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Hydrodynamic study of anadjustableheightpacked column operating

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  • 1. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME461HYDRODYNAMIC STUDY OF AN ADJUSTABLE HEIGHT PACKEDCOLUMN OPERATING ON THE PRINCIPLE OF ANAIR LIFT PUMPAdel OUESLATI 1 *, Ahmed HANNACHI2, Mohamed EL MAAOUI31College of technology, Department of Chemical engineering, Mogran, 6227,Zaghouan –Tunisia2National school of Engineers, Gabes, Department of chemical engineering, Omar ibn Elkhattab, Zrig, 6072- Tunisia3Faculty of sciences of Tunis, Department of chemistry, Elmanar 1002- TunisiaABSTRACTA setup consisting of a glass column packed with calibrated glass rings has beenachieved. It operates on the principle of an air lift pump. It was designed for the best contactbetween air and water. Performances of this system were determined by measuring thedisplaced water flow rates for different submersion depths and various air flow rates. Westudied the pressure drop versus the immersion depth in the column. The results show that thepressure loss is described by a second order polynomial equation. Efficiency was calculatedfor different conditions. The study shows that the proposed system can be set easily, has lowpower consumption, provides a good mix between phases and is very important for manyapplications where heat and mass transfer are involved.Keywords: air lift pump, porous media, packed column, efficiency1. INTRODUCTIONThe pumping system of water by air lift consists of the injection of compressed air atthe base of a pipe in order to drive the liquid therein. The only source of energy, used forpumping, is compressed air. A two-phase mixture is water-air, of lower density than thesurrounding liquid. Upward movement is initiated, and causing a stream of water.INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERINGAND TECHNOLOGY (IJMET)ISSN 0976 – 6340 (Print)ISSN 0976 – 6359 (Online)Volume 4, Issue 2, March - April (2013), pp. 461-478© IAEME: www.iaeme.com/ijmet.aspJournal Impact Factor (2013): 5.7731 (Calculated by GISI)www.jifactor.comIJMET© I A E M E
  • 2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME462Air lift pumps are widely used in aquaculture (Parker et al., 1987 [1)), in bioreactors(Chisti V., G. Trystam [2], 1992) in geothermal wells (Reley DJ, Parker GJ [3], 1982) inunderwater exploration (JL Mero [4]; Stenning and Martin [5], 1968), the extraction ofsludge in wastewater treatment (Casey TJ [6], 1992 and storck [7], 1975).Pumping systems air lift type are considered effective for low head conditions compared tocentrifugal pumps and other pumps (Lee, 1997 [8]; Kumar [9], 2003 and Oh [10], 2000).However, this efficiency was defined by Nickelin [11], (1963) and used by Rienemann [12],(1987) is given by the following relationship:η ൌρಽ୥୕ై୞ై୕ృ୔భ୐୬ቀౌబౌభቁ(1)With:ρ௅: Density of the liquid, g: Acceleration due to gravity, Q୐, Qୋ: volume flow rates of theliquid displaced and gas respectively, Z୐: Pressure head (m), P଴, Pଵ: Gas pressuresrespectively at the top and bottom of the column.Here we see that for values of QG and P0 data, QL and P1 will depend on factors thatinfluence the hydrodynamics of the system. The hydrodynamic in a gas-liquid contactor isvery complex. Characterization begins with the determination of gas flow regimes. Severalauthors have defined flow regimes co-updraft gas. Five regimes, two-phase flow water-airvertical co-current, observed by Roumy [13] (1969): Bubbles, separate dense bed of bubbles,slugs, annular and churn.In general, the transition from one regime to the other takes place byvarying one or both of air flows and water. But in the case of an air lift, the gas flow rate andthe initial height of the liquid, in the riser, that secure the flow rate of the circulating liquidand the flow regime.The pressure drop (P0 - P1) resulting mainly to gravity and viscous forces. Theydepend closely on speeds of fluids and therefore, they are dependent on the flow regime.Correlations for determining the pressure drop have been established by Govier GW and AzizA. [14] (1972), Govier and Radford,[15] (1957) for bubbles regimes, by Friedel [16],(1979) in the case of slug regime and by kern [17], (1975) if the flow is like churn andannular.The gas hold up is the ratio of the volume of gas contained in the mixture biphasic onthe useful volume of the column. It is the sum of the dynamic gaseous fraction and the staticgaseous fraction.Experiments carried out by several authors (Wallis (1969) [18], Nicklin [19] (1962))showed that the value of gas holdup varies with superficial gas velocities.It has an effect on the flow rate of the liquid and interfacial area (Merchuk [20],1981).For flow rates of gas and liquid, gas retention is variable from one point to another inthe column.It also depends on the design of the closed loop air lift system including theconnections between the riser pipe and tube down comer (Merchuk [21], 1994). The authorreports elucidated the effect of sections of the riser and the down comer of the gas holdupvalue. Nakoryakov [22] (1986), Rienemann [12] (1987) and Merchuk [21] (1994) showedthat the gas holdup depends on the diameter risers and the effects of viscosity, surface tensionand Reynolds number . The authors confirmed that if the tube diameter is much less than 6mm and if the gas flow is cut, surface tension prevents the rise of gas bubbles.
