DE3018665A1 - METHOD AND DEVICE FOR REGENERATING SULFURIC ACID - Google Patents

Description

BAYER AKTIENGESELLSCHAFT * 5090 Leverkusen, Bayerwerk BERTRAMS AKTIENGESELLSCHAFT CH-4132 Muttenz Zentralbere ich

Patents, trademarks and licenses Je / bc / c | ^{^: _Λ Χ%} \

Method and device for the regeneration of sulfuric acid

The present invention relates to a method and a device for the regeneration of contaminated Sulfuric acid in several stages with indirect heating in apparatus from or at least coated with E-mail. Glass-like coatings that are acid-resistant and subject to thermal cycling are under enamel withstand, understood.

The present invention relates to a process for the regeneration of contaminated sulfuric acid with indirect heating in apparatus made of or at least coated with enamel, which is characterized in that the acid is first reduced to an outlet concentration of 60 in an indirectly heated, single-stage, possibly also multi-stage pre-concentrating unit concentrated up to 80% by weight sulfuric acid, then passed into a high concentration unit and there at temperatures between approx. 16O ⁰ C to 25O ⁰ C and a pressure between approx. 30 Torr and 100 Torr up to a concentration of 90% by weight Concentrated 98.3 wt .-% sulfuric acid and then fed to a cleaning stage in which a temperature between about 220 ⁰ C and 330 ⁰ C is maintained and the acid is cleaned under atmospheric pressure or reduced pressure.

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The present invention also relates to a device for carrying out the method, the inventive The device is characterized in that, for heating the acid in the pre-concentrating unit, wfi.:transfer-heated Heat exchangers (15) made of enamelled double-walled tubes are provided and the first preconcentration stage (A) a, by a metallic material, e.g. a Tantalum tube bundle, formed heat exchanger (5) with a downstream evaporation tank (6), a downstream one second preconcentration stage (B), e.g. through enamel double-jacket pipes Heat exchanger (15) made of steel with a downstream evaporation tank (16) and the preconcentration stages (A, B) downstream high concentration stage (C) one through enamel double-jacket pipes made of steel formed heat exchanger (24) with downstream evaporation tank (25) and the downstream cleaning stage (D) a radiation-heated quartz glass tube (36), a heating zone (38) for heating the acid, a downstream one Reaction zone (39) and a downstream post-reaction zone

(46).

It has been shown that with enamel-coated systems it is possible to maintain the high temperatures mentioned and thus to concentrate the acid up to the azeotropic point. In an additional step, a complete cleaning of the acid is carried out in a separate apparatus, for example made of quartz or enamel-coated; and by heating the concentrated acid at 290 to 35O ⁰ C in compliance with certain residence times under reduced or atmospheric pressure, if necessary with addition of an oxidizing agent, preferably nitric acid.

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Enamel coatings are applied to steel at high temperatures. As a result of the different coefficients of thermal expansion of the steel and enamel layers, a compressive stress is created in the enamel layer when it cools, which is desirable and necessary to prevent cracks in the enamel layer. At a room temperature of 25 ^° C., the compression of the enamel layer corresponding to this compressive stress is approximately 0 _f 0015 m / lm. With decreasing temperature, the compressive stress increases as temperature increases accordingly, so that it usually is at a mean temperature of about 400 ⁰ C 0th One would therefore assume that there is always a compressive stress in the enamel layer must be present, the use of enamelled parts would be up to a temperature of about 400 ⁰ C is possible under ideal conditions under the normal conditions of use, however, carry out various factors in the known Evaporation devices mean that even at low average temperatures the compressive stress becomes 0 or even negative, which almost inevitably leads to the dreaded hairline cracks. These factors include the transfer of heat from the steel side through the enamel layer to a medium to be treated. As a result of the known temperature profile during the transfer of heat, the enamel layer has a lower mean temperature than the steel layer, which reduces the compressive stresses in the enamel layer compared to the steady state. This phenomenon is further negatively influenced by uneven heating or cooling of the enamel-coated wall, for example due to the fact that the heat supply from the steel side is uneven, or that the cooling on the side of the enamel layer as a result

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from incrustations of this layer or as a result of uneven Flow of the medium to be heated or its partial evaporation takes place unevenly. According to the invention therefore the enamel layers are used in such a way that that there is still a residual compressive stress in them. This residual stress according to the invention prevents the formation of hairline cracks, so that the enamel layer is perfectly protected against corrosion by the hot, highly concentrated Acid protects. The residual compressive stress or residual compression according to the invention that occurs under all operating conditions is to be maintained, should be as large as possible under the respective operating conditions, but preferably not less than 0.0003 m / 1 m. In very special cases there are also smaller residual compressive stresses permissible, but only if there is no deformation or other negative influences on the enamel layer are feared. In such cases, however, there should also be a residual compressive stress of approx. 10%, i.e. a compression of 0.00015 m / 1 m in the enamel layer.

