Department of Chemical Engineering Unive

how to make flow sheet

1. Process F lowsheeting P. Balasubramanian Department of Chemical Engineering Universiti Teknologi PETRONAS Malaysia

2. Introduction The flowsheet is a key document in process design It shows the arrangement of the equipment selected to carry out the process; the stream connections; stream flow-rates and compositions; and the operating conditions . The f lowsheet is a diagrammatic model of the process 7/9/2012 2

3. Introduction The flow-sheet will be used by the specialist design groups as the basis for their designs This will include piping, instrumentation, equipment design and plant layout It will also be used by operating personnel for the preparation of operating manuals and operator training During plant start-up and subsequent operation, the flow-sheet forms a basis for comparison of operating performance with design 7/9/2012 3

4. Introduction The flow-sheet is drawn up from material balances made over the complete process and each individual unit Energy balances are also made to determine the energy flows and the service requirements Manual flow-sheeting calculations can be tedious and time consuming when the process is large or complex C omputer-aided flow-sheeting programs are being increasingly used to facilitate this stage of process design 7/9/2012 4

5. Flowsheet presentation The process flowsheet is the definitive document on the chemical process The presentation in the process flowsheet must be clear, comprehensive, accurate and complete 7/9/2012 5

6. Types of flowsheet Block diagrams Pictorial representation Presentation of stream flow-rates Layout Precision of data Basis of the calculation Batch processes Services Equipment identification 7/9/2012 6

7. Block diagrams It is the simplest form of presentation Each block can represent a single piece of equipment or a complete stage in the process Only a limited use as engineering documents The blocks can be of any shape use a mixture of squares and circles, drawn with a template 7/9/2012 7 process feed product Fig.1 Block diagram for a chemical plant.

8. Pictorial representation On the detailed flow-sheets used for design and operation, the equipment is normally drawn in a stylized pictorial form 7/9/2012 8

9. 7/9/2012 9 Fig. 2. Example for pictorial representation.

10. Presentation of stream flow-rates The data on the flow-rate of each individual component, on the total stream flow-rate, and the percentage composition can be shown in a tabular form E ach stream line is numbered and the data tabulated at the bottom of the sheet Alterations and additions can be easily made 7/9/2012 10

11. Information to be included Essential information Stream composition, either: flow-rate of each individual component in kg/h stream composition as a weight fraction Total stream flow-rate in kg/h Stream temperature in degrees Celsius Nominal operating pressure Optional information Molar percentages composition Physical property data, mean values for the stream, such as: density , kg/m 3 viscosity , mN s/m 2 Stream name, example “ACETONE COLUMN BOTTOMS”. Stream enthalpy in kJ/h 7/9/2012 11

12. Layout The sequence of the main equipment items shown symbolically on the flow-sheet follows that of the proposed plant layout Equipment should be drawn approximately to scale The table of stream flows and other data can be placed above or below the equipment layout and the n ormal practice is to place it below 7/9/2012 12

13. Precision of data The total stream and individual component flows do not normally need to be shown to a high precision on the process flow-sheet A t most one decimal place is sufficient Imprecise small flows are best shown as trace (in parts per million) A trace quantity should not be shown as zero 7/9/2012 13

14. 7/9/2012 14 Fig. 3 Flow sheet-polymer production Source: Coulson & Richardson’s CHEMICAL ENGINEERING Vol. 6, Fourth ed., Chemical Engineering Design

15. 7/9/2012 15 Fig. 4 Flow sheet-simplified nitric acid process Source : Coulson & Richardson’s CHEMICAL ENGINEERING , Vol. 6, Fourth ed., Chemical Engineering Design

16. 7/9/2012 16 Fig. 5 A typical Flow sheet Source : Coulson & Richardson’s CHEMICAL ENGINEERING , Vol. 6, Fourth ed., Chemical Engineering Design

17. Basis of the calculation Show the basis used for the flow-sheet calculations O perating hours per year R eaction and physical yields, and D atum temperature used for energy balances list of the principal assumptions used in the calculations (to know about any limitations). 7/9/2012 17

