GB2456017A

GB2456017A - Combustion analyser and method for combustion analysing a sample

Info

Publication number: GB2456017A
Application number: GB0725379A
Authority: GB
Inventors: Jeroen Abraham Van Der Meer; Maurice Stephan Van Doeselaar; Louis Marie Smeets
Original assignee: Thermo Fisher Scientific Inc
Current assignee: Thermo Fisher Scientific Inc
Priority date: 2007-12-31
Filing date: 2007-12-31
Publication date: 2009-07-01
Also published as: GB0725379D0

Abstract

A combustion analyser comprises a combustion chamber and a trapping device 60 that comprises a trapping material 64 that has an affinity for the component of interest. The trapping material may comprise an adsorbent (e.g. synthetic carbon). The component of interest may be released from the adsorbent by the application of increased temperature (e.g. from heater 66). The trapping device may comprise a first end in switchable fluid communication with (i) a combustion products line 76 and (ii) an analysis line 78; a second end in switchable fluid communication with (i) a waste line 80 and (ii) purge carrier gas line 82. This arrangement may allow adsorbed material to be de-adsorbed without having to traverse the whole length of the trapping device; this may facilitate the desorption of strongly bound materials. The trapping material may be transferred to the combustion chamber to release the component of interest by combustion. The device may be used in the analysis of refinery products (e.g. gasoline, diesel, petrol). Alternatively, the use of a trap and purge device between a combustion chamber and a detector is disclosed.

Description

Intellectual Property Office mm For CroaLIv arid Innovrrlrori App1ui(ion No GBO72S379 2 RTM Date 17 April 2008 The following terms ai-e registered trademarks and should be read as such wherever they occur in this document: Therrno Fisher Scientific Ten ax UK Intellectual Property Office is an operating name of The Patent Office Combustion analyser and method for combustion analysing a sample

Field of the invention

The invention relates to an apparatus and method f or combustion analysis of a sample.

Background of the invention

Combustion analysers are used to determine the concentration of one or more components of a sample, by combusting the sample and analysing the gaseous products for specific oxides. Typically, the carbon, chlorine, sulphur and/or nitrogen content of the sample is measured by detecting C02, HC1, SO2 and NO, respectively.

A schematic illustration of a typical combustion analyser is shown in figure 1. The combustion analyser 10 comprises a sample introduction stage 20, a combustion stage 30, a conditioning stage 40, and a detection stage 50. The sample introduction stage 20 comprises a sample introduction apparatus 22, to which are connected a supply of a sample 24, a supply of oxygen 26 and a supply of argon 27. The sample introduction apparatus 22 introduces a liquid, gaseous or solid sample into a combustion tube 32 in a suitable form for combustion to take place. A further supply of oxygen 25 may be provided, directly into the combustion tube 32. The combustion tube 32 is heated by an electric heater 34, so that the sample is delivered into an oxygen-rich atmosphere at high temperature, typically around 1000°C. The sample is thereby converted into various combustion products, such as C02, H20, SO2, NOR, etc. The combustion products leave the combustion tube 32 and pass through the conditioning stage 40, where processes such as cooling, filtering, drying, etc. take place. The conditioned products then pass through one or more dedicated detectors 52, 54, in which properties of the components of the combustion products may be detected. For example, CO2 may be detected by absorption of infrared radiation, using a non-dispersive infrared (NDIR) detector; SO2 may be detected by fluorescence with ultraviolet light, using a light sensor; and NO can be detected from de-excitation processes following its reaction with ozone (03) to form excited NO2, using a chemilurninescence light sensor. The detected signals are indicative of the respective amount of each component of the combustion products and can therefore be related to the composition of the original sample. Finally, the detected combustion products are passed out of the detection stage 50, as waste products 56.

The performance of such a combustion analyser 10 -in terms of its suitability, reliability, accuracy and robustness -depends strongly on the performance of the detector(s). A number of drawbacks can be identified with the detectors generally employed in combustion analysers.

The detection limits achievable in practice are relatively high, while the market is calling for lower and lower limits.

US-A1-2006/0284073 relates to a method and apparatus for analysing combustion products, in which, after combustion of a sample, all gases from the combustion chamber are removed and collected in a reservoir. The reservoir comprises a gas displacer, which is then operated to force the collected gases out of the reservoir under pressure to a detector for measurement of the combustion product(s) of interest. The gases may be displaced to the detector at a faster rate than the rate at which they are produced in the combustion chamber and collected in the reservoir. A stronger signal may thereby be detected, reducing the effect of noise and helping to improve measurement accuracy.

However, detectors tend to suffer from matrix effects.

That is, the detector signals may be affected to some extent by the bulk composition of the combustion gases -mainly carbon dioxide and oxygen -which, in turn, are dependent on the composition of the sample, in particular in terms of its C:H ratio. Furthermore, a number of the detectors employed are not sufficiently selective and tend to exhibit crossover errors. For example, the UV fluorescence detector used to detect sulphur dioxide is also somewhat sensitive to nitrogen monoxide.

