Mass and Heat Transfer

1. MOMENTUM, HEAT, AND
MASS TRANSFER
Group Members:
Arslan Maqbool Subhan Ali
329400 328888
M. Umair Tauqeer
329662
NUST School of Chemical and Material Engineering

2. HEAT TRANSFER CHARACTERIZATION AND IMPROVEMENT IN AN
EXTERNAL CATALYST COOLER FLUIDIZED BED
Jiantao Li, Xiuying Yao, Lu Liu,
Xudong Zhong, Chunxi Lu
a. State Key Laboratory of Heavy
Oil, China University of Petroleum
(Beijing), Beijing 102249, China
b. Luoyang R&D Center of
Technology, SINOPEC Engineering
(Group) Co. Ltd., Luoyang 471003,
China

3. TABLE OF CONTENT
1. Introduction
2. Experimental setup
3. Measurement Methods
 Measurement of heat transfer parameter
 Measurement of flow parameters
4. Results and discussions
 Total heat transfer coefficient
 Regional heat transfer coefficients
5. Hydrodynamics in the dense heat transfer region
6. Intensification of heat transfer
7. Conclusion

4. Introduction:
 Gas–solid fluidized beds are widely used in chemical reaction processes and other
applications
 They provide a high degree of gas–solid contact and uniform temperature profiles
within the bed
 As an example, external catalyst coolers have been employed to remove heat from
reaction regeneration systems in fluid catalytic cracking (FCC) units
 Gas–solid fluidized bed coolers incorporate several vertical heat tubes and the bed-to-
wall heat transfer in these units is an important factor used to evaluate performance

5. Continued…
 This heat transfer process proceeds via radiation, gas convection and particle convection
 This heat transfer process proceeds via radiation, gas convection and particle convection
 Particle convection is often more important than gas convection because particles have a
greater heat capacity
 Although the radiation component is typically insignificant when the bed temperature is
below 773 K

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Experimental setup:
 A pilot scale cold mode system was built to
investigate the bed-to-wall heat transfer in a
gas–solid fluidized bed, as shown in Fig. 1
 This apparatus primarily consisted of a
fluidized bed for heat transfer experiments, a
riser, a gas–solid separator and a fluidized
bed for particle recirculation 6
Fig. 1. Experimental setup

7. Measurement methods:
Measurement of heat transfer parameter
 The locations of the axial measurement points at which the water temperatures inside the
tube were monitored using four Pt-100 pole thermocouples are provided in Fig. 2
 The bed temperatures at four axial heights were also measured using a pole thermocouple
capable of simultaneously determining the bed temperatures at five radial positions, as
indicated in Fig. 3
 During each experiment, the above mentioned water temperatures (TF), wall temperatures
(Tw) and bed temperatures (Tb) were collected simultaneously
 From these data, the bed-to-wall heat transfer coefficient was calculated as:
ℎ =
𝐶𝑤𝑎𝑡𝑒𝑟𝑚𝑤𝑎𝑡𝑒𝑟 𝑇𝐹,𝑖𝑛 −𝑇𝐹,𝑜𝑢𝑡
A (𝑇𝑤−𝑇𝑏)

8. Fig. 2. Schematic diagram showing
the axial measurement points
Fig. 3. Schematic diagram showing the radial
measurement points

9. Measurement of flow parameters
 Nine pressure sensors were used to monitor the pressure at different heights, as shown
in Fig. 2.
 In addition, an optical fiber probe was positioned axially in the dense region to permit
the effects of the gas–solid flow on heat transfer characteristics to be examined
 This effect can be used to obtain the local solids holdup, εs, based on the equation:
εs = 0.023 + 0.00468 ∗ e1.092U

10.  The constants were obtained using a separate calibration device in which the solids
holdup could be controlled from 0 to 0.55 by adjusting the gas velocity, as shown in Fig. 4
 The local bubble frequency, fb, in the dense region was determined by analyzing voltage
signals returned by the probe, and Fig. 5 shows typical signals obtained from the dense
region.
 The number of bubbles, nb, during the sampling time, t, can be determined based on the
critical voltage value and fb could be calculated as:
fb = nb/t
where nb and t are the number of bubbles and the sampling time,
respectively.
Continued…

