CA1058599A - Nox control in catalyst manufacture - Google Patents

Abstract

TITLE:
NOX CONTROL IN CATALYST MANUFACTURE

ABSTRACT OF THE DISCLOSURE

An improved process for suppression of oxides of nitrogen arising from heat treatment of catalyst materials containing a source of such oxides is provided which com-prises separately introducing the catalyst material and urea into the heating zone and then effecting heat treatment.

Description

CASE 25,720 lC~S~3S99 Thi's invention relates to a process for abating oxides o~ nitrogen that emana-te upon heat treatment of the catalyst materlals.- More particularly, the invention relates to such a process wherein the catalyst material and urea are separately introduced into the heat treatment zone and abate-ment results by reaction o~ the oxides o'f nitrogen with the .
urea thus introduced at reactive temperatur~ arising from' such heat treatment.
' A continuing con~ern over environmental pollution, particularly air'pollution is reflected in the~provisions for ever-increasingly stric~ limitations being enacted into law.~ l A major source of air pollubion arises ~rom exhaust gases .

emanating from combustion of petroleum products such as opera-tion of internal combustion engines, burnin~ in conjunction with heat and power generation, and industrial processes.
Considera~ble progress has been made ln redu~c;ing air pollution by provlding cleaner fuels and emlssion controls~. ~ large 'contribution in this control of noxious emissLons~has been ~' - provided by the catalyst industry'ln~the form of catalyst materials which provide cleaner fuels and convert noxlous ~umes to innocuous gases that are normally present in the air.
In preparing catalyst ma-terials for these and other uses, a convenient method involves providing a porous support or carrier material upon which various activators and/or pro-moters are carried. In providing the support material as well .
as the activator and/or'promoter therefor~ it is often neces-sary to employ nitric acid~ or salts of nitric acid. Such ~"
usages upon subsequent heat treatment of the catalyst ma-terial gives rise to copious fumes of oxides of nitrogen which are highly noxious air pollutants, and some provision for abate-ment of such emissions is required.
In many applications of catalyst materials, it is desirable that they be in the form of particles of consistent -lOS85~9 speciric size having a desired level o~ porosity. This is necessary to avoid excessive pressure drops across`a catalyst bed while providing the necessary diffusion to ef~ect the de-sired reaction. ~ccordingly,~the catalyst materials pre-pared for use in these applications are formed into the de-sired particAulate size and shape in conjunction with their manufacture. In the norma~ preparation of Ghe particulates, a small amount of fines, or undersized particles, will de-velop~ but the amount is w~thin limits and does not seriously affect processing.
One method of abating the emisslons o~ oxides of ;~
nitrogen is to pass them through specially installed scrub-bers which can remove pollutants. Scrubbing equipment is generally extremely buIky and costly to install and service.
Installation often is diffi~cult to achieve in existing plants because of space limitations with the result that redesign and reconstruction o~ the p~lant may be necessary. In many in-stances products arising ~rom the scrubbing operation may .
give rise to disposal problems and other dif~lculties.
` An alternative~method for abating emlssions of ox~
ides of nitrogen is to provide the catalyst material with a content of urea when it also contains a source of such oxides.
Subsequently, when the cat,alyst material is hea-t treated to temperatures at which oxides o~ nitrogen are evolved, the 25~ oxides are abated by the urea contained by the catalyst materi-al. Thus the catalyst material itself contains the reactant necessary for the abatement of the oxides of nitrogen that arise. However, because the urea is in contact with the catalyst material being heat treated, it becomes deposited within the pore volume of the particulate catalyst material.
It is believed that nitric acid and nitrates decompose accord ng to the equation 2HN03 ~ 2 :

