Commercial gases, purification

Although chemisorbents are not used as extensively as physical adsorbents, a number of commercially significant processes employ chemisorption for gas purification. 

A process development known as NOXSO (DuPont) (165,166) uses sodium to purify power plant combustion flue gas for removal of nitrogen oxide, NO, and sulfur, SO compounds. This technology reHes on sodium metal generated in situ via thermal reduction of sodium compound-coated media contained within a flue-gas purification device, and subsequent flue-gas component reactions with sodium. The process also includes downstream separation and regeneration of spent media for recoating and circulation back to the gas purification device. A full-scale commercial demonstration project was under constmction in 1995. 

Although copper catalysts were known to be highly active for this reaction for many years, it was not until the late 1960s that gas purification processes for synthesis gas were introduced that would allow the commercial use of these catalysts, which require very low sulfur, chlorine, and phosphoms feed impurity levels to maintain catalyst activity. 

Of the removal processes that have attained commercial status, the current favorite employs a shiny of lime or limestone. The activity of the reagent is promoted by the addition of small amounts of carboxylic acids such as adipic acid. The gas and the shiny are contacted in a spray tower. The calcium salt is discarded. A process that employs aqueous sodium citrate, however, is suited for the recoveiy of elemental sulfur. The citrate solution is regenerated and recycled. (Kohl and Riesenfeld, Gas Purification, Gulf, 1985, p. 356.)... 

Consequently, two semicommercial pilot plants have been operated for 1.5 years. One plant, designed and erected by Lurgi and South African Coal, Oil, and Gas Corp. (SASOL), Sasolburg, South Africa, was operated as a sidestream plant to a commercial Fischer-Tropsch synthesis plant. Synthesis gas is produced in a commercial coal pressure gasification plant which includes Rectisol gas purification and shift conversion so the overall process scheme for producing SNG from coal could be demonstrated successfully. The other plant, a joint effort of Lurgi and El Paso Natural Gas Corp., was operated at the same time at Petrochemie Schwechat, near Vienna, Austria. Since the starting material was synthesis gas produced from naphtha, different reaction conditions from those of the SASOL plant have also been operated successfully. 

Iron oxide is one of the oldest media used for H2S removal from process gas. Its use is limited by the necessity of periodically replacing the solid adsorbent (3), as the adsorbent cannot be regenerated completely and gradually loses its effectiveness. Hot-gas purification processes using iron oxide include the Appleby-Frodingham Process and the METC Fixed-Bed Process (1). In these processes the iron oxide removes the H2S at elevated temperatures (700-1000°F) and is also regenerated at these temperatures. None of these or other hot-gas purification processes is commercial today. 

Fig. 2.13. Commercial metal glove box. An aluminum glove box with a recirculating gas-purification system. (Reproduced by permission of the copyright ow ner Vacuum Atmospheres Corp., North Hollywood, Calif.)...

Commercial gas-adsorption processes (see Table I) can be divided into bulk separations, in which about 10 weight percent or more of a stream must be adsorbed, and purifications, in which usually considerably less than 10 weight percent of a stream must be adsorbed. Such a differentiation is desirable to make because in general different process cycles are used for the different categories, as will be discussed later. 

Since 1978, the use of zeolites to solve environmental pollution and energy conservation problems showed promise and was expected to increase. Natural zeolites have many commercial uses in coal gasification and natural gas purification, selectively adsorb molecules from water or air, purify sludge effluents to potable standards, extract trace amounts... 

Fluorination of nitrophenyl esters was carried out in stainless steel and Hastelloy autoclaves. Sulfur tetrafluoride used in fluorination was treated with mercury (16) to remove bromine, a consistent contaminant of the commercially available gas. Purification was essential for high yields. The presence of sufficient anhydrous hydrogen fluoride to ensure the presence of a liquid phase also appears to be important in obtaining reasonably good yields. Fluorination experiments are summarized in Table VI. 

Main commercial gas separation and purification processes using synthetic zeolites [100]. 

Although the monolith reactor has already found a number of applications in gas-phase reactions (e. g., the catalytic purification of exhaust gases from automobiles), it is not currently applied in commercial gas-liquid reactions in fine chemicals production. 

Purity of the carrier gas is very important in modern GC equipment designated for trace analysis. Consequently, it is essential that the gas purifiers, such as the traps containing various adsorbents, be inserted in the gas tine before the injection port. The same requirement usually applies for purification of the combustion gases for the flame ionization detector. The role of these adsorbent traps is to remove even the trace quantities of water, oxygen and organic impurities present in commercial gas cylinders, and thus minimize both the system contamination and chemical alteration of an injected sample. 

In commercial operation catalyst life, as well as activity, must be considered. In the earlier installations of methanol synthesis plants, there was little gas purification of the scale and effectiveness that is presently used, and as a result the catalysts or operating conditions that were used were necessarily more rugged and severe than is presently the case. In other words, the fact that recent methanol installations operate under milder temperature and pressure conditions is attributable not only to improved catalysts but also to the fact that the synthesis gases presently employed have been detoxified to the extent that catalysts that have long been known are now practicable. 

Fluid-fluid systems are widely used in chemical, petroleum, pharmaceutical, hydrometaflurgical, and food industries. Commercially important examples of gas-liquid mass transfer with or without reaction include gas purification, oxidation, halogenations, hydrogenation, and hydroformylation to name but a few. Important liquid-liquid reactions include nitration, phase transfer catalysis (PTC), cyclization, emulsion polymerization, homogenous catalyst screening, enzymatic reactions, extraction, precipitation, crystallization, and cell separation. 

The simultaneous absorption of two gases that react with the solvent at different rates has been studied by Ouwerkerk. The specific system which he selected for analysis was the selective absorption of HjS in the presence of CO2 into amine solutions. This operation is a feature of several commercially important gas purification processes. Bench scale experiments were conducted to collect the necessary pi sico-chemical data. An absorption rate equation was developed for H2S based on the assumption of instantaneous reaction. For CO2 it was found that the rate of absorption into diisopropanolamine (DIPA) solution at low CO2 partial pressures can best be correlated on the l is of a fast pseudo-first-order reaction. A computer program was developed which took into account the competition between H2S and CC>2 when absorbed simultaneously, and the computer predictions were verified by experiments in a pilot scale absorber. Finally, the methodology was employed successfully to design a large commercial plant absorber. 

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