EARLY DETECTION OF SMOLDERING POWDERS IN POWDER DRYING SYSTEMS COMPRISING A CO GAS DETECTION SYSTEM

Abstract

The invention concerns a powder drying system (1) comprising a carbon monoxide (CO) gas detection system adapted for detection of CO gas from smoldering powders in a powder drying system component, such as a spray dryer chamber (200), a fluid bed (500) or a bag filter (400), which CO gas detection system comprises at least one inlet CO gas detector (3) arranged on at least one gas inlet of a powder drying system component such as to provide at least one inlet CO gas content measurement, at least one outlet CO gas detector (3) arranged on at least one gas outlet of a powder drying system component such as to provide at least one outlet CO gas content measurement, and an analyzing unit (5) adapted for receiving the at least one inlet CO gas content measurement, receiving the at least one outlet CO gas content measurement and comparing the sum of the at least one inlet CO gas content measurement and the sum of the at least one outlet CO gas content measurement while compensating for dilution, mixing, and time delay of the outlet CO gas content measurement. At least the at least one outlet CO gas detector (3) comprises an IR laser transmitter and is adapted for detecting over a measurement volume (6) and is arranged on the at least one gas outlet in such a way that said measurement volume (6) extends directly inside a gas flow (20) in said at least one gas outlet.

Claims

1. A powder drying system comprising a CO gas detection system adapted for detection of CO gas from smoldering powders in a powder drying system component, such as a spray dryer chamber, a fluid bed, or a bag filter, wherein the CO gas detection system comprises: at least one inlet CO gas detector arranged on at least one gas inlet of the powder drying system component such as to provide at least one inlet CO gas content measurement; at least one outlet CO gas detector arranged on at least one gas outlet of the powder drying system component such as to provide at least one outlet CO gas content measurement; and an analyzing unit adapted for receiving the at least one inlet CO gas content measurement from the at least one inlet CO gas detector, receiving the at least one outlet CO gas content measurement from the at least one outlet CO gas detector and comparing the sum of the at least one inlet CO gas content measurement and the sum of the at least one outlet CO gas content measurement while compensating for dilution, mixing, and time delay of the outlet CO gas content measurement such as to provide a differential measurement, C.sub.outlet, indicative of the CO gas content from smoldering powders in the powder drying system component, wherein at least the at least one outlet CO gas detector comprises an IR laser transmitter and is adapted to detect over a measurement volume and is arranged on the at least one gas outlet in such a way that said measurement volume extends directly inside a gas flow in said at least one gas outlet.

2. A powder drying system according to claim 1, wherein the at least one inlet CO gas detector also comprises an IR laser transmitter and is adapted to detect over a measurement volume and is provided on the at least one gas inlet in such a way that its measurement volume extends directly inside a gas flow in said gas inlet.

3. A powder drying system according to claim 1, wherein at least the at least one outlet CO gas detector and/or at least the at least one inlet CO gas detector comprises an IR laser transmitter, an IR receiver, the IR laser transmitter and the IR receiver being arranged on positions being one of mutually offset in a radial and/or longitudinal direction, mutually opposite and diametrically opposite such that its measurement volume extends between said positions.

4. A powder drying system according to claim 1, wherein the measurement volume of the outlet CO gas detector and/or the measurement volume of the inlet CO gas detector extends over a length of at least 1 meter.

5. A powder drying system according to claim 1, wherein the at least one outlet CO gas detector and/or the at least one inlet CO gas detector further comprises at least one purging device arranged and adapted to purge the outlet CO gas detector prior to providing the outlet CO gas content measurement and/or the inlet CO gas detector prior to providing the inlet CO gas content measurement.

6. A powder drying system according to claim 1, wherein the measurement volume of the outlet CO gas detector and/or the measurement volume of the inlet CO gas detector may extend along any one of: a radial direction of the gas inlet or gas outlet, a longitudinal direction of the gas inlet or gas outlet perpendicular to the radial direction, or a direction being inclined with respect to the longitudinal direction and/or the radial direction of the gas inlet or gas outlet.

