Analytical Instrumentation

Optimise Ethylene and Propylene Testing with Stable Flow Rt®-Alumina BOND PLOT Columns

Mar 22 2010

Author: Jaap de Zeeuw *, Bill Bromps, Tom Vezza, Rick Morehead, Jan Pijpelink, and Gary Stidsen on behalf of Restek Corp

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The petrochemical industry depends on alumina PLOT columns for purity testing of ethylene and propylene, products that represent billions of dollars to the industry annually. The value of alumina is that it is one of the most selective adsorbents for light hydrocarbons available, allowing all the unsaturated C1-C5 isomers to be separated with the highest degree of resolution. Selectivity is influenced by deactivation and different deactivation salts are used to optimize separations for polar and nonpolar compounds. Although alumina PLOT columns are not new to the petro market, flow retention reproducibility has always been difficult to achieve and most commercially available columns show large variation in flow and retention, as well as poor loadability. This can cause significant problems with integration, calibration, and, ultimately, quantification. New PLOT column technology from Restek increases flow reproducibility, which results in highly predictable column-to-column retention times. By using new Rt®-Alumina BOND PLOT columns, accurate purity values can be obtained quickly and reliably, allowing better process stream management.

Precision Coating Ensures Consistent Flow and Predictable Retention Times
With traditional PLOT column technology, it is difficult to control stationary phase particle deposition, resulting in variable coating thicknesses along the length of the column. The positions in which the layer is thicker act as a flow restriction for the whole column (Figure 1). Since the number and intensity of these flow restrictions will vary column-to-column, highly variable flow rates are observed. In contrast, the process used to produce Rt®-Alumina BOND PLOT columns results in extremely consistent coatings, which, in turn, provide uniform flow characteristics and predictable retentions times. The data in Table I clearly demonstrate that both phase thickness, as measured by capacity factor (K), and coating efficiency (plates-per-meter) are extremely reproducible.

To demonstrate the effect of coating uniformity on column flow behavior, flow resistance (F) can be calculated using the retention time of an unretained compound (Equation 1). When F values are calculated for a set of columns of the same phase and configuration, tested under the same conditions, variation in flow, as indicated by F, can be used to assess column coating uniformity. As shown in Figure 2, Rt®-Alumina columns have extremely consistent flow characteristics, which ensures that highly predicable retention times are achieved.

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