18. Batch processes Flow-sheets drawn up for batch processes normally show the quantities required to produce one batch If a batch process forms part of an otherwise continuous process, it can be shown on the same flow-sheet, providing a clear break is made when tabulating the data between the continuous and batch sections; the change from kg/h to kg/batch. A continuous process may include batch make-up of minor reagents, such as the catalyst for a polymerization process. 7/9/2012 18

19. Services and Equipment identification The service connections required on each piece of equipment should be shown and labeled The service (utility) requirements for each piece of equipment can be tabulated on the flow-sheet Each piece of equipment shown on the flow-sheet must be identified with a code number and name The identification number (usually a letter and some digits) will normally be that assigned to a particular piece of equipment as part of the general project control procedures , and will be used to identify it in all the project documents The easiest code is to use an initial letter to identify the type of equipment followed by digits to identify the particular piece For example , H-heat exchangers, C-columns , R-reactors 7/9/2012 19

20. Manual flowsheet calculations The stream flows and compositions are calculated from material balances; combined with the design equations that arise from the process and equipment design constraints T wo kinds of design constraints External constraints Internal constraints External constraints not directly under the control of the designer, and which cannot normally be relaxed Examples Product specifications, possibly set by customer requirements Major safety considerations, such as flammability limits Effluent specifications, set by government agencies 7/9/2012 20

21. Manual flowsheet calculations Internal constraints determined by the nature of the process and the equipment functions The process stoichiometry, reactor conversions and yields Chemical equilibria Physical equilibria , involved in liquid-liquid and gas/ vapour -liquid separations Azeotropes and other fixed compositions Energy-balance constraints where the energy and material balance interact, as for example in flash distillation Any general limitations on equipment design 7/9/2012 21

22. Computer aided flowsheeting Full simulation programs, which require powerful computing facilities capable of carrying out rigorous simultaneous heat and material balances, and preliminary equipment design: producing accurate and detailed flow-sheets Simple material balance programs requiring only a relatively small core size 7/9/2012 22

23. Simulation packages 7/9/2012 23 acronym type source ASPEN Steady state Aspen Tech Aspen DPS DESIGN II Steady state WinSim HYSYS Steady state dynamic Hyprotech PRO II Steady state SimSci-Esscor DYNSIM dynamic CHEMCAD Steady state Chemstations Table 1. Commercial packages for process flowsheet simulation.

24. Process simulation program Sequential-modular programs: E quations describing each process unit (module ) are solved module-by-module in a stepwise manner and iterative techniques used to solve the problems arising from the recycle of information They simulate the steady-state operation of the process and can be used to draw-up the process flow sheet, and to size individual items of equipment, such as distillation columns Equation based programs T he entire process is described by a set of differential equations , and the equations solved simultaneously not stepwise, as in the sequential approach Equation based programs can simulate the unsteady-state operation of processes and equipment 7/9/2012 24

25. Example The hydrodealkylation (HDA) process converts toluene to benzene in the presence of a large excess of hydrogen Main reaction: dealkylation of toluene to benzene and methane C 6 H 5 -CH 3 +H 2 C 6 H 6 +CH 4 Secondary reaction: formation of naphthalene as by-product 2C 6 H 6  C 12 H 10 + H 2 7/9/2012 25

26. Example The reaction, globally exothermic, takes place in an adiabatic Plug Flow Reactor at pressures of 25 to 35 bars and temperatures between 620 and 720 o C Large excess of hydrogen , typically 5:1 molar ratio, prevents the formation of coke The reaction conversion is typically 60-80%, because at higher value the selectivity drops rapidly 7/9/2012 26

27. Example 1 7/9/2012 27 Fig. 6 UOP HDA process for benzene production Source: Alexandre C. Dimian , Integrated design and simulation of chemical processes, Elsevier, 2003. FEHE: feed effluent heat exchanger

28. Example 1 Problem analysis Input/Output streams Reactor system Reactor-Separation-Recycle system Separation system Control of flowsheet specifications Transformation of real units in simulation units Degrees of freedom analysis Thermodynamic issues Tear streams and computational sequence 7/9/2012 28

29. Example 1 Input/Output streams I nput streams: Toluene of 100% purity and Hydrogen with 5% CH 4 At steady state, material balance must be consistent. That is, Mass input = Mass output Output streams: Benzene product, lights gases from the stabilization column, and heavies from the distillation column A gaseous purge stream (methane is entered as impurity with the hydrogen feed 7/9/2012 29

30. Example 1 7/9/2012 30 Reaction Separation H ydrogen Toluene Benzene Lights Heavies Purge (to remove methane from hydrogen) Fig.7 Input-output streams for HDA process.

