Constant Pressure vs Constant Flow Packing

[M. Farooq]

Hello,

Does anyone have experience with packing less than 5 micron particles in the forum? The particles I have are polymeric and are pretty challenging to pack apparently because of shear thickening effect. A suggestion by colloidal rheologist was to use really low flow rates (it was counter intuitive) and then increase the flow rate (or pressure) once the column is filled. By this approach the column was tightly packed (extruded material came out as solid rod - other approaches to pack the column material and extrude as a rod did not work very well) but the peaks were split!

The peak splitting does not make much sense! Any suggestions regarding packing a shear thickening material would be appreciated.

Regards.

[tom jupille]

There is definitely an "art" to it.

When I was working with PS-DVB resin-based columns in the late 70's, the packing was done with a constant-pressure (pneumatic amplifier) pump. The trick is 2-fold:

1. limit the pressure (the exact pressure depends on the rigidity of the material. We had to run a series of test packings with different pressures for each new batch of material).

2. Once the flow stabilizes at the chosen pressure, quickly disconnect the column, wipe off the excess resin that extrudes out and put on the end fitting. If you do it too fast, the column will be "overpacked" and the pressure will be too high. If you wait too long, the bed will recompress in operation, which will result in a headspace and peak shape problems (including peak splitting). As with the packing pressure, the timing has to be worked out by "successive approximations" (that sounds a lot better than "trial and error"!).

Even then, these columns have a maximum flow rate and pressure above which the bed will compress, which often sets up a "runaway" compression and pressure increase.

[M. Farooq]

This material is essentially incompressible (to say sort of). It is good till 10,000 psi. So I don't have these problems of the material extruding out. I think the resins of the previous years are quite different than modern ones especially the % cross-link quite high these days. There are two challenging some aspects:

Small particle size and secondly shear thickening. The dense suspension of the particles becomes a rock (like corn starch)! I am wondering if this is what happens when the column is packed and this lead to relatively lower efficiency. So from a rheological perspective...column packing art is nothing but a high pressure filtration! But the tons of factors affecting the cake quality so formed inside is quite complex

There is definitely an "art" to it.

When I was working with PS-DVB resin-based columns in the late 70's, the packing was done with a constant-pressure (pneumatic amplifier) pump. The trick is 2-fold:

1. limit the pressure (the exact pressure depends on the rigidity of the material. We had to run a series of test packings with different pressures for each new batch of material).

2. Once the flow stabilizes at the chosen pressure, quickly disconnect the column, wipe off the excess resin that extrudes out and put on the end fitting. If you do it too fast, the column will be "overpacked" and the pressure will be too high. If you wait too long, the bed will recompress in operation, which will result in a headspace and peak shape problems (including peak splitting). As with the packing pressure, the timing has to be worked out by "successive approximations" (that sounds a lot better than "trial and error"!).

Even then, these columns have a maximum flow rate and pressure above which the bed will compress, which often sets up a "runaway" compression and pressure increase.

[rhaefe]

I have packed a few PS-DVB columns in the past myself. Tom mentioned already packing pressure as an important parameter. In my experience (mainly RP packings, PS-DVB based, up to 85% X-linked) suspension and packing solvents plus packing temperatures are critical as well.
For some columns we used different pressures as well as different packing solvents during the packing process.

Split peaks are mainly caused by (1) a partially obstructed frit, (2) an imperfection on the top of the column bed or (3) an imperfect or broken column bed.

[M. Farooq]

I have packed a few PS-DVB columns in the past myself. Tom mentioned already packing pressure as an important parameter. In my experience (mainly RP packings, PS-DVB based, up to 85% X-linked) suspension and packing solvents plus packing temperatures are critical as well.
For some columns we used different pressures as well as different packing solvents during the packing process.

Thanks. I resolved the issue of split peaks by using a different batch. I don't use solvents, although the particle can tolerate solvents, they are not recommended. Could you mention a generic procedure that you used in terms of pressures and packing temperatures for 85% cross-link PS-DVB?

Regards