Measurement and enhancement of gas-liquid mass transfer in milliliter-scale slurry reactors

Due to the limited availability of chemical reactants in the early process development of pharmaceuticals and fine chemicals, and sometimes the high-cost of catalyst, it is increasingly popular to use milliliter-scale slurry reactors with reaction volumes of 20 ml or less to screen catalyst candidates for three-phase reactions. To ensure the success of catalyst screening, it is advantageous to run reactions under kinetically controlled conditions so that the activities of different catalysts can be compared. Because catalysts with small particle sizes are used in slurry reactors, the reactions are susceptible to gas-liquid mass transfer limitations. This work presents an efficient way of enhancing gas-liquid mass transfer in milliliter-scale reactors through the use of magnetically driven agitation with complex motion. In the reactor described here, gas-liquid mass transfer coefficients can be doubled over those obtained with the agitation technique used in commercial milliliter-scale units. In addition, the reactor can achieve the top range of mass transfer coefficients obtained in a full-scale reactor. This work also presents the first measurements of gas-liquid mass transfer coefficients in milliliter-scale reactors, which are two orders-of-magnitude smaller than systems for which mass transfer coefficients have been reported earlier. Both physical and chemical absorption techniques are used.     

Preparation of monolithic catalysts

Monolithic catalysts can be attractive replacements for conventional catalysts in randomly packed beds or slurry reactors. The conventional procedures for preparing catalysts, however, cannot simply be applied to monolithic catalysts. Different procedures are discussed on how to put a coat layer of a catalyst support material like alumina, silica, or carbon on a monolith body by either filling the pores in that support or by putting a layer on that support. Different methods to apply an active phase to the support are discussed as well. Finally, methods to convert ready-made catalysts into monolithic catalysts are presented.     

Fast gas–liquid–solid reactions in monoliths: A case study of nitro-aromatic hydrogenation

Monolith catalyst supports are attractive as fixed bed reactors that, at the scale of the catalyst dimension, exhibit the mass transfer characteristics of slurry reactors. This paper presents a reactor design study for the single-pass conversion of dinitrotoluene in a loop configuration with an external heat exchanger. The advantage of such a loop system is the elimination of a solvent, which in turn allows more reaction heat to be recovered. The advantages of using a monolith are the low pressure drop at high recycle ratio, while maintaining good mass transfer characteristics. The modelling includes internal diffusion limitation, external mass transfer characteristics, heat effects, maldistribution and flow stability. The optimal design is found at the lowest hydrodynamic stable flow rates, where the mass transfer is fastest and the residence time in the column maximal.     

Comparison of different reactor types for low temperature Fischer-Tropsch synthesis: A simulation study

The commercially established slurry bubble column and fixed-bed reactors for low temperature Fischer-Tropsch synthesis were compared with novel micro- and monolith-reactors by mathematical modeling. Special attention was paid to the influence of catalytic activity on the reactor efficiency and the losses by mass and heat transfer resistances. The simulation results show that a micro-structured reactor exhibits the highest productivity per unit of catalyst volume followed by slurry bubble column reactor and monolith reactor. The fixed-bed reactor that was assumed to operate in the trickle-flow regime has a particularly low catalyst specific productivity due to severe mass transfer resistances. However, caused by a very low ratio of catalyst and reactor volume the micro-reactor has only a similarly low productivity per unit of reactor volume as the fixed-bed reactor. In contrast, the reactor specific productivity of slurry bubble column reactor and monolith reactor is up to one order of magnitude higher.     

规整结构催化剂及反应器研究进展

介绍了一种新颖的规整结构催化剂。它在废气排放处理等气相催化反应中已表现出优于常规催化剂的优良催化性能,在气液固多相催化反应领域的研究也表明它能够应用于更多的催化反应。将这种催化剂和反应器结构特性与常规催化反应中应用的固体催化剂和反应器进行了比较,结果表明它有可能替代浆态床和固定床反应器,具有良好的应用前景。     

Design of monolithic catalysts for multiphase reactions

Monolith reactors are emerging as an attractive alternative for gas-liquid-solid reactor applications. The use of monolithic catalysts in new reactors as well as in retrofit designs should be based on an optimal choice of monolith geometry and operating conditions.In this contribution, we illustrate through fundamental modeling of the transport-kinetic interactions in a monolith catalyst how such an optimal design may be evolved. We also highlight the potential benefits a monolith catalyst has as compared to a pellet-based trickle bed reactor.     

