A perturbation unit and a reactive distillation column using the same

Abstract

The application discloses a kind of perturbation unit and the reaction rectifying tower of application unit, the perturbation unit is suitable for gas-liquid-solid mass transfer reaction and is arranged in reaction rectifying tower, comprising: liquid injection pipe, it is fixed on upper tray and forms injection liquid stream at outlet, the injection liquid stream is used to drive a impeller to rotate and form dispersed phase droplet in the process of impact with impeller;Gas-lift agitating mechanism, the main body is gas-lift pipe structure, upper end is fixedly connected with impeller, lower end penetrates this layer tray and receives the rising gas phase from lower tray, the rising gas phase is below the liquid level of this layer tray and forms bubble mixing with liquid phase;The gas-lift agitating mechanism is rotated under the driving of impeller and carries out three-phase disturbance to liquid phase, the filler and / or catalyst solid particles suspended in liquid phase and gas phase.The application can not only effectively increase gas-liquid-solid three-phase contact and then improve gas-liquid mass transfer efficiency and reaction rate, but also effectively reduce operating cost.

Description

Technical Field

[0001] This invention relates to the field of petrochemical gas-liquid mass transfer technology, and particularly to a disturbance unit and a reactive distillation column using the unit. Background Technology

[0002] A reactive distillation column is a gas-liquid mass transfer device that combines reaction and distillation processes. Reactive distillation technology is widely used in etherification, esterification, and alkylation reactions. Compared to traditional processes that use reactors and distillation columns independently, the use of reactive distillation columns can significantly reduce operating and investment costs. Reactive distillation columns can be plate columns or packed columns. Plate columns are widely used due to their simple structure and high adaptability.

[0003] Traditional reactive distillation technologies employ homogeneous catalysts, which present challenges in separating them from the reaction products. Furthermore, most homogeneous catalysts are acidic or alkaline, causing severe corrosion to the equipment. In recent years, the use of heterogeneous catalysts has significantly improved the solutions to equipment corrosion and catalyst separation difficulties. However, for gas-liquid-solid three-phase reactive distillation, achieving uniform mixing and efficient mass transfer between the gas, liquid, and solid phases between trays remains a technical challenge. To ensure good contact and reaction space between the gas, liquid, and solid phases, the arrangement of the catalyst and the internal structure of the column require rational design and optimization.

[0004] Chinese patent application CN108261792A discloses a novel pneumatically driven stirred catalytic distillation apparatus, primarily addressing the problem of uneven three-phase mixing of gas, liquid, and solid phases between trays in existing catalytic distillation columns. Each tray employs a separate side-mounted pneumatic stirrer, with the stirring paddle suspended from the top of the tray. Each tray is equipped with a riser and a downcomer. The riser inlet is located on the opposite side of the pneumatic pump mounting side of each tray, and a gas distributor is connected to the upper part of the riser. Gas from the lower tray is evenly sprayed into the liquid-solid mixture on the tray through the gas distributor. A filter screen is installed at the top of the downcomer to prevent particulate or micro-particle catalyst from falling into the lower tray. The downcomer inlet is located at the bottom of the tray, extending below the liquid surface of the adjacent lower tray. This technical solution effectively solves the aforementioned problems and can be used in catalytic distillation. However, the existing technology only uses a stirring device in the liquid phase, and the power for stirring comes from the outside, resulting in high energy consumption; the riser and gas distributor still use the traditional fixed type, and the gas-liquid mass transfer efficiency still needs to be improved.

[0005] Therefore, there is an urgent need for a disturbance unit that can effectively improve the mass transfer efficiency of gas-liquid-solid three-phase contact and a reactive distillation column that uses this unit, while also effectively reducing operating costs.

[0006] The information disclosed in this background section is intended only to enhance the understanding of the overall background of the invention and should not be construed as an admission or in any way implying that the information constitutes prior art known to those skilled in the art. Summary of the Invention

[0007] The purpose of this invention is to provide a disturbance unit and a reactive distillation column using the unit, which can not only effectively increase the gas-liquid-solid three-phase contact and thus improve the gas-liquid mass transfer efficiency and reaction rate, but also effectively reduce operating costs.

