Abstract:
Processes using multiple expansion turbines for efficient recovery of power from a plurality of very high pressure streams of superheated vapor are disclosed. Beneficially, processes of the invention use at least two classes of expansion turbines. Processes according to this invention are particularly useful for recovery of power from very high pressure streams of superheated steam in an olefins manufacturing process. Such streams are typically produced by thermal cracking of suitable petroleum derived feed stocks, and the olefins being produced and purified are typically ethylene and/or propylene.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    This invention was made with government support under United States Department of Energy Cooperative Agreement No. DE-FC07-O1ID 14090. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The field of this invention relates to use of multiple expansion turbines for efficient recovery of power from a plurality of very high pressure streams of superheated vapor. More particularly, these power recovery processes employ at least two classes of expansion turbines. Processes according to this invention are particularly useful for recovery of power from a plurality of very high pressure streams of superheated vapor generated in manufacturing petrochemicals. High pressure streams of superheated steam are typically produced during thermal cracking or pyrolysis of suitable petroleum derived feed stocks. For example, superheated steam is generated where olefins, typically ethylene and/or propylene, are produced. 
       BACKGROUND OF THE INVENTION 
       [0003]    As is well known, olefins, or alkenes, are a homologous series of hydrocarbon compounds characterized by having a double bond of four shared electrons between two carbon atoms. The simplest member of the series, ethylene, is the largest volume organic chemical produced today. Olefins including, importantly, ethylene, propylene and smaller amounts of butadiene, are converted to a multitude of intermediate and end products on a large scale, mainly polymeric materials. 
         [0004]    Commercial production of olefins is almost exclusively accomplished by pyrolysis of hydrocarbons in tubular reactor coils installed in externally fired heaters. Thermal cracking feed stocks include streams of ethane, propane or hydrocarbon liquids ranging in boiling point from light straight-run gasoline through gas oil. 
         [0005]    This endothermic process is carried out in a plurality of large pyrolysis furnaces with the expenditure of large quantities of heat which is provided in part by burning the methane produced in the cracking process. After cracking, the reactor effluent is cooled and put through a series of separation steps involving cryogenic separation of products such as ethylene and propylene. The total energy requirements for the process are thus very large and ways to reduce the net energy use of olefins manufacturing facilities are of substantial commercial interest. 
         [0006]    Each of the plurality of pyrolysis furnaces produces a byproduct stream of very high pressure superheated steam. These streams are typically combined and directed to one or more multi-stage expansion turbines which produce power for use in the cryogenic separation system. The efficient conversion of the multiple very high pressure superheated steam streams into mechanical energy is crucial for the economic production of olefins. Processes which allow more efficient conversion of very high pressure steam into mechanical energy or electricity, such as the process of the present invention, beneficially reduce the net energy use of the olefins manufacturing facility. 
         [0007]    It is therefore a general object of the present invention to provide an improved process which overcomes the aforesaid problem of prior art methods for recovery of power from a plurality of very high pressure vapor streams, and generate turbine exhaust streams at a plurality of pressures 
         [0008]    An improved method for recovery expansion power should exhibit higher efficiency thereby providing lower net energy use and therefore lower variable costs of operation. 
         [0009]    Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims. 
       SUMMARY OF THE INVENTION 
       [0010]    Economical processes are disclosed for the use of multiple expansion turbines for efficient recovery of power from a plurality of very high pressure streams of superheated vapor. More particularly processes are disclosed for recovery of power using at least two classes of expansion turbines. Processes according to this invention are particularly useful for recovery of power from very high pressure streams of superheated steam generated in the manufacture of light olefins by the pyrolysis of hydrocarbons in a plurality of furnaces. Heat is removed from the furnaces and/or reactor effluent streams at least in part by the formation and removal therefrom of a plurality of very high pressure steam streams. 
         [0011]    Processes of the invention comprising a power generation system employing a plurality of steam expansion turbines wherein high-pressure steam is expanded to produce power and generate turbine exhaust streams at a plurality of pressures. More particularly, this invention comprises power recovery processes employing at least two classes of expansion turbines, which comprise: (a) expanding a first stream of superheated vapor at first inlet conditions, including temperature and pressure, to obtain at least one first expanded stream of superheated vapor at first intermediate conditions using at least one primary class expansion turbine to thereby recover a first amount of power; (b) combining two or more vapor streams into a single very high-pressure superheated vapor stream; (c) cooling the resulting single very high-pressure stream from step (b) by indirect heat exchange with at least a portion of the first expanded stream from step (a) to provide all or a portion of the first stream of superheated vapor for expansion in step (a), and a resulting heated first expanded stream at second intermediate conditions including a second intermediate temperature; (d) expanding at least a portion of the resulting heated stream from step (c) at second expansion inlet conditions to obtain at least one second expanded stream of superheated vapor at third intermediate conditions using at least one secondary class expansion turbine to thereby recover a second amount of power. 
