Abstract:
An apparatus and method useful for partially condensing vapor in the upper section of a fractionation tower to separate and remove a lighter gaseous fraction from a condensed liquid component, such as nitrogen from natural gas. A downflow, knockback condenser is disclosed that utilizes a vapor riser to introduce a flow of vapor into a headspace above a vertical tubular heat exchanger, thereby establishing a downflow of condensed liquid and a lighter gaseous fraction through the heat exchange tubes.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to an improved method and apparatus useful for partially condensing vapor in the upper section of a fractionation tower to separate and remove a gaseous fraction from a condensed liquid component, and more particularly, to separate and remove nitrogen from natural gas.  
         [0003]     2. Description of Related Art  
         [0004]     Systems and methods for removing nitrogen from natural gas have previously been disclosed, for example, in U.S. Pat. Nos. 5,141,544, 5,257,505 and 5,375,422.  
         [0005]     A significant part of the process for removing nitrogen from natural gas involves the fractionation of liquid methane from the rejected nitrogen. The liquid methane is sometimes referred to as “LNG,” but in the Nitech™ process invented by Applicant, the liquid is re-vaporized and becomes the sales gas product. The desirable LNG or sales gas exits the fractionation tower at the bottom. The nitrogen vent or waste gas exits the upper part of the tower after passing through the condenser. Only two major pieces of equipment are involved in the process: the fractionation tower and the condenser. The invention disclosed herein relates to improvements in the design and operation of the condenser.  
         [0006]     In the previous designs as disclosed in Applicant&#39;s prior art patents, the gas enters at the bottom of the tubes and exits at the top, whereas with the present design, the gas enters at the top of the tubes and exits at the bottom. The performance improvement arises from the fact that some of the gas entering the tubes is condensed, regardless of gas flow direction. In the vapor up-flow models, the liquid must exit by flowing downward or counter-current to the gas flow. While this was anticipated in the design of the prior art condensers, Applicant learned from their use that the “falling” liquid creates a film that negatively affects the heat transfer coefficient, requiring more condenser surface area to be installed with each condenser application and adding complexity to the estimation of condenser performance.  
       SUMMARY OF THE INVENTION  
       [0007]     A downflow, knockback condenser and method of use are disclosed herein that are particularly useful for partially condensing a vapor stream so that a lighter gas fraction can be efficiently removed and separated from the liquid that is condensed from the vapor stream. The term “lighter” refers to the actual density of the vapor constituent as compared to the liquid constituent density that may be present at any point in the apparatus. The subject apparatus and method are particularly useful for separating gaseous nitrogen from condensed natural gas liquid.  
         [0008]     A principal distinction between this invention and condensers disclosed in the prior art is the provision and use of a vapor riser to introduce vapor captured from the fractionation section of a tower into headspace above a tubular heat exchanger section to thereby establish downflow or countercurrent cooling of the vapor within the tubes of the condenser to partially condense it into a condensed liquid fraction from which a remaining uncondensed gaseous fraction is then separated and removed as an undesired component.  
         [0009]     According to one preferred embodiment of the invention, apparatus is disclosed that is useful for partially condensing vapor in the upper section of a fractionation tower to separate a lighter gaseous fraction from a condensed liquid component. The apparatus preferably comprises a substantially cylindrical shell and a condenser section having upper and lower tube sheets attached transversely to the inside of the shell. The tube sheets support a plurality of spaced-apart, vertically oriented, heat exchange tubes extending between the upper and lower tube sheets to provide fluid communication through the tubes. Refrigerant inlet and outlet ports are desirably disposed so as to establish a generally upward flow of refrigerant around the heat exchange tubes between the lower and upper tube sheets. A vapor riser provides fluid communication between a space in the fractionation tower disposed below the liquid trap plate and a headspace disposed above the upper tube sheet, thereby establishing an upward flow of vapor through the riser and a downward flow of vapor, condensed liquid and an uncondensed, lighter gaseous fraction through the heat exchange tubes. A vapor outlet port is preferably disposed below the lower tube sheet to receive the lighter gaseous fraction and any remaining vapor exiting the lower tube sheet. Liquid collection and recovery apparatus disposed below the lower tube sheet and below the vapor outlet port receive liquid condensed from the vapor.  
