Page 242 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Enthalpy of BFW = 483.0 kJ/kg

If x is the amount of saturated MP steam obtained by desuperheating, then an enthalpy balance gives

(1000)(3204.3) + (x – 1000)(483) = (x)(2779.1) x = 1185.2 kg

Cost of natural gas to produce 1000 kg of sat HP steam (assuming a 90% boiler efficiency) is

Total cost of MP steam (without power production) = $28.32 + $0.83 + $0.439
= $29.59/1000 kg

Note: This is almost identical to the cost for HP steam.

Low-Pressure Steam (5.2 barg)

The calculation procedures for evaluating the cost of low-pressure steam are identical to those given
above for medium-pressure steam and the results are given below.

Total cost of LP steam (with power production) = $32.25 – $5.94 + $0.95 + $0.439
= $27.70/1000 kg

Total cost of LP steam (without power production) = $28.03 + $0.82 + $0.439 = $29.29/1000 kg

Waste Heat Boilers

When steam is generated from within the process—in a waste heat boiler, for example—the savings to the
process are usually calculated from the avoided cost of using an equivalent amount of natural gas in the
boiler system. If we assume that the boiler efficiency is 90%, then for every GJ of energy saved by
producing steam within a process unit, the boiler facility saves ($11.1)/(0.9) = $12.33 in natural gas
costs.

Hot Circulating Heat-Transfer Fluids. Again, the greatest cost for these systems is the fuel that is
burned to heat the circulating heat-transfer fluid. Typical efficiencies (based on the lower heating value,
LHV, of the fuel) for these heaters range from 60% to 82% [ 1]. With air preheating economizers, the

efficiency can be as high as 90%. Example 8.7 illustrates the use of efficiencies in fired heaters.

Example 8.7