Main research projects




Effective models and software for dynamic simulations of catalytic monolith reactors (XMR)

XMR is a versatile software package for effective dynamic simulations of interconnected monolithic reactors and adsorbers, developed at the research group Monolith. Modular structure of the software enables easy linking of various reaction kinetics modules for individual catalytic converters used in the automotive industry (TWC, DOC, NSRC/LNT, SCR). Heterogeneous 1D and 2D(1D+1D) models of monolith channel are available. System decomposition and optimised numerical methods with implementation in Fortran enable effective solution of model equations. Both global kinetics and microkinetics can be used to describe the reaction mechanisms and rates. XMR can be used as a stand-alone application, or linked to Matlab Simulink (ExACT), or linked to StarCD (esafter-1d) as a part of fully-3D simulation of the exhaust gas aftertreatment system.

Effects considered in the model:
  • Convection of gas in the monolith channel
  • Mass and heat transfer from flowing gas to the external surface of the washcoat
  • Internal diffusion in the washcoat (full solution only in the 2D(1D+1D) model)
  • Adsorption of selected components
  • Catalytic reactions in the washcoat
  • Accumulation of mass and heat
  • Heat transport by convection in the bulk gas and conduction in the solid phase
Channel
Figure: Scheme of the processes in a monolith channel.

The model results serve for:
  • Prediction of the dynamic evolution of the outlet concentrations and temperature
  • Evaluation of instantaneous and overall (integral) conversions and cumulative emissions
  • Calculation of spatio-temporal concentration and temperature profiles inside the monolith
  • Parametric studies, testing of different monolith configurations, design of the exhaust gas aftertreatment system
  • Optimisation of the operating conditions and control strategy
  • Better understanding of dynamic processes inside the monolith


Periodic lean/rich operation of NOx storage and reduction catalytic converter (NSRC, lean NOx trap, LNT)

Economical Diesel and lean-burn gasoline engines work in excess of air, which provides better efficiency and lower fuel consumption compared to gasoline engines operated at the stoichiometric conditions. However, the reduction of nitrogen oxides (NOx) is difficult in the excess of oxygen - the reducing components like carbon monoxide (CO), hydrocarbons (HC) and hydrogen (H2) are preferably oxidised by oxygen and most of nitrogen oxides remain unreduced. One possible way to lower the emissions of nitrogen oxides is to use the NOx storage and reduction catalyst (NSRC, called also lean NOx trap, LNT) and operate it under periodically alternating lean and rich conditions.

Catalytic washcoat of the NSRC contains special NOx storage components (mainly the alkaline earth or alkali metals). In the course of the longer lean phase (excess of oxygen), the nitrogen oxides are adsorbed on the washcoat surface, forming surface nitrites and nitrates. The stored nitrogen oxides then need to be catalytically reduced (preferably to nitrogen) under the excess of reducing components (CO, H2 and HC) within a short rich phase (enrichment of the fuel mixture).

Lean phase
Figure: Lean phase - scheme of the processes on the catalyst surface.
 
 
Rich phase
Figure: Rich phase - scheme of the processes on the catalyst surface.

Reactions included in the NSRC model:
  • CO, HC and H2 oxidation
  • NO reduction
  • NO/NO2 reversible transformation
  • NOx storage, differentiated adsorption sites
  • Desorption and reduction of the stored NOx
  • Water gas shift and steam reforming (H2 formation)
  • Dynamic NH3 formation and re-oxidation
  • Oxygen storage effects
  • Water condensation in pores during cold start
The reaction kinetics parameters are evaluated from the data obtained in transient lab experiments with the samples of industrial catalytic monoliths. Dynamics of the NSRC processes are studied in a wide range of operating temperatures, focusing on NOx conversion, NH3 selectivity, CO and HC conversions, and effectivity of the regeneration with individual reducing agents (CO, H2, HC). The combined NSRC-SCR, DOC-NSRC, and DOC-NSRC-SCR aftertreatment systems are also being investigated, by both experiments and simulations.

Lab scheme
Figure: Scheme of the lab apparatus for transient experiments with catalyst samples.


