GB2294211A

GB2294211A - Nitrogen generation process

Info

Publication number: GB2294211A
Application number: GB9520834A
Authority: GB
Inventors: Andrew Gilbert Chalmers; Gary George Hosmer
Original assignee: Domnick Hunter Ltd
Current assignee: Domnick Hunter Ltd
Priority date: 1994-10-22
Filing date: 1995-10-11
Publication date: 1996-04-24
Also published as: JPH08231209A; DE19538997A1; GB9520834D0; GB9421308D0

Description

1 NITROGEN GENERATION PROCESS 2294211 This invention relates to a nitrogen

generation process.

GB-B-2152834 discloses a process for obtaining a nitrogen rich product gas in which feed gas containing oxygen and nitrogen is supplied to an adsorber vessel containing carbon molecular sieve (CMS) for the separation of nitrogen and oxygen molecules. The vessel is first pressurised by the feed gas while product gas, nitrogen rich due to the action of the molecular sieve, is withdrawn through a non-return valve to a nitrogen reservoir. The vessel is then vented to exhaust waste gas therefrom. This cycle is repeated, and product gas can be withdrawn from the reservoir as required.

In order to give a substantially continuous flow of product gas two vessels may be connected in parallel between the feed gas supply and the reservoir, one vessel being on-line and pressurised by connection to the supply, while the other vessel is off-line and is being vented. The functions of the vessels are then reversed by appropriate control of suitable valves. A parallel vessel process may be operazed in two basic ways, as a so-called equalising system or as a non-equalising system. In an equalising system on-line pressurisation of the first vessel is completed while the second vessel is vented, the vent valve of the second vessel is closed and the two vessels are then placed in direct communication so that pressure between them is rapidly equalised. Product gas is usually drawn from the reservoir into one or both of the vessels after this step. The second vessel is then placed on-line to be pressurised from the feed gas supply while the first vessel is vented. In a non-equalising system the vessels are alternately pressurised and vented without any direct communication being effected between the vessels.

It is recognised that an equalising system can operate more efficiently than a non-equalising syszem, but that it is more complex than a non-equalising system and has other disadvantages. Thus, the rapid gas flow into the off-line vessel stresses the molecular sieve material in that vessel and can damage it; suitable protection can be incorporated but this adds expense a-nd complication. A greater number of valves and more sophistizated control are required in an equalising system.

The invention seeks to improve the efficiency of a nonequalising nizrogen generation system.

Accordingly,:he invention provides a cyclic process for obtaining nitrDgen rich product gas from a feed gas containing oxygen and nitrogen, utilising sim-lar first and second vessels connected in zarallel between a -":eed gas supply line and a product gas delivery line and each containing CMS capable of separating nizrogen and oxygen mclecules, in which, during a first phase c--" each cycle:

(a) feed gas is supplied at a controlled rate and pressure to the first vessel and product gas is delivered from the first vessel to the delivery line, (b) a r-:rtion of the producz gas is passed as purge gas at reduced pressure to the Second vessel, (c) the second vessel is vented to exhaust during a firs.: period of the first phase and the exhaust is closed during a second period of te first phase; and durinc a second phase of eac. cycle (d) feed gas is supplie-m at a controlled rate and pressure to the second. --2ssel and product gas is deli-.-er=--- from the second vessel to the delivery line, 1 (e) a portion of the product gas is passed as purge gas at reduced pressure to ihe first vessel, (f):he first vessel is -jented to exhaust during a first pericd of the second phase and the exhaust is closed during a second period:f the second phase; and during the whole of the zycle, the only connection made between the vessels is that f:r the purge gas.

In operation, the purge gas 'elps to regenerate the bed of molecular sieve material in tne off-line vessel more rapidly than simply by allowing that vessel to be vented to exhaust. This enables the exhaust valve to be closed 'earlier in the cycle than would otherwise be che case, so assisting gradual pressurisazion of the off-line vessel before the changeover between the two phases of the =ycle places this on-line. Some pressurising of the regenerat--ng bed occurs naturally due to de-gassing of the CMS.

