Gas manufacture

Abstract: A method of manufacturing a breathable, life-supporting cryogenic liquid mixture including the steps of: producing a stream of treated natural air having substantially all the CO 2 and H 2 O removed therefrom; admixing a quantity of mixing gas comprising nitrogen present in a proportion greater than that present in natural air thereby to produce a product gas mixture having a desired oxygen/nitrogen ratio. Liquefying this said mixture in a heat exchanger against a suitable chilling fluid.

Description: The present invention relates to the manufacture of breathable life-supporting gas mixtures for use in cryogenic cooling apparatus. When cryogenic liquids such as nitrogen are used as refrigerants there is always an associated hazard of asphyxiation if people enter a region containing a high concentration of the vaporised liquid. Alternatively, if liquid oxygen were used, the danger of enhanced combustibility is present. Consequently, the use of a cryogen which would be breathable and life-supporting when vaporised and pose no enhanced fire risk is clearly desirable. Such cryogens are known but present methods of production present many problems. For example, mixtures are often made in a batch process, by mixing the pure components by weight which has intrinsic problems of accuracy of the mixture composition and of ensuring that adequate mixing has taken place. DE-A-20 24 397 discloses a process for the manufacture of cryogenic liquid air but without any indication of the need to adjust the composition of the air. It is an object of the present invention to provide a method of manufacturing a breathable, life-supporting cryogenic liquid mixture which reduces and possibly eliminates the above-mentioned problems. Consequently, the present invention provides a continuous or substantially continuous method of manufacturing a cryogenic liquid mixture of a composition which when vaporised is breathable and life supporting comprising the steps of claim 1. In one mode the mixing gas comprises a portion of the compressed, moisture and carbon dioxide reduced natural air which has been treated so as to remove at least a portion of the oxygen therefrom. In an alternative mode the mixing gas comprises a source of nitrogen other than the compressed, moisture and carbon dioxide reduced natural air. Advantageously, the method includes the step of monitoring the oxygen concentration of the product mixture and admixing the mixing gas in accordance with the monitored oxygen concentration. Preferably, the product mixture is chilled to a sub-cooled temperature. Advantageously, the chilling step comprises the step of passing the product gas through a heat exchanger and heat exchanging with a quantity of cryogenic chilling fluid. The method may include the step of controlling the flow of said product mixture in accordance with a final product demand. Advantageously, the method includes the step of monitoring the temperature of the product gas after the chilling step and controlling the flow of said cryogenic chilling fluid so as to maintain the final temperature of the product mixture within a given temperature range. Conveniently, the source of natural air comprises an air separation apparatus. Conveniently, said mixing gas comprises nitrogen from an air separation apparatus. Alternatively, said mixing gas comprises vaporised liquid nitrogen at pressure. Conveniently, the cryogenic chilling fluid comprises nitrogen from a cryogenic air separation apparatus. Advantageously, the mixing gas could comprise the vaporised cryogenic chilling fluid. The present invention will now be more particularly described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of an apparatus suitable for carrying out the present invention; Figure 2 is a schematic representation of an apparatus suitable for generating the desired gas flows for introduction into the apparatus of Figure 1; and Figure 3 illustrates the integration of the present invention with a commercial air separation unit (ASU). Referring to Figure 1, an apparatus 10 for producing a breathable, life-supporting cryogenic gas comprises a source of natural air A and nitrogen N, chilling means 12 for condensing a mixture of the gases and storage means 14 for the temporary storage of the condensate before use thereof. Air A to be supplied to the apparatus is first treated to form treated natural air by removing substantially all of any moisture and carbon dioxide from it and compressing it in a manner well known to those skilled in the art. If necessary, a particle filtration step may also be employed. A typical example of an apparatus suitable for carrying out this process step comprises a series arrangement shown schematically in Figure 1 and indicated by the reference 24 comprising, in sequence, a standard filter 16, compressor 18 and separator system 20 as used to pre-treat normal atmospheric air (by removal of water vapour and carbon dioxide therefrom) being fed to an air separation unit (ASU). Indeed, this pre-production process step may be carried out by such an arrangement when sized to provide air to both the present invention and a standard air separation unit. Details of the integration of the present apparatus 10 with a standard ASU 22 are disclosed later herein. Alternatively, the arrangement 24 may be provided for the sole purpose of supplying compressed, CO 2 and moisture free treated natural air A to the present apparatus 10. A still further alternative is