FI67355B - FOERFARANDE OCH ANORDNING FOER REGENERERING AV SVAVELSYRA - Google Patents

Description

RÄSFH [B] ^ NOTICE.SU

l J 1 'utlAggningsskrift O / 00 0 ^ ^ (51) r * .iki / iwca.3 C 01 B 17/88, 17/90 FINLAND — FINLAND pi) i ^ ttw ^ -P «« ianÄ »ini 801880 ( 22) H «k« mHpihfI - AmMinlngadag 1 1. 06.80 (23) AlkupiM— GIWgKettdag 1 1.06.30 (41) Tullut JulklMksI - Kh »offMtftg ^ ^ 2 8 0 (» »exam · and registrar-managed * j. KuL | uikmiatn ^ ',,', patent * och rcgiittntjf ralaan Ameku uöagd och uti ^ krtfMo pubttcorad 30.11.84 (32) (33) (31) Pietty • Tuoikw.ie ^ ftrtJ prior ** 13.06.79

Switzerland-Schweiz (CH) 5513 / 79-0 16.05 .80 Federal Republic of Germany Förbunds-republiken Tyskiand (DE) P 3018665-3 (71) Bayer AktiergeselIschaft, 5090 Leverkusen, Bayerwerk,

Federal Republic of Germany-FörbundsrepubΠken Tysk1 and (DE),

Bertrams Aktiengesel1schaft, CH — 4132 Muttanz-1,

Sve i ts i-Schwe i z (CH) (72) Hansruedi Forter, Ormalingen, Hans L. Kiihnlein, Fiillinsdorf,

Hans Rudolf KCJng, Frenkendorf, Sve i ts i - Schwe i z (CH),

Jurgen Groening, Siegburg, Joachim Maas. Bergneim,

Karl-Heinz Schultz, Krefeld, Federal Republic of Germany1ta-Förbunds Repub 1 I ken Tyskland (DE) (74) Oy Koi ster Ab (5b) Method and apparatus for the regeneration of sulfuric acid - Förfarande och anordning för Regenerating av svavelsyra

The present invention relates to a method and apparatus for regenerating impure sulfuric acid in several steps by indirect heating in equipment made of enamel or at least coated with enamel. Enamel is understood to mean vitreous coatings which are acid-resistant and can withstand the stress caused by temperature fluctuations.

The invention relates to a process for the regeneration of impurity-containing sulfuric acid in an indirectly heated, enamelled coating apparatus. The process according to the invention is characterized in that the acid is initially concentrated to an outlet concentration of 60-80% by weight in an indirectly heated single-stage, possibly also multi-stage pre-concentration unit, after which it is passed to a high-concentration unit at 90 ° C to 90% C and a pressure of 4-13.3 x 10J Pa and finally passed to a purification step in which the temperature is maintained at 220-350 ° C and in which the acid is purified at atmospheric pressure or reduced pressure, the enamel layer Γ 2 67355 still having a residual content of at least 10% under all operating conditions. compressive stress.

The invention further relates to an apparatus for carrying out the method according to the invention. The apparatus according to the invention is characterized in that it has a heat exchanger consisting of enamelled double jacketed tubes heated in a preconcentration unit for heating the acid in a preconcentration unit and in a first preconcentration step a heat exchanger consisting of enamel-double jacket tubes and an evaporation tank arranged downstream of the evaporating tank and the pre-concentrating steps; reaction zone and a rearranged post-reaction zone.

It has been found that by using enamel-coated equipment, it is possible to maintain said high temperatures and thus to concentrate the acid up to the azeotropic point. In a further step, the acid is completely purified in a separate apparatus, for example quartz or enamelled, by heating the concentrated acid to 290-350 ° C and maintaining this temperature for a certain time at either reduced or normal pressure, with the addition of an oxidizing agent, preferably nitric acid.

