GB2396571A

GB2396571A - Preventing refreezing of liquid condensate and associated overpressure

Info

Publication number: GB2396571A
Application number: GB0325189A
Authority: GB
Inventors: Roger Lee-Smith; Michael Sherwen; Duncan Guthrie; Trevor Brooks; Richard Smith
Original assignee: Genevac Ltd
Current assignee: Genevac Ltd
Priority date: 2002-10-31
Filing date: 2003-10-29
Publication date: 2004-06-30
Also published as: AU2003276415A1; GB0325189D0; WO2004039472A1; GB0225336D0

Abstract

Firstly disclosed is an apparatus and method in which a condensate is chill solidified then re-melted in a condenser vessel 12, the vessel 12 temperature being maintained above the condensate freezing temperature to prevent refreezing before the condensate liquid has drained from the vessel. Secondly disclosed is a condensing apparatus with a variable volume reservoir (32, fig.2) to accommodate additional condensate drain liquid and prevent overpressure in the event of the surface of the condensate liquid freezing. Thirdly disclosed is a condensing apparatus with a space accommodating capsule (50, fig.3), which implodes under overpressure in the event of the surface of the condensate liquid freezing to create additional volume for the condensate liquid. Finally disclosed is a condensing apparatus with melt heating (68, fig.4) of a local portion of any solid frozen layer which forms on top of a liquid condensate so as to provide an escape route for liquid in the event of overpressure.

Description

1 239657 1

C455/G

Title: Improvements in and relating to glass condensers Field of invention

This invention concerns glass condensers used to condense vapour from drying or evaporating processes.

Background to the invention

The use of glass condensers is attractive for the following reasons: À Glass is resistant to attack from almost all solvents; À Glass assemblies are typically much lower cost than the equivalent component fabricated from other chemically resistant materials (such as stainless steels such as Hastelloy); À Glass is transparent therefore the user can see what is occurring within the condenser.

Glass condensers are not new, but have been used for many years in laboratories and process industries typically in association with evaporation devices, to recover the solvent from the vapour/gas mixture from the evaporator. Examples of glass condensers are: À Water cooled glass condensers which are typically used in conjunction with laboratory evaporators such as Rotary Evaporators. Sometimes these condensers are used in conjunction with chiller units to provide lower condensing temperatures typically using water/ethylene glycol as a heat transport medium.

À Cold Anger type glass condensers which are typically cooled by the use of dry ice or liquid nitrogen, and are typically used in conjunction with laboratory evaporators such as Rotary Evaporators.

Condensers which incorporate interchangeable glass condensing units have also been proposed, in which condensate is collected in a removable glass jar or flask. During the evaporation/condensation process the condensate is collected in the jar which is immersed in a bath of water/ethylene glycol mixture which is chilled by a refrigeration unit. On completion of an evaporation process, the jar is removed to a safe location for defrosting in case of fracture and spillage, and an empty glass jar is installed, ready to handle the next evaporation process. This method is used in centrifugal evaporators supplied by Thermo Savant Corporation of the USA.

Problems occur when a glass condenser is used to condense a solvent that a) has a melting point above that of the condenser wall temperature, and/or b) expands when freezing.

Water is the most common of such solvents. When water is frozen in a condensing vessel that is cooled externally, the water will tend to freeze from the top down, trapping a volume of water below a layer of ice. When this trapped water eventually freezes the expanding ice has nowhere to go and this generates extreme pressure which is sufficient to break the wall of the glass vessel.

Sumunarv of the invention The invention provides different techniques to prevent glass condenser components breaking due to expansion of frozen solvent as it defrosts. Each technique can be used in isolation or in combination with one or both of the other techniques to achieve an acceptable life for the glass component before breakage occurs.

According to one aspect of the invention, in a condenser a temperature sensitive control system is provided to prevent a glass vessel within or forming the condenser from being chilled, after defrosting has occurred, until such time as all liquid solvent has drained therefrom.

The control system is arranged to take over and control the process, whether the defrosting is that which occurs at the end of an evaporation process or is a temporary intermediate period of defrosting that can occur before an evaporation process is completed.

