WO2003083433A2

WO2003083433A2 - Method for analysing a gas sample and apparatus therefor

Info

Publication number: WO2003083433A2
Application number: PCT/GB2003/001063
Authority: WO
Inventors: Clive Antony Marchant; Darell Green
Original assignee: Johnson Matthey Plc
Priority date: 2002-03-28
Filing date: 2003-03-13
Publication date: 2003-10-09
Also published as: AU2003214403A8; WO2003083433A3; AU2003214403A1; EP1540298A2

Abstract

The invention comprises a method for collecting a gas sample wherein a gas is collected in a sample conduit (10) of calibrated volume prior to adsortion on an adsorbent material (104) contained in a sample collection vessel (26). The apparatus of the invention comprises a sample conduit (10), means (20) to connect a source (22) of a gas to be sampled to the sample conduit, means (16) to introduce a carrier gas into said apparatus, and a sample collection vessel (26) comprising a container containing an absorbent material (104) which is capable of absorbing or adsorbing at least a portion of said gas to be sampled. A switchable valve means (34) is provided which, in the sampling mode configuration, connects the sample gas to the gas inlet end (12) of the sample conduit and the gas outlet end (14) of the sample to a vent (24) allowing the gas to be sampled to flow from the sample gas source through the sample conduit and out of the apparatus through the vent. In the loading mode configuration the carrier input (16) is connected to the inlet end (12) of the sample conduit and the gas outlet end (14) of the sample conduit is connected to the sample inlet of the sample collection vessel (26) such that carrier gas may flow through the sample conduit and into the sample collection vessel.

Description

Method for analysing a gas sample and apparatus therefor

The present invention relates to a method for analysing the composition of a gas sample, in particular to measuring the amount of one or more pre-determmed components contained within the sample, and to an apparatus for collecting the sample and performing said analysis

In many industrial and / or environmental processes it is necessary to determine accurately the composition of a gas or the analysis of a gas for a particular component or components It is of critical importance that the sample of the gas used for the analysis is correctly taken and accurately controlled in order to derive meaningful conclusions from the analysis Frequently the gas to be analysed is produced at a location remote from the analytical facility required to perform the analysis For example, the composition of production gas at an oil or natural gas well-head must be measured in order to determine certain characteristics of the well and drilling operation but the location of the well may make the provision of an on-site analytical facility difficult or expensive In these circumstances it is necessary to transport a representative sample of the gas to the analytical facility for analysis

Gas sampling has conventionally been done by two distinct types of method, bulk sampling and gas absorption In bulk sampling, a relatively large sample of gas may be collected, e g in a pressurised tank or cylinder The sample may then be transported to the location of the analysis and fed directly or indirectly (i e usually via a pressure reduction system) to the apparatus appropriate to the selected analytical technique, e g a gas-chromatography (gc) apparatus Limitations of this type of arrangement e in the necessity to transport the gas samples because the movement of pressurised gas containers is strictly controlled, particularly over long distances using e g air transportation Also, the cost of suitable gas containers may make this type of sampling unattractive when a large number of samples is required

As an alternative, it is known to collect gas samples by absorption of the gas onto an absorbent material such as an activated carbon using a capillary absorption transfer system (commonly referred to as CATS) In a typical method, the gas to be sampled is caused to flow through a bed of absorbent contained within a tube made from an impermeable material, known as a CATS tube The amount of gas absorbed is controlled by varying the rate of gas flow and the duration of flow The tube is then sealed and transported to the analytical apparatus The flow of gas through the tube is commonly achieved by collecting a sample of the gas in a syringe and then discharging the gas from the syringe through the CATS tube The sample is then admitted into a gc apparatus by a two-stage thermal desorption procedure in order to introduce the sample as a narrow band as required for gc analysis In the first stage, the gas sample is desorbed relatively slowly onto a smaller, known amount of an absorbent contained within a secondary absorption tube The amount and nature of this second absorbent is selected so that the appropriate amount of gas sample for injection into the gc apparatus is absorbed onto the secondary absorbent under known, controlled conditions Typically, the sample is desorbed from the primary absorbent onto the secondary absorbent until a volume equivalent to half the breakthrough volume of the secondary absorbent is absorbed The breakthrough volume for a particular system must be determined beforehand for the conditions used The sample gas is then desorbed balhstically from the secondary absorption tube in order to introduce the desired narrow sample band into the gc apparatus

The conventional CATS tube sampling systems suffer from a number of problems Firstly, environmental contamination of a sample may occur when the gas is collected in the CATS tube Also it has been difficult to control the flow rate of the gas through the CATS tube, leading to improper sampling and a consequent loss of accuracy in the analysis These factors also make the collection of duplicate samples very difficult because the conditions of sampling may not be reproduced exactly Furthermore the requirement to use a two-stage desorption method introduces complexity and further opportunity for analysis errors

