Separation of n-alkanes

The separation of n-alkanes from a kerosene or gas oil fraction by a molecular sieve can be performed in a liquid phase or in a gas phase process. In the gas phase processes there are no problems of cleaning the loaded molecular sieve from adherent branched and cyclic hydrocarbons. However, the high reaction temperature of the gas phase processes leads to the development of coke-contaminated sieves, which have to be regenerated from time to time by a careful burning off of the coke deposits. 

FIGURE 5-35. Separation of n-alkanes on two GPC columns. Column 100 A /i-styragel 7.8 mm ID x 30 cm. Mobile phase THF. Flow rate 0.5 mL/min. Detection refractive index. (Reprinted from reference 9 with permission.)... 

Molecular sieving by controlling access of molecules to the internal surfaces and by restricting molecular dilfusivity is particularly important in processes that require the separation of branched from linear alkanes or in resolution of the different isomers of xylene relevant dimensions of some important hydrocarbons of these types are given in Table 7.2. Small-pore zeolites such as Na-A are particularly important for the separation of n-alkanes and n-alkanols from their branched isomers, whereas medium-pore zeolites such as ZSM-5 show adsorption of p-xylene but very slow (or no) adsorption of o-xylene. Molecular sieving is also important in restricting the size of molecules that leave the pores of zeolites after catalytic reaction within them. This product diffusivity selectivity is described in detail for specific examples in the next chapter, but the intra-zeolitic isomerisation of xylenes in ZSM-5 to give predominantly p-xylene product is an excellent example. 

FIGURE 16. Reversed-phase separation of n-alkane standards on an LC-18-DB (5jum) column. The mobile phase was THF-water (85 15) (0.5 ml/min ) at 50 °C. The sample size was 5/il and contained equal weights of each C20, C22, C24, C26,... 

The separation of n-alkanes by clathration techniques is well documented. Methods such as urea and thiourea adduction as well as molecular sieves have been employed ... 

Figures 18 and 19 demonstrate that the separation of n-alkanes from a complex mixture is more clean-cut than their differentiation by thiourea adduction.

Occasionally the mound of unresolved components on the chromatogram supporting the superimposed n-alkane peaks confuse the true rt-aUcane profile. This has been overcome by separating off the n-alkanes using molecular sieves, prior to gas chromatography [493]. However, separation of n-alkanes in this way, or by urea complex formation... 

As further subfractionation facilitates subsequent studies at a molecular level, further separation into compound groups is applied. For example, the saturated hydrocarbon fraction can be treated with 5 A molecular sieves or urea for the removal of n-alkanes, leaving behind a fraction of branched and cyclic alkanes [7,8]. The procedure is described in the following text in detail. 

The titanosilicate version of UTD-1 has been shown to be an effective catalyst for the oxidation of alkanes, alkenes, and alcohols (77-79) by using peroxides as the oxidant. The large pores of Ti-UTD-1 readily accommodate large molecules such as 2,6-di-ferf-butylphenol (2,6-DTBP). The bulky 2,6-DTBP substrate can be converted to the corresponding quinone with activity and selectivity comparable to the mesoporous catalysts Ti-MCM-41 and Ti-HMS (80), where HMS = hexagonal mesoporous silica. Both Ti-UTD-1 and UTD-1 have also been prepared as oriented thin films via a laser ablation technique (81-85). Continuous UTD-1 membranes with the channels oriented normal to the substrate surface have been employed in a catalytic oxidation-separation process (82). At room temperature, a cyclohexene-ferf-butylhydroperoxide was passed through the membrane and epoxidation products were trapped on the down stream side. The UTD-1 membranes supported on metal frits have also been evaluated for the separation of linear paraffins and aromatics (83). In a model separation of n-hexane and toluene, enhanced permeation of the linear alkane was observed. Oriented UTD-1 films have also been evenly coated on small 3D objects such as glass and metal beads (84, 85). 