Such intermediate periods can range from a few seconds to upwards of 1 minute or more.

It has been found that glass condensers rarely break during an evaporative step in which solvent vapour condenses and freezes onto the walls of the vessel. But, in contrast, if the recovered frozen solvent is allowed to melt (defrost) and convert wholly, or even partially, to a liquid, and this liquid is allowed to refreeze in the vessel, the latter is almost certain to break if the freezing continues below a thin surface layer of the solvent.

According to a second aspect of the invention, a variable volume reservoir is provided into which liquid trapped in the base of a condensing vessel can expand as upper layers of liquid in the vessel freeze over.

Typically an expansible closed reservoir is connected by a pipe to the condensing vessel, the pipe communicating with a region of the interior thereof which in general will contain the last of the liquid to freeze.

Typically the pipe connection will be to the lowest part of the vessel.

Preferably the pipe is designed to ensure that the freeze-front travels from the vessel towards the variable volume reservoir.

Preferably the maximum expanded volume of the reservoir is such as to be able to accommodate solvent expansion under the worst case situation.

Preferably the variable volume reservoir is adapted to return to its initial smaller volume once the solvent is defrosted, thereby to expel liquid therefrom back into the vessel.

The variable volume reservoir may include a drain, so that any liquid remaining therein will drain therefrom independently of the drainage from the condensing vessel, to prevent solvent entrapment in the reservoir after the vessel has been completely defrosted and drained. The variable volume reservoir is preferably resiliently expansible and typically comprises a closed chamber formed at least in part by bellows between a fixed end wall and a movable end wall acted on by spring means tending to close the bellows, and a pipe connecting the reservoir to the vessel extends through the fixed wall.

According to a further aspect of the invention, a resiliently collapsible closed capsule may be located within the condensing vessel in a region which will be the last to freeze, and as the pressure in the liquid below upper freezing layers increases, the capsule collapses (implodes) so as to decrease the volume it occupies and thereby increase the volume available to the liquid below the freezing interface, thereby to prevent excessive forces building up within the lower regions of the vessel.

Typically the closed capsule includes spring means to provide some resistance to the crushing force and to reinstate the capsule to its normal size when defrosting occurs leading to drainage.

In addition or alternatively the closed capsule may be pressurised with air or an inert gas to provide the crushing resistance and restoring force.

According to a still further aspect of the invention, an escape path is provided through which liquid can flow from an entrapment region at the bottom of a condensing vessel to a region above the level of the solvent in the vessel, and means is provided to prevent liquid in the escape path from freezing.

In one embodiment, a strip of material having good thermal conduction properties is placed in contact with the external wall of the condensing vessel, to extend from the bottom of the

s vessel at least to a region thereof which will normally be above the maximum height to which liquid will rise in the vessel in use.

A thermally conductive compound (liquid gel or paste) can be used to improve the thermal contact between the strip and the wall.

The strip of thermally conductive material may be fitted with or comprise a resistive heating element, through which an electric current can flow to heat the strip and the elongate region of the vessel in contact therewith, and therefore the material within that region of the vessel.

Temperature monitoring means may be provided such as a thermistor or a thermocouple and electrical energy may be supplied to the heating element as required, by a control system, so as to maintain a constant temperature in the region of the strip, in the vessel, as measured by the temperature monitoring means. Typically this temperature is chosen to ensure that a small column of solvent immediately adjacent to the heated region of the vessel wall will remain unfrozen at least until all the liquid solvent in the lower end of the condensing vessel has frozen (or sufficient of the liquid has become frozen that further freezing will not create damaging pressures and forces within the glass vessel).

Since the condensing vessel is made from glass, it is a further aspect of the present invention to provide a viewing window over at least some of the height of the vessel through which the interior can be viewed. In order to prevent heat transfer from the environment the glass vessel is typically surrounded by an insulating sleeve or jacket and this needs to be cut away if an elongate viewing window is to be provided. To prevent heat transfer through the window an evacuated double glazed unit may be employed, through which the interior of the vessel can be viewed. The double glazed unit may be separate from and fitted over the window region of the glass walled vessel or may be integrally formed with the glass wall of the vessel.

