Abstract:
A distillation tower for use in a petrochemical or petroleum operation effects liquid and vapor separation by using centrifugal force applied to the feed introduced into a ring from a tangential inlet. The feed is separated into a liquid component that flows into the bottoms section of the tower and a vapor component that flows upwardly through the core of the ring to a wash zone in the tower. De-entrainment devices are provided in the core so that the vapor swirling upwardly impacts the devices and any remaining entrained liquid is separated from the vapor. As a result, overflash with decreased resid can be collected from the wash zone and used as feed suitable for a fluid catalytic cracking operation, for example.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application relates to and claims priority to U.S. Provisional Patent Application No. 60/902,879, entitled “Core De-Entrainment Devices for Vessels with Tangential Inlets,” filed on Feb. 23, 2007. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to devices for separating vapor and liquid. In particular, the invention relates to separating vapor and entrained liquid in a hydrocarbon distillation tower in which a feedstream is introduced into a flash zone. 
     2. Discussion of Related Art 
     Separation units, such as atmospheric distillation units, vacuum distillation units and product strippers, are major processing units in a refinery. Atmospheric or vacuum distillation units separate crude oil into fractions according to boiling point so downstream processing units, such as hydrogen treating or reforming units, will have feedstocks that meet particular specifications. Crude oil separation is accomplished by fractionating the total crude oil at essentially atmospheric pressure and then feeding a bottoms stream of high boiling hydrocarbons, also known as topped crude, from the atmospheric distillation unit to a second distillation unit operating at a vacuum pressure. 
     The vacuum distillation unit typically separates the atmospheric unit bottoms into gas oil vapors based on boiling point, including light gas oil, heavy gas oil, vacuum gas oil, and vacuum reduced crude. The vacuum reduced crude is also known as residuum or “resid” and leaves the vacuum distillation unit as a liquid bottoms stream. 
     In atmospheric or vacuum distillation, lighter hydrocarbons are vaporized and separated from relatively heavier hydrocarbons so that they can be fed downstream for catalytic processing. The bottoms separated from crude oil by an atmospheric distillation unit are fed to a flash zone in the lower portion of the vacuum distillation unit. Although the heavier hydrocarbons do not vaporize, they may be carried into the lighter hydrocarbons due to entrainment. The entrained heavier hydrocarbons are typically contaminated with metals, such as vanadium or nickel, which can poison the downstream catalytic processing, such as hydrotreating, hydrocracking, or fluid catalytic cracking. 
     If the entrainment of the heavier components can be significantly reduced or eliminated, a significant improvement in the quality of the feed for hydroconversion units, catalytic cracking units, and vacuum towers producing valuable gas oil distillates or lube oil distillates can be realized, in both yield and quantity. 
     Various methods of reducing entrainment of residuum from the flash zone have been developed. Many distillation towers use inlet horns for introducing the feedstream to the flash zone. One type of inlet horn uses a tangential entry for the vapor-liquid feed that opens into a peripheral open bottomed horn. The horn can be an annular or arcuate channel defined by an outer peripheral wall and an internal arcuate wall spaced from the tower peripheral wall and having a closed top. Thus, the feedstream swirls through the horn and the liquid and vapor components impact the walls from centrifugal force and separate since the force on the more dense liquid is substantially greater than the centrifugal force acting on the vapor. The separated liquid flows downward due to gravity to the stripping zone for collection in the bottom portion of the tower. The vapor component also flows downwardly within the horn and then out of the horn toward the lower pressure flash zone and is swept upwardly through the core toward the wash zone of the tower. 
     One example of a peripheral horn is shown in U.S. Pat. No. 4,770,747 in which the inlet horn has angularly disposed vanes connected between the walls of the channel so that vapor-liquid separation takes place evenly along the arc length of the horn. 
     Another example of an inlet horn is shown in U.S. Pat. No. 4,315,815 in which corrugated vanes are disposed in the horn for utilizing the centrifugal motion to create turbulence in the stream in the inlet horn. In this case, the turbulence causes a portion of the fine particle size bituminous material to impinge on the surfaces of the inlet horn and recombine with the fluid so that vaporized solvent and steam can be withdrawn. 
