Lecture 1.1 Process Dynamics and Control Introduction

Process Dynamics and Control

1. Introduction to Process Dynamics & Control Lecture 1.1 Objective

2. INTENDED LEARNING OUTCOMES By the end of this lecture, you should be able to: Explain the objective of a chemical plant Discuss the requirements to meet the chemical plant objectives Define a control system Discuss the aim of a control system Discuss the classification of variables in a control system

3. Introduction to process Dynamics & control In chemical engineering, we have a number chemical units : Reactors Distillation Columns Pumps Compressors Cooling Tower Etc. To constitute a chemical plant, we need to assemble few of the mentioned chemical units.

4. Objective of a chemical plant It receives raw material and uses available sources of energy to produce the desired products in the most economical way. Chemical Plant Raw Materials Products

5. Requirements to meet the plant objectives To meet the objectives of a chemical plant, we need to satisfy some of the following requirements: Safety: A reactor which is designed to operate within 100 KPa. To maintain the pressure, we need some external intervention so that the reactor operates below the safety limit (pressure below 100 KPa). Product Specification: We need to ensure the quantity and the quality of the product.

6. Requirements to meet the plant objectives Environmental Requirements There are several environmental policies, laws, and regulations which are enforced to maintain: Concentration of chemicals in the effluent stream SO2 concentration in the stream which are rejected to the atmosphere Wastewater concentration (BOD, COD, etc.) which are rejected to the bodies of water

7. Requirements to meet the plant objectives Operational Constraints A chemical plant has a specific constraints/limitation inherent to the operation. Examples: Distillation Column – should not be flooded Tanks – should not overflow or go dry Catalytic Reactor – should not exceed the upper limit temperature

8. Requirements to meet the plant objectives Economics Control operating conditions at minimum operating cost and maximum profit. Example: Minimize the use of energy Optimize the use of raw materials Optimize human intervention

9. To maintain the mentioned requirements, some external intervention is required. That external intervention is nothing but the control system Therefore, to maintain all requirements of a chemical plant, we need to devise a control system . Requirements to meet the plant objectives

10. objective of process control The primary objective of process control is to maintain a process at a desired operating conditions, safely and efficiently, while satisfying environmental, economic, and product quality requirement.

11. The influence of external disturbances The stability of the chemical process The performance of the chemical process Therefore, the aim of the control system: Suppress the influence of external disturbances Ensure stability of the chemical process Optimize the performance of the chemical process ISSUES TO BE DEALT BY THE CONTROL SYSTEM

12. External Disturbance Consider a heating tank system The liquid is entering with a flowrate, and a temperature, The outlet has a flowrate, F and a temperature, T The liquid inside the tank has a height of h To heat the liquid inside the tank, a coil is used wherein steam is entering with a flowrate,  

13. External Disturbance In an ideal circumstance, when the Fi, T, Fst , remains the same, then a control system is not necessary. BUT this is not the case in reality! The input variables always change with time, so we need a control system.

14. External Disturbance Objectives of the control system: Maintain the liquid temperature T at its desired value Maintain the height of the liquid h at its desired value  

15. External Disturbance HOW? Maintain the liquid temperature T at its desired value Maintain the height of the liquid h at its desired value  

16. External Disturbance HOW? Example for objective 1, Attach a thermocouple that will measure the temperature of the liquid inside the tank T The temperature is then compared with the desired value By comparing the two temperature, an error is computed, e = The error signal goes to a controller The controller produce/calculates control action which is implemented by the control valve  

17. External Disturbance e = Suppose: increase steam flowrate reduce steam flowrate Here, the disturbance are the feed flowrate and inlet temperature  

18. External Disturbance Input Effect Response of the Controller T T Input Effect Response of the Controller The controller suppress the effect of external disturbance

19. Case 1: Stable/Self Regulating Process Initially, the system operates at steady state (SS) x is disturbed at t = x returns automatically to SS No need of external intervention   Stability of the chemical process

20. Case 2: Unstable Process Initially, the system operates at steady state (SS) x is disturbed at t = x does not return to SS There is a need of external intervention and control system   Stability of the chemical process

21. Main Objectives: Safety Production Specifications Economic Objective Example: Jacketed CSTR Optimization of the performance of a chemical process Exothermic Reaction: A = reactant B = desired product C = undesired product Economic Objective: Profit is a function of: Revenue of the sales of product B Cost of reactant A Cost of coolant  

22. CLASSIFICATION OF VARIABLES Controlled Variable (CV)

23. Liquid Tank System Liquid input flowrate, Liquid output flowrate, Liquid has a height of h Tank cross-sectional area, A   Objective: Maintain the height of the tank to a desired value, Controlled variable (CV) = h   Input Variables: Output Variables: , h  

24. Liquid Tank System Controlled Variable (CV) Manipulated Variable (MV) h & Controlled Variable (CV) Manipulated Variable (MV) h Input Variables: ( Output Variables: , h  

25. Heating Tank System The liquid is entering with a flowrate, and a temperature, The outlet has a flowrate, F and a temperature, T The liquid inside the tank has a height of h To heat the liquid inside the tank, a coil is used wherein steam is entering with a flowrate,   Input Variables: , , ( Output Variables: , T , h   Objective: Maintain the height of the tank, h to a desired value, Maintain the temperature of the tank, T to a desired value,  

26. Heating Tank System Objective: Maintain the height of the tank, h to a desired value, Maintain the temperature of the tank, T to a desired value,   Input Variables: , , ( Output Variables: , T , h   Controlled Variable (CV) Manipulated Variable (MV) T h , Controlled Variable (CV) Manipulated Variable (MV) T h

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