Abstract:
A reaction vessel for conducting low temperature reactions is disclosed, the reaction vessel comprising: a circulation chamber with an entry port for the supply of a cooling medium at a temperature below ambient to said circulation chamber and an exhaust port; and a reaction chamber and a centrifugal fan arranged concentrically-inside the circulation chamber so that gas in said circulation chamber is forced radially outwards by the operation of said fan, wherein the movement of the gas past the reaction chamber maintains the temperature of reactants inside the reaction chamber at a temperature below ambient. A chiller for providing a gas supply at a temperature below ambient is also disclosed which may be used with the above reaction vessel.

Description:
[0001]    The present invention relates to an apparatus and method for providing low temperature reaction conditions in continuous flow reactions (flow chemistry). It is particularly, but not exclusively, concerned with apparatus and methods which provide for reaction conditions which are significantly below 0° C., and may be as low as −78° C. 
         [0002]    There is considerable demand in the flow chemistry field for apparatus which is able to maintain a constant temperature in a reaction vessel whilst a reaction takes place in that vessel. Various approaches have been used to achieve this, ranging from “chilled liquid baths” to hot air heater systems such as that disclosed in Canadian patent application CA 2,586,262A. 
         [0003]    However, as the application areas for continuous chemistry has expanded, a demand has emerged for systems which can keep a reaction vessel at a temperature below ambient (often considerably below ambient, down to −78° C.) Examples of reactions that require a temperature below ambient and their required temperatures are: organolithium reactions (−40° C. to −78° C.) and Grignard reactions (0° C. to −50° C.). 
         [0004]    The currently available apparatus for achieving the required reaction conditions for low temperature reactions is a refrigeration system such as that provided by the FTS product “Maxi-Cool” an example of which is illustrated in  FIG. 1 . This system involves at least one refrigeration unit which has a heat exchanger in which a coolant liquid flowing in a heat transfer circuit is cooled by heat exchange with the refrigerant in the refrigeration circuit. The coolant liquid flowing in the heat transfer circuit is fed to the reactor vessel where it passes in a “jacket” around an enclosed portion of the reactor vessel in which the reactants are mixed, thereby cooling the reactants by conduction through the wall of the reactor vessel before returning to the refrigeration unit. A schematic of such a system is shown in  FIG. 2 . 
         [0005]    Using a single stage refrigeration unit such as that described above it is possible to maintain the temperature of reactants in the reactor vessel at approximately −30° C. In order to maintain the temperature of reactants in the reactor vessel at lower temperatures, multiple stage (cascade) refrigeration units are used as illustrated in the schematic diagram of  FIG. 2 . Accordingly, in order to maintain reaction temperatures of −70° C. or below, it is necessary to use a two stage refrigeration unit of this type. 
         [0006]    The two stage refrigeration unit shown schematically in  FIG. 2  has two heat transfer circuits  102  and  104  comprising a compressor  110  and an expansion valve  112 . The first heat transfer circuit  102  has a condenser  114  and an evaporator  116  which is formed in conjunction with the condenser  118  of the second heat transfer circuit  104 . The two heat transfer circuits  102  and  104  operate at successively lower temperatures and coolant flowing in a cooling liquid circuit  106  is cooled by passing through a heat exchanger  120  with the second heat transfer circuit  104 . The coolant is pumped around the cooling liquid circuit  106  by a circulation pump  108  so that it passes through a jacket  132  of a reaction vessel  130  which surrounds a reaction chamber  134  in which the low temperature reaction is conducted. 
         [0007]    Refrigeration units such as those discussed above are generally large units (the FTS Maxi Cool product measures approximately 600 mm×700 mm×1200 mm) and are therefore not particularly convenient for experiments done in typical research laboratories as space has to be found for the units and insulated connections provided between the units and the apparatus in which the reaction is being conducted. Further, conducting reactions which are made up of a plurality of separate low temperature reactions (such as the lithiation of epoxides) can require two or more refrigeration units, with attendant pressures on space. 
         [0008]    Also, refrigeration units such as those discussed above, when operated at low temperatures, have to use special heat transfer fluids (such as MultiTherm ULT-170 (Registered Trade Mark) supplied by Multi-Therm LLC) in the refrigeration circuit which can be chilled to the desired temperature whilst continuing to flow around the refrigeration circuit. In order to provide for efficient heat transfer between the coolant in the refrigeration circuit and the contents of the reactor vessel, it is necessary for the reactor to provide for flow of the coolant around the surfaces of the reactor vessel. However, this arrangement can cause difficulties due to the toxicity (due to vapour inhalation) of the hydrocarbon heat transfer fluids used and is also messy when disconnecting the reactor, when the coolant in the reactor will need to be collected or disposed of. 
         [0009]    Typical coolants used in systems of this type generally have high specific heat capacities. Whilst this allows effective transfer of heat from the reaction vessel to the refrigeration unit for a relative low volume flow of coolant, it has the disadvantage that it is difficult for these systems to respond quickly to changes in desired temperature of the reaction vessel. 
         [0010]    This means that it can take considerable time (30 minutes to 2 hours) for the reaction vessel to be cooled to the required temperature in advance of a reaction being carried out, or for changes in the desired temperature to be achieved during the reaction being carried out, or for the system to react to the effects of exo- or endothermic reactions changing the temperature of the reaction vessel. 
         [0011]    In order to address the problem of control of temperature, it is known to arrange the system so that the coolant supplied from the refrigeration unit(s) is supplied at a temperature which is appreciably below the required temperature for the reaction vessel, and to provide an additional mechanism, which can be more accurately and rapidly controlled, to supply heat to the fluids immediately prior to entering the volume around the reaction vessel (such as a conductive heater) to raise the temperature from that of the coolant to the desired reactor temperature. Such an arrangement is not only wasteful of resources, but can be complicated to set up and control accurately and increases the demands on the space around the reaction vessel. 
         [0012]    Further disadvantages of refrigeration units such as those currently in use are their cost (typically in the range £12,000 to £25,000), the noise generated by the compressors which achieve the cooling and the additional heating effect on the laboratory from running these units. 
         [0013]    The present invention seeks to provide methods for conducting low temperature reactions and apparatuses in which low temperature reactions can be conducted which address or avoid one or more of the disadvantages of the existing methods and apparatuses discussed above. 
         [0014]    At their broadest, aspects of the present invention provide methods and apparatuses in which the temperature of a reaction vessel can be maintained at a required low temperature by arranging a flow of compressed gas across a cooling substrate, the resultant cooled gas being subsequently used to maintain a low temperature in the reaction vessel. 
