Title of Invention	"AN IMPROVED SINGLE COLUMN PRESSURE SWING ADSORPTION (PSA) PROCESS FOR THE SEPARATION OF HELIUM"
Abstract	The present invention relates to an improved single column pressure swing adsorption (PSA) process for the separation of helium. This invention particularly relates to the use of single column with an improved PSA cycle and a gas storage (surge) vessel to store the product, which is used subsequently for the regeneration / purging of the PSA column.

Full Text

The present invention relates to an improved single column pressure swing adsorption (PSA) process for the separation of helium. This invention particularly relates to the use of single column with an improved PSA cycle and a gas storage (surge) vessel to store the product, which is used subsequently for the regeneration / purging of the PSA column. The demand for high purity helium is increasing for several applications e.g. refrigeration plants, as shielding gas during welding and in the chemical industry, as inert gas in space technology, as a respiration gas during diving operations, as a carrier gas in chromatography, for detection of leakage and for other applications as well. For these applications, helium is required with a high degree of purity. The principle source of helium is natural gas. Helium can also be found in geothermal springs and monazite sands. A conventional way to recover helium from natural gas is to use a cryogenic process. In a typical cryogenic process the water, and carbon dioxide present are first removed by scrubbing followed by fractionation of the natural gas into a hydrocarbon stream and a crude helium stream by low temperature heat exchanger. The gas stream is further cooled to about 116 °K to liquefy the remaining hydrocarbons. The resulting crude helium is then reduced in temperature to about 77 °K to remove any trace hydrocarbons present and to produce a helium stream containing small amounts of nitrogen, argon, neon and hydrogen. The helium stream is pressurized to 17.3 MPa at 77 K and nitrogen and argon are separated. Final purification of this helium to grade A helium is generally achieved by using Pressure Swing Adsorption (PSA) processes. In pressure swing adsorption, a component of a gas stream is selectively adsorbed onto an adsorbent at high pressure. Other components that are less adsorbed get concentrated in the gas stream, pass through the adsorbent bed and are collected as product. After an adsorbent bed has become loaded with the adsorbed component, the adsorbent bed can undergo regeneration by reducing the pressure of the column and purging the column with the less adsorbed component. The prior art , US patents 5089048 and 4444572 ,on helium purification have mentioned that helium with a purity level over 99.9% by volume can be obtained using PSA, wherein feed mixture is essentially consisting of helium, nitrogen, argon, and oxygen. The process is only suitable for gas mixtures containing 50 to 95% volume helium in the feed mixture. US patent 5080694 discloses an efficient process wherein feed gas containing 5.4% volume helium was treated by pressure swing adsorption process to yield 99.9 % volume helium. In this process four adsorber beds with carbon molecular sieve as an adsorbent was used for removing nitrogen and methane. In another set of four beds, activated carbon was also used as adsorbent to remove higher molecular weight hydrocarbons and other contaminants. In the earlier PSA processes, there were four steps involved , namely : pressurisation, high pressure production, blow down and low pressure purge. In adsorptive gas separation applications it is desirable that one or more of the separated components is produced on a substantially continuous basis. This can be accomplished by using a multiple bed approach. This is generally the preferred procedure when the product gas is required on a large volume basis of more than 20,000 SCFH. Thus PSA processes with multiple beds and more than ten steps have been commercialised for separation of gas mixtures. This however, makes the equipment costs very high and also increases the complexity of the process . Since a large portion of the capital costs associated with multi- bed PSA , relates to the cost of processing vessels, associated piping, solenoid valves and the adsorbent for use in the adsorber, it will be appreciated that the capital costs can be significantly reduced by cutting down the number of adsorbent vessels and solenoid valves included in the system. Such a reduction can be achieved by using a single bed PSA system. In earlier single column PSA patents such as US 6183538 , a gas storage vessel was used to allow constant product flow. The gas was collected in a storage vessel during the adsorption step. Subsequently part of the gas was withdrawn as product and part used to purge the column. The main object of the present invention