Multidimensional gas chromatography for the characterization of permanent gases and light hydrocarbons in catalytic cracking process
Highlights
► Measurement of O2, N2, CO, CO2 and light hydrocarbons in one 12 min MDGC analysis. ► A practical, effective configuration employs of two 3-port planar microfluidic devices in series. ► PLOT capillary columns were employed for the separation of hydrocarbons. ► CO and CO2 were converted to methane using a post-column methanizer. ► CO, CO2, and hydrocarbons were detected using FID. ► Flow modulated TCD was employed to measure O2 and N2.
Introduction
The characterization of oxygen, nitrogen, carbon monoxide, carbon dioxide (permanent gases) and light hydrocarbons is an important analysis for petrochemical and chemical industries [1], [2], [3], [4], [5]. These compounds are often encountered in various critical chemical processes like catalytic cracking for example [6], [7], [8]. Gas chromatography is a technique of choice due to the high degree of volatility of the compounds involved. Unfortunately, without the employment of cryogenic chromatography, no single chromatographic stationary phase is capable of adequately separating all of these analytes [9]. For instance, on a porous polymer based stationary phase like divinyl benzene, oxygen, nitrogen, and carbon monoxide are perfectly co-eluted. Hydrogen, oxygen, nitrogen, methane, and carbon monoxide can be well separated using a molecular sieve stationary phase; however carbon dioxide and heavier hydrocarbons are irreversibly adsorbed. The presence of water in the sample as vapor can also deactivate the stationary phase. Carbon molecular sieve or carbon black has been used to separate these analytes, but inadequate separation was encountered for oxygen and nitrogen especially when the concentrations of the two compounds are at the percent level [10], [11], [12]. Further, there is evidence of elevated reactivity of the adsorbent toward double and triple bond hydrocarbons like propylene, methyl acetylene, and 1,3-propadiene.
To compensate for the lack of an appropriate stationary phase for the characterization of permanent gases and light hydrocarbons, a common chromatographic practice involves the use of a series of multi-port switching valves to selectively transfer the effluent of one column to another, or bypass certain columns at various stages in the analysis to prevent the columns from being contaminated by the analytes in the matrix as in the case of carbon dioxide or water on a zeolite stationary phase like molecular sieve. Under such a scheme, it is common for a system to have three to five multi-port rotary or slider valves and a number of packed or micropacked columns or porous layer open tubular columns involved to achieve the separation required [13], [14], [15]. While this approach performs adequately, there are some important constraints, like the requirement of a dedicated gas chromatograph as a custom-built analyzer. The use of multi-port switching valves can lead to degraded chromatography due to excessive void volume encountered from connections and tubing, the lack of inertness along the sample flow path, and port-to-port cross leaks from valve rotor wear. The requirement for an additional external oven to house the valve assemblies, and the need for a highly accurate pneumatic system to minimize valve-timing shifts over the course of normal use are further complicating factors. These constraints can substantially add to the overall cost of ownership and have a negative impact on reliability and availability of the instrument for analysis.
In the present article, we introduce a practical and effective integrated gas chromatographic system for the measurement of permanent gases and light hydrocarbons in one single analysis with the capability of addressing the above mentioned shortcomings. The system employs an arrangement involving the use of two three-port planar microfluidic devices in series of each other with built-in microfluidic gates and a mid-point pressure source to perform critical in-oven chromatographic tasks like column isolation and backflushing.
Section snippets
Experimental
An Agilent 6890N gas chromatograph (Agilent Technologies, Wilmington, Delaware, USA) was used for all analyses. The chromatograph was equipped with one split/splitless inlet (operated at 150 °C), one flame ionization detector (operated at 250 °C), and one flow modulated thermal conductivity detector (operated at 150 °C), a nickel based methanizer (operated at 375 °C) to convert carbon monoxide and carbon dioxide to methane, and a three-channel auxiliary pressure module. Two SilFlow three-port
Chromatographic system design
To achieve the separation of permanent gases and light hydrocarbons without the use of multi-port valves, two three-port planar microfluidic devices were integrated together with a mid-point pressure source as depicted in Fig. 1. Each three-port device with nuts and ferrules installed weighs a mere 8 g. Embedded 50 μm × 500 μm fluid logic gates are incorporated with the flow architecture of the planar microfluidic devices [17], [18], [19]. A fluid logic gate is an orifice in a flow path, implemented
Conclusions
Despite its importance and popularity, the actual hardware used to perform connections and the implementation of techniques involved with packed or micropacked columns are not fully compatible for use with capillary gas chromatography. Large thermal mass fittings can cause cold spots, leaks from graphitized/Vespel ferrules can negatively impact system reliability, and attempts to adapt the techniques to capillary column chromatography with commercially available products such as removable fused
Acknowledgements
Kaye Burns, Dan Martin, Mark Marinan, Jeff Mason, Dave Walter and Andy Szigety of Dow Analytical Technology Center were acknowledged for their support. Michelle Baker and Vicki Carter were recognized for their assistance.
Robert Shellie is the recipient of an Australian Research Council Australian Research Fellowship (project number DP110104923).
SilFlow and SilTite are trademarks of SGE Analytical Science, Ringwood, Australia. CP-PoraBOND Q, CP-Molsieve 5A, VF-Waxms are trademarks of Agilent
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