Removal of solvents from the gas phase

It was therefore a natural step to employ these techniques when the use of solvents on a large scale made the recapture of solvents from process efiluent air attractive economically. Our present concern with the quality of air is, of course, a much later development but carbon bed adsorption and air scrubbing are still two of the most frequently used methods of removing solvents from air (Fig. 2.1). To them, we can now add the low-temperature 

To put the requirements of solvent removal from air into perspective, it is useful to compare the purity levels that are required for a variety of purposes. For this comparison, all the concentrations in Table 2.1 have been reduced to parts per million (ppm) on a weight basis. 

To give satisfactory air pollution as far as ozone is concerned, photochemical oxidants which include most solvents should not exceed about 0.044 ppm in the atmosphere. 

While most of the available techniques for waste air purification can be considered, the following should be treated with caution  

High molecular ketones, alcohols, ethers Benzene, cyclohexane, dioxane, dimethyl sulphide, cyclohexanol Highly volatile solvents Ethanol, methanol, dichloromethane 

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Reactants and reagents can be conveniently loaded into the dry zeolite by adsorption. This can be accomplished by intimately mixing the solid or liquid reactant and the powdered zeolite, by absorption from the gas phase, or by diffusion in a solvent slurry containing the zeolite and dissolved reactant. The choice of solvent for the slurry method is critical. It must be volatile enough to be removable at a pressure and temperature that does not result in evacuation of the reactant or its decomposition. In addition, the reactant must have a greater affinity for the interior of the zeolite than for the slurry solvent itself. The lack of affinity for the interior of the zeolite is an acute problem for non-polar hydrocarbons that lack binding sites for the intrazeolitic cations. The use of fluorocarbons such as perfluorohexane as slurry solvents takes advantage of the fluorophobicity of many hydrocarbons and has alleviated this problem to some extent.29... 

C18 solid-phase extraction is used to fractionate polyphenolics for their identification and characterization. This technique can eliminate interfering chemicals from crude extracts and produce desirable results for HPLC or other analytical procedures. To obtain a sufficient volume for all analyses, several separations by solid-phase extraction may be performed. The individual fractions need to be combined and dissolved in solvents appropriate for HPLC analysis. In Basic Protocol 2, the application of a current of nitrogen gas for the removal of water from the C18 cartridge is an important step in the selective fractionation of polyphenolics into non-anthocy-anin and anthocyanin fractions. After the collection of non-anthocyanin polyphenolics, no additional work is necessary to elute anthocyanins bound to the C18 solid phase if anthocyanins are not to be determined. 

In 1982, Soga et al. 256> showed that exposure of acetylene to AsFs at low temperatures leads to rapid polymerization (in our experience this reaction can be explosively violent). The product is a solid polymer which is heavily arsenic-doped and has a conductivity several orders of magnitude lower than a conventional sample of polyacetylene saturation-doped from the gas phase. Aldissi and Liepins 2S7) have adapted this reaction to the preparation of soluble polyacetylene by adopting AsF3 as the reaction solvent. They claim that polymerization of acetylene with AsF5 is very rapid, giving a polymer which is soluble in common solvents. However, elemental analysis shows that the polyacetylene formed contains about one As atom per 10 CH units and this is not removed on repeated reprecipitations. It seems likely that the As atoms form part of the chain backbone, conferring sufficient flexibility to allow dissolution. It is claimed that films of soluble polyacetylene can be doped but very little information has been published. 

The height of an absorption column depends on the feed conditions, the product purity specifications, the solvent used and the extent of separation through the absorption equilibrium relationship, but also on the rate of separation. If the rate of mass transfer of the gaseous component from the gas phase into the liquid phase is slow, then the column needs to be longer to ensure that the required amount is removed. The rate of mass transfer depends on the mass-transfer coefficient, normally denoted kG or k. The value of the mass-transfer coefficient depends on the components in the gas feed and on the solvent used and is often determined experimentally. The type of packing used in the column will also have an impact on the column height as for distillation. 

By passing the effluent from the gas-phase reaction into a solvent at 253 K and then removing the aimnonia by warming up the solution or passing it through a colunrn with anhydrous copper(II) sulfate, solutions of chloramine in various solvents can be obtained. Almost pure (97%) NH2CI is obtained by the fractional low-temperature condensation of the effluent. Pure... 

We shall now consider briefly results obtained from the hydrogenation of 2-butyne in the liquid phase, using alcohol as solvent and Pd-BaS04 as catalyst (59). In this study cfs-2-butene was the only detectable product until the complete removal of the alkyne, the subsequent removal of the olefin by hydrogenation was accompanied by very fast isomerization. These results are in complete harmony with those obtained from the gas phase reaction and thus the mechanism of this reaction must be very similar in both the gas and liquid phases. 

The removal of a volatile solute from a relatively nonvolatile solvent may be accomplished by the operation of stripping whereby some chemically inert gas such as air is brought into intimate contact with the solution. The volatile material is then vaporized into the unsaturated gas and carried away, leaving behind the solute. The vaporized material must then be recovered from the gas phase by condensation (brought about by cooling and compressing), or adsorption. 

It is not uncommon practice in gas-absorption operations to employ a solvent that reacts with the solute being absorbed from the gas phase. The purpose of this practice is to promote the solute removal rate and to enhance the efficiency of the gas absorber. Acid gases such as H2S and CO2 are often contacted with solvents containing an alkaline component such as potassium or sodium hydroxide, or an ethanol amine. Conversely, the absorption of a basic solute such as ammonia can be promoted by reacting it with an acidic solvent. 

Usually the solvent temperature will be reduced as low as practical before entering the absorber in order to provide the greatest possible hydrocarbon removal from the gas stream. As hydrocarbon is absorbed in the top of the column, the liquid temperature increases, as does the Rvalue. As hydrocarbon is absorbed in the bottom of the column, the mol fraction and the partial pressure of solute in the gas phase decrease. In order to avoid an equilibrium pinch in the center of the absorber, the partially enriched solvent is collected on a trap tray and withdrawn from the column. This liquid is cooled in an external heat exchanger and... 

Radicals can be prepared from closed-shell systems by adding or removing one electron or by a dissociative fission. Generally speaking, the electron addition or abstraction can be performed with any system, the ionization potential and electron affinity being thermodynamic measures of the probability with which these processes should proceed. Thus, to accomplish this electron transfer, a sufficiently powerful electron donor or acceptor (low ionization potential and high electron affinity, respectively) is required. If the process does not proceed in the gas phase, a suitable solvent may succeed. 

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