ERT 422/4 Control system instrumentation

ERT 422/4 Control system instrumentation MISS. RAHIMAH BINTI OTHMAN (Email: rahimah@unimap.edu.my)

COURSE OUTCOMES

Process Control Systems

OUTLINES • Basic concepts of process dynamics and process control in bioprocess plant system. • The needs of control system. • Types of controller required in the selected plant processes • A case study.

CONTROL SYSTEM Process control is the method by which the input flow of processing plants is automatically controlled and regulated by various output sensor measurements. Process control can also describe the method of keeping processes within specified boundaries and minimising variation within a process. Process Control Refers to unsteady-state (or transient) process behavior. Process Dynamics

BASIC CONCEPTS OF PROCESS DYNAMICS • Safety - industrial plants operate safely so as to promote the well-being of people and equipment within the plant and in the nearby communities. • Environmental Regulations - Industrial plants must comply with environmental regulations concerning the discharge of gases, liquids, and solids beyond the plant boundaries. • Product Specifications and Production Rate. In order to be profitable, a plant must make products that meet specifications concerning product quality and production rate.

BASIC CONCEPTS OF PROCESS DYNAMICS • Why we need the control system? –cont’ • Economic Plant Operation - the plant operation over long periods of time must be profitable. Thus, the control objectives must be consistent with the economic objectives. • Stable Plant Operation. 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. Chapter 10

THE NEEDS OF CONTROL SYSTEM Justification of Process Control. • Increase product throughput • Increase yield of higher valued products • Decrease energy consumption • Decrease pollution • Decrease off-spec product • Increase Safety • Extended life of equipment • Improve Operability • Decrease production labor

1. Raw materials (eg. Media) preparation. 2. Preparation of the fermentation inoculum (microbial cells). 7. Preparation the product (packaging). TYPICAL BIOLOGICAL PROCESS 6. Product recovery from the fermentation broth. 3. Sterilization of the process. 4. Combine the media and the microbial cell in the bioreactor (inoculation). 5. Implement the fermentation step.

THE NEEDS OF CONTROL SYSTEM • Summary; In each of these steps, certain process conditions need to be maintained for acceptable operation, and this is accomplished by process control techniques. Fermentation process needs to be maintained for acceptable operation. With modern technology, bioprocess system go through a systematic events such as sterilization, filling a vessel, maintaining T, pH, DO concentration, emptying vessel & washing vessel.

STEPS IN CONTROL SYSTEM DESIGN • The design procedure consists of three main steps: • Select controlled, manipulated, and measured variables. • Choose the control strategy and the control structure • Specify controller settings & tuning

TYPES OF CONTROLLER IN PROCESS PLANT

CONTROL TERMINOLOGY • Controlled Variables (CV)- these are the variables which quantify the performance or quality of the final product, which are also called output variables (Set point). • Manipulated Variables (MV)- these input variables are adjusted dynamically to keep the controlled variables at their set-points. • Disturbance Variables (DV)- these are also called "load" variables and represent input variables that can cause the controlled variables to deviate from their respective set points (Cannot be manipulated). • Setpoint – the desired o specified value for the CV. • Sensor – the device that measures a process variable. • Final Control Element – the system that changes the level of the MV. The final control element usually involves a control valve and associated equipment or a variable speed pump. • Controller – a unit which adjusts the MV level to keep the CV at or near its setpoint.

Information from existing plants (if available) Management objectives Formulate control objectives Computer simulation Physical and chemical principles Develop process model Plant data (if available) Devise control strategy Process control theory Select control hardware and software Experience with existing plants (if available) Vendor information Install control system = Engineering activity = Information base Adjust controller settings NOTE: MAJOR STEPS IN CONTROL SYSTEM DEVELOPMENT FINAL CONTROL SYSTEM

CLASSIFICATION OF PROCESES CONTROL STRATEGIES Feedback Control Feedforward Control Cascade Control Ratio Control DESIRED OUTPUT

BACKGROUND • Normally a chemical or biochemical process has numerous inputs • and many outputs. • Consider the diagram below: The objective of a control system is to keep the cv’s at their desired values (or setpoints). This is achieved by manipulating the mv’s using a control algorithm.

