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Drying by fluidized bed that contains inert particles
International Journal of Mineral Processing, 6 (1979) 155--163
155
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
DRYING BY FLUIDIZED BED THAT CONTAINS INERT PARTICLES
A.H.N. MOUSA*
Department of Chemical Engineering, University of Khartoum (Sudan) (Received December 15, 1978; revised and accepted June 15, 1979)
ABSTRACT Mousa, A.H.N., 1979. Drying by fluidized bed that contains inert particles. Int. J. Miner. Process., 6: 155--163. The method of drying by a fluidized bed that contains inert particles was used to dry slurries of sodium chloride, limestone, clay, and a 50 -- 50 wt.% mixture of clay and limestone. The slurries were prepared by mixing specified weights of the material with a specified weight of distilled water. The slurry was fed into the air-fluidized bed either by the use of compressed air or the use of a sectorized feeder. The bed contains fluidized inert particles on which the slurry was dried. The inert particles used here were sand particles. The drying rate of the slurries depends on a number of factors. These factors are air mass velocity, air inlet temperature, latent heat of vaporization and the logarithmic-mean temperature difference. Combining these factors a general drying rate equation was obtained. The results were compared with method of drying by through,circulation ; drying by fluidized bed was far better.
INTRODUCTION
Drying generally refers to the removal of a liquid from a solid by evaporation. Mechanical methods for separating a liquid from a solid are n o t generally considered drying. The evaporation process can be caused by the use of indirect-heat-transfer devices in which the wet solids are separated from the heat-carrier medium by a wall, so the two phases are never in direct contact. The second m e t h o d used to cause evaporation is the use of direct-heat-transfer devices. Here there is direct contact between the wet solids and the h o t gases. The driers used can be batch or continuous devices: The driers most widely used in industry are continuous; the material is loaded and discharged continuously. The continuous driers are the most efficient for large plants. One of the widely used methods is the continuous drying by through-circulation. A new method, which is nowadays gaining ground is drying by fluidized beds that contain inert particles. *Present address : Department of Chemical Engineering, University of Kuwait, P.O. Box 5969, Kuwait.
156
The fluidized solid technique, first developed in petroleum refining, is now finding use in many industries. The technique is a versatile unit operation which offers ease of solid handling and an excellent means of heat transfer. The high heat transfer rate that can be achieved in fluidized systems is the main reason for their use in a variety of high thermal technological processes. One of these processes is drying by fluidization. As stated by Romankov (1971), fluidized-bed drying has been applied not only to granular materials but also to pastes, suspensions, solutions and molten materials. As a result many batch processes have been replaced by continuous processes, which are much faster and more economic. The fluidized-bed drying technique has simplified methods of manufacturing certain chemicals, for instance the usual operations of evaporation, crystallization, filtration, drying and pulverization are replaced by a single process of drying in a fluidized bed, which is much cheaper. Romankov and Rashkovskaya (1968) and Vanecek et al. (1965) discussed the special features and difficulties of operating fluidized-bed driers. Continuously operating driers of single chamber and multichamber design (Toei and Akao, 1968) have been used to dry limestone, dolomite foundary casting sands, blast furnace slag, phosphorites, and other heat-resistant inorganic materials. The technique is also widely used for drying coal (Aldridi, 1964). Vanecek et al. (1970) studied the operating conditions and drying performances of continuous fluidized-bed driers. Some materials are dried on inert bodies and the dry product is carried away from the drying chamber as a fine powder. This method of drying by using inert material, was first proposed by Schmidt (1955) and used by Frantz (1958) for evaporating sea water. Design equations that may be used to determine the drying rate and the dimensions of the drying equipment are required in order to develop methods of drying various materials in a fluidized bed. These equations should be derived on the basis of theoretical analysis of a physical model of the process and/or of experimental determination of the influence of the most important factors. The method of drying by through-circulation where the drying gases flow either upward or downward through a permeable bed of wet granular solids is widely used in industry. In industry the method is converted into a continuous process by using a conveyor as the bed. In this work the method of drying using a gas-fluidized bed that contains inert particles was investigated. Continuous feed of the material to be dried comes into an air-fluidized bed that contains sand particles. The sand particles are fluidized by the hot air. The wet material is dried on the surface of the h o t sand particles and it is carried out by the air and collected in cyclone.
