Distillation Self Study 4 Solutions PDF

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Distillation Self Study 4 Solutions...

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Chemical Engineering Design: Synthesis and Economics Self-study material 3 In this tutorial will be solved problems involving: 1) Shortcut distillation calculations 2) Numerical solution of Underwood equations 3) Non key composition estimates from Fenske 4) Distillation Sequences from Heuristics 5) Distillation Sequences from MVF method The arguments needed for the solution of the problems are in Lectures 1-5. 1. Shortcut distillation calculations You are given a 3 component feed, with q=0.8, which contains A, B and C with molar flows of 20:25:55 (total molar flow=100kmol /h). The relative volatilities of the components (with respect to C) are 3.5 ; 2.2 and 1.0. Two columns are needed to separate the stream into 3 relatively pure components. 1. List the 2 possible column sequences. 2. For the first column in the direct sequence, calculate (a) the product compositions, assuming a mole fraction of 0.01 for both the light key in the bottoms and the heavy key in the distillate. Assume that any non-key components are recovered 100% in the expected stream; (b) the minimum number of stages, Nm, at infinite reflux; (c) the minimum reflux ratio, Rm, for infinitely many stages; Hence, propose a column design based on the Gilliland correlation with a sensible reflux ratio and number of stages. 2. Numerical solution of Underwood equations Suppose the feed to a distillation column consists of 4 components with molar flow rates (kmol/h), mole fractions and relative volatilities given in the table below. Component A1 A2 A3 A4 Total

Feed Flow 300 300 250 150 1000

Feed Mol. Fraction 0.30 0.30 0.25 0.15 1

Relative Volatility 6.0 3.0 1.0 0.5

1.Designate the components A2 and A3 as the light and heavy keys . Estimate the minimum reflux ratio, assuming the feed is saturated liquid and assuming that the molar fraction of A2 in bottoms 0.05 and the molar fraction of A3 in distillates 0.017. 3. Distillation Sequence from Heuristics The following feed is to be split into relatively pure products: Component

Feed Flow Relative Volatility [kmol/h] A 10 2.0 B 20 1.5 C 10 1.2 D 60 1.0 Identify the sequences suggested by the distillation heuristics. Note those heuristics which are not applicable, on the basis of the information so far given. 4. Distillation Sequence from MVF Method Considering the previous feed: a) find the best sequence using the MVF criterion by the Branch and Bound method. Explain your result. b) Now suppose that component B is very corrosive. What is the best sequence? What is the extra cost in terms of added vapour flow?

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Chemical Engineering Design: Synthesis and Economics Self-study material 3: Solutions 1. Shortcut distillation calculations 1.1. Possible sequences:

1.2.a. Make a table of flows, letting A in bottoms (fAB) and B in dist. (fBD) be unknowns.

Composition specifications imply: F  FD  FB FD  20  0.01 FB  0.01 FD  FB  55   0.01 FB   25   0.01 FD 

 20  0.01 FB  FD     1 0.01 

Solution: Hence FD = 19.3878 kmol/h, FB = 80.6122 kmol/h and fAB = 0.8061 kmol/h; fBD = 0.1939 kmol/h Composition table is then:

1.2.b. Fenske with A and B as key components. N min

 0.99 0.31  ln   0.01 0.01 17.3   ln 3.5 / 2.2 

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1.2.c. Apply First Underwood taking xCD = 0. Note that q=(heat for vaporization 1mol feed)/(molar latent heat of vaporiz. of feed)=0.8. It corresponds to roughly 20% vapour in the feed (quite ideal VLE). Based on the saturated liquid and vapour enthalpies and assuming equal enthalpies of vaporization for the three components.

Need solution for in range 2.2...