31. Example 1 Reactor analysis: The stoichiometric approach is simple but sufficient for material balance purposes This modeling approach needs to know, ( i ) the conversion of the main reaction and (ii) selectivity of the secondary reaction Kinetic model must be considered when accurate kinetic data is available for either CSTR or PFR Industrial reactors are much more complex as the ideal models Therefore, kinetic models are not important in steady state flowsheeting at least in the early stages of process design Furthermore, kinetic modeling is important in operation level or plant wide control 7/9/2012 31

32. Example 1 Reactor-separator-recycle system: 7/9/2012 32 Fig. 8 Reactor-separator-recycle structure of HDA process.

33. Example 1 The Flash unit used for the separation of gas and liquid phases from a mixture. A vapour /liquid equilibrium model can simulate this operation To simulate the purge we place the unit Split modeled by Stream Splitter T he gas is recycled via a compressor Comp simulated by a Compressor unit. The simulation of the liquid separation system is more complicated. Use black-box model for the liquid separation system 7/9/2012 33

34. Example 1 Separation system: The simulation of the train of distillation columns 7/9/2012 34 Fig. 9 Liquid separation section of the HDA process. Stab: stabilizer, and Dist : distillation column

35. Example 1 Control of flowsheet specifications: The molar ratio hydrogen/toluene at the reactor inlet should be kept strictly at 5:1 Manipulated variable: The split-ratio of the purge Simulates the steady state behavior of a SISO (single input single output) feedback controller 7/9/2012 35

36. Example 1 7/9/2012 36 Fig. 10 Flowsheet controller in the HDA process.

37. Example 1 Transformation of real units in simulation units: Some real unit operations can find direct correspondence with the 'blocks' used in flowsheeting , as flashes, distillation columns, heat exchangers, and so on However, the equivalence could be difficult for many others A simple model may be satisfactory for a quite complex unit M odeling of real units can follow one of the following possibilities: 7/9/2012 37

38. Example 1 Decomposition in elementary simulation blocks Example : an azeotropic distillation column may be decomposed in reboiled stripping column, heat exchanger, three-phase flash separator and reflux splitter Aggregation of units Example : a heat exchanger and a flash vessel may be combined in a single flash block Black box units Examples : membranes, dryers, special separations, and so on Add-on user units This possibility involves the existence of a programming environment , including the access to physical properties and other routines 7/9/2012 38

39. Example 1 Furnace = heater Cross heat exchanger: two-side heat exchanger or a single side heater and cooler coupled by a common duty Single flash with duty: steam generator, cooler and the phase separator flash Degrees of freedom analysis : the degrees of freedom analysis in a model is the number variables that can be specified independently Where, N F = degrees of freedom N V = total number of variables involved in the problem N E = number of independent equations 7/9/2012 39

40. Example 1 Three cases: Case 1: N F = 0. The problem is exactly determined. A unique solution exists. Case2: N F > 0. the problem is underdetermined. At least one variable can be optimized. Case 3: N F < 0. The problem is overdetermined . No solution 7/9/2012 40

41. Example 1 Degrees of Freedom Analysis: The number of variables that must be set in order to solve the system of equations describing the model In the HDA process we might encounter some problems in simulating rigorously the distillation columns because these are involved in a recycle loop Imposing exact values for products could lead to failure of computations because of inconsistency in the material balance Specification of component recovery as the ratio between component flow rate in product and feed, gives always convergence 7/9/2012 41

42. Example 1 Thermodynamic issues: selection of thermodynamic models Estimation of physical properties for non-library components Equation of state model, as for example Peng -Robinson, for the whole flowsheet Computational sequence: The flowsheet must be decomposed in computational sequences if there are recycle loops and/or design specifications . The streams necessary to be initialized are called tear streams. 7/9/2012 42

