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Available online at http://arjournal.org APPLIED RESEARCH JOURNAL RESEARCH ARTICLE ISSN: 2423-4796 Applied Research Journal
Vol. 4, Issue, 5, pp.79-84, September, 2018
MODELING AND SIMULATION OF AIRLIFT BIOREACTOR WITH EXTERNAL FLOW LOOP USING COMPUTATIONAL FLUID DYNAMIC 1,2,*
Majid Hassanzadeganroudsari, 1Ali Taghvaie Nakhjiri, 1Amir Heydarinasab, 2Ali Raza
1 Department 2 College
of Chemical Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran. of Health and Biomedicine, Victoria University, Melbourne, Victoria, Australia.
ARTICLE INFO
ABSTRACT
Article History:
In this study, a modeling and comprehensive 2D simulation of airlift bioreactor with the external flow loop is presented with the purpose of achieving the required physical properties such as the velocity of gas and liquid phase and also viscosity. In order to simulate, an airlift bioreactor with a length of 1.75 m, a width of 0.5 m and a thickness of 0.08 m is applied. Fine mesh is used in this modeling and simulation to increase the accuracy and reduce the relative error of calculation. Simulation illustrates that the flow rate of gas at the inlet of the sparger is maximum and its amount decreases during the flow in bioreactor. Also, simulation suggests a direct relationship between the amount of gas hold up and its velocity of gas. According to the simulation, the highest dynamic viscosity value was 0.8 Pa/s and the lowest value was 0.1 Pa/s.
Received: 15, October, 2018 Final Accepted: 25, November, 2018 Published Online: 07, December, 2018 Key words:
Airlift bioreactor, Gas phase hold up, Dynamic viscosity, Liquid phase velocity.
© Copy Right, ARJ, 2018. All rights reserved
1. INTRODUCTION A bioreactor is referred to any device or system that provides an active biological and biomechanical environment. The design of bioreactors can be regarded as a very complex engineering process. The forces applied to bioreactors can be introduced under static or dynamic conditions. These forces include shear force, electrical force, magnetic force, mechanical-dynamical force, static-mechanical force and hydrostatic force. Bioreactor components include casing, piston, timer and compressor. The enclosure is a compartment in which the scaffolds contain the cell. This enclosure is enclosed in another area made by stainless steel and is compressed by the piston and stimulates the compression of the medium inside the enclosure [1]. The opening and closing of an electric valve is controlled by a timer. This valve locates along the air flow and causes application of cycle stimulation. Bioreactors are generally divided into two groups: rotating and nonrotating. The rotating bioreactors have a continuously rotating crop enclosure. This spin makes the cells grow evenly. The rotational speed can be adjusted to create a suspended state for cells. This action protects fragile tissues because it reduces shear stress and prevents contact between the cells and the bioreactor walls. Non-rotating bioreactors have a fixed and non-moving crop enclosure that allows them to cope with complex tissues. Also, it is easy to apply mechanical stress on the cultured tissues. In the last decade, standard stirred bioreactors have been successful in bio-processes. In industry to increase production capacity and reduce costs of production High volume bioreactors are needed. There are factors that limit the use of mechanical stirred bioreactors such as high energy costs, foaming, shear stress and PH correction. Therefore, many new bioreactors have been developed especially in sewage operations and the production of protozoa protein at high scales. These are non-mechanical stirred new bioreactors [2-4]. Bubble column and airlift bioreactors are those reactors in which the mixing operation is performed by the movement of bubbles into liquids. Airlift bioreactors are modified bubble column bioreactors and have been widely used in *Corresponding author: Majid Hassanzadeganroudsari, Email:
[email protected] Department of Chemical Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran. College of Health and Biomedicine, Victoria University, Melbourne, VIC, Australia.
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biotechnology, including aerosol fermentation for the production of various types of foods and sewage operations. The most important advantages of airlift bioreactors are low and gentle shear stress, low operating cost, and good mass transfer. Airlift bioreactors have different types, but two basic types currently use in industrial applications are airlift bioreactors with internal flow loop and airlift bioreactors with external flow loop. These bioreactors can be equipped with one or two spargers [5, 6]. The main purpose of this investigational paper is to render a comprehensive modeling and consequently a 2D numerical simulation for airlift bioreactor with the external flow loop to achieve the required physical properties such as the velocity of gas and liquid phase and also viscosity. In order to develop the model, COMSOL software is applied due to it's undeniable and brilliant capability such as high accuracy in solving the governing equations, memory adequacy and easiness to use [7-13].
