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Multi-component Separations Involving High-Recovery or Sharp Product Streams

Multi-component Separations Involving High-Recovery or Sharp Product Streams Feed : Species Moles/hr NBP, T C H 2 :Hydrogen(Component A) 18 -253

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Multi-component Separations Involving High-Recovery or Sharp Product Streams

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  1. Multi-component Separations Involving High-Recovery or Sharp Product Streams Feed: Species Moles/hrNBP, TC H2 :Hydrogen(Component A) 18 -253 C1_:Methane(B) 5 -161 C2o:Ethylene(C) 24 -104 C2_:Ethane(D) 15 -88 C3o :Propylene(E) 14 -48 C3+ :Propane(F) 6 -42 C4 :Heavies(G) 8 -1 Products: AB, C, D, E, F, G

  2. Definitions: (1) For sharp product streams, we normally use all- sharp or high-recovery separation sequences to separate the feed into products. In such sequences, each component being separated appears almost completely in one and only one product. (2) Key component in an all-sharp separation are commonly defined by: The light key (LK) is the lightest component in the bottoms and the heavy key (HK) is the heaviest component in the overhead.

  3. (A, B) H2 . C1 (C, D) (C) D E M E T H A N I Z E R D E E T H A N I Z E R C2 S P L I T T E R Feed . (D) (E, F) (E, F, G) (E) (C, D, E, F, G) D E P R O P A N I Z E R C1 S P L I T T E R (F) (G)

  4. (sequence a) (sequence b) A B C D E F G A B C D E F G A B (overhead) C B C D E F G (bottoms) A (LK)B (HK)C D E F G C D E F A B C D E F G C D E F G C D E F G E F (component B’s recovery fraction in the overhead) (component C’s recovery fraction in the bottoms)

  5. Questions: How do you synthesize these two industrial separation sequences ? Which sequence is “better” ? Does your ranking vary with feed conditions ?

  6. Possible Sequences for a 4-Component Feed A B C A B A B C D B C D C D A B C D B C D B C D C D (Direct sequence) Sequence (a) and (b)

  7. Possible Sequences for a 4-Component Feed A C A B C D C D A B B D Sequence (c)

  8. Possible Sequences for a 4-Component Feed A B A A B C D A B C B C A B C D A B C A B D C B D C (Indirect sequence) Sequence (d) and (e)

  9. SUBGROUPS FOR A FOUR-COMPONENT PROCESS FEED Process Feed First Separator Feeds to Subsequent Separators Products (A) (B) (C) (D) ( ) A A B B C B B C C D C D ( ) ( ) A B C D ( ) ( ) ( ) decreasing volatility

  10. UNIQUE SPLITS FOR A FOUR-COMPONENT PROCESS FEED Splits for First Separator Splits for subsequent Separators ( ) A B C A B C B C D B C D ( ) A B A B C D ( ) ( ) ( ) ( ) B C ( ) A B C D C D ( ) ( ) A B C D ( )

  11. Sequences of Two-Product Separators The Combinatorial Problem: Total number of possible sequences for N components(products) if only type of separator is used (e. g. ordinary distillation)= Number of unique feed and product groups = Number of unique splits =

  12. NUMBER OF SEPARATORS, SEQUENCES, SUBGROUPS, AND UNIQUE SPLITS FOR SIMPLE SEQUENCES USIGN ONE SIMPLE METHOD OF SEPARATION Number of S, G, U, Number of Separators in Number of Number of Number of Componentsa SequenceSequencesSubgroupsUnique Splits 2 1 1 3 1 3 2 2 6 4 4 3 5 10 10 5 4 14 15 20 6 5 42 21 35 7 6 132 28 56 8 7 429 36 84 9 8 1430 45 120 10 9 4862 55 165 11 10 16796 66 220

  13. n-Butylene Purification by Ordinary Distillation and Extractive Distillation Feed: Relative Volatility* SpeciesMole % ()I ()II A: Propane 1.47 B: 1-Butene 14.75 C: n-Butane 50.29 D: trans-Butene-2 15.62 E: cis-Butene-2 11.96 F: n-Pentane 5.90 *()I = adjacent relative volatility at 150 F for separation method I, ordinary distillation ()II = adjacent relative volatility at 150 F for separation method II, extractive distillation with furfural. (C4H3OCHO) Products: A, B, C, DE and F. 2.45 1.18 2.50 1.17(nC4/1-C4) 1.17(nC4/T-2-C4) 1.03

