Fluid catalysts circulation

Fig. 31. Original and modern methods of fluid-catalyst circulation. [Gunness, Chem. Eng. Progr. 49, 113 (1953). Reprinted by permission.]...

Figure 18. Original and modem methods of fluid-catalyst circulation. (Reproduced with permission from reference 50. Copyright 1953 American Institute of Chemical Engineers.)...

The MTO process employs a turbulent fluid-bed reactor system and typical conversions exceed 99.9%. The coked catalyst is continuously withdrawn from the reactor and burned in a regenerator. Coke yield and catalyst circulation are an order of magnitude lower than in fluid catalytic cracking (FCC). The MTO process was first scaled up in a 0.64 m /d (4 bbl/d) pilot plant and a successfiil 15.9 m /d (100 bbl/d) demonstration plant was operated in Germany with U.S. and German government support. 

Since the first fluid-bed catalytic cracking unit was commissioned in 1942, more than 300 additional units have been built. During this time, the process has evolved and has seen considerable improvement in mechanical constmction, reflabiUty, and process flow. A modern FCCU typically operates continuously for three to four years between turnarounds, during which time 10 kg of feedstock are processed and 7 x 10 ° kg of catalyst circulated. Early FCCU designs, (53) were complex compared with the compact configuration of more recent design (Fig. 1). 

In the fluid-catalyst process, finely divided catalyst powder is continuously circulated from reactor to regenerator and back again without mechanical means. The fluid process was originated by the Standard Oil Development Company, the research organization of the Standard Oil Company of New Jersey, in collaboration with The M. W. Kellogg Company and Standard Oil Company (Indiana). Other companies participating in the development were Anglo-Iranian Oil Company, Ltd., Shell Oil Company, The Texas Company, and Universal Oil Products Company. This process was first announced in 1941 (48). 

Catalyst residence time or process period). Catalyst residence time in a moving-bed or fluid unit, or process period in a fixed-bed unit, is the length of time a catalyst particle is used to crack oil in each cycle before it is regenerated. Catalyst residence time is equal to the ratio of the amount of catalyst in the reactor to the catalyst-circulation rate. In moving-bed units, it is essentially the same for all catalyst particles, whereas in fluid units there is a variation, as already discussed. 

Although instantaneous cracking rate is assumed to be directly proportional to oil partial pressure, the net effect of pressure in actual cracking operation is much less, particularly in a fixed bed, because of the increased coke deposition and more rapid activity decline at higher pressures (73). Even in catalyst-circulation processes the cracking rate is less than proportional to pressure for example, the cumulative reaction-velocity constant in fluid-catalyst operation appears to be proportional to about the 0.5 power of pressure. 

The carbon is almost completely removed during regeneration in the Houdry fixed-bed process, but not in circulating-catalyst processes. Thus, the carbon content of catalyst leaving the regenerator has been reported to be less than 0.5% in TCC units (158,241), and in the range from 0.3 to 1.0% (usually 0.3 to 0.7%) in fluid-catalyst units (305). 

A mixture of ethylene, air, and recycle gas is reacted in a vertical multitubular reactor containing a silver catalyst. These reactors employ a large number of vertical tubes contained in a shell much like a vertical shell and tube heat exchanger. The catalyst is packed inside the tubes and either a heat transfer fluid is circulated through the shell or boiling water on the shell side is used to remove the exothermic heat of reaction. 

There are basically two types of fixed-bed reactors (1) multitubular, in which tubes of approximately 1.5 to 4.0 cm in diameter are placed as a bundle within a shell through which a heat exchange fluid is circulated to control the temperature profile within the reactor and (2) adiabatic, in which the catalyst is placed directly inside a reactor (with no a priori limitation to the diameter), and heat... 

---