The present invention relates to the removal of carbon monoxide and hydrogen as well as carbon dioxide and water from air for the production of high purity nitrogen gas.
In many chemical processes, CO and H.sub.2 are undesired species because of their chemical reactivity. For example, the electronics industry requires high purity N.sub.2 (less than 5 ppb.sub.v CO and H.sub.2) for the production of semiconductor materials. Impurities present in the N.sub.2 during formation of the silicon wafers greatly increase chip failure rates. When air is subjected to cryogenic separation to produce N.sub.2, since N.sub.2 and CO have similar boiling points, CO present in the feed air to the cryogenic column will essentially end up in the product nitrogen. If no special provisions are taken to the distillation process, hydrogen enriches in the product N.sub.2 to approximately two and a half times its concentration in the feed air. Therefore, the production of high purity N.sub.2, i.e. CO and H.sub.2 -free N.sub.2 for the electronics industry requires either 1) removal of CO and H.sub.2 from ambient air prior to the distillation column or 2) post-treatment of the product N.sub.2. Often times, electronics customers require both pre- and post-treatment for added system reliability.
The current techniques for trace CO and H.sub.2 removal involve catalytic conversion of CO to CO.sub.2 and H.sub.2 to water with subsequent removal of CO.sub.2 and water impurities (or mere hydrogen adsorption).
In the conventional process for cryogenic separation of air to recover N.sub.2 and O.sub.2, feed air is compressed, then cooled to low temperature before introduction to a two stage distillation column. Unless water and CO.sub.2 are removed from the air before compression, these components will block heat exchangers employed for cooling the gas prior to distillation. The principal method for such removal is thermal swing adsorption (TSA) on molecular sieve. In the TSA system for CO.sub.2 and water removal, atmospheric air is compressed to about 100 psig followed by water cooling and removal of the thus condensed water. Then the air, which is then about 100.degree. F. (38.degree. C.), can be further cooled to 40.degree. F. (4.5.degree. C.) using refrigerated ethylene glycol. The bulk of the water is removed in this step by condensation and separation of the condensate. The gas is then passed to a molecular sieve bed or mixed alumina/molecular sieve bed system where the remaining water and CO.sub.2 are removed by adsorption. The sorbent beds are operated in a thermal swing mode with equal periods, such as four hours (maybe as long as 24 hours), being devoted to adsorption and to regeneration. By using two beds, one is operated for adsorption while the other is being regenerated and their roles are periodically reversed in the operating cycle. During the regeneration, part of the product gas (N.sub.2) or waste stream from the cold box is slightly compressed with a blower and heated to about 260.degree. C. The hot gas is passed through the bed being regenerated, perhaps for two hours, following which the regeneration gas is cooled to typically 4.5.degree. C. for the final two hours, so cooling the bed to that temperature. Regeneration is carried out in a direction counter to that of the adsorption step.
Alternatively, a pressure swing adsorption (PSA) system may be used. In this case, cycle times are shorter (feed steps are 5-30 minutes), but feed temperature, pressure and regeneration gas remains the same. In the case of PSA, the regeneration gas is not heated.
Such a system is effective for the removal of CO.sub.2, water and C.sub.3+ hydrocarbons from atmospheric air. However, conventional molecular sieve beds are not effective for the removal of CO or H.sub.2 The main conventional technique currently used to produce CO-free N.sub.2 includes oxidation of CO present in the ambient air to CO.sub.2 prior to feeding to the molecular sieve system. This additional catalytic conversion system adds to capital and operating costs of a standard N.sub.2 plant. Alternatively, such CO removal steps may be applied to the nitrogen obtained after the air separation process.
In one known method CO is removed from nitrogen using a Ni on alumina catalyst. The principle disadvantages with this material are high cost, the need for activation in reducing gas and the pyrophoric nature of the activated material. Good adsorbents for trace CO removal should preferably be less expensive, easily regenerable and not pyrophoric.
The oxidation of CO to CO.sub.2 and H.sub.2 to H.sub.2 O in the presence of O.sub.2 occurs readily at high temperatures (above 500.degree. C.). These reactions can be carried out at lower temperature, about 150.degree. C., in the presence of noble metal catalysts based on palladium or platinum (Anderson, H. C. and Green, W. J., Ind. Eng. Chem., 53, 645, 1961). This technique is currently used as a pre-treatment step for ambient air prior to the front-end adsorption system for CO.sub.2 and water removal on a cryogenic air plant. The main disadvantages of this removal technique include 1) the need to heat the air prior introduction to the catalyst bed, 2) an extra heat exchanger and an extra booster heater are required that result in increased plot space, and 3) the added system pressure drop and heat duty increase the power requirements of the system.