157 EXPERIMENTAL APPARATUS
The experimental apparatus is shown in Fig. 1A. It consists of a compressor which compresses air and delivers it to the bed. The quantity of air going to the bed is controlled by valve V1. An electric heater located after valve V~ heats the air to the required temperature by the use of a variable variac. A thermometer Ta and a manometer Pa are used to measure the temperature and pressure of the heated air. The velocity of the air is measured by an orifice meter. Inclined water manometers were used to measure the pressure drop across the orifice meter. An air distributor at the b o t t o m of the bed distributes the air evenly in radial directions. Then the air goes upwards through a calming section so as to have even distribution along the bed. The bed has an inside diameter of 10.2 cm. A thermometer T~ measures the inlet temperature of the air before it goes through the screen supporting the sand particles. Another inclined water manometer measures the pressure drop across the fluidized bed. The temperature of the air leaving the bed is measured by thermometer T2. Then the air goes through a cyclone where the dried particles are collected. On the top of the bed a feeding bottle is fixed. It contains the slurry to be dried. Compressed air is used to agitate the slurry and create enough pressure in the feed bottle to force the slurry into the fluidized bed. A valve on the vent line is used to control the pressure V NT
T~
~ COMPRESSEDAIR ~ FEEDINGBOTTLE WITH A CONTROLLING VALVE V1
C*LONE FLOW CALMER
ER
O R I F I C E ~ M E IT, ERp,
~11~1 =VALVE Vl ETERS
ELECTRICAL ' ' HEATER
/4
0MPRE AIR '
~
Fig. 1. A. Flow diagram of apparatus. B. Sectorized feeder with radial blades.
158 in the feed bottle. The flow rate of the slurry into the bed is controlled by valve V2. Another m e t h o d to feed the wet particles into the bed was the use of a sectorized feeder with radial blades. This was fixed on the top of the bed in place of the feeding bottle, this arrangement is shown in Fig. lB. This was used to feed sodium chloride slurry only. The height of the sand in the bed was 4.2 cm. The sand particles have a size range of 2800--2000 pm (mesh number 6-8). The slurry particles have a size range of 300--150 pm (mesh number 52-100). PROCEDURE Measured quantities of the material to be dried and distilled water were mixed and pored into the feeding bottle, with valve V2 closed. The flow rate of the fluidizing air was adjusted to the required value by valve V1. The inlet temperature T~ was adjusted to the required value by the variable variac, controlling the power input to the electrical heaters. When steady state was reached, valve V2 was opened slowly allowing the slurry to go into the fluidized bed. The a m o u n t of slurry going into the bed was adjusted so that the reading of the thermometer T2 was constant. The time taken to empty the feeding bottle was recorded. The above procedure was repeated for each experiment by changing the air flow rate only, keeping T~ and T2 constant at the same previous values. Then the material to be dried was changed and all these experiments were repeated. In case of drying the sodium chloride slurry, the feeding bottle was replaced by the sectorized feeder shown in Fig. 1. THEORETICAL ASPECTS The drying rate of wet particles is a function of a number of factors. These are: (1) The drying surface area (A). (2) The air mass velocity (G). (3) The latent heat of vaporization of water at the operating temperature
(x). (4) The logarithmic mean temperature difference between the slurry feed and the operating temperatures (A Tin). An equation relating these factors to the drying rate (R) can be written as: R oc
A Gn A
T1n
X C A Gn A
R =
Tln
where C and n are constants.
(1)
(2)
159 In these experiments, the temperatures were kept constant. T herefore A Tin and ~ will be constants. Also since the a m o u n t o f sand in the bed was constant, t h e r e f o r e (A) is constant. So eq. 2 can be written as: R = K G '~
(3)
R =logK+nlogG
(4)
or: log
where: K =
C A ,~ Tln
Equation (4) represents a straight-line equation similar to: y=a+bx
and so the least-square m e t h o d can be used t o find the constants K and n. CALCULATIONS AND RESULTS The gas density (p) was calculated from the pressure Pa and t e m p e r a t u r e Ta. From the orificemeter calibration the gas velocity (v) is obtained. Then the mass velocity (G) is calculated. The drying rate (R) is calculated by dividing the a m o u n t of water added to make the slurry by the time taken t o e m p t y the feed bottle. So for each experiment as the flow o f airis varied, values o f G and R can be calculated. The results o f these calculations are given in Table I for the different materials dried. The following factors were kept constant at the values given: t e m p e r a t u r e o f the gas entering the bed = 100 °C t e m p e r a t u r e o f the gas leaving = 67 °C t e mp er atu r e o f slurry = 27 °C drying surface area (A) = 4500 cm 2 ATln = 54.856 latent heat o f vaporization ~ = 560 cal/g The results o f the least-squares m e t h o d gave: n = 0.646 K = 0.523396
160 but : CA
A Tln
K..