A glass vessel condenser including one or more of the foregoing techniques can be used in conjunction with a Centrifugal evaporator, a Rotary evaporator, a Nitrogen blowdown evaporator, or a Vortex evaporator.

In the drawings: Fig 1 is a general arrangement of an in-situ refrigerated condenser; Fig 2 illustrates an external expanding reservoir for use with the condenser of Fig 1; Fig 3 illustrates an internal compressible capsule for use with the condenser of Fig 1; Fig 4 is a side elevation of a glass condensing vessel such as shown in Fig 1 having an external conductive strip arranged down the wall thereof; and Fig 5 is a plan view of the condensing unit of Fig 4 showing more detail of the conductive strip and temperature monitoring device and showing how a window can be provided for viewing the interior of the vessel.

Detailed description of the drawings

In Fig 1 a glass generally cylindrical condensing vessel 10 having a closed hemispherical lower end 12 is sleeved around its cylindrical section by a cooling jacket 14 through which a refrigerant fluid is circulated via inlet 16 and outlet 18. The jacket 14 and exposed hemispherical end 12 are encased in thermally insulating material 20 which may be contained in a casing (not shown).

The upper end of the cylindrical section of vessel 10 is closed by a lid 22 through which vapour and air from an evaporation chamber can pass via inlet pipe 24 and from which air/vapour is drawn by a pump (not shown) via outlet pipe 26.

A liquid drain 28 containing a flow control valve 30 for controlling the flow of liquid to waste, communicates with the bottom of the hemispherical end 12 of the vessel 10, to allow liquid solvent condensed in the vessel to be drained off when required.

In accordance with one aspect of the invention the temperature of the refrigerant and/or its supply to the jacket 14 is controlled so that after condensing is completed or partially completed and the solid contents of the vessel is allowed to melt and form a liquid, which collects in the base of the vessel, the temperature of the latter is not permitted to drop until valve 30 is opened and all the liquid has drained out of the vessel.

In accordance with another aspect of the invention an expansible reservoir 32 is shown fitted to the drain 28 in Fig 2. If 30 is closed, and following defrosting liquid 34 collects in 10 this can freeze over as at 36. Subsequent freezing of the liquid 34increases the pressure in the remaining liquid, thereby exerting unwanted forces on the glass wall of the vessel 10. However the increasing force in the liquid 34 transfer liquid into the expansible reservoir 32 causing the bellows section 38 of the reservoir to increase in volume against the action of the spring 40. The bellows section is conveniently formed from PTFE, and the two ends are closed by rigid end caps 42, 44, the latter having a through pipe 46 communicating with the drain 28 by way of a T-piece connector 48.

Opening the valve 30 when evaporation is complete relieves the liquid pressure and the captive liquid is expelled via 48 and 30, and as the frozen solvent 10 rises in temperature at the end of an evaporation step (or the complete evaporation process) the solvent melts and runs away via drain 28 and open valve 30.

Fig 3 shows an alternative arrangement in which a resiliently deformable capsule 50 is located within the lower end of the vessel 10, but so as not to block the exit to the drain 28. The capsule is composed of upper and lower end caps 52, 54, a bellows section 56 of PTFE and a helical spring 58 which is compressed if the pressure in the fluid 60 in the vessel increases.

The lower cap 54 is supported by feet such as 62 to allow liquid to enter drain pipe 28, when valve 30 is opened.

Pressure in liquid 60 can rise if the upper region 62 freezes over and then commences to freeze as can occur during, or at the end of, an evaporation process.

By selecting the spring 58 so that it will be readily compressed as the force acting on the end caps 52, 54 increases as the depth of ice in the layer 62 increases, and the liquid trapped below it has nowhere to go.

Figs 4 and 5 illustrate a further embodiment of the invention in which an escape path is provided through which liquid from the unfrozen body of liquid 64 below a frozen layer 66 (see Fig 4) can flow to relieve excess pressure in the liquid 64 below 66. The escape route is maintained, even when the body of liquid 64 begins itself to freeze, by heating a narrow strip of the wall of the vessel 10 from the hemispherical closed lower end to above the frozen later 66.

