MISS. RAHIMAH BINTI OTHMAN

MISS. RAHIMAH BINTI OTHMAN
ERT 422/4 Control system instrumentation MISS. RAHIMAH BINTI OTHMAN (

COURSE OUTCOMES CO APPLY basic concepts of process dynamics and process control in bioprocess plant system, IDENTIFY the needs of control system and JUSTIFY the types of controller required in the selected plant processes. INTRODUCE a case study as a reference.

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 Process Dynamics
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

TYPICAL BIOLOGICAL PROCESS
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.

Formulate control objectives Install control system
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

TYPE OF PROCESS CONTROL LOOP Feedforward-plus-Feedback Control
Feedforward Control Feedforward-plus-Feedback Control Cascade Control Ratio Control Split Range Control Differential 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

Some Possible Control Strategies:
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
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

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.

So, the disturbances are measured and passed to a 'FF' device that calculates the necessary coolant flowrate to compensate for any CV moves when the measured DV deviates from it's nominal value.

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.

FEEDFORWARD 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

Some Possible Control Strategies:
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.

FEEDFORWARD-PLUS-FEEDBACK CONTROL
Because of the difficulty of accounting for every possible load disturbance in a feedforward system, this system are often combined with feedback systems. Controller with summing functions are used in these combined systems to total the input from both the feedforward loop and the feedback loop, and send a unified signal to the final control element.  FC TC Process variable need to be controlled = Temperature FT Fluid in TT Y Steam Fluid out LCV-100

FEEDFORWARD-PLUS-FEEDBACK CONTROL
Example Figure below shows compressed gas vessel. Process variable that need to be controlled is pressure where the vessel should maintain pressure at 60 psi. By using pressure controlled through both the gas flow measurement into the vessel and vessel pressure itself, draw a feedforward-plus-feedback control loop system.  Y PIC PT FC V-100 Process variable need to be controlled = Pressure FT

PIPING AND INSTRUMENTATION DIAGRAM (P&ID)
Exercise Figure below shows the boiler system that used to supply hot steam to a turbine. This system need to supply 100 psi hot steam to the turbine where the PCV-100 will be opened when the pressure reached that desired pressure. With using pressure control through temperature and pressure measurement in the boiler, draw a feedforward-plus-feedback control loop system. BOILER Hot steam Water Process variable need to be controlled = Pressure

Answer 2 Figure below shows the boiler system that used to supply hot steam to a turbine. This system need to supply 100 psi hot steam to the turbine where the PCV-100 will be opened when the pressure reached that desired pressure. With using pressure control through temperature and pressure measurement in the boiler, draw a feedforward-plus-feedback control loop system.  TIC PIC Y TT PT BOILER Hot steam Water Process variable need to be controlled = Pressure

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
The normal block diagram representation of a cascade control loop is shown below,

CASCADE CONTROL Cascade Control
Cascade Control uses the output of the primary controller to manipulate the set point of the secondary controller as if it were the final control element. Reasons for cascade control: Allow faster secondary controller to handle disturbances in the secondary loop. Allow secondary controller to handle non-linear valve and other final control element problems. Allow operator to directly control secondary loop during certain modes of operation (such as startup).

CASCADE CONTROL Cascade Control (cont…)
Requirements for cascade control: Secondary loop process dynamics must be at least four times as fast as primary loop process dynamics. Secondary loop must have influence over the primary loop. Secondary loop must be measured and controllable.

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.

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.

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.

Ratio Control Ratio control is used to ensure that two or more flows are kept at the same ratio even if the flows are changing. FIC FF FT FT Water Acid 2 part of water1 part of acid

Ratio Control (cont…) Application: - Blending two or more flows to produce a mixture with specified composition. - Blending two or more flows to produce a mixture with specified physical properties. - Maintaining correct air and fuel mixture to combustion. FIC FF FT FT Water Acid 2 part of water 1 part of acid

Ratio Control (Auto Adjusted) If the physical characteristic of the mixed flow is measured, a PID controller can be used to manipulate the ratio value. For example, a measurement of the density, gasoline octane rating, color, or other characteristic could be used to control that characteristic by manipulating the ratio. FF Remote Set Point FIC Remote Ratio Adjustment FT AIC FT Physical Property Measurement Water Acid 2 part of water 1 part of acid

Split Range Control FC FT Valve A Valve B

Split Range Control The diagram shows pH adjustment; part of waste water treatment process. The process shall maintained at pH 6. When the process liquid states below pH 6, CV-102 will be opened to dosing NaOH to the tank TK-100. When the process liquid states above pH 6, CV-101 will be operated to dosing HCl. CV-102 TK-102 (base feed tank) pHIC pHT 1 CV-101 TK-100 (pH adjustment tank) TK-101 (acid feed tank)

Differential Control Meaning Systems Theory: In differential control, control action is based on the change (derivative) of the control error. The control error is defined as the difference between the set point and the process output. Explanation: Derivative control is used to provide anticipative action. This is done by using the current change (derivative) of the control error to estimate the error some time ahead and act anticipatively on this estimation.

