(702e) Application of Composite Polymeric Membranes for Efficient Regeneration of Physical Solvents

The most dominant CO₂ capture technology used for pre-combustion capture involves the application of physical solvents. Unlike chemical solvents, physical solvents do not react with the solute and they physically dissolve the acid gases, which are then stripped without the need to be heated by means of pressure swing techniques or combination of heat and pressure letdown. However, despite low energy required to regenerate the physical solvents and their high capacity to capture and separate acid gases from the syngas produced in a gasification plant, physical solvents have some disadvantages including CO₂ pressure loss and the energy required to pump the solvent to the high pressure absorber. 

The primary objective of this work is to evaluate the use of composite polymeric membranes, for the recovery of CO₂ from CO₂-rich solvent streams. To achieve this purpose, an experimental bench-scale setup was built to investigate and quantify CO₂ removal capacity from the rich solvent across different types of membranes. This setup primarily consists of a high pressure absorber where solvent is loaded and saturated with CO₂ and the membrane module where CO₂ diffuses through the membrane. In order to increase the driving force across the membrane, sweep gas is used (Nitrogen). To evaluate the absorption process, absorber temperature and pressure is measured and recorded continuously using a NI data logger and the LabView software. The absorber is equipped with a home-made cooling water coil and a relief valve to adjust the absorber temperature and pressure respectively. Two 400 W inline pencil heaters are used and controlled with a temperature controller to adjust the solvent temperature downstream of the membrane module. The solvent line temperature and pressure are also recorded continuously. CO₂ concentration in the sweep gas stream is measured using the Non-Dispersive Infrared CO₂ analyzer and Agilent 7850A GC- FID depending on the concentration of CO₂ in the sweep gas. Dimethyl ether of polyethylene glycol (Selexol) is used as the solvent since it is reputed to be one of the major physical solvents for CO₂ removal. To evaluate the effectiveness of different types of membranes, CO₂ permeation rate and membrane selectivity were measured for different membranes. To better understand the effect of system pressure and solvent flow rate on different experiment responses such as CO₂ flux, selectivity and % recovery, a two-factor two -level full factorial design  with two replicates an  three  center points were performed with the membrane that showed the most promising results in the screening study. 

The results of the screening study indicated that PDMS based membranes (PERVATECH and PERVAP 4060) shows higher CO₂ permeability compared to PVAOH based membranes (PERVAP 1211 and PERVAP 1201). The best membrane for further analysis and experiments to find the optimum operational conditions was chosen as PEVAP 4060 from SULZER due to its high CO₂ flux and selectivity compared to other membranes.

A statistical analysis was performed to identify the significant factors for each individual response such as permeation rate, leak rate and selectivity. For CO₂ flux, pressure appeared to be strongly significant. However, solvent flow rate had no significant effect on the rate of CO₂ permeation. The immediate conclusion from this observation is that the main mass transfer is controlled by the membrane. With respect to the solvent leak, the analysis of Pareto charts suggested pressure to be significant and solvent flow rate to be insignificant. As the system pressure increases, the liquid on the upper chamber of the membrane module forces itself more into the membrane and hence, solvent leak increases. However, increasing the solvent flow rate leads to higher liquid velocity on top of the membrane and shows no significant changes on the rate of solvent leak. Neither system pressure nor solvent flow rate found to be significant considering the selectivity as the experiment’s response. Finally, regarding the percent recovery, both the system pressure and solvent flow rate appeared to be significant. This is mainly because at elevated pressures, mole fraction of CO₂ in solvent increases and thus a higher driving force for CO₂ permeation exists. As a result of this, percent of recovery increases by pressure as confirmed by the Main Effects plots.  On the other hand, by increasing the solvent flow rate, more amount of CO₂ enters the upper membrane chamber but the mass transfer resistant through the membrane prevents more CO₂ from being transported. Thus, there is the condition of introducing more CO₂ to the upper chamber but still a slow mass transport process being controlled by the membrane which eventually decreases the separation ability of the membrane (percent recovery). 

In order to examine the chemical stability and structural integrity of the membranes after being exposed to the high pressure solvent, a series of post-experiment characteristic tests such as FTIR and DSC were performed. The results of these studies revealed no major changes.