Single-event Concept Research Articles

Single event kinetic modeling for paraffin hydrocracking over an industrial Ni–W silica–aluminum catalyst

Computer-aided fundamental kinetic modeling for paraffin hydrocracking based on graphical solution and the single event concept.

A kinetic model and parameters estimate for the synthesis of 2-phenyloctane: a starting material of bio-degradable surfactant

ABSTRACT In this article, we have proposed a kinetic model of a 2-phenyloctane formation from benzene and 1-octene in the presence of Y zeolite CVB760 solid acid catalyst. The typical elementary reaction involved in benzene alkylation consists of isomerisation of π-bond of 1-octene and attachment of octyl ion with benzene. Here, we have followed transition state theory and statistical thermodynamics to express the rate expression for the disappearance of 1-octene and formation of 1-octene and 2-phenyloctane isomers. In transition state theory, we have incorporated a single-event concept to account for a change in the relative position of the atom during the chemical reaction. Subsequently, we also estimate activation energies involved in the kinetic model using the regression method and experimental data. For this, a particle swarm optimisation (PSO) followed by Levenberg–Marquardt algorithm, called hybrid PSO, is adopted to measure the activation energies. The statistical methods involved in this investigation include analysis of variance, F-test and parity diagram and comparison between the proposed kinetic model and experimental data confirm the kinetic model reliability and optimal estimate of activation energies.

Fundamental kinetics model of acidity-activity relation for ethylene oligomerization and aromatization over ZSM-5 zeolites

The kinetics of ethylene oligomerization and aromatization (OA) over ZSM-5 with different Si/Al ratios was modeled by applying single-event concept combined with Brönsted kinetic model to establish a quantitative acidity-activity relationship. The NH3 desorption activation energy was related to the activation energy of the OA reactions and heat of de/protonation based on linear free-energy theory by introducing γ and δ as kinetics parameters for assessing the sensitivity of the OA reactions to the acid strength. A total of 36 parameters was estimated for the subtype elementary steps in each kinetics model. A hybrid genetic optimization algorithm was generated to obtain the kinetics model parameters, where the model can efficiently and accurately fit the changes in the product distribution obtained with the three catalysts for products in the reaction network. The regressed kinetics model can predict the influence of zeolites with the same topology and different acidity on the product distribution.

ChemInform Abstract: Kinetic Models of Methanol and Dimethyl Ether Conversion to Olefins over Zeolite Catalysts (Review)

AbstractReview: 89 refs.

Kinetic models of methanol and dimethyl ether conversion to olefins over zeolite catalysts (Review)

The review summarizes various approaches to the description of the olefins synthesis kinetics from methanol/dimethyl ether for two types of zeolite catalyst, ZSM-5 and SAPO-34. It has been shown that along with phenomenological models, microkinetic models derived with taking account of the detailed reaction mechanism and quantum-chemical calculations based on the single-event concept of the reaction rate and the transition state theory are being widely used now. Both power laws and the Hougen—Watson equations are proposed to describe the rates of elementary steps.

Single-Event MicroKinetics (SEMK) for Methanol to Hydrocarbons (MTH) on H-ZSM-23

A Single-Event MicroKinetic (SEMK) model was constructed for the Methanol to Hydrocarbon (MTH) reaction on ZSM-23 with a Si/Al ratio of 26. The experimental data were acquired at 400°C, atmospheric pressure using space times between 19.2 and 57.7kgcatsmol−1. Starting from a previously established fundamental reaction mechanism for MTH on ZSM-5, an extended reaction network, also involving primary carbenium ion alkylation/cracking reactions, was considered. The reaction network in terms of elementary steps for di-methyl ether (DME) and olefins formation from the aromatic hydrocarbon pool as well as from the alkene homologation cycle was implemented. The single-event concept together with thermodynamic constraints allowed to reduce the number of adjustable parameters to 33, from which only 2 activation energies for (s,p) and (t,p) alkylation/cracking, 6 alkene protonation heats, the total concentration of aromatic hydrocarbon pool and the stability difference between secondary and primary carbenium ion had to be determined by regression. The model explains well the product distribution with physically realistic parameter values. The estimated alkene protonation heats increase from 10.4kJmol−1 for ethene to 64.1kJmol−1 for hexene. A contribution analysis indicated that the alkene-homologation cycle is the dominant reaction pathway for olefins formation on ZSM-23.

