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The influence of feed quality to the process of catalytic cracking

According to the degree of influence to the catalytic cracking process all the feed quality parameters can be divided into the following groups:
1) parameters which affect the yield (i.e., the material balance) and the quality of cracking products: fractional and group chemical composition, the content of the hetero-organic compounds;
2) parameters which affect the reversible catalyst deactivation, i.e. density, coking properties and the content of sulfuric-acid tar;
3) parameters which affect the irreversible catalyst deactivation, i.e. metal content, especially vanadium and nickel.

According to the fractional content the feed shall correspond to the following requirements:
- practically full absence of light naphtha fractions, since they undergo insignificant cracking conversion, therewith the reactor is loaded inefficiently, besides the above fractions have negative impact on gasoline ON;
- limited fractional content available (up to10%) which has a boiling range up to 350 ° C;
- limited EBP (500 ... 620°C), which is caused by concentration of feed coke-formation compounds (tar and asphaltenes) as well as hetero-organic compounds and metals available in high-boiling fractions.Group chemical composition of feed has more significant influence on the yield and the quality of the product of catalytic cracking. The most of vacuum gas oils underwent the catalytic cracking process; the content of group components varies within its wide range: paraffin 15 ... 35 %, naphthenes 20…40 %, aromatics 15 ... 60%. The above data depends on the original oil type.
The best feed for catalytic cracking for the yield of end products (gasoline and LPG) is the one where paraffinic and naphthenic hydrocarbons predominate. Regarding to catalytic cracking, polycyclic ranges and feed tar output a little gasoline and a lot of heavy fractions and coke. Sulfur and oxygen compounds which are of the same type to their feed chemical composition have not significantly affect the material balance of catalytic cracking, but decrease the product quality. It should be noted herewith the polycyclic hydrocarbons and tar content are generally increased in feed. This takes place within increasing of hetero-organic compounds.

The components which reversibly deactivate cracking catalysts include polycyclic ranges, tar, asphaltenes and nitrogen compounds of the feed. The feed reversible deactivation ability can be indirectly valued by its density. As for quantitatively specification it should be valued by coking property, as determined by Conradson. As a rule, the higher feed coking property, the greater the yield of coke on the catalyst.

Typically, VGO (350…500°С) feed is mainly processed by catalytic cracking unit, hereby the coking property not more than 0, 3 ... 0.5% wt. If the generator has a power reserve upon the volume combustion of coke, in this case the feed with coking property (up to 2 ... 3%wt) can be treated. The coking property (up to 5 % wt) is permissible in the special refinery unit (equipped by the system of heat removal from the regenerator) designed for cracking of residual feed.
Reversible poisons which previously used in aluminosilicate catalysts units, were nitrogenous bases: they were firmly adsorbed on the catalyst active sites and disabled them. Due to their molecular sieve properties zeolite-containing catalysts are poisoned by nitrogen in a less degree than amorphous silica-alumina ones.
Organometallic compounds contained mainly in high-boiling fractions, especially residual oil, are considered to be permanently deactivating components of the cracking feed. The above compounds disable the catalyst active sites, they adversely affect its activity and the selectivity as well. Thus, with increasing of nickel and vanadium content, which are dehydrating metals, the yield of hydrogen and dry gas increases rapidly, and the yield of gasoline is significantly reduced.
Feed metal poisoning is standardized not more than 2 g / t particular in catalytic cracking units, in which no special techniques to capture or poison passivation of the metal content.
Feed with the coking property of more than 10% wt and metal content of 10 ... 30 g /t or more requires its mandatory pre-treatment action.
KNT group has its own Scientific Research Center (R&D center) (Ref. to section on SIC) and allows to monitor the product quality as well as to analyze samples of feed catalytic cracking during the procedure of microspherical cracking catalyst production (ref.to the product). Consequently, KNT group can produce a catalyst, taking into account the total parameters aggregate which characterizes feed quality according to the degree of influence on the process of catalytic cracking, i.e. the catalyst maximum appropriated to a particular type of feed.

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