Chemical and Bio-Process Control

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Chemical and Bio-Process Control

• James B. Riggs
• M. Nazmul Karim

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Chapter 1

• Introduction

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A Career in Process Control

• Requires that engineers use all of their chemical
  engineering training (i.e., provides an excellent
  technical profession that can last an entire
  career)
• Can become a technical Top Gun
• Allows engineers to work on projects that can
  result in significant savings for their companies
  (i.e., provides good visibility within a company)

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A Career in Process Control

• Provides professional mobility. There is a
  shortage of experienced process control
  engineers.
• Is a well paid technical profession for chemical
  engineers.

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Chemical Process Industries (CPI)

• Hydrocarbon fuels
• Chemical products
• Pulp and paper products
• Agrochemicals
• Man-made fibers

6
Bio-Process Industries

• Use micro-organisms to produce useful products
• Pharmaceutical industry
• Ethanol from grain industry

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Importance of Process Control for the CPI

• PC directly affects the safety and reliability of
  a process.
• PC determines the quality of the products
  produced by a process.
• PC can affect how efficient a process is
  operated.
• Bottom Line PC has a major impact on the
  profitability of a company in the CPI.

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Safety and Reliability

• The control system must provide safe operation
• Alarms, safety constraint control, start-up and
  shutdown.
• A control system must be able to absorb a
  variety of disturbances and keep the process in a
  good operating region
• Thunderstorms, feed composition upsets, temporary
  loss of utilities (e.g., steam supply), day to
  night variation in the ambient conditions

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Benefits of Improved Control
Old Controller
10
Benefits of Improved Control
Old Controller
New Controller
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Better Control Means Products with Reduced
Variability

• For many cases, reduced variability products are
  in high demand and have high value added (e.g.,
  feedstocks for polymers).
• Product certification procedures (e.g., ISO 9000)
  are used to guarantee product quality and place a
  large emphasis on process control.

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Benefits of Improved Control
Old Controller
New Controller
Improved Performance
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Maximizing the Profit of a Plant

• Many times involves controlling against
  constraints.
• The closer that you are able to operate to these
  constraints, the more profit you can make. For
  example, maximizing the product production rate
  usually involving controlling the process against
  one or more process constraints.

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Constraint Control Example

• Consider a reactor temperature control example
  for which at excessively high temperatures the
  reactor will experience a temperature runaway and
  explode.
• But the higher the temperature the greater the
  product yield.
• Therefore, better reactor temperature control
  allows safe operation at a higher reactor
  temperature and thus more profit.

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Importance of Process Control for the Bio-Process
Industries

• Improved product quality.
• Faster and less expensive process validation.
• Increased production rates.

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Driving a Car An Everyday Example of Process
Control

• Control Objective (Setpoint) Maintain car in
  proper lane.
• Controlled variable- Location on the road
• Manipulated variable- Orientation of the front
  wheels
• Actuator- Drivers arms/steering wheel
• Sensor- Drivers eyes
• Controller- Driver
• Disturbance- Curve in road

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Logic Flow Diagram for a Feedback Control Loop
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Temperature Control for a Heat Exchanger ChE
Control Example
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Heat Exchanger Control

• Controlled variable- Outlet temperature of
  product stream
• Manipulated variable- Steam flow
• Actuator- Control valve on steam line
• Sensor- Thermocouple on product stream
• Disturbance- Changes in the inlet feed temperature

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DO Control in a Bio-Reactor
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DO Control

• Controlled variable- the measured dissolved O2
  concentration
• Manipulated variable- air flow rate to the
  bio-reactor
• Actuator- variable speed air compressor
• Sensor- ion-specific electrode in contact with
  the broth in the bio-reactor
• Disturbance- Changes in the metabolism of the
  microorganisms in the bio-reactor

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Logic Flow Diagram for a Feedback Control Loop
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Comparison of Driving a Car and Control of a Heat
Exchanger

• Actuator Drivers arm and steering wheel vs.
  Control valve
• Controller the driver vs. an electronic
  controller
• Sensor the drivers eyes vs. thermocouple
• Controlled variable cars position on the road
  vs. temperature of outlet stream

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The key feature of all feedback control loops is
that the measured value of the controlled
variable is compared with the setpoint and this
difference is used to determine the control
action taken.
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In-Class Exercise

• Consider a person skiing down a mountain.
  Identify the controller, the actuator, the
  process, the sensor and the controlled variable.
  Also, indicate the setpoint and potential
  disturbances. Remember that the process is
  affected by the actuator to change the value of
  the controlled variable.

