Abstract:
Method and apparatus for contacting two liquid phases for liquid-liquid extraction in a vertical extraction vessel. A relatively heavier liquid phase, descending the extraction vessel at a relatively low volumetric flow rate, is dispersed into a continuous phase comprising a relatively lighter liquid rising through the extraction vessel at a relatively high flow rate. Sieve trays are provided with adjustable active areas and overlapping manways for personnel access.

Description:
BACKGROUND OF INVENTION  
       [0001]     The present invention relates to liquid-liquid extraction involving the dispersion of a relatively heavier liquid phase in a relatively lighter liquid phase, at elevated ratios of flow rates of light phase to heavy phase. The present invention also relates to mechanical improvements in liquid-liquid extraction equipment.  
         [0002]     In a typical form, liquid-liquid extraction processes perform mass transfer of target components from one liquid phase into a second liquid phase, typically to selectively recover valued components or eliminate undesirable components from one phase. The liquid phase containing the components to be extracted is called raffinate. The liquid phase that will extract the target components is called solvent, and when the solvent has completed the extraction, the solvent phase is called extract.  
         [0003]     In liquid-liquid extraction applications, two liquid phases are characterized by, among other properties, different relative solubilities of the target components being extracted, and different bulk densities. The two liquid phases must be brought into intimate contact with each other to make the mass transfer process efficient. A typical method of achieving intimate liquid-liquid contact entails breaking up a light liquid phase into small droplets and dispersing those droplets into a heavy liquid that is sustained as a continuous phase. This can be achieved using, for example, perforated sieve trays arranged successively in a preferably vertical liquid-liquid extraction vessel. In such a system the light and heavy liquids are typically introduced near bottom and top ends, respectively, of the extraction vessel, and the liquids flow countercurrently past and through one another because of differing respective liquid weights.  
         [0004]     Prior-art liquid-liquid extraction applications in sieve tray systems typically disperse the light liquid phase as droplets, as the light phase rises through sieve tray perforations. The light phase droplets pass through a continuous, heavy liquid phase above each sieve tray. This physical arrangement can be appropriate for applications in which light-phase flow rates are roughly equal to or substantially lower than heavy phase flow rates. The practice of dispersing a rising light phase also has been particularly favored to minimize maintenance and capacity problems associated with solids that may settle out of either liquid phase and block tray perforations by coming to rest on the tray surfaces.  
         [0005]     U.S. Pat. No. 4,247,521 to Forte et al discloses a method of liquid-liquid extraction in terms of applications to systems characterized by substantial excess in the flow of the heavy phase over that of the light phase. Forte et al also features increasing tray riser complexity as vessel and tray diameter increase.  
         [0006]     U.S. Pat. Nos. 5,047,179 and 5,049,319 to Nye address increasing active sieve tray area while also adding to vapor flow area to promote phase disengagement in vapor-liquid applications such as distillation. This can improve both fluid throughput capacities and vapor-liquid contact efficiency, for net gains in unit capacity and separation effectiveness.  
         [0007]     Bravo et al, “Sulfolane® RDC Tray Revamp,” AlChE Meeting, Chicago, Ill., November 1996, presents a case of retrofitting a liquid-liquid extraction vessel to replace the internals of a rotating disc contactor (RDC) with fixed sieve trays having five upcomers per tray.  
         [0008]     Zhu et al, “Hydrodynamic and Mass Transfer Performance of Multiple Upcomer Extraction Trays,” Canadian Journal of Chemical Engineering, August 1997, describes laboratory work investigating operation of sieve trays with three upcomers (risers).  
         [0009]     Each of the above patents and references is hereby incorporated by reference in its entirety.  
         [0010]     In the particular case of deasphalting lubricating oils, a rotating disc contactor (RDC) has been commonly used. RDC&#39;s have evidenced operating problems such as backmixing or premature flooding, which can be caused by excessive shear of the rotating discs that can create too fine a dispersal of process liquids. Results of very fine dispersion in a liquid-liquid extraction unit can include internal recirculation or entrainment of the dispersion. Both phenomena reduce extraction efficiency and capacity. Lube oil deasphalting typically operates at elevated pressures that may equal or exceed supercritical pressure, and RDC&#39;s have been prone to seal leakage where disc drive shafts penetrate the extraction vessels. In a number of RDC units seal leakage has been stopped or limited by welding the disc drive shaft seals, thus abandoning any advantages of agitation to enhance extractive mass transfer, and thereby limiting such columns to perform either at reduced throughput or reduced separation, or both.  
         [0011]     Liquid capacity, extraction efficiency, and product yield of RDC units are affected by excessive shear, with the potential impacts of light-phase backmixing and/or heavy-phase entrainment. Both phenomena interfere with phase separation and cause unproductive secondary phase contact, thus reducing extraction efficiency. One way to counteract backmixing and entrainment is to reduce liquid throughput rates, allowing added residence time for phase separation, but also reducing column capacity. Any of the three results i.e. operating with backmixing or entrainment, or operating at reduced liquid capacity reduces product yield.  
       SUMMARY OF INVENTION  
       [0012]     We have discovered a liquid-liquid extraction method and apparatus achieving intimate contact between two liquid phases by passing a relatively heavier liquid phase descending through perforated sieve trays in a liquid-liquid extraction vessel. The sieve trays generate droplets of the heavy liquid and disperse the heavy liquid droplets into a continuous, relatively lighter ascending liquid phase. This invention is designed to handle liquid traffic at varying flow rates and, in particular, it can pass a substantial excess of the light-phase flow rate compared to the heavy-phase flow rate. Another aspect of this invention addresses potential plugging of sieve tray perforations, both by proportionally slowing a rate of loss of tray perforation area and by facilitating restoration of degraded perforation area.  
         [0013]     This invention offers mechanical, operating, and economic improvements for sieve trays, as used in liquid-liquid extraction. Mechanical tray configuration is simplified, especially for large-diameter extraction vessels. Sieve trays can be provided with supplemental active area, and/or adjustable blanking strips as flow control elements that enable liquid throughput capacities to vary more widely with stable operation than prior art designs are capable of. The invention can facilitate prolonged operation between major cleaning turnarounds plus rapid, thus economical, cleaning and maintenance.  
