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Distillation Technology and Modelling Techniques

Commercial Part 1: Concepts in Distillation Instead of the single-stage batch pot described above, commercial In this review of distillation technology and modelling distillations are typically performed in a continuous manner, with multiple techniques, Konrad Miller and Katherine Shing offer a pot still equivalents (or ‘stages’) used broad survey of the science of distillation, including a to increase separation power. Consider conceptual framework of distillation; distillation column the set-up in Figure 1. Three pot are connected in a series of stages; components and, in part two, shortcut distillation design where the vapour from one stage rises methods such as the McCabe Thiele method and the and intermingles with the liquid in the stage above it, and liquid from a stage Fenske Underwood Gilliland algorithm. In part three, drains down to intermingle with the Konrad Miller will apply these principles to the production vapour in the stage below it. Since a series of connected pot processes of a modern brandy distillery. stills is an ineffi cient and needlessly complex design, distillation columns hat is distillation? Think back to water, so imagine your parents were are used industrially to achieve the Wyour childhood, when you fi rst boiling wine for a sauce. Water boils at same results. The internals of a distil- observed a pan of boiling water: you 100°C, but ethyl alcohol (the alcohol in lation column are comprised of a se- watched water vapour rise from the the wine) boils at 78.4°C. The liquid no ries of ‘plates’ or ‘trays’, each of which liquid surface at a temperature of ap- longer boiled at 100°C, but now some- allows for the thorough mixing of proximately 100°C (212°F). If you were where in between these two limiting vapour rising from a stage below and really curious, you might have turned values. As the liquid boils, ethanol, with liquid falling from a stage above. This the power up and noticed the liquid the lower boiling point, preferentially continuous contact promotes mass boil more vigorously with a resulting vaporises over water. The vapour leav- transfer between the phases and the increase in vapour fl ow. Now consider a ing the pan will be richer in ethanol and equilibrium between the liquid and va- pan containing a mixture of two fl uids. the liquid remaining will be richer in pour leaving each stage acts in exactly Throughout this discussion, we will be water. This is the basis of distillation – the same manner as the series of pot exploring the separation of ethanol and and indeed how it was discovered. stills idea shown in Figure 1. Figure 2

30 z Brewer and Distiller International February 2016 www.ibd.org.uk DISTILLATION l

Condenser

Cooling Water Re ux Drum Re ux

Pump

Overhead product Feed

Distillation Column Distillation plate Vapor Reboiler

Steam Condensate

Up owing liquid Bottoms Bottom Down owing liquid product Figure 1: Multiple pot stills operating to- liquid gether as a distillation column. Note the feed and product lines, distillation columns are a Figure 2: Vapour and liquid paths along trays in a distillation column. Note the column itself is continuous, steady-state process divided into two sections: Rectifying (above feed) and Stripping (below) illustrates the vapour and liquid paths conceptual shortcut for thermodynam- Alkane Species αi,n-C4 along the trays in a distillation column: ically well behaved mixtures), there are many instances where a species with C2 250 a higher boiling point is more volatile C3 57 Before we approach the internal work- than a lower boiling point species. i-C4 2.4 n-C4 1 ings of a distillation column in more In binary distillation (such as i-C5 0.4 detail, let us flesh out our notion of ethanol and water for spirits making), n-C5 0.009 distillation and discuss the motivation the ‘split’ of components is obvious: for learning more. Distillation can be ap- the lighter species (ethanol) partitions Table 1: Relative volatility for each species in an alkane feed: n-C4 as HK and i-C4 as LK. plied to separate liquid mixtures when- into the vapour top product known Note that relative volatility compared to n-C4 ever the ‘relative volatility’ of the species as distillate, and the heavier species decreases with increasing carbon count to be separated is greater than 1. (water) partitions into the liquid bot-