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Types of Feedback Controllers

• On-Off Control- e.g., room thermostat
• Manual Control- Used by operators and based on
  more or less open loop responses
• PID control- Most commonly used controller.
  Control action based on error from setpoint
  (Chaps 6-8).
• Advanced PID- Enhancements of PID ratio,
  cascade, feedforward (Chaps 9-11).
• Model-based Control- Uses model of the process
  directly for control (Chap 13).

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Duties of a Control Engineer

• Tuning controllers for performance and
  reliability (Chap 7)
• Selecting the proper PID mode and/or advanced PID
  options (Chap 6, 10-12)
• Control loop troubleshooting (Chap 2 8)
• Multi-unit controller design (Chap 14)
• Documentation of process control changes

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Characteristics of Effective Process Control
Engineers

• Use their knowledge of the process to guide their
  process control applications. They are process
  control engineers.
• Have a fundamentally sound picture of process
  dynamics and feedback control.
• Work effectively with the operators.

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Operator Acceptance

• A good relationship with the operators is a
  NECESSARY condition for the success of a control
  engineer.
• Build a relationship with the operators based on
  mutual respect.
• Operators are a valuable source of plant
  experience.
• A successful control project should make the
  operators job easier, not harder.

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Process Control and Optimization

• Control and optimization are terms that are many
  times erroneously interchanged.
• Control has to do with adjusting flow rates to
  maintain the controlled variables of the process
  at specified setpoints.
• Optimization chooses the values for key setpoints
  such that the process operates at the best
  economic conditions.

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Optimization and Control of a CSTR
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Optimization Example
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Economic Objective Function

• VB gt VC, VA, or VAF
• At low T, little formation of B
• At high T, too much of B reacts to form C
• Therefore, the exits an optimum reactor
  temperature, T

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Optimization Algorithm

• 1. Select initial guess for reactor temperature
• 2. Evaluate CA, CB, and CC
• 3. Evaluate F
• 4. Choose new reactor temperature and return to 2
  until T identified.

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Graphical Solution of Optimum Reactor
Temperature, T
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Process Optimization

• Typical optimization objective function, F
  F Product values-Feed costs-Utility costs
• The steady-state solution of process models is
  usually used to determine process operating
  conditions which yields flow rates of products,
  feed, and utilities.
• Unit costs of feed and sale price of products are
  combined with flows to yield F
• Optimization variables are adjusted until F is
  maximized (optimization solution).

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Generalized Optimization Procedure
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Optimization and Control of a CSTR
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In-Class Exercise

• Identify an example for which you use
  optimization in your everyday life. List the
  degrees of freedom (the things that you are free
  to choose) and clearly define the process and how
  you determine the objective function.

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Overview of Course Material

• Control loop hardware (Chap 2)
• Dynamic modeling (Chap 3)
• Transfer functions and idealized dynamic behavior
  (Chap 4-6)
• PID controls (Chap 7-10)
• Advanced PID controls (Chap 12-14)
• Control of MIMO processes (Chap 15-18)

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Fundamental Understanding and Industrially
Relevant Skills

• Fundamental Understanding-
• Laplace tranforms and transfer functions (Ch 4-5)
• Idealized dynamic behavior (Ch 6)
• Frequency response analysis (Ch 11)
• Industrially Relevant Skills-
• Control hardware and troubleshooting (Ch 210)
• Controller Implementation and tuning (Ch 7-9)
• Advanced PID techniques (Ch 12-14)
• MIMO control (Ch 15-18)

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Process Control Terminology

• Important to be able to communicate with
  operators, peers, and boss.
• New terminology appears in bold in the text
• New terminology is summarized at the end of each
  chapter.
• Review the terminology regularly in order to keep
  up with it.

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Overall Course Objectives

• Develop the skills necessary to function as an
  industrial process control engineer.
• Skills
• Tuning loops
• Control loop design
• Control loop troubleshooting
• Command of the terminology
• Fundamental understanding
• Process dynamics
• Feedback control

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Overview

• All feedback control loops have a controller, an
  actuator, a process, and a sensor where the
  controller chooses control action based upon the
  error from setpoint.
• Control has to do with adjusting flow rates to
  maintain controlled variables at their setpoints
  while for optimization the setpoints for certain
  controllers are adjusted to optimize the economic
  performance of the plant.