The GPC of a local crude (Bryan, Texas) sample spiked with a known mixture of n-alkanes and aromatics is shown in Figure 5 and the GPC of the crude is shown in Figure 6. The hydrocarbon mixture is used to calibrate the length of the species which separates as a function of retention volume. Ttie molecular length is expressed as n-alkane carboa units although n-alkanes represent only a fraction of the hydrocarbons in the crude. In addition to n-alkanes, petroleum crude is composed of major classes of hydrocarbons such as branched and cyclic alkanes, branched and cyclic olefins and various aromatics and nonvolatiles namely asphaltenes. Almost all of the known aromatics without side chains elute after n-hexane (Cg). If the aromatics have long side chains, the linear molecular size increases and the retention volume is reduced. Cyclic alkanes have retention volumes similar to those of aromatics. GPC separates crude on the basis of linear molecular size and the species are spread over 10 to 20 ml retention volume range and almost all of the species are smaller than the polystyrene standard (37A). In other words, the crude has very little asphaltenes. The linear... 

The sulfide fraction of non-biodegraded petroleums usually contains a complement of n-alkyl substituted sulfides, thiolanes and thianes, in addition to the terpenoid class. In the biodegraded Alberta oil sand bitumens, however, n-alkyl substituted sulfides are absent. The two classes of sulfides can be separated by thiourea adduction, the n-alkyl-containing molecules being adducted. Raney nickel reduction of the Bellshill Lake sulfides (1 11 yielded a series of n-alkanes showing a distribution quite similar to that of the n-alkanes in the saturate fraction of this oil. In some oils the thiolane and thiane concentrations are commensurate, e.g. Houmann, Figure 13 in others, the concentration of thianes may be small compared to thiolanes. 

L. Sojak, I. Ostrovsky, et al., Separation and identification of C14-C17 alkylbenzenes from dehydrogenation of n-alkanes by capillary gas chromatography using liquid crystals as the stationary phase, Ropa. Uhlie, 25(3) 149-157 (1983). 

Figure 5. Separation of C,4 C,- alkylbenzenes and o-dialkylbenzenes obtained4from dehydrogenation of n-alkanes in columns with Carbowax 2CM and MEAB l=l-butyl-2-butylbenzene, 2=l-propyl-2-pentylbenzene 3=i-ethyl-2-hexylbenzene, 4=l-methyl-2-heptylbenzene, 5=n-octylbenzene 6=l-butyl-2-pentylbenzene, 7=l-pro-pyl-2-hexylbenzene 8=l-etyl-2-heptylbenzene, 9=l-me-thyl-2-oktylbenzene, 10=n-nonylbenzene, ll=l-pentyl-2-pentylbenzene, 12=l-butyl-2-hexylbenzenev 13=l-pro-pyl-2-heptylbenzene, 14=l-ethyl-2-octylbenzene, 15=1-methyl-2-nonylbenzene, 16=n-decylbenzene, 17=l-pentyl-2-hexylbenzene, 18= 1-butyl-2-heptylbenzenev 19=l-pro-pyl-2-octylbenzene, 20= l-ethyl-2-nonylbenzene, 21=1-methyl-2-decylbenzene, 22=n-undecylbenzene ...

It has been shown that gas-Hquid chromatographic methods are particularly suitable for a quantitative characterization of the polarity of solvents. In gas-liquid chromatography it is possible to determine the solvent power of the stationary liquid phase very accurately for a large number of substances [98, 99, 259, 260]. Many groups of substances exhibit a certain dependence of their relative retention parameters on the solvation characteristics of the stationary phase or of the separable components. In determining universal gas-chromatographic characteristics, the so-called retention index, I, introduced by Kovats [100], is frequently used. The elution maxima of individual members of the homologous series of n-alkanes (C H2 +2) form the fixed points of the system of retention indices. The retention index is defined by means of Eq. (7-41),... 

The last method that was mentioned at the beginning of this section was called the complex formation method. This method is based on formation of complexes of crude oil compounds with other substances. The most popular methods of complex formation are complex formation with CO(NH2)2 and CS(NH2)2. For CO(NH2)2, it is typically to form complexes with w-alkanes and their derivates with relatively long paraffinic chains with normal structure. The formed complexes are crystallized from the sample. The separation of the n-alkane fraction from CO(NH2)2 can proceed by adding hot water to the crystallized complex. The CO(NH2)2 is very soluble in water whereas paraffins are insoluble. This is why two layers result in this separation the fraction of aqueous solution of CO(NH2)2 and the paraffin fraction. Because the analysis is done at room temperature where paraffins are usually solid, the paraffin plate can be easily taken off from the top of the analysis glass. The analysis with CS(NH2)2 is carried out in the same way as the analysis with CO(NH2)2. However, CS(NH2)2 forms a complex with iso-alkanes. By using both of these methods, a relatively exact separation of n-paraffin and iso-paraffin fractions is made possible. 