         [0012]    In a particularly useful aspect of the present invention, three or more vapor streams are combined in step (b) into a single very high-pressure superheated vapor stream. Advantageously, three or more of the vapor streams combined in step (b) into a single very high-pressure superheated vapor stream are derived from a petrochemical process. 
         [0013]    In another aspect of the present invention, the vapor comprises a light organic compound component containing from about 2 to about 4 carbon atoms, for example propane. The temperature differential between the second intermediate temperature and the first inlet temperature is advantageously no more than 100 Fahrenheit degrees. More advantageously, the temperature differential between the second intermediate temperature and the first inlet temperature is no more than 70 Fahrenheit degrees. 
         [0014]    In another particularly useful aspect, this invention comprises power recovery processes employing at least two classes of expansion turbines, which comprise: (a) expanding a first stream of superheated steam at first inlet conditions of temperature and pressure to obtain at least one first expanded stream of superheated steam at first intermediate conditions using at least one primary class expansion turbine to thereby recover a first amount of power; (b) combining three or more streams of very high pressure steam into a single very high-pressure superheated stream; (c) cooling the resulting single very high-pressure stream from step (b) by indirect heat exchange with at least a portion of the first expanded stream from step (a) to provide all or a portion of the first stream of superheated vapor for expansion in step (a), and a resulting heated first expanded stream at second intermediate conditions including a second intermediate temperature; (d) expanding at least a portion of the resulting heated stream from step (c) at second expansion inlet conditions to obtain at least one second expanded stream of superheated steam at third intermediate conditions using at least one secondary class expansion turbine to thereby recover a second amount of power. 
         [0015]    A particularly useful aspect of the present invention further comprises treating at least a portion of one or more second expanded stream of superheated steam from step (d) to thereby provide at least a portion of the resulting single very high-pressure stream of step (b). Beneficially, three or more of the vapor streams combined in step (b) into a single very high-pressure superheated vapor stream are generated in a process for thermal cracking of suitable petroleum derived feed stocks to produce olefins, and advantageously the olefins being produced are ethylene and/or propylene. 
         [0016]    In another particularly useful aspect of this invention three or more of the vapor streams combined in step (b) into a single very high-pressure superheated vapor stream are generated in a process for the manufacture of light olefins by the pyrolysis of hydrocarbons in a plurality of furnaces from which heat is removed in a plurality of very high pressure streams of superheated steam. The first inlet pressure is at least 900 psig, and/or the first inlet temperature is at least 800° F. In yet another useful aspect of this invention, the second intermediate temperature is no more than 100 Fahrenheit degrees below the first inlet conditions temperature. 
         [0017]    Yet another particularly useful aspect of the invention further comprises partially desuperheating the second expanded stream of superheated steam from step (d) by indirect heat exchange with a cooling medium to provide a supply of superheated low-pressure steam. More advantageously, the cooling medium is boiler feed water, and at least a portion of the heated boiler feed water is used to produce at least a portion of the very high pressure steam streams of step (b). 
         [0018]    Efficiency is improved in accordance with this invention for any power recovery steam cycle in which there are a relatively large number of steam generation and superheating units and where there are multiple pressure levels from which steam is expanded to produce power. The reheating of an intermediate-pressure steam, which has been extracted from an expansion turbine, in a single location by desuperheating a higher-pressure steam that has been superheated in the multiple steam superheating units is critical for best results. This invention avoids the complex and expensive system that would be needed to distribute and then re-collect the intermediate pressure steam if it were reheated in the multiple superheating units. 
         [0019]    For a more complete understanding of the present invention, reference should now be made to the embodiments illustrated in greater detail in the accompanying drawing and described below by way of examples of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0020]    The appended claims set forth those novel features which characterize the present invention. The present invention itself, as well as advantages thereof, may best be understood, however, by reference to the following brief description of preferred embodiments taken in conjunction with the annexed drawings, in which: 
           [0021]      FIG. 1  is a schematic diagram of a comparative power recovery process for the steam system in an olefins manufacturing thermal cracking unit. 
           [0022]      FIG. 2  is a schematic diagram of an embodiment of this invention in which intermediate-pressure steam is heated and low-pressure steam is desuperheated. 
           [0023]      FIG. 3  is a schematic diagram of another embodiment of this invention in which reheated intermediate-pressure steam is reheated and distributed to multiple expansion turbines. 
       
