I'm neutralizing glyc for some time now, and burning this upper layer after separation in classic oil burner. I would like to make this process of neutralization automated because it is sometimes difficult to match exact amount of acid. For neutralization I’m using sulfuric acid 98% concentrated.
If it is possible to measure Ph of glyc than it is easy to add sulfuric acid with dosing pump until Ph lowers to Ph7. I was thinking about using one circulation pump for circulating glyc inside one tank. Just before the pump (on the pump intake) I would use Ph probe to determine Ph. That information would go into the dosing pump which can add sulfuric acid unlit Ph reaches 7. That adding can be vary small and accurate so it would be more difficult to overdose glyc. At that point when Ph is 7 pump stops and separation has to occur.
What do you think about this process, has anyone tried something like that?
Would this process be easier if methanol is previously removed?
Is some other acid more suitable for this job?
This is quite possible. It's simply a pH control loop much like a temperature control loop. Although because of the pKa values of the soaps involved I would suggest going to pH of 6 instead of 7. You will get more top layer that way and the glycerin will have less soaps in it. I completely break the glycerin back to FFA and my glycerin is typically between 5.6 and 6.5 pH.
For a loop like this a PID controller would be the easiest to work with once you get it tuned to your mixing etc. A simple on/off controller is going to give you a big overshoot no matter how well mixed your fluid is.
As for ease of use... sulphuric acid is fine if that is what you have. Using a strong acid that completely disassociates like hydrochloric acid might be easier to tune the PID with but other than that there isn't really a difference. Use what is easier to obtain/cheaper.
If you are talking about breaking the soaps in the biodiesel glycerol by product then you are not trying to "neutralize" using acid. What you are trying to do is acidulate so that the soaps convert back to Fatty Acids.
If you only go to pH7 or even pH6 you are not likely to crack all of the soaps. We normally go down below pH5, although after separating off the fatty acid layer we then neitralize with a little caustic..
With regards to pH control loops, yes you can pH control the glycerine byproduct but pH loops are not as simple as tempereatre control as depending upon where abouts you are on the titration curve and where you are aiming to be you can end up with a big overshoot with only a very small amount of acid. For example you can start adding acid and very little happens the more and more acid you add you seem to get very little response so you start adding more acid then all of a sudden you get a huge jump, that is because you have reached the turning point in the titration curve. I guess all I am tryig to say is be careful when programming you PID loop controller as it is really easy to get a big overshoot.
Good description. Only thing I would add is that if you want to avoid the big 'dip' in pH once all the soaps have been neutralized/cracked just don't go all the way. Pick a pH slightly above it and control the pH there, just make sure the PID is tuned well enough that it doesn't drop past it at all.
Thank for your answers. I've already started to build machine. After some thinking I’ve decided to use oil pump from car engine for circulating glyc in side tank. This pump has a bit lower flow but is undestroyable. Do I need to heat glycerin first and if yes what should be the temperature of glyc before I start acid dosing?
I will dose acid faster at the beginning and when Ph reaches some lower point dosing will be slower.
How long will take glycerin to separate after I stop pump and dosing?
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