Ethanol distillation: the fundamentals 319

Ethanol distillation: the fundamentals

P.W. MADSON

KATZEN International, Inc., Cincinnati, Ohio, USA

Fundamentals of a distilling system

Certain fundamental principles are common toall distilling systems. Modern distillation systemsare multi-stage, continuous, countercurrent,vapor-liquid contacting systems that operatewithin the physical laws that state that differentmaterials boil at different temperatures.

Represented in Figure 1 is a typical distillationtower that could be employed to separate an idealmixture. Such a system would contain thefollowing elements:

a. a feed composed of the two components tobe separated,

b. a source of energy to drive the process (inmost cases, this energy source is steam, eitherdirectly entering the base of the tower ortransferring its energy to the tower contentsthrough an indirect heat exchanger called areboiler),

c. an overhead, purified product consistingprimarily of the feed component with thelower boiling point,

d. a bottoms product containing the componentof the feed possessing the higher boilingpoint,

e. an overhead heat exchanger (condenser),normally water-cooled, to condense the vapor

resulting from the boiling created by theenergy input. The overhead vapor, aftercondensation, is split into two streams. Onestream is the overhead product; the other isthe reflux which is returned to the top of thetower to supply the liquid downflow requiredin the upper portion of the tower.

The portion of the tower above the feed entrypoint is defined as the ‘rectifying section’ of thetower. The part of the tower below the feed entrypoint is referred to as the ‘stripping section’ ofthe tower.

The system shown in Figure 1 is typical forthe separation of a two component feedconsisting of ideal, or nearly ideal, componentsinto a relatively pure overhead productcontaining the lower boiling component and abottoms product containing primarily the higherboiling component of the original feed.

If energy was cheap and the ethanol-watersystem was ideal, then this rather simpledistillation system would suffice for theseparation of the beer feed into a relatively pureethanol overhead product and a bottoms productof stillage, cleanly stripped of its ethanol content.Unfortunately, the ethanol-water (beer) mixtureis not an ideal system. The balance of this chapter

will be devoted to a description of themodifications required of the simple distillationsystem in order to make it effective for theseparation of a very pure ethanol product,essentially free of its water content.

Figure 2 expands on Figure 1 by showing someadditional features of a distillation tower. Theseare:

a. The highest temperature in the tower willoccur at the base.

b. The temperature in the tower will regularlyand progressively decrease from the bottomto the top of the tower.

c. The tower will have a number of similar,individual, internal components referred toas ‘trays’ (these may also be described asstages or contactors).

d. Vapor will rise up the tower and liquid willflow down the tower. The purpose of thetower internals (trays) is to allow intimatecontact between rising vapors anddescending liquids correlated with separationof vapor and liquid.

Figure 3 shows a vapor-liquid equilibriumdiagram for the ethanol-water system atatmospheric pressure. The diagram shows molepercent ethanol in the liquid (X axis) vs molepercent ethanol in the vapor (Y axis). The plotcould also be made for volume percent in theliquid vs volume percent in the vapor and theequilibrium curve would only be slightlydisplaced from that shown in Figure 3. Molepercent is generally used by engineers to analyzevapor/liquid separation systems because it

Vapor

Reflux

Cooling water

Low boiling product

Feed mixture

Steam (energy)

Strip

pin

gR

ectif

yin

g

High boiling product

Condenser

Figure 1. Ideal distillation system.

relates directly to molecular interactions, whichmore closely describe the process occurring ina distillation system.

Analysis of the ethanol-water distillationsystem is mathematically straightforward whenusing molar quantities rather than the morecommon measurements of volume or weight.This is because of an energy balance principlecalled ‘constant molal overflow’. Essentially, thisprinciple states that the heat (energy) requiredto vaporize or condense a mole of ethanol isapproximately equal to the heat (energy)required to vaporize or condense a mole ofwater; and is approximately equal to the heat(energy) required to vaporize or condense anymixture of the two. This relationship allows thetower to be analyzed by graphic techniquesusing straight lines. If constant molal overflowdid not occur, then the tower analysis would

become quite complex and would not lend itselfeasily to graphic analysis.

Referring to Figure 3, a 45o line is drawn fromthe compositions of 0, 0 to 100% and 100%.This 45o line is useful for determining ranges ofcompositions that can be separated bydistillation. Since the 45o line represents thepotential points at which the concentration in thevapor equals the concentration in the liquid, itindicates those conditions under whichdistillation is impossible for performing theseparation. If the equilibrium curve contacts the45o line, an infinitely large distillation towerwould be required to distill to that compositionof vapor and liquid. Further, if the equilibriumcurve crosses the 45o line, the mixture hasformed an azeotrope. This means that even ifthe tower were infinitely large with an infiniteamount of energy, it would be impossible todistill past that point by simple rectification.