Ambient temperature processes for the removal of trace impurities from inert gases are also known in the art. U.S. Pat. No. 4,579,723 discloses passing an inert gas stream through a catalyst bed containing a mixture of chromium and platinum on gamma alumina followed by a second bed composed of gamma alumina coated with a mixture of several metals. These beds both convert CO to CO.sub.2 and H.sub.2 to water and adsorb the resulting impurities to form a high purity product (less than 1 ppm).
U.S. Pat. No. 4,713,224 teaches a one step process for the purifying gases containing trace quantities of CO, CO.sub.2, O.sub.2, H.sub.2 and H.sub.2 O in which the gas stream is passed over a material comprising elemental nickel and having a large surface area. If there is oxygen present, CO is oxidised to CO.sub.2, otherwise it is adsorbed. The specification is rather vague as regards the nature of the substrate on which the nickel is supported, referring to it merely as a `silica-based substrate`.
Processes for the ambient temperature oxidation of CO to CO.sub.2 are given in U.S. Pat. Nos. 3,672,824 and 3,758,666.
U.S. Pat. No. 5,110,569 teaches a process for removing CO, CO.sub.2, H.sub.2 O and optionally H.sub.2 from a feed stream (particularly air) comprising 1) initially removing water and carbon dioxide, 2) catalytic oxidation of CO to CO.sub.2 and H.sub.2 to H.sub.2 O and 3) removing the oxidation products. The resulting gas stream may then be purified by cryogenic distillation.
It is suggested in U.S. Pat. No. 4,944,273 that CO can be selectively adsorbed by zeolites doped with metals such as Ca, Co, Ni, Fe, Cu, Ag, Pt, or Ru. Based on this property, it is proposed there to use such doped zeolites in CO sensors, e.g. for use in sensors monitoring automobile exhaust gas systems. However, no demonstration of selectivity is shown in that specification. Also, the highest capacity for adsorbing CO demonstrated is in connection with the Na form of zeolite ZSM 8 and no CO adsorption is shown when the Co form of ZSM 5 or the Ru form of ZSM 8 are tested. Since the units in which adsorption was measured appear to be mis-stated, it is impossible to tell what adsorption capacity in absolute terms these adsorbents were found to have. However, for the purposes of U.S. Pat. No. 4,944,273 it would appear to be the change in electrical properties on exposure to CO that the zeolite exhibits that is important rather than adsorption capacity.
U.S. Pat. No. 4,019,879 discloses the use of a zeolite containing Cu.sup.+ ions for adsorbing CO selectively. However, the CO is recovered for use as a reagent from gas streams containing large concentrations of it and there is no indication that such an adsorbent would be effective to remove ppm levels of CO from a gas stream.
U.S. Pat. No. 4,019,880 describes the adsorption of CO on zeolites containing silver cations. The CO concentration can be reduced below as little as 10 ppm CO.
Forster et al, `Spectroscopic investigations on sorption and oxidation of carbon monoxide in transition metal ion-exchanged zeolites A: Studies on cobalt, nickel and copper forms` Zeolites, 1987, Vol. 7, Nov 517-521, discusses the adsorption of CO on the zeolites referred to in its title. Capacity for adsorption at low ppm levels is not discussed.
U.S. Pat. No. 5,110,569 teaches a process for removing trace quantities of carbon monoxide and hydrogen from an air stream along with larger quantities of carbon dioxide and water as a prelude to cryogenic distillation. The process is conducted by TSA or PSA using a three layer adsorption bed having a first layer for adsorbing water (suitably alumina, silica gel, zeolite or combinations thereof), a second layer of catalyst for converting carbon monoxide to carbon dioxide (suitably nickel oxide or a mixture of manganese and copper oxides) and a third layer for adsorbing carbon dioxide and water (suitably zeolite, activated alumina or silica gel). The second layer may include a catalyst for converting hydrogen to water and this may be supported palladium.
It is not disclosed that any catalyst is capable of both oxidising carbon monoxide to carbon dioxide and adsorbing the carbon dioxide produced. Nor is it disclosed that supported palladium can be used to convert carbon monoxide to carbon dioxide. Equally, it is not disclosed that the supported palladium used for oxidising hydrogen is capable of adsorbing the water produced.
It is also not apparently the intention that the carbon dioxide present initially should be adsorbed prior to the oxidation of the carbon monoxide.
In FR 2739304, carbon monoxide is first oxidised to carbon dioxide and the carbon dioxide produced together with carbon dioxide and water present initially are then adsorbed using conventional adsorbents. Thereafter, hydrogen is adsorbed on palladium supported on alumina. Metals that can be used in place of palladium are Os, Ir, Rh, Ru, and Pt. It is stated that hydrogen is not oxidised under these conditions. This casts doubt on whether U.S. Pat. No. 5,110,569 is correct in stating that hydrogen can be oxidised on supported palladium or other precious metals.