Aymen Assadi

Amine

Bouzaza Abdelkrim
email: abdelkrim.bouzaza@ensc-rennes.fr

Wolbert Dominique

Comparative study between laboratory and large pilot scales for VOC's removal from gas streams in continuous flow surface discharge plasma

Keywords: Scaling-up, Continuous reactor, surface discharge plasma, VOCs treatment, byproducts

This work investigated the performance of Isovaleraldehyde (3-methylbutanal) removal from gas streams using continuous flow surface discharge plasma at room temperature. The feasibility of pollutant removal using up-scaled reactor was systematically assessed by monitoring removal efficiency and mineralization at different operational parameters, such as specific energy, air flow rate and inlet concentration. Results show that increasing flow rate lead to improve the removal capacity. For example, when flow rate extends two times, the removal capacity varies from 0.6 to 1.1 g.h 1 . Moreover, when specific energy increased, both removal capacity and mineralization were enhanced. Additionally, a comparison between laboratory and pilot scales using surface discharge plasma system was carried out.

A methodology of scaling up the surface discharge plasma system was proposed. In this context, removal capacities were compared for different continuous reactors: two reactors at laboratory scale (planar and cylindrical reactor), and pilot unit. The results suggest that the plasma reactor scale-up for pollutant removal can be feasible.

Moreover, a special attention was given to high flow rate parameter (from 250 to 500 m 3 .h -1 ) and its effect on the reactor performance, which is innovative in comparison to the latter studies. Here, isovaleraldehyde (isoval) was chosen since this pollutant was the main molecule detected in the exhaust gases from animal quartering centers (ADEME, 2005).

Another goal of the present work is to investigate different reactors scales (laboratory and pilot) in order to see the possibility of the scale-up of the process.

Experimental setup

The used experimental setup was structured as follows: (i) continuous reactors, (ii) plasma system and (iii) analysis set-up. The experiments were carried out in ambient conditions, i.e. room temperature and pressure.

Two lab scale reactors were tested. The first one, planar reactor, consisted of a rectangular cross section (135 mm × 135 mm) and is 1 m length. It contained two plates, 4 mm thickness, which were arranged parallel to the length of the reactor and permit to hold up the two electrodes (Fig. 1b). The second one, cylindrical reactor, was composed principally of a glass tube (58 mm id and 100 cm length). It was covered by a copper grid forming the outer electrode. The glass tube, 4 mm thickness, acts as the dielectric media. The High Voltage (HV) electrode was a helicoidally wire shaped as a coil spring in close contact with the inner wall of the reactor (Fig. 1a). A detailed description of the geometry of the plasma reactor and electrical measurements was published elsewhere [START_REF] Guillerm | Removal of gas-phase ammonia and hydrogen sulfide using photocatalysis, nonthermal plasma, and combined plasma and photocatalysis at pilot scale[END_REF]. The unit comprised a pre-ltration box, a cooling bank, an electric heater (box no. 1), a vapor humidi er (box no. 2), a pollutant in ection area (box no. 3), an upstream pollution measurement box, a surface plasma treatment system, a downstream concentration measurement box (box no. 4); a fan (box no. 5) and nally an activated carbon ltration (box no. 6). The ventilation box consisted of a medium-pressure centrifugal fan. The used pilot was described in detail in our previous work (Assadi et al., 2014c).

Plasma system

The experimental set-up for surface discharge production was similar to the planar reactor. In fact, one side of the partition was provided with metallic grid forming the HV electrode. The total surface area of each lab reactor was 0.18m 2 . As seen in figure 3, the other side was formed of a copper sheet that served as a ground electrode. The two electrodes were separated by a dielectric sheet of glass (thickness 4 mm). These elements were tied together and this configuration was reproduced six times forming six parallel channels. The polluted gas flowed through these channels. The details concerning the surface discharge plasma system of the pilot unit were shown in figure 3. The air-isovaleraldehyde gas mixture was prepared by passing synthetic air (Air Liquide) through liquid isovaleraldehyde (Sigma-Aldrich, 97%). In fact, the pollutant (liquid) was firstly pressurized with air in a stainless steel tank (500 mL). Then, it was heated, vaporized and mixed with a zero-air ow in an especially designed Bronkhorst vaporization/mixing chamber (CEM). In these conditions, the inlet concentrations ranged from 2 to 10 mg.m -3 .

Analysis set-up

The experiment was carried out at room temperature and atmospheric pressure. The temperature and relative humidity were measured by a TESTO sensor. The gases (direct sampling or after concentration) were analysed by means of gas chromatography with a flame ionisation detector (FID) or mass spectrometry (MS). CO and CO 2 analysers were used to monitor the carbon monoxide and dioxide. Ozone formed in the plasma reactor was measured by bubbling it through a suitable liquid phase of KI. The analysis system was largely described in previous studies (Assadi et al., 2012(Assadi et al., , 2013(Assadi et al., and 2014a)).

