Research paperNumerical investigation on the flow characteristics of liquid oxygen and water in the structured packing
Introduction
Cryogenic distillation is currently prevalent in large-scale production of nitrogen and oxygen products in air separation systems, especially when the products are required for high purity or in liquid form [1]. During the cryogenic distillation process, the mixture of oxygen and nitrogen two-phase fluid spreads in the distillation column, accomplishing mass transfer and separation. In recent decades, the structured packing column, which has high specific area and void fraction, has gradually replaced the traditional tray column in cryogenic and also other chemical distillation fields due to its attractive performance of low pressure drop, high efficiency and high capacity [2]. The above advantages of structured packings allow reduction in the volume of the distillation column and in the energy consumption of the whole air separation unit.
The liquid hold-up and the interfacial area are key parameters to describe the hydrodynamic characteristics of multiphase flow inside the structured packing, which strongly affect the pressure drop and the gas-liquid interfacial heat and mass transfer in cryogenic and other chemical distillation processes. However, it is difficult to directly observe and measure the liquid hold-up and the interfacial area involved in a real packing since the packed bed is nontransparent and interlaced, especially under cryogenic insulation condition. At present, most of the published literatures about hydrodynamic investigations of the structured packing focus on water and other common fluids in chemical industry. Zakeri et al. [3] obtained the liquid hold-up of water in three different structured packings through performing a mass balance of the total liquid added to the system. Rocha et al. [4] presented a correlation of interfacial area of the structured packing based on the falling film of water on a inclined plate. Tsai et al. [5] adopted a chemical measurement based on the absorption of dilute CO2 into aqueous NaOH when NaOH solution flowed downwards along the packed column. The interfacial area of structured packings could be indirectly calculated by measuring the absorption rate of CO2.
Recently, with the help of advanced non-invasive measurement methods, it is possible to attain the images of liquid distribution in packings directly [6], [7], [8], [9]. Fourati et al. [6] measured the liquid hold-up distribution of water in four cross sections along longitudinal axis of a packed column containing Mellapak 250X through γ-ray computed tomography. Not only the actual liquid hold-up data of the packed column, but also the uniformity of liquid distribution in cross sections situated at different heights were clearly presented. Moreover, Aferka et al. [7] applied X-ray tomography to reveal the internal images of the irrigated packing of MellapakPlus 752.Y. The liquid hold-up and the interfacial area of water were also measured. However, the computed tomography technologies with high spatial resolution are still difficult to capture the gas-liquid-solid interface on account of the too small film thickness and the small differences in ray absorptivity between liquid and solid. Besides, this method cannot be applied in cryogenic conditions because of the existence of thermal insulation layers around packings, which may absorb most of the radiation.
Admittedly, the experimental method is still the most reliable way to study the structured packing. However, as stated above, it’s also complex, time consuming and only applied to just a few geometries and room-temperature fluids. Seldom measurements are adopted to study the flow features of cryogenic fluids like liquid oxygen and liquid nitrogen in the cryogenic structured packings. Currently, CFD simulation has become a powerful tool in the investigations of multiphase flow, in good complement with experimental works, enabling a deeper insight about local unsteady flow phenomenon inside the packings [10]. Actually, fully covered liquid film on the packing surface has been proven difficult to achieve because of interfacial interactions among gas, liquid and solid. The rivulet and even the droplet, which can only be represented by three-dimensional modelling, are the common flow patterns on the surface of metal sheets for fluid like water. The falling film of water and MEA aqueous solution on a inclined plate with a series of Weber number was simulated by Sebastia-Saez et al. [11]. The author found that the wetted area increased with the Weber number and the flow finally developed into full film flow when the flow rate was very large. The influence of surface textures on the wetted area was also studied and the result that stream-wise pattern enhanced liquid spreading was achieved. Subramanian et al. [12] presented the CFD simulation results of the development of liquid film flow on a inclined corrugated sheet with water, water-glycerol and silicon-oil falling. The influence of surface perforation and geometric structure on the flow pattern was investigated and the results were in accordance with experiments.
