Influence of channel geometry on hydrodynamics and mass transfer in the monolith film flow reactor

The hydrodynamics and the gas–liquid mass transfer as a function of the channel geometry have been investigated for the monolith film flow reactor. For the hydrodynamic studies, the liquid distribution and the flooding boundaries have been experimentally determined. The liquid distribution improved with increasing liquid flow rate. The flooding limits are in the range of other commercial structured packings and allow operation under industrially relevant conditions. Larger channel sizes and lower surface tension expand the operating window, while viscosity seems to have a minor impact. The gas–liquid mass transfer is a strong function of the surface to volume ratio defined by the channel dimensions. Co- and counter-current flow operation result in similar performance. Furthermore, shorter monoliths, with larger contribution of the inlet section have significant higher mass transfer due to the development of the concentration profile. The obtained k_GLa_V values of around 0.01 s⁻¹ are in the range of other commercial packings in counter-current flow operation. A three-dimensional single channel model describing the hydrodynamic and diffusion phenomena in the monolith is in good agreement with the experimental results. The flexibility in channel size and dimension allows tailoring the monolith reactor to the specific needs of the individual application.     

Simulations of photodegradation of toluene and formaldehyde in a monolith reactor using computational fluid dynamics

In this study, simulations were conducted on a monolith reactor for the photodegradation of toluene and formaldehyde. The monoliths in the reactor were treated as porous zones and the photocatalytic oxidation occurring on the monolith surfaces was modeled using Langmuir–Hinshelwood kinetics. A discrete ordinates model was used to simulate the light intensity with a novel approach, which involved an adjustable parameter—the absorption coefficient of the channel wall, for modeling the local light intensity across the porous media. The advantage of this approach was that despite its simplicity, it was able to capture and visualize the local light profile across the monolith channels and to integrate it into the reaction kinetics. Although it required a trial‐and‐error to determine the correct value of the channel wall absorption coefficient, the proposed model achieved a reasonable agreement between the simulation results and published experimental data. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Wall-flow filters with wall-integrated oxidation catalyst: A simulation study

Diesel soot abatement via diesel particulate filters composed of so-called wall-flow monoliths is well established. Today, due to recent improvements in the production technology full-featured catalyst functionality can be implemented in the filter walls.

This work focuses on a comparison of the reactor performance of the wall-flow filter and the conventional flow-through monolith. To this end a two-dimensional numerical model is set up for each of the two reactor configurations.

Concentration profiles in the wall-flow filter systematically change as a function of flow velocity.

At high flow velocities transport from the inlet channel into the porous wall is nearly entirely dominated by convection. This leads to uniform axial concentration profiles in the inlet and outlet channel and a steep gradient in the porous wall.

At low velocities radial transport into the porous wall is dominated by diffusive transport. This leads to a negligible radial concentration gradient between the inlet and the outlet channel.

Under most operating conditions relevant for an automotive exhaust catalyst the flow velocity is in an intermediate range with contributions of diffusive and convective transport.

The transition from entirely convection dominated transport at high space velocities to increasingly diffusion dominated transport at lower flow velocities is similarly found for first order kinetics and a generalized Langmuir–Hinshelwood–Hougen–Watson (LHHW) rate law.

Wall-flow filters show systematic conversion advantages over the conventional monolith for a first order reaction. For a reaction with LHHW-type kinetics this effect is not generally observed. It is one major result of this work that the relative performance of the two reactor configurations depends on the kinetics of the catalyzed reaction.     

Shouldn’t catalysts shape up?: Structured reactors in general and gas–liquid monolith reactors in particular

In this paper, the advantage of structuring catalytic reactors is discussed. In structured systems, the exact shape of all column internals is determined by design rather than chance. This results in two advantages for the engineer in charge of designing a reactor: (1) the structuring introduces additional characteristic length scales, leading to new degrees of freedom that allow decoupling of phenomena that otherwise would need a sub-optimal compromise and (2) full control over the entire geometry results in higher precision. Taking full control over the geometry boosts performance, especially if the catalyst is tailored to the reactor design.The monolith structure is the best-known example of a structured catalyst. We discuss the industrial benefits and practical pitfalls of this honeycomb of parallel capillaries that allows the advantages of microfluidics to be applied on an enormous scale. In this paper it is argued that the monolith is most suitable for processes that are (1) stable enough for packed-bed operation and (2) need better mass transfer than can be obtained in any conventional reactor, including the trickle bed and the stirred tank reactor. This includes several large-scale processes such as HDS. Fine chemical synthesis, where the objective of robust scale-up and predictability will never be met using stirred tanks, may equally benefit from the scalable ultra-high mass transfer that is obtained in monoliths.     

Transport and reaction in catalytic wall-flow filters

Diesel particulate filters composed of so-called wall-flow monoliths are well established devices for diesel particulate abatement. Recent improvements in production technology allow implementation of full-featured catalyst functionality in the filter walls.

From a reactor engineering point of view such wall-flow reactors with wall-integrated catalyst show fundamental differences compared to conventional flow-through monoliths. The complex interactions of convection, diffusion and reaction in the wall-flow monolith are studied by means of numerical simulation. A two-dimensional model for the flow in one pair of inlet/outlet channels with a generic first order reaction in the catalytic filter wall is developed. Concentration profiles in the reactor and a conventional flow-through catalyst are compared.

It is found that in the range of moderate reactor conversion concentration gradients along the inlet channel of the filter are small. Thus the reactor can be described by an approximate one-dimensional model, taking into account only the radial flux through the filter wall and assuming a constant inlet concentration in axial direction along the inlet channel.

