Distillation Technology and Modelling Techniques Part 3: Walkthrough and Analysis of a Continuous Brandy Distillery

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Distillation technology and modelling techniques Part 3: Walkthrough and analysis of a continuous brandy distillery

In this third and final part of the series, Konrad Miller presents an overview of a continuous distillery, with analysis of fermentation, distillation, and congener removal. As discussed in a prior article, distillation is a science with many applications; petroleum, commodity chemicals, and biofuels among them. However, the genesis of distillation was in the production of alcoholic beverages.

p until the late 19th century, both (0.6 mass fraction ethanol) off the Figure 1: A screw conveyer for handling grapes Uspirits and petroleum were pro- still. Like all distilled spirits, brandy duced via batch distillation, with multi- is colourless when produced; it gains are French Columbard (sweet, tropi- ple distillations taking place to achieve colour from oak maturation (tannin, cal fruit), Muscat (floral, ‘fruit loops’), higher purity as needed. As distillation colour, and flavour extraction). All and Barbera (red fruit). This fruit is technology has evolved, so has spirits brandy begins its life as grapes. From crushed to extract the juice, treated to production. This article walks through September to November, Califor- remove solids, and then fermented to the production process of a modern nia grapes are harvested for wine produce wine. brandy distillery, with concepts directly production. Just as there are different applicable to whiskey, rum, and vodka classes of crude oil and coal, grapes Ethanol modelling production. are divided into different varietals, Ethanol fermentations by Saccharomy- Brandy is wine distillate, typically each with their own characteristics. ces cerevisiae are well modelled by nu- produced at approximately 150 proof Key California brandy grape varietals trient (sugar) limited, product (ethanol)

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inhibited Monod kinetics. Equation 1 of ‘auto-pasteurisation’ and arrest fer- description of the column operation. delineates the governing fermentation mentation. Smaller fermentation tanks Before being fed to the column, equations, where ‘C’ is concentra- (<200,000 litres) are typically jacketed incoming feed is run through a heat- tion, subscript ‘c’ is cells, subscript with glycol to control the temperature. exchanger with the outgoing bottoms ‘s’ is sugar, subscript ‘p’ is product Larger tanks (>400,000 litres) lack to recover waste heat. This process (ethanol), ‘k’ refers to a kinetic rate pa- sufficient surface area for jacketing decreases the total steam demand on rameter, ‘µ’ is the specific growth rate, to be effective, so external shell-and- the column and allows the hot bottoms, ‘C*p’ is the maximum ethanol level tube heat exchangers are employed to which are rich in organic material, to yeast can grow in, and ‘Y’ refers to a control fermentation temperature. be sent to an anaerobic digester to yield coefficient: Prior to distillation, fermented produce environmentally friendly meth- wine is passed through a centrifuge ane to feed the steam boilers. Spiral Equation 1a: Cellular Concentration to remove residual grape solids and heat-exchangers are selected for this residual yeast hulls, as these can service, due to their high fouling resist- dCc = (rg – rd) foul the plate and frame exchangers ance and excellent countercurrent flow dt used in the distillery pre-heat train. patterns (Figure 5). Equation 1b: Sugar Depletion Red wines require additional process- While spirits distillations can be ing to remove the grape skins and approximated as a binary system dCs = Y s/c (– rg) – rsm seeds, which are the source of the (LK=ethanol, HK=water) for most dt eponymous red colour and associated purposes, it is in fact a complex Equation 1c: Ethanol Production tannins. White wines typically have multi-component mixture. Non-key their skins and seeds removed prior species are typically divided into two dCp = Y p/c (rg) to fermentation. categories- ‘Fusel Oils’ (alcohols dt with three or more carbons, such as Equation 1d: Cell Growth Rate Distillation column propanol, butanol, amyl alcohol, etc.) A typical spirits distillation column and ‘Heads’ (highly volatile impurities), Cp 0.52 CcCs r = µ *(1 – ) * might contain 70-100 trays, a column both of which are called ‘congeners’ g max * Cp Ks+Cs diameter of 0.5 to 2.5m and employ by the fermented beverage industry. Equation 1e: Cell Death Rate copper bubble cap trays above the The distillation column must be tuned rd = KdCc feed point and stainless-steel sieve to permit some of these impurities trays below the feed point. The high into the product spirit, as they impart Equation 1f: Cellular Maintenance number of stages allows for a powerful character, but care must be taken as Sugar Depletion fractionation of dilute components and they are unpleasant in excess. rsm = mCc minimises the required steam demand (as seen in the prior article, there is Side draw fusel oil Fermentable sugars are expressed an inverse relationship between tray Fusel oils are more viscous than as Residual Sugar (RS), or grams count and the required steam and ethanol or water, and impart a full fermentable sugars per 100 mL. When condenser duties for a given separa- mouthfeel to the spirit. Fusel oils are the above coupled differential equation). Figure 3 shows the ethanol- ‘Intermediate Keys’, meaning they tions are fed into MATLAB, the sugar water profile along the column height. accumulate between the Light Key and alcohol concentration can be Copper bubble cap trays are used for (ethanol) and Heavy Key (water). This generated and plotted: their high efficiency, excellent turn dynamic can lead to unsteady-state Care must be taken during fer- down, and ability to react away volatile accretion of fusel oils in the column mentation as the process is highly sulphur compounds, which are offen- – as the fusel oils accumulate they exothermic. Uncontrolled fermenta- sive to the nose and palate. Figure 4 is migrate up towards the product tions can peak at temperatures in a simplified process flow diagram for line, dropping the product draw tray excess of 45°C. This can lead to a kind a brandy distillation, and precedes a temperature. The operator responds

