Page 129 - Chemical engineering design

FUNDAMENTALS OF ENERGY BALANCES
from Figure 3.6 E P D 0.75
6 109
P r inlet D D 0.18
33.5
298
T r inlet D D 2.4
126.2
For these values the simplified equations can be used, equations 3.37a and 3.38a.
For N 2 D 1.4
1.4 1
m D ð 0.75 D 0.21
1.4
1 1
n D D D 1.27
1 m 1 0.21
0.21
1.5
without preheat T 2 D 298 D 223 K
6.0
Ž
D 50 C (acidic water would condense out)

0.21

1.5
with preheat T 2 D 673 D 503 K
6.0
Ž
D 230 C
From equation 3.31, work done by gases as a result of polytropic expansion
1.27 1 /1.27
1.27 1.5
D 1 ð 673 ð 8.314 ð 1
1.27 1 6.0
D 6718 kJ/kmol

Actual work D polytropic work ð E p
D 6718 ð 0.75 D 5039 kJ/kmol

10,410
Power output D work/kmol ð kmol/s D 5039 ð
3600
D 14,571 kJ/s D 14.6MW

Liquid streams

As liquids are essentially incompressible, less energy is stored in a compressed liquid than
a gas. However, it is worth considering power recovery from high-pressure liquid streams
(>15 bar) as the equipment required is relatively simple and inexpensive. Centrifugal
pumps are used as expanders and are often coupled directly to pumps. The design,
operation and cost of energy recovery from high-pressure liquid streams is discussed
by Jenett (1968), Chada (1984) and Buse (1985).