GEK Wiki / Filter-Pressure-Drop-vs-Char-Diameter
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Filter-Pressure-Drop-vs-Char-Diameter

This version was saved 15 years, 1 month ago View current version     Page history
Saved by jim mason
on February 5, 2009 at 1:11:38 pm
 

 

GEK Filter Pressure Drop vs. Char Diameter

 

Introduction

Our intent here was to determine a reasonable char diameter to recommend for the GEK filter canister.

 

A set of experiments were conducted to determine if an equation used to predict the pressure drop in a packed (non-moving) bed (the Ergun equation) would match experimental results. It did to a reasonable approximation. The chart provided below can therefore be used to estimate the amount of pressure drop to expect from the packed bed char filter (and only the packed bed of char, not the pressure drop of the entire system) at different flow rates.    Currently unknown is the effect and rate of clogging from tar or flow restriction from condensation in the filter.  These predictions and experimental results apply only for a fresh bed of dry char.

 

Recommendations

Interpretting the results, char sieved through 1/4" and then 1/8" wire mesh (5 mm) appears to be a reasonable recommendation for the filter. It will reach a pressure drop of 5inH2O at 100 m3/hr. Finer material may be reasonable if flow rates are in a lower range, say 10-30 m3/hr.

 

Predicted Pressure Drop

Assumes 9" (23 cm) packed bed height (filled filter) and char having 0.38 void space (measured for 1/4-1/8" sieved char). Bars show 20% error.

1/4-1/8" sieved char is ~5 mm

1/8-1/16" sieved char is ~2 mm

 

Note that pressure drop in the system is additive, the pressure drop predicted here is only for the effect of pressure drop across the filter media, not any of the plumbing/GEK or even filter canister.

 

Lacking more specific data on the properties of the produced gas, the properties of air at 25°C were used.  The predictions are likely a slight overestimate of the drop with gas at higher temperatures.

 

A quick summary of the Ergun equation:

  • Bed Length: Pressure drop is proportional to the length of the packed bed (longer bed, more pressure drop)
  • Superficial Velocity: The velocity the gas would have through an empty bed. Superficial velocity is just volumetric flow/bed cross-sectional area. Larger diameter tube, lower velocity.
  • Void Space: The empty space between particles, which plays a large role in the pressure drop. Void space will change as condensation occurs. A rough estimate for void space can be made by wetting the media, measure a volume of the media (say 250 mL), fill to the top with water, shake out any air. Drain the water and measure its volume. Void Space = drained volume/total volume. Small diameter particles will hold more water, so don't trust this for fine media (more specific draining process needed).
  • Equivalent Spherical Diameter (Dp): The diameter the particle would have if it was a sphere. Equation for determining Dp given in Flow through Packed Beds and Fluidized Beds [pdf] (R. S. Subramanian) as Dp=6*(Volume of Particle/Surface Area of Particle).
  • Fluid density and viscosity play a role, in the experiments and predictions below values for air were used. Whether values for gas are different enough to merit revisions should be explored.

The Ergun equation (from D. Thornhill, UW, linked above):

 

Results

The Ergun equation was tested with two sizes of sieved char, and various bed depths for one size of char.

Experimental Results (note that deeper beds better matched the prediction).

See the Excel file (bottom) for more detailed results.

 

Apparatus

The apparatus used to measure the pressure drop. Note that flow was under pressure, not vacuum, but the difference should be insignificant.

 

Char Media

Char used in the test. This '1/4-1/8" char' was passed through a 1/4" wire mesh and then fines removed with an 1/8" wire mesh.

Char used in the test. This '1/8-1/16" char' was passed through a 1/8" wire mesh and then fines removed with an 1/16" wire mesh.

 

Excel File with Results:

Packed Bed Pressure Drop vs. Char Size.xls

 

Additional Resources:

Air Property Calculator

 

 

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