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The concept for contact time used as a scale-up factor of the isothermal system was expanded as apparent contact time to the temperature gradient system where the reaction field within the thermal diffusion column (TDC) reactor is limited to a higher temperature zone. The apparent contact time in the temperature gradient system was defined as LS/F using three operational factors of pyrogen length L[m], cross-sectional area of reactor tube S[m2], and methane feed rate F[mol/min].The pyrogen temperature was set at 1400K. The dehydrogenative coupling of methane was carried out by varying single operational factors, and the following results were above 0.45mmol/min, the methane conversion shows a negative correlation with F. The methane conversion is kept constant when F is under 0.45mmol/min.(3) When L is lengthened from 0.195 to 0.360m, the methane conversion is increased remarkably.(4) Both product yields (gas and oil) and physical properties of products are related to the methane conversion.(5) When the LS/F value is less than 0.1m3min/mol, the methane conversion is increased with an increase in LS/F. Within the limited LS/F range, the apparent contact time could be applied as a scale-up factor of the TDC reactor.(6) It is considered that both recycling effects of ratio and zone within the TDC reactor lead to change in methane conversion.