Abstract:
A multi-tray distillation unit having a separator for de-entraining liquid particles which are picked up by an upwardly flowing gaseous stream which is flowing countercurrent to a descending liquid flow in the unit. The separator which is placed above the contact plate on which the gas contacts the liquid, comprises vertically spaced tiers of elongated, substantially horizontal open-topped liquid collector channels arrayed in parallel rows transversely to the flow of the gas. The liquid collector channels are vertically staggered to deflect the gas flow from a lower tier level through the gaps between the collection channels of an upper tier and around the channels so as to cause the entrained liquid particles to separate from the gaseous stream by inertia into the channels of the lower tier. The collected liquid is then returned through a liquid downcomer to a contact plate. An improved channel configuration has a rigidly-mounted, horizontally-elongated flow deflector extending downwards from an upper tier channel toward a lower tier channel, this provides gas flow deflection and increased efficiency of liquid particle de-entrainment. The flow deflector may be mounted directly on each upper tier channel. The channels may be formed from a single crimped and perforated metal sheet or the like, and adjacent formed tiers may be stacked to form a de-entrainment zone array. The lower tier channels may be formed from a single metal sheet having a channel bottom and sides, with a foraminous section extending upwardly from one side and a transverse section extending away from the lower channel, and terminating in a flow deflector adjacent to the bottom of a channel of an upper tier.