         [0015]    A first aspect of the present invention provides, at its broadest, a new reaction vessel for conducting reactions at low temperature. 
         [0016]    This aspect of the present invention provides a reaction vessel for conducting low temperature reactions, the reaction vessel comprising: a circulation chamber with an entry port for the supply of a cooling medium at a temperature below ambient to said circulation chamber and an exhaust port; and a reaction chamber and a centrifugal fan arranged concentrically inside the circulation chamber so that gas in said circulation chamber is forced radially outwards by the operation of said fan, wherein the movement of the gas past the reaction chamber maintains the temperature of reactants inside the reaction chamber at a temperature below ambient. 
         [0017]    The cooling medium used may be a gas, a liquid or a solid medium. An example of a liquid cooling medium is liquid nitrogen. An example of a solid cooling medium is crushed dry ice (solid carbon dioxide). An example of a gas cooling medium is the output of the chiller of the second aspect described below. Preferably any non-gaseous cooling medium used is chosen so that it is in gaseous form in the temperature and pressure conditions within the circulation chamber. 
         [0018]    The cooling medium may be fed to the reaction chamber from a pressured supply or by pumping (in the case of gaseous or liquid cooling medium) or by a mechanical feed arrangement, such as an augur (in the case of solid cooling medium). Preferably the rate of supply of cooling medium to the reaction chamber is controllable. 
         [0019]    Preferably the reaction vessel further comprises a return passage for returning gas from the outside of the circulation chamber to a central portion of the circulation chamber, thereby forming a pathway for re-circulation of gas around the circulation chamber and past the reaction chamber. This arrangement allows cooling gas within the circulation chamber to be circulated in that chamber so that a flow of cooling gas is constantly passing the reaction chamber and maintaining the temperature of the reaction chamber, but for it not to be necessary for this flow of gas to be continuously supplied to the reaction vessel from an outside source. Similarly, it is not necessary for the cooling medium to be continuously supplied to the reaction vessel from an outside source. Where the gas or cooling medium has been supplied from a pressurised source, this means that the gas or cooling medium in that pressurised source is not consumed as rapidly as if there were no re-circulation in the reaction vessel. Further, this arrangement helps to ensure a uniform temperature of the gas circulating in the circulation chamber. 
         [0020]    The gas in the circulation chamber which passes the reaction chamber will be referred to herein as “cooling gas” whether that gas is the cooling medium itself, or a gas which results from evaporation or sublimation of the cooling medium, or a separate gas which is cooled by the cooling medium. Similarly, the “cooling gas” may in fact be used to heat the reaction chamber as discussed below. 
         [0021]    Preferably the reaction chamber is located radially outside said fan, more preferably directly outside with no intervening components blocking the gas flow from the fan to the reaction chamber. This arrangement causes the action of the fan to force the cooling gas directly over the outer surface(s) of the reaction chamber. The action of the fan tends to agitate the gas in the circulation chamber, thereby reducing or removing any laminar flow and ensuring even temperature distribution of the gas leaving the fan and accordingly passing the reaction chamber. 
         [0022]    Preferably the reaction vessel is arranged such that the cooling medium is introduced to the circulation chamber at a position radially inside the fan. This allows the cooling medium to cool the gas inside the fan (for example by mixing with that gas or evaporating or subliming into gaseous form in that area) before the gas is forced radially outwards past the reaction chamber by the fan. This arrangement results in the gas passing the reaction chamber to be some of the coldest in circulation chamber and most easily controlled by controlling the rate of introduction of cooling medium. 
         [0023]    If solid or liquid cooling mediums are used, it is likely that at least some of the cooling medium will not immediately evaporate or sublime into gaseous form, but that liquid droplets or fine solid particles will be entrained from the cooling medium into the gas moving through the circulation chamber. This effect is advantageous as the entrained cooling medium will evaporate or sublime and continue to cool the gas within the circulation chamber as it moves within that chamber, thereby allowing a more consistent temperature of the gas within the circulation chamber to be maintained. 
         [0024]    Preferably the reaction vessel further comprises a temperature sensor arranged to measure the temperature of the reaction chamber. More preferably the temperature sensor is a contact temperature sensor which contacts an outer surface of the reaction chamber. 
         [0025]    In a development of this aspect, the reaction vessel may further comprise a pre-cooling unit contained within said circulation chamber which is arranged to conduct a reagent from outside the reaction vessel to said reaction chamber and to reduce the temperature of said reagent prior to its mixing with other reagents. By causing a reagent to pass through a pre-cooling unit before it is mixed, it is possible to ensure that reagents only come into contact with each other once they have reached the desired temperature and that the overall purpose of maintaining the reaction chamber at a low temperature is not defeated by the reagents being mixed at a higher temperature than that of the reaction chamber and reacting at that temperature. 
         [0026]    Preferably all of the reagents which are to be reacted in the reaction chamber pass through pre-cooling units before they are mixed and so there are a plurality of such units, thereby ensuring that all reagents entering the reaction chamber are already at or near the desired temperature. 
         [0027]    This arrangement may also assist or ensure that the temperature of the mixed reagents in the reaction chamber is constant, and that there is not a temperature gradient along the reaction chamber caused by reagents entering the reaction chamber at a higher temperature than the desired reaction temperature and being cooled in the reaction chamber itself. 
         [0028]    The pre-cooling unit(s) are preferably coils of tube which are placed in the circulation chamber or in the return passage so that the gas flow past the reaction chamber is not affected by these units, but they are in the flow pathway of the gas as it is re-circulated in the circulation chamber and so are cooled by the gas. Alternatively, the pre-cooling unit(s) may be located in the flow of the gas exiting the circulation chamber. 
         [0029]    In a further development of this aspect, the reaction vessel may further comprise a pre-mixing vessel having a plurality of inputs and an output which is connected to an input to the reaction chamber, such that reagents can be mixed in said pre-mixing vessel prior to entering said reaction chamber. Providing a pre-mixing vessel can ensure that a good mixture of reagents is achieved, and that once the reagents enter the reaction chamber the incidence of pockets where the concentration of one reagent is particularly high or low can be avoided, thus assisting complete reaction as the reagents pass through the reaction chamber. 
         [0030]    Preferably said pre-mixing vessel is located in said circulation chamber, and so is also maintained at a temperature below ambient by said circulation of gas in the circulation chamber. 
         [0031]    The pre-mixing vessel is preferably used with the pre-cooling unit(s) described above, in which case the reagent(s) from the pre-cooling unit(s) are fed into the pre-mixing vessel before entering the reaction chamber. 
