Abstract:
A refrigeration system having a partially decentralized operating control system wherein five defined separate controllers control either one or more than one defined variables.

Description:
TECHNICAL FIELD 
     This invention relates generally to refrigeration systems using a multicomponent refrigerant fluid and is particularly useful for controlling the operation of a cryogenic liquefier. 
     BACKGROUND ART 
     The employment of multicomponent refrigerant fluids in the operation of refrigeration systems such as cryogenic liquefiers has recently been receiving increased attention. In any refrigeration system it is important to operate that system in its proper mode in the face of disturbances and numerous control systems for so operating refrigeration systems are known. In the case where the refrigerant fluid for a liquefier is a multicomponent refrigerant fluid it would be highly desirable to have a control system which does not rely on adjusting the refrigerant mixture online as such adjustment requires the addition of equipment that would add capital expense and add to the complexity of the operation. 
     Accordingly, it is an object of this invention to provide an improved apparatus for controlling the operation of a refrigeration system such as a cryogenic liquefier. 
     It is another object of this invention to provide an improved apparatus for controlling the operation of a refrigeration system such as a cryogenic liquefier which is particularly useful when the refrigerant fluid employed in the system is a multicomponent refrigerant fluid. 
     SUMMARY OF THE INVENTION 
     The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention which is: 
     Apparatus for producing refrigerated product comprising: 
     (A) a compressor, a surge tank, means for passing refrigerant fluid from the compressor to the surge tank said means including a surge tank inlet valve, and means for passing refrigerant fluid from the surge tank to the compressor said means including a surge tank outlet valve; 
     (B) a multistage heat exchanger having an initial stage and a final stage, and means for passing refrigerant fluid from the compressor to the heat exchanger said means including an aftercooler having a cooling fluid input valve; 
     (C) a phase separator having a vapor exit and a liquid exit, means for passing refrigerant fluid from the initial stage of the heat exchanger to the phase separator, and means for passing refrigerant fluid from the liquid exit of the phase separator to the compressor said means including a first Joule-Thomson valve; 
     (D) means for passing refrigerant fluid from the vapor exit of the phase separator to a second Joule-Thomson valve, from the second Joule-Thomson valve to the final stage of the heat exchanger, and from the final stage of the heat exchanger to the compressor; 
     (E) means for passing product fluid to the heat exchanger, and means for recovering refrigerated product from the heat exchanger said means including a product valve; 
     (F) a first controller for regulating the product fluid production rate, said first controller manipulating the compressor; 
     (G) a second controller for regulating the pressure of refrigerant fluid passed to the compressor, said second controller adjusting the position of the second Joule-Thomson valve; 
     (H) a third controller for regulating product fluid liquid level, said third controller manipulating the product valve; 
     (I) a fourth controller for regulating the temperature of the refrigerant fluid downstream of the aftercooler, the temperature of the refrigerant fluid upstream of the first Joule-Thomson valve and the liquid level in the separator, said fourth controller controlling the position of the cooling fluid input valve and also adjusting the position of the first Joule-Thomson valve; and 
     (J) a fifth controller for regulating the pressure of refrigerant fluid discharged from the compressor, said fifth controller adjusting the position of the surge tank inlet valve and also adjusting the position of the surge tank outlet valve. 
     As used herein the term “controller” means a device that either directly manipulates or causes the manipulation of one or more pieces of plant equipment based on the value of one or more process measurements and the value of other inputs from either an operator or some other device. 
     As used herein the term “Joule-Thomson valve” means a valve that is used to provide cooling from the expansion of a gas. 
     As used herein the term “phase separator” means a vessel wherein incoming fluid is separated into individual vapor and liquid fractions, typically by gravity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of one preferred embodiment of the control system of this invention. 
     FIG. 2 is a logic diagram for the preferred control system illustrated in FIG.  1 . 
    
