AN APPARATUS FOR DETECTING SMALL CONCENTRATIONS OF ETHENE IN AIR

Abstract: An apparatus for detecting small concentrations of ethene in air, comprising a spherical glass vessel, having a flat bottom with a window portion, and a radiation detector in front of said window with a radiation ineterruptor interposed therebetween. A gas mixing tube extends in said vessel opposite to said bottom towards the centre of said vessel. Near its outer extremity said tube is provided with a constriction for sucking inwards air to be tested and ozone produced in an excess amount by an ozone source, and for mixing said gas flows. An exhaust connection is provided near said mixing tube, and serves to remove gas by maintaining a small negative pressure differential inside said vessel. Inside said mixing tube the mixing of the gases is completed, and a chemoluminescent reaction between the ozone and the ethene in the air to be tested is completed within said vessel before the gas flow leaves said vessel. The radiation produced by said reaction is reflected by a reflecting coating towards said window and is transformed in a pulsating electrical signal corrected for the dark current of of said detector. The correction for the dark current can be improved by using a signal amplifier controlled by a signal with the radiation interruption frequency. Preferably the ozone is produced by an ozonizator comprising coaxial tubes between which a corona discharge is maintained.

Description: An apparatus for detecting small concentrations of ethene in air. Vegetal products, in particular fruit, develop during storage ethene (CJ- ), and the presence of ethene in the environment of such products, in particular between densely packed fruit in crates and/or in closed transport and storage spaces, can give rise to an accelerated ripening and decay. On the other hand a controlled addition. of ethene can lead to a desired effect, e.g. efflorescence of flowers. However there do not exist apparatuses allowing to measure, outside a laboratory, small concentrations of ethene in air in a cheap manner as is required for monitoring stored vegetal products and for metering ethene for accelerating the ripening of such pro¬ ducts. The invention provides an apparatus for detecting small concentrations of ethene in air by a chemoluminescent reaction be- tween ethene and ozone, which is of a simple construction, and allows a continuous, reproducible, dependable and quick indication of the ethene content in the air to be tested. The apparatus of the invention is characterised by: - a substantially spherical glass vessel with a flat botton and a neck portion opposite said bottom; - a mixing tube fitting in said neck portion, the inner extremity of said tube ending in about the centre of said vessel, said mixing tube being provided, near the outer extremity, with a con¬ striction; - a first supply duct for the air to be tested, having an outer diameter which is smaller than the inner diameter of the outer extremity of said mixing tube, and being inserted into said outer extremity; - a second supply duct for ozone connected to an ozonisation means, and communicating with the annular space between said mixing tube and said first supply duct, said ozonisation means being adapted to produce an excess amount of ozone in respect of the ethene content in the air to be tested; - a discharge duct connected to a suction means and opening into said vessel near said neck portion, said suction means being 5 adapted to maintain a small negative pressure differential in said vessel in respect of the ambient air; a reflecting layer covering the outer wall of said vessel with the exception of a radiation window in said flat bottom; a radiation transducer behind said window adapted to produce 0 electrical signals in response to incident radiation quanta generated by the chemoluminescent reaction between ethene in the tested air and ozone produced by said ozonisation means; a rotating vane shutter arranged between said radiation window and said radiation transducer, said shutter being mounted on the 5 axis of an electric driving motor; a signal processing circuit connected to the output of said transducer and adapted to produce an electrical output signal corrected for the dark current of said transducer; and an opaque casing surrounding at least said vessel, said 0 shutter and said transducer. In said mixing tube the air to be tested is thoroughly mixed with an excess amount of ozone so as to ensure that all the ethene present in the air will react with ozone, so that the radi¬ ation produced by said reaction forms a direct measure of the amount 5 of ethene present in the air. This reaction starts already in said mixing ' tube, and will continue in the gas mixture flow leaving said mixing tube and flowing backwards towards the exhaust duct, the suction means being adjusted so that said reaction is completed before the gas mixture leaves the vessel. The reflecting layer on G the outer wall of said vessel ensures that the radiation produced by said reaction will substantially reach the radiation window. The radiation leaving the vessel is modulated by the rotating shutter so as to obtain a pulsating signal that can be amplified by an amplifier operating near the interruption frequency,. so that the greater . , part of the frequencies of the dark current of- said.