2. MATERIAL AND METHODS 2.1. Model Development The interface of a bubble flow creates a multiphase flow model for gas bubbles in a fluid. The physical interface provides the average gas phase concentration for each bubble. This interface is analyzed in order to obtain the fluid velocity, pressure and volume of the gas phase. The slow flow of gas velocity, ug, is calculated by using the following equation: =
+
(1)
In this equation, ul is the gas phase velocity and uslip is the relative velocities between gas and liquid, which is called, slip velocity. The slip velocity comes from the slip equation. The interface of bubble flow generates many slip models. The best slip equation for this bioreactor is stretch-compression slip model with a stretch coefficient for large bubbles. Previous studies established that the Reynolds number for a fully developed flow is 2×104 and thus the flow has been fully developed in present study [14]. The turbulence model for the bubble flow is quite similar to the single phase k- ε turbulence model, although, in the case of bubble flows, additional terms are added to turbulent equations. The additional reference term is called, Sk and is related to the bubble turbulence and is calculated by the following equation [5, 15]: =−
∇
(2)
An additional reference in ε equation is called Sε which is used to analyze the dispersion and loss of turbulence and is presented as follows: = ⁄
(3)
In this equation, Cε and Ck are the constants of the model. The values of Ck and Cε are strongly dependent on the problem, but can often be adjusted to achieve acceptable agreement between experimental data and simulation. Suggested values for Ck and Cε are in the ranges of [0.01,1] and [1,1.92], respectively. In order to achieve the turbulent transfer of bubbles, a drift velocity for the gas phase velocity is presented as follows: =
+
+
(4)
/ g
(5)
In this equation
=−
⁄
Using k-ε model, turbulent dynamic velocity is defined as [1]: =
⁄
(6)
In this equation, Cμ is a model constant.
3. RESULT AND DISCUSSION The experimental data used in the software simulation are presented in Table 1. Also, the angled and orthogonal geometries of simulation for the airlift reactor are shown in Figures 1 and 2. Meshing was performed in the simulation with the aim of reducing the error and increasing the computational accuracy. Hence, coarse meshing is applied to cover all of the bioreactor points.
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Table 1 Specifications of bioreactor used for modeling and simulation Name
Value
Unit
Reactor height (H)
1.5
m
Reactor width (W)
0.5
m
Reactor thickness (T)
0.08
m
Bubble diameter (db)
0.003
m
Radius (R)
0.02
m
Downcommer widths (L)
0.16
m
0.015 (m/s)
m/s
5 × 10
kg/m3s
pH
3.5
---
Optimal diameter ratio (DD/DR)
0.5
---
Aeration intensity (a)
2.27
---
Inlet gas velocity (Vg)
7
cm/s
Input speed (Vin) Slip velocity proportionality constant (CW)
Figure 1 Angled scheme of simulated airlift bioreactor.
Figure 2 The orthogonal scheme of simulated airlift Bioreactor.
One of the most important processes that directly affect computational accuracy is meshing. Obviously, increment of the mesh number can increase the accuracy of simulation and also decrease computational error. In this paper, Fine mesh is used to mesh the bioreactor so that it can cover all points of the bioreactor. Figures 3 and 4 illustrate the meshing procedure of airlift bioreactor in vertical and angular direction. The mesh should be adjacent to very tiny side points to fully control the gradient in bubble concentration at the bioreactor corners. One of the disadvantages of gas-liquid contact devices in the industry is the lack of proper contact between the gas and the liquid at the dead zones of the device, which usually occurs in the corners of bioreactors. Fine mesh increases the accuracy and precisely controls the amount of gas and liquid contact and it can help engineers to reduce the dead zones at industrial scale.
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Figure 3 Vertical meshing diagram of the airlift bioreactor.
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Figure 4 Angular meshing diagram of the airlift bioreactor.
Figure 5 and Figure 6 show the velocities of gas phase and liquid phase in the bioreactor, respectively. The maximum gas velocity is existed at the entrance of the sparger at the bottom of the bioreactor, which is about 7 cm/s and the lowest value is about 1 cm/s, which is seen above the bioreactor. Figure 6 demonstrates the magnitude of the velocity of liquid phase during the passage of the reactor, which is the maximum in the middle of the reactor and reaches to its minimum at the reactor's corners. Decrease in the liquid velocity at the corner of airlift causes a significant deterioration in the gas-liquid contact which eventuates in the reduction of airlift bioreactor efficiency. Figure 7 shows variations in the dynamic viscosity of turbulent flow in the liquid phase in Pascal per second. As can be seen from the figure, the maximum viscosity of the liquid phase is measured at about 0.8 Pa/s, while the minimum viscosity of the dynamic is about 0.1Pa/s. By increasing gas phase velocity, the diameter of the bubbles decreases and hence the contact area increases, which eventuates in increasing the process velocity. Increasing in the process velocity reduces turbulent dynamic viscosity.
Figure 5 Changes in gas phase velocity in the airlift bioreactor.
Figure 6 Changes in liquid phase velocity in the airlift bioreactor.
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Figure 7 Variations in the dynamic viscosity of turbulent flow in the liquid phase.
4. CONCLUSION The results of simulation indicate that the velocity of gas at the entrance of the sparger is higher than that of the inlet fluid, and the maximum gas velocity has been measured at the entrance to the sparger, whereas after passing through the bioreactor, the rate of gas is reduced and reaches a minimum at the top of the bioreactor. The liquid viscosity in the simulated bioreactor implies that the highest viscosity of the liquid phase occurs in the range of 0.6 to 1.4 m, and then viscosity occurs with a significant decrease, and at the corners of the reactor the viscosity of the liquid phase reaches its minimum. Increment in the velocity of gas phase reduces the bubbles diameter inside the bioreactor which positively encourages the contact area. Increasing in the process velocity reduces turbulent dynamic viscosity. The results of the simulation and try and errors generated in simulation and modeling indicate that the fine mesh has the ability to cover the bioreactor corners and reduce the relative error and increase the measurement accuracy.
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