Hydrodynamic study of anadjustableheightpacked column operating

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME
461
HYDRODYNAMIC STUDY OF AN ADJUSTABLE HEIGHT PACKED
COLUMN OPERATING ON THE PRINCIPLE OF AN
AIR LIFT PUMP
Adel OUESLATI 1 *
, Ahmed HANNACHI2
, Mohamed EL MAAOUI3
1
College of technology, Department of Chemical engineering, Mogran, 6227,Zaghouan –
Tunisia
2
National school of Engineers, Gabes, Department of chemical engineering, Omar ibn El
khattab, Zrig, 6072- Tunisia
3
Faculty of sciences of Tunis, Department of chemistry, Elmanar 1002- Tunisia
ABSTRACT
A setup consisting of a glass column packed with calibrated glass rings has been
achieved. It operates on the principle of an air lift pump. It was designed for the best contact
between air and water. Performances of this system were determined by measuring the
displaced water flow rates for different submersion depths and various air flow rates. We
studied the pressure drop versus the immersion depth in the column. The results show that the
pressure loss is described by a second order polynomial equation. Efficiency was calculated
for different conditions. The study shows that the proposed system can be set easily, has low
power consumption, provides a good mix between phases and is very important for many
applications where heat and mass transfer are involved.
Keywords: air lift pump, porous media, packed column, efficiency
1. INTRODUCTION
The pumping system of water by air lift consists of the injection of compressed air at
the base of a pipe in order to drive the liquid therein. The only source of energy, used for
pumping, is compressed air. A two-phase mixture is water-air, of lower density than the
surrounding liquid. Upward movement is initiated, and causing a stream of water.
INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING
AND TECHNOLOGY (IJMET)
ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
Volume 4, Issue 2, March - April (2013), pp. 461-478
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462
Air 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) in
underwater exploration (JL Mero [4]; Stenning and Martin [5], 1968), the extraction of
sludge in wastewater treatment (Casey TJ [6], 1992 and storck [7], 1975).
Pumping systems air lift type are considered effective for low head conditions compared to
centrifugal 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୐୬ቀ
ౌబ
ౌభ
ቁ
(1)
With:
ρ௅
: Density of the liquid, g: Acceleration due to gravity, Q୐, Qୋ: volume flow rates of the
liquid displaced and gas respectively, Z୐: Pressure head (m), P଴, Pଵ: Gas pressures
respectively 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 that
influence the hydrodynamics of the system. The hydrodynamic in a gas-liquid contactor is
very complex. Characterization begins with the determination of gas flow regimes. Several
authors have defined flow regimes co-updraft gas. Five regimes, two-phase flow water-air
vertical 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 by
varying one or both of air flows and water. But in the case of an air lift, the gas flow rate and
the initial height of the liquid, in the riser, that secure the flow rate of the circulating liquid
and the flow regime.
The pressure drop (P0 - P1) resulting mainly to gravity and viscous forces. They
depend 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 Aziz
A. [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 and
annular.
The gas hold up is the ratio of the volume of gas contained in the mixture biphasic on
the useful volume of the column. It is the sum of the dynamic gaseous fraction and the static
gaseous 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 in
the column.
It also depends on the design of the closed loop air lift system including the
connections between the riser pipe and tube down comer (Merchuk [21], 1994). The author
reports elucidated the effect of sections of the riser and the down comer of the gas holdup
value. Nakoryakov [22] (1986), Rienemann [12] (1987) and Merchuk [21] (1994) showed
that the gas holdup depends on the diameter risers and the effects of viscosity, surface tension
and Reynolds number . The authors confirmed that if the tube diameter is much less than 6
mm and if the gas flow is cut, surface tension prevents the rise of gas bubbles.