7. A powder drying system according to claim 1, further comprising a plurality of outlet CO gas detectors arranged on the same gas outlet and/or on outlets of different components of said powder drying system such as to provide a plurality of outlet CO gas content measurements, and/or a plurality of inlet CO gas detectors arranged on the same gas inlet and/or on inlets of different components of said powder drying system such as to provide a plurality of inlet CO gas content measurements.

8. A powder drying system according to claim 1, wherein the analyzing unit is adapted to compare the inlet CO gas content measurement and the outlet CO gas content measurement to obtain a differential measurement, C.sub.outlet(x), indicative of the CO gas content from smoldering powders in a powder drying system at a given time x by means of the relation: C.sub.outlet(x)=C.sub.outlet,measured(x)C.sub.outlet(x), where: $C_{\mathrm{outlet}}\left(x\right)=\frac{\left(\frac{c_{\mathrm{inlet}}\left(x-t_p\right){\mathrm{FLOW}}_{\mathrm{inlet}}}{V_{\mathrm{spraydyer}}}\right)t_s+c_{\mathrm{outlet}}\left(x-t_s\right)}{1+\left(\frac{{\mathrm{FLOW}}_{\mathrm{outlet}}{t}_s}{V_{\mathrm{spraydryer}}}\right)},$ and where: C.sub.outlet,measured(x) is the concentration of CO gas in ppm in the gas outlet according to an outlet CO gas content measurement as measured by means of the outlet CO gas detector at the time x, C.sub.outlet(x) is the calculated concentration of CO gas in ppm in the gas outlet at the time x, C.sub.outlet(x-t.sub.s) is the calculated concentration of CO gas in ppm in the gas outlet at the time x minus the sampling time t.sub.s, i.e. the latest previously calculated value of the concentration of CO gas in ppm in the gas outlet, C.sub.inlet(x-t.sub.p) is the concentration of CO gas in ppm in the gas inlet according to an inlet CO gas content measurement as measured by means of the inlet CO gas detector a number of seconds t.sub.p before the time x, V.sub.spraydryer is the volume of the spray dryer in m.sup.3, FLOW.sub.inlet is the air flow in m.sup.3/s in the main air inlet, FLOW.sub.outlet is the total gas flow out of the spray dryer, calculated as the sum of the gas flow in each of the main gas outlet, the gas outlet of the static fluid bed and the gas outlet of the vibro-fluidizer in m.sup.3/s, and t.sub.s is the sampling time.

9. A method for detecting CO gas from smoldering powder in a powder drying system component, such as for instance a spray dryer chamber, a fluid bed or a bag filter, of a powder drying system comprising a carbon monoxide (CO) gas detection system, the method comprising: providing an inlet CO gas content measurement by means of an inlet CO gas detector arranged on a gas inlet of the powder drying system component, providing an outlet CO gas content measurement by means of at least one outlet CO gas detector arranged on a gas outlet of the powder drying system component, receiving the inlet CO gas content measurement from the inlet CO gas detector, receiving the outlet CO gas content measurement from the at least one outlet CO gas detector, comparing the inlet CO gas content measurement and the outlet CO gas content measurement to compensate for dilution, mixing, and time delay of the outlet CO gas content measurement such as to provide a differential measurement, C.sub.outlet, indicative of the CO gas content from smoldering powders in the powder drying system component, wherein the at least one outlet CO gas detector comprises an IR laser transmitter and is adapted to detect over a measurement volume and is, prior to providing the outlet CO gas content measurement, arranged on the gas outlet in such a way that its measurement volume extends directly inside a gas flow in the gas outlet.

