Patent Publication Number: US-2023149874-A1

Title: Apparatus for distributing fluid in downflow reactors

Description:
TECHNICAL FIELD 
     The present subject matter relates in general to downflow reactors and in particular to an apparatus for distributing a polyphase fluid mixture to a catalyst bed in downflow reactors. 
     BACKGROUND 
     Reactors used in chemical, petroleum refining, and other industries generally have a fluid passing through a catalyst bed for performing various types of processes such as cracking, hydrotreating, etc. The fluid may be a polyphasic mixture of gas and liquid. In a downflow reactor, the gas and liquid flow in a concurrent manner from the top of the reactor to the bottom of the reactor. To ensure complete and efficient utilization of the catalyst, it is necessary for the gas and liquid mixture to be distributed throughout the catalyst bed. Generally, distribution of fluid in the reactor is achieved by using distributor plates or distributor trays. These distributors may be a plate with orifices with the distributor plate placed above the catalyst bed so that the fluid passes through the distributor plate before entering the catalyst bed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The detailed description is described with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the drawings to reference like features and components where possible. 
         FIG.  1    illustrates an example downflow reactor comprising an example apparatus for distributing a polyphasic liquid mixture, in accordance with an embodiment of the present subject matter. 
         FIG.  2    illustrates a top view of a section of an example distributor tray comprising a plurality of distributor units, in accordance with an embodiment of the present subject matter. 
         FIG.  3 ( a )  illustrates a cross-section of an example distributor unit in accordance with an embodiment of the present subject matter. 
         FIG.  3 ( b )  illustrates a top view of a section along the line A-A of the example distributor unit illustrated in  FIG.  3 ( a ) , in accordance with an embodiment of the present subject matter. 
         FIG.  3 ( c )  illustrates a top view of a section along the line B-B of the example distributor unit illustrated in  FIG.  3 ( a ) , in accordance with an embodiment of the present subject matter. 
         FIG.  3 ( d )  illustrates an enlarged view of an example dispersion section  374 , in accordance with an embodiment of the present subject matter. 
         FIG.  3 ( e )  illustrates a top view of a section along the line C-C of the example distributor unit illustrated in  FIG.  3 ( d ) , in accordance with an embodiment of the present subject matter. 
         FIG.  3 ( f )  illustrates the top view of a section along the line D-D of the example distributor unit illustrated in  FIG.  3 ( d ) , in accordance with an embodiment of the present subject matter. 
         FIG.  4 ( a )  illustrates another example distributor unit comprising the solid insert attached to the cap plate, in accordance with an embodiment of the present subject matter. 
         FIG.  4 ( b )  illustrates a top view of a section along the line A-A of the example distributor unit illustrated in  FIG.  4 ( a ) , and  FIG.  4 ( c )  illustrates a top view of a section along the line B-B of the example distributor unit illustrated in  FIG.  4 ( a ) , in accordance with an embodiment of the present subject matter. 
         FIG.  4 ( d )  illustrates another example distributor unit comprising a static mixer, in accordance with an embodiment of the present subject matter. 
         FIG.  5 ( a )  illustrates a cross-sectional view of another embodiment of the distributor unit comprising one tube, in accordance with an embodiment of the present subject matter. 
         FIG.  5 ( b )  illustrates a top view of a section along line A-A of the example distributor unit shown in  FIG.  6 ( a ) , in accordance with an embodiment of the present subject matter. 
         FIG.  6    illustrates the variation in sensitivity with superficial velocity for example 2, in accordance with an embodiment of the present subject matter. 
     
    
    
