Method for controlling vacuum pan

Abstract

In a process wherein heat is exchanged between the heat exchanger and the solution and crystals are generated and grown in the solution, the crystal size and the inter-crystal gap length in the solution are measured and the quantity of supply of the material is controlled on the basis of the measurement.

Description

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

The present invention relates to a method for controlling a vacuum pan used in the sugar manufacturing process.

2. DESCRIPTION OF THE PRIOR ART

The sugar manufacturing process consists of continuous process at the anterior stages and batch process at the posterior stages, both processes being intricately combined. Among the individual processes, the boiling process to cause the crystallization of sugar is the most difficult to automate.

The boiling process is performed in a vacuum pan and has been practiced for many years. Many attempts to automate the boiling process were made in vain due to the batch process which has complicated characteristics. And experienced staff usually operated the vacuum pan and the associated devices. There have been only a few successful cases, which were accompanied by some restrictions so that no definite method for automatically operating the vacuum pan has been established before now.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a method for automatically operating the vacuum pan without the difficulties encountered by the conventional automatic operation.

Another object of the present invention is to provide a method for controlling the vacuum pan according to which crystal growth is stably performed without the formation of false grains and the resolution of crystals taking place after seeding.

An additional object of the present invention is to provide a method for controlling the vacuum pan according to which the stable range in crystal growth is enlarged.

A further object of the present invention is to provide a method for controlling the vacuum pan according to which crystals are stably grown even if there is a disturbance owing to, for example, a change in the sort of supplied solution.

According to the present invention which has been made to attain such objects as described above, the quantity of supply of the solution is controlled by a signal obtained by comparing the reference signal from a programmer with more than two independent signals representative of the level of the solution in the vacuum pan, the inter-crystal gap length (or crystal content or consistency) or the average diameter of crystals instead of the level.

Moreover, the quantity of supply of the solution is controlled by comparing the reference signal with the preferentially selected one of the independent signals.

Further, the supply of the solution is controlled on the basis of the preferentially selected one of the components (detecting signals) concerning the control of the supply. The control is temporarily held when the deviation of any non-selected component from the reference signal exceeds a predetermined level and the supply is controlled on the basis of each selected component when the deviation of the selected component becomes equal to a predetermined level.

The present invention will be more clearly understood when the following description of the specification is read along with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a system for controlling the vacuum pan, embodying the present invention.

FIG. 2 is a graphical representation of the variations with time of variables important in the process of crystal growth.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The conventional method of controlling the vacuum pan will be described here with the aid of FIG. 1 of the attached drawings. In FIG. 1, parts within an enclosure 50 by the long-and-two-short-dash line is a means first proposed by the inventors claiming the patentability of this application and the other parts or elements are familiar to those skilled in the art. And the conventional vacuum pan was run according to the following steps of procedure.

1. Vacuuming

At first, an open valve 2 provided in a vacuum pan 1 in FIG. 1 is closed while the main vacuum valve 4 and the cooling-water valve 5 of a barometric condenser 6 communicating via a pipe 3 with the vacuum pan 1 are opened. As a result of these operations, vapor in the vacuum pan 1 is conducted into the barometric condenser 6 and condensed there into water by means of cooling water. The condensed water is evacuated and non-condensed vapor is ejected from the vacuum vessel by means of a vacuum pump (not shown). By this process the reduction of pressure in the vacuum pan 1 takes place.

2. Liquorfeed

When the pressure in the vacuum pan 1 reaches a predetermined value (about 400 mmHg Abs), the sugar solution, almost condensed up to saturation through the previous step, is sucked from a pan tank (not shown) through a pipe 7 and a valve 8 associated therewith into the vacuum pan 1. The valve 8 is closed as soon as the upper surface of the solution rises a little above a heat exchanger 9 in the pan 1.

