CHAPTER-one.pptx, introduction to process dynamics and control

introduction to process dynamics and control for chemical engineering includes; general introduction and classification of variables

1. CHAPTER-1 Introduction T o Process Dynamics and Control By: Tafere A.

2. Outline INTRODUCTION Process control Why is Control necessary? Example of controlled process Classification of variables in chemical process Elements of control system 2

3. Course Description This course combines the mathematical , physical and chemical concepts for application to process simulation and control . This is an introductory part for process control design and analysis. Whenever appropriate , MATLAB is used to demonstrate the behavior of the control system. 3

4. Process dynamics model The two main subjects of this course are process dynamics model and statical model . A static model is one, which is developed based on the steady state information , in which; - Nothing changes with time . Are typically represented with algebraic equation . A dynamic model; variables change with times (transient process behavior). Described by differential equations . So , control systems are needed to handle such changes in the process ( variable change ). 4

5. Introduction In any industrial plant the aim is : To produce standard and high quality products and sell them at prices which make profit . These purposes can be achieved in a successfully designed and controlled processes . The primary objective of process control is: To maintain a process at the desired operating conditions , safely and efficiently , while satisfying environmental and product quality requirements. Proper application of process control can actually improve the safety and profitability of a process. The subject of process control is : Concerned with how to achieve these goals . 5

6. Objectives The Operational Objectives of process controls are:- Safety Production Specifications Environmental Regulations Operational Constraints Economics Optimization : Combination of several variables together with most suitable conditions. Optimum conditions are important for; Continuity Quality Economics of processes. “main variable” 6

7. What is a process ? A process - is an operation or series of operations that uses resources to transform inputs into outputs on fluid or solid materials during which the materials are placed in a more useful state . “A process - is a series of operations involving the physical , chemical , or biological transformation of an input ( feed materials) to products for the purpose of achieving a desired product material .” In practice, the term process tends to be used for both the; Processing operation and Processing equipment . 7

8. Types of Chemical Processes – Batch Process 8

9. Types of Chemical Processes – Continuous Process 9

10. Con’t… The objective of a process is: To convert certain raw materials ( input feedstock ) into desired products (output) using available sources of energy in the most economical way. Many external and internal conditions affect the performance of a process. These conditions may be expressed in terms of process variables such as, temperature , pressure , flow , liquid level , weight, volume etc . 10

11. What is control ? The term Control means; Measuring the value of the controlled variable and applying the control signal to the system; To correct or limit deviation of the measured value from a desired value . Methods to force parameters in the environment to have specific values . It is very important in process industry to use control to operate processes in; Energy and raw materials are utilized safely , efficiently and economically . Chemical engineers need to master this subject in order to be able to d esign and operate the plants efficiently . 11

12. Why is Control necessary? Control - is necessary because during its operation , a chemical plant must satisfy several requirements enforced by its designers and the general technical , economic , and social conditions in the presence of ever changing external influences ( disturbances ). Such requirements are the following: Safety Production specifications Environmental regulations Operational constraints Economics 12

13. Safety The safe operation of a chemical process is a primary requirement for the well-being of the people in the plant and for its continued contribution to the economic development . Thus the operating pressures , temperatures , concentration of chemicals and so on should always be within allowable limits . 13

14. Production specifications A plant should produce the desired amounts and quality of the final products . For example; we may require the production of 2 million pounds of ethylene per day, of 99.5% purity . Therefore , a control system is needed to ensure that the production level and the purity specifications are satisfied . Product certification procedures (e.g., ISO9000) are used to guarantee product quality and place a large emphasis on process control . http ://www.iso.ch/iso/en/ISOOnline.openerpage 14

15. Environmental regulations Various federal and state laws may specify that the temperatures , concentrations of chemicals and flow rates of the effluents from a plant be within certain limits . Such regulations exist, for example on the amounts of SO 2 that a plant can eject to the atmosphere , and on the quality of the water returned to a river or lake . 15

