Experimental and numerical investigation of a static mixer for the production of biodiesel

Abstract

Currently, unless aided by subsidies or some form of government directive, biodiesel cannot compete against regular petro diesel because of lower pump costs. Attempting to lower production costs of biodiesel would be helpful in narrowing the price gap between these two competing fuels. The production of biodiesel involves mixing organic oil and a short chain alcohol which are not miscible in the early stages of reaction. The interfacial area between the two phases is believed to strongly affect the reaction rate leading to the development of numerous methods for the dispersion of short chain alcohol in organic oil. One such method is through the use of a static mixer which is a motionless inline pipe mixer with specially shaped baffles used to promote mixing. This study investigated the effectiveness of a Kenics static mixer as a dispersion tool in biodiesel production with methanol as the short chain alcohol and palm oil as the organic oil. This was performed by measuring the droplet sizes of methanol dispersed in palm oil for different flow parameters, namely the mixer length and flow rate. The moderately high concentration of methanol (20% volume fraction) presented an additional level of complexity in the experimental investigation and necessitated the development of a suitable droplet measurement technique which consisted of a modified borescope attached to a digital camera and coupled with a strobe light source. Experimental results showed that the static mixer was certainly an effective mixer, outperforming the conventional agitated vessel design seen in most biodiesel production plants but was not as effective as ultrasonic emulsifiers although this method requires the highest energy input. The experimental study was complemented by a parallel numerical investigation which was performed using the commercial computational fluid dynamics (CFD) code ANSYS-FLUENT. To account for the droplet size evolution, the CFD simulations were coupled to a population balance model with the appropriate droplet breakage and coalescence models. There was reasonable agreement between experimental results and numerical simulations. In addition, numerical simulations predicted that the static mixer was the most efficient mixer when accounting both droplet size reduction and energy requirements. The findings of this study suggest that the Kenics static mixer is an efficient mixer. It is, therefore, worth investigating the use of the Kenics static mixer as a continuous reactor for the production of biodiesel without any rotating parts and possibly at room temperature. This would present a novel design which could be attractive in achieving quality product at low operating cost.