There are various different types of distillation columns and all of
them can be used to make spirits. Some are more suited to vodka and some
suited to whisky, gin etc. Scroll to the bottom if you want to learn
which types Headlands Distilling Co. uses in the Wollongong distillery! 

There are two main categories of distillation columns, batch and continuous. 

Batch distillation

Batch refers to a distillation setup where there is one tank (called a
boiler, just a tank with a heat source) which will be filled with a
finite amount of material to be distilled and then closed shut. The
liquid will be heated, the more volatile product will be collected in a
more concentrated form out the top of the column, and the material left
in the boiler contains less of the more volatile product. Batch
distillation is what chemical engineers describe as a non-steady state
process. This is because the concentration of the more volatile product
in the boiler is constantly dropping. In order to maintain a constant
purity of product, parameters need to be changed during the run
(increase in reflux ratio), or else the purity of the product will keep
dropping. 

Example: 

100 litres of 7% alcohol and water is pumped into a boiler, the lid
is put on, heated until boiling and the vapour sent to a distillation
column. ~7.3 L of 96% alcohol will be collected out the top and 92.7 L
of water. 

This is usually the way microdistilleries in Australia and the rest of the world make vodka, whisky, gin, rum etc. 

Batch distillation allows products with different boiling points to
come off the still sort of in an orderly fashion. It isn’t quite this
simple, but it can be thought of that compounds with lower boiling
points will come off first. This allows the separation of heads, hearts
and tails cuts. The distiller can select which cuts to put into the
finished product. 

Continuous distillation

Continuous distillation systems can run 24/7 without ever stopping to
refill the boiler tank. Liquid called the “feed” which is a mixture of
two or more liquids to be separated (e.g. alcohol and water) is pumped
constantly into a distillation column. The more volatile alcohol will be
collected out the top of the column, and the higher boiling point water
will make its way to the bottom of the column. Continuous distillation
is capable of incredibly high throughput, industrial ethanol plants can
make millions of litres of ethanol a day using this method without ever
stopping. 

Large manufacturers of alcoholic spirits such as vodka from Absolut and Grey Goose use continuous distillation. 

Continuous distillation columns

 

When a batch distillation is carried out, such as in the previous
example with 7% alcohol and water, a lot of heat is used to boil the
mixture. In a distillery, the water is a waste product, so it is dumped
down the drain, along with the heat energy used to heat it up. 92.7 L of
boiling water will be dumped down the drain. 

Continuous distillation systems are much more energy efficient. The
waste boiling water out of the bottom of the distillation column can be
pumped past the incoming 7% alcohol feed using a heat exchanger to
recover most of the energy. A simple heat exchanger is a pipe inside a
pipe. One liquid flows through the inside pipe and the other through the
outside pipe. The hotter liquid heats up the other liquid. Hence, most
of the energy used to boil the water can be recovered by preheating the
feed. 

Next, continuous distillation systems also get free energy from the
condenser. When alcohol vapour flows up a distillation column, it is fed
into another heat exchanger, this time called a condenser, which serves
the same purpose as the heat exchanger described previously in reverse.
Cold liquid, usually water, is fed through the outside pipe of the
condenser, and the alcohol vapour passes through the inside tube. This
condenses the alcohol vapour, turning it into alcohol liquid. Instead of
wasting water or running a water cooling system, continuous
distillation columns pass the 7% alcohol feed through the outside pipe
of the condenser as the cooling liquid. This serves a few purposes- it
condenses the alcohol vapour, basically for free in terms of energy
usage. It preheats the feed to a higher temperature, so even less energy
is needed to get it boiling, a win win! 

If continuous distillation systems are so great energy wise, why
aren’t microdistilleries making all their vodka, whisky and gin with
them? Continuous systems take complicated control systems to work
properly, have lots of things that can go wrong such as pumps, valves,
clogged lines, are less flexible with different materials and can be
very expensive in terms of capital and design. This means that usually a
very large amount of product needs to be made to justify the initial
investment. 

Continuous distillation does not allow for the separation of heads,
hearts and tails in the one column like a batch distillation. Instead,
one product is separated at a time in each column. For instance in a
mixture of methanol, ethanol and water: top product of the first column
will be methanol and ethanol mixture, bottom product will be water. In
the second column, methanol will be the top product and ethanol will be
the bottom product. 

Stripping and rectifying columns

Continuous distillation columns comprise of two sections. The
stripping section and the rectifying section. The section of
distillation column below the feed is classified as the stripping
section. The purpose of the stripping section is to get all the alcohol
or other product out of the other liquid (water in case of alcohol
distillation). The stripping section needs to be long enough so no
alcohol comes out the bottom of the column. The section of column above
the feed point is classified as the rectifying column or rectification
column, same thing. The purpose of the rectifying column is to increase
the purity of the top product. In the case of vodka production, the
continuous rectifying column needs to be tall enough to obtain
approximately 96% alcohol, the maximum obtainable without vacuum or
molecular sieves. 

Types of column internals

Now that the two main types of distillation systems have been briefly
explained, the next step is the column internals, what is inside a
distillation column and how they relate to reflux.
When alcohol vapours pass up a distillation column, they will be at a
certain purity, for example 50% alcohol. If you take that alcohol and
distill it again, it will be at a higher percentage again, e.g. 80%.
Instead of collecting all the alcohol and distilling it over and over
again, you can simply condense the alcohol vapours at the top of the
column and either pump them back into the top of the column or use
gravity to feed them back into the top of the column, without having to
do any extra work. The liquid which you feed back into the distillation
column for additional purification is called reflux, and the amount of
liquid you feed back into the column compared to the amount you keep is
called the reflux ratio. 

