Unit 5 - Continuous Distillation - School of Life Sciences

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Unit 5 - Continuous Distillation - School of Life Sciences

MODULE: DISTILLINGUNIT: 5 CONTINUOUS DISTILLATIONABSTRACT: This unit is devoted to continuous distillation startingwith the Coffey Still and moving on to column stills for the productionof grain and neutral spirit. The theory of the continuous still isdescribed.LEARNING OUTCOMES: On completion of this unit you will be ableto:1. Describe the operation of the Coffey Still for the production ofgrain whisky spirit.2. Understand the basic theory of continuous distillation and theproblems associated with tray design.3. Understand the McCabe - Thiele approach to determining thenumber of trays in a column by means of operating lines.4. Appreciate the importance of reflux.5. Describe the column systems used in the production of neutralspirit for gin and vodka.6. Describe techniques which can reduce energy usage in the stillhouse.PREREQUISITE KNOWLEDGE: Basic scientific knowledge andterminology. Malting and Brewing Science 2, Section A, Unit 7 - TheProduction of Scotch Whisky.


Unit 5: Continuous distillationUNIT 5 - CONTINOUS DISTILLATIONCONTENT5.15.25.2.15.2.25.35.45.4.15.4.25.4.35.4.45.55.65.75.7.15.7.25.7.35.85.95.105.11IntroductionThe Development of Continuous DistillationBlended WhiskyThe Invention of the Continuous StillDescription of the Coffey StillTheory of Continuous DistillationA Column Distillation SystemOperating LinesReflux RatioEthanol:Water DistillationColumn DesignColumn Stills for Grain WhiskyNeutral Spirit ProductionSpecificationsExtractive DistillationDemethylisationEnergy ReductionSummary of Key PointsSelf Assessment QuestionsSelf Assessment AnswersPage34445101015202122232424252728303132© 2001 The International Centre for Brewing and Distilling 2


Unit 5: Continuous distillation5. CONTINUOUS DISTILLATION5.1 IntroductionThe previous unit dealt with batch distillation in pot stills. The scale ofoperations with pot stills is such that annual malt distillery outputs arein the range of 10mla (Glenfiddich) to 0.09 mla (Edradour) with about3 mla being an averaged sized distillery’s output.Continuous distillation lends itself to much larger scale operationswith outputs as high as 60 mla per annum, and the technique is usedin Scotland to manufacture grain whisky.The levels of annual production of malt and grain whisky in Scotlandare of the order of 180 mla and 280 mla respectively. You will not besurprised to learn that only a handful of grain distilleries are required.In 2001 the number of working malt distilleries was 90 and thenumber of working grain distilleries was 8 (one of which is muchsmaller than the other seven).Apart from the scale of operations and batch versus continuousprocessing, the fundamental differences between malt and grainspirit are:??Use of cooked unmalted cereals in the grist for grain spirit??In some grain distilleries mash solids go forward into the still??Grain whisky spirit is produced close to 94.8% abv, whereas maltspirit is produced at about 69% abv??Grain spirit contains much lower levels of congeners than maltspiritHowever continuous distillation is not limited to grain whiskyproduction. It is used for neutral spirit which is subjected to furthertreatment to produce gin and vodka. Neutral spirit, as the namesuggests, has no flavour or odour and, as its production is notcontrolled by the legal definition of Scotch Whisky, it can be distilledat a higher strength than grain spirit and commercial enzymes can beused rather than malt.Continuous distillation is also employed for the production ofAmerican and Canadian whiskies and light rums.© 2001 The International Centre for Brewing and Distilling 3


Unit 5: Continuous distillation5.2 The Development of Continuous Distillation.5.2.1 Blended WhiskyThe development of continuous distillation in the 1820 – 1830’splayed a vital role in the growth of the Scotch whisky industry. For itwas shortly after the introduction of the continuous still, that theconcept of making blends of malt and grain whisky was developed.Blending created whiskies which were blander than the malts of thattime and consequently they were more appealing to palates firstly inEngland and then around the world. The Scotch whisky industry isfounded on the marketing of blends and even today, although maltwhiskies command much interest and many column inches, sales ofsingle malts account for less than 10% of worldwide sales of Scotch.Grain whisky produced by continuous distillation is therefore a vitalpart of the scene, and without it Scotch whisky production wouldprobably have remained a cottage industry.5.2.2 The Invention of the Continuous Still.In 1827, mainly in response to the demand for a more economic andneutral flavoured spirit, which could supply the buoyant gin market inLondon, Robert Stein devised and patented the first continuous still.Stein was the owner of Kilbagie Distillery and the still was put intouse there. He later built a second still for Cameronbridge Distillery,which was owned by his cousin John Haig, a member of the famouswhisky family. But the Stein still was rather complicated and it wassuperseded in 1830 by a much simpler design. This was the work ofAeneas Coffey, a Dublin exciseman turned distillery owner.The Coffey Still, which is some times referred to as the Patent Still,consists of two tower sections, an analyser and a rectifier, which areboth built up a series of compartments separated by trays, or plates.The principle of its operation is the separation of the components of aliquid mixture by the countercurrent flow of liquid and vapour.Early Coffey stills were constructed in iron but this provedunsatisfactory as it delivered poor quality spirit. Copper was thenused for the trays and pipes, which were fitted into a wooden framewith considerable improvement, and eventually the whole unit wasmade in copper. Coffey stills remain in use in the whisky industry andthe design has remained fundamentally unchanged during the last170 years.The Coffey still opened the door to increased output of spirit at areduced cost. Many of the Lowland distillers, who supplied spirit to© 2001 The International Centre for Brewing and Distilling 4