US-A-5600075 describes a high pressure gas sample collection system in which a valve having six ports is operated to isolate two ports so that system process gas flows into the valve and in another position of the valve a sample loop can be filled which connects with a gas container which can be isolated and disconnected from the gas sampling system

US-A-5345828 describes a method and apparatus for obtaining a sample from an on-stream plant operation A six-port valve is operated to a filling position to enable plant process fluid to fill a buffer tank A purge gas flows to purge a receiving container Further operation of the valve enables the sample within the buffer tank to flow into the receiving container

US-A-3915013 describes a low pressure gas sample method and apparatus for introducing gas samples into a chromatography column comprising pressurising the gas in a gas sample loop to about the pressure of the chromatography column and then introducing the gas sample into the chromatography column

It is an object of the present invention to provide a gas sampling method and apparatus for the sampling of gases and a method for determining the composition of a gas which overcomes some, at least, of the problems associated with known methods and apparatus

In a first aspect of the invention, we provide a method of collecting a sample of a gas in a transportable sample collection vessel comprising i) Causing an amount of the gas to be sampled to flow through a sample conduit having a gas inlet aperture and a gas outlet aperture, said conduit defining a pre-determmed volume between said gas inlet and outlet apertures under defined conditions of pressure and temperature; ii) Isolating within said sample conduit a sample of the gas such that the sample conduit is isolated from the gas source; iii) Causing said sample of the gas to flow from said sample conduit into a sample collection vessel containing an amount of absorbent material which is capable of absorbing or adsorbing at least a portion of said gas under the prevailing conditions of temperature and pressure, such that said gas is adsorbed or absorbed by said absorbent material.

In a second aspect of the invention, we provide a method of analysing the composition of a gas comprising the steps of: i. Causing an amount of said gas to flow through a sample conduit having a gas inlet aperture and a gas outlet aperture, said sample conduit defining a pre-determined volume between said gas inlet and outlet apertures under defined conditions of pressure and temperature; ii. Isolating within said sample conduit a sample of the gas such that the sample conduit is isolated from the gas source; iii. Causing said sample of the gas to flow from said sample conduit into a sample collection vessel containing an amount of absorbent material which is capable of absorbing or adsorbing at least a portion of said gas under the prevailing conditions of temperature and pressure, such that said gas is adsorbed or absorbed by said absorbent material, iv. Transporting said sample collection vessel containing the gas sample to a sample inlet of an analytical apparatus which is capable of separating and/or determining the amount of at least one constituent of said gas sample and v. Desorbing said absorbed or adsorbed portion of the gas sample from said absorbent material such that it is caused to flow into said analytical apparatus.

In a third aspect of the invention we provide an apparatus for the collection of a gas sample comprising: (a) a sample conduit having a gas inlet end and a gas outlet end

(b) sample gas input means to connect a source of a gas to be sampled to the apparatus

(c) carrier gas input means to introduce a carrier gas into said apparatus

(d) at least one sample collection vessel comprising a container having a sample inlet aperture and a sample outlet aperture and having disposed therein an absorbent material which is capable of absorbing or adsorbing at least a portion of said gas to be sampled,

(e) means to connect said gas outlet end of said sample conduit to said sample collection vessel or to a sample exhaust

(f) switchable valve means having at least a first and a second position, wherein i. in said first valve position said sample gas input means is connected to said gas inlet end of said sample conduit and said gas outlet end of said sample conduit is connected to exhaust means such that the gas to be sampled may flow from the sample gas source through the sample conduit and out of the apparatus through the exhaust and ii. in said second valve position said carrier input means is connected to said gas inlet end of said sample conduit and said gas outlet end of said sample conduit is connected to said sample inlet of the sample collection vessel such that carrier gas may flow through the sample conduit and into the gas collection vessel.

The sample conduit is of a predetermined size so that a known volume of the gas to be sampled is contained within it at the conditions under which sampling is desired to take place. The volume of gas to be sampled depends upon the application for which the sample is used. Typically the sample volume is less than 100 cm³, usually less than 50 and especially less than 20 cm³. In a preferred embodiment sample volumes of between 1 μl and 10 cm³ are used, e.g. 5cm³. The sample conduit is preferably made to tolerances used for gas chromatography sample loops, i.e. preferably to within ± 1%, more preferably to less than ± 0.5% of the desired volume. In another preferred embodiment a larger sample conduit is used, having a volume of about 50 cm³. The sample conduit is very preferably calibrated so that the precise volume of gas sample taken is known. Calibration may be carried out using known methods, for example by using analysing a sample of standard calibrant gas of known composition or by the difference in weight between the empty tube and that of the tube filled with water or another fluid. Preferably the sample conduit is cross-calibrated using more than one calibration method.

