Inductive Heating of a Non-Conductive Reactor's Contents

[bogue]

TL;DR Summary

Can I heat a non-inductive reactor's contents if they are inductive? Example: A glass tube wrapped in an induction coil, filled with iron.

Hi!

I have been working with home-made reactor to do ambiant pressure, high temperature pyrolysis experiments that use an induction heating coil wrapped around the outside to heat the entire reactor. The contents are heated indirectly through their contact with the reactor. This worked well for experimental purposes, but I was wondering about modifying the reactor so that it is made of non-conductive, non-ferromagnetic material (ceramic, quartz, some sort of high temperature polymer), and then having conductive/ferromagnetic material (iron scrap?) inside the reactor for a more direct heat transfer approach. I understand that I will be losing some of the magnetic field from the new material's resistance/distance added, but couldn't I compensate by increasing the field strength?

Are there any applications like this that exist already, or magnetic permiable materials that are suitable for such a container? Thanks!

Reference: https://www.physicsforums.com/forums/materials-and-chemical-engineering.105/

 

[Baluncore]

Welcome to PF.

When the outer container is heated, it loads the electrical circuit, independent of the contents. Inductive heating generates electrical eddy currents in the electrically conductive material.

Inductive heating of the container can only indirectly heat the contents. It is simpler to inductively heat conductive material directly, through the crucible wall. There will be less energy required with direct heating, because the crucible will become a thermal insulator.

When you must heat a resistive material, it can be done with dielectric heating. The material is placed between capacitor electrodes, then high electric fields heat the material.

In both cases, the operating oscillator circulates power between electrically reactive capacitance and inductance, until the power is dissipated as real power, generating heat in the material.

 

[bogue]

Thank you for the response. Electrical engineering is not my field, and I appreciate all the help I can get.

If I am understanding you, there is no way to heat the contents within the container through induction, only indirectly by induction heating the inner wall of the container.

Is this true even if the container is permiable to the emf?

In my application the container is moving within the coil to provide mixing of the contents, and the contents are currently heated by the container, via an induction coil. I was hoping for a solution/material that could limit the container's heat/energy loses, while adding material to the container to allow for more direct heat transfer to the contents.

 

[Baluncore]

If I am understanding you, there is no way to heat the contents within the container through induction, only indirectly by induction heating the inner wall of the container.

You have misinterpreted my reply. If the container is not electrically conductive, and the material/charge in the furnace is electrically conductive, then the material will be heated directly, without any inductive heating of the container wall. That is the most efficient way to heat the material.

I was hoping for a solution/material that could limit the container's heat/energy loses, while adding material to the container to allow for more direct heat transfer to the contents.

You are correct. That is the most efficient way to do it.

 

[bogue]

Thank you for clarifying.

I don’t believe dielectric heating can work in this application, since I intend for the contents to be iron and flammable hydrocarbons.

I need to do some more research on high temperature materials that are non-conductive - but now I have some better direction.

 

[Baluncore]

I don’t believe dielectric heating can work in this application, since I intend for the contents to be iron and flammable hydrocarbons.

What is the aim of the process?
Do you want the inductive heat to pyrolyse the hydrocarbons?
Do you want to crack and vaporise the hydrocarbons, to extract as fuel?
Do you want to use the iron, or is it there only to generate local heat?

Why not use a resistive electrical hot wire element?

 

[bogue]

I’m researching plastic recycling and the aim of my process is depolymerization with iron oxide in various forms as the catalyst.

The reactor is shaken to mimic fluidization, and I would love to target fuels, but unfortunately my government wouldn’t like that. With the current proposal, I also can’t use a ‘catalyst’ …

So I will be calling it iron-reduction pretreatment and monomers/petrochemicals.

Is resistive electrical wiring robust enough to interact with the fluidization and the movement of the reactor? In industry I never see it on reactors/heated units, only pipes.

 

[Baluncore]

Iron oxide is not electrically conductive, but the iron metal will be.
Inductive heating of iron may change when the iron metal reaches the Curie temperature.
Resistive electrical wiring is NOT robust enough to use with fluidization.
It does help when we know what you are doing.

Dielectric heating of the polymer is used for welding plastics.
Direct dielectric heating of the plastic and oxide mix is possible for your process.

 

[Rive]

...having conductive/ferromagnetic material (iron scrap?) inside the reactor for a more direct heat transfer approach.

What you need to be careful about is, that inductive heating of 'disorganized' metal may result in disorganized heating, with a wide range of temperatures.
And as far as I know, plastics can be really sensitive to the right temperature.
You may not want some spots white hot, and others just lukewarm.

Is resistive electrical wiring robust enough to interact with the fluidization and the movement of the reactor? In industry I never see it on reactors/heated units, only pipes.

Pipes are really convenient forms for heating elements.

Of course, placing the heating element in metal pipes are just another convenient option if direct contact is desirable.

 

[bogue]

My apologizes, I didn't know what kind of help I would receive so I kept it brief. I will be more descriptive in the future.

The current setup is a modified paint shaker, strapped with a reactor containing high temperature flex hoses for outputs, filled with sand and input materials, heated by an induction coil. It was relatively cheap to purchase and modify and works excellent to mimic rapid fluidization/mixing for small scale pyrolysis experiments at ambient pressures.

I am interested in modifying it to replace the sand with scrap iron of particular composition/particle size. I understand the heating of the disorganized iron will not be uniform, but with adequate mixing and some modification to the induction controller setup, this might not be an issue. I can start with forms of iron that have known composition/inductive heating performance first before moving to mixtures that are more likely to be found in commercial waste/the environment (slag, scrap, rust, ore), and find the heating limitations that way.

