Page 229 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

No. 2 fuel oil is the final fossil fuel that is commonly used as an energy source in the chemical industry.
Until recently, it has been the highest-cost fossil fuel source. It is most readily available near coastal
regions where oil enters the country and refining takes place. The United States has become increasingly
dependent on imported oil, which may be subject to large upsets in cost and domestic availability.
Uncertainties in the availability of supplies, high storage costs, and large fluctuations in cost make this
source of energy least attractive in many situations. However, recently the cost of natural gas has
increased substantially to the point that No. 2 fuel oil is now a viable alternative to natural gas in many
plants.

Figure 8.1 shows that fuel costs have increased somewhat more rapidly and in a much more chaotic
fashion than the cost index (CEPCI) that we have used previously to correct costs for inflation. As a result
of the regional variations in the availability and costs of fossil fuels, along with the inability of the cost
index to represent energy costs, we take the position that site-specific cost and availability information
must be provided for a valid estimation of energy costs. We assume, in this text, that natural gas is the fuel

of choice unless otherwise stated.

The PFD for the toluene hydrodealkylation process (Figure 1.5) represents the “battery-limits” plant. The

equipment necessary to produce the various service or utility streams, which are used in the process and
are necessary for the plant to operate, are not shown on the PFD. However, the utility streams such as
cooling water and steam for heating are shown on the PFD. These streams, termed utilities, are necessary
for the control of stream temperatures as required by the process. These utilities can be supplied in a
number of ways.
1. Purchasing from a Public or Private Utility: In this situation no capital cost is involved, and
the utility rates charged are based upon consumption. In addition the utility is delivered to the
battery limits at known conditions.
2. Supplied by the Company: A comprehensive off-site facility provides the utility needs for many
processes at a common location. In this case, the rates charged to a process unit reflect the fixed
capital and the operating costs required to produce the utility.
3. Self-Generated and Used by a Single Process Unit: In this situation the capital cost for
purchase and installation becomes part of the fixed capital cost of the process unit. Likewise the

related operating costs for producing that particular utility are directly charged to the process
unit.

Utilities that would likely be provided in a comprehensive chemical plant complex are shown in Table

8.3.

Table 8.3 Utilities Provided by Off-Sites for a Plant with Multiple Process Units (Costs Represent

Charges for Utilities Delivered to the Battery Limit of a Process)