Method and system for use in controling a drying process

Technical field

The present invention relates to a method and system for use in controlling a drying process, wherein a bulk material is or will be subjected to the drying process.

Background of the invention

Drying processes are used in several industries. In the pharmaceutical industry, solid oral dosage forms, such as tablets and capsules, are often prepared by mixing different components, such as drug substances, excipients, binders, liquids etc., in a wet-mixing or granulation operation, subsequently drying the mixture in a drying operation such as fluidized bed drying, and a final operation such as tablet compression or capsule filling that produces the actual tablets or capsules.

In the drying operation, the moisture content (percentage of moisture) of the mixture should be reduced to a predetermined value. In the pharmaceutical industry, this value is selected on the basis of e.g. stability studies. There are different techniques for determining the moisture content, some of which have drawbacks in that they do not always account for all the moisture in the material. However, since the objective of the drying process is to reduce the moisture content of the material to a value determined using a specific analysis technique, it is clear that the term moisture content is to be interpreted as the moisture content determined by the technique in question.

For drying in a fluidized bed, the parameters of the drying process, such as the duration of the operation as well as the temperature, pressure, humidity, and flow rate of the drying medium, are often determined empirically or based upon previous experience. For example, after a predetermined period of time or when a state variable such as the temperature of the granulate reaches a predetermined value, the moisture content is checked using a suitable technique such as Loss On Drying (LOD), and if the material is hot dry enough, the drying process is continued.

Recently, various sensors, based primarily upon optical techniques, have been introduced in the pharmaceutical industry. Provided the sensor has been calibrated, such a

sensor makes it possible to determine indirectly the moisture content of the granulate without the need to perform an LOD analysis on a sample. The sensor is typically calibrated using LOD data obtained from several batches, and the calibrated sensor may then be used to determine indirectly the moisture content of subsequent batches. For example, the moisture content of the granulate can be determined indirectly from a near- infrared (NIR) absorbance spectrum.

The calibrated sensor thus makes it possible to control the drying process based upon the indirectly determined moisture content. For example, the drying process may be continued until the indirectly determined moisture content reaches a predetermined value. However, since the measurement and the calibration are carried out on different batches, batch-to-batch variability in the amounts and chemical and physical properties of the raw materials may cause the indirectly determined moisture content of a batch to differ from the moisture content determined using LOD. Furthermore, manufacturing separate batches for sensor calibration is time consuming and possibly also costly since expensive raw materials for these batches may be used up in the calibration process.

It is therefore of interest to develop a method of controlling the drying process that corrects for batch-to-batch variability in the amounts and chemical and physical properties of the raw materials to obtain a product in which the moisture content may be controlled with greater degree of accuracy. It is also of interest to develop a method of controlling the drying of different drug formulations without any need for manufacturing separate batches for calibration purposes.

Summary of the invention

An object of the invention is to provide a time efficient system and method of controlling a drying process.

Another object of the invention is to provide a system and a method of controlling a drying process, which correct for batch-to-batch variability in order to control the drying process with a greater degree of accuracy than with current techniques.

Yet another object of the invention is to provide a method and a system for use in controlling a drying process, which do not require separate batches for calibration purposes.

These and other objects, which will become apparent in the following, are achieved by means of a method and a system as defined in the appended claims.

The invention is based on the insight that, in practice, a small amount of material (sample material) can be dried in a shorter period of time than a large amount of material (bulk material), and that information obtained from drying the sample material can be used for controlling the drying process of the bulk material. The invention is also based on the insight that by taking a sample from the bulk material which is or will be subjected to the drying process, the sample, for a given moisture content, can be assumed to have substantially the same chemical composition as well as chemical and physical properties such as particle size and shape, porosity, crystallinity etc. as the bulk material.

