Multicomponent Distillation Problems Solution Essays - Essay for you

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Multicomponent Distillation Problems Solution Essays

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What Is Multicomponent Distillation? mobile wiseGEEK

wiseGEEK: What Is Multicomponent Distillation?

Multicomponent distillation is a chemical process in which a mixture of volatile compounds separates based on their boiling points. The process occurs in a distillation column, a vertical stacking of trays or stages upon which components in their liquid and vapor phases coexist. As the mixture moves up the column, high boiling compounds concentrate on lower stages, while low boiling compounds concentrate on higher stages.

Distillation columns use basic principles of liquid-vapor equilibrium mixtures. As heat is applied to a liquid, the liquid’s temperature rises until reaching the boiling point (BP). At the BP, additional energy does not cause a temperature increase; rather, the molecules use it to escape the liquid phase and become a gas.

The energy absorbed by the new gas molecule is no longer available to heat the next liquid molecule. As the liquid boils, it is also cooling from the absence of the energy keeping the gas molecules in a gas phase. This cooling will cause some of the gas molecules to condense into liquid again, releasing the energy of vaporization. The energy released is able to reheat the liquid. At equilibrium conditions, the rate of vaporization equals the rate of condensation.

In a multicomponent distillation column, an equilibrium mixture is established on each stage. There is a constant source of new energy supplied to the bottom of the column. This heat causes some of the gas-phase molecules of the most volatile compound on each stage, the one with the lowest BP, to rise to the next stage. At this higher stage, the mixture will attempt to come to equilibrium again. The molecules coming from below may have too high a BP to vaporize on this stage, so they accumulate in the liquid phase.

Eventually, the liquid in each stage becomes concentrated in one or more of the components. Side streams may be taken off at one or more stages. The liquid streams will be concentrated in one or more components, and additional distillations steps may be necessary.

Petroleum is typically distilled into fractions via multicomponent distillation. A fraction is a range of similar compounds with close BP that can be treated as a single compound. Gasoline is an example. A low BP gas may be taken off the top of the column and not require further processing.

The design of a multicomponent distillation requires complex calculations. The column design parameters include specifying the number of stages, the feed stage of the raw material, the stages at which product streams are removed, and the heat needed to drive the column. Specialized multicomponent distillation computer programs perform these calculations, but engineers still learn graphical solution methods to understand the process.

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Wolfram Demonstrations Project

Multicomponent Distillation Column with a Sidestream

Consider a distillation column operating at 101.325 kPa and separating an equimolar mixture of benzene ( ), toluene ( ), and -xylene ( ). The column has 20 stages, a partial reboiler, and a total condenser. The feed location is stage 13 counting from the top. The feed, a saturated liquid, has a flow rate equal to 100 kmole/hr. The column has a liquid sidestream at stage 7 counting from the top. You can set the draw rate of this sidestream as well as the reflux and reboil ratios of the column. The Demonstration plots the composition profile in orange, green, and blue for benzene, toluene, and -xylene, respectively. Plots of the temperature profile and liquid and vapor flow rates can also be displayed by choosing the appropriate tabs. Both composition and flow rate of the column's feed and product streams can be seen using the column output tab. The variables . . . and stand for feed, distillate, residue, and sidestream flow rates, respectively. You can easily check that the overall and components mass balance are verified.

Finally, two cases are studied and show perfect agreement with Aspen–HYSYS since the Demonstration solves the full steady-state MESH (mass, equilibrium, summation, and enthalpy) equations.



Until the advent of computers, multicomponent distillation problems were solved manually by making tray-by-tray calculations of heat and material balances and vapor-liquid equilibria. Even a partially complete solution of such a problem required a week or more of steady work with a mechanical desk calculator. The alternatives were approximate methods such as those mentioned in Sections 13.7 and 13.8 and pseudobinary analysis. Approximate methods still are used to provide feed data to iterative computer procedures or to provide results for exploratory studies.

The two principal tray-by-tray procedures that were performed manually are the Lewis and Matheson and Thiele and Geddes. The former started with estimates of the terminal compositions and worked plate-by-plate towards the feed tray until a match in compositions was obtained. Invariably adjustments of the amounts of the components that appeared in trace or small amounts in the end compositions had to be made until they appeared in the significant amounts of the feed zone. The method of Thiele and Geddes fixed the number of trays above and below the feed, the reflux ratio, and temperature and liquid flow rates at each tray. If the calculated terminal compositions are not satisfactory, further trials with revised conditions are performed. The twisting of temperature and flow profiles is the feature that requires most judgement. The Thiele-Geddes method in some modification or other is the basis of most current computer methods. These two forerunners of current methods of calculating multicomponent phase separations are.

