Sciencemadness Discussion Board - Concentrating H2SO4 without distillation (with flask ans some PTFE )

Concentrating H2SO4 without distillation (with flask ans some PTFE )

rockyit98 - 25-7-2019 at 17:01

about 2 years ago I discovered something very useful Accidentally on Purpose .
That is you can make up to 99%(I know H2O and H2SO4 Azeotrope is 98%) Concentrate H2SO4 by this simple setup.I know it seems like magic but it not.it use Graham's law of effusion (also called Graham's law of diffusion) https://en.wikipedia.org/wiki/Graham's_law coupled with Raoult's law and Dalton's law. since water is about 5 times lighter than Sulfuric Acid it will diffuse faster.
if you think i'm lying hold a pH paper above to check the Difference.

it uses 30-35% acid from battery suppler mix in with glass chips to prevent flash boiling .The opening of the flask is covered by 2-4 layers of plumbers PTFE tape if you use thick ones 1-2 will suffice.just try one layer first you can add more if necessary. secure them in place using CAPTON tape.

Use a sand heating to evenly and slowly heat the flask.otherwise PTFE layer will pop. i recommend flask with a big surface area opening(this is what i had in hand)
let the water escape leaving behind acid . i don't recommend pushing this to the extreme. note in hand don't boil the content in the flask just let it evaporate for safety resonances.
pros - all acid will concentrate in signal run
easy to clean just one flask
cons - this doesn't purify the acid
take bit more of time
need attention

BE CAREFUL!

draculic acid69 - 26-7-2019 at 06:00

So your saying heat dilute h2so4 in a flask to concentrate it.thats not really a new thing.

Sulaiman - 26-7-2019 at 06:38

The apparatus looks 'sensitive' to overheating,
but if 99% is correct then that is very interesting,
(whenever I've tried simple boiling I had to stop well before azeotropic due to the copious quantity of fumes)
(and I've not done a true distillation as I don't want to risk my Chinese glassware and/or my vacuum pump)
how confident are you that the concentration is greater than azeotropic ?

P.S. is the ptfe tape 'separating' water and acid ?
if so where does the energy come from ?
or is it just the pressure difference across the (assumed) semipermeable membrane ?
Whatever the mechanism - good thinking - great experiment :cool:

[Edited on 26-7-2019 by Sulaiman]

Tsjerk - 26-7-2019 at 07:37

I would love to see this repeated by someone else!

RedDwarf - 26-7-2019 at 08:53

I can believe this might work - membrane separation due to greater permeability of the much smaller water molecule, and much more likely to work in the vapour phase with less interaction between the acid and the water molecules - but I'll need to finish building my fume cabinet before i'd want to give it a try.
Just doing a quick google search shows a number of papers / articles on membrane separation involving sulfuric acid and water including:
https://patents.google.com/patent/WO2004074811A2
https://pdfs.semanticscholar.org/4da8/1b9d5a2d1342865c68c0d1...
https://inldigitallibrary.inl.gov/sites/sti/sti/3562835.pdf :
"Pervaporation was also applied with similar results for the concentration of sulfuric
acid. Decomposition of sulfuric acid is equilibrium limited requiring a recycle stream.
Concentration by distillation involves serious corrosion issues at the vapor-liquid interface. In
this work, Nafion-112® was used successfully as a membrane to remove water from sulfuric
acid using pervaporation. "

Excellent idea!

happyfooddance - 26-7-2019 at 09:33

Yeah, this is a really good idea, thanks for this

WGTR - 27-7-2019 at 06:56

Interesting. What is the width of your tape? Does it cover the mouth of the flask in one piece? The typical hardware store ptfe tape that we have here is 12mm wide.

Do you have a part number (etc) for the tape? There are three densities of ptfe tape available from what I can see at McMaster: https://www.mcmaster.com/#ptfe/=e44a82f26bce45b3ab895274af6a...

I figure that the widest possible tape would be the best option (about 5cm in this case). When you are heating the flask, is there condensation on the inside of the ptfe film, or is it dry?

[Edited on 7-27-2019 by WGTR]

RedDwarf - 27-7-2019 at 07:12

Here in the UK there are 3 types of ptfe tape :
yellow for gas ( heavier weight, not sure if it's thicker or denser or both)
white for water
pink (I've never seen this) which is apparently even heavier than the yellow.
Like WGTR all the tape I've ever come across has been 12 mm wide but I have managed to find a 50 mm online.

Further question : How long did you have to heat to remove what volume of water?

Edit- I'm guessing that you don't want to heat so much that you have condensation on the underside of the tape as that would reduce the surface area available for transport?

