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IVDistillation Sequencing
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Outline1. Basic Concepts of Distillation
Sequence Design2. Choice of Sequence and its
Operating Pressure.3. Performance of Distillation
Column (Sieve tray and packedtower)
4. Separation and Recycle forcontinues process
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IV.1.BASIC CONCEPT OF
DISTILLATION SEQUENCING
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
IV.1.1. Introduction Consider: Separation of a homogeneous
multi-component fluid mixture into anumber of products, whereas allseparations are carried out usingdistillation only.
If this is the case, how to choice thedistillation sequence?
For example: two simple columnsequences (direct and indirect)could be employed in separationof a three-component mixtureinto three relatively pureproducts.
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Direct Sequence vs Indirect Sequence
direct sequence indirect sequencethe lightest component is takenoverhead in each column
the heaviest component is taken asbottom product in each column
requires less energy for bothreboiling and condensation suppliedby utilities
requires more energy for bothreboiling and condensation suppliedby utilities
component A (light material) is onlyvaporized once
Component A (light material) isvaporized twice
can be more energy-efficient if thefeed to the sequence has a lowflowrate of the light material (A) anda high flowrate of heavy material (C)
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Alternative sequences for the separation of a four-product mixture.
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Number of possible distillation sequences using simple columns
The problem is that there may be significant differences in the capitaland operating costs between different distillation sequences that canproduce the same products.
In addition, heat integration may have a significant effect on operatingcosts (would be discussed next).
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
IV.1.2. Practical Constraints
1. Remove as early as possible:a. A particularly hazardous component safety considerationb. Reactive or heat-sensitive component to avoid problems of
product degradationc. Corrosive component to minimize the use expensive material of
construction2. The main component that difficult to be condensed should be
removed as early as possible using refrigeration system or highpressure system
3. Don’t take the final product from the bottom of column if:a. The component is decomposed in the reboilers (it can
contaminates the product)b. Polymerization inhibitors are used to inhibit polymerization of
some components when distilled. These polymerization inhibitorstend to be nonvolatile, ending up in the column bottoms
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IV.2.CHOICE OF SEQUENCE ANDITS OPERATING PRESSURE
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
IV.2.1. Heuristics of Choice of Sequence (Smith, R., 2005)
1. Component with its relative volatility close to unity or thatexhibit azeotropic behavior should be removed last.
2. The lightest components should be removed alone one byone in column overheads (use direct sequence).
3. A component composing a large fraction of the feedshould be removed first.
4. Favor splits in which the molar flow between top andbottom products in individual columns is as near equal aspossible.
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Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Example 4.2.1: Data for a mixture of alkanes to be separated bydistillation are as follows:
Use the heuristics to identify potentially good sequences that arecandidates for further evaluation!
The relative volatilitieshave been calculated onthe basis of the feedcomposition to thesequence, assuming apressure of 6 barg usingthe Peng–RobinsonEquation of State withinteraction parameters setto zero.
Different pressures can, inpractice, be used fordifferent columns in thesequence
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Solution: Alternative-1
Heuristic 1: Do D/E split last since this separation has thesmallest relative volatility.
Heuristic 2: Favor the direct sequence:
Heuristic 3: Remove the most plentifulcomponent first:
Heuristic 4: Favor near-equimolarsplits between top andbottom products:
All four heuristicsare in conflict here:Heuristic 1suggests doing theD/E split last, andHeuristic 3suggests it shouldbe done first.Heuristic 2suggests the A/Bsplit first andHeuristic 4 the C/Dsplit first.
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Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Solution: Alternative-2:Take one of the candidates and accept, say, the A/B split first.
Heuristic 1: Do D/E split last.
Heuristic 2:
Heuristic 3:
Heuristic 4:
Again the heuristics are in conflict• Heuristic 1 again suggests doing
the D/E split last, whereas againHeuristic 3 suggests it should bedone first.
• Heuristic 2 suggests the B/C splitfirst and Heuristic 4 the C/D splitfirst.
• There are 14 posible sequence• This process could be continued
and possible sequencesidentified for furtherconsideration.
• Some possible sequences wouldbe eliminated
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Quantitative measure as other consideration
Since heuristics (qualitative procedure) can be in conflict, aquantitative measure of the relative performance of differentsequences would be preferred
The vapor flow up the column as a physical measure can be readilycalculated. This provides an indication of both capital and operatingcost.
More vapor flow up the column, more heat duty required forreboiler and condenser, increase the operating cost of hot utility(steam) and cold utility (water or refrigerant)
A high vapor rate leads to a large diameter column, and alsorequires large reboilers and condensers, therefore the capital costincreases
Consequently, sequences with lower total vapor load would bepreferred to those with a high total vapor load.
But how is the total vapor load predicted?
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Prediction of the total vapor load minmin 1 RDV Underwood … (4.2.1)
Eq. (4.2.1) can also be written at finite reflux.Defining RF to be the ratio R/Rmin (typicaly R/Rmin = 1.1):
min1 RRDV F … (4.2.2)
Rmin can be calculated:
FHK
DHK
FLK
DLK
x
x
x
xR
1
1min … (4.2.3)
= relative volatility between the key componentsxDLK = mole fraction of light key in the distillatexFLK = mole fraction of light key in the feedxDHK = mole fraction of heavy key in the distillatexFHK = mole fraction of heavy key in the feed
where:
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Assuming a sharp separation:• only the light key and lighter than LK components in the overhead• only the heavy key and heavier than HK components in the bottoms
D
F
x
xR
FLK
DLK
1
1
1
1min … (4.2.4)
F = feed flow rateD = distillate flow rate
where:
Combining (4.2.4) and (4.2.2):
111
FF R
FDD
FRDV … (4.2.5)
thus:
1
.........
