Hydrodynamic Characteristics Effect of Foam Control in a Three-Phase Fluidized Bed Column

The present work was devoted to study the effect of operating parameters (e.g., superficial gas and liquid velocities, size of solid particles, volume percentage of particles loaded in column and type of particles) on foams, and to investigate the process of foam suppression. The experimental apparatus was operated in continuous mode for the two phases (i.e. air and a solution of aqueous anionic surfactant). Sands which were considered as hydrophilic behavior particles were used as solid phase. A specific surface treatment was performed on hydrophilic sands particles to transfer it into hydrophobic one. These two versions of sands were used in the experimental setup respectively to study their effect on suppression of foams. The average gas holdup for the entire column was measured by means of the local gas holdup which was computed from the pressure difference in each segment of the column. Local solid concentration along the column was measured experimentally by analyzing the samples of mixture drawn from the sampling ports. From the present Journal of Petroleum Research & Studies E 159 work it was found that: Flow regimes of multiphase system could be easily determined by utilizing local gas holdup profile measured by pressure drop transducer method. The transition from the homogenous to heterogeneous regime was advanced, from ug=4 to 2cm/s with increasing solid concentration from 10 to 20%v and decreasing average particle diameter from dp=(0.8) to (0.25) mm. When a mixture of water/surfactant was employed in the bubble column, foam could be present depending on the input operating parameters. The fluid mechanism of foam suppression with hydrophilic particles was enhanced by a direct attack on the foam by hydrophobic particles (i.e. hydrophobic particles were more effective in retaining liquid–destroying foam than the hydrophilic particles). It was found that hydrophobic particles of 0.25 mm average diameter and 10%v loading in the reactor could reduce foaminess fraction from 0.85 to 0.15 if the liquid velocity was 0.3 cm/s. The foaminess fraction could be reduced to 0.0 if the liquid velocity increased to 0.4 cm/s. The results of this study may have abroad applications in petroleum and petrochemical industries where liquid hydrocarbons are processed.


Introduction
Foams and foaming pose important questions and problems for the chemical industry in general. Foam can be desirable such as in bioreactors where it acts as a cushion preventing bursting bubbles from damaging the cells at the liquid surface [24]. In the oil industry, foams are used in under-balanced drilling, for reservoir clean-up and for enhanced oil recovery in porous sand [3].

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On the other hand, excessive foaming might create serious problems in many industrial processes. Foam can reduce throughput and separation performance or can even cause contamination of products due to takeover of foam from other vessels [15] [28]. In hydrocracking and other foaming reactors, the foam rises to the top because it has a higher gas fraction than the bubbly mixture from which it comes. The high gas hold-up in foams is undesirable in chemical reactors because it strongly decreases the liquid residence time and in hydrocracking reactors also promotes the formation of coke [10]. The hydrodynamics in a gas-liquid or gas-liquid-solid reactor are characterized by different flow regimes, namely, the homogeneous, transition, and heterogeneous regimes, mainly depending on the superficial gas velocity. The homogeneous regime exists at low superficial gas velocities and changes to the heterogeneous regime with an increase in the superficial gas velocity.
industrial interest for gas-liquid-solid processes is in the heterogeneous flow regime [21] [17]. The hydrodynamics, heat and mass transfer, and mixing behavior are quite different in different regimes [12].
It is reported that the basic factors affecting gas holdup are: superficial gas velocity, liquid properties, column dimensions, operating temperature and pressure, gas distributor design, and solid phase properties [25]. They found that the spatial variation of gas holdup is another important factor which gives rise to pressure

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variation and thus liquid recirculation. Since liquid recirculation plays an important role in mixing and heat and mass transfer. The superficial gas velocity is the dominant factor that influences gas holdup. With increasing superficial gas velocity, gas holdup increases, less pronounced in the heterogeneous regime than in the homogeneous regime, and numerous experimental studies reported these findings, it was found that increasing the liquid velocity significantly increased volumetric mass transfer coefficient but only slightly increased the gas holdup [10][11][12][13][14][15][16][17]. A slight increase in gas holdup with increasing superficial liquid velocity was reported by [18,15]. It was found that the influence of the liquid velocity on gas holdup became more pronounced at high pressures [14].
The effect of solid concentration on gas holdup has been investigated by a number of researchers [19-23] who concluded that an increase in solids concentration generally reduced the gas holdup.
Also It was reported that for low solids loading (<5 vol. %), the behavior of the slurry bubble column is close to that of a solid free bubble column [21].
The effect of surface-active agents on the phase holdups of a gas-liquid bubble column and three-phase fluidized bed (with glass beads of 1.2 and 5.0 mm) was studied by [24]. The results showed that the presence of surface-active agents increased the gas holdup in a bubble column by an average of 41% and increased the gas holdup in

