Frerich J. Keil

Methanol-to-hydrocarbons: process technology

This review presents methanol-to-hydrocarbons processes which have reached industrial applications, either on a commercial or on a pilot plant scale. The determination of kinetic expressions for various methanol conversion reactions is given. The processes discussed are: Mobils methanol-to-gasoline (MTG) plant in New Zealand, the fluidized bed MTG and methanol-to-olefins process; Mobils olefin-to-gasoline/distillate (MOGD) process; the MTO plant developed by UOP and Norsk Hydro; Haldor Topsoes TIGAS process. The developments of a liquid phase dimethyl ether synthesis (LP-DME) process by the Ahron University are also presented.

Efficient methods for finding transition states in chemical reactions : Comparison of improved dimer method and partitioned rational function optimization method

A combination of interpolation methods and local saddle-point search algorithms is probably the most efficient way of finding transition states in chemical reactions. Interpolation methods such as the growing-string method and the nudged-elastic band are able to find an approximation to the minimum-energy pathway and thereby provide a good initial guess for a transition state and imaginary mode connecting both reactant and product states. Since interpolation methods employ usually just a small number of configurations and converge slowly close to the minimum-energy pathway, local methods such as partitioned rational function optimization methods using either exact or approximate Hessians or minimum-mode-following methods such as the dimer or the Lanczos method have to be used to converge to the transition state. A modification to the original dimer method proposed by [Henkelman and Jonnson J. Chem. Phys. 111, 7010 (1999)] is presented, reducing the number of gradient calculations per cycle from six to four gradients or three gradients and one energy, and significantly improves the overall performance of the algorithm on quantum-chemical potential-energy surfaces, where forces are subject to numerical noise. A comparison is made between the dimer methods and the well-established partitioned rational function optimization methods for finding transition states after the use of interpolation methods. Results for 24 different small- to medium-sized chemical reactions covering a wide range of structural types demonstrate that the improved dimer method is an efficient alternative saddle-point search algorithm on medium-sized to large systems and is often even able to find transition states when partitioned rational function optimization methods fail to converge.

Modeling of diffusion in zeolites

Diffusion of adsorbed molecules in zeolites plays an important role in the use of zeolites as adsorbents in separation processes and in shape-selective catalysis. Computational chemistry is in a stage where diffusion phenomena, even for multicomponent diffusion, can be treated at a level of high accuracy. The present review presents most of the results on diffusion in zeolites obtained by classical Molecular Dynamics, dynamic Monte-Carlo approaches, Transition-State Theory, and the Maxwell-Stefan approach. Reactive and non-reactive conditions are considered.

Diffusion and reaction in porous networks

Abstract The present paper reviews three-dimensional random network models of catalyst support structures. Multicomponent diffusion in the individual pores of the network is described by the dusty-gas approach. In contrast to previous publications, the present network model can be applied to any common reaction kinetics. This becomes inevitable in order to make three-dimensional network models applicable to practical problems in the industry. The present model is closely connected to measurable quantities which may be obtained by standard equipment. Examples of pore structure optimizations with respect to various optimization criteria are given. Investigations of depositions within the pores by percolation theoretical methods are described. Results of simulations are compared to measurements in a single-pellet diffusion reactor. The importance of surface diffusion is stressed.

MODELING OF THREE-DIMENSIONAL PRESSURE FIELDS IN SONOCHEMICAL REACTORS WITH AN INHOMOGENEOUS DENSITY DISTRIBUTION OF CAVITATION BUBBLES. COMPARISON OF THEORETICAL AND EXPERIMENTAL RESULTS

During the last 50 years extensive experimental investigation has been carried out on the chemical effects of ultrasound, but limited work has been reported on modeling. This paper presents a new model in which a numerical calculation of the three-dimensional linear sound pressure field distribution in a commonly used sonoreactor containing three transducers is carried out. In this model the inhomogeneous three-dimensional time-dependent wave equation was solved using the finite difference approach. The modeled results are then compared with the experimentally measured values, and the agreement, in general, is found to be good. Further, our modeling studies have an advantage, since they clearly describe the continuous sound pressure field structure, unlike previously reported results in which some information is missing due to limited intermittent measured points.

Methanol to olefins—prediction of the performance of a circulating fluidized-bed reactor on the basis of kinetic experiments in a fixed-bed reactor

Abstract The conversion of methanol to olefins (MTO) over a zeolite catalyst is used as an example for the investigation of a circulating fluidized bed (CFB) reactor/regenerator system. A lumped-species reaction scheme has been developed which accounts for the most relevant components. Kinetic experiments were carried out in a standard fixed-bed Berty reactor over the 400°C to 500°C range at atmospheric pressure. The MTO process was performed in a circulating fluidized bed (riser dimensions: internal diameter 0.03 m, height 11 m) that is coupled with a regenerator for continuously burning off the coke deposits. Conversion of more than 98% and a very high olefin selectivity have been achieved. Based on the estimated kinetic parameters and on a CFB reactor model, methanol conversion and product distribution is predicted and compared with experimental data. The agreement is satisfactory for the whole temperature range examined.

Optimization of three-dimensional catalyst pore structures*

Optimization of the three-dimensional pore structure of a hydrodemetallation catalyst will be described. A random network model with different connectivities has been used. The influence of connectivity, diffusion coefficient, outer dimension of pellet and operating time on optimal pore structure has been investigated. Numerical methods employed will be discussed.

Molecular simulation of alkene adsorption in zeolites

The adsorption isotherms of various alkenes and their mixtures in zeolites such as silicalite-1 (MFI-type), theta-1 (TON-type), and deca-dodecasil 3R (DDR-type) were calculated using the grand canonical Monte Carlo (GCMC) approach. Additionally, the adsorption of alkene–alkane mixtures was simulated. The GCMC approach was combined with the configurational-bias Monte Carlo (CBMC) method. Effective Lennard–Jones parameters for the interaction between the oxygen atoms of all-silica zeolites and the sp2-hybridized groups of linear alkenes were determined using a united atom force field. They were adjusted to the experimental adsorption data of silicalite-1 (MFI). The inflection behaviour of the 1-heptene isotherm was investigated in detail. It is shown that, in the inflection region, the 1-heptene molecules alter their end-to-end length depending on their location. The occurrence of a maximum in the mixture adsorption isotherms is attributed to two effects: entropic effects and non-ideality effects. From the mixture simulations some general conclusions concerning the separation of hydrocarbons with silicalite-1 can be drawn. The transferability of the Lennard–Jones parameters to other zeolites was investigated. Simulations of adsorption isotherms in the zeolites theta-1 and DD3R and their comparison with experimental data indicate the possibility of transferring the parameters to other all-silica zeolites.

Scientific Computing in Chemical Engineering

Scientific Computing in Chemical Engineering gives the state of the art from the point of view of the numerical mathematicians as well as from the engineers. The application of modern methods in numerical mathematics on problems in chemical engineering, especially reactor modeling, process simulation, process optimization and the use of parallel computing is detailed.

REACTORS FOR SONOCHEMICAL ENGINEERING - PRESENT STATUS

Sonochemistry is a rapidly growing field of research and industrial application. Sonochemistry refers to the effects of sound and ultrasound on chemical reactions. Chemical activation and process intensification is provided through ultrasonically-induced cavitation in a liquid medium. The present review describes some fundamentals of cavitation related to Sonochemistry, sonochemical equipment, especially sonoreactors, and the present status of sonochemical reactor design. Some problems that should be investigated in more detail are listed.