Fluidized catalyst cracking unit

Figure 2.3.2 (Kraemer and deLasa 1988) shows this reactor. DeLasa suggested for Riser Simulator a Fluidized Recycle reactor that is essentially an upside down Berty reactor. Kraemer and DeLasa (1988) also described a method to simulate the riser of a fluid catalyst cracking unit in this reactor.

In 1971, LDHs containing different metal cations (such as Mg, Zn, Ni, Cr, Co, Mn and Al) with carbonate as interlayer anions, calcined at 473-723 K and partially or completely chlorinated, were reported to be effective as supports for Ziegler catalysts in the polymerization of olefins [8], with the maximum catalytic activity of polyethylene production observed for Mg/Mn/Al - CO3 LDH calcined at 473 K. Even earher, calcined Mg/Al LDHs were used to support Ce02 for SO removal from the emissions from fluidized catalytic cracking units (FCCU) [9,10]. Some transition metal oxides have also been... 

First we consider fluidized bed reactors in general, then fluidized combustors or regenerators and then provide specifics for a fluid catalyst cracking unit, FCCU, which consists of a riser or fluidized bed reactor, cyclone separator, steam stripper, spend catalyst transport, air-oxidizing regenerator, cyclone separator and a regenerated catalyst return. ... 

Fluidized-bed catalytic cracking units (FCCUs) are the most common catalytic cracking units. In the fluidized-bed process, oil and oil vapor preheated to 500 to SOOT is contacted with hot catalyst at about 1,300°F either in the reactor itself or in the feed line (called the riser) to the reactor. The catalyst is in a fine, granular form which, when mixed with the vapor, has many of the properties of a fluid. The fluidized catalyst and the reacted hydrocarbon vapor separate mechanically in the reactor and any oil remaining on the catalyst is removed by steam stripping. 

In the United States, approximately one-third of all processed crude oil, amounting to about 5 x 10° bbl/day, is catalytically converted over fluidized catalysts. Over 500 tons of catalyst are required daily, yielding sales that in 1987 were estimated at -250 million dollars (1). Thus, in terms of catalyst usage and product value, catalytic cracking is still the most important unit operation of the petroleumrefining industry. This year, the worldwide sales of catalysts to the petroleum, petrochemical, and chemical industry are expected to exceed 2.4 billion dollars, and catalyst producers are preparing themselves for the turn of the century when catalysts are projected to become a 5 billion dollars per year global business (2). 

An advanced cracking evaluation-automatic production (ACE Model AP) fluidized bed microactivity unit was used to study the catalyst and feed interactions. The fluidized bed reactor was operated at 980°F (800 K). Every feed was tested on two different catalysts at three cat-to-oil ratios 4, 6, and 8. Properties of laboratory... 

In catalytic cracking, a large amount of heat needs to be supplied at the reactor inlet to vapourize the feed and provide the heat of reaction. In commercial units, this heat is provided by the hot catalyst recirculated from the regenerator. High heat transfer rates are achieved when the fluidized catalyst is mixed with the feed. In some experimental units, feed and catalyst are injected at reactor temperature. The heat of reaction must then be supplied by an external heating element, at much slower rates of heat transfer. The product selectivity from such laboratory units cannot be expected to simulate that of commercial units... 

Fig. 1. A 1 MMTA fluid catalytic cracking unit with fast fluidized bed regenerator (courtesy Gaoqiao Petrochemical Company). 1, regenerator 2, first-stage FFB regenerator 3, second-stage FFB regenerator, 4, main fractionator, 5. FFB catalyst cooler 6, catalyst hoppers 7, cyclone...

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. 

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. 

Figure 1. Metal level trends on equilibrium fluidized cracking catalysts. Before 1992 data is based on Davison analysis of samples from cracking units in the USA and Canada, After 1992 the survey is world-wide (full symbols).

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