A process for providing nitrogen gas to a customer at a variable flow rate.

Abstract: A membrane nitrogen generator adapted to deliver a flow rate of nitrogen gas having a purity better or at least equal to P₀, which includes: (a) a source of compressed nitrogen containing gas having a purity lower than P₀, the source having a maximum flow rate Fm₁; (b) varying means connected to the source of a) which is adapted to vary the flow rate of the source between Fm₁ and zero; (c) a nitrogen membrane fed by the source of a) which is capable of delivering a low purity nitrogen gas which is enriched in nitrogen compared to the source and a nitrogen depleted gas which is depleted in nitrogen compared to the source, the low purity nitrogen gas having a purity P₁ which increases when the flow rate of the nitrogen containing gas source feeding the membrane decreases, the purity P₁ being lower than P₀ at the maximum flow rate Fm₁; (d) a source of high purity compressed nitrogen gas having a purity which is greater than P₀ delivering a high purity nitrogen gas having flow rate control means to control its flow rate from zero to a maximum flow rate value Fm₂; (e) mixing means adapted to mix the low purity nitrogen gas and the high purity nitrogen gas and delivering a nitrogen gas mixture; and (f) purity control means adapted to measure the purity of the nitrogen gas mixture and to generate a signal to the flow rate control means to increase the flow rate of the high purity nitrogen gas when the measured purity of said nitrogen gas mixture is smaller than or equal to P₀ and to decrease the flow rate of the high purity nitrogen gas when the measured purity of said nitrogen gas mixture is greater than P₀.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention provides a membrane gas generator which is used in conjunction with bulk nitrogen storage to be responsive to changes in customer demand patterns while avoiding undue increases in generator and operating costs. Description of the Background Membrane process are used with increasing frequency in the production of inert atmospheres from air, wherein the oxygen content in a feed air stream to the membrane is decreased by permeation from a high pressure side to a low pressure side. However, it is very difficult to size the generator for customer need, as the demand is not straightforward. For example, demand often changes considerably both in the very short term, i.e. at ten minute intervals, and in the long term, i.e., with the change of seasons and from year to year. Of course, this difficulty is not specific to membrane process, but is applicable to any small-on site plant serving the needs of a single customer. Although various methods have been proposed for overcoming this difficulty, all suffer from various disadvantages. For example, inert atmospheres have been produced at a pressure higher than required by customer need with storage of excess production in a buffer or storage tank during low demand periods with subsequent release during peak demand periods. While, these solutions have been somewhat helpful in adapting to customer demand, in most cases it remains necessary to use an oversized generator, which can be detrimental to the process economy when annual usage of the facility is low. Thus, a need exists for a membrane generator which exhibits increased flexibility and productivity and is responsive to changes in customer demand pattern while avoiding undue increases in generator and operating costs. Accordingly, it is an object of the present invention to provide a method for responding to changes in customer flow demand patterns without incurring an undue increase in generator and operating costs. The above object and others are met, according to the invention, by the method as defined in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a typical profile of flow rate with time for an oversized generator used in accordance with a conventional method. Figure 2 illustrates a conventional generator apparatus utilizing an oversized generator, and a flow controller. Figure 3 illustrates another conventional generator apparatus utilizing a mean consumption flow rate generator flow controller and a buffer tank. Figure 4 illustrates an alternative embodiment utilizing an undersized generator, flow controller and bulk storage tank. Figure 5 illustrates an embodiment of the present invention utilizing a generator having the capability of variable flow rate and purity, and a bulk storage tank. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance, with the present invention, a method is provided for responding to customer flow demand patterns without unduly increasing generator and operating cost requirement. Further, this is accomplished without using an oversized generator. The present invention provides a method for responding to changes in customer flow demand patterns for nitrogen without incurring an undue increase in generator and operating costs. The present invention provides a method for providing nitrogen gas to a customer at a variable flow rate, and at a given purity level regardless of the flow rate of the nitrogen gas. Generally, the present invention integrates bulk storage of nitrogen and/or liquid in gaseous form with the oxygen content of the total system production, i.e., the mixture of membrane generator product and bulk (when used). This is found to be very advantageous inasmuch as the flow capacity of the nitrogen membrane generator increases as the oxygen content increases. Thus, by mixing with the stored bulk nitrogen, which is essentially oxygen-free, it becomes possible to increase membrane generator production while keeping the mixture oxygen content under a specified limit. The present invention is of particular advantage when