KR101815753B1 - Self heat supply dehydrogenation reactor with heat source plate inside catalyst layer - Google Patents

Abstract

The present invention relates to a method for producing a polymer electrolyte fuel cell, which comprises a housing defining an inside of a reactor, a catalyst inlet formed at an upper end of the housing to store and input a catalyst, a reactor inlet formed at an upper side of the housing, A dehydrogenation catalyst layer disposed on the inside of the housing coaxially with the housing and defined by an inner screen and an outer screen, the dehydrogenation catalyst layer being filled with a dehydrogenation catalyst therein and undergoing a dehydrogenation reaction of the source gas, Wherein at least a portion of the one end of the one end is in contact with the inner screen and at least a portion of the other end of the one end is in contact with the outer screen and the other end is in contact with the outer screen, And is supplied into the dehydrogenation catalyst layer It relates to a self-heating dehydrogenation reactor containing a plurality of ten won plates to heat the charge gas, according to the present invention through the heat exchange plate of the heat source can supply the additional heat in the reactor can significantly reduce the preheating temperature.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a deuterium dehydrogenation reactor having a heat source plate,

The present invention relates to a dehydrogenation reactor useful for dehydrogenating various hydrocarbon raw materials, and more particularly, to a dehydrogenation reactor useful for dehydrogenating various hydrocarbons by using a heat source plate mounted inside a dehydrogenation catalyst layer, Dehydrogenation reactor.

The dehydrogenation of propane with propylene and isobutane with isobutene produces olefins which are more reactive than alkane feedstocks and which are easy to form coke at high temperatures used for dehydrogenation. The dehydrogenation reactor is a very large, long columnar vertical structure with a width of about 5 to 30 feet and a length of 10 to 100 feet or more. The general structure of this reactor is to inject a reactive gas into the inlet located at the bottom center of the vertical reactor where the gas flows up through the annular zone and passes radially outward through a porous catalyst bed or other suitable dehydrogenation catalyst Passes upwardly through the outer annular zone to be discharged from the top of the reactor outer part. These reactors are often referred to as "radial" reactors because the reactant gas flow through the catalyst bed is radial.

Generally, the radial flow reaction zone consists of cylindrical zones having various nominal cross-sectional areas, which are arranged vertically and coaxially to form reaction zones. Typically, the radial flow reaction zone includes a cylindrical reaction vessel having a cylindrical outer catalyst containing screen and a cylindrical inner catalyst containing screen coaxially disposed with the reaction vessel. The inner screen has a nominal inner cross-sectional area smaller than the outer screen and has a nominal inner cross-sectional area smaller than the reaction vessel. The reaction gas stream is introduced into an annular space present between the inner wall of the reaction vessel and the outer surface of the outer screen. The reaction gas stream passes radially through an annular space present between the outer screen and the inner screen through the outer screen and then through the inner screen. The collected stream into the cylindrical space inside the inner screen is withdrawn from the reaction vessel.

The propane dehydrogenation reaction is a process for producing propylene by separating a part of hydrogen from propane. This process is based on an endothermic reaction (ΔH ₀ ²⁹⁸ = +124 kJ / mole) and sufficient energy must be supplied during the reaction process in order for the reaction to proceed properly. The theoretical equilibrium conversion of propane increases with increasing reaction temperature. In order to increase the equilibrium conversion rate, the reaction gas is heated to 650 ° C or higher and an endothermic reaction proceeds.

Various techniques have been developed and applied in practice as an energy source for the propane dehydrogenation process, and the most common method is a fired heater. The furnace is installed upstream of the reactor to supply a certain amount of energy for the dehydrogenation process of the endothermic reaction. Propane, which is the main reaction gas, is injected into the high temperature heating furnace together with hydrogen before being introduced into the catalytic reactor, and is heated to a proper temperature through a heat exchange process.