A linear hearing element is shown at 68 to which electric current is supplied via leads 70, 72 and a temperature sensing device is shown at 74, for controlling the heating current to the element 68.

The relative positions and mounting of the heating element 68, and sensor 74 can be seen by comparing Fig 4 and Fig 5 which also shows a thermally conductive strip 76 running from th base of the vessel up the side to near the lid 22. The heating element and sensor are mounted on the external surface of the strip 76, whose good thermal conductivity ensures that a very small temperature gradient exists from one region to another along its length. The thermally conductive strip 76 may be sufficient on its own, without heating from element 68, to maintain a column of liquid condensate in the vessel up through any frozen layer such as 66.

By careful selection of the material for the strip 76, no separate heating element 68 may be required, and heating may be achieved by passing an electric current through some or all of the length of the strip 76.

A viewing window may be provided for each of the embodiments by discontinuing the insulating sleeve and insulating material, so as to leave a slot such as 78 (in Fig 5) clear of material, and permit the contents of the vessel to be seen through the exposed glass wall.

To prevent local heating of the vessel and its contents, the slot may be filled partially or wholly by an evacuated double-glazed unit which may be separate from the glass wall of the vessel, or may be integrally formed therewith.

Claims

C455/G CLAIMS

1. A method of preventing damage to a glass vessel forming part of or comprising a condenser, in which the vessel is chilled prior to and during a condensing step and in which in use solid condensed solvent collects on the inside of the vessel, wherein after a condensing step the condensate is allowed to defrost and melt and the liquid so formed is drained from the glass vessel before the latter is chilled again, wherein during the defrosting and drainage step the temperature of the vessel is maintained above the freezing temperature of the condensate to prevent the liquid re-freezing before it has drained from the vessel.

2. A condensing apparatus which includes or comprises a glass vessel which is chilled before and during use and in which a solvent is to be condensed so as to freeze onto the walls thereof in use, characterized by a temperature sensitive control system which in use after the glass vessel has been defrosted and the condensate melted to form a liquid to be drained from the vessel, prevents the temperature of the glass vessel from dropping sufficiently to allow the liquid to re-freeze before all the liquid condensate has drained from the vessel.

3. A condensing apparatus as claimed in claim 2 wherein the control system controls the temperature of the vessel after a defrosting step which occurs at the end of an evaporation process.

4. A condensing apparatus as claimed in claim 2 wherein the control system controls the temperature of the vessel after an intermediate defrosting step which occurs during an evaporation process, before evaporation is continued to completion.

5. A condensing apparatus as claimed in claim 4 wherein the duration of the intermediate defrosting step can range from a few seconds to upwards of some minutes.

6. A condensing apparatus which includes or comprises a glass vessel which is chilled before and during use and in which in use a solvent is condensed onto the inside surface thereof and resides thereon in a solid frozen state, characterized by a variable volume reservoir into which liquid formed by any defrosting of the frozen solvent at an intermediate point in, or at the end of, an evaporation process, and collected in the glass condensing vessel, can expand if upper layers of the liquid in the vessel freeze over and thereby trap the remaining liquid therebelow.

7. A condensing apparatus as claimed in claim 6 wherein the variable volume reservoir is an expansible closed chamber.

8. A condensing apparatus as claimed in claim 6 or 7 wherein the variable volume reservoir is connected to a region of the glass condensing vessel which in use, in general will be the last to freeze as the vessel is chilled.

9. A condensing apparatus as claimed in claim 7 or 8 wherein the connection is made to the lowest part of the vessel.

10. A condensing apparatus as claimed in any of claims 8 or 9 in which the connection is made via a pipe.

ll.A condensing apparatus as claimed in claim 10 wherein the design of the pipe is selected so that in use, in general the freeze-front travels from the glass vessel towards the variable volume chamber.

12.A condensing apparatus as claimed in any of claims 6 to 11 wherein the variable volume chamber is resiliently expansible to accommodate liquid due to expansion during the cooling, and the variable volume reservoir expels liquid back into the vessel as it returns to its initial volume once the glass vessel is defrosted.

