Naphtha cracking selectivity

CDTECH Butadiene C4+ from naphtha cracking Selective hydrogenation of C4 acetylenes in a distillation column to produce low acetylene feed for butadiene extraction 2 1998... 

The combination of low residence time and low partial pressure produces high selectivity to olefins at a constant feed conversion. In the 1960s, the residence time was 0.5 to 0.8 seconds, whereas in the late 1980s, residence time was typically 0.1 to 0.15 seconds. Typical pyrolysis heater characteristics are given in Table 4. Temperature, pressure, conversion, and residence time profiles across the reactor for naphtha cracking are illustrated in Figure 2. 

Zeolites are solid acid catalysts which are widely used in hydrocarbon processing, such as naphtha cracking, isomerization, dispropornation and alkylation. During reactions carbonaceous materials called coke deposit on the zeolite and reduces its activity and selectivity. Coke deposited not only covers the acid sites of the catalyst, but also blocks the pores, and restrain reactants from reaching the acid sites, leading to the decrease in the apparent reaction rate (1, 2). Here, we will mainly deal with the intracrystalline diffusivity of zeolites, and will discuss the relationship between it and the change in catalyst selectivity. 

The selectofoiming increases the octane number of the resultant naphtha by selectively cracking n-paraffins and increasing, therefore, the concentration of i-paraffins and aromatics. 

Selecting the naphtha type can be an important processing procedure. For example, a paraffinic-base naphtha is a better feedstock for steam cracking units because paraffins are cracked at relatively lower temperatures than cycloparaffins. Alternately, a naphtha rich in cycloparaffins would be a better feedstock to catalytic reforming units because cyclo-paraffins are easily dehydrogenated to aromatic compounds. Table 2-5 is a typical analysis of naphtha from two crude oil types. 

In the hydrocracking process, this phenomenon is exploited to shift catalyst selectivity from the naphtha to the distillate products. Here the wide separation of sites is exploited to minimize the potential for secondary cracking in initial products and intermediates. This, along with the introduction of escape routes for the primary product tends to preserve the higher molecular weight hydrocarbons, thereby producing more dishllates [49, 61, 62]. 

The demonstrated performance of ZSM-5 in over 35 cracking units is reviewed. The main features of ZSM-5 are its high activity and stability, favorable selectivity, metals tolerance and flexibility, particularly when used as an additive catalyst. ZSM-5 cracks and isomerizes low octane components in the naphtha produced by the faujasite cracking catalyst. As a result and olefins are produced and gasoline compositional changes occur which explain its increased research and motor octanes. A model was developed which predicts ZSM-5 performance in an FCC unit. 

Additional support for our observations was found when catalysts A-1 to A-3 were stndied. Catalyst A-1 was developed according to the old recommendations for a residue catalyst with a moderate zeolite surface area and a large active matrix snrface area. The catalyst did not give as good naphtha selectivity as expected when the North Sea long residue feed was cracked. An attempt to improve this was made with catalyst A-2 where the matrix surface was lowered, while the zeolite surface area was kept the same. The naphtha selectivity was however not improved, and it was concluded that the zeolite surface area was too low. So in catalyst A-3 the zeolite snrface area instead was increased compared with the base catalyst A-1. Now the naphtha selectivity increased, but the gas yields also increased dramatically. This catalyst did indicate that a possible way to go could be to increase the zeolite surface... 

From Figure 4.6 it can be seen that the coke yields showed different behaviors for the two types of catalysts. For the Type B catalysts the coke yield was almost unaffected by variations in the ZSA/MSA ratio. For the Type A catalysts, however, the coke yield decreased when the ZSA/MSA ratio increased, which means that more naphtha selective cracking gave decreased coke yield. This is also snp-ported by the coke yield as a fnnction of the zeolite snrface area, see Fignre 4.6b. By comparing catalyst A-1 with catalyst A-3 is it possible to see that the coke... 

Step 4. Fit rate coefficients for the cracking of C7 components to C6 and C5 -with C6 + C7 naphtha data. The rate of heptane cracking to C5- was arbitrarily set to 1 for the selectivity matrix thus k = kl31. The relative rate... 

