Induced pulsing in trickle beds — Particle shape and size effects on pulse characteristics

Periodic liquid feeding of the ON-OFF type is investigated — at sufficiently high frequencies to be classified as “fast” mode of induced pulsing — in the range of mean gas and liquid flow rates corresponding to the steady “trickling flow” regime. Two of the most common types of catalyst-support particles, i.e. porous spherical and cylindrical extrudates, are employed to study the imposed pulse characteristics. Detailed information is obtained, on the axial propagation and attenuation of pulses, from instantaneous, cross-sectionally averaged holdup measurements. Key fluid-mechanical parameters studied include, aside from dynamic holdup and pressure drop, pulse celerity and intensity, as a function of fluid feed rates (G,L) and liquid cyclic frequency. Similar published data, for 6 mm glass spheres, are employed for comparison; it is shown that, for the particles examined, particle size has a pronounced effect, but not as significant as that of particle shape. For particles of comparable size, the cylindrical shape is associated with much greater global dynamic holdup and pressure drop, and with increased pulse attenuation. Moreover, packed extrudates exhibit a significant increase of holdup in the axial direction, recently also observed in steady trickling flow. For spherical particles, both time-average holdup and pulse celerity are practically constant along the bed for fixed L,G. Pressure drop, global holdup and pulse celerity are not affected by cyclic liquid feeding frequency, for both spherical and cylindrical extrudate particles. Based on the pulse attenuation characteristics, for the three particle types examined, recommendations are made on preferred conditions for induced pulsing (from the fluid dynamics point of view) which would maximize benefits. Overall, it appears that spherical packings hold significant advantages over cylindrical extrudates of comparable size. Finally, in view of the observed significant decay of imposed pulses along the bed, care should be exercised to properly interpret data obtained in short laboratory reactors (where pulse attenuation is limited) for scale-up of the much longer industrial beds.     

A study of local liquid/solid mass transfer in packed beds under trickling and induced pulsing flow

Induced pulsing flow (by cyclic liquid feeding) in packed beds, operated in the trickling flow regime, is studied as a method of overall improvement of catalytic reactor operation. In this paper results are reported of experiments aimed at determining local and global liquid/solid mass transfer rates, mainly for the so-called fast mode of ON-OFF periodic liquid feeding, with frequencies of order 0.1 Hz. Such mass transfer data for the fast mode of induced pulsing are not available in the literature. Uniform 6 mm glass spheres and alumina cylindrical extrudates, of 1.5 mm diameter and a narrow distribution of lengths, are employed in the tests. For completeness, results are also reported for single-phase (liquid) and trickling flow through the same packed beds. A well-known electrochemical technique is employed to measure instantaneous local mass transfer coefficients by means of quite a few probes distributed throughout the bed. The hydrodynamic characteristics under the above conditions, reported in companion papers, are helpful in interpreting the new mass transfer data.There is a wide spread of the time-averaged local mass transfer rates, in all cases tested, apparently due to packing and flow non-uniformities. This spread is much smaller in the case of packed uniform spheres. In general, the benefits of cyclic liquid feeding are more evident in the packed bed of spheres than in that of cylindrical extrudates; for instance, with increasing mean liquid rate, induced pulsing tends to reduce the spread of local mass transfer coefficients, which suggests that more uniform fluids distribution is promoted. The imposed liquid pulses are reflected in the observed periodic variation of local mass transfer coefficients; the latter appear to decay along the bed in the same manner as the liquid pulses. Other trends of local mass transfer rates are identified and discussed in relation to measured variation of liquid holdup, under the same conditions. For packed spheres, the measured global mass transfer rates are in fair agreement with literature correlations obtained for the trickling flow regime, unlike the case of packed extrudates where significant deviation is observed.     