Another point of the invention is the whole Temperature control, the acid inevitably at such a predetermined speed to the enamelled heat exchanger surfaces so that no solid deposits take place on these surfaces can and at the same time so that no evaporation takes place on the heat exchanger surfaces, but just heating the acid. For this reason, according to the invention, several times that to be concentrated Amount of acid circulated in the apparatus and

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It is ensured in good time that the static pressure at the outlet of the last heat exchanger pipe is sufficiently high is to avoid evaporation at the given acid temperature. This temperature control must but in connection with the residual compressive stress according to the invention happen. For acid and heat carrier speeds of over approx. 0.8 m / sec, preferably 0.9 to 1.5 m / sec, the minimum 20% residual compression according to the invention of 0.0003 m / 1 m in the enamel

'^ shift not undercut. Generally speaking, the speed the acid in the heat exchanger tubes can be varied in the range between approx. 0.4 and 4 m / s. However, a speed of 0.8 to 1.2 m / sec is very particularly preferably set. at Any solids that may be present remain in the suspension at this rate.

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Although the high concentration stage with practically any acid concentration above 30, preferably above wt .-% H ₂ SO. Input concentration can be operated _{r it} is advantageous, especially in the case of highly dilute acids, to install a pre-evaporation system which, if the evaporation rate is high, preferably works in several stages at different pressures according to the principle of vapor utilization. The last evaporator stage of the pre-concentration is also with heat

^ ⁰ carriers heated. As a result, large temperature gradients and thus small heating surfaces and economical operation can be achieved. By suitable selection of the process stages, the type of heating and the structural design of the device according to the invention under the special conditions according to the invention, enamel materials can be used for the first time in such a way that it is possible in the last stage of the pre-concentrating unit and in the high concentration on metallic materials, which have only a limited resistance to corrosion, to do without completely. At the same time, the conditions according to the invention are also created to create sufficiently high temperature differences between the individual evaporator stages in order to be able to utilize the vapor heat economically.

The circulation expansion evaporator in particular has proven itself as an evaporator proven, in which in the heat exchanger itself only a warming up of the acid in the liquid phase is carried out, and the evaporation only takes place in the evaporation tank happens. This is especially important when

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the acids to be concentrated contain impurities which tend to stick to the evaporation surfaces crystallize out. This applies, for example, to ferrous acids. The solubility of iron sulfate in sulfuric acid absorbs in the range over 90% by weight H-SO. quickly and there is a risk that iron sulfate will stick to the evaporation surfaces. This danger exists not in the circulation expansion evaporator because no evaporation takes place on the heat exchanger surfaces and the precipitating iron remains in suspension. can. Instead of circulation flash evaporators however, depending on the quality of the acid, other types of evaporators can also be selected, e.g. falling film evaporators, heated evaporation tanks etc.

5 In the case of acids contaminated with organic components, the Organica distill off at least some of the vapors in the various evaporator stages, depending on the boiling point. In many cases, however, there is at least a residual proportion of organica which does not evaporate and therefore has to be oxidized at high temperatures. With the method according to the invention and in the device according to the invention, for example, trinitrotoluene can be ^{completely degraded at 320 °} C. with the addition of nitric acid as the oxidizing agent.

Lower reaction temperatures can also be maintained. With acids that again for the same Intended use can be used, for example, lower degrees of reaction and thus work-up be accepted because the impurities do not interfere with the previous process.

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For example, it is known to carry out the oxidation reaction with nitric acid in a cast iron residence tank to undertake. However, a tubular reactor made of quartz or optionally enamel has proven to be particularly advantageous proven, in which the acid is initially in the heating zone by indirect heat supply, e.g. by radiant heating, is brought to the reaction and oxidation temperature and then enters the reaction zone, where the acid in countercurrent with an oxidizing agent, previous preferably nitric acid, is brought into contact, whereby the organica are oxidized. Some of them resulting nitrosyl sulfuric acid (HNOSO.) reacts in the downstream post-reaction zone with the rest Organica present, so that a very pure acid can be drawn off at the end of the purification stage.

