Fluidized bed model

Designing a model fluidized bed which simulates the hydrodynamics of a commercial bed requires accounting for all of the mechanical forces in the system. In some instances, convective heat transfer can also be scaled but, at present, proper scaling relationships for chemical reactions or hydromechanical effects, such as particle attrition or the rate of tube erosion, have not been established. 

Example 5.4 Design a geometrically similar laboratory-scale cold model fluidized bed to simulate the hydrodynamics of a large-scale fluidized bed combustor. Also specify the operating conditions for the cold model. The combustor is a square cross section column with a width of 1.0 m and a height of 6 m. The fluidized bed combustor is operated at a temperature of 1,150 K, a superficial gas velocity of 1.01 m/s, and a bed height of 1.06 m. Particles with a density of2,630 kg/m3 and a diameter of677ptm are used for the combustor. The cold model is operated at a temperature of 300 K. Air is used for both the cold model and hot model fluidized beds. The physical properties of air are... 

Table E5.2. The Operating Conditions for the Hot Model and Cold Model Fluidized Beds...

What follows is a survey of the fluid-mechanical principles of fluidization technology, gas and solid mixing, gas-solid contact in the fluidized bed, typical industrial applications, and approaches to modeling fluidized-bed reactors. Further information is given in... 

Scala, F. and Salatino, P, Modelling fluidized bed combustion of high-volatile solid fuels. Chemical Engineering Science 2002, 57 (7), 1175-1196. 

The application of large scale computer simulations in modeling fluidized bed coal gasifiers is discussed. In particular, we examine a model wherein multidimensional predictions of the internal gas dynamics, solid particle motion and chemical rate processes are possible. 

FLUFIX, for modeling fluidized bed combusters (Chang et al 1989), at the Argonne National Laboratory (USA)... 

Boemer A, Qi H, Renz U, Vasquez S, Boysan F (1995) Eulerian computation of fluidized bed hydrodynamics - A comparison of physical models. Fluidized Bed Combustion - Volume 2 ASME 1995... 

Santana D, Rodriguez JM, Macias-Machin. A. Modelling fluidized bed elutriation of fine particles. Powder Technology 106 110-118, 1999. 

However, more recently the present authors (Werther and Reppenhagen, 1999 Reppenhagen and Werther, 1999b) have demonstrated that a strict observance of the above concept dramatically helps to overcome these discrepancies. They considered the attrition-induced loss flow of catalyst material from the cold model fluidized bed unit with external solids recirculation that is schematically shown in Fig. 21. The cyclone overflow is connected to a filter that collects... 

In recent years, several modified versions of the two-phase model were proposed for modeling fluidized bed reactors. They include a model proposed by Werther (1980) for catalytic oxidation of ammonia, in which the mass transfer process is expressed in terms of film theory, as described in Danckwerts (1970) a model proposed by Werther and Schoessler (1986) for catalytic reactions a model proposed by Borodulya et al. (1995) for the combustion of low-grade fuels a model proposed by Arnaldos et al. (1998) for vacuum drying and a model proposed by Srinivasan et al. (1998) for combustion of gases. The modifications include the consideration of axial mass transfer profile, the inclusion of a wake phase in addition to the bubble and emulsion phases, and the consideration of the growth of bubbles in the bubble phase. 

To be able to model fluidized bed reactors, one must first determine the hydrodynamic regime of operation. For an existing unit, this can usually be done experimentally. A rapid response pressure transducer and measurement of time-mean pressure profiles along the height of the reactor are usually sufficient to make this determination (1). Determining features of the signals are as follows ... 

Boemer A, Qi H, Renz U, Vasquez S, Boysan F (1995) Eulerian computation of fluidized bed hydrodynamics—a comparison of physical models. Fluidized bed combustion, vol 2. ASME Campbell CS (1990) Rapid granular flows. Atmu Rev Fluid Mech 22 57-92 Carlo AD, Bocci E, Zuccari F, DeU Era A (2010) Numerical investigation of sorption enhanced steam methane reforming process using computational fluid dynamics Eulerian-Eulerian code. Ind Eng Chem Res 49 1561-1576... 

The performance of fluidized-bed reactors is not approximated by either the well-stirred or plug-flow idealized models. The solid phase tends to be well-mixed, but the bubbles lead to the gas phase having a poorer performance than well mixed. Overall, the performance of a fluidized-bed reactor often lies somewhere between the well-stirred and plug-flow models. 