-.transducer i^s effectively suppressed. The constriction in the mixing tube acts, together with the first supply duct, as a jet pump sucking inwards the ozone supplied by the second supply duct, and providing an initial mixing of both gas flows, which mixing is completed inside the mixing tube. Further advantageous features of this apparatus are men¬ tioned in the appended subclaims. It is remarked that in US 3528779 (1970) an apparatus for detecting ozone in air using nitric oxide (NO) as the reaction gas for producing a chemoluminescent reation, and from US 3710107 (1973) a laboratory apparatus is known for detecting ozone in air in which ethene is used as the reaction gas, or for detecting nitric oxide by means of ozone as the reaction gas. Said prior publications do not disclose an apparatus for detecting ethene, and the apparatuses disclosed therein, although having same features in common with the apparatus of the invention, are not suitable for the present pur¬ poses, in particular since they do not produce a sufficient mixing and/or residence time inside the reaction vessel at higher flow rates as required for an apparatus for the present purposes. The invention will be elucidated -below by reference to a a drawing, showing in: Fig. 1 a diagrammatical section of an apparatus according to the invention; and Fig. 2 a diagraιτgτιatical section of a preferred ozonisation means for this apparatus. In Fig. 1 a preferred embodiment of the apparatus of the invention is diagrammatically shown. This apparatus comprises as the most important element a reaction chamber 1 with a substantially spherical wall 2 and a flattened terminal surface 3. To that end use can be made of a flat-bottomed laboratory flask. The wall beyond the flat portion 3 is provided with an outer reflecting layer 4. In front of the flattening 3 a tube 5 is inserted inwards through the wall; when using a glass laboratory flask said tube 5 can be inserted through the neck thereof. This tube 5 terminates substantially in the middle of the chamber 1. Laterally thereof a suction tube 6 opens into the chamber 1, which tube is to be connec¬ ted to a suction pump not shown. The tube 5 comprises, furthermore, a constriction 7 behind which an inner tube 8 ends, the latter being surrounded by an outer tube 9. The tube 8 is the supply tube for an air sample to be tested, and the tube 9 is connected to an ozone source. If a continuous sampling is to take place, a capillary tube 10 can be included in the tube 8 for ensuring a substantially constant flow rate. Of course also other means can be used for stabilising the flow rate. In the constriction 7 a flow acceleration occurs, and therein the air sample sucked in is well mixed with the ozone sucked in through the tube 9. The whole assembly is constructed in such a manner that an ample ozone excess in respect of the amount of ethene in the air sample is present. The air sample reacts in the chamber 1, and the reaction with ozone gives rise to the emission of radiation. There exists a fixed relationship between the number of radiation quanta and the number of moles of the reagents. The reflecting layer 4 ensures that the radiation can emerge only through the flat wall portion 3. Behind the flat wall 3 a sensitive radiation detector, in particular a photo-multiplier tube, is arranged, which is adapted to produce a current which is proportional to the number of radiation quanta, so that this current is a measure for the amounts of reagents taking part in the reaction. Since there is an ample excess of ozone, a complete conversion of the ethene in the gas mixture is ensured, and the intensity of the radiation phenomenon is determined by the amount of this component in the sample The radiation detector 11 produces also if no radiation is incident a so-called dark current, the (average) intensity of which depending, inter αliα, of the temperature, said current impeding the measurement of weak currents (i.e. low ethene concentrations). In order to suppress the effect of the dark current, the radiation can be periodically interrupted by means of a rotating vane shutter 12 which is driven by a motor 13. At least the shutter 12, the window 3 and the transducer 11 should be surrounded by an opaque enclosure shown schematically at 14. The pulsating signal thus obtained is sent to an amplifier 15 with a narrow transmission band around the shutter frequency so asto suppress the greater part of the dark current. In particular use can be made of a reference signal having the shutter frequency. The latter can, for instance, be obtained by means of an auxiliary radiation source 16 and an associated transducer 17 which are arranged at either sides of an auxiliary vane shutter 18 which is mounted on the shaft of the motor 13 as well, but outside the enclosure 14. The trans¬ ducer 17 is connected to a control input of the amplifier 15. The output of said amplifier is connected to a display or writing appa¬ ratus 19. The reaction speed is, in the case of ethene, rather low. In order to ensure that the reaction will be completely terminated before the gas flow leaves the reaction chamber 1, a relatively large reaction chamber 1 is used, e.g. with a volume of 1 1, and the suction connection 6 is provided so that a sufficient residence time in the chamber 1 is obtained. It can be advisable to increase the reaction speed by heating the air sample, e.g. by means of an electrical heater around or in the duct 8. For supplying a sufficient amount of ozone, advantageous¬ ly a corona ozonisator is used which is schematically shown in Fig. 2. The latter comprises two co-axial tubes 20 and 21, . which, at the mutually remote sides, are provided with metallic layers 22 and 23 resp., and air can be led through the intermediate space 24. The layers 22 and 21 are connected to a high-voltage source (Tesla- transformer), in order to obtain, in the space 24, a continuous corona discharge. The length of the tubes 20 and 21 determines the ozonisation effect. Such an apparatus can be constructed as a portable one. It is, then, possible to lead continuously a gas amount of, for instance, about 3 1/min through the chamber 1 (i.e. the contents thereof are refreshed 3 times/min), viz. 2,5 l/min air sample and 0,5 1/min ozone. In particular such an apparatus can be used for monitoring the ethene content in the air within storage spaces for fruit, flowers and the like, and the ventilation means used in such spaces can be controlled by such an apparatus so as to switch on said means only when the ethene concentration exceeds a given threshold value, which will lead to substantial energy savings, in particular in the case of cooled spaces. On the other hand, such an apparatus can also be used to control the supply of ethene to such spaces if the ripening of the products contained therein is to be accelerated by means of a controlled supply of ethene.

