Objective:
After reading this unit, you should able to Know what process control is.Know how many types of control configuration are.
Know the process control keywords.
Reading 1. What is process control?
A chemical plant is an arrangement of processing units (reactors, heat exchangers, pumps, distillation columns, absorbers, evaporators, tanks, etc.), integrated with one another in a systematic and rational manner. During its operation, a chemical plant must satisfy several requirements imposed by its designers and the general technical, economic, and social conditions in the presence of ever-changing external influences (disturbances). Among such requirements are: Safety, production specifications, environmental regulations, operational constraints and economics, to name a few.
The variables (flow rates, temperatures, pressures, concentrations, etc.) associated with a chemical process are divided into two groups:
Input variables. Which denote the effect of the surroundings on the chemical process.
Output variables. Which denote the effect of the process on the surroundings.
The input variables can be further classified into te following categories:
Manipulated (or adjustable) variables, if their values can be adjusted freely by the human operator or a conrol mechanism in order to keep the controlled variables at or near their set points.
Disturbances, if their values are not the result of adjustment by an operator or a control system.
The output variables are also clasified into the following categories:
Measured output variables, if their values are known by directly measuring them.
Unmeasured output variables, if they are not or cannot be measured directly.
Controlled variables (CVs): The process variables that are controlled. The desired value of a controlled variable is referred to as its set point. According to their direct measurability, the disturbances are classified into two categories: measured and unmeasured disturbances.
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Figure 9.1. Schematic of process control configurationTASK 1: MATCHING
Match a word or phrase in A with its meaning in B
1. The temperature in a room is maintained at 23 oC by the air conditioner.
A B
Manipulated variable 23oC
Controlled variable Outside temperature
Disturbance Win flow
Set point Temperature in the room
Electric power for air conditioner Door open times
3. A driver tries to maintain car on the proper lane.
A B
Manipulated variable Maintain car in proper lane.
Controlled variable Orientation of the front wheels
Disturbance Driver
Sensor Driver‘s arms/steering wheel
Set point (Control Objective) Location on the road
Controller Curve in road
Actuator Driver‘s eyes
4. Temperature heat exchanger
A B
Manipulated variable Outlet temperature of product stream
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Controlled variable Thermocouple on product stream
Disturbance Control valve on steam line
Sensor Changes in the inlet feed
temperature
Actuator Steam flow
Reading 2. Control configuration
Feedback control configuration: uses direct measurements of the controlled variables to adjust the values of the manipulated variables. The objective is to keeps the controlled variable at the desired levels (set points).
Inferential control configuration: uses secondary measurements (because the control variables cannot be measured) to adjust the values of the manipulated variables. The objective here is to keep the (unmeasured) controlled variables at the desired levels. One example is that, instead of measuring the vapor composition at the top of the column, which is difficult, we can measure the temperature at that column section.
Feedforward control configuration: uses direct measurement of the disturbances to adjust the values of the manipulated variables.
Cascade control configuration: refers to a process control strategy in which a process variable is controlled by manipulating the setpoint of a related process variable. In this particular strategy, the variable of interest is controlled by two control loops working in tandem. In the chemical process industries, a cascade control system is often used to reduce the effect of disturbances and upsets on the primary control objective.
A cascade control loop consists of a primary loop and a secondary loop. These loops might also be referred to as the outer loop and inner loop, respectively. The primary loop provides the secondary loop with a setpoint, or target, for a process related to the primary control objective. The primary loop is sometimes known as the master loop because it provides a setpoint that the secondary loop, or slave loop, must follow.
65 TASK 2. Gap Filling
Base on the reading above, write down the control configuration of the following scheme
…………. control configuration ………control configuration
………….control configuration
(Distillation column) ………….control configuration
Reading 3
Feedback control
Consider the generalized process shown in the below figure. It has an output y, a potential disturbance d, and an available manipulated variable m.
