DE1911200A1 - Nitric acid production - Google Patents

Abstract

Nitric acid especially of concentration greater than 70% wt., is prepared by absorbing NO2 from nitrous gases in H2O or aq. HNO3, by compressing the gases during the absorption and by washing the residual NO2 in the gas after absorption with 60-75% HNO3, then blowing this NO2 out of the wash HNO3 at the absorption pressure by a counter-current of nitrous gases from the combustion of NH3, the HNO3 being at temperature greater than during washing; the blowing gases and the NO2 blown out are then cycled to the absorption stage. NO2 remaining in the wash HNO3 may be removed by blowing secondary air; the HNO3 is then cooled and recycled to the wash step.

Description

"Process for the production of nitric acid with a concentration over 70% by weight ".

The invention relates to a method for producing nitric acid with a concentration in particular above 70% by weight due to the absorption of nitrogen dioxide from nitrous gases in water or aqueous nitric acid, with the nitrogen dioxide partial pressure in the absorption stage partly by compression of the nitrous gases, partly by one nitrogen dioxide cycle through the Absorption level is increased, in which the nitrogen dioxide remaining in the gas after absorption of preferably 60 to 75% strength by weight nitric acid (washing acid) washed out, from the washing acid again expelled and returned to the absorption zone.

The raw material for the production of nitric acid is generally ammonia, which is catalytically converted with Buft into nitrogen monoxide and water The nitrogen monoxide reacts with further oxygen to form nitrogen dioxide according to After cooling the gases with separation of the water of reaction in the form of acid condensate and after adding secondary air with the oxygen additionally required for acid formation, the gases enter the absorption zone, in which the nitrogen dioxide is absorbed in water or aqueous nitric acid with the formation of nitric acid according to The nitrogen monoxide is oxidized again to nitrogen dioxide according to equation (2).

In this zone practically all nitrogen oxides are absorbed up to Residual contents of about 1000 ppm NO + N02. Gas iuid acid are in countercurrent guided. Water is added to the last absorption stage, while in the In the first stage, when the nitrous gases enter, the product acid is drawn off. the in the case of condensation of the water of reaction accumulating acid condensates are added to the absorption at points of appropriate acid concentration.

The absorption effort depends on the desired product acid concentration, the required content of nitrogen oxides in the end gas, the temperature level in the Absorption and the absorption pressure or the nitrogen oxide and oxygen partial pressures.

The theoretically absorbable in each absorption stage according to equation (3) The amount of nitrogen dioxide is determined by the thermodynamic equilibrium for each Given absorption temperature.

The amount of nitrogen dioxide that can be absorbed increases with a given acid concentration and temperature with total nitric oxide partial pressure and degree of oxidation, i. the ratio of nitrogen dioxide to total nitrogen oxide content. The speed the oxidation reaction according to equation (2) increases according to Bodenstein with the square the nitrogen monoxide partial pressure and linearly with the oxygen partial pressure.

It is therefore obvious to carry out the absorption under pressure. Modern nitric acid absorption processes therefore take place under pressures of approx. 3 ata up to a maximum of 10 ata. Higher pressures are i. a. uneconomical because the decreased Absorption expenditure no longer justifies the increased energy costs for compressing the gases.

In principle, a distinction can be made between two procedures: The first Ammonia combustion and absorption take place under the same pressure, in the second the ammonia combustion takes place under lower pressure, and the nitrous gases become After adding the secondary air, it is then compressed to the higher absorption pressure.

The second variant has the advantage of better ammonia yield and to have lower platinum losses. In addition, during the excretion of the water of reaction less nitrogen dioxide absorbed, so that the nitrogen oxide content upon entry into the absorption remains higher. Due to the increased investment costs, this is The variant is generally not economical.

