Adsorbents for the Sorption Enhanced Steam-Methane Reforming Process

Transcription

1 Abstract Adsorbents for the Sorption Enhanced Steam-Methane Reforming Process Drazen Dragicevic & Marcus Ivarsson Department of Chemical Engineering, Lund University, Sweden August 27, 2013 Hydrogen can be produced industrially by steam reforming of methane. Besides hydrogen, carbon dioxide is formed as a by-product in the process. By adding a carbon dioxide adsorbent in the reactor, that is physically mixed with the catalyst, many advantages can be achieved. The adsorption of carbon dioxide contributes to a cleaner product stream, but also shifts the equilibrium towards higher hydrogen production. Other advantages are that the reaction can be performed at milder operating conditions, which would lower the costs of the production. The hydrogen can be further used to produce electricity in a combined heat and power plant. In this work, the goal was to find an adsorbent that fits as well as possible into the combined heat and power process. To distinguish which adsorbents that were of biggest interest a literature study was performed. Alumina and hydrotalcites proved to have best properties with respect to the requirements of the adsorbents, and were further investigated. Their properties were tested by using the characteristic methods; the Brunauer-Emmett-Teller theory (BET), temperature programmed desorption (TPD), Chemisorb- and fixed bed reactor experiments. The results showed that hydrotalcites had a slightly higher adsorption capacity, while alumina had faster adsorption kinetics within the temperature range of C. The major part of the adsorbed carbon dioxide bound through physisorption when alumina was used at lower temperatures, which is favorable when regenerating the adsorbent. The adsorbents were also implemented in the steam methane reforming process to investigate how their characteristics affected the production of hydrogen, at atmospheric pressure and a steam/carbon ratio of 4. The addition of adsorbents increased the production rate significantly until they were saturated. The outlet concentration of hydrogen was increased by 45 % at 300 C respectively 20 % at 400 C. Properties like adsorption capacity and adsorption rate were clearly reflected on the hydrogen production. To be able to tell which adsorbent that suits best for the application, parameters like cyclic stability and economy has to be investigated. Keywords: Adsorption, Sorption Enhanced Steam Methane Reforming, Combined Heat and Power. Introduction A common way to produce hydrogen is to react methane with steam to form hydrogen and carbon dioxide. This process involves two equilibrium reactions: Steam reforming and the water gas shift reaction. Equation 1 and 2 shows the two reactions. 3 (1) (2) The conventional process is performed by catalyzing the steam reforming reaction with a nickel-based catalyst at a temperature between 850 and 950 C to obtain a methane conversion of 80-90% [1]. The carbon dioxide in the

2 product flow has to be removed to increase the hydrogen purity. This can be accomplished by adsorbing the carbon dioxide using an adsorbent. If the adsorption of carbon dioxide is carried out in presence of the reforming and water gas shift reactions, several benefits can be accomplished. A simultaneous adsorption of the carbon dioxide enhances the hydrogen production rate, according to Le Chatelier s principle, and makes it possible to produce pure hydrogen at a lower temperature, typicallyy between 450 and 550 C [2]. This technique, named sorption enhanced steam methanee reforming (SE-SMR)) is for this reason an interesting way to t lower the operating costs for producing hydrogen. By integrating a fuel cell to the SE-SMR process, an efficient power heat plant can be designed. The depleted air from the fuel celll is humid, warm and can be used to regenerate the adsorbent in the reforming process. If the process has at least two reactors,, the reforming and regeneration can occur simultaneously. This makes it possible to run the process continuously, whichh minimizes the energy losses. Figure 1 describes how the reforming and regeneration procedures are carried out in the combined heat and power plant. Requiremen nts of the adsorbent The reforming process can be performed until the adsorbent is saturated, whichh makes it important to find an adsorbent with a high adsorption capacity. The adsorbent should also have a high adsorption rate. An adsorbent with a high adsorption rate can maintainn a high purity of the product, p withh a shorter adsorbent bed needed. The carbon dioxide can be removed from the adsorbent in several different ways. It can be removed by using a techniquee named pressure swing adsorption (PSA), where the pressure is lowered during the regeneration. When the pressure is decreased the reversible r bound carbonn dioxide are released from the adsorbent [3]. It can also be released by using a technique named thermal swing adsorption (TSA), wheree the adsorbent is regenerated by raising the temperature [4]. To keep the energy demand for the regeneration low, it s important i that the adsorbent has a weak bounding to the carbon dioxide. There aree two different types of bounding that t can occur; physisorption and chemisorption. Physisorption is the preferred interaction, where w the carbon dioxide interacts with weak vann der Waals forces to the surface of the adsorbent. Chemisorption involves a Figure 1. A combined heat and power plant where electricity is produced fromm hydrogen and the depletedd air regenerates the adsorbent.