It is also believed that nitrous acid reacts with urea according to the heat equation 2HN02 + ~NCONH2 ---i? 2N2~C02~3H20. Thus, at reaction temperature the two uolumes of nitrous acid combine with urea to provide six volumes of gaseous products. This voluminous gas generation appears to take place rapidly and exothermically causing large pressure increases within the pore structure of the particulate catalyst material. The sudden large pressure increases within the particulate catalyst material are b~lieved to be responsible for particl~e breakage and when urea is part of the oontent of the catalyst materials becomes excessive and intolerable.
Accordingly, there continues to exist the need for methods of abating oxides of nitrogen that arise in the production of catalyst materials and do not cause excessive fines production or~ other deficiencies of former methods. The provision for such a method would Gonstitute a notable advance in the art and fulfill a long-felt need.
It is a primary object of the present invention3 therefore, to provide a method of abating oxides of nitrogen that arise in the heat treat-ment of catalyst materials containing a source of such oxides, which method does not cause excessive production of fines when particulate catalyst materials are involved.
In accordance with the present invention, there is provided a process for abating oxides of nitrogen which arise during heat treatment in the manufacture of catalysts containing a source of said oxides which process comprises introducing the catalyst material requiring heat treatment at a temperature of 300C. and higher in said manufacture into a suitable heat treatment zone, separately introducing an effective amount of urea into said heat treatment zonej and thereafter effecting heat treatment at a temperature of 300C. and higher whereby oxides of nitrogen arising from said catalyst material are abated by reaction with the separately introduced urea.

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1058~;99 In a preferred embodiment o~ the inventive process, ' the urea ls deposited upon fines from prior preparations of catalyst'material and such treated fines are co-mingled with the catalyst material to be hea-t treated and separated there-from subsequent to heat treatment.
In a more preferred embodiment of the inventive processj the urea in the form of a mist is sprayed above the catalyst material directly onto the oxides of nitrogen that - arlse while avolding cont~ct o~ the urea mist ~ith the cata-lyst material. ' The present invention provides an effective method~
~of abating oxides of nitrogen arising from heat treatment of catalyst materials containing a source of such'oxides while avoiding'deficiencies of the former methods. When the catalyst msterial is processed in the form of particulates, the production of fines is'consistent wlth conventional pro-cessing, i.e. no excessive production of fines occurs. The -~
process is readily~carried out using existing installations , without the need for scrubbers and the space and servicing 20 ' - requirements associated therewith. The process is simple to~
perform and readily controlled.
It ls surprising that such a simple process should be so highly effective in vie~ o~ the prior teachings which requ-ire elaborate scrubbers or a urea content within the catalyst material being heat treated.
In carrying out the process of the present'inven-tion, the catalyst material is prepared in accordance with conventional procedures except ~or the heat treatment step.
The catalyst material will, of course, have a source of o-~0 xides of nitrogen as part of its composition at the time the heat treatment is to be conducted. In conducting the heat treatment, the catalys-t material and urea are separately in-troduced into the heat treatment'zone wherein a temperature L~

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- ~OS8S99 .
suf~icient to cause generation of oxides of nitrogen and to ef~ect reaction of urea therewith. The temperature at which reaction o~ urea with oxides of nitrogen occurs is essenti-.....
ally that at which the oxides of nitrogen arise. ~ccordingly, if the heat treatment conditions are suff~icient to generate oxides of nitrogen, the~ are inherently capable of effect-. ~ .
ing reaction of urea therewith. Generally the heat treat-ment is conducted at temperatures of 300C. and higher and effective.abatèmènt of oxi~es of nitrogen occ~rs. Even at ~ temperatures of a~out 150C., evldences of generatiorl of oxides of nitrogen~and of abatement thereof by~urea~are seen.
A typical~preparation of formed~catal~st material is one based on spra~-dried~powdèred alum1na obtalned by precipitation of sodium aluminate arid aluminum sulfate. The . -&lumina powder is mulled w1th water~and appropriate ~extru-sion`aids to form;an~eYtrudable paste. In~such mixture,~
nitr1c acid lS sometimes added~to modify properties of the~
resulting extrudates. After the extrudable pa~ste~has~been - obtained, it is extruded into desirable~orm and, if neces-; 20 ~sary,~cut to the`desired length.~ Typically, the pas~e is formed into cylinders of I~16 to l/4 inch diameter and cut to a~length to width~ratio of -from about 4:1 to l:l. The ex-t~udates are then subjected to heat trèatment to set their str~c-ture. Heat treatment generally consists of preliminary drying at low temperature~i.e. up to about 125C., ~nd o~
subsequent calcinatlon at temperatures of about 500C. and higher.
In preparing the`extrusion paste, a-s indicated, nitric acid may be present.~ In addi~tion, in some instances provision may also be made for actlvator or~promoter content in the extrusion mix. ~ In such instances, a metal compound in the form o~ a nitrate salt may be used. Accordingly, the extrudates as prepared may contain one or more sources of - ~ _ 5 _ - 105l~S99 .

