Multi-component Separations Involving High-Recovery or Sharp Product Streams
Feed: Species Moles/hr NBP, TC 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
(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.
Definitions:
DEMETHANIZER
C1
SPLITTERDEPROPANIZER
C2
SPLITTER
DEETHANIZER
Feed .(A, B)H2 . C1(C, D)
(C)
(D)
(C, D, E, F, G)(E, F, G)(E, F)(E)(G)(F)
(sequence a) (sequence b)
A(LK)B(HK)C D E F G A B(overhead) C
B C D E F G(bottoms)CD
EFGCD
EF
GEF(component Bs recovery fraction in the overhead)
(component Cs recovery fraction in the bottoms)ABCDEFGABCD
EFGAB
CD
EF
GC
D
E
F
How do you synthesize these two industrial separation sequences ?
Which sequence is better ?
Does your ranking vary with feed conditions ?Questions:
Possible Sequences for a 4-Component FeedABCD B C DCDABCD(Direct sequence)Sequence (a) and (b)ABCDBCDBCABCD
Possible Sequences for a 4-Component FeedABCD A BCDSequence (c) ACBD
Possible Sequences for a 4-Component FeedABCD A B CBC ABC(Indirect sequence)Sequence (d) and (e) ABCDABCABABCDD
FOR A FOUR-COMPONENT PROCESS FEEDSUBGROUPSProcess FeedFirst SeparatorFeeds to Subsequent SeparatorsProductsABCD()decreasing volatilityA AB BC BB CCD C D(A)
(B)
(C)(D)(())((()))
FOR A FOUR-COMPONENT PROCESS FEEDUNIQUE SPLITS Splits forFirst SeparatorABCD()A B C A BC
BCD
BCD ( )
( )
( )
(())Splits for subsequent Separators(())ABBCCDABCDABCD(())
Sequences of Two-Product SeparatorsThe 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 =
NUMBER OF SEPARATORS, SEQUENCES,SUBGROUPS, AND UNIQUE SPLITS FOR SIMPLESEQUENCES USIGN ONE SIMPLE METHOD OFSEPARATION Number of S, G, U,Number of Separators in Number of Number of Number ofComponents a Sequence Sequences Subgroups Unique 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
n-Butylene Purification by Ordinary Distillation and Extractive Distillation Feed: Relative Volatility* Species Mole % ()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.1.032.451.18
2.501.17(nC4/1-C4)1.17(nC4/T-2-C4)
How do you get to following industrial separation sequence?ABCDEFExtractiveDistillationSolvenetRecoveryCDEIIFEEDC3FCDE1-BUTENE COLUMNABDEPROPANIZERC3A1-BUTENEBC+S DE+ SolventDE2-BUTENESCn-BUTANE RECIRCULATED SOLVENTDEOILEREXTRACTIVEDISTILLATION COLUMNSOLVENT STRIPPER
ABCDEF
AB
CDEFA
B
CDE
F()IIC
DE
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.
Examples of Extractive Distillation : Mixture Solvent1-Butene(-6.3 C) and 1,3 Buradiene(-4.41C) Acetonitrile(81.6 C)Nitric Acid(83 C) and Water (100C) 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)
Example 1 Extractive DistillationNormal Boiling Points, C MCH 100.9 Toluene 110.6 Phenol Feed (Solvent) CH3C6H5: Toluene(T) CH3C6H11: MCH(M)181.75 extractive distillationMCH(M)Solvent Recovery Toluene(T) Phenol(P)(Recycled solvent)Phenol(P):C6H5OH(Make-upsolvent)
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 componentsOrdinary distillation only gives SN = 5Ordinary distillation plus extractive distillation with phenol gives
HEURISTICSHeuristics Used in Heuristic and Evolutionary Strategies forSynthesis of Separation SequencesType Separators Reference Applied Heuristics UsedLockhart (1947) Distillation 1, 6Harbert (1957) Distillation 2, 3Rod & Marek (1959) Distillation 4Heaven (1969) Distillation 1, 2, 3, 5 Rudd and his co- General 1, 2, 3, 6, 8 workers (1971-73) 12, 13King (1971) and General 1, 2, 3, 7, 11Thomp son and King (1972a, b)Stephanopoulos (1974), General 7 plus evol. and Stephanopoulos rules and Westerberg (1976)
HEURISTICSHeuristics Used in Heuristic and Evolutionary Strategies forSynthesis of Separation SequencesType Separators Reference Applied Heuristics UsedFreshwater & Henry (1975) Distillation 1, 2, 3, 5, 6Mahaec (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. rulesNath & Motard (1978) General 9, 10, 11, 14, 15 17, 19 plus evol. RulesDoukas & Luyben (1978) Distillation 1Hartmann (1979) and General 1, 2, 3, 6, 8, 16 Hartmann and 19 Hacker (1979)
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.