Unit 5: Continuous distillationthe London gin market, switched from distilling a wash produced froma mix of malt and unmalted barley in pot stills and installed newCoffey Stills. When blended whisky appeared on the scene, followingits invention by Andrew Usher in 1860, it boosted the demand forgrain whisky, and segregated the industry into three streams: pot stilldistillers, Coffey still distillers, and blenders.The rising importance and success of the grain whisky producers andthe blenders led to polarisation within the industry and the maltdistillers felt threatened. They argued that grain whisky and blends,some of which at that time contained only 10% malt whisky, shouldnot be allowed to use the appellation “whisky”.To resolve this a Royal Commission was established in 1908 toexamine definitions of spirits. The Commissions findings declaredthat both malt and grain spirit should be termed “whisky” and blendswere entitled to that description too.The number of grain distilleries has declined in recent times withlarger and better utilised plants. Unlike malt distilleries location wasnever an issue in determining the character, or the image, of theproduct so grain distilleries were, and still are, concentrated in theindustrial central belt of Scotland.KEY POINT: The Coffey Still, invented in 1830, is in use today ina form which has remained fundamentally unchanged during 170years.5.3 Description of the Coffey StillThe Coffey still is constructed of two columns, which can be as highas 60 feet, located side by side. The first one strips the alcohol fromthe wash feed and is known as the analyser, while the secondcolumn is known as the rectifier. A diagram of a Coffey still is shownin Figure 1.© 2001 The International Centre for Brewing and Distilling 5


Unit 5: Continuous distillationFigure 1. Coffey StillThe feeds into the still are wash entering the top of the rectifier andsteam entering the base of the analyser. The stills are divided into anumber of rectangular sections, separated by perforated trays, orplates,and in each of them there is countercurrent flow.The wash, at about 34°C, enters the top of the rectifier in a coppertube, which winds its way between the trays and moves from onesection to another via bends, which protrude from the still. In itsdescent the wash temperature rises as it contacts hot vapour fromthe analyser and by the time it reaches the bottom its temperature isabout 94°C. It is then directed to the top of the analyser and flows outof the pipe and cascades down the trays. It passes from tray to traythrough a series of downpipes,which create a flow across the trays,and at each tray it meets vapour forcing itself upwards through theperforations.The wash is stripped of its alcohol gradually as it descends andconversely the vapour becomes progressively richer in alcohol as it© 2001 The International Centre for Brewing and Distilling 6


Unit 5: Continuous distillationrises. Spent wash emerging from the base of the analyser willcontain no more than 0.05% abv.Returning now to the rectifier, the alcohol rich vapour (30 - 40% abv)entering at the bottom, rises through the trays heating thedescending wash pipe. The vapour is progressively condensed andliquids of different composition collect on the rectifier trays.Some of the vapours which escape from the top of the rectifier arecondensed and either returned as reflux or are mixed with the hotliquor (feints) emerging from the bottom and recycled to the analyser.To understand the process better we need to look in more detail atthe trays, which are shown in Figure 2.A. Analyser TrayB. Rectifier trayFigure 2. Analyser and Rectifier Trays© 2001 The International Centre for Brewing and Distilling 7


