Lecture 1-2 (1) - CHE811 Fluidizaton Engineering Dr M...

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Unformatted text preview: CHE811 Fluidizaton Engineering Dr. M. Shahzad Khurram Department of Chemical Engineering COMSATS Institute of Information Technology, Lahore Fluidizaton Fluidization is a process in which solids are caused to behave like a fluid by blowing gas or liquid upwards through the solid-filled reactor. Fluidization is widely used in commercial operations; the applications can be roughly divided into two categories, i.e., • physical operations, such as transportation, heating, absorption, mixing of fine powder, etc. and • chemical operations, such as reactions of gases on solid catalysts and reactions of solids with gases etc. The fluidized bed is one of the best known contacting methods used in the processing industry, for instance in oil refinery plants. Among its chief advantages are that the particles are well mixed leading to low temperature gradients, they are suitable for both small and large scale operations and they allow continuous processing. There are many well established operations that utilize this technology, including cracking and reforming of hydrocarbons, coal carbonization and gasification, ore roasting, FisherTropsch synthesis, coking, aluminum production, melamine production, and coating preparations. The application of fluidization is also well recognized in nuclear engineering as a unit operation for example, in uranium extraction, nuclear fuel fabrication, Synthesis Reactons The remarkable temperature uniformity of the fluidized bed has strongly recommended it as a vehicle for effecting catalytic reactions, especially highly exothermic and temperaturesensitive reactions. Successful applications in this area include the production of phthalic anhydride by the catalytic oxidation of naphthalene or ortho-xylene, the production of alkyl chloride, and the Sohio process for producing acrylonitrile. Why Fluidization Fluidized beds are used for gas-solid contacting processes primarily due to their favorable heat and mass transfer characteristics. Some of the prominent applications include coating, granulation, drying, and synthesis of fuels, base chemicals and polymers. Course Objectves To provide detailed knowledge of fluidization process with emphasis on properties & parameters governing the design of fluidized systems including fluidized bed reaction, gasification and combustion. Course Descripton Introduction to fluidization, Solids Characterization and Behavior, Classification of granular solids and powders, Fluidization Regimes: Dense Bed Fluidization, Bubbling Bed Fluidization, High Velocity Fluidization, Circulating Bed fluidization, Liquid Solid Fluidization, Particle to gas mass transfer, Particle to gas heat transfer, Fluidization Velocity, Elutriation and entrainment, Effect of Temperature and Pressure Fluid flow through the bed and pressure drop calculation, Fluidized Bed Scale-up, Two phase particle bed model, Three Phase Fluidization, Fluidization system design: Fluidized Bed Combustor/ Fluidized Bed Gasifier/ Fluidized Bed Reactor, Application of Fluidization. Recommended Books 1. Octave Levenspiel, “Fluidization Engineering”, 1991, Second edition, Imprint: Butterworth Heinemann, McGraw Hill 2. Jonghwun Jung, “Design and Understanding of Fluidized Bed Reactors”, 2009, VDM Verlag McGraw Hill 3. L G Gibilaro , “Fluidization Dynamics”, 2001, Publisher : Butterworth Heinemann . 4. O Molerus, “Heat Transfer in Fluidized Beds Book”, 1997, Wirth, K E Wirth, Chapman & Hall, McGraw Hill 5. Simeon N Oka, E J Anthony , “Fluidized Bed Combustion”, 2003, CRC Press History of Fluidizaton 1920 First Gasification fluidized bed was patented 1940 First large scale fluidized bed was commercialized for catalytic cracking of petroleum hydrocarbons. History of Fluidizaton In 1938, Exxon Research joined a consortium of large oil and processing companies to further develop the catalytic "cracking" process. They eventually came up with the concept of a moving bed of catalyst. The moving bed was termed "fluidized", as it moved and had properties analogous to a fluid. While regeneration still required shutting down the process, there were a number of benefits associated with the new process. When used for gasoline production this type of process delivered higher octane gasoline and increased production as compared to the previously mentioned Houdry units. The first production facility using the fluidized bed concept for the catalytic cracking of petroleum feed stocks came online at 2:25am on May 25, 1942. The startup of the Exxon Catalytic Cracking Unit in Baton Rouge, Louisiana, 1942 a. Products Cyclone Flue gas Riser reactor b. Flue gas Products Cyclone Reactor Regenerator Overflow well Catalyst stripper Catalyst stripper Steam Steam Regenerator Reactor feed Air Steam Air Reactor feed Figure 12.10 Commercial FCC riser reaction designs (a) Exxon, (b) UOP. Fluid Cat Cracker (Chevron) Stacked Fluid Cat Cracker (UOP) Shell Cat-Cracker All-riser Cracking FCC Unit Gasoline from Other Petroleum Fractions The Houdry process, in operation since 1937, was already available. However, because it used fixed beds of alumina catalyst requiring intermittent operations to regenerate deactivated catalyst, and because of the complicated arrangements for controlling bed temperatures, this process was unsuited for large-scale production. With war threatening in Europe and the Far East around 1940, the United States anticipated a need for vast quantities of high-octane aviation gasoline, so it urged its chemical engineering community to find new ways of transforming kerosene and gas oil into this critical fuel. In parallel with these efforts, research engineers at the Standard Oil Development Company (now Exxon) were trying to develop a pneumatic conveying system for the catalytic cracking of kerosene. However, they were plagued with mechanical problems and problems due to excessive pressure drop in long tubes. At this time, Professors Lewis and Gilliland, on the basis of experiments carried out at the Massachusetts Institute of Technology, confirmed that a completely pneumatic circuit of fluidized beds and transport lines could operate stably, and suggested that one be used WAR TIME In theINVENTIONS summer of 1940, Britain was under air attack from bases in the now Germanheld France and Belgium. Using 100octane gasoline, British fighter aircraft performed better and were able to match and finally outperform the attacking German airforce. The British minister of fuel and power at that time, Geoffrey Lloyd, indicated later that, "... without the 100-octane we should not have won the Battle of Britain". The Battle of Britain was a decisive air battle fought in the skies above England in mid- After the catalytic cracking units came online in 1942, production of 100-octane and many other fuels increased dramatically. By 1945, there were 34 cracking units online were producing 240,000 barrels per day - roughly 45% of US production. FBR for Synthetc rubber The Philippines and Indonesia were the principal source of raw materials for the US rubber industry. With those natural rubber supplies cutoff, the US had to increase its production of synthetic rubber. The new source for compounds used to produce. New FBRs were coming synthetic rubber came from the fluidized bed reactors online due to the acceleration of FBR processing necessary for wartime production. Current Applicatons forinFluidized Bed Reactors FBRs are everywhere the Process Industries. They find broad use in the petroleum and petrochemical industries, as well as numerous chemical industries. Petroleum Applications Petrochemical Applications Other Applications Industrial applicatons of fluidized beds Industrial applicatons of fluidized beds After fluidized bed technology was first applied in the Winkler process for coal gasification in 1930’s, then the development of fluidized bed technology growths very fast. In the recent years, fluidized bed technology has been applied in various industrial processes such as Fluid catalytic cracking, Thermal cracking Gas solid reaction Adsorption Ore extraction Drying Incineration of solid waste waste treatment and bio process. Gasoline from Coal & petroleum Winkler gas generator First large scale Fluid catalytic cracking Heat Exchanger Rapid quenching Fluidized beds have been used extensively for heat exchange because of their unique ability to rapidly transport heat and maintain uniform temperature. Indirect heat exchange between Coarse particles and gas Steam generation from hot ash Solidificaton of a melt to make granules To spread urea on fields from the air requires coarse granules in a narrow size range. For this purpose following solidification process was developed. Sprayed molten urea falls as droplets through a tall tower while cold air passes upward through the tower, cooling and solidifying the droplets. Solidification and granulation of molten urea Drying of solids The fluidized bed dryer is used extensively in a wide variety of industries because of its large capacity, low construction cost, easy operability and high thermal efficiency. Mixed flow, short retention time Multistage dryers Various designs of driers For pharmaceutical materials Two stage salt dryer For temperature sensitive materials Various designs of driers Coatng of objects and growth of partcles When a salt solution, such as sodium glutamate is injected or sprayed into a hot fluidized bed of dry particles, such as sodium chloride, the surfaces of the particles become wet. Subsequent drying of the liquid layer then gives an efficient coating process. Design for particle coating and growth Heater for coarse solids Adsorpton When very dilute components are to be removed from large flow of carrier gas, then continuous multistage fluidized adsorption process can be superior to conventional fixed bed processes. Recovery of dilute CS2 from air Removal of Dichloro ethane from foul gas Multistage fluidized