Sciencemadness Discussion Board - modifying a microwave for extractions

modifying a microwave for extractions

NEMO-Chemistry - 11-6-2016 at 03:24

I have come across a lot of recent papers regarding the use of microwave ovens for extraction, the method seems to give very good results from the papers i have read on it.

One or two show drawings of the flask/heating vessel inside the microwave oven, the condenser etc is shown as exiting the top of the oven presumably through a small hole.

I expect these are specially made ovens?? i have an old microwave oven and my question is. if i modify the oven and drill a hole through the top so i can get a still head (or whatever), through the top of the oven, hole do i seal around the whole to make sure its safe and no microwave escape?

I thought of sealing with silicon caulking and then foil, a bit of a pain to take apart for cleaning but east enough to redo when needed.

Anyone know of a better way?

PHILOU Zrealone - 11-6-2016 at 05:01

In one of the physico-chemistry departement of the Free University of Brussels where I did my end-study work, they used normal microwave oven to study the effect of microwaves on reactions.
So yes you can drill a hole in the top of the oven to allow for a condenser/ a distillation column / or a propeller to pass into the reactor inside the oven.

Usualy/practicaly the loss of microwaves through the hole are minor since the microwave is designed to get convergent/focus inside the core of the oven...once it goes outside it is divergent.

[Edited on 11-6-2016 by PHILOU Zrealone]

NEMO-Chemistry - 11-6-2016 at 06:59

Quote: Originally posted by PHILOU Zrealone

In one of the physico-chemistry departement of the Free University of Brussels where I did my end-study work, they used normal microwave oven to study the effect of microwaves on reactions.
So yes you can drill a hole in the top of the oven to allow for a condenser/ a distillation column / or a propeller to pass into the reactor inside the oven.

Usualy/practicaly the loss of microwaves through the hole are minor since the microwave is designed to get convergent/focus inside the core of the oven...once it goes outside it is divergent.

Great so basically just dont try to look through the gaps in the hole

. I appreciate the reply, i wasnt keen on cooking my bits and bobs if you know what i men

.

[Edited on 11-6-2016 by PHILOU Zrealone]

zed - 19-7-2016 at 17:41

Folks sometimes use elaborate precautions to make sure errant microwaves don't escape their containment.

A second, non-functioning micro-wave oven, can serve this purpose. Just bore a hole in the second oven, turned upside-down (or sideways) and affix that second oven, over yer escape hole.

Hopefully this second oven will provide enough space for your equipment. And, provide shielding against escaping microwaves.

Back in the day, it was rumored that stray microwaves had a very deleterious effect on the human cornea.

[Edited on 20-7-2016 by zed]

RogueRose - 19-7-2016 at 19:35

not to hijack the thread I am wondering if it is possible to stand the microwave on its end/side so as to allow for taller vessels to fit in the chamber. This will not allow the container being heated to spin/turn but it is possible to place a magnetic stirrer on the outside of the inner chamber to stir the liquids in beakers/flasks.

Any thoughts on this?

zed - 22-7-2016 at 15:22

Dunno. My Microwave ovens seem to focus low. Items placed on the floor of the oven, heat faster than those placed in higher areas.

semesa - 23-7-2016 at 00:24

be sure to note if the articles you are looking at specify single-mode microwave ovens or multi-mode(aka "domestic microwaves").

Some procedures require the specific type of oven but there are a few that can use either(with minor changes to the procedure.) Since single mode microwaves are significantly more expensive(and only used for specialized purposes i.e. research laboratories) it will often be specified in the "procedure and apparatus" section .

wg48 - 26-7-2016 at 16:22

I was curious as to how larger a hole you can get make in the MO's internal wall with only say 10mW escaping. As I had recently got my maths software going on an old XP pc I gave the calculation a try.

I assume a metal pipe was attached to the internal wall protruding outwards from a hole with the same diameter as the internal diameter of the the pipe. I calculated the attenuation of a the pipe assuming it was a circular waveguide (ref: http://learnemc.com/practical-em-shielding )

Shown below is a graph of the power attenuation for pipes of internal diameters of 13, 20, 25.5, 32 and 39mm (standard copper pipe), plotted for 25, 50, 75 and 100mm lengths. The graph,s accuracy is best in the lower right area.