Condenser Cooling water

Overhead product

Reflux liquid

Vapor

Feed

Hig

her

tem

perature

Low

er

tem

perature

Thermal energy

Vapor

Boiling liquid

Trays (contacting devices)

Bottoms product

Figure 2. Typical distillation relationships.

Consider a very simple system consisting of apot filled with a mixture of ethanol in water (abeer) containing 10% by volume ethanol (3.3mole %). This composition is identified in thelower left portion of Figure 3. A fire could bekindled under the pot, which would add thermalenergy to the system. The pot would begin toboil and generate some vapor. If we gathered asmall portion of the vapor initially generated andmeasured its ethanol content; we would findabout 24 mole % ethanol (53 volume %). If wecondense this vapor (note: there will be only asmall amount of this vapor), boil it in a secondpot and again collect a small amount of the initialvapor generated, this second vapor wouldcontain about 55 mole % (83 volume %) ofethanol (see Figure 3). If we should continuethis simplified process to a third and fourth

collection of small amounts of vapor, analysiswould reveal that each successive portion ofvapor would become richer in ethanol.

Thus we have created a series of steps bywhich we keep increasing the ethanol contentof the analyzed sample, both liquid and vapor.Unfortunately, this oversimplified process isidealized; and practically speaking, isimpossible. However if we had supplied ouroriginal pot with a continuous supply of ethanol-water feed and vapor generated in the first potwas continuously condensed and supplied to thesecond pot, etc. then the process becomes similarto the industrial distillation tower operationshown in Figure 2.

How far can this process be extended? Couldwe produce pure ethanol by continuouslyextending our process of boiling and reboiling?

100

0

Azeotrope

194° proof

(10 Volume %)

10 20 30 40 50 60 70 80 90 100

90

80

70

60

50

40

30

20

10

0

Mole % ethanol in liquid

Mole

%

ethanol in

vapor

Equilibrium Curve

Figure 3. Vapor/liquid equilibrium for the ethanol/water system at atmospheric pressure.

The answer is, no! We would finally reach apoint in one of the downstream pots, where thevapor boiling from the liquid was of the samecomposition as the liquid from which it wasbeing generated. This unfortunate consequencelimits our ability to produce anydrous ethanolfrom a dilute ethanol-water feed. What we finallyencounter in our simplified process is theformation of an azeotrope. This is a concentratedsolution of ethanol and water that when boiledproduces a vapor with a composition identicalto the composition of the liquid solution fromwhich it originated.

In summary then, we are limited in ethanol-water purification in any single multistagedistillation tower to the production of azeotropicethanol-water mixtures. These azeotropic

solutions of ethanol and water are also knownas constant boiling mixures (CBM) since theazeotropic liquid will have the same temperatureas the azeotropic equilibrium vapor being boiledfrom itself. Without some sort of drastic processintervention, further ethanol purificationbecomes impossible. The question thenbecomes: What can we do to make it possible toproduce anhydrous ethanol? Methods of doingso will be covered later in this chapter.

Figure 4 depicts the structure of the distillationprocess by dividing the vapor/liquid equilibriuminformation into three distinct zones of processand equipment requirements: stripping, rectifyingand dehydration. This division is the basis forthe design of equipment and systems to performthe distillation tasks.

0

0 10 20 30 40 50 60 70 80 90 100

10

20

30

40

50

60

70

80

90

100

Mole % ethanol in liquid

Azeotrope 194° proof

(10 volume %)

Mole

%

ethanol in

vapor

Equilibrium curve

Rectifying system

Stripping system

Stilla

ge

Spir

it

Anhydrous

product

Dehydration

system

Figure 4. Structuring the distillation system strategy.

Considerations in preliminary design

The engineer, given the assignment of designinga distillation tower, is faced with a number offundamental considerations. These include:

a. What sort of contacting devices should beemployed? (e.g. trays or packing). If traysare chosen, what type will give the mostintimate contact of vapor and liquid?

b. How much vapor is needed? How muchliquid reflux is required? (What ratio of liquid:vapor is required?)

c. How much steam (energy) will be required?

d. What are the general dimensions of thedistillation tower?

DISTILLATION CONTACTORS

Trays are the most common contactor in use.What are the functions expected of traycontactors in the tower? Figure 5 depicts a singletray contactor in a distillation tower and showsthe primary functions desired:

- mixing rising vapor with a falling fluid

- allow for separation after mixing

- provide path for liquid to proceed down thetower

- provide path for vapor to proceed up thetower

Enriched

vaporLiquid

Stripped

liquid

Vapor

V/L mixing

V/L separation

Figure 5. Distillation tray functions.

Figure 6 depicts a perforated tray contactor withcertain accoutrements required to control theflow of liquid and vapor and to assure theirintimate contact. Another type of tray contactingdevice, the disc-and-donut or baffle tray isshown in Figure 7. The characteristics of thistype of contactor make it especially useful fordistilling materials such as dry-milled grain beer,which would foul ordinary trays such asperforated, valve, etc.