16. Operational constraints The various types of equipment's used in a chemical plant have constraints inherent to their operation . Constraints should be satisfied throughout the operation of a plant . For example; Pumps must maintain a certain net positive suction head ; Tanks should not overflow or go dry Distillation columns should not be flooded The temperature in a catalytic reactor should not exceed an upper limit , since the catalyst will be destroyed . Control systems are needed to satisfy these operational constraints . 16

17. Economics The operation of a plant must conform with the market conditions , that is the availability of raw materials and the demand of the final products . Furthermore, it should be as economical as possible in its utilization of; Raw materials Energy Capital and Human labor. Consequently, it is required that the operating conditions are controlled at given optimum levels of minimum operating cost , maximum profit and so on . 17

18. Why is control necessary? All this requirements need for continuous monitoring for the operation of a chemical plant and external intervention ( control ); To guarantee the satisfaction of the operational objectives . To optimize operations. To interfere with situations when an unusual or dangerous situation occurs. 18

19. How is control done A process Control is accomplished through a rational arrangement or Can be controlled either by ( human beings manually ) and by necessary instrumentation (automatically) . Equipment ( measuring devices , valves , controllers , computers ) and; Human intervention ( plant designers , plant operators ), which together constitute a control system . 19

20. What is Process control Process control : is the action of monitoring ; changing of the process parameters , technology and equipment’s based on the results of process output . A process control is the act of controlling a final control element to change the manipulated variable to maintain the process variable at a desired set point . In process control , the basic objective is: To regulate the value of some quantity of interest at some desired value regardless of external Influences . To regulate means to maintain that quantity at some desired value ( reference value or set point ) regardless of external influences. 20

21. Con’t… Process control: Required to maintain safe operations , quality products and Profit. Safety: - The primary purpose of process control system . - Personnel safety , environmental safety and equipment safety . Quality: - Process control systems are central to maintaining product quality . Profit: When safety and quality concerns are met , process control objectives can be focused on profit . 21

22. Why do we need process control? Structure of chemical process plant is very complex. Any chemical plant consist of various process units which are inter connected with one another in systematic manner. Main objective of any plant is; To convert certain raw materials into desired product using available sources of energy. Other objective:- Safety , product specification , environmental regulations , operation constraints , economics . These all parameters are control by arrangement of various equipment like measuring devices , valves , controller . 22

23. Process control is the tool that enables manufacturers : To keep their operations running within specified limits and To set more precise limits to maximize profitability , ensure quality and safety . Importance of process control are; To Reduce variability/maintain product quality To help processes operate efficiently/resourcefully To ensure the safe operation of processes (loss prevention) To meet environmental regulations ( discharge). To meet operational constraints inherent to the operation of equipment's used in a chemical plant ( over flow, dry, flooded ). Economics ( Minimum operating cost and maximum profits ). NEEDS ( IMPORTANCE ) OF PROCESS CONTROL 23

24. Con’t … Increase productivity/Production rates. Increase stability Optimize the performance Minimize the influence of External Disturbance. Safety Reduce energy So, the primary objective of process control is; To maintain a process at the desired operating conditions , safely and efficiently , while satisfying environmental and product quality requirements. 24

25. How does a control system fulfill the above needs? 25

26. Reduce (Suppressing) the influence of external disturbances. Promote (Ensuring) the stability of a chemical process, and Enhance (Optimizing) the performance of a chemical process . A control system can meet the above mentioned process operation ( operational objectives of a process control ) by any combination of the following: 26

27. 1. Suppress the influence of external disturbances Suppressing the influence of external disturbances on a process is; The most common objective of a controller in a chemical plant . Such disturbances denote the effect that the surroundings (external world) have on a reactor , separator , heat exchanger , compressor , etc., and; To introduce a control mechanism that will make the proper changes on the process to cancel the negative impact that disturbances may have on the desired operation of a chemical plant. 27

28. Objectives : Achieve Set-point T = T s h = h s Fi - is flow rate ( /min) Ti- is the inlet temperature ( O F), of entering liquid into the tank. Fs - is the steam mass flowrate in lb/min used to heat the liquid F, T = the flow rate outgoing liquid and temperature of the stream leaving the tank respectively. The tank is considered to be well-stirred , ( temperature of liquid in the tank is uniform and is equal to the temperature of the effluent ). After reaching steady-state from start-up, Possible disturbances include: Changes in the feed flowrate, Fi Changes in feed temperature Ti Changes in ambient temperature How to achieve the objective?   Suppressing the effect of disturbances Consider the tank heater system shown in Figure; Cause changes in F, T. 28