The sample conduit is made from material which does not absorb the gas as it passes through the conduit. Suitable materials include those commonly used for sample loops for gas analysis apparatus such as gas chromatography (gc) equipment (for example stainless steel) and may be easily selected by the skilled person.

The gas to be sampled may be introduced into the apparatus of the invention by means of a conduit having at the inlet end a suitable connector for e.g. a gas syringe or a gas pressure reduction apparatus for reducing the pressure of gas contained within a pressurised container or for introducing a sample of a process gas directly from a process plant apparatus. The apparatus may further comprise a valve or similar flow control means for controlling the flow of gas into the apparatus via the sample gas input means. The flow of gas into the apparatus and through the sample conduit may be induced by creating a pressure difference between the gas inlet and the exhaust. This can be achieved by introducing gas at a pressure above ambient, the pressure being sufficient to ensure that the gas flows through the apparatus. Alternatively, the gas flow through the apparatus may be induced by reducing the pressure at the exhaust, e g by connecting to a pump or vacuum pump It is important to collect samples of gas at a known pressure and so if a pressure difference is applied to induce flow through the sample loop then the vacuum or raised pressure should be released to allow the pressure in the sample conduit to equilibrate with ambient pressure before the gas in the sample conduit is isolated Alternatively means for measuring the pressure of the gas within the sample conduit may be provided so that subsequent samples may be taken at similar pressure

The apparatus may further comprise one or more filters or traps to remove unwanted components from the gas stream before it enters the sample conduit For example, a liquid trap may be provided to remove liquid components or to condense out condensable components of the gas stream, such as water, if present Unwanted solids may be removed from the gas stream by placing appropriate filters in the gas path As a further example the gas flow path may pass through a bed of one or more chemical absorbents to remove constituents which are undesirable in the gas sample or which are toxic or corrosive to the sampling or subsequent analytical apparatus For well-head samples, it is usually desirable to remove sulphur compounds and this can be achieved by placing a bed of a sulphur absorbent, such as zinc oxide, such that the gas flows through the sulphur absorbent before entering the sample conduit

The carrier gas is normally selected to be inert towards the sample gas and the absorbent Suitable carrier gases may easily be selected by the skilled person and those gases commonly used in gas chromatography may be suitable for use in the method and apparatus of the invention As an example, nitrogen, of appropriate purity may be a suitable carrier gas for a variety of gas samples

The sample collection vessel comprises a container having a sample inlet aperture and a sample outlet aperture An absorbent material, which is capable of absorbing or adsorbing at least a portion of said gas to be sampled, is disposed within the container in such a way that gas flowing into the container through the inlet must flow through the absorbent material before flowing out of the container through the outlet aperture

Preferably the container takes the form of a tube of an impermeable, inert material such as glass, silica, copper, stainless steel (optionally lined with e g glass) The container should preferably be of a material which conducts heat sufficiently to enable the collected gas sample to be thermally desorbed from the absorbent, preferably by rapid thermal desorption methods The absorbent material is packed into the container so as to fill the container along at least a portion of its length, i e so as to fill the container or form a plug of absorbent within it The container may also contain other materials such as inert packing materials The type of container used is selected to be compatible with the thermal desorption apparatus to be used Most commercial thermal desorbers specify the particular type and dimension of desorption tube which may be used The container, for example a CATS tube, may be re-used after a conditioning process which removes all adsorbed material from the adsorbent Alternatively the container may be disposed of after one or more uses

The sample container may be disposed within a protective housing, made, for example of metal or a plastics material The housing serves to protect the vessel from breakage during sampling or transport of the sample The housing may also protect the contents from contamination or from light, heat or other radiation The housing preferably comprises an inlet and outlet port which are contiguous with the sample inlet and outlet of the sample collection vessel The inlet port is preferably configured and arranged to be connectable directly to the sample conduit gas outlet or a part of the apparatus which connects thereto The outlet port may be configured to be connectable to a sample inlet of an analytical apparatus The inlet and outlet ports are preferably of a different size and shape so that they are readily identifiable and to provide only a single point of connection to either the gas sampling apparatus or the analytical apparatus This feature may help to ensure that samples are collected and removed from the collection vessel in the desired direction It is generally preferred when sampling using a CATS tube system, that the gas sample is desorbed from the absorbent and flows out of the tube in the opposite direction from the direction of flow into the tube during sample collection The tubes may be marked with an arrow or similar mark in order to provide guidance as to the required direction of gas flow When a housing is provided, the user may apply a label or other identifying mark to the housing without risking contamination of the sample by adhesive etc where the tube is removed from the housing before desorption and analysis It is preferred that the inlet and outlet ports may be sealed or capped to protect the sample from external contamination