C = 1.1874
• 1 0 -3
So the drying rate eq. 2 becomes: 1.1874
• 1 0 -3 A G 0"646 A T l n
(5)
R =
TABLE I V a r i a t i o n s of t h e d r y i n g rate (R) w i t h t h e mass velocity (G). Material
Water added g
Time
R
G
min
g/rain
g / ( m i n e m ~)
5.25 4.60 4.80 5.15 4.45 4.55 3.90 3.38 3.45 4.00 3.45 3.00
40.100 35.900 34.000 31.500 30.000 27.000 25.400 23.700 22.200 20.600 19.000 17.600
Sodium Chloride
10.5 6.9 7.2 9.0 7.8 5.7 7.8 4.2 6.9 6.0 13.8 7.5
2.00 1.50 1.50 1.75 1.75 1.25 2.00 1.25 2.00 1.50 4.00 2.50
Limestone
50 50 50 50 50
10.5 11.0 13.5 15.0 17.0
4.76 4.55 3.70 3.33 2.94
25.997 24.487 23.720 22.957 22.170
Clay
50 50 50 50 50
8 9.5 10.5 11.0 12.5
6.25 5.263 4.761 4.545 4.00
25.924 24.418 23.655 22.132 21.005
50 -- 50% mixture of lime s t o n e a n d clay
50 50 50 50 50
10.5 11.0 13.0 15.0 15.5
4.762 4.545 3.846 3.333 3.226
25.346 23.822 22.289 21.153 19.884
161
DISCUSSIONS
The drying rate equation obtained above i.e. eq. 5 will be compared with the through-circulation drying equation. The continuous through-circulation dryers operate on the principle of blowing hot air through a permeable bed of wet material passing continuously through the dryer. The most widely used type is the horizontal conveying-screen dryer. The drying rates for continuous through-circulation dryers are high because of the large area of contact and short distance of travel for the internal moisture. Since the drying rates of the through-circulation are high they will be compared with the drying rate equation obtained in this work. Perry (1973a) discusses continuous through-circulation dryers and gives the following equation (eq. 20-48): G0.59 h = 0.11 Dp0.41 (6) where: h = heat transfer coefficient, B t u / ( h r ) (ft. 2) (°F) G = mass velocity of air, l b / ( h r ) (ft. 2) Dp = diameter of particles to be dried, ft. The drying rate equation for through-circulation is given by Perry (1973b) (eq. 20-12) as: ha (7) RTC ( T - Ts) pkd where: RTC = drying rate, lb w a t e r / ( h r ) (lb.dry solids) a = heat transfer area per cubic foot of bed, 1/ft. p = bulk density of dry material, lb/ft. 3 d = thickness of bed, ft. T = air temperature, °F Ts = evaporating-surface temperature, °F = latent heat of vaporization, Btu/lb Combining eq. 6 with eq. 7 and substituting the values used in this work we get: 0.351 W G 0"59 ATln RTC = (8) where W is the weight of the dry material used to form the slurry. For each experiment eq. 8 was used to calculate the drying rate. These calculated values are shown in Table II under the column RTC.
162 TABLE II Comparison of the drying rate (R) using fluidized bed that contains inert material and the drying rate (RTC) calculated from the through-circulation drying equation Materal
R g/min
RTC g/rain
Sodium Chloride
5.25 4.60 4.80 5.15 4.45 4.55 3.90 3.36 3.45 4.00 3.45 3.00
3.72 3.07 3.09 3.17 2.67 2.57 2.12 1.75 1.73 1.92 1.58 1.31
Limestone
4.76 4.55 3.70 3.33 2.94
1.03 0.95 0.758 0.669 0.579
Clay
6.25 5.263 4.761 4.545 4.00
1.47 1.19 1.06 0.97 0.83
50 -- 50% mixture of limestone and clay
4.762 4.545 3.846 3.333 3.226
1.123 1.033 0.841 0.706 0.659
CONCLUSION Table II s h o w s t h e d r y i n g rates (R) f o u n d in this w o r k and the d r y i n g rates {RTc ) calculated b y the e q u a t i o n o f t h r o u g h - c i r c u l a t i o n d r y i n g process. When these d r y i n g rates are c o m p a r e d it is clear t h a t t h e rate o f d r y i n g b y fluidized b e d (R) is always greater t h a n d r y i n g b y t h r o u g h - c i r c u l a t i o n . So it can be c o n c l u d e d t h a t d r y i n g by fluidized bed t h a t c o n t a i n s inert particles is far b e t t e r t h a n d r y i n g b y t h r o u g h - c i r c u l a t i o n .
