DENATURED BIO-ETHANOL FEEDSTOCK DESULFURIZATION BY ADSORPTION ONTO A NICKEL CONTAINING SOLID. Introduction

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1 DENATURED BIO-ETHANOL FEEDSTOCK DESULFURIZATION BY ADSORPTION ONTO A NICKEL CONTAINING SOLID Michel THOMAS, Sandra MONTPEYROUX, Karine SURLA IFP Energies nouvelles, Solaize, France Introduction Hydrogen can be produced from a large variety of feedstocks, including fossil fuels as well as renewable sources such as biomass. The large-scale hydrogen production processes, such as for ammonia and methanol syntheses, use often natural gas, or methane, to produce hydrogen either by steam reforming, partial oxidation or auto-thermal reforming. Other hydrocarbon sources, such as liquid hydrocarbons and coal, can also be used to produce hydrogen through gasification processes. In the refining industry, hydrogen is produced by reforming reaction (production of aromatics) and used in hydro-treatment and hydro-desulfurization units. The small-scale hydrogen production processes are a response to the problem of transport and storage of hydrogen. Small units, less costly and more mobile, make it possible to have a hydrogen source that is close to the installation that requires it. It is the case for example for electronic, metals processing and oil hydrogenation industries. Among renewable sources, ethanol appears to be a very promising hydrogen source due to its low toxicity, easy handling, transportation and storage and its wide availability. Moreover, using bioethanol produced from biomass sources has almost no impact on the greenhouse effect [1]. IFP Energies nouvelles is developing a new decentralized hydrogen production system from bio-ethanol liquid feedstocks []. The conversion of bio-ethanol to hydrogen is performed using a catalytic auto-thermal reformer (ATR) working under pressure which combines the exothermic reaction of ethanol with oxygen (partial oxidation) and the endothermic reaction of ethanol with water (steam reforming). The reactions, leading to synthesis gas production (syngas : mixture of H, CO and CO ), can be described as follow: C H OH O CO + 3H CH 5OH + H O CO + 4H At the outlet of this reforming section, the hydrogen-rich effluent gas contains impurities, in particular carbon monoxide CO. In order to purify this syngas and to increase the hydrogen yield, the conversion of carbon monoxide is performed using the water gas shift reaction (WGS): CO + H O CO + H A pressure swing adsorption unit is then used to purify the reformate gas. Using adsorption and desorption cycles over adsorbent beds (zeolite LTA 5A) allows to remove contaminants. A pure hydrogen stream (purity of % vol.) is then produced. For legal reasons, the ethanol that is not used in the food sector is denatured by the addition of a chemical substance, called the denaturing agent, and that makes it unfit for consumption [3]. This denaturing agent can be a natural gas condensate, or, more often, a gasoline. In this case, this ethanol feedstock very often contains undesirable substances such as sulfides or mercaptans (condensate), or thiophenic compounds (gasoline). Typical denatured ethanol has to contain at least 9,1 vol. % of ethanol, a denaturant amount comprised between and 5 vol. %, and no more than 1,7 vol. % of aromatics and 10 ppm mass of total sulfur. The presence of these sulfur-containing compounds is very disturbing because they can rapidly poison the catalysts used either in the ATR reactor or in the WGS one. Therefore the feedstock has to be desulfurized before its reforming. The aim of this work is to study the removal of thiophenic compounds from denatured bioethanol, by using adsorption in liquid phase over a nickel based solid, and without hydrogen.

