过 程 控 制

1. Reasons for Automating a Chemical Process（ 1 ） The process, whether it is being carried out in bench-scale equipment, a pilot plant, or a full production scale plant, can be run continuously without attention from operators or chemical technicians. This reduces manpower requirements and therefore lowers labor costs.（ 2 ） The reduction in need for operating personnel results in an elimination or decrease in human errors.（ 3 ） Chosen adherence to optimum conditions results in an improvement in overall process quality.（ 4 ） Necessary adjustments to operation can be made from on central location often resulting in a reduction in the space requirements for the process unit.（ 5 ） Safety in operation is increased by providing warning of abnormal conditions and automatically taking corrective action. In addition, automatic control eliminates the need for personnel to be in the immediate vicinity of hazardous equipment or conditions.

Detecting device

Measuring device

Controlling device

Final control element

2. What is Automatic Control?

Elements contained in manual control system:

--Mercury bulb

--Mercury column and scale

--Observer

--rheostat

Elements contained in automatic control system?

(1) Flow(2) Pressure(3) Liquid Level(4) Composition Control

3. What is Controlled in a Chemical Process?

pH,redox potential氧化还原电势 ), conductivity,etc.

Component analysis

Chromatography(色谱分离法 ) 、 spectroscopy（光谱法）

Physical property analysis

density, boiling point, color, viscosity,etc.

Chemical propertiesanalysis

Control in one form or another is an essential part of any chemical engineering operation. In all processes there arises the necessity of keeping flows, pressures, temperatures, compositions, etc., within certain limits for reasons of safety or specification. Such control is most often accomplished simply by measuring the variable it is required to control (the controlled variable), comparing this measurement with value at which it is desired to maintain the controlled variable(the desired value or set point) and adjusting some further variable(the manipulated variable) which has a direct effect on the controlled variable until the desired value is obtained.

4. The Control Loop

A control system may be more simply

represented in the form of a block diagram.

This shows how information flows around the control

loop and the function of each constituent section.

Each component is represented by a block which

denotes the relationship between the variable

entering the block and the variable leaving.

Arrows denote the flow direction of information.

5. Block Diagram(方块图 )

It can be seen that the control loop is appropriate as

information passes around a closed loop of components.

This form of control is called closed-loop(闭环 ) or

feedback(反馈 ) (referring to the feed-back of information

from the controlled variable to the comparator).

The control loop shown in figure 6. 2 and 6.3 could

equally well consist of electronic(电子 ) or pneumatic(气动 ) components or a mixture of both. The choice of which

to use is dictated by consideration of cost, accuracy and

safety.

Although pneumatic mechanisms were used almost

universally for many years, electronic installations are

now rapidly gaining in popularity.

6. The Open-loop Control(开环控制 ) Another type of control is occasionally employed which does not require the feed-back of information concerning the controlled variable. This is termed feed-forward(前馈 ), predictive or open-loop(开环 ) control. A possible arrangement is shown in figure 6.4. It is assumed that the inlet water temperature remains constant. The heat input to the water is adjusted directly by measurement of the water flow rate. This method has the advantage of anticipating the effect on temperature of variations in water flow rate and that temperature will not have to change from its desired value before corrective action can be taken (as with the feedback arrangement). The difficulty is that in order to design such a predictive system it is necessary to determine first how the temperature at Y will respond to changes in both water and steam flow rates. This becomes a considerable problem with more complex systems.

Measuring element

ProcessControllerDesired value

Depending on how control loop time lags are compensated for

leads to the following classification of controllers.

On-off control action( 通 -断控制 ): a control mechanism having

only two discrete values of output, fully on or fully off.

Proportional control action(比例控制行为 ): the controller

output signal is proportional to the deviation（偏差） .

Proportional-plus-reset control action(比例复位控制 ): the

controller output signal is proportional to the time integral(积分 ) of the deviation.

Rate control action(速率控制 ): the controller output signal is

proportional to the rate of deviation change.

Unit 29 The Modes of Control Action (I)

This is the simplest form of automatic control

and is sometimes referred to as two-position

control. The term on-off is really a misnomer

（误称） , for it is possible to have two-position

control without having the positions either fully

on or completely off. However, since most two-

position control systems are on or off, the term

is normally applicable.

1. On-Off control

Some chemical processes cannot tolerate the variation and continuous cycling produced by on-off control. A smoother control action can be obtained with proportional control. In proportional control a fixed linear relationship exists between the value of the controlled variable and the position of the final control element.The proportional controller moves the final control element to a definite position for each value of the controlled variable. This mode of action is illustrated in figure 6.17.

2. Proportional Control

--Proportional band (比例带 )

In the proportional-plus-reset mode of control, reset is automatic. In this mode of control as soon as the controlled variable deviates above or below the set point (i.e., as soon as some offset develops), there is a gradual and automatic shift of the proportional band to bring the variable back to the set point. Thus while proportional control is limited to a single valve position for each value of the controlled variable, proportional-plus-reset control changes valve position to accommodate load changes.

3. Proportional-Plus-Reset Control

Many chemical processes contain multiple

resistances (阻抗 ) and capacitances (容抗 )

resulting in a sizable lag(迟滞 ) between the

time that a change occurs and the time that the

measured change is applied to the control

mechanism.

This problem can partially be overcome by

using a control action that is proportional to the

rate of change of the deviation rather than to

the magnitude of the deviation.

4. Rate Control—anticipatory action