Production of nitric oxide gas mixtures

Abstract: A method of producing a stream of nitric oxide-containing gas suitable for treatment of medical conditions, comprising the steps: (a) contacting a feed gas comprising oxygen and nitrogen, oxygen and ammonia or oxygen, nitrogen and ammonia with a catalyst which effects the reaction of ammonia and oxygen to form nitric oxide at a temperature in the range of about 300 to about 1200°C, thereby producing a gas mixture containing nitric oxide and nitrogen dioxide; (b) passing said gas mixture through a bed of adsorbent which preferentially adsorbs nitrogen dioxide, thereby producing a substantially continuous stream of substantially nitrogen dioxide-free product gas.

Description: This invention relates to the production of nitric oxide gas mixtures and more particularly to a portable apparatus and processes for the production of nitric oxide-oxygen-nitrogen gas mixtures for use in medical applications. Nitric oxide has recently been found to play an important role in life processes in humans and animals. For example, it helps maintain blood pressure by dilating blood vessels, and kills foreign invaders in the body's immune system. Studies indicate that extraordinary benefits may be obtained by administering small dosages of nitric oxide to patients who suffer from certain illnesses or diseases. Of particular interest is the prospect of reducing pulmonary vasoconstriction in paediatric patients with congenital heart disease complicated by pulmonary artery hypertension by having the patients inhale oxygen-enriched air containing very small concentrations of nitric oxide. Nitric oxide is a relatively stable gas when it is in the pure state or mixed with an inert gas, such as nitrogen or argon. However when it is mixed with oxygen it reacts rapidly with the oxygen to form nitrogen dioxide, a substance that is highly toxic to humans. The nitrogen dioxide reacts with water to form nitric and nitrous acids, which, when inhaled can cause severe pulmonary oedema, acid pneumonitis or even death. Because of the highly toxic character of nitrogen dioxide, nitric oxide that is intended for inhalation use by humans is generally purified to remove any nitrogen dioxide that is initially in the nitric oxide product as a result of the manufacturing process, and the purified product is stored and shipped in an oxygen-free environment to prevent the subsequent generation of nitrogen dioxide in the storage or shipping container. Nitric oxide is generally administered to a patient by diluting a nitrogen-nitric oxide concentrate gas containing about 1000 ppm nitric oxide with oxygen or oxygen-enriched air carrier gas to produce an inhalation gas containing nitric oxide in the desired concentration range (usually about 0.5 to 200 ppm, based on the total volume of the inhalation gas). The concentrate is generally provided in large cylinders which are cumbersome and inconvenient for a patient to use in his home or while he is travelling. U. S. Patent No. 5,396,882 discloses a device for the generation of nitric oxide by subjecting a stream of air to an electric arc discharge. This method involves the use of complex and expensive equipment and produces the nitric oxide on an intermittent basis. Furthermore, the electric discharge also results in the production of ozone, which must be removed from the gas mixture prior to administering the gas to the patient. The invention is concerned with the provision of a portable apparatus and methods for the production of a continuous stream of a gaseous mixture of nitric oxide without producing ozone as a by-product. In accordance with the invention, a continuous stream of nitric oxide-containing gas for use as an inhalant by medical patients is generated at the site at which it is to be used. In a broad embodiment of the invention, the nitric oxide-containing gas is produced by contacting a mixed stream comprising oxygen and nitrogen or oxygen and ammonia or oxygen, nitrogen and ammonia through a bed of catalyst at a temperatures of at least 300°C and up to about 1200°C, the catalyst being effective to cause reaction between the oxygen and one or both of nitrogen and ammonia to produce nitric oxide. The gaseous product exiting the catalyst bed is subjected to a purification step to remove nitrogen dioxide therefrom. The process is preferably carried out on a substantially continuous basis to produce a substantially continuous stream of nitric oxide-containing gas which is substantially free of nitrogen dioxide and contains a substantially constant concentration of nitric oxide. The nitric oxide synthesis reaction is preferably carried out at a temperature of at least about 500°C, and is most preferably carried out at a temperature in the range of about 500 to about 1200°C. In a preferred embodiment the reactant gas is air. In another preferred embodiment the reactant gas is a combination of an oxygen-containing gas, preferably selected from air, oxygen-enriched air, nitrogen-enriched air, or substantially pure oxygen, and ammonia. When the reactant gas is a combination of oxygen-containing gas and ammonia, the synthesis reaction is carried out under conditions that effect the substantially complete reaction of the ammonia in the reactant gas. The nitric oxide synthesis catalyst is preferably a Group VIII noble metal catalyst, preferably a noble metal catalyst, such as platinum, palladium, iridium, rhodium or combinations of these noble metal catalysts. The Group VIII noble metal