10. A powder drying system according to claim 1, wherein at least the at least one inlet CO gas detector and/or outlet CO gas detector comprises an IR laser transmitter and an IR receiver and at least one reflector, the IR laser transmitter and the IR receiver being arranged in the same position and the at least one reflector being arranged in a position being one of offset in a radial and/or longitudinal direction, mutually opposite and diametrically opposite with respect to the position of the IR laser transmitter and/or the IR receiver such that the measurement volume of the at least one inlet and/or outlet CO gas detector system extends between said positions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0068] The invention will be described in more detail below by means of non-limiting examples of presently preferred embodiments and with reference to the schematic drawings, in which:

[0069] FIG. 1 shows a schematic view of the main components of a spray drying system in an embodiment of the first aspect of the invention, different exemplary positions of a CO gas detection system according to the invention being indicated;

[0070] FIG. 2 shows a schematic view of a CO gas detection system in an embodiment of the invention, where the CO gas detection system is arranged on an outlet of the spray drying system;

[0071] FIG. 3 shows a cross sectional view of a CO gas detection system according to FIG. 2; and

[0072] FIG. 4 illustrates schematically the steps of a method according to an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0073] FIG. 1 shows a schematic view of the main components of a powder drying system comprising a powder processing unit which by way of example in the embodiment shown is in the form of a spray drying system 1. In a manner known per se, the spray drying system 1 comprises a spray dryer with a drying chamber 200 and a process gas supply device 300, typically including an gas disperser. A gas inlet 201 is provided for intake of gas to the process gas supply device 300 and further to the drying chamber 200. At the lower end of the drying chamber 200, an outlet 203 for dried material is provided, and furthermore, for some types of powder drying systems, a further gas inlet 201 may be present. The drying chamber 200 also incorporates atomizing means, such as nozzles and/or a rotary atomizer wheel.

[0074] In the shown spray drying system 1, a static fluid bed 700 with air inlet 701 is provided and an after-treatment unit in the form of vibrating or static fluid bed 500 is provided. At one end, the vibrating or static fluid bed 500 comprises an inlet 503 at which it receives dried material from the outlet 203 of the drying chamber 200 for further treatment of the material, which is then to be collected at an outlet 504 at the other end of the vibrating or static fluid bed 500. The fluid bed 500 further comprises gas inlets 501 and 501 as well as a gas outlet 502. Further upstream or downstream equipment may be present as well.

[0075] Furthermore, the powder drying system comprises in addition to the spray drying system 1 a filter unit 400, to which spent process gas with particles entrained in the process gas is conducted. The filter unit 400 thus comprises a gas inlet 401 for spent process gas from one or more of the upstream operational units, a plurality of bag filters and a clean air outlet 601. The filter unit 400 may form part of a series of powder recovery units including further filter units and cyclones or bag filters, or any combination thereof. Furthermore, a cleaning arrangement 600 is shown in FIG. 1.

[0076] A number of conveying lines connect the operational units with each other in a manner known per se and will not be described in detail.

[0077] The general configuration of the CO gas detection system of a spray drying system according to the invention will now be described in more detail with particular reference to FIGS. 2-3.

[0078] According to the invention the spray drying system 1 further comprises a carbon monoxide (CO) gas detection system adapted for detection of CO gas from smoldering powders in a spray drying system component, such as for instance the spray dryer chamber 200, the fluid bed 500 or the bag filter unit 400.

[0079] Generally, the CO gas detection system comprises at least one CO gas detector arranged on a gas outlet, herein denoted outlet CO gas detector, and at least one CO gas detector arranged on a gas inlet, herein denoted inlet CO gas detector. In FIG. 1 various exemplary and non-limiting possible positions of outlet CO gas detectors and inlet CO gas detectors are indicated. Since the at least one outlet CO gas detector and the at least one inlet CO gas detector is of identical or similar structure, both are indicated on FIG. 1 by the reference numeral 3, and FIGS. 2-3 illustrate an embodiment of a CO gas detector which by way of example is arranged on a gas outlet 202, but which may just as well have been arranged on a gas inlet.

