Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B21/00—Nitrogen; Compounds thereof
  • C01B21/20—Nitrogen oxides; Oxyacids of nitrogen; Salts thereof
  • C01B21/24—Nitric oxide (NO)
  • C01B21/26—Preparation by catalytic or non-catalytic oxidation of ammonia
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B21/00—Nitrogen; Compounds thereof
  • C01B21/20—Nitrogen oxides; Oxyacids of nitrogen; Salts thereof
  • C01B21/24—Nitric oxide (NO)
  • C01B21/26—Preparation by catalytic or non-catalytic oxidation of ammonia
  • C01B21/267—Means for preventing deterioration or loss of catalyst or for recovering lost catalyst

Definitions

• the present invention relates to the manufacture of nitric acid, and relates in particular to improving the yield of the nitric acid manufacturing process.
• Nitric acid is mainly used to make nitrogen fertilizers and organic fertilizers in the form of ammonium nitrate, potassium nitrate or nitrophosphate. Nitric acid is also used in explosives, the plastics industry and the chemical industry.
• HNO3 Nitric acid
• the synthesis of HNO3 comprises three stages (Ostwald process):
• the first step is an oxidation of ammonia: 4 NH 3 + 5 O 2 ⁇ 4 NO + 6 H 2 O (1)
• the ammonia oxidation step is carried out in the presence of a platinum-based catalyst with yields of the order of 93-98% depending on the operating conditions chosen.
• the yield is here expressed in moles of NO produced over the number of moles of ammonia introduced.
• the second stage is a condensation of the combustion water:
• the third step is an absorption or fixation of nitric dioxide and tetraoxide: 3 NO 2 + H 2 O - 2 HNO 3 + NO (4) and N 2 O 4 + Vz O 2 + H 2 O 2 HNO 3 (5)
• the catalyst used in current processes is generally formed by one or more platinum webs or by a bundle of fibers held between two webs.
• Platinum alloy with 5 to 10% rhodium improves mechanical strength, improves yield and reduces catalyst activity loss.
• Palladium or iridium may also be used in the catalyst alloys. However, palladium improves the yield but lowers the mechanical strength, while the irridium improves the mechanical strength, but lowers the yield.
• the liquid ammonia is vaporized by cooling water, then filtered after overheating and sent to an air-ammonia mixer. Filtered atmospheric air is compressed, then divided into two streams, the primary air that goes to the air-ammonia mixer and the secondary air that goes into a denitcher.
• the air-ammonia mixture is directed to a reactor where it flows from top to bottom.
• a special device ensures its distribution on the surface of platinum cloths (usually platinum-rhodium), which are based on special steel supports, generally ring-shaped.
• the gas after combustion contains oxides of nitrogen, nitrogen and oxygen. Its heat is recovered in heat exchangers (recovery boiler).
• the gas and the acid are directed to the trays of the absorption column, which is equipped with cooling coils. Process water is introduced at the top and the acid at the desired concentration is withdrawn at its base.
• This acid goes to the denitcher equipped with trays. There is steam depletion (stripping) by secondary air. The gases leaving the head of the absorber are sent into a gas-gas exchanger, then into an expansion turbine and finally to a chimney.
• the single working pressure is generally between 6 and 10 bar.
• the process comprises three main steps:
• Air necessary for the formation of the acid after compression in a compressor C and cooling in a first exchanger el, enters a temperature of 200 0 C in a mixer which also receives suitable proportions of the vaporized ammonia in a second exchanger e2.
• nitrous gases mainly NO, diluted by the air nitrogen and the reaction water
• a recovery boiler located immediately after the catalytic furnace, then into a third exchanger e3 where they heat the tail gas and cool to a temperature slightly above their dew point.
• nitrous gases containing all of the ammonia oxidation water enter a refrigerant consisting of a pre-cooler the purpose of which is to condense the bulk of the water and then into a special condenser (with parallel) whose purpose is to complete the condensation of water and to oxidize NO to NO 2 .
• a refrigerant consisting of a pre-cooler the purpose of which is to condense the bulk of the water and then into a special condenser (with parallel) whose purpose is to complete the condensation of water and to oxidize NO to NO 2 .
• the special condenser receives a mixture of air and NO 2 from a denitrogen and a reactor.