  • 3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME463Air lift pumps that we present above are exclusively formed by a vertical tube inwhich there is water at a given height and air injection at the base. Our work is to use an airlift system wherein the tube is filled with packing. Although packed columns are very wellknown, the combination air lift and packing has not been addressed. Merchuk [21], (1994)studied the air lift bioreactors unlined but improved turbulence by design. There are otherresearches achieved a packed column reactor where liquid and gas are sent by two differentpumps (Barrios QEM [23], 1987; Sicardi S., G. Baldi, V. Specchia, I. Mazzarino [24], 1984,A. LARA Marquez [25], 1994).The packed column is used for the mixing and intimate contact between the phases.The rest of the studies, which are related to our work, concerned to conventionalpacked columns in which we seek to characterize the concurrent up flow of air and water.The parameters involved are flow regimes, gas and liquid hold up, liquid and gas flow ratesand pressure drop. Flow regimes are studied by JL Turpin, R. L. Huntington, [26], (1967);Y. Sato et al. [27], (1974), Nakamura et al. [28], (1978);, Barrios [23], (1987) and Lara et al.[29], (1992). The studies have shown that the gas flow depends on flow regimes and fluidcharacteristics of packing. Since the regime depends on the characteristics of solids, so wecannot make flow regimes maps similar to that of a biphasic system.On the gas holdup, studies by Moustiri [30], (2002); Therning [31], (2001), LaraMarquez [29], (1992), Abraham [32], (1990) and Barrios [23], (1987) showed that theoverall retention of the gas increases as a function of the superficial velocity gas anddecreases with that of the liquid without offering an explanation of the effect of solid packingon the gas holdup. The pressures drop in a packed column where the flow is co-currentupward can be described by the modified formula Ergun whatever regime (Maldonado JG, G.Hebrard, D. Bastoul, Roustan, JL Westrelin S. Baig, [33], 2004). The same authors havedefined sleep velocity and their effects on the mass transfer.This work is a study of co-flow updraft of a water-air mixture in a packed columnvertical operating on the principle of an air lift pump. The column is filled with glass rings.The water is flowing in a closed loop. In addition to its large surface area, the glass rings arecharacterized by a high void fraction which maintains a low pressure drop. Possibleapplications are expected in the field of air humidification or stripping. The tests areperformed in ambient conditions. Only two parameters are adjustable: the gas flow rate andthe initial height of the liquid in the riser. Water flow generated, the gas holdup, pressuredrop and pump efficiency are determined to evaluate the performance of assembly.According to the trends observed in the experimental study a physical interpretation isproposed.2. MATERIALS AND METHODS2.1. The experimental setupFig. 1 shows a schematic diagram of the setup. The system main components are:evaporator (1), down comer (2), water heater (3), cyclone (4), compressor (5), water flowmeter (6), water make up Tank (7), air heater (8), air flow meter (9), temperature control (10),swirl chamber (11), vapor condensers (12) and (13), Inlet cooling water (14), outlet coolingwater (15), pure water Tank (16), water level control (17), Temperature sensor (18), RelativeHumidity (HR) sensor (19) and pressure manometers.