is to provide an improved single column pressure swing adsorption (PSA) cycle for the separation of helium from mixtures with oxygen and nitrogen. Another object of the present invention is to split the adsorption step into two parts. In the first half of the adsorption step, the product is withdrawn as in conventional PSA. In the second part of the adsorption step, the product is stored in the surge/storage vessel for subsequent use as purge gas in the regeneration step. Still another object of the present invention is purging of the column by stored gas in the surge vessel collected in the second half of the adsorption step which improved the column performance compared to the case of purging with gas stored in first part of the adsorption step. Yet another object of the present invention is collection of product in storage vessel in second part of the adsorption step which allows higher pressures to be attained in the storage vessel. The purge is thus delivered at higher pressure into the column during regeneration and is able to strip off the strongly adsorbed component from the bed more efficiently. This improves the column performance both with respect to product purity and recovery. In the drawing accompanying this specification Figure - 1 represents the schematic line diagram of a single column PSA unit. Accordingly, the present invention provides, an improved single column pressure swing adsorption (PSA) process for the separation of helium, which comprises: a) pressurizing the adsorption column containing adsorbent by the feed gas mixture for a specified time, and withdrawing the product from the other end of the column for a specified time, (Adsorption I) b) collecting the product in a surge vessel (Adsorption II), c) terminating the feed flow into the column and depressurizing the column from the feed end, d) evacuating the column from the feed end by drawing a vacuum, e) allowing the gas collected in the surge vessel to purge the column from the product end while continuing the evacuation for a specified time, f) terminating the evacuation by closing the feed end while continuing to pass gas from the surge vessel into the column for a specified time so that column pressure rises and column becomes ready for the next cycle. In an embodiment of the present invention, the adsorbent may be a microporous solid of the type of zeolite , activated carbons , molecular sieves . In another embodiment of the present invention the gas feed mixture may contain helium in the range of 1 to 50 wt%, the balance being nitrogen, or nitrogen and oxygen. In yet another embodiment of the present invention the feed may be a mixture of helium and air. In still another embodiment of the present invention the flow rate of feed gas mixture during pressurization and adsorption step may be varied up to 5 L/min. In still another embodiment of the present invention the pressure of the column during pressurization and adsorption may be varied between 1.0 kg/cm2 to 10.0 kg/cm2. In still another embodiment of the present invention the experimental temperature may vary from 15°C to 50°C. In still another embodiment of the present invention the feed mixture may be fed to the column either from the top or the bottom of the column. In still another embodiment of the present invention adsorption step can be a single step or split into two steps. In still another embodiment of the present invention the product for purging/regeneration of the column may be stored in the surge tank either in the first part or the second part of the adsorption step. In still another embodiment of the present invention purge gas for the regeneration of the column may be pure helium or the product stored in the surge vessel. In still another embodiment of the present invention purge gas for the regeneration of the column may be stored in the surge tank either in the first part or the second part of the adsorption step. In still another embodiment of the present invention the regeneration of the adsorbent may be carried out by evacuation alone or evacuation with purge gas or solely with purge gas without evacuation. In still another embodiment of the present invention blowdown gas may be vented or collected in a separate surge tank. In still another embodiment of the present invention repressurisation of the column may be done with either with purge gas stored in surge vessel followed by final repressurisation by feed gas introduction or solely by feed gas or by using gas from blowdown tank. The detailed steps of the process are: The separation of a feed mixture containing He, O2 and N2 may be carried out by feeding the gas mixture at a rate of 1 to 4 LPM for a specified time to an adsorption column with its outlet end closed so that column pressure rises to 3 to 5 kg/cm2, this specified period being termed as the "Pressurisation step"' opening the column outlet while