Philipp Pelz

• Fouling in heat exchangers is a major challenge in many industrial processes due to increased energy costs, environmental pollution and shortened cleaning cycles. One strategy to avoid the growth of deposits is the use of alternative materials such as polyether ether ketone (PEEK) instead of the conventionally used stainless steel (SS). In this thesis, the fouling behavior of PEEK-based heat exchangers is therefore compared qualitatively and quantitatively with SS heat exchangers. The behavior of crystallization (calcium sulfate) and organic (whey protein concentrate, WPC) fouling are considered. The experimental studies show that PEEK has a lower susceptibility to fouling and better cleanability compared to SS under various process conditions, making it a promising material for preventing fouling. By modeling the fouling behavior of WPC, a deeper understanding of the process was achieved and predictions were made for the fouling behavior of non-tested operating conditions. In calcium sulfate solutions, PEEK exhibited asymptotic fouling behavior, with a fouling factor half that of SS, while talc-filled PEEK (TKT) had higher fouling rates due to increased surface roughness. Cleaning experiments conirmed the advantage of PEEK over SS, as deposits could be removed more quickly from the surface compared to SS. PEEK also showed a significant reduction in the effects of whey protein deposits, with up to a 40 percent reduction of heat resistance compared to stainless steel. The cause of the reduction in thermal resistance was the removal of the deposit layer caused by evaporation, so that the thermal resistance remained limited to PEEK surfaces and longer operating times were possible. Scale-up experiments confirmed that these results are scalable and showed consistent behavior. A mechanistic and an empirical model were developed to predict the deposition behavior. Both models were able to predict the experimental data well. The mechanistic model took into account protein denaturation and boiling effects and was able to extrapolate from the data better than the empirical model, even though significant deviations were observed during validation. The empirical model showed higher accuracy in predicting behavior under untested operating conditions within the limits of the experimental data compared to the mechanistic model. Symbolic regression further correlated the model parameters with operating conditions. These results show that PEEK heat exchangers reduce the costs associated with fouling and improve sustainability, making them a viable alternative to conventional materials in energy-intensive processes. The investigations show that PEEK is a competitive alternative to conventional stainless steels and can contribute to optimize processes.