Process and apparatus for drying a granular free flowing bulk material

Process and device for convectively drying grannular bulk material in a silo. A hot gas jet is vertically blown upward through a jet nozzle of jet aperture (orifice), a tube having a wall closed over is entire length is inserted into the bulk material coaxially with the axis of the vertical gas jet, the tube being lowered up to a particular distance above the bottom of the silo, in such a way that continuously an amount of the descending bulk material is entrained by the recycling gas jet and is blown into the vertically introduced upwardly directed jet of hot drying gas and the tube is of such a length that it projects with its top above the bulk material.

A process for conveniently drying and optionally roasting and 
peeling/grinding a granular free-flowing bulk material and a device for 
carrying out the process. 
The invention relates to a process for convectively drying a granular 
free-flowing bulk material, according to which the bulk material is passed 
into a silo and a vertically introduced, upwardly directed jet of hot 
drying gas is introduced into the silo via a jet nozzle or jet aperture 
(orifice), in such a way that granules are continously entrained by part 
of said gas jet along a straight upward path to above the bulk material 
and fall down as a fountain onto the surrounding bulk material which 
slowly descends in the silo, whereas another part of the gas jet is 
laterally deflected and flows downward as a recycle stream. 
A similar process is known from U.S. Pat. No. 2,786,280. Therein the gas 
jet is freely blown into the bulk material present in the silo via a jet 
nozzle or jet aperture (orifice) disposed in the bottom of the silo. A 
portion of the gas jet flows upward along a straight path, entraining the 
bulk material granules being at its path and causes a cylindrical channel 
bounded by the surrounding slowly descending bulk material; another 
portion of the gas jet, as applicants' investigation taught, deflects 
laterally and then flows downward (recycle stream); the remainder of the 
introduced gas jet (in practice often half to three quarters of it) 
deflects laterally on its way up and flows away upward through the 
surrounding bulk material. 
This process gives a high drying efficiency, especially thanks to the 
laterally deflected and upwardly flowing gas. 
When this process is applied to bulk material where the resistance 
controlling the moisture transport is in the granules, for instance to 
granular agricultural products, it only gives a constant drying rate at 
the initial stage with as a result that with bulk material of this kind 
the granules can easily damage, for instance get cracked, in consequence 
of which the useful value of bulk material dried with this known process 
is strongly reduced. 
Moreover, the system allows of little variation. The filling level of the 
silo for instance cannot be altered and alteration of the output of the 
air jet is hardly possible as well. 
It has now been found that by controlling the amount of laterally deflected 
and upwardly flowing drying gas a constant drying rate can be achieved. 
This can be explained in that the granules which are entrained by the 
upward flowing portion of the gas jet along a straight path give up the 
moisture present in the outer layer to the driving gas, whilst--as the 
moisture transport controlling resistance is within the granules--the 
moisture distribution in the core of the granules hardly changes and that 
thereupon the granules when they have fallen to the surrounding bulk 
material and slowly descend steadily with this bulk material, get 
sufficient opportunity for levelling out the unequal distribution of 
moisture which has been obtained in the gas jet, and those granules are 
then reintroduced into the vertically introduced gas jet by means of the 
recycle stream and are entrained by the portion of the gas jet flowing 
upward along a straight path, whereupon the whole cycle is repeated. By 
controlling the amount of laterally deflected and upwardly flowing gas and 
the recycle stream, same can be attuned to the nature of the bulk material 
and the drying result aimed at. 
The process according to the invention is characterized, in that during the 
drying of bulk material, where the moisture transport controlling 
resistance is within the granules, a tube having a wall closed along its 
entire length is inserted into the bulk material coaxially with the axis 
of the vertical gas jet, the tube being lowered up to a predetermined 
distance above the bottom of the silo, in such a way that continuously an 
amount of the descending bulk material is entrained by the recycling gas 
jet and is blown into the vertically introduced upwardly directed jet of 
hot drying gas, and the tube is of such a length that it projects with its 
upmost end above the bulk material. 
With this embodiment i.a. the following advantages are obtained. 
Thanks to the tube inserted into the bulk material phenomena occurring in 
the silo can accurately be controlled. Much greater variations in the 
output of the drying gas can be allowed than is the case with the process 
according to U.S. Pat. No. 2,786,280 and even halve the output of the gas 
compared to the output required in the process according to U.S. Pat. No. 
2,786,280, whereas the pressure at which the gas is blown-in may be 
smaller and may amount to about 2/3 of the pressure in the known process. 
Furthermore, maximum limits are no longer set to the filling level of the 
silo; in the process according U.S. Pat. No. 2,786,280 a maximum applies 
for the filling level, because when the filling level becomes too high, no 
continuous gas flow occurs anymore and all the gas is spread through the 
bulk material. 