oxides o~ nitro~en whi.ch require abatement durlng heat treat-ment. At low temperature drying condi:tions, evolution o~
oxides o~ nitrogen may not occur and abatement may, there-fore, be unnecessary. However, if oxides of nitrogen do ' arise at the drying conditions employed, they may be abated according to the process o~ the invention. Usually, the o-xides of nitrogen arise during high temper~ture heat treat-ments, such as calcination and'it is in con~unction there-with that the process of ~he invention is'particularly use-~;. 10 . ~ul. ' ' :
~' ' In the above dlscussion, it is indicated that in preparing the extrudates and setti~ng their structure by heat treatment, the necessity o~ abatement of oxides of nitrogen due to incorporation of nitric acid or metal nitrate salts in the extrusion mixture~may arise. ~It also~'frequently - occurs that provision for~activators and/or promoters is made after the extrudates have been prepared and set in structure by heat trea'tment.~ In~such ;n~stances~the calc-ined extrudates are treated~with suitable activators and/or~pro-moters, generally by-lmpregnation of~aqueous solutions of soluble~compounds~that upon subsequent heat treatment pro-vide the desired activator or promoter content. In many in-stances'one or more of the soluble compounds is a nitrate salb and will give rise to oxides o~ nitrogen in the sub-sequent heat treatment. Accordingly, the process of the present invention is applicable ~or-abating the oxldes o~
nitrogen that will arise upon subsequent heat treatment~
generally a second calcination.' The present lnvention-is not concerned with the p~re-paration of a catalyst material per se but is only concerned with abatement of oxides of nitrogen -that arise in con~unc-tion with the heat treatment thereof. Accordingly, prepara-tion of the catalyst materials will follow conventional pro-: , . .

