Minimizing Solvent Losses

Solvent Extraction. Extraction processes, used for separating one substance from another, are commonly employed in the pharmaceutical and food processing industries. Oilseed extraction is the most widely used extraction process on the basis of tons processed. Extraction-grade hexane is the solvent used to extract soybeans, cottonseed, com, peanuts, and other oilseeds to produce edible oils and meal used for animal feed supplements. Tight specifications require a narrow distillation range to minimize solvent losses as well as an extremely low benzene content. The specification also has a composition requirement, which is very unusual for a hydrocarbon, where the different components of the solvent must be present within certain ranges (see Exthaction). 

These characteristics combine to yield a system that has low heat and pumping requirements, is relatively noncorrosive, and suffers only minimal solvent losses (less than 1 Ib/MMscf). 

An exothermic reaction with the stoichiometry A 2B takes place in organic solution. It is to be carried out in a cascade of two CSTR s in series. In order to equalize the heat load on each of the reactors it will be necessary to operate them at different temperatures. The reaction rates in each reactor will be the same, however. In order to minimize solvent losses by evaporation it will be necessary to operate the second reactor at 120 °C where the reaction rate constant is equal to 1.5 m3/kmole-ksec. If the effluent from the second reactor corresponds to 90% conversion and if the molal feed rate to the cascade is equal to 28 moles/ksec when the feed concentration is equal to 1.0 kmole/m3, how large must the reactors be If the activation energy for the reaction is 84 kJ/mole, at what temperature should the first reactor be operated ... 

The effluent thus obtained is introduced into the extractive distiliatioii column operai ing between 80 and 130°C at 0.015.10 Pa absohite, in which the solvent enters at the top in a countercurrent stream. The mixture of dimethylacetaxnlde and styrene, removed at the bottom, is separated by simple reboiling at about 120 C under partial vacuum. The soh em drawn off at the bottom is rec>cled after cooling. A fraction of this solvent must be purified in an evaporator to remove the polymers and heavy products formed, and to minimize solvent losses. The total yield of the operation is as high as 87 per cent weight. 

While efficiency is a factor in equipment selection, mechanical considerations often provide the determining criteria. One must always be sure to minimize solvent losses, and concern about entrainment and emulsion formation can dictate the mode of operation if not the choioe of contactor. For example, power input for mixing may be titrated or the less viscous phase chosen to be coiuinuons to ensure good phase disengagement. 

Solvent is recovered in the second distillation column, which is a vacuum steam-distillation column, with reflux of the organic phase to get high-purity aromatics. The final column is a multistage extraction column, where water is used to wash the solvent from the raffinate. There is a closed cycle for water to minimize solvent loss, and a small amount of water can be tolerated in the extraction solvent. 

The vapor overheads of both the absorber and stripper are condensed with cooling water, generating the respective butanes and butenes products. Each column has a small reflux flow that washes the overhead product to minimize solvent losses. 

Pollution prevention tactics employed range from limiting/eliminating solvent use to seeking engineering solutions to minimize solvent loss/release. Table X presents some representative approaches to evaluating present solvent usage. 

The basic principles are the same for the preparation of both the cellulose acetate dope and the cellulose triacetate dope with the exception of the particular solvent mixture used for each. The flake, the solvent mixture, and a filtering aid are added to a heavy-duty mixer. The solution is prepared in a fully enclosed system to minimize solvent losses and also to meet strict exposure levels regarding the workers. In the case of cellulose acetate, the main solvent is acetone, which is highly flammable. Therefore, the vapor-air ratio must be maintained at a level that meets safety regulations. Strict fire codes are maintained in the dope-preparation department as well as in the fiber-spinning department. 

Some samples may contain volatile components that evaporate during a run, changing the composition of the sample. In this case, measurements have to be carried out at low enough temperatures or the sensor tool has to be covered to minimize solvent loss. When using concentric cylinders (which are more convenient than cone-and-plate and parallel plates to reduce solvent loss), a thin film of a nonvolatile liquid may be poured on top of the sample. 

Experimental transdermal patches have also been fabricated by flanking a solution or gel containing the test drug and other excipients between an impermeable backing laminate and a rate-controlling membrane. A pressure-sensitive adhesive coated on the membrane ensures an intimate patch-skin cmitact. The patch is kept in a sealed aluminum pouch to minimize solvent loss [31, 62, 64]. 

ILs are low-melting salts that are attractive for a number of applications as they are relatively nonvolatile, nonflammable, environmentally benign, and exceptionally thermally stable [18], In addition, there are numerous combinations of cations and anions that can be used to produce ILs, and thus chemical and physical properties of ILs can be tuned, which is needed to design an energy-efficient liquid absorbent for CO2 capture. The mechanism for CO2 capture in ILs is often based on physisorption and involves a weak association between the IL and CO2 molecules [19], Once the CO2 has been removed from the gas mixture, it can be released from the ILs (which would be reused) by either a decrease in pressure or an increase in temperature [18], While the viscosity of ILs minimizes solvent loss from the gas stream, this attribute also limits mass transfers, and they often suffer from low rates of absorption. To overcome these shortcomings and increase the capacity of simple ILs, amine-functionalized ILs have been developed, which allow higher rates of sorption to be achieved at pressures relevant to flue streams [ 19,20]. A number of reports have also demonstrated high CO2/N2 selectivity in polymerized ILs, which exhibit enhanced CO2 solubility relative to the monomeric ILs [21]. 

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