Seeing as it’s an excellent question, I figured I’d share the main differences I’m aware of when using these catalysts.
Both catalysts can be used to produce high-quality Biodiesel because they both do roughly the same thing; react with the triglycerides to break them apart so that the methanol can bond with the fatty acids and make Biodiesel. But, there are some differences….
Reactivity With Oil:
For the most part, both NaOH & KOH will react with the oil the same way. However, NaOH is typically a purer chemical than KOH and as such, I’ve noticed over the years that it tends to pack a little more “punch” to the reaction. Not to say that KOH doesn’t react well, it does. It’s just that in my experience, the NaOH seems to do it slightly better (assuming you can get it to dissolve..which leads to our next point).
Dissolving In Methanol:
Sodium Hydroxide (NaOH) is a royal pain in the butt to get it to dissolve in Methanol. It just LOVES to keep on hanging around…in solid form. Potassium Hydroxide (KOH) however, will begin dissolving almost on contact with methanol and dissolves extremely fast by comparison.
To give you an example, I can dissolve about a pound of KOH in under 15 minutes in 5 gallons of methanol. NaOH? Oh….maybe 30-40 minutes later a portion of that pound of NaOH will still be hanging out, just slowly taking it’s time to dissolve. It’s for this reason that most people tend to use Potassium Hydroxide over Sodium Hydroxide. The chances of KOH dissolving quickly are greatly increased which means they can get that batch of Biodiesel going quicker.
That said, I have noticed that the KOH tends to like to leave a residue in methoxide tanks more often than NaOH does. I don’t know why, it just does. Might have something to do with the fact that we have to add so much more of it to get the same effect as Sodium Hydroxide…which is discussed our next point.
Amounts Required For Biodiesel Production:
I’m not going to pretend to be a chemist, but suffice it to say, to make Biodiesel, you’ll always need more KOH in grams per batch than NaOH. Some have told me that there’s a long chemical explanation as to why this is necessary. Densities, molar ratios, stuff like that. Whatever the reason, it will always require more KOH than NaOH to make a given batch of biodiesel. That’s just the way it is.
How much more? Well, here’s an example:
For 50 gallons of oil with a 5% free fatty acid content, it’ll likely require
about 2175 grams of NaOH. However, it will take roughly 3030 grams of KOH for the same oil. It’ll dissolve faster, but you use more of it.
Cost Of Catalyst:
On average, KOH costs more than NaOH. For example, our 55 lb bag of KOH runs $137.50. That’s $2.50/lb. Our 50 lb bag of NaOH runs $70, or $1.27/lb. So, not only do you use more KOH when you make Biodiesel, but it costs more too. It’s the premium we pay for faster dissolving times.
Effects On Glycerin:
One of the biggest differences of using NaOH vs. KOH can be seen in the glycerin layer. Glycerin from NaOH reacted oil tends to be much thicker and can even solidify into a thick gooey mess. Glycerin from KOH reacted Biodiesel on the other hand is nice & liquid; even when it’s allowed to cool down to extremely low temperatures (I’ve seen Biodiesel glycerin from KOH reacted fuel still liquid at well below 32 Deg. F.)
Effects On Soap Making:
When making soap from Biodiesel glycerin, it’s typically much easier to make bar soap from NaOH-based glycerin (because it likes to harden naturally). However, if liquid or gel soap is what you’re after, then KOH-based glycerin is much easier to use.
KOH based glycerin CAN be made into soap bars and NaOH based glycerin CAN ALSO be made into liquid soap, but it does take more effort to get the desired result.
Effects On Methanol At Ambient Temperatures:
KOH will cause methanol to boil much more violently at hotter temperatures than NaOH. I’ve seen KOH added to methanol that was over 100 Deg. F and it boiled like crazy (which can be potentially dangerous). However, NaOH doesn’t boil nearly as violently when added to methanol at hot temperatures (probably because it takes so much longer to dissolve…or at least it FEELS that way when you’re using it).
So, a good rule of thumb is that if the ambient temps are above 80-90 Deg. F where you’re making Biodiesel, NaOH may be the better choice if boiling methanol is an issue with your equipment. If it’s much cooler though, KOH may be the way to go as it dissolves so much faster in methanol.
So which one do I recommend?
Well, that depends on what you’re after. If you’re planning to make bar soap out of your glycerin, or if you don’t mind waiting for it to dissolve, or if you’re on a tight budget, then go with NaOH.
If, however, you need a faster dissolving catalyst, plan on making liquid soap out of the glycerin, or the budget isn’t nearly as tight, go with KOH.
I’ve also heard (but never seen proven) that KOH based glycerin is a little easier to compost or get rid of because it’s supposedly not as toxic as NaOH based glycerin. That’s only what I’ve heard though, so consult a chemist or someone with more understanding of the toxicity of one versus the other before making that kind of decision.
I personally like to use KOH, even when it’s warmer. I just like the fact that it dissolves faster. Yeah, I know I’ll use more and I know it costs more, but for me, I’d rather be sure that it all got dissolved and that I was able to make a great batch of Biodiesel rather than piddling around waiting for NaOH to dissolve. I also live in Utah and when it gets cold around here, KOH based glycerin is MUCH easier to handle.