[0080] Generally, the CO gas detection system comprises an outlet CO gas detector 3 arranged on an gas outlet, such as on an outer surface thereof, of a powder drying system component such as to provide an outlet CO gas content measurement. Referring to FIGS. 2 and 3, the CO gas detection system more particularly and by way of a non-limiting example comprises an outlet CO gas detector 3 arranged on the gas outlet 202 of the drying chamber 200 of the spray drying system 1. More particularly, the outlet CO gas detector 3 is arranged on an outer surface 2021 of the gas outlet 202 of the drying chamber 200. Generally, an outlet CO gas detector 3 may alternatively or additionally be arranged on a second or further gas outlet of the same or a second spray drying system component, such as those described above and/or such as indicated in FIG. 1.

[0081] The CO gas detection system may further comprise at least one inlet CO gas detector arranged on a gas inlet, such as on an outer surface thereof, of a powder drying system component such as to provide an inlet CO gas content measurement. The CO gas detection system more particularly and by way of a non-limiting example comprises an inlet CO gas detector arranged on the gas inlet 201 of the drying chamber 200 of the spray drying system 1. More particularly, the inlet CO gas detector is arranged on an outer surface of the gas inlet 201 of the drying chamber 200. Generally, one or more inlet CO gas detectors may, alternatively or additionally, be arranged on one or more gas inlets of other spray drying system components, such as those described above and/or such as indicated in FIG. 1. In a preferred embodiment, the inlet CO gas detector measuring IR radiation.

[0082] For instance, the outlet CO gas detector 3 and/or the inlet CO gas detector, respectively, may in some embodiments be mounted on flanges welded to the gas outlet and/or gas inlet, respectively.

[0083] The CO gas detection system further comprises an analyzing unit 5. The analyzing unit 5 is adapted for receiving the inlet CO gas content measurement from the inlet CO gas detector and for receiving the outlet CO gas content measurement from the at least one outlet CO gas detector 3. The analyzing unit 5 may thus comprise a receiver. The analyzing unit 5 is furthermore adapted for comparing the inlet CO gas content measurement and the outlet CO gas content measurement to compensate for dilution, mixing, and time delay of the outlet CO gas content measurement such as to provide a differential measurement, C.sub.outlet, indicative of CO gas from smoldering powders in a spray drying system component, i.e. in the embodiment illustrated in the spray drying chamber 200. The analyzing unit 5 is thus electrically and/or optically connected to the inlet CO gas detector and the outlet CO gas detector 3 by means of a respective wired or wireless connection 13 (FIG. 2).

[0084] The analyzing unit 5 is furthermore adapted for, in case the result of the comparison shows an indication of CO gas from smoldering powders in a spray drying system component, taking an appropriate action. Such an appropriate action may comprise setting off an alarm, e.g. in the form of an acoustic or a visual signal, and/or automatically shutting down the spray drying system and/or activating fire-preventing means.

[0085] The analyzing unit 5 may thus, and as is shown on FIG. 2, form an integrated or stand-alone part of a safety system, which also comprises an alarm unit, e.g. in the form of an acoustic or a visual signaling unit. The alarm unit may alternatively be built into the analyzing unit 5. In any event, the analyzing unit is adapted for triggering the alarm unit and the alarm unit is adapted for emitting an alarm when the differential measurement, C.sub.outlet, indicative of CO gas from smoldering powders in a spray drying system component is above a predetermined threshold value, typically being close to 0. Furthermore, the alarm unit may also be triggered if the analysis shows an erroneous function of the CO gas detection system, particularly to emit a signal dedicated to the purpose, which is a feature not being possible in the prior art system. The analyzing unit 5 is thus electrically and/or optically connected to the alarm unit by means of a wired or wireless connection.

