Three phase fluidization
Download as PPTX, PDF4 likes5,969 views
This document discusses three phase fluidization, which involves suspending a granular solid in a fluidized state using gas and liquid passing through it. Three phase fluidized beds are used for various chemical reactions and industrial applications. Key parameters for three phase fluidized beds include bed expansion, phase holdups, minimum fluidization velocity, and gas holdup. Correlations are presented for predicting minimum fluidization velocity and gas holdup based on dimensionless groups. The gas holdup is influenced by variables like liquid and gas velocities, liquid properties, particle properties, and bed dimensions.
Three phase fluidization
- 2. Introduction • Fluidization has been a pivotal topic of research among the researchers for the past three decades. Since the Winkler process for gasification of coal was on track, the focus on fluidization has been consistently gaining significant importance in industrial phase as well as in academic research. • The concept of fluidization in Winkler process was further developed into various successful applications such as FCC, Metallurgical process, Sohio process, synthesis of polymers, combustion and in the recent years for the treatment of waste water. Analogous to such industrial developments, considerable academic research has also been carried out in the field of fluidization.
- 3. Three Phase Fluidization • The three-phase fluidized bed is a type of system that can be used to carry out variety of multiphase chemical reactions. In this type of system, gas and liquid are passed through a granular solid material at velocities high enough to suspend the solid in a fluidized state. • Three-phase fluidized beds enjoy widespread use in multiphase chemical reactions, a few of which include methanol production, conversion of glucose to ethanol, various hydrogenation and oxidation reactions, hydro treating and catalytic cracking in petroleum refinery, conversion of heavy petroleum and synthetic crude and coal liquefaction. • Three-phase fluidized beds are used in a wide range of industrial applications including processing of hydrocarbons, in particular in the upgrading of heavy oil and high molecular weight feedstock’s, aerobic wastewater treatment, and direct coal liquefaction.
- 4. Parameters • Hydrodynamic properties of three-phase fluidized beds are important for analyzing its ultimate performance. These include mainly bed expansion behavior, phase holdups and pressure drop. All the three phase holdups (∈g, ∈s, ∈l) should add to give unity. Bed expansion is important in determining the size of the system while the phase holdups are essential in mixing and studying the overall performance of the system.
- 5. Minimum Fluidization Velocity • For a gas-liquid-solid system, the minimum liquid fluidization velocity is the superficial liquid velocity at which the bed becomes fluidized for a given superficial gas velocity. Above the minimum liquid fluidization velocity, there is good contact between the gas, liquid, and solid phases. Concentration and temperature gradients are minimized, which is important if the solid phase is a catalyst, a reactant, or an adsorbent, and if the reaction is endothermic or exothermic as temperature gradients may degrade reaction selectivity, product quality, and catalyst activity. • Most studies on the minimum liquid fluidization velocity in gas-liquid solid systems have been conducted with solids whose density is at least twice the liquid density.
- 6. • It is essential in considering such three-phase systems to be able to predict the minimum gas and liquid flows needed to initiate and sustain full fluidization conditions. • Co-current upward gas-liquid fluidization of coarse solids (dp> 1 mm) is actuated primarily by the motion of the liquid at low gas velocities (Ug <0.2 m/s for air and water as the gas and liquid). For such systems, it is seen that the minimum liquid fluidization velocity, Ulmf, for a given modest gas superficial velocity is predicted quite well by a gas-perturbed liquid model, in which it is assumed that the solid particles are fully supported by the liquid, the role of the gas being simply to occupy space, thereby increasing the effective velocity of the liquid. • Bubble-induced flow can be neglected under these conditions. Although the liquids involved in all cases were Newtonian, a recent study has shown that the same model can be successfully adapted to non-Newtonian liquids.
