Amina Bouchemoua

Pedro Brantuas

Jean Michel Herri
email: herri@emse.fr

EQUILIBRIUM DATA OF CO 2 -BASED SEMI-CLATHRATES FROM QUATERNARY AMMONIUM SOLUTIONS

Keywords: gas hydrate, quaternary ammoniums salts, CO 2, CH 4, N 2, phase equilibrium

CO 2 mitigation represents a major industrial and scientific challenge of this century, therefore CO 2 capture and sequestration is a route to solve a part of the problem, especially for the industries in which the gases to be treated are well localized. CO 2 capture by using hydrate formation has revealed to present an energetic interest because it is less intensive than other methods. In this process the cost of the separation is due to compression of gases to reach the gas hydrate formation conditions at which water and gas forms a solid that encapsulates preferentially CO 2 . In fact, the hydrate formation requires high pressures and low temperatures, which explains the use of thermodynamic promoters to decrease the operative pressure. Quaternary ammoniums salts represent an interesting family of components because of their thermodynamic effect, but also because they can generate crystals that are easily handable. In this work, we have made experiments concerning the equilibrium of pure gas hydrates (CO 2 , N 2 and CH 4 ) in presence of the tetra-n-butyl ammonium bromide (TBAB) and compared these data to literature. We have applied the Clausius-Clapeyron equation, to the results obtained and the literature data, thus observing that the hydrates have a pure component behavior. In the specific case of N 2 -TBAB semi-clathrates, observations showed that the pressure is first order depending on the temperature only and independent from the TBAB concentration. For the CO 2 -TBAB and CH 4 -TBAB semiclathrates the results are different. In fact, the equilibrium pressure is clearly dependent on both the temperature and TBAB concentration, for that reason the constant B from the Clausius-Clapeyron equation becomes a function of the TBAB concentration. Another point of our investigation is the effect of other types of quaternary ammoniums to find an additive that allows the formation of hydrates close to room temperature and atmospheric pressure.

INTRODUCTION AND LITERATURE SURVEY

CO 2 is one of the most important greenhouse gases due to its global warming potential and long atmospheric residence time [START_REF] Forster | Changes in Atmospheric Constituents and in Radiative Forcing[END_REF]. A great challenge is to reduce the CO 2 emissions by increasing the efficiency of the process and developing new technologies. The capture and sequestration is an interesting route to solve part of the problem. The capture of Carbone dioxide by gas hydrate formation is a new process for separating CO 2 from flue gases, by crystallizing liquid water with compressed flue gas to form a solid structure in which CO 2 is encapsulated preferentially. After precipitation, the hydrate slurry is dissociated and Table 1 -Structure of gas hydrates [START_REF] Sloan | Clathrate Hydrates of Natural Gases. 2 nd Ed[END_REF].

purified CO 2 is released at gas state and ready for sequestration. To decrease the formation pressure, additives from the quaternary ammonium family where the most commonly used is tetra-n-butyl ammonium bromide (TBAB) which also presents an interesting property to generate hydrate slurries that is naturally anti agglomerant [START_REF] Darbouret | Etude rheologique d'une suspension d'hydrates en tant que fluide frigoporteur diphasique; Resultats experimentaux et modelisation[END_REF].

Gas Hydrates

The clathrates are ice-like compounds; they correspond to a re-organization of the hydrogen links to form a solid in which the crystallographic structure is based on H-bonds links. The clathrates of water are also qualified improperly of "porous ice" because the crystal structure shows that the water molecules built a network of cavities in which could be captured gases, volatile liquids or other small molecules. The clathrates hydrates, called gas hydrates, have been studied intensively in respect to their occurrence in deep sea pipelines where they cause hard problems of flow assurance. The gas hydrates can exhibit three different structures called SI, SII and SH [START_REF] Sloan | Clathrate Hydrates of Natural Gases. 2 nd Ed[END_REF] which are a combination of different types of polyhedral sharing faces between them. Jeffrey [START_REF] Jeffrey | Hydrate inclusion compounds[END_REF] suggested the nomenclature to describe each polyhedral: e is the number of edges of the face, and f is the number of faces with e edges. Currently, three different structures have been established precisely, called I, II and H (table 1). Numerous hydrate formation data, for pure carbon dioxide and its binary mixtures in the presence of water, have been reported in the literature. Kang et al [START_REF] Kang | Hydrate phase equilibria of the guest mixture containing CO 2 , N 2 and tetrahydrofuran[END_REF] presented data on the three phase HLV equilibrium for CO 2 -N 2 gas mixture. Fan and Guo [START_REF] Fan | Hydrate Formation of CO 2 -Rich Binary and Quaternary Gas Mixtures in Aqueous Sodium Chloride Solutions[END_REF][START_REF] Chen | Experimental and modeling studies on the hydrate formation of CO 2 and CO 2 -rich gas mixtures[END_REF] [START_REF] Hachikibo | Phase Equilibrium Studies on gas Hydrates Formed from Various Guest Molecules and Powder Ice[END_REF], obtained hydrate equilibrium lines of pure carbon dioxide, methane and their mixtures. Most recently, Servio et al [START_REF] Bergeron | CO 2 and CH 4 mole fraction measurements during hydrate growth in a semi-batch stirred tank reactor and its significance to kinetic modeling[END_REF] developed a new experimental technique to measure the mole fraction of gases in the hydrate phase and presented some equilibrium data for CO 2 -CH 4 gas mixture. Nevertheless, very few data have been presented to report the equilibrium that means a measurement of both the gas and the hydrate composition.

From the results of these different studies, it is evident that the principal difficulty to envisage a separation based on hydrate crystallization is the required high formation pressure. To reduce this pressure, it is necessary to use thermodynamic additives such as quaternary ammonium salts [START_REF] Duc | CO 2 Capture by Hydrate crystallization -A potential Solution for Gas emission of Steelmaking Industry[END_REF].

Quaternary ammonium semi-clathrates

Quaternary ammonium salts form, in presence of water, and without any gas, a semi-clathrates hydrate crystal, even at atmospheric pressure [START_REF] Mcmullan | Hydrates of the tetra n-butyl and Tetra i-amyl Quaternary Ammonium Salts[END_REF]. They have been extensively studied in reason of a direct application for air conditioning as a phase change material [2, 14, 15, 16, 17, 18 and 19]. Tetra butyl ammonium bromide salt forms at least four different structures (figure 1) with hydration number of 24, 26, 32 and 36 [START_REF] Mcmullan | Hydrates of the tetra n-butyl and Tetra i-amyl Quaternary Ammonium Salts[END_REF].

A B

Bu4PBr, mol % The crystallographic structure of semi-clathrates hydrates has been determined by McMullan and Jeffrey [START_REF] Bergeron | CO 2 and CH 4 mole fraction measurements during hydrate growth in a semi-batch stirred tank reactor and its significance to kinetic modeling[END_REF] and completed by Shimada [START_REF] Shimada | Tetra-nbutylammonium bromide-water (1/38)[END_REF]. The TBAB semi-clathrates are different from pure water gas hydrate because bromides participate in the water network and also N + cations which are located at the intersection of four cavities [START_REF] Shimada | Tetra-nbutylammonium bromide-water (1/38)[END_REF]. Figure 2 describes the unit cell indicated by solid lines. Bromide atoms and water molecules form the cage structure. Tetra-n-butyl ammonium is located at the centre of four cages, were the dashed lines represent the broken edges of former gas hydrates cavities. Oyama et al [START_REF] Oyama | Phase diagram. latent heat. and specific heat of TBAB semi-clathrate hydrate crystals[END_REF] reported some thermal properties of TBAB semi-clathrates. They determined the phase diagram of semi-clathrates hydrates under the conditions of atmospheric pressure, and also measured latent and specific heats capacity. From the phase diagram, the congruent melting points of two different TBAB semi-clathrates structures were determined (12°C and 9.39°C for semi-clathrates with respectively a hydration numbers of 26 and 38).

Pure Gas TBAB semi-clathrates

Figure 3: Structure of gas TBAB semi-clathrates [START_REF] Shimada | Tetra-nbutylammonium bromide-water (1/38)[END_REF].

Davidson [START_REF] Davidson | Water: a comprehensive treatise[END_REF] suggested that these semiclathrates crystals do not encage gas molecules, but Shimada et al [START_REF] Shimada | Tetra-nbutylammonium bromide-water (1/38)[END_REF] and Duc et al [START_REF] Duc | CO 2 Capture by Hydrate crystallization -A potential Solution for Gas emission of Steelmaking Industry[END_REF] have given opposite results or opinions. Shimada et al [START_REF] Shimada | Tetra-nbutylammonium bromide-water (1/38)[END_REF][START_REF] Shimada | Separation of gas molecule using tetra-nbutyl ammonium bromide semi-clathrate hydrate crystals[END_REF] has supposed that TBAB semi-clathrates can encage small gas molecules.

In fact, based on the structure analyze of pure TBAB semi-clathrates (formula (n-C 4 H 9 ) 4 N + Br 38H 2 O), they supposed that the structure can encapsulate the gas in the free cavities (Figure 3, [START_REF] Shimada | Tetra-nbutylammonium bromide-water (1/38)[END_REF]). Duc et al [START_REF] Duc | CO 2 Capture by Hydrate crystallization -A potential Solution for Gas emission of Steelmaking Industry[END_REF] have experimentally confirmed that semi-clathrates can encapsulate around 40 m 3 STP of gas per cubic meter of solid.

In 2008, Lin et al [START_REF] Delahaye | Phase equilibrium and dissociation enthalpy for semiclathrate hydrate of CO 2 + TBAB[END_REF] investigated equilibrium conditions and dissociation enthalpy of hydrates formed from CO 2 -TBAB (tetra-n-butylammonium-bromide) water mixtures. Differential Thermal Analysis (DTA) was used for hydrateliquid-vapor (H-L-V) with a TBAB concentration range from 4.43 to 9.01 wt% and a CO 2 pressure range from 0.3 to 2.5 MPa. The results showed that the presence of TBAB allowed decreasing the formation pressure of CO 2 hydrates by approximately 74% and 87% at 283K and 279 K, respectively. Moreover, the reduction of pressure depends on the TBAB concentration. The dissociation enthalpy and the composition of hydrate formed from 9.01 wt% TBAB solution were determined from both the DTA and the Clapeyron equation. Deschamps and Dalmazzone [START_REF] Makino | Thermodynamic stabilities of tetra-nbutyl ammonium chloride + H 2 , N 2 , CH 4 , CO 2 , or C 2 H 6 semiclathrate hydrate systems[END_REF] studied the dissociation enthalpies and the phase equilibrium of TBAB-gas semi-clathrates hydrates by differential scanning calorimetry (DSC) under pressure. Pure gases such as N 2 and CO 2 and gas mixtures such as N 2 -CO 2 and CH 4 -CO 2 were studied. They presented the phase diagram of N 2 -TBAB semi-clathrates hydrates for different gas pressures, in the TBAB concentration range from 17.0 to 40.0 wt%. Enthalpies of dissociation of TBAB semi-clathrates hydrates of pure gas and gas mixtures were determined as a function of the pressure. Arjmandi et al [18] have proposed equilibrium data for pure gas semi-clathrates of TBAB by using visual observation of the dissociation. They present phase boundaries data on high pressure TBAB (10% wt), TBAB (10% wt)-Nitrogen, TBAB (42.7% wt)-Carbon dioxide, TBAB (5, 10, 20, 30% wt)-Methane, and TBAB (5, 10, 43% wt)-natural gas semi-clathrates. Tetra-n-butyl ammonium chloride (TBACl) hydrate also has empty cages that can be used for gas capture like TBAB. Makino et al [START_REF] Deschamps | Dissociation enthalpies and phase equilibrium for TBAB semiclathrate hydrates of N 2 . CO 2 . N 2 + CO 2 and CH 4 + CO 2[END_REF] have shown that TBACl can be used with H 2 , N 2 , CH 4 , CO 2 and C 2 H 6 . The TBACl + CO 2 hydrate is more stable than the corresponding hydrate with TBAB with the hydrate being stable between 16 and 20°C in the same pressure range of TBAB.