     DETAILED DESCRIPTION 
     The present subject matter relates in general to downflow reactors and in particular to an apparatus for distributing a polyphase fluid mixture to a catalyst bed in downflow reactors. 
     In a downflow reactor, fluid having reactants enters from the top of the reactor and products are removed from the bottom of the reactor. The fluid may be polyphasic, comprising at least one liquid phase reactant and one gas phase reactant. Gas and liquid reactants are mixed and passed over a catalyst bed for performing several processes such as hydrotreating, cracking, desulfurization, etc. When the mixture is passed over the catalyst, it is important that the gas-liquid mixture is distributed uniformly over the catalyst bed to ensure efficient utilization of the catalyst. Efficient utilization of the catalyst is important for ensuring a uniform rate of reaction, increasing productivity, and yield. This may also allow increased use of catalyst before regeneration or replenishment. Furthermore, in hydroprocessing reactors involving exothermic reactions, uneven distribution of liquid or gas can lead to excess release of heat in certain regions compared to other regions. The high temperatures further accelerate the reaction rate, resulting in development of hot spots in the reactor, reducing the overall catalyst life. 
     Generally, distribution of gas and liquid over the catalyst bed is achieved using a distributor plate or distributor tray. Typically, a sieve plate distributor plate and a chimney distributor is used to distribute the gas and liquid reactants over the catalyst bed. The sieve tray distributor plate may be a plate with orifices that is placed over the catalyst bed horizontally. Conventional sieve tray distributor plates have orifices on a plate through which liquid and gas flows. Gas and liquid pass through the orifices and enter the catalyst bed. Since the orifices are spread throughout the plate, it helps with distribution of the gas-liquid mixture. However, the distribution of fluid through these plates is even if the plate is not completely level. The plate may go out of level over time, i.e., the plate may get inclined such that one side of the plate may be at a slightly lower level than another side. In such cases, more fluid flows through the portion of the plate that is at a lower level than the portion of the plate that is at a higher level. In other words, the plate is very sensitive to out-of-levelness. Conventional chimney distributors include pipes (called downpipes), which pass through orifices on the distributor plate. The gas enters the downpipes from near the top end of the downpipes and flows downwards through the pipes. The liquid collects over the tray and enters the downpipes through smaller apertures on the side of the downpipe and then flows downwards with the gas. The minimum size of these apertures on the side of the downpipe is about 6 mm. The apertures are susceptible to clogging when the liquid is sludgy, turbid, or has scales, requiring that the distributor plate be removed and cleaned. This increases reactor downtime and increases operational costs. 
     In addition, conventional distributors require a large number of orifices and specific designs to ensure uniform distribution. If the distributor plate becomes out-of-level, it results in some regions of the catalyst bed receiving more of the gas-liquid mixture than in other regions, making for non-uniform distribution of the fluid over the catalyst bed. In exothermic reactions, if the distributor plate does not provide uniform distribution of fluid to the catalyst bed, for example because of the distributor plate being not level, there is poor heat exchange, which leads to the formation of hot spots in the catalyst bed. This reduces reaction rate and reduces product yield. 
     The present subject matter overcomes these, and other problems associated with current distributors in downflow reactors. The present subject matter relates to an apparatus for distributing a polyphase fluid mixture over a catalyst bed in a downflow reactor. The apparatus comprises a distributor tray comprising a plurality of distributor units. 
     In one implementation, a distributor unit comprises an inner tube disposed on an orifice in the distributor tray. The inner tube comprises a first aperture disposed on a side portion of the inner tube to allow liquid to enter the inner tube. A solid insert is disposed in a top portion of the inner tube, the outer diameter of the solid insert being smaller than the inner diameter of the inner tube. The solid insert results in formation of a narrow passage around it in the top portion of the inner tube to allow gas to pass through. An outer tube is disposed concentric to the inner tube forming an annular portion between the inner tube and the outer tube. The outer tube comprises a slot disposed on a bottom portion of the outer tube. The slot allows liquid from the distributor tray to enter the annular portion between the inner tube and the outer tube. A ring-like cover is disposed on a top portion of the inner tube inside the outer tube to enclose the annular portion on the top while leaving the top end of the inner tube open to allow entry of gas. A support structure is disposed on the outer tube at its top portion to extend over the outer tube and a gas inlet is disposed on the support structure to allow gas to enter the distributor unit. The top end of the support structure is covered by a cap plate disposed on the support structure. The gas thus enters the inner tube through the support structure and mixes with the liquid that enters the inner tube through the first aperture. In an example, a venturi insert may be disposed within the inner tube and below the first aperture to further facilitate mixing of liquid and gas. 
     The apparatus of the present subject matter allows improved distribution of the liquid-gas mixture to the catalyst bed compared to conventional distributor trays. During operation, gas flows via the narrow passage in the inner tube at high velocity causing reduced pressure. This allows liquid entering via the slots in the outer tube to rise in the annular region. Thus, because the liquid flow is dependent on gas flow, the apparatus of the present subject matter has low sensitivity to out-of-levelness of the distributor tray. Hence, even if the distributor tray is not level, fluid flow to the catalyst bed is uniform. The apparatus can be used over a wide range of fluid flow regimes by changing the size of the solid insert in the inner tube. For example, in low gas flow conditions, the narrow passage may be made narrower than in a high gas flow condition, increasing gas velocity and thus providing a greater pressure drop, which increases the amount of liquid entering via slots in the outer tube and the height to which the liquid rises in the annular portion. This increases the dependence of liquid flow on gas flow, even in low gas flow conditions. This is advantageous in low gas flow conditions, as the low pressure can be created in the annular portion using the solid insert, resulting in lower sensitivity to out of levelness. As will be understood, low gas flow conditions and high gas flow conditions may correspond to different gas flow rates used in the reactor, which may depend on reactor configuration and processing parameters. 