3. Concentration

The steam valve 10 of the heat exchanger 9 is opened so that the heating of the solution in the pan is initiated. In order to prevent the heat exchanger 9 from appearing above the solution of sugar due to the reduction in volume of the solution owing to the steam heating, a supply valve through which additional sugar solution is supplied into the vacuum pan is so controlled as to maintain the surface of the solution at a constant level. The inside of the pan 1 is kept at a pressure of 50 to 150 mmHg Abs and at temperatures from 50.degree. to 60.degree.C.

4. Seeding

When the sugar solution is so far condensed till it reaches a supersaturated condition, crystal nuclei (seeds) are added to the solution through a valve 12.

There are other methods of producing crystals, such as

I. Natural seeding in which condensation is performed to excess so as to form natural crystals, and

II. Shock seeding in which inhalation of air with or without a small amount of seeds is used to form crystals by means of external shock.

However, it is difficult with the above methods to form a desired number of crystals. Therefore, according to the method preferred at present, there are added to the solution seeds equal in number to crystals desired to be formed. Moreover, instead of shock seeding, a method has been proposed in which seeds in the form of alcoholic slurry are mixed into the sugar solution in an air-tight condition.

The present invention can be applied to any one of the above described method.

5. Crystal growth

In the vacuum pan wherein the operation of seeding has been completed, the surface area of any crystal is very small in comparison with the speed of vaporization of the solvent and the crystallization speed is slow so that either the formation of false grains or the resolution of crystals once formed takes place. Therefore, the growth of crystals is very unstable. In order to improve the stability of crystallization, the following processes may be recommended.

1. To decrease the quantity of steam fed to the heat exchanger and to limit the speed of solvent vaporization. In this case, however, with a vacuum pan having no stirrer therein, the vaporization speed can not be rendered slower than a limit attainable with a vacuum pan having a stirrer.

2. To supply hot water through a valve 13 corresponding to the difference between the speed of vaporization of solvent and that of crystallization.

3. To decrease cooling water so as to increase the pressure within the vacuum pan. As a result, the speed of vaporization is suppressed, the temperature of the solution rises, and the degree of supersaturation is leveled down, so that the speed of crystallization decreases.

When the unstable phase just after seeding shifts to the rather stable phase of crystal growth, the above described processes are cancelled and the sugar solution is supplied again. Crystals of sugar grow gradually and the volume of massecuite (a slurry composed of crystals and molasses) also increases gradually.

In this stage, the surface area of any crystal increases, self-equilibrium is established and a quite stable condition is reached. Namely, as the surface area of each crystal increases, the speed of crystallization of sugar is accelerated. The crystallization speed depends on the degree of supersaturation; the speed is quickened with the rising of the degree of supersaturation and slows down with the fall thereof. Thus, the change in crystallization speed tends to decrease that in the degree of supersaturation. However, care must be taken of at this stage since adhesion, twinning and conglomeration of crystals are easily caused.

6. Boiling-down

When the diameter of each crystal reaches a predetermined dimension, the supply of sugar solution is stopped. Then, heating is continued to perform the boiling-down. This step is necessary to make the solution have such a fluidity as is suitable for the centrifugal separation as the next step and to increase the rate of crystallization.

If a desired consistency is reached, the valve 10 of the heat exchanger 9 is closed and the valve 2 is opened to reduce the pressure in the vacuum pan 1.

7. Discharge

The discharge valve 14 of the vacuum pan 1 is opened and the massecuite is discharged to be treated by the next separation process. The massecuite is then divided through centrifugal separation into crystals and molasses. Thereafter, the separated crystals are passed through a dryer and a cooler and stored in silos or sugar bins and the sugar is packed and sent to the market.

8. Washing of the pan

A washing steam valve 15 and a washing water valve 16 are opened and the inside of the vacuum pan 1 is cleaned with steam and hot water. Thus the vacuum pan 1 is prepared for the following boiling step.

