METHOD FOR PURIFYING A CONTINUOUS FLOW OF HELIUM AND/OR NEON GAS

Abstract: Continuous production of ultra-pure helium and neon gases is achieved by utilising the fact that these gases have the highest ionisation and excitation potentials of all chemical substances. The helium/neon gas stream to be purified is excited by, for example, a corona discharge not only to directly ionise some of the impurities but also to produce rare-gas ions and metastable atoms. These ions and metastables subsequently collide with and ionise impurities. A polarising field is used to draw the impurity ions out of the main gas flow and into a subsidiary flow bled from the main flow. In a preferred embodiment, the polarising field is established between a central rod-shaped anode(l3) and a surrounding tubular cathode(l0) through which the main gas flow passes. The cathode is made of a porous material so that positive impurity ions can be removed by the radial outwards flow of gas therethrough.

Description: The present invention relates to a method of purifying a continuous flow of helium and/or neon gas; in particular, but not exclusively, the invention is concerned with the continuous production of ultra pure helium or neon (or mixtures thereof) where 'ultra pure' means that contamination with any impurity gas or gases is less than one part in ten million by volume. Helium and neon gases of this purity have many uses including as sample carrier gases in modern analytical techniques, filling gases for discharge tubes and lasers, and purge gases in the semiconductor industry. Many processes are known for purifying continuous streams of gases. US Patent No 4,313,739 (Douglas-Hamilton) describes one process which is based on preferential ionization of contaminant molecules, and involves the steps of passing a stream of the gas to be purified along a predetermined flow path; ionising impurities in the gas; and removing the positive impurity ions from the main gas flow. In this process, the impurities, are ionised by using a corona discharge to generate ions and electrons in the gas which then exchange charge with impurity molecules. Of course, this method only works where the impurity molecules have a lower ionisation potential than the molecules of the gas to be purified. Furthermore, the efficiency of the overall process is poor, not least because the ions and electrons required to ionise the impurity are continuously removed from the gas flow by the polarising field. It is an object of the present invention to provide an improved method for the production of ultra pure helium and neon gases on a continuous basis and in a manner relatively easy to implement. According to one aspect of the present invention, there is provided a method of purifying a continuous flow of helium or neon gas or a mixture thereof, said method involving the operations of passing a stream of the gas to be purified along a predetermined flow path, ionising impurities in the gas, and removing the positive impurity ions from the main gas flow by setting up a polarising field across said stream to cause migration of said impurity ions out of the main gas flow, characterised in that ionisation of said impurities is effected by exciting the gas stream with an electrical discharge such as to produce rare-gas metastable atoms that serve to ionise said impurities by losing energy thereto, said metastable atoms providing the predominant impurity-ionisation process. The effectiveness of this purification method rests in the high excitation potentials of noble gas metastables. In particular, the metastable excitation potential of helium or neon is greater than the ionisation potentials of any impurities. In addition, these metastables, being neutral, are not removed from the gas stream by the polarising field. Furthermore, the noble gas metastables readily lose their stored energy to impurity molecules they encounter by collision leaving the noble-gas as uncharged, unexcited atoms. The ionising effect of noble gas 'metastable' atoms is, of course, itself a known effect (see UK Patent Specification No 882,997) though not previously used in the purufication of noble gases. The excitation of the gas stream will generally also result in the direct production of impurity ions and also in the production of noble gas ions, which, in turn, cause ionisation of impurities. However, these latter processes are considerably less efficient at ionising impurities than by generation of metastables. A helium/neon gas purifier embodying the invention will now be described by way of non-limiting example with reference to the accompanying diagrammatic drawing, the sole Figure of which is a longitudinal cross-section through the purifier. Although the purifier to be described is suitable for purifying helium or neon gases individually or in a mixture, for ease of explanation the following description will be given in terms of the purification of a helium gas flow only. As shown in the drawing, the purifier comprises a tubular member 10 defining a main gas flow path between an inlet 11 for impure helium and an outlet 12 for purified helium. The tubular member 10 is formed of a porous conducting material such as sintered metal or sintered metallized glass or quartz. The tubular member 10 constitutes a cathode of the purifier, the complementary anode 13 being formed by a metal rod positioned coaxially in the tubular cathode 10 insulating end seals 14. A polarising field is set up between the anode l3 and cathode l0 by a DC polarising source l5. This source l5 applies a potential between the anode and cathode which is substantially less than would cause an arc or discharge in helium at the pressure existing within the purifier during operation (generally slightly above atmospheric pressure); the applied potential is, however, sufficient to cause a rapid