         [0014]     A key feature in one embodiment of this invention is simplified access, wherein manway panels in sieve trays are more or less in alignment with one another, which can facilitate movement of personnel and materials during entry into a vessel. The invention also is particularly well suited to retrofit applications to improve performance and reliability of existing extraction units. Of note, retrofit candidates include units using mechanical internals such as rotating disc contactors (RDC&#39;s) and passive packing systems, and typically high-fouling services such as deasphalting of lubricating oil (lube oil) feedstocks can be beneficially converted to use the methods of this invention.  
         [0015]     One embodiment of the invention is a liquid-liquid extraction vessel having a cylindrical shell with an inside diameter of at least 1.5 m and an array of sieve trays vertically spaced in the shell. There is a perforated deck in each tray, and a single riser or a pair of parallel risers. The risers have top and bottom sections, and the bottom section has a cross-sectional flow area larger than a cross-sectional flow area of the top section. An exterior manway is formed in a wall of the vessel adjacent to at least one of the trays. A manway hatch is formed in the perforated deck of each tray for personnel access to each of the trays. The manway hatches in each tray preferably overlap in plan with a manway hatch in an adjacent tray.  
         [0016]     The risers can be alternating peripheral single-pass risers, alternating midsection-peripheral two-pass risers, or alternating midsection-peripheral three-pass risers. The vessel can also include blanking strips removably or adjustably secured to the perforated decks. The perforated decks can be assembled from a plurality of panels. The risers can include a perforated restriction plate between the top and bottom sections.  
         [0017]     In another embodiment the invention provides a liquid-liquid extraction method for contacting a relatively heavy liquid phase with a relatively light liquid phase. The method includes introducing a feed stream of the heavy phase at an upper inlet of a liquid-liquid extraction vessel comprising a plurality of successive, vertically arrayed trays including at least one perforated deck per tray and at least one riser per tray. The risers include respective top and bottom sections, and the bottom riser sections have larger transverse cross-sectional areas than respective top riser sections. The method includes introducing a feed stream of the light phase into a lower inlet of the extraction vessel at a volumetric flow rate greater than that of the heavy phase. The heavy phase is passed through perforations in the decks of successive trays to disperse droplets of the heavy phase into respective cross-flow zones below the decks. The heavy phase is collected on respective upper surfaces of the successive decks. The light phase is passed through respective cross-flow zones into adjacent disengagement zones and through the respective risers to discharge into succeeding cross-flow zones.  
         [0018]     The method can also include contacting the heavy and light phases above an uppermost one of the trays by distributing the heavy-phase feed stream adjacent the upper inlet across an upper distribution zone, passing the light phase upwardly from the at least one riser of the uppermost tray to countercurrently contact the heavy phase in the upper distribution zone, passing the light phase upwardly from the upper distribution zone into an ultimate disengaging zone to separate heavy-phase droplets into the upper distribution zone, and discharging the light phase essentially free of entrained heavy phase as an effluent from an upper outlet of the extraction vessel in communication with the disengaging zone.  
         [0019]     The method can similarly include contacting the heavy and light phases below a lowermost one of the trays by distributing the light-phase feed stream adjacent the lower inlet across a lower distribution zone, passing the heavy phase downwardly from the lowermost tray into the lower distribution zone to countercurrently contact the light phase in the lower distribution zone, passing the heavy phase downwardly from the lower distribution zone to an accumulation zone to coalesce the heavy phase, and discharging the heavy phase lean in entrained light phase as an effluent from a lower outlet of the extraction vessel in communication with the accumulation zone.  
         [0020]     The upward flow of the light phase through respective trays can be constrained with flow restrictions in the risers. The flow restrictions can be a restrictive cross-sectional area of the at least one top riser of the tray, or a perforated restrictor plate between the top and bottom riser sections. The method preferably includes alternating the configuration of the risers on the successive trays. The risers can be single-pass peripheral risers, midsection-peripheral two-pass risers, or midsection-peripheral three-pass risers.  
         [0021]     The ratio of the volumetric flow rates of the light phase feed to the heavy phase feed is greater than 1:1, preferably greater than 1.5:1, more preferably from 5:1 to 15:1, and especially from 6:1 to 10:1. The heavy liquid phase can be solvent and the light liquid phase a raffinate, or the heavy liquid phase raffinate and the light liquid phase solvent. In one particular embodiment of the method, the heavy-phase feed stream is lubricating oil feedstock containing asphaltenes, the light-phase feed stream is a solvent selected from aliphatic or cycloaliphatic hydrocarbons having from 3 to 5 carbon atoms, and the ratio of the volumetric flow rates of the light phase to the heavy phase is in a range from 6:1 to 10:1.  
         [0022]     In one preferred embodiment of the method, removable blanking strips are secured to the tray decks to block a first portion of the tray perforations and leave a second portion of the perforations unobstructed for said heavy phase passage. When needed or desired to increase the rate of the heavy phase, at least one of the blanking strips is removed to pass the heavy phase through unobstructed perforations of the first portion. Additionally or alternatively, adjustable blanking strips are secured to the tray decks to selectively block and unblock at least a portion of the tray perforations, and adjusted to increase or reduce the rate of passage of the heavy phase through the respective portions of the tray perforations.  
         [0023]     Another embodiment of the invention provides a liquid-liquid extraction unit for contacting a heavy liquid phase with a light liquid phase. The unit includes means for introducing a feed stream of the heavy phase for downward flow at a volumetric flow rate entering an upper inlet of a liquid-liquid extraction vessel comprising a plurality of successive, vertically-arrayed trays including at least one perforated deck per tray and at least one riser per tray. The risers include respective top and bottom sections, wherein the bottom riser sections have larger transverse cross-sectional areas than respective top riser sections. The unit also includes means for introducing a feed stream of the light phase into a lower inlet of the extraction vessel, for upward flow as a continuous phase at a volumetric flow rate greater than the heavy phase flow rate, means for passing the heavy phase through perforations in the decks of successive trays to disperse droplets of the heavy phase into respective cross-flow zones below the decks, means for collecting the heavy phase on respective upper surfaces of the successive trays, and means for passing the light phase through respective cross-flow zones into adjacent tray disengagement zones and through the respective risers to discharge into succeeding cross-flow zones.  
