04 Fakultät Energie-, Verfahrens- und Biotechnik

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Open Access
Fouling during solution polymerization in continuously operated reactors
(2021) Zander, Christian; Nieken, Ulrich (Prof. Dr.-Ing.)
Specialty polymers are mostly produced in discontinuous processes in tank reactors due to the need for flexibility in the production of this product class. Milli-structured, continuously operated reactors are promising alternatives for process intensification to increase energy efficiency and space-time-yield, reduce time-to-market for new products and maintain flexibility. A major obstacle for the transfer of batch processes to such reactor systems is the formation of fouling deposits, which grow and block the reactor. To overcome this obstacle, knowledge of the mechanisms of the formation of fouling deposits is essential. In this thesis, fouling during the polymerization of N-Vinylpyrrolidone (NVP) in aqueous solution is studied both experimentally and in simulations to gain insight into the underlying mechanism, find a model-based description of this mechanism and make suggestions how to prevent or at least decrease the formation of fouling deposits. First, results from experiments in different kinds of tank and tubular reactors are presented. In all these reactor systems, fouling deposits are formed by an insoluble polymer gel, which adheres strongly to metal surfaces. Initially, the polymer gel is formed in regions with increased local residence time, e.g. in dead-water zones of static mixer elements, at baffles of tank reactors or at walls of tubular reactors without mixer elements. Once fouling deposits have been formed, they grow by reaction and lead to clogging of tubular reactors systems. Since a polymer gel is formed, side reactions that lead to high-molecular and branched polymer chains must play an important role for the formation of deposits. Kinetic models that are based on a recently suggested reaction mechanism and predict microstructural property distribution are presented and validated using continuously stirred tank reactor (CSTR) experiments. The results confirm the suggested reaction mechanism in which creation and propagation of terminal double bonds lead to branched or crosslinked polymer chains. Although gelation of the bulk phase does not occur, fouling deposits are formed at the baffles of the tank reactor and in other poorly mixed regions of the reactor. This observation emphasizes the importance of the flow pattern and diffusive mass transport for the formation of fouling deposits. To demonstrate the interplay of the flow pattern, the reaction and diffusive mass transport, simulations using a transient CFD solver including a reduced version of the reaction kinetics model together with a model for diffusive mass transport are presented. The mass transport model is able to describe diffusive transport of statistical moments and is, therefore, consistent with the reaction kinetics model. Simulations in different two-dimensional geometries confirm that regions with increased local residence time lead to the formation of polymer gels. These regions, e.g. regions close to reactor walls or dead-water zones, cause concentration gradients, which induce mass transport between such regions and the bulk phase. Due to their lower diffusion coefficients in comparison to low molecular species, polymer molecules accumulate in these regions, which increases the viscosity locally. Because of the viscosity gradients, the flow pattern is distorted and the size of regions with increased residence time expands. The combination of an increased residence time, high polymer and low monomer contents promotes the formation of polymer gels by side reactions. Together with the adhesion of macromolecules on metal surfaces, this seems to be the relevant mechanism for the formation of fouling deposits. Therefore, strategies to decrease fouling should focus on surface modifications, which reduce adhesion of macromolecules, as well as the elimination of dead-water zones and viscosity gradients.
Open Access
Modeling of the branching point distribution during the polymerization of N‐vinylpyrrolidone
(2022) Welzel, Stefan; Zander, Christian; Hungenberg, Klaus‐Dieter; Nieken, Ulrich
To gain insights into the microstructure of polyvinylpyrrolidone (PVP), a detailed reaction mechanism is developed, which characterizes the polymer along the property coordinate chain length, terminal double bonds (TDB), and branching points. For practical purposes, calculations with three property coordinates are unfeasible, and model reduction is needed. Here, a reduced model with only one single property coordinate without significant loss of accuracy is derived. In the first step, the coordinate TDBs are reduced by a linear relationship between TDBs and chain length. As the parameters of this relation are state dependent, they are dynamically adjusted from a parallel calculated 0D model. In a second step, the pseudodistribution approach is used to reduce the 2D distribution to chain length as the only property coordinate and calculate moments of branching points as a function of chain length. A 2D class model is set up for validation. To demonstrate the benefits of the model, the chain length distribution and moments of branching points are calculated for different average residence times and monomer concentrations in a stirred tank reactor. In a future publication, the model will be validated by experimental data in terms of chain length distribution and branching points.
Open Access
Wall layer formation in continuously operated tubular reactors for free‐radical polymerizations
(2023) Welzel, Stefan; Zander, Christian; Nieken, Ulrich
Polymer fouling is a major problem for the operation of continuous reactors. Therefore, it is important to understand and quantitatively describe the mechanisms leading to formation of fouling deposits. In this work, a CFD model for the radical polymerization of N-vinylpyrrolidone is presented, where the reaction kinetics, a viscosity model, and a transport model for polymer moments are determined from independent experiments. The model is compared to experimental obtained residence time distributions in capillary reactors over a wide range of concentrations. Model predictions are in good agreement with experimental findings.
Open Access
Modeling strategies for the propagation of terminal double bonds during the polymerization of N‐vinylpyrrolidone and experimental validation
(2020) Zander, Christian; Hungenberg, Klaus‐Dieter; Schall, Thomas; Schwede, Christian; Nieken, Ulrich
Based on a recently suggested reaction mechanism, which involves the production and propagation of terminal double bonds (TDBs), kinetic models for the polymerization of N‐vinylpyrrolidone in aqueous solution are developed. Two modeling strategies, the classes and the pseudodistribution approach, are applied to handle the multidimensional property distributions that result from this reaction mechanism and to get detailed structural property information, e.g., on the chain length distribution and the distribution of TDBs. The structural property information is then used to develop reduced models with significantly lower computational effort, which can be used for process design, on‐line applications or coupled to computational fluid dynamic simulations. To validate the derivations, the models are first compared against each other and finally to experimental results from a continuous stirred tank reactor. The evolution of monomer conversion and molecular weight average data as well as molecular weight distributions can be represented very well by the models that are derived in this article. These results support the correctness of the reaction mechanism predicted by quantum mechanical simulations.

04 Fakultät Energie-, Verfahrens- und Biotechnik

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