3 chemical reaction between the surface and the adsorbate, resulting in a higher energy input needed during the regeneration. The cyclic stability is also of importance. An adsorbent with good adsorption properties would not be economically preferable if the stability is low. An expensive adsorbent can still be interesting if it can be used for a longer time. The desired properties of the adsorbents can be summarized by the following list: Choice of adsorbent High adsorption capacity Fast adsorption kinetics Major part of the adsorption capacity should occur through physisorption Low energy input during the regeneration Good cyclic stability Reasonable cost Before analyzing the effect of adding an adsorbent to the SE-SMR process, the right candidates had to be chosen. The following adsorbents where studied: activated carbons, zeolites, metal-organic frameworks (MOFs), calcium oxide, zirconates, orthosilicates, hydrotalcites and alumina. Activated carbons, zeolites and MOFs are negatively affected when the temperature and humidity are increased. The selectivity of carbon dioxide adsorption by zeolites are significantly lowered when non polar species are present, while the MOFs are adsorbing through molar weight and decompose at temperatures over 400 C. The calcium oxide is majorly adsorbing through chemisorption and need high regeneration temperatures. High regeneration temperatures affect the cyclic stability negatively. The most interesting adsorbents for application within the SE-SMR process were hydrotalcites, zirconates, orthosilicates and alumina. Hydrotalcites have proved to have a good stability, and interesting adsorption properties which can be modified by promoting with various alkaline and alkaline earth metals. The major part of the adsorption is occurring through physisorption, which means that the energy input during regeneration will be preferable. That was the reason why hydrotalcites were chosen for further studies. The zirconates and orthosilicates showed interesting capacity, but the adsorption rate and the stability were not favorable. Alumina has been used like the most common catalyst carrier, and has proved to be selective against adsorption of carbon dioxide. The properties could be modified within the temperature range of C by promoting with alkaline and alkaline earth metals. Alumina was chosen due to better adsorption rate and cyclic stability in humid environment. Method The hydrotalcites and alumina were further studied by using the characterization methods; BET, TPD, Chemisorb- and fixed bed measurements. The adsorbents that performed best were used to investigate the effect of the adsorption when producing hydrogen. Before performing the experiments the choice of particle size was determined by considering gradients in the bed. Temperature gradients, bypassing and axial dispersion could be avoided by preparing adsorbents with small particle sizes. A to small particle size would create a pressure drop in the reactor witch had to be avoided. When performing the hydrogen production test the bed dilution also had to be considered. Preparation of the adsorbents Hydrotalcites with ratios 30:70 and 70:30 wt% of magnesium:aluminum oxides in the ground structure were obtained from Sasol. They are further mentioned MG30 and MG70, which indicates the magnesium content. The obtained powder had a to small particle size when considering the phenomena s in the

4 process. The powder had to be tableted followed by a grinding and sieving procedure to obtain particles with a diameter between mm. The alumina was obtained from BDH with a particle size of 2-4 mm. It was grinded and sieved to obtain the same particle size like the hydrotalcites. After the right particle size was obtained the sieved adsorbents were activated. The activation was performed by increasing the temperature from ambient to 500 C with a heating rate of 10 C/ min in a Thermolyne 6,000 furnace oven. The temperature was held constant at 500 C for 4 hours, followed by a cooling procedure. The adsorbents were finally promoted with potassium carbonate from Sigma-Aldrich that had a purity above 99%. The promotion was performed according to Nataraj s dry impregnation method [5]. Before promoting the adsorbents the pore volumes had to be determined by using a BET-equipment. BET-measurements A Micromeritics ASAP 2400 was used to perform the BET-measurements, which explains the physical adsorption of gas molecules on a solid surface at different pressures. The experiments were performed to receive information of the samples morphology, such as specific surface area, pore volume and average pore diameter. The results were obtained by adsorption and desorption of nitrogen. Adsorption capacity measurements The CO 2 adsorption capacities were measured by using three different methods; chemisorption, temperature programed desorption (TPD) and fixed bed adsorption. The chemisorb provided with adsorption isotherms for the desired partial pressures of carbon dioxide. The TPD-measurements gave information regarding the adsorption capacity and how strong the carbon dioxide was bound to the adsorbent, while the fixed bed was mainly performed to study the adsorption of the promoted adsorbents Chemisorb measurements The CO 2 adsorption capacities of unpromoted alumina, hytrotalcite with MG30 and MG70 were measured by using a Micromeritics ASAP 2010C. Before starting the measurements a degasing procedure was performed, where approximately g of activated sample was heated to 450 C and evacuated for 24 hours. The main purpose of degasing the samples was to remove water. After the degasing procedure, the samples were analyzed according to table 1. Table 1. The programed analysis scheme used in the Micromeritics ASAP 2010C. Procedure Time (min) Evacuation 10 Helium flow 20 Evacuation 10 Leak test Evacuation 10 Analysis The adsorption isotherms were obtained by measuring the CO 2 adsorption capacity at 14 different pressures, between bar, at a specific temperature (300, 400 or 500 C). The carbon dioxide used in the measurements was ordered from Aga gas AB and had a purity of mol%, while the Helium was ordered from Air products with a purity of mol%. TPD measurements A Micromeritcs TPD/TPR 2900 was used during the TPD measurements to study the adsorption capacity and how strong the carbon dioxide was bound at different temperatures. When performing the measurements two different gases were used. Pure helium (99.995%) from Air products was used as carrier gas, while a mixture containing 5 mol% of carbon dioxide in helium was used as analysis gas. The amount of carbon dioxide

