Phenols mixer-settlers

The previous chapters have demonstrated that liquid-liquid extraction is a mass transfer unit operation involving two liquid phases, the raffinate and the extract phase, which have very small mutual solubihty. Let us assume that the raffinate phase is wastewater from a coke plant polluted with phenol. To separate the phenol from the water, there must be close contact with the extract phase, toluene in this case. Water and toluene are not mutually soluble, but toluene is a better solvent for phenol and can extract it from water. Thus, toluene and phenol together are the extract phase. If the solvent reacts with the extracted substance during the extraction, the whole process is called reactive extraction. The reaction is usually used to alter the properties of inorganic cations and anions so they can be extracted from an aqueous solution into the nonpolar organic phase. The mechanisms for these reactions involve ion pah-formation, solvation of an ionic compound, or formation of covalent metal-extractant complexes (see Chapters 3 and 4). Often formation of these new species is a slow process and, in many cases, it is not possible to use columns for this type of extraction mixer-settlers are used instead (Chapter 8). 

In this process developed by Lurgi [17], the phenolic effluent is contacted with the solvent in a multistage mixer-settler countercurrent extractor (Fig. 10.8). The extract, containing phenol, is separated into phenol and solvent by distillation and solvent is recycled to the extractor. The aqueous raffinate phase is stripped from solvent with gas, and the solvent is recovered from the stripping gas by washing with crude phenol and passed to the extract distillation column. 

The dephenolated raffinate is then fed to the top of the solvent stripper to recover the solvent by stripping with gas. To remove the solvent from the stripper overhead gas, the latter is washed with raw phenol, and the solvent-free gas is recycled to the stripper. The extract from the mixer/settler, containing the phenols and other extracted components, is fed to a two-stage distillation unit to produce highly pure ether overhead and solvent-free raw phenol at the bottom. Depending on the bottom temperature of the distillation column, the raw phenol may contain up to S % water. The raw phenol is partly exported as a product and partly used as wash liquor for the solvent scrubber. The ether-laden bottmn product from the solvent scrubber is recycled to the lower stage of the distillation column to expel the ether. 

The treatment stage therefore includes a mixer and a settler and extracts 50 to 60% of the phenols in spent caustic treatment. To reach the rates of 90 to 96% that may be required, there must be three or four stages with feed input countercunent to the two phases. 

---