Process for the manufacture of nitric oxide

Abstract: A process for the production of nitric oxide comprising contacting aqueous nitric acid solution with gaseous sulphur dioxide in a gas-liquid contact zone thereby producing a product gas comprising nitric oxide, wherein the nitric acid and sulphur dioxide are introduced into the contact zone at a nitric acid to sulphur dioxide molar ratio of at least 0.7:1, thereby producing a nitrous oxide- and nitrogen-depleted nitric oxide product gas.

Description: This invention relates to the manufacture of high purity nitric oxide (NO) by the reaction of aqueous nitric acid with gaseous sulphur dioxide. 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 or nitric oxide releasing substances to patients who suffer from certain illnesses or diseases. Of particular interest is the prospect of reducing pulmonary vasoconstriction in paediatric and other 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. The final dosage nitric oxide product is produced by mixing very high purity, ie pharmaceutical grade nitric oxide with inert ingredients and/or other active substances. Nitric oxide can be produced by a variety of methods. A method which is particularly useful because of the ready availability of the reactant materials is the reaction of aqueous nitric acid solution with gaseous sulphur dioxide. This reaction is discussed in the article "A Chemical Exchange System for Isotopic Feed to a Nitrogen and Oxygen Isotope Separation Plant" appearing in Separation Science and Technology, vol 24, (5 & 6), pp 415-428 (1989). This reaction can be carried out by contacting the reactants in a trickle bed reactor, with aqueous nitric acid being introduced at the top of the bed and the gaseous sulphur dioxide being introduced at the bottom of the bed. In the above article It is suggested that the reaction be conducted with a nitric acid flow of slightly in excess of the stoichiometric amount for the reaction to help prevent the passage of sulphur dioxide out of the reactor. The gaseous reaction product, which is predominantly nitric oxide, contains as impurities water vapour and small amounts of other nitrogen oxides. The principal nitrogen oxide impurities are nitrogen dioxide (NO 2 ) and nitrous oxide (N 2 O). To meet the required standards for pharmaceutical-grade nitric oxide, the product gas must be substantially free of nitrogen dioxide and nitrous oxide. DE-C-818 641 discloses a process for the production of nitric oxide by the reduction of nitric acid with sulphur dioxide. Because of the importance of the emerging medical uses of pharmaceutical-grade nitric oxide, manufacturing improvements which will facilitate the production of pharmaceutical-grade nitric oxide are continually sought. The present invention is concerned with a nitric oxide manufacturing technique which provides such an improvement. In accordance with the invention, there is provided a process for the production of pharmaceutical grade nitric oxide that comprises less than 5 ppm nitrogen dioxide, as set out in claim 1. The invention generally provides an improvement to a nitric oxide manufacturing process which results in the production of a gaseous nitric oxide product which contains little or no nitrous oxide. The improved process comprises contacting aqueous nitric acid solution with gaseous sulphur dioxide, for example in a trickle bed gas-liquid contact zone, at a temperature in the range of about 50 to about 120°C and at a pressure typically in the range of about 0.5 to about 5 bar (bar, absolute), with the nitric acid entering the contact zone at a nitric acid to sulphur dioxide molar ratio of at least about 1:1. The upper limit of the nitric acid to sulphur molar ratio is not critical and, in general, is determined primarily by economics. It is generally not economically feasible to conduct the reaction at nitric acid to sulphur dioxide molar ratios greater than about 10:1. The nitric acid and sulphur dioxide enter the gas-liquid contact zone at a nitric acid to sulphur dioxide molar ratio of at least 1:1 and preferably in the range of at about 1:1 to about 1.5:1. The weight ratio of water to nitric acid in the aqueous nitric acid solution entering the gas-liquid zone is in the range of about 1:1 to about 5:1. The nitric oxide gas product is pressurised to at least about 20 bar and chilled to a temperature not higher than about -40°C, and more preferably to a temperature in the range of