EFFECT OF VISCOSITY ON HYDRODYNAMIC PROPERTIES OF PULSING FLOW IN TRICKLE BEDS

New data on pulsing flow onset, properties of pulses (frequency, celerity, length), liquid holdup and pressure drop are presented for aqueous glycerol solutions of viscosity 6.7 and 20.2 mPa s and compared with similar measurements from an air-water (1.0 mPas) system. With the exception of viscosity, all other physical properties of the liquid phase are kept constant and fairly close to those of water, thus allowing a direct assessment of the effect of viscosity. Pulse formation and propagation with viscous liquids is examined on the basis of time records from a conductance type technique. A striking effect due to increased liquid viscosity is the reduction of the pulsing flow regime; in particular, the pulsing-to-bubbling transition boundary is shifted towards higher gas flow rates. Pulse frequency and celerity appear to decrease only slightly with increasing liquid viscosity, whereas the two-phase pressure gradient increases significantly. Liquid holdup also tends to increase with viscosity. Moreover, holdup with viscous liquids tends to increase significantly with the liquid flow rate, whereas an insignificant effect is found for water. A new correlation for estimating liquid holdup is proposed, and a simple model for predicting pulsing flow characteristics is modified in order to take account of the aforementioned effects.     

Onset of pulsing in gas-liquid trickle bed filtration

When liquid suspensions containing low concentration of fine solids are treated in catalytic packed bed gas-liquid-solid reactors, which are operated in trickle flow or near the transition between trickle and pulse flow, plugging develops and increases the resistance to two-phase flow. Also due to obstruction, such accumulation of fines in the catalyst bed shifts progressively the flow pattern from trickling to pulsing flow. The progressive onset of pulsing flow along the packed bed was estimated using a sequential approach based on combining a “large time-scale” unsteady-state filtration solution of two-phase flow with a “short time-scale” solution of a linear stability analysis of two-phase flow. Space-time evolution and two-phase flow of the deposition of fines in trickle bed reactors under trickle flow regime was described using a one-dimensional two-fluid model based on the volume-average mass and momentum balance equations and volume-average species balance equation for the fines. The model hypothesized that plugging occurred via deep-bed filtration and incorporated physical effects of porosity and effective specific surface area changes due to the capture of fines, inertial effects of phases, and coupling effects between the fines filter rate equation and the interfacial momentum exchange force terms. The transition between trickle flow and pulse flow regimes was described from a stability analysis of the solution of the transient two-fluid model around an equilibrium state of trickle flow under pseudo steady state conditions. The impact of liquid superficial velocity, viscosity and surface tension, gas superficial velocity and density, feed fines concentration, and fines diameter on the transition between trickle and pulse flows in the presence of fines deposition was analyzed.     

Structure of trickle-to-pulse flow regime transition and pulse dynamics at elevated temperature in slow-mode cyclic operation

The effects of temperature and pressure on the structure of the trickle-to-pulse flow regime transition in slow-mode cyclic operation in trickle-bed reactors were reported. The relationship between liquid holdup and liquid velocities at the trickle-to-pulse flow transition in cyclic operation, the shock wave behavior as a function of bed depth, as well as the pulsing flow regime properties were investigated for Newtonian (air-water) and non-Newtonian (air-0.25% carboxymethylcellulose (CMC)) liquids. At a given temperature, the breakthrough, plateau and decay times of the shock wave were found to decrease with bed depth. The pulse velocity and pulse frequency for pulsing flow regime both in cyclic operation and in natural pulsing (constant-throughput operation) were observed to increase with temperature. However, increasing the reactor pressure led to increased pulse frequency and decreased pulse velocity. Analysis of the transition liquid holdups for natural pulse flow and cyclic operation revealed that the liquid holdup decreased with temperature and pressure. The transition liquid holdups and superficial liquid pulse velocities in symmetric peak-base cyclic operation surpassed those in constant-throughput operation for given temperature, pressure and gas velocity, giving rise to wider trickle flow regime area in cyclic operation. The behavior of both Newtonian and power-law non-Newtonian fluids was similar regarding the effect of temperature, pressure and gas velocity.     

Multiple hydrodynamic states in trickle flow: Quantifying the extent of pressure drop, liquid holdup and gas-liquid mass transfer variation

It is well established that pressure drop and liquid holdup under trickle flow conditions are functions of the flow history. However, the extent of possible variation of these and other critical hydrodynamic parameters has not been fully quantified. In this study, specifically defined prewetting procedures are used as limiting cases for hydrodynamic hysteresis. These are:

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Non-prewetted.

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Levec prewetted: the bed is flooded and drained and after residual holdup stabilisation the gas and liquid flows are introduced.

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Kan_L prewetted: the bed is operated in the pulse flow regime (by increasing liquid velocity) after which liquid flow rate is reduced to the desired set point (all at the desired gas flow rate).

