dari chemical engineering design principles, practice and economics of plant and process design

7/26/2019 Dari Chemical Engineering Design Principles, Practice and Economics of Plant and Process Design

1/5

Centrifugal pumps are effective inline mixers for blending and dispersing liquids.Various proprietary motor-driven inline mixers are also used for special applications;see Perry et al. (1997).

Stirred Tanks

Mixing vessels fitted with some form of agitator are the most commonly used type ofequipment for blending liquids and preparing solutions.

Liquid mixing in stirred tanks is covered in several textbooks; see Uhl and Gray

(1967), Harnby et al. (1997), Richardson et al. (1999), and Tatterson (1991, 1993).A typical arrangement of the agitator and baffles in a stirred tank and the flowpattern generated are shown in Figure 10.54. Mixing occurs through the bulk flow ofthe liquid and, on a microscopic scale, by the motion of the turbulent eddies createdby the agitator. Bulk flow is the predominant mixing mechanism required for theblending of miscible liquids and for solids suspension. Turbulent mixing is important

Figure 10.53. Static mixer (Kenics Corporation).

Figure 10.54. Agitator arrangements and flow patterns.

614 CHAPTER 10 EQUIPMENT SELECTION, SPECIFICATION, AND DESIGN


2/5

in operations involving mass and heat transfer, which can be considered as sheacontrolled processes.

The most suitable agitator for a particular application will depend on the type omixing required, the capacity of the vessel, and the fluid properties, mainly thviscosity.

The three basic types of impeller that are used at high Reynolds numbers (loviscosity) are shown in Figures 10.55a, b, and c. They can be classified according tthe predominant direction of flow leaving the impeller. The flat-bladed (Rushtonturbines are essentially radial-flow devices, suitable for processes controlled bturbulent mixing (shear-controlled processes). The propeller and pitched-bladturbines are essentially axial-flow devices, suitable for bulk fluid mixing.

Paddle, anchor, and helical ribbon agitators (see Figures 10.56a, b, and c) and othspecial shapes are used for more viscous fluids.

The selection chart given in Figure 10.57, which has been adapted from a similchart given by Penny (1970), can be used to make a preliminary selection of thagitator type, based on the liquid viscosity and tank volume.

For turbine agitators, impeller to tank diameter ratios of up to about 0.6 are use

with the depth of liquid equal to the tank diameter. Baffles are normally used, timprove the mixing and reduce problems from vortex formation. Anchor agitators a

Disc-mounted flat-blade turbine

Hub-mountedflate-bladeturbine

Hub-mountedcurved-bladeturbine

Shrouded turbineimpeller

(a)

(c)(b)

Figure 10.55. Basic impeller types. (a) Turbine impeller. (b) Pitched-bladed turbine. (c) Maripropeller.

10.11. MIXING EQUIPMENT 61


3/5

used with close clearance between the blades and vessel wall, anchor to tank diameterratios of 0.95 or higher. The selection of agitators for dispersing gases in liquids isdiscussed by Hicks (1976).

Agitator Power Consumption

The shaft power required to drive an agitator can be estimated using the followinggeneralized dimensionless equation:

Np KRebFrc (10:12)

where

Np power number P

D5N3r,

Re Reynolds number D2Nr

m

,

Fr Froude number DN2

g,

P shaft power, W;K a constant, dependent on the agitator type, size, and the agitator-tank

geometry;

(a) (b)

(c)

Figure 10.56. Low-speed agitators. (a) Paddle. (b) Anchor. (c) Helical ribbon.



4/5

r fluid density, kg/m3;m fluid viscosity, Ns/m2;N agitator speed, s1 (revolutions per second) (rps);

D agitator diameter, m;g gravitational acceleration, 9.81 m/s2.

Values for the constantKand the indicesbandcfor various types of agitator, tanagitator geometries, and dimensions can be found in the literature; see Rushton et a(1999). A useful review of the published correlations for agitator power consumptio

Anchor, helical ribbon

Paddle

Turbine

Propeller (420 rpm)or turbine

Turbine orpropeller(1750 rpm)

Propeller(1150rpm

)orturbine

Tank volume, m3

Liquid

viscosity,

Ns/m2

101 100 101 102102

101

100

101

102

103

Figure 10.57. Agitator selection guide.

10.11. MIXING EQUIPMENT 61


5/5

and heat transfer in agitated vessels is given by Wilkinson and Edwards (1972); theyinclude correlations for non-Newtonian fluids. Typical power curves for propeller andturbine agitators are given in Figures 10.58 and 10.59. In the laminar flow region, theindex b 1; and at high Reynolds number, the power number is independent of the

Froude number; index c 0.An estimate of the power requirements for various applications can be obtained

from Table 10.15.

Side-Entering Agitators

Side-entering agitators are used for blending low-viscosity liquids in large tanks,where it is impractical to use conventional agitators supported from the top of thetank; see Oldshue et al. (1956).

Where they are used with flammable liquids, particular care must be taken in thedesign and maintenance of the shaft seals, as any leakage may cause a fire.

For blending flammable liquids, the use of liquid jets should be considered as an

intrinsically safer option; see Fossett and Prosser (1949).

10.11.3. Solids and Pastes

A great variety of specialized equipment has been developed for mixing dry solids andpastes (wet solids). The principal types of equipment and their fields of application are

Figure 10.58. Power correlation for single three-bladed propellers baffled [from Uhl and Gray(1967) with permission]. p D blade pitch, D impeller diameter, DT tank diameter.


dari chemical engineering design principles, practice and economics of plant and process design

Documents