Chemical Engineering Lab Report: FLUIDISATION PDF

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AbstractThe aim of this experiment was to determine the key parameters of a pilot scale fluidized bed reactor by calculating the experimental and theoretical values of the superficial velocity and voidage at minimum fluidization. The theoretical values of superficial velocity calculated was 0. 491ms...

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Abstract The aim of this experiment was to determine the key parameters of a pilot scale fluidized bed reactor by calculating the experimental and theoretical values of the superficial velocity and voidage at minimum fluidization. The theoretical values of superficial velocity calculated was 0.491ms-1 and the voidage was 0.568 when Al2O3 grade 54 was used. The experimental values of superficial velocity calculated was 0.184 ms-1 whereas the voidage calculated was 0.423. It was concluded that the 54 grade was used in the experiment since it has the closest values. However, due to the significant differences between the theoretical and experimental of both values, the result is inaccurate. Therefore, further experiments and precaution steps are necessary to identify the grade of aluminium oxide used accurately.

1 Introduction and Theory The minimum fluidization velocity (umf) is known as the superficial gas velocity at which the upward flowing gas imposes a large enough drag force to overcome the downward force of gravity. The gas particles impose frictional forces which are equal and opposite drag force on the gas. Hence, the gas velocity around the particles are affected caused by the drag forces (Fan, 2018). The design of fluidized beds is more complex compared to other types of reactors, such as packed-bed and stirred-tank reactors. It is also susceptible to erosion and particle attrition caused by the moving particles. This may increase operating cost especially when losses of solid particles are expensive catalyst. However, the fluidized beds have been used widely in industry such as spanning coal gasification, energy production, fluid catalytic cracking and pharmaceuticals (Dechsiri, 2004).The high usage of fluidized beds is due to the distinct advantages of superior heat transfer, the ability to move solids easily like a fluid and the ability to process materials with a wide particle size distribution. In a fluidized bed, the heat transfer rate is five to ten times greater than that in a packed-bed reactor. The ability to move solids like a fluid allows catalyst to be added or removed from the reactor without switching off the system (Cocco, Karri and Knowlton, 2014). The objective of this experiment is to determine the requirement of both experimentally and theoretically key parameters for the design of an air-fluidized catalytic cracker. The grade of the Al2O3 powder will be determined from the observation of a pilot scale fluidized bed in an air-fluidized catalytic cracker using aluminum oxide powder (Al2O3) as a catalyst. The superficial velocity (umf) and voidage at minimum fluidization (Ɛmf) were determined experimentally and compared to the calculated theoretical values. The experiment was also carried out to find the relationship between fluidized bed behavior and heat transfer within the bed (University of Bath, 2019). Voidage (Ɛmf) is determined by the fraction of unoccupied volume within the bed particles (Adkins, 2017).The bed voidage at minimum fluidization can be calculated as shown in Equation 1 where ΔP is the pressure drop over the bed, ρs is the density of solid (Al2O3 powder), ρg is the density of fluid (air), l is the height of bed and g is the acceleration due to gravity. △ 𝑃 = (1 − Ɛ𝑚𝑓 )(𝜌𝑠 − 𝜌𝑔 ) 𝑙𝑔 𝑬𝒒(𝟏) In this experiment, it is assumed that none fluidized voidage is approximately equal to the voidage at minimum fluidization (University of Bath, 2019). Thus, the minimum fluidization can be obtained by using Equation 2 where 𝜌𝑝𝑜𝑢𝑟 is the approximate pour density and 𝜌𝑠 is the density of solids. Ɛ𝑚𝑓 ≈ Ɛ = 1 −

𝜌𝑝𝑜𝑢𝑟 𝜌𝑠

𝑬𝒒(𝟐)

Equation 3 shows the Ergun equation which is used to obtain superficial velocity (umf) where µ is the viscosity of the fluid and d is the average particle diameter.

