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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin Design and Analysis II LECTURE 4: SEQUENCING OF SEPARATION TRAINS Daniel R. Lewin Department of Chemical Engineering Technion, Haifa, Israel Ref: Seider, Seader and Lewin (1999), Chapter 5

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin2 Assess Primitive Problem Steps in Process Design and Retrofit Development of Base-case Plant-wide Controllability Assessment Detailed Design, Equipment sizing, Cap. Cost Estimation, Profitability Analysis, Optimization Detailed Process Synthesis - Algorithmic Methods SECTION B

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin3 Section B: Algorithmic Methods

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin4 Introduction Almost all chemical processes require the separation of chemical species (components), to: purify a reactor feed recover unreacted species for recycle to a reactor separate and purify the products from a reactor Frequently, the major investment and operating costs of a process will be those costs associated with the separation equipment For a binary mixture, it may be possible to select a separation method that can accomplish the separation task in just one piece of equipment. However, more commonly, the feed mixture involves more than two components, involving more complex separation systems

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin5 Instructional Objectives Be familiar with the more widely used industrial separation methods and their basis for separation. Understand the concept of the separation factor and be able to select appropriate separation methods for liquid mixtures. Understand how distillation columns are sequenced and how to apply heuristics to narrow the search for a near- optimal sequence. Be able to apply systematic methods to determine an optimal sequence of distillation-type separations.. When you have finished studying this unit, you should:

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin6 Example 1. Specification for Butenes Recovery

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin7 Design for Butenes Recovery System 100-tray column C3 & 1-Butene in distillate Propane and 1-Butene recovery Pentane withdrawn as bottoms n-C4 and 2-C4=s cannot be separated by ordinary distillation ( =1.03), so 96% furfural is added as an extractive agent ( 1.17). n-C4 withdrawn as distillate. 2-C4=s withdrawn as distillate. Furfural is recovered as bottoms and recycled to C-4

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin8 Separation is Energy Intensive Unlike the spontaneous mixing of chemical species, the separation of a mixture of chemicals requires an expenditure of some form of energy Separation of a feed mixture into streams of differing chemical composition is achieved by forcing the different species into different spatial locations, by one or a combination of four common industrial techniques: ESA – energy separating agent the creation by heat transfer, shaft work, or pressure reduction of a second phase that is immiscible with the feed phase (ESA – energy separating agent) MSA – mass separating agent the introduction into the system of a second fluid phase (MSA – mass separating agent). This must be subsequently removed. solid phase the addition of a solid phase upon which adsorption can occur barrier the placement of a membrane barrier

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin9 Common Industrial Separation Methods Separation Method Phase of the feed Separation agent Developed or added phase Separation principle Equilibrium flash L and/or V Pressure reduction or heat transfer V or Ldifference in volatility DistillationL and/or V Heat transfer or shaft work V or Ldifference in volatility Gas Absorption V Liquid absorbent Ldifference in volatility StrippingL Vapor stripping agent Vdifference in volatility Extractive Distillation L and/or V Liquid solvent and heat transfer V and Ldifference in volatility Azeotropic Distillation L and/or V Liquid entrainer and heat transfer V and Ldifference in volatility

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin10 Common Industrial Sep.Methods (Cont’d) Separation Method Phase of the feed Separation agent Developed or added phase Separation principle Liquid-liquid Extraction LLiquid solvent Second liquid Difference in solubility Crystalli- zation LHeat transfer Solid Difference in solubility or m.p. Gas adsorption VSolid adsorbent Solid difference in adsorbabililty Liquid adsorption LSolid adsorbent Solid difference in adsorbabililty MembranesL or VMembrane difference in permeability and/or solubility

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin11 Common Industrial Sep.Methods (Cont’d) Separation Method Phase of the feed Separation agent Developed or added phase Separation principle Supercritical extraction L or V Supercritical solvent Supercritical fluid Difference in solubility LeachingSLiquid solvent LDifference in solubility DryingS and LHeat transfer VDifference in volatility

