ArticleThe effect of multi-orifice plate configuration on bubble detachment volume☆
Introduction
Bubble column is a kind of efficient multiphase contactor and reactor which has been applied in many industries such as chemical, coal liquefaction, metallurgy and waste water treatment [1], [2]. The work performance of bubble column has a close relation with bubble size distribution (BSD). Small bubbles can provide higher interface area concentration which is beneficial to mass, momentum and energy transfer between phases [3]. Besides the terminal rise velocity decreases as bubble size decreases [4]. According to Besagni, when bubble size is large, the lift coefficient is negative and bubbles move toward the center of bubble column, so in large bubble case the homogeneous flow regime is destabilized [5]. Therefore the performance of bubble column can be improved effectively by reducing bubble size.
Due to the significant influence of BSD on the work performance of bubble column, many researches have paid attention on this issue. It has been proved that BSD is affected by many factors including bubble formation, bubble coalescence and break-up rate, mass transfer and gas–liquid physical property [6], [7], [8]. A number of expressions have been proposed to calculate BSD based on the physical property of gas/liquid system, gas superficial velocity and bubble column diameter [9], [10]. According to population balance model which is an effective method of analyzing BSD [11], [12], [13], BSD is depends on the balance between the coalescence rate and the breakup rate under most conditions. Guédon and Besagni concluded that the prevailing regime can be distinguished in the homogeneous flow regime and heterogeneous flow regime. The homogeneous flow regime is further distinguished into “pure-homogeneous” flow regime and “pseudo-homogeneous” flow regime. In the homogeneous flow regime associated with low superficial gas velocity, it is agreed generally that there is no coalescence-induced bubble [5], [14]. Besides, it has been reported that bubble coalescence rate decreases dramatically and even to zero in inorganic solution [15], [16]. Under non-coalescence condition, the BSD right behind the gas sparger can represent for the BSD in the entire bubble column reactor [17]. The aeration performance of gas sparger plays a crucial role on BSD. Additionally, gas sparger has influence on other character parameters of bubble column. Many researchers have studied the relationship between gas sparger and the transition gas flow rate which separates the homogenous regime and the heterogeneous regime [18], [19]. Wilkison studied the column design for engineering application. He concluded that the gas sparger with small diameter orifice in laboratory can result in a higher gas hold up compared to the one with large orifice diameter in industry. So the effect of gas sparger on gas hold up is non-ignorable [20].
Due to the importance of gas sparger, many researchers have paid attention to it. In practical application, multi-orifice plate is a kind of widely used sparger due to the advantages in both design and operation: simplicity of construction, lack of mechanically operated parts, low energy input requirements, etc. The work principle of multi-orifice plate is to homogenize gas flow into each active bubble orifice and reduce the average gas velocity through orifice which leads to the decrease of bubble detachment volume (BDV). For the same gas flow rate, the average BDV decreases with the number of active bubble orifices. However, during bubble formation, the bubbles generated from two adjacent orifices can coalesce with each other. The coalescence efficiency of bubbles at adjacent orifices increases with decreasing the pitch of orifices [21]. Therefore it is crucial to determine the suitable number of open orifices. When the number of open orifices is significantly small, the gas flow rate through each orifice is too high which leads to a large BDV, but when the number of open orifices is much larger than the number of active bubble orifices (NABO), the pitch of orifices is so close that bubble coalescence at adjacent orifices occurs more frequently.
For the single-orifice gas sparger, BDV can be predicted accurately under different conditions refer to orifice diameter, gas chamber volume, gas flow rate and liquid physical property [22], [23], [24], [25]. However, the bubble formation behavior of multi-orifice plate is obviously distinctive from the one of single-orifice spargers. CFD simulation is usually regarded as a kind of effective method to study the multi-orifice plate. Guan proposed a new method to establish the relationship between chord length distribution and bubble size distribution [26]. The experimental results of Guan provided a benchmark data for validation of CFD simulation. Dhotre made a significant attempt to simulate the flow pattern on the upstream and downstream of the distributor. He analyzed the effects of opening area and hole diameter on the flow patterns of bubble column and gave a design guidance of distributor based on bubble column diameter and height, superficial gas velocity and liquid velocity [27]. Shi proposed a new inlet model for CFD simulation which is able to achieve a good balance between simulation accuracy and computational cost [28]. Bahadori investigated 2- and 3-D simulations and analyzed the relation of gas hold up and the number of open orifice [29]. In addition, the bubble characteristic of multi-orifice plate has been studied by experimental methods [30] and some regressive calculation expressions were proposed for predicting the Sauter diameter generated by multi-orifice plate [31], [32]. As far as the authors can see, there is few mechanism analysis for the relation of BDV and multi-orifice plate configuration. According to the theoretical model proposed by Loimer et al., the minimum gas volume flux above which NABO increases is obtained according to orifice diameter, gas density and liquid surface tension [33]. However the NABO predicted by Loimer model is proved to be not absolutely applicable in this work.
