Page 65 - Chemical process engineering design and economics

50 Chapter 2

hundreds of pounds, must be supported by steel structures and pipe hangers, fur-
ther adding to the cost of the plant. Another factor will account for this cost.
Because a typical plant will contain many items of machinery, such as
pumps, compressors, and mixers, a plant will require an electric power distribution
system. Electricians need wiring, switches, and other electrical equipment to con-
nect the machinery to the electrical system.
To maintain the production rate, product quality, and plant safety requires a
data acquisition and control system. This system consists of temperature, pressure,
liquid level, flow rate, and composition sensors. Computers record data and may
control the process. Modern chemical plants use program logic controllers (PLC)
extensively. According to Valle-Riestra [20], instrumentation cost is about 15% of
purchased equipment cost for little automatic control, 30% for full automatic con-
trol, and 40% for computer control.
Another factor is needed to estimate the cost of buildings to house the proc-
ess and the various services required to operate the plant such as offices, mainte-
nance shops, and laboratories. Process buildings, as described by Valle-Riestra
[20], are mostly open structures rather than enclosed structures and are preferred
for safety as well as economic reasons. Toxic or flammable gases or liquids re-
leased accidentally will dissipate more quickly in an open structure. A frequent
arrangement is an open process tower five decks in height and constructed with I-
beams [20]. Besides reducing the floor space occupied by equipment, the structure
allows for gravity flow.
Auxiliary facilities provide services that are necessary for the operation of
the process. Examples of these facilities are steam, electrical power, air, cooling
water, refrigeration, and waste treatment. To account for this cost requires deter-
mining whether the facility will be dedicated or shared. If the facility is dedicated
solely for the use of a single process, then its cost is assigned to the process. On
the other hand, if other processes share the facility, then its cost is divided accord-
ing to usage.
A plant is divided into four areas: the process area, storage, utilities, and
services, as illustrated in Figure 2.5. The process area is called "battery limits" and
the other areas auxiliary facilities. Battery limits derives from the time when oil
refineries contained several stills in a row, resembling a gun battery. The battery
limit contains all the equipment assigned to the process, but Valle-Riestra [20]
pointed out that a process unit is not always physically located in one area of a
plant.
Because the factor methods for calculating the depreciable capital cost are
rapid methods and not based on a detailed design, many small items of equipment
are knowingly omitted. Also, there are uncertainties in design and economic pro-
cedures, and bad weather, strikes, and other unforeseen events may cause delays.
To correct for uncertainties and unforeseen events requires using a contingency
factor or safety factor.
Engineers design, implement the design, and monitor the progress of con-
struction. They organize the total construction effort. Besides chemical engineers,

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