Unit 5: Continuous distillationThe analyser trays are linked by downcomer pipes, arranged so thatthere is a weir effect determining the liquid level on the tray. Thevapour pressure is such that the vapour bubbles into the liquidthrough the perforations, while preventing the liquid from fallingthrough them. When the vapour enters the liquid some of the lessvolatile components condense, thus the vapour which passesthrough becomes richer in the more volatile components.Simultaneously the latent heat of condensation causes the morevolatile components in the liquid to vaporise, thus stripping the liquidof alcohol as it descends from tray to tray.The rectifier tray has a wash coil snaking its way across and thendown the column. Again there is a rising vapour stream and adescending liquid one, created by condensation of vapour on theoutside of the wash coil and supplemented by the refluxing of someof the condensed vapour leaving the rectifier. The temperaturegradient causes the less volatile components in the vapour tocondense in the lower trays and the more volatile ones in the highertrays, which are at lower temperatures.KEY POINT: In both Coffey still columns there arecountercurrent flows. In the analyser hot wash descends andsteam rises; the result at the top of the column is a vapourenriched in alcohol. This vapour stream then rises in therectifier, progressively condensing on the outside of thedescending wash coil, such that liquids of differing compositioncollect on the rectifier trays.The beauty of the Coffey still is that the temperature gradient in thewash coil creates different liquid profiles at each tray. A typicalconcentration of compounds in a rectifier is shown in Figure 3. Theprofile is a reflection of the equilibrium temperature reached on eachtray. This is controlled by measuring the temperatures at the bendsand adjusting flow rates.© 2001 The International Centre for Brewing and Distilling 8


Unit 5: Continuous distillationFigure 3. Typical Concentrations of Compounds in the RectifierIn Figure 3 collection of spirit at about 94% abv is taken at plate 32. Astream rich in iso amyl alcohol (fusel oil) is collected from plate 6,which is redistilled separately in a packed column to separate the isoamyl alcohol from ethanol. Iso amyl alcohol is sold as a byproductand is used mainly in the perfume industry.Congeners such as propanol, butanol and iso amyl alcohol are lessvolatile than ethanol at high ethanol concentrations but more volatileat low ethanol concentrations. Each congener will concentrate in thesection where the ethanol concentration is such that the relativevolatilities are equal.The flow rate for the removal of fusel oil is one which balances theinput of fusel oil to the column. It can be seen from Figure 3 that thisdraw off will remove some butanol but no propanol. The amount ofbutanol in the spirit can be altered by adjusting the flow rate from thefusel oil collection tray; this has the effect of moving the butanol peakup or down the column (it also alters the level of iso amyl alcohol inthe spirit).© 2001 The International Centre for Brewing and Distilling 9


Unit 5: Continuous distillationKEY POINT: In the Coffey Still rectifier the alcohol rich streamat about 94% abv is drawn off about six trays from the top. Afusel oil stream is removed about six trays from the bottom ofthe column.The highly volatile compounds, which concentrate on the highertrays, are collected as vapour. These include ethanol, which iscondensed and fed back to the still, and some sulphur compounds,which are vented to the atmosphere.The spent wash emerging from the analyser contains a very low levelof ethanol (less than 0.05%) but it also contains insoluble solidsincluding grain fragments and yeast together with soluble but nonvolatilesubstances. The more important of these latter substancesare oligosaccharides, unfermentable dextrins and glycerol, which isproduced during fermentation. The spent wash is subjected toseparation processes and sold as animal feed.5.4 Theory of Continuous Distillation.5.4.1 A Column Distillation SystemA modern approach to continuous distillation uses cylindrical columnstills manufactured in stainless steel.Figure 4 shows a continuous distillation system, which can best bedescribed in terms of a two component feed – an ethanol and watermixture.© 2001 The International Centre for Brewing and Distilling 10


Unit 5: Continuous distillationVapourCondenserCooling waterRectifying sectionFeedReflux liquidTraysOverhead productStripping sectionSteamVapourBoiling liquidWasteproductFigure 4. A Continuous Distillation System.The feed enters the tower at a middle level. The energy to drive theprocess is steam, which enters at the base of the tower. An overheadwater cooled condenser condenses the vapour; some of this isreturned as reflux and the remainder goes forward as the ethanol richproduct stream. The waste stream, which is water rich, leaves fromthe base of the column.The highest temperature is at the base and there is a temperaturegradient with the temperature falling as the tower is ascended. Thetower is fitted with a number of perforated internal trays linked bydowncomer pipes, the purpose of which is to allow intimate contactbetween the rising vapour and the falling liquid. The section of thetower above the feed is known as the rectifying section and thesection below it as the stripping section.© 2001 The International Centre for Brewing and Distilling 11


