Pages From Chapter 10 Mixing and Agitation-cc05230cc2912d6536a3d533ce19a9f3

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10

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MIXING AND AGITATION

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suspended particles, and many solid-solid applications. However, many other applications invariably involve experimental work followed by scale-up. These include liquid-liquid, gas-liquid, and fast competitive chemical reactions. Scale-up is addressed here, and, as we cover scaleup, the reader will discover that an understanding of mixing fundamentals is essential to the proper handling of scale-up. This introduction would be incomplete without a short discussion of the place of this chapter in the toolbox of the practicing engineer. Today’s engineer is faced with the daunting task of separating the truly practical and immediately useful design methods from the voluminous available literature. For example, the recent Handbook of 1. Gases with gases Industrial Mixing (Paul et al., 2004) is comprised of 1,377 2. Gases into liquids: gas dispersion pages devoted only to the topic of Mixing and Agitation. 3. Gases with granular solids: fluidization, pneumatic Some of the coverage in that tome can be used with a conveying, drying http://www.download-it.org/learning-resources.php?promoCode=&partnerID=&content=story&storyID=19976 minimum of effort; however, much of the coverage 4. Liquids into gases: spraying and atomization includes a literature survey with little emphasis on sifting 5. Liquids into liquids: dissolution, emulsification, the ‘‘truly useful’’ from the ‘‘mundane and ordinary.’’ It is dispersion our intent here to sift through the entire literature in the 6. Liquids with granular solids: solids suspension, mass field of Mixing and Agitation and present only that transfer, and dissolution material which is most useful to the busy practicing engineer 7. Pastes with each other and with solids and to present worked examples that apply the design 8. Solids with solids: mixing of powders methods. In addition to the Handbook of Industrial Mixing there Interaction of three phases—gases, liquids, and solids— are at least 20 Mixing and Agitation books listed in the may also occur, as in the hydrogenation of a vegetable oil in References. In today’s electronic world there are also many the presence of a suspended solid nickel catalyst in a web sites of equipment vendors that provide very valuable hydrogen-sparged, mechanically agitated reactor. vendor design information. Among those sites are Three of the processes involving liquids—numbers 2, 5, www.chemineer.com, www.clevelandmixer.com, and 6 in the preceding list—employ the same equipment; www.lightnin-mixers.com, www.proquipinc.com, namely, tanks in which the liquid is circulated and subjected www.philadelphiamixers.com, and to a desired level of shear. Mixing involving liquids has been www.sulzerchemtech.com. All of these mentioned sites most extensively studied and is most important in practice; contain product information, but the Chemineer site (to a thus, fluid mixing will be given most coverage here. Many great extent) and the Lightnin site (to a lesser extent) contain mixing process results can be designed a priori, by using the useful design-oriented technical literature. The annual mixing literature without resorting to experimental studies. Chemical Engineering Buyer’s Guide is a good source for These include agitator power requirements, heat transfer, vendor identificatrion. liquid-liquid blending, solids suspension, mass transfer to ixing—the movement of fluids and solids to enhance a process result—is accomplished by means of an agitation source. For example, the sun is the agitation source for mixing in the earth’s atmosphere. Similarly, an air compressor and/or a mechanical mixer is the agitation source in any municipal wastewater treatment plant to enhance the process results of (1) solids suspension and (2) oxygen absorption from sparged or entrained air. In its most general sense, the process of mixing is concerned with all combinations of phases, of which the most frequently occurring are

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. 469–492) devotes a chapter to their design.

