EVAPORATION OF LIQUIDS AND RECIRCULATION OF PURIFIED GAS

Abstract: A liquid sample (3) hold in an open container (2) and including a volatile solvent, is evaporated by continuously removing inert gas rich in vapour by means of a suction tube (5) whose open end is held close to the top surface of the liquid. Vapour in the gas is then extracted in a refrigerated condensing vessel (6), from whence the purified gas is fed by a pump (7) for recirculation back into a sealed chamber (1) in which the container is mounted. In a modification, the inert gas is fed directly through a nozzle (17) into the container (2) while the gas/vapour mixture is extracted from the chamber (1) for recirculation, after passing through a condenser (6) or the like.

Description: EVAPORATION OF LIQUIDS AND RECIRCULATION OF PURIFIED GAS Field of invention This invention concerns evaporation of liquids. Background to the invention Directing a stream of air or an inert gas such as nitrogen onto the surface of a volatile solvent, is a technique used for evaporating such volatile liquids, but it suffers from the disadvantage that large volumes of inert gas or air, contaminated with vapour, must be disposed of. This can be expensive if the liquid is inflammable or toxic. In one arrangement illustrated in Figure 1 of the accompanying drawings an inert gas, which for convenience will be generally referred to herein as nitrogen, is blown onto the surface of the solvent in one or more tubes. The πibes are usually located in a housing which is not hermetically sealed and the spent nitrogen is ducted to an area, such as a fume hood, so that the nitrogen contaminated with solvent vapour is led away in a manner which is considered safe. As mentioned, this technique uses considerable quantities of nitrogen and generates large volumes of contaminated nitrogen which can be difficult to dispose of. It is an object of the present invention to provide an improved method for exposing volatile liquids to an inert gas during an evaporation process, which reduces the problem identified above. Summary of the Invention According to one aspect of the present invention there is provided a method of evaporating a liquid sample including a volatile solvent, comprising the step of continuously removing inert gas which is rich in solvent vapour from near the surface of the liquid. This allows much easier collection of any toxic or inflammable material. The gas may be withdrawn by a suction tube which is preferably maintained close to the surface of the liquid, as the liquid level drops due to evaporation. Advantageously the evaporation rate may be increased by imparting an orbital motion to the container holding the liquid sample, causing the latter to spin around in the form of a vortex. Preferably a liquid sample is contained within a chamber, and the volume of permanent gas recirculated to the chamber is progressively reduced, thereby to assist evaporation. The invention also extends to an apparatus for performing the aforesaid method. According to another aspect of the invention there is provided an apparatus for effecting the evaporation of a volatile solvent, wherein an inert gas, which is held in a sealed environment connected to at least one container containing the solvent, is recirculated via a condensing or absorbing device for absorbing or extracting solvent vapour entrained in the recirculating inert gas. Preferably the container holding the liquid sample is mounted in a vortex evaporator, known per se. Other aspects of the invention are defined in the appended claims. Brief description of the invention The invention will now be described, by way of example only, with reference to the accompanying drawings in which: - Figure 1 is schematic view of a known device in which an inert gas, such as nitrogen, is blown down onto the surface of a liquid to be evaporated; Figure 2 shows apparatus in accordance with the invention for evaporating a volatile solvent; Figure 3 shows apparatus closely similar to Figure 2 but operating in a different manner; and Figure 4 shows a modification of the apparatus of Figure 3. Detailed description of the drawings. In Figure 2 a liquid sample 3 in a solvent mixture is contained in a tube container 2 inside a sealed chamber 1. A suction tube 5 is placed with one end 11 in the container 2 at a position close above the top surface of the sample 3. This tube passes through the chamber 1 through a sealing means 12, through a further sealing means 13, and finally into a sealed refrigerated condensing vessel 6 in which the solvent vapour is stripped from the vapour/inert gas mixture. A pipe 8 connects the refrigerated condensing vessel 6 to a pump 7 which draws the purified inert gas (eg air or nitrogen) from the vessel 6 and recirculates it through a further pipe 9 back to the chamber 1 again through a sealing means 14. From here the gas passes into the top of the container 2, as shown at 10, mixing with solvent vapour 4 which has been evaporated and passing back into the tube 5. A heater 16, mounted around the pipe 9 near its inlet to the chamber, enables the cooled gas to be heated to a suitable temperature. A pressure control system 15 allows the pressure in the system to be held at any required level, by venting gas to atmosphere or admitting gas into the system from a suitable source, eg a gas cylinder via a pressure regulator. In operation the pump reduces the pressure in the refrigerated vessel 6, which causes the vapour 4 to be drawn through the tube 5 into the vessel 6 together with some inert gas, as above described. The temperature of the vessel 6 is maintained at a low enough value to condense the vapour, and any permanent gas passes through the vessel into the pump 7. Gas extracted by the pump 7 could be discharged to the atmosphere because it has been stripped of undesired vapours in the refrigerated vessel 6 or, as in this illustration, may be