         [0032]    In a further development of this aspect, the reaction vessel may further comprise a quench vessel located in said circulation chamber, which is connected to the output of said reaction chamber and has an input for supply of a quenching solution, thereby allowing the reaction in said reaction chamber to be quenched prior to exiting the reaction vessel and whilst at a reduced temperature. This arrangement avoids or reduces the possibility of unreacted reagents reacting once the mixture has left the reaction vessel and passes through a warmer region (such as the ambient temperature of the laboratory). 
         [0033]    A particularly preferred form for the reaction chamber itself is an elongate tube which is wound into a cylindrical configuration inside said circulation chamber. Such a tube is typically 0.1 mm-5.0 mm in internal diameter with preferred tubes having diameters of 3.0 mm. Smaller diameter tubes from 0.050 mm upwards may be preferred in the case of highly reactive materials as the small channels will provide the required rate of mass transfer by Brownian motion to achieve adequate mixing of the reactants. Such a tube reaction chamber may be made from plastic fluoropolymer, allowing the tube to be wound into the cylindrical configuration, for example in a helical manner. Such a configuration need not be precisely cylindrical, but may be polygonal in cross section. The cylindrical configuration of the reaction chamber may be made up of a plurality of concentric cylindrical configurations, such as successive windings of the reaction chamber. 
         [0034]    An elongate and thin reaction chamber is particularly preferred as it can reduce or avoid a temperature gradient arising across the cross-section of the reaction chamber as the contents of the reaction chamber get further from the inner surface of the reaction chamber and so further from the outer surface of the reaction chamber which is being maintained at the desired temperature. An elongate reaction chamber is also preferred as it provides for a large surface area on the outside of the reaction chamber for the cooling gas to pass over and maintain the surface at the desired temperature. 
         [0035]    In one particular arrangement, the circulation chamber has a plurality of longitudinal support members arranged equidistant from the longitudinal axis of said circulation chamber and said reaction chamber is supported by said support members, for example by being wound around those support members, or by being connected to those support members. 
         [0036]    In embodiments of the reaction vessel, the reaction chamber is arranged around such support members in a weave pattern. The weave pattern allows good flow of the cooling gas over the surface area of almost the entire reaction chamber, whilst reducing or eliminating the need for further support members or connections to hold the reaction chamber in place. The weave arrangement also presents an obstructed passage for gas forced outwards from the fan thereby causing further agitation of the flow from the fan and assisting in ensuring a uniform temperature of the outer surface of the reactor, since there are few, if any, straight routes through the gaps in the weave pattern. The reaction chamber is also designed to be easily removable so that reaction chambers having different internal volumes may be exchanged. Also should a reaction chamber become blocked then it may be easily replaced. 
         [0037]    As the reaction vessel has a circulation chamber in which a cold gas is circulating to maintain the temperature of the reaction chamber at a desired temperature which is below that of the surrounding environment, it is preferable that the reaction vessel further comprises an insulating layer arranged around the outside of said circulation chamber, which reduces the heat transfer from the surrounding environment to the circulation chamber and so improves the efficiency and temperature stability of the reaction vessel. 
         [0038]    One particular example of an insulating layer is an outer shell enclosing a vacuum chamber between the outer shell and the outside of the circulation chamber. Additional or alternative forms of insulation may be used and different insulation methods and principles may be combined. 
         [0039]    Although the reaction vessel of this aspect has been specifically described in relation to the conduct of low temperature reactions, the skilled person will appreciate that reaction vessels according to this aspect may also be used for reactions in which elevated temperatures are desired, in which case the gas supplied to the circulation chamber would be at a temperature above ambient. Many of the preferred and optional features of this embodiment would also be useful if the reaction vessel was used in this manner. 
         [0040]    The reaction vessel of this aspect is suitable for use with the chiller of the second aspect described below, but is not limited to such use. 
         [0041]    A second aspect of the present invention provides a chiller for providing low temperature reaction conditions in a reaction vessel, the chiller comprising: an insulated vessel having a cavity and an entry port and an exit port to said cavity, wherein said entry port is connected to a regulator which is arranged to lower the pressure of a gas input to said regulator from a pressurised source to a supply pressure above atmospheric; and said exit port is arranged to be connected to the reaction vessel, said cavity containing, in use, one or more pieces of a coolant substrate having a temperature below ambient, such that when pressurised gas is input into the apparatus, it passes over said coolant substrate and is supplied to said reaction vessel at a temperature below ambient. 
         [0042]    The chiller may also comprise the coolant substrate. A particularly preferred coolant substrate is dry ice (solid carbon dioxide). Carbon dioxide sublimes from solid form (dry ice) to gaseous form at −78° C. and so can be used to chill the gas passing through the chiller to this temperature, which is lower than the temperature required for most low temperature reactions. 
         [0043]    Dry ice is also generally available in most existing laboratory environments and so no additional storage or other arrangements are required to use the chiller of this aspect in a laboratory. 
         [0044]    Since dry ice sublimes and does not have a stable liquid phase (at the temperatures and pressures of relevance), the gaseous carbon dioxide formed by the sublimation can be entrained in the flow of pressurised gas, and no residue is left in the cavity. This means that the passage of the gas through the cavity is not obstructed by melted residue. 
         [0045]    Furthermore, by using dry ice as the coolant substrate, the dry ice is constantly subliming as it cools the pressurised gas passing over it. This means that the surface of the dry ice is constant at the sublimation temperature of the dry ice, in contrast to alternative non-consumed coolant substrates, in which the temperature of the pieces of coolant substrate will rise as the gas is cooled, since they will have an unchanging volume. 
         [0046]    However, alternative coolant substrates can include metal or ceramic substrates which are cooled to a pre-determined temperature before being placed in the chiller. These substrates are re-usable and can be removed from the chiller after use and re-cooled for a future use. 
         [0047]    Preferably said regulator forms part of the chiller, for example being attached to the outside of the chiller. This allows the chiller to operate as a stand alone unit and to be connected to a variety of pressurised gas sources, including (but not limited to) gas cylinders and pressurised supply networks. 
         [0048]    In order to allow the coolant substrate to be placed in and/or removed from the cavity of the chiller, the cavity preferably has a sealable opening at the top of the chiller which is sufficiently wide for the pieces of coolant substrate to pass through. Clearly, if the substrate is consumed in the chilling process, then there is no need to remove the substrate unless there is substrate remaining after the reaction(s) has been completed. 
         [0049]    In a preferred embodiment of the chiller of this aspect, the entry port to the cavity is located near the top of said cavity and the exit port is located near the bottom of said cavity, but not at the lowest point of said cavity (the terms “top” and “bottom” being with reference to the usual position of the chiller in use). 
