View
244
Download
0
Category
Preview:
Citation preview
8/17/2019 Chapter 6-Agitated Liquid
1/48
CHAPTER 6-AGITATED LIQUIDS
Introduction and Definition
Purpose of Agitation & Mixing
Agitated EquipmentTypes of Impeller
Flow Pattern in Agitated Vessel
Standard turbine design
8/17/2019 Chapter 6-Agitated Liquid
2/48
AGITATION & MIXING OF LIQUID
DEFINITIONS
• Agitation: It refers to the induced motion of a “homogenous” material in a
specified way. (eg: in a circulatory pattern in some container)
• Mixing:
It is the random distribution, into and through one another, of
two or more initially separate phases
PURPOSES OF AGITATION / MIXING
• Suspending solid particles
• Blending miscible liquids• Dispersing a gas through the liquid
• Dispersing a second liquid to form an emulsion or suspension
• Promoting heat transfer
8/17/2019 Chapter 6-Agitated Liquid
3/48
AGITATION AND MIXING
8/17/2019 Chapter 6-Agitated Liquid
4/48
AGITATION AND MIXING
8/17/2019 Chapter 6-Agitated Liquid
5/48
AGITATION AND MIXING
8/17/2019 Chapter 6-Agitated Liquid
6/48
AGITATION AND MIXING
8/17/2019 Chapter 6-Agitated Liquid
7/48
INTRODUCTION TO MIXING
Mixing is one of themost common operationscarried
out in the chemical, processing and allied industries.
The term "mixing" is applied to the processes used to
reduce the degree of non-uniformity, or gradient of a
property in a system such asconcentration,viscosity,
temperature and so on.
Mixing is achieved by moving material from one region to
another. It may be of interest simply as a means of
achieving a desired degree of homogeneity but it may alsobe usedto promote heat and mass transfer, often
where a system is undergoing a chemical reaction.
8/17/2019 Chapter 6-Agitated Liquid
8/48
TYPE OF MIXING
Single-phase liquid mixing
Mixing of immiscible liquids
Gas-liquid mixing
Liquid-solids mixing
Gas-liquid-solids mixing
Solids-solids mixing
8/17/2019 Chapter 6-Agitated Liquid
9/48
INTRODUCTION TO MIXING
1. Single-phase liquid mixing:
Two or moremiscible liquidsmust be mixed to give a
product of a desired specification.
This isthe simplest type of mixing as it involves neither
heat nor mass transfer, nor indeed a chemical reaction.
Example:
1.The use of mechanical agitation to enhance the rates of heat
and mass transfer between the wall of a vessel, or a coil, andthe liquid (brine solution= HCl+H2O).
2.In the blending of petroleum products of different viscosities.
8/17/2019 Chapter 6-Agitated Liquid
10/48
MIXING
2. Mixing of immiscible liquids:
Whentwo immiscible liquids are stirred together, one phase
becomes dispersed as tiny droplets in the second liquid whichforms a continuous phase.
Example:Liquid-liquid extraction, a process using successive
mixing and settling stages.
The liquids are brought into contact with a solvent that willselectively dissolve one of the components present in the mixture.
Vigorous agitation causes one phase to disperse in the other and,
if the droplet size is small, a high interfacial area is created for
interphase mass transfer.
When the agitation is stopped, phase separation takes place, but
care must be taken to ensure that the droplets are not so small
that a diffuse layer appears in the region of the interface; this can
remain in a semi-stable state over a long period of time and
prevent effective separation from occurring.
8/17/2019 Chapter 6-Agitated Liquid
11/48
MIXING
3. Gas-liquid mixing: Numerous processing operations involving chemical reactions,
such as aerobic fermentation, wastewater treatment,
oxidation of hydrocarbons, and so on, require good contacting
between a gas and a liquid.
The purpose of mixing here isto produce a high interfacialarea by dispersing the gas phase in the form of bubbles into
the liquid.
Generally, gas-liquid mixtures or dispersions are unstable and
separate rapidly if agitation is stopped.