The post-reaction zone is with the reaction zone directly connected. In the purification stage D, namely in the reaction zone, the concentration of the Hochkon Acid leaving the centering unit to a small extent decreased, increased or kept constant.

When nitric acid is added to oxidize the organic components, the water is already highly concentrated Acid introduced. Likewise, water is formed in small proportions during the oxidation of the organic Components. Due to this low water supply, the already highly concentrated acid in your Concentration will be lowered. In the case of acids contaminated with up to 2-3% by weight of organica, the concentration decrease 2-3% by weight, in exceptional cases it can also be higher, e.g. 5-6% by weight.

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The embodiment preferred according to the invention consists in removing this introduced proportion of water by adding more heat to the reaction zone, i.e. to withdraw the acid with the concentration obtained in the high concentration unit after the post-reaction zone.

Another possibility is to carry out a further high concentration in the reaction zone. With this mode of operation, a very pure and at the same time highly concentrated acid is obtained. These However, from an economic point of view, the approach is likely to be of secondary importance in most cases Be meaning.

In the drawing, the heating zone and the subsequent, de reaction zone and the post-reaction zone are in the same Apparatus accommodated. This is the preferred embodiment according to the invention.

However, this need not necessarily be the case. A variant of the method and device consists, for example, in that the reaction zone and heating zone consist of separate apparatus.

As already stated above, the preconcentration unit consists of one, but usually at least two individual stages connected in series,

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which are operated at different pressures. Depending on the concentration and composition of the processed For acids, the pre-concentrating unit has 1 to 3 stages and only in special cases possibly also in still formed several stages. In its preferred embodiment, the pre-concentrating unit is multi-stage formed, the vapors are so guided / that the vapors from the each evaporator stage working at higher pressure to heat the evaporator stage working at lower pressure from the acid side, the preceding stage can be exploited.

It has been shown to be beneficial at a three stage trained preconcentration unit in the different 5 those stages according to the invention have the following ranges set:

1st stage

Concentration of the acids to be worked up: 15 to 35% by weight H ₂ SO ₄ ;

Concentration of the emerging acid: 20 to 42% by weight H ₂ SO ₄ ;

Temperature range: 30 to 65 ° C;
Pressure range: 30 to 100 torr.

2nd stage

Concentration of the acids to be worked up: 20 to 42% by weight H ₂ SO ₄ ;

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Concentration of the emerging acids: 28 to 54% by weight; H ₂ SO ₄ ,

Temperature range: 65 to 100 ⁰ C; Pressure range: 150 to 500 torr.

3rd stage

Concentration of the acids to be worked up: 28 to% by weight H ₂ SO ₄ ;

Concentration of the emerging acids: 60 to 80% by weight H ₂ SO ₄ ;

Temperature range: 120 to 125 ° C; Pressure range: 500 to 1500 Torr.

In the case of a two-stage preconcentration unit, the following conditions apply according to the invention set:

1st stage

Concentration of the acids to be worked up: 25 to% by weight H ₂ SO ₄ ;

Concentration of the emerging acids: 34 to 61% by weight H ₂ SO ₄ ;

Temperature range: 40 to 100 ⁰ C; Pressure range: 30 to 200 torr.

2nd stage

Concentration of the acids to be worked up: 34 to% by weight H-SO .;

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Concentration of the emerging acids: 60 to 80% by weight; H ₂ SO ₄ ;

Temperature range: 125 to 225 ° C; Pressure range: 500 to 1500 Torr.

Under special circumstances, e.g. when the acid is already is obtained in a sufficiently high concentration can, if necessary, on several stages of the pre-concentrating unit can be dispensed with, so that this can also be operated in one stage. For such cases, according to the invention the following conditions are well proven:

A step

Concentration of the acid to be worked up: 30 to 60% by weight H ₂ SO ₄ ;

Concentration of the emerging acid: 60 to 80% by weight H ₂ SO ₄ ;

Temperature range: 60 to 160 ⁰ C;
Pressure range: 30 to 200 torr.

A particularly advantageous embodiment of the method or the device required for its implementation is shown below with reference to the drawings for example described.

Figure 1 shows the general process scheme with a two-stage preconcentration unit and the scheme of the device and

Figure 2 shows the scheme of an embodiment.

