GB2349588A

GB2349588A - Vacuum evaporator with fluid bath heating.

Info

Publication number: GB2349588A
Application number: GB0001976A
Authority: GB
Inventors: Michael Cole
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-02-09
Filing date: 2000-01-31
Publication date: 2000-11-08
Also published as: WO2000047976A1; GB0001976D0

Abstract

A vacuum evaporator for concentration of samples contained in a plurality of sample tubes 10 comprises a heat source and a non-volatile heat conducting fluid 26 surrounding the sample tubes for conducting heat to the tubes. The vacuum may be applied by means of a vacuum chamber or by evacuating each tube individually (Fig. 3). The heat conducting fluid is preferably in the form of a fluidised mass of particles, e.g. aluminium. Fluidisation may be achieved either by means of a gas flowing through the particles or by vibration.

Description

2349588 1 Title: l=roved heating method for EvaDorators

Field of invention

This invention concerns stationary and rotational evaporators, such as centrifugal and vortex evaporators.

Back-ground to the invention In a centrifugal evaporator, sample holders such as glass tubes containing liquid are spun around in a vacuum chamber.

The centrifugal force acting on the liquid samples not only retains the latter in the tube which typically is pivoted so that they occupy a nonvertical attitude during rotation. The centrifugal force can also assist in preventing unwanted boiling and spitting as the liquids evaporate under the reducing pressure.

A vortex evaporator typically consists of a vacuum chamber in which liquid samples, generally containing dissolved solids, are held in containers such as glass tubes, which in turn are frequently held in racks in heated aluminium blocks and subjected to orbital motion. This motion causes the liquid in the tubes to spin round inside the tubes and form a vortex. This motion increases the surface area of contact between the liquid and the wall of the container thereby increasing the amount of heat transferred to the liquid and increasing evaporation rate when the samples are subjected to vacuum by evacuation of the chamber. An example of such a device is shown in Figure 1.

In use the vacuum pressure has to be carefully controlled (usually manually) so that it is low enough to promote rapid evaporation but not too low to cause bumping or spitting of the liquids.

2 Secondly, if the sample tubes themselves are held in an array, for example, in a rack or holder, such as that illustrated in Figure 2, they cannot be mounted intimately in heat exchange blocks of aluminium or other suitable material, which will transfer heat efficiently and uniformly to all the tubes in the array so as to procure evaporation of the samples at a uniform rate.

Whilst the array can be surrounded by a heat exchanger, heat will tend to be preferentially transferred to the outer tubes in the. array while those in the middle of the array will receive much less heat.

A method has been suggested to heat samples held in such racks by means of an infrared radiant heater mounted above the samples, as illustrated in Figure 2. However if some samples dry more quickly than others, for example due to differences in composition, and the heat is still applied to dry the tubes which still contain liquid, any cooling due to evaporation will have ceased in the dry sample tubes and their temperature will rise considerably above that to which they were heated whilst they contained liquid. Under such conditions the temperature of the dry tubes can easily rise above the level at which the remaining sample material is damaged or even destroyed.

This is particularly important in cases in which samples are not of uniform composition, as for instance when samples are provided from a purification process by liquid chromatography using a solvent gradient. Here samples can be dissolved in a wide range of mixtures of water and acetonitrile. In this case water evaporates much more slowly than acetonitrile and samples dissolved in acetonitrile-rich mixtures dry more quickly than those in water-rich mixtures.

It is an object of the present invention to provide an improved method of and apparatus for heating samples in evaporators.

Surnmaa of the invention According to the present invention in an evaporator in which samples are carried in a 3 plurality of spaced apart open topped containers and subjected to vacuum, heat is conveyed to the latter by conduction from a surrounding heat source through a non-volatile heat conducting fluid which completely surrounds the containers.

Preferably vacuum is applied to the plurality of containers in that they are placed in a vacuum chamber.