         [0050]    This arrangement causes the pressurised gas to generally flow from top to bottom of the cavity, and the coolant substrate to be located at the bottom of the cavity (due to gravity). Accordingly, the pressurised gas passes over the coolant substrate immediately prior to exiting the cavity and there is no (or a minimal) temperature differential between the coolant substrate and the exit port. Furthermore, any stagnant or slow moving gas in the cavity will be at its coldest at the bottom of the cavity and so, if entrained in the gas flow through the cavity, will not appreciably increase the temperature of that gas flow. 
         [0051]    By not placing the exit port at the lowest point of the cavity, it is possible to avoid or reduce the possibility of the exit port being blocked, either by the coolant substrate itself or by a foreign object which has got into the cavity, and will naturally gravitate to the lowest point. 
         [0052]    Although the chiller and the reaction vessel of the above two aspects are not required to be used in conjunction with each other, they have features which are complementary. 
         [0053]    Accordingly, a further aspect of the present invention provides an apparatus for conducting reactions at low temperature, the apparatus comprising: a pressurised gas source; a chiller according to the second aspect above connected to said gas source via said regulator, and a reaction vessel according to the first aspect above connected to said chiller, such that gas flows from said pressurised source, is lowered in temperature in said chiller and passes to the entry port of the circulation chamber of the reaction vessel. 
         [0054]    In this aspect of the present invention the chiller may include none, any or all of the preferred and optional features of the second aspect described above and the reaction vessel may include none, any or all of the preferred and optional features of the first aspect. 
         [0055]    Preferably the apparatus further comprises a controller which is arranged to control the flow of gas to the chiller from the gas source or the flow of cooled gas from the chiller to the reaction vessel in order to maintain the temperature of the reaction chamber at a predetermined temperature. This controller may have an input by which a user can input the desired temperature of the reaction chamber, and is preferably connected to a temperature sensor which detects the temperature of the reaction chamber to provide for feedback control of the temperature of the reaction chamber. 
         [0056]    In particularly preferred embodiment of this aspect, the chiller is connected to the reaction vessel via a valve and the controller controls said valve to supply cooled gas to said reaction vessel in a pulsed manner, and may vary that pulsed manner to control the temperature of the reaction chamber accordingly. 
         [0057]    Further aspects of the present invention relate to methods of maintaining a temperature in a reaction chamber below the ambient temperature and of performing a reaction in a reaction chamber which is maintained at a temperature below ambient during the reaction. 
         [0058]    Preferably the methods of these aspects are carried out using the apparatuses of the above aspects, but they are not limited to such apparatuses. 
         [0059]    A further aspect of the present invention provides a method of maintaining a temperature in a reaction chamber of a reaction vessel below the ambient temperature, wherein the reaction vessel has a circulation chamber in which said reaction chamber is located, the reaction chamber containing, in use, the reagents to be reacted, and the method comprises the steps of introducing a cooling medium to said circulation chamber to produce cooling gas in said circulation chamber; and circulating said cooling gas within said circulation chamber to maintain the temperature of the reaction chamber at a predetermined target temperature. 
         [0060]    The method preferably includes the additional steps of: measuring the temperature of the reaction chamber; and controlling the flow of cooling medium to the reaction vessel to maintain the temperature of the reaction chamber at said target temperature. 
         [0061]    The flow of cooling medium may be controlled by varying a continuous flow rate of the cooling medium or by providing a pulsed flow of cooling medium and controlling the pulses to control the total flow. 
         [0062]    A further aspect of the present invention provides a method of maintaining a temperature in a reaction chamber of a reaction vessel below the ambient temperature, comprising the steps of: placing one or more pieces of a coolant substrate having a temperature below ambient inside an insulated vessel having an entry port and an exit port; feeding a dry gas under pressure through said entry port into said insulated vessel, over said coolant substrate and through said exit port to cool the dry gas; and feeding the cooled dry gas from the exit port to the reaction vessel to cool said reaction chamber. 
         [0063]    Preferably the coolant substrate is dry ice. Preferably the dry gas is nitrogen. 
         [0064]    Preferably the dry gas is supplied at high pressure and the method further comprises the step of regulating the supply of dry gas to a gauge pressure (i.e. a pressure above that of the surrounding atmosphere) of between 0 and 0.5 bar prior to feeding it to said entry port. At this pressure, the additional pressure above the local atmospheric pressure forces the gas through the insulated vessel to the reaction vessel (where the gas may be exhausted into the surrounding environment), but the pressure is not such that the various items of apparatus need to be specifically engineered to cope with high pressure gases (and in doing so, they may also avoid having to comply with any safety requirements associated with pressurised sealed vessels), or that there is a safety risk from the pressure of the gas. 
         [0065]    Preferably the reaction vessel has a circulation chamber in which said reaction chamber is located, the reaction chamber containing, in use, the reagents to be reacted, and the method further comprises the steps set out in the method of the previous aspect. 
         [0066]    In order to maintain the temperature of the reaction temperature at the predetermined target temperature, the method preferably further includes the steps of: measuring the temperature of the reaction chamber; and controlling the flow of cooled dry gas to the reaction vessel to maintain the temperature of the reaction chamber at said target temperature. 
         [0067]    A preferred way of controlling the flow is to provide a pulsed flow of said gas to the reaction vessel and to adjust the pulses to control the flow. 
         [0068]    A further aspect of the present invention provides a method of performing a reaction in a reaction chamber at a temperature below the ambient temperature, comprising the steps of: maintaining the temperature of said reaction chamber at a predetermined target temperature below ambient by carrying out a method according to either of the previous two aspects, including none, any or all of the optional and preferred features of that aspect; and feeding reagents to be reacted in said reaction to said reaction chamber. 
         [0069]    Preferably the method of this aspect further comprises the step of, prior to said step of feeding, passing at least one of said reagents through a pre-cooling unit contained within said reaction vessel, wherein the temperature in said pre-cooling unit is also maintained below ambient by said cooling gas. This allows the reagents to be cooled to at or near the target temperature before they come into contact with each other, so the overall purpose of maintaining the reaction chamber at a low temperature is not defeated by the reagents being mixed at a higher temperature than that of the reaction chamber and reacting at that temperature prior to their entry into the reaction chamber. 
         [0070]    Preferably all of the reagents which are to be reacted in the reaction chamber are passed through pre-cooling units before they are mixed and so there are a plurality of such units, thereby ensuring that all reagents entering the reaction chamber are already at or near the target temperature. 
         [0071]    This additional step may also assist or ensure that the temperature of the mixed reagents in the reaction chamber is constant, and that there is not a temperature gradient along the reaction chamber caused by reagents entering the reaction chamber at a higher temperature than the target temperature and being cooled in the reaction chamber itself. 
