(121e) On-Line Analysis of Nitrogen Containing Compounds in Steam Cracking Effluent

The petrochemical industry is strongly interested in finding new ways to economically apply heavy feedstocks in the steam cracking process. To accomplish this, it is of utmost importance to investigate more thoroughly how the steam cracking process reacts to the high amount of impurities present in these feedstocks. In this work the main focus is on the nitrogen containing compounds that are considerably present is feedstocks such as crude or shale oil. The composition of heavy crude oils and shale oils varies substantially depending on the origin of the mixture. In particular, they contain an increased amount of nitrogen containing compounds compared to the conventionally used sweet crude oils. As nitrogen compounds have an influence on the operation of thermal processes occurring in coker units and steam crackers, and as some species are considered as environmentally hazardous, a detailed analysis of the reactions involving nitrogen containing compounds under pyrolysis conditions provides valuable information. Therefore, a novel method for on-line analysis of nitrogen containing compounds has been developed and validated in a steam cracking pilot plant by feeding pyridine dissolved in heptane. The former being a representative compound for one of the most abundant classes of compounds present in crude and shale oil based feedstocks. The composition of the reactor effluent was determined via an in-house developed automated sampling system followed by immediate injection of the sample on a GC × GC coupled with a time-of-flight mass spectrometer (TOF-MS), flame ionization detector (FID) and nitrogen chemiluminescence detector (NCD). A novel method for quantitative analysis of nitrogen containing compounds using NCD and 2-chloropyridine as an internal standard has been developed and demonstrated. Consequently, the application of the NCD can be considered as a substantial improvement. The experimentally determined minimum detection level of 1.5 pgN/sec obtained using a reference mixture gives an indication that it is possible to trace even ppm and ppb concentrations of nitrogen containing compounds in the analyzed samples. The described work demonstrates the accuracy, repeatability and reproducibility of the methodology for on-line quantification of nitrogen containing products in an abundant hydrocarbon matrix. The high separation power of the GC × GC combined with a sensitive selective detector significantly goes beyond the present state of the art, resulting in a more detailed product detection and quantification. The described methodology is crucial for a proper identification of the formed nitrogen containing products and their subsequent tracing in the downstream ethylene unit equipment.