Page 42 - Design of Simple and Robust Process Plants

26 Chapter 2 Simple and Robust Plant Design
A system is complex when it is built up of a plurality of interacting elements of a variety
of kinds, in such a way that in the holistic result no evidence can be traced of the char-
acteristics of single elements.

The factors that determine complexity are (Scuricini, 1988; Wieringa and Stassen,
1993):

. the number of components;
. the number of interconnections;
. the number and level of interactions; and
. the variety of components.

From an operational perspective can be added:
. the number of measurement readings;
. the DOFs for the operator; and
. the surveyability of the state of the system.
The above are explained by example of a unit operations which have as their objec-
tive the achievement of a certain reaction or separation. These operations are com-
posed of a set of components. For example, a distillation column often has a tower,
reboiler, condenser, reflux drum, and pumps, with installed spares for feed, bottom
and reflux/distillate flows next to its piping and instruments. The total number of
equipment pieces can be 10 ± or even more if we add cross-exchangers, additional
reboilers, and vent condenser. With regard to the design, we already have quite a
few different components, with several interconnections adding to the complexity.
For a standard distillation in its operational step there are six controlled variables,
all of which are DOFs: feed, pressure, top and bottom specifications and the levels
in the reflux drum and the bottom of the tower. Depending on the control strategy,
each controlled variable will have a selected manipulated variable, which include set-
point settings. The total number of manipulated variables, which include manual
valves, for start-up/shut-down and exchange of pumps is much greater. The total
number of DOFs for the operator will add to the complexity.
A process plant is a set of unit operations connected to each other. A simple pro-
cess plant can have sequential unit operations (Figure 2.4) where the flow goes from
unit A to unit B, and successively to unit C. This might be compared with the toy
train that a child play with: as more and more carriages are connected to the train it
does not add to the complexity of the operation, because for the train driver the
operation is still the same. The sequential operation might be compared with the
situation shown in Figure 2.5, where the same main flow line exists from unit A to
B to C. Now, there are also a number of cross-connections between the different
units, and clearly these interconnections will add to the complexity. All these con-
nections are a source for disturbance and interaction. From the operation perspec-
tive the DOFs increase, but so too does the number of disturbances and interaction
increase for each unit. In the sequential operation example there were only distur-
bances from the upstream and downstream units.