METHOD OF GRANULATING MOISTURE-ABSORBING POWDER MATERIAL

Abstract: A method of granulating moisture-absorbing powder material includes maintaining the powder material in a fluidised state by blowing hot air through it in a fluidisation chamber and spraying a binder in liquid form onto it to cause agglomeration of the powder material into granules. The moisture-content of the powder material is continuously monitored by an infra-­ red absorption moisture meter 3 and the rate of supply of binder is controlled in dependence on the moisture content thereby ensuring that the rate of granulation is always at the optimum value.

Description: The present invention relates to a method of granulating moisture-absorbing powder material of the type which includes maintaining the powder material in a state of agitation in a chamber and adding a binder in liquid form to the powder material thereby causing agglomeration of the powder material into granules. In a known granulating method of the type in which starting powder material is fluidised in a stream of hot air and, while adding a binder liquid to the fluidised powder material, the powder particles are induced to adhere and agglomerate to produce granules by the addition of a binder liquid, the moisture content of the powder material which permits adequate fluidisation to be achieved has a range depending on the properties of the powder material. If the moisture exceeds the upper limit of the range, the flowing movement is rapidly slowed down and large lumps are produced. If the moisture content increases further, the flowing movement is stopped and the powder material solidifies to agglomerate form and the granulating apparatus is rendered inoperable. In this type of conventional granulation it has been the usual practice to use a control method which includes detecting the temperature or humidity of the exhaust air and controlling the temperature of the hot air or the feed rate of the binder in dependence thereon. When using a batch method, it has been usual to set the feed time of the binder by means of a timer while maintaining the temperature and flow rate of the hot air constant. When granulating a moisture-­ absorbing powder material, the range of moisture content within which the proper flowing movement of the powder is maintained is extremely small with an upper limit of about 3% D.B. (3% liquid with respect to the dry powder) in the case of amino acid-containing flavouring material and about 5% D.B. in the case of powder soy and exceeding this upper limit of the moisture content has the effect of rapidly impairing the fluidity of the powder and then terminating the flowing movement. In the conventional methods it has been impossible to detect small changes in the exhaust air temperature or humidity and it has been impossible to maintain constant the amount of hot air supplied due, for example, to the clogging of the filter incorporated in the apparatus. Thus, it has been necessary to operate with an extremely low moisture content in order to avoid the danger of terminating the flowing movement of the powder material within the apparatus and agglomerating the powder material due to, for instance, the repeated addition of the binder liquid and intermediate drying. Thus the conventional methods have suffered from the disadvantage that since it is impossible to detect an increase in the moisture content which would impair the flowing movement it has been necessary to monitor the fluid state by visual inspection and to use a relatively low moisture content which results in a decrease in the growth rate of granules which in turn necessitates a longer granulating time and makes it difficult to increase the product yield with respect to the desired granule size. It is an object of the invention to provide a method of granulating moisture-absorbing powder material which not only prevents the occurrence of any substantial deterioration in the flowability of the powder material and produces granules of a stable particle size in a continuous operation but also reduces the granulation time and increases the product yield. According to the present invention a method of the type referred to above is characterised by continuously monitoring the moisture content of the agitated powder material with moisture content detecting means arranged within the chamber or a space communicating directly therewith and controlling the growth rate of the granules by controlling the rate of supply of binder or the amount of binder supplied in dependence on the said moisture content. The method in accordance with the invention enables the moisture content of the powder material to be accurately measured and maintained substantially at the optimum value whereby the granulation of the powder material is performed both efficiently and in the minimum of time to produce granules of a desired particle size. The powder material is preferably maintained in a state of agitation by blowing hot air through it thereby fluidising it. The temperature of the hot air is preferably monitored and maintained substantially constant with the aid of a temperature sensor and temperature regulating means. In the preferred embodiment the binder is sprayed onto the powder through a spray nozzle and the rate of binder supply is varied in dependence on the measured value of the moisture content by adjusting the speed of the binder feed pump or adjusting a regulating valve in the binder supply line. The moisture content of the powder material is preferably brought up to a value substantially equal to or slightly less than the value at