     As the liquid travels first into the slot in the outer tube and then into the first aperture or a second aperture disposed above the first aperture in the inner tube, where the apertures are placed 90° away on a circumference of the inner tube from the slot, solid particles present in the liquid will deposit on the distributor tray. This prevents clogging of the apertures leading to longer use of the distributor tray between maintenance compared to conventional distributors. This reduces reactor downtime and leads to reduced operating costs. The venturi insert disposed in the inner tube allows greater mixing of gas and liquid before the mixture enters the catalyst bed. In an example, there may be a static mixer disposed near the exit of the inner tube, which improves mixing further. The enhanced mixing improves reaction rate and thus the product yield. Furthermore, the apparatus allows even liquid flow exiting the inner pipe even if the liquid flow to the distributor plate is uneven or is pulsed. 
     In another embodiment, the distributor unit comprises a tube disposed on the distributor tray. A first aperture is disposed on a lower portion of the tube near the distributor tray to allow liquid to enter the distributor unit. A solid insert is disposed within the tube so that a lower portion of the  soli  insert is adjacent to the first aperture. The lower portion of the solid insert has corrugated edges and an upper portion of the solid insert has rounded edges. A cap plate is disposed on a support structure disposed on the tube and a gas inlet is disposed on the support structure to allow gas to enter the distributor unit. 
     Aspects of the present subject matter are further described in conjunction with the appended figures. It should be noted that the description and figures merely illustrate the principles of the present subject matter. It will thus be appreciated that various arrangements that embody the principles of the present subject matter, although not explicitly described or shown herein, can be devised from the description and are included within its scope. Moreover, all statements herein reciting principles, aspects, and implementations of the present subject matter, as well as specific examples thereof, are intended to encompass equivalents thereof. 
       FIG.  1    illustrates an example downflow reactor comprising an example apparatus for distributing a polyphasic liquid mixture, in accordance with an embodiment of the present subject matter. The downflow reactor  100  comprises a fluid inlet  110 . The fluid may comprise a mixture of gas and liquid. The fluid passes through an apparatus  120  for distributing the mixture and enters the catalyst bed  130 , where catalytic hydroprocessing reactions, such as hydrotreating, cracking, etc., occur. The apparatus  120  may be a distributor tray  140  comprising a plurality of distributor units  150 . In an example, after the reactions are complete, the resulting products may pass through a second apparatus  160  for distributing the product mixture to a second catalyst bed  170 . The second apparatus  160  may be the same as the apparatus  120  or may be a conventional distributor. The product gas and liquid may be removed from the bottom of the reactor from a gas outlet  180  and a liquid outlet  190 . 
       FIG.  2    illustrates a top view of a section of an example distributor tray comprising a plurality of distributor units, in accordance with an embodiment of the present subject matter. The distributor tray  140  comprises a plurality of distributor units  150 , referred to singly as distributor unit  150 . The plurality of distributor units  150  may be disposed in any arrangement on the distributor tray depending on the required flow characteristics. In an example, the plurality of distributor units  150  may be arranged in a triangular pitch. The distributor unit  150  comprises an inner tube  210  and an outer tube  220  concentric to the inner tube. 
       FIG.  3 ( a )  illustrates a cross-section of an example distributor unit, in accordance with an embodiment of the present subject matter. The distributor unit  150  comprises an inner tube  210  disposed vertically on an orifice  310  of the distributor tray  140 . The inner tube  210  comprises a first aperture  314  disposed on a side portion of the inner tube  210  at a height above the distributor tray  140  to allow liquid to enter the inner tube  210 . In another example, the inner tube  210  may comprise two first apertures  314 , with the two first apertures  314  disposed 180° from each other on a circumference on the inner tube  210  at the same height. In an example, a second aperture  318  may be disposed on a side portion of the inner tube  210  above the first aperture  314 . In another example, the inner tube  210  may comprise two second apertures  318  disposed 180° from each other on the circumference on the inner tube  210 . In various examples, the first aperture  314  and the second aperture  318  may have a circular cross-section or cross-section of other shapes. The apertures  314  and  318  may be sized to offer required resistance to liquid flow, depending on the operational characteristics of the reactor  100 . 
     A venturi insert  322  may be disposed within the inner tube  210  below the first aperture  314 . In another example, a static mixer (not shown in the figure) may be disposed below the venturi insert  322 . The static mixer may comprise twisted ribbons, in an example. The inlet to the venturi insert  322  is placed such that the liquid entering the inner tube  210  from the first aperture  314  enters the venturi insert  322  and further mixes with the incoming gas that comes in from the top of the inner tube  210 . 
     A solid insert  326  may be disposed coaxially on a top portion of the inner tube  210 . In an example, the solid insert  326  may be cylindrical in shape with a hemispherical top portion and smooth edges on the bottom portion. The solid insert  326  forms a narrow passage  330  around it in the inner tube  210 , where gas flows with a high velocity and thus creates reduced pressure. The solid insert  326  may be held in position using support rod  328  that attaches the solid insert  326  to the top end of the inner tube  210 . The support rod  328  may pass through the inner tube  210  and the solid insert  326  to hold the solid insert  326  firmly. In an example, several support rods  328  may be disposed at multiple positions around the solid insert  326  with spacing provided between them to allow inflow of gas. The solid insert  326  may be disposed so that it is adjacent to an inner slot  334  on the inner tube  210 . The inner slot  334  may be fluidically connected to the narrow passage  330 . In an example, there may be two inner slots  334  diametrically opposite to each other. The size of the inner slot  334  is such that it offers low resistance to fluid flow. 
