GB2121171A - Cryometer and method of cryometric measurement - Google Patents

Abstract

A sample of a liquid solution is introduced into an annular chamber 13 of a cryometer cell 10 via a duct 17 and over a lip 18. Gas at a temperature substantially below the crystallisation temperature of the liquid is admitted by a jet nozzle 15 and temperature changes are sensed by a thermometer 14. The gas is chemically and physically inert with respect to the liquid and the walls, 11, 12 of the cell 10 do not readily transfer heat to or from the liquid sample. The cell 10 has a drain 16 and a stand pipe allowing emission of gas after passing through the liquid. The crystallisation temperature is indicated as being attained when the thermometer senses that temperature changes have substantially ceased. In further embodiments, a bi- conical chamber may be provided and the cell walls may be cooled so as to have the same temperature from moment to moment as the liquid. <IMAGE>

Description

SPECIFICATION Cryometer and method of cryometric measurement This invention relates to a cryometer and a method of cryometric measurement. A cryometer is an instrument which is used to determine the concentration of a solute dissolved in a crystallisable liquid solvent by measuring the depression of the temperature of crystal formation of the solution below the corresponding temperature for the pure solvent.

For the purposes of this invention, for any given solvent and solute the temperature of crystal formation (freezing point/melting point) is that temperature at which the liquid phase of the solution is in equilibrium with crystals of the solid phase of the solution, the mass of crystals neither increasing nor decreasing with time. Such measurement may, for example, have application in periodic sampling of supplies of fluid for monitoring the concentration of impurities (solutes) in same.

It is known to take a sample of a liquid, the purity of which is to be measured, and freeze it either partly or completely. Subsequently, applying a regulated rate of heat input and stirring so as to maintain uniform temperature distribution, the temperature of crystal formation is measured by noting when the last few crystals disappear and the rate of temperature increase is no longer made slower by transfer of latent heat.

In order to use a heating rate sufficiently large to deal with an acceptable hourly throughput of samples the heat input must be at such a rate that the last few crystals are incapable of controlling the temperature of the remanent liquid by melting and transferring their latent heat to the liquid.

Consequently, the temperature of crystal formation estimated in this manner is ill-defined, approximate and invariably too high.

Graphical manipulation of the recorded -temperature time relationship can yield a closer approximation to the true temperature of crystal formation.

Fig. 5 is a graph of conditions in a solution during application of heat, uniformly applied, the crystals melting progressively.

During the section between points A and B the crystals melt and dissolve, and the absorption of latent heat is sufficient to control the rate of temperature rise to a value that is approximately uniform.

In section C to D, there are no crystals at all; and the rate of rise of temperature is uncontrolled by evolution of latent heat.

Between B and C there are still some crystals dissolving; but they are so small in number and mass that they cannot hold the rate of temperature rise as they did between A and B.

Using the presence of the last few crystals as a criterion, point C would be taken as the temperature of crystal formation (melting point).

The error in so doing can be reduced by prolonging the reiatively straight portions AB and CD so that they intersect at E, which is a better approximation to the true temperature of crystal formation. However, this graphical correction, which has to be applied to each determination, renders the method unsuitable for automatic indication of the result, as required, for example, in processing plants in continuous use by semiskilled operators.

Likewise it is known to estimate the temperature of crystal formation by a cooling method. A typical cooling graph is shown in Fig.

6. The liquid solution cools at a uniform rate from point P to point , where crystals form spontaneously. However, at , the liquid has already "supercooled", i.e. has descended below the true temperature of crystal formation. In consequence, at , the first crystals "seed" others and crystal growth rapidly spreads, the resulting evolution of latent heat causing the temperature to rise along the curve QR. At point R, the spontaneous formation of crystals has finished and the solution cools along the line RST until completely solid.

In this case, it is usual to take point R, the point of a temporary arrest temperature as the temperature of crystal formation (freezing point) but this is only approximate and is invariably lower than the true temperature of crystal formation, as after supercooling less solvent remains in the liquid phase compared to the original solution composition. The reai value may be obtained graphically by extrapolating the line RS backwards to cut the line PQ at point X. This is reasonably accurate, but the method obviously does not lend itself to automatic indication of the temperature of crystal formation.

An object of the present invention is to provide a cryometer and a method of using same to measure the temperature of crystal formation of a solution directly without recourse to graphical manipulation or other manual intervention that would otherwise be required for obtaining an acceptable degree of accuracy.

With this object in view, the present invention provides a cryometer comprising a cell having inlet means for admission of a sample of a liquid, means for injection of an inert cooling gas capable of cooling the liquid to substantially below the crystallisation temperature of the liquid, and a temperature sensing device for recording the temperature at which a substantially steady reading is attained, the cell being provided with walls of material and/or construction giving low heat conductivity.

Preferably the cell includes an annular chamber defined between an inner surface of the walls and a circular lip of an inlet passage, the gas injection means being disposed to inject the gas substantially tangentially into the annular chamber so as to entrain the liquid in a circulating flow.

The inlet passage may be connected by a valve to a source of liquid to be sampled, and have a branch for release of gas which has passed through the sample, preferably with a filter or other means for preventing escape of any of the liquid entrained by the gas.

