quote:

Originally posted by greiswig:
3. I'm wondering if I could have my cake and eat it, too: has anyone tried prewashing after the acid stage with glycerine from a non-water-wash batch, to suck out water, neutralize remaining FFA, etc.?

Well, I ran an acid/base/base reaction this weekend, and still had to reprocess. But it was a "near miss" after the second base stage.

The 200l of oil was actually pretty good to begin with: titrated at 8g/l KOH. Figuring a grand total of 19% methanol by volume, I used 35% of that in the acid stage, along with 190ml of acid. After 3 hours, the titration was down to 4.5.

I then added 5 gallons of byproduct from the first stage of a previous base/base reaction, and mixed for about 30 minutes. As mentioned, I was hoping that this would generate a clean separation, pull out water, and neutralize remaining acids.

After 3 hours of settling, it still wasn't very clear where the water/glycerine layer ended and the oil layer began, perhaps because the acid process turns the oil so dark. But I think there’s more to it than just that: I don’t think they separate nearly as easily as biodiesel and glycerine do. I don’t know whether electrostatic separation might help this or not, and I don’t know how much time people typically allow for settling. Next time, I’ll probably let it sit overnight and see if that gives better results, and I might be able to time it right to allow me to run the acid stage at a higher temperature: normally I run it a bit on the low side (start between 125-130F), because I know I’ll gain 10 degrees when I introduce the base-methanol mix.

One other thing that I tried is listed in the equipment forum, but I’ll mention it here: even after the base stages, it can be difficult to tell where the glycerine/biodiesel layer is, because they can be so similar in color. Reasoning that biodiesel is a good insulator, and that byproduct should be a reasonable conductor, I put two straight pins through my output tube, about 1mm apart. I attached these to an ohmmeter. Using this setup, it’s much easier to gauge where the transition layer starts, so I end up wasting less.

Anyway, after settling, I drained off 5 gallons of whatever had settled to the bottom. I mixed up what was left, and titrated: new titration was at 3.5. I then ran the remaining methanol and base as though it were a two-stage process. In spite of whatever methanol was lost in draining, I very nearly passed after the second stage. I reprocessed with 4l of methanol and 120g of KOH, and got a pass after an hour.

Yield was much better than I had achieved previously, which I think I attribute to not accidentally discarding oil or biodiesel because I couldn’t see the layer change until it was WAY past the transition.

So I’m making progress, but I am still not achieving the easy results that others seem to. I am referring to other people apparently needing to only run the acid stage for 3 hours, titrate, NOT drain, run a single base stage, and pass within a couple of hours. I don’t know what the difference is.