Especially when producing nitric acid with concentrations Above 65% by weight, the absorption expenditure increases in the usual production processes so-called. Medium acid very strong, so that the economic limit to the application this method is about 70 wt .-%. The equilibrium curves for these acid concentrations are at the achievable nitrogen oxide partial pressures (e.g. 0.6 ata for an absorption pressure of 10 ata) with very high degrees of oxidation, i.e. very low nitrogen monoxide partial print, so that with only small amounts of absorbable nitrogen dioxide according to equation (3) The effort required to reoxidize the nitrogen monoxide in each absorption stage according to equation (2) uneconomical because of the long venceil time required becomes large, In addition, the partial pressure level decreases steadily in the entire absorption zone except for the values required by the permissible air pollution, so that in particular, the residual absorption also due to the small amount of process water when higher product acid concentrations are generated, it is greatly increased now known method in which the nitrogen oxide content in the absorption stage thereby is increased that one stops the absorption at a certain nitrogen oxide content and leave remaining nitrogen oxides with higher percentage nitric acid of e.g. 60 - Washes out 70% by weight. The nitrogen oxides dissolved in the form of nitrogen tetroxide will then blown out of the scrubbing acid by means of secondary air and get with it this before the absorption zone in the main nitrous gas stream. This will reduce the residual absorption replaced by the less complex washing zone and the nitrogen dioxide content continues increased in the absorption zone. When calculating this procedure, the result is however, certain disadvantages: In the processes in which ammonia combustion and absorption take place under approximately the same pressure, the secondary air is together with the combustion air condensed to the milage pressure.

At this pressure, however, the absorption capacity of the secondary air is for Nitrogen oxides only limited in the blow-out stage. A significant increase in the nitric oxide content absorption cannot therefore be achieved. In addition, the effort outweighs the Preheating and cooling the washing acid save money in the absorption stage.

A significant increase in the amount of nitrogen oxides in the cycle according to this principle one obtains however with the processes in which the combustion takes place under normal pressure. In the case of pressureless degassing, the possible loading is the secondary air with nitrogen oxides is considerably higher. The nitric oxide content of the in The absorption of the incoming gases can be reduced to about double the amount in this way Originally increased0 While here of course the absorption expenditure as well when producing nitric acid with concentrations above 70% by weight to a fraction that which can be reduced in other processes are also due to the effort to preheat and cool the scrubbing acid the increased costs for the pressureless Combustion part as well as the increased energy costs for the compression of the nitrous Gases opposite, there in addition, the nitrogen oxide gases circulated need to be condensed.

The procedure just described is a prerequisite for one Process in which nitric acid is absorbed by the absorption of nitrogen dioxide from nitrous gases with a concentration above the azeotropic point of the nitric acid-water mixture (about 68 wt. O / o) is generated, this is then in a rectification stage in Highly concentrated nitric acid separated from e.g. 99% by weight and azeotropic acid and the latter for concentrating on the feed concentration of the rectification is returned to the absorption stage.

The invention is based on the object, the method of the opening mentioned type to lead in such a way that the absorption effort and the energy expenditure to compress the gases is reduced.

According to the invention, this object is achieved in that the nitrogen dioxide from the scrubbing acid at absorption pressure and in relation to the temperature in the Washing stage of increased temperature by the coming from the ammonia combustion stage Nitrous gas stream is blown out in a degassing stage arranged upstream of the absorption stage will.

The method according to the invention has compared to the known method the following advantages: The amount of N02 in the circuit can be increased significantly. The absorption expenditure can consequently be reduced to a fraction of that of the known ones Reduce processes required. The effort for the additional process steps on the other hand is low. If one reduces the absorption pressure compared to the two known processes, this results in a further reduction in energy costs. When applying the invention to two-pressure processes, most of the must in the circuit guided nitrogen oxides are no longer compressed, which increases energy costs to reduce the compression of the gases. In addition, the heat balance is teased because the sensible warmth of the nitrous gases is used to preheat the loaded scrubbing acid that would otherwise have to be discharged with cooling water.

In principle, two embodiments of the invention are conceivable: 1. The acid produced is incorporated into the washing acid cycle. The concentration of the washing acid is equal to the product acid concentration in order to maintain the mass balance in Equalize the wash acid circuit or fill up the wash acid circuit when starting up to be able to. Another advantage is that product acid and washing acid are in one To be able to blow out the apparatus with secondary air.

2. The product acid is in a separate washing circuit via a degassing tower with secondary air and a second washing stage, the washing acid runs separately from it Via an extraction tower with nitrous gas and the first scrubbing stage immediately behind the absorption zone. This mode of operation has the advantage, especially with high product acid concentrations, that the washing acid concentration is optimally chosen regardless of the product acid can be, maintaining the thermodynamic equilibrium in the purging and washing stage becomes independent of the product acid. The degree of oxidation of the gas in both stages can be selected to be lower.

