Hydrogen separation-purification using

Figure 15. Hydrogen separation-purification using hydrides...

Gas Reduction. The use of a gaseous reduciag agent is attractive because the metal is produced as a powder that can easily be separated from the solution. Carbon dioxide, sulfur dioxide, and hydrogen can be used to precipitate copper, nickel, and cobalt, but only hydrogen reduction is appHed on an iadustrial scale. In the Sherritt-Gordon process, the excess ammonia is removed duting the purification to achieve a 2 1 ratio of NH iNi ia solution. Nickel powder is then precipitated by... 

Hydrogen sulphide is used in the preparation of metal sulphides, oil additives etc., in the purification and separation of metals, as an analytical reagent and as raw material in organic synthesis. Physical properties are summarized in Table 8.11 and effects of temperature on vapour pressure are shown in Figure 8.4. 

An existing industrial application of the ultrapure hydrogen separated by dense Pd alloy membranes is for electronics industry. In the fabrication of silicon chips, hydrogen acts as a carrier to transport small quantities of vaporized chemical compounds required to "dope" the chip to the surface of the silicon wafer [Philpott and Coupland, 1988]. The hydrogen used must be of a very high purity. The membrane units are used not only for gas purification but also for hydrogen recovery from hydrogen>rich gas streams. 

Since the only by-product is hydrogen, the purification of the precursors can be performed by evaporating all volatile components from the reaction mixture. The catalyst used remains in the precursor. An important advantage over ammonolysis reactions of chlorosilanes is the possibility of synthesizing highly cross-linked insoluble polymeric precursors, which are usually difficult to separate from solid by-products. 

This section will provide an overview of the principles of hydrogen separation and purification using membranes. More detailed discussions of the theory governing membrane separation processes can be found elsewhere.1... 

The first commercial metal membranes for hydrogen separation and purification were made of palladium alloyed with 23-25 wt % silver. These membrane were of the unsupported type and tubular in shape. Nevertheless, the wall thickness was substantial by current standards—typically at least 100- an thick. Advances in drawing thin-walled metal tubes has allowed for palladium-silver tubular membranes to be made with much thinner walls, about 20- an thick. Composite membranes are also usually at least 25-/an thick. REB Research and Consulting (Oak Park, MI) provides tubular composite metal membranes consisting of a palladium coating over a tantalum base metal, although other group 4 or 5 base metals may be used. 

There are several methods for the preparation of mercaptans and sulfides. It is our intention here to emphasize only the most facile methods that are safe to operate commercially, use inexpensive raw materials, produce minimum by-products, and allow for simple and easy by-product handling. While the sources of sulfur for thiolation can be many, the most common ones are hydrogen sulfide (H2S), elemental sulfur, and carbon disulfide (CS2). H2S is a component of sour natural gas and can be separated for use in the manufacture of mercaptans and other sulfur chemicals. Additionally, H2S is generated in the many hydrode-sulfurizing units in a refinery. With minimal purification this could be used for the manufacture of thiochemicals. In many instances H2S is manufactured by either the Girdler or the Folkins processes ... 

In the following sections some aspects of (potential) applications of sc-fluids in the fine chemical industry with respect to product separation/purification and catalytic reactions are discussed. Earlier industrial applications of supercritical fluid reactions, for example the Haber-Bosch process for the synthesis of ammonia, synthesis of methanol from hydrogen and carbon monoxide, or the polymerization of ethene will not be discussed. An extensive overview on the use of sc-fluids in the synthesis of bulk chemicals is given in the book edited by fessop and Leitner [12],... 

The cost of production for a steam methane reformer to produce 5 MMS-CFD of high purity hydrogen is shown in Table 4. This estimate is based on a cylindrical reformer furnace using a hydrogen PSA for hydrogen separation and purification. Capital cost is third-quarter 1996, US Gulf Coast [8]. 

Hydrogen is used in a large number of chemical processes, and may be used as a fuel itself or as a reactant in the production of synthetic fuels such as in the Fischer-Tropsch hydrocarbon synthesis process, for example. In applications where hydrogen purification is required, membranes can be used for hydrogen separation. Other hydrogen purification methods include pressure swing adsorption and cryogenic separation. 

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