Page 342 - Design of Simple and Robust Process Plants

328 Chapter 8 Instrumentation, Automation of Operation and Control
8.4.7
Determination of Feasible Pairing Options in Respect of Plant Wide Control and
Unit Control

Different types of control functions can be recognized, all of which have their place
in the control design of the process plant. These can be divided into:

± Inventory control, mainly level and pressure control.
± Capacity control, which includes product distribution.
± Quality control for products.
± Constraint control, mostly executed at MBC level but also at basic control
level (see Figure 9.14 in Chapter 9) for floating capacity control.
± Economic control, like set points for: conversion control of reactors and qual-
ity set points for recycle streams. These are normally calculated at the optimi-
zation level and implemented at the basic control level.
The design of the control configuration reaches it critical point now, when CVs,
MVs, DVs and measurement variables have been preliminarily identified. The final
identification will take place during the design of the control configurations, as de-
tailed analysis might lead to alternative CVs or MVs to achieve the same control
objective. There are two separate points which must be addressed and are differen-
tiated as plant wide control and unit control.

8.4.7.1 Plant wide process control
This is extensively discussed by Luyben et al. (1998). Plant wide control approach is
also a step-wise procedure which in essence starts with the external (site) situation.
This places constraints on the control design, utility wise, as well as on the feed and
product sides. In a similar way, plant control design places constraints on unit con-
trol design which must be respected. The design steps as set out by Luyben et al.,
and which fit into this sequential step for selecting feasible loop pairings, are as
follows:
± Establish an energy management system. Crucial in this respect is to avoid
propagation of disturbance in the energy systems which are transferred to
the different units in the process. This can lead to total plant swings ± which
are to be avoided. The determination of operational conditions of the plant
utility levels and provisions for back-up supply are essential. It was concluded
previously that the rejection of disturbances outside the plant is a good solu-
tion, as most utility systems are designed to cope with fast load variations.
The impact and priority within the site load shedding systems need to be
reflected in the control design of the system.
± Set production rate includes also the required product distribution. The
production rate for batch plants is determined at the front end of the process,
which is designed for feed-forward control with some limited feed-back
actions. Recycles are collected and processed for next batches. For continuous
processes, the production rate might be set by a flow rate between reaction