The purity of nitrogen, flow rate of the gas, and feed air temperature and pressure all affect the performance of the membrane separation system. By altering these variables, you can deliver nitrogen with precisely the right purity and flow for your applications.
Nitrogen purity can be regulated (up to a maximum of 99.9%) by adjusting the input air pressure and temperature. In most cases, the greatest system efficiency is achieved at purities of 95-99%, levels adequate for most cleanroom applications. See Figure 1.
The flow rate of the enriched nitrogen product is a function of the purity required. The higher the nitrogen purity required, the lower the flow rate produced, assuming constant feed air temperature and pressure. To produce a higher purity nitrogen product, more oxygen must permeate the membrane. You increase oxygen permeation by increasing the pressure difference across the membrane. This transports more gas, including more of the total oxygen content of the feed air, through the membrane. With more oxygen permeating the membrane, the oxygen level in the nitrogen product gas is reduced and purity improved.
Feed air temperature also affects the performance of the system. The higher the feed air temperature, the higher the feed air flow rate required, assuming constant air pressure and nitrogen purity. As feed air temperature rises, the membrane permeability increases, requiring an increased feed air flow rate to maintain product flow. However, feed air temperature also affects the membrane material. High feed air temperature shortens the life of the membrane. See Figure 2 for the relationship between feed air temperature and flow rate.
When the feed air temperature increases, the feed air flow required rapidly escalates. Therefore, the system must be designed to provide required feed air flow rate of the maximum anticipated feed air temperature, or an air conditioner must be used to control the feed air temperature.
Dew point is the temperature at which a given mixture of water vapor and gas is saturated. The dew point and trace contaminants of the nitrogen-enriched product gas are dependent on the water level and quality of the feed air. Operating in the purity range of 95-99% nitrogen, saturated feed air results in a product gas that contains less than 5 ppm water, depending on feed air conditions. The atmospheric dew point equivalent is -65°C (-85°F).
Water level in the feed gas is dependent on temperature and pressure. Therefore, if the feed air pressure is reduced, the dew point of the product may increase. If the temperature is increased, the feed dew point increases and the product stream dew point increases. Changes in dew point are minimized by using a refrigerated air dryer to condition the feed air.
Carbon dioxide in the product stream is typically less than 0.01% operating at 99% nitrogen, 135 psig and 25°C, based on feed air containing .03% carbon dioxide.