Enamel coating is performed on steel at high temperature. Due to the different coefficient of thermal expansion of the steel and the enamel layer, a pressure stress is created in the enamel layer as it cools, which is desirable and necessary for preventing the cracking of the enamel layer. At room temperature (25 ° C) against this compressive stress, the compression of the enamel layer is about 0.0015 m / 1 m. The decrease in temperature has the effect of increasing the compressive stress and correspondingly decreasing the temperature, usually about 40 ° C, at 0 ° C. , that the 3 67355 enamel layer must always be under stress, so that under ideal conditions the enamelled parts could be used at temperatures up to about 400 ° C. However, under normal production conditions, due to various factors, the pressure stress in known evaporator systems reaches a value of 0 or even a negative value even at lower temperatures, which almost necessarily leads to the formation of the dreaded hair cracks. Such factors include e.g. heat transfer from the steel side through the enamel layer to the medium to be treated. Due to the known temperature change in heat transfer, the enamel layer has on average a lower temperature than the steel layer, as a result of which the pressure stresses in the enamel layer decrease compared to the unchanged state. This phenomenon is further negatively affected by uneven heating or cooling of the enamelled wall due to, for example, uneven heat transfer from the steel side or uneven cooling of the enamel layer due to uneven flow or partial evaporation of the medium due to deposits in the enamel layer. According to the invention, the enamel layers are therefore manufactured in such a way that there is still a compressive stress remaining. This residual tension according to the invention prevents the formation of hair cracks, whereby the enamel layer provides excellent protection against the corrosive effect of hot, concentrated sulfuric acid. The residual compressive stress or compression according to the invention, which must prevail under all production conditions, must be as high as possible under all production conditions and not less than 0.0003 ml / 1 m. In very special cases, lower residual compressive stresses may also be used, but only when there is no fear of deformation or other negative effects of the enamel layer. Also in these cases, the residual pressure stress should be about 10%, i.e. the compression of the enamel layer should be 0.00015 / 1 m.

According to the invention, further noteworthy is the way in which temperature changes occur, in which the acid must be passed past the enamel layer at a predetermined rate so that no solids accumulate on the surfaces and at the same time no evaporation occurs on the heat exchanger surfaces but only acid heating. For this reason, according to the invention __ Γ ______ 67355, the apparatus recycles many times the amount of acid to be concentrated, while ensuring that the static pressure as the acid exits the last heat exchanger tube is high enough to prevent evaporation at a predetermined acid temperature. However, this temperature control must take place taking into account the residual pressure stress according to the invention. When the acid and heat carrier transfer rate is above about 0.8 m / s, preferably 0.9 to 1.5 m / s, the minimum compression of the enamel layer of 20% according to the invention is not exceeded by 0.0003 / 1 m. In general, the acid velocity in heat exchanger tubes can be about 0.4 to 4 m / s. However, the speed is particularly preferably 0.8 to 1.2 m / s. At this rate, any solids remain in suspension.

Although the high concentration step can be carried out at virtually any starting concentration of H 2 SO 4 of more than 30% by weight, preferably more than 40% by weight, it is advantageous to switch on a pre-evaporator, which is particularly multi-stage, especially in the case of relatively dilute acids. , where different pressures are at different stages and the device operates on the principle of utilization of steam heat. The last evaporator stage is also heated with heat carriers. In this way, large temperature drops and thus small heating surfaces and economical performance can be achieved. With a suitable choice of process steps, heating method and construction of the apparatus according to the invention, taking into account the special conditions according to the invention, the enamel material can be used for the first time so that metal materials with very limited corrosion resistance can be completely discarded in the final preconcentration stage and in the high concentration unit. At the same time, the method according to the invention provides the conditions for sufficiently high temperature differences between the individual evaporator stages, whereby the heat of the steam can be economically utilized.

A rotary evaporator has proved to be particularly suitable as an evaporator, in which only the heating of the acid as a liquid phase is carried out in the heat exchanger and the evaporation takes place only in the evaporation tank. This is especially important when the acids to be concentrated contain impurities that tend to crystallize on the evaporator surfaces. This is especially the case when the acid is e.g. iron-containing. The solubility of ferrous sulphate in sulfuric acid decreases rapidly in the range of more than 90% by weight, and there is a risk of ferrous sulphate adhering to the evaporator surfaces. This danger does not occur when using a rotary expansion evaporator, because no evaporation occurs on the heat exchanger surfaces and the precipitated ferrous sulphate can thus remain in suspension. However, depending on the quality of the acid, other evaporator cores can be used instead of a rotary evaporator, e.g. a well evaporator, heated evaporator tanks, etc.

