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4-Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin1
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
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
4-Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin3
Section B: Algorithmic Methods
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
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:
4-Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin6
Example 1. Specification for Butenes Recovery
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Pentane withdrawn as bottoms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n-C4 withdrawn as distillate.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
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:
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
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 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
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
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
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
Leaching S Liquid solvent
L Difference in solubility
Drying S and L Heat transfer
V Difference in volatility
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
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
4-Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin13
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:
IIII
II
CC
CCSF
21
21
/
/ (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:
V and L ideal for
/
/
2
12,1
2
1
22
11s
s
P
P
K
K
xy
xySF (5.2)
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)sL
sL
P
PSF
22
112,1
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, b , where:
II
IIII
SF21
212,1
/
/
b (5.5)
4-Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin15
Relative volatilities for equal cost separators
Ref: Souders (1964)
4-Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin16
Sequencing of Ordinary Distillation Columns
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.
Use a sequence of ordinary distillation (OD) columns to separate a multicomponent mixture provided:
4-Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin17
Algorithm to Select Pressure and Condenser Type
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, NS, to produce P products:
)!1(!
)]!1(2[
PPPNs (5.7)
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
4-Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin19
Example 2 – Sequences for 4-component separation
4-Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin20
Example 2 – Sequences for 4-component separation
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:
4-Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin22
Class Exercise
Design a sequence of ordinary distillation columns to meet the given specifications.
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.
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.
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
4-Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin26
Sequencing of V-L Separation Systems
When simple distillation is not practical for all separatorsin a multicomponent mixture separation system, othertypes of separators must be employed and the order ofvolatility or other separation index may be different foreach type.
For example, if P = 3, and ordinary distillation, extractivedistillation with either solvent I or solvent II, and LLextraction with solvent III are to be considered, then T =4, and applying Eqns (5.7) and (5.8) gives 32 possiblesequences (for ordinary distillation alone, NS = 2).
(5.8)sPT
s NTN 1
If they are all two-product separators and if T equals thenumber of different types, then the number of possiblesequences is now given by:
4-Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin27
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 forseparations with acceptable volatilities.
4-Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin28
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
4-Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin29
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 columnheight, H.
Compute tower diameter, D (using Fair correlation for floodingvelocity, or HYSYS Tray Sizing Utility).
Estimate installed cost of tower (see Unit 6 and Chapter 9).
Size and cost ancillary equipment (condenser, reboiler, refluxdrum). 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
Set distillate and bottoms column pressures using
4-Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin30
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
1st Branch of Sequences
Species
Propane A
1-Butene B
n-Butane C
trans-2-Butene D
cis-2-Butene E
n-Pentane F
(A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II
4-Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin31
Sequence Cost, $/yr
2-(8,9-21) 888,200
2-(8,10-22) 860,400
2nd Branch of Sequences
Species
Propane A
1-Butene B
n-Butane C
trans-2-Butene D
cis-2-Butene E
n-Pentane F
(A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II
4-Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin32
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
3rd Branch of Sequences
Species
Propane A
1-Butene B
n-Butane C
trans-2-Butene D
cis-2-Butene E
n-Pentane F
(A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II
4-Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin33
Sequence Cost, $/yr
4-14-15 1,115,200
4th Branch of Sequences
Species
Propane A
1-Butene B
n-Butane C
trans-2-Butene D
cis-2-Butene E
n-Pentane F
(A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II
4-Separation TrainsDESIGN AND ANALYSIS II - (c) Daniel R. Lewin34
Lowest Cost Sequence
Sequence Cost, $/yr
2-(8,10-22) 860,400
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