EP0000515A1 - Process for the separation of sulphur dioxide from a gas stream and installation for carrying out the process - Google Patents

Abstract

Process for separating SO2 from a gas stream to obtain sulfuric acid using the nitrogen oxide process, in which the gas containing SO2 is first passed through a pretreatment zone and then through a denitrification zone, followed by a SO2 processing zone, and then through a nitrogen oxide absorption zone, in which it flows SO2 processing zone is brought into contact with circulating thin acid, while in the absorption zone the nitrogen oxides released in the denitrification zone are taken up by sulfuric acid and nitrous acid with a content of 70 to 85% by weight H2SO4 is removed from the absorption zone and transferred to the denitrification zone becomes. A measuring and regulating device (285) is provided to automate the method, with which a) the NO content in the gas flow in the absorption zone or downstream of the absorption zone or simultaneously both in the absorption zone and downstream thereof is measured, and b) when a certain NO is exceeded Limit or if a steepness limit of the increase per unit of time of the NO content in the gas stream at the NO measuring point or points is metered into the thin acid circuit of the SO2 processing zone, a nitrogen-oxygen compound in a certain amount, so that a change in the acid concentration in the absorption zone is practically avoided.

Description

The invention relates to a method for separating SO ₂ from a gas stream containing it at least temporarily in a concentration that is inadmissibly high for discharging into the ambient air with the production of sulfuric acid by the nitrogen oxide method, in which the latter contains the SO ₂ -containing gas

first a denitrification zone or a pretreatment zone forming a pre-area of a SO ₂ processing zone and then the denitrification zone, then the main area of the SO ₂ processing zone and then a nitrogen oxide absorption zone, wherein it flows through

in at least one of the two areas of the SO ₂ processing zone, H ₂ SO _{4 is brought} into contact with thin acid circulated through the relevant area of this zone at concentrations below 70% by weight (55 ° Be),

while in the absorption zone the nitrogen oxides released in the denitrification zone are taken up by sulfuric acid and from the absorption zone nitrous acid containing 70 to 85% by weight (55 to 63.5 ° Be) H ₂ SO _{4 is} removed and transferred to the denitrification zone becomes.

• - 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.

The term "nitrose process" is used in the second reference.

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.

There are usually nitrogen oxide-sulfuric acid plants. from several cascaded packed towers with different acid cycles. High nitrogen oxide losses occur both with an excess of NO and with an excess of NO ₂ in the exhaust gases. Optimal absorption of nitrogen oxides requires a correct NO: NO 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 particularly advantageous to operate a nitrogen oxide-sulfuric acid system in such a way that the exhaust gases have a barely noticeable but nevertheless recognizable yellowing. Several measures are known for increasing the NO ₂ content of the exhaust gases and reducing 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, in need Known nitrogen oxide-sulfuric acid systems in general by experienced operating personnel in order to be able to operate the system with low nitrogen oxide losses,

It is known that a loss of nitrogen oxides from the system by adding nitric acid or concentrated nitrogen gases, such as those resulting from the catalytic combustion of ammonia, must be compensated for (make-up) in known nitrogen oxide-sulfuric acid plants the addition of nitric acid or the addition of strong nitrous gases either at the top of the denitrification tower (DE-OS 26 09 505) or in front of a production tower which is sprinkled with acid of more than 70% by weight H ₂ SO ₄ , carried out. The need for the addition of nitric acid or strong nitrous gases can be recognized 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 the towers of a system which are sprinkled with an acid of more than 70% by weight H ₂ SO ₄ (absorption acid) and give off gas with more gaseous nitrogen compounds than are contained in the inlet gas of the tower are counted as denitration zones . So-called "production towers" of a PETERSEN tower that meet this condition are therefore by definition considered to belong to the denitrification zone. If she. Acid in the outlet of a tower contains nitrous or free of Ni. trose is not used as a criterion for the designation "Der trierurm" or "Denitrierapparat". The upstream gas drying tower explained in more detail is sprinkled with acid under 70% by weight H ₂ SO ₄ and is therefore not included in the denitrification zone, even if it releases gaseous nitrogen oxides.

In known nitrogen oxide-sulfuric acid systems, 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, the control of the ratio NO: N0 ₂ in the exhaust gases, is done in most cases in known nitrogen oxide-sulfuric acid plants by adding more or less water in the acid cycle through the denitrification zone and Nitric 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 inertia towards corrective measures. If, for example, a yellow colored exhaust plume arises after the system due to a drop in the SO ₂ content of the inlet gases, this requires corrective measures that take effect only after several hours and make this yellow plume disappear again.

anwendbar sein, da die hierbei auftretender Temperaturdifferenzen zur Steueurng des Systems viel zu geing waren.As already stated in GB-PS 759 056 (Saint-Gobain) in 1956, the difference between the temperatures - 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-PS 1,486,757 (Jensen) from 1924 to determine nitrogen oxide losses and compensate for them by introducing nitric acid into the first SO 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 is also dependent on the daily fluctuations in the outside temperature. otherwise this method would also be used in intensive processes for the production of sulfuric acid, such as that of DE-OS 26 09 505 and the process of the present

be applicable, since the temperature differences that occurred in this connection were much too small for the control of the system.

• (a) den NO -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-OS 26 09 505 empfohlen wird, keineswegs die Reaktivität der Dünnsäure in der SO₂-Verarbeitungszone in einem System nach der DE-OS 26 05 909 beeinflussen.

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

• (a) measure the NO 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-OS 26 09 505, would in no way influence the reactivity of the thin acid in the SO ₂ processing zone in a system according to DE-OS 26 05 909.

DE-PS 1,140,909 (Ruhr-Schwefelsäure GmbH) describes an adjustment of the ratio of NO: NO ₂ and the content of NO + NO ₂ in the gas phase by means of 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.

Changes in the nitrous content would take place according to this DE-PS with the necessary speed during the operation of the system. However, the acid which circulates in the system according to this DE-PS is a strong acid with a content of 80% by weight H ₂ SO ₄ and no thin acid of less than 70% by weight H ₂ SO ₄ , such as it circulates in the S0 ₂ processing zone of DE-OS 26 09 505.

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-OS 26 09 505 and which consists in a decrease in the reactivity of the thin acid circulating through the SO ₂ processing zone of the system . This problem did not occur in previous processes. The invention further aims to achieve an improved, very rapid adaptation of the NO: NO ₂ ratio in the exhaust gas emitted from the system, so that this exhaust gas virtually eliminates the yellow flag caused by the presence of NO ₂ in the exhaust gas leaves 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 described 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-OS 25 10 294 and 26 09 505.

