Method and apparatus for drying thermosensitive materials

A method of drying thermosensitive materials includes spraying such materials onto inert bodies circulating about a closed space in a flow of a heat-transfer agent at an increased temperature. In the upper part of the flow of the heat-transfer agent, occupying not more than 1/3 of its cross section, the heat transfer agent is rotated at an axial velocity 2 to 3 times higher than the velocity of the heat transfer agent in the rest of the flow. An apparatus for carrying out this method has a cylindrical chamber with a tapered bottom accommodating axially of the chamber a blind tube with a base defining with the walls of the bottom an annular passage for introducing a heat transfer agent thereto. The chamber has a mesh and a propeller mixer secured thereunder on a shaft arranged at an angle of 60.degree.-90.degree. to the axis of the chamber at a distance from this axis which is less than R, where R is the radius of the chamber.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to methods of drying thermosensitive materials and 
apparatus for carrying out the methods. 
DESCRIPTION OF THE PRIOR ART 
There are known apparatus for drying thermosensitive materials, for example 
an apparatus with a fluidized bed (cf., Stabnikov V. N., et al, "Protsessy 
i apparaty pischevykh proizvodstv" Moscow, the Agropromizdat Publishers, 
1985, p. 360). 
The apparatus comprises two chambers arranged one above the other and 
separated by a gas distribution grid. The upper chamber is connected to a 
gas outlet pipe, a hopper for a moist material to be dried, and a pipe for 
discharging the dry material. 
The moist material is fed from the hopper onto the gas distribution grid. 
Hot gases forced through the holes in the grid from the lower to the upper 
chamber at a high velocity act to keep the material in a fluidized state. 
The dry material is evacuated through the discharge pipe. 
This prior art apparatus features a rather low capacity, since during 
drying thermosensitive materials the temperature of the heat-transfer 
agent should not be too high, as the product being dried tends to burn. 
Conversely, drying at low temperatures necessitates a longer contact of 
the material with the heat-transfer agent and affects the quality of the 
end product. 
There is also known a method of drying thermosensitive materials in a 
fluidized bed (cf., Sazhin B. S. "Osnovy tekhniki sushki", Moscow, the 
Khimia Publishers, 1984, p. 91). 
A moist material is admitted from a charging hopper to a chamber, where it 
is entrained by a heat-transfer agent fed through axial nozzles. 
In this chamber the material circulates until the dry particles, being 
lighter in weight, are carried by the heat-transfer agent to a cyclone. 
This method, however, fails to obtain a high quality material with uniform 
moisture content throughout its mass, because the particles of the 
material travel different paths. The method also fails to dry suspensions 
and paste-like materials having high moisture content. 
There is also known a method of drying thermosensitive materials and an 
apparatus for carrying out the method (cf., USSR Inventor's Certificate 
No. 1,192,764, Int. Cl. A 23 B 5/02, published Oct. 17, 1984). 
The method involves spraying thermosensitive materials onto inert bodies 
circulating about a closed space in a flow of heat-transfer agent at a 
temperature below the temperature of decomposition of such materials. 
The apparatus for carrying out the method comprises a cylindrical chamber 
having an outlet pipe and a tapered bottom in which there is arranged 
axially of the chamber a blind tube having a base defining with the walls 
of the bottom an annular passage whereto the heat-transfer agent is 
admitted. The walls of the chamber accommodate nozzles for feeding moist 
thermosensitive materials to the chamber. 
These methods and apparatus are disadvantageous, since they fail to yield 
an end product of high quality with low moisture content through the mass 
of the material. 
The method is impossible to carry out continuously for drying highly 
adhesive materials, and for this reason it is characterized by low 
efficiency. 
SUMMARY OF THE INVENTION 
The present invention aims to provide a method of drying thermosensitive 
materials by preselecting optimum thermal and gas-dynamic conditions, as 
well as to provide an apparatus for carrying out the method, through 
arranging inside the housing additional elements ensuring high quality of 
the dry thermosensitive materials of various adhesive capacities and high 
efficiency of the drying process. 
The aims of the invention are attained in the proposed method of drying 
thermosensitive food products by spraying such products onto inert bodies 
circulating about a closed space in a flow or stream of heat-transfer 
agent at increased temperatures. According to the invention, in the upper 
part of the flow occupying not more than 1/3 of the horizontal 
cross-section of the flow, the heat-transfer agent is rotated at an axial 
velocity 2-3 times higher than the velocity of the heat-transfer agent in 
the rest of the flow. 