         [0024]     A further embodiment of the invention provides a liquid-liquid extraction vessel. The vessel has an upper inlet to the extraction vessel to introduce a feed stream of a heavy phase at a volumetric flow rate, and a plurality of successive, vertically-arrayed trays including at least one perforated deck per tray and at least one riser per tray. The risers include respective top and bottom sections. The bottom riser sections have larger transverse cross-sectional areas than respective top riser sections. The tray is imperforate in an area of the riser bounded between attachments of the respective top and bottom riser sections to the tray. A lower inlet to the extraction vessel is provided to introduce a feed stream of a light phase at a greater volumetric flow rate than the heavy phase. Perforations in the tray decks pass the heavy phase downward and disperse droplets of the heavy phase into a continuum of the light phase. Cross-flow zones are provided below the respective tray decks to pass the heavy-phase droplets downwardly therethrough. Collection zones are provided below the respective cross-flow zones to coalesce the heavy-phase droplets on respective upper surfaces of successive decks. The vessel has disengagement zones under the bottom sections of the risers to receive the light-phase from the cross-flow zones and disengage entrained heavy phase droplets.  
         [0025]     The vessel can have an upper distributor in communication with the upper inlet to distribute the heavy phase feed stream in the extraction vessel, an upper distribution zone adjacent the upper distributor to contact the heavy and light phases in counter-current flow above an uppermost one of the trays, an ultimate disengaging zone above the upper distribution zone to separate droplets of heavy phase from the light phase, and an upper outlet in communication with the disengaging zone to discharge light phase effluent from the extraction vessel. Similarly, the vessel can also include a lower distributor in communication with the lower inlet to distribute the light phase in the extraction vessel, a lower distribution zone adjacent the lower distributor to contact the heavy and light phases in counter-current flow below a lowermost one of the trays, an accumulation zone disposed below the lower distribution zone to coalesce the heavy-phase droplets, and a lower outlet in communication with the accumulation zone to discharge heavy-phase effluent from the extraction vessel.  
         [0026]     The vessel can also include flow restrictions in the risers to constrain the upward flow of light phase through respective trays. The restrictions can be a restrictive cross-sectional area of the top risers, or a perforated horizontal restrictor plate disposed in each riser. The risers can have directional passages to direct the flow of the light phase from the riser top sections laterally into respective cross-flow zones. The directional passages can be a horizontal slot formed between a transverse cap attached above an open-ended riser stack and a top edge of the open riser. Alternatively, the directional passages can be a plurality of side-facing openings in a vertical wall of a closed-ended riser stack. The risers can be single-pass peripheral risers alternatingly disposed between opposite sides on the successive trays, or midsection-peripheral two-pass or three-pass risers alternatingly disposed on the successive trays.  
         [0027]     The vessel can also include blanking strips removably secured to the perforated deck surfaces to selectively block portions of the perforations, or adjustably secured to the perforated deck surfaces to selectively block and unblock portions of the perforations. Further, the vessel can include personnel access hatches in the perforated tray decks wherein access panels of adjacent trays overlap in plan.  
         [0028]     Furthermore, the present invention in another embodiment provides a method for converting a rotating disc contactor to a sieve tray liquid-liquid extraction unit. Existing extraction vessel internal components from the rotating disc contactor are selectively removed. The internals can be trays, packing, rotating discs, agitating internals, upper and lower feed distributors, or the like. At least one tray is installed that includes at least one perforated deck and at least one riser, wherein the risers include respective top and bottom sections, the bottom riser sections have larger transverse cross-sectional areas than respective top riser sections, and the trays are imperforate in an area of the riser bounded between attachments of the respective top and bottom riser sections to the tray. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0029]      FIG. 1  is a simplified cut-away elevation of an extraction vessel with an alternating midsection-peripheral two-pass riser arrangement according to an embodiment of the present invention.  
         [0030]      FIG. 2  represents a two-pass sieve tray  111   a  from the extraction vessel of  FIG. 1  in schematic perspective and partially cut away with a central riser according to an embodiment of the invention.  
         [0031]      FIG. 3  represents two-pass sieve tray  111   b  from the extraction vessel of  FIG. 1  in schematic perspective and partially cut away with peripheral risers according to an embodiment of the invention.  
         [0032]      FIG. 4  shows simplified configurations of sieve trays and flow paths in an extraction vessel with the sieve trays of  FIGS. 2-3 .  
         [0033]      FIG. 5  is a cross-section from  FIG. 1  as seen along the lines  5 - 5 , showing an embodiment of the upper distributor.  
         [0034]      FIG. 6  is a cross-section from  FIG. 1  as seen along the lines  6 - 6 , showing an embodiment of the lower distributor.  
         [0035]      FIG. 7  shows a perforated tray deck in plan according to an embodiment of the invention with blanking strips in position to occlude some of the perforations.  
         [0036]      FIG. 8  shows the perforated tray deck of  FIG. 7  with the blanking strips moved to allow use of the perforations that were occluded in  FIG. 7 .  
         [0037]      FIG. 9  shows a cross-section of two-pass sieve trays in elevation.  
         [0038]      FIG. 10  shows a plan view of the sieve tray arrangement of  FIG. 9  to illustrate relative proportions of tray bottom risers.  
         [0039]      FIG. 11  represents a perspective view, partially cut away, of a simplified embodiment of a sieve tray with a peripheral single-pass riser.  
         [0040]      FIG. 12  is a plan of a sieve tray with midsection-peripheral three-pass risers according to the invention wherein the risers comprise a plurality of pipes or conduits of reduced diameter.  
         [0041]      FIG. 13  is an elevation of a plurality of the sieve trays of  FIG. 12  in an alternating arrangement in an extraction unit.  
         [0042]      FIG. 14  is a plan of a sieve tray with midsection-peripheral three-pass risers according to the invention wherein the risers comprise a plurality of pipes or conduits of enlarged diameter with restriction orifices.  
         [0043]      FIG. 15  is a plan of a sieve tray with midsection-peripheral three-pass risers according to the invention wherein the risers comprise a plurality of conduits of non-circular cross-section with restriction orifices.  