1 is a schematic diagram showing a general dehydrogenation system in which heat is supplied using a heating furnace. Referring to FIG. 1, propane and hydrogen, which are reaction gases, pass through a heating furnace, are subjected to a heat exchange and heating process, and then are injected into a dehydrogenation catalytic reactor. Conventionally, a heating furnace used as a preheating device is a method of supplying a gas to a U-shaped pipe and heating several burners around the pipe. Since the heating furnace having such a structure can be heated at a high temperature of 600 to 700 ° C. or higher, it is suitable as a preheating device for the dehydrogenation process, but it is difficult to control the temperature and has a high risk in a process using a combustible gas. In addition, selective heating is not possible depending on the position and the area of the reactor, and a temperature gradient is generated between the inside and the outside of the propane pipe, and a locally heated portion is generated, thereby generating a hot spot of 700 ° C or more inside the heating apparatus. Thermal cracking is a side reaction. Methane, ethane, ethylene, etc. are produced by thermal cracking between propane and propylene. This side reaction is one of the most important parameters to control the heating condition of the furnace because it decreases the yield of propylene and is the main cause of reduction of the process performance.

Therefore, as the heating temperature of the heating furnace increases, the conversion of propane increases, but at the same time, the selectivity decreases due to thermal cracking, thereby reducing the yield of propylene and increasing the unit cost of the process. In addition, there is a problem that a huge initial investment cost and maintenance cost for insulation treatment of the pipe connecting the heating furnace and the reactor occur.

In order to overcome the problems of the prior art described above, the object of the present invention is to provide an apparatus and a method for controlling a preheating temperature by supplying a heat source plate inside a catalyst layer in which a catalytic reaction proceeds, And to maintain the entire process to be an isothermal reaction, thereby improving the process yield and reducing the operation, maintenance, and repair costs.

According to one aspect of the present invention for achieving the above object,

A catalyst inlet formed in the upper end of the housing for storing and introducing the catalyst therein, a reactor inlet formed on one side of the upper portion of the housing for supplying a fluid reactant into the reactor, A dehydrogenation catalyst layer disposed on the inside of the housing coaxially with the housing and defined by an inner screen and an outer screen, the dehydrogenation catalyst layer being filled with a dehydrogenation catalyst therein and undergoing a dehydrogenation reaction of the source gas, And a reaction gas outlet for recovering the reactant stream from the reaction gas outlet,

A plurality of heat source plates disposed between the inner screen and the outer screen for heating the source gas supplied into the dehydrogenation catalyst layer by contacting at least a portion of the one end with the inner screen and at least a portion of the other end with the outer screen, source dehydrogenation reactor comprising a source column.

The dehydrogenation reactor for self-heating according to the present invention can supply additional heat to the inside of the reactor through the heat exchange by the heat source plate, so that the preheating temperature can be remarkably lowered. In addition, since the temperature of the reactor is not locally heated and the temperature gradient between the inside / outside of the reactor is small, the heat cracking of the propane can be prevented and the heat lost by the endothermic reaction May be added to the reactor itself to increase the dehydrogenation yield and propylene selectivity. In addition, the manufacturing cost can be reduced by reducing the reaction temperature reduction problem and the heat insulation treatment cost due to the heat loss of the piping and the reactor.

1 is a schematic diagram showing a general dehydrogenation system in which heat is supplied using a heating furnace.
Figure 2 is a schematic longitudinal cross-sectional view of a reactor illustrating a heat source plate disposed within a screen of a reactor according to one embodiment of the present invention.
3 is a partial cross-sectional view of a reactor illustrating a heat source plate disposed within a screen of a reactor according to one embodiment of the present invention.
Figure 4 is a partial cross-sectional view of a reactor illustrating a heat source plate disposed within a screen of a reactor according to another embodiment of the present invention.
Figure 5 is a partial cross-sectional view of a reactor illustrating a heat source plate disposed within a screen of a reactor according to another embodiment of the present invention.

The present invention will now be described in more detail with reference to the accompanying drawings. Although the terms used in the present invention have been selected as general terms that are widely used at present, there are some terms selected arbitrarily by the applicant in a specific case. In this case, the meaning described or used in the detailed description part of the invention The meaning must be grasped. Like reference numerals refer to like elements throughout the specification.

Although the drawings illustrate specific shapes of the dehydrogenation reactor of the present invention, such a dehydrogenation reactor may have various shapes suitable for the specific environment in which it is performed in a particular application, And the like. Moreover, the numbers in the figures represent a simple schematic diagram of the dehydrogenation reactor of the present invention, only major components being shown. Other pumps, moving pipes, valves, hatches, access outlets and other similar components have been omitted.

The use of these components to modify the described dehydrogenation reactor is well known to those skilled in the art and does not depart from the scope and spirit of the appended claims.