By absorbent material, we mean a material which can absorb or adsorb certain compounds, whether by absorption or adsorption When we refer to the absorbent material being capable of absorbing or adsorbing at least a portion of the gas to be sampled, we mean that it may be capable of absorbing (and/or adsorbing) all of the gas or of each component of the gas (if the gas is a mixture) or only part of the gas or of only some (but not all) components of the gas Typically the absorbent material is selective, i e it absorbs some components preferentially When the method and apparatus is used to collect tagged samples of gas from a well-head, for example, it is important to know the amount of chemical tagging compound contained within a precise volume of gas and in this case the absorbent material would be selected so that all of the tagging compound is absorbed while much of the bulk of the gas sample can pass through the absorbent material The absorbent or adsorbent material may comprise an activated carbon material, optionally treated to selectively enhance its adsorptive properties, a molecular sieve, e g a carbon molecular sieve such as those sold under the Carboxen™ or Ambersorb™ brands such as Carboxen™569 or Ambersorb™ XE347, graphitised carbon black materials such as those sold as Carbopack™ and Carbotrap™, polymeric adsorbents such as the Chromosorb™ and Tenax™ materials, silica, diatomites, activated aluminas, zeohtic materials or other material which is adapted to adsorb a compound which is required to be trapped within the container An example of a suitable absorbent for a chemical tagging compound used in well-head analysis is a graphitised carbon black such as Carbopack B™ 60-80 mesh The absorbent is selected according to the nature of the gas sample or component thereof which is to be collected More than one type of absorbent may be used within the sample vessel When more than one type of absorbent is used the absorbent materials may be mixed or, preferably, disposed in discrete plugs of adsorbent along the tube in such a way that the least adsorptive material (for the particular gas compound to be trapped) is disposed nearer the gas inlet side of the tube in order to prevent the more adsorptive/absorptive mateπal(s) from irreversibly adsorbing the gas compound The amount of the materials used in such an arrangement should be provided on the basis of the break-through volume of each material in order for each successive bed to be protected from irreversible adsorption

The amount of absorbent mateπal is selected according to the components of the gas which are to be collected and the amount required to be collected It is greatly preferred that the amount of absorbent used is sufficient to absorb all of the desired component or components of the gas sample collected in the sample conduit The skilled person can select an appropriate absorbent and amount of absorbent for the particular gas and gas components used in any particular application Normally this is done by determining the break-through volume of the gas compound which is to be trapped on the selected absorbent The mass of absorbent used is then dependent upon the calculated volume required, the physical properties of the absorbent and the dimensions of the container For example, when it is desired to collect a sample of a trace component (such as of perfluoro-compounds used as tracers or taggants) in a 5 cm³ volume of the gas (such as natural gas), the amount of a carbon absorbent used may be sufficient to provide a 10 mm long plug of absorbent in a 80mm long 3mm (internal) diameter silica tube For a tube of these dimensions it is preferred that the absorbent plug is less than 40 mm long, more preferably < 30mm, especially up to about 25 mm In another embodiment, a commercially available 3 5 inch (89mm) stainless steel tube having a internal diameter about 5mm may be used This tube may contain a greater mass of absorbent than the silica tube described previously, e g about 0 5 - 1 5 g, typically about 1g This type of tube is specified in some ASTM standard sampling methods and is suitable for use with larger sample conduits, e g about 50cm³ The switchable valve means preferably comprises at least one multi-way cross-over valve which may be operated to connect the gas source, the sample conduit, carrier gas inlet, sample vessel and exhaust in at least two configurations. The valve may be operable manually, hydraulically, pneumatically or by a mechanical or electrical impulse. When the gas to be sampled is flammable or the apparatus is used in an environment where a fire or explosion hazard exists, electrical operation may need to be strictly controlled and therefore may not be preferred.

In the first valve position, referred to hereinafter as "sample mode", the sample gas input means is connected to the gas inlet end of the sample conduit and the gas outlet end of said sample conduit is connected to exhaust means such that the gas to be sampled may flow from the sample gas source through the sample conduit and out of the apparatus through the exhaust. In this mode the sample gas flows through the sample conduit so that the predetermined volume of sample gas is contained within the conduit. The sample gas then flows to an exhaust which is preferably at atmospheric pressure so that the pressure within the sample conduit is equalised with atmospheric pressure. In sampling mode the carrier gas inlet may be connected to the sample collection vessel so that carrier gas flows through the absorbent bed, or to a purge or exhaust.