• (a) der Gehalt an NO im Gasstrom in der Absorptions- zone oder stromab der Absorptionszone oder gleichzeitig sowohl in der Absorptionszone als auch stromab derselben gemessen wird, und dass
• (b) beim Ueberschreiten eines bestimmten NO-Grenz- . gehalts 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 SO₂-Verarbeitungszone eine weiter unten näher definierte Stickstoff-Sauerstoff-Verbindung in einer bestimmten Menge eindosiert wird, die einerseits aus- reicht um die Ueberschreitung das genannten NO-Grenzgehalts oder Steilheitsgrenzwertes zu beseitigen, aber andererseits nahe an den genannten Grenzwerten verbleibt, wobei eine Aenderung der Säurekonzentration in der Absorptionszone praktisch vermiedea wird.

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

• (a) the NO content in the gas stream is measured in the absorption zone or downstream of the absorption zone or simultaneously both in the absorption zone and downstream thereof, and that
• (b) when a certain NO limit is exceeded. content or, if a steepness limit value is exceeded, the increase per unit time of the NO content in the gas stream at the NO measuring point or points in the thin acid circuit in at least one of the two areas of the SO ₂ processing zone A defined amount of nitrogen-oxygen compound, which is defined further below, is metered in, which on the one hand is sufficient to eliminate the exceeding of the NO limit or steepness limit mentioned, but on the other hand remains close to the mentioned limit values, with a change in the acid concentration in the Absorption zone is practically avoided.

dass sie diesem optimalen Betrieb entsprechen.The specific amount of the NO limit or the steepness limit 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 in 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 the measuring devices for checking the limit values mentioned above

that they correspond to this optimal operation.

It has been found that an opalescent NO: NO ₂ ratio can be achieved in the absorption zone or in the aogases 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 containing 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 nitric oxide. Connections relatively quickly. During the consumption of the nitrogen content of the thin acid, a surprisingly strong reaction is triggered, causing an · acceleration. SO _{2 development is} brought about.

The inventive measures do not represent a "make-up" as an answer. indicates an increase in the NO content due to an insufficient NO content in the system. Rather, the increase in the NO content as it is poured in accordance with the invention shows above all one in the present system Decrease in the activity of the thin acid circulating in one of the sectors or in both sectors of the SO 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 into SO ₃ or sulfuric acid, surprisingly even if there is a lack 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 SO ₂ content of the entry 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.

eingeführt werden, und zwar so, dass auf den vorzugsweise mit Füllkörpern gefüllten Reaktionsapparaten diesor Zone sine Vermischung zwischen zugesetztem Flüssigkeit und Dünnsäure stattfindet.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 (eg nitric acid) can either be mixed directly with the thin acid or mixed with the packing material.

are introduced, in such a way that the reaction is preferably filled with packing apparatus diesor Zone sine mixing between added liquid and thin acid takes place.

"Nitrogen-oxygen compounds" are those compounds which normally occur or are used in the nitrogen oxide process, that is to say NO, NO ₂ , N ₂ O ₃ , nitrose 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 O ₃ or forms nitric acid with nitric acid.

Gaseous contain substances which bound to oxygen, nitrogen, are thus NO- or NO ₂ - and N ₂ O ₃ - containing gases, fliasige substances are in particular nitric acid itself or nitrous 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 in each corrective measure is only very small compared to the total amount of nitrous oxide circulating in the system.

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 indicated under (a) requires the addition of about 100 to 1000 ml of nitric acid or the like. Nitrogen-oxygen connection to the "thin acid" circulating through the SO ₂ processing zone. Such small amounts bring about 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 in the system. The latter would require the addition of a substantially larger amount 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 added containing nitrogen-oxygen compound is increased to the extent that a steepness limit of the increase per unit time of the NO content in the gas stream in or downstream of the absorption zone is exceeded. We prefer to use nitric acid 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 the substance mentioned is preferably interrupted when the NO content of the gas stream downstream of the absorption zone drops below a predetermined value. d is lower than the aforementioned limit of NO; or the addition of said substance is reduced if the slope falls below a predetermined steepness limit value per unit of time of the 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 NO₂-Konzentration des Gasstroms stromab der Absorptionszone gemessen werden und
• (b) bei Ueberschreiten eines bestimmten NO₂-Gehalt oder bei Ueberschreiten einer Steilheitsgrenze des Anstiegs des N0₂-Gehalts im Gasstrom an der NO₂-Meßstelle die Eindosierung der genannten Substanz in den Dünnsäurekreislauf der SO₂-Verarbeitungszone unterbrochen.werden.

Auch kann hierbei:

• (c) bei Ueberschreiten eines zweiten, höhereri Grer wertes des N0₂-Gehalts im Gasstrom stromab der Absorptionszc oder bis Ueberschreiten einer zweiten, höheren Steilheitsgre des Anstieges des NO₂-Gehalts im Gasstrom stromab der Absorp tionszone die Menge der aus der letzteren Zone ausfließender nitrosehaltigen Säure gesteigert und entsprechend aus der Denitrierzone stammende nitrosearme oder nitrosefreie Säure in die Absorptionszone eingeführt werden.

With particularly advantageous operation:

• (a) additionally measure the NO ₂ concentration of the gas stream downstream of the absorption zone and
• ₂ measurement point unterbrochen.werden (b) at exceeding of a certain NO ₂ content at or exceeding of a threshold slope of the rise of the N0 ₂ content in the gas stream on the NO, the metered addition of the said substance into the dilute acid cycle of the SO ₂ -processing zone.

You can also:

• (c) if a second, higher value of the N0 ₂ content in the gas stream downstream of the absorption zone is exceeded, or until a second, higher steepness of the increase in NO ₂ content in the gas stream downstream of the absorption zone, the amount of those flowing out of the latter zone is exceeded nitrous acid increased and correspondingly from the denitrification zone low or low nitrosic acid are introduced into the absorption zone.