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In a two-stage pre-concentration unit, a high concentration unit and a subsequent purification stage, 30% by weight crude sulfuric acid contaminated with organica is to be regenerated to _{98.3% H 3} SO _4.

In Figure 1, A and B are two stages of the pre-concentrating unit, with C the high concentration unit and with D the purification stage with reaction zone and Designated post-reaction zone.

With the centrifugal pump 2, raw acid (3333 kg / h, 4000 TOC, 30% H ₂ SO ₄ ) is fed to the heat exchanger 3, in which the raw acid is preheated by removing heat from the product acid. The dilute acid, preheated to 47 ^° C., is then fed into the circulation of the first Α stage at 4. The flow of raw acid is regulated by keeping the level in the evaporation tank 6 of the first stage constant. The sulfuric acid in circulation is heated in the heat exchanger 5 (e.g. tantalum tube bundle); exclusively in the evaporation tank 6, water is evaporated according to the amount of heat absorbed, thereby avoiding incrustations on the heating surfaces.

The first stage A is heated with the heat of condensation of the vapors (line 18) from the second stage B.

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A stage is operated under vacuum (50 torr, 41.5% H ₃ SO _4, 5O ⁰ C). The practically acid-free vapors from the first stage A (924 kg / h, 5O ⁰ C, 50 Torr) pass over the droplet separator 1, in which entrained droplets of sulfuric acid retained by the line 8 to a mixing condenser 9, where the vapors by injecting cooling water be condensed. The light organics distilled off with the vapors are separated as far as possible from the remaining condensate in the downstream separation vessel 10.

The vacuum is generated and the non-condensed organic vapors and inert gases are drawn off e.g. with a water ring vacuum pump 11.

As construction materials for the circulation and the evaporation tank For example, glass fiber reinforced plastic, PVC, graphite, glass or enamel are possible.

The first stage A can also be done with a falling film evaporator carried out, whereby the sulfuric acid is concentrated in one pass. In both cases it is the heat exchanger 5 made of a metallic material, here made of tantalum. Instead of a metallic one However, it is also possible to use graphite, glass or enamel.

The effluent from the first stage A pre-concentrated sulfuric acid (2409 kg / h, 50 ⁰ C, 41.5% H ₂ SO ₄₎ is fed through the conduit 13 of the centrifugal pump 14 in the circulation of the second stage B. The feed volume is regulated by keeping the level in the evaporator 16 of the second stage B constant.

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The sulfuric acid circulated by the centrifugal pump 19 is in the heat exchanger 15, consisting of enamelled double-walled pipes connected in series,
heated to then evaporate exclusively in the evaporation tank 16, whereby incrustations on the enamelled wooden surface are avoided.

The heating of the heat exchanger 15 of the second stage B takes place with a heat transfer oil, the heat transfer medium being guided in the jacket space of the enamelled double jacket pipes in countercurrent to the sulfuric acid. The concentration and the pressure in the evaporation tank 16 are chosen so that the acid concentration in the vapors without
additional column kept practically at zero and
the heat of condensation of the vapors in the first stage A can be used (75% H ₃ SO ₄ , 1 bar, 185 ⁰ C).

The vapors (1076 kg / h, 1 bar, 100 ^° C.) produced in the evaporation tank 16 pass through the droplet separator 17,
in which entrained sulfuric acid droplets are retained, through line 18 into the heat exchanger 5 of the first stage A, where they are condensed; the resulting condensate (h 1076 kg / 1 bar, 95 ⁰ C) at 21 finally led into the separation vessel 20th

The vapors distilled off with the vapors (e.g. all with a boiling point lower than 185 ° C)
and in the heat exchanger 5 condensed or crystallized organica are separated from the condensate. the
Inert gases are discharged through the exhaust system.

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The intermediate acid running off from the second stage B is fed through the line 22 with the centrifugal pump 23 to the rectification column 26 of the high concentration stage C. The pump 23 can be omitted with a suitable arrangement of the second and third stage A, B. The feed rate (1333 kg / h, 185 ° C, 75% H ₂ SO ₄ ) is regulated by keeping the level in the evaporation tank 25 of the high concentration stage C constant.

In the stripping column 27, the liquid undergoes a mass transfer with the vapors flowing up from the circulation evaporator 25, the liquid being enriched with higher-boiling components and the vapor with lower-boiling components. If the intermediate product concentration is selected correctly (75% H ₂ SO ₄ ), the exchange of substances and heat causes complete absorption of the H ₂ SO ₄ component in the vapors. If the final concentration of the second stage B is chosen too high (eg greater than 75% H ₃ SO ₄ ), the rectification column of stage C must be expanded with a rectifying column to which water has to be added.