Relative volatility (aij) of species i tom product known as bottoms. The ity chemical purification, food/flavour (more volatile) and j (less volatile) is situation is more complex for multi- production and of course distilled spir- defined as the ratio of the volatility (ki) component distillation. Separations its. The completeness and rigor with – yi of species i and j, where ki= /xi, where are usually specified between two which distillation is understood allows yi and xi are the mole fractions of spe- ‘key’ components: a ‘Light Key’ (LK), for the design and fabrication of full- cies i in the vapour and liquid phases, and a ‘Heavy Key’ (HK). The LK is the scale distillation columns without the respectively. These values are calcu- least volatile component that is en- need to pilot separations on smaller lated from thermodynamic equations riched in the distillate, or the ‘heavi- columns first, as is often required for outside the scope of this paper, but are est of the lights’; the HK is the most less-understood unit operations such strong functions of temperature, pres- volatile component that is enriched as membrane separations, resin sepa- sure, and composition. in the bottoms, or the ‘lightest of the rations, crystallisation, etc. The relative volatility is an expres- heavies’. This conceptualisation al- Let us return to the components sion of how well two species split lows us to think of a multicomponent of a distillation apparatus. In broad when vaporised: a high relative volatil- distillation as a pseudo-binary distil- terms, a distillation column is com- ity indicates that there is a significant lation between the LK and the HK. prised of three major components: enrichment upon vaporisation, mean- Table 1 provides an example of rela- ing the vapour phase contains more tive volatility values versus N-Butane 1. The column of component i than the liquid phase. for the alkane feed to a distillation The column offers surface area for A relative volatility of 1 indicates that column: mass transfer between the boiling no separation is possible; the volatility (bubble point) liquid streams falling of each component is the same. Pure Robust and well understood down the column and the saturated component boiling points are often So why is distillation useful, and where (dew point) vapour traveling up the used as a proxy for relative volatil- is it applied? Distillation is one of column. As the vapour and liquid make ity, i.e., low boiling species are more the most robust and well understood contact, the more volatile components volatile than high boiling point species. separations technologies, with appli- are enriched in the vapour and the While this is often true (and a handy cation to petroleum refining, commod- less volatile components are enriched www.ibd.org.uk Brewer and Distiller International February 2016 z 31 l DISTILLATION

reboiler is a large shell and tube heat vapor exchanger with liquid bottoms on bubble bubble the shell side and steam on the tube caps caps liquid side. Energy drawn from the steam weir to the bottoms liquid drives vapori- liquid sation, which provides the vapour upflow and heat to drive the separa- downcomer tray tion. The most common reboiler is a ‘thermosiphon’ reboiler; fed from panel

downcomer vapor the standing pool of liquid at the

liquid bottom of the column. The reboiler is on the same level or below as the active tray area liquid reservoir at the bottom of the column, so head pressure pushes weir liquid liquid from the bottom of the column to the reboiler. As the material in tray downcome r the reboiler flashes off, the level in the reboiler drops, and more liquid

pane l surges in, hence the siphoning action

vapor downcome r of this system. liquid Figure 5 depicts a typical thermo- siphon reboiler. Other reboiler types include kettle reboilers where liquid is pumped into the heat exchanger, Figure 3: Bubble Cap plates in a column. Note the liquid and vapour paths along the column and direct fired reboilers where the in the liquid. Column internals can be down from above to one side of the reboiler is effectively a furnace and divided into two categories: tray, then travels across the tray to fall combustion drives the distillation. Ma- Plates: A distillation plate (also onto the tray below. Weirs are installed terial leaving the reboiler is typically known as a tray) is a flat disk which next to the downcomers to ensure a known as the ‘boilup’, which enables supports a liquid level. Vapour paths minimum liquid level on the tray. the vapour-liquid equilibrium along the in the tray allow the vapour from the Packing: Packing is a more recent height of the column. tray below to rise and bubble through alternative to plates. The liquid/va- the standing liquid. Trays come in pour contacting surface area is not 3. The condenser three major types: Bubble Cap trays broken into discrete trays but rather The condenser is the mirror image have very high efficiency, very high is comprised of a continuous geom- of the reboiler. The reboiler drives turndown, high pressure drop, and etry to promote constant exchange vapour up the column while the are expensive; Valve trays have good between the phases. Packing is used condenser drives liquid down the efficiency, good turndown, moderate due to its low pressure drop and abil- column. If the feed to the column is pressure drop, and are moderately ity for high mass transfer efficiency liquid (as is typical), then liquid falls priced; and Sieve trays have low ef- vs. plates. Additionally, packing can down the trays below the feed point ficiency, low turndown, low pressure be applied to newer technologies to enable vapour-liquid contact, but drop, are good for high solids, and are such as reactive distillation. Unlike no liquid is available above the feed inexpensive. a column, there are no ‘stages’ that point. The condenser addresses this All of the above trays are classified act akin to a pot still. A distillation by returning an often substantial as ‘crossflow’ trays, where liquid falls column with plates is typically speci- portion of the condensed vapours as fied by a number of trays, whereas ‘’. Reflux is as critical to the packed columns are usually speci- operation of the distillation column fied by ‘HETP’ (Height Equivalent to a as boil-up. The condenser itself ) or ‘HTU’ (Height of is typically a shell and tube heat- a Transfer Unit) and ‘NTU’ (Number exchanger with cooling water (or of Theoretical Units), e.g. a distilla- some other coolant) in the tubes and tion column requiring 55 trays might condensing vapours in the shell. The require 55 NTUs with an HETP of 2 condenser may be above the column meters, for a total packing depth of with gravity draining reflux back 110 meters. Packing can be broadly onto the trays, or it may be below divided into two categories: random the column with reflux pumped back packing and structured packing. An to the top. Figure 6 shows a typical example of structured packing is condenser. shown in Figure 4. Putting all these pieces together, Figure 7 describes the overall action of 2. The reboiler a distillation column. With a concep- The reboiler is analogous to the tual understanding of the mechanics Figure 4: Structured packing segments prior to installa- stovetop in our boiling pot exam- of distillation we can, in part 2 of this tion. Note the complex packing pattern, which maximises ple; it provides the heat to drive the article, review methods for modelling contact area separation in the column. The typical the behaviour of a distillation column.

