Page 185 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

follows.
• Preliminary feasibility estimates (order-of-magnitude or study estimates) are made to compare
many process alternatives.
• More accurate estimates (preliminary or definitive estimates) are made for the most profitable
processes identified in the feasibility study.
• Detailed estimates are then made for the more promising alternatives that remain after the
preliminary estimates.
• Based on the results from the detailed estimate, a final decision is made whether to go ahead with
the construction of a plant.

This text focuses on the preliminary and study estimation classification based on a PFD as presented in
Chapter 1. This approach will provide estimates accurate in the range of +40% to –25%.

In this chapter, it is assumed that all processes considered are technically sound and attention is focused
on the economic estimation of capital costs. The technical aspects of processes will be considered in
later chapters.

7.2 Estimation of Purchased Equipment Costs

To obtain an estimate of the capital cost of a chemical plant, the costs associated with major plant
equipment must be known. For the presentation in this chapter, it is assumed that a PFD for the process is

available. This PFD is similar to the one discussed in detail in Chapter 1, which included material and
energy balances with each major piece of equipment identified, materials of construction selected, and the
size/capacity roughly estimated from conditions on the PFD. Additional PFDs and equipment summary
tables are given for several processes in Appendix B.

The most accurate estimate of the purchased cost of a piece of major equipment is provided by a current
price quote from a suitable vendor (a seller of equipment). The next best alternative is to use cost data on
previously purchased equipment of the same type. Another technique, sufficiently accurate for study and
preliminary cost estimates, utilizes summary graphs available for various types of common equipment.
This last technique is used for study estimates emphasized in this text and is discussed in detail in Section
7.3. Any cost data must be adjusted for any difference in unit capacity (see Section 7.2.1) and also for any
elapsed time since the cost data were generated (see Section 7.2.2).

7.2.1 Effect of Capacity on Purchased Equipment Cost

The most common simple relationship between the purchased cost and an attribute of the equipment
related to units of capacity is given by Equation 7.1.

(7.1)