10.1. A BASIC STIRRED TANK DESIGN Figure 10.1 gives a ‘‘typical’’ geometry for an agitated vessel. ‘‘Typical’’ geometrical ratios are: D=T ¼ 1=3; B=T ¼ 1=12 (B=T ¼ 1=10 in Europe); C=D ¼ 1 and Z=T ¼ 1. This so-called typical geometry is not economically optimal for all process results (e.g., optimal C/D for solids suspension is closer to C=D ¼ 1=3 than to C=D ¼ 1); as appropriate, the economical optimum geometry will be 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 ‘‘square’’—that is, the batch height equals the vessel diameter (Z=T ¼ 1). The vessel bottom 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

OFF-CENTER ANGLED SHAFT ELIMINATES VORTEXING AND 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 guideline: (1) vendors normally supply a 108 angled riser (2) at the vessel top, 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 shaft pointing 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 ß 2010 Elsevier Inc. All rights reserved. DOI: 10.1016/B978-0-12-372506-6.00010-1

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274 MIXING AND AGITATION Top View not Intended to Correspond Exactly to Side View

Platecoil Baffle

Rotated Harp Tube Bank Baffle

Rotated Platecoil Baffle

45⬚

45⬚

Harp Tube Bank Baffle

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T

1.5 x d1, Typical Helical Coils Attached to Wall Baffles

2 x d1

Wall Baffles, Four Total T/12

d1 = T/30, Typical

Typical Tube Row Spacing = d1 d1 = T/30, Typical Z = T, Typical

T/3, Typical

T/3

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

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10.2. VESSEL FLOW PATTERNS

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

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(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. INTERNAL HEAT TRANSFER SURFACES The CBIs will handle up to 200% more gas without flooding Heat transfer surfaces—helical coils, harp coils, or platecoils—are than will the 6BD, and the gassed power draw at flooding drops often installed inside the vessel and jackets (both side wall and only about 30%, whereas with a 6BD, the drop in power draw bottom head) so that the vessel wall and bottom head can be used exceeds 50%. as heat transfer surfaces. Figure 10.1 gives a suggested geometry for (g) The sawtooth (or Cowles type) impeller is the ultimate at helical coils and harp coils. investing its power as shear rather than flow. It is used extensively for producing stable liquid-liquid (emulsions) and dense gas-liquid (foams) dispersions. It is often used in conjunction IMPELLER SPEEDS with a larger diameter axial-flow impeller higher on the shaft. With 1750 rpm electric motors, standard impeller speeds (Paul et Lower NRe limit:  10. al., 2004, p. 352) are 4, 5, 6, 7.5, 9, 11, 13.5, 16.5, 20, 25, 30, 37, 45, (h) The helical ribbon impeller and the Paravisc (l) are the impellers 56, 68, 84, 100, 125, 155, 190, 230, 280, 350, and 1750. In addition, of choice when turbines and anchors cannot provide the neces1200 rpm electric motors are readily available. sary fluid movement to prevent stratification in the vessel. The turbine lower viscosity limit, for a Newtonian fluid, is determined primarily by the agitation Reynolds number IMPELLER TYPES (Re ¼ ND2 r=m). For 6BD and 4BF turbines, Fasano et al., Twelve common impeller types are illustrated in Figure 10.2. Im(1994, p. 111, Table 1) say Re > 1, and Hemrajani and Tatterpellers (a) through (i) and (k) in Figure 10.2 are available worldson (in Paul (2004), 345) say Re  10, although Novak and http://www.download-it.org/learning-resources.php?promoCode=&partnerID=&content=story&storyID=19976 wide. Impellers (j) (the Intermig) and (l) (the Coaxial [Paravisc Rieger (1975, p. 68, Figure 5) indicate a 6BD is just as effective Outside and Viscoprop inside]) are available only from Ekato. for blending as a helical ribbon above Re  1. Using Re ¼ 5 as Key factors to aid in selection of the best impeller to enhance the 6BD lower limit with T ¼ 8000 , D ¼ 3200 , N ¼ 56rpm, SG ¼ 1, desired process result(s) are as follows: the upper viscosity limit for a 6BD is about m ¼ ND2 r=Re ¼ (56=60)(0:0254  32)2 (1, 000)=5 ¼ 120 Pa  s ¼ 120,000 cp. (a) The three-bladed Marine Propeller (MP) was the first axialThus, with this system, the helical ribbon is the impeller of flow impeller used in agitated vessels. It is often supplied with choice for m > (100,000 cP. Lower NRe limit: ¼ 0. fixed and variable speed portable agitators up to 5 hp with (i) Anchor impellers are used for an intermediate range of impeller diameters (D) up to 600 . Above D ¼ 600 , marine propel0:5 > Re > 10 because they are much less expensive than helical lers are too heavy and too expensive to compete with hydrofoil ribbons and they sweep the entire vessel volume; whereas a impellers. They are usually applied at high speeds (up to turbine leaves stagnant areas near the vessel walls for Re < 10. 