     An outer tube  220  may be disposed on the distributor tray  140  concentric to the inner tube  210  forming an annular portion  342  therebetween. The outer tube  220  comprises slot  338  disposed on a bottom portion of the outer tube  220  and in contact with the distributor tray  140 . The slot  338  allows liquid from the distributor tray  140  to enter the annular portion  342  between the outer tube  220  and the inner tube  210 . In an example, two slots  338  may be disposed 180° from each other on a circumference of the outer tube  210 . In another example, the slot  338  may be disposed at 90° from the first aperture  314  and the second aperture  318 . As the liquid travels upward in the annular portion  342  and horizontally, for example, at 90°, around the circumference of the inner tube  210  in the annular region, most of the undesired particles or scales get settled on the distributor tray  140 . This prevents clogging of the first aperture  314  or the second aperture  318 . The sizes of the slot  338  and the diameter of the outer pipe  220  may be sized to provide calculated resistance for the liquid flow through the annular portion  342 , depending on the capacity of the reactor  100 . 
     A cover  346  may be disposed on top of the inner tube  210  inside outer tube  220  to cover the annular portion  342  while leaving the top end of the inner tube  210  open. The cover  346  may be for example of ring shape. In an example, the solid insert  326  may be attached to the cover  346 . A cap plate  350  may be disposed on a support structure  354  disposed on the outer tube  220 . The cap plate  350  covers the distributor unit  150  on the top. In an example, the support structure  354  may be metal rods or bars welded for holding the cap plate  350  in position above the outer tube  220 . A gas inlet  358  may be disposed on the support structure  354  to allow gas to enter the distributor unit as shown by arrow  358 . 
     During operation, liquid accumulates on the distributor tray  140  forming a liquid level  362 . Gas enters the distributor unit  150  via the gas inlet  358  and enters the inner tube  210  via the narrow passage  330  (shown by dotted arrows in  FIG.  3 ( a ) ). As the density of gas is low, the gas path volume is to be reduced to create a pressure drop that can lift the liquid in the annular portion  342 . The use of solid insert  326  in the upper region of the inner pipe accomplishes this by creating the narrow passage  330 . As a result, there is increase in liquid level inside the annular portion  342  compared to the liquid level on the distributor tray  140 . Since the inner slot  334  is fluidically connected to the annular portion  342  and the narrow passage  330 , the low pressure is transmitted to the annular portion  342  and causes a rise in the liquid in the annular portion  342  to an annular liquid level  366 , because gas will not flow into the annular portion  342  as the high velocity in the narrow passage  330  causes low pressure in the narrow passage  330 . The liquid in the annular portion  342  enters the inner tube  210  via the first aperture  314  (liquid flow is shown by solid arrows in  FIG.  3 ( a ) ). The presence of the venturi insert  322  increases the mixing between the gas and liquid. In an example, if the annular liquid level  366  is high, the liquid may also enter the inner tube  210  via the second aperture  318 . If the annular liquid level  366  rises further, the liquid may enter the inner tube  210  via a third aperture (not shown in the figure). Any number of apertures of any shape may be disposed on inner tube  210  to allow liquid to enter. It is preferred that the apertures are immersed in the liquid, otherwise the lower pressure in the annular portion  342  compared to the pressure in the inner tube  210 , may not be maintained. For higher liquid flow rates, it is preferred that the liquid passes through the lower end of the inner slot  334 . However, at the lower end of the inner slot  334 , because they are close to the narrow passage  330 , the low pressure in the annular portion  342  is not affected much even if they are not immersed in liquid. The location of the solid insert  326  allows for the low pressure region to be created just at the inner slot  334  and into the annular portion  342 . This assures low pressure is directly delegated to the annular portion  342 , without reducing the gas path too much. This is especially advantageous for relatively low gas flow rates. 
     For low liquid flow rates, the size of an aperture in conventional distributors is very small, which results in clogging of the liquid aperture for sludgy liquids. However, in the present subject matter, since the first aperture  314  is disposed on the inner pipe  210 , which is located inside the outer pipe  220 , the chances of clogging of the aperture is reduced as the particles settle on the tray rather than entering the inner pipe  210 . 
     In an example, a pipe-piece structure  370  may be disposed below the first aperture  314  and the venturi insert  322 . The pipe-piece is held in place by means of orifice plate  371 . The pipe-piece structure  370  may be disposed on a bottom portion of the inner tube  210  above the orifice  310 . The pipe-piece structure  370  also reduces a cross-sectional area of fluid flow. The shape of the pipe-piece structure  370  may be any shape that provides a constriction in the fluid flow path in the inner tube  210 . The pipe-piece structure  370  is sized to provide calculated resistance to flow and shaped to provide a constriction in the path of fluid flow. The presence of this resistance to flow increases the liquid level in the annular portion  342  of the distributor unit  150 . This resistance to liquid flow acts in series to the resistance caused by the first aperture  314  through which the liquid flows. The outlet of the pipe-piece structure  370  is shaped to splash the gas-liquid mixture onto the portion below. In a conventional distributor downpipe, for example in a chimney distributor, the minimum size of the aperture for liquid flow is about 6 mm. Further reduction in aperture size to increase resistance to liquid flow in case of low liquid flows is not recommended as it results in clogging of the aperture. The required resistance to flow in the present subject matter is provided by using resistance to flow in the inner tube  210  by sizing the pipe-piece structure  370  appropriately without clogging due to the reasons discussed above. In addition, some resistance to liquid flow may also be given by slot  338 . 
