Removal of carbon dioxide from biogas Mirsada Nozic Department of Chemical Engineering,, und University, P. O. ox 14, SE-1 und, Sweden The aim of this degree thesis is to simulate the absorption with water part of one combined technique for upgrading of biogas with the absorption by water and PS (Pressure Swing dsorption). The method of the simulation of process is partly to bring up the necessary relations for each unit that is included in the technique (scrubber, flash, and stripper) and to connect together the units in one common Matlab-code. The two Matlab-codes are brought up, one for counter-current absorption and one for co-current absorption in the scrubber. The methane losses, a recirculation of gas and design K- parameter for the scrubber respective the stripper has been studied. The different parameters of process can have an effect on these variables and the best result is the combination of parameter value. It means that is impossible to indicate the smallest value of each one of these parameters because they affect each other. Introduction The usage of biogas as vehicle fuel has significantly increased in the last years. Consequently the demand for a calculations model for one technique for upgrading of biogas to vehicle fuel is increasing as well. n interesting technique is absorption with water which is the most common technique in Sweden. The aim of this study is to create a Matlab code for simulation of absorptions part of one combined technique with the absorption by water and PS. The target with the simulation is to mae the image that show how different process s parameters affect for the process important design s parameters and to find the conditions witch give the smaller methane losses. The thesis has been carried out in cooperation with iomil, a company with the long experience of production and upgrading of biogas. t present four different techniques for upgrading of biogas are used commercially in Sweden: o bsorption with water o PS (Pressure Swing dsorption) o bsorption with Selexol TM o Chemical absorption with amines The absorption with water or water scrubbing is the most common technique. The technique is used in such a way that the carbon dioxide absorbs better in water due to better solubility than methane. ecause the solubility of carbon dioxide increases with pressure so the separation occurs at high pressure []. The simplified schema for process is shown in figure below (figure1). Upgrading of biogas The biogas is a produced by the anaerobic decomposition of organic matter. It is primarily composed of methane (CH 4 ) and carbon dioxide (CO ) with smaller amounts of hydrogen sulphide (H S), ammonia (NH 3 ) and nitrogen (N ). Usually, the mixed gas is saturated with water vapour [1]. iogas can be used for all applications designed for natural gas. Not all gas appliances require the same gas standards. The usage of biogas as vehicle fuel has significantly increased in the last years. or an effective use of biogas as vehicle fuel it has to be enriched in methane. This is primarily achieved by carbon dioxide removal which then enhances the energy value of the gas to give longer driving distances with a fixed gas storage volume [1]. igure 1. bsorption with water Usually the biogas is pressurized and fed to the bottom of the absorption column where water is fed on the top and so the absorption process is operated counter-currently. The co-current flow is also possible but it is seldom used. In the column, carbon dioxide is absorbed by water and gas out of the column is enriched in methane. The water
which exits the column wits absorbed carbon dioxide and a smaller amount of methane which is partly soluble in water leads to the flash tan there the gas is regenerated by de-pressuring and returned to the absorption column. The regeneration of water is made by stripping with air in the desorption column, the stripper. part from carbon dioxide, the gas which exits the stripper contains methane losses []. igur. Process schema of the upgrading plant with bsorption with water and PS Water scrubbing can be used for the removal of hydrogen sulphide since hydrogen sulphide is also soluble in water. Modeling of bsorption with water - process bsorption with water is purely physical process. It means that it is the absorption without chemical reaction. The mass transfer from the gas to the liquid phase can be described by the two film theory. It is the approximated model which always assume the steady state, but because the simply mathematics expressions it is relatively easy to understand and it give the good accuracy. ccording to the two film theory the resistance to the mass transfer can describes with one or two stagnant films, the gas and the liquid film. ecause the solubility of the gas follows the Henry s law and Henry s constant of carbon dioxide is large, it means that the solubility of carbon dioxide is small and concentration gradient in the liquid phase is large. ccording to the two films theory it result in that the significant resistance for the mass transfer is in the liquid phase and the gas film resistance and gas film itself can be neglected. If the process is controlled by the rate of mean transfer through the liquid film, such system is called for liquid phase controlled system. With these conditions, the al transfer rate of the component (methane) respective component (carbon dioxide) from the gas to the liquid phase in the differential volume at the absorption column (scrubber) is described by equation (1) and (). d p + dv H d p + dv H t t t t (1) () In order the modeling of flash tan requires following equations (for the indexes see the nomenclature and the process schema in figure ): y p = C H (3) T T y p = C H (4) ( C 1 C ) ( 1 y ) = ( C1 C ) y (5) The stripper wors as the convert scrubber and the al transfer rate of component respective component from the liquid to the gas phase in the differential volume at the desorption column is described by equation (6) and (7).
