Hi-

I was wondering if someone might have some insight into our process design problem. We are hoping to implement an acid pretreatment step in our plant in order to accommodate more marginal feedstocks.

We will be looking to process four 500 gallon batches during the work day (8 hours). I would like to capture the additional methanol and acid for reuse. But, my initial tests show that even after 24 hrs settling, there is significant concentrations of methanol still suspended in the oil layer (+5% by volume). I am wondering if there is a more efficient way of separating methanol/acid that can be utilized in an industrial-scale process?

Or, can I assume that the methanol-laden oil is fit to be transesterified, and that any residual acid is contained in the methanol layer that has settled?

If anyone has experience with industrial-scale acid pretreatment, I would love to pick your brain. Any other insights would be appreciated!

Thanks,
Nick

Disclaimer: I have no experience with industrial scale biodiesel yet.

I guess what I'm really wondering is what you're trying to achieve. If you could remove the acid/methanol layer you'd still have water accumulating in it with no realistic way to remove the water from the acid/methanol. The water is produced by the esterifcation process - a mole of FFA's being esterified produces a mole of water.

I don't see the point to removing the methanol: the next step it to add methanol + base for transesterification. The only thing that seems to have a function is removing the acid for use with the next batch, but it's going to become more and more diluted with water.

I suppose I could conceive of a process where you removed the acid/water/methanol layer and distilled off the methanol, then the water to recover the acid, but I can't imagine it making sense in 500 gallon batches.

Have you considered esterification with glycerine? By using glycerine you can run the reaction hot enough to remove the water as it's produced. If you're running the batches quickly enough behind each other you may be able to use the by-product of a batch to neutralize the acid of the next batch, cutting down on both the acid needed to acidulate the byproduct and the base needed for transesterification.

Hi David-

Thanks for your thoughts. Our goal is to recycle the methanol/acid phase and to eliminate the need to neutralize residual sulfuric acid in the base-catalyzed stage.

The water formation is predictable, and can be managed simply. Short of fractionization or a mole sieve column, the easiest way to deal with water is dilution. Recycling the methanol/acid in this way will reduce costs significantly, with very little material investment.

I haven't looked into glycerolysis for our process. I should give it some thought. But, I know of no one who is utilizing this method on an industrial scale. Maybe there is no good reason for this, maybe there is. In any case, acid esterification is the task at hand, for the moment.

-Nick

ecofox,
while I have no experience in the industry side of BD making...I do have lots of homebrew experience in acid/base processing.

it seems you're concerned about the 5% left in the oil/BD mix after draining. on the couple of batches where I drained, I usually don't, this 5% is still available in the base stage.
the first time I drained I didn't use enough methanol during the base stage and need several stages to pass the 27/3 test. the next batch that I drained I added about 3.5 gallon( about 1/2 of the initial methanol qty) to the first base stage quantity( 8% of the initial oil volume). this 2nd batch passed 27/3 in 1 base stage.

so what does this mean.
1. for high level of titration 14+ KOH homebrew.. draining is needed
2. the amount to drain seems to be about 1/2 of the acid stage methanol. if you're following my method.
3. if the oil's inital titration requires draining add about amount drained from #2 in methanol back for the first base stage.

other notes
I have heard my method works using 2 acid stages on oil that titrated at 80KOH.
thanks rasza.

will this work for you?? I'd like it hear if it does..

my method
titration done using homebrew 0.1% KOH solution.
Starting titration - 3 = A
A * 0.2ml/L = mls of H2SO4 to used per liter of oil
use 12% of the oil volume to determine the methanol for the acid stage(s)
use 8% of the oil volume for the base stage.
if it reaches 3 titration, start the base stage. using 8 or more + titration for KOH. I like to 9g/L instead of 8g/L

if it doesn't reach 3, settle for 12 hrs, drain and retitrate. repeat an acid stage if necessary.

on both of my drained batches( 25KOH and 27KOH) i didn't need a 2nd acid stage.
at this point I'm not sure at what titration a 2nd acid stage would be required.


good luck

-dkenny

Here's a question I don't know the answer to:

Is the acid miscible in the feedstock?