  • 4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME464The experiments were performed on a vertical cylindrical column made up in threeglass tubes 0.072 m diameter and 0.4 m length. The total height of the column and thepacked bed are 1.2 m and 1.0 m respectively. The characteristics of the solid packing areshown in Table 1. The column is provided with equidistant pressure sensors in order tomeasure the local pressure at different heights. Four polypropylene disc diffusers, with 67circular pores of 5 mm each, are arranged at the ends of the glass tubes. They have adouble role: first they change the fluid flow direction, second they prevent the exit ofsolid packing out of tube. Only one air jet nozzle is used in the experiments. It has adiameter of 3 mm.At the input of the column, a swirl chamber, stainless steel, is designed for theinjection of water and air. At a height equal to 1.02 m of the column, water is recycledthrough a down comer and the air continues its path to the cyclone and condensers.Water droplets separated at the cyclone are routed to the swirl chamber. A make-up tankof water is placed to keep a constant liquid level in the down comer.The water flow rate is measured by an orifice, with piezometers, placed betweenthe down comer and the riser. His uncertainty is less than 5%.The compressor used of 2 kW power, Michelin type and 25 liters of tank, providedwith a flow controller valve. The air flow meter is air float type; brand Tubux whosemeasuring range is between 0 and 25 m3/ h and the uncertainty of 4%.The setup is designed in a manner that the amount of water evaporated will bereplaced, automatically, by the same amount of liquid water issued from the tank (7).The riser will be used as an evaporator chamber. It is insulated by a transparentpolyethylene layers. The airflow humidified by passing through water level in evaporatorchamber then leaves from the outlet pipe in the direction of condensers. The water levelin the evaporator chamber is controlled by the level of water make up tank (7) and anelectric heater (3) of 2 kW power. The inlet water flow rate in the evaporator is measuredby a calibrated orifice (6).The physical properties of the packing particles are given in table.1.Table 1: Physical characteristics of the packing particlesType of packing particles Glass ringsDensity : ρS 2.187 (kg/m3)Average diameter: dp 0.008 (m)Averagelength : lp 0.008 (m)fixed bed porosity: ε 77.3Form Factor: φ 0.681
  • 5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME465Fig. 1. Schematic diagram of the set upFig.1: Schematic diagram of the experimental setup: evaporator (1), down comer (2),water heater (3), cyclone (4), compressor (5), water flow meter (6), water make up Tank (7),air heater (8), air flow meter (9), temperature control (10), swirl chamber (11), vaporcondensers (12) and (13), Inlet cooling water (14), outlet cooling water (15), pure water Tank(16), water level control (17), Temperature sensor (18), Relative Humidity (HR) sensor (19).The submersion ratio Sr is defined by this expression:ܵ௥ ൌ௓ೄ௓ೄ ା௓ಽ(2)Where:Z s: submerged depth (initial liquid height), The design for air lift pumps has typically beenbased on data derived from performances within the limits of S r (40% - 90%) (CHO Nam –Cheol, Hwang in ju, Lee chae-Moon, Park jung-won [34]). The total head, L is given by thefollowing equation:Z s + Z L = L (3)For this study, the total head is 1.02 m, so Z s, can any value between 0.4 and 0.9 m.
  • 6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME4662.2. Measuring the pressure dropTo measure the pressure loss due to the fluid flow between the ends of packedcolumn, we used a differential manometer connected to the tail and the head of the column.Another differential pressure gauge is intended for measuring the pressure at the head of thecolumn with respect to that of the atmosphere. The measurement error by differentialmanometer is equal to േ2 mm.2.3. Measuring the global gas hold upThe gas hold up is defined as the volume occupied by gas in the packed columncrossed by a diphasic mixture in continuous operation. It called also void fraction. It ismeasured by set a level of liquid in the packed column, which corresponds to a volume V0 ofthe liquid. Then, we inject a gas flow rate. Once the steady state is reached, flows into andfrom the packed column are cut by closing the corresponding valves. The new liquid volumeV1 in the packed column is noted. The void fraction is then:߳ீ ൌ௏బ ି௏భ௏బ(4)3. RESULTS AND DISCUSSION3.1. Average water flow rateThe results of measurements are plotted in Fig. 2.Fig. 2. Effect of the Gas flow rate on the water flow rate for many submersion ratiosThe examination of Fig.2 shows that there is a jump, in all the curves. This isattributed to movement of the packing which is subject only to his weight. Under the effect offlow, the mixture of gas and liquid induce a movement of packing, the void fraction increasesand the liquid flow rate increases also. Unless the presence of the perforated discs all thepacking would be ejected.This jump, which has a great influence on the liquid flow rate and pumping efficiency,becomes more important if submersion ratio, Sr, is also important.
  • 7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME467Fig. 3. Liquid flow rate is a function of submersion ratio SrFig.3 is a graphical representation of liquid flow rate as submergence ratio. We findthat the liquid flow rate increases with the gas flow. This curve confirms the relationshipbetween the liquid flow rate and the submergence ratio is linear when QG= 1.265 Nm3/h. But,when the gas flow rate increases, the relationship becomes nonlinear. Some authors like toshow the effect of the superficial gas velocity on the superficial liquid velocity.In order to find a model that includes the operating parameters (gas and liquid flow rates,submergence ratios) with the characteristics of the system (tube section, etc.), we representedthe flow rate ratio (QG / QL) based on liquid flow rate QL for different submersion ratios (Fig.4a). We got a parabolic relation in the range of liquid flow rates (0-105 L.h-1).(a) This study (b) D. Moran study [35]Fig. 4. Effect of Liquid flow rate on the Gas liquid flow rate ratioOutside this range, the curve is no longer parabolic, due to change of void fraction inthe bed. It is worthy to say that the curves of fig. (4a) of this study are similar to that obtainedby D. Moran [35] (Fig. 4b) with other conditions and type of air lift pump.