The fluid may be a non-volatile liquid such as a silicone, a pseudo-fluid comprised of a mass of small particles of an inert solid such as aluminium, polypropylene or polytetrafluoethylene, or a slurry which is a mixture of solids and liquids.

Where a pseudo-fluid formed from particles is employed, the particles should be shaped to be free flowing and of a size which will allow them to flow between the stacked tubes yet be subjected to vacuum without risk of being sucked into the vacuum pump.

The fluid is typically heated to a temperature below the maximum permissible sample temperature, and is caused to flow around and to make contact with the sample contamers, to which it imparts, heat, but the temperature of the samples will not exceed the temperature.of the fluid.

As samples dry, the evaporative cooling effect reduces and eventually ceases, and they quickly i --ach the fluid temperature, but thereafter do not absorb any more heat, so remain at that temperature.

Samples containing evaporating liquid will continue to absorb heat and will therefore continue to evaporate.

The samples in this case are being heated by conduction where heat flow is a function of temperature difference between two points. When a samples reaches the same temperature as the heat source, eg an alurninium. block, the heat flow will stop. This contrasts with the known radiant heating process as illustrated in Figure 2, in which the temperature of the heat source is usually much higher than the safe sample temperature, and no significant 4 reduction of heat flow occurs when a sample dries.

It may not always be possible to guarantee that the solid particles of pseudo-fluids will flow between the sample tubes, especially if the latter are closely spaced. Preferably therefore the pseudo-fluid particles are made of a good therinal conductor such as aluminium or silver, which will reliably conduct heat with minimal loss into the centre of the rack, and from one sample to another, even without significant particle movement.

Where the sample containers are so closely spaced that pseudo-fluid flow is restricted or even prevented, the particles are preferably packed around and in between the containers so that there are continuous lines of touching particles throughout the matrix and the temperature limiting feature of the arrangement is still obtained if the particles are of a good heat conducting material such as aluminium or silver, so that temperature gradients in the matrix are kept to the minimum.

Instead of the sample containers being placed in a vacuum chamber, as aforesaid, vacuum may alternatively be applied individually to each container in an array by means of sealing 1 _Cl caps and tubes connected by a manifold to a source of vacuum. An example of an array of such sample containers is illustrated in Figure 3.

Where the containers are individually evacuated, the conducting fluid may be a liquid, gas or vapour. The fluids may be heated and then forced past the containerl Preferably the sample array is mounted in a gas-tight enclosure so that the fluid can be recirculated through heating means. Good performance can be achieved by choosing a vapour which condenses at the temperature to which the containers need to be heated. The pressure may be varied in the enclosure to adjust the condensing temperature of the vapour.

If solid particles are used in the heat conducting pseudo-fluid, they may be made more effective either by being fluidised by blowing gas through them in the case in which vacuum is applied individually to the containers, or by vibration, if the particles are in a vacuum chamber.

The invention also extends to a method of heating in an evaporator wherein, in order to heat a liquid sample contained in a tube, the tube is immersed in a heat conducting nonvolatile fluid.

Other features of the invention are defined in the appended claims.

The invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 4 is a section in elevation of the main part of the vortex evaporator shown in Figure 1; Figure 5 is a perspective view corresponding to Figure 4; and Figure 6 shows diagrammatically how the evaporator part is rotated.

Referring first to Figures 4 and 5, a rectangular array of say 36 sample tubes, one of which is indicated at 10, are mounted in appropriate tightly fitting holes in upper and lower plastic. tray holders 12, 14. The base of the tubes rest on a plastic mat 16 supported within a cylindrical container 18, akin to a cake tin.

Extending from each of the four sides of the upper cay holder 12 is a spacer bar, one of which is indicated at 20, whose extremity is downturned to bear against the respective inside cylindrical wall of the container 18.

Resting firmly against the top of the tubes 10 is a gauze 22 which is stretched over the upper edges of the walls of the container and is secured thereto by suitable fixings, one of which is indicated at 24.

