Unit 5 - continuous distillation

Question 1

What is continuous distillation ideal for?

Answer 1

Grain whisky, grain neutral alcohol,
light rums and brandy

Question 2

How can the columns operate?

Answer 2

At normal atmospheric pressure.

At higher pressure, where higher temperatures are also used in the distillation system.

In a reduced pressure or vacuum distillation where operating temperatures are lower.

Question 3

What are the columns called in a two step distillation?

Answer 3

stripping column & rectifying column

Question 4

In scotch whisky production, what are the columns called?

Answer 4

the analyser and rectifier

Question 5

What are the different parts of the columns?

Answer 5

Columns
steam inlet
plates
feed
stillage outlet
condenser

Question 6

What is the most common method for designing these types of stills?

Answer 6

McCabe-Thiele method
This method calculates the theoretical number of plates needed in a distillation column to achieve the required purity of distillate.

Question 7

Typically, how many plates will a column still have?

Answer 7

30 plates

Question 8

What are the different types of plates?

Answer 8

Bubble cap trays

Sieve trays

Valve trays

Disc and doughnut

Question 9

Describe a thermocompressor

Answer 9

uses high pressure steam from a boiler to vaporise the liquid flowing out of the bottom of the stripping column.

Question 10

Describe a reboiler

Answer 10

steam passes through a tube.
Liquid running out of the bottom of the stripping column flows over the tube and is converted into steam.

Question 11

Describe the different compounds on each plate

Answer 11

Ethanol – We want to maximise the concentration of ethanol in the spirit.

Isobutyl alcohol (or isobutyl ethanol)– This compound has an unpleasant, solvent flavour. Highest around plate 14

Isoamyl alcohol – Also known as fusel oil, isoamyl alcohol has a very unpleasant solvent and pear drop flavour concentrations highest around plates 4-6.

Question 12

How do we remove fossil fuel from the column still?

Answer 12

remove fusel oil from the rectifying column by withdrawing a portion of the liquid from plates 4-6. This liquid is then processed using a fusel oil decanter or a small packed distillation column.
The liquid is then diluted with water. Fusel Oil forms a layer on top of the decanter, removed and sent to a storage tank
Remaining FO liquid in the decanter is returned to sC with the hot feints.

Question 13

Describe the Stripping Column

Answer 13

Starts with fermented liquor at approx 34C & between 7-14% ABV
FL is pre heated to approx 85C using heat recovered from the vapour cooling process
FL enters near the top of the stripping column
At the top of the SC, the vapour contains between 10-25%abv
Vapour is diverted to the bottom of the rectifying column
Less volatile components (spent wash or stillage) flow down the stripping column
Stillage (containing almost no alcohol) is collected at the bottom of SC
Stillage is boiled using a reboiler or thermocompressor
The steam produced to the bottom of the stripping column
Stillage leaving the reboiler or column for further processing

Question 14

Describe the rectifying column

Answer 14

Vapour from SC enters at bottom of RC
Less volatile fractions called hot feints flow to the bottom of the RC & are directed to the top of the SC
The vapour is condensed into a liquid called reflux
FL from fermentation can be used to condense these vapours which also pre heats the FL before entering the SC
Some reflux is returned to the top of RC to extract as much alcohol as possible
Rest of reflux is called cold feints & is collected separately
Often, hot and cold feints are mixed & returned to the top of the SC
Spirit is continuously collected from a single plate near the top of the rc called the spirit plate which contains 92-96% abv.
At a plate further down the RC the liquid contains high levels of isomayl alcohol or fusel oil.
Water is added to the liquid in a fo decanter to separate IA from liquid. IA is removed & liquid returned to RC
hot feints collect in the base of the RC.
The feints are recycled to the top of SC for further distillation.

Question 15

What is the purpose of continuous distillation in spirits production?

Answer 15

Enable higher volumes of distillate to be produced efficiently with the correct flavour required
Produces a lighter distillate than batch pot still process

Question 16

What’s is a “balanced operation”

Answer 16

Once the columns are operating in a balanced controlled way, the produce continuous streams of spirit fusel oil, and stillage.

It’s called a BO because the combined flow rates of liquid and vapour entering the system are equal to or in balance with the combined flow rates of liquid and vapour leaving the system.

Also called steady state operation.

Question 17

What are the inputs and outputs of continuous distillation?

Answer 17

Input - Fermented liquor
Outputs - Stillage (SC), Cold feints (RC), fusel oil (RC)

Question 18

Describe the steam inlet

Answer 18

typically used as a source of heat to run the distillation column. This steam may be from a boiler or from stillage heated using a re boiler and thermocompressor

Question 19

Describe what happens when a thermocompressor is used:

Answer 19

The boiling liquid (stillage) from the base of the stripping column flows into a tank connected to the thermocompressor venturi.

High pressure steam is fired at high velocity into this venturi, thus reducing the pressure on the inlet connected to the tank containing the boiling liquid.

The reduction in pressure causes more steam to be produced from the stillage (liquids boil at lower temperatures when pressure is reduced) and drawn into the venturi.

The velocity of this steam is increased as it combines with the high pressure steam.

The steam mixture flows through a tube where the diameter increases reducing velocity and increasing pressure.

The steam then flows back into the base of the stripping column and rises up through the column.

Question 20

Describe what happens when a re boiler is used:

Answer 20

A liquid level is maintained in the base of the column, which overflows into the reboiler.

Steam converts this liquid to a mixture of liquid and vapour.

This vapour/liquid stream then feeds back into the column base, above the liquid level.

As the feed to the stripping column contains solids, reboilers can block if not cleaned regularly.