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin12 Selecting Separation Method (1) The development of a separation process requires the selection of: Separation methods ESAs and/or MSAs Separation equipment Optimal arrangement or sequencing of the equipment Optimal operating temperature and pressure for the equipment separation method Selection of separation method largely depends of feed condition – Vapor: partial condensation, distillation, absorption, adsorption, gas permeation (membranes) Liquid: distillation, stripping, LL extraction, supercritical extraction, crystallization, adsorption, and dialysis or reverse osmosis (membranes) Solid: if wet drying, if dry leaching

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin13 Selecting Separation Method (2) separation factor single stage of contacting The separation factor, SF, defines the degree of separation achievable between two key components of he feed This factor, for the separation of component 1 from component 2 between phases I and II, for a single stage of contacting, is defined as: (5.1) C = composition variable, I, II = phases rich in components 1 and 2. SF is generally limited by thermodynamic equilibrium. For example, in the case of distillation, using mole fractions as the composition variable and letting phase I be the vapor and phase II be the liquid, the limiting value of SF is given in terms of vapor-liquid equilibrium ratios (K-values) as: (5.2)

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin14 Selecting Separation Method (3) For vapor-liquid separation operations that use an MSA that causes the formation of a non-ideal liquid solution (e.g. extractive distillation): (5.4) In general, MSAs for extractive distillation and liquid-liquid extraction are selected according to their ease of recovery for recycle and to achieve relatively large values of SF. If the MSA is used to create two liquid phases, such as in liquid-liquid extraction, the SF is referred to as the relative selectivity, , where: (5.5)

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin15 Relative volatilities for equal cost separators Ref: Souders (1964)

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin16 Sequencing of Ordinary Distillation Columns in each column is > The reboiler duty is not excessive. The tower pressure does not cause the mixture to approach the T C of the mixture. Column pressure drop is tolerable, particularly if operation is under vacuum. The overhead vapor can be at least partially condensed at the column pressure to provide reflux without excessive refrigeration requirements. The bottoms temperature for the tower pressure is not so high that chemical decomposition occurs. Azeotropes do not prevent the desired separation. Use a sequence of ordinary distillation (OD) columns to separate a multicomponent mixture provided:

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin17 Algorithm to Select Pressure and Condenser Type

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin18 Number of Sequences for Ordinary Distillation Equation for number of different sequences of P 1 ordinary distillation (OD) columns, N S, to produce P products: (5.7) P# of Separators Ns Ns

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin19 Example 2 – Sequences for 4-component separation

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin20 Example 2 – Sequences for 4-component separation

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin21 Identifying the Best Sequences using Heuristics Remove thermally unstable, corrosive, or chemically reactive components early in the sequence. Remove final products one-by-one as distillates (the direct sequence). Sequence separation points to remove, early in the sequence, those components of greatest molar percentage in the feed. Sequence separation points in the order of decreasing relative volatility so that the most difficult splits are made in the absence of other components. Sequence separation points to leave last those separations that give the highest purity products. Sequence separation points that favor near equimolar amounts of distillate and bottoms in each column. The reboiler duty is not excessive. The following guidelines are often used to reduce the number of OD sequences that need to be studied in detail:

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin22 Class Exercise Design a sequence of ordinary distillation columns to meet the given specifications.

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin23 Class Exercise – Possible Solution Guided by Heuristic 4, the first column in position to separate the key components with the greatest SF.

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin24 Complex Columns for Ternary Mixtures Ref: Tedder and Rudd (1978) In some cases, complex rather than simple distillation columns should be considered when developing a separation sequence.

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin25 Regions of Optimality ESI 1.6 ESI 1.6 As shown below, optimal regions for the various configurations depend on the feed composition and the ease-of-separation index: ESI = AB / BC

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin26 Sequencing of V-L Separation Systems When simple distillation is not practical for all separators in a multicomponent mixture separation system, other types of separators must be employed and the order of volatility or other separation index may be different for each type. For example, if P = 3, and ordinary distillation, extractive distillation with either solvent I or solvent II, and LL extraction with solvent III are to be considered, then T = 4, and applying Eqns (5.7) and (5.8) gives 32 possible sequences (for ordinary distillation alone, N S = 2). (5.8) If they are all two-product separators and if T equals the number of different types, then the number of possible sequences is now given by:

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin27 Example 3 (Example 1 Revisited) Speciesb.pt.( C)Tc ( C)Pc, (MPa) PropaneA ButeneB n-ButaneC trans-2-ButeneD cis-2-ButeneE n-PentaneF For T = 2 (OD and ED), and P = 4, N S = 40. However, since 1-Butene must also be separated (why?), P = 5, and N S = 224. Clearly, it would be helpful to reduce the number of sequences that need to be analyzed. Need to eliminate infeasible separations, and enforce OD for separations with acceptable volatilities.

