Handbook of Solvents - George Wypych - ChemTech - Ventech!

1440 Aydin K. Sunol and Sermin G. Sunol
Figure 21.1.13. Supercritical leaching and options for supercritical solvent regeneration.
centration and particle size distribution dependent. These processes are usually physical/mechanical
in nature. The feed preparation may be carried out in a unit designed
particularly for the purpose or the function may be tied to another function within a single
unit. For example, the fines, either nucleated through temperature and/or pressure perturbation,
or particles suspended in the supercritical fluid can be readily separated through cyclones.
Concentration (or isolation) tasks are usually necessary with streams where the product
concentration is very low (dilute). Adsorption and extraction are typical technologies
utilized here. Energy considerations dominate the technology selection subject to product
contamination and stability constraints. Thus, the base technology depends on the availability
of the Mass Separating Agent (e.g., adsorbent or solvent) and its efficient recycle. Supercritical
extraction of organic compounds from aqueous streams is a typical example of
isolation step.
The candidate technologies for purification are many. Distillation, the work-horse of
the chemical processes, leads the pack. Most of the synthesis effort to date has concentrated
on the product purification step. This step is often the last step for liquid products especially
in the chemical and petrochemical industries. The biochemical industry utilizes membrane
and chromatographic processes more than the other industries due to the thermal stability
and purity requirements. In the electronic industry, membrane processes are more prevalent
due the ultra-purities necessary. Supercritical fractionation of alcohol water systems with
the aid of a dense gas is an example of a purification step.
Crystallization, drying, and blending are typical operations necessary to polish (finish)
the product to its final form. Particle nucleation from a supercritical fluid solution is an
example of finishing. Each supercritical separation technique is identified on this generic
process.