Cracking atmospheric residue

When discussing the suitability of the ARCO pilot unit for cracking atmospheric residues, this cannot be done without touching on the question about how to prepare the catalysts for testing. An equilibrium catalyst used in a commercial residue FCCU contains significant amounts of metal contaminants, especially nickel and vanadium. Fresh catalysts must therefore be impregnated with these metals and deactivated before the catalysts can be nsed in the pilot unit. We have shown that this... 

Resid Cracking - Atmospheric Residue (AR) - Crude selection to avoid excessive asphalthenes and metals... 

The two greatest challenges facing the refiner cracking atmospheric residue include management of the increased levels of Conradson carbon levels and the contaminant metals in the feedstocks. 

Beyond 340°C in the reboiler, the residue begins to crack thermally. If the distillation is stopped at this point, the residue is called the atmospheric residue. In order to continue, the distillation is conducted under a low pressure, vacuum", so as to reduce the temperature in the reboiler. 

The visbreaking process thermally cracks atmospheric or vacuum residues. Conversion is limited by specifications for marine or Industrial fuel-oil stability and by the formation of coke deposits in equipment such as heaters and exchangers. 

Figure 10.8 presents a variant of the FCC process, the RCC (Residue Catalytic Cracking) capable of processing heavier feedstocks (atmospheric residue or a mixture of atmospheric residue and vacuum distillate) provided that certain restrictions be taken into account (Heinrich et al., 1993). 

At the end of the 1970s Statoil cracked a North Sea atmospheric residue for the first time in M. W. Kellogg s circulating pilot nnit in Texas [1]. This pilot unit was qnite large, with a capacity of one barrel a day. The test in this pilot nnit was very snccess-ful and showed that North Sea atmospheric residnes were very suitable feedstocks for a residue fluid catalytic cracker, and that North Sea atmospheric residnes gave very promising prodnct yields. 

Some years later Statoil decided to start a project within catalytic cracking in order to learn more abont residue fluid catalytic cracking in general, and particnlarly abont catalysts suitable for this process. The project started as a prestudy for the residue fluid catalytic cracker unit (FCCU) that Statoil was planning to bnild at the Mongstad refinery in Norway. The intention was to crack North Sea atmospheric residue directly, without first using a vacuum gas distillation tower followed by cracking... 

One important question we have asked ourselves many times since we managed to crack the North Sea atmospheric residue in the ARCO pilot unit for the first time was if the results are reliable. Does the pilot unit show the same trend and ranking as expected for the commercial FCCU, and can the yields from the pilot unit be used for modeling Initially there was no answer to these qnestions since no commercial data were available for comparison. The only possibility was to compare the tests done in the ARCO pilot unit with the tests done in the pilot nnit at M. W. Kellogg s some years earlier. 

It has been observed that catalysts aimed for cracking of North Sea atmospheric residues need a high RE content. A fully RE exchanged catalyst was able to crack... 

The ARCO pilot unit has shown to be a versatile tool for studying cracking of North Sea atmospheric residues. The yields from this unit were comparable with the yields from the FCCU at the Statoil Mongstad refinery on one occasion. 

To completely optimize the residue catalyst, other parameters than the different surface areas also must be optimized. For a catalyst cracking North Sea atmospheric residues, the pore size distribution also must be optimized. Pores in the mesopore range that is, pores with diameters between 50 and 500 Angstrom, are most important for precracking of resid molecules [21,23]. The possibility to make nickel and vanadium inactive is also important to optimize. 

The zeolite to matrix surface area ratio can be used for optimization of catalysts for catalytic cracking of atmospheric residues. For North Sea long residues this ratio should be as large as possible, but the ratio should not exceed an upper limit. For the main catalyst type (A) used in this investigation the upper limit of the ZSA/ MSA ratio was around 3.5. There is also a lower limit for the matrix surface area. If the matrix surface area is lower than this limit, the catalyst will not be able to crack all the heavy components in the residue feed, and the coke on the matrix will increase dramatically. This will prevent the catalyst from working properly. Different type of catalysts must be optimized individually, as well as different type of long residues. 

This process is used to produce light gases, naphtha, distillate fuel, heavy fuel oil, and petroleum coke by cracking heavy residual products such as atmospheric and vacuum resids. Both delayed coking and fluid coking processes are utilized. 