Therefore, in accordance with one aspect of the invention, a method of controlling a drying process is provided. The method comprises:

- providing bulk material which is or will be subjected to the drying process,

- taking a sample from the provided bulk material,

- measuring simultaneously a first and a second property of the sample while drying the sample, the second property being the weight of the sample, wherein a change in weight is related to a change in moisture content of the sample,

- establishing, based on the measurements of the first and second properties of the sample, a relationship between the first property and the moisture content of the sample,

- subjecting the provided bulk material to the drying process,

- measuring on the bulk material during the drying process the same first property that is or was measured on the sample, and

- controlling the drying process of the bulk material based on the measurement of the first property of the bulk material and on the established relationship between the first property and the moisture content of the sample.

Contrary to the known methods of using calibrated sensors, the present invention enables sensor calibration to be carried out in real time as drying of the bulk material

proceeds and does therefore not require special batches to be processed for calibration purposes. The sample measurements are used for subsequently controlling the drying process of the bulk material.

The present invention, by means of its real time calibration, corrects for batch-to-batch variability to increase the accuracy with which the drying process can be controlled. Furthermore, the invention allows consecutive use of the same drying equipment for different drug formulations, without any need for manufacturing separate batches for calibration purposes.

The invention can be used independently of other means of calibration, and may be relied on for controlling a drying process. Alternatively, the invention may be used as a complement or as a means of fine-tuning a pre-existing calibration. For instance, traditional sensors and methods, which can be used to obtain one measure of the moisture content of the bulk material, can be supplemented by the method according to the present invention in order to reduce the effects of batch-to-batch variability on the moisture content of the product at the end of the drying operation.

According to at least one embodiment of the invention, the first property is absorption of radiation, such as absorption of radiation at a specific wavelength or absorption of radiation at a number of different wavelengths that are suitably combined and/or weighted, wherein measuring the first property comprises irradiating the sample and bulk material, respectively, and detecting the emitted radiation therefrom. Thus, the absorption of radiation of the sample can be related to the moisture content of the sample.

The relationship between the moisture content of the sample, Xs ₃ and the first property of the sample, P _1S , may be expressed using a mathematical function (possibly nonlinear), Fs,

In general, the relationship between the moisture content of the bulk material, X _B , and the first property of the bulk material, P _1B , may be expressed using another mathematical function (possibly non-linear), F _B ,

XB-FB(P IB)

Since, for a given moisture content, the chemical composition of the sample may be assumed to be identical to the chemical composition of the bulk material, and since the sample may be assumed to have the same chemical and physical properties as the bulk material, the mathematical function can be assumed to be the same for the sample as for the bulk material so that F _S =F _B and P _1S =Pi _B - The moisture content of the bulk material may thus be determined using the known relationship between the moisture content of the sample and the first property of the sample.

The radiation may, for instance, be microwave radiation, terahertz radiation, far infrared radiation, mid infrared radiation, near infrared (NIR) radiation, visible radiation, or ultraviolet radiation.

It should be noted that choosing absorption of radiation as said first property is just one alternative. Another alternative is to have the dielectric constant as said first property. For instance, a probe is inserted into the material to be measured upon and an electromagnetic pulse is sent along the probe. The time taken for the pulse to travel down the probe and be reflected back from its end is measured. The propagation velocity depends on the dielectric constant of the material in contact with the probe. As the moisture content of the material changes, the dielectric constant also changes and thus also the velocity of the pulse.

A continuous or repeated monitoring of the first property, and thereby indirectly the moisture content, of the bulk material is enabled, whereby the drying process can be terminated when the moisture content, determined indirectly, reaches a predetermined value.

The measurements of the first property and the second property, i.e. the weight, may be performed continuously or at discrete points in time while drying the sample. In the case of measurements at discrete points in time, the weight measurement should be performed at substantially the same discrete points in time as the measurements of the first property. If the weight measurements are not performed at substantially the same points in time as

the measurements of the first property, the value of the first property at points in time when the weight is measured may be calculated via interpolation or possibly extrapolation. Similarly, if the measurements of the first property are not performed at substantially the same points in time as the weight measurements, the value of the weight at points in time when the first property is measured may be calculated via interpolation or possibly extrapolation.

