Evaluate SMB Chromatography for Your Separation

Here's a quick way to determine whether the technique suits your processing needs.

By Bruce Pynnonen, Shawn Feist, Yogesh Hasabnis, David Albers and Timothy Frank, The Dow Chemical Co.

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Once a satisfactory separation has been achieved, pulse test data can be interpreted to determine profile advancement factors. We define the profile advancement factor as normalized liquid flow within each zone:

fk = Qk tstep /Vcolumn(1)
where fk is the profile advancement factor for Zone k, Qk is the liquid flow rate within Zone k, tstepis the step time for the process and Vcolumn is the total empty volume of a column or column section. The basic procedure involves the following steps [7]:

1. Start with Zone III. From the pulse test chromatogram, choose a BV value that includes a large fraction of fast eluter but only a small fraction of slow eluter (at the leading edge of the second peak). The goal is to select a value that achieves high recovery of fast eluter in the raffinate while minimizing contamination by the slow eluter. Set f3, the profile advancement factor in Zone III, equal to this BV value.

2. Go to Zone IV. Choose a BV value that includes some of the fast eluter but only a small fraction of this component (at the leading edge of the first peak). The goal is to select a value that prevents fast eluter from moving forward into Zone I but is as large as possible to minimize the required amount of fresh elution solvent that needs to be added to Zone I. Set f4 equal to this value.

3. Then address Zone I. Choose a BV value that includes a majority of the slow eluter and almost all of the fast eluter (at the trailing edge of the first peak). The goal is to choose a value that prevents slow eluter from falling back into Zone IV but is as small as possible to minimize the required amount of elution solvent. Set f1 equal to this value. This procedure can be visualized as the mirror image of the procedure used to select f3, by interpreting the chromatogram from right to left instead of left to right.

4. Choose a maximum face velocity, the maximum velocity of total liquid flow at the entrance to a column. Normally this doesn't exceed about 10 cm/min (about 3 gal/min per ft2 of cross-sectional area). A study of face velocity effects may be conducted in the course of running pulse tests. Maximum velocity will determine the step time.

5. Now turn to Zone II. Determine a value for f2 by the process material balance. The value should fall between those of f4 and f3 such that f4 < f2 < f3 < f1.

6. Find corresponding flow rates for each zone from Eq. 1 and the process material balance.

PROCESS ECONOMICS
If pulse test results indicate a proposed separation is technically feasible, the next step is to evaluate process economics. In large-scale commodity separations the major cost comes from the need to isolate the product from one of the effluent streams and recover and recycle the elution solvent [5]. A typical process dilutes the solute by a factor of two or more. The magnitude of this dilution effect will decrease as the difference between the profile advancement factors f1 and f4 is reduced; flow rate values obtained from a pulse test may be used to estimate the amount of dilution. If this analysis suggests the process is economically attractive, then we recommend performing a mini-plant study. Use the flow rates estimated from the pulse test analysis for the startup flow rates and step time.

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