NL72707C - - Google Patents

Abstract

<PICT:0644371/III/1> <PICT:0644371/III/2> Nitrogen dioxide (NO2) is recovered in concentrated form from a dry gas mixture containing nitrogen, oxygen and a small amount of NO2, by passing the mixture over dry adsorbent particles and heating the mass of NO2-containing solid adsorbent particles to desorption temperature by forcing through the mass a current of pure NO2 maintained at desorption temperature and recovering the NO2 so desorbed. In applying the process to the production of NO2 in concentrated form from a gaseous mixture comprising water vapour, oxygen and nitric oxide (NO) in low concentration, such as that produced in the fixation of atmospheric nitrogen, e.g. by the process of Specification 598,534, or by the oxidation of ammonia, the gas mixture is cooled in a tower B (Fig. 1) by a water or calcium nitrate solution spray, dehydrated in a bed E of a solid adsorbent, passed into a catalyst chamber F, wherein the NO content of the gas is oxidized to NO2 by contact with an adsorbent saturated with NO2, and the resulting NO2-containing gas is cooled at G before passing to an adsorber H containing solid adsorbent, the adsorbed NO2 being subsequently desorbed and recovered and the adsorbent being prepared for re-use. The adsorbent used in E, F and H may be silica gel, the residues resulting from acid leaching vermiculite or zeolites &c., alumina gel, titania gel or activated carbon, and may have cooling coils embedded therein. In the desorption step of Fig. 1, a mixture of air and NO2 is withdrawn from one end of adsorber H and circulated by pump J through a heat-interchanger I back into the opposite end of H, the transference of heat in this way, liberating about 70 per cent of adsorbed NO2, is continued until the bed has reached 165 DEG C., when pure (98 per cent) No2 is withdrawn and condensed or passed into a water tower for production of nitric acid, the NO produced being returned at E to the adsorption process. Further (25 per cent) NO2 is desorbed from H by circulating air, drawn through a drying bed R, in the circuit J, I, H and the gaseous mixture withdrawn therefrom is converted into nitric acid. Bed E is then dried by pumping air through R, I (at 180 DEG C.), valve 4 and E, and is cooled simultaneously with bed H by passing compressed air through R, G, H and E, venting to the atmosphere through valve 6. In continuous operation, a plurality of drying beds E and adsorption beds H are used, one bed each of E and H being operated in circuit while the others are being treated for re-use. In the embodiment shown in Fig. 3 comprising four communicating chambers U, V, W, X, through which finelydivided adsorbent particles are continuously being showered, the cool, dry gas mixture, after being oxidized, is passed into chamber U in counter-current flow to the adsorbent particles which remove the NO2 content of the gas. During its fall in chamber V, the NO2-laden adsorbent is heated to 165 DEG C. by contact with NO2 circulated through a pump J and heater I, and desorbed NO2 is bled off to a point for use. Further desorption occurs as the adsorbent descends through chamber W by contact with air (at 165 DEG C.) in countercurrent flow, and finally the adsorbent is cooled in chamber X by cool dry air and returned to a hopper Y. In a further modification (Fig. 4, not shown), desorption is effected without the use of a pump. The Specification as open to inspection under Sect. 91 comprises also effecting adsorption under pressure and desorption by using steam instead of hot NO2, by evacuation, or by passing a heating fluid through coils embedded in the adsorption beds, and using two adsorption beds (H) in series. This subject-matter does not appear in the Specification as accepted.