Cooler for mixtures of crystals and crystallizing liquid in the production of sugar and related products

A cooler for mixtures of crystals and crystallizing liquid in the production of sugar and related products comprises a bundle of vertical pipes for the passage of such a mixture from a bottom chamber to a top chamber. The vertical pipes are surrounded by a cooling medium, and in the interior of the pipes plungers are provided which have a somewhat smaller diameter than the inner wall of the pipes so as to form a slot-like passage along the inner wall of each pipe. The plungers are moved up and down through the pipes. The cooler is especially adapted for promoting crystallization by a rapid drop of temperature.

BACKGROUND OF THE INVENTION 
The invention relates to a cooler for mixtures of crystals and 
crystallizing liquid in the production of sugar and related products, such 
products, which during crystallization show a similar behaviour as sugar, 
including particularly other saccharides, such as fructose and glucose. 
In the crystallization of easily soluble materials, the solubility of which 
increases with increasing temperature, crystallization is often effected 
by cooling a mixture of crystals and solution. This mixture will in the 
following be referred to as massecuite. 
Such mixtures of crystals and solution frequently have a high apparent 
viscosity (consistency). This is e.g. the case in the crystallization of 
saccharose, fructose and glucose. The cooling meets with difficulties 
because the high viscosity results in a poor transfer of heat and because 
frequently a layer of crystals grows up on the cooling surfaces, thereby 
additionally reducing the heat transmission. 
The cooling is frequently effected in containers with stirring mechanisms 
and with a movable or stationary cooling surface, which is cooled by 
water, air or other cooling media. The cooling is performed either 
batch-wise or by causing the massecuite to flow continuously through one 
or more containers provided with cooling equipment. It has been found in 
practice that the limiting factor in the crystallization process is the 
obtainment of the sufficient temperature drop, and it is often difficult 
and expensive to build in the sufficient cooling surface area. 
In order to obtain optimum crystallization with maximum yield it has been 
found by experiment that in the crystallization of the last product in the 
saccharose production it is useful to obtain a rapid cooling of the 
massecuite from the temperature of 70.degree.-80.degree. C. at which it is 
produced to 60.degree. C., whereafter a slower cooling in traditional 
equipment is advantageous. 
Coolers are known in the form of a pipe cooler having substantially 
vertical pipes through which a fluid is pumped and which opens into 
chambers at their bottom and top ends and are surrounded by a cooling 
medium, e.g. circulating water. However, these known coolers are not 
suitable for the cooling of mixtures of crystals and crystallizing liquids 
of the type here concerned, because a layer of of crystals is rapidly 
formed on the inner wall of the pipes, whereby the pipes can easily get 
clogged. 
SUMMARY OF THE INVENTION 
It is the object of the invention to remedy this drawback. According to the 
invention, plungers are provided in the pipes, which plungers in the whole 
or practically the whole pipe length fill out part of the cross-section of 
each pipe and therefore form a slot-like passage along the pipe wall, the 
plungers being coupled to means for moving them up and down through the 
pipes. 
By the use of such plungers an increased transmission of heat is obtained 
owing to the slot-like section of the flow passage through the pipes, and 
it has been found that the movement of the plungers up and down through 
the pipes keeps the cooling surfaces clean and further increases the heat 
transmission. 
In order to obtain maximum effect, the plungers should at any moment extend 
the whole way through the pipes and to achieve this the plungers should 
preferably be longer than the pipes, their ends moving up and down in the 
bottom and top chambers, whereby an efficient stirring of the massecuite 
in the bottom and top chambers is at the same time obtained. 
In a particularly advantageous embodiment of the invention, the cooler 
comprises two cooler sections connected in parallel, the bottom and top 
chambers of which are respectively interconnected, the plungers of one 
section moving up while the plungers of the other section are moving down. 
As a result of this arrangement the total effect of the plungers in the 
chambers has no displacement effect, whereby heavy pressure variations are 
avoided.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the embodiment illustrated, the cooler comprises two cooler sections 
connected in parallel. The massecuite to be cooled is supplied through an 
inlet pipe 14 and passes through a connecting chamber 20 into bottom 
chambers 21 of two parallel coolers 22. The coolers comprise a number of 
pipes 11 surrounded by a cooling medium and opening at their top ends into 
top chambers 24, which are interconnected through a connecting chamber 25, 
to which an outlet pipe 15 is connected. On the cooling medium side 
baffles 23 are provided which increase the speed of flow, and a 
circulation pump 16 is used for increasing the speed of flow of the 
cooling medium and thereby the coefficient of transmission. In each pipe 
11 there is provided a cylindrical plunger 17, e.g. consisting of a pipe 
into which sand may be filled for balancing. The diameter of the plunger 
is somewhat smaller than the diameter of the pipe wall so that an annular 
slot is formed. All plungers in each cooler are interconnected and are 
moved up and down by means of a hydraulic cylinder 8. 
The movement can be so controlled that the plungers of one of the coolers 
are moved up, while the plungers of the other cooler are being moved down. 
Movement is thus produced at the bottom of the coolers, whereby the 
deposition of crystals is avoided and the pressure at the inlet is kept 
substantially constant. 
The movement of the plungers of the two coolers may also take place 
simultaneously in the same direction, or the apparatus could be 
constructed with one cooler only. Thereby heavy pressure variations are 
produced. This is in some cases unacceptable, but in the cases where such 
pressure variations are acceptable the heat transmission will be somewhat 
higher than when the plungers are moved in counter phase. 
The plungers can be constructed in various ways: as smooth pipes, as pipes 
with distributed scrapers, or as pipes having a diameter varying along the 
length of the pipes. For sugar manufacturing massecuite plungers in the 
form of ordinary smooth pipes have been found equally efficient as other 
forms. 
EXAMPLE 
In a cooler of the type illustrated working at an inlet temperature of the 
massecuite of 75.degree. C., an outlet temperature of 64.degree. C. and a 
temperature of the cooling medium of 40.degree. C. in the processing of 
the afterproduct in sugar manufacturing a heat transmission coefficient of 
0.67 kcal/.degree.C. min.m.sup.2 has been measured. The stroke of the 
plungers was 30 cm, the pipe diameter 15 cm, the plunger diameter 11 cm 
and the pipe length 200 cm.