~058599 cedures usin~; conventional ingredients eY~cept for the pro-vision for abatement of oxides of nitrogen as desc-ribed herein.
When a catalyst material'Ls subjected to heat treat- -~ .
ment, such'treatment ma~ be carried out in a variety of , heating devices in accordance with conventional procedures.
These devices may lnclude muffle furnaces,'rotflr~ ca].clners, direct-fired kilns, and other types of heal;i.n~r' ovens that provide the necessary temperature of` trea't;ment. In'any of`
these devicesj'the heat tr~atment takes place in what ls con-~ veniently referred to as a heating zone. The catalyst materi-` al is introduced into the heating zone and subsequent~ly , reaches the desi.red temperature o~ hea,t treatment during iLs residence therein. Provision~can be readil,y made to intro-; duce other materials~into the heating zone along with the 15 ~ catalyst material.~
; One embodiment~of the present invention :involves in-troducing sDlid urea,~preferably as prills lnto~the heatins~ ~ ~
, - zone with~the catalyst~materi.al. Thi~s~ is effectively done~b~ ~' co-mlngling~the urea prills with the ~catalyst materials.~ ~s ~' the temperature~of the~materials in the heating zone ;lncrease ~3, and oxides of nitrogen are ~enerated, the'urea becomes'acti~re ~in the ab'atement reaction and converts;oxides of nitrogen to i~nocuous gases. ` ,' ''' A pre~erred~embodiment of the present invention is to lntroduce urea into the heating zone in a f~orm in which lt ls carried on fines from previous catalyst preparatlons. The urea lS impregnated~upon the fines -to deposit an ef~ective ' amount thereon and then dried. The thus-treated fines are then co~mingled wlth the catalyst material to be heat treated ~ -and again effective abatement'of the oxldes of` nitrogen is obtained.' In a more preferred emhocliment of the present in-vention, the urea is~introduced ~n~o the heating zone in the form of a mist directly onto the f'umes o~ ox:i,des o~ nitrogen , -- 7 --l~)S8S99 as they arise at a point o~ contact sufficiently removed ~rom the catalyst material' so that the urea is out of con-tact with the catalyst material. Alternatively, the urea mist may be introduced at a distance beyond the point of formation of the fumes but prior to the point where they are discharged to the atmosphere.
If the temperatu~e o~ the heating zone is adequate, the urea may be introduced in the form of a mist of an a-queous solut~ion thereof. ~lternatively, the urea may be introduced as a mist o~ solid or molten urea to minimize lowering o~ the temperature. The rate of indroduction of -the mist should be sufficient to minimize or eliminate the brown oxides o~ nitrogen that are emanated. ~he mist may be supplied rrom a single source or from a battery of sources in lateral or tandem arrangement. The mist may be introduced by use of any suitable means such as a ~steam aspirator, an air jet, a spray nozzle? an atomizer, and'such other devices ' that are normally used ~or provIding a mist. It is posslble ~- to introduce the mist at~a suitable point within the heat treating device ~ltsel~ or at a suitable point upon the exit~
gas stream. ~' As indicated in the equation above, one mole of urea is ef~ective for 2 moles of nitrous or nitric acid. Effec-tive'reduction of oxides of nitrogen will generally be ob- '-~
served using from about 0.'25 to 5 moles, preferably 0.5 to 4 moles of urea per mole of nitrate ion present in -the cat-alyst material to be heat treated.
The invention is more fully lllustrated in the ex-amples which follow wherein all parts and percentages are by weight unless otherwise specified.

Calcined alumina extrudates formed as cylinders of 1/8 inch diameter from precipitated alurnina and having a~ pore ` 1051~599 volum,e o~ 0.7~ cc. per gram were employed as carrier. ~n aqueous solution in the amount of 220 cc. and contalning 53.7 grams of (N~I4)2 M2 07 (ammonium dimolybdate), 8,8.5 grams of Ni~N03)2. 6H20 and 26.5 grams of 86.~ H2P04 was employed to impregnate 300 grams of'carrier. Impregnation was e~-fected by spraying using~a pressuriged spray gun directed on the extrudates rotating in a ~allon jar. Sprayin~, was for five minutes and the extrudates were rota~ted for an additional hour. The wet extrudates ~ere allowe~ to stand overnight and then dried àt 250F. for 2 hours.
The dried extrudates were co-mingled with 36.5 grams,~
of urea in solid crystalline ~orm in preparation for calcina-tion. The solid urea crystals were added to the 'extrudates and the mixture was rotated in a gallon jar to ef~ect uniform co-mingling. The resulting composition was then calcined at - 1200F. for 1.5 hours,:during which time no evolution of o-xides of nitrogen was detected,. The resulting catalyst materi-, àl had a composition corresponding to 6% N10, l~ MoO~, 3.2 ~, ~ P and the balance alumlna. Catalyst breakage was minimal.
20 ' ' COMPARATIVE EXAMPLE A ~ , , - The procedure of ExampIe 1 was,~ollowed in every material detail except that no provision for'urea co-mingling was made., Instead the dried extrudates without urea provision were sub,~ected directly to calcination. Copious evolution of oxides of nitrogen occured during calcination of the impreg-nated support.
, COMP~RATIVE EXAMPLE B
The procedure of Example 1 was followed eXcept that 36.5 grams of urea were incorporated in the impregnating solution and thus urea was present within the porosity o~ the support. Co-mingling of the solid urea was eliminated since the urea was present within the support porosity. 'No evolu-tion o~ oxides of nitrogen was detected dùring calcination _ 9 _ 1~5~S99 , of the impre~nated support. However, excesslve breakage of the resulting catalyst material occurred, resulting in pro-duction Qf fines that amounted to about three times the amount obtained in Example 1.