• 3) Dimerization of Nitrogen Dioxide and Formation of Nitric Acid The nitrous gases from the condenser pass through a fourth exchanger e4 where they cool to 0 ° C. by heating the tail gas at approximately 20 ° C., then enter bottom of a plate column traversed from top to bottom by an 80% nitric acid solution cooled to a temperature slightly below 0 ° C. In this column, NO 2 dimerizes to N 2 O 4 and dissolves in the acid.
• the nitrous gases which contain only very small amounts of nitrogen oxide NOx (about 200 ppm by volume) and which have been called tail gas, are heated successively through the exchangers e4, el, and e3. They finally relax in a recovery turbine that provides about 70% of the energy needs of the compressor.
• the solution of nitric acid and N2O4, after reheating in a fifth exchanger e5, is introduced into a second denitron which also receives air from the compressor C.
• This enriched NO 2 air is sent to a reactor at the same time 60% acid and the N2O4 compound reflux from a refrigerant on the reactor vent. It is in this latter reactor that the acid is formed at 80% of HNO3.
• the flow of this acid at 80% is separated into two parts: one returns to the circuit of the dissolution column (plate column), the other corresponds to the production of 73% HNO3 acid (average of 60% and 80% acid yields provided by the single-process).
• the yield of the oxidation step depends on several parameters such as the speed of the gases, the composition of the catalyst
• the operating conditions temperature, pressure
• the efficiency of the oxidation step also depends on the design of distribution or mixing of the fluids. In general, the yield: increases with the temperature in the usual zone of use, except at low speeds; depends on the ratio O2 / NH3 only when it deviates significantly from the value 1.75; - decreases with pressure; increases with residence time except at high temperatures; increases with the purity of the catalyst and with its rhodium richness (preferably up to 10%).
• the yield increases especially by decreasing the flow of NH3 and operating at low pressure.
• the present invention therefore aims to provide a method of manufacturing nitric acid, improved yield, which allows a larger production capacity and / or lower production costs.
• the present invention aims to design a process for producing nitric acid whose yield is close to
• the object of the present invention is also to allow a longer catalyst life by keeping the same high efficiency longer, thereby improving the profitability of the production unit.
• the present invention also aims to provide a nitric acid manufacturing process that is simple, fast (with the fewest possible steps), easy to implement, and easily adapts to the manufacturing devices of nitric acid existing in the industry.
• the non-production of 100% yield during the oxidation step is mainly related to the thermal or catalytic decomposition of NH3. Indeed, a portion of the ammonia introduced does not participate in obtaining HNO 3 , and - is decomposed into nitrogen and hydrogen according to the reaction:
• the present invention therefore aims to reduce this thermal or catalytic decomposition of NH3.
• the decrease in the activity of the catalysts as a function of the time of use is mainly related to the consumption of platinum, this consumption increasing with the temperature and the thermal shocks (stops, starts) and with the pressure.
• ammonia if it is non-synthetic source, may contain significant amounts of sulfur, which must be previously collected, so that their amount does not exceed 1 or 2 ppm.
• the air used for the oxidation of ammonia is generally filtered to remove the gaseous impurities of SO 2 .
• the Applicant Company has now found that, surprisingly, the addition of a sulfur-containing compound in a small quantity during the nitric acid production process makes it possible to increase the yield of the nitric acid production, allows to increase the lifetime of the platinum-based oxidation catalyst, thus making it possible to improve the profitability of the production unit. This effect is particularly pronounced if the sulfur compound is dimethyldisulphide (DMDS).
• DMDS dimethyldisulphide
• the subject of the present invention is therefore a process for the production of nitric acid, in which a gaseous mixture comprising air and ammonia is passed over a catalyst comprising platinum to effect oxidation. catalytic ammonia, and wherein a sulfur compound is added to the gas mixture.
• the sulfur compound is preferably dimethyl disulfide or dimethyl disulphide (DMDS).
• said catalyst comprises at least one platinum web, platinum preferably being rhodium-plated to form a Pt-Rh alloy.
• the sulfur compound is added to the gaseous mixture before passing over the catalyst.
• the sulfur compound is added to the gaseous mixture by injection at at least one injection point.