The sulfuric acid circulated by the centrifugal pump 29 is in the heat exchanger 24, consisting of series-connected enamelled double-jacket pipes, heated to then evaporate exclusively in the evaporation tank 25, whereby incrustations on the enamelled heating surface are avoided. The heating of the heat exchanger 24 takes place with a heat transfer oil, the heat transfer medium in the shell space of the enamelled double-shell pipes is conducted in countercurrent to the sulfuric acid.

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It has surprisingly been found that enamelled steel is resistant to sulfuric acid, especially to very highly concentrated sulfuric acid (98.3%) chemically very resistant under the conditions according to the invention is. This is all the more surprising since it is known that hairline cracks occur in enamel at higher temperatures, it is therefore essential to work under the special conditions according to the invention. On the other hand, may certain temperatures or temperature differences are not fallen below or exceeded. For this reason has proven to be useful to run the high concentration stage C under negative pressure. As operationally safe the following condition has been found: vacuum in the evaporator body (25) 30 to 100 Torr, preferably 50 to 70 Torr,

T5 temperature 160 to 250 ⁰ C, boiling temperature of the 98.3% acid 240 ⁰ C, heat carrier temperature at the end of the heat exchanger (24) 310 ⁰ C, acid temperature at this point 260 ⁰ C, heat carrier temperature at the inlet of the heat exchanger (24) 290 ⁰ C and acid temperature at this point 24O ⁰ C. The vapors formed in the evaporating vessel 25 have, depending on the final concentration, a different high acid content. This acid is exchanged in the rectification column 26 by mass transfer with the incoming sulfuric acid.

The vapors of stage C arrive with an acid content corresponding to the equilibrium of the acid applied via the droplet separator 28, in which entrained sulfuric acid droplets are retained, through line 30 into a mixing condenser 31,

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where the vapors (313 kg / h, 50 Torr, 185 ° C) are condensed by injecting water. With the vapors distilled off (for example, all with a boiling point below 240 ⁰ C) and 31 Mischkcndensator condensed or crystallized Organica be separated from the rest of the condensate as far as possible in the downstream separation vessel 32nd The vacuum is generated and the uncondensed organic vapors, the exhaust gases of the oxidized organica and the other inert gases are drawn off by means of the water ring vacuum pump 33.

In the case of acid contaminated with organic substances, a reaction between the organica and the SO ₃ gas in the evaporation tank usually takes place. In order to avoid the reduction of the SO ₃ to SO ₂ and the undesirable sulfurous acid H ₃ SO ₃ in the condensate, an oxidizing agent, preferably HNO ₃ , is added.

When iron-containing acids of the saturation condition (with 98.3% H ₂ SO ₄ and 240 ⁰ C for about 20 ppm Fe) is exceeded, in general, in the high-concentrating stage C, which is a departure of iron (II) sulfate has the consequence. Because of the constant circulation of the sulfuric acid, the iron sulfate remains in suspension and has no opportunity to settle in the evaporator.

In order to prevent the enamel from being rubbed off by precipitated iron sulphates, the rotational speed in the jacketed pipes and in the circulation lines are limited to 1 m / s.

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The high concentration stage is preferably carried out in enamelled steel, with the rectifying column glass is also possible as an alternative active ingredient. The circulation pump can also be made of other material be designed as enamelled steel, preferably in silicon casting.

The highly concentrated, but only partially purified sulfuric acid (1020 kg / h, 98% H ₂ SO ₄ , 24O ° C, 2000 ppm TOC) running off from the high concentration stage C is fed or sprinkled with acid via the IQ line 34 by means of the metering pump 35 , standing quartz glass tube 36 supplied. In this quartz tube, the concentrated sulfuric acid at normal pressure is close to the boiling point warmed up (for example from 24O ° C to 32O ⁰ C).

One embodiment variant is at the tube inlet (quartz glass tube 36 above) and in a possible rectifying column 41 to drive under reduced pressure. With the correct choice of negative pressure, there will be in the cleaning zone normal pressure due to the liquid column in the quartz tube 36. Another option is to use the upper part of the heating zone 38, in order to increase the heat transfer, to apply the sulfuric acid as a falling film.