Reference numeral 17 designates an orifice; 18 a pressure-difference detector to detect the pressures of steam before and after the orifice 18; 19 an arithmetic unit to measure the flow rate of steam on the basis of Bernoulli's theorem in response to the output of the detector 18; and 20 a programmer for the quantity of steam flow. A signal from the programmer 20 is applied through a selector 21 to a steam flow controller 22 and the aperture of the valve 10 is controlled in response to the deviation of the output of the arithmetic unit 19 from the reference signal of the programmer 20. A pressure detector 23 measures the absolute pressure in the vacuum pan 1, a temperature measuring element 24 measures the boiling point of the sugar solution in the vacuum pan, and a pressure-temperature transducer 25 obtains from the absolute pressure in the pan the corresponding temperature in accordance with the pressure-temperature characteristic of the solvent (water in this case). A supersaturation detector 26 receives the outputs of the element 24 and the transducer 25 to obtain the degree of supersaturation. A cooling water controller 27 receives the output signal of the pressure detector 23 to control the cooling water valve 5. An electromagnetic flow meter 28 delivers an output proportional to the flow rate of sugar solution to be applied to an arithmetic unit 29, the output signal of which, representative of the flow rate of sugar solution, is applied to a sugar-solution supply controller 30.

Now, the principle of the present invention will be described. By maintaining the degree of supersaturation of the sugar solution in the vacuum pan at values of from 1.00 to 1.25, i.e. at a quasi-stable condition, during seeding, crystals can be stably grown from added nuclei. The degree of supersaturation indicates the condition of the crystals growth during the time from the step of seeding to the middle point of the step of crystal growth.

After the middle point, the consistency of the solution is the most important indication of the occasion for boiling-down because of the following reasons.

1. During that time, the level of the solution rises and the consistency becomes smaller, so that the speed of the circulation of the massecuite slows down. The decrease in the circulation speed causes the degradation of the heat transfer capacity of the heat exchanger and therefore the time of boiling down is prolonged and the quantity of solvent vaporization decreases in a vacuum pan using natural convection with the result that the circulation is further decelerated. Moreover, the decrease in the circulation speed tends to cause the adhesion, twinning and conglomeration of crystals.

2. If, however, the crystal content is diminished, the circulation speed of massecuite increases while the inter-crystal gaps increase to leave more room for the formation of false grains. Thus, the uniformity of the diameters of produced crystals is adversely affected. The experiment has it that the gap should be diminished gradually during the step of boiling-down up to about 0.2 mm in the end.

However, the gaps between the crystals are not constant and

1. during seeding that quantity of sugar solution enough for the heat exchanger to be completely immersed in the solution which is needed in the step of condensation, determines the gap length. The gap length is usually larger than in the end of the seeding step so that it must be gradually lessened to a required value.

2. When the diameter of each crystal is smaller, the formation of the false grains tends to be suppressed even if the crystal content is small. Moreover, since the speed of crystal growth can be increased too, crystals can be produced at a high rate of circulation. Therefore, the above described matters have to be taken into consideration in order to grow crystals under an optimal condition.

FIG. 2 gives the summary of the foregoing consideration, which shows an optimal model of crystal growth. The model has been obtained as follows. It has been proved that it simulates a practical process of sugar boiling with a high accuracy.

It is here assumed that the crystal is a cube. And it follows that

Q = N(a + g).sup.3 (1)

where Q is the total volume of the massecuite in the pan, N the number of the total crystals, a the length of each edge of each crystal, and g the inter-crystal gap length. The ratio .PHI. of the net volume of the total crystals to the total volume is expressed by the formula ##EQU1## where .PHI. is the crystal content.

The crystal content cannot be determined by direct measurement but it is well known that it can be indirectly measured by means of a consistency meter. Namely, the consistency can be given by the expression ##EQU2## where x is the consistency, x.sub.o the reading on the consistency meter when .PHI. = 0, k .sub.1 and k.sub.2 constants. It is known empirically that the size of each crystal and the total volume can be expressed respectively by linear expression whose value increases with time, and the inter-crystal gap length is optimal as in the model shown in FIG. 2 so that the model can be obtained by substituting quantities from the curves in FIG. 2 into the above mentioned formulae.

