What Are Membranes? A membrane is a selective barrier. This simply means that different gases, vapors, or liquids move through the membrane at different rates. An example of a membrane is a balloon. Helium passes through a balloon over time, causing the balloon to deflate instead of oxygen and nitrogen passing through and further inflating the balloon. An example of a barrier is a plastic soda bottle. Only after a long period of time will the CO2 in the soda pass through the plastic bottle resulting in a flat soda. What is a membrane device and a membrane cartridge? Membrane devices include the membrane materials, seals to isolate the product streams from each other and the feed stream, and the pressure vessel housing all these necessary components. Membrane cartridges are all the components listed above less the pressure vessel. Cartridges allow the membrane devices to be taken apart, inspected, and replaced by the end user without replacing the pressure vessel component. Designs utilizing cartridges are advantageous when it is difficult or expensive to replace the pressure vessel or to remove the vessel from auxiliary equipment such as piping. Membrane Materials Membranes may be made from polymers, metals, ceramics, glasses, carbon, etc. Air Products' membranes are made from polymers. Membrane Material Geometry Membrane surface area per unit volume is an important specification in selecting a membrane separator. PRISM® membranes use hollow fiber membranes to achieve surface area per unit volume ratios greater than 5000 m²/m³. While other geometries are easier to fabricate and assemble compared to hollow fiber-based devices, they are not typically able to achieve high surface area per unit volume. The other common geometries include plate and frame (300 m²/m³) and spiral wound (600 m²/m³). How Hollow Fiber Membranes Work Membranes work on the principle of diffusion of gases in a polymer.  High-pressure gas mixtures are fed into the devices, forcing the gas either down the bores of the hollow fibers or along the outside surface of the hollow fibers, depending on the specific application. If the gas is fed down the bores of the hollow fibers, then the pressure outside the fibers is maintained at a lower value to establish a partial pressure gradient across the wall of the hollow fibers. Since membranes use the principle of selective permeation to perform a desired separation, some gas components will not permeate quickly through the wall of the hollow fibers and thus accumulate in the high-pressure gas leaving the device. Other gas components will quickly permeate through the wall of the hollow fibers and accumulate in the low-pressure gas leaving the device. Each gas component in a feed stream has a characteristic permeation rate, which is a function of the component's ability to dissolve in and diffuse through the membrane material. Examples of 'fast gases' are hydrogen and water vapor, which diffuse quickly through the membrane. Examples of 'slow gases' are nitrogen and methane, which remain on the high-pressure side of membrane and permeate through the wall of the fiber much more slowly. Membrane Permeability How Are PRISM Membranes Made? Air Products' membranes are made of polymers, which are extruded into thousands of very small hollow fibers. A typical membrane device contains thousands of fibers. These fibers together form the microscopic membrane filter, and are directly embedded into a thermoset resin, which is permanently bonded either directly to a suitable pressure vessel or to a component forming a removable cartridge to be inserted into a pressure vessel. The pressure vessel may be made from carbon steel, stainless steel, or ABS plastic, depending upon the application. When cartridge designs are used, Air Products provides complete fabrication drawing of the mating pressure vessel. Benefits of Membranes Membranes are simple, passive devices No maintenance required on membranes Minimal maintenance on membrane system, consisting of replacement of upstream filtration elements Field-proven in a wide variety of applications Easily expandable systems by the addition of membrane separators Significant savings available vs. other technologies Lightweight Looking for a membrane system to use for educational experiments? Contact Perfected Experiments to purchase a university-style system using two of our membranes.