Accordingly, it would be desirable to provide an improved or alternative apparatus and method for combustion analysing a sample. This invention aims to provide an apparatus and a method for combustion analysing a sample which avoid or reduce some or all of the above problems.

Summary of the invention

According to a first aspect of the invention, there is provided a combustion analyser for combustion analysing a sample, the analyser comprising: a combustion chamber for receiving and combusting a sample to form combustion products comprising a component of interest and unwanted components; and a trapping device for receiving the combustion products, the trapping device comprising a trapping material having an affinity for the component of interest for removing the component of interest from the combustion products for at least temporary storage of the component of interest in the trapping device.

When a sample is combusted, the combustion products include components which it may be desired to measure, such as C02, HC1, SO2, NO,, etc. (depending on the sample), and the term "component of interest" refers to one or more of these. Where the component of interest is only one such gas, or a selection of such gases, the remaining gases, along with oxygen and any (other) carrier gas, are not of interest for measurement and the term "unwanted components" refers to these. By passing the combustion products through the trapping device, the component of interest may be removed or separated from the unwanted components by means of the trapping material. The trapping device thus acts as a component separator, by the trapping material trapping and at least temporarily storing the component of interest. The unwanted components are preferably passed out of the trapping device to waste, leaving behind the component of interest in a relatively concentrated state. Subsequent processing and/or analysis of the component of interest may then take place without the interference or effect of the unwanted components, which have been separated from the component of interest. This allows for the possibility of lowering the achievable detection limit for the component of interest, reducing -if not eliminating -matrix effects, and reducing crossover errors. Thus the trapping device can improve the performance of the detection system used in a combustion analyser.

The trapping material has an affinity for the component of interest. This means that the trapping material is effective to remove the component of interest from the combustion products and at least temporarily store the component of interest. As stated above, the component of interest may comprise more than a single substance. In that case, a trapping material, which exhibits an affinity for more than one substance, would be chosen.

The trapping material having an affinity for the component of interest does not necessarily mean that it is selective only to the component of interest; i.e., that such removal and storage by the trapping material is to the exclusion of all other substances in the combustion products. However, it is preferable for the trapping material to be selective, or substantially selective, to the component of interest, so that substantially all of the unwanted substances present in the combustion products remain in the combustion products as unwanted components and may be passed out of the trapping device, untrapped.

Preferably, the trapping material comprises a sorbent, by which the component of interest may be trapped. For example, the trapping material may comprise an adsorbent on the surface of which the component of interest may be trapped. The adsorbent may comprise one or a combination of synthetic carbon, activated carbon and graphitised carbon black, or any other suitable adsorbent material having an affinity for or selectivity to the component of interest (which may be one or more gases). The use of carbon which has been made synthetically is advantageous, since naturally occurring carbon tends to contain sulphur and other substances, which can undesirably interfere with the measurement of the component of interest.

Alternatively, or additionally, the trapping material may comprise an absorbent in the bulk of which the component of interest may be trapped. The absorbent may comprise one or a combination of a molecular sieve, a zeolite, and a siliceous substance (i.e., a substance comprising a form of silica). A combination of two (or more) trapping materials may be used, in particular if selectivity/affinity towards specific components of interest is desired.

An advantage of the above forms of trapping material is that it is relatively straightforward to release and purge the component of interest from the trapping material, when it is desired to conduct subsequent processing/analysis on the component of interest. Preferably, the trapping material removes and stores the component of interest under a first condition, such as a first operating temperature for the trapping material, and releases the component of interest under a second condition, such as a second, higher operating temperature for the trapping material.

Preferably, the trapping device comprises a heater for heating the trapping material up to the second temperature under the second condition. One advantage of this is that it is possible to heat the trapping material and release the trapped component of interest so that a relatively high, sharp peak is provided at the detector. Advantageously, the first temperature is room temperature or in the region of room temperature, so that the first condition of the trapping device is achieved substantially passively (i.e. by the device being neither heated nor actively cooled).

Preferably, the heater comprises a heater controller, arranged to control the temperature of the trapping material, so that a desired release temperature may be provided in the trapping device under the second condition.

Such a controller may be arranged for fast heating of the trapping material, for quick release of the component of interest. Alternatively, the controller may be arranged for slow heating of the material, for a more gradual release of the component of interest.

Preferably, under the second condition, a purge carrier gas supply provides a flow of purge carrier gas through the trapping device. As the component of interest is given off from the trapping material, the purge carrier gas helps to carry the component of interest away from -and therefore purge -the trapping material. The component of interest may thus be transferred for subsequent analysis/detection in a relatively concentrated form in the purge carrier gas.