  • 8. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME468Fig.5a. Effect of mass gas flow rate on the mass liquid flow rate (Our results and there obtainedby Khalil [36] and Parker[1]))Fig.5b. Effect of mass gas flow rate on the mass liquid-gas ratiosThe fig. 5a shows a comparison between results of this study with the analogousresults obtained by other authors (parker [1] and Khalil [36]).The latters were related to otherair lift pumps and other conditions, in which there is no packing, but in a same submergenceration (S r = 0.55). We observe the same trends. In our case, it is obvious to note that packingcauses the decrease of the liquid flow rate.
  • 9. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME469Parker [1] considered the pumping efficiency can be described by liquid- gas massratio versus mass gas flow rate. He obtained a parabolic curve similar to that obtained in thisstudy (Fig. 5b). The same author recommended plotting a dimensionless liquid flow rate [QL/ (Ar*(2*g*Zs) ½)] versus gas-liquid flow rate ratios (QG / QL). With, Ar is riser section, g isthe gravity and Z s is the submersion depth.Fig. 6.Effect of gas-liquid flow rate ratios on the dimensionless liquid flow rateThe curves, obtained in (fig. 6), show that a model can describe the relation betweenthe operating parameters and setup characteristics. This model has been given by thefollowing expression:‫ݕ‬ ൌ ߙ. ‫ݔ‬ିఉ(5)Where:‫ݕ‬ ൌொಽ஺ೝඥଶ௚௓ೄ(6)‫ݔ‬ ൌொಸொಽ(7)386.04 ൑ ߙ ൑ 701.4 (8)1.314 ൑ ߚ ൑ 1.618 (9)
  • 10. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME470Table 2, gives the values of α and β for each submerged ratio value.Table 2: fitted equation for each submerged ratioS rொಽ஺ೝ‫כ‬ሺଶ‫כ‬௚‫כ‬௓ೄሻభ/మൌ ݂ሺொಸொಽ): Fitted equation R 20.5ொಽ஺ೝ‫כ‬ሺଶ‫כ‬௚‫כ‬௓ೄሻభ/మൌ 386.04 ‫כ‬ ቀொಸொಽቁିଵ.ଷଵସ0.96560.6ொಽ஺ೝ‫כ‬ሺଶ‫כ‬௚‫כ‬௓ೄሻభ/మൌ 701.4 ‫כ‬ ቀொಸொಽቁିଵ.଺ଵ଼0.97910.7ொಽ஺ೝ‫כ‬ሺଶ‫כ‬௚‫כ‬௓ೄሻభ/మൌ 435.9 ‫כ‬ ቀொಸொಽቁିଵ.ହହଵ0.98063.2. Pressure drop across the height of the fixed bedExperiments to measure the pressure drop through the bed were carried out fordifferent initial liquid heights using a differential manometer. The scope is to determine theeffect of gas flow rate, submerged depth and bed porosity on pressure drop.Fig. 7.Effect of gas flow rate en drop pressure for dry packingFig. 7 shows the effect of air flow rate and the dry packing on the pressure drop. Thetests are achieved in ordinary conditions. The obtained results can be fitted by a linearequation indicated in fig. 7. The pressure drop increases linearly with the gas flow rate andhis maximum value is less than 450 Pa in the test conditions.
  • 11. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME471(a) (b) (c)Fig. 8.Effect of air flow rate, submersion ratio and the fluid velocity on the pressure drop.Fig. 8a shows the experimental curves relating the pressure drop per unit bed heightversus air flow rates for different submersion depth. The analysis of the results shows theinfluence of the submerged depth and the gas flow rate on the pressure drop.Fig. 8b shows the variation of pressure drop per unit height of packed column,against the submersion depth for different gas flows. The curves obtained have a positiveslope. For QG = 0.758; 1.265 and 1.517 Nm3/h all the curves are linear and in agreementwith the fact that when the liquid height, Zs, (i.e. Sr) increases, air flow encounter moredifficulties to cross the packed column. So, ∆P increases.The lines are classified according to increasing QG. This is consistent with the fact that ∆P isof the form:∆ܲ ൌ ݂. ߩ௙. ܷ௙ଶ. ቂሺଵିఌሻయష೙ఌయ ቃ௅ఃయష೙ ௗ೛(10)(Leva [37], 1959)) where, ߩ௙ and ܷ௙ are the density and the superficial velocity of the fluidmixture respectively. Φ and ݀௣ are the shape factor and diameter of packing respectively. nis a constant.f is the friction factor, which is related to Reynolds number, Re, by the following equation:݂ ൌ௕ோ௘೙ (11)Where, b is constant and the Reynolds criterion based on grain size is:ܴ݁ ൌ ܷ௙ߩ௙݀௣ / ௙(12)µf is the fluid viscosity (pa /s).