From the above description, it can be deduced that

1 The variations of two variables with time must be independently specified so as to determine the condition in the vacuum pan. Namely, in this embodiment, the speed of crystal growth and the rate of increase in volume of massecuite have to be prescribed. Both the speed and the rate can be independently determined.

2 The change in consistency with time is generally considered to vary linearly but in fact it varies non-linearly. The reason is that the boiling of sugar solution is performed with non-uniform inter-crystal gap length.

As described in the previous embodiment, two independent programs are necessary to specify the condition in the vacuum pan. It is theoretically preferable to control in program the average size of crystals and the crystal content. The crystal content, as described above, is related to the inter-crystal gap length and can be controlled according to the gap length and the average size by directly measuring the gap using an industrial television system or pattern recognition system. Here, the average diameter of crystals and the crystal content are controlled with the level and the consistency of sugar solution in the vacuum pan. This will be explained by reference to FIG. 1, in which are shown a level detector 31 for detecting the level of the sugar solution in the pan, a consistency meter 32, a transducer 33 for obtaining the consistency of the solution from the output of the consistency meter 32, a programmer 34 for controlling the level of the solution, and a programmer 35 for controlling the consistency of the solution.

In addition, the present invention has the following features, which were devised to increase the range of stability.

1. The vacuum pan forms a part of the mutually interrelated, complicated systems and therefore the control system is given priority. Usually, the quantity of sugar solution supply is controlled by the programmer 34 related to the level of the solution in the vacuum pan and the level controller is actuated according to the deviation of the output of the level detector 31 from the reference signal from the programmer 34 to operate the sugar-solution supply controller 30 through the level controller.

However, if the output of the transducer 33 is smaller than the reference value of the programmer 35 during the time when the degree of supersaturation or the consistency is a better indication of control after seeding, respectively, then the signal from the programmer 35 is selected by means of the selector 37 prior to the signal from the programmer 34 so as to control the quantity of supply of sugar solution. Thus, there is provided a control system almost free from disturbances and with ease of programming by employing the method in which the control of consistency has priority to that of the level of the solution. The control of consistency consists of control to provide additional sugar solution successively, control to dilute the solution in the vacuum pan and control to continue boiling the solution with the program stopped for a moment. And by performing any one of these control operations can be controlled the inter-crystal gap length under an optimal condition. In FIG. 1 are further shown a supersaturation controller 38, a consistency controller 39 which delivers an output according to the deviation of the output of the transducer 33 from the output of the programmer 35, and a switch 40 selectively switched over when the consistency is a better indication of control than the degree of supersaturation after seeding. The programmer 35 controls the hot water valve 13.

2. Moreover, according to the present invention, when the level of the solution is deviated from the range defined by the used program, the progress of the program is temporarily interrupted. At this time, the solution is supplied to the pan in accordance with the programmed value of the supersaturation or consistency. Thereafter, the program of the level control program is resumed when the level of the massecuite is gradually restored and reaches the predetermined programmed value. This method of control is especially effective in the case where the program of consistency control can no longer be continued since the concentration of the sugar solution to be supplied is too thick to render control of consistency prior to the control of the level of the solution. For the consistency, control can be resumed when the condition suitable for consistency control is reached again by vaporizing the water of the solution with the level of the solution kept constant.

With the conventional non-automatic vacuum pan 1.5 .times. 1 to 2 man-hours for control for soft sugar, 1.5 .times. 2 to 4 man-hours for fine granulated sugar and 1.5 .times. 6 to 10 man-hours for remelted sugar were required so as to produce sugar from a solution having a purity of 90 to 100 %. On the other hand, using the automatic control method according to the present invention, a 0.5 .times. 1 to 9 man-hours can complete the whole process of boiling. It is quite natural to decrease the amount of labor where automatic operation is employed, but the reason for reducing the running time is probably as follows. The boiling process is performed nearly at the upper limit of the speed of crystal growth and therefore continuous control at a constant rate is very easy with an automatic operation while the control is stepwise with manual operation with the result that the accuracy of control is degraded to prolong the time of operation.

