Abstract:
Systems and methods are disclosed for a gas production system to efficiently maintain a pressure of a gaseous output stream within a tight pressure range when the system changes normal operation to backup operation. In one preferred embodiment, during normal operation a backup vaporizer is kept in cold standby by directing a small portion of a liquefied gas stream away from the main heat exchanger to the backup vaporizer. In this way, the backup vaporizer is able to respond immediately to a shutdown of the main gas production system. During normal operation, the output of the backup vaporizer is recombined with the gaseous output stream to any avoid loss of product thereby increasing efficiency.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/587,688, filed Jul. 14, 2004, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD OF INVENTION 
     The present invention provides a process for production of pressurized gas and, more specifically, provides a fast response, cost-efficient backup system for producing pressurized gas. 
     BACKGROUND OF THE INVENTION 
     Industrial gas consumers frequently request a relatively tight control in pressure variations of pressurized gas produced from a gas production facility. It is desirable that pressure variations remain within these relatively tight limits regardless of disruptive events that inevitably occur at the industrial facility, at least upon occasion. For example, such events may include stopping operation of the air separation unit for scheduled as well as non-scheduled plant shutdowns. 
     Similarly, it is also optimal to other users of pressurized gas to minimize pressure variations and to maintain the pressures in a desired range. 
     In modern air separation units, internal compression processes may be utilized to directly obtain gases under pressure at the cold box outlet. The liquefied gas is extracted from a distillation column, a separator, or a vessel. The liquefied gas may then be compressed by a pump and vaporized under pressure to produce high-pressure gaseous product, e.g., high-pressure gaseous oxygen. 
     When the normal production of gaseous products stops for any cause such as, for instance, purity upset, scheduled or non-scheduled shutdowns, or other reasons, the delivery of the gaseous products may be maintained by a backup system that may include one or more liquid storage tanks, pumps, and a backup vaporizer of various types. The switch over from normal mode to the backup mode has, in the prior art, generally produced a pressure fluctuation of the gaseous product in the pipeline connecting the air separation unit to the consumers. 
     To satisfy the customer requests regarding pressure fluctuations, mainly during air separation unit upset or shutdown, several possible solutions have been proposed. Each solution has advantages, but also has significant disadvantages. 
     One solution would provide a high-pressure gaseous buffer tank installed down stream of the back-up vaporizer. This method provides a very fast response time, but is a capital-intensive solution. 
     Another proposed solution would involve providing a high-pressure liquid tank installed upstream of the backup vaporizer. This solution provides a relatively fast response time, but is also capital intensive and is limited in the range of operating pressure permitted by this solution. 
     Another proposed solution would involve running a backup vaporization pump at an extremely reduced rate to minimize the start-up time of the backup pump, and the vaporizer. This method has a very fast response time, but it is liquid and energy consuming. 
     Consequently, improved systems and method are needed to minimize pressure fluctuations that occur during air separation unit upset or shutdown while simultaneously considerably reducing the capital investment required to effect such systems and methods. It would be desirable to have a simpler system, that is low in energy consumption, useable at all operating pressures, and which has a very fast response time. Those of skill in the art will appreciate the present invention that addresses the above and other problems. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       For a further understanding of the nature and objects of the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein 
         FIG. 1  provides an illustration of a backup system for producing gaseous product in accord with the present invention. 
         FIGS. 2 and 3  present schematic representations of one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 1 , there is shown gas production system  10  which comprises a main and backup gas production system in accord with the present invention. 
     During normal operation of gas production system  10 , liquid such as liquid oxygen and/or other product(s) is separated and extracted in air separation unit  11 . Air separation unit  11  may comprise one or more distillation columns  12 , heat exchangers, vaporizers, pumps, valves, or other separators, vessels, or components that may normally be utilized for this purpose by one of ordinary skill in the art. The liquid so extracted is then normally compressed by pump  14  and subsequently vaporized under high pressure in main heat exchanger  16 . 
     A backup system for gas production in system  10  is provided downstream of valve  20 . Valve  20  controls liquid flow to the backup or transfer flow line. In accord with one embodiment of the present invention, system  10  does not rely for backup only on having backup pump(s)  34  in cold standby but also maintains backup vaporizer  40  in a cold stand-by. Therefore, in accord with a preferred method of the present invention, valve  20  and/or valve  28  of the bypass line  30  may be partially open during normal operation to permit a small portion of the liquid flow therethrough. Valve  38  may also be open during normal operation to thereby maintain backup vaporizer  40  in a cold stand-by. 
