Fluid cracking operating conditions

Figure 2.3.1 (Wachtel, et al, 1972) shows the ARCO reactor that tried to simulate the real reaction conditions in a fluid cracking unit. This was a formal scale-down where many important similarities had to be sacrificed to get a workable unit. This unit was still too large for a laboratory study or test unit, but instead was pilot-plant equipment that could still give useful empirical results Since this serves a very large industry, it may pay off to try it, even if it costs a lot to operate.

The feeds to these types of units are usually atmospheric and vacuum residua. The products include feeds for the production of transportation fuels, fuel oils, olefins, etc. However, the operating conditions of the reactor, whether it is a fluid catalytic cracking unit or a fixed-bed unit, is dependent upon the desired product slate and the properties of the feed. 

Higher-boiling feed stocks are more readily cracked and, therefore, heavy gas oils require less-severe operating conditions than light gas oils. This requirement is met in Houdry fixed-bed units by the use of lower oil partial pressure or lower catalyst activity, and in moving-bed and fluid units by the use of higher space velocity, lower temperature, or lower catalyst/oil ratio. 

Vanadium, while not the only contributor to fluid cracking catalyst (FCC) deactivation, frequently dictates the amount of fresh catalyst added to the FCC unit to mmntain activity. Improvements have been made to both zeolites and matrices to minimize the effect of vanadium [1]. Another method of protecting the catalyst from vanadium deactivation is to use traps that prevent the vanadium from contacting the catalyst in the first place. Vanadium traps have frequently shown more promise in laboratory testing than has been realized commercially[2,3]. Sulfur, present in commercial operations, has been known to interfere with previous traps ability to capture vanadium. Recently it has been shown vanadium traps can be designed to perform successfully under commercial conditions. 

Table 10.2. Comparison of typical operating conditions for the two principal applications of fast fluidization fluid catalytic cracking and circulating fluidized bed combustion [56, 67].

Another variant of the severe hydrotreatment process is the substitution of wax for lubricant distillate as feedstock. The wax recovered from conventional solvent dewaxing units is essentially a pure alkane feedstock containing a high proportion of linear alkanes. With this type of feedstock and under appropriate operating conditions, the isomerisation reaction can be made to predominate over cracking reactions. Unconverted wax can be removed by conventional methods to yield a base oil that is exclusively composed of isoalkanes and that resembles synthetic polyal-phaolefin base fluids more closely than the hydrocracked base oils described in Section 1.5.2. A comparison of some of these base fluid properties is shown in Table 1.4. 

In the last 10 years, as a result of more efficient operating methods, carbonate stress corrosion cracking has occurred in the fractionator overheads in fluid catalytic crackers. The problem is most severe at a pH greater than 9 and a carbonate concentration above 110 ppm. It has also occurred at a pH between 8 and 9 when the carbonate concentration is above 400 ppm. Therefore, most refiners are now specifying postweld heat treatment of carbon steel when such conditions are anticipated. 

---