2 Experimental Model Feed In this study, experiments have been carried out with synthetic model feeds, composed of pure ethanol (98 vol. %), toluene ( vol. %) as the aromatic component of the denaturant and possible adsorption competitor, and thiophene (100 and 0 ppm "S") as the sulfur component. Solid Characterisation The solid used is mainly composed of a mixture of nickel and nickel oxide deposited on a silica and alumina support. This type of solid is known to be efficient in the removal of sulfur compound such as thiols and mercaptans directly from gasoline [4], with formation of nickel sulfide Ni 3 S. It is provided by Axens under the name AxTrap-405 (trilobe extrudates, diameter of 1,6 mm), supplied in a reduced and stabilized form. Stabilization permits simplified handling and loading of nickel catalysts which are normally pyrophoric [5]. The stabilization process involves the addition of carbon dioxide gas on the reduced solid, so it will contains around -3 mas. % of sorbed CO, and due to the hygroscopic nature of alumina, it could also have -3 mas. % of sorbed water. An activation step for removal of CO and H O is then at least necessary before using it. This solid has been characterized by classical methods [6, 7]. The specific BET surface area (S BET ) as well as the mesoporous volumes (V mp ) of this solid have been determined by adsorption of nitrogen at 77 K by using a Micromeritics ASAP-010 Physisorption Analyzer. Mean mesopore diameter (D mp ) is obtained from BJH treatment of the desorption branch of the nitrogen adsorption isotherm. Macroporous volume (V MP ) and particle density (ρ P ), including pore volume, have been determined by mercury intrusion by using a Micromeritics AutoPore-IV Mercury Porosimeter. Solid density is obtained by helium pycnometry by using a Micromeritics AccuPyc II 1340 Pycnometer. Thermogravimetry analysis (TGA) has been performed with a Netzsch thermobalance (Iris TG 09 F1), connected with a mass spectrometer (Balzers Thermostar) to follow desorption of H O and CO (mass of 18 and 44), from ambient temperature to 500 C, under Helium at 3 NL.h -1 and with a heating ramp of 10 C.min -1, with a sample mass of around 40 mg. Temperature Programmed Reduction (TPR) has been performed from ambient temperature to 900 C at 5 C.min -1, with a mixture of 10 vol.% H /Ar at a flow-rate of 50 Ncm 3.min -1.g -1 and with a sample mass of around 0,7 g, by using a Micromeritics AutoChem II 90 apparatus. Elemental composition (Si, Al, Ni) has been determined by Fluorescence-X. Test Setup The efficiency of the solid to remove thiophene from the ethanol feed has been evaluated with the dynamic "breakthrough curve" technique [8]. All the experiments have been performed on a laboratory scale breakthrough curve unit comprising : - stock tanks ( L), one for the solvent (pure ethanol), one for the feed (ethanol + thiophene + toluene), placed on two balances in order to follow their mass during all the experiment - HPLC pumps (Gilson model 307), allowing a liquid flow-rate comprised between 0,5 and more than 10 cm 3.min -1, and the injection of the liquid feed always in a liquid state (15-0 bar) - a three-ports valve, for the injection of either the pure solvent or the feed - a stainless steel column (internal diameter : 1 cm length : 7 cm volume : 1, cm 3 ), placed in an oven (380 C max.), and connected to the 3-ports valve on one end, and to a fraction collector on the other - a liquid fraction collector (Gilson model 06) allowing the sampling of the liquid effluent at the outlet of the column at different intervals of time (60 to 80 sampling vials of ml)

3 - a cooling bath (Lauda RK40) used to cool the effluent after the column outlet and the before the sampling device, in order to avoid or to reduce the possible evaporation of ethanol in the sampling vials Once the experiments is finished, the effluent collected in each sampling vial is analysed by gas chromatography in order to determine the thiophene concentration versus time (Agilent gas chromatograph 6890 with an automatic liquid sampler (injector system) serie 7683 Agilent PONA OV1 capillary column 50 m, 0, mm i.d., 0,5 µm film thickness, 10 min at 30 C and heating at 3 C.min -1 to 310 C F.I.D. detector at 300 C). After subtraction of the connecting lines volume, the amount of thiophene contained in the column is calculated by a simple mass balance between the amount injected in the feed and