20


Table 2. Geometric characteristics of bead packing in cutaway sample pieces Porosity


Overlap


1.40 1.45 1.50 1.55


Designed Geometrical analysis


0.575 0.538 0.501 0.463


0.561 ±0.033 0.542 ±0.008 0.511 ±0.051 0.442 ±0.027


121.3 107.3 94.3 82.2


Pore Flat to Flat Distance (µm) Designed Geometrical analysis


118.3 ±6.8 108.1 ±6.2 96.2 ±5.5 78.5 ±4.5


The number of pores analyzed were n = 63, 64, 71 and 80 for α = 1.40, 1.45, 1.50 and 1.55, respectively. The errors given are ei= 2σ where σ is the standard deviation.


Table 3. Performance characteristics of the four tested columns α


μ1


1.40 1.45 1.50 1.55


(ml)


1.278 1.078 0.991 0.917


θr exp


1.11 1.00 0.99 0.99


ϒ1


0.407 0.617 0.935 1.134


The values in Table 3 highlight the extremely good agreement between the design and experimental void volumes in all but one column (α = 1.40). It can also be seen that there is a linear relationship between the overlap factors and peak skewness. It is generally thought that a lower porosity and greater internal surface area would result in a more symmetrical RTD profile and greater chromatographic resolution [8]. In this instance, the designed specific surface areas of the α = 1.40 and α = 1.55 columns were 9.39 and 11.13 mm-1


Figure 5. Typical residence time distribution profiles of two columns.


, respectively, while the skewness values for the latter column was significantly


higher. A possible reason for this unexpected result could involve the printer resolution, where micro-rugosities on the bead surface are produced during the printing process – see Figure 4. In a column with greater bead overlap, and therefore smaller pores, the rugosity could have greater consequences in terms of band broadening. This finding suggests that extra-particle porosity alone is not a significant parameter in column performance but other factors as small imperfections in the outer shape of the beads will also influence column behaviour. Efforts to improve the chromatographic performance of 3D printed columns must therefore focus on ensuring homogenous flow channels instead of achieving the lowest porosity that the 3D printing process would allow.


Figure 6. Comparison of theoretical and experimentally determined void volumes and peak skewness of RTD profiles.


Conclusions


We have demonstrated the ability of 3D printing to create whole columns containing an ordered lattice of beads, with only subtle changes in column packing parameters, requiring fine control of bead positioning and overlap. The experimentally determined mean residence volumes were consistent with the design void volumes, indicating a good agreement between the CAD models and 3D printed artefacts.


Contrary to conventional wisdom, our findings have shown that a low extra- particle porosity and a high internal surface area do not automatically imply more symmetrical RTD profiles, for the case of 3D printed, ordered packings.


Due to the precision, scale and versatility that 3D printing offers in the production of chromatography columns, we expect this approach to revolutionise the field of packed bed research, enabling greater insights into the effects of morphological features of packed beds on performance.


Figure 4. Microscope images of cutaway sample parts displaying samples with; (a) α = 1.40, (b) α = 1.45, (c) α = 1.50, (d) α = 1.55 at a magnification of 5x.


References


[1] Vissers J.P.C., Hoeben M.A., Laven J., Claessens H.A., Cramers C.A., Hydrodynamic aspects of slurry packing processes in microcolumn liquid chromatography. Journal of Chromatography A, 2000, 883: p. 11-25. [2] Zimina T., Smith R.M., Highfield J.C., Myers P., King B.W., Study of the flow development during the slurry packing of microcolumns for liquid chromatography. Journal of Chromatography A, 1996, 728: p. 33-45. [3] Gritti F., Guiochon G., Mass transfer kinetics, band broadening and column efficiency. Journal of Chromatography A, 2012, 1221: p. 2-40. [4] Baranau V., Hlushkow D., Khirevich S., Tallarek U., Pore-size entropy of random hard-sphere packings. RSC Publishing - Soft Matter, 2013, 9: p. 3361-3372. [5] Schure M.R., Kroll D.M., Davis H.T., Simulation of ordered packed beds in chromatography. Journal of Chromatography A, 2004, 1031: p. 79-86. [6] Malkin D.S., Wei B., Foigel A.J., Staats S. L., Wirth M.J., Submicrometer plate heights for capillaries packed with silica colloidal crystals. Analytical Chemistry, 2010, 82: p. 2175-2177. [7] Fee C.J., Nawada S., Dimartino S., 3D printed porous media columns with fine control of column packing morphology. Journal of Chromatography A, 2014, 1333: p.18-24. [8] Guiochon G., Felinger A., Shirazi D.G, Fundamentals of Preparative and Nonlinear Chromatography, 2nd ed. 2006, Amsterdam: Elsevier.


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