Unit 5: Continuous distillationIt can be seen that this system closely resembles the Coffey still.Although Figure 4 shows only one column, in practice this is usuallysplit into two to reduce the height. The only significant difference isthat in the Coffey Still there is a wash coil causing condensation inthe rectifier, whereas in the column all the descending liquid in therectifying section is provided by reflux.KEY POINT: The significant difference between a Coffey Stilland a column still system is that in the latter all the descendingliquid in the rectifying section is provide by reflux.Figure 5 shows the vapour/liquid equilibrium diagram for anethanol/water mixture at atmospheric pressure. Note that molepercentages are used, as in chemical engineering this is thepreferred way of studying vapour / liquid systems.Azeotrope 97.2% abvEquilibrium curveMole % ethanol in vapour(10% abv)Mole % ethanol in liquidFigure 5. Vapour / Liquid Equilibrium Diagram.© 2001 The International Centre for Brewing and Distilling 12


Unit 5: Continuous distillationThe diagonal straight line in the diagram represents points where theconcentration in the vapour equals the concentration in the liquid;conditions under which distillation could obtain no separation of thecomponents. The equilibrium curve for ethanol:water meets thediagonal at 89 mole percent of ethanol. This is called the azeotrope(or constant boiling mixture) and indicates the maximum separationwhich can be achieved by distillation.Returning to the other end of the curve, let us consider what happenswhen a mixture containing 3.3 mole % ethanol (10% abv) is boiled ina pot. A vapour will be produced with 24 mole % ethanol (53% abv).If this is condensed and reboiled in a second pot, the vapour will be55 mole % ethanol (83%abv) and if this process is continued one willmove up the equilibrium curve producing more concentrated ethanolsolutions until the azeotrope is reached.This series of steps is oversimplified because as vapour is drawnfrom the pot so the circumstances change. However if the pot had acontinuous feed of the ethanol:water mixture and the vapourgenerated in the first pot was continually condensed, and thensupplied to the second pot, then the process would closely approachwhat happens in a distillation tower, with the trays replacing the pots.Now we need to examine the tray in more detail. Figure 6 shows across section of the perforated tray.Figure 6. Sieve Tray.The operation of such a tray is a complex balance. Low vapour flowrates result in pulsating liquid flow or dumping of liquid onto the tray© 2001 The International Centre for Brewing and Distilling 13


Unit 5: Continuous distillationbelow. At high vapour flow rates the bubbles carry liquid as spray, ordroplets, to the plate above (this in called entrainment). At low liquidflow rates vapour forces the liquid back from the slots and passesthrough them as a continuous stream. High liquid flow rates causethe trays to flood.Each type of tray has a region of satisfactory operation defined interms of vapour and liquid flows.Sieve trays are cheap to manufacture but carry the disadvantagesmentioned. One solution to this is the bubble cap tray shown inFigure 7.A. Bubble cap B. ValveC. Disc-and-DonutFigure 7. Alternative Tray Designs.Bubble caps (A) have risers which ensure that a level of liquid ismaintained on the tray at all flow rates. The vapour passes up therisers and escapes, either through slots in the cap or by flow over the© 2001 The International Centre for Brewing and Distilling 14


Unit 5: Continuous distillationserrated edges of the cap. Clearly this is a much more expensive trayto manufacture.Floating valves (B) offer another alternative with the valve movementbeing controlled by the vapour flow. If the flow drops then the valvefalls preventing liquid weepage. Valve trays fall between sieves andbubble caps in cost.However neither bubble caps or valve trays are suitable for a feedcontaining solids, because there is too much danger of blockage.Sieves can be used in these conditions and another option is the discand donut tray (C). Here the liquid is made to cascade from theoutside to the centre, and back to the outside of the column and thevapour has to force its way through the curtains of liquid.KEY POINT: The main concerns in tray design are theproblems caused by solids and the need to avoid the dumpingand entrainment of liquid.The condensation and evaporation which occurs on the tray is similarto that already described in the Coffey still section.5.4.2 Operating Lines.In order to calculate the number of trays required in a column it isnecessary to carry out mass balances at the top and bottom of thecolumn.Consider the situation shown in Figure 8, and note that in this Figurethe trays are numbered from the top to the bottom of the column.© 2001 The International Centre for Brewing and Distilling 15


Unit 5: Continuous distillationV n L n-1y n x n-1V n+1y n+1L nx nFigure 8. Column Tray Mass BalancesThe feed F, an ethanol:water mixture, produces distillate D and awaste stream B. The mole fractions of ethanol in these liquidstreams are x F ,x D and x B respectively.Vapour leaving a typical plate n has a flow rate V n and compositiony n , and the corresponding liquid descending to it has a flow rate ofL n-1 and composition x n-1 .At plate n the vapour arriving has a flow rate V n+1 and compositiony n+1 . Similarly, the liquid descending has a flow rate L n andcomposition x n .The mass balance at the top of the column with reference to ethanolis:© 2001 The International Centre for Brewing and Distilling 16


