A fluidized bed unit will be selected where floor space is at a premium because of its lower space requirements. The convenience of multiple units in a staged or stacked arrangement can offer significant savings in terms of floor area and can minimize external conveying equipment requirements. If it is necessary to isolate the drying system from the surrounding atmosphere, the lack of rotating seals or moving parts can be quite advantageous. The use of an expensive or limited supply of special drying gas indicates profitable use of the fluidized bed dryer. The lack of moving parts in the fluidized bed dryer yields an attractive installation in troublesome maintenance areas, Particular reference is made to areas in which corrosion and erosion problems are prevalent. For installations of this type, the fluidized bed dryer has a noticeable lack of trunnion rolls, tires, open gearing, chain drives, and moving parts of all kinds. Due to its compact size and relative light weight, it is a unit which can be fabricated in alloy materials without burdensome costs. For instance, in the fertilizer field it has been customary to manufacture rotary dryers of carbon steel and depend on slow corrosion rates. Now it is considered preferable to purchase stainless or alloy steel fluidized bed units. The resulting installations are much more satisfactory in terms of cleanliness, lack of corrosion, ease of cleaning with corrosive liquids, and over-all general appearance. Limitations of Fluidized Bed Drying
There are limitations to the application of this drying principle. Extremely high operating temperatures present somewhat unwieldy mechanical design conditions. It is, for instance, much more difficult to apply a 2400’ F. inlet air temperature to the fluidized bed dryer than to
I
20
INDUSTPIAL A N D E N G I N E E R I N G CHEMISTRY
a rotary dryer. Handling characteristics of some materials can preclude the use of fluidized bed equipment. If the material, in the course of drying, goes through a molten or liquid state, exheme care should be exercised in the selection of fluidized bed equipment. Large differences in bulk density between the wet feed material and the dry product require special consideration. Any e l v e n t of complete incompatibility between the wet and dry states will usually lead to the selection of other equipment for the drying operation. Siring the Dryer
Figure 1 indicates the typical arrangement of a fluidized bed dryer. In flowing through this unit, the air acts as the vehicle for both heat and mass transfer and is also the fluidizing medium. The required air flow rate is functionally dependent on each of these requirements. The functional design of the fluidized bed dryer requires certain basic information. The designer needs : i n l e t and outlet moisture rcquiremds -required residence time i n l e t and outlet air temperature -operational limits --maximum and minimum fruidizing velocity and limitations on operational bed depth, if any
In addition, knowledge of the relationship between bed temperature and retention time requirements can be most useful in terms of minimizing the combination of capital requirements and operational costs. The usual approach to the design problem is to select one combination of retention time and operating bed temperature (usually based on the minimum bed temperature for thermal efficiency), and design around these conditions. This is done in the calculation shown opposite. The resulting simplified design shows that size of the fluidized bed dryer can be controlled by sejeral different parameten. For instance, if the autlet air humidity is excessive, it is necessary to dilute the discharge moisture by using more air. This will typically require a larger diameter dryer. It should be noted that in general, the air flow rate is based on the heat load and that the diameter of the dryer is properly selected to distribute this air flow rate at an acceptable fluidizing spatial velocity. This example also demonstrates that there is a need for basic information such as the degree of bed expansion as a function of fluidizing velocity as well as the most satisfactory fluidizing velocity to be used within the acceptable range. This point bears further comment. The fluidizing velocity within any given system is a fairly wide range limited on both high and low ends by channeling, slugging, elutriation, or some combination of these. The equipment manufacturer usually resorts to pilot scale testing to get the “feel” of the particular system before finalizing commercial design. Laboratory testing is highly recommended in the early
phases of fluidized bed consideration. It is through this method that the influence of material characteristics on the operation of the fluidized bed is tested and demonstrated. The operational limits of the system are found. The laboratory test is aimed at such information as maximum and minimum fluidization velocities, the most satisfactory operational velocity, tendencies toward caking, if any, retention time requirements, maximum and minimum operating bed temperatures, and the general applicability of the technique to the particular ' system. The value of wrh tmts cannot he overemphasized.