463
Air lift pumps that we present above are exclusively formed by a vertical tube in
which there is water at a given height and air injection at the base. Our work is to use an air
lift system wherein the tube is filled with packing. Although packed columns are very well
known, 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 other
researches achieved a packed column reactor where liquid and gas are sent by two different
pumps (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 conventional
packed 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 rates
and 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 fluid
characteristics of packing. Since the regime depends on the characteristics of solids, so we
cannot make flow regimes maps similar to that of a biphasic system.
On the gas holdup, studies by Moustiri [30], (2002); Therning [31], (2001), Lara
Marquez [29], (1992), Abraham [32], (1990) and Barrios [23], (1987) showed that the
overall retention of the gas increases as a function of the superficial velocity gas and
decreases with that of the liquid without offering an explanation of the effect of solid packing
on the gas holdup. The pressures drop in a packed column where the flow is co-current
upward 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 have
defined 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 column
vertical 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 are
characterized by a high void fraction which maintains a low pressure drop. Possible
applications are expected in the field of air humidification or stripping. The tests are
performed in ambient conditions. Only two parameters are adjustable: the gas flow rate and
the initial height of the liquid in the riser. Water flow generated, the gas holdup, pressure
drop and pump efficiency are determined to evaluate the performance of assembly.
According to the trends observed in the experimental study a physical interpretation is
proposed.
2. MATERIALS AND METHODS
2.1. The experimental setup
Fig. 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 flow
meter (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 cooling
water (15), pure water Tank (16), water level control (17), Temperature sensor (18), Relative
Humidity (HR) sensor (19) and pressure manometers.

464
The experiments were performed on a vertical cylindrical column made up in three
glass tubes 0.072 m diameter and 0.4 m length. The total height of the column and the
packed bed are 1.2 m and 1.0 m respectively. The characteristics of the solid packing are
shown in Table 1. The column is provided with equidistant pressure sensors in order to
measure the local pressure at different heights. Four polypropylene disc diffusers, with 67
circular pores of 5 mm each, are arranged at the ends of the glass tubes. They have a
double role: first they change the fluid flow direction, second they prevent the exit of
solid packing out of tube. Only one air jet nozzle is used in the experiments. It has a
diameter of 3 mm.
At the input of the column, a swirl chamber, stainless steel, is designed for the
injection of water and air. At a height equal to 1.02 m of the column, water is recycled
through 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 tank
of 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 between
the 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, provided
with a flow controller valve. The air flow meter is air float type; brand Tubux whose
measuring 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 be
replaced, 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 transparent
polyethylene layers. The airflow humidified by passing through water level in evaporator
chamber then leaves from the outlet pipe in the direction of condensers. The water level
in the evaporator chamber is controlled by the level of water make up tank (7) and an
electric heater (3) of 2 kW power. The inlet water flow rate in the evaporator is measured
by a calibrated orifice (6).
The physical properties of the packing particles are given in table.1.
Table 1: Physical characteristics of the packing particles
Type of packing particles Glass rings
Density : ρS 2.187 (kg/m3
)
Average diameter: dp 0.008 (m)
Averagelength : lp 0.008 (m)
fixed bed porosity: ε 77.3
Form Factor: φ 0.681

465
Fig. 1. Schematic diagram of the set up
Fig.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), vapor
condensers (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 been
based 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 the
following 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.

466
2.2. Measuring the pressure drop
To measure the pressure loss due to the fluid flow between the ends of packed
column, 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 the
column with respect to that of the atmosphere. The measurement error by differential
manometer is equal to േ2 mm.
2.3. Measuring the global gas hold up
The gas hold up is defined as the volume occupied by gas in the packed column
crossed by a diphasic mixture in continuous operation. It called also void fraction. It is
measured by set a level of liquid in the packed column, which corresponds to a volume V0 of
the liquid. Then, we inject a gas flow rate. Once the steady state is reached, flows into and
from the packed column are cut by closing the corresponding valves. The new liquid volume
V1 in the packed column is noted. The void fraction is then:
߳ீ ൌ
௏బ ି௏భ
௏బ
(4)
3. RESULTS AND DISCUSSION
3.1. Average water flow rate
The results of measurements are plotted in Fig. 2.
Fig. 2. Effect of the Gas flow rate on the water flow rate for many submersion ratios
The examination of Fig.2 shows that there is a jump, in all the curves. This is
attributed to movement of the packing which is subject only to his weight. Under the effect of
flow, the mixture of gas and liquid induce a movement of packing, the void fraction increases
and the liquid flow rate increases also. Unless the presence of the perforated discs all the
packing 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.