    
    
       [0024]    It should be noted that only essential expansions and heating/cooling steps are shown in these schematic diagrams. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0025]    Hydrocarbon cracking processes have been commonly employed in the petroleum and allied industries for several decades, and many commercial cracking processes have been the subject of much academic and commercial interest. Cracking consists of breaking down the hydrocarbon molecules into smaller molecules, usually at a higher temperature. There are generally two types of cracking, thermal cracking and catalytic cracking, which utilize either the effect of temperature alone or in combination with the active sites of a catalyst. 
         [0026]    In a conventional thermal cracking unit, the hydrocarbon feedstock is gradually heated in a tubular furnace. The thermal cracking reaction takes place mainly in the portion of the tubes receiving the maximum heat flow, and the desired temperature is determined by the nature of the hydrocarbons to be cracked. 
         [0027]    In general, a cracking unit includes a plurality of pyrolysis furnaces. Each furnace includes a tubular or plug-flow reactor through which feedstock flows and in which the feedstock is thermally decomposed. A pyrolysis furnace is designed to transfer heat to internal reactor tubes which are conventionally arranged in three sections: a convection section, in which the hydrocarbon feedstock is preheated and very high pressure steam is superheated; a radiant section, in which the preheated hydrocarbon feedstock is thermally decomposed to olefins, diolefins, and aromatics; and a quench section where the cracked gas furnace effluent from the radiant section is cooled through the generation of very high pressure steam. 
         [0028]    The literature is replete with disclosures of suitable pyrolytic furnaces for the thermal cracking of hydrocarbons. For example, U.S. Pat. No. 5,271,809 in the name of Hans-Joachim Holzhausen. Pyrolytic furnaces advantageously comprise a radiation zone including burners and cracking tubes in the radiation zone consisting of parallel, vertically extending linear tube sections joined to one another by tube elbows located in the bottom region of the radiation zone. At least four cracking tubes are combined into groups uniformly arranged in the radiation zone, each group of cracking tubes being united in an outlet tube via manifold tube sections wherein the linear tube sections and the manifold tube sections of the individual groups are arranged in one row in the transverse direction of the pyrolytic furnace. 
         [0029]    When light olefins and monoaromatic compounds are to be produced, the necessary temperature is and generally ranges from about 1,440° F. to about 1,600° F., depending on the type of feedstock to be cracked, but is limited by the operating conditions of the process and by the operating complexity of the furnaces, which use supplementary heating energy. 
         [0030]    Suitable light hydrocarbon fraction or fractions may be advantageously chosen from the group consisting of light paraffins, such as ethane, propane and the butanes, and heavier hydrocarbons such as gasolines, naphthas and gas oils, and even certain higher-boiling but strongly paraffinic or naphthenic fractions, such as the paraffins or slack wax or the hydrocarbon recycles. These hydrocarbon fractions may come from different units of the refinery, for example the atmospheric distillation, visbreaking, hydrocracking, oil manufacturing or olefin oligomerization units, or from the effluents of the conversion unit itself. Additionally, the various fractions may be injected either alone or in combination with steam and optionally other gases such as hydrogen or light gases. 
         [0031]    Each olefins-producing pyrolysis furnace typically produces one or more streams of high-pressure superheated steam as a byproduct of the furnace operation. The steam is typically generated through the quenching of hot furnace effluent gases, and then superheated in the convective section of the pyrolysis furnace. The maximum temperature to which the steam is superheated is typically limited by the maximum inlet temperature of the expansion turbine to which the superheated steam is fed. The maximum inlet temperature of the expansion turbine is in turn a function of the design and metallurgy employed in the turbine. The maximum inlet temperature of steam turbines is typically in the range of 980 to 1000° F. 
         [0032]    The superheated steam streams are beneficially combined and enter one or more multi-stage expansion turbines. These turbines produce mechanical and/or electrical power which is beneficially used in the recovery and purification of olefins. 
         [0033]    The recovery and purification of light olefins such as ethylene and propylene from the furnace effluent is an energy-intensive process. A typical ethylene recovery and purification section comprises a cracked gas compressor to compress the quenched furnace effluent stream to a relatively high pressure, typically between 200-500 psig. At least a portion of the mechanical energy required for cracked gas compression is produced through the expansion of the very-high pressure steam generated in the pyrolysis furnaces. 