Results and discussion

Experimental parameters were defined as follows: (C in ) and (C out ) represented inlet and outlet concentration of pollutant (mg/m 3 ) respectively.

The specific energy (SE) was defined as the energy deposited per unit volume of the gas flow: SE (J.L -1 ) = P/(Q*1000)

(1)

where P was the input power (W) and Q denoted the total gas flow rate ( m 3 .s 1 ).

To evaluate the plasma process, the following parameters are employed:

The removal capacity (R) which was calculated from:

. . (%) 100 in C R Q IRE (2)
where IRE(%) isovaleraldehyde removal was defined as

-out in IRE % = × 100 % in C C C (3)
The

Pilot unit efficiency

The reactor was already being ushed under surface discharge for 1hour when we started sampling for analysis. Indeed, the inlet and outlet gas were then sampled manually.

The experiments which were repeated two times; showed a good reproducibility with 5% standard deviation. This standard deviation was represented by vertical bars in the experimental results in all figures.

Moreover, the performance of pollutant removal using the pilot unit (500 m 3 .h -1 ) was systematically assessed by monitoring the removal efficiency, the mineralization and ozone formation at different operational parameters, such as specific energy, air flow rate and inlet concentration.

Removal capacity of Isoval

The removal capacity of isovaleraldehyde was investigated by studying the effect of specific In fact, the energy cost for conversion was higher for lower isoval initial concentrations, exceeding 2.9 keV/molecule for the lowest concentration investigated (3 mg/m 3 ). On the other hand, we noted that energy cost was flow rate independent because when we increased the flow rate we must increase also the power in order to keep SE constant.

CO and CO 2 formation

When increasing the specific energy, the discharges snatched more electrons from the gasphase inducing a higher concentration of reactive species in the plasma (Fig. 5). So, more reactive species were available to induce by-product mineralization.

Comparison of reactors

In the next step of this investigation, we compared results obtained at lab scale with the removal capacity of pilot unit. A methodology was proposed to scale-up surface discharge reactors, employing purely laboratory information, avoiding the need of using adjustable parameters for each different application of the same process. For this purpose, isovaleraldehyde removal was studied previously in a laboratory scale reactor.

As explained above, under our experimental conditions, many parameters were kept the same for the two reactors studied: residence time, feed of Isoval (flow rate × inlet concentration), relative humidity and temperature. Indeed, feed of Isoval and residence times for lab and pilot scale were kept constant and equal to 1.2 g.h -1 and 0.6s respectively (4 m 3 .h -1 for the cylindrical reactor , 10 m 3 .h -1 for the planar reactor and 250 m 3 .h -1 for pilot unit).

Conclusion

The removal of isovaleraldehyde was carried out on surface discharge at pilot scale with high flow rates.

The results showed that the increment's effect of specific energy seemed to be signi cant on the average removal capacity and on the overall selectivity to CO 2 . Additionally, the increase of flow rate leaded to improve the removal of pollutant due to enhancement of mass transfer step.

The removal capacities of isovaleraldehyde with three different scale reactors were compared and the feasibility of the scale-up process was demonstrated due to continuity of the experimental results obtained. Thus, the present study confirmed that the scale-up of the surface discharge plasma could have been possible from the laboratory reactor study. Thus, an industrial design of plasma reactor could be possible by using only data lab.

  (VOCs) had adverse effects on environment and human health. They are emitted from different outdoor sources (motor vehicles, incomplete combustion in industrial processes) (Le Cloirec, 1998) as well as from indoor sources (ADEME, 2005). Various methods have been developed for indoor air cleaning and VOCs A c c e p t e d M a n u s c r i p t Page 3/3 removal such as thermal and catalytic incineration (Li and Gong, 2010), absorption (Hsu and Lin, 2011), adsorption (Kim and Ahn, 2012), condensation (Tan et al., 2005), biofiltration (Zehraoui et al., 2012), ozonation (Ogata et al., 2010) and photocatalysis (Hussain et al., 2011; Assadi et al., 2012). Although several chemical methods were used or studied for the effective treatment of VOCs, the environmental applications of non-thermal plasma (NTP) started at the beginning of the XX th century with the use of plasma-generated ozone for water depollution. Later on, this type of treatment process was successfully developed for air treatment (Khacef et al., 2006). The most significant advantage of NTP generated in ambient air was the production of highly reactive oxidizing radicals such as O and O , and also O 3 . Such species were produced at room temperature and at low energy cost compared to any alternative methods (Malik et al., 2011; Nunez et al., 1993). As a result, non-thermal plasma could easily break most chemical bonds of molecular pollutants at low temperatures and convert gas pollutants into end-products including CO, CO 2 , H 2 O (Cho et al., 2012). On the other hand, many researches on NTP technologies for processing gases show that NTP is very effective for gas treatment at low VOCs concentrations (Kim, 2004; Hammer, 2014). In fact, our previous research proved that some VOCs in gas streams can be degraded efficiently by surface discharge plasma at laboratory scale pilots with two different geometries (Assadi et al., 2014a). This present paper was to extend previous research at lab scale (Assadi at al., 2014a; Assadi et al., 2014b) by adding a new investigation about pilot scale of surface plasma application.

