EP0163184A2 - Method for regulating a vacuum boiling process for preparing sugar crystals - Google Patents

Abstract

The method entails adjusting the consistency of the composition during the boiling and crystallisation process by controlling the input of syrup as a function of present values for the consistency of the composition, and continually measuring the level of the composition in the boiling apparatus after the seeding during the controlled crystallisation process, and altering the specified value for the consistency as a function of the measured level in accordance with an exponential function Rhspec. = K1 . exp (K2 . hN) + K3, where, in each case when an initial level hNA is reached in the boiling apparatus before the seeding, the actual value RhA of the consistency of the composition is measured, where the initial point of the specified value plot which determines the control is automatically adjusted, according to the measured actual value RhA, by changing the constants K1 and K3 as a function of the measured actual value RhA, using the relation K1 = a . RhA + b and K3 = c . RhA - d.

Description

The invention relates to a method for regulating the vacuum cooking process for the production of crystalline sugar, the adjustment of the consistency of the filling compound during the cooking and crystallization process by regulating the syrup supply as a function of predetermined values for the consistency of the filling compound, after inoculation the level of the filling mass in the cooking apparatus is continuously measured and, depending on the level measured value, the setpoint value for the consistency is changed in accordance with a predetermined course until a maximum level is reached.

Crystalline sugar is usually produced in cooking apparatus in which syrups pressed or extracted, then cleaned and pre-concentrated from sugar-containing plants are thickened under the action of heat and water so that a supersaturated sugar solution is formed. This supersaturated sugar solution will Usually with the finely ground sugar (slurry) or with a crystal suspension (crystal base) from the production process (inoculated) so that the sugar can crystallize from the supersaturated solution on the presented crystals. As the crystals crystallize, further pre-concentrated syrup is continuously drawn into the apparatus, with condensate, ie water, being able to be drawn in to maintain the optimum state of supersaturation for crystal formation. The addition of further filler mass and possibly of condensate is dependent on the consistency of the filler mass measured continuously. As soon as the maximum level in the cooking apparatus is reached, each syrup and condensate intake is stopped and the entire filling is boiled with additional heat until a predetermined maximum value for the consistency is reached. After the boiling process has ended, the crystal suspension is separated into crystals and effluent by means of centrifuges.

The term consistency in the present case includes the so-called rheological value of the filling of the cooking apparatus, which is described at the beginning of the syrup intake by the viscosity of the crystal-free syrup and in the further course of the cooking process by the consistency of the filling compound consisting of mother syrup and crystals. An automatic control method is known from DE-AS 22 14 850, in which this rheometer value is used as a reference variable for automatic control of the cooking process. After thickening a basic filling quantity to the degree of supersaturation required for crystal formation, which is based on empirical tests by a certain; If the rheometer value can be determined, the filling is inoculated and after the inoculation while the syrup and condensate are continuously being drawn in. The setpoint of the reference variable "rheometer value" changes depending on the increase in level according to a predetermined program until the maximum level is reached and the other Syrup and condensate intake is stopped and the filling is boiled down with further heat supply until a predetermined maximum value for the "rheometer value" command variable is reached. After the cooking process has ended, the crystal suspension is separated into crystals and drain using centrifuges.

The goal of all automatic control methods is to keep the degree of supersaturation of the filling mass in the cooking apparatus constant after further vaccination with the addition of syrup in a narrow range in order to achieve the fastest possible, but even, crystal growth of the inoculants, but on the other hand to further increase it to prevent the degree of supersaturation, which would namely not only lead to irregularities in the crystal structure of the existing crystals, for example twin formation, but would also lead to spontaneous crystal formation from the supersaturated solution in the further course of the cooking process, so that at the time of boiling in the filling mass there are crystals with considerable grain differences. However, the fine grain fraction is undesirable. For this reason, the crystal content of the filling compound must be limited, since otherwise the impermeability of the crystal slurry in the centrifuge means that the drain from the crystal cake cannot be adequately separated and that the crystal cake cannot be washed again. However, this means a lower crystal yield from the pre-concentrated syrup used and a correspondingly higher return of the process to the manufacturing process. The fine grain fraction contained in the centrifuged crystals must then be separated off in elaborate screening plants, dissolved again and returned to the crystallization process.

