Treatment system for phenol-containing wastewater in coal chemical industry

Abstract

The utility model relates to the technical field of phenol wastewater treatment, in particular to a coal chemical phenol-containing wastewater treatment system which is characterized in that an inner pipe is arranged in an outer pipe, a preset distance is formed between the inner wall of the outer pipe and the outer wall of the inner pipe to form a water channel, one end of the inner pipe is a sealed end, and a water containing space is formed between the sealed end and the corresponding end of the outer pipe; the water containing space is communicated with the water channel, the other end of the inner pipe is connected with the end, away from the sealed end, of the outer pipe in a sealed mode, and the inner pipe is a ZnO (at) PDA / POMs nanocomposite inner pipe. One of the other end of the inner pipe and the water containing space is connected with an outlet of the phenol wastewater storage tank, and the other end of the inner pipe and the water containing space is connected with an inlet of the ground tank. Through the arrangement of the ZnO (at) PDA / POMs nanocomposite inner pipe, the catalytic degradation rate of the phenol wastewater is increased, meanwhile, catalytic degradation is conducted on the phenol wastewater, harm to the human body is reduced, and the service life of the phenol wastewater is prolonged. And the pollution to the atmospheric environment is reduced.

Description

Technical Field

[0001] This application relates to the field of phenol wastewater treatment technology, and in particular to a treatment system for phenol-containing wastewater from coal chemical industry. Background Technology

[0002] Phenolic wastewater is one of the most harmful and widespread industrial wastewaters in the world today. Phenol has the hazard of being teratogenic, carcinogenic, and mutagenic, posing a great threat to aquatic life and humans.

[0003] On the one hand, traditional methods for removing phenol from wastewater include chemical oxidation, electrolysis, adsorption, and liquid membrane methods. However, these methods suffer from drawbacks such as high investment costs, limited resources, high energy consumption, and a tendency to generate secondary pollution. Furthermore, they fail to meet the landscape requirements of river restoration projects involving phenol-containing industrial wastewater and are ill-suited to the trends of modern urban development. On the other hand, phenolic compounds are protozoan poisons, toxic to living organisms. They can be inhaled through skin and mucous membranes or ingested through the mouth. Upon contact with proteins in the protoplasm of cells, they form insoluble proteins, rendering the cells inactive. They have a particularly strong affinity for the nervous system, causing neurological disorders.

[0004] To address the problems of low degradation rate, complex treatment, and serious harm to human health and the environment in the degradation of phenol wastewater from coal chemical industry, we propose a more practical system for degrading phenol wastewater from coal chemical industry. Summary of the Invention

[0005] Based on this, this application provides a treatment system for phenol-containing wastewater from coal chemical industry, in order to solve the technical problems of low degradation rate, complex treatment, and serious harm to human health and environment of phenol wastewater in coal chemical industry in the prior art.

[0006] The technical solution to the above-mentioned technical problems in this application is as follows:

[0007] A treatment system for phenol-containing wastewater from coal chemical industry includes a phenol wastewater storage tank, a catalytic reaction unit, and an underground tank, wherein:

[0008] The catalytic reaction device includes an outer tube and an inner tube. The inner tube is disposed inside the outer tube, and there is a preset distance between the inner wall of the outer tube and the outer wall of the inner tube to form a water channel. One end of the inner tube is a sealed end, and there is a water-containing space between the sealed end and the corresponding end of the outer tube. The water-containing space is connected to the water channel. The other end of the inner tube is sealed to the end of the outer tube away from the sealed end. The inner tube is a ZnO@PDA / POMs nanocomposite material inner tube.

[0009] The other end of the inner tube and the water-containing space are connected, one to the outlet of the phenol wastewater storage tank and the other to the inlet of the ground tank.

[0010] Preferably, in the above-mentioned coal chemical phenol-containing wastewater treatment system, a first pressure gauge is installed at the outlet of the phenol wastewater storage tank, and a second pressure gauge is installed at the inlet of the ground tank.

[0011] Preferably, the above-mentioned coal chemical phenol-containing wastewater treatment system includes at least two sets of the catalytic reaction devices, and the at least two sets of the catalytic reaction devices are arranged in parallel.

[0012] Preferably, in the above-mentioned coal chemical phenol-containing wastewater treatment system, each group of catalytic reaction devices includes at least two catalytic reaction devices, and at least two catalytic reaction devices are arranged in series.

[0013] Preferably, in the above-mentioned coal chemical phenol-containing wastewater treatment system, the phenol wastewater storage tank is equipped with a liquid level sensor.

[0014] Preferably, in the above-mentioned coal chemical phenol-containing wastewater treatment system, a metering pump is installed at the outlet of the phenol wastewater storage tank.

[0015] Preferably, the above-mentioned coal chemical phenol-containing wastewater treatment system further includes a gas-liquid separator and a waste gas treatment buffer tank. The other end of the inner pipe and the water-containing space, one of which is connected to the inlet of the ground tank, is connected to the inlet of the gas-liquid separator. The gas outlet of the gas-liquid separator is connected to the waste gas treatment buffer tank, and the liquid outlet of the gas-liquid separator is connected to the inlet of the ground tank.

