Chemical Industry Digest. September 2012

CMYK

Ethanol

AUTHORS

Introduction

In a distillery unit, sugar in molasses is fermented to pro-duce ethanol. Figure 1 shows the typical process flowdiagram in a distillery. Fermentation process will pro-

duce an aqueous solution with alcohol content lesser than12 to 15% since the fermenting yeast is destroyed at thesealcohol concentrations. To produce ethanol of higher con-centration, the aqueous solution must be distilled. Usingmulti pressure distillation, the aqueous solution producesmixtures that are progressively closer to the azeotropic ra-tio of 95.5/4.5%.

We all know ethanol water forms a positive azeotropehaving 95.63% ethanol and 4.37% water (by weight). Etha-nol boils at 78.4oC, water boils at 100oC, but the azeotropeboils at 78.2oC, which is lower than either of its constitu-ents. Indeed 78.2oC is the minimum temperature at whichany ethanol-water solution can boil at atmospheric pressure.No number of distillations, however, will ever result in a dis-tillate that exceeds the azeotropic ratio. The further purifi-cation of ethanol up to 99.80% is done by dehydration us-ing molecular sieve dehydration technology.

This technology was introduced more than two decadesago to dehydrate ethanol. Earlier systems used for this pur-pose operated in liquid phase and used thermal swing re-generation process, which made them very energy consum-ing. Further development in the adsorbents saw introduc-tion of vapour phase operation with pressure swing regen-eration system. This proved to be highly energy-efficient. Themolecular sieve has proved to be the ideal solution to bringdown energy consumption and ensure high level of drynessin the final ethanol product.

Molecular Sieve Dehydration Technologyfor Ethanol Dehydration

Nilesh Patil, Dr V S Patil

Today many European countries areusing 10-15% blend of anhydrousethanol in petroleum. Molecular sievedehydration technology, an adsorptionphenomenon, is the technology used toproduce anhydrous ethanol. Thetechnology was first used in ethanoldrying in the early 80s.

The very precise pore size of molecularsieves enables them to select and sievemolecules of specific size and polarity.In adsorption, molecules diffuse fromthe bulk of the fluid to the surface of thesolid adsorbent forming a distinctadsorbed phase. These adsorbedmolecules adhere to the surface ofadsorbents due to weak cohesive forcescalled van der Waals forces. Separationby adsorption relies on one componentbeing more readily adsorbed than theother. For e.g. in an ethanol watermixture, water is more readily adsorbedon zeolite than ethanol.

The article elaborates the recent ad-vances of this technology along with allthe improvements for the production ofanhydrous alcohol from ethanol.

Nilesh P Patil is Senior Engineer - Process Design, PrajIndustries Ltd. He has 4 years of experience in functional

areas of process design, process development, trouble-

shooting of distillation columns, mass transfer operations

etc. He is an Associate Member of the Institution of

Engineers, Indian Institute of Chemical Engineers, and

the National Foundation of Indian Engineers.

Dr Vilas S Patil is Reader - Chemical Engi-neering, Department of Chemical Technol-

ogy, North Maharashtra University, Jalgaon.

He has 25 years of experience in mass trans-

fer operations, waste water treatment, corro-

sion engineering, transport phenomena etc.

He is a Life Member Institution of Engineers.

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Chemical Industry Digest. September 2012

CMYK

Ethanol

A molecular sieveis a synthetic adsor-bent. The vapourphase pressure swingregeneration systememploys molecularsieve beds which actas an adsorbent.These beds are madeof zeolite with an effec-tive pore size openingof about 3. In theethanol dehydrationprocess, the rectifiedspirit (hydrated etha-nol) vapours are al-lowed to pass througha column packed withfreshly activated mo-lecular sieve bed. Asthe rectified spiritvapour enters thesieve bed, water is dif-fused and adsorbedwithin the pores of theadsorbent structure ina thin layer. As morealcohol enters the col-umn, it passesthrough this layer to aslightly lower level

where another incremental amount of water is adsorbed.This continues until a point is reached where all possiblewater adsorption from this slug of alcohol is accom-plished or till the molecular sieve is saturated.