5 that was adsorbed and desorbed was measured through deviation in thermal conductivity. The test started by degasing the sample with a helium flow at 590 C, followed by decreasing the temperature to the experimental (300, 400 or 500 C). The flow was then switched to the analysis gas until the adsorbent was saturated by carbon dioxide. After the saturation the flow was switched back to pure helium, making it possible to measure how much of the carbon dioxide that was physisorbed. The chemisorbed CO 2 was determined by implementing a temperature ramp where the temperature was increased to 590 C, with a heating rate of 30 C/min. Construction of the rig The rig was built and dimensioned to suit a space velocity of 3,000 h -1, which is a common rate when producing hydrogen from steam reforming of methane [6]. It was used both during the fixed bed adsorption measurements and when hydrogen was produced from steam reforming of methane. Figure 2 shows a flowsheet of the system used for the fixed bed adsorption measurements. The difference between the processes for fixed bed measurements and hydrogen production was the ingoing flow and the reactor content. Fixed bed measurements The testing procedures started by loading the reactor with preheat material and adsorbent. Alpha-alumina was chosen as preheat material because of its low active surface area and good mechanical properties. The amount of alpha-alumina and adsorbent in the reactor was 45 and 8 gram respectively. The adsorption capacities were calculated by integrating the area between the breakthrough curves obtained from the adsorption tests. Hydrogen production The testing procedure started by loading the reactor with 35 g of the preheating material alpha-alumina, and a physical mixture containing 5 g of the catalyst FCR-HC14 and 8g of the adsorbent. The inlet flow of water and methane was set to 400 respectively 100 ml/min to achieve a steam carbon ratio of 4 and gas hourly space velocity (GHSV) of 3000 h -1. The gas mixture was preheated at 120 C and reacted endothermically at 300 and 400 C in the reactor. The outlet concentrations of hydrogen, methane, carbon monoxide and carbon dioxide were then analyzed with the IR-instrument. Figure 2. A flowsheet of the rig used in the fixed bed measurements.

6 Results and discussion BET measurements The BET- measurements showed that the surface area of the unpromoted adsorbents were highest for alumina and decreased with increased content of magnesium, see table 2. The activation at 500 C lowered the surface area of alumina while it increased the surface area of the hydrotalcite MG70. The activation was also performed at 400 C, which resulted in a higher surface area compared to the activation at 500 C. The activation did also affect the pores by increasing the pore volume and average pore diameter. The adsorbents were promoted after the activation, which resulted in a lower pore volume and higher average pore diameter. The increase in average pore diameter indicated that the small pores were first filled with the potassium carbonate salt. Chemisorption measurements The results obtained from the chemisorption tests showed that the adsorption capacity was increased with the partial pressure of carbon dioxide for all samples. Another clear trend was that the adsorption capacity was decreased with increased temperature in the range C. The highest capacity was obtained for the hydrotalcite MG70 sample, followed by hydrotalcite MG30 and alumina. Figure 3 shows the adsorption isotherms for the unpromoted adsorbents. TPD measurements The adsorption capacities obtained from the TPD-measurements were, like in the chemisorption measurements, highest for the hydrotalcite samples and decreased with increased temperature. The adsorbent used in the SE-SMR process would be regenerated many times and the adsorption capacity was therefore not the most important parameter. A more important parameter was the adsorption capacity obtained from reversible bound carbon dioxide. The reversible bound carbon dioxide was most common for alumina at 300 C but decreased at higher temperatures, where the hydrotalcite showed a higher capacity, see table 3. Hydrotalcite MG70 promoted with 0, 10 and 20 wt% K 2 CO 3 were also studied to see how the degree of promotion affected the adsorp- Table 2. The results obtained from the BET-measurements. BET surface area (m 2 /g) Pore volume (cm 3 /g) Average pore diameter (Å ) Alumina Alumina activated Alumina 20 wt% K 2 CO MG MG30 activated MG30 20 wt% K 2 CO MG MG70 activated 400 C MG70 activated 500 C MG70 10 wt% K 2 CO MG70 20 wt% K 2 OC