As used herein, the term "fluid" means a gas, liquid, or gas or liquid containing a dispersed solid or a mixture thereof. The fluid may be in the form of a gas containing dispersed droplets.

As used herein, the term "reaction zone" means the space in the dehydrogenation reactor where the reaction gas is in contact with the catalyst on the catalyst bed.

Herein, the direction of the flow of solids through the device by downward, downward, or gravity directions, i.e., cross-flow gas, is oriented so that the use of the terms 'lower', 'downward', 'upper' and 'upward' As shown in FIG.

As used herein, the term " inner "or" inner "refers to the direction of the radial center of the circle, which is the cross-section perpendicular to the direction of gravity of the annular reactor.

As used herein, the term " outer "or" outer "refers to the direction of the radial circumference of the circle, which is a section cut perpendicular to the gravitational direction of the annular reactor.

The term "screen " herein has a broad meaning, including means suitable for limiting the catalyst to the catalyst bed while permitting flow of the reaction gas stream across the catalyst bed.

The numbers in the figures represent a simplified schematic diagram of the dehydrogenation reactor according to the invention, only the main components being shown. Other pumps, moving pipes, valves, hatches, access outlets, and other similar components have been omitted. The use of these accessories to modify the dehydrogenation reactor described is well known to those skilled in the art and does not depart from the scope and spirit of the appended claims.

FIG. 2 is a schematic longitudinal sectional view of a self-heat supplying dehydrogenation reactor according to an embodiment of the present invention. FIG. Referring to FIG. 2, the self-heat supplying dehydrogenation reactor 100 of the present invention includes a housing 10 defining an interior of a reactor, a catalyst inlet 20 formed at an upper end of the housing 10, A reactor inlet 11 formed at one side of the upper portion of the housing for supplying a fluid reactant to the inside of the reactor and a condenser 12 disposed coaxially with the housing 10 inside the housing 10, And a reaction gas outlet (12) formed at a lower end of the housing (10) for collecting a reactant stream from the inside of the reactor, wherein the dehydrogenation catalyst layer (30) The dehydrogenation catalyst layer 30 is defined by an inner screen 31 and an outer screen 32 and a plurality of heat source plates 40 are arranged radially in the space between the inner screen 31 and the outer screen 32 . In the present invention, at least a part of one end of the heat source plate 40 is in contact with the inner screen 31 and at least a part of the other end is in contact with the outer screen 32. This configuration can increase the cross-sectional area of the heat source plate 40 as compared with a configuration in which the heat source plates 40 are spaced apart from the inner screen 31 and the outer screen 32, thereby increasing the amount of heat supplied. In addition, the heat source plate 40 can be stably installed in the reactor 100.

The dehydrogenation reactor 100 is configured to contact the reactant gas stream and the catalyst particles that are transferable to the annular catalyst bed by the gravity stream in a radial stream. Referring to FIG. 2, the dehydrogenation reactor 100 includes an outer cylindrical housing 10, and a dehydrogenation catalyst layer 30 including a catalyst bed accommodated therein is an annular reaction zone, And is separated by a reaction gas flow passage 50 at the center. The housing 10 is formed in a vertical cylinder shape, and a raw material gas inlet 11 is formed on an upper side of the housing 10 to introduce a raw material gas containing propane, hydrogen, and the like, and a reaction gas outlet 12 is formed on a bottom surface of the housing 10 The completed reaction gas is discharged.

The heat source plate 40 disposed in the dehydrogenation catalyst layer 30 supplies the heat transferred from the heat source (not shown) to the raw material gas in the dehydrogenation catalyst layer 30. That is, the heat generated from the heat source is transferred to the outer wall or inside of the heat source plate 40, and the raw material gas supplied into the dehydrogenation catalyst layer 30 is heated by the heat transferred to the outer wall or the inside of the heat source plate 40. The heat generated by the heat source moves to the outer wall or inside of the heat source plates 40 so that the material gas contacting the outer surface of the heat source plate 40 is heated by the heat existing on the outer wall or inside of the heat source plates, It is a principle.