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Kan_G prewetted: the bed is operated in the pulse flow regime (by increasing gas velocity) after which gas flow rate is reduced to the desired set point (all at the desired liquid flow rate).

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Super prewetted: the bed is flooded and gas and liquid flows are introduced once draining commences.

It is shown that the upper limiting case for pressure drop is the Kan_L mode of operation. The lower limiting cases are the non-prewetted and Levec prewetted modes (these coincide). Pressure drop may vary by as much as 700% even for prewetted beds. Liquid holdup is different in all five prewetting modes. The upper limiting case is the Kan_G mode of operation, while the lower limiting case is the non-prewetted mode (Kan_G holdup is approximately 160% that of non-prewetted mode holdup at ). At low gas velocities the Kan_L holdup can be 400% of that of the non-prewetted beds. Importantly, the lower limiting case for prewetted beds is the Levec mode. Holdup in the Kan_G mode may be as much as 130% of the holdup in the Levec mode (at ).The effect of hydrodynamic multiplicity of the volumetric mass transfer coefficient is measured by the desorption of oxygen from water into nitrogen. In this case the different prewetting procedures result in three distinct regions, the upper region being the Kan and Super prewetted beds, the intermediate region being the Levec prewetted bed and the lower region being the dry bed. Mass transfer coefficients in the upper region can be as much as 600% of that of the lower region and 250% of that of the intermediate region. Evidently, prewetting (and even pulsing flow prewetting) does not guarantee that the bed is operating at the maximum values of pressure drop, holdup and mass transfer coefficient. Evidence of operation in between the limiting cases is presented. These non-limiting cases can be reached in multiple ways.     

滴流床反应器内脉冲流下动持液量实验

对气液强相互作用下滴流床反应器内的流体动力学进行了分析讨论 .实验测定了脉冲流流型下的床层平均动持液量 .考察了气液流率、液体黏度、填料材料等因素对动持液量的影响 .根据对滴流床反应器内流体流动机理的分析及实验结果 ,提出了关联脉冲流型式下动持液量的关联式 .该关联式能很好地关联实验数据 ,可用于预测温和型脉冲流下的动持液量     

An investigation of liquid maldistribution in trickle beds

The influence of liquid maldistribution at the top of the packing on flow characteristics in packed beds of gas and liquid cocurrent downflow (trickle beds) is experimentally investigated. Particular attention is paid to the effect of gas and liquid flow rates on flow development. Tests are made in the trickling and pulsing flow regimes. A uniform, a half-blocked and a quarter-blocked liquid distributor is tested. Packings of various sizes and shapes are employed. Data are presented on pressure drop and liquid holdup as well as trickling to pulsing flow transition. Diagnosis of radial and axial liquid distribution is made by means of conductance probes. The effects of liquid foaming, bed pre-wetting, top-bed material, and blockage midway the bed on liquid distribution are also examined. Overall, liquid waves in the pulsing flow regime have a beneficial effect, promoting uniform liquid distribution in the bed cross section.     

A study of particle shape and size effects on hydrodynamic parameters of trickle beds

Uniform-spherical and cylindrical-extrudate particles are employed to study air-water downflow in a packed bed of 14 cm i.d. The effect of particle shape, neglected in the literature so far, is shown to be very significant. A packed bed of extrudates displays significantly greater global dynamic liquid holdup h_d and pressure drop, as well as a trickling-to-pulsing transition boundary at higher gas flow rates, compared to beds of spheres of comparable size. Moreover, packed extrudates exhibit a significant increase of holdup, h_d, in the axial flow direction, a trend reported for the first time as there are no similar data available in the literature; on the contrary beds of spherical particles are characterized by practically constant h_d in the axial direction. Although an explanation for this h_d axial variation is not obvious, one might attribute it to the anisotropy and non-uniformity of interstitial voids of packed cylindrical particles. For beds of uniform spheres, in the diameter range examined (3-6 mm), the effect of size on both dynamic holdup and pressure drop, although quite pronounced, is not as significant as the effect of particle shape. An extensive survey of literature data, obtained with similar spherical particles, suggests that small bed diameters have an appreciable influence on trickling-to-pulsing transition boundary. Comparisons are reported with literature methods for predicting the measured parameters; discrepancies between data and predictions may be partly due to the inadequacy of a single “equivalent” diameter to represent both shape and size of non-spherical particles; predictive methods performing best are also identified.     