It has been found that remarka----y high and unexpected increases Jn -L. eff 'cie-icy can be attain-ad using the process of the invention. One accepted meas'-re of performance for nitrogen generation systems is known as zhe Q-factor, and is given, for a speci-fied output purity at a given pressure and temperature, by the --at-'3 of flow rate of p=duct gass W. hr') to the volume (m') of CM5 used. Percentage =provements in Q-factor ranging -rom 45..i tc 100% may be achiev=--- by the invention. Indeed, the -nvent-JDn provides a non-equall.sing sys,:em that can be operated at the same or higher efficiency thar an equalising system,.aithout the disadvantages t=--- -are aztendant on equalising systems.

-he amcunt of purge gas that 's used depends on the outlet purity --ha-is required and = other operating parameters. A purge gas volume per minute of at least about 52k, preferably at 1 east about 10%, more preferaL----i at least about 20%, especially 1 at least about 30%, of the total volume of CMS contained in the first and second vessels is preferred in this invention. Preferably, the purge gas volume per minute is not more than about 90%, more preferably not more than about 80%, especially not more than about 75% of the total volume Of CMS. The purge gas volume can be equal to from about 30% to about 75% of the volume of CMS contained in the vessel. It should be mentioned that use of purge gas in an equalising system is known from US-4439219; however the volume per minute suggested in this document is lower at 55 to 25% of the CMS volume.

The period during which a vessel is vented to exhaust during the off-line phase is also dependent on other operating parameters. At present the preferred period is from 30% to 80% of the total time of the second phase. More accurate control of closing of the exhaust during the second phase may be achieved by monitoring the level of oxygen in the exhaust gas by incorporating an oxygen analyser in the exhaust line. During the initial part of the second phase the exhaust gas is oxygen rich as the CMS regenerates and releases the oxygen trapped while on-line. For given operating requirements a figure can be established for the optimum oxygen content at which the exhaust valve should close, and the analyser can used to generate a valve control signal when that figure reached.

be is Depending on volume requirements it will be understood that a system used in the process of the invention may comprise an equal plurality of first and second vessels, all connected in parallel between the feed gas supply line and the product gas delivery line and each first -vessel having a purge gas connection to a respective one of the second vessels.

The or each purge gas connection is preferably made between the delivery sides of respective first and second vessels. In this way all that is required is a connection line with a pressure restrictor, desirably a variable restrictor, therein.

Co:.i--iecticn between other parts of the system is zheoretically possible, but additional valves and controls woul- be required.

The invention extends also to all novel concepts disclosed herein, either alone or in combination.

In order that the invention may be better understood, a spezific embodiment of apparatus for operating the process of the invenzion will now be described in more detail, by way of example cnly, with reference to the accompanying drawing, which is a schematic diagram of a nitrogen generating system.

The apparatus shown in the figure comprises two -=irst vessels 1, 2 and -wo second vessels 3, 4. Each vessel c=tains a bed of --MS s--ich as that supplied by Carbotech Industrieservice under pr::duct reference CMS-F. This is an ac:zivated coke mazerial in which the pore sizes have been modified so that a seraratic- of oxygen and nitrogen can be made, an oxygen mo-ecule being slightly smaller than a nitrogen r.)lecule. The ma=-rial is well known in the nitrogen generaticn field.

The four -.-essels are connected in parallel between a source 5 of::omprezsed air and a delivery line 6 that is c:-,nnected to.a reservoir. not shown. Vessels 1 and 2 are connected to the source by way of a solenoidoperated on/off supply valve 7 and to --he de-ivery line by way of respective non-re:urn valves 8 an-- 9. ','=-ssels 3 and 4 are connected to the source by way of a s:leno- ---operated on/off supply valve 10 and tc the delivery li:-i=- by -,:ay of respective non-return valves 11 and 12. The supply s--'-- of each vessel can be connected to -.-ent (e.g. to at-.-zsphe--=-) through a respective solenoid- operated on/off ex.aust -.=-lve 13 to 16.

