The Basics of Reaction Kinetics for Chemical Reaction Engineering

Applied Catalysis, 1985

The elementary kinetics involved in the published low coverage ammonia synthesis potential energy diagram [G. Ertl, "Catalysis, Science and Technology", 4, 273 (1983)] have been extrapolated, by calculation, to industrially relevant temperatures (% 723 K) and pressures (> 100 atmospheres) where it has been found that the rates of ammonia synthesis predicted by it are 105 times too low. The problem is that the reported nitrogen atom well depth is too great (even in the presence of potassium promoter) so that the prediction of the kinetics is to a totally nitrided iron surface.

Chemical Engineering Science, 1989

The kinetics of ammonia synthesis under non-steady conditions on promoted industrial iron catalysts are not weil understood. Although classic models, such as the one proposed by Temkin and Pyzhev [Acra Physicochem. 12, 327-356 (1940)], fit steady-state data and are widely used in commercial reactor design, they fail to describe cycling kinetic behavior. A phenomenological adsorption/desorption (A/D) model is developed, the parameters of which are fit to experimental data from a differential reactor. This model predicts observed experimental integral reactor forced-cycling and steady-state behavior for several temperatures and pressures. Moreover, the model obeys thermodynamic equilibrium constraints. The existence of nitrogen storage in the bulk phase of iron catalysts is confirmed, but we show that it does not directly affect either steady-state or time-dependent ammonia production. Also, several potential pitfalls in kinetic modeling are exposed.

Catalysis Today, 1992

ABSTRACT

Catalysis Letters, 2009

Activity of cobalt and iron catalysts in ammonia synthesis was determined under a pressure of 10 MPa and at the temperature range of 673-823 K, in a sixchannel integral steel reactor. The catalytic ammonia decomposition was studied in a differential reactor under the atmosphere of low concentration of ammonia (\6%) in the temperature range of 673-823 K under atmospheric pressure. The determined values of the activation energy for the ammonia synthesis reaction over cobalt and iron catalysts are 268 and 180 kJ/mol, respectively, whilst for the ammonia decomposition reaction they are equal to 111 and 138 kJ/mol. The cobalt catalyst showed lower activity than a commercial iron catalyst in ammonia synthesis reaction. The cobalt catalyst turned out to be more effective in ammonia decomposition reaction than the iron one.

The increasing interest in ammonia decomposition is due to the fact that this compound can be used advantageously as a hydrogen carrier, allowing the development of single-step hydrogen generation systems. With the aim of developing efficient reactors for ammonia decomposition, e.g. for fuel cell applications, it is imperative to investigate the kinetics and reaction mechanism in depth. The main goal of this work is to develop reliable kinetic models that are able to predict the performance obtained using integral reactors, e.g. monoliths. In this case, an almost complete NH 3 conversion is obtained, with a high H 2 concentration at the exit of the reactor. The operating conditions, mainly the gas composition, are very different along the reactor. In addition, the temperatures needed to attain such large conversions are usually high. The kinetic models developed in this contribution are based on the Langmuir isotherm, considering that all the adsorbed species can be kinetically relevant, that the slow step or steps can be partially reversible, and that the surface can be considered as energetically uniform, i.e. ideal. Among other conclusions, the results obtained indicate that the variable kinetic orders and apparent activation energies frequently reported in the literature can be direct consequences of the data analysis and can therefore also be explained without considering any change in the controlling step with the reaction temperature or in the hydrogen or ammonia concentration.