migration of any positive ions present to the cathode l0. In order to ionise impurities in the gas flowing through the cathode l0, a pulse source l6 is provided for supplying high frequency (l00 kHz to 50MHz), high potential, short duration (less than one microsecond) pulses between the anode and cathode whereby to set up an ionising corona discharge between the electrodes. Such an arrangement is disclosed in UK Patent Specification No. l,597,623; and will not, therefore, be further described herein except to note that since to initiate corona discharge at least one ion or electron must be present, it is prudent to provide some means for generating the required ion or electron (although background radiation could be relied upon). Consequently, the polarising source could be made adjustable to cause an initial discharge before being set back to its normal operating potential. Surrounding the tubular cathode l0 is a cylindrical jacket l7 for collecting gas which, in operation, passes radially outwards through the porous cathode l0. This jacket l7 has an exhaust port l8 adjacent the inlet end of the main gas flow path defined by the cathode l0. A diffuser l9 is provided in the main gas flow path adjacent to inlet ll to establish streamline flow down the inside of the cathode l0. In operation, impure helium typically, one part per million impurity) is supplied to the inlet ll at a slight over-pressure relative to the surrounding atmosphere to establish a streamlined main gas flow coaxially down the inside of the cathode l0 in the direction of arrow A to the outlet l2. Due to the porous nature of the cathode l0, a small subsidiary gas flow is set up that passes radially outwards through the cathode and then in the direction of arrow B to the exhaust port l8. The corona discharge established between the anode l3 and cathode l0 by the pulse source l6 causes ionisation of any impurity atoms or molecules within the main gas stream either directly or by first generating helium metastable atoms and helium ions which subsequently collide with and ionise any impurities. Ultra-violet raditation emitted from the transitions of the helium ions and metastables will also produce ionisation of impurities by photo-ionisation. The predominant impurity-ionising process will generally be that due to the collision of helium metastables with impurity atoms and molecules. The polarising field set up between the anode l3 and cathode ll by the polarising source l5 causes positive impurity ions to migrate to the cathode where they pass out of the main gas stream through the porous cathode ll as part of the subsidiary gas flow. The purified helium typically with an impurity level of less than one part in ten million exits the purifier through the outlet l2 while the impurities are exhausted through exhaust port l8. The actual dimensions of the purifier will depend upon the desired flow rate of helium and the degree of purity sought. However, typical dimensions would be a tube l0 mm in internal diameter with a length of 0.3 metres. An ion current of 4.3 microamperes would be needed to remove all of a one part per million impurity in helium flowing at one millilitere per second at N.T.P. This assumes that all of the ion current is carried by impurity ions and that the impurities are completely removed from the main helium flow after the impurity ions encounter the cathode. In practice, ionising efficienes of l to l0% are more usual. The purifier can be thought of as analogous to a distillation column with gaseous diffision processes tending to hinder the smooth operation of the separation. For a cathode l0 mm in internal diameter and an anode 2 mm diameter and with the flow rate and impurity level quoted in the preceding paragraph, each 5 to l0 mm section of the purifier corresponds to a plate of a distillation column and 67% of impurity will be removed in each of the sections. Thus, five sections should theoretically remove about 99% of the impurity. A typical purifier is in fact 0.3m or more in length to extend the range of purification and the maximum flow rate. The structural and electrical characteristics of the purifier are preferably such as to avoid the development of ion densities in the purifier in the plasma region. In plasma, ions and electrons are closely associated in a gaseous cloud and the ions cannot be so readily moved to the electrodes by the applied polarising field. For a l0 mm section of purifier, with a cathode having an internal diameter of l0 mm, the polarising field current should not exceed one microampere if plasma conditions are to be avoided. Furthermore, whilst the use of a porous cathode is a particularly convenient and efficient way of removing the impurity atoms and molecules, it would be possible to use a non-porous cathode and then to separate off from the main gas flow, the gas stream that pases directly over the cathode and within which the neutralised impurities will be concentrated.

Claims: A method of purifying a continuous flow of helium or neon gas or a mixture thereof, said method involving the operations of passing a stream of the gas to be purified along a predetermined flow path, ionising impurities in the gas, and removing the positive impurity ions from the main gas flow by setting up a polarising field across said stream to cause migration of said impurity ions out of the main gas flow, characterised in that ionisation of said impurities is effected by exciting the gas stream with an electrical discharge such as to produce rare-gas metastable atoms that serve to ionise said impurities by losing energy thereto, said metastable atoms providing the predominant impurity-ionisation process.

Inventors: LOVELOCK  JAMES EPHRAIM, LOVELOCK JAMES EPHRAIM
Applicants: HEWLETT-PACKARD COMPANY
Application year: 1987
Main classification: C01B23
IPC codes: C01B23, B01D53