recirculated to continue to evaporate the sample. If the gas from pump 7 is discharged to atmosphere, fresh gas is introduced to the chamber 1 via the pressure control means 15. Suction by the pump 7 can, and preferable does, result in the pressure within the chamber 1 dropping as the evaporation process continues, in that the volume of air or nitrogen introduced by the control means 15 is arranged to be less than the volume of air or nitrogen withdrawn by the pump 7. The reduced pressure in the chamber 1 assists in the evaporation of the solvent in the container 2 in a manner known per se. The efficiency of the process is enhanced if the tube 5 is progressively lowered as the liquid level drops due to evaporation and held a few millimetres above the sample. This can be achieved manually using a means for lowering the tube 5 or raising the sample 3. Automatic positioning can also be used, using for example optical means to sense the position of the top of the liquid sample and using a sensor system to keep the bottom of the suction tube a few millimetres above the level. Electrical proximity means could also be used to sense the level. In the example shown, a single sample tube container is illustrated. The process can be applied equally to arrays of many tubes and blocks with numerous wells, for example microtitre plates. Although not shown in Figure 2, the sample tube 2 may be subjected to an orbital motion without alteration of the lateral orientation of the tube, such that the liquid 3 inside the tube moves around the tube in the form of a vortex. Such an arrangement is known per se as a vortex evaporator. The resultant agitation of the liquid in the tube increases the evaporation rate, allowing a more rapid extraction of vapour-rich gas to take place through the suction tube 5. In place of a refrigerated condensing vessel 6, there could be substituted any suitable absorbent material, such as molecular sieves, activated charcoal, silica gel etc. In this case the gas passing back into the vessel 6 will not have been cooled, so that the heater 16 may not be required. Referring now to Figure 3 , there is shown a modification of the apparatus of Figure 2 which operates in a different manner. In this arrangement the same reference numerals are used as in Figure 2. However, the heater 16 is here disposed around the external part of the tube 5 between the sealed chamber 1 and the condensing vessel 6. Moreover, the end of the tube 5 does not extend to a position close above the surface of the sample, but is instead shaped in the form of a nozzle 17. In this arrangement the flow of gases and vapours in the tube 5 and pipes 8 and 9 is opposite to that shown in Figure 2, the operation of the arrangement being as follows: The pump 7 sucks inert gas (typically nitrogen) mixed with solvent vapour out. of the sealed container 1 through the pipe 9 and passes it through the refrigerator condenser 6 or other device for absorbing or extracting the solvent vapour. The purified gas is then ducted into the tube 5 which terminates in the nozzle 17. The resultant jet of gas passing over the liquid 3 in the mbe 2 tends to pick up and remove the layer of saturated vapour 4 above the liquid. The contaminated nitrogen 10 then escapes from the mbe container 2 and is sucked out of the chamber 1 by the action of the pump 7, as above mentioned. A pressure control system 15 in the pipe 9 maintains the pressure in the system at the required level by adding or removing nitrogen to or from the system. The heater 16 is used to warm the gas emerging from the condenser 6. The mbe 5 and pipe 9 are sealed at 12, 13 and 14 where they enter and leave the container 1 and the condenser 6. Although, again, only a single sample mbe container 2 is illustrated, the system can be applied equally to a chamber housing a plurality of tubes, usually arranged in an array, and is equally applicable to arrangement in which the containers are replaced by blocks containing a plurality of wells, for example microtitre plates. The or each sample mbe container 2 is preferably subjected to an orbital motion without alteration of the lateral orientation of the mbe, such that the liquid 3 inside the mbe moves around the mbe in the form of a vortex, such an arrangement being known as a vortex evaporator. As with the apparatus of Figure 2, the condenser 6 could be replaced by a suitable absorbent material, eg molecular sieves etc, in which case the heater 16 may again not be required. Referring now to Figure 4, there is shown a further modification of the apparatus of Figure 3, in which the same reference numerals are used for similar pans. Here there are three containers 2 containing liquid samples 3, only the left-hand one being referenced. The top of each container is secured to a manifold 20 by gas-tight seals 22. The manifold is divided into upper and lower compartments 24, 26, the lower of which has apertures 28 extending into the outer regions of each container 2. The upper compartment 24 is similarly provided with a passageway 30 for each container which passes through the lower companment from the centre at the top of the container. The lower compartment 26 is connected by the pipework 5 to the heater 16, while the upper compartment chamber 24 is connected by the pipework 9 to a pump, such as the pump 7 shown in Figures 2 and 3, which in turn is connected to the condenser 6. The operation of the modified apparatus is similar to that for Figure 3. Thus inert gas mixed with solvent vapour is drawn through the passageways 30 and the upper compartment 24 by the pump 7 and is conveyed from there to the condenser 6. At the same time purified gas, warmed by the heater 16, is ducted into the lower compartment 26, and from thence through the apertures 28 and into each container 2, so as to purge the saturated vapour above each liquid sample and enable it to be extracted through the passageways 30.