29. Con’t… The control Objectives of the stirred tank heater are to : To keep the effluent temperature T at a desired value Ts and To keep the volume of the liquid in the tank at a desired value Vs . => Control action is needed to keep T and V at the desired values . 29

30. 1. To maintain the temperature of effluent ‘T’ at desired temperature ‘Ts’ The operation of the heater is disturbed by external factors such as; Changes in the feed flow rate Fi and temperature Ti . Or; If nothing changed , then after attaining T=Ts and V=Vs, We could leave the system alone without any supervision and control . 30

31. 1. To maintain the temperature of effluent ‘T’ at desired temperature ‘Ts ’. A thermocouple measures the temperature T of the liquid in the tank. Then T is compared with the desired value T s , yielding a deviation, ε = T s – T . The value of the deviation ε is sent to a control mechanism, which decides what must be done in order for the temperature T to return back to the desired value T . 31

32. 1. To maintain the temperature of effluent ‘T’ at desired temperature ‘Ts’ If ε > 0 , which implies that T < Ts , the controller opens the steam valve, so that more heat can be supplied . On the contrary, the controller closes the steam valve when, ε < 0 , or T > Ts . It is clear that when T = Ts (i.e., ε = 0), then, the controller does nothing . 32

33. A possible control configuration This control system is called Feedback control because: The variable of direct importance (T in this case) is measured. Corrective action is taken after the effect of the disturbance has been felt by the system. The desired value is called the Set Point ( decided or set based on process requirement ). The temperature of the liquid, T, is measured. T is compared with the desired value Ts . The difference called the error ( e = –T) is sent to a controller . The controller takes a corrective action based on the error.   33

34. Conclusion; Measure T Compare measured T with T s Compute error : e = T s - T e > 0; T s > T (increase F st ) e < 0; T s < T (reduce F st ) Fig. Feedback ,control in a stirred tank heater 34

35. Alternative configuration for the temperature control In Feedforward control: Feed forward control does not wait until the effects of the disturbances has been felt by the system , but acts appropriately before the external disturbance affects the system anticipating what its effect will be. We realize that we can use a different control arrangement to maintain T= Ts, when , changes .   35

36. Example-2: Objective:- To keep the volume at its set point or; The liquid level h s we measure the level of the liquid in the tank and we open or close the effluent flow rate . Therefore; h is controlled output F is manipulated variable Fi and Ti are disturbance inputs . 36

37. II. To maintain the height of liquid ‘h’ in the tank at desired level ‘ hs ’ In Figure we see a control action to keep h = hs, when Ti or Fi, changes . So that tank will not overflow or go dry . A level measuring device measures the height h of the liquid in the tank. Then, h is compared with the desired value , yielding a deviation; ε = h .   37

38. II. To maintain the height of liquid ‘h’ in the tank at desired level ‘ hs ’ The value of the deviation ε is sent to a control mechanism which decides what must be done in order for the height h to return back to the desired value hs . It may open or close the valve that affects the effluent flow rate F. 38

39. II. To maintain the height of liquid ‘h’ in the tank at desired level ‘ hs ’ If ε > 0 , which implies that h < hs , the controller opens the steam valve, so that more heat can be supplied. On the contrary, the controller closes the steam valve when ε < 0 or T > Ts . It is clear that when T = Ts (i.e., ε = 0), the controller does nothing . 39

40. 2 . Ensure the Stability of a Process Consider the behavior of the variable x shown: At time t = t o , the constant value of x is disturbed by some external factors . As the time progresses the value of x returns to its initial value to stays there. If x is a process variable like temperature , pressure , concentration , flow-rate , etc., This means; The process is stable or self-regulating; needs no external intervention for its stabilization no control mechanism is needed to force x to return to its initial value . Fig1. Response of a stable system x returns to steady-state without an intervention in a self-regulating process. 40

