884化工原理的電子版本課件雙語chapter4.2f

1、1Chapter 4Particle-fluid two-phase flow2A spray-fluidized dryer under construction3FluidizationDefinition A system of fluid flow with particles suspended in it. This system behaviors like a pure fluid. With this technology, transport and chemical reaction can be strengthen greatly.Minimum fluidized

2、velocity umf and critical voidage mf minimum fluidization 4Fluidizationoperation range of fluid beds operation rangeumf u utubDP0LuABCDEABCDEbed heightLog uLog pbumftutmfaggregateparticulateemulsified phaseFbubble phase5Minimum fluidized velocityumf is the transition point from fixed to fluidized be

3、d. It can be found from the data of pb u curve. At this point, the forces on the particle groups, not the single particle balance, and the pressure drop flowing through the bed, pb equals to the difference of gravity and buoyancy per unit area bed. It seems that the Eugan Eq. is applicable. Combine

4、the two Eqs. and replace dea with (AdeV) to give6Minimum fluidized velocityIf Rep1000，the first term on left hand can be ignored, then ， note：the Eugan Eq. is correct in theory but with great error. Therefore, empirical Eqs are used in practical applications.particle geometry and bed voidage can be

5、estimated byIf deV is small and Rep10, the second term on left hand can be ignored, then 7empirical equations for umf Many empirical equations have been suggested and most well known is the Eq. below For 10 Remf 10%pb，and 350 Pa) can distribute the gas homogeneously and avoid the occur of big bubble

6、s and ernal parts：preventing or breaking big bubblessize distribution：small particles with wide size distribution tend to give homogeneous fluidization.porous plateporous capsdistribute pipes fluidfluidfluid11Justification of Particulate & Aggregative fluidizationParticulate fluidizati

7、oncharacters particle and voidage are homogeneous in bed, bed expands homogeneously with velocity, bed surface and pressure drop are stable.It is ideal fluidizing state. Usually found in bed with small density difference between solid-fluid, e. g. liquid fluidized bed.Aggregative fluidizationcharact

8、ers a two-phase heterogeneous structure. One is a continuous phase with homogeneous particle and voidage distribution and another is a bubbling phase with very few particles. This is called also bubbling fluidization. Usually found in bed with large density difference between solid-fluid, e. g. gas

9、fluidized bed.12Justification of Particulate & Aggregative fluidizationUnder special conditions, gas fluidized beds can be particulate as low density particles fluidized in high pressure gas; or liquid fluidized bed as aggregative as heavy metal particles in water.For particulateFor aggregativeFroud

10、e number under minimum fluidized condition, D is bed diameter A criterion to justify the fluidizing state is given by Bomero and Johanson aswhereThere are many more criteria can be found in literature with their own limits.13 The particles are divided into 4 groups according to their fluidizing beha

11、viorsGeldarts classification of powders14Geldarts classification of powdersGroup A Powders in this group exhibit a particulate expansion before bubbling occurs；in other words，umb/ umf 1. A maximum bubble size does appear to exist，and most bubbles rise more rapidly than the interstitial gas.Group B m

12、edium-range size and density. Bubbling starts at incipient fluidization (umb/ umf =1)There is no evidence of a maximum bubble size，and most bubbles rise more quickly than the interstitial gas Group C All cohesive powders，very difficult to fluidizeGroup D large or very dense particles. In the corresp

13、onding beds，all but the largest bubbles rise more slowly than the interstitial gasIf gas is admitted through a central orifice, powders of group D can be made to spout 15Bed height and TDHFluidizing Number The fluidizing state and quality are related with this number.The ratio of operation and minim

14、um fluidized velocities, u/umfExpanding ratio RThe ratio of operation and minimum bed heights, L/ LmfDifferent fluidizing state may gives different bed expanding rule.16Bed height and TDHTDH（Transport Disengaging Height）： The height that particles flying upward and fall back to bed by gravity.The pa

15、rticles over TDH will be elutriated.TDH is very important to design the height of the bed. LTDHheightParticle density in fluid17Fluidization In a broad sense ut u. The fluidizing number can be as high as several hundreds. The elutriated particles can be collected with a cyclone and sent back to the

16、bed. So called recycling fluidized beds. This system includes 3 zones：dense phase, dilute phase and particle transfer.Example fluidizing oil catalytic cracking systemGasified crude oil and catalyst particles flow upward in riser reactor with high velocity，and the cracking process is finished in 5 to 7 seconds. The catalyst is collected in a cyclone, then sent to a regenerator through a