Solvent recovery is a form of waste reduction. In–process solvent recovery still is widely used as an alternative to solvent replacement to reduce waste generation. It is attractive, like end–of–pipe pollution control, since it requires little change in existing processes. There is widespread commercial availability of solvent recovery equipment which is another attraction. Availability of equipment suitable for small operations, especially batch operations, make in–process recovery of solvents economically preferable to raw materials substitution.
Commercially available solvent recovery equipment include:
Carbon adsorption of solvent, removal of the solvent by steam, and separation of the solvent for reuse in the operation. Carbon must be regenerated, two or more units are required to keep the operations continuous. Chloric acid formation from chlorinated solvents, carbon bed plugging by particulates, and buildup of certain volatile organics on the carbon and corrosion can be a problem.
Distillation and condensation can be used to separate and recover solvent from other liquids. Removal efficiency can be very high using this process and can be used for solvent mixtures as well as single solvents.
Dissolving the solvent in another material such as scrubbing. Solvents must be then recovered from the resulting solution, through distillation but efficiency of removal is often not high using this method.
Adsorption processes are useful and versatile tools when it comes to waste solvent recovery unit as they can be applied with high efficiency at relatively low cost in cases in which the desired component presents either a fairly small or a fairly high proportion of the stream. The applicable adsorbents vary according to different purposes.108,109 Adsorbents with low polarity (activated carbon, etc.) tend to adsorb nonpolar compounds, whereas ones with high polarity (e.g., silica, alumina) have higher affinity to adsorb polar substances. However, some adsorbents operate via specific binding sites (e.g., molecular sieves, molecularly imprinted polymers) rather than simple hydrophilic-hydrophobic interactions. It is worth mentioning that adsorption cannot easily be installed in a continuous configuration and is usually either a one-bed batch process or a twin-bed process with one bed in the adsorption, whereas the other one in the regeneration phase.
In organic solvent recycling, the most frequent issue is the removal of water content. Even traces of water can cause unexpected solubility problems, side reactions, or the decomposition of a reactant. There are various processes to recover wet solvents such as distillation methods or fractional freezing, whereas adsorptive methods are advantageous due to their low energy consumption. Molecular sieves (with pore size 3 or 4 Å), silica, and alumina are widely used for solvent drying.110,111 The polarity of the solvent affects the efficiency of water removal, which decreases with increasing polarity of the solvent. With the proper choice of adsorption technique, residual water content between 1 and 100 ppm is usually a realistic target.