REVIEW STUDY ON AIRLIFT PUMPING SYSTEMS

Multiphase Science and Technology, 24 (4): 323 362 (2012) REVIEW STUDY ON AIRLIFT PUMPING SYSTEMS P. Hanafizadeh & B. Ghorbani Center of Excellence in Design and Optimization of Energy Systems, School of Mechanical Engineering, College of Engineering, University of Tehran, P.O. Box: 11155-4563, Tehran, Iran Address all correspondence to P. Hanafizadeh E-mail: hanafizadeh@ut.ac.ir Airlift systems are widely used in various fields such as in the petrochemical and oil industries. Because the main part of the flow through the pipe of these systems is formed by gas liquid two-phase flow, analysis of such systems are accompanied with problems of two-phase flow modeling. Several effective variables are involved in airlift systems; therefore, a comprehensive method is needed when considering these parameters. This paper reviews the state of the art in the field of airlift systems. All of the related articles have been divided into two categories; namely, two-phase and three-phase systems based on the operation of the airlift systems in the fluids. Moreover, the paper describes the airlift pump operation as a kind of pumping system and evaluates developments in the modeling of these systems. In particular, the paper also concentrates on airlift history and background, structure, types, and applications. KEY WORDS: airlift system, multiphase flow, gas liquid flow, submergence ratio, flow regimes, flow patterns 1. INTRODUCTION Airlift pumps are simple devices used for rising liquids and mixtures of liquids and solid particles. Their operation is based on the use of buoyancy force to pump the liquid and solid particles through a partially submerged vertical upriser pipe in the fluid, which should be pumped. Despite its lower efficiency, the airlift pump has some practical advantages, such as lower initial cost, lower maintenance, easy installation, ability to resist clogging, small space requirements, simplistic design and construction, ease of flow rate regulation, and versatility in many applications over ordinary mechanical pumps. 1.1 Structure The airlift pump consists of a vertical pipe divided into two parts. The part that is located between the bottom and the air-injection point is called the suction pipe (L e ) and the part that is located between the air-injection port and discharge part is called the uprise pipe (L s + L d ), as shown in Fig. 1. The air-lifting operation depends on the injection of air 0276 1459/12/$35.00 c 2012 by Begell House, Inc. 323

324 Hanafizadeh & Ghorbani NOMENCLATURE A AFR ALS C D E f g GBD Gen H L LDN ṁ n p PDN Q R Re S t tube cross-sectional area air flow rate airlift system chamber diameter effectiveness friction factor gravity gas bubble disease generation water height upriser tube length lift dimensionless number mass flow rate efficiency pressure pump dimensionless number volumetric flow rate regulator Reynolds number entropy time TGP total gas pressure v velocity Z height Greek Symbols α submergence ratio ε gas void ratio η efficiency ν kinematic fluid viscosity ρ fluid density Σ surface tension number σ surface tension ψ exergy Subscripts a atmosphere G gas L liquid l lift s submerged sf single phase T s bubble in still fluid 0 base of the riser or gas at the bottom of the pipe, which is partially submerged in the liquid that should be lifted. An air compressor is used to provide the required compressed air or gas at the chosen pressure and flow rate. 1.1.1 Submergence Ratio An important parameters that plays an significant role in airlift pump operation is the submergence ratio, which is defined as follows as the ratio of the immersed depth to the total length of the upriser pipe: α = Submergence depth Total lift height = L s + L e L s + L e + L d Multiphase Science and Technology

Review Study on Airlift Pumping Systems 325 FIG. 1: Schematic of an airlift pumping system. For example, for an airlift pump with a dynamic lift of 0.3 m and a submergence depth of 1.2 m, the submergence ratio would be 1.2:1.5, or just 0.8 and the percent of the submergence ratio would be 80%. 1.2 Operation Airlift systems (ALSs) are simple devices that work with compressed air or gas. Their operation is based both on the effect of the buoyancy force of the bubbles and the differential pressure between the injection and outlet points in the pump. The gas phase is injected into the bottom of the pipe, and because the gas has a lower density than the liquid it rises up quickly in the pipe. The pressure and inertia of the ascending gas forces the liquid phase to move in the same direction. The dominant forces acting on the air water mixture in the airlift pump are gravitational, inertia, and buoyancy forces. The buoyancy force acts as the lifting force while the gravity force opposes it. If the driving forces are large enough, the liquid is lifted along the pipe and expelled at a level higher than the submerged level. Otherwise, the liquid is only lifted up to the level where these acting forces will be equal. It has been observed that various conditions may happen depending Volume 24, Number 4, 2012

326 Hanafizadeh & Ghorbani on the two-phase flow regime in the upriser pipe. In most of the literature, gas liquid two-phase flow regimes are generally divided into four different flow patterns; namely, bubbly, slug, churn, and annular regimes. In the bubbly flow regime, the air water mixture is lighter than the single-phase water, and therefore it is displaced to the upper level. The inertia force of the injected air bubbles displaces the surrounding water and moves it up to the upper level of the pipe. Observations have shown that the magnitude of the inertia force of the bubbles is not high enough to be able to move the surrounding water a significant distance. It has been concluded that the bubbly flow cannot be utilized in the pumping process. The main reason for this postulate is that in this regime the driving force is not enough to raise the water against the gravity direction. Thus, the bubbly flow can be used only with high submergence ratios in the airlift pump. In contrast, for the other flow patterns, particularly regarding slug and churn flows, large air bubbles act as pneumatic pistons and push the trapped slug water between them along the pipe. The air bubble not only pushes the front water but also drags water behind it due to the suction created by the quick movement of bubbles. In the annular flow regime, the inertia of high-velocity air forcing the water to move upward is directly related to the air water interface shear tension. The higher air velocity is necessary to create sufficient friction force; consequently, the airlift pump acts with low efficiency in the annular flow. It can be seen that three of the main regimes namely, slug, churn, and annular flows push the water rather than displacing it; therefore, they are more applicable than the bubbly flow regime in ALSs, especially at low submergence ratios. 1.3 Types of Airlift Systems The air-lifting operation also depends on the shape of the upriser pipe and the injection port of air or gas at the bottom of the pipe. From the viewpoint of the shape of the upriser pipe, there are mainly three different types of ALSs; i.e., ordinary, step, and tapered. 1.3.1 Ordinary Type The ordinary type is the simplest ALS, in which the upriser pipe has a constant crosssectional area over all of its length. Different parts of the ordinary airlift pump are schematically depicted in Fig. 2. 1.3.2 Step Type As can be seen in the Fig. 3, a riser tube of this type has some steps in its length. The idea of using this type of upriser tube was initially introduced by Kumar et al. (2003). Subsequently, Hanafizadeh et al. (2010, 2011c) studied the operation of this type of ALS in more detail and found why this shape of the upriser improves the operation of the ALS. Hanafizadeh et al. (2010, 2011c) also proposed an optimum height and diameter ratio for this type of pumping system. The literature in the field of ALSs Multiphase Science and Technology

Review Study on Airlift Pumping Systems 327 FIG. 2: Schematic of the ordinary-type ALS with its components. FIG. 3: Schematic of the step-type ALS. Volume 24, Number 4, 2012

