Separation of olefins and paraffins

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. 

Polyimides as Membrane Materials for Separation of Olefins and Paraffins... 

The separation of olefin and paraffins, particularly the C2 (ethane/ethene) and C3 (propane/propene) pairs is an extremely important and demanding separation in die petrochemical industry (56). It is currently performed via cryogenic distillation and is thus energy (and capital) intensive. Thus, the use of zeolites to perform this separation has been studied intensely. While many zeolites have been investigated to selectively adsorb the olefin, PSA-type approaches are not currently used for this separation. More recently, small-pore zeolites have been reported wherein a clear kinetic separation is observed in that the diffusion of propylene is dramatically faster than that of propane (57, 58). This potentially represents a significant breakthrough in the field. 

Each olefin is more soluble than the paraffin of the same chain length, but the solubiHty of both species declines as chain length increases. Thus, in a broa d-boiling mixture, solubiHties of paraffins and olefins overlap and separation becomes impossible. In contrast, the relative adsorption of olefins and paraffins from the Hquid phase on the adsorbent used commercially for this operation is shown in Figure 2. Not only is there selectivity between an olefin and paraffin of the same chain length, but also chain length has Httie effect on selectivity. Consequentiy, the complete separation of olefins from paraffins becomes possible. 

Anhydrous silver hexafluorophosphate [26042-63-7] AgPF, as well as other silver fluorosalts, is unusual in that it is soluble in ben2ene, toluene, and xylene and forms 1 2 molecular crystalline complexes with these solvents (91). Olefins form complexes with AgPF and this characteristic has been used in the separation of olefins from paraffins (92). AgPF also is used as a catalyst. Lithium hexafluorophosphate [21324-40-3] LiPF, as well as KPF and other PF g salts, is used as electrolytes in lithium anode batteries (qv). 

A few companies, eg, Enichem in Italy, Mitsubishi in Japan, and a plant under constmction at Eushun in China, separate the olefins from the paraffins to recover high purity (95—96%) linear internal olefins (LIO) for use in the production of oxo-alcohols and, in one case, in the production of polylinear internal olefins (PIO) for use in synthetic lubricants (syn lubes). In contrast, the UOP Olex process is used for the separation of olefins from paraffins in the Hquid phase over a wide carbon range. 

It is very difficult to separate the mixtures of olefins and paraffins produced in the above processes because of similar physical properties of the vapors. To solve this problem, the membrane method holds much promise. When using this method the correct choice of the membrane material is very important. 

Polyphenylene oxides are quite promising materials for separation and removal of olefins and paraffins. In terms of their... 

Shimazu A, Hachisuka H, and Ikeda K. Separation, dissolution and diffusion characteristics of olefins and paraffins using fluorine-containing polyimide membranes. Kagaku Kogaku 1996 60(4) 262-263. 

In the early 1970s, Union Oil developed and patented a chromatographic system based on the principle of a simulated moving bed (SMB) [6-8]. A schematic of a SMB unit is shown in Figure 1.4. Streams of the mobile phase (the desorbent ) and of the feed to separate are continuously injected into the column while streams of the less retained (the raffinate ) and the more retained components (the extract ) are continuously withdrawn, all at constant flow rates. The rotary valves switch periodically the positions in the columns where these streams enter or exit. The operation of SMB imits is discussed in detail in Chapter 17. Manufacturing facilities have been built and are operated for the fractionation of various petroleiun distillates, for example, the selective separation of p-xylene, o-xylene and ethylbenzene from the C7-C8 aromatic fraction of light petroleum reformates, the separation of olefins from paraffins in feed mixtures of hydrocarbons having 10 to 14... 

The SMB technology was developed by UOP and its major field of application is in the area of binary separations. For example, SMB has been used in the chemical industry for several separations known as SORBEX processes [1-3], which include, among others, the PAREX process for p-xylene separation from a Cs aromatic fraction [4], the OLEX process for the separation of olefins from paraffins, the SAREX process to separate fructose from glucose [4] and the MOLEX process [5]. Simulated moving bed is being used particularly for separation of enantiomers from racemic mixtures or from the products of enantioselective synthesis [6,7]. It has been used for the production of fine chemicals, and petrochemical intermediates, such as Cg-hydrocarbons [8], food chemistry such as fatty acids [2], or certain sugars from carbohydrate mixtures [8] and protein desalination [9]. 

Olex process for separation of olefins from a mixture of olefins and paraffins. 

In olefins separation the main industrial target is the separation of ethylene/ ethane and propylene/propane mixtures. Both separations are performed on high scale by distihation, but the relative volatiles of olefins and paraffins are so smaU that large columns are needed. Steigelmann and Hughes [55] presented the first results of a bench- and phot-scale study of ethylene and... 

