Optical window for monitoring samples

Optical window for monitoring samples

NDC Infrared Engineering Ltd.

C.152424

Technical Field

The present invention relates to optical sampling windows in apparatus for processing materials, e.g. dryers and granulators, and especially for use in monitoring material being processed using optical measuring techniques, for example infrared spectroscopy. As used in the present specification and claims, 10 the terms “optical” and “radiation” are intended to refer to non-visible parts of the spectrum, for example infrared radiation, in addition to visible radiation.

Background Art

Pharmaceuticals and other materials are often dried in fluid bed dryers and it is 15 known to monitor the progress of the drying using an infrared spectrometer (commonly referred to as a gauge). The gauge projects a beam of infrared radiation through a window in the wall of the dryer and measures the light reflected by the material being dried. The radiation directed into the dryer includes (a) a wavelength that is absorbed by water and (b) a wavelength that is 20 not absorbed by water or the material. The intensity of radiation reflected back by the material being dried in the wavelength that is absorbed by water will be reduced as a result of absorption by water m the material in comparison to the intensity of the reflected radiation in wavelengths that are not absorbed. The intensity of light at that wavelength increases as the amount of water in the 25 material decreases. By comparing the intensity of the radiation at the absorbed wavelength with that at non-absorbed wavelengths, it is possible to measure the amount of water in the pharmaceutical and assess when it is dry. The window should have a high transmission for radiation in all the wavelengths being used for measurement.

Suitable infrared gauges for such an application are well known and are manufactured by the Applicant Company. An example of such a gauge is

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ז described in published PCT application no. W098/22806. However, the precise details of the gauge do not form part of the present invention.

WOOl/25153 describes a radiation sensor mounted in a cylindrical body for sensing radiation from a source. The sensor is mounted in a channel with the axis of the cylindrical body lying at right angles to the axis of the channel. The cylindrical body can be rotated through 180’ to a position facing away from the radiation source so that the sensor can be cleaned.

JP-11-288870 describes a system for cleaning lenses. Two sets of lenses are provided on a rotatable housing. When one set of lenses is in use, the other is located at a cleaning station. The housing can be rotated to bring the lenses that were previously in use into the cleaning station and vice versa for the other set of lenses. Such an arrangement doubles the cost of the lenses.

GB-1567031 describes a sensor for sensing the turbidity of a liquid and includes a light source and a light sensor. The light source and light sensor can be moved into a retracted position past wiper blades to clean any deposits on the light transmitting housing containing the sensor and/or light source.

GB-2332756 describes a camera attachment for location in front of the camera lens. It consists of a disc that is rotated at high speed to remove any moisture that may gather on it.

WO89/09388 describes a near infrared analysis device of material held in a test chamber, which has an opening through which infrared radiation can pass. A ribbon of infrared transparent material is pressed against the opening to form a window. Between successive samples, the ribbon is advanced so that a fresh piece of ribbon forms the window in each successive test.

US-5,161,055 describes a rotating inspection window that spins at such a speed that water or other debris falling on the window is slung off by centrifugal force.

US-4,260,262 describes a grain quality analyser in which the grain is successively exposed to radiation of different wavelengths, including infrared.

CH-591078 describes a device for measuring the purity of a liquid flowing through a pipe. Part of the pipe is replaced by a light-transmitting sleeve that enables the liquid flowing through the sleeve to be analysed photometrically. Wiper blades are provided on the inside of the sleeve and the sleeve is rotated so that the blades wipe the inside of the sleeve clean.

JP-11118714 describes a system for measuring turbidity having a light source and a light sensor. A disc is rotated between the source and the sensor.

JP-1100272 describes a vapour deposition apparatus having an energy source that decomposes reaction gas. The radiation source is located within a cylindrical transparent window that is rotated so that part of its circumference is located in a vapour deposition chamber and part in a cleaning chamber.

JP-8278248 describes a powder treatment apparatus having a light transmitting window in the form of a rotatable cylinder that can be rotated past a wiper to clean the drum surface.

US-5324949 discloses an apparatus for measuring samples in a test chamber by means of near infrared radiation. The chamber has a window that is covered by a film through which the radiation can both enter and leave the chamber. In order to prevent material from one sample that has been deposited on the window from contaminating the optical signal for the next sample, the film covering the window is part of a strip and the strip is advanced between sample measurements so that a fresh strip part covers the window for each sample.

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US4622465 describes an arrangement for detecting the turbidity of a liquid flowing in a pipe by placing a sensing element in the pipe. The sensing element has a passageway through which the liquid flows than has a pair of opposed windows through which radiation can be passed from a radiation source on one side of the pipe to a sensor on the other side of the pipe. The attenuation in the radiation provides a measure of the turbidity of the liquid in pipe.

One severe disadvantage that limits the use of infrared gauges in the monitoring of materials being dried in fluid bed and other dryers is that the material can be sticky, particularly when moist. It can therefore adhere to the window and prevent radiation from passing into the interior of the dryer. Since, in these circumstances, it is not possible to monitor the water content of the material being dried, the drying may have to be stopped in order to allow the window to be cleaned. The present invention is intended to overcome the above problem.

Disclosure of Invention

According to the present invention there is provided an optical window that can be secured in a wall of a material-processing apparatus to derive information about the stage of processing of the material in the apparatus, as defined in the accompanying claims.

The present invention also comprises a material processing apparatus, e.g. a granulator or a dryer such as a fluid bed dryer, that includes such an optical window and a method of processing material using the above window to monitor the progress of the processing.