about -40 to a temperature of about 1 to about 5°C above the boiling point of nitric oxide, thereby producing a substantially moisture-free nitric oxide gas product containing not more than about 10 ppm nitrogen dioxide. In a most preferred aspect of this embodiment and generally, nitric oxide product gas is preferably compressed to a pressure greater than about 35 bar, for example about 35 to 50 bar, and cooled to a temperature of less than about -50°C, for example a temperature in the range of about -50 to a temperature of about 1 to about 5°C above the boiling point of nitric oxide. Moisture is condensed from the pressurised nitric oxide gas product by cooling the gas to a temperature in the range of about 1 to about 5°C. Preferably, then nitrogen dioxide and additional moisture are removed from the partially dried product gas by chilling it to a temperature in the range of about -40 to a temperature about 1 to about 5°C above the boiling point of nitric oxide. In another preferred embodiment the nitric oxide product gas is first scrubbed with water or dilute aqueous nitric acid to remove nitrogen dioxide from the product gas, then the nitrogen dioxide-depleted product gas is compressed to a pressure in the range of 30 to about 50 bar and cooled to a temperature in the range of about 1 to about 5°C to remove moisture from the product gas, and then the nitrogen dioxide-depleted and moisture depleted gas product is chilled to a temperature in the range of about -40°C to a temperature of about 1 to about 5 °C above the boiling point of nitric oxide to remove additional moisture and nitrogen dioxide. The dried, nitrogen dioxide-depleted nitric oxide product gas is passed through an adsorbent which selectively adsorbs nitrogen dioxide, thereby reducing the concentration of nitrogen dioxide in the product gas to less than about 5 ppm. For a better understanding of the invention, reference will now be made, by way of exemplification only, to the accompanying drawing illustrating a preferred method of carrying out the invention. The principal reaction occurring in the process of the invention is: 2HNO 3 + 2H 2 O + 3SO 2 → 2NO ↑ + 3H 2 SO 4 The gas product produced in the reaction comprises predominantly nitric oxide (NO). Nitrogen dioxide (NO 2 ) is also generally produced in amounts up to about 3 vol.% of the gas product. Other products that may be produced are nitrous oxide (N 2 O) and nitrogen (N 2 ). Since pharmaceutical-grade nitric oxide must be substantially free of other nitrogen oxides, it is important to conduct the reaction under conditions that minimise the production of nitrogen and nitrogen oxides other than nitric oxide. As can be seen from the above reaction, the stoichiometric molar ratio of nitric acid to sulphur dioxide for nitric oxide production is 2:3. When the molar ratio of nitric acid to sulphur dioxide is less than the stoichiometric value, nitrous oxide and nitrogen are produced as byproducts of the process. Thus, to avoid the formation of these byproducts, it is important that the ratio of nitric acid to sulphur dioxide throughout the reaction zone be at least 2:3. It has been discovered that very little or no nitrous oxide and nitrogen are produced in the reactor when nitric acid and sulphur dioxide are introduced into the reaction zone at nitric acid to sulphur dioxide molar ratios of at least 1:1. Conducting the reaction at nitric acid to sulphur dioxide ratios at or above this value ensures that the molar ratio of nitric acid to sulphur dioxide in all parts of the reaction zone will be stoichiometric or greater, and thus the production of nitrous oxide and nitrogen is substantially eliminated. In the drawing, auxiliary equipment, including valves, compressors and heat exchangers, that is unnecessary for an understanding of the invention have been omitted for the purpose of simplicity. Illustrated in the drawing is a system for producing nitric oxide which includes, as major equipment components, a gas-liquid contact reactor, A, an optional gas scrubber ,B, a nitrogen dioxide removal unit C, and an optional gas adsorption system D. Reactor A can be any type of gas-liquid contact reactor, but it preferably comprises a glass, glass-lined, or stainless steel trickle bed column, 2. The column 2 may be filled with random packing or structured packing, and it is provided with suitable heat exchange means, such as a cooling jacket 4, to remove heat from the reaction zone. The