The Response of Nitriding Chemistry to Different Initial Gas Compositions (original) (raw)
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Parameter Optimization of Nitriding Process Using Chemical Kinetics
Metallurgical and Materials Transactions A, 2016
Using the dynamics of chemical kinetics, an investigation to search for an optimum condition for a gas nitriding process is performed over the solution space spanned by the initial temperature and gas composition of the furnace. For a two-component furnace atmosphere, the results are presented in temporal variations of gas concentrations and the nitrogen coverage on the surface. It seems that the exploitation of the nitriding kinetics can provide important feedback for setting the model-based control algorithms. The present work shows that when the nitrogen gas concentration is not allowed to exceed 6 pct, the N ad coverage can attain maximum values as high as 0.97. The time evolution of the N ad coverage also reveals that, as long as the temperature is above the value where nitrogen poisoning of the surface due to the low-temperature adsorption of excess nitrogen occurs, the initial ammonia content in the furnace atmosphere is much more important in the nitriding process than is the initial temperature.
Nitrogen Availability Of Nitriding Atmosphere In Controlled Gas Nitriding Processes
Archives of Metallurgy and Materials, 2015
Parameters which characterize the nitriding atmosphere in the gas nitriding process of steel are: the nitriding potential KN, ammonia dissociation rate α and nitrogen availabilitymN2. The article discusses the possibilities of utilization of the nitriding atmosphere’s nitrogen availability in the design of gas nitriding processes of alloyed steels in atmospheres derived from raw ammonia, raw ammonia diluted with pre-dissociated ammonia, with nitrogen, as well as with both nitrogen and pre-dissociated ammonia. The nitriding processes were accomplished in four series. The parameters selected in the particular processes were: process temperature (T), time (t), value of nitriding potential (KN), corresponding to known dissociation rate of the ammonia which dissociates during the nitriding process (α). Variable parameters were: nitrogen availability (mN2), composition of the ingoing atmosphere and flow rate of the ingoing atmosphere (FIn).
The Effects of Modifying the Activity of Nitriding Media by Diluting Ammonia with Nitrogen
Materials, 2021
This paper discusses the issue of the effects of modifying the activity of nitriding media by diluting ammonia with nitrogen and the concomitant variation in the degree of ammonia dissociation on the layer’s growth kinetics and their phase composition. To understand and quantify the effects of the variation in the main parameters that influence the layer growth kinetics, the experimental programming method was used and mathematical models of interactions between influence and kinetics parameters were obtained for two metallic materials: Fe-ARMCO and 34CrAlMo5 nitralloy steel. It was concluded that the nitriding operating temperature and the degree of nitrogen dilution of the ammonia have statistically significant influences on the kinetics of the nitrided layer. In the same context, it was analytically proved and experimentally confirmed that the ammonia degree dissociation from the gaseous ammonia-nitrogen mixture, along with the dilution degree of the medium with nitrogen, signifi...
2021
In order to study the relationship and effect of nitrogen gas in the reducing gases used in the reducibility tests of iron oxides, under isothermal conditions, a test scheme was executed using ammonia gas, such that its decomposition of the gas in the reactor produced a mixture of H2 and N2 gases. Furthermore, the addition of 6% NH3 in a 28% H2 and 68% N2 gas stream was planned to obtain a gas composition of 70% N2 and 30% H2. This would allow comparing the reducibility curves between both conditions, assuming that the possible difference between both conditions to compare the volume changes of the reduced samples. The difference to be studied will be based on the estimation and comparison of the rate of formation of metallic iron in the stages of reduction of Hematite / Magnetite / Wustite (FeO), as well as the effects of nitrogen absorbed by the fresh metallic iron produced, or present. in iron catalysts to produce ammonia, from the reducing gas mixture, on the volume change of th...
Nitriding in ammonia-nitrogen gaseous mixtures, after the simultaneous saturation with Ti and Al
International Journal of Surface Science and Engineering, 2022
The paper addresses aspects of nitriding in ammonia-nitrogen gaseous mixtures, after simultaneous saturation with Ti and Al using solid powdery media. The effects quantification of the nitriding conditions variation, i.e., temperature, the ammonia dissociation degree and dilution degree with nitrogen, on the layer's growth kinetics, simultaneous saturated with Ti and Al and subsequently nitrided, in the case of two metallic materials, Fe-ARMCO and 34CrAlMo5 steel, could be ensured using the second-order non-compositional experimental programming method. It was concluded that, in the case of ammonia dilution with nitrogen, the variation of the ammonia dissociation degree in the range 20÷70%, in the context of simultaneous dilution of ammonia with nitrogen, statistically significantly influences the layers growth kinetics with increasing nitriding temperature. On the other hand, for a dilution degree of ammonia with about 30% nitrogen and an ammonia dissociation degree of about 45%, the most intense layers growth kinetics was highlighted if the nitrided layer has been previously saturated simultaneously with Ti and Al.