I
The preceding functional calculations result in dryer diameter, as well as the active and settled bed heights. From these, the over-all dryer height is readily determined. It is typical to supply a plenum chamber height of approximately four feet for convenient access to the bottom side of the distribution plate as well as to all parts of plenum chamber. The over-all equipment height includes the plenum chamber, the height of the active bed, and sufficientfreeboard to allow the desired degree of dust disentrainment. A convenient rule of thumb is to allow one diameter of fkee board above the top of the active bed of material. A minimum of five feet of freeboard is necessitated by surging and splashing of the bed. The dust entrainment characteristic of the material is another factor which can be at least generally evaluated in the laboratory test. The preceding example results in one particular dryer size selection which is based on certain operating conditions. If the inlet air temperature is increased, the air flow rate requirement is decreased accordingly. When the air rate requirement is decreased, the cross ~ectional area (or diameter of the dryer) is decreased. This leads to the conclusion that the highest permissible inlet air temperature should be used. The only limitation to this approach from the functional standpoint is when the air rate has been reduced to the point that excessive exhaust humidities are realized ; then the system becomes unworkable. At this point, the mass transfer driving force is reduced to zero and moisture condensation results in operational difficulties. Notice that the bed volume is based on the volumetric material throughput rate and the required residence time. Since the bed area is predicated on the air flow rate requirement, the relationship between residence time and bed volume simplifies to a linear relationship between residence time and bed height. The height of the active fluidized bed is the primary determinant of fan or blower pressure. A linear relationship also exists between pressure drop across the fluidized bed and the settled bed height. One of the requirements of the air stream is that it fluidize the active bed of material. In this condition, the residual volume of dry solids within the dryer essentially rests on the air stream or, conversely, it should be expected that the pressure drop across the active fluidized bed on the air stream will be equivalent to the hydrostatic head of material (in the static bed) in the unit. The height of the settled bed multiplied by the bulk density of material represents a back pressure which must be overcome by the fan or blower. In the preceding example, the required air pressure drop through the material is:
43.8 ft.
70 Ib.
7 x - ft.'-
psi. 144 0.s.f.
X
27.7 in. of H 2 0 0.s.i. 49 in. of H a
The actual pressure drop would be expected to he about ninety percent of this value. It is evident that the air flow rate is dependent upon either fluidizing velocity, heat transfer, or mass transfer requirements. The designer must select an adequate VOL. 5 5
NO. 7
JULY 1 9 6 3
21
SOLIDS D I S C H A R G E A R R A N G E M E N T S SSUL IRANSMIITER
ADJUSTABLE WEIR
DISTRIBUTION PLATE
PLATE
PLENUM CHAMBER
ROTARY LOCK
CONTROLLER
Figure 3. Ovcrrpom Lvcir and r o r v lock. An altmaft discharge is wually provided ncm the diirm'butiun plafe for dumping the entire
Figurr 4. ConfroNed soli& discharge. In rlnr fairly rlabmote onongcmcnt the achurl bcd level can be cffecticlivrly conhollrd by inrtru-
bed
nimrcrlion
air supply to satisfy all three. Caution is to be exercised with reference to the outlet air humidity. For instance, it is possible that for a high temperature gradient across the air stream, sufficient quantities of air can be supplied for fluidization (in fact, the diameter of the unit can be adjusted so, that any quantity of air is sufficient) and sufficient heat input supplied to the unit, but the moisture removal requirement would result in a supersaturated outlet air condition. Needless to say, this will result in moisture condensation in the freeboard and duct work with all its attendant problems. The previously mentioned "feel" for the fluidized bed system involving the particular commodity is very important in determining the height to diameter ratio. There appears to be a wide range of acceptable operating conditions between the two extremities of a wide shallow bed and a narrow deep bed. When a smaller diameter unit with a deeper bed is selected, a lower volume of air is required although it must be supplied at a higher pressure. Since operating static pressure is generally more costly than additional air volume, the natural tendency is toward larger diameter and shallower beds. The past practice has been to observe two rather general limitations in this regard. Operational difficulties may be encountered when the active fluidized bed depth is less than a nominal 9 inches or greater than approximately one bed diameter. Much has been said and much more can be said regarding the different characteristics of the fluidized bed at different bed depths. Yet it should suffice to say that for commercial equipment (considered to be at least several feet in diameter), operational difficultieswould be expected with both extremities. In general, it seems that the deeper the fluidized bed (in relation to its diameter), the more agglomeration occurs between air bubbles and consequently the larger is the degree of splashing at the top of the bed as the air disengages
from the solids. In this discussion it is presumed that the fluidized bed unit is operating in the higher end of the fluidization range. A condition of aggregate fluidization or dilute phase fluidization exists. The bed of solids is usually maintained in a violent condition of agitation.
