Field of the invention

The present invention relates to removal of nitrogen oxides (NOx) from a gas stream by means of selective catalytic reduction.

Prior art

Abatement of NOx contained in a gas stream is required for environmental reasons. The symbol NOx and the term“nitrogen oxides” are commonly used to denote mono-nitrogen oxides, namely nitric oxide (NO) and nitrogen dioxide (NO ₂ ). Said nitrogen oxides are known to harm the environment and be potent greenhouse gases. In some cases, removal of nitrous oxide (N ₂ O) may also be required, possibly in combination with the removal of NOx.

A gas stream containing NOx and possibly N ₂ O to be removed is originated from combustion processes and from several processes of interest. A noticeable example is the production of nitric acid, wherein a NOx-containing tail gas is produced. Generally, the nitrogen oxides and N ₂ O must be removed to below a target level in the purified gas, e.g. as required by the applicable law to discharge the gas.

Removal of NOx via selective catalytic reduction (SCR) is based on reduction of the nitrogen oxides to environmentally acceptable compounds, in the presence of a suitable catalyst and possibly in the presence of a reductant. A catalyst which is suitable for SCR is termed“deNOx catalyst”. A common reductant is ammonia and selective catalytic reduction with ammonia as reductant is commonly termed “ammonia SCR”. Reactions involved in the ammonia SCR process are termed “ammonia SCR reactions”.

An SCR process is typically performed by passing the gas through one or more catalytic bed(s) contained in one or more converter(s). Ammonia may be provided e.g. by injecting liquid or gaseous ammonia in the gas stream upstream a converter or directly in the converter. In a multiple-bed process, ammonia may be injected at an intermediate stage, e.g. added to the effluent of a first catalytic bed before it enters the subsequent catalytic bed.

In ammonia SCR, excess ammonia may be required. The term of excess ammonia refers to ammonia being in excess relative to the stoichiometric amount that reacts with the NOx in the ammonia SCR reactions. Feeding excess ammonia is needed to target very low residual emission of NOx e.g. less than 40 ppm, for example 20 ppm. In the presence of excess ammonia, one issue is to avoid ammonia slip, i.e. that unreacted ammonia escapes the process and remains in the purified gas. Ammonia slip poses risks of explosions in downstream operations due to possible formation of ammonium nitrate with residual NOx upon cooling of the gas.

The prior art of catalytic NOx abatement relies mainly on the following options of deNOx catalyst:

V2O5/ T1O2-WO3 Vanadium deNOx catalyst;

Cu-SSZ13, Cu-ZSM-5 Copper zeolite deNOx catalyst;

Fe-Fer, Fe-ZSM5, Fe-BEA, Fe-Y Iron-loaded zeolite deNOx catalyst;

Pt / AI2O3 Platinum supported alumina deNOx catalyst;

Ce / T1O2 Cerium supported titania deNOx catalyst;

Cu / Ce0 ₂ Copper supported ceria deNOx catalyst.

All the above catalysts are effective for the reduction of NOx in the presence of a suitable reductant such as ammonia. Flowever they also have drawbacks.

Vanadium and copper zeolite deNOx catalysts are ineffective towards N2O abatement, and even entail significant N2O byproduct formation when operated in typical ammonia SCR conditions. Flence, they are undesirable in general, and inapplicable when very low emissions levels of both NOx and N2O are targeted.

In ammonia SCR, the Vanadium deNOx catalyst has a further disadvantage in that it does not catalyse reactions involving ammonia other than SCR. Therefore, it lets any excess ammonia slip from the catalyst bed.

Iron-loaded zeolite deNOx catalyst is considered the current state-of-the-art for NOx and N20 abatement from nitric acid tail gas. For example zeolite may be of the Ferrierite (FER) type or of the FMI type (e.g. ZSM-5 zeolite).

Iron-loaded (also termed iron-laden or iron-exchanged) zeolite denotes a catalyst wherein iron (Fe) is intentionally loaded into the zeolite support through an ion exchange procedure. In this step, expendable elements/ions are replaced by Fe ions in sites of the zeolite support. The result is a homogeneous distribution of Fe throughout the support. The iron exchange process can be performed either on a virgin zeolite powder, i.e. before catalyst shaping, or after the shaping of the virgin zeolite powder (e.g. extrusion in pellets).