Organic substances are distilled from acids containing organic impurities in part at different stages of the evaporator, always according to their boiling point with steam. In many cases, however, some organic matter is always not evaporated and must therefore be oxidized at high temperatures. With the process according to the invention and the apparatus according to the invention, for example, trinitrotoluene can be completely decomposed at 320 ° C by adding nitric acid as an oxidant. Lower reaction temperatures can also be used. When purifying acids again for the same use, small amounts of reaction products, for example, can also be left in them, since the impurities do not interfere with the previously carried out process.

It is known to carry out the oxidation reaction with nitric acid in a cast iron residence tank. However, it has proved particularly advantageous to use a quartz or possibly enamel tubular reactor in which the acid is first indirectly heated by heat, for example by radiant heating, to the reaction and oxidation temperature and then subjected to a reaction zone where the acid is contacted countercurrently with oxidants. This results in the formation of partially nitrosyl sulfuric acid (HNOSO 2), which reacts with the remaining organic substances in the post-reaction zone, so that a very pure acid is obtained at the end of the purification step.

The post-reaction zone is directly related to the reaction zone. In the reaction zone of purification step D, the concentration of the acid leaving the high concentration unit may be lower, the same or higher than the concentration of the acid introduced into the unit.

When nitric acid is added to oxidize organic matter, nitric acid comes into contact with the already highly concentrated acid.

6 67355

Similarly, the oxidation of organic matter produces small amounts of water. These small amounts of water can lower the concentration of highly concentrated acid. The concentration of the acid containing 2-3% by weight of organic impurities can be reduced by about 2-3% by weight, in exceptional cases also more, e.g. 5-6% by weight.

In a preferred embodiment of the invention, this amount of water is removed by introducing an additional amount of heat into the reaction zone, i. the acid extracted after the post-reaction zone has the same concentration as the acid in the high concentration unit.

Another possibility is to carry out a second high concentration in the reaction zone. This method gives a very pure and at the same time highly concentrated acid. However, this method is unlikely to be of great economic importance in most cases.

In the drawing, the heating zone and the associated reaction zone as well as the after-reaction zone are in the same apparatus. This is a preferred embodiment of the invention.

However, this need not be the case. For example, in one method and apparatus variation, the reaction zone and the heating zone are different devices.

As already mentioned above, the preconcentration unit consists of one or, more usually, several successively connected stages with different operating pressures. Depending on the concentration and composition of the acid to be treated, the pre-concentration unit is 1-3 steps and only in special cases multi-steps. If the preconcentration unit in its preferred embodiment is multi-stage, then the evaporative vapors are passed to the entire preconcentration unit so that the vapors from the evaporator stage operating at a higher pressure are used to heat the lower stage operating at a lower pressure.

According to the invention, it has proved advantageous to adjust the different steps of the three-stage preconcentration unit to the following ranges: Step 1 Concentration of the acid to be treated: 15-35% by weight of H 2 SO 4;

Outgoing acid content: 20-42% by weight H 2 SO 4 'Temperature range: 30-65 ° C;

Pressure range: 30-100 torr.

67355 Step 2 Concentration of acid to be treated: 20-42% by weight H2SO4;

Outgoing acid content: 28-54% by weight Temperature range: 65-100 ° C;

Pressure range: 150-500 torr.

Step 3 Concentration of acid to be treated: 28-54% by weight I- ^ SO ^;

Outgoing acid content: 60-80% by weight Temperature range: 120-125 ° C;

Pressure range: 500 - 1500 torr.

According to the invention, the two-stage preconcentration unit has the following conditions: Step 1 Concentration of the acid to be treated: 25-50% by weight;

Outgoing acid content: 34-61% by weight K 2 SO 4; Temperature range: 40-100 ° C;

Pressure range: 30-200 torr.

Step 2 Concentration of acid to be treated: 34-61% by weight of H2SO4;

Outgoing acid content: 60-80% by weight of H 2 SO 4; Temperature range: 125-225 ° C;

Pressure range: 500 - 1,500 torr.