Two embodiments are illustrated in the drawing.

These show: FIG. 1 a flow diagram of a plant in ammonia combustion and absorption take place under approximately the same pressure, and FIG. 2 shows a flow sheet a plant in which the ammonia combustion and absorption at different Printing take place.

In the system according to Fig.l, the air required for the process after cleaning in an air filter 1 in a compressor 2 on one Pressure of 2 - 10 ata, e.g. 4 ata, compressed. After branching off the secondary air volume the mixture is mixed with ammonia gas. If the ammonia is available in liquid form, it is previously evaporated in an ammonia evaporator 5. The air-ammonia mixture After fine filtering, it enters a burner 61 in which there are platinum-rhodium catalysts the conversion to nitrogen monoxide and water at temperatures between 8000C and 950 ° C takes place. The reaction gases are then cooled, using the thermal energy partly in a steam generator 7 and partly for preheating the end gas in a heat exchanger 8 is used. In a condenser gas cooler 9, most of the reaction water is precipitated with the formation of a 20 to 40% acid condensate, which means a pump 10 at a point of corresponding concentration in an absorption column 12 is funded.

The cooling medium is the loaded scrubbing acid from a scrubbing column 13, which is preheated in the cooler 9 before entering a blow-off column 91.

The nitrous gases then pass through the blow-out column 11, in which the loaded scrubbing acid from the column 13 to a residual content of e.g. -0.5 to 2.0% by weight Nw2Q is freed from nitrogen tetroxide, whereby the nitrogen oxide content of the main gas increases to a multiple of the original.

Before entry into the blow-out column 11 is appropriate dimensioning the gas clears the oxidation of NO to NO2 so far that gas and scrubbing acid are in thermodynamic equilibrium so that neither acid decomposes nor nitrogen dioxide is absorbed with the formation of acid.

The one required to blow out the nitrogen tetroxide Heat of desorption is caused by the sensible heat of the gases, the heat of oxidation and the condensation the heat released by the remaining water vapor.

The purge column 11 is expediently a packed column carried out, 'because the proportions between gas and acid without acid circulation a Allow hydrodynamically favorable operation.

The largely nitrogen oxide-free scrubbing acid is partly after cooling in a cooler 15 by means of a pump 16 in the first stage of the washing column 13, partly after the remaining tetroxide has been inflated with secondary air in a degassing stage 18 and cooling in a cooler 19 by means of a pump 20 in the second stage of the Wash column 13 returned.

In Fig. 1, the scrubbing acid is degassed together with secondary air with the product acid from the absorption column 12.

This assumes that the concentration of the washing acid is equal to the Product acid concentration is. The advantage of joint degassing is next to that Avoidance of a further degassing column a better utilization of the secondary air as well as simplified operation of the system, especially when starting up and shutting down: The washing circuit can be topped up from the product acid tank or the washing acid can be emptied into this.

Of course, the concentration of the scrubbing acid circuit can be can also be chosen different from the product acid concentration, which in particular is advantageous at very high product acid concentrations.

After leaving the blow-out column 11, the nitrous gases enter the absorption column 12, in which nitrogen dioxide with the formation of nitric acid is absorbed. Gas and acid flow in countercurrent. Process water is at the top of the column respectively.

Abandoned condensate from a column 14. The addition of the acid condensate from the cooler 9 takes place at a point of appropriate concentration. The product acid is withdrawn from the sump.

The absorption column 12 is expediently used as a sieve tray column with internal cooling coils or as a packed column with circulation stages and external cooling carried out. The after absorption in the. Gas remaining Nitrogen oxides, the amount of which corresponds approximately to the amount of nitrogen oxide carried in the circuit, are then washed out in the washing column 13.

The wash column 13 is designed as a two-stage packed column. On the first stage, the scrubbing acid from the blow-out stage 11 is about 0.5 to 2 wt .-% N204 after cooling in the Eühler 15, on the second stage the washing acid abandoned from the degassing tower 18. The loaded acid is from the sump Column 13 withdrawn.

As indicated in Fig. 1 with a dashed line, the washing acid the second stage of the scrubbing column 13 are conducted in a separate circuit, in which the wash acid concentration is equal to the product acid concentration.

