Bitumen Mixing tank

[shubhamw]

Hi all!

I have a tank designed for dissolving rubber in bitumen .

Bitumen is heated at 180 degree celsius.
The tank diameter is 2.3 metres and height is 1.5 metres, the tank is conical bottom.
Now, the problem is that when we are adding the rubber through a screw conveyor at a rate of 30 kg/minute,
the rubber powder is accumulating on the same spot where it is being added .
We are using a jetfoil/paddle type agitator with 2 paddles one on the top and one on the bottom, with three blades each , designed to throw the bitumen in opposite directions to create a cycle. There is also one baffle along the length and there are coils for hot oil circulation along the circumference also.
The agitator is attached to a gearbox with 15:1 ratio, the motor rpm is 990 rpm. So, an output of 66rpm
Please advise how this problem of agitation can be improved so that the rubber is quickly dissolved.

 

[LittleInch]

Your description is quite good, but would be greatly enhanced with a plan and section showing where the agitators are relative to the injection point and what the flow pattern / swirl is.

Questions are: where in the tank and how is the rubber being added? (above liquid on in the liquid?) what do you mean by a baffle along the length? length of what - I had assumed this was a vertical cylindrical tank? what is viscosity at 180 C?

How much rubber is added per tank in total or is there a contant throughput of bitumen - if so at what flow rate?

Can you see inside the tank in operation through observation windows?

As much info as possible may get you the assistence you are looking for, but currnetly too many unknowns / conflicts.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[shubhamw]

agitators are relative to the injection point and what the flow pattern / swirl is. : the agitator is mounted in the centre that is 1.15 metres from either side.The rubber is being dosed from 200 mm from the tank beginning about 900mm away from the agitator moving outwards from the centre. There is considerable vortex formation also. Is it good?

Questions are: where in the tank - from the top we are dosing rubber , so it is falling on top of bitumen

and how is the rubber being added? (above liquid using a screw conveyor)

what do you mean by a baffle along the length? : one single baffle whose height is same as the cylindrical vessel that is 1.5 metres.
for better mixing/flow obstruction.

I had assumed this was a vertical cylindrical tank? - yes it is a cylindrical tank indeed
what is viscosity at 180 C? - 70 - 100 centipoise

How much rubber is added per tank in total = 600 kgs and 4400 Litres of bitumen by volume so total batch of 5,000 Litres

Can you see inside the tank in operation through observation windows? : Negative

Please ask more questions.

 

[geesamand]

This sounds like a very challenging application. I have seen basic paddle style mixers be outperformed by *orders of magnitude* when the correct mixer impeller type / size / speed is implemented.

What is the viscosity of this bitumen? Are there dead zones in the tank?

A scale lab model may be very well justified, especially if the throughput of this process affects your profitability and maximum throughput directly improves profit. Look for a mixing vendor who has the experience, technology, and lab equipment to support this kind of work.

David

 

[geesamand]

I should also mention that I've seen difficult mixing applications improve by a factor of 10x and even 100x when scale models are used in the lab environment. My company's lab has left customers simply flabbergasted with the magnitude of improvement over tradition hydrofoil or pitched blade turbine performance. Typical investment in the lab test is under $20k.

Of course if you're looking for maybe 5% improvement and anything better won't add value, then the small investment in lab testing may not be worthwhile.

David

 

[LittleInch]

Have you considered adding the rubber into a pumped circualtion line instead, posisble even upstream of an axial pump or some other high shear device which create large turbulence by sucking from the bottom of the tank and re-injecting into the top?

You might need to experiment with the relative flow rates of bitumen and rubber to make it work more effectively, but at least the two would be mechanically agited in a closed pipe this way.

Just a thought.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[MJCronin]

I didn't know that rubber disolves in bitumen......

Are you mixing up some kind of fuel for a boiler ? (They used to burn tire chunks along with coal....limited sucesss)

Perhaps the "screw conveyor/agitator" combination is not the best idea.

Perhaps you could entrain the ground rubber into diesel fuel and use an eductor to inject it into the tank.

This would, of course, dilute the bitumen.... But you could operate the tank at a somewhat lower temperature, thus preventing the rubber from charring.

 

[shubhamw]

@ Littleinch - Yes, a high hear mixer is connected to the bottom of the tank.
It recirculates the bitumen back into the tank . Flowrate is 900 Litres per minute.Its same as centrifugal pump.

Injection of rubber inline is a good idea . But you need to suggest more properly what you are trying to say.

Rubber is used for road paving along with bitumen and mixed with chips for bettter road life. Rubber adds elasticity .