Chemical Process Control PDF

Chemical Process Control CHE 4623 – Dr. Amjad A. Shaikh & Dr. Zin Eddine Dadach 1 Content What is process control Example of control systems Objectives of process control systems Components of process control syste...

Chemical Process Control CHE 4623 – Dr. Amjad A. Shaikh & Dr. Zin Eddine Dadach 1 Content What is process control Example of control systems Objectives of process control systems Components of process control systems Instrumentation used in process control Static characteristics of instrumentation 2 Overview LO1 Define the terms and discuss the role and importance of process control systems and its components in chemical plants. Explain the importance of process measurement instrumentation and define static characteristics of sensors. 3 TYPICAL EXAMPLES OF SYSTEM TO BE CONTROLLED HUMAN BODY COMPANY CHEMICAL PLANT 4 TYPICAL EXAMPLES OF SYSTEM TO BE CONTROLLED Ex#1: HUMAN BODY BEHAVIOR Certainly you've noticed that on a hot day, especially when you're active, your skin perspires. This is your body's way of CONTROLLING ITS TEMPERATURE (cooling itself). when you run down the street to catch a bus, your body responds by increasing your heart rate and your breathing rate TO SUPPLY YOUR HARDWORKING MUSCLE CELLS WITH MORE OXYGEN ( PROCESS CONTROL)  THERE ARE BILLIONS OF THESE EXAMPLES IN OUR BODY 5 TYPICAL EXAMPLES OF SYSTEM TO BE CONTROLLED EXAMPLE #2 :COMPANY TO MAKE PROFIT ANY BUSINESS COMPANY SHOULD BE WELL ORGANIZED AN EFFICIENT ORGANIZATION OF ANY COMPANY DEPENDS ON ITS DECISIONS 6 TYPICAL EXAMPLES OF SYSTEM TO BE CONTROLLED THE CONTROL SYSTEM OF A BUSINESS COMPANY? THE MANAGER THE MANAGER DIFFICULT JOB IS TO MAKE DECISIONS FOR THE FUTURE OF THE COMPANY GOOD DECISIONS  PROFIT BAD DECISIONS  OUT OF BUSINESS 7 EXAMPLES OF PROCESS CONTROL 8 EXAMPLES OF PROCESS CONTROL  Steam is used in the tank to warm up the fluid The fluid is heated and the steam is partially condensed  There are two control loops  Temperature of the tank Flow of heated fluid 9 EXAMPLES OF PROCESS CONTROL Heating up the temperature in the tank is a process that has the specific, desired outcome to reach and maintain a design value for the temperature (e.g. 80°C), kept constant over time. The desired temperature (80°C) is the set point. The controller will manipulate the valve of hot water to maintain the room temperature at 800C. 10 PROCESS CONTROL ROOM 11 PROCESS CONTROL ROOM 12 OBJECTIVES OF PROCESS CONTROL Process control is an interdisciplinary branch of engineering and mathematics, that deals with the behavior of dynamic systems. The main objective of a process control system is to transform dynamic systems into safe steady state systems To do this: design value is introduced as set point in a controller which manipulates a valve or a motor to obtain the desired effect on the controlled variable of the system. 13 OBJECTIVES OF PROCESS CONTROL 1. Safety 2. Environmental Protection 3. Equipment protection 4. Smooth Operation and production rate 5. Product Quality 6. Profit 7. Monitoring and Diagnosis 14 OBJECTIVES OF PROCESS CONTROL 1. Safety. It is imperative that industrial plants operate safely so as to promote the well being of people and equipment within the plant and in nearby communities. 2. Environmental Regulations. Industrial plants must comply with environmental regulations concerning the discharge of gases, liquids and solids beyond the plant boundaries. 3. Product Specifications and Production rate. In order to be profitable, a plant must make products that meet specifications concerning product quality and production rate. 15 OBJECTIVES OF PROCESS CONTROL 4. Economic Plant Operation. It is an economic reality that the plant operation over long periods of time must be profitable. Thus, the control objectives must be consistent with the economic objectives. 5. Stable Plant Operations. The control system should facilitate smooth, stable plant operation without excessive oscillation in key process variables. Thus, it is desirable to have smooth, rapid set-point changes and rapid recovery from plant disturbances such as changes in feed composition. 