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Deaerators are used to remove dissolved, corrosive gases from boiler feed water. They heat incoming makeup water for injection into the boiler, and return the condensate to a temperature which minimizes the solubility of these gases.

Separation Process Engineering, 2nd Edition

Separation Process Engineering, 2nd Edition Description

The Comprehensive Introduction to Standard and Advanced Separation for Every Chemical Engineer

Separation Process Engineering, Second Edition helps readers thoroughly master both standard equilibrium staged separations and the latest new processes. The author explains key separation process with exceptional clarity, realistic examples, and end-of-chapter simulation exercises using Aspen Plus.

The book starts by reviewing core concepts, such as equilibrium and unit operations; then introduces a step-by-step process for solving separation problems. Next, it introduces each leading processes, including advanced processes such as membrane separation, adsorption, and chromatography. For each process, the author presents essential principles, techniques, and equations, as well as detailed examples.

Separation Process Engineering is the new, thoroughly updated edition of the author's previous book, Equilibrium Staged Separations. Enhancements include improved organization, extensive new coverage, and more than 75% new homework problems, all tested in the author's Purdue University classes.

  • Detailed problems with real data, organized in a common format for easier understanding
  • Modular simulation exercises that support courses taught with simulators without creating confusion in courses that do not use them
  • Extensive new coverage of membrane separations, including gas permeation, reverse osmosis, ultrafiltration, pervaporation, and key applications
  • A detailed introduction to adsorption, chromatography and ion exchange: everything students need to understand advanced work in these areas
  • Discussions of standard equilibrium stage processes, including flash distillation, continuous column distillation, batch distillation, absorption, stripping, and extraction
Sample Content Table of Contents Preface xv Acknowledgments xvii About the Author xix Nomenclature xxi Chapter 1: Introduction to Separation Process Engineering 1

