Energy efficiency of membrane vs hybrid membrane/cryogenic processes for propane recovery from nitrogen purging vents: A simulation study

Abstract

When pure propane contained in pipelines or storage tankers has to be flushed out by nitrogen fluxes for maintenance purpose, propane recovery is obviously of interest both for environmental issue and for propane valorization. A low temperature condensation is generally used for that purpose and is attractive because it allows to recover propane under liquid state. However, the energy required is very important to reach a high recovery separation target. An alternate approach can be the combination of a nitrogen or propane permselective membrane with a conventional condensation step to improve the global efficiency of the separation process. The present work has been intended to investigate this approach and evaluate to which extent a hybrid process can bring an added value. This study shows first a map of membrane separation performance of propane over nitrogen from data of the open literature. Interestingly it shows that either N2-selective or conversely, propane-selective membranes can be used and also that propane–selective membrane will require higher area for atmospheric down-stream pressures (1 bar) compared to down-streams kept at lower pressures (0.1 bar). The selection of the most nitrogen and propane selective membrane is used in order to simulate hybrid processes where separations performances (purity of the propane condensed versus energy required) are compared to the baseline cryogenic standalone process. For low propane contents in the feed mixture (xin,C3H8 = 0.5 and 5% mol/mol), it is shown that the use of a propane selective membrane module with a vacuum system seems to be the most energy efficient process while reaching a high purity (>98%). Based on rigorous process simulations, other hybrid processes cases were performed considering SSZ-13 zeolite membranes and the different separation performances are discussed. It is shown that a membrane/cryogenic process can indeed be less energy demanding to recover propane from N2-vents. However, according to the recovery target the choice of the membrane type can vary from an organic to a zeolite membrane.