Fluidized solids technique

The feedstock is sprayed onto a fluidized bed of hot coke particles which is agitated by the gaseous products rising through the bed in the reactor. The fluidized solids technique permits the use of higher temperatures (than delayed coking) but without the usual overabundance of coke formation because of the shorter contact times with the result that higher yields of liquid products are produced. 

Fluid coking is a continuous process that employs the fluidized solids technique for the conversion of heavy low-grade feedstocks to lighter, more valuable products. The feedstocks charged to a fluid coker may be any type of resid or residuum where the carbon residue falls into the range 5-50 wt% or those materials having an API gravity less than 20. 

In contrast to delayed coking, fluid coking is a continuous process which uses the fluidized solids technique to convert vacuum residue to more valuable products, and coke formed during this kind of coking is a byproduct of the process... 

The development took only three years from the initial concept to the first commercial operation. This paper reviews the development of the Fluidized Solids Technique, which revolutionized petroleum refining and created a new field of chemical engineering. 

This discussion has focused on the basic developments leading to successful commercialization of the Fluidized Solids Technique however, many areas were important to success of the project and were pursued vigorously at the same time. Some of these are listed in Table 3. For example, the availability of special equipment had to be assured, including cyclones, slide valves and expansion joints. Also, metals and refractories had to be tested and methods of fabrication developed. A large supply of catalyst was needed. Fortunately, the natural clay type catalyst used initially was readily available, having been used for clay treating of lubricating oils, etc. It was soon found that a synthetic silica-alumina catalyst was much better, with the result that a whole new industry was started to supply it. Twenty years later the silica alumina catalyst was displaced by the more active zeolites. 

Catalytic cracking to make high-octane gasoline was the first application of the new Fluidized Solids technique, and provided the impetus for commercial development. Since then, a great many other applications have been explored and many of them carried through to commercial use. Table 4 lists some of these. Fluid coking, as one example, fluidizes particles that are coated with a sticky liquid. 

Eluid coking. Continuous process that uses the fluidized-solids technique to convert residue feedstock to more valuable products. The use of a fluid bed permits the coking reactions to be conducted at higher temperatures and with shorter contact times than in delayed coking. These conditions result in lower yields of coke and higher yields of liquid products (Parrish et al., 1996 Eurimsky, 2000 Kamienski et al., 2007). 

Thus fluidization is a technique which enables solid particles to take on some ot the properties ot a fluid. For example, solids fluidized by a gas will adopt the shape ot the container in which they are held and can be made to flow, under pressure, trom an oritice or overflow a weir. It the wall ot the bed is punctured by a series ot apertures aligned vertically the fluidized solids will behave just as it the bed were tilled with liquid a stream ot solids will issue trom each aperture, that trom the highest point in the bed will travel only a short horizontal distance whereas the stream trom the lowest aperture will travel turthest. Gibilaro (2001) reters to a demonstration rig in the Department ot Chemical Engineering at UCL in which a plastic toy duck buried in a bed ot sand exchanges place with a brass duck on the bed surtace when the bed is fluidized with air. ... 

In measurement and visualization of gas-solid flow in fast fluidization, various techniques have been developed and used, such as pressure gradient, pressure fluctuation, capacitance probe, optical fiber probe, momentum probe, laser Doppler velocimeter, night-television, and video camera. The resulting data on local gas and solid velocity, solids concentration and its... 

Larachi et al. (1996) applied their radioactive particle tracking technique to determine the time-averaged solids velocity in a three-phase fluidized bed. Techniques which are similar to CARPT, such as positron emission particle tracking and radioactive particle tracking, have been applied successfully to systems involving particulate solids. 

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. 

Both of the above processes have normally been carried out over supported metal halide catalysts at elevated temperature and pressure. One of the most diflBcult problems has been removing the large quantity of heat generated at the surface of the catalyst by the reaction. If temperature is not adequately controlled in oxychlorination, a serious loss in selectivity will result, and catalyst volatilization will occur. In some cases fluidized solids or moving-bed techniques have been used, but these have generally met with difficulties owing to the volatile nature of the more active metal halide catalysts, such as copper chloride and the corrosive nature of the system. 

One of the more interesting methods for continuous countercurrent ion exchange is the use of fluidized bed techniques for continuous circulation of the resin. Figure 1.6 shows the Dorrco Hydro-softener. In the fluidized bed, a solid phase is suspended in a liquid or gas. Consequently, the solid behaves like a fluid and can be pumped, gravity fed, and handled very much like a liquid. The fluidized resin moves down through the softener on the right and is then picked up by a brine-carrier fluid and transferred to the regenerator on the left. 

The removal of solid constitutes is performed to avoid clogging of the packet bed eolumns. Expanded bed or fluidized bed techniques can be successfully applied instead to treat solid-containing mixtures and, henee, skip the removal of solids. However, these options mean reduction of the column efficiency. Actual column technique shall be selected considering peculiarity of each particular separation step. 

In an Oak Ridge study [61] various types of fluidized-bed reactors were compared. Systems investigated were (a) a sodium-cooled fast reactor, (b) a gas-cooled system, (c) an organic-moderated reactor, (d) a heavy-water-moderated reactor, and (e) a light-water system. A detailed study of this latter system was carried out to compare its characteristics and performance with solid-fuel heterogeneous pressurized-water reactors. The results indicated that both the light-water-moderated and organicmoderated fluidized reactors showed promise, while the gas-cooled, the D20-cooled, and the fast (unmoderated) reactors were found to be less satisfactory for application of the fluidized-bed technique. 

The fact that gases have a simple equation of state makes possible the use of absorptiometry with polychromatic beams to give information about the state of a gas under conditions (in detonation waves,16 boundary layers,17 or supersonic flow18) transient or difficult of access. Temperature measurements19 have also been made. The technique is a unique method for studying the fluidization of a finely divided solid by a gas. Bed density profiles, which reveal the character and effectiveness of fluidization, have been readily determined20 without disturbing the system as probes would inevitably do. 

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