    
     The numerals and symbols used in the Drawings are the same for the common elements. 
     DETAILED DESCRIPTION 
     The invention employs a partially decentralized control system. A centralized controller is a controller that takes into account the interaction between all of the variables. It modulates all of the manipulated variables simultaneously based on the present and past values of all controlled variables, usually according to a rule that optimizes the future trajectory of the controlled variables. A fully de-centralized control system uses distinct controllers that act independently of each other. Each controller monitors a single controlled variable and modulates a single manipulated variable. The advantages of a fully centralized control system are that it can usually achieve better control performance because it accounts for interactions and, because of the way the control algorithms are designed, it can provide a more uniform control philosophy for dissimilar processes. The disadvantages of a fully centralized controller are that it can be difficult to tune and maintain and it can also be more expensive to implement. A fully decentralized control system, while much easier to operate, does not provide the performance desirable for top level units. 
     The invention will be described in detail with reference to the Drawings. In FIGS. 1 and 2 the following variables are identified as indicated below. 
     FI 1 —Product Fluid Production Rate 
     PI 1 —Cold Refrigerant Fluid Pressure 
     LI 1 —Product Fluid Liquid Level 
     LI 2 —Liquid Level in Intermediate Separator 
     TI 1 —Temperature in Intermediate Separator 
     TI 2 —Temperature of Refrigerant Fluid Downstream of Aftercooler 
     PI 2 —Pressure of Refrigerant Fluid Discharged From the Compressor 
     MV 1 —Compressor Control Such As Slide Valve Position or Guide Vane Position 
     MV 2 —Position of Second Joule-Thomson Valve 
     MV 3 —Position of Product Valve 
     MV 4 —Position of First Joule-Thomson Valve 
     MV 5 —Position of Cooling Fluid Input Valve 
     MV 6 —Position of Surge Tank Inlet Valve 
     MV 7 —Position of Surge Tank Outlet Valve 
     Multicomponent refrigerant fluid  1 , which is preferably a multicomponent refrigerant fluid disclosed in U.S. Pat. No. 6,076,372—Acharya et al., is compressed by passage through compressor  2 , which may be a single stage or multi-stage compressor, to a pressure generally within the range of from 14.7 to 300 pounds per square inch absolute to form pressurized refrigerant fluid  3 . The pressurized refrigerant fluid is cooled and partially condensed in aftercooler  4  by indirect heat exchange with cooling fluid  5  provided through cooling fluid input valve  6 . Cooled refrigerant fluid  7  is fed to separator A wherein the liquid and vapor are separated for the purpose of being evenly distributed into the passages of the downstream heat exchanger. 
     Liquid  8  and vapor  9  from separator A are combined to form stream  10  and fed to initial stage  11  of a multistage heat exchanger where the combined stream is cooled against returning low pressure refrigerant fluid. In the embodiment of the invention illustrated in FIG. 1, the multistage heat exchanger comprises initial or warm stage  11  and final or cold stage  21  along with one intermediate stage  17 . As the high pressure refrigerant is cooled more of the vapor condenses. The mixed-phase high pressure refrigerant fluid  12  is passed into an intermediate separator B, which has a vapor exit and a liquid exit, where the vapor and liquid phases are separated and fed to the different refrigerant loops. The liquid  13 , comprising mostly heavy refrigerant, is passed out from the liquid exit and fed to the warm loop, and the vapor  14 , comprising mostly light refrigerant, is passed out from the vapor exit and fed to the cold loop. Liquid refrigerant fluid  13  is flashed to a low pressure across first Joule-Thomson valve  15 . The resulting expansion cools the stream before it is introduced as stream  16  to intermediate stage  17 . Vapor refrigerant fluid  14  from separator B is also fed to intermediate stage  17 . Both refrigerant fluid streams  16  and  14  exchange heat with each other and with the returning low pressure refrigerant resulting in refrigerant streams  18  and  19  respectively. 
     High pressure refrigerant stream  19  is fed to phase separator D wherein any refrigerant that condensed in heat exchanger  17  is separated from vapor. The two phases are then recombined to form stream  20  which is passed to cold or final stage  21 . In final stage  21  the high pressure refrigerant stream is cooled by indirect heat exchange with returning low pressure refrigerant. Downstream of final stage  21  the high pressure refrigerant stream is flashed to a low pressure across second Joule-Thomson valve  22  whereby significant cooling results. Resulting cooled and generally partially condensed refrigerant fluid  23  is passed to phase separator E wherein the liquid is separated from vapor to ensure even distribution in the subsequent heat exchange. The liquid  24  and vapor  25  from separator E are recombined to form stream  26  which is passed to final stage  21  to provide refrigeration for the refrigeration of the product fluid. 
     Product fluid  27  is provided to the multistage heat exchanger wherein it is cooled and may be totally or partially liquefied by indirect heat exchange with the refrigerant fluid. And suitable fluid may be used as the product fluid in the practice of this invention. Preferably the product is an industrial gas among which one can name oxygen, argon and nitrogen. Gas mixtures can also be employed as the product fluid. Typically the product fluid is cooled to a temperature within the range of from 70K to 150K. Cooled product fluid  28  is passed through product valve  29  and recovered as refrigerated product  30 . 
     Low pressure refrigerant fluid  31  emerging from final stage  21  is recombined with refrigerant fluid  37  from phase separator C, which received refrigerant fluid stream  18  from intermediate stage  17 , to form low pressure returning refrigerant stream  33  which passes through intermediate stage  17  and initial stage  11  for providing cooling to both the high pressure refrigerant stream and to the product fluid stream prior to returning to compressor  2  as stream  1 . 
     A surge tank  34  is tied into the process to serve two purposes. The first purpose is to provide surge capacity for the compressor train. The second purpose is to serve as hold-up capacity for refrigerant during turn-down or shutdown. Surge tank inlet valve  35  can be opened to allow material to flow from the compressor discharge in line  37  to surge tank  34 . This reduces the circulating mass in the cycle. Surge tank outlet valve  31  can be opened to allow material to flow from the surge tank in line  38  to the return stream for passage to the compressor suction. 
     There are several process design variations which may be used in the practice of the invention including but not limited to: 
     The compressor could be any type suitable to the compression of the mixed refrigerant (e.g. oil flooded screw, centrifugal, etc.); 
     The compressor could be single or multi-staged and can comprise a single train or multiple trains in parallel; 
     The aftercooler could be water-cooled, could use chilled water, or could be a separate refrigeration cycle; 
     Two or more of the heat exchanger stages could be combined into a single block; 
     The re-combining of material in the warm and cold loops could take place in one of the separators; 
     There could be additional minor process streams to facilitate the transfer of liquid or vapor from one point to another; and 
     Small compressors and pumps could be added to facilitate the transfer of liquid or vapor from one point to another. 
     To better gauge the relative dynamic merits of potential control structures, a linear quadratic regulator problem was solved for each structure to determine sets of optimal controller tunings with respect to the following objective function:        J   =       ∫     t   =   0     T            (         y   T        Qy     +       u   T        Ru       )             t                                
     where y represents the vector of controlled variables and u represents the vector of manipulated variables. The outputs y were scaled according to their allowable ranges and the inputs were scaled according to their spans. For simplicity Q and R were taken to be identity matrices. The Q and R matrices designate the relative weighting of deviations in controlled and manipulated variables. The use of identity matrices sets the relative importance of each variable to be equal. T was selected to be ½ hour. Initial states were taken to lie on the unit sphere. To provide a baseline for comparison, the problem was solved first for a fully centralized controller. The partially de-centralized control structure was significantly more simple than the fully centralized structure and did not sacrifice much in the way of dynamic performance. This control system of this invention is illustrated in FIG.  2 . 
     In the control structure of this invention there are five independent controllers. The first controller GC 1  monitors the production rate, compares it to the desired production rate, and adjusts the compressor operation accordingly (the actuator type is dependent on the type of compressor employed). The second controller GC 2  manipulates the second or cold Joule-Thomson valve position to maintain the cold-end pressure at its setpoint. A third controller GC 3  uses the product valve to control the product liquid level. Controller GC 4  adjusts the first Joule-Thomson valve position and the cooling fluid valve position simultaneously to control the liquid level in separator B, the temperature of separator B, and the temperature of refrigerant fluid leaving the aftercooler. The fifth controller GC 5  monitors the compressor discharge pressure and manipulates the surge tank inlet and outlet valve positions based upon the desired value of the discharge pressure. 
     To better understand the merits of the invention, one can compare the dynamic performance of several control structures and weigh the dynamic performance against simplicity. Table 1 lists the performance, as measured by J, versus the simplicity, as measured by the minimum number of tuning parameters involved, for several control systems. The base-line control system is the one that is fully centralized (all manipulated variables are used to regulate all controlled variables). 
     As can be observed from Table 1 the control structure of the invention offers an excellent trade-off between performance and simplicity. In moving from the fully centralized control structure to the invention, the performance is only degraded by 14% while the number of tuning parameters is decreased by more than three-fold. Further simplifications to the control system result in performance sacrifices greater than 44% and only simplify the system an additional 33%. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Performance and Complexity 
               