         [0072]    Preferably the method further comprises the step of, prior to the step of feeding said reagents to said reaction chamber, mixing said reagents in a pre-mixing vessel contained within said reaction vessel, wherein the temperature in said pre-mixing vessel is also maintained below ambient by said cooling gas. 
         [0073]    Mixing the reagents in a pre-mixing vessel can ensure that a good mixture of reagents is achieved, and that once the reagents enter the reaction chamber the incidence of pockets where the concentration of one reagent is particularly high or low can be avoided, thus assisting complete reaction as the reagents pass through the reaction chamber. 
         [0074]    The step of pre-mixing is preferably combined with the step of pre-cooling described above, in which case the reagent(s) from the pre-cooling unit(s) are fed into the pre-mixing vessel before entering the reaction chamber. 
         [0075]    Preferably the method further comprises the step of quenching said reaction by adding a quenching solution to the output of said reaction chamber before the output from said reaction chamber exits said reaction vessel and the temperature of said output rises above said target temperature. 
         [0076]    This additional step avoids or at least reduces the possibility of unreacted reagents reacting once the mixture has left the reaction vessel and passes through a warmer region (such as the ambient temperature of the laboratory). 
     
    
     
       BRIEF DESCRIPTION OF FIGURES 
         [0077]    Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: 
           [0078]      FIG. 1  shows a known recirculating chiller unit produced by FTS and has already been discussed. 
           [0079]      FIG. 2  depicts a known system for conducting low temperature reactions utilising one or more refrigeration units and has already been discussed. 
           [0080]      FIG. 3  shows a cross-section through a chiller according to an embodiment of the present invention. 
           [0081]      FIG. 4  shows the lid of the chiller of an embodiment of the present invention. 
           [0082]      FIG. 5  shows a perspective view of a reaction vessel according to an embodiment of the present invention with the outer casing and supporting structure removed. 
           [0083]      FIG. 6  shows the same perspective view as  FIG. 5  with the reactor chamber removed. 
           [0084]      FIG. 7  shows the same perspective view as  FIG. 5  with the temperature sensor in place. 
           [0085]      FIG. 8  shows a plan view of the reaction vessel of  FIG. 5 . 
           [0086]      FIG. 9  shows a cross-section through the complete reaction vessel of the illustrated embodiment along the line marked A-A in  FIG. 8 . 
           [0087]      FIG. 10  shows the outer casing and supporting structure of the reaction vessel shown in  FIG. 5 . 
           [0088]      FIG. 11  shows a complete assembled reactor vessel according to an embodiment of the present invention. 
           [0089]      FIG. 12  shows an apparatus for conducting reactions at a low temperature according to an embodiment of the present invention. 
           [0090]      FIG. 13  is a graph of temperature measurements taken from an apparatus illustrated in  FIG. 12 . 
       
    
    
     DETAILED DESCRIPTION 
       [0091]    Preferred embodiments of the present invention will now be described. In particular, the embodiments of the present invention include a chiller as shown in  FIGS. 3-4 , a reaction vessel as shown in  FIGS. 5-11  and the overall apparatus for conducting reactions at a low temperature as shown in  FIG. 12 . The skilled person will appreciate that whilst the chiller and reaction vessel of these embodiments are described as being used together, each can also be used with alternative additional components. 
         [0092]      FIG. 3  shows a cross-section through a chiller  10  according to an embodiment of an aspect of the present invention. The chiller  10  comprises a cavity  12  which can be of any size and shape but typically, as shown, is essentially cylindrical with a capacity of approximately 2 litres. The cavity  12  contains, in use, a plurality of lumps of a coolant substrate  14 . These lumps of coolant substrate are preferably irregularly shaped so that, when placed in the cavity  12 , they do not fit perfectly and have gaps between them to allow for gas flow through the coolant substrate as a whole and around the individual lumps  14  as indicated by the arrows G. 
         [0093]    The cavity  12  has an entry port  16  located near the top of the cavity and an exit port  18  located near the bottom of the cavity to allow for flow of gas through the cavity and past the coolant substrate. Preferably the exit port  18  is located close to, but not directly at, the lowest point of the cavity when the chiller is placed on a level surface, so that the exit port  18  is not blocked by any dirt or other solid matter that may end up in the cavity  12 . 
         [0094]    Whilst the chiller  10  shown in  FIG. 3  has the entry port  16  at the top of the cavity  12  and the exit port  18  at the bottom, and this is the preferred arrangement for thermal circulation reasons, it is not necessary for this arrangement to be adopted. Indeed, the entry and exit ports could be reversed. It is preferable that the entry and exit ports are located at opposite ends of the cavity  12  so that the gas passing through the cavity  12  has to pass over the maximum amount of coolant substrate  14  before exiting the cavity  12 , thereby achieving the greatest cooling effect possible on the gas. 
         [0095]    The preferred coolant substrate is dry ice (solid carbon dioxide/CO 2 ). However the coolant substrate may be made from different substances, depending on the desired output temperature of the chilled gas and the resources available. For example, for reactions in conditions only slightly below ambient, and above 0° C., ice could be used as the coolant substrate. Alternatively, the coolant substrate could be metal or ceramic which has been cooled to a predetermined temperature prior to being placed in the chiller. 
         [0096]    The advantages of using dry ice as the coolant substrate are that it has a well-defined and unchanging surface temperature (−78° C.) at which the solid CO 2  sublimes to gaseous CO 2 . In contrast, a non-expendable coolant substrate, such as a metal or a ceramic, will have a varying surface temperature over the time it is in use as heat is transferred to it from the passing gas. Further, since CO 2  sublimes directly to a gaseous form, the evaporated CO 2  can be immediately entrained in the gas flow over the remaining coolant substrate and out of the chiller, and so does not block the exit port  18 . In contrast, if ice was used as the cooling substrate, it would melt and the resultant liquid water would have to be periodically removed from the cavity  12  to ensure that the gas flow was not obstructed. 
         [0097]    It has been further found that using dry ice as the coolant substrate allows a highly uniform temperature of the cooled gas exiting the chiller and being conducted to the reaction vessel, and that an exit gas temperature of approximately −76° C. can be achieved even after the gas has travelled up to 1 metre along an insulated pipe from the exit port  18 . This can only be achieved if the exit port  18  has a minimum diameter of 3.5 mm. As a result of having an exit port  18  with this minimum diameter, it has been found that the flow of gas over the dry ice not only results in a cooling, by conduction, of the gas itself, but in the entraining of small particles of dry ice. 