Description:
FIELD OF THE INVENTION 
   The present invention relates to liquid-gas phase separation techniques. In particular, it relates to de-entrainment of liquid particles suspended in an upwardly flowing gas stream, such as encountered in a distillation tower or the like. 
   BACKGROUND OF THE INVENTION 
   Typically, distillation unit operations are conducted in countercurrent multi-tray towers in which the liquid flows across the trays and down the column by way of downcorners, while the gas passes upward through the trays, which may contain contact plates, bubble caps, sieves or similar contact devices. Mass transfer occurs in the liquid-gas mixtures near the bottom of each tray. Phase separation, necessary to secure overall countercurrent flow of gas and liquid, takes place by gravity in the spaces between the trays. 
   During the flow of the vapor and the liquid through the unit, liquid particles or droplets become entrained in the vapor phase of the upwardly flowing gaseous stream. The entrained liquid is difficult to separate from the flowing stream, and various techniques have been employed to provide phase separation. Various mechanisms may be exploited in the separation. Micro-droplets may be coalesced to form larger drops and liquid particles may be separated by centrifugal force, gravity, or impingement on solid surfaces. Demister screens and flow deflectors of varied design have proven effective in phase separation. Many of these devices, however, are expensive to fabricate and operate, and create excessive pressure drops. 
   U.S. Pat. No. 4,361,469 (Trutna) teaches co-current distillation components employing a multi-level array of rectangular horizontal channels (or U-shaped troughs) adapted for liquid separation, collection and distribution. The liquid collected in the channels is discharged to a downcorner that delivers the liquid to the tray of a lower stage. The de-entrainment devices may be installed above one or more distillation trays, as required by the particular application. In arrangements of this type, it is expected that most of the liquid swept up between one set of channels will continue to be entrained upward by the gas. The main liquid collection mechanism seems to be eddy flows past the upper edges of the channels. While this method exhibits a low pressure drop, the relative amount of recovered liquid particles is lower than desired. This deficiency is believed to be related to the relatively unrestricted flow path of the vapor stream with entrained particles as it passes through the array of channels. 
   SUMMARY OF THE INVENTION 
   According to the present invention, an improved separation system for de-entraining liquid particles from an upwardly flowing gaseous stream, passes a gaseous stream through a de-entrainment zone comprising multilevel tiers of elongated, U-shaped, liquid collector channels arrayed in parallel rows transverse to the gaseous stream flow. The channels are arranged so that they are substantially horizontal, that is, horizontal or with only a slight inclination so that they can effectively function to collect the liquid. The tiers are spaced apart vertically with staggered parallel rows of collector channels to deflect the flow of the gaseous stream from a lower tier level through the open gaps between the channels and around the channels in an upper tier level, thereby permitting liquid particles to separate from the gaseous stream by gravity into lower level channels for collection and distribution via liquid downcorners. A rigid, elongated flow deflector extends parallel with each channel of an upper tier in the region between the tiers; this deflector extends in a downward direction from the channels of the upper tiers towards the channels of the next adjacent lower tier to deflect the gaseous stream flow and increase the efficiency of liquid particle collection in the collector channels. The deflector may be attached directly to the channels of the upper tier or, alternatively, to the channels of the lower tier to provide the desired deflection of gas flow and of separated liquid droplet flow. 
   The method for increasing the de-entrainment efficiency of the liquid particle from the gaseous stream passes the stream through a de-entrainment zone that includes multilevel tiers of elongated, substantially horizontal, U-shaped, liquid collector channels arrayed in vertically-spaced, parallel rows transverse to gaseous stream flow. The tiers are arranged with staggered parallel rows of collector channels to deflect the gaseous stream flow from a lower tier level around the channels in an upper tier level through open gaps between channels, so as to permit liquid particles to separate from the gaseous stream by gravity into lower level channels for collection and distribution. Flow deflectors extend parallel with each channel of the upper tiers in the region between the tiers; the deflectors extend in a downward direction from the upper channels towards the channels of the next adjacent lower tier to deflect the gaseous stream flow and increase the efficiency of liquid particle collection in the collector channels. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described below in more detail with reference to preferred forms of the invention, given by way of example only, with reference to the accompanying drawings in which like numerals are used to designate like parts throughout and in which: 
       FIG. 1  is a schematic vertical axial cross sectional view showing a distillation column for a multi-level array of liquid collectors arrayed in a de-entrainment zone; 
       FIG. 2  shows a schematic cross sectional view of a multi-tiered array of collector channels with an affixed deflector according to the present invention; 
       FIG. 3  shows a similar view of an alternative array of collector channels; 
       FIG. 4  shows a view of a fabricated array of collector channels; 
       FIG. 5  presents a schematic cross sectional view of another alternative array of collector channels; 
       FIG. 6  shows a schematic cross sectional view of yet another alternative array of collector channels; 
       FIG. 7  shows a cross section of a unit with collector channels with control for liquid drainage; 