The diameter of the tube inserted into the bulk material in accordance with 
the invention has to be such that granules of the bulk material which are 
entrained by the gas jet are shot into the tube as much as possible and 
are then entrained at high speed to above the bulk material present in the 
silo. In practice this means that the inner diameter of the tube must at 
least be equal to the diameter of the jet nozzle or jet aperture (orifice) 
through which the gas flow is introduced into the silo. On the other hand 
the inner diameter of the tube preferably should not be larger than twice 
the average diameter of the straight upward path along which without 
application of a tube granules of bulk material are entrained by the gas 
jet to above the bulk material. 
Preferably, the inner diameter of the tube inserted into the bulk material 
is nearly equal to the average diameter of the straight upward path along 
which without application of a tube granules of bulk material are 
entrained by the gas jet to above the bulk material. In practice this 
means that the inner diameter of the tube preferably amounts to 0.8-1.2 
times the average diameter of this straight upward path. With such an 
inner diameter of the tube inserted into the bulk material, the least 
disturbances in the flow occur and the most favourable control of the 
drying process is attained. 
The diameter of the straight upward path along which without application of 
a tube inserted into the bulk material granules of bulk material are 
entrained by the gas jet to above the bulk material depends on the nature 
of the bulk material which is dried in the silo, on the diameter of the 
silo and on the diameter of the nozzle or orifice through which the gas 
jet is introduced into the silo. 
In practice this diameter is established experimentally, for instance by 
filling the silo to be used according to the invention with the bulk 
material to be dried in this silo up to the maximal level at which with 
the available drying gas output--without tube inserted into the bulk 
material--still an upward gas jet along a straight path is realized in 
which granules of the bulk material are entrained to above the level of 
the surrounding bulk material, by blowing the maximum available jet of 
drying gas into the silo via the nozzle or orifice in the bottom of the 
silo and measuring the kinetic pressure over a cross-section. 
For an accurate determination of the diameter of the straight upward path 
along which, without application of a tube inserted into the bulk 
material, granules of bulk material are entrained by the gas jet to above 
the bulk material, such a test can suitably be carried out in a silo which 
is halved by means of a transparent sheet running through the axis of the 
silo, blowing half the amount of gas in at the same filling level with 
bulk material. The cross-section of the straight upward path is then 
visible through the transparent sheet and can easily be measured. 
The distance between the lower end of the tube inserted into the bulk 
material and the bottom of the silo which is kept in the process according 
to the invention, determines to a high degree the flow pattern which 
occurs in the process according to the invention in the silo and therewith 
the drying result aimed at.

FIG. 1 shows the flow pattern occurring in practice in the silo in the 
process according to U.S. Pat. No. 2,786,280. The flow of the gas is 
indicated by half arrows; the movement of the granular bulk material is 
indicated by complete arrows. 
This flow pattern can be explained as follows. 
The gas entering the silo at O via a nozzle or orifice with a particular 
momentum looses part of its momentum on its way through the silo; the rate 
decreases as a function of the distance from O. This decrease of the rate 
is attended by a gradual increase of the static pressure along the axis of 
the silo (the gas jet) which increase of the static pressure is 
restricted, however, by dissipation losses and according as the 
dissipation losses increase even turns into a decrease of the static 
pressure. Consequently, at a particular point (A) along the axis of the 
gas jet a pressure peak occurs. 
The static pressure which occurs along the axis of the gas jet causes part 
of the gas from the gas jet to deflect laterally. Below point (A) this 
laterally deflected gas flows back to O where it mixes with gas freshly 
introduced at O. This recycling gas entrains granules from the surrounding 
bulk mass and takes them into the gas jet. Above the point (A) the 
laterally deflected gas flows upward into the surrounding bulk material 
mass. Because granules are continuously entrained by the recycling gas to 
the gas jet at the lower end of the bulk mass, the bulk mass slowly 
descends. At the top it is replenished by bulk granules which have been 
entrained by the gas jet to above the bulk material and fall there as a 
fountain onto the surrounding bulk material. 
By inserting a tube into the bulk material coaxially with the axis of the 
vertical gas stream, a part of the flow of gas through the surrounding 
bulk material is avoided. 
For a tube having an inner diameter which is substantially equal to the 
average diameter of the straight upward path along which--without 
application of a tube--granules of bulk material are entrained by the gas 
jet, the following cases may occur. 
