The food and pharmaceutical industries continuously strive to improve the efficiency and safety of the chemical processes in manufacturing their products. To help them do so, Prof. Juray De Wilde’s team has developed a unique type of pilot reactor.
The pouring rain fails to douse Prof. De Wilde’s enthusiasm as we approach the building that houses his laboratory: ‘The tests with the new pilot have begun’, he says, excited by the plume of white smoke rising from a brand new metal chimney. Inside, researchers including Axel de Broqueville, with whom he developed this type of reactor, are huddled around a metal tank in the middle of a jumble of wires and tubes. ‘It’s one of our two pilot reactors, a type that’s unique in the world’, Prof. De Wilde says. ‘Here we transform liquid milk into powdered milk.’ What excites him is the sight of the escaping water vapour: the drying process seems to be working.
A professor at the UCLouvain Louvain School of Engineering Institute of Mechanics, Materials, and Civil Engineering (iMMC), Prof. De Wilde explains, ‘We’re working on modelling and sizing reactors.’ Obviously, ‘reactor’ is to be understood in the term’s chemical sense, as the volume in which chemical reactions take place (see box). ‘The objective is to carry out reactions as efficiently as possible but also in the most controlled and safest possible way.’
Multiple phenomena occur in reactors and one has to take into account what’s happening on very different scales. The work of UCLouvain researchers consists of fundamentally modelling what takes place in reactors, in order to identify the slow steps for given processes and remedy them with new reactor concepts which allow for unique operating conditions. ‘For each phenomenon that takes place,’ Prof. De Wilde says, ‘we must have equipment which enables us to study it specifically; there are too many parameters to be able to study the whole directly.’ The comprehensive models which couple descriptions of different phenomena must nevertheless be validated. With the help of industrial partners, his team designed and built two pilot reactors allowing for detailed measurements for validating simulation models used to scale up two new technologies of industrial importance: the structured fixed-bed reactor and the high-G fluidised bed reactor.
Intensifying gravity
The principle of fluidised bed reactors is based on the injection of a fluid under a bed of particles in order to mix them and impart fluid properties to the particles. It’s like injecting a pressurised gas or liquid under a bed of sand. The fluid will lift and disperse the grains of sand. The law of gravity limits the proper functioning of these reactors, because if a gas passes too quickly through a bed of particles, they tend to fly away; gas and particles will mix not properly, which will require their subsequent separation. It’s therefore advantageous for the particles not to be driven away by the fluid.
How to intensify gravity? By centrifugal force, the researchers say: a powerful force capable of keeping particles close to the reactor wall even if a gas passes through at high speed. But this would require turning the reactor at high speed, which is hard to imagine given the size of industry reactors. ‘So we imagined rotating the injected fluid instead, and to do that we inject it tangentially through slots in the reactor’s cylindrical wall,’ Prof. De Wilde explains. ‘So we manage to generate a rotary bed at high rotational speed and great centrifugal force. It’s like operating under 10 or 100 times gravity.’ What does this change for reactions? If gases and particles have very different velocities, the transfers of matter and heat between the two are more efficient. But it also allows you to work with much finer particles, because the centrifugal force prevents them from flying away. ‘So we can further increase the fluid-particle contact and envisage new applications with ultrafine particles.’
Powdered milk
This type of reactor allows for essential applications in the pharmaceutical and food industry, which the iMMC laboratory has worked on. Specifically, the technique makes it possible to coat particles of 40 microns with a one-micron layer, for example to mask a bad taste. Such coating also makes it possible to regulate an active ingredient’s rate of release in the body. And this must be done precisely, on an immense number of particles and with a minimum of agglomeration or with controlled agglomeration! Quite a challenge. So is mixing different particles within agglomerates which must all have the same composition! ‘Industrial partners have asked us to produce food powders using this technique. And we’re now working with a dairy company that would like to produce powdered milk much more efficiently, it’s the only form for exporting milk over vast distances.’ Hence the plume of white smoke that rose above the rotary bed pilot reactor.
Structure
The chemical industry’s workhorse, however, remains the fixed-bed reactor. In its centre is a fixed solid structure containing a catalyst on which fluid circulates and with which it reacts. ‘It’s the catalyst that drastically and selectively accelerates reactions,’ says Prof. De Wilde. But the use of particles introduces limitations. Pressure loss and heat transfer are very important aspects. The reactions are either very endothermic (they consume heat) or exothermic (they emit heat), which requires either supplying or removing this heat continuously, which is done via exchange through the reactor wall and across the bed. It’s often this heat exchange that determines reactor size and production capacity. ‘If we want to improve heat transfer,’ Prof. De Wilde says, ‘the price to pay is an increase in pressure loss. In our laboratory, we are working on bed structure, which makes it possible to modify this relationship and therefore significantly increase the production capacity for the same reactor size.’
Validating the transfer and reaction models, particularly for methane (natural gas) steam-reforming, is the raison d'être of the second pilot reactor, which will soon be operational at UCLouvain. This is a technique for producing hydrogen or a mixture of hydrogen and CO (synthetic gas) used for producing ammonia and methanol, two of the most important chemical intermediates for industry, as one is used for producing fertilisers, the other to form the basis of various chemical syntheses.
Henri Dupuis
See also : Boosting chemical reactions with a hybrid catalyst
Chemical reactors
In chemistry, a reactor is an enclosure in which a chemical reaction, which is a process for transforming matter, occurs. There are many types. Slow chemical reactions must be carried out in the presence of catalysts, substances which will accelerate the reactions selectively. In a fixed-bed reactor, catalyst particles are stacked in the reactor in a ‘bed’ which remains stationary. The fluid with the reagents crosses this bed. The same happens in a fluidised bed reactor, but here the catalyst particles are in motion, allowing for continuous insertion and evacuation. In general, the particles are finer than in a fixed bed. In many processes, controlling reaction heat is critical, both for optimising the conversion of reagents and for safety – reactor design must take this into account.
A glance at Juray De Wilde's bio
By being drawn to science, Juray De Wilde may have produced a synthesis of his parents' passions: his father is a philosopher, his mother a violinist at the Ghent Opera House. Art and philosophy aren’t foreign to science, no? ‘I embarked on the adventure of the civil engineering entrance exam. I succeeded ... so I continued!’ He graduated from the University of Ghent, his hometown, in 1995. He doesn’t regret his choice: ‘The profession of engineer is a fine job, it requires taking responsibility, there’s no room for mistakes, and you’re confronted with limitations in practice, so you have to find a way.’ After earning his PhD in 2001, he completed a postdoc at Princeton and Ghent, then joined UCLouvain in 2005. In 2012-18, he was responsible for the Materials and Process Engineering Division (IMAP). Since 2019 he has been president of the FNRS Thematic Doctoral School in Process Engineering. In 2015-16, invited by the United States Department of Energy, he received a nomination from the Oak Ridge Institute for Science and Education to collaborate with the National Energy Technology Laboratory and various American universities, including Columbia University. Prof. De Wilde is co-author of the chemical reactor ‘bible’ Chemical Reactor Analysis and Design (John Wiley & Sons, 2010).