Simulation,Optimization And Control Of VPSA Process For CO₂ Capture From Dry Flue Gas

The greenhouse effect that caused by excessive discharging of carbon dioxide is becoming the focus of all nations in world.‘The Paris Agreement’which China signed in Sep.2016 has clearly pointed out that the emission of greenhouse should be controlled effectively.Among all the CO₂ produced every year,discharging via the form of flue gas has accounted for 40%,which is the major source.Finding an efficient and low energy consumption way to capture CO₂ from dry flue gas has already been the key in greenhouse gas capture.Meanwhile,owing to its low energy consumption,highly degree of automation,easy operation and simple device,pressure swing adsorption has caught widely attention in the field of CO₂ capture from dry flue gas.This thesis mainly focus on simulation,optimization and control about the process of CO₂ capture from dry flue gas via pressure swing adsorption.On the one hand,a more simple and convenient two-bed one-stage VPSA operation was built and validated via experiment which could help the work closer to reality.On the other hand,lowest energy consumption oriented optimization and product purity control under disturbances of feed composition variation were carried out based on model and newly-built process.This could enhance the separation’s economy efficiency and investigated major factors’effect on product purity and recovery.Experiment for N₂/CO₂ separation adopting the vacuum pressure swing adsorption process based on the two-bed one-step experimental set-up using silica gel as adsorbent was firstly carried out.The VPSA mathematical models and parameters were verified by comparing results between experiments and simulation.Based on these result,energy consumption was set as the objective to be minimized in the optimization process.The optimization process was built in software gPROMS and constraints were given according to real industrial production condition.The bed mathematical models were discretized in 1-D over space and employed the reduced space successive quadratic-programming algorithm to minimize the energy consumption.By adjusting operate variables like opening of valves and flowrates of pumps,energy consumption has been successfully reduced from 623.64kWh/t to419.99kWh/t by adjusting valve coefficient and flowrate,which could help enhancing the economy efficiency of the VPSA process.Afterwards,the PID control strategy was established and embedded into the VPSA process to tackling the feed composition changes in real industrial process.Constraints on amplitude and various ranges of control variables were given according to real industrial conditions and inherent property of the VPSA process.Transient impulse of CO₂ concentration in feed dropped from 15%,the highest possible concentration in reality,to the lowest possible concentration of 12%.The competence of the controller was investigated by comparing product’s purity under open-loop and closed loop condition.By analyzing gas phase contribution distribution under initial condition,open-loop condition and closed-loop condition,the effect of controller on process in bed was well studied.Finally,random disturbances was introduced into bed and inspected product’s purity under open and closed-loop condition to make the simulation work more reality.The results confirmed that by adding the PID controller into the system,impact of outside disturbances on the VPSA system could be well weakened as well as maintain product purity at set point of 90%with relatively higher recovery.