Applications of membrane unit operations in ethylene process

This work, analyzing a traditional ethylene process, identifies specific applications in this process where membrane technologies can be used to replace typical technologies resulting in some benefits. Design data are provided for several cases along with a discussion on pros and cons of various options referring to a typical cracking plant installed in Europe. Gas separation by means of membrane operations was proposed to remove hydrogen from the compressed gas of an ethylene cycle. Three different schemes for membrane operation were analyzed for H2 purification. A high concentration (99%) and/or recovery (99%) of H2 were achieved depending on feed composition, stage-cuts, pressures and the number of membrane stages. Membrane application reduces significantly the energy required in the cryogenic distillation ''cold train and hydrogen/methane separation'', allowing less severe operating conditions with respect to the typical cycle. Gas separation was also proposed for oxygen-enriched air to be used instead of air in combustion and/or decoking phases. Hydrocarbon removal from water streams is proposed by means of membrane contactors, able to put in contact two phases in a very low volume of the membrane unit. Depending on the operating temperature, a removal ranging from 90% to almost 100% was achieved. Membrane contactors were also used for acid gas removal from a furnace effluent, obtaining an abatement of 97% for CO2 and 99% for H2S. In a typical ethylene cycle, coke particles are present in gaseous streams. Water scrubbing is a possibility to capture the finest fraction. In this context, by treating this water using microfiltration, it was possible to recover 90% of the water allowing its re-use in the plant. An exergetic analysis, provided for some cases of interest, showed the better efficiency of the membrane operations when compared to conventional industrial systems.