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MECHANISTIC CONSIDERATIONS OF ACOUSTIC DEWATERING TECHNIQUES
R. E. BEARD and H. S . MURALIDHARA
B a t t e l l e Columbus D i v i s i o n , Columbus, Ohio
Two s o l i d / l i q u i d s e p a r a t i o n processes commonly u s e d i n t h e c h e m i c a l and m i n e r a l p r o c e s s i n g i n d u s t r i e s a r e f i l t r a t i o n and c e n t r i f u g a t i o n . F i l t r a t i o n i n v o l v e s pass ing t h e suspens ion th rough a f i l t e r medium t o r e t a i n t h e s o l i d p a r t i c l e s . C e n t r i f u g a t i o n u t i l i z e s a r o t a t i o n a l f i e l d t o i n c r e a s e t h e p a r t i c l e s e t t l i n g r a t e . U n f o r t u n a t e l y , t h e e f f i c i e n c y o f b o t h p r o c e s s e s d rops s i g n i f i c a n t l y as t h e p a r t i c l e s i z e decreases.
It has been shown t h a t a c o u s t i c f i e l d s can be used t o i n c r e a s e t h e dewa te r ing r a t e , as w e l l as t o i n c r e a s e t h e s o l i d s c o n t e n t o f t h e f i n a l p roduc t . However, t h e mechanisms i n v o l v e d a r e n o t y e t under - s tood. I n t h i s paper, t h e t h e o r e t i c a l aspec ts o f a c o s u t i c dewa te r ing a r e cons ide red . The r e s u l t s s h o u l d l e a d t o a b e t t e r u n d e r s t a n d i n g o f t h e mechan isms i n v o l v e d i n a c o u s t i c d e w a t e r i n g processes f o r i n d u s t r i a l a p l i c a t i o n s .
INTRODUCTION
U l t r a s o n i c energy was o r i g i n a l l y s u g g e s t e d many y e a r s ago f o r dewater ing . I t i s e s p e c i a l l y a p p e a l i n g as a means o f dewa te r ing hea t s e n s i t i v e m a t e r i a l s , s u c h a s f o o d a d d i t i v e s a n d p h a r m a c e u t i c a l s , where f a s t d e w a t e r i n g c o u l d p r e v e n t d e t e r i o r a t i o n o f t h e p r o d u c t s i f damaging h e a t l e v e l s c o u l d be avoided.
Dewater ing i s d e f i n e d as r e m o v i n g m o i s t u r e f r o m a p r o d u c t w i t h o u t v a p o r i z a t i o n ; i . e . , s o l i d - l i q u i d s e p a r a t i o n w i t h o u t a p h a s e change . D r y i n g , by c o n t r a s t , r e f e r s t o removal o f w a t e r f r o m a p r o d u c t b y p r o d u c i n g a p h a s e c h a n g e . Cons ide rab ly more energy i s r e q u i r e d f o r d r y i n g , s i n c e a s i g n i f i c a n t amount o f energy i s consumed i n s u p p l y i n g t h e hea t o f v a r p o r i z a t i o n .
dewa te r ing i n c l u d e : The a d v a n t a g e s o f u s i n g u l t r a s o n i c s f o r
0 F a s t e r dewa te r ing r a t e , 0 Lower p r o c e s s i n g tempera ture , and 0 Maintenance o f p r o d u c t i n t e g r i t y .
Recent i n t e r e s t i n energy c o n s e r v a t i o n has spu r red a s e a r c h f o r more e f f e c t i v e a n d e c o n o m i c a l d e w a t e r i n g methods. Dewate r ing i s o f t e n used as an i n t e r m e d i a t e p rocess p r i o r t o some f o r m of t he rma l d r y i n g . Obv ious ly , i t i s d e s i r a b l e t o remove as much w a t e r f r o m a p r o d u c t as i s e c o n o m i c a l l y f e a s i b l e b e f o r e t h e f i n a l e n e r g y - i n t e n s i v e d r y i n g stage. U l t r a s o n i c s shows c o n s i d e r a b l e p romise as
1072 - 1985 ULTRASONICS SYMPOSIUM
an economical means o f remov ing wa te r f r o m c e r t a i n t ypes o f p roduc ts , e s p e c i a l l y i n comb ina t ion w i t h o t h e r techn iques .
BACKGROUND
C a n d i d a t e s y s t e m s f o r dewa te r ing a r e h i g h m o i s t u r e c o n t e n t sys tems , i n w h i c h t h e space b e t w e e n s o l i d s ( p a r t i c l e s , f i b e r s , e t c . ) t o be dewatered i s c o m p l e t e l y f i l l e d w i t h mo is tu re , and t h e system has t h e c h a r a c t e r i s t i c s o f a l i q u i d o r semi -so l i d . The wa te r may be p r e s e n t i n a number o f fo rms:
0 bu lk , 0 p o r e ( c a p i l l a r y bound), 0 chemisorbed, 0 hydrogen bonded.