−

△𝑃

2

µ(1 − Ɛ𝑚𝑓 ) 𝑢𝑚𝑓 + 1.75 = 150 Ɛ3𝑚𝑓 𝑑 2

𝜌𝑓 (1 − Ɛ𝑚𝑓 ) 2 𝑢𝑚𝑓 Ɛ3𝑚𝑓 𝑑

𝑬𝒒(𝟑)

𝑙 To calculate the heat transfer coefficient (h), Equation 4 was used where Q is rate of heat transfer, A is the cross-sectional area for heat transfer and 𝛥T is the temperature difference between the heater and the bed (Coulson & Richardson, 2002). ℎ=

𝑄 𝐴𝛥𝑇

𝑬𝒒(𝟒)

2 Methods Two experiments were carried out to determine the mass and heat transfer in a fluidized bed catalytic reactor. After the main power was switched on, the air bleed control valve was increased and decreased gently to obtain a levelled bed. The initial bed height, pressure drop and air flow rate was then recorded. The mass transfer experiment was carried out by adjusting the air bleed control valve to 7 increasing flow rates and 6 flow rates in the reverse order. Three readings of the bed height were recorded at each flow rate to account for fluctuations. The pressure drop and rotameter readings were recorded at each flow rate too. The heat transfer experiment was carried out by choosing two voltage set points between 10-20V and 20-30V. Three different flow rates were repeated at each selected voltage. The flow rates used are below, close to and above the minimum fluidization point. At each flow rate, three different temperature measurements were also obtained: the bed temperature (T1), heater temperature (T2) and the air inlet temperature (T3). Sufficient time was left for each temperature to stabilize after the power input or air flow rate was adjusted. Other measurements such as the current supplied, pressure drop, bed height and rotameter readings were also recorded at each flow rate. After the data were recorded, the variable transformer was turned off and the air flow was left open for 10 minutes to cool the system. The main power was switched off only after the air bleed valve was left fully opened. 3 Results and Calculations The theoretical value of Ɛmf and umf were calculated using formulas from Equation 1, 2 and 3. In Equation 4, the theoretical value of Ɛmf was calculated by using Equation 2. The following calculations area for Al2O3 54 grade Ɛ𝒎𝒇 ≈ Ɛ = 1 −

1720 = 0.568 3982.6

𝑬𝒒(𝟒)

According to the literature, the viscosity of air at 20°C and 101.3kPa is 1.785x10-5 Pa s, the density of Al2O3 at 25°C is 3982.6 kg m-3 and the density of air for dry atmosphere in standard atmospheric conditions is 1.225 kg m-3 (Belu, 2019) as recorded in Table 1. By substituting the values from Table 1 into Equation 3, the theoretical value of umf was obtained as 0.491 ms-1 as shown in Equation 5. Table 1: Values used to calculate the superficial velocity Viscosity of Air, µ (Pa s) 1.785E-05 3.20E-04 Average particle diameter, d (m) Density of Al2O3, ρs (kg m-3) 3982.6 -3 1.225 Density of Air, ρ (kg m )

0=

(150)(1.785𝑋10−5)(1 − 0.568)2 0.5683(320𝑥10−6)2

𝑢𝑚𝑓 +

(1.75)(1.225)(1 − 0.568) 2 𝑢𝑚𝑓 − (1 − 0.568)(3982.6 − 1.225)(9.81) (0.5683)(320𝑥10−6)

𝑬𝒒(𝟓)

𝑢𝑚𝑓 = 0.491 𝑚𝑠 −1 By using similar formulas, the theoretical value of Ɛ𝒎𝒇 and 𝑢𝑚𝑓 for Al2O3 100 grade is 0.608 and 0.127 ms-1. The experimental value for Ɛ𝒎𝒇 and 𝑢𝑚𝑓 was obtained from Figure 1 and Figure 2 respectively.