EXPERIMENTAL PROCEDURE AND SET-UP Experimental set-up

An experimental apparatus (figure 4) has been built to investigate the gas hydrates formation and dissociation with and without TBAB. The thermodynamic equilibrium conditions have been obtained by determining pressure and temperature, but also the composition of all liquid and hydrate phases. The experimental set-up is a stainless steel high pressure batch reactor (Autoclave) with a measured volume of 2.51L±0.03. It is surrounded with a double jacket connected to an externally cooler (HUBERT CC-505) with a controller CC3 of 0.02 K precision. Two sapphire windows of (12 x 2 cm) mounted on both sides of the reactor allow the visual observation of the hydrates. A Pyrex cell is located in the stainless steel autoclave in which the pressure can be raised up to 10 MPa. The Pyrex cylinder is filled with a volume of 1 L of water containing TBAB at concentration between 5 wt% and 40 wt%. The liquid is injected in the pressurized reactor by using a HPLC pump (JASCO-PU-1587). A four vertical-blade turbine impeller ensures stirring of the suspension during crystallization. Temperature is monitored by two Pt100 probes in the bulk and the gas phases. Pressure is measured by mean of a pressure transducer (range: 0-10 MPa) with a precision of ±0.05MPa. The data acquisition unit (P, T) is connected to a personal computer. A classical valve is used to take a sample of 1ml of liquid. It is analyzed by refractometry to determine either the TBAB or the TBACL concentration.

Experimental procedure

The hydrate is obtained by crystallization of pure gases (N 2 , CO 2 , and CH 4 ) with a liquid phase containing water and the salt. At the beginning, the reactor is closed and vacuumed with a vacuum pump then the cell is flushed three times with N 2 , CO 2 or CH 4 depending on the gas studied. It erases any trace of other gases used in a previous experiment. Then, the reactor is vacuumed again. After, the reactor is pressurized with the gas. The gas phase is stirred and cooled down and then maintained at the operative temperature (typically in the range of 278 to 286 K). The temperature need to be higher than the pure TBAB semiclathrates equilibrium temperature, and lower than pure water gas hydrate equilibrium temperature to be sure that we form gas-TBAB semi-clathrates and not pure water gas hydrates or pure TBAB semi-clathrates. The stirrer is stopped and the liquid solution (1L) is injected in the reactor by using the HPLC pump. An increase of temperature and pressure is simultaneously observed, firstly because the liquid is at ambient temperature, and also due to the gas compression resulting from the reduction of the gas volume by liquid. The stirrer is started the pressure decreases due to gas dissolution in the liquid. After a while (from some minutes to several hours, nucleation being a stochastic phenomena), crystallization begins. This is accompanied by a increase of temperature (figure 5). During crystallization (exothermic process), the pressure decreases due to gas consumption by hydrates and the temperature returns to the operative temperature. After a while, the system reaches the equilibrium (end of crystallization), the pressure and the temperature reach constant values. We can proceed to gas hydrate dissociation by two different ways. First way is called isochoric heating. The temperature of the reactor is heated by steps of 1K (figure 6). During each step, the pressure increases due to gas hydrate dissociation and reaches a constant value which represents the thermodynamic equilibrium.

The second method, called isothermal depressurization consists in depleting the pressure by opening the reactor and waiting for a new equilibrium point. Each step of depressurization is approximatively 0.2MPa. The temperature is controlled and maintained at constant value.

During the dissociation, the liquid is analyzed by refractometry to evaluate the quaternary ammonium concentration. This procedure is the same in the case of TBACl. Figure 6: typical example of the evolution of pressure and temperature during dissociation of pure gas -TBAB semi-clathrates

RESULTS AND DISCUSSIONS N 2 -TBAB semi-clathrates

In this section we present a comparison between the equilibrium data from literature and two experimental points from our work: by using Differential Scanning Calorimetry, Deschamps and Dalmazzone [START_REF] Shimada | Separation of gas molecule using tetra-nbutyl ammonium bromide semi-clathrate hydrate crystals[END_REF] have reported 5 equilibrium points with TBAB concentration of 40% Mass.

Two additional points at a same TBAB concentration (40 % Mass TBAB) have been reported by Duc et al [START_REF] Duc | CO 2 Capture by Hydrate crystallization -A potential Solution for Gas emission of Steelmaking Industry[END_REF] by using the same experimental procedure of this publication. Lin et al [START_REF] Delahaye | Phase equilibrium and dissociation enthalpy for semiclathrate hydrate of CO 2 + TBAB[END_REF] By plotting ln P versus 1/T (figure 7), we observe a linear behavior which allows to suppose that a common structure is crystallized whatever the temperature. But, more surprising, it reveals also that the pressure is first order dependent on the temperature only, and not on the TBAB concentration. Normally, the equilibrium pressure is dependent also on the salt concentration because the chemical potential of water varies in proportion of the salt concentration. Here, it seems to not be the case, or at least, it seems that the equilibrium pressure is dependent on the salt concentration at an order that cannot be evaluated in respect to the precision of the data. This observation is different from the ones we will show in the next part of the work about CO 2 and CH 4 pure gas semi-clathrates. We will see later that, for CH 4 and CO 2 pure gas semi-clathrates, the equilibrium pressure is clearly dependent on both the temperature and TBAB concentration. But, for N 2 pure gas semi-clathrates, the equilibrium pressure is first order on the temperature only. The literature survey allows completing our experimental results (Table 3) with the data of Arjmandi et al [18]. They give a complete work of the influence of the pressure-temperature dependency over TBAB concentration of 5%, 10%, 20% and 30% wt. From our experimental results (Table 3), we also evaluated the B value for each x TBAB , and we report on figure 9 B versus x TBAB (A value is assumed to be A=-26999). We can see that, up to 1/x TBAB value of 15, experimental results from Arjmandi et al [18] and our data fit well. It signifies we get the same equilibrium data. But, for higher values, which correspond to x TBAB <0.05, we can see a difference between the B values, and so a difference between our equilibrium points and data reported by Arjmandi et al [18]. This difference can be due to a mistake in the measurements. But, we will see in the next part about the CO 2 -TBAB that semi -clathrates can present different polymorphs at a same TBAB concentration. So, the difference between the equilibrium points from our work and from literature could also be explained by a difference between the polymorphs that have been crystallized in both the laboratories.

0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5
0,00334 0,00336 0,00338 0,0034 0,00342 0,00344 0,00346 0,00348 0,0035 ln P (P in MPa)

1/T (T in K) CH4,

CO 2 -TBAB semi-clathrates

We present our experimental data on Table 4.

The literature survey allows completing with experimental points from Lin et al [START_REF] Delahaye | Phase equilibrium and dissociation enthalpy for semiclathrate hydrate of CO 2 + TBAB[END_REF]. They give a complete work of the influence of the pressure-temperature dependency over TBAB concentration of 4%, 7% wt and 9%wt. It can be completed with data from Deschamps and Dalmazzone [START_REF] Makino | Thermodynamic stabilities of tetra-nbutyl ammonium chloride + H 2 , N 2 , CH 4 , CO 2 , or C 2 H 6 semiclathrate hydrate systems[END_REF], Arjmandi et al [18] and Duc et al [START_REF] Duc | CO 2 Capture by Hydrate crystallization -A potential Solution for Gas emission of Steelmaking Industry[END_REF]. Each of these authors has given data for a TBAB concentration of 40% wt. The figure 10 assumes a common structure and plot ln P versus 1/T. We can conclude that data can be fitted assuming a linear shape (y=A•x+B) and constant slope A=-27337. It confirms that the semiclathrates structure remains constant over temperature. The A value is much closer to the value obtained for CH 4 hydrate (A=-26999), and we can suspect the structures to be the same. The B value can be interpolated under the following form Equilibrium data from literature.

This B value calculated from equilibrium (P,T) data from literature have been compared to the ones calculated from our experimental results. In figure 12 we can see that TBACl hydrates are more stable than TBAB hydrates at higher pressures and temperatures, while TBPB hydrates are in the same temperature range of TBAB although at a slightly lower pressure. Interesting is the difference between the results for TBACl where the same solution has a different aspect depending on the initial value of CO 2 in the system.

CONCLUSIONS

In this paper, we presented new experimental data about equilibrium of pure gas hydrates (CO 2 , N 2 and CH 4 ) in presence of the tetra n-butyl ammonium Bromide (TBAB) and we compare our results to literature Three gas-TBAB semi-clathrates were studied in this work CO 2 -TBAB, N 2 -TBAB and CH 4 -TBAB. In the case of N 2 -TBAB semi-clathrates, observations show that the pressure is first order depending on the temperature only and not dependent on the TBAB concentration.

For the CO 2 -TBAB and CH 4 -TBAB semiclathrates the results are different. In fact, the equilibrium pressure is clearly dependent on both temperature and TBAB concentration We compare the experimental results for CH 4 to experimental results of Arjmandi [18] we observe that data fit well for x TBAB >0.05. But for higher values corresponding to x TBAB <0.05, the Arjmandi's data didn't fit our results, this difference can be due to experimental errors or the presence of different polymorphs at same TBAB concentration what we observe in the case of CO 2 -TBAB semi-clathrates Our studies concerning other salts than TBAB are on the beginning phase, therefore we do not have enough data to realize the same study we did in regards of the gas -TBAB semi clathrates. Still this gives us good indications in what to look for in the future and to continue to analyze these types of hydrates.

Figure 1 :Figure 2 :

 12 Figure 1: A) T, X -Phase diagram of tetra-nbutyl ammonium salts (respectively fluoride, chloride, bromide and iodide) hydrates [20]. B) T, X -Phase diagram of tetra-n-butyl phosphonium bromide. [21] The values along the lines of composition are the hydration numbers

Figure 4 :

 4 Figure 4: Experimental set-up

Figure 5 :

 5 Figure 5: typical example of the evolution of pressure and temperature during crystallization of pure gas -TBAB semi-clathrates

Figure 7 N 2 -

 72 Figure 7 N 2 -TBAB semi-clathrates. Experimental equilibrium data from literature and additional point from this work

Figure 8 CH 4 -

 84 Figure 8 CH 4 -TBAB semi-clathrates. Equilibrium data from Arjmandi et al [18].

Figure 9 CH 4 -

 94 Figure 9 CH 4 -TBAB semi-clathrates. Value of the B constant in the equation ln P = -26999/T + B.

  Figure 10 CO 2 -TBAB semi-clathrates.Equilibrium data from literature.

Figure 11 CO 2 -Figure 12

 11212 Figure 11 CO 2 -TBAB semi-clathrates -Value of the B constant in the equation ln P = -27337/T + B.

Table 3 CH

 3

	4 -TBAB semi-clathrates.
	Experimental equilibrium data from literature and
	additional point from this work

Table 4

 4 Arjmandi et al, 2007; Duc et al, 2007, Deschamps and Dalmazzone, 2010

	CO 2 -TBAB semi-clathrates.
	Experimental equilibrium data from literature and
	additional points from this work


Jean-Michel Herri
email: herri@emse.fr

Amina Bouchemoua

Matthias Kwaterski

Amara Fezoua

Yamina Ouabbas

Ana Camairao

Pedro Brantuas

GAS HYDRATE EQUILIBRIA FOR CO 2 -N 2 AND CO 2 -CH 4 GAS MIXTURES, EXPERIMENTS AND MODELLING

Keywords: gas hydrates, gas mixtures, thermodynamics

CO 2 capture in industry is regarded as a possible tool that is suitable for reducing the global carbon emissions. The gases emitted by industry are by definition localized at the plants, like steelmaking plants, gas or coal power plants, chemical plants or natural gas production plants.