As described above, with a vacuum pan according to the present invention, all the requirements for automatic operation can be satisfied and the labor of the supervisor can be decreased to a great extent. The present invention can be applied to such processes involving crystallization as the productions of some drugs and of sodium glutamate.

Claims

I claim:

1. In a method for controlling a vacuum pan according to which heat is exchanged between a heat exchanger and a massecuite to generate and grow crystals in the massecuite in a batch operation, the improvement comprising (a) measuring the inter-crystal gap length of the crystals grown in the solution and controlling the quantity of the supply of the solution on the basis of the measured inter-crystal gap length to effectively grow crystals, and (b) measuring the value of at least one other parameter indicative of the progress of the crystal growing process, comparing the measured inter-crystal gap length with a programmed value for the inter-crystal gap length provided by a first program, comparing the measured value of said other parameter with a programmed value for said other parameter provided by a second program, automatically preferentially selecting either said first program or said second program for controlling said supply quantity, and controlling said supply quantity on the basis of the deviation of the measured value from the programmed value of the parameter corresponding to the selected program.

2. A method for controlling a vacuum pan as claimed in claim 1, wherein when the deviation of the measured value of at least one of the parameters corresponding to the non-selected programs from the corresponding programmed value exceeds a predetermined level, the progress of the program is suspended; and when the deviation of the measured value of the parameter corresponding to the selected program becomes equal to a predetermined level, the control of the supply quantity of the solution is resumed on the basis of the selected program.

3. A method for controlling a vacuum pan as claimed in claim 1, wherein the average diameter of the crystals grown in the solution is used as said other parameter.

4. A method for controlling a vacuum pan as claimed in claim 1, wherein the level the solution is used as said other parameter.

5. A method for controlling a vacuum pan as claimed in claim 1, wherein the average diameter of the crystals grown in the solution is used as a component for controlling the supply quantity of the solution.

6. In a method for controlling a vacuum pan according to which heat is exchanged between a heat exchanger and a massecuite to generate and grow crystals in the massecuite in a bath operation, the improvement comprising (a) measuring the crystal content of the massecuite and controlling the supply quantity of the solution on the basis of the measured quantity to effectively grow crystals, and (b) measuring the value of at least one other parameter indicative of the progress of the crystal growing process, comparing the measured crystal content of the massecuite with a programmed value for the crystal content of the massecuite provided by a first program, comparing the measured value of said other parameter with a programmed value for said other parameter provided by a second program, automatically preferentially selecting either said first program or said second program for controlling said supply quantity, and controlling said supply quantity on the basis of the deviation of the measured value from the programmed value of the parameter corresponding to the selected program.

7. A method for controlling a vacuum pan as claimed in claim 6, wherein when the deviation of the measured value of at least one of the parameters corresponding to the non-selected programs from the corresponding programmed value exceeds a predetermined level, the progress of the program is suspended; and when the deviation of the measured value of the parameter corresponding to the selected program becomes equal to a predetermined level, the control of the supply quantity of the solution is resumed on the basis of the selected program.

8. A method for controlling a vacuum pan as claimed in claim 6, wherein the level of the solution is used as said other parameter.

9. In a method for controlling a vacuum pan according to which heat is exchanged between a heat exchanger and a massecuite to generate and grow crystals in the massecuite in a batch operation, the improvement comprising (a) measuring the consistency of the massecuite and controlling the supply quantity of the solution on the basis of the measured quantity to effectively grow crystals, and (b) measuring the value of at least one other parameter indicative of the progress of the crystal growing process, comparing the measured consistency of the solution with a programmed value for the consistency of the solution provided by a first program, comparing the measured value of said other parameter with a programmed value for said other parameter provided by a second program, automatically preferentially selecting either said first program or said second program for controlling said supply quantity, and controlling said supply quantity on the basis of the deviation of the measured value from the programmed value of the parameter corresponding to the selected program.