6.7

B

Log (ΔP) (-)

A

6.5

y = 0.7952x + 6.6591 R² = 0.8728

6.3 Increasing Flow Rate Decreasing Flow Rate

6.1

C

Linear (Increasing Flow Rate)

y = 0.8858x + 6.6758 R² = 0.9278

5.9

Linear (Decreasing Flow Rate)

5.7 5.5

-1

-0.9

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

Log (uc) (-)

Figure 1: Log-log plot of pressure drop against superficial velocity. Figure 1 showed that as superficial velocity increases, the pressure drop increases as well. Two bestfit linear lines were plotted where one was the increasing flow rate and the other was the decreasing flow rate. The R2 value from the best-fit linear line of decreasing flow rate is 0.9278 whereas the R2 value of increasing flow rate is 0.8728. The experimental value of 𝑢𝑚𝑓 was determined by the intersection of both linear lines plotted which is at -0.184 of the x-axis. Thus, the 𝑢𝑚𝑓 is 0.184 m s-1. Log ( ΔP/Mean Bed Height) (-)

7.5 y = 0.686x + 7.5251 R² = 0.7868

7.4 7.3 7.2

Increasing Flow Rate

7.1

Decreasing Flow Rate 7 y = 0.8099x + 7.5601 R² = 0.9015

Linear (Increasing Flow Rate) Linear (Decreasing Flow Rate)

6.9 6.8 6.7

-1

-0.8

-0.6

-0.4

-0.2

0

Log (uc) (-)

Figure 2: Log-log plot of pressure gradient within a bed as a function of superficial velocity.

Figure 2 also displayed that the pressure gradient increases with an increasing superficial velocity. The two best-fit linear lines were based on both the increasing and decreasing flow rate. The R2 value for the line with increasing flow rate is 0.7868 whereas the R2 value for the decreasing flow rate is 0.9015. The difference between these two R2 values is 0.1147. The calculation of Ɛ𝑚𝑓 was displayed in Equation 6 with the similar formula in Equation 3. Hence, the Ɛ𝑚𝑓 value is 0.423 2

(1 − Ɛ𝑚𝑓 )(3982.6 − 1.225)9.81 = 150

(1.785𝑋 10−5 )(1 − Ɛ𝑚𝑓 ) 1.225(1 − Ɛ𝑚𝑓 ) 0.1842 0.184 + 1.75 3 Ɛ𝑚𝑓 (320𝑥10−6 ) Ɛ3𝑚𝑓 (320𝑥10−6 )2

𝑬𝒒(𝟔)

Ɛ𝑚𝑓 = 0.423 The heat transfer coefficient was calculated with Equation 4 from Introduction. An example of calculation for the heat transfer coefficient with a voltage of 14V and a current of 0.2A was shown in Equation 7. 14 𝑥 0.2 ℎ= = 54.69 𝑬𝒒(𝟕) 0.0016 𝑥 (58 − 26) 4 Discussion As plotted in Figure 1, the pressure drop increases with the superficial velocity until the bed expands and increases the porosity. With an increasing flow rate between point A and point B, the frictional drag forces caused the particles to rearrange, affecting the voidage. This was shown by the decrease in gradient after point A. The pressure decreases slightly after point B which was due to the rearrangement of particles and remained approximately constant then increased again after point C. The error in significant increment after point C might caused by insufficient time given for the pressure drop to equilibrate. The trendline of decreasing flow rate indicated the process of reformation of the fixed bed particles. In Figure 2, the bed height used to calculate the pressure gradient may have affected the accuracy of the graph. This was because the difficulty in determining the ambiguity bed height has a high level of uncertainty due to the large fluctuations. The oscillation in the bed increases as the flow rate increases. Each reading of bed height was taken three times to have a more reliable bed height. This result could have been improved by recording the oscillations and review the video frame by frame to determine the average bed height. In Figure 3, it was clear that the 26V have a higher heat transfer coefficient than the 14V since there was a larger heat transfer rate. A gradual decline in value of heat transfer coefficient should be observed as the superficial velocity increases (Chen, 1999). However, the plot in Figure 3 displayed otherwise. This suggested that inadequate time was left for the system to equilibrate between measurement points. Another possible source of error was how the range of flow rate used may not be broad enough to show this trend. To improve this result, a significant time interval between each measurement should be taken. The theoretical and experimental values are recorded in Table 2. Table 2: The theoretical and experimental values of 𝑢𝑚𝑓 and Ɛ𝒎𝒇 . Grade Voidage, Ɛ𝒎𝒇 Superficial Velocity, 𝑢𝑚𝑓 (ms-1)