         [0034]    The ethylene contained in the compressed cracked gas is then typically recovered and purified through cryogenic distillation. While the design of the ethylene recovery and purification section admits of many variations, it typically contains a deethanizer tower to separate C3 and heavier material from the ethylene-containing stream, a demethanizer tower to separate methane and lighter material from the ethylene-containing stream, and a C2 splitter tower to separate ethylene from ethane. Such distillation steps are typically cryogenic in nature, that is they are carried out at temperatures below ambient temperature. They therefore demand significant amounts of process refrigeration. At least a portion of the mechanical energy required for providing the process refrigeration is produced through the expansion of steam generated in the pyrolysis furnaces. 
         [0035]    In the course of its extensive work in this field, the applicants have found that use of multiple expansion turbines increases the efficiency in the recovery of power from a plurality of very high pressure streams of superheated vapor generated and superheated among a relatively large number of furnaces. More particularly, power recovery processes in accordance with the invention employ at least two classes of expansion turbines. Reheating of intermediate-pressure exhaust or extraction vapor from the primary class of expansion turbines is critical for improving the efficiency of the overall power recovery system. 
         [0036]    This invention is useful for power generation systems from a plurality of streams at very high pressure that use any working fluid, though steam is by far the most common. Processes of this invention are particularly suitable for use in the thermal cracking of hydrocarbons, for example, using steam streams from a plurality of thermal cracking furnaces. 
       BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
       [0037]    While this invention is susceptible of embodiment in many different forms, this specification and accompanying drawings disclose only some specific forms as an example of the use of the invention. In particular, a preferred embodiment of the invention for recovery of mechanical and/or electrical power from a plurality of high pressure superheated vapor streams is illustrated and described. The invention is not intended to be limited to the embodiment so described, and the scope of the invention will be pointed out in the appended claims. 
         [0038]    The apparatus of this invention is used with certain conventional components the details of which, although not fully illustrated or described, will be apparent to those having skill in the art and an understanding of the necessary function of such components. Various values of compositions, flow rates, temperatures, and pressures are given in association with a specific example described below; those conditions are approximate and merely illustrative, and are not meant to limit the invention. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0039]    This invention represents an improved, more energy-efficient method for utilizing high-pressure superheated vapor generated from multiple sources to generate mechanical energy through the use of expansion turbines. It can be utilized with any high-pressure superheated vapor, but a common use would be in a steam system where the vapor is water vapor. For ease of understanding the invention will be described in terms of an improved steam system for the generation of power within an olefins manufacturing complex. It should be noted that the concept and methods of this invention are not limited to this application. 
         [0040]    In olefins manufacture, high-pressure steam is generated in a number of cracking furnaces. The number of cracking furnaces in a particular olefins unit will depend on many factors, including the capacity of the olefins manufacturing unit, the capacity of the furnaces, and the design of the furnaces. Typically between four and 12 furnaces are utilized within an olefins manufacturing complex. Each of these furnaces produces a stream of superheated steam as a byproduct of the olefin-producing process. These streams are typically combined and then directed to steam turbines to produce mechanical energy. The mechanical energy thus produced is typically used to compress the olefin-containing gas and to drive machinery designed to provide refrigeration to the olefins process. 
         [0041]      FIG. 1  depicts a schematic diagram of a portion of a conventional steam system for an olefins manufacturing unit. This schematic contains only the major heat transfer and power production steps that are required to understand the basic operation of such a steam system, and to allow comparison with the current invention. Those skilled in the art will recognize that olefins unit steam systems admit to many variations in design, but most contain the steps outlined in  FIG. 1 . 
         [0042]    Very high pressure superheated steam is generated by the multiple furnaces and combined into a single very high-pressure steam header line depicted as stream  1 . The temperature and pressure of stream  1  can vary significantly between units. Stream  1  is typically at a pressure of at least 900 psig and a temperature of at least 900° F. 
         [0043]    The entirety of this very high pressure steam is typically directed to steam turbine  2 . This steam turbine expands the very high-pressure steam to produce power for other parts of the process. Typically, the power derived from steam turbine  2  would be used to drive a cracked gas compressor to compress the cooled olefin-containing furnace effluent gas to a higher pressure. Turbine  2  is shown as an extracting turbine, with two stages (stage  2   a  and stage  2   b ) which are typically mechanically coupled. High-pressure steam (typically at about 600 psig) is recovered from stage  2   a  as stream  3 . 