Single-Event Microkinetics for Methanol to Olefins on H-ZSM-5

A single-event microkinetic (SEMK) model was developed for the conversion of methanol to olefins (MTO) and used in the assessment of experimental data obtained on H-ZSM-5 with a Si/Al ratio of 200. The experiments were performed at temperatures from 643 to 753 K, space times between 0.5 and 6.5 kgcat·s mol–1 and at atmospheric pressure. Dimethyl ether (DME) and primary olefins formation through aromatic hydrocarbon pool and higher olefins formation via the alkene homologation cycle, was implemented in terms of elementary steps. The single-event concept, in combination with thermodynamic constraints allowed a significant reduction of the number of adjustable parameters. A further reduction was achieved by calculation of the single-event pre-exponential factors based on statistical thermodynamics. Physicochemical constraints along with Boudart’s criteria were used to limit the parameter space. Twenty one activation energies of kinetically significant reaction families and eight protonation enthalpies corres...

Extension of the Single-Event Microkinetic Model to Alkyl Substituted Monoaromatics Hydrogenation on a Pt Catalyst

The Single-Event MicroKinetic (SEMK) methodology, which had been successfully applied to benzene hydrogenation on a Pt catalyst, has now been extended toward substituted monoaromatics, that is, toluene and o-xylene. The single-event concept combined with thermodynamic constraints allowed to significantly reduce the number of adjustable parameters. In addition to the number of unsaturated nearest neighbor carbon atoms, H-atom addition rate and equilibrium coefficients were assumed to depend on the carbon atom type, that is, secondary or tertiary. This leads to three additional reaction families compared to benzene hydrogenation. Gas phase toluene and o-xylene hydrogenation experiments were performed on 0.5 wt % Pt/ZSM-22 in a temperature range from 423 to 498 K, a total pressure range from 1 to 3 MPa, H2 inlet partial pressures between 100 and 600 kPa and aromatic inlet partial pressures between 10 and 60 kPa. A simultaneous regression of the SEMK model to an experimental data set consisting of 39 toluene and 37 o-xylene hydrogenation experiments resulted in activation energies of H additions to tertiary carbon atoms that are 10.5 kJ mol–1 higher than to secondary carbon atoms. This can be related to the steric hindrance experienced during H addition to a carbon atom bearing a substituent. The presence of a substituent on the aromatic ring was found not to affect the chemisorption enthalpies. The reaction path analysis has been carried out via differential contribution analysis and identified that the hydrogenation first occurs at secondary carbon atoms prior to the hydrogenation of the tertiary carbon atoms in the hydrogenation sequence. This is in line with the distribution of hydrocarbon species on the catalyst surface.

Fundamental Kinetic Modeling of Catalytic Reforming

A fundamental kinetic model for the catalytic reforming process has been developed. The complex network of elementary steps and molecular reactions occurring in catalytic reforming was generated through a computer algorithm characterizing the various species by means of vectors and Boolean relation matrices. The algorithm is based on the fundamental chemistry occurring on both acid and metal sites of a Pt−Sn/Al2O3 catalyst. The number of rate coefficients for the transformations occurring on the metal sites was reduced by relating them to the nature of the involved carbon atoms. The single event concept was applied in the development of rate expressions for the elementary steps on the acid sites. This approach allows obtaining rate coefficients that are independent of the feedstock, owing to their fundamental chemical nature. The Levenberg−Marquardt algorithm was used to estimate the rate coefficients. The estimation was based on data reported from a previous naphtha reforming study in a fixed bed reactor with Pt−Sn/Al2O3 as a catalyst. The agreement between the experimental and estimated yields is excellent. The statistical tests were also satisfied. The kinetic model was used in pseudo-homogeneous and heterogeneous reactor models simulating an industrial three-bed adiabatic catalytic reformer with centripetal radial flow.