Claims: C l a i m s 1. An apparatus for detecting small concentrations of ethene in air by a chemoluminescent reaction between ethene and ozone, charac teri sed by: - a substantially spherical glass vessel (1) with a flat bottom (3) • and a neck portion opposite said bottom (3); - a mixing tube (5) fitting in said neck portion, the inner extremity of said tube (5) ending in about the centre of said vessel (1), said mixing tube (5) being provided, near its outer extremity, with a constriction (7); - a first supply duct (8) for the air to be tested, having an outer diameter which is smaller than the inner diameter of the outer extremity of said mixing tube (5), and being inserted into said outer extremity; - a second supply duct (9) for ozone connected to an ozonisation means, and communicating with the annular space between said mixing tube (-5) and said first supply duct (8), said ozonisation means being adapted to produce an excess amount of ozone in res¬ pect of the ethene content of the air to be tested; - a discharge duct (6) connected to a suction means and opening into said vessel (1) near said neck portion, said suction means being adapted to maintain a small negative pressure differential in said vessel (1) in respect of the ambient air; - a reflecting layer (4) covering the outer wall of said vessel with the exception of a radiation window in said flat bottom (3); - a radiation transducer (11) behind said window (3) adapted to produce electrical signals in response to incident radiation • quanta generated by the chemoluminescent reaction between ethene in the tested air and ozone produced by said ozonisation means; - a rotating vane shutter (12) arranged between said radiation window (3) and said radiation transducer (11), said shutter (12) being mounted on the axis of an electric driving motor (13); _ a signal processing circuit (15) connected to the output of said transducer (11) and adapted to produce an electrical output signal corrected for the dark current of said transducer (11); and - an opaque casing (14) surrounding at least said vessel (1), said shutter (12) and said transducer (11). 2. The apparatus of claim 1, characterised by an assembly of an auxiliary light source (16), a light-sensitive means (17) and a second rotating vane shutter (18) mounted en the axis of said driving motor (13), ' said assembly being placed outside said opaque casing (14), the output of said light-sensitive means being connected to a control input of said processing circuit (15) for supplying to the latter a signal with the same frequency as the radiation interruption by said first shutter (12) to be mixed with the output signal of said transducer (11). 3. The apparatus of claim 1 or 2, characterised in that a gas flow stabilising means (10), in particular a capillary tube, is connected to said first supply duct (8) in order to maintain a substantially constant supply flow rate of the air to be tested. 4. The apparatus of any one of claims 1..3, characterised in that said ozonisation means comprises two co-axial tubes (20, 21) defining therebetween a flow channel (24), and being provided at the surfaces not contacting said channel (24) " with metallic electrode layers (22, 23) connected to a high-voltage source adapted to pro¬ duce corona discharges between said electrode layers (22, 23), one end of said channel (24) communicating with the ambient air, and the other end being connected to said second supply duct (9). 5. The device of any one of claims 1..4, characterised in that said vessel (1) is a commercially available flat-bottomed laboratory flask provided with an additional tube connection (6) serving as the discharge duct or connection therefore.

Inventors: HOUPT PIETER MARIUS, HOUPT, PIETER, MARIUS
Applicants: NIEAF SMITT B V, NIEAF-SMITT B.V.
Application year: 1985
Main classification: G01N21
IPC codes: G01N21, G01N33