11. Fig. 4. The solids holdup as a function of voltage Fig. 5. Voltage signal acquired using a fiber optic
probe in the dense region

12. Total heat transfer coefficient
 Fig. 6 presents two different profiles of the total heat
transfer coefficient, ht, as the superficial gas velocity, ug,
was increased at different external solid mass flux values,
Gs
 At Gs ≤ 23 kg/(m2.s), the total heat transfer coefficient
initially increased sharply and then underwent slight
fluctuations as the superficial gas velocity was continually
increased
 In contrast, at Gs > 32 kg/(m2.s), the total heat transfer
coefficient first increased very rapidly and then rose more
slowly
 It is also evident that the external solid mass flux affected
the total heat transfer coefficient differently at different
superficial gas velocities
Results and discussions:
Fig. 6. Effect of operational conditions on the
total heat transfer coefficient

13. Regional heat transfer coefficients
(Division of heat transfer regions)
 Fig. 7 presents two typical axial profiles for the dimensionless wall temperature, Tw∗, under
different operational conditions
 As shown in Fig. 8, the height corresponding to the average value of the dimensionless wall
temperature could be used as a dividing line between the dense and the dilute heat transfer
regions
 Fig. 9 shows that the dividing lines determined by Zhang’s method and the present
technique are similar, with a relative error of less than 4.5% of the axial height
corresponding to the dividing line
 This result indicates that the present method is valid

14. Fig. 7. Axial Tw∗ profiles under
different operational conditions
Fig. 8. Divisions of the heat transfer
regions in the axial direction
Fig. 9. Comparison of the dividing
lines between the dense and dilute
regions as
determined by different methods

15. (a) Effect of the superficial gas velocity
 Fig. 12(a) summarizes the radial distribution of the bubble
frequency, fb, in the dense heat transfer region. This value was
high in the bed center (r/R = 0) and low near the bed wall (r/R
= 1)
 From Fig. 12(b) it is apparent that the bubble frequency
increased rapidly at first and then slowly decreased as the gas
velocity increased in the bed center
Hydrodynamics in the dense heat transfer region

16. (b) Effect of the external solid mass flux
 Fig. 15 (a) presents the effect of the external solid mass flux
on bubble frequency near the heat transfer surface (r/R =
0.69)
 At ug < 0.3 m/s, the bubble frequency near the heated
surface was almost unaffected by the external solid mass flux
 In addition, the radial mixing was poor at low gas velocities
(Fig. 15 (b) and some particles directly exited the bed without
contacting the tube (referred to herein as solid heat transfer
bypass)

17. Intensification of heat transfer
 The relationship between hydrodynamics and heat
transfer characteristics in the bed demonstrates that a
high bubble frequency and intensive radial mixing of
the particles both enhance the bed-to-wall heat
transfer
 Further increases in heat transfer can be obtained
using two strategies to improve the flow of the gas–
solid phase in the fluidized bed
 This Includes:
(1) Increasing the frequency of bubbles
(2)Increasing the radial mixing of particles
Fig. 16. Schematic diagram showing
the heat transfer intensification
technique
used in this work

18. Conclusion:
 A pilot scale experimental unit was designed and constructed to examine the effects of
hydrodynamics on bed-to-wall heat transfer and to explore methods of intensifying the
heat transfer between the bed and tube surface in a dense gas–solid fluidized bed with
external solids circulation
 The axial profile of the dimensionless wall temperature, Tw∗, was employed to divide the
fluidized bed into dense and dilute heat transfer regions to assess regional heat transfer
mechanisms, and heat transfer loads in these regions were compared at various
operational conditions
 In the dense heat transfer region, the bubble frequency was correlated with the bed-to-
wall heat transfer because of enhanced packet renewal on the heat transfer surface

19.  The radial mixing was poor at low gas velocities, such that some particles left the bed
without transferring heat to the tube, known as solid heat transfer bypass, and this bypass
effect could be enhanced by increases in the external solid mass flux
 Based on the observed hydrodynamics and heat transfer characteristics, a baffle was
designed and installed in the bed and this device increased the total heat transfer
coefficient by more than 70%
 The effect of installing baffles was more pronounced at low superficial gas velocities and
high solid mass fluxes, as a result of reductions in weakened solid heat transfer bypassing
Continued…

20. Thank you