Page 347 - Design of Simple and Robust Process Plants

8.4 Control Design 333

TC

LC

Fig. 8.16. Flash unit with basic control loops.

This expression is the constitutive part of the balance equation. The expression elim-
inates all the terms of the balance equation like the components terms, and only
leaves the essential terms for the operation. It shows for a flash that only three vari-
ables affect the rate of energy transfer between the entropy current and the flash
temperatures; these are the considered potential dominant variables. The I s flow
term can be manipulated, while the temperatures are the intensive variables. In this
example the temperature in the flash is dominant, as the up-front temperature is
not available for control in this scheme. The control scheme with the dominant vari-
ables is shown in Figure 8.16.
The following loop pairing rule was presented by evaluation of the power release
term for a unit:

Whenever a flow appears as an important energy carrier pair this flow with the corre-
sponding intensity belonging to the unit.

When this rule is applied to other thermal processes like a distillation column, the
energy carrier (heat duty of the reboiler) is paired with a corresponding intensity
(selected column temperature). Identical expressions for thermal processes can be
generated that are also applicable for single component irreversible reactions such
as A ® B that can easily be expressed as a thermal process (e.g., a fired heater). In a
fired heater, the loop pairing is the energy flow and a (intensity) process tempera-
ture.
For multi-component reactions the component production terms must be includ-
ed in the equation. This has been described extensively in the referred article of Tyr-
Øus with several general conclusions for irreversible and equilibrium reaction sys-
tems.
The methodology has demonstrated its usefulness by its application to a coupled
reactor system as a FCC unit. An entire process (Tennessee Eastman process) was
successfully studied with this methodology to design the dominant pairings.