Simulation of structured catalytic reactors with enhanced thermal conductivity for selective oxidation reactions

The replacement of conventional-packed beds of pellets with “high conductivity” honeycomb catalysts in industrial externally cooled multitubular fixed-bed reactors is investigated by modeling and simulation for the oxidation of methanol to formaldehyde and for the epoxidation of ethylene to ethylene oxide, which involve a consecutive and a parallel reaction scheme, respectively. Results suggest that near-isothermal operation of the fixed-bed reactors can be achieved using monolithic catalyst supports based on relatively large volume fractions of highly conductive materials. Pressure drops are reduced to less than 1%. The selectivity is favored by the excellent control of the intraporous diffusional resistances resulting from the thin catalytic washcoats. Reactor designs based on larger tubes are feasible at the expense of greater volume fractions of catalyst support. A critical aspect is represented by the restrictions on the specific load of catalyst per reactor volume resulting from the poor adhesion of very thick catalyst layers onto metallic surfaces. Such a difficulty can be circumvented by maximizing the geometric surface area of the monolith (e.g. minimizing the honeycomb pitch), enhancing the catalytic activity (e.g. increasing the load of active components), and increasing the coolant temperature (if the selectivity is not adversely affected).     

Modeling of a metal monolith catalytic reactor for methane steam reforming-combustion coupling

A novel metal monolith reactor for coupling methane steam reforming with catalytic combustion is proposed in this work, the metal monolith is used as a co-current heat exchanger and the catalysts are deposited on channel walls of the monolith. The transport and reaction performances of the reactor are numerically studied utilizing heterogeneous model based on the whole reactor. The influence of the operating conditions like feed gas velocity, temperature and composition are predicted to be significant and they must be carefully adjusted in order to avoid hot spots or insufficient methane conversion. To improve reactor performance, several different channel arrangements and catalyst distribution modes in the monolith are designed and simulated. It is demonstrated that reasonable reactor configuration, structure parameters and catalyst distribution can considerably enhance heat transfer and increase the methane conversion, resulting in a compact and intensified unit.     

Activated Carbon Monolith Catalysts (ACMC): A New and Novel Catalyst System

Monolithic catalysts have been widely used in gas–solid phase systems for a variety of environmental applications. More recently they are being investigated for use in three phase systems however there are few catalysts available commercially for these applications. Monoliths offer several advantages over traditional three phase slurry and trickle bed systems, in that they can be easily separated from the system, offer low pressure drop and high rates of adiabatic cooling, and are mostly linear when scaling up. In this chapter, we present a new and novel three phase catalyst, Activated Carbon Monolith Catalyst (ACMC), utilizing a newly developed integrated activated carbon monolith as an alternative to granular catalysts.     

Parallel synthesis and fast screening of heterogeneous catalysts

We are presenting an effective method to prepare and test heterogeneous catalysts much faster than by the conventional way. A catalyst array was prepared via an incipient wetness method by combination of different amounts of Pt, Zr, and V on Al₂O₃ by means of an automatic liquid handler. For catalytic testing for methane oxidation a ceramic monolith reactor module, the channels of which contain the different catalyst compositions, was developed in which up to 250 catalyst compositions can be prepared and tested in parallel. Gas samples from each channel of the monolith were analysed sequentially by a mass spectrometer by moving the QMS inlet capillary into the channels using a three-dimensional positioning system which works at high temperatures. By comparison of the testing results with experiments carried out in flow reactors it is shown that the monolithic reactor is an efficient tool for fast screening of heterogeneous catalysts. This revised version was published online in June 2006 with corrections to the Cover Date.     

Water removal by reactive stripping for a solid-acid catalyzed esterification in a monolithic reactor

Solid-acid catalysts are attractive replacements for processes using conventional homogeneous catalysts. In the esterification of an alcohol with a carboxylic acid, however, the side product water strongly inhibits the activity of a solid-acid catalyst. Since an esterification is an equilibrium limited reaction, full conversion is not possible unless one of the products is removed. A novel reactor type, utilizing a solid-acid catalyst coated monolith, is presented in which water can be removed from the liquid reaction mixture by means of reactive stripping. In this manner the inhibition is eliminated and complete conversion can be reached. The advantages of this reactor concept are demonstrated by both experiments and reactor modeling.     

甲烷催化燃烧与十二烷脱氢反应的间接热耦合

将甲烷催化燃烧的Pd-Zr/SBA-15/Al2 O3/FeCrA1金属整体式催化剂和十二烷催化脱氢的Pt-Sn-Li/Al2 O3/FeCrA1金属整体式催化剂分别填充在套管式反应器的内管和环隙中,在电炉伴温条件下研究了这两个反应的热耦合.实验结果表明,炉温略低于催化剂床层温度,说明吸热反应所需的热量来源于放热反应,...     

