Olefins membrane-distillation

In membrane distillation, two liquids (usually two aqueous solutions) held at different temperatures are mechanically separated by a hydrophobic membrane. Vapors are transported via the membrane from the hot solution to the cold one. The most important (potential) applications of membrane distillation are in water desalination and water decontamination (77-79). Other possible fields of application include recovery of alcohols (e.g., ethanol, 2,3-butanediol) from fermentation broths (80), concentration of oil-water emulsions (81), and removal of water from azeotropic mixtures (82). Membrane (pervaporation) units can also be coupled with conventional distillation columns, for instance, in esterifications or in production of olefins, to split the azeotrope (83,84). 

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. 

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. 

As documented in Chapter 5, zeolites are very powerful adsorbents used to separate many products from industrial process steams. In many cases, adsorption is the only separation tool when other conventional separation techniques such as distillation, extraction, membranes, crystallization and absorption are not applicable. For example, adsorption is the only process that can separate a mixture of C10-C14 olefins from a mixture of C10-C14 hydrocarbons. It has also been found that in certain processes, adsorption has many technological and economical advantages over conventional processes. This was seen, for example, when the separation of m-xylene from other Cg-aromatics by the HF-BF3 extraction process was replaced by adsorption using the UOP MX Sorbex process. Although zeolite separations have many advantages, there are some disadvantages such as complexity in the separation chemistry and the need to recover and recycle desorbents. 

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. 

Whilst the enhancement of unwanted side reactions through excessive distortion of the concentration profiles is an effect that has been reported elsewhere (e.g., in reactive distillation [40] or the formation of acetylenes in membrane reactors for the dehydrogenation of alkanes to olefins [41]), the possible negative feedback of adsorption on catalytic activity through the reaction medium composition has attracted less attention. As with the chromatographic distortions introduced by the Claus catalyst, the underlying problem arises because the catalyst is being operated under unsteady-state conditions. One could modify the catalyst to compensate for this, but the optimal activity over the course of the whole cycle would be comprised as a consequence. 

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... 

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. 

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]. 

The fact that silver ions form complexes with olefins has led to at least one patent for separation of unsaturated from saturated hydrocarbons (13). Very recently, however, Amoco was forced by economics to shelve plans to replace distillation columns in olefin plants with membrane-based separators involving hollow fibers of cellulose acetate saturated with solutions of silver nitrate (37). 

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. 

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