Facing the variety of gases to be treated with regard to their quantities, qualities (mainly the CO 2 content, but also the presence of minor impurities such as H 2 S, SO 2 , NO X ….), and conditions of pressure and temperature, different strategies and technologies need to be developed to minimize the cost of the process. Hydrate technology could be used as an alternative approach to remove green house gases, and this is the route we try to develop here. A preliminary costing has revealed the process to be competitive for high concentrated mixtures of CO 2 containing N 2 [1] such as found in exhaust gases of steel making plants at atmospheric pressure. This work presents a set of experimental data on the hydrate liquid vapour equilibria encountered in the mixtures of CO 2 -N 2 and CO 2 -CH 4 with pure water. We present in detail our experimental procedure by which the gas composition can be measured directly, whereas the hydrate composition is to be calculated from a mass balance. Furthermore we have tried to validate our experimental data by using the classical van der Waals and Platteeuw model [2] with internal parameters found in the literature. These parameters are the so-called macroscopic parameters (i.e. macroscopic parameters from table 3 which refer to a classical thermodynamic approach) and the so-called Kihara parameters (referring to a statistical thermodynamic approach). Due to large deviations between the modelled values obtained in the way described above and the experimental results, we have re-fitted the internal parameters, essentially by retaining a set of macroscopic parameters from Handa and Tse [3] and re-fitting the Kihara parameters from our experimental results. Finally the new set of parameters is validated against experimental data from other sources available in the literature, or falsified against other sources.

INTRODUCTION

CO 2 capture in industry is a challenging task. It is seen as a possible tool for making a significant contributing to the reduction of the global carbon emissions. The gases emitted by industry are by definition localized at the plants, like e.g. steelmaking plants, gas or coal power plants, chemical plants or natural gas production plants. For that reason it is envisaged to employ suitable industrial processes to remove those industrial gases that have an impact on the global warming before being emitted into the atmosphere. However, in designing processes for removal of these green house gases, it is very important to consider the quantities to be treated. In case of steelmaking plants for example, the emissions can be in the order of several cubic meters of CO 2 per second. In power plants, the concentration of CO 2 is generally low, typically in the range of 5-15%, but it can be several tens of percents in steelmaking plants or in some cases of natural gas production.

Facing the variety of gases to be treated with regard to their quantities, qualities (mainly the CO 2 content, but also the presence of minor impurities such as H 2 S, SO 2 , NO X ….), and conditions of pressure and temperature, different strategies and technologies need to be developed to minimize the cost of the process. Hydrate technology could be an alternative approach to remove green house gases and this is the route we try to develop in this work. A preliminary costing has revealed the process to be competitive for high concentrated mixtures of CO 2 containing N 2 (Nuyeng et al., 2007) such as found in exhaust gases of steel making plants at atmospheric pressure. Currently we are working on the accurate modeling of hydrate equilibria in the presence of multiple gas components. The respective routines are to be implemented into process simulation software allowing for the precise evaluation of different sizing and costing schemes of capture. This work presents a set of experimental data on the hydrate liquid vapour equilibria in systems comprised of the binary gas mixtures CO 2 -N 2 and CO 2 -CH 4 , respectively, and pure water. We present in detail our experimental procedure by which the gas composition can be measured directly, whereas the hydrate composition is to be calculated from a mass balance. Furthermore we have tried to validate our experimental data by using the classical van der Waals and Platteeuw model (1959) with internal parameters found in the literature. These parameters are the so-called macroscopic parameters (i.e. macroscopic parameters from Table 2 which refer to a classical thermodynamic approach) and the so-called Kihara parameters (referring to a statistical approach). Due to large deviations between the modeled values obtained in the way described above and the experimental results, we have re-fitted the internal parameters, essentially by retaining a set of macroscopic parameters from Handa and Tse (1986), and re-fitting the Kihara parameters from our experimental results. Finally the new set of parameters is validated or falsified against experimental data available in the literature.

HYDRATES STRUCTURES

Three different hydrate structures have been identified experimentally, which are denoted by sI, sII and SH. They differ by their crystallographic structure in which water is organized in a three dimensional network which provides internal cavities of different polyhedra cavities called 5 12 , 5 12 6 2 , 5 12 6 4 , 4 3 5 6 6 3 and 5 12 6 8 (e f describes a polyhedra: e is the number of edges of the face, and f is the number of faces with e edge). In Table 1 the SI and SII are described more precisely structure that can be only formed in respect to the nature of the Martian gases. Sloan (1998, p. 33).

(2) Variation in distance of oxygen atoms from centre of cages (Sloan, 1998, p. 33).

(3) For ethane hydrate, from (Udachin, 2002). (4) For tetrahydrofuran hydrate, from Udachin (2002).

EXPERIMENTAL PROCEDURE AND APPARATUS

An experimental apparatus (figure 1) has been built to investigate the thermodynamic equilibrium conditions of gas hydrates (pressure and temperature) and to determine the composition of all existing phases (gas, liquid and hydrate). The experimental set-up consists of a stainless steel high pressure batch reactor (Autoclave) with a double jacket connected to an external cooler (HUBERT CC-505) equipped with a CC3 controller maintaining the temperature with a precision of 0.02 K. Two sapphire windows of (12 x 2 cm) mounted on both sides of the reactor enable to detect the occurrence of a hydrate phase by direct visual observation. A Pyrex cell is located in the stainless steel autoclave in which the pressure can be raised up to 10 MPa. The Pyrex cylinder is filled with 800 ml to 1000 ml (or 0.8 l to 1 l) of water containing LiNO 3 as an anionic tracer at a concentration of approximately 10 to 15 ppm (weight fraction). The liquid is injected in the pressurized reactor by using a HPLC pump (JASCO-PU-1587). A four vertical-blade turbine impeller ensures stirring of the suspension during crystallization. The temperature is monitored by two Pt100 probes, one in the liquid bulk, and the other one in the gas phase (Prosensor instrument, precision of 0.02 K). The pressure is measured by means of a pressure transducer (Keller instrument, range: (0-10) MPa, precision of 0.05 MPa). The (T, P) data acquisition unit is connected to a personal computer. The composition of the gas phase is determined in-line by using a gas chromatograph after sampling by a ROLSI instrument. This tool collects a controlled volume of gas (some µm 3 ) which is directly injected into the loop of the gas chromatograph (VARIAN model CP-3800 GC). The precision in gas composition is 2% (see [START_REF] Herri | Gas Hydrate Equilibria for CO2-N2 and CO2-CH4 gas mixtures-Experimental studies and Thermodynamic Modelling[END_REF] Figure 1 Experimental set up A classical valve is used to take a sample of 1 ml of liquid which is directed to a DIONEX ionic exchange chromatograph (off-line) to measure the tracer (LiNO 3 ) concentration. The tracer is an ionic element which is not incorporated into the hydrate structure but concentrated in the liquid phase during crystallization. The gas mixtures are prepared by injecting each gas directly into the reactor. The mixtures are analyzed by gas chromatography to obtain the exact composition of each gas mixture (see [START_REF] Herri | Gas Hydrate Equilibria for CO2-N2 and CO2-CH4 gas mixtures-Experimental studies and Thermodynamic Modelling[END_REF].

The hydrate is obtained by crystallization of gas mixtures (CO 2 with N 2 or CH 4 ) in presence of a liquid phase (water + LiNO 3 (10 ppm weight fraction)).

Experimental procedure

Initially the reactor is closed and evacuated by means of a vacuum pump. Subsequently, the cell is flushed three times with nitrogen (or CH 4 , depending on the experiment) to eliminate any trace of other gases (e.g. from a preceding experimental run). After this cleaning procedure, the reactor is evacuated again. At the beginning of the actual experimental run, the reactor is pressurized with the first gas (generally this has been CO 2 because the maximum pressure in the CO 2 bottle is about 5 MPa). Subsequently the second gas (N 2 or CH 4 ) is injected until the operative pressure is reached (up to 10 MPa depending on the experiments). The gas mixture is stirred and cooled down, and then maintained at the operative temperature (typically in the range from 0 to 10 °C) The stirrer is then stopped and the liquid solution (1 l) is injected in the reactor by using the HPLC pump. Upon injection of the solution an increase of both temperature as well as pressure is observed simultaneously, firstly because the liquid is at ambient temperature, and also as a consequence of the gas compression resulting from the reduction of the gas volume by the liquid injection. Subsequently, the stirrer is started and a decrease of the pressure is detected due to partial dissolution of the gaseous components in the liquid phase. After a while (ranging from some minutes to several hours, since nucleation being a stochastic phenomenon), crystallization (exothermic process) starts accompanied by a sudden increase of temperature that depends on the intensity of the crystallization. During the formation of the solid, the pressure decreases due to the gas consumption to form hydrates. While the crystallization takes place, the gas phase is sampled with the ROLSI© instrument and analyzed by in-line gas chromatography. The liquid phase is sampled to be analyzed off-line by ion exchange chromatography. After a while, the system reaches equilibrium (end of crystallization), and correspondingly the values of pressure and temperature approach constant values. The gas hydrate dissociation is operated at constant volume and started by heating the reactor in increments of 1°C (Fig 3). After each increment of temperature, the pressure increases due to gas hydrate dissociation and reaches a constant value which represents the thermodynamic equilibrium. In the same way as in the case of the crystallization steps, the gas and the liquid phases are sampled to determine the compositions of the phases at equilibrium (the method to evaluate the experimental results is presented in detail in [START_REF] Herri | Gas Hydrate Equilibria for CO2-N2 and CO2-CH4 gas mixtures-Experimental studies and Thermodynamic Modelling[END_REF]. The method outlined above results in the determination of the complete set of quantities charaterising the phase equilibrium condition. In other words, the approach enables the experimental determination of the tuple of values of temperature, pressure, and the compositions of the gas and hydrate phases, respectively ..

MODELLING

The so-called van der Waals and Platteeuw model (van der Waals and Platteeuw, 1959) has been explained in many publications and books on the subject (see, e.g., [START_REF] Sloan | Clathrate hydrates of natural gases. 2 nd Ed[END_REF]. Our objective is not to present the details of the model equations again, but to recall the input data which we focused on in this work. In the case of hydrates, in thermodynamic equilibrium, the equality of the chemical potentials of water in the ice, liquid as well as in the hydrate phase, respectively, can be written by introducing a reference state which is a hypothetical phase β corresponding to a hydrate with empty cavities.



