NTHU Department of Chemical Engineering Chapter 1 Introduction

國立清華大學化學 程學系 NTHU Department of Chemical Engineering Chapter 1 Introduction Professor Shi-Shang Jang Chemical Engineering Department National Tsing-Hua University Taiwan March , 2015

Overview Why this Course? üOur world is Dynamic and is govern by many factors. üWe have to control some entities for safe and happier world.

Overview Why we need Process Control ? • Every Chemical Process requires conversion of feed material to product using different chemical and physical operations. • Strong need to control processes for more efficiency and economically viability has arose due to stronger business competition, tougher environmental and safety regulations and rapidly changing technological trends. • Process Control plays a crucial task in deciding the product quality and has a very high significance in today’s chemical industry.

Overview Process Control in our daily life • Bathroom water heater maintains hot water temperature in safe zone for our comfort. • Refrigerator controls temperature of the food items kept in it and avoid their spoilage. • Traffic over roads and highways is controlled by traffic light or there can be accidents and jams over the roads.

Overview -- 5 W in Process control • What -- makes a change to bring the process on target • When -- dynamic process • Where -- Everywhere • Why -- performance, profit, safety, … • Who -- people

Overview cont. . What is Process Dynamics ? • Process Dynamics is study of a system in un-steady (or transient) state. • Dynamics study is required to study situations like plant start-ups and shutdowns, unusual process disturbances and planned transitions from one product grade to another. • Dynamics sometimes plays an important part in deciding the control strategies for a process.

Overview cont. . Dynamics of Process - bus and bicycle Though, a bus and a Bicycle may move with same speed, but they will follow different trajectory at a turn on the road. Therefore, Study at the transient state is necessary to develop a successful control strategy.

Overview-Cont… Input and Output of a Process: Given a process, there must exist some inputs no matter material or energy, that is used to be converted into products(output) met with some specifications, such as purity or temperature. Matter Process Energy Product

The Feedback Control Loop Control Calculation Sensor Final Element

Overview- tank example Valve ØA separating process Valve Separating tank Valve Pump ØWhat variable should be controlled in this process? • Feed flow rate • Liquid level • Tank pressure

Main Features of Process Control Closed Loop Control 1. Temperature Element 2. Temperature Transmitter 3. Distributive Control System 4. Valve Positioner & I/P 5. Actuator 6. Valve Body

Main Features of Process Control Terminologies • Controlled variables (CV): Process variables that should be maintained at fixed values to get better control, or should be adjusted to some new more appropriated values. • Manipulated variables (MV): The variables that is used to “affect” the process. The specification of the output that to be controlled is termed (CV) in process control. • Disturbance variables (DV): input variables to the plant, those variables may or may not be measurable, but however, the change of those variables may affect the normal operations of the plant, that reflects back the change of CV’s.

Main Features of Process Control – example : water heater Disturbance Variable Water Flow Tap Opening Gas Flow Manipulated Variable Temperature Control Variable

Main Features of Process Control – example : tank Valve ØA separating process Valve Separating tank Valve Pump ØCVs: ØMVs: • Feed flow rate • Liquid level • Vapor flow rate • Tank pressure • Liquid flow rate

Illustrative Example – A petroleum blending

Illustrative Example – A Blending Process Objective: To maintain the tank composition CV: composition, X MV: flow rate of pure A, X 2 DV: flow rate of mixture and composition x 1, w 1

Illustrative Example – A Blending Process – SIMULINK Expression

Illustrative Example – A Blending Process – Scenario Simulation – step disturbance (shift) ; w 1=1. 1 m 3/min

Heart of Chemical Plant – Control room

Display of a DCS Computer

1 -9 Feed-forward vs Feedback • Feed-forward control: To prevent process upset due to disturbances, by predicting the potential disturbance and performing control in advance. • Feedback control: To compensate the effect of the disturbance by measuring the controlled variable (CV) and then considering the difference for control. ØFeedback control is the most common used in the industries because the disturbances are basically unpredictable. However, feed-forward control is sometimes useful in some special cases.

Control Strategies -- Feedback • Feedback control: To compensate the effect of the disturbance by measuring the controlled variable (CV) and then considering the difference for control. Feedback control is the most common used in the industries because the disturbances are basically unpredictable. However, feedforward control is sometimes useful in some special cases.

Control Strategies -- Feed-forward • Feed-forward control: To prevent process upset due to disturbances, by predicting the potential disturbance and performing control in advance.

Statistical Process Control v. s. Process Control • Statistical Process Control is referred to routine monitoring of key variables within specific limits. • Alert and action should be done in case of excursions of key variables beyond their limits. • Statistical process control is a different area from automatic process control addressed in this course

Benefits of Process Control • Improves plant performance by reducing the variation of the key variables. • When the variation has been reduced, the desired value of the controlled variable can be adjusted to increase profit. • Process Control provided higher safety to plant and human resource. • Process Control can reduce scrap and waste products and thus would give more profit and longer life for the equipment. • Process control helps in determining inventory in hand so would give almost exact stock position.

Process control can bring --

Homework #1 Text p 10 1 -1 1 -2 Due 3/11