8/17/2019 Chapter 6-Agitated Liquid
12/48
MIXING
4. Liquid-solids mixing:
Mechanical agitation may be used to suspend particles in a liquidin orderto promote mass transfer or a chemical reaction.
The liquids involved in such applications are usually of low
viscosity, and the particles will settle out when agitation ceases.
5. Gas-liquid-solids mixing: In some applications such ascatalytic hydrogenation of
vegetable oils, slurry reactors, froth flotation, evaporative
crystallization, and so on, the success and efficiency of the
process is directly influenced by theextent of mixing between
the three phases.
8/17/2019 Chapter 6-Agitated Liquid
13/48
MIXING
6. Solids-solids mixing: Mixing together of particulate solids, sometimes referred to as
blending, is a very complex process in that it is very dependent,
not only on the character of the particles — density, size, size
distribution, shape and surface properties.
Mixing of sand, cement and aggregate to form concreteand of the ingredients in gunpowder preparation are examples of
the mixing of solids.
Other industrial sectors employing solids mixing include food,
drugs, and the glass industries.
8/17/2019 Chapter 6-Agitated Liquid
14/48
MIXING
Miscellaneous mixing applications:
Mixing equipment may be designed not only to achieve a
predetermined level of homogeneity, but also toimprove heat
transfer.
For example, the rotational speed of an impeller in a mixing
vessel is selected so as to achieve a required rate of heattransfer, and the agitation may then be more than sufficient
for the mixing duty.
Excessive or overmixing should be avoided as it is not only
wasteful of energy but may be detrimental to product quality.
It is therefore important to appreciate that overmixing mayoften be undesirable because it may result in both excessive
energy consumption and impaired product quality.
8/17/2019 Chapter 6-Agitated Liquid
15/48
MIXING
In mixing, there are two types of problems to be considered
— how to design and select mixing equipment for a givenduty, andhow to assesswhether a mixer is suitable for a
particular application. In both cases, the following aspects of
the mixing process should be understood:
(i) Mechanisms of mixing.(ii) Scale-up or similarity criteria,
(iii) Power consumption,
(iv) Flow patterns.
(v) Rate of mixing and mixing time.
(vi) The range of mixing equipment available and its
selection.
8/17/2019 Chapter 6-Agitated Liquid
16/48
A BASIC STIRRED TANK DESIGN
Amount of energy required for achieving a needed amount of
agitation or quality of mixing are based on;
Size of vesel
Dimensions and arrangement of impellers, baffles and other
internals factors.
The internal arrangements depend on the objectives of theoperation: whether it is to maintain homogeneity of a reacting
mixture or to keep a solid suspended or a gas dispersed or to
enhance heat or mass transfer.
A basic range of design factors, however, can be defined to coverthe majority of cases, for example as in Figure7.5.
8/17/2019 Chapter 6-Agitated Liquid
17/48
A BASIC STIRRED TANK DESIGN
8/17/2019 Chapter 6-Agitated Liquid
18/48
THE VESSEL:
A dished bottom requires less power than a flat one. When a
single impeller is to be used, a liquid level equal to the diameter isoptimum (DT=H), with the impeller located at the center for an
all-liquid system.
BAFFLES:
Baffles are needed to preventvortexing and rotation of the
liquid mass as a whole. A baffle width one-ten the tank
diameter,WB = DT /10; a lengthextending from one half the
impeller diameter,D/2,from the tangent line at the bottom to the
liquid level.
IMPELLER TYPES:
Typically, impeller is placed from the bottom vessel withZ A= D. A
basic classification is into those that circulate the liquidaxially
and those that achieve primarilyradial circulation.
A BASIC STIRRED TANK DESIGN
8/17/2019 Chapter 6-Agitated Liquid
19/48
TYPES OF IMPELLERS
A rotating impeller in a fluid imparts flow and shear to it, the
shear resulting from the flow of one portion of the fluid past
another. The flows are in the axial or radial directions so that impellers
are classified conveniently according to which of these flows is
dominant.
Those generate currents parallel with the axis of the impeller
shaft are calledaxial-flow impellerand those that generatecurrents in a radial or tangential direction are calledradial
flow impeller.