43. Example 1 7/9/2012 43 Fig. 11 Process Simulation Diagram of the HDA process.

44. Example 1 Three recycle loops may be identified : heat integration around the reactor recycle of hydrogen, and recycle of toluene We have three loops but only two tear streams Simulation procedure Once the process simulation diagram known, the following approach can be followed to run a simulation Draw the flowsheet Input the components Select the thermodynamic options Analyze the recycles and identify the tear streams Supply data for input and tear streams Supply specifications for the simulation units ( blocks) Run and make converge the simulation Analyze the results 7/9/2012 44

45. Process Simulation and Process Synthesis The design of chemical process involves synthesis and analysis Process synthesis is the overall development of a process flowsheet by combining individual steps(equipment and operating conditions) into an optimal arrangement. Process analysis breaks down the flowsheet to evaluate the performance of each individual element as well as how the overall process would perform, typically by a process simulator The flowsheet is synthesized with the use of a process synthesis model and simulation tool 7/9/2012 45

46. Process synthesis model source: CEP magazine, October 2005, 25-29 7/9/2012 46 reactor Separation and recycle Heat exchange network utilities Fig. 12 The onion model of process design.

47. 7/9/2012 47 Example 2 Fig. 13 Process synthesis.

48. 7/9/2012 48 Fig. 14 Reactor separator recycle system.

49. Hierarchical approach to process design Batch vs continuous Input-output structure of the flowsheet Recycle structure of the flowsheet General structure of the separation system Vapor recovery system Liquid recovery system Heat exchanger network 7/9/2012 49

50. Layer 1: Reactor Synthesis of a new process flowsheet should start at the heart of the chemical process. That is the reactor system What is the right reactor model(CSTR, PFR) and what are its operating conditions(isothermal, adiabatic, constant outlet temperature, vacuum and so on)? How should the product conversion and yield be determined? Is a catalyst needed in the reactor system modeling? 7/9/2012 50 reactor Raw materials Valuable products

51. Layer 2: Separation and recycle Products and any by-products formed in the reactor needed to be separated from unconverted reactant for further purification, while the unconverted raw material is recycled back to the reactor Separation system: vapor separation system and liquid separation system Flash column is used for phase separation: for example, mixture of liquid and vapor is separated into the phases Vapor separation system: condensers, flash tanks, absorbers, adsorbers , and gas separation membranes These unit operations are normally used to purify a vapor recycle stream before it re-enters the process A purge stream is always used to avoid undesired contaminant build-up 7/9/2012 51

52. Layer 2 Liquid separation systems include Distillation (including extractive distillation) Solvent extraction Stripping Filtration (including membrane separation) Centrifugation and so on 7/9/2012 52

53. Layer 2 7/9/2012 53 Reactor system flash Vapor separation system Liquid separation system feed purge products byproducts Fig. 15 The overall separation scheme consists of vapor, liquid and flash separations

54. Recycle system: Tear stream concept 7/9/2012 54 A B C D E F R 1 R 2 Recycle stream Tear recycle stream Unit operation in simulator Fig. 16 The tear stream concept is used in recycle simulation.

55. Tear Recycle stream Use the concept of a tear stream in modeling of a recycle loop In Fig. 16, the recycle stream after unit F is considered as two separate tear streams R 1 and R 2 Step 1: Perform the simulation for the units A and B Step 2: Assume initial value for R 1 and perform the simulation of unit C Step 3: Perform the simulation for the units D , E and F Step 4: After unit F converges, compare the flowrate of stream R 2 with the initial value for R 1 Step 5: If the values of R 1 and R 2 agree to within a specified tolerance, it is likely that the simulation model has converged. The calculated value of R 2 is used in place of R 1 in unit C and the simulation is rerun Step 6: if tear streams R 1 and R 2 do not agree to within the specified tolerance, the initial guess for R 1 must be revised and perform the simulation(without connecting the recycle stream to unit C ) until the convergence is achieved. 7/9/2012 55

