Processes Using Mixed Solvents

Physico-chemical solvents are in certain respects superior also to purely physical COS absorbents. In the case of methanol, for instance, Henry s coefficient for COS absorption is only about half that for H2S. For all practical purposes, this means that the solvent rate has to be doubled if not only H2S but COS as well has to be eliminated completely (Fig. 2.7). Since, in the case of mixed solvents, COS is converted by hydroljrsis to H2S and this reaction proceeds very rapidly, physico-chemical gas cleaning units are designed only for H2S removal. Such processes are used to good effect at ambient temperature and pressures of more than 15 bar. 

A combined effort at both reducing steam demand and obtaining an acid gas with higher sulfur contents led to the development of a selective version of the Sulfinol process which had the DIPA replaced by MDEA. However, with this M-Sulfinol process, as it is called, good selectivity is in this case achieved at the expense of higher residual sulfur contents in the clean gas. If the residual H2S content is required to be less than 10 ppm, much of the selectivity is lost, since for instance for 1 ppm residual H2S the wash liquor loop becomes so large that 

As the solubility of hydrogen and carbon monoxide is practically independent of the prevailing temperature (see Fig. 2.7), the rate at which these two gas components are dissolved at ambient temperature economically justifies an interim flash stage followed by H2 and CO recompression, above all for large plants and high pressures. 

Bgure 2.15 shows the process flow diagram for a single-stage Amisol unit. Having been cleaned in the absorber at something like 40 bar, the gas has to be fed to a water scrubber to avoid major solvent losses which would otherwise be 

The scrubber effluent, which contains some methanol, is fed to a distillation column to expel the methanol overhead and obtain clean water at the bottom. This distillation column operates at a slightly elevated pressure to ensure that the overhead vapors are condensed in a separate reboiler at the regeneration column and can thus provide part of the heat required by that column. The remaining regeneration heat demand may be covered by waste heat at temperatures above 90 °C. 

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This is an oversimplified treatment of the concentration effect that can occur on a thin layer plate when using mixed solvents. Nevertheless, despite the complex nature of the surface that is considered, the treatment is sufficiently representative to disclose that a concentration effect does, indeed, take place. The concentration effect arises from the frontal analysis of the mobile phase which not only provides unique and complex modes of solute interaction and, thus, enhanced selectivity, but also causes the solutes to be concentrated as they pass along the TLC plate. This concentration process will oppose the dilution that results from band dispersion and thus, provides greater sensitivity to the spots close to the solvent front. This concealed concentration process, often not recognized, is another property of TLC development that helps make it so practical and generally useful and often provides unexpected sensitivities. 

When butadiene is produced in olefins plants or in refinery crackers, they come mixed with relatively large volumes of the other C4 family. Sometimes the other C4S need not be separated from each other, for example if they are going to be used for allcylation plant feed. In that case, the butadiene can be separated from the other C4S by extractive distillation. This process uses a solvent that will preferentially dissolve butadiene, ignoring the other components in the stream. 

Liquid sulfur dioxide-benzene process a mixed-solvent process for treating lubricating-oil stocks to improve viscosity indexes also used for dewaxing. 

The process using single solvent addition (flowchart shown in Figure 3.8) involves the separation of mixed plastics using two primary steps [Lynch and Nauman, 1989] ... 

Benzene, toluene, and a mixed xylene stream are subsequently recovered by extractive distillation using a solvent. Recovery ofA-xylene from a mixed xylene stream requires a further process step of either crystallization and filtration or adsorption on molecular sieves. o-Xylene can be recovered from the raffinate by fractionation. In A" xylene production it is common to isomerize the / -xylene in order to maximize the production of A xylene and o-xylene. Additional benzene is commonly produced by the hydrodealkylation of toluene to benzene to balance supply and demand. Less common is the hydrodealkylation of xylenes to produce benzene and the disproportionation of toluene to produce xylenes and benzene. 

Phillips Petroleum Company developed an efficient slurry process used for the production of both HDPE and LLDPE (Eig. 6). The reactor is built as a large folder loop containing long mns of pipe from 0.5 to 1 m ia diameter coimected by short horizontal stretches of pipe. The reactor is filled with a light solvent (usually isobutane) which circulates through the loop at high speed. A mixed stream containing ethylene and comonomers (1-butene,... 

Extractive distillation, using similar solvents to those used in extraction, may be employed to recover aromatics from reformates which have been prefractionated to a narrow boiling range. Extractive distillation is also used to recover a mixed ben2ene—toluene stream from which high quaUty benzene can be produced by postfractionation in this case, the toluene product is less pure, but is stiU acceptable as a feedstock for dealkylation or gasoline blending. Extractive distillation processes for aromatics recovery include those Hsted in Table 4. 

Solvent separation, using the propane deasphalting process, is another procedure by which asphalts of the straight reduced type may be manufactured. This is a physical separation process used to recover high viscosity lube fractions from a given vacuum residuum. When mixed with the residuum, the solvent preferentially dissolves the oil and precipitates the asphalt. 

In some cases, the solids themselves are subjected to extraction by a solvent. For example, in one process used to decaffeinate coffee, the coffee beans are mixed with activated charcoal and a high-pressure stream of supercritical carbon dioxide (carbon dioxide at high pressure and above its critical temperature) is passed over them at approximately 90°C. A supercritical solvent is a highly mobile fluid with a very low viscosity. The carbon dioxide removes the soluble caffeine preferentially without extracting the flavoring agents and evaporates without leaving a harmful residue. 

Researchers studying polypeptide and polypeptide hybrid systems have also processed vesicles using two solvents. This method usually involves a common organic solvent that solubilizes both blocks and an aqueous solvent that solublizes only the hydrophilic block. The two solvents can be mixed with the polypeptide or polypeptide hybrid system at the same time or added sequentially. The choice of organic solvent depends heavily upon the properties of the polypeptide material, and commonly used solvents include dimethylformamide (DMF) [46, 59], methanol (MeOH) [49], dimethyl sulfoxide (DMSO) [50, 72], and tetrahydrofuran (THF) [44, 55]. Vesicles are usually formed when the organic solvent is slowly replaced with an aqueous solution via dialysis or removed through evaporation however, some vesicles have been reported to be present in the organic/aqueous mixture [49]. 

In order to obtain good mixing of ethene with the catalyst, the original Ziegler-Natta processes used hexane as a solvent. Although the solvent is almost completely recovered, the use of a hazardous material such as hexane detracts from the greenness of the process. Since the catalyst is highly moisture sensitive it needs to be deactivated at the end of the process by addition of water or alcohol, and this produces a small waste... 

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