Claims: CLAIMS 1. A method of evaporating a liquid sample including a volatile solvent, comprising the step of continuously removing inert gas which is rich in solvent vapour from near the surface of the liquid. 2. A method according to claim 1 in which the gas is cooled in a condenser wherein the vapour condenses, allowing the purified gas to be recirculated. 3. A method according to claim 1 or claim 2 in which the evaporation rate is increased by imparting an orbital motion to the container holding the liquid sample, causing the latter to spin around in the form of a vortex. 4. A method according to any one of claims 1 to 3 in which the liquid sample is contained within a chamber, and the volume of gas recirculated to the chamber is progressively reduced, thereby to assist evaporation. 5. Apparams for performing the method according to any one preceding claim in which the liquid is held in a container, and said gas is withdrawn by a suction mbe whose open end is maintained close above the top surface of the liquid as the liquid level drops in the container due to evaporation. 6. Apparams according to claim 5 as dependent upon claim 2 in which the mbe is connected to a refrigerated condensing device through which the permanent gas is passed by a pump for recirculation. 7. Apparams according to claim 5 or claim 6 in which the container is mounted in a vortex evaporator. 8. Apparams according to any one of claims 5 to 7 further comprising a vacuum chamber in which the container is mounted and means for progressively reducing the volume of gas which is recirculated to the container by the pump. 9. Apparams for effecting the evaporation of a volatile solvent, wherein an inert gas, which is held in a sealed environment connected to at least one container containing the solvent, is recirculated via a condensing or absorbing device for absorbing or extracting solvent vapour entrained in the recirculating inert gas. 10. Apparams according to claim 9 in which the inert gas is returned directly into the upper end of the container. 11. Apparams according to claim 10 in which the inert gas is directed into the container through nozzle means which directs the gas directly onto the surface of the solvent in the container. 12. Apparams according to any one of claims 9 to 11 in which a pressure control means introduces or extracts inert gas, in order to maintain a predetermined pressure in the recirculating system. 13. Apparams according to any one of claims 9 to 12 in which the inert gas is recirculated by means of a pump. 14. Apparams according to any one of claims 9 to 13 in which a gas reservoir means is provided in which inert gas is stored under pressure for reuse as required, with pressure controlling means limiting the pressure of the gas leaving the reservoir to a desired level. 15. Apparams according to any one of claims 9 to 14 in which the container is mounted in a vortex evaporator. 16. Apparams according to any one of claims 5 to 15 in which the inert gas is nitrogen. 17. Apparams according to any one of claims 5 to 16 in which a heater is provided in the recirculating system downstream of the condensing device. 18. Apparams according to any one of claims 9 to 15 in which the container is open- topped for accommodation in the sealed environment. 19. Apparams according to any one of claims 9 to 15 in which a plurality of containers are connected to a manifold through which the inert gas is recirculated. 20. Apparams according to claim 19 in which the manifold is divided into two compartments, one being connected to a pump for extracting the mixture of the inert gas and solvent vapour and the other feeding recirculated inert gas from the condensing or absorbing device. 21. A method of, or apparams for, evaporating a liquid sample substantially as herein described with reference to, and as shown in, Figures 2 to 4 of the accompanying drawings.

Inventors: COLE MICHAEL, COLE, MICHAEL
Applicants: COLE MICHAEL, COLE, MICHAEL
Application year: 2000
Main classification: G01N1
IPC codes: B01D3, G01N1