Download - Physical modelling for airlift loop tanks
Physical modelling for airlift loop tanks†
JA Trilleros,1* R Dıaz2 and P Redondo2
1Departamento de Ciencia de los Materiales e Ingenierıa Metalurgica, Facultad de Ciencias Quımicas, Universidad Complutense deMadrid, Madrid 28040, Spain2Facultad de Ciencias Experimentales y de la Salud, Universidad San Pablo CEU, Boadilla del Monte, Madrid 28668, Spain
Abstract: Airlift loop tanks (ALT) are used in a many industrial applications where close contact
among the different phases is essential. We can find such examples in the chemical, petrochemical,
biochemical and minerals processing industries. The ALT principle is based on creating a zone within
the tank through which gas rises. In this way a difference in density is obtained between the gas–liquid–
solid riser zone and the liquid–solid downcomer zone; this density difference induces circulation of the
liquid and solid phases through both zones. When the gas velocity is increased, gas bubbles inside the
liquid phase are also transported into the downcomer. This circulation creates a good mixture of all
three phases and efficient suspension of solid in the liquid. A main parameter that characterizes ALT
operation is the circulation velocity, which can be quantified by the riser superficial liquid velocity.
This parameter is a function of the operational adjustable variables such as gas flow-rate, liquid and
solid densities, fractional holdup, solid size and tank geometry. The following variables were deter-
mined by experimental work: pressure losses in ALT, superficial solid velocity in riser and downcomer,
superficial liquid velocity in riser and solid holup in riser. Finally the relationship between these
variables was established.
# 2003 Society of Chemical Industry
Keywords: airlift; three phase; mixing; superficial velocity; holdup
NOTATIONQ Volumetric flow-rate
U Phase superficial velocity (m s�1)
e Fractional holdup
Subscripts:D Referred to downcomer
G Referred to gas phase
L Referred to liquid phase
R Referred to riser
S Referred to solid phase
Keys to assay notation:1st Tube diameter -D- (-44- for 44mm, -82- for
82mm, -125- for 125mm, -240- for 240mm
and -WT- for assays without tube)2nd Tube length -L- (-1050- for 1050mm and
-630- for 630mm)
3rd Tank without tube -WT-4th Solid type (-PL- for the polystyrene, -LGS-
for little glass spheres and -BGS- for bigglass spheres)
5th Solid loads on tank, volumetric percentages
(-1- for 1%, -2- for 2%, -3- for 3%, -4- for4%, -5- for 5%, -6- for 6%, -7- for 7%)
Example: D44L1050PL1 Tube (diameter 44mm,
length 1050mm; solid (polystyrene, load 1%)
1 INTRODUCTIONThis equipment essentially is described as a
cylindrical tank with a central airlift tube. The aim
of these systems is mixing and homogenization of
the several phases that take part in the process. To
achieve this objective, an injection of gas is made at
the bottom of the tank promoting an ascending
column of bubbles into the central airlift tube,
which drag the denser phases to the free surface of
the tank. In this upper zone the gas phase leaves the
tank and the denser phases return to the bottom of the
tank.
The optimization of the mixing time and the energy
consumption in this tank depends on several opera-
tional variables such as gas flow-rate, superficial
velocity of gas at the airlift tube, suspended solids
concentration and the promoted velocities of liquid
and solid; it also depends on design variables such as
solid density and size, and diameter and length of the
central airlift tubes.1–4
2 EXPERIMENTAL SETUPThe experimental work was done on the pilot plant
scale, by simulation with the system air–water–solid,
and the solids were of different densities and sizes. A
single tank of cylindrical shape and a truncated cone as
base was employed for all the experimentation. The
(Received 15 April 2002; revised version received 24 June 2002; accepted 30 July 2002)
* Correspondence to: JA Trilleros, Departamento de Ciencia de los Materiales e Ingenierıa Metalurgica, Facultad de Ciencias Quımicas,Universidad Complutense de Madrid, Madrid 28040, SpainE-mail: [email protected]† Paper presented at the Process Innovation and Process Intensification Conference, 8–13 September 2002, Edinburgh, UK
# 2003 Society of Chemical Industry. J Chem Technol Biotechnol 0268–2575/2003/$30.00 146
Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 78:146–150 (online: 2003)DOI: 10.1002/jctb.716
dimensions of this tank were 1250mm total height and
420mm diameter, and the truncated cone was 250mm
high with a 60° inclined wall.
