Simulation of CO2 Separation from Flue Gas by Cryogenic Process

Abstract

Separating carbon dioxide (CO2) from flue gas is challenging due to the energy penalty. Developing a cryogenic or low-temperature CO2 separation method helps overcome the energy penalty associated with CO2 separation. This study aims to develop an energy process to treat coal-fired power plants’ flue gas. In this research work, as a first step, a preliminary comparative study is carried out between the theoretically modified Linde and external cooling processes for the separation of CO2 by liquefaction. The study results reveal that CO2 separation by the external cooling process is better than the modified Linde process. A separation unit, heat exchanger, and compressor are essential components for CO2 separation by external cooling. Generally, three separation methods, namely gas-liquid separation, flash separation, and column distillation are used in industrial applications. So, as a second step, a sensitivity analysis is carried out for these three methods using Aspen Plus. This is to determine an appropriate method for separating CO2 from a gas stream containing N2 and CO2. The study results reveal that the "RadFrac" column distillation model is a rigorous model for simulating all types of multistage vapor-liquid fractionation operations. The column distillation model provides a better model for CO2 separation compared to the other two methods. As a third step, a sensitivity analysis of the heat exchanger is conducted using the Aspen Exchanger Design and Rating for the N2-CO2 mixture. The study results reveal that the heat transfer coefficient increases with an increase in CO2 concentration in the hot fluid. The pressure drop decreases with an increase in CO2 concentration in the hot fluid. A fourth step is a sensitivity analysis of a compressor using Aspen Plus for an N2-CO2 mixture at various CO2 concentrations (10–90%). Compressor load decreases with an increase in CO2 concentration in the feed gas. As a fifth step, a cryogenic column distillation separation process is developed to separate CO2 from N2-CO2 mixtures. The findings show that the energy penalty is low for highly concentrated CO2 feed gas in this process. Coal-fired power plants emit 13-15% CO2 and other pollutants. As a result, the developed cryogenic column distillation separation process consumes more amount of energy. The membrane-cryogenic column distillation separation method overcomes this difficulty. Therefore, as a sixth step, membrane modeling for hollow fiber membranes is performed, and an in-house computer program is developed using MATLAB software. The effects of the number of tubes, length of the tube, feed pressure, and permeate pressure on purity and recovery are studied. The membrane model is optimized using multi-objective JAYA algorithm. The objective function for optimization is obtained from the Response Surface Method (RSM) in Design Expert software. The study results reveal that hybrid processes have lower energy penalties than stand-alone conventional processes. As a seventh step, the hybrid process's energy savings are further improved by utilizing waste heat available in the hybrid process's compression train. For this purpose, the organic Rankine cycle (ORC) is coupled to the hybrid separation system (membrane-cryogenic CO2 capture). This arrangement improves separation system efficiency. The energy, exergy, economic, and environmental analyses (4E) of the hybrid separation process are carried out. The energy penalty of the membrane-cryogenic distillation CO2 separation process with ORC is 1.33 MJ/kg of CO2. The exergy loss of the membrane-cryogenic distillation CO2 separation process with ORC is 9.05 kW for capturing 19.9 kg/hr CO2 from the flue gas's 106.28 kg/hr flow rate. The specific capital and operating cost of the membrane-cryogenic distillation CO2 separation process with ORC is 91.94 million / kg of CO2, respectively. The indirect CO2 emissions of the membrane-cryogenic distillation CO2 separation process with ORC is 5.24 kg/h.