       FIG. 8  shows a cross section of a unit with a simplified liquid downcorner configuration. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a multi-tray distillation unit in which all the liquid on the tray is entrained upwards, captured in channels after separation from the ascending gas flow and then directed to a preceding contact tray. Distillation column  10  includes an outer shell or wall  11  which may have any suitable horizontal cross section such as circular, square, rectangular, oval, or others. Downcorner passage  13  is provided at one wall of shell  11  by a wall  14 . Additional downcorner passages  15 ,  16  and  17  are defined by shell  11  and walls  18 ,  19  and  20 , respectively. 
   Adjacent the lower end of downcorner passage  13 , a liquid-gas contact tray plate  22  is disposed transversely across the column interior, and similar or identical plates  23 ,  24  are disposed transversely across the column interior at the lower ends of downcorner passages  15  and  16 , respectively. Above each plate  22 ,  23 ,  24  there is disposed a gas-liquid separator zone, each separator  26 ,  27 ,  28  consisting of plural vertically spaced tiers or levels  31 ,  32 ,  33  of horizontally spaced parallel open-topped rectangular channels (referenced only in the lowest tier  28  only, for clarity). 
   Excessive vapor flows at the channel exit which can interfere with collection and delivery of liquid can also be prevented by providing a separate sealed downcorner for each channel row or stage. This alternative is advantageous where large variations in the liquid-gas ratio are encountered. 
   The fractionating column shown in  FIG. 1  may have any number of superimposed stages, only three stages being shown. Feed vapor enters at the bottom of the column and flows upwardly as indicated by arrows  51 . The vapor contacts liquid which is delivered to tray  24  through downcorner  16  and a highly agitated mixture of vapor and liquid flows upward through the column. Liquid is separated from the vapor at separator  28 , and the vapor then continues up the tower past tray  23 , to which liquid is supplied through downcorner  15  by way of liquid seal pot  15   a.  The separated vapor flows upwardly past plate  22 , to which liquid is supplied through downcorner passage  13  by way of liquid seal  13   a,  and the resulting gas-liquid mixture flows upwardly, through separator  26 . Assuming that separator  26  is at the upper end of the column, the separated vapor continues upwardly leaving the column as indicated by arrows  55  and feed liquid to the column is introduced through downcorner  13 . If separator  26  is not at the upper end of the column the separated vapor  55  continues upwardly through the column and the liquid supply through downcorner passage  13  is supplied from a separator higher up in the column. The liquid separated by separator  26  flows downwardly through downcorner passage  15  and liquid seal  15 a to plate  23 . The liquid separated by separator  27  flows downwardly through downcorner passage  16  and seal  16 a to plate  24 . The liquid separated at separator  28  flows downwardly through downcorner passage  17  either out of the column or to a plate further down in the column. 
   As indicated in  FIG. 1 , the channels of each separator  26 ,  27 ,  28  are substantially horizontal so that they can function effectively to collect the separated liquid particles and secure a relatively uniform liquid depth over the lengths of the channels but they are given a slight downward slope toward the downcorner in order to facilitate liquid flow from the collector channel into the downcorner. The channels are preferably sloped downwardly toward the downcorner with a slope of the order of five to ten percent, to facilitate liquid discharge from the channels to the downcorners, while maintaining the desired relatively uniform liquid depth over the lengths of the channels. Thus, the channels are typically at an inclination of no more than 10 percent from the horizontal plane, preferably five to ten percent from the horizontal. For convenience of description, this orientation is referred to here as “horizontal” and the tiers are referred to as “horizontal” tiers notwithstanding the preferred slight departure from true horizontal disposition. 
   When using the normal open type downcorner,  13 ,  15 ,  16  and  17  of  FIG. 1 , the channels have the outlets from the discharge ends of the channels reduced by skirts  60  to approximately the height of liquid in the channels in order to prevent excessive vapor flow from the column, through the outlets and into the downcorners. If excessive gas flow from the column into the downcorners takes place through the outlets of the lower channels, there is a possibility that reverse gas flow back into the column from the downcorner, through the outlet(s) of the upper channels of the tier, holding up liquid in the upper channel(s). At design conditions, when there is a substantial amount of entrained liquid captured in the lower channel(s), the opening below the skirt will be mainly filled with outflowing liquid, thus blocking gas from exiting from the lower channel into the downcorner. At low liquid entrainment/capture rates, this opening may be completely filled with liquid and hence some gas will get through into the downcorner and then find its way back into an upper channel, holding up the liquid in the upper channel(s). If, however, there is not much liquid to be separated from the gas, hold-up of liquid in the upper channel(s) of the tier may present no significant practical problems. 
   In general, the proportion of the exit area of the channel outlet which is blocked by the skirt should, in principle, be greatest for the lowest tier, and decrease to zero for the highest tier. The optimal proportion of the outlet blocked on the lowest tier will increase to some degree as the number of tiers increases, since (due to the additional tiers) the pressure drop across the whole channel array (bottom to top) will increase, which tends to increase the pressure difference which tends to drive liquid and gas from the end of the lowest channel, under its skirt, and into the sealed downcorner zone. Thus, as the number of tiers increases, a proportionately smaller open area under the skirt is necessary or, at least, desirable, in order to drain the liquid from that channel. A design in which all the channels in the upper half of the tiers have no skirts, and all the channels in the lower half of the tiers have skirts blocking the upper half (50%) of the channel outlets, has been found to be satisfactory under a range of flow conditions. In a four-tiered construction, for example, the lower two tiers out of four might have skirts blocking the upper halves of the channel outlets while the uppermost two tiers might have channel outlets with no skirts. In the three-tiered separation zone of  FIG. 1 , the lowest tray might have a skirt blocking the upper half of the channel outlet, the uppermost tray have no skirt and, depending on expected liquid and gas flow rates and entrainment rates, the intermediate tray might have either no skirt or, if the added complication of fabrication be accepted, a skirt blocking less than one half the outlet area. Standard hydraulic calculations and observations of working devices may be used to optimize the size of the skirts for various geometries and operating conditions. 