FEEDBACK CONTROL • Distinguishing feature: measure the controlled variable • It is important to make a distinction between negative feedback and positive feedback. • Negative Feedback – desirable situation where the corrective action taken by controller forces the controlled variable toward the set point • Positive feedback – controller makes things worse by forcing the controlled variables farther away from the set point.

FEEDBACK CONTROL SYSTEM; EXAMPLE • A basic feedback control system is shown in Figure (1). • The objective is to control the temperature of the outlet stream of the shell and tube heat exchanger. • The temperature is CV. • The MV is coolant flow. • Typical DV’s would include inlet temperature, inlet flow, ambient temperature, etc.

FEEDBACK CONTROL SYSTEM; EXAMPLE (cont’) • If the CV is not at setpoint then the objective of the controller is to • adjust the MV to ensure that the desired level of operation is obtained. • It is easier (believe it or not) to visualise the control system in terms of • a block diagram. • A possible block diagram for the feedback control system is; Block Diagram For The Feedback Control System • Note that the feedback controller is ‘driven’ by the error between the • actual process output and the setpoint. • Generally, the feedback controller is of the Proportional-Integral- • Derivative (PID) type.

FEEDBACK CONTROL SYSTEM; (Example: Blending System) • * Notation: • w1, w2 and w are mass flow rates • x1, x2 and x are mass fractions of component A

Assumptions: • w1 is constant • x2 = constant = 1 (stream 2 is pure A) • Perfect mixing in the tank 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

Control Question. Suppose that the inlet concentration x1 changes with time. How can we ensure that x remains at or near the set point, xsp ? • Some Possible Control Strategies: • Method 1. Measure x and adjust w2. • If x is too high, w2 should be reduced • If x is too low, w2 should be increased • Can be implemented by a person (manual control) • More convenient and economical using automatic control

FEEDBACK CONTROL SYSTEM; (Example: Blending System)

FEEDBACK CONTROL SYSTEM; Advantages & Disadvantages • Advantages: • Corrective action is taken regardless of the source of the disturbance. • Reduces sensitivity of the controlled variable to disturbances and changes in the process. • Disadvantages: • No corrective action occurs until after the disturbance has upset the process, that is, until after x differs from xsp. • Very oscillatory responses, or even instability…

FEEDFORWARD CONTROL • A feedforward control law is used to compensate for the effect that • measured DV’s may have on the CV. • The basic idea is to measure a disturbance directly and take control • action to eliminate its impact on the process output. • How well the scheme will work depends on the accuracy of the • process and disturbance models used to describe the system • dynamics. • Feedforward control actually offers the potential for perfect control. • However, because of Plant Model Mismatch (PMM) and • unmeasured / unknown disturbances this is rarely achieved in practice. • Consequently, feedforward control is normally used in conjunction with • feedback control. • The feedback controller is used to compensate for any model errors, • unmeasured disturbances etc. and ensure offset free control.

FEEDFORWARD CONTROL SYSTEM; EXAMPLE

FEEDFORWARD CONTROL SYSTEM; EXAMPLE • The objective is to maintain the temperature of the reaction mass at • the desired value when subjected to changes in inlet concentration • (Cin) and temperature (Tin). • CV is reactor liquid temperature • MV is the coolant flowrate to the heat exchanger • DV’s are inlet concentration and inlet stream temperature. • The feedforward control loop may be configured as follows; Here, 'FF' represents the feedforward control algorithm, 'CT' and 'TT' are symbols used to describe the composition and the temperature transmitters.

FEEDFORWARD CONTROL SYSTEM; EXAMPLE So, the disturbances are measured and passed to a 'FF'device that calculates the necessary coolant flowrate to compensate for any CVmoves when the measured DV deviates from it's nominal value.

FEEDFORWARD CONTROL SYSTEM; EXAMPLE • Feedforward control: a block diagram description; Gp(s) is a symbol used to represent the process dynamics. This is the relationship between the coolant flow (the MV) and the temperature (the CV). This could be a 1st order plus dead-time transfer function. Gd(s) is a symbol used to describe the mathematical relationship between inlet concentration and reactor temperature. The feedforward controller calculates the appropriate MV to ensure the CV remains at SP.

