CHE 415 - Fluidization Formal Report 2006

Ryerson University Department of Chemical Engineering CHE 415 Unit Operations II Lab Report Experiment #8: Fluidization Experiment Performed on: October 26, 2006 Report Submitted to Dr. By Group # 2 Section # 021 Group Members: 1. (Leader) 2. (Inspector) 3. (Data Reporter) Date Report Submitted: November 2, 2006. Marking Scheme Formatting Answer to all 6 questions in each Report Section / 10 General Appearance; Grammar and Spelling / 5 Complete and Informative Tables and Graphs / 15 Contents Accuracy and Precision of Results / 20 Comparison with Literature Data / 10 Influence of Procedural Design on Results / 10 Logic of Arguments / 20 Sample Calculations / 10 _____ Total: / 100 Table of Contents 22 33 66 77 88 1010 1010 1212 1313 1414 1616 16 Introduction In this experiment, the objective was to determine the type of fluidization possible in a tower filled with plastic balls (a fluidized bed system), and to investigate the effect a counter-current flow of water might have on the minimum fluidization velocity. Fluidization is a process where the solid bed of particles is converted into an expanded, suspended mass that closely resemble a liquid. To determine the minimum fluidization velocity, air at different velocities was sent through the bed of particles to convert it from a fixed bed state to a fluidized state. The instance the bed of particles became fluidized; this was the determined to be the minimum fluidization velocity. This was also repeated with a counter-current of water being sent from the top of the tower onto the bed of particles. Based on our results, the minimum fluidization velocity with no counter-current of water added was determined to be at least 17.7 m/s, and this gave a percent error of ~50%. The minimum fluidization velocity with a counter-current of water being used was determined to be 49.3 m/s, and this gave a percent error of 46.8%. The theoretical minimum fluidization velocity for this unit operation was 34.14 m/s. Upon comparing the results between operating with no water and with water added, there is a noticeable effect with water added, as the overall fluidization velocities were higher than if there was no water added. Therefore, it can be concluded that this experiment is viable in determining the minimum fluidization velocity and also the effect of counter-current water flow on the system. Theoretical Background In this experiment, a fluidization in a gas-solid system is investigated. Fluidization is a process where the solid bed of particles is converted into an expanded, suspended mass that closely resembles a liquid (Perry, 1997). Basically, for a fluidized bed system, the gas velocity of a system is the superficial velocity, U, which is the gas velocity through the tower/vessel if it was empty. When there is no gas velocity being sent through the bed the particles are at steady state where there are void spaces between the particles. At low gas velocity, the solids inside do not move, meaning it is a packed bed of solids. To create fluidization a gas, air for this experiment, is sent through a bed of particles. Fluidization occurs as the void spaces of the bed particles are filled with air and expands with increased air flowrate. As the gas velocity is increased, the pressure drop increases until the drag and buoyancy forces on the solid particles overcome its weight and any interparticle forces. When this happens, the bed is minimally fluidized, and the corresponding gas velocity is called the minimum fluidization velocity, Umf (Avidan, 2006). To measure the minimum fluidization velocity, pressure drop across the column must be known. The pressure drop across a fluidized bed is defined by the following equation (Geankoplis, 1993): (1) Where, P: Pressure drop across bed. g: Density of fluid passing through the bed. p: Density of particle LM: height of bed parallel to fluid flow. g: Gravitational force. : Minimum porosity at fluidization, where can be further described as (2) Where, LM being the height of bed at fluidization and Lo being height of bed prior to fluidization. To predict the minimum fluidization velocity, extrapolation of the Ergun equation, combined with the pressure drop equation (equation 1) can be used and it will give the following relationship: (3) Where, DP: The diameter of the particle. ?S: Particle sphericity. ?: Dynamic viscosity of the fluid. o: The gas velocity. To solve for o in the equation above can be tedious, but it has been proven that if the Reynolds number for the size of the particle, that is defined by (4) Where, is less than 20 then the first term of equation 1 is negligible. Thus, equation 