16 PROCESS AUTOMATION The process is “that portion of an automation operation which use energy measurable by some quality such as pressure, temperature, level, flow, (and many others) to produce changes in quality or quantity of some material or energy.” Input PROCESS Energy or Some Quality or Quantity Desired Material of the Result Material or Energy 17 PROCESS AUTOMATION Example of a Temperature Process The objective of this process is to maintain a constant water bath Water Bath temperature. Temperature Heating Element 18 PROCESS AUTOMATION Temperature Process Terminology This is a Temperature Process The measuring means is the thermometer. (Temperature Indicator- TI) thermocouple The process temperature is maintained at a desired point (Set Point – SP) Steam (Control Agent) is used to vary the temperature by opening and closing the control valve (Final Control Element) Water Bath Temperature Heating Element 19 PROCESS AUTOMATION Level Process The control objective is to maintain a constant liquid level of oil inside the tank (e.g. 100 gallons +/- 20 gallons). The hand valve is opened and closed as required to maintain the desired tank level. Oil Stock Level Indicator Oil Feed to next process 20 PROCESS AUTOMATION Level Process Terminology PROCESS: Level CONTROLLED VARIABLE: Level by Head pressure at bottom of tank CONTROL POINT: The level of oil in the tank (Set Point = 100 gallons) MEASURING MEANS: Level Indicator (Head Pressure) MANIPULATED AGENT: Volume of oil stock MANIPULATED VARIABLE: Flow rate of oil (gpm) Oil Stock Level Indicator Oil Feed to next process 21 PROCESS AUTOMATION Basic Model of a Process The process is maintained at the desired point (SP) by changing the FCE based on the value of the PV Manipulated Controlled Variable Variable FINAL CONTROL PROCESS Control Desired ELELMENT (Temperature, Agent (valve) Result pressure, level, flow) Measuring pH, conductivity, humidity, Means density, consistency, etc. (transmitter) Actuating Input Process Variable (PV) Process equilibrium (balance) is when the input energy maintains the output at a constant “desired” point. 22 PROCESS AUTOMATION Manual Control Open loop (or manual control) is used when very little change occurs in the Process Variable (PV) Manipulated Controlled Variable Variable FINAL CONTROL PROCESS Control Desired ELELMENT (Temperature, Agent (valve) Result pressure, level, flow) Measuring pH, conductivity, humidity, Means density, consistency, etc. (transmitter) Actuating Input Process Variable (PV) Corrective action is provided by manual feedback 23 THE FOUR BASIC STEPS OF A PROCESS CONTROL SYSTEM HOW DOES IT WORK? 24 Definitions of important variables of process control system Output variables: denotes the effects of the chemical process or system on the surroundings Measured variables: If their values are known by directly measuring them. Unmeasured: If their values are not or can not be measured directly. Input variables: denotes the effects of the surrounding on the chemical process or system Manipulated variables : If their values can be adjusted freely by a human operator or a control mechanism. Disturbances: If their values are not the result of adjustment by operator or control mechanism. 25 Class work 26 BASIC STEPS OF A PROCESS CONTROL SYSTEM THE FIRST STEP: TAKING THE INFORMATION IF WE DO NOT KNOW WHAT IS WRONG, HOW CAN WE CONTROL ? TAKING INFORMATION OF THE IMPORTANT VARIABLES ( Design Values) OF THE PROCESS. 27 BASIC STEPS OF A PROCESS CONTROL SYSTEM IN CASE OF TEMPERATURE PROCESS EXAMPLE  Temperature of the tank has to be controlled.  Temperature SHOULD FIRST BE MEASURED. THE EQUIPMENT FOR temperature MEASUREMENT IS : thermocouple 28 BASIC STEPS OF A PROCESS CONTROL SYSTEM THE SECOND STEP OF A PROCESS CONTROL SYSTEM: TRANSMISSION OF THE INFORMATION  LINK BETWEEN THE PLANT AND THE CONTROL ROOM)  THE MEASUREMENT OF THE CONTROLLED VARIABLE IS SENT TO THE CONTROLLER IN THE CONTROL ROOM. THE EQUIPMENT FOR TRANSMISSION IS THE TRANSMITTER Thermocouple is also a transmitter 29 BASIC STEPS OF A PROCESS CONTROL SYSTEM IN CASE OF TEMPERATURE PROCESS EXAMPLE THE ANALOG SIGNAL OF THE VALUE OF FB ( MEASURED VARIABLE) IS TRANSMITTED TO A/D CONVERTER THE RESULTING DIGITAL SIGNAL IS SENT TO THE CONTROLLER (digital or computer software)  WHY A/D CONVERTER? 