22
INDUSTRIAL AND ENGINEERING CHEMISTRY
.
Auxiliary Equipment
Fan or Blower.
Particular attention should be paid to the fan or blower curve. This equipment should be selected so that variations in air volume as well as pressure are possible over a fairly wide range. It is felt, at the present time, that approximately 20 to 25% excess air volume as well as pressure should be supplied to give the required degree of flexibility. Although blowers and compressors have been used for particular installations, many more installations successfully utilize a high pressure fan. The static pressure requirements depend upon the total air handling system and particularly upon the active bed of material. Generally it is expected that air will be needed at static pressures in the vicinity of 40 inches of water. Static pressurp of this magnitude are generated by high tip velocity of the fan wheel. In order to convert these high velocities to static pressure, it is necessary to pay careful attention to the angle of divergence of transition pieces between the dryer and the fan. Good design practice indicates total included angles for these transitions in the vicinity of 25 to 40 degrees'depending on the velocity head. Heating Fquipment. The air heating equipment is tailored to the individual task. For low temperature operations (up to about 300' F.) the usual steam heating AUTHOR Martin F. Quinn is th Chief
Engincm of the Process Equipment Division, General Ammican Trmfiurlation Curp., Chicago, Ill.
f-----l
DRYER SHELL
COWTROL IWRtMENl
-
I
I
~ V I l u l O l CONVNOR
HOPPER Figure 6. Matnial scol awongtnicnl. A soli& sed hos also b m OpPIicdto the matm'al dischmgcproblem, asshown h m
coil is applied between the fan and the dryer. For higher inlet air temperatures, it is typical to employ an oil or gas (or combination) furnace at this point. Since this location is downstream from the fan, it is necessary to design the combustion equipment for satisfactory operation under the static pressure conditions of the fan. If additional fans are utilized on the combustion equipment, such as for primary or secondary air, their performance characteristics should be carefully matched to the characteristics of the main air fan. The alternate is to utilize the normal combustion equipment at atmospheric (or slightly subatmospheric) pressures with the fan applied to the high temperature exhaust air stream from the combustion equipment. Since this arrangement imposes additional special requirements on the air handling fan, the first method has found wider industrial application. Air Distribution Plate. After the air has been delivered to the plenum chamber of the dryer at the required pressure and temperature levels, it must be distributed uniformly across the entire cross section of the dryer. An air distribution plate serves this purpose. The air distribution nozzle should be arranged so that dry solids cannot travel downward through the plate. The distribution plate should support the complete downward load of the bed of solids with the appropriate impact factor applied. It should be designed to withstand the total static head developed by the fan. Sufficient resistance must be incorporated to distribute the air uniformly across the cross section of the dryer. There have been wide variations in the approach to both the structural and functional aspects of this problem. One quite successful approach incorporates the use of a perforated distribution plate in which each perforation is fitted with an air distribution nozzle. The perforations are located very dose to each other so that the maximum air distribution results. Air pressure
drop is regulated by proper selection of the restricting openings in the nozzles (Figure 2). ' The mechanical features of the distribution plate generally include structural reinforcement on the bottom to provide adequate stiffness. The distribution plate is typically located in the horizontal plane. For drainage convenience and quick emptying requirements of certain units, plates have been installed at a slight inclination. Air Exhaust System. The air handling system beyond the dryer outlet connection is usually designed on the basis of the dust collection requirements. Depending on the condition of operation, dust loading in the outlet air stream can be comparatively high. Also, depending on the cost of the material being handled, as well as its toxicity, dust collection and scrubbing equipment downstream from the dryer range from the most rudimentary to the most complex systems. Air Scale. The solids handling system requires air seals. All of the normal wet material conveyors can be used to feed the fluidized bed dryer. The most convenient sealing arrangement, if applicable, is the use of a rotary lock between the conveyor and the dryer. Other techniques have been used ranging from sealing through pneumatic conveyors to utilizing a h o p p d l of wet material ahead of the feed conveyor as the seal. A small gas flow is to be expected through a hopper. Product Discharge. The solids discharge rate must be matched to the feed rate and several mechanisms have been effectively utilized to accomplish this objective (Figures 3 4 ) . It is recommended that the simplest configuration be employed wherever possible and this is usually the overflow weir assembly. Provisions can be made for manual adjustment of operational bed level. Controls. The fluidized bed dryer has three operational variables-feed rate, air flow rate, and air temperature. From the control standpoint, the dryer is a VOL 55
