Residue catalysts coke yield

The second group was characterized by well-performing catalysts with high naphtha yields combined with low yields of coke and gas. At that time this was rather unexpected, since it was commonly accepted in those days that a residue catalyst should have a medium zeolite content and a high matrix surface area [15]. Obviously more studies were necessary within this held. 

The surface area of the catalyst as well as the pore size distribution can easily be measured, and the zeolite and matrix surface areas of the catalyst can be determined by the t-plot method. The different FCC yields can then be plotted as a function of the ZSA/MSA ratio, zeolite surface area or matrix surface area, and valuable information can be obtained [9], The original recommendation was that a residue catalyst should have a large active matrix surface area and a moderate zeolite surface area [10,11]. This recommendation should be compared with the corresponding recommendation for a VGO catalyst a VGO catalyst should have a low-matrix surface area in order to improve the coke selectivity and allow efficient stripping of the carbons from the catalyst [12], Besides precracking the large molecules in the feed, the matrix also must maintain the metal resistance of the catalyst. 

The structure of the hydrocarbons produced can be modified by the use of catalyst. Catalytic cracking consumes less energy than the noncatalytic process and results in formation of more branch-chain hydrocarbons. On the other hand the addition of the catalyst can be troublesome, and the catalyst accumulates in the residue or coke. There are two ways to contact the melted polymer and catalysts the polymer and catalyst can be mixed first, then melted, or the molten plastics can be fed continuously over a fluidized catalyst bed. The usually employed catalysts are US-Y, and H-ZSM-5. Catalyst activity and product structure have been reported [7-11]. It was found that the H-ZSM-5 and ECC catalysts provided the best possibility to yield hydrocarbons in the boiling range of gasoline. 

From the operation results of the RFCC industrial apparatus it is seen that in comparison with the commonly used residual oil cracking catalyst CHZ-2, the CHZ-3 catalyst with the Si-rich zeolite as the active component increases the content of low-pressure residual oil by 8.02%, whereas it decreases the oil pulp yield by 1.34% under circumstances where the coke yield remains constant. Meanwhile, the light-oil component yield increases by 1.10%, whereas the combined yeild of liquefied gas + light oil increases by 1.73% if CHZ-3 catalyst is used, indicating that this catalyst has excellent activity-stability as well. 

Consequently, there are significant differences in FCC unit operation when residue is added to normal feed. Conversion falls and less gasoline is produced, as shown in Table 5.2, and the catalyst-to-oil ratio must rise as coke yields increase. The coke also has a different composition relative to that produced from normal feed not only because of the higher Conradson carbon levels and high-boiling compounds, which are absoibed by the catalyst particles, but also from the dehydrogenation activity of the metal impurities, which leads to polymerization reactions and contaminant coke formation. 

When cracking residue with coke selective REUSY-zeolite catalysts, the low A coke allows more flexible operation by increasing conversion and gasoline selectivity at a constant coke yield. Several different types of A coke are deposited during the reaction ... 

Another approach used to reduce the harmful effects of heavy metals in petroleum residues is metal passivation. In this process an oil-soluble treating agent containing antimony is used that deposits on the catalyst surface in competition with contaminant metals, thus reducing the catalytic activity of these metals in promoting coke and gas formation. Metal passivation is especially important in fluid catalytic cracking (FCC) processes. Additives that improve FCC processes were found to increase catalyst life and improve the yield and quality of products. ... 

Feed residue coke is the small portion of the (non-residue) feed that is directly deposited on the catalyst. This coke comes from the very heavy fraction of the feed and its yield is predicted by the Conradson or Ramsbottom carbon tests. 

The exponents vary somewhat with charge stocks. For East Texas light gas oil the values of x and y are —0.4 and 0.6, respectively. When other conditions are constant, any combination of these two variables which results in a given yield of coke will also result in given yields of gasoline, gas, and residual gas oil (20). Therefore, space rate and ratio of catalyst to oil are interchangeable variables. 

Figure 6.11 shows the product yields for each catalyst. The products are classified into four lumps, i.e. gas (carbon number 1-4), gasoline (5-11), heavy oil (above 12), and a carbonaceous residue referred to as coke. In the figure, PE oil represents the feed oil and contains a 34% gasoline fraction. The feed oil was effectively cracked by solid acid catalysts. The gasoline yield was highest with REY zeolite. HZSM-5(65) yielded the... 

Clearly, improvements in FCC catalyst metal resistance and activity retention and in coke selectivity will allow the refiner to increase (bottoms) conversion and increase the intake of lower-valued residual feedstocks. On the other hand, the RFCC operating constraints will in general have a bigger impact on the profitability of the unit than incremental yield improvements. 

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