Page 252 - Chemical engineering design

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CHEMICAL ENGINEERING
3. Product quality:
To maintain the product composition within the specified quality standards.
4. Cost:
To operate at the lowest production cost, commensurate with the other objectives.
These are not separate objectives and must be considered together. The order in which they
are listed is not meant to imply the precedence of any objective over another, other than
that of putting safety first. Product quality, production rate and the cost of production will
be dependent on sales requirements. For example, it may be a better strategy to produce
a better-quality product at a higher cost.
In a typical chemical processing plant these objectives are achieved by a combination
of automatic control, manual monitoring and laboratory analysis.

5.7.3. Automatic-control schemes
The detailed design and specification of the automatic control schemes for a large project
is usually done by specialists. The basic theory underlying the design and specification
of automatic control systems is covered in several texts: Coughanowr (1991), Shinskey
(1984) (1996) and Perry et al. (1997). The books by Murrill (1988) and Shinskey (1996)
cover many of the more practical aspects of process control system design, and are
recommended.
In this chapter only the first step in the specification of the control systems for a process
will be considered: the preparation of a preliminary scheme of instrumentation and control,
developed from the process flow-sheet. This can be drawn up by the process designer
based on his experience with similar plant and his critical assessment of the process
requirements. Many of the control loops will be conventional and a detailed analysis of
the system behaviour will not be needed, nor justified. Judgement, based on experience,
must be used to decide which systems are critical and need detailed analysis and design.
Some examples of typical (conventional) control systems used for the control of specific
process variables and unit operations are given in the next section, and can be used as a
guide in preparing preliminary instrumentation and control schemes.

Guide rules
The following procedure can be used when drawing up preliminary P and I diagrams:
1. Identify and draw in those control loops that are obviously needed for steady plant
operation, such as:
(a) level controls,
(b) flow controls,
(c) pressure controls,
(d) temperature controls.
2. Identify the key process variables that need to be controlled to achieve the specified
product quality. Include control loops using direct measurement of the controlled
variable, where possible; if not practicable, select a suitable dependent variable.
3. Identify and include those additional control loops required for safe operation, not
already covered in steps 1 and 2.