Population balance modeling of bubble size distributions in a direct-contact evaporator using a sparger model

The study of bubble size distributions in direct-contact evaporators was addressed both theoretically and experimentally. Recently developed models for calculating bubble coalescence and breakage frequencies in isothermal bubble columns were adapted to the population balance equation using the bubble mass as the internal coordinate which was discretized using an expansion of the number density function by impulse functions. A sparger model was developed based on experimental data for a non-coalescing system and using bubble formation models for isothermal and non-isothermal conditions. Bubble size distributions in a direct-contact evaporator operating in the quasi-steady-state regime for four different gas superficial velocities, including the homogeneous and heterogeneous regimes, together with the sparger model, were used for estimating the three empirical parameters from the population balance model, which were observed to be functions of the gas superficial velocity. In all cases considered, the population balance model fitted the experimental data rather well and the regressed parameters exhibit the physically expected behavior with changes in the gas superficial velocity.     

Thermal destratification of cryogenic liquid storage tanks by continuous bubbling of gases

This paper focuses on thermal destratification and pressurisation inside thermally stratified storage tanks by continuous gas bubbling. The primary purpose of doing these studies is to better understand the effect of bubble dynamics on thermal destratification and quantify the extent of destratification. The volume of fluid and interface compression method of OpenFOAM CFD code is utilised for the present analysis. Different values of inlet gas velocities (V_g), orifice diameters (d_o), and arrangement of the orifices in triangular and square fashion with different pitches (p/d_o) are considered. In addition, the effect of gravitational forces (g/g_e) on thermal destratification is also reported. For all these cases, the effectiveness of thermal destratification is quantified in terms of a newly defined parameter, the destratification index (I_d). For V_g = 1 m/s, the I_d value is maximum compared to lower V_g values. It is seen that when the gas velocity increased from 0.3 m/s to 1.0 m/s, the average effectiveness in thermal destratification (I_davg) and pressure at the ullage increased by 44.38% and by 64.81%, respectively. The I_davg and pressure at ullage increased by 96.29% and 14.91%, respectively, when the g/g_e ratio changed from 0.3 to 3. Compared to the triangular arrangement with p/d_o = 10, the calculated I_davg increased by 30.67% when gas inlets were arranged with a square pitch of 10. For p/d_o = 4, 6 and 8, the increments in I_davg are of the order of 12.86%, 19.43% and 21.92%, respectively, for gas inlets arranged in a square fashion as compared to the triangular arrangement. It is found that continuous bubbling with gas inlets arranged in square pitch p/d_o = 10 gives higher effectiveness in thermal destratification. Thus, by these studies, one can develop a thermal destratification mechanism with continuous bubbling for optimum performance. Also, these studies give an overall idea of sparger design for getting the correct gas flow rate for thermal destratification within the cryogenic liquid storage tanks.     

饱和蒸汽在过冷液面凝结特性的实验研究

实验研究了堆芯补水箱上部加装遮流板时，饱和蒸汽在过冷液面上直接接触冷凝的瞬态特性。研究表明，遮流板改变了蒸汽射流的方向，减弱了近液面层的波动，降低了蒸汽的凝结速率，使堆芯补水箱中热水层的厚度减小，系统压力响应时间缩短：与无遮流板相比，实验初期水表面凝结换热系数降低76％～92％，总凝结量减少67％以上。     

Effects of sparger geometry on the mechanism of flow pattern transition in a bubble column

Electrical resistance tomography (ERT) is used to measure void fraction wave characteristics and to identify flow pattern in a bubble column reactor (0.24 m diameter, 2.75 m height). The effects of column pressure and superficial gas velocities for different sparger geometry and for different flow pattern have been investigated. The ERT sensor can distinguish the void fraction disturbances in different flow regimes with a good clarity. The holdup derived from ERT is in good agreement with the hold-up values measured by pressure transmitters. Different flow regimes have been identified based on void fraction properties and wall pressure fluctuations. The spectral analysis of ERT measurements yields quantitative information, such as a characteristic time and a characteristic frequency of void fraction waves, which are closely related to flow structure in the prevailing regime. The experimental observations are compared with the literature.     

Design of a Pipe/Ring Type of Sparger for a Bubble Column Reactor

The sparger is an important accessory of bubble column reactors which governs the performance of the reactor. Specifically the sparger design becomes more important when the aspect ratio of the bubble column is low. The maldistribution and design of the sparger are of major concern and both these aspects are described in detail in the present work. Various methods for reducing maldistribution have been discussed and a simple method for its reduction is presented experimentally. Further a step‐wise design procedure for a pipe/ring type of sparger for bubble columns is presented together with a work example.     

