054402 Design and Analysis II

1
054402 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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Steps in Process Design and Retrofit

• Detailed Process Synthesis -Algorithmic Methods

• SECTION B

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Section B Algorithmic Methods
4
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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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..

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Example 1. Specification for Butenes Recovery
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Design for Butenes Recovery System
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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

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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 L difference in volatility
Distillation L and/or V Heat transfer or shaft work V or L difference in volatility
Gas Absorption V Liquid absorbent L difference in volatility
Stripping L Vapor stripping agent V difference in volatility
Extractive Distillation L and/or V Liquid solvent and heat transfer V and L difference in volatility
Azeotropic Distillation L and/or V Liquid entrainer and heat transfer V and L difference in volatility
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Common Industrial Sep.Methods (Contd)
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 L Heat transfer Solid Difference in solubility or m.p.
Gas adsorption V Solid adsorbent Solid difference in adsorbabililty
Liquid adsorption L Solid adsorbent Solid difference in adsorbabililty
Membranes L or V Membrane Membrane difference in permeability and/or solubility
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Common Industrial Sep.Methods (Contd)
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 Difference in solubility
Drying S and L Heat transfer V Difference in volatility
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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

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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

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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)

• 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.

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Relative volatilities for equal cost separators

• Ref Souders (1964)

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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 gt 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.

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Algorithm to Select Pressure and Condenser Type
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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
 P of Separators  Ns
2 1 1
3 2 2
4 3 5
5 4 14
6 5 42
7 6 132
8 7 429
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Example 2 Sequences for 4-component separation
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Example 2 Sequences for 4-component separation
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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.

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Class Exercise
Design a sequence of ordinary distillation
columns to meet the given specifications.
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Class Exercise Possible Solution
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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)

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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

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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.

• 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).

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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.

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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 1.17. 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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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

• 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

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1st Branch of Sequences
(A/B)I, (E/F)I, (B/C)I, (A/C)I , (C/B)II,
and (C/D)II
Sequence Cost, /yr
1-5-16-28 900,200
1-5-17-29 872,400
1-6-18 1,127,400
1-7-19-30 878,000
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
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2nd Branch of Sequences
(A/B)I, (E/F)I, (B/C)I, (A/C)I , (C/B)II,
and (C/D)II
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
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3rd Branch of Sequences
(A/B)I, (E/F)I, (B/C)I, (A/C)I , (C/B)II,
and (C/D)II
Sequence Cost, /yr
3-11-23-31 878,200
3-11-24 1,095,700
3-12-(25,26) 867,400
3-13-27 1,080,100
Species
Propane A
1-Butene B
n-Butane C
trans-2-Butene D
cis-2-Butene E
n-Pentane F
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4th Branch of Sequences
(A/B)I, (E/F)I, (B/C)I, (A/C)I , (C/B)II,
and (C/D)II
Sequence Cost, /yr
4-14-15 1,115,200
Species
Propane A
1-Butene B
n-Butane C
trans-2-Butene D
cis-2-Butene E
n-Pentane F
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Lowest Cost Sequence
Sequence Cost, /yr
2-(8,10-22) 860,400
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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 BB methods to
  determine an optimal sequence of
  distillation-type separations..