PROCESSES: BASICS OF PROCESS DESIGN - Biomass as a Sustainable Energy Source for the Future - page 224

5. Time Scales and Dynamics
The dynamic behavior of a process must match the dynamics of the supply
chain and be robust against external disturbances. If demand and supply change
with, e.g., a typical time constant of one week, a process should be able to
respond to these changes within a much shorter time (say, 1 day). For processes
delivering power to the power grid, the response time to load changes can be
much shorter. It is important to know the dynamics of a process unit, particu-
larly the response times of its outputs to changes in inputs as well as the occur-
rence of any inverse responses. In addition, also from a safety point of view, it is
essential to have information on the (dynamic) stability of a process unit. For
example, how far are normal operating conditions removed from runaway con-
ditions, where positive feedback mechanisms in the unit, such as heat release
by chemical reactions, may cause temperature and pressure to escalate to dan-
gerous levels?

6. Availability
Availability is the fraction of time over a specified time horizon a process unit is
capable of fulfilling its nominal processing functions. This availability depends
both on the reliability of the equipment (mean time to failure) and the mainte-
nance efforts to repair a unit that broke down (mean time to repair). The reliability
of a unit or of its components must be known to determine the availability of the
entire process. Acceptance in a design of new process equipment with improved
processing characteristics also requires sufficient reliability.

7.11 APPLICATION POTENTIAL

Process design requires understanding domain contents (e.g., physical, chemical, and
equipment knowledge) as well as a procedure to structure the design decision making.
The next chapters in this topic offer mainly domain contents. However, the ”

section at the end of some chapters will also ask questions about some underlying design
decisions and trade-offs in developing process unit concept(s), in accordance with the
design approach presented in this chapter. For instance, Chapter 17 on the use of syngas
for the production of synthetic transportation fuels presents design related questions on
FT synthesis.

“ problems

CHAPTER SUMMARY AND STUDY GUIDE

This chapter introduces a top-down hierarchical, multilevel approach to decision mak-
ing in process design. The design approach starts with the way of embedding a process
into a supply chain, as represented by an input

output diagram. Then, the process can
be split in subprocesses with intermediate feeds and products, and for each subpro-
cess, a network of processing functions is generated and presented in a function block
diagram. The next design step is to turn the network with processing functions into a

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