PROCESS FOR REMOVING NITROGEN OXIDES FROM A GAS

Abstract

A process of reducing the content of nitrogen oxides in a gas, comprising passing the gas over a catalyst suitable for selective catalytic reduction of nitrogen oxides and in the presence of a reducing agent, wherein the catalyst is a FER-zeolite obtainable with a process which does not include a step of iron loading and does not include a step of loading with any transition metal so that said FER-zeolite does not contain ion-exchanged iron and is not loaded with iron or any transition metal.

Claims

1-25. (canceled)

26. A process of reducing a content of nitrogen oxides (NOx) in a source gas, the process comprising: passing the gas over a catalyst suitable for selective catalytic reduction of NOx and in a presence of a reducing agent; wherein said catalyst includes an iron ferrierite (FER) zeolite catalyst obtainable with a process that does not include a step of iron loading and does not include a step of loading with any transition metal so that said FER-zeolite does not contain ion-exchanged iron and is not loaded with iron or any transition metal.

27. The process according to claim 26, wherein said FER zeolite catalyst includes a binder material.

28. The process according to claim 27, wherein the binder material includes one or more of: Al.sub.2O.sub.3, SiO.sub.2, ZrO.sub.2, or CeO.sub.2.

29. The process according to claim 27, wherein the binder material is in a concentration of 10% to 30% by weight.

30. The process according to claim 26, wherein the FER zeolite catalyst is loaded with at least one alkaline metal.

31. The process according to claim 26, wherein the FER zeolite catalyst has a Si/A1 ratio greater than 6.

32. The process according to claim 26, wherein said reducing agent is or includes ammonia.

33. The process according to claim 32, further comprising providing an ammonia-containing reducing agent, wherein the molar ratio of ammonia contained in the added reducing agent over NOx contained in the source gas is 0.5 to 2.5.

34. The process according to claim 26, further comprising a passage of NOx-containing gas in a catalytic bed containing said zeolite catalyst, wherein the space velocity in said catalytic bed is 5000 to 50000 h.sup.−1.

35. The process according to claim 26, wherein the content of NOx in the source gas is reduced by at least 50%.

36. The process according to claim 26, wherein the source gas has a temperature that is less than 500° C.

37. The process according to claim 26, further comprising reducing the content of N.sub.2O in the source gas.

38. The process according to claim 37, further comprising passing the gas over a N.sub.2O decomposition catalyst in at least one deN.sub.2O stage which is before or after a deNOx stage, and/or comprising a removal of N.sub.2O concurrent with the removal of NOx.

39. The process according to claim 38, further comprising removing N.sub.2O concurrent with the removal of Nox, and wherein the concurrent removal of N.sub.2O and NOx is performed in at least one catalyst bed containing said FER zeolite catalyst which is not loaded with iron and transition metals.

40. The process according to claim 38, further comprising removing N.sub.2O in a deN.sub.2O stage separate from removal of NOx in the presence of a N.sub.2O decomposition catalyst, wherein said N.sub.2O decomposition catalyst of the deN.sub.2O stage is iron-loaded zeolite catalyst or a FER zeolite catalyst which is not loaded with iron and not loaded with any transition metal.

41. The process according to claim 38 wherein at least 10% of N.sub.2O initially contained in the source gas is removed.

42. The process according to claim 26, wherein the source gas is a tail gas of a process for the synthesis of nitric acid.

43. A catalyst for use as a deNOx catalyst in a process of selective catalytic reduction of NOx, the catalyst comprising: an iron ferrierite (FER) zeolite catalyst obtainable with a process that does not include a step of iron loading and does not include a step of loading with any transition metal, so that said FER zeolite is not loaded with iron, does not include ion-exchanged iron and does not include any transition metal.

44. The catalyst according to claim 43 wherein said FER-zeolite catalyst is catalytically active for removal of NOx from a source gas having a temperature not greater than 500° C.

45. The catalyst according to claim 43 wherein said FER-zeolite catalyst is catalytically active for co-current removal of NOx and N.sub.2O from a source gas.

46. The catalyst according to claim 43 wherein said FER-zeolite catalyst does not contain bismuth.

47. The catalyst according to claim 43, further comprising a binder material.

48. The catalyst according to claim 47 wherein said binder material includes one or more of Al.sub.2O.sub.3, SiO.sub.2, ZrO.sub.2, or CEO.sub.2.

49. The catalyst according to claim 47, wherein the binder material is in an amount of 10% to 30% by weight.

50. The catalyst according to claim 43, wherein the FER zeolite has a Si/Al ratio greater than 6.

Description

DESCRIPTION OF PREFERRED EMBODIMENTS

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.Math.10.sup.3) h.sup.−1, preferably 7000 to 25000 (7 to 25.Math.10.sup.3) h.sup.−1 on the basis of the relevant gas flow rate and volume of catalyst.

[0046] The temperature of the process is preferably greater than 300° C. The pressure may be generally in the range 1 to 50 bar.

[0047] 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.

