Process for sterilizing loose material

Two-stage process for sterilizing loose material, such as flour, cocoa, feedstuffs, fillers for cosmetics and pharmaceuticals, and mould cultures, by means of sterilizing agents, such as superheated steam. In the first stage the loose material is vigorously mixed with the sterilizing agent by means of a mechanical agitator, while being brought to the sterilization temperature. In the second stage the loose material is allowed to remain virtually stationary for a predetermined length of time at the sterilization temperature.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process for sterilizing loose material by means 
of sterilizing agents, such as superheated steam, which are positively 
mixed with the continuously agitated material with exclusion of air to 
heat the material to the sterilization temperature. The invention also 
relates to an apparatus for carrying out such a process, comprising a 
sterilization container in which an agitator is rotatably mounted to 
agitate the material to be sterilized and which is provided with means for 
discharging and introducing sterilizing agents. 
2. Description of the Prior Art 
It has become increasingly customary to sterilize loose materials such as 
flour, cocoa, feedstuffs, fillers for cosmetics and pharmaceuticals and 
mould cultures, so that completely new fields of application have opened 
up for sterilisation. In particular, sterilisation must be adapted to high 
throughput rates for such bulk goods. 
Various continuously operating apparatus for sterilizing loose material are 
already known which contain stationary or movable baffles and inserts to 
ensure that the product which is being treated cannot leave the apparatus 
until it has been in it for the minimum length of time required for 
sterilisation. In spite of elaborate constructional measures, however, 
so-called short circuits cannot be prevented in practice, that is to say 
portions of the products being treated are liable to pass too rapidly 
through the apparatus so that they do not stay in it for the minimum time 
prescribed for sterilisation. If portions of the product are left 
unsterilized, the whole quantity of the product is, of course, unsterile 
since the non-sterilized portions contaminate the remainder. It is also 
difficult to empty such apparatus completely and clean them. 
For batchwise sterilisation of loose material using steam or gas as the 
sterilizing agent, it is known to loosen up the material vigorously and 
agitate it so as to fluidize it and at the same time add the sterilizing 
agent and thereafter discharge the sterilizing agent from the sterilized 
material at subatmospheric pressure while the material continues to be 
vigorously agitated and loosened up (German Offenlegungsschrift 
1,642,087). To ensure complete sterilisation, the material must be kept at 
the sterilisation temperature for a given time which varies according to 
the temperature. The higher the sterilisation temperature employed, the 
shorter is the necessary sterilisation time. Since, however, there is a 
limit to the sterilisation temperature which can be employed, the material 
must be kept in the sterilisation zone for a comparatively long period and 
at the same time thoroughly loosened up and agitated. Due to its 
comparatively long residence time in the structurally complicated dynamic 
sterilizer, the material is then liable to suffer mechanical damage. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to reduce the effort required for 
sterilisation of loose materials and particularly the mechanical effort 
and hence to reduce the mechanical stress to which the loose material is 
subjected. 
It has been found that in order to achieve economical and yet reliable 
sterilisation of loose material, it is essential that the sterilizing 
agent should briefly come into contact with every particle of the loose 
material so that the material is completely and as rapidly as possible 
heated to a uniform sterilisation temperature. All that is thereafter 
required is to keep the material at the sterilisation temperature for the 
necessary sterilisation time. 
The solve the given problem in the light of this finding, the invention 
proposes that in a process of the type described above, the material to be 
sterilized is kept practically stationary in a post-sterilisation zone at 
the sterilisation temperature for a predetermined length of time after it 
has been heated to the sterilisation temperature and, if desired, after 
removing the sterilizing agent from the material. 