The lower parts of the tubes are surrounded by a mass of pseudo-fluid particles 26 which extend up to about the level of the lower tray holder 14. The particles are of about 1 to 2 min diameter in size, not necessarily round, and are made of aluminium or aluminium 6 oxide which is a good heat conductor.

Normally in a vortex evaporator the container 18 is horizontally disposed, as shown. However, in order for the particles to move relative to the tubes, the container is given a slow backwards and forwards tilting motion about a single axis, in addition to the normal rapid orbital rotation which causes the liquid sample in each tube to form a vortex. Typically, a tilting action occurs within the range 1 to 60 times per minute, while the orbital rotation takes place at 100 to.1000 rpm.

Alternatively, as shown in Figure 6, the container is mounted on a spindle 28, which is driven about an eccentric path 30 so that it sweeps out the shape of a cone. As a result the container 18 remains inclined, rotating from one extreme position 18 to the opposite extreme position 18' (shown chain-dotted). Thus the particles 26 migrate or slush around the circular wall of the container in a kind of vortex motion, similar to that of the liquid samples as shown in Figure 1. As the particles are heated, by known means, the heat is transferred to the tubes and in turn to the liquid sample therein.

As with Figure 1, the whole device is situated within a vacuum chamber (not shown), and the apertures in the gauze 22 are large enough to enable evaporated vapour 32 from the tubes 10 to escape, but small enough to prevent the particles 26 from passing through.

The density of the particles 26 is sufficiently high to tend to cause the tubes 10 to float upwards, particularly when the outer tubes are subjected to a higher level of the particles at each rotation of the container 18. It is therefore important that the gauze 22 is sufficiently strong to hold down the tubes at all times.

In a modification (not shown) of the vortex evaporator illustrated and described with reference to Figures 4 to 6, evaporation is achieved or at least promoted by placing a suction tube inside each sample tube 10 so that the inserted end of the suction tube is positioned just above the top surface of the liquid in the sample tube. Evaporating vapour is thus withdrawn through the suction tube, while allowing ambient gas to flow into the sample tube to replace the volume of vapour extracted by the suction process.

7 An example of such a suction tube arrangement is described in co-pending UK Application No. 9918914.4 (ref: C1094/C), the entire disclosure of which is hereby incorporated by reference into the present specification.

In this modification the whole sample tube arrangement may again be placed within a vacuum chamber, as in Figure 1, or alternatively may be held at atmospheric pressure or at any other desired pressure.

8

Claims

1. An evaporator in which liquid samples are carried in a plurality of spaced apart opentopped containers and subjected to vacuum, wherein heat is conveyed to the containers by conduction from a surrounding heat source through a non-volatile heat conducting fluid which completely surrounds the containers.

2. An evaporator according to claim 1 in which vacuum is applied to the plurality of containers in that they are placed in a vacuum chamber.

3. An evaporator according to claim 1 or claim 2 in which the conducting fluid is a nonvolatile liquid such as a silicone, a pseudo-fluid comprised of a mass of small particles of an inert solid such as aluminium, polypropylene or polytetrafluoethylene, or a slurry which is a mixture of solids and liquids.

4. An evaporator according to claim 3 in which a pseudo-fluid formed from particles is employed, wherein the particles are shaped to be free flowing and of a size which will allow them to flow between the containers yet be subjected to vacuum without risk of being sucked into the source of vacuum, such as a vacuum pump.

5. An evaporator according to any one of claims 1 to 4 in which the fluid is heated to a temperature below the maximum permissible sample temperature, and is caused to flow around and to make contact with the sample containers to which it imparts heat, so that the temperature of the samples does not exceed the temperature of the fluid.

6. An evaporator according to any one of claims 3 to 5 in which the pseudo-fluid particles are made of a good thermal conductor, such as aluminium or silver, which will reliably conduct heat with minimal loss into the centre of the rack, and from one sample to another, even without significant particle movement.

