Published: October 12, 2011 r2011 American Chemical Society 8777 dx.doi.org/10.1021/ie201640w | Ind. Eng. Chem. Res. 2012, 51, 8777–8787 ARTICLE pubs.acs.org/IECR Biodiesel Process Intensification by Using Static Mixers Tubular Reactors E. Santacesaria,* ,† R. Turco, † M. Tortorelli, † V. Russo, † M. Di Serio, † and R. Tesser † † Department of Chemistry of the University of Naples “Federico II”, Complesso Universitario Monte S. Angelo, Via Cinthia 4, IT 80126 Naples, Italy ABSTRACT: Biodiesel is usually produced by reacting triglycerides, contained in vegetable oils, with methanol in the presence of KOH, NaOH, or related alkoxides as catalysts. In industry, the reaction is performed in stirred tank reactors and requires 12 h of reaction time being the reaction rate strongly affected by mass transfer limitation. We have recently shown, by using a Corrugated Plates Heat Exchanger Reactor, that a very high productivity (about 2 tons/day L) can be obtained by working at 60100 °C thanks to the presence of an intense local “micromixing”. Moreover, we have recently tested the performances obtained in a tubular reactor filled with stainless steel spheres of different diameters. By opportunely changing the spheres diameters it is possible to obtain microchannels in a range of 3001000 μm with an intense local micromixing. Again, thanks to micromixing we obtained very high productivities. However, in these last reactors the void portion of the reactor is low and the productivity per overall reactor volume is relatively low. It is possible to obtain better results, in terms of productivity, by filling the tubular reactor with stainless steel wool, being in this case the void fraction about 0.9. In the present work, some of the mentioned systems will be compared for their performances by using different amounts of KOH as catalyst (1 or 2% b.w. of oil). A dramatic change in biodiesel yield has been observed in all cases passing from 1 to 2% of catalyst independently of the reactants flow rate. These behaviors cannot be interpreted with the pseudomonophasic kinetic models, normally reported in the literature. At this purpose, for interpreting all the observed kinetic behaviors a new biphasic kinetic model, based on a reliable catalytic mechanism, has been developed. This model has been applied, first of all, to data reported in the literature related to runs performed in batch conditions with the scope of estimating the kinetic parameters, and then it has been applied to all the runs performed in continuous reactors with a satisfactory agreement. 1. INTRODUCTION Biodiesel is usually produced in industry by reacting vegetable oils with methanol in the presence of a homogeneous catalyst such as NaOH, KOH, or related alkoxides, at 60 °C (the methanol boiling point) and atmospheric pressure. The reagents are immiscible liquids and the interphase plays a fundamental role, as it will be seen in this paper. Many of the published works reporting kinetic data are related to batch reactors and are often affected by mass transfer limitation. 17 In a recently published paper and patent 8,9 we have shown that a very high productivity (2000 kg/day L) can be obtained by performing the transester- ification reaction in a continuous corrugated plates heat exchan- ger reactor (CP-HEX reactor) working in a range of temp- eratures between 60 and 100 °C. The high performances observed are due to a very active local micromixing, because, the conversion increases by increasing the overall liquid flow rate. Therefore, it derives that all the reactors favoring a local micro- mixing or more generally increasing the liquidliquid interfacial area can give high performances in this reaction. This observation is confirmed by many results, reported in the literature, in which the transesterification reaction is performed in the following: reactors containing a static mixer, microwave irradiated reactors, ultrasonic irradiated reactors, centrifugal contactors, rotating packed bed reactors, and jet flow stirred reactors. It is then possible to increase the liquidliquid interfacial area also by using microreactors. Microchannels of sizes less than 300 μm allow to improve very much mass, heat, and momentum transfer. 1012 The use of the different mentioned technologies and related performances have recently been reviewed by Qiu, Zhao, and Weatherley. 13 In other words, transesterification of triglycerides with methanol seems strongly promoted by the extension of the polar/apolar interface area. As the most com- monly used catalyst KOH reacts in methanol giving place to KOCH 3 according to the reaction K þ OH  þ CH 3 OH a K þ OCH 3  þ H 2 O ð1Þ The high interface area initially occurs to give the possibility to the anion CH 3 O  to react with a molecule of triglyceride so transferring the anion charge in the oil phase in the form of diglycerolate. 14 Higher is the initial interface area faster is the CH 3 O  disappearance and the formation in situ of the true catalyst, that is, a mixture of di- and monoglycerolate. 14 Equilib- rium (1) is clearly shifted to the right as a consequence of the methanolate disappearance. Then, the reaction rate abruptly declines, because of the ultimate formation of glycerolate anion. This anion migrates in the polar phase being insoluble in the oil phase and is poorly active in the reaction. According to us, this behavior is responsible also of a never explained experimental phenomenon observed in batch kinetic runs, that is, by using Special Issue: CAMURE 8 and ISMR 7 Received: July 28, 2011 Accepted: October 12, 2011 Revised: October 11, 2011