Units, fluid cracking

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.

An improved design undertaken by Sacony used high-velocity gases to replace the mechanical elevator systems as catalyst carriers. These so-called air-lift units improved upon the Thermofor process both in terms of economies and octane numbers. It was, however, only with the fluid cracking process that catalytic technology realized fully continuous production. 

By the early 1960s, fluid cracking had become the workhorse of the refining industi y. It was the central process m the production of over 70 percent of all high-octane fuel. From early 1940s to mid-1960s, capacities of fluid units have grown from less than... 

FIG. 17-24 UOP fluid cracking unit. (Reprinted with permission of UOP.)... 

Steam pre-treatment of fluid cracking catalysts has been conventionally employed to represent the deactivation occurring in a commercial FCC unit. Appropriate steam pre-treatment methods have been developed so that the activity and selectivity of the steam pre-treated catalyst is equivalent to a commercially deactivated catalyst (12). However, a unique steaming method may not be suitable for catalysts of varying compositions (12). Two steaming methods designed to simulate deactivation in a commercial unit of the two types of catalysts used in this work were employed. Super-D was treated for 8 hours at 732 C with a steam pressure of 2 atmospheres. The catalysts containing ZSM-5 were treated for 12 hours at 827°C with a steam partial pressure of 0.2 atmosphere. 

The growth of zeolite containing fluid cracking catalysts skyrocketed during the mid-1960 s. Figure 3, and today it is safe to say that without exception all fluid cracking unts in the United States employ some form of zeolite catalysts. 

Figure 3. Growth of zeolite catalyst use in U.S. fluid cracking units...

For industrial fluid cracking units most of the modeling work in the literature is based upon a highly empirical approach that helps in building units and in operating them, but does not elucidate the main features and characteristics of the units in order to help improve the design and control of such units, or to optimize their output. 

Analysis of the Riser Reactor of a Fluid Cracking Unit... 

Figure 1. Metal level trends in equilibrium fluid cracking catalysts (FCC). Data based on Davison analysis of samples from 96% of fluid units in the USA and Canada.

Miscellaneous units-fluid catalytic cracking, monoethanolamine (MEA) extraction. HF alkylation, boiler, propylene polymerization, propane deasphalting. 

Through a series of round robin tests conducted by participating laboratories, ASTM Committee D-32 on Catalysts has characterized a variety of catalyst materials using standard test methods. Materials include fluid cracking catalysts, zeolites, silicas, aluminas, supported metals, and a gas oil feedstock. Properties characterized include surface area, crush strength, catalytic microactivity, particle size, unit cell dimensions and metal content. These materials are available from the National Institute of Standards and Technology as reference materials. 

Of particular mention and of widespread interest throughout the petrochemical industry has been the Committee s success in obtaining round robin results on testing fluid cracking catalysts. Overcoming a natural desire not to share data or methods, industry representatives developed a standard method to determine the weight percent conversion of gas oil in a fixed bed microactivity unit. 

Commercial fluid cracking catalysts are predominantly particles with diameters ranging from 20 to 100 microns and densities of the order of 1.0 to 1.6 g./cc. (based on geometric volume of particles including pores). Gas velocities of 1 to 2.5 ft./second are ordinarily used in the reaction vessels of fluid cracking units. Within this range, the bulk density of the dense-phase fluidized bed is typically 40 to 60% of the bulk density of the packed static bed. 

Fm. 30. Flow diagram of fluid cracking unit—upflow design, [Murphree et al., Ind. Eng. Chem. 36, 768 (1943). Reprinted by permission.]... 

Fig. 32. Fluid cracking unit—downflow design. [Courtesy of Standard Oil Company (Indiana).]...

The relationship between the principal variables involved in an overall heat balance around the reactor and regenerator of a fluid cracking unit is illustrated in Figure 34. This plot shows the reactor temperatures obtainable at various combinations of feed-preheat temperature and... 

Fig. 34. Over-all heat balance in fluid cracking unit. [Murphree, et al., Trans. Am. Inst. Ckem. Engrs. 41, 19 (1945). Reprinted by permission.]...

Porous ceramic or micrometallic filters are very effective for recovering entrained fines from gas streams (228,252). Multiple installations are required because it is necessary to blow back each filter element periodically to dislodge the catalyst cake that builds up on the surface and leads to increased pressure drop. Filters have been used for catalyst recovery in other fluid-catalyst processes where high cost or other considerations justify extraordinary measures to minimize catalyst losses. However, this expedient has not been employed in commercial fluid cracking units because losses are readily controlled to a reasonable level by simpler means. In fact, intentional discard of catalyst is often practiced, in addition to normal losses, in order to maintain catalyst quality at a high level by permitting increased additions of fresh catalyst. 

Compressor drives. Compressors and blowers in fluid cracking units may be either steam or electrically driven (209). Steam turbines were preferred over electric-motor drives in the design of early units, partly because of fear of a power failure. However, under some circumstances, the economics favor the use of electric motors. Several units have been built with electrically driven main air blowers, and at least one refiner has selected electric motors for the air blower, gas compressor,... 

Instrumentation and control. Operation of a fluid cracking unit is simplified by the use of automatic controls. As a further aid, graphic panel-boards are sometimes employed which utilize small indicating instruments located at the appropriate positions in a simplified flow diagram of the process (68,309). Audible and visual alarms, as well as automatic controls for emergency shutdown of the unit, are often provided (202). 

The weight of catalyst in a vessel is determined by measuring the pressure differential between taps installed at the top and bottom. Density of the fluidized catalyst is determined in a similar manner from the differential pressure between taps located a measured distance apart in the dense phase. Location of the catalyst level can be determined from the combination of the density and the total weight of catalyst, or by the use of a series of pressure taps placed at intervals along the height of the vessel. A hot-wire probe has been used to locate the level in laboratory fluidized beds (250), but this technique has not been adopted for fluid cracking units. The method depends upon the fact that heat-transfer rate from the heated wire is much higher when immersed in the dense phase of fluidized solids than when in the dilute phase. 

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