163 ACKNOWLEDGEMENT
The author would like to thank Mr. Abdel Raheem Ahmed and Mr. Sayid Magaddam for their help in building the apparatus. REFERENCES Aldridi, R.J., 1964. Fluid bed dryers. In: The Encyclopedia of Chemical Process Equipment. Reinhold, New York, N.Y. Frantz, T.E., 1958. Ph.D. Thesis, Louisiana State University, La., U.S.A. Perry, R.H. and Chilton, C.H. (Editors), 1973a. Chemical Engineers Handbook, 5th ed. McGraw-Hill, New York, N.Y., p. 20-29. Perry, R.H. and Chilton, C.H. (Editors), 1973b. Chemical Engineers Handbook. 5th ed. McGraw Hill, New York, N.Y., p. 20-11. Romankov, P.G., 1971. Drying. In: J.F. Davidson and D. Harison (Editors), Fluidization. Academic Press, London, pp. 569--598. Romankov, P.G. and Rashkovskaya, N.B., 1968. Drying in a fluidized bed. Khimiya, Leningrad. Schmidt, A., 1955. Continuous drying of solutions. Chem. Zentralbl., 126: 3219--3220. Toei, R. and Akao, T., 1968. Multi-stage fluidized bed apparatus with perforated plates. Tripartitle Chem. Eng. Conf., (fluidization), Monterial. Vanecek, V., Drbohlave, R. and Markvart, M., 1965. Fluidized bed drying. Leonard Hill, London. Vanecek, V., Markvart, M., Drbohlave, R. and Hummel, R.L., 1970. Experimental evidence on operation of continuous fluidized bed driers. Chem. Eng. Prog. Symp. Ser., 66: 243--252.
155
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
DRYING BY FLUIDIZED BED THAT CONTAINS INERT PARTICLES
A.H.N. MOUSA*
Department of Chemical Engineering, University of Khartoum (Sudan) (Received December 15, 1978; revised and accepted June 15, 1979)
ABSTRACT Mousa, A.H.N., 1979. Drying by fluidized bed that contains inert particles. Int. J. Miner. Process., 6: 155--163. The method of drying by a fluidized bed that contains inert particles was used to dry slurries of sodium chloride, limestone, clay, and a 50 -- 50 wt.% mixture of clay and limestone. The slurries were prepared by mixing specified weights of the material with a specified weight of distilled water. The slurry was fed into the air-fluidized bed either by the use of compressed air or the use of a sectorized feeder. The bed contains fluidized inert particles on which the slurry was dried. The inert particles used here were sand particles. The drying rate of the slurries depends on a number of factors. These factors are air mass velocity, air inlet temperature, latent heat of vaporization and the logarithmic-mean temperature difference. Combining these factors a general drying rate equation was obtained. The results were compared with method of drying by through,circulation ; drying by fluidized bed was far better.
INTRODUCTION
Drying generally refers to the removal of a liquid from a solid by evaporation. Mechanical methods for separating a liquid from a solid are n o t generally considered drying. The evaporation process can be caused by the use of indirect-heat-transfer devices in which the wet solids are separated from the heat-carrier medium by a wall, so the two phases are never in direct contact. The second m e t h o d used to cause evaporation is the use of direct-heat-transfer devices. Here there is direct contact between the wet solids and the h o t gases. The driers used can be batch or continuous devices: The driers most widely used in industry are continuous; the material is loaded and discharged continuously. The continuous driers are the most efficient for large plants. One of the widely used methods is the continuous drying by through-circulation. A new method, which is nowadays gaining ground is drying by fluidized beds that contain inert particles. *Present address : Department of Chemical Engineering, University of Kuwait, P.O. Box 5969, Kuwait.
