CA1133680A - Catalytic process for synthesis of ammonia - Google Patents

Abstract

CATALYST FOR AMMONIA SYNTHESIS A B S T R A C T Hydrided titanium is employed as a catalyst for reacting nitrogen and hydrogen to yield ammonia. The catalyst is formed by first removing surface oxides from titanium metal with hydrogen gas at elevated temperatures and pressures and then reacting the free metal with hydrogen to produce titanium di-hydride. The catalyst is preferably associated with an inter-metallic compound of iron and titanium. The particle size of the combined catalyst may be reduced by one or more steps of hydriding and dehydriding the carrier alloy.

Description

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The present invention relates to the synthesis of ammonia from molecular nitrogen and hydrogen through the use of a new catalyst and novel processes based thereon. Ammonia is an important raw material in the chemical industry, particularly in the production of synthetic fertilizers. Agricultural research has shown that nitrogen is an indispensable ingredient of fertilizers for crops. The major source of that nitrogen at the present time is ammonia and one of the major goals of chemical technology in the fertilizer field is the production of ammonia at faster rates and with correspondingly lower costs.
Prior art methods for the production of ammonia from gaseous nitrogen and hydrogen have employed iron catalysts of various types However, such catalysts require high temperatures and pressures necessitating expensive equipment of relatively low capacity, and are rapidly consumed at the temperatures and pressures required by various degradation mechanisms.
Research on the synthesis of ammonia has also progressed in the direction of room temperature synthesis in aqueous ~lutions where either biological "nitrogen-fixation" conditions are simulated, or a metal salt or complex is used as a catalyst together with a reducing agent. Although aqueous studies are still at the fundamental research level and have not yet been commercialized, it has been found that the metals of heterogeneous catalysts effective to synthesize ammonia at high temperatures from gaseous nitrogen and hydrogen also catalyze the ammonia synthesis when present as a salt or complex in aqueous solutions.
Although gaseous nitrogen and hydrogen theoretically can be combined at room temperature and atmospheric pressure to produce ammonia at an equilibrium yield of 95.5% on thermo-dynamic grounds, there is no catalyst or method for ammonia ~' ' 1~33~

synthesis in the prior art that can be employed at room tempera~e to produce ammonia from gaseous nitrogen and hydrogen at com-mercially feasible rates. Therefore, it has been necessary to employ much higher temperatures in ~he synthesis of ammonia to achieve satisfactory reaction rates. The iron catalysts previously used produce no appreciable ammonia from gaseous nitrogen and hydrogen at temperatures below approximately 360~ C. Even higher temperatures are therefore necessary for acceptable yields.
However, higher temperatures in turn result in a drastic re-duction in the thermodynamic equilibrium yield of the reaction due to its exothermic nature. Reductions in equilibrium yields with increasing temperatures can only be partly compensated for by increasing the operating pressure, and the pressures needed closely approach the design limits of the equipment presently available for industrial application.
Prior art processes for the synthesis of ammonia from gaseous nitrogen and hydrogen over commercial iron catalysts operate at temperatures around 500~ C. The equilibrium yield of the reaction at this temperature with only one atmosphere of pressure is well below 0.2%. Higher pressures in the range of 150 atmospheres are therefore employed to compensate for the low yield at ambient pressure. Such high pressures result in high equipment and maintenance cost. Furthermore, equilibrium yields attained at such elevated pressures are usually less than 25~/D.
This means that a substantial portion of the effluents from reactor vessels must be recycled one or more times, adding sub-stantially to the cost of production both in the form of added equipment and longer operating times for separation and recycle of the product stream.
By reason of the foregoing thermodynamic and kinetic 1 336~3~
considerations, the cost of producing ammonia by prior art methods is quite high, involving relatively slow production rates and costly equipment. The cost of the iron catalyst it-self is also quite substantial, mainly because special manu-facturing processes must be utilized to improve the catalytic properties of the iron. Thus, additional components called promoters must be added to the iron and, in most cases, both the iron and the promoters must be supported on a special carrier in a manner to permit sufficient contact between those constituents and the gaseous reactants is quite expensive.
The foregoing disadvantages encountered with reactions between nitrogen and hydrogen over prior art catalysts are avoided through the use of the present invetion. The novel catalyst of this invention for the ~irst time allows the use of substantially lower operating temperatures and pressures in achieving commercially feasible production rates. ~t such temperatures and pressures, equilibrium yields fall within the range of 30 to 60% and therefore actual yields are greatly improved. This new catalyst is comprised of titanium dihydride, either alone or associated with a binary alloy of iron and titanium in a 1:1 ratio. In some circumstances the binary alloy may be used alone as the catalyst.
The reaction rates attainable with this new catalyst at any given temperature and pressure are at least an order o~
magnitude (factor of 10) and sometimes several orders of magnitude greater than reaction rates experienced with prior art catalysts and processes. Consequently, the cost of ammonia production is substantially lo~ered. Furthermore, equipment requirements are much less stringent and less costly by reason o~ the lower operating pressures that can be employed at the process temperature ~ ~ 3 ~

