Page 248 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Electric Power = P = Output power/ξ = (49.1)/(0.90) = 54.6 kW
dr
dr
Yearly Cost = (54.6)(0.06)(24)(365)(0.95) = $27,200/yr
e. Same as Part (d) with steam driven compressor. For 10 barg steam with exhaust at 0 barg, Table
8.5 provides a steam requirement of 8.79 kg steam/kWh of power. The shaft efficiency is about
35% (extrapolating from Figure 8.7).

Steam required by drive = (49.1)(8.79/0.35) = 1233 kg/h (0.34 kg/s)

–3
Cost of Steam = (1233)(24)(365)(0.95)(28.32 × 10 ) = $290,600/yr
f. P-101: Shaft power is 14.2 kW. From Figure 8.7 the efficiency of an electric drive is about 86%.

Electric Power = 14.2/0.86 = 16.5 kW

Yearly Cost = (16.5)(0.06)(24)(365)(0.95) = $8240/yr

Note: The cost of using steam to power the compressor is much greater than the cost of electricity even
though the cost per unit energy is much lower for the steam. The reasons for this are (1) the
thermodynamic efficiency is low, and (2) the efficiency of the drive is low for a small compressor.
Usually steam drives are used only for compressor duties greater than 100 kW.

8.7 Cost of Treating Liquid and Solid Waste Streams

As environmental regulations continue to tighten, the problems and costs associated with the treatment of
waste chemical streams will increase. In recent years there has been a trend to try to reduce or eliminate
the volume of these streams through waste minimization strategies. Such strategies involve utilizing
alternative process technology or using additional recovery steps in order to reduce or eliminate waste
streams. Although waste minimization will become increasingly important in the future, the need to treat

waste streams will continue. Some typical costs associated with this treatment are given in Table 8.3, and
flowrates can be obtained from the PFD. It is worth noting that the costs associated with the disposal of
solid waste streams, especially hazardous wastes, have grown immensely in the past few years, and the
values given in Table 8.3 are only approximate average numbers. Escalation of these costs should be
done with extreme caution.

8.8 Evaluation of Cost of Manufacture for the Production of Benzene via the

Hydrodealkylation of Toluene

The cost of manufacture for the production of benzene via the toluene HDA process is given in Example
8.10.

Example 8.10

Calculate the cost of manufacture without depreciation (COM ) for the toluene hydrodealkylation process
d