Figure 1 shows the experimental installation dia-
gram, consisting of the tank, the data-logger equip-
ment and the controller of the injected gas flow-rate.
The air flow-rates employed were selected and
injected into the bottom tank through 12 nozzles of
orifice diameter 1mm distributed on a circumference
of diameter 30mm. The air flow-rates were regulated
with a needle valve and were quantified by 12 previ-
ously calibrated rotameters.
2.1 Operational variablesThe operational variables employed in the experi-
mental work were the following.
2.1.1 Operational gas flow-ratesSeven gas flow-rates were selected for each of the
experimental assays (Table 1).
2.1.2 Solids concentrationFor each type of solid different concentrations of solid
loads were disposed in the tank. Only a part of this load
was suspended; the rest stayed at the bottom of the
tank.5
To observe the influence of the solid particles
density and size on the system, three different types
of solids were employed: cylindrical particles of poly-
styrene (density 1gcm�3), little glass spheres (di-
ameter between 0.25 and 0.42mm, density 2.6g
cm�3) and big glass spheres (diameter between 1 and
1.41mm, density 2.6gcm�3). The solid loads were
from 1 to 7% (volume) at seven levels with a 1%
increment.
2.2 Design variablesThe following design variables were modified in the
experimental work.
2.2.1 Density and size of the solid particlesThree types of solid particles were employed:
(i) Polystyrene: cylindrical particles with a diameter
and length of 3mm. The density of these particles
was 1gcm�3.
(ii) Little glass spheres: spherical particles with a
diameter between 0.25 and 0.42mm with a
density of 2.6gcm�3.
(iii) Big glass spheres: spherical particles with a
diameter between 1 and 1.41mm with a density
of 2.6gcm�3.
2.2.2 Configuration of the central airlift tubesThe configuration type of the central airlift tubes
promotes different gas holdups for each tube, depend-
ing on its diameter.6–8 The different lengths employed
for the central airlift tubes promote pressure variation
in the upper zone of the tank. Four distinct tube
diameters (44, 82, 125 and 240mm) and two lengths
of tubes (630 and 1050mm) were chosen as design
variables, and there was another configuration without
a central airlift tube.
3 EXPERIMENTAL TECHNIQUES ANDMEASUREMENTSThe experimental techniques employed for measuring
the variables were as follows:
(i) gas holdups at the riser by manometers and
overall airlift tank gas holdups by a level indicator
system;
(ii) solids concentration at several heights of the tank
using a conductivity technique;
(iii) velocity of the liquid phase by pulse thermal
tracer.
3.1 Gas holdup determinationThe gas holdup promotes a displacement of the liquid
free surface height as a consequence of the gas volume
flowing through the liquid bulk. For determining the
overall airlift tank gas holdup, the tank at the free
surface of liquid, in the absence of gas, was closed by a
methacrylate disk with an axial central tube of the
same material, the diameter of which was 195mm.
This arrangement is shown in Fig 2.
First, the central airlift tube corresponding to each
configuration was placed in the tank, second, the
quantity of solid required was added and third, the
tank was filled with water to the free surface of the
Figure 1. Experimental installation.
Table 1. Gas flow-rates
QG1 QG2 QG3 QG4 QG5 QG6 QG7
Volumetric flow-rate
(m3s�1�10�4)
1.35 1.78 2.51 3.81 4.84 6.00 7.70
J Chem Technol Biotechnol 78:146–150 (online: 2003) 147
Physical modelling for airlift loop tanks
methacrylate disk and axial tube. The volume differ-
ence between the initial and final levels of the free
surface achieved when the gas was injected from the
bottom of the tank is the overall tank gas holdup.
3.2 Riser gas holdupFor the riser gas holdup determination, corresponding
to the central airlift tube, manometer techniques for
measuring differential pressures were employed. The
pressure tapes were at 10mm from the upper and
lower ends of the airlift tubes. Once the central airlift
tube holdups and corresponding overall tank gas
holdups had been determined, the gas phase at the
separator was deduced through a gas-phase balance
considering, that there was no gas phase at the
downcomer.