selected. In addi~ion, the catalyst itself is free from many problems commonly encountered with prior art catalysts such as sintering and physical attrition. To the contrary, it has been found that the catalytic activity of the titanium catalyst employed in the present invention increases with aging in the presence of the hydrogen component of the reactant atmosphere.
This enhancement of activity is the result of cracks and fissures formed in the catalyst par~icles, both microscopically (surface cracks) and macroscopically (breakage into smaller particles), with the attendant increase in the active surface area of the catalyst. There is also much less poisoning or deactivation of the catalyst through smothering of adsorbing sites by the adsorption of impurities that normally exist in commerically available nitrogen and hydrogen reactants. With proper control of the process conditions, activation of the catalyst bed can continue simultaneously with the production reaction.
It is therefore an object of this invention to provide a novel process for the manufacture of ammonia from hydrogen and nitrogen with much higher yields than heretofore attainable.
A further object of the invention is to provide a catalytic process for making ammonia from gaseous hydrogen and nitrogen at substantially increased ratés of production.
Another object of the invention is to provide a commer-cially economic process for the production of ammonia from gaseous hydrogen and nitrogen at lower temperatures and pressures than heretofore possible, and to thereby reduce the cost of process equipment and operating and maintenance costs associated therewith.
A further object of the invention is a process for producing ammonia with a catalys~ which is resistant to poisoning . .

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by impurities that usually exist in commercially available hydrogen and nitrogen reactant gases.
A still further object of the present invention is to provide a catalytic process capable of yielding higher rates o~ ammonia prod~lction at ambient temperatures than heretofore possible.
Another object of the present invention is to provide a catalyst which will promote ammonia synthesis in aqueous solutions containing molecular hydrogen and nitrogen.
A still further object of the invention is the identifi-cation of a metal composition capable of weakening or breaking the chemical bonds of nitrogen and hydrogen molecules and the utilization of such composition as a catalyst for the production of ammonia from molecular hydrogen and nitrogen.
Yet another object of the invention is a catalyst which can be readily and economically associated with a bi-metallic alloy compound, and possibly other co~ponents, to further promote the catalytic reaction.
A still further object of the present invention is to pr~vide a commerical process for the catalytic production of ammonia wherein the cost of the catalyst itself is-substantially less than the cost of prior art catalysts for such processes.
The catalyst of the present;invention is comprised of titanium hydrides, primarily titanium dihydride which is a stable compound. More definitively, it has been found that titanium metal combines with hydrogen to form an extremely active catalyst for the production of ammonia from gaseous nitrogen and hydrogen.
Furthermore, the reaction appears to be enhanced when the hydrided titanium is associated with a binary alloy of iron and titanium, particularly as a hydrided titanium phase in the iron-titanium alloy (which itself may be converted to iron titanium hydride).