Table 7 shows the yield distribution of the C4 isomers from different feedstocks with specific processing schemes. The largest yield of butylenes comes from the refineries processing middle distillates and from olefins plants cracking naphtha. The refinery product contains 35 to 65% butanes olefins plants, 3 to 5%. Catalyst type and operating severity determine the selectivity of the C4 isomer distribution (41) in the refinery process stream. Processes that parallel fluid catalytic cracking to produce butylenes and propylene from heavy cmde oil fractions are under development (42). 

In Western Europe it is expected that new isomerization capacity may exceed alkylation installations since naphtha availability generally exceeds demand. By selecting isomerization over alkylation the octane number of the gasoline pool may be increased without increasing the volume. Moreover, olefinic charge stock avails for alkylation are considerably smaller in Europe since there are fewer catalytic cracking units per refinery than in the United States and Canada. It is predicted that C5, and to a lesser extent C5/C6 isomerization, will prevail over alkylation in Western Europe until more catalytic cracking units are installed and/or a shift in the demand for naphtha over fuel oil is experienced. 

Table III gives a range of the possible feedstocks that can be used to produce ethylene and the kinds and amounts of by-products that can be made from them. For our purposes we have selected a constant basis of 1 billion lbs/year ethylene production. The feedstocks illustrated in Table III include ethane, propane, n-butane, a full range naphtha, a light gas oil, and a heavy gas oil. The yields reflect high severity conditions with recycle cracking of ethane in all cases. For propane feed, propane recycle cracking has been included as well.

Both of these reactions have very important industrial uses (Section 14.3.9). In order to obtain alkene streams of sufficient purity for further use, the products of steam-cracking or catalytic cracking of naphtha fractions must be treated to lower the concentration of alkynes and alkadienes to very low levels (<5ppm). For example, residual alkynes and dienes can reduce the effectiveness of alkene polymerisation catalysts, but the desired levels of impurities can be achieved by their selective hydrogenation (Scheme 9.4) with palladium catalysts, typically Pd/A Os with a low palladium content. A great deal of literature exists,13,37 particularly on the problem of hydrogenating ethyne in the presence of a large excess of... 

Results showed that the two-stage TCH process could be used for upgrading Athabasca bitumen and for producing reformer naphtha feedstock, fuel oils, and catalytic cracking gas-oil feedstock. Product weight yields ranging from 86.4% to 93.0% were obtained. A 3 wt % CoO-15 wt % Mo03 on alumina catalyst was found to be sufficiently active to produce specification distillates. Comparison of various catalysts showed some differences in selectivities. However, extended life studies should be carried out to substantiate the differences. 

Reforming. The hydrotreated naphthas were reformed over a conventional platinum reforming catalyst in an attempt to maximize aromatics. The catalyst was Cyanamid AERO PHF-4 (0.3% Pt, 0.6% Cl). The intent was to operate the reformer at constant conditions in order to better compare naphthas. By operating at severe conditions, the expected hydrocracking activity of the catalyst would tend to purify the aromatics by selectively cracking away the paraffins. If the resultant reformate had a suitably high aromatic content, it could be fed directly to a hydrodealkylator. 

Cerqueira and co-workers203 confirmed the appearance of the of the tetrahedral aluminium and phosphorus in AlPO-like crystalline structures both in beta (BEA) and in MOR zeolites treated with phosphoric acid. 31P MAS,27Al MAS and TQM AS NMR spectra permitted the species present in the samples to be assigned. Possibly, besides the the Altet-f species, other Al species are also taking part in the activity and selectivity of the catalysts. The formation of Alocl o P can also contribute to the increase in the activity by preventing further dealumination. Dual zeolite additives have no impact on the quality of naphtha when compared to MFI-based additives, which are used in the fluid catalytic cracking processes. 

Score [Selective cracking optimum recovery] A process for making ethylene by cracking ethane or naphtha. It combines Exxon Chemical s low-residence time technology with Brown Root s cracking technology. Developed by Kellogg Brown Root in 1999 and planned to be used at Dow s refinery in Freeport, TX, in 2003. BP planned to use it when it expanded its Chocolate Bayou ethylene plant for completion in 2005. 

Linde AG Ethylene LPG, Naphtha, gas oils and hydrocracker residue Highly selective furnaces thermally crack hydrocarbons and efficiently recover products 30 1996... 

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