Patent Publication Number: US-11649199-B2

Title: Refrigeration recovery from reactor feed in a propane dehydrogenation system

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a divisional application from U.S. patent application Ser. No. 17/016,530 filed Sep. 10, 2020, now U.S. Pat. No. 11,306,045, which claims the benefit of U.S. Provisional Patent Application Ser. No. 62/898,439 filed Sep. 10, 2019, both of which are incorporated herein in their entireties by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a process for recovering propylene from a propane dehydrogenation (PDH) process, and more particularly relates to recovering refrigeration from reactor feed in a PDH process. 
     BACKGROUND 
     Propane dehydrogenation (PDH) is a process step in the production of propylene from propane. PDH is important to the petrochemical industry because propylene is the second most important starting product in the petrochemical industry after ethylene. The purpose of a deethanizer and chilling train systems in a PDH process is to separate the cracked gas into a methane-rich tail gas product, a C2 process stream and a C3 process stream. 
     It would always be beneficial to improve a PDH process, such as by reducing refrigeration power requirements and/or reducing the size and cost of the compressor system. In particular, the refrigeration power required to provide process cooling in the −5 to 30° C. range typically is high. Presently, the depropanizer column overhead stream is produced as a vapor product, which feed to the reactor system without recovery of refrigeration. Process cooling in the −5 to 30° C. range is provided through propylene refrigeration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a non-limiting, schematic illustration of a portion of a PDH process as described herein. 
     
    
    
     SUMMARY 
     In one non-limiting embodiment, there is provided a process for recovering propylene from a propane dehydrogenation process, where the process includes partially or fully condensing a depropanizer overhead stream against a relatively warm cooling medium at relatively high pressure; letting down the liquid overhead stream in pressure; and partially or fully vaporizing the liquid overhead stream against at least one process stream to provide refrigeration. 
     In another non-restrictive version, there is provided a system for recovering propylene from a propane dehydrogenation process, where the system includes a depropanizer column generating a depropanizer overhead stream, at least one condenser for partially or fully condensing the depropanizer overhead stream, at least one device for letting down the pressure of the depropanizer overhead stream as determined by back-pressure of a reactor, and at least one chiller for partially or fully vaporizing the depropanizer overhead stream. 
     DETAILED DESCRIPTION 
     In a typical propane dehydrogenation process, fresh propane-rich feed as well as the propane-rich recycle from a C3 Splitter passes through a Depropanizer column to remove C4+ components. The purified propane-rich stream is recovered as a vapor overhead product that feeds directly to the reactor system without refrigeration heat recovery. 
     In one non-limiting embodiment, it has been discovered that the Depropanizer column overhead stream may be partially or fully condensed against a relatively warm cooling medium such as cooling water at relatively high pressure. In one non-limiting embodiment, “relatively warm” is defined as between about 20° C. independently to about 50° C., alternatively between about 28° C. independently to about 34° C.; where the term “independently” when used with reference to a range means any threshold may be used with any other threshold to give a suitable alternative range. In another non-restrictive version “relative high pressure” is defined as between about 10 independently to about 20 barg; alternatively between about 12 barg independently to about 15 barg. Other cooling media besides water may include, but not necessarily be limited to, air cooling. 
     The liquid overhead stream is then let down in pressure (based on back-pressure from the reactor) and vaporized against process streams to provide refrigeration in the temperature range of about −10° C. independently to about +30° C.; alternatively of about −5° C. independently to about 10° C., and offset the requirement for refrigeration typically provided by a propylene or propane refrigeration system in the same temperature range. 
     In more detail, and with reference to  FIG.  1   , dry process gas  1  from a Process Gas Dryer (not shown—coming from the left of  FIG.  1   ) is chilled against Propane in First Deethanizer Feed Chiller  20  and sent to the First Deethanizer Feed Drum  22  as gas  2 . Liquid from First Deethanizer Feed Drum  22  is fed to Deethanizer column  24 . The overhead vapors go to Process Gas Compressor (PGC)  26 . The discharge gas  3  from Process Gas Compressor  26  is cooled and partially condensed against propane in Second Deethanizer Feed Chiller  28 , and is sent as stream  4  followed by chilling by propylene refrigerant in Third Deethanizer Feed Chiller  30  and sent to Second Deethanizer Feed Drum  32 . Liquid from Second Deethanizer Feed Drum  32  is fed to Deethanizer  24  column. Also shown is Deethanizer Reboiler  38 . 
     A pumparound on the Deethanizer Column  24  upper section  5  via pump  36  uses propane in Deethanizer First Side Cooler  46  and mid-level propylene refrigerant  6  in Deethanizer Second Side Cooler  48  to reduce the requirement for low-level refrigerant on an overhead condenser. 
     Liquid Propane  7  from the Depropanizer Column overhead stream (coming from the left of  FIG.  1   —Depropanizer not shown) is let down in pressure (as determined by reactor back-pressure), in one non-limiting embodiment by using valves  40 ,  42 , and  44 , and fully vaporized in the combination of Second Deethanizer Feed Chiller  28 , First Deethanizer Feed Chiller  20  and Deethanizer First Side Cooler  46 . Vaporized propane  9  from the chillers is routed to Cold Box  34  to be reheated and from the Cold Box  34  to the reactor as propane stream  10 . 
     Non-limiting examples of stream information are given in Table I, whereas non-limiting examples of duties for the chillers are given in Table II. 
     