the amount analysed in the effluent. The amount adsorbed on the solid can then be calculated with the knowledge of the interstitial liquid volume in the column, obtained from density measurements. The solid is introduced in the column (average around 18,5 g), and then either activated under nitrogen or reduced under hydrogene, with the following conditions : - gas flow-rate (N or H ) : nl.h -1.g -1 - temperature profile : 30 min at 100 C 30 min at 00 C 30 min at 300 C, with heating ramps close to 10 C.min -1 After this step and after cooling, pure ethanol is then slowly introduced in the column from the solvent stock tank through the HPLC pump. The column temperature is then again increased up to the chosen temperature for the test, and the denatured model feed is introduced in the column once the temperature is stabilized at the correct level. The feed flow-rate has been fixed to cm 3.min -1. The corresponding LHSV (Liquid Hourly Space Velocity) is 5,6 h -1. It corresponds to a contact time between feed and solid close to 5 min. The total duration of a test is 6 hours, and the maximum eluted feed volume is close to 70 cm 3, or around 33,6 column volumes. A new solid sample is used for each test. Results Solid Characterization a/ Porosity Solid surface area and porous volumes, determined by N adsorption at 77K and mercury intrusion/extrusion, are given in table 1. The corresponding curves are given figures 1 and. Table 1. Porous volumes from N and Hg porosimetries S Solid BET V mp D mp V MP ρ P (m.g -1 ) (cm 3.g -1 ) (nm) (cm 3.g -1 ) (g.cm -3 ) AxTrap ,433 8,6 0,037 1,69 Volume Liq. N Adsorbed (cm 3.g -1 ) 0,50 0,45 0,40 0,35 0,30 0,5 0,0 0,15 0,10 0,05 0,00 0,0 0,1 0, 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 Ads. Des. 1,0E+06 1,0E+05 1,0E+04 1,0E+03 1,0E+0 1,0E+01 0,50 0,45 0,40 0,35 0,30 0,5 0,0 0,15 0,10 0,05 0,00 1,0E+00 Volume Hg (cm 3.g -1 ) Intr. Extr. Relative Pressure Pore Diameter (nm) Figure 1 : N adsorption 77K Figure : Hg Intrusion/Extrusion curves

4 This solid appears to be essentially a mesoporous one. The bulk density, ρ B, of this solid is 0,817 g.cm -3, and its structural density, ρ S, determined by helium pycnometry, is 3,89 g.cm -3. From these data, one can calculate from the following equations the particle porosity (ε P ) equal to 0,566 as well as the inter-particle bed porosity (ε I ) equal to 0,517. The total bed porosity (ε B ) is therefore equal to 0,790. ρ P ρ B ε P = 1 ε I = 1 ε B = ε I + ( 1 ε I ). ε P ρ ρ S b/ Elemental Analysis The elemental composition of the solid is given in table. Table. Elemental composition Si Al Ni Solid (mas. %) (mas. %) (mas. %) AxTrap-405 4,0 ± 0,3 5,0 ± 0,4 53, ± 1,1 c/ Thermogravimetry analysis (ATG) Thermogravimetric curves are presented figures 3 (ATG) and 4 (ATG/MS). The mass loss occurs in two main steps, the first one from 30 to 05 C corresponding mainly to water desorption (,6 mass %), the second one from 05 to 400 C, corresponding mainly to carbon dioxide desorption (3,5 mass %). This confirms that CO is well chemisorbed on the surface of the solid. The total mass loss represents 6,1 mass %. P 0 0,0E+00 1,0E-08 0,0E+00-1,0E-04 Tg ,0E-04 -,0E-04-3,0E-04 dtg/dt (mg/g/ C) Tg dtg/dt M.S. Intensity (a.u.) 1,0E-09 1,0E-10 -,0E-04-3,0E-04-4,0E-04-5,0E-04 dtg/dt (mg/g/ C) M = 18 M = 44 dtg/dt ,0E Température 1,0E-11-6,0E Température Figure 3 : Tg and dtg curves (ATG) Figure 4 : dtg and M.S. curves (ATG-SM) d/ Temperature programmed reduction (TPR) A first experimental has been performed with the solid "as received", and a second one with the pre-activated solid in Argon (400 C) in order to removed the passivating agent before the reduction step. The hydrogen consumptions for both experiments are given in table 3 and the corresponding curves are shown figure 5. Table 3. TPR results Solid H (Ncm 3.g -1 ) H (Ncm 3.g -1 ) T < 450 C T > 450 C AxTrap-405 "as received" 67,75 81,3 AxTrap-405 "Ar pre-activated" 55,41 96,9

