Abstract:
A method of operating a nitrogen generator is provided, wherein the method includes providing a source of compressed air and operating a plurality of pneumatic valves with the compressed air. The method also includes operating at least one pneumatic timer to toggle the nitrogen generator between a production mode where compressed air is channeled to a nitrogen adsorber to produce nitrogen, and a regeneration mode where substantially oxygen-rich air in the nitrogen adsorber is exhausted into the atmosphere.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. provisional application No. 60/684,510 filed May 25, 2005, which is herein incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates generally to generators, and more specifically to pressure-swing-adsorption (PSA) nitrogen generators. Herein, the generators are generally referred to as nitrogen generators. However the disclosed embodiments also apply to generators of other gases, such as oxygen, methane, etc.  
         [0003]     Nitrogen is used for many applications. The most common general application is taking advantage of its inert property, typically to keep oxygen away from combustible products or products that degrade with exposure to oxygen and/or moisture. Systems are known that utilize combusted fossil fuel to produce a mixture consisting of approximately 88% N2 and 12% CO2 for use as an inert gas. However, the presence of CO2 caused a problem for many applications. Cryogenic (approx −320F) liquid nitrogen (LN2) has became increasingly available and has replaced most of the earlier nitrogen generators. Later, pressure swing adsorption (PSA) was commercialized, making it possible to produce high purity nitrogen at facilities, including remote locations. This alleviated the need to have LN2 tanks, piping, dependence on LN2 suppliers etc. PSA also eliminated heavy losses of nitrogen product due to heat transfer, and the hazards of handling cryogenic fluid.  
         [0004]     PSA systems use a carbon molecular sieve (CMS), which adsorbs oxygen and other molecules much more readily than nitrogen molecules. A bed of CMS in a pressure vessel is pressurized with standard compressed air. The CMS adsorbs the oxygen, while nitrogen flows through a port typically located in the opposite end from the compressed air inlet.  
         [0005]     After a certain length of time (2 minutes for example), the CMS has adsorbed about as much oxygen as it has capacity to adsorb. At that point, the purity of the nitrogen diminishes, as more and more oxygen molecules make their way through the CMS bed to the nitrogen outlet. Typical PSA systems use two CMS adsorber vessels. Vessel ‘A’ is pressurized and producing nitrogen, while vessel ‘B’ is depressurized and “regenerated”, similar to a regenerative dessicant air dryer. After a predetermined time period, valves are switched, so that vessel ‘B’ is pressurized and produces nitrogen, while vessel ‘A’ is regenerated. This is typically controlled by electromechanical timers, or via a programmable logic controller (PLC).  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0006]     In one aspect, a method of operating a nitrogen generator is provided, wherein the method includes providing a source of compressed air and operating a plurality of pneumatic valves with the compressed air. The method also includes operating at least one pneumatic timer to toggle the nitrogen generator between a production mode where compressed air is channeled to a nitrogen adsorber to produce nitrogen, and a regeneration mode where substantially oxygen-rich air in the nitrogen adsorber is exhausted into the atmosphere.  
         [0007]     In another aspect, a nitrogen generator is provided, wherein the nitrogen generator includes a source of compressed air, a plurality of pneumatic valves operated by the compressed air and configured to channel the compressed air, and a nitrogen adsorber fluidly coupled to at least one of the plurality of pneumatic valves. The nitrogen generator also includes at least one pneumatic timer to toggle said nitrogen generator between a production mode and a regeneration mode, wherein, during the production mode, the compressed air operates the plurality of pneumatic valves such that at least one pneumatic valve channels the compressed air to the nitrogen adsorber to produce nitrogen and, during the regeneration mode, the compressed air operates the plurality of pneumatic valves such that at least one pneumatic valve exhausts substantially oxygen-rich air in the nitrogen adsorber into the atmosphere.  
         [0008]     In a further aspect, a nitrogen adsorber is provided, wherein the nitrogen adsorber includes a first end, a second end and a body extending therebetween. The body includes a carbon molecular sieve to remove oxygen from compressed air and a desiccant material to remove water from the compressed air. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a schematic of a pneumatic control system.  
         [0010]      FIG. 2  is a cross-sectional view of an adsorber vessel that may be used with the system shown in  FIG. 1 .  
         [0011]      FIG. 3  is an illustration of internal components of the adsorber vessel shown in  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]     Described herein are methods and apparatus that reduce cost and complexity, and improve performance, of pressure swing adsorbtion (PSA) nitrogen generators. The ability to control timing of a control system is provided with a pneumatic system that obviates the need for a programmable logic controller (PLC) or electromechanical timer, and allows operation of the system without requiring electricity. Variants of the system described herein are used for dual bed PSA systems. However the primary application is for single bed (monobed) PSA systems. Also provided is a method of constructing vessels designed for easy maintenance and low cost as is a method of obtaining quality flow distribution of gas in a space-efficient and cost-effective manner.  
