diseño recipientes a presión y sistemas de agitación

7/21/2019 Diseño Recipientes a Presión y Sistemas de Agitación

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Escuela Politécnica Nacional

Carrera de Ingeniería Química

Diseño de plantas

Manual para el diseño de recipientes a presión y sistemas de agitación

Materiales



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Recipientes a Presión



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Sistemas de Agitación



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a) Propeller, Marine

type

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 6″. Above d = 6″, marine propellers 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 Relimit: ~ 200.

b) Hydrofoil 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/Di (0,4 > d/Di optimum> 0,6) is greater for hydrofoilsthan for higher shear impellers.


c) 6-blade disk

(turbine)

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

emulsions. 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.

Lower Relimit: ~ 5.

d) 4-blade 45°pitched

blade (4BP) turbine

The 4-blade 45°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/2below the free liquid surface), in conjunction with a lower 6BD or a concave blade disk impeller.


e) 4-blade flat blade

(4BF) impeller

The 4-blade flat blade (4BF) impeller is universally used to provide agitation as a vessel is emptied. It is

installed, normally fitted with stabilizers, as low in the vessel as is practical. An upper HE-3 or a 4BP is

often installed at about Hi /Di= 0,5 to provide effective agitation at high batch levels.

Lower Relimit: ~ 5.

f) 6-blade disk-style

concave blade

impellers (CBI)

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 flooding than will the 6BD, and the gassed power draw at flooding drops only about 30%,

whereas with a 6BD, the drop in power draw exceeds 50%.

g) Sawtooth (Cowlestype) The sawtooth impeller is the ultimate at investing its power as shear rather than flow. It is usedextensively 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.


h) Helical ribbon

impeller

The helical ribbon impeller and the Paravisc are the impellers of choice when turbines and anchors

cannot provide the necessary 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. 6BD

and 4BF turbines at Re > 1, are just as effective for blending as a helical ribbon above. The helical

ribbon is the impeller of choice for μ>100000 cP. Lower Re limit: = 0.

i) Anchor impeller Anchor impellers are used for an intermediate range of 0,5 > Re > 10 because they are much less

expensive than helical ribbons and they sweep the entire vessel volume; whereas a turbine leaves

stagnant areas near the vessel walls for Re < 10.

Lower Relimit: ~ 2.

j) Ekato Intermig

impeller

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/Di 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 Relimit: ~ 5.

k) Hollow-shaft

impeller

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) Paravisc According to Ekato (2000, p.85), “The paravisc is particularly suitable for highly viscous and

rheologically difficult media.…” With products that are structurally viscous or have a pronounced flow

limit or with suspensions having a low liquid content, the paravisc is used as the outer impeller of acoaxial agitator system.” The Ekato viscoprop is a good choice for the counter-rotating inner impeller.

There is not a lower Relimit. The coaxial, corotating agitator is an excellent choice for yield stress fluids

and shear thinning fluids.



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Transferencia de Calor



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diseño recipientes a presión y sistemas de agitación

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