Thin defect-free membranes

Several factors contribute to the successful fabrication of a high-performance membrane module. First, membrane materials with the appropriate chemical, mechanical and permeation properties must be selected this choice is very process-specific. However, once the membrane material has been selected, the technology required to fabricate this material into a robust, thin, defect-free membrane and then to package the membrane into an efficient, economical, high-surface-area module is similar for all membrane processes. Therefore, this chapter focuses on methods of forming membranes and membrane modules. The criteria used to select membrane materials for specific processes are described in the chapters covering each application. 

A variety of polymers and copolymers are used for gas separation membranes. To be suitable for gas separation, the polymer must have good permeability and selectivity and the material must be capable of forming a strong, thin, defect-free membrane with good chemical and thermal stability. Commercial gas separation membranes are based on modified cellulose, treated polysulfone or a substituted polycarbonate polymer. Membranes... 

Concerning the preparation of thin membranes directly on porous supports, a lower thickness limit seemingly exists for which a dense metal layer can be obtained. This thickness limit increases with increasing surfaee roughness and pore size in the support s top layer." " Clearly, this relation puts strong demands on the support quality in terms of narrow pore size distribution, and the amount of surface defects. Therefore both pore size and roughness of the support surface are often reduced by the application of meso-porous intermediate layers prior to deposition of the permselective metal layer. This procedure facilitates the preparation of thin defect-free membranes beeause it is relatively easier to cover small pores by filling them with metal. It is therefore conceivable that for a certain low Pd-alloy thickness and support pore size, the H2 flux becomes limited by the support resistance. ... 

An ideal pervaporation membrane should consist of an ultra thin defect free skin layer (dense layer) supported by a porous support. The skin layer is perm-selective and hence responsible for the selectivity of the membrane. However, the porous support also plays an important role in overall performance of the membrane. The effect of the porous support, of a composite membrane, on the permeation properties of the membrane is discussed in details in the composite membranes... 

A two-step membrane manufacturing process has been reported where a defect free Pd-alloy membrane is first prepared by sputtering deposition onto the perfect surface of a silicon wafer, for example. In a second step the membrane is removed from the wafer and transferred to a porous stainless steel support (see Figure 11.1). This allows the preparation of very thin ( 1-2 pm) defect-free membranes supported on macroporous substrates (pore size equals 2 pm). By this technique, the ratio of the membrane thickness over the pore size of the support may become less than 1, which is two orders of magnitude smaller than obtained by more conventional membrane preparation techniques. Tubular-supported palladium membranes prepared by the two-step method show a H2/N2 permselectivity equal to 2600 at 26 bars and hydrogen flux of 2477 mL(STP) min cm . Since the method enables the combination of macro-porous stainless steel supports and thin membrane layers, the support resistance is negligible. ... 

Porous substrates should have a smooth surface with constant and homogeneous characteristics (wettability) and a narrow pore size distribution. Pores much larger than average and grains broken out of the surface, or irregularities in the porous substrate, may result in defects in the separation layer applied on it. Therefore, surface modification or formation of intermediate layer(s) is necessary to prepare a thin and defect-free membrane. 

Rezac, M. E., and Koros, W. J. (1992). Preparation of polymer-ceramic composite membranes with thin defect-free separating layers. J. Appl. Polym. Sci. 46, 1927 1938. 

N. Peng, T.S. Chung, and J.Y. Lai, The rheology of Torlon solutions and its role in the formation of ultra-thin defect-free Torlon hoUow fiber membranes for gas separation. Journal of Membrane Science 326 (2009) 608-617. 

N. Peng and T.S. Chung. (2008). The effects of spinneret dimension and hollow fiber dimension on gas separation performance of ultra-thin defect-free Torlon hollow fiber membranes, J. Memb. Sci. 310 455-465. 

Most solution-cast composite membranes are prepared by a technique pioneered at UOP (35). In this technique, a polymer solution is cast directly onto the microporous support film. The support film must be clean, defect-free, and very finely microporous, to prevent penetration of the coating solution into the pores. If these conditions are met, the support can be coated with a Hquid layer 50—100 p.m thick, which after evaporation leaves a thin permselective film, 0.5—2 pm thick. This technique was used to form the Monsanto Prism gas separation membranes (6) and at Membrane Technology and Research to form pervaporation and organic vapor—air separation membranes (36,37) (Fig. 16). 

Synthesis of thin (<1 pm) defect-free and stable membranes... 

The cost of Pd-alloy membranes used for hydrogen separation may be reduced by depositing a thin Pd-alloy film on a suitable porous substrate to form a composite membrane. Almost all of the Pd-alloy membrane development efforts are, thus, focused on preparing thin yet defect-free Pd-alloy composite membranes (e.g., Hopkins, 2007 Coulter, 2007 Delft et al., 2005 Damle et al., 2005 Mardilovich et al., 2002). A detailed review of the Pd-alloy membrane research has been prepared by Paglieri and Way (2002) with an extensive bibliography of the palladium membrane research to date. An updated review has been recently prepared by Collot (2003) and Paglieri (2006). 

Nenoff, T.M., M.E. Welk, and F. Bonhomme, Defect-free thin film membranes for H2 separation and isolation, 2003 Annual National Hydrogen Association Meeting, Washington, March 2003. 

For Pd-Ag membranes the manufacture of defect-free thin layers, reproducible in production, and stability are major issues. In addition, sulfur resistance and the interaction of the membrane with the catalyst material is important. 

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