Thus, the weight or the moisture content may be related to the measurement of the first property. Since, in the case of measurement at discrete points in time, the first property of the sample was only measured for a number of discrete values of the moisture content, the aforementioned functions Fs and F _B can be constructed, for example via interpolation or extrapolation, so as to yield a value of the moisture content also for values of the first property that were not included in the measurement of the sample.

The change in moisture content is suitably detected by means of Loss On Drying (LOD) technique. In the pharmaceutical industry, the LOD technique may be used to determine the moisture content of a moist sample on a wet- weight basis (weight of moisture in sample divided by total weight of wet sample) by the use of a moisture balance, which has a heat source for heating and a scale. A sample is placed on the balance and made to dry until there are no further changes in its weight or made to dry for a specified period of time that has suitably been selected so that there is or will be no further change in its weight. It is assumed that there are no other volatile materials present, and the amount of moisture lost by evaporation may therefore be calculated based upon the readings from the scale.

According to at least one embodiment of the invention, the sample is dried until its weight is constant. When the weight of the sample remains unchanged, it is considered to represent a zero-level and any possible liquid or non-volatile component which is trapped in the sample is not regarded as moisture. Having established the zero-level, it is now possible, for a given point in time of the previously performed drying of the sample, to determine the moisture content of the sample (e.g. expressed as percentage of total or dry weight). Thereby, the relationship between the first property and the moisture content of the sample can be established.

According to at least another embodiment of the invention, if the initial moisture content of the sample is known before initiating the drying thereof, a direct relationship between the first property and the moisture content can be established as the drying proceeds. In this embodiment, the sample does not have to be dried until there are no further changes in its weight.

As an alternative to LOD, other gravimetric techniques may be used. One example is Dynamic Vapor Sorption (DVS). In DVS, the sample is provided in a chamber through which a gas with a known temperature, pressure, and humidity flows. The humidity of the gas can be varied. For instance, the gas may initially be provided with a humidity of 10% and the humidity then gradually reduced. The moisture content of the sample will be reduced as the humidity of the gas is reduced. As the sample is dried, the weight and said first property of the sample are measured.

A further alternative would be to use a combination of LOD and DVS, wherein heating elements are used in combination with a flowing gas to dry the sample. LOD and DVS may also be used in a combination where DVS first is used to dry the sample to a certain degree and LOD then is used to determine the amount of remaining moisture. With this latter, sequential combination, it is not necessary to employ DVS to obtain a completely dry sample. The sequential combination is advantageous because, using the DVS technique, it is easier to obtain approximately the same process conditions in the sample dryer as in the bulk dryer. LOD and DVS may also be used in a combination where LOD first is used to dry a part of the sample to establish an initial moisture content for a subsequent DVS analysis on the remainder of the sample.

The method according to the present invention may be used for end point control of the drying process, i.e. the point in time when the drying process should be terminated. According to at least one embodiment of the invention, the drying process of the bulk material is terminated when the moisture content reaches or passes a predetermined value. The point in time when the moisture content of the bulk material reaches this value is determined using a measurement of the first property of the bulk material and the relationship between the first property of the sample and the moisture content of the sample.

According to at least one embodiment of the invention, end point control comprises

- selecting a desired value of the moisture content of the bulk material,

- determining the current value of the moisture content of the bulk material based on the measurement of the first property of the bulk material and on the established relationship between the first property and the moisture content of the sample, and

- terminating the drying process of the bulk material when the determined value of the moisture content is equal to or lower than said desired value.