A particular advantage of the above arrangement is as follows. During conventional combustion of a sample, the combustion products typically pass out of the combustion chamber at a relatively slow speed (since a fast gas flow through the combustion chamber would reduce the residence time -and therefore the opportunity for complete combustion -of the sample in the chamber). A relatively low signal of a combustion product to be measured is thus detected, over a relatively long measurement time, leading to inaccurate measurements, especially through noise interference for trace samples. With the above arrangement, the combustion products can pass out of the combustion chamber at a relatively slow speed, to allow for substantially complete combustion of the sample in the combustion chamber, but, rather than being directed straight to the detector(s), the gases pass through the trapping device and the component(s) of interest is trapped therein. When desired -typically, when sample combustion is complete -the trapping material is made to release the component(s) of interest, which is carried off by the purge gas to the detector(s). The flow rate of the purge gas effectively determines the rate at which the component(s) of interest is supplied to the detector(s) and this rate may be controlled as desired. In particular, the rate may be set relatively higher than the flow rate of the combusted sample into the trapping device, so that the component(s) of interest is concentrated (from the trapping device) and provided in a relatively short period of time to the detection stage. The detected signal may thereby have an increased peak height and reduced peak width, resulting in a significantly higher signal-to-background (noise) ratio. Consequently, lower detection limits, especially for trace samples, can be achieved.

A purge gas controller may be used to implement the above step. The controller may be arranged to provide a substantially constant flow rate for the purge gas, so that the measurements made by the detector(s) are set against a generally constant background and not subject to the significant variations resulting from changes in flow rate and composition of the gases leaving the combustion chamber.

Greater detection stability and reproducibility may thereby be achieved.

The purge carrier gas preferably comprises an inert gas, such as argon or helium. The purge carrier gas may alternatively comprise a gas which is stable (i.e., preferably unreactive and non-dissociatable) under the second condition, such as nitrogen. In this way, the purge carrier gas may act simply as a transport medium for the component of interest, without affecting its detection. One particular benefit of this configuration applies in the analysis of samples containing sulphur. This is because the UV fluorescence detector used to detect sulphur dioxide can be quenched by exposure to oxygen (that is, the oxygen absorbs some of the UV fluorescence, reducing the intensity of the signal at the detector). With the above configuration, although the combustion products leaving the combustion chamber will include oxygen, the oxygen -as an unwanted component -is not permitted to pass on to the detector. Instead, the unwanted components may be passed to waste from the trapping device, while the component of interest is retained therein. Then, the component of interest is released from the trapping material and carried with a non-reactive purge carrier gas to the detector. In this way, in addition to helping to improve the detection limit of the analyser, this configuration helps to avoid the detrimental effect of oxygen on the detector. Indeed, the inventors have found that, by changing the effective carrier gas from oxygen (as the gas conventionally present from the combustion chamber) to, for example, helium (as the purge gas from the trap and release device), the signal-to-noise ratio at the detector may be increased by a factor of up to five, or more. This represents a significant improvement over known combustion analysers.

Also, since the purge carrier gas supply can be chosen and controlled independently of the gases from the sample and the combustion chamber, it is possible to provide better control and better stability to the combustion analyser. In particular, it is easier to obtain a stable, constant and reproducible gas flow to the detector(s).

Thus, embodiments of the invention provide a number of advantages, including improved detection limits due to the delivery to the detector being faster than the rate of production from the furnace; improved accuracy due to the removal of unwanted gases that can affect the detectors; and improved stability, reproducibility and accuracy due to the controlled purge gas delivery system.

Preferably, the trapping device has a first end and a second end, each respective end being in switchable fluid communication with two, respective gas flow lines. The gas flow lines may each connect directly into the respective end of the trapping device, via a respective switch or valve.

Preferably, the trapping device has at its first end a first port connected to a first multi-way valve, itself connected to first and second ones of the gas flow lines, and at its second end a second port connected to a multi-way valve, itself connected to third and fourth ones of the gas flow lines. In a first arrangement, the first line is a combustion products line for supplying the combustion products to the trapping device; the second line is a purge carrier gas line for supplying the purge carrier gas to the trapping device; the third line is a waste line for receiving from the trapping device the unwanted components and the fourth line is an analysis line for receiving from the trapping device the component of interest (along with the purge carrier gas) for subsequent analysis thereof.

Alternatively, and preferably, in a second arrangement, the first line is the combustion products line; the second line is the analysis line; the third line is the waste line; and the fourth line is the purge carrier gas line. In either case, under the first, trapping condition, the switches/valves are configured so that the first and third lines are in fluid communication with the trapping device and, under the second, purging condition, the switches/valves are configured so that the second and fourth lines are in fluid communication with the trapping device.

In the first arrangement, the component of interest passes (with the combustion products) into the trapping device via the first end and passes (with the purge carrier gas) out of the trapping device via the second end. In the second arrangement, the component of interest still passes (with the combustion products) into the trapping device via the first end, but passes (with the purge carrier gas) out of the trapping device back via the first end. This second, contraf low arrangement is preferred since some trapping materials may trap the component of interest so strongly/readily that the majority of the component of interest is stored and concentrated at or near the first end. Passing the purge carrier gas in the opposite flow direction (from the second end to the first end) thus allows for relatively quick release and purging of the component of interest from the trapping device, Conversely, flowing the purge carrier gas in the same direction as the combustion products would result in the component of interest moving relatively slowly to the second end, undergoing a process of being repeatedly trapped and released (adsorbed/absorbed and desorbed) on the way. This arrangement therefore leads to a longer retention time and detected signal peak widening compared with the contraf low arrangement.

