The purity of the nitrogen that is generated by the membrane system will be dependant on three controlled system variables, the general setting of which will be determined by the overall configuration of the generator package. The size and pressure setting of the low pressure feed air compressor and the capacity of the high pressure booster compressor determine the basic model of nitrogen generator. The three controlling variables that determine or fine tune the actual purity coming from the system are membrane pressure, membrane temperature, and membrane product flow and one should consider the effect of each separately.
The purity of the product being removed is directly proportional to the pressure of the air supplied to the membrane. The higher the pressure supplied (within design limit), the higher the purity of the nitrogen generated. This is normally controlled by the pressure setting of the low pressure feed air compressor. The outlet pressure of most LP compressors will vary slightly due to pulsation and the hysterises of the pressure controlling mechanism however, so a pressure regulator is inserted between the LP compressor and the membrane inlet, and set below the lower hysterisis limit to supply a steady state pressure to the membrane bundle. In general, increasing this setting will increase the nitrogen purity, and decreasing this setting will drop the nitrogen purity. Please keep in mind that increasing the setpoint to much may allow the pressure fluctuations of the LP control mechanism hysterisis to pulse the membrane fibers which would be detrimental to membrane performance.
Like the effect of system pressure, the product purity is directly proportional to the temperature of the membrane. The higher the air temperature supplied to the membrane the higher the product purity. For this reason, a heater unit is mounted directly in front of the membrane bundle. From a design perspective, this heater increases the air temperature slightly to assure a non saturated air supply (less than 100% humidity) to the membrane so liquid water that has developed during the low pressure compression and cooling in the feed air compressor does not clog the separators fibers. This heater is also used to maintain an acceptable membrane temperature for proper purity and a higher setpoint will, to a degree, produce a higher purity.
Caution: In the event that a low purity condition occurs, the setpoint of the heater and proper heater function should be verified. It is unwise however to increase the heater setpoint above design parameter to effect a purity increase as increasing the membrane temperature will cause excessive permeate to flow from the membrane permeate port and this will only lead to a drop in membrane pressure so no net purity benefit will be obtained. Membrane over temperature however will cause permanent damage.
Unlike the effects of pressure and temperature, the flow rate of gas through the membrane bundle is inversely proportional to the product purity. The higher the flow rate, the lower the purity. The general flow rate from the membrane is controlled by the capacity of the booster compressor via its size or rotational speed however like most compressors, the capacity varies with inlet pressure. Mounted on the outlet of the membrane bundle, the booster compressor inlet pressure regulator controls this variable. Increasing this regulators setpoint will force the booster to pump more gas, increasing the capacity of the generator but decreasing the purity of the gas being produced. Decreasing the setpoint of this regulator will lower the capacity of the generator and increase the purity of the product. Caution Some generators are equipped with boosters that accept pressures around standard atmosphere and some system may take up to 10 bar and beyond inlet pressures. Close attention should be paid to the nominal setpoint from the operating manual as too high an inlet pressure will overload the booster causing high power draw and relief valves to open, while too low an inlet pressure may cause excessive compression ratios and overheating which may lead to high oil consumptions.
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