Patent Publication Number: US-2003221818-A1

Title: Reflux condenser system for improved fluids separation

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates generally to condensers. In another aspect, the invention relates to reflux condensers employed at the top of distillation towers. In a further aspect, the invention relates to an alkylation unit employing an improved rectifier for separating n-butane from alkylate.  
       [0003] 2. Discussion of the Prior Art  
       [0004] For many years, reflux condensers have been employed at the top of distillation towers to condense high boiling point fluids mixed with the low boiling point vapors exiting the top of the tower. Referring to FIG. 1, a conventional reflux condenser  10  employed at the top of a distillation tower typically includes a condensing zone  12  having a lower condensing zone inlet  14  for receiving upwardly flowing fluids from the top of the distillation tower and an upper condensing zone outlet  16  for discharging gaseous fluids from the reflux condenser  10 . A U-tube bundle  18  is disposed in the condensing zone  12 . The open ends of the individual U-tubes  20  in U-tube bundle  18  fluidly communicate with a cooling fluid inlet  22  and a cooling fluid outlet  24  so that a cooling fluid can flow continuously through the U-tubes  20 . When the upwardly flowing gaseous fluids from the top of the distillation tower contact the cool U-tubes  20  in the condensing zone  12 , the high boiling point component(s) of the fluids condense.  
       [0005] In the conventional reflux condenser  10 , the individual U-tubes  20  of U-tube bundle  18  are supported relative to one another via a plurality of horizontally disposed plate-type baffles  26 . It has been discovered that the use of such plate-type baffles  26  in the reflux condenser  10  presents a number of drawbacks. For example, the velocity profile of the fluid flowing upwardly through the condensing zone  12  is non-uniform due to the configuration of the plate-type baffles  26 . Further, the flat upper surfaces of the plate-type baffles  26  present areas where “pooling” of condensed liquids can occur. The pooled liquids from the flat upper surfaces of the plate-type baffles  26  can drip off of the plate-type baffles  26  and become entrained in the high velocity fluids flowing upwardly through the reflux condenser  10 . The entrained liquids can then be carried out of the reflux condenser  10  through the condensing zone outlet  16 . When a significant amount of the entrained, condensed liquids exits the reflux condenser  10  through the condensing zone outlet  16 , the main function of the reflux condenser  10  (i.e., condensing and separating high boiling point fluids from low boiling point fluids) is frustrated.  
       OBJECTS AND SUMMARY OF THE INVENTION  
       [0006] Responsive to these and other problems, it is an object of the present invention to provide a condenser which more efficiently condenses and separates high boiling point fluids from low boiling point fluids.  
       [0007] A further object of the present invention is to provide a reflux condenser employing tube-supporting baffles that present little or no flat upper surfaces where condensed liquids can pool.  
       [0008] Another object of the present invention is to provide a reflux condenser that allows for a substantially uniform velocity profile of the fluids flowing upwardly therethrough.  
       [0009] Still another object of the present invention is to provide a reflux condenser that minimizes the maximum velocity of upwardly flowing fluids so that condensed liquids do not become entrained in the upwardly flowing fluids.  
       [0010] Yet another object of the present invention is to provide a reflux condenser which separates liquids entrained in gaseous fluids flowing through the reflux condenser before the gaseous fluids exit the reflux condenser.  
       [0011] It should be noted that not all of the above-listed objects need be accomplished by the present invention, and other objects and advantages of the invention will be apparent from the written description and drawings.  
       [0012] Accordingly, in one embodiment of the present invention, a condenser is provided that comprises a main body and a generally upright U-tube bundle. The main body defines a condensing zone. The U-tube bundle is disposed in the condensing zone and comprises a plurality of U-tubes and a plurality of rod-type baffles for supporting the U-tubes.  
       [0013] In accordance with another embodiment of the present invention, there is provided a distillation unit which comprises an elongated upright distillation column and a reflux condenser. The distillation column has a lower end and an upper end and defines a fractionation zone extending between the lower and upper ends. The reflux condenser is positioned above and rigidly coupled to the upper end of the distillation column and defines a condensing zone fluidly communicating with the fractionation zone via a condensing zone inlet. The condenser includes a heat exchange tube bundle disposed in the condensing zone and comprising a plurality of upright elongated heat exchange tubes and a plurality of rod-type baffles for supporting the heat exchange tubes.  
