EP0000515B1 - Process for the separation of sulphur dioxide from a gas stream and installation for carrying out the process - Google Patents

Description

The invention relates to a process for separating S0 ₂ from a gas stream which contains it at least temporarily in a concentration which is inadmissibly high for discharging into the ambient air with the production of sulfuric acid by the nitrogen oxide process, in the latter the gas containing SO ₂ first being a denitrification zone or a gas dewatering zone serving pretreatment zone and then the denitrification zone, then the main area of the SO ₂ processing zone, consisting of at least one full-body tower and then a nitrogen oxide absorption zone, wherein it flows through the relevant area of this zone in at least one of the two areas of the SO ₂ processing zone circulating dilute acid of concentrations below 70% by weight (55 ° Be) H ₂ SO _{4 is brought} into contact, while in the absorption zone the nitrogen oxides released in the denitrification zone are taken up by sulfuric acid and nitrous acid from the absorption zone t a content of 70 to 85 wt .-% (55 to 63.5 ° Be) H ₂ S0 _{4 is} removed and transferred to the denitrification zone.

• - Winnacker-Küchler, Chemische Technologie, Bd. II Carl Hauser-Verlag, München, 1970, Seite 38 ff.
• - Ullmann's Enzyklopaedie der Technischen Chemie, 1964, Bd. 15, Seite 432 ff.

The nitrogen oxide process for the production of sulfuric acid is over 100 years old. The process is described in the following books, for example:

• - Winnacker-Küchler, Chemical Technology, Vol. II Carl Hauser-Verlag, Munich, 1970, page 38 ff.
• - Ullmann's Encyclopedia of Technical Chemistry, 1964, Vol. 15, page 432 ff.

In the second reference, the term «nitrose process is used.

In order to keep the air clean, more and more relatively low concentrated SO ₂ gases have to be processed recently. Although the nitrogen oxide-sulfuric acid process is superior to the contact process for low concentration SO ₂ gases in terms of investment and operating costs, the process has so far not been able to establish itself for environmental protection purposes. One reason is likely that it has been relatively difficult to operate nitrogen oxide-sulfuric acid plants with exhaust gases of only very low concentrations of nitrogen oxides, and above all so that no yellow plume is visible in the ambient air at the discharge chimney.

• - vermehrte Zugabe von Salpetersäure,
• - Verminderung der Kühlung in den Produktionstürmen,
• - vermehrte Zufuhr von Wasser in die Produktionstürme.

Nitric oxide-sulfuric acid plants mostly consist of several cascaded packed towers with different acid cycles. High nitrogen oxide losses occur both with an excess of NO and an excess of NO _Z in the exhaust gases. Optimal absorption of nitrogen oxides requires a correct NO: N0 ₂ ratio. NO is an invisible gas. With incorrect control of a nitrogen oxide-sulfuric acid system, large amounts of nitrogen oxides can therefore be lost with the exhaust gas in a short time without a yellow flag becoming visible. It is known that it is advantageous to operate a nitrogen oxide-sulfuric acid system in such a way that the exhaust gases have a barely perceptible but nevertheless recognizable yellow color. There are several measures known to increase the N0 ₂ content of the exhaust gases and reduce the NO content in a nitrogen oxide-sulfuric acid system:

• - increased addition of nitric acid,
• - reduction of cooling in the production towers,
• - increased supply of water to the production towers.

A corrective action taken in one system packing tower always has repercussions on other packing towers and it is relatively difficult to keep the overall system in balance. For this reason, known nitrogen oxide-sulfuric acid systems generally require experienced operating personnel in order to be able to operate the system with low nitrogen oxide losses.

It is known, for example, that a loss of nitrogen oxides from the system must be compensated for by adding nitric acid or concentrated nitrous gases, such as those resulting from the catalytic combustion of ammonia (make-up). In known nitrogen oxide-sulfuric acid plants, the addition of nitric acid or the addition of strong nitrous gases is carried out either at the top of the denitrification tower (DE-A-26 09 505) or in front of a production tower which is covered with acid of more than 70% by weight H ₂ SO _{4 is} sprinkled. The need for the addition of nitric acid or strong nitrous gases can be seen in known nitrogen oxide-sulfuric acid systems above all by the nitrous content of the acid flowing out of the absorption zone into the bottom of this zone. It has the highest nitrous content of any acid in the system.

In the following description of the invention, all towers of a system which are sprinkled with an acid of more than 70% by weight H _Z S0 ₄ (absorption acid) and which release gas with more gaseous nitrogen compounds than are contained in the inlet gas of the tower are included in the denitrification zone. So-called “production towers of a PETERSEN tower system that meet this condition are therefore by definition considered to belong to the denitrification zone. Whether the acid in the outlet of a tower is nitrous or free of nitrose is not used as a criterion for the designation "denitrification tower" or "denitrification apparatus". The upstream gas drying tower explained below is sprinkled with acid under 70% by weight H _Z S0 ₄ and is therefore not included in the denitrification zone, even if it releases gaseous nitrogen oxides.

In known nitrogen oxide-sulfuric acid systems gene, nitric acid or other nitrogen oxide-containing substances are added at any point in the denitrification zone just defined. The regulation of the color of the exhaust gases of the system, that is to say the control of the ratio NO: N0 ₂ in the exhaust gases, is carried out in most cases in known nitrogen oxide-sulfuric acid plants by adding more or less water into the acid cycle through the denitrification zone and the nitrogen oxide absorption zone.

It is therefore necessary to continuously monitor the system with many analyzes of the density and the nitrous content of the circulating acids, and it is hardly possible to automate the regulation of the known system. Another disadvantage of known nitrogen oxide-sulfuric acid systems is their great slowness to corrective measures. If, for example, a yellow colored exhaust plume arises from the system due to a drop in the S0 ₂ content of the inlet gases, corrective measures are required that take effect only after several hours and make this yellow plume disappear again.

As stated in GB-A-759 056 (Saint-Gobain) already in 1956, the difference between the temperatures of the gases at the inlet and at the outlet of the series of reaction chambers of the SO ₂ processing zone provides what difference according to the US-A- 1 486 757 (Jensen) from 1924 to determine nitrogen oxide losses and to compensate for them by introducing nitric acid into the first S0 ₂ processing chamber, for example, is not a reliable criterion, since this temperature difference is not only due to the more or less correct functioning of the System but also depends on the daily fluctuations in the outside temperature. Otherwise, this method would also not be applicable to intensive processes for the production of sulfuric acid, such as, for example, that of DE-A-26 09 505 and the process of the present invention, since the temperature differences occurring in this case would be far too small to control the system.

• (a) den NO_x-Gehalt in der Mitte und am Eingang der S0₂-Verarbeitungszone zu messen und
• (ß) Aenderungen im NO_x-Gehalt durch Aenderungen in der Dichte der umlaufenden Säure auszugleichen, wozu Salpetersäure in die Denitrierungszone eingeführt werden soll. Jedoch würde eine solche Massnahme, welche auch in der DE-A-2609505 empfohlen wird, keineswegs die Reaktivität der Dünnsäure in der SO₂-Verarbeitungszone in einem System nach der DE-A-26 09 505 beeinflussen.

That is why it is recommended in the GB-PS mentioned

• (a) measure the NO _x content in the middle and at the entrance to the S0 ₂ processing zone and
• (ß) Compensate changes in the NO _x content by changes in the density of the circulating acid, for which purpose nitric acid is to be introduced into the denitrification zone. However, such a measure, which is also recommended in DE-A-2609505, would in no way influence the reactivity of the thin acid in the SO ₂ processing zone in a system according to DE-A-26 09 505.

DE-C-1 140 909 (Ruhr-Schwefelsäure GmbH) describes an adjustment of the ratio of NO: N0 ₂ and the content of NO + N0 ₂ in the gas phase by opposite changes in the density of the acid. Such changes are said to be the most effective measure to influence the oxidation rate of the process.

According to this patent document, changes in the nitrous content would take place with the necessary speed during the operation of the plant. However, the acid which circulates in the system according to this patent is a strong acid with a content of 80% by weight H _Z S0 ₄ , and not a thin acid of less than 70% by weight H ₂ SO ₄ , as in the SO ₂ processing zone of DE-A-26 09 505 circulates.