catalyst can be used in combination with other Group VIII metals, such as nickel, cobalt or iron. The gas exiting the portable nitric oxide synthesis reactor is treated to remove any nitrogen dioxide formed during the synthesis reaction. This is accomplished by adsorption using an adsorbent which preferentially adsorbs nitrogen dioxide from the nitric oxide synthesis gas. In the most preferred embodiment, the nitric oxide purification is carried out by adsorption using an adsorbent selected from silica gel, hydrophobic zeolites or mixtures of these. The purified nitric oxide-containing gas may be blended with air or oxygen to produce a gas mixture of the desired nitric oxide concentration. The invention provides a convenient method for providing a stream of air or oxygen-enriched gas containing a therapeutic amount of nitric oxide at the site at which it is to be used. The process can be practised, for example, in a hospital room or in a patient's home. For a better understanding of the invention, reference will now be made, by way of exemplification only, to the accompanying drawing which illustrates apparatus for carrying out a preferred embodiment of the invention. Only equipment and lines necessary for an understanding of the invention have been included in the drawing. The principal equipment units used in the system are a portable nitric oxide reactor, A and a portable nitric oxide gas purifier, B. Reactor A is any portable reactor vessel suitable for carrying out high temperature chemical reactions. Reactor A is packed with a noble metal catalyst of the group described above. Reactor A is provided with feed gas inlet line 2 on its inlet side and nitric oxide product gas line 4 on its outlet side. Feed gas line 2 may be connected to a source of air or other suitable oxygen- and nitrogen-containing gas or a gas mixture containing oxygen and ammonia, and optionally nitrogen, e.g. an air-ammonia gas mixture. Alternatively, a source of ammonia gas may be separately provided to the system via line 6. Line 6 may be attached to line 2, as illustrated, or it may be directly connected to reactor A. Heating means is also provided to heat the reactant gas or the catalyst bed to a temperature sufficiently high to effect the desired nitric oxide producing reaction in reactor A. As shown, the heating means takes the form of heat exchanger 8, which may be a furnace or other suitable heat exchange means. Alternatively, reactor A can be provided with heating means, such as an electrical resistance or induction heating coil. The nitric oxide product gas line 4 connects the outlet of the reactor A to cooler 10, which serves to cool the product gas to a suitable temperature, which is not critical but is generally in the range of about 0 to about 100°C. The product gas outlet end of cooler 10 is connected to the inlet end of purifier B. Purifier B is any portable gas adsorption unit, which contains an adsorbent substance which more strongly adsorbs nitrogen dioxide than nitric oxide. As noted above, gas adsorption units are used in this invention, because of their ability to adsorb nitrogen dioxide using a dry, inexpensive, particulate adsorbent material. These materials are easily adaptable for use in portable units and the adsorbent can be easily replaced when it becomes spent. The adsorbent used in this embodiment can be any substance which more readily adsorbs nitrogen dioxide than nitric oxide. Suitable adsorbents include molecular sieves of the FAU, MFI and MEL type structures, including zeolites that have been made alumina-deficient by dealumination, and molecular sieves that are directly synthesised without introducing alumina groups into the lattice structure. These alumina-deficient molecular sieves include dealuminated type Y zeolite (DAY), ZSM-5, ZSM-11 and ZMS-20, all having silicon to aluminium atomic ratios of at least about 100. Preferred zeolites of this class include aluminium-deficient zeolite 5A, alumina-deficient zeolite 13X, aluminium-deficient zeolite 4A and aluminium-deficient type Y zeolites. Most preferred zeolites are aluminium-deficient type Y zeolite, ZSM-5 zeolite, ZSM-11 zeolite, ZSM-20 zeolite and mixtures of these. Details of such adsorbents and their use in purifying nitric oxide are described in US Patent Specification No. 5,417,950, the contents of which are incorporated herein by reference. Adsorbents which are also useful in the invention include metal cation-free adsorbents, including silica gel, alumina, and metal cation-free synthetic zeolites, such as types A, X and Y zeolites, and natural zeolites, such as mordenite, faujasite, chabazite, etc. Preferred adsorbents of this type include silica, alumina and types A, X and Y zeolites. The most preferred adsorbent of this category is silica gel. The terms "metal cation-free" and "substantially free of metal cations" when use in reference to an adsorbent, mean that the adsorbent contains no more than trace amounts of metal cations. These adsorbents and their use in the purification of nitric oxide are disclosed in U S Patent Specification No. 5 514 204, the contents of which are incorporated herein by reference. Although the adsorbent in purifier B can be regenerated by subjecting it to pressure reduction or temperature increase, it is generally easier and more cost effective to discard the spent adsorbent and replace it with fresh adsorbent. Since the amount of nitrogen dioxide removed from the nitric oxide is very small, for example usually in the range of about 1 to about 