Matteo Maestri

Synergy of Homogeneous and Heterogeneous Chemistry Probed by In Situ Spatially Resolved Measurements of Temperature and Composition

The interaction between heterogeneous and homogeneous chemistries is a crucial issue for high-temperature catalytic processes. In particular, the assessment of the main routes that control the selectivity to the desired products is essential for the design and safe operation of reaction units. This assessment is particularly true for the ultrafast conversion of hydrocarbons in short-contact-time (SCT) reactors that play a pivotal role in the effort to cope with the worldwide growing demand for more efficient exploitation of energy andmaterial resources. Examples are the catalytically assisted combustion for gas turbines with ultralow emissions, the catalytic partial oxidation (CPO) of hydrocarbons to H2 or CO/H2 mixtures (i.e., syngas), and the oxidative dehydrogenation (ODH) of light alkanes to olefins. As a common feature, these processes operate in autothermal and compact reactors, with noble-metal catalysts (palladium, rhodium, and platinum). An enormous energy intensity is peculiar to the SCT autothermal conversion of hydrocarbons over noble metals. Strongly exothermic and endothermic reactions proceed on the catalyst surface at extremely high rates. As a consequence, sharp gradients of temperature (up to 200 8Cmm ) and concentration are established within the small reactor volumes. Temperatures ranging from 250 to 1100 8C are generally experienced within a few millimeters. Such a level of severity—in terms of power density and extent of temperature and concentration gradients—is typical of flames and gas-phase oxidation processes in general. For a catalytic process, however, these conditions represent a thoroughly unconventional kinetic regime. To best grasp the intensity and the speed of the involved phenomena, we need to think of SCT conversion of light alkanes as the catalytic equivalent of a flame. This analogy can clearly depict the complexity of the process and emphasize the related scientific issues: To what extent does a catalytic process “stick” to the catalyst surface at these very high temperatures, where the adsorption of species is thermodynamically unfavored? Can the gas-phase activation of C H bonds (e.g., the formation and propagation of radicals) cooperate or compete with the catalytic process? In this respect, it is largely accepted that in the case of CH4 CPO over rhodium the gas-phase paths are negligible at atmospheric pressure and the catalytic route dominates. Conversely, the SCT-ODH of short alkanes over platinum proceeds mainly in the gas phase, thus giving rise to the production of olefins and other hydrocarbon species. On the basis of these examples, we could conclude that either catalytic or gas-phase chemistry governs the SCT conversion of hydrocarbons, depending only on the stability of the C H bond in the gas phase. Herein, by using novel techniques for collecting spatially resolved temperature and concentration profiles, we show that the partial oxidation of short-chain alkanes over rhodium breaks the paradigmatic compartments of heterogeneous processes and gas-phase processes, revealing the real complexity of these “flame-like” processes. Specifically, we examine the reaction of C3H8 CPO (C3H8+ 3/2O2!3CO+ 4H2) as a case study, and we apply novel techniques for collecting spatially resolved gas-phase and solid temperature and concentration profiles within a rhodium-coated honeycomb monolith to monitor the evolution of a propane/air mixture fed at high flow rate. The temperature and the composition of the reacting system were monitored from the inlet reactor section where the mixture was fed to the outlet section of the catalytic unit where the syngas stream was delivered. The results are reported in Figure 1 and Figure 2 as spatially resolved profiles of temperature and molar fraction of reactants and products. In the first 5 mm of the honeycomb, a sharp drop of O2 and C3H8 concentration was observed, accompanied by the formation of total oxidation products (CO2 and H2O) and partial oxidation products (H2 and CO). Correspondingly, a hot spot formed on the catalyst surface (980 8C, measured by the pyrometer) and a steep rise was observed in the gas temperature (up to 945 8C, measured by the thermocouple). In line with the occurrence of the endothermic steam reforming reaction, the evolution of H2O showed a maximum. Moreover, the temperature of the solid surface and in the gas phase decreased toward the exit of the honeycomb. Qualitatively, this behavior is what our and other research groups have observed also in the case of a CH4CPO experiment on rhodium, and can be fully explained by the catalytic production of syngas on rhodium. Thus, integral measurements (i.e., measurements of temperature and composition collected exclusively at the reactor outlet) would have suggested the unique existence of a heterogeneous process. A purely catalytic process was, for instance, [*] Dr. A. Donazzi, D. Livio, Dr. M. Maestri, Prof. A. Beretta, Prof. G. Groppi, Prof. E. Tronconi, Prof. P. Forzatti Laboratory of Catalysis and Catalytic Processes Dipartimento di Energia, Politecnico di Milano Piazza Leonardo da Vinci 32, 20133 Milano (Italy) Fax: (+39)0223993284 E-mail: alessandra.beretta@polimi.it Homepage: http://www.lccp.polimi.it

Catalytic partial oxidation of CH4 and C3H8: experimental and modeling study of the dynamic and steady state behavior of a pilot-scale reformer

Abstract An investigation which combines kinetic tests, adiabatic reactor tests and mathematical modeling has been performed to gain insight on Catalytic Partial Oxidation (CPO) of hydrocarbons. A pilot-scale honeycomb reactor, equipped with sliding thermocouples, was assembled for testing the CPO of methane over a 2 (wt/wt) Rh/α-Al2O3 catalyst at atmospheric pressure. Both the cold start-up and the steady state performances of the reactor are presented and discussed. A one-dimensional (1D) mathematical model of the reactor was applied to the quantitative analysis of the observed performances. The model analysis was extended to the simulation of CPO of propane.

