Page 43 - Design of Simple and Robust Process Plants

2.2 The Level of Complexity 27

DOF DOF DOF

U.O. U.O. U.O.
A B C
U.O. = UNIT OPERATION
DOF = DEGREES OF FREEDOM
Fig. 2.4. Sequential operation.

DOF
DOF U.O.
B

U.O.
A DOF

U.O.
C
U.O. = UNIT OPERATION
DOF = DEGREES OF FREEDOM
Fig. 2.5. Sequential operation with interaction.

In the example of the distillation column the disturbances are introduced through
feed stream, heating and cooling media. Interaction is inherently part of the distilla-
tion system as the quality controllers of the products impact each other.
Based on the overall definition of complexity, a specific complexity can be com-
posed for a unit operation in a chemical process plant.
The level of complexity, C, of a unit in a chemical process is defined as a function of:
. M, the number of equipment accessible by the operator;
. N, the number of DOFs, including manual/actuated valves/switches and set
points of control loops;
. O, the number of measurement readings;
. P, the number of input and output streams, including energy streams;
. Q, the interaction in the unit requiring an operator intervention; and
. R, the number of external disturbances (for the unit) asking for action from
an operator.
Note: For Q, under interaction we understand that if, an operator manipulation is per-
formed to achieve a certain change in a controlled variable, at the same time another
controlled variable is strongly influenced that would require another operator manip-
ulation to offset this change. In case the interaction is properly de-coupled in a model-
based controller, it will no longer be regarded as an interaction discussion.
In formulae form: C unit = f (M) (N) (O) (P) (Q) (R), (2)