recycle, bypass, & purge calculations

CHE 31. INTRODUCTION TO CHEMICAL ENGINEERING CALCULATIONS

Lecture 12Recycle, Bypass, & Purge Calculations

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

LECTURE 12. Recycle, Bypass, & Purge Calculations

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

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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.



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Bypass Stream

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



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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.



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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 A portion of the product from the condenser is returned to the column as reflux R, and the rest is withdrawn as the final product D. Assume that V, R, and D are identical in composition since V is condensed completely.

Find the ratio of the amount refluxed R to the product withdrawn D.



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Overall Process

Total Balance: 10,000 = D + WBenzene Balance: 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 + DR = 2950 kg/hr

Ratio (R/D) = (2950/5050) = 0.58



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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.



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Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños SLIDE10

Overall and Single Pass Conversion

PROCESS

REACTOR

Input-Output 100 - 0Overall Conv. of A= = ×100 = 100%

Input 100

Input-Output 100 - 25SinglePassConv. of A= = ×100 = 75%

Input 100




Solution:

Basis: 100 lbm of fresh feed

Overall Process

1. Total Balance: P = 100 lbm

Mixing Point

2. Total Balance: 100 + R = T3. Sucrose Balance: 100(0.30) + R(xS)R = T(xS)T4. Inv. Sugar Balance: (0) + R(xI)R = T(0.05)




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 W 1 mol S 18 lbm W lbm W = 0.0526

1 mol S 342.35 lbm S 1 mol W lbmS

6. H2O Balance: T(xW)T – T(xS)T(0.90)(0.0526) = (R + 100)(xW)R

7. xR: (xS)R + (xI)R + (xW)R = 1.08. xT: (xS)T + 0.05 + (xW)T = 1.0




Solving equations 2-8 simultaneously:

R = 20.94 lbmT = 120.94 lbm

(xW)T = 0.698(xS)T = 0.252

(xW)R = 0.6858(xS)R = 0.0252(xI)R = 0.289

recycle, bypass, & purge calculations

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