9

7. An evaporator according to any one of claims 3 to 6 in which the sample containers are so closely spaced that pseudo-fluid flow is restricted or even prevented, wherein the particles are packed around and in between the containers so that there are continuous lines of touching particles throughout the matrix.

8. An evaporator according to any one of claims 1 and 3 to 7 in which vacuum is applied individually to each container in an array by means of sealing caps and tubes connected by a manifold to a source of vacuum.

9. An evaporator according to claim 8 in which the heat conducting fluid is a liquid, gas or vapour which is heated and then forced past the containers.

10. An evaporator according to claim 8 or claim 9 in which the array of containers is mounted in a gas-tight enclosure so that the fluid can be recirculated through heating means.

11. An evaporator according to claim 9 or claim 10 in which the fluid is a vapour which condenses at the temperature to which the containers need to be heated.

12. An evaporator according to claim 10 or claim 11 in which the pressure is varied in the enclosure to adjust the condensing temperature of the vapour.

13. An evaporator according to any one of claims 3 to 10 in which solid particles are used in the heat conducting pseudo-fluid, which are made more effective by being fluidised by blowing gas through them in the case in which vacuum is applied individually to the containers, or by vibration, if the particles are in a vacuum chamber.

14. A method of heating in an evaporator wherein, in order to heat a liquid sample contained in a tube, the tube is immersed in a heat conducting non-volatile fluid.

15. A method according to claim 14, wherein the fluid is a non-volatile liquid such as a silicone, a pseudo-fluid comprised of a mass of small particles of an inert solid such as aluminium, polypropylene or polytetrafluoethylene, or a slurry which is a mixture of solids and liquids.

16. A method according to claim 15 for use with a plurality of stacked tubes wherein, where a pseudo-fluid formed from solid particles is employed, the particles are shaped to be free flowing and of a size which will allow them to flow between the stacked tubes and be subjected to vacuum without risk of being sucked into the vacuum pump.

17. A method according to any one of claims 14 to 16 wherein evaporation of the liquid sample is achieved, or at least promoted, by inserting a suction tube into the top of a sample tube containing the liquid and withdrawing vapour from above the surface of the liquid while allowing ambient gas to flow into the sample tube to replace the volume of vapour extracted.

18. A method according to claim 16 or claim 17 wherein the evaporator is a vortex evaporator in which the stacked tubes are contained in a cylindrical container in which a pseudo-fluid is allowed to migrate while the container is rotated in an eccentric manner to cause the liquid samples to move inside the tubes in the form of a vortex.

19. A method according to claim 18, wherein the tubes are mounted in a tube holder in an array, and the top of the tubes are covered by a gauze extending over and secured to the cylindrical walls of the container.

20. A method according to clahn 19, wherein the tube holder has spacer bars extending to the container wall to maintain the holder centrally within the container.

21. A method according to any one of claims 18 to 20, wherein the pseudofluid is additionally vibrated to cause it to become fluidised.

22. A method according to any one of claims 18 to 21, wherein the pseudofluid is heated to a limited temperature below the maximum permissible sample temperature, and is caused to flow around and make contact with the sample tubes to which it imparts heat, 11 without permitting the temperature of the samples to exceed the temperature of the fluid.

23. A method according to any one of claims 15 to 22, wherein the pseudofluid particles are made of a good thermal conductor such as aluminium or silver, which will reliably conduct heat with minimal loss from one sample to another, without significant particle movement.

24. A method according to any one of claims 17 to 23, wherein the tubes are so closely spaced that pseudo-fluid flow is restricted or even prevented, the particles being packed around and in between the containers so that there are continuous lines of touching particles throughout the matrix, and the said temperature limiting feature of the arrangement is maintained, the particles being of a good heat conducting material such as aluminium or silver, so that temperature gradients in the matrix are kept to the minimum.

25. An evaporator, or a method of heating an evaporator, substantially as herein described with reference to, and as shown in, Figures 4 to 6 of the accompanying drawings.