Transfer of water from the vapour of rectified spirit tothe molecular sieve occurs through a zone where water(adsorbate) content is reduced from its inlet to its outletconcentration. This finite length of bed, where the adsor-bate transfer occurs, is known as amass transfer zone. Two beds are pro-vided in order to make the process con-tinuous. Whilst the active bed is underpressure carrying out dehydration, thesecond bed is under regeneration andis under vacuum. The swing of opera-tion from one bed to another is con-trolled with the help of control valvesand automation.

Principle of OperationThe very precise pore size of mo-

lecular sieves enablesthem to select and removemolecules of particularsize from a bulk mixturecontaining moleculeswith large sizes and lowpolarity.

Fig 2 shows that a mo-lecular sieve has a porousstructure containing uni-form cavities of a specificsize. The selection of mo-lecular sieves dependsmainly on the size of themolecules to be removed from the mixture of other mol-ecules. As shown in the figure the water (H

2O) molecule

having size 3 is easily getting adsorbed in the cavitiespresent in the molecular sieves, whereas the ethanol(C

2H

5OH) molecule being larger in size than the water

molecule cannot enter the cavities present in the molecu-lar sieves.

Molecular sieves:> Separate molecules by size and polarity> Have very strong affinity for water> Dehydrate under pressure, regenerate under vacuum.

Fig 2. Cavities in a molecular sieve

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Cyclohexane

RectifiedSpirit

Steam

DehydrationColumn

Absolute Alcohol

Steam

Decanter

CWR CWS

Alcohol

RecoveryColumn

Spent Lees

Fig 3. Process flow diagram for azeotropic distillation

Molasses

Fermentation

Distillation

Rectified Spirit

Dehydration

Anhydrous Alcohol

(96.50% V/V)

(99.80% V/V)

Fig 1. Process flow diagram in adistillery

Detail Process in a DistilleryProduction of Fuel-Ethanol from

Molasses

The water (H2O) moleculehaving size 3 is easilygetting adsorbed in thecavities present in themolecular sieves, whereasthe ethanol (C2H5OH)molecule being larger insize than the water mol-ecule cannot enter thecavities present in themolecular sieves.

53

Chemical Industry Digest. September 2012

CMYK

Ethanol

Comparison of various processes to purify ethanolFurther purification of ethanol to produce anhydrous

alcohol is done by using:

> Azeotropic distillation

> Extractive distillation

> Membrane separation

> Molecular sieve dehydration.

Azeotropic DistillationProcess flow dia-

gram for azeotropic dis-tillation is shown in Fig3. Some of the reasonsthat make azeotropicdistillation unfeasibleinclude:

Use of carcinogens

High energy require-ment

Traces of entrainer

Complex installa-tion process.

ExtractiveDistillation

Fig 4 outlines theprocess of extractivedistillation. Extractivedistillation is more ex-pensive and unfeasiblefor use due to:

Uses chemical com-pounds like glycolto extract moisture

Requires high pres-sure steam

Operating cost is

lower than azeotropic distillation but isstill higher than molecular sieve dehy-dration technology.

Membrane SeparationMembrane separation is more expen-

sive and unfeasible for use due to:

Unavailability of membranes of par ticular size

Requires high pressure source to pass the fluid through membrane

Membranes do not have the ability to separate the selective size molecules.

Molecular Sieve DehydrationFollowing points make molecular sieve the most at-

tractive option:

Lower operating costs

No external chemicals required

Highly automated plant

High alcohol recovery.