Under special conditions, e.g. when the acid already has a sufficiently high concentration, several steps in the pre-concentration can be omitted and the pre-concentration can be performed in one step. In such a case, the following conditions have proved advantageous according to the invention:

One step Content of acid to be treated: 30-60% by weight

Outgoing acid content: 60-80% by weight H2SO4; Temperature range: 60-160 ° C;

Pressure range: 30-200 torr.

In the following, a particularly preferred embodiment of the method and the equipment required therein will be described with reference to the drawings as an example.

Fig. 1 is a general method diagram and a hardware diagram including a two-stage pre-concentration unit, and Fig. 2 is a diagram showing an exemplary embodiment.

67355 Γ

In a two-stage pre-concentration unit, a high-concentration unit and an associated purification step, 30% by weight of crude acid containing organic impurities is regenerated to 98.3% sulfuric acid.

Figure 1 shows A: 11a and B the two steps of the pre-concentration unit, C the high concentration unit and D the purification step with the reaction zone and the post-reaction zone.

The centrifugal pump 2 pumps raw acid (3 333 kg / h, 4000 TOC, 30% I 2 SO 4) down the pipe 1 to the heat exchanger 3, where the crude acid is preheated by the heat of the product acid. The preheated (47 ° C) dilute acid is fed at step 4 into the first stage A circuit. The flow of crude acid is controlled by keeping the liquid level in the first stage evaporation tank 6 constant. The circulating sulfuric acid is heated in a heat exchanger 5 (e.g. a tantalum tube bundle); only in the evaporator tank 6 an amount of water corresponding to the amount of heat obtained evaporates, whereby the formation of deposits on the heating surfaces is avoided.

The first stage A is heated by the heat of condensation of the vapors of the second stage B (line 18). Step A is vacuum (50 torr, 41.5% H 2 SO 4, 50 ° C). The substantially acid-free vapors of the first stage A (924 kg / h, 50 ° C, 50 torr) pass through a droplet separator 7 which traps the entrained sulfuric acid droplets along line 8 to a mixing condenser 9 where the vapors are condensed with injected cooling water. The organic matter distilled with the vapors is separated, as far as possible, from other condensation in a separation vessel 10.

Vacuum and the suction of non-condensable organic vapors and inert gases are achieved, for example, by a water ring vacuum pump 11.

Suitable structural materials used in recycling equipment and the evaporation tank are, for example, fiberglass-reinforced plastic, PVC, graphite, glass or enamel.

The first step A can also be performed in a liquid film evaporator, whereby the sulfuric acid is concentrated in one pass. In either case, the heat exchanger 5 is made of metal, in this case tantalum strip. However, graphite, glass or enamel can also be used instead of metal.

The preconcentrated sulfuric acid leaving the first stage (2,409 kg / h, 50 ° C, 41.5% H 2 O 2) is fed via line 13 by means of a centrifugal pump to the second stage B circuit. The amount fed is controlled by keeping the liquid level of the second stage B constant.

The sulfuric acid recycled by the centrifugal pump 19 is heated in a heat exchanger 15 consisting of successively connected enamelled double jacket tubes and then evaporated only in the evaporating tank 16, thus avoiding deposits on the enamelled heating surface.

The heating of the heat exchanger 15 of stage B is carried out with heat transfer oil, which is led upstream of the sulfuric acid in the jacket space of the double jacketed pipes. The temperature and pressure in the evaporator tank are chosen so that the acid concentration in the steam can be kept practically zero without an additional condensing column and that the heat of condensation of the vapors of the first stage A is utilized (75% I, SO 2, 1 bar, 185 ° C).

The vapors generated in the evaporator tank 16 (1,076 kg / h, 1 bar, 100 ° C) pass through a droplet separator 17, in which the entrained sulfuric acid droplets are kept, via a line 18 to the heat exchanger 5 of the first stage A, where they condense; the condensate obtained is finally passed via line 21 (1,076 kg / h, 1 bar, 95 ° C) to a separation vessel 20.

In this separation vessel, the organic substances condensed with the vapors (e.g. all substances with a boiling point below 185 ° C) and condensed or crystallized in the heat exchanger 5 can be separated. Inert gases are discharged through the waste gas system.