[Edited on 27-7-2019 by RedDwarf]

rockyit98 - 27-7-2019 at 11:26

PTFE tape stretch easily You can even use 10mm ones if have to.it just matter of how many layers.
I used 20mm .
maybe can do phosphoric acid .I don"t got any.

(I was meant to get 90% phosphoric acid by heating mono calcium phosphate.got no time

)

Ubya - 27-7-2019 at 12:10

Quote: Originally posted by rockyit98

PTFE tape stretch easily You can even use 10mm ones if have to.it just matter of how many layers.
I used 20mm .
maybe can do phosphoric acid .I don"t got any.

(I was meant to get 90% phosphoric acid by heating mono calcium phosphate.got no time

)

i have 19mm ptfe tape, when stretched sideways it reaches maximum 6.5cm

Abromination - 27-7-2019 at 12:16

You might be on to something here, this is an idea most interesting. I wonder if you can find membranes even more effective then the teflon tape. My worry would be corrosion of the membrane otherwise, though.

RedDwarf - 27-7-2019 at 12:57

Without the fume cupboard I'm trying a less corrosive solvent mix (2017 Plum wine), ethanol is a smaller molecule than sulfuric acid but still significantly bigger than water. I've used strips of 12mm tape in parallel to cover neck of the flask (will try stretching tape next time if this isn't successful). Anyone got any scraps/offcuts of nafion they don't want?

WGTR - 27-7-2019 at 13:17

The first thing I suggest is trying this, but with water only, no acid. Measure how fast the water leaves the flask through the membrane at various temperatures and conditions.

happyfooddance - 27-7-2019 at 14:00

Quote: Originally posted by WGTR

The first thing I suggest is trying this, but with water only, no acid. Measure how fast the water leaves the flask through the membrane at various temperatures and conditions.

I thought this too, but a problem is that you are much limited as to how high of a temp you can achieve because of water's B.P.

I have a suspicion that a little higher temp is necessary to get reasonable permeation to make this effective.

WGTR - 27-7-2019 at 16:07

Here is a document that explains permeabilities of different materials to water, and gives a general discussion on the matter:

Attachment: PNNL-26070.pdf (3.3MB)
This file has been downloaded 631 times

I have a small glass screw-top vial in the oven right now. I covered the opening with some 12mm ptfe. As others mentioned, it stretched easily. Also, it sticks very well to the glass, and the Kapton tape sticks very well to it. It seems to have a good seal, and I did not see any pinholes under the microscope.

The vial has been in the oven at 95°C for about an hour, and water does seem to be permeating the membrane. I would expect the film to be bulging outward like a balloon, but it seems to be laying flat, with only a slight convex curvature.

happyfooddance - 27-7-2019 at 16:29

I don't know if the pores in the ptfe tape are the mechanism or if it's capillary spaces, a little research has led me to believe it's the latter, though. From wikipedia article on Knudsen diffusion (https://en.m.wikipedia.org/wiki/Knudsen_diffusion):

"Consider the diffusion of gas molecules through very small capillary pores. If the pore diameter is smaller than the mean free path of the diffusing gas molecules and the density of the gas is low, the gas molecules collide with the pore walls more frequently than with each other. This process is known as Knudsen flow or Knudsen diffusion."

It seems to me that a frit or a short column of well-packed celite would work better than this ptfe tape deal, and a vacuum to greatly increase the mean free path. Heat will still need to be applied because of the low vapor pressure of 98% H2SO4. This is cool and I hope to do an experiment soon, unfortunately I'm hella busy at the moment. I will try and report back as soon as I do, though.

Abromination - 27-7-2019 at 18:59

When I get the chance tomorrow, I will try this with PTFE in different conditions with water to observe the escapement levels in different conditions. Would steam diffuse through agar or gelatin membranes? I do not see either of these options as being very useful as I imagine they would quickly disintegrate. However, a drying tube packed with celite may be a better alternative to teflon as happyfooddance said.

To both concentrate and purify sulfuric acid by this method, I suppose you could add concentrated H2O2 to produce a piranha solution, destroying most all impurities.
It could be a safer alternative to distillation.
Bumping would of course still be a problem, however.

Abromination - 27-7-2019 at 22:14

Tomorrow morning, I will slowly heat a flask of 10% acetic acid with a teflon membrane secured by a thermometer adaptor. Ideally, I will be able to get a quite concentrated solution. I doubt that the acid vapors will be able to permeate the membrane, but we shall see. I also doubt that I will be able to concentrate the acid to the glacial state, but again we will see.

I’m a bit nervous to use this setup, it is almost like heating a closed system, but it will be done very slowly and carefully.
Hopefully if the pressure does get too high the teflon will just bust, but if not the joint will be quite loose and pointed away from anything.

