Page 167 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Example 6.2

It is necessary to provide a nitrogen stream at 80°C and a pressure of 6 bar. The source of the nitrogen is
at 200°C and 1.2 bar. Determine the work and cooling duty required for three alternatives.
a. Compress in a single compression stage and cool the compressed gas.
b. Cool the feed gas to 80°C and then repeat part a.
c. Repeat part b, except use two stages of compression with an intercooler.
d. Identify any conditions of special concern that occur.

Nitrogen can be treated as an ideal diatomic gas for this comparison. Use as a basis 1 kmol of nitrogen
and assume that the efficiency, ε, of each stage of compression is 70%.
For ideal diatomic gas: C = 3.5R, C = 2.5R, γ= C /C = 1.4, R = 8.314 kJ/kmol K, and assuming an
υ
p
p
υ
efficiency, ε = 0.70
Equations used: q = C ΔT, w = RT γ/(γ–1)[P /P ) (γ-1)/γ -1]/ε,
in
out
in
p

Figure E6.2 gives the process flow diagrams for the three alternatives and identifies stream numbers and
utilities.

Figure E6.2 Alternative Process Schemes for Compression of Nitrogen