Nitre and scale with 8% sap

[Brent]

Now that the new RO is getting set up and I've got a second syrup pan so I can clean one while boiling in the other ... I start dreaming about nitre build up.

Is it pretty proportional to time spend boiling or directly related to syrup produced. Will I get the same amount of nitre in 6 hours boiling even though with the RO I should make about 3 - 4 times as much syrup ???

[Thompson's Tree Farm]

Brent,
I have only one year experience with the RO and so I am no expert but my experience is this: I made 4x as much syrup per hour and probably got 4x as much sugar sand and build up per hour. I'd say the sand/nitre is in the product and is related to the amount of syrup produced, not the time boiling. I'm sure there are others with more experience who can comment on this.

[KenWP]

I would say it has to remain the same as some niter has to be removed with the water in the RO. You pass a certain amount of sugar through so a certain amount of niter has to be going thru also.

[Russell Lampron]

My experience has been that the nitre tends to stay suspended in the syrup and I can make alot more syrup before the pan gets dirty. When the evaporator is only boiling for about 3 hours at a time instead of 12 for the same amount of raw sap the nitre doesn't have time to burn on to the bottom of the pan.

Anything that has a larger molecule than water stays in the concentrated sap when you run it through an RO. Most of it ends up in the filter press.

[DrTimPerkins]

I made 4x as much syrup per hour and probably got 4x as much sugar sand and build up per hour. I'd say the sand/nitre is in the product and is related to the amount of syrup produced, not the time boiling.

This is correct. The sugar sand/niter come from dissolved material in the sap. Very little is removed by the RO. The formation of niter is related to the concentration of the liquid during the evaporation process. So you basically get the same amount of niter whether you use an RO or not. It may appear earlier in the process (further back in the evaporator system) when using RO though.

What often happens when people move to an RO is that they also downsize their evaporator, so the appearence is that niter builds up much faster. Basically you just have much less surface area for the niter-scale to build up on, so the niter problem seems worse if anything.

Tim P.
UVM PMRC

[Brent]

OK so do we understand yet if the nitre that sticks to the pans actually comes out of solution at the pan / liquid interface and bind to the pan or does it come out of solution in suspension and bind to the pan when it comes into contact with the surface?

This leads me back to a question I could not find anything on .... would running your solutions through a filter while boiling reduce the deposits on the pan surface. Some members here say they think it helps if they filter when they shut down at night.

[DrTimPerkins]

OK so do we understand yet if the nitre that sticks to the pans actually comes out of solution at the pan / liquid interface and bind to the pan or does it come out of solution in suspension and bind to the pan when it comes into contact with the surface?

Both processes happen simultaneously. In some cases it will stick to pan surfaces (forming niter scale). It can also bind to other particles forming sugar sand.


This leads me back to a question I could not find anything on .... would running your solutions through a filter while boiling reduce the deposits on the pan surface. Some members here say they think it helps if they filter when they shut down at night

This might help a little bit to cut down the amount of sugar sand, but probably not a huge amount. Another thing that helps is that there ceases to be any concentration going on when you shut down the evaporator, and some niter scale will redissolve into the partially boiled liquid overnight.

[Brent]

Thanks for the comments guys. It's going to be an interesting start up to the new season

[sapman]

When visiting a large sugarmaker this fall, they were running 20~% sap through. They make so much, that I believe he said they need to clean the pans with acid after every boil, including the flue pan.

Tim

[DrTimPerkins]

When visiting a large sugarmaker this fall, they were running 20~% sap through. They make so much, that I believe he said they need to clean the pans with acid after every boil, including the flue pan.

Tim

This practice is becoming more common among the larger producers.

[caseyssugarshack93]

Was that mike parker Sapman?

[sapman]

You got it, Nate. Very impressive setup! His RO is really something.

Tim

[caseyssugarshack93]

You got that right, the 3200 springtech with a 2 membrane upgrade i think he is pocessing like 4,000gph on that rig, As for niter, I noticed alot of niter build up with me boiling 18-20% in my 2x6 I think this year im going to try and clean the pans more offen to get the grade up some if it gets too bad, Only issue is that the wse pans are a pain to revese the flow but o well should get be by this year but not so sure next season

[lew]

As far as cleaning the pan each time, I think I do things a little different than most (not all). I have a 5x10 all flue rig that feeds a 3x7 flat pan. It is all fed 8% sap in the beginning. The flue pan feeds the 3x7 26 Brix sap and we finish continuously. When I fire up the 3x7 (oil fired) I stand by the pan waiting for the first bubbles to start appearing in the last half of the pan where the niter builds up. When the almost syrup first starts to boil, it will snap and pop and most of the niter will come free from the bottom of the pan. I have an old scoop with a fine brass mesh for a bottom. I keep scooping through the almost finished syrup until I get no more niter. I then take a large dowel with a stainless steel scrubby on the end and scrub the bottom of the pan to remove any things i missed. Works pretty good. Not 100%, but saves on the number of times that I have to take the pan off to clean. It's not an easy job to take that size pan off by yourself.