1750 rpm) in vessels up to 500 gal, with a viscosity limit of Lower NRe limit:  2. about 5000 cp. Lower NRe limit:  200. (j) The Ekato intermig impeller has reverse pitch on the inner and outer blades and they are almost always used with (b) The impeller shown is the Chemineer HE-3 hydrofoil, high multiple impellers. They are used at high D/T and promote a efficiency impeller, but all vendors have competitive impellers more uniform axial flow pattern than other turbine impellers. (e.g., Lightnin offers the A310 hydrofoil impeller). Hydrofoils They are advertised to be very effective for solids suspension, are used extensively for high flow, low shear applications such blending, and heat transfer in the ‘‘medium viscosity’’ range. as heat transfer, blending, and solids suspension at all speeds in Lower NRe limit not given by Ekato (9), perhaps  5. all vessels. The economical optimum D=T (0:4 > [D=T]optimum (k) The hollow-shaft self-gassing impeller can, if properly > 0:6) is greater for hydrofoils than for higher shear impellers. designed, eliminate the need for a compressor by taking the Lower NRe limit:  200. headspace gas and pumping it through the hollow shaft and (c) The 6-blade disk (the 6BD and, historically, the Rushton turdispersing it into the batch as it leaves the hollow blades. As bine) impeller is ancient; nevertheless, it still has no peer for indicated in the Ekato Handbook, ‘‘Handbook of Mixing some applications. It invests the highest proportion of its power Technology’’ (2000, p. 164), the ‘‘self-gassing’’ hollow-shaft as shear of all the turbine impellers, except those (e.g., the impeller is often used in hydrogenation vessels where the Cowles impeller) specifically designed to create stable emulsparged hydrogen rate drops to very low levels near the end sions. It is still the preferred impeller for gas-liquid dispersion of batch hydrogenation reactions. for small vessels at low gas rates, it is still used extensively for (l) According to Ekato (2000, p. 85), ‘‘The paravisc is particularly liquid-liquid dispersions, and it is the only logical choice for use suitable for highly viscous and rheologically difficult media. . . . ’’ with fast competitive chemical reactions, as will be explained in With products that are structurally viscous or have a proa later section of this chapter. Lower NRe limit:  5. nounced flow limit or with suspensions having a low liquid (d) The 4-blade 458 pitched blade (4BP) impeller is the preferred content, the paravisc is used as the outer impeller of a coaxial choice where axial flow is desired and where there is a need for a agitator system.’’ The Ekato viscoprop is a good choice for the proper balance between flow and shear. It is the preferred counter-rotating inner impeller. There is not a lower NRe limit. impeller for liquid-liquid dispersions and for gas dispersion from the vessel headspace (located about D/3 to D/2 below The coaxial, corotating agitator is an excellent choice for yield the free liquid surface), in conjunction with a lower 6BD or a stress fluids and shear thinning fluids. concave blade disk inpeller. Lower NRe limit:  20. (e) The 4-blade flat blade (4BF) impeller is universally used to 10.2. VESSEL FLOW PATTERNS provide agitation as a vessel is emptied. It is installed, normally The illustrations in Figure 10.3 show flow patterns in agitated fitted with stabilizers, as low in the vessel as is practical. An upper vessels. In unbaffled vessels with center mounting (Figure HE-3 or a 4BP is often installed at about C=T ¼ 12 to provide 10.3(a) ) much swirl and vortexing is produced, resulting in poor effective agitation at high batch levels. Lower NRe limit:  5

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