1.1. Importance of Separations 1

1.2. Concept of Equilibrium 2

1.3. Mass Transfer 4

1.4. Problem-Solving Methods 5

1.5. Prerequisite Material 7

1.6. Other Resources on Separation Process Engineering 8

1.7. Summary—Objectives 9

Chapter 2: Flash Distillation 12

2.1. Basic Method of Flash Distillation 12

2.2. Form and Sources of Equilibrium Data 14

2.3. Graphical Representation of Binary VLE 16

2.4. Binary Flash Distillation 21

2.5. Multicomponent VLE 29

2.6. Multicomponent Flash Distillation 34

2.7. Simultaneous Multicomponent Convergence 40

2.8. Size Calculation 45

2.9. Utilizing Existing Flash Drums 49

2.10. Summary—Objectives 50

Appendix: Computer Simulation of Flash Distillation 59

Chapter 3: Introduction to Column Distillation 65

3.1. Developing a Distillation Cascade 65

3.2. Distillation Equipment 72

3.3. Specifications 74

3.4. External Column Balances 76

3.5. Summary—Objectives 81

Chapter 4: Column Distillation: Internal Stage-by-Stage Balances 86

4.1. Internal Balances 86

4.2. Binary Stage-by-Stage Solution Methods 90

4.3. Introduction to the McCabe-Thiele Method 97

4.4. Feed Line 101

4.5. Complete McCabe-Thiele Method 109

4.6. Profiles for Binary Distillation 112

4.7. Open Steam Heating 114

4.8. General McCabe-Thiele Analysis Procedure 118

4.9. Other Distillation Column Situations 125

4.10. Limiting Operating Conditions 130

4.11. Efficiencies 133

4.12. Simulation Problems 135

4.13. New Uses for Old Columns 136

4.14. Subcooled Reflux and Superheated Boilup 138

4.15. Comparisons between Analytical and Graphical Methods 140

4.16. Summary—Objectives 142

Appendix: Computer Simulations for Binary Distillation 157

Chapter 5: Introduction to Multicomponent Distillation 161

5.1. Calculational Difficulties 161

5.2. Profiles for Multicomponent Distillation 167

5.3. Summary—Objectives 172

Chapter 6: Exact Calculation Procedures for Multicomponent Distillation 176

6.1. Introduction to Matrix Solution for Multicomponent Distillation 176

6.2. Component Mass Balances in Matrix Form 178

6.3. Initial Guess for Flow Rates 181

6.4. Bubble-Point Calculations 181

6.5. θ-Method of Convergence 184

6.6. Energy Balances in Matrix Form 191

6.7. Summary—Objectives 194

Appendix: Computer Simulations for Multicomponent Column Distillation 200

Chapter 7: Approximate Shortcut Methods for Multicomponent Distillation 205

7.1. Total Reflux: Fenske Equation 205

7.2. Minimum Reflux: Underwood Equations 210

7.3. Gilliland Correlation for Number of Stages at Finite Reflux Ratio 215

7.4. Summary—Objectives 219

Chapter 8: Introduction to Complex Distillation Methods 225

8.1. Breaking Azeotropes with Other Separators 225

8.2. Binary Heterogeneous Azeotropic Distillation Processes 227

8.3. Steam Distillation 234

8.4. Two-Pressure Distillation Processes 238

8.5. Complex Ternary Distillation Systems 240

8.6. Extractive Distillation 246

8.7. Azeotropic Distillation with Added Solvent 251

8.8. Distillation with Chemical Reaction 254

8.9. Summary—Objectives 258

Appendix: Simulation of Complex Distillation Systems 270

Chapter 9: Batch Distillation 276

9.1. Binary Batch Distillation: Rayleigh Equation 278

9.2. Simple Binary Batch Distillation 279

9.3. Constant-Level Batch Distillation 283

9.4. Batch Steam Distillation 284

9.5. Multistage Batch Distillation 285

9.6. Operating Time 291

9.7. Summary—Objectives 292

Chapter 10: Staged and Packed Column Design 301

10.1. Staged Column Equipment Description 301

10.2. Tray Efficiencies 309

10.3. Column Diameter Calculations 314

10.4. Sieve Tray Layout and Tray Hydraulics 320

10.5. Valve Tray Design 327

10.6. Introduction to Packed Column Design 329

10.7. Packed Column Internals 329

10.8. Height of Packing: HETP Method 331

10.9. Packed Column Flooding and Diameter Calculation 333

10.10. Economic Trade-Offs 341

10.11. Summary—Objectives 345

Chapter 11: Economics and Energy Conservation in Distillation 354

11.1. Distillation Costs 354

11.2. Operating Effects on Costs 359

11.3. Changes in Plant Operating Rates 366

11.4. Energy Conservation in Distillation 366

11.5. Synthesis of Column Sequences for Almost Ideal Multicomponent Distillation 370

11.6. Synthesis of Distillation Systems for Nonideal Ternary Systems 376

11.7. Summary—Objectives 380

Chapter 12: Absorption and Stripping 385

12.1. Absorption and Stripping Equilibria 387

12.2. Operating Lines for Absorption 389

12.3. Stripping Analysis 394

12.4. Column Diameter 396

12.5. Analytical Solution: Kremser Equation 397

12.6. Dilute Multisolute Absorbers and Strippers 403

12.7. Matrix Solution for Concentrated Absorbers and Strippers 406

12.8. Irreversible Absorption 410

12.9. Summary—Objectives 411

Appendix: Computer Simulations for Absorption and Stripping 421

Chapter 13: Immiscible Extraction, Washing, Leaching, and Supercritical Extraction 424