Sterilisation is therefore virtually carried out as a two stage process in 
which the material to be sterilized is rapidly and uniformly heated to the 
desired sterilisation temperature in the first stage and kept at the 
sterilisation temperature for the necessary length of time in the second 
stage. The material to be sterilized is vigorously mixed with sterilizing 
agent with the aid of a mechanical agitator to bring it to the 
sterilisation temperature, but in the post-sterilisation zone it is 
sufficient to ensure that the temperature of the material does not drop 
below the sterilisation temperature. No mechanical devices such as 
stirrers or mixers are therefore required in the post-sterilisation zone, 
and the energy consumption and wear and tear are therefore reduced 
accordingly and there is no further risk of mechanical damage to the 
material. 
In the post-sterilisation zone, the temperature is maintained mainly by 
preventing loss of heat by radiation. In addition, external heating may 
also be applied. The temperature of the material will then not drop below 
the desired temperature even in the interior of the mass of material. 
The sterilized material is preferably removed continuously from the 
post-sterilisation zone, regardless of whether the treatment with 
sterilizing agent in the main sterilisation zone is carried out 
continuously or batchwise. The residence time of sterilized material in 
the post-sterilisation zone can in this way be controlled as desired. 
It is particularly desirable to mix the material batchwise with sterilizing 
agent to heat it to the sterilisation temperature, that is to say after 
the material has been mixed with sterilizing agent and thus heated to the 
sterilisation temperature, it is also introduced batchwise into the 
post-sterilisation zone but removed continuously from this zone. 
Furthermore, to solve the given problem in an apparatus of the type 
described above, it is proposed to provide a second chamber downstream of 
the chamber which contains the stirrer or mixer, in which second chamber 
the material which has been heated to the sterilisation temperature is 
kept under sterilisation conditions for a predetermined length of time 
before it is removed from the apparatus. 
The first chamber preferably consists of any mixing vessel in which a 
mixing device is installed whereas the second chamber is a silo-type of 
container which is attached to the discharge opening of the mixing vessel 
and itself has a discharge opening at the bottom. When the material has 
been heated to the required temperature by the sterilizing agent, it is 
discharged, for example from the mixing vessel into the second chamber 
which is provided for post-sterilisation. It is generally introduced 
batchwise into the second chamber and it remains there for the necessary 
sterilisation time. The completely sterilized material is then discharged 
from the second chamber, preferably continuously. 
The capacity of the post-sterilizer provided for the post-sterilisation 
treatment is preferably equal to or greater than the capacity of the 
mixing vessel so that a number of batches of material from the mixing 
vessel can be kept in the post-sterilisation vessel for the necessary 
post-sterilisation time before the completely sterilized material is 
removed continuously. The capacity of the post-sterilizer need not 
necessarily be greater than the capacity of the mixing and sterilizing 
vessel preceding it, the necessary capacity depending rather on the extent 
to which the mixing vessel is filled. 
To ensure continuous and uniform removal of completely sterilized material 
without any mechanical conveyor devices, another feature of the invention 
provides that the post-sterilizer is a mass flow bin from which the 
sterilized material pours out of the opening at the bottom in such a 
manner that the various batches of sterilized material which are stacked 
in layers above one another in the bin leave it uniformly, that is to say 
layer by layer. This ensures that every portion of material remains for 
the same length of time in the post-sterilizer and no portions can be 
short circuited. 
To ensure that the material in the post-sterilisation zone will be kept at 
the sterilisation temperature, the post-sterilizer is thermally insulated, 
i.e. it is protected against loss of heat by radiation, as is also the 
mixing vessel preceding it. If no heat can radiate outwards, the mass of 
sterilized material will not cool down. However, since heat losses cannot 
always be completely prevented, another feature of the invention provides 
that both the post-sterilizer and the mixing vessel preceding it are 
adapted to be heated externally to compensate for the heat loss. If the 
layer of sterilized material adjacent to the wall of the post-sterilizer 
is kept at the sterilisation temperature, no heat will be lost from the 
interior of the mass of material by radiation and therefore once the 
material has been heated through completely to the sterilisation 
temperature, it can be kept at this temperature for a considerable period 
with minimum energy consumption. The sterilized material does not begin to 
cool down until it has left the apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A preferred embodiment of the apparatus comprises a cylindrical container 
1, with a horizontal axis in which a shaft 2 is mounted. The shaft 2 is 
driven by a motor 3 and has radial arms 4 attached to it along its length 
and over its circumference, with mixing devices 5 attached to the ends of 
the arms. 
