Distillation Control, Optimization, and Tuning

Fundamentals and Strategies

Lanny Robbins

• Publisher: CRC Press
• Year: 2016
• Print ISBN: 9781498770859
• eBook ISBN: 9781000218626
• Edition: 1
• Topic: Mathematics
• Subtopic: Mathematical Analysis
• Index: Mathematics

With a focus on the fundamentals and strategies of distillation columns, this book covers the process variables for continuous distillation columns, as well as four basic control strategies and the typical cases in which they are used. The author defines the inlet and outlet streams and process variables for a distillation column and then explains the overall concept of the separation and purification that is performed. Performance and product quality are described in terms of specification requirements, and tools and techniques for the optimization of quality performance are provided. Figures and graphs are included within the reference to illustrate concepts.

Chapter 1
Introduction and Overview

Welcome to the Strategy of Distillation Control, Optimizing Quality Performance, and Tuning Control Loops. This first chapter is a short summary of the topics covered in this course.

Learning Objectives

When you have completed this chapter, you should be able to

1. Understand the general organization of the content in the course

2. Know the course objectives

3. Know how to proceed through the course

1.1 Course Coverage

This book focuses on the fundamentals of process control of distillation columns. It covers the following topics:

1. The process variables for continuous binary distillation columns and four basic control strategies

2. The distillate and bottoms product quality performance objectives

3. The tuning of process control loops

When you finish this course, you will understand the fundamental separation and purification concepts to be achieved by a distillation column and the functional criteria that are critical for successful implementation of process control. Concepts for measuring and optimizing product quality performance will also be understood. You will also learn how process control loops for distillation columns can be tuned for stable operation with a balance between minimum variability from setpoint changes and excellent response to load disturbances. By approaching the subject this way, you will gain a fundamental understanding that can guide decisions for the design, operation, and troubleshooting of distillation process control systems.

This book is written from a perspective that was developed as the result of 37 years of industrial research at The Dow Chemical Company that focused on inventing, developing, and implementing industrial separation and purification processes. The work process for research and development is the scientific method, which will now be described.

The Scientific Method

1. Define the problem and the opportunity.

2. Search and understand the state of the art.

3. Develop a hypothesis, that is, a concept or a model.

4. Design and run experiments to test the validity of the hypothesis.

5. Evaluate, summarize, and document the results.

The main problem in distillation process control is to separate and purify chemical components in liquid and vapor streams while shedding the disturbances that are imposed on the distillation column. The opportunity is to separate the components from a feed stream into new vapor and liquid streams that have increased economic value at a cost that is competitive with other producers. Conducting research in a large corporation provides the opportunity to apply the results of improved performance to many distillation towers in many different businesses.

Distillation is a mature technology that is well developed and tested by the scientific method. Two comprehensive books have been written by Kister^1,2 on the design and operation of distillation columns. The control of distillation columns has been widely studied for many years, many papers have been written, and several books have been published on the subject such as the one by Buckley, Luyben, and Shunta.³ There is a large body of information to search and understand in the state of the art of distillation and process control.

During the last 20 years, there has been a continuing emphasis on improved product quality, performance, and reduced operating costs such as energy consumption, lost product, rework, maintenance, and labor cost per unit of product produced. Computer hardware and computer programs have been improved dramatically for simulating and modeling the distillation process and the dynamic response for the hypothesis in Step 3 in the scientific method. The chemical engineering fundamentals and mathematical hypotheses used to describe the design and performance of distillation columns are continually tested for their validity by plant operations in industry. Ultimately, the results are judged by the business that is responsible for the quality and profitability of the products produced by distillation and by the manufacturing department responsible for operating the process equipment.

1.2 Purpose

The purpose of this book is to present the fundamentals of process control of a distillation column as a separation and purification unit operation. This includes the critical concepts and functional criteria for the design, operation, and troubleshooting of distillation process control plus the concepts of measuring and improving product quality performance. There is a prevailing need to strike a balance between understanding the concepts that are critical to exercising good engineering judgment and understanding the intricate details of each hypothesis. The focus of this book is on achieving distillation product purity at low cost without dwelling on complex mathematical descriptions.

Often, there is more than one way to achieve the desired results, so conflicts can develop about which is the best way to approach a subject and what creates the most value. The cost of acquiring resources and developing everyone’s knowledge is generally too high. On the other hand, the risk of failing to meet production rates or product quality or of consuming excessive raw materials and energy has economic consequences for the business. Generally, the highest value is created by striking a balance.

1.3 Audience and Prerequisites

The material in this book can be useful for engineers, technicians, and plant operators concerned with the design, operation, and troubleshooting of process control systems for distillation columns. The course can also be useful for students who want to gain insights into the practical approach to distillation process control in industry and tuning control loops in a plant control room.

There are no specific prerequisites for taking this course. However, it would be helpful to have a basic understanding of the distillation unit operation and process control loop concepts. The only mathematics skills required are basic arithmetic and algebra.

1.4 Study Materials

This textbook is the only study material required. Additional references are provided in each chapter.

1.5 Organization and Sequence

This book is divided into 10 separate study chapters. Chapters 2, 3, and 4 deal with the distillation variables, and Chapter 5 covers distillation process control strategies. Chapter 6 describes some of the constraints on distillation variables and separation capabilities. Chapter 7 introduces the concepts that are critical to product quality and the measurements that evaluate performance criteria such as frequency of failure. Chapter 8 describes the concepts and nomenclature that are fundamental to PID control loops. Chapter 9 covers the concepts of tuning process controllers when they are operating in automatic output mode. Chapter 10 is about measuring the response of pr...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• The Author
• 1 Introduction and Overview
• 2 Distillation Control Variables
• 3 Separation Power
• 4 Distillate/Feed Material Balance Split
• 5 Distillation Control Strategies
• 6 Constraints
• 7 Optimizing Product Quality Performance
• 8 PID Feedback Control Loop
• 9 Closed-Loop Tuning of Controllers
• 10 Open-Loop Testing of Process Response
• Appendix: Solutions to Chapter Exercises
• Index