When the lower end of the tube is at less than half of the distance OA from 
the bottom of the silo, the gas jet blown-in through the nozzle or orifice 
sucks additional gas through the bulk material present around the tube, 
said gas entraining granules; the amount of sucked gas and therewith the 
amount of entrained granules is determined by the distance of the lower 
end of the tube to the bottom of the silo and by the output of the gas jet 
blown-in through the nozzle or orifice. 
Such a position of the tube is generally not wanted in practice. 
When the lower end of the tube is between half of the distance OA and the 
point A part of the blown-in gas jet is deflected laterally and sucked 
back to the gas inlet; besides additional gas is also sucked in through 
the bulk material present around the tube. This is a suitable position of 
the tube. 
When the lower end of the tube is at the level of point A only a recycle 
stream of laterally deflected gas occurs. 
The output of the gas jet through the tube is equal to the output of the 
gas introduced via the nozzle. Also this position of the tube is suitable. 
When the lower end of the tube is above point A, the incoming gas jet is 
divided into a jet going upward through the tube and a stream going upward 
through the bulk material present about the tube. Besides an unimpeded 
recycle stream occurs. The distribution between the gas jet through the 
tube and the gas stream upward through the surrounding bulk material and 
with it the concentration of the suspension of granules which goes upward 
with the gas jet is determined by the precise position of the tube. 
In general, when the lower end of the tube is above point A, the most 
favourable drying effect occurs. Such a position of the tube is therefore 
preferred. The precise position of the tube selected for drying a 
particular bulk material, also depends on by-effects which may be wanted. 
Because the granules move upward as a thin suspension in the gas jet, they 
also regularly collide, against each other and against the wall of the 
tube, effecting a grinding effect. The grinding effect is especially 
determined by the gas inlet velocity and by controlling this velocity the 
grinding effect can be controlled. This can be made use of for both drying 
the granules and removing the outer layer of the granules. 
This possibility is in particular important for the drying of granular 
agricultural crops, where often in addition to drying also grinding or 
peeling has to take place. 
A favourable circumstance is that in the process of the invention the 
ground portion is immediately separated from the remainder. This takes 
place by blowing away (with small particles) or segration (with large 
peelings), in which latter case the peelings as it were float above the 
surface of the surrounding bulk material. 
The silo which is applied in the process according to the invention may 
have any dimensions. In general the diameter has to be at least 15 cms, 
but the upper limit for the diameter is only determined by practical 
considerations. 
When the diameter of the silo, which in view of a particular drying 
capacity is desirable would become very large, a plurality of jets of 
drying gas can be applied in the silo, each gas jet giving the drying and 
grinding effect for part of the silo. 
A suitable selection of the diameter of the silo, in combination with the 
selection of the filling level makes it possible to adjust the total 
duration of a drying cycle. 
In the process of the invention air can be used as drying gas. In case the 
bulk material to be dried is sensitive to oxygen, instead of air an inert 
gas for instance nitrogen can be applied. 
The temperature at which the gas is heated depends on the properties of the 
bulk material that is dried and upon the result one wishes to achieve. 
The process of the invention can be applied with a wide variety of granular 
products which are used as bulk material. 
The most important among them are the granular agricultural products, 
varying from grains to for instance coffee beans and oleiferous seeds. In 
general one wishes to dry and optionally to peel under such conditions 
that no granule degradation occurs as a result of the granule temperature 
attained. 
This is reached when the temperature of the drying gas is not higher than 
200.degree. C., in particular 160.degree. C. 
Then the temperature of the granules does not become so high that 
degradation occurs. The most favourable temperature can simply be 
established experimentally. 
With oleiferous seeds it often is favourable to apply a higher gas 
temperature for effecting both drying and roasting of the granules. 
Sesame-seed for instance is roasted at temperatures of about 
100.degree.-110.degree. C. and then a gas temperature is to be applied 
giving such a temperature of the granules. For this purpose the 
temperature of the gas should be above 160.degree. C. and preferably above 
200.degree. C. The most suitable temperature can also here simply be 
established experimentally. 
For coffee-beans it is favourable when they are simultaneously dried and 
roasted. The temperature for roasting coffee is usually at about 
300.degree. C. The most favourable gas temperature again can be 
established experimentally. 
The process according to the invention can be performed both batch-wise and 
continuously. In the first case the silo is filled up to the desired 
level, whereupon a gas jet is introduced and this goes on until the charge 
is dry, whereupon the silo is emptied. In the second case the silo is 
filled up, a gas jet is introduced and thereupon more bulk material is 
continuously introduced, whilst simultaneously a corresponding amount of 
bulk material is withdrawn from the silo at another spot. 
