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<br />r, <br />....' <br /> <br />However, there exists a minimum solvent rate for adequate mixing and the <br />benefits of using improved solvents diminish with increasing distribution <br />coefficients. <br />The optimum solvent rate fer the Lurgi example is 30 1b.solvent/lOO lb <br />wastewater. Suitable solvents will usually be recovered for recycle by <br />distilling them from the extracted phenols. Assuming a value for the latent <br />heat of vaporization of the solvent of 133 Btu/lb (for n-buty1 acetate) and a <br /> <br />1--' <br />en <br />~ <br /> <br />20\ reflux in ~~e solvent still, the energy <br />6 <br />wastewater = 0.4 x 10 Btu/1000 gals. At a <br /> <br />consumed is 4,800 Btu/l00 lb <br />6 <br />cost of $3/10 Bru, the energy <br /> <br />cost is $1.20/1000 gals. In addition, the charges for cooling water, solvent <br /> <br />makeup and electricity are about $0.60/1000 gals so that the total utility ~~d <br /> <br />chemical costs do not exceed about $1.80/1000 gals. Thus, the total operatir.g <br /> <br />cost of solvent extraction, inclucinq amortization and maintenance, is about <br />6 6 <br />$5.00/1000 gals., or 6~/10 Btu product output.. <br /> <br />Solvent extraction is not a cheap process, although the costs are par~ly <br />offset by the value of the recovered phenol. Since the quality of the recovered <br />phenol is un~~own and probably not high, its value is probably limited to its <br />iuel value, about 4C/lb. The phenol recovered from 6,000 p~m has a val~e of <br />6 <br />about $2.0/1000 gals, or 2.4~/10 Btu, as compared to the total cost of solvent <br />extraction of 6~/l06 Btu. Thus, the value of recovered phenol is important. <br />Solvent extrac~ion is not a required treatment. It will be used ~hen it <br />pays to use it, which will be when the phenol concentration is high. Furthe~cre, <br />solvent extrac~ion is not suitable for obtaining very low concentration effluent <br />water. The examples shown on Tables 2-7 and 2-8 for Phenoso1van plant effluent <br />are examples of waeer treated by solvent extraction. They probably represene <br />the best water reasonably obtainable by solvent extraction. The water is, in <br />our opinion, just qood enough to feed to a cooling tower when diluted ~ith an <br />equal volume of river water~ <br />Bioloq1cal Oxidation <br />Every test reported shows that most of the organic material in coal <br />gasification wastewater is biodegradable. Biolcgical oxidation is a suitable <br />treatment. In Reference 4 we calculated the cost of air activated sludge <br />biological treatment and of high purity o~fgen activated sludge treatment. <br />The air activated sludge plants were siz.ed lion the assumption that the biodegradability <br />of coa.l convers.:.on wastewaters is identical W"ith t.hat of coke wastewater". <br /> <br />40 <br />