3.3 Determination of solids concentration byconductimetryThe equipment was constituted by four elements: a
personal computer, a square-wave generator, an
electronic board and two files of electrodes supported
on opposite sides of the inner tank wall.
The square-wave generator (ICL 8038) produced a
wave with an amplitude of 5V distributed between
�2.5 and þ2.5V at 500Hz frequency, which avoids
polarization. This wave crosses through the liquid
containing the solids suspended from one electrode to
another situated at the opposite side wall of the tank;
the liquid was a conductor and the solid a non-
conductor. The electrodes were distributed long-
itudinally on two files distributed on opposite sides
of the tank inner wall, and at each side were 11 sensors
at a distance of 9cm.
The computer software allowed control of the
square wave generated and measurement of the
potential signal resulting as a consequence of wave
travel through the liquid–solid suspension, and also
recorded the data corresponding to each assay. The
software allowed the selection of the measuring time,
intervals, units and data representation.
A conventional conductimeter for measuring the
continuous phase conductivity, which was the refer-
ence for equipment gain adjustment before each essay,
was also employed. The conductivity installation is
shown in Fig 3.
The electrical conductivity of the continuous phase
is constant when the temperature remains constant.
The conductivity of the liquid–solid suspension varies
with the quantity of solids suspended, the conductivity
being lower if the concentration of solids increases.9,10
The knowledge of the relationship between the
concentration of solids and the conductivity was
necessary. This experimental relationship was estab-
lished through a calibration process in a perfectly
mixed mechanically agitated tank. The diameter of
this tank was the same as the airlift tank diameter, and
the distances between electrodes (three on each file)
were also the same (90mm). The temperature must be
constant during the calibration process.
The conductivity technique was calibrated for each
type of solid, so each conductivity corresponds to the
percentage of solid particles suspended. The solid
suspended concentration was measured in the tank
cylindrical zone where there was no gas phase and the
distance between sensors was constant.
The conductivity was measured on each sensor at 5s
intervals, capturing 100 data for each sensor in a
period of 500s. There were 12 electrode sensors along
the height of the tank cylindrical zone. The total of
data recorded for each assay was 1200. The solids
suspended concentration corresponds to the conduc-
tivity measured, being the same at all heights of the
tank.
Figure 3. Conductimeter with acquisition data equipment.
Figure 2. System employed for overall tank gas holdup determination.
148 J Chem Technol Biotechnol 78:146–150 (online: 2003)
JA Trilleros, R Dıaz, P Redondo
3.4 Temperature measuring equipmentThe temperature measurements were made using five
thermocouples sited along the downcomer, in a
vertical file at 50mm from tank wall, and at regular
distances (240mm between each of them). The
acquisition data equipment employed had the follow-
ing five elements: five thermocouples distributed along
the tank, the data-logger, an RS-232-C connection
and a personal computer.
The experimental liquid circulation velocities were
determined by a pulse thermal tracer technique. As
thermal tracer, 2dm3 of hot water at 90°C were
thrown over the free surface of the tank where the gas
phase was disengaged. The circulation time was
identified as the time elapsed between the temperature
variation between the first thermocouple near the free
surface and the last thermocouple near the tank
bottom.
4 EVOLUTION OF VARIABLESFor the several configurations of the airlift loop tank,
the following evolutions of the distinct variables
against the gas superficial velocity were obtained.
4.1 Gas holdup evolution versus the gassuperficial velocityFigure 4 shows the evolution tendencies of the gas
holdup versus the gas superficial velocity.
For all the airlift central tube diameters a power law
was observed, except for the tubes of diameter 44mm.
On this graph can be observed a pronounced dis-
continuity between the values obtained in the tubes of
diameter to 82mm and those of 44mm, because the
gas holdup is smaller than expected in the latter case at
the first four values of the gas superficial velocity. The
hydrodynamic behaviour of this tube is different to all
the others, because the small diameter of this tube
generates larger pressure losses due to the greater
contraction, and as a consequence there is a smaller
pressure available to drag the solid particles.