56. Layer 3: Heat exchanger network The process heating and cooling loads are determined after the process structure within the two inner layers of the onion model has been finalized Design and model the heat exchanger network Use the tool of process integration Utility targeting Network design After a preliminary network has been synthesized, the process flowsheet will undergo a complete re-simulation for the verification of energy balances 7/9/2012 56

57. Layer 4: Utilities The selection of hot and cold utilities is another well established application of process integration Options to be explored for the placement of the heat pump and heat engine 7/9/2012 57

58. Example 2: Production of n-octane The onion model synthesis and simulation technique will be used to develop a process flowsheet for n-octane (C 8 H 18 ) production from ethylene(C 2 H 4 ) and i -butane(C 4 H 10 ). The component flowrates (some impurities) and stream specifications for the fresh feed are given Table 2. 7/9/2012 58 component Flow rate (kg- mol /h) specification Nitrogen N 2 0.1 T = 30 o C P = 20 psia Ethylene C 2 H 4 20 n-Butane C 4 H 10 0.5 i -Butane C 4 H 10 10 Table 2. Feed specifications for the production of n-octane.

59. Example 2: Production of n-octane Q1: Draw the input-output streams for n-octane production process. Q2: Develop process flowsheet for n-octane production process. 7/9/2012 59

60. Answer for Q1 7/9/2012 60 Reaction Separation purge n -octane N itrogen Ethylene n -Butane i - Butane Fig. Q1: Input-output streams for n -octane production process.

61. Example 2 Flowsheet development: Layer 1: Reactor Ethylene and i -butane react isothermally in a stoichiometric isothermal reactor at 93 o C to produce n-octane. The key limiting component is ethylene Overall conversion is 98% The pressure drop across the reactor is specified at 5 psi. The reaction is 7/9/2012 61

62. Example 2 Layer 2: Separation and recycle Flash column is added to the reactor effluent to separate unconverted raw materials from the desired product A pressure drop of 2 psi is introduced, while the operating temperature is maintained the same as that of the reactor The more volatile compounds (ethylene, i -butane and other impurities) are flashed to the top product stream together with a small portion of n-octane, while remaining n-octane leaves at the bottom An additional separation unit is needed to recover the n-octane from the top stream Distillation is then added to the flash column’s top product to recover n-octane 10 theoretical trays, operating pressure 15 psia Bottom product: n-octane Top products: volatile components 7/9/2012 62

63. Example 2 The unconverted raw material leaving at the top of the distillation column is recycled back to the reactor A purge stream is added before the stream is recompressed, reheated and sent back to the reactor Layers 3 and 4: the design of heat exchanger network and utility system will be handled simultaneously. 7/9/2012 63

64. Example 2 7/9/2012 64 Fig. 17 Preliminary flowsheet for the production of n-octane after completion of o nion model layers 1 and 2. Source: CEP magazine, October 2005, page 28.

65. Example 2 7/9/2012 65 Fig. 18 The complete flowsheet with a heat-integrated distillation column Source: CEP magazine, October 2005, page 28.

66. Outline of the hierarchical approach Hierarchical Approach is a methodological frame for the conceptual synthesis of process systems. The overall strategy is organized in a number of levels, each level being decomposed in a number of tasks. The approach presented below is organized in eight levels as follows (Douglas and Stephanopoulos, 1995 ): Level 0 - Input information Collect data on chemistry, raw materials, product specifications, economic constraints and legal regulations. Level 1 - Number of Plants Determine the number of plants for multi-step reactions. 7/9/2012 66

67. Outline of the hierarchical approach Level 2 - Input-Output Structure and Plant Connections Specify feed, exit and recycle streams for each plant. Specify interconnections between plants. Estimate minimum capital and operation costs. Level 3 - Recycle structure of the simple plants For each plant specify reactor type, recycle streams and flows. Set upper and lower limits for conversion by taking into account the cost of recycles . 7/9/2012 67

68. Outline of the hierarchical approach Level 4 - Separation systems of the simple plants For each plant specify the separation system and estimate its annualised cost. Additional hierarchical refinement reveals seven sub-levels. 4a. General separation structure: identify specific separation subsystems. 4b.Vapour separation and recovery system: separate gaseous products and recover valuable liquid components. 4c. Solid recovery system: recover valuable solids from solutions. 4d. Liquid separation system(s); separate products from liquid mixtures. 4e. Solid separation system: separate solid products. 4f. Combine separation systems in the whole flowsheet , and study interactions. 7/9/2012 68

