Dehydrocyclization, paraffins reforming

We have explored rare earth oxide-modified amorphous silica-aluminas as "permanent" intermediate strength acids used as supports for bifunctional catalysts. The addition of well dispersed weakly basic rare earth oxides "titrates" the stronger acid sites of amorphous silica-alumina and lowers the acid strength to the level shown by halided aluminas. Physical and chemical probes, as well as model olefin and paraffin isomerization reactions show that acid strength can be adjusted close to that of chlorided and fluorided aluminas. Metal activity is inhibited relative to halided alumina catalysts, which limits the direct metal-catalyzed dehydrocyclization reactions during paraffin reforming but does not interfere with hydroisomerization reactions. 

Of the main reactions, aromatization takes place most readily and proceeds ca 7 times as fast as the dehydroisomerization reaction and ca 20 times as fast as the dehydrocyclization. Hence, feeds richest in cycloparaftins are most easily reformed. Hydrocracking to yield paraffins having a lower boiling point than feedstock proceeds at about the same rate as dehydrocyclization. 

Increasing the octane number of a low-octane naphtha fraction is achieved by changing the molecular structure of the low octane number components. Many reactions are responsible for this change, such as the dehydrogenation of naphthenes and the dehydrocyclization of paraffins to aromatics. Catalytic reforming is considered the key process for obtaining benzene, toluene, and xylenes (BTX). These aromatics are important intermediates for the production of many chemicals. 

The predominant reaction during reforming is dehydrogenation of naphthenes. Important secondary reactions are isomerization and dehydrocyclization of paraffins. All three reactions result in high-octane products. 

Other reactions may also occur. These include carbon formation, hydrocracking or thermal cracking, dehydrocyclization of paraffins to naphthenes, and dehydrogenation of naphthenes to aromatics. These have been discussed in the deactivation of reforming catalysts, in Section 2. 

The product coming out of the reactor consists of excess hydrogen and a reformate rich in aromatics. Typically the dehydrogenation of naphthenes approaches 100%. From 0% to 70% of the paraffins are dehydrocyclized. The liquid product from the separator goes to a stabilizer where light hydrocarbons are removed and sent to a debutanizer. The debutanized platformate is then sent to a splitter where Cg and C9 aromatics are removed. The platformate splitter overhead, consisting of benzene, toluene, and nonaromatics, is then solvent extracted (46). 

Catalytic reforming rearranging hydrocarbon molecules in a gasoline-boiling-range feedstock to produce other hydrocarbons having a higher antiknock quality isomerization of paraffins, cyclization of paraffins to naphthenes (q.v.), and dehydrocyclization of paraffins to aromatics (q.v.). 

At the present time, the industrial production of benzene and its homologs is implemented by coal carbonization, dehydrocyclization of the usual paraffin hydrocarbons and dehydrogenation of cyclohexane hydrocarbons with catalytic reforming of directly distilled gasoline fractions. The petroleum refining industry is the main source for meeting the demand for benzene and its reserves can fully meet the increasing demand for this compound. 

For example, in the industrial process of reforming of gasoline fractions, one of the main target transformation channels is dehydrocyclization of linear C5+-paraffin, such as... 

Although the constant metallic activity is a small fraction of the initial activity, does not control the bifunctional mechanisms of the main reforming reactions (paraffin dehydrocyclization and isomerization). The acid function controls these reactions and is the function whose deactivation causes the end of the operation cycle. 

Coke Deposition on the Catalytic Functions and Their Deactivations. The acid and metal functions of the reforming catalysts are balanced to have the highest possible yield in the bifunctional reaction of paraffin dehydrocyclization. Coke is deposited on both acid and metal sites, decreasing their catalytic activities. It is important to know (under commercial conditions) the deactivation on both sites and which controls the reactions and fixes the catalyst s practical cycle. If the rate of reaction is several orders of magnitude higher on one site than on the other, the deactivation of the first site does not modify the rate of the bifunctional reaction and the deactivation of the second will control the whole reaction. If a reaction is so rapid on a catalytic site that it is under thermodynamic equilibrium, the deactivation of the site will not be noticed if the reaction is kinetically controlled, it is possible to follow the site deactivation by means of this reaction change. 

Catalytic reforming processes gasolines and naphthas from the distillation unit into aromatics. Four major reactions occur dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins to aromatics, isomerization, and hydrocracking. 

Figures 7A and 7B show the amoimt of aromatics (mol%) in equilibrium as a function of temperature, pressure, and H2/paraffin initial ratio for the dehydrocyclization of re-hexane and re-heptane, respectively. The longer the paraffin, the higher the equilibrium conversion to aromatic. The pressure has a strong influence. At the typical reforming pressures in modem reformers (below 15 atm), the equilibrium conversion of the paraffins to aromatics approach 100%.

The paraffin dehydrocyclization reaction is slower than the dehydrogenation of cyclohexanes, dehydroisomerization of cyclopentane, and isomerization of paraffins. Because the rate is not so high, it is not possible to reach the equilibrium conversion. The larger fraction of the transformation of the paraffins into aromatics occurs in the last reactor of the reforming unit, in which the largest fraction of catalyst is loaded, and the higher operation temperature is used. 

The isoparaffins dehydrocyclize at a lower rate than the n-paraffin wdth the same number of carbon atoms, due to the lower possibility of ring closure wdthout previous rearrangement. For example, the 2,2,4-trimethylpentane cannot dehydrocyclize. It has to be first rearranged to an iso-hexane or iso-heptane compoimd, which does not readily occur with commercial reforming catalysts. 

Catalyst Support Alumina. The support used exclusively in commercial naphtha reforming catalysts is alumina. As previously mentioned, the Pt/KL-zeolite might be used for dehydrocyclization of linear C6-C8 paraffins, but not for straight-run naphthas reforming. 

Straight-run gasoline is composed primarily of alkanes and cycloalkanes with only a small fraction of aromatics, and has a low ON of about 50. The ON is improved by catalytic reforming of n-paraffins and cycloalkanes into branched alkanes and aromatics. The main reactions are isomerization (w- to iso-), cycli-zation, dehydrogenation, and dehydrocyclization. The bifunctional catalyst has an acidic function to catalyze isomerization and cyclization and a dehydrogenation function that requires an active metal site. Typically, platinum is used as the metal and AI2O3 for the acidity. 

The chemistry of catalytic reforming includes the reactions listed in Table 18. All are desirable except hydrocracking, which converts valuable Cs-plus molecules into light gases. The conversion of naphthenes to aromatics and the isomerization of normal paraffins provide a huge boost in octane. H2 is produced by dehydrocyclization of paraffins and naphthene dehydrogenation, which are shown in Figure 15. 

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