Pages From Chapter 10 Mixing and Agitation-cc05230cc2912d6536a3d533ce19a9f3

http://www.download-it.org/learning-resources.php?promoCode=&partnerID=&content=story&storyID=19976

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 10---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------MIXING AND AGITATION

Mixingthe movement of fluids and solids toenhance a process resultis accomplished bymeans of an agitation source. For example, thesun is the agitation source for mixing in the

earths atmosphere. Similarly, an air compressor and/or amechanical mixer is the agitation source in any municipalwastewater treatment plant to enhance the process results of(1) solids suspension and (2) oxygen absorption fromsparged or entrained air.

In its most general sense, the process of mixing isconcerned with all combinations of phases, of which themost frequently occurring are

1. Gases with gases2. Gases into liquids: gas dispersion3. Gases with granular solids: fluidization, pneumatic

conveying, drying4. Liquids into gases: spraying and atomization5. Liquids into liquids: dissolution, emulsification,

dispersion6. Liquids with granular solids: solids suspension, mass

transfer, and dissolution7. Pastes with each other and with solids8. Solids with solids: mixing of powders

Interaction of three phasesgases, liquids, and solidsmay also occur, as in the hydrogenation of a vegetable oil inthe presence of a suspended solid nickel catalyst in ahydrogen-sparged, mechanically agitated reactor.

Three of the processes involving liquidsnumbers 2, 5,and 6 in the preceding listemploy the same equipment;namely, tanks in which the liquid is circulated and subjectedto a desired level of shear. Mixing involving liquids has beenmost extensively studied and is most important in practice;thus, fluid mixing will be given most coverage here. Manymixing process results can be designed a priori, by using themixing literature without resorting to experimental studies.These include agitator power requirements, heat transfer,liquid-liquid blending, solids suspension, mass transfer to

suspended particles, and many solid-solid applications.However, many other applications invariably involveexperimental work followed by scale-up. These includeliquid-liquid, gas-liquid, and fast competitive chemicalreactions. Scale-up is addressed here, and, as we coverscaleup, the reader will discover that an understanding ofmixing fundamentals is essential to the proper handlingof scale-up.

This introduction would be incomplete without a shortdiscussion of the place of this chapter in the toolbox of thepracticing engineer. Todays engineer is faced with thedaunting task of separating the truly practical andimmediately useful design methods from the voluminousavailable literature. For example, the recent Handbook ofIndustrial Mixing (Paul et al., 2004) is comprised of 1,377pages devoted only to the topic of Mixing and Agitation.Some of the coverage in that tome can be used with aminimum of effort; however, much of the coverageincludes a literature survey with little emphasis on siftingthe truly useful from the mundane and ordinary. It isour intent here to sift through the entire literature in thefield of Mixing and Agitation and present only thatmaterial which ismost useful to the busy practicing engineerand to present worked examples that apply the designmethods.

In addition to the Handbook of Industrial Mixing thereare at least 20 Mixing and Agitation books listed in theReferences. In todays electronic world there are also manyweb sites of equipment vendors that provide very valuablevendor design information. Among those sites arewww.chemineer.com, www.clevelandmixer.com,www.lightnin-mixers.com, www.proquipinc.com,www.philadelphiamixers.com, andwww.sulzerchemtech.com. All of these mentioned sitescontain product information, but the Chemineer site (to agreat extent) and the Lightnin site (to a lesser extent) containuseful design-oriented technical literature. The annualChemical Engineering Buyers Guide is a good source forvendor identificatrion.

10.1. A BASIC STIRRED TANK DESIGN

Figure 10.1 gives a typical geometry for an agitated vessel.

Typical geometrical ratios are: D=T 1=3; B=T 1=12 (B=T 1=10 in Europe); C=D 1 and Z=T 1. This so-called typicalgeometry is not economically optimal for all process results (e.g.,

optimal C/D for solids suspension is closer to C=D 1=3 than toC=D 1); as appropriate, the economical optimum geometry willbe indicated later. Four full baffles are standard; they extend the

full batch height, except baffles for dished bottoms may terminate

near the bottom head tangent line. Baffles are normally offset from

the vessel wall about B/6. The typical batch is squarethat is, the

batch height equals the vessel diameter (Z=T 1). The vesselbottom and top heads can be either flat or dished. For axial flow

impellers (discussed later) a draft tube, which is a centered cylinder

with a diameter slightly larger than the impeller diameter and about

two-thirds Z tall, is placed inside the vessel. Sterbacek and Tausk

(1965, p. 283) illustrate about a dozen applications of draft tubes,

and Oldshue (1983, pp. 469492) devotes a chapter to their design.

