Hello my name's Sarah Martin-Law
and I work for BOC. You may not have heard of BOC
but the gases we produce go into
hundreds of products we come across
every day.
My name is Keith Nicholson and I also
work for BOC
in the technical sales department. I'm
based here at Thame,
in the food technology centre. BOC
supplies hundreds of gases, mixtures and
pure, to our customers,
but in the main we supply air gases,
oxygen and nitrogen,
which are extracted from the atmosphere.
Whether it's frozen food,
fizzy drinks, hospital oxygen supplies,
hydrogen powered cars,
MRI scanners or party balloons, all of
them use gases produced by BOC.
In order to turn the air in the atmosphere into
useful gases
to package products, a series of processes
must take place.
This starts with the air being filtered
to remove contaminants, such as dust,
as these would damage equipment in the
plant. The pressure of the air is
increased to five and a half times
atmospheric pressure by a machine called a
compressor. This process causes the air to
warm up to about 75 degrees centigrade,
so the filtered and compressed air is cooled
down to six degrees centigrade
by passing through a heat exchanger,
similar to a radiator with cold water.
As the air cools down a lot of the water
vapour in the air condenses
and the free water is drained off. It is
important to remove
any remaining water vapour before the air
is cooled any further.
This is because it will freeze and the
ice crystals may block the pipes and
the equipment,
particularly the heat exchangers. Carbon
dioxide freezes
at -56.6 degrees centigrade, so it must
be removed as well because the air will
eventually be cooled
to around -200 degrees
centigrade. A heat exchanger is used to
cool the clean, dry air
further. Cold gases at -175
degrees centigrade
are lead off the distillation column and
into tubes at the bottom of
the heat exchanger. The air passes in the
opposite direction
and heat from it moves into the cold
gases. The air cools down to about minus a
hundred and seventy degrees centigrade
before it leaves the bottom of the heat
exchanger, while the cold gases warm up
to about 13 degrees centigrade.
In this way the air is made very cold.
Individual liquids can be separated from
a mixture of liquids
through fractional distillation. Fractional
distillation relies on each fraction
having a different boiling point
and it works best if a long distillation
column is used.
Nitrogen boils at -196 degrees
centigrade.
Oxygen boils at -183 degrees centigrade.
So we use 2 distillation columns to
separate these elements from the air.
The first distillation column is a
high-pressure column where pure nitrogen is
obtained.
When liquefied air is passed into the bottom,
the liquid nitrogen in it boils and rises up
the column as a gas.
The oxygen remains below it's boiling
point, so it falls as a liquid to the bottom
of the column.
Notice that there are several layers of trays inside the column.
These let the rising gaseous nitrogen come
into contact with the falling liquid oxygen,
which improves the separation of the two elements.
The nitrogen is drawn off at the top of the
column
and condenses to form liquid nitrogen.
The second distillation column is a low
pressure column.
It allows any nitrogen that might remain
in the oxygen-rich liquid from the
bottom of the first column
to be removed. This leaves almost pure
liquid oxygen
at the bottom of the column.
The boiling points of oxygen and argon are very
close.
This makes them very difficult to
separate, so therefore we use a third
column
to remove the argon from the oxygen stream.
The gases separated in the air separation
unit can be distributed in small
quantities
in pressurised steel cylinders. Much
larger quantities of gases
are distributed in vacuum-insulated bulk
tankers as liquids. These
tankers travel across the country to
food production factories,
making the kinds of frozen foods we buy in our
supermarkets and store in our freezers at
home.
I started my career as an apprentice in
electrical engineering
and spent a number of years in the
energy sector before joining BOC.
In my time with BOC I've been able
to gain technical qualifications
in parallel with my career development.
