• Vol 10, No 3 (2019)
  • Chemical Engineering

Computational Study of the Time-dependent Flow Field of a Water-Molasses Mixture Inside a Stirred Vessel

Suci Madhania, Tantular Nurtono, Sugeng Winardi, Yuswan Muharam, Widodo Wahyu Purwanto

Corresponding email: widodo@che.ui.ac.id


Cite this article as:
Madhania, S., Nurtono, T., Winardi, S., Muharam, Y., Purwanto, W.W., 2019. Computational Study of the Time-dependent Flow Field of a Water-Molasses Mixture Inside a Stirred Vessel. International Journal of Technology. Volume 10(3), pp. 561-570
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Suci Madhania -Department of Chemical Engineering, Faculty of Engineering, Universitas Indonesia -Chemical Engineering Department, Institut Teknologi Sepuluh Nopember, Kampus ITS Sukolilo,
Tantular Nurtono Chemical Engineering Department, Institut Teknologi Sepuluh Nopember, Kampus ITS Sukolilo, Surabaya 60111, Indonesia
Sugeng Winardi Chemical Engineering Department, Institut Teknologi Sepuluh Nopember, Kampus ITS Sukolilo, Surabaya 60111, Indonesia
Yuswan Muharam Department of Chemical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia
Widodo Wahyu Purwanto Department of Chemical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia
Email to Corresponding Author

Abstract
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Detailed information on the flow field in the operation of a mixing unit is necessary for the optimal design of the reactor. The flow field characteristic is an essential factor in obtaining an optimal stirred vessel design. The efficiency of the stirred vessel system depends on, for example, the stirred vessel geometry, the flow induced by the impeller, the working fluid properties and the operating condition. The aim of this study is to exhibit the time-dependent flow field of the mixing process inside a stirred vessel for different propeller rotational speeds using computational fluid dynamics methods. The working fluid in question is molasses and water, which is a miscible liquid. The stirred vessel is a conical-bottomed cylindrical vessel (D = 0.28 m and H = 0.395 m) equipped with a three-blade propeller (d = 0.036 m). The transient calculation was conducted using ANSYS Fluent version 18.2. The Mixture multiphase flow model coupled with the Reynolds-averaged Navier-Stokes Standard k-? (SKE) turbulence model was applied to capture the information on the time-dependent flow fields at various propeller rotational speeds inside the stirred vessel. The flow generated by the propeller was compared at 1000 rpm, 1300 rpm and 1500 rpm. The Multiple Reference Frame method was used to solve the moving domain and stationary domain multiple frames case. The results revealed the local velocity, flow pattern, molasses volume fraction value, density gradient distribution, power number and flow number. The profile of all the variables determines the optimal operating conditions for the degree of mixing required.

Computational fluid dynamics; Multiphase flow; Propeller; Stirred vessel; Turbulent flow

Introduction

The blending of water and molasses with different properties has related applications in the production of bioethanol. The water-molasses mixture includes the miscible liquid system. Homogenization of two mutually dissolvable liquids is achieved through molecular diffusion and convection, but stirring can speed up the homogeneous condition reaches. The stirred vessel is one type of mixing equipment used in an industrial process. The flow field information inside the stirred vessel is necessary for the optimal design of the reactor. The efficiency of the stirred vessel system depends on, for example, the stirred vessel geometry and the flow induced by the impeller. The flow generated by the impeller is influenced by various factors including the shapes and number of impeller (Muharam & Kurniawan, 2016), the impeller size, the impeller type (Zhao et al., 2011; Sossa-Echeverria & Taghipour, 2015), the location and layout of the impeller (Rahimi, 2005) and rotational speed.

The experimental study is the primary method used to describe the flow characteristic in a stirred vessel. Direct measuring techniques can disturb the flow field, while indirect methods may not be appropriate as they are often too expensive and take a long time to carry out. However, advancements in computer technology and mathematical modeling leads researchers to prefer to use computational fluid dynamics (CFD) over an experimental study. CFD has become a capable device for describing fluid flow and has also been successfully used to predict the mixing behavior of the miscible liquid system, e.g. in the mixing of ethanol and glycerol (Al-Qaessi & Abu-Farah, 2014), the homogenisation of two mutually dissolvable fluids with different properties (Derksen, 2011), the water-ethanol system (Orsi et al., 2013) and the blending of two miscible liquids with different densities and viscosities (Montante et al., 2016).