International Journal of Materials and Chemistry
Nitriding experiments for powder specimens of Mo and Fe were carried out using a solar furnace SF40 at PSA (Plataforma Solar de Almería) in Tabernas (Spain) in uncracked ammonia NH 3 gas (NH 3 gas with suppressed extent of dissociation by flowing) aiming at determining the range of linear velocity v of NH 3 gas flow to yield higher nitride phases, δ-MoN for Mo and ε-Fe 2 N for Fe. Standard solar exposure duration at a specified reaction temperature T was set to be 60 min over range of v between 1.14 mm•s-1 and 11.4 mm•s-1. By X-ray diffraction (XRD) analysis, presence of δ-MoN was detected besides γ-Mo 2 N and metallic Mo for Mo powder specimen heated to 900 ºC in NH 3 gas flow at v = 1.14 mm•s-1 but XRD peaks identifiable as δ-MoN became indiscernible when v was increased to 11.4 mm•s-1. On the other hand, for Fe powder specimen exposed to NH 3 gas flow at v = 1.14 mm•s-1 at T = 500 ºC, remnant metallic α-Fe was detectable by XRD at the downstream side of the specimen holder but no metallic α-Fe was detected at the upstream side of the specimen holder suggesting that chemical activity a(N) of N atom in uncracked NH 3 gas tended to decrease along the NH 3 gas flow path on going from the upstream side to the downstream side.
Ammonia Pyrolysis and Oxidation in the Claus Furnace
Industrial & Engineering Chemistry Research, 2001
The modified Claus process is commonly used in oil refining and gas processing to recover sulfur and destroy contaminants formed in upstream processing. In oil refining, in addition to the typical modified Claus plant feed of H 2 S and CO 2 , NH 3 is also often present. NH 3 is a process contaminant and must be destroyed in the front-end furnace of the modified Claus plant, otherwise, it poses a risk of poisoning the catalyst beds and plugging off downstream equipment because of the formation of ammonium salts. In this paper, the pyrolysis and oxidation of NH 3 was studied under Claus furnace temperatures and residence times. Experimental data was taken, and new reaction rate expressions were developed for NH 3 pyrolysis and oxidation. The derived rate expressions are outlined as follows: the NH 3 pyrolysis rate expression r ) A exp(-E a /RT)P NH 3 1.25 , where A is 0.004 21 mol s -1 atm -1.25 cm -3 and E a is 16.5 kcal mol -1 , and the NH 3 oxidation rate expression r ) A exp(-E a /RT)P NH 3 P O 2 0.75 , where A is 4430 mol s -1 atm -1.75 cm -3 and E a is 40.0 kcal mol -1 . The rate expression for NH 3 pyrolysis matched experimental data within 13% and matched well with published data. The rate expression for NH 3 oxidation matched experimental data within 10%. 10.
Surface and bulk changes in iron nitride catalysts in H2/CO mixtures
Journal of Catalysis, 1988
Preparation of the c-, E-, and y'-iron nitride phases was confirmed by Miissbauer spectroscopy, X-ray diffraction, and quantitative mass spectrometry of NH1 evolved during decomposition. Computer fitting of the s-nitride Miissbauer spectra with a distribution of hyperfine fields shows the conversion of iron with two nitrogen nearest neighbors (Fe 2nn) to Fe 3nn as the nitrogen content increases. The dynamic response of the nitrides to HJCO mixtures at reaction temperatures was followed by constant-velocity Miissbauer spectroscopy and transient mass spectrometry. The rapid decomposition of the iron nitrides in H, at 523 K occurs with surface reaction as the rate-limiting step, initially. At lower temperatures or after significant nitride decomposition, the data are best fit with a shrinking core model. For reaction at 473 K, the MBssbauer effect identified an a-Fe shell, a {-Fe2N core, and an .s-Fe,N transition region. Surprisingly, loss of the pure nitride phase is barely retarded for HjICO mixtures compared to Hz alone at 523 K. Mass spectrometric studies show that the freshly prepared nitride has a substantial hydrogen inventory, equivalent to a monolayer of NH, for <-FeZN. On exposure to synthesis gas, the nitride catalysts produce no methane until one to two monolayers of N have been removed, but carbon is deposited on the catalyst by the Boudouard reaction. Mass spectral measurements show no evidence for active nitrogen on the surface after the synthesis reaction has been established. Both M(issbauer spectroscopy and mass spectral measurements confirm, however, that following the initial loss of nitrogen, bulk carbonitrides form which lose their nitrogen very slowly as the reaction proceeds. These data suggest that differences in the performance of iron and iron nitride catalysts may be strongly influenced by the way surface carbon is deposited during reactor startup.
Gaseous Nitriding: In Theory and In Real Life
Expert systems for gaseous nitriding, be it simulators or controllers, are largely based on the Lehrer Diagram, which shows the correlation between nitrogen-iron phases, temperature, and the partial pressure ratio of ammonia and hydrogen. While this theory is widely held, in real-world applications, nitriding cycles do not always work in this way, and thus materials, parts and results do not match Lehrer Diagram's calculations. In this presentation we will identify additional parameters needed to complement the Lehrer Diagram by using a variety of processes and samples.
Ammonia synthesis kinetics: Surface chemistry, rate expressions, and kinetic analysis
Topics in Catalysis, 1994
The results of surface science studies of nitrogen adsorption and desorption on iron single crystals are summarized with respect to ammonia synthesis reaction kinetics. Analytical rate expressions for ammonia synthesis on high surface iron catalysts at industrial reaction conditions are presented and compared to the behavior of microkinetic models based on the surface science results. Microkinetic models based on uniform surfaces are successful in extrapolating "results from surface science studies at low surface coverages to describe the performance of iron catalysts at industrial ammonia synthesis reaction conditions. We suggest that the value of rate constant for recombination of nitrogen atoms on the surface is a weak function of surface coverage. Furthermore, modest variations in the heat of N2 adsorption are sufficient to achieve fractional ammonia orders that are relatively insensitive to the ammonia pressure.