Fig 5 shows that rectification in addition to water ad-

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>

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> >

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>>>

Solvent

RectifiedSpirit

Steam

ExtractiveColumn

AbsoluteAlcohol

Spent Lees

Steam

Solvent Recycle

RecoveryColumn

CWSCWR

Fig 4. Process flow diagram for extractive distillation

Adsorption50

60

80

Rectification

88 90 92 94 96 98

4.5

4

3.5

3

2.5

2

1.5

1

Hea

tin

g s

tea

m c

on

sum

pti

on

, k

g p

er k

ilo

gra

m o

f se

pa

rate

d w

ate

r

Ethanol concentration, % vol

Fig 5. Energy consumption in rectification and water adsorption process

Number of Plates inRectification Section

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Chemical Industry Digest. September 2012

CMYK

Ethanol

sorption can achieve significant energy savings. The plotdiagram of heating steam consumption vs. ethanol con-centration (% V/V) of end products from 88 to 96.5%has been developed on the basis of data collected fromseveral authors.

From the graph it is clear that as the alcohol concen-tration in a rectified spirit increases beyond 95%, the en-ergy consumption grows rapidly, approaching the infi-nite consumption at azeotrope alcohol concentration. Incontrast, the energy consumption of water adsorption isindependent of the alcohol concentration. The graph alsoshows that the energy consumption of alcohol dehydra-tion solution upto 95% concentration is less in rectifica-tion process, whereas above this concentration, the en-ergy consumption is more in rectification process thanin water adsorption.

It can be concluded that significant reduction in en-ergy consumption of bioethanol technology can beachieved by reconciling (congruenting) rectification of al-cohol to water adsorption. The graph also shows that thepotential energy savings can be obtained by combiningrectification of alcohol with water adsorption. In the clas-sic dehydration bioethanol technology, in which rectify-ing achieves 96.5% alcohol concentration, one kilogramof water separation consumes 4.0 kg heating steam. If thecongruent technology relationship between the rectifica-

tion and adsorption ofwater be set so that upto90% concentrations,rectification is used,while for further con-centration water ad-sorption is used, thenthe heating steam con-sumption will be only1.8 kg per kilogram ofseparated water, that is,about 55% less.

If 15% energy sav-ings are added from thewaste water recircula-tion between the mo-lecular sieves and therectification column, theused congruentbioethanol dehydrationtechnology can bringsignificant energy sav-ings. This technologynot only saves energybut also simplifies theequipment design. It is

known that greater is the rectified alcohol concentration,greater is the number of stages (plates) required in the tra-ditional rectification column. This, in turn, increases thetower height. Figure 5 shows that for 95% (V/V) alcoholconcentration, the column requires 50 plates, while for96.5% (V/V) alcohol concentration reach, the column re-quires 80 plates.

ConclusionMolecular sieve technology reduces energy require-

ment and leads to significant savings. This new technol-ogy allows rectification and significantly reduces the num-ber of plates in the rectification column and hence theheight of the column. This new technology brings abouttechnological ease in the design of the process and pro-cess equipments.

References1. Lyons T. P., Kelsall D. R., Murtagh J. E. The Alcohol Text-

book Nottingham: University Press, 1995, Page 259.

2. Bremers G., Birzietis G., Blija A., Skele A., Danilevics A.Bioethanol Congruent Dehydration. Proceedings of 8th In-ternational Scientific Conference Engineering for Rural De-velopment, Jelgava, 2009, Page 131-134.

3. Handbook of alcohol production: equipment, means of mechani-zation and Automation 1983, Page 480.

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Water to Recycle

Purge Revap

Product

98.8%EtOH

ProductTank

1/2 Ballast

TwoUnits

Condenser

Wire MeshSupport Screen

1/2 Ballast

1/4Ballast

PurgeVac for

regeneration

FeedSuper Heater

Sequencing: Adsorption cycle ends,bed is depressurized & flow isdiverted to the next bottle set foradsorption. Heat & pressure aretransferred to the next bottle & notlost.

Distillation/Rectification

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Fig 6. Process flow diagram for combined bioethanol dehydration technology

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