Recover and Reuse Solvents

The main distillation types include atmospheric, vacuum, steam, azeotropic, extractive, and pressure distillation [45]. AU of these distillation methods can be carried out in a batch or continuous marmer with the exception of extractive distillation, which is solely continuous by nature. Gomplex solvent systems often require the use of multiple distillation columns in series to purify certain solvents that are not easily separated. The energy consumption in distillation columns can therefore be quite large because of the continuous operation of condensers and reboilers over extended periods of time. In order to cut down on these costs, both vacuum and steam distillation can be employed ]45]. 

Some solvent mixtures can be very difficult and energy intensive to separate because of the closeness of boiling points and the formation of azeotropic mixtures [45]. Azeotropic or extractive distiUation can be used for azeotropic solvent mixtures and solvents which have very low relative volatihties ]43, 45]. Azeotropic and extractive distillation involves the addition of another solvent, known as an entrainer, which will form its own azeotrope with one of the components to be separated ]45]. However, the additional solvent required for azeotropic and extractive distillation can also generate more wastes depending on how easily the entrainer itself can be recycled and reused. 

There are several companies today which specialize in the manufacture of different distillation systems to recover solvents for further reuse within a pharmaceutical plant ]46, 47]. According to ProsGon , the major cost benefits that their customers realize when using their solvent recovery systems are reductions in solvent purchases, waste disposal costs, and transportation costs [46]. 

Transport through the membrane can be considered to occur by a solution-diffusion mechanism under the influence of a chemical potential driving force [48, 49]. The primary benefit of using PV systems is that they are essentially independent of the vapor-liquid equihbrium of solvent mixtures. Therefore PV can be used to overcome the separation barriers created by many azeotropic mixtures [48, 50]. 

Solvent mixtures that contain heat-sensitive compounds, are viscous, or have high boihng points can be separated using wiped-film evaporators (WFE) [57, 58]. A WFE operates by receiving a liquid feed into a column that contains several wiper blades. The walls of the WFE are heated at a constant temperature in order to vaporize the solvent film. As the solvent vapors migrate to the center of the WFE, they come into contact with a coohng unit that condenses the vapors, allowing them to flow down the condenser to the outlet receiver. The thin solvent film and reduced system pressure (in the case of vacuum operation) allows the solvents to be separated slowly at lower temperatures [57-59]. 

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