Single-Event Microkinetic Model for Fischer−Tropsch Synthesis on Iron-Based Catalysts

A single-event microkinetic (SEMK) model was developed for Fischer−Tropsch synthesis and applied to experimental data obtained on an iron-based catalyst in a temperature range from 523 to 623 K, total pressures from 0.6 to 2.1 MPa, and H2/CO inlet ratios from 2 to 6 mol/mol. The use of the single-event concept allowed a significant reduction of the number of adjustable parameters. The single-event pre-exponential factors were calculated based on statistical thermodynamics. The reaction enthalpies, as well as initial guesses for the activation energies, were obtained through the unity bond index−quadratic exponential potential (UBI-QEP) method. Ten activation energies of the kinetically relevant reaction families and four atomic chemisorption enthalpies remained as adjustable parameters, the latter corresponding to so-called catalyst descriptors. The SEMK model describes well the product distribution over a wide range of operating conditions with physically sound kinetic parameters. The reductive eliminati...

Mechanistic Kinetic Modeling of the Hydrocracking of Complex Feedstocks, such as Vacuum Gas Oils

A mechanistic kinetic model has been developed for the hydrocracking of complex feedstocks such as vacuum gas oil (VGO), based on an exhaustive computer-generated reaction network of elementary steps. The model utilizes a detailed composition of VGO, characterized by 16 different molecular classes up to C40. These classes are divided into 45 subclasses by distinguishing the isomers of a class according to the number of methyl branches, leading to 1266 groups of isomers/pure components. The frequency factors of the rate coefficients for the acid site steps are modeled using the single-event concept, and the activation energies are modeled based on the nature of the reactant and product carbenium ions. The saturation of polyaromatics is modeled according to the sequential hydrogenation of aromatic rings on the metal sites. The competitive chemisorption of aromatics and naphthenes is taken into account, based on the number of aromatic/naphthenic rings on the metal sites, and based on the corresponding gas-ph...

A Generalized Mechanistic Kinetic Model for the Hydroisomerization and Hydrocracking of Long-Chain Paraffins

A mechanistic kinetic model has been developed for the three-phase hydroisomerization and hydrocracking of long-chain paraffins in which rate-determining steps are assumed to occur on both acid and metal sites of the bifunctional catalyst. The frequency factors for the acid site elementary steps are modeled using the single-event concept and the corresponding activation energies using the Evans−Polanyi relationship. The rate coefficients for the dehydrogenation reactions on the metal sites of the catalyst are classified into five different classes depending on the nature (i.e., primary, secondary, or tertiary) of the carbon atoms forming the double bond. The model contains 14 independent parameters for the hydroisomerization and hydrocracking of heavy paraffins. These parameters are estimated from experimental data on the three-phase isothermal hydrocracking of n-hexadecane in a tubular reactor. A number of reactor simulations have been performed for various operating conditions and hydrocarbons of different chain lengths. The effect of the relative metal to acid activity of the catalyst on the isomerization and cracking selectivities and on the carbon number distribution of the products is discussed in detail.

Alkylation on Solid Acids. Part 2. Single-Event Kinetic Modeling

The reaction network of isobutane alkylation with butenes over proton-exchanged Y-zeolites was generated at the elementary-step level of carbenium-ion chemistry by means of the Boolean relation matrix approach. A significant reduction in the number of model parameters from 3130 to 14 was accomplished by expressing elementary-step rate coefficients in terms of the single-event concept together with the application of the Evans−Polanyi relationship, the stabilization energy concept, and thermodynamic constraints. The deactivation was expressed in terms of site coverage by higher oligomers. For parameter estimation, the complete kinetic model was embedded into a model for the fixed-bed reactor used in the experimental investigation. Nonlinear regression led to parameter values in line with the rules of carbenium-ion chemistry. The simulation of the transient behavior of the fixed-bed reactor illustrates how deactivation occurs in a narrow zone moving through the catalyst bed leading to a typical breakthrough in the product composition with process-time.

Single Event Kinetic Modeling of Complex Catalytic Processes

AbstractFor Abstract see ChemInform Abstract in Full Text.

