Residence Time Distribution of A Tubular Reactor

Abstract

Most flow reactors are neither ideal plug flow nor continuous stirred tank reactors. This makes it difficult in accounting for actual conversion obtained from such reactors thus causing much concern to the industrialists especially Chemical Engineers. The extent of departure from the two ideal limits of flow reactors remained unclear until the concept of residence time distribution (RTD) was developed and applied to real reactors, to assess the extent of non-ideality, plus their effects on reactor performance. To study the empirically generated RTD behavior, a tubular reactor was designed and built with improved features to ensure minimum axial dispersion. It was found that the RTD functions generated were generally of the shapes expected, with long tails beyond the peak concentration exit age, but with mean residence times ( ), and distribution variance fairly large, and sensitive to flow rates. While a range of experimental conditions were tried, the run with the highest turbulent Reynold Number (Re) revealed = 46s, which differed markedly from the expected (i.e., calculated) reactor space time ( ) of 17s. This significantly higher empirical was due to transport delay ( 15s) and delay due to actual axial dispersion (31s) as obtained under the peak of the E(t) curve. Furthermore, it was also found that variance of 60s 2 obtained in this work signified that the axial dispersion was high, despite high Re of 3.3 giving a calculated Peclet Number of 69. Also, when n-CSTR theoretical model was applied to that same run, an n value of 35 was estimated, which was good compared to the theoretical value of n = However, at lower flow rates and Re, the implied n in the n-CSTR model was too optimistic and high, with n =15. However, would have tallied better with the high observed axial dispersion. But that was a weakness of n-CSTR theory, not of the empirical results. Thus, every reactor is neither a CSTR nor PFR but a continuum exhibiting features of ideality of both reactors. It is recommended that RTD be applied as a tool not only for troubleshooting existing reactors but also in test running new ones as it would give insight into designing future reactors with improved performance. Keywords: Residence Time Distribution, Plug Flow Reactor, Tubular Reactor, Axial dispersion, Peclet number.