• Abstract and Figures
• Public Full-text

● DISTILLATION Distillation Technology and Modelling Techniques Commercial distillations Part 1: Concepts in Distillation Instead of the single-stage batch pot still 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 stills 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 Reux Drum Reux Pump Overhead product Feed Distillation Column Distillation plate Vapor Reboiler Steam Condensate Upowing liquid Bottoms Bottom Downowing 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 ai,n-C4 along the trays in a distillation column: ically well behaved mixtures), there are many instances where a species with C2 250 Relative volatility 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 liquid reservoir at the bottom of the active tray area r column, so head pressure pushes weir liquid liquid from the bottom of the column to the reboiler. As the material in downcome tray r the reboiler flashes off, the level in l the reboiler drops, and more liquid pane surges in, hence the siphoning action vapor downcome 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.

Load more

  4

Copy Link