SOLVENT RECOVERY, CONDENSATION

If regulations governing specific emission limit VOC concentrations to the low ppm range then, of course, vapor fractions such as those illustrated by the above tabulation will not be acceptable. It may, however, still be justified to consider VOC condensation as a precursor to a final abatement device such as an adsorption bed. Removing most of the solvent from a vent stream by condensation, can drastically reduce the size and cost of a downstream cleanup system. 

If solvent recovery is maximized by minimizing the temperature approach, the overall heat-transfer coefficient in the condenser will be reduced. This is due to the fact that a large fraction of the heat transfer area is now utilized for cooling a gas rather than condensing a Hquid. Depending on the desired temperature approach the overall heat-transfer coefficients in vent condensers usually range between 85 and 170 W/m K (ca 15 and 30 Btu/h-ft. °F). 

Perry s Chemical Engineers Handbook, 6th ed., McGraw-HiH, New York, 1984, pp. 5—59. 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

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In this system, solvent from a pressure-equalized reservoir (500 ml capacity) is introduced, under controlled flow, into a concentration chamber (Figure 10.3) [3], Glass indentations regulate the boiling of solvent so that bumping does not occur. This reservoir is surrounded by a heater. The solvent reservoir inlet is situated under the level of the heater, just above the final concentration chamber. This chamber is calibrated to 1.0 and 0.5 ml volumes. A distillation column is connected to the concentration chamber. Located near the top of the column are four rows of glass indentations which serve to increase the surface area. Attached to the top of the column is a solvent-recovery condenser with an outlet to collect, and hence recover the solvent. 

The clean moist soil was prepared for experiments by placing it in the 500 ml round bottom flask and spiking it with a solution of tetrachloroethylene to a concentration of either 9.28 or 957.3 ppm. The flask was sealed and rotated in an ice bath for 3 hr to homogenize the soil and the spike by tumbling. At the end of this tumbling period, the flask was attached to a water-cooled solvent-recovery condenser. The side leg of the condenser was placed in a chilled receiver containing approximately 10 ml of pesticide grade hexane, A Tenax trap was placed on the gas outlet port of the condenser to trap uncondensed vapors. 

The overhead temperatures of both the absorber and stripper are kept as low as possible to minimise solvent carryover. A temperature of about 311 K is typically used ia the high pressure absorber. The overhead temperature ia the stripper is set by the boiling poiat of the saturated complex solution and by the operating pressure of the stripper. At a stripping pressure of 0.166 MPa (1.7 atm), a temperature of 378 Kis used. The solvent-rich gas from the stripper is cooled to recover as much solvent as possible by condensation prior to the final aromatics-recovery section. Fiaal solvent recovery is accomphshed by adsorption on activated carbon (95). 

The vacuum plate drwer is provided as pari of a closed system. The vacuum dryer has a cylindrical housing and is rated for fiill-vacuum operation (typical pressure range 3-27 kPa absolute). The exhaust vapor is evacuated try a vacuum pump and is passed through a condenser for solvent recovery. There is no purge-gas system required for operation under vacuum. Of special note in the vacuum-drying system... 

Another example in the polymers industry is illustrated in Figure 17, which is a process aimed at the batch drying of waste residue with solvent recovery. In this application liquid or viscous waste solutions are pumped into a batch dryer where they are dried under vacuum to a solid granular residue. Vaporized water and solvent are recovered by condensation and then separated by gravity. The process scheme is flexible, offering a range of temperatures and vacuum levels for treating... 

Solvent Recovery We now focus attention on the operation of recovery. Volatile solvents vaporized during a manufacturing process may be recovered and used again. From the mixture of air and vapor, which is generally the form in which the solvent must be sought, the latter may be condensed to a liquid and trapped by the application of cold and moderate pressure the vapor-laden air may be passed... 

Of these, the most commonly used for air pollution control are the fixed-bed and canister units. Fixed beds are also used in solvent-recovery applications. Major process steps include adsorption, regeneration, and further treatment of the desorbed organic compounds. Typically, further treatment includes condensation and separation. 

The condensed reaction mixture is evaporated in film evaporator 16 under vacuum. The crude l-(2,6-dimethyl)-phenoxy-2-aminopropane hydrochloride is precipitated in tank 17 using HCl dissolved in organic solvent //, separated in centrifuge 18, and dried in tray drier 19. The final purification by crystallization from solvent III occurs in crystallizer M. Pure l-(2,6-dimethyI)-phenoxy-2-amino-propane hydrochloride is separated in centrifuge 21 and dried in tray drier 22. The plant is equipped with typical solvent recovery and storage facilities not shown in the figures. 

A flow diagram of an anhydrous phenol solvent extraction plant is shown in Figure 8. Raw distillate is passed through a tower in which it absorbs phenol from the recovery system vapor. The oil is then passed to the treating tower, generally a few sections above the bottom. Anhydrous phenol is introduced at the top of this tower. Phenolic water condensate from the solvent recovery system (about 9.5% phenol) is introduced at the bottom of the tower to effect reflux. A temperature gradient of 10° to 75° F. may be... 

The crude C4 mixture is charged to a 70 tray extractive distillation column T-l that employs acetonitrile as solvent. Trays are numbered from the bottom. Feed enters on tray 20, solvent enters on tray 60, and reflux is returned to the top tray. Net overhead product goes beyond the battery limits. Butadiene dissolved in acetonitrile leaves at the bottom. This stream is pumped to a 25-tray solvent recovery column T-2 which it enters on tray 20. Butadiene is recovered overhead as liquid and proceeds to the BDS reactor. Acetonitrile is the bottom product which is cooled to 100°F and returned to T-l. Both columns have the usual condensing and reboiling provisions. 

The extract is pumped from the bottom of D-l to a stripper D-2 with 35 trays. The stripped solvent is cooled with water and returned to D-l. An isoprene-acetonitrile azeotrope goes overhead, condenses, and is partly returned as top tray reflux. The net overhead proceeds to an extract wash column D-3 with 20 trays where the solvent is recovered by countercurrent washing with water. The overhead from D-3 is the finished product isoprene. The bottoms is combined with the bottoms from the raffinate wash column D-4 (20 trays) and sent to the solvent recovery column D-5 with 15 trays. 

The overhead from the second stage is heated by an exchange with hot solvent. The fired heater further raises the temperature of the solvent/demetallized oil mixture to a point above the critical temperature of the solvent. This causes the demetallized oil to separate. It is then flashed and steam-stripped to remove all traces of solvent. The vapor streams from the demetallized oil and asphalt strippers are condensed, dewatered, and pumped up to process pressure for recycle. The bulk of the solvent goes overhead in the supercritical separator. This hot solvent stream is then effectively used for process heat exchange. The subcritical solvent recovery techniques, including multiple effect systems, allow much less heat recovery. Most of the low grade heat in the solvent vapors from the subcritical flash vaporization must be released to the atmosphere requiring additional heat input to the process. 

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