In a second valve position, referred to hereinafter as the "loading mode", the carrier gas input means is connected to the gas inlet end of the sample conduit and the gas outlet end of the sample conduit is connected to the sample inlet of the sample collection vessel such that carrier gas may flow through the sample conduit and into the sample collection vessel. The sample gas source is isolated from the sample conduit in this configuration so that sample gas cannot flow from the gas source into the sample conduit. The sample gas flow may instead be directed to an alternative sample conduit or to an exhaust. In loading mode, the carrier gas sweeps the pre-determined volume of sample gas from the sample conduit and into the sample collection vessel where at least some components of the sample gas are absorbed as it passes through the absorbent. The remaining parts of the gas sample and carrier then preferably flow out of the collection vessel outlet to an exhaust.

The valve may also provide further configurations, for example to isolate the sample conduit from the sample collection vessel, the carrier gas and the sample gas source by closing the gas inlet and outlet ends. In such a configuration any carrier gas and sample gas flowing into the apparatus may be diverted to an exhaust.

It is preferable to purge the sample conduit with carrier gas before placing the valve in sampling mode so that any previous gas sample or other contaminant may be removed. This may be achieved by selecting loading mode so that carrier gas flows through the sample conduit. Since, in loading mode the carrier gas normally flows through the sample collection vessel, it may be preferable to remove the sample collection vessel during purging to avoid collection of residual impurities on the absorbent Alternatively the sample collection vessel may be in place for all or part of the purging so that the carrier flushes through the absorbent

In a further embodiment, the apparatus may comprise more than one sample collection vessel When more than one sample collection vessel is provided, they may each contain the same or a different absorbent (or combination of absorbents), depending on whether it is desired to collect duplicate samples, or to collect different components of the gas stream in each vessel The sample collection vessels may be arranged in series or in parallel, e g using a sample splitter arrangement

In a preferred embodiment, the apparatus comprises more than one sample conduit and at least one sample collection vessel associated with each sample conduit The sample conduits may be of the same volume or different, but are preferably the same The vessels may be the same or different in their size, nature of absorbent etc It is particularly preferred to provide identical sample vessels so that the apparatus may be arranged to collect duplicate samples under the same conditions This overcomes a disadvantage associated with prior methods of gas sampling in which it is very difficult to collect duplicate samples because the conditions of sampling may vary slightly between samples Duplicate samples may be useful to provide a means of calibrating or checking the analytical method used, by spiking one sample with a known amount of a calibrant gas or of a tracer to provide an internal standard

In an embodiment for collecting duplicate samples in which two sample conduits and two sample collection vessels are provided, each sample conduit being connectable to a respective sample collection vessel, the valve is arranged so that in sampling mode, gas from the sample gas source flows through each sample conduit before flowing out of the apparatus, and that in loading mode, the carrier gas flows though both sample conduits to sweep the sample gas contained therein into a respective sample collection vessel In this way each sample collection vessel contains a duplicate sample, collected under identical conditions

When the apparatus provides more than one sample collection conduit and vessel, the multi- way cross-over valve must be specified appropriately, for example when two conduits and two vessels are used, a ten-way valve or equivalent device or arrangement, such as two or more six-way valves, is required

In another embodiment, when two or more sample conduits are provided, each may be different in volume from the or each other sample conduit This provides the facility to select the size of the sample of gas to be collected When this facility is provided, the apparatus may comprise two or more multiport valves, each having connections to a gas source, a sample conduit, carrier gas inlet, sample vessel and sample exhaust in at least two configurations connections. The operator may then select the appropriate inlet positions associated with the valve which connects to the selected sample conduit. Alternatively a single valve, having more than one sample mode position and more than one loading mode position may be connectable to the inlet port of more than one sample conduit, the gas being diverted to the selected sample conduit in sample mode by moving the valve to an appropriate position in which the gas flows from the gas source to the selected sample conduit and the carrier gas being directed in loading mode to the sample conduit containing the gas sample.

When the facility to collect duplicate samples is provided, it may be advantageous to provide means to identify the sample containers which hold the secondary sample so that the samples collected from the first and second (or subsequent) sample conduits may be identified. This may be achieved by labelling, by providing each collection vessel with identifying markings or a particular colour (which may be designed to match a colour on the corresponding outlet port of the apparatus) or by sizing the inlet and outlet ports differently so that only the correct vessel may be connected to the intended outlet port of the apparatus.

In a further embodiment, the apparatus may be adapted for collection of a plurality of samples by providing an array of two or more sample collection vessels for each sample conduit and means by which gas from the or each sample conduit may be directed to one sample collection vessel when the apparatus is in loading mode and then in turn to each other sample collection vessel during the or each subsequent loading mode. The means may comprise a multi-way valve disposed between the outlet of the sample conduit and the array of sample collection vessels. Alternatively or in addition, each of the sample collection vessels may be moveable relative to the outlet of the sample conduit such that each sample collection vessel may be moved in turn to communicate with the outlet of the sample conduit. This may be achieved by mounting the array of sample collection vessels within a housing which is itself movable relative to the sample conduit outlet, such as, for example a cassette or carousel.