[0008] To achieve the above objectives, according to a first aspect of the present invention, a disturbance unit is provided, suitable for gas-liquid-solid mass transfer reactions and disposed in a reactive distillation column, comprising: a downcomer jet pipe fixed to an upper tray and forming a jet stream at the outlet, the jet stream driving an impeller to rotate and forming dispersed phase droplets during the impact with the impeller; and a gas-lifting stirring mechanism, the main body of which is a gas-lifting pipe structure, the upper end of which is fixedly connected to the impeller, the lower end of which penetrates the tray and receives rising gas phase from the lower tray, the rising gas phase being below the liquid surface of the tray and forming bubbling mixture with the liquid phase; the gas-lifting stirring mechanism rotates under the drive of the impeller and performs three-phase disturbance on the liquid phase, the packing and / or catalyst solid particles suspended in the liquid phase, and the gas phase.

[0009] Furthermore, in the above technical solution, the liquid downcomer may include: a vertical section, which is fixed on the upper tray and used to receive the liquid phase accumulated from the upper tray; and a horizontal section, which is connected to the vertical section and whose outlet is directly opposite the impeller blades.

[0010] Furthermore, in the above technical solution, the horizontal segment may include a first horizontal segment, a reduced-diameter segment, and a second horizontal segment, wherein the diameter of the second horizontal segment is smaller than the diameter of the first horizontal segment.

[0011] Furthermore, in the above technical solution, the gas lifting and stirring mechanism may include: a gas lifting main pipe, the upper end of which is fixedly connected to the impeller, and a bearing fixed to the tower plate at the lower end; a gas phase branch pipe, which is connected to the gas lifting main pipe and extends in the radial direction, and the portion of the gas phase branch pipe below the liquid surface is provided with a gas distributor, and the gas distributor has uniformly opened gas holes.

[0012] Furthermore, in the above technical solution, multiple gas phase branch pipes can be evenly arranged in a straight line, cross, or star shape along the circumference of the main gas riser.

[0013] Furthermore, in the above technical solution, the gas distributor can be set on the vertical part of each gas phase branch pipe and form an array. Each gas distributor is set horizontally. During rotation, the vertical part of the gas phase branch pipe and the overall structure formed by the gas distributor can create a stirring effect on the liquid phase.

[0014] Furthermore, in the above technical solution, the inlet of the downcomer jet pipe has a filter section for intercepting solid particles in the liquid phase of the upper tray.

[0015] Furthermore, in the above technical solution, the filter section can adopt a cylindrical structure and be adapted to the diameter of the inlet of the downcomer jet pipe.

[0016] Furthermore, in the above technical solution, an ultrasonic component for generating vibration and cavitation effects can be provided below the liquid surface of each tray.

[0017] Furthermore, in the above technical solution, the ultrasonic component may include: an ultrasonic transducer, which may be located on the inner wall of the tower and close to the filter section, and the vibration effect generated is used to drive away solid particles attached to the filter section; and an ultrasonic generator, which is connected to the ultrasonic transducer and located outside the tower.

[0018] According to a second aspect of the present invention, a reactive distillation column is provided, comprising a disturbance unit as described in any one of the preceding claims, wherein the disturbance unit is disposed in each tray.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1) The liquid jet pipe used in this invention has a vertical section and a horizontal section, and the horizontal section is made of reduced diameter, which can convert the descending liquid phase into a horizontal high-speed water flow. The high-speed water flow impacts the impeller, which can not only effectively drive the impeller to rotate, but also form dispersed phase droplets by impacting the impeller blades. The dispersed phase droplets can collide and mix with the gas phase above the liquid surface of the tray, which is beneficial to gas-liquid mass transfer and reaction.

[0021] 2) The riser pipe of the present invention is configured to be rotatable. During the rotation of the riser pipe, the gas phase will be thrown towards the wall of the riser pipe under the action of centrifugal force. This will create a low-pressure space in the center of the riser pipe, which can effectively promote the rise of the gas phase on the lower tray into the riser pipe, making the gas phase flow smoother and thus reducing the pressure drop of each tray space.

