16


Analytical Instrumentation


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


Jaap de Zeeuw *, Bill Bromps, Tom Vezza, Rick Morehead, Jan Pijpelink, and Gary Stidsen Restek Corporation, 110 Benner Circle, Bellefonte, PA, USA. * Restek Corporation, weerhaan 9, Middelburg, The Netherlands


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® BOND PLOT columns, accurate purity values can be obtained quickly and reliably, allowing better process stream management.


-Alumina


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.


(Equation 1) F =


retention time of unretained compound in uncoated tubing retention time of unretained compound in coated column


Reliable Impurities Analyses Result from Optimised Deactivations


The highly predictable retention times that result from uniformly coated Rt®


-Alumina BOND columns can


benefit petro labs by reducing the amount of time spent on review and manual integration. Manual processes can also be reduced by the highly selective deactivations, which provide sharp, fully-resolved peaks that can be accurately quantified. The analysis of impurities in propylene and ethylene are an excellent demonstration of column selectivity and how it differs based on the type of deactivation. Restek columns are


available with Na2SO4 or KCl deactivations, and both have been optimised to provide complete separation of hydrocarbon impurities.


Figure 2: Highly consistent column-to-column flow characteristics (F) of Rt® times and less manual integration.


-Alumina BOND PLOT columns result in predictable retention


-Alumina


Figure 1: Inconsistent coating thicknesses result in restrictions that cause significant variation in flow.


Partition Ratio, K (Mean±Std. Dev.)


Rt®


-Alumina BOND (Na2SO4) n = 85


Rt®


-Alumina BOND (KCl) n = 40


2.557 (0.332) 4.600 (0.441)


Efficiency, Plates-per-meter (Mean±Std. Dev.)


1,542 (59.6) 1,206 (113)


Results shown for 1,3-butadiene; Column dimensions: 50 m x 0.53 mm x 10 µm; Sample: refinery gas test mix, 1.0% in nitrogen; Inj.: 5-10 µL test mix; Split flow: 80 mL/min.; Tested isothermally at 130 °C (Na2SO4) or 100 °C (KCl); Detector: FID.


Table I: Tight manufacturing controls result in highly consistent column coatings.


February/March 2010


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