Abstract:
A centrifugal vapor/liquid separator separates the vapor and liquid in a flash of hydrocarbon and steam mixture, such that only the vapor stream is fed and processed further downstream. The design of the separator ensures that all partially wetted surfaces in the separator, except at the vapor outlet pipe, are well-wetted and washed by the non-vaporized liquid portion of the feed or by injection of external liquid thus ensuring that no coke deposition occurs inside the separator. The flash temperature in the separator therefore can be increased beyond the typical limit thus achieving a deeper cut into the feed and recovering a larger fraction of the feed as vapor for further downstream processing.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to Assignee&#39;s co-pending application entitled “THERMAL CRACKING OF CRUDE OIL AND CRUDE OIL FRACTIONS CONTAINING PITCH IN AN ETHYLENE FURNACE” and further identified as application Ser. No. 09/520,491, filed on Mar. 8, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to pre-heating hydrocarbon feedstock materials prior to cracking the feedstock. More specifically, the invention relates to the separation of the liquid and vapor components of the feedstock. 
     2. Description of Related Art 
     The concept of a flow-through cyclone, for separation of solids or liquids from a carrier gas, is well established in the literature. 
     Assignee&#39;s present olefins gas oil steam cracker plant uses a separator (or knock-out pot) to separate heavy hydrocarbon in the feed before the vaporizable fraction of the heavy feed enters the radiant tubes of a pyrolysis furnace. 
     The vane portion of the vapor/liquid separator design disclosed herein, which is used to impart the centrifugal force necessary for separation of the incoming gas and liquid phases, is similar to that originally designed by Assignee and which is currently used in Assignee&#39;s Catalytic Cracking Unit&#39;s (CCU) “third-stage separators”(TSS), for separating very fine (typically less than 20 micron) solid catalyst particles from flue gas exiting CCU regenerator vessels. This work focused on separation of dry cat cracking catalyst from a vapor stream, flow was downward through the vane assembly, then the gas would reverse and flow upward through the central hub. Catalyst fines drop out through the bottom of the separator. The vane design disclosed herein was selected since it provides a very smooth aerodynamic acceleration and spin to the incoming gas/liquid mixture necessary to achieve high separation efficiency and low pressure loss. The vane design is further distinguished by its lack of stagnant zones which would lead to areas of coke deposition. In addition, unlike conventional tangential entry type cyclone separators which typically feature a single, asymmetrical inlet slot or pipe opening, the vane itself is comprised of a series of vane elements or blades which are responsible for imparting a uniform centrifugal force to the incoming gas/liquid mixture along the entire circumference of the inlet section of the vapor/liquid separator. 
     SUMMARY OF THE INVENTION 
     A specially designed centrifugal vapor/liquid separator separates the vapor and liquid in a flash of hydrocarbon and steam mixture, such that only the vapor stream is fed and processed further downstream. The design of the separator ensures that all partially wetted surfaces in the separator, except at the vapor outlet pipe, are well wetted and washed by the non-vaporized liquid portion of the feed. The surface wetting requirement ensures that no coke deposition that would eventually lead to plugging of the separator occurs inside the separator. With the surface-wetting provision preventing coking, the flash temperature in the separator can be increased beyond the typical limit (limited because of the coking concern), thus achieving a deeper cut into the feed and enabling the recovery of a larger fraction of the feed as vapor for further downstream processing. 
     One application of the instant vapor/liquid separator is in pre-processing heavy olefins plant feed (crude or condensates) by flashing the hydrocarbon feed with steam at high temperature, then mechanically separating the non-vaporizable liquid fraction by this vapor/liquid separator so that only the vaporizable fraction of the feed is fed further downstream to be processed in the radiant tubes of a thermal pyrolysis furnace. The liquid, non-vaporizable portion contains heavy hydrocarbons such as pitch which are separated and sent to a coker, cat cracker, or other residue-processing units for further processing, or as fuel. 
     This particular invention further distinguishes itself by relying on uniformly wetted walls to mitigate coking that would reduce the service life of a normally operated, non-irrigated cyclone. The multiple-inlet type of vane design described herein is especially well suited to the creation and maintenance of a uniform film of irrigating liquid on the internal surfaces of the vapor/liquid separator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of a flow diagram of the overall process in a pyrolysis furnace which may be used with the instant invention. 
     FIG. 2 is an elevational view, partly in section, of a vapor/liquid separator according to the invention. 
     FIG. 3 is a plan view of FIG.  2 . 
     FIG. 4 is a perspective drawing of the vane assembly of the vapor/liquid separator of FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The heavy ends of crude oils and heavy natural gas liquids cannot be vaporized under typical ethylene furnace convection section conditions. They are normally removed by distillation, and only the lighter, vaporizable fraction from the distillation is used as olefin plant feeds. The feed preparation step of distilling off the heavy ends from the olefins plant feed require additional capital and operating cost. The present inventive apparatus and process integrates the heavy end separation step with the feed pre-heater section of the modified olefins furnace, allowing only the vaporizable fraction of the heavy feed to enter the cracking zone of the furnace. 