Carrying out the drying process under such conditions makes it possible to 
provide within a short period of time highly uniform drying of the 
thermosensitive materials of various adhesive capacity, while preventing 
their tendency to burn and decompose. In consequence, the method ensures 
high quality of the end product, while considerably intensifying the 
drying process. 
In order to speed up the process of drying and retain the quality of the 
dry material, it is advisable to carry out the process in a flow of 
heat-transfer agent fed at a rate of 21-27 m/s and a temperature of 
220.degree.-260.degree. C. with the temperature gradient being maintained 
lengthwise of the flow at 120-150 K/m; or, alternatively, at a rate of 
feeding the heat-transfer agent of 14-19 m/s and a temperature of 
170.degree.-190.degree. C. with the temperature gradient being maintained 
lengthwise of the flow at 90-120 K/m. 
For attaining the maximum efficiency of the drying process it is desirable 
that the time of circulation of the inert bodies amounting to a volumetric 
fraction 0.2-0.25 be within a range of 2 to 3 seconds. 
The aims of the invention are further attained by an apparatus for drying 
thermosensitive food products comprising a cylindrical chamber with an 
outlet pipe, a tapered bottom and nozzles for feeding a thermosensitive 
material, the bottom of the chamber having arranged axially with the 
chamber a blind tube with a base defining with the walls of the bottom of 
the chamber an annular passage for introducing a heat-transfer agent 
thereto. According to the invention, the cylindrical chamber has at the 
side of the outlet pipe a mesh and a propeller mixer disposed under the 
mesh and secured on a shaft arranged at an angle of 60.degree.-90.degree. 
to the axis of the chamber and at a distance from this axis which is less 
than R, where R is the radius of the chamber. 
The apparatus allows considerable invigoration or the drying process due to 
the provision of zones, where the inert bodies move at a higher speed. 
Such zones ensure favorable conditions of spalling the dry material of 
high adhesive capacity. 
Preferably, the shaft of the propeller mixer is spaced from the axis of the 
chamber at a distance of 0.6-0.7 R, whereas the distance from the shaft of 
the mixer to the mesh is within 0.65-0.75 R, the diameter of the blades of 
the propeller mixer being 0.4-0.5 R. 
For a more uniform spraying of the thermosensitive material onto the inert 
bodies, the nozzles for spraying this material are arranged about the 
perimeter of the tapered bottom of the chamber at the upper portion of the 
bottom. 
Alternatively, several additional propeller mixers on shafts can be 
arranged under the mesh about the perimeter of the cylindrical chamber, 
each such shaft can also carry two mixers. Such an arrangement ensures 
uniform admission of the inert bodies to the zones of increased velocities 
of the heat-transfer agent and provides favorable conditions for spalling 
the dry material without affecting the quality of the material. 
Described hereinbelow are various preferred examples for carrying out the 
method of drying thermosensitive materials according to the invention. 
EXAMPLE 1 
A heat-transfer agent, such as clean air, heated to a temperature of 
260.degree. C. and fed to the heating zone at a rate of 27 m/s ensures 
that inert bodies present therein and having a volumetric fraction of 0.20 
recirculate about a closed space for 2.3 seconds. The inert bodies have 
the form of fluoroplastic cubes with edge length of 3 mm. The 
heat-transfer agent in the upper portion of the flow occupying 1/3 of its 
horizontal cross-section is rotated at an axial velocity of 54 m/s. The 
temperature gradient lengthwise of the flow is maintained at 150 K/m. A 
moist material having high adhesive properties and dry matter content of 
29 mass per cent heated to a temperature of 50.degree. C. is sprayed onto 
the inert bodies circulated in the flow of heat-transfer agent. The inert 
bodies entering the zone of rotational movement of the heat-transfer agent 
having a speed exceeding the free-fall velocity are caused to vigorously 
interact. This gives rise to spalling of the dry material and for the 
material carried by the heat-transfer agent to be evacuated from the 
drying zone. 
Due to the action of centrifugal forces, the inert bodies return to the 
flow of the heat-transfer agent to continue circulation about the closed 
space. 
The end product is then separated from the heat-transfer agent. The content 
of dry matter in the end product is 96 mass per cent. The amount of 
moisture evacuated from the material in the course of drying amounts to 61 
kg/h. Solubility of the end product is 0.6 ml of wet sediment. 
