Purification acid gas

Acid Gas Purification Techniques for CO2 (carbon dioxide ) Benfield Process, Air Pollution Control Technology in Japan, Exhaust Gas Treatment Equipment, Kobe Steel, LTD., Tokyo, Japan, 2002 (nett21. unep.or.jp/CTT DATA/AIR/AIR 7/html/Air-208.html). 

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. 

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. 

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... 

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.)... 

Hydrochloric acid Production, purification, recovery, processing Bayonet heaters, heat exchangers, coils, condensers, HCl absorbers, synthetic HCl plants, acid coolers, gas coolers, chlorine burners, strippers, thermometer wells... 

HPC [Hot potassium carbonate] A generic name for a process for absorbing acid gases by the use of hot aqueous potassium carbonate. Developed by H. E. Benson and J. H. Field at the U.S. Bureau of Mines in the 1950s. Further developed by other organizations, it became the basis for the Benfield, CATACARB, and other gas purification processes. 

The acid gas stream from the purification process is sent to a sulfur recovery unit which is usually a Stretford or a Claus unit. The product from these units is elemental sulfur. 

Techniques for purification of acid gas streams by removal of H2S, COS and carbon dioxide are standard technology. Recovery of elemental sulfur from these acid gas streams by use of Claus or Stretford units is also conventional technology. These technologies are being practiced on a large scale by both petroleum refiners and natural gas processors. 

The redistillation of phosphorus is occasionally resorted to as a means of purification. On account of its excessive inflammability, the process demands great care and attention. Dumas recommends for the distillation of phosphorus a double U-shaped tube, into the first curve of which the phosphorus to be distilled is placed, the other curve serving in place of a receiver. The arrangement is afterwards connected with an apparatus from which issues a current of perfectly dry hydrogen, or carbonio acid gas. The phosphorus, when submitted to heat, volatilizes, and distils over with the gas, and is condensed in the second curve of the tube, from which it is removed after being melted in hot water. 

The exhaust gas must have a temperature of > 300 °C at the exit of the kiln to prevent condensation of sulfuric acid in the ducting. Energy can be saved by recirculating some of the gas to the combustion chamber of the kiln and mixing it with the fuel gases as a partial replacement for air. Alternatively, it can be used for concentrating the dilute acid (see Section 2.1.3.5). The gas then goes to the waste-gas purification system. 

The CNG process meets three objectives commonly acknowledged by experts in gas purification to be highly desirable of acid gas removal in synthetic fuels production from coal ... 

Rosseau, R.W., Kelly, R.M., and Ferrell, J.K., "Evaluation of Methanol as a Solvent for Acid Gas Removal in Coal Gasification Processes," Symposium on Gas Purification, AIChE Spring National Meeting, Houston, Texas, April 1981. 

Reactive absorption is probably the most widely applied type of a reactive separation process. It is used for production purposes in a number of classical bulk-chemical technologies, such as nitric or sulfuric acid. It is also often employed in gas purification processes, e.g., to remove carbon dioxide or hydrogen sulfide. Other interesting areas of application include olefin/paraffin separations, where reactive absorption with reversible chemical complexation appears to be a promising alternative to the cryogenic distillation (62). 

Industrial fluid-fluid reactors may broadly be divided into gas-liquid and liquid-liquid reactors. Gas-liquid reactors typically may be used for the manufacture of pure products (such as sulfuric acid, nitric acid, nitrates, phosphates, adipic acid, and other chemicals) where all the gas and liquid react. They are also used in processes where gas-phase reactants are sparged into the reactor and the reaction takes place in the liquid phase (such as hydrogenation, halogenation, oxidation, nitration, alkylation, fermentation, oxidation of sludges, production of proteins, biochemical oxidations, and so on). Gas purification (in which relatively small amounts of impurities such as C02, CO, COS, S02, H2S, NO, and... 

Sulfuric acid plants are located throughout the industrialized world, Fig. 2.2. Most are located near their product acid s point of use, i.e. near phosphate fertilizer plants, nickel ore leach plants and petroleum refineries. This is because elemental sulfur is cheaper to transport than sulfuric acid. Examples of long distance sulfur shipment are from natural gas purification plants in Alberta, Canada to acid plants near phosphate rock based fertilizer plants in Florida and Australia. A new sulfur-burning sulfuric acid plant (4400 tonnes of acid per day) is costing 75 million U.S. dollars (Sulfuric 2005). 

Gas Purification. There are many and varied examples using both natural and synthetic zeolites. The natural zeolites are Ukely to be chabazite, clinoptilolite, or mordenite. The latter two are useful for dealing with acid gas streams. 

Methanation as final purification for the raw gas from partial oxidation was proposed by Topsoe [739]. In this case the shift conversion is carried out in two stages with a special sulfur-tolerant shift catalyst followed by removal of hydrogen sulfide and carbon dioxide in an acid gas removal unit. Because of the potential danger of a sulfur break-through causing poisoning, the normal copper - zinc - alumina catalyst is usually not applied, which is surprising as the same risk exists in partial oxidation based methanol plants for the similarly composed methanol catalyst. 

The most important parameter in the absorption is the pressure. At high pressure (up to 15 bar) concentrated acid can be produced and tail gas purification can be dispensed with... 

The reaction product can either be directly poured onto cooling rollers or first purified by distillation (b.p. 513 to 515°C). If a non-discolored phosphorus(V) sulfide is required, organic impurity-free phosphorus and very pure sulfur have to be used. The former can be obtained by sulfuric acid extraction and the latter from natural gas purification. 

With the double contact process it is unnecessary to purify the tail gases to reduce their sulfur dioxide content still further, whereas tail gases from single contact plants have to be purified. This can be realized either by scrubbing with ammonia or with an aqueous solution of sodium sulfite and sodium hydrogen sulfite (Wellman-Lord process), absorption on activated charcoal (sulfacid process from Lurgi) or by oxidative gas purification such as in the peracidox process (oxidation of sulfur dioxide with hydrogen peroxide or peroxomonosulfuric acid). 

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