• the sulfur compound comprises at least one of the following compounds: elemental sulfur, carbon disulfide, mercaptan, hydrogen sulphide, thiophene compound, sulphide, disulfide, dialkylsulphide, dimethylsulphide.
• the sulfur compound comprises at least one organic polysulfide of formula:
• R - S (n) - R ' where n is an integer from 2 to 20 and where the radicals R and R', which are identical or different, each represent an organic radical each containing 1 to
• the sulfur compound is dimethyldisulphide or dimethyl disulfide or DMDS.
• the sulfur compound is added to the air upstream of the mixture with the ammonia and / or is added to the ammonia upstream of the mixture with the air, and / or is added to the gaseous air / ammonia mixture.
• the temperature of the gaseous mixture at an injection point is in the range from 100 ° C. to 500 ° C., preferably in the range from 150 ° to 250 ° C.
• the gaseous mixture comprising said sulfur compound is brought to a temperature in the range from 800 ° C. to 1000 0 C, preferably 820 0 C to 940 0 C during the catalytic oxidation of ammonia.
• the gaseous mixture comprising said sulfur compound is brought to a pressure of about 3 to 10 bar during the catalytic oxidation of ammonia.
• the flow rate of the gaseous mixture comprising said sulfur compound is adjusted so that its residence time is in the range of 10 -3 s to 10 -4 s during the catalytic oxidation of ammonia.
• the amount of sulfur compound added to the gaseous mixture is in the range from 5 to 500 ppm expressed by volume of sulfur relative to the volume of NH 3 introduced into the process, and preferably in the range from 10 to 100 ppm. expressed in volume of sulfur relative to the volume of NH3 introduced into the process.
• the sulfur compound is injected continuously into the gaseous mixture.
• one or more sulfur compounds from the catalytic oxidation step of ammonia are taken from the gas mixture, after passing said sulfur compound on said oxidation catalyst.
• the present invention also relates to the use of sulfur compound in an effective amount in a process for producing nitric acid by catalytic oxidation of ammonia, to increase the yield of said process.
• the nitric acid manufacturing process of the present invention is characterized by comprising adding a certain amount of sulfur compound.
• the sulfur compound may be of any type and may comprise carbon disulfide, mercaptans, hydrogen sulphide, thiophene compounds, sulphides, disulphides, dialkyl sulphides, especially DMS dimethyl sulphide or DMDS dimethyl disulphide or a mixture one or more of these compounds.
• the sulfur compound may comprise elemental sulfur, for example powder (sulfur flower) or else a mixture of elemental sulfur and at least one organic polysulfide of formula: R - S ( n ) - R 'where n is a an integer from 2 to 20 and wherein the radicals R and R ', which are identical or different, each represent an organic radical each containing 1 to 150 carbon atoms per molecule, these radicals being chosen from the group consisting of alkyl radicals, saturated or unsaturated radicals; , linear or branched or naphthenic type, aryl radicals, alkylaryl radicals and arylalkyl radicals, R 'may also represent the hydrogen atom.
• elemental sulfur for example powder (sulfur flower) or else a mixture of elemental sulfur and at least one organic polysulfide of formula: R - S ( n ) - R 'where n is a an integer from 2 to 20 and wherein the radicals R and R ', which are identical or different
• sulfur powder sulfur flower
• another sulfur compound for example an organic polysulfide as defined above
• 5 to 90% of polysulfide and 95 to 10% of elemental sulfur more particularly, still by way of non-limiting example, 20 to 50% polysulfide and 80 to 50% elemental sulfur.
• the process according to the invention preferably uses dimethyl disulphide.
• DMDS Dimethyldisulphide of the formula H3C-S-S-CH3, hereinafter referred to as DMDS, which may also be called dimethyl disulphide or methyl dithiomethane, is used in a large number of applications.
• DMDS is used as a sulphurization or pre-sulphurization agent in refineries to activate hydrotreatment catalysts.
• DMDS is also used in the petrochemical industry to protect steam cracking systems from coke and carbon monoxide formation. It can also be used as a synthesis intermediate in fine chemistry or metallurgy for its anti-corrosion properties.