I also, if it works, imagine it could also be used to overcome some other azeotropic mixtures. As long as the water passes through the membrane and the solute does not, it should work relatively decently.

05DDE7CD-B7A1-4BB6-B896-12B84FC300C5.jpeg - 1.8MB

RedDwarf - 28-7-2019 at 11:43

Last night's trial with the wine didn't work as the seams between the tape leaked with a water/alcohol mixture escaping. I've tried again today with a single piece of tape stretched across the entire opening (4 layers of the white tape).
At 55C I lost 4g of water over 2 hours (but I couldn't be sure that this wasn't mostly during the initial phase when i was struggling to get a constant low temp).
At 95C I lost 16g of water over 90 minutes. During this run the ptfe was bulging then going flat again regularly. I can't see a leak on the tape so this may just be the bulging stretching the pores and pressure being released this way.
Third run with heating just up a little further (temp showing as 97C) but membrane continuously bulging: 32g lost over 110 minutes

[Edited on 28-7-2019 by RedDwarf]

Abromination - 28-7-2019 at 13:38

Quote: Originally posted by RedDwarf

Last night's trial with the wine didn't work as the seams between the tape leaked with a water/alcohol mixture escaping. I've tried again today with a single piece of tape stretched across the entire opening (4 layers of the white tape).
At 55C I lost 4g of water over 2 hours (but I couldn't be sure that this wasn't mostly during the initial phase when i was struggling to get a constant low temp).
At 95C I lost 16g of water over 90 minutes. During this run the ptfe was bulging then going flat again regularly. I can't see a leak on the tape so this may just be the bulging stretching the pores and pressure being released this way.
Third run with heating just up a little further (temp showing as 97C) but membrane continuously bulging: 32g lost over 110 minutes

[Edited on 28-7-2019 by RedDwarf]

Do you know the concentration of ethanol before and after your second run? Even if it doesn’t work well with telfon, the idea is still there and celite packed in a drying tube may be an alternative. I am currently working with a 50mL sample of 10% acetic acid, I will have some better observations in a few minutes.

EDIT: Good results so far. I am slowly building up the temperature of my hotplate, the membrane is bulging out but the joint does not seem to be under stress. I smell little to no vinegar.

[Edited on 7-28-19 by Abromination]

RedDwarf - 28-7-2019 at 14:02

Quote: Originally posted by Abromination

Do you know the concentration of ethanol before and after your second run?

[Edited on 7-28-19 by Abromination]

The runs today were just with water. I'm going to try isopropyl alcohol/water mix tomorrow.

Abromination - 28-7-2019 at 14:17

Quote: Originally posted by RedDwarf

Quote: Originally posted by Abromination

Do you know the concentration of ethanol before and after your second run?

[Edited on 7-28-19 by Abromination]

The runs today were just with water. I'm going to try isopropyl alcohol/water mix tomorrow.

Alright, the wine part confused me. A problem I am realizing that was actually addressed by OP is that the diffusion through the membrane depends on the weight of the two liquids. If its too close together, I imagine they would diffuse out together which would explain why I can smell acetic acid a bit now.
With this issue, it may only be useful in a few applications.

UPDATE: it appears that most of what is escaping is water, although I can smell a faint aroma of vinegar. I can feel the water vapor escaping from the membrane, which when condensed only seems like a very dilute solution of acid.

Another thing to note is that acetic acid and water do not form an azeotrope, they just have close boiling points.

[Edited on 7-28-19 by Abromination]

[Edited on 7-28-19 by Abromination]

rockyit98 - 29-7-2019 at 08:47

it not the size that matter its the molecular weight!
why most one people trying to concentrate alcohol ? they must be reading between the lines of "Concentrating H2SO4 without distillation (with flask and some PTFE) ".which means they must be drunk already.

i don't think it's works, ethanol Molar mass is: 46.07 g/mol and shaped kinda linearly.acetic acid seems to work but try it any way

.
can any one find the efficiency of that .its my bad i ran out of the stuff.

[Edited on 29-7-2019 by rockyit98]

Abromination - 29-7-2019 at 10:06

Quote: Originally posted by rockyit98

it not the size that matter its the molecular weight!
why most one people trying to concentrate alcohol ? they must be reading between the lines of "Concentrating H2SO4 without distillation (with flask and some PTFE) ".which means they must be drunk already.

i don't think its works, ethanol Molar mass is: 46.07 g/mol and shaped kinda linearly.acetic acid seems to work but I try it any way

.
can any one find the efficiency of that .its my bad i ran out of the stuff.

