FR3022471A1 - METHOD FOR REGENERATING A DENITRIFICATION CATALYST CONTAINING IRON AND / OR MANGANESE OXIDES - Google Patents

Abstract

In order to regenerate a denitrification catalyst containing oxides of iron and / or manganese, the invention provides that: the catalyst is washed by circulating on the catalyst for at least fifteen minutes a washing solution which, before contact with the catalyst, contains neither sodium nor potassium nor calcium, and which, in contact with the catalyst, permanently has a pH between 8.5 and 10.5; the catalyst is then dried for at least one hour at a temperature of between 80 ° C. and 130 ° C. and then for at least four hours at a temperature of greater than or equal to 140 ° C.

Description

The present invention relates to a process for the regeneration of a denitrification catalyst containing iron and / or manganese oxides. BACKGROUND OF THE INVENTION The present invention relates to a method for regenerating a denitrification catalyst containing iron oxides and / or manganese. Denitrification catalysts are widely used to reduce emissions of nitrogen oxides, commonly referred to as NOx, in a wide range of industries, including the incineration and power generation industries. The catalysts used employ a reducing agent, most often ammonia, and the process is known as the SCR process, which is the acronym for "Selective Catalytic Reduction". The catalyst used typically employs a support base, such as alumina, titania, zirconium oxide or a zeolite, on which at least one Group VB or VIB metal is deposited. By far the most commonly used formulation is a structured catalyst having a titanium oxide base in anatase form on which active ingredients are deposited, most preferably vanadium (V205) and tungsten (WO3) oxides. In order to reduce the nitrogen oxides, such catalysts are combined with the introduction of ammonia so that the reactions between the ammonia and the nitrogen oxides (NOx) form nitrogen, thereby detoxifying the fumes.

Several catalyst formulations have been proposed. Among them, mention may be made of the vanadium and tungsten catalysts mentioned above, as well as the catalysts based on copper, iron or manganese, which are more active at low temperature but more sensitive to deactivation by the sulfur. These various catalysts operate, to denitrify fumes, at a temperature between 130 ° C and 400 ° C. The fumes to be denitrified generally contain sulfur oxides which, even present at concentrations of less than 50 mg / Nm3, will deposit on the catalysts, and all the more easily as the temperature is low: thus, the dioxide of Sulfur (SO2) and sulfur trioxide (SO3) negatively interact with catalysts, either by reaction with ammonia to deposit salts, or by direct reaction between sulfur and the catalyst metals. More specifically, these two mechanisms respectively consist of: - deposition of salts, in the form of ammonium salts, such as ammonium hydrogen sulphate or ammonium sulphate, or even ammonium sulphites and hydrogen sulphites, since Ammonia, used as a reagent for the destruction of nitrogen oxides, is present in high concentration; this phenomenon is all the more sensitive as the temperature is low and the content of sulfur oxides is high; For this reason it is often necessary to set a minimum temperature, below which it would be dangerous to operate the denitrification, depending on the concentration of sulfur oxides; and an attack of the active ingredients of the catalyst, in particular iron and manganese oxides, with the sulfur oxides or with sulfuric acid which is present in the form of aerosols, which will transform the oxides into sulfates , inactive. Thus, iron and manganese catalysts, which are very active at low temperatures, have a marked affinity for sulfur oxides which combine with metal oxides, such as Fe 2 O 3 and Mn 2 O 3, to give iron sulphate and sulphate. 10 of manganese, inactive. Moreover, all these catalysts, when they are subjected to the stream of fumes to denitrify, are exposed at the same time to various poisons, among which heavy metals and some metalloids, such as arsenic, and are also exposed to dust which, by pore blocking or by exchange reaction with other compounds, gradually deactivate the catalyst. Several proposals have been made for reactivating such catalysts. In particular, the catalyst can be heated to a sufficient temperature, generally above 300 ° C., in an attempt to desorb the poisons. This approach works well for ammonium hydrogen sulphate, which is quite volatile, and for certain poisons such as the chlorides of certain heavy metals. However, it is inefficient to restore an activity if the active element of the catalyst has been altered, as is particularly the case when an oxide, such as manganese oxide, has been transformed into manganese sulphate which is particularly stable at high levels. temperatures, temperatures of more than 500 ° C are necessary to retransform the sfflate oxide.