       [0014] In still another embodiment of the present invention, there is provided an alkylation unit comprising a reactor, a depropanizer, and a rectifier. The reactor is operable. to contact an iso-paraffin, an olefin, and an acid catalyst under reaction conditions sufficient to produce a reactor effluent comprising an alkylate, propane, and n-butane. The depropanizer fluidly communicates with the reactor and is operable to substantially separate the propane and the alkylate. The rectifier fluidly communicates with the depropanizer and is operable to separate at least a portion of the n-butane from the alkylate and return the resulting separated alkylate to the depropanizer. The rectifier includes an elongated upright distillation column and a reflux condenser. The condenser includes a main body defining a condensing zone and a heat exchange tube bundle disposed in the condensing zone. The heat exchange tube bundle comprises a plurality of elongated heat exchange tubes and a plurality of rod-type baffles for supporting the heat exchange tubes.  
       [0015] In yet another embodiment of the present invention, a process is provided that comprises the steps of: (a) fractionating a fluid mixture in an upright distillation column; (b) conducting a light fluid component of the fluid mixture to a reflux condenser rigidly coupled to the top of the distillation column, wherein the condenser includes a plurality of heat exchange tubes supported by a plurality of rod-type baffles; and (c) condensing a heavy fluid component of the light fluid component in the condenser, thereby providing a condensed liquid in the condenser. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES  
     [0016] Preferred embodiments of the present invention are described in detail below with reference to the attached drawing figures, wherein:  
     [0017]FIG. 1 is a partial sectional side view of a conventional reflux condenser, particularly illustrating the plate-type baffles that support the heat exchange tubes;  
     [0018]FIG. 2 is a side view of a distillation unit including an upright elongated distillation column and a reflux condenser coupled to the top of the distillation column;  
     [0019]FIG. 3 is a partial sectional side view of a reflux condenser constructed in accordance with the principles of the present invention, particularly illustrating a U-tube bundle comprising a plurality of rod-type baffles for supporting the U-tubes;  
     [0020]FIG. 4 is a sectional top view of the reflux condenser taken along line  4 - 4  in FIG. 3, particularly illustrating the components of the U-tube bundle and the mist extraction pad positioned proximate the outlet of the reflux condenser;  
     [0021]FIG. 5 is a top view of a group of rod-type baffles coupled to a single baffle ring;  
     [0022]FIG. 6 is a partial isometric view of a segment of a single U-tube leg and four adjacent vertically spaced rod-type baffles, particularly illustrating the positive four-point containment system provided by the rod-type baffles; and  
     [0023]FIG. 7 is a schematic diagram of an alkylation unit employing a rectifier that takes full advantage of the benefits provided by the novel reflux condenser described herein. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0024] Referring initially to FIG. 2, a distillation unit  100  is illustrated as generally comprising an elongated upright distillation column  102  and a reflux condenser  104 . Distillation column  102  presents a lower end  106  and an upper end  108  and defines a interior fractionation zone extending between lower and upper ends  106 ,  108 . Distillation column  102  includes an inlet  1   10  vertically positioned between lower and upper ends  106 ,  108  and fluidly communicating with the fractionation zone. In operation, a substantially gaseous heated fluid mixture enters the fractionation zone via inlet  1   10 . In the fractionation zone, the fluid mixture is fractionated into various components and withdrawn from the fractionation zone according to the boiling point of the components. Generally, heavier (i.e., higher boiling point) fluid components condense toward lower end  106  of distillation column  102  while the lightest (i.e., lowest boiling point) fluid components do not condense in the fractionation zone and flow upwardly out of upper end  108  and into reflux condenser  104 . Any heavy fluid components remaining in the light fluid component flowing through reflux condenser  104  are condensed in reflux condenser  104 . The condensed liquids in reflux condenser  104  are then allowed to flow downwardly into the fractionation zone of distillation column  102  by gravitational force.  