       - = ∆ ∑ ∑ - j i j i i β RT θ ν µ 1 ln H w (2)
In eq. ( 2) ν i is the number of cavities of type i per molecule of water and i j θ is the occupancy factor

( ] 1 , 0 [ ∈ i j θ
) of the cavities of type i by the gas molecule j. This last parameter is very important to define the thermodynamic equilibrium and to determine the hydrate properties. The occupancy factor is described by a model based on ideas considering the analogy between the gas adsorption in the 3-dimensional hydrate structure and the 2-dimensional Langmuir adsorption. It can be expressed as a function of the fugacity f j of the gas j as:

        - = ∆ ∑ ∑ - j j i j i i β P T f C RT ) , ( 1 ln H w ν µ (3)
Where i j C is the Langmuir constant of component j in the cavity i that describes the interaction potential between the encaged guest molecule and the surrounding water molecules evaluated by assuming a spherical symmetrical cage that can be described by a spherical symmetrical potential:

∫ ∞       - = 0 2 ) ( exp 4 dr r kT r w kT C i j π (4)
Where w is the interaction potential between the cavity and the gas molecule according to the distance r between the guest molecule and the water molecules over the structure. The interaction potential can be determined by different models such as e.g. the van der Waals and Platteeuw model (van der Waals and Platteeuw, 1959), the Parrish and Prausnitz model [START_REF] Parrish | Dissociation pressure of gas hydrates formed by gas mixtures[END_REF] or the so-called Kihara model. The latter, being the most precise [START_REF] Mckoy | Theory of dissociation pressures of some gas hydrates[END_REF], can be expressed as:

            + -       + = 5 4 5 6 11 10 11 12 2 ) ( δ δ σ δ δ σ ε R a r R R a r R z r w (5)               - + -       - - = - - N N N R a R r R a R r N 1 1 1 δ (6)
The gas parameters ε, σ and a are the so-called . In a good approximation, the aqueous phase can be regarded as ideal and the activity coefficient therefore be set to a fixed value of 1, resulting in ( ) A minimization algorithm has been implemented in the GasHyDyn Sofware (Java language) to determine the state variables, i.e., P, T, gas composition, and hydrate composition at equilibrium computationally by using kihara parameters and reference state parameters as input quatities, or inversely, to determine the set of these parameters from (P, T, gas composition, hydrate composition) experimental data (more details can be found in [START_REF] Herri | Gas Hydrate Equilibria for CO2-N2 and CO2-CH4 gas mixtures-Experimental studies and Thermodynamic Modelling[END_REF] The idea behind this work was to generate an autocoherent set of parameters. In fact, when taking a look at the literature, we can notice the variation in the numerical values of both Kihara parameters, but also reference state parameters that have been published. J mol -1 K -1 -38.12 -38.12

0 L w , , L w , L w , 0 0 0 T T b c c p P T p P p - + ∆ = ∆ - - - β β β (9)
β - L w , p b J mol -1 K -2 0.141 0.141 6011 0 , I w 0 , L w - ∆ = ∆ - - β β h h
, where 6011 is the enthalpy of fusion of Ice (J mol -1 ) (1) refers to Dharmawardhana et al. (1980) (2) refers to John et al (1985) (3) refers to Handa and Tse, 1986 [START_REF] Sloan | Clathrate hydrates of natural gases. 2 nd Ed[END_REF] The classical way to fit the Kihara parameters is firstly to suppose a set of reference state parameters, and secondly to benefit from a rich data base implementing not only (pressure and temperature) equilibrium data but also equilibrium data for gas mixtures. In this second case, the ideal case is to benefit from the most complete set of (pressure, temperature, gas composition and hydrate composition) data. Subsequently, in order to retrieve Kihara parameters, the work consists in supposing a hydrate structure (sI, sII or sH) and then to fit calculated equilibrium data with experimental data. As a result of the approach outlined above, the Kihara constants are dependent on the reference state parameters that have been selected. In the next part of the work, we will evaluate the performance of three models. Model 1 is the model implemented with the values for 3.04 0.60 1.25 For each line, an individual deviation called %D3 is evaluated which displays the difference between the experimental result and the corresponding value calculated by the model implemented with reference properties from (3) ADi is an average deviation referring to models i = 1, ..., 3 in which reference properties are from (1) Dharmawardhana et al ( 1980), ( 2 .38 For each line, an individual deviation called %D3 is evaluated which displays the difference between the experimental result and the corresponding value calculated by the model implemented with reference properties from (3) ADi is an average deviation referring to models i = 1, ..., 3 in which reference properties are from (1) Dharmawardhana et al (1980), (2) John et al (1985), (3) Handa and Tse (1986) (a) experimental equilibrium data from this study Table 4 and Table 5 give the comparison between our experimental results and the results of the model using the optimised set of Kihara parameters with the three sets of macroscopic parameters (Table 3) being implemented in the model from Dharmawardhana et al. (1980), John et al. (1985) and Handa and Tse (1986). For each set of macroscopic parameters, we have optimised the Kihara parameters, and the results are given in Table 6. Table 4 andTable 5 John et al. (1985) and Handa and Tse (1986) is presented. The models have been run with an optimised set of Kihara parameters which are recapitulated in Table 6. Table 4 shows the comparison of the modelling results with experimental data on (CO 2 -CH 4 ) mixtures (this study). All the models reveal to be very efficient, both with regard to the estimation of the equilibrium pressure as well as the hydrate composition. At that level of the presentation, it is difficult to underline/identify the best model description. The best model description seems to be the one using Kihara parameters that have been fitted after implementation of the reference properties from Handa and Tse (1986). The average deviation for this case amounts to 3% for the calculation of the equilibrium pressure, and a remarkable good evaluation of the gas composition, both for the CO 2 (deviation of 0.6%) and the CH 4 system (deviation of 1.25%), has been attained.

0 L w T v β - ∆ , 0 , L w , β - ∆ p c , β - L w , p b :
The situation changes completely if we take a look at the CO 2 -N 2 mixture. The Kihara parameters regressed on the data of the hydrate equilibrium involving N 2 as a single gas under the assumption of a SII structure is implemented in the model. The corresponding results are compared with our experimental results. Using the reference properties from Dharmawardhana et al. (1980) or John et al. (1985), the model fails in simulating the experimental data. With the reference properties from Handa and Tse (1986) as given Table 3, and optimised Kihara parameters from Table 6 (this study), we observe a good agreement between the model and the experiments, with respect to the evaluation of both, the equilibrium pressure (average deviation of 8.1%) as well as the hydrate composition. For the latter we observe an excellent evaluation of the CO 2 composition (average deviation of 4.4%) and at least a reasonable evaluation of the N 2 composition (average deviation of 10.4%).

Validation of the best set of parameters on experimental results from the literature for the N 2 -CH 4 equilibrium

The literature presents a large amount of experimental data giving the equilibrium pressure as a function of the temperature and gas composition, especially for pure gases and binary components. However, the literature is poor in presenting complete sets of equilibrium data: pressure, temperature, gas, as well as hydrate composition. Fortunately, the work of [START_REF] Jhaveri | [END_REF] presents such data for the system containing the binary gas mixture N 2 -CH 4 . As we have presented our own experimental results for the CO 2 -N 2 and CO 2 -CH 4 mixtures, the data of [START_REF] Jhaveri | [END_REF] are particularly interesting because they allow for "closing the composition triangle" with the data on the N 2 -CH 4 mixture. The comparison between the models is presented in Table 7. The best model continues to be the model 3 (i.e., the Kihara parameters fitted in this work in combination with the reference properties from Handa and Tse (1986)) with a reasonable average deviation of about 13% for the evaluation of the equilibrium pressure, and a deviation of about 10% for the evaluation of the hydrate composition. 13.58 10.66 9.13 For each line, an individual deviation called %D3 is evaluated which displays the difference between the experimental result and the corresponding value calculated by the model implemented with reference properties from (3) ADi is an average deviation referring to models i = 1, ..., 3 in which reference properties are from (1) Dharmawardhana et al. (1980), (2) John et al. (1985), (3) Handa and Tse (1986) (e) experimental data from Jhaveri and Robinson (1986)

Conclusion

We proposed a new set of Kihara parameters for the components CO 2 , CH 4 and N 2 (Table 6). They are associated with the classical van der Waals and Platteeuw model (1959) implemented with reference state parameters from Handa and Tse (1986) and completed with additional parameters from Sloan (1998) (Table 6). This set of parameters allows us to predict correctly the complex HLV phase equilibrium data (pressure, temperature, gas composition, hydrate composition). The calculated data are presented in this paper along with the original experimental results on the hydrate systems formed by the binary gas mixtures CO 2 -CH 4 and CO 2 -N 2 . Finally, the set of parameters is compared to literature data concerning the N 2 -CH 4 -mixture and reveals to correct. Perpective Our set of parameters needs to be confronted with other experimental data. This work has been presented in [START_REF] Herri | Gas Hydrate Equilibria for CO2-N2 and CO2-CH4 gas mixtures-Experimental studies and Thermodynamic Modelling[END_REF]. It reveals for the systems generated from the CO 2 -N 2 and the CO 2 -CH 4 mixtures, respectively, that our experimental data differ from the data of [START_REF] Seo | Hydrate Phase Equilibria for Gas Mixtures Containing Carbon Dioxide: A Proofof-Concept to Carbon Dioxide Recovery from Multicomponent Gas Stream[END_REF] and [START_REF] Kang | Hydrate Phase Equilibria of guest mixtures containing CO 2 . N 2 and Tetrahydrofuran[END_REF], but in turn, that the data of [START_REF] Seo | Hydrate Phase Equilibria for Gas Mixtures Containing Carbon Dioxide: A Proofof-Concept to Carbon Dioxide Recovery from Multicomponent Gas Stream[END_REF] also differs from the data of [START_REF] Kang | Hydrate Phase Equilibria of guest mixtures containing CO 2 . N 2 and Tetrahydrofuran[END_REF]. The results of this study raise the question of the validity of our experimental results, but also of the data of [START_REF] Seo | Hydrate Phase Equilibria for Gas Mixtures Containing Carbon Dioxide: A Proofof-Concept to Carbon Dioxide Recovery from Multicomponent Gas Stream[END_REF] and [START_REF] Kang | Hydrate Phase Equilibria of guest mixtures containing CO 2 . N 2 and Tetrahydrofuran[END_REF]. However, they may also raise the question of a possibly different stability diagram of such mixed gas hydrates.

  of the chemical potentials between the water in the hydrate or liquid phase and in the reference phase, means of relations originating from classical thermodynamics.

∆

  Kihara parameters and can be calculated from experimental data by fitting the model equations to corresponding hydrate equilibrium data.The reference conditions are the temperature T 0 = 273.15 K and the pressure P 0 = 0. The difference of the chemical potential of water in the phase under consideration (the liquid aqueous phase in our case, but it could be an ice or the vapour phase) and the (hypothetical) empty hydrate phase β, water in the ice phase is 1, and if water is present in the liquid phase L w a is given as the product of the mole fraction of water in the liquid phase, wx , and the activity coefficient of water in that phase,

  Dharmawardhana et al. (1980), model 2 is implemented with the set of values published byJohn et al. (1985) and model 3 is implemented with the values fromHanda and Tse (1986).

  )John et al (1985), (3)Handa and Tse (1986) (a) experimental equilibrium data from this study (b) and (c) experimental equilibrium data fromAdisasmito et al, 1991

Table 2

 2 Reference state parameters

			Unit	Structure I	Structure II
	L	,	0
	w

Table 3

 3 Reference state parameters

			Unit	Structure I	Structure II
	L	,	0
	w

Table 4

 4 Experimental data and comparison to models for CO 2 and CH 4 and CO 2 -CH 4 gas hydrates

			Experiment				Structure			Simulation
				Gas	Hydrate		Pressure	Hydrate
		ϑ	P eq	Molar fraction	Molar fraction		P eq		Molar fraction ± 0.06
		°C	MPa	CO 2	CH 4 CO 2 CH 4		MPa	%D3 CO 2 %D3 CH 4 %D3
	(a)	4.00	2.04	1.00 0.00 1.00 0.00	SI	2.00	1.92	1.00 0.00 0.00 0.00
	(a)	4.00	2.36	0.64 0.36 0.77 0.23	SI	2.32	1.72	0.76 1.16 0.24 3.83
	(a)	4.00	2.55	0.52 0.48 0.68 0.32	SI	2.47	3.15	0.67 1.09 0.33 2.28
	(a)	4.00	2.80	0.36 0.64 0.54 0.46	SI	2.74	2.18	0.53 1.17 0.47 1.34
	(a)	4.00	3.55	0.11 0.89 0.21 0.79	SI	3.39	4.56	0.21 0.20 0.79 0.06
	(a)	4.00	3.90	0.00 1.00 0.00 1.00	SI	3.88	0.59	0.00 0.00 1.00 0.00
	(b)	0.15	1.42	1	0	1	0	SI	1.27 10.25	1	0
	(b)	2.35	1.63	1	0	1	0	SI	1.64	0.83	1	0
	(b)	3.65	1.90	1	0	1	0	SI	1.92	0.93	1	0
	(b)	4.45	2.11	1	0	1	0	SI	2.11	0.17	1	0
	(b)	5.95	2.55	1	0	1	0	SI	2.55	0.04	1	0
	(b)	7.45	3.12	1	0	1	0	SI	3.11	0.29	1	0
	(b)	8.35	3.51	1	0	1	0	SI	3.53	0.70	1	0
	(b)	8.95	3.81	1	0	1	0	SI	3.87	1.50	1	0
	(b)	9.75	4.37	1	0	1	0	SI	4.37	0.01	1	0
	(c)	0.25	2.68	0	1	0	1	SI	2.63	1.94	0	1
	(c)	1.45	3.05	0	1	0	1	SI	2.97	2.67	0	1
	(c)	3.55	3.72	0	1	0	1	SI	3.70	0.62	0	1
	(c)	5.15	4.39	0	1	0	1	SI	4.37	0.46	0	1
	(c)	6.45	5.02	0	1	0	1	SI	5.05	0.61	0	1
	(c)	7.75	5.77	0	1	0	1	SI	5.83	0.97	0	1
	(c)	9.15	6.65	0	1	0	1	SI	6.90	3.80	0	1
	(c)	10.45	7.59	0	1	0	1	SI	8.11	6.80	0	1
	(c)	11.55	8.55	0	1	0	1	SI	9.34	9.25	0	1
	(c)	12.55	9.17	0	1	0	1	SI	10.73 17.06	0	1
	(c)	13.25 10.57	0	1	0	1	SI	11.87 12.34	0	1
	AD1									2.22	1.15	2.46
	AD2									3.16	1.67	3.14
	AD3