In addition, numerous communications or patents propose to wash the catalyst with acidic solutions. For example, US 3,258,430 or Jung Bin Lee, Korean Journal of Chem. Eng. 2011 or Yanke Yu in Cheng Chern, Catalysis Communications, vol 39, Sep 5, 2013. These various disclosures propose washing generally with dilute sulfuric acid.

If such an acidic wash is sufficient to restore activity to the most commonly used catalysts, i.e. those based on vanadium, this wash can not be used for iron oxide catalysts and / or manganese, because the acidic washing solution tends to transform the metal oxides into inactive salts and moreover soluble. The known washing processes are therefore unsuitable for catalysts containing iron and / or manganese oxides.

The object of the present invention is to provide a process for the regeneration of catalysts based on iron oxides and / or manganese, which preserves, or even improves, the qualities of the catalyst. For this purpose, the subject of the invention is a process for the regeneration of a denitrification catalyst containing iron and / or manganese oxides, in which: the catalyst is washed by circulating over the catalyst for at least fifteen minutes; minutes a washing solution which, before contact with the catalyst, contains neither sodium, potassium nor calcium, and which, in contact with the catalyst, permanently has a pH of between 8.5 and 10.5; the catalyst is then dried for at least one hour at a temperature of between 80 ° C. and 130 ° C., and is then penalized for at least four hours at a temperature greater than or equal to 140 ° C. The idea underlying the invention is to reactivate the catalyst by washing with a basic solution whose pH is between 8.5 and 10.5 and which contains neither alkaline nor alkaline earth. In practice, such a washing solution may be an ammoniacal solution, a solution of ammonium carbonate or ammonium carbamate, for example. It is important to note that alkaline solutions containing potassium or sodium can not be used, the latter being poisons known to the catalysts. You can not use lime either. Solutions containing calcium would react to form calcium sulphate, which is very sparingly soluble and would block the pores of the catalyst. In contrast, the washing solution used for the invention advantageously consists of other alkaline solutions, especially solutions containing ammonia or capable of releasing, such a washing solution having a beneficial triple action, namely: the wash solution physically cleans the catalyst, removing it from its soluble salts, such as NH4HSO4 and (NH4) 2SO4, particularly, and entraining the dusts that would be trapped therein; the washing solution converts the salts, such as manganese sulphates and iron sulphates into the corresponding hydroxides, which, during drying and, if appropriate, subsequent calcination, are then converted in their turn into oxides, which are catalytically active; and the washing solution leads to an additional solubilization of certain residues, for example certain residues of the manufacture, which cleans the pores, opens them and thus increases the BET surface, the catalytic activity being favorably impacted by this increase in the BET surface.