328 Hanafizadeh & Ghorbani indicates that a maximum amount of liquid is lifted if the pump operates in the slug regimes; thus, the best efficiency for this type of pump always occurs in this flow regime. An increase in the cross section of the upriser can stabilize the slug flow regime, and therefore the efficiency of the pump will be improved with this diameter. Therefore, it seems that an increase in pipe diameter may postpone the transition of the slug flow pattern to a churn flow pattern and increase the efficiency of the pump. The best position of the step type is where the transition of the flow regime from slug flow to churn flow occurs in the pipe. It is obvious that larger outer diameters can destroy the slugs, and therefore reduce the efficiency of the pump. This indicates that a very large outer diameter not only does not improve the efficiency of airlift pump but also deteriorates the operation of the pump. Therefore, it could be concluded that the outer diameter of step type also has an optimum value that can retain the flow pattern in slug regime. 1.3.3 Tapered Type The tapered type was also introduced for the first time by Kumar et al. (2003). Studies on the step type showed that sudden enlargement in the cross section of the upriser pipe can cause some head losses because of the separation of flow, sudden expansion of the fluid, and creation of secondary flow. In the case of sudden enlargement, only a fraction of the decrease in kinetic energy can be recovered as an increase in pressure energy; the remainder is dissipated as heat. Thus, it seems logical to increase the diameter of the upriser tube continuously in a smoother manner. This leads exactly to the tapered type of upriser, which is shown in Fig. 4. An increase in the cross-sectional area along the upriser pipe due to tapered geometry can increase the efficiency of the pump. Hanafizadeh et al. (2010) observed that the water mass flow rate increases with the increase of the tapering angle; however, at a tapering angle around 3 the rate of increase in the output discharge decreases. This can be attributed to the flow regime and operating conditions in the airlift pump. Therefore, the rate of increase in the output discharge, and consequently the efficiency of the pump, does not continuously increase with enlargement of the cross-sectional area of the pipe. The drawback of this geometry is only its complex manufacturing requirement. 1.4 Types of Air-Injection Ports From the point of view of the air-injection port, airlift pumping systems can be categorized into two main types; i.e., external and internal type. 1.4.1 External Type In the external type of air-injection port, the air intake line is placed outside the riser tube; thus, the intake line does not have any direct contact with the fluid. This type is Multiphase Science and Technology

Review Study on Airlift Pumping Systems 329 FIG. 4: Schematic of a tapered airlift pump: 1, overhead collecting tank; 2, tapered upriser pipe; 3, compressor; 4, air injector; 5, water reservoir. more convenient than the internal type; however, an external system cannot always be used (see Fig. 5). 1.4.2 Internal Type In the internal type of air-injection port, the air intake line is situated inside the riser tube; thus, the intake line has contact with the fluid. This type can also be used in some special cases, such as in a depressurized oil well. Because of the internal existence of intake, this type can have many different shapes. Also, because of the direct contact between the intake and fluid in the riser, by using this type some changes would occur in the flow regimes. These changes can improve or worsen the system operation depending on the shape of the riser, air intake, and also the operating conditions. Although external airline systems are fundamentally more efficient, internal airline pumps are more frequently used because of their versatility and ease of assembly (see Fig. 5). Volume 24, Number 4, 2012

330 Hanafizadeh & Ghorbani FIG. 5: Schematic of the internal and external ALS. 2. HISTORY AND BACKGROUND The first airlift pump was invented by the German engineer Carl Emanuel Löscher in 1797. The first practical application of this technology in the United States was not seen until 1846, where it was used in Pennsylvania in the oil industry (Castro et al., 1975). Due to both the complexity of modeling two-phase flow in an upriser pipe of a pump and the special application of the airlift pump in various industries, evaluating the performance of these pumps has remained a topic of great interest to researchers. Various investigations may be found in the literature, which have tried to simulate the behavior of airlift pumps. Kato et al. (1975) developed a simple model for consideration of airlift pumps. This model was derived by applying a momentum balance throughout the upriser, assuming that the void fraction remains constant. The mean void fraction model was limited, mainly because only one flow regime (slug flow) was assumed to exist throughout the upriser. As a consequence, flow transitions were not considered, which limited the applicability of the model to wells up to 11 m deep. Therefore, the correlations proposed for calculating the volume fraction and friction losses were of limited accuracy. Sorour (1984) carried out an experimental investigation on the transition from bubbly to slug flow in a vertical annulus, and showed the parameters that affect the transition from bubbly to slug flow are the void fraction, number of holes, and bubble size. Then, Sorour and Elbeshbeeshy (1986) carried out investigations of the hydrodynamic characteristics of bubbly flow in a vertical annular channel, and obtained parameters affecting the void fraction and pressure fluctuations in vertical annular channels. Multiphase Science and Technology

Review Study on Airlift Pumping Systems 331 Apazidis (1985) considered the influence of bubble expansion on the performance and stability of an airlift pump, and introduced an airlift pump as a self-control system. It had been shown by Hjalmars (1973) that an occasionally observed breakdown in the self-control mechanism, which leads to instability, is due to the fact that the control mechanism is delayed. This investigation had been carried out with the assumption of a single-phase flow of an ideal incompressible liquid. Apazidis (1985) considered the stability conditions of an airlift pump within the frame of a more general flow model (namely, a separate two-phase flow of compressible gas and incompressible liquid) that took into account the effects of the expansion of gas bubbles during their lift and the relative velocity; i.e., the difference in the velocity of the gas bubbles and the liquid. Assuming one-dimensional (1D) flow and isothermal expansion of air bubbles, according to Boyle s law, and neglecting the wall friction and the velocity of water outside the close neighborhood of the lower pipe opening, Apazidis (1985) wrote down separate continuity and momentum equations for each phase. Combining these equations, Apazidis (1985) eliminated forces due to mutual hydrodynamic drag and also used two additional equations, one being an empirical relationship expressing the relative velocity in terms of bubble dimensions and bubble concentration, and the other being the equation of state for gas. Decomposing the variables in the stationary and small time-dependent perturbated quantities Apazidis (1985) obtained a system of ordinary differential equations of the first order with initial conditions for the stationary parts and a system of partial differential equations of the first order with boundary conditions for the perturbations. Solving the system for the stationary values and satisfying the initial conditions, Apazidis (1985) found values of the rise; i.e., the height of the upper pipe opening over the free water level in the basin as a function of the air and water fluxes. The solution of the boundary value problem for the perturbations gave values of the critical rise (i.e., values of the rise at which instability sets in) as function of the air flux alone. The problem was treated as 1D separate two-phase flow of compressible gas and incompressible liquid. Finally, Apazidis (1985) showed that an increase in the flow stability is obtained by the air bubble size reduction. This increase was less in the case of longer tubes. Apazidis (1985) also showed that at higher flow rates the values of the rise become practically independent of the bubble size, and thus independent of the relative velocity. Clark and Dabolt (1986) presented an analytical approach to investigate the function of the airlift pump. They used a drift-flux model combined with an approximate relationship to predict pressure loss, substituted into the total pressure differential. Integration of the resulting equation provides an explicit formula for the calculation of lift. Morrison et al. (1987) showed the effect of using four and eight diffuser ports as a method of injection on the performance of airlift pumps. The experimental data showed that the eight-port diffuser is more efficient than the four-port one. The data indicated that the efficiency was greatly affected by the air flow rate (AFR) and injection method. Volume 24, Number 4, 2012

