Page 128 - Chemical engineering design

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CHEMICAL ENGINEERING
separation, depends on the efficient recovery of the energy of compression. The energy
recovered by expansion is often used to drive the compressors directly; as shown in
Figure 3.14. If the gas contains condensible components it may be advisable to consider
heating the gas by heat exchange with a higher temperature process stream before
expansion. The gas can then be expanded to a lower pressure without condensation and
the power generated increased.
An interesting process incorporating an expansion turbine is described by Barlow (1975)
who discusses energy recovery in an organic acids plant (acetic and propionic). In this
process a thirteen-stage turbo-expander is used to recover energy from the off-gases. The
pressure range is deliberately chosen to reduce the off-gases to a low temperature at the
Ž
expander outlet ( 60 C), for use for low-temperature cooling, saving refrigeration.
The energy recoverable from the expansion of a gas can be estimated by assuming
polytropic expansion; see Section 3.13.2 and Example 3.17.
The design of turboexpanders for the process industries is discussed by Bloch et al.
(1982).

Example 3.17
Consider the extraction of energy from the tail gases from a nitric acid adsorption tower,
such as that described in Chapter 4, Example 4.4.
Gas composition, kmol/h:

O 2 371.5
N 2 10,014.7
NO 21.9
NO 2 Trace
Ž
H 2 O saturated at 250 C
Ž
If the gases leave the tower at 6 atm, 25 C, and are expanded to, say, 1.5 atm, calculate
the turbine exit gas temperatures without preheat, and if the gases are preheated to
Ž
400 C with the reactor off-gas. Also, estimate the power recovered from the preheated
gases.

Solution

For the purposes of this calculation it will be sufficient to consider the tail gas as all
nitrogen, flow 10,410 kmol/h.

P c D 33.5atm, T c D 126.2K

Figure 3.6 can be used to estimate the turbine efficiency.

10,410 1
Exit gas volumetric flow-rate D ð 22.4 ð
3600 1.5
3
' 43 m /s