According to at least one embodiment of the invention, end point control comprises

- selecting a desired moisture content of the bulk material, - determining, based on the established relationship between the first property and the moisture content of the sample; a value of the first property of the bulk material that corresponds to said desired moisture content, and

- terminating the drying process of the bulk material when said determined value is reached for the first property of the bulk material. According to at least one embodiment of the invention, end point control comprises

- identifying a reference weight of the sample, which is related to a desired moisture content,

- determining which value of the measured first property of the sample was or would have been (e.g. by interpolation) obtained simultaneously with said reference weight of the sample, and

- terminating the drying process of the bulk material when said determined value is reached for the first property of the bulk material.

The present invention is not only useful in end-point control, but also for other control objectives. According to at least one embodiment of the invention, the act of controlling the drying process of the bulk material comprises controlling heating elements in a drying vessel or the temperature, pressure, humidity, or flow rate of the gas flowing into the drying vessel containing the bulk material, or the rate of flow or chemical composition of the bulk material flowing into the drying vessel or into the manufacturing process stream upstream of the drying vessel based on the measurement of the first property of the bulk material and on the established relationship between the first property and the moisture

content of the sample. The drying vessel may e.g. be an oven, a microwave dryer or a convection dryer such as a fluidized bed dryer or a tumble dryer. For example, for a fiuidized bed dryer, it may be desirable to decrease the gas flow rate as the bulk material dries so as to avoid material carry-over. In other cases, it may be desirable to decrease the temperature of the air flowing into the vessel as the moisture content of the bulk material decreases in order to control the temperature of the bulk material.

According to at least one embodiment of the invention, said measurements of the first property and the weight of the. sample are performed simultaneously while subjecting the bulk material to its drying process. Thus, while the bulk material is subjected to the drying process in a drying vessel, a sample may be removed and measured upon to establish a relationship between the moisture content of the sample and the first property of the sample, as previously explained. Another possibility is to remove a sample from the bulk material before the bulk material is subjected to the drying process, and to dry the bulk material while the sample is measured upon. Apart from drying the bulk material while performing the measurements on the sample, the first property of the bulk material may also be measured simultaneously with measuring the first property and the weight of the sample. The relationship between the weight and the first property of the sample may be established as these measurements progress, which may therefore provide useful information also of the progress of the drying process of the bulk material. Since the sample is of smaller volume and mass than the bulk material, in practice, the sample will dry in a shorter period of time. Thus, the previously described strategy for end point control may still be used even though the sample and bulk material are simultaneously subjected to their respective drying processes.

Alternatively to the previously described embodiment, the sample may be measured upon before subjecting the bulk material to the drying process. However, since the sample should be an adequate representation of the bulk material, and since the bulk material may, as time passes, change its chemical and physical properties so that they become different from the chemical and physical properties of the sample material, it is advisable to have a small time gap between the drying of the sample and the drying of the bulk material.

The bulk material may either be provided as a batch or in the form of a continuous flow of material. If the bulk material is provided as a batch, it is generally sufficient to establish the relationship between the weight and the first property of the sample once. If the bulk material is provided in the form of a continuous flow, it is advisable to repeat the measurements and the establishment of the relationship between the first property and the moisture content at regular intervals in case the chemical composition, except the moisture content, or the chemical and physical properties of the bulk material have changed.

According to a second aspect of the invention, a system is provided for use in controlling a process of drying bulk material in a drying vessel. The system comprises: a sample dryer for drying a sample taken from the bulk material, means for measuring a first property of the sample while the sample is dried in the sample dryer and of the bulk material while the bulk material is dried in the drying vessel, means for measuring a second property of the sample while the sample is dried in the sample dryer, the second property being the weight of the sample, wherein a change in weight is related to a change in moisture content of the sample, and a control unit operatively connected to said means for measuring the first and the second property, wherein, based on input from said means for measuring the first and the second property, the control unit is arranged to control or provide information related to the progress of the drying process of the bulk material.

According to at least one embodiment, the control unit is adapted to use said input in order to establish a relationship between the first property and the moisture content of the sample.

According to at least one embodiment, the control unit may be operatively connected to the drying vessel, whereby the control unit may perform direct control, such as end- point control or control of other parameters, such as the temperature or the flow rate of the gas flowing into the drying vessel. According to at least another embodiment, the control unit may provide a visual or audio signal according to which operating personnel may take appropriate action to control the drying process.