13.A condensing apparatus as claimed in any of claims 6 to 12 wherein the variable volume chamber includes a drain, so that any liquid remaining therein will drain therefrom independently of the drainage from the condensing vessel, to prevent solvent entrapment in the former after the vessel has been completely defrosted and drained.

14. A condensing apparatus as claimed in claim 12 or 13 wherein the variable volume chamber is formed at least in part by bellows between a fixed end wall and a movable end wall acted on by spring means tending to close the bellows, and a pipe connecting the chamber to the vessel extends through the fixed wall.

15. A condensing apparatus which includes or comprises a glass vessel which is chilled before and during use and in which in use a solvent is to be condensed on the inside surface thereof and reside thereon in a solid frozen state, characterized by a resiliently collapsible closed capsule within the glass condensing vessel, located in a region in which liquid will collect if the frozen condensate defrosts, and which in practice will be the last of the region to freeze if the vessel is chilled and the liquid freezes, and wherein as the pressure in liquid below any upper freezing layer increases the capsule will collapse (implode) under any increasing liquid pressure so as to decrease the volume it occupies and thereby increase the volume available to the liquid trapped below the freezing layer, thereby to prevent excessive forces building up within the glass vessel below such layer.

16. A condensing apparatus as claimed in claim 15 wherein the closed capsule includes spring means to provide resistance to the crushing force and to reinstate the capsule to its normal size when defrosting occurs.

17. A condensing apparatus as claimed in claim 15 or 16 wherein the closed capsule is pressurised with air or an inert gas to provide resistance to crushing and a restoring force.

18.A condensing apparatus which includes or comprises a glass vessel which is chilled before and during use and in which a solvent is to be condensed onto the side thereof and which can melt and form a liquid condensate which if chilled can re-freeze if it has not

drained form the vessel, characterised by means forming an escape path through which liquid can flow from an entrapment region at the bottom of the vessel below any freezing interface in the liquid to a region above the level of a frozen layer of condensate in the vessel, and wherein the said means forming the escape path, or additional means, serves to prevent liquid in the escape path from freezing.

19. A condensing apparatus as claimed in claim 18 wherein the escape path is created by a strip of thermally conductive material located in thermally conductive contact with the external wall of the condensing vessel, to extend from the bottom of the vessel at least to a region thereof which in use will be above the maximum height to which liquid condensate will rise in the vessel, to create a locally unfrozen region through which the liquid can pass even if the upper region of liquid in the vessel becomes frozen and would otherwise prevent expansion of liquid thereabove.

20. A condensing apparatus as claimed in claim 19 wherein a thermally conductive compound (liquid gel or paste) is used to improve the thermal contact between the strip and the wall of the vessel.

21. A condensing apparatus as claimed in claim 19 or 20 wherein the strip of thermally conductive material has fitted thereto, or comprises, a resistive heating element, and means is provided to cause an electric current to flow through the element to heat the strip and the elongate region of the vessel in contact therewith, and therefore the condensate within that region of the vessel.

22. A condensing apparatus as climed in claim 21 further comprising temperature monitoring means, such as a thermistor or a thermocouple, and the current supply means is adapted to supply electrical energy to the heating element as required, by a control system, so as to maintain a constant temperature in the liquid in the vessel in the region of the strip as measured by the temperature monitoring means.

23. A condensing apparatus as claimed in any of claims 6 to 22 wherein a viewing window is provided over at least some of the height of the glass vessel through which the interior can be viewed.

24. A condensing apparatus as claimed in claim 23 wherein the window is formed by an evacuated double glazed unit fitted over the outside of the glass vessel.

25. A condensing apparatus as claimed in claim 23 wherein the window is formed by an evacuated doubled glazed region forming an integral part of the vessel wall.

26. A condensing apparatus as claimed in any of claims 2 to 25 in combination with a Centrifugal evaporator, a Rotary evaporator, a Nitrogen blowdown evaporator, or a Vortex evaporator.

27. A condensing apparatus constructed arranged and adapted to operate substantially as described with reference to and as illustrated in the accompanying drawings.