• DMDS Dimethyl disulphide
• DMDS has never been used in a nitric acid production process. Its effect in the present invention is quite unexpected, since it runs counter to the widespread misconception that sulfur compounds are detrimental to the production of nitric acid. Without the Applicant being held to any explanation, it may be that under the operating conditions of the HNO3 manufacturing process, the DMDS is decomposed into different chemical species which are in equilibrium because of their short residence times in the region. installation, improving the efficiency of the catalyst, the yield of nitric acid and reducing the loss of ammonia by decomposition.
• the subject of the present invention is a process for producing nitric acid in which a gaseous mixture comprising ammonia, air and optionally oxygen on a catalyst composed of rhodium-plated platinum wire, and in which a sulfur compound, preferably DMDS, is added to the gaseous mixture.
• a gaseous mixture comprising ammonia, air and optionally oxygen on a catalyst composed of rhodium-plated platinum wire, and in which a sulfur compound, preferably DMDS, is added to the gaseous mixture.
• the sulfur compound for example DMDS
• DMDS may be added in admixture with the air before mixing with the ammonia.
• DMDS can also be added to ammonia before mixing with air.
• DMDS can be added directly to the air-ammonia mixture. According to the method of the invention, it is possible to use only one of these possibilities of addition or to combine two or three of these different possibilities of addition of sulfur compound.
• the sulfur compound may be added by injection at one or more injection points of the process.
• the temperature at the injection point depends on the flow retained for the injection. In general, the temperature of the gas mixture at a sulfur compound injection point is of the order of 100 to 500 ° C, preferably of the order of 150 to 250 ° C.
• the gaseous mixture thus additive by the sulfur compound is then brought to the reaction temperature of the oxidation step, that is to say at a temperature of the order of 820 and 940 ° C.
• the pressures are in the range of 3 to 10 bar and the flow rates are adjusted to have residence times of the order of 10 " 3 to 10- 4 s.
• DMDS can be added continuously to maintain an optimal level of sulfur.
• the DMDS is preferably added to the gaseous mixture, either in the air (optionally supplemented with oxygen) upstream of the mixture with NH 3, or in the NH 3 upstream of the mixture with air, or directly in the gaseous mixture with air. Nlrb, preferably in a single injection point located on the device of the method.
• the addition of DMDS is preferably during the normal course of the reaction although it is also possible to add it during the catalyst activation step (approximately 24 to 48 hours).
• the quantities of DMDS injected into the gaseous mixture range from 5 to 500 ppm expressed as a volume of sulfur relative to the volume of NH 3 introduced into the unit, and preferably from 10 to 100 ppm expressed by volume of sulfur relative to the volume of NH 3. introduced into the production unit.
• DMDS is preferably injected continuously.
• the sulfur compound passing through the platinum catalyst may not remain intact and decompose into other sulfur compounds.
• the process of the invention may comprise the removal or recovery of the sulfur compound (s) downstream of the oxidation step. This makes it possible to avoid possible interactions of the sulfur with other catalysts used in the rest of the process.
• catalysts used to carry out the catalytic decomposition of N 2 O are sometimes placed downstream of Pt-Rh webs or directly replace the usual support ring of Pt-Rh webs in the process device. The contact of these N2O decomposition catalysts with sulfur compounds could indeed be detrimental to them.
• N 2 O decomposition catalysts By way of example of N 2 O decomposition catalysts, mention may be made of cobalt oxide spinels, those using magnesium oxide or zirconium dioxide, an alloy with iron, or a ZrO 2 and Al 2 O 3 mixture. impregnated with a zirconium salt, etc.
• the sulfur compounds from the oxidation step are therefore removed by any means possible during the process. It is thus avoided that sulfur compounds pass into the rest of the process device which is not dedicated to the oxidation of ammonia, and where the activity of such sulfur compounds, in addition to being useless, could be harmful. .
• DMDS is a non-toxic liquid product that can be easily vaporized under the conditions of the process (boiling point of about 110 0 C). It allows to improve effectively means the productivity of the new or used catalyst implemented and without requiring additional step of purification of the finished product.
• the product obtained according to the process of the invention is free of sulfur compound, which allows its direct use in any subsequent process.
• the nitric acid synthesis pilot used operates with the following operating parameters:
• the DMDS is added directly to the air / NH ⁇ mixture at 150 ° C. via a metering pump with mass flow rates ranging from 0.3 to 5 g / h. A one-day stabilization period is observed between each increase in DMDS flow rate before measuring the efficiency of the oxidation step.

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