Paraffin separation membrane-distillation

The rest of this Chapter is organized as follows. Section 10.2 outlines a typical olefin/paraffin separation scheme in a conventional ethylene plant, which includes the selected distillation colunms for retrofitting with a membrane unit. Section 10.3 describes the mathematical modeling of the membrane unit and the various assumptions that were made. Section 10.4 presents the procedure of simulating a retrofitted HMD system and the preliminary techno-economic analysis that was carried out. Section 10.5 covers the formulation of MOO problem, which includes selection of objectives, decision variables and constraints in the optimization problems studied. In Section 10.6, results from the optimization of two objectives for various cases are presented and discussed. Finally, conclusions of this study are given in Section 10.7. 

Xu, L., Rungta, M., Brayden, M.K. et al. (2012) Olefins-selective asymmetric carbon molecular sieve hollow fiber membranes for hybrid membrane-distillation processes for olefin/paraffin separations. Journal of Membrane Science, 423 24, 314—323. 

Though offering good olefin/paraffin separation performances at laboratory scale, those materials were subject to a loss of solvent by evaporation, which resulted in dramatic fall of olefin/paraffin separation performances. In order to maintain the separation performances of those membranes, the feed had thus to be saturated with vapor in order to prevent the drying of the membrane. This is a critical point as, at industrial scale, propylene/propane mixtures are dehydrated before being fractionated by distillation. 

Concurrently with the work on carbon dioxide and hydrogen sulfide at General Electric, Steigelmann and Hughes [27] and others at Standard Oil were developing facilitated transport membranes for olefin separations. The principal target was the separation of ethylene/ethane and propylene/propane mixtures. Both separations are performed on a massive scale by distillation, but the relative volatilities of the olefins and paraffins are so small that large columns with up to 200 trays are required. In the facilitated transport process, concentrated aqueous silver salt solutions, held in microporous cellulose acetate flat sheets or hollow fibers, were used as the carrier. 

Shape selectivity was demonstrated by the separation of /i-paraffins from a naphtha distillate using a zeolite-CaA membrane [21]. The remaining fluid is enriched in branched parrafins and has a high octane number. 

The separation of olefin/paraffin gas mixtures is one of the most energy-intensive processes in the petrochemicals industry, because it is mainly performed by cryogenic distillations. Membrane processes using the concept of facilitated transport have been considered as an intriguing alternative to cryogenic distillation, as they can simultaneously improve both permeability and selectivity. Silver ions incorporated in liquid membranes act as olefin... 

Another very large potential application of membranes in ethylene plants is replacing the C2 and C3 splitters. An example of a possible process design is shown in Fig. 7.15. In this example, a two-step membrane system equipped with propylene-permeable membranes is used to split a 50/50 propylene/propane overhead stream from a depropanizer column into a 90% propylene stream and a 90% propane stream. Both streams could then be sent to distillation units for polishing, but the size of columns required would be much reduced. For this design to be feasible, membranes with an olefin/paraffin selectivity of 5 to 10 are required. Many other designs that combine membranes and distillation columns to achieve good separation are possible [23]. 

A recent study estimated that about 10,000 BTU of energy is used armually for olefin-paraffin distillation. The distillation process is used commercially in this separation process. However, membrane separation with low energy consttmption and with relative ease in operation, can be significantly competitive with the distillation process [6]. Therefore, carbon membranes can contribute greatly to the petrochemical industry. 

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