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Augsburg University of Applied Sciences | Faculty of Mechanical and Process Engineering Brno University of Technology | Faculty of Mechanical Engineering.

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Presentation on theme: "Augsburg University of Applied Sciences | Faculty of Mechanical and Process Engineering Brno University of Technology | Faculty of Mechanical Engineering."— Presentation transcript:

1 Augsburg University of Applied Sciences | Faculty of Mechanical and Process Engineering Brno University of Technology | Faculty of Mechanical Engineering | Institute of Process and Environmental Engineering Projektování a řízení procesů (KPJ) Conceptual Design of Distillation, Absorption and Stripping Systems Prof. Dr.-Ing. Marcus Reppich Room D5/249  Important notice: These documents are to be used exclusively for study purposes, they are made available to participants of the lecture Conceptual Design of Distillation, Absorption and Stripping Systems (Projektování a řízení procesů, KPJ) at the Institute of Process and Environmental Engineering at the Brno University of Tech- nology only. Cover image: Copyright by BASF SE

2 Conceptual Design of Distillation, Absorption and Stripping Systems Timetable and Contents of Lectures and Exercises Lectures P 09Fundamentals of Binary Distillation P 10Types of Distillation Columns P 11Design of Distillation Columns Exercises Assignment date C 10 Design of a Multicomponent Distillation System Using the Process Simulation Software CHEMCAD (group work of two students, elaboration of a final project report) C C Due date

3 Conceptual Design of Distillation, Absorption and Stripping Systems General Requirements for Project Final Reports Each group must submit a written final report. Your report should be written in Czech, English or German language, single-column, 1 inch margins, 11-point size Arial. The report must have a title page, a table of contents, the assignment, main content, references, if necessary, an appen- dix, and should comprise maximum 20 pages. The final report in PDF format and the cc6 file must be submitted by both to the instructor Ing. Bohuslav Kilkovský, Ph.D. and the lecturer Prof. Dr.-Ing. Marcus Reppich on or before the due date. Submission deadline is 6:00 p.m. December 15, To facilitate the grading of your written report and to ensure that your final report is clear and concise, please follow the suggested organization listed below. The title page should include the assignment number the title of the CHEMCAD project the assignment date and the due date group member names and addresses course name, and instructor's and lecturer´s name Table of contents Assignment

4 Conceptual Design of Distillation, Absorption and Stripping Systems General Requirements for Project Final Reports The main content of your report should at least include the following: Introduction (maximum 2 pages) Present a brief discussion to explain what the report is about. Summarize the given multicomponent separation problem. Discuss the background of your CHEMCAD project. Your approach (about 5 to 7 pages) Describe the basis of your CHEMCAD implementation (selection of thermodynamic model, selection of unit operations, boundary conditions, etc.). Describe your approach using CHEMCAD in detail, indicate assumptions made and their justifica- tion. Consider the final recommended design with appropriate data. You may present your results in any way you want as long as you believe it's convincing and clear. Include graphs, tables, and figures that are essential for understanding your approach. Validation of your approach (about 2 to 4 pages) Prove or demonstrate the effectiveness and/or efficiency of your approach. You may do this through various CHEMCAD case studies. Indicate possible sources of error. Conclusions and recommendations (maximum 2 pages) Summarize the conclusions of your project together with a clear statement of the recommendations. Discuss any problems occured. References List all the citations referenced in your paper. Appendix (if necessary: sample calculations, results of case studies, specification sheets, etc.)

5 Conceptual Design of Distillation, Absorption and Stripping Systems Recommended Literature E. J. Henley, J. D. Seader, D. K. Rooper: Separation Process PrinciplesHoboken: John Wiley & Sons 2011 P. C. Wankat:Separation Process EngineeringUpper Saddle River: Pearson Education 2012 W. L. McCabe, J. C. Smith, P. Harriot: Unit Operation of Chemical Engineering New York: McGraw-Hill 2005 M. F. Doherty, M. F. Malone: Conceptual Design of Distillation Systems New York: McGraw-Hill 2001 R. Smith:Chemical Process Design and Integration Chichester: John Wiley & Sons 2008 J. Benítez:Mass Transfer OperationsHoboken: John Wiley & Sons 2009 R. W. Rousseau (Ed.):Handbook of Separation Process Technology New York: John Wiley & Sons 1987 Chemstations, Inc.:CHEMCAD Version 6 User GuideHouston: Chemstations, Inc P. Ditl:Difúzně separační pochodyPraha: Nakladatelství ČVUT 2008 Resources for students on the server: dpee.fme.vutbr.cz  Výuka

6 1Fundamentals of Binary Distillation In binary distillation, a feed mixture of two components is separated into two products, an overhead distillate and a bottom product, whose compositions differ from that of the feed. Continuous distillation is a multistage, countercurrent sepa- ration process. Column containing the equivalent of N theo- retical stages is fed near its center at stage f with a steady flow of feed of definite composition. Assuming the feed is a liquid at its boiling point the feed flows down the stripping section to the bottom of the co- lumn, in which a definite level of liquid is maintained. Li- quid from the bottom stage 1 flows by gravity to partial re- boiler, which generates vapor and returns it to the bottom stage of the column. The liquid bottom product richer in the less volatile component is withdrawn from the reboiler. The vapor passes up the entire column. A total condenser in which the overhead vapor richer in the more volatile com- ponent leaving the top stage N is completely condensed to a bubble-point liquid distillate and a liquid reflux that is re- turned to the top stage. Inside the column, the liquids and vapors are always at their bubble and dew points respecti- vely, so that the highest temperature is at the bottom, the lowest at the top. For a binary mixture it is ordinarily possible by this method to separate the solution into its components, recovering each in any desired purity. Overhead vapor Top stage Total condenser Reflux Rectifying section (Enriching section) Stripping section (Exhausting section) Feed Bottom of column Partial reboiler N 1 f Source acc. to Gmehling, J.; Brehm, A.: Grund- opertionen. Weinheim: WILEY-VCH 1996 Bottom stage Feed stage Distillate Bottom product Top of column Boilup | Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems | 6 |

7 1Fundamentals of Binary Distillation Applications and Distillation Equipment © BASF SE Applications: the most widely used large-scale method for separating homogeneous fluid mixtures in the chemical and petrochemical industry if no azeotropes are encountered, overhead and bottom products may be obtained in any desired purity suitable for the separation of liquid mixtures of components having similar boiling points into their individual components (at low relative volatility, but >1,05) Equipment: Tray Columns (stagewise contact between the phases on individual trays) Packed Columns (continuous contact bet- ween the phases on the surface of a pack- ing material) | Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems | 7 |

8 1Fundamentals of Binary Distillation Overview of Calculation Methods Types of Binary Distillation Calculation  Energy requirements and heat exchanger design for a given adiabatic separation process (calculate the heat duties of the condenser and reboiler, specify the heating steam consumption and coolant requirements, thermal-hydraulic design of the condenser and reboiler)  Determination of main dimensions of the distillation column: estimating the number of equilibrium stages required for a given separation, the column height and the column diameter for a desired pressure drop (H = f(N), D) Historical Review of Calculation Methods until 1970s: simplified, partially graphical design procedures for tray columns separating binary mixtures: Ponchon-Savarit (1921/22), McCabe-Thiele (1925) approximate calculation methods for the solution of multicomponent, multistage sepa- ration problems (Shortcut methods): Fenske (1932), Gilliland (1940), Underwood (1946) design of packed columns based on NTU/HTU concepts: Chilton, Colburn (1935) in the present:complex mathematical matrix methods allow to find exact solutions of nonlinear equation systems: Wang-Henke (1966), Naphtali-Sandholm (1971) commercial process simulation software allowing design and rating calculations of tray and packed columns operating at steady or unsteady state conditions (ASPEN , CHEMCAD  ) | Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems | 8 |

9 1Fundamentals of Binary Distillation Assumptions for an Approximate Calculation of the Binary Distillation Assumptions and Simplifications: 1)The two components have equal and constant molar enthalpies of vaporization (latent heats). 2) The component heat capacity changes and the heat of mixing are negligible compared to the heat of vaporization (considering ideal behaviour of binary mixtures). 3) The distillation column, the condenser and the reboiler are well insulated so that heat losses to environment are negligible. 4)The pressure is constant throughout the column, no pressure drop occurs. The above assumptions lead to the concept of constant molar overflow. This approach assumes that the amount of molecules which evaporate and which condensate in each stage are the same or nearly the same. That means, that all liquid and vapor molar flow rates in the rectifying section are constant and that all liquid and vapor molar flow rates in the stripping section are constant but not the same as those in the rectifying section. Further requirements are: 5)Kinetic and potential energies are negligible. 6)The distillation column is operated at continuous steady state conditions. 7) The streams leaving each stage are assumed to be in vapor-liquid equilibrium. The liquids and vapors are always at their bubble points and dew points, respectively. | Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems | 9 |

10 Balances for Continuous Distillation Column Material and energy balances for the entire column: Overall mole balance Light component mole balance Distillate flow rate Bottoms flow rate Overall energy balance Material and energy balances for a total condenser: Mole balance Energy balance If the overhead product and liquid reflux have the same composition, temperature and pressure then Condenser duty Assuming that the condenser removes latent heat only and the condensate and the returned reflux is liquid at its bubble point, we get Important: All mole fractions refer to light component. 1Fundamentals of Binary Distillation Approximate Calculation of the Binary Distillation Material and Energy Balances | Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems | 10 |

11 1Fundamentals of Binary Distillation Approximate Calculation of the Binary Distillation Material and Energy Balances Balances for Continuous Distillation Column Balance for the column and partial reboiler: Energy balance Reboiler duty If the distillate and reflux have the same composition, temperature and pressure, then ; Assuming the overhead vapor leaves the column as saturated vapor and liquid reflux enters the column at its bubble-point, we get and therefore | Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems | 11 |

12 The mathematical-graphical McCabe-Thiele Method can be used to determine the number of ideal stages N needed for a given separation of a binary mixture (to produce a distillate and a bottom product ) and column operating pressure. The method uses material balances around certain parts of the column and the equilibrium curve. Rectifying section including the total condenser: Overall mole balance Light component mole balance Rectifying section operating line (Upper operating line) Vapor-liquid equilibrium on stage j (3) Stripping section including the partial reboiler: Overall mole balance Light component mole balance Stripping section operating line (Lower operating line) Vapor-liquid equilibrium on stage i N N -1 j +1 j N +1 i -1 i 1 0 (1) (2) (4) (5) 1Fundamentals of Binary Distillation Approximate Calculation of the Binary Distillation McCabe-Thiele Method for Trayed Towers | Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems | 12 |

13 The quantities of the liquid and the vapor streams change abruptly at the feed tray f since the feed may consist of liquid, vapor, or a mixture of both. The feed-stage condition influences the change in slope of the operating lines: a) Feed is a saturated liquidb) Feed is a saturated vapor Definition of thermal condition of the feed stream: Mole balance around the feed stage(6) Energy balance around the feed stage Parameter q(7) According to eq. (7) the parameter q is defined as the ratio of increase in liquid molar flow rate across the feed stage to the molar feed rate. The value of q can be estimated from the energy to convert one mol of the feed to saturated vapor divided by the molar enthalpy of vaporization of the feed. Molar enthalpy of the feed RS SS f + 1 f - 1 f f + 1 f - 1 f 1Fundamentals of Binary Distillation Approximate Calculation of the Binary Distillation McCabe-Thiele Method for Trayed Towers: Thermal Conditions of Feed | Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems | 13 |

14 1Fundamentals of Binary Distillation Approximate Calculation of the Binary Distillation McCabe-Thiele Method for Trayed Towers: Thermal Conditions of Feed Possible feed-stage conditions: a)Saturated liquid b)Saturated vapor c)Subcooled liquid d)Superheated vapor e)Partially vaporized (liquid + vapor) The parameter q can be used to determine the point of intersection of the operating lines for the rectifying and stripping sections. Substracting equation (1) from (4) gives: Effect of thermal condition of feed on slope of q-line (8) Combining (6) and (7) with the ligth component mole balance yields: The equation for the q-line, dividing the column into the rectifying and stripping section, becomes: (8) 01 1 q < 0 q = 0 0 < q < 1 q = 1 q > 1 Eq. (8)       | Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems | 14 |

15 In 1925, McCabe and Thiele published an approximate graphical method for combining the equilibrium curve for a bi- nary system with operating lines to estimate the number of equilibrium stages required for a desired degree of separa- tion of the feed. 1)Draw the equilibrium curve according to eq. (3), e.g.: 2)Draw the rectifying section operating line (1) resp. (2): a)Locate the distillate product composition on the diagonal line b)Determine the slope from the selected reflux ratio or the ordinate intercept 3)Draw the q-line according to eq. (8): a)Locate the feed composition on the diagonal b)Determine the parameter q from eq. (7) 6)Step off the theoretical stages, changing operating lines at the feed stage The staircase can be stepped of from the top to the bottom, starting from the point on the diago- nal. Alternatively, the stages can be stepped of from the bottom (from the partial reboiler) at the point to the top. c)Calculate the slope 4)Determine the point of intersection S of the rectifying section operating line (2) and the q-line (8) 5)Draw the stripping section operating line (4) resp. (5) a)Locate the bottoms product composition on the diagonal b)Join with the point of intersection S Fundamentals of Binary Distillation Approximate Calculation of the Binary Distillation McCabe-Thiele Method for Trayed Towers: Graphical Construction S | Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems | 15 | 0 N p=const.

16 1Fundamentals of Binary Distillation Approximate Calculation of the Binary Distillation McCabe-Thiele Method for Trayed Towers: Limiting Conditions For a given specification, the reflux ratio can be selected anywhere from the minimum to an infinite value. As a result, there are two important limiting conditions that need to be considered for distillation: a) Minimum Number of Equilibrium Stages N min at Total Reflux The entire overhead vapor is condensed and returned to the top stage as reflux. Furthermore, all liquid leaving the bottom stage is vaporized and returned as boilup to the column. Thus, no products are withdrawn from the column and there is no feed: At this limiting condition, both the rectifying and stripping operating lines coincide with the diagonal line, see equations (2) and (5) and neither the feed composition nor the q-line influences the staircase construction. The minimum number of equilibrium stages N min can be determined graphically or by the Fenske equation: The infinite reflux ratio corresponds to the minimum number of equilibrium stages required for the separation, thus to minimum capital costs, at infinite operating costs (heat for the reboiler, condenser coolant, power for the reflux pump) | Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems | 16 |

17 1Fundamentals of Binary Distillation Approximate Calculation of the Binary Distillation McCabe-Thiele Method for Trayed Towers: Limiting Conditions b) Minimum Reflux Ratio r min As the reflux ratio decreases from the limiting case of total reflux, the intersection of the two operating lines and the q-line moves from the diagonal toward the equilibrium curve. The number of equilibrium stages required increases because the operation lines move closer and closer to the equilibrium curve, thus requiring more and more stairs to move from the top of the column to the bottom. Finally, a limiting condition is reached when the point of intersection S´ is on the equilibrium curve and therefore an infinite number of stages is required. The slope of the rectifying operating line is given by eq. (2): The minimum reflux ratio r min can be determined graphically from the ordinate intercept of the rectifying operating line or by the Underwood equation (that can be applied to saturated- liquid feed, q = 1): The minimum reflux ratio corresponds to the need for an infinite number of stages at a minimum boilup ratio, thus at minimum operating costs necessary for the separation  Pinch Point | Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems | 17 |

18 1Fundamentals of Binary Distillation Approximate Calculation of the Binary Distillation McCabe-Thiele Method for Trayed Towers: Optimum Reflux Ratio An distillation column must be operated between the two limiting conditions of minimum reflux r min and total reflux, with the corresponding number of equilibrium stages re- quired varying from infinity to the minimum number N min. The reflux ratio to be used for a new design should be the optimum, the one for which the total annual cost C tot of the distillation, which is the sum of the installed capital and operating costs, will be the least. The reflux ratio influences both the number of stages required (and thus the installed capital cost C cap  1/r) and the energy requirements (and thus the operating costs C op  r). The total cost must pass through a minimum at the optimum reflux ratio, that frequently occurs in the range of 1,05 · r min < r opt < 1,5 · r min at high energy costs at high costs of construction materials A first estimate of the optimum reflux ratio can be obtained from r opt = 1,2 · r min. C cap C op C tot total installed annual capital cost annual operating and maintenance cost (heating and cooling costs) total annual cost r min r opt r C cap (r) C op (r) C tot (r) C [€/a] | Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems | 18 |

19 1Fundamentals of Binary Distillation Approximate Calculation of the Binary Distillation McCabe-Thiele Method for Trayed Towers: Additional Remarks For a highly nonideal binary mixture, the operating line for the rectifying section is tangent to the equilibrium curve. The slope of this tangent operating line cannot be reduced any further because it would then cross over the equilibrium curve. In this case, the minimum reflx ratio r min can be determined graphically from the ordinate intercept of the tangent: Using an energy balance around the entire distillation column it is possible to show that the reflux ratio r and the boilup ratio s are related by the following equation: | Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems | 19 |


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