Solvent adsorption cleaning solvents

In the dry cleaning business or in vapor degreasing plants, losses of hydrocarbon or chlorinated hydrocarbon vapors occur from the use and recovery of solvents. These losses can be prevented by the use of activated adsorbent systems to capture the organic vapors from the vents or hoods of these devices. Activated carbon is one of the best adsorbents for this purpose because of its selectivity for organic vapors, and can provide up to 1000 m /g (ca. 8 acres/oz) of adsorption area. Solvent present in high concentrations in air may also be recovered by cooling the air to liquid nitrogen temperatures to condense the solvent. 

The cleaned solvent is collected in a 20-m tank and, via sintered stainless steel filters, transferred to a larger storage tank for reuse. As the metal filters may become very radioactive, provision is made for back washing and remote replacement. Final solvent polishing by adsorption on anion-exchange resin has been found advantageous at Hanford [S4]. 

The basic principles of some generally accepted gas cleaning processes for solvent removal are given in Table 22.1.2. For more than 70 years adsorption processes using activated carbon, in addition to absorption and condensation processes, have been used in adsorptive removal and recovery of solvents. The first solvent recovery plant for acetone was commissioned for economic reason in 1917 by Bayer. In the decades that followed solvent recovery plants were built and operated only if the value of the recovered solvents exceeded the operation costs and depreciation of the plant. Today sueh plants are used for adsorptive purification of exhaust air streams, even if the return is insufficient, to meet environmental and legal requirements. 

To prevent an excessive temperature increase across the bed due to the heat of adsorption, inlet solvent concentrations are usually limited to about 50 g/m. In most systems the solvent-laden air stream is directed upwards through a fixed carbon bed. As soon as the maximum permissible breakthrough concentration is attained in the discharge clean air stream, the loaded adsorber is switched to regeneration. To reverse the adsorption of the solvent, the equilibrium must be reversed by increasing the temperature and decreasing the solvent concentration by purging. 

The adsorption of solvents on activated carbon " is controlled by the properties of both the carbon and the solvent and flie contacting conditions. Generally, the following factors, which characterize the solvent-containing waste air stream, are to be considered when selecting the most well suited activated carbon quality for waste air cleaning ... 

In other words, moisture will have a negative impact on adsorption of cleaning solvents water will compete with solvent for the available adsorption area, and It will win under some circumstances ... 

Consequendy, the facilities present in Figure 4.40 must be augmented. The expanded facilities are noted in Figure 4.41 as being within the dashed box, and include (1) the ability to dilute the aqueous stream containing soluble solvent with clean water, and (2) the ability to dry water from the recovered solvent by using another adsorption bed. The second bed has 3 A mole sieves ". ... 

Often substituted for one another to delay or resist negative environmental consequences, trichloroethylene and n-propyl bromide have similar performance as cleaning solvents (Ref 2, Chapter 2), good adsorption on activated carbon (Table 4.10) similar, compatibility in or with water (Table 4.10), and aggressive reactivity with water to produce... 

The complicated application involves use of lip vents attached to an open-top vapor degreaser in which each of the six solvents noted above is being independently used. The assumed feed and exit concentrations are 20 x and 1 x the exposure limit respectively. Complications arise from the intersection of three factors (1) the fictitious feed and exit concentrations to and from the adsorber are different for each of the six solvents (because they have different posted exposure limits. Footnote 55), (2) each solvent has a different adsorption vs. concentration response (Figure 4.17), and (3) each solvent has a different solubility (Table 4.10) in and with water (condensed steam). For example, the range of solvent concentrations for n-propyl bromide or trichloroethylene is 200 ppm to 10 ppm, and that for CFC-113 is 1000 ppm to 20,000 ppm. Solubility data of many cleaning solvents in water, and of water in cleaning solvents, are available in Appendix 1, Table A1-5. 

Consequently, for a few solvents whose purchase price is high (such as CFC-113), the adsorption process appears to produce profit since it is not charged for what will become an asset. But for every cleaning solvent, the actual book-kept operating cost of the adsorption process is understated because credit for the recovered solvent byproduct is given in the cost accounting. 

The purpose of this appendix is to provide a scientific and engineering basis for evaluating the performance of activated carbon adsorbents when removing cleaning solvent vapors from air. One may also project adsorption isotherms for solvents where there is no empirical data. 

Measured adsorption data for four cleaning solvents (methylene chloride, trichloroethylene, perchloroethylene, and CFC-113) are plotted in Figure A2-l . ... 

Airborne Solvent Level, ppm Figure A2-1 Measured Adsorption Isotherms for Four Cleaning Solvents... 

Adsorption data of each of these four cleaning solvents can be described by all three of the adsorption isotherm equations above. This is shown in Figures A2-2 (perchloroethylene) through A2-5 (CFC-113). 

Suppose that an existing gas absorber using clean solvent has its feed rate G doubled over the previous design value. A quick scan of the Kremser plot shows that to maintain the same effluent concentration or solute recovery as before, the solvent flow rate L would likewise have to be doubled. Similar considerations apply to adsorption, extraction, and leaching. 

The pore structure of the activated carbon must be matched to the solvent and the solvent concentration for each waste air cleaning problem. Figure 21.1.11 shows Freun-dlich adsorption isotherms of five typical solvents as a function of the solvent concentrations in the gas stream. It is clear that different solvents are adsorbed at different rates according to the intensity of the interacting forces between solvent and activated carbon. 

Nevertheless, for contaminated samples it is still necessary to carefully cany out a number of clean up procedures. Among them it should be cited classical liquid adsorption chromatography in columns, solid-phase extraction, gel-permeation chromatography, reduction of solvent volume and solvent exchange, which imply an important timing of the process. Their characteristics and operational procedure have been described in air monitoring, and will not be further detailed here. 

---