It is a main object of the invention to solve a problem which occurs in particular in the production of sulfuric acid according to the intensive process described in DE-A-26 09 505 and in a decrease in the reactivity of the thin acid circulating through the S0 ₂ processing zone of the system consists. This problem did not occur in previous processes. The invention further aims to achieve an improved, very rapid adaptation of the NO: N0 ₂ ratio in the exhaust gas emitted from the system, so that this exhaust gas leaves the system practically without formation of a yellow flag caused by the presence of N0 ₂ in the exhaust gas while avoiding inadmissible losses of colorless NO from the system.

Another object of the invention is to achieve an improved emission of gases from the system mentioned, in which the nitrogen oxide content is below a legally prescribed maximum limit (e.g. 400 ppm).

The invention explained below also primarily solves the problem of correcting the composition of the exhaust gases of a nitrogen oxide-sulfuric acid system within a few minutes and also enabling automation of the overall system.

The invention is preferably applied to the method described in the introduction, which is described in particular in DE-A-25 10 294 and 26 09 505.

• (a) der Gehalt an NO im Gasstrom in der Absorptionszone oder stromab der Absorptionszone oder gleichzeitig sowohl in der Absorptionszone als auch stromab derselben gemessen wird, und dass
• (b) beim Ueberschreiten eines bestimmten NO-Grenzgehalts oder bei Ueberschreiten eines Steilheitsgrenzwertes des Anstiegs je Zeiteinheit der NO-Gehalts im Gasstrom an der oder den NO-Messstellen in den Dünnsäurekreislauf mindestens eines der beiden Bereiche der S0₂-Verarbeitungszone eine weiter unten näher definierte Stickstoff-Sauerstoff-Verbindung in einer bestimmten Menge eindosiert wird, die einerseits ausreicht um die Ueberschreitung des genannten NO-Grenzgehalts oder Steilheitsgrenzwertes zu beseitigen, aber andererseits nahe an den genannten Grenzwerten verbleibt, wobei eine Aenderung der Säurekonzentration in der Absorptionszone praktisch vermieden wird, und wobei für den Fall, dass eine gasförmige Stickstoff-Sauerstoffverbindung verwendet wird, diese zuerst in Dünnsäure gelöst und letztere dann, wie oben beschrieben, in den Dünnsäurekreislauf eindosiert wird.

According to the invention, the tasks described above are achieved and the stated objectives are achieved by a method of the type described at the outset, which is characterized in that

• (a) the content of NO in the gas stream in the absorption zone or downstream of the absorption zone or simultaneously both in the absorption zone and downstream thereof is measured, and that
• (b) at least one of the two areas of the S0 ₂ processing zone defines one defined in more detail below when a certain NO limit content is exceeded or when a steepness limit value of the increase per unit time of the NO content in the gas stream at the NO measurement point (s) in the thin acid circuit is exceeded Nitrogen-oxygen compound is metered in in a certain amount, which is sufficient on the one hand to exceed the NO limit or steepness limit mentioned eliminate, but on the other hand remains close to the mentioned limit values, practically avoiding a change in the acid concentration in the absorption zone and, if a gaseous nitrogen-oxygen compound is used, first dissolve it in thin acid and then the latter as described above , is metered into the thin acid circuit.

The specific amount of the NO limit value or the slope limit value cannot be specified in general, but depends on the system and in particular on the size of the various reaction apparatuses (towers). In the production of sulfuric acid using the nitrogen oxide process and in particular Petersen's intensive tower process, it is always necessary to determine the conditions under which the operation of a system is carried out optimally and to set the measuring devices for checking the specified limit values so that they correspond to this optimal operation.

It has been found that an optimal NO: NO ₂ ratio can be achieved in the absorption zone or in the exhaust gases of the absorption zone within a few minutes, and that it is also possible to automate the entire system if the nitric acid or other liquid or gaseous substances which contain nitrogen-bound nitrogen, depending on the NO content of the gases leaving the absorption zone, are metered into the thin acid or the SO ₂ processing zone sprinkled with thin acid. The thin acid loses added nitric acid or other compounds containing nitrogen oxide relatively quickly. During the consumption of the nitrogen content of the thin acid, a surprisingly strong reaction is triggered, which leads to an acceleration of the SO _z processing.

The inventive measures do not represent a pure "make-up" in response to an increase in the NO content due to an insufficient NO _x content in the system. Rather, the increase in the NO content as measured according to the invention shows the present case System primarily a decrease in activity (= ability of the thin acid of the interface between liquid and gas phase to bring about the reaction of S0 ₂ to sulfuric acid) of the circulating thin acid in one of the sectors or in both sectors of the S0 ₂ processing zone.

By adding relatively small amounts of nitric acid or the like nitrogen-oxygen compound, the activity of the thin acid in the SO ₂ processing zone is just sufficiently stimulated to continue the conversion of SO _{2 to} S0 ₃ or sulfuric acid, and surprisingly even then, if there is a deficiency of nitrogen oxides in the overall balance of the system. This makes it possible to operate the system practically at a minimum level of NO _x , thus avoiding any excess of NO. The stimulation mentioned is far greater than was to be expected from the slight increase in the NO _x content in the system, as occurs as a result of measures (a) and (b).

Changes as they occur in the system, especially those that occur due to fluctuations in the S0 ₂ content of the inlet gas, are compensated for within a few seconds, and the stimulating effect of every small addition of nitrogen-oxygen compound lasts about 10 to 15 minutes, depending on the circulating one Amount of thin acid.

The measures defined under (a) and (b) above are therefore not primarily intended to increase the NO _x content of the system in such a way that a deficiency in the overall balance of NO _X in the system is compensated for (make-up) , but as I said, to stimulate the activity of the thin acid circulating in the SO ₂ processing zone each time a decrease in this activity is indicated by the measure (a) mentioned.

It is essential for the invention that the nitric acid or the nitrogen oxides are brought into the liquid phase (the thin acid). The liquid containing nitrogen-oxygen compound (for example nitric acid) can either be mixed directly with the thin acid or introduced into the SO ₂ processing zone provided with packing elements, in such a way that mixing of the added substances in this zone, which is preferably filled with packing elements, occurs Liquid and thin acid takes place.

“Nitrogen-oxygen compounds” are those compounds that normally occur or are used in the nitrogen oxide process, ie NO, NO ₂ , N ₂ O ₃ , nitrous or nitric acid and possibly also solid “chamber crystals”. Nitrous acid is immediately oxidized to nitric acid in the nitrogen oxide process, or releases N ₂ 0 ₃ or forms nitric acid with nitric acid.

Gaseous substances which contain nitrogen-bound nitrogen are therefore NO- or N0 ₂ - or N ₂ 0 ₃ -containing gases, liquid substances are in particular nitric acid itself or nitrous sulfuric acid.

The amount of metered substance containing nitrogen-oxygen compound is preferably increased, to the extent that the NO content of the gas stream increases in or after the absorption zone.

In simpler systems, however, the inflow rate of substance containing nitrogen-oxygen compound can also be kept constant as long as the NO content of the gas stream in or downstream of the absorption zone exceeds the limit content of NO.

The amount of nitrogen-oxygen compound added to the pretreatment part or the main part of the SO ₂ processing zone with each corrective measure is compared to the total amount of nitrous oxide circulating in the system only very small.

In a system in which 100 tons of acid circulate and in which the content of nitrose in the acid phase is 3% by weight, about 3 tons of nitrose circulate in the system. The corrective measure carried out according to the invention in response to an increase in the NO content measured at the points specified under (a) requires the addition of about 100 to 1,000 ml of nitric acid or the like nitrogen-oxygen compound to the circulating through the SO ₂ processing zone. Thin acid ». Such small amounts result in the desired correction effect in less than a minute, and in no way constitute a make-up of losses occurring in the overall balance of NO _x in the system. The latter would require the addition of significantly larger amounts of nitrous acid or nitric acid, whereby such an addition, as is customary in the known intensive processes, preferably takes place in the denitrification zone.

In another mode of operation, the amount of substance containing nitrogen-oxygen compound added is increased to the extent that a steepness limit value of the increase per unit time of the NO content in the gas stream in or downstream of the absorption zone is exceeded. Nitric acid is preferably used as the nitrogen-oxygen compound.

In addition, the nitrogen-oxygen compound preferably consists of nitrogen oxides generated by ammonia combustion.

The substance mentioned can be formed in particular by absorption of nitrogen oxides in sulfuric acid.

