Abstract:
In a pressure swing adsorption process for the production of nitrogen enriched product gas from air, when the process is halted during a pressure equalization step, that is, with valves open to permit the flow of gas from one vessel to a second vessel, then the gas remaining in the vessels is exhausted to atmosphere by opening a further valve. This results in rapid return to the required purity of nitrogen enriched product gas on start-up of the process. Preferably, subsequent to opening the further valve, another valve is opened, thereby permitting nitrogen-rich product gas from a reservoir to flow through the vessels, thereby purging said vessels with nitrogen enriched product gas.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to processes for the separation of a gaseous mixture and in particular to a process for the production of nitrogen rich gas from air. 
     Processes are known for the separation of nitrogen from air employing an adsorbent material which has the ability to effect separation as between the two major components of air by virtue of its more rapid adsorption of oxygen that nitrogen. The adsorbent is typically molecular sieve carbon, for example, as described in Japanese patent publication (Kokoku) No. 18675/1977. 
     In operation, a bed of the adsorbent is put through a cycle of operations which includes an adsorbent step during which time air is passed through the bed and most of the oxygen and a proportion of the nitrogen and substantially all the carbon dioxide and water vapour in the feed are adsorbed, and a nitrogen-rich product gas is supplied from the outlet of the bed; and a desorption step during which time the outlet of the bed is closed, the bed is vented to atmospheric pressure through its inlet and/or evacuated through its inlet so that the adsorbed gases are substantially removed from the bed, thereby preparing it for the next adsorption step. 
     In practice, two adsorbent beds are employed and operate on similar cycles which are sequenced to be out of phase with one another by 180° so that when one bed is on during its adsorption step, the other bed is on its desorption step and vice-versa. 
     However, although the known nitrogen producing systems are satisfactory for the the supply of product gas on a continuous basis, should they have to be closed down, then after a fresh start-up a considerable time is required before the product gas reaches an acceptable purity. Normally, the process is halted (closed down) during a pressure equalisation step to make a fresh start-up easier. However, if the apparatus is allowed to stand for a long time, it has been found that oxygen which was originally adsorbed by the molecular sieve carbon is desorbed and on start-up the oxygen dilutes the nitrogen product gas. As a result, the time after start-up before an acceptable purity of product gas can be supplied is prolonged. This can necessitate the discharge of enriched nitrogen of insufficient purity from the apparatus prior to an acceptable level of purity being obtained. Thus, when the known nitrogen producing systems are used on an intermittent basis, their operating efficiency is low. 
     SUMMARY OF THE INVENTION 
     It is an aim of the present invention to provide a process for the separation of a gas mixture to provide a product gas which when closed down and then restarted very quickly produces product gas of an acceptable purity. 
     In particular, it is an aim of the present invention to provide a process for the supply of nitrogen-rich gas from air which when closed down and then restarted very quickly produces nitrogen-rich product gas of an acceptable purity. 
     According to the present invention, a process for the separation of a gas mixture comprises the steps of: 
     (a) repeatedly performing a cycle of operations in employing a first vessel containing adsorbent which absorbs one component of a gas mixture more readily than another or the other component of the gas mixture, the cycle of operations comprising: 
     (i) passing the gaseous mixture under pressure through the vessel whereby said one component is adsorbed and a gas stream relatively lean in the adsorbed component flows out of the bed as a product gas; 
     (ii) regenerating the adsorbent by desorbing gas therefrom and causing a gas stream relatively rich in the desorbed gas to flow out of the vessel; 
     (b) repeatedly performing such a cycle of operations employing a second vessel containing said adsorbent, the cycles being phase relative to one another such that at no time is step (i) or step (ii) in one cycle performed simultaneously with the corresponding step in the other cycle; 
     (c) passing the product gas stream into a reservoir; and 
     (d) at intervals between successive adsorption steps the two beds are placed in flow communication with each other; 
     wherein when the process is halted during the intervals between successive adsorption steps when the two beds are placed in flow communication, then any gas remaining in the vessels is exhausted from the said vessels countercurrently to the flow of product gas thereby to depressurise the vessels. 
     Preferably, subsequent to depressurisation of the vessels, the vessels are purged with product gas. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     An embodiment of the invention will now be described by way of example, reference being made to the Figures of the accompanying diagrammatic drawings in which: 
     FIG. 1 is a graph showing isothermic equilibrium adsorption of nitrogen and oxygen by molecular sieve carbon; 
     FIG. 2 is a graph showing the adsorption rate of oxygen and nitrogen by molecular sieve carbon; and 
     FIG. 3 is a diagrammatic flow sheet of an apparatus for the separation of a gas mixture. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Referring first to FIG. 3, a nitrogen producing apparatus includes a raw material gas (air) feed pipe extending to inlet valves, 1, 2. Valve 1 controls the flow of air to a first vessel 16 containing beds of molecular sieve carbon whilst valve 2 controls the flow of air to a second vessel 16&#39; also containing beds of molecular sieve carbon. The molecular sieve carbon has an isothermal equilibrium adsorption to oxygen and nitrogen as shown in FIG. 1. 
     Valve 3 controls the flow of nitrogen-rich product gas leaving vessel 16 whilst valve 4 controls the flow of nitrogen-rich product gas leaving vessel 16&#39;. The outlets from valves 3, 4 are joined to a product gas outlet pipe 20 in which are positioned valves 9 and 10. Also connected to the pipe 20 is an analyser 18 for measuring any oxygen present in the product gas. The pipe 20 terminates at a reservoir 11. The flow of product gas from reservoir 11 to the outlet fom the apparatus is controlled by an adjustable valve 12. A pipe 22 extends from reservoir 11 to the outlet of vessel 16&#39; between the vessel 16&#39; and valve 4. The pipe 22 includes a valve 15. 
     A first pipe 24 extends between the inlets to vessel 16, 16&#39; and includes two valves 7, 8. A pipe 9 communicates with pipe 24 intermediate said valve 7, 8 and extends to atmosphere via a silencer 17. 
     A second pipe 26 extends between the inlets to vessel 16, 16&#39; and include a valve 5. An exhaust pipe extends from said second pipe 26 and includes a valve 14. A pipe 28 extends between the outlets of the vessels 16, 16&#39; and includes a valve 6. 
     
