(709a) Solvent Recovery by Steam-Less Adsorption Processes

Solvents are commonly recovered by adsorption based techniques like thermal swing adsorption (TSA) and vacuum swing adsorption (VSA), with activated carbon commonly used as the adsorbent. Steam is commonly used for regeneration for TSA systems because the adsorbent bed can be heated rapidly and the steam displaces air minimizing explosion hazards. However, if the solvent is water-miscible, then the effluent during steam regeneration is a solvent-water mixture that requires a downstream distillation unit to recover the solvent. The downstream distillation is particularly expensive if the solvent-water mixture (e.g., water and isopropanol) form an azeotrope.

In this study, alternative processes are developed which use novel PSA-VSA-TSA cycles without the addition of steam while meeting environmental standards for the effluent gas purity and recovering solvent for reuse. If the feed is oxygen-free (e.g., solvent and nitrogen) there is no safety limit on the amount of solvent vapor present in the feed. For this scenario, a TSA process using a combination of hot feed and nitrogen for regeneration was developed. The solvent-rich effluent gas is cooled to condense out some of the solvent and the remaining gas saturated with solvent is recycled. If the feed is an air-solvent mixture, the solvent concentration must be kept at or below ¼ of the lower explosion limit. Hence, the previous TSA process is modified to first remove oxygen prior to thermal regeneration. The effluent during thermal regeneration is again cooled (with condensation of solvent) and recycled. Because steam is not added in either process, the expensive distillation steps are eliminated.

A VSA process for air-solvent mixtures is also developed that regenerates the bed at low pressures with small amount of nitrogen purge. The resulting effluent stream during vacuum regeneration is small and highly concentrated. Further, if the feed is a dilute solvent-nitrogen mixture, PSA followed by TSA can be used. PSA first produces pure nitrogen and a concentrated mixture (without any safety hazard since oxygen is absent) and the concentrated mixture is then fed to a TSA system. The TSA operation leads to further concentration. A condenser now condenses most of the solvent and the remaining saturated solvent-nitrogen mixture is recycled to the PSA feed.

Simulations using the commercial ADSIM simulator are presented for the recovery of isopropanol in nitrogen and for the recovery of dilute isopropanol in air.