         [0044]    A portion of stream  3  is directed as stream  4  to stage  2   b  of the turbine and withdrawn as stream  5 . Stream  5  is typically recovered at as low a pressure as feasible (typically under vacuum) and condensed against a near-ambient cooling medium. 
         [0045]    Another portion of stream  3  is directed as stream  6  to the high-pressure steam header. Portions of the high-pressure steam from the header, depicted as streams  7  and  8 , can be directed to other steam turbines, depicted as  9  and  10 . It is understood that more or fewer turbines can be fed by the high-pressure steam header, depending on the needs of the olefins process. In order to simplify the  FIG. 1 , only two turbines are depicted. 
         [0046]    A further portion of the high pressure steam can be directed as stream  11  to one or more heat exchangers to provide heating to one or more units in the olefins process. While a single exchanger  12  is shown in  FIG. 1 , it is understood that it may represent multiple heat exchangers in a commercial olefins facility. The condensate stream  13  from exchanger  12  is withdrawn as shown and at least a portion is typically re-used as boiler feed water for the process. A final portion of the high-pressure steam can be exported as stream  14  to another process or otherwise used within the olefins unit. 
         [0047]    In  FIG. 1  steam turbine  10  is shown as an extracting turbine, with two stages (stage  10   a  and stage  10   b ) which are typically mechanically coupled. Low-pressure steam (typically at about 65 psig) is recovered from stage  10   a  as stream  15 . 
         [0048]    A portion of stream  15  directed as stream  16  to stage  10   b  of the turbine and withdrawn as stream  17 . Stream  17  is typically recovered at as low a pressure as feasible (typically under vacuum) and condensed against a near-ambient cooling medium. It should be noted that this turbine could produce more than two expanded steam streams, each at different pressure levels. In practice, turbine  10  could, for example, provide power to drive a refrigeration compressor in a commercial olefins unit. 
         [0049]    Another portion of stream  15  is directed as stream  18  to the low-pressure steam header, along with stream  19 , the expanded high-pressure steam from turbine  9 . The majority of the low-pressure steam is typically withdrawn as stream  20  and used for process heating needs in exchanger  21  as shown. The single exchanger  21  in  FIG. 1  would typically represents a number of separate exchangers in the commercial unit. The condensate stream  22  from exchanger  21  is withdrawn as shown and at least a portion is typically re-used as boiler feed water for the process. A further portion of the low-pressure steam can be exported as stream  23  to another process or otherwise used within the olefins unit. 
         [0050]      FIG. 2  depicts a preferred embodiment of the present invention, wherein reheat of the high-pressure steam and desuperheating of the low-pressure steam is accomplished. Very high-pressure superheated steam from each of the olefins cracking furnaces is combined as shown and directed to the very high-pressure steam header stream  30 . It is a characteristic of the current invention that stream  30  is superheated in the furnaces to a significantly higher temperature than the corresponding stream  1  of  FIG. 1 . Stream  30  is partially de-superheated in the reheat exchanger  31 . The resulting very high-pressure steam stream  32  exits exchanger  31  at a temperature roughly similar to that of stream  1  of  FIG. 1 . The maximum temperature of stream  32  is typically limited by the design and metallurgy of the downstream expansion turbine  33 . 
         [0051]    Stream  32  is directed to steam turbine  33 , which provides similar functionality as turbine  2  of  FIG. 1 . Turbine  33  is shown as an extracting turbine, with two stages (stage  33   a  and stage  33   b ) which are typically mechanically coupled. High-pressure steam (typically at about 600 psig) is recovered from stage  33   a  as stream  34 . A portion of stream  34  is directed as stream  35  to stage  33   b  of the turbine and withdrawn as stream  36 . Stream  36  is typically recovered at as low a pressure as feasible (typically under vacuum) and condensed against a near-ambient cooling medium. 
         [0052]    Another portion of stream  34  is directed as stream  37  to the reheat exchanger  31  where it is reheated against the desuperheating very high-pressure steam stream  30 . The reheated high-pressure stream  38  is directed to the high-pressure steam header as shown. It is a characteristic of this invention that the high-pressure steam stream  38  entering the high-pressure steam header of  FIG. 2  is at a higher temperature than the corresponding high-pressure steam stream  6  in the conventional steam system of  FIG. 1 . 
         [0053]    Portions of the high-pressure steam from the high-pressure steam header, depicted as streams  39  and  40 , can be directed to other steam turbines, depicted as  41  and  42 . It is understood that more or fewer turbines can be fed by the high-pressure steam header, depending on the needs of the olefins process. In order to simplify the  FIG. 2 , only two turbines are depicted. 
         [0054]    A further portion of the high-pressure steam can be directed as stream  43  to one or more heat exchangers to provide heating to one or more units in the olefins process. While a single exchanger  44  is shown in  FIG. 2 , it is understood that it may represent multiple heat exchangers in a commercial olefins facility. The condensate stream  45  from exchanger  44  is withdrawn as shown and at least a portion is typically re-used as boiler feed water for the process. A final portion of the high-pressure steam can be exported as stream  46  to another process or otherwise used within the olefins unit. 
