Method and apparatus for the purification and recirculation of gases
This application is a continuation of application Ser. No. 092,460, filed Sept. 3, 1987, now expressly abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a method for the purification and recirculation of gases which are fed to a process plant, preferably a plant for the production of metal powder or a plant for compacting metal powder under elevated pressure. 
     The gases exiting the plant are at a reduced pressure and are conveyed in a closed loop of filters and pressurizing means for the mechanical and/or chemical purfication of the gases as well as for repressurization. In addition, the invention relates to an apparatus suitable for carrying out this method. Adsorbers and reactors for the mechanical and chemical purification of the gases may be provided as filters. 
     U.S. Pat. No. 4,629,407 discloses a plant for the production of metal powder which is equipped with a gas recycling system. In order to raise the purified gases to the required pressure for atomizing the metal powder (between 8 and 160 bar), a multi-stage compressor is required. The cost for this compressor is quite high as far as investment and operating costs are concerned, especially if the gases to be purified and recirculated must be brought to pressures which exceed 40 bar. 
     OBJECT AND SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved method for the purification and recirculation of process gases and an apparatus suitable for carrying out the method which is inexpensive as far as investment costs are concerned. 
     In the process according to the present invention, the recirculating gas exiting the plant is filtered or purified, then liquefied by the application of liquid nitrogen. The liquefied gas is then fed to an evaporator by a high-pressure pump where it is changed into the gaseous phase and returned to the plant. The plant, according to the invention, no longer requires the cost of a multi-stage compressor. For liquefying the gas liquid nitrogen is used as the refrigerant which is readily available and inexpensive. While the plant, according to the invention, has higher operating costs than a plant utilizing compressors, it operates substantially faster profitable since the cost ratio of liquid nitrogen to the process gas, e.g. argon, is approximately 1 to 8, as calculated on a gas basis, i.e. 0.84 kg of liquid nitrogen are required for liquefying 1 kg of argon. 
     For liquefying argon it is advantageous to keep the liquid nitrogen as a refrigerant in a liquefying device at a pressure above atmospheric pressure, preferably about 3 bar. At this pressure, liquid nitrogen has a boiling point of about 88° K. This temperature is above the solidification point of argon which is 84° K. while argon is liquefied at a temperature of at least 90° to 97° K. At these temperatures, a liquefication pressure of about 1.3 to 2.5 bar is required. At a liquid nitrogen temperature of 88° K., it is assured that the pressure does not drop below atmospheric pressure during the liquefication of the argon. Thus, the inlet pressure for the subsequent argon pump is always above 1 bar (abs), and no evaporation or cavitation occurs in the pump. 
     Further advantages and details of the invention will be explained by reference to one embodiment of the invention shown schematically in the drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing is an elevational schematic of the process of the present invention shown in a metal powder production plant. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the illustrated embodiment, the method of the present invention can be performed in a plant, to which an operating gas can be fed at high pressure and from which the gas emerges at a reduced pressure. The plant may, for instance, be a tower which serves for atomizing liquid metal for the production of a metal powder, or it may be an isostatic powder press. In the production of metal powder, high-pressure gas (between 8 and 160 bar) is introduced via a nozzle (not shown) which is located in the tower 1. In the nozzle, a melt dripping down is atomized with the gas. The heat of melting from the melt is given off to the gas in this process. As will be appreciated atomizing plants of this type are commercially known to those of ordinary skill in this art. 
     The hot gas which is loaded with metal powder impurities exits the tower 1 and flows into a filter system which consists of a cyclone 3, a cyclone bag filter 4 as well as a fine filter 5. Between the first cyclone 3 and filter 4 a valve 2 and a gas cooler 6 are located. The above described components 2 to 6 must be adapted to the plant 1 in question as required. It is essential that the operating gas, preferably argon in the case of metal atomizing plants, is free of dust after it leaves the fines filter 5. 
     The fines filter 5 is followed by a compression device 7 wherein the gas is elevated to a pressure greater than 1.3 bar. The compression device 7 is advantageously designed as a single-stage or two-stage Roots blower. The blower 7 is followed by the cooler 8 for removing the heat of compression. 
     The operating gas is transported to a liquefier 12 via line 9 through valve 11. Optimally, a gas purifier 10 can be connected in line 9 to remove impurities such as O 2 , CO 2 , CO, H 2 , H 2  O, N 2 . The description of such a purifier is found in U.S. Pat. No. 4,629,407, incorporated herein by reference. 
     The liquefier 12 comprises a liquefacient insert 14 located in a tank 13, the lower end of which leads to a collecting vessel 16. Liquid nitrogen is contained in the liquefier 12 and the liquid nitrogen level 17 is controlled by a level control (not shown) in such a manner that the liquefacient insert 14 is below the surface of the liquid. In order to prevent noncondensable gases from interfering with the liquefaction, a small portion of the argon to be liquefied is drawn-off with noncondensable components from the liquefier 12 to the atmosphere via valve 15. 
     The liquefier 12 is preceded by a cooler 18, through which operating gas line 9 and nitrogen line 19 are brought. Nitrogen evaporating in the liquefier 12 is removed through line 19. In cooler 18, the cold nitrogen cools the argon to be liquefied which flows through line 9 before it enters the liquefacient insert 14. A line 20 serves to feed the liquid nitrogen into the liquefier 12. 
     Liquefied gas is transported from the collecting tank 16 into a supply tank 22 by means of a pump 21. The pump 21 is designed as a centrifugal or reciprocating pump and is switched on only if the liquefied gas in the supply tank 16 exceeds a selected level. The supply tank 22 is maintained at elevated pressures, for example, 5 bar. 
     The liquefied operating gas is removed from the supply tank 22 by means of a high-pressure pump 23, elevated to the proper pressure, then fed to an evaporator 24. The liquefied operating gas is converted into a high pressure gas in evaporator 24 then stored in a tank 25 at the desired pressure. The gas is recycled to the plant 1 via line 26 through valve 27. 
     To ensure the operation of the pump 21 when argon is the operating gas, the pressure in the collecting tank 16 must not fall below atmospheric pressure. It is advantageous that a slight overpressure, i.e. a pressure between 1 and 1.5 bar (abs), prevail at this point. This assumes argon temperatures of about 87° to 91° K. Since liquid nitrogen has a temperature of about 77° K. at atmospheric pressure, the mentioned temperatures in the region of the collecting tank 16 can be obtained only if the nitrogen bath is under pressure, so that the boiling point is shifted to higher temperatures. An advantageous temperature has been found to be 88° K. if a temperature of about 91° K. is to prevail in the collecting tank 16. The liquid nitrogen in the tank 13 assumes a temperature of about 88° K. if the nitrogen bath is at a pressure of about 3 bar. Control mechanisms which assure the maintenance of this pressure and the pressure in the liquefacient insert 14 of the liquefier 12, are known in detail and are therefore not shown. 
     The invention in its broader aspects is not limited to the specific described embodiments and examples and departures may be made therefrom within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantages.