Adsorption and Electrothermal Desorption of Volatile Organic

Article pubs.acs.org/EF

Adsorption and Electrothermal Desorption of Volatile Organic Compounds and Siloxanes onto an Activated Carbon Fiber Cloth for Biogas Purification Sylvain Giraudet,* Benoit Boulinguiez,† and Pierre Le Cloirec Chimie et Ingénierie des Procédés, Ecole Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226, 11 Allée de Beaulieu, CS 50837, 35708 Rennes Cedex 7, France ABSTRACT: Although biogases mainly consist of a mixture of carbon dioxide and methane, traces of volatile organic compounds are present, and these undesirable compounds must be removed during the purification process. Adsorption onto an activated carbon fiber cloth (ACFC) was investigated and, in particular, the feasibility of electrothermal desorption. Five compounds were chosen, and their desorption was assessed by monitoring the electric resistance of the material as a function of the temperature. The results were confirmed by thermogravimetric analysis. Toluene, dichloromethane, and isopropanol were entirely desorbed at 420 K, whereas siloxane D4 and ethyl mercaptan (ethanethiol) were partially removed. These conclusions were confirmed by dynamic adsorption measurements. Cycles of adsorption followed by electrothermal desorption were first carried out for a single component (toluene). Although there was a loss of adsorption capacity between the first and second cycles, a steady performance was reached, shedding light on the complete reversibility of the adsorption of toluene. On the contrary, for the mixture of five organic compounds, a constant loss of adsorption capacity was measured after 3 cycles, which was attributed to the incomplete regeneration at 420 K of two organic compounds (ethanethiol and siloxane D4) from the ACFC from one cycle to another. The electrical consumption for the electrothermal desorption was highest at the beginning of the desorption and rapidly decreased as the temperature within the filter reached its set point. An average consumption was estimated at 1500 W kg−1 of activated carbon fiber cloth.

1. INTRODUCTION The production of biogas is increasing worldwide because it constitutes a renewable source of energy (heat and/or electricity), which is, moreover, produced locally. Anaerobic digestion (and its last step, methanogenesis) leads to the production of a biogas whose composition mainly depends upon the raw organic substrates.1−3 Three main types of organic waste are used for biogas production: landfills, wastewaters, and agricultural residues.2,3 For all of these sources, traces of volatile organic compounds (VOCs) have been detected and quantified in biogases, with the most common chemical species being sulfur, aromatic, aliphatic, siloxane, and halogenated compounds.4,5 Although the concentrations of these VOCs are low (less than 1% of the total volume of emissions, i.e., less than 2 g Nm−3),6 their presence in the product is detrimental to the subsequent use of the biogas (corrosion, abrasion of mechanical parts,7 or deterioration of the separation membrane), and thus, treatments are required prior to the separation of carbon dioxide and methane.5,8 Adsorption onto activated carbon fiber cloths (ACFCs) appears to be a promising process because its efficiency has been proven for a wide range of VOCs in either liquid or gas phases.9−16 More precisely, the separation of the main VOCs from biogases has been investigated recently,17 and Boulinguiez and co-authors showed, from adsorption isotherms and kinetics, that the adsorption of five VOCs, representative of the trace compounds in biogases, was more favorable on ACFC than on granular activated carbons. This benefit was observed for the lowest concentrations, which are of interest for the purification of biogases. The faster kinetics of adsorption was related to the large © 2014 American Chemical Society

external surface area of the ACFC, and its adsorption capacity was explained by the narrow micropore size distribution. Among other advantages of such materials (uniform micropore size distribution, large external surface area, etc.), the electrothermal regeneration of the fibers is crucial. In contrast to fixed beds packed with activated carbon granules that require an external heat source (hot nitrogen or steam) to remove the VOCs, ACFCs benefit from the continuity of the fibers and are efficiently warmed to high temperatures using the Joule effect. A difference in electric potential is simply applied between the two ends of the medium. Sufficient temperatures of desorption are then reached.10,12 Furthermore, this regeneration in situ enables VOCs to be recovered without a separation step.11 This process was economically assessed for the reuse of isobutane under controlled operating conditions18 in combination with a condenser for the recovery of n-butane, isobutene, and 1,1,1,2tetrafluoroethane.19 In these cases, recovery was only possible if the concentration in the gas phase was sufficiently high during regeneration of the filter for condensation to occur, as indicated by the simulation of the entire adsorption−desorption process.20 Thus, attempts to optimize the process were also made through a desorption that could be easily controlled via the electric power applied, as shown for the desorption of methylethylketone,21 or through controlling the resistance of the filter as an indicator of the complete regeneration.22 Received: March 17, 2014 Revised: May 12, 2014 Published: May 20, 2014 3924

