7- Column & Hybrid Stills Flashcards
Heat sources
Some directly heated (Armagnac), most use steam
Indirect steam
External heat exchangers- reboilers most common
Steam injection
Large operations, adds water-has to be removed, Maillard reactions minimized (advantage for light or neutral aromas)
Managing reflux
Internal structures designed to manage reflux, most fitted with rectification plates, some use packed columns
Advantage over pot stills
Can make spirit highly rectified as 96% abv, can run continuously
Rectification plates
2 features:
1- force gas & liquid to interact more than pot stills, unlike pot stills can control reflux tightly, vapours rising up forced through layer of liquid on plate
2- allow liquid to flow down via downcomer (pipe), when liquid rises it overflows, water flows down & ethanol goes up in vapour, double separation, # of plates have cumulative effects, with 40 plates water/ethanol mix can reach 97.3% abv
Packed columns
Filled with bits of material (metal or glass), create large SA, vapours rise through & partly condense on surfaces, rate of condensation increases as temp decreases with height, each small pocket reflux can occur, less ordered reactions but so many is possible to make highly rectified spirit, used in trad Chinese stills for baijiu, not widely used elsewhere, some in multiple column distillation (de-methylisers)
Condensers
Not every column still needs but common, worm tub condensers rare but variation in Armagnac, shell & tube condensers & dephlegmators widely seen
Running continuously
Example with 40 plates, preheated fermented liquid enters 1/2 way up, heat input comes from steam injectors at base
Reaching dynamic equilibrium
Techniques vary, fractions are forced to keep moving up & down, in early stages fractions on each plate constantly changing, over time movement never stops but changes in conc on each plate decreases, with constant conditions you can know conc of each fraction on any plate with high accuracy
Methanol
Group-1 fraction, boils at 64.7 C (148 F), very volatile, more conc on every plate & rises, most conc at top of still
Iso-amyl alcohol
Group-3 fraction, boils at 131 C (268 F), reaches highest conc on lower plate where it’s least volatile, on higher plates is not conc & small amounts that rise up flow back down (no fraction stops part way, vapour contains multiple fractions)
Separation
Once in equilibrium each fraction reaches highest conc depending on boiling point, most volatile/lowest boiling points are most conc at top, because fractions spread across height of still is possible to take off heads, hearts, tails & liquid waste simultaneously
Maintaining equilibrium
Can be maintained indefinitely as long as conditions do not change
Temperature
Temp gradient constant otherwise composition of liquid on plates will change, if temp rises then some less volatile fractions rise in greater amounts, if temp falls these fractions flow down in greater amounts, to keep temp constant liquid entering preheated
Composition of the fermented liquid
Needs to be same or composition of fractions on plate will change
Flow rates
Volume entering must be = to volume leaving or composition of fractions on plate will change
Rectification plate designs
2 common types: Bubble cap plate & Sieve plate