Cryogenic oxygen plants are
oxygen generating plants in the cryogenic process. We all know that there
are basically two processes used in gas plants to produce pure oxygen-one is
the cryogenic process and the other is the non cryogenic process.
While the cryogenic process is suitable for large scale gas plants as well
small sized gas plants, the non cryogenic process involving the PSA
technique is more suitable for small sized plants. Cryogenic gas plants are
suitable for producing oxygen for industrial and medical uses and available
in large, bulky sizes and also small sized cylinders. In other words,
cryogenic oxygen plants are available in low purity which is approximately
95% and high purity which is almost 99.6+%. There are oxygen-only plants,
and multi-product configurations like oxygen, nitrogen, argon etc.
What is Cryogenic Oxygen?
Cryogenic oxygen is oxygen in liquid state and it is very cold. In
cryogenic oxygen plants, oxygen is produced, stored and maintained in a
liquid state at a substantially constant pressure.
The Cryogenic Process
Commercial oxygen produced in cryogenic gas plants use the cryogenic
distillation process originally developed in 1895. This cryogenic process
produces oxygen that is 99+% pure. The steps involved are as follows:
Compression
- The air is compressed using a multi-stage compressor, which is then
passed through a water-cooled after cooler to condense any water. Before
compression, air is pretreated to remove impurities. Once the impurities
are removed, the air is submitted to fractional distillation, where the
components are separated in several stages. In the cryogenic process, a
cryogenic section is required to provide the low temperatures required
to liquefy the gas components. Once the liquid oxygen is separated, it
is purified and stored.
- The air passes through a molecular sieve adsorber, containing zeolite
and silica gel-type adsorbents. These adsorbents adsorbs the carbon
dioxide, heavier hydrocarbons, and any remaining traces of water vapor.
Separating
- Air is separated into its major components in this step. The portion
of the pretreated air is diverted through a compressor, where its
pressure is cooled and allowed to expand to nearly atmospheric pressure.
This expansion rapidly cools the air using the cryogenic technique.
- The air stream which is part liquid and part gas enters the base of
the high-pressure fractionating or distillation column. As the air moves
up the column, it loses additional heat. The oxygen continues to
liquefy, leading to the formation of oxygen-rich mixture in the bottom
of the column, and other gases like nitrogen and argon flow to the top
as a vapor. The oxygen at the top is the liquid oxygen mixture, also
known as crude liquid oxygen, which is almost 99.5%.
Purifying
The oxygen at the bottom is about 99.5% pure. Newer cryogenic distillation
units are used to recover more of the argon/nitrogen from the low-pressure
distillation column, and this improves the oxygen purity to about 99.8%. If
higher purity is needed, additional fractionating columns may be added to
further refine the oxygen product.
Distributing
The purified cryogenic oxygen produced is distributed to the end users in
gas pipelines from nearby air separation plants or transported in cylinders
and storage tanks to specified destinations. It is transported in large,
insulated tanks, usually made of two shells and the air is evacuated between
the inner and outer shell to obstruct heat loss.
Diagrammatic representation
Oxygen Grades at Cryogenic Oxygen
Plants
The Compressed Gas Association, formed in USA with the intention to work
for the development and promotion of safety standards and safe practices in
the industrial gas industry, establishes grading standards for both gaseous
oxygen and liquid oxygen. These standards are based on the amount and type
of impurities present.
Gas grades are called Type I and range from A to F. The grade A stands for
99.0% pure and grade F stands for 99.995% pure. Liquid grades are called
Type II. These also range from A to F. However, the types and amounts of
impurities in liquid grades are different from the gas grades. The most
commonly produced grades of oxygen are Type I Grade B and Grade C and Type
II Grade C which is 99.5% pure. They are used in steel making and in the
manufacture of synthetic chemicals.
Applications of Cryogenic
Oxygen
- Combustion
- Glass industry
- Metal fabrication
- Pulp and paper industry
- Chemical industry
- Medical
- Waste water treatment.
Equipment required in cryogenic oxygen plants
- Suction Filter
- Air Compressor
- After Cooler
- Chilling Unit
- Oil Absorber
- Moisture Separator
- Molecular Sieve Battery
- Expansion Engine
- Air Separation Column
- Liquid Oxygen Pumps
Buyer's guide
- Size: Large, Medium, Small, Portable
- Capacity
- Purity
- Air Capacity
- Power Connected
- Power Consumed
|
- Power Supply
- Starting pressure
- Working Pressure
- Areas Required
- Assembly Height
- Weight
- Air Separation Column
|
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Cryogenic
Oxygen Plant