dx.doi.org/10.1021/ef500600b | Energy Fuels 2014, 28, 3924−3932

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2.2. ACFC. A commercial adsorbent was used in this study. THC515 is an ACFC manufactured by Dacarb (Asnières, France) and a woven ACFC made from polycrylonitrile and activated at 1173 K in a CO2 stream. The supplied samples were all washed and soaked in ultrapure water overnight to remove fines, particles, and pollutants slightly adsorbed during the production process. Then, they were dried overnight at 423 K to eliminate water molecules and other contaminants. The physical characteristics of this adsorbent were obtained from the analysis of the physisorption isotherm of nitrogen at 77 K, as described elsewhere.35 Briefly, the surface area was 1576 ± 92 m2 g−1, and the total pore volume was 0.81 ± 0.07 cm3 g−1. THC515 was predominantly microporous, with a micropore size distribution narrowly dispersed around 0.53 nm. 2.3. Electrothermal Desorption. An experimental setup was designed to determine the electrothermal behavior of ACFCs (Figure 1). This device consisted of two bars of refractory ceramics with

In the past, several studies have pointed out the most significant characteristics of ACFCs in terms of thermal resistance.10,23,24 Both texture and surface properties have been highlighted. The larger the surface area, the higher the resistance because the smallest cross-section for the electric current is available. On the other hand, the presence of oxygenated surface groups has been shown to increase resistance because these groups have an influence on the density of the π electronic cloud of the carbon skeleton, thus decreasing the conductivity of the fibers.10,25,26 Fewer studies have focused on the significant effect of adsorbed molecules on the resistivity of the material. For example, methylethylketone (butan-2-one) at equilibrium with a gas phase at 1000 ppm lowered the resistivity by 12%.12 Likewise, when saturated with n-hexane, a 20% decrease was observed in comparison to the virgin ACFC.27 Generally, siloxanes were proven to follow the same trend.28 Similar conclusions were obtained for carbon monoliths, whose resistivity decreased linearly with the amount of toluene adsorbed.29 For granular activated carbons, the resistance is dependent upon the temperature, the amount adsorbed, and the type of compound (alcanes or polar compounds).30−32 On the other hand, the type of porosity of the adsorbent and its graphitic degree were shown to impact the desorption of physisorbed benzene.33 Two objectives were proposed for this study. First, the electrothermal desorption of VOCs from ACFC was investigated, and the influence of the operating conditions was thoroughly studied. Special attention was paid to the effect of the nature of the gas phase (carrier gas) and the adsorbed molecules. The desorption of VOCs was studied in situ as a function of the temperature. To confirm the conclusions, the observations were compared to results obtained by thermogravimetric analysis. In a second part, the efficiency of the adsorption process was investigated in dynamic conditions. For this purpose, cycles of adsorption steps followed by electrothermal desorption were measured for a fixed bed of ACFC. The amounts of VOCs desorbed were compared to those adsorbed, and the comparison was made in light of the experimental observations for the in situ desorption of the first part of the study. Overall, the aim of our study was to assess the feasibility of the adsorption process for the removal of VOCs in biogases as well as the reversibility of the reaction using electrothermal desorption for ACFC.

Figure 1. Experimental setup for electrothermal desorption. removable parts, so that, when screwed, the contact between the electrodes and the carbon fibers was ensured (in addition to the conductive tape). A continuous power generator supplied a difference in potential between the two ends of the ACFC. Pieces of ACFC of 0.1 × 0.1 m were chosen to achieve an accurate and homogeneous dispersion of heat over the entire surface. The thickness of one layer of the ACFC was 0.43 mm. This experimental setup was placed in a glovebox, and the atmosphere was controlled (ambient air, dry air, nitrogen, carbon dioxide, and helium). The electric power (voltage and current) and temperature were recorded. The temperature distribution was obtained simultaneously by a thermocouple at the center of the adsorbent piece and an infrared thermal camera (Flir A40). To evaluate the accuracy of the measurements, multiple independent experiments were carried out with increasing electric currents, and it was shown that, above 0.4 A, the relative standard deviation was below 5%. 2.4. Dynamic Adsorption. An experimental unit was designed for the adsorption of the mixture of VOCs and the following electrodesorption. Figure 2 presents the process with its two main parts: the production of the synthetic biogas and the adsorption unit.