Unit 5: Continuous distillationThis is the equation of the top operating line giving the desiredrelationship between the liquid and vapour compositions. Asimplification is then applied, which is that conditions of constantmolar flow apply i.e. the liquid and vapour flow rates are constant ineach section of the column.With this assumption:Using the overall mass balance, V = L + D, and writing r =L/D theequation of the top operating line becomes:Where r is the ratio of liquid returned to the column as reflux to theproduct flow rate; this is referred to as the reflux ratio.Examining the lower section of the column, the mass balance is:Applying the same assumptions about constant molar flow, using theoverall mass balance V = L - B and substituting s for V/B, theequation of the bottom operating line can be written:It should be noted that because of the introduction of the feedbetween the stripping and rectifying sections the flow rates in the twosections are not equal.© 2001 The International Centre for Brewing and Distilling 17


Unit 5: Continuous distillationIn 1925 McCabe and Thiele, two graduate students at MIT, proposedthat these equations represent straight lines which can be drawn onthe equilibrium diagram. Consider the simple equilibrium diagram inFigure 9.Figure 9. Operating Line for the Rectifying Section.Here we can draw the operating line for the rectifying section startingfrom the point on the diagonal which represents the composition ofthe product stream, or liquid distillate.The composition of the liquid reflux, x D and the reflux ratio, r areknown.Substituting in the mass balance equation for the top of the columnwe get:The operating line for the rectifying section can now be drawnstarting at the point y 1 ,x D with a slope of r / (r+1).© 2001 The International Centre for Brewing and Distilling 18


Unit 5: Continuous distillationLiquid and vapour phases are in equilibrium on each tray so x 1 is theliquid composition on tray 1, corresponding to y 1 the vapourcomposition. Each tray is represented by a step between theoperating and equilibrium lines, which is shown in Figure 10.Mole fraction in liquidComposition of liquid distillateFigure 10. Analysis of the Rectifying SectionA similar approach can be adopted for the bottom of the column,giving an operating line for the stripping section, which can be"stepped off" in the same way. This is shown in Figure 11.Mole fraction in vapourMole fraction in vapourComposition of liquidbottomsMole fraction in liquidFigure 11. Analysis of the Stripping Section.© 2001 The International Centre for Brewing and Distilling 19


Unit 5: Continuous distillationWhen the rectifying and stripping lines are put together theintersection gives the point at which the feed is introduced, Figure 12.Mole fraction in vapourComposition of liquidbottomsMole fraction in liquidFigure 12. A Multistage Distillation ProcessIn this example there are three trays below the feed and two above it.5.4.3 Reflux RatioAs the reflux ratio is increased the slope of the operating line for therectifying section moves towards unity and the number of theoreticaltrays is reduced. This is shown in Figure 13.yGradient =r minr min + 1XMinimumXMaximumFigure 13. Minimum and Maximum Tray Numbers© 2001 The International Centre for Brewing and Distilling 20


Unit 5: Continuous distillationThe opposite effect is observed when the reflux ratio is decreased,for the number of theoretical trays increases. The maximum isreached when the intersection point of the operating lines iscoincident with the equilibrium curve.KEY POINT: The number of trays in a column is determined byobtaining the operating lines for the rectifying and strippingsections and then "stepping off" the areas between theselines and the equilibrium curve.The number of trays has a significant impact on the capital cost of acolumn and in practice values of r are employed which are 1.1 – 1.5times the minimum reflux ratio.5.4.4 Ethanol : Water DistillationA more detailed example of the McCabe – Thiele approach to anethanol : water mixture is shown in Figure 14.Typical: 14 rectifying stages8 stripping stagesEquilibrium curveMole % ethanol in vapourOperatingline strippingOperating linerectification95% abvBeer feed(10% abv)Mole % ethanol in liquidFigure 14. Ethanol : Water Distillation© 2001 The International Centre for Brewing and Distilling 21