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Figure 9.2. (a) process and (b) corresponding control loop
The disturbance d (also known as load or process load or process load) changes in an unpredictable manner and our control objective is to keep the value of the output y at desired levels. A feedback control action takes the following steps:
1. Measures the value of the output (flow, pressure, liquid level, temperature, composition) using the appropriate measuring device. Let ym be the value indicated by the measuring sensor.
2. Compares the indicated value ym the the desired value ysp (set point) of the output. Let the deviation (error) be ysp ym
3. The value of the deviation is supplied to the main controller. The controller in turn changes the value of the manipulated variable m in such a way as to reduce the magnitude of the deviation . Usually, the controller does not affect the manipulated variable directly but through another device (usually a control valve), known as the final control element.
The Figure 9.2 b summarizes pictorially the foregoing three steps.
The system in Figure 9.2a is known as open loop, in contrast to the feedback-controlled system of Figure 9.2 b, which is called closed loop. Also, when the value d or m changes, the response of the first is call open-loop response which that of the second is the loop response. The origin of the term closed-loop is evident from Figure 9.2 b.
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Figure 9.3. Simplified representation of feedback loops
All the example above indicate that the basic hardware components of the feedback control loop are the following:
1. Process: The material equipments along the physical or chemical operations which take place (tanks, heat exchangers, reactors, separators, etc.).
2. Measuring instruments of sensors: for example, thermocouples (for temperature), bellows, or diaphragms (for pressure or liquid level), orifice plates (for flow), gas chromatographs or various types of spectroscopic analyzers for composition), and so on.
3. Transmission lines: used to carry the measurement signal from the sensor to the controller and the control signal from the controller to the final control element. These lines can be either pneumatic (compressed air or liquid) or electrical.
4. Controller: also includes the function of the comparator. This is the unit with logic that decides by how much to change the value of the manipulated variable. It requires the specification of the desired value (set point).
5. Final control element usually, a control valve or variable-speed metering pump. This is the device that receives the control signal from the controller and implements it by physically adjusting the value of the manipulated variable.
Each of the elements above should be viewed as a physical system with an input and an output.
Consequently, their behavior can be described by a differential equation or equivalently by a transfer function. In the following sections of this chapter we take a closer look at the dynamics of these hardware elements
Cascade control
Cascade control can improve control system performance over single-loop control whenever either: (1) disturbances affect a measurable intermediate or secondary process output that directly affects the primary process output that we wish to control; or (2) the gain of the secondary process, including the actuator, is nonlinear. In the first case, a cascade control system can limit the effect of the disturbances entering the secondary variable on the primary output. In the second case, a cascade control system can limit the effect
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of actuator or secondary process gain variations on the control system performance. Such gain variations usually arise from changes in operating point due to setpoint changes or sustained disturbances.
A typical candidate for cascade control is the shell and tube heat exchanger of Figure 7.4
Figure 9.4. Cascade control of effluent temperature via steam flow control.
If feed flow and temperature variations are significant, then these disturbances can be at least partially compensated by using the exchanger pressure rather than the steam flown as the secondary variable in a cascade loop, as shown in Figure 9.5
Figure 9.5. Cascade control of effluent temperature via shell side pressure control
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TASK 3: Please list all the control configurations and then the controlled variables and magnitude variables.
Figure 9.6 …… ….Distillation column Figure 9.7 …………..Reactor with steam jacket
Figure 9.8……… Evaporator control strategy
Figure 9.9………Mixing process
TASK 4: How to find good keywords
Find the keywords in reading paragraphs 2 and 3
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TASK 6: SummaryIn about 5-8 sentences, summarize the main idea in paragraphs 2 and 3 TASK 11: Glossary
Search your knowledge, look up your dictionary, internet or ask your instructor to clarify the definition and Vietnamese meaning of the following terminologies.
No Terminology Definition Vietnamese
1 absorbers
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30 tanks31 thermocouple 32 valve