[0022] 3) The spiral rising gas phase in the main gas lift pipe of this invention has a "centrifugal outward throwing" effect on the gas phase reaching the top. Therefore, the overall structure of the main gas lift pipe and the gas phase branch pipe can be designed in a shape similar to an inverted "mountain". The flow rate of the gas phase will not be significantly lost. Such an overall structure not only allows the gas distributor's pores to be distributed in the middle or lower part of the liquid phase, making the "bubbling mixing" effect better, but also allows the vertical part of the gas phase branch pipe and the horizontally arranged gas distributor to penetrate below the liquid surface. Its overall structure can more fully stir the liquid phase and maximize the disturbance of the gas-liquid-solid three phases, thereby obtaining better gas-liquid mass transfer efficiency and accelerating the gas-liquid reaction rate.

[0023] 4) The present invention sets an ultrasonic transducer below the liquid surface. On the one hand, ultrasonic waves can effectively avoid the blockage of filter screen packing and / or catalyst particles. On the other hand, the "cavitation effect" of ultrasonic waves can further enhance the gas-liquid-solid three-phase mass transfer and reaction effect.

[0024] 5) This invention utilizes the gravity of the liquid phase itself and the increased flow rate as power to drive the rotation of the gas lift pipe and form agitation, without the need for additional external energy, which can significantly reduce the operating cost of the reactive distillation column of this invention.

[0025] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it according to the contents of the specification, and to make the above and other objects, technical features and advantages of the present invention easier to understand, one or more preferred embodiments are listed below and described in detail with reference to the accompanying drawings. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the disturbance unit in the reactive distillation column of the present invention.

[0027] Figure 2 This is a top view schematic diagram of the air-lifting and stirring mechanism in the disturbance unit of the present invention.

[0028] Explanation of key figure labels:

[0029] 1-Reactive distillation column, 10-Training plate, 11-Downcomer, 111-Vertical section, 112-First horizontal section, 113-Reduced diameter section, 114-Second horizontal section, 12-Main gas riser, 120-Gas branch pipe, 121-Gas distributor, 13-Bearing, 14-Impeller, 15-Cylindrical filter section, 16-Ultrasonic transducer, 160-Ultrasonic generator. Detailed Implementation

[0030] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments.

[0031] Unless otherwise expressly stated, throughout the specification and claims, the term "comprising" or its variations such as "including" or "comprises" shall be understood to include the stated elements or components without excluding other elements or other components.

[0032] In this document, for ease of description, spatial relative terms such as “below,” “under,” “down,” “above,” “above,” “upper,” etc., are used to describe the relationship of one element or feature to another element or feature in the accompanying drawings. It should be understood that spatial relative terms are intended to encompass different orientations of an object in use or operation, in addition to those depicted in the figures. For example, if an object in the figure is flipped, an element described as “below” or “under” another element or feature would be oriented “above” that element or feature. Thus, the exemplary term “below” can encompass both the downward and upward orientations. An object may also have other orientations (rotated 90 degrees or other orientations), and the spatial relative terms used herein should be interpreted accordingly.

[0033] In this document, the terms "first," "second," etc., are used to distinguish two different elements or parts, and are not used to define specific positions or relative relationships. In other words, in some embodiments, the terms "first," "second," etc., can also be used interchangeably.

[0034] This invention provides a reactive distillation column in which a disturbance unit is incorporated to agitate the gas, liquid, and suspended packing and / or catalyst particles in the liquid phase. The packing particles promote gas-liquid mass transfer, improving its efficiency, while the catalyst particles accelerate the gas-liquid reaction. This invention drives an impeller to rotate via a top-down liquid phase. This rotation not only drives the gas lift and agitation mechanism, causing the gas phase to rise rapidly in the main lift pipe and exit through the branch pipes from the gas distributor, mixing with the liquid phase accumulated on the trays in a bubbling manner, but also creates a gas-liquid-solid agitation effect. This effectively solves the mixing problem of the gas-liquid-solid three-phase system when using heterogeneous catalysts in existing technologies. This invention utilizes the gravity of the liquid phase and the increased flow rate as the driving force to rotate the lift pipe and create agitation, eliminating the need for additional external energy and significantly reducing the operating cost of the reactive distillation column.

[0035] Example 1

[0036] like Figure 1 , Figure 2 As shown, this embodiment provides a disturbance unit suitable for gas-liquid-solid mass transfer reactions and installed in a reactive distillation column, including a liquid downcomer 11 and a gas upcomer stirring mechanism.

