Dehydrogenation, of naphthenes

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. 

Aromatization. The two reactions directly responsible for enriching naphtha with aromatics are the dehydrogenation of naphthenes and the dehydrocyclization of paraffins. The first reaction can he represented hy the dehydrogenation of cyclohexane to benzene. 

It should be noted that both reactions leading to aromatics (dehydrogenation of naphthenes and dehydrocyclization of paraffins) produce hydrogen and are favored at lower hydrogen partial pressure. 

Catalytic reformers are normally designed to have a series of catalyst beds (typically three beds). The first bed usually contains less catalyst than the other beds. This arrangement is important because the dehydrogenation of naphthenes to aromatics can reach equilibrium faster than the other reforming reactions. Dehydrocyclization is a slower reaction and may only reach equilibrium at the exit of the third reactor. Isomerization and hydrocracking reactions are slow. They have low equilibrium constants and may not reach equilibrium before exiting the reactor. 

The second and third reactors contain more catalyst than the first one to enhance the slow reactions and allow more time in favor of a higher yield of aromatics and branched paraffins. Because the dehydrogenation of naphthenes and the dehydrocyclization of paraffins are highly endothermic, the reactor outlet temperature is lower than the inlet temperature. The effluent from the first and second reactors are reheated to compensate for the heat loss. 

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. 

Side chains are broken free of ring structure dehydrogenation of naphthenic rings containing nine or more carbon atoms is common... 

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. 

Dehydrogenation of naphthenes containing C5 and C6 ring compounds. Examples of these reactions are shown below ... 

Besides dehydrogenation of naphthenes and the isomerization of both naphthenes and... 

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). 

In reforming processes, naphtha fractions are reformed to improve the quality of gasoline (Speight, 1999). The most important reactions occurring during this process are the dehydrogenation of naphthenes to aromatics. This reaction is endothermic and is favored by low pressures and the reaction temperature lies in the range of 300-450°C (570-840°F). The reaction is performed on platinum catalysts, with other metals, e.g., rhenium, as promoters. 

Catalytic Reforming A catalytic reaction of heavy naphtha(1) used to produce high-octane gasoline. The byproducts are hydrogen and light hydrocarbons the primary reaction is dehydrogenation of naphthenes to produce aromatics. Some reshaping of paraffins to produce aromatics and some isomerization of paraffins to produce isoparaffins also occur. 

On the other hand, the above may also mean that the initial thermal dehydrogenation of naphthenic compounds in FCC feed is a step which determines the quantity of hydrocarbons which can be converted to aromatics by hydrogen transfer. 

At higher cracking temperature other reactions, i.e. cyclization and dehydrogenation of naphthenes and hydrocarbon pyrolysis with production of benzene and ethylene, butadiene and hydrogen are also possible. 

In the first group, the production of aromatics is a complementary objective to the refinery processing of gasoline fractions to raise the aromatic content, which evidently links these refining functions. Catalytic reforming processes are used to convert paraffins to naphthenes (cycloparaffins) to be followed by dehydrogenation of naphthenes to aromatics (Chap. 18). Since aromatization of naphthenes is an easier process to accomplish than cycloalkylation, the emphasis in refinery operations is on maximization of the second step in this sequence, when there is an adequate supply of naphthenes. The demand for the aromatics component of gasoline will compete with the feedstock aromatic need from this source. 

The reactions of major importance in the octafining process are isomerization of naphthenes and aromatics, hydrogenation of aromatics, dehydrogenation of naphthenes, disproportionation of aromatics, dealkylation of aromatics, and hydrocracking of saturates (Figure 3). The last three reactions, of course, result in loss of product xylenes. These reactions, like the desired isomerization reactions, are carbonium-ion catalyzed. 

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. 

Houdry Litol Production of benzene from toluene 50 600 Co, Mo Hz Hydrogenation of unsaturated compounds hydrocracking of non-aromatics desulfurization, dealkylation and dehydrogenation of naphthenes lead to higher benzene yields... 

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