     In this way, a small portion of the produced liquid is diverted from the main flow through valve  18  and main heat exchanger  16  through valve  20 . This small portion may typically be less than five percent of the produced liquid and may often be less than or much less than one percent. This extracted liquid which is already compressed at the appropriate pressure by pump  14  is transferred through bypass line  30  to the inlet of backup vaporizer  40  where it is permanently vaporized. The so vaporized gas passes through normally open valve  42  to the produced gas at outlet  50  of the plant and is thereby recombined with the gaseous product coming from main heat exchanger  16  through valve  22 . Thus, the diverted liquid is not lost and is therefore efficiently utilized. 
     The valves used herein may comprise a variety and combinations of valves known to one of ordinary skill in the art including, but not limited to solenoid valves, mechanical valves, which are automatically controlled, manually controlled, or programmable. The valves may further comprise stop valves which shut off or, in some cases, partially shut off the flow of fluids. The involved valves may in addition or alternatively include check valves. 
     The system may also comprise the use of a regulator or similar apparatus which serve to regulate the flow and pressure (not shown), as well as actuators to open and close the valves. 
     Except for bypass line  30 , the path downstream of valve  20  is the regular transfer line to liquid storage tank  26  which may be used to store sufficient liquid in liquid storage tank  26  for the time the backup system is to be used. The constant and/or controlled flow through the path downstream of valve  20  and through bypass line  30  maintains the transfer line full of liquid, and maintains backup vaporizer  40  in cold stand-by, thereby allowing the backup system to react immediately to any flow to be vaporized. 
     At the moment air separation unit  11  is tripped, which also causes shut-off of production valve  22  at the outlet of main heat exchanger  16 , process product pump  14  is maintained and/or functions in normal pumping operation. All the liquid which is normally compressed and directed through main heat exchanger  16  is then re-routed to backup vaporizer  40  with an appropriate valve sequence, a preferred embodiment thereof is discussed hereinafter. Some of the liquid inventory in distillation column  12  or other vessel is utilized, which may typically be for only a few moments, until backup pumps  34  are started and fully loaded to deliver the product. 
     It will be noted that for the case of liquid oxygen being vaporized to produce high-pressure gaseous oxygen and/or a cold box architecture sometimes called “side by side” type, the liquid inventory in main vaporizer  16  is not spoiled by the liquid falling from the low pressure distillation column in the case of trip. Thus, the liquid can be used as a clean source of liquid to be vaporized in backup vaporizer  40 , again increasing the efficiency of the backup system. 
     The process of changing from normal to backup operation may be described by a series of steps of operation or change in operation for the various valves, pumps, vaporizers, and so forth. A presently preferred embodiment of this process is subsequently described but it will be understood that many variations thereof are conceivable in accord with the present invention depending of the particular type and construction of the installation, circumstances requiring backup operation, and the like. 
     For main or normal operation, valves  18 ,  38 ,  42 ,  32  may preferably be open. Valves  20 ,  24 , and  28  may be partially open to keep liquid at the desired pressure within the transfer lines for the backup system, as discussed above. Valve  22  is controlled as desired for maintaining output pressures as necessary and the like. Normally operating process pump  14  is on. Backup pump  34  is in cold standby. Backup vaporizer  40  is in cold standby, as discussed above. 
     When an event occurs that requires shutting down air separation unit  11 , an initial step towards backup operation may involve turning on backup vaporizer  40 , and closing valves  18 ,  22 , and  24 . Valves  20  and  28  may also be opened at this time. Valve  38  is controlled in a variable open position as necessary for maintaining the desired output pressures and/or other purposes. Process pump  14  remains on and backup pump  34  remains in cold standby during this initial step toward changing from normal to backup operation in this embodiment of operation. 
     In a subsequent step for changing from normal to backup operation, both process pump  14  and backup pump  34  are temporarily simultaneously on. Valve  36  is opened as backup pump  34  is turned on. In this embodiment, clean liquid inventory from air separation unit  11  may be utilized. 
     In yet another subsequent step for changing to backup operation, valve  32  may then be controlled for maintaining output product pressure as necessary. 
     In a final step for changing to backup operation in accord with one method of the invention, pump  14  is turned off after the clean liquid inventor is exhausted and the switch over to backup operation is complete. 