[0037] The downcomer 11 is fixed to the upper tray and forms a jet stream at the outlet. This jet stream drives the impeller 14 to rotate and forms dispersed phase droplets during the impact with the blades of the impeller 14. Specifically, the trays of the reactive distillation column can be circular or square. When a circular structure is used, the diameter D1 of the tray can range from 800 to 5000 mm. (Reference) Figure 1 The downcomer 11 can be positioned at a distance of 0.3D1 to 0.45D1 from the center of the tray. Preferably, but not limitingly, the downcomer 11 has an L-shaped structure, including a vertical section 111 and a horizontal section. The vertical section 111 is fixed to the upper tray and is used to receive the liquid phase accumulated from the upper tray. The vertical portion of the pipe passes through the tray 10, and its length L1 can range from 100 to 300 mm. A portion of 0.3L1 to 0.5L1 in length is located above the tray 10; this portion can be configured as a cylindrical structure with a filter (i.e., filter section 15) to prevent packing and / or catalyst particles on the tray from flowing to the lower tray with the descending liquid. The remaining 0.5L1 to 0.7L1 length of the vertical section 111 is located below the tray 10. The horizontal section communicates with the vertical section 111, and the outlet of the horizontal section faces the blades of the impeller 14. Further reference. Figure 1 , Figure 2 The horizontal section includes a first horizontal section 112, a reduced-diameter section 113, and a second horizontal section 114. Through the reduced-diameter setting, the diameter of the second horizontal section 114 is smaller than the diameter of the first horizontal section 112. This reduced-diameter setting accelerates the liquid phase movement, resulting in a stronger impact on the impeller 14. Specifically, the horizontal section is located below the tray, and its length L2 can be designed to be 300–600 mm. The diameter d2 of the reduced-diameter second horizontal section 114 is 0.5–0.7 times the diameter d1 of the first horizontal section 112. The length L3 of the reduced-diameter section can be designed to be 100–200 mm. The length L4 of the second horizontal section 114 can be designed to be 50–150 mm.

[0038] The downcomer jet is designed with an L-shaped structure, which has a vertical section and a horizontal section. The horizontal section is narrowed to convert the descending liquid phase into a horizontal high-speed water flow. The high-speed water flow impacts the impeller, which not only effectively drives the impeller to rotate, but also forms dispersed liquid droplets by impacting the impeller blades. The dispersed liquid droplets can collide and mix with the gas phase above the liquid surface on the tray, which is beneficial to gas-liquid mass transfer and reaction.

[0039] Further as Figure 1 , 2As shown, the main body of the gas-lifting stirring mechanism is a gas-lifting pipe structure, with the upper end fixedly connected to the impeller 14 and the lower end passing through this layer of tray and receiving the rising gas phase from the lower tray. This rising gas phase forms bubble mixing with the liquid phase below the liquid level of this layer of tray 10. The gas-lifting stirring mechanism rotates under the drive of the impeller 14 and perturbs the liquid phase, the packing and / or catalyst solid particles suspended in the liquid phase, and the gas phase in a three-phase manner. Preferably but not limitedly, the gas-lifting stirring mechanism may include a main gas-lifting pipe 12 and gas-phase branch pipes 120. Among them, the upper end of the main gas-lifting pipe 12 is fixedly connected to the impeller 14 and can rotate with the impeller. A bearing 13 fixed to the tray is provided at the lower end of the main gas-lifting pipe 12. The main gas-lifting pipe 12 can rotate stably under the support of the bearing 13, avoiding axial movement and radial runout. The main gas-lifting pipe 12 is spatially connected to the lower tray. Since the main gas-lifting pipe 12 rotates, under the action of centrifugal force, the gas phase will be thrown towards the wall surface of the main gas-lifting pipe, so a low-pressure space will be formed in the center of the main gas-lifting pipe, which can effectively promote the rising of the gas phase on the lower tray into the main gas-lifting pipe 12, making the gas phase rise more smoothly and thus reducing the pressure drop in the space of each tray.