     The problem with the prior art devices is that the inlet horns still allow an amount of vapor with entrained liquid to move up the tower through the core of the horn. In a typical tower, the overflash, which includes vacuum gas oils in the wash oil, is collected from the overflash collection tray and sent to the stripping zone for further processing. It typically has a high percentage of resid and is not suitable for certain feed applications, especially for fluid catalytic cracking (FCC). In order to more effectively use the overflash, especially for FCC feed, it is desirable to further reduce the entrainment of resid. Additionally, more effective de-entrainment will improve the reliability of the wash zone. Excessive resid entrainment in the wash zone accelerates formation of coke, forcing sub optimal operation and shutdown. Higher quality overflash, such as would be acceptable for FCC, can increase wash oil rates and virtually eliminate the risk of coking, allowing units to operate at higher temperatures and higher efficiencies. 
     Thus, there is a need for a separation device in which entrainment of resid can be significantly reduced. 
     BRIEF SUMMARY OF THE INVENTION 
     Aspects of embodiments of the invention relate to providing a de-entrainment device that effectively separates entrained liquid from vapor. 
     Another aspect of embodiments of the invention relates to a separation assembly that provides a low cost method of reducing entrainment to provide overflash that can be upgraded for additional uses, such as FCC feed. 
     This invention is directed to a liquid-vapor separation assembly for use in a distillation tower comprising a tower having a peripheral wall with at least one tangential inlet therein and a central hollow core. An interior wall is spaced from the peripheral wall and is connected thereto by a top wall so that a channel with a closed top and an open bottom is defined between the peripheral wall and interior wall. A vapor flow path extends from the inlet, through the channel, under the interior wall and up into the core. At least one de-entrainment device extends from the interior wall into the core of the tower to form an obstruction in the vapor flow in the core. 
     The interior wall can be arcuate or annular. A pair of tangential inlets can be formed in the peripheral wall. 
     The de-entrainment device can be a barrier or a partition. The barrier or partition can be a wall, solid or porous, disposed vertically or at a non-vertical angle, and can have channels to direct separated liquid away from the vapor flow. The de-entrainment device can be formed as a crinkled mesh screen or could have elongated V-shaped channels. The de-entrainment device can include a plurality of evenly spaced partitions extending inwardly in the core from the interior wall, and the partitions can be disposed at different vertical levels. 
     The invention is also directed to a distillation tower comprising an outer shell with at least one inlet, wherein a flash zone is disposed in a lower portion of the shell and a wash zone is disposed in an upper portion of the shell and a fluid flow path extends from the inlet through the flash zone and into the wash zone. An annular ring is disposed within the shell in communication with the inlet and adjacent to the flash zone. The annular ring is formed as an open bottom channel and defines a hollow central core leading to the wash zone. At least one de-entrainment device is disposed within the core, wherein the de-entrainment device is a barrier in the flow path, so that a mixed phase feed flowing from the inlet through the channel separates into a liquid that flows downward toward the flash zone and a vapor that flows upward through the core and impinges on the de-entrainment device to effect separation of liquid remaining entrained in the flowing vapor. 
     The inlet can be tangential and can comprise a pair of opposed tangential inlets. 
     The de-entrainment device can be a partition extending from the annular ring into the core. The partition can include V-shaped or other shaped channels, a crinkled wire mesh screen, a plurality of barrier walls spaced around the annular ring at different vertical heights, or a plurality of barrier walls each disposed at a non-vertical angle. 
     The distillation tower can be combined with a refinery operation. 
     The invention is also directed to a method of separating liquid entrained in vapor in a fluid stream in a distillation tower comprising introducing a feed stream into a distillation tower at a flash zone through a tangential inlet, directing the feed stream into an annular ring formed of an open bottom channel so that some liquid separates from vapor in the fluid stream, wherein the separated liquid flows downwardly into the tower and the vapor flows upwardly through a core of the annular ring, and impeding the vapor flow through the core of the annular ring to cause entrained liquid to separate from the upwardly flowing vapor. 