[0086] Referring to FIGS. 2 and 3, the outlet CO gas detector 3 is an IR detector suitable for detecting over a measurement volume 6. The outlet CO gas detector 3 is arranged on the outlet 202 in such a way that its measurement volume 6 extends directly inside a (process) gas flow 20 in the outlet 202. More particularly, the outlet CO gas detector 3 comprises an IR laser transmitter 7 and an IR receiver 8. The IR laser transmitter 7 and the IR receiver 8 are arranged on opposite points, here but not mandatorily always on diametrically opposite points, on the outlet 202 such that its measurement volume 6, i.e. the volume through which the IR laser beam 15 of the IR detector extends when in operation and measuring, extends over a length D between the opposite points of the outlet 202 through the inside 2022 of the outlet 202. The IR laser transmitter 7 and the IR receiver 8 may furthermore be electrically and/or optically connected by means of a wired or wireless connection 12.

[0087] In alternative embodiments the outlet CO gas detector 3 may also comprise one or more IR reflectors, which reflectors may be arranged together with the IR laser transmitter, e.g. in the same housing, or apart from the emitter, e.g. in a different housing than the IR laser transmitter, such as in the housing of the IR receiver, or to redirect the laser light over one or more segments along one or more of the longitudinal direction, the radial direction and the cross sectional direction of the gas outlet. The same applies to the inlet CO gas detector.

[0088] The outlet CO gas detector 3 may further comprise at least one purging device 9 arranged and adapted for purging the outlet CO gas detector 3 prior to providing the outlet CO measurement. The purging device 9 is connected to the outlet CO gas detector 3, particularly to the IR laser transmitter 7 and to the outlet IR receiver 8, by means of an air tube 4.

[0089] The inlet CO gas detector may also be substantially identical to the outlet CO gas detector 3. The inlet CO gas detector may preferably comprise an IR laser transmitter and is adapted for detecting over a measurement volume.

[0090] The inlet CO gas detector is then provided on the inlet 201 in such a way that its measurement volume extends directly inside a (process) gas flow in the inlet 201. More particularly, the inlet CO gas detector system comprises an IR laser transmitter and an IR receiver. The IR laser transmitter and the IR receiver are arranged on opposite points, possibly but not mandatorily diametrically opposite points, on the inlet 201 such that its measurement volume, i.e. the area in which the IR laser beam of the IR detector extends when in operation and measuring, extends over a length between the opposite points of the inlet 201 through the inside of the inlet 201. The IR laser transmitter and the IR receiver may furthermore be electrically and/or optically connected by means of a wired or wireless connection.

[0091] The inlet CO gas detector may further comprise at least one purging device arranged and adapted for purging the inlet CO gas detector prior to providing the inlet CO gas content measurement. The purging device is connected to the inlet CO gas detector, particularly to the IR laser transmitter and to the inlet IR receiver, by means of air tubes.

[0092] The measurement volume 6 of the outlet CO gas detector 3 and the measurement volume of the inlet CO gas detector may extend along any one of a radial direction R (FIG. 2) of the gas inlet or gas outlet as shown in FIG. 3, along a longitudinal direction L (FIG. 2) of the gas inlet or gas outlet perpendicular to the radial direction R and a direction being inclined with respect to both the longitudinal direction L and the radial direction R of the gas inlet or gas outlet. The measurement volume 6 of the outlet CO gas detector 3 and the measurement volume of the inlet CO gas detector may extend over a length of more than 1 meter, such as to provide for a measurement volume being sufficiently large to enable the desired sensitivity and precision of the measurements.

[0093] The measurement principle employed by the invention is thus adsorption of an IR laser beam at specific wavelengths emitted from the IR laser transmitter 7. The adsorption will be proportional to the number of molecules of CO in the process gas. Thus, from the adsorption as well as from the temperature and pressure of the process gas, the concentration of CO gas may be calculated based on analysis of the resulting adsorption spectrum, for instance in the manner described further below.

[0094] In a manner known per se to the skilled person within IR based CO gas content measurements, the outlet CO gas detector 3 may further comprise a first transmitter 10 for measuring process gas temperature for providing a process temperature compensation to the CO gas content measurement to compensate for the effects on the measurements caused by elevated temperatures caused by the IR laser beam, or in particular from elevated process temperatures compared to room temperature, which may otherwise shift the adsorption spectrum obtained. The outlet CO gas detector 3 may further comprise a second transmitter for 11 measuring process gas pressure for providing process pressure compensation to the calculation to compensate for the effects on the measurements caused by elevated process pressures compared to atmospheric pressure. Analogously, the inlet CO gas detector may further comprise a first transmitter for providing process temperature compensation and/or a second transmitter for providing process pressure compensation.