               
                 of Control Alternatives 
               
             
          
           
               
                   
                   
                   
                 Complexity (# of 
               
               
                   
                 Structure 
                 Performance (J) 
                 Tuning Params) 
               
               
                   
                   
               
             
          
           
               
                   
                 Fully Centralized 
                 20.48 
                 30 
               
               
                   
                 Invention 
                 23.35 
                 9 
               
               
                   
                 decentralized 
                 44.81 
                 6 
               
               
                   
                   
               
             
          
         
       
     
     There are many possible ways to implement each of the five controllers that are involved in the invention. The most straightforward way to implement the controllers that have single inputs and single outputs is as PI type controllers. This is because the algorithms for PI control are well established, the software needed to implement them is readily available, and tuning methods are simple and well known. 
     The fourth controller, which controls the liquid level in separator B, the temperature of separator B, and the temperature of the refrigerant leaving the aftercooler using the first JT valve position and the aftercooler input valve position can be any form of multi-variable controller or could be single input, single output controllers that interact via overrides. 
     Controller GC 5  can also take advantage of available multi-variable control algorithms, but the most straightforward implementation of this controller is to use two solenoids. One solenoid opens the surge tank inlet valve when the compressor discharge pressure exceeds its target value by a few pounds per square inch. The other solenoid opens the surge tank outlet valve when the compressor discharge pressure is lower than a few pounds per square inch below its target value. 
     Although the invention has been described in detail with reference to a certain particularly preferred embodiment, those skilled in the art will recognize that there are other embodiments of the invention within the spirit and scope of the claims.