         [0044]      FIG. 16  depicts a plan view of a portion of a sieve tray showing blanking strips and a manway access hatch integrated in relatively large panels of the three-pass tray deck of the  FIG. 15  design.  
         [0045]      FIG. 17  depicts a plan view of another portion of the three-pass sieve tray of  FIG. 15 .  
         [0046]      FIG. 18  is an enlarged view of area  18  in  FIG. 16 , depicting the manway hatch in an open position.  
         [0047]      FIG. 19  is a cross-sectional view along the lines  19 - 19  of  FIG. 16 , showing an elevation of the tray to illustrate tray deck panels and tray supports in section.  
         [0048]      FIG. 20  is a cross-sectional view along the lines  20 - 20  of  FIG. 17 , showing an elevation through a lateral riser and riser panel supports.  
         [0049]      FIG. 21  illustrates a simplified retrofit conversion of a rotating disc contactor (shown on the left, prior art) to a tray column using a tray design of the present invention (shown on the right). 
     
    
     DETAILED DESCRIPTION  
       [0050]     In the figures, where like elements refer to like numbers,  FIG. 1  shows an embodiment of the present invention in a preferred arrangement of internals of an extraction vessel  110 , depicting generally the flow and control of the heavy and light liquid phases.  
         [0051]     The extraction vessel  110  comprises a plurality of successive, horizontal trays  111   a ,  111   b  of alternating configurations, vertically spaced between an upper vessel inlet  105  for a heavy-phase feed  100  and a lower vessel inlet  120  for a light-phase feed  115 . Both configurations of the trays  111   a ,  111   b  include respective decks  113   a ,  113   b  with at least one area on each deck having perforations  112 . The decks  113   a ,  113   b  are intersected by corresponding vertically aligned risers  114   a ,  114   b , respectively. An upper vessel outlet  130  for a light-phase effluent  125  is connected through a top wall of the vessel  110 , and a lower vessel outlet  140  for a heavy-phase effluent is connected through a bottom wall of the vessel  110 .  
         [0052]     An upper distribution zone  170  is disposed above an uppermost one of the trays  111   a  (or  111   b ). An upper distributor  109  is connected to the upper inlet  105 , and has a plurality of distribution devices  107  arrayed across a horizontal section of the vessel  110  adjacent the upper distribution zone  170 . An ultimate disengaging zone  175 , in communication with the upper outlet  130 , is disposed above the upper distribution zone  170  and the upper distributor  109 . A lower distribution zone  180  is disposed below a lowermost one of the trays  111   a  (or  111   b ) adjacent the lower inlet  120 . A lower distributor  123  is attached to the lower inlet  120  and oriented across a horizontal section of the vessel  110  adjacent the lower distribution zone  180 . An accumulation zone  185 , in communication with the lower outlet  140 , is disposed below the lower distribution zone  180 .  
         [0053]     The heavy liquid feed  100  enters the vessel  110  via the upper inlet  105 , and the heavy-phase effluent  135  discharges from the lower outlet  140 . The light liquid feed  115  enters the lower inlet  120 , and the light-phase effluent  125  discharges from the upper outlet  130 . The heavy phase  100  and the light phase  115  are substantially immiscible, and flow past and through one another in traversing the extraction vessel  110 . In passing through the vessel  110 , the heavy-phase liquid  100  is dispersed beginning with discharge from the distribution heads  107  of the upper distributor  109 . The heavy liquid  100  passes through the perforations  112  in successive tray decks  113   a ,  113   b . The perforations  112  in sieve tray decks  113   a ,  113   b  disperse the descending heavy-phase liquid as droplets  150  beneath each tray  111   a ,  111   b . Concurrently, the light-phase liquid  115  is distributed across the vessel section beginning with discharge from the lower distributor  123 . The light phase liquid  115  passes horizontally beneath respective trays  111   a ,  111   b , rising past trays  111   a ,  111   b  via the respective intersecting risers  114   a ,  114   b.    
         [0054]     The light and heavy-phase effluents  125 ,  135  collect and discharge as follows. The light phase  115  flows upwardly through the extraction vessel  110 , the trays  111   a  (or  111   b ), and into the ultimate disengaging zone  175 . There a majority of entrained heavy-phase droplets  150  settle back downward into the upper distribution zone  170 . Substantially free of entrained heavy phase droplets  150 , the light phase  115  discharges as effluent stream  125  via the upper outlet  130 . The heavy phase flows downward, progressively dispersing through and coalescing on trays  111   a ,  111   b . Droplets  150  from the lowermost tray  111   b  (or  111   a ) pass through the lower distribution zone  180  and coalesce in the accumulation zone  185  as a continuous heavy phase effluent  135 . The heavy phase effluent  135  discharges via the lower vessel outlet  140 .  
         [0055]      FIGS. 2 and 3  provide perspective views of tray configurations  111   a  and  111   b , respectively. The risers  114   a ,  114   b  comprise top riser sections  116   a ,  116   b  and connected bottom riser sections  117   a ,  117   b , respectively. The bottom riser sections  117   a ,  117   b  have larger transverse cross-sectional areas than the adjoining top riser sections  116   a ,  116   b . Adjoining the perforated area of each respective tray deck  113   a ,  113   b , a restrictor plate  118  connects each bottom riser section  117   a ,  117   b  to a corresponding top riser section  116   a ,  116   b  through the respective tray deck  113   a ,  113   b . The decks  113   a ,  113   b  are imperforate above the bottom riser section  117   a ,  117   b  in an area outside the top riser section  116   a ,  116   b . Openings  119  in vertical walls of the top riser sections  116   a ,  116   b  direct the flow of light phase  115  toward the bottom riser sections  117   a ,  117   b  of succeeding trays  111   a ,  111   b  in an upward flow path of the light phase  115 . The restrictor plates  118  successively limit flow rates of light phase  115  through the risers  114   a ,  114   b  of each tray  111   a ,  111   b , facilitating stable flow distribution through the vessel. The plates  118  aid the disentrainment of heavy-phase droplets  150  from the light phase to control heavy-phase recirculation.  