ENERGY ANALYSIS

In addition to the selection of the basic contactingdevice, the energy requirement must be

Inlet weir

Outlet weir

Downcomer

area

Downcoming

liquid

Stripped

liquid

Enriched

vapor

Vapor

V/L mixing

V/L separation

Perforated

area

Figure 6. Perforated trays.

established. This is accomplished by analyzingthe vapor/liquid equilibrium data from Figure4, for the liquid:vapor flow ratio to perform acontinuous series of steps within the limits ofthe equilibrium curve. Table 1 demonstrates a

Disc

Donuts

Descending

liquid

Stripped stillage

Vapor

Enriched

vapor

Descending

liquid

simplified procedure to calculate theapproximate energy requirement from theliquid:vapor flow ratio that will be employed inthe tower design. Repetition of this type ofcalculation for different conditions produces a

Figure 7. Disc-and-donut trays.

Table 1. Simplified calculations for steam requirements for ethanol distillation.

Example 1. Calculate the steam required (lbs/gallon of product) to strip 100 gpm of a 10% volume beer (90 gpm water/10 gpm ethanol).

L/V* = 5.0 (typical for a 10% volume beer) or L = 5 V

L = 90 gpm • 500 lbs/hr = 45,000 lbs/hr = 5 • V gpm

Therefore, V = 9,000 lbs/hr (steam)

And: 9,000 lbs/hr (steam) • hr = 15 lbs steam/gallon of product 10 gpm (product) 60 min

Example 2. Calculate the steam required to strip 100 gpm of a 5% volume beer (95 gpm water/5 gpm ethanol).

L/V = 6.33 (typical for a 5% volume beer) or L = 6.33 • V

L = 95 gpm • 500 lbs/hr = 47,500 lbs/hr = 6.33 • V gpm

Therefore, V = 7,500 lbs/hr (steam)

And: 7,500 lbs/hr (steam) • hr = 25 lbs steam/gallon of product 5 gpm (product) 60 min

*L and V are liquid and vapor flow rates, respectively, expressed in lb-mole per hr. Note: at base of column use simplifying assumption of water (L) and steam (V). Therefore: L (lb-mole/hr) = L (lbs/hr)

V(lb-mole/hr) V(lbs/hr)

design chart like that shown in Figure 8 for theethanol-water system. Such a graph is usefulwhen calculations are needed to ascertaintechnical and economic feasibility andpreliminary conditions for the design.

Figure 9 demonstrates how the liquid:vaporflow ratio, in connection with the number ofstages (theoretically ideal trays) required for aspecified separation between ethanol and water,is graphically determined. Note that the stagesare constructed by drawing straight linesvertically and horizontally between theequilibrium curve (previously determinedexperimentally) and the operating lines. For anethanol stripper/rectifier, there are two operatinglines: one for the rectification section and onefor the stripping section. The operating linesrepresent the locus of concentrations within thedistillation tower of the passing liquid and vaporstreams. The operating lines for a given tower

are based on the energy input, as calculated andrepresented in Figure 8. Because of the principleof constant molal overflow, the operating linescan be represented as straight lines. If constantmolal overflow was not valid for the ethanol/water distillation, then these lines would becurved to represent the changing ratio of liquidflow to vapor flow (in molar quantities)throughout the tower. The slope of the operatingline (the ratio of liquid flow to vapor flow) isalso called the internal reflux ratio. If the energyinput to a tower is increased while the beer flowremains constant, the operating lines will movetoward the 45o line, thus requiring fewer stagesto conduct the distillation. Likewise if the energyinput is reduced (lowering the internal refluxratio), the operating lines will move toward theequilibrium curve, reducing the degree ofseparation achievable in each stage and therefore

Beer concentration (volume %)

Lbs steam

/gallon (anhydrous ethanol basis

)

30

25

20

15

10

5

0

0 5 10 15

Constraints:

1. 190° proof product

2. 0.02% (wt) bottoms

3. Saturated feed

4. Direct steam

Theoretical minimum

Practical minimum

Figure 8. Steam requirements ethanol stripper/rectifier.

requiring more stages to conduct the distillation.The calculations underlying the preparation ofFigure 9 go beyond the scope and intent of thistext, but have been included for continuity. Thedashed lines represent the graphical solution tothe design calculations for the number oftheoretical stages required to accomplish adesired degree of separation of the feedcomponents. Figure 9 is referred to as a McCabe-Thiele diagram. For further pursuit of thissubject, refer to the classical distillation textbookby Robinson and Gilliland (1950).

