Particle-level membrane reactors

A second option is to apply the membrane on the particle level (millimeter scale) by coating catalyst particles with a selective layer. As a third option, application at the microlevel (submicrometer scale) is distinguished. This option encompasses, for example, zeolite-coated crystals or active clusters (e.g., metal nanoparticles). Advantages of the latter two ways of application are that there are no sealing issues, it is easy to scale-up, the membrane area is large per unit volume, and, if there is a defect in the membrane, this will have a very limited effect on the overall reactor performance. Because of these advantages, it is believed that using a zeolite... 

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. 

In conventional industrial multi-phase reactors, the heterogeneous catalyst can be organized as a packed (or fixed) bed of catalyst particles (e.g., in trickle-bed reactors or in submerged up-flow reactors), as catalyst particles suspended or fluidized in one of the two phases (in the hquid phase of a three-phase reactor, as for example in a slurry-stirred reactor and a slurry-bubbling reactor) or finally as a structured catalyst (e.g., monolith and membrane reactors). Structured catalysts are regular solid structures which reduce randomness through a well-defined structure and shape at a reactor level. The selection of the most appropriate traditional multi-phase... 

Ideally, the axial velocity through the cross-flow unit should be greater than about 4-6 m/s to minimize the boundary layer of particles near the membrane surface. The wax permeate flow from the filter is limited by a control valve actuated by a reactor-level controller. Hence, a constant inventory of slurry is maintained within the SBCR system as long as the superficial gas velocity remains constant. Changes in the gas holdup due to a variable gas velocity are calculated... 

A low pressure UV lamp (11W, Amax = 253.7 mn) was positioned vertically inside the quartz glass cylinder in the middle of the photocatalytic zone. Air was supplied from a porous titanium plate directly below the membrane module. The purpose of the aeration was to provide dissolved oxygen for photoreaction, to fluidize the IIO2 particles and to create sufficient turbulence along the membrane surface. The reaction temperature was controlled by using cooling water. Permeate was withdrawn from the system with the help of a suction pump. A water level sensor was used to maintain a constant level of solution in the reactor. Additionally, the exterior wall of the reactor was covered with a reflecting aluminum foil to improve the efficiency of UV utilization. 

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