   Referring to  FIG. 2 , an array of collector channels is shown schematically in partial cross section, showing the deflectors. The individual channels  31 A,  31 B,  31 C,  32 A,  33 A, etc. are each in the form of an elongated open top channel of rectangular cross section having a bottom  39  and sidewalls  40 ,  41 . The upper tiers at least are provided with deflectors  45 A, B, C and, for convenience, the channels may be constructed likewise. The channels are spaced apart vertically in the tiers and horizontally, as indicated in  FIG. 2 , to provide a de-entrainment zone with an overall horizontal open gap area between channels of at least 20%, preferably 30–60%, most preferably 35–50%, of the total horizontal area of the tier (channels plus gaps). The expected upward gas-liquid flow pattern is indicated by the array of arrows  46 . The up-flowing gas-liquid typically is in the form of a two-phase mixture of aerated liquid, foam, spray or gas having entrained liquid droplets. Because of the changes in flow direction, the gas-liquid mixture can be thrust against the channels to separate the liquid which is then collected in the channels underneath, so that a satisfactory separation of the liquid from the up-flowing gas stream occurs. 
   As shown in  FIG. 2 , the deflectors  45 A,  45 B,  45 C, are relatively small compared to the height of the channels. The deflectors are less than 50% of the height between tiers, preferably about 10%–45%. The deflectors can form an integral, downward extension of a sidewall or be independently mounted in the desired position, as later explained. The deflector(s) may be positioned anywhere on the bottom of the channel. In a preferred form, all channels in the same row have deflectors similarly positioned. In still another preferred form, all rows either have deflectors asymmetrically positioned or, as shown in  FIG. 2 , in an alternating mirror-image position. The symmetry of the deflectors may be even further modified as would be apparent to one of ordinary skill in the art. 
   Vapor stream  51  with entrained liquid flows upwards between the channels and impacts with the channel walls. The de-entrained liquid will collect on the bottom of the channel. Due to deflector configuration and position, the liquid will collect more readily around the deflector, and deposit more freely into the channel below. The deflector partially directs the flow of the gas/liquid current moving past the channel and, in addition, aids in the more complete collection of the liquid by increasing the accuracy of the droplet fall path, allowing a much greater percentage of the de-entrained liquid to fall directly into the channel below. The deflectors may be affixed to the bottom portion of the channel and in parallel to the line of the collector, preferably being oriented within 0–45° of vertical. The deflectors may have a variety of forms, such as serrated or perforated plates or curved surfaces. 
   As shown by vapor flow through the channels, the deflectors alter the path of the vapor flow. This has the added effect of maximizing the de-entrainment properties of the channel by inertial separation of the entrained liquid particles, which may allow for fewer overall channels to be used in a distillation tower. When the deflector is positioned at one edge of the channel, it may be advantageous to provide a slope in the bottom portion of the channel towards the deflector. This slope may be in addition to the lateral (transverse) sloping of the channels as depicted in  FIG. 1 . The deflector itself may also be at an angle from the vertical. By angling the deflector, more control may be gained over the vapor flow and the dripping location of the de-entrained liquid. 
   As shown in  FIG. 3 , multiple deflectors may be used in multiple locations on the bottom of channels  131 A,  132 A,  133 A, etc of the upper tiers. In this embodiment two deflectors  145 A, B are affixed to the bottom of channel  132 B and similar deflectors are also affixed to the other channels, at least in the upper tiers. Here both deflectors are at opposing sides of a channel&#39;s bottom, but they may be positioned in any location that will optimize gas flow characteristics, liquid particle separation and liquid collection, allowing the collected liquid to be deposited into the center of the channel below. 
   The positioning of a deflector over an lower channel may in practice be off-center. This may be due to the predicted vapor flow movement pushing a droplet in a certain direction as it falls. Therefore, in some instances, the deflectors may be positioned such that they are not aligned over the channel below, but so that the dripping liquid will be blown into the appropriate position by the gas flow around and over the channel. 
   The deflectors in one form may be longitudinal strips extending continuously along the length of the channel. Alternatively, the deflectors may be added in selected locations along the channel length. This would further provide flexibility in directing vapor flow. The deflectors themselves may also have a modified shape that facilitates the collection of liquid or the vapor flow, such as tapering thickness. 
   Referring to  FIG. 4 , a technique is shown for fabricating channel arrays  231 ,  232  by bending one piece of thin sheet metal. The deflectors  245  are integrally formed from the same piece of sheet metal or the like used to form the channels themselves. As shown in  FIG. 4 , the deflectors, which are relatively small compared to the height of the channel side wall, form a downward extension of a sidewall into the region between the tiers. The deflectors may, however, be positioned at any point on the bottom of the channel instead of being at one of the edges. Perforations  250  are formed in the portions of the sheet extending above the channels to permit gas/liquid flow between tiers, while permitting the channels in a tier to be formed from the single sheet of material by suitable perforation and bending. 