2. EXPERIMENTAL SECTION 2.1. VOCs. Five VOCs from various chemical species were selected to represent the VOCs encountered in biogases. These were toluene (aromatic), isopropanol (alcohol), dichloromethane (halogenated compound), ethanethiol (sulfur compound), and octamethylcyclotetrasiloxane (siloxane D4). All chemicals were provided by Sigma-Aldrich at analytical-grade purity. The main properties are presented in Table 1.

Table 1. Physical and Chemical Properties of the Studied VOCs34

VOC toluene isopropanol dichloromethane ethanethiol (ethyl mercaptan) siloxane D4

formula C7H8 C3H8O C2H2Cl2 C2H6S C8H24O4Si4

molecular weight (g mol−1)

boiling point at atmospheric pressure (K)

vapor pressure at 298 K (kPa)

92.1 60.1 84.9 62.1

383.8 356.2 313.2 308.2

3.0 4.4 47.4 58.9

296.6

448.9

0.1

Figure 2. Experimental setup for dynamic adsorption (production of biogas and the adsorption unit). 3925


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A biogas was produced by mixing methane and carbon dioxide (45:55%, v/v) and then loading it with traces of VOCs. The relative amounts of liquid VOCs were mixed prior to the experiment, and the homogeneous liquid phase was placed in a reservoir, which was then put under pressure (2.03 × 105 Pa). The flow of liquid was controlled by a mass flowmeter (Bronkhorst μ-Flow) before entering a vaporization cell in a flow of carbon dioxide. The concentrations of VOCs in the gas were thus accurately controlled. The composition of the mixture is detailed in Table 2.

conditions for the adsorption and desorption cycles are presented in Table 2. These operating conditions were obtained from an experimental design that was carried out, for each VOC, using one to five layers of ACFC and superficial velocities ranging from 2.78 × 10−2 to 0.14 m s−1. The gas phase at the outlet of the adsorption unit was analyzed using a gas chromatograph, a flame ionization detector (Agilent 6890N), and an automatic sampling pump. Thus, the breakthrough curves were recorded with concentrations measured every 600 s. The VOCs were separated by combining a ramp of the carrier gas (H2, from 25 to 35 psi) and a ramp of the temperature (from 313 to 423 K).

Table 2. Operating Conditions for the Experimental Unit condition

value

carrier gas, CO2/CH4 toluene isopropanol dichloromethane ethanethiol siloxane D4 adsorbent number of layers total thickness superficial velocity duration of the adsorption step duration of the electrothermal desorption temperature of desorption

ratio 55:45 (v/v) 360 × 10−3 g m−3 95 × 10−3 g m−3 180 × 10−3 g m−3 90 × 10−3 g m−3 70 × 10−3 g m−3 Dacarb THC515 5 2.15 × 10−3 m 2.78 × 10−2 m s−1 15000 s 9000 s 420 K

3. RESULTS AND DISCUSSION During the desorption tests, the temperature was shown to be homogeneously distributed over the entire ACFC. For this

For the adsorption unit, the layers of ACFC were rectangular (0.05 × 0.04 m), so that the copper bands could be placed on each side (using a conductive adhesive). However, only a circular section of the filter (0.035 m diameter) was open to the flow of gas, as shown in Figure 3. All

Figure 5. Influence of the surrounding gas phase, namely, ambient air, dry air, nitrogen, helium, and carbon dioxide.

purpose, infrared thermography was used36 and confirmed that the temperature at the filter center was representative of all temperatures, as illustrated in Figure 4. In fact, when the

Figure 3. Plan view of the adsorption filter.