Hydrodynamics in a pilot‐scale cocurrent trickle‐bed reactor at low gas velocities

Hydrodynamic data obtained from laboratory‐scale trickle‐beds often fail to accurately represent industrial‐scale systems with high packing aspect ratios and column‐to‐particle diameter ratios. In this study, pressure drop, liquid holdup, and flow regime transition were investigated in a pilot‐scale trickle‐bed column of 33 cm ID and 2.45 m bed height packed with 1.6 mm × 8.4 ± 1.4 mm cylindrical extrudates for air‐water mass superficial velocities of 0.0023 – 0.094 kg/m²s and 4.5 – 45 kg/m²s, respectively, at atmospheric pressure. Significant deviation was observed from pressure drop and liquid holdup correlations at low liquid flows rates, corresponding to gravity‐driven flow limit. Likewise, liquid saturation is overestimated by correlations at high liquid flow rates, owing to significantly reduced wall effects. Lastly, trickle‐to‐dispersed bubble flow and trickle‐to‐pulsing flow regime transitions are reported using a combination of visual observations and analysis of the magnitude of local pressure fluctuations within the column. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2560–2569, 2018     

Influence of temperature on fast-mode cyclic operation hydrodynamics in trickle-bed reactors

Despite the merits of periodic operation praised in the academic literature as one of the process intensification strategies advocated for trickle-bed reactors (TBRs), there is still reluctance to implement it in industrial practice. This can partly be ascribed to the lack of engineering data relevant to the elevated temperature and pressure characterizing industrial processes. Currently, the hydrodynamics of trickle beds under cyclic operation, especially in fast mode at elevated temperature and pressure, remains by and large terra incognita. This study proposes exploration of the hydrodynamic behavior of TBRs experiencing fast liquid flow modulation at elevated temperature and moderate pressure. The effect of temperature and pressure on the liquid holdup and pressure drop time series in terms of pulse breakthrough and decay times, pulse intensity and pulse velocity was examined for a wide range of superficial gas and liquid (base and pulse) velocities for the air-water system. The pulse breakthrough and decay times decreased, whereas the pulse velocity increased with temperature and/or pressure. The pressure drop was attenuated with increasing temperature for a given superficial gas, and base and pulse superficial liquid velocities. Experimental pulse velocity values were compared to the Giakoumakis et al. [2005. Induced pulsing in trickle beds—characteristics and attenuation of pulses. Chemical Engineering Science 60, 5183-5197] correlation which revealed that it could be relied upon at elevated temperature and close to atmospheric pressure.     

Modulation of liquid holdup along a trickle bed reactor with periodic operation

Temporal variations of the liquid holdup in a mini-pilot scale trickle bed reactor cold-mockup, induced by an ON-OFF liquid flow modulation strategy of operation, are explored at different axial positions. The reactor is packed with porous beads of γ-Al₂O₃ and the liquid holdup is approximately estimated with a conductimetric technique, using probes that mimic the packing. The effects of the liquid and gas superficial velocities, the bed depth and the cycling parameters, cycle period and split, on the liquid holdup modulation are examined for a wide range of conditions. For slow and intermediate cycle periods, the liquid holdup time dependence observed during the dry period is represented by an exponential function. The characteristic value of the decay is correlated to the examined variables. The correlation allows reconstruction of the liquid holdup time dependence along the column.     

Impact of gravity on the bubble‐to‐pulse transition in packed beds

A model based on two‐phase volume‐averaged equations of motion is proposed to examine the gravity dependence of the bubble‐to‐pulse transition in gas‐liquid cocurrent down‐flow through packed beds. As input, the model uses experimental correlations for the frictional pressure drop under both normal gravity conditions and in the limit of vanishing gravity, as well as correlations for the liquid‐gas interfacial area per unit volume of bed in normal gravity. In accordance with experimental observations, the model shows that, for a given liquid flow, the transition to the pulse regime occurs at lower gas‐flow rates as the gravity level or the Bond number is decreased. Predicted transition boundaries agree reasonably well with observations under both reduced and normal gravity. The model also predicts a decrease in frictional pressure drop and an increase in total liquid holdup with decreasing gravity levels. © 2013 American Institute of Chemical Engineers AIChE J 60: 778–793, 2014     