         [0010]     According to a particularly preferred embodiment of the invention, a condenser apparatus is disclosed that is useful for removing gaseous nitrogen from nitrogen-containing natural gas, the apparatus comprising a substantially cylindrical shell, a plurality of substantially vertical heat exchange tubes disposed inside the shell, a riser introducing vapor comprising natural gas and nitrogen into a headspace above the tubes a refrigerant flowing upwardly through the shell around the tubes and sufficiently cooling the tubes to condense natural gas passing downwardly through the tubes, thereby liquefying the natural gas and separating it from the gaseous nitrogen, an outlet disposed below the tubes for receiving gaseous nitrogen exiting the tubes, and a receptacle disposed below the outlet for receiving liquefied natural gas exiting the tubes.  
         [0011]     According to another preferred embodiment of the invention, a method for partially condensing a vapor to separate a lighter gaseous fraction from a condensed liquid fraction, the method comprising the steps of providing a condenser having a substantially cylindrical, vertically oriented shell; upper and lower tube sheets attached transversely to the inside of the shell, the tube sheets supporting a plurality of spaced-apart, vertically oriented, heat exchange tubes extending between the upper and lower tube sheets, and providing fluid communication through the tubes; providing refrigerant inlet and outlet ports disposed in the shell so as to establish a generally upward flow of refrigerant around the heat exchange tubes between the lower and upper tube sheets; providing a vapor riser providing fluid communication between a space in the shell disposed below the lower tube sheet and a headspace disposed above the upper tube sheet; establishing an upward flow of vapor through the riser and a downward flow comprising vapor, condensed liquid fraction and lighter gaseous fraction through the heat exchange tubes, the refrigerant having sufficient cooling capacity to condense a desired liquid fraction from the vapor while passing through the heat exchange tubes; and separately recovering the lighter gaseous fraction from the condensed liquid fraction collected below the heat exchange tubes.  
         [0012]     Through use of apparatus and method disclosed herein, one is able to achieve more predictable condenser performance, improved plant flexibility; higher sales gas recoveries, and lower capital costs. Greater predictability in condenser performance is particularly significant for meeting performance guarantees required by gas plant owners, especially for larger plants, where specific component performance plays a significant role in overall plant design.  
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0013]     The apparatus of the invention is further described and explained in relation to the drawing, which is a simplified cross-sectional elevation view of a preferred downflow knockback condenser of the invention that is preferably welded to the top of a conventional fractionation tower, the lower portion of which is broken away.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]     The drawing depicts the upper portion of a fractionation tower  10  in which the upper portion of shell  12  contains a preferred embodiment of downflow, knockback condenser  14  of the invention. Fractionation tower  10  is preferably made of conventional materials capable of operating at the temperatures and pressures needed for a particular application, and has a nominal diameter ranging from about 18 to about 120 inches, depending upon plant size and throughput. Generally speaking, the fractionation section of fractionation tower  10  is disposed below section  60 , and is broken away to facilitate enlargement of the upper section of the tower in which condenser  14  resides. As shown in the embodiment depicted in the drawing, section  60  of fractionation tower  10  is separated by liquid distribution plate  54  from the gas and condensed liquid recovery zones disposed between section  60  and condenser  14 . Liquid distribution plate  54  allows rich vapor  56  rising upwardly from the fractionation section to enter the condenser section of the tower, and distributes condensed liquid recovered from condenser  14  as further described below to pass downwardly as reflux liquid into the fractionation section of the tower, as indicated by arrow  58 , countercurrent to the upwardly rising rich vapor  56 .  
         [0015]     As used herein, the term “condenser section” collectively refers to Zones A, B and C and shown in the drawing. In Zone A, rich vapor rising upwardly from the fractionation section through liquid distribution plate  54  enters riser  32  and is directed upwardly into the headspace designated as Zone B above condenser  14 . From Zone B, as indicated by arrows  62 , the rich vapor flows downwardly through upper tube sheet  16  into the plurality of substantially vertical heat exchange tubes  20 , which are cooled by refrigerant entering shell  12  through refrigerant inlet  24  as shown by arrow  26 . The refrigerant flows around heat exchange tubes  20  through spaces  22  and, as it absorbs heat from tubes  20 , eventually rises to a point where it exits outlet  28  as indicated by arrow  30 . As condensed liquid and an uncondensed gaseous fraction exit downwardly from tubes  20  through lower tube sheet  18  into Zone C, the gaseous fraction exits shell  12  through outlet  44  as indicated by arrow  46 , and the condensed liquid is collected on liquid trap plate  40 . From liquid trap plate  40 , the condensed liquid received into Zone C from condenser  14  flows downwardly through opening  50 , through reflux liquid return seal leg  48 , as shown by arrow  64 , where it is discharged from end  53  into reflux seal pan  52  in Zone A. From reflux seal pan  52 , the condensed reflux liquid spills over, as shown by arrow  66 , onto liquid distribution plate  54 , from which it is returned to the fractionation section as indicated by arrow  58 .  