EXAMPLE 2 
A heat-transfer agent, viz., clean air, heated to a temperature of 
190.degree. C. is fed to the drying zone at a rate of 19 m/s to ensures 
that inert bodies occupying 0.25 of the volume of the drying zone 
circulate about the closed volume for 2.5 seconds. The inert bodies are 
generally fluoroplastic cubes with 5 mm edge size. The heat-transfer agent 
in the upper portion of its flow occupying 1/3 of its horizontal cross 
section is rotated at an axial velocity of 57 m/s. The temperature 
gradient lengthwise of the flow is maintained at 120 K/m. The Feblu 
concentrate having the initial dry matter content of 18.5 mass per cent 
and having high adhesive capacity is heated to 70.degree. C. to be sprayed 
onto the inert bodies circulating in the flow of heat-transfer agent. 
While entering the zone of rotational movement of the heat-transfer agent 
exceeding in flow speed the free-fall velocity of the inert bodies, the 
inert bodies tend to vigorously interact. This results in spalling of the 
dry product, and it is carried from the drying zone by the heat-transfer 
agent. The inert bodies return to the flow of heat-transfer agent to 
recommence circulation due to the action of centrifugal forces. The end 
product is separated from the heat-transfer agent. It contains 95.4 mass 
per cent dry matter. The amount of moisture removed from the material 
during drying is 96.4 kg/h. The solubility of the end product is 0.65 of 
wet sediment. 
EXAMPLE 3 
A heat-transfer agent, viz., clean air heated to a temperature of 
220.degree. C. and fed to the drying zone at a rate of 21 m/s ensures that 
the inert bodies of 0.2 volumetric fraction present therein are 
recirculated about a closed space for 2 seconds. The inert bodies are 
fluoroplastic cubes of 4 mm edge length. The heat-transfer agent in the 
upper portion of its flow occupying 1/4 of its horizontal cross section is 
rotated at an axial velocity of 42 m/s. The temperature gradient 
lengthwise of the flow is maintained at 120 K/m. Whole milk substitute 
heated to a temperature of 70.degree. C. is sprayed onto the inert bodies 
circulating in the flow of heat-transfer agent. The inert bodies entering 
the zone of rotational movement of the heat-transfer agent having a speed 
exceeding the free-fall velocity of the inert bodies are caused to 
vigorously interact. Spalling of the dry product takes place, and the dry 
product is evacuated from the drying zone by the heat-transfer agent. By 
virtue of the centrifugal forces the inert bodies are returned again to 
the flow of heat-transfer agent to continue circulation about the closed 
space. The end product is separated from the heat-transfer agent, and its 
quality is tested for solubility to preferably be 0.7 ml wet sediment. The 
end product contains 98 mass per cent dry matter. The amount of moisture 
removed from the material after drying is 145 kg/h. 
EXAMPLE 4 
A heat-transfer agent, viz., clean air heated to a temperature 170.degree. 
C. and fed to the drying zone at a rate of 14 m/s provides that the inert 
bodies present therein and having a volumetric fraction of 0.25 circulate 
about a closed space for 3 seconds. The inert bodies are generally 
fluoroplastic cubes measuring 3 mm in edge length. The heat-transfer agent 
in the upper portion occupying 1/3 of its horizontal cross section is 
rotated at an axial velocity of 42 m/s. The temperature gradient is 
maintained lengthwise of the flow at 90 K/m. A substitute of whole milk 
heated to a temperature 70.degree. C. is sprayed onto the inert bodies 
circulating in the flow of heat-transfer agent. The inert bodies entering 
the zone of rotational movement of the heat-transfer agent having a 
velocity exceeding the free-fall velocity thereof are caused to vigorously 
interact. This causes spalling of the dry product and the product is 
carried out of the drying zone by the flow of heat-transfer agent. The end 
product is separated from the heat-transfer agent to be checked for 
quality in terms of solubility amounting to 0.72 ml of wet sediment. The 
end product contains 97 mass per cent dry matter. The amount of moisture 
removed from the material after drying amounts to 120 kg/h. 
