Proceedings World Bioenergy 2010

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in the adsorber is separated, carbon dioxide is adsorbed entirely and pure methane is obtained. Adsorption lasts for about 950 sec and is ended when breakthrough of carbon dioxide occurs. Figure 3: TSA process with desorption by indirect heating with hot water Desorption is achieved by heating the adsorbent indirectly with hot water. Thereby, water is heated and is let through tube bundles placed inside the adsorber. As a consequence, heat exchange between the hot water and the cold adsorbent takes place and heats the adsorbent. During the desorption step methane is desorbed at first. Methane comes to the most part from the space between the adsorbent grains (porosity approx. 45%) as well as from the space under the adsorber, since adsorption capacity of the adsorbent with regard to methane is quite low. The peak of carbon dioxide desorption occurs after methane desorption, but after this peak the flow rate of carbon dioxide decreases steadily and in the end remains at a very low level. Desorption lasts about 2000 s what is twice as long as adsorption time. Referring to temperature, the temperature increase during the adsorption step indicates the arrival of the adsorption layer. Adsorption is an exothermic process, so heat is released and it heats the adsorbent. This phenomenon can be clearly seen with the steep rise of the temperature of 30°C. Although cold water is applied during the entire adsorption step, its coolness is not sufficient in order to carry off the released heat. Nevertheless, cold water application has an effect because after the peak the temperature decreases very fast and this allows adsorption to take place for a prolonged time period. Temperature during desorption increases steadily but it takes about 2000 s in order to get to a temperature of 70°C. Regarding the following cycles of this experiment, adsorption capacity in each of them decreased to one third of the capacity in the first cycle. The reason for this lies in the only partial desorption of carbon dioxide. Desorption by heating with hot water does not allow complete desorption of the entire adsorbed gas components; only partial desorption, correlated to the equilibrium and adsorption capacities at certain temperatures, is possible. Desorption by indirect heating with water is considered as a very inefficient way of desorption due to long desorption time as well as low desorption efficiency. 7.2 Desorption by indirect heating with hot water and afterwards purging Figure 4 depicts the TSA process with desorption 82 world bioenergy 2010 carried out by indirect heating with hot water and afterwards purging with nitrogen. This process is very similar to that described above with one difference occurring not until the end of the desorption step. During the adsorption step separation performance is excellent as well and also here adsorption lasts for about 950 s. Within the first 2000 s of desorption, desorption is also carried out indirectly by application of hot water functioning as heat exchange medium. The characteristics of desorbed gases are similar too, with methane desorbing at first followed by carbon dioxide. Figure 4: TSA process with desorption by indirect heating with hot water and afterwards purging After 2000 s of indirect heating, no more desorption of methane or carbon dioxide takes place. As already explained above, complete desorption is not achievable by indirect heating and therefore purge gas (N 2) was applied, for about 200 s. Desorption with purge gas allows complete desorption due to changes in partial pressure and the correlated adsorption capacity at equilibrium. As a consequence, still adsorbed gas components should theoretically be desorbed. Figure 4 shows clearly that application of purge gas leads to further desorption of gas components. Obviously, the desorbed gas consists for the most part of carbon dioxide but also contains small amounts of methane. Regarding temperature, during the desorption step of indirect heating with hot water, temperature increases steadily but during purging temperature decreases again, although the purge gas is preheated to 75°C. The reason for temperature decrease is the desorption process itself which is of endothermic nature. Desorption of this remarkable amount carbon dioxide and methane needs energy and this causes temperature decrease. Referring to the following cycles of this experiment, adsorption capacity in each of them decreased only by approximately 1 w% compared to the capacity in the first cycle. Desorption by indirect heating with water is considered as a rather inefficient. However, application of purge gas led to further desorption of methane and carbon dioxide, what causes extended adsorption capacities (compared to desorption only with hot water) in the following cycles and therefore improves the entire process efficiency. 7.3 Desorption by indirect heating with hot water and simultaneous direct heating with purge gas Figure 5 illustrates the TSA process with desorption carried out by indirect heating with hot water and simultaneous direct heating with purge gas.