                       TABLE 1______________________________________(underline shows that the valve is opened)Valve  Time →______________________________________1      Absorption2                           Absorption45                   P.E.*              P.E.*6                   P.E.*              P.E.*7                           Depressurisation8      Depressurisation______________________________________ Note: *is pressure equalisation 
    
     As shown with reference to table 1, a two step cycle is repeatedly performed by each vessel 16, 16&#39;. The two steps are called herein &#34;adsorption&#34; and &#34;desorption&#34;. These steps are typically performed at the same temperature as one another. The adsorption and desorption steps are all of equal time and phased such that whenever one of the beds is performing an adsorption step the other is being regenerated or desorbed. 
     Moreover, there is an interval between one pair of adsorption and desorption steps and the next pair of adsorption and desorption steps. Throughout this interval the two beds are placed in communication with one another by opening the valves 5 and 6 to perform what is referred to herein as a pressure equalisation step. 
     Assuming that vessel 16 is in its adsorption step, then raw material gas such as air passes through air feed pipe under pressure and hence through valve 1 which is open and into the vessel 16, where oxygen is adsorbed preferentially by the beds of molecular sieve carbon within vessel 16. Normally, the pressure in the vessel 16 during the adsorption step is in the range of 1 to 10 kg/cm 2  g and preferably within the range 2 to 9 kg/cm 2  g. Nitrogen rich product gas then leaves vessel 16 and passes through valve 3 which is open and through valves 9 and 10 and into reservoir 11. 
     As shown in table 2, this adsorption step usually takes approximately 56 seconds. 
     
                       TABLE 2______________________________________Absorption zoneCycle time(second) Bed 16&#39;        Bed 16______________________________________0 to 4   pressurisation by                   depressurisation by    pressure equalisation                   pressure equalisation 4 to 60 pressurisation by raw                   depressurisation to one    material gas and                   atmospheric pressure    adsorption60 to 64 depressurisation by                   pressurisation by pressure    pressure equalisation                   pressure equalisation 64 to 120    depressurisation to one                   pressurisation by raw    atmospheric pressure                   material gas and                   adsorption______________________________________ 
    