Table 5

 5 Experimental data and comparison to models for CO 2 -N 2 gas hydrates

			Experiment				Structure		Simulation
				Gas		Hydrate		Pressure	Hydrate
		ϑ	P eq	Molar fraction	Molar fraction		P eq	Molar fraction ± 0.06
		°C	MPa CO 2	N 2	CO 2	N 2		MPa %D3	CO 2 %D3	N 2	%D3
	(a)	0.25 6.10	0.16 0.84 0.66 0.34	SI	5.79	5.01	0.59 10.20 0.41 19.62
	(a)	1.35 6.20	0.16 0.84 0.66 0.34	SI	6.49	4.70	0.59	9.86 0.41 18.89
	(a)	2.25 6.40	0.19 0.82 0.66 0.34	SI	6.73	5.13	0.62	6.08 0.38 11.60
	(a)	3.35 6.60	0.20 0.80 0.58 0.42	SI	7.41 12.26	0.63	7.13 0.37 10.02
	(a)	0.75 5.90	0.25 0.75 0.75 0.25	SI	4.38 25.74	0.71	5.42 0.29 16.43
	(a)	1.55 5.90	0.26 0.75 0.73 0.27	SI	4.82 18.29	0.71	3.08 0.29	8.33
	(a)	2.85 5.90	0.26 0.74 0.70 0.30	SI	5.57	5.54	0.71	0.17 0.29	0.41
	(a)	3.75 6.00	0.27 0.74 0.70 0.30	SI	6.29	4.76	0.70	0.58 0.30	1.38
	(a)	4.65 6.30	0.29 0.71 0.67 0.33	SI	6.62	5.04	0.72	6.59 0.28 13.43
	(a)	4.95 6.40	0.30 0.71 0.69 0.31	SI	6.84	6.87	0.72	3.58 0.28	8.01
	(a)	5.25 6.40	0.30 0.71 0.72 0.29	SI	7.17 12.06	0.71	0.38 0.29	0.96
	(a)	5.45 6.50	0.30 0.70 0.70 0.31	SI	7.27 11.80	0.72	2.96 0.28	6.74
	(a)	2.25 6.10	0.20 0.80 0.67 0.33	SI	6.29	3.04	0.64	4.25 0.36	8.64
	(a)	2.85 6.20	0.22 0.78 0.65 0.35	SI	6.44	3.93	0.66	1.26 0.34	2.33
	(a)	6.95 5.30	0.56 0.44 0.85 0.16	SI	5.17	2.47	0.87	2.40 0.13 13.08
	(a)	7.95 5.60	0.59 0.42 0.82 0.18	SI	5.79	3.47	0.87	6.14 0.13 27.80
	AD1									39.18	10.31	25.20
	AD2									70.81	53.67	127.3
	AD3									8.13	4.38	10

Table 6

 6 Kihara parameters regressed from experimental results of this study, and Kihara parameters from literature

		Kihara parameters regressed from experimental results of this study and implemented in
				model 1,2,3 with macroscopic parameters from table 3
		(1)Dharmawardhana et al, 1980 -(2) John et al, 1985 -(3) Handa an Tse, 1986
		k ε	CO 2 σ	a	k ε	CH 4 σ	a	k ε	N 2 σ	a
	Model 1	170.00	2.9855	0.6805	157.85	3.1439	0.3834	126.98	3.0882	0.3526
	Model 2	164.56	2.9824	0.6805	154.47	3.1110	0.3834	166.38	3.0978	0.3526
	Model 3	171.41	2.9830	0.6805	158.71	3.1503	0.3834	138.22	3.0993	0.3526
					Kihara parameters from literature
	Sloan, 1998	168.77	2.9818	0.6805	154.54	3.1650	0.3834	125.15	3.0124	0.3526
	Sloan, 2007	175.

.405 2.97638 0.6805 155.593 3.14393 0.3834 127.426 3.13512 0.3526

  present the experimental points (pressure, temperature, gas composition, and hydrate composition) on the left part. The mid-column indicates the structure that is presented as the most stable one from the model. The right part of the table shows the results of the simulation for the best set of reference parameters (which turns out to be the parameters fromHanda and Tse (1986): we present the calculated values of pressure and composition, and additionally the deviation between calculated and measured results. At the bottom of the table the average deviation from the three sets of macroscopic parameters fromDharmawardhana et al. (1980),

Table 7

 7 Experimental data fromJhaveri and Robinson (1986) in comparison with results obtained from model calculations for N 2 -CH 4 gas hydrate

			Experiment				Structure			Simulation
				Gas		Hydrate		Pressure		Hydrate
		ϑ	P eq	Molar fraction	Molar fraction		P eq			Molar fraction
		°C	MPa	N 2	CH 4	N 2	CH 4		MPa %D3	N 2	%D3 CH 4 %D3
	(e)	0.05	2.64	0.00	1.00	0.00	1.00	SI	2.58	2.40	0.00
	(e)	0.05	3.62	0.16	0.84	0.07	0.94	SI	2.99 17.40	0.04 33.01	0.96	2.30
	(e)	0.05	4.31	0.31	0.69	0.10	0.90	SI	3.51 18.45	0.10	0.81	0.90	0.09
	(e)	0.05	5.35	0.53	0.47	0.20	0.80	SI	4.73 11.67	0.21	6.97	0.79	1.74
	(e)	0.05	6.55	0.65	0.36	0.35	0.65	SI	5.75 12.26	0.31 12.52	0.69	6.74
	(e)	0.05	7.75	0.73	0.28	0.43	0.58	SI	6.74 12.97	0.39	7.79	0.61	5.76
	(e)	0.05 10.64	0.82	0.19	0.62	0.38	SI	8.37 21.36	0.52 15.94	0.48 26.01
	(e)	0.05 11.65	0.88	0.12	0.71	0.29	SI	10.07 13.57	0.65	8.89	0.35 21.77
	(e)	0.05 12.77	0.90	0.10	0.77	0.24	SII	10.61 16.93	0.77	1.00	0.23	3.24
	(e)	4.25	3.86	0.00	1.00	0.00	1.00	SI	3.98	3.15	0.00
	(e)	4.25	5.20	0.44	0.56	0.18	0.82	SI	6.59 26.66	0.17	5.87	0.83	1.29
	(e)	4.25	8.11	0.63	0.37	0.31	0.69	SI	9.07 11.86	0.31	0.19	0.69	0.08
	(e)	4.25 10.34	0.74	0.26	0.47	0.53	SI	11.50 11.24	0.43	7.72	0.57	6.85
	(e)	4.25 12.06	0.78	0.22	0.56	0.44	SI	12.70	5.28	0.49 12.48	0.51 15.89
	(e)	4.25 13.32	0.93	0.07	0.81	0.19	SII	18.90 41.92	0.84	3.37	0.16 14.38
	(e)	4.25 14.59	0.94	0.06	0.86	0.14	SII	19.51 33.69	0.87	0.86	0.13	5.26
	(e)	4.25 16.21	1.00	0.00	1.00	0.00	SII	22.23 37.13	1.00
	(e)	6.65	5.14	0.00	1.00	0.00	1.00	SI	5.17	0.57	0.00
	(e)	6.65	7.14	0.35	0.65	0.09	0.91	SI	7.69	7.76	0.13 40.58	0.87	4.06
	(e)	6.65	8.37	0.46	0.54	0.22	0.78	SI	9.06	8.29	0.19 15.14	0.81	4.37
	(e)	6.65 15.55	0.75	0.25	0.55	0.45	SI	16.16	3.95	0.46 15.68	0.54 19.16
	(e)	6.65 20.67	0.84	0.16	0.68	0.32	SI	20.58	0.43	0.61 10.86	0.39 23.07
	(e)	6.65 25.23	0.91	0.09	0.80	0.20	SII	25.62	1.53	0.82	2.83	0.18 11.47
	(e)	6.65 32.42	1.00	0.00	1.00	0.00	SII	30.62	5.55	1.00
	AD1									27.50		20.04		23.67
	AD2									66.01		123.7		61.82
	AD3

ACKNOWLEDGEMENTS

This work has been supported by ANR in the framework of the SECOHYA project. The authors thank all the members of the GasHyDyn team for their constant support and especially the technical staff: Alain Lallemand, Fabien Chauvy, Richard Drogo and Albert Boyer.


Laurent Godet
email: godet@mnhn.fr

Patrick Le Mao

Cindy Grant

Frédéric Olivier

Marine invertebrate fauna of the Chausey archipelago: an annotated checklist of historical data from 1828 to 2008

Keywords:

HAL is

Introduction

Fauna and flora inventories are critical components in designing conservation policies. However, historical data areoften lacking along the world's coastlines and it is often difficult for scientists to assess long-term impacts of human activities. European countries such as Denmark (e.g. [START_REF] Petersen | Valuation of the sea. The animal communities of the sea-bottom and their importance fot marine zoogeography[END_REF][START_REF] Petersen | On the animal communities of sea-bottom in the Shagerrak, the Christiana fjord and the Danish waters[END_REF], Sweden (e.g. [START_REF] Molander | Animal communities on soft bottom areas in the Gullmar fjord[END_REF], the United Kingdom (e.g. [START_REF] Holme | The bottom fauna of the Great West Bay[END_REF][START_REF] Holme | The bottom fauna on the English Channel[END_REF], and France (e.g. [START_REF] Cabioch | Contribution à la connaissance des peuplements benthiques de la Manche occidentale[END_REF]) are among those benefiting from historical data on invertebrate marine fauna. In France, the analysis of works by early naturalists has led to updated checklists of marine species for several coastal areas (e.g. De [START_REF] De Montaudoin | Contribution to a synopsis of marine species richness in the Pertuis Charentais Sea with new insights in soft-bottom macrofauna of the Marennes-Oléron Bay[END_REF][START_REF] Bachelet | An updated checklist of marine and brackish water Amphipoda (Crustacea: Peracarida) of the southern Bay of Biscay (NE Atlantic)[END_REF][START_REF] Dauvin | Liste actualisée des espèces d'Annélides Polychètes présentes en Manche[END_REF] and has permitted the assessment of long-term changes in the spatial distribution of invertebrate species or habitats (e.g. [START_REF] Godet | Before and after wasting disease in common eelgrass Zostera marina along the French Atlantic coasts: a general overview and first accurate mapping[END_REF].

The Chausey archipelago (Normand-Breton Gulf, France, Fig. 1) can be considered as a 'living museum' of French marine biology which merits conservation due to its high scientific value [START_REF] Godet | L'évaluation des besoins de conservation d'un patrimoine naturel littoral marin. L'exemple des estrans meubles de l'archipel de Chausey[END_REF]. However, with the exception of [START_REF] Godet | L'évaluation des besoins de conservation d'un patrimoine naturel littoral marin. L'exemple des estrans meubles de l'archipel de Chausey[END_REF], no comprehensive review has been conducted in this area. In this paper, we present the first checklist of the marine invertebrates of the Chausey archipelago.