41. 2. Ensure the Stability of a Process In contrast to the above behavior; the variable y shown, does not return to, its initial value after it is disturbed by external influences. Processes whose variables follow the pattern indicated by y , (curves a, b, c) are called unstable processes and require external control for the stabilization of their behavior . Un Stability process. A process is said to be unstable if its output becomes larger and larger (either positively or negatively ) as time increases . Examples: The explosion of a hydrocarbon fuel with air is such an unstable system. Riding a bicycle is an attempt to stabilize an unstable system and we attain that by pedaling , steering and leaning our body right or left . Fig 2 . Response of a unstable system y never returns to steady-state in three different unstable processes (A, B, C) 41

42. Example: Controlling the Operation of an Unstable Reactor Consider a continuous stirred tank reactor (CSTR) where an irreversible exothermic reaction A B takes place. The reaction mixture is cooled by a coolant medium that flows through a jacket around the reactor.   42

43. The heat removed by the coolant is a linear function of the temperature T (curve B). At steady state , the heat produced by the reaction should be equal to the heat removed by the coolant , thus yielding the steady states Pl, P2, P3 at the intersection of the curves A and B . The steady states p1 and P3 are called stable while the P2 is unstable. To understand the concept of stability let us consider the steady state P2. 43

44. Assume that we are able to start the reactor at the temperature T 2 , and the concentration C A that corresponds to this temperature . Consider that the temperature of the feed T i increases . This will cause an increase in the temperature of the reacting mixture, say T2’ 44

45. At T2’, the heat released by the reaction (Q2’) is more than the heat removed by the coolant, Q2” . Thus leading to higher temperatures in the reactor and consequently to increased rates of reaction . Increased rates of reaction produce larger amounts of heat released by the exothermic reaction which in turn lead to higher temperatures and so on. 45

46. Therefore, we see that an increase in Ti takes the reactor temperature away from the steady state P2 and that the temperature will eventually reach the value of the steady state P3 . Figure 1 . Dynamic response of a CSTR: (a) and (b) indicate the instability of the middle steady state . (a) (b) 46

47. Sometimes we would like to operate the CSTR at the middle unstable steady state for the following reasons : The low temperature steady state causes very low yields because the temperature is very low . The high temperature steady state may be very high causing unsafe conditions , destroying the catalyst for a catalytic reactor , or degrading the product B , etc. In such cases we need a controller which will ensure the stability of the operation at the middle steady state .   47

48. 3. Optimize the Performance of a Chemical Process Optimization: is a major requirement to achieve maximum profit . Safety and the satisfaction of the production specifications are the two main operational objectives for a chemical plant. Once these are achieved, the next goal is; How to make the operation of the plant more profitable . Conditions that affect the operation of the plant do not remain the same , it is clear that we would like to be able to change the operation of the plant ( flow rates, pressures, concentrations, temperatures ) in such a way that an economic objective (profit) is always maximized . This task is undertaken by the automatic controllers of the plant and its human Operators . 48

49. Applications Process industries Petroleum • Chemical • Steel • Power Food Goods manufacturing • Automobile parts • Refrigerators • Electronic equipment's like, T.V and Radio 49

50. Con’t… Transport system • Railways • Airplanes • Free missiles • Ships Power machines • Machine Tools • Compressors and Pumps • Prime movers • Electrical power – Supply Units 50

51. Classification of variables in chemical process A process variable is a condition of the process fluid that can change the manufacturing process in some way. Common process variables include: Pressure , Flow , Level , Temperature , Density , pH ( acidity or alkalinity ), Liquid interface (the relative amounts of different liquids that are combined in a vessel), concentration , Mass and Conductivity. 51

52. Con’t… The variables ( flow rates , t emperatures , pressures , concentrations , etc .) associated with a chemical process are classified into two : Input Variable (manipulated and disturbances ): - – This variable shows the effect of the surroundings on the chemical process . – Normally refers to those factors that influence the process. Output variables (measured and unmeasured) – Which denote the effect of process on the surroundings. Are Called controlled variables. 52

