Page 53 - Chemical process engineering design and economics

Production and Capital Cost Estimation 39

process units based on previous experience. If experience is lacking, Cevidalli and
Zaidman [7] propose using Equation 2.1.

K N
L = ———— —— (2.1)
(l+p) n m b

This formula is a modification of a formula originally proposed by Wessel
[8]. Cevidalli and Zaidman [7] examined several processes to determine the effect
of production rate, process complexity, and degree of automation on the operating
labor cost. In Equation 2.1, L is the number of hours required to produce one kilo-
gram of product.
The process-productivity factor, K, is given in Table 2.4, which lists three
process types: batch, continuous (normally automated), and continuous (highly
automated). According to Table 2.4, a continuous, highly-automated process is the
most efficient. We expect that the operating efficiency of the process will improve
as engineers and technicians become more experienced in operating the plant. The
improvement in operating efficiency is the yearly fractional increase in productiv-
ity, p. The base year for computing the operating labor is 1952. Thus, n is the
number of years since 1952. By assuming that the fractional increase in labor pro-
ductivity is 0.02, Cevidalli and Zaidman [7] found that the calculated operating
labor using Equation 2.1 agrees with the actual labor requirement for several proc-
esses by 40%. This error is not unreasonable for an economic estimate.
Operating labor also depends on the the plant capacity, m, in kg/h. Table 2.4
shows that the exponent, b, in Equation 2.1 depends on the plant capacity. The
exponent is 0.76 if the plant capacity is less than 5670 kg/h (12500 Ib/h) and 0.84
if it is greater than 5670 kg/h. The economy of scale is evident in Equation 2.1,
because the operating labor required to produce a kilogram of product decreases as
the plant capacity increases. As shown in Table 2.1, once we calculate the operat-
ing labor we can calculate the operating supervision and maintenance labor.
The complexity of a process, as determined by the number of process units,
N, also affects the operating labor required. The greater the number of process
units the more complex the process is and the greater the operating labor. The
number of process units is the most difficult term to evaluate in Equation 2.1.
Bridgewater [9] defines a significant process unit as a unit that achieves a chemi-
cal or physical transformation of major process streams or any substantial and
necessary side streams. Examples of process units are fractionation and filtration.
Use the following guidelines for determining the number of process units:

1. Ignore the size of a process unit and multiple process units of the same type in
series, such as the number of evaporators for multi-effect evaporation or the
number of Continuously Stirred Tank Reactors (CSTRs).

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