The three main types of impeller (low to moderate viscosity) are :
1. Propellers,
2. Turbines,
3. High – efficiency impeller.
For high viscosity= Helical impellers and anchor agitators
8/17/2019 Chapter 6-Agitated Liquid
20/48
1. Propeller:
A propeller is an axial-flow, high speed impeller for liquids of low
viscosity. The direction of rotation is usually chosen to force the liquid
downward, and the flow currents leaving the impeller continue
until deflected the floor of the vessel.
Because of the persistence of the flow currents, propeller agitators
are effective in very large vessels.
For deep tank- two or more maybe mounted on the same shaft.
T Type: Standard 3-blade marine
propeller with square pitch (commonin used).
-four blade, toothed/other designed.
8/17/2019 Chapter 6-Agitated Liquid
21/48
AGITATION AND MIXING
8/17/2019 Chapter 6-Agitated Liquid
22/48
2.Turbines.
3 types;
Type 1: The turbine with flat vertical blades extending to the
shaft is suited to the vast majority of mixing duties up to 100,000CP or so at high pumping capacity. The currents it generates
travel outward to the vessel wall and then flow either upward or
downward. Such impellers are sometimes called paddles.
Type 2/3: Create zones of high shear rate.Good in dispersing gas in a liquid (gas is
forced at high shear rate to flow
radially to the blade tips)
8/17/2019 Chapter 6-Agitated Liquid
23/48
AGITATION AND MIXING
8/17/2019 Chapter 6-Agitated Liquid
24/48
AGITATION AND MIXING
8/17/2019 Chapter 6-Agitated Liquid
25/48
3.High- efficiency impeller.
. Variations of the pitched-blade turbine have been developed to
provide more uniform axial flow in addition to radial flow forbetter mixing, as well as to reduce the power required for a given
flow rate.
.These impeller are widely used to mix low or moderate viscosity
liquids, but they are not recommended for very viscous liquids or
for dispersing gases.
. Eg: A310 fluid foil impeller
8/17/2019 Chapter 6-Agitated Liquid
26/48
Highly viscous liquids impeller.
Use for liquid with viscosities more than 20 Pa.s
-diameter helix approximately to inner diameter of tank
Provide good agitation near the floor of the tank;. No vertical motion
. Promotes good heat transfer to/from the vessel.
a) Double-flight helical-ribbon impeller b) Anchor impeller
8/17/2019 Chapter 6-Agitated Liquid
27/48
AGITATION AND MIXING
8/17/2019 Chapter 6-Agitated Liquid
28/48
MIXING EQUIPMENT
The wide range of mixing equipment available
commercially reflects the enormous variety of mixing duties
encounteredin the processing industries.
It is reasonable to expect therefore thatno single item of
mixing equipmentwill be able to carry out such a range of
duties effectively.
This hasled to the development of a number of distinct
types of mixer over the years.
Thechoice of a mixertype and its design is therefore
primarily governed by experience. In the following
sections, the main mechanical features of commonly used
types of equipment together with their range of applications
are described qualitatively.
8/17/2019 Chapter 6-Agitated Liquid
29/48
MECHANICAL AGITATION
This is perhaps the most commonly used method of mixing
liquids, and essentially there are three elements in such devices.
Vessels
These are often vertically mounted cylindrical tanks, up to10 m
in diameter, which typically are filled to adepth equal to
about one diameter, although in somegas-liquid contacting
systems tall vessels are used and the liquid depth is up to aboutthree tank diameters; multiple impellers fitted on a single
shaft are then frequently used.
The base of the tanks may beflat, dished, or conical, or
specially contoured, depending upon factors such as ease of
emptying, or the need to suspend solids, etc., and so on.
For the batch mixing ofviscous pastes and doughs using
ribbon impellers and Z-blade mixers, the tanks may be
mountedhorizontally.
8/17/2019 Chapter 6-Agitated Liquid
30/48
Baffles
To prevent gross vortexing, which is detrimental to mixing,
particularly in low viscosity systems, baffles are often fitted to the
walls of the vessel.