At least a 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 content of the nitroseha sulfuric acid removed from the absorption zone and metered into the SO ₂ processing zone by the addition of nitrogen-oxygen compound 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 SO₂-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 nacheinan einen, vorzugsweise am einen, vorzugsweise unteren Ende der Denitrierungszone in diese eintritt und vom/oberen Ende der letzteren zur SO₂-Verarbeitungszone und von der letzteren zur Stickoxid-Absorptionszone und schließl: vom einen, vorzugsweise oberen Ende der letzteren Zone ins Freie führt,

• (e) eine Kreislaufleitung für Dünnsäure durch die SO₂-Verarbeitungszone, und schließlich
• (f) eine Kreislaufleitung für Säure vom in
  
  flußrichtung ersten Sumpf der Stickoxid-Absorptionszone zum oberen Ende der Denitrierungszone und vom Sumpf der letztere zurück zum/oberen Ende eines Reaktionsapparats der Stickoxid Absorptionszone, wobei die erfindungsgemäße Anlage dadurch gekennzeichnet ist, daß
• (g) an die Dünnsäure-Kreislaufleitung durch die SO Verarbeitungszone eine Zuleitung für Stickstoff-Sauerstoff-Verbindung in flüssiger oder gasförmiger Phase angeschlossen daβ
• (h) eine Meßeinrichtung zum Messen des Gehalts an NO im Gasstrom, die an die Absorptionszone oder an die Leitu (d) für den Gasstrom stromab der Absorptionszone oder gleich zeitig an die Absorptionszone und an die Leitung stromab der Absorptionszone angeschlossen ist, und daß
• (i) eine Regeleinrichtung, welche die Zugabe von S stoff-Sauerstoff-Verbindung durch die Zuleitung in Abhängigk von den durch die Meßeinrichtung ermittelten NO-Gehalten dos Eine weiter verfeinerte Ausführungsform der erfindungsgemäßen Anlage ist dadurch gekennzeichnet, daß eine NO₂-Meβeinrichtung zum Messen des NO₂-Gehalts im Gasstrom, die an die Absorptionszone oder an die Leitung für den Gasstrom stromab der Absorptionszone angeschlossen ist, vorgesehen ist, und daß die Regeleinrichtung
• (k) eine übergeordnete Regelvorrichtung umfaβt, 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.

In a further aspect, the invention relates to a system 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, which system comprises at least one after the other:

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

with at least one gas-liquid reaction apparatus per zone, which is provided with a sump serving to receive liquid emerging from the lower end of the apparatus concerned,

• (d) a line for the gas stream which enters one after the other, preferably at one, preferably the lower end of the denitrification zone and from 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 SO ₂ processing zone, and finally
• (f) a recycle line for acid from in
  
  flow direction first sump of the nitrogen oxide absorption zone to the upper end of the denitrification zone and from the sump the latter back to the / upper end of a reactor of the nitrogen oxide absorption zone, the plant according to the invention being characterized in that
• (g) a feed line for nitrogen-oxygen compound in the liquid or gaseous phase is connected to the thin acid circulation line through the SO processing zone
• (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 at the same time to the absorption zone and to the line downstream of the absorption zone, and that
• (i) a control device which detects the addition of S-oxygen compound through the feed line depending on the NO contents determined by the measuring device A further refined embodiment of the system according to the invention is characterized in that a NO ₂ 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 umfaβt which the supply of nitrogen-oxygen compound through the feed line according to (g) in dependence on by the NO ₂ - means the measured 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 compounds in the form of nitrous sulfuric acid can be connected to the bottom of a reaction apparatus of the nitrogen oxide absorption zone.

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

eine der Denitrierzone vorgeschaltete Vorbehandlungszone mit Reaktionsapparat und Sumpf umfassen, wobei die Leitung für

Gasstrom zuerst in diese Vorbehandlungszone vorzugsweise an deren unterem und vom anderen Ende derselben zu einen, vorzugsweise unteren Ende der Denitrierzone führt und wobei eine separate, durch die Vorbehandlungszone von oben nach unten führende Kreislaufleitung für Dünnsäure sowie eine Ausgleichsleitung zwischen den Sümpfen der Vorbehandlungszone und der Denitrierzone vorgesehen ist, und wobei eine Zweigleitung aus der genannten Zuleitung zum oberen Ende der Vorbehandlungszone vorgesehen sein kann.Such a preferred system according to the invention

include a pretreatment zone upstream of the denitrification zone with a reactor and a sump, the line for

Gas flow first into this pretreatment zone, preferably at its lower end and from the other end thereof, to one, preferably lower end of the denitrification zone and with a separate circuit line for thin acid leading from top to bottom through the pretreatment zone and a compensation line between the sumps of the pretreatment zone and the denitration zone is provided, and a branch line from said feed line to the upper end of the pretreatment zone can be provided.

oxide in Schwefelsäure dienenden Kolonne vorgesehen sein,und eine Ueberführungsleitung kann den Fuß der Kolonne mit dem

oberen Ende eines in Gasstromrichtung ersten

der SO₂-Verarbeitungszone zwecks Zuführen

Schwefelsäure in die letztere Zone verbinden.An ammonia incineration plant can also be used with at least partial absorption of those developed therein

Oxide in sulfuric acid serving column, and a transfer line can the bottom of the column with the

upper end of a first in the gas flow direction

the SO ₂ processing zone for feeding

Combine sulfuric acid in the latter zone.

Such a system can further comprise a pretreatment zone of the type described above upstream of the denitration zone, a feed line for nitrogen oxide-containing gases the upper end of the column of the ammonia combustion system with 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 nitrogen sump of the nitrogen oxide absorption zone between the uptake of the nitrogen-oxygen compound in the form of nitrogenous sulfuric acid and the bottom of a reaction apparatus, and a branch line can lead from the latter to the upper end of the column of the ammonia combustion plant.

mit einem weiteren Unterzweig an das obere Ende der

lungszone angeschlossen sein.This branch line from the supply line according to (g)

with another sub-branch to the top of the

zone.

• Fig. 1 eine erste, besonders einfache Ausführungsform einer erfiudungsgemäβen Anlage,
• Fig. 2 eine zweite, der in Fig. 1 geaeigten ähnli- che Ausführungsform, aber mit Vorbehandlungszone, wobei Stickstoff-Sauerstoff-Verbindung aus der Stickoxid-Absorptions-zone nur in die genannte Vorbehandlungszone eingeführt wird;
• Fig. 3 eine dritte Ausführungsform mit Vorbehandlung zone, wobei Stickstoff-Sauerstoff-Verbindung sowohl in die Voi behandlungszone als auch in den Hauptbereich der SO₂-Verarbeitungszone eingeführt wird, und außerdem eine Meßeinrichtung für NO₂ vorgesehen ist;
• Fig. 4 eine weitere, der in Fig. 3 gezeigten ähnlich Ausführungsform mit Vorbehandlungszone sowie mit einer Ammonia verbrennungsanlage zur zusätzlichen Stickoxiderzeugung; und schließlich
• Fig. 5 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.

