Method and Device for Obtaining Sugar Crystals From a Sugar Solution

The invention relates to a method and a device for obtaining sugar crystals from a sugar solution by concentrating the solution in a multi-stage evaporator and by supplying the thick juice through a buffer vessel to a discontinuous boiling pan, in which the sugar is allowed to crystallize out. In the known method the juice emanating from the multi-stage evaporator in a concentration of about 65 Brix is concentrated in a boiling pan, in which the temperature of the thick juice is slightly higher than the value corresponding to the boiling temperature of the juice at the absolute pressure prevailing in the boiling pan. During the concentration self-evaporation will occur so that the concentration of the juice increases. When a given level of juice in the boiling pan is attained, the supply of heating vapour is opened so that the juice is further concentrated. When the juice is sufficiently concentrated, seed granulate is added so that crystallisation can start. During crystallization sub-saturated juice is added, while the supply of heat continues. When finally the concentration has reached the maximum value, the boiling cycle terminates and the boiling pan can be emptied.

The invention has for its object to provide a semi-continuous method in which considerable energy saving can be ensured and a number of additional advantages are obtained.

According to the invention the thick juice emanating from the multi-stage evaporator is concentrated in a continuously operating pre-concentrator to a concentration at which seed granulate can be added in the boiling pan, the pressure of the concentrated thick juice being increased and the juice being fed into a buffer vessel, in which the juice is subjected to a higher pressure than the pressure at which it is concentrated, the temperature being maintained and the contents of the vessel being thoroughly stirred, while the boiling pan is filled from the buffer vessel and seed granulate is added simultaneously. With such a method the first part of the boiling cycle, which has hitherto been carried out in a boiling pan, is performed in the pre-concentrator. In the pre-concentrator the working temperature and pressure correspond with those at which in the boiling pan the juice would be concentrated to the same concentration. The value of this concentration may be of the order of 85 Brix. Since the largest quantity of water is evaporated in the concentrator, the developed vapour is available for use elsewhere in the system, for example, for heating the boiling pan. This means an important saving of energy. With the known methods the boiling pan was heated by vapour from the first or second stage of the multi-stage evaporator. In accordance with the invention the heating steam can be derived from the concentrator, which can be heated by vapour from the second or third evaporation stage. A direct consequence of the method described is that less injection water is needed in the condensation devices.

Since only crystallisation rather than concentration takes place in the boiling pan, a much higher capacity can be attained with the same contents of the boiling pan. Moreover, the peak load of the steam withdrawal is also more favourable. An additional advantage is that a change-over from the known method to the method embodying the invention requires only small investment because a majority of the existing apparatus can be employed.

According to the invention it is advantageous to regulate the quantity of heating steam supplied to the pre-concentrator in dependence upon the temperature of the concentrated juice, the pressure of the inspissated juice being kept at a constant value, whilst the adjusted temperature and the pressure of the regulators are determined as state magnitudes associated with the thick juice of the desired concentration and the level in the concentrator is kept constant. With such a regulating system the correct values can be readily adjusted with any desired production concentration in accordance with the purity of the juice.

As stated above, the steam developed from the concentrator may be used elsewhere in the system.

The invention furthermore relates to a device for carrying out the method described above, said device comprising a multi-stage evaporator and a discontinuous boiling pan.

According to the invention the multi-stage evaporator has arranged after it a continuously operating pre-concentrator, whose juice outlet is connected with a buffer vessel, in which a higher pressure may prevail than in the pre-concentrator and which is provided with means capable of maintaining the temperature of the vessel. Since a higher pressure prevails in the buffer vessel than in the pre-concentrator, the over-saturated juice will not crystallize out provided the temperature of the vessel does not drop.

In accordance with the invention the pressure in the buffer vessel can be raised in a simple manner by establishing an open communication between the space above the fluid level and the environment. Then atmospheric pressure prevails in the buffer vessel, which materially exceeds the pressure in the pre-concentrator.

According to the invention a displacer pump may be included in the conduit between the pre-concentrator and the buffer vessel. It can thus be ensured in a simple manner that the pressure in the buffer vessel exceeds that in the concentrator.

In accordance with the invention the conduit between the last stage of the multi-stage evaporator and the pre-concentrator may include a displacer pump. This provides the advantage that fluctuations of the operational conditions of the pre-concentrator will not affect the multi-stage evaporator.

According to the invention the buffer vessel may be constructed in a spherical shape. This provides an advantageous ratio between the heat emitting surface and the contents of the buffer vessel. In this case a minimum amount of heat is required for maintaining the temperature of the vessel. Moreover, the vessel has no dead corners in which over-saturation could occur to an extent such that crystals would form spontaneously.

In accordance with the invention the buffer vessel may be provided with an agitator, whose driving gear is connected with a regulating device controlling the supply of sub-saturated juice to the buffer vessel, so that with an increase of the absorbed power of the driving gear beyond a given value sub-saturated juice is fed to the vessel. In this way a very effective protection against an undersirable increase in over-saturation is obtained. If the over-saturation increases and the concentration rises, the power absorbed by the driving gear will increase and as soon as a critical value is attained, the over-saturation can be eliminated by feeding sub-saturated juice.

