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**LECTURE 4: SEQUENCING OF SEPARATION TRAINS**

LECTURE FOUR DESIGN AND ANALYSIS II 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 DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains Daniel R. Lewin, Technion

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**Steps in Process Design and Retrofit**

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

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**Section B: Algorithmic Methods**

LECTURE FOUR DESIGN AND ANALYSIS II Section B: Algorithmic Methods DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains Daniel R. Lewin, Technion

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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 DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**Instructional Objectives**

When you have finished studying this unit, you should: 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.. DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**Example 1. Specification for Butenes Recovery**

DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**Design for Butenes Recovery System**

Propane and Butene recovery 100-tray column C3 & 1-Butene in distillate 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. Pentane withdrawn as bottoms 2-C4=s withdrawn as distillate. Furfural is recovered as bottoms and recycled to C-4 DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**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: 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) the introduction into the system of a second fluid phase (MSA – mass separating agent). This must be subsequently removed. the addition of a solid phase upon which adsorption can occur the placement of a membrane barrier DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**Common Industrial Separation Methods**

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 L difference in volatility Distillation Heat transfer or shaft work Gas Absorption V Liquid absorbent L Stripping Vapor stripping agent Extractive Distillation Liquid solvent and heat transfer V and L Azeotropic Distillation Liquid entrainer and heat transfer DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**Common Industrial Sep.Methods (Cont’d)**

Separation Method Phase of the feed Separation agent Developed or added phase Separation principle Liquid-liquid Extraction L Liquid solvent Second liquid Difference in solubility Crystalli-zation Heat transfer Solid Difference in solubility or m.p. Gas adsorption V Solid adsorbent difference in adsorbabililty Liquid adsorption Membranes L or V Membrane difference in permeability and/or solubility DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**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 Leaching S Liquid solvent L Drying S and L Heat transfer V Difference in volatility DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**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 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 DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**Selecting Separation Method (2)**

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) DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**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) 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, b , where: (5.5) 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. DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**Relative volatilities for equal cost separators**

Ref: Souders (1964) DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**Sequencing of Ordinary Distillation Columns**

Use a sequence of ordinary distillation (OD) columns to separate a multicomponent mixture provided: in each column is > 1.05. The reboiler duty is not excessive. The tower pressure does not cause the mixture to approach the TC 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. DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**Algorithm to Select Pressure and Condenser Type**

DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**Number of Sequences for Ordinary Distillation**

Equation for number of different sequences of P 1 ordinary distillation (OD) columns, NS, to produce P products: (5.7) P # of Separators Ns 2 1 3 4 5 14 6 42 7 132 8 429 DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**Example 2 – Sequences for 4-component separation**

DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**Example 2 – Sequences for 4-component separation**

DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**Identifying the Best Sequences using Heuristics**

The following guidelines are often used to reduce the number of OD sequences that need to be studied in detail: 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. DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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

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**Class Exercise – Possible Solution**

Guided by Heuristic 4, the first column in position to separate the key components with the greatest SF. DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**Complex Columns for Ternary Mixtures**

In some cases, complex rather than simple distillation columns should be considered when developing a separation sequence. Ref: Tedder and Rudd (1978) DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**Regions of Optimality ESI 1.6 ESI 1.6**

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

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**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. (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: 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, NS = 2). DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**Example 3 (Example 1 Revisited)**

Species b.pt.(C) Tc (C) Pc, (MPa) Propane A -42.1 97.7 4.17 1-Butene B -6.3 146.4 3.94 n-Butane C -0.5 152.0 3.73 trans-2-Butene D 0.9 155.4 4.12 cis-2-Butene E 3.7 161.4 4.02 n-Pentane F 36.1 196.3 3.31 For T = 2 (OD and ED), and P = 4, NS = 40. However, since 1-Butene must also be separated (why?), P = 5, and NS = 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. DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**Example 3 (Example 1 Revisited)**

Adjacent Binary Pair ij at 65.5 oC Propane/1-Butene (A/B) 2.45 1-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 DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**Estimating Annualized Cost, CA**

For each separation, CA 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: Set distillate and bottoms column pressures using 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, CTCI. Compute annual cost of heating and cooling utilities (COS). Compute CA assuming ROI (typically r = 0.2). CA = COS + r CTCI DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**(A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II**

1st Branch of Sequences Sequence Cost, $/yr 900,200 872,400 1-6-18 1,127,400 878,000 1-7-20 1,095,600 Species Propane A 1-Butene B n-Butane C trans-2-Butene D cis-2-Butene E n-Pentane F DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**(A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II**

2nd Branch of Sequences Sequence Cost, $/yr 2-(8,9-21) 888,200 2-(8,10-22) 860,400 Species Propane A 1-Butene B n-Butane C trans-2-Butene D cis-2-Butene E n-Pentane F DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**(A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II**

3rd Branch of Sequences Sequence Cost, $/yr 878,200 1,095,700 3-12-(25,26) 867,400 1,080,100 Species Propane A 1-Butene B n-Butane C trans-2-Butene D cis-2-Butene E n-Pentane F DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**(A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II**

4th Branch of Sequences Sequence Cost, $/yr 1,115,200 Species Propane A 1-Butene B n-Butane C trans-2-Butene D cis-2-Butene E n-Pentane F DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**Lowest Cost Sequence Sequence Cost, $/yr 2-(8,10-22) 860,400**

DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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**Separation Trains - Summary**

On completing this unit, you should: 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.. Next week: Azeotropic Distillation DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 4 - Separation Trains

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