Enzyme-enhanced CO₂ absorption

Carbonic anhydrase (CA) is an enzyme that enhances the hydrolysis of CO₂ in the natural environment and originates among other life forms than human beings. By means of breeding modifications, the enzyme is made applicable for chemical absorbents and conditions that are used in post-combustion CO₂ capture from flue gases. By using CA it is possible to use compounds like tertiary amines, which have a low heat of regeneration but a low reaction rate. The latter makes it difficult to apply these tertiary amines directly as it leads to unrealistically large absorption columns. However, the use of CA increases the CO₂ absorption into aqueous tertiary alkanolamine solutions by enhancing the CO₂ hydration reaction by such an extent that feasible column dimensions can be obtained. The mechanism and kinetics of the CA catalyst in various absorbents used or proposed for use in post-combustion carbon capture have been investigated. Experiments showed that the CO₂ hydration reaction is first order in both CO₂ and water. Its rate constant, k*H₂O, is a function of the enzyme concentration and can be described with a Langmuir-Hinshelwood-like equation. The newly introduced kinetic constants on their turn are dependent on temperature and pKa value of the chemical absorbent. Moreover, it has been demonstrated that the chemical and physical properties of the absorption liquids are not affected by the addition of CA. To investigate the potential and applicability of the enzyme-catalyzed absorbents, process simulations have been carried out for a complete post-combustion case in which the enzyme kinetics were implemented. From these simulations it was demonstrated that a system consisting of 30wt% MDEA aqueous solution with 1kg/m³CA can be compared to the reboiler duty of a 30wt% MEA system at identical absorber heights.