[Beweller]

I don't suppose anyone is going to want to try this, but a technique that can reduce scale deposits in boilers is recycling scale.

The basis of the technique is that the recycled scale serves as nucleation sites for the developing scale, preventing the concentration build-up at the heat exchange surface. The larger scale particles are relatively easy to remove from the liquid (compared to the very small particles that result from spontaneous nucleation). (The scouring action of the recycled scale particles may also be involved.)

The composition of niter deposits I have seen always seem to contain "sugar". This sugar, which must be partially charred, may be the bonding agent that causes the scale to adhere to the pan. This suggests the following possible sequence of events.

When a bubble nucleates and grows, the resulting bare spot on the pan will quickly rise in temperature. The thinner the metal (and probably the lower the thermal diffusivity) the faster and higher the temperature will rise. When the bubble breaks away from the surface, liquid rushes in, quenching the hot spot. The liquid is monentarily exposed to a higher than average temperature, leading to charring.

If this mechanism is correct, there should be a difference in the niter deposits and syrup color that correlates with pan metal type and thickness,
although the effect of thickness might be weak and require large changes to be observable. It also suggests that steam heated evaporators should have smaller niter deposits and make lighter syrup.

This mechanism might also have a part in producing the effects of air injection. I do not recall discussions of the effect of air injection on niter build up.

[DrTimPerkins]

This mechanism might also have a part in producing the effects of air injection. I do not recall discussions of the effect of air injection on niter build up.

Although some producers will swear that air injection reduces niter (scale) build up on pans, the only scientific study of this including extensive measurements of scale thickness was done at UVM Proctor Maple Res Center. in paired evaporators boiling from the same sap or concentrate source, we found that the amount of scale (niter adhered to pans) was identical. There was similarly not any less loose sugar sand to be filtered from the syrup.

The reason for this is pretty straight-forward. Precipitation of niter from solution occurs largely due to concentration. If you concentrate to a certain level, the salts (primarily calcium malate) must come out of solution. It just doesn't matter whether there is air injection or not. Air injection doesn't affect the concentration or the precipitation. What it does affect however is the appearance of the scale. With air injection you are making very light syrup. As a consequence, you make very light colored niter. With dark syrup you make the same amount of niter, but it appears very dark in color, and to our eyes, appears thicker. The color doesn't fool the "thickness" gauge though.

[Jeff E]

Thanks guys for 'diving in'.

I appreciate the depth of understanding of the processes you discuss. It helps in our planning and scheming!

I noticed significant niter increase (even in the flue pan) when I ran my sap into the evaporator at 12+%. I resolved to not go over 10% as the evaporator (a 3x12 Leader American) boiled much more cosistantly at this percentage, making better syrup with consistant drawoffs.

I have a better set up for reversing the flow under fire this year, so I hope to keep the niter problem in check this spring.

[Haynes Forest Products]

Thank you Dr Perkins for elaberating on what Beweller was asking/proposing NOW...............Does the niter come out of suspension when it hits other niter particels floating in the boiling sap? Or when it hits niter that has bonded to the Pans. Does it act like sugar in high consentrations that start to bond to other single crystals and then build from there. Like rock candy in the bottom of the class container it seams as though the crystals dont float around until they hit another one they build from a crystal that is already bonded to the glass wall..............YIKES Im confused.

[Brent]

BeWeller

I think I have seen what you're talking about. It seem that when the pan somehow sheds some of the nitre and the flakes are floating around, like a cup of them in the last chamber of the syrup pan, the nitre does not seem to re-form so fast. No sceince, just seems to be what was happening. Going to have to watch that more this year.

[Brent]

Lew you point out another thing that I have seen. I would guess that as the pans cool at the end of a boil, some moisture penetrates the layer of nitre. When you heat it, if you can heat it fast, that moisture would hit a boil and make a bubble of steam under the scale and pop it off ..... I think.

This is kind of an admission that in seasons past I did not clean every night.

I also saw this when trying to clean the pans this summer. I had been leaving them soaking in various "cleaner" solutions. When I blasted them with my weed killing flame thrower, lots of the scale just popped off.

[Haynes Forest Products]

Because of the rate of expansion and contraction the scale is not able to expand and contract like the metal pan. The scale/niter is formed when the pan is hot and when it cools down it contracts/shrinks and the scale doesnt and it pops off. Not all the scale pops off only the real thick stuff. I THINK!!!

[DrTimPerkins]

Does the niter come out of suspension when it hits other niter particels floating in the boiling sap? Or when it hits niter that has bonded to the Pans. Does it act like sugar in high consentrations that start to bond to other single crystals and then build from there.

The process of scale formation is quite complicated and not terribly well understood in maple systems. The partially boiled sap/syrup becomes supersaturated with mineral "salts" at some point, and these salts will nucleate and deposit onto any surface, particularly heated surfaces, forming scale. Some scale niter will break off to form "sugar sand" particles, and some deposition can also occur to these particles. In addition, sugar can become entrained (captured) in the scale.