Separation of solvents from residues

Solvents for recovery are frequently contaminated with solutes that have a negligible vapour pressure and are waste materials for disposal, possibly even for land-fill if they are solid or highly viscous and have a high flash point after treatment. These solvents can arise in different ways. 

The recovery of solvents from such mixtures poses four problems. 

There have been many accidents diuing solvent recovery operations due to the triggering of exothermal reactions causing structural damage to the recovery equipment. 

Commercially available laboratory equipment can be used to test crude solvent under the most severe conditions possible on a given plant. Both temperature and exposiue time may combine to lead to an exotherm. If in the laboratory an exotherm is found the temperatiue should be reduced until no 

Many chemical reactions have activation energies in the range 20-30 kcal/mol. This means that, in the temperature band 100-180 C the rate of reaction doubles for each 10 C increase in temperature. A 20 °C margin thus gives a safety factor of about 400%. 

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Achieving the required separation of solvent from residue at the safe operating temperature is likely to involve the use of reduced pressure, particularly towards the end of a batch when the mole fraction of volatile solvent becomes low and that of the involatile residue becomes high. Because this situation is present all the time in a continuous operation, it is likely to be under vacuum. This presents no insuperable problem for handling solvents with high boiling points since it is still possible to condense their vapours with cooling water or ambient air with an adequate temperature diiference in the condenser. 

Recovery techniques exist for the separation of solvents from solids, residues, liquids, including water, and from gases. These are achieved by such technologies as decanting, distillation, liquid-liquid extraction, condensation, adsorption and absorption. In many instances several processes are used in series to achieve the most efficient recovery, or desired quality, from the recovered solvent. A summary of the principal technologies used in solvent recovery is discussed below. 

Recovery of solvent is possible by batch distillation of the mixture of liquids and solids yielding solvent of very good purity. Batch systems are preferred to continuous processes as the former can limit the quantity of solids to be removed and are therefore easier to handle. The simplest form of distillation consists of the heated vessel, condenser and one or more collection tanks. Clearly there is only one theoretical plate in this system and this is only suitable for the recovery of a single solvent or for initial separation of a mixture of solvents from residues. Further treatment would be required to separate the individual components of a mixture. 

Separation of catalysts from high-value products such as fine chemicals or pharmaceuticals is often accomplished by precipitating the catalyst from the product solution. Recycling of these catalysts is feasible, provided that they do not decompose. In industry, catalyst recovery by means of catalyst precipitation is applied only in relatively small batch processes. An example of such a process is the production of (—)-menthol (id) in which an Rh-BINAP isomerization catalyst converts the allylic amine substrate into (R)-citronellal (after hydrolysis of the enamine) in high yield (99%) and with high enantioselectivity (98.5% ee). After distillation of the solvent (THF) and product, the catalyst is recovered from the residue by precipitation with -heptane. 

Supercritical extraction has been used increasingly in recent years for specialized processes. These processes include separation of drugs from plants, oils from vegetable seeds, impurities from labile materials, and chemical feedstocks from coal and petroleum residual. The utility of supercritical extraction processes stems principally from the enhanced solubility characteristics of CO2 near its critical point and the ease with which the solvent can be recovered for recycle. 

The one-phase liquid system is more frequently encountered since many organic reactions are carried out in solution. Direct fractional distillation may separate the product, if it is a liquid, from the solvent and other liquid reagents, or concentration or cooling may lead to direct crystallisation of the product if this is a solid. However, it is often more appropriate, whether the required product is a liquid or solid, to subject the solution to the acid/base extraction procedure outlined above and considered in detail on p. 162. This acid/base extraction procedure can be done directly if the product is in solution in a water-immiscible solvent. A knowledge of the acid-base nature of the product and of its water solubility is necessary to ensure that the appropriate fraction is retained for product recovery. In those cases where the reaction solvent is water miscible (e.g. methanol, ethanol, dimethylsulphoxide, etc.) it is necessary to remove all or most of the solvent by distillation and to dissolve the residue in an excess of a water-immiscible solvent before commencing the extraction procedure. The removal of solvent from fractions obtained by these extraction procedures is these days readily effected by the use of a rotary evaporator (p. 185) and this obviates the tedium of removal of large volumes of solvent by conventional distillation. 

The use of brine as a solvent in the hydrometallurgical separation of lead from its ores was extensively studied by Lyon and Ralston (H8). Saturated sodium chloride solution and neutral ammonium acetate solutions were found to be good solvents for lead chloride and lead sulfate. Lead oxide and lead carbonate became soluble if the brine was first acidified with either sulfuric or hydrochloric acid. The dissolved lead was recovered electrolytically (S18). Marsden (M13) used this method in a process to recover lead from zinc plant residues. About 80% recovery of the lead was obtained by leaching at ambient temperature and the recovery of lead was increased to 98% when hydrochloric acid was added to the brine. 

In a first step of the process propane, butane, pentane, or hexane is used to extract vanidyl porphyrins from residual oil. The liquid extract of hydrocarbon and porphyrins is next subjected to supercritical CO2 exU action between 90and 130 °Fand 1,075 to 8,000 psi. At almost all combinations of these pressures and temperatures the C3 through C(, paraffins are miscible in CO2. With a supercritical extraction column we wonder what ratio of C02-lo-extract would result in purified oil and what happens to the vanidyl porphyrins. In the process diagram and description, the supercritical C02-liquid solvent extract is expanded to an unspecified different pressure and temperature. But, it is important to relate that pentane and hexane are miscible with CO2 at room temperature and 800 psi so that the expansion would have to be to conditions higher in temperature and lower in pressure to separate CO2 and hexane. For propane the miscibility conditions extend to much lower pressure and so the separation of propane from carbon dioxide becomes problematic. 

Separation of lithium from other alkali metals in order to separate lithium from the other alkali metals, they are all converted into the chlorides (by evaporation with concentrated hydrochloric acid, if necessary), evaporated to dryness, and the residue extracted with absolute alcohol which dissolves the lithium chloride only. Better solvents are dry dioxan (diethylene dioxide, C4Hg02) and dry acetone. Upon evaporation of the extract, the residue of lithium chloride is (a) subjected to the flame test, and (b) precipitated as the phosphate after dissolution in water and adding sodium hydroxide solution. 

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