Page 312 - Design of Simple and Robust Process Plants

298 Chapter 8 Instrumentation, Automation of Operation and Control
The selection of specifically the intermediate states of the units/sections should fit
in a start-up/shut down sequence of the overall process. In the process plant, units
(or combinations of units) can be identified which can run independently from over-
all processing. The presence of simultaneous inverse operations within system
boundaries makes it possible to create a stationary state, like heating versus cooling
and separation and re-mixing (Fusillo and Powers, 1987, 1988). Examples include a
refrigeration unit which has an evaporator and a condenser in the unit, and a distil-
lation column operating at total reflux. It was Verwijs et al. (1995) who introduced
the terms ªreversibleº and ªirreversibleº unit operations.
A reversible unit operation is defined as ª¼ a process system that can be operated
stand alone, without any process streams fed into or exiting subsystem due to the
presence of inverse operationº. This opens the possibility of conditioning a process
system during start-up. Conditioning in this aspect should be understood as bring-
ing the process system or unit close to its planned operational conditions to accom-
modate a controlled start-up. As a pre-requisite, Verwijs et al. (1996) mentioned the
simultaneous operation of the inverse process functions which can be achieved by
recycling material. An example to be mentioned here is a distillation column where
the top and bottom products are recycled to the feed, while there is no fresh feed.
Another example is an absorber±stripper combination where the solvent is recycled
over the absorber and stripper with its heaters and coolers active, while there is no
feed stream fed to this section. A reactor system for an equilibrium reaction can
often be operated as a reversible unit by recycling the outlet streams.
Irreversible unit operations are started up by adding feed into the system, where-
upon the unit has to start. An example of such a unit is an irreversible reaction
system as shown by Verwijs et al. (1992) in an overall flowsheet for an exothermal
hydration reaction (Figure 8.4). Other examples of irreversible operations next to
irreversible reactions are drying filtering.
By distinguishing between reversible and irreversible units operations, a guide
can be made for planning the sequential order to start a process plant. Reversible
units are put into operation first.
Before the overall sequential order for start-up/stop of a process is discussed, the
start of a reversible and irreversible unit operations are discussed.

8.3.2.1 Start-up of a reversible unit operation
The reversible unit selected is a distillation column of which start-up is described
step-wise.

1. Fill the distillation unit with normal feed from the feed point.
2. Take the top condenser in operation and put the reflux level loop in operation
(total reflux situation).
3. Start the reboiler and build up the level in the reflux drum at the expense of
level in the bottom. The loss of level in the bottom is compensated by adding
some more feed.
4. Bring the unit to its operational conditions with at least one product stream
(preferably the most critical) at its required quality.