Solvent Recovery by Steamless Temperature Swing Carbon Adsorption Processes

Abstract

Adsorption with activated carbon followed by regeneration with steam is the most common technique for solvent recovery from gas streams. The resulting steam−solvent gaseous mixture when condensed can result in either two separate layers (immiscible) or a single liquid phase (miscible or miscible with an azeotrope). For miscible systems that form azeotropes, the downstream distillation system can easily be more expensive than the adsorbers. An alternative is to use hot nitrogen instead of steam for desorption. By adding a short oxygen/water removal step to the adsorption cycle, the need for distillation is avoided. The model system studied for the azeotropic case was recovery of 0.5 mol % isopropanol (IPA) from air with hot nitrogen for regeneration. Since the nitrogen cost dominates, nitrogen should be recovered and recycled as much as possible. Two- and four-bed adsorption schemes were studied, and the four-bed system, which recovers almost all of the nitrogen, was best. A preliminary total cost comparison showed that solvent recovery of IPA with steam regeneration has total costs that are ∼14% higher than for IPA recovery with hot nitrogen regeneration if the nitrogen is recovered and recycled.