which the fluidisability of the powder material is significantly impaired. It is preferred that the moisture content of the powder material is monitored directly by detecting a change in the characteristic of the powder material within the chamber. The moisture content is preferably monitored in a non-contact manner and in one embodiment the characteristic which is monitored is the infra-red absorptivity. Further features and details of the invention will be apparent from the following description of one specific embodiment which is given by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram showing the general construction of a granulating apparatus employing a method according to the present invention; Figure 2 is a detailed sectional elevation of the major components of the moisture content sensor shown in Figure 1; and Figures 3 and 4 are graphs showing the comparative measurements taken in granulating methods controlled conventionally and in accordance with the invention. Figure 1 shows a granulating apparatus 1 for containing the processing powder material within which are a spray nozzle for spraying a liquid binder onto the powder material and an infra-red absorption-type moisture meter 3 for detecting continuously and in a non-constact manner the moisture content of the powder material through a detecting tube 4. The moisture meter 3 is connected to a moisture content regulator which is operable in response to the output signal of the moisture meter 3 and which in turn is connected to a speed setting means or controller 6 of a binder feed pump 7 whose speed may be varied by the speed controller 6 to vary the amount of binder liquid supplied to the nozzle 2. The powder material is maintained in a state of agitation in the granulating apparatus and in this case is fluidised by hot air which is blown by a fan 8 into a hot air chamber 12 and then through an opening in the bottom of the granulating apparatus which accommodates a gas dispersing plate 13 in which a plurality of apertures 13a is formed. The air is drawn from the atmosphere by a fan 8 and then heated in a heat exchanger 9 which is heated by steam. A temperature sensor 10 detects the temperature of the air leaving the heat exchanger 9 and produces an output signal. A steam regulating valve 11 is operated in dependence on the difference between this output signal and a predetermined reference signal to supply a sufficient volume of steam to maintain the temperate of the air substantially constant. A bag filter 14 situated within the upper portion of the granulating apparatus 1 collects fine powder which is entrained in the air leaving the fluidised powder and then drops back down into the fluidised bed. An exhaust fan 15 discharges the air cleaned by the filter 14 to the atmosphere. As may be seen in Figure 2, the moisture meter 3 communicates with the fluidised bed via the inclined tube 4. Within the tube 4 there are a main purge pipe 4a and a bypass purge pipe 4b. In use, purging air is supplied through these pipes from a source of air which is not shown. In use, air drawn in by the fan 8 is heated to the desired temperature and forced into the fluidisation chamber 1a of the granulating apparatus 1 from the hot air chamber 12 through the holes 13a in the gas dispersing plate 13 and the powder material within the fluidisation chamber 1a is maintained in a fluidised state. Purging air is supplied through the main purging tube 4a into the detecting tube 4 and blown into the fluidised powder material present in the vicinity of the communication with the fluidisation chamber. Thus, powder material is prevented from entering into the detecting tube 4 but moves across its open end. The purging air supplied from the bypass purging tube 4b removes the powder material which tends to deposit on the bottom of the detecting tube 4. The interface between the fluidised powder material and the air in the tube 4 thus apparently remains substantially stationary in relation to the moisture meter 3 and satisfies the requirement for a measuring surface for measuring the degree of attenuation of infra-red rays projected at and reflected from the material. The absorption of light energy by the material is substantially proportional to its moisture content. The moisture content of the powder material detected by the moisture meter 3 is represented by an electrical signal which is applied to the moisture content regulator 5 and the speed setting means 6 is operated in dependence on the difference between the detected moisture content and a desired value thereof corresponding to the type of the powder material. Binder liquid is mixed with spraying air at a rate determined by the rotational speed of the binder feed pump 7 which is in turn determined by the output of the speed setting means 6 and is atomised and sprayed from the spray nozzle 2 thus inducing flowing of the powder material within the desired ranges of temperatures and moisture content and thereby causing granulation of the powder material. Measurements have been made relating to the fluidised bed granulation of moisture-absorbing powder material such as powder soy or amino acid-containing seasoning and these have shown: (1) The growth rate of the particles is determined substantially only by the moisture content of the powder material irrespective of the amount of solid matter contained in the added binder liquid. (2) The amount of the solid matter in the added binder liquid has an effect on the strength of particles and the percentage of the particulate material which reverts to powder during the finishing and drying