  14. How do you get to following industrial separation sequence? 1-BUTENE COLUMN DEPROPANIZER A B C D E F AB C3 A C D E FEED II 1-BUTENE B C+S CDE DE + Solvent DE Extractive Distillation Solvenet Recovery 2-BUTENES RECIRCULATED SOLVENT n-BUTANE C3 C F EXTRACTIVE DISTILLATION COLUMN DEOILER SOLVENT STRIPPER

  15. A B C D E F A B C D E F A B C D E F C D E ( ) II

  16. Definition: Extractive distillation is a form of distillation involving the addition of a solvent which modifies the vapor-liquid equilibria of the components to be separated such the separation becomes easier. The added solvent has a volatility lower than the components to be separated (i.e., the added solvent has a boiling point higher than those of the components to be separated). Also, it is usually introduced near the top of a column.

  17. Examples of Extractive Distillation : Mixture Solvent 1-Butene(-6.3 C) and 1,3 Buradiene(-4.41C) Acetonitrile(81.6 C) Nitric Acid(83 C) and Water (100C) Sulfuric Acid(300 C) Meth1 Cyclohexane(100 C) and Benzene(80.1 C) N-Formylmorpholine(243 C) Isooctane(99.2 C) and doiuene (110.6 C) Phenol(181.75 C) Monomethyiamine(-6.3 C). Dimethylamine(7.4 C) and Trmethylamine(2.87 C) Water(100 C) Methy1 Cyclohexane(100 C) and Toluene(110.6 C) Phenol(181.75 C) Acetone(56.2 C) and Methanol (62.5 C) Water(100 C)

  18. Example 1 Extractive Distillation Normal Boiling Points, C MCH 100.9  Toluene 110.6  Phenol Feed (Solvent) CH3C6H5: Toluene(T) CH3C6H11: MCH(M) Toluene(T) MCH(M) extractive distillation Phenol(P):C6H5OH  181.75  Solvent Recovery (Make-up solvent) (Recycled solvent)   Phenol(P)

  19. If : 1. T type of two-product separators are allowed. 2. Any mass-separating agent is recovered for recycle in the separator following the one into which it is introduced. Then : For example : N = 4 components Ordinary distillation only gives SN = 5 Ordinary distillation plus extractive distillation with phenol gives

  20. HEURISTICS Heuristics Used in Heuristic and Evolutionary Strategies for Synthesis of Separation Sequences Type Separators ReferenceApplied Heuristics Used Lockhart (1947) Distillation 1, 6 Harbert (1957) Distillation 2, 3 Rod & Marek (1959) Distillation 4 Heaven (1969) Distillation 1, 2, 3, 5 Rudd and his co- General 1, 2, 3, 6, 8 workers (1971-73) 12, 13 King (1971) and General 1, 2, 3, 7, 11 Thomp son and King (1972a, b) Stephanopoulos (1974), General 7 plus evol. and Stephanopoulos rules and Westerberg (1976)

  21. HEURISTICS Heuristics Used in Heuristic and Evolutionary Strategies for Synthesis of Separation Sequences Type Separators ReferenceApplied Heuristics Used Freshwater & Henry (1975) Distillation 1, 2, 3, 5, 6 Mahaec (1976) and General 6, 11 plus evol. Mahalec & Motard rules (1977a, b) Seader & Weaterberg General 1, 2, 3, 9, 11, 12, (1977) 13, 18 plus evol. rules Nath & Motard (1978) General 9, 10, 11, 14, 15 17, 19 plus evol. Rules Doukas & Luyben (1978) Distillation 1 Hartmann (1979) and General 1, 2, 3, 6, 8, 16 Hartmann and 19 Hacker (1979)

  22. Heuristic Rules. 1. Remove components one-by-one as overhead products. 2. Save the most difficult separation for last. 3. Favor 50-50 splits. 4. Sequence with the minimum total vapor flow. 5. Make high recovery fractions last. 6. Separate the more plentiful components first. 7. Choose the cheapest as the next separator. 8. Remove the thermally unstable and corrosive material early. 9. Perform least-tight separation first. 10. Favor sequences with the smallest product set. 11. Avoid separations using a mass-separating agent (MSA). 12. Remove a MSA from one of the products in another, subsequent separation product.

