Functional process steps can be used for rapid investment cost for start-up time estimations. In this article I explain the use of functional process steps.
What are functional process steps?
Functional process steps are the steps that fulfill a single function in a process design. Examples of functional process steps are:
- heating
- mixing
- reaction
- distillation
- packaging
A function consists in general of one unit operation. If, however, several of the same unit operations fulfill a function, for instance a number of reactors in series, then that is counted as one functional process step. Process designs based on functional process steps can be quickly made and represented in block-flow diagrams. The definition of functional process steps is also very useful for rapid investment cost estimates and for rapid start-up time estimates of new commercial scale processes.
Rapid Investment Cost Estimate
For the rapid investment cost estimate Bridgwater derived a cost correlation based on hundreds of process projects. For process capacities of > 60 kton/year the investment cost is given by:
CDPC = 4.3 x 103 x N x (Q/S)0.675 It is valid for process capacities > 60 kton/year
CDPC = Capital investment of Process Inside battery limits ($)
N = Number of functional process steps
Q = Plant production capacity (ton/year)
S = Reactor single pass conversion fraction of Process Input
Bridgewater published this correlation in 1974. So, an inflation correction factor is needed to get a current estimate. The inflation correction factor from 1974 till 2024 is 5.4 [2].
For the start-up time for new commercial scale processes Merrow derived from a large data set of the Rand corporation, the following correlation [3].
tstart-up = 3.3. + 3.7 N – 3.2 F + S
The start-up time tstart-up starts when for the first time the real feed is put into the process and ends when steady state production of the product is reached. It is measured in months.
N is the number of new functional process steps in the process. And ‘new’ is defined as not applied before at commercial scale for that feed, or that product, or that catalyst.
F is the fraction of all process streams with known compositions. F ranges between 0 and. 1.
S is the factor for solids processing. If no solids are processed S = 0. S = 0.7 months if the product is a refined solid, such as a plastic. S = 10.8 months, if the feed is a raw solid.
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