     After a predetermined interval, valves 1 and 3 are closed and valves 5 and 6 are opened so that the two vessels 16, 16&#39; are in communication at their inlets and outlets and gas will flow from vessel 16 to vessel 16&#39;. This equalisation step usually takes from 1.5 to 5 seconds. 
     Whilst the vessel 16 was in the adsorption mode, the vessel 16&#39; was in its desorption mode. Valve 8 was open, allowing oxygen rich waste gas to pass countercurrent through vessel 16&#39; and along pipe 19 and through silencer 17 to atmosphere. The pressure in vessel 16 can be reduced to atmospheric pressure by passing the waste gas directly out to atmosphere as shown. Alternatively, a vacuum pump can be incorporated to reduce the pressure in vessel 16 to sub-atmospheric pressure, usually between 60 to 300 torr. 
     As explained previously, the three steps of adsorption, pressure equalisation and desorption are repeated continuously for each bed at 180° out of phase whilst the apparatus is in operation. Nitrogen rich product gas can be withdrawn from reservoir 11 by opening valve 12 at any time during the cycle of operations. 
     When the apparatus is closed down during a pressure equalisation step and is allowed to stand for a long time, the oxygen and nitrogen remaining in the adsorption vessel come to equilibrium with the molecular sieve carbon according to the relationship shown in FIG. 1. As a result, the oxygen which was initially adsorbed becomes desorbed and the composition percentage oxygen in the gas phase exceeds that found throughout the vessel at the end of a 1.5 to 5 second equalisation. 
     As explained with reference to the prior art, when it is desired to shut down the apparatus, this is done during a pressure equalisation step and valves 1, 2, 3, 4, 7 and 8 are closed but valves 5 and 6 remain open. Thereafter, according to the present invention, valve 14 is opened and gas remaining in vessels 16, 16&#39; is exhausted to atmosphere. Subsequently, valve 15 is opened permitting some of the nitrogen-rich product gas from the reservoir 11 to pass through vessels 16, 16&#39; to purge the vessels with nitrogen-rich product gas. Finally, valves 14, 15 are closed. 
     The valve 9 is used for adjusting adsorption pressure in the vessels 16, 16&#39; whilst valve 10 is a two-way valve which leads product gas into the reservoir 11 when the percentage oxygen specification is correct but diverts it to waste when the percentage oxygen specification is incorrect. 
     Table 3 shows the preferable time sequence of the apparatus in which a vacuum step is carried out. 
     
                       TABLE 3______________________________________Absorption zoneCycle time   Bed 16&#39;        Bed 16______________________________________0 to 4  pressurisation by                  depressurisation by   pressure equalisation                  pressure equalisation4 to 9  pressurisation by raw                  depressurisation to one   material gas and                  atmospheric pressure   adsorption 9 to 60   pressurisation by raw                  evacuation   material gas and   adsorption60 to 64   depressurisation by                  pressurization by pressure   pressure equalisation                  equalisation64 to 69   depressurisation to one                  pressurisation by raw   atmospheric pressure                  material gas and adsorption 69 to 120   evacuation     pressurisation by raw                  material gas and adsorption______________________________________ 
    
     EXAMPLES 1 AND 2 AND CONTROL TESTS 1 AND 2 
     Air was used as a raw material gas. The adsorption conditions are as follows: 
     Absorption pressure--7 kg/cm 2  g 
     Pressure of the zone at the time when the pressure equalisation had been completed--3.5 kg/cm 2  g 
     Pressure of the zone at the time the depressurising step had been completed--0 kg/cm 2  g 
     Control tests 1 and 2 relate to the prior art process. Examples 1 and 2 relate to the present process. 
     The results are shown in table 4. 
     
                       TABLE 4______________________________________                       Time necessary                       for attainingExample &amp;                   nitrogen gas ofcontrol                     99.5% by volumetest    Operation method    (minutes)______________________________________Control When the apparatus was re-                       30test 1  started after 2 hour haltControl When the apparatus was re-                       50test 2  started after 29 hour haltExample 1   When the gas remaining in                       7   the zone was discharged after   halt of the apparatus and the   apparatus was allowed to   stand for 21 hoursExample 2   When the gas remaining in                       0   the zone was discharged and   part of the nitrogen enriched   product was passed through the   zone after halt of the apparatus   and the apparatus was allowed   to stand for 22 hours______________________________________ 
    
     It was apparent from Control tests 1 and 2 that the time necessary for attaining the permissible oxygen level in the nitrogen enriched gas became prolonged as the halt time became longer. In Examples 1 and 2 according to the present invention, the time necessary for attaining the permissible oxygen level was significantly shortened.