Sciencemadness Discussion Board - Sulfuric acid distillation: heating routes?

Sulfuric acid distillation: heating routes?

Funkerman23 - 3-7-2017 at 00:14

Hello!

Most of us have seen Nurdrage's video on distilling it, and there was a thread here on quartz distilling flasks that could do this with the right flame.

What I want to know is; for those of you who have distilled Sulfuric acid to its azeotrope, what was your preferred heating method?

Did a sand bath& burner work well or would a High temp mantle from
glas-col be better? Is the Quartz really needed or did you feel safe with Borosilicate?
I am asking for more of a consensus and opinions thread than anything else.
Thanks, in anycase and good day.

Sulaiman - 3-7-2017 at 05:03

I have not distilled sulfuric acid yet but I have concentrated it by boiling, almost the same as a simple distillation.
I did it in a cheap borosilicate beaker ... that developed three tiny radial cracks around the bottom where the base meets the walls,
I think that it was caused by uneven heating on my hotplate,
so I suggest a sand or hot air bath or rbf/mantle combination.

You need to take care that no cold condensate drips onto hot glass
so everything from the flask to the still head should be thermally insulated, I believe.

I have not even titrated the (presumed) concentrated acid yet so take my words with caution

The drain cleaner as purchased boiled at c325 ^oC and I stopped the boiling when the b.p. reached 331 ^o due to choking clouds of acid vapour

I suspect that obtaining the azeotropic concentration by simple distillation will be difficult to achieve,
and fine fractional distillation would be difficult to do due to heat losses,
96 - 97 % seems straight forward but I bet that 98% is not easy.

P.S. use an air condenser rather than a Leibig etc. as I believe that the temperature difference between the inner and outer walls causes enough differential thermal expansion along the tubing length that it may crack the glass, although YouTube videos often show such an arrangement used.

[Edited on 3-7-2017 by Sulaiman]

JJay - 3-7-2017 at 05:40

There are several videos on YouTube of people distilling sulfuric acid in borosilicate glass with a heating mantle. I did see one video of someone doing it in borosilicate glass over direct flame, but that's probably not a good idea. You can heat quartz with a direct propane flame (like a Bunsen burner or a camping stove), but direct heating can be dangerous with borosilicate glass, which has a much higher coefficient of expansion and a maximum working temperature of around 550 C. I don't see why sulfuric acid couldn't be distilled in borosilicate over a wire gauze, although care should be taken to avoid hot spots and overheating (and obviously this should be performed only outside or in a fume hood).

The major drawback of sand baths is the amount time it takes to heat them up safely, not to mention the relatively low rate of heat transfer; blasting one with maximum heat from the start is likely to break a flask. I think mixing in some copper or iron pieces might greatly improve heat transfer.

It goes without saying that distilling sulfuric acid is very dangerous. I think the risk is lower in quartz, and it's much easier to just distill over a flame if you're doing your distillation out of doors. But it can be done in borosilicate glass with a heating mantle.

[Edited on 3-7-2017 by JJay]

LD5050 - 3-7-2017 at 20:13

I used to concentrate drain cleaner all the time by simple distillation with cheap eBay borosilicate glass. For a heating mantle I would make one frome the heating wire from an old stove I had taken apart. I would mold plaster of Paris around a RBF and then wrap the wire around the soft wet plaster and then I would dry it in the oven and connect it up to a power source and it would work great. I have distilled H2SO4 around 5-10 times now without ever any issue other than cracking one of my condensors because I made the stupid mistake of running cold water through it while distilling the H2SO4.

I was thinking tho what about distilling it under vacuum? It should reduce the bp considerably but idk it seems like a recipe for disaster if the RBF ever imploded.

My suggestion would be to find a good enough heat source take a cheap RBF and distill outside incase of equipment failure. As long as you are at a safe distance if anything ever happened you should be ok and the only damage would be a broken flask.

