IMPROVED VACUUM CONTROL IN EVAPORATORS

Abstract: An evaporator, more especially a vortex evaporator, wherein vacuum pressure is gradually reduced to evoke sample evaporation at a pre-observed rate which does not cause spitting, the sample temperature is monitored, and pressure is reduced substantially to that lowest level at which spitting does not occur at the monitored temperature. Alternatively, or additionally, the pressure may be monitored by a fast-acting transducer, such as a crystal pressure transducer (14) or a microphone, and the pressure reduction controlled at least in part from signals derived from the transducer.

Description: Title: Improved Vacuum Control in Evaporators Field of invention This invention concerns evaporators and in particular methods of controlling vacuum levels in evaporators during evaporation of liquid samples, so as to minimise loss of sample material due to spitting. Background to the invention 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 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 vortex evaporators the vacuum pressure has to be carefully controlled (usually manually) so that it is low enough to promote rapid evaporation but not low enough to cause bumping or spitting of the liquids. In order to deal with this disadvantage, an operator has to watch an evaporator during its operation and attempt to control the vacuum manually to achieve a reasonable evaporation rate. However it is nearly impossible to ensure that no spitting occurs and even if only limited spitting occurs, samples loss and cross-contamination of samples may take place. It is an object of the present invention to provide an improved pressure control method for an evaporator which overcomes this disadvantages. Summary of the invention The invention provides a first method of controlling pressure in an evaporator, in which vacuum pressure is gradually reduced so as to provide sample evaporation at a rate which from previous observation will not cause spitting, by monitoring the sample temperature, and reducing the pressure to the lowest level at which it is known from previous experimentation that spitting will not occur from the liquid at its monitored temperature. As evaporation cools the liquid, the pressure can be reduced correspondingly. Preferably the pressure to which a sample may safely be exposed at any given temperature is determined from a look-up table (or tables) containing data from previous experimentation using the liquid (or liquids) it is proposed to evaporate. This data can be stored in digital form in a computer memory associated with the evaporator control system. Preferably the data is called up by the operator before starting an evaporation run, and the evaporator control system is programmed to control the pressure to the lowest safe value for the given liquid using the temperature-pressure information stored in the memory from earlier monitoring. The invention also provides a second method of controlling the pressure in an evaporator, in which vacuum pressure is gradually reduced so as to provide a controlled liquid sample evaporation, and in which the vacuum pressure is monitored by means of a sensitive and fast-acting pressure transducer and controlled at least in part from signals derived from the transducer. Typically the transducer comprises a microphone. It has been found that when spitting occurs the pressure in the vacuum chamber momentarily increases. However before any spitting actually occurs, it has also been observed that as the pressure is reduced to a level near to that at which spitting will occur, small, short lived pressure rises occur (henceforth referred to as pressure spikes). These spikes are probably due to the formation and collapse of bubbles of vapour which escape from the surface of the liquid but which are not violent enough to cause liquid to be ejected and spitting to occur. In a preferred version of this second method in which the evaporator is mounted in a vacuum chamber, the chamber pressure is first reduced until pressure spikes are detected in the transducer output whereupon the pressure is held constant (or increased by a small amount and then held constant), until the pressure spike signals cease, at which point the pressure is steadily again further reduced until pressure spike signals are again detected, when the above procedure is repeated. It may be advantageous to combine the two methods using stored pressure/temperature information to allow the chamber pressure to be pumped down rapidly to a safe level, and then more slowly to the level at which pressure spike signals are detected. It is to advantage to follow this procedure since it has been found that pressure spikes do not appear immediately after the pressure is reduced to the level at which they can occur, and it is advisable to lower the pressure slowly as it approaches that level, whereas it can safely be lowered at a much faster rate at pressures well above the pressure spike level. The two methods either alone or together may be used to control the vacuum in an evaporator in which the surface area of the sample liquid in each tube is increased by orbital motion which creates a vortex. Such a vortex evaporator, in which the samples are held in tubes mounted in racks, is illustrated in Figure 2. Infrared heat is applied to the samples from above, as shown. Instead of the evaporator being contained in a vacuum chamber, vacuum may alternatively be applied individually to each tube, and the space outside the tubes open to atmosphere or any other medium which may be desirable. This arrangement makes it easier to apply heat to the samples, although it may present difficulties in designing reliable vacuum sealing means for each individual sample tube. An example of individual suction being applied via a manifold 8, to a rack of sample tubes 12 is shown in Figure 3, which is a schematic view in elevation. Mounted in the vacuum tube 10 of each sample tube 12 is a crystal pressure transducer 14 which transmits a signal corresponding to the instantaneous pressure in that tube to a central control for determining the. vacuum pressure to be applied to the manifold 8. Where the samples in the tubes are of similar composition, a single transducer may instead be disposed in the manifold 8. Likewise, where the evaporator is mounted in a vacuum chamber, a single pressure transducer enables the pressure spikes to be detected.

Claims: CLAIMS 1. A method of controlling pressure in an evaporator, in which vacuum pressure is gradually reduced so as to provide liquid sample evaporation at a rate which from previous observation will not cause spitting, by monitoring the sample temperature, and reducing the pressure to the lowest level at which it is known from previous experimentation that spitting will not occur from the liquid sample at its monitored temperature. 2. A method according to claim 1, wherein, as evaporation cools the liquid sample, the pressure is reduced correspondingly. 3. A method according to claim 1 or claim 2, wherein the pressure to which a sample is exposed at any given temperature is determined from a look-up table (or tables) containing data from previous experimentation using the liquid (or liquids) to be evaporated. 4. A method according to claim 3, wherein the said data is stored in digital form in a computer memory associated with the evaporator control system. 5. A method according to claim 3 or claim 4, wherein the data is called up by the operator before starting an evaporation run, and the evaporator control system is programmed to control the pressure to the lowest safe value for the given liquid using the temperature-pressure information stored in the memory from earlier monitoring. 6. A method of controlling the pressure in an evaporator, in which vacuum pressure is gradually reduced so as to provide a controlled liquid sample evaporation, and in which the vacuum pressure is monitored by means of a sensitive and fast-acting pressure transducer and controlled at least in pan from signals derived from the transducer. 7. A method according to claim 6, wherein the transducer comprises a crystal transducer or a microphone. 8. A method according to claim 6 or claim 7, in which the evaporator is mounted in a vacuum chamber, and wherein the chamber pressure is first reduced until pressure spikes are detected in the transducer output whereupon the pressure is held constant (or increased by a small amount and then held constant), until the pressure spike signals cease, at which point the pressure is steadily again further reduced until pressure spike signals are again detected, when the above procedure is repeated. 9. A method according to any of claims 1 to 5 in combination with the method of any of claims 6 to 8. 10. A method according to claim 9, wherein stored pressure/temperature information is used to allow the vacuum pressure to be pumped down rapidly to a safe level, and then more slowly to the level at which pressure spike signals are detected. 11. A method according to any of claims 1 to 10, when used to control the vacuum in an evaporator in which the surface area of the liquid sample in each tube is increased by orbital motion which creates a vortex. 12. A method according to claim 11, in which the vacuum is applied individually to each sample tube, and the space outside the tubes is open to atmosphere or any other medium.

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