     Furthermore, the ability to flash the hydrocarbon with dilution steam at a temperature higher than that typically achievable in a vacuum column (750° F.), results in a higher fraction of the crude oil being recovered as olefins plant feed than that recovered through the atmospheric/vacuum distillation columns, thus reducing the yields of the lower value heavy end pitch. This is achieved through a non-coking vapor/liquid separator design according to the instant invention. The vapor/liquid separator can be operated over a wide range of temperatures, e.g., 500-900° F. Optimal conditions are determined by acceptable coking over a desired temperature range. 
     The convection section of a typical olefin pyrolysis furnace is modified such that heavy hydrocarbon feeds can be fed directly to the cracking furnace. Heavy hydrocarbon feeds include desalted crude oils, heavy natural gas liquids, long and short residues which contain heavy tail-end hydrocarbons that cannot be completely vaporized under normal operating conditions in the convection section of an olefins pyrolysis furnace. 
     Referring now to FIG. 1 which is a schematic view of an ethylene furnace  10 , the heavy hydrocarbon feed  11  enters through first stage preheater  12  of the convection section A of ethylene furnace  10  at a temperature of about 285° F. and at a pressure of 300 psig. A small amount of dilution steam  13  (saturated steam at ˜100 psig) is fed into the convection section preheater tubes until it is heated to a temperature ranging from about 650-900° F. at a pressure of about 70-80 psig, at which point the mixed hydrocarbon and steam  14  is fed into a vapor/liquid separator  20 . The vapor/liquid separator  20  removes the non-vaporized portion  15  of the mixed hydrocarbon feed and steam  14 , the non-vaporized liquid  15  being withdrawn and separated from the fully vaporized hydrocarbon  16 . Depending on the heavy hydrocarbon feed  11 , different processing schemes may be employed. 
     The vaporized portion  16  of the mixed hydrocarbon feed and steam  14  subsequently fed through a vaporizer/mixer  17 , in which the hydrocarbon vapor  16  mixes with superheated steam  18  to heat the mixture  19  temperature to about 950-1050 ° F. external to the furnace  10 . The mixture  19  is then heated further in the second stage preheater portion  21  of the convection section A of the olefins pyrolysis furnace  10  and is subsequently fed into the radiant section B,  22  of the pyrolysis furnace  10  where the hydrocarbon mixture  19  is thermally cracked. 
     The conditions of the hydrocarbon/steam mixture  14  at the entrance of the vapor/liquid separator  20  are dependent on the heavy hydrocarbon feed  11  properties, with the requirement that there always be enough non-vaporized liquids  15  (between 2-40 vol % of feed, preferably 2-5 vol %) to wet the internal surfaces of the vapor/liquid separator  20 . This wetted wall requirement is essential to prevent coke formation and deposition on the surface of the separator  20 . The degree of vaporization (or vol % of non-vaporizable liquid  15 ) can be controlled by adjusting the dilution steam/feed ratio and flash temperature of the hydrocarbon/steam mixture  14 . 
     The vapor/liquid separator  20  described herein permits separation of the liquid  15  and vapor  16  phases of the flash mixture in such a manner that coke solids are not allowed to form and subsequently foul either the separator  20  or the downstream equipment (not shown). On account of its relatively compact construction, the wetted- wall vapor/liquid separator  20  design can achieve a higher temperature flash than that in a typical vacuum crude column, thus effecting the recovery of a higher vaporized fraction  16  of the feed  11  for further downstream processing. This increases the fraction of hydrocarbon feed  11  which can be used for producing higher valued products  23 , and reduces the fraction of heavy hydrocarbon liquid fraction  15  having a lower value. 
     Referring now to FIGS. 2 and 3, the vapor/liquid separator  20  is shown in a vertical, partly sectional view in FIG.  2  and in a sectional plan view in FIG.  3 . The vapor/liquid separator  20  comprises a vessel having walls  20   a , an inlet  14   a  for receiving the incoming hydrocarbon/steam mixture  14 , a vapor outlet  16   a  for directing the vapor phase  16  and a liquid outlet  15   a  for directing the liquid phase  15 . Closely spaced from the inlet  14   a  is a hub  25  having a plurality of vanes  25   a  spaced around the circumference of the hub  25 , preferably close to the end nearest the inlet  14   a . The vane assembly is shown more clearly in the perspective view of FIG.  4 . The incoming hydrocarbon/steam mixture  14  is dispersed by splashing on the proximal end of the hub  25  and, in particular, by the vanes  25   a  forcing a portion of the liquid phase  15  of the mixture  14  outwardly toward the walls  20   a  of the vapor/liquid separator  20  thereby keeping the walls  20   a  completely wetted with liquid and preventing any coking of the interior of the walls  20   a . Likewise, the outer surface of the hub  25  is maintained in a completely wetted condition by a liquid layer that flows down the outer surface of hub  25  due to insufficient forces to transport the liquid  15  in contact with the surface of hub  25  to the interior of the walls  20   a . A skirt  25   b  surrounds the distal end of the hub  25  and aids in forcing any liquid transported down the outer surface of the hub  25  to the interior of the walls  20   a  by depositing said liquid into the swirling vapor. The upper portion of the vapor/liquid separator  20  is filled in at  20   b  between the inlet  14   a  and hub  25  to aid wetting of the interior of walls  20   a  as the vapor/liquid mixture  14  enters the vapor/liquid separator  20 . As the liquid  15  is transported downward, it keeps the walls  20   a  and the hub  25  washed and prevents the formation of coke on their surfaces. The liquid  15  continues to fall and exits the vapor/liquid separator  20  through the liquid outlet  15   a . A pair of inlet nozzles  26  is provided below the vapor outlet tube  16   a  to provide quench oil for cooling collected liquid and reduce downstream coke formation The vapor phase  16  enters the vapor outlet duct  16   a  at its highest point  16   c , exits at outlet  16   a  and proceeds to a vaporizer  17  for further treatment prior to entering the radiant section B  22  of the pyrolysis furnace  10  as shown in FIG. 1. A skirt  16   b  surrounds the entrance  16   c  to the vapor duct  16  and aids in deflecting any liquid  15  outwardly toward the separator walls  20   a.    