   Vapor with entrained liquid flows upwards between the channels and impacts with the channel edges and deflectors. The liquid de-entrained at those points by inertial separation will collect on the bottom of the channel. The liquid will, however, collect more readily around the deflector, and deposit more freely into the channel below. The deflector therefore not only aids in better separation of the entrained liquid droplets from the greater extent of direction changes in the gas flow but it also promotes better collection of the liquid and deposition of the separated droplets by improving the falling accuracy of the droplets into the lower collection channels. This allows a greater amount of the de-entrained liquid to fall directly into a lower channel, than if the liquid were left to collect on its own on the bottom of the channels. 
   In the embodiment of  FIG. 5  channel arrays  331 ,  332  are formed in a similar way by bending and perforating thin sheet metal. Perforations  350   a,    350   b  in the vertical portions of the sheet extending above the collection channels permit fluid flow between the tiers. In this case, the deflectors  345   a,    345   b  in the inter-tier region are mounted not on the upper channels but rather, on the lower channels although they retain the appropriate disposition in the region between the tiers, downwardly towards the lower channel so as to improve separation and collection efficiency. The deflectors are mounted on the lower channels by bending the sheet transversely above the perforated sections  350   a,    350   b,  to provide a deflector integrally formed from the same piece of sheet material (metal or the like). Thus, the channels of the lower tier provide the mounting for the deflectors above. Various foraminous structures, such as an open wire grid, may be substituted for the perforated plate configuration. 
   The top and bottom tiers of the array may have special features.  FIG. 6  shows some advantageous treatments for the top channels  431   a,    431   b  and bottom channels  432   a,    432   b  of a two-tier array. A cap type deflector extends over the top of the uppermost channels to assist liquid capture by causing a change in flow direction of the gas flow exiting the upper channels through the foraminous region above the upper tier channels. Each deflector has a horizontal portion or flap  459   a,    459   b,  with a dependent ridge  460   a,    460   b,  which causes the gases leaving perforations  450  to undergo a change of direction which tends to throw the entrained liquid particles out of the flowstream. The bottom tier includes its own type of gas flow deflectors in the form of dependent flaps  461 a,  461 b,  461 c to direct the vapor flow in a preferred pattern. These bottom tier flaps are, in this case, longer, and oriented differently to the vertical deflectors  445   a,    445   b,  on the bottoms of the upper tier. 
   In order to provide a protected path to return the separated liquid from the channels downwards to a contact plate, a sealed downcorner configuration may be adopted, as shown in  FIG. 7 . The contact plates  72 ,  73 ,  74 ,  75  in the column are shown with separation zones comprising two trays  76 ,  77  (indicated only for two zones) each with its collector channels, which empty into sealed drains  78  through channel outlets  79 . The channels are provided with deflectors (not shown in the figure) which deflect the gas flow and direct the liquid particle collection, as previously described. Skirts  80  partly block the outlets from the lower channels in each zone to preclude reverse gas flow into the sealed downcorners. The separated liquid from the channels flows into the drains and overflows lip  81  of liquid seal pan  82  and from there falls into downcorners  83  to the contact plate in the preceding contact zone. A simpler construction is shown in  FIG. 8  in which the channels  87 ,  88  under contact plates  84 ,  85 ,  86  are sealed into the downcorners  89  (one indicated) with the downcorner discharging onto the contact plate of the same contact zone. As with  FIG. 7 , the channels are provided with deflectors (not shown) which deflect the gas flow and direct the liquid particle collection. The openings through the downcorner are sized to allow passage of the liquid which was collected in the channels, without excessive gas accompanying the liquid. This configuration has the marginal disadvantage of reducing the increase in column capacity since the liquid is returned to the same contact zone rather than the zone below but this device is still capable, however, of improving deentrainment. 
   EXAMPLE 1  
   De-entrainment apparatus is fabricated in five tiers according to the configuration of  FIG. 2 . The channels are 0.8 inch wide and 1 inch high, with a vertical spacing of 0.5 inch between tiers. The channels are supported in a de-entrainment zone having a rectangular cross section of 39 sq. in. total area. The rigid deflectors extend downward from the channels 0.35 inch. 
   The channels are closed at one end and collected liquid is withdrawn at an open end for distribution below. If an additional 30% area is assumed for downcorners, liquid feed zones or other hardware, a total device area of 51 in 2  or 0.35 ft 2  is arrived at. Flooding loads become 17 gpm/ft 2  liquid, and 0.41 C-factor. 
   For the channels with the fitted deflectors, the flooding air rate is about 290 CFM at a liquid feed rate of 6 gpm. Here the flood point is defined as the gas velocity at which more than 6% of the liquid feed is carried up through the channel array without being captured (de-entrained). For the channel area alone, the flooding C-factor is 0.70 ft/s at 22 gpm/ft 2 . If the area is increased by 30% to account for downcorners, the estimated flooding C-factor becomes 0.54 ft/s at 17 gpm/ft 2  liquid. This capacity is about double the capacity of conventional trays. 
   EXAMPLE 2  
   For comparison, a base case apparatus has identical channel configuration, except without deflector means and spacing between tiers is reduced to 0.4 inch to achieve comparable pressure drop across the de-entrainment zone. The flooding air rate is about 220 cfm at a liquid rate of 6 gpm. This flooding air rate is about 30% lower than seen with the improved deflector-equipped channel of Example 1. 
   De-entrainment testing employs air as the carrier gas and isoparaffin hydrocarbon liquid as the dispersed liquid phase. The results of comparative de-entrainment test are given in the following tables. 
   
