karlovitz number and, thus, investigate the Peters-Borghi diagram unexplored zone.

Plateau ~ 35ms

Plateau ~ 35ms

Experimental setup

Innovative Turbulent Vessel:

• Previously unexplored experimental points of high pressure and high temperature

• Testing the inflammability limits with increasing dilution

• Flow field characterization (turbulent and pseudo-laminar conditions)

• Investigation of well-known monocomponent fuels and characterized gasoline surrogate Full mesh grid: 56x50 Full mesh grid: 20x20