         [0055]    Alternatively, the high-pressure steam export can be taken as stream  47  at a point before the high-pressure steam reheater. This could be advantageous if the external high-pressure steam users are not equipped to utilize the hotter high pressure steam represented by stream  46 . 
         [0056]    In  FIG. 2  steam turbine  42  is shown as an extracting turbine, with two stages (stage  42   a  and stage  42   b ) which are typically mechanically coupled. Superheated low-pressure steam (typically at about 65 psig) is recovered from stage  42   a  as stream  48 . 
         [0057]    A portion of stream  48  is directed as stream  49  to stage  42   b  of the turbine and withdrawn as stream  50 . Stream  50  is typically recovered at as low a pressure as feasible (typically under vacuum) and condensed against a near-ambient cooling medium. It should be noted that this turbine could produce more than two expanded steam streams, each at different pressure levels. In practice, turbine  42  could, for example, provide power to drive a refrigeration compressor in a commercial olefins unit. 
         [0058]    Another portion of stream  48 , stream  51 , is combined with the expanded superheated steam stream  52  from turbine  41  and the combined stream  53  enters the desuperheater exchanger  54 . It is a characteristic of this invention that the low-pressure steam streams  51  and  52  are at a higher temperature than the corresponding low-pressure steam streams  18  and  19  in the conventional steam system of  FIG. 1 . 
         [0059]    Stream  53  is at least partially desuperheated in exchanger  54  to produce the low-pressure steam stream  55 . Cooling for the desuperheater exchanger  54  can be supplied by any suitable cooling medium. For example, a relatively cool boiler feed water stream  56  could be used as the cooling medium to produce a relatively warmer boiler feed water stream  57 , thereby recovering heat within the steam system and improving the overall efficiency of the process of the present invention. 
         [0060]    The low-pressure steam stream  55  from the desuperheater exchanger enters a low-pressure steam header as shown. The majority of the low-pressure steam is typically withdrawn as stream  58  and used for process heating needs in exchanger  59  as shown. The single exchanger  59  in  FIG. 2  would typically represents a number of separate exchangers in a commercial unit. The condensate stream  60  from exchanger  59  is withdrawn as shown and at least a portion is typically re-used as boiler feed water for the process. A further portion of the low-pressure steam can be exported as stream  61  to another process or otherwise used within the olefins unit. 
         [0061]      FIG. 3  depicts an alternate configuration of the high-pressure steam reheat section of the present invention. It is similar in function to the reheat section of  FIG. 2 , but the high pressure steam to the second stage of the first turbine is reheated before entering the second stage. 
         [0062]    Very high-pressure superheated steam from each of the olefins cracking furnaces is combined as shown and directed to the very high-pressure steam header stream  70 . Stream  70  is partially de-superheated in the reheat exchanger  71 . The resulting very high-pressure steam stream  72  exits exchanger  71  at a temperature roughly similar to that of stream  1  of  FIG. 1  and stream  32  of  FIG. 2 . 
         [0063]    Stream  72  is directed to steam turbine  73 . Turbine  73  is shown as an extracting turbine, with two stages (stage  73   a  and stage  73   b ) which are typically mechanically coupled. High-pressure steam (typically at about 600 psig) is recovered from stage  73   a  as stream  74 . If desired, a portion of the high pressure steam can be exported from the process a stream  75 . The remainder of the steam is directed as stream  76  to the reheat exchanger  71  where it is reheated against the desuperheating very high pressure steam. 
         [0064]    A portion of the reheated stream  77  is directed as stream  78  to stage  73   b  of the turbine and withdrawn as stream  79 . Stream  79  is typically recovered at as low a pressure as feasible (typically under vacuum) and condensed against a near-ambient cooling medium. 
         [0065]    Another portion of stream  77  is directed as stream  80  to the high-pressure steam header as shown. For simplicity the remainder of the steam process is not depicted in  FIG. 3 , but it is understood that it can be similar in nature to that of  FIG. 2  (where stream  80  of  FIG. 3  corresponds to stream  38  of  FIG. 2 ), or it can be of a different configuration. 
         [0066]      FIGS. 2 and 3  depict two configurations which utilize the concept of recovering superheat from a combined vapor stream in order to re-heat at least a portion of a lower-pressure vapor stream which has been extracted from an expansion turbine. Those skilled in the art will recognize that, once the basic concept is grasped, other configurations can be developed, and all such configurations are covered within the scope of this invention. 
       EXAMPLE OF THE INVENTION 
       [0067]    The following Example will serve to illustrate a certain specific embodiment of the herein disclosed invention. This Example should not, however, be construed as limiting the scope of the novel invention as there are many variations which may be made thereon without departing from the spirit of the disclosed invention, as those of skill in the art will recognize. 
       General 