332 Hanafizadeh & Ghorbani 3. MODELING AND SIMULATION 3.1 Two-Phase Systems 3.1.1 Theoretical and Analytical Works Reinemann et al. (1990) investigated the effects of the tube diameter on vertical slug flow for a small diameter airlift pump in terms of the Reynolds and surface tension numbers. For a given tube diameter, imposing the gas flow rate and the submergence ratio, Reinemann et al. (1990) derived the liquid flow rate using the system of equations summarized below. Some of these equations had been derived before (Bendiksen, 1985; Collins et al., 1978; Griffith and Wallis, 1961; Nicklin et al., 1962; Nicklin, 1963) ε = Q G 1.2 ( Q L + Q G) + V T s where Q G and Q L are the volumetric gas and liquid flow rates, respectively; V is the velocity; and the subscript T s denotes the bubble in still fluid. It is worth mentioning that the gas volumetric flow rate can be converted to the mass flow rate by considering the flow pressure and temperature. The submergence ratio is defined by the following equation: (1) = Z s Z l + Z s (2) where Z is the height, and subscripts l and s denote lift and submerged, respectively. Performing a static pressure balance on a vertical tube that is submerged in fluid yields ρgz s = ρg (1 ε) (Z l + Z s ) (3) It is assumed that the weight of the gas is negligible relative to the weight of the liquid. If the fluid in the tube is moving, an extra pressure drop due to frictional losses must be added to the right-hand side of Eq. (3). The single-phase frictional pressure drop can be calculated based on the mean slug velocity as P sf = f (Z l + Z s ) ρv 2 m 2D where f is friction factor, which is calculated by (4) where Re is the Reynolds number f = 0.316 Re 0.25 (5) Re = D (Q L + Q G ) νa (6) Multiphase Science and Technology

Review Study on Airlift Pumping Systems 333 where ν is the kinematic liquid viscosity. The single-phase frictional loss in Eq. (4) must then be multiplied by (1 ε), the fraction of the tube occupied by the liquid slugs, to obtain the total frictional pressure drop in the riser tube. The frictional effects in the liquid film around the gas bubble have been shown to be small compared to those in the liquid slug, and are therefore neglected (Nakoriakov et al., 1986). Finally, by including the frictional effects in the pressure drop and dividing both sides by ρg(z l + Z s ), we have [ = (1 ε) 1 + f ] ( Q 2 L + Q ) 2 G (7) where ε is gas void ratio, α is the submergence ratio, and f is friction factor. Dimensionless forms of the volumetric liquid and gas rates are defined by Q L = Q G = Q L A (gd) 1/2 (8) Q G A (gd) 1/2 (9) where A is tube cross-sectional area, g is gravity, and D is the diameter. When the effects of viscosity can be neglected, as is the case for an air water system, the bubble Froude number in the still fluid can be expressed as a function of the surface tension parameter alone, and the value of V T s can be computed as follows (Nickens and Yannitell, 1987; White and Beardmore, 1962): V T s = 0.352 ( 1 3.18Σ 14.77Σ 2) (10) where Σ is surface tension number Σ = σ ρgd 2 (11) where σ is the surface tension and ρ is the fluid density. The liquid flow rate can be calculated using the algorithm described in Fig. 6. It is usual to define the efficiency of the airlift pump as the net work done in lifting the liquid divided by the work done by the isothermal expansion of the air (Nicklin, 1963) Q L Z l ρg n = (12) Q G P a ln (P 0 /P a ) where P a is the atmospheric pressure and P 0 is the pressure at the base of the riser. Nicklin (1963) observed a difference between single-bubble and bubble-train slug flow in air water systems at low Reynolds numbers and found that the velocity profile coefficient approaches a value of 2.0 for low-re flow in air water systems, while a single Volume 24, Number 4, 2012

334 Hanafizadeh & Ghorbani FIG. 6: Algorithm for computing the liquid flow rate. gas slug rises in a moving liquid stream. This indicates a laminar velocity profile in the liquid ahead of the gas slug. When a series of gas slugs rise concurrently with a series of liquid slugs the velocity profile coefficient remains at a value of 1.2 for Re values as low as 500. This indicates turbulent flow in the liquid slugs. It was believed that this difference had been the result of the vorticity generated in the liquid film surrounding the gas slugs in their wake. It had been shown that including this effect and the effects of surface tension on the bubble rise velocity allows the airlift pump theory previously described by Nicklin (1963) to be extended to lower tube diameters. It had also been shown that airlift efficiency and an optimal submergence ratio increase in this range of tube diameters. Nicklin (1963) believed that his proposed theory can be used with confidence to design small-diameter airlift pumps. Multiphase Science and Technology

Review Study on Airlift Pumping Systems 335 Zenz (1993) used various correlations to simulate airlift pumps and showed that in practice an ALS operates with more efficiency in the slug flow regime. Zenz (1993) made some graphical comparisons, which implied that the water rate falls off sharply when the air rate is either less (bubbly) or greater (annular) than near the point of optimum slugflow operation. Zenz (1993) mentioned the Chamberlain (1957) data in this study and believed that an excellent correlation had been shown. De Cachard and Delhaye (1998) proposed a linear stability method to consider the stability of small diameter airlift pumps. A detailed study of the stable operation of small-diameter airlift pumps had been published previously by De Cachard and Delhaye (1996), in which detailed experiments including differential pressure and void fraction measurements were carried out on a 10-mm-diameter setup. Based on the obtained results, it was shown that existing models were not appropriate for small-diameter airlifts, particularly because they overpredict the frictional pressure drop in slug flow. A new steady-state airlift model was proposed. The pressure gradient in the riser was predicted by a combination of specific models describing slug and churn flow. These models were based on the available literature on two-phase flow. The particular structure of slug flow was accounted for by a cellular model. The model proposed represents an accurate analysis tool for the design of small diameter (up to 40 mm), tall (length-to-diameter ratio greater than 250) airlifts. De Cachard and Delhaye (1998) is the continuation of the aforementioned study and deals with the prediction of instabilities. De Cachard and Delhaye (1998) noticed the same variation of the influencing parameters might stabilize or destabilize the performance of the pump depending on the values of the other parameters, and showed that airlift instabilities are due to density wave oscillations in the two-phase flow. Depending on the liquid flow inertia, friction terms, and gas flow compressibility term, the density waves were sustained or not sustained. The effect of the gas compressibility term was preponderant. The objectives of the linear stability analysis were considered to be achieved. Actually, the unstable behaviors observed within the linear stability domain were attributed to some nonlinear effects. This was certainly correct when both stable and unstable regimes were observed. It should also be correct for the always unstable points observed near the marginal linear stability. In this case, the finite perturbations inherent to the system were sufficient to bring it into a neighboring, unstable state, and the flow cannot be stabilized. The linear analysis performed predicted the complex and interacting effects of the geometrical parameters and the gas flow rate well. Subcritical instability had only been observed in regions adjacent to the linear stability boundary. Thus, it was possible, using empirically defined safety margins, to predict airlift stability in a conservative manner. Such an empirical criterion had been derived for engineering purposes. The stability prediction, for a given airlift operating point within the linear stability domain, was based on the first (lowest) oscillatory frequency predicted by the linear analysis for this point. If this frequency is far enough from the equivalent frequency (first oscillatory mode) at the linear stability boundary, the point is predicted as stable. Volume 24, Number 4, 2012

