Problem5-24 - 5—24 Small amounts of an inorganic salt...

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Unformatted text preview: 5—24. Small amounts of an inorganic salt contained in an organic fluid stream can be removed by contacting the stream with pure water as illustrated in Figure 5.24. The process requires that the organic and aqueous streams be contacted in a mixer that provides a large surface area for mass transfer. and then separated in a settler. If the mixer is efficient. the two phases will be in equilibrium as they leave the settler and you are to assume that this is the case for this problem. You are given the following information: a) Organic stream flow rate: 1000 lbmfmin b) Specific gravity of the organic fluid: pm; i pHEG = 0.37 c) Salt concentration in the organic stream entering the mixer: (c)! )m, = 0.0005 mnlesfliter d) Equilibrium relation for the inorganic salt: [CA jag = Keg {-351ng where Km! = 60 Here (CA jag represents the salt concentration in the aqueous phase that is in equilibrium with the salt concentration in the organic phase. [cring _ In this problem you are asked to determine the mass flow rates of the water stream that will reduce the salt concentration in the organic stream to 0.1, 0.01 and 0.001 times the original salt concentration. The aqueous and organic phases are to be considered completely immiscible. i.e_, only salt is transferred between the two phases. In addition, the amount of material transferred is so small that the volumetric flow rates of the two streams can he considered constant. organic agile-nus .. phase organic —_— phase Figaro 5.24. Liquid—liquid extraction 5-24. In this problem no information is given or required about the stream between the mixer and the settler. thus the appropriate control volume is the one illustrated in Figm‘e 5.24a. we assume that the system operates at steady-state and that no chemical Two-Phase systems (is Equilibrium Stages 5—2? X 31 fig ucflufi phase I 1 organic {lg—"k __ phasfl Figure 5.36:1. Control volume for mixer—settler reaction occtu's. thus the macroscopic mole balance for the inorganic salt takes the form Inorganic salt: IcAy-udA = t] (1) A E' Here we have used tan in place of VA .u with the idea that diffilsiye effects are negligible at the enu‘ouces and exits of the system. When Eq. 1 is applied to the control volume illustrated in Figure 5 .24a. we obtain Inorganic salt: —{CA)1Q1 — (£24th + {CEIth + (CA)4Q4 = D (2) We are given that (fall = 9- (Cals = Kgqifi‘ah (3} and this allows us to express Eq. 2 in the form Kegifl'ahQs = (€14)st — (53494 (4) We are also given that the Tcoltuuetric flow rates can be constrained by the approximations Q1 = Q3: Q: = Q4 (5) and this allows us to express the yohuuetric flow rate of the stream entering the mixer- settler system as 91 = W (.5) Egg (CA )4 In terms of the mass flow rates of water and the organic phase entering the system. we have a soiutiou to our problem given by 5—28 Chapter 5 - _ _ le2 {CA}: _ m1 — p191 — pEKeql:{cA)4 1] a) and this solution can also be expressed as m mg — {Ema}: _ I] (B) (pmg/pH20)Keq licsotth To compute the answers requested in this problem. we make use of the appropriate munbcrs to obtain mHEo m1 = {CA)2!((CA)4 = This problem was particularly simple because the volumetric flow rates of the aqueous and organic phases were approximated as constants and the salt concentration in the aqueous stream enteiingr the system was zero. If these simplifications are not made. problems of this type become much more complex as indicated in the following problem. ...
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