     The level of liquid in the annular portion  342  is more stable compared to the liquid level outside the outer pipe  220  as it is controlled by the gas flow rate. Hence, the amount of liquid entering the inner tube  210  is more even, without much pulsing, or uneven flow, as the outer pipe  220  helps in dampening the liquid waves on the tray plate. This results in even flow of liquid into the inner tube  210  through the first aperture  314 . 
     The liquid flow from the annular portion  342  to the inner tube  210  is only due to the liquid head in the annular portion  342  and not because of the low pressure within the inner tube  210 , as there is no pressure difference within the inner tube  210  and the annular portion  342  because there is no or minimal flow of gas through the inner slot  334  As a result, the liquid flow is only due to the liquid head, which reduces the liquid flow through the second aperture  318  placed above the first aperture  314 . This increases the operating range of distributor unit  150 , as the apparatus  120  can operate either only through the first aperture  314  or through both the first aperture  314  and second aperture  318 , without much deviation when liquid level passes above the first or second apertures  314  and  318 . 
     As the liquid level depends on the gas flow through the narrow passage  330  and the resistance offered by the pipe-piece structure  370 , when the distributor tray  140  is out of level, an increase in the liquid flow through a distributor unit  150  that is lower than another distributor unit  150 , causes lower gas flow through the narrow passage  330 , as the pressure drop across the plurality of distributor units  150  is same on a distributor tray  140 . Because of the presence of the flow resistance due to the pipe-piece structure  370 , when the distributor tray  140  is out of level, the distributor units  150  that are lower send more liquid, resulting in lower gas flow, as the overall pressure drop across the plurality of distributor units  150  is equal. This leads to less gas flow into the lower distributor unit  150 , which causes less liquid level to be built up in the annular portion  342  resulting in lesser liquid flow. Hence, the distributor tray  140  has lower sensitivity to out-of-levelness. 
       FIG.  3 ( b )  illustrates a top view of a section along the line A-A of the example distributor unit illustrated in  FIG.  3 ( a ) , in accordance with an embodiment of the present subject matter. The outer tube  220  is disposed concentric to and outside the inner tube  210  forming the annular region  342 . The solid insert  326  is disposed in the inner tube  210  forming the narrow passage  330  in the inner tube  210 . The inner tube  210  has two inner slots  334  that open into the annular region  342 . 
       FIG.  3 ( c )  illustrates a top view of a section along the line B-B of the example distributor unit illustrated in  FIG.  3 ( a ) , in accordance with an embodiment of the present subject matter. The first aperture  314  is disposed on the inner tube  210 . Two slots  338  are disposed on the outer tube  220  such that the slots  338  are disposed 180° from each other on the circumference of the outer tube  220  and at 90° from the first aperture  314 . 
       FIG.  3 ( d )  illustrates an enlarged cross-sectional view of an example dispersion section  374 , in accordance with an embodiment of the present subject matter. In an example, a dispersion section  374  may be disposed below the exit of the inner tube  210 . The dispersion section  374  allows the exiting gas-liquid mixture to be spread out before entering the catalyst bed. The dispersion section  374  may comprise dispersion slots  378 . The dispersion slots  378  may be placed at the exit of the inner tube  210 . In one example, the width of the dispersion slots  378  may be such that it is equal to the space between them. An upper sieve plate  382  may be attached below the dispersion slots  378 , such that there is a height h between the upper sieve plate  382  and the dispersion slots  378 . A splash plate  384  may be placed on the upper sieve plate  382 , such that the top of the splash plate  384  is at the same height h, above the top of the upper sieve plate  382 . 
     The pipe-piece structure  370  may be shaped to direct fluids on the splash plate  384 . The pipe-piece structure  370  is intended to be used along with splash plate  384 . As the fluid mixture is directed by the pipe-piece structure  370 , fluid hits the splash plate  384  dispersing liquid through the dispersion slots  378 . A part of the liquid is dispersed and passes through the dispersion slots  378 . Another part of the liquid passes through the upper sieve plate  382  and is dispersed on a lower sieve plate  320  of larger diameter, where it is spread more widely. The lower sieve plate  320  has sieve plate wall  323  on the edges with sieve plate slots  324 . This allows a certain amount of liquid to be held on the lower sieve plate  320 , allowing uninterrupted flow of liquid 
       FIG.  3 ( e )  illustrates a top view of a section along the line C-C of the example distributor unit illustrated in  FIG.  3 ( d ) , in accordance with an embodiment of the present subject matter. The splash plate  384  is disposed on the upper sieve plate  382  comprising sieve orifices  325 . 
       FIG.  3 ( f )  illustrates the top view of a section along the line D-D of the example distributor unit illustrated in  FIG.  3 ( d ) , in accordance with an embodiment of the present subject matter. The lower sieve plate  320  comprises sieve plate slots  324  and lower sieve orifices  321 . The lower sieve plate  320  allows the fluid to spread more before it enters the catalyst bed 
       FIG.  4 ( a )  illustrates another example distributor unit comprising the solid insert attached to the cap plate, in accordance with an embodiment of the present subject matter. The solid insert  326   b  is disposed within the inner tube  210 . The top of the solid insert  326   b  may be removably attached to the cap plate  350 , for example using bolts. This allows for replacing the solid insert as required, for processes or reactors that require varying gas flows. For example, for low gas flows, a sold insert  326   b  with a larger diameter may be used. The dispersion section  374  may be disposed below the exit of the inner tube  210 . 
       FIG.  4 ( b )  illustrates a top view of a section along the line A-A of the example distributor unit illustrated in  FIG.  4 ( a ) , and  FIG.  4 ( c )  illustrates a top view of a section along the line B-B of the example distributor unit illustrated in  FIG.  4 ( a ) , in accordance with an embodiment of the present subject matter. Referring to  FIG.  4 ( b ) , the solid insert  326   b  may be disposed within the inner tube  210 . The inner tube  210  has two inner slots  334 . The outer tube  220  is disposed concentric to and outside the inner tube  210 . Referring to  FIG.  4 ( c ) , the first aperture  314  is disposed on the inner tube  210 . Two slots  338  are disposed on the outer tube  220  such that the slots  338  are disposed 180° from each other on the circumference of the outer tube  220  and at 90° from the first aperture  314 . 