467
Fig. 3. Liquid flow rate is a function of submersion ratio Sr
Fig.3 is a graphical representation of liquid flow rate as submergence ratio. We find
that the liquid flow rate increases with the gas flow. This curve confirms the relationship
between 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 to
show 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 represented
the 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 ratio
Outside this range, the curve is no longer parabolic, due to change of void fraction in
the bed. It is worthy to say that the curves of fig. (4a) of this study are similar to that obtained
by D. Moran [35] (Fig. 4b) with other conditions and type of air lift pump.

468
Fig.5a. Effect of mass gas flow rate on the mass liquid flow rate (Our results and there obtained
by Khalil [36] and Parker[1]))
Fig.5b. Effect of mass gas flow rate on the mass liquid-gas ratios
The fig. 5a shows a comparison between results of this study with the analogous
results obtained by other authors (parker [1] and Khalil [36]).The latters were related to other
air lift pumps and other conditions, in which there is no packing, but in a same submergence
ration (S r = 0.55). We observe the same trends. In our case, it is obvious to note that packing
causes the decrease of the liquid flow rate.

469
Parker [1] considered the pumping efficiency can be described by liquid- gas mass
ratio versus mass gas flow rate. He obtained a parabolic curve similar to that obtained in this
study (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 is
the gravity and Z s is the submersion depth.
Fig. 6.Effect of gas-liquid flow rate ratios on the dimensionless liquid flow rate
The curves, obtained in (fig. 6), show that a model can describe the relation between
the operating parameters and setup characteristics. This model has been given by the
following expression:
‫ݕ‬ ൌ ߙ. ‫ݔ‬ିఉ
(5)
Where:
‫ݕ‬ ൌ
ொಽ
஺ೝඥଶ௚௓ೄ
(6)
‫ݔ‬ ൌ
ொಸ
ொಽ
(7)
386.04 ൑ ߙ ൑ 701.4 (8)
1.314 ൑ ߚ ൑ 1.618 (9)

470
Table 2, gives the values of α and β for each submerged ratio value.
Table 2: fitted equation for each submerged ratio
S r
ொಽ
஺ೝ‫כ‬ሺଶ‫כ‬௚‫כ‬௓ೄሻభ/మ
ൌ ݂ሺ
ொಸ
ொಽ
): Fitted equation R 2
0.5
ொಽ
ൌ 386.04 ‫כ‬ ቀ
ொಸ
ொಽ
ቁ
ିଵ.ଷଵସ
0.9656
0.6
ொಽ
ൌ 701.4 ‫כ‬ ቀ
ொಸ
ொಽ
ቁ
ିଵ.଺ଵ଼
0.9791
0.7
ொಽ
ൌ 435.9 ‫כ‬ ቀ
ொಸ
ொಽ
ቁ
ିଵ.ହହଵ
0.9806
3.2. Pressure drop across the height of the fixed bed
Experiments to measure the pressure drop through the bed were carried out for
different initial liquid heights using a differential manometer. The scope is to determine the
effect of gas flow rate, submerged depth and bed porosity on pressure drop.
Fig. 7.Effect of gas flow rate en drop pressure for dry packing
Fig. 7 shows the effect of air flow rate and the dry packing on the pressure drop. The
tests are achieved in ordinary conditions. The obtained results can be fitted by a linear
equation indicated in fig. 7. The pressure drop increases linearly with the gas flow rate and
his maximum value is less than 450 Pa in the test conditions.

