ON-LINE SAMPLING AND GAS CHROMATOGRAPHIC METHOD FOR A MICRO-SCALE GAS-LIQUID OXIDATION REACTOR

摘要

The development and application of a novel 5-cc micro reactor system for gas-liquid hydrocarbon oxidation kinetics studies under commercially-relevant conditions is described. A unique system feature is the use of a novel automated method for direct sampling and analysis of the liquid phase. This is accomplished by means of a custom-designed sampling valve that is integrated into the micro reactor body and connected to d GC system for direct injection of the vaporized liquid sample via heated capillary transfer tubing. The advantages of this approach over conventional manual sampling include: (1) the effect of sample size on the instantaneous reactor volume is negligible; (2) post-reaction of the sample once it exits the reactor is minimized; (3) the sample statistics are improved over conventional sampling methods; (4) light-boiling components and dissolved gases are directly captured for subsequent analysis; (5) reactor operation is more safe than commercially-available laboratory reactors owing to the small volume (5 cc or less); and (6) unattended operation with precise intermittent sampling of the liquid phase is possible. The sampling concept Was first demonstrated using a custom-designed test vessel whose geometry mimicked a 5 cc stirred gas-liquid microreactor. Two modes of liquid sampling were evaluated: (1) flashing of the liquid sample into a calibrated loop of a VICIR Valco six-port gas sampling valve; (2) flashing of the liquid sample directly into the injection port of a HP 5890 Series II gas chromatograph. A variation of the second mode involved the addition of a ten-foot heated section of capillary tubing between the exit of the vessel and the GC injector. The test samples consisted of an aqueous mixture of ca. 1 % each of CI to C4 alcohols, and a crude reaction product obtained from a commercial-scale cyclohexane oxidation reactor. It was found that the ability to fill the Valco valve sampling loop was dependent upon the pressure developed by the flashing liquid. For a fixed vessel pressure, the amount of vapor in the loop reaches a maximum and then decreases as the pulse valve duration increases. This occurs because the pressure created by the flashing liquid greatly reduces the amount of sample that can exit the valve. In the case of the direct injection methods, this effect was not observed. Hence, the total GC area counts were reproducible within 0.1 % for a fixed value of the pulse valveduration. The variation that uses the transfer line is particularly useful for applicationswhere the reactor must be located some distance from the GC system. Other detailson the valve design and test results are also provided.