Further details of the invention are 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, the nitrogen-oxygen compound being introduced from the nitrogen oxide absorption zone only into the aforementioned pretreatment zone;
• Fig. 3 shows a third embodiment with pretreatment zone, wherein nitrogen-oxygen compound is introduced both in the Voi treatment zone and in the main area of the SO ₂ processing zone, and also a measuring device for NO _{2 is} provided;
• FIG. 4 shows another 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
• 5 shows an embodiment similar to that according to FIG. 4, but in which, similar to the plants according to FIGS. 2 and 3, nitrogen-oxygen compound-containing acid from the absorption zone both into the pretreatment zone and into the main part of the SO ₂ . Processing zone can be introduced.

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".

und nachfolgendem zweiten Turm 4 und auf diesen die in eine und nachfolgendem zweiten Turm 4 und auf diesen die in eine einzigen Turm vereinigten Füllkörperschichten 5 und 6 ver Stickoxid-Absorptionszone. Aehnlich wie bei der in der DT-OS 26 09 505 beschriebenen Anlage wird das zu behand SO₂-haltige Abgas über die Einlaßleitung 102 in das unter Ende des Denitrierturmes 2 eingeleitet und gelangt über

Leitung 32 vom oberen Ende des Turmes 2 in das obere Ende des Turmes 3 der S0₂-Verarbeitungszone, vom unteren Ende Turmes 3 durch die Gasleitung 42 in das untere Ende des Turmes 4 derselben Zone und vom oberen Ende des Turmes 4 durch die Gasleitung 52 in das untere Ende des Turmes mit der Füllkörperschicht 5 und anschliessend in das untere Ende der Füllkörperschicht 6 des gleichen Turmes und vom oberen Ende der letztgenannten Füllkörperschicht über die Abgasleitung 72 mit Hilfe des Gebläses (Ventilators) 167 ins Freie.The S0 ₂ processing zone follows the denitriding tower 2 in the gas flow direction with a first one

and subsequent second tower 4 and on this the one and subsequent second tower 4 and on this the united in a single tower packed layers 5 and 6 ver nitrogen oxide absorption zone. Similar to the system described in DT-OS 26 09 505, the SO ₂ -containing exhaust gas to be treated is introduced via the inlet line 102 into the bottom end of the denitrification tower 2 and passes through

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 filler layer 5 and then into the lower end of the filler layer 6 of the same tower and from the upper end of the latter filler layer via the exhaust line 72 with the aid of the blower (fan) 167 outside.

dieses Turmes im Sumpf 31 von welchem die Dünnsäure mitte der Pumpe 45 durch die Leitung 43 und den Wärmeaustausche auf das obere Ende der Füllkörperschicht des Turmes 4

gepumpt wird. Der Dünnsäurekreislauf durch die SO₂-Verarb tungszone wird durch Abpumpen der Dünnsäure aus dem Sumpf des Turmes 4 über die Leitung 33 geschlossen. Bevorzugt

als Dünnsäure in diesem Kreislauf Schwefelsäure von etwa 65 Gew. -% H₂SO₄ verwendet.As in the aforementioned known system, a in the system shown in FIG. 1, thin acid is circulated through the thin acid line 33 by means of a pump 35 via a heat exchanger 34 to the upper end of the packing layer of the tower 3 and collects at the bottom

of this tower in the sump 31, from which the thin acid is pumped 45 through line 43 and exchanges heat to the upper end of the packed layer of tower 4

is pumped. The thin acid cycle through the SO ₂ processing zone is closed by pumping the thin acid out of the bottom of the tower 4 via line 33. Prefers

sulfuric acid of about 65% by weight H ₂ SO _{4 is} used as the thin acid in this circuit.

entweder Wasser oder auch Säure in an sich bekannter Weise herangezogen werden. Die entsprechenden Ventile und Leitungen sind jedoch aus der Figur 1 der besseren Uebersichr halbar weggelassen.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. To regulate the density

either water or acid can be used in a manner known per se. However, the corresponding valves and lines have been omitted from FIG. 1 for better clarity.

Similar to the known system, the vapor 51 of the common tower of the nitrogen oxide absorption zone, which is provided below the packing layer 5, is passed via line 54 with the aid of the pump 25, using 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 filler layer 6 flows directly onto the filler layer 5, while the gas to be treated from the filler layer 5 directly into the filler layer 6. In this case, therefore, no gas seal is required between the two filler 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 P through line P.

oxid-Absorptionszone (Türme 5 und 6) wird eine Säure zwisch 70 und 85 Gew.-% H₂SO₄ (Absorptionssäure) verwendet. Vorzug weise beträgt die Konzentration der Absorptionssäure 72 bis 80 Gew.-% H₂SO₄. Wie beim bekannten Stickoxid-Schwefelsäure Verfahren wird auch im Verfahren der Erfindung in der Denit zone Stickoxid aus der zugeführten Säure freigesetzt und ge langt über den Gasweg in die Absorptionszone, wo das gasför Stickoxid von Schwefelsäure absorbiert wird. Gemäß Figur 1 findet dieser Vorgang wie folgt statt: die nitrosefreie Absorptionssäure, welche aus der Füllkörperschicht des Turmes in den Sumpf 21 fließt, wird im Kühler 64 gekühlt und gelan über die Pumpe 65 und die Leitung 63 auf die Füllkörperschi 6 und fließt dann in.die Füllkorperschicht 5 und von dort i den Sumpf 51 und danach über die Leitung 54 zur Pumpe 25 ur. über den Säureerhitzer 24 und die Leitung 23 zurück zur Fül perschicht des Turmes 2, wo die Stickoxide, welche in den Schichten der Türme 5 und 6 aufgenommen.worden sind, wieden das Gas abgegeben werden.The denitrification step is thus carried out in known catfish in the packed layer of the tower 2. The process step of SO ₂ processing takes place in the packed layers of towers 3 and 4 and the nitrogen oxide absorption step in the packed layers of the towers

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 in the known nitrogen oxide-sulfuric acid process, nitrogen oxide is also released from the supplied acid in the denite zone in the method of the invention and reaches the absorption zone via the gas path, where the gas-form 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 into the sump 21, is cooled in the cooler 64 and can be pumped through the pump 65 and the line 63 onto the packed ski 6 and then flows into. the packing layer 5 and from there i the sump 51 and then via line 54 to the pump 25 ur. Via the acid heater 24 and the line 23 back to the filling layer of the tower 2, where the nitrogen oxides which have been taken up in the layers of the towers 5 and 6 are released again.