         [0013]      FIG. 1  is a block diagram of a time control system  100 . As shown in  FIG. 1 , compressed air,  101 , is supplied to system  100 . A small amount of compressed air diverts to a pressure regulator  117 , which reduces pressure downstream of  117  to, for example, 80 psig. In the preferred embodiment, valve  117  is a pneumatically operated spring return valve which supplies pressure to a timer circuit when pressure is not being supplied via a pressure switch  115 . Pressure switch  115  in the preferred embodiment is a spring operated compressor unloaded valve, but may be a pneumatic or electrically operated pressure switch. When a nitrogen receiver tank  111  is “full” (at desired storage pressure), switch  115  stops applying pressure to valve  117 , and energizes the timer circuit.  
         [0014]     Pneumatic timers  121  and  122  allow independent control of production and regeneration time for an adsorber vessel  105 . Timers  121  and  122  may be a single device, electromechanical, or other types of timers, however in the preferred embodiment timers  121  and  122  are fully pneumatic devices with an adjustable valve control dial that regulates a length of time prior to switching output.  
         [0015]     A pulse valve  118  and a shuttle valve  119  start the system in the regeneration mode. This may be accomplished alternately by spring-loading valve  120  or other means. Adsorber  105  may be started in production cycle, however starting in the production cycle is not recommended for optimal carbon life and performance.  
         [0016]     When valve  117  has first supplied pressure to the circuit, a pulse valve  118  supplies pressure for a small length of time (one second for example). This switches shuttle valve  119  to position A, applying pressure to valve  120 , labeled in  FIG. 1  as port  14  for descriptive purposes. This passes pressure to port ‘B’ of valve  120 , applying pressure to valve  110 , which allows nitrogen to flow through, or “purge”, adsorber vessel  105 . This nitrogen purge flow is an optional feature that improves system performance. An orifice  109  is a fixed orifice in the preferred embodiment, but may also be a throttling valve or a length and diameter of tubing that will give the desired flow rate for a given system design.  
         [0017]     The amount of nitrogen purge flow, as a function of nitrogen production, is an important variable. In one embodiment, the purge/production ratio is less than 0.05. Additional variables such as carbon molecular sieve (CMS) type, operating pressure, adsorber geometry will all affect the purge/production ratio.  
         [0018]     The essential feature of the regeneration mode is that valve  103  is in the position that exhausts adsorber  105  contents into the atmosphere. These contents are oxygen-rich air. The oxygen and other molecules desorb from the CMS when pressure is removed. The optional flow nitrogen described above assists in flushing oxygen from the CMS.  
         [0019]     Once the proper regeneration time has expired, for example one minute, timer  122  switches and passes air from its power port to its output port. Switching of timer  122  passes pressure to valve  120  port  12 , which allows pressure to be applied to a valve  103 . This starts the “production” cycle which allows compressed air to enter adsorber  105 . Nitrogen-rich gas flows past the CMS, through a check valve  106 , a flow control valve  107 , and a backpressure regulator  108 . When a sufficient backpressure is achieved, for example 100 psig, regulator  108  begins to open and fill nitrogen receiver  111 .  
         [0020]     Once timer  121  switches to allow pressure to flow from power port to output port, pressure is applied to shuttle valve  119 , which switches valve  120 , initiating the regeneration cycle and the cycle repeats. This continues until pressure switch  115  reaches its setpoint, and applies pressure to valve  117 , which allows the timing circuit to exhaust and deenergize. This indicates that the nitrogen receiver is full, and stops generation of nitrogen to conserve compressed air.  
         [0021]     A primary advantage of this system is the elimination, in the preferred embodiment, of electric power. This obviates the need for an electrician and the expense and inconvenience of wiring in typical locations. It also can allow operation in a remote site or one with non-standard voltage where a compressor is present, but possibly not a generator or supply of power. The system can safely be operated in hazardous areas where combustible gases may be present.  
         [0022]      FIG. 2  is a schematic view of an adsorber vessel that may be used with system  100 . The vessel consists of a pipe or tube,  234 , which retains the internal pressure. The wall thickness of tube  234  is determined in accordance with well known hoop stress equations. A top head  231  and a bottom head  239  also serve to retain pressure, and are designed similarly per well known head equations.  
         [0023]     The vessel also includes a top piping port  232  and a bottom piping port  236 . Ports  232  and  236  can be piped with normal production flow coming in the top, and flowing downward to the bottom, or reversed. In either case, flow reverses during the regeneration cycle.  
