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FLUID SOLIDS OPERATIONS INTRODUCTION Fluid bed reactors became important to the petroleum industry with the development of fluid catalytic cracking (FCC) early in the Second World War. Today FCC is still widely used. The following section surveys the various fluid bed processes and examines the benefits of fluidization. The basic theories of fluidization phenomena are also reviewed. F'LUID BED PROCESSES Catalytic Cracking Cracking is a process for breaking down large, high bo
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  FLUID SOLIDS OPERATIONS INTRODUCTION Fluid bed reactors became important to the petroleum industry with the development of fluid catalytic cracking (FCC) early in the Second World War. Today FCC is still widely used. The following section surveys the various fluid bed processes and examines the benefits of fluidization. The basic theories of fluidization phenomena are also reviewed. F LUID BED PROCESSES Catalytic Cracking Cracking is a process for breaking down large, high boiling petroleum molecules to produce light gases, gasoline, jet fuel, and heating oils. Cracking srcinally was a thermal process relying on high temperature to cause reaction. In the early fixed bed catalytic process, the catalytic process operated at lower temperatures and produced a more valuable product distribution. Because the reaction was endothermic and deposited carbon on the catalyst, cyclic operation was necessary to permit frequent regeneration and reheating of the catalyst. To meet these reaction needs, a fluid bed process was developed in which very fine catalyst particles were circulated between a reactor and a regenerator vessel. This provided much more satisfactory uniform operation and now most catalytic cracking is carried out in fluid bed processes. Fluid cat cracking required identifying stable operating regimes for beds of fine catalyst at high gas flow rates. Highly efficient cyclone and electrostatic systems had to be developed for catalyst recovery. Finally, the principles of pressure 26  Fluid Solids Operations 27 balancing and standpipe/transfer line flow had to be developed for the catalyst circulation scheme. Fluid Hydroforming As the demand for high octane gasoline increased, reforming processes were developed. In reforming, low octane normal paraffins are isomerized and/or aromatized. One process developed was a fluid bed process, called fluid hydroforming. In concept, it was similar to cat cracking, employing a reactor and regenerator with solids circulation for regeneration and heat balancing. However, fluid hydroforming faced some additional problems in the area of gas-solids contacting. Due to the nature of the process, fluidization was not inherently good in hydroforming, but at the same time the reaction required better contacting than FCC to obtain high yields of high octane gasoline. Thus, a large and ultimately successful program was carried out to improve fluidization and contacting. This work led to some of the earlier theories explaining the behavior of fluidized beds as chemical reactors. While there are hydroformers still operating, reforming today is generally carried out in fixed bed units using platinum catalysts, because of their superior product yield and distribution. Fluid platinum catalyst processes are not feasible because catalyst losses would be too great. Fluid Coking Coking is a thermal process for converting heavy, residual oils into lighter products and solid carbon. In the earliest coking process, called delayed coking, after heating and partial vaporization, the residuum is passed into a coking drum which fills up with solid coke deposits. This coke must then be drilled out. The coke can be deposited on particles of seed coke in a fluidized bed, and the coke product is in the form of freely flowing granules. Fluid coking also employs two beds with particles circulating between the coking reactor and a burner vessel, where some coke particles are burned to produce the necessary heat. Fluid coking is very insensitive to poor gas-solids contacting, but has one problem not faced by cat cracking or hydroforming. If the heavy residual oil is fed too fast to the reactor, the coke particles will become wetted and stick together in large unfluidizable lumps. Correct control of feed rate is necessary to prevent this bogging.  