[0095] Furthermore, in some embodiments where the CO gas detection system comprises a purging device 9, the purging device 9 may also be arranged and adapted for purging a measurement range 6 (FIG. 3) inside the gas outlet before providing the outlet CO gas content measurement and/or be arranged and adapted for purging a part of a measurement range inside the gas inlet before providing the inlet CO gas content measurement.

[0096] Still further, the outlet CO gas detector 3 and/or the inlet CO gas detector, such as comprising the IR laser transmitter and or IR receiver, may in some embodiments be cooled, such as air cooled or even water cooled. For instance, the purging units may also serve as air cooling units. Alternatively, a separate cooling device may be provided.

[0097] Referring now to FIG. 4 exemplary embodiments of a method according to the invention will be described.

[0098] Prior to step 1000 a spray drying system with a CO gas detection system according to the invention is provided by mounting said CO gas detection system on the spray drying system. In step 1000, an inlet CO gas content measurement is provided by said at least one inlet CO gas detector. In step 1200 an outlet CO gas content measurement is provided by means of said at least one outlet CO gas detector. In step 1300 the inlet CO gas content measurement from the inlet CO gas detector is received by the analyzing unit 5. In step 1400 the outlet CO gas content measurement from the at least one outlet CO gas detector is received at the analyzing unit 5. In step 1500 the inlet CO gas content measurement and the outlet CO gas content measurement is compared by means of the analyzing unit 5 to compensate for dilution, mixing, and time delay of the outlet CO gas content measurement such as to provide a differential measurement, C.sub.outlet, indicative of CO gas from smoldering powders in a spray drying system component. Finally, in step 1100, prior to the step 1200 of providing the outlet CO gas content measurement, the at least one outlet CO gas detector is provided as an outlet CO gas detector comprising an IR laser transmitter and being adapted for detecting over a measurement volume and is arranged on the gas outlet in such a way that its measurement volume extends directly inside a gas flow 20 in the inside 2022 of the gas outlet 202.

[0099] The method may furthermore comprise one or more of the following optional steps.

[0100] A step 900 of, prior to the step 1000 of providing the inlet CO gas content measurement, providing the inlet CO gas detector as an inlet CO gas detector optionally comprising an IR laser transmitter and being adapted for detecting over a measurement volume, and arranging the inlet CO gas detector on the gas inlet in such a way that its measurement volume extends directly inside a (process) gas flow in the inside of the gas inlet 201.

[0101] In embodiments where at least the outlet CO gas detector comprises an IR laser transmitter and an IR receiver, a step of arranging the IR laser transmitter and the IR receiver on opposite points, optionally diametrically opposite points, on the gas outlet 202, such as on an outer surface 2021 of the outlet 202, such that its measurement volume extends between the opposite points of the gas outlet 202.

[0102] In embodiments where the inlet CO gas detector comprises an IR laser transmitter and an IR receiver, a step of arranging the IR laser transmitter and the IR receiver on opposite points, optionally diametrically opposite points, on the gas inlet 201, such as on an outer surface of the inlet 201, such that its measurement volume extends between the opposite points of the gas inlet 201.

[0103] An optional step of purging the outlet CO gas detector prior to the step 1200 of providing the outlet CO gas content measurement.

[0104] An optional step of purging the inlet CO gas detector prior to the step 1300 of providing the inlet CO gas content measurement.

[0105] An optional step of cooling, preferably air cooling, the outlet CO gas detector and/or a step of cooling, preferably air cooling, the inlet CO gas detector.

[0106] An optional step of providing a plurality of outlet CO gas detectors, arranging the plurality of outlet CO gas detectors on the same gas outlet and/or on outlets of different spray drying system components, and a step of providing a plurality of outlet CO gas content measurements.