TOWER SIZING

The goal of the design effort is to establish thesize of the distillation tower required. Table 2shows the basic procedure to determine the

diameter required for the given distillation tower.Since all of the distillation ‘work’ is done by thetrays, the tower is actually the ‘container’ tosurround the vapor and liquid activity that is‘managed’ by the trays. Tower diameter designis, therefore, actually the design of the necessarytray diameter for proper vapor/liquid interactionand movement.

The f factor (vapor loading) is an empiricallydetermined factor that depends primarily upontray type and spacing, fluid physical properties,froth stability and surface tension at theoperating conditions of the system. The propervalues for f are determined by field observations.In summary then, the f factor can be describedas an adjusted velocity term (units are ft/sec) thatwhen multiplied by the square root of the densityratio of liquid to vapor, will give the allowablevapor velocity in the empty tower shell, such

14

100

0 10 20 30 40 50 60 70 80 90 100

90

80

70

60

50

40

30

20

10

0

Typical L / V=5.0

Typical: 14 rectifying stages

8 stripping stages

1

2

3

4

5

6

7

8 1110 12 139

Equilibrium curve

Operating line

rectification

Operating line

stripping

190° proof

Mole % ethanol in liquid

Stillage

(0.02 wt %)

Beer feed

(10 volume %)

Mole %

ethanol in vapor

Figure 9. Vapor/liquid equilibrium stage analysis.

Table 2. Calculations for tower sizing (base of stripper).

Example: Calculate tower diameter required for a 10% volume beer at 100 gpm (15 lbs steam/gallon of product)

W Vapor flow rate = 9,000 lbs/hr (steam)P Operating pressure at base = 1.34 ATMMAVG Average MW of vapor = 18 lbs/lb-moleρL Liquid mixture density = 59.5 lbs/ft3 (227 °F)T Absolute operating temperature = 687 °RD Tower inside diameter in inchesf Vapor loading factor = 0.05-0.3

Sizing equation: f

16.45=

687

59.5•18•1.34•f

9000•0.2085=

T

ρ•M•P•f

W•0.2085=DLAVG

Assuming f = 0.16 (specific to tray design and spacing), the tower diameter is:

inches 41.125=0.16

16.45=D

Tower diameter calculation utilizes the following equation. Values for the terms are indicated above.

T

ρ•M•P•f

W•0.2085=DLAVG

The final design equation can be derived beginning with the fundamental equation:

ρρ-ρ•=u

V

V L f

Where u = average vapor velocity in empty tower shell (ft/sec)ρL = liquid density (lbs/ft3)ρV = vapor density (lbs/ft3)f = tower vapor loading factor (ft/sec)

(Note: For most cases ρL is much greater than ρV, so that ρL – ρV ≅ ρL. For example, water (steam) at 212oF and atmosphericpressure: ρL = 59.8 lbs/ft3 and ρV = 0.0373 lbs/ft3. Then ρL – ρV = 59.7627 lbs/ft3 ≅ 59.8 lbs/ft3, which results in a negligible0.06% error.)

Consequently, ρ ρ •u

V

Lf≅

Imposing the equation of continuity: W = A•ρV•u or u = W/A•ρV

Where A = tower cross-section area (ft2)ρV = vapor density (lbs/ft3)u = average vapor velocity in empty tower shell (ft/sec)

and W = vapor mass flow (lbs/sec)

Substituting for u in the equation above:

ρρ•

ρV •A

W

V

L

f≅ then ρ•ρ•

WAor ρ•ρ•

A

W

v

L

vLf

f ≅≅ρV ρL ρV

ρL ρV

that liquid entrainment and/or vapor phasepressure drop in the tower will not be excessive.

Excessive vapor velocity will first manifestitself by causing excessive liquid entrainmentrising up the tower, causing loss of separationefficiency. Ultimately the excessive entrainmentand pressure drop will cause tower flooding.

To achieve a well-balanced tower design, theforegoing analysis must be performed at eachstage of the tower, from bottom to top.Composition changes, feed points, draws, etc.,each can cause a different requirement. Thetower must be examined to locate the limitingpoint.

Similar analyses, with empirically-observedperformance coefficients, are applied to vaporpassing through the trays and through the liquid,and to the movement and control of liquidpassing through downcomers and across thetrays. These analytical procedures are beyondthe scope of this text. Reference should be madeto the aforementioned text by Robinson andGilliland for further information.

Considerations in optimizing distillationsystem design

Optimizing the technical and economic designof distillation equipment and similar gas and

vapor/liquid mass transfer systems involves anumber of interrelated parameters. The positive/negative balance of a variety of contactingdevices with different capacities and efficienciesfor promoting vapor/liquid mass transfer mustbe taken into consideration. Along with thetechnical issues considered in such designs,economical operation is essential not only in thereduction of energy and other direct costs, butalso in relation to investment and return oninvestment from the operation being considered.In this respect, distillation towers are notindependent process-wise, as consideration mustalso be given to other auxiliaries such asreboilers, condensers, pumps, controls andrelated equipment.