The inlet passage may enter the cell axially of the annular chamber from one side of the cell, and communicate with the annular chamber over the circular lip, between the lip and a wall of the cell disposed opposite the inlet passage.

Control means, such as a flow restrictor valve, may be provided to control the rate of admission of the gas.

Means may be provided whereby the cell wall is maintained at the same temperature as the liquid from moment to moment. Such means may comprise a temperature sensor in the cell wall having an output connected, together with an output from the temperature sensor in the cell, into an electronic comparator which acts through a control device to control the exposure of the exterior surfaces of the cell wails to a cooling medium, for example the same gas as is used to contact the liquid.

The invention also provides a method of cryometic measurement comprising introducing into a sample of a liquid, within a cell which has minimal heat transfer to or from the liquid, a gas which is non-reactive with the liquid and is at a temperature below the crystallisation temperature of the liquid thereby to effect spontaneous crystallisation of liquid directly contacting the gas, and continuously measuring the temperature of the liquid in the cell until a substantially steady temperature reading is attained.

The inert gas may be introduced tangentiaily into an annular chamber containing the liquid so as to entrain the liquid in a circulating flow. The gas may also be introduced into the liquid as a jet of small bubbles.

The gas is preferably subsequently separated from the liquid without loss of liquid by entrainment of droplets in the gas and the cell walls may be maintained at substantially the same temperature as the liquid from moment to moment.

The term "inert" as applied to the gas means that it should not only be chemically un-reactive with the liquid, but that it also should not enter any physically bound system with the liquid, for example by dissolving into the liquid, or by entering into inter-crystalline structures. The presence of the gas must not affect the composition of the liquid solution due to contact between the gas and liquid.

The invention will be described further, by way of example, with reference to Figs. 1 to 4 of the accompanying drawings, in which: Fig. 1 is a cross-section of a preferred embodiment of the cryometer of the invention; Fig. 2 is a cross-section of the same cryometer on line Il-Il of Fig. 1; Fig. 3 is a vertical cross-section of a second embodiment of the cryometer of the invention; and Fig. 4 is a plan view of the lower half of the cryometer of Fig. 3 when the upper half above the line IV-IV is removed.

Referring to Figs. 1 and 2, a preferred embodiment of the cryometer of the invention comprises a cell 10 in the form of two hollowed blocks 1 2 of a suitable non-metallic material, such as polyfluoro-ethylene-propylene or polytetrafluoro-ethylene, either of which may be reinforced with fibrous or particulate filler, e.g.

glass fibre or certain silicates. These blocks 1 2 are clamped together between two aluminium alloy blocks 11 so as to define an annular chamber 13.

Projecting into the periphery of the chamber 13 are a thermometer 14, and a jet-pipe 1 5 formed of a similar non-metallic material as mentioned in relation to the blocks 12. The jet pipe 1 5 is provided with a small orifice for discharge into the chamber 1 3. Both the thermometer 14 and the jet-pipe 15 are arranged approximately tangentially to the periphery of the annular chamber 13. There is also a drain port 1 6 at the lowest part of the periphery.

At the centre of the annular chamber 13 there is a re-entrant duct 1 7 formed with a re-curved lip 18. The duct 17 connects with a vertical standpipe 1 9 terminated by a filter 20 or other means to forbid the entry of contaminants and with a side branch 21 and valve 22, whereby a sample of liquid may be introduced into the base of the standpipe 19 and may flow or be forced through the duct 1 7 into the annular chamber 13.

In use, a sample of liquid is introduced into the chamber 13 from the side branch 21 at a temperature above the temperature of crystal formation of said liquid and the sample valve 22 is closed. Inert gas at a temperature low enough to compel crystals to form spontaneously in the liquid sample is blown into the chamber 13 through the jet pipe 1 5. The sample liquid is thus entrained and rapid circumferential motion of the combined gas and liquid is engendered. This is particularly important in assisting prevention of supercooling and the prevention of early formation of crystals which do not immediately dissolve. Such crystals are not desirable as they significantly increase the proportion of impurity in the remaining liquid and further depress the freezing point so that the eventual steady temperature is erroneously low.

In the present case, small bubbles of cold gas emerge into the entrained liquid and surround themselves momentarily with thin shells of crystal. These crystals dissolve quickly, abstracting heat from the liquid and providing a bed of seed crystals of large surface area. As the liquid cools, the gas bubbles remain longer and travel further before their shells of crystallised material melt and dissolve into the ambient liquid.

At a suitable temperature below that of the inlet sample and above the temperature of crystal formation of the liquid the flow rate of the injected gas is reduced. This is accomplished by restricting the flow by operating, by mechanical or pneumatic means, a flow-controi valve in the pipeline supplying the gas in response to a command from a logic component of temperature sensing means of which the thermometer 14 forms part. The temperature of the gas may be reduced at the same time by similar automatic control of its cooling means, These adjustments in flow rate and temperature are made in accordance with preliminary trial and/or operating experience.The crystallisation temperature is indicated as being attained when the thermometer 14 senses that temperature changes have substantially ceased indicating an equilibrium between the latent heat evolved by the liquid as it crystallises and the abstraction of heat by the cooling gas. In some cases, however, there is an equilibrium between crystal formation and dissolution.

