In catalytic reforming

Powerforming is one tecnique used for aromatics chemical production. Powerforming uses a platinum catalyst to reform virgin naphthas. The principal reaction is the conversion of naphthenes in virgin naphthas to aromatics e.g., isomerization and dehydrocyclization reactions also occur in catalytic reforming. 

In order to produce more paraxylene than is available in catalytic reformate, a xylenes-isomerization plant is sometimes included in the processing scheme. The isomerization step uses the effluent (filtrate) from the paraxylene crystallization step as feed. The filtrate contains about 7-9 percent of paraxylene. The isomerization unit brings the concentration back to its equilibrium value of about 20 percent. 

The catalysts generally used in catalytic reforming are dual functional to provide two types of catalytic sites, hydrogenation-dehydrogenation sites and acid sites. The former sites are provided by platinum, which is the best known hydrogenation-dehydrogenation catalyst and the latter (acid sites) promote carbonium ion formation and are provided by an alumina carrier. The two types of sites are necessary for aromatization and isomerization reactions. 

Abstraction of a hydride ion from a tertiary carbon is easier than from a secondary, which is easier than from a primary position. The formed car-bocation can rearrange through a methide-hydride shift similar to what has been explained in catalytic reforming. This isomerization reaction is responsible for a high ratio of branched isomers in the products. 

The process consists of a reactor section, continuous catalyst regeneration unit (CCR), and product recovery section. Stacked radial-flow reactors are used to minimize pressure drop and to facilitate catalyst recirculation to and from the CCR. The reactor feed consists solely of LPG plus the recycle of unconverted feed components no hydrogen is recycled. The liquid product contains about 92 wt% benzene, toluene, and xylenes (BTX) (Figure 6-7), with a balance of Cg aromatics and a low nonaromatic content. Therefore, the product could be used directly for the recovery of benzene by fractional distillation (without the extraction step needed in catalytic reforming). 

B 977 525 354 2.44 14 Blistering was detected with ultrasonic examination in catalytic reformer piping. Metallography indicated surface decarburization and blistering at non-metallic inclusions, with intergranular cracks growing from the blisters. Cr content was 1.10 percent. 

Most of the toluene and xylenes have their origin in catalytic reforming or olefins plants. From there, the processing schemes vary widely from site to site. The schematic in Figure 3-6 captures most of the variations, although its hard to portray that some plants separate the BTXs from each other early in the scheme while others do it at varying places downstream of an aromatics recovery unit. 

Figure 2-15 Reactions in catalytic reforming of n-heptane to 2,2,3-trimethylbutane and toluene.

The octane number improvement obtained by isomerization of paraffin hydrocarbons is not great since the amounts of the more highly branched paraffins formed at equilibrium are small at the temperatures employed in catalytic reforming (5). Naphthene isomerization, on the other hand, plays a more important role in reforming. In most naphthas about 50% of the naphthene hydrocarbons are of the cyclopentane type (4) so that in order to obtain the maximum aromatic formation, isomerization of these rings to cyclohexane rings must be promoted by the catalyst. 

Catalytic reforming92-94 of naphthas occurs by way of carbocationic processes that permit skeletal rearrangement of alkanes and cycloalkanes, a conversion not possible in thermal reforming, which takes place via free radicals. Furthermore, dehydrocyclization of alkanes to aromatic hydrocarbons, the most important transformation in catalytic reforming, also involves carbocations and does not occur thermally. In addition to octane enhancement, catalytic reforming is an important source of aromatics (see BTX processing in Section 2.5.2) and hydrogen. It can also yield isobutane to be used in alkylation. 

Of all the reactions taking place in catalytic reforming, the dehydrogenation of cyclohexanes occurs by far the most readily. Isomerization... 

The thermodynamics of the more important reactions in catalytic reforming can be discussed conveniently by referring to the equilibria involved in the various interconversions among certain of the C hydrocarbons. Some thermodynamic equilibrium constants at 500°C., a typical temperature in catalytic reforming, and heats of reaction are given in Table I. The equilibrium constants in Table I apply when the partial pressures of the various components are expressed in atmospheres. 

The conversion of cyclohexanes to aromatics is a highly endothermic reaction (AH 50 kcal./mole) and occurs very readily over platinum-alumina catalyst at temperatures above about 350°C. At temperatures in the range 450-500°C., common in catalytic reforming, it is extremely difficult to avoid diffusional limitations and to maintain isothermal conditions. The importance of pore diffusion effects in the dehydrogenation of cyclohexane to benzene at temperatures above about 372°C. has been shown by Barnett et al. (B2). However, at temperatures below 372°C. these investigators concluded that pore diffusion did not limit the rate when using in, catalyst pellets. 

The principal source of toluene is catalytic reforming of refinery streams. This source accounts for ca 79% of the total toluene produced. An additional 16% is separated from pyrolysis gasoline produced in steam crackers during the manufacture of ethylene and propylene. The reactions taking place in catalytic reforming to yield aromatics are dehydrogenation or aromatization of cyclohexanes, dehydroisomerization of substituted cyclopentanes, and the cyclodehydrogenation of paraffins. The formation of toluene by these reactions is shown. 

In catalytic reforming, a low octane naphtha cut (typically a straight run or hydrocracked naphtha) is converted into high-octane aromatics, including benzene, toluene, and mixed xylenes. Aromatics are separated from the reformate by using a solvent such as diethylene glycol or sulfolane and then stripped from the solvent. Distillation is then used to separate the benzene-toluene-xylene into its components. 

We consider the situation in which coke is oxidized from a fixed catalyst bed by oxygen in low concentration in a nitrogen stream, as in catalytic reforming. It is important to be able to predict temperatures attained during burning in order to achieve a quick bum without sintering metal crystallites on the catalyst. 

This paper presents a simulation of coke burning valid for all inlet conditions, and capable of handling any sequence of switches in these conditions. Sample results are presented for conditions of interest in catalytic reforming. 

Kapner, R. S. and Lannus, A., "Thermodynamic Analysis of Energy Efficiency in Catalytic Reforming," Energy (Oxford),... 

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