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The alloy itself m~y also be used as a catalyst under some circumstances.
The specific alloy associated with the hydrided titanium in the preferred embodiment has a titanium to iron mole ratio of 1.0 and is available from the International Nickel Company This alloy is further described in a book entitled Constitution of Binary Alloys, First Supplement, as authored by -. P. Elliot and published by McGraw-Hi~l, New York, New York, 1965, and in a paper of Reilly, et al. referenced more fully below. This alloy is formed from relatively pure iron and titanium by a melting process requiring temperatures in the range of 1500 to 1900~ C. Although this alloy can be made from com-mercial grade iron, the purity of electrolytic iron is preferred.
The combined catalyst may be made by the same process as the binary alloy since an alloyed mixture containing more than 1 mole of titanium for each mole o~ iron will produce a composition cor.sis~ing of two phases, namely, the intermetallic compound having a titanium to iron mole ratio of one and pure titanium, the latter being converted to titanium dihydride in an initial hydriding step. Titanium dihydride is a stable compound which remains as such throughout the production process and is continually activated (decontaminated) as long as it is in contact with significant amounts of hydrogen.
The following range of compositions for hydrided titanium associated with iron-titanium 1 to 1 compound have been determined to have significant catalytic activity in combining molecular hydrogen and nitrogen to produce ammonia and are listed in the order of increasing activity for this reaction: 0.1 moles hydrided Ti to l mole FeTi, 1 mole hydrided Ti to l mole FeTi, and 2 moles hydrided Ti to 1 mole FeTi. Thus, the greater the ~ ~ 3 3 ~ ~

amount of titanium associated with the alloy, the greater the reaction rate and yield ~or ammonia synthesis. Combined catalyst compositions richer in titanium dihydride than a mole ratio oE

