Two Phase Gas-solid Pressure Drop in the Riser of by gvl14091


									Two Phase Gas-solid Pressure Drop in the Riser of a
Circulating Fluidised Bed

D Acharya, Non-member

                     Data on the static pressure distribution for the gas-solid system in the riser of a circulating fluidized bed have been
                     collected to obtain the experimental pressure drop. Theoretical pressure drop equations based on mechanical energy
                     balance as well as expanded packed bed analogy have been derived. Since, the solids friction factor data from literature
                     gave large deviations between experimental and theoretical pressure drop, the solids friction factor values were
                     determined experimentally. Axial voidage distribution correlations were developed for the dense and dilute phases
                     which provided the voidage values required by the theoretical pressure drop equations. The predictions from the pressure
                     drop models agreed well with the experimental data.

                     Keywords: CFB; Riser; Dense and dilute phase; Voidage; Static pressure

NOTATION                                                                           ∆P         :   pressure drop, N/m2
   Dfb , DP      :   riser and particle diameters, respectively, m                 ε          :   gas void fraction
   fg , fs       :   gas and solid skin friction factors, respectively             µg         :   gas viscosity, N/m2
   Ldl , Ldn     :   dilute and dense phase heights, respectively, m               ρg , ρs    :   gas and solids densities, respectively, g/cm3
   LT            :   riser height, m                                            INTRODUCTION
   M g , Ms      :   gas and solid mass flow rates, respectively,               In recent years, it has been observed that bubbling fluidized
                     kg/s                                                       bed boilers used in thermal power plants have shown
   Q g , Qs      :   volumetric flow rate of gas and solid                      decreased efficiency in steam generation. This is because of
                                                                                partial combustion of coal particles in the bubbling fluidized
                     respectively, m3/s
                                                                                bed. To remove this inherent deficiencies and also to obtain
   Ref           :   gas phase Reynolds number, DfbUg ρ f / µ f                 enhanced rate of heat transfer from the combustion of coal to
                                                                                water tubes of the boiler, the concept of circulating fluidized
   Res           :   solid phase Reynolds number dpUs ρ s / µ f ,
                         d           i
                      dP V g − U P ρ f / µ f
                                                                                bed (CFB) has come into existence.
                                                                                The CFB consists of two fluidized beds, namely, a fast
   Ret           :   Reynolds number               for     solids-settling      fluidized bed (FFB) and a slow fluidized bed (SFB). In the SFB
                     d pU t ρ f / µ f                                           the solids are fluidized using the minimum fluidization
   Vg , Vs       :   gas and solids velocities, respectively in riser,          velocity from where the solids enter the FFB also known as
                     m/s                                                        riser via a solids recirculation line. The solids from the riser
                                                                                are recirculated into the SFB through a cyclone separator and
   Vdl , Vdn     :   dilute and dense phase solids velocities,                  a solids return leg. Therefore, FFB (or riser) is the main
                     respectively, m/s                                          equipment where high rate of combustion of coal is achieved
                                                                                because of recirculation of solids.
   Ug            :   superficial velocity of gas, m/s
                                                                                The data on two phase pressure drop in the riser are required
   UP            :   particle velocity in riser, m/s
                                                                                to determine the horse power of the blower for its
   Ut            :   solids settling velocity, m/s                              specification.
   W             :   solids mass flux, kg/m2s                                   LITERATURE SURVEY
   Z             :   riser height, m                                            In order to develop pressure drop models for gas-solid flow in
                                                                                the riser, the flow pattern for the two phase flow must be
D Acharya was with the Department of Mechanical Engineering, MGM                known. It has been reported1-2 that axial or longitudinal
College of Engineering, Nanded 431 602, Maharashtra.                            voidage distribution in the riser showed an S shaped curve,
This paper was received on October 23, 2003. Written discussion on the paper    indicating a point of inflection between the dense phase (above
will be entertained till March 31, 2005.                                        distributor) and a dilute phase above the dense phase. This