The bottoms of the CD, also known as atmospheric residue, are charged to a second fired heater where the typical outlet temperature is about 750-775°F. From the second heater, the atmospheric residue is sent to a vacuum tower. Steam ejectors are used to create the vacuum so that the absolute pressure can be as low as 30-40 mm Hg (about 7.0 psia). The vacuum permits hydrocarbons to be vaporized at temperatures below their normal boiling point. Thus, the fractions with normal boiling points above 650°F can be separated by vacuum distillation without causing thermal cracking. In this example (Fig. 18.14), the distillate is condensed into two sections and withdrawn as two sidestreams. The two side-streams are combined to form cracking feedstocks vacuum gas oil (VGO) and asphalt base stock. 

However, gas oil is used in refineries to produce diesel transport fuel and has consequently a high value and a product slate too poor to make it an attractive feedstock relative to naphtha. In recent years, many gas oil crackers have been reconfigured to crack lighter feedstock or heavier feedstock such as atmospheric residual fuel (b.p. > 360°C). 

Resid hydrotreating catalysts, developed in 1987 have been used together with a scale-and iron-removing catalyst for (1) the cracking and desulfurization of atmospheric residue, (2) the pretreatment of RFCC and (3) the cracking of vacuum residue. Approximately 7000 tons of industrial catalysts have been used in commercial units so far. 

The deactivation curve of the catalyst system in the cracking and the deep desulfurization of atmospheric residue are illustrated in Figure 9 and 10, respectively. The curves for the pilot and commercial units in both cases almost fall on each other. Those figures reveal, therefore, that the pilot test results correlate with the commercial operation results. The information concerning the... 

Several of the commercial simulation programs offer preconfigured complex column rigorous models for petroleum fractionation. These models include charge heaters, several side strippers, and one or two pump-around loops. These fractionation column models can be used to model refinery distillation operations such as crude oil distillation, vacuum distillation of atmospheric residue oil, fluidized catalytic cracking (FCC) process main columns, and hydrocracker or coker main columns. Aspen Plus also has a shortcut fractionation model, SCFrac, which can be used to configure fractionation columns in the same way that shortcut distillation models are used to initialize multicomponent rigorous distillation models. 

Cracking properties of a Si-VPl-5 (Si/Al=0.4 in the gel) and the reference catalyst were also investigated by Micro Activity Tests (MAT), using a short contact time MAT (SCT-MAT) which is a modified version of the ASTM MAT with better correlation to a commercial unit 1 g of atmospheric residue was fed to the reactor during 30 seconds at 524°C. Liquid products were collected at 0°C and analyzed on a HP 5880 Sim Dist GC, while gas products were collected over a saturated KCl-solution and injected on a HP 5880 Refinery Gas Analyzer. 

We attempted to compare our results to earlier reports for catalytically active Si-VPI-5 samples, however, relevant results for a direct comparizon were not found. Martens et al. [6] and Kraushaar-Czametzki etal. [13] reported their results for Pt/Si-VPI-5 bifunctional catalysts in the isomerisation of n-decane [6], and in the hydroconversion of n-heptane, respectively. Davis et al. also used metals supported on VPI-5 to test catalytic reactions [12]. Our main interest were the cracking capability and the selectivity to useful fuel fractions, using the Si-VPI-5 catalysts in reactions of trans-decalin (model compound) or atmospheric residue. 

H-Oil cracking appears to significantly increase the readiness of an atmospheric residue to precipitate asphaltenes. The increased readiness can be explained on a molecular and colloidal... 

Table VI. H-Oil Processing of Kuwait Atmospheric Residue to Yield Steam Cracking Feedstocks...

The different thermal behavior of the different groups of sample may be demonstrated using the TGA curves of the samples No. 8 vacuum residue Kuwait (Fig. 4-12), No. 10 vacuum residue Arabian Heavy (Fig. 4-13), No. 14 atmospheric residue Arabian Light (Fig. 4-14), No. 17 atmospheric residue Kuwait (raw material for sample No. 8) (Fig. 4-15), No. 21 visbreaker residue base Venezuela (Fig. 4-16), and No. 22 residue from thermal cracking, base Iranian Heavy (Fig. 4-17). 

The distillation curves of the vacuum residues and bitumens are shown in Figs. 4-18 to 4-20. Due to the mode of manufacture, none of the samples possesses a distillable fraction. The crackable parts amount to 60 or 70 wt% maximum. Fig. 4-21 shows the distillation curves of an atmospheric residue and the corresponding vacuum residue produced therefrom (origin Kuwait). It may clearly be seen that the vacuum distillation in the refinery has been performed at a temperature in the vicinity of the cracking region. 

The data from the simulated cracking process may also be assembled in groups. Statistical evaluation of the groups A vacuum residues and bitumens (samples 1-13), B atmospheric residues (samples 14-17), and C products from conversion processes which can be cracked under the given experimental conditions (samples 19-22) is shown in table 4-34 ... 

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