NO. 7 J U L Y i 9 6 a
23
t PRESSURE TRANSMITTER 4
--DRYER
SHELL
ADJUSTABLE WEIR
-
n
U FLUIDIZED BED ;DENSE)
* FlUlDlZED
BED (DILUTE)
-ii
DISTRIBUTION PLATE
I PLENUM CHAMBER
ROTARY LOCK
Figure 3. Over-jow weir and rotary loch.. h ulternate discharge is usuallv provided near the distribution plate for dumbing the entire lied
air supply to satisfy all three. Caution is to Le exercised with reference to the outlet air humidity. For instance, it is possible that for a high temperature gradient across the air stream, sufficient quantities of air can be supplied for fluidization (in fact, the diameter of the unit can be adjusted so that any quantity of air is sufficient) and sufficient heat input supplied to the unit, but the moisture removal requirement would result in a supersaturated outlet air condition. Needless to say, this will result i n moisture condensation in the freeboard and duct work with all its attendant problems. The previously mentioned "feel" for the fluidized bed system involving the particular commodity is very important in determining the height to diameter ratio. There appears to be a wide range of acceptable operating conditions between the two extremities of a wide shallow bed and a narrow deep bed. When a smaller diameter unit with a deeper bed is selected, a lower volume of air is required although it must be supplied at a higher pressure. Since operating static pressure is generally more costly than additional air volume, the natural tendency is tokvard larger diameter and shallower beds. The past practice has been to observe two rather general limitations in this regard. Operational difficulties may be encountered when the active fluidized bed depth is less than a nominal 9 inches or greater than approximately one bed diameter. Much has been said and much more can be said regarding the different characteristics of the fluidized bed at different bed depths. Yet it should suffice to say that for commercial equipment (considered to be at least several feet in diameter), operational difficulties would be expected with both extremities. In general, it seems that the deeper the fluidized bed (in relation to its diameter), the more agglomeration occurs between air bubbles and consequently the larger is the degree of splashing at the top of the bed as the air disengages 22
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
CONTROLLER Figure 4
"8'c SOLIDS DISCHARGE Controlled solids discharge
In
[hi3
/uzrl)
riaborate
ntiungrment the actual bed level can be effaticel) Lontrollrd b j znstru-
mentation
from the solids. I n this discussion it is presumed that the fluidized bed unit is operating in the higher end of the fluidization range. A condition of aggregate Huidization or dilute phase fluidization exists. The bed of solids is usually maintained iri a \G)lent condition of agitation. Auxiliary Equipment
Particular attention should be paid This equipment should be selected so that variations in air volume as ~ 7 d as l pressure are possible over a fairly wide range. It is felt, at the present time, that approximately 20 to 25% excess air volume as well as pressure should be supplied to give the required degree of flexibility. Although blowers and compressors have been used for particular installations, many more installations successfully utilize a high pressure fan. The static pressure requirements depend upon the total air handling system and particularly upon the active bed of material. Generally it is expected that air will be needed at static pressures in the vicinity of 40 inches of water. Static pressures of this magnitude are generated by high tip velocity of the fan wheel. In order to convert these high velocities to static pressure, it is necessary to pay careful attention to the angle of divergence of transition pieces between the dryer and the fan. Good design practice indicates total included angles for these transitions in the vicinity of 25 to 40 degrees, depending on the velocity head. Heating Equipment. The air heating equipinenr is tailored to the individual task. For low temperature operations (up to about 300 F.) the usual steam heating Fan or Blower.
to the fan or blower curve.