       [0068]    To demonstrate several beneficial aspects of the present invention, both the comparative process depicted in  FIG. 1  and the embodiment of  FIG. 2  were simulated using commercially available process simulation software. 
       Comparative Example 
       [0069]    Following is an example of a conventional steam system configuration for an olefins manufacturing unit. The design of this conventional steam system is similar to that shown in  FIG. 1 , and all stream and unit numbers in this example refer to those in  FIG. 1 . Very high-pressure steam at a temperature of 980° F. and a pressure of 1800 psig is generated from multiple furnaces. High-pressure steam is extracted from turbine  2  at 600 psig, while the low-pressure header operates at 50 psig. No high-pressure steam is exported in this case. 
         [0070]    Turbine  2  generates approximately 112,000 HP. Turbine  10  represents the combination of two separate refrigeration turbines which generate a total of 33,800 HP. Turbine  9  represents a number of smaller turbines which combined generate approximately 4,900 HP. 
         [0071]    Stream flows and conditions for this example are given in Table 1. Stream numbers correspond to those of  FIG. 1 . A total of 941,500 lb/hr of very high pressure steam is used. 
       Example of the Present Invention 
       [0072]    Following is an example of a steam system configuration of the present invention for an olefins manufacturing unit. This novel steam system incorporates the high-pressure steam reheat and low-pressure steam desuperheating functions contained within the process of this invention. The steam system of this example is similar to that shown in  FIG. 2 , and all stream and unit numbers in this example refer to those in  FIG. 2 . Very high-pressure steam at a temperature of 1090° F. and a pressure of 1800 psig is generated from multiple furnaces. High-pressure steam is extracted from turbine  33   a  at 605 psig, and experiences a 5 psi pressure drop across the reheat exchanger so that the high-pressure header operates at 600 psig. The low-pressure header operates at 50 psig. 
         [0073]    No export steam was taken through either streams  45  or  47 . The amount of steam withdrawn as stream  37  was set so as to maintain a temperature of 980° F. in stream  38 . In addition, the amount of low-pressure steam produced by both the current and previous examples was kept constant. 
         [0074]    Further, in recognition that the olefins furnaces can provide a finite duty for steam generation, the furnace convective bank duty required to generate the very high pressure steam stream of the present invention (stream  30 ) was approximately equal to that required for the previous example (stream  1 ). Stream  30  contains 917,000 lb/hr of steam at 1800 psig and 1090° F. 
         [0075]    Note that although the total convective bank furnace duties in these two examples are approximately equal, there are differences in how the duty is utilized to generate steam. In the invention of the present invention, boiler feed water is preheated by superheated expanded steam in exchanger  54  of  FIG. 2 . Therefore, compared with the comparative example, in the process of the present invention the furnace convection section provides relatively less preheating of the boiler feed water and relatively more superheating of the very high pressure steam. The result is that slightly less very high pressure steam is generated, but it is at a higher final temperature. 
         [0076]    Stream flows and conditions for this example are given in Table 2. Stream numbers correspond to those of  FIG. 2 . 
         [0077]    The improved efficiency of the present invention is manifested in increased power production in turbines  33  and  42  as compared to turbines  2  and  10 . Table 3 compares the turbine power generation results from the above examples. It is clear that the higher efficiency of the system of the present invention ( FIG. 2 ) produces over 4000 HP more power than a conventional steam system, from similar furnace steam duty. 
         [0078]    It should be noted that the improved efficiency of the process of this invention can be manifested in a number of ways. One is the ability to produce more usable power from the same furnace steam duty, as demonstrated above. It may be desirable instead to produce similar power as the conventional system, but with reduced furnace steam duty or through the export of high-pressure steam. These and other methods of taking advantage of the increased efficiency of the present invention will be apparent to those skilled in the art. 
         [0079]    An Example has been presented and hypotheses advanced herein in order to better communicate certain facets of the invention. The scope of the invention is determined solely by the scope of the appended claims. 
         [0080]    For the purposes of the present invention, “predominantly” is defined as more than about fifty percent. “Substantially” is defined as occurring with sufficient frequency or being present in such proportions as to measurably affect macroscopic properties of an associated compound or system. Where the frequency or proportion for such impact is not clear, substantially is to be regarded as about twenty percent or more. The term “a feedstock consisting essentially of” is defined as at least 95 percent of the feedstock by volume. The term “essentially free of” is defined as absolutely except that small variations which have no more than a negligible effect on macroscopic qualities and final outcome are permitted, typically up to about one percent. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Comparative Example 
               