     [0025] Referring now to FIG. 3, reflux condenser  104  includes a main body  112  and a generally upright U-tube bundle  114 . Main body  112  defines a condensing zone  116  having a lower condensing zone inlet  118  which fluidly communicates with the upper end of the fractionation zone defined by distillation column  102  (shown in FIG. 3) and an upper condensing zone outlet  120  through which fluids exit condensing zone  116 . Upright U-tube bundle  114  is disposed in condensing zone  116  and extends at least partly into the flow path of fluids flowing from condensing zone inlet  118  to condensing zone outlet  120  so that the fluids flowing through condensing zone  116  contact the outer surface of the plurality of individual U-tubes  122  of U-tube bundle  114 . As used herein, the term “U-tube” denotes a continuous tube formed generally in the shape of a “U” and having a pair of open ends. As used herein, the term “upright U-tube bundle” denotes a group of individual U-tubes and a support system that supports the U-tubes in a manner such that the sides (i.e., legs) of the individual U-tubes are substantially upright and are open at their top ends.  
     [0026] Main body  112  of reflux condenser  104  further defines a cooling fluid manifold  124  which is positioned generally above, and fluidly isolated from, condensing zone  116 . Cooling fluid manifold  124  is divided into an inlet portion  126  and an outlet portion  128 . Inlet portion  126  receives a cooling fluid via a cooling fluid inlet  130  defined by main body  112 . Outlet portion  128  discharges the cooling fluid through a cooling fluid outlet  134  defined by main body  112 . Inlet portion  126  fluidly communicates with the open inlet ends of U-tubes  122  and outlet portion  128  fluidly communicates with the opposite open outlet ends of U-tubes  122 . Inlet and outlet portions  126 ,  128  are fluidly isolated from one another, except for the fluid flow communication provided therebetween by U-tubes  122 .  
     [0027] Referring to FIGS. 3 and 4, U-tube bundle  114  generally comprises U-tubes  122 , a plurality of rod-type baffles  134 , and a plurality of vertically spaced, generally horizontally disposed baffle rings  136   a - e  (shown in FIG. 3). Baffle rings  136   a - e  and rod-type baffles  134  cooperate to rigidly support U-tubes  122  relative to main body  112 . As used herein, the term “rod-type baffle” shall denote an elongated baffle member whose axial cross section shows an outer surface with no substantially flat portions. Generally, rod-type baffles  134  will be rods having substantially cylindrical or elliptical axial cross sections.  
     [0028] Referring now to FIG. 5, a group of laterally spaced, parallelly extending rod-type baffles  134   a  are associated with each baffle ring  136   a . Preferably, the ends of baffles  134   a  are rigidly coupled to baffle ring  136   a  so that baffles  134   a  extend chordally across the open center baffle ring  136   a . Referring to FIGS.  3 - 5 , it is preferred for the separate groups of parallel baffles  134  that are coupled to adjacent vertically spaced baffle rings  136  (e.g., rings  136   a  and  136   b ) to extend substantially perpendicular to one another. Thus, the groups of baffles  134  that are coupled to every other vertically spaced baffle ring  136  (e.g., rings  136   a  and  136   c ) extend substantially parallel to one another. It is preferred for the groups of rod-type baffles  134  that are coupled to every other vertically spaced baffle ring  136  (e.g., rings  136   a  and  136   c ) to contact each U-tube  122  on generally opposite sides of U-tube  122 , thereby forming a positive four-point containment system for supporting each U-tube  122 .  
     [0029] Referring now to FIG. 5, the positive four-point containment system formed by four vertically spaced rod-type baffles  134   a - d  is illustrated as supporting a section of one leg of U-tube  122 . As used herein, the term “positive four-point containment system” shall denote a system for supporting a heat exchange tube cooperatively employing at least four rod-type baffles that contact the tube and are axially spaced along the tube, wherein adjacent axially spaced baffles extend substantially perpendicular to one another and alternating axially spaced baffles contact generally opposite sides of the tube.  
     [0030] Referring again to FIGS.  3 - 6 , employing rod-type baffles in such a positive four-point containment system provides reflux condenser  104  with minimal flow-induced U-tube  122  vibration, uniform fluid velocity profile in condensing zone  116 , enhanced heat transfer due to turbulence caused by rod-type baffles  134 , no flat baffle surfaces on which condensed liquids can pool, and reduced entrainment of condensed liquids due to low fluid velocities.  