  23. Heuristic Rules 13. A separation method using a MSA cannot be used to isolate. 14. Favor distillation. 15. Separate first the components which might undergo undesirable reactions. 16. Set splits fractions of the key components to pre-specified values. 17. Avoid extreme processing conditions. 18. Favor ambient operating pressure. Nishida, Stephanopoulos, Westerberg(1980)

  24. Heuristic Synthesis of High-Recovery or Sharp, Multi-component Separation Sequences (Nadgir and Liu, 1983; Liu, 1987) Classification of Heuristics Method Heuristics (M Heuristics) : Favor the use of certain separation methods under given problem specifications Design Heuristics (D Heuristics) : Favor specific separation sequences with certain desirable properties Species Heuristics (S Heuristics) : Based on the property differences between the species to be separated Composition Heuristics (C Heuristics) : Related to the effects of feed and product composition on separation costs Nadgir, V. M. and Y. A. Liu, “Studies in Chemical Process Design and Synthesis. 5. A Simple Heuristic Method for Systematic Synthesis of Initial Sequences for Multi-component Separation,” AIChE Journal, 29, 926-934 (1983). Liu, Y. A., “Process Synthesis: Some Simple and Practical Developments,” Chapter 6, in Recent Development in Chemical Process and Plant Design, Y. A. Mebee, Jr. and W. R. Epperly, editor, Wiley, NY (1987), pp. 147-168 and 245-260.

  25. A Simple, “Rank-Ordered,” Heuristic Method (Nadgir and Liu, 1983) a. Decide the separation method to be used : M1: Favor ordinary distillation M2: Avoid vacuum and refrigeration b. Identify the forbidden splits, and essential first and last separations : D1: Favor smallest product set S1: Remove corrosive and hazardous components first (also : reactive component, monomer) S2: Perform difficult separations last c. Synthesize the initial separation sequences : C1: Remove most plentiful component first C2: Favor 50/50 split d. Remove products and recycle stream as distillates. If not possible, take vapor from reboiler.

  26. “Rank-Ordered” Heuristics: (1) The heuristics are to be applied one by one in the given order. Higher-ranked heuristics appear first. (2) If any heuristic is not important in, or not applicable to, the synthesis problem, the next one in the method is considered. (3) If two heuristics give different recommendations regarding the next split, we should follow the guideline suggested by the higher-ranked heuristic. For example, heuristic C1 overrules heuristic C2.

  27. Heuristic M1 (Favor Ordinary Distillation) a. All other things being equal, favor separation methods using only energy separating agent (e.g., ordinary distillation), and avoid using separation methods (e. g., extractive distillation) which require the use of species not normally present in the processing, i. e., the mass separating agent (MSA). b. If the separation factor or relative volatility of the key components < 1.10, then the use of ordinary distillation is not recommended. or c. An MSA may be used provided it improves . d. When an MSA is used, remove it immediately follow the separator into which it is used. In other words, always try to remove MSA early.

  28. Heuristic M1 (Favor Ordinary Distillation) A LLK Light Component B LK Light Key (e. g. 98% of B appears in overhead) C HK Heavy Key (e. g., 97% of C appears in bottoms) D HHK E HHK Increasing Normal Boiling Point Heavy Components or

  29. Minimum  required for consideration of extractive distillation or Minimum  required for consideration of L/L extraction Sounders, M., CEP, 60 (2) , 75-82 (1964)

  30. Example 2 Extractive distillation. Add nonvolatile component to modify’s B HNO3 S HNO3 B C H2O C H2O H2O C S H2SO4 S H2SO4 e. g., B = HNO3 C = H2O S = H2SO4

  31. Example 3 Azeotropic Distillation. Add component that forms an azeotrope with one or more of feed component volatile BCS ternary heterogeneous azeotrope S-rich C-rich B+C azeotropa B+C azeotrope B C B ethanol C water e. g., B = Ethanol C = Water S = Benzene recycle

  32. Extraction  B B+S a B/C distillation  B+C S C  C(+B)    B+C   C   C+S (+B)    B S

  33. Reactive Distillation Add reactive component to modify ’s B S B C C C+S e. g., B, C = xylenes:  = 1.03 S = organometallic: B, CS:  30 B : meta-xylene C : para-xylene S : sodium cumene

  34. Heuristic M2 (Avoid Vacuum and Refrigeration) a. All other things being equal, avoid excursions in temperature and pressure, but aim high rather than low. b. If vacuum operation of ordinary distillation is required, liquid- liquid extraction with various solvents might be considered. c. If refrigeration is required, cheaper alternatives to distillation such as absorption might be considered.