         [0056]     With reference to  FIG. 4 , the light phase  115  moves upward through the extraction vessel  110  and traverses successive cross-flow zones  160   a ,  160   b  below respective perforated decks  113   a ,  113   b . Exiting the cross-flow zones  160   a ,  160   b , the light phase  115  passes into adjacent disengagement zones  165   a ,  165   b  under respective bottom riser sections  117   a ,  117   b . Moving up the risers  114   a ,  114   b , the light phase  115  discharges from the openings  119  in the top riser sections  116   a ,  116   b , and traverses succeeding cross-flow zones  160   a ,  160   b.    
         [0057]     Flowing generally counter to the passage of the light phase  115 , the heavy phase  100  passes downward, dispersed by tray perforations  112  as droplets  150  into the successive cross-flow zones  160   a ,  160   b . At each succeeding tray  111   a ,  111   b , droplets  150  collect and tend to coalesce on upper surfaces of the respective decks  113   a ,  113   b . The coalescing permits light phase liquid  115  entrained with the heavy-phase droplets  150  to separate and avoid being recirculated downward through tray perforations  112 . The heavy-phase droplets  150  pass through the successive cross-flow zones  160   a ,  160   b  and downwardly out of a lowermost cross-flow zone below the lowermost tray.  
         [0058]     The present invention aligns single risers  114   a  on selected trays  111   a  in central positions along the middle areas of the respective trays  11   a . On alternate selected trays  111   b , a pair of peripherally disposed tray risers  114   b  are offset horizontally from the middle areas of the respective trays  111   b , and oriented in parallel with respect to the single risers  114   a . The positions of tray risers  114   a ,  114   b  are thus alternated from tray midsection or preferably center to off-center or preferably peripheral on successive trays  111   a ,  111   b , respectively. This alternation of positions directs the flow of the light phase  115  through successive cross-flow zones  160   a ,  160   b , in turn, more or less transversely inward and then transversely outward from the central risers  114   a  in succession. As used herein, the number of passes of a riser arrangement refers to the number of cross-flow zones separated by risers in each tray, e.g. two in the case of the  FIG. 1  embodiment.  
         [0059]     The perforations  112  on the tray decks  113   a ,  113   b  are of a size and number to pass the heavy phase  100  at the desired volumetric flow rate, dispersing the heavy phase  100  into droplets  150 . Droplet sizes can be consistently formed in a range of diameters by sizing the perforations  112 . Selecting a droplet sizing range and number of perforations  112  on the tray decks  113  enables extraction to operate over a range of flow rates of the heavy phase  100 .  
         [0060]      FIG. 5  illustrates a configuration of the upper distributor  109  as a ladder configuration of piping segments  108  and attached distribution heads  107 , sized to proportionally distribute the volumetric flow of the heavy liquid  100  across the upper distribution zone  170  shown in  FIG. 1 .  FIG. 6  depicts the lower distributor  123  as preferably a ladder configuration of perforated piping segments  124  to distribute the volumetric flow of the light phase. The light-phase feed stream  115  enters via the extraction vessel lower inlet  120  beneath the lowermost tray  11   a ,  111   b , and the lower distributor  123  distributes the light phase across the lower distribution zone  180 .  
         [0061]     One embodiment of the present invention addresses tendencies for some process liquid combinations to form deposits on trays  111   a ,  111   b , for example in “dirty” services such as solvent deasphalting of lube oil feedstocks. Deposits accumulating over tray perforations  112  progressively reduce flow capacity for the heavy phase  100  by occluding available perforation area. Occlusions can be formed by fouling materials such as scale comprising sediment, precipitation, corrosion, or combinations thereof. Accordingly, a surplus of perforations  112  is preferably designed into the respective tray decks  113   a ,  113   b  to provide capacity to compensate for a loss of perforation area. However, excessive total active perforation area can cause a loss of control of liquid flow rates and poor phase separation. Therefore, the total active perforation area is positively managed in this embodiment.  
         [0062]     As shown in  FIGS. 7-8 , a plurality of blanking strips  127  are adjustably secured on the tray decks. The strips  127  can selectively uncover or cover surplus areas of perforations  112  provided. The blanking strips  127  encompass a distributed portion of the total perforations  112  of the tray decks  113   a ,  113   b , preferably in a range of 20 to 50 percent of total deck perforated area. A plurality of positioning slots  128  are provided as cutouts in each blanking strip  127 . Mounting holes  129  for the blanking strips  127  also penetrate the tray decks  113   a ,  113   b , and hold-down fasteners  131  are concentrically joined, e.g. by threaded engagement, through the slots  128  and mounting holes  129  to secure the strips  127  to the decks  113   a ,  113   b  in a variable positional pattern.  
         [0063]      FIGS. 7-8  depict a plurality of blanking strips  127  in closed position and open position, respectively. The tray perforations  112  under a continuous part of the strips  127  appear “closed” in  FIG. 7 . In “open” position in  FIG. 8 , the perforations  122  in each strip  127  have been moved into alignment to fully expose a matching pattern of the tray perforations  112  by sliding the strip  127  via the slots  128  after releasing or loosening the fasteners  131 , if necessary. The strip perforations  122  can be slightly larger in diameter than the tray perforations  112 .  
         [0064]     The blanking strips  127  are adjusted to selectively cover and uncover a plurality of surplus perforations  112  on the decks  113   a ,  113   b . This facilitates regulation of the flow of the heavy phase  100  through successive trays  111   a ,  111   b , and facilitates rapid maintenance in response to plugging of active perforations. The strips  127  are adjusted to vary total available perforation area by sliding the strips laterally within a range of movement allowed by the positioning slots  128 . Stabilizing the strips  127  with the fasteners  129  secures a selected total amount of exposed tray perforations  112  for stable operation. Individual trays can be independently adjusted to utilize differing portions of total installed perforation area. Alternatively, blanking strips  127  are disconnected and removed, in which case it is not necessary for the strips to include the perforations  122 .  
         [0065]     The above-described options for adjusting or removing blanking strips  127  enable clear tray perforations  112  to optionally be progressively placed in service when other perforations  112  become fouled. Coupling the excess tray perforation area with the blanking strips  127  also affords an expanded range of operating liquid flow conditions. This aspect allows a given vessel diameter to accommodate broader ranges of design flow rates than are typically possible with fixed active passage areas for a dispersedphase fluid. In retrofit applications, as further described below, the blanking strips facilitate designing for a set of target conditions, and having a degree of built-in flexibility to “tune” the resulting retrofit for optimal performance.  