The metering of said substance is preferably interrupted when the NO content of the gas stream downstream or absorption zone drops below a predetermined value which is lower than the aforementioned limit NO content; or the addition of said substance is reduced if the drop below a predetermined steepness limit value per unit time of NO content in the gas stream downstream of the absorption zone, the latter steepness limit value roughly corresponding to the steepness limit value according to (b).

• (a) zusätzlich die N0₂-Konzentration des Gasstroms stromab der Absorptionszone gemessen werden und
• (b) bei Ueberschreiten eines bestimmten N0₂-Gehalts oder bei Ueberschreiten einer Steilheitsgrenze des Anstiegs des N0₂-Gehalts im Gasstrom an der N0₂-Meßstelle die Eindosierung der genannten Substanz in den Dünnsäurekreislauf der SO_z-Verarbeitungszone unterbrochen werden.

With particularly advantageous operation:

• (a) additionally measure the N0 ₂ concentration of the gas stream downstream of the absorption zone and
• (b) If a certain N0 ₂ content is exceeded or if the slope of the increase in the N0 ₂ content in the gas stream at the N0 ₂ measuring point is exceeded, the metering of the substance mentioned into the thin acid circuit of the SO _z processing zone is interrupted.

• (c) bei Ueberschreiten eines zweiten, höheren Grenzwertes des NO₂-Gehalts im Gasstrom stromab der Absorptionszone oder bis Ueberschreiten einer zweiten, höheren Steilheitsgrenze des Anstieges des N0₂-Gehalts im Gasstrom stromab der Absorptionszone die Menge der aus der letzteren Zone ausfließenden nitrosehaltigen Säure gesteigert und entsprechend aus der Denitrierzone stammende nitrosearme oder nitrosefreie Säure in die Absorptionszone eingeführt werden.

You can also:

• (c) at exceeding of a second, higher limit value of the NO ₂ content in the gas stream downstream of the absorption zone or to exceeding of a second, higher steepness limit of the rise of the N0 ₂ content in the gas stream from the absorption zone downstream of the amount of the effluent from the latter zone nitrosehaltigen acid increased and correspondingly from the denitrification zone low or low nitrosic acid are introduced into the absorption zone.

At least part of the substance introduced into the S0 ₂ processing zone can consist of nitrous sulfuric acid removed from the absorption zone, in particular the nitrogen-oxygen compound content of the nitrous sulfuric acid removed from the absorption zone and metered into the S0 ₂ processing zone by adding Nitric acid can be increased.

In a particularly preferred embodiment of the method according to the invention, a pretreatment zone through which the gas stream is passed before being introduced into the denitrification zone is connected upstream of the denitration zone and a part or the total amount of nitrogen-oxygen compound containing substance is added initiated this pre-treatment zone. The portion of the stated amount of substance introduced into the pretreatment zone can be branched off from a nitrous sulfuric acid flowing from the absorption zone into the SO ₂ processing zone. At least part of the amount of substance to be introduced into the pretreatment zone can consist of gaseous nitrogen oxides produced by ammonia combustion, which are introduced into the gas stream before it enters the pretreatment zone.

In this case, too, the nitrogen-oxygen compound content of the nitrogen-oxygen compounds removed from the absorption zone and partly into the main area of the SO ₂ processing zone and partly into the pretreatment zone can be increased by adding nitric acid.

Furthermore, the density of the acid emerging from the denitrification zone can be kept constant by adding thin acid or water, while the density of the thin acid flowing through the main area of the SO ₂ processing zone in the circuit can be kept constant by adding acid from the pretreatment zone or water.

• (a) eine Denitrierungszone
• (b) eine S0₂-Verarbeitungszone und
• (c) eine Stickoxid-Absorptionszone

mit je Zone mindestens einem Gas-Flüssigkeits-Reaktionsapparat, der mit einem für die Aufnahme von aus dem unteren Ende des betroffenen Apparates austretender Flüssigkeit dienenden Sumpf versehen ist,

• (d) eine Leitung für den Gasstrom, welche nacheinander am einen, vorzugsweise unteren Ende der Denitrierungszone in diese eintritt und vom einen, vorzugsweise oberen Ende der letzteren zur S0₂-Verarbeitungszone und von der letzteren zur Stickoxid-Absorptionszone und schließlich vom einen, vorzugsweise oberen Ende der letzteren Zone ins Freie führt,
• (e) eine Kreislaufleitung für Dünnsäure durch die S0₂-Verarbeitungszone, und schließlich
• (f) eine Kreislaufleitung für Säure vom in Gasstromflußrichtung ersten Sumpf der Stickoxid-Absorptionszone zum einen, vorzugsweise oberen Ende der Denitrierungszone und vom Sumpf der letzteren zurück zum einen, vorzugsweise oberen Ende eines Reaktionsapparats der Stickoxid-Absorptionszone, wobei die erfindungsgemäße Anlage dadurch gekennzeichnet ist, daß
• (g) an die Dünnsäure-Kreislaufleitung durch die S0₂-Verarbeitungszone eine Zuleitung für Stickstoff-Sauerstoff-Verbindung in flüssiger oder gasförmiger Phase angeschlossen ist, daß
• (h) eine Meßeinrichtung zum Messen des Gehalts an NO im Gasstrom, die an die Absorptionszone oder an die Leitung (d) für den Gasstrom stromab der Absorptionszone oder gleichzeitig an die Absorptionszone und an die Leitung stromab der Absorptionszone angeschlossen ist, und daß
• (i) eine Regeleinrichtung, welche die Zugabe von Stickstoff-Sauerstoff-Verbindung durch die Zuleitung in Abhängigkeit von den durch die Meßeinrichtung ermittelten NO-Gehalten dosiert, vorgesehen sind.

In a further aspect, the invention relates to a plant for separating S0 ₂ from a gas stream which contains it at least temporarily in a concentration which is inadmissibly high for discharging into the ambient air while obtaining sulfuric acid by the nitrogen oxide process, which plant comprises at least one after the other:

• (a) a denitrification zone
• (b) an S0 ₂ processing zone and
• (c) a nitrogen oxide absorption zone

with at least one gas-liquid reaction apparatus per zone, with one for the absorption of liquid emerging from the lower end of the apparatus concerned serving swamp is provided

• (d) a line for the gas stream, which successively enters the denitrification zone at one, preferably lower end of this and from one, preferably the upper end of the latter to the SO ₂ processing zone and from the latter to the nitrogen oxide absorption zone and finally from one, preferably leads to the upper end of the latter zone,
• (e) a circuit line for thin acid through the S0 ₂ processing zone, and finally
• (f) a circulation line for acid from the first bottom of the nitrogen oxide absorption zone in the gas flow direction to the one, preferably the upper end of the denitrification zone and from the bottom of the latter back to the one, preferably upper end of a reaction apparatus of the nitrogen oxide absorption zone, the system according to the invention being characterized in this that
• (g) a feed line for nitrogen-oxygen compound in the liquid or gaseous phase is connected to the thin acid circuit line through the S0 ₂ processing zone that
• (h) a measuring device for measuring the content of NO in the gas stream, which is connected to the absorption zone or to the line (d) for the gas stream downstream of the absorption zone or simultaneously to the absorption zone and to the line downstream of the absorption zone, and that
• (i) a control device which doses the addition of nitrogen-oxygen compound through the feed line depending on the NO contents determined by the measuring device.

• (k) eine übergeordnete Regelvorrichtung umfaßt, welche die Zufuhr von Stickstoff-Sauerstoff-Verbindung über die Zuleitung gemäß (g) in Abhängigkeit vom durch die NO₂-einrichtung gemessenen NO₂-Gehalt im Gasstrom unterbricht oder vermindert.

A further refined embodiment of the system according to the invention is characterized in that a NO _z measuring device for measuring the NO ₂ content in the gas stream, which is connected to the absorption zone or to the line for the gas stream downstream of the absorption zone, is provided, and that Control device

• (k) a higher-level control apparatus comprises that the supply of nitrogen-oxygen compound through the feed line according to (g) in dependence on the ₂ -einrichtung measured by the NO NO interrupts or reduces ₂ content in the gas stream.

In preferred embodiments of the above system, the feed line for taking up the nitrogen-oxygen compound in the form of nitrous sulfuric acid can be connected to the bottom of a reactor of the nitrogen oxide absorption zone.