cooling jacket 4 is provided with a cooling water inlet and discharge lines 6 and 8, respectively. Located at or near the top of column 2 is an aqueous nitric acid feed line 10, and at or near the bottom of the column is a sulphur dioxide gas inlet line 12. Also located at the top of column 2 is a product gas discharge line 14, and also located at the bottom of the column is a waste liquid discharge line 16. In the embodiment of the invention illustrated in the drawing, the downstream end of the line 14 is connected to the inlet of the gas scrubber B. Scrubber B is provided with a scrubbing liquid inlet and discharge lines 17 and 18, respectively. The outlet end of the scrubber B is connected by a conduit to the inlet end of a compressor 20. On its outlet end, the compressor 20 is connected via a conduit to the feed gas inlet end of a water condenser 22. The condenser 22 is provided with a condensate drain line 24. The gas outlet end of the condenser 22 is connected to an inlet manifold 26 of a nitrogen dioxide freezing unit C. The manifold 26 is connected to the inlets of nitrogen dioxide freezers 28 and 30. Flow between the manifold 26 and the freezers 28 and 30 is controlled by valves 32 and 34, respectively. The freezers 28 and 30 are provided with nitrogen dioxide discharge lines 36 and 38, respectively, which join and discharge to a downstream waste disposal system. The gas outlet end of the nitrogen dioxide freezers 28 and 30 are connected to a manifold 40. Flow between the freezer 28 and the manifold 40 is controlled by a valve 42, and flow between the freezer 30 and the manifold 40 is controlled by a valve 44. A line 46 connects the manifold 40 to a feed gas manifold 48 of a gas adsorption unit D. The manifold 48 is connected to lines 50 and 52, which, in turn, are connected to the inlets of adsorption vessels 54 and 56, respectively. Flow through the lines 50 and 52 is controlled by valves 58 and 60, respectively. The nonadsorbed gas outlet end of the vessels 54 and 56 are connected to a nonadsorbed gas manifold 62. Flow between the vessels 54 and 56 and the manifold 62 is controlled by valves 64 and 66, respectively. The manifold 62 is connected to a nonadsorbed gas discharge line 68. A desorbed gas manifold 70 connects the lines 50 and 52. Flow between the lines 50 and 52 and the manifold 70 is controlled by valves 72 and 74, respectively. The manifold 70 is connected to a desorbed gas discharge line 76. In carrying out the process of the invention, an aqueous nitric acid solution containing about 1/6 to about 1/2 by weight nitric acid is introduced into the reactor 2 through the line 10. The nitric acid solution is evenly distributed across the top of the reactor to establish a uniform flow of nitric acid downwardly through the column 2. At the same time, gaseous sulphur dioxide is introduced into the bottom of the column 2 through the line 12. The sulphur dioxide flows through a gas distributor (not shown) positioned in the bottom of the column 2 to establish an even flow of sulphur dioxide upwardly through the column 2. The nitric acid and sulphur dioxide enter the column 2 at a nitric acid to sulphur dioxide molar ratio of at least 1 to 1. As the reactants pass through the column 2 they come into intimate contact and react to produce nitric oxide gas. Because the ratio of nitric acid to sulphur dioxide everywhere in the column 2 is significantly greater than the stoichiometric ratio, very little or no nitric acid is reduced to nitrous oxide or nitrogen. Furthermore, the ratio of water to sulphur dioxide entering the column 2 is sufficient to minimise nitrogen dioxide formation during the reaction stage of the process. Sulphuric acid waste, together with excess nitric acid and water leave the column 2 via the line 16 and are sent downstream to a spent acid treatment facility (not shown). The reaction between the nitric acid and the sulphur dioxide is exothermic. It is desirable to maintain the reaction temperature below the boiling point of the aqueous solution. Accordingly, the column 2 is provided with a cooling means capable of maintaining the temperature in all parts of the column 2 at about 50 to about 120°C. This can be accomplished by, for example, passing a coolant, such as cool water, through the cooling jacket 4, which surrounds the column 2. The coolant enters and exits the jacket through the lines 6 and 12, respectively. The overhead gas product from the column 2, which is