The sample dryer may suitably be a device which holds the sample and which, optionally, can be put under conditions reflecting those present in the drying vessel when the bulk material is dried. The accuracy with which the moisture content of the material may be determined from a measurement of the first property may be affected by the consistency between the conditions used for drying the sample and those used for drying the bulk material. The degree to which this accuracy depends on the consistency between the conditions used for drying the sample and those used for drying the bulk material may depend on the specific choice of the first property.

If the conditions in the sample drier, such as temperature, pressure, humidity etc., are substantially different from the conditions in the bulk drying vessel, it may be suitable to adjust the measurement results by compensating for the differences in the conditions. For example, the first property P ₁ at the temperature of the sample material Ts ₁ P _1TSJ may be calculated from the first property at the temperature of the bulk material T _B , P _ITB , using a mathematical function (possibly non-linear), G,

so that the established relationship between the moisture content of the sample material and the first property of the sample material may still be used even though the temperature of the sample material is different from the temperature of the bulk material. Thus, the moisture content of the bulk material may be calculated using

XB=FS(P ₁ TS)=FS(G(P ₁ TB))

In the simplest case, the difference in the operating conditions does not affect the measurements results, in which case SO that PIT _S =P _ITB -

According to at least one embodiment, said means for measuring the first property comprises a unit for measuring the first property of the sample in the sample dryer and another unit for measuring the corresponding first property of the bulk material in the drying vessel, wherein the control unit is operatively connected to and receives input from

both units. According to at least one alternative embodiment, said means for measuring the first property comprises a unit which is movable between the sample dryer and the drying vessel for measuring the first property of both the sample and the bulk material.

According to at least one embodiment, said unit (or units) comprises a radiation source for irradiating the sample and bulk material, respectively, a detector for detecting the emitted radiation from the sample and bulk material, respectively, and means for outputting information related to the detected radiation to the control unit. The radiation used may be selected from the group consisting of microwave radiation, terahertz radiation, far infrared radiation, mid infrared radiation, near infrared radiation, visible radiation, and ultraviolet radiation.

According to at least one embodiment, the means for measuring the second property comprises a scale for measuring the weight of the sample and means for outputting information related to the measured weight to the control unit.

It should be noted that the second aspect of the invention encompasses any embodiments or any features described in connection with the previously described aspect of the invention, as long as they are compatible with the system of the second aspect.

Brief description of the drawings

Fig. 1 illustrates schematically parts of a pharmaceutical processing system in which at least one embodiment of the present invention has been implemented.

Detailed description of the drawings

In Fig. 1, one of the shown parts of a pharmaceutical processing system 10 is a granulation vessel 12 in which a drug substance is mixed with a filler (excipient) and a binding substance, such as water or other liquid. Another part is a drying vessel 14 in which the mixed pharmaceutical material is dried to obtain a desired liquid content. The drying medium, such as dry air, is arranged to flow in through the bottom portion of the drying vessel 14, to absorb moisture from the bulk material to be dried, and to flow out at the upper portion of the drying vessel 14 (illustrated by arrows below and above the drying vessel 14).