Heuristic Rules13. 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)
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. ASimple Heuristic Method for Systematic Synthesis of Initial Sequences forMulti-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.
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 lastc. Synthesize the initial separation sequences : C1: Remove most plentiful component first C2: Favor 50/50 splitd. Remove products and recycle stream as distillates. If not possible, take vapor from reboiler.
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.
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. orc. 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.
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
IncreasingNormalBoilingPoint
orHeavy Components
Minimum required for consideration of extractive distillationorMinimum required forconsideration of L/L extraction
Sounders, M., CEP, 60 (2) , 75-82 (1964)
Add nonvolatile component to modify s Example 2 Extractive distillation.B HNO3SHNO3 B C H2OC H2OS H2SO4 H2O C S H2SO4e. g., B = HNO3 C = H2O S = H2SO4
Add component that forms an azeotrope with one or more of feed component Example 3 Azeotropic Distillation.e. g., B = Ethanol C = Water S = Benzene volatileS-richC-richrecycleBCS ternary heterogeneous azeotropeB+C azeotropeC waterBCB ethanolB+Cazeotropa
ExtractionBB+SB+CCC(+B) B+C C+S(+B)SBSCa B/C distillation
Reactive Distillation Add reactive component to modify sCSBBCC+Se. g., B, C = xylenes: = 1.03 S = organometallic: B, CS: 30
B : meta-xylene C : para-xylene S : sodium cumene
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.
Relative Costs of Cooling and Heating at DifferentTemperatures (Berthouex and Rudd, 1977)Roomtemperature Cooling waterAmmonia refrigerantLiquid nitrogen$/K calTemperatureLow pressuresteamHigh pressuresteamFuel oilBurnerHeatingCooling
-100-200 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"Avoid: High P and T or Low P and T"Pressure of operation, atm100101.00.1
Heuristic D1 (Favor Smallest Product Set)When multi-component products are specified, favor sequencesthat 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.
An Example for Heuristic D1Feed : Normal Boiling Species Mole% Point, TC A 25 140 B 25 160 C 25 180 D 25 200202020
An Example for Heuristic D1Good Sequences for Different Product Sets : a. Products : A, BC, D
b. Products : A, C, BDABCDA
BCDB
DProduct Set for Separation: (A, BC, D)ABCDA
BCDB
CDC
DBDProduct Set for Separation: (A, B, C, D)C
Heuristic S1Remove corrosive and hazardous components first. (Essential first separations) also reactive components and monomersHeuristic 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)
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.70A20B10CA 70
B 20C 10BC70A20B10CA 70B 20
C 10AB
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) (hydrogenchloride) chloride)
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 Species Mole/hr Point, TC T A: HCl 1 -85 B: Benzene 5 80 C: Kerosene 1 214 D: Keryl Benzene 1 250 E: Heavy Ends RelativeFlow Rate()165134 36
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)
An Example for Heuristics S1, S2 and C1 :[See Rudd, et al. (1973), pp.197-199, Problem 9]
IV. Initial Separation SequenceABCDA(HCl)
BCDB(Benzene)
CDC(Kerosene)D(Keryl Benzene)
BCD
Heuristic C2 (Favor 50/50 Split)If the component compositions do not vary widely, sequenceswhich give a more nearly 50/50 or equimolal split of the feedbetween the distillate (D) and bottoms (B) products should befavored, provided that the separation factor or relative volatilityis reasonable for the split.F(Feed)100D(overhead) 50, 40, 5
B(Bottoms) 50, 60, 95
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) highestWhere or Such that or
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 mayuse
More on CES (Coefficient of Ease of Separation)SeparatorFeed100Overhead, De. g., 70, 60, 50, 30, 20Bottoms, Be. g., 30, 40, 50, 70, 80 Split ratio D/B or B/DB/DD/B (Boiling-Point Difference) or CES Close to one, 50/50 splitThe larger the or (-1), the easier the separation
Examples of the Synthesis of Sharp or High-Recovery Separation Sequences Example 1: Multicomponent Distillation Sequencing for Thermal Cracking of Hvdrocarbons
1. Feed Normal Boiling Species Moles/Hr Point, T C T CES A: Hydrogen 18 -253 B: Methane 5 -161 C: Ethylene 24 -104 D: Ethane 15 -88 E: Propylene 14 -48 F: Propane 6 -42 G: Heavies 8 -1