Petre and Larachi [13], [14] divided the structured packing into various representative elementary units (REUs) to simplify the intricate inner geometry. REU that describes the geometric periodicity provides an approach to reduce cost of calculation and is widely used in CFD works. Basden [15] carried out single-phase and two-phase CFD simulations in periodic geometry of structured packing. The periodic boundary conditions were employed on its both sides, accounting for the interaction between REUs. His results proposed that liquid viscosity has slight effect on the liquid hold-up and the wetted area, which is in contrast to Sidi-Boumedine’s research finding [16]. Furthermore, Sebastia-Saez et al. [17] and Chen Jiangbo et al. [18] constructed different REUs for studying the liquid hold-up and the interfacial area. Besides, multi-scale simulation has been proven to be an effective tool to evaluate various aspects of performance of the structured packing comprehensively [19]. Dan Yu et al. [20] examined the flow characteristics of a novel wave-like polyline-arc structured packing with multi-scale simulation of water. The variation of average liquid film thickness, wetted area, liquid hold-up and pressure drop of this new packing were analyzed under different liquid and gas loads. Amini et al. [21], [22], [23] conducted a series of studies on different scales to investigate the flow and the mass transfer performance of a high capacity structured packing. The local flow on the packing surface was simulated by micro-scale VOF method, while the mass transfer efficiency and the pressure drop of the whole column were studied using macro-scale Eulerian-Eulerian model.
As stated above, the flow characteristics of liquid in the structured packing were well revealed by experiments and most CFD simulations, while all of these studies were carried out on water or other fluids at room temperatures, which played a significant role on the development of the structured packings in the past. The hydrodynamic investigations of liquid oxygen (LOX) and liquid nitrogen (LN2) in cryogenic distillation process have not been reported. In cryogenic distillation, the operation temperature is low and the packed column needs to be coated with thick insulation material, making many measuring instruments unable to be used. Owing to the difficulty of cryogenic experiments, the performance test of the structured packings used in cryogenic distillation is often carried out by room temperature fluids like water. However, the real flow features of LOX and LN2 in the structured packing are hardly known. Since LOX and LN2 have quite different physical properties compared to water, such as lower viscosity, surface tension and contact angle, their liquid hold-up and interfacial area in the structured packing should be diverse from those of water. This could lead to considerable deviation in the calculations of wet pressure drop and mass transfer coefficient [24], [25], [26]. Therefore, studies on revealing the flow characteristics of cryogenic fluids in the structured packing are particularly required, helping the precise optimization of the cryogenic distillation in the future.
In this work, as typical fluid in the cryogenic distillation, the two-phase flow of liquid oxygen (LOX) was considered and a three-dimensional CFD model combining REUs method was built to study its flow characteristics in the structured packing. The simulation results of LOX were compared with those of water. The influence of liquid load on the liquid hold-up, the interfacial area and the average liquid film thickness of LOX was investigated. The equivalent contact angle, which is found to be a significant parameter of surface textures determining the liquid dispersion, was studied to reveal its relationship with the hydrodynamic parameters of LOX on the packing surface.
Section snippets
Hydrodynamics model
The flow inside the structured packings is typically two-phase flow, so the simulations were carried out using the VOF method for multiphase-flow analysis. Not only widely used in normal temperature conditions, but the VOF method has also been applied in simulations of cryogenic two-phase flows [27], [28]. The VOF method solves a single set of momentum equations throughout the computational domain and is capable of tracking the interface between two immiscible fluids, using the concept of the
Validation of the hydrodynamics model
In order to verify the established model, the liquid hold-up and the interfacial area of water and LOX were simulated and compared with the reported correlations. The widely accepted correlations of liquid hold-up and interfacial area are shown in Table 2 and Table 3 respectively, both of which are suitable for Mellapak 250Y structured packing. The above results are plotted against the liquid load in Fig. 4 and Fig. 5. In present study, the liquid load, which is volume flow rate of liquid per
Conclusion
In this work, three-dimensional CFD simulations on the flow characteristics of water and LOX were carried out using VOF method in constructed REUs of Mellapak 250Y structured packing. The effects of liquid load and equivalent contact angle θ on the flow pattern, the liquid hold-up and the interfacial area of water and LOX were analyzed. Differences of flow characteristics between water and LOX due to their different physical properties were compared. The main conclusions are as follows:
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Rivulet
CRediT authorship contribution statement
Chenjie Gu: Methodology, Software, Formal analysis, Writing - original draft. Ruiping Zhang: Software, Validation. Xiaoqin Zhi: Supervision, Software, Writing - review & editing. Shaolong Zhu: Writing - review & editing. Limin Qiu: Supervision, Conceptualization, Project administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is supported by the Nation Key R&D Program of China (No. 2017YFB0603701) and the Fundamental Research Funds for the Central Universities (No. 2020QNA4006).
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