The:Ieli-,-.=-ry side of vessel 1 is connected to the --;elivery side of .-esse- 3 by a purge gas line 17 which incorporates variable flc-,; res:rictors 18, 19. The delivery side a-l vessel 2 is connected to the delivery side c-' vessel 4 by a purge gas line 20 which includes variable flow restrictors 21, 22.

The compressed air supply 5 is zonnected to the supply valves 7 and 10 by a line which includes filters and/or dryers 23 as required, ensuring that only clean, dry a--r reaches the vessels.

The solenoids operating the supply and exhaust valves are controlled in the desired sequence by a timing circuit, not shown, the design of which will apparent to z'hose skilled in the art from the description of iperation that follows.

After start-up and stabilisat-:)n of the system a typical operating cycle will be as -:.-oll:ws. At the siart of a first phase of the cycle supply valve 7 will open, supply valve 10 will close, exhaust valves 15 -=--ld 16 will o=en and exhaust valves 13 and 14 will rema-n in their already closed conditions. Vessels 1 and 2 w----1 thus be placed on-line and vessels 3 and 4 will be taken Compressed air is fed into vessels 1 and 2 at a =on--i-olled 'Llow ra--e. As the air flows through the CMS oxyge--mc-ecules are captured with the result that a nitrogen rich prod--:::t gas --2lows f--3m the delivery side of zhe vessels. The ma-:rity cf this gas passes on delivery lire 6 to the res=-rvc:.r, where --t -s stored under pressure for use as required. Part of the product gas, however, passes at a reduced pressure con----olled by the restrictors 18, 21 through =he ---,rge gas line-s 17, 20 to the delivery side of the vessels 3 =--d Vessels --- and 4 are ccnnected_zo ven-z by t-e open exhaust valves 15 and 16 and they are ac:Drd--'ng--Iy depressurising. The purge gas assists that opera--ioy-- and as tha:: gas is relatively nitrogen ric' its passage throug- the off-line -.-essels 3 and 4 assists regeneration of:he CM--: in thcse ves-zels by purging oxygen f rom:he CMS. Af ter a se: period of zhe --first phase has elapsed:he exhaust va-,ves- ---= and -_6 are closed. The continuing flows of purge gas thus assist the natural pressurising of the respective vessels as the CMS continues to de-gas.

At the end of the f irst phase of the cycle the second phase commences, the supply valve 7 closing and exhaust valves 13 and 14 opening to take vessels 1 and 2 off-line. Supply valve 10 opens and vessels 3 and 4 are thus placed on-line to receive feed gas. As the vessels are additionally pressurised by the purge gas the pressure shock on the CMS from opening the supply valve 10 is reduced.

Vessels 3 and 4 deliver product gas to the reservoir and purge gas to the vessels 1 and 2, which are venting through exhaust valves 13 and 14. The CMS in vessels 1 and 2 is thus regenerated and after a set period of the second phase the exhaust valves 13 and 14 are closed so that the purge gas commences to pressurise vessels 1 and 2. The second phase finishes, and the first phase of the following cycle commences as described.

The following specific examples illustrate the efficiency gains that can be achieved using the invention. Test apparatus was constructed as shown in the drawing, but using a group of five first vessels and a corresponding group of five second vessels. Each vessel contained 25 litres of the aforesaid CMS. Compressed air was supplied:o the apparatus at 7 barg and the apparatus was controlled to provide outlet gas at various oxygen concentration levels. Obviously, the lower the oxygen content the higher is the desired nitrogen level. The results are shown at Part A of the following table, which show the Q-factor as already defined, together with a further accepted measure of performance, the air/N,.ratio, defined as the volume of feed air required to produce one volume of product gas. High Q-factor figures indica:e more efficient performance; low air/N2 ratio figures also indicate higher efficiency.

To provide a comparison with a known non-equalising system the purge connections 17, 20 were removed from the test apparatus so that no purging could occur, and exhaust valve timing was changed so that exhaust valves were not closed until their associated supply valve was opened. Thus, one group of vessels was open to vent during the whole of the time that the other group was on-line. Apart from these differences, the control apparatus was run as described in the preceding paragraph. The results are shown at Part B of the table.

