Membrane high pressure steam

High-pressure steam may also serve as a driver for turbines. Chlor-alkali plants, which are usually placed in areas of low electrical power cost, are less likely than most other types to justify the use of steam-turbine drives but still may use them as a backup source of power. Furthermore, one of the ways to cope with a major electrical failure is to use steam to operate critical drives imtil all systems are shut down or electrical power is restored. An example of a critical service is the caustic circulation pump on the emergency vent scrubber. A spare pump is always necessary, and it should have an independent source of power. One way to accomplish this is with a steam-turbine drive. Other services may also be considered critical for personnel safety or process security. The latter is especially true in a membrane-cell plant, where some systems are vital for the protection of the membranes from damage. 

The high-pressure gas (retentate) stream leaves the membrane steam reformer at 625°C and 30 bar, while the H2-rich permeate stream leaves the membrane steam reformer at 555°C and 1.5 bar. Pure nitrogen from an air separation plant is supplied as a sweep gas on the permeate side of the membrane. 

A fixed-bed reactor often suffers from a substantially small effectiveness factor (e.g., 10 to 10 for a fixed-bed steam reformer according to Soliman et al. [1988]) due to severe diffusional limitations unless very small particles are used. The associated high pressure drop with the use of small particles can be prohibitive. A feasible alternative is to employ a fluidized bed of catalyst powders. The effectiveness factor in the fluidized bed configuration approaches unity. The fluidization system also provides a thermally stable operation without localized hot spots. The large solid (catalyst) surface area for gas contact promotes effective catalytic reactions. For certain reactions such as ethylbenzene dehydrogenation, however, a fluidized bed operation may not be superior to a fixed bed operation. To further improve the efficiency and compactness of a fluidized-bed reactor, a permselective membrane has been introduced by Adris et al. [1991] for steam reforming of methane and Abdalla and Elnashaie [1995] for catalytic dehydrogenation of ethylbenzene to styrene. 

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