Power Consumption in Uncovered Unbaffled ... - ACS Publications

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Power consumption in uncovered-unbaffled stirred tanks: influence of viscosity and flow regime Francesca Scargiali, Antonio Busciglio, Franco Grisafi, Alessandro Tamburini, G. Micale, and Alberto Brucato Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie402466w • Publication Date (Web): 20 Sep 2013 Downloaded from http://pubs.acs.org on September 23, 2013

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Industrial & Engineering Chemistry Research

Power consumption in uncovered-unbaffled stirred tanks: influence of viscosity and flow regime. Francesca Scargiali*, Antonio Busciglio, Franco Grisafi, Alessandro Tamburini, Giorgio Micale, Alberto Brucato

Dipartimento di Ingegneria Chimica, Gestionale, Informatica e Meccanica Università di Palermo, Viale delle Scienze, Ed.6, 90128 Palermo (Italy)

Abstract: Notwithstanding the increasing industrial interest towards unbaffled tanks, available

experimental information on their behaviour is still scant, even for basic quantities such as mechanical power drawn. In this work the influence of Reynolds and Froude numbers on power consumption characteristics is presented for unbaffled stirred tanks operating both in non-aerated conditions (sub-critical regime) and in aerated (super-critical) conditions, i.e. when the free surface vortex has reached the impeller and the gas phase is ingested and dispersed inside the reactor. Experimental results obtained at various liquid viscosities show that power numbers obtained in subcritical conditions do line up quite well on a smooth Np versus Re function, with no need to involve the Froude number in the correlation. At rotational speeds involving air entrapment and dispersion inside the reactor (super-critical regime), a steep reduction of power number is observed. A novel overall correlation for power number prediction, able to deal with both subcritical and super-critical regimes is finally proposed.

Keywords: Mixing, Multiphase reactors, Unbaffled tanks, Power number, Bioreactors Corresponding author: Francesca Scargiali, Dipartimento di Ingegneria Chimica, Gestionale, Informatica e Meccanica Università di Palermo, Viale delle Scienze, Ed.6, 90128 Palermo (Italy) e-mail address: [email protected] tel: +39 09123863714 1

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1. INTRODUCTION Low viscosity liquid mixing is typically carried out in stirred cylindrical vessels equipped with swirl-breaking baffles (typically 4 baffles at 90° from each other) aimed at improving mixing performance. As a matter of fact in the absence of baffles (unbaffled tanks) the liquid tends to mainly move along circular trajectories. As a result, only small relative velocities between impeller and fluid and weak radial flows are generated which, in turn, create a poor radial/axial mixing. If a free surface is present (unbaffled uncovered tanks) a pronounced vortex is formed whose depth depends on the stirrer rotational speed and at high speeds may reach the stirrer blades, giving rise to gas ingestion and dispersion inside the liquid1,2. Despite being poorer mixers than baffled vessels, unbaffled stirred tanks are enjoying a growing interest in the process industry, as they provide significant advantages in a number of applications where the presence of baffles is undesirable for some reason3-4. This is for instance the case of crystallizers, where the presence of baffles may promote particle attrition5, or of precipitation processes, where baffles could suffer incrustation problems6, as well as in food and pharmaceutical industries, where vessel cleanness is a topic of primary importance7. As concerns bioreactor applications, when shear sensitive cells are involved, mechanical agitation and especially sparging aeration (and associated bubble bursting) can cause cell death8-11. In unbaffled vessels, at low agitation speeds, the required oxygen mass transfer may well take place through the free surface deep vortex which takes place when agitation is started. This feature clearly makes unbaffled vessels potentially advantageous for shear sensitive cultures (e.g. animal cell or filamentous mycelia cultures) as well as for foaming gas-liquid systems, provided that process rates, and relevant gas consumption needs, are compatible with the relatively small gas transfer rates achievable2.

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Moreover, in the aerated regime (the free surface vortex has reached the impeller and a gas phase is ingested and dispersed inside the reactor) unbaffled tanks may be a convenient alternative to sparged gas-liquid reactors. This is especially true when the use of a sparger can create practical problems, or simply add undue costs, as for instance when a solid phase that might cause sparger holes blockage is present (e.g. three-phase catalytic reactors), or when dealing with hazardous gaseous reagents (e.g. chlorinations and hydrogenations), for which the need for an external recycle circuitry is conveniently avoided. These advantages are shared with other self-ingesting devices, that however imply an increased geometrical complexity and often also larger power dissipation rates12-15. Notwithstanding the increasing industrial interest towards unbaffled tanks, available experimental information on unbaffled tank behaviour and relevant scale-up criteria are still scant, even for basic quantities such as power consumption. As a matter of fact, to the authors’ knowledge only Rushton et al.16 and Rushton17 included some experimental results on unbaffled stirred tanks in their studies on power consumption and scale-up criteria, even if they didn’t specify whether their data were related to un-gassed (sub-critical) or gassed (super-critical) conditions. More recently Yoshida et al.18 studied experimentally the progress of mixing of Newtonian liquids in relation to the impeller power characteristics for an unbaffled agitated vessel with an angularly oscillating impeller. Rao et al.19 and Rao and Kumar20 reported experimental information about power consumption and scaleup criteria of surface aerators both baffled and unbaffled and proposed a correlation to find power consumption in aerated conditions for unbaffled systems. In the present work the influence of viscosity on power consumption in unbaffled tanks operating both in sub-critical conditions and in super-critical conditions is reported and discussed.

