Solvent-switching, batch distillation

As mentioned earlier, a major cause of high costs in fine chemicals manufacturing is the complexity of the processes. Hence, the key to more economical processes is reduction of the number of unit operations by judicious process integration. This pertains to the successful integration of, for example, chemical and biocatalytic steps, or of reaction steps with (catalyst) separations. A recurring problem in the batch-wise production of fine chemicals is the (perceived) necessity for solvent switches from one reaction step to another or from the reaction to the product separation. Process simplification, e.g. by integration of reaction and separation steps into a single unit operation, will provide obvious economic and environmental benefits. Examples include catalytic distillation, and the use of (catalytic) membranes to facilitate separation of products from catalysts. 

Manipulation of the operating conditions such as reflux ratio or pressure during a batch distillation can be useful. In addition, the feed to the batch distillation may vary during the process. A common application is to replace one solvent with another in the presence of a heavy nonvolatile product, as may be encountered in pharmaceutical production. One option for switching solvents is to use simple distillation repeatedly Initially a portion of the first solvent is removed by boiling. Then the second solvent is added, and a simple distillation removes more of the first solvent along with some of the second. Repetition of the latter step can be used to reduce the concentration of the first solvent to very small levels. 

D17. A nonvolatile solute is dissolved in 1.0 kmol of methanol. We wish to switch the solvent to water. Because the solution is already concentrated, a first batch distillation to concentrate the solution is not required. We desire to have the solute in 1.0 kmol of solution that is 99.0 mol% water and 1.0 mol% methanol. This can be done either with a constant-level batch distillation or by diluting the mixture with water and then doing a sinple batch distillation. VLE data (ignore the effect of the solute) are in Table 2-7. Do a constant-level batch distillation fromxjyf jj i = 1.0 (pure methanol) to Xj = 0.01. Find the moles of water added during the constant-level batch... 

The distillation of the solvent and catalytic regrouping are conducted in reactor 12. For this purpose, from collector 11 the product of hydrolytic cocondensation is sent by nitrogen flow into reactor 12, the agitator is switched on and the reactor is heated to 36-40 °C by sending vapour into the jacket. At this temperature most of the solvent is distilled within 3-4 hours. Cooler 16 is switched into the direct operation mode. After cooling, methylenechloride vapours are collected into collector 17, poured into batch box 5 and re-ised in the process of hydrolytic cocondensation. 

From the top part of separator settling box 5 silanol continuously enters one of two condensator tanks 6 (the diagram shows one), switched as they fill. These enameled apparatuses with gate agitators and water vapour jackets are used for the distillation of the solvent and partial condensation of the product. If it is necessary to obtain modified varnishes, it is possible to introduce various organic additives into apparatus 6 from batch box 7 at this stage. As one of condenser tanks 6 accumulates, silanol is clarified before the distillation of the solvent, i.e. separated from traces of moisture by settling at 80-90 °C. 

After supplying silanol, the heating of apparatus 17 is switched on and the solvent is distilled, sending air through the bubble at the speed of 3 m3/h at 150 °C. The solvent vapours are condensed in cooler 11 and collected in receptacle 18. The distillation of the solvent occurs simultaneously with the condensation of silanol. During the distillation the temperature in apparatus 17 is increased to 170-200 °C, and the speed of air supply is raised to 30 m3/h. The mixture is periodically sampled to determine the viscosity and polymerisation time of the polymer. After the distillation air supply is stopped, the reactor receives a necessaiy amount of the solvent from batch box 10 and the mixture is cooled to 60 °C. The solution (var-... 

If the tests are positive, the product from collector 11 is filtered in nutsch filter 12 and sent into batch box 13. The distillation of the solvent and polycondensation of the product of hydrolytic condensation is carried out in reactor 16, a vertical cylindrical apparatus with a jacket, agitator and direct cooler 15. The reactor receives from batch box 13 toluene solution of the product of hydrolytic condensation the agitator is switched on the mixture is heated to 120-130 °C and the mixture of toluene and butyl alcohol is distilled into receptacle 17. After the distillation, the temperature in the reactor is increased within 1.5-2 hours to 250 °C at 250-260 °C and continuous agitation polycondensation is conducted. The distilled butyl alcohol, water and partially toluene are condensed in cooler 15 and collected in receptacle 17. After 3-4 hours of polycondensation the contents of the reactor are cooled to 60 °C and analysed. 

For example, an ABE plant was established at Germinston, South Africa in 1937 and ran successfully until 1983, first producing solvent from starch but switching to molasses. The fermentation and distillation recovery process operated in batch mode. The fermentation produced approximately 20gl of mixed solvents from 55 to 60 g 1 of substrate with solvent yields of about 0.35 g g sugar. The butanol acetone molar ratio is typically 2 1 [178]. 

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