The heat is transferred by thermal radiation, ie the quartz tube 36 is surrounded by a concentric radiation jacket and this in turn is surrounded by an electrical resistance heater 37 (or possibly another heat source, for example flue gases). The thermal energy is transferred by radiation from the heater to the radiant tube, and from this the inwardly emitted thermal radiation is absorbed by the quartz and the acid.
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- ν-η

If necessary, a sulfuric acid not concentrated to the azeotropic point (for example 90 to 97% by weight H ₂ SO ₄ ) up to 98% by weight or 98.3% by weight can be concentrated in the heating zone 38. In the case of strong vapors, it is advisable to provide the heating and, in this case, the concentration zone with built-in components, most suitably with packings.

Those that arise when concentrating in the heating zone 38 Depending on the initial concentration, vapors have pressure

IQ and temperature of the acid, a different level of acidity. This acid must be exchanged in a rectifying column 41 by mass transfer with a liquid to be dispensed. Water can be used as the feed liquid, and the washing water that has run off, enriched with sulfuric acid, can be returned to the cleaning stage or fed into the high concentration stage together with the dilute acid. The dilute acid can also be used as the feed liquid (if the concentration is below 75% by weight H ₂ SO ₄ ).

The vapors produced during concentration pass through the rectification column 41, in which the aforementioned mass transfer takes place, into a condenser 42. The vacuum pump is used to generate the vacuum, as well as to extract the non-condensed organic vapors, the exhaust gases of the oxidized organics and other inert gases 43. The preheated, concentrated, but still unpurified sulfuric acid (1020 kg / h, 97% by weight H ₂ SO ₄ , 320 ° C.) flows from the heating zone 38 into the reaction zone 39.

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The reaction zone, consisting of the lower part of the quartz tube 36, is from a concentric radiation jacket and this in turn from an electrical resistance, heater 37 (or possibly another heat source, e.g. flue gases). The heat energy is through Radiation from the heater is transferred to the radiant tube and from this the inwardly emitted thermal radiation is emitted absorbed by quartz and acid.

In the lower part of the quartz tube 36, the organica is oxidized by means of an oxidizing agent added at 44 (e.g. 65% nitric acid), which in vapor form in the lower part of the cleaning column e.g. is introduced via a perforated plate to generate small, evenly distributed vapor bubbles. In the case of very heavily contaminated sulfuric acids, it is advisable to provide reaction zone 39 with internals, most expediently with random packings. If necessary, the built-in components can be extended over the entire quartz tube v / earth. The addition of nitric acid can also reduce the undesired properties of the product acid Form nitrosyl hydrogen sulfate. This is partially in a postreactor 40 through reaction with the Organica dismantled again.

The series connection of the heating zone and possible concentration zone with the cleaning zone has proven to be an advantage. _{The SO 3} gases produced during concentration in the lower part of the heating zone 38 are reabsorbed by the colder acid in the upper part of the quartz glass tube 36. Those rising from the reaction zone

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Neither reduced nor reduced NO gases can be used in the

Ji

zone continue to react with the organica. A strict separation According to the invention, there is no provision between the heating zone and the reaction zone.

_{The not completely reduced NO 3} gases emerging from the purification stage D and the reaction gases can be passed via a pressure reducing valve in the line 45 into the evaporation tank 25 of the high concentration stage C, where the excess NO gas with the organica

Ji

can react and at the same time the possible SO ^ reduction at least partially prevented.

The hot concentrated sulfuric acid (320 ° C., 98.3% H ₂ SO ₄ ) emerging from the post-reactor 40 is cooled in a mixing cooler 46 installed next to the post-reactor using secondary circuit acid of the same concentration and purity. Before being fed into the mixed cooler 46, the secondary cycle acid is cooled in a heat exchanger 3 by the raw acid and by a downstream water-cooled heat exchanger 47. The product acid is removed from the secondary circuit and, if necessary, passed into a plate separator 54, where the iron sulfate in suspension is separated off.

In purification stage D, the sulfuric acid has an input concentration of 90 to 98% by weight, an output concentration from 90 to 98.3% by weight and a temperature between 225 and 330 ° C.

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The heating of the heat exchangers 15 and 24 of the second and third stage B ₇ C takes place by means of a central heating system consisting of a high-temperature heater 48, a burner 49, a circulation pump 50, an air preheater 51, a combustion air fan 52 and the chimney 53 Compared to flue gas heating, the heating system has the advantage that heavy oil containing sulfur can also be used as fuel. The two heat exchangers 15 and 24 can be connected in series or in parallel on the heat carrier side.