If you assume a 1kW power in the oven and 10W incident on the entrance to the pipe an attenuation of 0.001 is required to reduce the exiting power to 10mw. That can only be achieved with a 15mm dia (int 13mm)at least 50mm long or a 22mm dia (int 20mm) at least 75mm long.

Irritatingly not very big for fitting glassware thru. Note that nothing conductive can protrude thru the pipe and potentially glass will decrease the attenuation. The 10mW could be pushed to 100mW on the assumption no part of your body is immediately next to the hole.

I do not know details of the graphs accuracy but it does give some indication of the problem. I will check the calculation and assumption again on an other evening.

Mush - 19-10-2016 at 09:26

E. Texas students modifying microwave for chemistry research
https://www.youtube.com/watch?v=kGejN8J1o8U

Maroboduus - 19-10-2016 at 15:28

If you're worried about your safety, couldn't you use aluminum foil around the glassware exiting the microwave?

Or just wear a foil lab coat?

Maybe that guy down the street with the foil hat ISN'T crazy, but just forgets to take it off sometimes after doing microwave chemistry.

wg48 - 12-11-2016 at 05:15

One problem with using a standard microwave oven for heating solutions is the on off power control. The power is so high during the on period solutions boil very vigorously and tend to bump.

This can be solved with a water bath which is almost mandatory when heating test tube aqueous solutions. But small water baths boil vigorously and bump too. There is a way to reduce the on power. In most MOs there is an approximately 1uF capacitor in the doubler circuit of its power supply. I see no reason why this could not be reduced. The effect would be to reduce the width of the half wave voltage pulse supplied to the magnetron and thus its output power durring the On period. This is not the on off control from the front panel which would still work.

Between 1uF and 0.1uF the output power should be approximately linear. So a second identical capacitor in series should with the original half the power for example.

I have some 0.15uF and 1 uF caps I will check this mod out this weekend.

Of cause if you let any solution get too hot in a test tube particularly after it has boiled once it can bump. So a water bath for test tube heating will still be needed and a pinch of sodium bicarbonate to eliminate its bumping of the bath.

Edit: I should also add that small (test tube) sized loads and conducting liquids reflect most of the microwave energy which can reduce the life of the magnetron and cause arcing. That effect can be reduced by putting a container of water in the oven with what your heating. The volume of the water relative to what your heating adjustes the amount of power absorbed between them. By reducing the ON power the damage caused by a mismatched load is reduced. A water bath also eliminiates that problem.

[Edited on 12-11-2016 by wg48]

[Edited on 12-11-2016 by wg48]

wg48temp9 - 12-9-2019 at 11:43

I recently found the following graph showing how current of a domestic microwave oven magnetron varies with the value of the capacitor in the doubling circuit. The magnetron current determines the output power of the magnetron. Typically the value of the cap is between 0.9uF and 1.1uF. By using two such capacitors in series the oven power is halved. The reduces the chance that a small mass will be overheated during an on cycle of the on/off power control and makes the oven more robust against powering the oven with light load or a load that does not absorb as strongly as water. Even 0.1uF can be used to reduce the power to about 10W.

From http://www.radio.walkingitaly.com/radio/radiosito/za_fatti/vel_luce/Microwave(Sharp)%20Training.pdf

[Edited on 9/12/2019 by wg48temp9]

Pumukli - 13-9-2019 at 12:00

Hm, it was a useful post for me. I saved an old but working MW oven for chemistry but it is 800W. Now I have some chance to reduce its power fairly safely. Thank you!
Btw. the given link was also informative!

wg48temp9 - 13-9-2019 at 18:26

Quote: Originally posted by Pumukli

Hm, it was a useful post for me. I saved an old but working MW oven for chemistry but it is 800W. Now I have some chance to reduce its power fairly safely. Thank you!
Btw. the given link was also informative!

A useful effect is that the peak microwave power will not be significantly reduced. The usual MO power supply pulses the magnetron at the mains power frequency. Reducing the capacitor's value will reduce the duration of the pulses while the peak power remains approximately constant.

That means that a miniature neon tube or small florescent tube can still be used as a microwave detector while the average microwave energy is reduced the peak power is still able to initiate the discharge in a neon or florescent tube while the danger is reduced of having 1000W. That is useful if you want to experiment with resonant structures or direct coupling to the magnetron.