         [0016]     The design, structure and operation of a preferred embodiment of downflow, knockback condenser  14  of the invention is further described and explained below in relation to a computer simulation of a preferred method of the invention wherein rich vapor containing natural gas (methane) and nitrogen is partially condensed to separate and remove the gaseous nitrogen from the condensed natural gas liquid. The reference numerals used below generally relate to the structures and flows as described above in relation to the drawing.  
         [0017]     Zone A contains both vapor and liquid. The vapor enters Zone A from section  60  of the fractionation tower via liquid distribution tray  54  disposed below liquid trap plate  40 . The liquid enters Zone A from condenser  14  above via the reflux liquid return seal leg  48 . The Zone A vapor component is expected to exist at the temperature, pressure and composition given below, and is at the dew point of the rich vapor, meaning that any reduction in temperature at the same pressure will create liquid condensate. In a computer simulation of fractionation tower  10  as operated with the downflow knockback condenser  14  of the invention, the Zone A vapor and liquid conditions are as follows:  
                                                                                                             Zone A Vapor (Entering)                Temperature (deg. F.)   −245.46           Pressure (psia)   315.00            Component (mole %):                Nitrogen   97.54           Methane   2.46            Zone A Liquid (Entering)                Temperature (deg. F.)   −247.52           Pressure (psia)   315.00            Component (mole %):                Nitrogen   98.4570           Methane   1.5430                      
 
         [0018]     The liquid in Zone A provides the reflux for fractionation tower  10  to minimize the amount of methane that is vented with the nitrogen waste gas through outlet  44 . The vapor from Zone A proceeds upward through the vapor riser  32  into Zone B. Entrance  34  to vapor riser  32  is preferably cut obliquely on a  60  degree bias to provide greater entrance area to riser  32  and thereby reduce the entrance velocity and associated pressure losses of the rich vapor. Reducing the velocity at entrance  34  allows less liquid, in the form of droplets, to enter riser  32 . Some liquid droplets entering riser  32  will not significantly impair the performance of fractionation tower  10  or condenser  14 , but neither does it help. The entrance of riser  32  is desirably spaced approximately one foot from the underside of liquid trap plate  40  to reduce the vapor velocity at the lower or bottom face of liquid trap plate  40 . Lowering this velocity will help minimize the heat transfer across the plate. Heat transfer across liquid trap plate  40  is not desirable because it will reduce the overall effectiveness of condenser  14 , and should be minimized. Upper end  36  of vapor riser  32  is desirably extended about six inches above upper tube sheet  16 . This extension will help in more evenly distributing the vapor flow across upper tube sheet  16 .  
         [0019]     The section between upper tube sheet  16  and lower tube sheet  18  is the principal heat exchanger section of condenser  14 . A primary point of distinction between this invention and my prior inventions is that we have reversed the flow direction of the vapor to be cooled through the heat exchange section. In the previous designs as disclosed in my prior art patents, the gas enters at the bottom of the heat exchange tubes and exits at the top, whereas with the present design, the gas enters at the top of heat exchange tubes  20  and exits at the bottom.  
         [0020]     The Zone B vapor conditions are substantially the same as in Zone A but there is no liquid present. In reality, the temperature in Zone B is slightly lower than in Zone A and the computer predicts a slight temperature decrease and a lower pressure due to the vertical elevation difference between Zone A and Zone B. The temperature differences here are insignificant in the overall operation of the unit, but the pressure drop is significant, as is further explained below. Any temperature reduction in riser  32  is beneficial, but a conservative approach plans for minimal temperature decrease and only as predicted by the computer simulations.  
         [0021]     The Zone B vapor conditions are as follows:  
                                                                 Zone B Vapor                                    Temperature (deg. F.)   −245.56           Pressure (psia)   314.09            Component (mole %):                Nitrogen   97.54           Methane   2.46                      
 
         [0022]     Condenser  14  is desirably mounted on the top of fractionation tower  10  approximately  70  feet from grade. Condenser  14  is preferably a shell and tube heat exchanger configured with substantially vertical tubes  20  supported at the ends by the upper and lower tube sheets  16 ,  18 , respectively. Heat exchange tubes  20  provide the heat transfer surface between the refrigerant, on the shell side, and the process vapor on the tube side. The shell side of the exchanger is isolated from the tube side as a different process fluid is present on that side. The refrigerant used on the shell side of the condenser is preferably LNG created from a tower bottom source. The refrigerant desirably enters condenser  14  through a nozzle at inlet  24  in shell  12  and exits shell  12  through a nozzle at outlet  28 .  