continuing the feeding of the gas mixture into the column outlet while continuing the feeding of the gas mixture into the column for a specified period so that feed mixture flows through the column, the specified period now being termed as "Adsorption Step", maintaining the pressure at constant level in the column by a Back Pressure Regulator (BPR), allowing withdrawal of the product for the first half of the Adsorption step, collecting the product during the second half of the Adsorption Step in a surge vessel, terminating the feed flow into the column and releasing the pressure of the column from the feed end while keeping the product end closed for a specified time called "Blow down Step'" evacuating the column from the feed end by pulling a vacuum for specified time called "Evacuation Step", allowing the gas collected in the surge vessel to flow through the column from the product end while continuing the evacuation for a specified time called the "Evacuation and Purge Step"' terminating the evacuation by closing the feed end while continuing to pass gas from the surge vessel into the column so that column pressure rises for a specific time called "Purge Pressurisation Step" The said process for purifying/removing helium from gas mixtures of helium, nitrogen, and oxygen can be carried out according to the steps as given below 1- Pressurising the column containing adsorbent by allowing gaseous feed mixture to flow into the top of the column while keeping the column outlet at the bottom closed. The pressure of the column rises and the step, which is called the Pressurisation step is terminated after a specified time. 2- Feed mixture continues to flow through column at higher pressure and the column outlet at bottom is opened so that product is withdrawn at a constant flow rate for a fixed time. 3- Continuing to pass the feed mixture from the top of the column but collecting the product withdrawn from the bottom in a surge tank for a specified time. 4- Feed flow is stopped and column is depressurised to ambient pressure counter current to feed flow, the gas mixture of feed composition is removed from the void spaces of the adsorbent bed and this step is called "Blow Down Step". 5- Column is further depressurised counter currently by evacuating the adsorbent bed from the top. During this step strongly adsorbed component starts desorbing from the adsorbent. This step is called "Evacuation Step". 6- Purge gas from the storage vessel is now introduced into the column counter-currently from the bottom to strip off the strongly adsorbed components form the adsorbent bed. Evacuation is continued during this step. 7- Evacuation is terminated and product flow from surge tank is continued. The column pressure rises. In adsorptive gas separation applications it is desirable that one or more of the separated components is produced on a substantially continuous basis. This can be accomplished by using a multiple bed approach. This is generally the preferred procedure when the product gas is required on a large volume basis of more than 20,000 SCFH. Thus PSA processes with multiple beds and more than ten steps have been commercialised for separation of gas mixtures. This however makes the equipment costs very high and also increases the complexity of the process . Since a large portion of the capital costs associated with multi bed PSA relates to the cost of processing vessels, associated piping, solenoid valves and the adsorbent for use in the adsorber, it will be appreciated that the capital costs can be significantly reduced by cutting down the number of adsorbent vessels and solenoid valves included in the system. Such a reduction can be achieved by using a single bed PSA system. The purification/ separation of helium from gas mixtures containing nitrogen and oxygen may be carried out in a metallic insulated adsorption column "A" of 34 mm internal diameter and 60 cm height depicted in Figure-1 of the drawing accompanying the specification. The column is loaded with suitable adsorbent. The column has SS screens at the top and bottom to support the adsorbent. To avoid entrainment of the adsorbent dust which may be generated during high and low pressure cycling, two NUPRO microfilters (0.24 microns) F 23 and F 24 are installed at the top and the bottom of the column. Solenoid valves SV 0 to SV 5 are fixed at various positions as shown in Figure -1 to control the timing of different steps and flow direction of various streams. Cycle sequencing of the PSA process is performed by a tinner operated by data acquisition and control software, which controls the solenoid valve operation. During "Pressurisation Step" the gaseous feed mixture is fed to the top of the adsorbent column through Line 11 and 12. Except for solenoid valve SV 4 all the other valves are closed. Feed mixture of desired composition is obtained by precisely controlling the