Thanks to the action of centrifugal forces on the inert bodies, the latter 
are returned to the flow of heat-transfer agent to continue circulation in 
the closed space.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The proposed apparatus for drying thermosensitive materials with reference 
to FIG. 1 comprises a cylindrical chamber 1 with a tapered bottom 2 
accommodating axially of the chamber a blind tube 3 having a base 4 
defining with the walls of the bottom 2 an annular passage 5 for 
tangentially introducing a heat-transfer agent. 
Arranged about the perimeter of the tapered bottom 2 in its upper portion 
are nozzles 6 for spraying the thermosensitive material onto inert bodies. 
Such an arrangement of the nozzles 6 ensures feeding of the material to 
the zone of the maximum temperature of the heat-transfer agent. 
The moist material is heated and dried without its properties being 
affected. The thermosensitive material starts drying at a temperature 
insignificantly exceeding the temperature of adiabatic evaporation of 
clean liquid. Fast rate drying proceeds under soft conditions. The process 
of drying is considerably intensified with the aforedescribed arrangement 
of the nozzles 6. 
At the side opposite to the tapered bottom 2 the cylindrical chamber 1 is 
provided with an outlet pipe 7 for evacuating the dry material by the flow 
or stream of the heat transfer agent. 
The chamber 1 has at the side of the outlet pipe 7 a mesh 8 overlying a 
propeller mixer 9 secured on a shaft 10 arranged at an angle of 60.degree. 
to 90.degree. to the longitudinal axis of the chamber 1 and at a distance 
from this axis which is less than R, where R is the radius of the chamber 
1. In order to maintain the most favorable conditions for the drying 
process to proceed, it is preferable that the shaft 10 of the propeller 
mixer 9 be spaced from the longitudinal axis of the chamber 1 at a 
distance 0.6 to 0.7 R, the distance from the shaft 10 to the mesh 8 be 
within a range of 0.65 to 0.75 R, whereas the diameter of the propeller 
mixer 9 be 0.4 0.5 R. Alternatively, several additional propeller mixers 
11 can be arranged on shafts 12 (FIG. 2), preferably a pair of such mixers 
11, 13 per each shaft 12. The diameter of the blades of all these mixers 
11, 13 is advisably less than the diameter of one mixer 9, whereas the 
area of accelerated speed of the flow of the heat-transfer agent in the 
upper portion of the cylinder is not to be in excess of 1/3 of the 
horizontal cross section of the flow. 
The shafts 10, 12 are connected to a motor 14 secured to the wall of the 
chamber 1 by means of brackets 15. The chamber 1 has an inlet pipe 16 for 
charging inert bodies thereto, and a cover cap 17. The apparatus operates 
in the following manner. Inert bodies in the form of fluoroplastic cubes 
having edges 2-5 mm in length are charged through the intake pipe 16 to 
the interior of the chamber 1. A heat-transfer agent, viz., clean air 
heated to a temperature 170.degree.-260.degree. C., is fed along the 
annular passage 5 at a rate of 14 to 27 m/s. 
The flow of heat-transfer agent formed by the blind tube 3 and walls of the 
tapered bottom 2 and chamber 1 acts to elevate the inert bodies, which 
initiate circulation about the closed space. By adjusting the feed rate of 
the heat-transfer agent circulation of the inert bodies is maintained 
within 2 to 3 sec. Desirably, the temperature of the heat-transfer agent 
is invariable at the outlet of the pipe 7, so that the temperature 
gradient lengthwise of the flow would be 90-150 K/m. 
The mixer 9 secured on the shaft 10 is rotated by the motor 14. This mixer 
produces, at the top of the flow of heat-transfer agent, occupying not 
more than 1/3 of its horizontal cross section, an area of increased speed 
of the heat-transfer agent exceeding the free-fall velocity of the inert 
bodies. The axial velocity of the heat-transfer agent in this area exceeds 
by 2 to 3 times its speed in the rest of the flow. As the movement of the 
inert bodies is stabilized, the nozzles 6 are engaged to spray onto the 
inert bodies a thermosensitive food product preliminarily heated to a 
temperature not exceeding its decomposition temperature. The 
thermosensitive material is carried on the inert bodies by the flow of 
heat-transfer agent to the zone of the propeller mixer 9. While passing 
the blades of the mixer 9, the inert bodies tend to collide therewith to 
result in spalling of part of the dry material. 