       
         
           
               
             
               
                 TABLE I 
               
             
            
               
                   
               
               
                 EXAMPLE STREAM INFORMATION 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Stream Number 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 9 
                 10 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Propane 
               
               
                   
                   
                 Gas 
                   
                 Gas 
                 From 
                 From 
                 Propane 
                 Propane 
                 from 
               
               
                   
                 Gas to 
                 from 
                 Gas to 
                 from 
                 36 to 
                 46 to 
                 from 
                 from 
                 cold 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Description 
                 Units 
                 20 
                 20 
                 28 
                 28 
                 46 
                 48 
                 Depropanizer 
                 chillers 
                 box 34 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Vapor Fraction 
                   
                 1.00 
                 0.78 
                 1.00 
                 0.80 
                 0.00 
                 0.00 
                 0.00 
                 1.00 
                 1.00 
               
               
                 Temperature 
                 ° C. 
                 10.0 
                 −1.8 
                 29.7 
                 −1.6 
                 14.2 
                 4.3 
                 42.3 
                 −5.7 
                 38.0 
               
               
                 Pressure 
                 kg/cm 2   
                 13.9 
                 13.7 
                 21.9 
                 21.7 
                 12.8 
                 12.8 
                 15.0 
                 4.0 
                 3.9 
               
               
                 Mass Density 
                 kg/cum 
                 15.8 
                 20.1 
                 18.1 
                 24.0 
                 497.6 
                 513.2 
                 462.6 
                 8.6 
                 6.8 
               
               
                 Total Mass Flow 
                 kg/hr 
                 121969 
                 121969 
                 75698 
                 75698 
                 500000 
                 500000 
                 194605 
                 194605 
                 194605 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE II 
               
             
            
               
                   
               
               
                 EXAMPLE DUTIES 
               
            
           
           
               
               
               
            
               
                   
                 Duties 
                 Gcal/h 
               
               
                   
                   
               
               
                   
                 First Deethanizer Feed Chiller 20 
                 4.7 
               
               
                   
                 Second Deethanizer Feed Chiller 28 
                 4.0 
               
               
                   
                 Deethanizer First Pumparound Cooler 46 
                 3.1 
               
               
                   
                 Cold Box (propane) 34 
                 3.6 
               
               
                   
                   
               
            
           
         
       
     
     Use of the propane feed to the reactor for refrigeration recovery as described herein substantially reduces the requirements for refrigeration from a propylene or propane refrigeration system, and could reduce the size and cost of the compressor system. In one non-limiting embodiment the refrigeration requirements may be reduced by about 50% or more, alternatively by about 30% or more. In a different non-restrictive version the reduction in the cost of the compressor system may be about 30%, alternatively about 20%. 
     In the foregoing specification, the invention has been described with reference to specific embodiments thereof. However, the specification is to be regarded in an illustrative rather than a restrictive sense. For example, equipment, processes, and operating conditions falling within the claimed or disclosed parameters, but not specifically identified or tried in a particular example, are expected to be within the scope of this invention. 
     The present invention may be practiced in the absence of an element not disclosed. In addition, the present invention may suitably comprise, consist or consist essentially of the elements disclosed. For instance, there may be provided a process for recovering propylene from a PDH process, where the process consists essentially of or consists of: partially or fully condensing a depropanizer overhead stream against a relatively warm cooling medium at relatively high pressure; letting down the liquid overhead stream in pressure; and partially or fully vaporizing the liquid overhead stream against at least one process stream to provide refrigeration. 
     There may also be provided a system for recovering propylene from a propane dehydrogenation process, where the system consists essentially of or consists of a depropanizer column generating a depropanizer overhead stream; at least one condenser for partially or fully condensing the depropanizer overhead stream; at least one device for letting down the pressure of the depropanizer overhead stream as determined by back-pressure of a reactor; and at least one chiller for partially or fully vaporizing the depropanizer overhead stream. 
     The words “comprising” and “comprises” as used throughout the claims, are to be interpreted to mean “including but not limited to” and “includes but not limited to”, respectively. 
     As used herein, the word “substantially” shall mean “being largely but not wholly that which is specified.” 
     As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     As used herein, the term “about” in reference to a given parameter is inclusive of the stated value and has the meaning dictated by the context (e.g., it includes the degree of error associated with measurement of the given parameter). 
     As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.