C o n v e n t i o n a l d e w a t e r i n g t e c h n i q u e s s u c h as f i l t r a t i o n and c e n t r i f u g a t i o n a r e l i m i t e d t o remov ing o n l y t h e b u l k water . T y p i c a l l y , i n a c o l l o i d a l suspension, a s i g n i f i c a n t q u a n t i t y o f water w i l l remain a f t e r f i l t r a t i o n o r c e n t r i f u g a - t i o n . G r e g u s s has shown t h a t c a p i l l a r y and f i r m l y b o u n d w a t e r c a n b e r e l e a s e d b y t h e a p p l i c a t i o n o f u l t r a s o n i c s i n m a t e r i a l s l i k e s i l i c a g e l , a l t hough i t i s u n c l e a r whether t h i s i n v o l v e s a phase change ( d r y i n g ) .
Exper imen ta l
T h i s s e c t i o n summarizes t h e r e s u l t s o b t a i n e d i n t h e c a n d i d a t e s y s t e m f o r d e w a t e r i n g by u l t r a s o n i c s and vacuum f i l t r a t i o n , i n d i v i d u a l l y , and i n comb ina t ion t h e r e o f , f o r coa1:water s l u r r y .
Procedure
F i g u r e 1 i s a s c h e m a t i c d i a g r a m o f t h e dewa te r ing appara tus f o r h i g h m o i s t u r e s u s p e n - s ions . A c y l i n d e r 5 cm i n d iamete r and 130 cm l o n g i s connected th rough a s t a i n l e s s s t e e l g r i d wh ich a c t s as a f i l t e r d i s c t o a Buchner f u n n e l where s u c t i o n can be a p p l i e d . The rubber gaske ts sea l t h e c e l l a t t h e base. U l t r a s o n i c energy i s a p p l i e d b y a t r a n s d u c e r and a wave g u i d e o r h o r n d e s i g n e d f o r t h e s e e x p e r i m e n t s . The s t a i n l e s s s t e e l c y l i n d r i c a l ho rn i s immersed i n t h e s l u r r y t o c o u p l e t h e u l t r a s o n i c energy f o r dewater ing .
0090-5607/85/0000-1072 $1.00 0 1985 IEEE
v*cum
r / 1
Figure 1. Batch experimental dewatering apparatus for high-moisture suspensions. Upper right: Ultra- sonic power generation equipment f o r batch apparatus.
The main components of the ultrasonic equipment used for these experiments are shown in Figure 1. The ultrasonic power generator produces a nominal 2 0 kHz electrical signal. The power level can be externally controlled. The converter, booster and horn are the three functional parts of the transducer. The piezoelectric converter converts the electrical power signal to mechanical vibration in the booster. The booster and horn are mechanical wave guides and amplifiers. The horn transmits the ultrasonic vibration energy from the booster to the suspension being dewatered.
Each of the dewatering experiments was performed on a 50 g suspension. Coa1:water slurries were prepared on a 1:l basis using the following coal particle size distribution:
-200 x 270 mesh - 25 percent by weight -270 x 325 mesh - 25 percent by weight -325 x 0 mesh - 50 percent by weight.
The prepared sample was transferred into the cylindrical portion of the apparatus, at the bottom of which a filter paper (Whatman 41) was placed on top of the stainless steel grid. The sample was subjected to vacuum or ultrasonics, or a combination thereof for different intervals of time. After the experiment the moisture in the cake was determined by moisture balance. The accuracy of the moisture balance is in the range o f 23 percent. In order to check the results, the
volume of water removed was also collected and weighed.
Results
Initial experiments were run with coal slurry of 50 percent moisture content. The results are shown in Figure 2. It was observed that the moisture remaining in the cake decreased with time. At 50 watts input electrical load power for 12 minutes, the final moisture present in the cake was 15 percent. As a base case, the experiment was repeated with vacuum only for comparison. Solids concentration achieved with vacuum only gave 30 percent moisture product. This definitely demonstrates that ultrasonics application improved the rates of dewatering.
80 L 70 t
Coal :water slurry
U l t r a s o n i c s + Suc t ion 0
Suc t ion V
I-
O 2 4 6 8 10 12 14 16 18
T i m e , minutes
Figure 2. Dewatering results f o r coa1:water slurry with experimental batch apparatus.
THEORETICAL CONSIDERATIONS
The first stage of dewatering consists of agglomeration of suspended particles, leaving regions of water free of solids. The free water is then selectively removed from the suspension via filtration. The agglomeration of the particles also squeezes the water present between the particles and helps to prevent blinding of filters during a filtration process, thereby enhancing dewatering rate.
The primary mechanisms responsible for this agglomeration due to ultrasonics are: (1) particle-fluid covibration, described by Stoke's Law, (2) inter-particle attraction according to Bernoulli's Law, and (3) particle migration caused by radiation pressure. The relative contribution of each mechanism, as will be shown, depends on the vibration frequency as well as the particle size and density, fluid viscosity and other properties of the system to be dewatered.
