Study On The Preparation And Properties Of Modified Polystyrene Used On Selective Separation Of Aromatic Hydrocarbons

The separation of aromatics hydrocarbons from aliphatic hydrocarbon ischallenging due to their close boiling points and azeotropes formation. The traditionalseparation methods are extraction and extractive distillation with organic solventssuch as N-formyl morpholine, N-methyl pyrrolidone or sulfolane. The regeneration ofsolvents needs to remove them from the extract and raffinate phase, partial residualsolvents in aliphatic phase needs to be removed using large amounts of water, whichcause an additional investment and energy consumption. In addition, two kinds ofmethods can not obtain high selective factors for feeds with low aromatic content(<20%). Recently, ionic liquids, which are considered to be promising as replacementsfor the aforementioned organic solvents, have been studied widely. However, mostionic liquids have unfavorable chemical properties which disable their use on biggerscales in industries. These limitations are mainly due to instability as a result ofcorrosiveness and moisture instability as well as insolubility in water. Anotherdisadvantages of ionic liquids is its high viscosity and high price. Permeability is anemerging method of aromatic/aliphatic separation. Permeability through polymericmembranes is mainly dependent on solubility and diffusion differences between thecomponents of mixture. The membranes based on polyethylene, polypropylene havedisplayed good film-forming ability and large flux, but swelling of membrane insolvents and low selectivity have been some problems difficult to be solved. Althoughthe membranes based on aromatic polyurethane， polyether amide and polybenzoxazole can not be dissolved by common solvents and displayed highseparation factors, the major roadblocks in pervaporation are the low flux and harshoperation conditions. For all these reasons, the industry has always been eager to lookfor a viable alternative to the conventional aromatics/aliphatics separation processes.Polystyrene have excellent mechanical properties, good chemical and thermalstability. Their excellent performances provide wide applications in chromatography,adsorption, ion-exchange resins and imprinted polymer fields. As a type of absorbentfor potential separation of aromatics/aliphatics, polystyrene has a lower raw materialcost, simple operation and recovery. Compared with traditional separation methods,the separation of aromatics/aliphatics using polystyrene based on selective swelling inaromatic hydrocarbons, mainly depends on the strong π-π interaction between thephenyl group of the styrene and aromatics. In addition, due to the introduction ofpolar groups, polystyrene still can swell in aromatics, but can not do in aliphatics.In this dissertation, we aim at the design and preparation of polystyrene modifiedwith polar groups that can be used on the separation of aromatics/aliphatics. Severalkinds of polystyrene with different structure and composition have been prepared andthe separation effects on a variety of aromatic/aliphatic hydrocarbons have beenstudied.In chapter1, we have made a brief introduction of separation methods ofaromatics/aliphatics; then we reviewed the methods of modification of polystyrene.In chapter2, we have prepared the low molecular weight sulfonate polystyrene.Their properties were fully characterized by nuclear magnetic resonance, Fouriertransform infrared spectroscopy and elemental analyses. Modified polystyrene wasapplied to extract toluene from toluene/n-heptane solution.The results indicates thatwith increasing content of sulfone groups, the distribution coefficient and selectivefactor of toluene obviously increased. This kind of material displayed excellentthermal abilities and good regeneration capacities.In chapter3, we have prepared cross-linked polystyrene beads modified with ester,amide or sulfone groups. The influencing factors of particle size of beads were studied and the effect of modified group content on swelling ratio and selective factor wasobserved. The results indicate that the crosslinked polystyrene beads modified withamide groups displayed high swelling ratios in the range of43-54%. The beadsmodified with sulfone groups which was prepared using post-sulfonylation revealedhiger separation factors. A comparison of the experiments with three kinds ofsulfonating agent led to the conclusion that the order of reactivity of sulfonatingagents is：benzenesulfonyl chloride〉4-toluene sulfonyl chloride〉methanesulfonylchloride. The separation factors of sulfonated polystyrene with4-toluene sulfonylchloride and benzenesulfonyl chloride were also higher than that of methanesulfonylchloride. The swelling ratio was24%and the separation factor was3.27when thetoluene content was13wt.%in the feed mixture.In chapter4, the cross-linked polystyrene beads with channels were successfullyprepared by occupation and etch of CaCO3whiskers. These beads were sulfonated andthe distribution of sulfur was characterized by EDAX. Results indicate that thechannel structure facilitated the uniform distribution of sulfone groups. Comparedwith that of nonchannel polystyrene beads, the saturated swelling time of channelbeads obviously decreased. These beads were applied to separate benzene/hexane andresults indicate that swelling ratio was31%and selective factor was6.37.