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

ein Behälter 80 vorgesehen, in welchem sich Salpetersäure

eine Schwefelsäure mit einem hohen Gehalt an Nitrose

Salpetersäure befindet. Die Salpetersäure wird durch die

83 in den Behälter 80 geleitet.According to the invention, the system according to FIG.

a container 80 is provided in which there is nitric acid

a sulfuric acid with a high content of nitrose

Nitric acid located. The nitric acid is by the

83 passed into the 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 discharged into the container 80 through the line 82. The latter container is now according to the invention via the line 37, in which the pump 85 and the valve 46 are provided, connected to the upper end of the first tower 3 of the SO ₂ processing zone.

zone abströmenden Gase. Sobald der NO-Gehalt über einen zulässigen Wert ansteigt, oder falls der Anstieg der

an NO über eine festgelegte Geschwindigkeit hinaus erfolgt, wird die Pumpe 85 in Betrieb gesetzt,'und stark Salpetersäurehaltige Schwefelsäure fließt über das/Ventil 46 und die Leitung 37 in die Füllkörperschicht des Turmes 3.An analyzer 285 continuously measures the NO Ge from the packed layer 6 of the tower, the

zone outflowing gases. As soon as the NO content rises above a permissible value, or if the rise in the

If NO occurs beyond a defined speed, the pump 85 is started, and sulfuric acid containing high nitric acid flows via the valve 46 and the line 37 into the packed layer of the tower 3.

As soon as the 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, the pump 85 is deactivated. Instead of switching the pump 85 on or off, a technically improved increase or decrease in the flow of strongly nitric acid-containing sulfuric acid can of course also be brought about. 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.

der Gase in die Füllkörperschicht 5 des

zwischen den Füllkörperschichten 5 und 6 der

zu messen. An diesen Stellen ist der Stickoxidgehalt selbst verständlich entsprechend höher als am Austritt der

zone. Jedoch kann auch eine Messung innerhalb der

zone als Regelgröße herangezogen werden. Es ergibt sich

eine Verkürzung der Totzeit. Es ist auch möglich, und in sonderen Fällen zweckmäßig, eine NO-Messung gleichzeitig

verschiedenen Stellen der Stickoxid-Absorptionszone durchz führen.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 adjust the nitrogen oxide content on

the gases in the packing layer 5 of the

between the packing layers 5 and 6 of the

to eat. At these points, the nitrogen oxide content is of course correspondingly higher than at the outlet of the

Zone. However, a measurement within the

zone can be used as a controlled variable. It follows

a reduction in dead time. It is also possible and, in special cases, expedient to have an NO measurement at the same time

perform various points in the nitrogen oxide absorption zone.

It was further found that it is advantageous to add liquid or gaseous St according to the invention which contain nitrogen-bound nitrogen as far as possible in the system. This measure allows relatively small amounts of added nitrogenous subs to have a major influence on the NO content of the exhaust gases from the system. The invention thus makes it possible to automate a nitrogen oxide-sulfuric acid system.

form der erfindungsgemäßen Anlage verwirklicht. In der

nach Fig. 2 werden in an sich aus der DT-OS 26 09 505 beka Weise feuchte SO₂-haltige Gase in einem der Denitrierzone geschalteten Vorbehandlungsturm 1 mit Dünnsäure in Kontaktbracht, wobei den Gasen ein Teil ihres Wasserdampfgehalte zogen wird. In der in Fig. 2 dargestellten Anlage übernim Füllkörperschicht 1 diese Funktion. Die aus der Füllkörpe schicht des Turmes 1 in den Sumpf 71 dieses Turmes abflie Dünnsäure wird nun von der Pumpe 75 über.die Leitung 73

den die Säure erwärmenden Wärmeaustauscher 74 und weiter die Leitung 73 in das obere Ende des Turmes 7, dessen

perschicht eine Säureentwässerungszone darstellt, gefördeThis measure is shown in FIG. 2

realized form of the system according to the invention. In the

2, in a manner known per se from DT-OS 26 09 505, moist SO ₂ -containing gases are brought into contact with thin acid in a pretreatment tower 1 connected to the denitration zone, the gases being drawn part of their water vapor content. In the system shown in FIG. 2, this function takes place in the packing layer 1. The thin acid flowing out of the packing layer of tower 1 into the sump 71 of this tower is now pump 75 via line 73

the acid heating heat exchanger 74 and further the line 73 in the upper end of the tower 7, the

layer represents an acid drainage zone

Turm 1 der Vorbehandlungszone stellt erfindungsgemäß einen Vorbereich der SO₂-Verarbeitungszone dar.In the heat exchanger 74, the acid. additionally be warmed so that the water release of the acid in the

According to the invention, tower 1 of the pretreatment zone represents a pre-area of the SO ₂ processing zone.

The separated from the thin acid circuit of the main area of the SO ₂ processing zone thin acid circuit for the pretreatment zone via line 73 is now closed in that the thin acid, which is sprinkled on the upper end 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 heat exchanger 74 increases the release of water in the packed layer of tower 7. The evaporated water leaves the system as water vapor 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 SO ₄ ). ;

By regulating the supply of heating medium to the heat exchanger 74, the amount of water released can be controlled, whereby a desired concentration of the acid in the circuit via the packed layers of the towers 7 and 1

die Denitrierzone geschaltet ist, zum Regeln des gesamten Systems. Dieser mit Füllkörperschicht versehene Turm 1 oder ein entsprechender Gas-Flüssigkeits-Reaktionsapparat anderen Typs, welcher mit Dünnsäure berieselt wird und sich gasseitig am Anfang des Systems befindet, wird also mit der Gesamtmenge der flüssigen oder gasförmigen Stoffe, welche an Sauerstoff gebundenen Stickstoff enthalten, beschickt. Die Füllkorperschicht des Turmes 7 ist an ihrem unteren Ende über die Leitun 72 mit dem Ventilator 167 verbunden und erhält aus der Füllkörperschicht des Absorptionsturmes 6 trockene Abgase aus der Absorptionszone. Diese Abgase entziehen der Säure in der Füllkörperschicht des Turmes 7 Wasser und gelangen zusammen mit dem Wasserdampf über die Leitung 705 in die Atmosphäre.According to the invention, nitric acid or sulfuric acid or nitrous or nitric nitric acid-containing sulfuric acid is now conveyed from the container SO via the pump 85 through the line 37 to the valve 86 and further via the line 78 to the upper end of the packed layer of the tower and sprayed into the packed layer of the tower 1. when the NO analyzer 285 opens the valve 86 and starts the 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. 2, uses the tower 1 sprinkled with thin acid, which is on the gas side

the denitrification zone is switched to regulate 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 to the fan 167 via the conduit 72 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.

