Direct Air Capture: Desorption of CO2 and H2O from amines

Abstract

Zero Emission Fuels(ZEF) is a start up, and ZEF is building a micro plant which is ableto produce bio-methanol from solar energy and air. CO2and H2O are absorbed from theair, the H2O is split into H2which is fed together with the CO2in the methanol synthesisreactor. The reaction product is distilled and Grade AA MeOH is the product. Thisresearch the focus is on the continuous Direct Air Capture unit. The component consistsof 2 sub components, the absorber and the desorber. In the absorber the amine is broughtin contact with the air and absorbs CO2and H2O. As pure CO2and H2O molecules areneeded, the amine is desorbed in the desorber.This research specifically focuses on the desorption part of the DAC, amines(PEI andTEPA) are used to capture the CO2and H2O. The CO2is captured in two ways, via thecarbamate and the bicarbonate route. Carbamate is formed with primary and secondaryamines, bicarbonate is formed with the help of H2O and protonated primary, secondaryor tertiary amines. Bicarbonate is formed at loadings higher than 0.4 [mol CO2/ mol Namine], as the absorption capture capacity is low due to the low CO2partial pressure inthe air, this means that bicarbonate is not formed in the range of ZEF. After absorption30 wt% H2O and the CO2loading is expected to be 1.5 [mol CO2/ mol N amine].A VLE experiment is done to find the equilibrium between CO2in the gas phase andCO2in the amine, using the apparatus is was also possible to find the H2-Amine VLE.The experiment done with a 30 wt% failed due to high viscosity after which the solutionwas not homogeneous anymore. Adding more H2O up to 70 wt% solves the problem.However, the VLE is not accurate enough in low loading range(ZEF range. But, usingtwo isotherms, at 80 and 120°C the heat of absorption can be calculated. The results isa heat of absorption of -73.6 and -68.2 [kJ/mol CO2] for TEPA and PEI, respectively.To find the total desorption of CO2and H2O a desorption experimental setup isbuilt, experiments are performed at three pressures(50 100 and 200 mbar) and increasingtemperatures steps(80,90,100,110,120°C) . As the H2O evaporated at low temperature(<80°C). At higher temperatures than 90°C only CO2was present in the gas phase, hencethe partial pressure is equal to the total pressure. The output of CO2increases withdecreasing pressure and increases with temperature. The maximum output is equal to0.9 [mol CO2/ kg amine] using TEPA. The output of TEPA is twice as high as the outputof PEI.Using the experimental data from both tests, an energy model is made find the energypenalty of the desorption component. A base case study, a simple one-stage flash vesselat T = 120°C and p = 100 mbar, a total energy penalty of 18.1 and 33.6 MJ/ kg CO2,TEPA and PEI respectively. By altering the pressure and temperature the energy penaltycan be reduced to 16.6 [MJ/ kg CO2]. As the CO2demand is known, the correspondingneeded power is equal to 159 [W]. A design is suggested to put two flash vessels in parallelto reduce the H2O output of the system. At at temperature of 100 and 110°C with aconstant pressure of 100 mbar, a minimum of 15 [MJ/ kg CO2] is reached, a reductioni
of 17% compared to the base case. Using a pinch analysis 93 [W] of the total 159 [W]can be retrieved by heat integration. Furthermore, the integration of a specially designedsolar boiler could provide the remained 66 [W] with heat