Particle Entrainment in a Counter-Current Flow Reactor

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Discussion Overview

The discussion revolves around the challenges of particle entrainment in a counter-current flow reactor, specifically a rotating kiln designed for processing solid feedstock. Participants explore the relationship between gas flow velocity and the entrainment of solid particles, considering factors such as particle properties and flow dynamics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the relationship between gas flow velocity and solid particle entrainment, seeking empirical or fundamental relationships.
  • Another participant draws a parallel to fluidized bed reactors, prompting clarification on the role of gravity in the kiln setup.
  • A participant describes the kiln's configuration, noting the inclination and the potential for fine, low-density particles to be entrained by the gas flow, affecting residence time and mass transfer.
  • Concerns are raised about calculating the maximum gas velocity that can be used without causing particle entrainment, highlighting the importance of particle properties such as density and size distribution.
  • One participant suggests Stokes' law as a possible reference for understanding particle behavior in gas flows, particularly for small, spherical particles.
  • Another participant recommends consulting textbooks on cement production for further insights into the problem.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the calculation of maximum gas velocity and the factors influencing particle entrainment. There is no consensus on a straightforward method to address the issue, and multiple viewpoints on the relationship between gas flow and particle behavior are presented.

Contextual Notes

The discussion highlights the complexity of the problem, including the need for specific particle characteristics and flow dynamics, which may not be fully resolved in the current exchanges.

davidgrant23
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Hi there,

I am currently looking to build a rotating kiln to provide the necessary heat/res. time/gases to react a solid feedstock. The problem that I can envision, however, is that the solid (which is fed as small particles) may become entrained in the counter-current gas flow inside the rotating kiln.

Is there some fundamental or empirical relationship between the velocity of the counter-current gas flow and the rate of entrainment of the solid particles?

Cheers
 
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Your query sounds a little bit like a fluidized bed.

Where does gravity act in relation to your kiln?

Can you elaborate on what you visualize as entrained?
 
anorlunda said:
Your query sounds a little bit like a fluidized bed.

Where does gravity act in relation to your kiln?

Can you elaborate on what you visualize as entrained?

Hi Anorlunda,

The rotating kiln is inclined slightly away from horizontal (1-5 degrees). The solid particles are fed at the elevated end and travel towards the lower end through rotation of the kiln. While they are traveling a counter-current gas flow is passed through the kiln.

The problem I can envision is that my feedstock (which are fine, low density particles) may become entrained in the counter-current gas flow if the gas velocity is sufficiently high. This will affect the residence time inside the kiln, as well as the mass transfer characteristics. What I'm not sure of, however, is how to take the gas velocity inside the kiln (which I can calculate) and combine that information with the flow properties of the solid particles (density, PSD, shape etc.) to calculate what is the MAXIMUM gas velocity I can run at without having the particles get picked up by the counter current flow and exit the reactor via the gas outlet (instead of the solid outlet).

This problem differs from a fluidized bed reactor as in that case the gas is passed vertically through a stationary bed (inducing fluidization). In this case the particles are traveling horizontally and the flow is counter-current.

Hopefully this cleared up the problem a little.

Cheers,
Dave
 
That sounds very non-trivial. I'm not sure there is a straightforward way to calculate that max velocity.
The size and density of the solid particles would be critical parameters. centimeters? MIcrons?

I'm afraid your question is over my head. Hopefully, others can help.
 
If your gas is flowing upward, the particles settling downward, the particles are well separated, the particles are roughly spherical, and the particles are small, then google Stokes' law. You might also google pneumatic conveyor for cases where flow velocities are higher.
 
Try to check textbooks on cement production.
Regards,
 

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