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As I understand, the methanol vapor can heat to temperatures above the pure methanol boiling point because of liquid-liquid interaction between glycerol and methanol (both polar). In my methanol recovery rig I monitor vapor temp at the inlet to the condenser. I get very little liquid condensate until the vapor temp hits about 140 deg. F. Condensate flows pretty steadily as the vapor temp rises from 150 to about 170. The condensate rate drops significantly at 175-180 deg. F. I haven't tried running the temp any higher. The resulting still bottoms is glycerol that will not pour unless heated.
csp97 - You lucky dog! I wish I had access to something like Hysys! Could you run a Hysys simulation of a glycerol-methanol batch distillation and post the output to this forum? I'd like to see how the overhead (MeOH) flowrate and cumulative condensate volume relate to vapor temp. We could all use this info in picking a stopping point for methanol recovery. If you need some starting points, these might be convenient for most of us: Concentration: 60% w/w glycerol, 40% w/w MeOH Amount: 10 gals of mixture (about 15.38 kg methanol, 23.07 kg glycerol) Heat input: 1500 watts Thanks a million in advance, from all methanol moonshiners. Regards, brbd 1986 VW Golf 1.6NA 2003 VW Jetta TDI Wagon |
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No, there is a big difference between the pot temperature and the temp at the high spot of your distiller.
If you have 0.03% glycerol in your methanol vapour the temperture will still be close to 65 C. If you see the vapour temperature going up you are probably distilling water. |
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brbd
Here’s a run at the 60/40 wt% mixture you suggested. The temperatures are the temperatures of the liquid and the temperature of the vapor leaving the liquid. I think neutral may have a point that the temperature at the top of the still will be less. This may be due to heat losses through the still and overhead piping. You can’t predict a top/vapor temperature unless you know how much heat you’re losing. To make the percentages easy, the mixture is 100 pounds total, 60# of glycerol, 40# of methanol. The results are for pounds of material recovered as distillate as a function of temperature. Temp, F...Lbs MeOH...Lbs Glyc 153.6......0.00........0.00 159.3......0.00........0.00 168.5......1.14........0.00 174........10.89.......0.00 177........14.61.......0.00 181........18.49.......0.00 189........23.90.......0.00 202........29.13.......0.00 232........34.65.......0.04 280........37.74.......0.28 300........38.34.......0.53 325........38.84.......1.12 |
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I was curious as to the effect of water in the glycerol mixture. I see a lot of people recommending keeping the still temperature below x degrees or water will be distilled along with the methanol. I ran the simulation again with a mixture consisting of 60# of glycerol, 39# of methanol, and 1# of water. For the same temperatures as above, here are the pounds of material distilled as a function of temperature for the water containing mixture.
Temp, F....MeOH....H20....Glyc 153.6......0.77....0.17...0.00 159.3......1.20....0.77...0.00 168.5......4.07....0.89...0.00 174........9.16....0.94...0.00 177........13.97...0.96...0.00 181........18.73...0.98...0.00 189........24.18...0.98...0.00 202........29.03...0.99...0.01 232........33.97...0.99...0.05 280........36.82...0.99...0.28 300........37.42...1.00...0.54 325........37.90...1.00...1.14 The vapor/liquid equilibrium states that most of the water is distilled by 174 degrees when only 25% of the initial methanol is recovered. This is saying that you can’t make a good methanol/water separation with only 1 stage of separation, ie a pot still. If you are recovering methanol, you will also be recovering some water if any is present. An actual distillation column with multiple stages of separation (trays or packing) is required to make a clean split between methanol and water. |
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csp97
All good information in the tables and very useful to have handy. However you still seem to be not quite on the ball re temperature of the vapour. You say: "You can’t predict a top/vapor temperature unless you know how much heat you’re losing." This is not correct. There is no way that the temperature of the vapour at the top of the condenser can be anything other than close to methanol boiling point while methanol is the main material coming over. It can't be lower or it would condense on the walls and not rise up there. It can't be higher because there is no heater up there to supply heat to it. If you look at the table above you will see that you need to get to 230 F to get most of the methanol off. At this point there is almost nothing coming over except methanol. The temp must be 65 C at the top. Anyone wishing to distil methanol from a water-containing glycerine will need a fractionating column and would be well advised to have a thermometer at the corner at the top of the distillation head. Depending on how much heat is being put in you will see one of two things when most of the methanol is off. Either there will be not enough methanol rising to keep the top of the equipment hot, in which case the temperature will fall and the flow will completely stop, or, if there is enough heat input the water will start to come over and the temperature will have to rise to 100 C. You see the value of the thermometer - either way it tells you when to stop. You say "experience beats theory". Well that is both theory and my experience. |
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Ok, help me out here. My only experience, unfortunately, is with the 150,000 bpd crude and vacuum towers and the two 50,000 bpd naphtha fractionators I’ve got to screw with every day at work.