Yeah, I was kind of starting to think that. Not sure why I didnt edit that right away.
Anyways, acetic acid should work although there still is a decent escapement of acid through the membrane. Also, you definitely need a larger opening and SA of the membrane if you are going to release any large amount of vapor put into the system. It will pop the joint, as expected.
Is there a certain way you layer different pieces of the teflon over the flask mouth? I think lattice layers of tape could help prevent unwanted escapement, but I’m not certain. I to am almost out, I plan to look for some tape with a wider width.
It would also help to find the compound with the lowest molecular weight that can stay in. If ethanol is 46.07g/mol and acetic acid is 60.052g/mol there should be something in between, probably closer to acetic acid.

rockyit98 - 29-7-2019 at 10:24

must be H2SO4 has low Vapor pressure (higher boiling point) than water.
i'm going to watch https://www.youtube.com/watch?v=-n8CBYoKU_c to get better understanding of things later.
The separation of ethanol water mixture with a PVA (https://pdfs.semanticscholar.org/8f20/1d2ba346a7ea18612265b0...)

[Edited on 29-7-2019 by rockyit98]

RedDwarf - 29-7-2019 at 12:27

Quote: Originally posted by rockyit98

My assumption is that molecular size vs pore size in the membrane that is the mechanism here, but I'm happy for you to prove me wrong

In the meantime I'll continue to work on seeing if the membrane can be used to prevent passage of larger molecules.

RedDwarf - 29-7-2019 at 12:41

My isopropanol / water mixture test didn't turn out as hoped. I left the mixture in the flask at room temperature for over 12 hours without any loss, but within 15 minutes of heating I'd got a strong smell of alcohol outside the flask (confirmed by reduction of K permanganate solution on a strip of filter paper held over the membrane). Analysis of the starting and finishing densities (using a density bottle) showed that what had been lost was mostly alcohol.
During this heating stage I noticed that the membrane was very bowed (so even more stretched than previously) and it appeared as if this was just the outermost layer separated from the other 3 so there may have been a leak in one or more layers.
I'm going to try a number of different membranes/supports to try and avoid over stretching the membrane (and consequentially the pore size):
- I've ordered some gas (yellow) tape that should be thicker and thus more resistant to stretching.
- I've cut a few pieces of an old goretex coat to see if this membrane (supported on a permeable nylon mesh) might also work.
- I'm going to use some lightweight permeable nylon above white ptfe layers to prevent stretching, so that pore size should be more constant with temperature and pressure.

Abromination - 29-7-2019 at 13:09

Quote: Originally posted by RedDwarf

Quote: Originally posted by rockyit98

My assumption is that molecular size vs pore size in the membrane that is the mechanism here, but I'm happy for you to prove me wrong

In the meantime I'll continue to work on seeing if the membrane can be used to prevent passage of larger molecules.

I don't think I can! Molecular size is the main factor here as larger molecules diffuse slower, although weight matters as well because heavier molecules move more slowly.

I am sorry that the isopropanol run did not work, but it is still good information. It would be nice to know the pore size of teflon also. The problem is as we stretch it, the pores change size.

You would be surprised how little I have found in permeation of vapors, Fick’s law does not help to much as we know little of the variables in this circumstance.

RedDwarf - 29-7-2019 at 13:26

Just to be clear, I'm not suggesting that Graham's law (and hence molecular mass) isn't part of the effect here, but I believe there is also a pore size issue, with pore size varying according to the delta P on the membrane. Out of interest does anyone know if membrane separation/effusion has ever been used for isomer separation (same mass different shape), I know it has been used for isotope separation (same shape, (slightly!)different mass) ?

Abromination - 29-7-2019 at 16:02

According to Wikipedia, “When a diffusion process does not follow Fick's laws (which happens in cases of diffusion through porous media and diffusion of swelling penetrants, among others),[3][4] it is referred to as non-Fickian.”

Fick’s law refers to diffusion without a membrane. If you you were to fill a container with a gate in the middle that nothing could pass through with a solution on one side and pure water on the other, the diffusion of the ions across the container when the gate is removed is what Fick’s law describes.

I guess that leaves it to being covered by Grahm’s law, although I hate to say it.

[Edited on 7-30-19 by Abromination]

Johnny Cappone - 23-12-2020 at 13:04

I would really love to try this procedure.
One of the problems I have when concentrating battery acid, perhaps the biggest one, is that big cloud of whitish mist produced in the final moments of the process. However, I confess that I am a little confused. Okay, normally just heating the acid, without using any membrane, works well. The heat leaves the molecules agitated and the water, having a lower boiling point, escapes. The temperature gradually rises to 290-320 degrees, where we finally reach the 98% azeotrope.

What is being said is that, with the membrane, I could perform this whole process at 95 degrees, is that it?