  • 12. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME472For QG = 2.023 Nm3/h a change in the slope is observed at ܵ௥ ؆ 0.6. This phenomenon couldbe attributed to the displacement of the packing rings when they are submitted to a high gasvelocity. The new curve branch is linear and shows a significant increase in the slope.We see, in a first approximation, that:- When the submersion ratio is very important, ∆P has approximately a parabolic form.- For a lower value of Sr (Sr = 0.6) the parabolic form is more flat.- For Sr more lower, (Sr = 0.5), we have a parabolic branch very close to a linear form.Considering that a parable is described by the following formula: ൌ ܽ. ‫ݔ‬ଶ൅ ܾ. ‫ݔ‬ ൅ ܿ .We know that the parable is more and more flat when the coefficient, a, is smaller andsmaller.To a fixed liquid immersion depth, the pressure drop increases with increasing gas flow,which can be attributed to the growth of the water flow. These experiments show theimportance of the degradation of energy by friction. The pressure drop is given by theexpression of Ergun [38], (1952):∆௉ு್೐೏ൌ ‫ܣ‬ሺଵିఌሻమఌయ೑ሺΦௗುሻమܷ௙ ൅ ‫ܤ‬ଵିఌఌయఘ೑ሺΦௗುሻܷ௙ଶ(13)This equation can be written as follows:∆௉ு್೐೏ൌ ‫ܥ‬ଵ. ௙. ܷ௙ ൅ ‫ܥ‬ଶ. ߩ௙. ܷ௙ଶ(14)With:‫ܥ‬ଵ ൌ ‫ܣ‬ሺଵିఌሻమఌయଵሺΦௗುሻమ (15)And‫ܥ‬ଶ ൌ ‫ܤ‬ଵିఌఌయଵሺΦௗುሻ(16)Fluid velocity:Uf = (UG + UL) = (QG + QL) / Ar (17)According to this equation, the pressure drop increases with the superficial liquid velocity UL.Trends illustrated in Fig. 8c are described by the second order polynomial equation. We notethat the viscosity µf and density, ρf depend on the temperature, so that the value of thepressure loss depend on the thermodynamic conditions of the measurements. Fortunately, allthe experimental values in this work were obtained at constant temperature (27°C). It shouldbe noted that each submersion ratio corresponds to a hydrostatic pressure, ܲௌ௧, which is equalto :ܲௌ௧ ൌ ߩ௅. ݃. ܼ௦ (18)Where:ߩ௅: Liquid density (kg. m-3),݃:Gravity (m. s-2),
  • 13. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME473Thus the air must have a pressure greater than (ܲௌ௧ ൅ ∆ܲ) for flowing in the packedcolumn. Below this value, the gas does not pass through the packed column; therefore wecannot talk about pressure loss. So, the curves of pressure drop versus fluid velocity fordifferent submersion ratios (Sr= 0.5; Sr=0.6 and Sr=0.7) do not go through the origin. Theyhave the following equations in the following table 3:Table 3: fitted pressure drop equation for each submerged ratioSr Fitted Pressure drop equationR20.5∆௉ுಳ೐೏ൌ 0 ‫כ‬ ܷ௙ଶ൅ 5326.7 ‫כ‬ ܷ௙ ൅ 1138.70.96960.6∆௉ுಳ೐೏ൌ 117966 ‫כ‬ ܷ௙ଶെ 10601 ‫כ‬ ܷ௙ ൅ 1837.40.98170.7∆௉ுಳ೐೏ൌ 214499 ‫כ‬ ܷ௙ଶെ 18464 ‫כ‬ ܷ௙ ൅ 2214.20.98630 (dry packing)∆௉ுಳ೐೏ൌ 0 ‫כ‬ ܷ௙ଶ൅ 1575.6 ‫כ‬ ܷ௙ ൅ 41.430.9384It is instructive to say that we can find easily the effect of liquid flow rate on thepressure drop. For a submersion depth of 60cm and for a gas flow rate of 2.023 Nm3/h, thecharacteristics of the bed change due to the porosity variation explained above, the pressurewill increase rapidly. A jump is observed for submersion depth of 60cm and 2.023 Nm3/h ofgas flow rate (fig. 8a) the same jump is observed at a point having the coordinates 70 cm assubmergence depth and 1.5 Nm3/h as gas flow rate (fig. 8b). So the jump depends on the gasflow rate and the submergence depth.3.3. Gas hold upThe global gas hold up profiles obtained in the fixed bed at different gas flow ratesand liquid heights submersions were determined. The same tendencies are observed at allsubmersion ratios (Fig. 9).Fig. 9. Effect of gas velocity on the gas holdup