336 Hanafizadeh & Ghorbani Samaras and Margaris (2005) analyzed the transformation of flow regime maps into a selected coordinate system for airlift pumps. Their work presents very simple maps, directly showing the measured data and the flow regime transitions. Samaras and Margaris (2005) indicated four different flow regimes (slug, churn, annular, and wispy annular) as the applicable regimes in airlift pumps. In fact, Samaras and Margaris (2005) found a new flow regime map that was appropriate for airlift pump performance and regime transition and transformed the Hewitt and Roberts (1969) regime map for representing the flow performance of an airlift pump. Samaras and Margaris (2005) used gas and liquid superficial velocities as coordinates for the map. They also proposed another map that gave the void fraction versus gas superficial velocity and claimed this map had the advantage of finding the void fraction in an airlift pump from the gas superficial velocity and flow regime, which usually exist in experimental data. Hanafizadeh et al. (2011a) presented a new definition for the efficiency of an ALS. For a device that does not involve input or output work, such as an airlift upriser pipe, it is logical to define the second law efficiency in terms of the ratio of output to input availabilities. Thus, given the definition of the second law efficiency in this case, the efficiency of the airlift pump can be stated as follows: η 2nd law = ṁair out ψ airout + ṁ waterout ψ waterout ṁ airin ψ airin + ṁ waterin ψ waterin (13) where ṁ is the mass flow rate and ψ is the exergy of the flow. However, this kind of efficiency is typically defined for devices that include heat interaction between both the inlet and outlet flows. Therefore, it seems that the second law efficiency is not quite appropriate for noting the state of efficiency for an ALS. Since the main goal of an airlift pump is related to the outlet water mass flow rate and the main source of energy is in the inlet air mass flow rate, combining the first and second law efficiencies can lead to redefining efficiency for ALSs. In this study, the efficiency of the ALS have been defined as η airlift = ṁwater (ψ waterout ψ waterin ) ṁ airin ψ airin (14) while in most previous corresponding literature, the efficiency of an ALS was described as the ratio of the outlet water flow rate to the inlet AFR. This efficiency is defined as η old = ṁwater out ṁ airin (15) The main drawback to such a definition is that the efficiency is not between 0 and 1, and it is independent of the injected air pressure and temperature. The new definition of efficiency for an ALS is in the aforementioned range and takes all of the effective variables into account. Multiphase Science and Technology

Review Study on Airlift Pumping Systems 337 3.1.2 Numerical Works Nenes et al. (1996) presented a mathematical model for the simulation of an airlift pump based on the interspersed continua approximation for two-phase flow systems, together with an algorithm that selects the appropriate friction correlation for specific flow regimes. Their model can either predict the water or AFR for an ALS. Nenes et al. (1996) solved the problems previously mentioned in relation to the study done by Kato et al. (1975). First, the flow regimes within the upriser were allowed to vary and were predicted by using an appropriate flow regime map. Second, the properties of the two-phase mixture within the upriser were allowed to vary by dividing the upriser into subsections or cells. In each of these cells, the mean properties (such as the pressure and void fraction) were considered representative of the cell and differed from the properties of adjacent cells. A momentum balance was applied to each cell, using correlations for friction losses and void fraction. Eventually, a system of algebraic equations was obtained and was solved using the two known pressure boundary conditions. Nenes et al. (1996) showed that the predictions based on the hydrodynamic model are significantly better in comparison to the mean void fraction model. This is because the hydrodynamic model takes into account the gas compressibility itself (in the momentum and continuity equations) and all the effects that result from this (i.e., multiple flow regimes). Nenes et al. (1996) also found that mean air-volume fraction model might give better predictions if a single flow regime were predominant along the upriser. However, for water wells of moderate to large depths, the compressibility effects of the gas phase are large, which among other things leads to multiple flow regimes. Saito et al. (1999) investigated the hydrodynamics of a gas-lift system in deep sea applications. They obtained a maximum liquid flow rate for each pump for the range of gas injection rates studied. Moreover, the liquid-phase hold-up was adequately modeled based on a gas-phase Froude number and by applying 1D drift-flux analysis. Abed (2003) used a mathematical model to find operational criteria for the performance of airlift pumps. In this study little information was available on pump performance with a wide range of operating conditions. In addition, little had been reported on the limits of the operation; that is, the minimum AFR that required to obtain the output of the water discharge and the corresponding AFR. All published theoretical analysis of airlift pumps had been based on two-phase slug flow. In fact, the flow is composite starting as bubbly flow and ending as annular flow. In the range between the minimum and maximum AFR the flow could be considered as slug flow, which means the pump should be operated between these two limits. The Stenning and Martin (1968) model, with the modification suggested by Parker (1980), was used for the performance estimation of airlift pumps in this study. Abed (2003) also modified the calculation program of Stone (1987) for this study, although the procedure described by Clark and Dabolt (1986) allowed explicit calculation of the lifted water discharged. From the characteristics curves, it was found that the output water flow rate increases with an increase of the Volume 24, Number 4, 2012

338 Hanafizadeh & Ghorbani submergence ratio and the inner diameter. Also, pumps that have smaller diameters are working in a narrow range of operating conditions and an increase of the inner diameter increases the operating range. At the same time, an increase of the riser tube length increases the operating range and consequently needs more air supplied. The maximum AFR supplied increases when the inner diameter increases and submergence ratio decreases. The maximum output water flow rate is greatest when the submergence ratio is 0.8 and decreases when the submergence ratio decreases. Abed (2003) showed that for all of the investigated types, the maximum output water is independent of the riser tube length but depend on the inner diameter and submergence ratio; however, the maximum AFR supplied depends on the riser tube length, submergence ratio, and inner diameter of the riser. Hanafizadeh et al. (2010) developed a new numerical approach, called the physical influence scheme (PIS), to simulate two-phase flow in an airlift pump upriser pipe. This method couples the continuity and momentum equations and enforces the role of the pressure directly into the continuity equations. Hanafizadeh et al. (2010) presented a novel collocated finite-volume formulation to model two-phase flow in airlift pumps. The formulation utilized the PIS to predict gas and liquid velocities along the pipe. The analysis was focused on the upriser part. In this regard, Hanafizadeh et al. (2010) solved the flow governing equations and applied suitable boundary and initial conditions. Similar to the work of Markatos and Singhal (1982) and Markatos (1986), Hanafizadeh et al. (2010) also treated 1D unsteady flow by considering variations of the properties in which the variation axis defined along the upriser was included. For simplicity, it was assumed that the operating conditions were steady and the flow in both phases was isothermal. Hanafizadeh et al. (2010) the rate of convergence was dramatically high because the PIS scheme provides strong coupling between the pressure and velocity variables in the set of discretized governing equations. Hanafizadeh et al. (2010), the pressure gradient was also calculated using the integral method (Nenes et al., 1996) and was compared with the computational fluid dynamics simulation of Markatos and Singhal (1982). The results showed good agreement with the reference pressure distributions. Hanafizadeh et al. (2011c) numerically considered the effect of step-type geometry on the performance of the airlift pump and observed an improvement in the performance of the step airlift pump in comparison with ordinary type. Hanafizadeh et al. (2011c) also considered the effect of the height of the steps and secondary diameter for the steps on this improvement and realized that there are optimum heights and diameters for steps in which the pump performance is maximized in the constant AFR. For means of verification, the numerical results were compared with experimental data (White, 2001). The comparison showed that the numerical results were in good agreement with the experimental data. Hanafizadeh et al. (2011c) showed that the optimum height of step is decreased when the injected AFR increases, and they concluded that it can be described by the fact that an increase in the AFR advances the transition of the slug flow regime to churn flow. An increase in the pipe cross-sectional area can postpone the flow pattern Multiphase Science and Technology