Dimensional similitude for scale-up of hydrodynamics in a gas-liquid-(solid) EL-ALR

This study tests the scaling approach for gas-liquid-(solid) external loop airlift reactor (EL-ALR) hydrodynamics based upon geometric and dynamic similitude with a limited number (6) of dimensionless groups. A sixth dimensionless group for sparger characteristic (Scp) is developed. Scp is a convenient parameter to characterize fluid flow performance of EL-ALR and a large cold-flow 0.86 m diameter column. To our knowledge, there is more ambiguity concerning its definition for EL-ALR and a large cold-flow 0.86 m diameter column especially when different sparger structures are used. In this study, reliability of the modified Reynolds and Scp are proposed for such an EL-ALR and a large cold-flow 0.86 m diameter column used different sparger structures. The Scp is based on the pitch and angle between hole normal vector and horizontal line as the characteristic parameters. Experiments were carried out in two systems in which all six dimensionless groups were matched: a 55 wt% aqueous glycerol solution with ceramic catalyst particles in an industrially operated large cold-flow 0.86 m diameter column (system 1) and silicone oil with cylindrical aluminum particles in Taishan Scholar Lab (TSL) EL-ALR with a riser (0.47 m diameter and 2.5 m height) and two downcomers (0.08 m diameter and 2.5 m height) (system 2) with air as the gas in both cases. The micro-conductivity probe and the 3D Laser Doppler Anemometer (LDA) techniques were, respectively, implemented to measure local gas holdup (α_Gr) and Peclet number (Pe) in the riser over a wide range of operation conditions. Although Peclet number was slightly different for the two systems, trends were similar. Gas holdups were always slightly higher for system 1. The dimensionless transition velocities from dispersed to coalesced flow were similar. Differences between the two systems are significant, but generally less than 15%, so the dimensionless similitude approach gives a reasonable basis for estimating global hydrodynamic parameters under the present operating conditions. The differences between the two systems are attributed to the complex coalescence behavior of liquid mixtures and the different sparger structures chosen, suggesting that additional dimensionless groups are needed to fully characterize the local dynamic bed behavior. This agreement proves that the proposed modified dimensionless numbers can be well adapted for engineering purposes and used to compare the flow performance between the two systems.     

Hydrodynamics investigation of laboratory-scale Internal Gas-lift loop anaerobic digester using non-invasive CAPRT technique

Internal gas-lift loop reactor (IGLR) was used as an anaerobic digester and its hydrodynamics were studied using Computer Automated Radioactive Particle Tracking (CARPT) Technique. This paper deals with the experimental study on a laboratory-scale digester. An anaerobic digester is a three-phase system consisting of gas, liquid, and solids; however solid–liquid slurry was treated as a single phase due to smaller size and lower density of solids. The effect of various geometric and operating variables on the hydrodynamics was studied. The superficial gas velocity was maintained at very low values and IGLR was operated in bubbly flow regime, which is suitable for operation of anaerobic digesters. The flow pattern and liquid velocity profile was obtained and effect of gas superficial velocity, draft tube diameter, type of sparger on liquid velocity and dead volume was studied in detail. Mean circulation times were calculated and compared for different digester configurations. Results showed that the increasing gas velocity increases the liquid velocity, decreases circulation time but does not offer any significant advantages in reducing the dead volumes. The configuration with draft tube diameter to tank diameter ratio of 0.5 showed good liquid circulation throughout the digester volume and low mean circulation time implying better mixing.     

Spargers for controlled bubble size by means of the multiple slot disperser (MSD)

Sparging technology is crucial in the dispersion of gases in liquids. In this work, it was demonstrated that an effective, controllable sparger can be made by assembling an array of flat parallel slot-nozzles which may offer new options for sparger design and operation. To illustrate the technique, a compact, ‘multiple slot disperser’ (MSD) having a slot width of and a total slot length of 1.26 m was assembled from a series of 5-mm-thick graphite plates. In water containing a low concentration of frother, a dense three-dimensional bubble plume was produced. The MSD generated consistently narrow bubble size distributions with well-defined median bubble diameters in the range 2.6-3.1 mm, equivalent to a range of gas flow rate from 11 to 26 std l/min. The bubble sizes were readily predictable from established single slot bubble size correlations. The method of construction also allows for simple maintenance, repair and replacement of individual components as needed.     

Bubble size distribution in the sparger region of bubble columns

It is well known that the gas distributor can play an important role on the evolution of the bubble size distribution (BSD) in gas-liquid reactors, strippers and absorbers. Therefore, the main subject of the present work was to study the influence of sparger design and process parameters on the BSD in the sparger region of the considered apparatus. For this purpose, both detailed measurements and prediction of the size of bubbles produced at the sparger were carried out in three different experimental apparatuses.The unique set of BSD curves were obtained by analyzing a large amount of bubbles with a measurement based on image analysis technique.Additionally, Colella's model of BSD evolution in bubble columns was further developed by implementing a detailed physical model for predicting the initial BSD at the sparger where the model input is only based on design/process parameters. A validation of the model was carried out using data from two different columns. The comparison between calculated and experimental BSD shows good agreement.