To achieve extremely low end gas contents below 200 ppa NO + N02 the scrubbing acid is brought to temperatures by means of brine before being fed into the second stage cooled below 0 ° C to -20 ° G 0 The required cooling energy can either be from a separate Saltav system or by evaporating the ammonia at normal pressure upset will. In this case, the ammonia gas must be compressed to the system pressure.

During the washing process, the washing acid is warmed up by the heat of absorption on. In the case of very large amounts of nitrogen dioxide circulated, the acid is therefore to be intercooled several times in the wash column.

The gases leaving the scrubbing column 13 still contain considerable amounts Amounts of acid fumes. In a post-washing column 14, these are with water or acid condensate washed out. The practically nitrogen oxide-free gases are then in the cooler 8 preheated and in an expansion turbine 3, which is used to drive the air compressor 2 serves, relaxed.

Fig. 2 shows the flow diagram of a plant in which ammonia combustion and absorption take place at different pressures and the produced (over-azeotropic) Acid in a rectification stage into high percentage (e.g. 99 / above) nitric acid and about azeotropic acid is separated, which is used to concentrate to the circulating concentration is returned from the rectification stage to the absorption stage.

Air and ammonia gas are mixed and at about atmospheric pressure in the burner 4 on platinum-rhodium networks at approx. 800-950 ° C to nitrogen monoxide and Water decomposes.

The reaction gases are then cooled in the cooler 7, the Heat energy for steam generation is used. The removal of the residual heat and the Condensation of the major part of the water of reaction with formation of an approx. D / oigen Acid condensate takes place in the condenser 25. According to the water balance, a part of the condensate discharged. After entering the with nitrogen oxides loaded secondary air are the nitrous gases in a compressor 26 to a Pressure of approx. 4-7 ata compressed. The heat of compression is used to preheat the end gas in a heat exchanger 27. The precipitation of the residual water takes place in a Condenser 28, in which the loaded washing acid is preheated to e.g. 100C.

The gases then pass into a degassing tower 29 in which the loaded washing acid to a residual content of approx. 0.5-2% by weight N2 ° 4 of dissolved Nitrogen oxides is freed, the nitrogen oxide content of the nitrous gases of approx. 7 % By volume increases to 14 to 21% by volume. The largely nitrogen oxide-free washing acid is partly after cooling in a cooler 30 by means of a pump 31 in the lowest Returned to the stage of a scrubbing column 32, partly in a degassing column 33 practically completely freed from dissolved nitrogen oxides by means of secondary air and gradually Cooling in a cooler 34 by means of a pump 35 on the second stage of the wash column 32 abandoned.

The main gases then pass through an absorption column 36, in which part of the nitrogen oxides is absorbed with acid formation.

The condensate that is not discharged from the condenser is fed into this column 25, which previously passed through the scrubbing column 42, the condensate from 28 and the one with nitrogen oxides loaded azeotropic acid from the washing tower 32 corresponding to the amount of acid drained abandoned from a rectifying column 37. Nitric acid runs in the bottom of the column with over-azeotropic concentration, which after blowing out the nitrogen oxides by means of Secondary air in a degassing tower 38 in the rectification column 37-in high percentage Nitric acid vapor, which is precipitated in a condenser 38, and azeotropic Acid is separated. The latter arrives after cooling in coolers 39 and 40 by means of a pump 41 on the top step of the washing tower 32.

In the washing tower 32 approximately azeotropic composition is created by means of acid practically all nitrogen oxides washed out of the gas. Remaining acid vapors are in a Column 42 precipitated by means of approx. 29 'above condensate. The residual gases are after preheating in the heat exchanger 27 in an expansion turbine 43, which for Serves drive of the nitrous gas compressor, relaxed.

Claims (16)

