Figure 1. A single radial inlet leads to flow maldistribution.
CFD simulations indicated that dead zones with very low surface velocities were occurring in certain sections of the module. Within these dead zones, fresh gas feed isnt brought to the membrane surface quickly enough, so the CO2 flux through the membrane is reduced (Figure 2). Based on these results, it was estimated that the dead zones collectively cut the overall efficiency of the membrane module by approximately 50%. The simulation also showed that the CO2 was essentially depleted from the feed gas well before reaching the outlet end of the module a large portion of the membrane was doing little work.
Figure 2. The normalized CO2 flux of the original module indicates large dead zones at low feed rates.
A revised module was designed with four tangential inlets evenly spaced around the circumference of the housing tube to improve the flow distribution around the circumference (Figure 3). A similar change was made at the outlet end of the module.
Figure 3. Changing the inlet and outlet configurations eliminates problems in the original module.
A CFD simulation of the revised design showed improved performance (Figure 4). Compared to the original design, the maximum localized CO2 flux is approximately the same. However, the average flux across the entire membrane surface is approximately 4% to 5% higher at intermediate flow rates. This performance enhancement is attributed to the reduction of dead zones in the revised design. Because of the swirling flow effect created by the tangential inlet/outlet ports more of the membranes surface is engaged in separating CO2.
Figure 4. Even at low feed rates, CO2 is evenly distributed over the media surface in the revised design.
Obviously, operating at high pressure and velocity makes the most of the membrane surface area. Further improvements in membrane module performance could be achieved by increasing internal mixing to reduce the occurrence of dead zones.
Both the original and revised membrane module designs were tested in the laboratory under similar conditions. The relative performance of the two module designs closely matched the trends predicted by the CFD models.
ExxonMobil has used polymeric membranes in production facilities for gas separation but hasnt yet used ceramic-based membranes in such operations. Further research and development work is needed before ceramic membrane materials will become available for commercial-scale gas separation modules.
A potent tool
These results demonstrate that the performance of membranes used for CO2 separation can be substantially enhanced by using CFD to evaluate multiple design alternatives. CFD simulation not only is faster than the alternative of building and testing but offers substantially greater quantities of diagnostic information that can be used to improve the design. As larger and more complex membrane modules are developed for commercialization, CFD is expected to play a key role in their optimization.
Paul J. Rubas is an engineering associate at ExxonMobil in Fairfax, Va., and Kevin Geurts is a senior engineering specialist at ExxonMobil in Houston. E-mail them at firstname.lastname@example.org and email@example.com.