Mono-naphthenes

Saturated cyclic hydrocarbons, normally known as naphthenes, are also part of the hydrocarbon constituents of crude oils. Their ratio, however, depends on the crude type. The lower members of naphthenes are cyclopentane, cyclohexane, and their mono-substituted compounds. They are normally present in the light and the heavy naphtha fractions. Cyclohexanes, substituted cyclopentanes, and substituted cyclohexanes are important precursors for aromatic hydrocarbons. 

Examples of rate-selective adsorption are demonstrated using silicalite adsorbent for separation of Ciq-Cm n-paraffins from non- -paraffins [40, 41] and Ciq-Ch mono-methyl-paraffins from non-n-paraffins [42-45]. Silicalite is a ten-ringed zeolite with a pore opening of 5.4A x 5.7 A [22]. In the case of -paraffins/non-n-paraffins separation [40, 41], n-paraffins enter the pores of silicalite freely, but non-n-paraffins such as aromatics, naphthenes and iso-paraffins diffuse into the pores more slowly. However, the diffusion rates of both normal -paraffins and non-n-paraffins increase with temperature. So, one would expect to see minimal separation of n-paraffins from non-n-paraffins at high temperatures but high separation at lower temperature. 

Many organic substances 7 are soluble in liquid sulphur dioxide, e.g. many alcohols, ether, resins, carbon disulphide, chloroform, benzene and alkaloids.8 It has been shown that under ordinary working conditions di-olefines are soluble and mono-olefines insoluble in liquid sulphur dioxide. This difference in solubility may be advantageously utilised in the refining of mineral lubricating oils, but it does not appear possible to separate naphthenes from paraffins by this method.9... 

Along these lines a more or less complete separation is possible of paraffins, naphthenes with 1 ring, 2 rings, etc., benzene derivatives, mono-aromatics with 1 ring, 2 rings, etc., as shown in Table VI. The physical data for each fraction enable the corresponding amounts of structural compounds present to be calculated, as will be discussed below. 

Within the middle distillate fractions, the concentration of n-paraffins decreases regularly from Cuto C2o and since the number of possible Ci5 isomers exceeds four thousand, its not surprising that few additional iso-paraffins have been identified. Mono- and di-cycloparaffins with five or six carbons per ring constitute the bulk of the naphthenes in the middle distillate boiling range, decreasing in concentration as the carbon number increases substituted three ring naphthenes are also present. 

Saturated constituents contribute less to the vacuum gas oil than the aromatics but more than the polar constituents that are now present at percentage rather than trace levels. The vacuum gas oil itself is occasionally used as heating oil but most commonly it is processed by catalytic cracking to produce naphtha or by extraction to yield lubricating oil. Within the vacuum gas oil saturates, the distribution of paraffins, /iso-paraffins and naphthenes is highly dependent upon the petroleum source. The bulk of the vacuum gas oil saturated constituents consist of /Iso-paraffins and naphthenes. The naphthenes contain from one to more than six fused rings and have alkyl substituents. For mono- and di-aromatics, the alkyl substitution typically involves one long side chain and several short methyl and ethyl substituents. 

The aromatics in vacuum gas oil may contain one to six fused aromatic rings that may bear additional naphthene rings and alkyl substituents in keeping with their boiling range. Mono- and di-aromatics account for about 50% of the aromatics in vacuum gas oil samples. 

Turova-Polyak and co-workers have carried out extensive studies of naphthene isomerization with AICI3, particularly of the substituted cyclopentanes. The conversion of mono- and disubstituted cyclopentanes to cyclohexanes was reported as an analytical technique for the determination of cyclopentanes in mixture with paraffins (411). Ethyl-cyclopentane at room temperature gave an 18-20% yield of cyclohexane derivatives (412). At 140-145°, an 85% yield of 1,3,5-trimethyl-cyclohexane was obtained. This work was also extended to 1,1-dimethyl-cyclopentane (410), up to 95% of which was converted to methyl-cyclohexane at 115°. Similar conversions of alkylated cyclopentanes were also reported by Shulkin and Plate (375). These researches parallel similar work done in the United States. 

Modifications of the conditions listed above were tried within the ranges given above but these resulted in lower conversions (35%-53% mono-cyclic aromatics). More severe hydrocracking (higher conversion to naphthenes) and more effective nitrogen removal would almost surely have yielded more aromatics from reforming. 

The number of naphthenic rings in aromatic compounds is determined by the assignment of Z values, where every naphthenic ring indicates a deficiency of two hydrogens. The distribution of compound types indicates that monoaromatic compounds consist largely of mono-. 

We can t see the organic molecules as they wiggle in and out of these channels. But we can measure how quickly or slowly they diffuse into ZSM-5. From diffusion studies (Ref. 11), we know that only straight chain and mono-methyl paraffins and olefins, certain one-ring aromatic and naphthenic molecules diffuse at useful rates through ZSM-5. The less bulky the molecule, the faster the diffusion rate. Larger molecules either diffuse in slowly, and react at a lower rate, or they are completely excluded. We call this reactant shape selectivity. 

From the work described in this chapter, it can be seen that the thread joining several generations of base stocks has been one of increasing saturated hydrocarbon levels and decreasing levels of mono-, di-, and polyaromatics driven by the need to improve both VI and oxidation stability. The accompanying changes on the saturates side of the equation have been to decrease polycyclic naphthenes and increase those of their mono- and dicyclic counterparts, for the same reasons. 

Figure 21 Coking capacity of hydrocarbons on mono- and bimetallic catalysts as a function of their number of carbon atoms. For each catalyst the values are relative to that of n-heptane. Curve 1 = n paraffins on Pt/Al203 2 = n-paraffins on Pt-Re-S/Al203 3 = n-paraffins on Pt-Ce/Al203 A = naphthenes of...

The possible reactions of n-pentane on the bifunctional catalysts were presented in Equations (2) through (5). The possible types of reactions of n-hexane are the same with the addition of aromatiza-tion leading to benzene. It was shown that on mono- as well as on bimetallic catalysts, naphthenes with rings of five carbon atoms... 

As expected from the above discussion, the amounts of cracked products and methane increase by a factor of 2 to 2.5. However, the DHC products (naphthenes and toluene) increase by a factor of 12. Under the experimental conditions, DHC can be both mono- or bi-([metal + alumina]-) functional. The rate of dehydrogenation of MCH to toluene (not shown) actually decreases for the bimetallic catalyst, from 45 to 12 mol/h/kg. 

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