A CFD Model for a Fixed Bed Reactor for Fischer Tropsch Reaction using Ansys

Chitranshi, Vidushi

A CFD Model for a Fixed Bed Reactor for Fischer Tropsch Reaction using Ansys

Thesis / Dissertation

2023

Abstract

Fischer-Tropsch (FT) is a process which can convert synthesis gas derived from natural gas, coal or even biomass to a variety of products including saturated and unsaturated hydrocarbon chains, while keeping the emission of greenhouse gases minimum. Among various types of reactors used for commercial FT, fixed bed tubular reactors are among the most common type of reactor. However, there is a big challenge faced by these tubular reactors. FT is a highly exothermic process and therefore, heat removal in these reactors is needed to be highly efficient to avoid a thermal runaway. To improve the heat transfer in any reactor, it is necessary to estimate the heat production correctly. Therefore, the kinetics in the FT system needs to correctly represent the heat transfer behaviour in the system. This requires an effective description of the reaction kinetics. FT is a polymerisation reaction, so the rate expressions must be able to retain the chain reaction behaviour. This is not possible with a lumped approach model which is used by most researchers in literature. Therefore, a partial equilibrium approach was employed, where thermodynamic and kinetic models were coupled, and the reaction rates depended on the concentration of reactants as well as products. The kinetic model employed in the current project was taken directly from the work of Davies and Moller. The abovementioned partial equilibrium kinetics was used to develop a CFD model for the FT reaction system. This model was reproduced using COCO simulator for a plug flow reactor for same operating conditions as Ansys to compare the results from both the softwares. The results showed a close agreement and hence, assured that the CFD model could be used for further testing. The other challenge with the FT in FBRs is the heat dissipation. To avoid the thermal runaway, some innovations in reactor design have been studied in literature. In terms of heat transfer capabilities, when shell and tube heat exchangers are compared with the plate and frame heat exchangers, various sources in literature claim that the latter is found to be more effective. However, plate type reactors have not yet been explored in detail for their heat transfer capabilities. Taking an idea from this, the CFD model developed for the tubular reactors was adapted for plate type reactors. The heat transfer capabilities of the plate type reactors were compared relative to the tubular reactors. The tube reactor and plate type reactor were compared on the basis of two criteria. One criterion was based on physical similarity between the reactors. It included having equal Reynolds Number and equal surface area available per unit volume for both type of reactors. For a plate with plate spacing t and a tube with diameter D, the latter condition resulted in the expression, D = 2t. The factors that Reynolds number for a packed bed depends on were all same for both the geometries, so by default, the Reynolds Number was identical for both cases. The other criterion was based on catalyst packing. It included having equal tube-to-particle diameter ratio for both geometries. For tube reactors, diameter is an important parameter that determines the heat dissipation behaviour, so a parametric study was carried out to study the effect of a diameter and plate spacing on heat transfer behaviour for the same set of operating conditions. It was found that the plate type reactor had a hotspot temperature which was less than the hotspot temperature of corresponding tube reactor at all plate spacings. This indicated that the heat dissipation in a plate type reactor is better than in the corresponding tube reactor. Since the tube reactors observed higher temperatures than corresponding plate reactors, the CO conversion observed in the tube reactors was higher. When the product distributions for the two geometries were compared at isothermal conditions, the results almost overlapped for the two geometries. But when they were compared for non- isothermal conditions, significant differences were observed. This showed that heat dissipation mechanisms in the system had a huge role in bringing out different performances for the two geometries. Effect of temperature and conversion on the product distribution were also studied. On the basis of tube to particle diameter ratio criterion, tube reactor was found to outperform the plate reactors in terms of temperature control when compared using the tube-to-particle diameter ratio. Therefore, the superiority of one reactor over the other was dependent on the criterion they are being compared for. The plate type reactor was then represented in PFR model by tuning the heat transfer coefficient of the tubular model in COCO. The difference between the CO conversions achieved between the plate type reactor in Ansys and the representative model in COCO was found to be very little. Hence, the plate type reactor representation could be successfully achieved in COCO. There can be a lot of further research that can be done using the current model. The areas of reaction kinetics and reactor design were highlighted in this regard. The current model can be extended to a larger number of species, for a better representation of the FT product spectrum. Formation of liquids was completely neglected in the current project. It can be taken into account as presence of liquid can affect the FT reactor system by imposing internal and external mass transfer limitations to the reactions. The current model can also be used to study the HTFT process and also to study the isomeric products in the LTFT which were assumed to be not present in the current project. In the areas of reactor design, the geometry of catalyst particles can be included in the reactor geometry. This model can also be used for plates of other shapes and sizes to study the effect of shape and size on heat transfer capabilities. Different types of corrugated plates are used in the Plate and Frame Heat exchangers nowadays. The corrugations increase the surface area available and also increase mixing. Using the current model, such modifications can be studied for their effect on the reactions in a reactive system.

Keywords

Engineering

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