These take the form of thin strips aboutone-ten of the tank
diameter in width, and typicallyfour equi-spaced bafflesmay
be used.
In some cases, thebaffles are mounted flush with the wall,
although occasionally a small clearance is left between the wall
and the baffle to facilitate fluid motion in the wall region.
Baffles are, however, generallynot required for high viscosity
liquids because the viscous shear is then sufficiently great to
damp out the rotary motion. Sometimes, the problem ofvortexing
is circumvented by mounting impellers off-centre.
8/17/2019 Chapter 6-Agitated Liquid
31/48
Impellers
Figure 7.20 shows some of the impellers which are frequently used.
Propellers, turbines, paddles, anchors, helical ribbons and
screwsare usually mounted ona central vertical shaftin acylindrical tank, and they are selected for a particular duty
largely on the basis of liquid viscosity.
By and large, it is necessary to move from a propeller to a turbine
and then, in order, to a paddle, to an anchor and then to a helical
ribbon and finally to a screw as theviscosity of the fluids to bemixed increases. In so doing thespeed of agitation or
rotation decreases.
Propellers, turbines and paddlesare generally used with
relatively low viscosity systems and operate at high
rotational speeds. A typical velocity for the tip of the blades of aturbine is of the
order of3 m/s, with apropeller being a little fasterand the
paddle a little slower.
8/17/2019 Chapter 6-Agitated Liquid
32/48
8/17/2019 Chapter 6-Agitated Liquid
33/48
These are classed as remote-clearance impellers, having
diameters in the range (0.13-0.67) x (tank diameter).
Typical design take asD=0.5DT. Furthermore, minor
variations within each type are possible. For instance,
Figure 7.20b shows a six-flat bladed Rushton turbine,
whereaspossible variations are shown in Figure 7.21.
a six-flat bladed Rushton turbine
Hence it is possible to have retreating-blade turbines,angled-blade turbines, four- to twenty-bladed turbines, and
so on. For dispersion of gases in liquid, turbines are usually
employed.
8/17/2019 Chapter 6-Agitated Liquid
34/48
Propellers are frequently of the three-bladed marine type
and are used for in-tank blending operations with low
viscosity liquids, and may be arranged as angled side-entryunits, as shown in Figure 7.22.
For large vessels, and when the liquid depth is large
compared with the tank diameter, it is a common practice
to mount more than one impeller on the same shaft. With
this arrangement the unsupported length of the propeller
shaft should not exceed about 2 m.
8/17/2019 Chapter 6-Agitated Liquid
35/48
In the case of large vessels, there is some advantage to be
gained by using side- or bottom-entry impellers to avoid the
large length of unsupported shaft, though a good gland ormechanical seal is needed for such installations or
alternatively, a foot bearing is employed.
Despite a considerable amount of practical experience, foot
bearings can be troublesome owing to the difficulties oflubrication, especially when handling corrosive liquids.
8/17/2019 Chapter 6-Agitated Liquid
36/48
In comparing propellers and turbines, the following features
may be noted:
Propellers:
(a) are self-cleaning in operation,
(b) can be used at a wide range of speeds,
(c) give an excellent shearing effect at high speeds,
(d) do not damage dispersed particles at low speeds,
(e) are reasonably economical in power, provided the pitch is
adjusted according to the speed,
(f) by offset mounting, vortex formation is avoided,
(g) if horizontally mounted, a stuffing box is required in theliquid, and they are not effective in viscous liquids.
8/17/2019 Chapter 6-Agitated Liquid
37/48
8/17/2019 Chapter 6-Agitated Liquid
38/48
Shrouded turbine
(a) are excellent for providing circulation,
(b) are normally mounted on a vertical shaft with the stuffing boxabove the liquid,
(c) are effective in fluids of high viscosity,
(d) are easily fouled or plugged by solid particles,
(e) are expensive to fabricate,
(f) are restricted to a narrow range of speeds, and
(g) do not damage dispersed particles at economical speeds,
Open impellers
(a) are less easily plugged than the shrouded type,
(b) are less expensive, and
(c) give a less well-controlled flow pattern.