[0107] An optional step of providing a plurality of inlet CO gas detectors, arranging the plurality of inlet CO gas detectors on the same gas inlet and/or on inlets of different spray drying system components, and a step of providing a plurality of inlet CO gas content measurements.

[0108] An optional step of providing the outlet CO gas detector and/or the inlet CO gas detector with process temperature compensation and/or process pressure compensation prior to the step 1200 of providing the outlet CO gas content measurement and/or prior to the step 1000 of providing the inlet CO gas content measurement.

[0109] In the following the mathematical model lying behind the comparison performed by the analyzing unit 5 and forming part of the method performed by the powder drying system according to the invention, and more particularly forming part of the step of comparing of the method according to the invention will be described.

[0110] A rise in the CO gas concentration in the inlet air, C.sub.inlet, will over time result in an elevated CO gas concentration in the outlet air, C.sub.outlet. By mathematically modelling the spray drying system as a perfectly mixed tank plus a plug flow, t.sub.p, i.e. a time delay, it becomes possible to calculate the CO gas concentration in the outlet air based on the CO gas concentration in the inlet air. The calculated CO gas concentration in the outlet air, C.sub.outlet, is indicative of the naturally occurring fluctuations of the CO gas concentration in reaction to which the alarm system should not set off. Rather, the alarm system should only be set off in reaction to a rise in CO gas concentration originating from smoldering of powders in the spray drying system. The rise in CO gas concentration, or differential measurement, C.sub.outlet, originating from smoldering may be expressed as:

C.sub.outlet=C.sub.outlet,measuredC.sub.outlet (1),

[0111] where C.sub.outlet, measured is the CO gas concentration in the outlet according to an outlet CO gas content measurement measured by the outlet CO gas detector.

[0112] To calculate C.sub.outlet the following model may be set up. First of all, the amount of CO gas accumulated per time unit in the spray drying system may be expressed as:

C.sub.inlet*FLOW.sub.inletC.sub.outlet*FLOW.sub.outlet (2),

and as

[00002]$\begin{array}{cc}\frac{{\mathrm{dC}}_{\mathrm{spray}.Math..Math.\mathrm{dryer}}}{\mathrm{dt}}{V}_{\mathrm{spray}.Math..Math.\mathrm{dryer}}.& \left(3\right)\end{array}$

[0113] In the above equations FLOW.sup.let is the air flow in m.sup.3/s in the main air inlet 201, FLOW.sub.outlet is the total gas flow in m.sup.3/s out of the spray drying system and V.sub.spraydryer is the volume of the spray drying system in m.sup.3.

[0114] The air flow in m.sup.3/s in the main air inlet 201, FLOW.sub.inlet, is assumed to be constant over time, is equal to the flow in the main air inlet in the case that the static fluid bed and the VIBRO-FLUIDIZER have air inlets separate from the main air inlet, and is equal to the sum of the flow in each of the main air inlet, the air inlet of the static fluid bed and the air inlet of the VIBRO-FLUIDIZER if a common air inlet is used.

[0115] The total gas flow out of the spray drying system, FLOW.sub.outlet, is calculated as the sum of the gas flow in each of the main gas outlet, the gas outlet of the static fluid bed and the gas outlet of the VIBRO-FLUIDIZER in m.sup.3/s.

[0116] The volume of the spray dryer in m.sup.3, V.sub.spraydryer, may be determined experimentally by introducing CO gas into the spray dryer. A theoretical value may be obtained as t.sub.p*FLOW.sub.inlet, where t.sub.p is an assumed time delay of the gas flow through the spray drying system, i.e. the time it is assumed the gas flow takes to flow from the inlet CO gas detector through the spray dryer to the outlet CO gas detector.