Thus, the catalyst regenerated by the process according to the invention has an activity comparable to or even greater than that of the original new catalyst. In addition, the invention prohibits the washing of the catalyst in an acid medium, the acid of such a medium attacking the oxides by itself while a subsequent heat treatment at a moderate temperature, typically below 350 ° C. , would not be enough to transform the sulphates into oxides. The invention also differs from a "simple" washing in a neutral medium, especially with water: since acid sulphates are present on the catalyst, the washing of the latter can only acidify such a neutral medium, lowering the pH well below 7, and metal salts would be extracted, thereby leading to surface depletion and loss of catalyst integrity. According to advantageous optional features of the process according to the invention: after having dried the catalyst, it is subjected for at least two hours to calcination in an oxidizing atmosphere and at a temperature of between 350 ° C. and 550 ° C .; before washing the catalyst, it is mechanically dusted by blowing; the washing solution is an ammoniacal solution; the washing solution is an ammonium carbonate solution; the washing solution is an ammonium carbamate solution; The circulation of the washing solution on the catalyst is carried out with a specific watering rate of greater than 8 m 3 / m 2 / h. The invention will be better understood on reading the description which follows, given solely by way of example. In order to regenerate a denitrification catalyst containing iron oxides and / or manganese, the detailed steps are carried out successively below. A preliminary step, optional but beneficial, consists in mechanically dusting the catalyst by blowing, with, for example, air or dry steam. Then, during a washing step, the catalyst is contacted with a basic wash solution which is circulated over the catalyst. In practice, this circulation of the basic solution can be carried out in situ, using a pump. Alternatively, the catalyst to be regenerated may also be taken out and washed outside the reactor in which it is normally installed and used to denitrify fumes. It is necessary to avoid washing with a solution that would become acidic, otherwise the iron and / or manganese oxides of the catalyst could be attacked. In addition, since acidic salts, such as NH4HSO4, are often present on the catalyst, the wash solution tends to acidify on contact with the catalyst and becomes frankly acidic, i.e. 6, if the pH of the wash solution was not sufficiently high before contact with the catalyst, for example if the wash solution was water. According to the invention, it must therefore be ensured that the circulating washing solution is permanently basic with a pH of at least 8.5. The duration of the washing with the basic solution is important, in the sense that the salts present on the catalyst must be solubilized or converted into hydroxides. In practice, a washing time of at least fifteen minutes is required. Conversely, an excessively long wash time, for example greater than two hours, should be avoided as this would over-weaken the catalyst, which generally contains binders, such as clays, which are hydrophilic: temporarily, the catalyst could then soften and deform. As indicated above, the washing solution used should initially not contain sodium, potassium or calcium. More generally, the solution should not contain any substances or salts, which would not be decomposable at less than 350 ° C. For this reason, an ammonium carbamate solution or an ammonium carbonate solution are acceptable, as is an ammonia solution. Advantageously, the specific watering rate of the catalyst with the washing solution must allow sufficient irrigation of the channels of this catalyst and good wetting of their walls. This specific watering rate corresponds to the circulating flow rate of the washing solution, brought back to the cross section of the catalyst to be regenerated. Preferably, the specific watering rate is at least 8 m3 / m2 / hour. The washing step is followed by a drying step, during which the catalyst is dried, first by wearing it for at least one hour at a temperature between 80 ° C and 130 ° C, preferably between 90 ° C and 120 ° C and then for at least four hours at a temperature greater than or equal to 140 ° C. Optionally, this drying step is followed by a calcination step, during which the catalyst is, in an oxidizing atmosphere, in particular an air atmosphere, brought to a temperature of between 350 ° C. and 550 ° C., during a period between two and four hours, although a longer time is not detrimental. During the drying step, as well as, if appropriate, during the calcination step if it is carried out, the hydroxides resulting from the action of the basic solution on the catalyst during the step of Washing, such as Mn (OH) 2, Fe (OH) 2 and Fe (OH) 3, are oxidized and converted to oxides, such as Mn 2 O 3 and Fe 2 O 3. The drying step, as well as the calcination step where appropriate, are also aimed at giving the catalyst sufficient strength.

It will be understood that, according to the process according to the invention, elements such as iron or manganese sulphate, which are, within the catalyst, the product of the oxides of iron and manganese oxide being attacked by the oxides. Sulfur, instead of being rinsed off, are converted to hydroxides and then to oxides which are finely divided, present on the surface of the catalyst and very active. Conversely, a simple washing with water or an acid wash would eliminate these metals and deplete the catalyst on the surface, which is detrimental to its activity. Similarly, it is important to note that simply heating the catalyst without the presence of a basic element, such as ammonia, is not sufficient to regenerate the catalyst.

