(1b) Characterization of Specialized Mixing Elements Applied in a Novel Single-Screw Extrusion Reactor Designed to Process and Enhance Mixing in Highly Viscous Reactive Systems.

Current flow processes generally maintain dilute reagent conditions to avoid fouling and blockages in the reactor. Consequently, there is potential to advance towards more sustainable flow processes by reducing or even fully eliminating the amount of solvent used in these processes. Nevertheless, decreasing the solvent content from reactive systems often increases its viscosity, resulting in reduced control over the molar flow rates, and heat- and mass-transfer limitations inside the reactor [1].

Extruders, like single screw extruders (SSE) and twin screw extruders (TSE), are highly advantageous for the transportation and mixing of high viscosity and solid materials. However, implementing liquid/liquid and liquid/solid chemical synthesis in extrusion reactors is still challenging, as it often results in the formation of byproducts and lower reaction kinetics compared to reactions in solution due to a reduced mixing efficiency [2].

In literature, Mixing Elements (ME's) were previously applied in polymer chemistry to enhance polymer blending and processability inside extruders by inducing shear stresses and elongational flow [3]. These literature ME’s are roughly divided into two categories based on their geometry: Pin mixers (Normal and Pineapple) and Slotted Flight Mixers (Dulmage and Saxton). Figure 1 showcases examples of each category. However, experimental micromixing characterization of these ME’s remains scarce in literature with the exception of the research performed by Rożeń et al. [4], [5], [6]. Furthermore, this micromixing characterization was exclusively performed in aqueous media. Hence, there is no unambiguously conclusion on which ME-geometry provides the most optimal macro- and micromixing performance at high viscosities.

In conclusion, solvent-reduced synthesis in extrusion processes faces challenges related to mixing due to the high viscosity of the reaction mixture. Therefore, these challenges will be tackled in this research by investigating macro- and micromixing properties of four distinct mixing elements applied in a specialized single-screw extrusion (SSE) reactor platform. The SSE-reactor platform, visualized in Figure 2, was designed and manufactured in-house. Thus, the primary goal of this research is to determine the most optimal ME-geometry to enhance mixing performance in high viscosity media. In addition, experimental characterization techniques to quantify the macro- and micromixing performance in extruders will be adapted for applications in high viscosity media.

Methodology

The macro- and micromixing behavior in the SSE platform equipped with ME’s is experimentally characterized at several constant viscosities ranging between 1 mPa.s to 100 Pa.s. Mixtures with viscosities up to 1 Pa.s will be obtained using solutions of glycerol in water, while mixtures with viscosities ranging between 1 Pa.s and 100 Pa.s will be obtained using solutions of polyvinylalcohol in water. Furthermore, ME-geometry (Pin, Pineapple, Dulmage, Saxton) and screw configuration (T-ME-T, ME-ME-T, ME-T-ME with T = transport element and ME = mixing element, Figure 3) and screw rotational speed (0 to 400 RPM) are studied.

Macromixing refers to the mixing that occurs at macro scales which affects the distribution of reactants and heat within the reactor [7]. Residence time distribution (RTD) is applied to quantify macromixing, more specifically axial dispersion. A step signal of an inert tracer or salt is injected into the feed of the SSE-ME platform. The concentration of salt is measured at the exit of the reactor by an inline conductometer. The macromixing efficiency can be quantified using the mean residence time (), variance () and the dispersion number (D ) where a larger value of the dispersion number represents a larger amount of axial dispersion.

On the other hand, micromixing occurs at the molecular level, where reactants come into contact and interact to initiate chemical reactions influencing the kinetics, selectivity and yield of the reaction [7]. The well-known Villermaux-Dushman protocol will be employed to characterize micromixing. This technique employs two competing reactions: a neutralization reaction (R1) and a redox reaction (R2), both of which consume acid-protons ( ) [8]. First of all, the protocol is upscaled to higher viscosities in batch and benchmarked against the literature reported (batch) protocols with glycerol and sucrose. Next, the upscaled protocol is applied in the SSE-ME platform, equipped with an inline UV-VIS spectrophotometer where the effect of the aforementioned parameters on the micromixing efficiency is studied.

Results and Discussion

Firstly, the effect of the screw rotational speed and feed flowrate on the axial dispersion or macromixing performance was studied (Figure 4). When no rotational speed is applied, the mixing elements behave as static mixers and the transport elements provide no additional forward axial movement of the fluid. When increasing the rotational speed from 0 to 100RPM, there is a decrease in axial dispersion due to the forward axial movement of the fluid imposed by the rotation of the helical transport screw. However, when a rotational speed between 100 and 400 RPM is applied to the mixing screw, the axial dispersion increases when increasing the rotational speed, indicating the occurrence of backmixing. Furthermore, increasing the viscosity of the feed results in increased axial dispersion and backmixing at the same rotational speed and feed flowrate. Lastly, applying higher feed flowrates reduces the amount of backmixing and induces more plug-flow behavior for all viscosities. These results suggested that the distance between the transport elements and the barrel had to be decreased to prevent fluid moving backwards in the reactor thus decreasing backmixing. Further experiments are being conducted to determine the most optimal ME-type and ME-configuration for macromixing.

In addition, the Villermaux-Dushman protocol was successfully upscaled to a viscosity of 1 Pa.s in a batch reactor. Batch micromixing results indicate that reactor volume, injection time and stirrer speed are important factors influencing micromixing. It was concluded that higher stirring speed are required, for the 1 Pa.s mixtures compared to 1 mPa.s, to achieve the same degree of micromixing. In the succeeding experiments, the protocol will be scaled up further to a viscosity of 10 Pa.s. Once the protocol is fully optimized, it will be applied to study micromixing in the SSE-ME platform.

Conclusion

Macromixing was studied in the SSE-ME platform at different viscosities (0.001 to 1 Pa.s), rotational speeds and feed flowrates. The results suggested design improvements to reduce backmixing in the reactor. Upcoming experiments will involve studying the macromixing performance of other ME-types and ME-configurations.

Furthermore, the Villermaux-Dushman protocol was successfully performed in batch at 0.001 Pa.s and 1 Pa.s. The protocol will be scaled up further to 10 Pa.s before micromixing tests in the SSE-ME platform will be executed.

References

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[6] A. Rozeń, R. A. Bakker, and J. Bałdyga, “Effect of Operating Parameters and Screw Geometry on Micromixing in a Co-Rotating Twin-Screw Extruder,” Chemical Engineering Research and Design, vol. 79, no. 8, pp. 938–942, Nov. 2001, doi: 10.1205/02638760152721163.

[7] S. Gobert, S. Kuhn, L. Braeken, and L. Thomassen, “Characterization of Milli- and Microflow Reactors: Mixing Efficiency and Residence Time Distribution,” Org Process Res Dev, vol. 21, Feb. 2017, doi: 10.1021/acs.oprd.6b00359.

[8] J. M. Commenge and L. Falk, “Villermaux–Dushman protocol for experimental characterization of micromixers,” Chemical Engineering and Processing: Process Intensification, vol. 50, no. 10, pp. 979–990, Oct. 2011, doi: 10.1016/J.CEP.2011.06.006.