Page 239 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

stations (valves). In the former case, credit will be taken for generating power; in the latter case, credit
will not be taken. The procedure for calculating the cost of steam at different pressure levels is given in
Example 8.6.

Example 8.6

Determine the cost of producing high-, medium-, and low-pressure steam using a natural gas fuel source.
For medium- and low-pressure steam production, assume that steam is produced at the highest pressure
level, and consider both the case when this steam is sent through a turbine to make electricity and when it
is simply throttled through a valve.

Again the approach taken here is to assume that the fixed capital investment associated with the initial
purchase of the steam generation facilities has been accounted for elsewhere. The analysis given below
accounts only for the operating costs associated with steam (and power) production. The source of fuel is
assumed to be natural gas that costs $11.10/GJ. See Table 8.3.

High-Pressure Steam (41.0 barg)

Basis is 1000 kg of HP steam generated at 45.3 bar and 400°C h 44.3 barg, 400°C = 3204.3 kJ/kg.

Conditions at the header are 41 bar saturated (T = 254°C). Note that the steam is generated at a higher
sat
pressure and superheated for more efficient expansion, but that desuperheating will be assumed at the
process user.

Assume boiler feed water comes from a deaerator that operates at exhaust steam header pressure of 0.7

barg and T = 115°C (10 psig) h BFW = 483.0 kJ/kg.
sat

ΔH BFW-HP Steam = (3204.3 – 483.0) = 2721.3 kJ/kg

Energy required to produce HP steam = (2721.3)(1000) = 2.721 GJ

Because this HP steam is superheated, we can produce more than 1000 kg of saturated steam from it. In
order to desuperheat this steam, BFW is added to produce saturated steam at 41.0 barg (h = 2797.6
kJ/kg). See Figure E8.6.

Figure E8.6 Sketch of Desuperheating Process for HP Steam