The heat transferred to the heat source plate 40 by the heat source connected to the upper portion of the heat source plate 40 serves to supplement the heat energy as much as the heat lost by the reaction gas due to the dehydrogen endothermic reaction, Increase the temperature to such an extent that an optimized endothermic reaction can occur. Such a heat exchange method using the heat source plate 40 is different from the direct heat exchange method using a heating furnace installed in a conventional reactor, in which the raw material gas is heated by heat transferred by a heat source, to be. That is, heat generated by the heat source connected to the upper portion of the heat source plate 40 is transmitted to the outer wall of the heat source plate 40, and heat transferred to the outer wall of the heat source plate 40 is supplied into the dehydrogenation catalyst layer 30 It is possible to further supplement the heat lost by the dehydrogenation endothermic reaction by heating the reaction gas around the heat source plate 40. By such an indirect heat exchange method, the preheating temperature by the heating furnace can be remarkably lowered, and further heat is continuously supplied into the reactor from the heat source plate 40 by the reduced preheating temperature to supplement the heat of reaction. And the heat lost by the endothermic reaction occurring in the dehydrogenation catalyst layer 30 is further self-supplied in the reactor 100.

As a heat source usable in the present invention, an electric heater, a hot gas, a flame, a metal oxide catalyst-based heat generating material (HMG), or steam may be used, It is not. For example, high-pressure gas or steam may be generated outside the reactor and connected to the upper portion of the heat source plate 40 through a heat insulating pipe. Also, the electric heater can be used by inserting an electric heater into the reactor and connecting the electric supply line of the electric heater to the outside through the explosion-proof pipe.

The reactor 100 according to the present invention is characterized in that the optimum reaction temperature at which the maximum yield can be achieved by controlling the preheating temperature by the heating furnace and the internal heating temperature condition by the heat source plate 40 is maintained in the entire process . The temperature of the outer wall of the heat source plate 40 for this purpose is 550 to 850 ° C. If the temperature is less than 550 ° C, it is difficult to compensate for the heat energy lost by the endothermic endothermic reaction. If the temperature exceeds 850 ° C, there is a problem that the power loss of the heat source is large and the probability of the hydrocarbon pyrolysis reaction as a side reaction increases.

3 is a partial cross-sectional view of a reactor illustrating a heat source plate disposed within a screen of a reactor according to one embodiment of the present invention. 3, the heat source plate 40 is disposed radially spaced apart from each other by a predetermined distance in an annular reaction space defined by an inner screen 31 and an outer screen 32, And the other end is in contact with the outer screen 32.

Referring to FIG. 3, the heat source plates 40 are spaced apart from each other, and the width w of the heat source plate 40 is preferably 0.2 to 10 cm. When the width (w) is less than 0.2 cm, the thermal energy lost by the endothermic endothermic reaction is difficult to compensate. If the width w exceeds 10 cm, the catalytic reaction does not proceed smoothly, and the power loss of the heat source is large and the hydrocarbon thermal decomposition reaction There is a problem that probability increases.

It is preferable that the number of the heat source plates 40 mounted inside the screen is 5-20. In the case where the number of units to be mounted is less than 5, the thermal energy lost by the endothermic endothermic reaction is difficult to compensate. If the number exceeds 20, the catalytic reaction does not proceed smoothly, and the power loss of the heat source is large and the hydrocarbon pyrolysis reaction There is an increasing problem.

The heat source plate 40 may be formed of steel that does not deform at high temperatures of 800 DEG C or higher or other materials that are commonly accepted in the field to which the present invention belongs.

For example, in FIG. 3, a plurality of heat source plates 40 are radially disposed between the inner screen 31 and the outer screen 32, and one Although the cross-section of the heat source plate 40 is shown as being rectangular, the cross-section may be triangular, rhombic, trapezoidal, or curved-rectangle. Further, it may be hollow in which the inside is empty. It may also be configured such that the cross-sectional area increases from one side in contact with the inner screen 31 on the transverse section of the reactor 100 to the other end in contact with the outer screen 32. It is also possible that the cross-sectional area of the upper part of the reactor is smaller than the cross-sectional area of the lower part, and the cross-sectional area increases from the upper part to the lower part.

Figure 4 is a partial cross-sectional view of a reactor illustrating a heat source plate disposed within a screen of a reactor according to another embodiment of the present invention.

4, a plurality of heat source plates 40 radially disposed between the inner screen 31 and the outer screen 32 are arranged diagonally on each of the transverse sections of the reactor 100, and one heat source plate 40 are trapezoidal in cross-section.

Figure 5 is a partial cross-sectional view of a reactor illustrating a heat source plate disposed within a screen of a reactor according to another embodiment of the present invention.

5, a plurality of heat source plates 40 radially disposed between an inner screen 31 and an outer screen 32 are arranged diagonally on a transverse plane of the reactor 100, and one heat source plate 40 Is a cross-sectional shape in which a plurality of rhombs are connected to one another, and one vertex of each rhombus is connected to each other. In this arrangement, the cross-sectional area is wider than other arrangements, and the heat efficiency is good and the heater is subdivided so that precise temperature control is possible. In the embodiment according to FIG. 5, the total number of individual plates having a rhombic cross section constituting the heat source plate 40 may be from 15 to 100.

As described above, the heat source plate 40 according to the present invention is installed inside the screen and supplies additional heat to the reaction gas through indirect heat exchange inside the catalyst filling layer. As a result, heat lost by the endothermic reaction in which the dehydrogenation reaction proceeds in the dehydrogenation catalyst layer 30 is dehydrogenated can be further supplemented, thereby inducing the reactor 100 to be an isothermal reaction as a whole. Further, since heat is further supplied to the inside of the reactor from the heat source plate 40 to supplement the heat of reaction, the preheating temperature by the heating furnace can be significantly lowered and heat cracking by the reactor using the conventional heating furnace Side reaction does not occur and the dehydrogenation performance can be increased. The dehydrogenation reaction can proceed under the condition that the heat loss is minimized in the source gas introduced into the reactor 100, and the entire source gas can uniformly absorb the energy.

According to an embodiment of the present invention, the raw material gas is injected into the inlet 11 and passes through the dehydrogenation catalyst layer 30 surrounded by the screen, and the product is mixed at the center of the dehydrogenation catalyst layer 30 And finally discharged to the discharge port 12. At this time, the raw material gas such as propane and hydrogen introduced into the annular dehydrogenation reactor 100 through the raw material gas inlet 11 flows into the dehydrogenation catalyst layer 30 through the outer screen 32, The reaction proceeds.

The dehydrogenation catalyst layer 30 is formed with a catalyst distribution pipe 33 at an upper portion thereof and a catalyst exhaust pipe 34 at a lower portion thereof to fill the dehydrogenation catalyst particles 35 with a moving bed. The dehydrogenation catalyst layer 30 has an annular reaction zone defined by the inner screen 31 and the outer screen 32. The inner and outer screens 31 and 32 formed on the inner and outer sides of the catalyst bed are large enough to allow the reaction gas stream to pass through without any flow resistance or a large pressure drop so that the accommodated dehydrogen catalyst particles 35 can not pass therethrough, And a screen or porous body having a mesh size small enough to be placed. Many such screens or porous bodies are known and, alternatively, the screens may comprise punch plates, perforated plates or perforated pipes. The size of the pores should be such that the flow of the reaction gas is facilitated through the screen, while the passage of the catalyst particles is inhibited. The holes of the perforated plate are formed in the form of a circle, a square, a rectangle, a triangle, a narrow horizontal or vertical slot, and the like. The screens used in the present invention are not limited to cylindrical screens. The screens may comprise a group of planar plates interconnected to form a catalyst particle retaining structure, such as a cylinder.

In addition, the present invention is not limited to the case where the direction of movement of the raw material gas flows from the upper portion of the reactor 100 to the lower portion thereof. It is also possible that the raw material gas flows into an inlet (not shown) disposed at one side of the lower portion of the reactor 100 and is subjected to a series of dehydrogenation reactions and then discharged to an outlet (not shown) disposed at one side of the upper portion of the reactor 100 .

The reaction gas introduced into the reactor 100 flows through the screen and the catalyst bed and reacts with the dehydrogenation catalyst to produce the product fluid, and also usually the gas. The reactor uses a screen through which the gas flows to keep the catalyst inside. The dehydrogenation catalyst particles 35 supplied from the upper catalyst distribution pipe 33 are moved downward by gravity and taken out continuously from the catalyst exhaust pipe 34. The catalyst thus taken out is sent to a regenerator (not shown). On the other hand, in the inner space of the inner screen 31 of the dehydrogenation catalyst layer 30, a reactive gas trapping region 50 is formed. The dehydrogenation reaction gas passing through the dehydrogenation catalyst layer 30 is collected in the reactive gas trapping region 50 and is sent downstream through the reactive gas outlet 12 for further processing.