7 Adsorption Capacity (mol/kg) 0,35 0,3 0,25 0,2 0,15 0,1 0, ,2 0,4 0,6 0,8 1 Partial Pressure of Carbon Dioxide (bar) Alumina 300 C Alumina 400 C Alumina 500 C MG C MG C MG C MG C MG C MG C Figure 3 Adsorption isotherms for the unpromoted adsrobents. tion properties. A promotion of 10 wt% increase the capacity drastically, but a further promotion up to 20 wt% had no significant effect. The promotion also increased the proportion of reversibly bound carbon dioxide. Fixed bed measurements The fixed bed measurements were performed to study the adsorption properties for the promoted alumina and hydrotalcite samples. Unfortunately, one of the MG30 batches showed aberrant characteristics and was not included in this report. The aberrant characteristics were probably obtained because of different tableting technique. Figure 4 shows a breakthrough test, in this case for promoted alumina, hydrotalcite MG30 and hydrotalcite MG70 at 400 C. Table 3 The TPD results obtained for the unpromoted samples. Adsorption (mol/kg) The alumina had the fastest adsorption rate, resulting in a high slope in the beginning of the test. A high adsorption rate makes it possible to use a larger part of the adsorption capacity and reduces the slip of carbon dioxide. The promoted hydrotalcites had higher adsorption capacity but lower adsorption rate compared to promoted alumina at 400 C. Table 4 shows the adsorption capacities calculated from the fixed bed adsorption measurements. The table shows that promotion affected the adsorption capacity positively for both alumina and the hydrotalcite samples. Alumina had a higher adsorption rate compared to the hydrotalcites, both when it was promoted and unpromoted. The table also shows that the adsorption capacities of the promoted hydrotalcite samples increased with Physisorbed (mol/kg) Chemisorbed (mol/kg) Adsorptiondesorption (mol/kg) Alumina 300 C Alumina 400 C Alumina 500 C MG C MG C MG C MG C MG C MG C

8 the temperature. This can be compared with the promoted alumina, which capacity decreased with the temperature. The results were in good agreement with Oliveira et al s. study of alkiline promoted hydrotalcite samples [7]. Outlet CO2 Concentration (mol%) Breakthrough Curves for Promoted Adsorbents at 400C No adsorbent 5 Alumina 20 wt% K2CO3 HTC MG30 20 wt% K2CO3 HTC MG70 20 wt% K2CO Time (s) Figure 4. Break through curve for promoted adsorbents at 400 C. Table 4. The adsorption capacities calculated from the fixed bed adsorption measurements. Test Adsorption capacity (mol/kg) Alumina 300 C Alumina 400 C Alumina 20 wt% K 2 CO C Alumina 20 wt% K 2 CO C HTC MG C HTC MG30 20 wt% K 2 CO C HTC MG30 20 wt% K 2 CO C HTC MG C HTC MG C HTC MG70 20 wt% K 2 CO C HTC MG70 20 wt% K 2 CO C Hydrogen production Experiments on hydrogen production were carried out to investigate the influence when adding an adsorbent to the steam reforming process. Figure 5 shows how the outlet concentration of hydrogen varied with time at a temperature of 300 C. The black line shows the result obtained when no adsorbent was present. The red line represents the results when physically mixing 8 grams of promoted alumina with 5 grams of catalyst, and the blue line represents when 8 grams of MG70 was physically mixed with 5 grams of catalyst. The addition of promoted adsorbents had a significant influence on the outlet hydrogen concentration, which increased with almost 45 percent until the adsorbent was saturated. The promoted alumina had a faster adsorption rate compared to the promoted hydrotalcite MG70, which resulted in a sharper peak. The area of the peak was larger for the promoted alumina at 300 C, which was a result of the higher adsorption capacity see table 4. Outlet Concentraion of Hydrogen (mol%) Hydrogen Production at 300 C Alumina 20 wt% K2CO3 HTC MG70 20 wt% K2CO3 No adsorbent Time (s) Figure 5. The effect of adding promoted alumina and hydrotalcite MG70 when producing hydrogen at a temperature of 300 C. The hydrogen experiments were also studied at 400 C, see figure 6. An increase in temperature had a significant impact on the hydrogen production. The production of hydrogen at 400 C was increased with 20 %