using hollow fiber tube-fed modules in the counter-current operating mode where a surprising phenomenon is observed. Notably, the oxygen content increase due to the production increase is much less than expected for shell-fed modules operated in the perfect counter-current mode. Generally, there is no specific relationship between customer yearly demand and generator capacity as optimal generator capacity is very dependent on the customer demand profile. However, in general, optimum generator capacity is equal to about 2 to 4 times average hourly demand, computed as yearly demand divided by 8,760. Further, the present membrane generator generally utilizes a flow rate controller which is downstream of both the generator and the bulk storage means and in fluid connection with each. Preferably, in accordance with the present invention, the flow rate controller or control means for controlling the flow rate of the source of high purity compressed nitrogen gas is an on/off-type valve. While the bulk storage means may be any means for storing product, it is preferably a tank, and the product stored is preferably nitrogen in liquid form. Generally, the purity control means of the present invention is adapted to measure the purity of the low purity nitrogen gas and generate a signal to the flow rate control means to increase the flow rate of the high purity nitrogen gas when the measured purity of the nitrogen gas mixture is smaller than or equal to P o and to decrease the flow rate of the high purity nitrogen gas when the measured purity of the nitrogen gas mixture is greater than P o . Moreover, in accordance with the present invention, a computer means may be used in connection with the varying means and/or the purity control means to manage control of the same. As note above, it is preferred that the nitrogen membrane comprises a plurality of hollow fibers forming a bundle which is encased in a substantially cylindrical shell forming altogether a module. For example, the present membrane nitrogen generator may have a module which contains an input for feed gas from the nitrogen containing gas source, a first output for the permeate gas which permeates through the walls of the fibers of the bundle and a second output for the non-permeate gas or gas mixture which does not permeate through the fibers, wherein the input and first output are located on the shell of the model in such a way that the feed and the permeate gas circulate substantially counter current in the shell of the module. In general, P 0 will have a value of about 0.01% O 2 to about 10% O 2 . Preferably, however, P o will have a value of about 0.1% O 2 to about 3% O 2 . Moreover, r 1 will have a value, in general, of about 0.01% to about 10% O 2 . Preferably however, P 1 will have a value of about 0.1% O 2 to about 3% O 2 . Further, Fm 1 will generally have a value of about 1 to about 10,000 Nm 3 /h, with a preferred value of about 10 to about 2,000 Nm 3 /h. Fm 2 will generally have a value of about 1 to about 10,000 Nm 3 /h with a preferred value of about 5 to 1,000 Nm 3 /h. In general, a feed source is used which is upstream of the separator equipment. As a feed source, compressing means is generally used. Further, control means is used at an outlet of the feed source in order to achieve flow control. The control means may be, for example, a variable frequency electric power source feeding the compressing means, such as the motor of an air compressor. Thereby, a variable flow output may be obtained through a variable rotation speed. The control means may also be a sliding vane with electric or pneumatic controls, being part of a screw compressor casing, such as a Sullair compressor, achieving variable flow output through internal recirculation. Further, the control means may be the combination of an air receiver at the outlet of the compressing means with control means to start and stop the compressing means according to the feed demand. Generally, the nitrogen membrane in fluid connection with the source of compressed nitrogen-containing gas is downstream of the feed source at all times. The source of high purity compressed nitrogen gas (d) is located on a side branch, while mixing means (e) is located downstream of both nitrogen membrane c) and source of high purity compressed nitrogen gas (d). Moreover, purity control means (f) is downstream of (e). Figures 1-5 will now be explained in more detail. Figure 1 illustrates a typical profile of flow rate with time for an oversized generator used in accordance with a conventional method. This illustrates the inefficiency resulting from the use of an oversized generator, thus rendering such a process uneconomical. Figure 2 illustrates a conventional generator apparatus, which encompasses an oversized generator, and a flow controller. Figure 3 illustrates a conventional generator apparatus, which encompasses a mean consumption flow rate generator, flow controller and a buffer tank. Figure 4 illustrates an alternative embodiment which encompasses an undersized generator, flow controller and bulk storage tank. Figure 5 illustrates an embodiment of the present invention, which utilizes a generator having the capability of variable flow rate and purity, and a bulk storage tank. Generally, the storage capacity of the bulk storage means is selected based primarily upon the back-up function played by the bulk storage means in the event of generator failure. That is, a determination is made as to expected need in case of generator failure and a storage capacity is selected to meet that need. As an example, however, storage capacity will generally be about 1 to 3 days of customer average demand. Furthermore, the bulk storage means may be any type of apparati conventionally used. The material of which the