69. Outline of the hierarchical approach Level 5. Process Integration 5a. Pinch Point Analysis: Heat Exchanger Network (HEN) design for optimal heat and power saving. 5b. Water Minimization: design an efficient system for the recycling of water. 5c. Solvent Minimization: design an efficient system for the recycling of solvents . Level 6. Evaluate Alternatives 6a. Examine alternative design decisions. 6b. Examine alternatives for the reactor design. 6c. Consider more efficient separation systems . 7/9/2012 69

70. Outline of the hierarchical approach Level 7. Hazop analysis 7a. Identify sources of hazards and risks. 7b. Perform hazard and operability study. Level 8. Control system synthesis 8a. Plantwide Control: develop the control strategy of the whole plant. 8b. Control structure for units: design control structures for individual units. 7/9/2012 70

71. Mass balances with recycle streams Law of conservation of mass Mass neither be created nor destroyed although it is transformed from one form to another form That is, mass input = mass output Mass balances for three simple units Mixer Separator Reactor 7/9/2012 71

72. Mass balances with recycle streams Mixer(MIXR) Separator(SEPR) 7/9/2012 72 MIXR separator Where, sf i is the split fraction: The fraction of component i going to the overhead stream

73. Mass balances with recycle streams Reactor(REAC) Stoichiometry N moles of A react, then ( b / a ) N moles of B also react and form ( c / a ) N moles of C and ( d / a ) N moles of D Limiting reactant here is A and conversion of A is x 7/9/2012 73 Reactor

74. Mass balances with recycle streams The equation for each of the components is 7/9/2012 74

75. Example 3: without recycle A stream in a refinery is at 100 psia and 75 o F and contains the following : Your task is to separate this stream into five streams, each of which is a relatively pure stream of one component. 7/9/2012 75 component flow rate ( lb mol / h) propane 100 i -Butane 300 n-Butane 500 i -Pentane 400 n-Pentane 500 total 1800

76. Example 3 7/9/2012 76 Fig. 19 Distillation train.

77. Example 3 7/9/2012 77 Fig. 20 Simplistic distillation train.

78. Example 3 7/9/2012 78 Fig.21 Distillation train modeled using simple SEPR units.

79. Example 3 7/9/2012 79 Stream number 1 2 3 4 5 6 7 8 9 split fraction SEPR-1 SEPR-2 SEPR-3 SEPR-4 C3 0.99 1 1 1 i-C4 0.005 0.99 1 1 n-C4 0.005 0.99 1 i-C5 0.005 0.99 n-C5 0.005 Flow rate lb mol /h C3 100 99 1 1 i-C4 300 1.5 298.5 297 1.5 1.5 n-C4 500 500 2.5 497.5 495 2.5 2.5 i-C5 400 400 400 2 398 396 2 n-C5 500 500 500 500 2.5 497.5

80. Example 4: with recycle Consider a simple process in which the reactor can only convert 40 percent of the feed to it (perhaps due to equilibrium constraints), but the separation of reactant A and product B is complete and the unreacted A is recycled. Solve the problem using (a) sequential method, and (b) tear stream approach. 7/9/2012 80 MIXR REAC SEPR feed product S1 S2 S3 S4 S5 1 A 0 B A B 40 % conversion per pass Recovery fraction 100 % A 0 % B Fig. 22 Process with recycle and 40 % conversion per pass. Ans : S5 = 1. 5 mol

81. Example 5: Process flowsheet simulation using simple mass balances Solve mass balances for a process consisting of a feed stream, a mixer in which the feed stream is mixed with the recycle stream, and a reactor, followed by a separator where the product is removed and the reactants are recycled. The process takes hydrogen and nitrogen (in a 3:1 ratio) to make ammonia. The reactor is limited by equilibrium and 25 percent conversion per pass in the reactor. Use stoichiometric feed and determine the molar flow rate of components in each stream using Excel 7/9/2012 81 Mixer reactor separator 1 2 3 Stoichiometry 4 5 6 N 2 = 100 H 2 = 300 98 %NH 3 0.5 %N 2 0.5 %H 2 Fig. 23 Ammonia process with a recycle.