Apart from multi-orifice plate configuration, the gas chamber condition also has a significant influence on BDV. According to Hughes, the bubble formation type is classified according to the capacitance number defined by Eq. (1), Bubble formation is under constant flow condition for Nc < 1 and under constant pressure conditions for Nc > 10 [34]. Gaddis proposed an accurate calculation for bubble detachment volume under constant flow condition. The application range of calculation can be up to transition to the jetting regime [35]. Under constant pressure condition, Park considered that the gas chamber volume can be divided into three levels and proposed different BDV calculation expressions [22]. Weeping is a kind of disadvantage phenomenon for aeration which occurs under some specific conditions. The volume of gas space in gas chamber is reduced by weeping phenomenon [35].
In this work, the influences of gas chamber condition and gas chamber volume on BDV are studied experimentally with multi-orifice plate sparger. A set of experiments have been conducted to study the relation between multi-orifice plate configuration and the average BDV. A theoretical model is improved to analyze bubble detachment volume of multi-orifice plate. In addition, the effect mechanism of gas flow rate and sparger configuration are discussed and a design criteria for the number of open orifice is proposed.
Section snippets
Experimental facility
The schematic of test facility is shown in Fig. 1. This test facility is designed to conduct visual experiment. The main test section located on an aluminum alloy shelf is a cuboid pool which is 1 m tall and has a rectangular cross section (200 mm × 150 mm) which can protect bubble from wall disturbance. Stalinite, a kind of hard and transparent glass, is selected to be made as the visual faces and other two side faces are made of stainless steel. In Fig. 1, the portion marked by blue color is
Theoretical Model
The model of Wenxing Zhang has been proved to be accurate to predict the bubble formation at single-orifice plate [37], but this model should be modified in order to be applied for multi-orifice plate. The improved theoretical model in this work employs a more suitable method to calculate the gas flow resistance through orifice and improves the force balance acting on bubble. In addition, the variational number of active bubble orifice can be simulated in the improved model. Fig. 7 displays the
Experimental results
In Fig. 9, the multi-orifice plate is 2 mm orifice diameter, 6 mm pitch of orifices and equipped on the 3# gas chamber. There are two gas chamber conditions in Fig. 9, Fig. 10, Fig. 11 which are called “chamber filled with water” and “chamber without water” respectively. The chamber condition is related with weeping phenomenon. “Chamber without water” and “chamber filled with water” represent the beginning and end of weeping respectively. In order to analyze the separate influence of gas
Conclusions
This work pays attention on a specific type of gas sparger which is composed of a multi-orifice plate and a large gas chamber. The relation between sparger construction and aeration performance is studied by experimental measurements and theoretical analysis. In order to characteristic the aeration performance of multi-orifice plate, a new parameter, EBT, is proposed and verified by the image processing method.
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The TPNABO of high NABO becomes smaller at the end of weeping, because the
Nomenclature
- a
Bubble radius, m
- asc
Projected radius of spherical cap bubble, m
- b
Multi-orifice plate thickness, m
- C
Circularity degree of bubble projection
- Cc
Shrinkage coefficient of section
- CD
Drag force coefficient
- Cg
Orifice coefficient for gas flow
- FB
Buoyancy force, N
- Fi
Inertial force, N
- Fd′
Overall drag force, N
- Fσ
Surface tension force, N
- g
Acceleration due to gravity, m·s−2
- h
Distance from lower surface of bubble to orifice, m
- k
The number of active bubble orifice in experimental results
- kb
Resistant coefficient
- M
Molecular
Subscripts
- f
The following bubble
- g
Gas
- L
Liquid
- P
The previous bubble
- sc
Spherical-cap bubble
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Supported by the Fundamental Research Funds for the Central Universities (HEUCFM181203).