滴流床反应器内脉冲流宏观流体力学的特性参数

引　言在石油化工工业的加氢处理中 ,滴流床反应器常操作在接近脉冲流流型区域[1] .脉冲流下气液流速都比较高 ,所以该操作方式适合催化剂活性高、反应速率快的反应[2 ] ,而且气液流量大有利于强放热反应的反应热从反应器移出。流体力学参数如脉冲速度、脉冲频率、持液量、气液分布、压力降等对于该类反应器的工业设计及操作具有很重要的意义 .Ｓａｔｏ等[3] 较早地对脉冲流的特性进行过定性的实验研究 .Ｂｌｏｋ等[4 ] 以及Ｔｓｏｃｈａｔｚｉｄｉｓ等[5] 通过电导法也对脉冲流宏观特性进行过研究 .本文用不同的实验方法对滴流床反应器内脉…     

Trickle Flow Multiplicity: The Influence of the Prewetting Procedure on Flow Hysteresis

The existence of multiple hydrodynamic studies (MHS) in trickle flow is a well-known phenomenon. It is also known that different prewetting procedures result in major differences in MHS when the hydrodynamic variables pressure drop, liquid holdup and gas–liquid mass transfer are considered. Given a certain prewetting procedure one still has the option to perform flow hysteresis cycles to achieve an even wider variety of MHS. Although numerous studies have been performed on trickle flow hysteresis, none have attempted to decouple the hysteresis behaviour from the prewetting procedure followed. Accordingly there are numerous hysteresis possibilities that have not been investigated. In this work a single liquid and gas cycle were performed for four distinct prewetting procedures described here as a dry bed, a Levec type prewetted bed, Kan prewetted bed (achieved by increasing either the liquid or the gas flow rate until the pulsing flow regime is reached) and a Super prewetted bed. Pressure drop, liquid holdup and gas–liquid mass transfer are the hydrodynamic parameters studied to quantify the various MHS. It is shown that the shape and extent of the hysteresis cycle are strongly dependant on the prewetting procedure. In terms of flow structure, similar hysteresis trends on the Kan Liquid and Super prewetting modes indicate that these modes are hydrodynamically similar. The additional measurement of the hysteresis behaviour of gas–liquid mass transfer proofs that neither holdup nor pressure drop can be used as an indicator of the distribution uniformity.     

Residence time distribution and liquid holdup in Kenics KMX static mixer with hydrogenated nitrile butadiene rubber solution and hydrogen gas system

A Kenics^® KMX static mixer that has curved-open blade internal structure was investigated to study its hydrodynamic performance related to residence time distribution and liquid holdup in a gas/liquid system. The static mixer reactor had 24 mixing elements arranged in line along the length of the reactor such that the angle between two neighboring elements is 90°. The length of the reactor was 0.98 m with an internal diameter of 3.8 cm and was operated cocurrently with vertical upflow. The fluids used were hydrogen (gas phase), monochlorobenzene (liquid phase) and hydrogenated nitrile butadiene rubber solution (liquid phase). In all the experiments, the polymer solution was maintained as a continuous phase while hydrogen gas was in the dispersed phase. All experiments were conducted in the laminar flow regime with the liquid side hydraulic Reynolds number in the range of 0.04-0.36 and the gas side hydraulic Reynolds number in the range of 3-18. Different polymer concentrations and different operating conditions with respect to gas/liquid flow rates were used to study the corresponding effects on the hydrodynamic parameters such as Peclet number (Pe) and the liquid holdup (ε_L). Empirical correlations were obtained for the axial dispersion coefficient (D_a) and liquid holdup in liquid system alone and for the gas/liquid system separately. It was observed that the Peclet number decreased with the introduction of gas in to the reactor while in the liquid system alone, an increase in viscosity decreased the Peclet number. The liquid holdup was empirically correlated as a function of the physical properties of the fluids used in addition to the operating flow rates.     