The Canadian Journal of Chemical Engineering, 1992

For the purpose of developing a simple and quantitative description of the rate of reactions occurring in the Thermal DeNOx process, the kinetics of the homogeneous reaction between NO and NH, with excess of O2 has been studied in an isothermal plug flow reactor. Temperature windows of NO reduction were observed with the optimum temperature decreasing with increasing residence time. The degree of reduction of NO increases with residence time at lower temperatures but is sensitive to residence time only at very short residence times at higher temperatures. The degree of conversion of NHI increases with both temperature and residence time. A simple model has been proposed to describe the reaction rates: The two rate constants in the model were estimated based on the experimental data obtained in the present plug flow reactor at about 4% (vol.) oxygen: k,,, = (2.21 5 0.33) x 10'' exp (-(38160 +_ 1 7 0 ) / T ] , I/s k,. = (2.45 F 0.49) x exp { -(29400 k 2 5 0 ) i T ) . mT/mol . s It is verified that the kinetic model can give a satisfactory prediction of the experimental results under different conditions. Afin de mettre au point un modele de description simple et quantitatif de la vitesse des reactions survenant dans le processus thermique DeNOx, on a ktudit la rkaction homogkne entre NO et NH, avec exces de 0: dans un rkacteur a Ccoulement piston isotherme. Les gammes de temperatures pour la rkduction du NO ont it6 observees, et on a constate que la temperature optiniale diminuait avec I'augmentation du temps de sejour. Le degre de reduction du NO augmente avec le temps de sejour aux tem$ratures infkrieures mais cette sensibilitk s'observe seulernent h des temps de sejour trks courts et des temphtures klevees. Le degre de conversion du NH, augmente a la fois avec la tenipkrature et le temps de se.jour. On propose un modkle simple pour decrire les vitesses de reaction: rNO = k,,, "H,I k,. "H,I "01 r N H , = -k,,, "Hjlk,-"H3I "01 Les deux constantes de vitesse du modkle ont it6 estimees B partir des donntes experimentales obtenues avec le rkacteur 170)/Tj, 11s icoulement piston utilisi ici B environ 4% (volume) d'oxygkne: k,>, = (2.21 k, = (2,45 _+ 0.49) X 10" exp On a pu dklnontrer que le modkle cinetique peut predire de manikre satisfaisante les resultats expkrimentaux dan:, 0,33) x 10'' exp { -(38160 -(29400 k 250)/T], m'/mol . s di ffkrentes conditions.

Ammonia synthesis optimization is a topic of high interest in industry as the market continues to expand and demand increases. This proposed process is designed to produce 1,016 metric tons/day of ammonia at a feed of 5,500 kmol/hr while maintaining the best compromise between production and purity. Simulated in ASPEN with an adiabatic Gibbs reactor, optimal production is achieved at 100 bar reactor pressure and a 7.25% purge stream, resulting in 98.96% product stream purity. The simulated process is comparable to conventional ammonia synthesis plants. Further economic optimization is focused on compression costs and reactor efficiency. A new ruthenium-based catalyst with higher activity at lower total pressures can be employed enabling the process to run at significantly lower pressures while maintaining high ammonia conversion. Installing this catalyst into a multi-bed radial plug-flow reactor results in an attractive combination of high production and reduced costs that can be custom made for expansion, retrofit, or grassroots projects.

Chemical Reaction Engineering: Essentials, Exercises and Examples presents the essentials of kinetics, reactor design and chemical reaction engineering for undergraduate students. Concise and didactic in its approach, it features over 70 resolved examples and many exercises. The work is organized in two parts: in the first part kinetics is presented focusing on the reaction rates, the influence of different variables and the determination of specific rate parameters for different reactions both homogeneous and heterogeneous. This section is complemented with the classical kinetic theory and in particular with many examples and exercises. The second part introduces students to the distinction between ideal and non-ideal reactors and presents the basic equations of batch and continuous ideal reactors, as well as specific isothermal and non-isothermal systems. The main emphasis however is on both qualitative and quantitative interpretation by comparing and combining reactors with and without diffusion and mass transfer effects, complemented with several examples and exercises. Finally, non-ideal and multiphase systems are presented, as well as specific topics of biomass thermal processes and heterogeneous reactor analyses. The work closes with a unique section on the application of theory in laboratory practice with kinetic and reactor experiments. This textbook will be of great value to undergraduate and graduate students in chemical engineering as well as to graduate students in and researchers of kinetics and catalysis.

Catalysts, 2019

In the development of catalytic materials, a set of standard conditions is needed where the kinetic performance of many samples can be compared. This can be challenging when a sample set covers a broad range of activity. Precise kinetic characterization requires uniformity in the gas and catalyst bed composition. This limits the range of convecting devices to low conversion (generally <20%). While steady-state kinetics offer a snapshot of conversion, yield and apparent rates of the slow reaction steps, transient techniques offer much greater detail of rate processes and hence more information as to why certain catalyst compositions offer better performance. In this work, transient experiments in two transport regimes are compared: an advecting differential plug flow reactor (PFR) and a pure-diffusion temporal analysis of products (TAP) reactor. The decomposition of ammonia was used as a model reaction to test three simple materials: polycrystalline iron, cobalt and a bimetallic p...