Theoretical 54 0.568 0.491

100 0.608 0.127

Experimental 0.423 0.184

According to Table 2, the experimental and theoretical value of voidage of grade 54 showed the most similar values. The difference between the values is 0.145 which is a 26% difference from the theoretical value. However, from the experimental value of superficial velocity, it is calculated to have more similar values to the grade 100. The difference between the two velocities shows a 45% difference. Hence, it was assumed that the 54 grade was used since it has a smaller difference. The difference in theoretical and deduced superficial value could be explained due to high uncertainty of the measurements taken throughout our experiments. Conclusion The experimental value of the voidage and superficial velocity calculated was 0.423 and 0.184 ms-1, whereas the theoretical value of voidage and superficial velocity calculated was 0.568 and 0.491 ms-1. This showed a voidage difference of 0.145 and a velocity difference of 0.307. The difference in experimental and theoretical values suggests that there were a few errors in this experiment which are worth accounting for. The possible errors must be considered when assessing the comparison to both the values. However, it was deduced that the aluminium oxide used in the system was assumed to be of grade 54 since the experimental value was the closest to the theoretical value obtained. More precautions should have been taken to reduce errors that may propagate into an error for the final calculated value of 𝑢𝑚𝑓 and Ɛ𝒎𝒇. Further experimentations are also required to determine the grade of aluminium oxide used accurately and efficiently. References Fan, L., 2018. Powder Technology. [ebook] Elsevier, pp.454-485. Available from: https://www.sciencedirect.com/science/article/pii/S0032591017308008 [Accessed 30 Oct. 2019].

Cocco, R., Karri, S. and Knowlton, T. (2014). Introduction to fluidization. [ebook] American Institute of Chemical Engineers (AIChE), pp.1-29. Available at: https://www.aiche.org/sites/default/files/cep/20141121.pdf [Accessed 25 Oct. 2019]. University of Bath, 2019 CE20225 Chemical Engineering Skills and Practice 2, Student Lab Book 2019-20, pp34-35. Coulson, J.M., and Richardson, J.F., 2002. Coulson and Richardson’s Chemical Engineering. Volume 2 Fifth Edition. Oxford, Butterworth-Heinemann Adkins, W., 2017. [online] Sciencing. Available from: https://sciencing.com/calculate-voidage-6136680.html [Accessed 26 Oct. 2019].

Belu, R., 2019. Industrial Power Systems with Distributed and Embedded Generation - 9.2.2 Air Density, Temperature, Turbulence, and Atmospheric Stability Effects.. [online] App.knovel.com. Available from: https://app.knovel.com/hotlink/pdf/id:kt011W01Y1/industrial-power-systems/air-densitytemperature [Accessed 10 Oct. 2019]. Dechsiri, 2004. Particle transport in fluidized beds: experiments and stochastic models. [online] p.23. Available from: https://www.rug.nl/research/portal/files/9807330/c2.pdf [Accessed 26 Oct. 2019].

Chen, J., 1999. Fluidization, Solids Handling, and Processing. [ebook] Available from: https://www.sciencedirect.com/topics/chemical-engineering/heat-transfer-coefficient [Accessed 15 Oct. 2019].

