Chemical Process Dynamics and Control Chapter 1 Lecture Notes

l o r t n o C & n o i t a t n e m u r t s n I , s c i m a n y D s s e c o r P l a c i m e h C

Introduction to Chemical Process Dynamics, Instrumentation & Control


Chapter Objectives End of this chapter, you should be able to: 1.

Under nders stand tand the the rol role of of pro proces cess dy dynami amics an and control in industry

2.

Understand general concepts

3.

Classify va variables

4.

Under nders stand tand the the purp purpos ose e of proce roces ss cont contro roll

5.

Under nders stand tand contr ontrol ol aspec spects ts of comple mplete te chemical plant

6.

Under nders stand tand har hardw dwa are for for proc rocess ess con contr trol ol sys yste tem m


Role of process dynamics and control in industry Illustration with examples • Example 1 – 1 – a a simple process where dynamic response is important • Example 2 – 2 – use use of a single feedback controller • Example 3 – 3 – simple simple but typical chemical engineering plant


Example 1 – A gravity-flow tank Under steady state conditions, the flow rate out of the tank must equal the flow rate into the tank. What would happen dynamically if we changed F o? How will h(t ) and F (t ) vary will time?


Example 2 – Heat Exchanger

We want to control the temperature of oil leaving the heat exchanger.


How to control? A thermocouple is inserted in a thermowell in the exit oil pipe. Thermocouple wires are connected to a “temperature transmitter” that converts the millivolt output into a 4- to 20 mA signal. This signal sent to a temperature controller. The temperature controller opens the steam valve if more steam is needed or closes it a little if the temperature is too high.


Components of control loop • A sensor • A transmitter • A controller • A final control element Process control deal with: • What type of controller to be used? • How it should be “tuned”?


Example 3 - A typical chemical plant


Concepts of Process Control Another simple example:


Block diagram


Need for control Performance requirements for process plants have become increasingly difficult to satisfy. Key factors for tightening product quality specifications: • Stronger competition • Rapidly changing economic conditions • Tough environmental and safety regulations • Modern plants are complex and highly integrated • It is difficult to prevent disturbances from propagating from one unit to other interconnected units.

Process control has become increasingly important due to increased importance on safe and efficient plant operation.


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The term process dynamics refer to unsteady state (or transient) behavior. Dynamic studies provide us the behavior of the process under unsteady-state conditions Gain knowledge about the process behavior.


Objectives of Process Control • Maintain a process at the desired operating conditions, safely and efficiently • Satisfy product quality and environmental requirements


Process control applications • Large-scale integrated processing plants such as oil refineries or ethylene plants require thousands of process variables such as temperature, pressure, flow, level and compositions are measured and controlled. • Large number of process variables, mainly flow rates, can be manipulated. • Feedback control systems compare measurements with their desired values and then adjust the manipulated variables accordingly.


Representative process control problems • Foundation of process control is process understanding. What is a process? • The conversion of feed materials to useful products using chemical and physical operations – PROCESS. • Common processes can be continuous, batch or semi-batch.


Continuous Processes


Tubular Heat Exchanger

Control problem: The exit temperature of the process fluid is controlled by manipulating the cooling water flow rate. Disturbances: Variations in the inlet temperatures and process fluid flow rate.


Continuous stirred tank reactor (CSTR)

Control problem: If the reaction is highly exothermic, it is necessary to control the reactor temperature by manipulating the flow rate of the coolant in a jacket or cooling coil. Disturbances: The feed conditions (composition, flow rate, and temperature).


Thermal cracking furnace Control Problem: The furnace temperature and amount of excess air in the flue gas to be controlled by manipulating the fuel flow rate and the fuel/air ratio. Disturbances: The crude oil composition and the heating quality of the fuel.


Multi-component distillation column

Control Problem: Distillate composition can be controlled by adjusting the reflux flow rate or the distillate flow rate. Disturbances: The feed conditions


Process variables Three important types: (Control Terminology) 1. Controlled variables - these are the variables which quantify the performance or quality of the final product, which are also called output variables. 2. Manipulated variables - these input variables are adjusted dynamically to keep the controlled variables at their set-points. 3. Disturbance variables - these are also called "load" variables and represent input variables that can cause the controlled variables to deviate from their respective set points.


Process variables • Specification of controlled variables, manipulated variables and disturbance variables is a critical step in developing a control system


Batch and semi-batch processes


Control problems • Batch or semi-batch reactor: The reactor temperature is controlled by manipulating a coolant flow rate. • Batch digester: The end point of the chemical reaction is indicated by Kappa number, a measure of lignin content. It is controlled to a desired value by adjusting the digester temperature, pressure, and/or cycle time. • Plasma etcher: The unwanted material on a layer of a microelectronics circuit is selectively removed by chemical reactions. The temperature, pressure and flow rates of etching gases to the reactor are controlled by adjusting electrical heaters and control valves.


