Inside a Reactor You Can’t Open: CFD Simulation for Trickle-Bed Chemical Reactors
Chemical reactors are among the most process-critical pieces of equipment in the chemical industry and among the least observable. Conditions inside an operating trickle-bed reactor routinely reach temperatures of 160°C or more, with pressures in some configurations exceeding 50 bar. The catalyst bed is packed and opaque. The fluid dynamics of gas and liquid flowing simultaneously through a porous medium are extraordinarily complex. And a design decision you can’t validate until the reactor is built and running is an expensive mistake to make.
This is the challenge that COA-CFD and its partner Biosimo AG are tackling together using Computational Fluid Dynamics simulation to model what no experiment can easily observe.
The Reactor and the Chemistry
Biosimo AG is developing a sustainable chemical process to produce acetic acid from bio-based ethanol, a ‘drop-in’ chemical that meets industry specifications for direct replacement of fossil-derived equivalents, at an estimated 20% cost reduction. The process uses a trickle-bed reactor: a vertical vessel packed with a heterogeneous catalyst through which gas (oxygen) and liquid (ethanol) flow simultaneously in parallel downward flow.
The reaction is exothermic. Heat management is critical. Localised hot spots can trigger side reactions that reduce yield and degrade the catalyst. Wetting efficiency of the catalyst particles matters too: dry catalyst does not participate in the reaction, and uneven fluid distribution across the reactor cross-section directly affects conversion rates.
To build a commercial-scale reactor with optimal energy management and yield, Biosimo needs to understand the fluid dynamics inside the vessel before committing to a physical prototype. Building multiple hardware prototypes to explore design space is expensive and time-consuming. Simulation offers a faster, lower-cost path to validated design.
The CFD Challenge: Three Phases, One Simulation
Modelling a trickle-bed reactor in CFD is a demanding problem. Three phases are present simultaneously: gas, liquid, and solid catalyst. The porous bed spans the full length of the reactor, requiring either a direct geometric representation of thousands of individual catalyst particles or an effective continuum model of the porosity. The density contrast between gas and liquid phases is large, creating numerical stability challenges. And the chemical kinetics nine candidate models were developed and validated during the first year of the project must be coupled to the fluid dynamics to correctly represent the composition changes, heat release, and mass transfer occurring within the reactor.
It is impossible to build a robust flow dynamic model without a strong and effective representation of the chemical reaction. The reaction changes the composition, and therefore the physical and flow properties, of everything flowing through the reactor.
The COA-CFD team at Engineering Software Steyr (ESS) has approached this progressively:
- Initial simulations used an effective porosity model treating the catalyst bed as a continuous porous medium rather than resolving individual particles to represent gas flow through the reactor
- A subsequent approach used Lattice Boltzmann Method (LBM) with direct geometric modelling of the porous structure, which performed well for single-phase flow
- Current work is exploring Lagrangian Differential Dynamics (LDD), treating gas as a passive background field and directly modelling the liquid phase addressing the challenge of simulating the two-phase system across the required density range
What the Simulation Will Deliver
The target workflow, currently in preparation, will allow reactor engineers to:
- Define porous bed geometry by specifying sphere diameter and packing fraction (automatically meshed by script)
- Set up tank geometry and inlet conditions
- Run liquid flow through the porous bed and extract contact time, the key metric for estimating wetting efficiency and reactor performance
- Model chemical reactions and heat transfer within the same simulation framework
Output data will include fluid distribution maps across the reactor cross-section, temperature profiles (essential for hot spot identification and heat management design), wetting efficiency estimates, and ultimately conversion rate predictions. The simulation will also be able to confirm whether heat generated by the exothermic reaction can serve as a feedstock pre-heater a circular energy recovery mechanism that improves overall process efficiency.
Why This Matters for Chemical Engineering
Trickle-bed reactors are widely used in the chemical and petrochemical industries for hydrogenation, oxidation, hydrodesulfurisation, and a range of catalytic processes. The CFD modelling framework being developed within COA-CFD is not specific to the Biosimo process: it is designed to be generalisable to other reactor geometries, chemistries, and operating conditions.
The target users are engineers and plant operators designing or evaluating reactor systems particularly in speciality chemicals, pharmaceutical intermediates, and emerging bio-based processes where physical prototyping is costly and the design space is large. The COA-CFD platform provides cloud-based access to this capability without requiring in-house HPC infrastructure or specialist CFD code development.
Reactors that can’t easily be opened, observed, or probed can now be understood from the inside through simulation before the first weld is made.
The collaboration between Biosimo AG and ESS continues through the final phase of the project, with validation simulations underway and alignment ongoing on experimental data for model verification.