An introduction device for nitric acid can be provided in the last-mentioned feed line.

Such a preferred plant according to the invention may comprise a pretreatment zone with reaction apparatus and sump upstream of the denitration zone, the line for the gas stream first leading into this treatment zone, preferably at the lower end and from the other end thereof, to one, preferably lower end of the denitration zone, and one separate, leading through the pretreatment zone from top to bottom circulation line for thin acid and a compensating line between the sumps of the pretreatment zone and the denitrification zone is provided, and a branch line from said feed line to the upper end of the pretreatment zone can be provided.

An ammonia combustion plant can also be provided with a column which serves at least partially to absorb the nitrogen oxides developed therein in sulfuric acid, and a transfer line can connect the base of the column with one, preferably the upper end of a first apparatus in the gas flow direction of the SO ₂ processing zone for the purpose of supplying sulfuric acid containing nitrogen oxide connect to the latter zone.

Furthermore, such a system can comprise a pretreatment zone of the type described above upstream of the denitrification zone, a feed line for nitrogen oxide-containing gases connecting the upper end of the column of the ammonia combustion system to the part of the line introducing the gas stream into the preferably lower end of the pretreatment zone.

The feed line mentioned under (g) can be connected to the bottom of a reactor of the nitrogen oxide absorption zone for the purpose of taking up the nitrogen-oxygen compound in the form of nitrous sulfuric acid, and a branch line can lead from the latter to the upper end of the column of the ammonia combustion plant.

This branch line from the feed line according to (g) can be connected to the upper end of the pretreatment zone with a further sub-branch.

• Fig. 1 eine erste, besonders einfache Ausführungsform einer erfindungsgemäßen Anlage,
• Fig. 2 eine zweite, der in Fig. 1 gezeigten ähnliche Ausführungsform, aber mit Vorbehandlungszone, wobei Stickstoff-Sauerstoff-Verbindung aus der Stickoxid-Absorptionszone nur in die genannte Vorbehandlungszone eingeführt wird ;
• Fig. 3 eine dritte Ausführungsform mit Vorbehandlungszone, wobei Stickstoff-Sauerstoff-Verbindung sowohl in die Vorbehandlungszone als auch in den Hauptbereich der S0₂-Verarbeitungszone eingeführt wird, und außerdem eine Meßeinrichtung für N0₂ vorgesehen ist ;
• Fig. 4 eine weitere, der in Fig. 3 gezeigten ähnliche Ausführungsform mit Vorbehandlungszone sowie mit einer Ammoniakverbrennungsanlage zur zusätzlichen Stickoxiderzeugung ; und schließlich
• Fig. eine Ausführungsform ähnlich derjenigen nach Fig. 4, in welcher aber, ähnlich wie bei den Anlagen nach Figuren 2 und 3, Stickstoff-Sauerstoff-Verbindung enthaltende Säure aus der Absorptionszone sowohl in die Vorbehandlungszone als auch in den Hauptteil der SO₂-Verarbeitungszone eingeführt werden kann. In der in Fig. 1 beschriebenen einfachsten Ausführungsform der erfindungsgemäßen Anlage eines Stickoxid-Schwefelsäure-Systems umfaßt diese die üblichen mit Füllkörperschichten versehenen Gas-Flüssigkeits-kontaktapparate, die im folgenden der Einfachheit halber als « Türme bezeichnet sind.

Further details of the invention will be apparent from the following description in conjunction with the accompanying drawings, in which:

• 1 shows a first, particularly simple embodiment of a system according to the invention,
• FIG. 2 shows a second embodiment, similar to that shown in FIG. 1, but with a pretreatment zone, wherein nitrogen-oxygen compound from the nitrogen oxide absorption zone is only introduced into said pretreatment zone;
• 3 shows a third embodiment with a pretreatment zone, the nitrogen-oxygen compound being introduced both into the pretreatment zone and into the main area of the SO ₂ processing zone, and a measuring device for NO _{2 is also} provided;
• FIG. 4 shows a further embodiment, similar to that shown in FIG. 3, with a pretreatment zone and with an ammonia combustion system for additional nitrogen oxide generation; and finally
• Fig. An embodiment similar to that of FIG. 4, but in which, similar to 2 and 3, nitrogen-oxygen compound-containing acid from the absorption zone can be introduced both into the pretreatment zone and into the main part of the SO ₂ processing zone. In the simplest embodiment of the system according to the invention of a nitrogen oxide-sulfuric acid system described in FIG. 1, it comprises the usual gas-liquid contact apparatuses provided with packing layers, which for the sake of simplicity are referred to below as “towers”.

The denitrification tower 2 is followed in the gas flow direction by the S0 ₂ processing zone with a first tower 3 and subsequent second tower 4, and by this the packing layers 5 and 6 of the nitrogen oxide absorption zone combined in a single tower. Similar to the system described in DE-A-26 09 505, the SO ₂ -containing exhaust gas to be treated is introduced via the inlet line 102 into the lower end of the denitrification tower 2 and passes via line 32 from the upper end of the tower 2 into the upper end of the tower 3 of the S0 ₂ processing zone, from the lower end of the tower 3 through the gas line 42 into the lower end of the tower 4 of the same zone and from the upper end of the tower 4 through the gas line 52 into the lower end of the tower with the packing layer 5 and then into the lower end of the packing layer of the same tower and from the upper end of the latter packing layer via the exhaust line 72 with the aid of the blower (fan) 167 to the outside.

As in the aforementioned known system, thin acid is circulated in the system shown in FIG. 1 through the thin acid line 33 by means of a pump 35 via a heat exchanger 34 to the upper end of the packed layer of the tower 3 and collects at the lower end of this tower in Sump 31 from which the thin acid is pumped up by means of the pump 45 through the line 43 and the heat exchanger 44 to the upper end of the packed layer of the tower 4. The thin acid cycle through the S0 ₂ processing zone is closed by pumping off the thin acid from the sump 41 of the tower 4 via line 33. Sulfuric acid of about 65% by weight H ₂ SO ₄ is preferably used as the thin acid in this circuit.

The density of the circulating acid in the SO ₂ processing zone is measured by the density measuring device 233 at the outlet of the thin acid from the sump 41. Either water or acid can be used in a manner known per se to regulate the density. The corresponding valves and lines have been omitted from FIG. 1 for the sake of a better overview.

Similar to the known system, from the sump 51 of the common tower of the nitrogen oxide absorption zone, which is provided below the packing layer 5, nitrous sulfuric acid is fed via line 54 with the pump 25 into the heat exchanger 24 and after heating in the latter through the acid line 23 sprayed onto the upper end of the packing layer of the denitrification tower 2. after passing through the packed layer of tower 2, the denitrified acid collects in the sump 21 of this tower and is pumped through line 121 to heat exchanger 64 and from the latter through line 63 to the upper end of packed layer 6 of the tower of nitrogen oxide. Pumped up absorption zone.

The acid from the packing layer 6 flows directly onto the packing layer 5, while the gas to be treated passes from the packing layer 5 directly into the packing layer. In this case, no gas seal is required between the two packing layers.

Denitrated sulfuric acid with a content of 70-85% by weight of H ₂ SO ₄ can be branched off from line 63 via line 95 and valve 96 into the acid container 90 and can be removed from the system via the removal line at P.

The denitrification step therefore takes place in a known manner in the packed layer of the tower 2. The S0 ₂ processing step takes place in the packed layers of towers 3 and 4 and the nitrogen oxide absorption step in the packed layers of towers 5 and 6 S0 ₂ processing zone (towers 3 and 4) a sulfuric acid of less than 70 wt .-% H ₂ SO _{4 is} used (thin acid). In the denitrification zone (tower 2) and in the nitrogen oxide absorption zone (towers 5 and 6) an acid between 70 and 85% by weight H ₂ SO ₄ (absorption acid) is used. The concentration of the absorption acid is preferably 72 to 80% by weight of H ₂ SO ₄ . As with the known nitrogen oxide-sulfuric acid process, nitrogen oxide is also released from the supplied acid in the denitration zone in the method of the invention and passes via the gas path into the absorption zone, where the gaseous nitrogen oxide is absorbed by sulfuric acid. According to FIG. 1, this process takes place as follows: the nitrous-free absorption acid, which flows from the packed layer of the tower 2 into the sump 21, is cooled in the cooler 64 and reaches the packed layer via the pump 65 and the line 63 and then flows into the Packing layer and from there into the sump 51 and then via the line 54 to the pump 25 and via the acid heater 24 and line 23 back to the packing layer of the tower 2, where the nitrogen oxides which have been taken up in the layers of the towers 5 and 6, be given back to the gas.