The movement of the housing or of the sample tubes within it or the operation of a multi-way valve to divert the sample gas to a selected vessel (hereinafter referred to as "switching of the sample vessels") is preferably actuatable by mechanical, hydraulic, pneumatic or electrical means and is preferably co-ordinated with the operation of the apparatus between sampling and loading modes. In a preferred embodiment the switching of the sample vessels is automated, using a timer or other stimulus such as a change in temperature or pressure or in response to a programmed instruction, e.g. from a computer program, so that a number of samples may be collected automatically and /or sequentially in a programmed manner. This type of arrangement may be useful for collecting process gas samples at different stages in a chemical process By providing at least two sample conduits which are each connected to a switchable array of sample tubes, a plurality of duplicate samples may be taken

More than one sample from the sample conduit may be collected in the same sample collection vessel This may be required when sampling a process stream for example by taking different samples at different times The analysis is then done on the total sample collected in the tube When collecting more than one sample in a single collection vessel, it is preferred that carrier gas is not run continuously through the collection vessel between samples in order to avoid blowing the collected sample through the adsorbent The or each sample collection vessel is preferably sealed after the sample has been collected, to avoid contamination

The apparatus may incorporate a timer so that the time of flow of sample gas and carrier gas may be controlled to ensure that a repeatable procedure is used for each sample The time of flow of sample gas into the sample conduit should be sufficient to allow the gas in the sample conduit to equilibrate The time of flow of carrier gas must be sufficient to sweep the whole of the gas from the sample conduit, yet not long enough to allow the sample to permeate completely through the adsorbent bed The time of flow required depends upon the size of sample conduit and the nature of the gases used and should normally be pre-optimised using a control sample The operation of the apparatus may be controlled by a microprocessor or other control device or it may be operable manually

The apparatus may additionally incorporate flow control valves, pressure relief valves and vents and flow meters as appropriate

The method and apparatus of the invention may be used for a variety of gas sampling applications An example of such an application is in the tracing of well-fluids in a gas or oil production well where a small amount of a tracer compound such as a perfluorocarbon is injected into the well at one or more locations and then the appearance of tracer in gas drawn from a different part of the well is monitored by taking gas samples and analysing for the tracer compound In this type of application, the absorbent used is selected to adsorb the tracer compound selectively when a stream of the production gas is passed over it

The apparatus is particularly suited for taking small samples of gas with great precision because the use of a calibrated volume sample conduit ensures that the amount of sample taken is controlled

Embodiments of the method and apparatus according to the invention will be further described, by way of example only, with reference to the accompanying drawings, which are - Fig 1 A schematic representation of an apparatus in loading mode according to a first embodiment of the invention,

Fig 2 A schematic representation of an apparatus in sampling mode according to a second embodiment of the invention, Fig 3 A flow chart showing the flow of gas in the apparatus of Fig 2 in sampling (3A) mode and loading (3B) mode,

Fig 4 A schematic longitudinal sectional view of a preferred form of CATS tube for use with the apparatus of the invention

Fig 5 A schematic representation of an apparatus in sampling mode according to a third embodiment of the invention,

The apparatus shown in Fig 1 comprises a 5 cm³ (± 0 02 cm³) sample conduit loop 10, having an inlet end 12 and an outlet end 14 Carrier gas enters the apparatus via carrier gas inlet 16 and is controlled by a metering valve 18 The sample gas inlet line 20 allows flow into the apparatus from a sample gas source, in this example a gas syringe 22 Line 24 leads to a vent or exhaust at ambient pressure and temperature A CATS tube 26 containing an absorbent material is connected such that, in the configuration shown (loading mode), a gas flow path exists along line 28 from the carrier gas inlet 16 via the sample loop 10 Needle valve 30 and flow-meter 32 are connected in-line and indicate the flow of gas through line 28 The outlet 27 from the CATS tube leads to a vent A 6-way cross-over valve 34 connects the carrier inlet 16 to the sample conduit inlet 12 and the sample conduit outlet 14 to loading line 28 The gas from the syringe 22 is directed to a vent via valve 34 and line 24 6-way valve 34 is arranged to be able to connect, in sampling mode, sample inlet 20 to sample loop inlet 12, sample loop outlet 14 to vent, and carrier gas inlet 16 to the sampling line 28 so that the gas sample from source 22 may flow through sample loop 10 and carrier gas flows through the CATS tube 26