A hierarchical approach to chemical reactor engineering: an application to micro packed bed reactors

Hierarchical modeling is applied for the investigation of micro packed bed reactors. This method allows for the use of Computational Fluid Dynamics (CFD) simulations in the analysis of representative complex geometries, where a full-scale CFD simulation of the entire reactor is not possible. Detailed and computationally demanding analyses are used to study a selected number of conditions and phenomena. Then, lumped parameters are derived from CFD results by means of engineering correlations. These parameters are incorporated in simplified reactor models based on macroscopic conservation equations. We provide evidence for the potential of the approach by using as a show-case a micro packed bed reactor in the context of highly exothermic selective oxidation processes. This reactor configuration consists of catalytic particles packed in the channels of a honeycomb matrix, which is expected to strongly enhance the radial heat transfer. In particular, we first focus on the analysis of energy transfer mechanisms by CFD and their interpretation via a 1D model and we provide an assessment of existing correlations with respect to the unconventional configuration (2 mm channel equivalent diameter and 0.8 mm sphere diameter). These correlations are then implemented in a pseudo-continuous (i.e. macroscopic) 2D model to allow for a systematic investigation of the capabilities of the micro packed bed reactor in dealing with the selective oxidation of o-xylene to phthalic anhydride. We found that due to the enhanced radial heat transfer micro packed bed reactors allow for quasi-isothermal operations, thus extending the range of operating conditions possible without occurring in adverse thermal behavior of the reactor. On a more general basis, we prove that the hierarchical approach to chemical reactor engineering is an effective tool to bring the application of fundamental modeling at a level of complexity relevant to full-scale applications, otherwise not possible because of the impractical computational costs.

Cell agglomeration algorithm for coupling microkinetic modeling and steady-state CFD simulations of catalytic reactors

Abstract We propose the application of a Cell Agglomeration (CA) algorithm for coupling detailed microkinetic models with multi-region steady-state Computational Fluid Dynamics (CFD) simulations of catalytic reactors. This numerical methodology – originally developed for dynamic CFD simulation with detailed gas-phase kinetics – is herein applied in the context of steady-state microkinetic CFD simulations of catalytic reactors by exploiting the particular structure of the governing equations of the adsorbed species, which are characterized by the absence of the transport term. The potentialities of the method are assessed by the analysis of different reactor geometries and microkinetic mechanisms in a wide range of operating conditions. Our tests show this method to allow for a reduction of the computational time up to an order of magnitude. Thus, the CA algorithm turns out to be a useful tool to enable the detailed and fundamental simulation of catalytic devices in steady-state conditions.

Design and testing of an operando-Raman annular reactor for kinetic studies in heterogeneous catalysis

In this work, we present a novel experimental tool that integrates in situ Raman spectroscopy and an annular reactor for the operando-Raman kinetic analysis of heterogeneous catalytic reactions. The proposed configuration can monitor via Raman spectroscopy the catalytic surface under kinetically limited reaction conditions, with reliable product analysis, thus retaining the main features of both Raman spectroscopy and kinetic investigation in an annular reactor. We report a thorough description of the key constraints in developing online Raman spectroscopic tools for kinetic investigations. These constraints are considered in the design, assembly and testing of the experimental method by minimizing the mutual invasiveness of the Raman spectroscopy and of the annular reactor configurations. Show-cases of dry reforming and partial oxidation of CH4 on Rh catalysts are used to establish proof of concept of the method, demonstrating the acquisition of time-resolved Raman spectroscopic data under kinetically relevant conditions. Experiments both on clean and coked Rh surfaces reveal that well-structured graphitic deposits are likely to form during DR. During CPO, instead, the presence of O2 and H2O limits the formation of organized graphitic-like carbonaceous species. On a more general basis, this reactor allows a detailed structural characterization of a catalyst material during the reaction and at conditions of temperature, pressure and composition relevant to catalysis. Therefore, it is an important breakthrough for the simultaneous collection of spectroscopic and kinetically relevant data for the investigation of the structure–activity relationship in heterogeneous catalysis.

Catalysis engineering: From the catalytic material to the catalytic reactor

This chapter deals with the application of chemical reaction engineering and computational fluid dynamics (CFD) for the analysis and assessment of the interactions between mass and heat transport and chemical reactions. In the first part of the Chapter, we review fundamental concepts of chemical reaction engineering, by showing the potential impact of transport phenomena at the macroscale on the observed functionality of the catalytic material. This includes both the effect of the distribution of the residence times in the reactor and the impact of internal and external transport phenomena. In the second part, we illustrate modern approaches to catalytic reaction engineering based on CFD simulations. In particular, we present the algorithms to couple microkinetic models and kinetic Monte Carlo (kMC) simulations with CFD. The potentialities of the method are assessed by means of a showcase of the CFD-based analysis of a spectroscopic cell for operando experiments. This example clearly shows that transport artifacts in standard equipment may lead to an erroneous interpretation of the experiments if not properly accounted for.