Elsevier

Chemical Engineering Science

Volume 202, 20 July 2019, Pages 447-461
Chemical Engineering Science

Numerical investigation of liquid dispersion by hydrophobic/hydrophilic mesh packing using particle method

https://doi.org/10.1016/j.ces.2019.03.046Get rights and content

Highlights

  • The velocity inlet model for MPS Method were developed to simulate jet flow.

  • The hydrophobic and hydrophilic surfaces on wall were reproduced accurately.

  • The mechanism of liquid dispersion using mesh packing was investigated.

  • The liquid dispersion under hydrophobic/hydrophilic conditions were analyzed.

Abstract

Liquid dispersion in chemical processes is significant for increasing mass or heat transfer. A particle-based method, the moving particle semi-implicit (MPS) method, was employed in this study to simulate the process of liquid dispersion by mesh packing. The computational framework includes a designed inlet flow model, hydrophilic/hydrophobic surface boundary conditions, and a surface tension model. This study mainly focuses on the effect of hydrophobic/hydrophilic mesh packing on liquid dispersion performance. The mechanism of liquid dispersion is systematically investigated in basic situations as a liquid passing through a single wire, an orthogonal wire and a single aperture. The results indicate that hydrophobic and hydrophilic mesh packing have different effects on the formation of the free surface, which is the key to the mechanism of the liquid dispersion process. Hydrophobic mesh packing can disperse a liquid into several fine columns with detected droplets, which further develop into fine droplets, whereas hydrophilic mesh packing will guide a liquid to converge into a column at the bottom of the mesh packing. In addition, the surface area density and dispersion range of a liquid are quantitatively analyzed to illustrate liquid dispersion performance.

Introduction

Liquid dispersion is a significant approach used in chemical processes to increase mass or heat transfer (Science and Britain, 1995). As an efficient gas–liquid mass transfer device, a rotating packed bed (RPB) can provide a high centrifugal force to allow liquid to flow into porous materials (packing zone) for liquid dispersion and full contact between gas and liquid. Packing zone is the main work space of RPBs, which accounts for up to 70% of the effective mass transfer area of a whole RPB (Guo et al., 2014). The liquid introduced into the rotating packing zone will rapidly develop into thin films and tiny droplets near the inner edge and then synchronously moves with the packing, the gas-liquid mass or heat transfer is also going on at the same time (Luo, 2017, Zheng et al., 2016). Improving of liquid dispersion performance of the packing could enlarge the interface area between liquid and gas and enhance the mass and heat transfer of RPBs.

Extensive experimental studies have been carried out to study the flow regularity within RPBs and enhance RPB mass transfer efficiency (Burns and Ramshaw, 1996, Sang et al., 2017a, Sang et al., 2017b, Yan et al., 2012, Liu et al., 2017). Some studies show that the mass transfer efficiency in an RPB can be affected by the packing surface wettability. Zheng et al. (2016) prepared a surface-modified nickel foam packing (SNP), which is hydrophobic, and a nonmodified nickel foam packing (NNP), which is hydrophilic. They experimentally, separately investigated the mass transfer performance in an RPB with these two kinds of packing materials. The results show that the RPB with SNP had a higher mass transfer performance because of enhanced liquid dispersion. Considering that the mass transfer coefficient of mesh packing is the highest among various shapes (Chen et al., 2006), Zhang et al. (2017) carried out visual experiments to observe the liquid dispersion created by a surface-modified stainless mesh (SSM) and nonsurface-modified stainless mesh (NSM). The results show that the SSM is more conducive to forming a liquid dispersion than the NSM, and the mass transfer performance in an RPB with SSM packing is better than that with NSM packing.