         [0066]     In one exemplary embodiment of the invention, the heavy-phase feed stream  100  is a lubricating oil feed-stock with asphaltene compounds as a component fraction of the feedstock.  FIGS. 9-10  depict approximate proportional riser layouts for this application using the tray configurations of  FIGS. 1-3 . In this embodiment the light-phase feed stream  115  is a solvent, preferably from the homologous family of propane through pentane aliphatic and cycloaliphatic hydrocarbons, to selectively extract non-asphaltene fractions of the feed stream  100 . In this application, a ratio of volumetric flow rate of the light phase feed  115  to the heavy phase feed  100  is greater than 1:1, preferably from 5:1 to 15:1, and more preferably from 6:1 to 10:1.  
         [0067]     The deasphalting produces a light-phase effluent  125  as an extract carrying a major portion of the solvent feed stream, e.g. more than 50 percent, and with dissolved heavy-phase constituents comprising a major portion of the lubricating oils and other non-asphaltene fractions from the feedstock. The heavy-phase effluent  135  from the process is treated raffinate carrying a major portion of the asphaltenes from the feedstock, e.g. more than 50 percent, with minor fractions including unrecovered lubricating oils and a portion of the solvent.  
         [0068]     By dispersing the minor phase, here the downward-flowing heavy phase  100 , the present embodiment provides, first, supplemental perforations for active area of the tray decks  113   a ,  113   b  and, second, adjustable blanking strips  127  for matching useable active area with actual operating flow rates. These innovations make the extraction column fundamentally more flexible in total capacity and capable of operating longer periods between turnarounds for removing deposits from the occluded perforations.  
         [0069]     In other embodiments, the methods of the invention are implemented while reversing the roles of the liquid phases, i.e., the heavy liquid feed stream  100  can be a solvent, and the light liquid feed stream  115  can be a raffinate. In this embodiment, the ratio of the volumetric flow rate of the light phase  115  to the volumetric flow rate of the heavy phase  100  has the same relative relationship described for the lubricating oil feedstock case. The heavy-phase effluent  135  leaving the lower outlet  140  of the extraction vessel  110  would thus be the extract and, correspondingly, the light-phase effluent  125  leaving the upper outlet  130  of the extraction vessel  110  would be the treated raffinate. The methods of this invention are designed to accommodate higher volumetric flow rates of the light phase  115  than of the heavy phase  100 , independent of the process function of either liquid phase.  
         [0070]     The tray installations discussed above and depicted in  FIGS. 1-4  and  9 - 10  generally embody an alternating sequence of single and dual risers  114   a ,  114   b  in a two-pass mode. Another configuration can use a single-riser tray design of single-pass mode, such as shown in  FIG. 11 . A plurality of single-pass sieve trays  111   c  can be vertically arrayed in an extraction vessel (not shown), wherein each tray has one riser  114   c  vertically intersecting the tray  111   c  and laterally disposed near a tray edge. Successive single-pass trays  111   c  can be stacked vertically, wherein each successive tray is rotated 180 degrees in a horizontal plane with respect to an adjacent tray. The successive rotations orient respective risers  114   c  adjacent opposite sides of a vertical axis of an extraction vessel (not shown) with respect to a riser of an adjacent tray. The single-pass risers  114   c  include a top riser section  116   c  and a bottom riser section  117   c , attached respectively above and below a plane of the tray  111   c , and conjoined at the tray surface by a restrictor plate  118  in the plane of the tray  111   c . The top riser section  116   c  can include a solid vertical surface  152  more or less concentric with and adjacent the stripping vessel wall (not shown), a perforated vertical surface  154  transecting a cord of the tray  111   c , and a top solid surface  156 , with all three riser surfaces connected to form the top riser section  116   c  enclosed above and laterally. The transecting surface  154  is perforated with a row of openings  119  adjacent a top edge of the surface  154 . An area of the tray  111   c  includes a deck  113   c  with a plurality of perforations  112 . A cross-flow zone  160   c  is disposed beneath each tray perforated deck  113   c , and adjacent a disengagement zone  165   c  that is beneath the tray  111   c  under and/or within the bottom riser section  117   c.    
         [0071]     In this single-pass configuration the heavy-phase liquid  100  follows a flow path generally axially downward through successive trays  111   c . The continuous light-phase liquid  115  follows a flow path laterally through cross-flow zones  160   c  beneath successive trays  111   c , then upward through respective tray risers  114   c , and discharging generally horizontally through the top riser openings  119  into succeeding cross-flow zones  160   c . In these flow paths the heavy phase  100  is repeatedly collected, coalesced, and dispersed by successive trays  111   c , falling successively through the light phase  115  in single-pass contact in the respective cross-flow zones  160   c.    
         [0072]     Another sieve tray configuration embodiment integrates aspects of riser elements from  FIGS. 2-3  in a three-pass mode as depicted in  FIGS. 12-13 . As above, a plurality of three-pass sieve trays  111   d , such as shown in plan and elevation in  FIGS. 12-13 , can be vertically arrayed in an extraction vessel  110 , wherein each successive three-pass tray  111   d  is rotated 180 degrees in a horizontal plane with respect to an adjacent tray  111   d.    
         [0073]     The three-pass tray  111   d  uses dual riser banks  114   d ,  114   e , wherein the risers  114   d ,  114   e  vertically intersect respective trays  111   d . A first riser bank  114   d  is laterally disposed near a tray edge. A second riser bank  114   e  is offset horizontally from and aligned generally parallel to the first riser bank  114   d . Successive tray rotations orient the respective edge risers  114   d  on opposite sides of a vertical axis of the extraction vessel  110  with respect to the edge riser of an adjacent tray. The three-pass risers  114   d ,  114   e  include a top riser section  116   d  and bottom riser sections  117   d ,  117   e , respectively above and below a plane of the tray  111   d . The top and bottom riser sections  116   d ,  117   d ,  117   e  are connected through openings  158  in the tray surface. The bottom risers  117   d ,  117   e  form transverse channels beneath the trays  111   d.    