In a known manner, acid is additionally pumped from the sumps of the towers to the top of the same tower in order to increase the mass exchange between the gas and the acid. However, the corresponding devices are not shown to simplify the scheme; only those acid lines are shown in the diagram which have an acid bring about exchange between different packing layers.

According to the invention, a container 80 is now provided in the system according to FIG. 1, in which there is nitric acid or a sulfuric acid with a high content of nitrous and / or nitric acid. The nitric acid is passed through line 83 into container 80.

A line 82 provided with a valve 56 is connected to the sump 51 of the nitrogen oxide absorption zone and opens into the container 80. Nitrous acid-containing sulfuric acid can be drained through line 82 into container 80. The latter container is now connected according to the invention via line 37, in which the pump 85 and the valve 46 are provided, to the upper end of the first tower 3 of the SO ₂ processing zone.

An analyzer 285 continuously measures the NO content of the gases flowing out of the packed layer of the tower of the absorption zone. As soon as the NO content rises above a permissible value, or if the concentration of NO rises above a predetermined speed, the pump 85 is started and strongly nitric acid-containing sulfuric acid flows via the valve 46 and the line 37 into the packed layer of the Tower 3.

As soon as analyzer 285 indicates that the NO content of the gases leaving the absorption zone has dropped below a set value, or when the rate of NO removal exceeds a fixed value, pump 85 is deactivated. Instead of switching the pump 85 on or off, it is of course also possible to bring about an increase or decrease in the flow of strongly nitric acid-containing sulfuric acid, which is improved in terms of control technology. The dead times in the control system are only a few minutes. Nevertheless, it is advisable to take appropriate control measures to prevent the concentrations from swinging back and forth. Such control measures are well known.

In the system according to FIG. 1, the analysis device 285 is located at the outlet of the packing layer of the absorption tower 6. It is also possible to measure the nitrogen oxide content at the entry of the gases into the packing layer 5 of the absorption tower or between the packing layers 5 and 6 of the absorption zone. At these points, the nitrogen oxide content is of course correspondingly higher than at the exit of the absorption zone. However, a measurement within the absorption zone can also be used as a controlled variable. The dead time is then reduced. It is also possible, and in particular cases expedient, to carry out an NO measurement simultaneously at different points in the nitrogen oxide absorption zone.

It was also found that it is advantageous to carry out the addition of liquid or gaseous substances according to the invention which contain nitrogen-bound nitrogen as far as possible in the front of the system. This measure makes it possible to exert a large influence on the NO content of the exhaust gases of the plant with relatively small amounts of added nitrogenous substances. The invention thus makes it possible to automate a nitrogen oxide-sulfuric acid system.

This measure is implemented in the embodiment of the system according to the invention shown in FIG. 2. In the system according to FIG. 2, in a manner known per se from DE-A-26 09 505, moist SO ₂ -containing gases are brought into contact with thin acid in a pretreatment tower 1 upstream of the denitrification zone, part of their water vapor content being removed from the gases . In the system shown in FIG. 2, the packing layer 1 takes over this function. The thin acid flowing out of the packing layer of the tower 1 into the sump 71 of this tower is now pump 75 via line 73 through the acid-heating heat exchanger 74 and further through line 73 into the upper end of the tower 7, the packing layer of which is an acid dewatering zone represents, promoted.

The acid can be additionally heated in the heat exchanger 74 so that the water release of the acid in the packed layer of the tower 7 is increased. According to the invention, the tower 1 of the pretreatment zone shown in FIG. 2 represents a pre-area of the SO ₂ processing zone.

The separated from the thin acid circuit of the main area of the S0 ₂ processing zone thin acid circuit for the pretreatment zone via line 73 is now closed in that the thin acid, which is trickled at the top of the tower 7 on the packing layer, through the latter layer and through the the lower end of the tower 7 gas-liquid seal 77 flows back into the packing layer located in the tower 1 below.

The amount of water absorbed by this acid during its circulation in the packed layer of the tower 1 is thus passed directly to the end of the system, i.e. led into the packed layer of the tower 7 and released there to the exhaust gas of the system supplied via line 72.

When passing through the pretreatment zone in tower 1, the thin acid has already warmed up; the additional heating in the heat exchanger 74, as stated, increases the release of water in the packed layer of the tower 7. The evaporated water leaves the system together with the SO ₂ -free exhaust gases via line 705.

This dewatering measure, by means of which the gas stream is dried in the pretreatment zone, means that less or no water vapor is transported further in the gas stream, thereby making it possible to maintain a sufficient acid concentration in the packing layer of the denitrification tower 2 and in the packing layers 5 and 6 of the nitrogen oxide absorption zone (e.g. 75% by weight H ₂ S0 ₄ ).

By regulating the supply of heating medium to the heat exchanger 74, the amount of water dispensed can be controlled, as a result of which a desired concentration of the acid in the circuit can be set via the packed layers of the towers 7 and 1. As already explained above, the acid flowing out of the packing layer of the tower 7 passes directly to the packing layer of the tower 1 via the gas seal 77.

According to the invention, nitric acid or sulfuric acid or nitrous or nitric acid nitric acid is now conveyed from the container 80 via the pump 85 through the line 37 to the valve 86 and further via line 78 to the upper end of the packed layer of the tower and sprayed into the packed layer of the tower 1 when that NO analyzer 285 opens valve 86 and activates pump 85. As a result, the advantages of the method according to the invention already described are realized in a fully automatic manner. This preferred embodiment of the invention, as shown in the system according to FIG., Therefore uses tower 1 sprinkled with thin acid, which is connected on the gas side in front of the denitrification zone, for regulating the entire system. This tower 1, which is provided with a packing layer, or a corresponding gas-liquid reaction apparatus of another type, which is sprinkled with thin acid and is located on the gas side at the beginning of the system, is thus charged with the total amount of the liquid or gaseous substances which contain nitrogen-bound nitrogen . The packing layer of the tower 7 is connected at its lower end via line 72 to the fan 167 and receives 6 dry exhaust gases from the absorption zone from the packing layer of the absorption tower. These exhaust gases remove water from the acid in the packed layer of the tower 7 and, together with the water vapor, reach the atmosphere via line 705.

So far, it has been customary to control the NO: N0 ₂ ratio in the exhaust gas of the system or within the absorption zone of the nitrogen oxide-sulfuric acid system to change the concentration of the acids in certain circuits. This was done by adding water or adding dilute acid to more concentrated acid. In contrast to this, according to the invention, changes in the acid concentrations for the purpose of regulating the system can be dispensed with. It is sufficient to keep the density of the acids at the outlet of the denitrification zone and in the circuit of the SO ₂ processing zone constant. This constant keeping is possible by known automatic density control devices. As can be seen from FIG. 2, the density measuring device 221 is located on the drain line 121 from the sump 21. This measuring device controls the valve 36, so that some thin acid is introduced into the packed layer of the denitrification tower 2 via the pump 25 and the line 23 reached. the density measuring device controls the addition of thin acid in such a way that a constant acid concentration is maintained at the outlet of the denitration zone, ie at the outlet of the packing layer of the denitrification tower 2.

A compensation line 133 connects the sumps 71 and 31 and thus ensures a compensation of the acid level in both sumps.

In the further embodiment of a plant according to the invention shown in FIG. 3, the total amount of nitrogen-oxygen compound or substance containing it is not passed into the pre-area of the SO ₂ processing zone, that is to say the pre-treatment tower 1, but only part of the compound mentioned or substance, while the remaining part of the latter is introduced into the upper end of the tower 3, that is to say into the main area of the SO ₂ processing zone, as in the system according to FIG. 1. The ratio of the partial quantities which on the one hand reach the preliminary area and on the other hand reach the main area of the SO _Z processing zone can be controlled by corresponding actuation of valves 46 and 86.

The advantageous acceleration of the reactions occurring during the 7Verarbeitung ₂ SO is the greater, the larger the contact surface between gas and liquid in the reaction apparatus of the SO ₂ -processing zone is.

