Page 241 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

It is assumed that letting steam down through a turbine from the high-pressure header to the medium-
pressure header will generate electrical power.

The theoretical steam requirement (kg steam/kWh) for this situation is found by assuming an isentropic
expansion of the steam from the HP condition to the medium-pressure level. From the steam tables, we
have the following information:

h 44.3 barg, 400°C = 3204.3kJ/kg and s 44.3 barg, 400°C = 6.6953 kJ/kg K

By interpolating at constant specific entropy, we get that the outlet temperature is 212°C and the outlet
enthalpy = 2851.0 kJ/kg.

Δh = (3204.3 – 2851.0) = 353.3 kJ/kg = theoretical work

Therefore, 1000 kg of HP steam produces 353.3 MJ or 98.14 kWh of electricity. Assuming a turbine
efficiency of 75%, the output power is (0.75)(98.14) = 73.6 kWh.

Credit for electricity = (73.6)(0.06) = $4.42

The actual outlet enthalpy of the steam is 3,204.3 – (353.3)(0.75) = 2939.3 kJ/kg. This is still superheated
steam. Desuperheating the steam to 10.3 barg and saturated conditions (h = 2779.1 kJ/kg) will generate x

kg of MP steam from the 1000 kg of HP steam, where

(1000)(2939.3) + (x – 1000)(483.0) = (x)(2779.1) x = 1069.8 kg

Therefore, the cost of natural gas to produce 1,000 kg of MP steam (assuming a 90% boiler efficiency) is

We can find the cost of electricity for the blower by using the ratio of the natural gas usage from the high-
pressure steam case. Therefore,

Total cost of MP steam (with power production) = $31.37 – $4.42 + $0.92 + $0.439 = $28.31/1000 kg

For the case when power production is not implemented, the HP steam is throttled to the pressure of the
MP header through a let-down station, which is essentially an irreversible, isentropic process through a
valve. The superheated steam is then desuperheated at the process user.

Enthalpy of HP steam (at 44.8 barg and 400°C) = 3204.3 kJ/kg

Enthalpy of saturated MP steam = 2779.1 kJ/kg