         [0098]    These entrained particles subsequently sublime to gas and, in doing so, maintain a low temperature of the cooled gas. 
         [0099]    The cavity  12  has an opening  20  at the top of the chiller  10  to allow the coolant substrate  14  to be placed into the cavity  12 , and the cavity  12  to be cleaned if required. This opening  20  can be sealed, for example by a screw-in lid  22  as shown in  FIG. 3 , both to seal the cavity  12  at a pressure above atmospheric and to insulate the cavity  12 . The lid  22  is insulated. 
         [0100]      FIG. 4  shows a perspective view of the upper portion  30  of the chiller  10  with the flask portion removed. In the embodiment shown, two exit ports  18  are provided to promote uniform gas flow through the chiller  10  and reduce the amount of stagnation in the lower portions of the chiller  10 . Further exit ports can be provided, although additional ports will require additional pipes  32  which conduct the chilled gas from the exit ports  18  to the reactor vessel, and the space at the top of the chiller  10  may restrict the number of such pipes  32 . The exit ports  18  are formed as angled cuts through the end of pipes  32  which conduct the chilled gas from the inside of the chiller  10  to the reactor vessel. This configuration of the exit ports  18  minimises the risk of all of the exit ports  18  being blocked by the coolant substrate  14 . 
         [0101]    The cavity  12  is insulated by an outer shell  24 . In the preferred embodiment shown in  FIG. 3 , this outer shell  24  encloses a vacuum chamber  26  which surrounds the cavity  12  and insulates it from the surrounding air. Alternatively or additionally, the outer shell may be made from insulating material (material with a low heat conductivity) such as a solid foam. The inside surface  28  of the cavity  12  may have a reflective coating to further insulate the contents of the cavity  12 . 
         [0102]    Typical chillers according to embodiments of the present invention have cavities with capacities of between 1 and 5 litres, particularly between 2 and 3 litres. 
         [0103]    A chiller with a capacity of 2 litres or similar according to an embodiment of the present invention can therefore be provided which can be set on the laboratory bench adjacent the reactor vessel in which the low temperature reaction is being conducted. This is not only convenient in that it avoids pipes carrying coolant from running across other parts of the laboratory, but also minimises the importance of insulation of such pipes and/or the temperature gained by the coolant between its point of lowest temperature and the reaction vessel. 
         [0104]    Furthermore, if it is desired to conduct multiple low temperature reactions in series, multiple chillers can be provided without significant constraints on space. The output from the reaction chamber of one chilled reaction vessel can be fed into the reaction chamber of a second chilled reaction vessel, and each reaction vessel cooled by a supply of chilled gas from a separate chiller according to an embodiment of the invention. 
         [0105]    As shown in  FIG. 12 , the entry port  16  is connected by a pipe  34  to a regulator  32  which is connected to the pressurised gas source  36 . The regulator  32  acts to reduce the pressure of the gas from the pressurised gas source  36  to a lower pressure. In the present embodiment, the regulator  32  acts to reduce the pressure from approximately 10 bar in the pressurised gas source (the pressure supplied from a typical pre-regulated compressed gas cylinder—nitrogen in cylinders is typically stored at 230 bar, but regulated at the cylinder exit to 10 bar) to 0.5 bar gauge pressure (i.e. the overpressure above atmospheric) for supply to the chiller  10  and onwards through the apparatus. 
         [0106]    There are two principal reasons for the use of the regulator  32 . The first is that, in many jurisdictions (including the UK), there are safety regulations which place requirements on any sealed vessel which is intended to contain a gas above a certain pressure (the applicable UK regulations are the Simple Pressure Vessels (Safety) Regulations 1991, which set a limit of a gauge pressure of 0.5 bar, above which the vessel must satisfy additional requirements as to its construction and certification). Since the apparatus of this embodiment uses the pressure of the supplied gas to cause flow through the apparatus, but does not require pressures above 0.5 bar gauge pressure to achieve this flow, the chiller can be made simpler, cheaper and safer by regulating the pressure of the gas supply at the start of its flow through the apparatus. 
         [0107]    The second reason for regulating the pressure of the gas supply is to provide for efficient use of the gas supply. If the gas supply was connected to the apparatus and not regulated, then the volume flow rate of gas through the apparatus would be significantly greater. This would not only deplete the pressurised gas source  36  faster (necessitating more regular changes of the gas source where it is an individual cylinder, or more regular refills of a larger store), but would also reduce the cooling effectiveness of the gas in the reaction vessel, since the gas would spend a shorter time in contact with both the coolant substrate  14  and the surfaces of the reactor chamber  56  in the reaction vessel  50 . 
         [0108]    Therefore, whilst regulation of the input gas to a gauge pressure of less than 0.5 bar is applicable in the UK, alternative pressures may be selected for use of the apparatus in different countries, but it is advantageous in any event that the regulator reduces the pressure of the pressurised gas source to a gauge pressure of less than 1.0 bar, more preferably to a gauge pressure of less than 0.7 bar. 
         [0109]    Operating at a gauge pressure of 0.5 bar, and maintaining a demand temperature of −40° C. for the reaction chamber, an apparatus such as that shown in  FIG. 12 , and described below, can operate using only approximately 0.2 kg of dry ice per hour. 
         [0110]    Any appropriate source of pressurised gas can be used with the chillers and reaction vessels of the present invention. Ideally the pressurised gas is dry, so that there are no problems with icing when the gas is cooled below 0° C. The preferred source of pressurised gas is compressed nitrogen. Compressed nitrogen is readily available in cylinder form and already in use in most laboratories and is dry and does not contain any component which will solidify or condense in the temperature range of reactions generally being considered. Compressed nitrogen is also stored in larger quantities by some companies. Alternatively, compressed dry air may be used. 
         [0111]    As shown in  FIG. 12 , the exit port  18  from the cavity  12  is connected by an insulated pipe  98  to the reaction vessel  50 . 
         [0112]      FIG. 5  shows a perspective view of a reaction vessel  50  according to an embodiment of the invention with the outer casing and supporting structure removed.  FIG. 6  is the same perspective view, but with the reactor chamber removed to show further detail of the inner components.  FIG. 7  is the same perspective view as  FIG. 5 , showing the temperature sensor.  FIG. 8  is a plan view of the reaction vessel  50  with the outer casing removed.  FIG. 9  shows a cross-section through the reaction vessel  50  along the line marked A-A in  FIG. 8  and showing the reaction vessel  50  in complete form. 
