A molecular sieve battery is an important tool used in oxygen/nitrogen gas
plants for producing oxygen, nitrogen and other pure gases through the
cryogenic process. The main purpose of using a molecular sieve battery is to
remove impurities from the processed gas
Components of Molecular Sieve Battery
The battery consists of two vessels. Both the vessels are filled with a
molecular sieve of 1.5/3 mm type 13-X. This is regenerative type capacity
400m3/hr to 2000 m3/hr and 12 hour regeneration cycle. The molecular sieve
helps to absorb carbon dioxide and moisture from process air. Besides carbon
dioxide and moisture, molecular sieve battery also removes acetylene and
other hydrocarbons from the process air. This helps in removing the
requirement for separate acetylene absorbers. This also eliminates recurring
caustic soda costs. For all these reasons, the battery is very useful in gas
plants. During the process of purification and adsorbing, one vessel will be
online and the other will be regenerated by dry nitrogen. All molecular
sieve batteries are provided with a regenerating electric heater,
temperature sensors & control panel with temperature controller.
The Working Process
As already mentioned, the vessels are filled with Molecular Sieve of Type
13-X. As the process air passes through the battery, the molecular sieve
will absorb water vapor and CO2 from the air. After about 10 hours or so,
the molecular sieve becomes saturated, and it will have to be regenerated.
That is why there is a regenerating heater with the battery. If hot air at
low pressure is passed through the saturated molecular sieve, the sieve will
discharge the carbon dioxide and water vapor and is ready for use again. The
absorbing capacity of the sieve falls rapidly if the process air is warm.
Therefore, it is always necessary that the compressed air entering into the
battery is below 15 Degree C. And secondly the molecular sieve is thoroughly
cooled after regeneration, before the process air is passed through it
again.
The waste Nitrogen passes through an Electrical Heater, which then passes
through one vessel to be regenerated. The temperature at the vessel's inlet
rises rapidly, while the outlet temperature rises very slowly. If the outlet
temperature reaches 170 Deg.C, the molecular sieve is regenerated. This
process takes about 2-3 hours and is known as the heating cycle. The heater
must be put off if the inlet temperature exceeds 300 Deg.C. and put on again
after 10 minutes. The Cold Nitrogen continues to pass through the vessel
which cools the sieve. This is known as the cooling cycle process and
continues for about 4 hours. After cooling, the vessel is ready for use
again when needed for purifying the process air.
How to optimize performance?
It is to be noted that oil vapor destroys the molecular sieve battery and
hence it is necessary that the carbon filter is in good condition and the
process air is as cool as possible so that the maximum amount of oil and
moistures moves out before the air enters the vessel. While changing from
one vessel to the other, pressurize the vessel very slowly.
This can be done by opening the valve A-7 or A-8. If the main valves are
opened rapidly, the battery may break up because of propelling of high
pressure air in to the Molecular sieve in the vessel. When charging fresh
Molecular sieve, it should be lowered slowly through a pipe, to prevent
breaking on falling at the bottom. Satisfactory working of the Molecular
Sieve battery will prevent many problems. Therefore, maintain and operate
this equipment carefully.
Flow Diagram of Molecular Sieve Battery
Buying Tips
Technical specifications to consider are as follows:
- Type of Absorbent
- Size of Molecular Sieve
- Quantity of air handled
- Heating Time
- Cooling time
- Changeover Time
- Heating Inlet Temperature
- Heating Outlet Temperature
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- Cooling Outlet Temperature
- Main Air Inlet Temperature
- Air Pressure
- Max. Permissible Carbon dioxide in Atm. Air
- Max. Moisture content
- Heater capacity
- Heater voltage supply
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Molecular
Sieve Battery
