CO2 absorption using phase–split blended aqueous amine solution

https://doi.org/10.1016/j.petlm.2016.11.002

“These are blended aqueous amine solvents that form a single phase solution prior to amine–CO₂ reactions, but separate into two immiscible liquid phases during or after reaching equilibrium with amine–CO₂ reactions. Phase–split amine solutions are also known as phase–change solvent or bi–phasic solvent. These two phases are usually termed CO₂ lean and CO₂ rich phases, respectively denoting phases that is lighter and heavier. This peculiar property has prompted further research for such blended amine solutions because only the CO₂ rich phase will be sent to the regenerator thus, likely reduces the regeneration energy compared to that of 5 kmol/m³ MEA [117], [142], [143], [144]. The lower amine circulation rate to the regenerator means less amount of heat input (GJ/hr) is required per liter of amine solution. This therefore, makes the phase–split amine system very promising process for CO₂ capture. Fig. 10 depicts an ideal process description of a typical CO₂ capture plant with phase–split aqueous amine solution. The process is similar to the conventional configuration of single phase blended aqueous amine solution shown earlier in Fig. 6. However, a liquid–liquid phase separator is installed upstream of the cross exchanger (L/R exchanger) to facilitate the separation of the CO₂ lean and rich phases (by differences in density) before sending to the absorber and the regenerator, respectively. This is depicted in Fig. 10 (neglecting the dotted process route). For high pressure CO₂ capture process systems (like pre–combustion process and natural gas processing) a flash drum is installed upstream of the liquid–liquid phase separator (Fig. 10). This will both step down the pressure of the CO₂ saturated amine solution to the stripper pressure and also remove some of the absorbed CO₂.

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Fig. 10. Typical process flow diagram of CO₂ capture using phase–split amine based solvents.

After regenerating the CO₂ rich amine phase, the amine is mixed with the CO₂ lean phase before sending them to the absorber. The advantage of this technology is that only a portion of the CO₂ rich amine solution is sent to the regenerator (stripper) for desorption, which can lead to reduced regeneration energy.

Hu stated that bi–phasic amine solutions would be made up of at least one activator and another solvent in the mixing ratio of 20% and 80%, respectively [145]. Bruder and Svendsen found out that the bi–solvent blends of 2-(Diethylamino)ethanol (DEEA)/3-(Methylamino)propylamine (MAPA) mixed in 5 kmol/m³ and 2 kmol/m³ ratio can form two immiscible liquid phases after CO₂ absorption [146]. The solvent also had a higher cyclic loading compared to that of 5 kmol/m³ MEA. They also observed that the viscosity of the CO₂ rich phase was very high and affected its pumping to the regenerator. The CO₂ rich amine solution viscosity is an essential parameter because its significant increase will limit heat and mass transfer (both in the cross exchanger and regenerator) and pumping difficulties. This can then affect the desired solvent regeneration efficiency.

Zhang et al. studied a bi–phasic amine solution consisting of 3 kmol/m³ DMCA (N,N-dimethylcyclohexylamine), 1 kmol/m³ MCA (N-methylcyclohexylamine), and 1–1.5 kmol/m³ AMP tri–solvent blend [147]. Based on their study, they discovered that the CO₂ loading (mol/kg) of their novel tri–solvent blend was 150% higher than the single solvent MEA. It is also important to note that they used continuous stirred tank reactor for the desorption process instead of the conventional process. In addition, they also discovered high viscosity of the CO₂ rich phase which in reality is not desirable in large scale operation.

Xu et al. discovered that bi–solvent blends of 1,4-Butanediamine (BDA) and diethylaminoethanol (DEEA) of 2 kmol/m³ and 4 kmol/m³ concentration ratio can form two immiscible liquid phases after amine–CO₂ reactions which was attributed to the limited CO₂ solubility in DEEA and fast CO₂ absorption rate of BDA [148]. They also reported that this BDA–DEEA blend had higher cyclic loading (46%) and cyclic capacity (48%) compared to that of 5 kmol/m³ MEA. More recently, Ye et al. found out another bi–phasic blended amine solution consisting of TETA and DEEA to have a higher CO₂ loading (40%), faster reaction rate and lower estimated energy savings (30% lower) compared to MEA [144]. Phasic–split blended aqueous amine solutions (bi–solvent and tri–solvent) have shown superiority in terms of regeneration energy savings, higher cyclic loading and capacity compared to single solvent MEA for post–combustion CO₂ capture, even without significant modification in the process configuration. However, high viscosity occasionally noticed in the CO₂ rich phase must be addressed to avoid the associated problems.

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