28 Pressure Safety Design Practices FIOR Fluid Iron Ore Reduction (FIOR) is a process for reducing ore to iron with a reducing gas in a fluid bed. For thermodynamic efficiency, iron ore reduction requires counter current flow of ore and reducing gas. This is achieved in FIOR in a multiple bed reactor. Precautions are necessary to prevent significant back mixing of solids between beds, since this would destroy counter current staging. Other Fluidization Processes Several other processes that are aimed at the manufacture of gasoline from coal have been applied over the years. The main reactor in these processes uses three phase fluidization in which solid coal particles, gases, and liquids are all contacted at very high temperatures and pressures. Fluid bed dryers and fluid cokers are also used in synthetic fuels manufacture. Fluid bed boilers have also been applied as a cure to sulfur dioxide air pollution from power plants. Various schemes have been developed in which combustion of a sulfur containing fuel takes place in a fluidized bed of particles which absorb or react with sulfur dioxide. The particles are usually regenerated to recover sulfur, which often has enough by-product value to make a significant contribution to process economics. Several important applications of fluid beds exist outside the petroleum industry. Fluid bed roasting of pyritic ores is widely used in the metallurgical industry. Calcination of lime is a commercial process. There are also fluidization processes for various nuclear processing steps. Fluid bed processes have been subject to many problems and uncertainties in development and scale up from bench-scale reactors. The fluidization behavior of each process seems different and very often does not meet expectations based on experience with earlier plants. With hindsight fluid cat cracking seems to be an ideal system from the point of view of easy operation and straightforward scale up. BEHAVIOR OF FLUIDIZED BEDS Qualitative Aspects of Fluidization When a fluid flows upward through a bed of solid particles, pressure drop across the bed increases as the flow rate increases. Eventually the pressure drop equals the weight of the bed (per unit horizontal area) at which point the particles are  Fluid Solids Operations 29 suspended by the flowing fluid, and the bed is said to be fluidized . In this state, the particles in the bed are able to move freely instead of resting on each other, and the bed behaves much like a liquid. Waves can be generated and propagated along the surface of the bed. A large low density solid object will float on the surface of the bed. Particles will flow like a liquid between vessels and will stream in a jet from a hole in the side of the bed. As the fluid flow rate is increased beyond the minimum fluidizing velocity, the bed expands. The total pressure drop across the bed remains essentially constant, but the pressure drop per unit height decreases as shown in Figure 1. At flow rates above the minimum for fluidization, bed characteristics depend greatly on whether the fluid is a liquid or a gas. For most liquid fluidized systems, the bed expands uniformly, with particles being well dispersed throughout. For most gas fluidized beds the gas flow in excess of minimum fluidization will pass up through the bed as bubbles. These bubbles have many of the properties of gas bubbles in liquids. The bubbles contain very few particles. Most particles remain in the emulsion phase, packed closely together as they were at minimum fluidization conditions. Fluidized beds have a number of properties which make them desirable in certain reaction and processing steps. They permit the use of fine particles, which have very large surface areas for catalysis and gas-solids contacting. A packed bed of fine particles would have a prohibitively high pressure drop at reasonable gas velocities. Solids are well mixed so that temperatures throughout the bed are uniform even when very exothermic or endothermic reactions are occurring. In addition, heat transfer between the fluid bed and exchanger surfaces is high The ease of handling fluidized solids makes feasible the continuous addition and removal of solids. A number of disadvantages are also associated with fluidized beds, although proper design can often minimize them. Perhaps of greatest concern is the problem of contacting. The gas in bubbles is not in intimate contact with solids, and the bubbles have comparatively short residence times in the reactor, thus, bypassing often occurs. The seriousness of this depends on the size and velocity of the bubbles, the design of the reactor, and the kinetics of the desired reaction. Flow patterns, particularly for solids, approach those in stirred t nks rather than those in plug flow reactors, an undesirable feature if high conversion is desired. This, of course, can be remedied by staging several fluid beds in series. Sticky or agglomerating materials often prove hard to handle in fluid beds. Entrainment of fie particles can cause serious solid losses and air pollution if efficient recovery devices are not provided. For some low pressure processes, such as burners, pressure drop may be substantially higher for a fluid bed (because the gas supports the solids) th n for the more conventional process. Bubbles make

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Jul 23, 2017
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