bulk storage means is constructed is determined by service conditions, such as cryogenic temperature and service pressure. The present invention will now be further illustrated by reference to certain examples which are provided solely for purposes of illustration and are not intended to be limitative. Example 1 In a design according to the present invention, the unit is sized for 50 m 3 /h eg, and product O 2 content is increased to 2.775% O 2 during consumption surges to that generator production is boosted to 62.27 m 3 /h so that bulk consumption is reduced to 6.29 m 3 for each 8 hrs 10 min sequence. Comparative Examples 1-3 I. One conventional solution for meeting this customer need would be to size the generator for 100 m 3 /h, and have a generator control set a reduced flow at 40 Nm 3 /h level. This is the design disclosed by US 4,806,132. However, this addresses only part of customer need, and does not address total need adequately in practice, due to the high cost of sizing the generator at 100 m 3 /h, and due to the lowered product pressure available while operating the generator in a reduced flow mode. II. Another conventional solution for meeting customer need would be to size the unit for average flow during customer operation, i.e., 52 Nm 3 /h, with storage excess product for four hours, in a buffer tank, with delivery in addition to generator production during consumption storage. This approach would improve system capital cost, but at the expense of added energy consumption for higher pressure storage in buffer tank. III. Another conventional solution would be to size the unit for a given flow, e.g., 50 Nm 3 /h or less, and draw peak demands from an on-site bulk storage. This would decrease cost of generator but would dramatically increase operating cost. However, this kind of design is often necessary, when the customer insists on having an installed back-up system for safety reasons. If unit is sized for 50 Nm 3 /h, bulk consumption will be 8.33 m 3 for each 8 hrs. 10 min. sequence. A summary of the above solutions is indicated below in Table I: Solution Generator Size Bulk Consumpition Comp. Ex. 1 100 0 Comp. Ex. 2 52 0 Comp. Ex. 3 50 8.37 present invention 50 6.29 While this phenomenon is of moderate importance on small modules, it is particularly striking and surprising for large size industrial modules as noted below in Table II: production flow % O 2 ideal module % O 2 industrial module 100 0.1 0.1 172 0.5 0.47 228 1 0.87 317 2 1.65 574 5 3.93 This table, based on industrial size module performance, shows that increased production flow on an existing plant, assuming that sufficient air feed is available, has much less impact on product purity than would be expected from computer simulation. This helps boosting system production in case of customer consumption surge when advantage is taken of installed pure nitrogen bulk back-up and mixing such pure nitrogen with membrane generator production the purity of which is set at all times at a value such that unit production is increased while keeping total product oxygen content under specified value. Example 2 Customer need: 2% O 2 in product with flow pattern as follows Basic flow is 50 Nm 3 /h at 2% O 2 content. Surges of 100 Nm 3 /h consumption at 2% O 2 occur during 10 mins every 4 hrs. Then the consumption stops for 4 hrs and then starts all over again. Average flow is then 26.53 Nm 3 /h. In this particular case, the invention brings the benefit of a 25% reduction in liquid consumption at essentially the same capital cost as solution b. (Except an increase in necessary compressor size, but this is small in relative value). Example 3 Another example, seen in some industrial application, exhibits a flow pattern similar to one as shown below: In the first period, customer requires a flow of 100 m 3 /h inert at a maximum O 2 content of 2%. In a second period, flow need falls down to 25 m 3 /h at a purity of 0.1% O 2 max. The particulars from the four designs described above are summarized below in Table III: solution generator size bulk a) 1. 100 at 2% 100 m 3 per 8 hrs a) 2. 100 at 0.1% (equivalent to 356 at 21%) 0 b) not applicable c) 25 at 0.1% 300 m 3 per 8 hrs invention (equivalent to 89 m 3 /h at 2%) 44 m 3 per 8 hrs Thus, the generator design according to the invention would save 85% of bulk consumption compared to solution C), while having a reduction on generator size over solution a) 1 and a)2. Hence, in accordance with the present invention, a membrane gas generator for example a nitrogen generator, is provided having increased flexibility with high productivity, which avoids expensive reliance upon assigned generators in meeting variations in customer demand. Additionally, it is to be understood and hereby explicitly disclosed that it is equivalent to the presently claimed apparatus and process to employ any apparatus and/or process which satisfy the following two conditions: a) produced gas is deliverable in bulk form as a standby source for customer needs in case of generator failure, and b) gas can be produced by an on-site generator with a strong correlation between production flow and purity. For example, the above two conditions specifically apply to the production of either nitrogen or oxygen in the present invention. Those skilled in the art will appreciate that the permeable membranes employed in the practice of the invention will commonly be employed in membrane assemblies typically positioned within enclosures to form a membrane system, as understood with reference to the invention, a membrane system comprises a membrane module or a number of such modules, arranged for either parallel or series operation. The membrane modules can be constructed in convenient hollow fiber form, or in spiral wound, pleated flat sheet