4.2 Evolution of the riser solid holdup versus thegas superficial velocityFigure 5 shows the tendencies for the evolution of the
riser solid holdup versus the superficial velocity of gas.
The riser solid holdup increases with increase in the
superficial velocities of the gas.
It can be observed in Fig 5 that the riser solid holdup
depends on the concentration and density of solid, and
also depends on the central airlift tube diameter and
length. For glass particles the diameter was smaller
when the solid holdup was larger, and the solids
holdup was larger for polystyrene particles than for
glass particles, owing to the lower density of poly-
styrene particles. When the tank was operated with the
same particles, the larger rise solid holdups were
obtained when the central airlift tube diameter was the
smallest (44mm), and smaller riser solid holdups were
obtained when this diameter was the largest (240mm).
The tube length had a variable influence over the
maximum riser solid holdup reached for the same tube
diameter and type of solid.
4.3 Riser liquid superficial velocity versus gassuperficial velocityThe evolution of the liquid superficial velocity versusthe gas superficial velocity, both on the central airlift
tube, is presented in Fig 6. As general rule, the
tendency observed in all cases is a larger induced liquid
superficial velocity when the central airlift tube
diameter is smaller, and also the liquid superficial
Figure 4. Gas holdup versus gas superficial velocity.
Figure 5. Riser solid holdup versus gas holdup.
Figure 6. Riser liquid superficial velocity versus gas superficial velocity.
J Chem Technol Biotechnol 78:146–150 (online: 2003) 149
Physical modelling for airlift loop tanks
velocity is larger when the gas superficial velocity
increases. When the assays were done with polystyrene
particles, the liquid superficial velocity was larger than
the corresponding cases with glass particles. The
liquid superficial velocity is smaller when the solid
holdup increases and is larger when the particle
diameter is smaller because the number of particles
that can be suspended is greater.
4.4 Riser pressure losses versus the phasesuperficial velocitiesFigure 7 shows the tendencies for the evolution of riser
pressure losses versus the sum of the superficial
velocities of the three phases.
The total pressure loss, sum of the pressure losses
obtained at the entrance and exit of the central airlift
tubes, is plotted versus the sum of the superficial
velocities of the three phases. There is a parabolic
tendency: the pressure losses increase when the sum of
superficial velocities have larger values. The length of
the central airlift tube is not an influencing variable on
the pressure losses. The pressure losses are larger on
tubes of smaller diameter, because of the greater
friction produced by diameter reduction and also the
greater friction resulting from the suspended solids
increment at smaller diameters for the central airlift
tube.
With polystyrene particles the pressure losses are
larger than in the corresponding cases with glass
particles. Also, it can be seen that the pressure losses
are smaller when the particles have a smaller diameter.
5 CONCLUSIONSLarge-scale experiments have been carried out to
evaluate the performance characteristics of an ALR
tank and obtain more fundamental information on the
relevant hydrodynamic flow variables. The intercon-
nections between the design variables (tank height,
draft tube height, riser diameter), the liquid and solid
properties, the size of the solid particles and the
observable hydrodynamic variables in an ALR tank are
presented in Fig 4 for the gas holdups, Fig 5 for the
riser solid holdups and Fig 6 for the riser liquid
superficial velocities.
The superficial liquid velocity is a function of the gas
and solid holdups and the gas input rate or riser gas
superficial velocity. From Figs 4, 5 and 6 it can be
deduced, for a draft tube (44mm diameter) and a solid
configuration (little glass spheres and 3% solid load),
the relationship for the hydrodynamic variables shown
in Fig 8.
The liquid circulation velocity in an ALR tank can
be calculated by making an overall momentum
balance taking into account the following items:
hydrostatic head difference (downleg minus upleg),
entry and exit losses in draft tube and acceleration loss
across the gas injection into the draft tube. Figure 7
shows the overall pressure loss (entry and exit) versussuperficial velocities of the three phases.
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Figure 7. Total pressure losses versus the sum of the superficial velocitiesof the three phases.
Figure 8. Diagram showing the relationship between the hydrodynamicvariables.
150 J Chem Technol Biotechnol 78:146–150 (online: 2003)
JA Trilleros, R Dıaz, P Redondo