13.1. Extraction Processes and Equipment 424

13.2. Countercurrent Extraction 428

13.3. Dilute Fractional Extraction 435

13.4. Single-Stage and Cross-Flow Extraction 439

13.5. Concentrated Immiscible Extraction 443

13.6. Batch Extraction 444

13.7. Generalized McCabe-Thiele and Kremser Procedures 445

13.8. Washing 448

13.9. Leaching 452

13.10. Supercritical Fluid Extraction 454

13.11. Application to Other Separations 457

13.12. Summary—Objectives 457

Chapter 14: Extraction of Partially Miscible Systems 468

14.1. Extraction Equilibria 468

14.2. Mixing Calculations and the Lever-Arm Rule 471

14.3. Single-Stage and Cross-Flow Systems 474

14.4. Countercurrent Extraction Cascades 477

14.5. Relationship between McCabe-Thiele and Triangular Diagrams 485

14.6. Minimum Solvent Rate 486

14.7. Extraction Computer Simulations 488

14.8. Leaching with Variable Flow Rates 489

14.9. Summary—Objectives 492

Appendix: Computer Simulation of Extraction 499

Chapter 15: Mass Transfer Analysis 501

15.1. Basics of Mass Transfer 501

15.2. HTU-NTU Analysis of Packed Distillation Columns 504

15.3. Relationship of HETP and HTU 511

15.4. Mass Transfer Correlations for Packed Towers 513

15.5. HTU-NTU Analysis of Absorbers and Strippers 521

15.6. HTU-NTU Analysis of Co-current Absorbers 526

15.7. Mass Transfer on a Tray 528

15.8. Summary—Objectives 531

Chapter 16: Introduction to Membrane Separation Processes 535

16.1. Membrane Separation Equipment 537

16.2. Membrane Concepts 541

16.3. Gas Permeation 544

16.4. Reverse Osmosis 558

16.5. Ultrafiltration 573

16.6. Pervaporation 579

16.7. Bulk Flow Pattern Effects 588

16.8. Summary—Objectives 595

Appendix: Spreadsheets for Flow Pattern Calculations for Gas Permeation 603

Chapter 17 Introduction to Adsorption, Chromatography, and Ion Exchange 609

17.1. Sorbents and Sorption Equilibrium 610

17.2. Solute Movement Analysis for Linear Systems: Basics and Applications to Chromatography 621

17.3. Solute Movement Analysis for Linear Systems: Thermal and Pressure Swing Adsorption and Simulated Moving Beds 631

17.4. Nonlinear Solute Movement Analysis 654

17.5. Ion Exchange 663

17.6. Mass and Energy Transfer 672

17.7. Mass Transfer Solutions for Linear Systems 678

17.8. LUB Approach for Nonlinear Systems 687

17.9. Checklist for Practical Design and Operation 692

17.10. Summary—Objectives 693

Appendix: Introduction to the Aspen Chromatography Simulator 708

Appendix A: Aspen Plus Troubleshooting Guide for Separations 713 Answers to Selected Problems 715

PPT - Distillation V Multicomponent Distillation PowerPoint Presentation

Distillation V Multicomponent Distillation PowerPoint PPT Presentation

2- Constant Relative Volatility Method
  • Before in Lewis, during calculations you need to apply equilibrium relation but to do so you need temperature of that plate which is dependent on composition so trial and error needed.
  • In this method to get red of this difficulty will use relative volatility instead of k-values in relating the vapor and liquid composition (which are in eqm).

  • So, no more trial and error will be done on T.

    2- Constant Relative Volatility Method
    • What happens here is that for each component get average relative volatility (constant) and work with it a long the tower. So, Temperature will not be included in calculations.
  • Finally, this method will be like Lewis-Matheson in steps but without trials on T only the difference that equilibrium relation will be.

    Where is constant for each component along the tower.

    3- Shortcut Methods:a)Hengstebeck’s
    • It’s also called Pseudo binary system method.
  • System is reduced to an equivalent binary systemand is then solved by McCabe-Thiele method graphically.

  • The only method that solves the multi-component systems graphically (in our course of course).

  • Used as for preliminary design work.

    • Using the concept of light and heavy key components
  • So, we can consider the system as a binary system where it’s desired to separate the light key from the heavy key.

    According to that the molar flow rate of the non-key components can be considered constant. ****

    Also the total flow rates of vapour and liquid are considered constant.

    The method used for these calculations was developed by R.J.Hengstebeck, that’s why it’s called Hengstebeck’s method.

    Hengstebeck’s Method

    V=total molar vapour flow rate in the top section

    L=total molar liquidflow rate in the top section

    V’=total molar vapour flow rate in the bottom section

    L’=total molar liquidflow rate in the bottom section

    yni=mole fraction of component “i” in vapour phase on tray “n”

    xni=mole fraction of component “i” in liquid phase on tray “n”

    uni=molar vapour flow rate of component “i” from stage “n”

    lni=molar liquid flow rate of component “i” from stage “n”

    di=molar liquid flow rate of component “i” in top product

    wi=molar liquid flow rate of component “i” in bottom product

    Hengstebeck’s Method

    To reduce the system to an equivalent binary system we have to calculate the flow rates of the key components through the column (operating line slope is always L/V)

    The total flow rates (L and V) are constant, and the molar flow rates of non key components are constant, then we can calculate the molar flow rates of key components in terms of them.

    NOTE: The total flow rate is constant and the molar flow rates of non key components are constant, this does not mean that the molar flow rates of key components are constant as the mass transfer is equimolar.

    Hengstebeck’s Method

    If Le and Ve are the estimated flow rates of the combined keys.

    And li and ui are flow rates of the non-key components lighter than the keys in the top section.

    And l’i and u’i are flow rates of the non-key components heavier than the keys in the bottom section.

    Then slope of top section operating line will be Le/Ve

    And slope of bottom section operating line will be L’e/V’e

    Hengstebeck’s Method

    The final shape of the x-y diagram will be as shown

    We need to calculate:

    2) Estimate the number of ideal stages needed in the butane-pentane splitter defined by the compositions given in the table below. The column will operate at a pressure of 8.3 bar, with a reflux ratio of 2.5. The feed is at its boiling point.

    Compositions of feed, top and bottom products are shown in table below:

    Equilibrium constants were calculated and found to be:

    Light key will be:

    Heavy key will be: