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MODULE 12: “Heat and Mass Exchange Networks Optimization”

Program for North American Mobility in Higher Education Introducing Process Integration for Environmental Control in Engineering Curricula. MODULE 12: “Heat and Mass Exchange Networks Optimization”. 1. PURPOSE OF MODULE 12. What is the purpose of this module?

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MODULE 12: “Heat and Mass Exchange Networks Optimization”

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  1. Program for North American Mobility in Higher EducationIntroducing Process Integration for Environmental Control in Engineering Curricula MODULE 12: “Heat and Mass Exchange Networks Optimization” 1

  2. PURPOSE OF MODULE 12 What is the purpose of this module? This module is intended to convey and illustrate the basic principles and methodology of heat and mass networks optimization. It is applied to chemical engineering, especially touching the petroleum and paper industry. At the end of the module, the student should be able to understand the main concepts of the heat and mass exchange network and apply it to real world context. 2

  3. STRUCTURE OF MODULE 12 What is the structure of this module? Module 12 is divided in 3 “tiers”, each with a specific goal: • Tier 1: Basic concepts • Tier 2: Application examples • Tier 3: Open-ended problems in a real world context These tiers are intended to be completed in order. Students are quizzed at various points, to measure their degree of understanding, before proceeding. Each tier contains a statement of intent at the beginning, and a quiz at the end. 3

  4. Tier I BASIC CONCEPTS 4

  5. TIER 1 - STATEMENT OF INTENT The goal of Tier 1 is to provide the basic principles and solution methods for heat and mass exchange networks optimization with emphasis on retrofit, heat transfer and mass transfer analogy and optimization techniques. 5

  6. TIER 1 - CONTENTS Tier 1 is broken down into three sections: 1.1 Optimization of heat exchanger networks (HEN) by Pinch Analysis 1.2 Optimization of mass exchange networks 1.3 Application of optimization techniques to heat and mass exchange networks analysis At the end of this tier there is a short multiple answer Quiz. 6

  7. 1.1 OPTIMIZATION OF HEAT EXCHANGER NETWORKS (HEN) BY PINCH ANALYSIS 7

  8. 1.1 OPTIMIZATION OF HEAT EXCHANGER NETWORKS (HEN) BY PINCH ANALYSIS • Principles of Pinch Analysis • Methodology • Special problems in heat exchangers network design • Pinch analysis and energy integration • Special case of heat exchange • Retrofit design • Pinch software 8

  9. INTRODUCTION One important goal in our industry today: Minimize the utilities consumption (fuel, steam and cooling water) Methods based on thermodynamic analysis, that have the objective of minimizing the utilities consumption, are based on fundamental concepts that help to understand the problem of heat exchange. 9

  10. WHAT IS PINCH TECHNOLOGY? Pinch Technology provides a systematic methodology for energy saving in processes and total sites. The methodology is based on thermodynamic principles 10

  11. Utilities Heat Exchanger Network Separator Reactor Site-wide Utilities WHAT IS THE ROLE OF PINCH TECHNOLOGY IN THE OVERALL PROCESS DESIGN? The Onion Diagram • The design of the process starts with the reactors (the core) • Once feeds, products, recycle concentrations and flowrates are known, the separators (the second layer) can be designed • The basic process heat and material balance is now in place and the heat exchanger network (the third layer) can be designed • The remaining heating and cooling duties are handled by the utility systems (the fourth layer) Pinch Analysis starts with the heat and material balance for the process at this boundary 11

  12. PROCESS DATA EXTRACTION OF HOT AND COLD STREAMS FROM PROCESS FLOWSHEET SIMULATION DETERMINATION OF ENERGY TARGETS (NEEDS FOR HEATING AND COOLING) DATA EXTRACTION UTILIZATION OF HEURISTICS TO CONCEIVE A HEAT EXCHANGER NETWORK TO REACH ENERGY TARGETS AT A MINIMUM COST TARGETING DESIGN OPTIMIZATION THE PHASES OF PINCH ANALYSIS 12

  13. DATA EXTRACTION • Extraction of information required for Pinch Analysis from a given process flowsheet ant the relevant heat and material balance • Data extraction is THE KEY link betweenprocess and pinch analysis • The quality of data extraction has a direct influence on the quality of the final result of the analysis 13

  14. WHAT ARE WE SEARCHING FOR? • Thermal data must be extracted from the process • This involves the identification of process heating and cooling duties 14

  15. DEFINITIONS (1-2) • Hot streams are those that must be cooled or available to be cooled. e.g. product cooling before storage (heat sources) • Cold streams are those that must be heated. e.g. feed preheat before a reactor (heat sinks) • Utility streams are used to heat or cool process streams when heat exchange between process streams is not practical or economic (e.g cooling water, air, refrigerant) 15

  16. DEFINITIONS (2-2) For each hot and cold stream identified, the following thermal data is extracted: • TS : supply temperature, the temperature at which the stream is available (oC) • TT : target temperature, the temperature the stream must be taken to (oC) • ΔH : enthalpy change of streams (kW) • CP: heat capacity flow rate CP = Cp * M (kW/oC = kJ/oC kg * kg/s) 16

  17. TYPICAL STREAM DATA 17

  18. NOTION OF ΔTmin (1-2) • ΔTmin is the minimum temperature difference, imposed in the system; under this value, heat exchange between two streams is not possible • Thus, the temperature of the hot and cold streams at any point in exchangers must always have at least a minimum temperature difference (ΔTmin) • The selection of ΔTmin value has implications for both capital and energy costs 18

  19. NOTION OF ΔTmin (2-2) • In each temperature interval, each cold and hot stream has to be separated at least by ΔTmin. The principle of modified temperatures has to be introduced: • for a cold stream : Tmodified = T + (ΔTmin/2) • for a hot stream : Tmodified = T - (ΔTmin/2) 19

  20. COMPOSITE CURVES • Composite curves consist of temperature-enthalpy profiles of heat availability in the process (the hot composite curve) and head demands in the process (the cold composite curve) • Composite curves allow to determine and visualize the pinch point and the energy targets (heating and cooling demands) 20

  21. HOW TO DO IT? - A stream with a constant CP value is represented by a straight line running from TS to TT - When there are a number of hot and cold streams, the construction of hot and cold composites curves involves the addition of the enthalpy changes of the streams in the respective temperature intervals See Fig. (a), (b) 21

  22. T (oC) Cooling required QCmin Heating required QHmin Internal recuperation of heat Hot composite curve TPINCH Cold composite curve Pinch point H (kW) RESULT 22

  23. PINCH GOLDEN RULES • Do not transfer heat across pinch • Do not use cold utilities above the pinch • Do no use hot utilities below the pinch 23

  24. SUMMARY • The composite curves provide overall energy targets BUT... • They do not clearly indicate how much energy is supplied by different utility levels SOLUTION... • The utility mix is determined by the Grand Composite Curve (GCC) 24

  25. GRAND COMPOSITE CURVE • It shows the utility requirements in both enthalpy and temperature terms • It is used to optimize the utilities network when the utilities are available at different quality levels • It is useful for integrating special equipments: cogeneration, heat pump, etc. 25

  26. QHmin T Heat sink Pockets of heat recovery Pinch point Heat source ΔH QCmin GRAND COMPOSITE CURVE 26

  27. DESIGN A HEAT EXCHANGER NETWORK (HEN) • Application of heuristics to design a heat exchanger network with the objectives of: Reaching energy targets Respecting pinch rules 27

  28. DEVELOP A HEN FOR A MAXIMUM ENERGY RECOVERY (MER) (1-2) • Divide the problem at the pinch: above the pinch and below the pinch • Design hot-end, starting at the pinch: • Pair up exchangers according to CP and number of streams “N” constraints • Immediately above the pinch, pair up streams such that CPHOT CPCOLD , NHOT NCOLD • Add heating utilities as needed (QHmin) 28

  29. DEVELOP A HEN FOR A MAXIMUM ENERGY RECOVERY (MER) (1-2) • Design cold-end, starting at the pinch: • Pair up exchangers according to CP and number of streams “N” constraints • Immediately above the pinch, pair up streams such that CPHOT CPCOLD , NHOT NCOLD • Add heating utilities as needed (QCmin) 29

  30. MINIMUM NUMBER OF HEAT EXCHANGERS (Umin) The minimum number of heat exchangers in a network is given by Umin = Nstream + Nutilities - 1 where Nstream is the total number of streams and Nutilities the total number of utilities in the heat exchanger network 30

  31. SPECIAL PROBLEMS INHEN DESIGN • Introduction on a same stream of: • Splitting • Mixing • Elimination of loops More opportunities More complex Frequently the only way of getting Umin 31

  32. NOTION OF OPTIMAL ΔTmin • At the beginning, an arbitrary Tmin is fixed • The goal is to find an optimal Tmin for a minimum cost • The total cost is function of the utility cost and the heatexchanger cost • Utility cost = f(Qc, Qh)  it is an energetic cost • Heat exchanger cost = f(exchange area)  it is a capital cost 32

  33. ESTIMATION OF THE ENERGY COST Energy cost = (Costcold utility X Qc) + (Costhot utility X Qh)  where the cost unit is $/kW and Qc unit is kW 33

