Chemical Processing - September 2007 - (Page 60)

Retentate (residue) outlet Retentate (residue) outlet >> making it work Fee a CO2 and CH4 gas mixture, is injected into the annular space between the membrane surface and the module housing through a small tube perpendicular to the module’s outer surface. As the feed gas mixture travels along the length of the annulus, the CO2 passes through the membrane into the cylindrical space inside the support. It then exits through a separate permeate outlet at the downstream end. The remaining gas mixture reaches a residue outlet at the downstream end of the annulus, consisting of another small tube attached perpendicular to the module’s outer surface. 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 isn’t 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. >> Even distribution Retentate Retentate Retentate Retentate 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 240000 96.6% Inlet Feed rate (sccm) Average CO2 ﬂux (% of maximum) Inlet Inlet Inlet 6000 64.7% 24000 82.5% 120000 95.7% Figure 4. Even at low feed rates, CO2 is evenly distributed over the media surface in the revised design. Because of the swirling flow effect, more of the membrane’s surface is engaged in separating CO2. 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. 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 membrane’s surface is engaged in separating CO2. 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. 60 • September 2007 Both the original and revised membrane module designs were tested in the laboratory under similar condiRetentate Retentate Retentate tions. The relative performance of the Retentate two module designs closely matched the trends predicted by the CFD models. ExxonMobil has used polymeric membranes in production facilities for gas separation. As yet, the company has not yet employed ceramic-based membranes in such operations. Further research and development work still is necessary before ceramic membrane materials will become available for commercial-scale gas Inlet Inlet Inlet Inlet separation modules. Feed rate a (sccm) tool potent 6000 24000 120000 240000 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% These results demonstrate78.3% the performance 93.9% that of Average CO2 ﬂux 66.3% 90.5% (% of maximum) membranes used for CO 2 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 actual physical models but additionally 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. CP 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 paul.j.rubas@exxonmobil.com and kevin. r.geurts@exxonmobil.com. www.chemicalprocessing.com http://www.chemicalprocessing.com

Table of Contents Feed for the Digital Edition of Chemical Processing - September 2007

Contents From the Editor Field Notes In Process Energy Saver Compliance Advisor Succeed at Simulation Rethink Your Approach to Process Safety Avoid Blending Blunders Get the Right Cartridge or Bag Filter Wireless Proponents Take HART Membrane Boasts Material Benefits Process Puzzler Plant InSites Chem Show Product Preview ISA Product Preview Equipment & Services Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - September 2007

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