2 to 1 do not appear to be commercially available at the present time due thedifficulties experienced in the manufacture of corresponding iron-titanium alloys. However, it is believed that even higher catalytic reaction rates may be attainable if greater quantities of free titanium are associated with the iron-titanium compound or other alloys. Therefore, the scope of this dis-closure is intended to cover the full range of free titanium compositions. Based on the foregoing, the preferred catalyst of this invention is that having 2 moles of hydrided titanium associa-ted with a carrier comprised of a binary alloy of titanium and iron in a mole ratio of 1 to 1.
The catalysts of the invention are active for ammonia formation at all temperatures around and above 180 C. and at all pressures at and above atmospheric, the higher the temperature ~, or the pressure or both, the greater the reaction rates attainable.
The upper limits of temperature and pressure for production reactions are set onl~ by the physical design parameters of the equipment employed.
Although hydrided titanium appears to exhibit increased catalytic activity when alloyed with or otherwise associated with the iron-titanium bi-metallic compound, and although the bi-metallic compound shows some catalytic activity itself, the catalytically more active component is considered to be titanium dihydride. For further information on the independent properties of each of these compounds, particular reference is made to the article entitled "Formation and Properties of Iron Titanium Hydride" by J. 3. Reilly and R. H. Wiswall, Jr., of Brookhaven 113368~:) National Laboratory, published in Inor~anic Chemistry, Vol. 13, No. 1, 1974, at pages 218-222, and to the book by W. M. Mueller, et al., entitled Metal Hydrides as published by Academic Press, New York, New York., 1968.
The titanium metal, either alone or as associated with the bi-metallic alloy, is first reacted with hydrogen to clean the material and form titanium dihydride, preferably as a separate series of steps prior to use of this compound as the catalyst in the ammonia production reaction. The hydriding steps first remove reaction inhibiting oxides and other impurities on the catalyst particles and then produce titanium dihydride as well as iron titanium hydrides when associated with the bi-metallic alloy.
The catalyst composition should be ordered from the manufacturer in particulate form of relatively small size (less than 16 mesh). As received, the particles are coated with an oxide layer. In this state, the metal will not form its hydride, which is the compound active as a catalyst. These activation steps also remove other surface and internal impurities such as carbon and nitrogen compounds and adsorbed gases other than nitrogen and hydrogen. Initial cleaning of the particles and formation of the catalyst is preferably accomplished by subjecting a particulate bed of titanium to hydrogen gas at temperatures in the range of 200 C. to 400~ C. and pressures in the range of 150 to 200 psia for an extended period. The reactant gases are then passed through the bed for the production reaction.
Where the catalyst is associated with the iron-titanium 1 to 1 binary alloy, the initial immersion is followed by out-gassing and then alternately pressurizing the catalyst bed with hydrogen at much higher pressure and outgassing the hydrogen so _ g _ . ~ ~ 3 ~
that the alloy is successively hydrided and dehydrided. This process breaks up the supported catalyst particles into smaller particles and also produces multiple cracks in the surface of each individual particle, th~rehy greatly increasing the reactive surface area of the catalyst bed. The dehydriding cycle is carried out at approximately 200 C. or greater, preferably about 400 C. with outgassing by pure helium purging at approximately atmospheric pressure, and the hydriding cycle is generally carried out at ambient temperature (20 to 25 C.) and 1,000 psia.
Alternatively, and preferably due to the possibility of impurities in the helium, outgassing may be accomplished by drawing a slight vacuum of 1 or 2 inches of water in the reactor vessel containing the catalyst bed. This size reduction process is preferably continued until the average particle size is 200 mesh or less.
Following the steps for preparation of the catalyst, a gaseous feed stream comprised of nitrogen and hydrogen is con-tinually passed over the catalyst bed in a production reaction carried out at a temperature and pressure selected for the highest or other desired level of yield of ammonia in the product.
Although significant yields of the product are attainable at temperatures as low as 180 C. and pressures as low as atmo-spheric, commerial yields generally require higher temperatures in the range of 275 to 325 C. and higher pressures in the range of 500 to 1500 psia (30 to 100 atm.). Even greater temperatures and pressures would further increase reaction rates, but such operations are limited at the present time by restrictions imposed by design parameters of the process equipment available. In addition, higher temperatures could also have adverse effects on the ammonia synthesis reaction by increasing the rate of ammonia dissociati~n into nitrogen and hydrogen to lmacceptable levels In other words, excessively high tempera~ures could reverse the synthesis reaction because of thermodynamic limit-ations arising from the exothermic nature of the reaction.
~lowever, at temperatures and pressures within the ranges specified above, yields approaching 100% of theoretical are attainable.
When the parital pressure of the hydrogen in the reactor vessel is equal to or greater than the equilibrium dissociation pressure o~ iron titanium hydride and the catalyst is associated with the 1 to 1 binary alloy, that alloy phase will be converted to iron titanium hydride which is a stable compound only under such conditions. When the catalyst bed is outgassed, this hydride compound reverts to ~he binary alloy, aiding in the break up of the particles. However, it is believed that the catalytic reaction itself would not be inhibited by the hydrided form of the inter-metallic compound if such pressures were employed in the production process. The partial pressure of hydrogen to be used at a given temperature to achieve the hydrided state of the alloy can be determined from the equilibrium dis-sociation pressure of iron titanium hydiide at that temperature, the latter relationship being set forth in the literature. For the specific temperature and pressure relationship utilized in this invention, see particularly the article entitled "Formation and Properties of Iron Titanium Hydride" referenced above.
The preferred processes for both making the combined form of the catalyst and subsequently producing ammonia will now be described. A catalyst bed consisting of as purchased particles of titanium and iron alloyed at a ratio of 3 to 1 is charged into a conventional reactor vessel, such as that presently used in the production of ammonia with prior art catalysts. This composition contains a titanium metal phase associated with an irorl-titanium 1 to 1 compound phase with two moles of free titanium ~or every mole of carrier compound.
The reactor is then heated and outgassed by drawing a vacuum at ~00 C. for approximately six to eight hours to desorb and expel contaminant gases. Following the outgassing and while maintaining the vessel at 400 C., the reactor is pressurized with hydrogen to 200 psia and maintained at temperature and pre-ssure until the formation of titanium dihydride has been completed~
which requires approximately four to six hours. The catalyst forming process also removes the oxide films and other adsorbed impurities from the catalyst so as to enhance diffusion of hydrogen into the alloy, as well as to permit adsorption of the reactant gases during the production reaction. The initial hydrogen treatment is best carried out with the hydrogen gas confined to the reactor vessel in a static condition, instead of utilizing any type of flow regime with pure hydrogen which could result in excessive temperatures from the hydride formation reaction. Upon removal of the oxide films, titanium dihydride begins forming in the presence of the hydrogen with the evolution of sufficient heat to raise the reactor temperature significantly.
When the reactor temperature levels off and starts to fall back to that maintained externally (40~ C.), formation of the titanium dihydride catalyst is complete.
A process to enhance the activity of the supported catalyst thus formed is then commenced by allowing the reactor to cool to near ambient temperature (20 to 25 C.) while con-tinuouslydrawing a vacuum over the catalystbed to outgas the hydrogen. Upon reaching ambient temperature, the reactor is then pressurized again with hydrogen to a pressure above the ~ ~ 3 ~7~equilibrium dissocia~ion pressure of the hydride form of the iron-titanium alloy at the prevail,ing temperature. A hydr~gen pressure of l,000 psia is sufficient to accomplish this hydriding step at the usual ambient temperatures encountered. After such pxessuriæation has been maintained for approximately one-half hour, the reactor is again heated to approximately 400 C. and immediately allowed to cool upon reaching the temperature while maintaining a slight vacuum throughout the heating and cooling cycle to outgas the hydrogen in a dehydriding s~ep. These hydriding and dehydriding cycles break the catalyst p~rticles without destroying the integral bond between the titanîum di-hydride phase and the intermetallic compound phase, and are preferably repeated until the desired particle size is attained.
This usually requires three to four cycles, depending on the original particle size and the dimensions of the catalyst bed.
The catalyst is then ready for the production reaction.
Following the last dehydriding step of the size reduction process, the reactor is heated to 300 C. and pressurized with hydrogen to 80 atm. A reactant composition comprised of l mole of nitrogen to 3 moles of hydrogen is then introduced into the reactor ,~nd the product stream drawn off on a continuous basis at a flow rate determined by space velocity (ratio of feed rate to total weight of catalyst) which should not e~ceed 500 cubic meters per hour per ton of catalyst as determined with reference to standard conditions of temperature and pressure. A variety of other feed compositions, such as hydrogen to nitrogen mole ratios of 2 to 1 or 1 to 1 may also be employed within the scope of this invention. However, it is desirable to always maintain sufficient hydrogen in the feed stream to achieve continuous activation of the catalyst.

