1. Introduction
Due to the high energy consumption and decreasing oil supply, the alternative fuels such as natural gas and biomass can be considered as the main sources of energy in the future. Currently, one of the technologies to convert such biomass to more valuable products, syngas, is gasification process. However, these sources including natural gas contain acid gases like carbon dioxide (CO2) and hydrogen sulfide (H2S). These gases are required to remove because of the decrease in product heating values, operational problems, corrosion and environmental concern. The requirement for the minimum level of contaminants in the gas cleaning of CO2 and H2S concentrations from many criteria are less than 5% and 10 ppb, respectively. The basic tools to do preliminary study is to apply process simulation. Nowadays the effective technologies used for acid gas removal is absorption both physical and chemical absorptions. The chemicals commercially used for acid gas removal are monoethanolamine (MEA) and methyldiethanolamine (MDEA) for chemical absorption and dimethylether of polyethylene glycol (DMPEG) or Selexol for physical absorption process. The advantages of these chemicals are that they can capture CO2 and H2S at the same time with various conditions; then they can be used in single acid gas removal unit. Therefore, the data for solubility, physical properties and so on are available from literature. Much of the experimental work on acid gas removal has been studied. The simulation work of acid gas removal from natural gas and gasification processes is very interesting.
Therefore, the objective of this work is to simulate and compare the absorption process for acid-gas treating by using various solutions at different operating conditions in order to meet the requirement of product gas.
2. Simulation
The simulation on acid gas cleaning process by means of absorption column with structured packing is investigated. The study used the ASPEN Plus software package to the process modelling based on aqueous MEA solution for gas processing in natural gas. In this work, the effects of solution concentration, liquid flow rate, temperature and pressure for chemicals used are investigated for the optimal design in the future.
2.1 Simulation of Physical Solvent
The physical solvent used is Selexol. Because there is no chemical reaction, this solvent combined less strongly with CO2 and H2S. The equilibrium concentration of the absorbing agent in the liquid phase is strongly dependent on the partial pressure of that component in the gas phase. The advantage of using such solvents is that these gases can be separated from them in the regenerator by reducing the pressure, resulting in much lower energy consumption.
2.2 Simulation of Chemical Solvent
For chemical absorption, the chemical reactions illustrating the process have been used in simulation. However, the reactions can be simplified by assuming the following conditions:
1. The equilibrium curve is linear over the range of concentrations encountered.
2. The partial pressure of the inert gas is essentially constant over the length of the column.
3. The solute contents of gaseous and liquid phases are sufficiently low that the partial pressure and liquid concentration values may be assumed proportional to the corresponding values when expressed in terms of moles of solute per mole of inert gas (or of solvent).
3. Results and Discussion
The absorption column is practical for acid gas removal. It is used for simulation to study the effect of parameters such as types of absorbing agents, concentration of solutions and liquid flow rate on CO2 and H2S removal from feed gas. The simulation results are based on the data of experimental work from the previous study at 0.9-2.5 MPa absolute and 25-40 °C and concentration of inlet acid gases.
3.1 Effect of Chemicals Used
The effects of the chemicals (MEA, MDEA and Selexol) on acid gas cleaning are studied for CO2 and H2S at the various conditions. The simulation results show that MDEA and MEA are excellent to remove CO2 and MDEA. Also, Selexol is very outstanding in capturing H2S. Moreover, the product gas consists of 3.64 vol% CO2 and 27.24 ppb H2S for MEA, 0.74% CO2 and 0.02 ppb H2S for MDEA and 7.45% CO2 and 1.12*10-6 ppb H2S for Selexol. From the requirement, the product gas should have less than 5% for CO2 and 10 ppb for H2S. Therefore, MDEA is the suitable chemical for this application because it can capture both gases at the required level.
3.2 Effect of Liquid Flow Rate in the Packed Column
It is evident that the liquid flow rate has a direct relationship with the efficiency of acid gas removal in that the efficiency increases as the liquid flow rate increases. This can be explained by the fact that applying a higher liquid flow rate to the system results in a larger driving force for acid gas removal, and consequently a higher rate of removing acid gas by the chemical per unit of time. This leads to a higher overall mass transfer coefficient that is directly proportional to the mass transfer values.
From the viewpoint of liquid phase, at a higher liquid flow rate, the amount of free-amine molecules in the system becomes larger or the system has more capacity to absorb acid gas from the gas phase, thus enhancing the mass transfer. However, an increase in solution flow rate leads to a higher circulation rate and regeneration costs; thus, may not improve the overall system efficiency. Maximizing liquid flow rates may not lead to optimum operating conditions. Therefore, an optimum flow rate has to be determined.
3.3 Effect of Solution Concentration
The effect of solution concentration is an interesting aspect since it affects the design of the process. The results of solution concentration of MEA at 3, 4 and 5 M are studied for both CO2 and H2S removal. At flow rate of 150 kmol/h, the amount of CO2 left in the product gas for 3M MEA is 5.03% and that for H2S is 39.6 ppb. For MDEA solution, the corresponding amount of CO2 remained in the product gas for 3M is found to be 4.00% and that for H2S is found to be 0.05 ppb. Also, the results of MDEA can meet the requirement for the criteria for the amount of acid gas in the product gas. From the results, it is clear that the efficiency of the process can increase as solution concentration increases because the amount of free molecules has increased per unit volume, resulting in more acid gases removed. However, the higher concentration can cause more equipment corrosion and the cost of chemical used as well as liquid viscosity. However, for the case of Selexol, there is no data for effect of concentration because normally Selexol is used as relatively pure chemical.
3.4 Effect of Temperature
The temperatures used in this study are 25 and 40 °C and the effects of system temperature are simulated for all chemicals. At flow rate of 150 kmol/h, MDEA provides better performance among them. In addition, theoretically, at higher temperature, gas has lower solubility because of higher kinetic energy to escape to gas phase. However, these two temperatures show little difference for solubility. Therefore, the practical operating temperature can be at 40 °C especially for the tropical countries.
3.5 Effect of Pressure
The effects of absolute pressure at 0.1, 0.9 and 2.5 MPa in the column are reported for both CO2 and H2S removal with MEA. It is evident that solubility of gases in solution increases at higher pressure corresponding to Henry’s law. Therefore, the operation at 2.5 MPa offers the best solubility for MEA especially at the flow rate of 150 kmol/h. However, the increasing pressure means that extra equipment will be needed and leads to higher capital cost for acid gas removal.
4. Conclusion
Among MEA, MDEA and Selexol, MDEA is the most effective chemical to capture both CO2 and H2S at the studied conditions. The effects of liquid flow rate, solution concentration, temperature and pressure on removal performance are also studied and investigated. From the results, it is evident that the higher liquid flow rate can remove more gas because it has a higher driving force because of the higher amount of free amine molecules per unit of time. Consequently, a higher rate of removing acid gases by the chemicals can be achieved.
For higher solution concentration, the amount of free-amine molecules in the system becomes larger or the system has more capacity to absorb acid gases. However, it is noted that increasing solution concentration or liquid flow rate can lead to a higher circulation rate, regeneration costs, more equipment corrosion, and cost of chemical used. In addition, high pressure and low temperature can provide higher solubility for acid gases but these conditions need some equipment to fulfill their requirement and the cost will be higher.
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