Review Study on Airlift Pumping Systems 339 transition, and hence the slug flow can be abided in the upriser pipe. Hanafizadeh et al. (2011c) also concluded that setting the step to the optimum height can improve the efficiency of the pump. Also, the best position of the step is where the transition of flow regime from the slug flow to the churn flow occurs in the pipe. The maximum amount of liquid is lifted if the pump operates in the slug regime; thus, the best efficiency for this type of pump always occurs in this flow regime. An increase in the cross-sectional area along the upriser pipe due to the step geometry can increase the efficiency of the pump. The results showed that the outer diameter of the step also has an optimum value that can retain the flow pattern in the slug regime. 3.1.3 Experimental Works Khalil and Mansour (1990) carried out experimental work to study the effect of introducing a surfactant in the pumped liquid, and proved that the use of a surfactant in small concentrations always increases the capacity and efficiency of the pump. Furthermore, Khalil and Mansour (1990) found that the pump performance is a function of the volumetric AFR, submergence ratio, air-supply pressure, and surfactant concentration. Khalil and Mansour (1990) also carried out experimental tests to study the effect of the injection method on the airlift pump performance. Their results showed that the initial bubble size and bubble distribution in the main lift riser greatly influence pump performance, where good homogenous mixtures formed in the riser reduce slip and consequently increase airlift pump efficiency. Khalil et al. (1999) studied airlift pump performance experimentally for different submergence ratios using different air-injection footpiece designs. For this purpose, an airlift pump with a 200-cm-long, 2.54-cm-diameter riser was designed and tested. Nine different air-injection footpiece designs were used at four submergence ratios with different air-injection pressures (from 0.2 to 0.4 bar). An area of 10 2 cm 2 was chosen and divided into nine injection hole arrangements (1, 2, 3, 4, 6, 15, 25, 34, and 48 holes) to cover the whole experimental range. Four submergence ratios were used for this work; i.e., 0.75, 0.7, 0.6, and 0.5. The following conclusions can be deduced from the obtained results: The performance of the airlift pump depends on several factors, such as the footpiece design and submergence ratio. The initial bubble size and distribution in the riser section affected the airlift pump performance and a marked improvement in pump performance was obtained by using multiple-hole injectors. (An improvement in the efficiency of 21% was reached by using a three-hole disk compared with the single-hole disk efficiency at H/L 0.75.) The air injector design had a considerable effect on the water discharge as well as on the whole performance of the airlift pump. Volume 24, Number 4, 2012

340 Hanafizadeh & Ghorbani It was found to be possible to increase the quantity of pumped water by using a suitable distribution of holes in the injection disk for a certain submergence ratio. Iguchi and Terauchi (2001) studied the effect of the pipe-wall wettability on the transition among bubbly and slug flow regimes in air water two-phase flow in a vertical pipe. They changed the advancing contact angle of an acrylic pipe by coating its inner wall with a hydrophilic substance or liquid paraffin. Some differences were observed between the pipe walls with different wettability. Flow pattern maps were developed for different pipe-wall wettability. Furukawa and Fukano (2001) investigated experimentally the effects of the liquid viscosity on the flow patterns of upward air liquid two-phase flow in a vertical pipe. They used three different liquids including water and glycol solutions. They proposed flow pattern maps for each liquid viscosity. It was found that flow pattern transitions strongly depend on the liquid viscosity. Guet et al. (2002) studied experimentally an upward air water bubbly flow in a pipe. Special attention was paid to the transition from bubbly flow to slug flow. In their study, the focus was on low-to-moderate pipe Reynolds numbers based on the superficial liquid velocity, for which no bubble breakup occurred because turbulent eddies. For such conditions, there are only a few experimental studies available in the literature (see, for instance, Nakoriakov et al., 1996). When bubble breakup is absent, the initial bubble size can be expected to be an important parameter. Guet et al. (2002) expected at the start of their investigation that also the initial bubble concentration distribution had an impact on the performance of the technique. Therefore, in their water air experiments three different bubble inlet devices were used with symmetric and non-symmetric initial bubble concentration distributions and small and large initial bubbles. The effect of the initial concentration distribution and initial bubble size on the transition from low-re bubbly flow to slug flow was the main topic of Guet et al. (2002) investigation. They believed that the postponement of this transition has a favorable influence on the performance of the gas-lift system, and the longer the flow remains in the low-re bubbly flow regime the higher is the performance of the technique. This was different from later results (for example, Hanafizadeh et al., 2011b). They also believed that the pipe diameter had a significant influence on the transition from bubbly to slug flow. In their experiments the pipe diameter was 72 mm, a value realistic for many gaslift applications. The height of the pipe was 18 m. They showed experimentally that the efficiency of the gas-lift technique at low liquid flow conditions was very dependent on the inlet device used for injecting the gas. When small bubbles were generated during gas injection, the transition from bubbly flow to slug flow occurred at higher values of the void fraction than for large bubbles. Large values of the void fraction under bubbly flow conditions led to large values of the liquid flow. This is not the case when large bubbles are generated during injection because large bubbles would lead to slugflow conditions already at low values of the void fraction. In addition, it was known Multiphase Science and Technology

Review Study on Airlift Pumping Systems 341 that slug flow has had a very detrimental effect on the efficiency of the gas-lift technique. Guet et al. (2002) showed that the transition from bubbly flow to slug flow as a function of the injected bubble size could reliably be predicted by the Taitel et al. (1980) criterion, combined with the critical void fraction relationship of Song et al. (1995). In practical applications, when the size of the injected bubbles is known, the critical void fraction (for the transition from bubbly flow to slug flow) can be calculated with the critical void-fraction relationship of Song et al. (1995). The Taitel et al. (1980) criterion could then be used to calculate the optimal influence of the injected gas flow rate on the liquid flow rate. Hitoshi et al. (2003) found experimentally that the gas-injection point has a significant effect on the discharged water. They concluded that as the gas-injection point increases above 45 pipe diameters the discharged water decreases. Kumar et al. (2003) reported a simple way to improve performance of an airlift pump. They fitted a pump with a tapered upriser pipe. The main reason for the improvement was discussed as the adverse pressure gradient in the tapered upriser pipe on the two-phase flow regime transitions and slug flow parameters. The use of a stepped upriser was explored in their work as well. Kumar et al. (2003) carried out a hydrodynamic investigation to understand the mechanism of improving performance in the pump due to the tapered upriser pipe fitted to the pump. They also observed that the water output in the case of tapered upriser pipe is more sensitive to the AFR than that for the uniform sized pipe, which means tapered upriser pipes with smaller taper angles were found to result in greater improvement in airlift performance. The water output in the case of tapered upriser pipes was observed to be more sensitive to the AFR than that for uniform-sized pipes. A tapered upriser pipe could be implemented in practice by employing stepped upriser pipes in successive size ranges but with smaller diameters, where better performance is noticed. The slug height was found to be smaller in the case of tapered upriser pipes than for the uniform sized tubes. A tapered pipe with increasing size seemed to result in a higher water hold-up, which as such is beneficial for airlift. Dare and Oturuhoyi (2007) introduced the lift dimensionless number (LDN) and pump dimensionless number (PDN) to capture all of the flow parameters. They observed that airlift pumps with smaller riser pipe diameters yield higher lifts. Dare and Oturuhoyi (2007) also found that fluid with better adhesive properties produced higher lifts, and concluded that for all cases the lift increased by increasing the submergence. Kassab et al. (2009) evaluated the performance of an airlift pump under predetermined operating conditions and optimized the related parameters. For this purpose, an airlift pump was designed and tested. Experiments were performed for nine submergence ratios and three risers of different lengths with different air-injection pressures. Moreover, the pump was tested under different two-phase flow patterns. A theoretical model was proposed in this study taking into account the flow patterns at the best efficiency range where the pump is operated. Kassab et al. (2009) showed that the pump capacity Volume 24, Number 4, 2012

