Paraffins Conversion

Two distinct kinds of Catalytic Dewaxing are in use today. One involves the boiling point conversion of the paraffin components of waxy feeds by selective hydrocracking and the other by selective hydroisomerization. 

An example of the first kind of process that has been in use since the late 1970 s is MLDW a very durable technology that handles feedstocks across the entire lube boiling range, utilizing the zeolite ZSM-5. This unique 

These new processes differ from earlier commercial forms of hydroisomerization technologies that utilized amorphous bifunctional acidic, noble metal catalysts supported on fluorided aluminas and silica-aluminas. The amorphous catalyst processes were the first to illustrate that hydroprocessed waxy feeds could be manipulated into valuable isoparaffins with exceptional properties.  

For example, thermal diffusion of a wax isomerate derived from the hydroisomerization of a heavy slack wax using a fluorided alumina catalyst reveals the profile shown in Table 4. Notably, there are samples from most ports with both high VI and low pour point, testifying to molecules present that are not common in mineral basestocks derived from separations processes or even in basestocks made from hydroprocessing by only ring conversion methods. 

Feed heavy grade slack wax Catalyst R-fluorided alumina 

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Secondary alcohols (C q—for surfactant iatermediates are produced by hydrolysis of secondary alkyl borate or boroxiae esters formed when paraffin hydrocarbons are air-oxidized ia the presence of boric acid [10043-35-3] (19,20). Union Carbide Corporation operated a plant ia the United States from 1964 until 1977. A plant built by Nippon Shokubai (Japan Catalytic Chemical) ia 1972 ia Kawasaki, Japan was expanded to 30,000 t/yr capacity ia 1980 (20). The process has been operated iadustriaHy ia the USSR siace 1959 (21). Also, predominantiy primary alcohols are produced ia large volumes ia the USSR by reduction of fatty acids, or their methyl esters, from permanganate-catalyzed air oxidation of paraffin hydrocarbons (22). The paraffin oxidation is carried out ia the temperature range 150—180°C at a paraffin conversion generally below 20% to a mixture of trialkyl borate, (RO)2B, and trialkyl boroxiae, (ROBO). Unconverted paraffin is separated from the product mixture by flash distillation. After hydrolysis of residual borate esters, the boric acid is recovered for recycle and the alcohols are purified by washing and distillation (19,20). 

Fig. 3. The Shell middle distillate synthesis (SMDS) process. HPS = heavy paraffin synthesis. HPC = heavy paraffin conversion.

Light hydrocarbons (Ci to C4) and aromatics (mainly Ce to Ce) were produced by ZSM-5 due to the the conversion of olefins and paraffins. Thus,these results provide evidence for cracking of olefins, paraffins and cyclization of olefins by ZSM-5 at 500 C. The steam deactivated ZSM-5 catalyst exhibited reduced olefin conversion and negligible paraffin conversion activity. 

Paraffin conversion to naphthenes is very unfavorable (last column of Table IV). For paraffins to be converted to naphthenes by ring closure, naphthenes must be at very low concentrations. If appreciable naphthenes exist, such as at short catalyst contact times, naphthene ring opening to paraffins can occur. Again, equilibria improve with carbon number. Eight-and nine-carbon paraffins behave quite similarly. 

Figure 2 shows that there is a limited conversion of higher paraffins on either zeolite. There is seme evidence to suggest that C7/C8 paraffin conversion is higher with ZSM-5 and C9/C10 with REHY. However, it would appear that ZSM-5 is not, under the present conditions, particularly effective in paraffin modification in the presence of a relatively high concentration of olefins. 

With the possible exception of the SRC II straight-run naphthas, these Ce-Co reformates could be fed to a hydrodealkylator without first being extracted. However, it must be noted that hydrocracking, as evidenced by paraffin conversion, was not nearly as active as expected. In the event that a coal-derived naphtha contained a substantial portion of paraffin, particularly C6 paraffin, the aromatic content of the resultant reformate would be significantly less. 

The paraffin-conversion reaction was carried out in a flow reactor of 8 mm inner diameter. The reaction gas composed of paraf-c v (f -- "-o 1 w — —atios was fed at a tern—... 

Fig. 24. Coke yield versus shape selectivity of paraffin conversion for various zeolite catalysts. (Reproduced from Ref. 266 with permission from the authors.)...

The PACOL process (paraffin conversion to olefin) produces n-olefins by dehydrogenation of paraffin over a heterogeneous platinum catalyst. The Pacol process is more selective than thermal cracking and produces smaller amounts of byproducts. 

If we view broadly the past and present work cited and reported above, we can summarize certain observations that lead to an understanding of some of the factors involved in catalytic selectivity of paraffin conversion. 

The rate of light paraffin conversion over a Pt catalyst (Oleflex type process) can be expressed as a... 

The paraffin dehydrogenation reaction scheme is shown in Fig. 2. Paraffins are dehydrogenated to form mono-olefins with the double bond distributed according to thermodynamics (less than 10% in the a position). The extent of the reaction is largely controlled by thermodynamic equilibrium, and typical paraffin conversion levels are limited to 10-15%. The reaction is typically carried out at low pressure to enhance the equilibrium in favor of olefin production. 

This catalytic system, as well as systems based on Mo/V/Te/Nb mixed oxides which have been developed by Mitsubishi (65), also represent an example of catalyst characterized by multifunctional properties. The rutile structure is the matrix to host vanadium ions as solid solutions, while the antimony oxide is present as a dispersed microcrystalline oxide. Vanadium is the component which is more active in paraffin conversion, while the high selectivity to the desired product is due to the presence of dispersed, separate phase, antimony oxide. 

Table 3 shows the catalytic properties of these samples. One can see from the data, that all catalysts synthesized on the basis of the mechanochemically treated V2O5 show an increased selectivity towards maleic anhydride and higher specific rate of n-butane and n-pentane oxidation as compared to those obtained in traditional synthesis. The best effect in the improvement of selectivity can be reached by increase of the relative exposure of (001) plane at the VOHPO4.O.5.H2O surface which is known to be transformed into (200) plane of (VO)2P207. The low paraffins conversion over VPO-E samples at the given reaction conditions can be directly connected with their low specific surface area. The comparison made between samples VPO-El and VPO-E2 shows that the precursor synthesis using V2O5-E needs to be optimized in order to improve the catalytic performance. 

It is evident that the above approach was followed to arrive at a high selectivity towards middle distillates, the prerequisite being a second stage which can convert the heavy wax fraction in the HPS effluent very selectively into middle distillates, the Heavy Paraffin Conversion (HPC) stage (see Fig. 2). In the HPC the waxy product of the HPS is hydro-isomerized and hydrocracked to give the maximum yield of middle distillates. 

Compared to SMDS, this simplified process employs a different catalyst in the synthesis stage and does not include a Heavy Paraffinic Conversion stage for producing the finished middle distillate fractions. The syncrude product is a broad boiling range of hydrocarbons (Table 4), and the relative amounts of individual products can be varied by adjusting the reaction conditions. Alternatively, syncrude products can be processed in existing refineries into finished transportation fuels. 

Shape selective catalysis with molecular sieve zeolites has progressed in its first thirty years to become an established branch of catalytic science. Since the first demonstration of selective n-paraffin conversion over 5A molecular sieves, increased insight into how these catalysts function has created opportunities for the development of a number of new industrial processes. 

Paraffin conversion to valuable BTX products over HZSM-5 catalyst is due to... 

FIGURE 10.7 Coke yield versus shape selectivity for paraffin conversion. 

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