Combustion of methane lean mixtures in reverse flow reactors: Comparison between packed and structured catalyst beds

The scope of this work is to compare systematically the performance of particle beds and monolithic beds in catalytic reverse flow reactors used for combustion of lean methane/air mixtures, using alumina-supported palladium as catalyst. Different values of gas surface velocity (0.1–0.3 m/s), particle diameter (3–6 mm, for particle bed), cell density (200–400 cpsi, for structured bed) and catalyst/inert ratio (0.4–1) were used for the simulation of the combustion of 3500 ppm methane in both kinds of reverse flow reactor. An unsteady one-dimensional heterogeneous model has been developed and solved using a MATLAB code. The model, physical parameters and transport properties used had been experimentally validated in a previous work, operating with a particle bed reverse flow reactor. Results obtained indicate that the reverse flow reactor is more stable when the catalyst particle beds are use, although the difference with the monolith bed decreases as surface velocity increases. In contrast, pressure drops in the bed are higher for the particle bed.     

New Catalyst Synthesis and Multifunctional Reactor Concepts for Emerging Technologies in the Process Industry

In order to promote the introduction of emerging technologies in the process industry, it has been established the two types of supporting R&D activities can be effectively pursued in parallel: (1) new catalyst synthesis methods that eliminate or minimize mass transfer limitations; and (2) multi-functional reactors by integrating catalysis, heat transfer and/or separation. Catalyst Synthesis: In this paper, several methods of catalyst tailoring will be described that can minimize mass transfer limitations at industrially relevant conversion levels. Three (3) specific examples have been selected to demonstrate what can be achieved: (1) micro-engineered catalyst that enables enhanced inter-phase transfer; (2) new mesoporous catalysts with ultra large pores to accommodate slowly diffusing reactants; and (3) customized zeolites of extremely small particles to achieve high effectiveness factors while retaining the virtues of shape selectivity. Multi-functional Reactors: Applying process intensification principles, mature high-volume petrochemical processes can be improved dramatically, beyond the expected progress. This will be described using three (3) specific examples: (1) intra-reactor oxidative reheat for the production of styrene, by staging endothermic and exothermic reactions in series; (2) simultaneous operation of endothermic, dissociative adsorption of methane with exothermic, oxidative removal of carbon during catalytic partial oxidation; and (3) catalytic distillation for the production of ethers, ethyl benzene and the selective hydrogenation of highly unsaturated components in olefins streams.     

Optimal design of axial noble metal distribution for improving dual monolithic catalytic converter performance

In practical applications, monolithic catalytic converters are operated at non-isothermal conditions. In this case, the active metal distribution along the length of the converter may influence its performance. Indeed, better conversions can be achieved by controlling the distribution of the same quantity of active material. In this study, we used a one-dimensional catalyst model to predict the transient thermal and conversion characteristics of a dual monolithic catalytic converter with Platinum/Rhodium (Pt/Rh) catalysts. The optimal design of a longitudinal noble metal distribution of a fixed amount of catalyst is investigated to obtain the best performance of a dual monolithic catalytic converter. A micro-genetic algorithm with considering heat transfer, mass transfer, and chemical reactions in the monolith during the FTP-75 cycle was used. The optimal design for the optimal axial distribution of the catalyst was determined by solving multi-objective optimization problems to minimize both the CO cumulative emissions during the FTP-75 cycle, and the difference between the integral value of a catalyst distribution function over the monolith volume and total catalytic surface area over the total monolith volume.     

The selective hydrogenation of butyne-1,4-diol by supported palladiums: a comparative study on slurry, fixed bed, and monolith downflow bubble column reactors

The present study was carried out to asses performance of a Pd-monolith downflow bubble column (DBC) reactor, and compare it with that of the slurry and the fixed bed DBC. The selective hydrogenation of butyne-1,4-diol to cis-2-butene-1,4-diol over palladium catalyst was chosen as a model reaction. In principle, the monolith DBC allowed the reaction to take place under kinetic control regime. Comparison with DBC employing 5% Pd/C powder and 1% Pd-on-Raschig ring catalysts revealed a better performance of the monolith DBC (1% Pd loading) with advantage of smaller reaction volume and intensified reaction rate. In the monolith DBC, improved hydrogen transport was possible, as the interface between bubbles and the channel wall was very thin, thus, the length of the diffusion path was very short. In addition, the interfacial surface area at both gas–liquid and liquid–solid interface in the monolith was also very high. The reaction kinetics was well represented by the Langmuir–Hinshelwood mechanism. As an alternative to conventional three-phase reactors, the monolith DBC was simple due to its inherent characteristic operation and no specially designed device.