         [0113]    Reaction vessel  50  is generally cylindrical and has a base  51  on which a set of concentric components are mounted. Viewed as a whole in  FIG. 9 , the reaction vessel  50  comprises, from the inside outwards, a central chamber  52 , a centrifugal fan  54 , a reaction chamber  56  which is an elongate tube wound around a series of posts  58 , an exhaust cavity  60 , an inner shell  62 , an insulating vacuum cavity  64 , an outer shell  66 , and a support casing  68 . The volume contained inside the inner shell  62  will be referred to as a circulation chamber  70 . 
         [0114]    A cooling medium, which may be liquid nitrogen, crushed dry ice or cold gas from a chiller (preferably a chiller such as that described in the embodiment above) is supplied to supply port  94  in the reaction vessel  50  which feeds into the central chamber  52  where it cools the gas within that central chamber, for example by evaporating or subliming, or by mixing with gas within the central chamber. 
         [0115]    The size and position of the supply port  94  may differ in particular depending on the form of the cooling medium being used. 
         [0116]    The centrifugal fan  54  is driven by an electric motor  72  and causes gas from the central chamber to be forced outwards past the reaction chamber  56  to the exhaust cavity  60 . A return passage  74  allows gas arriving in the exhaust cavity  60  to be returned to the central chamber  52  thereby creating a circulation pathway indicated on  FIG. 9  by arrows F of cold gas around the circulation chamber  70  and a constant flow of cold gas past the reaction chamber  58 . Exhaust ports  90  are connected to the exhaust cavity  60  and allow a flow of gas out of the circulation chamber  70  through an exit cavity  92  to exit ports  95  to the outside, thereby keeping the pressure inside the circulation chamber  70  constant. 
         [0117]    The centrifugal fan  54  is made from a conductor such as aluminium (or any other metal, although metals with lower specific heat capacities are preferable as they allow for more rapid changes of operating temperature), which ensures that it is at uniform temperature and once it has been initially cooled to the temperature of the gas circulating in the circulation chamber  70  it does not affect the temperature of the gas passing through it. 
         [0118]    The fan  54  rotates at high speed (preferably between 1,000 and 5,000 rpm, preferably between 2,000 and 4,000 rpm and typically at 2,500 rpm) and as well as forcing circulation of the gas inside the circulation chamber  70  agitates the gas to ensure that the flow in the circulation chamber is relatively uniform over all parts of the reaction chamber  56 , and is non-laminar, thereby ensuring uniform temperature distribution of the outer surface of the reaction chamber  56  (and consequently along the length of the inside of the reaction chamber  56 ). 
         [0119]    The reaction chamber  56  is an elongate tube through which the reagents flow whilst the reaction is being conducted. By using an elongate tube as the reaction chamber  56 , the time for which the reagents are mixed in the reaction chamber  56  can be controlled by controlling the flow rate of reagents into the chamber or by changing the reaction chamber for one having a different internal volume. The elongate tube provides for a large surface area of the reaction chamber  56 , which provides for the maximum area for heat transfer between the cold gas in the circulation chamber  70  and the reaction chamber  56 . This ensures that the reaction chamber  56  as a whole as well as the contents of the reaction chamber  56  are maintained at the desired temperature by the cold gas, and that there is no temperature gradient within the reaction chamber  56 . Furthermore, the reaction chamber  56  can be wound in a helical manner to form a cylindrical structure as shown in  FIGS. 5 ,  7  and  9 , thereby allowing the reaction chamber  56  to be arranged in a compact manner and the concentric arrangement of the components of the reaction vessel achieved. 
         [0120]    As shown in  FIGS. 5 ,  7  and  9 , the reaction chamber  56  is wound in a weave pattern around the posts  58 . This weave arrangement maximises the flow of the cold gas in the circulation chamber  70  over the surface of the reaction chamber  56 , allowing the cold gas to pass between the coils of the reaction chamber  56 , without the need to provide additional structural support to ensure separation of the coils of the reaction chamber  56 . The weave arrangement also interrupts the direct flow of the gas from the centrifugal fan  54  and further ensures that the gas flow over the reaction chamber  56  is agitated and non-laminar. 
         [0121]    In the reaction vessel  50  shown in  FIGS. 5-11 , the central chamber  52  contains two mixing vessels: a pre-mixing vessel  76  and a quench vessel  78 . As these mixing vessels are within the circulation chamber  70 , they are also cooled to the desired reaction temperature by the passage of the gas around the circulation chamber  70 . 
         [0122]    The pre-mixing vessel  76  allows combination and mixing of the reagents prior to their passing through the reaction chamber  56 . An output  80  from the pre-mixing vessel  76  is therefore connected (connection not shown) to the reaction chamber  56  and the pre-mixing vessel  76  has a plurality of inputs  82  for the supply of reagents. 
         [0123]    The quench vessel  78  allows the reaction being carried out in the reaction chamber  56  to be quenched before the reaction mixture leaves the low temperature environment of the reaction vessel  56 , thereby preventing incomplete reactions suddenly being accelerated by the passage of the reaction mixture to an area of higher temperature. The quench vessel  78  has an input (not shown) which is connected to the output end of the reaction chamber  56 , an input  84  which can be connected to a supply of quench solution and an output  86 , which is the ultimate output of the reaction vessel  50 . The quench solution is also passed through a pre-cooling tube (as described below for reagents) to ensure the quench solution is at the temperature of the reactor. 
         [0124]    Just as it is undesirable for the reaction mixture to leave the low temperature environment of the reaction vessel  50  before the reaction has either completed or been quenched, it is also undesirable for the reagents or quench solution to be mixed at a higher temperature than the reaction is intended to be carried out at (e.g. room temperature) before they enter the pre-mixing vessel  76  or the reaction chamber  56  as any reaction (which may be different to the desired reaction, or may occur at an undesirable rate) is likely to commence immediately on mixing. 
         [0125]    Accordingly, the reaction vessel  50  may also have a plurality of pre-cooling coils (not shown) which are coils of tubing contained within the exit cavity  92 . The cool gas exiting the exhaust cavity  60  via the exhaust ports  90  enters this exit cavity  92  and cools the reagents flowing in the pre-cooling coils before they enter the pre-mixing vessel  76  or the reaction chamber  56  (in an arrangement in which there is no pre-mixing vessel). In alternative arrangements, the pre-cooling coils may pass through other parts of the circulation chamber, for example in the central chamber  52 , particularly if the neither of the mixing vessels  76 ,  78  is provided in that chamber, or the exhaust cavity  60 . 
         [0126]    Reagents and the quench solution enter the reaction vessel  50  through input tubes  97 A and  97 B, and the output from the quench vessel  78  exits the reaction vessel  50  through output tube  97 C. In the embodiment shown, there are two input tubes  97 A for the input of reagents, a single input tube  97 B for the quench solution and a single output tube  97 C, but alternative arrangements are envisaged in which there are further input or output tubes depending on the type of reaction to be conducted in the reaction vessel  50 . 