In the past, it was customary to change the concentration of acids in certain circuits in order to regulate the NO: NO ₂ ratio in the exhaust gas of the system or within the absorption zone of the nitrogen oxide-sulfuric acid system. This happened through Add water or add 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 known keeping constant is possible through 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 /, which causes some thin acid to flow into the packing layer of the denitrification tower via the pump 25 and the line 23 2 arrives. The density meter controls the addition of thin acid in such a way that a constant acid concentration is maintained at the exit of the denitration zone, ie at the exit 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 of the substance containing it is not passed into the pre-area of the SO ₂ processing zone, that is to say the pretreatment tower 1, but only part of the compound mentioned or substance, while the remaining part of the latter, as in the plant according to FIG. 1, is introduced into the upper end of the tower 3, that is to say into the main area, of the SO ₂ processing zone. 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 ₂ processing zone can be controlled by actuating the valves 46 and 86 accordingly.

The advantageous acceleration of the reactions occurring during SO ₂ processing is all the greater, the larger the contact area between gas and liquid in the reactors of the SO ₂ processing zone

Sauerstoff-Verbindung in den Vorbereich und iin den Hauptbereich der SO₂-Verargbeitungszone, also die Füllkörperschichten der Türme 1 und 3 wird nun eine viel größere Oberfläche entsprechend der Summe der Füllkörpervolumen der Türme 1 und 3 berieselt und eine entsprechend höhere Beschleunigung des ganzen Verfahrens der SO₂-Verarbeitung erzielt.By simultaneously initiating

Oxygen connection in the pre-area and in the main area of the SO ₂ processing zone, i.e. the packed layers of towers 1 and 3, is now sprinkled with a much larger surface area corresponding to the sum of the packed volumes of towers 1 and 3 and a correspondingly higher acceleration of the whole process of SO ₂ processing achieved.

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 ₂ content of the gases inside or after the absorption zone is also measured. 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 NO ₂ content exceeds a specified value, the addition of substances which contain nitrogen-bound nitrogen is continuously reduced or reduced. In addition, if the NO ₂ content is too high or the NO ₂ concentration rises too quickly, nitrous acid-containing sulfuric acid is removed from the absorption zone with the valve 56 open, and a corresponding amount of nitrous-free or low-nitrous acid is passed into the absorption zone. The analyzer 255 continually measures the NO ₂ concentration of the gases at the outlet of the packing layer 6. As soon as the NO ₂ content rises above a predetermined level, or if the NO ₂ concentration rises too rapidly, the pump 85 is shut down. As a result, the addition of strongly nitric acid-containing sulfuric acid from the container 80 into the packing layer of the tower 3 is interrupted. From lyser 255 also controls the pump 55. 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 the line 53 to the packing layer 5 of the absorption tower is directed. The nitrous-free absorption acid flushes the nitrous acid out of the packing layer 5 into the sump 51 and via line 82 into the container 80

If the supply of / 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 S0 ₂ processing zone. Rather, a quicker and more powerful effect is achieved if the nitrogen oxides are first dissolved in sulfuric acid and the nitrogen-containing sulfuric acid thus obtained is introduced into the SO2 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 system. 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,

optimaler Weise durchzuführen.Finally, an overall system is shown in FIG. 5, which combines all the features of the embodiments of the systems and permits them according to FIGS. 1 to 4 /, depending on the type and SO ₂ content of the gases to be processed and depending on whether temporarily more Ammonia or more nitric acid is to be consumed,

to perform optimally.

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

Example

Der Rest besteht aus Stickstoff.A gas stream containing SO ₂ 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 14 m ³ . Polyethylene bodies according to DT-OS 24 16 955 are used as filling material. The filling has a surface area of 2 3 _c a. 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 of} gas. The sprinkling of layer 5 of the absorption tower is reinforced by a pump and a line, which are not shown in FIG. 3. This pump, not shown, conveys acid from the sump 51 onto the layer 5, so that together with the acid flow shown in FIG. 3, the filling results in a trickle of 4 liters / Nm ^{3 of} gas.

The acid temperature at the outlet of heat exchange 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 SO. Processing in the layers of towers 3 and 4 is all the better the more nitrogen oxides are contained in the gas stream.

One liter of nitrous acid per Nm ^{3 of} gas is passed into the layer of tower 2 via line 23. The higher the nitrous content of the acid, the more SO ₂ -containing 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. To regulate the nitrous content of the acid in the sumpi 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 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 Contains 300 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 SO 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 SOA can be used ₂ into the absorption zone, which increases the NO content of the system's exhaust gases.

nahmen nicht angewendet werden - der Nitrosegehalt der Säure welche in die Schicht des Turmes 2 geleitet wird, gesteigert und/oder die Menge der Säure, welche indie Schicht des

2 geleitet wird, erhöht werden. Bei der normalen Berieselung der Schicht des Turmes 2 von 1 liter/Nm³ Gas muß der Nitrose gehalt im Sumpf 51 auf ca. 3 Gew.-% HN0₃ angehoben werden,

500 Nm³/h Gas verarbeitet werden müssen.In order to be able to process the increased amount of SO ₂ without a large loss of nitrogen oxide, the invention must - provided that

were not used - the nitrous content of the acid which is passed into the layer of the tower 2 increased and / or the amount of acid which is in the layer of the

2 is 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 approximately 3% by weight of HNO ₃ ,

500 Nm ³ / h of gas must be processed.

hoben werden. Hierzu würde der Zusatz von Wasser oder

vermindert oder abgestellt. Es würde aber mehrere Stunden

dauern, bevor die NO₂-Konzentration im Abgas wieder unter

absinkt.When adding nitric acid per se into the layer of the denitrification tower 2, one must proceed step by step because it takes many minutes for a change in the nitrogen oxide content of the gases to occur downstream of the absorption zone. A too high addition of nitric acid or a decrease in the SO ₂ content of the inlet gases leads to an increase in N0 ₂ and thus to a yellow coloration of the exhaust gases of the system. To remove this yellow coloration, the concentrate of the acid would then have to be in the denitrification zone and the absorption zone

be lifted. The addition of water or

reduced or turned off. But it would take several hours

last before the NO ₂ concentration in the exhaust gas drops below

sinks.