In this application, the.material which is processed to become a final product, has been referred to as "bulk material" in order to avoid confusing it with the "sample" which is primarily used as part of the inventive method of control and generally not intended to be processed into a final product. A connecting part in the form of a pipe 16 allows the mixed material in the granulation vessel 12 to be transferred to the drying vessel 14. The granulation vessel 12 has one or more inlets (not shown) for receiving the material to be mixed, and has also an outlet 18 from which the mixed material may enter into the pipe 16. Similarly, the drying vessel 14 has an inlet 20 connected to the pipe 16 for receiving the mixed materials, and possibly one or more outlets (not shown) for outputting the sufficiently dried materials for further processing. The outlet 18 of the granulation vessel 12 is arranged on a vertically higher level than the inlet 20 of the drying vessel 14, thereby allowing the gravity to act on the mixed materials for transporting it through the inclined pipe 16. However, means for promoting transport through the pipe 16 other than gravity may be provided. A first valve 22 is provided at the outlet 18 of the granulation vessel 12 and a second valve 24 is provided at the inlet 20 of the drying vessel 14. The valves 22, 24 control the material flow. For instance, before the materials in the granulation vessel 12 have been mixed to a desired degree, at least the first valve 22 is closed so as to prevent material from leaving the granulation vessel 12. A third valve 26 is provided on the pipe 16 for bleeding off a small portion of the mixed material through a conduit 28 to a sample dryer 29 in which the sample will be dried. The sample dryer 29 contains a sample holder 30 for holding the sample while it is dried. A device is arranged to measure a first property of the sample. In the figure this is exemplified as a first probe 32, such as a NIR probe, which projects through the wall of the sample dryer 29 in order to measure the absorption of radiation of the sample. Another device is arranged to measure the weight of the sample. In the figure this is exemplified as a-scale-34 upon- which- the sample-holder 30 Tests in order-to-weigh the-sample: Also, a heating element 35 whose temperature may be controlled by the control unit 40 is arranged below the sample holder 30 to heat the sample and promote drying thereof. Signals are transmitted from the first probe 32 and the scale 34, via wires 36, to a control unit 40

which establishes the relationship between the absorption of radiation of the sample and the moisture content of the sample. It should be noted that even though the figure illustrates the first probe 32 and the scale 34 as two separate units, as an alternative, they could be incorporated in one unit. Said first property is also measured on the bulk material, which in Fig. 1 is illustrated with a second probe 42 which is of the same or corresponding type as the first probe 32. The second probe 42 is mounted to a window or through the wall of the drying vessel 14 in order to measure the absorption of radiation of the bulk material being dried inside the drying vessel 14. Signals from the second probe 42 are transmitted, via a wire 44, to the control unit 40.

In. the following, a non-limiting example of the operation of the shown set-up will be discussed. As the bulk material is transferred from the granulation vessel 12 to the drying vessel 14 through the connecting pipe 16, the third valve 26 is opened temporarily for a short period of time in order to bleed off a small sample of the bulk material. The sample is led through the conduit 28 to the sample holder 30. The opening and closing of the third valve 26 may be operated manually. Alternatively, an actuator operatively connected to the control unit 40 may control the opening and closing of the third valve 26, just like the control unit 40 may be involved in opening the other valves 22, 24. As an alternative to using the third valve 26 to bleed off material, the sample may be taken directly from the drying vessel 14 or from the granulation vessel 12 by means of a sample collector (manual or controlled by the control unit 40) and then be transferred to the sample holder 30. Instead of having a connecting pipe 16, another alternative would be to use a carrier unit, such as a container provided with wheels, in order to move the material from the granulation vessel to the drying vessel, wherein the sample may be taken directly from the carrier device. Instead of having a granulation vessel 12 and a drying vessel 14, it is possible to have a single vessel which is adapted for carrying out both a granulation process and a subsequent drying process, in which case the sample may be taken directly from that single vessel at any suitable time.

When the bulk material has been transferred to the drying vessel 14, the drying process is initiated. While the bulk material is subjected to this drying process, the sample