         [0127]    Power to the motor  72  driving the fan  54  is supplied through power lead  91 . 
         [0128]    The outer shell  66  and the vacuum cavity  64  insulate the circulation chamber  70  from the surrounding environment. 
         [0129]    Base  51  has similar insulating properties, but uses solid insulation. The skilled person will appreciate that alternative arrangements may be provided to insulate the circulation chamber. However, insulation of at least part of the reaction vessel  50  using a vacuum cavity  64  is particularly advantageous as, when provided in conjunction with either a transparent outer shell  66  and support casing  68  (e.g. made from glass or transparent plastic) or viewing windows in the outer shell  66  and support casing  68 , it allows visual inspection of the circulation chamber  70  and in particular the reaction chamber  56  whilst the reaction is being carried out. 
         [0130]    To connect the support casing  68  and the base  51  of the reaction vessel, the base  51  has a number of locking lugs  67  which interact with the support casing  68  to hold the base and support casing together. 
         [0131]    A temperature sensor  88  is provided which senses the temperature of the reaction chamber  56 . The preferred arrangement uses a platinum thin film PT100 sensor, which has been found to offer a lower cost solution than a thermocouple, and also provide a greater degree of noise immunity. 
         [0132]    The temperature sensor  88  is inserted into an entry port  90  in the outer shell  66  and positioned so as to abut the outer wall of the reaction chamber  56 . In this arrangement the temperature sensor  88  does not contact the reactants flowing in the reaction chamber  56 . However, an extremely accurate temperature measurement of the temperature of the fluid passing through the reaction chamber  56  can be achieved due to the direct contact with the wall of the reaction chamber. 
         [0133]    The entry port  90  for the temperature sensor and the temperature sensor  88  interact so as to seal the entry port when the temperature sensor is in place and thus prevent chilled gas circulating in the circulation chamber  70  from escaping through the entry port  90 . A cover or similar mechanism may be provided for sealing the entry port  90  if the temperature sensor is not connected. 
         [0134]    The temperature sensor  88  provides an electrical output through cable  99  which forms an input into a controller  40 , as shown in  FIG. 12 , which is arranged to control the temperature of the reaction chamber  56  at a desired temperature for the reaction being conducted. 
         [0135]    The desired temperature can be set by a data entry into the controller  40 . This may be means of a manual input device such as a dial or buttons, or through entry of an appropriate value on a computer which is connected to or forms part of the controller  40 . 
         [0136]    In the embodiment shown, the controller  40  controls the temperature of the reaction chamber  56  by controlling the input of gas to the chiller  10 . In alternative embodiments, where the cooling medium is not supplied to the reaction vessel  50  from a chiller  10 , the controller  40  may control the temperature of the reaction chamber  56  by controlling the flow of cooling medium to the supply port  94 . 
         [0137]    This may be achieved in a number of ways. The preferred way of controlling the temperature where a chiller  10  is used is to control the amount of dry gas being input to the chiller  10  and thereby control the amount of cool gas exiting the chiller and entering the reaction vessel  56 . In the embodiment illustrated, this is achieved through the controller  40  controlling a valve  45  which permits, restricts or prevents flow of the dry gas into the chiller  10  to the reaction vessel  50 . Arrangement of the valve  45  in the gas flow prior to the chiller  10  is preferred as it avoids the problems with icing of the valve(s) in the cold part of the gas flow. In particular, the flow of gas through the valve  45  may be pulsed, and the overall amount of gas flowing may be controlled by changing the time ratio or proportion of the “on” and “off” pulses to each other. 
         [0138]    Similar pulsed arrangements may be used for the supply of liquid or gaseous cooling medium to the reaction vessel  50  from other sources, particularly where the sources are pressurised. Alternatively, for a liquid cooling medium, a pump may be used which is controlled by the controller  40  to deliver known quantities of cooling medium in a continuous adjustable rate or through a pulsed arrangement. A further alternative, for a solid cooling medium, is a augur filled with said solid cooling medium, the augur being controlled by the controller  40  to deliver known quantities of the cooling medium in a continuous, adjustable rate or in a quantised manner, for example by using a stepper motor to drive the augur. 
         [0139]      FIG. 13  shows a graph of temperature measurements taken during testing of the apparatus shown in  FIG. 12 . In particular,  FIG. 13  shows three traces which represent the measured temperatures at the chiller  10  and two readings from temperature sensors located in the reaction vessel  50 . Except where the readings substantially overlap (readings up to 55 minutes on the x-axis of  FIG. 13 ), the upper reading is that from the chiller  12 . 
         [0140]    The temperature profiles of  FIG. 13  show the performance of the apparatus, starting from an ambient temperature of 25° C., in response to a series of demand temperatures, as set by the controller  40 , of respectively −70° C. (from time zero on the x-axis of  FIG. 13 ), −60° C. (from time 45 minutes), −40° C. (from time 55 minutes), −20° C. (from time 75 minutes) and 0° C. (from time 110 minutes). 
         [0141]    From  FIG. 13  it can be seen that the apparatus as a whole reaches an operating temperature of −70° C. in approximately 20 minutes from the demand for that temperature being set. This is a considerable improvement over known chiller apparatuses, where such temperatures in a reaction vessel can only be achieved 30-120 minutes from the low temperature being demanded. 
         [0142]    The effect of pulse control of the gas input to the chiller can be seen in the “saw-tooth” temperature readings at each of the steady temperatures. 
         [0143]    Furthermore, each demand temperature is reached without appreciable overshoot in either the positive or negative direction, and so the apparatus of this embodiment demonstrates the high degree of temperature control that can be achieved by using an apparatus according to the above embodiment. 
         [0144]    In order to increase the response rate of the apparatus to demands for increases in the temperature in the reaction chamber, it is possible to provide an intermediate heater between the chiller and the reaction vessel, rather than relying on heating from the surrounding environment to raise the temperature. However, as the response times achievable are generally comparatively rapid, such an arrangement is generally not necessary. 
         [0145]    The rapid responses and accuracy of the temperature control result from the use of chilled gas as a coolant, which is much more controllable than a liquid coolant, and in particular has a lower unit heat capacity, so the gas in the circulation chamber of the reaction vessel is able to change temperature rapidly without using an additional heater. Also, by arranging the temperature sensor as a contact sensor on the outer wall of the reaction chamber, the controller is able to accurately control the input of chilled gas to regulate the temperature of the gas circulating in the reaction vessel.