Separation trins

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


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


Section B: Algorithmic Methods


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


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:


Example 1. Specification for Butenes Recovery


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Pentane withdrawn as bottoms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n-C4 withdrawn as distillate.

2-C4=s withdrawn as distillate.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


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


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


Common Industrial Sep.Methods (Cont’d)

Separation Method

Phase of the feed

Separation agent



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


Common Industrial Sep.Methods (Cont’d)

Separation Method

Phase of the feed

Separation agent



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


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



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)



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)


Relative volatilities for equal cost separators

Ref: Souders (1964)


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:


Algorithm to Select Pressure and Condenser Type


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


Example 2 – Sequences for 4-component separation


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:


Class Exercise

Design a sequence of ordinary distillation columns to meet the given specifications.


Class Exercise – Possible Solution

Guided by Heuristic 4, the first column in position to separate the key components with the greatest SF.


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.


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


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:


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.


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


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


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


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



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



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



Lowest Cost Sequence

Sequence Cost, $/yr

2-(8,10-22) 860,400


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

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