Abstract
Pd-based membranes have potential applicability in the PDH process to selectively remove hydrogen. In the current work, H2 flux values and coke formation kinetics applying representative gaseous feed mixtures under varying operating conditions are evaluated and modelled. From these experiments, it is clear that coke formation is very likely under the operating conditions required for an integrated catalyst and membrane system, i.e. temperatures of 450–500 °C and low hydrogen to propene ratios. However, coking could be limited at lower operating temperatures, and a decrease to at least 300 °C, or preferably, to 250 °C is required to obtain a sufficiently stable membrane operation in the conditions observed in a non-integrated sequential reactor-membrane process design. In the sequential reactor-membrane process, the effect of steam content on catalyst and membrane activity and stability were investigated. Results show that steam is required to obtain good catalyst stability, but that the amount of produced H2 is independent on steam content between 7% and 20%. A stable membrane performance is obtained at 200 °C at hydrogen recovery factor (HRF) values varying from 38% to 50%. The developed model of membrane deactivation capitalizes on the observations of a critical H/C ratio beyond which the H2 flux is stable, at a given temperature, and similarly a critical temperature at a given H/C ratio that limit the coking process. The model fits these critical ratios and predicts well the temporal behaviour.