The modeling of fluidized beds remains a difficult problem since the usual assumptions made for the heat and mass transfer processes in coal combustion in stagnant air are no longer vaUd. Furthermore, the prediction of bubble behavior, generation, growth, coalescence, stabiUty, and interaction with heat exchange tubes, as well as attrition and elutriation of particles, are not well understood and much more research needs to be done. Good reviews on various aspects of fluidized-bed combustion appear in References 121 and 122 (Table 2). 

In the chemical engineering domain, neural nets have been appHed to a variety of problems. Examples include diagnosis (66,67), process modeling (68,69), process control (70,71), and data interpretation (72,73). Industrial appHcation areas include distillation column operation (74), fluidized-bed combustion (75), petroleum refining (76), and composites manufacture (77). 

The first commercial fluidized bed polyeth)4eue plant was constructed by Union Carbide in 1968. Modern units operate at 100°C and 32 MPa (300 psig). The bed is fluidized with ethylene at about 0.5 m/s and probably operates near the turbulent fluidization regime. The excellent mixing provided by the fluidized bed is necessary to prevent hot spots, since the unit is operated near the melting point of the product. A model of the reactor (Fig. 17-25) that coupes Iduetics to the hydrodynamics was given by Choi and Ray, Chem. Eng. ScL, 40, 2261, 1985. 

Wen, C. Y. and L. H. Chen, "Flow Modeling Coneepts of Fluidized Beds," n Handbook of Fluids in Motion, N. P. Cheremisinoff (Editor), Butterworth Publishers, Ann Arbor, MI, 1983. 

The effectiveness of a fluidized bed as a ehemical reactor depends to a large extent on the amount of convective and diffusive transfer between bubble gas and emulsion phase, since reaction usually occurs only when gas and solids are in contact. Often gas in the bubble cloud complex passes through the reactor in plug flow with little back mixing, while the solids are assumed to be well mixed. Actual reactor models depend greatly on kinetics and fluidization characteristics and become too complex to treat here. 

Knowledge of these types of reaetors is important beeause some industrial reaetors approaeh the idealized types or may be simulated by a number of ideal reaetors. In this ehapter, we will review the above reaetors and their applieations in the ehemieal proeess industries. Additionally, multiphase reaetors sueh as the fixed and fluidized beds are reviewed. In Chapter 5, the numerieal method of analysis will be used to model the eoneentration-time profiles of various reaetions in a bateh reaetor, and provide sizing of the bateh, semi-bateh, eontinuous flow stirred tank, and plug flow reaetors for both isothermal and adiabatie eonditions. 

Viswanathan et al. (V6) measured gas holdup in fluidized beds of quartz particles of 0.649- and 0.928-mm mean diameter and glass beads of 4-mm diameter. The fluid media were air and water. Holdup measurements were also carried out for air-water systems free of solids in order to evaluate the influence of the solid particles. It was found that the gas holdup of a bed of 4-mm particles was higher than that of a solids-free system, whereas the gas holdup in a bed of 0.649- or 0.928-mm particles was lower than that of a solids-free system. An attempt was made to correlate the gas holdup data for gas-liquid fluidized beds using a mathematical model for two-phase gas-liquid systems proposed by Bankoff (B4). 

As mentioned in Section 11.3, fluidized-bed reactors are difficult to scale. One approach is to build a cold-flow model of the process. This is a unit in which the solids are fluidized to simulate the proposed plant, but at ambient temperature and with plain air as the fluidizing gas. The objective is to determine the gas and solid flow patterns. Experiments using both adsorbed and nonadsorbed tracers can be used in this determination. The nonadsorbed tracer determines the gas-phase residence time using the methods of Chapter 15. The adsorbed tracer also measures time spent on the solid surface, from which the contact time distribution can be estimated. See Section 15.4.2. 

An overly simplified model of fluidized-bed combustion treats the solid fuel as spherical particles freely suspended in upward-flowing gas. Suppose the particles react with zero-order kinetics and that there is no ash or oxide formation. It is desired that the particles be completely consumed by position z = L. This can be done in a column of constant diameter or in a column where the diameter increases or decreases with increasing height. Which approach is better with respect to minimizing the reactor volume Develop a model that predicts the position of the particle as a function of time spent in the reactor. Ignore particle-to-particle interactions. 

Marmo, L., Rovero, G., and Baldi, G., Modeling of catalytic gas-solid fluidized bed reactors, Catal. Today, 52, 235-247 (1999). 

Literature references and the measurement of the contact time distribution in a large, cold-flow model of a gas-fluidized bed are reported in... 

The reactor is a gas-fluidized bed for which the fractional tubularity model is usually appropriate. 

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