[0048] 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 N.sub.2O in the source gas. A step of removing N.sub.2O may be performed before or after a removal of NOx. A removal of N.sub.2O 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 N.sub.2O by passing the NOx- and N.sub.2O-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.

[0049] Therefore a FER-zeolite catalyst according to the invention may be active for co-current removal of NOx and N.sub.2O. Particularly preferably, said FER-zeolite catalyst may be active for co-current removal of NOx and N.sub.2O 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.

[0050] The process of removing NOx and possibly N.sub.2O, 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.

[0051] Removal of N.sub.2O may also be performed in the presence of a N.sub.2O decomposition catalyst. Said N.sub.2O decomposition catalyst may be the same catalyst used for deNOx process or different. Said N.sub.2O 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 N.sub.2O is known to occur in the presence of an iron zeolite catalyst. A possible explanation of the N.sub.2O 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 deN.sub.2O activity.

[0052] When N.sub.2O is also removed, at least 10% of N.sub.2O initially contained in the source gas is preferably removed, more preferably at least 30% and even more preferably at least 50%.

[0053] An embodiment of a process for removing NOx and N.sub.2O as well includes a deN.sub.2O stage followed by a deNOx stage with intermediate addition of ammonia-containing reducing agent. To this purpose, one or more deN.sub.2O catalytic beds (deN.sub.2O stage) are arranged upstream one or more de-NOx catalytic beds (deNOx stage). An advantage of this embodiment (deN.sub.2O-first setup) is a saving of N.sub.2O decomposition catalyst because further N.sub.2O abatement occurs in the deNOx stage, up to the target N.sub.2O abatement level.

[0054] In a deN.sub.2O-first setup, a preferred feature is that removal of N.sub.2O in the deN.sub.2O stage is not greater than 90%, preferably not greater than 80%. An advantage is that a residual content of N.sub.2O helps destroying the NOx in the subsequent deNOx stage.

[0055] Another embodiment includes a deN.sub.2O 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 deN2O 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.

[0056] A process which includes the removal of N.sub.2O may also include the addition of a reducing agent for N.sub.2O. Said reducing agent for N.sub.2O may include a hydrocarbon, carbon monoxide (CO), hydrogen (H.sub.2) or a mixture thereof.

[0057] A particularly preferred application of the invention concerns the treatment of a tail gas in a nitric acid production process.

[0058] The industrial process for the synthesis of nitric acid involves the catalytic oxidation of ammonia to produce a gas containing N.sub.2O 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.sub.2 in water and produce nitric acid, whilst N.sub.2O 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.sub.2O 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.sub.2O from the tail gas may be required to meet the applicable environmental requirements.

[0059] The abatement of N.sub.2O and NOx upstream of the tail gas expander is termed tertiary abatement. N.sub.2O 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.sub.2O 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.sub.2O formation during the oxidation of ammonia are called primary abatement. Abatement of N.sub.2O and/or NOx performed after the expansion (i.e. downstream of the expander) is termed quaternary abatement.

[0060] 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.

[0061] 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.

[0062] 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).

[0063] 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 2 mm outer diameter×10 mm length.

[0064] 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.

[0065] A FER zeolite according to a preferred embodiment is exchanged with 0.1% wt of Na.sub.2O and 0.6% wt of K.sub.2O. 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.

[0066] The following is an exemplary composition of a FER zeolite catalyst, not iron-loaded, according to the present invention.

TABLE-US-00001 % wt Fe 0.06 K 0.42 Na 0.35 Mg <0.05 Si 21.6 Al 3.9

EXAMPLE

[0067] The following table 1 compares a process for concurrent NOx and N.sub.2O abatement with a catalyst according to the invention having the above exemplary composition and a prior-art process for concurrent NOx and N.sub.2O 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).

[0068] The test was performed in a lab-scale test plant with a single catalytic bed. Ammonia was fed as reductant with NH.sub.3/NOx ratio of approximately 1.1 (i.e. 10% excess of ammonia). Feed conditions were typical of nitric acid plant with upstream N.sub.2O abatement, e.g. a tertiary N.sub.2O abatement step, or secondary N.sub.2O abatement step. Process conditions were optimized for NOx reduction and concurrent N.sub.2O abatement. In the table, the symbol ppm denotes parts per million in volume; the symbol GHSV denotes gas hourly space velocity in the test catalyst bed.

TABLE-US-00002 TABLE 1 Catalyst bed inlet Same for invention Catalyst bed outlet Abatement Catalyst bed outlet Abatement and prior art Invention Invention Prior art Prior art P (bar) 10 10 T (° C.) 430 430 Molar composition NOx (ppm) 402 29 93% 39 90% (NO/NO2 = 2.3) N.sub.2O (ppm) 302 103 66% 24 92% NH.sub.3 (ppm) 442 0 100%  0 100%  O.sub.2 (% mol) 3% 3% 3% H.sub.2O (% mol) 0.4%   0.4%   0.4%   N.sub.2 Balance Balance Balance GHSV (h.sup.−1) 14 000  

[0069] The example shows: a higher NOx abatement for invention than prior art, 93 vs 90%. N.sub.2O 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.