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MIXING AND SEPARATION OF MICROAGAE ASSISTED BY VIBRATION
Nikolai Kozlov1,2, Dominique Pareau2, Moncef Stambouli2
1 Laboratory of Vibrational Hydromechanics,
Perm State Humanitarian Pedagogical University,
24, Sibirskaya av., 614990, Perm, Russia
2 Engineering Process and Material Lab Ecole Centrale Paris,
Grande Voie des Vignes, 92295, Chatenay-Malabry Cedex, France
Abstract. The possibility of application of average vibrational flows for enhancement of biotechnological process is studied experimentally. A vertical cylindrical container with an annular working layer is filled with an aqueous fine suspension and set at rotational vibration (librations) around its axis. Six activators are placed regularly on the inner side of the outer wall and have constant width along their whole length. Thus the geometry is axi-symmetric and two-dimensional. At vibration, an average flow of a regular vortical structure, periodic along the azimuth, is excited. The intensity of vibration is characterized by the pulsation Reynolds number. Vibrational mixing provides efficient homogenization of a microalgae suspension, preventing sedimentation.
The application of the effect for separation of microalgae is discussed.
Key words: rotational vibration, separation, mixing, microalgae, Stokes boundary layer, average flow.
INTRODUCTION
Vibrations bring multiple advantages and new solutions to technological processes. For instance, traditionally mixing is done with a rotating stirrer, which can be very efficient at high rotation rates. But its drawback is high shear stress, which is undesirable for some products, e.g. microalgae. The cells of the latter can be mechanically damaged by
© Kozlov N., Pareau D., Stambouli M., 2013
Kozlov N., Pareau D., Stambouli M. Mixing and separation of microagae
a stirrer, and this would result in a change of the product chemical properties.
Separation is an important step in many technologies. Solid-liquid separation aims to recover valuable solids from a suspension, recover valuable molecules from permeate, clean the liquid from solid intrusions, or separate and reuse different phases of a suspension.
The following approaches are described in literature in the context of an assisted separation process: Table 1.
Table 1
Acoustic vibration 1. Membrane vibrations or tangential oscillations of liquid - reduced fouling and higher flow rate.
Ultrasonic vibration 2. Acoustic streaming - controlling the particles motion. 3. Standing wave - particles accumulation and assisted sedimentation.
Rotationally generated shear 4. Intensive Taylor vortices are used to remove particles from the membrane surface. 5. RO filtration with a rotating disk module.
Ultrasound. Acoustic waves exert forces on solid particles in a liquid [1]. A standing ultrasonic wave can be used for particles accumulation in the nodes, thus allowing enhanced sedimentation [2]. In [3] it is proposed to apply pulsed ultrasounds to control the migration of microparticles in an acoustic streaming.
Membrane processes are widely used in different biological and chemical technologies, for example: biomass harvesting, water purification, desalination, etc. Membrane fouling is a critical issue. A viable idea to address this problem consists in producing a strong shear on the membrane surface.
Vibratory processes. The rotational membrane oscillations allow augmenting the critical transmembrane pressure and the maximum permeate flux [4]. It means the possibility of having a higher flow rate at acceptable fouling. It is achieved due to the reduced cake formation as the membrane surface is washed by the tangential oscillating flow. This principle is used in a commercial product VSEP [5]. Translational vibration of submerged hollow fibers is proposed in [6] to generate the shear and the secondary flows.
Non-vibrational methods. Another way to clean the membrane is to
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produce high shear via rotation. The inner permeable cylinder (membrane) rotating relative to the outer coaxial cylinder can be washed by the Taylor vortexes [7, 8]. The Taylor-Couette flow is applied to assist the filtration process of the red blood cells and to separate plasma from the whole blood. Also, a rotating disk module can be applied for reducing membrane fouling [4]. In this case the membrane is stationary, and a single rotating disk produces the shear. These two methods refer to the reverse osmosis.
Vibrational mixer. In [9] was experimentally studied the fluid motion in a cylindrical cavity of the square cross-section, when the cavity performed rotational oscillations about its axis. The average flow was excited, which consisted of a periodic vortex pattern and assured an intensive mixing of the total liquid volume. In the present work it is proposed to apply this effect to design a vibrational mixer, configuration of which is oriented for the use in biotechnological processes.