[0117] Due to the assumed perfect mixing in the spray dryer, the CO gas concentration in the outlet air is equal to the CO gas concentration in the spray drying system (dC.sub.spray dryer/dt=dC.sub.outlet/dt). Due to the assumed time delay, t.sub.p, of the gas flow through the spray drying system, the inlet CO gas concentration measured by the inlet CO gas detector at the time t.sub.p before a given point of time, x, is used for calculating the outlet CO gas concentration of the spray drying system at the time x:

[00003]$\begin{array}{cc}\frac{{\mathrm{dC}}_{\mathrm{outlet}}\left(x\right)}{\mathrm{dt}}=\frac{C_{\mathrm{inlet}}\left(x-t_p\right){\mathrm{FLOW}}_{\mathrm{inlet}}-C_{\mathrm{outlet}}\left(x\right){\mathrm{FLOW}}_{\mathrm{outlet}}}{V_{\mathrm{spray}.Math..Math.\mathrm{dryer}}},& \left(4\right)\end{array}$

[0118] where C.sub.inlet(x-t.sub.p) is the concentration of CO gas in ppm measured by the inlet CO gas detector in the inlet at a number of seconds t.sub.p before the time x.

[0119] Using sampling steps of a small size, t.sub.s, one obtains based on equation (4) above:

[00004]$\begin{array}{cc}\frac{C_{\mathrm{outlet}}\left(x\right)-C_{\mathrm{outlet}}\left(x-t_s\right)}{t_s}=\frac{C_{\mathrm{inlet}}\left(x-t_p\right){\mathrm{FLOW}}_{\mathrm{inlet}}-C_{\mathrm{outlet}}\left(x\right){\mathrm{FLOW}}_{\mathrm{outlet}}}{V_{\mathrm{spray}.Math..Math.\mathrm{dryer}}},& \left(5\right)\end{array}$

[0120] Thus, by rearranging equation (5), the following relation for the calculated CO gas concentration in ppm in the outlet air, C.sub.outlet, at a time x is arrived at:

[00005]$\begin{array}{cc}{C}_{\mathrm{outlet}}\left(x\right)=\frac{\left(\frac{c_{\mathrm{inlet}}\left(x-t_p\right){\mathrm{FLOW}}_{\mathrm{inlet}}}{V_{\mathrm{spraydyer}}}\right)t_s+C_{\mathrm{outlet}}\left(x-t_s\right)}{1+\left(\frac{{\mathrm{FLOW}}_{\mathrm{outlet}}{t}_s}{V_{\mathrm{spraydryer}}}\right)},& \left(6\right)\end{array}$

[0121] where C.sub.outlet(x-t.sub.s) is the concentration of CO gas in ppm at the time x minus t.sub.sthe sampling timei.e. the latest previously calculated value of C.sub.outlet.

[0122] Equation (6) may thus be used for comparing the inlet CO gas content measurement and the outlet CO gas content measurement. The result may then be inserted into equation (1) together with the outlet CO gas content measurement, C.sub.outlet, measured(x), obtained by the outlet CO gas detector at the time x to calculate the differential measurement, C.sub.outlet(x), indicative of the rise in CO gas concentration originating from smoldering at the time x, thus achieving:

C.sub.outlet(x)=C.sub.outlet,measured(x)C.sub.outlet(x) (7).

[0123] Based on the thus calculated differential measurement, C.sub.outlet, indicative of the rise in CO gas concentration originating from smoldering it may, for each calculation made, be determined whether an alarm should be set off and/or other suitable action be taken.

[0124] It should be noted that the above description of preferred embodiments serves only as an example, and that a person skilled in the art will know that numerous variations are possible without deviating from the scope of the claims.

[0125] For instance, and optionally, as methane (CH.sub.4) has adsorption wavelengths being similar to those of CO, the method and/or device according to the invention may also be employed to detect CH.sub.4 gas in a gas inlet or gas outlet of a spray drying component. The results of such a CH.sub.4 gas detection may also be used to take into account the effects caused by the CH.sub.4 gas interfering with the CO gas measurements, at least in case reasonably large quantities of CH.sub.4 gas are present.