One alternative with the first arrangement is to supply the purge carrier gas and the combustion products through the same gas flow line into the trapping device; i.e., providing a single line from the combustion chamber to the trapping device and supplying the purge carrier gas from an introduction apparatus into and through the combustion chamber. This is beneficial where the same gas supply is used for the background/carrier gas during combustion of the sample in the combustion chamber as well as during purging of the component of interest from the trapping device.

In some embodiments, the waste line could be the same line as the analysis line. In this arrangement, under the first condition, the detectors may be switched off or otherwise disabled.

An alternative technique for releasing the component of interest from the trapping material, once stored thereby, -12 -involves removing the trapping material from the trapping device, placing it in the combustion chamber, and combusting or heating it. The component of interest and trapping material can thus be analysed like a solid sample, with the gases formed or released being passed on to a detector for measurement. This applies particularly where the trapping material is carbon-based, preferably synthetic carbon, since it can be ensured that substantially all of the component(s) of interest is released, as the trapping material itself is combusted to carbon dioxide and also transferred out of the combustion chamber. The disadvantages are that the trapping material needs to be replaced for each sample, the analysis time is longer, and the error in the analysis tends to be increased.

According to another aspect of the invention, there is provided a method of combustion analysing a sample comprising the steps of: combusting the sample to form combustion products comprising a component of interest and unwanted components; passing the combustion products to a trapping device comprising a trapping material having an affinity for the component of interest; using the trapping material to remove the component of interest from the combustion products and to store at least temporarily the component of interest in the trapping device.

Preferably, the trapping step takes place over a first period of time and the releasing step takes place over a second period of time, the first period being longer than the second period.

According to another aspect of the invention, there is provided the use of a trap and purge device between a combustion chamber and a detector of a combustion analyser.

The term combustion products is used here to mean any -13 -substances present in the combustion analyser during or following the combusting step and this may include the sample and other substances, such as oxygen or a carrier gas, and their respective constituents, both in pre-combustion and post-combustion forms, whether fully or incompletely cornbusted.

Other preferred features and advantages of the invention are set out in the description and in the dependent claims which are appended hereto.

Brief description of the drawings

The invention may be put into practice in a number of ways and some embodiments will now be described, by way of non-limiting example only, with reference to the following figures, in which: Figure 1 shows a schematic layout of a typical combustion analyser; Figure 2 shows a schematic side view of a trap and purge device in a trapping configuration; and Figure 3 shows a schematic side view of a trap and purge device in a purging configuration.

Description of preferred embodiments

Referring to figures 2 and 3, there is shown a schematic side view of a trap and purge device 60 according to one embodiment of the invention, in a trapping configuration and a purging configuration, respectively.

The trap and purge device 60 comprises a trapping chamber 62 substantially filled with a trapping material 64. The trapping chamber 62, thus the trapping material 64, may be heated by a heater 66, which in this embodiment is an electrically heated element extending helically around the trapping chamber.

The trapping chamber 62 has an inlet/outlet tube 68,70 at each end, each tube being connected to a respective two-way valve 72,74. Each two-way valve 72,74 is connected to a further two gas flow tubes 76,78;80,82, respectively. The two-way valves 72,74 are switchable so that the inlet/outlet tubes 68,70 can be configured in fluid communication with one or other of the gas flow tubes 76,78;80,82 connected to the valves. In this embodiment, the gas flow tube 76 forms part of a combustion products line from a combustion chamber (not shown), the tube 78 forms part of an analysis line to a detector (not shown), the tube 80 forms part of a waste line (not shown), and the tube 82 forms part of a purge carrier gas line from a purge carrier gas supply (not shown) The trap and purge device 60 is installed in a combustion analyser, which may take numerous forms, including the general form of the analyser schematically shown in figure 1. The trap and purge device 60 is located downstream of the combustion chamber (not shown in figures 2 and 3), from which combustion products may flow along the combustion products line, and upstream of the detector(s) (not shown), to which a component of interest may flow along the analysis line (as indicated previously, the term, component of interest, includes one or more substance to be detected) . Preferably, the trap and purge device 60 is also located downstream of any gas conditioning stages, so that the combustion products may be dried, filtered and/or cooled before entering the device.

In operation, the trapping and purging procedures take place at different stages and these will now be described.

In the trapping stage, the trap and purge device 60 is in its trapping configuration, as shown in figure 2. That is, -15 -the valve 72 is switched to open the path from the combustion products line, through the gas flow tube 76 and inlet/outlet tube 68, to the trapping chamber 62 and the valve 74 is switched to open the path from the trapping chamber, through the inlet/outlet tube 70 and the gas flow tube 80, to the waste line. The gas flow tubes 78,82 are closed off. In addition, the heater 66 is switched off.