By simultaneously introducing a nitrogen-oxygen compound into the pre-area and into the main area of the S0 ₂ processing zone, i.e. the packed layers of towers 1 and 3, a much larger surface corresponding to the sum of the packed volumes of towers 1 and 3 is now sprinkled and one correspondingly achieved higher acceleration of the whole process of S0 ₂ processing.

In the plant shown in FIG. 3, in a further embodiment of the invention, as already mentioned, in addition to the NO content in an analyzer 255, the NO _Z content of the gases is measured inside or after the absorption zone. This measuring device is coupled to a control device of a known type (not shown) which is superior to the control described above, which is based on the measurement of the NO content. As soon as the N0 ₂ content exceeds a specified value, the addition of substances containing nitrogen-bound nitrogen is interrupted or reduced. In addition, if the NO ₂ content is too high or the NO ₂ 7 concentration rises too quickly, nitrous acid-containing sulfuric acid is removed from the absorption zone by opening valve 56 beyond the normal level and a corresponding amount of nitrous-free or low-nitrous acid is passed into the absorption zone. The analyzer 255 continuously measures the N0 ₂ concentration of the gases at the outlet of the packing layer 6. As soon as the NO _z content rises above a fixed value, or if the NO ₂ concentration rises too rapidly, the pump 85 is shut down. This will add strong nitric acid containing sulfuric acid from the container 80 interrupted in the packed layer of the tower 3. The pump 55 is also controlled by the analyzer 255. If the NO ₂ content increases too sharply at the outlet of the packing layer 6, this pump is started and also the valve 56 is opened, whereupon nitrous acid flows into the tank 80 and from the tank 90 nitrous-free acid via line 53 to the packing layer of the Absorption tower is directed. The nitrous-free absorption acid flushes the nitrous acid from the packing layer 5 into the sump 51 and via the line 82 into the container 80. If the supply of nitrogen-bound nitrogen is not in the form of nitric acid but in the form of gaseous nitrogen oxides, it is advantageous not to introduce the gaseous nitrogen oxides directly into the packed layers of towers 3 and 4 of the SO ₂ processing zone. Rather, a faster and more powerful effect is achieved if the nitrogen oxides are first dissolved in sulfuric acid and the nitrogen-containing sulfuric acid obtained in this way is introduced into the SO _z processing zone.

4, 108 denotes a device for the catalytic oxidation of ammonia. The nitrogen oxides formed are cooled in a heat exchanger 104 and then flow through a column 10 from bottom to top. That part of the nitrogen oxides which is not absorbed in the column 10 passes via a line 12 into the line 102 and thus into the main gas stream of the plant. Acid can be passed from line 78 via valve 106 into column 10, in which it is saturated with nitrogen oxides. This acid now passes through the sump 101 and the line 115 to the pump 116 and via line 117 and the valve 118 into the packed layer of the tower 3. Finally, an overall system is shown in FIG. 1 which shows all the features of the embodiments of the systems according to FIGS 4 combines and permits, depending on the type and S0 ₂ content of the gases to be processed and depending on whether more ammonia or more nitric acid is to be used temporarily, the treatment of the SO ₂ -containing in any of the sub-systems shown in the previous figures To carry out exhaust gases in accordance with the method according to the invention in an economically optimal manner.

The following exemplary embodiment illustrates the implementation of the method according to the invention in the system shown in FIGS. 3 and 5.

example

A SO ₂ -based gas stream contains the following molar amounts per Nm ³ (1 normal cubic meter ₌ 1 m ³ at 0 ° C and 1,013 bar):

The rest is nitrogen.

A controllable volume flow of this gas is introduced into the packing layer of the tower 1 of a plant according to FIG. 3. The volume of the packed layers of towers 1 and 7 is 1 m ³ each. The layers of towers 2 to 6 each contain 2.6 m ³ packing. The total of all filling volumes is therefore 15 m ³ . Polyethylene bodies according to DE-A-24 16 955 are used as filling material. The filling has a surface area of approx. 300 m ² / m ³ . The acid circulation in the packed layers of towers 1 and 7 takes place as shown in FIG. 3 and amounts to 2 liters / Nm ^{3 of} gas. The layer of the tower 2 is sprinkled with 1 liter / Nm ^{3 of} gas in the manner shown in FIG. 3. The layers of towers 3 and 4 each have a sprinkling of 3 liters / Nm ³ gas. The sprinkling of layer 5 of the absorption tower is increased by a pump and a line, which are not shown in FIG. 3. This pump, not shown, conveys acid from the sump 51 to the layer 5, so that together with the acid flow shown in FIG. 3, the filling is sprinkled with 4 liters / Nm ^{3 of} gas.

The acid temperature at the outlet of heat exchangers 74 and 24 is 63 ° C. The acid temperature after the heat exchangers 34, 44 and 64 is between 30 and 40 ° C. Relative to a temperature of 15 ° C, the pumped acids have the following liter weight:

The nitrogen oxides absorbed in layers 5 and 6 of the absorption tower are passed with the acid via the sump 51 through the pump 25 via line 23 to the layer of the tower 2, in which these nitrogen oxides are reacted with the SO ₂ as NO in the Gas flow arrive, which leaves the layer of the tower 2 through line 32. The S0 ₂ processing in the layers of towers 3 and 4 is the better, the more nitrogen oxides are contained in the gas stream.

Via the line 23, one liter of nitrous acid is passed into the layer of the tower 2 per Nm ^{3 of} gas. The higher the nitrous content of the acid, the more SO ₂ -containing gas can be processed in the system. If the acid's nitrous content is too high, however, the nitrogen oxide content of the system's exhaust gases increases. In order to regulate the nitrous content of the acid in the sump 51, nitric acid, which was fed to the container via line 83, is passed from the container 80 by means of the pump 85 via the valve 46 through the line 37 to the layer of the tower 3.

The nitrogen oxides released in the layer of this tower 3 and the subsequent tower 4 pass through the gas line 42, 52 into the layer 5 of the absorption tower and are taken up there by the sulfuric acid and trickle into the sump 51.

With a gas volume of 400 Nm ³ / h and with a nitrous content of the acid in the sump 51 of 2% by weight, calculated as HN0 ₃ , an exhaust gas from the overall system in line 705 is obtained, which contains 50 to 100 ppm NO and 200 to 300 contains ppm N0 ₂ . If instead of 400 Nm ³ / h a gas quantity of 500 Nm ³ / h is fed into the system, or if the S0 ₂ content in the inlet gas increases while the gas quantity remains the same, the system can no longer process the S0 ₂ completely and S0 is reached ₂ into the absorption zone, which increases the NO content of the system's exhaust gases.

In order to be able to process the increased amount of SO ₂ without large losses of nitrogen oxide, the nitrous content of the acid which is passed into the layer of tower 2 must be increased and / or the amount of acid which is in - unless the measures according to the invention are used the layer of the tower 2 is directed, increased. With the normal sprinkling of the layer of tower 2 of 1 liter / Nm ^{3 of} gas, the nitrous content in the sump 51 must be increased to approx. 3% by weight of HN0 ₃ if 500 Nm ³ / h of gas have to be processed.

When adding nitric acid to the layer of the denitrification tower 2, it is necessary to proceed step by step, because it takes many minutes for the nitrogen oxide content of the gases to change downstream of the absorption zone. A too high addition of nitric acid or a decrease in the S0 ₂ content of the inlet gases leads to an increase in NO ₂ and thus to a yellow coloring of the exhaust gases of the system. To remove this yellowing, the concentration of the acid in the denitrification zone and the absorption zone would then have to be increased. For this purpose, the addition of water or thin acid would be reduced or stopped. However, it would take several hours before the NO ₂ concentration in the exhaust gas drops below 400 ppm again.

If, on the other hand, the process is carried out in accordance with the invention, both an increase and a decrease in the SO ₂ concentration of the inlet gases and also a change in the volume flow of the SO ₂ -containing gases can be compensated for without a greater loss of nitrogen oxide via the exhaust gases. An increase in the S0 ₂ content in the gas to be treated causes an increase in the NO content of the gases measured continuously in the analyzer 285 after the absorption zone above the normal value of 100 ppm to, for example, 150 ppm. An automatic control device then activates the pump 85, as a result of which a sulfuric acid, which contains 20% by weight of nitric acid, is passed from the container 80 through the lines 78 and 37 into the packed layers of the towers 1 and 3. The valves 86 and 46 are regulated so that 0.4 liters per minute flows to the layer of tower 1 and 0.6 liters per minute to the layer of tower 3. After only two minutes, the NO content at the exit of the absorption zone drops below 100 ppm and the pump 85 is deactivated. The nitric acid added to the thin acid in the two circuits through towers 1 and 3 consumes after about 15 minutes and is released in the form of gaseous nitrogen oxides. These nitrogen oxides are taken up by the acid in the absorption zone, as a result of which the nitrous content of the acid in the sump 51 increases.