Unit 5: Continuous distillationThe operating lines are shown and although the detailing of the stepsat the top and bottom of the equilibrium curve are too close togetherto be seen, the diagram indicates 14 rectifying steps and 8 strippingones.5.5 Column DesignNow that a number of column features have been discussed, theapproach to column design can be considered.The essential stages in column design are listed below.??Identify the compositions of the feed, the required compositions ofthe product and waste streams and the rate of product production??From experience choose a reflux ratio for the separation??Define the operating conditions of pressure and steam andcooling requirements??Determine the operating lines for each section??Adjust the operating lines for different feed temperatures andvapour loads and find the most efficient arrangement??Find the theoretical number of trays by “stepping up” theoperating lines??Decide on the type of tray (choice will be limited if solids arepresent in the feed) and its efficiency rating. This latter figure willindicate the number of trays needed over and above thetheoretical, reflecting the fact that in practice equilibrium is notreached on the trays??Calculate the diameter for the top of the column using the vapourflow rate arrived at earlier??Decide on the tray spacing, which will decide the height of thecolumn. This is deduced by calculating the maximum vapour flowwhich can be achieved without flooding the trays??Calculate the diameter for the stripping section of the column. Ifthe column is to be in one piece then the larger of the twodiameters will be used.It is clear from the above process that experience has to be fed intotheory when designing a column, and, of course, in the case ofseparation of ethanol and water mixtures, a great deal of experienceis available from existing columns. Most designs are likely to err onthe side of safety in terms of output and performance, which is just aswell as columns have a long life and conditions and requirementscan change over time.KEY POINT: Column design is a blend of theoretical calculation andexperience gained from working columns in similar applications.© 2001 The International Centre for Brewing and Distilling 22


Unit 5: Continuous distillation5.6 Column Stills for Grain WhiskyThe application of column separation can be considered as a modernalternative to the Coffey still.An example of this is shown in Figure 16, which describes a twocolumn system.24BEER STRIPPERCONCENTRATORCONDENSER403585FUSEL OILDECANTER CONDENSERSURGETANKFigure 16. Column Distillation of Grain Whisky© 2001 The International Centre for Brewing and Distilling 23


Unit 5: Continuous distillationHere we see a wash, or beer, feed 24 trays above the steam entrypoint at the bottom of the stripping column, or concentrator. Therectifier has 40 trays with the product stream drawn off at tray 35 andfusel oil removed at tray 5. Propanol could be drawn off at tray 8 ifnecessary. The condenser system at the top of the rectifier providesreflux, allows some vapour to be vented and separates heads, whichare recycled to the stripping column. Feints are returned to thestripping section from the base of the rectifier.If the columns are built in stainless steel, then it is necessary toprovide some sacrificial copper in order to bring about thosereactions which remove unwanted compounds. This is done bypassing the feints stream through a tank packed with small coppercuttings.5.7 Neutral Spirit Production5.7.1 SpecificationsNeutral spirit is the spirit used in the production of gin, vodka andsome other drinks. It is odourless, tasteless, and is continuouslydistilled in column stills all over the world. Escaping the legaldefinitions attached to Scotch Whisky, it is most economically madeusing commercial enzymes rather than malt. The starch source isusually of cereal or sugar origin, but even whey and potatoes areused.There are legal definitions for neutral spirit though they are generallyset at easily attainable standards. Producers or purchasers of neutralspirit for high quality gin or vodka will have much tighterspecifications than those demanded by regulations.The main parameters of the European Union definition are comparedwith a typical product specification in Table 1.© 2001 The International Centre for Brewing and Distilling 24


Unit 5: Continuous distillationParameterg/100 laEuropeanUniondefinitionNeutral spiritspecificationfor gin andvodkaStrength %abv >96 >96Acids (as acetic acid)


Unit 5: Continuous distillationIn general applications of extractive distillation a solvent is added to adistillation column to change the relative volatility of key componentsin the feed mixture.The technique of extractive distillation as applied to alcohol is alsoknown as hydroselection. It utilises the fact that some of the higherboiling point congeners, which are normally less volatile than ethanolbecome more volatile in the presence of water.KEY POINT: Extractive distillation is employed to removecongeners with boiling points above that of ethanol. It utilisesthe fact that some compounds become more volatile in thepresence of water.The extractive distillation column is fitted with trays just like thecolumns already discussed. The alcohol feed, at say 94% abv, entersthe column at about three-quarters height, with steam entering thebase, and water entering the top of the column. Most of thecongeners rise up the column and are condensed as heads. Thedilute alcohol stream (say 15% abv) emerges from the base of thecolumn.By adjusting the feed rate and the water and steam flows most of thecongeners with boiling points close to ethanol can be made to rise upthe column. The optimum conditions will vary for each congener andsome degree of compromise is needed. For example less water andmore steam is requires to remove iso amyl alcohol than propanol.After extractive distillation, it is then necessary to carry out strippingand rectification to achieve the high strength of neutral spirit required.This is done in the conventional manner with the feed containingcongeners such as the low boiling point compounds not removed inextractive distillation, methanol, diacetyl and 2,3 pentanedione, andrelatively low concentrations of iso amylalcohol, isobutanol andpropanol.This column delivers a water waste stream, which can be recycled tothe extractive distillation column, a high strength product drawn offnear the top of the rectifying section and fusel oil streams from itslower trays. The heads are condensed, with venting of the morevolatile compounds. A proportion of the condensed liquid is used asreflux.© 2001 The International Centre for Brewing and Distilling 26