JJay - 4-7-2017 at 01:27

I guess I should mention that I'm planning on building a furnace for heating my crucibles and small flasks, and it might be used for sulfuric acid. I considered constructing a heating mantle, but I think it's easier to build a furnace.... I'm going to make a mica cylinder that will with a small gap wrap around a 500 mL flask. I'll coil a nichrome heating element around the cylinder, regulating power by a triac dimmer (could additionally use a temperature probe and an Arduino, but I don't think I'll need anything like that right away). I still haven't decided exactly how I'm going to insulate it; the obvious options are rockwool and refractory brick.

The maximum working temperature will be only around 800 C, but that should be sufficient for most things and is certainly hot enough to distill sulfuric acid. A quartz or fired porcelain cylinder would be able to attain even higher temperatures.

LD5050 - 4-7-2017 at 06:10

Jay I'm curious about this furnace you are talking about. How do you figure it would be easier to build than a heating mantle? I'm not questioning you I'm just asking because maybe I would be interested in building one myself. Do you have any reference pics I could look at to see what you mean? Also is this something that could be used with borosilicate?

JJay - 4-7-2017 at 06:30

I'm going to be posting pictures of it after I build it... I already bought most of the materials I need for it, and they are sitting in a storage locker across town. The rest I can get at a home improvement or specialty insulation store. I think the design could be used with borosilicate as long as care is taken not to let it get too hot. I selected this design mainly because I have tried building heating mantles, and I always had a terrible time wrapping the wire around the base of the flask.

I'm planning on using the furnace to dehydrate sodium bisulfate to sodium pyrosulfate and then crack it to produce oleum. But I think it would work fine for distilling sulfuric acid.

The main problem with vacuum distilling sulfuric acid would be getting sulfuric acid vapor in your vacuum pump oil; you'll definitely want to protect your pump if you try it.

Dlavroht - 13-7-2017 at 04:20

Quote: Originally posted by LD5050

I have distilled H2SO4 around 5-10 times now without ever any issue other than cracking one of my condensors because I made the stupid mistake of running cold water through it while distilling the H2SO4.

Can you explain a little more about how you cracked your condenser? I thought the whole point of one was to bring gases down to a liquid state with a colder substance, so how could they crack by using cold water? What was it made out of? I'm asking because I plan on making a distiller for H2SO4 soon, and I don't want to make mistakes that other people have made.

[Edited on 13-7-2017 by Dlavroht]

ave369 - 13-7-2017 at 10:34

Borosilicate round-bottom flask covered in asbestos cord to prevent refluxing on walls, camping gas heater and a wide berth while the apparatus is on: this is my choice. Usually, a flask cracks after two to four such distillations, but it does not suddenly crack while the flame is on and burst acid everywhere: rather, the apparatus finishes its work, cools, I disassemble it and find a tiny crack in the flask.

NEMO-Chemistry - 14-7-2017 at 05:29

I used a pyrex glass cooking pan filled with some sand and stuck a normal round bottomed flask in that. i used 2 liebeg (200mm) condensers joined together and no water for cooling.

Bum twitched all the way through it! That was on a large gas camping hob.
part of me thinks maybe a vacuum to lower the temp would help, but then you have the whole vac risk to worry about.........

[Edited on 14-7-2017 by NEMO-Chemistry]

NeonPulse - 17-7-2017 at 22:59

Quote: Originally posted by LD5050

Jay I'm curious about this furnace you are talking about. How do you figure it would be easier to build than a heating mantle? I'm not questioning you I'm just asking because maybe I would be interested in building one myself. Do you have any reference pics I could look at to see what you mean? Also is this something that could be used with borosilicate?

Here's one similar which I had just finished building. The thought never crossed my mind that I could distill sulfuric with it..... I do have one of them replacement mantle sleeves which I want to dope up with hotter wire which will do it.

JJay - 18-7-2017 at 03:09

That furnace looks awesome! I started my first attempt at one by wrapping nichrome around a mica core, but mine isn't going to look as nice and I don't think it will be very durable. I am planning on putting rockwool around it, but I definitely like your approach. What did you use to fasten your heating element to the sides of the furnace?

I'm not too sure about its application in this furnace, but I do think that the mica foil I bought rolls up into nice tubes, so I think it could be used to make a very cheap and simple tube furnace that can reach 800 C. Of course, that doesn't help much for sulfuric acid....

tsathoggua1 - 18-7-2017 at 09:42

I'm not surprised your posterior was twitching, situated on a propane burner

that would make most people jump.

