Pore Size Analysis of Hollow Fiber Membranes by Porometry

The superior mass-transfer characteristics of the hollow fiber construction, which is a considerably large lumen surrounded by a large area of a thin porous membrane wall has caused it being used in a number of commercial applications in broadly differing fields such as medical (blood fractionation), water reclamation (purification and desalination), gas separation, and techniques using pervaporation.

Other applications of this membrane type are in the biochemical industry or bioseparation and bioreactors. Specifically its advantageous characteristics when compared with conventional filtration and separation systems are high volume efficiency, modest energy requirements, low operating costs and two modes of operation and low operating costs. These benefits are offset to a certain extent by more-frequent fouling and initial capital expense.

The challenge faced by those requiring to determine the pore size distribution through the walls is to determine a technique which can functionally transfer a fluid radially through a narrow fiber, making pore size measurements across a flat sheet is simple by comparison. The challenge of the analysis of hollow fibers has been overcome by an exclusive preparation technique which involves sealing an individual fiber into a special sample holder.

Sample Preparation

Acrylic tube sample holders, 8 mm outer diameter and 30 mm length were prepared by drilling the bore to 1.5mm and countersinking each end.

Each of the sample fibers were cut and opened by inserting a TFE coated wire down the lumen (center of the tube). This wire is in place during the following procedure: the sample was glued and sealed inside the acrylic holder using quick-drying epoxy resin. The loose end of the fiber was sealed with the glue. The wire was re-moved after the glue had started to harden in 2 or 3 minutes.

Sample mounted and glued in acrylic sample holder. Sealed fiber end is shown arrowed.

Figure 1. Sample mounted and glued in acrylic sample holder. Sealed fiber end is shown arrowed.

Analysis

Four polymeric hollow fiber sample identified as 1, 2, 3, 4, each having an outside diameter of 1 mm and a wall thickness of 100 µm were prepared as above. The completed sample holders were installed in an external sample manifold in place of the usual sample holder assembly and block, and analyzed on a Porometer 3G z using Porofil wetting fluid. The 3G z was equipped with both 10 and 100 ml/min sensors, and both ranges were used. Around 256 data points were measured over the selected pressure or pore size range.

External Sample Manifold. The red arrow indicates the attachment point for the acrylic sample holder.

Figure 2. External Sample Manifold. The red arrow indicates the attachment point for the acrylic sample holder.

Results

Figure 3 shows the measured data of flow versus pressure for wet and dry runs for all four samples are shown. The mean flow pore sizes were determined similarly at the pressure intersection of half the dry flow data with the wet flow curve. These bubble point values and these values are presented in Table 1.

Measured "wet" and "dry" flow versus pressure. The differences between all the samples is already evident.

Figure 3. Measured "wet" and "dry" flow versus pressure. The differences between all the samples is already evident.

Table 1.

  SAMPLE 1 SAMPLE 2 SAMPLE 3 SAMPLE 4
Maximum Pore Size (microns) 1.3861 1.1132 0.7506 0.5173
Mean Flow Pore Size (microns) 0.2384 0.9801 0.1547 0.1828
Minimum Pore Size (microns) 0.1460 0.7503 0.0905 0.1076
Bubble Point Pressure (bar) 1.3861 0.5749 0.8527 1.2371
Bubble Point Flow Rate (l/m) 0.1407 0.0647 0.0080 0.0518

 

The pore size distributions were determined from the measured pressure assuming a zero contact angle. Number distributions were determined based on the internal geometric surface of the sample fibers from gross dimensions. These distributions are graphically represented in Figures 4 and 5.

Differential percent flow versus pore size. Differences between sub-micron pores are highly resolved.

Figure 4. Differential percent flow versus pore size. Differences between sub-micron pores are highly resolved.

Differential pore number percent area versus pore size. Some similarities between samples 3 and 4 can now be seen.

Figure 5. Differential pore number percent area versus pore size. Some similarities between samples 3 and 4 can now be seen.

Conclusions

A new sample preparation technique has been employed successfully to demonstrate the sub-micron resolution of the Porometer 3G on what are usually considered to be "difficult" samples. This ability to measure pore size distributions in the (membrane) wall of a single hollow fiber is of significant value to manufacturers and users of these materials.

This information has been sourced, reviewed and adapted from materials provided by Quantachrome Instruments.

For more information on this source, please visit Quantachrome Instruments.

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