If not, life becomes much easier.

quote:

Posted 31 October 2007 05:31 PM Hide Post
Hi David-

Thanks for your thoughts. Our goal is to recycle the methanol/acid phase and to eliminate the need to neutralize residual sulfuric acid in the base-catalyzed stage.

How much acid are you using? What level of FFA's are you looking to process?

My guess is that the acid would be far more miscible in the water/methanol layer, so most of it would be in that layer, rather than the esterified feedstock.

quote:

The water formation is predictable, and can be managed simply. Short of fractionization or a mole sieve column, the easiest way to deal with water is dilution. Recycling the methanol/acid in this way will reduce costs significantly, with very little material investment.

I'm afraid I'm not following you with this one. Do you plan to dilute the water in more methanol, and, presumably, acid? Unless there's a way to leave a significant portion of the water in the feedstock it's just going to keep accumulating in the methanol/acid/water layer

quote:

I haven't looked into glycerolysis for our process. I should give it some thought. But, I know of no one who is utilizing this method on an industrial scale. Maybe there is no good reason for this, maybe there is. In any case, acid esterification is the task at hand, for the moment.

Glycerolosis IS esterification, of course. You're just attaching the FFA's to a different alcohol, just not the one you want it to end up with.

Hopefully I can let you know how it works with 100 gallon batches in a few weeks. In the meantime, good luck with methanol esterification



-Nick[/quote]

quote:

Here's a question I don't know the answer to:

Is the acid miscible in the feedstock?

That is precisely what I would like to know. As I posted over at BiodieselNow, I was under advisement that potassium sulphate created as a neutralization product is undesirable. This came from a local university professor that we are working with. I didn't think to ask further, but I will now. In general, it seems that reusing the acid catalyst would be more efficient than creating additional byproducts. It is also more cost effective- with reductions in both acid and base catalyst used. So, determining whether acid rises/drops with the methanol phase would be important.

We are using 5% sulfuric acid by weight of FFA. The oil we currently process has around 2-4% FFA, but we are looking at sources in the 6-12% range.

quote:

I'm afraid I'm not following you with this one. Do you plan to dilute the water in more methanol, and, presumably, acid?

That's correct. Surprisingly it won't take much dilution to keep water at an acceptable level.

Perhaps you could start a new thread with your glycerolysis experience. I would be interested to hear the results.

Thanks,
Nick

I was just looking up some older posts by neutral that suggest that 1/3 of the excess methanol remains in the biodiesel and the rest in the glycerin layer since the glycerin is polar(?) and the biodiesel is not.

That raises the question of how that relates to the acid pre-treatment.

Since there is little or no glycerin where does the methanol end up? In the remaining feedstock, the esters or both?

What are the proportions between the feedstock/esters and the methanol/water/acid layer?

quote:

Since there is little or no glycerin where does the methanol end up? In the remaining feedstock, the esters or both?

I would presume most of the methanol ends up with the glycerine because it's fully miscible in it.

I don't know for sure about the acid, but there's no question the water and methanol (miscible in each other) will mix with the oil to any significant degree. My guess is the acid will stay with the water and methanol.

quote:

What are the proportions between the feedstock/esters and the methanol/water/acid layer?

The original poster was talking about 10% FFA's, so we're looking at 90% oil. If it fully esterifies we have just under 90% oil, just under 10% esters, and a few percent of (excess) methanol and water.

--- David

quote:

Originally posted by David Miller:

quote:

What are the proportions between the feedstock/esters and the methanol/water/acid layer?

The original poster was talking about 10% FFA's, so we're looking at 90% oil. If it fully esterifies we have just under 90% oil, just under 10% esters, and a few percent of (excess) methanol and water.

--- David

What I meant was is the excess methanol distributed between the ester and feedstock or is it just in the ester?

quote:

What I meant was is the excess methanol distributed between the ester and feedstock or is it just in the ester?

If it were solely suspended in the esters, and not the triglycerides, we would be looking at a concentration of >100% methanol in esters. I don't think this is the case, however. So, methanol must be suspended in both esters and triglycerides.

The question is, what is the saturation point? And where do we see the excess acid catalyst?

-Nick