Pervaporation hydrocarbon separation

Figure 13 Combined distillation/pervaporation systems for (a) propylene/ propane splitting and (b) aromatic/aliphatic hydrocarbon separation. (Part a from Ref. 234 part b from Ref. 235.)...

Thus, the permeation of hydrocarbons in polymer membranes is governed by the basic regularities typical of permeation of low MW penetrants, modified however by certain peculiarities related to the stmcture and shape of hydrocarbon molecules. We will now discuss the physicochemical regularities of hydrocarbon separation and removal using polymer membranes, by trying to reveal the relationship between the chemical stmcture of polymers and their separation properties with respect to mixtures containing hydrocarbons. It follows from literary data that mbbery polymers are mainly used in gas/vapor separation processes for selective separation of hydrocarbon vapors from their mixtures with air as well as in pervaporation processes for the removal of hydrocarbons from their aqueous solutions. In practice, glassy polymers are used for separation of olefins and paraffins as well as for separation of aromatic, ahcyclic, and aliphatic hydrocarbons. 

A number of hydrocarbon separations have been intensely studied and piloted in recent years and commercialization is expected soon. Pervaporation is expected to be one of a number of proven options for sulfur and benzene removal from fuels and olefin/ paraffin separations. These plants will use robust, specially engineered polymer membranes, installed in large-scale tubular modules. 

One of the most active areas of research involves the use of pervaporation for or-ganic/organic separations. The industrial interest is extremely high and several authors have reported the use of pervaporation for separation of aromatic/ahphatic hydrocarbon mixtures at high temperature (Matsui and Paul, 2003), as well as the separation of binary organic mixtures involving methyl tert-butyl ether (MTBE) and methanol (Yoshida and Cohen, 2003). As an example, Sulzerchemtech has already reported the possibility for industrial recovery of methanol and ethanol from organic mixtures. 

Membrane Pervaporation Since 1987, membrane pei vapora-tion has become widely accepted in the CPI as an effective means of separation and recovery of liquid-phase process streams. It is most commonly used to dehydrate hquid hydrocarbons to yield a high-purity ethanol, isopropanol, and ethylene glycol product. The method basically consists of a selec tively-permeable membrane layer separating a liquid feed stream and a gas phase permeate stream as shown in Fig. 25-19. The permeation rate and selectivity is governed bv the physicochemical composition of the membrane. Pei vaporation differs From reverse osmosis systems in that the permeate rate is not a function of osmotic pressure, since the permeate is maintained at saturation pressure (Ref. 24). 

C. Streicher, P. Kremer, V. Tomas, A. Hubner and G. Ellinghorst, Development of New Pervaporation Membranes, Systems and Processes to Separate Alcohols/Ethers/ Hydrocarbons Mixtures, in Proceedings of Seventh International Conference on Pervaporation Processes in the Chemical Industry, R. Bakish (ed.), Bakish Materials Corp., Englewood, NJ, pp. 297-309 (1995). 

S. Matsui and D.R. Paul, Pervaporation Separation of Aromatic/Aliphatic Hydrocarbons by a Series of Ionically Crosslinked Poly(n-alkyl acrylate) Membranes, J. Membr. Sci. 213, 67 (2003). 

Krishna and Paschek [91] employed the Maxwell-Stefan description for mass transport of alkanes through silicalite membranes, but did not consider more complex (e.g., unsaturated or branched) hydrocarbons. Kapteijn et al. [92] and Bakker et al. [93] applied the Maxwell-Stefan model for hydrocarbon permeation through silicalite membranes. Flanders et al. [94] studied separation of C6 isomers by pervaporation through ZSM-5 membranes and found that separation was due to shape selectivity. 

Is the current separation the most suitable or should other methods of separation be considered For example, distillation is commonly the workhorse in the chemical industry and is often the immediate choice. Other separation methods, such as pervaporation, can be sometimes more effective especially if the volatility differences are small. Pervaporation is a membrane-based process with the difference that the permeate appears as a vapor, thus permitting solute recovery and recycle. For example, benzene can be recovered from hydrocarbon streams using this method in fairly high concentrations and in a usable form ready for recycle. Many alternative separation methods must be considered, and one should not simply bank on past experience or expertise. 