illustrated in Figure 2 from which it will be appreciated that streams A and N may be generated by filtering natural air in filter 26, compressing it in compressor 28 and directing it to a first filtration device 30 for the removal of CO 2 and H 2 O before splitting the flows such that a portion is directed to a further separation device 31 for the removal of at least some of the oxygen therefrom and, hence, produce stream N. Such separation devices 30, 31 may comprise simple membrane or adsorption apparatus well known to those skilled in the art and therefore not described further herein. Further possibilities also present themselves for example, stream N maybe created by vaporising liquid nitrogen generated as a product or by-product of another related or unrelated process. One such example of this alternative is illustrated in Figure 3, which will be discussed in detail later herein. It will be appreciated that the present invention is aimed at providing a cryogenic liquid having an oxygen concentration not significantly higher and preferably less than that of natural air and is, therefore, not a fire hazard. Consequently, whilst stream N is described as being a nitrogen stream, it will be appreciated that, as its function is to dilute the oxygen concentration of natural air A, it might itself contain components other than nitrogen. Indeed, as long as the other components present no further hazard to the user, their presence may be tolerated. Consequently, mixing gas N may comprise substantially pure nitrogen or a nitrogen rich mixture of gases. Referring now once again to Figure 1, streams A and N are passed through stop valves V A and V G respectively before being mixed together in a ratio determined by the flow-rate of the nitrogen through a flow control valve V B thereby to produce a gaseous product mixture P G typically at about 286K and 3.5barg. Once mixed, mixture P G is passed along supply pipe 30 having, in flow series, a flow indicator/controller 32, an oxygen analysis controller 34, vent valve V E and a control valve V F . A high/low analysis switch 36 is operably connected to the oxygen analyser/controller 34 so as to initiate operation over valves V E and V F at the inlet thereof. In operation, the flow of product mixture P G through the apparatus 10 is controlled by operating valve V C so as to allow the desired flow rate to exit chiller 12. Flow indicator/controller 32 being operably linked to valve V C via line 38 acts to detect the flow rate of product mixture P G through pipe 30 and also initiates control over valves V A , V B and V G as to cause the product supply rate to match the output demand. Under normal operating conditions, the oxygen concentration of the product mixture is monitored by oxygen analysis controller 34 which initiates control over valve V B so as to alter the ratio of natural air A to mixing gas N in the supply. In the event that the oxygen percentage falls outside pre-determined boundaries, high/low oxygen analysis switch 36 operates to initiate control over valves V E and V F so as to prevent any defective product being supplied from the chiller by causing the defective product to be vented via valve V E . The flow control step may be temporarily overridden during venting of defective gaseous product P G . Under normal operation, product gas P G is passed into chiller 12 which takes the form of for example, a cryogenic heat exchanger and is heat exchanged with a cryogenic cooling fluid C F typically at about 79K and 2barg, thereby to liquefy the product gas. Once liquefied, liquid product gas P L is directed through outlet pipe 33 to liquid product buffer 14 via a further purity control system 40, which prevents any defective liquid product being supplied to the buffer 14. The purity control system 40 conveniently comprises a further oxygen analyser 42 and valves V P and V W which, in operation, collectively act to allow or inhibit flow of liquid to the buffer 14 as and when necessary. Out of specification liquid may be vented to atmosphere via valve V W or directed for disposal in any one of a number of ways. Whilst it will be appreciated that the chilling medium C F may comprise any one of a number of fluids having a sufficiently low temperature to facilitate liquefaction of the product gas, it has been found that waste or product nitrogen from a cryogenic air separation apparatus 22 is particularly suitable for this purpose. Liquid nitrogen has sufficient chilling capacity to provide a significant degree of product sub-cooling which is extremely desirable if the product is to be stored for any length of time. Also, its high chilling capacity facilitates the production of product at a rate somewhat higher than might be achievable with some alternative chilling systems. The liquid nitrogen L N may be supplied from a liquid store 52 or a liquefier and is typically passed to the chiller 12 at about 79K and 2barg. A flow control valve V D is provided in inlet line 54 and acts to allow or inhibit the flow of chilling nitrogen in response to signals from a controller 56 operably connected for receiving level signals from differential pressure monitor 64 positioned for monitoring the level of chilling liquid in the boiling passage of the chiller 12 which is arranged with generally vertical passages therethrough. The set-point of the level controller is determined by a temperature signal from temperature monitor 58 positioned for monitoring the temperature of product P L as it exits