iv distillation sequencing - ydhermawan's blog · pdf file1/4/2017 · a high...

Chemical Plant Design – 1210384 Chapter-4

Department of Chemical Engineering - UPN “Veteran” Yogyakarta of 18

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




IV.1.BASIC CONCEPT OF

DISTILLATION SEQUENCING


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.




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)


Alternative sequences for the separation of a four-product mixture.




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).


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




IV.2.CHOICE OF SEQUENCE ANDITS OPERATING PRESSURE


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.




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


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.




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


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?




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:


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)




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


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




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


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.




Solution of Example 4.2.3:


IV.3.PERFORMANCE OF

DISTILLATION COLUMN(Plate/Tray Column and Packed Column)




Distillation Tray and Packing

Distillation Tray Distillation Packing


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

---




IV.4.SEPARATION AND RECYCLE SYSTEM

FOR CONTINUES PROCESS


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)




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.


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.




3. Recycling byproducts for improved selectivity

Consider: If a byproduct is formed via areversible secondary reaction thenrecycling the byproduct can inhibit itsformation at source.


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.




5. Feed impurities

If the impurity hasan adverse effect onthe reaction orpoisons the catalyst


5. Feed impurities (continued)

I if the impurity doesnot have a significanteffect on the reaction,then it could perhapsbe passed throughthe reactor and beseparated




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.


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.




7. Reactor heat carrier.

The introduction of an extraneous component as a heatcarrier effects the recycle structure of the flowsheet.


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.

iv distillation sequencing - ydhermawan's blog · pdf file1/4/2017 · a high...

Documents