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
SLIDE
2
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
SLIDE
3
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
SLIDE
4
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rest is withdrawn as the final product D. Assume that V, R,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withdrawn D.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
SLIDE
5
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
SLIDE
6
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
SLIDE
7
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
SLIDE
8
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
SLIDE
9
LECTURE 12. Recycle, Bypass, & Purge Calculations
Overall and Single Pass Conversion
 Input-Output 
 100 - 0 
Overall Conv. of A = 
=
×100 = 100%



Input

PROCESS  100 
 Input-Output 
 100 - 25 
Single Pass Conv. of A = 
=
×100 = 75%



Input

REACTOR  100 
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
SLIDE
10
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
SLIDE
11
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
SLIDE
12
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
SLIDE
13