THE DRYING OF RAW SUGAR IN FLUIDISED AND SPOUTED BEDS

Manufacturing - Engineering THE DRYING OF RAW SUGAR IN FLUIDISED AND SPOUTED BEDS R. H. Weiland, G. Low and L. S. Leung Department of Chemical Engineering, University, of Queensland, Queensland, Australia ABSTRACT The batch drying of raw sugar has been studied in fluidised and spouted beds. Sugar with moisture greater than 0,6% can only be fluidised if it is mixed with an appropriate amount of drier sugar. Sugar with moisture as high as 1,5% can be easily spouted. In both cases drying occurs in the falling rate period and drying rates are independent of bed height and air flow rate. Typical drying times are 1 minute and 2 minutes for fluidised and spouted beds, respectively. Spouted beds are highly stable and can sustain shock loads while requiring about the same or slightly higher air rates as fluidised bed driers. Lower air pressures are required since the entire weight of the bed does not have to be supported. In the production of raw sugar the sugar is usually dried in a rotary drum drier from an initial moisture content of between 0,6% and 2% to about 0,5% for convenient handling and storage. Sugar enters the drier at about 56 C and leaves at about 30-35 C. In the literature nothing seems to have been reported on the application of either fluidised or spouted bed techniques to raw sugar drying, although fluidisation has been applied to the cooling of refined sugar.l Typical fluidised and spouted bed driers are sho\yn in Fig. 1. Possible advantages of using fluidised or spouted bed driers include the short retention /, Feed -Distributor Plate, y+prifice Product Air in Fluidized Bed Air in Spouted Bed FIGURE 1. Typical Fluidised and Spouted Beds. 1561 I a+' 3,; 93,, 1,")! +

1562 MANUFACTURING - ENGINEERING times needed, the small floor space required, the ability to stack the units one above the other and the absence of the dynamic loads which occur with rotating drum equipment. Possible disadvantages are the carryover of fines produced by attrition and the variation of drier performance with particle size, although the lattei- is more pronounced in fluidised driers than other types. The purpose of this work is to examine and compare the use of fluidised and spouted bed driers from the points-of-view of drying rates, maximum permissible moisture content in the sugar and stability of operation. EXPERIMENTAL PROCEDURE Equipment The batch fluidised bed (Fig. 2) consisted of a 15 cm internal diameter perspex column on a 60" conical base. A suitable arrangement for feeding sugar to the bed and for allowing the air to escape from the top was provided together with a 3,75 cm diameter hole situated near the base of the column to enable easy sampling of the sugar. A piece of fine filter cloth was used as the gas distributor. Funnel Probe ~hermometers 15 cm Diameter Perspex Column Sampling Port Filter Cloth Distributor ( - ~ i In r FIGURE 2. Experimental Fluidised Bed. A Shaw Hygrometer (time constant less than 1 sec) with a recorder output was used to measure the outlet humidity of the air. The probe was inserted at the top of the column. Wet and dry bulb thermometers were used to measure the humidity and the temperature of the inlet air. The outlet air and the bed temperature were also measured. Drying air was supplied by an Apex 2-stage centrifugal blower.

. R. H. WEILAND, G. LOW AND L. S. LEUNG 7 30 cm Diameter Perspex Column Air Supply FIGURE 3. Experimental Spouted Bed. The batch spouted bed was 30 cm diameter with a 60" cone bottom and is shown in Fig. 3. Drying air entered through a 7,5 cm diameter orifice plate at the bottom of the cone. Samples were taken through a 3,75 cm diameter hole near the bottom of the column. The relative humidity of the air was measured prior to entering the bed using wet and dry bulb thermometers. No other moisture measurements were made on the air stream. The moisture content of all sugar samples was measured by drying the samples to "constant weight" in an air oven at 103 to 105 C. Procedure The charge of sugar was prepared by mixing in sufficient water to achieve the desired initial moisture content. Several days were allowed for the water to be completely absorbed. The ability of the sugar to be fluidised or spouted was determined by visual observation and by finding the maximum moisture content above which stable fluidisation or spouting could no longer be achieved. This was done by mixing wet sugar with varying amounts of dry material (i.e. in equilibrium with the drying air) to find the dilution required to permit the bed to be fluidised or spouted. For initial moisture below 0,6% no dilution was required for fluidisation. In the batch fluidised drying studies the initial moisture content of the sugar was determined by oven drying a wet-sugar sample. The relative humidity of the leaving air was used to infer the moisture content in the sugar during the drying process. In the batch spouted drying work, sugar samples were taken at 30 sec intervals, the moisture content being directly measured by oven drying. i!,, '\ i y,," 5' 'I.