         [0074]     The connections joining the top and bottom risers  116   d ,  117   d ,  117   e  (see  FIG. 13 ) have a diameter or cross-section sufficiently reduced to inhibit the flow of the light-phase fluid and thus can avoid the need for restrictor plates. Alternative three-pass trays depicted in  FIGS. 14-15  use relatively larger top risers  116   d ,  116   e , and the orifices in the respective restrictor plates create a pressure drop to regulate the light-phase flow rate.  
         [0075]     As shown in  FIGS. 12-15 , the top risers can optionally comprise one sectional stack  116   e  ( FIG. 15 ) or a plurality of discrete, separated stacks  116   d  having various crosssections ( FIGS. 12-14 ). Alternative riser cross-sectional shapes can be used as determined by consideration of fluid mechanics, accessibility, or fabrication economics. As shown in  FIG. 13  (and  FIG. 19  discussed below), each top riser section  116   d  includes an open vertical duct  160  with an imperforate cap  162  attached by brackets  164 . The caps  162  direct the light-phase fluid laterally and inhibit entry of heavy-phase fluid. The caps  162  can have an outwardly sloped upper surface to inhibit accumulation and facilitate movement of the heavy-phase fluid on top of the caps  162 . For clarity, caps  162  are omitted from the plan views of  FIGS. 12 and 14 - 17 .  
         [0076]     An area of the three-pass tray  111   d  includes a plurality of panels connected edgewise to one another and to adjacent tray risers  114   d ,  114   e , to form an integral deck  113   d  with a plurality of perforations  112 . A cross-flow zone  160   d  is disposed beneath each tray perforated deck  113   d , and adjacent a disengagement zone  165   d  that is beneath the tray  111   d  adjacent the bottom riser section  117   d.    
         [0077]      FIGS. 16-18  depict selected details of an embodiment of a three-pass tray configuration to illustrate overall use of tray area for blanking strips and installation of access hatches. For general orientation,  FIG. 16  corresponds approximately to an upper-left quadrant of a tray  111   d  as represented in  FIG. 15 , and  FIG. 17  to the opposite upperright quadrant. In  FIG. 16  blanking strips  127  are more or less uniformly allocated across a perforated tray deck area  113   d , in contrast to a distribution such as shown in  FIGS. 7-8  favoring margins of the tray decks  113   a ,  113   b.    
         [0078]     In  FIG. 16 a  pair of relatively large manway access hatches  171  are mounted in the perforated tray deck  113   d . The deck  113   d  also includes a plurality of fixed panels disposed transversely between an edge riser  114   d  and a horizontally offset riser  114   e , extending between the hatches  70  and an outer edge of the deck  113   d . Blanking strips  127  can also be allocated to the manway hatches  170 . The hatches  171  are each attached to the tray deck  113   d  along respective rows of laterally opposed hinge elements  172  for easy opening, and the hatches  171  are fixed in closed position, as illustrated in  FIG. 16 , by a row of hatch fasteners  174  along adjoining edges of the hatches  171 . In  FIG. 17  relatively smaller manway hatches  176  are preferably disposed in panels of the perforated deck  113   d  between a horizontally offset second tray riser  114   e  and a tray edge.  
         [0079]     The manway hatches  171 ,  176  of  FIG. 16  and  FIG. 17 , respectively, can be installed in alternating successive trays, and the respective trays can be installed in an extraction column in alternating 180-degree rotation in respect to one another. As a result of this installation pattern, manway hatches  171  and  176  will be oriented to overlap in more or less vertical alignment from tray to tray in an extraction column to facilitate personnel access.  
         [0080]      FIG. 18  depicts one of the relatively larger manway access hatch pairs  171  in an open position, in reference to area  18  of  FIG. 16 . According to the tray-to-tray orientation described above, the open manway hatches  171  of  FIG. 18  more or less align above the relatively smaller access hatches  176  of a tray of the configuration of  FIG. 17  so that personnel can move serially between the trays. It will be appreciated that the installation of the manway hatches  171 ,  176  in the perforated decks is possible due to the presence of just one or two risers in each tray and the overall diameter of the trays.  
         [0081]      FIG. 19  shows the interlocking tabs  178  between the adjacent decking panels of  FIG. 16 . An anchor ring  180  is attached to a wall of the extraction vessel  110  to facilitate positioning and supporting the tray  111   d .  FIG. 20  shows a stabilizing bracket  182  structurally connecting the discrete top riser stacks  160  and the anchor ring  180 . The interlocking tabs  178  allow the perforated deck to be assembled using multiple panels for perforated decks  113   a - d  and risers  114   a - e . The stabilizing bracket  182  facilitates structural stiffening of the trays and a uniform elevation.  
         [0082]     The invention can be advantageously applied to retrofit existing liquid-liquid extraction units without constructing substantially new units. In a preferred embodiment, this is done by selectively replacing respective internal components of an existing system with the inventive sieve tray components described above. This application of the invention is of practical and economic interest in existing facilities for which the present invention offers improvements to liquid carrying capacities, extraction efficiencies, maintainability, or a combination thereof, compared to existing methods. As noted above, for example, prior art rotating disc contactors (RDC) can be retrofit candidates due to historical difficulties with the mechanical reliability of RDC (e.g. leakage around rotating components) and process performance (e.g. phase entrainment/flooding or low efficiency due to inoperable RDC rotors, both conditions requiring reduction of throughput to maintain product quality).  
         [0083]     As shown generally in  FIG. 21 , an RDC unit  200  can be converted to a trayed column  220  using the principles of the present invention with relatively few major steps: This involves removing the RDC rotor shaft  202  and its appurtenances. Then, the RDC stators  204 , which partially bridge an annular space between an extraction vessel wall  210  and the rotor shaft  202 , are cut back to a reduced annular dimension (but remaining attached to the vessel wall) and used as structural support rings  180  for attachment of trays  111   a ,  111   b . The conversion illustrated in  FIG. 21  can be referred to as a “1 for 2” retrofit since a new sieve tray is installed on every other stator  204 . An important benefit to fabrication costs in such a case is that cutting or welding on the vessel wall  210  can be avoided.  