comprised substantially of nitric oxide, but also contains up to about 3 volume percent water and nitrogen dioxide as impurities, is next treated to remove these impurities. The gas product passes through the line 14 and, in the embodiment illustrated in the drawing, enters the scrubber B. In the scrubber B, the gas is contacted with water or a dilute aqueous solution of nitric acid, which enters the scrubber B through the line 16. The water or nitric acid solution scrubs nitrogen dioxide from the gas product, and the scrubbing liquid, together with the scrubbed out nitrogen dioxide leaves the scrubber B through the line 18 and is sent to a waste disposal plant ( not shown). As noted above, the scrubber B is not a necessary part of the invention, but it is useful when it is desired to reduce the burden on the nitrogen dioxide freezing unit C. The nitric oxide product gas is next compressed to a pressure of at least about 30 bar, and preferably to a pressure of at least about 35 bar in compressor 20 The compressed gas is then passed through the cooler (condenser) 22, which is maintained at a temperature in the range of about 1 to about 5°C. Water in the product gas is condensed and removed from the condenser 22 through the condensate line 24. The condenser 22 is likewise not a necessary part of the system of the invention, but is advantageous in that it, too, reduces the burden on freezing unit C. The compressed product gas next enters the manifold 26 of the nitrogen dioxide freezing unit C. In the embodiment illustrated in the drawing, the unit C comprises the pair of freezers 28, 30, which are operated out of phase, such that one freezer is on line while the other freezer is being regenerated. The unit C can comprise a single nitrogen dioxide freezer, if desired. In such case, the unit C will be operated on an intermittent basis and a holding tank (not shown) can be provided in the line 14 to collect nitric oxide product gas produced when the unit C is not in operation. When the unit C is operating with the freezer 28 in service, the valves 32 and 42 are open and the valves 34 and 44 are closed, and when the freezer 30 is in service, the valves 34 and 44 are open and the valves 32 and 42 are closed. Product gas passing through the freezer in service is cooled to a temperature below the freezing point of nitrogen dioxide (-11.2°C), but above the freezing point of nitric oxide (-163.6°C), typically to a temperature in the range of about -40°C to about 1 to 5°C above the boiling point of nitric oxide, and preferably to a temperature in the range of about -50°C to about 1 to 5°C above the boiling point of nitric oxide. During this step, substantially all of the nitrogen dioxide and any water vapour remaining in the product gas are frozen and removed from the product gas. While the freezer 28 is in nitrogen dioxide freezing service, frozen nitrogen dioxide in the freezer 30 is thawed by, for example, heating the frozen nitrogen dioxide by circulating a warm gas or liquid through the freezer, or by means of an electric heater, or by a combination of these techniques. The thawed nitrogen dioxide is removed from the freezer 30 via the line 38 and sent to a downstream waste disposal unit (not shown). When a predetermined amount of nitrogen dioxide accumulates in freezer 28, this freezer is taken out of service for regeneration, and freezer 30, which has completed its regeneration step, is put into nitrogen dioxide freezing service. The dry and nitrogen dioxide-depleted product gas passes out of the freezer unit C through the valve 42 in the manifold 40. This gas, which now generally contains less than about 10 ppm nitrogen dioxide, is further purified to remove most of the residual nitrogen dioxide remaining in the gas. The embodiment illustrated in the drawing includes an adsorption plant for further purifying the nitric oxide product gas. In this embodiment, the nitric oxide product gas leaving the freezer unit C passes through the line 46 to the adsorption plant D. The plant D is typically a cyclic adsorption unit, such as a pressure swing adsorption (PSA) unit. The adsorption plant D comprises the pair of adsorption vessels, 54 and 56, which are operated out of phase, such that one adsorption vessel is in adsorption service while the other is being regenerated. Each of the vessels 54 and 56 contains an adsorbent which adsorbs nitrogen dioxide. Details of the adsorption step, including the particular