**6. Conclusions**

The design of a multitubular fixed bed reactor train for the production of acrylic acid involves simultaneous consideration of various influencing parameters, including shell side heat transfer, shell side pressure drop, tube-side hotspot formation, tube side pressure drop, catalyst interfacial gradients and selection of inlet conditions. For both the propylene oxidation reactor (R-101) and acrolein oxidation reactor (R-102), the heat transfer fluid flow rate affected the minimum temperature the most whilst the maximum temperature was affected by the heat transfer fluid inlet temperature the most. Some of the energy carried in the molten salt recycle was utilized to preheat the feed to reactor R-101 by including an inert packed zone of approximately 0.8 m at the front of the reactor. This eliminated the need for a separate preheating exchanger before the reactor. Allowing for some reduction of the molten salt temperature at the front end of reactor R-101 also limited the overall temperature rise in this unit for both the process and thermal fluid. Dilution of catalyst was investigated as a means of regulating the bed temperature in the acrolein oxidation reactor R-102. However, employing solid inert material up to 45% did not significantly reduce the temperature rise or peak temperature in the reactor. An alternative strategy of cold air injection at the inlet of the reactor was successful in mitigating hotspot formation and allowing for a gradual increase in process side temperature, albeit with a higher final temperature than in the case of catalyst bed dilution. The accurate representation of the process performance on the Aspen Plus® (AspenTech, Bedford, MA, USA) platform allowed for probing of alternative energy utilization strategies. Owing to the elimination of the fired preheater from the front end of the process, the reduction in energy was almost 7 MW. The use of the cold air injection in the second reactor reduced thermal fluid pumping power requirements by over 5 kW.

**Author Contributions:** Conceptualization, D.L.; methodology, K.C.P.; software, K.C.P.; validation, K.C.P. and D.L.; formal analysis, K.C.P.; investigation, K.C.P.; resources, K.C.P.; data curation, K.C.P.; writing—original draft preparation, K.C.P. and D.L.; writing—review and editing, D.L.; visualization, K.C.P.; supervision, D.L.; project administration, D.L.; funding acquisition, D.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work is based on research supported by the National Research Foundation of South Africa, unique grant number 120019. The authors also acknowledge financial support from ESKOM.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