In addition, having attained an accelerated velocity and curvilinear path 
of travel in this area, the inert bodies initiate vigorous mutual 
interaction and engagement with the mesh 8 to result in further spalling 
of the dry material. Under the action of centrifugal forces the inert 
bodies are thrown to the periphery of the chamber 1 at a high velocity to 
travel downwards and then again be elevated by the flow of heat-transfer 
agent. 
The spalled particles of the dry product are evacuated by the flow of 
heat-transfer agent from the apparatus. Thereafter, the dry product is 
separated from the heat-transfer agent the heat-transfer agent being 
recyclable back to the apparatus. 
In the absence of rotation of the flow of heat-transfer agent the inert 
bodies would have been left pressed to the wall, since the velocity of the 
flow is higher than the free-fall velocity of these inert bodies. 
The propeller mixer 9 is so disposed as to reduce the residence time of the 
thermosensitive material in the drying zone upon attaining thereby a 
desired moisture content. 
This prevents burned taste and denaturation of thermosensitive food 
products, and, in other words, improves the quality of the dry product. 
This is especially important when drying highly adhesive materials. The 
blades of the mixer 9 and mesh 8 ensure favorable conditions for such a 
material to spall and be evacuated from the drying zone. 
The provision of several additional propeller mixers 11 mounted on shafts 
12 about the perimeter of the chamber 1 ensures a more uniform passage of 
the inert bodies through the zone where the velocity of the flow of 
heat-transfer agent is high. 
In the case of mounting a pair of mixers 11, 13 on one shaft 12 a pulse 
resulting from rotation of the mixers is continuously transmitted to the 
flow of heat-transfer agent lengthwise of the shaft to compensate for 
dissipation of the energy of the flow. 
This facilitates spalling of the dry product and invigorates the process of 
drying. 
The Table hereinbelow represents conditions of arrangement of the mixer 9 
on the shaft 10 in the chamber 1, and solubility of a milk substitute 
dried according to the proposed method. 
TABLE 
__________________________________________________________________________ 
Angle of Distance Solubi- 
mixer from the lity of 
shaft to 
Distance from 
mesh to 
Diameter 
dried milk 
the axis 
chamber axis 
the of the 
substitu- 
of the 
to the shaft 
mixer mixer 
te in ml 
chamber, 
of the mixer, 
shaft, 
blades, 
of wet 
Test No. 
degrees 
m m m sediment 
1 2 3 4 5 6 
__________________________________________________________________________ 
1 0 0.50 0.50 0.20 2.20 
2 30 0.50 0.60 0.30 1.60 
3 60 0.60 0.65 0.40 0.80 
4 80 0.70 0.70 0.45 0.78 
5 90 0.75 0.75 0.50 0.78 
6 90 0.80 0.80 0.60 1.68 
7 100 0.80 0.80 0.70 1.80 
__________________________________________________________________________ 
It follows from the above table that at the angle of less than 60.degree. 
the quality of an product deteriorates sharply. 
This is because the flow of heat-transfer agent downstream of the mixer 9 
is redestributed so that its velocity equalizes cross-wise, and the force 
of impact of the inert bodies against the mesh 8 is insufficient for 
spalling of the dry product. Therefore, the product remains to dwell in 
the drying zone, whereby its quality is affected. 
An increase in the angle to over 90.degree. results in that the blades of 
the mixer 9 act to direct the flow downwards thus disturbing the stability 
of spraying of the thermosensitive material onto the inert bodies. 
The arrangement of the shaft 10 of the mixer 9 at a distance of 0.6-0.7 R 
from the axis of the chamber 1, and 0.65-0.75 R to the mesh 8 at a 
diameter of the blades of the mixer 0.4-0.5 R determines the travel path 
of the inert bodies in the part of the volume of the upper zone of the 
flow of heat-transfer agent. 
In this travel path the product is chipped off the inert bodies because 
these bodies collide with each other and with the wall of the chamber 1. 
A reduction in the distance from the mixer to the wall of the chamber 1 or 
to the mesh 8 makes this travel path shorter to result in incomplete 
spalling of the dry product and the tendency of the product to overheat. 
Conversely, an increase in this distance causes redestribution of the 
flow, whereby the inert bodies are delivered to the mesh 8 at velocities 
insufficient for providing an impact power thereagainst for spalling the 
remainder of the dry product, which also affects the quality of the dry 
product. 
This invention can find application in the food industry, such as for 
making whey and blends, in microbiology for making fermentation 
preparations, in the chemical industry, medical practice, and elsewhere.