As described above, in the self-heat supplying dehydrogenation reactor according to the present invention, the heat source plate is applied to the inside of the screen where the dehydrogenation reaction proceeds, so that there is no locally heated point and the temperature gradient between the inside / It is possible to reduce the production cost by reducing the reaction temperature reduction problem and the heat insulation treatment cost due to the heat loss of the piping and the reactor .

While the invention has been described in connection with various specific embodiments, it is to be understood that various modifications thereof will become apparent to those of ordinary skill in the art upon reading the specification. Accordingly, the invention as described herein is intended to embrace such modifications as fall within the scope of the appended claims.

10: housing 11: source gas inlet
12: reaction gas outlet 20: catalyst inlet
30: dehydrogenation catalyst layer 31: inner screen
32: outer screen 33: catalyst distribution pipe
34: catalyst exhaust pipe 35: dehydrogen catalyst particle
40: heat source plate 50: reaction gas collecting region

Claims (14)

A catalyst inlet formed at an upper end of the housing and adapted to store and input a catalyst therein, a reactor inlet formed at an upper side of the housing to supply a fluid reactant into the reactor, A dehydrogenation catalyst layer disposed coaxially with the housing, the dehydrogenation catalyst layer being defined by an inner screen and an outer screen, the dehydrogenation catalyst being filled with a dehydrogenation catalyst to cause a dehydrogenation reaction of the source gas, and a dehydrogenation catalyst layer formed at a lower end of the reactor, And a reaction gas outlet for recovering the reaction gas, wherein the dehydrogenation reactor comprises:
At least a portion of which is in contact with the inner screen and at least a portion of the other end is in contact with the outer screen to heat the source gas supplied into the dehydrogenation catalyst layer, A self heat provision dehydrogenation reactor comprising heat source plates.
The heat exchanger according to claim 1, wherein a heat source is connected to the upper part of the heat source plates, heat generated from the heat source moves to the outer wall or inside of the heat source plates, Characterized in that the supplied raw material gas is heated.
2. The dehydrogenation dehydrogenation system according to claim 1, wherein the heat source is an electric heater, a hot gas, a flame, a metal oxide catalyst-based heat generating material (HMG) Reactor.
The reactor according to claim 1, wherein the temperature of the outer wall of the heat source plate is 550 to 850 ° C.
The reactor according to claim 1, wherein the width w of the heat source plate is 0.2 to 10 cm.
The reactor according to claim 1, wherein the number of the heat source plates mounted in the screen is 5-20.
2. The dehydrogenation catalyst according to claim 1, wherein the dehydrogenation catalyst layer is formed with a catalyst distribution pipe at an upper portion thereof and a catalyst exhaust pipe at a lower portion thereof, and a dehydrogenation catalyst supplied from the upper portion is moved downward by gravity and discharged, , And a reactive gas trapping region is formed in the inner space of the inner screen.
The dehydrogenation reactor as claimed in claim 1, wherein the heat source plate is made of steel.
The reactor as claimed in claim 1, wherein the cross-section of one of the heat source plates is a rectangle, a triangle, a rhombus, a trapezoid, or a curved-rectangle.
The reactor as claimed in claim 1, wherein the heat source plate has a hollow shape with an empty interior.
The reactor as claimed in claim 1, wherein the heat source plate increases in cross-sectional area on the cross-section of the reactor from one side in contact with the inner screen to the other end in contact with the outer screen.
The reactor as claimed in claim 1, wherein the heat source plates increase in cross-sectional area from the upper portion to the lower portion of the reactor.
2. The dehumidifying apparatus according to claim 1, wherein the heat source plates are disposed radially between the inner screen and the outer screen, respectively, and are arranged diagonally on the transverse section of the reactor, and one heat source plate has a trapezoidal cross- .
[2] The apparatus of claim 1, wherein the heat source plates are disposed radially between the inner screen and the outer screen, respectively, and are arranged diagonally on the transverse section of the reactor, wherein the cross section of one heat source plate has a plurality of rhombs interconnected Wherein one of the vertexes of the rhomboid is connected to each other.

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