design of control system for complete plants

8/18/2019 Design of Control System for Complete Plants

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Design of control system for complete

plants

The TEAM113CH0080 SAI SWAROOP MANDAL

113CH0081 SANJUKTA NAYAK

113CH0082 SIDHARTHA NAYAK

113CH0083 VAIBHAV NAYAK

113CH0084 ABHISHEK PADHY

113CH0086 SIDHANT PATRA113CH0087 ANKIT PATNAIK

113CH0088 JAYADEV PRADHAN

113CH0089 BIBHU PRASAD


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Designing control systems for complete chemical plants is the ultimategoal of a control designer. The problem associated with these situations isquite large and complex. It requires or involves large number oftheoretical and practical considerations such as quality of controlledresponse; stability; the safety of the operating plant; the reliability ofcontrol system; the range of control system and ease of startup,shutdown, or changeover; the ease of operation and the cost of thecontrol system.

As studied from the previous chapters of process control the variousequations which we simplify are mostly linear in nature. The difficultiesaggravated by the fact that chemical process are largely non-linear,imprecisely known, multivariable systems with many interactions.

The measurements and manipulations are limited to relatively smallnumber of variables, while control objectives may not be clearly stated or even known at the beginning of the control system design.

Process Design and Process Control


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Traditionally, one undertakes the design of the control system for achemical plant only after a process flowsheet has been synthesizedand designed or even constructed to a significant detail. This allowsthe control designer to know

1. What units are in the plant and their sizes?

2. How they are interconnected?

3. The range of operating conditions.

4. Possible disturbances, available measurements and manipulation.

5. What problem may arise during startup and shutdown of the plant?

The above information are crucial and of course necessary to designan effective control system for plant.

What if the design of plant is finalized?


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In these cases the controller is forced to work with a largely specifiedsystem.

The task is to design a control system that ensures satisfactory

operation of the whole plant. However, this may not always bepossible because of following reasons

1. The process doesn’t possess a sufficient no of manipulatedvariables.

2. Very strong interactions exist between the processing units.

3. It is not possible to cope up with all external disturbances.

4. The time lag(dead time) may be significant or the process gainsmay be too high or too low.

5. And if the process is inherently unstable.

From the previous points discussed, it is evidently clear that a certain degree of coordination and cooperation between process engineers and control designersis required.

Ideally, process designers should have a good exposure in process controlproblems and process control designers should have a good exposureunderstanding of process design aspects. It is close and intricate interplaybetween design and control that makes a chemical engineer the most suitableperson to undertake the design of a control system for chemical plant.

When the chemical process were simpler, with very few interactions with units,mostly serial in architecture and without strong requirements of operationaloptimality, the design of control system was much simpler. In situations of real livesnow, the picture is completely different. Rising costs in energy and raw materials,strong competition in market, improved safety and pollution standards and manymore such constraints have forced designers to construct complex plants that arehighly integrated, with many and strong interactions among the processing units.


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Example 1: Effect of intermediate storage tanks on process control

Referring to above diagram the following statements are given:

Consider the following process where the reactor effluent stream iscondensed and stored in large tank. Material from the tank is fed todistillation column where desired product is separated from unreactedreactant, which flows back to second storage tank before it is again fedback to the reactor. The two intermediate storage tanks decouple the twoprocessing units (i.e. reactor and distillation column ).

Thus any variations in the composition, temperature or flow of the reactor’seffluent stream are damped out in tank 1and don’t disturb operation ofthe column. Similarly, tank 2 absorbs any changes in the flow or thecomposition of columns bottom product, leaving the operation of the

reactor undisturbed. The decoupling between reactor and column allowsthe control designer to synthesize a control system for each unitindependently.

There are several drawbacks in design which includes reactor effluent iscooled and then it is heated up again, the two intermediate storage tanksincrease significantly the fixed capital expenditure.


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Example 2: Effect of feed effluent heat exchange on reactor control

Continued


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Example: Poor process designs leading to control problems.


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Continued Example

Hydroealkylation of Toluene plant toproduce benzene : A Case Study


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Material Balance control for the Hydroealkylation plant:

The purpose of the control system for hydroealkylation plant is toensure production of benzene with

1. The desired quality(99.9% by mole)

2. The desired rate

The product quality must meet with the desired specifications;otherwise it must be reprocessed or wasted. The part of control systemwhich ensures that the product quality is maintained at the desiredlevels is known as product quality control.

Although a plant is usually designed for nominal production rate, adesign tolerance is always incorporated because the marketconditions may require an increase or decrease in production level.

This is known as material balance control, because its purpose is todirect the control action in such a way as to make the inflows equal tothe outflows and achieve a new steady state material balance for theplant.


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Material Balance controllers for the Hydroealkylation Plant

Product quality control for the Hydroealkylation plantThe material balance control system is designed for a single purpose: to adjustthe plant’s production rate at the level dictated by the management and keep itat this level. In other words the material balance control is not designed tomaintain the operating variables in certain units at constant values (set points)against the changes in various disturbances.

The basic purpose of additional control system is to ensure the desired productquality. This can be accomplished by

1. Regulating the operation of benzene column against disturbances changesentering the column.

2. Cancelling the effects of disturbances changes in upstream units which whenleft unattended will propagate and affect product quality.


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Benzene Column control

Continued


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Continued

Comments on the control of design forcomplete plants


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Continued:


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The introduction of digital computers for process control has allowed theimplementation of optimizing control strategies in chemical plants. The usualmode of implementation is that of supervisory control. Fig 25.11(b) showssupervisory control implementation of an optimizing control strategy for the

simple process of 25.11(a).Optimizing control is used extensively today and its future looks even morepromising. It is particularly beneficial for large plants where even smallimprovements in the operating cost are multiplied by large productionthroughputs, thus leading to significant economic savings on yearly basis.

Normally there will be tremendous number of alternative control systems for acomplete plant. Initially the control designer uses for empirical arguments toeliminate the bad ones. Progressively he or she uses more and more complexanalysis for further screening and elimination of alternatives. The designer mayneed to resort to steady state or dynamic simulation of the plant and itscontrollers in order to evaluate the alternative control systems.

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design of control system for complete plants

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