Type of paper: Report

Topic: Stress, Pressure, Experiment, Flow, Water, Bed, Air, Size

Pages: 10

Words: 2750

Published: 2020/10/10

Summary

The laboratory work is designed to study the properties of the upward flow of fluid and gas through a column packed with glass beads of different size. The experimental rig Armfield CEL-MKII is applied. The upward flow generates fluidization, namely the state when the glass beads suspend in the flow. The pressure drops of water and air fluidization, as well as the behaviour of glass beads of different sizes are studied. The Carman-Kozeny equation was applied for calculation pressure drop in the system. The calculated values of the fluidization pressure drop correspond well with the experimental values.

Introduction

The processes that involve solid phase are typically troublesome due to low kinetic characteristics and therefore the necessity of high pressure and temperature application arises (Coulson, Richardson & Sinnott 2009). Fluidization is a process that allows bringing solid matter to fluid-like condition. As a result, various processes are enabled to run at lower pressures and temperatures, and more efficient yields and kinetic characteristics are reached (Sundmacher, Kienle & Seidel-Morgenstern 2005).
Fluidization by gas and liquid flow is widely applied in mineral resource, chemical, pharmatheutical and food industry (ed. Scala 2013). The operations for particles classification by size and density, washing of soils, gas and water treatment systems (Epstein 2002).
For fluidization understanding, basic concepts of hydraulics are required. For example, viscosity, kinetic characteristics of flow, pressure drop and the equations for its calculation (Darby 2001).
The application of fluidization at heat energy processes for carbon dioxide reduction has been performed in the work of Chameera, Britt & Lars-Andre (2014). The minimum fluidization velocity and pressure drop were determined. The similar experiments will be performed at the laboratory.

Equipment
The fluidised flow is studied with an experimental rig Armfield CEL-MKII. The experimental is designed to measure and observe the flow through fixed and fluidised beds filled with solid granules. The experimental setup is presented on Figure 1: 1 – unit control tools (unit and pump switch, instrumentation), 2 – flow control regulators; 3 – column cap; 4 – water overflow. It consists of three test columns; one is used with water (column W), and two – with air (marked A). Water and air column 1 are packed with granules of the same size. Thus, it is possible to observe the difference between particulate and aggregative fluidisation. Air columns 1 and 2 are used with different granules.
Figure 1: Armfield CEL-MKII

Experiment A. Voidage of granular material

The experiment aims to determine the bed voidage of two samples of the granular materials. The granular materials are 0.625 mm and 0.250 mm glass beads.
Equipment: weight balance, 0.5 l measuring cylinder (dry).

The voidage is determined using the following formula:

ε=1-Mass of ParticlesDensity×Total Bed Volume
The measuring cylinder is weighted, and the balance is tared. The cylinder is filled with dry glass beads to the certain volume and weighted. The procedure is performed for 0.625 and 0.250 mm glass beads. The density of glass beads is 2500 kg/m3 (or 2.5 g/cm3)

Experiment B. Flow through fixed and fluidised beds using water as the fluidising medium

The experiment aims to investigate the characteristics associated with water flowing vertically through the bed with glass beads. There are four tasks set for the test:
1. Determination of the head loss across the bed;
2. Perform verification of the Carman-Kozeny equation;
3. Observe the onset of fluidisation and difference between the fixed and fluidised bed;
4. Compare the predicted fluidisation onset with the measured pressure drop.

Water column is filled to 300 mm height with glass beads of 0.625 mm

According to Carman-Kozeny equation, the pressure drop:
△PL ⋅ DpρVsm2 ∙ ε31- ε=150 1- εRe+1.75,
Δp – the pressure drop; L – height of bed, 0.3 m; μw – viscosity of water, 0.001 N/sm2; νw – kinematic viscosity, 10-6 m2/s, ρw- water density, 1000 kg/m3, ρ- granule density, 2500 kg/m3, ε-bed voidage (obtained from the experiment A), Re – Reynolds' number based on superficial velocity (dimensionless, calculated as DpVsmρw/μw).

The average superficial velocity is calculated as:

Vsm=Q∙10-3A.

A – bed cross sectional area.

The pressure drop ∆Pρwg=h∙10-3 (g = 9.81 N/m2).

Thus, the equation for the pressure drop is:

h= 150L 1- ε2∙ Vsm ∙ μωDp2 ∙ ε3 ∙ρw ∙g+1.75L Vsm21- ε Dp ∙ ε3g x103mmH2O

The pressure drop can be predicted by the equation:

∆P=L1-ερs- ρwg
h=L 1-εpwρs- ρwx103mm H2O
The flow regulator is set at minimum flow, and the water pump is switched on. The water flow rate should be set at 0.05 l/min, controlling the flow reading at the display. The flow is increased until the bed is fully fluidised and the pressure drop remains stable. While changing the setting, allow the conditions to stabilize and record the flowrate and the differential pressure from the display.

Experiment C. Flow through fixed and fluidised beds using air as the fluidising medium

This part of the experiment aims to investigate vertical air flow through the Armfield CEL-MKII bed of granular material. The tasks are the same as in experiment B. In the experiment, the glass beads used are 0.625 mm.
The Carman-Kozeny equation is applied, as in experiment B. However, when converting pressure drop value from Pascals to mm of water, the air density has to be taken into account (ρa- density of air):
ρaρw∆Pρwg=h∙10-3

Therefore, the equation for the pressure drop is:

h= ρaρw ∙ △Pρw⋅g ∙ LρVsm2Dp∙Re ∙ 1- εε3 ∙ 150L1- εVsmDp+1.75 x103mm H2O
h= 150L 1- ε2∙ Vsm ∙ μαDp2 ∙ ε3 ∙ ρw ∙g+175L Vsm21- ε ραDp ∙ ε3 ∙ ρw ∙ g

The pressure at fluidization is calculated the same as in Experiment B:

h=L 1-εpwρs- ρax103mm H2O
The regulator is closed in front of the air columns. The air supply is connected to the inlet and the rear of the filter regulator. The air flow is slowly adjusted in increment of 5 l/min. The rate is increase until fully fluidised and the pressure drop remains stable at several readings. At each setting the readings are taken after the conditions have stabilized. The state of bed is observed and recorded.