NeonPulse - 18-7-2017 at 21:49

I cut a groove in the firebricks for the element and used small bits of 1mm kanthal wire bent into little staples. They seem to have worked ok but I'm yet to run this furnace to its potential. I still want to better insulate the outside of the firebricks with some ceramic kaowool. After running it as a test at 430c for a half hour it was quite warm where the thinnest sides of brick are. I have since sealed up the joins with stove putty and have cut the sections of kaowool ready and it will be sandwiched between ply and aluminium. Any heat felt is a loss in efficiency. I hope to push it to 900-1000c this is within the elements limits so it should be possible.

JJay - 18-7-2017 at 22:06

Nice... I like your design a lot. I will probably give something like that a try after I destroy my mica furnace.

Fyndium - 14-9-2020 at 01:09

I was planning on distilling sulfuric acid under aspirator vacuum. At maximum theoretic aspirator vacuum of 32mbar it should take 337C BP down to 161C which is well achievable, even with some liquid heating bath. I have 1L 3-neck flask which I usually use for vacuum distillations and I was gonna use my ordinary setup to bleed air through capillary to eliminate bumping.

Just in case of implosion I was gonna build a temporary blast shield around the still from plywood and glass. It doesn't have to be resistant to corrosion, but to stop the splashes and fragments and give time to spray Na2CO3 solution or just plain water with hose to it. This is carried in a technical workspace where there is good drainage so pretty crude methods could be used in case necessary. I would never do it indoors or living quarters, though.

Fyndium - 14-9-2020 at 12:08

Ok, I attempted to distill sulfuric acid. I used avocado oil up to 220C with some smoking, but the acid didn't want to distill. I did not have probe in the flask, only in the bath and the still head. Water boiled at 26C for control. I used bleeder for zero bumping. Does the acid have different vapor pressure than water, aka does normal pressure boiling point calculation give how accurate temps for other substances than water?

macckone - 14-9-2020 at 13:25

sulfuric acid is hard to distill.
It also has hydrates that impact the boiling point.
My guess is you had insufficient vacuum for the temperature of the acid.

clearly_not_atara - 14-9-2020 at 19:46

If you heated the oil bath to 220 C and it was smoking, are you sure you left it hot long enough? Sounds like you would have stopped that pretty quickly. But you need to let the flask sit in the oil bath for a while to come to temperature.

Also, while some Internet sources give the smoke point of avocado oil as 270 C, the actual smoke point of any sample of oil will depend highly on how purified it is. Avocado oil has an excellent flavor and often isn't fully refined, reducing the smoke point dramatically.

But you can buy soybean oil or safflower oil that is specifically prepared for high-heat cooking, which will have a higher smoke point. It doesn't taste good, but it should do a great job of distilling sulfuric acid :p

monolithic - 15-9-2020 at 03:16

Quote: Originally posted by Fyndium

Ok, I attempted to distill sulfuric acid. I used avocado oil up to 220C with some smoking, but the acid didn't want to distill. I did not have probe in the flask, only in the bath and the still head. Water boiled at 26C for control. I used bleeder for zero bumping. Does the acid have different vapor pressure than water, aka does normal pressure boiling point calculation give how accurate temps for other substances than water?

Boiling water at 26 C means a vacuum of about 60 torr which means sulfuric acid wouldn't boil until about 236 C with this same vacuum applied. Common rule, at least for rotovaps, is that bath temperatures should be about 20 degrees higher than your boiling point. Your oil bath wasn't hot enough -- it would have to be around 256 C.

Fyndium - 15-9-2020 at 05:41

What calculator/table do you use to determine boiling point? I got mine from here:

http://www.trimen.pl/witek/calculators/wrzenie.html

And I'm not sure if it's correct. It states that water should boil at 26C in 32mbar. 60 torr is about 80mbar?