The intermediate acid obtained from the second stage B is passed via line 22 by means of a centrifugal pump 23 to the rectification column 26 of the high concentration stage C. By means of a suitable arrangement of the second and third stages A and B, the pump 23 can be omitted. The amount of acid fed in (1333 kg / h, 185 ° C, 75% I 2 SO 4) is controlled by keeping the level of the evaporation tank 25 of the high concentration step C constant.

In the distillation column 27, a metabolism takes place between the liquid and the steam flowing out of the rotary evaporator 25, whereby the high-boiling constituents and the lower-boiling constituents are enriched in the liquid. With the right choice of intermediate concentration (75% H2SO4), complete absorption of Η2εθ ^: η is obtained by metabolism and heat exchange. If the final concentration of the second stage B is chosen to be too high (e.g., more than 75% I 2 SO 2), the stage C rectification column must be supplemented with a water-cooled amplifier column.

The sulfuric acid recycled by the centrifugal pump 29 is heated in a heat exchanger 24 with sequentially connected enamelled double jacket tubes, and then evaporated only in the evaporator tank 25, thus avoiding deposits on the enamelled heating surfaces. The heating of the heat exchanger 24 is carried out with heat transfer oil, which is led countercurrently to the jacket space of the double jacket pipes enamelled with sulfuric acid.

Surprisingly, it was found that the enamelled steel is chemically very resistant to sulfuric acid, in particular to quite highly concentrated sulfuric acid (98.3%) under the conditions according to the invention. This is all the more surprising given that cracks appear in the enamel at high temperatures, so it is necessary to work under the special conditions of the invention. On the other hand, certain temperatures or temperature differences must not be below or exceeded. For this reason, the high concentration step C is conveniently performed under reduced pressure. The following design has proven to be reliable: in the evaporator vessel (25), a vacuum of 30-100 torr, preferably 50-70 torr, a temperature of 160-250 ° C, a boiling point of 98.3% acid is 240 ° C, the temperature of the heat carrier in the heat exchanger (24) at the end of 310 ° C, the acid temperature at the same point 260 ° C, the temperature of the heat carrier when introduced into the heat exchanger (24) 290 ° C and the acid temperature at the same point 240 ° C. The vapors generated in the evaporator vessel (25) have a different acid content depending on the final concentration. This acid is exchanged in the rectification column 26 by exchange with sulfuric acid flowing to the column.

The vapors of stage C with an acid content in equilibrium with the donated acid are passed through a droplet separator 28, where the entrained sulfuric acid droplets are retained, via line 30 to a mixing condenser 31, where the vapors (313 kg / h, 50 torr, 185 ° C) are condensed with a water jet. Organic substances distilled with vapors (eg all boiling below 240 ° C) and condensed or crystallized in the mixing condenser 31 are separated from the other condensate, if possible, in a separation vessel 32 connected downstream of the condenser. To provide a vacuum and to oxidize non-condensable organic vapors,

II

A water ring vacuum pump 33 is used to remove the gases and other inert gases emitted by 11 67355.

The reaction between the organic substances of the acid containing organic substances and the SO 2 gas in the evaporation tank usually takes place. To avoid reduction of SO 2 to SC 2 and undesired I 2 SO 2 in the condensate, an oxidant, preferably nitric acid, is added.

In the case of ferrous acids, in the high concentration step C, the saturation state is generally exceeded (in a 98.3% acid at 240 ° C about 20 ppm Fe), as a result of which ferrous sulfate precipitates. Because sulfuric acid is in constant motion, ferrous sulfate remains in suspension and cannot precipitate in the evaporator.

In order to avoid the enamel-consuming effect of ferrous sulphate, the rotational speed in double-jacketed pipes and circulating pipes can be limited to 1 meter / s.

The high concentration step is preferably carried out in an apparatus made of enamelled steel, whereby my rectification column may alternatively be made of glass. The recirculation pump can also be made of a material other than enamelled steel, preferably silicon iron.

The highly concentrated, freshly partially purified sulfuric acid leaving the high concentration step C (1,020 kg / h, 98% I 2 SO 2 2,000 ppm TOC) is fed via line 34 by means of a dosing pump 35 to an acid-filled or flushed vertical quartz tube 36. In this quartz tube the concentrated sulfuric acid is heated at normal pressure to near boiling point (e.g. from 240 ° C to 320 ° C).