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2. EXPERIMENTAL The experimental apparatus involved an uncovered-unbaffled transparent cylindrical stirred tank with internal diameter T = 190mm and total height of 300 mm, stirred by a six bladed "Rushton turbine" with D=T/3 placed at C=T/3 from tank bottom. The vessel was filled with deionized water up to an eight H=T under no agitation conditions. A schematic diagram of the experimental apparatus is reported in Fig. 1. It is a geometrical configuration identical to the well known standard configuration, with the only difference that no baffles are provided here, that might therefore be termed as the u-standard configuration. The stirrer shaft was driven by a 1200W DC motor (Mavilor MSS-12), equipped with tacho and speed control unit (Infranor SMVEN 1510) so that rotational speed was maintained constant within 0.1%. Rotational speeds ranged from 100 to 1300 rpm in order to explore different fluid-dynamics regimes occurring inside the unbaffled stirred reactor. Liquid viscosity was increased by adding weighted amounts of polyvinylpyrrolidone (PVP) to distilled water in order to obtain liquid viscosities ranging from 10-3 to 15.2*10-3 Pa*s. PVP amounts used to obtain relevant liquid viscosities at 20 °C are reported in Table 1. A solution of glycerine and water was used to obtain the highest liquid viscosity of 40.7 *10-3 Pa*s. In all runs, temperature inside the reactor was maintained at 20 oC. This was obtained by adjusting the initial temperature and exploiting the circumstance that the temperature increase during each single run was always less than 0.2 oC. Liquid viscosity was measured by means an Ubbelhode capillary viscometer. A static frictionless turntable and a precision scale were employed for measuring the mechanical power dissipated by the impeller at various agitation speeds for each liquid viscosity investigated (Fig. 1). Details of this inexpensive yet precise apparatus may be found in Brucato et al.21. 4


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3. RESULTS AND DISCUSSION 3.1 Fluid dynamic regimes In Fig. 2 typical snapshots obtained at different agitation speeds with deionized water, are reported. As it can be seen, at the lowest rotational speeds (Figs. 2 a -c) no gas dispersion is present inside the reactor and a vortex appears on the free surface with a depth increasing as rotational speed increases ( sub-critical regime). At the highest velocities (Figs. 2 d and e) vortex bottom has already overcome the impeller leading to bubble injection in the liquid phase (clearly visible in the images) thus resulting in the formation of a gas-liquid dispersion (super-critical regime). Notably, bubbles ingested by the liquid phase are radially entrained by the impeller stream towards vessel wall creating a gas dispersion as well as to the well known gas-cavities behind impeller blades. Then, while moving upwards under the effect of gravity, they undergo a centripetal acceleration towards the central vortex, due to their smaller density with respect to the liquid phase. Notably, at the impeller speed at which vortex bottom reaches the impeller plane (Fig. 2c) still no bubbles are ingested in the liquid phase: this implies that in order to reach the super-critical regime, characterized by a significant gas ingestion, agitation speed needs to be further increased.

3.2 Power consumption The specific power dissipation values obtained at the various agitation speeds by the static frictionless turntable are reported in Figure 3-a for the smallest viscosity (distilled water , 10-3 Pa*s) and for the highest viscosity (water-glycerol solution, 41.7*10-3 Pa*s) here investigated. As it can be seen, at each agitation speed power requirements of the higher viscosity fluid are significantly larger than those of the low viscosity fluid, a result somewhat expected. For both liquids in Fig.3-a the expected steep increase of power dissipation with agitation speed is also observed. However, 5



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power dissipation appears to follow two different dependencies on agitation speed, due to the different flow regimes: in the sub-critical regime the specific power dissipation follows a power law with an exponent of about 3, while beyond some critical agitation speed (indicated in the figure by arrows) a net reduction of the power law exponent is observed, likely due to the presence of the dispersed gas phase inside the reactor, as confirmed by visual observation. This may well depend on the formation of gas pockets (called cavities) behind stirrer blades, as observed in sparged gasliquid reactors by many authors22-24. Another phenomenon that was visually observed and is likely to play a role (if not the major role) in mechanical power demand, is that when increasing velocity in supercritical unbaffled tanks, an increasingly large portion of the impeller blades falls within the vortex and is therefore not in contact any more with the liquid phase. This regards both front and back sides of the blades and the uncovered portion clearly becomes unable to contribute any more to torque. Conversely, the portion of the blade immersed in the liquid, the sole contributing to torque and power drawn, becomes smaller and smaller the larger the agitation speed, which might justify the regular, monotonically decreasing, power number trends observed. Notably this phenomenon has no counterpart in the case of baffled tanks, marking therefore a significant difference between the two vessel types. The same data of Fig.3a are reported in Fig. 3b in terms of power number (Np) values, defined as

Np =

P ρ L N 3 D5

(1)

where P is agitation power, ρL is liquid density, N is agitation speed (s-1) and D is impeller diameter. As it can be observed there, at sub-critical agitation speeds power number is a weak function of Re, as already observed by Rushton et al.16. After critical conditions are achieved, Np dependence on Reynolds number changes to a power law close to a simple reverse proportionality. 6 ACS Paragon Plus Environment

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In practice, in this regime the already quoted phenomena affecting power demand give rise to a quite simple dependence of Np on Re, which implies in turn a simple proportionality between P and N2. Clearly when choosing a correlation for power number prediction in unbaffled uncovered stirred reactors, it is very important to specify whether sub-critical or super-critical conditions are met. The power numbers obtained in all experimental runs are reported in Fig. 4 as a function of Reynolds number. As it can be seen, all power numbers obtained do follow the general trends already depicted in Fig.3b. As regards the data obtained in subcritical conditions these do line up quite well on a smooth Np versus Re power law, as it can be better appreciated in Fig.5, where only the subcritical regime data are reported, together with the best fit power law (solid line):

Np=19.5*Re-0.3

(2)

that can be considered to hold for 200

Power Consumption in Uncovered Unbaffled ... - ACS Publications

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