Aromatics/aliphatics separation

The membrane used depends on the nature of the organics. Poly(vinyl alcohol) and cellulose acetate [14] have been used to separate alcohols from ethers. Polyurethane-polyimide block copolymers have been used for aromatic/aliphatic separations [17]... 

Based on the principles of n-complexation, we have already developed a number of new sorbents for a number of applications. These include sorbents for (a) olefin/paraffin separations [9-12], (b) diene/olefin separation or purification (i.e., removal of trace amounts of dienes from olefins) [13], and (c) aromatics/aliphatics separation and purification (i.e., removal of trace amounts of aromatics from aliphatics [14]. Throughout this work, we have used molecular orbital calculations to obtain a basic understanding for the bonding between the sorbates and sorbent surfaces, and further, to develop a methodology for predicting and designing n-complexation sorbents for targeted molecules (e.g. Ref 11). 

Matuschewski, H. and Schedler, U. 2008. MSE-modified membranes in oiganophihe pervaporation for aromatics/aliphatics separation. 224 124—131. 

Studies on ternary liquid-liquid equilibria have centred on specific challenges facing the chemical, petrochemical, pharmaceutical and biochemical industries. A summary of the ternary liquid-liquid equilibria data (selectivities, capacities) for aliphatic-aromatic separations with ionic hquids is presented in Table 1. The aromatic content (expressed as a percentage) is included to provide an indication of the region in which the selectivity and capacity values are determined (since these are a function of the overall composition). Selectivity and capacity values of traditionally or commercially employed solvents have been included for comparative purposes. The favourable characteristics required for solvents suitable for aromatic-aliphatic separations are the following large selectivity and capacity values, high solubihty of the aromatic components in the ionic hquid, poor solubihty of the ahphatic components in the ionic liquid and the availability of fairly simple and inexpensive means to recover the ionic hquid from the extract and raffinate phases. 

Meindersma et al. [ 17] conducted a comprehensive study on the selection of ionic liquids for aromatic-aliphatic separations by investigating the extraction efficiency of ionic liquids as a function of cation, anion, aJkyl chain length, temperature and composition. As in the study of Selvan et al. [45], it was observed that a shorter alkyl group on the cation was desirable for higher selectivities (but lower capacities). 

The high selectivity of [emim][Tos] is due to the aromatic ring present in the tosylate anion, which allows for greater discrimination in an aromatic-aliphatic separation, favouring the former. 

The desulfurization of liquid fuels using pervaporation has been increasingly investigated over the last few years [84]. As middle distillates contain mainly aromatic sulfur compounds, desulfurization membranes tend to make use of developments in aromatic-aliphatic separation. The most frequently used membrane materials investigated for the desulfurization of liquid hydrocarbon mixtures are polyurea-polyurethane, polysiloxane. Nation, cellulose triacetate, and poly-imide [84]. In addition to a range of processes for the desulfurization of naphtha fractions patented by ExxonMobil, Transionics, and Marathon Oil, only the S-Brane process developed by W. R. Grace and Sulzer has been tested beyond the laboratory scale [84]. 

Aromatics/aliphatics separation is accomplished by solvent extraction. A number of solvents have been used (Bailes, 1983). Although these separation processes are efficient, they are energy intensive, and more importantly, the solvents (such as sulfolane) increasingly pose as environmental hazards. Another possible separation technique is fractional distillation. It is, however, difficult because of the close relative volatilities. For benzene/cyclohexane, the mixture has a minimum azeotrope at about 53%. Therefore, acetone is added as an entrainer and a complex hybrid system (distillation combined with extraction in this case) can be used for separation (Stichlmair and Fair, 1998). 

Because of the importance of aromatics/aliphatics separation and the problems associated with solvent extraction, possible alternatives have been studied. These include liquid membranes (Li, 1968 1971 Goswami et al., 1985), pervaporation (Hao et al., 1997), and the use of liquid inclusion complexes (Atwood, 1984). No selective sorbents are known for aromatics/aUphatics separation. It is, however, certainly possible to develop such sorbents based on jr-complexation. In the benzene molecule, the carbon atom is sp hybridized. Hence, each carbon has three sp orbitals and another Pz orbital. The six Pz orbitals in the benzene ring form the conjugative n bond. The Pz orbitals also form the antibonding n orbitals, which are not occupied. When benzene interacts with transition metals. 

For liquid mixtures of type (3), there are a number of different types of mixtures aromatic-aliphatic separation sulfur-containing organics from other petroleum constituents olefins from paraffins, etc. 

Also Meindersma Haan presented a conceptual process design for the separation of aliphatic/aromatic hydrocarbons, in which the authors concluded that ILs which show a high aromatic distribution coefficient, Danm = 0.6 m/m, with a reasonable aromatic/ aliphatic selectivity, Samm/aUf = 40, could reduce the investment costs of the aromatic/aliphatic separation to about M 25 to 30 and the annual costs to M 16 to 17 respect to total investment costs in the typically applied sulfolane extraction process (Meindersma de Haan, 2007). 

Meindersma, G. W. de Haan, A. B. (2007). Conceptual process design for Aromatic/ Aliphatic Separation with Ionic Liquids. Proceedings of European Congress of Chemical Engineering (ECCE-6). Copenhagen, 16-20 September 2007. 

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