'With the previously known method, attempts have now been made to achieve this narrow range of the degree of supersaturation required for uniform crystal formation, that, in addition to a linearly increasing rheometer value, which, based on empirical studies, only takes into account the filling mass consisting of mother syrup and crystals with increasing crystal growth in the filling mass, condensate, ie water, is also drawn in in addition to the syrup intake in order to prevent the degree of supersaturation being exceeded here which would lead to fine grain formation. In such an energy-intensive process as the boiling of sugar, each addition of additional water during the cooking and crystallization process results in an additional expenditure of energy. Attempts have therefore been made to design automatic controls based on the previously known method, but in order to reduce the energy requirement, the condensate intake to support the control has been omitted. However, this immediately resulted in the undesirable fine grain formation described above. In order to avoid fine grain formation, attempts were made, for example, to achieve greater homogeneity of the contents of the apparatus and thus more uniform crystal growth by additionally installing agitators. Furthermore, efforts were made to ensure this through the use of quickly responding measuring, regulating and control systems by means of a linear link between the syrup intake and the reference variable. However, these measures did not lead to the desired success.

Optimization would in itself only be possible by precisely maintaining a characteristic crystal distance, the crystals forming during the cooking process having to serve as "regulators" for the process. However, since there are no sensors for this purpose, the invention is based on the object of creating a control method based on the previously known method, namely the regulation of the cooking process as a function of the consistency of the filling compound, but avoiding the previously known disadvantages additional condensate intake to support the control to a higher crystal yield with a significant reduction in fine grain part leads.

vorgegeben ist, daß jeweils bei Erreichen eines Anfangsniveaus h_NA im Kochapparat vor dem Impfen der Ist-Wert Rh_A der Konsistenz der Füllmasse gemessen wird, daß entsprechend dem gemessenen Ist-Wert Rh_A der Anfangspunkt der für die Regelung maßgeblichen Sollwertkurve durch Veränderung der Konstanten k₁ und k₃ in Abhängigkeit vom gemessenen Konsistenz-Ist-Wert Rh_A automatisch angepaßt wird und zwar nach der Beziehung

und

The object of the invention is achieved in that an exponential function is used to change the target value for the consistency as a function of the level measurement value

It is specified that when an initial level h _{NA is reached} in the cooking apparatus, the actual value Rh _{A of} the consistency of the filling compound is measured before inoculation, and that, according to the measured actual value Rh _A, the starting point of the setpoint curve relevant for the control by changing the constants k ₁ and k _{3 is} automatically adjusted as a function of the measured actual consistency value Rh _A and that according to the relationship

and

The advantage of this control method is that by specifying the exponential function given above for guiding the rheometer setpoint, the crystallization process can be carried out in such a way that the crystals produced have a much narrower grain distribution spectrum than in the previously customary methods, with another advantage the additional supply of water to compensate for excess concentrations during the regulated crystallization process can be completely dispensed with. The constant k ₂ given in the formula is an apparatus constant in which the characteristics of the relevant cooking apparatus and the corresponding sensor (rheometer, V / K transmitter, radiometric density measuring section) are taken into account. This constant is determined by a series of test runs, in each of which the parameter k ₂ , which specifies the steepness of the expnential function, is varied until an optimization of the crystal yield

and uniformity of the crystals produced is achieved. The cooking process is now divided into two phases, namely a first controlled phase in which a minimum filling quantity kept constant by appropriate syrup intake is thickened by water evaporation to the degree of supersaturation, which then enables crystal formation by seeding with crystallization nuclei. As already explained above, this degree of supersaturation is determined by an empirically determined rheometer value. After the vaccination, the regulated cooking phase then takes place, ie the further syrup is fed into the cooking apparatus up to the predetermined maximum filling level, depending on the predetermined course for the rheometer setpoint. An exponential curve that only takes into account the apparatus constant k ₂ would presuppose that the initial value of the rheometer setpoint for each brew corresponds exactly to the rheometer value at the time of vaccination. However, this cannot be achieved in practice, since the consistency of the syrup at the time of vaccination can be different due to different starting compositions. The transition from the controlled phase to the regulated phase will therefore result in a "jump" in the rheometer setpoint in a number of cases, which would immediately lead to disturbances in the crystallization process. Accordingly, in the exponential curve for the course of the rheometer setpoint during the regulated phase, a correction possibility is provided by the constants k ₁ and k ₃ , which also take into account the state of the filling of the cooking apparatus at the time of the transition from the controlled phase to the regulated phase. These constants are determined immediately before the transition to the regulated phase using auxiliary functions as a function of the actual rheometer value measured at that point in time and then used in the exponential curve. With the modern technical aids of data processing, it is possible to feed the rheometer initial value RhA practically synchronously as a data signal to the control circuit according to correspondingly constructed computing modules, which then accordingly the prescribed calculation program, the input measurement signal "Rh _A" transform accordingly and then k as a constant _1, and k ₃ in the control block for guiding the Rheometersollwertes enter, so that the rheometer Rh _A equal to _R heometerwert Rh _to at the start point of the exponential curve.