In one method variant, the tube (the top of the quartz-glass tube 36) is introduced and, if necessary, fed to the rectification column 41 under reduced pressure. When suitably selected, the vacuum will be at the normal pressure in the cleaning zone due to the liquid column of the quartz tube 36. Alternatively, the sulfuric acid may be passed to the top of the heating zone 38, from where it may be drained as a film to improve heat transfer.

Heat transfer takes place by means of thermal radiation, i.e. the quartz tube 36 is surrounded by a concentric radiation sheath, which in turn is surrounded by an electric heating element 37 (or possibly another heating source, e.g. flue gases) is used. Thermal energy is transferred as radiation from the heating point 12 67355 to the radiant tube and from this as emitting thermal radiation inside the quartz tube and is absorbed by the acid.

In heating zone 38, sulfuric acid (e.g., 90-97% I 2 SO 4) not concentrated to the azeotropic point can be concentrated to 98% or 98.3%. When a large amount of steam is formed, it is preferred that the heating and, in this case, the concentration zone below the column of liquid contains a filler, suitably filler bodies.

The vapors generated during concentration in the heating zone 38 have different acid concentrations depending on the acid inlet concentration, pressure, and temperature. This acid must be metabolized in a rectification column 41, where it donates the acid it contains to the receiving liquid. Water can be used as the receiving liquid, in which case the removed washing water enriched with sulfuric acid is returned to the purification stage or fed to the high concentration stage together with dilute acid. The receiving liquid may also be a dilute acid (when the H 2 SO 4 content is ai], e 75% by weight).

The vapors generated during the concentration are passed through a rectification column 41, where said metabolism takes place, to a condenser 42. A vacuum pump 43 is used to create a vacuum and to extract the exhaust gases and other inert gases generated by oxidised organic substances. , 97% by weight (SO 2, 320 ° C) flows from the heating zone 38 to the reaction zone 39.

The reaction zone comprises a lower part of a quartz tube 36 surrounded by a concentric radiation jacket, which in turn is heated by an electric heating element (or possibly another heat source such as flue gas). The heat energy generated by the heating is transferred to the radiant tube and as emitting heat radiation inside the quartz tube, where it is absorbed by the acid.

In the lower part of the quartz tube 36, the oxidation of the organic substances takes place with an oxidant added at 44 (e.g. 65% nitric acid), which is passed as steam through a perforated plate to be in the form of small evenly distributed vapor bubbles. If the acid is highly impure, it is preferable to use a filler, suitably fillers, in the reaction zone. If necessary, topping

II

67355 13 may be in the entire quartz tube. Upon addition of nitric acid, some of the less desirable nitrosyl hydrogen sulfate may also be formed in the product acid. This is again decomposed into a post-reactor by reaction with organic substances.

It has proved advantageous to connect the heating zone and the possible concentration zone as well as the purification zone in sequence. The SO 2 gases generated by the concentration in the heating zone 38 are again absorbed in the upper part of the quartz glass tube 36 by the colder acid. Unreduced NOx gases rising from the reaction zone can further react with organic substances in the heating zone. According to the invention, there is no clear boundary between the heating and reaction zones.

The incomplete reduced NO 2 gases from the purification step D as well as the reaction gases can be passed through a pressure relief valve in line 45 to the evaporation tank of the high concentration stage C, where the excess NO 2 gas can react with organic substances while at least partially preventing SO 2 reduction.

Concentrated sulfuric acid (320 ° C, 98.3% H 2 SO 4) leaving the trunks from the after-reactor 40 is cooled in a stirred condenser 46 built next to the after-reactor with secondary circulating acid of the same concentration and purity. Prior to feeding to the stirred-cooler 46, the secondary circulating acid is cooled in a heat exchanger 3 by crude acid and a water-cooled heat separator 47 connected thereto. The product acid is taken from the secondary circuit and, if necessary, passed to a plate separator 54 where the suspended ferrous sulfate is separated.

The sulfuric acid introduced into the purification step D has a concentration of 90 to 98% by weight, and the sulfuric acid obtained therefrom has a concentration of 90 to 98.3% by weight and a temperature of 225 to 330 ° C.