1. EXPERIMENTAL SETUP
The central part of the experimental setup is a cylindrical cavity with an annular layer 1, partially separated by activators 2 regularly placed on the outer wall into six equal domains (Fig.1). The outer radius of the annulus Ro = 50.0 mm, and the inner Ri = 30.0 mm. The activators are
half-cylinders of constant diameter d = 16.0 mm. The spacing between the activators 2a is chosen so that each domain measures in the azimuth direction twice as in the radial one, a » Ro - Rt. This aims to provide homogeneous mixing as the shape of vortices 3 is close to a circle (Fig. 1 and Fig.4a) for the slow flow (see 2. Experimental results). These vortices are excited due to rotational vibration of the cavity.
The photo of the experimental setup is shown on Fig.2. The transparent cylindrical container 1 with an annular cavity is fixed on the platform 2 and filled with a working liquid. The stepper motor McLennan 34HSX-208 3 transmits rotation to the eccentric disk 4 via a belt drive. The motor is controlled by the drive MSE570-2. The connecting rod 5 transforms rotation of the disk to rotational oscillations of the platform and the cavity. The vibration frequency f varies from 0 to 20 Hz approximately. The DC alimentation of the motor is done with the help of the power supply Isotech IPS 303DD 6, for higher intensity of vibration two power supplies are connected in a series. The clock signal for the motor is provided by a sound generator Hameg 7. The amplitude of vibration j0 is set by positioning the axe of the connecting rod on the
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eccentric disk and varies approximately from 0.05 to 0.2 rad.
Water with small amount of tracer particles is used for visualization and measurement of the flow, the cinematic viscosity being n = 1.0 cSt. Aqueous suspension of micro-algae Chlorella is used for the study of concentration and sedimentation. The biosuspension is obtained during a cultivation process and contains the microorganisms in their natural environment.
Fig. 1. Schematics of the cell, top view
Fig.2. Experimental setup
The velocity measurements were done on the PIV setup manufactured by Dantec, data being processed using the Flow Manager software. Photo registration was used for studying the flow pattern, also. Microal-
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gae concentration was measured by spectroscopy, using Varian visi-ble/UV spectrometer.
2. EXPERIMENTAL RESULTS
Rotational vibration creates tangential oscillations of the cavity wall relative to the liquid. The liquid is displaced by oscillating activators, which move together with the cavity wall. The width of the annulus is not constant as it is narrower in places where the activators are placed. For that reason the azimuth gradient of the pulsation velocity amplitude is present. As a consequence, the oscillations of the liquid lead to generation of the average flows in the thin viscous boundary layers near the walls. This principle is known as Schlichting mechanism [10]. The average mass force acting in the boundary layers involves in rotation the liquid in the whole layer.
Flow regimes
The hydrodynamic study allows distinguishing five different flow modes (Table 2). To characterize the flow and vibration intensity, following [9], we use the pulsatory Reynolds number Rep = jRo2W Iv
and the dimensionless frequency w=W(Ro -R)2 Iv . Here W = 2pf.
The map of different modes is shown on Fig.3. In the present state of the research the thresholds are determined approximately.
Table 2
О l 3 cd * ^ Flow mode
1. < 160 Laminar flow. Vortex structure symmetric relative to an activator. (Fig.4a, b)
2. > 160, < 270 Next type of symmetry, rotation. One global vortex in the inner part. Spatial period equal to the one of the activators. (Fig.4c)
3. > 270, < 350 Period doubling, global vortex break. (Fig.4d)
4. > 350, < 550 Appearance of non-stationary flow (turbulent zones). (Fig.4e)
5. > 550 Fully turbulent flow. (Fig.4f
The flow patterns are shown on Fig.4. Each picture (except b showing the PIV result) is a superposition of 10 to 20 consecutive photographs. A few shaded areas are due to the laser beam refraction on the cylindrical walls of the cavity and the activators.