     Impeding the vapor flow may include providing barriers in the core of the annular ring, providing angled partitions protruding from a wall of the annular ring into the core, or providing crinkled wire mesh screens protruding from a wall of the annular ring into the core. 
     These and other aspects of the invention will become apparent when taken in conjunction with the detailed description and appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic side view in section of a distillation tower in accordance with the invention; 
         FIG. 2  is a plan view of the inlet area of the distillation tower showing the inlet and annular ring of  FIG. 1 ; 
         FIG. 3  is a plan view of an inlet area showing a velocity profile of an inlet and annular ring; 
         FIG. 4  is a top view of a de-entrainment device in accordance with the invention; 
         FIG. 5  is a front view of another de-entrainment device in accordance with the invention; 
         FIG. 6  is a side view of another de-entrainment device in accordance with the invention; 
         FIG. 7  is a top view of a de-entrainment device installed in the core; 
         FIG. 8  is a side perspective view of a de-entrainment device installed in the core; 
         FIG. 9  is a front view of a modified de-entrainment device; 
         FIG. 10  is a side perspective view of another de-entrainment device installed in the core; 
         FIG. 11  is a schematic side view in section of a conventional inlet area of a distillation tower; and 
         FIG. 12  is a plan view of the inlet area of  FIG. 11 . 
     
    
    
     In the drawings, like reference numerals indicate corresponding parts in the different figures. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The de-entrainment device disclosed herein can be used in various systems that relate to separation devices, particularly devices for separating vapor streams that have entrained liquid droplets. While the device is described in the context of a processing unit in a refinery, especially with respect to crude oil processing, liquid entrainment reduces separation efficiency in other hydrocarbon and non-hydrocarbon systems in which feed entries are flashed. Typical systems include product strippers or towers that are fed a partially vaporized stream. It will be understood that this device can be used in various settings. 
     The components representative of a conventional distillation tower  100  are schematically shown in  FIG. 11 . The tower  100  is formed of a shell  102  made of a peripheral wall.  FIG. 11  shows two zones within the tower  100  including the flash zone  104  and the wash zone  106 . The flash zone  104  has a tangential inlet  108  that leads to an annular ring  110 . As seen in  FIG. 12 , the annular ring  110  defines a central hollow core  112 . 
     Mixed phase feed, which is formed of vapor and liquid and is typically heated, enters the tower  100  through the inlet  108 . The feed experiences centrifugal action in the annular ring  110 , as illustrated by the arrow in  FIG. 12 , which separates much of the liquid from the vapor. The separated liquid L then moves down the tower  100  by gravity and is collected and treated in the bottom, or stripping section, of the tower  100 . 
     The vapor V plus entrained liquid, called resid, moves up the tower  100  through the core  112  of the annular ring  110  to the upper wash zone  106  for rectification and collection of the distilled separated products. Specifically, the vapor travels through the overflash collection tray  114 , also called a chimney tray, and into the wash zone  106 . The wash zone  106  is filled with packing and is designed to remove entrainment. The captured entrainment is washed down by injecting wash oil from an injection device  116  at the top of the wash zone  106 . The wash oil is typically vacuum gas oil. Part of the wash oil is vaporized, and rises upward in the tower  100  to another collector tray  1118 , if desired. The remainder wash oil, including the captured entrainment, drop into the overflash collection tray  114  and are removed via the overflash outlet  120 . The remainder stream is called the overflash. It may contain 30%-50% resid. While a significant portion of the overflash is gas oil, the resid content, including metals and carbon, is too high to be used as feed for most FCC units. The overflash is typically sent to the stripping section of the tower  100  along with the liquid portion from the feed. 
     The distillation tower  10  in accordance with this invention has similar basic components, as seen in  FIG. 1 . The tower  10  is formed of a shell  12  which has an inner peripheral arcuate wall. Two zones are shown within the tower  10  including the flash zone  14  and the wash zone  16 . The flash zone  14  has a tangential inlet  18  that leads to an annular ring  20 . The annular ring  20  is formed as on open bottom channel with side walls and top wall. As seen in  FIG. 2 , the annular ring  20  defines a central hollow core  22  with an interior wall  23 . While the ring  20  is shown as entirely annular it is also possible to use segments of the arcuate wall. Additionally, it is also possible to use more than one inlet  18 ; for example, a pair of opposed tangential inlets can also be utilized, as shown in  FIG. 3  (one inlet designated  18 ). 
