Page 275 - Chemical engineering design

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Q D plant capacity, tonne per year
s D reactor conversion CHEMICAL ENGINEERING
Reactor conversion is defined as:
mass of desired product
s D
mass reactor input
Timms, IChemE (1988) gives a simple equation for gas phase processes; updated to
1998:
C D 9000 N Q 0.615 6.5

where the symbols are the same as for equations 6.3 and 6.4.
In US dollars
0
C D 14,000 N Q 0.615 (6.5a)
0
Where C D captial cost in US dollars
Example 6.3

Estimate the capital cost for the nitric acid plant shown in Figure 4.2, Chapter 4.

Solution

Number of significant processing steps 6.
Capacity 100,000 tonne per year
C D 9000 ð 6 ð 100,000 0.615 D 64.2 ð 10 6 6.5

say, £65 million.

0
C D 14,000 ð 6 ð 100,000 0.615 D 99.8 ð 10 6
say, $100 million.
Clearly, step counting methods can only, at best, give a very approximate idea of the
probable cost of a plant. They are useful in the conceptual stage of process design, when
comparisons between alternative process routes are being made.

6.6. THE FACTORIAL METHOD OF COST ESTIMATION
Capital cost estimates for chemical process plants are often based on an estimate of the
purchase cost of the major equipment items required for the process, the other costs
being estimated as factors of the equipment cost. The accuracy of this type of estimate
will depend on what stage the design has reached at the time the estimate is made, and
on the reliability of the data available on equipment costs. In the later stages of the
project design, when detailed equipment specifications are available and firm quotations
have been obtained, an accurate estimation of the capital cost of the project can be
made.