What factors contribute to high yield?

[gwalker]

In a given quantity of VO to be reacted with KOH, what causes better or worse yields?

eg, Does adding more catalyst optimise the yield and conversely, does being a bit short reduce the yield?

The same goes for Methanol.

Will 25% give a better result than 22%?

Will 18% produced more glycerol and less biodiesel than 20%?

If you want to err on the side of getting maximum yield:

would 25% Methonal and a couple of extra grams per litre of KOH (ie base + titration +2) be a good policy?

George

[tillyfromparadise]

Hello George
In a single stage method the main things that affect yield are titration and water content.
It has been my experience that high conversion results in lower yield and low conversion will maximise yield.

[gwalker]

Hello George
In a single stage method the main things that affect yield are titration and water content.
It has been my experience that high conversion results in lower yield and low conversion will maximise yield.

Tilly,
I must be confused with the term 'high conversion'. What I thought it meant was: more biodiesel and less glycerine.
George

[tillyfromparadise]

Hello george

You can think of conversion as what percent of the oil has been turned into Methyl esters.
Yield is how much fuel you have from each litre of oil reacted, this includes methyl esters as well as mono di and tri glycerides and whatever else might be in the fuel.
Less glycerine would = lower conversion.

[marki]

Hello george

You can think of conversion as what percent of the oil has been turned into Biodiesel.
Yield is how much fuel you have from each litre of oil reacted..
Less glycerine would = lower conversion.

I think you're getting a bit mixed up there Tilly. Your definitions of conversion and yield are the same......

[TroyH]

yeah a little confusing there, however I think what Tilly is getting at is that you can get a high conversion of oil without it all being converted to product (i.e. making lots of soap). I'm not so sure about the whole high conversion = low yield thing. You would need higher conversion to have higher yield, unless you had less soap made.

However, in a true catalytic reaction, the yield is not affected, only the rate. This is because they affect the forward and reverse reactions equally, so equilibrium is not altered.
In our case, where there are alternate reactions going on, that use the catalyst as an actual reagent, then things get a bit more complicated. So more catalyst should only affect yield due to the amount of soap formed, no actual direct affect on the transesterification.

[gwalker]

So more catalyst should only affect yield due to the amount of soap formed, no actual direct affect on the transesterification.

So, are you saying that once there is enough catalyst, you don't need or want any more?

What about Methanol? Does more Methanol (eg 25%) convert more VO to BD producing less Glycerine?
George

[tillyfromparadise]

Hello Marki

I think you're getting a bit mixed up there Tilly. Your definitions of conversion and yield are the same......

Conversion is what percent of your oil is converted into Biodiesel (methyl esters) or probably more correctly what percent of the fuel you are testing is methyl esters.
Some of the oil will not be converted into methyl esters but will still be present and considered part of the yield.
I have changed the word biodiesel to Methyl esters in my post above as I can see that it could be confusing.

[tillyfromparadise]

Hello Troy

yeah a little confusing there, however I think what Tilly is getting at is that you can get a high conversion of oil without it all being converted to product (i.e. making lots of soap).

Conversion is how much of your fuel is methyl esters.
Yield is how much fuel you have at the end of the reaction. The fuel will consist of methyl esters and mono, di and Triglycerides + other assorted things.
Conversion and yield are different.

I'm not so sure about the whole high conversion = low yield thing. You would need higher conversion to have higher yield, unless you had less soap made.
However, in a true catalytic reaction, the yield is not affected, only the rate. This is because they affect the forward and reverse reactions equally, so equilibrium is not altered.
In our case, where there are alternate reactions going on, that use the catalyst as an actual reagent, then things get a bit more complicated. So more catalyst should only affect yield due to the amount of soap formed, no actual direct affect on the transesterification.

Exactly, if you add enough NaOH/KOH to drive the reaction to very high conversion you make extra soap and reduce yield.
Most people either wash or settle the soap out of the biodiesel so is usually not considered a part of the yield

[tillyfromparadise]

Hello George
Yes, if you use excess Catalyst to what is required for very high conversion you will make more soap.
Within reason, increasing methanol or NaOH/KOH will increase conversion. One test I performed suggested that increasing NaoH/KOH will produce more soap and so lower yield than increasing methanol does. But I would need to do more testing to be sure.

So, are you saying that once there is enough catalyst, you don't need or want any more?
What about Methanol? Does more Methanol (eg 25%) convert more VO to BD producing less Glycerine?
George

Typically we call the byproduct layer glycerine. It mainly consists of glycerine, soap, Methanol and NaOH/KOH
Higher conversion will produce more actual glycerine.
The titration has an effect on how much soap is produced, and there is a side reaction that is also producing soap. The more methanol used in the reaction the more that will end up in the "byproduct layer"
Adding excess NaOH to what is required will make extra soap and lower yield, but it does not lower conversion.