Figure 4. Infrared thermography of the THC515 ACFC and temperature distribution along the horizontal and vertical lines (Tave = average temperature). 3926


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0.63. Table 3 presents the results of the modeling and clearly shows the similar properties of the ACFC in every gas. Considering the difference between dry and humid air, the resistivity was slightly influenced by the presence of adsorbed water inside the porosity of the ACFC. In comparison to the resistivity of air, 1.12 × 109 Ω m, the resistivity of water with various contents of solutes ranged from 106 to 109 Ω m.34,40 Therefore, when adsorbed into the porous network, water molecules should decreased the resistivity of the whole ACFC by reducing the insulating void fraction. Likewise, CO2 was adsorbed and slightly decreased the resistivity of the material (Figure 5). However, considering the experimental errors, the influence of the surrounding gas phase was not significant. The presence of adsordable species did not alter the resistance of the ACFC. 3.2. Monitoring the Desorption of VOC. Using the experimental unit presented in Figure 1, the resistivity of the THC515 fiber cloth was measured as a function of the temperature and the amount of VOCs adsorbed. In this study, adsorption capacities were chosen between zero coverage and saturation of THC515. For this purpose, adsorption isotherms (previously reported17) were used. For each VOC, the resistance of the material was determined for four different loadings. The influence of the adsorbed molecules could be observed from the comparison between the raw ACFC and the loaded materials. As shown in Figure 6, two types of behavior were found. First, toluene, dichloromethane, and isopropanol were entirely desorbed at 420, 370, and 380 K, respectively. On the contrary, ethanethiol and siloxane D4 were not completely removed from the ACFC, regardless of the amount of VOC adsorbed. Either byproduct was produced in the solid, which could not be removed, or higher temperatures were required for complete regeneration. In more detail, dichloromethane and isopropanol were the organic molecules removed from the ACFC at the lowest temperatures. In fact, above 380 K, the resistivity of the material reached that of the virgin filter, indicating the complete regeneration of the adsorbent.41 To confirm this conclusion, thermogravimetric analysis was carried out in earlier works.17 The mass of the loaded sample was recorded as the temperature increased. The differential mass exhibited a peak when the compound was removed from the porosity of the adsorbent. Isopropanol was totally desorbed at 480 K, irrespective of the kinetic effects in differential thermogravimetry (DTG). The desorption peak occurred at 370 K, in agreement with the in situ desorption curve in Figure 6. The same behavior was observed for dichloromethane. Furthermore, toluene had its own desorption behavior. The interactions of toluene molecules with the carbon surface are enhanced by π−π electronic interactions in comparison to isopropanol or dichloromethane. Consequently, the temperature required for a complete removal of this aromatic compound was higher. From the DTG curve,17 the desorption was complete at higher temperatures, up to 620 K. The peak was wider, and the maximum was close to 470 K. However, as illustrated in Figure 6, the in situ regeneration with various loadings of toluene showed that the resistivity of the material could only return to the virgin material at temperatures just above 420 K, which is most often considered the limit for safe handling of the electrothermal desorption process. Thus, it was concluded that toluene constituted the boundary compound for complete regeneration using this technique.

Table 3. Modeling the Resistance of the ACFC in Various Atmospheres atmosphere uncontrolled (relative humidity of approximately 40%) dry air helium nitrogen carbon dioxide

RT0 (Ω)

αH (K)

RSS

r2

32.3 ± 0.2

616 ± 12

0.23

>0.99

37.2 ± 0.5 35.6 ± 0.5 37.0 ± 0.2 36.5 ± 0.5

554 ± 23 642 ± 28 608 ± 16 623 ± 20

0.50 0.63 0.37 0.44

>0.99 0.99 >0.99 >0.99

temperatures were plotted along the horizontal and vertical lines, deviations below 10% were obtained. 3.1. Influence of the Gas Phase. The thermal behavior of the ACFC was assessed in various atmospheres. In industrial conditions, the adsorbent is regenerated under inert gases to prevent excessive warming and ignition risks (nitrogen, helium, or carbon dioxide). However, carbon dioxide, for instance, is adsorbed and can then be desorbed using electrothermal regeneration.37 Consequently, the influence of the surrounding gas phase was studied. The resistance of the ACFC was determined as a function of the temperature. The comparison between five conditions is shown in Figure 5. Very similar curves were obtained, demonstrating the negligible influence of the surrounding gas phase. This result confirms the validity of previous reports on the thermal behavior of ACFCs. In fact, the resistivity of ACFCs measured in ambient air (uncontrolled conditions) could be extrapolated to the actual filters used under different conditions. ACFCs are made of carbon fibers with low electric resistance. These fibers, with diameters in the micrometric range, are gathered in wires that are much thicker, in the millimetric range. Although these materials possess insulating interstices between the fibers, the response to an electric current is a homogeneous warming of the ACFC.10,12 Desorption will depend upon the temperature reached when the ACFC is exposed to the electric power. At a given temperature, the resistance is determined from eq 1