30 BASIC STEPS OF A PROCESS CONTROL SYSTEM THE THIRD STEP :THE CONTROLLER MAKE DECISION THE THIRD STEP IS THE CONTROLLER IN THE CONTROL ROOM THE CONTROLLER: 1) RECEIVE THE INFORMATION FROM THE PLANT 2) COMPARE IT WITH THE SET POINT 3) CALCULATE THE DIFFERENCEε BETWEEN THE SET POINT AND THE INFORMATION. 4) MAKE A DECISION FOR ACTION TO BE TAKEN IN THE PLANT. 31 BASIC STEPS OF A PROCESS CONTROL SYSTEM IN CASE OF TEMPERATURE PROCESS EXAMPLE THE CONTROLLER WILL FIRST COMPARE T ( MEASURED VARIABLE) TO ITS SET POINT TSP. THE CONTROLLER WILL THEN CALCULATE THEIR DIFFERENCE ε =( TSP-T) THIS DIFFERENCE ε IS MULTIPLIED BY A FACTOR K DEPENDING ON THE TYPE OF CONTROLLER ( P, PI OR PID TO BE STUDIED LATER) 32 Block Diagram of process control system 33 THE MEASURING INSTRUMENTS IN PROCESS CONTROL WHAT ARE THEY? 34 REVIEW OF MEASURING INSTRUMENTS Transmitters Strain gauge Pressure Piezo-electric Pressure Transmitter Pneumatic Capacitance 3-15 PSI Bourdon Tube Electrical Mechanical Floats Current Guided Wave Level Transmitter 4 – 20 mA Weight (load cell) Level 0 – 20 mA Ultrasonic Differential Pressure 10 – 50 mA Differential Pressure Cell Voltage 0–5V 1–5V Head meters 0 – 10 V Flow (orifice, venturi) Coriolis, velocity, Digital Flow Transmitter Mass, ON/OFF Field Bus ModBus ProfiBus Thermocouples HART Temperature RTDs / Thermistors Temperature Transmitter Filled Systems Bi-metallic 35 STATIC CHARACTERISTICS OF INSTRUMENTS The performance characteristics of an instrument are mainly divided into two categories: i) Static characteristics ii) Dynamic characteristics In this course we will be studying only static characteristics 36 STATIC CHARACTERISTICS OF INSTRUMENTS The set of criteria defined for the instruments, which are used to measure the quantities which are slowly varying with time or mostly constant, i.e., do not vary with time, is called ‘static characteristics’ The various static characteristics are: i) Accuracy ii) Precision iii) Sensitivity iv) Linearity v) Reproducibility vi) Repeatability vii) Resolution viii) Threshold ix) Drift x) Stability xi) Tolerance xii) Range or span We will consider the definition of some of these.. 37 STATIC CHARACTERISTICS OF INSTRUMENTS Accuracy: It is the degree of closeness with which the reading approaches the true value of the quantity to be measured. Precision: It is the measure of reproducibility i.e., given a fixed value of a quantity, precision is a measure of the degree of agreement within a group of measurements. Sensitivity: The sensitivity denotes the smallest change in the measured variable to which the instrument responds. It is defined as the ratio of the changes in the output of an instrument to a change in the value of the quantity to be measured. Linearity: The linearity is defined as the ability to reproduce the input characteristics symmetrically and linearly. 38 STATIC CHARACTERISTICS OF INSTRUMENTS Reproducibility: It is the degree of closeness with which a given value may be repeatedly measured. It is specified in terms of scale readings over a given period of time. Repeatability: It is defined as the variation of scale reading & random in nature. a) zero drift: If the whole calibration gradually shifts due to slippage, permanent set, or due to undue warming up of electronic tube circuits, zero drift sets in. Resolution: If the input is slowly increased from some arbitrary input value, it will again be found that output does not change at all until a certain increment is exceeded. This increment is called resolution. Threshold: If the instrument input is increased very gradually from zero there will be some minimum value below which no output change can be detected. This minimum value defines the threshold of the instrument. 39 STATIC CHARACTERISTICS OF INSTRUMENTS Stability: It is the ability of an instrument to retain its performance throughout is specified operating life. Tolerance: The maximum allowable error in the measurement is specified in terms of some value which is called tolerance. Range or span: The minimum & maximum values of a quantity for which an instrument is designed to measure is called its range or span. 40