82. Example 5: Solution 7/9/2012 82

83. Example 6 Flowsheet simulation using chemical reaction equilibrium: the reactor conversion is changed to be the equilibrium conversion, which may not be 25 percent. The equilibrium equation is given by Recalculate the molar flow rate of components in each stream using tear stream concept. Data K p = 0.05 at 589 K, P = 220 atm 7/9/2012 83

84. Example 6: Solution 7/9/2012 84

85. Example 7 Flowsheet simulation in a reactor-separator-recycle system including phase equilibrium: Process ammonia synthesis Ammonia is condensed Conversion per pass in the reactor is 25 % Phase separation K values: nitrogen 4.8, hydrogen 70, ammonia 0.051, and carbon dioxide 0.32 Flow rates of nitrogen, hydrogen and carbon dioxide into the process are 100, 300, and 1 mol /time Because of carbon dioxide, we add a purge stream as 1 % of the recycle stream. 7/9/2012 85

86. Example 7 Separator model: Rachford -Rice equation Draw the simplified flowsheet for ammonia synthesis and perform mass balances with vapor-liquid equilibria 7/9/2012 86 Where, z i is the mole fraction of the species i into the flash K i is the K value for the species i (vapor pressure/total pressure) i s the fraction of feed that goes out as vapor x i is the mole fraction of species i in the liquid phase y i is the mole fraction of species i in the vapor phase V is the vapor flow rate L is the liquid flow rate F is the feed flow rate

87. Example 7: Solution 7/9/2012 87 mixer reactor Flash separator separator Fig. 24 Ammonia process with vapor-liquid equilibria and a purge stream. 1 feed 5 product 7 purge 2 out of mixer 3 reacting 4 out of reactor 6 8

88. Example 7: Solution 7/9/2012 88 constraints

89. Basic steps in flowsheet synthesis Synthesis of a chemical process Gathering information This step helps to uncover existing process alternatives Literature survey Representing alternatives Flowsheet and different aggregations Representing processes using tasks(mixing, composition change, heating, reaction, separation) Flow of heat in a process (temperature versus amount of heat transferred) Representing the changes in composition space is useful for the synthesis of reactor network 7/9/2012 89

90. Basic steps in flowsheet synthesis Criteria for assessing preliminary designs How much is a design worth to our company? Need to assess the performance of a design alternative and a value for that performance Process performance: use equations from first principles (mass and energy balances to establish stream flows, temperature and pressure) Value of a design (profitable or not) Economic evaluation: cost of equipment and the costs associated with the purchase of utilities Environmental concerns: pollution control, safety, flexibility: process design requires the manufacture of specified products in spite of variation in the feedstocks and the operating conditions Controllability: ability to operate the process satisfactorily while undergoing dynamic changes from one operating condition to another or recovering from disturbances 7/9/2012 90

91. Basic steps in flowsheet synthesis Generating and searching among alternatives The availability of a concise representation is essential for the generation and description of the alternatives 7/9/2012 91

92. Conclusion In this lecture, the general concepts for the flowsheet synthesis are discussed. The steady state simulation of process flowsheet is presented with illustrative examples. 7/9/2012 92

93. References Alexandre C. Dimian , Integrated Design and Simulation of Chemical Processes , Elsevier, 2003. R. Smith, Chemical Process: Design and Integration , Wiley, 2005. D. Chwan Yee Foo, Z. A. Manan , M. Selvan , M. L. McGuire, Integrate Process S imulation and Process S ynthesis, Chem. Eng. Prog . October 2005, 25-29. Bruce A. Finlayson, Introduction to Chemical Engineering Computing , Wiley, 2006. L. T. Biegler , I. E. Grossmann, A. W. Westerberg, Systematic Methods of Chemical Process Design , Prentice Hall, 1997. Max S. Peters, Klaus D. Timmerhaus , Plant design and economics for Chemical Engineers, McGraw Hill, fourth edition, 1991 . 7/9/2012 93