100 ppm, the adsorbent will generally have a useful life of many months. Water vapour in the product gas stream will generally be adsorbed onto the adsorbents. The nitrogen dioxide reacts with adsorbed water to form nitric acid or nitrous acid, which may destroy the microstructure of the adsorbent and shorten its useful life. Accordingly, it is preferable to remove moisture from the nitric oxide product gas prior to adsorptive removal of nitrogen dioxide therefrom. This can be accomplished by inserting a dryer bed upstream of the nitrogen dioxide adsorbent. Any conventional drying agent, such as alumina, can be used in the dryer bed. The outlet end of purifier B is connected to a product gas line 12. In the embodiment illustrated in the drawing, oxygen blending line 14 and moisture injection line 16 are connected to line 12. In the process of the invention as practised in the system illustrated in the drawing, a feed gas mixture, which may be an oxygen- and nitrogen-containing gas, such as air, or a mixture of ammonia and an oxygen-containing gas, enters the system through line 2. Alternatively, ammonia gas may be separately introduced into the system through line 6. The mixture is passed through heat exchanger 8, where it is heated to the desired reaction temperature, for example about 500°C. The heated gas mixture enters reactor A and passes through the bed of catalyst contained in the reactor. As the hot gas mixture contacts the catalyst the ammonia and oxygen react to form a mixture of nitrogen oxides, the mixture being comprised predominantly of nitric oxide and nitrogen dioxide. The temperature in reactor A and the period of time that the reaction gas mixture remains in contact with the catalyst is desirably such that substantially all of the ammonia in the feed is converted to nitrogen oxides. It can be appreciated that the quantity of nitric oxide in the product gas on will depend, to a great extent, upon the amount of ammonia in the gas, and to a lesser extent, on the amount of nitric oxide produced by reaction of nitrogen and oxygen. Under the conditions prevailing in reactor A, ammonia and oxygen react much more readily than nitrogen and oxygen, and since the reaction between ammonia and oxygen is quantitative, the amount of nitrogen oxide in the product gas can be controlled by regulating the concentration of ammonia in the feed gas. The product gas, comprised predominantly of air, oxygen-enriched air or oxygen, and containing nitric oxide and nitrogen dioxide, leaves reactor A through line 4 and next enters separator B. As noted above, separator B is an adsorber. These units are all well known and need not be described in detail. The separator is an adsorber unit, since adsorption avoids the need for liquid handling or complex and expensive equipment. As the product gas passes through separator B substantially all of the moisture and nitrogen dioxide is removed from the gas. If a hydrophobic adsorbent, such as dealuminated type Y zeolite, is used in separator 8, moisture will not be substantially removed from the product gas. The gas exiting separator B, now substantially free of nitrogen dioxide, can be diluted with air or oxygen, if desired. This is accomplished by introducing the air or oxygen into the gas through line 14. Moisture can be introduced into the product gas by means of line 16, which may consist of a water spray or other suitable means. Alternatively, moist air can be introduced into the product gas through line 14, in which case line 16 is unnecessary. Since oxygen reacts rapidly with nitric oxide to form nitrogen dioxide, it is important that the product gas in line 12 be administered to the patient within a very short time, for example within two seconds or so, after the gas exits separator B. The gas can be administered to the patient using any standard equipment, and such equipment forms no part of this invention. The apparatus used in the invention is portable. The feed gas is introduced into line 2 from pressurised storage containers, such as gas bottles, or, if air is used as the principal gas vehicle a blower can be use to provide the air at the desired pressure. In the latter case, ammonia can be introduced into the air from a pressurised container. Portable units are preferably kept as simple and as compact as possible to minimise servicing requirements. Conventional equipment may be used to monitor and automatically to regulate the flow of gases within the system so that it can be fully automated to run continuously in an efficient manner. The invention is further illustrated by the following hypothetical example in which, unless otherwise indicated, parts, percentages and ratios are on a volume basis. A gas mixture comprising 5.39 cc/min of ammonia, 6.74 cc/min of oxygen and 4.52 l/min of nitrogen is passed through a catalytic reactor at ambient pressure and 850° C. The catalyst is a five-layer platinum alloy gauze having a diameter of about 2.5 cm and containing 90% platinum, 5% rhodium and 5% palladium. The product gas issuing from the reactor is cooled to ambient temperature and dried by passage through a condenser and an alumina drying bed. The effluent from the drying bed is passed through a silica gel adsorption bed. The purified gas will contain about 998 ppm nitric oxide and generally less than about 1 ppm nitrogen dioxide. The above example illustrates the beneficial use of the system of this invention for producing an oxygen-based nitric oxide-containing gas for administration to patients.