The costants a, b, c and d of the auxiliary functions for adaptation to the initial value of the exponential curve can in turn be determined empirically by means of a corresponding trial test.

If the regulated crystallization process is carried out over the specified exponential curve for the rheometer setpoint after the above-mentioned costs have been specified, the crystallization process can, as already explained above, be carried out without the addition of condensate and furthermore offers the advantage that the previously usual boiling of the filling compound without further syrup addition can be omitted up to a predetermined rheometer end value. In the method according to the invention, the crystallization process is also completed when the predetermined filling level is reached, with regard to an optimal, i.e. uniform grain size distribution, the irregularities in the crystal formation which are possible with the previously known method are avoided.

wobei k₄ proportional der zur Verfügung stehenden Kochzeit ist und k₅ der zum Impfzeitpunkt gemessenen Temperatur der Füllmasse im Kochapparat entspricht. k₄ ist hierbei der Temperaturgradient, durch den implizit die Verdampfungsleistung, d.h. die Kochzeit bzw. das angelegte übersättigungspotential vorgegeben wird, während über die Konstante k_S wiederum der stoßfreie.übergang beim Umschalten von der gesteuerten zur geregelten Phase bewirkt wird, da hier der Temperaturanfangswert für die geregelte Phase vorgegeben ist. Die Konstante k₄ ist so vorgegeben, daß innerhalb der für den betreffenden Kochapparat zur Verfügung stehenden Kochzeit die erforderliche Wasserverdampfung auch geleistet wird. Hierbei ist zu berücksichtigen, daß üblicherweise nicht ein Kochapparat in einer Kochstation vorhanden ist, sondern daß mehrere Kochapparate eine Kochstation bilden, so daß trotz des chargenweisen Betriebes für den einzelnen Kochapparat sich für die gesamte Kochstation insgesamt eine quasi kontinuierliche Betriebsweise ergibt. Die Temperatursollwertführung entsprechend der Erfindung berücksichtigt hierbei, daß die für die Kristallisation so wichtige Anfangsphase, in der die zur Verfügung stehende Kristalloberfläche noch relativ gering ist, eine möglichst hohe Kochtemperatur vorgegeben werden kann, um ein zu hohes Übersättigungspotential zu vermeiden. Mit zunehmendem Wachstum der Kristalle in der Füllmasse kann dann durch eine geregelte Absenkung der Temperatur der Füllmasse die Verdampfungsleistung erhöht werden- Dies geschieht in der Regel über eine entsprechende Regelung des am jeweiligen Kochapparat anliegenden Vakuums. (Kaskadenregelung)In one embodiment of the method according to the invention it is further provided that the temperature of the filling compound is additionally regulated during the controlled operation of the crystallization process according to a temperature setpoint which is changed depending on the level measurement value, specifically according to the relationship

where k _{4 is} proportional to the cooking time available and k ₅ the temperature measured at the time of inoculation corresponds to the filling mass in the cooking apparatus. k ₄ is the temperature gradient that implicitly specifies the evaporation power, i.e. the cooking time or the applied supersaturation potential, while the constant k _S in turn causes the bumpless transition when switching from the controlled to the regulated phase, since this is the initial temperature value is specified for the regulated phase. The constant k ₄ is predetermined so that the required water evaporation is also achieved within the cooking time available for the cooking apparatus in question. It should be taken into account here that usually not one cooking apparatus is present in a cooking station, but that several cooking apparatuses form a cooking station, so that despite the batch operation for the individual cooking apparatus, the entire cooking station as a whole has a quasi-continuous operating mode. The temperature setpoint control according to the invention takes into account that the initial phase, which is so important for the crystallization and in which the available crystal surface is still relatively small, the highest possible cooking temperature can be specified in order to avoid an excessive saturation potential. With increasing growth of the crystals in the filling mass, the evaporation capacity can then be increased by a controlled lowering of the temperature of the filling mass. This is usually done by corresponding regulation of the vacuum applied to the respective cooking apparatus. (Cascade control)