d C dv t t + + + H C p (6) p d + = dk H t t (11) d C dv t t + + + H C p (7) It is assumed that the raw gas content only methane and carbon dioxide. Design parameter K Parameter K is introduced in the material balances for the scrubber and the stripper because the easily dimensioning of the columns. K is defined as dk dv (8) ecause the same parameter should be used for both methane and carbon dioxide, the rewriting of the mass transfer coefficient of carbon dioxide is introduced. ccording to the turbulence model which is the most reliable empirical relation, the mass transfer coefficient for carbon dioxide can be written as D D 3 (9) With the value of diffusivities at C the following relation is received =.9 (1) 1 p d + = 1.9 dk H t t (1) With the same reasoning, parameter K is introduced in the material balances of the stripper. Numerical solution of the model In the numerical solution or the simulation of the process model, the each unit that is included in the technique (scrubber, flash, and stripper), is connected together in one common Matlab-code. The two Matlab-codes are brought up, one for counter current absorption and one for cocurrent absorption in the scrubber. The gas recirculation from the PS and the dryer is added to the code as the constant percent of the gas which enter the unit. The Matlab-code is iterated until the system is converged. The simulation begins with input of the different process parameters which the user inputs from the eyboard. The bloc schema that shows which parameters has been inputted and which parameters are received, is presented in the figure 3. The material balances for methane and carbon dioxide in the counter-currently scrubber can be written as igure 3. The bloc schema for the simulations input and received parameters
Result The important variables to study are the loss of methane, a recirculation of gas and design parameter K for the scrubber respective the stripper. The loss of methane is important both from the economic and the environment point of view. or these reasons it is necessary to eep it as low as possible. The different parameters of process can have an effect on these variables. The base case is chosen and one parameter at a time is varied and its effect is studied. The process s parameters that can be varied are the liquid (water) flow, the pressure in the flash tan, the air flow in the stripper, the amount of stripped carbon dioxide, and the amount of absorbed methane for counter current absorption, and the methane fraction in the gas out of the scrubber for cocurrent absorption. The influence of the pressure in the flash tan on the methane losses and the recirculation of the gas for the counter current absorption are shown in diagram below (figure 4 and 5). Methane losses (%) 6 5 4 3 1 3 4 5 6 7 8 P flash (bar) igure 4. The influence of the pressure in flash tan on the methane losses Recirculation of gas (%) 6 55 5 45 4 35 3 5 3 4 5 6 7 P flash (bar) igure 5. The influence of the pressure in flash tan on the recirculation of gas It is important to have the low water flow, the low flash pressure, and to strip the smaller amount of carbon dioxide to reduce the loss of methane. It is impossible to indicate the smallest value of each one of these parameters because they affect each other, e.g. extremely low flash pressure demands larger water flow. In other words, it is important to find a combination of the parameter values that will give the best answer. The best result with regard to the methane losses is.5 % for the counter current absorption and.9 % for the cocurrent absorption for the process that has been the basis of this thesis with the raw gas flow of 36 Nm 3 /h. Conclusion The choice of the process s parameter values has some limitations. The minimum liquid flow is controlled by the conditions which should been filled, the amount of absorbed methane for the counter current absorption and the methane fraction in the gas out of the scrubber for cocurrent absorption. If the liquid flow is too small, the condition can not been filled and no response for the K scrubber is determined. The pressure in the flash tan is controlled by the liquid flow and recirculation of gas. The extremely low pressure demands larger liquid flow and larger recirculation of gas. The methane losses increase with the amount of striped carbon dioxide because the K stripper is increased and with it, the amount of striped methane is also increased. Over- respective under dimensioned scrubber can be simple regulated by the change of the amount of absorbed methane for the counter current absorption and the methane fraction in the gas out of the scrubber for cocurrent absorption. The regulation of the stripper is preformed by the amount of striped carbon dioxide. In other words, it is important to find the combination of the process s parameter values which will give the best answer for the variable which is more interesting to eep as low as possible. Nomenclature Methane (CH 4 ) Carbon dioxide (CO ) C ir G Gas phase iquid phase System without the chemical reaction t Top of column b ottom of column Mass transfer coefficient, m/s Molar rate of gas, mol/s iquid rate, m 3 /s V Volume of the column, m 3 a Specific surface area, m /m 3 C i Molar concentration of component i, mol/m 3
D Diffusion coefficient, m /s H Henry s constant, Pa m 3 /mol K Design variabel, s/m 3 p, p T Total pressure, Pa y i Mol fraction of component i in the gas phase, dimensionless References [1] Jarvis, Å. (4) iogas renewable energy from organic waste, The Swedish iogas ssociation, Stocholm [] Dahl,. (3) uality fuse of biogas as the vehicle fuel, Swedish Gas Centre (SGC), Rapport 138, Malmö Received for review ebruary 8, 6