1564 MANUFACTURING - ENGINEERING I In all drying experiments, wet sugar was mixed with dry material before drying commenced. Results are reported on the basis of wet sugar; the dry material was ignored since its only purpose was to 'permit fluidisation or spouting. RESULTS, DISCUSSION AND CONCLUSIONS Fluidising and Spouting Characteristics of Raw Sugar It was found that sugar having a moisture greater than 0,6% could not be satisfactorily fluidised. To fluidise sugars with high moistures, they had to be mixed with a proportion,,of already dry material. On the other hand, sugars with moisture as high"a~ 2,5 to 1,7% could be stably spouted and dried. Only a small amount of premixing with dry material was required. Fluidised and spouted beds are being examined here for use as continuous driers. For such operations, the moisture of the contents of the bed will be equal to that of the desired product, usually between 0,2% and 0,6%. Since both types of equipment can be stably operated under these conditions, either one can be used. However, the spouted bed is stable under much more severe conditions and so is well able to accept shock loads of very wet feed without adversely affecting its operation. In addition the spouted bed drier used about the same or only slightly higher air rates to form and hold the spout as those required for fluidisation of the same diameter bed. In contrast to a fluidised bed, the entire bed weight does not have to be supported by the air flow in a spouted bed; hence, delivered air pressures are lower. Although feed sugar contains no fines some would be expected to be produced by attrition in fluidised and spouted beds. It has been found that 1. 0 ~ ' I I 0 10 20 30 40 50 60 FIGURE 4. Drying curves for raw sugar in a fluidised bed at 20 C; O,A for Brand 1 at air velocities of 1,5 m/sec and 0,86 m/sec respectively; O,A for JA sugar at the same fluidising velocities.

0 10 20 30 40 50 60 FIGURE 5. Drying curves for raw sugar in a 15 cm fluidised bed at 56 C; O,A for Brand 1 at air velocities of 1,5 mlsec and 0,86 mlsec respectively; 0, A for JA sugar at the same fluidising velocities. under the most vigorous fluidising conditions less than 0,2% of the bed weight is carried over as fines in a one-minute interval. Work is in progress to determine attrition in spouted beds. Drying Characteristics Dimensionless moisture content, y* = (y - ye) / (yi'- ye) is shown plotted against time for the fluidised drying of raw sugar with initial temperatures 0 20 40 60 80 100 120 FIGURE 6. Drying curves for Brand 1 sugar in a spouted bed at 56 C and an air velocity of 3,5 m/sec.

1566 MANUFACTURING - ENGINEERING FIGURE 7. 1,37 m/sec. 0 20 40 60 80 100 120 Drying curves for Brand 1 sugar in a spouted bed at 56 C and an air velocity of at 20 C and 56 C in Figs. 4 and 5, respectively. The equilibrium moisture, ye, is calculated from a correlation given by Miller and Wright.2 Here y is the moisture at any time and yi the initial moisture content. Corresponding results for spouted bed drying at 20 C and two air velocities are given in Figs. 6 and 7. It can be seen that for both fluidised and spouted bed drying, the rate of air flow, i.e. the intensity of fluidising or spouting, does not have a measureable effect on the drying rate. This tends to support the contention that drying rates are controlled by the heat transfer and diffusional processes taking place inside single sugar crystals and not by interfacial processes such as evaporation from the surface of particles. One should therefore operate such equipment at the minimum air rates consistent with bed stability and easy control. A drying time of one minute for fluidised beds and two minutes for spouted beds is sufficient to bring wet sugar very close to its equilibrium moisture content with the drying air; JA sugar required a slightly longer drying time than Brand 1. The equipment is thus small and compact and in addition requires no moving parts. Maintenance costs can therefore be expected to be low. Of the two methods studied, the spouted bed drier seems preferable because of its greater stability, its resistance to shock loads and the lower air pressure requirements. The only obvious possible disadvantage is that attrition rates, i.e. fines production, may be greater than in a fluidised drier. The batch data reported here should be useful in the design of continuous driers. ACKNOWLEDGEMENTS The authors wish to acknowledge financial support by the Sugar Research Institute, Mackay for this work and the useful discussions with Dr P. G. Wright of the Institute.

R. H. WEILAND, G. LOW AND L. S. LEUNG 1567 - REFERENCES 1. Neuzil, L., Herdina, hl. and Valter, V. (1970). Listy Cukrov, 68:3. 2. Miller, K. F. and Wright, P. G. (1971). The equilibrium relative humidity of raw sugar, Proc Queens Soc Sug Cane Tech, 38:83-88. EL SECADO DEL AZUCAR CRUD0 POR LOS SISTEMAS DE "FLUIDIZACION" Y CHORRO MOVIL R. H. Weiland, G. Low y L. S. Leung RESUMEN El secado del azljcar crudo se ha estudiado por 10s sistemas de "fluidizacidn" y de chorro mdvil. El azljcar con humedad mayor de 0,6% solo puede fluidizarse si se le mezcla con una corta proporcidn de azljcar seco. El aziicar con humedad hasta de 1,5% puede ficilmente llevarse a su fase fluida en un chorro mdvil. En ambos casos el secado ocurre en el period0 de descanso y las ratas de secado son independientes de la altura del lecho y de la rata de flujo. Uno y dos minutos son 10s tiempos tipicos de secado para 10s mbtodos de fluidizacidn y chorro mdvil. Los chorros mdviles son muy estables y soportan cargas variables, con la misma o ligeramente superior rata de aire que la utilizada en 10s secadores que utilizan el sistema de fluidizacidn. En el sistema chorro m6vil se necesita menor presidn de aire debido a que no se necesita sostenertodo el peso del lecho.