         [0024]     A CMS bed  237  performs the separation of nitrogen and argon from other constituents in the air, which is described above. A desiccant material  238 , typically activated alumina, retains free water in the compressed air to prevent it from reaching the CMS material. Water degrades CMS and prevents oxygen from being retained. During the regeneration cycle, desiccant material  238  is also regenerated. A thin sheet of inert material  235  separates CMS bed  237  and dessicant material  238 . In one embodiment, material  235  is a fibrous mat material which is sometimes colloquially referred to as “coconut”. Components used in this construction consist of inexpensive and off-the-shelf pipe, end-caps, and clamps. Welding and costly machining is eliminated, compared to known designs.  
         [0025]     One of the features of this monobed construction style, in addition to the use of only one vessel versus the typical use of two vessels, is the combination of CMS and desiccant in the same vessel. Known systems use a separate vessel for the desiccant. This feature significantly reduces system complexity, cost and size.  
         [0026]     Another cost-reduction feature is the use of clamp fittings  230  that retain heads  231  and  239 . The preferred embodiment are clamp fittings used in fire sprinkler systems, manufactured by Victaulic Co., Anvil Corp. (Gruvlok™), and others. These clamp fittings use a rubber or other elastomer seal, compressed by the fitting, to provide an airtight seal, depicted by item  233 . Grooves cut or rolled into the pipe and head allow the clamp to retain the heads. These fittings provide significant cost reduction compared with the typical use of ANSI flanges. In addition, they provide a method of quick access into the contents of the adsorber vessel, reducing labor during fabrication and maintenance operations. ANSI flanges take many more large bolts (typically 4, 8, 12, 16 or more bolts per closure). Typically desiccant must be changed every 3-4 years, while the CMS can last a decade or more. The clamps also typically have a smaller diameter than ANSI flanges, allowing more compact system packaging.  
         [0027]     Another feature described herein is the placement of desiccant  238  on top of CMS bed  237 . The placement of desiccant allows the more frequent changing of the desiccant material to be performed without disturbing the CMS or removing the adsorber vessel. The desiccant is typically removed utilizing a vacuum device. Conversely it is possible to turn the vessel over from the preferred orientation and remove the CMS while leaving desiccant intact, on the less frequent occasions where this is necessary.  
         [0028]     An additional benefit of this construction is that there is not a requirement for welding. This allows fabrication without the need for a welding machine or operator. It also obviates the need for welding qualifications and inspection of welds and certain construction codes. These aspects significantly reduce construction costs.  
         [0029]      FIG. 3  is a close-up cross-section of the head region illustrated in  FIG. 2 . Item  344  is the clamp, and  343  is the head. Item  340  is a thick section of the previously described “coconut” material (or other inert material). This material serves as a gas-distribution system, allowing the material to distribute evenly across the cross-sectional area without excessive pressure drop. The means presently known in the field typically involve a complex assembly of metal standoffs and perforated fabricated assemblies. These other designs typically use a much more significant volume. The embodiments disclosed herein, by comparison, improve air consumption efficiency.  
         [0030]     Still referring to  FIG. 3 , a mesh screen  342  prevents CMS and/or desiccant material from flowing into the process piping, which would cause damage to other components, and degradation of the adsorber performance. Item  341  is a perforated plate with holes larger than screen  342 . Plate  341  is typically sheet metal, but may be of plastic or other materials. Items  341  and  342  may be a single device with perforations. However, it is believed that the use of two devices lends to superior performance, where item  342  catches fine particles, but item  341  blocks larger particles, helping to keep screen  342  from clogging. Item  341  is firmly attached to head  343 , by tack-welding, screws, rivets, or other common means.  
         [0031]     The primary result of the embodiments described herein is the production of a low-cost efficient means for producing nitrogen. The means disclosed herein greatly reduce the cost of producing systems with small capacity. There are many markets with a need for low cost, reliable units. These include tire inflation, food preservation (displacing oxygen which degrade food), beverage production, especially alcohol, beverage dispensing, blanketing of tanks that have chemicals and petroleum products, and many others. In addition, the embodiments described herein enables nitrogen generators to be effective and productive in many more markets by reducing costs and eliminating the requirement for electrical power.  
         [0032]     As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural said elements or steps, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features.  
         [0033]     Although the apparatus and methods described herein are described in the context of a carbon molecular sieve (CMS) and a pressure-swing-adsorption (PSA) nitrogen generator, it is understood that the apparatus and methods are not limited to CMS or PSA nitrogen generators. Likewise, the CMS and PSA nitrogen generator components illustrated are not limited to the specific embodiments described herein, but rather, components of the CMS and PSA nitrogen generator can be utilized independently and separately from other components described herein.  
         [0034]     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.