Development of Mimic Enzyme-based Membrane and Membrane Contactor for CO2 Capture

Abstract

Emission of greenhouse gases especially CO2 has become a major environmental issue. In order to control and reduce the CO2 emissions, carbon capture and storage (CCS) is an important tool. Post combustion CO2 emission is the largest point source of CO2 emissions. Both membrane technology and CO2 capture by absorption are the most investigated techniques in the field of post combustion CO2 capture but due to high gas volume and low partial pressure of CO2 in post combustion emissions, capturing CO2 by both these technologies is an economical and operational challenge. Research is going on in the field of membrane technology to develop highly permeable and CO2/N2 selective membrane to develop a robust and inexpensive separation process. At the same time advancements in the field of fast reacting and highly selective absorbents are also taking place to make the CO2 capture process more efficient and economically viable. One approach to address these challenges is to look at the nature’s way of CO2 separation at low pressure i.e studying the respiratory system of animals and use a similar approach. Enzyme (carbonic anhydrase) promoted CO2 capture system is a bio-mimicking process that achieves efficient CO2 separation by mimicking the mechanism of the mammalian respiratory system. Enzymatic membranes for CO2 separationist is a fast and efficient process but due to high cost of enzyme, its limited lifetime, and loss of catalytic activity by thermal and chemical poising, naturally occurring carbonic anhydrase is not considered to be a feasible choice for industrial application. The metal organic compounds that mimic the catalytic ability of enzymes have gained much needed attention due to their high activity and thermal/chemical stability. These are low molecular compounds with a metal atom and organic ligands. The active site of these compounds is similar to that of carbonic anhydrase enzyme and their CO2 hydration is also comparable to that of naturally occurring carbonic anhydrase. After a detailed literature study, the metal organic complex of Zinc metal as catalyst active site and cyclen ligands were selected for this work due to its high catalytic activity, high thermal/chemical stability. Zinc-cyclen also referred as mimic enzyme can be used to promote CO2 hydration in a chemical absorbent or facilitate CO2 transport in a membrane. The objective of this work is to evaluate the performance of mimic enzyme in a facilitated transport membrane and membrane contactor for CO2 capture at post combustion conditions. Mimic enzyme synthesised in lab was characterized with the help of H1NMR and ESIMS to validate its existence. This mimic enzyme was tested for dissociation constant (pKa) to determine the pH at which catalyst is activated. The effects of catalyst concentration in solution and operating temperature on pKa of catalyst were studied to study the speciation at different pH levels of solution. The zinc metal in mimic enzyme is coordinated by four amine groups of cyclen and one water/hydroxyl group is temporary attached to it. Depending on the pH of solution, the catalyst shifts between water and hydroxyl ligands. The pH of solution must be maintained over pKa of mimic enzyme to ensure high catalytic activity. Keeping in view that mimic enzyme requires aqueous environment to operate, highly water swollen PVA membrane was selected for this work. A mimic enzyme promoted facilitated transport membrane was developed and optimized for post combustion CO2 capture. This composite membrane has a thin dense polyvinyl alcohol (PVA) selective layer containing a low molecular weight mimic enzyme and a polysulfone (PSf) ultrafiltration porous support. The membrane morphology was studied by scanning electron microscope (SEM). The effect of mimic enzyme loading and the pH value of the membrane casting solution was investigated. The optimal Zn-cyclen loading of 5 μmol/g PVA. Furthermore, hydrophilic carbon nanotubes were added to the membrane to improve the separation performance of membrane.The influence of humidity on the performance of the membrane was also studied by conducting experiments at variable relative humidity levels (i.e., 50-100%). This membrane showed a CO2 permeance of 0.98[m3 (STP)/(m2 bar hr)] and a CO2/N2 selectivity of 120, which is significantly higher than that of a PVA membrane without mimic enzyme operating under the same conditions. A membrane contactor based CO2 separation process promoted by mimic enzyme was also investigated. This work presents a CO2 membrane absorption process using a K2CO3 solvent promoted by mimic enzyme in a membrane contactor. The mass transfer resistances in the membrane, gas and liquid phases in the membrane contactor were determined. The effects of gas film and liquid film resistances on the overall mass transfer coefficient were studied by varying the gas and liquid velocities. A tubular, hydrophobic porous glass membrane contactor with pore size of 200nm was used to study the CO2 absorption in potassium carbonate (0.5M K2CO3) solution promoted by different concentrations of mimic enzyme. The kinetic rate constant for absorption of CO2 in the K2CO3 solvent promoted by mimic enzyme was increased by 10 fold compared to the experiment without mimic enzyme. The significantly improved CO2 separation performance demonstrates a novel approach to the effective enhancement of CO2 absorption by using a low cost, chemically stable mimic enzyme. Furthermore, computational fluid dynamic based modelling is used to simulate the membrane contactor containing mimic enzyme for post combustion CO2 capture. In short this work consists of experimental and modelling study for the development and testing of mimic enzyme promoted membrane separation processes for post combustion CO2 capture.