Dear Dan,

your question is not really clear ? can you explain more also can you show the type of model you are dealing with.

Mohamed Omar

Thank you for your reply.

I uploaded a quick sketch of the system, since the industrial scale equipment and design details are protected by our company in agreement with the vendor.

In big terms, we have a vessel, in the shape of truncated cone. The Rotor -Stator impeller is positioned at the base. The fluid intake is marked with blue line and the output is marked with red (The RSD takes fluid via it's Z axis and then it accelerates it radially and pumps it back in the vessel via a set of channels).

With green I marked solid particles, that need to be kept in suspension.

My question was related to Zwietering's method of computing the minimum impeller speed in order to achieve suspension of the solid particles. Most of the articles I read are about classical impellers, without any casings or screens, such as PBTs, Hydrofoils, etc. Would it be justified to approach such a problem (RSD at the bottom of the tank) using Zwietering's method ?

I know, the problem could be simulated but I'm looking for a quick approximation right now.

Thanks in advance

Dear Dan,

Thank you for sending the sketch of the system. you basically asking if the correlation proposed by Zwietering to determine the critical hold speed or minimum hold speed, Njs is valid for your model. The Equation you provided above defines the expression for determining the Njs, where the density difference (in the given equation) plays a great influence on the minimum fluidization angular speed (Njs). The particle size, viscosity, and concentration, on the other hand, have a lower impact on determining the minimum speed.

The Mass fraction or concentration of solid (X), can be expressed in terms of percentage of solid mass. Although recent studies support the original findings of Zwietering. However, some researchers claim that there is still a limited range of test parameters.

So the answer to your question, can you use the Zwietering correlation, the answer is yes as long as you are aware of the limitations and applicability of Zwietering's in finding the minimum speed:

• The values of "S" must be previously known; (From the D/T and C/T relationship, it is possible to determine the value of S)

• The variable D/T, in turn, has a significant value in determining "S".

• In some cases, not applicable outside of experimental conditions (T ~ 0.1 m - 0.6 m; D/T ~ 1/6 - 1/2; C/T ~ 1/20 - 1/2);

• Applies to free and hindered sedimentation conditions, that is, low to moderate concentrations of solids by mass (X ~ 0.5% - 20%);

• The particle diameter must be contained in the range of 140 to 520 micrometers.

also recently, many correlations for obtaining Njs have been developed for different geometries and operating conditions. it is worth looking at those correlations.

thanks for reply !