is dried in the sample holder 30. Since the sample is of a considerably smaller amount and volume than the bulk material, the sample will dry in a shorter period of time than the bulk material. The heating element 35 is switched on to increase the temperature in the sample dryer 29. The heating element 35 may suitably be controlled by the control unit 40 by means of a wire 37. While drying the sample, a first property, in this example the absorption of NIR radiation, is measured by means of the first probe 32. Also, a second property, namely the weight of the sample, is measured by means of the scale 34. As the sample becomes drier and drier due to evaporation of moisture, the weight of the sample will be reduced. The sample is continued to be dried and measured upon until there are no further changes in its weight. The sample is then considered to have zero moisture content. Signals relating to the weight measurement and the absorption measurement are sent via the wires 36 to the control unit 40. The control unit 40 may also be adapted to control the operation of the first probe 32 and scale 34, e.g. to start/stop, to change the sampling frequency, etc.. The control unit 40 may suitably comprise a computer or a microprocessor. The control unit 40 establishes the relationship between the absorption of radiation of the sample and the moisture content of the sample. Since the first probe 32 and the scale 34 will provide signals to the control unit 40 over a certain period of time, a clock or timing device inside or connected to the control unit 40 may be useful. Such a timing device could match those signal which are associated with measurements made at the same point in time by the first probe 32 and the scale 34, respectively. Thereby, for one or a plurality of measurements from the first probe 32 there will be corresponding measurements from the scale 34.

The control unit 40 also receives signals from the second probe 42 relating to the absorption of radiation of the bulk material, the information of which is interpreted by the control unit 40 in view of the relationship between the absorption of radiation and the moisture content of the sample, whereby the moisture content of the bulk material in the drying vessel 14 may be determined. According to how the control unit 40 is programmed, it may control the drying process appropriately. This is shown in the figure in the form of a wire 46 between the control unit 40 and the drying vessel 14. The control unit 40 may control e.g. the temperature, humidity, pressure, and/or the flow rate of gas into the drying

vessel 14. For end-point control, e.g. if the bulk material should be dried to a desired moisture content, the control unit 40 may be programmed to stop the drying process when the absorption of radiation of the bulk material (measured by the second probe 42) corresponds to the desired moisture content (based upon the signals from the first probe 32 and the scale 34). As an alternative to direct control, the control unit 40 may provide a visual or audio signal, wherein the operating personnel may take appropriate action to stop the drying process. Likewise, the personnel may monitor the progress of the drying process by interpreting output data, such as moisture content of the bulk material, from the control unit 40, and then manually perform appropriate control operations, e.g. change the temperature or the flow rate of the air entering the dryer.

The control unit 40 is herein illustrated as having wires 36, 37, 44, 46 connected to the different components. However, the control unit 40 may also be operatively connected to said components by other means, such as radio control. Another alternative, would be for one or more memory means to be connected to the first probe 32 and/or the scale 34, wherein, after the sample measurements are completed, the memory means is/are moved and docked to a docking station of the control unit 40.

As an alternative to using the first probe 32 and the second probe 42 as illustrated in Fig. 1, it is conceivable to use only one probe. In such case, the probe is first used in the sample holder 30 during the drying of the sample. Then, when the sample has been dried, the probe is moved to the drying vessel 14 in which the bulk material is still subjected to drying.

As an alternative to using a heating element 35 underneath the sample holder 30 as illustrated in Fig. 1, a heat radiation source such as an infrared lamp may be used to heat the sample. In order to prevent the first probe 32 from being saturated by the radiation from the infrared lamp, the infrared lamp may temporarily and briefly be switched off at points in time while the first probe 32 collects spectra from the sample, without substantially interfering with the drying process.

It should be noted that the term "moisture" is not limited to water, but can be other liquids as well, or a combination thereof. For instance, in pharmaceutical manufacturing, some materials may during a processing step be mixed with ethanol.

It should also be noted that even though the description has been focused on pharmaceutical manufacturing the invention is not limited to this. On the contrary, the invention is applicable to other kinds of industry in which bulk material is to be subjected to a drying process. Furthermore, the invention is not limited to be used solely in the exemplified pieces of equipment. For instance, instead of controlling a drying process in a fluidized bed vessel, the invention may be used to control the drying process in an oven or furnace, or other drying equipment.

From what has been described above, it should now be clear that the present invention, in a general sense, allows measurement to be made on a sample taken from a bulk material hi order to predict the moisture content of the bulk material subjected to a drying process.