3 can be rearranged to solve for fluidization velocity over the entire range that is written as, (5) Through theoretical evaluation, an equation was designed by Wen and Yu [2], which stated that (6) this simplifies equation (5) to (7) Experimental Method This fluidization experiment was studied by using a fluidization tower with about 63 cm diameter containing small plastic balls with a diameter 2.0 cm, acting as the solid bed of particles (fluidized material). The experiment itself was divided into two sections. The first section involved the determination of the minimum fluidization velocity, and the second section was to investigate whether the counter-current flow of water affects that minimum fluidization velocity. To determine the minimum fluidization velocity, the blower voltage was adjusted from the start until the bed of plastic balls inside the tower was suspended by the air flow. Adjustment of the air velocity was done by adjusting the blower voltage frequency on the blower speed controller panel. In order to change the frequency on the controller panel, the frequency adjustment menu was opened, followed by pressing the button “Local/Hand”, then the “Start” button and then select the value desired by pressing the “+/-“ button. Measurements from the main panel such as humidity of air, pressure drop in the U-tube and the air flowrate were recorded, as well as frequency of the blower voltage on the blower speed controller panel. The relationship between the air pressure and height of the bed was also investigated, as was done in this experiment, by changing the frequency of the blower voltage and recording the height of the bed in each trial. To investigate the effect of liquid water counter flowrate on minimum fluidization velocity, a constant water flow rate was chosen for the first run. This liquid counter flowrate was the additional force against which the air flowrate must overcome to introduce minimum fluidization, thus air flow rate was to be adjusted so that minimum fluidization velocity could be achieved, just like in the previous part. Within this water flowrate, frequency of the blower controller was also being varied, and the height of the bed to the corresponding frequency adjustment was also recorded. This was done to examine the pressure-height relationship of the counter-current effect on fluidization process. Experimental Apparatus Figure 1: Schematic Diagram of Fluidized Bed Reactor Results and Discussion One of the main objectives of this experiment was to determine the minimum fluidization velocity and compare it to the theoretical value. When air was blown through from the bottom of the tower, the particle bed would begin to fluidize at the minimum fluidization velocity. The density of one of the plastic balls was 267.068 kg/m3. After operation with air only through the bed of plastic balls, the system was set up where counter-current water flowed downward onto the packed bed at a high velocity. The theoretical minimum fluidization velocity was found to be 34.14 m/s. The data taken from the experiment is shown on Table 1 and 2 in the appendix, which also included the pressure change and the fluidization height. From these values, the experimental velocities were found. The plots of pressure change versus velocity were graphed as follows: Figure 2: Log Plot of Pressure vs. Velocity for Air Only Figure 3: Log Plot of Pressure vs. Velocity for Air and High Water Flowrate The experimental minimum fluidization velocity determined was based on the curve of the graphs. As can be seen from the previous two figures, the experimental curves demonstrated a non-linear relationship. Also, as the air flow rate increased so did the pressure drops. In Figure 2, the fluidization velocities ranged from 17.7 m/s to 1400 m/s with 17.7 m/s being the minimum fluidization velocity. These velocities gave a percentage error from 48.8 to more than 100%, which was a large deviation. From Figure 3, with counter-current water flow added, the fluidization velocities ranged from 49.3 m/s to 2490 m/s with 49.3 m/s being the Umf. These velocities had a range of percent error from 46.8 to more than 100%. Based on the results, the minimum fluidization velocities from both operations, the air velocities from counter current required a higher air velocity than with air only. Although, as a consequence, the deviation between the experimental minimum fluidization velocity data and theoretical minimum fluidization velocity values were shown that the higher error in the performance of this experiment. Thus, the velocities found for minimum fluidization velocities could not be taken as the true values. Error