Page 208 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Talbe 7.10 Relative Costs of Metals Using Carbon Steel as the Base Case

To account for the cost of different materials of construction, it is necessary to use the appropriate
material factor, F , in the bare module factor. This material factor is not simply the relative cost of the
M
material of interest to that of carbon steel. The reason is that the cost to produce a piece of equipment is
not directly proportional to the cost of the raw materials. For example, consider the cost of a process
vessel as discussed in the previous section. Just as the bare module cost was broken down into factors
relating to the purchased cost of the equipment (Tables 7.6 and 7.8), the purchased cost (or at least the

manufacturing cost) can be broken down into factors relating to the cost of manufacturing the equipment.
Many of these costs will be related to the size of the vessel that is in turn related to the vessel’s weight,
W vessel . An example of these costs is given in Table 7.11.

Talbe 7.11 Costs Associated with the Manufacture of a Process Vessel

From Table 7.11, it is clear that the cost of the vessel is proportional to its weight. Therefore, the cost
will be proportional to the vessel thickness, and thus the pressure factor derived in the previous section is
valid (or at least is a reasonably good approximation). The effect of different MOCs is connected to the
factor β RM . Clearly, as the raw material costs increase, the total manufacturing costs will not increase
proportionally to β RM . In other words, if material X is 10 times as expensive as carbon steel, a vessel

made from material X will be less than 10 times the cost of a similar vessel made from carbon steel. For
example, over the last 15 years, the cost of stainless steel has varied between 4.7 and 7.0 times the cost of
carbon steel [16]. However, the cost of a stainless steel process vessel has varied in the approximate
range of 2.3 to 3.5 times the cost of a carbon steel vessel for similar service.

Materials factors for the process equipment considered in this text are given in Appendix A, Tables
A.3–A.6, and Figures A.18 and A.19. These figures are constructed using averaged data from the