Biodiesel Technologies
Falling Film Biodiesel Methanol Evaporative Separator

The production of biodiesel fuel requires the separation of methanol or ethanol - depending on which alcohol is used - from the post transesterification product. Often, large quantities of the alcohol is used, to drive the reactions, and therefore must be recovered for various reasons part economics and part generally responsibility. One process engineering approach proffered for the separation of the alcohol has been by Flash Equilibrium Distillation. While this separation process is effective and perhaps provides satisfactory performance for when and where ethanol is used as the preferred alcohol, the performance of such separation process may not be optimal for the use of methanol which is toxic.  The situation is such that while very small quantities of the alcohols is left after the flash distillation process, the residual methanol may still be significant enough to require additional processing.

An effective process equipment that proffers all the stated requirements for removing the final residuals of the methanol is Falling Film Methanol Evaporation Separator. Obviously, an efficacious separator for this situation - removal of small quantities of dissolved high volatility chemicals must provide large surface area for the evaporation to take place, allow heating the  biodiesel to leverage the temperature dependent volatility, and also ensure the prevention of the re-condensation of the methanol back into the biodiesel. The Falling Film Biodiesel Methanol Evaporation Separator provides large surface area of the biodiesel from which the methanol evaporation can occur, allows for heating by applying heat to the equipment wall as well as variable heating, allows for passing inert gas or air at the core to collect the methanol and therefore prevent the methanol from condensing back into the biodiesel as well as allows for both co-current and counter-current flow of the gas to provide the most efficacious operation.

The preferred design of such equipment is as follows: Pipe inside a pipe geometry is implemented, the biodiesel shall flow from the top down under gravity, non-reactive gas or air flows upwards through the inner pipe the inside wall of which provides the film wall, hot water or heated air flows through the annulus either downwards or upwards as the designer may prefer. This configuration just about accomplishes the object of the process equipment for the methanol separation. Preferably, the outlet of the inner pipe is connected to a Cooler Condenser which enables the recovery of the methanol or even ethanol in the gas passing through the core of the inner pipe. The alcohol is then recycled into the production process.

By this arrangement, as the biodiesel flows down the inner-wall of the inner pipe it is exposed to gas that is carrying the vapor of the alcohol towards the top when it enters the pipe and none further down the length of the separator. So further down where the gas is free of the alcohol and the incoming hot gas is hottest, the alcohol remnants receives enough heat of vaporization and gets evaporated readily into the gas. As the gas continues to flow upwards, more of the alcohol is collected and convected upwards. Because the even the feed biodiesel is heated immediately on entry into the separator, with the result that the alcohol even at that region is evaporating the alcohol vapors already in the core gas does not condense back into the biodiesel fluid as required. Effectively then for a fairly long separator the quantities of the methanol in the biodiesel fluid can be reduced to regulatory allowed amounts.

The length needed to support any degree of separation, of course, is also dependent on the inner-diameter of the inner-pipe that provides the support for the film, because the larger the diameter the more the mass transfer surface available for the evaporation of the methanol.

Although the presentation seem quite applicable, designing to support the formation of falling films is not so simple and depends on several factors including the "wettability" of the inner-wall of the inner pipe by the falling film fluid, the flowrate of the film fluid, and the thermophysical properties of the fluid. The region of flow is also very important, as the transition from laminar flow to turbulent flow can be very abrupt depending, again, on several factors. While laminar flow is much more orderly, turbulent flow causes radial good mixing and therefore higher rates of evaporation and hence shorter separator. However, that situation is also often accompanied by droplet knifing and entrainment by the gas.

However, the adoption of this separator for the removal of the last traces of methanol in a biodiesel fuel still provides an effective means of ensuring a good quality final product of the biodiesel process. Because ethanol is not as toxic as methanol, there must be a focused effort at separating the methanol more than there is focus on the removal of ethanol.

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