       FIG.  4 ( d )  illustrates another example distributor unit comprising a static mixer, in accordance with an embodiment of the present subject matter. In another example, the pipe-piece structure  370  may be disposed below the first aperture  314  and a static mixer  322   b . In an example, the static mixer  322   b  may comprise twisted ribbons. Any other static mixers known in the art may be used. The inlet to the static mixer  322   b  is placed such that the liquid from the first aperture  314  enters in each of the openings of the static mixer  322  and mixes with the incoming gas in a series of rotations. The thickness of the ribbon of the static mixer  322   b  may be varied so that there is no significant pressure loss because of the static mixer  322   b . The static mixer  322   b  may be placed near an exit of the inner tube  210  to create a swirling motion of the gas-liquid mixture exiting the inner tube  210 . 
     The pipe-piece structure  370  may be disposed on a bottom portion of the inner tube  210  above the orifice  310 . The pipe-piece structure  370  reduces a cross-sectional area of fluid flow. The shape of the pipe-piece structure  370  may be any shape that provides a constriction in the fluid flow path in the inner tube  210 . The pipe-piece structure  370  is sized to provide calculated resistance to flow and shaped to provide a constriction in the path of fluid flow. The presence of this resistance to flow increases the liquid level in the annular portion  342  of the distributor unit  150 . This resistance to liquid flow acts in series to the resistance caused by the first aperture  314  through which the liquid flows. The outlet of the pipe-piece structure  370  is shaped to splash the gas-liquid mixture onto the portion below. In a conventional distributor downpipe, for example in a chimney distributor, the minimum size of the aperture for liquid flow is about 6 mm. Further reduction in aperture size to increase resistance to liquid flow in case of low liquid flows is not recommended as it results in clogging of the aperture. The required resistance to flow in the present subject matter is provided by using resistance to flow in the inner tube  210  by sizing the pipe-piece structure  370  appropriately. In addition, the required resistance to liquid flow may also be given by slot  338 . 
       FIG.  5 ( a )  illustrates a cross-sectional view of another embodiment of the distributor unit comprising one tube, in accordance with an embodiment of the present subject matter. In another embodiment, the distributor unit  500  comprises a tube  510  disposed on an orifice  514  on a distributor tray  140 . A first aperture  518  may be disposed on a lower portion of the tube  510  near the distributor tray  140  to allow liquid to enter the distributor unit  500 . In an example, there may be two first apertures  518  disposed at 180° from each other on a circumference of the tube  510  at the same height. A solid insert  522  may be disposed within the tube  510  so that a lower portion of the solid insert  522  is adjacent to the first aperture  518 . The lower portion of the solid insert  522  may have corrugated edges  526  and an upper portion of the solid insert  522  may be hemispherical or may have rounded edges  530 . The solid insert  522  may be held in position using support rod  528  that attaches the insert to the tube  510 . The support rod  528  may pass through the tube  510  and solid insert  522  to hold the insert  522  firmly. The support rod  528  may be present at multiple positions around the solid insert  522  while leaving sufficient space for allowing flow of gas and liquid. A cap plate  534  may be disposed on a support structure  538 , the support structure  538  being disposed on the tube  510 . A gas inlet  542  may be disposed on the support structure  538  to allow gas to enter the distributor unit  500 . The distributor unit  500  may comprise a second aperture  546  disposed on a portion of the tube  510  above the first aperture  518 . The second aperture  546  allows liquid to enter the tube  510 . A top slot  550  may be disposed near a top portion of the tube  510  to allow liquid to enter the tube in cases where the liquid level  554  on the distributor tray  140  becomes high or increased flow conditions. 
     In an example, the first aperture  518  may be disposed 1 to 4 inches above the distributor tray  140 . The first and second apertures  518  and  546  may be sized as to offer required resistance to liquid flow, depending on the operational characteristics of the reactor. The solid insert  522  effectively reduces the flow path, creating a passage  562  between the solid insert  522  and the tube  510 . The feed gas passes through the passage  562  at high velocity creating low pressure in the region. Liquid from the distributor tray  140  is pulled to this low pressure region and the corrugated edges  526  result in increased mixing with the gas. 
     An orifice insert  558  may be disposed at the exit of the tube  510 , which reduces the flow area, thereby enhancing mixing of gas and liquid. In an example, the orifice insert  558  may be disposed 1 to 3 inches below the solid insert  522 . In an example, the orifice  514  may be part of the orifice insert  558 , and in this case the orifice  514  may be disposed higher or lower in the tube  510  and may be different from a tray orifice where the tube  510  is attached to the tray  140 . In another example, a venturi insert may be disposed below the first aperture  518 . In an example, the dispersion section  374  as discussed above may be disposed below the exit of the tube  510 . The distributor tray  140  comprising a plurality of distributor units  500  may used to distribute a gas-fluid mixture in a reactor. 
       FIG.  5 ( b )  illustrates a top view of a section along line A-A of the example distributor unit shown in  FIG.  5 ( a ) , in accordance with an embodiment of the present subject matter. The solid insert  522  is adjacent to the two first apertures  518 , the two first apertures  518  disposed at 180° from each other on the circumference of the tube  510 . 
     During operation, the low pressure in the passage  562  causes liquid to be pulled into the tube  510  via the first aperture  518 . As a result, the driving force for liquid flow is increasing gas velocity compared to liquid head above the tray  140 . This can help in providing a lower requirement of the height of the distributor unit  500  for the same sensitivity to out-of-levelness. 
     The volumetric flow rate of liquid is given by 
         Q   L =ƒ( H   0.5 )+ g ( Q   G )
 
