Study On Purifying The Methane Of The Biogas By Pressure Swing Adsorption

The main composition of biogas is methane, account for the total volume about55%-65%. The rest is mainly CO2. In recent years, the countries all over the world get attention about the industrialization using of the biogas, particularly in Europe, they has used methane as car fuel. But, because of the biogas with higher CO2, it will not only reduce the heat value of the biogas, and increase the cost of transportation and storage. So it is very important to realize the industrialization by effective removal of CO2in the biogas.There are many methods in separating the methane and carbon dioxide in biogas. The method of pressure swing adsorption, because of its low cost, simple auto control, easy operation, won’t cause secondary pollution to the environment etc, and it is considered to be prospect method in separating methane and carbon dioxide.According to the previous PSA application in air, hydrogen gas mixture, the experiment designed and manufactured a manual two-tower pressure swing adsorption device to separate the methane and carbon dioxide of biogas, using this device, to choose the best adsorbent, and explore the process parameters of the best adsorbent. On this basis, in order to realize the circulation of PSA and get the continuous product gas, design and manufacture three towers automatic control PSA device, and using this device, test and optimize the process parameters of the selected adsorbent, and through the experiments to determine the optimum parameters of adsorbent. Research conclusions are as follows:1Refer to the method of separating air, nitrogen and argon by PSA, design and manufacture a manual two-tower PSA device.2According to the literatures, measure the adsorption isotherms of common adsorbents (the silica gel,13X molecular sieve,5A molecular sieve) in different pressure (0.1MPa-0.8MPa), and the results show that:5A molecular sieve in0.7MPa, get the highest methane concentration, about90.24%;13X molecular sieve in0.74MPa, get the second place methane concentration, for75.95%; silica gel in0.7MPa, get the minimum methane concentration, about70.48%, so choose5A molecular sieve is the adsorbent to separate the methane and carbon dioxide of biogas.3Doing the experiments with the two-tower PSA device, measure the breakthrough curves of5A molecular sieve in0.7MPa, the flow rates were8.7L· min-1,16.5L· min-1,22.1L· min-1,27.4L· min-1,34.1L· min-1,40.6L· min-1. The results show that with the increase of flow, the shorter the breakthrough time, get the lower concentration of methane. When the flow is 22.1L· min-1, get a series of the largest concentration of methane, at83.5%. The limit breakthrough time is200s.4In order to get the time of recovering the effective gas when the tower has finished the adsorption, measure the depressurization curve after the tower has finished the adsorption in different flows, it shows that when the time is more than10s, the carbon dioxide in gas has increased; its concentration has been more than that in material. The mixed gas has not been as recycling gas for other towers, it will reduce the concentration of product. So getting the time of effective gas recovery, namely pressure time, should be controlled within10s.5The regeneration property of selected adsorbent by vacuum has been studied. Measure the curve of taking off the adsorbent which has been finished the adsorption and depressurization in different flows, the experimental results show that in every flow, the adsorbent can regeneration completely, use the vacuum pump after a period of vacuum, found out that the gas from the adsorption tower has not included carbon dioxide. The longest vacuum suction time, namely regeneration time is560s. And in flow22.1L· min-1, the time of the adsorbent regeneration is560s.6In order to realize the5A molecular sieve process cycle and get continuous product gas methane, need to let the device achieve automatic control and increase the number of adsorption tower. So in the original device, and on the basis of design and manufacture a set of automatic control three-tower PSA device.7For the realization of the three-tower PSA technology automatic cycle. Need to make sure the adsorption time and pressure time of the adsorbent. Separately measure in0.7MPa adsorption pressure, the flow22.1L· min-1, the adsorption time is170s/180s/190s/200s, the pressure time is5s/10s, the depressurization point is O.1MPa, regeneration time is560s. Get the methane concentration from every tower after the process cycle of three-tower PSA, the result is in adsorption time is180s, the pressure time is10s, the methane concentration is the highest, the average of three towers is87.71%, the highest is88.87%. The condition is the best technical parameters to realize the removal of carbon dioxide in biogas by the three-tower PSA.