adsorber to remove solvents and odours materials from foul air Synthesis reactons High demand of fluidized bed is due to its better temperature control ability in reaction zone Phthalic Anhydried Fischer-Tropsch Synthesis The synthesis of hydrocarbons from H2 and CO gases is strongly exothermic and proceeds in a narrow temperature range around340 o C. Synthol circulating solids reactor Acrylonitrile by the Sohio process Acrylonitrile production Maleic anhdride production Polymerizaton of Olefins In this process, reactant gas (ethylene with its comonomers, butene and higher) is fed at a rate of 3 to 6 times the minimum fluidizing velocity into a bed of polyethylene particles kept at 75-100 oC and 20 atm. Unipol process for making polyethylene Cracking of Hydrocarbons Fluid Catalytic cracking (FCC) With catalyst, vaporized heavy hydrocarbons crack into lowermolecular-weight compounds. The FCC process does this efficiency and simply by making the catalyst regeneration step supply the heat for the reaction. FCC model Riser cracking FCC units In riser cracking FCC units, feed oil is sprayed into the fast upflowing lean-phase stream of regenerated catalyst. Practically all reactions occurs in this upflow riser. Fluid andto produce flexi coking Fluid coking coking process both gas oil and close to spherical coke particles between 20 and 100 mesh from a pitched feed. Exxon’s fluid coker and flexi-coker process Thermal cracking In contact with a hot surface, naphtha petroleum fractions crack to produce ethylene and propylene, which are useful starting materials for organic syntheses and polymerizations. The cracking reaction is highly endothermic and proceeds as follows, BASF fluidized coke unit Lugri sand cracking unit Thermal cracking of hydrocarbons to produce olefins K-K Process BASF process Thermal cracking in solid circulation systems to produce olefins Fluidized combuston of coal An alternative for low grade coal and oil shale fines, fuels that cannot be burned efficiently in conventional boiler furnaces. Bubbling bed type fluidized bed coal combustor (FBC) Circulating solid type fluidized bed coal combustor Incineraton of solid waste Incineration of municipal solid waste chain grate or inclined grate incinerators are being used, countercurrent or crosscurrent modes of contacting are sometime troublesome because of the noxious odors of the flue gas. This problem can be avoided with fluidized bed incineration. Fluidized bed Incinerators FB=fluidized bed, FG=flue gas, FR=freeboard, IB=ignition burner, LS=lime stone, MB=moving bed, PA=primary air, RE=residue, SA=secondary air, SN=sand, SP=spreader, SW=solid waste. Gasificaton of coal and coke Various processes for the gasification of coal A=air, AS=ash, C=coal, CH=char, FB=fluidized bed, GA=gasifier, G=product gas, H2=hydrogen, He=helium, O=oxygen, PN=pneumatic conveyer, RE=regenerator, S=steam, W=water Various processes for the gasification of coal A=air, AS=ash, C=coal, CH=char, FB=fluidized bed, GA=gasifier, G=product gas, H2=hydrogen, He=helium, O=oxygen, PN=pneumatic conveyer, RE=regenerator, S=steam, W=water Actvaton of carbon Charcoal is formed and activated by low temperature (800-900 oC) endothermic gasification with hot combustion gas of wood, peanut shells. The fluidized bed for this operation is generally a multistage unit. Multistage gives a more uniform residence time distribution for the solids. Reactors for activation of charcoal from Sawdus Charcoal t Pitch bead s A=air, CG=hot combustion gas, E=ejector, FS=feed solids, FU=fuel gas, OG=off-gas, PS=product solid, S=steam. Gasificaton of solid waste In gasification of solid municipal solid waste the clean up of combustion gases is much simpler and cheaper because volume of gas produced is much smaller than that from incinerators. Gasifiers for solid waste Tsukishima process Pyrox process Calcinaton articles of limestone and dolomite can be calcined in a fluidized bed by burning fu irectly in the bed. As this reaction is endothermic and gas –solid leaves at 1000 o o to recover much of the heat, multistage is used as shown in figures. Reactors for calcination Particulate limestone Powdery limestone Lime slurry Alumina fines A=air, FS=feed solids, FU=fuel, OG=off-gas, PS=product solid Roastng sulfides ores Matellurgical roasters for sulfide ores Dorr-Oliver type BASF type Silicon for the semiconductor and solar cell industries Fluidized beds are used to produce pure silicon for semiconductors and photovoltaic industries. Following are the FB system to carry 6 step production of silicon. Reactor producing SiHCl3 Reactor producing pure silicon from SiHCl3 Reducton of Iron ore Direct reduction of iron ore is being carried out by different fluidized bed system. Iron ore reduction processes A=air, CP=coke particles, FO=feed oil, FS=feed solids, GR=gasifier, H2=hydrogen, HC=hydrocarbon, O=oxygen, OG=off-gas, PS=product solid, RG=reducing gas, S=steam, W=water Iron ore reduction processes A=air, CP=coke particles, FO=feed oil, FS=feed