The flow conditions in stirred tanks are mostly turbulent due to the presence of the impeller; therefore, the selection of the turbulence model should be appropriately considered as a way of resolving the effect of turbulence inside the system unit that operates at a high Reynolds number flow (Daryus et al., 2016). Commonly encountered forms of turbulence model include the Direct Numerical Simulation (DNS), Large Eddy Simulation (LES) and Reynolds-Averaged Navier-Stokes (RANS) models. Time-resolved and full-length-scale Navier-Stokes equations without any models or assumptions are solved directly by DNS. In LES, large eddies are resolved directly, while the effects of small eddies are modeled using subgrid-scale stress. However, the equations in RANS calculate the average of large eddies and some assumptions should be applied. Meanwhile, in real conditions, the tremendous computational costs of the LES and DNS still pose a significant barrier and there is a greater focus on mean flow characteristics as opposed to detailed turbulence. Therefore, the RANS model is commonly used. The RANS model comprises the standard k-? (SKE) model (Launder & Spalding, 1974), the renormalization k-? (RNG) model (Yakhot & Orszag, 1986) and the realizable k-? (RKE) model (Shih et al., 1995).

In CFD, a stirred tank system is included in the multi-reference frame category due to the presence of the impeller, which is a moving part. There are several approaches to handling multi-reference frame cases, among others the Sliding-Mesh (SM) and the Multiple Reference Frame (MRF) method. The SM is an unsteady approach to treat the moving and stationary frame interaction, while MRF is a steady state condition (Luo et al., 1994).

Power number (Npand Flow number (NQ) are important factors for characterising the impeller inside the stirred vessel. Zadghaffari et al. (2009) used the power number in his studies on mechanical agitation inside the stirred vessel. The power number can be obtained using both the torque applied in the impeller and by integrating the turbulence energy dissipation rate over the tank volume (Ge et al., 2014). However, based on Singh et al. (2011), torque-based prediction is more accurate than prediction based on the turbulence energy dissipation rate.

Up to now, the main progress of CFD study on the mixing process inside a stirred vessel has been achieved in the context of a top-entry installed propeller configuration. However, relatively few studies have been undertaken with regard to a side-entry configuration, e.g. CFD simulation on the mixing of crude oil in flat-bottomed cylindrical storage tanks (Dakhel & Rahimi, 2004), the influence of propeller layout on the mixing of the crude oil system inside a stirred tank (Rahimi, 2005), the intensity of solid-liquid mixing inside a stirred tank with various impeller layouts (Wu, 2011), and the mixing of pseudoplastic solutions (Sossa-Echeverria & Taghipour, 2015).

This study aims to present detailed information on the time-dependent flow fields generated by a three-blade propeller at three different rotational speeds for the water-molasses mixing process in a side-entry configuration stirred tank using the ANSYS Fluent CFD method. This research is a continuation of previous research that has reviewed the mixing effectiveness of different propeller installments (Madhania et al., 2017) and the mixing phenomena associated with different computational solution strategies (Madhania et al., 2018).


Conclusion

The time-dependent flow field of two mutually dissolved liquids in a stirred vessel was predicted using ANSYS Fluent 18.2 version at three different propeller rotational speeds: 1000, 1300 and 1500 rpm. The RANS SKE was used as the turbulence model, and the MRF method was used for counting the moving-stationary frame interaction. The well-documented time-dependent flow fields of local velocity, flow pattern, molasses volume fraction value, density gradient, power number and flow number were elucidated. The flow pattern formed a non-symmetric double-loop circulation pattern near the propeller. The propeller vicinity velocity showed a maximum value of 3.83 ms-1. The predicted molasses volume fraction value, density gradient and power number all fell as a function of time at the different propeller rotational speeds under the mixing process, which was in contrast to the flow number, which was the opposite of the other variables. As the rotational speed increased, so the gradient further decreased. The profile of all the variables can be applied to determine the optimal operating conditions for mixing water and molasses with respect to the degree of mixing required.

Acknowledgement

The United States Agency for International Development (USAID) supports the publication of this research/article through the Sustainable Higher Education Research Alliance (SHERA) Program for Universitas Indonesia’s Scientific Modeling, Application, Research and Training for City-centered Innovation and Technology (SMART CITY) Project, Grant #AID-497-A-1600004, Sub Grant #IIE-00000078-UI-1, contract number 0142/UN2.R3.SC/HKP.05.01/2018.

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