Fluid catalysts fluidization

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.

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. 

Fluidize. In general to convert to a liquid state but in recent technology the term refers to processes in which a finely divided solid is caused to behave like a fluid by bringing it into suspension in s moving or liquid. The solids so treated are frequently catalysts and hence the term "fluid catalysts . In such a case the fluidized catalyst is brought into intimate contact and causes a desired reaction in the suspending liquid or gas mixture. Local over-... 

Weight space velocity has been universally adopted in the case of the fluid-catalyst process because the extent of cracking is dependent upon the amount of catalyst, whereas the volume of a given amount of catalyst may vary considerably with different fluidization conditions. In correlations, it is often more convenient to use reciprocal space velocity, WJWol hour, which is zero with no catalyst, under which condition the conversion and yields of cracked products are likewise essentially zero, whereas space velocity is infinite. 

Currently available data for the flow properties of the fluidized catalyst bed are fragmentary, since the local motion of the emulsion phase is diflicult to measure experimentally. Therefore, it is useful to clarify the flow properties of the bed in terms of our knowledge of bubble columns. First, the fluid-dynamic properties of the bubble columns will be explained then, the available data will be adapted to apply to fluid catalyst beds. The reader will be able to picture an emulsion phase of carefully prepared catalyst particles operating in intense turbulence for fluidized beds under conditions of practical interest. This turbulence distinguishes the flow properties of fluid catalyst beds from those of widely studied teeter beds. 

Early attempts to approximate gas-solid contacting in fluid catalyst beds were based on the assumption either of isothermal plug flow of the fluidizing gas through the bed with the catalyst uniformly distributed or of isothermal complete mixing of the gas within the bed. The simple dispersion model, falling between the above two cases, was also used (G8, R4). Evidence from both large-scale and laboratory observations (G9a, L12),... 

The direct contact model has some difiiculties, however. In fluidized beds, gas bubbles of very low solid content are usually considered to exist in the dense phase (H14, K13, T19). Also, the cloud layer is negligibly thin, due to small (/ r for the usual fluid catalyst beds, according to equa-ticMis of Davidson and Harrison (D3) and Murray (M47). The streamlines of gas phase through a bubble have been observed to pass through the cloud, but not through the bubble wake (R17). Thus there seems little possibility of believing that the bubble gas is in direct contact with a substantial amount of catalyst in the bubble phase (see also Secticxi VI,A). Furthermore, the direct contact model is applied to the data by Gilliland and Knudsen, and v in Eq. (7-9) is calculated to fit the data. Calculation (M26) shows that the volume of catalyst, with an apparent density the same as for the emulsion, which contacts the bubble gas freely exceeds the volume of bubble gas itself (v/ib = 3.3, 2.0, and 1.5, respectively, for Uc. = 10, 20, and 30 cm/sec). This seems to be unsound physically. 

Moving-bed reactor (Fig. 1.3b)—vessel where solid particles (either reactant or catalyst) are continuously fed and withdrawn. The gas flow is maintained to allow the downward movement of the particles. Fluidized-bed reactor (Fig. 1.3c)—vessel filled with fine particles (e.g., smaller than 500 p.m) that are suspended by the upward flowing fluid. The fluidized bed provides good mixing of the particles and, consequently, a uniform temperature. 

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. ... 

In general a good catalyst should not lose its activity during operation, and the loss by attrition should be small. Natural catalysts are softer and are therefore destroyed more rapidly than most synthetic catalysts. Obviously, the catalyst for the Fluid process must exhibit such a size distribution that it fluidizes properly. For a natural catalyst, the percentage of 0 to 80 microns material should be kept above 60 (preferably 76) and the 0 to 40 microns material above 16 per cent (preferably 26). Cyclone separators often behave erratically, but will usually retain particles larger than 10 microns in both reactor and regenerator service. Natural fluid catalyst contains some particles that are oblong rather... 

One disadvantage of fluidized heds is that attrition of the catalyst can cause the generation of catalyst flnes, which are then carried over from the hed and lost from the system. This carryover of catalyst flnes sometimes necessitates cooling the reactor effluent through direct-contact heat transfer hy mixing with a cold fluid, since the fines tend to foul conventional heat exchangers. 

In fluid catalytic cracking, a partially vaporized gas oil is contacted with zeoflte catalyst (see Fluidization). Contact time varies from 5 s—2 min pressure usually is in the range of 250—400 kPa (2.5—4 atm), depending on the design of the unit reaction temperatures are 720—850 K (see BuTYLENEs). 

Carbonyl sulfide can be either a starting or intermediate material (108—110), or it can be used as a fluidizing gas in a carbon fluid-bed process (111). Making carbon disulfide from boiler flue gas by catalyticaHy reducing SO2 with CO to COS, and then converting COS to CS2 over an alumina catalyst has been proposed (112). 

Activated alumina and phosphoric acid on a suitable support have become the choices for an iadustrial process. Ziac oxide with alumina has also been claimed to be a good catalyst. The actual mechanism of dehydration is not known. In iadustrial production, the ethylene yield is 94 to 99% of the theoretical value depending on the processiag scheme. Traces of aldehyde, acids, higher hydrocarbons, and carbon oxides, as well as water, have to be removed. Fixed-bed processes developed at the beginning of this century have been commercialized in many countries, and small-scale industries are still in operation in Brazil and India. New fluid-bed processes have been developed to reduce the plant investment and operating costs (102,103). Commercially available processes include the Lummus processes (fixed and fluidized-bed processes), Halcon/Scientific Design process, NIKK/JGC process, and the Petrobras process. In all these processes, typical ethylene yield is between 94 and 99%. 

GLS Fluidized with a Stable Level of Catalyst Only the fluid mixture leaves the vessel. Gas and liquid enter at the bottom. Liquid is continuous, gas is dispersed. Particles are larger than in bubble columns, 0.2 to 1.0 mm (0.008 to 0.04 in). Bed expansion is small. Bed temperatures are uniform within 2°C (3.6°F) in medium-size beds, and neat transfer to embedded surfaces is excellent. Catalyst may be bled off and replenished continuously, or reactivated continuously. Figure 23-40 shows such a unit. 

As mentioned in Section 2.2 (Fixed-Bed Reactors) and in the Micro activity test example, even fluid-bed catalysts are tested in fixed-bed reactors when working on a small scale. The reason is that the experimental conditions in laboratory fluidized-bed reactors can not even approach that in production units. Even catalyst particle size must be much smaller to get proper fluidization. The reactors of ARCO (Wachtel, et al, 1972) and that of Kraemer and deLasa (1988) are such attempts. 

This reaction is carried out in tall fluidized beds of high L/dt ratio. Pressures up to 200 kPa are used at temperatures around 300°C. The copper catalyst is deposited onto the surface of the silicon metal particles. The product is a vapor-phase material and the particulate silicon is gradually consumed. As the particle diameter decreases the minimum fluidization velocity decreases also. While the linear velocity decreases, the mass velocity of the fluid increases with conversion. Therefore, the leftover small particles with the copper catalyst and some debris leave the reactor at the top exit. 

The essential feature of a Jluidized-bed reactor is that the solids are held in suspension by the upward flow of the reacting fluid this promotes high mass and heat transfer rates and good mixing. Heat transfer coefficients in the order of 200 W/m-°C between jackets and internal coils are typically obtained. The solids may be a catalyst, a reactant (in some fluidized combustion processes), or an inert powder added to promote heat transfer. 

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