4 February / March 2018
Industrial Applications of Offline and Online Comprehensive Gas Chromatography
by Melissa N. Dunkle*, Pascal Pijcke, Mubasher Bashir, and Matthijs Ruitenbeek Dow Benelux BV, Herbert H. Dowweg 5, 4542NM, Hoek, The Netherlands Email:
MNDunkle@Dow.com Phone: +31115673772
Comprehensive GC, or GCxGC, is used regularly at Dow Benelux BV, where both offline and online approaches are being utilised in research and development and in routine applications. This technical article describes the offline GCxGC analysis of hydrocarbon streams with comparison of the accuracy and precision to the detailed hydrocarbon analysis (DHA). Also discussed in this article is how a seven parallel reactor setup was coupled to online GCxGC for the evaluation and optimisation of catalyst formulations.
Introduction
The spectrum for hydrocarbon streams is extensive, ranging from light streams of two to four carbon numbers (C2-C4) up to heavy streams, which can extend to C15 and higher carbon numbers. Accurate determination of the hydrocarbon composition of such streams is essential to maximise operating margins; however, analysis of each hydrocarbon stream has different method requirements (Figure 1).
For the lighter hydrocarbon streams, a one-dimensional gas chromatographic (1D GC) method is suffi cient, where the PIONA or DHA methods are typically utilised. The PIONA, or Paraffi n (P), Isoparaffi n (I), Olefi n (O), Naphthene (N), Aromatic (A) analyser, quantifi es each compound class as weight percent in the sample [1]. The DHA method, or Detailed Hydrocarbon Analysis, not only provides the quantifi cation of the compound classes as weight percentage in the sample, but also provides quantitative information for the individual components [2]. Having such detailed information is extremely important as the input data for modelling operating margins.
For heavier hydrocarbon streams, the level of compositional complexity increases. Consequently, a 1D GC method is not suffi cient to separate all components from one another, and as such, two-dimensional GC is required [3]. By utilising comprehensive GC (GCxGC), separation based on carbon number and group type can be achieved, affording the same level of detail as the
Figure 1: Spectrum of hydrocarbon streams.
DHA, but for heavier hydrocarbon streams. However, there are many considerations for the successful implementation of GCxGC, including column selection, detector selection, and modulator, to name a few.
There are two approaches for column selection in GCxGC; a normal column set or a reversed column set can be utilised [4]. With a normal column set, a non-polar stationary phase is selected for the fi rst dimension separation, and a polar stationary phase is utilised for the second dimension. For the reversed column set, a polar stationary phase is installed for the fi rst dimension separation, while a non-polar stationary phase is used in the second dimension. No matter which column set is selected, the fi rst dimension column will be longer than the second dimension and operated at a slow temperature gradient (1-5˚C) with typical total run times between 45-120 min, while the separation on the second dimension column will be typically between 3-10 sec [5]. This,
in turn, requires extremely fast detection after the second dimension column, limiting detector options to those capable of operating at 100 Hz. To transfer the sample from the fi rst column to the second, a modulator is utilised. Flow and thermal modulators are commercially available, and they both serve the same purpose; to collect the fractions eluting from the fi rst dimension column, re-inject the fractions on to the second dimension column, and trap eluents from the fi rst column during the launch of the preceding fraction onto the second column [6].
Typically, GCxGC is utilised as an offl ine analytical technique (i.e. not in process); however, Dow has coupled an analytical GCxGC instrument to a seven parallel reactor setup for the online analysis of complex reaction mixtures generated by catalysis research and development. This article will detail how both offl ine and online GCxGC are utilised for quantifi cation of hydrocarbon compositions.
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