Light-off curves are computed for the wall-flow and for the conventional flow-through monolith. Significantly better conversion is found for the wall-flow configuration. This can be explained by mass transfer limitation in the conventional flow-through monolith.     

Monoliths in catalytic oxidation

Catalytic combustion is useful to avoid emission of nitrogen oxides, to combust fuel gas of different calorific levels, and to combust low contents of badly smelling or hazardous gaseous compounds. After dealing with some characteristics of catalytic combustion it is argued that catalytic combustion to a final temperature lower than about 800°C calls for a rapid transport of thermal energy out of the reactor. A fluidized bed in which combustion has been successfully performed is dealt with as well as a reactor filled with metal bodies sintered to each other and to the wall of the reactor. To achieve a sufficiently high catalytically active surface area a thin layer of silicone rubber is applied to the surface of the metal bodies and subsequently pyrolyzed to a highly porous layer of silica. To raise the thermostability alumina can be added to the silica layer.

To establish a final temperature above 900°C the homogeneous gas-phase combustion can be ignited by a solid catalyst or the reaction can be performed completely catalytically. Since the combustion reaction proceeds very rapidly at elevated temperatures, a large gas flow can be utilized, which calls for a reactor exhibiting a low-pressure drop. For catalytic combustion monoliths and gauzes are appropriate. The chemical composition of ceramic and metallic monoliths is dealt with as well as the cell densities and wall thicknesses of commercial monoliths. The application of active components to the surface of the walls of monoliths is subsequently discussed. Since monoliths do not allow radial mixing, a homogeneous gas mixture has to be fed to the monolith to prevent very high temperature levels moving randomly over the channels of the monolith and deactivating the catalyst.

With monoliths in gas turbines often catalytic ignition is used. To limit the temperature a fraction of the fuel feed is injected into the homogeneous combustion chamber. A number of alternatives of transporting the fresh fuel to the homogeneous combustion zone is mentioned. The cause of the catalyst temperature being higher than that of the gas flow is dealt with as well as the low volatility at elevated temperatures required for the catalytic components. Selection of the catalytically active materials is discussed and the procedure to bring the gas flow at the light-off temperature of the catalyst.

Monolithic combustors used in radiant heaters display often an oscillatory behavior. After dealing with the cause of the oscillations, prevention by means of a flame arrestor is mentioned.     

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.     

Fischer–Tropsch synthesis on ceramic monolith-structured catalysts

This paper reports recent research results about the impact of different catalyst bed configurations on Fischer–Tropsch (FT) synthesis product distributions. A powdered CoRe/γ-alumina catalyst with a particle size ranging from 60 to 100 mesh was prepared and tested in a packed bed reactor. The same catalyst was ball milled and coated on a ceramic monolith support structure of channel size about 1 mm. The monolith catalyst module was tested in two different ways, as a whole piece and as well-defined channels. Steady-state reaction conversion was measured at various temperatures under a constant H₂/CO feed ratio of 2 and a reactor pressure of 25 bar. Detailed product analysis was performed. Significant formation of wax was evident with the packed particle bed and with the monolith catalyst that was improperly packed. By contrast, wax formation was not detected in the liquid product by confining the reactions inside the monolith channel. This study presents an important finding about the structured catalyst/reactor system, in that the product distribution highly depends on how the structured reactor is set up. Even if a catalyst is tested under identical reaction conditions (T, P, H₂/CO ratio), hydrodynamics (or flow conditions) inside a structured channel may have a significant impact on the product distribution.     

Additive manufacturing of sodalite monolith for continuous heavy metal removal from water sources

Herein, we present a simple strategy for preparing monolithic sodalite adsorbents via sequential additive manufacturing and post-treatments. In detail, the method includes (i) 3D printing of cylindrical monoliths using clay as the base material; (ii) thermal activation of the 3D-printed clay monoliths by calcination (to produce reactive alumina and silica species and enable mechanical stabilization); (iii) conversion of the activated clay monoliths to hierarchical porous sodalite monoliths via hydrothermal alkaline treatment. Parametric studies on the effect of calcination temperature, alkaline concentration and hydrothermal treatment time on the property of the resulting materials (such as phase composition and morphology) at different stages of preparation was conducted. Under the optimal conditions (i.e., calcination temperature of 850℃, NaOH concentration of 3.3 mol·L^-1, reaction temperature of 150℃, and reaction time of 6 h), a high-quality pure sodalite monolith was obtained, which possesses a relatively high BET surface area (58 m²·g^-1) and hierarchically micro-meso-macroporous structure. In the proposed application of continuous removal of heavy metals (chromium ion as the model) from wastewater, the developed 3D-printed sodalite monolith showed excellent Cr³⁺ removal performance and fast kinetics (~98% removal efficiency within 25 cycles), which outperformed the packed bed using sodalite pellets (made by extrusion).     

多相光化学反应器的辐射能传递模型

以双通量模型为基础模拟多相光化学反应器中的辐射能传递行为 ,考虑透过光催化剂颗粒的光子总数修正了双通量模型 ,采用比较简单的模型说明了多相光化学反应器内量纲 1光辐射能分布和光催化剂的量纲 1吸收光子总数分布 ,举例讨论了半导体超细粉末二氧化钛为光催化剂的气固相光化学反应器在不同光催化剂粒径、不同反应器厚度条件下反应器内量纲 1光辐射能分布和光催化剂的量纲 1吸收光子总数分布