operations is increased with a decrease in the amount of the solid matter. (3) If the finished and dried particles are screened in e.g. a 100-mesh sieve and the yield of the particles which do not pass through the sieve is compared with that obtained in a conventional method, for the same amount of added binder solid matter, the product yield is increased if the granulation is performed at a higher moisture content of the powder material. In other words, it has been found that by suitably determining the moisture content of the powder material at the end of the addition of the binder or the rate of increase of the moisture content during the addition of the binder, it is possible to control the particle size of the product as desired and hence it is possible to improve the yield of the product of the desired particle size. Figures 3 and 4 are graphs of measurements made during experiments conducted in accordance with the above method used with a batch-type granulating apparatus compared with a conventional method. The points indicated by the symbol X are from the method in accordance with the invention and those indicated by the symbol O are from the conventional method. The graph of Figure 3 shows the results obtained when granulating powder soy, and plots a comparison between the moisture contents and the yields of plus 100-mesh sieve product, that is to say the yield of particles which do not pass through a 100-mesh sieve, against the amount of added binder liquid. It has been found that there is a definite correlation between the moisture content and the yield of plus 100-mesh sieve product and that the yield can be increased by 35% by increasing the moisture content at the end of the operation by 2% D.B., that is to say by 2% based on the dry weight of the powder. The graph of Figure 4 shows the results obtained when granulating amino acid-containing seasonings and plots the yields of plus 100-mesh sieve products and moisture contents against time. It has been found that by setting the moisture content at the end of the operation of 28% D.B., which is just below the content at which insufficient fluidisation occurs, it is possible to reduce the operating time and the addition of binder liquid to about one half and increase the yield by 18% by comparison with the conventional method. The operating conditions in the experiments were as follows: Apparatus used: batch-type fluidised bed granulating apparatus incorporating an agitator Fluidised bed area : 0.2m² Hot air temperature : 80°C Fluidising air velocity: 0.82m/sec Binder : 20% potato starch Binder liquid addition : (conventional method) 5-minute spraying at 70 g/min alternating with 1-minute in-process drying for 0-48 minutes (cycle spraying) followed by 110 g/min for 48-60 minutes (continuous spraying) (Method according to the invention) 1. Maximum binder feed rate, 110 g/min 2. Moisture content regulator setting, 28% D.B. In the described embodiment, the material to be granulated is agitated by blowing hot air throught it to fluidise it However, this can also be effected by the use of a mechanical agitator or the like incorporated in the granulating apparatus.

Claims: 1. A method of granulating moisture-absorbing powder material including maintaining the powder material in a state of agitation in a chamber and adding a binder in liquid form to the powder material thereby causing agglomeration of the powder material into granules, characterised by continuously monitoring the moisture content of the agitated powder material with moisture content detecting means (3,4) arranged within the chamber or a space communicating directly therewith and controlling the growth rate of the granules by controlling the rate of supply of binder or the amount of binder supplied in dependence on the said moisture content. 2. A method as claimed in claim 1 characterised in that the powder material is maintained in a state of agitation by blowing hot air through it thereby fluidising it. 3. A method as claimed in claim 2, characterised in that the temperature of the hot air supplied to the chamber is monitored and maintained substantially constant. 4. A method as claimed in any one of the preceding claims, characterised in that the binder is sprayed onto the powder through a spray nozzle (2) and that the rate of binder supply is varied in dependence on the measured value of the moisture content by adjusting the speed of the binder feed pump (7) or adjusting a regulating valve in the binder supply line. 5. A method as claimed in any one of claims 1 to 4, in which the moisture content of the powder material is brought up to a value substantially equal to or slightly less than the value at which the fluidisability of the powder material is significantly impaired. 6. A method as claimed in any one of the preceding claims characterised in that the moisture content of the powder material is monitored directly by detecting a change in a characteristic of the powder material within the chamber. 7. A method as claimed in any one of the preceding claims, characterised in that the moisture content is monitored in a non-contact manner by an infra-red absorption moisture meter (3). 8. A method as claimed in claim 7 characterised in that the moisture meter (3) communicates with the chamber via a detecting tube (4) and that air is blown through the detecting tube (4) into the chamber thereby substantially preventing powder material from entering into the detecting tube (4).

Inventors: NAGASAWA, KAZUMASA, SHIBATA, TAKEO
Applicants: OKAWARA MFG
Application year: 1986
Main classification: B01J2
IPC codes: B01J2