Since the paint shaker is rather violent with its movement, fixed pipes, connected electrical wires, and adding any mass to the reactor is very limited. The setup is built for induction heating, but perhaps I can work on a new design with dielectric heating.

 

[bogue]

Currently, the shaker reactor is not designed to hold pressure, and pyrolysis vapours flash off as depolymerization occurs. In part of making this new design where the container is non-conductive and the contents are, I am hoping to puzzle out a material that is checks all the boxes for this experiment (though I understand it is a tall order).

Ideally, the new container would have the following characteristics.

1. Compatible high temperatures (the higher the better).
2. Compatible with violent shaking/fluidization.
3. Compatible with induction heating of contents.
4. Capable of holding pressure (the higher the better).
5. Fairly compatible with hydrocarbons.
6. Minimal absorbance of induction energy.
7. Relatively cheap and accessible.

Silicon and PTFE have temperature limitations. Ceramics or borosilicate glass might work, but I have a feeling there is a better metal alloy or composite material out there that would be ideal.

 

[Baluncore]

Ceramics or borosilicate glass might work, but I have a feeling there is a better metal alloy or composite material out there that would be ideal.

The common or kitchen microwave oven is a dielectric heater. It has a metal cavity with an external microwave energy source, a magnetron, that does not like to be shaken.

Ultrasonic cleaners are another way of fluidising a charge. They would not require a liquid when using powdered iron oxide, but would then be classed, or described as, a "non-traditional" processing technique.

Combine the two, and you have a possible continuous process occurring within a waveguide or cavity. The magnetron could be fixed and isolated, with energy coupled into the processing cavity through a quarter-wave flanged gap.

I would be concerned that, in the presence of air, with insufficient iron oxide, an explosion might "flash-off" in the fine plastic dust cloud. As an anaerobic chemical process, it might produce a metallic iron powder suitable for 3D printing sintered metal parts.

 

[bogue]

There is always concern of an explosion, but precautions are taken.

The entire system has a sufficient barrier, is flushed with nitrogen and no more than 200g of feedstock is entered at a time. The volitiles are removed and condensed constantly, and any new input iron-material is going to be sufficient size to mitigate dust explosions, but I am still designing a resonable particle size/surface area to mitigate the explosion without reducing catalytic effectiveness.

Your suggested process with a megnetron and ultrasonic cavitation is intriguing. Would the waveguide need to be attached/exposed to the process conditions? I am unfamiliar with magnetron design specifics but am working on that. Also, am I understanding you properly, in that the ultrasonic cavitation is capable of agitating/mixing powders/solids in the absence of liquids? I haven't seen an application like that before; I will do some more research.

Even if it doesn't, another project I am working on is with hydrothermal depolymerization and ultrasonic caviatation could play a roll in that - although I hadn't considered it much before.

 

[berkeman]

There is always concern of an explosion, but precautions are taken.

Great. Can you say what those precautions are? Thanks.

 

[Baluncore]

Can you say what those precautions are?

Avoid sifting flour by the light of a candle.
https://en.wikipedia.org/wiki/Dust_explosion#Protection_and_mitigation

Also, am I understanding you properly, in that the ultrasonic cavitation is capable of agitating/mixing powders/solids in the absence of liquids?

A fine powder, suspended in a gas, will either clump together and settle, or behave like a liquid, depending on the electrostatic charge on the powder surface.

 

[bogue]

Great. Can you say what those precautions are? Thanks.

Oh, of course - I’ve been treating this as a conversation with experts and less of a public thread. Safety is important. My apologizes.

The experimental reactors in the facility I research at undergo a process safety assessment before commissioning - typically Layer of Protection Analysis, Hazard and Operability, Process Hazard & Risk Assessment, or something equivalent.

The reactors are isolated from other personal and labs by fire walls and distance. The facility is well outside of city limits, in a low population rural area. The experimental reactor section of the facility is built with fire suppression systems (sprinklers and emergency deluge, excellent ventilation, and blast proof enclosures for each reactor system). Each reactor has its own additional ventilation sized appropriately, and fire extinguisher.

The reactor itself is programmed with heating control loops and limitations (auto shutoff above 650C), kill switches at the controls and the entrance to the experimental reactor section. It’s electrically grounded, contains appropriately designed pressure safety valves, and is is thin walled to break easily without pressurizing above relief pressures. Hot surfaces are covered, and moving parts are shielded.

Only trained operators are allowed to touch a reactor, and they are locked out when not in use. Another operator is always aware of the experiment I am running, and close by. No one is allowed to work alone. All experiments are designed with safety limitations in mind, and research plans have multiple peer reviewers familiar with the system prior to commencing.

PPE is mandatory. I wear nomex coveralls, steel toe/steel shank shoes, heat proof gloves, a hardhat and safety glasses, and keep a half face respirator with appropriate cartridges at my station during operation.

There’s lots of other precautions and paperwork to do during setup, operating and cleaning, as well as techniques to stay alert and monitor the controls appropriately, but those are usually custom to each experiment.

 

[bogue]

Avoid sifting flour by the light of a candle.
https://en.wikipedia.org/wiki/Dust_explosion#Protection_and_mitigation


A fine powder, suspended in a gas, will either clump together and settle, or behave like a liquid, depending on the electrostatic charge on the powder surface.

I’m still a bit confused on this. You’re describing fluidization with a flow of gas, and he was talking about fluidization with ultrasonic cleaners.

 

[Baluncore]

Agitation by mechanical shaking, or agitation by using ultrasonics. Both have a gas surrounding the material charge.
The shaker agitates coarse particles, while ultrasonics agitates a fine dust.