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 the NO content of the gases measured continuously in the analyzer 285 after the absorption zone to exceed the normal value of 100 ppm to, for example, 150 ppm. An automatic control device then starts the pump 85, whereby a sulfuric acid, which contains 20% by weight of nitric acid, is passed from the container 80 through the lines 78 and 37, respectively, 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 must 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 necessary for SO 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 NO ₂ content of the gases increases downstream of the absorption zone and the NO content falls far below 100 ppm from. The N0 ₂ content of the exhaust gases in the absorption zone is continuously measured by the analyzer 255. If the N0 ₂ content per minute increases by more than 30 ppm per minute, and / or if the NO ₂ content reaches over 200 ppm, an automatic control means. device (not shown) the pump 85 stopped, provided that it was in operation at that time. With an increase of more than 50 ppm per minute and / or an NO _Z content above 300 ppm per minute and / or an NO ₂ content, an automatic control device opens the valve 56 and switches on the pump 55. As a result, nitrous 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 drawn off is 0.3 liters per / Nm ^{3 of} 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 line 83, a nitric acid content of 20% is produced in container 80.

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

The control devices described make it possible to keep the N _x O _y 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 ₂ concentration is subject to frequent fluctuations in the range of ± 70% of the daily average. The liter weight (or density) of the acid circuits is regulated as follows:

The liter weight of the circuit over the packing layers of the 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 packing layer of the tower 7 and therefore increases the liter weight. - The liter weight in the acid cycle of the packed layers of the towers 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. 3. 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. For example, we manufacture them from Thermo Electron Corporation, Waltham, Mass., USA and sell them as "NO _x chemiluminescent source analyzer for automative emission" or as "NO-NO _x chemiluminescent analyzer", eg "Model 44".

A circuit to be used for control is, for example, in the article "Automation Control Technology" in Meyer. Handbook on technology, Biographical Institute Mannheim General Publishing House 1971, pages 729 - 736. If, for example in Fig. 1 of this article, a NO or N0 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 SO ₂ from a gas stream containing it at least temporarily in a concentration that is impermissibly high for discharging into the ambient air, with the production of sulfuric acid by the nitrogen oxide process,] in which the gas containing SO ₂ first contains a denitrification zone or a preliminary area SO ₂ -processing zone and then forming pre-treatment, the denitration zone, then the main part of the SO ₂ -processing zone, and then a nitrogen oxide absorption zone passes through, wherein in at least one of the two portions of the SO ₂ -processing zone with by the concerning region of this 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 de-nitration zone are taken up by sulfuric acid and from the absorption zone contains nitrosic acid with a content at 70 to 85 wt .-% (55 to 63.5 ° Be H ₂ SO ₄ removed and transferred to the denitrification zone is characterized in that (a) the content of NO in the gas flow in the absorption zone or downstream of the absorption zone or at the same time both in the absorption zone and downstream thereof: will, and that (b) when a certain NO- is exceeded

content or if a steepness limit of the increase per unit time of the NO content in the gas stream is exceeded

or the NO measuring points in the thin acid cycle at least one of the two areas of the SO ₂ processing zone

Substance-oxygen compound (as defined) is metered in in a certain amount, which is sufficient on the one hand