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin28 Example 3 (Example 1 Revisited) Adjacent Binary Pair ij at 65.5 o C Propane/1-Butene (A/B) Butene/n-Butane (B/C)1.18 n-Butane/trans-2-Butene (C/D)1.03 cis-2-Butene/n-Pentane (E/F)2.50 Splits A/B and E/F should be by OD only ( 2.5) Split C/D is infeasible by OD ( = 1.03). Split B/C is feasible, but an alternative method may be more attractive. Use of 96% furfural as a solvent for ED increases volatilities of paraffins to olefins, causing a reversal in volatility between 1- Butene and n-Butane, altering separation order to ACBDEF, and giving C/B = Also, split (C/D) II with = 1.7, should be used instead of OD. Thus, splits to be considered, with all others forbidden, are: (A/B…) I, (…E/F) I, (…B/C…) I, (A/C…) I, (…C/B…) II, and (…C/D…) II

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin29 Estimating Annualized Cost, C A For each separation, C A is estimated assuming 99 mol % recovery of light key in distillate and 99 mol % recovery of heavy key in bottoms. The following steps are followed: Estimate number of stages and reflux ratio by FUG method (e.g., using HYSYS.Plant “Shortcut Column”). Select tray spacing (typically 2 ft.) and calculate column height, H. Compute tower diameter, D (using Fair correlation for flooding velocity, or HYSYS Tray Sizing Utility). Estimate installed cost of tower (see Unit 6 and Chapter 9). Size and cost ancillary equipment (condenser, reboiler, reflux drum). Sum total capital investment, C TCI. Compute annual cost of heating and cooling utilities (COS). Compute C A assuming ROI (typically r = 0.2). C A = COS + r C TCI Set distillate and bottoms column pressures using

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin30 SequenceCost, $/yr , , ,127, , ,095,600 1 st Branch of Sequences Species PropaneA 1-ButeneB n-ButaneC trans-2-ButeneD cis-2-ButeneE n-PentaneF (A/B…) I, (…E/F) I, (…B/C…) I, (A/C…) I, (…C/B…) II, and (…C/D…) II

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin31 SequenceCost, $/yr 2-(8,9-21)888,200 2-(8,10-22)860,400 2 nd Branch of Sequences Species PropaneA 1-ButeneB n-ButaneC trans-2-ButeneD cis-2-ButeneE n-PentaneF (A/B…) I, (…E/F) I, (…B/C…) I, (A/C…) I, (…C/B…) II, and (…C/D…) II

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin32 SequenceCost, $/yr , ,095, (25,26)867, ,080,100 3 rd Branch of Sequences Species PropaneA 1-ButeneB n-ButaneC trans-2-ButeneD cis-2-ButeneE n-PentaneF (A/B…) I, (…E/F) I, (…B/C…) I, (A/C…) I, (…C/B…) II, and (…C/D…) II

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin33 SequenceCost, $/yr ,115,200 4 th Branch of Sequences Species PropaneA 1-ButeneB n-ButaneC trans-2-ButeneD cis-2-ButeneE n-PentaneF (A/B…) I, (…E/F) I, (…B/C…) I, (A/C…) I, (…C/B…) II, and (…C/D…) II

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin34 Lowest Cost Sequence SequenceCost, $/yr 2-(8,10-22)860,400

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4 - Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin35 Separation Trains - Summary Be familiar with the more widely used industrial separation methods and their basis for separation. Understand the concept of the separation factor and be able to select appropriate separation methods for liquid mixtures. Understand how distillation columns are sequenced and how to apply heuristics to narrow the search for a near- optimal sequence. Be able to apply systematic B&B methods to determine an optimal sequence of distillation-type separations.. On completing this unit, you should: Next week: Azeotropic Distillation Next week: Azeotropic Distillation

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