Tubular and multitubular reactor modelling: identifying and eliminating hotspots
Many of the world's reactor applications involve fixed-bed catalytic reaction.
Fixed-bed catalytic reaction is often complicated to model, because of the many phenomena that need to be taken into account in order to mode accurately. These include:
- rate-limiting diffusion of reactants and products to and from the catalyst surface
- the catalytic reaction itself; many such reactions involve a lot of species, and reaction rates must be accurately quantified in order to predict the selectivity at different conditions correctly
- heat transfer through the bed and from the bed to the outside world.
In fact, to gain maximum benefit from modelling requires highly detailed models that take into account all of the phenomena shown in the diagram below (click to enlarge).
gPROMS facilities for FBCR
In addition to the general facilities of the gPROMS environment, PSE provides a number of proven tools, capabilities and techniques aimed specifically at the modelling of large-scale catalytic reaction systems.
In particular gPROMS has the modelling facilities and numerical solution power to be able to handle the resulting large-scale sets of equations.
- The Advanced Model Library for Fixed-Bed Catalytic Reaction (AML:FBCR) comprises technology-leading models that represent many years' accumulated modelling expertise.
- State-of-the-art model validation tools allow estimation of multiple model parameters from steady-state and dynamic experimental data, and provide rigorous model-based data analysis.
- The gPROMS–CFD Hybrid Multitubular interface provides ultimate accuracy in the modelling of mutitubular reactors by linking a CFD model of the shell-side fluid hydrodynamics to a gPROMS model of the tube-side catalytic reaction.
- Model-based experiment design capabilities can design experiments that generate the maximum amount of parameter information from the minimum number of experiments.
In many cases as important as the tools are the services to ensure rapid project completion and deployment of fit-for-purpose models. PSE provides expertise in a number of forms:
- ModelCare model configuration, validation and execution services provide rapid project execution and transfer of know-how to customer personnel. ModelCare reactor projects follow a well-proven and cost-effective model-based engineering project methodology.
- PSE's expert Consulting services analyse and formulate customers' modelling requirements, execute model-based analyses, interpret results and advise on action – for example, appropriate experiments and measurement techniques.
- Model-Based Innovation services help guide customers' experimentation and integrate R&D effort with engineering design and operational improvement. This speeds up R&D programmes and provides high-quality parameter information for design and risk analysis.
Typical reactor configurations
Any configuration of fixed catalyst bed can be modelled using the AML:FBCR. Typical configurations are:
Tubular and annular reactor
The PSE tubular reactor model is a 2-D (axial and radial) or 3-D (axial, radial and catalyst penetration) model of a catalyst-filled tube.
Reaction kinetics and other relevant rate information (supplied by you or via PSE's reaction characterisation service) are implemented within the generalised tube model framework and fitted to plant or laboratory data under well-defined conditions in order to gain maximum predictive accuracy.
With rate constants determined by validation against the appropriate laboratory or pilot data, the model is capable of highly accurate prediction of temperatures and compositions throughout the bed.
Typical results are shown on the right.
Once the complexity of the reaction has been captured in a catalyst bed (tube) model, a number of tubes can be assembled into a multitubular reactor model, taking the shell-side cooling effects into account.
This is done in one of the following ways:
- 1-D shell-side model. This is the simplest and fastest approach, and generally gives sufficiently accurate results for design and trouble-shooting purposes.
- CFD shell-side model (right). Where a very high degree of predictive accuracy is required, gPROMS can be linked to a CFD package for calculation of shell-side fluid dynamics and heat transfer coefficients, using PSE's Hybrid gPROMS—CFD Multitubular option.
The figure on the right shows a temperature colour map from a STAR-CD–gPROMS hybrid model of a Gas-to-Liquid (GTL) multitubular reactor.
The figures on the right (click to enlarge) shows the results from a hybrid Fluent-gPROMS model.
The line plots show the temperature along the length for several tubes at different radial positions in the reactor, for a reactor with two cooling compartments.
The tube temperature peaks in certain tubes just after the baffle in an area of low flow near the cooling outlet nozzle. Tubes at the same radial oposition in the reactor are clearly operating at quite different temperatures, leading to non-uniform conversion across the reactor.
The associated temperature colour maps graphically demonstrate the non-uniform temperature distribution across the shell-side cooling fluid.
A hot-spot can clearly be seen, with potential for catalyst burn-out in the adjacent tubes in that area.
This can be designed out by considering alternative baffle arrangements, or graduated packing of catalyst and inert in the tube.
Reducing the magnitude of the temperature peak allows the whole reactor to run hotter, improving conversion. It also helps prolong catalyst life significantly, thus reducing catalyst cost and lost production due to downtime.
The approach is described in much greater detail in the Hydrocarbon Processing article
Advanced reactor models in other environments
The tubular and multitubular models can be inserted into any CAPE-OPEN compliant steady-state flowsheeting package – for example, Aspen Technology's Aspen Plus® and Hysys® simulators – using PSE's Unit Object for CAPE-OPEN – the gO:CAPE-OPEN product.
This means that the same model can be used for both dynamic and steady-state analysis.
All the services described here can be supplied under PSE's ModelCare.
Services can include a full validation against laboratory and pilot plant data in accordance with our reactor modelling methodology.
ModelCare helps ensure rapid implementation and a robust and accurate solution, as well as transfer of know-how to your modelling personnel.
The AML:FBCR's 2-D tubular reactor model takes into account detailed diffusion and reaction effects within the reactor, allowing designs to be optimised rapidly. Tube models can be combined with a shell model into multitubular reactor models that provide detailed analysis of both shell and tube side effects.
Modelling the complexity of reactions
The complex interactions between the various components of a reaction system govern the equipment requirements and operating envelope. In order to capture the effects of these interactions, it is sometimes necessary to model systems in great detail.
The AML:FBCR models can include reactions in the bulk fluid, in the film and on the catalyst surface, as well as the rate-limiting diffusion of reactants and products across the film.
The AML:FBCR tubular reactor model is a 2-D (axial and radial) model of a catalyst-filled tube.
Reaction kinetics and other relevant rate information (supplied by you or PSE) are plugged into the generalised tube model framework and fitted to plant or laboratory data for additional accuracy.
With well-defined rate constants, the model is capable of highly accurate prediction of temperatures and compositions throughout the tube.