flow rates of each gas (nitrogen and helium) by the mass flow controllers MFC 6 and MFC 7 placed on individual gas lines as shown in Figure-1. During Pressurisation Step , pressure of the column rises. Pressure at the top of the column is monitored by digital pressure transducer PT 9 which is interfaced with a computer to continuously acquire pressure data . This step is followed by Adsorption step where gas feeding to the column via Line 11 and 12 is continued. Product is withdrawn via Line 13 and 14. Solenoid valves SV 4 and SV 5 are open during this step. Pressure in the column is maintained by back pressure regulator BPR 21. Product is analysed on line with an on- line gas chromatograph fitted with automatic gas sampling valve. Product withdrawal is stopped after a specified time and product is now routed for storage in the surge vessel 25 via Line 15 and Line 19 by opening solenoid valve SV 0 and closing SV 5. Sudden drop in the column pressure is avoided during this step by back pressure regulator BPR 22. After the adsorption step, the column pressure is depressurised to ambient pressure counter currently by opening Solenoid valve SV 1 and closing all the other solenoid valves. In this step , the gas mixture of feed composition is removed from the void spaces of the adsorbent bed via Lines 12, 16 and 17. This step is called "Counter Current Blow down". This step is followed by the Evacuation step where in the adsorbent bed is evacuated counter current to feed flow by an vacuum pump 28 The column is evacuated via Lines 12, 16 and 18. Solenoid Valve SV 2 is open and rest of the solenoid valves are closed. While the column is still under evacuation step, part of the gas collected in the surge vessel 25 is allowed to flow via Line 19, 20 and 13 into the column to strip off the strongly adsorbed component. The flow of the gas is counter current to feed flow, thus avoiding contamination of the unused bed. The flow of the gas is controlled by mass flow controlled MFC 8 placed on Line 20. Solenoid valves SV 3 and SV 2 are open. Column evacuation is now stopped by closing SV 2 but purge flow from the surge vessel is continued by keeping SV 3 open. The purge flow repressurises the column to an intermediate pressure. The cycle steps are now repeated. The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention. Example 1: A feed input gas containing 57.5 % helium by weight and 42.5 % nitrogen by weight is fed to a single column pressure swing adsorption unit as shown in FIG 1. The adsorbent bed contains 347.5 gms of a molecular sieve 13X from Universal Oil Products UOP. The input gas is fed at a pressure of 3.3 kg/cm2 and a flow rate of 2.6 L/min. The step sequence of the seven step PSA cycle and its timings are set out in Table-1. The time for adsorption -1 is 20 sees. The sequence of on-off operation of solenoid valves is described in Table-2. The flows of various streams entering or leaving the adsorption column are depicted in Figure-2 During pressurisation step, column pressure rises to 4.3 kg/cm2 and this decreases to 3.5 kg/cm2 when column is in the Adsorption step. Product is withdrawn at a flow rate of 2.4-2.6 L/min. The helium content in the product is analysed by Gas Chromatograph and is of the level of 99.99%. In the adsorption-ll step the product is directed to the surge vessel. The pressure of the surge vessel rises to 2.5 Kg/cm2. The column then goes through the blowdown step followed by evacuation. While evacuation is continued the purge is introduced from the surge vessel to flow through the column at 3.0 L/m. This flow is controlled by MFC-3. Evacuation is then stopped, while purge flow is continued, so that column pressure rises. The flows of various streams obtained during adsorption, blowdown and purge steps were measured by using a wet gas flow meter. Product yield is calculated from the following equation % Product Yield = (Vol. of product withdrawn/Total vol. of feed input)* 100 In this example the product yield observed was 40%. Example II This example describes the purification of crude helium from feed mixture containing 57.5% heHum and 42.5 % nitrogen in a single column PSA unit using 13X UOP as adsorbent. In this example the time for adsorption step I was 40 sec. compared to 20 sec adsorption time cited in Example-l. The column pressure rises to 4.5 bar and stabilises at 2.5 bar in adsorption step. The pressure of the surge vessel however rises to 3.5 kg/cm2 as compared to Example I, where a rise of 2.5 kg/cm2 was noticed. The difference in the Surge Vessel pressure may be attributed to the higher time allowed for adsorption I step. The column goes through the seven step PSA cycle. The helium content analysed in the product is 97.3 wt% and yield obtained is 54.8%. Example III This example describe the purification of crude helium from feed mixture containing 35.7 wt% helium and 64.3 wt% air in a single column. In earlier examples a model mixture of helium and nitrogen was used and feed and molecular sieve 13X(UOP) was used as an adsorbent. In this example the adsorbent used was molecular sieve 5A (UOP). The time for adsorption step I was 30 seconds and adsorption step II was 10 seconds. The column pressure in this example rises to around 6.0 bar and stabilises at 3.8 bar in adsorption step. The column goes thorough seven step PSA cycle. The yield of the product obtained was 30.2 % and helium content in the product was around. 