9 when adding the adsorbents. The promoted alumina had still a higher adsorption rate compared to the hydrotalcite. The adsorption capacity for the hydrotalcite was higher than the alumina s at 400 C, which resulted in a larger peak. Outlet Concentraion of Hydrogen (mol%) Hydrogen Production at 400 C Alumina 20 wt% K2CO3 HTC MG70 20 wt% K2CO3 No adsorbent Time (s) Figure 6.The effect of adding promoted alumina and hydrotalcite MG70 when producing hydrogen at a temperature of 400 C. When the adsorbents were saturated, equilibrium was reached that was strongly temperature dependent. The reaction temperature was 5 C higher during the hydrotalcite experiment, which resulted in a higher equilibrium. Except the small difference in temperature the hydrogen production could have been affected by a difference in heat exchange surface. When performing the hydrogen production the reactor contained the same amount of material, but the α-alumina had a higher density than hydrotalcites and γ- alumina which created a difference in cavity and heat exchange surface. The hydrogen production was more affected by small temperature differences at lower temperatures than at high. Another explanation can be that carbon was produced; because alumina has Lewis acidic sites while the promotion creates Brönstedt basic sites which don t charge compensate each other. The carbon was not noticed when the reactor was cleaned, which indicates that the equilibrium differences are probably due to the heat exchange surface. The conversion of methane was also increased when an adsorbent was added to the system. Conclusion The aim with this thesis was to find an implementable adsorbent for the SE-SMR process. Alumina and hydrotalcites turned out to be the two best alternatives due to the requirements of the adsorbents. As unpromoted, the hydrotalcites showed a slightly higher adsorption capacity within a temperature range of C. Interestingly, alumina had the highest proportion of reversibly bound carbon dioxide at a low temperature, which is an important parameter for an adsorbent that will be recycled many times. Alumina also showed the highest adsorption rate. A high adsorption rate results in a cleaner product and makes it possible to utilize a larger part of the adsorption capacity. The adsorption capacity was significantly increased for both alumina and the hydrotalcites when they were promoted with potassium carbonate. Promoted alumina had the highest capacity at 300 C but hydrotalcites were better at higher temperatures. Alumina showed the fastest adsorption kinetics even if the adsorbents were promoted. An addition of adsorbents to the steam reforming process improved the hydrogen production significantly, and the adsorption properties were clearly reflected. Adsorbents with higher capacity increased the outflow concentration of hydrogen during a longer time, while adsorbents with higher kinetics reached a high hydrogen production level fastest. The small differences in the reactor temperature had an influence on the conversion of methane, and the level of hydrogen in the outflow. The levels of methane conversion and hydrogen in the outflow did not reach the same level at the equilibrium, which could be described by the difference in heat exchange surfaces of the adsorbents. In order to designate which adsorbent that is most suitable for the process; further work has to be carried out considering cyclic stability and the economical aspect.

10 Reference list 1. Halabi, M.H., et al., A novel catalystsorbent system for an efficient H 2 production with in-situ CO 2 capture. International Journal of Hydrogen Energy, (5): p Reijers, H.T.J., et al., Hydrotalcite as CO 2 Sorbent for Sorption-Enhanced Steam Reforming of Methane. Industrial & Engineering Chemistry Research, (8): p Sircar, S., Pressure Swing Adsorption. Industrial & Engineering Chemistry Research, : p Hedin, N., L. Andersson, L. Bergström and J. Yan, Adsorbents for the postcombustion capture of CO 2 using rapid temperature or vacuum swing adsorption. Applied Energy, : p Nataraj, S., J.R. Brzozowski, B.T. Carvill, T.R Gaffney, J.R Hufton, S.G. Mayorga, Materials selectively adsorbing CO 2 from CO 2 containing streams. European patent specification No. EP B1, issued 28 april Rase, H.F., Chemical Reactor Design for Process Plants. John Wiley & Sons, New York, 1977, Second Edition. 7. Oliveira, E.L.G., C.A. Grande, and A.E. Rodrigues, CO2 sorption on hydrotalcite and alkali-modified (K and Cs) hydrotalcites at high temperatures. Separation and Purification Technology, (1): p