     EXAMPLE 1 
     A 70% scale, cold-flow clear plastic and metal model using water and air was tested and refined in the laboratory. In the cold-flow test model, the vapor/liquid separation was so effective that no liquid phase was detected at the vapor outlet, and visual observation showed that the internal surfaces of the model vapor/liquid separator remained well-irrigated by an active flow of the incoming liquid phase over these surfaces. Such liquid coverage is required to prevent run-limiting coke formation. 
     The important data for sizing include vapor rate, density and viscosity. Liquid rate, density and surface tension are also checked as a comparison with the performance of the air/water model and to estimate the drop sizes reporting to the separator. 
     The inlet pipe size recommended (eight inch diameter) was chosen to provide a calculated liquid drop size. 
     The vane assembly sizing was determined and sized to give a velocity through the vanes of 80-100 Ft/Sec. For Assignee&#39;s current design, 18″ Schedule 80 pipe and twelve vanes attached to a ten inch diameter Schedule 40 pipe, the estimated velocity is 88 Ft/Sec through the 30° flat section of the vanes. This vane assembly is shown in FIG.  4 . 
     Position of the vane assembly  25   a  relative to the entrance  14   a  and ‘filling’ in of the top head  20   b  of separator  20  was guided by computational fluid dynamics modeling. The intent was to remove areas of potential recirculation to reduce coking tendencies. The internal shape of the head  20   b  was formed to follow the stream lines of the gas so the walls  20   a  would remain washed by liquid that was pushed into the main body of the separator  20 . 
     The distance of the hub  25  extension below the vanes  25   a  was picked based on estimation of the liquid drop size that would be captured before the drop had moved more than half way past the hub  25 . Significant liquid  15  will be streaming down the hub  25  (based on observations with the air/water model) and the presence of a ‘skirt’  25   b  on the hub  25  will introduce liquid droplets into the vapor phase well below the vanes  25   a , and collection will continue below the skirt  25   b  of hub  25  due to the continued swirl of the vapor  16  as it moves to the outlet tube  16   a.    
     The hub skirt  25   b  was sized to move liquid from the hub  25  as close as possible to the outer wall  20   a  without reducing the area for vapor  16  flow below that available in the vanes  25   a . As a practical matter, about 20% more area for flow has been provided than is present at the vanes  25   a.    
     The distance between the bottom of the hub  25  and the highest point  16   c  of vapor outlet tube  16   a  was sized as four times the vapor outlet tube  16   a  diameter. This was consistent with the air/water model. The intent is to provide area for the vapor to migrate to the outlet  16   a  without having extremely high radial velocities. 
     The distance from the entrance  16   c  of the vapor outlet tube  16   a  to the centerline of the horizontal portion of vapor outlet pipe  16   a , has been chosen as roughly three times the pipe diameter. The intent is to provide distance to keep the vortex vertical above the outlet tube  16   a —not have it disturbed by the proximity of the horizontal flow path of the vapor  16  leaving outlet tube  16   a . The position and size of the anti-creep ring  16   b  on the vapor outlet tube  16   a  are somewhat arbitrary. It is positioned close to, but below, the lip and is relatively small to allow room for coke to fall between the outer wall  20   a  and the ring  16   b.    
     Details of the separator  20  below the outlet tube  16   a  have been dictated by concerns outside the bounds of this separator. As long as nothing is done to cause liquid to jet above the inlet  16   c  to the outlet tube  16   a , there should be no impact to separation efficiency. 
     Chief areas of coking concern involve sections with vapor recirculation, or metal not well washed with liquid. The area  20   b  inside the top head may be shaped or filled with material to approximate the expected recirculation zone. The inside of the hub  25  is another potential trouble point. If coke were to grow and fall over the inlet  16   c  to vapor outlet tube  16   a , a significant flow obstruction could occur (such as a closed check valve). For this reason, a cage or screen  25   c  of either rods or a pipe cap may be used. This would not prevent the coke from growing, but would hold most of it in place so that a large chunk is not likely to fall. Areas under the vane skirts and the skirts  16   b  on the vapor outlet tube  16   a  are also ‘unwashed’ and coke growth in these areas is possible. 
     The lab model on which these design rules have been ‘validated’ has been tested over a wide range of flow conditions as shown in TABLE 1 below. Air rates ranged from 50-150 Ft/Sec at the inlet and water was tested at 1-4.5 gpm. Over all these conditions, losses were below the measurable range. At water flows less than 1 gpm (estimated at 0.5-0.75 gpm) the wetting of the separator outer wall  20   a  was less than complete. Streamers of water ran down the plexiglass, with ‘dry’ areas between. In terms of gpm water per inch of circumference, at 1 gpm water the separator walls  20   a  were washed at a rate of 0.032 gpm/inch. The design data oil rate, 4,116 Lb/Hr at 41.1 Lb/Ft{circumflex over ( )}3, or 12.5 gpm would give a wash rate of 0.246 gpm/inch. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Low Air/ 
                 High Air/ 
                 Plant Design 
               