25 1 RS 0.9 20 Abv 0.8 0.7 15 Ethanol 0.6 Water 10 0.5 0.4 Concentration 5 Mole fractions 0.3

0 0.2 0.1 -5 0 0 50 100 150 200 250 300 93 89 85 81 77 73 69 65 61 57 53 49 45 41 37 33 29 25 21 17 13 9 5 1 Time, hours Stage number

Figure 2: Timeline of an model fermentation: RS = Residual Sugars, Figure 3: Typical ethanol/water profile along a brandy column, generated g/100mL; Abv = Alcohol by volume, % via ChemCAD SCDS model www.ibd.org.uk Brewer and Distiller International May 2016 z 3 l DISTILLING

part fusel oil draw to three parts Primary condenser Vent condenser water), which functions as a fusel Bottoms oil antisolvent due to unfavourable van der Waals interactions (water is Feed dominated by hydrogen bonds, while Reflux the large carbon backbone in fusel oils Heads promotes London dispersion forces). This pushes the composition into the Product draw two-phase envelope shown in Figure 6. The less-dense organic phase is decanted off to remove fusel oils, while Pre-heater Fusel oil draw the aqueous phase is returned to the column to conserve ethanol.

Final product heads The level of heads in the final product must also be carefully titrated – too low, and the spirit lacks a certain sweet, fruity character; too much, and the spirit becomes harsh and biting. Heads are comprised of lighter-than- Reboiler light key (LLK) components, such as methanol, acetaldehyde, and ethyl acetate. These components are removed by means of a dual condenser Figure 4: Simplified process flow diagram of a spirits distillation column train. The first (primary) condenser is by increasing the reflux ratio, driving of a stage between the feed and the maintained around 77°C, just below the fusel oils back down the column. product draw. Operators analyse dif- the condensation point for ethanol at This results in a cooling of the bottom ferent trays on the column for fusel oil 1 atmosphere. The second (vent) con- trays, which is indicative of product content, and adjust the fusel oil draw denser is maintained at 16°C. This is loss (ethanol) out the bottoms. The accordingly. designed such that ethanol condenses operator responds by increasing the The fusel oil draw is shunted off to in the primary condenser, while the steam to the column, thereby increas- a single-stage liquid-liquid extractor majority of the heads are condensed ing the boilup ratio. This pushes the known as a fusel oil decanter. In-line in the vent condenser. All material fusel oils back up the column. This to the extractor, it is mixed with water condensed in the primary condenser increases until sufficient fusel oils (typically in a volumetric ratio of one (about 95% of the vapour leaving the accumulate to form second liquid layer on the distillation trays (Fig- Residue curve at 1.00atm, Binodal plot at 25.00C Binary Azeotrope ure 6). This two-phase liquid system Isoamyl alcohol = 131.20C (Ethanol, water) = (91.3%, 8.7%) at 78.21C significantly impedes mass transfer Ethanol = 78.29C (Isoamyl alcohol, water) = (17.7%, 82.3%) at 94.77 in the column and can lead to column Water = 100.00C flooding or ‘crashing’. It also leads to 10 10 excess fusel oils in the product, which results in an unpleasant ‘wet card- 20 20 board’ odour in the spirit. To combat this, a slight side draw 30 30 (called a fusel oil draw) is taken off 40 40

Mole percent (water) 50 50 Mole percent (ethanol)

60 60

70 70

80 80

90 90

10 20 30 40 50 60 70 80 90 Mole percent (Isoamyl alcohol)

Figure 5: Flow patterns in spiral heat exchangers promote Figure 6: Residue curve between water, ethanol, and isoamyl alcohol via NRTL model on good heat transfer (due to counter-current operation) and ChemCAD. Note the existence of two liquid phases in the absence of ethanol. This phenomenon resist fouling well can cause two-phase formation on column, and is exploited in the fusel oil decanter