471
(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 height
versus air flow rates for different submersion depth. The analysis of the results shows the
influence 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 positive
slope. For QG = 0.758; 1.265 and 1.517 Nm3/h all the curves are linear and in agreement
with the fact that when the liquid height, Zs, (i.e. Sr) increases, air flow encounter more
difficulties to cross the packed column. So, ∆P increases.
The lines are classified according to increasing QG. This is consistent with the fact that ∆P is
of the form:
∆ܲ ൌ ݂. ߩ௙. ܷ௙
ଶ
. ቂ
ሺଵିఌሻయష೙
ఌయ ቃ
௅
ఃయష೙ ௗ೛
(10)
(Leva [37], 1959)) where, ߩ௙ and ܷ௙ are the density and the superficial velocity of the fluid
mixture respectively. Φ and ݀௣ are the shape factor and diameter of packing respectively. n
is 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).

472
For QG = 2.023 Nm3
/h a change in the slope is observed at ܵ௥ ؆ 0.6. This phenomenon could
be attributed to the displacement of the packing rings when they are submitted to a high gas
velocity. 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 and
smaller.
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 the
importance of the degradation of energy by friction. The pressure drop is given by the
expression of Ergun [38], (1952):
∆௉
ு್೐೏
ൌ ‫ܣ‬
ሺଵିఌሻమ
ఌయ
೑
ሺΦௗುሻమ
ܷ௙ ൅ ‫ܤ‬
ଵିఌ
ఌయ
ఘ೑
ሺΦௗುሻ
ܷ௙
ଶ
(13)
This equation can be written as follows:
∆௉
ு್೐೏
ൌ ‫ܥ‬ଵ. ௙
. ܷ௙ ൅ ‫ܥ‬ଶ. ߩ௙. ܷ௙
ଶ
(14)
With:
‫ܥ‬ଵ ൌ ‫ܣ‬
ሺଵିఌሻమ
ఌయ
ଵ
ሺΦௗುሻమ (15)
And
‫ܥ‬ଶ ൌ ‫ܤ‬
ଵିఌ
ఌయ
ଵ
ሺΦௗುሻ
(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 note
that the viscosity µf and density, ρf depend on the temperature, so that the value of the
pressure loss depend on the thermodynamic conditions of the measurements. Fortunately, all
the experimental values in this work were obtained at constant temperature (27°C). It should
be noted that each submersion ratio corresponds to a hydrostatic pressure, ܲௌ௧, which is equal
to :
ܲௌ௧ ൌ ߩ௅. ݃. ܼ௦ (18)
Where:
ߩ௅: Liquid density (kg. m-3
),݃:Gravity (m. s-2
),

473
Thus the air must have a pressure greater than (ܲௌ௧ ൅ ∆ܲ) for flowing in the packed
column. Below this value, the gas does not pass through the packed column; therefore we
cannot talk about pressure loss. So, the curves of pressure drop versus fluid velocity for
different submersion ratios (Sr= 0.5; Sr=0.6 and Sr=0.7) do not go through the origin. They
have the following equations in the following table 3:
Table 3: fitted pressure drop equation for each submerged ratio
Sr Fitted Pressure drop equation
R2
0.5
∆௉
ுಳ೐೏
ൌ 0 ‫כ‬ ܷ௙
ଶ
൅ 5326.7 ‫כ‬ ܷ௙ ൅ 1138.7
0.9696
0.6
∆௉
ுಳ೐೏
ൌ 117966 ‫כ‬ ܷ௙
ଶ
െ 10601 ‫כ‬ ܷ௙ ൅ 1837.4
0.9817
0.7
∆௉
ுಳ೐೏
ൌ 214499 ‫כ‬ ܷ௙
ଶ
െ 18464 ‫כ‬ ܷ௙ ൅ 2214.2
0.9863
0 (dry packing)
∆௉
ுಳ೐೏
ൌ 0 ‫כ‬ ܷ௙
ଶ
൅ 1575.6 ‫כ‬ ܷ௙ ൅ 41.43
0.9384
It is instructive to say that we can find easily the effect of liquid flow rate on the
pressure drop. For a submersion depth of 60cm and for a gas flow rate of 2.023 Nm3/h, the
characteristics of the bed change due to the porosity variation explained above, the pressure
will increase rapidly. A jump is observed for submersion depth of 60cm and 2.023 Nm3/h of
gas flow rate (fig. 8a) the same jump is observed at a point having the coordinates 70 cm as
submergence depth and 1.5 Nm3
/h as gas flow rate (fig. 8b). So the jump depends on the gas
flow rate and the submergence depth.
3.3. Gas hold up
The global gas hold up profiles obtained in the fixed bed at different gas flow rates
and liquid heights submersions were determined. The same tendencies are observed at all
submersion ratios (Fig. 9).
Fig. 9. Effect of gas velocity on the gas holdup