Material and Methods

Fifty-two manuscripts, three zoological collections, and a series of recent personal observations related to marine invertebrate fauna of Chausey Archipelago were examined in this study. All sources are numbered in chronological order (included in the caption of the Table 1).

We compiled all data on marine invertebrate species in a checklist (Table 1) including: i) species names with classification by Phylum, Class, and Order; ii) the source of the original data; and iii) information on the storage of type specimens in the collections of the French Museum of Natural History.

The species names and classification have been updated according to the European Register of Marine Fauna [START_REF] Costello | European Register of Marine Species. A check-list of the marine species in Europe and bibliography of guides to their identification[END_REF]. Specimens identified to the genus level (e.g. Capitella sp.) are mentioned only if they are the single members of this genus having been reported in Chausey. Finally, collection catalogues available at the French Museum of Natural History were examined to identify potential holotypes (H) and syntypes (S) (column 'T' -Table 1) that were collected from Chausey for the following groups: Mollusca, Crustacea, Polychaeta, and Octocorallia.

Results

A total of 14 phyla and 769 species have been reported from Chausey (Table 1), among which Arthropoda (245 species), Mollusca (197 species) and Polychaeta (146 species) covered more than 75% of the total species number (Fig. 2). Table 1. Updated checklist of the marine invertebrate species reported in Chausey. Holotypes (H) and Syntypes (S) collected in Chausey are mentioned in the'T' (Type) column. Tableau 1. Liste actualisée des invertébrés marins recensés à Chausey. Les Holotypes (H) et Syntypes (S) récoltés à Chausey sont mentionnés dans la colonne 'T' (Type).

Eteone longa (Fabricius, 1780) 30,48,51 Eulalia mustela Pleijel, 1987 52 Eulalia viridis (Johnston, 1828) 35 9,27,35,49 Gibbula magus (Linnaeus, 1758) 5,7,9,27,30,35,48,49,52 Gibbula pennanti (Philippi, 1846) 9,27,35,48,49 Gibbula tumida (Montagu, 1803) 6,9 Gibbula umbilicalis (da Costa, 1778) 6,9,27,35,46,48 Haliotis tuberculata (Linnaeus, 1758) 5,9,14,16,17,27,29,35,48 Forest, 197832,33,35 Macropodia rostrata (Linnaeus, 1767) 9,30,32,33,36,46,48 Macropodia tenuirostris (Leach,1814) 9 Maja brachydactyla Balss, 192117,29,32,33,46,49 Necora puber (Linnaeus, 1767) 29,32,33,35,36,46,48,49 Pagurus bernhardus (Linnaeus, 1758) 12,30,32,33,48 Pagurus cuanensis Bell, 1845 32,33,35,48

Phylum Bryozoa Class Gymnolaemata

Order Cheilostomatida Beania mirabilis Johnston, 1840 13

Gulf and 32% of the 255 species in the English Channel [START_REF] Dauvin | Mise à jour de la liste des espèces d'Amphipodes (Crustacea : Peracarida) présents en Manche[END_REF].

The high-level of species richness in Chausey is partly due to a rich naturalist history spanning 180 years. However, it also corresponds to particular environmental conditions. Chausey is located in the Normand-Breton Gulf, dominated by a complex and fragmented mosaic of benthic habitats [START_REF] Retière | Contribution à la connaissance des peuplements benthiques du golfe normano-breton[END_REF]. Additionally, it is situated in the Western part of the English Channel, which has greater spatial heterogeneity than the Eastern part. This location explains the presence of both Lusitanian and Boreal species. For example, according to the geographical-ecological distribution typology of [START_REF] Lincoln | British Marine Amphipoda: Gammaridea[END_REF], the Gammaridea reported in Chausey are essentially composed of temperate species (56%), but coldtemperate (21%), and warm-temperate species (18%) are also present (three Gammaridea species are not reported in the typology of Lincoln).

In addition to listing the most common and widespread species reported in Chausey, comments are included below for five different categories of species.

Rare and localized species on a regional scale

Parazoanthus anguicomus

The report of this species from Chausey is only the second record in the Normand-Breton Gulf, the first record being from the bay of Saint-Malo [START_REF] Girard-Descatoire | Inventaire de la faune et de la flore sur les fonds rocheux de la baie de Saint-Malo-Dinard. Rapport ADMS[END_REF].

Clymenura tricirrata

Initially described in the Marseille area (France) as Leiochone tricirrata (see [START_REF] Bellan | Leiochone tricirrata n.sp., une maldanidae nouvelle de la région de Marseille[END_REF], this species was not reported by [START_REF] Dauvin | Liste actualisée des espèces d'Annélides Polychètes présentes en Manche[END_REF] in the English Channel. It has been recently recorded in the Pertuis Charentais Bay (De Montaudouin & Sauriau, 2000) and, in 2007, at five stations in Normandy near Granville, the Havre de Lessay, Cherbourg, and the Cap Lévi [START_REF] Olivier | Acquisition des données biologiques (Invertébrés de substrat meuble) complémentaires à la définition du réseau de surveillance en application de la Directive Cadre sur l'Eau[END_REF].

Euclymene droebachiensis

Initially mentioned by [START_REF] Godet | L'évaluation des besoins de conservation d'un patrimoine naturel littoral marin. L'exemple des estrans meubles de l'archipel de Chausey[END_REF] as Euclymene sp., upon reexamination, the specimens collected by this author correspond to Euclymene droebachiensis as described by [START_REF] Garwood | Family Maldanidae. A guide to the species in waters around the British Isles[END_REF]. This record has been confirmed by new specimens collected by Grant (in prep). According to [START_REF] Dauvin | Liste actualisée des espèces d'Annélides Polychètes présentes en Manche[END_REF], this species has not been reported from the English Channel.

Schistomeringos caeca

This species, also not recorded by [START_REF] Dauvin | Liste actualisée des espèces d'Annélides Polychètes présentes en Manche[END_REF] for the English Channel, has been recently reported from the Eastern part, in Wimereux (France) and Belgium [START_REF] Müller | Faune et flore du littoral du Nord, du Pas-de-Calais et de la Belgique : inventiare[END_REF]. The record of [START_REF] Godet | L'évaluation des besoins de conservation d'un patrimoine naturel littoral marin. L'exemple des estrans meubles de l'archipel de Chausey[END_REF] is thus a new report for the Western part of the English Channel, and corresponds to two individuals collected in a Lanice conchilega bed from the Western side of the Archipelago in 2005.

Fimbriosthenelais zetlandica

The collection of this species in Chausey (as Sthenelais papillosa) surprised [START_REF] Benard | [END_REF], who mentioned that the species has only been previously reported in South Africa. However, according to [START_REF] Pettibone | Partial revision of the genus Sthenelais Kinberg (Polychaeta: Sigalionidae) with diagnoses of two new genera[END_REF], this species seems to have a large distribution in the North and South Atlantic, stretching from Norway, Skagerrak, Shetland Islands, Great Britain, and the Isle of Man, to Northwest Africa (Cape Verde Islands) and South Africa (False Bay) where it is found between 33 to 558 meters depth. The data from [START_REF] Benard | [END_REF] may constitute the first record for the English Channel, and the first report of the species in the intertidal zone.

Hesiospina similis

This species, collected in Chausey by [START_REF] Benard | [END_REF], was not reported by [START_REF] Dauvin | Liste actualisée des espèces d'Annélides Polychètes présentes en Manche[END_REF] in the English Channel. Initially described in Japan, this species has been recorded in the Bay of Galway (O'Connor & Shin, 1983).

Corophium bonelli

The presence of this boreal species is of special interest, considering its scarcity and the retreat of its distribution in the Western English Channel [START_REF] Lincoln | British Marine Amphipoda: Gammaridea[END_REF]. This retreat may correspond with the progression of the alien species, Corophium sextonae, and probably with the current global warming context.

Stegocephaloides christianiensis

The unique specimen collected by [START_REF] Godet | L'évaluation des besoins de conservation d'un patrimoine naturel littoral marin. L'exemple des estrans meubles de l'archipel de Chausey[END_REF], in an intertidal Lanice conchilega bed of the archipelago, corresponds to the first report from the English Channel according to a recent checklist of amphipods [START_REF] Dauvin | Mise à jour de la liste des espèces d'Amphipodes (Crustacea : Peracarida) présents en Manche[END_REF]. In France, this species has been reported from the southern Bay of Biscay [START_REF] Bachelet | An updated checklist of marine and brackish water Amphipoda (Crustacea: Peracarida) of the southern Bay of Biscay (NE Atlantic)[END_REF], in the deep sea (Cap Ferret canyon, 346-1098 m) and on the continental shelf (Capbreton canyon, 1000 m). From the Arctic Ocean to the Western Mediterranean Sea, this species is known to be subtidal in distribution (depth range about 40-750 m; [START_REF] Lincoln | British Marine Amphipoda: Gammaridea[END_REF].

Terrestrial species seldom reported in marine inventories

Halophiloscia couchi, Armadillidium album,

Miktoniscus patiencei

With the exception of Ligia oceanica, halophytic woodlice are rarely reported by marine biologists according to whom they are terrestrial species. yet, Halophiloscia couchi, Armadillidium album and Miktoniscus patienci are true coastal species, restricted to salt-marshes and high tide marks along cliffs, or in shingle and boulder beaches [START_REF] Hopkin | A key for the woodlice of Britain and Ireland[END_REF][START_REF] Livory | Les crustacés isopodes de la Manche[END_REF]. Halphiloscia couchi is common along European shores and on the main island of Chausey archipelago. Armadillidium album has recently been recorded in Chausey and in numerous parts of the coast of the Cotentin peninsula [START_REF] Livory | Les crustacés isopodes de la Manche[END_REF]. Miktoniscus patiencei is found much less-frequently, with very scattered records, mainly distributed along the English Channel and the South Irish coasts. It is very rare on French shores, and has been only recently been reported on the main island of Chausey archipelago [START_REF] Livory | Les crustacés isopodes de la Manche[END_REF].

Halorates reprobus

The first report of this species in France is from Chausey (Gadeau de Kerville, 1894b). This spider is a strictly coastal species, inhabiting the zone from the high tide mark to the Fucus serratus belt.

Enopolognatha mordax

This spider has seldom been reported by benthic biologists, despite being common in similar intertidal areas of the region, such as at the Bay of the Mont-Saint-Michel [START_REF] Pétillon | Responses of spider communities to salinity and flooding in a tidal salt marsh (Mont St.Michel Bay, France)[END_REF]. Three specimens of E. mordax were collected by [START_REF] Godet | L'évaluation des besoins de conservation d'un patrimoine naturel littoral marin. L'exemple des estrans meubles de l'archipel de Chausey[END_REF] in Lanice conchilega beds of the Western part of the archipelago.

Invasive species

Some species, introduced in European waters during the 19 th Century or at the beginning of the 20 th Century, are now extensively established in the Normand-Breton Gulf and are present in the Chausey archipelago.

Haliplanella luciae

H. luciae was introduced to Europe, possibly from Japan, in the 19 th Century probably via imported mussels and oysters, but also from transoceanic transport on ship hulls [START_REF] Gollasch | Transoceanic dispersal of benthic macrofauna: Haliplanella luciae (Verrill, 1898) (Anthozoa, Actinaria) found on a ships hull in a shipyard dock in Hamburg harbour, Germany[END_REF]. Today, although this species has been reported from several sites, long-term settlements have not been detected in European seas [START_REF] Gollasch | Transoceanic dispersal of benthic macrofauna: Haliplanella luciae (Verrill, 1898) (Anthozoa, Actinaria) found on a ships hull in a shipyard dock in Hamburg harbour, Germany[END_REF].