SIZING TOWERS

In determining optimum diameter and height oftowers for distillation, absorption, stripping andsimilar mass transfer operations, design factorsare affected by whether the installations will beindoors or outdoors. With indoor installations,building height limitations, as well as floor levelaccessibility, are an important factor in thedesign. Where there are height limitations, towersmust be increased in diameter to provide forreduced tray spacing, which in turn will require

Use the Ideal Gas Law to express the vapor density.

T•RM•P

=ρ AVGV Then by substitution one obtains

T•Rρ•M•P

•

W=A

LAVG f

Using the Universal Gas Constant R = 0.73 (ft3)(atm)/(lb-mole)(oR) the equation becomes:

Tρ•M•P

••1.17

W=A

LAVG f Now D•0.7854=4D• π

=A 22

and T

ρ•M•P•

W•1.0879=

T ρ•M•P

••0.9192

W=D

LAVGLAVG

2

ff

Adjusting units:

T ρ•M•P

•

W•

60

12• 1.043=D

LAVGf

Therefore,

T ρ•M•P

•

W•0.2085=(inches) D

LAVGf

M•P AVG

ρ•M•P LAVG D2•

ρ•M•P LAVG

ρ•M•P LAVG ρ•M•P LAVG

ρ•M•P LAVG ρ•M•P LAVG

lower vapor velocities. With outdoorinstallations, literally the ‘sky is the limit’, andrefinery and petrochemical towers of 200 feetin height are not uncommon.

In either case, indoors or outdoors, theinterrelated tower diameter and tray spacing arelimited by allowable entrainment factors (ffactors) (Katzen, 1955). If outdoors, towerheights and diameters must be related tomaximum wind loading factors in the specificplant location and may be complicated byallowance for earthquake factors.

TRAY AND PACKING SELECTION

Vapor/liquid contacting devices may be of twodistinct types, namely packed or tray (staged)towers. In packed towers, the transfer of materialbetween phases occurs continuously anddifferentially between vapor and liquid through-out the packed section height. By contrast, intray towers, the vapor/liquid contact occurs onthe individual trays by purposely interruptingdown-flowing liquid using downcomers toconduct vapor-disengaged liquid from tray totray and causing the vapor/liquid contact to occurbetween cross-flowing liquid on the tray withvapor flowing up through the tray. In otherwords, the vapor/liquid contact is intermittentfrom tray to tray, and is therefore referred to asbeing stagewise. Thus, for any given separationsystem, the degree of vapor/liquid contact willbe greater with a greater height of the packedsection, or in the case of tray towers, a greaternumber of trays used.

It is generally considered that packing-typeinternals may be used with relatively clean vaporand liquid systems where fouling is not aproblem. Economics indicate that packing isapplicable in small and modest sized towers. Asthe towers become larger, packing becomescomplicated by the need for multiple liquidredistribution points to avoid potential vapor/liquid bypassing and reduction in efficiency.Structured packings (Fair et al., 1990; Bravo etal., 1985) are designed to minimize theseproblems by reducing the height requirement andcontrolling, to some extent, the distribution ofliquid. However, high fabrication andspecialized installation costs would indicate thatthese are applicable only for relatively low-volume, high-value product processing.

Trays of various types are predominant invapor/liquid contacting operations, particularlyon the very large scale encountered in thepetroleum and petrochemical industries, in largescale operations of the chemical processindustries and in the large scale plants of themotor fuel grade ethanol industry.

The venerable bubble cap tray, with a widevariety of cap sizes, designs and arrangementsto maximize contact efficiency, has fallen outof favor during the past few decades because ofthe relatively high cost of manufacture andassembly. Valve trays of several types have takenover in operations requiring a relatively widevapor handling capacity range (turndown). Thishas been extended by use of different weightsof valves on the same tray. Specialty trays suchas the Ripple, Turbogrid, tunnel cap and othersdesigned to improve contact under certainspecific circumstances have been used to alimited extent.

The long established perforated tray is acontacting tray into which a large number ofregularly oriented and spaced small circularopenings have been drilled or punched. Thesetrays are commonly referred to as ‘sieve trays’because of the original practice of putting themaximum number of holes in any given trayarea. This original design produced a fairlyinefficient operation at normal loading, and avery inefficient operation with decreased vaporloading. About 50 years ago, engineers beganto suspect that the design approach had been inerror, and that the hole area in the trays shouldbe limited by the hole velocity loading factor toobtain maximum contact by frothing, asindicated in Figure 10.