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a three-phase fluidized bed by an average of 37%. The presence of electrolyt or impurities was also found to increase the gas holdup [25].
Foam control agents are usually cracked under the severe condition present in the reactors. For example, in hydrocracking reactor, anti-foam agents are exposed to hydrogen pressures over (100 bar) and temperature of    C 140 or higher. Anti-foaming agents tend to crack into different chemical products which contaminate the liquid and gas in the reactor, such cracking of anti-foaming agents also tends to increase the operating costs of the overall process [24]. Prior literature [2,26] on the use of particles to destroy foam described effects of hydrophobic particles which attack the foam. Adhesion of air bubbles to Teflon-coated glass beads were observed to be fluidized in water [1]. The phenomenon of bubble adhesion to the non-wettable particle leads to a decrease in the apparent density of the particle, which in turn is responsible for a larger bed expansion and smaller gas holdup compared with wettable particle systems. Non-wettable particles can be used to thin or reduce foam layers, and consequently reduce foam formation [7]. The characteristics of water-air-solid fluidization with non-wettable (hydrophobic) particles and were studied and classified of the flow pattern according to the motion of at the bottom of the column through a (25 m) head centrifugal pump (stream ® ) and adjusted by a needle valve and calibrated flow meter.
The foaming system is selected to give a maximum foaminess in the operating column. To specify desired foaming system; different types of alcoholic aqueous solution were tested using the shaking test bottle [2]. Light gas oil, heavy gas oil (VGO), tetra butanol, propanol, and glycerin were prepared in different concentrations with water., Although the foaming system which gave maximum foaming (i.e., Al-Doura refinery Al-Doura refinery According to [5], the particle size and density of the particles to be added are preferably selected so as to provide a minimum fluidization velocity ( mf U ) which is less than the desired superficial velocity ( l V ), and to provide particle settling velocity in the liquid phase ( t U ) which is greater than the superficial liquid velocity [10]. Thus, the particle size and density are preferably selected so as to This advantageseously serves to cause the particles to expand the fluidized bed in bubbly liquid mixture below the foam. In the present work, sands which happen to be hydrophilic were used as solid phase.
The properties of sand particles are shown in Table (2). In the present work gas and liquid superficial velocities were specified according to   The foaminess is calculated as follows [2]:

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production from sparger, a higher number of small bubbles per unit volume is existed in the lower section of the column which means higher gas holdup.
The effect of solid particles on stability of the bubbly flow regime is shown in fig. (3-7), the inception of the transition region in fig. (4-7) start at lower gas velocity compared to the system shown in fig. (3). This early inception is attributed to bubble coalescence    The experimental results are presented in fig. (14), show this time for 10% volume fraction of the hydrophilic and hydrophobic sands with a mean size of (700-900) ) . Again, we observe a better m  In fig. (18), the foam fraction as a function of the gas velocity at a liquid velocity of 0.152 cm/s are compared for two different volume fractions (10% and 20%) of the hydrophobic sand with a mean size of (700-900) µm. As expected, the foam suppression was favored by the presence of the highest solid fraction, because rate of foam destruction

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is proportional to the density of hydrophobic particles/ liquid suspension. Fig. (19-a, b, c) and (20-a, b, c) show , the concentration profile of (200-300) μm for 10% solid loading hydrophilic and hydrophobic particles respectively. These smaller particles of fig. (19) are 2. The transition from the homogenous to heterogeneous regime is advanced with increasing solid concentration and decreasing particle diameter.
3. Foam appears above a bubble mixture when the superficial gas velocity is greater than a critical value. At any fixed gas velocity g U , foam may be eliminated with increasing of l U .