Claims: A method of producing a stream of nitric oxide-containing gas for treatment of medical conditions at the site at which it is to be used, comprising the steps: (a) providing a portable apparatus comprising a portable reactor vessel and a portable purifier; (b) contacting a feed gas comprising oxygen and nitrogen, oxygen and ammonia or oxygen, nitrogen and ammonia with a catalyst in the reactor vessel which effects the reaction of ammonia and oxygen to form nitric oxide at a temperature in the range of about 300 to about 1200°C, thereby producing a gas mixture containing nitric oxide and nitrogen dioxide in the reactor vessel; (c) passing said gas mixture through a bed of adsorbent which preferentially adsorbs nitrogen dioxide, thereby producing a substantially continuous stream of substantially nitrogen dioxide-free product gas in the purifier. A method according to Claim 1 in which the feed gas is air. A method according to Claim 2 in which the feed gas comprises air and ammonia and the product gas is substantially free of ammonia. A method according to any preceding claim further comprising blending the substantially nitrogen dioxide-free product gas with oxygen. A method according to any preceding claim further comprising introducing moisture into the substantially nitrogen dioxide-free product gas. A method according to any preceding claim in which the catalyst is a Group VIII noble metal. A method according to Claim 6 in which the catalyst is platinum, palladium, iridium, rhodium or mixtures thereof. A method according to any preceding claim in which the adsorbent is an exchangeable metal cation-free adsorbent selected from silica gel, alumina, zeolites and mixtures thereof. A method according to Claim 8 in which the adsorbent is an aluminium-deficient adsorbent selected from dealuminated type Y zeolite, ZSM-5 zeolite, ZSM-11 zeolite, ZSM -20 zeolite and mixtures of these. Portable apparatus for providing a continuous stream of oxygen-nitric oxide gas mixture to a patient by a process of any one of Claim 1 to 9 comprising: (a) a catalytic chamber, in the form of a portable reactor vessel, containing a catalyst which converts a feed mixture comprising oxygen and nitrogen, oxygen and ammonia or oxygen, nitrogen and ammonia to nitric oxide at a reaction temperature in the range of about 300 to about 1200°C, the catalytic chamber having a gas inlet for feed gas and a gas outlet for nitric oxide-containing product gas; (b) means for heating the feed gas to the reaction temperature; (c) a portable adsorption vessel for purifying the nitric oxide-containing product gas by removing nitrogen dioxide therefrom, the adsorption vessel containing an adsorbent which more strongly adsorbs nitrogen dioxide than nitrogen, oxygen and nitric oxide, and having a gas inlet in fluid communication with the gas outlet of the catalytic chamber and a gas outlet for purified nitric oxide-containing product gas; (d) dispensing means in fluid communication with the gas outlet for purified converted gas for administering purified nitric oxide-containing product gas to a patient; and (e) means for moving gas through the catalytic chamber, the adsorption vessel and the dispensing means.

Inventors: SHEU LIEN-LUNG, SHEU, LIEN-LUNG
Applicants: AGA AB, AGA AKTIEBOLAG
Application year: 1997
Main classification: C01B21
IPC codes: C01B21, A61K33, A61M16