Martin F . Quinn is t h Chief Engineer of the Process Equipment Division, General AmeriLan 7'ransfiorlalion CorP., C h i c u p , Ill.
AUTHOR
ttermining
optimum operating
conditions for
D producing acceptable quality material is complex.
The procedure described in this paper is simple and straightforward. Three process variables are considered, and three responses are measured. Two of the responses are subjective-i.e., they are measurements of subjective judgments on interval scales. The third response is a quantitative measurement on a ratio scale. The specific experimental design used for the experiment is known as a replicated central composite design. No claim is made that the choice of the design is unique; however, it is a design which is extremely useful in this type of application.
STATISTICAL APPROACHES TO EXPERIMENTAL DATA
SUBJECTIVE RESPONSES I N
The Experiment
The total experiment was conducted for the specific purpose of creating an excellent pie crust recipe. In this article, however, only part of the total experiment is discussed. In particular, the three variables are the amounts of water, flour, and shortening, designated as XI, XZ,and XZ; and the three responses are flakiness, gumminess (toughness), and specific volume (cc. per gram), designated as YI,Yz, and Ys,respectively. The actual amounts of the variables, XI, Xz, and XS, have been coded so that the smallest amounts appear as - 1 in the design space, and the largest amounts appear as +l. The center of the experimental region is denoted as (0, 0, O), and represents a set of conditions which according to experience is best at the present time. While this is not a necessary part of designing experiments of this type, the center point of the design usually fills this role. Flakiness and gumminess are qualitative or subjective ratings based on an evaluation by a panel of trained judges. Each pie crust made was judged by the panel, and given a flakiness score and gumminex, score using a 1 to 10 scale. A high score on flakiness (above 7) and a low score on gumminess (below 3.75) was considered good. Specific volume is a continuous variable, and the range of interest was 2.200 to 2.400. because the responses were not expected IO be linear in the experimental space, an experimental design was needed which would yield estimates of the curvature aspects of each response. For this experiment, only the quadratic types of curvature needed to be considered. The mathematical model to be evaluated for each response, then, is as follows:
PROCESS
I NV ESTIGAT ION This procedure has been used with considerable success. It is simple, economical, and service to the chemist is rapid.
A computer does
all calculations
H. SMITH
A. ROSE
The experimental design to be used for this experiment should have two properties-i.e., it should provide estimates of the flu's in the above mathematical model which are orthogonal to each other; also, it should provide an independent estimate of the variability inherent in baking pie crusts. (Continued on next p o p )
Figure 7.
Tlu dasign com'ded of 32 exprrimcntl VOL 5 5 NO. 7
JULY 1963
25
Contour surfaces were printed out, lines were drawn through X P
ln u)
W
-1.3390
6.5623 6.6795 6.7711 6.8370 6.8773 6.89B
6.8494 6.W1 6.9252
6.8529 6.9140 6.9495
-1.3330 6.8650 6.9341 6.9777 6.9956 6.9878 6.9544 6.8954 6.8107 6.7004
-0.9998 6.8134 6.8929 6.9469 6.9152
-0.6665 6.7529 6.0429 6.9072 6.9459
6.9778 6.9548 6.9062 6.8320
6.9590 6.9464 6.9082
-0.3333 6.6835 6.7839 6.8586 6.9078 6.9312 6.9291 6.9013 6.8478 6.7688