  34. ESTIMATION OF HEN CAPITAL COST (1-3) The capital cost of a HEN depends on 3 factors: • the number of exchangers • the overall network area • the distribution of area between the exchangers Capital cost =  + .A  where A is the exchange area and , ,  are economical and technical factors 34

  35. ESTIMATION OF HEN CAPITAL COST (2-3) Using a temperature-enthalpy diagram and the composite curves, the estimation of the exchange area can be obtained by: Amin = (1/ TLM * qj/hj) COMPLETER.....mettre le i! where i: enthalpy interval j: jth stream TLM: log mean temperature difference or LTMD qj: enthalpy change of the jth stream in the interval i hj: transfert coefficient of jth stream 35

  36. T (oC) HEN AREAmin = A1 + A2 + A3 +...+ Ai Enthalpy intervals in the composite curves A1 A2 A3 A4 A5 H (kW) ESTIMATION OF HEN CAPITAL COST (3-3) Estimation of exchange area 36

  37. OPTIMAL ΔTmin • To arrive to an optimum Tmin, the total annual cost (the sum of total annual energy and capital cost) is plotted at varying values (see next page). Three key observations can be made: • an increase in Tmin values result in higher energy costs and lower capital costs • a decrease in Tmin values result in a lower energy costs and higher capital costs • an optimum Tmin exists where the total annual cost of energy and capital costs is minimized 37

  38. Total cost Annualized cost Energy cost Capital cost Optimum Tmin Tmin ENERGY-CAPITAL COST TRADE OFF (OPTIMAL ΔTmin) 38

  39. RETROFIT DESIGN • For a new process: the application of pinch concepts is relatively easy: • low uncertainty for data extraction • low constraints in the process • For an existing process: the application of pinch concepts is more complicated: • technical, geographical and economical constraints 39

  40. DATA EXTRACTION FOR A RETROFIT DESIGN • Data is extracted from the existing process and indeed from a simulation that has to be validated on-site • Validate a simulation is difficult: it can take up to one year! The cost is too high! • Data are less reliable and the quality of the pinch analysis decreases 40

  41. HEN IN RETROFIT DESIGN • There is already in the process violation of the golden rules • Some exchangers are already installed, used or not, have to be taken into account  important for the investment/capital cost • The geographical constraints are important for fitting of equipment in a limited space 41

  42. OPTIMAL ΔTmin IN RETROFIT DESIGN • New factors have an influence on the determination of the optimum ΔTmin: • Geographical constraints that have an impact on the capital cost • Investments already realized for the actual network • Preservation of the efficiency of the actual network • In some cases, we can use Δtmin in the actual HEN or use a ΔTmin from similar processes 42

  43. Industrial sector Experience Tmin values o Oil refining 20 – 40 C o Petrochemical 10 – 20 C o Chemical 10 – 20 C Low temperature o 3 – 5 C processes OPTIMAL ΔTmin IN RETROFIT DESIGN 43

  44. PINCH SOFTWARES • Super Target (Linhoff March) • Pinch Express (Linhoff March) • Aspen Pinch (Aspentech) • Hint (Angel Martin, freeware) • available on www.heatintegration.com • These softwares include the basic concepts of pinch analysis and optimization tools can be integrated 44

  45. 1.2 OPTIMIZATION OF MASS EXCHANGE NETWORKS 45

  46. 1.2 OPTIMIZATION OF MASS EXCHANGE NETWORKS • Heat transfer and mass transfer analogy • Equipment configurations • The three types of mass exchange networks analysis 46

  47. HEAT TRANSFER AND MASS TRANSFER ANALOGY • There is an analogy between the exchange potentials (temperature differences and concentration differences) and the quantities that are exchanged (enthalpy and mass) • Parameters such flux, transfercoefficient, exchangerate and other nondimensional numbers appear in the two fields, have similar roles, but the way they are expressed are sometimes really different 47

  48. HEAT TRANSFER AND MASS TRANSFER ANALOGY Source:Manousiouthakis, 1999 48

  49. MASS EXCHANGE NETWORK • Mass exchange operations are important to limit or eliminate sources of industrial pollution • In process integration, mass exchange operations are used to transfer selectively some undesirable species starting from process streams (called rich streams) to mass separating agents (MSA) that act as receiving streams (called lean streams) 49

  50. MASS EXCHANGER • Definition: a mass transfert unit by direct or indirect contact that use a MSA (lean phase) to remove selectively some compounds (for example pollutants) from a rich phase (for example a waste stream) • Mass exchangers are present in processes of absorption, adsorption, liquid-liquid extraction, desorption, etc. 50

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