However, the increase in nitrose is not yet sufficient to bring about a reaction in the layer of the denitrification tower which is adapted to the increased quantity of SO ₂ in the gas stream. After about 12 minutes, the NO content of the exhaust gases rises again above 150 ppm, while the NO ₂ content drops from a maximum of 250 ppm below 150 ppm. The analyzer 285 switches the pump 85 on again and again the NO content of the gases after the absorption zone drops below 100 ppm after only two minutes, as a result of which the pump 85 is switched off.

This process of switching the pump 85 on and off is repeated until the nitrous content in the sump 51 has adapted to the increased amount of SO ₂ in the gas stream entering through line 102, which has to be processed in the system. The distance between two switching periods becomes longer, the closer the nitrous content in the sump 51 approaches the value required for S0 ₂ processing.

If, on the other hand, the amount of gas that has to be processed in the system becomes smaller, or the concentration of S0 ₂ in the gas to be treated drops, then the N0 ₂ content of the gases increases downstream of the absorption zone and the NO content falls far below 100 ppm from. The analyzer 255 continuously measures the NO ₂ content of the exhaust gases in the absorption zone. If the NO ₂ content per minute increases by more than 30 ppm per minute and / or if the NO ₂ content reaches over 200 ppm, the pump 85 is stopped by an automatic control device (not shown), if the same at this time was in operation. With an increase of more than 50 ppm per minute and / or an N0 ₂ content of more than 300 ppm per minute and / or a NO ₂ content of more than 300 ppm opens an automatic control device, the valve 56 and switches the pump 55 a. As a result, nitrous acid-containing sulfuric acid flows into the container 80, while from the container 90 nitrous-free sulfuric acid reaches the packing layer 5 by means of the pump 55 via the line 53. The amount of acid added and withdrawn is 0.3 liters per / N _M ³ gas. After about 3 minutes, the N0 ₂ content of the gases after the absorption zone drops below 200 ppm and the automatic control device closes the valve 56 and puts the pump 55 out of operation. By adding nitric acid through the lei device 83, a nitric acid content of 20% is produced in the container 80.

As already explained, the acid of the container 80 is reintroduced into the system during periods of entry of gases with a high SO ₂ content into the system.

Corresponding to the amount of S0 ₂ processed in the system, sulfuric acid is produced, which flows out of the container 90 as production via the drain line (P).

• Das Litergewicht des Kreislaufes über die Füllkörperschichten der Türme 1 und 7 wird durch Regelung der Wärmezufuhr im Wärmeaustauscher 14 bei 1,5 kg/Liter gehalten. Vermehrte Wärmezufuhr verstärkt die Wasserabgabe in der Füllkörperschicht des Turmes 7 und erhöht daher das Litergewicht. Das Litergewicht im Säurekreislauf der Füllkörperschichten der Türme 3 und 4 der S0₂-Verarbeitungszone wird durch geregelten Zusatz von Säure aus dem Sumpf 71 bei 1,56 kg/Liter gehalten. Die entsprechenden Leitungen und Ventile sind in Fig. nicht dar- gestellt. Das Litergewicht am Austritt des Denitrierturmes 2 wird durch geregelten Zusatz von Dünnsäure aus dem Sumpf 31 auf die Füllkörperschicht des Turmes 2 bei 1,67 kg/Liter gehalten.

The control devices described make it possible to keep the N _x Oy content (sum of the NO and NO ₂ content) of the exhaust gases of the system constantly below 400 ppm, even if the amount of gas to be processed or the SO _z concentration are frequent Fluctuations in the range of ± 70% of the daily average. The intermediate weight (or density) of the acid cycles is regulated as follows:

• The liter weight of the circuit over the packed layers of towers 1 and 7 is kept at 1.5 kg / liter by regulating the heat supply in the heat exchanger 14. Increased heat supply increases the water release in the packed layer of the tower 7 and therefore increases the liter weight. The liter weight in the acid cycle of the packed layers of towers 3 and 4 of the SO ₂ processing zone is kept at 1.56 kg / liter by the controlled addition of acid from the sump 71. The corresponding lines and valves are not shown in FIG. The liter weight at the outlet of the denitrification tower 2 is kept at 1.67 kg / liter by controlled addition of thin acid from the sump 31 onto the packed layer of the tower 2.

Analyzers as used in plants according to the invention are well known. We are, for example, manufactured by Thermo Electron Corporation, Waltham, Mass., USA and marketed as "NO _x chemiluminescent source analyzer for automat ⁱ ve emission" or as "NO-NO _x chemiluminescent analyzer", eg "Model 44".

A circuit to be used for control is described, for example, in the article "Automation Control Technology in Meyers Handbook on Technology, Blographisches Institut Mannheim, Allgemeine Verlag 1971, pages 729-736. If, for example in Fig. 1 of this article, a NO or NO ₂ analyzer of the type described above is used instead of the thermocouple, the valve shown is controlled via the electrical transmitter MU and controller R. A more detailed representation of the switching part from the electrical transmitter MU to a valve or a pump is illustrated in the left half of Fig. 8 of the same article.

Claims (28)

1. A process for separating S0₂ from a current of gas which contains this substance, at least intermittently, in a concentration which is impermissibly high for discharge into the ambient atmosphere, with attendant production of sulfuric acid by the nitrogen oxide process, in which latter process the S0₂-containing gas initially flows through a denitration zone or first through a pretreatment zone forming a first sector of an S0₂-processing zone and then through the denitration zone, thereafter through the main zone of the S0₂7processing zone, and subsequently through a nitrogen oxide absorption zone, in the course of which flow it is brought into contact in at least one of the two sectors of the S0₂- processing zone with dilute acid having a H₂SO₄- concentration of less than 70 % by weight (55° Be) which acid is circulated through the sector concerned of this latter zone, whilst in the absorption zone the nitrogen oxides released in the denitration zone are absorbed by sulfuric acid, and nitrose-containing acid having a H₂S0₄-concentration of 70 to 80 % by weight (55° to 63.5 Be) is drawn off from the absorption zone and fed into the denitration zone, which process further comprises :

(a) measuring the content of NO in the current of gas at a point in the absorption zone or downstream of the absorption zone or simultaneously at points both in the absorption zone and downstream thereof, and

(b) introducing a nitrogen-oxygen compound (as defined) into the dilute acid cycle of at least one of the two sectors of the S0₂-processing zone when a specific limit value of the NO-content or of the steepness of an increase per unit of time of the NO-content in the current of gas at the NO-control point or points is exceeded, in an amount which is sufficient, on the one hand, to eliminate the increase in the said NO-content limit value or slope limit value, but, on the other hand, which remains close to the said limit values, thus virtually avoiding a change in the acid concentration in the absorption zone, and, if a gaseous nitrogen-oxygen compound is used, dissolving this compound firstly in dilute acid and then introducing it, as described above, into the dilute acid cycle.

2, The process of claim 1, wherein the added amount of nitrogen-oxygen compound in increased proportionately to the increase in the NO-content of the current of gas in or downstream of the absorption zone.

3. The process of claim 1, wherein the rate of flow of added nitrogen-oxygen compound is kept constant as long as the NO-content of the current of gas in or downstream of the absorption zone exceeds the said limit value of NO.

4. The process of claim 3, wherein the added amount of nitrogen-oxygen compound is increased at the same rate as a limit value of steepness of an increase per unit of time of the NO-content in the current of gas in or downstream of the absorption zone is exceeded.

5. The process of claim 1, wherein the added amount of nitrogen-oxygen compound is kept constant as long as a limit value of the steepness of the increase per unit of time of the NO-content in the current of gas in or downstream of the absorption zone is exceeded.

6. The process of any one of claims 1 to 5, wherein the nitrogen-oxygen compound consists of nitric acid, or of nitrogen oxides which are produced by the oxidation of ammonia.