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

LEVEL CONTROL IN A TANK Control objective: to maintain the level in the tank The setpoint: desired level in the tank Controlled variable (CV): the level in the tank Manipulated variable (MV): exit flow from the tank Disturbances variable (DV): changes in the inlet flow rate Sensor: level indicator on the tank (LT) Final control element: the control valve on the outflow line Controller – level controller (LC) – compares the measured level with the setpoint for the level in the tank and makes a change to the control valve on the exit flow from the tank.

Control Diagram Of A Tank With A Level Controller

AEROBIC FERMENTATION PROCESS
Control objective: to maintain a specified dissolved oxygen (DO) conc. in the fermentation reactor so that the cells in the process have adequate oxygen levels. The setpoint: desired DO concentration Controlled variable (CV): the DO conc. in the fermentor Manipulated variable (MV): air flow rate to the fermentor Disturbances variable (DV): changes in the rpm of the mixer impeller Sensor: DO sensor-transmitter (AT) Controller – DO controller (AC) – compares the measured DO with the setpoint value and sets the air flow rate to the fermentor. Final control element: variable speed air compressor

Schematic of a Bio-reactor with a Dissolved Oxygen Controller

FED-BATCH BIOREACTOR Control obj:
to maintain a specified cell concentration in the bioreactor. What is the CV, MV, DV, sensor & final control element?

Control objective: to maintain a specified cell concentration in the bioreactor.
The setpoint: desired cell concentration Controlled variable (CV): the cell conc. in the bioreactor Manipulated variable (MV): the feed rate of glucose and nutrients mixture to the bioreactor. Disturbances variable (DV): changes in the concentration of glucose in the feed. Sensor: turbidity meter (AT) which provides a measurement that correlates with the cell conc. in the broth. Controller – controller (AC) – compares the measured cell conc. with the setpoint value and sets the glucose feed rate. Final control element: variable speed pump

Selection of Controlled, Manipulated & Measured Variables
Process variables can be classified into input variables and output variables. Definition of input variables: physical variables that affect the output variables. Input variables can be divided into manipulated variables and disturbance variables. Manipulated variables are typically flow rates Common disturbance variables include the feed conditions to a process and the ambient temperature. The output variables are process variables that typically are associated with exit streams (e.g. compositions, temperatures, levels and flow rates).

Selection of Controlled Variables
Guideline 1. All variables that are not self-regulating must be controlled. -Non self-regulating variable: an output variable that exhibits an unbounded response after a sustained disturbance. -must be controlled in order for control process to be stable. Guideline 2. Choose output variables that must be kept within equipment and operating constraints (e.g., temperatures, pressures, and compositions).

Guideline 3. Select output variables that are a direct measure of product quality (e.g., composition, refractive index) or that strongly affect it (e.g., temperature or pressure). Guideline 4. Choose output variables that seriously interact with other controlled variables. Guideline 5. Choose output variables that have favorable dynamic and static characteristics.

Selection of Manipulated Variables
Guideline 6. Select inputs that have large effects on controlled variables. Guideline 7. Choose inputs that rapidly affect the controlled variables. Guideline 8. The manipulated variables should affect the controlled variables directly rather than indirectly. Guideline 9. Avoid recycling of disturbances.

Selection of Measured Variables
Guideline 10. Reliable, accurate measurements are essential for good control. Guideline 11. Select measurement points that have an adequate degree of sensitivity. Guideline 12. Select measurement points that minimize time delays and time constants

QUIZ 1 Exercise 1 LT 1 LIC 1 FC L3 Where LT 1 and LIC 1 to control PCV-100 (failure close); PCV-100 close when level reached L 3 PCV-100 open when level below L3 PCV-100 L2 TK-100 L1 FC V-100 L5 PCV-101 LT 2 LIC 2 L4 Where LT 2 and LIC 2 to control PCV-101 (failure close); PCV-101 close when level reached L5 PCV-101 open when level below L5

Answer LIC 1 LT 1 LIC 1 FC L3 Where LT 1 and LIC 1 to control PRV-100 (failure close); PRV-100 close when level reached L 3 PRV-100 open when level below L3 LT 1 PRV-100 L2 TK-100 L1 LIC 2 V-100 FC L5 LT 2 LIC 2 LT 2 PRV-101 L4 Where LT 1 and LIC 1 to control PRV-101 (failure close); PRV-101 close when level reached L5 PRV-101 open when level below L5

Prepared by, MISS RAHIMAH OTHMAN
Thank you Prepared by, MISS RAHIMAH OTHMAN

MISS. RAHIMAH BINTI OTHMAN

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