- 7. The gas-perturbed liquid model • This model, applied to three-phase fluidization involving Newtonian liquids, equates the liquid-buoyed weight of solids per unit bed volume to the frictional pressure gradient given by the Ergun packed bed equation applied to the liquid-solids part of the incipiently fluidized bed. where αmf is the gas hold-up divided by the total fluid (gas+liquid) hold-up at minimum fluidization. By means of the two approximations by Wen and Yu relating the minimum fluidization voidage, εmf, to the sphericity, ϕ, i.e.,
- 8. • In the absence of gas flow, i.e., for αmf =0 , Eq. (la) becomes equivalent to the Wen and Yu equation for minimum liquidsolid fluidization. For three-phase (gas-liquid- solid) fluidization, however, an estimate of αmf is required in order to solve Eq. (1) or (la). A good estimate at minimum fluidization is provided by the empirical equation of Yang et al. for co current upward flow of gas and non-foaming liquid through fixed beds of solids, which can be written as (2)
- 9. • Eq. ( 1 ) or Eq. ( 1 a), together with Eq. (2), must be solved simultaneously or iteratively for αmf and Ulmf.
- 10. Gas Holdup • The gas holdup is one of the most important characteristics for analyzing the performance of a three-phase fluidized bed. For chemical processes where mass transfer is the rate limiting step, it is important to estimate the gas holdup since this relates directly to the mass transfer. • Although gas holdup in three phase fluidized beds have received significant attention as summarized in various reviews, in most of the previous work air, water, and small glass beads have been used as the gas, liquid, and solids, respectively. This combination limits the generality and usefulness of the results. The gas holdup in such systems is often considerably lower than for pilot-plant or industrialscale units.
- 11. • Industrial-scale units operate with high gas holdup and contain small bubbles: these conditions result from effects of both high reactor pressures and low surface tension liquids. • With the use of high viscous and low surface tension liquids, the gas hold up is enhanced. • Higher liquid viscosity exerts higher drag on the gas bubble; the same is done by lower surface tension of liquid due to formation of surface tension gradient on the bubble surface. A higher drag results in lower bubble rise velocities and hence higher holdup. With the lower surface tension of the liquid finer bubbles are formed. • The gas holdup characteristic depends upon the bubble size and its dispersion in the bed.
- 12. • The gas holdup was determined from the pressure drop measurements using Eq. (1). • Previous studies on three-phase fluidized beds have identified ten variables (UL, Ug, μL, σL, ρL, Δρg, ρp, dp, hs, Dc) which are expected to influence the gas holdup significantly. The gas density has been incorporated in the buoyancy (Δρg). Using these ten significant variables which involve three fundamental dimensions (mass, length and time), seven independent dimensionless groups can be formed according to Buckingham Pi theorem. Keeping in mind the advantage of using groups that are familiar in multiphase flow, two sets of seven independent dimensionless groups has been developed by rearrangement as;
- 13. The combinations Weber number (We), Reynolds number (Re), and Froude number (Fr) or Morton number (Mo) and Eötvös number (Eo) are used to characterize the multiphase flow of bubbles or drops moving in a surrounding fluid. Morton number (Mo) is the combination of Weber number (We), Reynolds number (Re), and Froude number (Fr).
- 14. • The R-square value of the developed equation is 0.972. • The R-square value of the developed equation is 0.997. • It is precise enough for the prediction of gas holdup and has been used to optimize the operating conditions for finding highest possible gas holdup in the experimental domain.
- 15. Other correlations • Dakshinamurthy(1972) • Begovich and Watson(1978) a=2.12, b= 0.41 a=2.65, b= 0.6
- 16. • Liquid hold up by Kato (1984)
- 17. Pressure Drop
- 18. References • Fluid Bed Technology in materials processing by C.K.Gupta. • Minimum liquid fluidization velocity in two and three phase beds by H. Miura and Y. Kawase. • Minimum liquid fluidization velocity in gas-liquid-solid fluidized beds of low-density particles by C. L. Briens, A. Margaritis and J. Hay. • Determination of optimum gas holdup conditions in a three-phase fluidized bed by genetic algorithm by H. M. Jena, G. K. Roy and S. S. Mahapatra. • Minimum fluidization velocities for gas-liquid-solid three-phase systems J.P. Zhang, N. Epstein , J.R. Grace.