Estimate average particle diameter, mass transfer coefficients, and mixer stage efficiency for extraction of benzoic acid from water into toluene for Example 13-7.

Example 13-7

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Design a baffled mixing vessel and a horizontal settler for extraction of benzoic acid from water (diluent) into toluene (solvent). The mixing tank (Figure 13-31) should have H+/dtank = 1.0, and there should be a 1-in. (0.0254 m) head space (vapor space at top). The flow rate of the aqueous feed is 0.006 m/s, and the volumetric solvent-to-feed ratio = 0.2. A six-blade flat turbine impeller with impeller diameter d; = 0.25 dtank and impeller width w;= d;/5 is operated at 1000 rpm. Simple settling experiments show that this is a type I (rapid settling) system. A. For a mixture residence time of 1.0 minutes, find the mixer dimensions and estimate the volume fraction of the dispersed phase d and power P required. B. Set L/d settler = 4, and use Stokes law to size the horizontal settler. Data at 20C (Treybal, 1980): Water Solution Toluene Solution Density p 998 kg/m 865 kg/m Viscosity 0.95 x 103 kg/(m s) Diffusivity D 2.2 10-9 m/s 0.59 x 10-3 kg/(m s) 1.5 x 10-9 m/s Molecular wt 18.02 92.14 D is the diffusivity of benzoic acid in water or toluene. Interfacial tension, = 0.0222 N/m, and distribution coefficient = Cextract/Craffinate = 20.8, Cextract = kmol benzoic acid/m extract, and Craffinate = kmol benzoic acid/m raffinate. Food and solvent in Propeller or turbine Motor Balles V Emulsion out