For very small particles, particle-fluid covibration, according to Stoke's Law, is the principle mechanism of agglomeration. As collisions occur, the average particle size
1985 ULTRASONICS SYMPOSIUM - 1073
increases unti 1 the particles become almost stationary in the fluid. At this point, inter- particle attraction described by Bernoulli's Law becomes the dominant mechanism.
At the vibration frequencies at which Stoke's force and Bernoulli's force are significant, radiation pressure is generally negligible. However, at higher frequencies the radiation pressure tends to predominate.
T h e following sections describe the contribution of each of these forces to the agglomeration process involved in dewatering a high moisture suspension. The total contribution is obtained by the superposition of all of these forces; however, one mechanism generally pre- dominates over the others at any one time.
Stoke's Force
If fluid flows by a rigid, spherical particle, momentum is transferred to the particle within the boundary layer between the sphere and fluid. This produces a force on any particle much smaller than the wavelength. The oscillatory motion of the fluid in a sound wave produces an alternating force according to Stoke's law (Table 1, eq. A.l) which is in dynamic equilibrium with the inertial force of the particle.
TABLE 1 . Phyrlcrl
N U of Fa=* Caul. Eqmtlon of hpnltud. o f Forcl Olrectlin o f F o x @
P.r.llrl to lOYrcC &*I ..I,
SYMBOL DEFINITIONS r = radius of p a r t i c l e U = maximum p a r t i c l e v e l o c i t y np = v i s c o s i t y of f l u i d
P = dens i ty of p a r t i c l e dp - diameter of p a r t i c l e up = p a r t i c l e v e l o c i t y ( instantaneous)'$ = a,.ea of cross section
Up = maximum f l u i d v e l o c i t y L = dis tance between 2 p a r t i c l e s ; rad ius of small p a r t i c l e
rad ius of l a r g e p a r t i c l e
# = frequency
particle = f l u i d v e l o c i t y ( instantaneous) cf = sonic velocity in
Bernoulli's Force
Consider the hydrodynamic attraction that occurs between two spherical particles that are smaller than a wavelength and close to each other. If the particle velocity due to Stoke's force is much less than the fluid velocity in the sound field, a hydrodynamic flow pattern will form around two particles. For laminar flow (particle diameter larger than /(p fu), the continuity of volume flow through the constric- tion between the particles causes an increased flow velocity there, which lowers the hydrostatic pressure in the constriction. The resulting pressure difference is obtained from Bernoulli's equation. The net pressure produces a force (Table 1, eq. A.2 and 6.1) between the particles.
This force will come into play mainly during the latter phases of the agglomeration process, because maximum relative velocity is obtained when the majority of the particle population has accumulated enough mass to stay fixed in the sound field.
Radiation Force
Another force appears within a suspension subjected to sound waves if there is a discon- tinuity of energy density. If the particles are much smaller than the wave length and can be considered as sources of scattering, such scattering leads to a nonuniformity of energy density in the immediate vicinity of the parti- cle. The resulting force on a small rigid particle is given by Equation 6.2 (Table 1). In standing waves, a maximum force occurs at the positions of maximum energy density, according to
Xm = Xva 2 8 .
where Xva i s the coordinate of any particular velocity antinode. The force is directed toward the velocity antinode. Note that the force due to radiation pressure on a small sphere i n a sound field achieves appreciable magnitude only at high frequencies.
CONCLUSIONS
We have shown that ultrasonics can signifi- cantly increase dewatering rate and decrease the final moisture content of high moisture suspen- sions when used in combination with conventional dewatering techniques. Other advantages of this technology are listed below:
1.
2.
3.
4.
5.
0 Applicability to heat sensitive materials
0 Maintenance of product integrity by virtue of low temperature and non-intrusive applicator High efficiency of operation due to low input energy requirement
dewatering equipment, such as vacuum f i 1 ters.
0
e Compatibility with conventional mechanic
REFERENCES
Bell, D. J. and Dunhill, P., Mechanisms for the Acoustic Conditioning of Protein Precipi- tates to Improve Their Separation by Centrifugation, Biotechnology and Bioengineering, Vol. 26, July 1984. Bolt, R. H. and Heuter, T. F., Sonics, John Wiley & Sons, Inc., NY, 1955. Muralidhara, H. S., and Ensminger, D., Acoustic Dewatering and Drying, State of the Art Review. Int. Journal of Orvino . , " Technology, & (December 1985). Muralidhara, H. S., Senapati, N., Ensminger, O., and Chauhan, S . P . , Electro Acoustic Dewatering. A novel Solid/Liquid Separation Technique Accepted for publication in AIChE Symposium Series, flocculation, sedimentation, and consolidation. Gregass, P. "The Mechanism and Possible
1074 - 1985 ULTRASONICS SYMPOSIUM
Applications o f Drying by Ultrasonic Irradiation", Ultrasonics, April-June, 1963, pp 83-86.
1985 ULTRASONICS SYMPOSIUM - 1075
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