     As explained above, mixed phase feed, formed of vapor and liquid, enters the tower  10  through the inlet  18  and becomes separated under the centrifugal action in the annular ring  20 . The separated liquid L then moves down the tower  10  by gravity and is treated in the bottom, or stripping section, of the tower  10 . 
     The vapor V plus any entrained resid moves up the tower  10  through the core  22  of the annular ring  20 . The vapor travels through the overflash collection tray  24 , also called a chimney tray, and into the wash zone  16  for entrainment removal. The captured entrainment is washed down by injecting wash oil from an injection device  26  at the top of the wash zone  16 . The wash oil is typically vacuum gas oil. Part of the wash oil is vaporized, and rises upward in the tower  10  to another collector tray  28 , if desired. The remainder wash oil, including the captured entrainment, or overflash is dropped into the overflash collection tray  24  and is removed via the overflash outlet  30 . The overflash is typically sent to the stripping section of the tower  10  along with the liquid portion from the feed. 
     As seen in  FIGS. 1 and 2 , in this distillation tower  10 , additional de-entrainment devices are used to reduce or eliminate resid entrainment in the vapor before it reaches the wash zone. Specifically, at least one de-entrainment device  40  is disposed within the core  22  of the annular ring  20 . As seen in  FIG. 2 , in a preferred embodiment, a plurality of de-entrainment devices  40  are spaced within the core  22 . The de-entrainment devices  40  are disposed to take advantage of the areas within the core that experience the highest velocities in the rising vapor. 
       FIG. 3  shows a map of a velocity profile taken at the inlet nozzle elevation projected at the X-Y (horizontal) plane of the ring  20 . The highest velocities occur in the channel of the annular ring  20  near the inlet (in this case, near both inlets). Within the core  22 , the highest velocities occur at the interior core wall  23  and diminish toward the center of the core  22 . This is the effect of the centrifugal forces of the spinning vapor within the channel of the ring  20  and the vapor that flows under the interior wall  23  into the core  22 . The de-entrainment devices  40  take advantage of the very high velocities near the wall  23  to de-entrain droplets of resid that could not be removed by centrifugal action alone. The devices  40  can be arranged in various configurations within the core  22 , as described below. While the devices  40  could extend entirely across the diameter of the core  22 , it is not necessary as the velocity of the vapor at the center of the core  22  is greatly diminished. The devices  40  are effective when extending outwardly a defined distance from the wall  23  so as to act as barriers to the upwardly swirling vapor. For example, the devices  40  can extend outward from the wall  23  about 2 to 4 feet into the core  22 . 
     The devices  40  can be formed as any type of barrier or partition that captures the liquid entrained with the rising vapor and channels the liquid from the core to prevent re-entrainment with the swirling vapor. In the most simple form, the barrier or partition can be a wall with downwardly extending channels. As seen in  FIG. 4 , the channels  42  may be shaped as elongated chevrons or V-shaped elements. The devices could also be formed of crinkled wire mesh screens  44 , as seen in  FIG. 5 .  FIG. 6  shows the barrier formed as a wall  46  disposed at an angle to vertical. The number, dimensions, elevation, angle, and impact on velocity profile can vary depending on application. For example, the devices  40  can be evenly spaced around the core  22 . The devices  40  could also be disposed in a vertically staggered or offset arrangement. 
       FIG. 7  shows one embodiment of the de-entrainment devices  40  in which a partition  50  extends outwardly from the interior wall  23  of the core  22  and has V-shaped channels  42  disposed in a staggered fashion across the partition  50 . In this case, the partition  50  is made such that vapor can flow through portions of the partition  50 . For example, the partition  50  may be made of screen or of an open framework or merely horizontal supports. The partition  50  has a front  52  and a back  54  offset from the front so that the channels  42  can extend across the full width of the partition  50  while still allowing the vapor represented by the arrow to flow through the partition  50 . As can be understood by  FIG. 7 , the vapor flowing through and past the partition  50  will impinge on the channels  42 , which will cause liquid to separate and flow downwardly toward the bottom of the core  22 . By this, the separated liquid will not become re-entrained with the swirling vapor. 
