Page 34 - Coulson Richardson's Chemical Engineering Vol.6 Chemical Engineering Design 4th Edition

INTRODUCTION TO DESIGN
Process stream
Consider a single-phase stream, containing C components. 17
Variable Number
Stream flow-rate 1
Composition (component concentrations) C
Temperature 1
Pressure 1
Stream enthalpy 1
Total, N v D C C 4
Relationships between variables Number

Composition 1 1
Enthalpy 2 1

Total, N r D 2

Degrees of freedom N d D N v N r D C C 4 2 D C C 2
(1) The sum of the mass or mol, fractions, must equal one.
(2) The enthalpy is a function of stream composition, temperature and pressure.
Specifying (C C 2) variables completely defines the stream.

Flash distillation
The idea of degrees of freedom in the design process can be further illustrated by consid-
ering a simple process unit, a flash distillation. (For a description of flash distillation see
Volume 2, Chapter 11).

F , P , T , (x ) i 2
2
2
2
F , P , T , (x ) i 1
1
1
1
q F , P , T , (x ) i 3
3
3
3
Figure 1.7. Flash distillation
The unit is shown in Figure 1.7, where:
F D stream flow rate,
P D pressure,
T D temperature,
x i D concentration, component i,
q D heat input.
Suffixes, 1 D inlet, 2 D outlet vapour, 3 D outlet liquid.