  35. Fuel oil Burner $/K cal High pressure steam Cooling Low pressure steam Heating  Room temperature Cooling water Temperature Ammonia refrigerant Liquid nitrogen Relative Costs of Cooling and Heating at Different Temperatures (Berthouex and Rudd, 1977)

  36. 100 Avoid: High ”P” and ”T” or Low ”P” and ”T" Pressure of operation, atm 10 1.0 0.1 -200 0 100 200 300 400 500 600 Atmospheric boiling point, C Recommended Ranges of Pressure and Temperature for Separation Operations (Souders, 1964) Favor: High ”P” and Low ”T” Low ”P” and High ”T" -100

  37. Heuristic D1 (Favor Smallest Product Set) When multi-component products are specified, favor sequences that yield these products directly or with a minimum of blending, unless separation factors or relative volatilities are appreciably lower than those for a sequence which requires additional separators and blending.

  38. An Example for Heuristic D1 Feed : Normal Boiling SpeciesMole% Point, TC A 25 140 B 25 160 C 25 180 D 25 200 20 20 20

  39. An Example for Heuristic D1 Good Sequences for Different Product Sets : a. Products : A, BC, D b. Products : A, C, BD Product Set for ”Separation“: (A, BC, D) A B C D A B C D B D C Product Set for ”Separation“: (A, B, C, D) A B C D A B C D B C D BD C D

  40. Heuristic S1 Remove corrosive and hazardous components first. (Essential first separations) also reactive components and monomers Heuristic S2 (Perform Difficult Separations Last) a. All other things being equal, perform the difficult separations last, b. Separations where the relative volatility of the key components is close to unity should be performed in the absence of non-key components. (Essential last separations)

  41. Heuristic C1 (Remove Most Plentiful Component First) A product composing a large fraction of the feed should be separated first, provided that the separation factor or relative volatility is reasonable for the separation. A 70 B 20 C 10 70A 20B 10C  B C A B A 70 B 20 C 10 70A 20B 10C

  42. An Example for Heuristics S1, S2 and C1 : [See Rudd, et al. (1973), pp.197-199, Problem 9] Industrial Sequence for Separating Chlorination and Alkylation Products in the Manufacture of Detergent I. Reactions : (Chlorination) (Alkylation) (kerosene) (chlorine) (keryl (hydrogen chloride) chloride) C12H5Cl + C12H25 - + HCl (keryl (benzene) (keryl benzene) (hydrogen chloride) chloride)

  43. An Example for Heuristics S1, S2 and C1 : [See Rudd, et al. (1973), pp.197-199, Problem 9] Industrial Sequence for Separating Chlorination and Alkylation Products in the Manufacture of Detergent II. Reaction Products to be Separated into Pure Components Normal Boiling SpeciesMole/hr Point, TC T A: HCl 1 -85 B: Benzene 5 80 C: Kerosene 1 214 D: Keryl Benzene 1 250 E: Heavy Ends ( ) Relative Flow Rate 165 134 36

  44. An Example for Heuristics S1, S2 and C1 : [See Rudd, et al. (1973), pp.197-199, Problem 9] III. Key Questions : a. Species: Any corrosive and hazardous components? Split A/BCD (essential first separation) b. : Any difficult separations? Split C/D (essential last separation) c. C1: Any plentiful components? Split B/CD (desirable early separation)

  45. An Example for Heuristics S1, S2 and C1 : [See Rudd, et al. (1973), pp.197-199, Problem 9] IV. Initial Separation Sequence A(HCl) B C D A B C D B(Benzene) C D C(Kerosene) D(Keryl Benzene)

  46. C D B

  47. Heuristic C2 (Favor 50/50 Split) If the component compositions do not vary widely, sequences which give a more nearly 50/50 or equimolal split of the feed between the distillate (D) and bottoms (B) products should be favored, provided that the separation factor or relative volatility is reasonable for the split. D(overhead) 50, 40, 5 B(Bottoms) 50, 60, 95 F(Feed) 100

  48. Coefficient of Ease of Separation (CES) If it is difficult to judge which split is closest to 50/50 and with a reasonable separator factor or relative volatility, then perform the split with the value of the coefficient of ease of separation (CES) highest Where or Such that or

  49. Fractional recoveries of light-key(LK) and heavy-key(HK) components in sharp or high-recovery separations : Overhead Bottoms LK dLK 0.98 bLK 0.02 HK dHK 0.02 bHK 0.98 As an approximation, To simplify the calculations of CES in sharp separations, we may use

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