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XO, depending on sensory character- 0.004 istics and target styles), it is decanted out, charcoal filtered for smoothness, cut to 80 proof, and bottled. Thus concludes our review of distil- 0.003 lation technology. We have reviewed the fundamental concepts around distillation, how to apply shortcut methods to understand the inner 0.002 workings of a column – and finally seen the technology in action during Weight fractions Weight our walkthrough of a brandy distillery. From its medieval roots in spirits production to state of the art petroleum 0.001 column, distillation has evolved with our society, and no doubt will continue to be the ‘top shelf’ choice in chemical separations for years to come. 0 93 89 85 81 77 73 69 65 61 57 53 49 45 41 37 33 29 25 21 17 13 9 5 1 Acknowledgements Stage number The author would like to thank Profes- Methanol Acetaldehyde Ethyl acetate N-propanol Isobutanol sor Katherine Shing for technical Sec-butanol Isoamyl alcohol direction, Andrew Curtis for his distillation training, David Warter for his Figure 7: Congener profile in a column. Fusel oils accumulate in the middle, while heads comments, and Kim MacFarlane for partition to the top of the column. Generated via SCDS model with ChemCAD help in editing. column) is returned to the top tray as the varietal distilled. GBD will have reflux, for an effective reflux ratio of variable mouthfeel depending on the Sources around 20. Figure 7 shows a typical distillation technique and the varietal. • Blanch, Harvey W., and Douglas S. Clark. column profile of the various conge- Brandy will have a brown sweet Biochemical Engineering. New York: M. Dek- ners in a spirits column. note from the oak. It will have less ker, 1996. Print. Finally, product is taken near the burn and a more round mouthfeel. • Fogler, H. Scott. Elements of Chemical Reaction Engineering. Upper Saddle River, top of the column. A distiller is typi- The floral and red notes will generally NJ: Prentice Hall PTR, 1999. Print. cally on site to analyse the spirit, both decrease, but the apple note comple- • Kister, Henry Z. Distillation Design. New for taste and chemical markers. The ments the oak well.” York: McGraw-Hill, 1992. Print. distiller adjusts the product proof, Prior to the installation of new dis- • Kister, Henry Z. Distillation Troubleshoot- steam duty, reflux ratio, fusel oil draw, tillation columns, ChemCAD computer ing. Hoboken, NJ: AIChE, 2006. Print. heads draw, and product draw plate modelling is applied to optimise the • Peynaud, Emile. Knowing and Making until the desired sensory characters columns with respect to flavour profile Wine. New York: J. Wiley, 1984. Print. are achieved. This spirit is known as and operational efficiency. Table 1 • Prausnitz, John M., Rudiger N. Li- ‘GBD’, or grape brandy distillate. It be- shows the material balance around a chtenhaler, and Edmundo G. De Azevedo. comes brandy after being aged in oak typical spirits column. Molecular Thermodynamics of Fluid-phase Equilibria. Vol. 3. Upper Saddle River, N.J: for at least two years. David Warter, a Prentice Hall PTR, 1999. Print. chemical engineer and spirits produc- And finally • Seader, J. D., and Ernest J. Henley. Sepa- tion expert, describes the sensory From the distillery, brandy is trans- ration Process Principles. New York: Wiley, characteristics of GBD and brandy ported to a barrelling facility, where 2006. Print. thusly: the GBD is cut to ~120 proof, 60% ABV • Smith, J. M., Van Ness H. C., and Mi- “GBD normally has prominent and aged for several years to smooth chael M. Abbott. Introduction to Chemical fresh fruit notes (normally apple/pear out harsh notes and gain the more Engineering Thermodynamics. New York: for white grapes and cherry/raspberry familiar brown colour of aged spirits. McGraw-Hill, 1996. Print. for red grapes). Sometimes the GBD Once the distillers determine how to will have floral notes depending on blend the aged brandy (VS, VSOP, or The author Konrad Miller is a process Component Feed Product Bottoms Fusel Oils Heads engineer from the United States. A licensed Profes- Ethanol 133 125 0 2 6 sional Engineer, he gradu- Water 1381 110 1264 6 1 ated from the University Methanol 0.02 0.01 0 0 0.01 Acetaldehyde 0.1 0.01 0 0 0.09 of California, Berkeley with a BS in Ethyl Acetate 0.04 0.01 0 0 0.03 chemical engineering, and from the N-Propanol 0.04 0.03 0 0.01 0 University of Southern California with Isobutanol 0.02 0.02 0 0 0 an MS in chemical engineering. His Sec-Butanol 0.02 0.1 0 0.01 0 work as a process engineer in the Isoamyl Alcohol 0.09 0.07 0 0.02 0 alcohol industry has included process Table 1: Typical column material balance. Generated via SCDS model with ChemCAD. All units in design, plant simulation, fermentation, kg-mole/hr and distillation. www.ibd.org.uk Brewer and Distiller International May 2016 z 5