The method of collecting a gas sample, using the apparatus of Fig 1 is performed by first purging the sample loop 10 in loading mode by allowing carrier gas to flow through the sample loop 10 and then to a vent Then the valve 34 is operated to place the apparatus in sampling mode, whereby sample inlet 20 is connected to sample loop inlet 12, sample loop outlet 14 to vent, and carrier gas inlet 16 to the sampling line 28 so that the gas sample from source 22 may flow through sample loop 10 and carrier gas flows through the CATS tube 26 Gas from the gas syringe 22 is injected into the apparatus through line 20 and flows through the sample loop 10 and out to a vent The flow of sample gas is maintained for sufficient time (e g 4 minutes) to fill the sample loop 10 and allow it to equilibrate at atmospheric pressure and temperature The time of flow, when optimised, should be the same each time a similar sample is taken When the sample loop contains the gas sample at equilibrium, the valve 34 is operated to switch to loading mode in which the carrier inlet 16 is connected to the sample conduit inlet 12 and the sample conduit outlet 14 to loading line 28. In this mode, the sample gas in loop 10 is swept out of the loop 10 and through the CATS tube 27, where the gas, or the desired component of the gas, is adsorbed onto the adsorbent contained within the tube. The carrier gas is allowed to flow for sufficient time to ensure that all of the sample flows from the loop 10 and through tube 27. The time of flow should be less than that required to move the sample from the adsorbent out of the tube (the breakthrough time). Preferably the time of flow is less than half of the breakthrough time. This time may be predetermined on a control sample by known methods. Normally, using a 5 cm³ sample it is sufficient to allow the carrier gas to flow for 4 minutes. Again, the time should be the same for each similar sample taken. The CATS tube containing the sample may be removed from the apparatus, preferably capped and labelled, and then transported for desorption and analysis. A fresh CATS tube may then be coupled to line 28 if another sample is required.

The apparatus shown schematically in Fig 2 comprises two sample loops (40a & 40b) and two CATS tubes (42a & 42b) but is otherwise very similar to the apparatus shown in Fig 1. The source of gas to be sampled is, in this example, a gas cylinder 44. A pressure let-down system 46, pressure regulation valve 48 and ball valve 50 are provided to control the entry of gas into the apparatus. A pressure relief valve may also be provided to protect the system from damage in the event of over-pressurisation. A scrubber 52 is provided to remove H₂S from the in-coming gas before it enters the apparatus. The carrier gas line 54 is also provided with a pressure relief valve 56 and pressure gauge, a flow controller 58 and a ball valve 60. Lines 62, 64 and 66 all connect to a vent. The needle valve 68 and flow-meter 70 operate as described previously. The carrier gas line contains a splitter 55 to produce two carrier gas streams 54a and 54b.

The 10-way cross-over valve 72 is operable to switch between sampling mode and loading mode. In sampling mode, in the configuration shown (sampling mode), the gas source 44 is connected via the pressure let-down system 46 and gas scrubber 52 to sample loop inlet 74a, through the sample conduit 40a and out through sample loop outlet 76a, then through the second sample loop 40b via inlet 74b and outlet 76b and then through the flow-meter to vent line 62. The carrier gas, in this mode flows to the CATS tubes 42a, 42b and then to the vent 64a, 64b. When the valve 72 is operated to switch to loading mode, the sample gas is routed to the vent 62. The carrier gas lines 54a and b flow through sample loops 40a and 40 b respectively and then into a respective CATS tube 42 a or b.

Fig 3 shows flow charts of the gas flows in the apparatus of Fig 2 in sampling mode (Fig 3A) and loading mode (Fig 3B). Using the apparatus shown in Figs 2 and 3 duplicate gas samples may be collected at the same time and under identical conditions The method of using the apparatus is as described for the basic apparatus in Fig 1 , with obvious modifications for dealing with two CATS tubes instead of one

A preferred form of a CATS tube is shown in detail in Fig 4 For clarity, the parts are shown in slightly exploded form, whereas in practice they may be intended to fit together more closely The CATS tube 100 is a silica tube, 80mm long and having an internal diameter of 3mm and an outside diameter of 5mm It is generally disposed within a brass housing 102 The adsorbent bed 104 is 10mm deep and comprises a graphitized carbon black compound (Carbopack™ B, which has a mesh size 60/80, density 0 36 gem^"3, surface area 100 m²g^"1 and is suitable for use with low hydrocarbon gases) The adsorbent bed is held in place with an inert packing of glass fibres or glass frit The silica tube is held at each end within housing 102 by a toroidal PTFE support 108 Each end of the housing terminates in an internally threaded end portion 110 of wider diameter than the mam body portion An externally threaded end-piece 112 is screwed into end portion 110 End-piece 112 carries an externally threaded nozzle 114 defining a passage 116 through which gas may enter and leave the tube 100 A brass end cap 118 is provided to close passage 116 so that the tube may be sealed when not in use Nozzle 115 is of a different diameter from nozzle 114 at the opposite end of the housing This is to facilitate correct sampling so that the gas flows through the tube in one direction during sampling and out of the tube in the opposite direction during discharge The gas flows into the tube in the direction of the arrow and, preferably, out of the tube in the opposite direction