[0040] Further as Figure 1 , 2 shown, the gas-phase branch pipes 120 are connected to the main gas-lifting pipe 12 and extend in the radial direction. In this embodiment, it specifically adopts the method of first extending horizontally and then extending vertically downward, so that the vertically downward extending part is fully immersed below the liquid level. A gas distributor 121 is provided on the part of the gas-phase branch pipe 120 below the liquid level, and pores are evenly opened on the gas distributor 121. Due to the rotation of the main gas-lifting pipe 12, the gas phase in it spirally rises and is thrown towards the gas-phase branch pipes 120 after rising to the top. Preferably but not limitedly, the gas-phase branch pipes 120 can be designed as multiple. The multiple gas-phase branch pipes can be evenly arranged circumferentially along the main gas-lifting pipe 12. They can be arranged in a straight line. When arranged in a straight line, the main gas-lifting pipe 12 and the gas-phase branch pipes 120 as a whole are in an inverted "mountain" shape. They can also be arranged in a cross shape (refer to Figure 2 ) or a star shape. The gas distributor 121 is arranged on the vertical part of each gas-phase branch pipe 120 and forms an array. Each gas distributor 121 is arranged horizontally. During the rotation process, the overall structure formed by the vertical part of the gas-phase branch pipe 120 and the gas distributor 121 is in the structure of a "stirring rod", thus forming sufficient stirring of the liquid phase. Further, the diameter d3 of the pores opened on the gas distributor 121 can be 3 - 10 mm, the spacing L6 of the pore diameters ranges from 20 - 50 mm, the length L5 of the gas-phase branch pipe 120 can be designed as 100 - 800 mm, and the diameter d4 of the gas-phase branch pipe 120 is 25 - 100 mm. The number of gas distributors is 2 - 4, and the distance range between the gas distributors can be 100 - 300 mm.

[0041] The spiraling gas phase in the main gas riser allows the gas phase channel of this invention (i.e., the overall structure of the main gas riser and the gas phase branch pipe) to be designed in an inverted "mountain" shape, without significant loss of gas phase flow rate. The inverted "mountain" shape structure not only allows the pores to be distributed in the middle or lower part of the liquid phase, resulting in better "bubbling mixing" effect, but also allows the vertical part of the gas phase branch pipe 120 and the horizontally arranged gas distributor 121 to extend below the liquid surface. The overall structure provides more thorough stirring of the liquid phase, maximizing the disturbance of the gas-liquid-solid three phases, thereby obtaining better gas-liquid mass transfer efficiency and accelerating the gas-liquid reaction rate.

[0042] Preferred, but not limited, such as Figure 1 As shown, to further enhance the three-phase disturbance effect, this embodiment also includes an ultrasonic component below the liquid surface of each tray 10 for generating vibration and cavitation effects. This ultrasonic component may include an ultrasonic transducer 16 and an ultrasonic generator 160. The ultrasonic transducer 16 is located on the inner wall of the tower and near the filter section 15; the generated vibration effect is used to dislodge solid particles adhering to the filter section 15. Further as... Figure 1 As shown, the ultrasonic generator 160 is connected to the ultrasonic transducer 16 and is located outside the tower.

[0043] The operation process of the disturbance unit in this embodiment is as follows:

[0044] 1) The liquid phase accumulated on the upper tray descends to the current tray through the downcomer. The descending liquid flows out horizontally through the horizontal section. After being accelerated through the narrowing section, the velocity of the outflowing liquid increases, and the kinetic energy of the liquid increases, forming a jet state.

[0045] 2) The accelerated ejected liquid phase impacts the impeller blades, causing the entire gas lifting and stirring mechanism, including the main gas lifting pipe and the gas phase branch pipe, to rotate (at the same time, the gas phase ejected from the gas distributor forms a bubbling mixture with the liquid phase), thereby driving the gas-liquid-solid three-phase mixture on the vertical part of the gas phase branch pipe and the stirring plate of the gas distributor to enhance the contact between the gas, liquid, and solid phases.

[0046] 3) While stirring and disturbing, the ultrasonic transducer installed below the liquid surface and near the inlet of the downcomer pipe emits ultrasonic waves. On the one hand, this can effectively prevent the filter screen from being blocked by packing and / or catalyst particles. On the other hand, the "cavitation effect" of the ultrasonic waves can further enhance the gas-liquid-solid three-phase mass transfer and reaction.

