Optimization and evaluation of radially interconnected versus bifurcating flow distributors using computational fluid dynamics modelling.

Two main groups of flow distributors, viz. "bifurcating distributors" (BF) and "radially interconnected distributors" (RI), as well as some hybrid distributors were investigated. Computational fluid dynamics was used to evaluate the performance of the distributors and to establish the design yielding the most uniform velocity field and the smallest variance for the bands emerging from the distributor. A minimum channel width of 100 µm was considered to allow the use of micro-milling techniques for chip fabrication. The main factors that influenced the values of band variances were identified. The performance of the distributors was found to correlate most strongly with the volume of the flow distributors. The separation bed should be positioned immediately after, but not against the flow distributor. It was concluded that BF distributors perform best in terms of band variance. The values of band variances for the BF distributor decreased with increasing angle between bifurcation branches and the lowest value of about 0.01 mm(2) was found for α=175°. Both BF and RI flow distributors were found to perform reasonably well when imperfections were present in the structure. However, severe blockages (exceeding 75% of the cross-sectional area and length) of channels in, especially, BF flow distributors may jeopardize their performance.

Errors involved in the existing B-term expressions for the longitudinal diffusion in fully porous chromatographic media Part I: computational data in ordered pillar arrays and effective medium theory.

Numerically solving the effective diffusion in a simplified representation of a chromatographic bed, it was found that the B-term expressions that have up to now been used in the literature, and which can all be reduced to either Deff=(gamma mDm+k'gamma sDs)/(1+k') or Deff=(gamma meDm+k''Dpart)/(1+k''), can no longer be considered to be unconditionally valid. This could be demonstrated by showing that the simulated diffusion data are in agreement with the mathematically sound effective medium theory (EMT), whereas the B-term expressions used up to now in literature are in conflict with the EMT, a theory that is widely accepted in all other fields of science. It is also shown that the use of the existing B-term expressions can lead to very large measurement errors (up to a 100% and more) for the determination of the stationary phase diffusion coefficient gamma sDs from peak parking experiments. The representation of the B-term diffusion should in the future hence be based on the Deff expressions that can be derived from the EMT. These are physically sound and are also more accurate than the classical B-term expressions used up to now.