342 Hanafizadeh & Ghorbani and efficiency are functions of the air mass flow rate, submergence ratio, and riser pipe length. The following concluding remarks can be deduced from the Kassab et al. (2009) study: As the submerged ratio increased, the maximum efficiency of the pump increased at the same AFR. The airlift pump lifted the maximum amount of liquid if operated in the slug or slug-churn regimes. The maximum efficiency did not occur at the maximum water mass flow rate. The best efficiency points were always located in the slug or slug-churn flow patterns. For the same submergence ratio, varying the length of the riser pipe affected the airlift pump performance. Hanafizadeh et al. (2011b) analyzed experimentally the gas liquid upward two-phase flow regimes in the upriser pipe of airlift pump by using the image processing technique. The experiments of this study on two- phase flow were conducted in the experimental setup that is shown schematically in Fig. 7. Hanafizadeh et al. (2011b) detected four main flow regimes (namely, bubbly, slug, churn, and annular) in the airlift and found the slug flow regime to be the most appropriate one for this type of pumping system (see Fig. 8). The details of flow properties are given as follows: Bubbly flow: In bubbly flow air is distributed as discrete bubbles in continuous water. The bubbles may be small and spherical or large with spherical and flat ends. Slug flow: In slug flow the diameters of the air bubbles are approximately the same as that of the upriser pipe and their lengths may have noticeable variations. The nose of the bubble has the form of a spherical shape. While the slugs move upward, the liquid film that separates the air slugs from the pipe wall descend slowly. The water that is entrapped between two large slugs is forced to move upward along with slugs. Small air bubbles can be found in the wake of the slug or in the water phase. Churn flow: Large slugs break down and the churn flow is formed. The nature of the churn flow regime is transitory and oscillatory due to the irregularity of the air flow. Annular flow: In annular flow the air occupies the core of the pipe and water film is formed on the pipe wall, which is dragged upward by air due to the interface friction. The surface of the water film becomes wavy due to interaction of the phases. In high AFRs, these waves may break up to small droplets in the air phase. This case is recognized as a wispy annular regime in corresponding literature studies. Multiphase Science and Technology

Review Study on Airlift Pumping Systems 343 FIG. 7: Schematic of the test rig (Hanafizadeh et al., 2011b). FIG. 8: Photograph of different flow patterns in the airlift pump. The observations show that the magnitude of the inertia force of the bubbles is not high enough to move the surrounding water great distances upward. It can be concluded that the bubbly flow cannot carry on the pumping process. In this regime the driving Volume 24, Number 4, 2012

344 Hanafizadeh & Ghorbani force is not high enough to raise the water against the direction of gravity; therefore, the bubbly flow can be used only in the high submergence ratios of the airlift pump. In the other flow patterns, particularly in the slug and churn flows, air slugs act as pneumatic pistons and push the trapped water between them up along the pipe. The air slug not only pushes the front water but also drags water behind it due to the suction created by the quick movement of slug. In the annular flow, the inertia of high-velocity air forces the water to move upward owing to air water interface friction. The high air velocity is necessary to create sufficient friction; consequently, the airlift pump acts with low efficiency in the annular flow. It can be seen that three main regimes (namely, slug, churn, and annular) push the water rather than displace it; therefore, they are more applicable than the bubbly regime in ALSs, especially at low submergence ratios. As the air mass flow rate is increased, the air bubbles coalesce together and small slugs develope in the upriser pipe. At the upper part of the pipe, slugs grow whose size will be greater than the ones at lower part of the pipe. It is observed that water is trapped between the formed slugs and is pushed to the top of the pipe, where the pumping action begins. Further increase in the AFR causes the slugs to start to grow and break down to churn flow. An increase in the AFR causes the air to move upward from the core of the pipe and the film of the water is formed on the wall pipe, which causes the annular flow regime to emerge. In the annular flow regime the outlet water mass flow rate comes down in the airlift pump. Hanafizadeh et al. (2011b) found that increase of submergence ratio, increases the efficiency of the airlift pump. This concept is completely in accordance with the physics of the airlift pump because with high submergence ratios the level of water that must be lifted by the injected air is low; therefore, less power is needed to lift the water. Hanafizadeh et al. (2011b) observed that in high submergence ratios, the efficiency of the pump increases with a decrease in the AFR, while in low submergence ratios this trend is inverse and there is not any significant difference in the efficiency values of the pump. Hanafizadeh et al. (2011a) used an exergy analysis to model an airlift pump. They calculated the entropy generation in different flow regimes in an airlift pump and discovered the least entropy generation in the slug flow regime. An exergy analysis was used to study the effects of some parameters on the performance of the ALS. The experimental data were collected from an airlift with a diameter of 50 mm and length of 6 m. The results were used to investigate both the entropy generation and efficiency in the ALS. It was observed that the flow regime in the upriser pipe has the dominant role in the efficiency of the pump; thus, it was concluded that the best flow regime for airlift pump is the slug flow regime. It was also shown that lower entropy generation occurs in lower inlet air pressure and flow rate. In addition, it was demonstrated that using high submergence ratios decreases the entropy generation, and hence increases the efficiency of the ALS. Hanafizadeh et al. (2011a) found entropy generation for different flow patterns, as shown in Fig. 9. Multiphase Science and Technology

Review Study on Airlift Pumping Systems 345 FIG. 9: Entropy generation for different flow patterns: (a) in terms of the AFR for different submergence ratios; (b) in different flow patterns for a submergence ratio of 0.58; (c) in different flow patterns for a submergence ratio of 0.67; (d) in different flow patterns for a submergence ratio of 0.75. Using the new definition of efficiency based on the exergy analysis causes the effect of pressure and temperature of the inlet injected air to be introduced directly into the efficiency of the ALS. Because the ALS operates nearly in adiabatic condition the effect of pressure is more superior to the effect of temperature. The exergy view considers the pressurized air as an available energy, which leads the ALS to operate and lift the water to the upper level. Moreover, this method probes the loss of energy that happens due to the interaction of the phases, both with each other and with the walls. However, this consideration is internally seen when the first and second laws of thermodynamics are written down for the ALS control volume. Therefore, consideration of the aforementioned effects does not confront the complexity of the interaction of two-phase flow. Thus, it seems that the exergy efficiency is more accurate than the old definition of the efficiency, while it enjoys the simplicity of exergy analysis. Volume 24, Number 4, 2012

346 Hanafizadeh & Ghorbani Ahmed and Badr (2012) designed a dual air injector in order to increase the efficiency of the airlift pump. This injector was tested for different water submergence ratios. They found that the efficiency of the new design is at least 30% higher than that of a conventional design. Ahmed and Badr (2012) also proposed a model for analysis of the pump performance, which was validated with the obtained experimental data. Their model predicted pump performance within the range of ±15% of the obtained experimental data. The designed dual injector introduced the compressed air both in the radial and axial directions at the bottom part of the riser pipe. Ahmed and Badr (2012) considered the water flow rate and the pump efficiency as functions of the airflow rate for three air-injection modes; i.e., radial, axial, and combined (50% radial and 50% axial) injection. They observed that the efficiency of the pump was improved with the newly designed dual-injection port. Fan et al. (2013) presented an experimental and theoretical analysis to obtain the performance of an airlift pump for artificial upwelling of ocean water. Their experiments were performed at one submerged depth, with four different air-injection nozzle designs and various injected air-volume flow rates. Their results showed that the pump capacity and efficiency are functions of the geometrical parameters of the pipe, air volume flow rate, air-injection method, and vertical distribution of water density. Fan et al. (2013) found that the upwelling efficiency increases with the increase of the pipe diameter due to the reduction of the frictional loss, kinetic energy, and power demand of the sea surface rise. They also observed that the air injector design has a considerable effect on the upwelling efficiency. Hanafizadeh et al. (2013) introduced a new definition of the performance characteristic for a gas liquid lifting pump. This definition was obtained based on the actual physical behavior of the pump and the measured experimental data during its operation. Hanafizadeh et al. (2013) used a typical gas-lift pump (6 m high; 0.05 m diameter) to obtain the experimental data. They examined several charts and suggested the definition for the performance characteristic of the pump. Also, they considered the effect of some important parameters such as the slip ratio, submergence ratio, and two-phase flow regimes on the performance of the pump. Hanafizadeh et al. (2013) showed that for a constant injected air pressure and constant submergence ratio, decreasing the two-phase density causes a decrease in pump s head. It is obvious from their study that the head decreases by increasing the submergence ratio. 3.2 Three-Phase Systems 3.2.1 Theoretical and Analytical Works Giot (1982) proposed a model for the investigation of a three-phase airlift pump used in the pumping of metallic nodules. In order to lift metallic nodules from the seabed, compressed air is injected in the pipeline at a specific depth. Due to the pressure Multiphase Science and Technology