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Kozlov N., Pareau D., Stambouli M. Mixing and separation of microagae
At weak vibration, a periodic vortex pattern is formed in the annulus. Each vortex is in contact with an activator and washes its surface in the outward direction. On the equal distance from two neighboring activators the vortices come into contact and are directed inward. The spatial period of the vortex pattern is equal to the period of the activators positioning (Fig.4a). PIV measurements confirm this result (Fig.4b). The secondary small vortices are observed on the inner wall, opposite the activators, and on the outer wall between the activators.
Fig.3. The map of the flow modes. The numbers 1-5 correspond to the table 2
With increase of the vibration intensity, Rep, in the threshold 1-2 on
Fig.3, a bifurcation occurs when a half of the vortices (either even, or odd) are united into one large-scale vortex, having the shape of a cog wheel. All of the grouped vortices have the same rotation direction, this latter becoming the direction of the big vortex. Meanwhile the other half of the vortices are oppressed and localized in close proximity of the activators (Fig.4c).
With the further Rep increase, in the threshold 2-3, the large-scale
vortex structure changes, inside it breaks in smaller vortices forming a new pattern with the doubled spatial period (Fig.4d). In the threshold
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a: 2 Hz, 0.050 rad
b: 2 Hz, 0.050 rad
c: 4.5 Hz, 0.050 rad
d: 4 Hz, 0.072 rad
e: 8 Hz, 0.072 rad
f 4 Hz, 0.103 rad
Fig.4. Flow structure, f and j0 are presented under the pictures
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3-4 the average flow is no more stationary, the streamlines found in smaller modes begin breaking (Fig.4e). In the threshold 4-5 the average flow becomes chaotic (Fig.4f), no stationary streamlines are observed at all.
As the frequency number tends to zero, the thresholds have a trend to coincide in one point. At high frequency number it is possible to expect some asymptotic law (Fig.3).
Intermixing
Vibrations provide efficient intermixing of the suspension. The average flows provide homogeneous distribution of the particles in the volume and assure their continuous motion by means of a regular vortex pattern.
a b c
Fig.5. Concentration restitution after sedimentation due to vibrational mixing. Time elapsed between a and b is 35 min, between b and c -79 min; f = 3.5 Hz, j0 = 0.075 rad
In the experiments with micro-algae at sufficiently intensive vibration, Rep = 309 and w = 8800 , the deposited particles go up to the surface. For this the suspension was left for the natural sedimentation during 50 hours. At the end of this period the microalgae concentration approximately 7 cm below the surface of the suspension was equal 42.5% of the initial one. Then the vibration was set and during 5 hours and 30 minutes the concentration was restored to 81.8% from the initial one, taken at homogenous particles distribution. On Fig.5 are shown three consecutive photos. The concentration front is distinctly seen on the pictures a and b, and the color of the suspension is nearly homogeneous on the picture c, though a slight vertical gradient is seen.
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At vibration the particles concentration in the proximity of the surface is maintained on the initial level during a 6 hours experiment. The vibrational mixing efficiently prevents sedimentation.
3. PERSPECTIVES OF APPLICATION Separation
It is proposed to use the designed mixer for intensification of membrane filtration. Nonuniformity of the annulus width, and consequently velocity inhomogeneity in the Stokes layer, provides oscillations of the liquid relative to the cavity walls and to the membrane surface. This might be compared with a vibrating membrane, when an oscillatory flow would wash away the particles and make the fouling layer thinner. Pulsatory flows would assure the particles repulsion from the membrane surface, and the average flow would carry the oscillating particles away and would mix them with the liquid in the coaxial layer. The use of nontranslational vibration excludes the presence of strictly perpendicular flows on the membrane surface. As the flow is mainly directed tangentially, it creates the conditions for reduced fouling.
An advantage of the average motion is that it produces the constant renewal of the solution, which might simplify the delivery of the liquid and maintain the particles concentration near the wall smaller, resulting in reduced fouling.
Other applications
The proposed mixer can be used as a base of a vibrational bioreactor. In the configuration there are no sharp edges, as for instance an impeller edges, which allows reducing shear stress. At the same time, the mixing can be efficient due to the average vibrational flows. The use of an annulus allows increasing the ratio of illuminated surface to the volume (or the layer thickness), and also to place light sources both outside and inside with respect to the layer of a biological culture. These properties can be useful for a phototrophic cultivation of microalgae sensitive to mechanical stress.