ρL (1) el where R is the resistance (Ω), ρ is the electric resistivity (Ω m), L is the length between the electrodes (m), l is the width (m), and e is the thickness of the material (m). Nonetheless, because carbonaceous materials are semiconductors, the electric resistance will depend upon the temperature. In a first approximation, a linear relationship was used to describe the drop in the resistivity of ACFC with the temperature.38 A more suitable expression was then developed for thermoresistance with an exponential function of the temperature (eq 2)39 R=

⎛ ⎛1 R 1 ⎞⎞ = exp⎜⎜αH⎜ − ⎟⎟⎟ R T0 T0 ⎠⎠ ⎝ ⎝T

(2)

where RT0 is the resistance at the reference temperature T0 (Ω), T is the temperature (K), T0 is the reference temperature (K), and αH is the Hart thermal coefficient (K). According to eq 2, the resistivity of THC515 ACFC was accurately described with coefficients of determination (r2) greater than 0.99 and a residual sum of squares (RSS) less than 3927


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Figure 6. Thermal resistivity of THC515 as a function of the temperature and VOC loading (qe in mmol kg−1 at 298 K, prior to the desorption): (a) toluene, (b) isopropanol, (c) dichloromethane, (d) ethanethiol, and (e) siloxane D4.

On the other hand, the other VOCs, ethanethiol and siloxane D4, were not entirely removed from the porosity during the electrothermal desorption process. In fact, up to 420 K,

desorption of these compounds was incomplete, as evidenced by the differences in thermal resistivity between the loaded and virgin materials. With regard to ethanethiol, the explanation for 3928


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3.3. Adsorption−Desorption Cycles. Taking into account the previous results, multiple cycles of adsorption and desorption of toluene were measured using the experimental setup for dynamic adsorption (Figure 2). The temperature for regeneration was set at 418 K, which is sufficient for the complete desorption of toluene in batch conditions (Figure 6). The dynamic adsorption−desorption cycles were aiming at confirming the reversibility of this process. Moreover, toluene was chosen because it was desorbed at the highest temperature in comparison to dichloromethane or isopropanol. Figure 7 shows the breakthrough curves for the 5 consecutive cycles. For all of them, the inlet concentration of toluene was 250 mg m−3 and the superficial velocity of the gas was 8.3 × 10−2 m s−1. The desorption lasted 2000 s, and the current was adjusted to achieve a constant temperature within the filter of 418 K. A loss of adsorption capacity was observed after the first cycle. In fact, 5.4 mol kg−1 was adsorbed, and only 4.5 mol kg−1 was recovered, which corresponded to a recovery rate of 75%. However, for the following cycles, a steady state was reached and the adsorbed mass of toluene was consistent for all adsorption cycles (4.5 mol kg−1). The complete desorption of toluene was thus confirmed in dynamic conditions. Next, the adsorption of the synthetic biogas was considered with a mixture of VOCs. First, the breakthrough curves of these compounds were monitored (Figure 8). Dichloromethane and ethanethiol were poorly adsorbed, and the breakthrough times for the compounds were estimated at 3700 and 5000 s, respectively. The amounts adsorbed were 0.21 and 0.18 mol kg−1 for dichloromethane and ethanethiol, respectively, after 60 h. On the contrary, siloxane D4 had a longer breakthrough time (122 400 s or 34 h), which illustrates its better adsorption ability onto the ACFC (1.23 mol kg−1 adsorbed after 60 h). In fact, the adsorption of siloxanes was the main objective because these are the most problematic compounds for the valorization of biogases as a result of the formation of SiO2 and the deterioration of motors.8 These results agree with those concerning the affinity of the VOC for THC515, which were determined from the adsorption of single components in batch reactors.17 This agreement between the adsorption capacities for pure components and those measured

Figure 7. Cycles of adsorption and desorption of toluene.

this behavior has already been given by the chemical transformations of this sulfur compound on the carbon surface during thermal desorption. Solid deposits of elemental sulfur (S0) were experimentally observed that would definitely impede the adsorption capacities as well as the electric resistivity of the adsorbent.35,42 Likewise, during oxidation, sulfur and disulfur compounds were identified. Further evidence was provided by the DTG curve,17 which clearly revealed the partial desorption at 420 K. A constant mass was measured at higher temperatures, meaning that the deposit of elemental sulfur was stable and could not be removed from the surface. The same mechanisms were put forward for siloxane D4. Silica deposits might result from its thermal decomposition, as indicated by the permanent modification of the ACFC resistance (Figure 6). In addition, the DTG curve17 clearly illustrated that desorption would occur over a large range of temperatures, from 320 to 650 K.