         [0084]     The present invention is advantageously used with a ratio of volumetric flow rates of light-phase to heavy-phase in a general range from 1.5:1 to 15:1. As described above for lubricating oil extraction, for example, narrower values of this range of ratios are determined by a process of selecting a particular solvent to effect an extraction from a particular raffinate, considering the design performance specification for the extraction. Design specifications typically entail, for example, defining production rates and a degree of separation; balancing capital and operating costs; achieving target product purities while minimizing downstream/upstream processing costs (e.g. for waste disposal and solvent regeneration); and other factors pertaining to construction and performance of processes.  
         [0085]     Accordingly, designing a particular application for this invention can involve specifying a number of trays, tray spacing, sizes of risers, and a schedule of tray areas and perforations to satisfy a design specification, e.g. liquid flow rates, feed conditions, and separation performance. A design practice will account for respective physical and chemical properties of particular light and heavy phases for such application.  
       EXAMPLE 1  
       [0086]     The present invention can be compared to alternative methods for liquid-liquid extraction in applications both as grassroots process designs and as retrofits to existing systems. For deasphalting of lubricating oil (lube oil) feedstocks, the prior art has used rotating disc contactors (RDC&#39;s) and packed-bed extraction vessels. Table 1 compares the present invention with RDC&#39;s using estimates of performance for selected operating parameters.  
                             TABLE 1                           Performance Estimates for Solvent Deasphalting In Present       Invention Relative to Rotating Disc Contactor Baseline                Parameter   Performance of Present Invention                       Liquid Capacity   20≧35% throughput increase           Extraction Efficiency   2 times RDC           Product Yield   0.5-5 volume percent increase           Operating Cost   Lower           Investment Cost   Lower                      
 
         [0087]     RDC&#39;s consume energy to drive the disc contactors, which are not required in the present invention. Also, given an advantage in separation efficiency and capacity for the present invention, an RDC unit must consume extra power to increase liquid pumping rates to yield equivalent product quantity and quality. Therefore, the present invention incurs lower operating costs by operating at reduced liquid throughput loads and by avoiding the disc-drive power costs, when comparing the two technologies at common bases of liquid throughput and product quality.  
         [0088]     Similarly, given the advantage in separation efficiency and capacity for the present invention versus an RDC, the RDC unit requires a proportionally larger vessel volume. The added volume is needed for an RDC to operate at higher gross liquid rates and contact times required to achieve a quantity and quality of product equal to the present invention. RDC&#39;s also feature added rotating disc drive equipment not needed in the present invention. Therefore, by eliminating the extra vessel size and RDC drive components, the present invention offers savings in initial costs of fabrication and construction, yielding lower capital costs.  
       EXAMPLE 2  
       [0089]     To illustrate a range of retrofit options, Table 2 lists six cases of design dimensions for a lube oil solvent deasphalting application using the design of  FIGS. 1-4 , with reference to  FIGS. 9-10  for simplified diagrams of sieve trays relating to dimensions in Table 2. The cases are for a potential retrofit of an existing RDC unit.  
                                                                                         TABLE 2                           RDC RETROFIT CASES                Heavy liquid flow = 93 m 3 /h           Common Bases:   Light liquid flow = 306 m 3 /h            Parameter   Option 1   Option 2   Option 3   Option 4   Option 5   Option 6                    Vessel ID, m   3.6   3.6   3.6   3.6   3.6   3.6       Vessel Area, m 2     10.18   10.18   10.18   10.18   10.18   10.18       Riser Area, m 2     2.75   3.26   3.56   3.87   4.28   5.00       Riser Area, % of   27   32   35   38   42   49       Vessel       Deck Area, m 2     7.43   6.92   6.62   6.31   5.90   5.18       Perforation   0.042   0.056   0.084   0.168   0.248   0.112       Area, m 2         Perforation   6   8   9.5   13   19   25       Diameter, mm       Perforation   1498   1124   1183   1266   874   229       Count Per Tray                  
 
         [0090]     A common basis in Table 2 is that each option at least meets the product specification of the RDC unit targeted for retrofit. In meeting the product specifications, the inventive options offer design-capacity increases in fluid throughput ranging from 33 percent to 108 percent of a nominal historical capacity of the RDC unit. In each case, the inventive options also offer a maximum capacity rated at about a 20-35 percent margin over the design basis capacity for each option. The combined gain in design capacity and design margin in the extraction unit also reflects a collateral benefit in undertaking such retrofit in the form of a potential for further improving overall plant productivity, for example by debottlenecking upstream or downstream systems in ways that may have been infeasible with existing extraction performance.  
       EXAMPLE 3  
       [0091]     An extraction vessel conceptually similar to that shown in  FIG. 1  is used in a liquid-liquid extraction process to remove asphaltene compounds from a lubricating oil raffinate feed stream, using propane as a solvent. Table 3 provides ranges of values for the compositions of the feed, extract, and raffinate product streams.  
         [0092]     A lube oil feed stream is introduced as the heavy-phase liquid  100 , entering the upper inlet  105  at a temperature of 100-250° F. The feed contains 50-90 volume percent asphaltene constituents. Simultaneously, propane is introduced at the lower inlet  120  at a flow rate of 5 to 15 times the volumetric flow of the feed and a temperature of 100-200° F. The extraction is operated with a temperature differential of 0-40° F. with respect to the oil feed temperature. The vessel  110  is operated at 2.75-4.8 MPa (400-700 psig). The raffinate product  135  is withdrawn at the bottom outlet  140  of the vessel  110  and contains a major fraction of the asphaltenes. An extract phase  125  is withdrawn from the top outlet  130  and contains a major fraction of the lube oil from the feed.  
                                             TABLE 3                                       Volume Percent Composition of Process Streams                Component   Feed   Extract   Raffinate                       Asphaltenes   50-90   nil   50           Lube Oil Fraction   10-50    1-11   nil           Propane   nil   89-99   50                      
 
         [0093]     The invention is described above with reference to nonlimiting examples provided for illustrative purposes only. Various modifications and changes will become apparent to the skilled artisan in view thereof. It is intended that all such changes and modifications are within the scope and spirit of the appended claims and shall be embraced thereby.