The invention also relates to a device for performing the process according 
to the invention, consisting of a silo which at its top is in 
communication with the open air, with in its bottom at least a supply 
nozzle or orifice for a gas jet, connected to a supply main for hot gas, 
which device is characterized in that in the silo at the level of each 
supply nozzle or orifice for a gas jet a tube having a wall closed over 
its entire length is disposed whose axis coincides with the vertical axis 
running through said supply nozzle or orifice, said tube ending at a 
certain distance above the bottom of the silo. 
The silo may for instance be open at its top, but is preferably provided at 
its top with a cover having a gas discharge channel which may suitably be 
provided with a dust collector, for instance a cyclone; in this manner 
problems with environmental pollution are avoided. 
The latter embodiment moreover offers the possibility for sucking the gas 
jet, which is introduced via the supply nozzle or orifice when using the 
device, through the silo by means of a suction pump connected to the gas 
discharge channel instead of blowing it into the silo by means of a 
compressor. In this way drying is effected at a certain sub-atmospheric 
pressure which in general is favourable. 
A baffle is suitably disposed above each tube disposed in the silo, 
preferably spaced above the tube in such a way that when the device is in 
operation the jet of gas and entrained granules coming from the tube are 
deflected by it. In this way a good distribution of the falling bulk 
material over the surrounding bulk material mass is obtained and moreover 
it is avoided that granules are discharged from the device with the gas 
jet. 
The device is preferably provided with means with which the position of the 
tube in the silo can be adjusted. This makes it possible to adapt the 
position of the tube to the bulk material which is treated, to the output 
of the gas jet and to the effect one wishes to achieve. 
The latter possibility offers important attendant advantages especially if 
the device is constructed in such a way that the gas jet is sucked through 
the silo. 
Then the silo can for instance be filled up by lowering the tube up to the 
bottom and sucking the bulk material from below through the tube into the 
silo. When the silo is filled up to the desired level in this way, 
thereupon the tube inserted in the silo can be slowly pulled up, whilst 
gas is continuously sucked through the silo, and then when the correct 
position of the tube in the silo is reached, the cycle patterns essential 
of drying occur of itself. 
In the following examples some experiments are described, which illustrate 
the process according to the invention and the effect achieved with it. 
EXAMPLE I 
Some drying tests were carried out in a silo in the shape of a cylindrical 
vessel (inner diameter 30 cms), having a conical bottom (inclination 
45.degree.), which at its apex was provided with an inlet (jet aperture) 1 
for drying air having a diameter of 27 mms, and in which a tube having a 
length of 1.5 m and an inner diameter of 53 mms was adjustably mounted, 
the axis of the tube coinciding with the axis of the silo. 
The inlet for drying air was connected to a supply main for air, in which a 
control valve, a flow meter and an electric heater were mounted. 
In all the tests 58.3 kgs of rice having a humidity based on bone-dry rice, 
according to NEN 3090 of 30% by weight were passed into the silo. 
Via the supply nozzle air of a temperature of 160.degree. C. was blown into 
the silo. 
The position of the tube in the silo and the air output were varied as 
follows: 
TABLE A 
______________________________________ 
distance lower end of tube to 
Test gas output (kg/s) 
bottom (cm) 
______________________________________ 
1 0.030 2 
2 0.034 8 
3 0.030 10 
4 0.034 12 
5 0.046 20 
______________________________________ 
The tests were continued until a humidity of about 16% based upon (0.16 kg 
H.sub.2 O per kg of dry rice, corresponding to 14% based upon wet rice, 
which is the maximum admissible humidity for storing rice, at which the 
grow of mould is avoided) was reached. 
The course of drying was followed by sampling rice from the silo from time 
to time and determining the humidity. 
The result of these tests is represented in FIG. 2. 
In all cases it is achieved thanks to the tube positioned in the silo that 
the drying rate is constant. In consequence practically no damaging of the 
rice (cracking of the rice) occurs during drying. 
EXAMPLE II 
In the same silo as applied in the tests of example I, a further drying 
test was carried out. 58.5 kgs of rice having a humidity of 30%, based 
upon the dry rice were passed into the silo. The tube inserted in the silo 
was adjusted in such a way that the distance of the lower end of the tube 
to the bottom amounted to 8 cms and in this test 0.046 kgs/s of air having 
a temperature of 160.degree. C. was blown into the silo via the supply 
nozzle. 
The test was continued until a rest humidity was reached of about 16% (dry 
basis). 
During the test rice was sampled from to time, both the humidity and the 
temperature being determined. 
The results of these measurements are represented in FIG. 3. 
Also now a constant drying rate was reached. The temperature of the rice 
grains increased, in spite of the gas temperature of 160.degree. C., only 
to 65.degree. C. at a humidity of 16%, a temperature at which not yet 
degradation of the grains occurs; a yield of unbroken dried grains was 
obtained of 59-63%.