Figure 8. Breakthrough curves for the synthetic biogas. 3929


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Figure 9. Adsorption−desorption cycles for the biogas.

The major compound of interest for biogas purification was siloxane D4. During the first and second cycles, all compounds broke through, except siloxane D4, and the process achieved its objective of the complete elimination of siloxane D4. Moreover, siloxane was desorbing because this VOC was observed during electrothermal regeneration. However, during the third adsorption step, siloxane was measured in the outlet gas before the end of the adsorption period. The outlet concentration was 10% of the inlet content (7 mg m−3). This observation confirmed the loss of adsorption capacity and, thus, the partial desorption of this siloxane. In fact, the complete desorption of siloxane could not be reached at 418 K. Therefore, from one cycle to another, the ACFC was not totally regenerated and the adsorption capacity was decreasing. This process led to a significant loss of adsorption capacity and therefore, after 2 cycles, the capacity was not sufficient to ensure the complete removal of siloxane D4. Considering the cost of the process, the voltage and current were recorded during the electrodesorption and the electrical consumption was plotted against the temperature inside the filter and against the elapsed time (Figure 10). The initial power was 17 W, but this was rapidly reduced to 14 W (in less than 15 min) as the temperature reached 410 K. The electric power was averaged over the entire desorption, and the electrical consumption per mass of the adsorbent was close to 1500 W kg−1.

Figure 10. Electrical consumption during regeneration as a function of the temperature inside the adsorbent.

in mixtures for the VOCs in biogas was also highlighted by Gaur et al.43 Dynamic multicomponent adsorption sheds light on the affinity between the VOCs and the carbonaceous material. In fact, dichloromethane, ethanethiol, and especially isopropanol had outlet concentrations exceeding their inlet contents. This particular behavior was linked to the better affinity of siloxane D4, which removes the other compounds as it is adsorbing progressively in the porosity of the adsorbent. A similar observation was reported by Matsui and Imamura for the competing adsorption of two different siloxanes (D5 and D4).44 Then, three adsorption−desorption cycles were measured for the mixture of VOCs. The duration of the adsorption was 15 000 s (4.2 h), and the regeneration lasted 9000 s (2.5 h), during which the electric current through the ACFC was controlled to reach a constant temperature within the bed of 418 K. The results for the multicomponent adsorption and desorption are shown in Figure 9. The periods of time without experimental data represent the cooling of the ACFC, prior to the following adsorption step.

4. CONCLUSION The feasibility of the electrothermal regeneration of ACFC was assessed for the removal of undesirable VOCs from biogases. First, this consisted in measuring, in situ, and in controlled conditions, the resistance of the material with different amounts of organic compounds adsorbed. Two types of desorption behavior were clearly revealed. Among the five VOCs considered, toluene, dichloromethane, and isopropanol were shown to be completely removed at 420 K. On the contrary, ethanethiol and siloxane D4 were more problematic because they were partially desorbed, and reduced adsorption capacities after regeneration were predicted. The previous observations were confirmed by the breakthrough curves of the VOC mixture on ACFC for multiple cycles. During the adsorption step, the amounts of VOC adsorbed 3930


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ranged from 0.18 to 3.01 mol kg−1 for ethanethiol and toluene, respectively. Siloxane D4 was adsorbed in large amounts onto the ACFC (1.23 mol kg−1). However, after 3 cycles of adsorption and desorption, siloxane D4 broke through the filter, whereas it was not present at the filter outlet during the first and second adsorption cycles. A loss of the adsorption capacity of the ACFC was clearly observed, and this difficulty must be overcome before this adsorption process can be considered an efficient purification step for the production of biogases. The electric power required for the electrothermal desorption was estimated at 1500 W kg−1.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +33-223238015. Fax: +33-223238120. E-mail: [email protected]. Present Address †

Benoit Boulinguiez: MT-Energie GmbH, Ludwig-ElsbettStraße 1, 27404 Zeven, Germany.

Notes

The authors declare no competing financial interest.

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