OFF-CENTER ANGLED SHAFT ELIMINATES VORTEXINGAND SWIRL

For axial flow impellers, the effect of full baffling can be achieved in

an unbaffled vessel with an off-center and angled impeller

shaft location. J. B. Fasano of Chemineer uses the following guide-

line: (1) vendors normally supply a 108 angled riser (2) at the vesseltop, looking along the vessel centerline, move up (a) 0.19T and then

(b) 0:17LS to the right (3) position the agitator with the angled shaftpointing left. Vendors can help to provide optimum positioning.

An offset impeller location, illustrated in Figure 10.3(b) will

not totally eliminate vortexing, but it will eliminate most swirl, give

273

Copyright 2010 Elsevier Inc. All rights reserved.DOI: 10.1016/B978-0-12-372506-6.00010-1

Buy this file: http://www.download-it.org/learning-resources.php?promoCode=&partnerID=&content=story&storyID=19976


Platecoil Baffle

RotatedPlatecoil Baffle

Harp Tube BankBaffle

Rotated HarpTube Bank Baffle

4545

Top View notIntended toCorrespond

Exactly to SideView

T

1.5 x d1, Typical

Helical Coils Attached toWall Baffles

2 x d1

d1 = T/30, Typical

Wall Baffles,Four Total

T/12

Typical Tube RowSpacing = d1

d1 = T/30, Typical

T/3,Typical

T/3

Z = T,Typical

Figure 10.1. Agitated vessel standard geometry showing impeller, baffles, and heat transfer surfaces.

274 MIXING AND AGITATION



good top-to-bottom turnover, and keep the vortex from reaching

the impeller.

INTERNAL HEAT TRANSFER SURFACES

Heat transfer surfaceshelical coils, harp coils, or platecoilsare

often installed inside the vessel and jackets (both side wall and

bottom head) so that the vessel wall and bottom head can be used

as heat transfer surfaces. Figure 10.1 gives a suggested geometry for

helical coils and harp coils.

IMPELLER SPEEDS

With 1750 rpm electric motors, standard impeller speeds (Paul et

al., 2004, p. 352) are 4, 5, 6, 7.5, 9, 11, 13.5, 16.5, 20, 25, 30, 37, 45,

56, 68, 84, 100, 125, 155, 190, 230, 280, 350, and 1750. In addition,

1200 rpm electric motors are readily available.

IMPELLER TYPES

Twelve common impeller types are illustrated in Figure 10.2. Im-

pellers (a) through (i) and (k) in Figure 10.2 are available world-

wide. Impellers (j) (the Intermig) and (l) (the Coaxial [Paravisc

Outside and Viscoprop inside]) are available only from Ekato.

Key factors to aid in selection of the best impeller to enhance

desired process result(s) are as follows:

(a) The three-bladed Marine Propeller (MP) was the first axial-

flow impeller used in agitated vessels. It is often supplied with

fixed and variable speed portable agitators up to 5 hp with

impeller diameters (D) up to 600. Above D 600, marine propel-lers are too heavy and too expensive to compete with hydrofoil

impellers. They are usually applied at high speeds (up to

1750 rpm) in vessels up to 500 gal, with a viscosity limit of

about 5000 cp. Lower NRe limit: 200.(b) The impeller shown is the Chemineer HE-3 hydrofoil, high

efficiency impeller, but all vendors have competitive impellers

(e.g., Lightnin offers the A310 hydrofoil impeller). Hydrofoils

are used extensively for high flow, low shear applications such

as heat transfer, blending, and solids suspension at all speeds in

all vessels. The economical optimum D=T (0:4 > [D=T]optimum> 0:6) is greater for hydrofoils than for higher shear impellers.Lower NRe limit: 200.

(c) The 6-blade disk (the 6BD and, historically, the Rushton tur-

bine) impeller is ancient; nevertheless, it still has no peer for

some applications. It invests the highest proportion of its power

as shear of all the turbine impellers, except those (e.g., the

Cowles impeller) specifically designed to create stable emul-

sions. It is still the preferred impeller for gas-liquid dispersion

for small vessels at low gas rates, it is still used extensively for

liquid-liquid dispersions, and it is the only logical choice for use

with fast competitive chemical reactions, as will be explained in

a later section of this chapter. Lower NRe limit: 5.(d) The 4-blade 458 pitched blade (4BP) impeller is the preferred

choice where axial flow is desired and where there is a need for a

proper balance between flow and shear. It is the preferred

impeller for liquid-liquid dispersions and for gas dispersion

from the vessel headspace (located about D/3 to D/2 below

the free liquid surface), in conjunction with a lower 6BD or a

concave blade disk inpeller. Lower NRe limit: 20.(e) The 4-blade flat blade (4BF) impeller is universally used to

provide agitation as a vessel is emptied. It is installed, normally

fittedwith stabilizers, as low in thevessel as is practical.Anupper

HE-3 or a 4BP is often installed at about C=T 12to provide

effective agitation at high batch levels. Lower NRe limit: 5

(f) The 6-blade disk-style concave blade impellers (CBI) [the

Chemineer CD-6, which uses half pipes as blades, is shown] are

used extensively and economically for gas dispersion in large

vessels (in fermenters up to 100,000 gal) at high gas flow rates.