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Higher space velocities and corresponding feed rates are also possible, but m~y give lower yields and would require higher pumping energy inputs. Nevertheless, faster throughput and lower yields may be more economical overall depending on the optimum parameters of the separation and recycle equipment employed to handle the product downs~ream of the reactor vessel.
The product stream leaving the catalyst bed will con-tain the nitrogen and hydrogen reactants and the ammonia product.
If desired, the ammonia can be separated from the product stream in conventional fashion and the reactants recycled to the reactor vessel. One such separation scheme involves cooling the product s~ream to a temperature in the range of 25 C. to 100 C., which is usually low enough to totally condense the ammonia product and then passing the steam through a condensate separator and recycl-ing ~he gas effluent consisting of the uncombined nitrogen and hydrogen back to the reactor. ~ctual condensate temperature in this case would be determined by the process economics, taking into account the cooling, heating and pumping operations required, as well as the partial pressure of the ammoina in the product stream. Some of the ammonia product might also be recycled, depending of course on the parameters of the separation equipment.
Although but a single embodiment of the present invention has been described, other embodiments and variations will occur to those skilled in the art. For example,- the hydrogen and nitrogen reactant may be contacted with the catalyst while in physical states other than a gas. Thus, aqueous solutions and other carriers containing free nitrogen and hydrogen molecules may be passed over or in contact with the catalyst and the hydrogen and nitrogen thereby reacted to produce ammonia. All such processes are within the contemplation of the present invention.