• Fig. 1 einen Kochapparat mit Regelkreis,
• Fig. 2 schematische Kochkurven mit unterschiedlichen Anfangswerten, die den Zusammenhang zwischen Rheometerwert und der Füllhöhe im Kochapparat in % angeben,
• Fig. 3 die Kristallwachstumskurve bei einer Regelung gemäß der Erfindung im Vergleich zur Regelung nach dem Stande der Technik.

The method according to the invention is explained in more detail using an exemplary embodiment. The schematic drawings show:

• 1 is a cooking apparatus with control loop,
• 2 schematic cooking curves with different initial values, which indicate the relationship between the rheometer value and the filling level in the cooking apparatus in%,
• Fig. 3 shows the crystal growth curve in a control according to the invention compared to the control according to the prior art.

The optimal operating points for the crystal size distribution and yield were determined on an existing cooking apparatus. A maximum cooking time of 4.5 hours, a heating volume of 35% of the level output signal and a boiling point at 80% of both the level in the cooking device and the rheometer value were specified. The cooking apparatus was used to produce an RZ 2 product, which was produced by boiling an RZ 1 process with a dry matter content of between 70 and 78%. The constant k ₂ was then determined by experiment with regard to an optimum yield and crystal size distribution, giving a value of 0.0185.

The temperature control was also determined by experiments, the constants k ₄ = −0.1333 and the constant k _{5 being} 86.67, ie the initial temperature of the cooking mass, for the course of the temperature setpoint curve.

und

The following values resulted for the auxiliary functions for determining the constants k ₁ and k ₃ :

and

• a)Kristallausbeute
• b)Kristallgrößenverteilung
• c)Kochzeit (Chargendauer)

bei einem Kochapparat ohne Rührwerk und einem vorkonzentrierten Sirup mit einem Zuckergehalt von 82 Gewichtsprozent bezogen auf die Trockensubstanz folgende Werte:

• a)Kristallausbeute: lineare Führung: durchschnittliche Ausbeute = 40% exponentielle Führung: durchschnittliche Ausbeute = 50%.
• b)Kristallgrößenverteilung:(nach RRSB): ausgehend von einem Kristallfuß mit n = 3,3 und d^' = 0,42 mm lineare Führung: n = 3,4, d' = 0,89 mm exponentielle Führung: n = 4,8, d' = 0,98 mm

In a comparison of the control method according to the invention with the usual control method, which had a linear _'linking rheometer and Niveaumeßwert revealed the factors concerning

• a) Crystal yield
• b) crystal size distribution
• c) Cooking time (batch duration)

in the case of a cooking apparatus without a stirrer and a pre-concentrated syrup with a sugar content of 82 percent by weight, based on the dry matter, the following values:

• a) Crystal yield: linear guidance: average yield = 40% exponential guidance: average yield = 50%.
• b) Crystal size distribution: (according to RRSB): starting from a crystal base with n = 3.3 and d ^' = 0.42 mm linear guidance: n = 3.4, d' = 0.89 mm exponential guidance: n = 4, 8, d '= 0.98 mm

• c)Kochzeit (Chargendauer): ausgehend von gleicher Kristallqualität und Kristallausbeute konnte die Kochzeit um 30% verringert werden.

This result is shown in the crystal growth curves shown in FIG. 3.

• c) Cooking time (batch duration): based on the same crystal quality and crystal yield, the cooking time could be reduced by 30%.

In Fig. 1, the control according to the inventive method is shown in the form of a block diagram. A cooking apparatus 1 is connected in its lower area to a syrup supply line 2. A control valve 3 is installed in the syrup supply line 2 and its position can be adjusted via a positioner 4. In the lower area of the cooking apparatus there is a heating chamber 5 charged with heating steam, via which the heat of vaporization is introduced into the filling of the cooking apparatus. In the upper area is the interior of the cooking apparatus 1 connected to a vacuum line 6, the vacuum acting on the interior of the cooking apparatus and thus the evaporation capacity being able to be regulated via an adjusting flap 7 which is provided with an actuator 8.