The chamber is preferably a duct through which the cleaning fluid can be passed to clean the radiation-transmission element located adjacent to and in contact with the interior of the duct. The fluid will generally be a liquid solvent that will dissolve the material sticking to the radiation-transmitting element, i.e. the material being processed. It is conceivable that the element might be able to be cleaned by a gas, for example a blast of dry air. If a solvent is used, the duct

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WO 03/025550 is preferably also connectable to a source of gas to dry the element after cleaning and before it is returned to its normal (first) position transmitting radiation into and out of the apparatus.

In addition, the cleaning of the optical radiation-transmitting element may be brought about by a wiper member that passes over and wipes the element as it is moved between the first and the second positions. In extreme cases; the moveable body may be moved to expose the radiation-transmitting element to the outside thereby allowing it to be cleaned manually.

The material processing apparatus is preferably a dryer and the property being measured is preferably the content of a volatile material, e.g. water, in the material being dried.

The invention is applicable to the processing of pharmaceuticals and foodstuffs, e.g. milk products such as milk powders and dried cheese; it can be used to remove not only water but also other volatile materials, e;g. solvents.

Brief Description of the Drawings

A sampling window according to the present invention will now be described, by way of example only, with reference to the following drawings in which:

Figure 1 is a sectional view taken through the wall of a dryer and through an optical window of the present invention;

Figure 2 is a sectional view of the dryer wall and the window of the present invention taken along line Π-II shown in Figure 1; and

Figure 3 is a front elevation, of the optical window of Figures 1 and 2 viewed in the direction of arrow “A” in Figure 1 but without the infrared gauge 30.

Description of the best mode for implementing the invention

Referring initially to Figure 1, there is shown the wall 10 of a dryer. Materia] to be dried is located in a drying compartment 12 (only part of the compartment is

WO 03/025550 shown). The wall 10 has an opening 14 and an optical sampling window 15 according to the present invention extends across the opening. The window includes a socket 23 holding a sphere 24 containing a bore 26. The bore 26 is closed at one end by a radiation-transmitting element 28 which is located in contact with the dryer compartment 12, as shown in Figure 1. PTFE seals 25 close the gaps between the spherical holder 24 and the wall 10.

An infrared spectrometer (“gauge”) is indicated schematically by block 30 and is located on the outside of the dryer compartment 12 on the axis 32 of the bore 26; the gauge can transmit infrared radiation in the direction of arrow A through the bore 26 and through the optical element 28 into the interior compartment 12 of the dryer. Likewise, radiation reflected by material within the compartment 12 can pass back along the same path in the direction of arrow B to the gauge 30. The radiation should contain at least two wavelengths, which may be transmitted by the gauge simultaneously or sequentially, namely a wavelength that is substantially absorbed by water and a wavelength that is not substantially absorbed by water. In this way, as is known, the moisture of the material within the dryer can be monitored by the gauge by assessing the relative intensities of the reflected radiation at the two wavelengths. The drying process is stopped when the moisture content of the material has dropped to the desired level.

Materials within the compartment 12 can adhere to the outside of the radiationconductive element 28 and obscure the optical path of the gauge 30. In order to clean this adherent material off, the sphere 24 may be rotated so that the lip 34 of the seal 25 wipes over the optical element 28 and removes adhering material. Since the seal 25 is made of PTFE, the friction between the lip and the element is minimised while allowing close contact between the lip and the element to clean it. Other alternative arrangements for wiping the element can be used.

The above arrangement only removes large adhering particles but does not remove small particles and films. Accordingly, the sphere can be rotated

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'׳ though 52° by a shaft 38 (see Figure 1) to the position shown in outline 31 in Figure 2, in which the element 28’ is located in a chamber 40, which runs annually around the sphere. The seal 25 along with seal 27 isolate the chamber 40 from the inside of the drying chamber and the bore (respectively).

The chamber 40 is formed in two parts that are held together around the sphere 24 by a clamp 42, which can be released by the handle 56.

Water or other solvent for the material being dried in the dryer is passed into 10 the annular chamber 40 through a nipple 44 and passes out of the chamber through a second nipple 46 as shown by arrows C in Figure 2, cleaning the element as it passes through the chamber. After the solvent has passed, a drying gas, e.g. warm air, is passed through annular chamber 40 to dry the element 28. Once the cleaning has been accomplished the sphere 24 is rotated back to the 15 position shown in Figure 1 in which the bore is aligned with the optical axis 32.

The materials used for making the sphere 24 and the element 28 should be compatible with the material being dried and with any regulations governing the processing of such material. In the case of pharmaceuticals, the sphere 24 is 20 preferably made of stainless steel and the element 28 is preferably made of sapphire.

It will be appreciated that the holder for carrying the radiation-transmitting element may have a shape other than a sphere that will allow it to rotate within 25 a socket, for example it may be cylindrical. Alternatively the element may be provided on a wheel (not shown) that can be rotated or on a slide (not shown) that can be slid to bring the element 28 into contact with a cleaning chamber.

The solvent and air are supplied to and drained from the annular chamber 40 30 via lines (not shown) extending from the nipples 44, 46 to an outside solvent reservoir and discharge line.

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In the initial stages of drying the material in the dryer, when the material .is at its most sticky, the dryer may advantageously be operated with the sphere 24 in the position shown in outline 31 in Figure 2, since the amount of water present will be known to be high and so a reading from the gauge 30 is unnecessary; in 5 position 31, the material will self-evidently not adhere to the element 28. After a time in which the operator knows through experience that the material will be partially dry, he/she can rotate the sphere to the position shown in Figure 1 in which the element 28 is in contact with the drying compartment for monitoring the drying procedure in the final stages of drying, when the material will be 10 less sticky. Such a regime will reduce the number of times that the element 28 needs to be washed in the course of a drying cycle.