(297a) A Continuous Biodiesel Production Using Calcium Methoxide with Microwave Irradiation

Introduction

With the growing price of fuel and the increasing popular desire for renewable and non-petroleum fuels, alternative liquid fuels such as vegetable oil biodiesel are gaining a great deal of attention.[1]  Commercial biodiesel is produced from the transesterification of vegetable oils with methanol in the presence of a base catalyst.  The majority of biodiesel producers currently use a homogeneous base catalyst (e.g. NaOH).  In spite of their popularity, homogeneous catalysts have numerous undesirable characteristics such as the addition of water to the fuel (and therefore potential soap formation), difficult downstream separation, and difficult recycling of the catalyst back into the reaction. 

Because of the problems associated with homogeneous catalysts, heterogeneous catalysts are a promising alternative, as they have none of the above mentioned processing difficulties and can be used with comparable performance.  In addition, a solid catalyst could be easily incorporated into a continuous reactor design, which would be economically desirable due to the increased production rate.  This investigation solely uses calcium methoxide which is desirable due to its superior basicity, low cost, and its ability to readily form stable suspensions [2].  There has also been increased interest in the use of microwave heating instead of the conventional heating methods.  Previous work has shown that microwave reactors are very effective in heating catalytic organic reactions, such as transesterification [3].  Recently, work has been presented that demonstrates the successful use of microwaves in heating the biodiesel reaction with homogeneous base catalysts such as KOH and NaOH [4].

Materials and Methods

The reactor setup is as shown in Figure 1:  The premixed reactants and catalysts are pumped via a peristaltic pump into a 2 L continuously stirred round bottom flask.  This vessel is then heated via microwave radiation at a maximum of 1200 W, during which the transesterification will occur.  Because of the pressure in the vessel, the reacted biodiesel will exit through a tube out of the reactor into a collection vessel. 

Figure 1: The current reactor set up involves the premixed reaction mixture (6:1 molar ratio of methanol to oil, and 2 wt% calcium methoxide, entering the reactor and being heated using microwave radiation.  The reacted biodiesel is then pumped up the center pipe to a collection vessel.

            The average residence time for the CSTR with a fixed reaction volume is determined only by the flowrate from the pump.  This pump used is an electric Masterflex^® L/S peristaltic pump from Cole-Parmer Model # 7569-00.  It is fitted with an L/S 15 drive head with translucent yellow Tygon^® 4040 Fuel-Grade tubing.  This tubing was chosen because it is advertised as being inert to most fuels, oils, and lubricants.  This pump is capable of operating from 1 to 100 rpm, 1.7 mL/min to 170 mL/min respectively. 

            The microwave used here is a MARS 5 (Microwave Accelerated Reactor System) by CEM corporation.  This microwave utilizes and in situ fiber optic temperature probe which is used to monitor the reaction vessel temperature.  The temperature of the reaction is then kept at the desired setpoint, which for this investigation was kept at 60°C.

Results and Discussion

            It was found that the optimum reactant preparation method is as follows:  First, 2 wt% (with respect to the oil) calcium methoxide is mixed in the methanol using a traditional stir bar at ~1000 rpm for 30 minutes.  This mixture is then mixed in an ultra-sonic bath for another 30 minutes.  This was found to effectively break up any visible catalyst clusters that may have formed.  This can then be continued to be mixed with a stir bar until ready for use.  When the reactor is ready, the oil can then be added and allowed to mix thoroughly with the methanol/catalyst suspension.

            Due to the small inner diameter of tubing used, the peristaltic pump is easily clogged by even small undispersed catalyst clusters.  In addition, these clusters hide the catalysts effective surface area, decreasing the overall activity of the 2 wt% catalyst.  Because of this, proper and complete mixing of the catalyst is imperative.  The well mixed reactants can then be pumped into the microwave reactor, which has been prefilled with reactant and preheated to 60°C.

            The product biodiesel is then analyzed using a novel HPLC ELSD (High Pressure Liquid Chromatograph with an Evaporative Light Scattering Detector).  Preliminary results have shown that with a 10 minute average residence time, there is limited (<10%) conversion of the soybean oil to biodiesel.  This is assumed to be mainly a cause of the constant dilution of the products with the reactants that is inherent in any single-pass continuously-stirred tank reactor.  Further work has been done with a total product recycle.  This configuration recycled the product stream back through the reactor a total of five times.  Preliminary analysis shows that the conversion continued to increase in each cycle.

References

[1] Ragauskas AJ, et al, (2006) Science (Washington, DC, United States) 311: 484

[2] S. Gryglewicz. Rapeseed oil methyl esters preparation using heterogeneous catalysts. Bioresource Technology 1999, 70, 249-253

[3] R. J. J. Jachuck, D. K. Selvaraja and R. S. Varma. Process intensification: oxidation of benzyl alcohol using a continuous isothermal reactor under microwave irradiation. Green Chem. 2006, 8, 29?33

[4] Leadbeater NE, Stencel LM (2006) Energy & Fuels 20: 2281