The procedure o~ Example l was again Eollowed ex-cept that granular urea added was in the lorm o~ prills.
Substantially the same results were obtained as in Example 1.
EXAMP~E ~
~; 10 Fines screened ~rom previous support preparation in the amount of 100 grams were lmpregnatéd with 74 cc. of an aqueous solution containing 36.5 grams of urea. The impreg-- nated fines were dried at 150C. to remove the aqueous ~ medium.
The procedure of Example 1 was again folIowed ex cept that the urea-lmpregnated fines were co-rningled with the dried extrudates in place of the crystalline solid urea.
No evolution of oxides of nitrogen was;détected during the -~ calcination step. The~fines were~then separated from the 2Q catalyst material and the yield of unbroken catalyst materlal~
was improved by the use o~ the urea-impregnated fines.

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m e procedure of Example 3 was followed except that the irnpregnated support was not dried after impregnation, the urea-impregnated fines being co-mingled with the wet ex-trudates. Upon subsequent calcination no evolution of oxides of nitrogen was detected and breakage of catalyst material was rninimal.

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A catalyst support is prepared following conventional procedures using a precipitated alumina-powder, processing the powder as an aqueous slurry in a mix-muller, using added nitric acid to increase denslty of the resulting support, ~058599 and extruding the resulting mix. The extrudates are then heat treated -to remove water therefrom and to set the sup-port structure. This heat treatment is carried out using a conventional calciner. The exhaust gàses that emanate from ~ the calciner contain copious brown fumes of oxides of nitro-gen when no provision is made for abatement of such fu~es.
At a point near the discharge end of the calciner, which operates at about 1000 to 1300F., a mist of urea in the form o~ a 30 weight p~cent aqueous solution is intro-duced above the extrudates being heated therei~ so that no contact of mist therewith occurs. The exhaust gases travel countercurrent to -the flow of extrudates and discharge -to-ward the ~eed end of the calciner. The mist of urea thus .
introduced eliminates all evidence o~ the typical brown fumes of oxides of nitrogen.
EXAMPLE_6 Impregnated extrudates are prepared as-in Example 1.
When a portion~of the extrudates are calci.ned at 1200F. for 1.5 hours, copious brown fumes emanate from the ca~ciner.
~ When provision is made for abatement of fumes of oxides of nitrogen following the procedure of Example 5, no evidence of the typical brown fumes of oxides of nitrogen is se,en. Catalyst breakage is minimal.

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Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for abating oxides of nitrogen which arise during heat treatment in the manufacture of catalysts containing a source of said oxides which process comprises introducing the catalyst material requiring heat treatment at a temperature of 300°C. and higher in said manufacture into a suitable heat treatment zone, separately introducing an effective amount of urea into said heat treatment zone, and thereafter effecting heat treatment at a temperature of 300°C. and higher whereby oxides of nitrogen arising from said catalyst material are abated by reaction with the separately introduced urea.

2. The process of claim 1 wherein said urea is introduced in the form of a mist.

3. The process of claim 1 wherein said urea is introduced in solid form.

4. The process of claim 2 wherein said mist is an aqueous solution.

5. The process of claim 2 wherein said mist is pulverized solid urea.

6. The process of claim 3 wherein said urea is in prill form.

7. The process of claim 3 wherein said urea is carried on catalyst fines.

8. The process of claim 1 wherein said heat treatment is calcination.

9. The process of claim 1 wherein said catalyst material is one based on an alumina support.

10. The process of claim 1 wherein said catalyst material is one based on an alumina support and contains molybdenum and nickel activators.