156
The fluidized solid technique, first developed in petroleum refining, is now finding use in many industries. The technique is a versatile unit operation which offers ease of solid handling and an excellent means of heat transfer. The high heat transfer rate that can be achieved in fluidized systems is the main reason for their use in a variety of high thermal technological processes. One of these processes is drying by fluidization. As stated by Romankov (1971), fluidized-bed drying has been applied not only to granular materials but also to pastes, suspensions, solutions and molten materials. As a result many batch processes have been replaced by continuous processes, which are much faster and more economic. The fluidized-bed drying technique has simplified methods of manufacturing certain chemicals, for instance the usual operations of evaporation, crystallization, filtration, drying and pulverization are replaced by a single process of drying in a fluidized bed, which is much cheaper. Romankov and Rashkovskaya (1968) and Vanecek et al. (1965) discussed the special features and difficulties of operating fluidized-bed driers. Continuously operating driers of single chamber and multichamber design (Toei and Akao, 1968) have been used to dry limestone, dolomite foundary casting sands, blast furnace slag, phosphorites, and other heat-resistant inorganic materials. The technique is also widely used for drying coal (Aldridi, 1964). Vanecek et al. (1970) studied the operating conditions and drying performances of continuous fluidized-bed driers. Some materials are dried on inert bodies and the dry product is carried away from the drying chamber as a fine powder. This method of drying by using inert material, was first proposed by Schmidt (1955) and used by Frantz (1958) for evaporating sea water. Design equations that may be used to determine the drying rate and the dimensions of the drying equipment are required in order to develop methods of drying various materials in a fluidized bed. These equations should be derived on the basis of theoretical analysis of a physical model of the process and/or of experimental determination of the influence of the most important factors. The method of drying by through-circulation where the drying gases flow either upward or downward through a permeable bed of wet granular solids is widely used in industry. In industry the method is converted into a continuous process by using a conveyor as the bed. In this work the method of drying using a gas-fluidized bed that contains inert particles was investigated. Continuous feed of the material to be dried comes into an air-fluidized bed that contains sand particles. The sand particles are fluidized by the hot air. The wet material is dried on the surface of the h o t sand particles and it is carried out by the air and collected in cyclone.
157 EXPERIMENTAL APPARATUS
The experimental apparatus is shown in Fig. 1A. It consists of a compressor which compresses air and delivers it to the bed. The quantity of air going to the bed is controlled by valve V1. An electric heater located after valve V~ heats the air to the required temperature by the use of a variable variac. A thermometer Ta and a manometer Pa are used to measure the temperature and pressure of the heated air. The velocity of the air is measured by an orifice meter. Inclined water manometers were used to measure the pressure drop across the orifice meter. An air distributor at the b o t t o m of the bed distributes the air evenly in radial directions. Then the air goes upwards through a calming section so as to have even distribution along the bed. The bed has an inside diameter of 10.2 cm. A thermometer T~ measures the inlet temperature of the air before it goes through the screen supporting the sand particles. Another inclined water manometer measures the pressure drop across the fluidized bed. The temperature of the air leaving the bed is measured by thermometer T2. Then the air goes through a cyclone where the dried particles are collected. On the top of the bed a feeding bottle is fixed. It contains the slurry to be dried. Compressed air is used to agitate the slurry and create enough pressure in the feed bottle to force the slurry into the fluidized bed. A valve on the vent line is used to control the pressure V NT
T~
~ COMPRESSEDAIR ~ FEEDINGBOTTLE WITH A CONTROLLING VALVE V1
C*LONE FLOW CALMER
ER
O R I F I C E ~ M E IT, ERp,
~11~1 =VALVE Vl ETERS
ELECTRICAL ' ' HEATER
/4
0MPRE AIR '
~
Fig. 1. A. Flow diagram of apparatus. B. Sectorized feeder with radial blades.
