Pressure drop

[renqi]

The pressure drop shown by the HPLC system is different from the pressure drop calculated by ERGUN equation? Why is this so when all parameters were constant.

[tom jupille]

Exactly which parameters did you use?

Remember that the packing particles in an HPLC column are not monodisperse. You can see quite different pressures from two systems using the same nominal particle size column depending on the particle size distribution.

[renqi]

actually i have done an experiment using HPLC. the pressure that is indicated on the instrument is smaller than the pressure calculated from the ERGUN equation.

for the ERGUN equation, the parameters are:
viscosity of solvent = 1.4083 mPa.s
void fraction of the bed = 0.35
particle diameter = 5micometer
length of column = 250mm
volumetric flow rate = 0.5 ml/mim
diameter of column = 4.6mm

bt nw i still cant figure why is that so.

[tom jupille]

How much different were the observed and predicted values?

Remember, "the model is not the reality".

[HW Mueller]

Not knowing the equation, it still seems that another discrepancy may arise when the temp. at which the viscosity ref. was established differs from the experimental temp.

[tom jupille]

Other possible source is the 1 significant-figure values for the particle size and flow rate. Given that pressure is related to the square of the particle size, and is directly related to flow, the pressure could be off by more than 30% just from those variations.

[DR]

does the void fraction of the bed take particle porosity into account, or does it assume perfectly solid spheres?