5 0,00 T.C.D. Signal (a.u.) -0,5-0,50-0,75-1,00 Pre-Activated As Recieved -1, Temperature Figure 5 : TPR curves (pre-activated and as received samples) An important hydrogen consumption is observed with the "as received" solid, without any pre-activation, in the temperature range C, while the hydrogen consumption is weaker with the pre-activated solid, in the range C. In the temperature range C, the hydrogen consumption is quite similar. The first important peak observed at the mean temperature of 65 C with the "as received" solid can be partially explained by the methanation reaction between hydrogen and the passivating agent, carbon dioxide: CO + 4 H CH 4 + H O The fact that no hydrogen consumption is observed at lower temperatures (< 00 C) indicates that the chimisorbed CO protects well the solid surface. Breakthrough Experiments The main parameters which have been taken into account are: - solid pre-treatment, consisting either in an activation step under nitrogen or a reduction step under hydrogen, at temperatures comprised between 10 and 300 C - adsorption temperature, comprised between 30 and 160 C, with both activated and reduced solids - influence of the presence or not of the possible aromatic inhibitor (toluene) in the feed - thiophene (Tphn) concentration in the feed a/ Adsorption temperature influence with an activated or reduced solid at 300 C A first set of experiments performed with a feed containing toluene ( %) and thiophene (100 ppm "S") shows that a minimum adsorption temperature of 10 C is required to observe a significant sulfur adsorption on the activated solid at 300 C. The sulfur breakthrough point occurs after elution of 17 column volumes at 10 C, and is not observed after elution of 34 column volumes at 160 C. At lower adsorption temperatures, sulfur breakthrough is quite immediate. Results are given in table 4 and corresponding curves on figure 6. Table 4. Influence of adsorption temperature with activated solid at 300 C T Ads 30 0,6 0, ,6 / 3,4 0,14 / 0, ,8 4, > 33,6 > 8,10 A second set of experiments carried out with the same feed but with a reduced solid at 300 C shows that the sulfur breakthrough occurs after elution of around 0 column volumes when

6 adsorption temperatures are in the range from 30 to 10 C, while it is not observed at 160 C. At low or moderate temperatures ( 80 C), reduction appears therefore more efficient than activation. At higher temperatures, no conclusions can be drawn as sulfur breakthrough is not observed in both cases. Results are given in table 5 and corresponding curves on figure 7. Table 5. Influence of adsorption temperature with reduced solid at 300 C T Ads 30,1 5, ,5 / 30,4 6,96 / 7, ,7 4, > 33,6 > 8,10 1,0 1,0 0,9 0,9 0,8 0,8 C/C0 Thiophene 0,7 0,6 0,5 0,4 0,3 0, 30 C 80 C 10 C 160 C C/C0 Thiophene 0,7 0,6 0,5 0,4 0,3 0, 30 C 80 C 10 C 160 C 0,1 0,1 0, Eluted Volume / Column Volume 0, Eluted Volume / Column Volume Figure 6 : Effect of adsorption temperature with activated solid at 300 C Figure 7 : Effect of adsorption temperature with reduced solid at 300 C b/ Effect of activation or reduction temperature The third set of experiments has been performed with the same feed as previously, but with lower activation temperatures. With an activated solid at 10 or 150 C, sulfur breakthrough is observed immediately whatever the adsorption temperatures below 10 C. Sulfur adsorption becomes significant only when activation and adsorption temperatures are both equal to 150 C. In this last case, sulfur breakthrough is not observed after elution of 34 column volumes. With an activated solid, both activation and adsorption temperature are important, and 150 C appears to be the lower activation and adsorption temperature in order to observe a significant sulfur concentration decrease. Results are given in table 6 and corresponding curves on figure 8. Table 6. Influence of activation and adsorption temperatures T Act T Ads ,6 0, ,6 0, ,6 0, > 33,6 > 8,10 The forth set of experiments performed in the same conditions but with a reduced solid indicates that a reduction temperature of 150 C is required to observe sulfur adsorption. Indeed, if the solid is reduced at 10 C, sulfur breakthrough is also observed immediately at 10 C, but its efficiency increases dramatically when it is reduced at 150 C, sulfur breakthrough being observed after elution of 4 column volumes with an adsorption temperature of only 80 C, and after more