According to the current art and literature, iron-loading of the zeolite is considered an essential step to ensure a proper abatement activity of the catalyst, especially towards concurrent NOx and N2O. It is claimed that the iron is the active material for NOx and N2O abatement, and the monomer form of iron is active at lower temperatures. The deN ₂ 0 and deNOx mechanism proposed in literature over Fe- zeolite are both based on the redox cycle Fe ^IN and Fe" wherein the desorption of adsorbed oxygen acts as the rate determining step. Literature data point out the iron in the zeolite as active species in the relevant reactions.

A drawback of the iron-loaded zeolite catalyst is the high cost. The cost of the catalyst is mainly due to the expensive iron-loading (ion-exchange) process. This process requires a significant amount of a raw material (the precursor of iron to be exchanged); it produces waste water which needs treatment at an additional cost; it requires dedicated equipment items. In addition, the process is very sensitive to parameters and must be carried out under carefully controlled conditions. This increases the manufacturing cost of the catalyst. A method for reducing the content of NOx and N2O in gases, in the presence of an iron-loaded zeolite catalyst is disclosed in US 2008/0044331.

Summary of the invention

The invention aims to provide an efficient and cost-effective SCR process for reducing the content of NOx in a gas.

The aim is reached with a process of reducing the content of nitrogen oxides (NOx) in a source gas, comprising a passage of the source gas over a catalyst suitable for selective catalytic reduction of NOx and in the presence of a reducing agent, wherein the catalyst is a zeolite catalyst and the zeolite of the catalyst is a FER zeolite which is not loaded with iron and is not loaded with any transition metal.

Another aspect of the invention is a catalyst for use in a deNOx process, according to the claims.

Preferred embodiments of the process and of the catalyst are stated in the dependent claims.

The process of the present invention makes use of a ferrierite (FER) zeolite catalyst wherein the FER zeolite is not iron-loaded.

Said FER zeolite catalyst is a catalyst obtainable with a process wherein no iron and no transition metal is loaded into the zeolite. Particularly, no ion exchange procedure to load iron or any transition metal into the zeolite structure is performed during the manufacturing process of the catalyst.

The term of transition metal denotes any element within the period 4, period 5, period 6 of periodic table of elements. In a preferred embodiment the zeolite is not loaded with elements of the period 4 of periodic table and more preferably is not loaded with any of Fe, Cu, V, Ni, Co and Mn.

The FER zeolite may be loaded with at least one alkaline metal. Preferred alkaline metals for this purpose are sodium and potassium. For example, in a preferred embodiment the FER zeolite is subject to an exchange process with Na20 and K20. The FER zeolite used in the present invention, accordingly, does not contain ion-exchanged iron. Although not loaded with iron, the FER zeolite of a catalyst according to the invention may contain small amounts of iron, although in a different form than ion-exchanged iron which is found in the iron-loaded zeolites. Particularly, the catalyst of the present invention may contain impurities of iron which have typically the form of agglomerates of iron; in contrast the ion- exchanged iron is in the form of atoms of iron bonded to the zeolite.

The content of iron in a catalyst according to the invention may also be smaller compared to the iron content of most iron-loaded zeolites.

In a catalyst according to the invention, iron may be found for example as FeaC . In a catalyst according to the invention, iron (e.g. Fe2C ) may be heterogeneously dispersed contrary to the homogeneous dispersion found in iron-loaded zeolites, due to the lack of a proper Fe-exchange step during the manufacturing process. Iron may also be detected as minor impurity in the zeolite structure. Traces of transition metals may also be present.

In a preferred embodiment, said FER-zeolite catalyst does not contain Bismuth.

