Designing a High Performance Membrane Device for Gas Separations

Generon® IGS values its roll as an innovator and developer of high performance membrane products. Much of what we produce is designed for the efficient generation of N2 from compressed air but general concepts of membrane and device design that we use at Generon® are applicable to a vast array of membrane products.

What Membrane Type?

For gas separations, most commercial suppliers of membranes have chosen hollow fiber membranes as the design of choice. Flat sheet membranes can also be used but one can get 5 to 20x the volumetric productivity with a hollow fiber shape. Hollow fibers can provide high surface area with a minimal amount of volume or membrane weight. Generon’s® most commonly used module, the 6800, is roughly 1.2 cubic feet in volume but contains over 4300 square feet of surface area or ~3600 ft2/ft3. The best flat sheet membrane device may contain ~400 ft2/ft3. Hollow fiber membranes are more difficult to produce but the 10x greater surface area factor has made it the configuration of choice for membrane based gas separations.

Generon® Fiber

The size of the hollow fiber is typically determined by a trade-off between module productivity and pressure drop considerations. As one goes to finer hollow fibers, the surface area per unit volume goes up but so does the resistance to flow for the gas being separated. Commercial modules are typically bore-side fed, feed air is passed through the bore of the fiber and the “fast” gases are permeated through the wall of the fiber to the shell-side of the membrane. The smaller the bore of the fiber and longer the module length, the greater the pressure drop for the feed gas passing down the length of the module to the product end. Other factors are important as well, such as the permeation rate of fast gas from the fiber and the pressure and temperature of operation. Generon® has settled for a standard fiber size which is on the order of the size of your hair for our air separation modules. Our modules for air dehydration have fibers that are 50% larger because the gas permeation rates and the module flowrates are considerably larger. Air dehydration modules are also typically less than 36” in length while the air separation modules are up to 72” in length. This optimization of fiber size and module length is done in an attempt to provide the best balance of performance for the customer within the limits of the manufacturer’s fiber extrusion technologies.

Module Design Issues

Once the hollow fiber membranes are developed, it is important to use them effectively in the module. Conventional module designs are patterned after shell and tube heat exchangers. The best performance is achieved by forcing the permeate stream to run counter-current to the direction of the feed stream as it passes down the length of the module. In this way the maximum partial pressure driving force for the fast gas is achieved down the length of the module. External sweep stream streams are sometimes used to further enhance the separation by minimizing the amount of fast gas partial pressure on the shell-side of the fiber. This is typically done for gas separations that have high differences in the permeation rates for the two gases being separated and requires a relatively inexpensive source of sweep gas. Air and natural gas dehydration are separations that typically use a sweep stream on the permeate side of the membrane.

Outside of these cases where a sweep stream is preferred, the design must have a counter-current flow pattern to be commercially competitive. In the early 80s, most module designs were cross-flow, meaning that the permeate stream flowed perpendicular to the feed stream. Simple baffling of these designs, lead to significant improvements in module performance. As Generon® and others became more sophisticated in our designs, the packing of fiber in the modules became more important. Although modules in the late 80s had baffles to give macroscopic counter-current flow patterns, there was still significant channeling and by-pass in these designs. It became important to control local flow patterns in the device to avoid these inefficiencies. To do this, the fiber packing in the device has to be uniform. While not trivial in devices that can contain over 1.5 million fibers, our solution has been the controlled weaving of our fiber in a fabric belt that can be rolled up into the module bundle. Spacing between the fibers is inherently controlled in this way which leads to an improvement in module performance and a significant decrease in module to module variability. Today’s devices operate at near 100% counter-currency. In separating air into a 99% N2 stream, we have improved the module design to give more than 35% increase in flow from the earlier cross-flow designs and are 15% more productive than the counter-current designs in which fiber spacing was not controlled effectively.

The next area of emphasis for module design will be the management of the permeate side pressure drop. Just as there is pressure drop down the bore of the fiber for the high pressure feed stream, there is also a pressure drop associated with the permeate gas flow down the shell-side of the fibers. While not an obvious problem, this backpressure suppresses gas from permeating through the fiber wall. New membrane materials and applications with inherently higher permeate flowrates need module designs that accommodate these higher flows without incurring significant pressure drop. Small increases in permeate backpressures can lower the module’s productivity by 3% per psi. Membrane materials of the near future may have 4 to 5 times the permeate flowrates of existing modules which already have 1-2 psi of permeate backpressure. Decreasing localized permeate channeling and maximizing counter-current flow patterns tend to increase this pressure drop but Generon® has developed new designs (USP 6136073) that allow for uniformly placed permeate flow channels to carry the permeate away at minimal pressure drop while maintaining high levels of counter-currency.

Module Operation Issues

Customers want membrane products that match their needs. Generon® has designed a large number of membrane based systems that range from 0.1 to over 5000 scfm of enriched nitrogen. Typical systems target N2 purities around 97%, but we have designed and produced membrane products that deliver anywhere from 86% N2 to 99.999% N2. While performance is paramount, Generon® is constantly working to ensure the durability of our product line. Modules are designed with pressure capabilities up to 24 barg and temperatures up to 55C. The module designs have also passed stringent pressure cycling and vibration testing before they go to the customer.

Customers find membrane systems easy to use. They are impressed with the rapid response from a membrane system. It typically takes less than a minute to reach steady state performance purity and flow for the modules. Some applications such as controlled atmosphere for food storage require the O2 purity to change over time or change as a function of the type of produce being stored. Membrane systems are very versatile in that one can get different purities quickly and with the same system. Systems can even have modules staged to deliver different purities and flows for various applications at the same time. The compact size (about 35% of the space requirement of a competing PSA or pressure swing adsorption process), low weight and simple use are all virtues of the membrane technology that we strive to enhance with each new generation of membrane products.