Vol 85, January 2005                                                                                                                        219
complex flow pattern has made the theoretical prediction of                                                                         K
the pressure drop very difficult. There are few attempts to
propose empirical equations3 for static pressure distribution or
semi-theoretical equations4-5 for prediction of pressure drop in
                                                                       A Blower               P7                                                G
the riser. Most of the reported works used riser (FFB) having                                                           V7
                                                                       C Orifice Meter
diameter more than 5.0 cm and height more than 2.0 m using             D Distributor          F
excessive fine particles with high solids circulations rates.          E Solids Transfer line                           V6
Since, the voidage distribution depends upon, gas flow rates in        F Fast Bed Column
riser, riser height, riser diameter, solids recirculation rates,       G Cyclone Separator
                                                                       H Solids Return Leg     P5
particle size etc, the correlations on voidage distribution            I Slow Bed Column
                                                                                                                        V4                  H
reported for large CFB may not be applicable for shallow CFB           K Air Exit              P4
(diameter less than 5 cm and height around 2 m). There is              P0 to P7 Pressure Taps
scope for use of shallow CFB for thermal captive power                 V0 to V7 Quick Opening P3
                                                                                Valves                                  V2
plants, bench scale studies for heat transfer and combustion of        W Air Control Valves   P2
coal.                                                                                                                   V1
In the present work, data on static pressure distribution ob-                                           V0
tained in the riser of a shallow CFB system for low solids                                                                   E                  D
circulation rate have been analysed and the experimental                                                P0
pressure drop data were compared with those predicted by the
theoretical pressure drop models derived.                                             W

EXPERIMENTAL DETAILS                                                                                                     C
A schematic diagram of the experimental set-up is shown in
Figure 1. The riser or FFB (F) is made of perspex, 4.6 cm in                                    A
diameter and 2.0 m height (measured from the distributor to
gas exit). The riser is provided with butterfly valves V0 to V7
and pressure tappings P0 to P7 . The slow bed column (I ) is
connected to a cyclone separator (G) through a solids return        Figure 1 Schematic diagram of the experimental set-up for voidage and
leg (H ). The solids from the slow bed (I ) are re-circulated in    static pressure measurement in circulating fluidized bed (CFB)
the riser through solids transfer line (E ). The gas flowrates
were measured by orificemeters. Solids flowrates in the solids      Table 1 Experimental details for study on axial voidage and static
transfer line (E ) were measured by segmental orifice meter.        pressure distribution in CFB

Experimental Procedure                                               Operating Conditions                               Material Characteristics

                                                                                                             Material            Density,       Size,
Initially static beds of solids were maintained in both the riser
                                                                                                                                  g/cm2         cm
and slow bed column. Then the solids in the riser were
fluidized using high air velocity and the flow rate of air in the    Range of superficial
slow bed was kept just above minimum fluidisation velocity.            air velocity in riser,
The fluidized solids from the riser were recirculated into the         290 cm/s–975 cm/s                     Non-coking coal      1.723         0.0780
same bed through cyclone separator, solids return leg, slow
                                                                     Range of superficial
bed column and solids transfer line.
                                                                       air velocity in slow bed,
Data on voidage distribution were obtained by simultaneously           8.7 cm/s–49 cm/s                      Bauxite ore          2.650         0.0754
closing the valves V0 to V7 and collecting the solids deposited      Mass velocity of recirculated
on these. Static pressure distribution data were collected from       solids, 10.81 kg/(m2s)
manometer readings of tappings P0 to P7 .                             to 33.25 kg/(m2s)                      Limestone            2.870         0.0427
Visual observations confirmed that the FFB consisted of a
dense phase, a dilute phase in the annulus and a thin film of       RESULTS AND DISCUSSION
downflowing solids at wall region surrounding the dilute            Effect of Riser Height and Air Velocity on Voidage
phase.                                                              Distribution, ε
The solids used were limestone, bauxite ore and non-coking          Figures 2 and 3 show the relation between ε and riser height, z,
coal. The fluidizing medium was air. Table 1 presents the           for the system bauxite-air, and non-coking coal-air. The curves
experimental details for the studies on axial voidage and static    show a change in slope around a riser height of 120.0 cm. The
pressure distribution in CFB.                                       curves are S shaped as reported for large CFB. The change in

220                                                                                                                              IE (I) Journal—MC
                                                                                             Correlation for the Axial Voidage
                                                 848 cm/s
                                                 930 cm/s, gas vel
                                                                                             An attempt was made to calculate ε from the correlations
                                                                                             given in literature and it was found that the calculated values
                                                 986 cm/s
                                                                                             of ε are much higher than those obtained in this work. The
                                                 Particle dia = 0.0754 cm                    present CFB being shallow, the flowrates of gas and
                               0.95              Solid rate = 12.02 g/s                      re-circulated solids were much less. Therefore, using the
                                                                                             experimental data on voidage, the correlations proposed are:
                                                                                             For the dense phase

                                                                                                 ε dn = 0.312 ( z )0.119 Re f / Re s            j   0.076
                               0.85                                                          For the dilute phase
  Axial voidage distribution