The invention is based on the very unexpected finding that a FER zeolite catalyst without iron loading exhibits a deNOx activity which is comparable (or even superior) to that of the much more expensive iron-loaded zeolite catalysts. This finding is contrary to the current belief that iron loading is responsible for de-NOx activity of the zeolite catalysts. The applicant has found that a catalyst according to the invention tends to be less active for N2O abatement, compared to an iron- loaded zeolite catalyst. This reduced activity for N2O abatement is however overcompensated by the advantage of a comparable activity towards NOx abatement at a much less cost. The delS O performance can be matched to iron- loaded zeolite by minor adjustment of operating parameters to reach same near complete abatement levels. No ammonia slip was detected. The catalyst of the invention is available at a competitive cost due to the absence of loading with iron or transition metals in the manufacturing process.

The catalyst of the invention preferably includes an inorganic binder material. Preferred inorganic binders include AI2O3, S1O2, Zr0 ₂ , Ce0 _2. A preferred concentration of binder is 10% to 30%wt. A preferred type of FER zeolite has a Si/AI ratio greater than 6. Said ratio is more preferably in the range 8 to 9, most preferably it is 8.8 or around 8.8. These preferred embodiments show the best compromise between mechanical stability and chemical activity.

According to embodiments of the invention, a removal of N2O may also be performed co-current with the removal of NOx or separately.

It has to be noted that the FER-zeolite according to the invention is in itself the catalyst for removal of NOx and possibly of N2O from the source gas, i.e. the FER-zeolite is not merely a support for a catalyst. Preferably said FER-zeolite is active for removal of NOx and possibly of N2O without a pre-treatment.

Preferably the process of removal of NOx (and possibly of a co-current removal of N2O) is performed in the presence of only the inventive FER-zeolite, i.e. without any other catalyst.

The invention is applicable to a process for removing NOx and to a process for removing NOx and N2O (e.g. concurrent NOx reduction and N2O decomposition). The source gas may be a flue gas, e.g. a gas produced by a combustion process, or another process gas which needs NOx and possibly N2O abatement before a further use or before discharge into atmosphere. An interesting application of the invention relates to treatment of a tail gas in a nitric acid production process.

A process according to the invention may be performed under temperature, pressure, space velocity and composition similar to those of a process using iron- loaded FER zeolite catalyst. The invention allows reaching a target concentration of NOx and N2O as low as 50 ppm.

The invention is also applicable to a revamping procedure, e.g. revamping of a nitric acid plant. Revamping a nitric acid plant may be required to meet a target concentration of NOx and N2O in the tail gas discharged to the atmosphere.

Description of preferred embodiments

A process of reducing the content of nitrogen oxides (NOx) in a source gas, according to the invention, may be carried out in a deNOx stage including a catalytic bed or a plurality of catalytic beds. The catalytic bed or beds may be contained in one pressure vessel or in a plurality of pressure vessels. In a multi bed embodiment, two or more beds may be contained in the same pressure vessel or each catalytic bed may have a separate vessel, according to various embodiments. Any catalytic bed used in the invention may have any of axial flow, radial flow, mixed axial-radial flow.

A process according to the invention is carried out in the presence of a reducing agent. Preferably, said reducing agent is a nitrogen-containing reducing agent. Particularly preferably, said reducing agent is or includes ammonia. The reducing agent is preferably gaseous. Ammonia may be in excess over the stoichiometric amount for reduction of NOx.

A process according to the invention may include a step of adding an ammonia- containing reducing agent. Preferably, the molar ratio of ammonia contained in the added reducing agent over NOx contained in the source gas is 0.5 to 2.5, preferably 0.8 to 2.0 and more preferably 0.9 to 1.5.

For example, a process according to an embodiment may include the step of providing at least one stream of an ammonia-containing reducing agent, which is brought into contact with the source gas. An ammonia-containing reducing agent may be mixed with the source gas before admission into a catalytic bed. In a multiple-bed embodiment, the reducing agent may be added at an intermediate step, e.g. added to the effluent of a first catalytic bed before admission into the next catalytic bed.

A process according to the invention may include setting a proper space velocity in the one or more catalytic bed(s) where the process is performed. A preferred space velocity through a catalytic bed containing a catalyst according to the invention is 5000 to 50000 (5 to 50 10 ³ ) IT ¹ , preferably 7000 to 25000 (7 to 25 10 ³ ) IT ¹ on the basis of the relevant gas flow rate and volume of catalyst.