The hole vapor loading factor (or perforationfactor) is defined as the vapor velocity throughthe perforations adjusted by the square root ofthe vapor density at the specific tower locationof a given perforated tray. With the paralleldevelopment of separation processes in thepetroleum refining, chemical and ethanolindustries, the modern approach has developedto what is now called ‘perforated tray’ design.The Fractionation Research Institute of theAmerican Institute of Chemical Engineersdiverted its efforts from bubble cap studies toperforated tray testing; and have established abasis for the design of perforated trays with highefficiency and wide capacity range (RaphaelKatzen Associates, 1978).

Where foaming or tray fouling (caused bydeposition of solid materials in the tower feed)can be an operational problem, novel designssuch as the baffle tray may permit extendedoperating time between cleanings. Baffle traysmay take a number of different forms. They canbe as simple as appropriately spaced,unperforated, horizontal metal sheets coveringas much as 50-70% of the tower cross sectionalarea; or they may take the form of a series ofvertically spaced, alternating, solid disc-and-donut rings (see Figure 7). Towers up to 13 ftdiameter are in operation using this simple disc-and-donut design concept.

Although system-specific data have beendeveloped for each type of tray, it is difficult tocorrelate tray loading and efficiency data for awide variety of trays on a quantitative basis.Each system must be evaluated based uponempirically-derived loading factors for vaporand liquid operations within the tower.

AUXILIARIES

Energy input is of prime importance in towerdesign, particularly in ethanol stripping andrectification units. In aqueous and azeotrope-forming systems, direct steam injection has been

common practice to maintain simplicity.However, the need to reduce the volume ofwaste going to pollution remediation facilitieshas minimized use of this simple steam injectiontechnology to avoid the dilution effect of thesteam being condensed and added to the stillage.Direct steam injection transfers both the energyand the water into the process. By imposing aheat exchanger (reboiler) between the steam andthe process, only the energy is transferred intothe tower. The condensate water is returned in aclosed loop to the boiler, thus reducing thebottoms outflow from the process. Reboilers arethus growing in acceptance, and several typesmay be employed. Kettle and thermosyphonreboilers are preferred where fouling is not aproblem. Where fouling can occur, high velocity,forced-circulation, flash heating reboilers arepreferred. Figure 11 depicts the reboiler energytransfer by a forced-circulation reboiler ascompared to Figure 1 which depicts direct steaminjection.

Thermocompression injection of steam hasalso been utilized where low pressure vapors areproduced from flash heat recovery installationsand where higher pressure motive steam is alsoavailable.

Condenser design would appear to be simple.However, in many cases, water limitationsrequire adapting condenser designs to the use

Inlet weir

Outlet weir

Downcomer

area

Downcoming

liquid

Stripped

liquid

Enriched

vapor

Froth zone

Vapor

V/L mixing

V/L separation

Figure 10. Perforated tray frothing.

of cooling tower water with limited temperaturerise and minimal scale-forming tendencies. Onthe other hand, where water is extremely scarce,air-cooled condensers are used.

Energy conservation

The increasing cost of thermal energy, whetherprovided by natural gas, fuel oil, coal or biomass,is fostering an increased emphasis on heatrecovery and a reduction in primary thermalenergy usage (Fair, 1977; Petterson et al., 1977;Mix et al., 1978). Conventional bottoms-to-feedheat exchangers are now being supplementedwith recovery of overhead vapor latent heat bypreheating feed streams and other intermediateprocess streams. Techniques of multistage

distillation (similar to multiple effectevaporation) are also practiced. Pressure-to-atmospheric, atmospheric-to-vacuum, orpressure-to-vacuum tower stages are utilized,with the thermal energy passing overhead fromone tower to provide the reboiler heat for thenext one. Two such stages are quite commonand three stage systems have also been utilized(Katzen, 1980; Lynn et al., 1986).

Furthermore, the modern technique of vaporrecompression, commonly used in evaporationsystems, is also being applied to distillationsystems. Such a system can provide forcompression of overhead vapors to a pressureand temperature suitable for use in reboiling alower pressure stripping tower. However, thecompression ratios required for such heatrecovery may consume almost as much

Condenser Cooling water

Low boiling product

Note: See Figure 1 for

comparison to direct

steam injection

Reflux

Vapor

Feed mixture

Rectif

yin

gS

trip

pin

g

Steam (energy)

High boiling product

Condensate

return to boiler

Reboiler

Figure 11. Energy transfer by a forced-circulation reboiler.

electrical energy as would be saved in thermalinput. Alternative systems, using vaporrecompression as an intermediate stage devicein the distillation system, have also beenproposed.

Control systems

Control systems can vary from manual control,through simple pneumatic control loops to fullyautomated distributed control (Martin et al.,1970). High level computer control hasfacilitated the application of sophisticatedcontrol algorithms, providing more flexibility,reduced labor and higher efficiency with lowercapital investment. Such systems, when properlyadapted to a good process design, have provenmore user-friendly than the control techniquesutilized in the past.