                                                                                                 ε dl = 0.437 ( z )0.068 Re f / Re s            j   0.061

                                                                                             Pressure Drop Models
                                                                                             For predicting two phase gas-solid pressure drop in the riser of
                                                                                             CFB two approaches have been made.
                                                                                             Pressure Drop Equation based on Mechanical Energy Balance
                               0.70                                                              ∆PT = ∆P1 + ∆P2 + ∆P3 + ∆P4                                                     (3)
                                                                                             where ∆P1 is potential energy; ∆P2 , skin frictional energy;
                               0.65                                                          ∆P3 , kinetic energy; and ∆P4 , the momentum of gas-solid
                                      20    40      60    80 100 120 140     160   180 200   mixture.
                                                          Riser height, cm

   Figure 2 Axial voidage distribution in riser of CFB for bauxite-air system                            d                  i
                                                                                                 ∆P1 = M g + M s g LT / Q g + Q s           d           i                        (4)

                                                                                             where M g = V g ε Aρ g ; Ms = Vs ( 1 − ε ) A ρ s ; Q g = Vg ε A ; and
                               0.95               388 cm/s
                                                                                             Qs = Vs ( 1 − ε ) A .
                                                  469 cm/s, Gas vel
                                                  545 cm/s                                       ∆P2 = ∆P fg + ∆P fs                                                             (5)
                               0.90               non-coking coal-air,
                                                  Particle dia = 0.078
                                                  Solid rate = 12.02 g/s                                    e       2
                                                                                                 ∆P fg = 2 f g LT U g ρ g / D fb    j                                            (6)
                                                                                                 ∆P fs = ∆P fs ( dense phase ) + ∆P fs ( dilute phase )
Axial voidage distribution

                               0.80                                                                     e           2
                                                                                                                                    j           e
                                                                                                      = 2 f s Ldn V dn ρ s / D fb + 2 f s Ldl V dl ρ s / D fb         j          (7)

                                                                                                 ∆P3 = V g2 ρ g / 2                                                              (8)
                                                                                                 ∆P4 = W V sl / 2                                                                (9)

                                                                                                                b               g
                                                                                             where W = V s 1 − ε av ρ s ; and V sl = mean slip veocity
                                                                                             = Vg − Ut .


                                       20   40      60     80 100 120 140    160 180   200
                                                                                                        o               b
                                                                                                 ∆P1 = U g ρ g + 1 − ε av V s ρ s g LT  g       t           / U g + b1 − ε av g V s
                                                          Riser height, cm                                                                                                      (10)
   Figure 3 Axial voidage distribution in riser of CFB for non-coking coal-                  where U g = V g ε .
   air system
                                                                                             In equation (6), fg gas friction factor is given by Blasius
   the slope of the curve is marked by a point of inflection indi-                           equation,
   cating a dense phase above grid and a dilute phase above the                                                     − 0 .25
   dense phase.                                                                                  f g = 0.316 Re f                                                               (11)

   Vol 85, January 2005                                                                                                                                                         221
Equation (11) is valid for Ref < 100 000                                                                                                                     Equation (3)
                                                                                                                                             50              Equation (17)
For evaluation of ε av :                                                                                                                                     Equation (3) using Exp.
                                                                                                                                                             Solids friction factor,
                                                                                                                                                             non-coking coal-air system

                           z                                      z
                          120                                 190
      ε av = ( 1 / 90 )         ε dn dz + ( 1 / 70 )                  ε dl dz                      (12)
                          30                                 120

                                                                                                          Theoretical pressure drop, g/cm2
where dense phase height starts from 30 cm above grid and
extends upto 120 cm and dilute phase height is from 120 cm                                                                                   40
upto 190 cm (entry level to cyclone separator).
The actual solids velocities in the dense and dilute regions (Vdn
and Vdl ) can be obtained modifying Yang’s equations as given                                                                                35
by equations (13) and (14), respectively.
For dense phase:

      eV gn −V dn j / U t                         e                               j
                                     = 1+             f sn V dn   / 2 g D fb          ε 4.7
                                                                                        dn         (13)

