Catalyst coated membrane requirements

However, it is not certain, for example, how to deal with impurities such as chloride in terms of recovery, even though it is well understood which particular part of catalyst-coated membrane requires attention. 

Membrane Reactor. Another area of current activity uses membranes in ethane dehydrogenation to shift the ethane to ethylene equiUbrium. The use of membranes is not new, and has been used in many separation processes. However, these membranes, which are mostly biomembranes, are not suitable for dehydrogenation reactions that require high temperatures. Technology has improved to produce ceramic and other inorganic (90) membranes that can be used at high temperatures (600°C and above). In addition, the suitable catalysts can be coated without blocking the pores of the membrane. Therefore, catalyst-coated membranes can be used for reaction and separation. 

The hybrid sulphur process requires electrolysers which are not described in chemical engineering economics literature. A specific approach has been developed by collecting data from literature and constructors of alkaline electrolysers (Mansilla, 2008). Electrolyser characteristics are also considered (catalyst coating, membranes). 

The acidic electrolyte requires a precious metal catalyst hence platinum supported on carbon particles serves as electrocatalyst, with typical Pt loadings of ca. 0.1 mg/cm at the anode side and ca. 0.4 mg/cm at the cathode side. Platinum nanoparticles, typically a few (3-5) nanometers in diameter, are deposited on various carbon substrates (e.g., 20 0%Pt/C) by wet chemical processes and then further processed in combination with solubilized ionomer material and binder(s) (PTFE) to yield an ink, which then is applied either to the electrolyte membrane surface (CCM, catalyst-coated membrane) or to the GDL to form the GDE. 

As described above, dense ceramic membranes are made of composite oxides with a large number of oxygen vacancies in the crystaUine lattice. Such materials are inherently catalytic to the oxidation and dehydrogenation reactions. Therefore, dense ceramic membrane may serve as both catalyst and separator, and catalyst is not required in the membrane reactor. As shown in Fig. 7.5a, the lattice oxygen directly takes part in the chemical reactions. Since the chemical reactions take place on the membrane surface, it is required to have a very porous membrane surface so as to contain a sufficient quantity of active sites. This can be achieved in the membrane preparation process, or by coating a porous membrane material after the preparation. The main potential problems for this are that the membrane may not have sufficient catalytic activity, and the catalytic selectivity cannot be modulated with respect to the considered reactions. 

As is obvious, many potential hurdles discussed in the previous sections do not apply to appHcation of zeolite membranes at the micro- and particle levels. Issues Hke scale-up and high-temperature sealing do not play a role here. Additionally, coated catalyst particles do not require a change of reactor, but only replacement of the catalyst. Application of zeoHte membranes at these levels is therefore considered to be easier and their implementation will probably occur earlier. 

Apply this catalyst ink onto one side of fhe membrane (Na+ fype). Two coats are typically required for adequate catalyst loading. 

Palladium and some of its alloys are catalytically active to many dehydrogenation reactions. In other cases where other catalysts arc required, they arc either impregnated in the porous support (for those composite membranes), deposited on the membrane surfaces as a coating or packed as pellets inside the membrane element. 

In parallel with the development of the membrane reformer system, a new concept membrane module, which has a palladium alloy membrane coated on the porous support tube with catalytic activity has been developed (Nishii, 2009). This membrane module is expected to provide a more compact reactor because the reactor does not require a separate catalyst. It is also expected that this module can be manufactured at low cost by applying the industrially-established mass production process used to make oxygen sensors for combustion control in vehicles with internal combustion engines. 

Ferreira-Aparicio et al. reported the development of a laboratory-scale membrane reactor for the partial dehydrogenation of methylcyclohexane into toluene in a membrane reactor [527]. A platinum/alumina catalyst containing 0.83 wt% platinum was put into a porous stainless steel tube, which had been prior coated with a palladium membrane by electroless plating. At 350 °C reaction temperature and a pressure of 1.4 bar at the reaction side, 99% of the hydrogen product could be separated through the membrane, which had a thickness of 11 pm. However, the sweep stream required on the permeate side was more than 20 times higher than the hydrogen permeate flow rate that could be achieved. 

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