32 z Brewer and Distiller International February 2016 www.ibd.org.uk DISTILLATION l

Bottom Section of top vapor Distillation Column fraction to condenser Tray liquid reux Downcomer

Liquid + Vapor V

Mixture L L downcome r Trays LC downcome r recti cation Steam Feed

Thermosyphon downcome r Reboiler Liquid Feed tray downcomer downcomer Trays Condensate strippin g downcomer V = vapor L = liquid Bottoms Liquid Bottoms LC = level control Product downcomer from Reboil Figure 5: Sketch of a thermosiphon reboiler. Note the liquid inlet at the bottom of the column, and the vapour is above the column liquid level. The vapour is discharged below the bottom to stage, and provides the upflow which drives separation Reboiler

shell-side Figure 7: Major flows and components of a tray column uid in tube bundle with Sources tube sheet straight tubes tube sheet • Blanch, Harvey W., and Douglas S. Clark. Biochemical Engineering. New York: M. Dek- ker, 1996. Print. • Fogler, H. Scott. Elements of Chemical

outlet plenum Reaction Engineering. Upper Saddle River, NJ: Prentice Hall PTR, 1999. Print. • Kister, Henry Z. Distillation Design. New York: McGraw-Hill, 1992. Print.

inlet plenum • Kister, Henry Z. Distillation Troubleshoot- ing. Hoboken, NJ: AIChE, 2006. Print. • Peynaud, Emile. Knowing and Making Wine. New York: J. Wiley, 1984. Print. • Prausnitz, John M., Rudiger N. Li- bafes shell chtenhaler, and Edmundo G. De Azevedo. shell-side Molecular Thermodynamics of Fluid-phase tube-side uid out tube-side Equilibria. Vol. 3. Upper Saddle River, N.J: uid in uid out Prentice Hall PTR, 1999. Print. • Seader, J. D., and Ernest J. Henley. Sepa- ration Process Principles. New York: Wiley, Figure 6: A shell and tube exchanger. Most distillation condensers are in this configuration 2006. Print. • Smith, J. M., Van Ness H. C., and Mi- The authors: of Southern California. She earned chael M. Abbott. Introduction to Chemical Konrad Miller is a process engineer her BS in from Engineering Thermodynamics. New York: from the United States. A licensed pro- Washington State University and her McGraw-Hill, 1996. Print. fessional engineer, he graduated from PhD in Chemical Engineering from the University of California, Berkeley Cornell University. Her research inter- Image Credits with a BS in Chemical Engineering and ests include classical and statistical • Figures 2 (Continuous Binary Fractional from the University of Southern Califor- thermodynamics, computer simula- Distillation), 3 (Bubble Cap Trays), 6 (Straight- nia with an MS in Chemical Engineer- tion, and fluid phase equilibria. tube Heat Exchanger 1 Pass), 7 (Tray Distilla- ing. His work as a process engineer tion Tower) by H. Padleckas, licensed under Creative Commons. in the alcoholic beverage industry has Acknowledgements • Figure 4 (Structured Packing for Distilla- included , plant simula- Konrad Miller would like to thank An- tion Column) by Luigi Chiesa, licensed under tion, fermentation, and distillation. drew Curtis for his distillation training, Creative Commons. Katherine Shing is a Professor of David Warter for his comments, and • Figure 5 (Thermosyphon Reboiler) by mbey- Chemical Engineering at the University Kim MacFarlane for help in editing. chok, licensed under Creative Commons. www.ibd.org.uk Brewer and Distiller International February 2016 z 33