       FIG. 8  shows a side view of partition  56  formed as an open framework with the channels  42  supported thereon. As can be understood, any suitable supporting device can be used to support the channels in a desired configuration. In this case, a single wall is provided but multiple staggered walls, similar to those shown in  FIG. 7 , could also be provided. 
       FIG. 9  shows another configuration in which a partition  60 , which again could be a screen, mesh or other permeable wall, has a series of vertically oriented channels  62  with flanges  64  disposed along their length to catch liquid and direct it to the channels as the vapor swirls around and upwardly through the core  22 . The channels  62  could be grooves or simply elongated elements that would naturally direct liquid flow downward due to gravity. The flanges  64  could be cap elements upon which the vapor impinges and the liquid separates and clings to before naturally flowing downwardly due to gravitational forces. By catching and directing the separated liquid downwardly, re-entrainment can be diminished. 
       FIG. 10  illustrates another modification of a partition in which a screen wall  70  has a series of elongated curved drainage elements  72  supported thereon. The drainage elements  72  can be half-pipes supported on the screen wall  70  or could be grooves cut into another type of supporting wall. 
     These embodiments are intended to illustrate the various forms the drainage channels can take along with the various types of support partitions suitable for use in the core. With any of these configurations, multiple offset layers of channels, as seen in  FIG. 7 , could be used. Additionally, the channels could be vertically offset as well. The channels can extend to the lower edge of the interior core wall  23  or could extend below the core wall  23  to ensure that the liquid is directed to a point beneath the upwardly flowing vapor to prevent re-entrainment. It is even possible to provide drainage formations in the interior core wall  23  in communication with the de-entrainment devices  40  in order to direct liquid away from the swirling vapor. 
     The assembly can be configured based on the particular desired application. Factors that will impact the design parameters include the type, quantity, and desired distillation rate of the feed to be distilled. Other factors include the vapor-liquid feed ratio, the vapor and liquid density, and the flash zone pressure. 
     Various changes to the design details can be made and remain within the scope of the invention. The design dimensions, which may vary, include the radius of the tower, the radius of the channel, the height of the channel, the angle between the de-entrainment devices, the number of devices, and particular configuration of the devices. 
     The liquid-vapor separation assembly can be used for distillations conducted at superatmospheric, atmospheric, and subatmospheric pressures. It is applicable to the distillation of feeds such as petroleum crude oils, natural gas condensate, residua, heavy oils, and the like. The assembly may also be used for the distillation of feeds other than petroleum and petroleum refining derived liquids and liquid-vapor mixtures. For example, suitable feeds include mixtures of organic and inorganic solvents, organic oxygenates and mixtures thereof, and mixtures of distillable natural products. 
     The vapor will have both horizontal and vertical velocity components. The de-entrainment devices can be chosen and designed to handle both components. For example, to improve drainage, the de-entrainment devices and/or the drainage channels may be placed at a non-vertical angle. The devices also may have integral gutters or other drainage configurations. 
     If the de-entrainment devices become coked or plugged, the distillation tower function will not be affected. Since the remaining open area of the core is sufficient for the passage of vapor, operation of the tower will not be adversely affected. In particular, even if the de-entrainment devices become coked or plugged, the distillation tower will not need to be shut down. Additionally, there will be virtually no pressure drop if the de-entrainment devices become plugged. 
     Another benefit of the assembly disclosed herein is that the swirl velocity of the vapor will be reduced, which will improve the vapor distribution of the packed bed above the flash zone. Also, it is desirable to reduce the velocity as high vapor velocities can re-entrain the collected liquid. 
     Various modifications can be made in our invention as described herein, and many different embodiments of the device and method can be made while remaining within the spirit and scope of the invention as defined in the claims without departing from such spirit and scope. It is intended that all matter contained in the accompanying specification shall be interpreted as illustrative only and not in a limiting sense.