Batch blend time in square stirred tanks
Introduction
Stirred tanks are often used in industry for blending and for homogeneous liquid phase reactions. In both cases, impeller and motor design requires calculation of blend time based on the impeller characteristics and the tank geometry. Many workers have measured and correlated blend time, particularly for cylindrical tanks. A well-accepted correlation is available for this geometry; however, almost no information is available for the square tank which is widely used in the water treatment industry (Clark et al., 1994). The objective of this work is to validate the blend time correlation for a range of impeller geometries in cylindrical tanks, and to define a blend time correlation equation for square tanks.
Section snippets
Experimental setup
A diagram of the equipment used to measure the blend time in the stirred tank is shown in Fig. 1. Experiments were carried out in cylindrical tanks with tank diameters of 0.14 and , and in square tanks with side lengths of 0.178 and . Four baffles of standard width were used in the cylindrical stirred tank with flat bottom. No baffles were used in the square tanks. The corners act as virtual baffles, as shown by the bold lines in Fig. 2. This prevents formation of a surface
Results and discussion
For all impellers tested, , the product of blend time and impeller rotational speed, was averaged from 12 runs in the big tanks and 6 runs in the small tanks. The power numbers in the square tank are taken to be the same as those for the cylindrical stirred tank. The power number for the paddle impeller is calculated from Sano and Usui (1985), others are standard literature values which were verified by Zhou and Kresta (1996). The results, shown in Fig. 5, agree very well with the published
Conclusions
The blend time correlation for cylindrical tanks can also be used for square tanks if the side dimension of the tank is used as the “diameter” and the correlation constant is increased from 5.8 to 6.0. This suggests that the square tank is a somewhat less efficient mixing geometry than the cylindrical tank.
Notation
C off bottom clearance of the impeller, measured to the bottom of the blades, m normalized concentration at probe i D impeller diameter, m H height of fluid, m number of blades N rotational speed of the impeller, rps power number Re impeller Reynolds number, dimensionless t time, s T diameter of cylindrical tank, m length of the side of the square tank, m W width of impeller blade, m Greek letters blade angle, deg blend time, s variance of normalized concentration
Acknowledgements
The authors would like to acknowledge Dallas Chapple, who provided technical support for the blend time measurements, as well as Lightnin and NSERC, who funded this project through a Strategic Grant.
References (6)
On impeller circulation and mixing effectiveness in the turbulent flow regime
Chemical Engineering Science
(1997)- Clark, M.M., Srivastava, R.M., Lang, J.S., Trussell, R.R., Mccollum, L.J., Bailey, D., Christie, J.D., Stolarik, G.,...
- et al.
Blending of miscible liquids
Cited by (29)
Experimental determination and computational prediction of blend time in the USP dissolution testing Apparatus 1
2023, Chemical Engineering Research and DesignExperimental mass and heat transfer at a serpentine tube heat exchanger located at the wall of a square stirred tank reactor
2023, Applied Thermal EngineeringTurbulent cylinder-stirred flow heat and momentum transfer research in batch operated single-phase square reactor
2022, International Journal of Thermal SciencesCitation Excerpt :The number of blades is considered to define its power number, although in turbulent regime this parameter needs more investigation [4], because different turbulence quantities were reported for specific regions of the flow like near impeller and bulk. Clearance was found to play a role in the mean flow obtained in cylindrical reactors [6]. He et al., 2019 [7] reported a floc morphology dependence on the clearance in baffled square tanks, which are used as a mean to enhance mixing.
Mass and heat transfer intensification at the wall of a square agitated vessel by chemically active semicylindrical turbulence promoters
2021, Alexandria Engineering JournalCitation Excerpt :Although turbulence promoters have been widely used to augment the rate of heat and mass transfer in flow ducts such as heat exchangers, continuous flow electrochemical reactor, and continuous membrane processes, little work has been investigated their use in improving the rate of heat and mass transfer in batch stirred tank reactors [20], where diffusion-controlled exothermic reactions take place. Square agitated vessels have the advantage over the cylindrical vessels in that they are simpler in design, self-baffled by virtue of their sharp corners [21], and consume less mechanical power [22]. Semicylindrical turbulence promoters were chosen to conduct the present study owing to their high efficiency compared to other geometries [23,24].
Concept of a swirling diffuser in batch blending tanks
2020, Chinese Journal of Chemical EngineeringCitation Excerpt :Therefore, characterizing each one of them would be aimless for the purpose of this work. The more so that in the problem bibliography for some time past one can distinguish two currents: first one undertaking general questions like experimental techniques (e.g. [1,2]) or the influence of the mixing chamber shape (e.g. [3,4]); second one devoted to the specialized aspects of the different technical branches (e.g. [5,6]). This paper contains a proposal of the first kind.