2. Products : C, D, E, F and G.
3. Key Questions : a. Products: Any multi-component products ? b. Species: Any corrosive and hazardous components ? c. T: Any difficult separations ? d. Moles/hr: Any plentiful components ? e. CES: Any easy and balanced (50/50) splits ?AB,7292 23.057 14.640 1.141 4.019.618.1616Example: CESA/BCDEFG =
1. M1 (favor ordinary distillation) and M2 (avoid vacuum and refrigeration): Use ordinary distillation with refrigeration at high pressure.
2. D1 (favor smallest product set): Avoid splitting AB (a single product).
3. S1 (remove corrosive and hazardous components first): Not applicable.
4. S2 (perform difficult separations last): Split C/D and E/F last,
5. C1 (remove most plentiful components first): Not applicable.
6. C2 (favor 50/50 split): Split AB/CDEFG with the largest CES = 19.6 and AB as a single product.
CES A B C D E F GAB
CDEFG19.618.1 Initial Separation Sequence
For separating CDEFG, splits C/D and E/F are performed last so that the remaining splits to be chosen are CD/EFG and CDEF/G. Split CD/EFG is done first since it has a larger CES of 28.7: CD/EFG CDEF/G f 28/39 8/59 T 40 41CES 28.7 5.6
ABCDEFGAB
CDEFGEFGCDCDEFEFGDifficult SplitDifficult Split24 C15 D14 E 6 F 8 G16C6C? CD/EFG? CDEF/GThis sequence performs splits C/D and E/F last are it is exactly the same as the one being practiced in the industry.
The second sequence can be obtained by making the split ABCD/EFG first, which has the second largest CES of 18.1.ABCDEFGABCD
EFGSingle product?16C(last)6C(last)
Another Initial Separation Sequence :ABCDEFGABCD
EFGAB
GCDCD16CEFEF6CLast SeparationsCES=19.6(large)
CES=18.1(2nd largest)[See King, C. J., Separation Processes, 2nd Edition, McGraw Hill (1980), P.718]
Example 2: Multi-component Distillation Sequencing for n-Butylene Purification by Ordinary and Extractive Distillation.
1. Feed: Relative Volatility* Species Mole % ()I ()II (CES)I (CES)II A: Propane 1.47 B: 1-Butene 14.75 C: n-Butane 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, ordinary extractive distillation with furfural (C4H3OCHO)
2. Products: A, B, C, and F. (Example) (CES)I,A/BCDEF =
(CES)II,ABCDE/F = 2.45 2.1631.18 1.17 3.29 1.70 1.510 35.25
2.5050.301.033.4859.404DE
3. Key Question : a. Products: Any multi-component products ? b. Species: Any corrosive and hazardous components ? c. Mole%: Any plentiful components ? d. Relative Volatilities: Any difficult separations ? e. (CES): Any easy and balanced (50/50) splits ?
1. M1 (favor ordinary distillation): Use extractive distillation for C/D and ordinary distillation for all other splits.2. M2 (avoid vacuum and refrigeration): Use refrigeration at high pressure.3. D1 (favor smallest product set): Avoid splitting DE (a single product)4. S1 (remove corrosive and hazardous components first): Not applicable.5. S2 (perform difficult separations last): Split C/DE last (difficult extractive distillation), so that the added solvent (furfural) can be recovered at the end without having to worry about its presence as a contaminant in earlier splits.6. C1 (remove most plentiful components first): C(50.30 mole %) dominates the feed, but it is not separated first due to S2.Note: Heuristics are ranked in their relative importance according to their given order, e. g.,Heuristic S2 overrides Heuristic C1
Separation Sequencing for Example 2
7. C2 (favor 50/50 split): (a) Split ABCDE/F first with the largest (b)Favor AB/CDE over A/BCDE (CES)I = 9.404 (Split C/DE last) (Split C/DE last, avoid splitting DE) A/BCDE AB/CDE f 1.47/92.63 16.22/77.88 (-1)100 145 18 CES 2.301 3.749ABCDEFA 1.47B 14.75C 50.30D 15.62E 11.95
FInitialSeparationSequenceABCDEFABCDE
FABABC
DE