     
       
             
           
             
             
             
           
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Channels with Deflectors 
             
           
        
         
             
               Liquid feed 
               Gas Rate 
               Uncaptured Liquid 
             
             
               (gpm) 
               (cfm) 
               (% feed) 
             
             
                 
             
           
        
         
             
               3.6 
               200 
               3.3 
             
             
               3.6 
               300 
               8.9 
             
             
               3.6 
               400 
               15.6 
             
             
               6 
               300 
               6.8 
             
             
               11 
               200 
               2.1 
             
             
                 
             
           
        
       
     
   
   
     
       
             
           
             
             
             
           
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               Channels without Deflectors 
             
           
        
         
             
               Liquid feed 
               Gas Rate 
               Uncaptured Liquid 
             
             
               (gpm) 
               (cfm) 
               (% feed) 
             
             
                 
             
           
        
         
             
               3.6 
               200 
               5.0 
             
             
               3.6 
               300 
               14.9 
             
             
               6 
               200 
               4.4 
             
             
               6 
               300 
               14.0 
             
             
               11 
               200 
               3.7 
             
             
                 
             
           
        
       
     
   
   The improved channel/deflector design generally provides 20–30% less uncaptured liquid than the same configuration without deflectors. Besides being used in a CoFlo™ type of tray, such as the Trutna design, the improved channel arrays may enhance the capacity of conventional trays, operatively mounted immediately below at least a portion of the trays, i.e. at the top of the free area above the bubbling area. There they could capture entrained liquid, which would otherwise be carried upward to the tray above. These channels are less prone to flooding than conventional de-entrainment devices such as mesh or vane packs. Using only two or three tiers of channels may provide sufficient entrainment catch for such applications, with lower cost and pressure drop. A path of liquid to drain down out of the ends of the channels, such that the liquid is protected from the upwardly vapor, is preferred. One means of achieving this would be to seal the ends of the channels into a baffle, with this plate extending downward close to a wall of the vessel. A liquid head can build up in this protected volume. An opening, possibly incorporating a sealing device such as a seal pan, at the bottom of this protected zone allows liquid to drain out of this protected zone and preferably towards the downcorner leading to the tray below. 
   The improved channel arrays may be used in a wide variety of applications where de-entrainment of liquid from a vapor stream is desired. These improved channel arrays may be substituted for more conventional de-entrainment devices, such as mesh pads and vane packs. Preferred applications are those where the liquid load is greater than 1 gpm/ft 2  in order to take advantage of the liquid-capturing capability of the channel array.