10. A method for controlling a vacuum pan as claimed in claim 9, wherein when the deviation of the measured value of at least one of the parameters corresponding to the non-selected programs from the corresponding programmed value exceeds a predetermined level, the progress of the program is suspended, and when the deviation of the measured value of the parameter corresponding to the selected program becomes equal to a predetermined level, control of the supply quantity of the solution is resumed on the basis of the selected program.

11. A method for controlling a vacuum pan as claimed in claim 9, wherein the average diameter of the crystals grown in the solution is used as said other parameter.

12. A method for controlling a vacuum pan as claimed in claim 9, wherein the level of the solution is used as said other parameter.

13. In a process for controlling a vacuum pan in which a raw solution is heated to cause said raw solution to reach a supersaturated condition, crystal seeds as nuclei are added to the raw solution to form massecuite, and the massecuite is furthher heated and additional raw solution or water or both are supplied to the massecuite to thereby generate and grow crystals in the massecuite, the improvement comprising:

1. controlling the supply of raw solution to the vacuum pan with reference to the deviation between the signal produced by a level control programmer and the detected level of the massecuite in the vacuum pan,

2. after a predetermined time period or when the level of the solution in the vacuum pan reaches a predetermined level after addition of the seeds, preferentially controlling the supply of raw solution and the supply of water to the vacuum pan with respect to the deviation between the detected value of the supersaturation of the massecuite in the vacuum pan and a signal from a consistency control program, the preferential control of step 2 being in preference to controlling the supply of raw solution by reference to the deviation between the detected value of the liquid level and the signal produced by the level control programmer, the preferential control of step 2 being automatically selected by means of a selector, and

3after another predetermined time period or when the level of the massecuite in the vacuum pan reaches another predetermined level after the addition of the seeds, preferentially controlling the supply of raw solution and the supply of water to the vacuum pan by reference to the deviation between the detected value of the consistency of the massecuite and a signal from the consistency control programmer, the preferential control of step (3) being in preference to control by reference to the deviation of the detected value of the liquid level of the massecuite and the signal from the level control programmer, the preferential control of step (3 being automatically selected by means of a selector.

14. The process according to claim 13, further comprising stopping the progress of the programs in the liquid level controller and the consistency controller when the detected level of the massecuite in the vacuum pan is different from the programmed value of the liquid level by a predetermined value, allowing heat exchange to be carried out while controlling the supply of raw solution and water with respect to the signals produced by said liquid level control programmer and said consistency control programmer at the time of the stopping of said programs, and resuming the progress of the programs in the level control programmer and the consistency control programmer when the level of the massecuite reaches a predetermined level.

15. In a process for controlling a vacuum pan in which a raw solution is heated to cause said raw solution to reach a supersaturated condition, crystal seeds as nuclei are added to the raw solution to form massecuite, and the massecuite is further heated and additional raw solution or water or both are supplied to the massecuite to thereby generate and grow crystals in the massecuite, the improvement comprising:

1. controlling the supply of raw solution to the vacuum pan with reference to the deviation between the signal produced by a level control programmer and the detected level of the massecuite in the vacuum pan, and

2. after the addition of the seeds, automatically selecting by means of a selector the deviation between the detected value of the supersaturation of the massecuite in the vacuum pan and a signal from a consistency control programmer in preference to the deviation between the signal from the level control programmer and the detected level of the massecuite in the vacuum pan for controlling the quantity of raw solution supplied to the vacuum pan, said consistency control programmer further controlling the quantity of warm water supplied to the vacuum pan.

16. The process according to claim 15, further comprising stopping the progress of the programs in the liquid level controller and the consistency controller when the detected level of the massecuite in the vacuum pan is different from the programmed value of the liquid level by a predetermined value, allowing heat exchange to be carried out while controlling the supply of raw solution and water with respect to the signals produced by said liquid level control programmer and said consistency control programmer at the time of the stopping of said programs, and resuming the progress of the programs in the level control programmer and the consistency control programmer when the level of the massecuite reaches a predetermined level.