     Thus, a method is also provided for operating gas production system  10  which comprises a normally operating gas producing system and a backup gas production system. The method provides that changeover from a main or normally operating gas production system to a backup gas production system occurs in a way that minimizes pressure fluctuations and maintains efficient operation. In one embodiment, the method may comprise producing a liquefied gas stream in a normally operating air separation unit  11 , pumping the liquefied gas stream with at least one normally operating pump  14  into at least one normally operating heat exchanger  16 . Other steps may comprise vaporizing the liquefied gas stream in the normally operating heat exchanger  16  to produce an output product stream. In one embodiment, the method comprises diverting a small portion of the liquefied gas stream, e.g., less than five percent. The method may further comprise directing at least a portion of the diverted liquefied gas stream into backup heat exchanger  40  to maintain backup heat exchanger  40  in a cold standby mode. In a preferred embodiment, the output of backup heat exchanger  40  is combined with the output product stream. The method may further comprise providing that a pressure at an inlet of backup heat exchanger  40  is approximately equal to a pressure at an inlet of main heat exchanger  16  during normal operation to maintain backup heat exchanger and the liquid transfer lines thereto in a cold startup mode for immediate operation and so that gas vaporized in backup heat exchanger  40  is at the desired regulated pressure. The method may further comprise changing from normal operation to backup operation by shutting off flow of the liquefied gas stream to normally operating or main heat exchanger  16 , and diverting all of the remaining liquefied gas stream to backup heat exchanger  40 . Other steps may comprise at least temporarily continuing to pump all of the liquefied gas stream with at least one normally operating process pump  14 . The method may further comprise subsequently turning on at least one backup pump  34 . In one embodiment, the method may further comprise providing that normally operating pump  14  and backup pump  34  are temporarily simultaneously on during the changing from normal operation to the backup operation. In one presently preferred embodiment, the method may further comprise storing at least a portion of the diverted liquefied gas stream in liquid storage tank  26 . The method may further comprise providing bypass line  30  around liquid storage tank  26  to connect with backup heat exchanger  40 . 
     The present invention may include additional or fewer valves, tanks, pumps, separators, vessels, and flowlines, variations in connections, locations, arrangement, and/or other equipment and interrelated components. 
     Preferably the process and apparatus also include the use of an apparatus which monitors pressure and/or flow in part or all of the system. Such apparatus are readily known and used by one skilled in the art for similar and related applications (not shown). 
     Further, the process and apparatus may use dew point monitoring technology to ensure the purity of the product or gases prior to usage (not shown). 
     The apparatus also preferably has at least one component such as a computer, programmable logic device or other component known or used by one skilled in the art for recording and/or storing data about the pressure, flow, and/or purity of the gas and/or liquid which is analyzed during the process. The data logging and reporting maybe accomplished by components which are known to one skilled in the art. 
     The apparatus also preferably has at least one unit for displaying or reporting data. The data may be displayed on a variety of components such as a CRT, LED screen, computer monitor, paper printout and other types of displaying means known or used by one skilled in the art (not shown). The apparatus may also have sound and/or light components and alarms to indicate when certain processes occur, when the desired environment is reached, or when there is a problem or failure with the gas, liquid media, pressure, flow or other parameters measured or monitored by one skilled in the art (not shown). 
     Preferably the apparatus also has a component for storing the data such as a mainframe computer, hard drive, portable computer unit, or the like known or used by one skilled in the art (not shown). 
     For the purposes of the description of this invention, any terms to be utilized such as “upper”, “lower”, “right”, “left,” “vertical”, “horizontal”, “top”, “bottom”, and other related terms shall be defined as to relation of embodiments of the present invention as it is shown and illustrated in the accompanying  FIG. 1 . Further, for purposes of the description of this invention, the terms “upper portion”, “lower portion”, “top”, “bottom”, and the like shall be defined to mean an upper portion and a lower portion and not specific sections. However, it is to be understood that the invention may assume various alternative structures and processes and still be within the scope and meaning of this disclosure. Further, it is to be understood that any specific dimensions and/or physical characteristics related to the embodiments disclosed herein are capable of modification and alteration while still remaining within the scope of the present invention and are, therefore, not intended to be limiting. 
     Thus, it will be understood that many additional changes in the details, materials, steps, processes, and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above and/or the attached drawings.