Selective scrubbing

The quenched gas passes to an H2S removal stage where it may be assumed that H2S is selectively scrubbed down to 15 parts per million with substantially nil removal of CO2. Solution regeneration in this process is undertaken using the waste low-pressure steam from another process. The scrubbed gas, at 35°C and saturated, has then to undergo CO conversion, final H2S removal, and CO2 removal to allow it to meet the product specification. 

As no chemical reaction between CO2 and tertiary amines occurs, the latter (e.g., MDEA) are used in selective scrubbing of H2S. To absorb CO2, primary or secondary amines in the aqueous phase are required, which can react with CO2 directly... 

Another problem is the detoxification of the hazardous wastes that are already present in the environment. Efforts are thwarted by the problan of how less toxic are the detoxification products in themselves Thus, in incineration, for instance, what are the toxicides of all the final combustion products Called products of incomplete combustion, or PlCs, these are the myriad by-products and coproducts of the competing reactions that occur during combustion, and for that matter, during any chemical conversion, more or less. Can they ever be fully detected and analyzed Or if selectively scrubbed, what is to be the disposition of the absorbed materials What is called detoxification may be merely a further dispersion throughout the ecosphere, and a process of trading one set of problems for another. 

The vapor stream from the meal and oil distillation systems consists of solvent vapor, water vapor (from the stripping steam) and air that has entered the system entrained in the voids of the press cake. Some additional air unavoidably gains entry into the vacuum systems. The vapor stream from the final condenser consists of air saturated with solvent and water vapors. The solvent vapors are selectively scrubbed in a packed tower by a counterflow of special mineral oil. The mineral oil is recycled after being heated, stripped of solvent, and cooled. The hexane vapor remaining in the vent stream should be below the lower explosive limit (1.3% by volume). Nevertheless, overall rapeseed plant losses per tonne flow through the extractor are typically much higher than they are for soybeans. This problem should be researched and, if possible, overcome. 

If required, relatively pure carbon monoxide can be recovered by cryogenic separation (i.e. condensation at very low temperatures), use of selective scrubbing solutions (for example CUAICI4 in toluene, the COSORB process) or solid adsorbents, and by other methods. Some CO recovery techniques are also applicable to the CO-rich (c. 70% molar) off-gases from basic oxygen furnaces and the leaner off-gases from air blast furnaces produced in steel manufacture, though operations of this type are still limited. 

This can be illustrated by the work of Cornelisse et al (38) who have considered the selective absorption of HpS into secondary or primary amines in the presence of CO. This is of commercial importance in the selective scrubbing of a number of industrial gas streams. The reactions involved can be written (following previous notations)... 

These reactions are used industrially for scrubbing operations and for selective removal of H2S from natural gas containing CO2. 

As stated, gas absorption is a process whereby gas and liquids are brought into intimate contact and a component of the gaseous phase becomes dissolved in the liquid. With the proper selection of scrubbing liquid, the gas contaminant will have... 

Corrosion resistance it is critical to select construction materials such that neither the gas and solute nor the scrubbing liquid will corrode the packing materials. Ceramic and plastic materials, are commonly selected for this reason. 

Gilbert, W., Selecting Materials For Wet Scrubbing Systems, Pollution Engineering, Aug. 1973, p. 28. 

Refiners can address the sulfur issue in stages, but decisions should be made that will leave the door open for further reductions. If hydrotreating is selected, the design can include oversized reactors, connections for a spare compressor, or connections for adding amine scrubbing inside the recycle loop. Some process or catalyst changes can buy time, some can solve the problem. 

The composition of the gas mixture, which is introduced into the tube bundle reactor (tubes of 6-12 m length and 20-50 mm diameter, filled with the Ag catalyst) consists of 15-50 vol % ethylene, 5-9% oxygen, as much as 60% methane as dilution gas, and 10-15% carbon dioxide. The reaction therefore proceeds above the upper explosion limit. The ethylene conversion runs up to 10% per cycle through the reactor. The ethylene oxide selectivity amounts to 75-83 % maximum. The formed ethylene oxide is recovered by scrubbing with water and the newly formed carbon dioxide is separated from the cycle gas, e.g., by hot potash washing process. 

SOLINOX SO,. Linde NO,] A process for removing both NOx and SOx from fluegases. The SOx is removed by scrubbing with tetra-ethylene glycol dimethyl ether, circulated in a packed tower (the Selexol process). The NOx is destroyed by Selective Catalytic Reduction ( SCR). The sorbent is regenerated with steam the SOx is recovered for conversion to sulfuric acid. Developed by Linde in 1985 and used in a lead smelter in Austria and several power stations in Germany. In 1990 it was announced that it would be used at the titanium pigment plant in The Netherlands operated by Sachtleben. 

The purity of the product is determined not only by the inherent selectivity of the solvent system for the component sought, but also on the phase ratio, the concentration of the contaminants, and the performance of any scrubbing of the extract. 

As no solvent is perfectly selective for a given solute, the extract often contains higher concentrations of impurities that should be removed. This is done by "scrubbing" the extract in a second stage, usually involving the original solvent under such conditions that the desired solute is largely retained in the extract while the impurities are washed out. 

It is also doubtful that the industry will be in a position for many years to come to undertake sulfur removal from residual fuels solely to improve product quality. A number of consumer industries demand low sulfur fuel oils, but these special requirements can at present be met more appropriately by selection of crude rather than by adoption of desulfurization processes. In general industrial use, it is corrosion and atmospheric pollution that are the main disadvantages of high sulfur content. But there is no sign yet of the development of a cheap desulfurization process, the cost of which can be substantially offset by the gain in efficiency resulting from permissible lower stack temperatures or by the elimination of flue gas scrubbing equipment previously necessary for reduction of sulfur dioxide content. 

Pervaporation. Vapor arbitrated pervaporation is used to remove ethanol from whiskey by selective passage of the alcohol through a membrane. Whiskey flows on one side of a membrane. A water-vapor stream flows on the other side and sweeps away the ethanol that permeates the membrane. Thus alcohol reduction and selective retention of flavor and aroma components can be achieved using membranes with a particular porosity. The ethanol can be recovered by condensing or scrubbing the vapor stream. Pervaporation systems operate at or slightly above atmospheric pressure (Fig. 

Sherds from the Persian Period Stratum of Tell el-Hesi. The ceramic materials came from field I, areas 1, 2, 3, 11, 21, 31, in phases A, B, C of stratum V (Persian period). Field procedures called for the excavated sherds to be kept in buckets by loci. The buckets were filled with well water, and the sherds were allowed to soak overnight. Any remaining dirt on the sherds was scrubbed off with nylon brushes and clean well water. Washed sherds were dried in the sun, and representative types were selected to be registered, labelled, and drawn. 

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