Single Event Kinetic Modeling of Complex Catalytic Processes

Kinetic models for complex processes may contain an excessive number of rate parameters. Rather than simplifying the reaction scheme the approach illustrated here develops it in terms of the elementary steps of cation chemistry, using a computer algorithm. The resulting scheme, although gigantic, thus consists of a limited number of types of steps, generally involving series of homologous species. The rate coefficients of these steps are modeled based upon transition state theory and statistical thermodynamics. The single event concept explicits the effect of structure on the entropy contribution to the rate coefficient of the transformation, while the enthalpy effect is calculated using the Evans‐Polanyi relation. Together with thermodynamic constraints this approach drastically reduces the number of independent parameters to be determined from the experimental data. The approach is applied to the methanol‐to‐olefins process on ZSM5 and to a process with complex feedstock like the catalytic cracking of vacuum gas oil. In the latter case an additional problem is the requirement of an adequate feedstock definition. Other processes catalyzed by acid catalysts, eventually loaded with metals, like hydrocracking, catalytic reforming, alkylation and, more generall, processes involving series of homologous components can be dealt with along the same lines.

Modelling of n-hexadecane hydroisomerization and hydrocracking reactions on a Mo/Hβ-alumina Bi-functional catalyst, using the single event concept

Hydrocracking of a long chain paraffin, hexadecane, was carried out on a Mo/Hβ-alumina bi-functional catalyst with a weak dehydrogenating function. It was found through an isomerization selectivity analysis that the behavior of the system is away from ideal hydrocracking conditions. To implement for this system the single event concept, until now applied only to systems for which the ideal hydrocracking assumption was valid, a hybrid model that includes the more general case in which reactions on the acid sites are not rate determining was developed. This model was called the single-event-lumped-parameter hybrid (SELPH) model, for it considers fundamental rate constants for the reactions that take place on the acid sites using the single event concept, and lumped rate constants for the reactions on the metal sites. The kinetic coefficients at 533 K were estimated and the agreement between the calculated and experimental data was satisfactory.

Single Event Kinetic Modeling of the Methanol-to-Olefins Process on SAPO-34

The MTO process on SAPO-34 was modeled in a fundamental way using the single event concept combined with the Evans−Polanyi relationship. This approach together with a number of thermodynamic constraints leads to a drastic reduction of the number of independent parameters. These were determined from the experimental data using a combination of a hybrid genetic algorithm, sequential quadratic programming, and the Levenberg−Marquardt optimizer. The effect of the structure of reactant and product on the rate coefficients of the various types of elementary steps was illustrated. The effect of operating variables such as temperature and the methanol partial pressure was investigated using the kinetic model. The deactivation of the catalyst was modeled in terms of the amount of C6+ olefins formed and trapped inside the SAPO-34 cavities. The effect of deactivation on the evolution with process time of the conversion and product yields of the MTO process was illustrated by the simulation of an isothermal fixed bed...

Analysis of Fundamental Reaction Rates in the Methanol-to-Olefins Process on ZSM-5 as a Basis for Reactor Design and Operation

The paper reports on work aiming at collecting more insight into the methanol-to-olefins process and its product distribution and, more generally, into the behavior of carbenium ion elementary steps of methylation, olgomerization, and β scission on zeolite catalysts. The single-event concept for the rate coefficient, derived from transition-state theory and statistical thermodynamics, and the Evans−Polanyi relationship for the activation energy drastically reduced the number of parameters to be derived from the experimental data from 726 to 33. The number of single events and the heats of formation required for this approach were calculated from the reacting structures and ab initio molecular orbital calculations. The kinetic parameters were inserted into a reactor model to generate sets of rate and yield profiles in an isothermal fixed-bed reactor in the range of 380−480 °C. Finally, for commercial applications, a multibed adiabatic reactor concept was explored.

Kinetic modeling of acid-catalyzed oil refining processes

In this paper the kinetic modeling of acid-catalyzed oil-refining processes is based upon the elementary steps of carbenium-ion chemistry. This completely avoids the lumping of species and reactions practised today. A number of reasonable assumptions, the introduction of the single-event concept, the Polanyi relation and thermodynamic constraints keep the number of kinetic parameters within tractable limits. The approach is applied to the simulation of a riser reactor for the catalytic cracking of gasoil.