adsorbent used in the units, form no part of the invention. These details are disclosed in our copending European Patent Application No. 691 154. When the vessel 54 is in adsorption service and the adsorbent in the vessel 56 is being regenerated, the valves 58, 64 and 74 are open, and all the other valves are closed. Product gas passing through the line 46 at an elevated pressure enters the vessel 54 through the line 50. As the gas passes through the bed in the vessel 54 nitrogen dioxide is adsorbed from the gas. The purified gas passes out of the vessel 54 through the manifold 62, and is sent to downstream storage or to other treatment facilities through the line 68. Meanwhile the vessel 56 is being countercurrently depressurised through the line 52, the valve 74 and the line 76. The desorbed gas is sent to a downstream disposal facility (not shown). As the adsorption step proceeds, a nitrogen dioxide adsorption front forms in the vessel 54 and moves cocurrently through the adsorption vessel. When the adsorption front reaches a predetermined point in the vessel 54, adsorption in this vessel is terminated. The valves 58, 64 and 74 are closed and the valves 60, 66 and 72 are opened, and the vessel 54 enters the bed regeneration phase while the vessel 56 enters the adsorption phase of the cycle. The product gas exiting the adsorption plant D has a nitrogen dioxide content of less than about 5 ppm. It will be appreciated that it is within the scope of the present invention to utilise conventional equipment to monitor and automatically 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 example in which, unless otherwise indicated, parts, percentages and ratios are on a volume basis. EXAMPLE A glass reactor three feet long and having an inside diameter of one inch and packed with stainless steel random packing was used for this example. Sulphur dioxide was passed upwardly through the reactor at a flow rate of 49.4 millimoles/min and aqueous nitric acid containing 25 weight percent nitric acid was passed downwardly through the reactor at a flow rate of 15.53 g/min, which amounts to a nitric acid to sulphur dioxide molar ratio of 1.25 to 1. The reactor was operated at about 100°C and slightly above atmospheric pressure. During the run, the concentration of nitrogen in the gaseous effluent from the reactor varied from about 50 to 1500 ppm, the concentration of nitrous oxide varied from about 20 to about 50 ppm and the balance was water vapour and nitric oxide. Only trace amounts of nitrogen dioxide were detected in the effluent. The example illustrates that when reacting nitric acid with sulphur dioxide with a stoichiometric excess of nitric acid relative to the concentration of sulphur dioxide, very small amounts of nitrogen and nitrous oxide are produced in the reaction.

Claims: A process for preparing a pharmaceutical grade nitric oxide that comprises less than 5 ppm nitrogen dioxide and is substantially free of other nitrogen oxides, said process comprising contacting an aqueous nitric acid solution having a weight ratio of water to nitric acid in a range of about 1:1 to 5:1 with gaseous sulphur dioxide in a molar ratio of nitric acid to sulfur dioxide of at least about 1:1 in a gas-liquid contact zone at a temperature of about 50°C to about 120°C, thereby producing a product gas, compressing the product gas to a pressure of at least about 20 bar and cooling to about 1°C to about 5°C to condense moisture therefrom, cooling the partially dried product gas to a temperature not higher than about -40°C, thereby freezing nitrogen dioxide therefrom, and passing the resultant product gas through an adsorbent which selectively adsorbs nitrogen dioxide, thereby reducing the concentration of nitrogen dioxide therein to less than about 5 ppm. A process according to Claim 1 in which the product gas is compressed to a pressure of at least about 35 bar and cooled to a temperature not greater than about -50°C. A process according to Claim 1 or Claim 2 in which prior to compressing the product gas, nitrogen dioxide is removed from the product gas by scrubbing it with water or a dilute aqueous nitric acid solution. A process according to Claim 2 or 3 in which the weight ratio of water to nitric acid in the nitric acid solution is in the range of 2.5:1 to 3.5:1.

Inventors: SHEU LIEN-LUNG, SHEU, LIEN-LUNG
Applicants: AGA AB, AGA AKTIEBOLAG
Application year: 1996
Main classification: C01B21
IPC codes: A61M16, C01B21