FEEDBACK CONTROL SYSTEM; (Example: Blending System) – Feedforward system • * Notation: • w1, w2 and w are mass flow rates • x1, x2 and x are mass fractions of component A

Assumptions: • w1 is constant • x2 = constant = 1 (stream 2 is pure A) • Perfect mixing in the tank 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

Control Question. Suppose that the inlet concentration x1 changes with time. How can we ensure that x remains at or near the set point, xsp ? • Some Possible Control Strategies: • Method 1. Measure x and adjust w2. • If x is too high, w2 should be reduced • If x is too low, w2 should be increased • Can be implemented by a person (manual control) • More convenient and economical using automatic control

Method 2. Measure x1 and adjust w2. • Measure disturbance variable x1 and adjust w2 accordingly • Thus, if x1 is greater than , we would decrease w2 so that • If x1 is smaller than , we would increase w2.

Method 3.Measure x1 and x, adjust w2. • This approach is a combination of Methods 1 and 2. • Method 4. Use a larger tank. • If a larger tank is used, fluctuations in x1 will tend to be damped out due to the larger capacitance of the tank contents. • However, a larger tank means an increased capital cost.

FEEDFORWARD CONTROL SYSTEM; Advantages & Disadvantages • Distinguishing feature: measure a disturbance variable • Advantage: • Correct for disturbance before it upsets the • process. • Disadvantages: • Must be able to measure the disturbance. • No corrective action for unmeasured • disturbances.

CASCADE CONTROL • Cascade control is widely used within the process industries. • Conventional cascade schemes have two distinct features: • There are two nested feedback control loops. There is a secondary control loop located inside a primary control loop. • The primary loop controller is used to calculate the setpoint for the inner (secondary) control loop. • Cascade control is used to improve the response of a single • feedback strategy. • The idea is similar to that of feedforward control: to take • corrective action in response to DV's (which are not necessarily • measured) before the CV deviates from setpoint. • The secondary control loop is located so that it recognises the • upset condition sooner than the primary loop.

CASCADE CONTROL; EXAMPLE • Cascade control of a CSTR • Figure (2) shows a conventional feedback control scheme on a CSTR. • Here temperature is being controlled using coolant flowrate to a • cooling jacket.

CASCADE CONTROL; EXAMPLE • Figure(3) shows a cascade control • scheme on the same CSTR. • The idea of the cascade strategy is to • improve the control of temperature • specifically with regard to changes • in coolant temperature. • Thus the inner loop controller takes • control action to mitigate the effect • of coolant temperature disturbances • on the temperature of the reaction • mixture.

CASCADE CONTROL; EXAMPLE • The normal block diagram representation of a cascade control • loop is shown below,

Ratio Control • The objective of a ratio control scheme is to keep the ratio • of two variables at a specified value. • Thus, the ratio (R) of two variables (A and B); • Is controlled rather than controlling the individual variables. • Typical ratio control schemes include: • • Maintaining the reflux ratio for a distillation column. • • Maintaining the stoichiometric ratio of reactants to a • reactor. • • Maintaining air/fuel ratio to a furnace.

Ratio Control • Implementation: method I • The flowrate of the two streams is • measured and their ratio calculated • using a 'divider' (just a piece of • extra electronics). • The output of the divider is sent to • the ratio controller (which is • actually a standard PI controller). • The controller compares the actual • ratio with that of the desired ratio • and computes any necessary change • in the manipulated variable.

Ratio Control • Implementation: method II • Here one stream is under standard • feedback control. • The flow of the second stream is • measured and sent to a 'multiplier' • (again just a piece of extra • electronics) which multiplies the • signal by the desired ratio yielding • the setpoint for the feedback control • law.

INDUSTRIAL PROCESS CONTROL EXAMPLES • Flow Controller • Level Control in a tank • Aerobic Fermentation Process • Fed-Batch Bioreactor

FLOW CONTROLLER • Control objective: to maintain the desired flow rate • The setpoint: desired flow rate • Controlled variable (CV):the outlet flow rate • Manipulated variable (MV):the inlet flow rate of the process stream • Disturbances variable (DV):changes in the upstream pressure for the process stream • Sensor:combination of an orifice plate and a device that measure a pressure drop across the orifice, which directly related to the flow rate. • Final control element:the control valve in the line • Controller : flow controller (FC) – compares the measured flow rate with the specified flow rate (flow setpoint) and opens/closes the control valve accordingly.

FLOW CONTROLLER Flow control loop

ERT 422/4 Control system instrumentation