Solvent vapors, recovery

The rotary bed adsorber (also called adsorption wheel) provides a truly continuous TSA system. It uses a shallow wheel-shaped adsorption bed that continuously turns about an axis inside a fixed supporting frame. A section of the wheel is continuously used for adsorbing impurities from a gas while the other section is continuously regenerated by heating it with an impurity free gas. The adsorbent is made from a honeycomb-shaped alumina substrate that can be coated with layers of silica gels, activated carbons, or zeolites [14], It has been used for gas dehumidification, solvent vapor recovery, VOC removal, and deodorization of a gas stream. 

Carbon dioxide-methane separation Solvent vapor recovery Hydrogen and carbon dioxide recovery from steam-methane reformer off-gas Hydrogen recovery from refinery off-gas Carbon monoxide-hydrogen separation Alcohol dehydration Production of ammonia synthesis gas Normal-isoparaffin separation Ozone enrichment... 

Figure 22.3(b) is a schematic drawing of a two-column embodiment of a PSA process for solvent vapor recovery, (i) The solvent-laden feed gas is passed through the carbon column at a superambient pressure level (P ) in order to produce a solvent-free effluent gas at pressure P. (ii) The column is then countercurrently depressurized to a near ambient pressure level (Pp) to desorb a part of the adsorbed solvents. It is then (iii) countercurrently purged at pressure Pd with a part of the solvent-free gas produced by step (i) in order to further desorb the solvent, (iv) Finally, the column is countercurrently pressurized from Pp to P with another part of the solvent-free gas produced during step (i). The... 

Figure 22.3 Schematic drawings of solvent vapor recovery systems (a) conventional thermal swing adsorption, (b) conventional pressure swing adsorption, and (c) rotary adsorber.

The hydrophobic nature of the activated carbons is best suited for the solvent vapor recovery applications because most industrial waste gas streams containing organic solvents are saturated with water vapor. Activated carbons can retain a large fraction of their dry adsorption capacities for organic compounds in presence of high humidity. Table 22.3 shows a few examples of this behawor [22]. Most polar adsorbents (zeolites, alumina, and silica gels) will not be effective for this appHcation. 

Hirose, T. andKuma, T. (1990). Honeycomb rotor continuous adsorber for solvent vapor recovery and dehumidification. 2nd Korea-Japan Symposium on Separation Technology. 

One alternative is to blanket the dryer with an inert gas. Because of the operating expense of an open-loop system, this usually involves recycle and solvent vapor recovery from the drying gas. These steps add to both capital and operating costs of the fluidized-bed dryer and offset some of its advantage in energy and floor space requirements. 

Gas-phase applications of activated carbon in the USA are mainly concentrated towards granular carbons. Total consumption in 1994 accounted for somewhat <20% of the total market. The main applications are air purification (it includes industrial gas purification), solvent vapor recovery, automotive evaporation control systems, and others, of which the two first constitute the main share of the total. 

Put in the necessary hooding, ducting, and equipment for an afterburner system which will bum the organic solvent vapors to a less polluting emission, but with no solvent recovery. 

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... 

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. 

Different designed solvent recovery systems are used. As an example there is the solvent system that consists of fixed bed adsorbers containing activated carbon and a distillation system. The carbon adsorbs the solvent vapors. Then the beds are steamed in sequence to remove the solvent. The solvent and steam are condensed into a large tank. The distillation system is then used to distill the solvent from the water to a purity of 99.99% so that it can be reused. Because of the high cost of solvent, complex monitoring equipment is used to insure a high rate of recover. 

Contact Condensers Spray condensers, jet condensers, and barometric condensers all utilize water or some other liquid in direct contact with the vapor to be condensed. The temperature approach between the liquid and the vapor is very small, so tne efficiency of the condenser is high, but large volumes of the liquid are necessary. If the vapor is soluble in the liquid, the system is essentially an absorptive one. If the vapor is not soluble, the system is a true condenser, in which case the temperature of the vapor must be below the dew point. Direct-contact condensers are seldom used for the removal of organic solvent vapors because the condensate will contain an organic-water mixture that must be separated or treated before disposal. They are, however, the most effective method of removing heat from hot gas streams when the recovery of organics is not a consideration. 

NOXIOUS GAS REMOVAL. Gaseous pollutants can be removed from air streams either by absorption, adsorption, condensation, or incineration. A list of typical gaseous pollutants that can be treated with these four methods is given in Table 9. Generally, condensation is not utilized as a method for removing a solvent vapor from air or other carrier gas unless the concentration of the solvent in the gas is high and the solvent is worth recovery. Since condensation cannot remove all of the solvent, it can only be used to reduce the solvent concentration in the carrier gas. 

---