A sample to be combustion analysed is supplied to the combustion chamber, along with a supply of oxygen and possibly also a carrier gas, and the sample is cornbusted to produce combustion products, which include one or more constituent oxides (at least one of which is the component of interest) and a number of residual gases/fluids, such as oxygen, water vapour, and carrier gas (the unwanted components). The combustion products pass out of the combustion chamber and preferably undergo gas conditioning, to dry, filter and/or cool them. In any case, the combustion products cool off relatively quickly after leaving the hot combustion chamber, as they pass along the combustion products line. The combustion products line may be configured to be sufficiently narrow and long for the combustion products to reach the trapping device at a relatively low temperature, preferably room temperature or thereabouts. Alternatively/additionally, further cooling may be provided by, for example, a fan or a Peltier element.

In any case, the combustion products pass into the tube 76 (as shown by the arrow in figure 2), through the valve 72 and the inlet/outlet tube 68, and into the trapping chamber 62.

The combustion products pass through the trapping chamber 62, from its first end to its second end. Since the trapping chamber 62 is substantially filled with trapping -16 -material, the combustion products follow a random path through, over, around and/or past the trapping material, during which process the component of interest is trapped and accumulated by the trapping material. By the time the combustion products reach the second end of the trapping chamber 62, substantially all of the component of interest is removed from the combustion products and stored by the trapping material. The remaining, unwanted components of the combustion products flow out of the tube 70, through the valve 74 and gas flow pipe 80, and into the waste line (as shown by the arrow in figure 2) to be disposed of appropriately. Once all of the combustion products have passed through the trapping chamber 62 -with the component of interest being separated from the unwanted components and retained in the chamber -the trapping stage is complete.

In the purging stage, the trap and purge device 60 is in its purging configuration, as shown in figure 3. That is, the valve 74 is switched to open the path from the purge carrier gas line, through the gas flow tube 82 and inlet/outlet tube 70, to the trapping chamber 62 and the valve 72 is switched to open the path from the trapping chamber, through the inlet/outlet tube 68 and the gas flow tube 78, to the analysis line. The gas flow tubes 76,80 are closed off. In addition, the heater 66 is switched on, to heat the trapping chamber 62 and thus the trapping material 64.

A flow of a purge carrier gas is provided along the purge carrier gas line. The purge carrier gas passes through the gas flow tube 82 (as shown by the arrow in figure 3) , valve 74 and inlet/outlet tube 70, into the trapping chamber 62, then passes through the inlet/outlet tube 68, valve 72 and gas flow tube 78, into the analysis -17-line, which leads to one or more detectors. While only the purge carrier gas flows to the detector(s), the generated detection signal(s) represents a general, background measurement.

When the heater 66 is switched on, the temperature of the trapping material 64 is raised to a level sufficient for the trapping material to release the component of interest therefrom. The desired/required temperature for such release to take place depends on the type of trapping material employed, but is generally in the region of 200°C or so to 400°C or so, but may be as high as 600°C or more for some trapping materials. In any case, due to the relatively high temperature, the trapped component of interest is released by the trapping material 64 and purged out of the trapping device 60, to the detector(s) (as shown by the arrow in figure 3) with the flow of the purge carrier gas. In this way, the detector(s) receives only the purge carrier gas with the component of interest at a relatively high concentration. Since the component of interest is concentrated at the trapping material 64 and is released upon heating of the trapping material to a suitable temperature, it is possible to obtain a relatively sharp, high peak detected signal at the detector(s) . This can help to reduce the lower detection limit for a sample component.

This can also remove substantially all matrix effects, since the unwanted, potentially interfering components are disposed of before detection. Depending on the selectivity of the trapping material, this can also help to reduce the effect of crossover errors, by trapping only the substance, or substances, of interest for a particular detection regime.

An advantage of this arrangement is that the component -18-of interest may be trapped from the combustion products over a relatively long period of time, concentrated in the trapping device, then released to the detector(s) in a relatively short period of time, to provide a higher signal-

to-background ratio. Depending on the component of

interest, the flow rate of the purge gas can be controlled to enhance this advantage further. With a relatively fast flow of purge gas, a higher, sharper signal peak for the component of interest may be detected, resulting in a more accurate measurement. On the other hand, with a relatively slower flow of purge gas, the concentration of the component of interest in the gases flowing to the detector(s) will be higher (also helping to increase the signal-to-background ratio).

It will be noted that, in this embodiment, the combustion products pass through the trapping chamber in a first direction and the purge carrier gas passes through it in a second, opposite direction. This contraf low configuration is advantageous and especially so when employing a trapping material having a particularly high affinity for the component of interest. Such a material would trap the component of interest so strongly that the majority of it would be concentrated at the end into which the combustion products were supplied. Were the purge carrier gas to be supplied from the same end during the purging stage, the component of interest would travel relatively slowly to the other end of the trapping chamber 62, as it would be continually trapped and released by the trapping material 64 on its way. By supplying the purge carrier gas from the opposite end, the component of interest can be released from the trapping material 64 and carried relatively quickly out of the trapping chamber 62 from the same end through which it entered. This can help to reduce the retention time in the trapping device 60 and thus to keep the detected signal peak width relatively small.