             
          
           
               
                   
                   
                 Steam Flow 
                 Temperature 
                 Pressure 
               
               
                   
                 Stream 
                 (lb/hr) 
                 (Deg F.) 
                 (psia) 
               
               
                   
                   
               
             
          
           
               
                   
                 1 
                 941,475 
                 980 
                 1,815 
               
               
                   
                 5 
                 511,524 
                 126 
                 2 
               
               
                   
                 6 
                 429,951 
                 710 
                 615 
               
               
                   
                 7 
                 97,090 
                 710 
                 615 
               
               
                   
                 8 
                 332,861 
                 710 
                 615 
               
               
                   
                 11 
                 0 
                 N/A 
                 N/A 
               
               
                   
                 14 
                 0 
                 N/A 
                 N/A 
               
               
                   
                 17 
                 158,385 
                 126 
                 2 
               
               
                   
                 18 
                 174,476 
                 310 
                 65 
               
               
                   
                 19 
                 97,090 
                 392 
                 65 
               
               
                   
                 23 
                 0 
                 N/A 
                 N/A 
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Example of This Invention 
               
             
          
           
               
                   
                   
                 Steam Flow 
                 Temperature 
                 Pressure 
               
               
                   
                 Stream 
                 (lb/hr) 
                 (Deg F.) 
                 (psia) 
               
               
                   
                   
               
             
          
           
               
                   
                 30 
                 917,000 
                 1,089 
                 1,815 
               
               
                   
                 32 
                 917,000 
                 980 
                 1,810 
               
               
                   
                 36 
                 505,714 
                 126 
                 2 
               
               
                   
                 37 
                 411,286 
                 712 
                 620 
               
               
                   
                 38 
                 411,286 
                 980 
                 615 
               
               
                   
                 39 
                 76,086 
                 980 
                 615 
               
               
                   
                 40 
                 335,200 
                 980 
                 615 
               
               
                   
                 43 
                 0 
                 N/A 
                 N/A 
               
               
                   
                 46 
                 0 
                 N/A 
                 N/A 
               
               
                   
                 47 
                 0 
                 N/A 
                 N/A 
               
               
                   
                 50 
                 139,720 
                 126 
                 2 
               
               
                   
                 51 
                 195,480 
                 511 
                 65 
               
               
                   
                 52 
                 76,086 
                 622 
                 65 
               
               
                   
                 55 
                 271,566 
                 308 
                 62 
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Turbine Power 
               
             
          
           
               
                 Comparative Example 
                 Example of This Invention 
               
               
                 (FIG. 1) 
                 (FIG. 2) 
               
             
          
           
               
                 Turbine Number 
                 Power (HP) 
                 Turbine Number 
                 Power (HP) 
               
               
                   
               
             
          
           
               
                 2a + 2b 
                 110923 
                 33a + 33b 
                 108882 
               
               
                 9 
                 4889 
                 41 
                 4888 
               
               
                 10a + 10b 
                 33800 
                 42a + 42b 
                 40269 
               
               
                 Total 
                 149612 
                 Total 
                 154039