Unit 5: Continuous distillationThe collected fusel oil is distilled in a column packed with stainlesssteel Pall rings to provide a large surface area. Ethanol can becollected at the top of the column and an iso amyl alcohol / waterstream from the base. This latter mixture is condensed and allowedto separate in a storage tank. The iso amyl alcohol can be decantedfrom the top of the tank.5.7.3 DemethylisationWith some feedstocks the methanol content may still be too high forthe specifications set for some vodkas. We now have a different setof circumstances, namely the need to separate ethanol from themore volatile methanol.This can be achieved in a standard column but in this case thebottom stream will be the high strength ethanol and the top streamwill be methanol.This system in conjunction with extractive distillation and rectificationis shown in Figure 17.CONDENSER CONDENSER CONDENSERCONDENSERCONDENSERSURGETANKEXTRACTIVE DISTILLATION COLUMNRECTIFIERSTRIPPINGSECTIONREBOILERDEMETHYLYSERFigure 17. Extractive Distillation and Demethylisation© 2001 The International Centre for Brewing and Distilling 27


Unit 5: Continuous distillationKEY POINT: To obtain a neutral spirit of high quality it isnecessary to remove methanol. This is done in a conventionalcolumn but in this case the higher boiling point of ethanolmeans that it emerges from the base of the column and vapourrich in methanol emerges from the top.5.8 Energy ReductionHeat recovery, or energy conservation, has become of increasedimportance in the last few decades. Some techniques which arecommonly used are described.The bottoms stream from a stripping section will be in the region of100°C and a thermocompressor can be used to extract heat from it.A typical thermocompressor is shown in Figure 18.Figure 18. A Typical Thermocompressor© 2001 The International Centre for Brewing and Distilling 28


Unit 5: Continuous distillationHigh pressure steam from the boiler house passes through a needlevalve and, as a result, there is a drop in pressure. This creates apartial vacuum in the flash vessel. The bottoms stream enters theflash vessel and boils at a lower temperature because of the reducedpressure. The steam generated by this is drawn from the flash vesselto augment the steam flow and this is directed into the base of thestripping section.If the bottoms stream is spent wash with grain solids in it there is anadded bonus as the temperature of the spent wash is reduced.The second technique employs a reboiler and is shown in Figure 19.Vapour fromanother columnFigure 19. A ReboilerThe vapour stream from the top of a rectifier is at a high temperatureand can be used to provide some of the heat needed for anothercolumn in the system. The vapour is condensed in the shell of areboiler, which provides the heat source for the other column. Forthis to work there must be a difference of about 10°C between thevapour dew point and the boiling point of the liquid in the secondcolumn.This technique is most often applied in a full neutral spirit still systemby altering the operating pressures of the stills. This affects theboiling points so that the vapours of one column can be used to heatanother. Alteration of pressure needs to be built into the design of thecolumns as they will be smaller at high pressure and larger undervacuum.© 2001 The International Centre for Brewing and Distilling 29


Unit 5: Continuous distillation5.9 SUMMARY OF KEY POINTS1. The Coffey Still, invented in 1830, is in use today in a form whichhas remained fundamentally unchanged during 170 years.2. In both Coffey Still columns there are countercurrent flows. In theanalyser hot wash descends and steam rises; the result at the topof the column is a vapour enriched in alcohol. This vapour streamthen rises in the rectifier, progressively condensing on the outsideof the descending wash coil, such that liquids of varyingcomposition collect on the rectifier trays.3. In the Coffey Still rectifier the alcohol rich stream, at about 94%abv, is drawn off about six trays from the top. A fusel oil stream isdrawn off about six trays from the bottom of the column.4. The significant difference between a Coffey Still and a column stillsystem is that in the latter all the descending liquid in therectifying section is provide by reflux.5. The main concerns in tray design are the problems caused bysolids and the need to avoid the dumping and entrainment ofliquid.6. The number of trays in a column is determined by obtaining theoperating lines for the rectifying and stripping sections and then"stepping off" the areas between these lines and the equilibriumcurve.7. Column design is a blend of theoretical calculation andexperience gained from working columns in similar applications.8. Extractive distillation is employed to remove congeners withboiling points above that of ethanol. It utilises the fact that somecompounds become more volatile in the presence of water.9. To obtain a neutral spirit of high quality it is necessary to removemethanol. This is done in a conventional column but in this casethe higher boiling point of ethanol means that it emerges from thebase of the column and vapour rich in methanol emerges from thetop.© 2001 The International Centre for Brewing and Distilling 30


