Analysis of Fluid Flow and Heat Transfer inside a Batch Reactor for Hydrothermal Carbonization Process of a Biomass

Abstract

This work analyzes the heat transfer and fluid flow within a batch reactor for hydrothermal carbonization (HTC) of raw olive pomace (ROP). The autoclave is partially filled with a mixture of ROP and distilled water and hence it is considered as a dispersed medium. The reactor is heated through its lateral surface, whereas the bottom wall and the upper surface of the mixture are thermally insulated. Under the effect of heat and pressure, the fluid moves inside the reactor, while particles are subject to other forces. Additionally, the biomass (ROP) is decomposed into very fine particles to produce a solid product (hydrochar). COMSOL Multiphysics software is used for the analysis of heat transfer and fluid dynamics. Chemical kinetics of the reactions are modeled by a basic kinetics model. Numerical results are validated using experimental data carried out in similar operating conditions. They are in good agreement since the deviation between them does not exceed 6%. Isotherms, velocity fields, and isobars are evaluated within the reactor as well as velocity and distribution of particles. These amounts are influenced by the imposed heat flux at the lateral wall (q0). Also, it has been shown that the temperature and pressure values reached are above those required by the HTC process and, consequently, a HTC reactor could be designed with optimal operating conditions.

Research Needs and Pathways to Advance Hydrothermal Carbonization Technology

“…The application of CFD to HTC has thus far been quite limited. Only four relevant works have been published [54,[64][65][66] since 2020, each with interesting findings but also significant simplifications. Another recent work [67] neglected instead HTC reactions, and discussed how natural convection affects the inner behavior of a hydrothermal reactor depending on the employed heating scheme.…”

Computational Modeling Approaches of Hydrothermal Carbonization: A Critical Review

“…Most recently, Chater et al [43] conducted a computational study to analyze the CFD behavior of raw olive pomace during HTC and build on the results obtained from the ANSYS simulation made by Mendecka et al [40]. They instead use COMSOL to investigate the heat transfer and fluid-dynamics, coupled with a simplified kinetics model and experimental results.…”

“…Their study further emphasizes the variability in feedstock that causes difficulties in proposing accurate numerical models, suggesting that a good approach to improve the work on this area could be to create more computational heat transfer models that are designed for many different types of biomass, both lignocellulosic and non-lignocellulosic. The scope of the primary numerical models which were previously discussed are designed for cellulose [39], olive pomace [40,43], pinewood particles [12], and avocado stones [35]. This leaves much room to develop computational models for the HTC of various biomass sources, processing wastes, and agriculture residues.…”

“…In terms of the particle transport behavior of the solid biomass phase during the heat-up phase, the COMSOL model developed by Chater et al [43] demonstrated that the solid phase was randomly dispersed throughout the reactor during this phase (600 s). After this, the particles gravitated to the top of the reactor due to their low bulk density, where they experienced much lower velocities.…”

Mixing Due to Natural Convection in a Hydrothermal Reactor: A CFD Study

“…The biomass is included as a dispersed continuous phase, but the gas phase and evaporation phenomena are not taken into account. Chater and colleagues 7 also used COMSOL to replicate the HTC of raw olive pomace in a batch unstirred reactor. In their model, particles were considered as discrete, spherical entities that do not collide, and the gas phase was disregarded.…”