Control problems • Kidney dialysis unit: The blood flow rate is maintained by a pump, and “ambient conditions”, such as temperature of the unit, are controlled by adjusting a flow rate.


Control Terminology(2) • set-point change - implementing a change in the operating conditions. The set-point signal is changed and the manipulated variable is adjusted appropriately to achieve the new operating conditions. Also called servomechanism (or "servo") control. • disturbance change - the process transient behavior when a disturbance enters, also called regulatory control or load change. A control system should be able to return each controlled variable back to its set-point.


Il l u s t r a t i v e E x a m p l e: B l en d i n g s y s t em

Notation:

Assumptions:

• w1, w2 and w are mass flow rates

• w1 is constant • x2 = const. = 1 (stream 2 is pure A

• x1, x2 and x are mass • Perfect mixing in the tank fractions of component A


B l en d in g s y s t em Control Objective:

Keep x at a desired value (or “set point”) xsp, despite variations in x1(t ). Flow rate w2 can be adjusted for this purpose. Terminology:

• Controlled variable (or “output variable”): x • Manipulated variable (or “input variable”): w2 • Disturbance variable (or “load variable”): x1 • Design Questi on What value of w2 is required to have x  x sp ?


Overall balance:

0  w1  w2  w

(1-1)

Component A balance:

w1x1  w2 x2  wx  0

(1-2)

(The overbars denote nominal steady-state design values) At the design conditions, x  x sp . Substitute in Eq.1-2, x  x sp and x2 for w2 : xSP  x1 w2  w1 1  xSP

 1, then solve Eq. 1-2 (1-3)

Equation 1-3 is the design equation for the blending system.


• If our assumptions are correct, then this value of w2 will keep x at x sp . • But what if conditions change?

Control Question. Suppose that the inlet concentration x1 changes with time. How can we ensure that x remains at or near the set point x sp ? As a specific example, if x1  x1 and w2  w2 , then x > xSP .


Some Possible Control Strategies Method 1. Measure x and adjust w2. • Intuitively, if x is too high, we should reduce w2; • Manual control vs. automatic control • Proportional feedback control law

w2  t   w2  Kc  xSP  x t  

(1-4)

• K c is called the controller gain • w2(t ) and x(t ) denote variables that change with time t

• The change in the flow rate, w2  t   w2 , is proportional to the deviation from the set point, xSP – x(t ).


Control Method 1


Method 2 M easur e x and adju st w 1 2

• Thus, if x1 is greater than x1 , we would decrease w2 so that w2  w2 . • One approach : Consider Eq. (1-3) and replace x1 and

w2 with x1(t ) and w2(t ) to get a control law: w2  t   w1

xSP  x1  t 

1  xSP

(1-5)

• Because Eq. (1-3) applies only at steady state, it is not clear how effective the control law in (1-5) will be for transient conditions.


Control Method 2

Method 3. Measure x 1 and x, adjust w 2 . • This approach is a combination of Methods 1 and 2.


Control Method 4 Use a larger tank. • If a larger tank is used, fluctuations in x 1 will tend to be damped out due to the larger capacitance of the tank contents. • However, a larger tank means an increased capital cost.


Classification of Control Strategies Table. 1.1 Control Strategies for the Blending System Method

Measured Variable

Manipulated Variable

Category

1

x

w2

FB

2

x1

w2

FF

3

x1 and x

w2

FF/FB

4

-

-

Design change


Feedback Control • Distinguishing feature: measure the controlled variable. • It is important to make a distinction between negative feedback and positive feedback . Engineering Usage vs. Social Sciences Advantages: • Corrective action is taken regardless of the source of the disturbances. • Reduces sensitivity of the controlled variable to disturbances and changes in the process.


Feedback Control Disadvantages: • No corrective action occurs until after the disturbance has upset the process, that is, until after x differs from x sp. • Very oscillatory responses, or even instability…


Feedforward Control Distinguishing feature: measure a disturbance variable Advantage: • Correct for disturbance before it upsets the process. Disadvantage: • Must be able to measure the disturbance • No corrective action for unmeasured disturbances


Justification of Process Control Specific Objectives of Control • Increased product throughput • Increased yield of higher valued products • Decreased energy consumption • Decreased pollution • Decreased off-spec product • Increased Safety • Extended life of equipment • Improved Operability • Decreased production labor


Economic Incentives - Advanced Control


Hierarchy of process control activities (days-months)

(hours-days)

(minutes-hours)

(seconds-minutes )

5. Pl anning and Scheduling

4. Real-Time Optimization

3b. Mu lti variab le and Cons traint Control

3a. Regulatory Control

(< 1 second )

2. Safety, Environment and Equipment Protection

(< 1 second )

1. Measurement and Actuation

Process


Major steps in control system development

Chemical Process Dynamics and Control Chapter 1 Lecture Notes

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