An odour standard of n-alkane vapours is used to calibrate sensitivity and specificity of the zNose . Specificity is what allows the instrument to recognize known chemicals and/or chemical groups (odour signatures) and to deliver the appropriate alarms. The zNose is an ultra-fast GC which separates and measures the concentration of the individual chemicals of an odour directly, typically in 10 seconds. Individual chemicals are recognized by their retention time relative to the retention times of the linear-chain alkanes. Tabulating the retention times and detector counts (cts) provides a complete and quantitative measure of any odour or fragrance. 

Recent advances in isotope analysis include the ability to analyse isotopically smaller proportions of individual compounds by the use of more sensitive online isotope ratio mass spectrometers (cf. Merritt Hayes 1994 Merritt et al. 1994) and stable carbon isotope determination of individual n-alkanes, isoprenoids and biomarkers in petroleums is now a standard tool. Despite early work on collection of chromato-graphically separated individual n-alkanes followed by combustion and isotope determination (Welte 1969) it was for a number of years impossible to analyse isotopically long chained alkanes with the same comfort and ease as the... 

The suitability of a stationary phase for a particular application depends on the selectivity and the degree to which polar compounds are retarded relative to what their retardation would be on a completely non-polar stationary phase. Since retention time is a function of temperature, flow-rate, stationary phase type and loading or film thickness it cannot be used to relate the retention characteristics of one column to another. Various retention index methods have been described such as evaluating the partition and separation properties of solute-stationary phase systems. Kovats (1958) devised a system of indexing chromatographic retention properties of a stationary phase with respect to the retention characteristics of n-alkanes, alkanes being used as reference materials since they are non-polar, chemically inert and soluble in most common stationary phases [8-10]. The retention index (RI) for the n-alkanes is defined as... 

Separation and Sub-fractionation of Alkanes Saturated hydrocarbons were separated from the neutral oil by silica-gel (60-120 mesh, dehydrated at 150°C for 5 h) chromatography in a 1 m X 30 mm i.d. column eluted with distilled n-hexane. n-Alkanes were separated from iso-octane solutions of total alkanes by adsorption for one week on 5 X molecular sieve (freshly dehydrated for 2h h at U00°C). Washing with iso-octane, followed by Soxhlet extraction, freed the molecular sieve from inwanted non-adsorbed compo mds n-alkanes were recovered by desorption after refluxing the molecular sieve for several hours with n-hexane. For the Kuwait crude and fluidized-bed tar, the molecular-sieve treatment was preceded by urea-adduction of n-alkanes and thiourea-adduction of branched-chain alkanes. 

N.M.R. Spectroscopy. For comparing mixtiires of saturated hydrocarbons from different fuels and for monitoring the effectiveness of silica-gel column chromatography in the separation of normal alkanes from non-normals by 100 and, especially, 220 MHz H N.M.R., the emphasis is on the chemical-shift profile (j 0) and peak areas rather than on spin-spin coupling constants (Table III). Although the 100 MHz spectra indicate that the n-hexane-soluble part of Montan wax in CCli has rather similar hydrogen distributions to the chloroform-soluble part,about of Montan wax was soluble in n-hexane presumably the n-hexane-insoluble fraction contains all the alkanes, as well as polycyclic aromatics. The spectra of n-hexane- and chloroform-soluble fractions of Turkish asphaltite indicate hydrogen distributions of about 7.8 and 12.2 Hy, 21.2 and 22.0 H, U6.0 and U3.3 Hg, and 25 and 22.5/ Hy. 

Availability of H N.M.R. spectra at 220 MHz (Figure I) increases the chemical-shift separation and so enhances the differences between distinct kinds of hydrogen. For all the fuel extracts examined, removal of n-alkanes by molecular sieve from the total alkanes causes small changes in the methyl absorptions as a result of changing proportions of spin-spin triplets (from C 3-CH2-), doublets (from OT3-CH-), and singlets (from CH3-Cc) ... 

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