The temperature of the process is preferably greater than 300°C. The pressure may be generally in the range 1 to 50 bar.

Preferably, the temperature of the source gas is not greater than 500 °C and more preferably is in the range 200 °C to 500 °C or even more preferably 300 °C to 500 °C. Accordingly, the process of removal of NOx is performed at 500 °C or less and preferably at 200 °C to 500 °C or 300 °C to 500 °C. A particularly preferred range is 380 °C to 450 °C.

Accordingly, a FER-zeolite catalyst according to the invention may be active for removal of NOx at the above mentioned ranges of temperature. A process according to the invention may include also a step of reducing the content of N2O in the source gas. A step of removing N2O may be performed before or after a removal of NOx. A removal of N2O may also be performed concurrently with a removal of NOx in the same catalytic bed. Particularly, an embodiment includes a concurrent removal of NOx and N2O by passing the NOx- and N ₂ 0-containing gas over a catalyst which is not loaded with iron and transition metals. This can be made in one or more catalyst bed(s) containing the described catalyst.

Therefore a FER-zeolite catalyst according to the invention may be active for co current removal of NOx and N2O. Particularly preferably, said FER-zeolite catalyst may be active for co-current removal of NOx and N2O at a temperature not greater than 500 °C, preferably 200 °C to 500 °C, more preferably 300 °C to 500 °C and even more preferably 380 °C to 450 °C.

The process of removing NOx and possibly N2O, in the various embodiments of the invention, may be carried out in one or more catalyst beds which contain exclusively the catalyst of the invention, wherein the FER zeolite catalyst is obtained without iron loading and transition metals loading. Removal of N2O may also be performed in the presence of a N2O decomposition catalyst. Said N2O decomposition catalyst may be the same catalyst used for deNOx process or different. Said N2O decomposition catalyst may be a conventional iron-loaded zeolite catalyst or a FER zeolite catalyst according to the invention which is not loaded with iron and transition metals. Decomposition of N2O is known to occur in the presence of an iron zeolite catalyst. A possible explanation of the N2O decomposition is given in the above mentioned prior art US 2008/0044331 , starting from [0030] As stated above, the applicant has noted that also a catalyst according to the invention has a significant delS O activity.

When N2O is also removed, at least 10% of N2O initially contained in the source gas is preferably removed, more preferably at least 30% and even more preferably at least 50%.

An embodiment of a process for removing NOx and N2O as well includes a delS O stage followed by a deNOx stage with intermediate addition of ammonia- containing reducing agent. To this purpose, one or more deN ₂ 0 catalytic beds (deN ₂ 0 stage) are arranged upstream one or more de-NOx catalytic beds (deNOx stage). An advantage of this embodiment (deN ₂ 0-first setup) is a saving of N2O decomposition catalyst because further N2O abatement occurs in the deNOx stage, up to the target N2O abatement level.

In a deN ₂ 0-first setup, a preferred feature is that removal of N2O in the deN ₂ 0 stage is not greater than 90%, preferably not greater than 80%. An advantage is that a residual content of N2O helps destroying the NOx in the subsequent deNOx stage.

Another embodiment includes a deN ₂ 0 stage downstream of a deNOx stage (deNOx-first setup). A deNOx bed in lead position is advantageous particularly when a deNOx catalytic bed and a deN20 catalytic bed are contained in the same pressure vessel (dual bed reactor). In that case, the mixing of reducing agent and process gas can be performed outside the pressure vessel; the dual bed reactor has a smaller volume and the construction of the reactor is simpler. A process which includes the removal of N ₂ O may also include the addition of a reducing agent for N ₂ O. Said reducing agent for N ₂ O may include a hydrocarbon, carbon monoxide (CO), hydrogen (H ₂ ) or a mixture thereof.

A particularly preferred application of the invention concerns the treatment of a tail gas in a nitric acid production process.