     Where, H=liquid height above the lowest aperture,
 
Q L =is volumetric liquid flow rate
 
Q G =volumetric gas flow rate
 
f(H 0.5 ) is a function of h 0.5  
 
g(Q L ) is an another function of Q L  
 
     When the tray  140  is out-of-level, the lower distributor unit passes more liquid due to the increased height. This causes less gas to flow through that particular distributor unit, which in turn lowers the amount of liquid passing through the distributor unit as the pressure drop is reduced due to lower gas flow. Thus, as the liquid flow depends on increasing gas velocity, the distributor apparatus has lower sensitivity to out of levelness. 
     EXAMPLES 
     The disclosure will now be illustrated with working examples, which are intended to illustrate the working of disclosure and not intended to take restrictively to imply any limitations on the scope of the present disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice of the disclosed methods and compositions, the exemplary methods, devices and materials are described herein. It is to be understood that this disclosure is not limited to particular methods, and experimental conditions described, as such methods and conditions may apply. 
     Example 1 
     The following is to illustrate the advantage of using the solid insert  326  in the distributor unit  150  comprising the inner tube  210  and the outer tube  220 . This was compared to a conventional distributor tray without a chimney distributor unit. The distributor trays are subject to be out-of-level with the horizontal, such that the higher distributor is 1 cm above the lower distributor. The sensitivity due to 1 cm out of levelness is defined as: 
     
       
         
           
             
               
                 
                   
                     % 
                     ⁢ 
                         
                     
                       Sensitivity 
                       lq 
                       
                         1 
                         ⁢ 
                            
                         cm 
                       
                     
                   
                   = 
                   
                     
                       
                         ( 
                         
                           
                             Q 
                             
                               l 
                               ⁢ 
                               o 
                               ⁢ 
                               w 
                             
                           
                           - 
                           
                             Q 
                             
                               h 
                               ⁢ 
                               i 
                               ⁢ 
                               g 
                               ⁢ 
                               h 
                             
                           
                         
                         ) 
                       
                       
                         
                           ( 
                           
                             
                               Q 
                               
                                 l 
                                 ⁢ 
                                 o 
                                 ⁢ 
                                 w 
                               
                             
                             + 
                             
                               Q 
                               
                                 h 
                                 ⁢ 
                                 i 
                                 ⁢ 
                                 g 
                                 ⁢ 
                                 h 
                               
                             
                           
                           ) 
                         
                         / 
                         2 
                       
                     
                     * 
                     100 
                     ⁢ 
                     % 
                   
                 
               
             
             
               
                 
                   QL 
                   = 
                   
                     
                       f 
                       ⁢ 
                       
                         ( 
                         
                           H 
                           
                             0 
                             . 
                             5 
                           
                         
                         ) 
                       
                     
                     + 
                     
                       g 
                       ⁢ 
                       
                         ( 
                         
                           Q 
                           G 
                         
                         ) 
                       
                     
                   
                 
               
             
           
         
       
     
     where, Q low  is volumetric liquid flow through lower distributor unit, and Q high  is volumetric liquid flow rate through higher distributor unit. 
     Table 1 shows a comparison of the sensitivity to out of levelness of the distributor apparatus of the present subject matter and conventional apparatus. The sensitivity to out of levelness at high flow rate and low flow rate is the least with the distributor unit of the present subject matter. The addition of the solid insert  326  and the additional resistances, such as venturi insert  370 , reduces the sensitivity further. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Comparison of sensitivity to out of levelness 
               
               
                 of different distributor apparatus. 
               