Experiment D. Effect of granule size using air as the fluidising medium

The experiment is meant to assess the difference in behaviour of the glass beads of different size. As in previous experiments, pressure drop is measured and calculated, fluidization observed. The results obtained for two columns are compared.
The theory and calculations involved are the same as in experiment C. The size of granules applied is 0.250 mm, and the flow rate is adjusted at steps 0.5 l/m because the smaller granules are fluidized at smaller velocity.

Results

Experiment A. Voidage of granular material

The experimental results are presented in Table 1.

Experiment B. Flow through fixed and fluidised beds using water as the fluidising medium
The results are presented in Table 2. The pressure drop curves are depicted on Figures 1 and 2.

Experiment C.

The results are presented in Table 3. The pressure drop curves are depicted on Figures 3 and 4.

Figure 3: The relationship between air flow and the measured pressure drop (glass beads size 0.625 mm).
.
Figure 4: The relationship between water flow and the predicted pressure drop (glass beads size 0.625 mm).

Experiment D.

The results are presented in Table 4. The pressure drop curves are depicted on Figures 5 and 6.

Figure 5: The relationship between air flow and the measured pressure drop (glass beads size 0.25 mm).
Figure 6: The relationship between water flow and the predicted pressure drop (glass beads size 0.25 mm).

Discussion

Experiment A.
The voidage of 0.625 mm glass beads: ε0.625=1-138.1g2.5gcm3×96cm3=0.42.
The voidage of 0.250 mm glass beads: ε0.250=1-151.7g2.5gcm3×97cm3=0.37.

The voidage of beads with smaller size is smaller since they are packed more tightly in a column.

Experiment B.
The average superficial velocity is calculated as:
Vsm=Q∙10-3A.
A – bed cross sectional area; bed diameter is 0.05 m. Therefore, A = πd24=3.14∙0.0524=0.002 m2. Thus, for flow rate 0.1 l/min, Vsm=0.1∙10-30.002∙60=0.0008m/s.

The pressure drop is calculated as described in the experimental section:

h=0.31-0.4210002500- 1000∙103mm H2O=260.1 mm H2=26.1 mBar
The predicted fluidization point is 26.1 mBar. However, the fluidization is observed at 34.8 mBar; the difference in 8.7 mBar is a value of initial pressure at the column (before the experiment). Therefore, the Carman-Kozeny equation works well for prediction of the pressure drop in the water fluidized bed.
The graphical interpretation of measured and predicted pressure drop (Figure 1 and 2) show the fluidization point at 0.5 l/min as a line kink.

Experiment C.

For air fluidized system the pressure drop:
h=0.31-0.4210002500- 1.23∙103mm H2O=435 mm H2=43.5 mBar
The calculated value of 43.5 mBar is close to the experimentally observed (45.2 mBar). Therefore, the Carman-Kozeny equation provides precise value of the pressure drop for air fluidization. However, it provides different results of pre-fluidization area, which is depicted on Figures 3 and 4.

Experiment D.

The pressure drop in a bed packed with glass beads 0.25 mm:
h=0.31-0.3710002500- 1.23∙103mm H2O=435 mm H2=47.2 mBar
The experimental value for fluidization pressure is 41.2 mBar, which is close. The observed difference might be attributed to unstable flow rate.

The measured and calculated value are different for the pre-fluidization state.

The pressure drop values for two sizes of glass beads are close, namely 43.5 and 47.2 mBar for glass beads of 0.625 and 0.25 mm, respectively. However, in the experiment with 0.625 mm granules the fluidization takes place at 43.2 l/min, while 0.250 mm glass beads fluidize at 9 l/min. Therefore, the system with smaller granules is more efficient.

Conclusions

The laboratory experiments were designed to study the fluidization of the packed bed column with water and air. As the water or air flow through the bed increase, the pressure drop proportionally increases. When the flow rate allows lifting the granules, the fluidization is observed. After this, the flow rate will increase the height of the bed, and the pressure drop remains stable.
The pressure drop at the bed was measured, and the fluidization pressure was calculated and measured experimentally. The obtained experimental values are close to the calculated values.

Reference List

Chameera, KJ, Britt MH & Lars-Andre, T 2014 ‘Experimental and Theoretical Study of Minimum Fluidization Velocity and Void Fraction of a Limestone Based CO2 Sorbent’, Energy Procedia, vol. 63, pp. 1432-1445.
Coulson, JM, Richardson, JF, & Sinnott, RK 2009, Chemical engineering, Pergamon: Oxford.
Darby, R. 2001. Chemical engineering fluid mechanics. Marcel Dekker: New York,.
Epstein, N 2002, 'Applications of Liquid-Solid Fluidization', International Journal of Chemical Reactor Engineering, vol. 1, iss. 1, pp. 1542-6580.
Sundmacher, K, Kienle, A, & Seidel-Morgenstern, A 2005, Integrated chemical processes: synthesis, operation, analysis, and control, Wiley-VCH: Weinheim.
Scala, F (ed), 2013 Fluidized Bed Technologies for Near-Zero Emission Combustion and Gasification, Woodhead Publishing, PA.

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