Having said that, where the trapping material does not exhibit such a high affinity, or simply if desired, the reverse configuration may alternatively be employed. That is, the gas flow tubes 76,78 on one side of the trapping chamber 62 could form part of a combustion products line and a purge carrier gas line, respectively, and the gas flow tubes 80,82 on the other side of the chamber could form part of a waste line and an analysis line, respectively, so that gases flow through the chamber in the same direction for both the trapping and purging stages.

It will be appreciated that the operation and effectiveness of the trapping device 60 depends on the choice of trapping material 64 to be used. On the one hand, the trapping material should preferably have a high affinity for, and more preferably also selectivity to, the component of interest at relatively low temperatures (preferably, in the region of room temperature), to ensure that substantially all of the component of interest is trapped thereby. On the other hand, the trapping material should preferably readily release the component of interest at higher temperatures, to ensure that substantially all of the component of interest can be recovered at relatively high concentration and in an acceptable time.

The trapping material 64 is preferably a sorbent material and may be an adsorbent for retaining the component of interest on its surface, or an absorbent for retaining the component of interest within its bulk. Preferably, the sorbent material is able to sorb a component of interest at a relatively low temperature and thermally desorb the -20 -component at a relatively high temperature. One adsorbent material which is currently preferred as the trapping material 64 is synthetic carbon. Synthetic carbon is manufactured by Thermo Fisher Scientific Inc., by vapour deposition of carbon at high temperatures onto porous silicate material, which is then dissolved to leave porous synthetic carbon. Synthetic carbon may be used to trap, for example, 502 at room temperature up to around 50°C and will release it at temperatures above approximately 4000C.

Synthetic carbon may also be used to trap NOR, C02, and HC1 Other adsorbent materials may be employed and include activated carbon (also known as activated charcoal or active carbon) and graphitised carbon black, such as Carbotrap X. Such a carbon-based adsorbent has been found to be effective to trap substantially all of the sulphur dioxide in combustion products at room temperature and to allow it to be recovered at an elevated temperature of around 400°C.

Absorbent materials which may be used include molecular sieves, zeolites and forms of silica (i.e., siliceous substances, such as silica gel). These materials are able to trap, for example, SO2 and then to release it at temperatures of around 400°C or more for carbon-based absorbents and of around 600°C or more for siliceous absorbents.

The trapping material 64 may be provided in a number of forms, including as powder, pellets, beads or rods, but preferably in granular form. The trapping material 64 substantially fills the trapping chamber 62 and is preferably kept in place by quartz/glass wool plugs at either end of the chamber. In order to allow for fast heating and (passive) cooling of the trapping material (for trapping and releasing of the component of interest), the -21 -trapping chamber 62 is preferably of relatively small dimensions, such as of diameter of 1-5 mm and length of 10-mm. of course, the dimensions chosen will depend on the desired component release response rate and the type of trapping material used.

The trapping of the component of interest is generally cumulative over time, so, depending on the desired detection limit and/or accuracy of the analysis, the duration of the trapping stage may be chosen from a few seconds up to many minutes. Similar considerations apply to the flow rate chosen for the combustion products through the trapping chamber 62. The flexibility of the operational configuration of the combustion analyser is one of its advantages.

The duration of the purging stage is also subject to variation and may be affected by the release rate of the component of interest from the trapping material 64, the flow rate of the purge carrier gas through the trapping chamber 62, and detector constraints. As to the purge carrier gas, any suitable gas may be used. Preferably, the gas is stable and non-reactive at the temperatures reached for purging the component of interest from the trapping material 64 and does not interfere with the measurements taken at the detector(s). One possible gas is nitrogen. A noble gas is, however, preferred; in particular, argon or helium. Thus, potentially interfering, unwanted components are removed and disposed of from the trapping device 60 before detection, and the component of interest is transferred for detection in a preferably non-interfering, inert carrier gas, which may be chosen and controlled independently of the gases from the sample and combustion chamber.

-22 -As mentioned above, the arrangement of the various gas lines into and out of the trapping chamber 62 may be varied, so that the trapping stage and purging stage gas flows are in the same direction, instead of in contraf low. In such an arrangement, with the combustion products and the purge carrier gas entering the trapping chamber 62 from the same side, these could be arranged to do so via a single gas line. That is, the purge carrier gas could be supplied into and through the combustion chamber and along the combustion products line, into the trapping chamber 62. Indeed, where the combustion stage of the analysis takes place under a background flow of a carrier gas (such as argon), the same gas supply could also be used for the purging stage. In one embodiment, the purging stage could then be initiated simply by switching the valve 74 from the waste line to the analysis line and switching the heater 66 on.

One further alternative embodiment is as follows. The trapping stage is as before, with the component of interest being retained by the trapping material 64 in the trapping chamber 62 at low temperature, while unwanted combustion products are disposed of to waste. The trapping material 64 is a carbon-based material, preferably synthetic carbon.

