Page 69 - Coulson Richardson's Chemical Engineering Vol.6 Chemical Engineering Design 4th Edition

52
CHEMICAL ENGINEERING
Let flow of ethylene to block A be X and to block B be Y, and the HCl recycle be Z.
Then the total mols of DCE produced D 0.98X C 0.95Y, allowing for the yields, and
the mols of HCl produced in block C
D 0.98X C 0.95Y 0.995 D Z a
Consider the flows to and product from block B
C H Block
2 4
O 2 B DCE
(Z) HCL

The yield of DCE based on HCl is 90 per cent, so the mols of DCE produced
0.90Z
D
2
Note: the stoichiometric factor is 2 (2 mol HCl per mol DCE).
The yield of DCE based on ethylene is 95 per cent, so
0.9Z
D 0.95Y
2
0.95 ð 2Y
Z D
0.9
Substituting for Z into equation (a)gives
0.9
Y D 0.98X C 0.95Y 0.995 ð
2 ð 0.95
Y D 0.837X b
Total VC produced D 0.99 ð total DCE, so

0.99 0.98X C 0.95Y D 200 kmol/h
Substituting for Y from equation (b)gives X D 113.8kmol/h

and Y D 0.837 ð 113.8 D 95.3kmol/h

HCl recycle from equation (a)

Z D 0.98 ð 113.8 C 0.95 ð 95.3 0.995 D 201.1kmol/h
200
Note: overall yield on ethylene D ð 100 D 96 per cent
113.8 C 95.3

2.15. PURGE

It is usually necessary to bleed off a portion of a recycle stream to prevent the build-up of
unwanted material. For example, if a reactor feed contains inert components that are not