Detailed 3D modelling of porous catalytic materials

Novel methods for the modelling of transport, reactions and transformations in porous materials are being developed. Effects of the internal porous structure on the macroscopic properties of the porous catalysts and membranes are examined. 3D digital reconstruction methods are employed. On the basis of electron micrographs (SEM,TEM) and X-ray microtomography the porous medium is reconstructed into the form of a discrete phase function, represented by a 3D matrix. Within this 3D domain the following processes are simulated: material transformations during the preparation stage (agglomeration of primary particles, wetting and capillary effects during impregnation of active metals, drying and crystallisation of nanoparticles, calcination and sintering etc.), and reaction with transport in the final porous catalyst. The proposed approach involves linking of mathematical models at different scales, from molecular dynamics in smallest pores to overall description of the system by effective parameters (diffusivity, effectiveness factor). The aim is to identify the steps leading to optimisation of the porous structure.
Figure: Example of calculated CO reaction rate profile in a 3D digitally reconstructed porous Pt/γ-Al2O3 catalyst layer. CO oxidation with diffusion limitations, source of reactants at the top (z1 plane), section edge 10 μm. Only macropores can be seen, small pores and individual Pt crystallites are below the resolution.


Complex dynamic behaviour of three-way catalytic converters (TWC)

The development of new and advanced three-way catalytic converters demands not only experimental work, but also detailed modelling and simulation studies. The models become more complex, when all the important physical and chemical phenomena are considered. Particularly the use of mechanistic kinetic models (microkinetics) and the incorporation of diffusion effects in porous catalyst structure play key role in the understanding and optimisation of TWC converter performance. For a certain range of operating conditions the complex reaction schemes (together with mass transport effects) lead both to multiplicities (hysteresis) and to oscillations of the concentrations and temperature in the reactor.
Figure: Example of a periodic-quasiperiodic-chaotic map in the microkinetic model of TWC.


Effective diffusivities and pore-transport characteristics of washcoated monolith for automotive catalytic converter

Structured catalyst supports are widely used in automotive exhaust gas converters. Small sized channels are contained in monoliths to provide large surface area. Typically, both metal and ceramic monoliths are used. The active catalyst is supported (washcoated) onto the monolith by dipping it into slurry containing the catalyst precursors. A commonly used washcoat material is γ-Al2O3 with a surface area of 100 - 200 m2/g. The excess of the deposited material (washcoat) is then blown out with hot air and monolith is calcinated to obtain the finished catalyst. This process gives thin washcoat layer; however, it also results in a variation in thickness around the channel perimeter. Although the washcoat layer is thin, pore diffusion can affect monolith performance and thus need to be included in any realistic mathematical model. Therefore, it is necessary to have reliable information on the mass transport rate in the porous medium (e.g. effective diffusivities of exhaust gases in the washcoat layer). In our laboratory we use an approach inspired by the chromatographic method. The chromatographic method for evaluation of effective diffusion coefficients in porous materials is well established. Porous particles can be packed into the column in two ways: either a wide bed is packed with particles (with column/particle diameter ratio larger then 20), or in an arrangement known as single-pellet string column (SPSC). In SPSC particles are packed one by one into a column with diameter that exceeds only slightly (10-20%) the particle dimension. SPSC is usually used for spherical or cylindrical porous particles and could be also used for porous solids with other shapes. Effective diffusivities are evaluated from response (chromatographic) signals of columns packed with particles of the tested porous material. Analysis of outlet peaks is based on the transport processes inside the column. Matching of column response peaks with model equations is done in the time-domain. Chromatographic system consists of calibrated mass flow-meter/controllers for carrier and tracer gases, a six-way sampling valve for tracer gas with sampling loop (0.273 ccm), chromatographic column (SPSC) and thermal conductivity detector (TCD). Metal capillaries, with 1 mm inner diameter, are employed for connecting the system components. Two columns with lengths LI = 100 cm and LII = 50 cm and identical column diameters (8 mm).
washcoat
Figure: Typical distribution of catalytic washcoat (B) on the walls (A) of square monolith channels.
washcoat
Figure: Scheme of the chromatographic setup.
1 - Tracer gas source
2 - Carrier gas source
3 - Sampling valve
4 - Chromatographic column
5 - Thermal conductivity detector (TCD)
6 - Digital data logger
7 - Computer
F - Calibrated mass flow-meter/controllers