solids, GR=gasifier, H2=hydrogen, HC=hydrocarbon, O=oxygen, OG=off-gas, PS=product solid, RG=reducing gas, S=steam, W=water Biofluidizaton The cultivation of microorganisms appears to be one of the more interesting applications of fluidization. Following is the system to produce soy sauce. Wheat bran is fluidized by sterilized air after treatment and pasteurization Fluidized bed cultivator to produce threadlike fungus Reference 1. Kunii, D. and Levenspiel, O., "Fluidization engineering," 2nd ed., Butterworth-Heinemann, (1991). Predicton of behavior of Fluidized bed “The arrival time of a space probe traveling to Saturn can be predicted more accurately than the behavior of a fluidized bed chemical reactor!.” Even though the above quotation (Geldart, 1986) is almost 20 years old it remains true in the new millennium of fluidization engineering. The difficulties in prediction stem in part from the complexity and ambiguity in defining the fundamental parameters such as size, shape and density of the particles. These parameters play an important role in the calculation and prediction of dynamic behavior in fluidized beds. Most physical properties of the particles are estimated indirectly, such as estimating particle shape by the bed voidage. All factors are explicitly and implicitly significant in the estimation of the behavior of fluidization operations. Although new technology is helping us to understand and give more precise prediction in fluidization, more research is still needed. Characterizaton of Partcles • particle size • particle shape • Sphericity • mechanical properties • charge properties • microstructure. Particle Size analysis In many powder handling and processing operations particle size and size distribution play a key role in determining the bulk properties of the powder. Describing the size distribution of the particles making up a powder is therefore central in characterizing the powder. In many industrial applications a single number will be required to characterize the particle size of the powder. This can only be done accurately and easily with a mono-sized distribution of spheres or cubes. Real particles with shapes that require more than one dimension to fully describe them and real powders with particles in a range of sizes, mean that in practice the identification of single number to adequately describe the size of the particles is far from straightforward. Particle size distribution Liquid Solid system Smooth expansion of the bed beyond minimum fluidization (umf) Gas-Solid System instabilities with bubbling and channeling of gas observed beyond umf Fluidization Regimes When the solid particles are fluidized, the fluidized bed behaves differently as velocity, gas and solid properties are varied. It has become evident that there are number of regimes of fluidization, as shown in Figure 2.1. Fixed Bed When the flow of a gas passed through a bed of particles is increased continually, a few vibrate, but still within the same height as the bed at rest. This is called a fixed bed (Figure 2.1A). Only A powder at low gas velocity Minimum Fluidization (umf) With increasing gas velocity, a point is reached where the drag force imparted by the upward moving gas equals the weight of the particles, and the voidage of the bed increases slightly: this is the onset of fluidization and is called minimum fluidization (Figure 2.1B) with a corresponding minimum fluidization velocity, Umf. Bubbling Fluidization Increasing the gas flow further, the formation of fluidization bubbles sets in. At this point, a bubbling fluidized bed occurs as shown in Figure Slugging Bed As the velocity is increased further still, the bubbles in a bubbling fluidized bed will coalesce and grow as they rise. If the ratio of the height to the diameter of the bed is high enough, the size of bubbles may become almost the same as diameter of the bed. This is called slugging. Only narrow beds Turbulent Fluidizaton If the particles are fluidized at a high enough gas flow rate, the velocity exceeds the terminal velocity of the particles. The upper surface of the bed disappears and, instead of bubbles, one observes a turbulent motion of solid clusters and voids of gas of various sizes and shapes. Beds under these conditions are called turbulent beds. Fast and Pneumatic fluidization With further increases of gas velocity, eventually the fluidized bed becomes an entrained bed in which we have disperse, dilute or lean phase fluidized bed, which amounts to pneumatic transport of solids Minimum Fluidization Velocity (umf) When a fluid is passed upwards through a bed of particles the pressure loss in the fluid due to frictional resistance increases with increasing fluid flow. A point is reached when the upward drag force exerted by the fluid on the particles is equal to the weight of the particles in the bed. At this point the particles are lifted by the fluid, the separation of the particles increases and the bed become flu...
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