the heat exchanger 12. Control is initiated to increase the level and thereby flow rate of chilling fluid if the temperature of the product P G rises above a pre-determined level. Additional control devices may include a signal select device 62 which acts to decrease the flow if the chilling fluid exits the chiller at a temperature below a predetermined level as detected by temperature monitor 60 positioned to monitor the temperature of the chilling fluid as it exits the heat exchanger 12 and a low level switching system comprising the level transmitter 64 monitoring the fluid level on the chilling fluid side of heat exchanger 12 being operably connected to a low level switching device 66 having control over valves V A to V D inclusive to close the apparatus down should the chilling fluid level in the boiling passage of the chiller drop below a predetermined valve. A further safety feature in the form of low temperature detector 68 may be provided being operably connected to valve V D for the control thereof should the temperature of the spent chilling fluid C F fall below a pre-determined value, thereby indicating a system failure. A manually operated switch 70 may also be provided to override either or both of these systems and initiate control over the flow of chilling fluid C F . Waste chilling fluid C F is passed for disposal or further use via line 72 and flow indicator 74. If liquid nitrogen is being used as the chilling fluid, the waste gas which is typically at 276K and atmospheric pressure may be compressed in compressor 76 and used as the source of nitrogen rich gas N. The apparatus shown in Figure 1 also includes a pressure relief valve 77, several non-return valves 78, and temperature indicators 79 and 80 which respectively display the temperature of the gas mixture and the temperature of the liquid nitrogen upstream of their respective entries into the chiller 12. Referring now to Figure 3, from which it will be appreciated that the present invention lends itself to integration with a commercial ASU suitably sized so as to provide surplus treated air A and nitrogen product or by-product which forms the chilling fluid C F of the present invention and possibly the source of mixing gas N. Integration is fairly simple and involves the selection of air filter 126, compressor 128 and CO 2 /H 2 O remover 130 stages having sufficient capacity to provide pre-treated air to both the ASU 22 and air inlet A of the present invention. Preferably, the ASU is operated for the production of oxygen product 150 and the nitrogen 52 is sent for use as the chilling fluid C F of the present invention. It will however be appreciated that one might still integrate the present invention with an ASU designed for the production of nitrogen product. However, it will be appreciated that some loss of product will result. Other features of the apparatus shown in Figure 3 are the same as that shown in Figure 1 and are indicated by the same reference numerals as used therein.

Claims: A continuous or substantially continuous method of manufacturing a cryogenic liquid mixture of a composition which when vaporised is breathable and life supporting, comprises the steps of: compressing a quantity of natural air; removing substantially all of any moisture from the natural air; removing substantially all of any carbon dioxide from the natural air; providing a quantity of mixing gas comprising nitrogen present in a proportion greater than that present in natural air, admixing a portion of the mixing gas with at least a portion of the compressed moisture and carbon dioxide reduced natural air, thereby to produce a product mixture having an oxygen concentration of between 14% and 20%, and chilling the product mixture to a cryogenic temperature, thereby to liquefy the product mixture. A method as claimed in Claim 1, in which the mixing gas comprises a portion of the compressed, moisture and carbon dioxide reduced natural air which has been treated so as to remove at least a portion of the oxygen therefrom. A method as claimed in Claim 1 or Claim 2, including the step of monitoring the oxygen concentration of the product mixture and admixing the mixing gas in accordance with the monitored oxygen concentration. A method as claimed in any one of the preceding Claims, in which the product mixture is chilled to a sub-cooled temperature. A method as claimed in any one of Claims 1 to 4, including the step of controlling the flow of said product mixture in accordance with a final product demand. A method as claimed in any one of Claims 1 to 5, in which said chilling step comprises the step of passing the product gas through a heat exchanger in heat exchange relationship with a quantity of cryogenic chilling fluid. A method as claimed in Claim 6, including the step of monitoring the temperature of the product gas after the chilling step and controlling the flow of said cryogenic chilling fluid so as to maintain the final temperature of the product mixture within a given temperature range. A method as claimed in Claim 6 or Claim 7, in which the mixing gas is taken from the cryogenic chilling fluid downstream of passage of the cryogenic chilling fluid through the heat exchanger. A method as claimed in Claim 8, in which the cryogenic chilling fluid is liquid nitrogen.

Inventors: PAIGE CATHARINE SARAH, PAIGE, CATHARINE SARAH
Applicants: BOC GROUP PLC, THE BOC GROUP PLC
Application year: 1996
Main classification: F25J1
IPC codes: F25J3, F25J1, B01D53, B01D8