         [0023]     The approximate conditions of the refrigerant entering inlet  24  of condenser  14  are as follows:  
                                                                 Inlet Refrigerant                                    Temperature (deg. F.)   −254.75           Pressure (psia)   21.88           Vapor mole fraction   0.095            Component (mole %):                Nitrogen   4.00           Methane   84.81           Ethane   8.62           Propane   2.17           I-Butane   0.09           N-Butane   0.24           I-Pentane   0.02           N-Pentane   0.04           N-Hexane   0.00724           N-Heptane   0.0019                      
 
         [0024]     The approximate conditions of the refrigerant exiting condenser  14  at outlet  28  are as follows:  
                                                                 Exit Refrigerant                                    Temperature (deg. F.)   −247.47           Pressure (psia)   18.57           Vapor mole fraction   0.641            Component (mole %):                Nitrogen   4.00           Methane   84.81           Ethane   8.62           Propane   2.17           I-Butane   0.09           N-Butane   0.24           I-Pentane   0.02           N-Pentane   0.04           N-Hexane   0.0072           N-Heptane   0.0019                      
 
         [0025]     It should be noted that the temperature is slightly higher on the exiting stream, but, and this is of greater significance, that the vapor fraction is much greater on the exiting stream. Because the temperatures of the refrigerant streams entering and exiting the heat exchanger are lower than the vapor inside the vertical tubes  20 , heat will be transferred from the process vapor from Zone B into the refrigerant.  
         [0026]     The fluid next passes from Zone B into Zone C through condenser  14 , where the temperature is reduced. As stated before, the condition of the vapor in Zone B is at the dew point, which means that any reduction in temperature will produce condensate from the entering vapor.  
         [0027]     The conditions of the fluid stream entering Zone C from condenser  14  are as follows:  
                                                                 Zone C Entering Vapor and Liquid Mixture                                    Temperature (deg. F.)   −247.55           Pressure (psia)   314.30           Vapor mole fraction   0.25            Component (mole %):                Nitrogen   97.54           Methane   2.46                      
 
         [0028]     It should be noted that the vapor fraction of the Zone C fluid is now only 25 percent vapor, whereas it was 100 percent vapor upon entering condenser  14 . This refrigeration effect is the tool that is used to yield a “clean” vent stream as required to provide high efficiency separation of nitrogen from the incoming natural gas streams.  
         [0029]     Vented nitrogen exits the condenser at outlet  44  and has approximately the following conditions:  
                                                                 Vented Nitrogen Exiting Outlet 44                                    Temperature (deg. F.)   −247.55           Pressure (psia)   314.30           Vapor mole fraction   1.00            Component (mole %):                Nitrogen   99.0           Methane   1.0                      
 
         [0030]     This particular simulation indicates methane content in vent stream  30  of 1.0 percent. The amount of nitrogen coming into fractionation tower  10  on this particular design and upstream of the tower is approximately 21 percent. Of particular importance is the absence of ethane and heavier components. The process of the invention as disclosed herein vents none of these heavier hydrocarbon compounds to the atmosphere, thereby providing a significant advantage of this process over other nitrogen rejection processes.  
         [0031]     Completing the circuit, the vapor part of the fluid stream exiting from heat exchange tubes  20  at the lower tube sheet exits the unit at vapor fraction outlet  44 , from which liquid is preferably shielded by liquid barrier  42 , and the condensed liquid component falls to liquid trap plate  40  where it flows by gravity through inlet  50  into reflux liquid return seal leg  48 , and from there into reflux seal pan  52 . The purpose of the seal leg  48  is to provide a liquid head created by standing liquid in the seal leg to offset the pressure loss in moving the vapor from Zone A into Zone B and eventually into Zone C. The pressure drop through the total circuit is critical and is held to approximately 0.70 psi. The standing liquid in seal leg  48  creates this differential by using gravity and the higher density of the liquid component as compared to the same compounds as vapor. Reflux seal pan  52  provides a liquid trapping mechanism to prevent flow of the vapor in Zone A from flowing directly up seal leg  48  and bypassing condenser  14 . Under normal operating conditions, the liquid level is anticipated to be approximately 1 foot deep on top of liquid trap plate  40 .  
         [0032]     Other alterations and modifications of the invention will likewise become apparent to those of ordinary skill in the art upon reading this specification in view of the accompanying drawings, and it is intended that the scope of the invention disclosed herein be limited only by the broadest interpretation of the appended claims to which the inventor is legally entitled.