The process of Air separation is regarded as a common mechanical process
that is employed to extract all or particularly one of the essential air
components. The three fundamental constituents are Oxygen - 20.9%, Nitrogen -
78.1% and Argon - 0.9%. These three particular gases form the bloodline of
modern industries such as the chemical product, metallurgy, petrol refining, and
steel product industries and thus, they are the primary users of the Air
Separation Unit (ASUs) of the world. For example, argon is required for
welding, nitrogen is unquestionably used as an inert gas in metals, and food
processing and oxygen is required for steel production. The residual gases
exist in minute amounts and are generally not extracted. In large scale, ASUs or
air separation units, Krypton, Xenon, and Neon is often extracted in tiny
amounts.
The cryogenic air separation process exploits the different boiling
condensing points of the air components to facilitate the separation through
distillation at cryogenic temperatures.
The main elements of air possess the below-mentioned boiling and
condensing points at atmospheric pressure:
Ø Nitrogen (320.4° F)
Ø Oxygen (297.3° F)
Ø Argon (302.5° F)
Air is a simple and abundant mixture, hence
distilling and liquefying air contributes to a process to separate Argon,
Nitrogen, and Oxygen. All worldwide ASUs function through this process.
Components of a Basic ASU
§ Main Air Compressor
or MAC
The job of MAC is to compress atmospheric air,
mostly to 60 to 90 PSIG, and deliver it to the central system. Electric motors
drive these compressors. The inbuilt Interstage coolers remove heat generated
in each particular stage of compression, which is mostly of 2 to 3.
§ Front End Clean Up
The modern ASUs consist of a Prepurifier Unit
(PPU) that eliminates moisture, Carbon dioxide, and most of the hydrocarbons.
CO2 and moisture need to be removed to prevent the forming of ice and dry ice
that can form later. A Prepurifier Unit typically constitutes a chiller for
cooling the air till 40-55F, also a condensate separator for removing water and
vessels that are filled with mole sieve and desiccant material to absorb the
contaminants while the air passes through. One of the beds is always in line
with the process, and the one is regenerated using the heated waste Nitrogen to
eliminate the accumulated waste. The beds switch automatically every 5 to 8
hours. The air flowing through PPU is CO2 free and contains little moisture.
Some older, preceding ASUs facilitate reversing of the heat exchangers for
accomplishing the front end clean up. The systems precisely include exclusive
heat exchangers made from cryogen to chill down the CO2 and moisture, thus
letting the clean air to flow towards the process of distillation. The heat
exchanger passes are swapped every 3 to 10 minutes through an array of check
and butterfly valves. While one pass removes the contaminant waste, the other
one gets regenerated by the growing waste gases. The removal of the CO2 and
moisture through the reversing of the heat exchangers is practical both in
operating and capital costs.
§ Coldbox
This component has inbuilt distillation
columns, heat exchangers made from cryogen, piping, and connected valves.
Because most parts of the system are cold, they are reinforced inside of the
coldbox and sheathed in insulation. The Coldbox can be of cylindrical or
rectangular shape and are generally tall, can be than 200 feet in height
depending on the Argon system type and capacity.
Latest Coldboxes are constituted using perlite
insulation that makes it easy and light to remove and install, as and when
required. Older coldboxes are mostly filled with Rockwool that is packed by
hand as fourteen pounds per cubic foot. This made them time-consuming and
tedious to install or remove.
§ Expander
Most ASUs except for some consist of
expanders. Expanders supply with the required iciness to the distillation
column systems. Waste Nitrogen, air or Nitrogen is fuelled to expander that
turns the wheel and transfers energy to the generator, compressor oil brake. The
energy transfer cools the gas down. With the continuation of the process, the
expanders outlet temperature gradually reaches the design temperature level,
while the column system gets cooled down.
§ Liquid Argon
Systems
The Liquid Argon Systems are commonly 2 types.
Generally, plants are not equipped with Argon separation equipments. In those
cases, Argon is exited from the ASUs with the wasted gases. The first kind uses
a coarse Argon column, which concentrates Argon at 2 to 3% of Oxygen content
from the 88-92% of the low-pressure oxygen column. The coarse Argon is heated
and then merged with hydrogen before entering the catalytic reactor. It is
where the O2 and H2 combine to form water. The wet form of Argon is dried and
cooled again to the level of cryogenic temperatures. Next to this, the N2 and
H2 are removed in the distillation and separator column, distinctively.
The Cryogenic Argon system is solely
dependable on distillation to start with the purification. The columns are more
than 200 feet tall, to fit in the large volume of trays for separating Argon
from O2. Most of the new plants employ Cryogenic Argon System for avoiding the
usage of Hydrogen and Argon compressor in the procedure. The drawback to this
is the recovery time becomes for achieving purity after it starts up over and
again within the next 48 hours.
Setting up of an Air Separation Unit
Scope of Work
This is the standard procedure followed by Air
Separation Units for the production of liquid nitrogen, liquid argon, and
liquid oxygen. The unit consists of centrifugal air compressors, fixed
absorption purification beds, packed with molecular sieves for removal of
carbon dioxide, and water moisture.