The industrial process for the synthesis of nitric acid involves the catalytic oxidation of ammonia to produce a gas containing N ₂ O and nitrogen oxides. Said oxidation of ammonia is typically performed over platinum-rhodium (Pt-Rh) catalytic gauzes. The so obtained gas is subjected to a subsequent step of absorption wherein the gas is contacted with water to absorb NO ₂ in water and produce nitric acid, whilst N ₂ O is not absorbed. The absorption step is performed in an absorber, which is typically an absorber column. The absorption step delivers a liquid product stream containing nitric acid, and a gas containing N ₂ O and residual NOx, which is termed tail gas. Said tail gas is at pressure above atmospheric and may be work-expanded in a suitable expander for energy recovery before being discharged into the atmosphere. Abatement of NOx and N ₂ O from the tail gas may be required to meet the applicable environmental requirements.

The abatement of N ₂ O and NOx upstream of the tail gas expander is termed tertiary abatement. N ₂ O does not play a role in the formation of nitric acid and, therefore, may also be removed in the previous process steps. Removal of N ₂ O from the gas after the oxidation of ammonia and before the absorption stage is referred to as secondary abatement, whilst measures aimed to avoid N ₂ O formation during the oxidation of ammonia are called primary abatement. Abatement of N ₂ O and/or NOx performed after the expansion (i.e. downstream of the expander) is termed quaternary abatement.

A process of removing NOx according to the present invention can be carried out downstream of an absorber in a tertiary or quaternary position, namely upstream or downstream of a tail gas expander respectively. A tertiary abatement is preferred because it enables use of pellet catalysts rather than monolith.

The deNOx catalyst of the present invention can be produced, for example, by any of the following: co-extrusion with additives and/or binder(s), or 3D printing of raw materials, or co-precipitation of binders and zeolite.

The catalyst can be shaped in different forms, depending on the application (e.g. tertiary vs quaternary abatement in a nitric acid process), which may also affect the catalytic performance (activity).

The catalyst may be in pellet form or monolith or foams. A preferred shape of the catalyst is a cylindrical or multilobe extrudate; a particularly preferred shape is a trilobe. Trilobe is preferred because it is more active due to the lower diffusion limitation than monolobe. Moreover the applicant has found out that the trilobe is a particularly preferred compromise between activity and mechanical resistance. The size of the extrudate is preferably about 2mm outer diameter x 10mm length.

A process for making the catalyst of the present invention may include the steps of: i) providing the raw materials, e.g. Al, Si and Na precursors; ii) mixing and hydrothermal treatment to obtain a powder; iii) mixing with binder and additives; iv) extrusion to the final shape, e.g. pellets. The process does not include any step of loading the zeolite with iron, such as ion exchange.

A FER zeolite according to a preferred embodiment is exchanged with 0.1 %wt of Na ₂ 0 and 0.6 %wt of K2O. A final content of about 0.1 %wt of Na and about 0.5 %wt of K was observed in the zeolite after the ion-exchange process.

The following is an exemplary composition of a FER zeolite catalyst, not iron- loaded, according to the present invention.

%wt

Fe 0.06

K 0.42

Na 0.35 Mg <0.05

Si 21 .6

Al 3.9

Example

The following table 1 compares a process for concurrent NOx and N2O abatement with a catalyst according to the invention having the above exemplary composition and a prior-art process for concurrent NOx and N2O abatement with a conventional iron-loaded FER zeolite catalyst (iron ferrierite catalyst). The conventional catalyst was a Fe-FER catalyst containing Fe 0.54%; K 0.13%; Na 0.26%; Mg<0.05%; Si 22.7%; Al 3.8% (%wt).

The test was performed in a lab-scale test plant with a single catalytic bed. Ammonia was fed as reductant with NFh/NOx ratio of approximately 1.1 (i.e. 10% excess of ammonia). Feed conditions were typical of nitric acid plant with upstream N2O abatement, e.g. a tertiary N2O abatement step, or secondary N2O abatement step. Process conditions were optimized for NOx reduction and concurrent N2O abatement. In the table, the symbol ppm denotes parts per million in volume; the symbol GFISV denotes gas hourly space velocity in the test catalyst bed.

Table 1

The example shows: a higher NOx abatement for invention than prior art, 93 vs 90%. N2O abatement of the inventive process was lower than prior art, 66 vs 92%. No ammonia slip for the inventive process despite 10% excess in feed as in prior art.