158 in the feed bottle. The flow rate of the slurry into the bed is controlled by valve V2. Another m e t h o d to feed the wet particles into the bed was the use of a sectorized feeder with radial blades. This was fixed on the top of the bed in place of the feeding bottle, this arrangement is shown in Fig. lB. This was used to feed sodium chloride slurry only. The height of the sand in the bed was 4.2 cm. The sand particles have a size range of 2800--2000 pm (mesh number 6-8). The slurry particles have a size range of 300--150 pm (mesh number 52-100). PROCEDURE Measured quantities of the material to be dried and distilled water were mixed and pored into the feeding bottle, with valve V2 closed. The flow rate of the fluidizing air was adjusted to the required value by valve V1. The inlet temperature T~ was adjusted to the required value by the variable variac, controlling the power input to the electrical heaters. When steady state was reached, valve V2 was opened slowly allowing the slurry to go into the fluidized bed. The a m o u n t of slurry going into the bed was adjusted so that the reading of the thermometer T2 was constant. The time taken to empty the feeding bottle was recorded. The above procedure was repeated for each experiment by changing the air flow rate only, keeping T~ and T2 constant at the same previous values. Then the material to be dried was changed and all these experiments were repeated. In case of drying the sodium chloride slurry, the feeding bottle was replaced by the sectorized feeder shown in Fig. 1. THEORETICAL ASPECTS The drying rate of wet particles is a function of a number of factors. These are: (1) The drying surface area (A). (2) The air mass velocity (G). (3) The latent heat of vaporization of water at the operating temperature
(x). (4) The logarithmic mean temperature difference between the slurry feed and the operating temperatures (A Tin). An equation relating these factors to the drying rate (R) can be written as: R oc
A Gn A
T1n
X C A Gn A
R =
Tln
where C and n are constants.
(1)
(2)
159 In these experiments, the temperatures were kept constant. T herefore A Tin and ~ will be constants. Also since the a m o u n t o f sand in the bed was constant, t h e r e f o r e (A) is constant. So eq. 2 can be written as: R = K G '~
(3)
R =logK+nlogG
(4)
or: log
where: K =
C A ,~ Tln
Equation (4) represents a straight-line equation similar to: y=a+bx
and so the least-square m e t h o d can be used t o find the constants K and n. CALCULATIONS AND RESULTS The gas density (p) was calculated from the pressure Pa and t e m p e r a t u r e Ta. From the orificemeter calibration the gas velocity (v) is obtained. Then the mass velocity (G) is calculated. The drying rate (R) is calculated by dividing the a m o u n t of water added to make the slurry by the time taken t o e m p t y the feed bottle. So for each experiment as the flow o f airis varied, values o f G and R can be calculated. The results o f these calculations are given in Table I for the different materials dried. The following factors were kept constant at the values given: t e m p e r a t u r e o f the gas entering the bed = 100 °C t e m p e r a t u r e o f the gas leaving = 67 °C t e mp er atu r e o f slurry = 27 °C drying surface area (A) = 4500 cm 2 ATln = 54.856 latent heat o f vaporization ~ = 560 cal/g The results o f the least-squares m e t h o d gave: n = 0.646 K = 0.523396
160 but : CA
A Tln
K..
C = 1.1874
• 1 0 -3
So the drying rate eq. 2 becomes: 1.1874
• 1 0 -3 A G 0"646 A T l n
(5)
R =
TABLE I V a r i a t i o n s of t h e d r y i n g rate (R) w i t h t h e mass velocity (G). Material
Water added g
Time
R
G
min
g/rain
g / ( m i n e m ~)
5.25 4.60 4.80 5.15 4.45 4.55 3.90 3.38 3.45 4.00 3.45 3.00
40.100 35.900 34.000 31.500 30.000 27.000 25.400 23.700 22.200 20.600 19.000 17.600
Sodium Chloride
10.5 6.9 7.2 9.0 7.8 5.7 7.8 4.2 6.9 6.0 13.8 7.5
2.00 1.50 1.50 1.75 1.75 1.25 2.00 1.25 2.00 1.50 4.00 2.50
Limestone
50 50 50 50 50
10.5 11.0 13.5 15.0 17.0
4.76 4.55 3.70 3.33 2.94
25.997 24.487 23.720 22.957 22.170
Clay
50 50 50 50 50
8 9.5 10.5 11.0 12.5
6.25 5.263 4.761 4.545 4.00
25.924 24.418 23.655 22.132 21.005
50 -- 50% mixture of lime s t o n e a n d clay
50 50 50 50 50
10.5 11.0 13.0 15.0 15.5
4.762 4.545 3.846 3.333 3.226
25.346 23.822 22.289 21.153 19.884
161
DISCUSSIONS
The drying rate equation obtained above i.e. eq. 5 will be compared with the through-circulation drying equation. The continuous through-circulation dryers operate on the principle of blowing hot air through a permeable bed of wet material passing continuously through the dryer. The most widely used type is the horizontal conveying-screen dryer. The drying rates for continuous through-circulation dryers are high because of the large area of contact and short distance of travel for the internal moisture. Since the drying rates of the through-circulation are high they will be compared with the drying rate equation obtained in this work. Perry (1973a) discusses continuous through-circulation dryers and gives the following equation (eq. 20-48): G0.59 h = 0.11 Dp0.41 (6) where: h = heat transfer coefficient, B t u / ( h r ) (ft. 2) (°F) G = mass velocity of air, l b / ( h r ) (ft. 2) Dp = diameter of particles to be dried, ft. The drying rate equation for through-circulation is given by Perry (1973b) (eq. 20-12) as: ha (7) RTC ( T - Ts) pkd where: RTC = drying rate, lb w a t e r / ( h r ) (lb.dry solids) a = heat transfer area per cubic foot of bed, 1/ft. p = bulk density of dry material, lb/ft. 3 d = thickness of bed, ft. T = air temperature, °F Ts = evaporating-surface temperature, °F = latent heat of vaporization, Btu/lb Combining eq. 6 with eq. 7 and substituting the values used in this work we get: 0.351 W G 0"59 ATln RTC = (8) where W is the weight of the dry material used to form the slurry. For each experiment eq. 8 was used to calculate the drying rate. These calculated values are shown in Table II under the column RTC.
