Capacity: The plant is designed to generate 42,000 SCFH or 36.5 TPD nitrogen gas, at <3 ppmv impurity, at 110 psi(g) pressure. Impurities less than 3 ppmSCF Measured at: 14.7 Psi, 70°FNominal Flow Rating 400 Nm3/hr (15,220 SCFH) Pressure Range 5-10 bar(G) / (73-145 psig)Ambient Conditions: DB = 70°FWB= 60°FBAR=13.25 PSIALast Run: 2004
Power Consumption: Indicative only: 400kW, all inclusive, There are other filterpresses installed but unused (PP 1000x1000, 800x800, 630x630, Setz Orion 316SS)
The process used to separate air into the crude oxygen and pure nitrogen products of the plant consists of using low-level refrigeration to liquefy the air and then using column distillation to separate the nitrogen. Before the air is cooled to liquefaction, water and carbon dioxide are frozen out in the HE-21 and HE-22 Reversing Exchangers. Hydrocarbons are removed in the D-24 Adsorber.
Refrigeration requirements are highest during plant cool-down, when it is necessary to remove heat from the plant and build a liquid inventory. After the plant is cooled down, the only refrigeration required is that which is necessary to compensate for the warm end temperature differences at the exchangers, heat leakage into the plant, and the heat of reactivation.
REFRIGERATION BY EXPANSION
Refrigeration for the process is obtained by compressing a gas, then converting the work done to compress the gas into refrigeration. This is done by expansion (dropping the pressure on the gas).
By one method, air is expanded through an expansion turbine where it is made to do work. By doing work, the air or gas gives up some of its energy, which lowers its temperature and heat content.
A second method of expansion is Joule-Thomson expansion, in which the pressure of a gas is dropped by restricting its flow. Ordinarily a valve is used to restrict the flow so that the pressure drop can be controlled. However, this type of expansion also occurs throughout the plant due to friction losses in the piping. Joule-Thomson expansion does not lower the heat content or energy of the gas, but it does drop the temperature of the gas so that the gas can be used to cool a warmer stream.
A third type of cooling occurs when a liquid is expanded to a lower pressure. In this case, cooling occurs because the boiling point of a liquid is lowered when the pressure on it is dropped. If the liquid is at its boiling point when it is expanded, some liquid immediately flashes (vaporizes) to cool the rest of the liquid to the new boiling point. FLOW DESRIPTION
Air is filtered through F-10 Inlet Air Filter, compressed by CP-11 Air Compressor, passes into T-12 Surge Drum and into HE-21 and HE-22 Reversing Exchangers. As the air passes through the exchangers it is cooled to almost saturation temperature, and water and carbon dioxide are frozen out and deposited in the exchangers. The cooling media in the exchangers are the waste crude oxygen, product nitrogen, and reheat crude oxygen streams.
From the exchangers, the air passes through the D-24 Adsorber and into C-25 Rectification Column. Some of the air is withdrawn and liquefied by heat exchange against product nitrogen, expander exhaust, and re-heat crude oxygen in HE-23 Air Liquefier and is then introduced at tray 7 in the C-25 Rectification Column. The balance of the air entering the column passes up through the column trays and boils up the liquid in the trays. The net effect of the boil-up is that the more volatile nitrogen is boiled out of the liquid, and the exchange of latent heat condenses the oxygen out of the air passing up through the trays. As boil-off continues up through the column, the vapors increase in nitrogen purity until finally the vapors leaving the top tray of the column are pure nitrogen vapors.
Most of the pure nitrogen vapors pass up through the RB-31 Re-Boiler Condenser, where they condense by boiling the crude oxygen in the condenser section of the column. The remainder of the pure nitrogen vapors are withdrawn from the HP column as product gas. The product gas is piped through the HE-23 Air Liquefier, where it is used to sub-cool the crude oxygen stream and liquefy the air feed stream, and is then warmed to approximately ambient temperature in HE-21 and HE-22 Reversing Exchangers. From the exchangers, the product gas passes to the delivery point. Most of the condensed nitrogen flowing from the RB-31 Re-Boiler Condenser is returned to the column as reflux. The liquid nitrogen returned to the HP column as reflux is used to condense oxygen vapors rising up through the column and to replenish the liquid in the trays down through the column. As the overflow of liquid from the trays spills back down through the column, nitrogen is boiled out of the liquid and oxygen is condensed. This results in an increasing oxygen concentration in the trays down through the column.
The crude oxygen liquid collected in the sump of the HP column is transferred to the condenser section of the column to submerge the RB-31 Re-Boiler Condenser. The column condenser is kept at a lower operating pressure than the HP column so that when the liquid is expanded as it enters the condenser, its temperature drops to the boiling point corresponding to the lower pressure. This drop in temperature provides the temperature difference across the condenser required to condense the nitrogen vapors.
The boil-off occurring in the condenser becomes the waste oxygen stream of the plant. This stream is passed back through the HE-23 Air Liquefier and back through the reheat passages of HE-22 Reversing Exchanger to keep the proper temperature profile in the exchanger for clean-up of CO2 and water. The reheat stream is warmed to about -155 degrees F in the reheat passages and then sent through the expander, where it is cooled again.
After passing through the expander, the waste oxygen stream is warmed in the HE-23 Air Liquefier and is then directed back through the switching passages of HE-21 and HE-22 Reversing Exchangers. Passing through the exchangers, the waste oxygen picks up the CO2 and water and is vented from the plant to the atmosphere.
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