53. For the CSTR reactor:   We have: input variables: C Ai , T i , F i , T ci , F c , F output variables: C A , T, F, T co , V 53

54. Con’t … F can be considered either as input or output. If there is a control valve on the effluent stream the variable F is an input. Otherwise, F is an output variable. 54

55. The input variables can be further classified into the following categories : Manipulated (or adjustable) variables If their values can be adjusted freely by the human operator or a control mechanism. Typically flow rates of streams entering process that we can change in order to control the plant. Disturbances (load) variables If their values are not the result of adjustment by an human operator or a control system. It is an undesired change in one of the factors that can affect the process variable. These are also called "load“ variables and represent input variables that can cause the controlled variables to deviate from their respective set points . 55

56. The output variables are also classified into: Measured/ Controlled output variables : If their values are known by directly measuring them. Example ; Flow rates , compositions , temperatures , levels , and pressures in the process that we will try to control either trying to ; Hold them as constant as possible or Make them follow some desired value . Unmeasured output variables : Variables in the process that are not controlled / measured directly . 56

57. Example -2: Suppose that the inlet stream in the CSTR system comes from an upstream unit over which we have no control . Then, C Ai , F i , T i are disturbances. If the coolant flow-rate is controlled by a control valve, then; F c is a manipulated variable, while T ci is a disturbance. If the flowrate of the effluent stream is controlled by a valve , then F is a manipulated variable , otherwise it is an output variable .  With respect to the output variables we have the following: T, F, T co , and V are measured outputs The concentration C A can be measured variable if an analyzer (gas chromatograph, infrared spectrometer, etc.) is attached to the effluent stream . 57

58. Example: For the tank heater system: Disturbance inputs: F i and T i Manipulated inputs can be: F st , F Measured outputs can be: V, T 58

59. Con’t… Disturbances , based on their direct measurability or not , can be further classified into two categories: Measured e.g. the disturbances F i and T i of the stirred tank heater and unmeasured disturbances e.g. the feed composition for a distillation column, extraction unit, reactors and the like . 59

60. Con’t… 60

61. 61

62. Con’t Error: In process controls, error is defined as: Error = set point - process variable. Set point: The set point is where you would like a controlled process variable to be . Set point variable - is the one that is set by operator , master controller or computer as a desired value for a controlled variable. It is also called reference value . 62

63. Con’t … Set point; is a value for a process variable that is desired to be maintained . - For example, if a process temperature needs to kept within 5 °C of 100 °C, then the set point is 100 °C . A temperature sensor can be used to maintain the temperature at set point . If the temperature reading is 110 °C, then the controller determines that the process is above set point and signals the fuel valve of the burner to close slightly until the process cools to 100 °C . 63

64. Example :- Input variable – , , , Output variable – F, T Set point – These input variables are adjusted dynamically to keep the controlled variables at their set-points . Manipulated – Disturbance – , , ,   64

65. What are the basic elements of process control? The process itself , the sensor that measures the process value , the final control element that changes the manipulated variable and the controller . The process Processes have a dynamic behavior that is determined by physical properties which cannot be altered without making a physical change to the process . 65

66. Sensors Measure the value of the process output called Process Variable (PV) such as temperature, pressure, mass, flow and level . Final Control Element The physical device that receives commands from the controller that manipulate the resource. Controller Provides the signal to the final element . 66

67. The Manipulated Variable (MV) – is the measure of resource being fed into the process , for instance how much thermal energy. 67

68. A Final Control Element (FCE) – is the device that changes the value of the manipulated variable . The Controller Output (CO) – is the signal from the controller to the final control element. The Process Variable (PV) – is a measure of the process output that changes in response to changes in the manipulated variable . The Set Point (SP) – is the value we wish to maintain the process variable at . 68

69. Examples of controlled processes 1. Controlling the temperature of a water stream by controlling the amount of steam added to the shell of a heat exchanger . 69

70. Con’t… 2. Operating a jacketed reactor isothermally by controlling the coolant that flows through the jacket of a jacketed reactor. 70

71. Con’t… 3. Controlling the height of fluid in a tank to ensure that it does not overflow . 71

72. Questions ? 72