After the trapping stage, the combustion chamber 62 is sealed off from the combustion analyser and opened for access to the charged trapping material, which is removed from the trapping chamber and transferred into the combustion chamber. The combustion chamber is then operated as if the charged trapping material were a typical, solid sample, so that the trapping material is fully combusted to carbon dioxide and the trapped component of interest (e.g., sulphur dioxide) is released. All gases in the combustion chamber are passed therefrom along a gas line switched to -23 -the detector(s), where normal sample' analysis can take place to measure the concentration of the substance(s) in the component of interest.

This technique may be desirable in some applications, but it does tend to increase the error in the analysis and to increase the overall analysis time. Furthermore, the trapping material is used up in each analysis cycle and synthetic coal, for example, is relatively expensive.

One further alternative is to rinse the trapping material, once charged with the component of interest, with an (organic) solvent, to remove the component of interest.

The solvent may subsequently be provided to the combustion chamber to be cornbusted and analysed, as above.

While the above embodiments use a single trapping chamber 62, it may be desirable to use a dedicated trapping chamber for each respective substance which forms part of the component of interest. In practice, however, it is difficult to find different trapping materials which exhibit the necessary selectivity to the individual substances.

In addition, it will be readily appreciated that the precise arrangements of apparatus components described above are not the only way the invention may be put into practice and all variations, modifications and replacements of features are intended to form part of the invention. For example, the arrangement of two-way valves 72,74 connecting the gas flow tubes 76,78;80,82 to the inlet/outlet tubes 68,70 may be changed so that the gas flow tubes connect directly into the trapping chamber 62 and are each provided with a respective shut-off valve. Also, the valve 72 could be a three-way valve, additionally allowing direct connection between the combustion products line and the analysis line. Furthermore, the heater 66 need not be a -24 -coiled heating element, but may take many forms, as will be well understood.

As to the feasibility of any particular type and form of trapping material, the skilled person will readily appreciate that a straightforward series of analyses, using known, standard samples which will produce known concentrations of one or more substances of interest may be conducted. Measurements can be taken from the combustion products which have passed out of the trapping chamber to waste, to check if and how much substance of interest has been allowed to pass through the trapping chamber without being trapped, to test the affinity of the proposed trapping material. Similar measurements at the detector(s) can be taken from the gases comprising the substance of interest, to test the efficiency of release of the substance by the trapping material. Such feasibility analyses are standard procedures and will be well understood.

For example, such feasibility analyses conducted on TenaxTM (a 2,6-diphenylene oxide polymer) as a possible trapping material for sulphur dioxide revealed that it was able to adsorb substantially all SO2, but did not release all of the 502 at its maximum operating temperature of around 300°C. As such TenaxTM is not currently considered to be suitable for this particular application.

Other combinations, modifications, or alterations to the features of the above embodiments will be readily apparent to the skilled person and are intended to form part of the invention.

The invention may be employed for various applications in, for example, the chemical, refinery, hydrocarbon, petrochemical, and food and beverage sectors. The invention may be used in the analysis of solid, high-viscosity, liquid -25 -or gaseous samples. In particular, the invention may be used in the analysis of refinery products, such as gasoline and diesels.

Claims

-26 -CLAIMS: 1. A combustion analyser for combustion analysing a sample, the analyser comprising: a combustion chamber for receiving and combusting a sample to form combustion products comprising a component of interest and unwanted components; and a trapping device for receiving the combustion products, the trapping device comprising a trapping material having an affinity for the component of interest for removing the component of interest from the combustion products for at least temporary storage of the component of interest in the trapping device.
2. The combustion analyser of claim 1, wherein the trapping material comprises an adsorbent for adsorption of the component of interest thereon.
3. The combustion analyser of claim 2, wherein the adsorbent comprises synthetic carbon.
4. The combustion analyser of claim 2, wherein the adsorbent comprises one of activated carbon and graphitised carbon black.
5. The combustion analyser of claim 1, wherein the trapping material comprises an absorbent for absorption of the component of interest therein.
6. The combustion analyser of claim 5, wherein the absorbent comprises one of a molecular sieve, a zeolite and a siliceous substance.

