Distillation Control

An Engineering Perspective

Cecil L. Smith

• Publisher: Wiley
• Year: 2012
• Print ISBN: 9780470381946
• eBook ISBN: 9781118259696
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Chemical & Biochemical Engineering
• Index: Technology & Engineering

Learn to Design the Best Control Configuration for Any Distillation Column

Today, distillation is by far the most common separation technique used in the chemical and petroleum industries. All distillation columns need to be carefully controlled in order to meet specified production and quality levels. Distillation Control enables readers to do this by approaching the subject from a process to develop, analyze, and troubleshoot all aspects of column controls. Readers are efficiency and effectiveness and minimizing coats.

Distillation Control begins with a chapter dedicated to underlying principles, including separation processes, reflux and boilup ratios, and composition dynamics. Next, the author covers such critical topics as:

• Composition control
• Pressure control and condensers
• Reboilers and feed preheaters
• Application of feedforward
• Unit optimization
• Complex towers

As readers progress through the text, they'll discover that the best control configuration for a distillation column is largely determined using steady-state process characteristics. The stage-by-stage separation models that the author sets forth for column design, therefore, provide information that is essential in developing the optimal control configuration.

In addition to its clear explanations, Distillation Control is filled with clear diagrams and illustrations that clarify complex concepts and guide readers through multi-step procedures.

Engineers as well as other professionals working in process facilities that use distillation to separate materials will fin that this book enables them to implement the latest tested and proven distillation control methods to meet their particular processing needs.

1

PRINCIPLES

A distillation column obtains separation through energy. Consequently, it seems intuitive that a product composition must be controlled by manipulating a term relating to energy. When the composition of both product streams from a two-product tower must be controlled, this suggests the following approach:

• Control the distillate composition by adjusting the reflux.
• Control the bottoms composition by adjusting the boilup.

For most columns, this control configuration exhibits a substantial degree of interaction, which translates to operational problems in the field.

An alternate approach is as follows:

• Control the composition of one of the products (distillate or bottoms) by adjusting an energy term (reflux or boilup).
• Control the composition of the other product by adjusting the respective product draw.

For most applications, the degree of interaction is much lower.

With this approach, one of the compositions is being controlled by directly adjusting a term in the column material balance. Consequently, this presentation begins with various material balances (entire tower, condenser only, reboiler only). The discussion proceeds to component material balances for binary distillation, followed by an examination of the relationship between energy and separation. The primary objective is to provide insight into the nature of distillation and make the case that controlling one of the product compositions by adjusting a product draw is not only possible but is likely to be the appropriate approach for most towers.

This chapter reviews the general principles of distillation that are relevant to process control, including

• material balances, energy, and separation;
• composition control, through either energy terms or product flows;
• the stage-by-stage separation models for multicomponent distillation and their utility in control analyses;
• tray towers and packed towers;
• column dynamics.

1.1. SEPARATION PROCESSES

A simple separation process splits a feed stream into two product steams. In a pure separation process, no molecules are created, rearranged, or destroyed. That is, every molecule in the feed stream appears unchanged in one of the product streams.

Examples of industrial separation processes include the following:

• adsorbers
• centrifuges
• crystallizers
• cyclones
• decanters
• distillation columns
• dryers
• evaporators
• filters
• mist extractors

Every separation process relies on some principle to separate the molecules. Some separate by phases—a filter separates solids from liquids, a mist extractor separates liquids from gases, a decanter separates two immiscible liquids. Some separate by forcing a phase change—a dryer vaporizes a component such as water, leaving the nonvolatile solids behind. Distillation separates components based on their differences in volatility.

Separation processes, and distillation in particular, can become quite complex. Multiple feeds are possible. Multiple product streams are very common in distillation applications. Considerations such as energy conservation often add complexity to improve overall energy efficiency. Even reactive distillation systems are now occasionally incorporated into plant designs.

1.1.1. Binary Distillation

A binary separation process is one for which the feed contains only two components. Most presentations begin with such processes, as they are the simplest cases. Binary separations are occasionally encountered in practice, but most industrial columns are multicomponent.

A binary distillation example commonly used in textbooks is a column whose feed is a mixture of benzene and toluene. At atmospheric pressure, benzene boils at 80.1°C; toluene boils at 110.8°C. Consequently, benzene is more volatile than toluene. If a mixture of benzene and toluene is heated to its bubble point, the benzene vaporizes preferentially to the toluene. If the mixture is 50% benzene and 50% toluene, the vapor will contain more than 50% benzene and less than 50% toluene.

In distillation, the terms “light” and “heavy” are used to distinguish the components. But as used in distillation, these terms do not reflect weight, density, and so on. The light component is the more volatile; the heavy component is the less volatile. This notation is also reflected in the subscripts that designate the components:

x_L = mole fraction of light component in a liquid stream or phase;

x_H = mole fraction of heavy component in a liquid stream or phase;

y_L = mole fraction of light component in a vapor stream or phase;

y_H = mole fraction of heavy component in a vapor stream or phase.

1.1.2. Stages

A stage provides an arrangement where a vapor phase is in equilibrium with a liquid phase. The more volatile components concentrate in the vapor phase. The less volatile components concentrate in the liquid phase. The relationship between the vapor composition and the liquid composition is governed by the vapor–liquid equilibrium relationships for the various components.

A flash drum is a separation process that consists of a single stage. The feed is a superheated liquid that partially vaporizes (or flashes) within the flash drum. The two phases are separated to provide a vapor stream and a liquid stream. These are assumed to be in equilibrium as per the vapor–liquid equilibrium relationships.

Such single-stage separations are only viable when a crude separation is required between materials of significant difference in volatility. In distillation columns, a separation section provides a sequence of stages whereby liquid flowing down the section is successively contacted with the vapor flowing up the section. One approach is to use t...

Table of contents

• Cover
• Title page
• Copyright page
• PREFACE
• 1 PRINCIPLES
• 2 COMPOSITION CONTROL
• 3 PRESSURE CONTROL AND CONDENSERS
• 4 REBOILERS AND FEED PREHEATERS
• 5 APPLYING FEEDFORWARD
• 6 UNIT OPTIMIZATION
• 7 DOUBLE-END COMPOSITION CONTROL
• 8 COMPLEX TOWERS
• Index