In an effective embodiment the boiling pan may be provided with a mechanical agitator. In the novel method no heating steam need be supplied, when the juice is fed to the boiling pan. Hitherto steam has been constantly supplied in order to ensure adequate circulation. In order to ensure a satisfactory circulation in the novel method, even if no steam is supplied, it is important to provide a mechanical agitator. If such a stirring device were not employed, sub-saturated juice as well as heating steam could be supplied. Like hitherto a circulation would then be produced due to the heat produced by the supply of steam.

According to the invention the buffer vessel may be double-walled, while the chamber enclosed between the walls may be provided with one or more connecting stubs. The space can thus be exhausted so that a satisfactory thermal insulation is obtained. It is, as an alternative, possible to feed steam into the chamber between the walls so that heat losses to the outside can be completely compensated for.

The invention will be described more fully hereinafter with reference to the drawing and a process shown schematically. In the drawing the various apparatus forming part of the system embodying the invention are illustrated schematically.

Reference numeral 1 designates the last stage of a multi-stage evaporator. The penultimate stage thereof is designated by 2. The vapour evolving in the evaporator 2 is fed through the conduit 3 to the evaporator 1. The juice concentrated in the evaporator 2 is supplied through a conduit 4 to the evaporator 1, from where the juice flows through the conduit 5 and a displacer pump 6 to a pre-concentrator 7. The pre-concentrator comprises an evaporation body 8, to which steam is supplied via the conduit 9. On the top side the evaporator communicates through the conduit 10 with the vapour separator 11, from which the developed vapour can escape through the conduit 12, whereas through conduit 13 the fluid is supplied to the evaporation body 8. The vapour escaping through the conduit 12 may be utilized elsewhere in the system, for example, for heating boiling pans. The conduit 5 includes a regulator 14, which keeps the level in the separator 11 constant. The conduit 9 includes a regulator 15, which controls the steam supply in accordance with the temperature in the conduit 13. The conduit 12 includes a regulator 16, which maintains the pressure prevailing in the separator 11. With the conduit 13 communicates a conduit 17, which includes a displacer pump 18. Through the conduit 17 the juice inspissated in the pre-concentrator 7 is fed to a buffer vessel 19. The buffer vessel has a spherical shape and is provided with a mechanical agitator 20. There is furthermore provided a heating element 21. Through a conduit 22 the vessel can communicate with the open air. With the aid of the heating element 21 the contents of the vessel are maintained at the desired temperature, the steam being supplied to the heating element 21 through a regulator 23, which responds to the temperature in the vessel. The conduit 17 communicates with a conduit 24 including a regulator 25. The regulator 25 is controlled by the power absorbed by the agitator 20, which means that at an increase in the absorbed power beyond a given value the regulator 25 feeds juice of lower concentration than the juice already contained in the vessel 19 from the conduit 5 to the vessel. Moreover, the regulator 25 is acted upon by a direct measurement of the concentration of the contents of the vessel 19. Consequently, the regulator 25 ensures that the concentration in the vessel 19 will not rise to a value at which crystallisation could occur. Through a conduit 26 the vessel 19 communicates with a boiling pan 27. The boiling pan 27 is provided with a heating element 28, to which steam is supplied through a conduit 29. The conduit 29 includes a regulator 30, which responds to over-saturation of the juice in the boiling vessel. The vapour developed in the boiling pan 27 is conducted away through a conduit 31, which communicates with a condensation device 32. The conduit 31 includes a regulator 33, which maintains the vacuum in the boiling pan. The boiling pan 27 is furthermore provided with a mechanical agitator 34.

The device operates as follows. Through the displacer pump 6 and the conduit 5 the juice concentrated, for example, to a concentration of 65 Brix is fed from the multi-stage evaporator to the pre-concentrator 7. In this pre-concentrator the juice is thickened to a concentration of, for example, 85 Brix. The thickened juice is fed from the concentrator through the conduit 17 and the displacer pump 18 to the buffer vessel 19. In the buffer vessel 19 prevails a higher pressure than in the pre-concentrator 7, so that evaporation will no longer occur and the concentration will, therefore, not increase further. In order to avoid condensation the heat loss is compensated for by the heating element 21. The uniform temperature is maintained by means of the agitator 20, which serves in addition as a further safeguard gainst crystallisation. If the concentration should nevertheless increase, the power absorbed by the agitator 20 would rise to a very high value so that the regulator 25 would feed juice of 65 Brix from the conduit 5 to the buffer vessel. The concentration would then drop immediately. At the beginning of a boiling cycle the juice of 85 Brix stored in the buffer vessel is fed to the boiling pan 27, whilst smultaneously seed granulate is added. Only a small quantity of water need be evaporated in the boiling pan 27 in order to initiate crystallisation. The major quantity of water to be evaporated in order to raise the concentration from 65 to 85 Brix is evaporated in the pre-concentrator, the developed vapour being conducted away through the conduit 12 for utilization at a further place. As compared with the prior-art methods an important saving of energy is thus obtained. Moreover, the capacity of the available boiling pans can be raised because the cycle in the boiling pan can be much shorter. Because a considerably smaller quantity of vapour is required for the condensation device 32, the consumption of cooling water of the condensation device is materially reduced.