90 wt% Main Advantages of the Present Invention are that 1- high purity helium at purity levels fo 95 to 99% can be obtained by PSA in a single column unit. 2- yield of helium product exceeds 50% 3- This purification can be carried out in a single column 4- The purity levels of products are higher than obtained in a double column PSA unit under almost identical conditions. 5- The product yield of 60% and product purity of 64-65 wt%. are obtained in double column.Though the product yields are higher in double column compared to single column, the product purity obtained is lower. It is worth mentioning here that the marginal improvement in the yield of the product is at the cost of more complex unit with increase in number of associated piping, fittings, solenoid valves and amount of adsorbent for the double column 6- The main advantage of existing system is that by incorporating a surge vessel and by increasing the number of cycle steps, a significant improvement is obtained in a single bed PSA system with respect to both yields and purity in the separation of helium from nitrogen and from air. We Claim: 1. An improved single column pressure swing adsorption (PSA) process for the separation of helium, which comprises a) pressurizing the adsorption column containing adsorbent by the feed gas mixture for a specified time, and withdrawing the product from the other end of the column for a specified time, (Adsorption I) b) collecting the product in a surge vessel (Adsorption II), c) terminating the feed flow into the column and depressurizing the column from the feed end, d) evacuating the column from the feed end by drawing a vacuum, e) allowing the gas collected in the surge vessel to purge the column from the product end while continuing the evacuation for a specified time, f) terminating the evacuation by closing the feed end while continuing to pass gas from the surge vessel into the column for a specified time so that column pressure rises and column becomes ready for the next cycle. 2. An improved process as claimed in claim 1 wherein the adsorbent used is microporous solid of the type of zeolite, activated carbons, molecular sieves. 3. An improved process as claimed in claims 1 to 2 wherein the gas feed mixture contain helium in the range of 1 to 50 wt%, the balance being nitrogen, or nitrogen and oxygen. 4. An improved process as claimed in claims 1-3 wherein the flow rate of feed gas mixture during pressurization and adsorption step is varied up to 5.0 L/min. 5. An improved process as claimed in claims 1-4 wherein the pressure of the column during pressurization and adsorption varied between 1.0 kg/cm2 to 10.0 kg/cm2. 6. An improved process as claimed in claims 1-5 wherein the experimental temperature vary from 15°C to 50°C. 7. An improved process as claimed in claims 1 to 6 wherein the feed mixture fed to the column either from the top or the bottom of the column. 8. An improved process as claimed in claims 1 to 7 wherein the regeneration of the adsorbent is carried out by evacuation alone or evacuation with purge gas or solely with purge gas without evacuation. 9. An improved process as claimed in claims 1 to 8 wherein blowdown gas is vented or collected in a separate surge tank. 10. An improved process as claimed in claim 1 to 9 wherein re pressurisation of the column is done with either with purge gas stored insurge vessel followed by final re pressurisation by feed gas introduction or solely by feed gas or by using gas from blowdown tank. 11. An improved single column pressure swing adsorption (PSA) process for the separation of helium substantially as herein described with reference to the examples accompanying this specification.

Documents:

246-DEL-2003-Abstract-(25-07-2008).pdf

246-del-2003-abstract.pdf

246-DEL-2003-Claims-(25-07-2008).pdf

246-del-2003-claims.pdf

246-del-2003-correpsondence-others-(28-07-2008).pdf

246-DEL-2003-Correspondence-Others-(25-07-2008).pdf

246-del-2003-correspondence-others.pdf

246-del-2003-correspondence-po.pdf

246-del-2003-description (complete)-25-07-2008.pdf

246-del-2003-description (complete).pdf

246-del-2003-drawings.pdf

246-DEL-2003-Form-1-(25-07-2008).pdf

246-del-2003-form-1.pdf

246-del-2003-form-18.pdf

246-DEL-2003-Form-2-(25-07-2008).pdf

246-del-2003-form-2.pdf

246-del-2003-form-3.pdf

246-del-2003-petition-138-(28-07-2008).pdf