Simulation And Optimization Of Biogas Decarbonization By Activated MDEA Process

Biogas utilization is now mainly focused on its low grade thermal energy in our country. Facing great crisis for fossil energy depletion and global warming, high-end utilization ways of biogas are being explored. After purification, clean biogas can substitute natural gas or be used as vehicle fuel. The volume content of CO₂ in biogas can be as high as 30～40v/v%, and it should be below 3v/v% after purification to reach the standard of compressed natural gas as vehicle fuel. Comparing many kinds of decarbonization methods and based on the characteristics of biogas, activated methyldiethanolamine（MDEA） process was used. Activated MDEA process was simulated and then optimizd to for energy conservation.This article established the model of biogas decarbonization by activated MDEA process using Aspen Hysys. Biogas yield was 10,000m³/d. The composition of lean absorbing liquid was 35wt% MDEA, 3.5wt% PZ and 61.5wt% H2 O. After absorption treatment, the volume content of CO₂ in product gas was about 2.5v/v%. Related parameters of each operating unit, material balance calculation and energy balance calculation were given. Simulation results give the conclusion that, under proper conditions, biogas after activated MDEA process is feasible to meet the requirements of compressed natural gas as vehicle fuel.The parameter sensitivity analysis showed effects of major operating parameters on decarbonization and system energy consumption. Optimal ranges of these parameters were suggested by modeling analysis. And two operating factors which had significant influence on the system energy consumption were determined. One was the flow ratio of amine solution; the other was the temperature difference between inlet and outlet liquid of the regeneration tower.Three suggestions for process improving were put forward for energy conservation. The first method was grading absorption, which means the process changed from conventional process into a two-stage absorption & two-stage desorption process, the second was the concourse of the two streams containing the condensate separated from regeneration gas and the lean amine solution, and the last method was to install an internal cooler at the bottom of the absorption tower. The two-stage absorption & two-stage desorption process was not able to reduce energy consumption. The second method turned out that it worked. Compared to the conventional process, heat flow of the reboiler, cooler and condenser decreased 16.35%, 7.47% and 28.14%, respectively. This improvement corresponds to adding one more tray at the top of the regeneration tower, which means that the rich amine solution was regenerated more completely. At the foundation of the second method, an internal cooler was installed at the bottom of the absorption tower. Then, the flow ratio of amine solution was reduced. Moreover, heat flow of the reboiler and cooler decreased 2.82% and 32.12%, respectively. Heat flow of the condenser was almost unchanged. In addition, with total operation cost as target, economic model for biogas decarbonization by activated MDEA process was established to evaluate economy of the process.