Review Study on Airlift Pumping Systems 347 difference, buoyancy, and compressed air expansion in the pipe line, the three-phase mixture is lifted up to the sea surface. Hu et al. (2012) established a theoretical model based on the momentum theory. They considered the mixture flow governing equations for airlift and obtained the relationship among the volumetric fluxes of air, water, and solid phases by numerical approach, theoretical analysis, and empirical formulas. Hu et al. (2012) evaluated the accuracy of their model with the experimental data obtained from a river sand airlift pumping system and found that the proposed model was more accurate for modeling of air water than solid air water. Also, their theoretical evaluation matched well with the experimental observation at moderate air volumetric flux. 3.2.2 Numerical Works Pougatch and Salcudean (2008) simulated deep sea air lifting with a mathematical model of the three-phase flow in an upward pipe and studied the influence of the pipe diameter on the airlift efficiency. They found that lifting efficiency increases with the increase of the pipe diameter due to the reduction of the influence of wall friction on the flow. 3.2.3 Experimental Works Fujimoto et al. (2004) investigated experimentally the performance of a small ALS for transporting solid particles (see Fig. 10). The flows of liquid air solid particle threephase mixtures arise in ALSs transporting slurries or relatively large solid particles (Kato et al., 1975; Weber and Dedegil, 1976; Yoshinaga and Sato, 1996; Hatta et al., 1999; FIG. 10: Schematic of the Fujimoto et al. (2004) ALS. Volume 24, Number 4, 2012

348 Hanafizadeh & Ghorbani Fujimoto et al., 2003). Fujimoto et al. (2005) also studied experimentally as well as numerically the performance of airlift pumps for transportation of solid particles. Fujimoto et al. (2003) investigated experimentally the pump performance of a small airlift pump. The supplied gas flux, gas-injection point, particle size, and material density of the particles were systematically changed as parameters. Also, the critical boundary at which the solid particles can be lifted along the pipe was studied. In Fujimoto et al. (2004), the performance of a small airlift pump in transporting solid particles was investigated. The uprisers had a local S-shaped portion above or below the gas injector. The objective of this study was to investigate the flow characteristics in the S-shaped portion. In so doing, three types of S-shaped pipes were used. Their final results are summarized below: 1. The particle velocity changed temporally as well as spatially. The particle motion was very complex in nature because of several factors such as flow pulsation, radial distribution of the liquid axial velocity, gravity, and pipe wall. 2. The utilization of locally bent uprisers caused a reduction of pump performance, although it could be inevitable in an actual use. The larger inclined angle of S- shaped pipes gave rise to larger reduction of performance. 3. A bed of particles was formed in the inclined pipe portion when the S-shaped pipe was set below the gas injecting point. Reduction of pump performance occurred due to momentum transfer from the liquid to the particles in the bed. There was also the possibility of pipe clogging by the particles. Such operating conditions should be avoided in actual use. Kassab et al. (2007) developed a theoretical model based on the control volume approach to predict airlift pump performance in three-phase flow, in which the effects of the submergence ratio and the size of solid particles on the performance of the pump were investigated. Kassab et al. (2007) concluded that the performance of the airlift pump for lifting solid particles and liquid strongly depends on the flow pattern in which the pump operates. In the case of an airlift pump conveying solid particles there was no definition of the efficiency in the literature by this study. Therefore, in order to evaluate the performance of the airlift pump conveying solid particles, a new parameter called the effectiveness of the pump, was introduced as the ratio between the mass flow rate of the lifted solid particles and the mass flow rate of the injected air and can be written as E = ṁsolid ṁ air (16) Kassab et al. (2007) experimentally evaluated the relationship between the air mass flow rate and the effectiveness at a submergence ratio of 0.5. They observed that the effectiveness increases rapidly with an increase of the air mass flow rate up to 4 kg/h. As the Multiphase Science and Technology

Review Study on Airlift Pumping Systems 349 airflow rate increases beyond this value, the effectiveness of the pump decreases. Their results revealed that the maximum effectiveness does not exist at the point of maximum solid mass flow rate. Meanwhile, it is noted that the values of effectiveness of the airlift pumps were low. However, this can be compensated for because no other pump can be used in the airlift pump applications such as recovery of underwater objects where the most important parameter is the safety of the objects. The following concluding remarks can be obtained from their work: The Kassab et al. (2007) modified model based on the original model proposed by Yoshinaga and Sato (1996) for a uniform solid particle was in good agreement with the experimental results for coarse particles, which represent more real engineering applications. The mass flow rate of the solid particles increased as the submergence ratio increased at the same airflow rate. The mass flow rate of the solid particles increased with the decrease of the particle size. The performance of the airlift pump, lifting water, and solid particles depended on the flow pattern in which the pump operates. The proposed model for prediction of airlift pump performance operating in threephase conditions could be used to predict the performance of airlift pumps lifting liquid only by setting the value of the solid mass flow rate in the model to zero. This model could be used in optimizing the design and operating conditions of airlift pumps operating in three-phase flow. It could also be used to select the best geometric parameters for each application and operating condition. 4. APPLICATIONS This type of pumping has low efficiency but great advantages in utilization over mechanical pumps because of lower initial and maintenance costs, easy installation, small space requirements, simplistic design and construction, and ease of flow rate regulation. These advantages, accompanied by the absence of moving mechanical parts, dictate that the airlift pump can be used for pumping different fluids that are corrosive, abrasive or slurries, explosive, toxic, sandy or salty, viscous liquids like hydrocarbons in the oil industry, shaft and well drilling, under-sea mining, and biorectors. They also do not suffer from lubrication or wearing out problems, and can be used in sludge removal in sewage treatment plants (Storch, 1975). Moreover, they are easy to use in irregularly shaped wells where other deep well pumps do not fit. Recently, airlift pumps have been used to pump boiling liquids where there is a change in phase from liquid to gas (Abu-Mulaweh Volume 24, Number 4, 2012

350 Hanafizadeh & Ghorbani et al., 2011). Airlift pumps also have excellent potential for use in cages, floating raceways, closed or recirculating systems, pond de-stratification or aeration (Wurts et al., 1994), return-activated sludge, waste-activated sludge, aerobic digester supernatant return, underwater explorations, raising coarse particle suspensions in the dredging of river estuaries and harbors, the mining of minerals from ocean beds, and the recovery of coal in mine shafts. They also have a wide range of applications in chemical and nuclear areas. In petroleum fields gas-lift pumps are employed to raise oil from weak wells (Khalil et al., 1999). 4.1 Oil Industry The early use of the airlift pump was focused on the coal mining industry because of its ability to extract minerals from deep mine shafts. The first practical application of the airlift pump in the United States was in a Pennsylvania oil field in 1846 (Johnson, 2008). As the primary oil reserves have played out, producing oil by using the traditional technology of pump jacks has become difficult. Shallow stripper well production is generally uneconomical for several reasons (Bennett, 2004) such as the following: The low oil output of stripper wells; High labor costs for normal pump-jack maintenance; Significant wear-and-tear pump-jack equipment; Low reliability and significant downtime for repairs of pump jacks; and The corrosive shallow wells environment destroys the equipment. In 1997, Energy, Inc., developed four generations of airlift pumps Gen 1, Gen 2, Gen 3, and Gen 3.1 which solved the problems facing traditional pump-jack equipment; namely, reliability and corrosion. Their design was safer, environmentally friendly, and required less maintenance in comparison to pump jacks, and potentially would allow thousands of old abandoned stripper wells to become economically feasible again because of low operating costs (Bennett, 2004). They tested four generations of airlift units on three oil lease sites (Stone, Lahr, and Wilson, Indiana state, U.S.A) from 1998 to 2004 and the results were compared with the results of the pump jacks previously installed on the aforementioned fields. The tested wells had similar characteristics and were outfitted with traditional pump-jack equipment. They collected maintenance and downtime logs recorded during the operation of the pump-jack, Gen 1, Gen 2, Gen 3, and Gen 3.1 pumping units. The results are summarized in Table 1, which gives the downtimes among the various pumping units. The results show that the airlift Gen 3.1 unit operates with reliability four times better and two and a half times in the required repair time than the standard pump-jack Multiphase Science and Technology

