1. Field of the Invention
The present invention is generally directed to an improved apparatus and method for intermittent manufacturing nitrogen gas, separated from air by using a permeable membrane in an automatic and unattended process. Particularly, the improved method and device adds a gas flow measuring device in a gas conduit downstream from the permeable membrane. The gas flow measuring device indirectly monitors nitrogen gas purity by measuring normal gas flow and detects deviation from desired nitrogen purity levels by measuring changing gas flow from the permeable membrane. The nitrogen and vent gas flow rates from the permeable membrane relate to the nitrogen purity so monitoring either or both the nitrogen and/or vent gas flow indirectly monitors the nitrogen gas purity. The automated process allows non-skilled people to use nitrogen gas from an automatic nitrogen producing apparatus. These people need assurances that the nitrogen gas meets purity requirements.
2. Description of the Prior Art
Nitrogen manufacture from air by separating the oxygen and nitrogen has been accomplished by selective absorbent materials, distillation of liquid air, and membrane separation. These processes produce nitrogen for industrial uses such as chemical manufacture, inert gas welding, purging of explosive environments prior to electric arc cutting or welding, and food preservation. Also, these processes are mostly continuous nitrogen production to the industrial process or to continually fill large storage containers. The prior art is referenced in U.S. Pat. No. 5,588,984.
This U.S. Pat. No. 5,588,984, describes a method and apparatus to intermittently manufacture and dispense nitrogen. Air is filtered, compressed, and enters a module containing a permeable membrane that selectively separates nitrogen from the air and discharges vent oxygen and other gases. Automated temperature and pressure monitors allow the permeable membrane to separate air components. A discharge hose permits use of the nitrogen product for a variety of intermittent applications including vending for inflation of tires, filling portable nitrogen vessels, and use in manufacturing processes.
During the use of this apparatus, debris, moisture, or oil collecting in the membrane may affect the permeable membrane efficiency of separating nitrogen from air. The membrane may need cleaning or other possible malfunctions from the air compressor, temperature, or pressure controls would change the apparatus efficiency or nitrogen purity. In large production and continuous manufacturing of nitrogen for storage or commercial use, a direct purity analysis method uses oxygen analyzers to indicate and monitor nitrogen purity. These oxygen analyzers directly determine the percent of oxygen, using electrochemical fuel cells thereby relating nitrogen purity to oxygen purity. One oxygen analyzer, manufactured by Teledyne, uses an expensive replaceable cell, needs monthly calibration, and is warranted for only six months while having a one year life. The analyzer is not rugged and would present a reliability problem for intermittent nitrogen production. An information sheet concerning these analyzers is enclosed. Oxygen gas analyzers usually require monitoring by manufacturing personnel to maintain the purity of nitrogen in constant flow nitrogen manufacture.
For automatic, intermittent operation by users of nitrogen, unfamiliar with the apparatus, a method and device for signaling an alarm and/or automatic shutdown of the nitrogen manufacturing apparatus is needed if the nitrogen purity is unacceptable. To replace the direct oxygen purity analysis equipment, an indirect monitor device using simple, reliable, and long life flow indicators, pressure monitors, and/or flow switches are used in intermittent apparatus operation down stream of the permeable membrane to monitor the purity of nitrogen gas. These gas flow devices do not require maintenance/and last appreciably longer than one year.