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RECYCLE PURGE LESSONS QUESTIONS AND ANSWERS...

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CHE 31. INTRODUCTION TO CHEMICAL ENGINEERING CALCULATIONS

Lecture 12

Recycle, Bypass, & Purge Calculations

Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños

LECTURE 12. Recycle, Bypass, & Purge Calculations

Recycle Stream

Recycle stream is a term denoting a process stream that returns material from downstream of a process unit back to the process unit.

Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños

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LECTURE 12. Recycle, Bypass, & Purge Calculations

Bypass Stream

Bypass stream is a stream that skips one or more stages of the process and goes directly to another downstream stage.

Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños

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LECTURE 12. Recycle, Bypass, & Purge Calculations

Purge Stream

Purge stream is a stream bled off to remove an accumulation of inerts or unwanted material that might otherwise build up in the recycle stream.

Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños

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LECTURE 12. Recycle, Bypass, & Purge Calculations

Example 12-1. Distillation of Benzene and Toluene

A distillation column separates 10,000 kg/hr of a mixture containing equal mass of benzene and toluene. The product D recovered from the condenser at the top of the column contains 95% benzene, and the bottom W from the column contains 96% toluene. The vapor V entering the condenser from the top of the column is 8000 kg/hr. A portion of the product from the condenser is returned to the column as reflux R, and the rest is withdrawn as the final product D. Assume that V, R, and D are identical in composition since V is condensed completely. Find the ratio of the amount refluxed R to the product withdrawn D. Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños

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LECTURE 12. Recycle, Bypass, & Purge Calculations

Example 12-1. Distillation of Benzene and Toluene

Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños

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LECTURE 12. Recycle, Bypass, & Purge Calculations

Example 12-1. Distillation of Benzene and Toluene

Overall Process Total Balance: Benzene Balance:

10,000 = D + W 10,000(0.50) = D(0.95) + W(0.04)

Solving simultaneously, D = 5050 kg/hr ; W = 4950 kg/hr Total balance around the separator: 8000 = R + D R = 2950 kg/hr Ratio (R/D) = (2950/5050) = 0.58

Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños

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LECTURE 12. Recycle, Bypass, & Purge Calculations

Example 12-2. Conversion of Sucrose to Glucose and Fructose

Refined sugar (sucrose) can be converted to glucose and fructose by the inversion process C12H22O11 + H2O =======> C6H12O6 (G) + C6H12O6 (F) The combined glucose and fructose is called inversion sugar. If 90% conversion of sucrose occurs on one pass through the reactor, what would be the recycle stream flow per 100 lbm fresh feed of sucrose? What is the concentration of inversion sugar in the recycle and product streams? Assume that the two streams are identical in composition. Refer to the flowchart for the other details of the process.

Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños

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LECTURE 12. Recycle, Bypass, & Purge Calculations

Example 12-2. Conversion of Sucrose to Glucose and Fructose

Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños

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LECTURE 12. Recycle, Bypass, & Purge Calculations

Overall and Single Pass Conversion

 Input-Output   100 - 0  Overall Conv. of A=  =    ×100 = 100% Input 100    PROCESS  Input-Output   100 - 25  Single PassConv. of A=  =    ×100 = 75% Input  REACTOR  100  Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños

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LECTURE 12. Recycle, Bypass, & Purge Calculations

Example 12-2. Conversion of Sucrose to Glucose and Fructose

Solution: Basis: 100 lbm of fresh feed Overall Process

1. Total Balance:

P = 100 lbm

Mixing Point

2. Total Balance: 3. Sucrose Balance: 4. Inv. Sugar Balance:

100 + R = T 100(0.30) + R(xS)R = T(xS)T (0) + R(xI)R = T(0.05)

Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños

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LECTURE 12. Recycle, Bypass, & Purge Calculations

Example 12-2. Conversion of Sucrose to Glucose and Fructose Reactor + Separator

5. Sucrose Balance:

T(xS)T – T(xS)T(0.90) = (R + 100)(xS)R

Calculate for the pound of water consumed per pound of sucrose consumed:

1 mol S lbm W  1 mol W     18 lbm W  = 0.0526  1 mol S   342.35 lbm S   1 mol W  lbm S     6. H2O Balance:

T(xW)T – T(xS)T(0.90)(0.0526) = (R + 100)(xW)R

7. xR: 8. xT:

(xS)R + (xI)R + (xW)R = 1.0 (xS)T + 0.05 + (xW)T = 1.0

Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños

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LECTURE 12. Recycle, Bypass, & Purge Calculations

Example 12-2. Conversion of Sucrose to Glucose and Fructose

Solving equations 2-8 simultaneously:

R = 20.94 lbm T = 120.94 lbm (xW)T = 0.698 (xS)T = 0.252 (xW)R = 0.6858 (xS)R = 0.0252 (xI)R = 0.289

Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños

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