Chemical Processing - September 2007 - (Page 59)

Inlet Feed rate (sccm) Average CO2 ﬂux (% of maximum) Inlet Inlet Inlet 30% 20% 10% 0% 6000 64.7% 24000 82.5% < > 120000 240000 95.7% 96.6% mixed on the feed side of the membrane. First, the feed >> Dead zones gas flow rates should be evenly distributed across the membrane surface to avoid the formation of dead zones 100% that quickly become depleted of CO2. Within these local90% Retentate Retentate Retentate Retentate ized dead zones, the membrane surface becomes inactive 80% because it’s not replenished with a fresh gas mixture. Sec70% ond, turbulence is beneficial near the membrane surface 60% because it helps maintain a sufficient concentration of 50% CO2. Third, the so-called “stage cut” effect, in which the 40% mixed gas feed rate is too low to take advantage of the 30% full surface of the membrane, should be minimized. If, 20% for example, most of the CO2 gas is depleted near the 10% 0% inlet of the module, then the remaining membrane surInlet Inlet Inlet Inlet face effectively becomes inactive. At higher feed-gas flow Feed rate rates, this effect is reduced. 6000 24000 120000 240000 (sccm) In the past, designers of membranes used scale models to evaluate the performance of each design. However, Average CO2 ﬂux 66.3% 78.3% 90.5% 93.9% (% of maximum) there are several problems with this approach. It’s timeconsuming and expensive to build and test each design Figure 2. The normalized CO2 flux of the original module indicates large dead zones at being considered. Also, while physical tests generally low feed rates.. do a good job of measuring the overall performance of the membrane module, they often can’t capture the a membrane module that had been built and tested in flow details inside that play such an important role in the laboratory. Predictions from this CFD simulation the module’s performance. As a result, engineers often matched the laboratory test results. have to guess at the reason behind the performance of a Feed inlet particular design and rely on intuition and experience in optimizing the design Feed inlet an effort to improve it. The original module design (Figure 1) consists of a In recent years ExxonMobil engineers have been us- membrane on a hollow tubular support inserted into a ing CFD to improve membrane tube Membrane tube CFD cylindrical housing of slightly larger diameter. The feed, Housing module design. A Annular gap simulation provides fluid velocity, pressure, temperaappropriture, gas composition and other variables, as Permeate ate, throughout the solution domain for problems with Seal >> Revised module design outlet complex geometries and boundary conditions. As part of Seal Feed inlet the analysis, an engineer may change the geometry of the system or the boundary conditions and then observe the effect of the changes on fluid flow patterns or distribuRetentate Retentate other gas composition. CFD tions of outlet variables such as (residue) outlet Retentate (residue) Feed inlet (residue) outlet overcomes the traditional difficulties of membrane modFeed inlet ule design by making it possible to quickly and inexpenFeed inlet Feed inlet sively evaluate design alternatives. It also gives a host of Feed inlet diagnostic information. Membrane tube The performance of several proposed laboratoryFeed inlet scale module designs was evaluated using Fluent CFD Seal software from Ansys, Inc., Canonsburg, Pa. One of Seal Feed inlet the most challenging parts of the process was modelHousing tube ing the rate at which CO2 molecules pass through the Feed inlet membrane surface. This was accomplished by writing Retentate Feed inlet Retentate Retentate Retentate a user-defined function Retentate that moves CO 2 mol(UDF) (residue) outlet 100% ecules across the membrane based on the pressure and 90% CO2 concentration gradients. The performance of the Figure 3. Changing the inlet and outlet configurations eliminates problems in the origi80% UDF was validated by simulating the performance of nal module. www.chemicalprocessing.com 70% 60% 50% 40% September 2007 • 59 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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