Elminius modestus

This antipodean cirripede was first introduced to European waters in the 1950s, probably via ships and flying boats during World War II [START_REF] Bishop | Establishment of an immigrant barnacle in British coastal waters[END_REF]. It was reported in Brittany at the beginning of the 1950s [START_REF] Goulletquer | Open Atlantic coast of Europe-A century of introduced species into french waters[END_REF]. In the Normand-Breton Gulf, larvae were abundant in plankton samples collected in 1952 [START_REF] Franc | La répartition du zooplancton dans les eaux de Dinard[END_REF].

Balanus improvisus

This species, native to the North-West Atlantic, was introduced to France in the 19 th Century [START_REF] Goulletquer | Open Atlantic coast of Europe-A century of introduced species into french waters[END_REF]. Today, it has a wide distribution in the Normand-Breton Gulf, where it was first reported in 1864 [START_REF] Ansted | The Channel Islands[END_REF].

Corophium sextonae

The origin of this species is unknown. According to [START_REF] Turquier | A propos des Corophium (Crustacea, Amphipoda) des côtes françaises de la Manche[END_REF], Crawford suggested that this species is a non-native species for the English Channel, mainly because it is totally absent from early specimen collections. However, [START_REF] Turquier | A propos des Corophium (Crustacea, Amphipoda) des côtes françaises de la Manche[END_REF] encountered the species in several sites along the coasts of Normandy and Brittany and concluded that its absence from the historic collection probably arose from its potential confusion with C. acherusicum.

In addition to these species, there are other exotic and potentially very invasive species, mainly introduced via shellfish culture. [START_REF] Noël | [END_REF] considered the unpublished record of Lubet in 1978 as dubious, because the spatial range of this species is much more southern. However, a specimen was also collected in Jersey in 1994 [START_REF] D'udekem D'acoz | Inventaire et distribution des crustacés décapodes de l'Atlantique nord-oriental, de la Méditerranée et des eaux continentales adjacentes au nord de 25[END_REF]. This last record reflects that even if there are no population settled in the Normand-Breton Gulf, isolated specimens can be found in this area, perhaps due to the exchanges between this area and the Bay of Biscay for shellfish farming.

Eriphia verrucosa

Ruditapes philippinarum

This species, native to the Western Pacific Ocean, has been introduced to several European countries and has since been naturalized in Italy, France, and Britain [START_REF] Goulletquer | History, present conditions and future of the molluscan fisheries of North America and Europe. Marine molluscan production trends in France: from fisheries to aquaculture[END_REF]. In Chausey, this species is cultivated, and the archipelago is the main national production site [START_REF] Toupoint | Does Manila clam cultivation affect habitats of the engineer species Lanice conchilega (Pallas, 1766)?[END_REF].

Crassostrea gigas

Since the mid-1990s, this oyster species has become invasive on the intertidal reefs of northern Brittany, especially near Brest and the Bay of Saint-Brieuc. In the Bay of Mont-Saint-Michel, this species has become invasive on Sabellaria alveolata reefs [START_REF] Dubois | Effects of epibionts on Sabellaria alveolata (L.) biogenic reefs and their associated fauna in the Bay of Mont Saint-Michel[END_REF] and on the sea walls of Granville harbour. considered as an additional record of Halichondria sp. (Ecosub, 2006). Confusions between H. panicea and H. bowerbanki, the later being the only species positively identified in the Rance basin and the surrounding areas of Saint-Malo (C. Lévi, pers. com.), are possible. Additional sampling would be necessary to perform microscopic examinations of the collected specimens in order to confirm the species.

Caryophyllia inornata (Duncan, 1878)

Identification by visual observation of C. inornata by Ecosub (2006) has been rejected, and not integrated in this list. It is very similar to the much more common C. smithii, which has also been reported in Chausey. young specimens of C. smithii are highly-variable in form [START_REF] Zibrowius | Les Scléractiniaires de la Méditerranée et de l'Atlantique nord-oriental[END_REF], and may be virtually indistinguishable from C. inornata [START_REF] Manuel | The Anthozoa of the British Isles -a colour guide[END_REF]. Consequently, visual identification by diving cannot be accepted, and additional data are required to confirm the potential presence of C. inornata in Chausey.

Trapania maculata Haefelfinger, 1960

The identification of T. maculata by Ecosub (2006) to the species level has been rejected and only taken into account to a genus level. T. maculata is a rare Mediterranean dorid nudibranch, first recorded in the UK at Portland in May 1974 [START_REF] Brown | Trapania maculata Haefelfinger, a doridacean nudibranch new to the British fauna[END_REF]. The data in the English Channel could be due to confusion with T. pallida (DORIS, 2008). Additional data are therefore still required to confirm the report from Chausey.

Antalis entalis

The existing data on this species in the Normand-Breton Gulf are relative to dead specimens (shells), and thus with some potential misidentifications [START_REF] Marshall | Additions to "British Conchology[END_REF]. The data of [START_REF] Locard | Prodrome de malacologie française[END_REF] and Lubet (1968) should therefore be verified.

Chtamalus stellatus & Chtamalus montagui

According to [START_REF] Southward | On the taxonomic status and distribution of Chtamalus stellatus (Cirripedia) in the north-east Atlantic region with a key to the common intertidal barnacles of britain[END_REF], only C. montagui should be found in the archipelago. Indeed, this was the only species found during a personal investigation in 2009 on the 'Grande Ile'. Additional data are still required to validate the reports of C. stellatus in the archipelago.

Callianassa subterranea (Montagu, 1808) & Callianassa tyrrhena While C. subterranea was reported by [START_REF] Fauvel | Histoire Naturelle de la presqu'île du Cotentin -Chapitre III : La Faune[END_REF] and [START_REF] Bertrand | Les Crustacés Malacostracés de la région dinardaise[END_REF] in Chausey, they did not mention C. thyrrenea, the only species of the genus found by recent authors in Chausey and the Normand-Breton Gulf. A misidentification is most likely responsible for this discrepancy. Even if C. subterranea has been reported in the nearby region of Jersey (Ngoc-Hô, 2003), we rejected the mentions of the species in Chausey.

Liocarcinus zariquieyi (Gordon, 1968) As with [START_REF] Noël | [END_REF][START_REF] D'udekem D'acoz | Inventaire et distribution des crustacés décapodes de l'Atlantique nord-oriental, de la Méditerranée et des eaux continentales adjacentes au nord de 25[END_REF] contests the record of [START_REF] Benard | [END_REF], and both these authors consider its listing as a potential confusion with L. pusillus. There are no other available data in Chausey, and the record of this species in Chausey has been rejected and not integrated in our data.

Macropodia parva van Noort & Adema, 1985[START_REF] D'udekem D'acoz | Inventaire et distribution des crustacés décapodes de l'Atlantique nord-oriental, de la Méditerranée et des eaux continentales adjacentes au nord de 25[END_REF] wondered about the validity of the original description of M. parva and its distinction from M. rostrata. Because of this problem we considered the observations M. parva of [START_REF] Livory | Rubrique Chausey[END_REF]Livory ( & 1997) ) as M. rostrata, which is very common in Chausey.

Xantho poressa (Olivi, 1792) This species, reported in Chausey archipelago by Lubet (1968), is mainly a Mediterranean species with a very restricted Atlantic distribution [START_REF] D'udekem D'acoz | Inventaire et distribution des crustacés décapodes de l'Atlantique nord-oriental, de la Méditerranée et des eaux continentales adjacentes au nord de 25[END_REF]. [START_REF] Noël | [END_REF] attribute Lubet's record to Xantho incisus, which is fairly common in the Normand-Breton Gulf. The two species are very similar [START_REF] D'udekem D'acoz | Inventaire et distribution des crustacés décapodes de l'Atlantique nord-oriental, de la Méditerranée et des eaux continentales adjacentes au nord de 25[END_REF], and thus we reported the record of Lubet (1968) as Xantho sp.

Thracia pubescens (Pulteney, 1799) The only other data relative to this species in the Normand-Breton Gulf are early testimonies from the end of the 19 th Century in Guernsey (Duncan, 1841[START_REF] Chambers | Channel Island Marine Molluscs, An Illustrated Guide to All the Species from Jersey, Guernsey, Alderney, Sark and Herm[END_REF]. [START_REF] Jeffreys | British conchology or an account of the mollusca which now inhabit the British Isles and the surrounding seas. Volume III. Marine Shells, comprising the remaining conchifera, the Solenoconchia and Gasteropoda as far as Littorina[END_REF] and [START_REF] Locard | Prodrome de malacologie française[END_REF] did not take into account this record. In Chausey, additional data would be welcome: whereas adults are unmistakable, young specimens are very similar to T. papyracea, a species present in Chausey.

Figure 1 .

 1 Figure 1. Location map of the study site.Figure 1. Carte de localisation du site d'étude.

Figure 1 .

 1 Figure 1. Location map of the study site.Figure 1. Carte de localisation du site d'étude.

Figure 2 .

 2 Figure 2. Total number of marine invertebrates reported in Chausey, sorted by phyla.Figure 2. Nombre total d'invertébrés marins recensés à Chausey, classés par phyla.

Figure 2 .

 2 Figure 2. Total number of marine invertebrates reported in Chausey, sorted by phyla.Figure 2. Nombre total d'invertébrés marins recensés à Chausey, classés par phyla.

Sabellaria alveolata (Linnaeus, 1767) 17 Sabellaria spinulosa Leuckart, 1849 22 Subclass Archiannellida Archiannelida sp. 30 Saccocirrus papillocercus Bobretzky, 1872 48,52 Phylum Mollusca Class Polyplacophora

