Establishing the Maximum Carbon Number for Reliable Quantitative Gas Chromatographic Analysis of Heavy Ends Hydrocarbons. Part 3. Coupled Pyrolysis-GC Modeling

The purpose of this research work is to determine the maximum single carbon number (SCN) which can be reliably quantified using High Temperature Gas Chromatography (HTGC) analysis of heavy oil hydrocarbons, accounting for (i) thermal cracking risk and (ii) the non/incomplete elution. To that end, an in-house coupled numerical Pyrolysis-GC model has been developed, capable of calculating the degree of elution and of simulating the migration, partitioning, and pyrolysis conversion of a mixture of 11 heavy n-alkanes spanning the range from nC₁₄H₃₀ to nC₈₀H₁₆ throughout the GC column. On the basis of this model and using a commonly used column configuration and temperature program, two conclusions have been made: (i) half of the mass injected of nC₈₀ thermally decomposed before nC₇₀ has eluted, suggesting a possible coelution of both nC₇₀ and the pyrolysis products of nC₈₀ and therefore making the HTGC analysis of nC₇₀ and heavier n-alkanes no longer reliable, and (ii) alkanes heavier than nC70 take progressively longer to elute completely from the column, compromising the resolution of the peaks, i.e., nC₇₀ takes 2.5 min and nC₈₀ takes 8.5 min. Moreover, nC₈₀ remained 12.9 min in the isothermal plateau before complete elution, implying that the nC₈₀ peak will be overlooked and masked by the FID plateau signal, in combination with column bleed products. Therefore, in the case study the maximum reliable SCN which can be quantitatively analyzed with HTGC will be the lighter components than nC₇₀.