Membrane Pervaporation Since 1987, membrane pervaporation has become widely accepted in the CPI as an effective means of separation and recovery of liquid-phase process streams. It is most commonly used to dehydrate liquid hydrocarbons to yield a high-... 

In another study by Nishiyama et al. [53], the Vapour-phase Transport method was applied on alumina supports. No permeation of 1,3,5-triisopropylbenzene (kinetic diameter 0.85 nm) could be observed through the 10 pm thick membrane. Mordenite has parallel channels with an elliptical pore dimension of 0.65 x 0.7 nm. Pervaporation of benzene-p-xylene (molar ratio 0.86) at 22°C resulted in a separation factor of 164 (total flux 1.19 10" mol.m s ). The theoretical value based on the gas-liquid equilibrium amounts to 11.3. Apparently, the mordenite-based membrane shows high selectivity for aromatic hydrocarbons. 

Streicher C, Kermer P, Tomas V, Hubner A, and EUinghorst G. Development of new pervaporation membrane, system and processes to separate alcohols/ethers/hydrocarbone mixture. In Bakish. R., ed. Proceedings of the 7th International Conference on Pervaporation Process in the Chemical Industry. Englewood, NJ Bakish Material Corporation, 1995 297-309. 

In oil processing, separation of aromatic isomers Cg (ethylbenzene 7b= 136°C,p-xylene 7b= 138.3°C, m-xylene Ty, = 139.1°C, >-xylene T], = 144.4°C) is required. According to the literary data, the following isomers of hydrocarbons are separated p-xylene/m-xylene, p-xylene/o-xylene, -hexane/2,2-dimethylbutane, -hexane/3-methylpentane, and n-butane/f-butane [8,83,130-137]. Pervaporation method is the most effective for this purpose. To separate the isomers, membranes based on various polymers were used. Good separation for aU isomer mixtures was attained by the polyimide Kapton film (fip = 1.43-2.18) but parylene films and cellulose acetate also exhibited a relatively high separation factor (fip = 1.22-1.56 and /3p = 1.23-1.56, respectively). Temperatures >200°C were required to obtain a reasonable flux through the polyimide film and a pressure of about 20 atm was necessary to keep the feed stream liquid [8]. 

The polymer materials mainly used for the membranes are glassy polymers, the first and foremost polyimides. The use of glassy polymers having a rigid ensemble of macromolecules results in high separation effectiveness. Separation effectiveness in pervaporation processes is characterized by the separation factor, /3p, which is determined by the diffusion component, /3d, and the sorption component, /3s [8,55]. Let us consider the effect of chemical composition of polymer membranes on their transport properties with respect to aromatic, alicyclic, aliphatic hydrocarbons and analyze ways to improve these properties. 

Ruckenstein E. Emulsion pathways to composite pol3mieric membranes for separation processes. Colloid Polym Sci 1989 267 192-191. Park JS and Ruckenstein E. Selective permeation through hydrophobic-hydrophihc membranes. J Appl Pol Sci 1989 38 453 61. Wang Y, Hirakawa S, Wang H, Tanaka K, Kita H, and Okamoto K. Pervaporation properties to aromatic/non-aromatic hydrocarbon mixtures of cross-linked membranes of copoly(methacrylates) with pendant phosphate and carbamoylphosphonate groups. J Membr Sci 2002 199 13-27. 

Matsui S and Paul DR. Pervaporation separation of aromatic/aliphatic hydrocarbons by crossUnked poly(methyl acrylate-co-acryUc acid) membranes. J Membr Sci 2002 195 229-245. 

Separating hydrocarbons Various Continuous pervaporation Various Embryonic... 

The first effort in the development of pervaporation membranes was aimed at their potential to separate organic mixtures, especially those of hydrocarbons [2j. Following the results of the latest research [27], and the operation of pilot plants [29], this goal may be reached in the next couple of years. 

Matsui, S. and Paul, D.R. 2003. Pervaporation separation of aromatic/aliphatic hydrocarbons by a series of ionically crosslinked poly(n-alkyl acrylate) membrane. J. Memh. Sci. 213 67-83. 

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