Distillation Design and Control Using Aspen Simulation

William L. Luyben

• Publisher: Wiley-AIChE
• Year: 2013
• Print ISBN: 9781118411438
• eBook ISBN: 9781118510094
• Edition: 2
• Topic: Technology & Engineering
• Subtopic: Chemical & Biochemical Engineering
• Index: Technology & Engineering

Learn how to develop optimal steady-state designs for distillation systems

As the search for new energy sources grows ever more urgent, distillation remains at the forefront among separation methods in the chemical, petroleum, and energy industries. Most importantly, as renewable sources of energy and chemical feedstocks continue to be developed, distillation design and control will become ever more important in our ability to ensure global sustainability.

Using the commercial simulators Aspen Plus® and Aspen Dynamics®, this text enables readers to develop optimal steady-state designs for distillation systems. Moreover, readers will discover how to develop effective control structures. While traditional distillation texts focus on the steady-state economic aspects of distillation design, this text also addresses such issues as dynamic performance in the face of disturbances.

Distillation Design and Control Using Aspen ™ Simulation introduces the current status and future implications of this vital technology from the perspectives of steady-state design and dynamics. The book begins with a discussion of vapor-liquid phase equilibrium and then explains the core methods and approaches for analyzing distillation columns. Next, the author covers such topics as:

• Setting up a steady-state simulation
• Distillation economic optimization
• Steady-state calculations for control structure selection
• Control of petroleum fractionators
• Design and control of divided-wall columns
• Pressure-compensated temperature control in distillation columns

Synthesizing four decades of research breakthroughs and practical applications in this dynamic field, Distillation Design and Control Using Aspen ™ Simulation is a trusted reference that enables both students and experienced engineers to solve a broad range of challenging distillation problems.

Chapter 1

Fundamentals of Vapor–Liquid Equilibrium (VLE)

Distillation occupies a very important position in chemical engineering. Distillation and chemical reactors represent the backbone of what distinguishes chemical engineering from other engineering disciplines. Operations involving heat transfer and fluid mechanics are common to several disciplines. But distillation is uniquely under the purview of chemical engineers.

The basis of distillation is phase equilibrium—specifically, vapor–liquid equilibrium (VLE) and in some cases vapor–liquid–liquid equilibrium (VLLE). Distillation can only effect a separation among chemical components if the compositions of the vapor and liquid phases that are in phase equilibrium with each other are different. A reasonable understanding of VLE is essential for the analysis, design, and control of distillation columns.

The fundamentals of VLE are briefly reviewed in this chapter.

1.1 Vapor Pressure

Vapor pressure is a physical property of a pure chemical component. It is the pressure that a pure component exerts at a given temperature when there are both liquid and vapor phases present. Laboratory vapor pressure data, usually generated by chemists, are available for most of the chemical components of importance in industry.

Vapor pressure depends only on temperature. It does not depend on composition because it is a pure component property. This dependence is normally a strong one, with an exponential increase in vapor pressure with increasing temperature. Figure 1.1 gives two typical vapor pressure curves, one for benzene and one for toluene. The natural log of the vapor pressures of the two components is plotted against the reciprocal of the absolute temperature. As temperature increases, we move to the left in the figure, which means a higher vapor pressure. In this particular figure, the vapor pressure P^S of each component is given in units of mmHg. The temperature is given in kelvin.

Figure 1.1 Vapor pressures of pure benzene and toluene.

Looking at a vertical constant-temperature line shows that benzene has a higher vapor pressure than toluene at a given temperature. Therefore, benzene is the “lighter” component from the standpoint of volatility (not density). Looking at a constant-pressure horizontal line shows that benzene boils at a lower temperature than toluene. Therefore, benzene is the “lower-boiling” component. Notice that the vapor pressure lines for benzene and toluene are fairly parallel. This means that the ratio of the vapor pressures does not change much with temperature (or pressure). As discussed in a later section, this means that the ease or difficulty of the benzene/toluene separation (the energy required to make a specified separation) does not change much with the operating pressure of the column. Other chemical components can have temperature dependences that are quite different.

If we have a vessel containing a mixture of these two components with liquid and vapor phases present, the vapor phase will contain a higher concentration of benzene than will the liquid phase. The reverse is true for the heavier, higher-boiling toluene. Therefore, benzene and toluene can be separated in a distillation column into an overhead distillate stream that is fairly pure benzene and a bottoms stream that is fairly pure toluene.

Equations can be fitted to the experimental vapor pressure data for each component using two, three, or more parameters. For example, the two-parameter version is

The C_j and D_j are constants for each pure chemical component. Their numerical values depend on the units used for vapor pressure (mmHg, kPa, psia, atm, etc.) and on the units used for temperature (K or °R).

1.2 Binary VLE Phase Diagrams

There are two types of VLE diagrams that are widely used to represent data for two-component (binary) systems. The first is a “temperature versus x and y” diagram (Txy). The x term represents the liquid composition, usually in terms of mole fraction. The y term represents the vapor composition. The second diagram is a plot of x versus y.

These types of diagrams are generated at a constant pressure. Be...

Table of contents

• Cover
• Title Page
• Copyright
• Dedication
• Preface to the Second Edition
• Preface to the First Edition
• Chapter 1: Fundamentals of Vapor–Liquid–Equilibrium (VLE)
• Chapter 2: Analysis of Distillation Columns
• Chapter 3: Setting Up a Steady-State Simulation
• Chapter 4: Distillation Economic Optimization
• Chapter 5: More Complex Distillation Systems
• Chapter 6: Steady-State Calculations for Control Structure Selection
• Chapter 7: Converting From Steady-State to Dynamic Simulation
• Chapter 8: Control of More Complex Columns
• Chapter 9: Reactive Distillation
• Chapter 10: Control of Sidestream Columns
• Chapter 11: Control of Petroleum Fractionators
• Chapter 12: Divided-Wall (Petlyuk) Columns
• Chapter 13: Dynamic Safety Analysis
• Chapter 14: Carbon Dioxide Capture
• Chapter 15: Distillation Turndown
• Chapter 16: Pressure-Compensated Temperature Control in Distillation Columns
• Chapter 17: Ethanol Dehydration
• Chapter 18: External Reset Feedback to Prevent Reset Windup
• Index