The teehnique of desorption by simulated countercurrent flow displacement is also applied to other separation operations the separation of ethylbenzene from a mixture of aromatics and that of olefins from a mixture of olefins and paraffins. The composition of the zeolite adsorbent is adjusted in each case to optimize the effectiveness of the separation Na-Y or KSr-X zeohtes for ethylbenzene and Ca-X or Sr-X for olefins. The nature of the liquid desorbent also depends on the molecule to be separated. 

Although a lot of effort has been spent in the development of membranes for the separation of mixtures of nonpolar organic components no large-scale application has yet been reached. Of specific interest is the separation of olefins from paraffins, e.g. propene from propane, aromatics like benzene or toluene from aliphatic hydrocarbons or the separation of the xylene isomers. A number of different membranes are reported in the patent literature [27]. The first pilot plants are being operated and results reported for the separation of sulfur-containing aromatics from gasoline [28], or for the separation of benzene from a mixture of saturated hydrocarbons [29]. 

Silver fluorocomplexes are also used ia the separation of olefin—paraffin mixtures (33), nitration (qv) of aromatic compounds (34), ia the synthesis of (9-bridged bicycHcs (35), pyrroles (36), cyclo-addition of vinylbromides to olefins (37), and ia the generation of thioben2oyl cations (38). 

Natural gas and crude oils are the main sources for hydrocarbon intermediates or secondary raw materials for the production of petrochemicals. From natural gas, ethane and LPG are recovered for use as intermediates in the production of olefins and diolefms. Important chemicals such as methanol and ammonia are also based on methane via synthesis gas. On the other hand, refinery gases from different crude oil processing schemes are important sources for olefins and LPG. Crude oil distillates and residues are precursors for olefins and aromatics via cracking and reforming processes. This chapter reviews the properties of the different hydrocarbon intermediates—paraffins, olefins, diolefms, and aromatics. Petroleum fractions and residues as mixtures of different hydrocarbon classes and hydrocarbon derivatives are discussed separately at the end of the chapter. 

This chapter reviews the adsorptive separations of various classes of non-aromatic hydrocarbons. It covers three different normal paraffin molecular weight separations from feedstocks that range from naphtha to kerosene, the separation of mono-methyl paraffins from kerosene and the separation of mono-olefins both from a mixed C4 stream and from a kerosene stream. In addition, we also review the separation of olefins from a C10-16 stream and review simple carbohydrate separations and various acid separations. 

Mono-olefins (un) react with solid copper(I) halides to form unstable complexes of the type [CuX(un)] (X = Cl, Br), which dissociate into their constituents above 0° (67, 138). Dienes (e.g., butadiene, isoprene, pipery-lene, bicyclo[2,2,l]hepta-2,5-diene, and cyclopolyolefins) form more stable complexes of the type [Cu2X2(diene)J (1,63, 67,138,192), in which a copper atom is attached to each C C bond industrial processes to separate dienes from mono-olefins and paraffins are based on this difference in stability (8). Complexes of the type [Cu(un)]+, [CuCl(un)], and [CuCl2(un)] have been shown to exist in dilute acid solution (15, 67, 138). 

Olefin Separation. U.O.P. s Olex Process. U.O.P. s other hydrocarbon separation process developed recently—i.e., the Olex process—is used to separate olefins from a feedstock containing olefins and paraffins. The zeolite adsorbent used, according to patent literature 29, 30), is a synthetic faujasite with 1-40 wt % of at least one cation selected from groups I A, IIA, IB, and IIB. The Olex process is also believed to use the same simulated moving-bed operation in liquid phase as U.O.P. s other hydrocarbon separation processes—i.e., the Molex and Parex processes. 

Carbon dioxide removal by reactive absorption in amine solutions is also applied on the commercial scale, for instance, in the treatment of flue gas (see later in this chapter). Another possible application field of the technique is gas desulfurization, in which H2S is removed and converted to sulfur by means of reactive absorption. Aqueous solutions of ferric chelates (160-162) as well as tetramethylene sulfone, pyridine, quinoline, and polyglycol ether solutions of S02 (163,164) have been proposed as solvents. Reactive absorption can also be used for NOx reduction and removal from flue or exhaust gases (165,166). The separation of light olefins and paraffins by means of a reversible chemical com-plexation of olefins with Ag(I) or Cu(I) compounds in aqueous and nonaqueous solutions is another very interesting example of reactive absorption, one that could possibly replace the conventional cryogenic distillation technology (167). 

Tanaka, K., Taguchi, A., Hao, J., Kita, H. and Okamoto, K. (1996) Permeation and separation properties of polyimide membranes to olefins and paraffins. Journal of Membrane Science, 121, 197. 

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