Advanced Process Modelling for reaction systems

Tubular and multitubular reactor modelling: identifying and eliminating hotspots

Reactor operator's dilemma

Process modelling can bring significant benefits to reaction system design and operation in:

design optimisation and scale-up
throughput and quality enhancement
catalyst comparison and extension of catalyst life
controllability analysis
optimisation of operating conditions and policy
hot-spot determination and general troubleshooting
optimisation of feed and product grade change

See our Hydrocarbon Processing article: Optimize terephthaldehye reactor operations, describing LG Chem's design approach for a new multitubular reactor operation

Reactor design and operation are areas where process modelling can deliver major capital and operational savings.

To gain maximum benefit from modelling requires highly detailed models that take into account all the complexities of the reaction process.

Process Systems Enterprise's gPROMS^® environment is the reaction system modelling tool of choice for many of the world's largest chemical companies.

gPROMS has many proven capabilities for modelling of reaction systems, including sophisticated reaction modelling and advanced parameter estimation and optimisation capabilities, as well as the flowsheeting facilities necessary for optimising reactors within their operating context.

In addition to the toolls and services PSE provides for reaction modelling generally, we provide:

the gPROMS Advanced Model Library for Fixed-Bed Catalytic Reactors (AML:FBCR)
the Hybrid gPROMS-CFD Multitubular interface cater particularly for high-fidelity modelling of multitubular reactors.

PSE's 2-D tubular reactor model allows detailed modelling of 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 catalytic 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.

PSE 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.

Tubular reactor

The PSE 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.

Multitubular reactor

Once the complexity of the reaction has been captured in a tube model, a number of tubes can be assembled into a multitubular reactor model, taking the shell-side cooling effects into account 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 (below). 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.

Typical results

The figure shows the temperature along the length for two tubes in different parts of the reactor, and the non-uniform temperature distribution across a shell fluid cross section.

The tube temperature peaks around 0.3 m from the inlet, and one side of the reactor is clearly operating at higher temperature.

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 will allow the whole reactor to run hotter, improving conversion.

Typical data requirements for such a model include geometric data, catalyst selectivity information, composition vs temperature for various operating states and (optional) initial estimates for kinetic parameters. Advanced gPROMS models within CAPE-OPEN flowsheeting packages

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.

PSE ModelCare

PSE provides a comprehensive configuration service for fixed-bed catalytic reactors, which can include a full validation against laboratory and pilot plant data in accordance with our reactor modelling methodology. This means that PSE's wide expertise in modelling reacting systems can help ensure rapid implementation and a robust and accurate solution.

Models can be supplied in user-configurable or open source form, depending on your requirements.

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