The important for the regulation of measurement data once a Niveaumeßeinrichtun ^{g of} 9, a sensor 10 for measuring the viscosity or consistency of the cooking apparatus filling and a temperature sensor 11 is measured.

The position of the individual measuring elements is only indicated schematically and depends on the measuring principle on which the respective measuring device is based.

In the exemplary embodiment shown, the consistency is measured using a so-called rheometer. The measurement signal emanating from the rheometer is now fed to a control circuit 12 and processed there. Since a setpoint curve dependent on the measured rheometer initial value is to be specified, the actual rheometer value Rh _A measured at a certain point in time is fed to a computing module 13 containing the auxiliary function for determining the constants k ₁ and to a computing module 14 containing the auxiliary function for determining the constants k ₃ . The constants k ₁ and k ₃ determined by the arithmetic components are then fed to a setpoint computer 15, which determines the setpoint curve applicable to the respective batch with its starting point. As soon as the setpoint curve has been established, which is indicated schematically here by block 16, the rheometer 10 is switched over to this part of the control circuit so that, with simultaneous application of the measurement signal from the level sensor 9, the formation of the difference between the setpoint and actual value of the rheometer value required for the control Output signal is generated, which is connected to the actuator 4 of the syrup intake valve 3 and this is adjusted according to the sign of the output signal in the opening or closing direction.

The temperature sensor 11 is connected to a temperature controller 17, to which the level measurement value is supplied as a guide signal, with the aid of which the setpoint curve for the temperature control is changed depending on the level.

Fig. 2 shows schematically different cooking curves of the same cooking apparatus, i.e. with a constant k ₂ for the exponential function of the rheometer setpoint but different initial values, as it results from the adaptation process immediately before the start of the control phase, taking into account the measured rheometer actual value during the transition from the control to the control . In the diagram, the boiling phase up to the time of vaccination is marked with I and the crystallization phase after vaccination until the end of brewing with II. The same markings are also given in Fig. 1 with respect to the level of the cooking apparatus. The strongly drawn rheometer setpoint curve in FIG. 2 is additionally supplemented for phase I with the course of the rheometer value. For this phase, the cooking apparatus is filled with 35% of the maximum level for each batch, ie filled so high that the heating 5 is covered with syrup at a certain height. The water evaporating continuously from the syrup is now supplemented by continuous syrup addition while maintaining the level, so that this filling quantity is concentrated and has finally reached a certain degree of supersaturation. Based on many years of experience, it is known that a certain rheometer value indicates the most favorable degree of supersaturation for vaccination. As soon as this rheometer value is reached, in the diagram according to FIG. 2 this is the point 18, this measurement signal is first fed to the arithmetic modules 13 and 14 for determining the auxiliary function, the result signal of which is then fed to the setpoint calculator 15, which is the setpoint curve relevant for the control adapts that the initial value of the setpoint curve corresponds to the measured rheometer actual value at the time of vaccination, so that when switching from the control in phase I to the control in phase II, the transition is smooth.

Claims (3)

1. Process for controlling the vacuum cooking process for the production of crystalline sugar, wherein. elie. The consistency of the filling mass is adjusted during the cooking and crystallization process by regulating the syrup supply as a function of predefined values for the consistency of the filling mass and, after inoculation during the controlled management of the crystallization process, the level of the filling mass in the cooking apparatus is measured continuously and depending on the level measurement of the The setpoint value for the consistency is changed according to a predetermined course until a maximum level is reached, characterized in that an exponential function is used to change the setpoint value for the consistency as a function of the level measurement value

It is specified that when an initial level h _{NA is reached} in the cooking apparatus, the actual value Rh _{A of} the consistency of the filling compound is measured before inoculation, and that, according to the measured actual value Rh _A, the starting point of the setpoint curve relevant for the control by changing the constants k ₁ and k _{3 are} automatically adjusted depending on the measured actual value Rh _A , according to the relationship

and

2. The method according to claim 1, characterized in that in addition the temperature of the filling compound is regulated during the controlled management of the crystallization process according to a temperature setpoint which is changed as a function of the level measurement value, specifically according to the relationship

where k _{4 is} proportional to the cooking time available and k ₅ corresponds to the temperature of the filling mass in the cooking apparatus measured before the time of inoculation.

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