[0047] Example 2

[0048] This embodiment provides a reactive distillation column 1, in which each tray is equipped with the disturbance unit described in Embodiment 1. This embodiment can produce the same technical effects as Embodiment 1, and will not be described again here.

[0049] The foregoing description of specific exemplary embodiments of the present invention is for illustrative and explanatory purposes. These descriptions are not intended to limit the invention to the precise forms disclosed, and it will be apparent that many changes and variations can be made in accordance with the foregoing teachings. The exemplary embodiments were chosen and described in order to explain the specific principles of the invention and its practical application, thereby enabling those skilled in the art to implement and utilize various different exemplary embodiments of the invention, as well as various different choices and variations. Any simple modifications, equivalent changes, and alterations made to the foregoing exemplary embodiments should fall within the scope of protection of the present invention.

Claims

1. A disturbance unit, characterized in that, Suitable for gas-liquid-solid mass transfer reactions and installed in reactive distillation columns, including: The liquid jet pipe is fixed on the upper tray and forms a jet of liquid at the outlet. The jet of liquid is used to drive an impeller to rotate and form dispersed phase droplets during the collision with the impeller. The dispersed phase droplets collide and mix with the gas phase above the liquid surface of the tray. The gas-lifting and stirring mechanism is mainly a gas-lifting pipe structure. Its upper end is fixedly connected to the impeller, and its lower end penetrates the tray of this layer and receives the rising gas phase from the lower tray. This rising gas phase is below the liquid surface of this tray and forms bubbling mixture with the liquid phase. Driven by the impeller, the gas-lifting and stirring mechanism rotates and performs three-phase disturbance on the liquid phase, the suspended packing and / or catalyst solid particles in the liquid phase, and the gas phase. The gas-lifting and stirring mechanism includes: a main gas-lifting pipe, whose upper end is fixedly connected to the impeller, and whose lower end is equipped with a bearing fixed to the tray; a gas phase support... The gas phase branch pipe is connected to the main gas riser and extends radially and then vertically downward. The portion of the gas phase branch pipe below the liquid surface is equipped with a gas distributor with uniformly distributed gas holes. During the rotation of the main gas riser, under the action of centrifugal force, the gas phase is thrown towards the wall of the main gas riser, thus forming a low-pressure space in the center of the main gas riser, which promotes the rise of the gas phase on the lower tray into the main gas riser. During the rotation, the vertical portion of the gas phase branch pipe and the gas distributor together form a stirring effect on the liquid phase.

2. The disturbance unit according to claim 1, characterized in that, The liquid-falling jet pipe includes: The vertical section, which is fixed to the upper tray, is used to receive the liquid phase accumulated from the upper tray; The horizontal section is connected to the vertical section, and the outlet of the horizontal section is directly opposite the blades of the impeller.

3. The disturbance unit according to claim 2, characterized in that, The horizontal segment includes a first horizontal segment, a narrowed segment, and a second horizontal segment, wherein the diameter of the second horizontal segment is smaller than the diameter of the first horizontal segment.

4. The disturbance unit according to claim 1, characterized in that, The multiple gas phase branch pipes are evenly arranged in a straight line, cross, or star shape along the circumference of the main gas riser.

5. The disturbance unit according to claim 1, characterized in that, The gas distributors are arranged in an array on the vertical part of each gas phase branch pipe, and each gas distributor is arranged horizontally.

6. The disturbance unit according to claim 1, characterized in that, The inlet of the liquid dropper has a filter section for trapping solid particles in the liquid phase of the upper tray.

7. The disturbance unit according to claim 6, characterized in that, The filter section has a cylindrical structure and is adapted to the diameter of the inlet of the liquid jet pipe.

8. The disturbance unit according to claim 6, characterized in that, Each of the trays is equipped with an ultrasonic component below the liquid level to generate vibration and cavitation effects.

9. The disturbance unit according to claim 8, characterized in that, The ultrasound component includes: An ultrasonic transducer, located on the inner wall of the tower and close to the filter section, generates a vibration effect to drive away solid particles attached to the filter section. An ultrasonic generator is connected to the ultrasonic transducer and is located outside the tower.

10. A reactive distillation column, characterized in that, It includes a disturbance unit as described in any one of claims 1 to 9, wherein the disturbance unit is disposed in each tray.

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