滴流床反应器的脉冲特性

滴流床中气液流速较高时会产生脉冲流。脉冲特性可表示为：脉冲频率、脉冲速度、脉冲持液与脉冲间持液。这些参数可用气液流速、填料特性及气液物性来度量。在宏观物料衡算基础上推得脉冲速度的模型。脉冲持液就是反应器的动持液；脉冲持液与脉冲间持液之比近于1．5。发现脉冲频率完全受表观液速与临界液速的差值所控制。获得了三个关联式，涉及不同系统，也包含文献数据。     

滴流床反应器持液量的滞后现象

通过采用电容层析成像仪测定了不同气速、不同液速、不同操作方式以及不同床层位置的持液量滞后曲线,发现影响持液量滞后的最大因素是液体流速。在实验的基础上,分析了不同操作状态下产生滞后现象的原因,并提出了采用持液量滞后曲线终点来判断脉冲点的关联式：Re^＊LReG^＊0.42042=800。     

Trickle bed hydrodynamics and flow regime transition at elevated temperature for a Newtonian and a non-Newtonian liquid

Despite the hydrodynamics of trickle beds experiencing high pressures has become largely documented in the recent literature, trickle bed hydrodynamic behavior at elevated temperatures, on the contrary, largely remains terra incognita. This study's aim was to demonstrate experimentally the temperature shift of trickle-to-pulse flow regime transition, pulse velocity, two-phase pressure drop, liquid holdup and liquid axial dispersion coefficient. These parameters were determined for Newtonian (air-water) and non-Newtonian (air-0.25% Carboxymethylcellulose (CMC)) liquids, and the various experimental results were compared to available literature models and correlations for confrontation and recommendations. The trickle-to-pulse flow transition boundary shifted towards higher gas and liquid superficial velocities with increasingly temperatures, aligning with the findings on pressure effects which likewise were confirmed to broaden the trickle flow domain. The Larachi-Charpentier-Favier diagram [Larachi et al., 1993, The Canadian Journal of Chemical Engineering 71, 319-321] provided good predictions of the transition locus at elevated temperature for Newtonian liquids. Conversely, everything else being kept identical, increasingly temperatures occasioned a decrease in both two-phase pressure drop and liquid holdup; whereas pulse velocity was observed to increase with temperature. The Iliuta and Larachi slit model for non-Newtonian fluids [Iliuta and Larachi, 2002, Chemical Engineering Science 46, 1233-1246] predicted with very good accuracy both the pressure drops and the liquid holdups regardless of pressure and temperature without requiring any adjustable parameter. The Burghardt et al. [2004, Industrial and Engineering Chemistry Research 43, 4511-4521] pulse velocity correlation can be recommended for preliminary engineering calculations of pulse velocity at elevated temperature, pressure, Newtonian and non-Newtonian liquids. The liquid axial dispersion coefficient (D_ax) extracted from the axial dispersion RTD model revealed that temperatures did not affect in a substantial manner this parameter. Both Newtonian and power-law non-Newtonian fluids behaved qualitatively similarly regarding the effect of temperature.     

Rationalising MRI, conductance and pressure drop measurements of the trickle-to-pulse transition in trickle beds

Recent MRI data have shown that the transition from trickle to pulsing flow in trickle-bed reactors occurs over a range of liquid velocities at constant gas velocity. The transition is initiated by isolated local pulsing events, which increase in number with increase in liquid velocity until a maximum number exists. Above this liquid velocity, which we have termed the transition point, the individual pulses merge until a single macro-scale pulse is formed and the whole bed demonstrates pulsing flow. In this paper we compare the characterisation of the transition obtained using conductance and pressure drop measurements with that obtained using MRI. Using the insights gained from the 3-D MRI measurements, recorded with a data acquisition time of 280 ms, it is shown that the conductance and pressure drop measurements are sensitive to different stages of the evolution of the hydrodynamic transition, a factor important when using these different measurements in the development and validation of numerical and theoretical models. Conductance measurements identify unambiguously only the onset of the single macro-scale pulse regime, consistent with a determination of the transition point made by visual observation. In contrast, pressure drop measurements are sensitive to both the onset of formation of local pulses and the liquid velocity at which the maximum number of liquid pulses occurs. We also show how a combination of conductance and pressure drop measurements can be used to fully characterise the transition, thereby enabling translation of the insights gained by MRI into a robust measurement strategy for use on larger-scale reactors. Data are reported for a cylindrical column of length 70 cm and inner diameter 43 mm, packed with cylindrical porous γ-Al₂O₃ packing elements of length and diameter 3 mm. The bed was operated under conditions of co-current downflow of air and water, at ambient temperature and a pressure of 2 barg. Gas and liquid superficial velocities were in the range 25-300 and 0.9-, respectively.