Agitator design for solids suspension under gassed conditions

Abstract

Design of mechanical agitators for solids suspension under gassed conditions can be achieved using a constant ungassed torque criterion if the solids suspension task is more difficult than the gas dispersion task. When gas dispersion is the more difficult task, design for complete gas dispersion will ensure solids suspension. Evidence supporting this approach is very strong for radial-flow impellers, but less so for up-pumping and down-pumping axial-flow impellers.La conception d'agitateurs mecaniques pour la suspension de solides en conditions aeries peut Ctre effectuee a I'aide d'un critere de couple non aere constant lorsque la suspension des solides est plus difficile a realiser que la dispersion du gaz. Lorsque la dispersion du gaz est plus difficile a realiser. une conception basee sur la dispersion complete du gaz assurera la suspension des solides. Les preuves soutenant cette approche sont tres evidentes pour les turbines a ecoulement radial, mais moins pour les turbines a ecoulement axial a pompage ascendant eta pompage descendant.Keywords: gassed solids suspension, solids suspension, gas dispersion, three-phase agitation.gitator design for three-phase (gassolid-liquid) opera-A tion is a challenging and industrially-relevant task. Chapman et al. (1983) investigated this problem in detail, relating the just-suspended speed to liquid and solid physical properties in a Zwietering-like (1958) manner. For radial-flow impellers they found that the increase in justsuspended speed under aerated conditionswas linearly related to the volumetric gas flow rate. Suchsoa simple relation was not found for up-pumping or downpumping axial-flow impellers. Frijlink et al. (1990) related the increase in just-suspended speed under aerated conditions to the cavity structure that forms on aerated impeller blades and the associated drop in impeller power draw. They correlated their gassed justsuspended speed data in the following manner:where the just-suspended power number was defined in terms of the liquid density:The exponent n of this correlation was found to be 1.1 for down-pumping axial-flow impellers, and 2 for radial-flow and up-pumping axial-flow impellers. Recently, Pantula and Ahmed (1 997) found these same exponents and pointed out that an exponent of 2, as found for radial-flow and up-pumping axial-flow impellers, corresponds to a constant torque criterion; that is, the just-suspended torque under gassed conditions is equal to the just-suspended torque under ungassed conditions *Author to whom correspondence may be addressed. E-mail address: **Present address: Fluent Inc., Lebanon, NH 03766, USA kmyersl @engr.udayton.edu (qsg = q s o ' where torque is defined as M = P/27ch9. There is no apparent physical meaning for an exponent of 1.1 as was found for down-pumping axial-flow impellers.The present work examines the constant-torque design criterion including comparison of the difficulty of the solids suspension and gas dispersion tasks. In addition, solid property effects are studied, since they were not considered b...

Critical Impeller Speed for Suspending Solids in Aerated Agitation Tanks

“…It should also be mentioned that although the present correlation provides a way of predicting just suspension impeller speed, it does not contradict the findings of Pantula and Ahmed (1997), i.e., that the torque required for achieving solids suspension is independent of the gassing rate and is the same as that in ungassed conditions (see also Lehn et al, 1999). The correlation proposed by Frijlink et al (1990), i.e., also implies constant torque at just suspension conditions.…”

“…However, for situations where very light particles (in the order of liquid density) are used, solid suspension is no longer more difficult to achieve than gas dispersion. The correlation should not be applicable, since the gas dispersion is the primary concern (Lehn et al, 1999). Further study is needed to validate the correlation for light solids particles.…”

“…Over recent decades, solid suspension in agitated tanks has been extensively researched (e.g., Zwietering, 1958;Chapman et al, 1983;Frijlink et al, 1990;Buurman et al, 1986;Wong et al, 1987;Bujalski et al, 1988;Raghava Rao et al, 1988;Rewatkar et al, 1991;Drewer et al, 1994, Dutta andPangarkar, 1995;Ibrahim and Nienow, 1996;Ahmed, 1997, 1998;Myers and Bakker, 1998;Lehn et al, 1999;Wu et al, 2000Wu et al, , 2001. See also, for example, Nienow et al (1985) and Nienow (1992) for reviews on the subject.…”

Review on Mixing Characteristics in Solid‐Liquid and Solid‐Liquid‐Gas Reactor Vessels

“…Recently, studies on the solid suspension in a three-phase stirred contactor with some modern impellers, like Scaba 6SRTG, Chemineer HE3 and Lightnin A315 etc., have also been reported (Wong et al, 1987;Neale and Pinches, 1994;Lehn et al, 1999). The schematic of the various impellers studied is shown in Figure 10.…”

“…Also, the new impellers such as BX04 and Scaba 6SRTG may be used for the solid suspension in a three-phase system as they are less susceptible to the presence of gas and require less power compared to the conventional radial fl ow disc turbine. Lehn et al (1999) js P/V same) when P/V same) when P/V the system is gassed to ensure satisfactory operating conditions in terms of suspension. Overall, it can be concluded that, the most stable impeller, requiring the least specifi c energy dissipation rate for solids suspension and gas dispersion under the most demanding conditions, was found to be the mixed fl ow impeller pumping in upward direction.…”