Fig1

  Fig1: Scheme and sectional drawing of the cylindrical (a) and planar (b) reactors

Figure 2 :

 2 Figure 2: A functional diagram of pilot unit supplied by CIAT

Figure 3 :

 3 Figure 3: Photography of Plasma system used in pilot unit

  energy yield E (eV/molecule) =3.21. 10 3 *P/[Q* (C in -C out )] (4) where 3.21. 10 3 was constant taking account Avogadro's number, molecular weight of Isoval and joule to electron-volt Conversion. The selectivities of CO and CO 2 which were de ned as: ] and [CO 2 ] were respectively the concentrations of carbon monoxide and carbon dioxide detected in the effluent gas as a result of Isoval removal, and [Isoval] conv was the concentration of Isovaleraldehyde converted by plasma surface discharge.A c c e p t e d M a n u s c r i p t Page 7/7 Ozone concentration: Ozone was well known to be an inevitable byproduct in a plasma process. It could be formed by the following reaction: O° + O 2 +M O 3 +M (1) where O°was atomic oxygen and was generated by O 2 dissociation due to its impact with high energy electrons, and M could be either molecular oxygen or molecular nitrogen (Atkinson et al., 2003). Moreover, several mechanisms occurred when non-thermal plasma was present since plasma produces various species such as high energy electrons,

  energy, flowrate and inlet concentration of pollutant (Figure 4.a). Firstly, figure 4.a showed that the increase of flowrate improved the removal capacity. In fact, at flowrate equal to 250 m 3 .h -1 and 3 mg.m -3 of inlet concentration, when SE extended six times, the removal capacity increased from 0.2 to 0.45 g.h 1 . This result was similar to those reported for trimethylamine and isovaleric acid at laboratory scale (Assadi et al., 2014a), for toluene (Vandenbroucke et al., 2011), H 2 S (Chen and Xie, 2013) and for NO x (Khacef et al.,2013).. On the other hand, for a selected flow rate, it was interesting to report that experimental results showed an enhanced removal capacity when increasing inlet results were found in previous studies (Assadi et al., 2014a). For diluted effluent, the oxidation rate was directly proportional to the inlet concentration. The degradation occurred to fit a pseudo first-order kinetic. At this specific concentration range, many reactive species remained available for the reaction. On the other hand, the experimental results about the effect of air flow rate (Q) on isovaleraldehyde removal were illustrated in Fig.4.a. In our case, despite the decrease of residence time, an improvement in the removal capacity was noticed. Indeed, at SE= 4.5 J.L -1 and when Q increased from 250 to 500 m 3 /h, removal capacity increased two times. The gas-phase mass transfer rate influenced the treatment capacity by producing a concentration gradient between the bulk and the discharge zone. For instance under transitional conditions, the removal capacity depended on gas-phase mass transfer rate and plasma reaction rate(Assadi et al., 2014d). At higher flow rate, the process become a chemically step controlled. Thus, it could be suggested that the increase of air flow was helpful to the transfer of isovaleraldehyde molecules from bulk to discharge zone (Vandenbroucke et al., 2011).

Figure 4

 4 Figure 4.a: Removal capacity of Isoval and energy yield vs. specific energy at different flow rates and different inlet concentrations using pilot unit.

Figure

  Figure 4.b: Energy yield vs specific energy at different flow rates and different inlet concentrations using pilot unit

Figure 5 :

 5 Figure 5: Variation of the mineralization vs. specific energy at different inlet concentrations and different flow rates

FigureFigure 7 :

 7 Figure 6: Variation of the CO selectivity (%) vs. specific energy at different inlet concentrations and different flow rates.

Figure 8

 8 Figure8showed the removal capacities of Isoval using different scale reactors. The results showed that the removal capacities with Pilot scale were about six times higher than that of the laboratory reactors, regarding to flow rate and plasma surface involved. Moreover, at high value of flow rate, the mass transfer was not a limited step. Thus, previous results and present study confirmed that the scale-up of the surface discharge plasma

Figure 8 :

 8 Figure 8: Different reactors and scales comparison vs. specific energy: Feed of Isoval = 1.2 g.h -1 , residence time=0.6s

Figure 3 :FigureFigure 6 :

 36 Figures

Figure 7 :

 7 Figure 7: Variation of the ozone vs. specific energy at different inlet concentrations and different flow rates

Same results were reported in other studies at laboratory scale (Lee et al., 2014; Zhang et al., 2014; Xiao et al., 2014). It was well known that at

  laboratory scale, the decrease of ow rate and inlet concentration lead to an increase of mineralization.

Acknowledgment

The authors gratefully acknowledged the financial support provided by the French National Research Agency (ANR) for this research work.