     [0031] Referring again to FIGS. 3 and 4, a mist extraction pad  138  is preferably disposed in condensing zone  116  and covers condensing zone outlet  120 . Mist extraction pad  138  is made of a porous material through which the gaseous fluids exiting condensing zone  116  may pass. Mist extraction pad  138  is preferably operable to cause coalescence of liquid droplets (i.e., mist) entrained in the gaseous fluids exiting condensing zone  116  on elements of mist extraction pad  138 . The coalesced liquids can then drain downwardly from mist extraction pad  138 , through condensing zone  116 , and into distillation column  102  by gravitational force. Mist extraction pad  138  is preferably a commercially available wire mesh mist extraction pad such as, for example, the Metex Opti-Mesh™ mist eliminator available from Metex Corporation, Edison, N.J., U.S.A.  
     [0032] Referring again to FIGS.  2 - 4 , during operation of reflux condenser  104 , a cooling fluid (e.g., 40-125° F. water) continuously flows through U-tubes  122  while a light fluid component flows generally upwardly through condensing zone  116 . When the light fluid component contacts and is cooled by U-tubes  122 , a heavy fluid component of the light fluid component condenses in condensing zone  116 . The condensed fluid then flows downwardly from condensing zone  116  into distillation column  102  via condensing zone inlet  118 . In order to prevent the velocity of fluids flowing upwardly through condensing zone inlet  118  from being high enough to cause entrainment of the downwardly flowing condensed liquids therein, it is preferred for the minimum horizontal open area of condensing zone inlet  118  to be at least 50 percent of the maximum horizontal open area of condensing zone  116 . As used herein, the term “horizontal open area” shall denote the total area of an opening taken along a horizontal cross-sectional line through the opening. Preferably, the minimum horizontal open area of condensing zone inlet  118  is at least 75 percent of the maximum horizontal open area of condensing zone  116 , most preferably the minimum horizontal open area of condensing zone  118  is at least 80 percent of the maximum horizontal open area of condensing zone  116 .  
     [0033] Referring now to FIG. 7, an alkylation unit  200  is illustrated as generally comprising a reactor  202 , a catalyst regenerator  204 , a depropanizer  206 , and a rectifier  208 . In reactor  202 , an iso-butane stream and an olefin (e.g., butylene and/or propylene) stream are contacted in the presence of an acid catalyst (e.g., hydrofluoric acid) to thereby provide a deactivated acid catalyst and a reactor effluent. The deactivated catalyst can be cycled through catalyst regenerator  204  in order to reactivate the catalyst. The reactor effluent typically comprises an alkylate, propane, iso-butane, and normal-butane (n-butane). As used herein, the term “alkylate” denotes an alkylation reaction product primarily comprising C 5 +hydrocarbons that boil in the gasoline boiling range and have an end point between 375 and 450° F. The reactor effluent is sent to depropanizer  206  for separation of the propane and iso-butane from the alkylate. Depropanizer  206  can be any conventional fractional distillation tower typically used in alkylation units for separating propane from alkylate.  
     [0034] Rectifier  208  is fluidly coupled to depropanizer  206  and is operable to remove a “side draw” of fluid, primarily comprising alkylate and n-butane, from depropanizer  206 . Rectifier  208  generally includes a distillation column  210  and a reflux condenser  212 . Preferably, rectifier  208  is constructed in the same manner as distillation unit  100 , described above with reference to FIGS.  2 - 6 . In distillation column  210 , substantially all of the alkylate is separated from the n-butane. In reflux condenser  212 , any alkylate remaining in the upwardly flowing n-butane stream is condensed and drains back into distillation column  210  for return to depropanizer  206 .  
     [0035] The preferred forms of the invention described above are to be used as illustration only, and should not be used in a limiting sense to interpret the scope of the present invention. Obvious modifications to the exemplary embodiments, set forth above, could be readily made by those skilled in the art without departing from the spirit of the present invention.  
     [0036] The inventors hereby state their intent to rely on the Doctrine of Equivalents to determine and assess the reasonably fair scope of the present invention as pertains to any apparatus not materially departing from but outside the literal scope of the invention as set forth in the following claims.