I came to a conclusion that my pump was not quite to it's abilities because some of the water vaporized and ended up in the oil, making it a milky emulsion, hence, the vapor pressure comes down due to water. No worries, I disassembled and cleaned the pump and changed the oil immediately after the concentration process. I initially ran the aspirator, but it only brought the water bp down to 34-36C.

I need to improve it a bit by adding a graham condenser after the primary dimroth type condenser cooled with pumped air and ambient air by coil&outer wall, and immerse the condensate flask in a cryogenic bath in order to keep the water from evaporating into the pump. A cold trap had little help, it filled with condensed water multiple times.

I'm also aware of the temperature gradient, and I suspected it to be the reason alongside the insufficient vacuum.

For the matter, I might actually consider just distilling the acid via the Cody's lab way. Chinesium 1L flasks cost 5-10$ a piece so I can blast one with propane flame and if it happens to crack, it's not a biggie. The times prior my baths I exclusively used bare flames to heat everything, which would not be ideal, but the torture I exerted to the various glasses indicates that the statistical risk of cracking glassware seems to be lower than presumed, warranting a controlled risk taking to cut a lot of extra work and possibly costs too(a heating mantle).

[Edited on 15-9-2020 by Fyndium]

[Edited on 15-9-2020 by Fyndium]

monolithic - 15-9-2020 at 08:18

Quote: Originally posted by Fyndium

What calculator/table do you use to determine boiling point? I got mine from here:

http://www.trimen.pl/witek/calculators/wrzenie.html

And I'm not sure if it's correct. It states that water should boil at 26C in 32mbar. 60 torr is about 80mbar?

I came to a conclusion that my pump was not quite to it's abilities because some of the water vaporized and ended up in the oil, making it a milky emulsion, hence, the vapor pressure comes down due to water. No worries, I disassembled and cleaned the pump and changed the oil immediately after the concentration process. I initially ran the aspirator, but it only brought the water bp down to 34-36C.

I need to improve it a bit by adding a graham condenser after the primary dimroth type condenser cooled with pumped air and ambient air by coil&outer wall, and immerse the condensate flask in a cryogenic bath in order to keep the water from evaporating into the pump. A cold trap had little help, it filled with condensed water multiple times.

I'm also aware of the temperature gradient, and I suspected it to be the reason alongside the insufficient vacuum.

For the matter, I might actually consider just distilling the acid via the Cody's lab way. Chinesium 1L flasks cost 5-10$ a piece so I can blast one with propane flame and if it happens to crack, it's not a biggie. The times prior my baths I exclusively used bare flames to heat everything, which would not be ideal, but the torture I exerted to the various glasses indicates that the statistical risk of cracking glassware seems to be lower than presumed, warranting a controlled risk taking to cut a lot of extra work and possibly costs too(a heating mantle).

[Edited on 15-9-2020 by Fyndium]

[Edited on 15-9-2020 by Fyndium]

I use this one. https://www.sigmaaldrich.com/chemistry/solvents/learning-cen... In reality it's just an estimate. If you weren't seeing boiling then you weren't pulling a hard enough vacuum. If that's as good as your vacuum pump can do, then you have to get it to a higher temperature until it begins to distill.

Fyndium - 15-9-2020 at 10:44

Nice, it gives 8C as water BP at 32mbar. The .pl site gives 26C.

Since (afaik) it is cheapest way to determine the vacuum by testing at which temp water boils, it would be nice to know the actual, accurate boiling temp of water and what possible factors would affect it.

Engineeringtoolbox gives bp 25.7C, which is where .pl probably rounds it since it shows only full digits.

Sigma chart begins to differ at higher temps. 336C will turn into 208C from Sigma and 160C from .pl site. The estimate from reality probably doesn't differ more than few deg C, and reaching 208C temp should be no issue.

Vacuum pump, afaik, can generate only as much vacuum as the vapor pressure of it's oil is, and if it is contaminated by (hot) water, it supposedly affects the maximum vacuum reachable. Like said, I gotta test it out with new, clean oil and new setup to see if it's the actual mechanical limit. I'm highly disappointed though, since the pump is rated at 5Pa vacuum, but most likely this has more than one or two digits of deviation because 100€ Chinesium rotary vane pump can be basically whatever that looks like a pump.