-1.3350
-0.9998
0.6665 0.9998 1.3330
XI
4 -1.3330 -0.%98 -0.6665
-0.3333 0 . W
0.3333 0.6665 0.99911 1.3330
-. W a m
6.6711 6.7731
6.3521 6.5157 6.6538 6.7662
6.5723 6.6791 6.7602 6.8158 6.8456 6.8499
.1 2
.-'
6.3902 6,5435
-0.3333
4 9LL 0 . P
z :'i 0 .3g a 4%
-09339 5.9478 6.1627
6.2818 6.4398
0 . m
ln
-0.6668 6.0068 6.2113
-0.9998 -0.6665
0.oow
4 -
-0.9998 6.0568 6.2510 6.4195
-0.99 -0.6L-0.3333
0.om 0.3333 0.6666 0.9998 1.S330
I
6.7320
6.9177
6.9077
6.7441 6.5449 6.3200
6.7445 6.5557 6.3412
6.8443 6.7%
-0.6665
--
o.ow0 5.8799 6.1053
6.3050 6.4791 6.6276 6.7504 6.8475 6.9191 6.9649
0.00W 6.6052 6.7160 6.8012 6.8607 6.8946 6.9029 6.8855 6.8425 6.7738
0.3333 5,8032
0.6665 5.7176
6.0390 6.2491 6.4336 6.5925 6.7257 6.8333 6.9152 6.9715
5.9638 6.1844 6.3793
0.9333 6.5181 6.6393 6.7349 6.8048 6.8491 6.8678 6.8608 6.8282 6,7699
6.5485 6.6922 6.8102 6.9025 6.9692
-0.3333
-
6.8889
6.8611
6.7361 6.5577 6.3536
6.7188 6.5508 6.3571
In 6.9308 6.8245 6.6926 6.5350 6.3517
0.9998 5.6231 5.8797 6.1101 6.3160 6.4957 6.6497 6.7781
5.7868 6.0282
6.8809 6.9580
6.7373 6.8504 6.9380
0.6665 6.4221 6.5537 6.6597 6.7401 6.7948 6.8238 6.8273 6.8051 6.7572
0.9991 6.3172 6.4592 6.5756 6.6664 6.7315 6.7710 6.7849 6.7731 6.7356
1.9930 6.2034 6.3559 6.4827 6.5839 II 6.6594 6.7093 6.7336 6.7322 6.705j
0.6665 6.8957 6.9127 6.9042
0.9998 6.7803 6.8078 6.8097
1.9330 6.6566 6.69411
,;9;5;
6.8700
6.7859
..a749 6.7790 6.6575 6.5103
6.8101 6.7246
6.7365 6.6614 6.5607 6.4343 6.2823
6.6135 6.4768 6.3144
6.3375
1.3330
6.589 6.4 6.38 6.2
$1
.$ i:,, _
.;, *.'
L
~
The experimental design chosen was a replicated orthogonal central composite design consisting d 32 experlnents-i.e., 8 vertices of a cube, 6 added points, 2 center points. Each of these design points was replicated, thus giving a total of 32 experimental points. This design is shown geometrically in Figure 1. The design points are shown in Table I in their coded form; however, a few points are plotted in Figure 1. Determination of the a coordinate is shown in Table I1 where the formula shown ensures that the central composite design is orthogonal. Andydr of the Expwimenf
The thirty-two experiments were performed in random order, and the responses measured on each experiment. Then, the design matrix (Table I) and a matrix of the responses were placed on cards and the complete statistical analysis was done on the IBM 705 computer.
AUTHORS H . Smith is Head of thcDepmtmmt of M a t h a t i c s is Head Sfatislician al thc Winlon a d Sfaiisics; and A . ROSC Hill Technical Centcr, pioctn. &? Gamble Go., Cincinnati, Ohio. 26
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
The following statistical results were printed out for each response: Design matrix (Table I ) Response matrix
Anaiyris of variance for each resfionst Regrcsszon equation for each response
I
Analysis for determining th adequacy of 6/18 m a t h a t i c a l model (Equation 7) Contour map for each 7tSf07JSt
For example, consider the analysis for flakiness. By the method of least squares, the following regression equation is obtained for the mathematical model (Equation 1) :
Yl = 6.89462 + 0.06323Xi - 0.12318X2+ 0.15162Xa
- 0.11544Xf - 0.03997Xz - 0.11544X: -k
0.09375X1X2
- 0.34375X1X8
-
0.03125X2Xa (2)
This is the prediction equation for flakiness using the coded values for XI, X,, and X,. The analysis of variance was calculated for this model to determine if it was significant in a probability sense. In this experiment the Type I error probability wasset at a = 5%. (Continurd on page 28)