Review Study on Airlift Pumping Systems 351 TABLE 1: Comparison of the results for different airlift generations Pumping system Cumulative Downtime Days between Length of days tested (%) failures failure Pump jack 8465 6.9 90 6.2 Airlift Gen 1 1856 1.2 265 3.3 Airlift Gen 2 1472 4.9 113 5.5 Airlift Gen 3 1097 7.4 78 5.8 Airlift Gen 3.1 583 1.7 146 2.5 technology. High reliability and low maintenance costs make the airlift the natural and economical choice to replace the pump-jack technology in the stripper wells. In addition, the low required surface for installation and the environmentally friendly design of the ALS increase its popularity for well owners. Moreover, several airlift units could be run with one compressor, which saves in operational and capital costs. Also, using multiplestage units makes it practical to operate in deeper wells. They installed the six-stage airlift Gen 3.1 unit in the southern Illinois well at 1,600 feet depth in 2004. They concluded that the airlift pumping system is quite a practical technology that impacts the oil industry in several ways; namely, making stripper wells economical, making older wells open for use, and making wells environmentally friendly. Guet and Ooms (2006) studied the fluid mechanical aspects of the gas-lift technique especially in oil wells. They mentioned the reservoir and tubing pressure as two dominant factors in pressure drop occurring through the system operation. Instability generated by these systems also was considered in their study. Guet and Ooms (2006) proposed the system components that are important in the generation of gas-lift instability are the gas-lift valve, choke, volume of the annulus, and height of the tubing. Moreover, they believed that small perturbations could lead to extensive oscillations in the production flow. Guet and Ooms (2006) deduced that bottom pressure perturbations are sufficient enough to generate these instabilities. A major type of perturbation is the change in the void fraction due to the occurrence of hydrodynamic instabilities generated by void fraction waves or by the non-stationary character of certain flow regimes such as slug flow. Guet and Ooms (2006) also explained the whole process of developing these instabilities in the system and showed how the potential operating condition can be predicted by using some operation curves. Guet and Ooms (2006) allocated a part of their study to considering local flow phenomena such as the bubble relative velocity, void fraction radial profile, lift force coefficient as a function of the spherical equivalent bubble diameter, and flow pattern changes. Guet and Ooms (2006) presented Euler Euler modeling of vertical upward bubbly pipe flows as a method in which both the continuous and dispersed phases are considered as a continuum. Finally, they had some comments on how to optimize a gas-lift system, and stated that the idea of decreasing the bubble size could be taken into account to increase the gas-lift efficiency. Volume 24, Number 4, 2012

352 Hanafizadeh & Ghorbani Guerrero-Sarabia and Fairuzov (2013) considered linear and nonlinear analyses of flow instability in a gas-lift well. Their linear analysis was based on a modified gaslift stability criterion that is appropriate for saturated reservoirs. Guerrero-Sarabia and Fairuzov (2013) also used direct numerical integration to solve the governing equation and described the nonlinear dynamics and stability of the gas-lift system. The drift-flux theory was used for multiphase flow modeling and the results showed that the largest reduction in oil production takes place in the case of the most severe heading in the well. Guerrero-Sarabia and Fairuzov (2013) also described that an increase in the depth of the gas-injection port may result in heading and an increase in operating costs. Moreover, an increase in the separator pressure has a destabilizing effect in gas-lift systems. Salahshoor et al. (2013) considered the stabilization of gas-lift oil wells by a nonlinear model based on the neural network method. They recognized casing heading instability caused by dynamic interaction between the injection gas and multiphase fluid as the main source of the oscillations. Moreover, they captured the essential dynamics of casing heading instability in a nonlinear model structure by the neural network method. 4.2 Medical Applications Wicomb et al. (1985) used an ALS for perfusion storage of an isolated heart. At that time, there was an inadequate supply of donor hearts, resulting in high mortality in potential recipients awaiting transplantation (Cooper et al., 1982). Storage of donor hearts for periods of up to 24 48 h would enable transportation over long distances, and would therefore increase the donor supply available to any one center. 4.3 Description of the Preservation System 4.3.1 Apparatus A diagram of the preservation unit is shown in Fig. 11. The unit consists of two vertical chambers connected by a polyvinyl chloride pipe (T 2 ), leading from the lower chamber (C 1 ) to the upper chamber (C 2 ). The fluid capacity of C 2 is 250 ml and the fluid capacity of C 1 is 4 L. The total volume of fluid perfused in the system is 3 L. A filter of 20- µm pore size is inserted along the pipe to remove any particulate debris that may have accumulated in the perfusate. Compressed oxygen from a 4.5-kg gas cylinder, with a regulated flow rate, is supplied to the system at E. The heart is suspended in C 1, by the aorta, using heavy thread ligatures from an extension tube from C 2. An overflow pipe leads from C 2 into C 1, although this can be closed by a simple regulator mechanism (R), in which C 2 is open to the atmosphere via a breather port containing sterile cotton wool. With the regulator open, C 1 is also at atmospheric pressure. The preservation unit is maintained at a temperature below 10 C (but above 4 C) by placing it in a stainless steel, insulated box packed with ice. Storage of the heart below 3 C by this system had Multiphase Science and Technology

Review Study on Airlift Pumping Systems 353 been found to result in loss of viability. The entire device, including the preservation unit, ice, and box, weighs approximately 25 kg when fully loaded. 4.3.2 Energy Source Energy is provided to the system from a 4.5-kg pressurized oxygen cylinder containing 1.56-kg of liquid oxygen. The cylinder is fitted with a flow regulator that can regulate gas flow between rates of 0 and 1000 ml/min. Oxygen enters C 1 through a pipe (T 1 ) from the cylinder. 4.3.3 Fluid Gas Dynamics The design of the pumping device is based on the airlift principle used to lift water from wells. The direction of flow of gas and fluid is illustrated in Fig. 11. The flow through the vessels is determined by turning off the gas flow and timing the rate of fluid loss from the graduated C 2 chamber. A pressurized oxygen cylinder provides the driving force for the pumping mechanism. When the gas enters the system at E through T 1, the equilibrium between the gas and fluid in T 2 is destroyed because the density of the mixture of gas and fluid is less than that of the fluid alone; therefore, the fluid flows into T 2 from below, pushing the aerated column up to restore equilibrium in the system. The displaced fluid passes from C 1 via the filter to C 2. The fluid then perfuses through the heart at a flow rate ranging from 10 to 100 ml/min; the remaining fluid accumulates in C 2. Factors that influence the flow rate through the heart include the size of the organ, the coronary vascular resistance, and perfusion pressure and will be discussed subsequently. The perfusion pressure head to which the coronary arteries are FIG. 11: Portable hypothermic perfusion apparatus. Volume 24, Number 4, 2012