(7c) Hydrophobic Polymeric Solvents for the Selective Absorption of CO2 from Warm Gas Streams that also Contain H2 and H2O

Post-combustion capture of CO2 is typically accomplished by absorption from combustion off-gas into basic aqueous solutions, such as with alkanolamines and ammonia.  There are significant energy requirements of regenerating the absorbent and, in the case of ammonia, controlling NH3 emission and absorber cooling.  Proposed alternative processes include absorption into ionic liquids and membrane separation.  Evaluation of process energy demands is often difficult because varying process conditions are studied and can depend on the process and property models employed.

An approach to comparing energy requirements of different processes can be based solely on steady energy and entropy balances of the streams entering and leaving individual or lumped process units.  These two equations, with systematically varied entropy generation in the units, allow solving for two stream variables or heat/work amounts.  The analysis provides quantitative descriptions of the effects of irreversibilities as well as can reveal process sections for focused improvement.  

This method was previously used to evaluate different sulfur-iodine processes for hydrogen production [1].  In addition to showing reversible and realistic energy requirements, it also uncovered significant differences in property model predictions for the complex mixtures of the S-I system.

The current study uses the process model-free analysis for CO2 absorption systems using Aspen property models.  Several cases for NH3 capture with different absorber conditions are compared with MEA systems with similar levels of irreversibility.  The energy costs of NH3 recovery and CO2 processing are obtained.  Property modeling issues for the CO2 absorption systems have also been found.  Suggestions for applying the method to other CO2 processes will be made.

[1]. Process model-free analysis for thermodynamic efficiencies of sulfur–iodine processes for thermochemical water decomposition, J.P. O'Connell, P. Narkprasert, M.B. Gorensek, Int. J. Hydrogen Energy 34 (2009) 4033–4040.