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: trillero@quim.ucm.es† 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