The use of a CATS tube of the type described, which is adapted to contain the whole of the sample taken enables the sample to be desorbed in a single stage into the gas chromatography column For desorption, the silica tube 100 is removed from the housing 102 and inserted into a thermal desorber

Fig 5 shows an alternative embodiment of the apparatus which is adapted to collect five gas samples sequentially, each in duplicate The apparatus is in part similar to that shown in Fig 2 and like numerals refer to similar parts of the apparatus In this example, the gas source is a chemical process and gas enters the apparatus via a suitable process gas let-down unit 78 The sample loop outlets, 76a&b or the carrier gas conduits 54a&b may each be connected to a multi-way valve 80a&b The multi-way valve 80 has an inlet 81 and five outlets, each outlet connecting to a CATS tube 82a,b,c,d&e Each CATS tube outlet is connected to the vent, as before The multi-way valve is operable to connect any of the CATS tubes 82 to inlet 81 In this way the carrier gas or the gas sample may be caused to flow into the selected tube Operation of the valve 80 causes the carrier gas or gas sample to flow into a different selected CATS tube By switching the CATS tubes between each cycle of sampling mode and loading mode, five gas samples may be taken sequentially without the need to disconnect the used CATS tube and connect a fresh one between samples The use of a second multi-way valve 80b and a corresponding set of CATS tubes allows a duplicate of each sample to be taken

Claims

1 A method of collecting a sample of a gas in a transportable sample collection vessel comprising i causing an amount of the gas to be sampled to flow through a sample conduit having a gas inlet aperture and a gas outlet aperture, said sample conduit defining a predetermined volume between said gas inlet and gas outlet apertures under defined conditions of pressure and temperature, then II isolating within said sample conduit a sample of the gas such that the sample conduit is isolated from the gas source, and then in causing said gas to flow from said sample conduit into a sample collection vessel containing an amount of absorbent material which is capable of absorbing or adsorbing at least a portion of said gas sample under the prevailing conditions of temperature and pressure, such that at least a portion of said gas sample is adsorbed or absorbed by said absorbent material

2 A method of analysing the composition of a gas comprising the steps of i causing an amount of said gas to flow through a sample conduit having a gas inlet aperture and a gas outlet aperture, said conduit defining a pre-determmed volume between said gas inlet and gas outlet apertures under defined conditions of pressure and temperature, then n isolating within said sample conduit a sample of the gas such that the conduit is isolated from the gas source, then in causing said sample of gas to flow from said sample conduit into a sample collection vessel containing an amount of absorbent material which is capable of absorbing or adsorbing at least a portion of said gas sample under the prevailing conditions of temperature and pressure, such that at least a portion of the gas sample is adsorbed or absorbed by said absorbent material, and iv transporting said sample collection vessel containing the absorbed gas sample to a sample inlet of an analytical apparatus which is capable of separating and/or determining the amount of at least one constituent of said gas sample and v desorbing said absorbed gas sample from said absorbent material such that it is caused to flow into said analytical apparatus

3 An apparatus for the collection of a gas sample comprising (a) a sample conduit having a gas inlet end and a gas outlet end

(b) sample gas input means to connect a source of a gas to be sampled to the apparatus such that the gas to be sampled may flow into the apparatus

(c) carrier gas input means to introduce a carrier gas into said apparatus (d) a sample collection vessel comprising a container having a sample inlet aperture and a sample outlet aperture and having disposed therein an absorbent material which is capable of absorbing or adsorbing at least a portion of said gas to be sampled, (e) means to connect said gas outlet end of said sample conduit to said sample collection vessel or to an exhaust (f) switchable valve means having at least a first and a second configuration, wherein i in said first configuration said sample gas input means is connected to said gas inlet end of said sample conduit and said gas outlet end of said sample conduit is connected to an exhaust such that the gas to be sampled may flow from the sample gas source through the sample conduit and out of the apparatus through the sample exhaust and n in said second configuration said carrier gas input means is connected to said gas inlet end of said sample conduit and said gas outlet end of said sample conduit is connected to said sample inlet of the sample collection vessel such that carrier gas may flow through the sample conduit and into the gas collection vessel

An apparatus as claimed in claim 3, comprising two or more sample conduits each having at least one respective sample collection vessel associated therewith, the sample conduits being arranged such that the sample gas or the carrier gas may flow through each of them simultaneously either in parallel or in series

An apparatus as claimed in either claim 3 or claim 4, wherein more than one sample collection vessel is associated with each sample conduit and means are provided whereby the gas from the sample conduit may be directed to any one of said sample collection vessels