8/17/2019 Chapter 6-Agitated Liquid
39/48
8/17/2019 Chapter 6-Agitated Liquid
40/48
8/17/2019 Chapter 6-Agitated Liquid
41/48
FLOW PATTERN
The way a liquid moves in an agitated vessel depends on;
a) the type of impeller;
b) the characteristics of the liquid, especially its viscosity;c) the size and proportions of the tank, baffles and impeller.
.The liquid velocity at any point in the tank has three
components, and the overall flow pattern in the tank depends on
the variations in these three velocity components from point to
point.
.The first velocity component - radial and acts in a direction
perpendicular to the shaft of the impeller.
.
The second component- longitudinal and acts in a directionparallel with the shaft.
.The third component- tangential, or rotational, and acts in a
direction tangent to a circular path around the shaft.
8/17/2019 Chapter 6-Agitated Liquid
42/48
FLOW PATTERN
8/17/2019 Chapter 6-Agitated Liquid
43/48
ASSIGNMENT 2 – (4 MEMBERS/GROUP)
MIXING OPERATIONS AVAILABLE IN INDUSTRY
Choose 1 type of mixing operation available in industry andyour discussion shall includes;
1. AGITATION AND MIXING PROCESSES BACKGROUND
2. DESIGN OF MIXING TANK - Production Rate
- Tank Dimension (D and H), Agitated System, No of baffles,
3. THE FLOW PATTERNS
4. CALCULATION OF POWER CONSUMPTION
Date of Report Submission: wk 13 (13 Dec 2013)
Date of Presentation: 18 Dec 2013
8/17/2019 Chapter 6-Agitated Liquid
44/48
MIXING MECHANISMS
If mixing is to be carried out in orderto produce a uniform
mixture, it is necessaryto understand how liquids move
and approach this condition.
In liquid mixing devices, it is necessary that two
requirementsare fulfilled.
1. There must be bulk or convective flow so that there are no
dead (stagnant) zones.
2. There must be a zone of intensive or high-shear mixing in
which the inhomogeneities are broken down.
MIXINGMECHANISMS
8/17/2019 Chapter 6-Agitated Liquid
45/48
MIXING MECHANISMS
Both these processes are energy-consuming and ultimately
the mechanical energy is dissipated as heat; the proportion of
energy attributable to each varies from one application to
another.
Depending upon the fluid properties, primarily viscosity, theflow in mixing vessels may be laminar or turbulent, with a
substantial transition zone in between the two, and
frequently both flow types will occur simultaneously in
different parts of the vessel.
8/17/2019 Chapter 6-Agitated Liquid
46/48
Laminar mixing.
Laminar flow is usually associated with high viscosity liquids
(in excess of 10 N s/m2) which may be either Newtonian or
non-Newtonian. In laminar flow, mixing process occurs when the liquid is
sheared between two rotating cylinders. During each
revolution, the thickness of the fluid element is reduced, and
molecular diffusion takes over when the elements are
sufficiently thin. This type of mixing is shown schematically in Figure 7.3 in
which the tracer is pictured as being introduced
perpendicular to the direction of motion.
Finally, mixing can be induced by physically slicing the fluid
into smaller units and re-distributing them. In-line mixersrely primarily on this mechanism, which is shown in Figure
7.4.
Thus, mixing in liquids is achieved by several mechanisms
which gradually reduce the size or scale of the fluid elements
and then redistribute them in the bulk.
LAMINARFLOW
8/17/2019 Chapter 6-Agitated Liquid
47/48
LAMINAR FLOW
TURBULENTFLOW
8/17/2019 Chapter 6-Agitated Liquid
48/48
TURBULENT FLOW
Turbulent mixing.
For low viscosity liquids (less than 10 mN s/m2), the bulk flowpattern in mixing vessels with rotating impellers is turbulent.
The inertia imparted to the liquid by the rotating impeller is
sufficient to cause the liquid to circulate throughout the vessel
and return to the impeller.
Mixing is most rapid in the region of the impeller because of
the high shear rates due to the presence of trailing vortices,
generated by the impeller.
Recommended