DESIGN OF A CONTINUOUSLY STIRRED REACTOR WITH A HELICAL RIBBON IMPELLER USING CFD MODELLING FOR NON-NEWTONIAN FLUID MIXING

Abstract

The anaerobic digestion process for waste treatment consists of two main hydrolysis and fermentation stages. However, the main challenge for this kind of process is designing a reactor with optimum operation conditions to obtain a high conversion with low energy consumption. Black Jumper Ltd is designing the innovative Black Jumper waste treatment process with a novel method to produce biomethane and bioethanol using an organic fraction of municipal solid waste (OFMSW) as raw material. This work aims to design a continuously stirred reactor for the hydrolysis stage using a helical ribbon as an impeller. The main target is to achieve fluid uniformity inside the reactor with a low power requirement for the mixing process. CFD modelling is used to investigate the behaviour of the non-Newtonian fluids with 36% solids content inside the reactor. The main variables are the impeller geometry and location, impeller rotational speed, and feed rate. The reactor was designed based on what is recommended in the literature then optimised based on a sensitivity analysis outcome. The study concludes that, the fluid has assertive non-Newtonian behaviour and to limit its impact, the impeller rotational speed must be over 500 rpm. For the initial design, the fluid becomes uniform at 700 rpm rotational speed, and the power requirement is 32 kW/m3. Moreover, having anchor blade in the bottom of the impeller is useful to overcome the issue of mixing the fluid in the bottom of the tank. Therefore, the fluid uniformity could be reach at lower rotational speed. The impeller diameter has a huge impact on the mixing performance and the energy requirement. Having a larger impeller diameter (>30% of the tank diameter), reduces the energy requirement. Furthermore, more experiments are required for the missing data for the physical properties and the impact of the enzyme load. Also, the enzyme load needs to be modelled as it affects the mixing performance and the power consumption.