7 than 34 column volumes at 150 C. No definitive conclusion can be drawn at 150/150 C as sulfur breakthrough is not observed with activated or reduced solid. These observations indicate that thiophene adsorption on reduced nickel is an activated reaction. Results are given in table 7 and corresponding curves on figure 9. Table 7. Influence of reduction and adsorption temperatures T Red T Ads ,7 0, ,6 5, ,8 0, > 33,6 > 8,10 1,0 1,0 0,9 0,9 0,8 0,8 C/C0 Thiophene 0,7 0,6 0,5 0,4 0,3 0, Act. 10 C - Ads. 80 C Act. 150 C - Ads.80 C Act. 10 C - Ads. 10 C Act. 150 C - Ads. 150 C C/C0 Thiophene 0,7 0,6 0,5 0,4 0,3 0, Red. 10 C - Ads. 80 C Red. 150 C - Ads. 80 C Red. 10 C - Ads. 10 C Red. 150 C - Ads. 150 C 0,1 0,1 0, Eluted Volume / Column Volume 0, Eluted Volume / Column Volume Figure 8 : Effect of activation and adsorption temperatures on sulfur removal Figure 9 : Effect of reduction and adsorption temperatures on sulfur removal c/ Effect of toluene concentration The fifth set of experiments has been performed with or without toluene in the feed. As an aromatics, toluene could indeed possibly act as a sulfur adsorption inhibitor, leading to a decrease of thiophene adsorption and a lower efficiency in sulfur removal from ethanol. With a reduced solid at 300 C and an adsorption temperature of 80 C, the sulfur breakthrough occurs after the elution of more than 34 column volumes without toluene and 9 column volumes in presence of toluene ( vol. %). As it is difficult to conclude, two other experiments have been carried out with a higher flow-rate (F = 4 cm 3.min -1 VVH = 11, h -1 ) in order to treat a larger volume of feed over the solid within the same time. With these new conditions, sulfur breakthrough is observed after elution of 3 and 1 column volumes, respectively without and with toluene. These two values being close each other, one can conclude than toluene has no or little effect on sulfur adsorption. It has to be noticed than sulfur breakthrough occurs logically more rapidly when the flow-rate (or the feed velocity within the column) increases. Results are given in table 8. Table 8. Influence of toluene concentration T Red = 300 C T Ads = 80 C > 33,6 > 8, ,5 / 30,4 6,96 / 7, ,0* 5, ,8* 4,97 * : VVH = 11, h -1 C Tol (%) C Tphn (ppm S)

8 d/ Effect of thiophene concentration The sixth set of experiments has been performed with two thiophene concentrations in the feed, 0 and 100 ppm "S", in presence of toluene ( %). With a sulfur concentration of only 0 ppm "S", an activation or reduction temperature of 300 C and an adsorption temperature of 80 C, sulfur breakthrough occurs after elution of larger feed volumes : around 3 (100 ppm "S") and 3 (0 ppm "S") column volumes with the activated solid, 30 (100 ppm "S") and more than 34 (0 ppm "S") with the reduced solid. As sulfur breakthrough is not observed, it is impossible to check if the breakthrough volume is directly function of the sulfur concentration in the feed, and no extrapolation is therefore possible at this stage. Results are given in table 9. Table 9. Influence of thiophene concentration T Ads = 80 C C Tphn (ppm S) T Act/Red (Act) 3,5 1, (Act) 0,6 / 3,4 0,14 / 0, (Red) > 33,6 > 1, (Red) 8,5 / 30,4 6,96 / 7,5 Conclusions The aim of this work was to study and develop a pre-treatment adsorption step in liquid phase in order to desulfurize a denatured bio-ethanol feedstock prior to ATR and WGS reactions in order to produce hydrogen, with a nickel-based solid and without hydrogen. Experiments carried out on a laboratory scale with a model synthetic feed have shown that the use of a such a solid, containing both nickel and nickel oxide deposited onto an alumina-silica support, was efficient for this purpose. Nevertheless the pre-treatment of the solid is very important in order to improve its efficiency in the sulfur removal. A reduction step at a temperature of 150 C is necessary, and the adsorption temperature has to be at least of 80 C. However higher temperatures of reduction and adsorption lead to a better efficiency, thiophene adsorption being an activated reaction. By comparison, a simple activation step does not lead to a similar efficiency of the solid in the sulfur removal of such a feedstock. References 1. Ni, M., Leung, D.Y.C., Leung, M.K.H. Int. J. Hydrogen Energy, 007, 3, 388. Avenier, P., Nebois, C., Surla, K., Ambrosino, J.L., Boyer, C. Hypothesis Conference, Edinburgh 11-1 June, ASTM D Ma, X., Spague, M., Song, C. Ind. Eng. Chem. Res., 005, 44, Huber, F., Yu, Z., Lögdberg, S., Rønning, M., Chen, D., Venvik, H., Holmen, A. Catalysis Letters, 006, 110, Lowell, S., Shields, J.E., Thomas, M.A., Thommes, M. "Characterization of Porous Solids and Powders: Surface Area, Pore Size and Density", Kluwer Academic Publishers, Lynch, J. "Physico-Chemical Analysis of Industrial Catalysts", Technip Ed., Ruthven, D.M. "Principles of Adsorption and Adsorption Processes", John Wiley & Sons, Inc, 1984