For dilute phase:                                                                                                                            25

      eV gl −V dl j / U t                     e                                j ε 4dl.7
                                     = 1 + f sl V dl / 2 g D fb                                    (14)                                           38   40   42    44     46     48        50    52   54
                                                                                                                                                            Experimental pressure drop, g/cm2
For computing the solids friction factor, Yang’s equations can                                                   Figure 4 Comparison between experimental and theoretical pressure
be used.                                                                                                         drop in riser

              b g                        b gb
      f s ε 3 / 1 − ε = 0.0126 1 − ε R et / R es                           g   − 0.979
                                                                                                               Equation (17) considers the total pressure drop at incipient
                                                                                                               fluidisation and those due to skin friction of the gas and solids.
                                                          for Vg / Ut > 1.5                        (15)
                                                                                                               Figure 4 shows the comparison between ∆PT predicted by
                                                                                                               equation (17) and the experimental values. The deviation is
and                                                                                                            more than 40%. The solids friction factor, fs required in equa-
                                                                                                               tion (17) was obtained from Yang’s equations.
              b g                        b gb
      f s ε 3 / 1 − ε = 0.0410 1 − ε R et / R es                           g   − 1.021
                                                                                                               From analysis of theoretical pressure drop equations it was
                                                                                                               found that the values of fs applicable for shallow CFB should
                                                          for Vg / Ut < 1.5                        (16)
                                                                                                               be determined to minimize the percent deviation. Therefore,
where fs = fdn or fdl .                                                                                        using the experimental pressure drop data of this work in
                                                                                                               equation (17), the values of solids friction factor, fs were
The voidage ε dn or ε dl were calculated from the equations (1)
                                                                                                               calculated which were in the range of 3.15 × 10 − 4 to
and (2), respectively.
                                                                                                               4.82 × 10 − 4 . Using these values of fs , the ∆PT theoretical was
The theoretical pressure drop was calculated from equation (3)                                                 again calculated from equation (3) and now the deviation
and compared with the experimental data as shown in Figure 4.                                                  between ∆PT (theoretical) and ∆PT (experimental) was less than 10% as
The deviation is above 30%. This is because Yang’s equations (15)                                              shown in Figure 4.
and (16) for solids friction factor, fs , are valid for large CFB
having riser diameter more than 5.0 cm and height more than                                                    CONCLUSION
2.0 m.
                                                                                                               Experimental data on axial voidage and static pressure
Pressure Drop Equation for an Expanded Packed Bed                                                              distribution in a shallow CFB were obtained for low solids
The riser of the CFB may be considered as an expanded                                                          circulation rates. To predict voidage distribution, correlations
packed bed during the two phase flow. The total pressure                                                       have been proposed for dense as well as dilute phase. Using the
drop, ∆PT , can be written as                                                                                  solids friction factor data from literature-correlations, the
                                                                                                               theoretical pressure drop models developed in this work gave
              e            je             j 2
      ∆PT = 1 − ε mg ρ s − ρ g Lm + 2 f g U g ρ g LT / D fb                           j                        large percentage deviations for ∆PT (theoretical) . Therefore, using
                                                                                                               the experimental pressure drop data, the values of fs were
                                                       + e2 f V
                                                                      dn   ρ s Ldn     j/ D   fb   (17)        calculated for shallow CFB and these values of fs when used in

222                                                                                                                                                                                  IE (I) Journal—MC
equation (3) predicted              with a percent deviation below              3. Y Li and M Kwauk. ‘The Dynamics of Fast Fluidisation in ‘Fluidisation.’ (ed)
10%.                                                                            J R Grace and P Matsen, Plenum Press, New York, 1980, p 537.

ACKNOWLEDGEMENT                                                                 4. C Brereton, J R Grace and J Yu. ‘Axial Gas Mixing in a CFB.’ in ‘Circulating
                                                                                Fluidised Bed Technology II.’ (ed) P Basu and J F Large, Pergamon Press,
The author wishes to thank Shri S S Choudhary of the
                                                                                Oxford, 1988, p 307.
Department of Mechanical Engineering, MGM College of
Engineering, Nanded for his guidance during this work.
                                                                                5. W C Yang. ‘A Model for the Dynamics of a Circulating Fluidised Bed Loop’
REFERENCES                                                                      in ‘CFB Technology II’, (ed) P Basu and J F Large, Pergamon Press, Oxford,
                                                                                1988, p 181.
1. C Brereton and L Stromberg. ‘Some Aspects of the Fluid Dynamic
Behaviour of Fast Fluidised Beds.’ in ‘Circulating Fluidised Bed Technology.’
                                                                                6. K E Wirth. ‘Steady State Diagram for CFB.’ in ‘Circulating Fluidised Bed
(ed) P Basu, Pergamon Press, Toronto, 1986, p 133.
                                                                                Technology III.’ (ed) P Basu, M Horio and M Hasatani, Pergamon Press,
2. K Smolders and J Baeyens. ‘Hydrodynamic Modelling of Axial Voidage           Oxford, 1991, p 99.
Profile in the Riser of a Circulating Fluidised Bed.’ Can Journal of Chemical
Engineering, vol 79, June 2001.                                                 7. W C Yang. ‘Powder and Bulk Solids Technology.’ 1977, p 89.

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