exceeding the stated NO limit or

to eliminate value, but on the other hand close to the

Limit values remain, whereby a change in the acid concentration in the absorption zone is practically avoided. 2. The method according to claim 1, characterized in that the amount of metered substance containing St: substance-oxygen compound is increased w: to the extent that the NO content of the gas stream increases in or after the absorption zone. 3. The method according to claim 1, characterized in that the inflow rate of nitrogen-containing substance is 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. 4. The method according to claim 1, characterized in that the amount of metered nitrogen-oxygen compound-containing substance is increased to the extent that a steepness limit of the increase per unit time of the NO content in the gas stream in or downstream of the absorption zone is exceeded. 5. The method according to claim 1, characterized in that the amount of zudosie: nitrogen-oxygen compound-containing substance is kept col as long as a steepness limit of the ascent; per unit time of the NO content in the gas flow in or downstream of the absorption zone is exceeded. 6. The method according to any one of claims 1 to 5, characterized in that the .Nitrogen-oxygen compound consists of nitric acid or a nitrogen oxides generated by ammonia combustion. 7. The method according to any one of claims 1 to 6, characterized in that said substance is formed by absorption of nitrogen oxides in Sch acid. 8. The method according to any one of claims 2 coder 3, characterized in that the metering of said substance is interrupted when the NO content of the gas stream downstream of the absorption zone drops below a predetermined value which is lower than the aforementioned limit of NO. 9. The method according to any one of claims 4 or 5, characterized in that the metering of said substance is reduced if a predetermined steepness limit of the drop per unit time of the NO content in the gas stream downstream of the absorption zone is undershot, the latter steepness limit being approximately the steepness limit according to (b). 10. The method according to any one of claims 1 to 9, characterized in that (a) additionally measuring the NO ₂ concentration of the gas stream downstream of the absorption zone; (b) If a certain NO ₂ content is exceeded or if the steepness of the increase in the NO ₂ content in the gas stream at the NO ₂ measuring point is exceeded, the metering of the substance mentioned into the thin acid circuit of the SO ₂ processing zone is interrupted. 11. The method according to claim 10, characterized in that (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 NO ₂ content in the gas stream downstream of the absorption zone, the amount of the effluent from the latter zone increased nitrous acid and Gürd introduced low or low nitrosic acid from the denitration zone. 12. The method according to any one of claims 1 to 11, characterized in that at least a portion of the substance introduced into the SO ₂ processing zone consists of nitrous sulfuric acid removed from the absorption zone. 13. The method according to claim 12, characterized in that the content of the nitrous sulfuric acid removed from the absorption zone and to be metered into the SO ₂ distribution zone of nitrogenous oxygen compound is increased by adding nitric acid. 14. The method according to any one of claims 1 to 12, characterized in that upstream of the denitrification zone is preconditioned by a separate thin acid circuit pretreatment zone, dure which the gas stream is passed before introduction into the denitrification zone and that part or the total amount of nitrogen Substance containing oxygen compound is introduced into this pretreatment zone. 15. The method according to claim 14, characterized in that the portion of the total amount of substance introduced into the pretreatment zone is branched off from one of the absorption zone into the S0 ₂ processing zone flowing nitrous sulfuric acid. 16. The method according to any one of claims 14 and 15, characterized in that at least part of the amount to be introduced into the pretreatment zone consists of gaseous nitrogen oxides produced by ammonia combustion, and that these nitrogen oxides are introduced into the gas stream before it enters the pretreatment zone 17. The method according to claim 15, characterized in that the nitrogen-oxygen compounds content of the nitrous sulfuric acid to be introduced from the absorption zone and partly into the hair zone of the SO ₂ processing zone and tails into the pretreatment zone is increased by adding nitric acid . 18. The method according to any one of claims 1 to 17, characterized in that the density of the acid emerging from the denitrification zone is kept constant by adding thin acid or water. 19. The method according to any one of claims 1 to 18, characterized in that the density of the thin acid flowing through the main area of the SO ₂ processing zone in circulation is kept constant by adding acid from the pretreatment zone or water. 20. Plant for the separation of SO ₂ from a gas stream which contains it at least temporarily in a concentration which is impermissibly high for discharging into the ambient air with the production of sulfuric acid by the nitrogen oxide process, which plant comprises at least one after the other: (a) a denitrification zone (2) (b) an SO ₂ processing zone (3,4) and (c) a nitrogen oxide absorption zone (5,6)
with at least one gas-liquid reaction apparatus per zone, which is provided with a sump (21; 31; 41; 51;) serving to receive liquid emerging from the lower end of the apparatus concerned, (d) a line for the gas stream (102, 22, 32, 42, 52, 72, 705) which enters the denitrification zone (2) one after the other at the lower end and from the upper end of the latter to the SO ₂ processing zone (3 , 4) and leads from the latter to the nitrogen oxide absorption zone (5,6,) and finally from the upper end of the latter zone to the outside, (e) a circuit line (33, 43) for thin acid through the SO ₂ processing zone, and finally (f) a circulation line for acid (54, 23) from the first sump (51) of the nitrogen oxide absorption zone (5,6) in the gas flow direction to the upper end of the denitrification zone (2) and from the sump (21) of the latter back (121, 63) to the upper end of a reaction apparatus (6) of the nitrogen oxide absorption zone (5, 6,), characterized in that (g) a feed line (37, 117) for nitrogen-oxygen compound in liquid (37) or gaseous (117) phase is connected to the thin acid circulation line (33, 43) through the SO ₂ processing zone, that (h) a measuring device (285) for measuring the content of NO in the gas stream, which is connected to the absorption zone (5,6,) or to the line (d) for the gas stream downstream of the absorption zone or simultaneously to the absorption zone (5,6, ) and connected to the line (72) downstream of the absorption zone (5,6,) and that (i) a control device (85), which doses the addition of nitrogen-oxygen compound through the feed line (37) as a function of the NO contents determined by the measuring device (285), is provided. 21. Plant according to claim 20, characterized in that a NO ₂ measuring device (255) for measuring the NO ₂ content in the gas stream, which is connected to the absorption the absorption zone is connected, gas is present, and that the control device (k) a higher-level control apparatus comprises that or reduces the supply of nitrogen-oxygen compound through the feed line (37, 117) in dependence on the ₂ -einrichtung measured by the NO NO interrupts ₂ content in the gas stream. 22. Plant according to one of claims 20 and 21, characterized in that the feed line (37, 117) for receiving the nitrogen-oxygen compound in the form of nitrous sulfuric acid at the bottom (51) of a reaction apparatus (5) of the nitrogen oxide absorption zone (5th , 6) is connected. 23. Plant according to claim 22, characterized in that an introduction device (83) for nitric acid is provided in the feed line (37). 24. Plant according to one of claims 22 and 23, which comprises a pretreatment zone (1) upstream of the denitration zone (2) with reaction apparatus and sump (71), the line (102, 22) for the gas stream first into this pretreatment zone (1 ) at its lower end and from the upper end of the latter to the lower end of the denitrification zone (2) and whereby a separate circuit line (73) for thin acid leading through the pretreatment zone from top to bottom and a compensation line (133) between the sumps of the Pretreatment zone (71) and the denitrification zone (21) is provided, characterized in that a branch line (78) is provided from the feed line (37) to the upper end of the pretreatment zone (1). 25. Plant according to one of claims 20 or 21, characterized in that an ammonia combustion plant (108) is provided with a column (10) serving for at least partial absorption of the nitrogen oxides developed therein in sulfuric acid, and that a transfer line (115, 117) is provided Bottom of the column (10) with the upper end of a first reactor (3) of the SO ₂ processing zone in the gas flow direction. (3,4,) purpose feeding nitrogen oxide-containing sulfuric acid into the latter zone. 26. Plant according to claim 25, which one of the denitrification zone (2) upstream pretreatment zone (1) with reaction apparatus and sump (71), a separate through the pre. Treatment zone (1) from top to bottom leading circuit line (73) for thin acid and a compensation line (133) between the sumps of the pretreatment zone (71) and the denitrification zone (21), the line (102,22) of the gas stream first into the Pretreatment zone (1) at its lower end and from the upper end of the latter to the lower end of the denitrification zone (2), characterized in that a feed line (112) for nitrogen oxide-containing gases the upper end of the column (10) of the ammonia combustion plant with the Gas flow in the lower end of the pretreatment zone introducing (102) line (102, etc.) connects. 27. Plant according to one of claims 25 or 26, characterized in that the feed line (37) for receiving the nitrogen-oxygen compound in the form of nitrous sulfuric acid to the bottom (51) of a reactor. (5.) the nitrogen oxide absorption zone (5,6) is connected and that a Zweigleit

28. Plant according to claim 27, characterized in that the branch line (78) from the feed line (37) with a further sub-branch is connected to that upper end of the pretreatment zone (1).

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