Order Counts the AAF Method
By Rick Da Tech
On the vegetableoildiesel.co.uk forum, they have come up with the AAF method. It basically is you reverse the process around. Add methoxide to the reactor first, then add hot oil slowly to the methoxide. It speeds up the reaction giving finished product faster than when the other method is used.
When using pump mixing and adding the methoxide to the oil through a venturi type arrangement as is done on most US design processors, Appleseed included, A maximum ratio is hit at about 20% methoxide. Greater than that and you get massive soap formation, Glop if your using NaOH. This of course does not apply to propeller mixed reactors with adequate mixing. With prop mixed reactors, you can typically dump all the methoxide into the hot oil at one time without effecting processing time or soap production.
Now we look at the twist. pump the hot oil into the reactor containing methoxide. If you go slowly, the oil added will react completely almost instantly in the presence of an extreme excess of both methanol and caustic. The key to adequate mixing when the methanol is added to the oil is to keep the local (like 1 cubic inch) ratio of methanol to glycerin to WVO to biodiesel the same as for the whole batch. Usually we have to fight the fallout of glycerin more and more as the reaction progresses. With the AAF method the glycerin does not fall out, rather it blends with the excess methanol, at least in the early phases. Then as the final portion of the oil is added, enough glycerin is produced to start the fallout. By this time the reaction is nearly complete.
Under the traditional process, water is created by the neutralization of FFA in a very fast reaction. It happens as soon as the methoxide comes into contact with the ffa. In this case the methanol and the hydroxide are dissolved into the WVO. When the water is released, it too is dissolved into the WVO. It stays dissolved in the oil acting as a catalyst for hydrolysis (soponification) until enough glycerin has been formed to attract it out preferentially out of the oil. In the reverse, the ffa is actually dissolved into the methanol when it comes in contact with the hydroxide. The water formed is released into the methanol and no longer available as a catalyst for hydrolysis. Also the soap is dissolved inside the methoxide. This is true until enough the glycerin starts to fall out. Hopefully all of the WVO has been added before this happens. Once the glycerin starts to overpower the methanol to clump up and fall out the reaction is nearing completion.
The overall result is that we can use less methanol and less caustic to reach the same level of conversion. The compromise balances we can reach using this process are near endless. What do you want to do? higher conversion, less reactants, faster conversion? Pick one or two and you can adjust the recipe and process to fit your needs.
The system was developed for a paddle mix reactor. It does not easily adapt to the pump mixed processors we see so much of in the US. The exception would be pump mixing with the return being a high pressure nozzle on the bottom of the tank. Traditionally we suck the mix off the bottom of the tank and dump it back in the top. Our circulation pump would need to be a diaphragm trash pump with wetted materials capable of withstanding methoxide and biodiesel. A little on the expensive side, but available.