7. The process of any one of claims 1 to 6, wherein the nitrogen-oxygen compound is formed by the absorption of nitrogen oxides in sulfuric acid.

8. The process of either of claims 2 or 3, wherein the addition of nitrogen-oxygen compound is interrupted when the NO-content of the current of gas downstream of the absorption zone falls below a fixed value which is lower than the limit- content of NO.

9. The process of either of claims 4 or 5, wherein the addition of nitrogen-oxygen compound is reduced when the NO-content in the current of gas downstream of the absorption zone falls below a predetermined limit value of the steepness of the increase in the NO-content per unit of time, the limit value of the steepness corresponding approximately to that defined under (b), supra.

10. The process of any one of claims 1 to 9, wherein (a) additionally the NO₂-concentration of the current of gas at a point downstream of the absorption zone is measured ; and (b) the addition of nitrogen-oxygen compound to the dilute acid cycle through the SO₂-processing zone is interrupted when a specific limit N0₂-content or a limit value of the steepness in the rise of the NO₂- content in the current of gas at the NO_z-control point is exceeded.

11. The process of claim 10, wherein (c) the amount of nitrose-containing acid flowing from the absorption zone is increased and nitrose- improverished or nitrose-free acid from the denitration zone is correspondingly introduced into the absorption zone whenever a second, higher limit value of the N0₂-content in the current of gas downstream of the absorption zone is exceeded or until a second, higher steepness limit in the rise of the NO_Z-content in the current of gas downstream of the absorption zone is exceeded.

12. The process of any one of claims 1 to 11, wherein at least a portion of the nitrogen-oxygen compound introduced into the S0₂-processing zone consists of nitrose-containing sulfuric acid taken from the absorption zone.

13. The process of claim 12, wherein the content of nitrose-containing sulfuric acid taken from the absorption zone and to be introduced into the S0₂-processing zone is increased by the addition of nitric acid to said nitrose-containing sulfuric acid.

14. The process of any one of claims 1 to 12, further comprising a pretreatment zone through which a separate cycle of dilute acid flows and through which the current of gas is passed before it enters the denitration zone, which pretreatment zone is arranged upstream of the denitration zone, a portion of the total amount of nitrogen-oxygen compound being introduced into this pretreatment zone.

15. The process of claim 14, wherein the portion of said substance introduced into the pretreatment zone is diverted from a nitrose-containing sulfuric acid which flows from the absorption zone into the SO₂-processing zone.

16. The process of either of claims 14 or 15, wherein at least a portion of the amount of nitrogen-oxygen compound to be introduced into the pretreatment zone consists of gaseous nitrogen oxides which are produced by the oxidation of ammonia and are introduced into the current of gas before this latter current enters the pretreatment zone.

17. The process of claim 15, wherein the content of nitrogen-oxygen compound in the nitrose-containing sulfuric acid taken from the absorption zone and being partly introduced into the main sector of the S0₂-processing zone and partly into the pretreatment zone is increased by the addition of nitric acid to the last-mentioned acid.

18. The process of any one of claims 1 to 17, wherein the density of the acid leaving the denitration zone is kept constant by the addition of dilute acid or water thereto.

19. The process of any one of claims 1 to 18, wherein the density of the dilute acid circulating through the main sector of the S0₂-processing zone is kept constant by the addition of acid from the pretreatment zone, or of water.

20. A plant for separating S0₂ from a current of gas which contains the same, at least intermittently, in a concentration which is impermissibly high for discharge into the ambient atmosphere, with attendant production of sulfuric acid by the nitrogen oxide process, which plant comprises, arranged in succession,

(a) a denitration zone (2),

(b) an S0₂-processing zone (3, 4), and

(c) a nitrogen oxide absorption zone (5, 6), each zone consisting of at least one gas-liquid reaction apparatus which is provided with a sump (21, 31, 41, 51) for taking up liquid which leaves the bottom of the apparatus concerned ; and furthermore

(d) a line for the current of gas (102, 22, 32, 42, 52, 72, 705) which is connected to one end of the denftration zone (2) and connects, in succession, the other end of this zone with the S0₂-processing zone (3, 4) and this latter zone with the nitrogen oxide absorption zone (5, 6) and finally leads from one end of this absorption zone into the surrounding atmosphere,

(e) a line for circulating dilute acid through the SO₂-processing zone (33, 43), and

(f) a line for circulating acid (54, 23) from the first sump (51), in the direction of flow of the current of gas, of the nitrogen oxide absorption zone (5, 6) to the top end of the denitration zone (2) and from the sump (21) of this zone (121, 63) to one end of a reaction apparatus (6) of the nitrogen oxide absorption zone (5, 6), said plant further comprising

(g) a feed line (37, 117) for nitrogen-oxygen compound in liquid (37) or gaseous (117) phase which feed line is connected to the line for circulating dilute acid (33, 43) through the S0₂- processing zone,

(h) a measuring device (285) for measuring the NO-content in the current of gas which device is connected to the absorption zone (5, 6) or to the line (d) for the current of gas downstream of the absorption zone or simultaneously to the absorption zone (5, 6) and to the line (72) downstream of the absorption zone (5, 6), and

(i) a control device (85) for adding nitrogen-oxygen compound through the feed line (37) contingent on the NO-contents determined by the measuring device (285).

21. The plant of claim 20, further comprising an NOg-measuring device (255) for measuring the N0₂-content in the current of gas, said NOg-measuring device being connected to the absorption zone (5, 6) or to the line (72) for the current of gas downstream of the absorption zone, and wherein said control device (k) comprises a superimposed control device which interrupts or reduces the supply of nitrogen-oxygen compound through the feed line (37, 117) contingent on the NO_Z-content in the current of gas measured by the N0₂-measuring device.

22. The plant of either of claims 20 or 21, wherein the feed line (37, 117) serves for adding nitrogen-oxygen compound in the form of nitrose-containing sulfuric acid and is connected to the sump (51) of a reaction apparatus (5) of the nitrogen oxide absorption zone (5, 6).

23. The plant of claim 22, wherein said feed line (37) comprises means (83) for introducing nitric acid thereinto.

24. The plant of either of claims 22 or 23, further comprising a pretreatment zone (1) upstream of the denitration zone (2) with reaction apparatus and sump (71), whilst the line (102, 22) for the current of gas passes firstly into this pretreatment zone (1) at the lower end thereof and from the top end of this latter to the lower end of the denitration zone (2), and a separate line (73) for circulating dilute acid through the pretreatment zone from the top of the latter downward, an equalising line (133) between the sump of the pretreatment zone (71) and the sump of the denitration zone (21) and a branch line (78) from the feed line (37) to the upper end of the pretreatment zone (1).

25. The plant of either of claims 20 or 21, further comprising an ammonia oxidising unit (108) with a column (10) for at least the partial absorption of the nitrogen oxides formed therein by sulfuric acid, and with a second feed line (115, 117) which connects the lower end of the column (10) to the upper end of a first reaction apparatus (3) of the S0₂-processing zone (3, 4), in the direction of flow of the current of gas, said second feed line being adapted for introducing nitrogen oxide-containing sulfuric acid into the latter zone.

26. The plant of claim 25, further comprising a pretreatment zone (1) upstream of the denitration zone (2) with reaction apparatus and sump (71), a separate line (73) for circulating dilute acid through the pretreatment zone (1) from the top downward, as well as an equalising line (133) between the sump (71) of the pretreatment zone and sump of the denitration zone (21), wherein line (102, 22) of the current of gas passes firstly into the pretreatment zone (1) at the lower end thereof and from the top end of the latter zone to the bottom and of the denitration zone (2), and a line (12) for gases which contain nitrogen oxides and connecting the top end of said column (10) of said ammonia oxidation unit (108) to that part of said line (102, etc.) through which the current of gas is introduced into the bottom end of the pretreatment zone.

27. The plant of either of claims 25 or 26, wherein said feed line (37) is connected to the sump (51) of a reaction apparatus (5) of the nitrogen oxide absorption zone (5, 6), and further comprises a branch line (78, 106) connecting the feed line (37) with the top end of the column (10) of the ammonia oxidising unit (108).

28. The plant of claim 27, wherein said branch line (78) of the feed line (g) comprises a secondary branch line being connected to the top end of the pretreatment zone (1).

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