It is also possible to use a special burner 49 in a substoichiometric amount of the NO gases contained in the exhaust gas Reduce atmosphere.

The method described and carried out with the device shown thus delivers up to the azeotropic level Point concentrated, perfectly purified sulfuric acid.

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030051/0664

Claims (9)

Claims 1. A process for the regeneration of contaminated sulfuric acid with indirect heating in apparatus made from or at least coated with enamel, characterized in that the acid is first reduced to an outlet concentration of between 60 to 80% by weight in an indirectly heated, single-stage, optionally also multi-stage pre-concentrating unit. H ₃ SO ₄ concentrated, then passed into a high concentrating unit and there at temperatures between about 160 ⁰ C and 25O ⁰ C and a pressure between about 30 Torr and 100 Torr up to a concentration of 90 wt .-% to 98.3 wt .-% H ₂ SO . concentrated and then one Purification stage supplied, in which a temperature between 220 ^° C. and 350 ^° C. is maintained and the acid is purified under atmospheric pressure or reduced pressure. 2. The method according to claim 1, characterized in that in a three-stage preconcentration unit, the acid in the first stage at temperatures between 30 ⁰ C and 65 ⁰ C, under a pressure between 30 Torr and 100 Torr to an outlet concentration between about 20 and 42% by weight H ₃ SO ₄ , in the second stage at temperatures between approx. 65 and 100 ^° C. under a pressure of 150 to 500 Torr to an exit concentration between approx. 28 and approx. 54% by weight H ₃ SO ₄ and in the third stage at temperatures between approx. 125 ^° C. and 225 ^° C. under a pressure of 500 torr to 1,500 torr to an exit concentration Le A 20 190 030051/0664 between about 60 and about 80 wt .-% H-SO _{4 is} brought. 3. The method according to claim 1, characterized in that carried out in a two-stage. Pre-concentrating the acid in the 1st stage at temperatures between approx. 40 and approx. 100 ⁰ C under a pressure between approx. 30 and 200 Torr to an exit concentration between approx. 34 and 61 wt .-% H ^ SO ₄ and in the second stage at temperatures between approx. 125 to 225 ° C and a pressure between approx. 500 and 1500 Torr is brought to an exit concentration between approx. 60 and 80% by weight of H ₃ SO ₄ . 4. The method according to one or more of the claims 1 to 3, characterized in that with a multi-stage pre-concentrating unit individual stages can be operated at different pressures. 5. The method according to one or more of the claims 1 to 4, characterized in that with a multi-stage pre-concentrating unit the vapors from the evaporator stage, which operates at a higher pressure, for heating the at low pressure stage can be used. 6. The method according to one or more of claims 1 to 5, characterized in that with the concentrated acid preheated crude acid is preconcentrated in two stages to about 75% by weight, with Le A 20 190 030051/0664 the vapors of the second stage operated at atmospheric pressure to heat the first, under Vacuum-operated stage are used, while the second preconcentration stage is the subsequent high concentration unit be heated by a heat transfer circuit. 7. The method according to one or more of claims 1 to 6, characterized in that the high concentration unit is operated under such conditions that the acid is concentrated to over 96% before entering the purification stage got. 8. Device for performing the method according to one or more of claims 1 to 6, characterized characterized in that heat carrier heated for heating the acid in the preconcentration unit Heat exchangers (15,24) made of enamelled double-jacket tubes are provided and the first preconcentration stage (A) a z. B. formed by a tantalum tube bundle heat exchanger (5) with a downstream evaporation tank (6), a downstream second preconcentration stage (B) one, e.g. through enamel double-jacket pipes made of steel formed heat exchanger (15) with downstream evaporation tank (16) and the preconcentration stages (A, B) downstream final concentration stage (C) one by means of enamel double-jacket pipes made of steel formed heat exchanger (24) with downstream evaporation tank (25) and the downstream Cleaning stage (D) a radiation-heated quartz Le A 20 190 030051/0664 glass tube (36), a heating zone (38) for heating the acid, a downstream reaction zone (39) and has a downstream post-reaction zone (40). 9. Apparatus according to claim 8 / containing in the Cleaning zone a device through which an oxidizing agent in the form of small drops or Bubbles of the high concentration stage supplied in countercurrent to the liquid contained in it can be. Le A 20 190 030051 / Ό664