474
Whatever, we can note that global gas holdup increases with increasing gas flow rates
and decreases with increased liquid flow rates. These variations have been already pointed
out 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. The
dynamic gas fraction is the volume of gas in the packed column which is renewed
continuously by the inlet gas throughput. But the static gas fraction corresponds to the
remaining gas in the packed column when the gas flow is cut off. It depends on the
characteristics 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 attributed
to the decrease in the drag force.
3.4. The slip velocity
The slip velocities are calculated from the following equation:
‫ܩ‬ ൌ
௎ಸ
ఌಸ
െ
௎ಽ
ଵିఌಸିఌೄ
(19)
Fig.10a. Effect of gas velocity on the Fig. 10b.Effect of gas velocity on the
Slip 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 submersion
ratios. So, the slip velocities increase with decreasing contact area. A comparison of this
study 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 attributed
to the importance of gas hold up in our case, which is related to the high gas velocities and
glass ring as packing used in this study.

475
3.5. Efficiency of the air lift pump
The term air lift pump efficiency was presented by equation (1). It is used by several
authors for system pumping evaluation. Fig. 11a shows that the efficiency of the air lift pump
increases with increasing gas flow rate up to a certain value where it becomes almost constant
and decreases after. The submergence ratio (Sr) effect is observed in this curve. The
efficiency decreases with the increase of submersion ratio.
(a) (b)
Fig. 11. Effect of the air flow rate on the efficiency of the air lift pump
The comparison of this study with that achieved by Khalil [36] shows easily the same curves
trends; even the two air lift pumps and the operating conditions are different (fig. 11b). So, it
is important to underline the gas energy loss caused by liquid flow rate, packing and the
connection between the riser and the down comer. Merchuk [21] showed that if, Ad and
Ar 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 liquid
efficiency. However, it should be interesting to announce that the setup is designed not for
very 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 many
applications.
4. CONCLUSIONS
In this study we determined, under ambient conditions: atmospheric pressure and
temperature 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, liquid
flow rate depends on gas flow rate, bed porosity and system design. When gas flow rate
increases, then liquid flow rate increases also. Besides, the submerged ratios increase the
liquid flow rate increase also. In the range of operating conditions tested the liquid flow rate
decreases with increasing of gas -liquid flow rate ratios. We found that liquid flow become
high enough when immersion depth is greater than 40%. Below this value, the pumping of
water in a granular medium by the air is impossible.

476
It is observed that the pressure drop per meter of packed bed increases with increasing
gas flow rate. It increases more intensively with the increase of submerged ratio. It can be
described by a second order polynomial equation. Average gas holdup and slip velocity
increase with superficial gas velocity but decreases with an increase of submerged ratio and
liquid velocity.
Finally this study shows that the pump efficiency increases with increasing gas flow
rate up to a maximum is reached. Then it decreases regardless on the gas flow. The packing
presence is really an obstacle to liquid flow. A model giving the liquid flow rate for a given
gas flow rate and for submergence ratio range, between 0.5 and 0.7, is proposed. As a
conclusion we can say that the studied set up is very interesting and may constitute the base
of several useful applications.
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