FNCHKLKBALKBA
RFFFFFFFFV … (4.2.6)
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Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Example 4.2.2:
Using the data (below) for a ternary separation of benzene, toluene,and ethyl benzene. Based on the vapor flow-up, determine whetherthe direct or indirect sequence should be used!
Symbol ComponentFlowrate(kmole/h)
RelativeVolatility
RelativeVolatilitybetwwenadjacent
componentsA Benzene 269 3.53
1.96B Toluene 282 1.80
1.80C Ethyl Benzene 57 1.00
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Solution of Example 4.2.2:
Direct Sequence: A/BC and B/C
180.1
1.15782282
196.1
1.157282269269V
1.7487.965
kmole/h8.1713
Indirect Sequence: AB/C and A/B
196.1
1.1282296269
180.1
1.157282269282269V
4.9001387
kmole/h4.2287
Hence we should use the direct sequence
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Direct and Indirect Sequence(example 4.2.2)
269 kmol/h
282 kmol/h
57 kmol/h 57 kmol/h
269 kmol/h282 kmol/h
269 kmol/h
282 kmol/h
V =1713.8 kmol/h V =2287.4 kmol/h
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Example 4.2.3:
Using the Underwood Equations, determine the best distillationsequence, in terms of overall vapor load, to separate themixture of alkanes in Example 4.2.1 into relatively pureproducts. The recoveries are to be assumed to be 100%.Assume the ratio of actual to minimum reflux ratio to be 1.1and all columns are fed with a saturated liquid. Neglectpressure drop across each column. Relative volatilities can becalculated from the Peng–Robinson Equation of State withinteraction parameters assumed to be zero (see Chapter 4).Determine the rank order of the distillation sequenceson the basis of total vapor load for all column pressuresfixed to 6 barg with relative volatility calculated fromthe feed to the sequence.
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Solution of Example 4.2.3:
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IV.3.PERFORMANCE OF
DISTILLATION COLUMN(Plate/Tray Column and Packed Column)
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Distillation Tray and Packing
Distillation Tray Distillation Packing
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Plate/Tray Column vs Packed Column
Plate/Tray Column Packed Column
Contact of vapor-liquid relativelygood
chanelling and backmixing could behappen
More liquid hold-up ---Easy to be cleaned ---
--- Small Pressure drop, prefer tovacuum operation
--- Cheaper for corrosive fluid--- Prefer to small diameter
Can be used for liquid that containssolid particles
Solid particle plugs the packed
--- Foaming liquid--- Lighter
Products can be taken from theside-stream
---
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IV.4.SEPARATION AND RECYCLE SYSTEM
FOR CONTINUES PROCESS
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
IV.4.1. Introduction
Do separation for some reasons:1. to achieve product specification2. to meet environment law
Material to be separated:1. reactants2. main product3. byproduct (could be sold4. waste (could not be sold)
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IV.4.2. Function of Process Recycles
1. Reactor conversion:Consider FEED PRODUCT with conversion of about 95%Incomplete conversion in the reactor requires a recycle forunconverted feed material.
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
2. Byproduct formation
Consider:1st : FEED PRODUCT + BYPRODUCT
or
1st : FEED PRODUCT
2nd : PRODUCT BYPRODUCT
Using a purge saves the cost of a separatorbut incurs raw material losses, and possiblywaste treatment and disposal costs. Thismight be worthwhile if the FEED-BYPRODUCT separation is expensive.
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3. Recycling byproducts for improved selectivity
Consider: If a byproduct is formed via areversible secondary reaction thenrecycling the byproduct can inhibit itsformation at source.
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
4. Recycling byproducts or contaminants that damagethe reactor
When recycling unconverted feed material, it is possiblethat some byproducts or contaminants, such as products ofcorrosion, can poison the catalyst in the reactor.
It is clearly desirable to remove such damagingcomponents from the recycle stream.
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5. Feed impurities
If the impurity hasan adverse effect onthe reaction orpoisons the catalyst
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5. Feed impurities (continued)
I if the impurity doesnot have a significanteffect on the reaction,then it could perhapsbe passed throughthe reactor and beseparated
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5. Feed impurities (continued)
As with its use toseparate byproducts,the purge saves thecost of a separation,but incurs rawmaterial losses.
This might beworthwhile if theFEED-IMPURITYseparation isexpensive.
Care should be taken to ensure that the resulting increase inconcentration of IMPURITY in the reactor does not have anadverse effect on reactor performance.
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
6. Reactor diluents and solvents.
An inert diluent such as steam is sometimes needed in thereactor to lower the partial pressure of reactants in the vaporphase.
Diluents and solvents are normally recycled.
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7. Reactor heat carrier.
The introduction of an extraneous component as a heatcarrier effects the recycle structure of the flowsheet.
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
7. Reactor heat carrier (continued)
This figure illustrates the use of the product as the heat carrier. This simplifies the recycle structure of the flowsheet and removes the
need for one of the separators. The use of the product as heat carrier is obviously restricted to situations
where the product does not undergo secondary reactions to unwantedbyproducts.