	Clelandella miliaris (Bocchi, 1814)		5	Filograna sp. Oken, 1815		46
	Diodora graeca (Linnaeus, 1758)		9,17,27,35	Megalomma vesiculosum (Montagu, 1815)		17,30,48
	Emarginula fissura (Linnaeus, 1758)	,49,52 5,6	Pomatoceros lamarcki (Quatrefages, 1865)		35,46
	Eumida punctifera (Grube, 1860) Gibbula cineraria (Linnaeus, 1758)	17	5,	Pomatoceros triqueter (Linnaeus, 1767)		22,52
	Eumida sanguinea (Oersted, 1842)	30,48		Sabella discifera Grube, 1874		46
	Eunereis longissima (Johnston, 1839)	30,50,52	Sabella pavonina Savigny, 1820		46,48,49
	Exogone (Exogone) naidina Örsted, 1845	48,52		Sabella spallanzanii (Viviani, 1805)		46
	Exogone (Parexogone) hebes (Webster & Benedict, 1884) 30,48,51,52	Salmacina dysteri (Huxley, 1855)		46,49
	Fimbriosthenelais zetlandica (McIntosh, 1876)	30		Spirorbis spirorbis (Linnaeus, 1758)		9
	Glycera capitata Oersted, 1842	30,52		Order Spionida
	Glycera gigantea Quatrefages, 1865	30,48		Aonides oxycephala (Sars, 1862)		30,48,52
	Glycera lapidum Quatrefages, 1865	52		Aonides paucibranchiata Southern, 1914		52
	Glycera oxycephala Ehlers, 1887	48,52		Dipolydora flava Claparède, 1870		48
	Goniada emerita Audouin & Milne-Edwards, 1832	14		Malacoceros ciliatus (Keferstein, 1862)		30
	Goniadella bobrezkii (Annenkova, 1928)	30,48,52	Malacoceros girardii Quatrefages, 1842		30
	Harmothoe extenuata (Grube, 1839)	9,48,52		Malacoceros tetraceros (Schmarda, 1861)		30
	Harmothoe impar (Johnston, 1838)	48		Malacoceros vulgaris (Johnston, 1826)		48
	Hediste diversicolor (O.F. Müller, 1776)	31,48		Microspio mecznikowianus (Claparède, 1869)		51
	Hesionura elongata (Southern, 1914)	52		Pseudopolydora antennata (Claparède, 1868)		30,48,52
	Hesiospina similis (Hessle, 1924)	30		Pygospio elegans Claparède, 1863		48,50,51
	Kefersteinia cirrata (Keferstein, 1862)	52		Scolelepis squamata (O. F. Müller, 1789)		30,48,52
	Lepidonotus squamatus (Linnaeus, 1758)	30		Spio decoratus Bobretzky, 187		48,50,51,52
	Malmgreniella arenicolae (de Saint Joseph, 1888)	30,48,50,52	Spio filicornis (O. F. Müller, 1776)		30,48,52
	Microphthalmus pseudoaberrans Campoy & Vieitez, 1982 52		Spio martinensis Mesnil, 1896		48,51,52
	Myrianida prolifera (O.F. Müller, 1788)	52		Spiophanes bombyx (Claparède, 1870)		30,48,50
	Neanthes irrorata (Malmgren, 1867)	17,30,48	Order Spioniformia
	Nephtys caeca (Fabricius, 1780)	9,30,48,52	Ampharete acutifrons (Grube, 1860)		48,50,51
	Nephtys cirrosa Ehlers, 1868	30,48,52	Ampharete baltica Eliason, 1955		14,30
	Nephtys hombergii Savigny, 1818	30,48,50,51,52	Amphicteis gunneri (M. Sars, 1834)		48
	Nephtys incisa Malmgren, 1865	30		Aphelochaeta marioni (de Saint Joseph, 1894)		48
	Odontosyllis sp. Claparede, 1863	30		Caulleriella alata (Southern, 1914)		30
	Palposyllis prosostoma Hartmann-Schröder, 1977	30,52		Chaetozone setosa Malmgren, 1867		30,48,50,51
	Parapionosyllis minuta (Pierantoni, 1903)	48,52		Cirriformia tentaculata (Montagu, 1808)		9,30,48,50,51,52
	Perinereis cultrifera (Grube, 1839)	48,52		Euphrosine foliosa Audouin & Milne-Edwards, 1832	14
	Pholoe inornata Johnston, 1838	30,48		Eupolymnia nebulosa (Montagu, 1818)		14,17
	Phyllodoce (Anaitides) mucosa Oersted, 1842	30,48,50	Lanice conchilega (Pallas, 1766)	1,17,30,35,48,51,52
	Plakosyllis brevipes Hartmann-Schröder, 1956	52		Melinna palmata Grube, 1870		14,30,51
	Platynereis dumerilii (Audouin & Milne-Edwards, 1832) 9,48		Pista cristata (O. F. Müller, 1776)		48,52
	Polynoe laevis Audouin & Milne Edwards, 1831	C3	H	Poecilochaetus serpens Allen, 1904		30,48
	Polynoe scolopendrina Savigny, 1818	9
	Protomystides bidentata Lagerhans	52
	Pseudomystides limbata de Saint Joseph, 1888	30
	Salvatoria swedmarki (Gidholm, 1962)	52
	Sigalion mathildae Audouin & Milne Edwards in Cuvier, 1829 14	S
	Sphaerosyllis bulbosa Southern, 1914	30,48,5252
	Sphaerosyllis glandulata Perkins, 1981	52
	Sphaerosyllis taylori Perkins, 1981	30,52
	Sthenelais boa (Johnston, 1832)	30
	Streptosyllis bidentata Southern, 1914 Streptosyllis campoyi Brito, Núñez & San Martín, 2000 52 30 Streptosyllis websteri Southern, 1914 48,52 Syllides sp. Örsted, 1845 52 Syllis amica Quatrefages, 1865 30 Syllis armillaris (O.F. Müller, 1776) 30 Syllis cornuta Rathke, 1843 30 Syllis garciai (Campoy, 1982) 48,52 Syllis hyalina Grube, 1863 30 Syllis pontxioi San Martín & López, 2000 48,52 Synmerosyllis (Pionosyllis) lamelligera Saint Joseph, 1887 52 Trypanosyllis coeliaca Claparède 1868 52 Websterinereis glauca (Clarapède, 1870) 48 Order Sabellida Bispira volutacornis (Montagu, 1804) 46,49 Fabricia sabella (Ehrenberg, 1835) 48		Order Neoloricata Callochiton septemvalvis (Montagu, 1803) Order Lepidopleurida Acanthochitona crinita (Pennant, 1777) Acanthochitona fascicularis (Linnaeus, 1767) Leptochiton asellus (Gmelin, 1791) Lepidochitona cinerea (Linnaeus, 1767) Leptochiton cancellatus (Sowerby, 1839) Class Gastropoda Subclass Prosobranchia Order Archeogastropoda Acmaea virginea (O. F. Müller, 1776) Ansates pellucida (Linnaeus, 1758) Calliostoma granulatum (Von Born, 1778) Calliostoma zizyphinum (Linnaeus, 1758)		52 9,35,52 9,27 52 5,27,30,35,52 30 27,30 27 6 5,7,9,27,35,46,49

Acknowledgments

We especially thank Dr. Guillermo San Martin for identifying the Syllidae, Dr. Danny Eibye-Jacobsen for the Ampharete, and Dr. Julien Pétillon for the spiders recently collected in the archipelago. Thank you to Dr. Philippe Bouchet and Dr. Bernard Métivier for their help in the actualisation of the names of the molluscs, Dr. Jean-Loup d'Hondt for the bryozoa, Dr. Claude Lévi for the sponges and Dr. Françoise Monniot for the tunicates. We thank Claude Nozères and Amélia Curd for improving the English of the manuscript.

Discussion

The high level of diversity (769 marine invertebrate species) is particularly remarkable when considering the relatively small size of the study area (5000 ha). Chausey thus hosts a significant portion of the total species diversity of the Normand-Breton Gulf and the English Channel.

Considering only Polychaeta and Amphipoda (two wellstudied taxonomic groups in the region) 146 polychaete species were found in Chausey, representing 55% of the 265 species of the Normand-Breton Gulf and 30% of the 493 species recorded in the English Channel [START_REF] Dauvin | Liste actualisée des espèces d'Annélides Polychètes présentes en Manche[END_REF], while the 82 amphipod species in Chausey represented 42% of the 194 species in the Normand-Breton

Mytilus edulis

The native mussel species of the Normand-Breton Gulf is Mytilus galloprovincialis, whereas M. edulis is a cultivated species. In Chausey, small M. edulis beds form on intertidal sandflats adjacent to mussel concessions.

Crepidula fornicata

This well-known, North American native species was reported in France for the first time in 1949 in Brest and Hemanville [START_REF] Goulletquer | Open Atlantic coast of Europe-A century of introduced species into french waters[END_REF]. It colonized the Normand-Breton Gulf in the 1970s [START_REF] Retière | Contribution à la connaissance des peuplements benthiques du golfe normano-breton[END_REF][START_REF] Dupouy | Le développement de la crépidule sur le gisement de coquilles Saint-Jacques de la baie de Saint-Brieuc[END_REF]) and now attains high-densities, especially in the bays of Saint-Brieuc, Mont Saint-Michel, Granville, and in the Chausey archipelago (Blanchard & Erhold, 1999).

Species whose identification or taxonomic position is problematic

Myriochele oculata

The specimens recently collected in Chausey [START_REF] Godet | L'évaluation des besoins de conservation d'un patrimoine naturel littoral marin. L'exemple des estrans meubles de l'archipel de Chausey[END_REF][START_REF] Nebout | Etude des peuplements benthiques Ouest et Nord Cotentin dans le cadre du contrôle de surveillance REBENT DCE[END_REF]Nebout & Olivier, in prep.) were first identified as Myriochele heeri Malmgren, 1867 with the fauna of [START_REF] Fauvel | Polychètes sédentaires[END_REF]. The reexamination of these specimens for the present study using the key proposed by [START_REF] Parapar | Oweniidae (Annelida, Polychaeta) from Icelandic waters, collected by the BIOICE project, with a description of Myrioglobula islandica n. sp[END_REF] resulted in their identification as Galathowenia oculata (referenced here as Myriochele oculata, following the European Register of Marine Fauna [START_REF] Costello | European Register of Marine Species. A check-list of the marine species in Europe and bibliography of guides to their identification[END_REF]). According to [START_REF] Martin | Revisión de las especies de Owenidae (Annelida, Polychatea) de la Península Ibérica[END_REF], the two species M. oculata and M. heeri have been repeatedly confused in the past.

Syllidae family

Syllids are small polychaetes of several millimetres, typically found in subtidal coarse sediments and rocky environments. Their identification is complex, mainly based on the presence of ventral cirri, the type of palps and chaetae.

During the last century, benthologists classically used Fauvel's fauna [START_REF] Fauvel | Polychètes errantes[END_REF] to identify syllids, but recently San Martin and other contributing authors totally revised this family. The Fauna Iberica Vol. 21 is dedicated to the family of Syllidae in areas of Spanish North Africa (San [START_REF] Martin | Annelida, Polychaeta II: Syllidae[END_REF] and we used this guide when identifying syllids. A great deal of confusion and revisions have been brought to light when compared with [START_REF] Fauvel | Polychètes errantes[END_REF]; we thus strongly recommend the use of Fauna Iberica fauna or recent scientific publications when examining Syllidae.

Ampharete baltica & Ampharete acutifrons

According to [START_REF] Dauvin | Liste actualisée des espèces d'Annélides Polychètes présentes en Manche[END_REF], Ampharete baltica is the unique polychaete of this genus reported in the English Channel. However, the specimens collected by [START_REF] Godet | L'évaluation des besoins de conservation d'un patrimoine naturel littoral marin. L'exemple des estrans meubles de l'archipel de Chausey[END_REF] in Chausey have been identified as A. acutifrons, as described by [START_REF] Holthe | Polychaeta terebellomorpha[END_REF]. Moreover, the identification of similar specimens collected in the Bay of the Mont-Saint-Michel as A. acutifrons has been confirmed by Eibye-Jacobsen (pers. comm.). The specimens collected in this bay and in Chausey were subadults (length < 40 mm), which has two consequences: the palae are not as thick and are clearly different from subsequent notochaete, as one would expect in adults, and the dorsal cirri on the dorsal edge of the last thoracic and all abdominal neuropodia are shorter than in adults (Eibye-Jacobsen, pers. comm.). Moreover, the presence of these dorsal cirri and the fact that the two groups of branchiae are well-separated from each other distinguish A. acutifrons from A. baltica (Eibye-Jacobsen, pers. comm.). According to us, the specimens collected in Chausey by [START_REF] Benard | [END_REF] and identified as A. baltica should be re-examined.

Spisula solida vs Spisula ovalis

On the CLEMAM website which is considered as a reference for valid taxonomic names of molluscs, only Spisula solida (L., 1758) is accepted. However, a note indicates that Spisula ovalis (Sowerby, 1817) is suggested as being distinct based on morphometric evidence [START_REF] Glémarec | Distinction de deux Mactridae des côtes atlantiques européennes: Spisula solida (Linné) et Spisula ovalis (Sowerby)[END_REF]. Our Spisula individuals may be related to S. solida when using the fauna of [START_REF] Tebble | British bivalve seashells[END_REF], but the shell is more elongated (lower H/L ratio) than in the description. When considering the description of Spisula ovalis by [START_REF] Glémarec | Distinction de deux Mactridae des côtes atlantiques européennes: Spisula solida (Linné) et Spisula ovalis (Sowerby)[END_REF], we undoubtedly concluded that our individuals belong to this latter species. To our knowledge, no works have focused on genetical differences between these two species. Without further research on Spisula phylogeny, we reasonably consider that the specimens collected in Chausey belong to the species Spisula ovalis.

Leptosynapta spp.

The identification of the species of this genus by several authors has been fraught with difficulties. An example can be seen in the analyses of specimens collected in Chausey by De Quatrefages (1854), initially described as Leptosynapta duvernea sp. nov. (= Leptosynapta inhaerens Müller, 1776). [START_REF]Les échinodermes de Bretagne. III. Distribution dans le golfe normano-breton[END_REF] re-examined these data and concluded that these specimens belonged to four different species: L. bergensis, L. cruenta, L. galliennii, L. inhaerens.

Species for which additional data are required

Halichondria spp.

The identification by visual observation of Halichondria to the species level has previously been rejected and only