Means are preferably provided to confirm that this arrest in temperature change is in fact the result of heat abstraction and latent heat contribution in approximate balance, and not a spurious near-equilibrium between extraneous heat injection and abstraction. Spurious nearequilibria are approximate and temporary. They are rejected by accepting as vaiid only those equilibria which remain within a narrow band of temperatures over a prolonged time. The criteria of "narrow" and "prolonged" are established by trial and prove to be adequate to distinguish.

Spurious equilibria become less iikely and more readily distinguished as heat transmission through the cell wall is reduced, making the evolution of latent heat of crystallisation dominant and extraneous heat flows less able to compete.

It has been found that by minimising the leakage of heat into the cell 10, for example through the walls 11, 12 the heat abstraction rate by the gas from the liquid can be made so small that latent heat evolution can maintain the temperature of the liquid within a few millidegrees of the temperature of crystal formation for periods of the order of one minute.

In this respect, in order to maintain the cell wall at the same temperature as the liquid from moment to moment so that no heat leakage occurs, there may be a temperature sensor in the cell wall having an output connected, together with an output from the thermometer in the cell, into an electronic comparator which acts through a control device to control the exposure of the exterior surface of the cell walls to a cooling medium such as the same gas as is used to contact the liquid.

Automatic recording of the temperature of crystal formation is thus facilitated without the need for manipulation, approximation, or computation.

Figs. 3 and 4 illustrate an alternative embodiment of the cryometer of the invention comprising a cell 30 having a conical dhamber 31 with a sample inlet duct and drain 32 at the bottom apex. A thermometer 33 extends into the chamber 31 and a gas inlet nozzle 34 is arranged tangentially of the chamber 31. An overflow sump 35 and drain 36 are provided to prevent overfilling of the chamber 31. An upper part 37 provides a conical upwardly converging funnel 38 leading to a gas vent 39. The tangential gas jet from nozzle 34 creates a vortex in the chamber 31 which ensures maximum gas/liquid contact.

The cryometer is used in the manner described with reference to Figs. 1 and 2 above.

The proportion of solute "impurity" in benzene, cyclo-hexane, and para-xylene can be obtained in the above-described manner, with carbon dioxide or nitrogen as the inert cooling gas. Similarly, the proportion of solute(s) in urea and phenol may be determined using dried and filtered air as the cooling gas. Alternatively, in the case of phenol, pure nitrogen may be used.

Claims (10)

Claims

1. A cryometer comprising a cell having inlet means for admission of a sample of a liquid, means for injection of an inert cooling gas capable of cooling the liquid to substantially below the crystallisation temperature of the liquid, and a temperature sensing device for recording the temperature at which a substantially steady reading is attained, the cell being provided with walls of material and/or construction giving low heat conductivity.

2. A cryometer according to claim 1 wherein the cell includes an annular chamber defined between an inner surface of the walls and a circular lip of an inlet passage, the gas injection means being disposed to inject the gas substantially tangentially into the annular chamber so as to entrain the liquid in a circulating flow.

3. A cryometer according to claim 2 wherein the inlet passage is connected by a valve to a source of liquid to be sampled, and has a branch for release of gas which has been passed through the sample, the inlet passage entering the cell axiaily of the annular chamber from one side of the cell and communicating with said chamber over the circular lip, between the lip and a wall of the cell disposed opposite the inlet passage.

4. A cryometer according to claim 1, 2 or 3 wherein means are provided for controlling the rate of inlet of the gas into the annular chamber.

5. A cryometer according to any preceding claim wherein the cell wall is maintained at substantially the temperature which the liquid has at the same moment.

6. A cryometer substantially as hereinbefore described with reference to and as illustrated in Figs. 1 and 2 or Figs. 3 and 4 of the accompanying drawings.

7. A method of cryometric measurement comprising introducing into a sample of a liquid within a cell which has minimal heat transfer to or from the liquid, a gas which is non-reactive with the liquid and is at a temperature below the crystallisation temperature of the liquid thereby to effect spontaneous crystallisation of liquid directly contacting the gas, and continuously measuring the temperature of the liquid in the cell until a substantially steady temperature reading is attained.

8. A method according to claim 7 wherein the inert gas is introduced tangentially into an annular chamber containing the liquid so as to entrain the liquid in a circulating flow.

9. A method according to claim 7 or 8 wherein the gas is introduced into the liquid as a jet of small bubbles.

10. A method according to any of claims 7 to 9 wherein the gas is subsequently separated from the liquid without loss or without substantial loss by entrainment.

1 1. A method according to any of claims 7 to 10 wherein the cell walls are maintained at substantially the temperature which the liquid has at the same moment.

1 2. A method of cryometric measurement substantially as hereinbefore described.

1 3. A procedure for determining the proportion of a solute in a solvent using a method according to any one of claims 7 to 12, or apparatus according to any one of claims 1 to 6.