How this calculator works
For linear flow, divide bed length by superficial velocity. For volumetric flow, calculate the cylindrical geometric bed volume and divide by flow. This is an empty-bed contact time. A packed column with void fraction epsilon has a fluid hold-up time of approximately epsilon times this value, before considering retention or dispersion.
Formula
t = L/u = (pi d^2 L/4)/Q. Packed-bed fluid hold-up time is approximately epsilon t.
Worked example
A 0.1 m bed at superficial velocity 0.001 m/s gives 100 s. A 1 m long, 0.1 m diameter column at pi/4000 m3/s gives 10 s.
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Frequently asked questions
Is this chromatographic retention time?
No. Adsorption and partitioning can make solute retention longer than nominal fluid residence.
Does porosity enter the calculation?
The output uses geometric empty-bed volume. Multiply by fluid-accessible void fraction when estimating hold-up time.
Can I solve for diameter with linear flow?
Diameter requires volumetric flow. Its solve mode uses the volumetric-flow fields.