Once again, the binary gas mixture passing through the adsorber is separated during the adsorption step, carbon dioxide is adsorbed up to 100% and pure methane is obtained. Adsorption lasts for about 950 s and is ended when breakthrough of carbon dioxide occurs. Figure 5: TSA process with desorption by indirect heating with hot water and simultaneous direct heating with purge gas Desorption is carried out by heating the adsorbent indirectly with hot water and at the same time directly with preheated purge gas (N 2). During the desorption step methane and carbon dioxide are desorbed quite simultaneously. The concentrations depicted in figure 5 are rather low but this is due to the high purge gas flow rate of 25 Nl/min. Desorption lasts for about 1000 s what is in the same time range as adsorption time. Referring to temperature, during the desorption step temperature increases faster compared to desorption ways described above; it takes only about 1000 s instead of 2000 s to reach a temperature of 70°C within the adsorber. The reason for this fast temperature increase is on the one hand the combined heating and on the other hand the high purge gas flow rate which enhances the heat transfer. The temperature profile shows one inflection point during the desorption step. The inflection point indicates the almost complete desorption of carbon dioxide. From that point on, supplied heat is used mainly for adsorbent heating, leading to a more steeply rise of the temperature profile. Regarding the entire experiment, adsorption capacity remained stable in each cycle. Desorption by indirect heating with hot water and at the same time direct heating with purge gas is a very effective way of desorption. This combined way of heating has two effects. First, temperature rises faster compared to only indirect heating, leading to a shorter time period of desorption. And second, desorption is very effective indicated by stable adsorption capacities in each cycle. 8 CONCLUSION This paper presented a novel process for biogas upgrading by means of temperature swing adsorption (TSA). TSA process experiments were performed in a laboratory test rig. Results clearly show that biogas upgrading by TSA is feasible. Separation performance is excellent since during the adsorption step carbon dioxide is adsorbed to up to 100% and pure methane is obtained. Experiments also investigated different ways of desorption. Thereby, desorption by any combination of direct and indirect heating is considered to be the best and most efficient way of desorption, in terms of desorption time as well as of desorption efficiency. One drawback arises from purge gas application. Nitrogen as purge gas causes dilution of the desorbed gas, making the valuable desorbed gas complicated to handle for further utilization. In order to avoid dilution, alternative purge gases such as carbon dioxide or methane have to be evaluated. 9 REFERENCES [1] P. Weiland, Biogas, Thieme Römpp Online (2010) [2] M. Persson, O. Jönsson and A. Wellinger, Biogas Upgrading to Vehicle Fuel Standards and Grid Injection, IEA Bioenergy (2006) [3] W. Urban, K. Girod and H. Lohmann, Technologien und Kosten der Biogasaufbereitung und Einspeisung in das Erdgasnetz. Ergebnisse der Markterhebung 2007- 2008, Fraunhofer UMSICHT (2009) [4] A. Petersson and A. Wellinger, Biogas upgrading technologies – developments and innovations, IEA Bioenergy (2009) [5] W. Kast, Adsorption aus der Gasphase, VCH (1988) [6] Resindion, Mitsubishi Chemical Corporation, Product Data Sheet and Material Safety Data Sheet (2004) [7] E. Buss, Gravimetric measurement of binary gas adsorption equilibria of methane-carbon dioxide mixtures on activated carbon, Gas Sep. Purif. Vol. 9 No. 3, Elsevier (1995) [8] H. Feichtner, Experimente und numerische Berechnungen zur Entwicklung eines Festbettverfahrens zur Abtrennung von Kohlendioxid aus Biogas durch Adsorption an einem polymeren Adsorbens, PhD Thesis, Vienna University of Technology (2007) 10 ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support of Klima- und Energiefonds. Dieses Projekt wurde aus Mitteln des Klima- und Energiefonds gefördert und im Rahmen des Programmes “ENERGIE DER ZUKUNFT“ durchgeführt. world bioenergy 2010 83
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