Solvent vapor recovery adsorption

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. 

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. 

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. 

Recoveiy of solvent vapor (dicfaloromethane, CH2CI2) fiom air by pressure swing adsorption (PSA) was studied, using two columns packed with high silica zeolite and resin as adsorbent Gravimetric measurements wete made for dichloromethane on the adsorbents. Computer calculations were carried out to simulate the experimental results using the Stop-Go method to show the calculated results coincide well with experimental results. The method is useful to predict the performance of a solvent recovery system operated by PSA... 

The response speed, associated with the adsorption of vapor molecules on free sites in the PPy films, was typically found to be faster than the recovery speed, consisting of the desorption and evaporation of the analyte molecules from the occupied sensing sites, see Fig. 7.10a. This became especially evident in nontemplated films. On average the response time of nontemplated and DG-structured films on exposure to solvent vapors was similar, but on purging with nitrogen the structured films outperformed the nontemplated deposits, see Fig. 7.10a. Curiously, the response times varied very little with different vapor concentrations. 

Carbon Adsorption—A recovery process that captures solvent vapors from air on activated carbon. The solvent is recovered (by desorption) from the carbon by injection of steam into the carbon bed and condensing the resultant solvent and water vapor. 

Butane adsorption it gives an indication of the applicability in solvent recovery and other gas-phase applications. The usual test consists in the determination of the adsorption of -butane at near ambient temperature, and for some applications such as gasoline vapor recovery in automotive vehicles the adsorption and desorption capacity are measured in a specific number of cycles. 

The efficiency of activated carbon adsorption units is 90 to 99%, depending on the specific solvent vapor used and the design of the carbon bed. If the gas adsorption system is equipped with an IR analyzer for monitoring the solvent vapor concentration, the discharge effluent can be automatically switched from one saturated bed to another fresh bed by the IR sensing device, and the recovery process is started. 

Solvent Recovery The largest current industrial use of pei vapo-ration is the treatment of mixed organic process streams that have become contaminated with small (10 percent) quantities of water. Pei vaporation becomes vei y attractive when dehydrating streams down to less than 1 percent water. The advantages result from the small operating costs relative to distillation and adsorption. Also, distillation is often impossible, since azeotropes commonly form in multicomponent organic/water mixtures. 

Smooth and uniform polymer surface after vacuum plays a key role to ensure good OFRR sensing performance. We have observed in experiments that toluene after vacuum is prone to leave a number of cavities of a few micrometers in diameter on the surface. These cavities will induce additional scattering loss for the WGMs in the OFRR, which greatly degrade the g-factor, and hence the detection limit of the OFRR vapor sensor. Moreover, these small cavities have different adsorption characteristics compared to smooth polymer surface. Vapor molecules may be retained for a longer time at the cavity, which increases the response time and recovery time. Acetone and methanol are found to be better candidates for solvents because they usually leave uniform and smooth surface after vacuum. 

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