Heating of the second and third stage B and C heat exchangers 15 and 24 is performed in a central heating system comprising a high temperature heater 48, a burner 49, a recirculation pump 50, an air preheater 51, a combustion air blower 52 and a flue 53. This heating system also has the advantage of heavy oil as fuel. Both heat exchangers 15 and 24 can be connected in series or in parallel on the heat carrier side.

6 7 3 5 5 14

The reduction of NO gases in the exhaust gas is also

X

possible to carry out in a special burner 49 with a stoichiometric amount of air.

By the method described, and by the equipment shown, concentrated, properly purified sulfuric acid is obtained up to the azeotropic point.

Claims (9)

67355 15 A process for the regeneration of impurity-containing sulfuric acid by indirect heating in an enamel-coated apparatus, characterized in that the acid is initially concentrated to an outlet concentration of 60-80% by weight in an indirectly heated single-stage, possibly also multi-stage, pre-concentrated to a high concentration unit where it is concentrated to a concentration of 90-98.3% by weight H „SO. at a temperature of 160-250 ° C and a pressure of 4-13.3 x 10J Pa and finally subjected to a purification step in which a temperature of 220-350 ° C is maintained and in which the acid is purified at atmospheric pressure or reduced pressure, the enamel layer still having at least 10% under all operating conditions: n residual compressive stress. Process according to Claim 1, characterized in that the acid is concentrated in a three-stage preconcentration unit in a first stage to an outlet concentration of 20 to 42% by weight of H 2 SO 4 at a temperature of 30 to 65 ° C and a pressure of 4 to 13.3 x 10 3 Pa, in a second stage to an outlet concentration of 28 to 54% by weight H_SO. at a temperature of 65- o 3 2 4 100. and a pressure of 20-66 x 10 Pa, and in a third step to an outlet concentration of 60-80% by weight at a temperature of 125-225 ° C and a pressure of 66-200 x 103 Pa. Process according to Claim 1, characterized in that the acid is concentrated in a two-stage preconcentration zone in a first step to an outlet concentration of 34 to 61% by weight at a temperature of 40 to 100 ° C and a pressure of 4 to 26.7 x 10 3 Pa, and in a second step to an outlet concentration of 60 to 80 wt% H9SO. at a temperature of 125-225 C and a pressure of 66-200 x 10J Pa. Process according to one or more of Claims 1 to 3, characterized in that in a multi-stage pre-concentration unit, different stages are operated at different pressures. Process according to one or more of Claims 1 to 4, characterized in that, in a multi-stage pre-concentration unit, steam from the higher-pressure evaporation stage is used to heat the lower-pressure stage. 16 6 7 3 5 5 Process according to one or more of Claims 1 to 5, characterized in that the crude acid preheated with concentrated acid is pre-concentrated to about 75% by weight, the vapors of the second normal pressure stage being used to heat the first vacuum stage and the second pre-concentrating stage followed by a high concentration. heated by a heat canister cycle. Process according to one or more of Claims 1 to 6, characterized in that the high-concentration unit is operated under conditions such that the acid is concentrated to more than 96% before it is introduced into the purification unit. Apparatus for use in carrying out the method according to one or more of claims 1 to 6, characterized in that it comprises heat exchangers (15, 24) consisting of enamelled double jacketed tubes for heating acid in a pre-concentration unit, the first pre-concentration step (A) comprising e.g. (5), followed by the evaporation tank (6), that the subsequent second preconcentration step (B) comprises, for example, a heat exchanger (15) of enamelled double-jacketed steel tubes and an evaporation tank (16) arranged downstream of the heat exchanger, and preconcentration steps C) comprises a heat exchanger (24) consisting of enamelled double jacketed steel tubes and an evaporation tank (25) arranged behind it, and that the next purification step (D) comprises a radiation-heated quartz glass tube (36), a heating zone (38) for heating the acid, n reaction zone (39) and the subsequent post-reaction zone (40). Apparatus according to claim 8, characterized in that it comprises in the purification zone a device through which the oxidant can be introduced into the high concentration stage in the form of small droplets or bubbles upstream of the liquid contained in this stage. Il 17 67355