Conclusion. The vibrational excitation of average flows in an annulus with activators is studied experimentally. It is shown that the efficient vibrational mixing of a microalgae (microparticles) suspension is possible. Vibrational mixing prevents sedimentation. Application of the studied phenomena for membrane separation is discussed.
The promising properties of the proposed technical solution are:
• possibility of choice between different flow modes: slow with
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Kozlov N., Pareau D., Stambouli M. Mixing and separation of microagae small shear stress or intensive;
• high level of organization of the flow allows suggesting high performance factor;
• predominance of the tangential flow is good for reduced membrane fouling.
Further investigation is necessary to find optimal geometrical configuration and parameters of vibration for separation.
The work is supported by an International research group project S-26/625 «Vibrational hydromechanics in application to modern biological and chemical processes», by White biotechnologies chair of Ecole Centrale Paris and project F-29 of Strategic program of PSHPU development.
BIBLIOGRAPHY
1. King L. V. On the acoustic radiation pressure on spheres // Proceedings Royal Society London. 1934. Vol. 147. P. 212-240.
2. Cappon H.J., Stefanova L.A., Keesman K.J. Concentration based flow control in acoustic separation of suspensions // Separation and Purification Technology. 2013. Vol. 103. P. 321-327.
3. Hoyos M., Castro A. Controlling the acoustic streaming by pulsed ultrasounds // Ultrasonics. 2013. Vol. 53. P. 70-76.
4. Frappart M., Jaffrin M., Ding L. H. Reverse osmosis of diluted skim milk: Comparison of results obtained from vibratory and rotating disk modules // Separation and Purification Technology. 2008. Vol. 60. P. 321-329.
5. New Logic Research, Inc. URL: http://www.vsep.com.
6. Kola A., Yun Y., Amy H. et al. Application of low frequency transverse vibration on fouling limitation in submerged hollow fibre membranes // Jl. of Membrane Science. 2012. Vol. 409-410. P. 5465.
7. Pederson C.L., Lueptow R.M. Fouling in a high pressure, high recovery rotating reverse osmosis system // Desalination. 2007. Vol. 212, Issues 1-3. P. 1-14.
8. Lueptow R.M. Rotating Filtration: A Practical Application of Taylor Couette Flow // 18th International Couette-Taylor Workshop, University of Twente, Enschede, The Netherlands, 24-26 June 2013. P. 12.
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9. Ivanova A.A., Kozlov V.G. Vibrational convection in nontransla-tionally oscillating cavity (isothermal case) // Fluid Dynamics. 2003. Vol. 38, No 2. P. 186-192.
10. Schlichting H. Boundary-Layer Theory. McGraw-Hill, New York, 1979. 817 p.
ПЕРЕМЕШИВАНИЕ И РАЗДЕЛЕНИЕ МИКРОВОДОРОСЛЕЙ ПРИ ПОМОЩИ ВИБРАЦИЙ
Николай Козлов, Доминик Паро, Монсеф Стамбули
Экспериментально изучается возможность применения средних вибрационных потоков для усовершенствования биотехнологических процессов. Вертикальный цилиндрический контейнер с кольцевым рабочим слоем заполняется мелкой водной суспензией и приводится во вращательные вибрации (либрации) вокруг своей оси. Шесть активаторов регулярно расположены на внутренней стороне внешней стенки и имеют постоянную ширину вдоль всей длины. Таким образом, геометрия осесимметричная и двумерная. При вибрациях возбуждаются средние потоки с регулярной вихревой структурой, периодичной вдоль азимута. Интенсивность вибраций характеризуется пульсационным числом Рейнольдса. Вибрационное перемешивание обеспечивает эффективную гомогенизацию суспензии микроводорослей, препятствуя оседанию. Обсуждается применение эффекта для отделения микроводорослей от суспензии.
Ключевые слова: вращательные вибрации, разделение, перемешивание, микроводоросли, пограничный слой Стокса, среднее течение.
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