Analysis One of the sources of error in this experiment was from observation errors when reading the measurements. Another source of error is related to the design of the fluidized bed reactor. It was observed that the air flow through the tower was not uniform with the majority of the air flowing through the middle of the bed particles; therefore, this could affect the accuracy in determining the minimum fluidization velocity. Also, related to the gas velocity, it was noted that there was a small initial range of blower frequencies which initially moved the bed of plastic balls. Therefore, this meant that there was not a specific initial gas velocity that would fluidize the bed. Another source of error was determining the water flowrate. The digital readouts for the water flow rate did not work properly; therefore, an accurate measurement of the water flow rate was not done for the experiment. Conclusions and Recommendations The experiment was done by fluidizing the particle column with air only at different air flow rates followed by the introduction of water flow from the top of the column to observe the counter current effects on the minimum fluidization velocity. It was determined that the minimum fluidization velocity for air only and with counter-current of water were 17.7 m/s and 49.3 m/s respectively. This gave an percent error, from the theoretical minimum fluidization velocity, for air only and with counter-current of water of 48.8% and 49.3% respectively. The high percentage could be due to the range of gas velocities which began to fluidize the solid bed of plastic balls. Overall this experiment was successful in determining the minimum fluidization velocity and the effect of counter-current water flow. A recommendation for this experiment would be to reduce the particle size of the plastic ball as this might improve uniformity of the bed of particles. Appendix Nomenclature Dp Diameter of particle NRe,mf Reynolds Number (at minimum fluidization velocity) rair Density of air m Viscosity of air mp Mass of particle vp Volume of particle rp Density of particle fs Sphericity of particle emf Porosity of particle uo Fluidization terminal velocity g Gravity Lmf Bed height vmf Minimum fluidization velocity CD Drag coefficient Constants Required for Calculations Dp = __0.02 m_______ mp = __0.0011 kg____ Vp = _4.12 x 10-6_m3_ rp = __267.068 kg/m3_ m = 1.85 x 10-5 kg/ms rair = 1.23 kg/m3__ fs = _1_ (for sphere)__ g = ___9.8 m/s2___ Cross-sectional area of the air flow passage: __0.9283 m2__ Vp = 4/3pr3 Raw Data: Table 1: Generated Data for Air in the System Only Table 1: Air flow only ? ? ? ? ? ? ? Frequency (Hz) ?P (in. H2O) ?P (Pa) Bed Height (scale) Humidity (%) void Space Air Velocity (m/s) log v log p 15 3 747.198 2.2 48.9 0.318182 1.77E+01 1.248998 0.477121 20 3.1 772.1046 4.5 49 0.666667 3.34E+02 2.523488 0.491362 25 3.2 797.0112 6.6 49.5 0.772727 7.62E+02 2.882172 0.50515 30 3.3 821.9178 9.4 49.6 0.840426 1.40E+03 3.145174 0.518514 Table 2: Generated Data for Air and Counter-Current high Water Flowrate Table 2:Maximum Water flow ? ? ? ? ? ? ? Frequency (Hz) ?P (in. H2O) ?P (Pa) Bed Height (scale) Humidity (%) void Space Air Velocity (m/s) log v log p 15 3 747.198 2.6 58.4 0.423077 4.93E+01 1.69278 0.477121 20 3.1 772.1046 5.8 59.6 0.741379 5.92E+02 2.772098 0.491362 25 3.3 821.9178 9.9 61 0.848485 1.51E+03 3.180116 0.518514 30 3.5 871.731 14 62.3 0.892857 2.49E+03 3.397023 0.544068 Sample Calculations: Void Space: ? = L - Lo = 2.2 - 1 = 0.2 L 2 Theoretical Minimum Fluidization Velocity: ?’mf = Dp2 g (?p – ?s) = (0.02m)2 (9.8m/s2)(267.068kg/m3 - 1.23kg/m3) 1650 ? (1650)(0.0000185kg/m-s) = 34.14 m/s Actual Fluidization Velocity (for part one, with air only): ?'mf = Dp2 g (?p – ?s) ?s2 ?3mf 150 ? (1 – ?mf) = (0.02m)2(9.8m/s2)( 267.068kg/m3 - 1.23kg/m3) x (0.2)3 (150)(0.0000185kg/m-s) (1 -0.2) = 3.755 m/s Reynolds Number for Verification NRe = [(33.7)2 + 0.0408 Dp3?(?p – ?)g]1/2 - 33.7 ?2 = [(33.7)2 + (0.0408)( 0.02m)3(1.23kg/m3)( 267.068kg/m3 - 1.23kg/m3)9.8m/s2)]1/2 (0.000018kg/m-s) 2 33.7 = 15.69 Percent Error Calculation for Only Air in the System: % Error = Theoretical – Experimental X 100 Theoretical = 34.14 –3.755 X 100 34.14 = 89.01% References Avidan, Amos A. et.al. “Fluidization,” Kirk-Othmer Encyclopedia of Chemical Technology, online edition, John Wiley & Sons, New York, 2000. P.792-793. Geankopolis, Christie J. Transport Processes and Unit Operations, 3rd Edition. PTR Prentice Hall Inc., New Jersey, 1993. Perry, H. and Green D.W. “Gas-Solid Operations and Equipment,” Perry’s Chemical Engineers Handbook, 7th edition, McGraw-Hill, New York, 1997. p. 17-2 and 17-4 (2) (2) (2) (2) (2) (2) (2)