The CBIs will handle up to 200% more gas without floodingthan will the 6BD, and the gassed power draw at flooding drops

only about 30%, whereas with a 6BD, the drop in power drawexceeds 50%.

(g) The sawtooth (or Cowles type) impeller is the ultimate at

investing its power as shear rather than flow. It is used exten-

sively for producing stable liquid-liquid (emulsions) and dense

gas-liquid (foams) dispersions. It is often used in conjunction

with a larger diameter axial-flow impeller higher on the shaft.

Lower NRe limit: 10.(h) The helical ribbon impeller and the Paravisc (l) are the impellers

of choice when turbines and anchors cannot provide the neces-

sary fluid movement to prevent stratification in the vessel. The

turbine lower viscosity limit, for a Newtonian fluid, is deter-

mined primarily by the agitation Reynolds number

(Re ND2r=m). For 6BD and 4BF turbines, Fasano et al.,(1994, p. 111, Table 1) say Re > 1, and Hemrajani and Tatter-son (in Paul (2004), 345) say Re 10, although Novak andRieger (1975, p. 68, Figure 5) indicate a 6BD is just as effective

for blending as a helical ribbon above Re 1. Using Re 5 asthe 6BD lower limit with T 8000, D 3200, N 56rpm, SG 1,the upper viscosity limit for a 6BD is about m ND2r=Re (56=60)(0:0254 32)2(1, 000)=5 120 Pa s 120,000 cp.Thus, with this system, the helical ribbon is the impeller of

choice for m > (100,000 cP. Lower NRe limit: 0.(i) Anchor impellers are used for an intermediate range of

0:5 > Re > 10 because they are much less expensive than helicalribbons and they sweep the entire vessel volume; whereas a

turbine leaves stagnant areas near the vessel walls for Re < 10.Lower NRe limit: 2.

(j) The Ekato intermig impeller has reverse pitch on the inner and

outer blades and they are almost always used with

multiple impellers. They are used at high D/T and promote a

more uniform axial flow pattern than other turbine impellers.

They are advertised to be very effective for solids suspension,

blending, and heat transfer in the medium viscosity range.

Lower NRe limit not given by Ekato (9), perhaps 5.(k) The hollow-shaft self-gassing impeller can, if properly

designed, eliminate the need for a compressor by taking the

headspace gas and pumping it through the hollow shaft and

dispersing it into the batch as it leaves the hollow blades. As

indicated in the Ekato Handbook, Handbook of Mixing

Technology (2000, p. 164), the self-gassing hollow-shaft

impeller is often used in hydrogenation vessels where the

sparged hydrogen rate drops to very low levels near the end

of batch hydrogenation reactions.

(l) According to Ekato (2000, p. 85), The paravisc is particularly

suitable for highly viscous and rheologically difficultmedia. . . .

With products that are structurally viscous or have a pro-

nounced flow limit or with suspensions having a low liquid

content, the paravisc is used as the outer impeller of a coaxial

agitator system. The Ekato viscoprop is a good choice for the

counter-rotating inner impeller. There is not a lower NRe limit.

The coaxial, corotating agitator is an excellent choice for yield

stress fluids and shear thinning fluids.

10.2. VESSEL FLOW PATTERNS

The illustrations in Figure 10.3 show flow patterns in agitated

vessels. In unbaffled vessels with center mounting (Figure

10.3(a) ) much swirl and vortexing is produced, resulting in poor

10.2. VESSEL FLOW PATTERNS 275


The publisher detailed in the title page holds the copyright for this document

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recorded or otherwise, without the writtenpermission of Spenford IT Ltd who are licensed to reproduce this document by the publisher

All requests should by sent in the first instance to

[email protected]

Please ensure you have book-marked our website.

www.download-it.org

Chapter extract

To buy the full file, and for copyrightinformation, click herehttp://www.download-it.org/learning-resources.php?promoCode=&partnerID=&content=story&storyID=19976

Pages From Chapter 10 Mixing and Agitation-cc05230cc2912d6536a3d533ce19a9f3

Documents

Transcript of Pages From Chapter 10 Mixing and Agitation-cc05230cc2912d6536a3d533ce19a9f3