Unit 5: Continuous distillation5.10 SELF ASSESSMENT QUESTIONS1. What are the key dates that feature in the invention and rapiddevelopment of the Coffey Still?2. What happens in the rectifier of a Coffey Still?3. What is a downcomer?4. What is fusel oil and what happens to it?5. What is the maximum strength of grain spirit that can beproduced and what alcohol strength might you expect in spentwash?6. Why are some vapours vented from rectifier columns?7. What are the main differences between Coffey Stills and columnstill systems?8. What is an azeotrope?9. What are the advantages and disadvantages of a bubble cap traycompared with a sieve tray?10. What are the key steps which need to be taken to calculate thenumber of trays in a column?11. What differences in the production process would you findbetween grain whisky and neutral spirit manufacture?12. How does a demethylisation column work?13. Name two techniques for energy reduction in a neutral stillhouse?© 2001 The International Centre for Brewing and Distilling 31


Unit 5: Continuous distillation5.11 SELF ASSESSMENT ANSWERSThat was quiet a long section, so there are more questions thanusual. This is a particularly important area so if you haveuncertainties take your time and re-read any pages which you feelyou may not have understood or remembered.1. What are the key dates that feature in the invention and rapiddevelopment of the Coffey Still?The first patent for a continuous still was that of Robert Stein in1827. Aeneas Coffey's patent was dated 1830.Another key date in the development of the Coffey Still was theinvention of blended whisky by Andrew Usher in 1860.2. What happens in the rectifier of a Coffey Still?There is a lot going on in the rectifier;??The wash coil enters at the top containing wash at about 34ºCand as the wash descends it is heated to about 94ºC.??Alcohol rich vapour from the analyser rises up the rectifiercondensing on the outside of the wash coil.??Each tray ends up with liquid of a different composition.??Spirit is drawn off near the top of the column and fusel oil fromnear the bottom.??Hot feints are withdrawn from the bottom of the rectifier and recycledto the analyser.??Vapours leaving the top of the rectifier are condensed; some isreturned as reflux and some is re-cycled to the analyser.3. What is a downcomer?A downcomer is a pipe which allows liquid to flow from one tray toanother. At the lower end is a weir system which prevents vapourrising through the downcomer. The upper end protrudes above thetray to retain a set depth of liquid on the tray.4. What is fusel oil and what happens to it?Fusel oil is a mix of higher alcohols, of which the main constituent isiso amyl alcohol. Ethanol is removed as a vapour from the mixture ina packed column. The water:iso amyl alcohol solution removed fromthe bottom of the column is allowed to separate into two layers in a© 2001 The International Centre for Brewing and Distilling 32


Unit 5: Continuous distillationholding tank. The top layer of iso amyl alcohol is decanted and soldas a byproduct to the perfume industry.5. What is the maximum strength of grain spirit that can be producedand what alcohol strength might you expect in spent wash?Maximum strength 94.8% abv.Alcohol in spent wash will be about 0.05% abv.6. Why are some vapours vented from rectifier columns?Because they contain sulphurous compounds.7. What are the main differences between Coffey Stills and columnstill systems?The Coffey Still rectifier contains a wash coil and condensation ofvapour on it provides most of the liquid in that section. In the rectifierof a column still the liquid all comes from reflux.A column still will be cylindrical, whereas a Coffey Still is usuallyrectangular and a column still will usually be made of stainless steelnot copper.8. What is an azeotrope?An azeotrope is a constant boiling mixture. The composition of theliquid and vapour are the same, so separation by distillation isimpossible.9. What are the advantages and disadvantages of a bubble cap traycompared with a sieve tray?The advantage is that with a bubble cap liquid cannot weep or dumponto the tray below. The disadvantages are that the bubble cap ismore expensive and cannot be used if solids are involved.10. What are the key steps which need to be taken to calculate thenumber of trays in a column?Determine the operating lines for the rectifying and stripping sectionsand then "step up" these to find the number of trays above and belowthe feed point.© 2001 The International Centre for Brewing and Distilling 33


Unit 5: Continuous distillation11. What differences in the production process would you findbetween grain whisky and neutral spirit manufacture?Neutral spirit has low congener levels and a higher alcohol strength.To achieve this an extractive distillation must be carried out, followedby rectification. If low methanol levels are required in the neutralspirit, as they tend to be for vodka, then a demethylisation columndistillation is also needed.12. How does a demethylisation column work?The feed is ethanol with traces of methanol. In the distillation columnmethanol is removed from the top as a vapour and ethanol from thebottom as a liquid.13. Name two techniques for energy reduction in a neutral stillhouse?Thermocompression.Reboiling using vapour from one column as a heat source foranother.© 2001 The International Centre for Brewing and Distilling 34

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