  • 14. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME474Whatever, we can note that global gas holdup increases with increasing gas flow ratesand decreases with increased liquid flow rates. These variations have been already pointedout by Heilman [39], (1968); Achawal [40], (1976); Barrios [23], (1987);Lara Marquez [29],(1992) and Gillot [41], (2005).The global gas hold up is the sum of the dynamic and static gas fractions. Thedynamic gas fraction is the volume of gas in the packed column which is renewedcontinuously by the inlet gas throughput. But the static gas fraction corresponds to theremaining gas in the packed column when the gas flow is cut off. It depends on thecharacteristics of the fixed bed such as porosity, shape and nature of the packing (Maldonado[33], 2005; Tung et al. [42], 1988).The decrease of the global gas holdup with increasing of submersion ratio is attributedto the decrease in the drag force.3.4. The slip velocityThe slip velocities are calculated from the following equation:‫ܩ‬ ൌ௎ಸఌಸെ௎ಽଵିఌಸିఌೄ(19)Fig.10a. Effect of gas velocity on the Fig. 10b.Effect of gas velocity on theSlip velocity (This study) Slip velocity (Maldonado [33] study)The fig.10a shows the increase of the slip velocities with the superficial gas velocities.Moreover, it appears that the high values of slip velocities are obtained with low submersionratios. So, the slip velocities increase with decreasing contact area. A comparison of thisstudy with that achieved by Maldonado [33] (fig. 10b), we conclude that the slip velocity,obtained in this, is greater than that obtained by Maldonado [33]. This greatness is attributedto the importance of gas hold up in our case, which is related to the high gas velocities andglass ring as packing used in this study.
  • 15. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME4753.5. Efficiency of the air lift pumpThe term air lift pump efficiency was presented by equation (1). It is used by severalauthors for system pumping evaluation. Fig. 11a shows that the efficiency of the air lift pumpincreases with increasing gas flow rate up to a certain value where it becomes almost constantand decreases after. The submergence ratio (Sr) effect is observed in this curve. Theefficiency decreases with the increase of submersion ratio.(a) (b)Fig. 11. Effect of the air flow rate on the efficiency of the air lift pumpThe comparison of this study with that achieved by Khalil [36] shows easily the same curvestrends; even the two air lift pumps and the operating conditions are different (fig. 11b). So, itis important to underline the gas energy loss caused by liquid flow rate, packing and theconnection between the riser and the down comer. Merchuk [21] showed that if, Ad andAr are the sections of down comer and riser respectively, the decrease of the ratio (Ad / Ar)have a negative effect on the gas holdup but also a negative effect on the pumping liquidefficiency. However, it should be interesting to announce that the setup is designed not forvery high pumping liquid flow rates but for high heat and mass transfer efficiency.Consequently, liquid flow rates recorded, in the experimental study, are very high for manyapplications.4. CONCLUSIONSIn this study we determined, under ambient conditions: atmospheric pressure andtemperature of 27°C, the effect of immersion depth and the gas flow on the liquid flow rate.Air flow rate have an important effect on the liquid flow. At a given submerged ratio, liquidflow rate depends on gas flow rate, bed porosity and system design. When gas flow rateincreases, then liquid flow rate increases also. Besides, the submerged ratios increase theliquid flow rate increase also. In the range of operating conditions tested the liquid flow ratedecreases with increasing of gas -liquid flow rate ratios. We found that liquid flow becomehigh enough when immersion depth is greater than 40%. Below this value, the pumping ofwater in a granular medium by the air is impossible.