P a t e n t a n 8 p r ü c h e: 1. Process for the preparation of nitric acid with a concentration in particular over 70% by weight through absorption of nitrogen dioxide from nitrous gases in water or aqueous nitric acid, the nitrogen dioxide partial pressure in the absorption stage partly by compressing the nitrous gases, partly by a, Nitrogen dioxide cycle is increased through the absorption stage, in which the after the absorption in the gas remaining nitrogen dioxide by preferably 60 to 75 % By weight nitric acid (washing acid) washed out, driven out of the washing acid again and is returned to the absorption zone, thereby g e -k e n n n z e i c h n e t that the nitrogen dioxide from the scrubbing acid at absorption pressure and in the ratio to the temperature in the washing stage (13,32) increased temperature by the dor Annuonia incineration level (6) incoming nitrous gas stream in one arranged upstream of the absorption stage (12, 36) Degassing stage (11,29) is blown out. 2. The method according to claim 1, characterized in that g e k e n n z e i c h n e t that all or part of the washing acid after blowing out most of the Nitrogen oxides by means of nitrous gas by blowing out with secondary air practically completely from dissolved nitrogen oxides is released and, after cooling, returned to the washing stage - (13,32) will. 3. The method according to claim 2, characterized in that g e k e n n z e i c h n e t that the wash acid concentration is equal to the product acid concentration. 4. The method according to claim 1 to 3, characterized g e k e n n z e i c h -n e t that the product acid generated in the absorption stage (12) in a second Washing circuit, consisting of a degassing tower operated with secondary air (18) and a second washing stage (13b), which in the gas stream after a first washing stage (13a) washes out any remaining nitrogen oxides, the one taking place in the washing stage (13b) loaded acid together with the product acid running off from the absorption stage (12) is degassed and partly returned to the washing stage (13) after cooling, partly is taken as product acid. 5. The method according to claim 1 to 4, characterized in that g e k e n n z e i c hn e t that the condensation heat is used to preheat the loaded scrubbing acid before it is blown out when separating the water of reaction and the heat of reaction during the oxidation of the NO to NO2 is used, both indirectly (9) and directly - <- ii) heat exchange. 6. The method according to claim 2 to 5, characterized in that g e k e n n -z e i G h n e t that the practically nitric oxide-free washing acid. by means of cold brine at temperature is cooled below 0 ° C to approx. 2000 and to the second stage (13b.) of the washing zone (13) is abandoned. 7. The method according to claim 6, characterized in that g e k e n n z e i c h n eE t that the temperature in the second washing stage (13b) through the use of cold brine is kept between 0 ° C and -200C. G. Method according to Claims 1 to 7, characterized in that it is possible to use it -n e t that the gases after passing the washing stage (13) in a further washing stage (14) for washing out nitric acid vapors with water or acid condensates in Be brought into contact. 9. The method according to claim 1, 2, 5 and / or 6, characterized g e -k e n It is not stated that over-azeotropic acid is produced in the absorption stage (36) is, this in a rectification stage (37) in high percentage, e.g. 9c'ige and azeotropic acid is separated and the latter is returned to the absorption stage (36) will. 10. The method according to claim 9, characterized in that g e k e n n z e i c h -n e t that the azeotropic drainage acid of the rectification stage (37) before use in the Absorption stage (36) is used to wash out nitrous gases in the washing stage (32) will. 11. The method according to claim 1 to 10, characterized in that g e k e n n -z e i c hn e t that by appropriate oxidation rooms for the nitrous gas before entry in the blowing and / or washing stage (11, 13; 23, 32) the degree of oxidation of the gases is adjusted so that thermodynamic equilibrium between gas and acid prevails. 12. The method according to claim 1 to 11, characterized in that g e k e n n -z e i c hn e t that nitrous gases are produced by the catalytic conversion of ammonia with air generated, the ammonia conversion at about the same pressure as the absorption or at lower pressure. 13. The method according to claim 1, characterized in that g e k e n n z e i c h -n e t that for the production of liquid nitrogen tetroxide the nitrous gases behind the blow-out stage below the probe temperature at the present nitrogen dioxide partial pressure be cooled. 14. The method according to claim 2 to 13, wherein the pressure in the ammonia combustion stage is equal to the pressure in the absorption stage, thus g e k e n n n e i c h n e t that. the secondary air with the nitrogen oxides behind the blow-out stage (11) and before the absorption stage (12) is added to the nitrous gas stream (Fig.l). 15. The method according to claim 2 to 13, wherein the pressure in the ammonia combustion stage is lower than the pressure in the absorption stage, thus g e k e n n e i c n e t that the secondary air with the nitrogen oxides upstream of the nitrous gas compressor is added (Fig. 2). 16. Device for performing the method according to one or more of claims 1 to 15, characterized 6 e k e n n n z e i c h -n e t t that the columns the washing and stripping stage (11, 13; 29, 32) as 11-body columns without internal circulation are.