1 PRESSURE FLUCTUATIONS IN A GAS-SOLID FLUIDIZED BED M. Hartman, O. Trnka, K. Svoboda Institute of Chemical Process Fundamentals Academy of Sciences of.

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PRESSURE FLUCTUATIONS PRESSURE FLUCTUATIONS

IN AIN A GAS-SOLID FLUIDIZED BEDGAS-SOLID FLUIDIZED BED

M. Hartman, O. Trnka, K. Svoboda

Institute of Chemical Process Fundamentals

Academy of Sciences of the Czech Republic

Rozvojova 135, 165 02 Prague–Suchdol, CR

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Fluidization

- a unit operation widely employed in the process industry.

- a bed of particulate solids made to behave like a liquid by the upward passage of the fluid (usually gas); but it has some unusual characteristics.

- recognized as an efficient means of contacting gas and particulate materials.

Fluidized bed (FB)

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Advantages of FBs

1. An efficient bed-to-surface heat transfer due to rapid particle movement (good mixing).

2. Nearly isothermal conditions can be achieved: „hot/cold“ spots are rapidly dissipated.

3. The easy circulation / transfer of solids between coupled reactors / vessels (e.g., a catalytic reactor and a regenerator).

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Some examples of industrial applications of FBs

Physical operations: Drying of solids,Adsorption,Coating, Granulation,Heat exchange, etc.

Chemical reactions: Catalytic cracking of hydrocarbons,Combustion of coal,Incineration of waste,Gasification of coal / waste,Roasting of sulfide ores,Calcination, etc.

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Disadvantages of FBs

1. Mechanical problems: the erosion of bed internals,

the elutriation of fines,

sticky materials can cause the

defluidization.

2. Our understanding of the physics of fluidization is still limited.

3. Reliable design and scale-up of the FB reactors is difficult to achieve.

4. The performance of FB units is affected strongly by the flow regime in the vessel, i.e., by the hydrodynamic state of the bed.

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The gas-solid flow in a FB exhibits very distinct structures:

1. Low-velocity fluidization with bubbles or slugs; the bubbling / slugging beds.

2. Medium-velocity fluidization with quick tongues of gas; the turbulent beds.

3. High-velocity fluidization with clusters or strands of particles; the entrainment / circulating beds.

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The basic premise

Quite detailed information on the system hydrodynamics (the state of FB) can be inferred from the study of pressure fluctuations within the FB.

Pressure fluctuations in the FBs (PFs)

- very intricate phenomena which are the source of much discussion,

- associated with the dynamics of the random motion of heterogeneities such as gas pockets and/or particle clusters in fluidized gas-solid suspensions,

- a very significant indicator of the dynamics of FBs.

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MEASUREMENT OF PRESSURE FLUCTUATIONS

- with the use of sensitive pressure transducers connected to a pressure probe inserted into the FB; A/D convertor;

computer.

- the sampling frequency for the fluctuating pressure signal: 512 Hz, the sampling period: 64 s.

- an 8-cm-ID glass cylinder; 5 m high column with a perforated plate distributor.

- the particulate materials: glass beads, 0.8 – 1.0 mm

limestone, 0.5 – 0.65 mm

ceramsite, 1.0 – 1.25 mm

- air used as a fluidizing gas at ambient temperature.

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1

2

3

4

5

6

78

910

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12

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Experiments at ambient T and P. Superficial gas velocity: 0 – 3.8 m/s.Sampling : for 64s at the frequency 512 Hz.

Experimental system

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Static (fixed) beds 1; 2-

Bubling beds 2+; 3; 4

Slugging 5

Turbulent beds 6; 7

Entrainment beds 8; 9;10

Overwiew of operation regimes of FBs

tUGas velocity increases from zero to (the terminal free fall velocity of particles).

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Evaluation of the successive time series of the PFs

- based upon the spectral analysis of the pressure fluctuations within the bed,

- by means of the fast (discrete) Fourier transform (FFT) of the fluctuating signal,

0 4time, sec.

P, Pa Pmean

Pmin

Pmax

f, Hz

a, Pa

0 64

we obtain an amplitude spectrum.

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The spectrum is then assorted in the degressive order of the amplitudes

and divided into two equal areas by the median fM.fM

a

0 fmax

Three quantities deduced from the spectra are defined as descriptors of the actual state of FB:

1. The divide ratio max1 ffM MfM is the median

of assorted spectrum

… The measure of dominancy of the spectrum

.166.0 M

66.0M

1M

For a signal of random noise;

For a signal of single dominant frequency.

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2. The square root of the spectral power of the PFs, E.

256

1

2256

4

iiaW WE Viewed as a measure of the energy

dissipated in the bed.

Pmean

Pmin

Pmax

minmax

max

PP

PPS mean

3.

MS The product defines the transition between thebubbling ( ) and the turbulent beds ( ) .5.0MS 5.0MS

expresses the symmetry of the sampled PF signal.

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Relative sizes and overlapping of the intervals of values of the quantity M for the static and bubbling beds

1; 2- : fixed bed; sporadic small bubbles particles at standstill;2+ : (gently) bubbling bed;3 : large bubbles;4 : exploding bubbles;5 : slugging.

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Essentials

An actual state/behavior of the FB can be described withthe aid of the three quantities: E(U), M(U), and U.

This triad is called an operation/fluctuation pointp(U)=[ E(U), M(U), U ] .

- When the superficial gas velocity, U, is gradually increased, p(U) moves along a trajectory called the characteristics of FB.

- By projection of the trajectory into the planes E x M and E x U we get a graph of the fluctuation characteristics.

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0,65

0,7

0,75

0,8

0,85

0,9

0,95

1

0 20 40 60 80 100 120 140

0

0,5

1

1,5

2

2,5

3

3,5

4

0 20 40 60 80 100 120 140

1,2- static beds

2+ bubbles

3 large bubbles

4 exploding bubbles

5 slugging

6 intermed. turbul.

7 full turbulence

8 entrainment starts

9 diluted bed

10 transport starts

Fluctuation characteristics

E(U)

U

M

Turbulent beds (6-7)

Bubbling beds (2+ … 5)

Entrainment beds (8-10) Ume 10

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Features of the fluctuation characteristics

- It describes the FB as an integral whole.

- The sections of the curve corespond to the real/actual operation regimes of the FB.

- The curve remains the same, provided that the bed material does not change.

- Changes in the bed material (such as the variation of its amount and granularity) result in the change of the fluctuation characteristics.

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C O N C L U S I O N S

- Pressure fluctuations are a significant indicator of dynamics of gas-solid FBs.

- Useful information can be extracted from PFs for monitoring the fluidization behavior using an adequate spectral analysis of the fluctuating pressure signal.

- The proposed method makes it possible to determine on line a number of changes in the FB by examining the alternations and deformations of the characteristic curves.

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T H E E N D