The studies above have provided credible and practical results to improve the liquid dispersion and increase the mass transfer performance in an RPB. Further mechanism analysis is needed to investigate liquid dispersion patterns using hydrophobic and hydrophilic packing. Computational fluid dynamics (CFD) can provide visible flow traces and detailed information inside the flow field. As one branch of CFD, particle-based methods such as smoothed-particle hydrodynamics (SPH) (Monaghan, 1992), macro-scale pseudo-particle method (MaPPM) (Wei and Jinghai, 2001), and moving particle semi-implicit (MPS) (Koshizuka and Oka, 1996) discretize continuum mechanics into particles, whose coordinates are updated every time step to match the shape of the fluid. Thus, particle-based methods have the advantage of simulating large deformation (Xiang and Chen, 2015, Xu and Deng, 2016), multiphase flow (Xiong et al., 2010, Xiong et al., 2011), free surface flow (Yanget al., 2016) and even large-scale flow (Xiong et al., 2013, Chen and Wan, 2019).

The MPS method was proposed by (Koshizuka and Oka, 1996) and was specifically applied to solve the incompressible Navier-Stokes equations for an incompressible fluid simulation. To date, the MPS method has been successfully used in many industrial fields (Natsui et al., 2014, Gambaruto, 2015). For example, the mixing process of two viscous liquids in an agitator and the liquid transformation in a rotating atomizer were analyzed using the MPS method (Sun et al., 2009a, Sun et al., 2017). Considering the hold-up phenomenon in a packed bed in blast furnaces, Kon et al. (2015) employed the MPS method to study the packed bed flow. For small-scale problems, Sun et al. (2009b) introduced a surface tension model, CSF (continuum surface force), to the MPS method and simulated binary collisions (Xi and Sun, 2018). More researchers have added different physical models to the MPS method for further application of the MPS method.

In this study, the MPS method with the surface tension model was employed to separately analyze the mechanism of liquid dispersion by mesh packing under hydrophobic and hydrophilic conditions. First, three basic flow patterns, single wire flow, orthogonal wire flow and aperture flow, were employed to investigate the wetting mechanism of the mesh packing. Then, the liquid dispersion process by hydrophobic and hydrophilic mesh packing with three initial jet velocities was simulated. Finally, comparisons of the surface particle number and liquid dispersion range between hydrophobic and hydrophilic conditions are shown, and the influence of wettability on the liquid dispersion performance is discussed.

Section snippets

Governing equations

The governing equations for incompressible flow motion are continuity, and the Navier-Stokes equation is shown as follows:DρDt+ρ·u=0ρDuDt=-p+μ2u+ρg+fswhere u is velocity [m/s], t is time [s], ρ is density [kg/m3], p is pressure [N/m2], μ is the kinetic viscosity coefficient [Pa·s], g is the acceleration of gravity [m/s2], fs is surface tension [N/m3] translated into a force per unit fluid volume, ∇ is the gradient, and ∇2 is the Laplacian.

Discretization

In the MPS method (Koshizuka and Oka, 1996), the

Flow mechanism of wire circumferential motion

The mechanism of liquid dispersion by hydrophilic/hydrophobic mesh packing, which has seldom been analyzed by experimental methods, was investigated using the MPS method in this study. The complex process of liquid dispersion by mesh packing was decomposed into three basic flow models: single wire flow, orthogonal wire flow, and single aperture flow. In this section, these basic flow models were simulated and analyzed to obtain the basic rules of liquid dispersion by mesh packing. The

Conclusion

In the liquid dispersion process by mesh packing, the wettability of the solid surface plays a significant role in affecting the dispersion performance. In this study, the moving particle semi-implicit (MPS) method with a surface tension model was employed to simulate the process of liquid dispersion using typical hydrophobic and hydrophilic mesh packing. A three-dimensional mesh packing model was established, and an inlet boundary condition was applied to simulate a continuous jet flow. The

Conflict of interest

There is no conflict of interest.

Acknowledgement

We thank De-lin Chai, Tong-sheng Wang, Kai Zhang, Yun-zhang Song, Ya-li Chen and Yong Zhang for helpful discussions. This work is supported by the Natural Science Foundation of China (NSFC) Project (No. 51576154).

References (36)

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