NITROUS OXIDE GAS PLANT
Nitrous Oxide is produced by heating Ammonium Nitrate to a temperature of 250 Deg.c. Ammonium Nitrate is decomposed in the process to give a mixture of Nitrous Oxide & super heated steam. At the same time impurities are produced including Ammonium Nitrate fumes, Nitrogen & other oxides of Nitrogen. The steam & impurities are removed by scrubbing with water, caustic soda and sulphuric acid in sequence Nitrogen present in traces is removed by bleeding from the top of storage vessels where Nitrous Oxide is stored, after compression by a compressor.
DESCRIPTION OF PROCESS :
Nitrous Oxide (N2O) is manufactured by thermal decomposition of Ammonium Nitrate.
NH4NO3 —————› N2O (Nitorus Oxide) + 2H2O (water)
Ammonium Nitrate is melted and decomposed by reaching to a temperature of approximate 250ºC to 260°C to give a mixture of Nitrous Oxide and super heated steam as per above reaction. At the same time impurities are produced including Ammonium Nitrate fumes, Nitrogen and Oxide of Nitrogen. The steam and impurities are removed by scrubbing in turn with water, caustic soda and finally acid scrubber.
From the decomposition pot the hot gas including impurities enters the primary scrubber. The scrubber is fitted with a small condensing coil mounted at its top to condense water from part to the effluent gas stream and this water provides a counter-current flow for recovery of ammonium nitrate fumes from the gas. N2O gas leaving the primary scrubber passes to a stainless steel water scrubber for the removal of ammonium nitrate fumes.
Finally, the gas passes through a mild steel caustic soda scrubber followed by a fiber glass/stainless steel acid scrubber. The circulation of caustic and water to the respective scrubber is maintained by means of pumps. The scrubber gas is then allowed to collect in a rubber gasholder (balloon). The gas from the gasholder is sucked by a multi stage water lubricated compressor, which compresses the gas to approx. pressure of 65 kgs/cm2 depending on ambient temperature.
The compressed gas passes through a gravity type water separator before being dried in Alumina Drier Battery. Finally Nitrous Oxide in liquid form is collected in a bank of three storage vessels. The storage vessels are kept cool by spraying chilled water on them.
During storage the impure nitrogen gas is being removed by venting out the gas after cooling down the storage vessels. The pure liquid nitrous oxide is then filled into cylinders by weight. For this, the cylinders are to be filled by keeping them on a weighing scale. Nitrous Oxide is a liquefiable gas and is stored in cylinders in liquid form only.
DESCRIPTION OF PLANT EQUIPMENTS
Melting and decomposition pot: Heating and decomposition of Ammonium Nitrate is carried out in the interconnected aluminum alloy pots made of Alloy 20 material, which are heated by electrical heaters. 1st pot is called melting pot charged with NH4NO3 (Ammonium Nitrate) manually. The rear pot is called decomposition pot, which produces N2O gas along with other impurities like ammonium nitrate fume, nitrogen and oxides of nitrogen.
- Primary scrubber: The hot gas from the decomposition pot containing about 1% undercomposed Ammonium Nitrate in the form of fume is recovered by passage through a stainless steel tower called primary scrubber mounted on the decomposition pot. The primary scrubber is packed for almost its full length with 12 mm, size porcelain rings. To avoid blockage by carry of un-decomposed Ammonium Nitrate from the pot, the bottom portion of the scrubber is packed with 25 mm size porcelain lessing rings for a length of approx. 100 mm. A portion of the main gas stream enters the condensing coil and steam is condensing to provide water for reflux. The rate of the gas flow to the condensing coil is automatically controlled by an orifice plate fitted in the main outlet line. With normal operation at a specific generation rate of approx. 8000 liters per hour per set of pots. The temperature at the top of the primary scrubber will be in the region of 100 – 110ºC. In case the above temperature is exceeded recovery of Nitrate fume will be reduced and the purity of gas will deteriorate. The restriction of the Primary Scrubber is indicated by the inclined tube manometer. The scrubber should be periodically dismantled and rings should be washed with caustic soda solution and finally rinsed with water and dried.
- Water Scrubber: The scrubber is fed with water saturated with Nitrous Oxide from overhead tank via condensing coil tank. The flow rate is controlled at 200 – 250 Ltrs., per hour Under normal operation the scrubber will be cold expect for a length of 150 mm from the bottom, the effluent water having a temperature of approx. 50o C.
- Caustic Scrubber : The gas stream coming out from the water scrubber is passed through a mild steel caustic scrubber packed with 25 mm MS lessing rings in which caustic soda solution containing potassium permanganate is circulated to remove residual traces of Ammonium Nitrate, Carbon-di-Oxide and Oxides of Nitrogen. Potassium Permanganate is added to the caustic solution to remove Nitric Oxide from the process stream by its oxidation and final absorption in caustic.
- Caustic Circulation and Pump and Tank: As described above caustic solution is circulated through the caustic scrubber by a pump. The circulation rate of caustic is maintained approx. 400 to 500 liters per hour.
- Acid Scrubber: The processed gas steam coming out from caustic scrubber passed through 1% Sulphuric Acid solution (battery water) which is sprayed in FRP/SS tower packed it with 25 mm PVC lessing rings.
- Rubber Gas Holder (Balloon): The processed gas after acid scrubber is collected in the rubber gas holder.
- Gas Compressor: The processed gas collected is being sucked by water lubricated Nitrous Oxide Compressor and compressed to a pressure of approx. 80 Kg/cm2 depending on ambient temperature.
- Water Separator : This is a gravity type separator kept after the gas compressor from which water is drained out meticulously every 10 to 15 minutes.
- Alumina Drier with regeneration arrangement: The compressed gas devoid of free water is now dried in a battery of alumina dryer. The battery consists of three vessels the two vessels being used alternately while the third one will serve as a guard drier. One of the two dryers when is in circuit the other one will be generated by hot air supplied from a blower and heater.
- Liquid Nitrous Oxide Storage Vessels: The dried liquid Nitrous oxide is collected in one of the storage vessels kept cool by spray of chilled water on the same. While one of the vessels is under filling, the stored Nitrous Oxide from the outlet of the other one is fed to the service system.
- Filling Unit: A complete set of filling unit is supplied to fill small and bulk cylinders from the storage vessel through Nitrous Oxide Pump.