The 3 liquefied elements are composed for
cryogenic storage. The description of the startup procedure is as follows:
Ø The cooling water from the compressor intercoolers gets circulated
throughout all the coolers.
Ø The air absorption purification unit is prepared by separating the
absorption bed that was to be initially used for the start-up procedure. The
block valves are setup for allowing the purification procedure to take place on
one bed while the other bed is regenerated.
Ø The central air compressor is manually flipped over while the main
starter of the electrical motor is disengaged. This permits the compressor
shaft to rotate freely. The vent valve for discharge gets entirely opened along
with the interstage drain valves.
Ø When there is confirmation of the instrumentation to be in working order,
and the electrical switch to the motor starter of the compressor is unlocked,
the circuit gets activated. All of the lubrication systems get confirmed.
Ø The HP column’s expansion valves are split open along with the expansion
valve of the low-pressure column.
Ø The manual rotation of the expander valve is done to ensure the procedure
operates freely, and the inlet air valves is shut tight while the outlet air
block valve gets opened. When all the instrumentation seems to be working
normally, and the electrical switch of the expander’s motor-generator gets
unlocked, the circuit is activated. The lubrication systems also start working
at the same time.
Ø The initiation of the air compressor is done by turning on the electrical
starter to the compressor’s motor. The unit is then allowed to blow the air
through the interstage drains and the discharge vent. After the confirmation of
the proper functioning of the compressor and that there are no issues with
instruments and lubrication, the drains are slowly closed in series one-by-one
in the order of increasing stages. The last to be gradually closed is the
discharge vent valve, and the compressor’s discharge enters the adsorption
purification system, while the speed of the entire of the discharge system
boosts up to operating pressure. The pressure of the discharge is kept at the
operating level employing the expansion valve on the HP column.
Ø After the normal operation of the air compressor is ensured and feeding
of the purification system and the HP column is in a steady, controlled state,
the turboexpander is placed on stream by gently opening the valve of the
suction block. The throttling of the expansion valve of HP column regulates the
compressor discharge pressure steadily to be at the operating level. After
checking the steady running of the turboexpander and the gradual decrease of
the discharge temperature, most of the discharge is sent to the expander by
smothering the HP column expansion valve. This is done for speeding up the
cooling down and increasing build-up of coarse oxygen liquid level within the
HP column.
Ø The above step goes on until the liquid level of the HP column reaches the maximum design level, and the LP column expansion is slowly opened for
allowing the crude oxygen generating from the HP column to chill down the LP
column. The procedure is continued until there is a built-up liquid level at the
sump of the LP pressure column. During this time the feed to the turboexpander
is decreased while slowly opening the HP expansion valve. Routine Purity
samples are taken during this time for confirming the established stream
purities – especially the purities of pure oxygen.
Ø After establishing the purities from the argon side column, it is put on
steam by opening the feed valve off of the LP column towards the argon
column.
Ø The ongoing streaming of the purity samples is continued and the empty
adsorption bed is regenerated during this time. The regeneration process can be
commenced as soon as there is 99% waste nitrogen in the column.
Ø Once liquid levels get controlled and design stream purities are
instituted and held to a fixed level, the transfer of the products, oxygen,
nitrogen and argon are transferred on to their distinctive storage tanks.
Importance of Cryogenic Air Separation
The cutting edge technology of cryogenic air
separating process possesses the capability to produce bulk quantities of the
products ranging from a moderate to a high purity level, in comparison to the
non-cryogenic systems. They are made up of membrane technologies with pressure
swing absorption that is utilized at the lower end of the production scale for
low purities. Also, Cryogenic ASUs control the bulk production of industrial
gases. They are electricity-intensive, which means large capacities of air
volumes are being crammed to high pressures through electric motor driven
compressors. As a result, fast but significant price fluctuations in
electricity are caused by the current deregulation of electricity markets. The process
economics of powerful manufacturing processes can be enhanced by the frequent
adjustment of the process outputs with respect to the price changes in
electricity.
The operational process can be further
improved by the advanced control and automation of the plant. The application
of advanced control has been incorporated in the few last decades. The first
utilization of the computer-assisted control system in an air separation unit
was employed in the early 1970s. Since those days, this proficient procedure
has been used to improve the efficiency, productivity, and utility of an air
separation plant.
Conclusion
After more than two decades, enterprises
have successfully dealt with a large number of technical issues related to
manufacturing and equipment design through the improvement of old techniques
and equipment, independent research, and technology introduction. This
technology has helped to increase efficiency and production to the capacity
that each ASUs in Indian steel industries produce nearly 1100 tonnes each day.
The huge investments and Joint Ventures to open Air Separation units have
increased tenfold. With the setting up of advanced ASU establishments, India
can finally take pride and boast about her budding industrial manufacturing and
gas industry taking off and reaching new heights.
No comments:
Post a Comment