Smart Reactors–Rethinking Reactor Design
- Xihua Hu, M.Sc., Institute of Thermal Separation Processes -
Reactor design for chemical and bioprocess engineering has not changed much in the past decades, owing it to the robustness of both applied materials and the processes themselves. However, the ever-increasing modern-day productions in bio-/chemical industries demand higher flexibilities in process design and operation due to fluctuating raw material qualities, energy sources, personalized products, shorter cycle times and various other individual requirements. To tackle this challenge, a new generation of reactors with added functionalities need to be conceptualized. With the emerging availability of new fabrication technologies and functional materials, “smarter” reactors can be developed. “Smart” as in both smartly designed, by increasing space-time yields of traditional processes or process cascades, and also smart in a literal sense, where the regulation of the reactor during operation is handled autonomously without external monitoring. Within the i3 Lab project “Smart Reactors”, researchers from process engineering and material science departments are working together to develop such reactor concepts.
The key to developing smart reactors is the application of state-of-the-art additive manufacturing and smart materials. Stimuli-responsive hydrogels are among those smart materials, which are able to change their macroscopic properties when triggered through a stimulus. By subjecting them to small changes in environmental conditions such as temperature, pH or the composition of the surrounding fluid, the gels are able to imbibe or expel liquid with several folds of its initial volume. This feature can be utilized to translate an inbound (bio)chemical signal of a reaction taking place and translate it into actuated process control.
An initial proof-of-concept is demonstrated here (Figure 1), where the reactor outlet of an exothermic emulsion polymerization is controlled by a temperature-responsive hydrogel. During regular operation, the hydrogel valve is triggered by the heat of reaction, thereby leading the flow towards the product outlet, whereas during an interruption of the reaction the product outlet is blocked, thereby forcing the reaction medium to leave through the other outlet. This way, the reactor is able to discern whether the polymerization is still progressing as required, thus securing product quality.
Apart from the valve-based design, other unconventional regulation tasks can be realized with additive manufacturing of stimuli-responsive hydrogels (Figure 2). Here, the gels are fabricated into the form of periodic open-celled structures (POCS) in order to control the fluid dynamics and mass transfer in gas-liquid reactors. By changing its bulk volume, the gel structures are able to control the bubble size of the gas flow, thereby influencing the solubilization of the gas into the liquid phase. This concept can potentially be applied to an exothermic gas-liquid reaction and a temperature-sensitive gel structure, which would enable an integrated negative feedback control loop of the reaction. Both examples have demonstrated the potential of applying smart materials in the process engineering discipline, which we will further pursue to develop reactor concepts, which are both smart in design and smart by design.
 X. Hu et al., “Smart reactors – Combining stimuli-responsive hydrogels and 3D printing,” Chem. Eng. J., p. 123413, Nov. 2019, doi: 10.1016/j.cej.2019.123413.
 X. Hu, C. Spille, M. Schlüter, and I. Smirnova, “Smart Structures—Additive Manufacturing of Stimuli-Responsive Hydrogels for Adaptive Packings,” Ind. Eng. Chem. Res., vol. 59, no. 43, pp. 19458–19464, Oct. 2020, doi: 10.1021/acs.iecr.0c03137.