Steam desorption

Basic arrangement of adsorptive solvent recovery with steam desorption.. . . 1515... 

Adsorption Adsorption of the solvents by passing the waste air through an adsorber filled with activated carbotL Solvent recovery by steam or hot gas desorption By steam desorption the resulting desorbate has to be separated in a water and an organic phase using a gravity separator (or a rectification step)... 

For solvent desorption direct steaming of the activated carbon bed is the most widely used regeneration technique because it is cheap and simple. Steam (110-130°C) is very effective in raising the bed temperature quickly and is easily condensed to recover the solvent as a liquid. A certain flow is also required to reduce the partial pressure of the adsorbate and carry the solvent out of the activated carbon bed. A flow diagram for steam desorption for toluene recovery is given in Figure 22.1.5. 

Figure 22.1.7. Options for solvent recovery by steam desorption (After references 2,3,12).

Steam desorption Increasing the bed temperature by direct steaming and desorption of solvent. The steam is condensed and solvent recovered. Most widely used regeneration technique for solvent recovery plants. Concentration range from 1-50 g/m ... 

Reduced pressure and low temperature steam desorption Steam desorption at low temperature and reduced pressure. Recovery of reactive solvents of the kehme-type. Minimizing of side-reactions like oxidizing, decomposing or polymerizing. 

The majority of solvents are effectively recovered by adsorption on activated carbon and, when this is the case, the operation of the plant is straightforward. But some solvents may decompose, react or polymerize when in contact with activated carbon during the adsorption step and the subsequent steam desorption. 

Unlike the side products of cyclohexanone, acetic acid the reaction product of the MEK (corrosive ) will be removed from the activated carbon during steam desorption. Adsorption performance is thus maintained even after a number of cycles. 

Adsorptive solvent recovery with steam desorption and condensation units with gravity separator and a stripper have become a standard practice in modem production plants. The solvents-laden air (toluene, xylene) is collected from emission points, e.g., rotogravure printing machines, drying ducts by means of a blower and passed through the recovery plant. 

Adsorptive reeovery with steam desorption was widely used for many years but its disadvantage is that the solvents are recovered in various mixtures containing large amounts of water. The recovery of solvents is complicated because most solvents produce azeotropes with water whieh are not easily separated and, consequently, the waste water fails to meet the more stringent environmental regulations. In the last 10 years the adsorbent regen-... 

Adsorptive removal on activated carbon and recovery by steam desorption. For simultaneous HjS and CS2 removal the Sulfosorbon-process uses adsorbers packed with two different activated carbon types... 

Extraction with Organic solvents Caustic soda solution Supercritical gases Exiiaetion with earbon disulfide or other solvents Percolation with eaustie soda e.g. extraction with supercritical CO2 Sulphur extraction Sulfosorbon process Phenol-loads activated earbon Organic compounds Desorbate treatment by distillation, steam desorption of solvent Phenol separation with subsequent purging S aration of C02/or-ganic compounds... 

Consider the following point of view. Use of methylene chloride (BP 40°C) and any similar highly volatile solvents, should not be attempted in open-top vapor degreasers. In addition to the normal losses of highly volatile solvent from an open-top vapor degreaser, the above information reveals that "black box" containment of emitted solvent vapor by activated carbon (with steam desorption) spawns three problems for the one it solves. 

KK. Gu, J., and Bart, H-J., "Heat and Mass Transfer in Steam Desorption of an Activated Carbon Adsorber," International Communications in Heat and Mass Transfer, Vol. 32, 2005, pages 296 to 304. 

Figure 22.1.4. Flow sheet of a solvent recovery system with steam desorption.

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