162 TABLE II Comparison of the drying rate (R) using fluidized bed that contains inert material and the drying rate (RTC) calculated from the through-circulation drying equation Materal
R g/min
RTC g/rain
Sodium Chloride
5.25 4.60 4.80 5.15 4.45 4.55 3.90 3.36 3.45 4.00 3.45 3.00
3.72 3.07 3.09 3.17 2.67 2.57 2.12 1.75 1.73 1.92 1.58 1.31
Limestone
4.76 4.55 3.70 3.33 2.94
1.03 0.95 0.758 0.669 0.579
Clay
6.25 5.263 4.761 4.545 4.00
1.47 1.19 1.06 0.97 0.83
50 -- 50% mixture of limestone and clay
4.762 4.545 3.846 3.333 3.226
1.123 1.033 0.841 0.706 0.659
CONCLUSION Table II s h o w s t h e d r y i n g rates (R) f o u n d in this w o r k and the d r y i n g rates {RTc ) calculated b y the e q u a t i o n o f t h r o u g h - c i r c u l a t i o n d r y i n g process. When these d r y i n g rates are c o m p a r e d it is clear t h a t t h e rate o f d r y i n g b y fluidized b e d (R) is always greater t h a n d r y i n g b y t h r o u g h - c i r c u l a t i o n . So it can be c o n c l u d e d t h a t d r y i n g by fluidized bed t h a t c o n t a i n s inert particles is far b e t t e r t h a n d r y i n g b y t h r o u g h - c i r c u l a t i o n .
163 ACKNOWLEDGEMENT
The author would like to thank Mr. Abdel Raheem Ahmed and Mr. Sayid Magaddam for their help in building the apparatus. REFERENCES Aldridi, R.J., 1964. Fluid bed dryers. In: The Encyclopedia of Chemical Process Equipment. Reinhold, New York, N.Y. Frantz, T.E., 1958. Ph.D. Thesis, Louisiana State University, La., U.S.A. Perry, R.H. and Chilton, C.H. (Editors), 1973a. Chemical Engineers Handbook, 5th ed. McGraw-Hill, New York, N.Y., p. 20-29. Perry, R.H. and Chilton, C.H. (Editors), 1973b. Chemical Engineers Handbook. 5th ed. McGraw Hill, New York, N.Y., p. 20-11. Romankov, P.G., 1971. Drying. In: J.F. Davidson and D. Harison (Editors), Fluidization. Academic Press, London, pp. 569--598. Romankov, P.G. and Rashkovskaya, N.B., 1968. Drying in a fluidized bed. Khimiya, Leningrad. Schmidt, A., 1955. Continuous drying of solutions. Chem. Zentralbl., 126: 3219--3220. Toei, R. and Akao, T., 1968. Multi-stage fluidized bed apparatus with perforated plates. Tripartitle Chem. Eng. Conf., (fluidization), Monterial. Vanecek, V., Drbohlave, R. and Markvart, M., 1965. Fluidized bed drying. Leonard Hill, London. Vanecek, V., Markvart, M., Drbohlave, R. and Hummel, R.L., 1970. Experimental evidence on operation of continuous fluidized bed driers. Chem. Eng. Prog. Symp. Ser., 66: 243--252.