Economic design

In integrating the technology discussed, the finalanalysis must be economic. Alternative systemsmust be compared on the basis of investmentrequirements, recovery efficiency and relativecosts of operation. Thus, any heat exchangersinstalled for heat recovery must show asatisfactory return on the investment involvedin their purchase and installation. In comparingalternative separation systems, the overallequipment costs must be compared againstenergy and other operating costs to determinewhich system offers the best return. Moderncomputer-assisted designs incorporateeconomic evaluation factors so economicoptimization can be determined rapidly.

Ethanol distillation/dehydration: specificsystems technology

Proven industrial technologies are available fordistillation of various grades of ethanol fromgrain, sugarcane, molasses and other feedstocks.Improvements have been made over the years,particularly during development of the motorfuel grade ethanol industry. In such installations,a key requirement is the minimization of totalenergy usage.

The operation that has been most subject tocritical comment is the distillation process. Manyrelatively new ‘authorities’ in the field havebased their criticism on technologies that goback 50-60 years, and have created anunwarranted condemnation of distillation as aviable process for low energy motor fuel gradeethanol production. Systems developed over theyears will be described to show that much ofsuch criticism is unwarranted and unjustified.

PRODUCTION OF INDUSTRIAL ETHANOL

Prior to the emphasis on motor fuel gradeethanol, the major ethanol product utilizedworldwide was high purity, hydrous industrialethanol, which is generally produced at astrength of 96o GL (192o US proof) (oGL =degress Gay Lussac = % by volume ethanol; USproof = 2 x % by volume ethanol). Efficientsystems have been in commercial operation formany years for the production of such highgrade ethanol from ethylene, grain, molasses andsulfite waste liquor. The basic distillation systemis shown in Figure12.

In the case of synthetic ethanol (outside thescope of this publication), the beer strippingtower is not required and the refining system isa simple three tower unit, which achieves 98%recovery of the ethanol in the crude feed as afirst grade product. The final product maycontain less than 5 ppm total impurities and hasa ‘permanganate time’ of more than 60 minutes.

For the production of industrial or beveragespirit products made by fermentation of grain,molasses or sulfite liquor, the system utilizes thefull complement of equipment shown in Figure12. The beer feed is preheated from the normalfermentation temperature in several stages,recovering low level and intermediate level heatfrom effluent streams and vapors in the process.This preheated beer is degassed and fed to thebeer stripper, which has stripping trays belowthe beer feed point and several rectifying traysabove it. The condensed high wines from thetop of this tower are then fed to the extractivedistillation tower, which may operate at apressure of 6-7 bars (87-101.5 psi). In this tower,most of the impurities are removed and carriedoverhead to be condensed as a low grade ethanol

stream, from which a small purge of heads(acetaldehyde and other low boiling impurities)may be taken while the primary condensate flowis fed to the concentrating tower. The purified,diluted ethanol from the bottom of the extractivedistillation tower is fed to the rectifying tower,which has an integral stripping section. In thistower, the high grade ethanol product, whetherindustrial or potable, is taken as a side draw fromone of the upper trays. A small heads cut isremoved from the overhead condensate. Fuseloils (mixtures of higher alcohols such as propyl,butyl, and amyl alcohols and their isomers, whichare fermentation by-products or ‘congeners’) aredrawn off at two points above the feed tray butbelow the product draw tray to avoid a buildupof fusel oil impurities in the rectifying tower. Theoverhead heads cut and the fusel oil draws arealso sent to the concentrating tower.

It should be noted that the rectifying tower isheated by vapors from both the pressurizedextractive distillation tower and the pressurizedconcentrating tower.

In the concentrating tower, the various streamsof congener-containing draws are concentrated.

A small heads draw is taken from the overheadcondensate, which contains the acetaldehydefraction along with a small amount of the ethanolproduced. This may be sold as a by-product orburned as fuel. A fusel oil side draw is taken athigh fusel oil concentrations through a coolerto a washer. In the washer, water is utilized toseparate the ethanol from the fusel oil, with thewashings being recycled to the concentratingtower. The decanted fusel oil may be sold as aby-product. The ethanol recovered from thecrude streams is taken as a side draw from theconcentrating tower and fed back to theextractive distillation tower for re-purificationand recovery of its ethanol content.

In an early version of this system, installedmore than 60 years ago for the production ofpotable ethanol from grain and from molasses,all towers were operated at atmospheric pressure.However, installations made within the past 40years utilize the multistage pressure system toreduce energy consumption to a level of about50% of the all-atmospheric system.