  • 16. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME476It is observed that the pressure drop per meter of packed bed increases with increasinggas flow rate. It increases more intensively with the increase of submerged ratio. It can bedescribed by a second order polynomial equation. Average gas holdup and slip velocityincrease with superficial gas velocity but decreases with an increase of submerged ratio andliquid velocity.Finally this study shows that the pump efficiency increases with increasing gas flowrate up to a maximum is reached. Then it decreases regardless on the gas flow. The packingpresence is really an obstacle to liquid flow. A model giving the liquid flow rate for a givengas flow rate and for submergence ratio range, between 0.5 and 0.7, is proposed. As aconclusion we can say that the studied set up is very interesting and may constitute the baseof several useful applications.REFERENCES[1] Parker G. J., The Effect of foot piece Design on the performance of small air lift pump,INT. J. HEAT & FLUID FLOW vol. 2 NO . 4.[2] Chisti V., G. Trystam, 1992, « Assure Bioreactor Sterility », Chem.Eng.Prog.,September 1992, pp. 80-85[3] Reley D.J., Parker G.J. « Flowing geothermal wells: Cerro Priet wells M.91 and Kraflawell KJ-9.1: Computer analysis compared with experimental data », Int. Conf. OnGeothermal Energy, Florence Italy, May 1982, pp. 187-196.[4] Mero J.L., »Seafloor Minerals: A Chemical Engineering Challenge », Chem. Eng.Prog., July 1 1968, pp.73-80[5] Stenning A.H., Martin C.B. "An analytical and experimental study of airlift pumpperformance", J.Eng.Power, 90, 1968, pp. 106-110[6] Casey T.J., "Water and Wastewater Engineering", Oxford University Press, 1992, pp.181- 185[7] Storch. B., 1975. Extraction of sludges by pneumatic pumping.In : second symposiumon Jet pumps and Ejectors and Gas lift Techniques, Churchill College, Cambridge,England, G. A. PP. 51-60.[8] Lee H S, Yoon C H, Kim I K et al., 1997. Overflow volume generated by air lift pump,The International Society of Offshore and Polar Engineers, The Proceedings of the 2ndISOPE OMS, 1997(1): 117–121.[9] Kumar E Anil, Kumar K R V, Ramayya A Venkata, 2003. Augmentation of airliftpump performance with tapered up riser pipe – an experimental study, the Institution ofEngineers (India). Journal of Mechanical Engineering Division, 84: 114–119.[10] Oh S K, 2000. A study on airlift pump for water circulation and aeration.Ph.DThesis.Pukyoung University, Korea. 19–27.[11] Nicklin, D. J. 1963 the air-lift pumps: theory and optimization. Trans. Instn chem.Engrs 41, 29-39.[12] Reinemann D J, Parlange, J. Y., Timmons, M. B., « Theory of small-diamter airliftpumps », Int. J. Multiphase Flow Vol. 16, No. 1, pp. 113-122, 1990.[13] Roumy, R., 1969. Structure des écoulements diphasiques eau-air. Etude de la fractionde vide moyenne et des configurations d’écoulement, CEA-R-3892. Commissariat àl’EnergieAtomique, France.[14] Govier G.W., Aziz A., The flow of complex mixtures in pipes, Van Nostrand ReinholdCo., 1972.
  • 17. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME477[15] Govier G.W., Radford B.A., Dunn J.S.C. "The upwards vertical flow of air-watermixtures", Can. J. Ch. Eng., Aug. 1957, pp. 58-70.[16] Friedel L. "Improved friction pressure drop correlations for horizontal and vertical two-phase pipe flow", European Two Phase Flow Group Meet., Ispra, Italy, paper E2, 1979.[17] Kern R., Chemical Engineering, McGraw-Hill, New York, 1975, pp. 145-151.[18] Wallis G.B., One dimensional Two-Phase Flow, McGraw-Hill, New York, 1969.[19] Nicklin, D. J., Wilkes, J. O. & Davidson, J. F. 1962 Two-phase flow in vertical tubes.Trans. Instn chem. Engrs 40, 61-68.[20] Merchuck J. C., and Y. Stein, AICHE J. 27, 377-381 (1981).[21] Merchuk J.C., N. Ladwa, A. Cameron, M. Bulmer, and A. Pickett, AICHE J. 40, 1105-1117 (1994).[22] Nakoryakov, V. E., Kashinsky, O. N. &Kozmenko, B. K. 1986 Experimental study ofgas-liquid slug flow in a small diameter vertical pipe.Int. J. Multiphase Flow 12, 337-355.