Gas Separation Membranes

Membrane separation technology is based on the interaction of specific gases with the membrane material by a physical or chemical interaction. Membrane processes are considered to be visible and effective technologies for the separation of gaseous mixtures at the industrial scale due to their high efficiency, simple operation, and low cost. Membrane processes encompass a wide range of applications in fluid separation and are now considered a new and emerging separation technology for industrial applications. For several important separation processes, membrane technology has now reached its initial stage of maturity.

Gas permeation is a technique for fractionating gas mixtures by using nonporous polymer membranes having a selective permeability to gas according to a dissolution–diffusion mechanism. The membrane gas separation process is driven by a pressure difference across the membrane. The membrane may be either in the form of a flat sheet or a hollow fiber. In general, hollow fibers are preferred as they achieve a higher effective membrane area within a given module volume.

A membrane is a layer of material which serves as a selective barrier between two phases and is impermeable to specific particles, molecules, or substances when exposed to the action of a driving force. Some components are allowed passage by the membrane into a permeate stream, whereas others are retained by it and accumulate in the retentate stream. Membranes can be of various thicknesses, with homogeneous or heterogeneous structures. Membrane can also be classified according to their pore diameter. There are three different types of pore sizes based on the IUPAC (International Union of Pure and Applied Chemistry) classification: microporous (d _p  < 2 nm), mesoporous (2 nm < d _p  < 50 nm), and macroporous (d _p  > 50 nm) [1, 2]. Membranes can be neutral or charged, and the transport through a membrane can be active or passive. The latter can be facilitated by pressure, concentration, chemical or electrical gradients. Membranes can be generally classified into synthetic membranes and biological membranes.

The escalating research in membrane fabrication for gas separation applications signifies that membrane technology is currently growing and becoming the major focus for industrial gas separation processes. Material selection and method of preparation are the most important parts in fabricating a membrane. Different preparation methods result in various isotropic and anisotropic membranes, which are related to different membrane processes. The commercial value of a membrane is determined by its transport properties—permeability and selectivity. The method of membrane fabrication can have considerable influence on its effectiveness and there are a range of techniques available to create membranes, such as melt-pressing, solution casting, phase inversion, sputtering, extruding, and interfacial polymerization. Membranes can be fabricated either in a hollow fiber or spiral wound format.

Hundreds of thousands of square meters of membrane are needed to perform the required separation of compounds in industrial plants. There are several efficient and economical ways to create a large surface area in a membrane package for effective compound separation. From an overall cost standpoint, not only the cost of membranes per unit area is crucial but also the cost of the containment vessel into which they are mounted. These packages are called membrane modules. The most important are: Plate-and-frame, Tubular, Spiral-wound and Hollow fiber.

Membrane-based gas separation (GS) systems are today widely accepted and, in some cases, used as unit operations for generation, separation, and purification of gases in gas, chemical, petroleum, and allied industries. There are several fields of application of membrane GS, and several membrane materials and membrane modular solutions are available today for the various fields of interest. However, the growth of large-scale industrial applications for GS is still far from reaching the real potential this technology offers. Together with the investigation of new materials with improved properties, a key component for widespread use of this technology is a better understanding and utilization of the unit operations already available on the market in integrated membrane systems, combining various membrane operations in industrial processes. The role of membrane engineering is crucial to overcome this hurdle.

Gas transport through polymers is an area of growing interest as materials with unique transport properties continue to find uses in new, specialized applications ranging from extended life tennis balls to natural gas systems. Membrane users (manufacturers and membrane scientists) require knowledge of membrane characteristics in order to choose an appropriate one for application in different processes. Understanding these characteristics will help to determine membrane casting conditions, control membrane quality, and develop membrane transport. Membrane mechanisms and characteristics include surface morphology, and various chemical and physical properties. An ideal characterization method should be non-destructive, accurate, repeatable, and fast and should maximize data. Many methods of characterization have been devised, which can be classified according to the physical mechanisms they exploit.