Page 68 - Coulson Richardson's Chemical Engineering Vol.6 Chemical Engineering Design 4th Edition

FUNDAMENTALS OF MATERIAL BALANCES
The solution of more complex material balance problems involving several recycle
loops is discussed in Chapter 4. 51
Example 2.13
The block diagram shows the main steps in the balanced process for the production of
vinyl chloride from ethylene. Each block represents a reactor and several other processing
units. The main reactions are:
Block A, chlorination
C 2 H 4 C Cl 2 ! C 2 H 4 Cl 2 , yield on ethylene 98 per cent
Block B, oxyhydrochlorination
1
C 2 H 4 C 2HCl C O 2 ! C 2 H 4 Cl 2 C H 2 O, yields: on ethylene 95 per cent,
2
on HCl 90 per cent
Block C, pyrolysis
C 2 H 4 Cl 2 ! C 2 H 3 Cl C HCl, yields: on DCE 99 per cent, on HCl 99.5 per cent
The HCl from the pyrolysis step is recycled to the oxyhydrochlorination step. The flow
of ethylene to the chlorination and oxyhydrochlorination reactors is adjusted so that the
production of HCl is in balance with the requirement. The conversion in the pyrolysis
reactor is limited to 55 per cent, and the unreacted dichloroethane (DCE) separated and
recycled.

Cl 2 A Recycle DCE
Chlorination
Ethylene C VC
Pyrolysis
B
Oxygen Oxyhydro-
chlorination
Recycle HCL

Using the yield figures given, and neglecting any other losses, calculate the flow of
ethylene to each reactor and the flow of DCE to the pyrolysis reactor, for a production
rate of 12,500 kg/h vinyl chloride (VC).

Solution
Molecular weights: vinyl chloride 62.5, DCE 99.0, HCl 36.5.
12,500
VC per hour D D 200 kmol/h
62.5
Draw a system boundary round each block, enclosing the DCE recycle within the
boundary of step C.