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Fabrizio Sarghini » 21.Liquid Mixing


Liquid Mixing

Food liquid mixtures could theoretically be sampled and analysed in the same way as previously described for solid mixtures, but complexity in flow pattern due to nonlinearity resulted in less investigational work on the mixing performance of fluid mixers.

Most of the available information concerns the power requirements for the most commonly used liquid mixers, like paddles or propeller stirrers, where the fluids to be mixed are collected together and then the stirrer is rotated.

Experimental measurements have been made in terms of dimensionless ratios involving all of the physical factors influencing power consumption, and the results have been correlated obtaining an equation of the form (1).

(1)

(1)


Liquid Mixing (cont’d)

where:

  • Re = (D2Nρ/μ) is the Reynolds number;
  • Po = (P/D5N3ρ) is called the Power number, relating drag forces to inertial forces, also known as Newton number;
  • Fr = (DN2/g is called the Froude number, relating inertial forces (V ρD2V2 see note for the velocity) and gravitational forces, (ρD3g) ;
  • D is the diameter of the propeller;
  • N is the rotational speed of the propeller (rev/sec);
  • ρ is the density of the liquid;
  • μ is the viscosity of the liquid;
  • P is the power consumed by the propeller.

[from J. Wei, J. L. Anderson, K. B. Bischoff Advances in Chemical Engineering, Volume 17, Academic Press (1991)]

Notice that the Reynolds number uses the product DN for the velocity, differing by a factor of π from the actual velocity at the tip of the propeller.


Liquid Mixing (cont’d)

In the study of stirred tanks, the Froude number governs the formation of surface vortices, and it correlates the effects of gravitational forces and it only becomes significant when the propeller disturbs the liquid surface.

Below Reynolds numbers of about 300, the Froude numbehasr  minimal effect, so that the previous equation becomes: (2).

In this case general formulae have not been obtained, so that the results are confined to the particular propeller configurations used in the experiment.

If experimental curves are available, then they can be used to give values for n and K in previous eqn. and the equation can be used to predict power consumption.

(2)

(2)


Liquid Mixing (cont’d)

 Stirred tank reactor

Stirred tank reactor


Liquid Mixing (cont’d)

Impeller types

Impeller types


Liquid Mixing (cont’d)

 Impeller types

Impeller types


Liquid Mixing (cont’d)

 High viscosity Impeller types

High viscosity Impeller types


Liquid Mixing (cont’d)

Adapted from Rushton J H, Costich E W & Everett H J. Power characteristics of mixing impellers.  Chem. Eng. Progr.,46, 1950.

Adapted from Rushton J H, Costich E W & Everett H J. Power characteristics of mixing impellers. Chem. Eng. Progr.,46, 1950.


Liquid Mixing (cont’d)

 Flow patterns in stirred tanks

Flow patterns in stirred tanks


Liquid Mixing (cont’d)

Multiple impeller configurations

Multiple impeller configurations


Liquid Mixing (cont’d)

 Anchor impeller configurations

Anchor impeller configurations


Liquid Mixing (cont’d)

Mixing in Pipelines

Most industrial mixing processes take place in tanks or vessels.

However, mixing often takes place also in the pipes connecting these process vessels, and sometimes the pipelines themselves serve as process vessels.

In many cases the pipe, especially when equipped with an internal static mixer device, is a better place to mix and it is more economical than a vessel.

In particular, it is convenient when fast blending is required or when long hold-ups in the mixing vessels are not desirable.

Liquid Mixing (cont’d)

Mixing devices in Pipelines:

  • Static mixer;
  • Tee mixer;
  • Impinging jet mixer;
  • Spray nozzle;
  • Empty pipe or duct, elbows, etc.;
  • In-line mechanical mixer.

Liquid Mixing (cont’d)

Pipeline mixing is convenient when:

  • The process is continuous;
  • The component feed rates are uniform;
  • Plug flow is preferred to backmixing;
  • Short residence time is desirable (long residence times requires special consideration involving slow reactions);
  • Solids are of consistent concentration and usually small particle size;
  • Gas phase continuous (bubble column or agitated tanks not applicable);
  • High pressure (seal concerns);
  • Limited space available, limited access–low maintenance desirable.

Liquid Mixing (cont’d)

 Tee jet mixer

Tee jet mixer


Liquid Mixing (cont’d)

Impinging jet micromixer
  from microinnova

Impinging jet micromixer from microinnova


Liquid Mixing (cont’d)

 Static mixer: helical mixers for high pressure and high viscosity applications 
. From: Aamhwamixr

Static mixer: helical mixers for high pressure and high viscosity applications . From: Aamhwamixr


Liquid Mixing (cont’d)

Static mixer

Static mixer


Liquid Mixing (cont’d)

 HEV Static mixer, chemineer

HEV Static mixer, chemineer


Liquid Mixing (cont’d)

 KMX Static mixer. From: Chemineer

KMX Static mixer. From: Chemineer


Liquid Mixing (cont’d)

 KMX Static mixer. From: Chemineer

KMX Static mixer. From: Chemineer


Liquid Mixing (cont’d)

 Stator Rotor mixer

Stator Rotor mixer


Liquid Mixing (cont’d)

 Stator Rotor inline mixer

Stator Rotor inline mixer


Liquid Mixing (cont’d)

 Stator Rotor batch mixer

Stator Rotor batch mixer


Liquid Mixing (cont’d)

Stator Rotor – stirred tank combined batch mixer

Stator Rotor – stirred tank combined batch mixer


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