               
                   
                 High Water 
                 Low Water 
                 Case 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Vapor Inlet 
                 50 
                 150 
                 83 
               
               
                 Velocity, Ft/Sec 
               
               
                 Vapor Vane 
                 58 
                 150 
                 88 
               
               
                 Velocity, Ft/Sec 
               
               
                 Vapor Rate, Lb/Hr 
                 1454 
                 4362 
                 45885 
               
               
                 Vapor Rate, 
                 303 
                 909 
                 1734 
               
               
                 ACFM 
               
               
                 Liquid Rate, Lb/Hr 
                 2250 
                 500 
                 4116 
               
               
                 Liquid Rate, 
                 4.5 
                 1 
                 12.5 
               
               
                 GPM 
               
               
                 Lb Liquid/ 
                 1.55 
                 0.11 
                 0.090 
               
               
                 Lb Vapor 
               
               
                 GPM Liquid/ 
                 0.0031 
                 0.0011 
                 0.0072 
               
               
                 ACFM Vapor 
               
               
                 GPM Liquid/Inch 
                 0.14 
                 0.032 
                 0.246 
               
               
                 Separator 
               
               
                 Circumference 
               
               
                   
               
             
          
         
       
     
     If the coking tendency of the separator walls  20   a  is controlled by the wash rate (liquid volumetric flow rate per circumferential inch), the plant design conditions should provide better washing than the lab model. Assuming the plant wash properties track those of the lab, opportunity will exist to operate with feeds having lower liquid volumes. The design data indicate a liquid flow that is ‘low’ on a weight basis and ‘high’ on a volume basis, when compared to the lab. However, the lab model showed no to visual problems with separation at liquid rates below 1 gpm or above the 4.5 gpm at which data was taken.