[23] Barrios Q.E.M., Etude de l’hydrodynamique des réacteurs en lit fixe avec écoulementde gaz et de liquide en co-courant ascendant, Thèse de doctorat, sciences pétrolières,Université Pierre et Marie Curie Paris VI, Ecole Nationale Supérieure du Pétrole et desMoteurs, Solaire, France, 1987, 342 p.[24] S. Sicardi, G. Baldi, V. Specchia, I. Mazzarino, Hydrodynamics on fixed bed reactorswith cocurrent upward flows, Ing. Chim. Ital. 20 (7/8) (1984) 66–69.[25] A. Lara Marquez, G. Wild, N. Midoux, A review of recent chemical techniques for thedetermination of the volumetric mass-transfer coefficient kLa in gas–liquid reactors,Chem. Eng. Process. 33 (1994) 247–260.[26] J.L. Turpin, R.L. Huntington, Prediction of pressure drop for two-phase, two-component concurrent flow in packed beds, AIChE J. 13 (6) (1967) 1196–1202.[27] Y. Sato, T. Hiroshe, T. Ida, Upward co-current gas–liquid flow in packed beds, KagakuKogaku 38 (1974) 534.[28] M. Nakamura, T. Tanahashi, A. Takeda, S. Sugiyama, Reprints of the 43rd AnnualMeeting of the Society of Chemical Engineers, 1978, p. 42.[29] A. Lara Marquez, F. Larachi, G.Wild, A. Laurent, Mass transfer characteristics of fixedbeds with concurrent up flow and down flow. A special reference to the effect ofpressure, Chem. Eng. Sci. 47 (1992) 3485–3492.[30] Moustiri S., G. Hébrard, M. Roustan, Effect of a new porosity packing onhydrodynamics of bubble columns, Chem. Eng. Process. 41 (5) (2002) 419–426.[31] Therning P., A. Rasmuson, Liquid dispersion and gas holdup in packed bubblecolumns at atmospheric pressure, Chem. Eng. J. 81 (2001) 69–81.[32] M. Abraham, S.B. Sawant, Hydrodynamics and mass transfer characteristics of packedbubble columns, Chem. Eng. J. 43 (1990) 95–105.[33] Garcia Maldonado J.G. , G. H´ebrard, M. Roustan, D. Bastoul, S. Baig, Hydrodynamicand mass transfer in a three phase fixed bed reactor with cocurrent gas–liquid upflow:the effect of different packings, in: Proceedings of the 17th IOA International WorldCongress Strasbourg, 2005.[34] CHO Nam-Cheol, HWANG In-Ju, LEE Chae-Moon, PARK Jung-Won, Anexperimental study on the airlift pump with air jet nozzle and booster pump, Journal ofEnvironmental Sciences Supplement (2009) S19–S23.[35] MORAN, D., “Carbon dioxide degassing in fresh and saline water. II: Degassingperformance of airlift”, Aquacultural Engineering 43 (2010) 120-127.
  • 18. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME478[36] Khalil M. F., K.A. Elshorbagy, S.Z. Kassab, R.I. Fahmy, « Effect of air injectionmethod on performance of an air lift pump”, INT. J. HEAT & FLUID FLOW (1999).[37] Leva, Fluidisation, McGraw Hill, N.Y., (1959)[38] Ergun S., Fluid flow through packed columns, Chem. Eng. Prog. 48 (3) (1952) 89–94.[39] Heilman W., V.W. Hofman, Zur hydrodynamic zweiphasigdurchstromterschuttschichten, in: Proceedings of the fourth European Symposium on Chemical ReactorEngineering, Brussels, Belgium, September 9–11, 1968.[40] S.K. Achawal, J.B. Stepanek, Holdup profiles in packed bed, Chem. Eng. J. 12 (1976)69–75.[41] S. Gillot, F. Kies, C. Amiel, M. Roustan, A. H´eduit, Application of the off-gas methodto the measurement of oxygen transfer in biofilters, Chem. Eng. Sci. 60 (2005) 6336–6345.[42] V. X. Tung and V.K. Dhir, “A hydrodynamic model for two -phase flow throughporous media”. Int. J. Multiphase Flow vol. 14, No. 1, pp. 47-65, 1988.[43] Adel Oueslati, Ahmed Hannachi and Mohamed Elmaaoui, “An Experimental Study onthe Airlift Packed Column with Adjustable Height and Many Air Injection Points”,International Journal of Advanced Research in Engineering & Technology (IJARET),Volume 4, Issue 1, 2013, pp. 42 - 49, ISSN Print: 0976-6480, ISSN Online: 0976-6499.[44] Ajay Kumar Kapardar and Dr. R. P. Sharma, “ Numerical and Cfd Based Analysis ofPorous Media Solar Air Heater”, International Journal of Mechanical Engineering &Technology (IJMET), Volume 3, Issue 2, 2012, pp. 374 - 386, ISSN Print: 0976 – 6340,ISSN Online: 0976 – 6359.