EXAMPLE: An extruded catalyst in the form of a structured catalyst, with square section channels approximately 3.5 mm in size, which is predominantly composed of iron oxide, manganese oxide and titanium dioxide as well as bentonite clay having served as extrusion binder, was prepared for catalytic denitrification at a temperature below 160 ° C. This catalyst was placed in contact with real fumes from an incinerator, after dedusting and removing most of the contaminants such as sulfur dioxide. This catalyst was left in place for about three months and was passed through with fumes whose composition is detailed in the table below: average peak unit dust 25 mg / Nm³ dry, 11% 02 SO2 200 mg / Nm3 dry, at 11% O2 HCl 50 mg / Nm3 dry, at 11% O2 temperature 140 135-150 ° C NOx 350 500 mg / Nm3 dry, at 11% O2 The values given above in spike columns are exceptional in the sense that they relate to catalyst exposure situations which have been brief compared to normal conditions which are associated with the values indicated in the "average" column: it should be noted that the exposure The catalyst at such particularly high pollutant peaks helped to deactivate the catalyst more than would have been the case under normal operating conditions. Catalyst activity was determined according to a standardized test, which is used commercially to monitor the activity of the catalysts. Similarly, the BET surface of the tested catalyst was also determined. These various determinations were made before exposure of the catalyst to the fumes, that is to say while the catalyst was new, and after exposure to fumes, and after regeneration of the catalyst. In accordance with the invention, regeneration of the catalyst first included blowing the catalyst to remove dust. Then, the catalyst was washed by flowing an ammoniacal solution through its channels for one hour. The solution used was a solution of ammonia in water, whose initial pH was 9.5. After contact with the catalyst to be regenerated, the solution had a pH greater than 8.5.

The table below groups together the activity and specific surface area values, the activity being dimensionlessly expressed by the formula -Ln (1-conversion) where "conversion" is the conversion rate of nitrogen oxides in the conditions of the test. catalyst new catalyst exposed catalyst regenerated BET surface area (in m2 / g) 56 32.2 64.1 activity 2.38 0.73 2.88 It can be seen that the regeneration according to the invention made it possible to restore the initial activity. catalyst, or even more because of the opening of its pores. Similarly, the BET surface area has also been recovered to a higher value than the initial one. On the other hand, the iron and manganese contents in the washing solution after its contact with the catalyst were measured: at the end of the washing step, this solution contains less than 1 mg / l of iron and less than 5 mg / I of manganese. Washing the same catalyst with water, under the same conditions, leads to a content of 200 mg / l of manganese, which results from the dissolution of the manganese sulphate, product of the attack of the catalyst by the oxides of sulfur. 25

Claims (7)

CLAIMS 1. A regeneration process of a denitrification catalyst containing iron oxides and / or manganese, wherein: - the catalyst is washed by circulating on the catalyst for at least fifteen minutes a washing solution which, before contact with the catalyst, does not contain sodium, potassium or calcium and, in contact with the catalyst, permanently has a pH of between 8.5 and 10.5; the catalyst is dried for at least one hour at a temperature of between 80 ° C. and 130 ° C., and is then penalized for at least four hours at a temperature greater than or equal to 140 ° C. 2. A process according to claim 1, wherein after drying the catalyst, it is subjected for at least two hours to calcination in an oxidizing atmosphere and at a temperature of between 350 ° C and 550 ° C. 3. A process according to one of claims 1 or 2, wherein before washing the catalyst, it is mechanically dusting by blowing. 4. A process according to any one of the preceding claims, wherein the wash solution is an ammonia solution. 5. A process as claimed in any one of claims 1 to 3, wherein the wash solution is an ammonium carbonate solution. 6. A process as claimed in any one of claims 1 to 3, wherein the wash solution is an ammonium carbamate solution. 7. A process according to any one of the preceding claims, wherein the circulation of the washing solution on the catalyst is carried out with a specific watering rate greater than 8 m3 / m2 / h.