Reflux, the liquid fed back into the top of the column needs to mix
intimately with the rising vapours. If the reflux is simply running down
the walls of the distillation column and the vapours are rising up the
inside without mixing, there will be very little purification taking
place. The column internals are designed to mix the reflux with the
rising vapour. 

All of these column internals can be used in either batch or
continuous mode. It should also be mentioned that a pot still, one used
to make whisky and flavour gin, is a type of distillation system without
any column internals. A pot still is the simplest type of still and
consists basically of just a boiler and condenser. 

Bubble caps

Copper bubble caps inside a distillation column 

Bubble caps are a type of distillation column internal which forms a
positive vapour seal at each plate. Distillation columns can contain as
little as 1 of these plates to 30+. The more plates, the higher purity
the product will be. The alcohol flowing down the column, called the
reflux, will form a pool of liquid on each plate. Alcohol vapour will
flow up from the plate below, through the inside of the little round cap
and bubble into the pool of liquid on the tray, mixing the vapour and
liquid intimately. Each bubble plate has a downcomer,
in the picture above you can see a drain hole, this is it. Liquid can
only pool up on the plate as high as the downcomer, all excess liquid
will flow down to the next plate. Bubble cap plates are excellent for
alcohol distillation, but have a some drawbacks. They are expensive.
They can’t handle much suspended solids in the column, in the case of
continuous distillation where the feed is pumped into the column. In
alcohol production, milled grain husks can still be present in the
alcohol to be distilled. Sieve tray column internals can handle solids
without the need to filter, where bubble caps and random packing will
quickly clog up. 

Sieve trays

Sieve trays are basically a metal plate with a bunch of holes cut in
them, like a sieve, also called a perforated plate. Part of the plate is
cut away so that liquid can flow down the column. Sieve trays are very
cheap and simple, can handle solids and are very efficient. However,
they can only operate in a small flow rate range. If you don’t have the
reflux and upcoming vapour rate perfectly calculated the sieve tray
won’t work very well at all. If  the reflux isn’t high enough, the plate
won’t have a liquid seal and the rising vapour will simply shoot up the
column without being mixed. Microdistilleries hardly ever use sieve
trays for this reason, choosing to use the more expensive bubble cap
option, which has a liquid seal at each plate by design, a more
foolproof option. Large petrochemical plants have teams of engineers
designing their column, so often choose sieve trays over bubble caps. 

Valve trays

Similar to a bubble cap plate, but instead of a solid cap with slots
in it, a moveable riser sits in place of the bubble cap. Once the
pressure beneath the riser (valve) reaches a certain level, the riser
lifts up and the vapours mix with the liquid on the tray. Valve trays
are sort of a cross between a sieve tray and a bubble cap plate. They
are cheaper than bubble caps in industrial installations, provide great
throughput and efficiency, but can’t run at the same range of flow rates
that bubble cap plates can. 

Random packing

Instead of having discrete plates inside a column, you can simply
pour a bunch of tiny metal, ceramic or plastic pieces into the column.
The reflux will flow down the pieces and mix with the rising vapours. 

In a properly designed randomly packed column, greater separation can
be obtained vs bubble caps, valve trays of sieve trays in the same
column height. 

Usually the random packing is small metal rings. Types include Pall Rings, Dixon Rings, Raschig Ring and Super Raschig Ring. 

You can’t simply pour random packing rings into a column and expect
it to work well though. If reflux is running down the walls of the
column, vapours won’t be mixing with it and little separation will
occur. Random packing columns need a plate called a distribution plate
or distributor every so often down the column, to ensure reflux is
running evenly over all the packed rings and not down the walls. The
distrubution plate is generally just a perforated metal plate with
central holes. 

Structured packing

Structured packing is like a porous piece of metal honeycomb with
channels designed in it to flow reflux down the column. They are more
expensive than random packing, but can often have higher efficiency than
random packing and less pressure drop. 

What type of column does Headlands Distilling Co. use?

For making Seacliff Vodka at our distillery in Wollongong, NSW
Australia, we first run a super energy efficient continuous
distillation. Our continuous still is tiny compared to industrial
versions, but it is packed full of tech! Pressure sensors, automated
valves, thermocouples galore, all plugged into an industrial computer
which controls everything called a Programmable Logic Controller (PLC)
and we are proud to say we built and programmed everything ourselves
from scratch. The stripping section of the column uses sieve trays,
because we still have a small amount of fine solids in the feed and
sieve trays handle solids very well. We preheat the feed with both the
first condenser and the bottoms waste water through a heat exchanger.
Using the 7% alcohol solution as the coolant in the condenser also gives
us free cooling, without wasting water or using electricity to run a
chiller. The feed is pumped about half way up the distillation column,
already very hot, practically for free. Above the feed we use random
packing, made of small stainless steel Pall Rings, with a plug of pure
copper mesh in the top of the column. The bottoms product (water with
some proteins, husks and other nutrients) is concentrated and given to a
local farmer as a nutrient rich animal feed. 

The next distillations are done in batch mode. The sieve trays are
removed from the column and repacked with distribution plates and more
Pall Rings. Batch mode allows us to take heads, hearts and tails cuts,
where continuous distillation doesn’t. 

For our gin and whisky, the still is rebuilt again into a hybrid pot
still mode which allows the alcohol to be taken off at a lower
percentage, carrying through more flavour.