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7. The combustion analyser of any of the preceding claims, wherein, under a first condition, the trapping material is arranged to remove and at least temporarily store the component of interest and, under a second condition, the trapping material is arranged to release the stored component of interest for subsequent analysis thereof.
8. The combustion analyser of claim 7, wherein the first condition is a first temperature of the trapping material and the second condition is a second, higher temperature of the trapping material, and the trapping device further comprises a heater for heating the trapping material to the second temperature.
9. The combustion analyser of claim 7, wherein the heater comprises a heater controller arranged to control the temperature of the trapping material.
10. The combustion analyser of any of claims 7 to 9, further comprising a purge carrier gas supply for providing a flow of purge carrier gas through the trapping device under the second condition for transfer of the component of interest out of the trapping device.
11. The combustion analyser of claim 10, wherein the purge carrier gas comprises an inert gas or a gas stable under the second condition, such as argon, helium, or nitrogen.
12. The combustion analyser of claim 10 or 11, wherein the purge carrier gas supply comprises a controller arranged to control a flow rate of the purge carrier gas.
13. The combustion analyser of any of claims 10 to 12, the trapping device further comprising: a first end in switchable fluid communication with a combustion products line for supplying the combustion products to the trapping device, and a purge carrier gas line for supplying the purge carrier gas to the trapping device; and a second end in switchable fluid communication with a waste line for receiving from the trapping device the unwanted components for waste disposal thereof, and an analysis line for receiving from the trapping device the component of interest for subsequent analysis thereof, wherein, under the first condition, the trapping device is configured in fluid communication with the combustion products line at the first end and the waste line at the second end and, under the second condition, the trapping device is configured in fluid communication with the purge carrier gas line at the first end and the analysis line at the second end.
14. The combustion analyser of claim 13, wherein the waste line and the analysis line are provided by the same line.
15. The combustion analyser of any of claims 10 to 12, the trapping device further comprising: a first end in switchable fluid communication with a combustion products line for supplying the combustion products to the trapping device, and an analysis line for receiving from the trapping device the component of interest for subsequent analysis thereof; and a second end in switchable fluid communication with a waste line for receiving from the trapping device the -29 -unwanted components for waste disposal thereof, and a purge carrier gas line for supplying the purge carrier gas to the trapping device, wherein, under the first condition, the trapping device is configured in fluid communication with the combustion products line at the first end and the waste line at the second end and, under the second condition, the trapping device is configured in fluid communication with the purge carrier gas line at the second end and the analysis line at the first end.
16. The combustion analyser of any of the preceding claims, wherein the trapping device is configured to allow removal of the trapping material therefrom.
17. A method of combustion analysing a sample comprising the steps of: combusting the sample to form combustion products comprising a component of interest and unwanted components; passing the combustion products to a trapping device comprising a trapping material having an affinity for the component of interest; using the trapping material to remove the component of interest from the combustion products and to store at least temporarily the component of interest in the trapping device.
18. The method of claim 17, further comprising the step of passing the unwanted components to waste from the trapping device.
19. The method of claim 17 or 18, wherein the trapping -30 -material comprises an adsorbent and the component of interest is adsorbed thereon.
20. The method of claim 19, wherein the adsorbent comprises synthetic carbon.
21. The method of claim 19, wherein the adsorbent comprises one of activated carbon and graphitised carbon black.
22. The method of claim 17 or 18, wherein the trapping material comprises an absorbent and the component of interest is absorbed therein.
23. The method of claim 22, wherein the absorbent comprises one of a molecular sieve, a zeolite and a siliceous substance.
24. The method of any of claims 17 to 23, wherein the step of removing and storing the component of interest in the trapping device takes place under a first condition, the method further comprising the step of releasing the component of interest from the trapping material under a second condition.
25. The method of claim 24, wherein the first condition is a first temperature of the trapping material and the second condition is a second, higher temperature of the trapping material, the method further comprising the step of heating the trapping material to the second temperature to release the component of interest from the trapping material.
26. The method of claim 25, further comprising the step of -31 -controlling the temperature of the trapping material with a heater controller.
27. The method of any of claims 24 to 26, further comprising the step of providing a flow of a purge carrier gas through the trapping device under the second condition to transfer the component of interest out of the trapping device.
28. The method of claim 27, wherein the purge carrier gas comprises an inert gas or a gas stable under the second condition, such as argon, helium, or nitrogen.
29. The method of claim 27 or 28, further comprising the step of controlling a flow rate of the purge carrier gas.
30. The method of any of claims 24 to 29, wherein the trapping step takes place over a first period of time and the releasing step takes place over a second period of time, the first period being longer than the second period.
31. The method of any of claims 27 to 29, or 30 when dependent on any of claims 27 to 29, the trapping device comprising a first end and a second end, wherein, under the first condition, the combustion products pass into the first end and the unwanted components pass out of the second end and, under the second condition, the purge carrier gas passes into the first end and along with the component of interest out of the second end.
32. The method of any of claims 27 to 29, or 30 when dependent on any of claims 27 to 29, the trapping device -32 -comprising a first end and a second end, wherein, under the first condition, the combustion products pass into the first end and the unwanted components pass out of the second end and, under the second condition, the purge carrier gas passes into the second end and along with the component of interest out of the first end.
33. The method of any of claims 17 to 23, further comprising the steps of: transferring the trapping material from the trapping device to the combustion chamber; and heating or combusting the trapping material in the combustion chamber to release the component of interest from the trapping material.
34. The use of a trap and purge device between a combustion chamber and a detector of a combustion analyser.
35. A combustion analyser substantially as herein described with reference to figure 2 or 3, or figure 1 in combination with figure 2 or 3.
36. A method of combustion analysing a sample substantially as herein described with reference to figure 2 or 3, or figure 1 in combination with figure 2 or 3.

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