Heavy feeds

Problems with heavy feeds, fotmation of polynuclear aromatics (coronene, ovalene, etc.)... 

J. L. Mauleon and J. C. CourceUe, "FCC Heat Balance Considerations with Heavy Feeds," presented at Katalistiks 6th MnnualFCC Symposium, Munich, Germany, May 1985. 

J. L. Mauleon, J. B. Sigaud, and G. Heinrich, "FCC Heat Balance Management with Heavy Feeds, MTC Approach," presented at JPIPetroleum Eefmery Conference, Tokyo, Japan, Oct. 1986. 

Cracking temperatures are somewhat less than those observed with thermal pyrolysis. Most of these catalysts affect the initiation of pyrolysis reactions and increase the overall reaction rate of feed decomposition (85). AppHcabiUty of this process to ethane cracking is questionable since equiUbrium of ethane to ethylene and hydrogen is not altered by a catalyst, and hence selectivity to olefins at lower catalyst temperatures may be inferior to that of conventional thermal cracking. SuitabiUty of this process for heavy feeds like condensates and gas oils has yet to be demonstrated. 

An active matrix provides the primary cracking sites. The acid sites located in the catalyst matrix are not as selective as the zeolite sites, but are able to crack larger molecules that are hindered from entering the small zeolite pores. The active matrix precracks heavy feed molecules for further cracking at the internal zeolite sites. The result is a synergistic interaction between matrix and zeolite, in which the activity attained by their combined effects can be greater than the sum of their individual effects [2J. 

If the object of debottlenecking is to run heavier feeds, multiple test runs may be needed with heavy feed added in stages. 

The regenerator review will include spent catalyst distribution, air distribution, and cyclones. If the test run with heavy feed indicates a temperature limitation, catalyst coolers, partial combustion, or riser quench should be considered. 

Gaseous emissions (CO, NO, SO, particulates) have been regulated at local and national levels. The quantity of these emissions is directly related to the quality of the FCC stocks, operating conditions, catalyst type, and mechanical conditions of the unit. Processing heavy feeds will release a greater amount of SO, NO, and particulates. 

The FCC process is used worldwide in more than 300 installations, of which about 175 are in North America and 70 in Europe. Figure 9.10 shows the principle of an FCC unit. The preheated heavy feed (flash distillate and residue) is injected at the bottom of the riser reactor and mixed with the catalyst, which comes from the regeneration section. Table 9.5 gives a typical product distribution for the FCC process. Cracking occurs in the entrained-flow riser reactor, where hydrocarbons and catalyst have a typical residence time of a few seconds only. This, however, is long enough for the catalyst to become entirely covered by coke. While the products leave the reactor at the top, the catalyst flows into the regeneration section, where the coke is burned off in air at 1000 K. 

The C4 Olex process is designed with the full allotment of Sorbex beds in addition to the four basic Sorbex zones. The C4 Olex process employs sufficient operating temperature to overcome diffusion limitations with a corresponding operating pressure to maintain liquid-phase operation. The C4 Olex process utilizes a mixed paraffin/olefin heavy desorbent. In this case it is an olefin/paraffin mix consisting of n-hexene isomers and -hexane. A rerun column is needed to remove heavy feed components such as Cs/C because they would contaminate or dilute the hexene/hexane desorbent. Table 8.5 contains the typical feed and product distributions. 

The other problem with this petroleum fraction is that it is deficient in hydrogen. We need C H = 7 16 for 2,2,3-trimethylbutane (TMB), but this ratio is greater than 1 1 for many heavy feeds. Therefore, we need to add hydrogen in the refining process, and we could describe them generically as the reaction... 

The FCC matrix plays a crncial role in precracking, vaporization, and internal diffusion of heavy feed molecnles on catalyst particles. Therefore, many efforts have been made to optimize the acidity and pore size distribution of the matrix to improve reaction performance. 

The choice of the appropriate catalyst system will have an impact on the potential formation of the heavy polymers and coke. Cracking the high molecular weight precursors catalytically will significantly reduce the possibility of thermally degrading these components. The zeolite activity should be optimized in combination with an active matrix selective to upgrading the heavy feed components. 

Another important catalyst characteristic is porosity. Particularly when heavy feeds are processed, high pore volumes and pore diameters are required to reduce pore diffusion limitations. These limitations occur when the intrinsic rate of reaction is high compared with the rate of diffusion of the reactants through the catalyst particle to the active surface. The catalyst is then not used effectively, and reaction rates and selectivity become functions of particle size. If the kinetics of the reaction are known, it is possible to estimate from theory the reaction rate or threshold above which a catalyst of known size will begin to exhibit diffusion limitations. 

Two possible future developments, should they become reality, could radically affect the U.S. ethylene industry. Lower cost imported heavier feedstocks and by-product value changes associated with lead free gasoline each would increase the attractiveness of using the heavy feeds. Acting together they would probably lead to future heavy feed domination of almost all new U.S. ethylene plant construction. 

The concentration of metals in the feedstock can also have a major impact on catalyst life. Figure 53 compares the relative catalyst lifetimes for a typical HDS catalyst processing the high-metals Maya residuum and an Arabian Heavy residuum. As is evident, a higher concentration of metals in the feedstock increases the rate of deactivation of both the intermediate period and the final pore-plugging phase. New catalyst systems are required to handle heavy feeds that have metal concentrations of this magnitude. 

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