A feed hopper connection 6 through which the material to be sterilized is 
introduced into the container 1 is provided at the top of the container 
and may be closed by a cover 7. A sterilizing agent such as steam or a gas 
can be introduced into the container through a pipe connection 8 and 
discharged from the container 1 through a pipe connection 9. 
Sterilisation of the loose material is carried out in container 1. To 
achieve this, the loose material is vigorously agitated and loosened up by 
means of the rotating mixing device and thereby mixed with the sterilizing 
agent introduced into container 1. Every particle of loose material in 
container 1 is thereby heated to the sterilisation temperature within a 
very short time. The sterilizing agent may be withdrawn through the pipe 
connection 9 and the heated material may be discharged from the container 
1. 
To empty the container 1, an aperture 10 covered with a closure flap (not 
shown) as situated at the bottom of the container 1 and opens into a 
downpipe 11 which in the illustrated embodiment opens into a mass flow bin 
12 which is situated underneath the container 1 and serves a 
post-sterilizer. In this mass flow container, which has no mechanical 
inserts, the material which has been heated to the sterilisation 
temperature remains virtually stationary in the form of layers 13 stacked 
above one another for the remainder of the required sterilisation time, 
the post-sterilizer being kept under sterilisation conditions. After 
expiry of the required post-sterilisation time, the sterilized material is 
continuously removed through an outlet 14 at the lower end of the mass 
flow bin 12. The sterilized material also leaves the post-sterilizer 
uniformly, that is to say the various layers 13, which may, for example, 
correspond to the individual batches introduced into the mass flow bin 12 
from the container 1 are emptied evenly one after another. The discharge 
aperture 14 may be provided with a closure member (not shown). 
In the illustrated embodiment, the container 1 in which sterilisation 
begins and the mass flow bin 12 which serves as post sterilizer are 
combined to form one structural unit and covered with an insulating jacket 
15 which is spaced apart from the external surface of the container 1, 
downpipe 11 and mass flow bin 12 so as to leave a cavity 16 between the 
external wall of the unit and the insulating jacket. The jacket therefore 
serves as a double jacket for a heating medium. An inlet pipe connection 
17 opens into the cavity 16 for the introduction of a heating medium such 
as steam, and an outlet connection 18 is provided to remove the cooled 
heating medium as condensate. Baffle plates (not shown) may be arranged 
within the cavity 16 to guide the flow of heating medium. If desired, the 
heating medium may flow through the cavity 16 in the reverse direction, 
i.e. from 18 to 17. 
In the illustrated embodiment, the container 1 which forms the sterilizer 
and the mass flow bin 12 which forms the post-sterilizer are combined to 
form a unit which may be provided with legs or some other framework to 
support it on a base, and in this form it may be transported to the 
required site as one structural unit. Alternatively, the sterilizer and 
post-sterilizer may be set up separately from each other, in which case 
the material may be transferred from the container 1 to the mass flow bin 
12 for example through a heat insulated, encapsulated conveyor device such 
as a conveyor screw. 
As the sterilized material is continuously removed through the discharge 
aperture 14, the layers 13 in the mass flow bin 12 gradually sink so that 
each layer reaches the emptying zone after expiry of the necessary 
sterilisation time and can be removed. This downward movement is the only 
movement carried out by the material or product in the post-sterilizer so 
that the material is not subjected to any mechanical damage during the 
period of post-sterilisation. Reliable sterilisation and controlled 
destruction of bacteria in loose materials of all kinds can thus be 
achieved with the simplest apparatus and under the most careful treatment 
of the loose material. 
Although in the example described above the loose material is sterilized 
batchwise in the first sterilisation stage and continuously removed from 
the post-sterilisation zone, the heating of the material to the 
sterilisation temperature in the first treatment stage may also be carried 
out continuously.