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It is possible, of course, to use various features of the specific embodiment described, such as any of the various compositions of tit~ni.um dihydride with intermetallic compounds of iror~ and titanium and various combinations of temperature and pressure, and such uses are intended to be covered by the present invention. It is also possible to combine titanium dihydride and/or the intermetallic compound with known catalytic-ally active metals for this reaction such as ruthenium and osmium, either in the form of mixtures or multi-component (e.g. ternary, quaternary or higher) alloys, or to associate the titanium dihydride with other intermetallic compounds, or to support the catalytic compositions on an inert carrier material or other substrate.
Furthermore, many other changes in the process steps are possible and such changes are within the scope of the disclosure. By way of further example, activation of the catalyst can be achieved, although at a slower rate, by exposure to the hydrogen in the feed stream itself, particularly if the product stream was to be recycled until the desired level of product was achieved. It is therefore ~o be understood that the foregoing specification merely illustrates and describes a preferred embodiment of the invention and that other embodiments are con-templated within the scope of the appended claims.

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Claims (11)

WHAT IS CLAIMED IS:

1. A method of making ammonia which comprises contacting nitrogen and hydrogen in synthesis proportions at synthesis conditions with a catalyst including an alloy of titanium and iron in a mole ratio of total titanium to total iron greater than 1.0 and then exposing such titanium iron alloy to hydrogen at hydriding conditions, the proportions of titanium and iron and the alloying conditions being such as to produce an alloy containing both an iron titanium intermetallic compound and free titanium in amounts effective to catalyze the conversion of nitrogen and hydrogen to ammonia and said hydriding conditions being such as to hydride at least a portion of the free titanium.

2. A method of making ammonia according to claim 1 wherein said alloy is exposed to hydrogen at a pressure at least equal to the equilibrium dissociation pressure of iron titanium hydride at the prevailing temperature of the alloy.

3. A method of making ammonia according to claim 1 wherein said intermetallic compound has a mole ratio of titanium to iron substantially equal to 1Ø

4. A method of making ammonia according to claim 1 wherein said hydriding conditions include exposing said alloy to hydrogen at an elevated temperature and pressure effective to remove oxides from exposed surfaces thereof.

5. A method of making ammonia according to claim 1 wherein the mole ratio of total titanium to total iron in said alloy is at least equal to 1:1.

6. A method of making ammonia according to claim 1 wherein said synthesis conditions include a catalyst temperature of at least 180° C. and the reactants at a total pressure of at least one atmosphere.

7. A method of making ammonia according to claim 1 wherein the proportion of hydrogen to the proportion of nitrogen is at least equal to a mole ratio of 3 to 1 and wherein said synthesis conditions include contacting the catalyst with a gaseous mixture of the reactants at a rate not exceeding 500 cubic meters of gas per hour per ton of catalyst as determined with reference to standard temperature and pressure conditions.

8. A method of making ammonia according to claim 1 wherein said hydriding conditions are such that substantially all of said free titanium is converted to titanium dihydride.

9. A method of making ammonia according to claim 1 wherein said hydriding conditions are such that said alloy contains both an iron titanium hydride phase and a separate titanium hydride phase.

10. A method of making ammonia according to claim 1 wherein preparation of the catalyst includes the steps of exposing granules of said alloy to hydrogen at a pressure at least equal to the equilibrium dissociation pressure of iron titanium hydride at the prevailing temperature of the granules to hydride the alloy, and outgassing said granules at dehydriding conditions effective to break said granules into smaller particles.

11. A method of making ammonia according to claim 10 wherein exposure of said granules to hydrogen includes a first step of exposing said granules to gaseous hydrogen at an elevated temperature and pressure effective to remove oxides from exposed alloy surfaces.