            
           
           
               
               
               
            
               
                   
                 Sensitivity at 0.8 
                 Sensitivity at 0.4 
               
               
                   
                 cm/s (%) 
                 cm/s (%) 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                 Distributor unit without solid insert 
                 6.9 
                 20 
               
               
                 and venturi insert 
               
               
                 Distributor unit with solid insert at 
                 4.5 
                 15 
               
               
                 upper region 
               
               
                 Distributor unit with solid insert at 
                 3.1 
                 8 
               
               
                 upper region and orifice or other 
               
               
                 pressure resistances in inner tube 
               
               
                 below liquid inlet. 
               
               
                   
               
            
           
         
       
     
     Example 2 
     The following is to illustrate the different benefits of the distributor unit  150  with an inner tube  210  and outer tube  220 , is compared with a conventional chimney distributor. The distributor trays are subjected to be out of level with the horizontal, such that the higher distributor is 1 cm above the lower distributor. The sensitivity due to 1 cm out of levelness is defined as 
     
       
         
           
             
               % 
               ⁢ 
                   
               
                 Sensitivity 
                 lq 
                 
                   1 
                   ⁢ 
                      
                   cm 
                 
               
             
             = 
             
               
                 
                   ( 
                   
                     
                       Q 
                       
                         l 
                         ⁢ 
                         o 
                         ⁢ 
                         w 
                       
                     
                     - 
                     
                       Q 
                       
                         h 
                         ⁢ 
                         i 
                         ⁢ 
                         g 
                         ⁢ 
                         h 
                       
                     
                   
                   ) 
                 
                 
                   
                     ( 
                     
                       
                         Q 
                         
                           l 
                           ⁢ 
                           o 
                           ⁢ 
                           w 
                         
                       
                       + 
                       
                         Q 
                         
                           h 
                           ⁢ 
                           i 
                           ⁢ 
                           g 
                           ⁢ 
                           h 
                         
                       
                     
                     ) 
                   
                   / 
                   2 
                 
               
               * 
               1 
               ⁢ 
               0 
               ⁢ 
               0 
               ⁢ 
               % 
             
           
         
       
     
     where, Q low  is volumetric liquid flow through lower distributor unit, and Q high  is volumetric liquid flow rate through higher distributor unit. Low value of sensitivity indicated better performance in uneven conditions of liquid depth on the tray. 
       FIG.  6    illustrates the variation in sensitivity with superficial velocity for example 2, in accordance with an embodiment of the present subject matter. The operational superficial velocity of liquid in typical hydroprocessing reactors ranges between 0.3 to 1.2 cm/s. The solid line  610  is for the distributor unit  150  of the present subject matter, the dotted line  620  is for a standard chimney distributor with a slot opening. The dashed line  630  shows sensitivity for a conventional chimney distributor with two apertures. The figure shows limit of less than 20% above flow of 0.3 cm/s for the distributor unit  150 , which is better than the acceptable limit of 20%. This is achieved, due to combined effect of using the solid insert  326  and additional inserts, such as the venturi insert  322  and the pipe-piece structure  370  to give calculated resistance to flow in the inner tube  210 . In contrast, the conventional distributor does not achieve a sensitivity below 20% for any flow rate and the distributor with two apertures achieves it only at high flow rates above about 0.7 cm/s 
     Example 3 
     Two distributors were compared to illustrate the advantage of having liquid flow depend on gas velocity in addition to the static liquid head. Table 2 shows the values of sensitivity and liquid height for a conventional chimney distributor compared to the distributor unit  500  comprising tube  510 . Equalizing the sensitivity, it can be seen that the distributor unit  500  of the present disclosure has liquid height of 3 cm compared to the conventional chimney distributor. The additional height can be utilized for catalyst bed. 
         Q   L =ƒ( H   0.5 )+ g ( Q   G )
 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Comparison of sensitivity to out of levelness of a conventional distributor 
               
               
                 and a distributor of the present disclosure with one tube. 
               
            
           
           
               
               
               
            
               
                   
                 Sensitivity at 0.5 
                 Liquid height for 0.5 
               
               
                   
                 cm/s (%) 
                 cm/s distributor (cm) 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                 Chimney distributor 
                 5 
                 8 cm 
               
               
                 Distributor with one tube 
                 4.9 
                 3 cm 
               
               
                 of present disclosure 
               
               
                   
               
            
           
         
       
     
     Although embodiments of the present subject matter are described in language specific to structural features, it is to be understood that the specific features and methods are disclosed as example embodiments for implementing the claimed subject matter.