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This method is for determining the composition of aromatic hydrocarbon types and their carbon number distribution in detergent alkylates. Samples are principally composed of mono-n-alkylbenzenes having the empirical formula CnH2n-6 and in a carbon number range from C15 C22. The method is particularly useful for determining the composition of the more hydrogen deficient aromatic types, CnH2n-8, etc., which may be present as impurities. However, it is also applicable to the characterization of heavy alkylate samples, which have a carbon number range from approximately C22 C40. Non-aromatic hydrocarbons in the finished product alkylate are not determined.
Calibration sensitivities are derived from concentrates which are representative of the average isomer distribution present in the alkylate. Since the sensitivities of some individual mass isomers differ from each other significantly and may vary due to changing processing conditions it is difficult to determine the absolute accuracy of the results. However, this method is useful to characterize impurities and indicate relative changes in concentration.
The hydrocarbon types are referred to by "Z number" or "negative constant" in the empirical formula subscript for hydrogen. Thus, "Z = 6" represents hydrocarbons with the formula CnH2n-6. The analysis yields arrays of Z numbers by carbon number expressed as mass percent.