Appendix Table 2: Data recorded for mass transfer experiment Bed Height (cm)

Mea n Bed Hei ght (cm )

Superf icial velocit y

11 .5

11 .5 11.5

0.279

11 .3

11 .3

11 .3 11.3

0.123

11.5

11 .3

11 .3

11 .3 11.3

0.175

22. 5

16.5

11 .4

11 .4

11 .4 11.4

0.234

23.6

26. 8

20.0

11 .5

11 .4

11 .4 11.4

0.279

24A

1.9

40. 8

20.0

11 .6

11 .6

11 .5 11.6

0.425

24A

4.0

44. 1

23.0

12 .1

13 .4

12 .8 12.8

0.459

24A

5.0

48. 7

24.5

12 .7

15 .2

13 .5 13.8

0.507

24A

4.0

44. 1

23.5

12 .5

14 .2

13 .1 13.3

0.459

24A

1.9

40. 8

19.0

11 .6

11 .6

11 .6 11.6

0.425

14XA

23.6

26. 8

18.0

11 .5

11 .6

11 .5 11.5

0.279

14XA

20.0

22. 5

14.5

11 .5

11 .5

11 .5 11.5

0.234

14XA

15.0

16. 8

10.5

11 .4

11 .4

11 .4 11.4

0.175

14XA

10.0

11. 8

6.5

11 .4

11 .4

11 .4 11.4

0.123

Rotam eter Readi ng (cm)

Flo w Rat e (l/m in)

Press ure Drop (bar)

1

2

14XA

23.6

26. 8

19.5

11 .5

14XA

10.0

11. 8

7.0

14XA

15.0

16. 8

14XA

20.0

14XA

Rotam eter Type

3

Log velo city 0.55 4 0.91 0 0.75 7 0.63 0 0.55 4 0.37 2 0.33 8 0.29 5 0.33 8 0.37 2 0.55 4 0.63 0 0.75 7 0.91 0

Log Press ure Drop

Pressur e Gradient (pa/m)

Log (Pres sure Gradi ent)

6.290

1695652 1.739

7.229

5.845

6194690 .265

6.792

6.061

1017699 1.150

7.008

6.217

1447368 4.211

7.161

6.301

1749271 1.370

7.243

6.301

1729106 6.282

7.238

6.362

1801566 5.796

7.256

6.389

1775362 3.188

7.249

6.371

1771356 7.839

7.248

6.279

1637931 0.345

7.214

6.255

1560693 6.416

7.193

6.161

1260869 5.652

7.101

6.021

9210526 .316

6.964

5.813

5701754 .386

6.756

Table 3: Data recorded for heat transfer experiment Rotameter Reading (cm) 10.0 23.5 3.0 10.0 23.5 3.0

Flow Pressure Voltage Current T1 rate Drop (V) (A) (°C) (l/min) (bar) 11.8 7.00 14 0.2 26 26.8 19.50 14 0.25 26 41.4 21.50 14 0.25 28 11.8 7.00 26 0.425 28 26.8 20.50 26 0.425 28 41.4 21.50 26 0.425 30

Bed Height (cm) T2 T3 (°C) (°C) 1 2 3 58 25 11.4 11.4 11.4 58 25 11.4 11.4 11.4 48 26 12.0 13.2 12.3 98 26 11.3 11.3 11.3 99 26 11.3 11.3 11.3 80 26 11.9 13.2 12.2

Mean Bed Height (cm) 11.4 11.4 12.5 11.3 11.3 12.4

Table 4: Heat transfer coefficient calculated Temperature Difference ,ΔT 32 32 20 70 71 50

Heat Transfer Coefficent (W m-2 Superficial Velocity Log K-1) (ms-1) velocity 54.6875 0.122916667 -0.910389 68.359375 0.279166667 -0.554136 109.375 0.43125 -0.365271 98.66071429 0.122916667 -0.910389 97.27112676 0.279166667 -0.554136 138.125 0.43125 -0.365271

Voltage (V) 14 14 14 26 26 26...