Recovery of Solvent

Fig. 23(D) shows a simple distillation apparatus with an adaptor fitted to the lower end of the condenser. This apparatus can also be used for the recovery of solvents, or for the concentration of a solution with collection of the distilled solvent.

The solvent used to form the dope is evaporated during the extrusion process and must be recovered. This is usually done by adsorption on activated carbon or condensation by refrigeration. For final purification, the solvent is distilled. Approximately 3 kg of acetone, over 99%, is recovered per kg of acetate yam produced. Recovery of solvent from triacetate extmsion is similar, but ca 4 kg of methylene chloride solvent is needed per kg of triacetate yam extmded. 

The filter cake can then be washed either by displacement or by reslurrying. Reslurrying is easily accompHshed using the stirring action of the rotor blades when the rotor is lowered into the cake. The cake may also be dried in situ by the passage of hot air through it, or may be steam distilled for the recovery of solvent. 

Enclosed agitated filters are useful when volatile solvents are in use or when the solvent gives off toxic vapor or fume. Another significant advantage is that their operation does not require any manual labor. Control can be manual or automatic, usually by timers or by specific measurements of the product. Most filters are made of mild steel, with the exposed surfaces protected by lead, tile, mbber lining, or by coating or spraying with other substances as necessary. Filtration areas up to 10 m are available and the maximum cake thickness is 1 m. Apphcations are mainly in the chemical industry for the recovery of solvents. 

Recovery of solvents or reagents, extraction and neutralization of mother Hquors, precipitation and separation of salts, and sembbing of gaseous effluents. 

The scale and complexity of an adsorption unit varies from a laboratory chromatographic column a few millimeters in diameter, as used for analysis, to a fluidised bed several metres in diameter, used for the recovery of solvent vapours, from a simple container in which an adsorbent and a liquid to be clarified are mixed, to a highly-automated moving-bed of solids in plug-flow. 

Allows recovery of solvent-laden rags, sludges, or recycling of still bottoms for reuse. 

The dewaxed oil and wax phases discharged from the centrifuges are transferred through heat exchange equipment to individual solvent recovery units for continuous recovery of solvent. 

Installations for the recovery of solvent usually consist of main and ancillary sections. The main installation comprises equipment for the recovery of the solvent which escapes from the powder on predrying. The ancillary installation recovers solvent from various production departments. Solvent recovery ducting is installed in rooms containing high solvent concentration. This improves the working conditions by removing ether-alcohol vapours from the atmosphere. 

The main installation may be used for the recovery of solvent either by condensation or by absorption. The ancillary installation usually deals with considerably dilute solvent vapours so that the best results are attained by absorption. 

The recovery of solvent from the water after soaking the powder and from the vapours evolved on soaking is a separate problem and will be dealt in a section on p. 620. 

In choosing a method for the recovery of solvent operational safety is a primary factor to be taken into account. While the machinery is in operation the greatest danger is associated with the mixture of air and alcohol-ether vapour which flows through the pipelines and appears in various parts of the plant. 

Recovery of solvent by the condensation of vapours. This is one of the oldest methods which once was widely used in many countries (France, U.S.A.) and well checked in practice. 

Recovery of solvent by isothermal compression. This method was proposed by Claude [14]. It was applied to the recovery of alcohol containing camphor which escapes during the manufacture of celluloid. With alcohol and ether this process entails compressing the vapours to 7 atm, thus causing the condensation of the alcohol and after that rapidly expanding them. Ether is condensed by intensive cooling. The necessary plant was very expensive and there was risk of explosion when the mixture of the air with alcohol and ether was compressed too rapidly. It never attained wide application. 

Absorption of solvent with sulphuric acid. This is another of the oldest methods for the recovery of solvent. It was first used for the recovery of alcohol and ether in the manufacture of artificial silk by the old Chardonnet process and was then widely applied in the manufacture of powder in Germany and Austria before and during World War I. The air containing alcohol and ether entered the tanks filled with sulphuric acid. The tanks were cooled from outside by spraying with water. 

Fig. 236. View of a drying room with an installation for the recovery of solvent at reduced pressure.

An installation for the recovery of solvent (chiefly alcohol) by condensation is connected with a drier operating at a reduced pressure. The installation works at atmospheric pressure. It consists of two condensers cooled with cold water in which ether and alcohol (containing some water and ether) are condensed. From 10 to 25 kg of 95% alcohol is recovered per 100 kg of powder according to the season (more in winter). 

The recovery of solvent from the water after soaking... 

The soaking of nitrocellulose powder involves recovery of the solvent (chiefly alcohol) which passes into the water. It is worth while recovering this solvent if the content of alcohol in the soaking water is not less than 5%. By applying the battery counter-current soaking system the alcohol content in the water may be increased to 15-20% and the recovery of solvent thus made more profitable. 

Beds usually are vertical adsorbers 45 ft high and 8-10 ft dia are in use. When pressure drop must be minimized, as in the recovery of solvents from atmospheric air, horizontal vessels with shallow beds are in common use. Process gas flow most often is downward and regenerant gas flow is upward to take advantage of counterflow effects. Upflow rates are at most about one-half the fluidizing velocity of the particles. Vertical and horizontal types are represented on Figure 15.17. 

Economic recovery of solvent from the water products can be readily accomplished by steam stripping. Small remnant concentrations (1 to 2 p.p.m.) may be removed by bacterial decomposition, either on storage or by use of trickle filters. 

Arafat, H. A., M. C. Hash, A. S. Hebden, and R. A. Leonard. 2001. Characterization and Recovery of Solvent Entrained During the Use of Centrifugal Contactors. Argonne National Laboratory Report ANL-02/08, Argonne, IL. 

One of the major uses of activated carbon is in the recovery of solvents from industrial process effluents. Dry cleaning, paints, adhesives, polymer manufacturing, and printing are some examples. Since, as a result of the highly volatile character of many solvents, they cannot be emitted directly into the atmosphere. Typical solvents recovered by active carbon are acetone, benzene, ethanol, ethyl ether, pentane, methylene chloride, tetrahydrofuran, toluene, xylene, chlorinated hydrocarbons, and other aromatic compounds [78], Besides, automotive emissions make a large contribution to urban and global air pollution. Some VOCs and other air contaminants are emitted by automobiles through the exhaust system and also by the fuel system, and activated carbons are used to control these emissions [77,78],... 

Tran and Mujtaba (1997) and Mujtaba et al. (1997) highlighted the operating features and limitations of BED processes for close boiling and azeotropic mixtures. However, the works were limited to the separation of only one key component in the distillate without due regard to the recovery of solvent or the separation of other components in the feed mixture. 

Mujtaba (1999) considered the conventional configuration of BED processes for the separation of binary close boiling and azeotropic mixtures. Dynamic optimisation technique was used for quantitative assessment of the effectiveness of BED processes. Two distinct solvent feeding modes were considered and their implications on the optimisation problem formulation, solution and on the performance of BED processes were discussed. A general Multiperiod Dynamic Optimisation (MDO) problem formulation was presented to obtain optimal separation of all the components in the feed mixture and the recovery of solvent while maximising the overall profitability of the operation. 

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