17. In a process for controlling a vacuum pan in which a raw solution is heated to cause said raw solution to reach a supersaturated condition, crystal seeds as nuclei are added to the raw solution to form massecuite, and the massecuite is further heated and additional raw solution or water or both are supplied to the massecuite to thereby generate and grow crystals in the massecuite, the improvement comprising:

1. controlling the supply of raw solution to the vacuum pan with reference to the deviation between the signal produced by a level control programmer and the detected level of the massecuite in the vacuum pan, and

2. after the addition of the seeds, automatically selecting by means of a selector the deviation between the detected value of the consistency of the massecuite in the vacuum pan and a signal from a consistency control programmer in preference to the deviation between the signal from the level control programmer and the detected level of the massecuite in the vacuum pan for controlling the quantity of raw solution supplied to the vacuum pan, said consistency control programmer further controlling the quantity of warm water supplied to the vacuum pan.

18. The process according to claim 17, further comprising stopping the progress of the programs in the liquid level controller and the consistency controller when the detected level of the massecuite in the vacuum pan is different from the programmed value of the liquid level by a predetermined value, allowing heat exchange to be carried out while controlling the supply of raw solution and water with respect to the signals produced by said liquid level control programmer and said consistency control programmer at the time of the stopping of said programs, and resuming the progress of the programs in the level control programmer and the consistency control programmer when the level of the massecuite reaches a predetermined level.

19. In a method for controlling a vacuum pan in which heat is exchanged between a heat exchanger and a raw solution to generate and grow crystals therein, the improvement comprising measuring the value of a first parameter indicative of the progress of the crystal growing process, comparing the measured value of the first parameter with a programmed value for the first parameter provided by a first program, measuring the value of at least one other parameter indicative of the progress of the crystal growing process, comparing the measured value of said other parameter with a programmed value for the other parameter provided by a second program, automatically preferentially selecting either said first program or said second program for controlling the supply of the solution fed to said vacuum pan, and controlling the supply of solution fed to said vacuum pan on the basis of the deviation of the measured value from the programmed value of the parameter corresponding to the selected program.

20. The process according to claim 19, wherein when the deveation of the measured value of at least one of the parameters corresponding to the non-selected programs from the corresponding programmed value exceeds a predetermined level, the progress of the programs is suspended; and when the deviation of the measured value of the parameter corresponding to the selected program becomes equal to a predetermined level, the control of the supply quantity of the solution is resumed on the basis of the selected program.

21. A method for controlling the liquid level, the amount of solution feed and the amount of solvent feed in a batch process for generating and growing crystals in a vacuum pan comprising measuring the liquid level of the solution in said vacuum pan; comparing the measured liquid level with a programmed value for the liquid level provided by a level control programmer; measuring the value of at least one other parameter indicative of the progress of the crystal growing process, comparing the measured value of said other parameter with a programmed value for said other parameter provided by a second program, automatically preferentially selecting said second program for controlling the amount of solution and solvent fed to said vacuum pan after a predetermined time period or when the liquid level in the vacuum pan reaches a predetermined value, and controlling the amount of solution and the amount of solvent fed to said vacuum pan on the basis of the deviation of the measured value from the programmed value of said other parameter.

22. The method according to claim 21, wherein said other parameter is the consistency of the solution.

23. The method according to claim 21, wherein said other parameter is the degree of supersaturation when the crystal growing process is in an unstable phase, and further wherein said other parameter is the consistency of the solution when the crystal growing process is in a stable phase.

24. The method according to claim 21 wherein said other parameter is the supersaturation of said solution.

25. The process according to claim 24, further comprising determining the value of the supersaturation of said solution by comparing the boiling point of said solution with the temperature in said vacuum pan.

26. The method according to claim 25, wherein the pressure in said vacuum pan is measured and the value of the temperature in said vacuum pan is determined by means of a pressure/temperature transducer.