At the risk of beating a dead horse, I’m imagining a simple still with glycerol and methanol. It’s toward the end of recovery and the temp of the liquid is 230F. A tiny bubble containing 99.9% methanol / 0.1% glycerol forms near the heating element. The temperature of the vapor bubble is also 230F, it’s surrounded by 230F liquid. The bubble rises through the 230F liquid and breaks the surface. The vapor temperature at the surface is also 230F or very near it. Now at some point between the surface of the liquid and the top of the still the temperature of this vapor drops from 230F(110C) to 149F(65C). What is the cause of such a large decrease in temperature? Where does this heat go? Why is 65C, the dew point of pure methanol important here? The vapor contains 99.9% methanol and 0.1% glycerol, a vapor mixture which has a dew point of 230F(110C). |
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The answer is as follows:
There is only one source of heat for the upper parts of the equipment and that is the condensing methanol. The methanol condenses on the insides of the vessel and gives up its latent heat of vapourization. If you do it in glass you will see the methanol running back down the inside. If you start off with a cold system and apply heat you will see the methanol climbing further and further up the glass as the pot heats up. Eventually the methanol reaches the top and starts to distil and run down the condenser. At this point you have liquid methanol and gaseous methanol in contact with one another. In that situation the temperature is fixed at the boiling point of methanol. Why? Because the top of the vessel cannot get hotter than 65 C as its only source of heat is the condensing methanol. It cannot get colder than 65 C if the methanol is distilling because if it did more methanol would condense on it and heat it up. So there we have it. Regardless of the temperature of the liquid in the pot the temp at the neck is 65 C. The presence of that tiny amount of glycerol will have negligible effect. |
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Condensing methanol is a source of heat? The only source of heat to the system is the heating element in the bottom. I don’t guess we’re ever going to be on the same page here, but that’s cool. Thanks for the discussion.
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dOES ANYONE HAVE ANY INFORMATION ON THE BOILING POINTS OF METHANOL/GLYCERIN/WATER MIXTURES AND BOILING POINTS AT VARYING PRESSURES AND VACUUMS?
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I AM ALSO LOOKING FOR INFORMATION ON THE HEAT VALUES FOR THE MIXTURE OF METHANOL/GLYCERIN AND WATER. ANY INFORMATION OUT THERE?
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csp97 - Thanks a million for all your work. These Glycerol/MeOH/H2O numbers are very enlightening. Could you generate a table of dew point-bubble point data for this three component system? It looks like glycerol has a big effect on the MeOH volatility, even though glycerol is essentially non-volatile in our typical MeOH recovery temp range. Also, with DewPt-BubblePt info I can try a McCabe-Thiele analysis to see how many stages are needed to recover 99+% MeOH (assuming I can understand McCabe-Thiele after 20+ years). Thanks in advance, brbd 1986 VW Golf 1.6NA 2003 VW Jetta TDI Wagon |
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Hello brbd, the data you have is excellent. Have you tried to run a simulation with water, methanol and glycerin mixture of the following composition: 33% methanol, 38% water and the balance glycerin. I am using the M-T method to design a tower with a once through flow but require the v-l equilibrium data and the enthalpy data.
The final composition of the residue is 85% wt basis glycerin with th remaider being water and the composition of the distillate is all methanol (99.99% recovery) and the remaider of the water. The number of stages required is only two as the relative volatility of water and methanol with respect to glycerin is very high. Looking forward to your reply. |
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Hi folks
The glycerol byproduct also has a surprisingly high proportion of biodiesel incorporated within it, thanks to its attraction to methanol and glycerol. Try VERY GENTLY diluting 100ml glycerol byproduct in 1000 ml of water (1:10) and see what floats on top. Do this with as little agitation as possible, because the included soap+glycerol emulsifies the whole lot VERY easily. Add the byproduct to the water, ultra slow, and leave for a day or 2 for the components to disperse and separate. Rover 75 + Skoda Fabia on B100 http://www.graham-laming.com Bicycle on G100 12,000 miles p.a. ( http://tinyurl.com/krppyc ) |
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GL,
Should I do this before I recover the Methanol?..........Roy |
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Hi bigroy
Do this before you recover methanol, to see what your starting content of biodiesel is - I think it may enough to influence the simulation figures. Rover 75 + Skoda Fabia on B100 http://www.graham-laming.com Bicycle on G100 12,000 miles p.a. ( http://tinyurl.com/krppyc ) |
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Hi Graham:
have distilled a methanol, glycerol, water misture in equal mole ratios to determine the evaporation rate of methanol and water from glycerol and the amount of heat required? 33% m, 33% w, 34 % glycerol. AHM |
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Graham, thanks for posting this Gem on your home page, I was looking for this very thing!
So here are some related questions: 1. We make soap from our glycerol after methanol recovery. Not knowing any better, I have been taking the recovery stage to 212 F, figuring (incorrectly) that if we reached the BP of water, the methanol must be gone. According to the chart posted above, at 212 F, my methanol / glycerol solution might still be 15-20 % methanol. Is this correct? Next question becomes, is our soap unsafe due to the presence of toxic methanol? We run the circulating pump throughout the recovery phase, but do not have a venturi (yet). OK taking it further, our local regulatory agency is concerned about methanol in the glycerol when we go to do composting in the field. Yes it breaks down, but methanol is also considered an air and water pollutant in this state, and with reason the regulators do not allow composting of methanol contaminated waste. Is csp97 saying we need to reach 400+ F to remove all of the methanol? This will never happen in my shop, we feel plenty hot at 212 F and I can't imagine the time and energy that would go into higher temps. Thanks for your feedback, I hope others find this revived thread useful! Cheers, Farmer |
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Here's a question...
the figures in CSP97's theoretical chart do not seem to jive with my experience. I believe that my glycerol byproduct is less than 30% methanol to start, yet I get methanol production at pot temps well below 189F. Similarly, when we recover methanol from fuel (each batch) we get methanol production starting at about 160F pot temp, and I firmly believe our fuel is well below 5% methanol. Anyone (besides me) care to revive this topic? Thanks, farmer |
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