The commercial installations utilizing themultistage pressure, or ‘pressure cascading’

Condenser

CW

Beer feed

Extraction

water

Condenser/

reboiler

Condenser/

reboiler

Preheater/

condenser

Reflux

Waste water

Washwater

Fusel oil

by-product

Fusel oil

decanter

Fusel oil

cooler

CW

CW

Heads by-product

Product ethanol

to storage

Stillage

SteamSteam

Steam

Reboiler Reboiler Reboiler

Atm

ospheric

rectif

yin

g tow

er

Pressuriz

ed h

eads a

nd f

usels

concentratin

g t

ow

er

Pressurized extractiv

e tow

er

Beer strip

per

Figure 12. Low energy-consuming high grade hydrous ethanol distillation.

technique operate with a steam consumption of3.0-4.2 kg of steam/liter (25-35 lb/gallon) of 96o

GL ethanol. This may be compared to about 6kg of steam/liter for earlier conventionaldistillation systems.

Production of anhydrous ethanol

Systems have been designed and installed forproduction of extremely dry and very pureanhydrous ethanol for food and pharmaceuticaluse, primarily in aerosol preparations. Thesesystems, as shown in Figure 13, yield ethanolcontaining less than 200 ppm water (99.98o GL),less than 5 ppm total impurities and more than60 minutes permanganate time.

The two tower dehydrating system has beenoperated in two super-anhydrous plants inCanada, and was used to produce motor fuelgrade ethanol (99.5o GL) in four installations inCuba (prior to the advent of the Castro regime).The dehydrating tower and the entrainer-recovery tower are operated at atmosphericpressure. Thus, they may utilize either lowpressure steam, hot condensate or hot wastestreams from other parts of the ethanol processto minimize steam usage. To simplify equipment

and minimize investment, a common condensingand decanting system is used for the two towers.

The entrainer used to remove water as a ternary(three component) azeotrope may be benzene,heptane (C6-C8 cut), cyclohexane, n-pentane,diethyl ether or other suitable azeotropic agents.The entrainer serves to create a three componentazeotrope that boils at a temperature lower thanany of the three individual components andlower than the ethanol/water binary (twocomponent) azeotrope. Therefore the ternarymixture will pass overhead from the tower,carrying the water upward. Upon condensing,the mixture separates in a decanter into anentrainer-rich layer and a water-rich layer.

The hydrous ethanol feed enters thedehydrating tower near the top. The feedcontacts the entrainer in the upper section of thetower. The three component mixture in thissection of the tower seeks to form its azeotrope,but is deficient in water and contains moreethanol than the azeotrope composition.Therefore, the ethanol is rejected downward inthe liquid and is withdrawn as an anhydrousproduct from the bottom of the tower. The waterjoins the entrainer, passing upward as vapor toform a mixture that is near the azeotrope

190° proof ethanol feed

from distillation

Dehydration

tower

Reboiler

Condenser

Entrainer

recovery tower

Anhydrous ethanol

to storage

CW

CW

Reboiler

Waste

water

Cooler

Decanter

Low pressure steam

or hot condensate

Condensate return

CW

Cooler

Figure 13. High grade anhydrous ethanol system.

composition for the three components. Thecondensed mixture separates into two layers inthe decanter and the entrainer-rich layer isrefluxed from the decanter back to the top ofthe tower. The aqueous layer is pumped fromthe decanter to the entrainer-recovery tower, inwhich the entrainer and ethanol are concentratedoverhead in the condenser-decanter system. Thestripped water, emerging from the base of thetower, may go to waste. If it has substantialethanol content, it may be recycled to the spiritunit, but this introduces the risk of traces of theentrainer in the hydrous ethanol which may notall be sent to the dehydration system. This systemoperates with a steam consumption of 1-1.5 kg/liter (8.3-12.5 lb/gallon) of anhydrous ethanoldepending on the quality of product required.As indicated above, a major part of theequivalent steam energy can be provided by hotcondensate and hot waste streams from the spiritunit.

References

Bravo, J.L. et al. 1985. Hydrocarbon Proc. Jan.p.91.

Katzen, R. 1955. Chem. Eng. Nov. p. 209.Fair, J.R. 1977. Chem. Eng. Prog. Nov. p. 78.Fair, J.R. et al. 1990. Chem. Eng. Prog. Jan. p.

19.Katzen, R. 1980. Low energy distillation

systems. Bio-Energy Conference, Atlanta, GA,April.

Lynn, S. et al. 1986. Ind. & Eng. Chem. 25:936.Martin, R. L. et al. 1970. Hydrocarbon Proc. March

1970, p. 149.Mix, T.J. et al. 1978. Chem. Eng. Prog. April p.

49.Petterson, W.C. et al. 1977. Chem. Eng. Sept. p.

79.Raphael Katzen Associates. 1978. Grain Motor

Fuel Alcohol Technical and EconomicAssessment Study. Prepared for: USDepartment of Energy HCG/J6639-01.

Robinson and Gilliland. 1950. Elements ofFractional Distillation, McGraw Hill Book Co.,Inc.