membrane assemblies, or in any other desired configuration. Membrane modules are contracted to have a feed air surface side and an opposite permeate gas exit side. For hollow fiber membranes, the feed air can be added either to the bore side or to the other surface side of the hollow fibers. Cellulose derivatives, such as cellulose acetate, cellulose acetate butyrate and the like; polyamides and polyimides, including aryl polyamides and polyimides, including aryl polyamides and aryl polyimides; polysulfones; polystyrenes and the like, are representative of membrane materials which can be used for the membrane separator. However, in accordance with the present invention, polyimides are preferred. As indicated above, the permeable membranes comprising the membrane system positioned within the insulated enclosure of the invention may be in any desirable form, with hollow fiber membranes being generally preferred. As noted, the membranes, as employed in the practice of the invention, may be in composite membrane form, in asymmetric form or in any such form that it is useful and effective for the particular gas separation being carried out using the system and process of the invention. The hollow fiber membranes are generally formed from a polymeric material which is capable of separating one or more fluids from one or more other fluids in a fluid mixture. The polymeric materials which may be used to prepare the hollow fiber membranes preferably include olefinic polymers, such as poly-4-methylpentene, polyethylene, and polypropylene; polytetrafluoroethylene; cellulosic esters, cellulose ethers, and regenerated cellulose; polyamides; polyetherketones and polyetheretherketones; polyestercarbonates and polycarbonates; polysulfones; polyimides; polyethersulfones; and the like. The hollow fiber membranes may be homogeneous, symmetric (isotropic), asymmetric (anisotropic), or composite membranes. The membranes may have a dense discriminating region which separates one or more fluids from one or more other fluids based on differences in solubility and diffusivity of the fluids in the dense region of the membrane. Alternatively, the membranes may be microporous and separate one or more fluids from one or more other fluids based on relative volatilities of the fluids. Hollow fiber membranes with dense regions are preferred for gas separations. Asymmetric hollow fiber membranes may have the discriminating region either on the outside of the hollow fiber, at the inside (lumen surface) of the hollow fiber, or located somewhere internal to both outside and inside hollow fiber membrane surfaces. In the embodiment wherein the discriminating region of the hollow fiber membrane is internal to both hollow fiber membrane surfaces, the inside (lumen) surface and the outside surface of the hollow fiber membrane are porous, yet the membrane demonstrates the ability to separate gases. In the embodiment wherein gases are separated, the preferred polymeric materials for membranes include polyestercarbonates, polysulfones, polyethersulfones, polyimides, and the polycarbonates. More preferred polymeric materials for gas separation membranes include polycarbonates and polyestercarbonates. Preferred carbonate and polyestercarbonate membranes for gas separation include those described in U.S. Pat. Nos. 4,874,401, 4,851,014, 4,840,646, and 4,818,254; the relevant portions of each patent incorporated herein by reference for all legal purposes which may be served thereby. In one preferred embodiment, such membranes are prepared by the process described in U.S. Pat. No. 4,772,392, the relevant portions incorporated herein by reference for all legal purposes which may be served thereby. Furthermore, in accordance with the present invention, either tube-fed or shell-fed hollow fiber membranes may be used, or either plate and frame or spiral wound film membranes may be used.

Claims: A process for providing nitrogen gas to a customer at a variable flow rate, but at a purity which is not less than a first purity P 0 , comprising the steps of : providing a membrane separator as a first source of nitrogen gas for supplying nitrogen gas at a second purity P 1 which is dependent upon the flow rate of the first source, P 1 being lower than P 0 when the flow rate is in a certain range which is above the nominal flow rate of the separator and up to a maximum flow rate and greater than P o at lower flow rates providing a second source of nitrogen gas for supplying nitrogen gas at a third purity P 2 , P 2 being greater than P 0 , measuring the second purity P 1 of nitrogen gas, supplied by the first source, comparing P 1 to P 0 , and when P 1 < P 0 , mixing a quantity of nitrogen gas from the second source with the nitrogen gas from the first source and delivering to the customer a gas mixture which purity is not less than P 0 , when P 1 ≥ P 0 , delivering directly the nitrogen from the first source to the customer, the flow rate of the nitrogen gas provided to the customer being adjusted as a function of its need. The process of claim 1, wherein the membrane separator comprises a plurality of hollow fibers forming a bundle which is encased in a shell forming altogether a module. The process according to claim 2, wherein the membrane separator is fed with pressurized nitrogen-containing gas by at least one compressor. The process of any of claims 1-3, wherein the second source of nitrogen gas is a tank containing nitrogen under pressure.

Inventors: BARBE CHRISTIAN, BRUGEROLLE JEAN-RENAUD, BARBE, CHRISTIAN, BRUGEROLLE, JEAN-RENAUD
Applicants: L AIR LIQUIDE SA POUR BR L ETU, L'AIR LIQUIDE, SOCIETE ANONYME POUR<BR>L'ETUDE ET L'EXPLOITATION DES PROCEDES<BR> GEORGES CLAUDE
Application year: 1993
Main classification: B01D53
IPC codes: B01D63, B01D53, C01B21