OUTLET GAS 02 CONCENTRATION PART A First phase (s) First period second phase (s) Second period second phase (s) Purge gas flow (volume/minute as % CMS volume) Q-factor Air/N2 ratio PART B - CONTROL First phase (s) Second phase (s) Q-factor Air/N2 ratio 0.001 0.10 70 10% 12 so 3 01k 72 4.5 0.50 1.00 50 4 0 % 3.5 40 60% 132 3.3 2.00 3.00 40 is 60% 174 2.9 so 40 7 0 5 210 2.7 120 90 90 60 60 120 90 90 60 60 is 42 68 85 120 140 14 5.5 4.25 3.8 3.4 3.2 The remarkable improvements in efficiency, measured either by Q-factor or by air/N2 ratio are clearly shown, and even further improvement is believed possible. These efficiency figures match or exceed those of similai-ly sized equalising systems, without any of the disadvantages that are associated with such systems. Specifically, the valve and control systems used in the invention are much simpler than those required for an equalising system and there is less stress on the CMS. It will be noted that the phase times in Part B of the table are longer than those in Part A. These longer times were necessary with the control apparatus in order to produce the efficiency f igures shown, and any shorter time would materially have reduced the efficiency. Shorter cycle times are in themselves beneficial, so adding to the advantages that are gained by the invention.

The example described with reference to the drawing shows two vessels in each of the first and second groups. Depending on the output volume requirement each group may contain only one vessel, or any number of vessels greater than two. Higher flows may require two or more parallel supply valves for each group of vessels.

The example also describes a simple connection to vent, e.g. atmosphere, through the exhaust valve from each vessel, and closure of relevant exhaust valves after a set period of each phase of the cycle. If required, each output from an exhaust valve may be connected to an oxygen analyser which continuously monitors the vent output. As soon as the analyser detects that the oxygen content has dropped to a given value during a vent period it may generate a signal which will cause the associated exhaust valve to be closed. If there are more than one vessel in each group then it is possible either to connect all exhaust valves of that group to a common analyser, or to provide an analyser to sample the exhaust from one valve only of the group, the analyser being operative to generate a signal that will effect simultaneous closing of all exhaust valves of the group.

Other modifications will be apparent to those skilled in the art.

Claims

CLAIMS:

1. A cyclic process for obtaining nitrogen rich product gas from a feed gas containing oxygen and nitrogen, utilising similar first and second vessels connected in parallel between a feed gas supply line and a product gas delivery line and each containing carbon molecular sieve capable of separating nitrogen and oxygen molecules, in which, during a first phase of each cycle:

(a) feed gas is supplied at a controlled rate and pressure to the first vessel and product gas is delivered from the first vessel to the delivery line, (b) a portion of the product gas is passed as purge gas at reduced pressure to the second vessel, (c) the second vessel is vented to exhaust during a first period of the first phase and the exhaust is closed during a second period of the first phase; and during a second phase of each cycle (d) feed gas is supplied at a controlled rate and pressure to the second vessel and product gas is delivered from the second vessel to the delivery line, (e) a portion of the product gas is passed as purge gas at reduced pressure to the first vessel, (f) the first vessel is vested to exhaust during a first period of the second phase and the exhaust is closed during a second period of the second phase; and during the whole of the cycle, the cnly connection made between the vessels is that for the purge gas.

2. A process according to claim 1 in which the purge gas volume per minute is equal to from 10% to 90% of the total volume of carbon molecular sieve contained in the first and second vessels.

3. A process according to claim 1 or claim 2 in which the period during which a vessel is vented to exhaust during the off-line phase is from 30% to 80% of the total time of the second phase.

4. A process according to any one of the preceding claims in which closing of the exhaust during the second phase is in response to a signal generazed by an oxygen analyser in the exhaust line when a given oxygen content in the exhaust gas is sensed.

5. A process according to any one of the preceding claims in first and second vessels is used, which an equal plurality of all said vessels being connected in parallel between the feed gas supply line and the product gas delivery line and each of said first vessels having a purge gas connection to a respective one of said second vessels.

6. A process according to any one of the preceding claims in which the or each purge gas connection is made between the delivery sides of respective first and second vessels.