Gases purification

The removal of impurities from a gas that is to be cooled or liquefied may be accomplished by several methods. The most common of these for cryogenic systems include those of refrigeration, physical adsorption, and chemical reaction. 

Stream after refrigeration would be inversely proportional to its vapor pressure. 

Under ideal gas behavior, the mole fraction of a component in the mixture is related to the partial pressure of that component by 

Since a perfect gas behavior was assumed in the derivation of Eq, (6.76), caution is advised in the use of this equation when the pressure of the gas mixture deviates appreciably from this assumption. For example, experimental measurements have shown that the actual water vapor content in air will be over four times that predicted by ideal gas behavior at a temperature of — 227K and a pressure of 20.2 MPa. Familiarity with these deviations is necessary if problems are to be avoided with this method of impurity removal. Data of this type are available as enhancement factors, defined as the ratio of the actual molar concentration to the ideal molar concentration of a specific impurity in a given gas. 

Example 6.15. Impure hydrogen gas with a volumetric composition of 85% hydrogen, 9% methane, and 6% ethane enters a refrigeration purifier at 300 K and 0.1013 MPa. Assuming that all the gases obey the perfect gas equation of state, determine the composition of the gas leaving the purifier at a temperature of 67 K and a pressure of 0.1013 MPa. 

In further purification, carbon dioxide, residual carbon monoxide, and sulfur compounds (only present in the synthesis gas from partial oxidation) have to be removed as they are not only a useless ballast but above all poisons for the ammonia synthesis catalyst. 

The raw synthesis gas produced by steam reforming of natural gas and light hydrocarbon feedstocks is free of sulfur. Any sulfur contained in the feedstock has to be removed of upstream of gasification to avoid poisoning of the sensitive reforming catalysts. This is usually performed by hydrodesulfurization and adsorption of the H2S by ZnO. As this is an essential part of the steam reforming process, it was already treated in Section 4.1.1. 

In partial oxidation processes there is no prior treatment of the feedstock and the total sulfur contained in the coal or hydrocarbon feed is converted in the gasification to H2S and a smaller amount of COS. As H S is soluble in the same solvents which can be used for C02 removal, selective absorption and/or desorption is a special problem 

Chemical solvents are best suited to scrubbing gases that have a relative low C02 partial pressure, whereas the physical solvents are more suitable for higher C02 contents in the raw gas. As both sour gases, C02 and H2S, have good solubility in the applied solvents, special process configurations are required for partial oxidation gases to recover separately a pure C01 fraction and an H2S-rich fraction suitable for sulfur disposal. 

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Kohl, A. L., and Riesenfeld, F. C., Gas Purification, Gulf Publishing Company, Houston, Texas 1979. 

The purpose of such scmbbing operations may be any of the following gas purification (eg, removal of air pollutants from exhaust gases or contaminants from gases that will be further processed), product recovery, or production of solutions of gases for various purposes. Several examples of appHed absorption processes are shown in Table 1. 

Gas purifications H2O/olefin-containing cracked gas, natural gas, air, synthesis gas, etc sHica, alumina, zeoHte... 

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

Gas purification processes fall into three categories the removal of gaseous impurities, the removal of particulate impurities, and ultrafine cleaning. The extra expense of the last process is only justified by the nature of the subsequent operations or the need to produce a pure gas stream. Because there are many variables in gas treating, several factors must be considered (/) the types and concentrations of contaminants in the gas (2) the degree of contaminant removal desired (J) the selectivity of acid gas removal required (4) the temperature, pressure, volume, and composition of the gas to be processed (5) the carbon dioxide-to-hydrogen sulfide ratio in the gas and (6) the desirabiUty of sulfur recovery on account of process economics or environmental issues. 

Synthesis gas preparation consists of three steps ( /) feedstock conversion, (2) carbon monoxide conversion, and (2) gas purification. Table 4 gives the main processes for each of the feedstocks (qv) used. In each case, except for water electrolysis, concommitant to the reactions shown, the water-gas shift reaction occurs. 

Steam-Reforming Natural Gas. Natural gas is the single most common raw material for the manufacture of ammonia. A typical flow sheet for a high capacity single-train ammonia plant is iadicated ia Figure 12. The important process steps are feedstock purification, primary and secondary reforming, shift conversion, carbon dioxide removal, synthesis gas purification, ammonia synthesis, and recovery. 

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. 

In general, plants using SO2 gas derived from metallic sulfides, spent acids, or gypsum anhydrite purify the gas stream before drying it by cold, ie, wet, gas purification. Various equipment combinations including humidification towers, reverse jet scmbbers, packed gas cooling towers, impingement tray columns and electrostatic precipitators are used to clean the gas. 

Plants that bum good quaUty elemental sulfur or H2S gas generally have no faciUties for purifying SO2. Before the advent of relatively pure Frasch or recovered sulfur, however, hot gas purification was frequentiy used in which the SO2 gas stream was passed through beds of granular soHds to filter out fine dust particles just prior to its entering the converter. 

Sulfur shipped as a soHd frequentiy becomes contaminated with dirt and scale during shipping and handling. In areas of the world where soHd sulfur is stiU handled, molten sulfur is frequentiy filtered prior to use as an alternative to, or in combination with, hot gas purification. Since the eady 1970s, most sulfur used in the United States and Europe has been shipped and handled as a Hquid containing very low ash concentrations, typically <0.005%. Using this type of raw material, neither sulfur filtration nor hot gas purification are essential, and are rarely used. 

Oxygen-enriched air is sometimes used in spent acid decomposition furnaces to increase furnace capacity. Use of oxygen-enriched air reduces the amount of inerts in the gas stream in the furnace and gas purification equipment. This permits higher SO2 throughput and helps both the heat and water... 

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. 

Ethanolamines. These are produced by the reaction of ethylene oxide and ammonia (see Alkanolamines). Approximately one-third of the production is used in detergents. Other appHcations include natural gas purification, cosmetics, metalworking, textiles, and chemical intermediates (282). 

Generahzed prediction methods for fci and Hi do not apply when chemical reaction occurs in the liqmd phase, and therefore one must use ac tual operating data for the particular system in question. A discussion of the various factors to consider in designing gas absorbers and strippers when chemical reac tious are involved is presented by Astarita, Savage, and Bisio, Gas Treating with Chemical Solvents, Wuey (1983) and by Kohl and Ricseufeld, Gas Purification, 4th ed., Gulf (1985). 

Gas purification or the removal of relatively small amounts of impurities such as CO2, CO, COS, SO2, H2S, NO, and others from air, natural gas, hydrogen for ammonia synthesis, and others... 

SOURCE From Kohl and Riesenfeld, Gas Purification, Gulf, 1985. 

FIG. 23-27 CO, in potassium carbonate solutions (<2) equilibrium in 20% solution, (h) mass-transfer coefficients in 40% solutions. (Data cited hy Kohl and Riesenfeld, Gas Purification, Gulf Fuhlishing, 1985.)... 

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.)... 

Wastes from petroleum refining, natural gas purification and pyrolitic treatment of coal Wastes from inorganic chemical processes Wastes from organic chemical processes... 

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