[0016] Preferably, in the above-mentioned coal chemical phenol-containing wastewater treatment system, the gas-liquid separator includes a tank and an agitator. The agitator is disposed in the tank, and the agitating part of the agitator is disposed in the tank. The tank is divided into an upper side and a lower side according to the liquid level in the tank. The inlet and gas outlet of the gas-liquid separator are disposed on the upper side of the tank and are arranged opposite to each other. The liquid outlet of the gas-liquid separator is disposed on the lower side of the tank.

[0017] Preferably, in the above-mentioned coal chemical phenol-containing wastewater treatment system, the bottom of the tank is inverted cone shape and is provided with a drain outlet.

[0018] Preferably, in the above-mentioned coal chemical phenol-containing wastewater treatment system, the inner pipe is detachably installed inside the outer pipe.

[0019] Compared with the prior art, this application has at least the following advantages:

[0020] This application discloses a treatment system for phenol-containing wastewater from coal chemical industry. The system comprises an outer pipe and an inner pipe, with the inner pipe located inside the outer pipe. One end of the inner pipe is sealed, and a water-containing space exists between the sealed end and the corresponding end of the outer pipe. Taking a specific embodiment as an example, phenol wastewater is transported to the inner pipe through the outlet of the phenol wastewater storage tank. The phenol wastewater entering the inner pipe (which has pores) flows into a water channel. During this process, the phenol wastewater undergoes catalytic degradation due to the inner pipe being a ZnO@PDA / POMs nanocomposite material, thus exhibiting its catalytic oxidation properties. After catalytic degradation, the wastewater flows into the water-containing space through the water channel and then into the ground tank. This application, through the ZnO@PDA / POMs nanocomposite material inner pipe, not only improves the catalytic degradation rate of phenol wastewater but also reduces harm to human health and pollution to the atmospheric environment. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structure of a treatment system for phenol-containing wastewater from coal chemical industry (first specific embodiment);

[0022] Figure 2 This is a schematic diagram of a treatment system for phenol-containing wastewater from coal chemical industry (a second specific embodiment);

[0023] Figure 3 A schematic diagram of two catalytic reaction units connected in parallel.

[0024] Figure 4 A schematic diagram of a structure in which two catalytic reaction devices are connected in series;

[0025] Figure 5 This is a schematic diagram of the inner tube structure;

[0026] Figure 6 This is a structural diagram of the outer tube and the annular sealing plate;

[0027] Figure 7 This is a schematic diagram showing the connection between the outer and inner pipes.

[0028] In the diagram: phenol wastewater storage tank 100, liquid level sensor 110, metering pump 120, catalytic reaction device 210, outer pipe 220, inner pipe 230, sealing end 240, water holding space 250, sealant 260, first pressure gauge 270, second pressure gauge 280, water channel 290, gas-liquid separator 300, tank body 310, sewage outlet 320, waste gas treatment buffer tank 330, ground tank 340, and annular sealing plate 350. Detailed Implementation

[0029] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The technical solutions of this application will be further described below with reference to the accompanying drawings of the embodiments, and this application is not limited to the following specific implementation methods.

[0030] It should be understood that the same or similar reference numerals in the accompanying drawings of the embodiments correspond to the same or similar components. In the description of this application, it should be understood that if terms such as "upper," "lower," "front," "rear," "left," "right," "top," and "bottom" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, they are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or component referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms describing positional relationships in the accompanying drawings are only for illustrative purposes and should not be construed as limiting this patent. Those skilled in the art can understand the specific meaning of the above terms according to the specific circumstances.

[0031] Please refer to Figures 1 to 7 A treatment system for phenol-containing wastewater from coal chemical industry includes a phenol wastewater storage tank 100, a catalytic reaction device 210, and an underground tank 340, wherein:

[0032] The catalytic reaction device 210 includes an outer tube 220 and an inner tube 230. The inner tube 230 is disposed inside the outer tube 220, and there is a preset distance between the inner wall of the outer tube 220 and the outer wall of the inner tube 230. The preset distance can be 10 cm or 15 cm, and this application does not limit it to form a water channel 290. One end of the inner tube 230 is a sealed end 240. There is a water-containing space 250 between the sealed end 240 and the corresponding end of the outer tube 220. The water-containing space 250 is connected to the water channel 290. The other end of the inner tube 230 is sealed and connected to the end of the outer tube 220 away from the sealed end 240. The inner tube is a ZnO@PDA / POMs nanocomposite material inner tube.

[0033] Specifically, the outer tube 220 in this application can be a 304 stainless steel outer tube. The inner tube 230 is a ZnO@PDA / POMs nanocomposite material inner tube, that is, the inner tube 230 is made of ZnO@PDA / POMs nanocomposite material. Specifically, ZnO@PDA / POMs nanocomposite material is mixed with porous fly ash to obtain a supported catalyst. The supported catalyst is sprayed onto the outer surface of a stainless steel tubular reactor. After drying, calcining, and cooling the stainless steel tubular reactor, the ZnO@PDA / POMs nanocomposite material inner tube 230 is obtained. For details, please refer to application number 202. Chinese Invention Patent No. 410418755.7, "A Porous Layer High-Throughput Tubular Reactor, Its Preparation Method and Application", describes the inner tube 230 in this application as the porous layer high-throughput tubular reactor described in Chinese Invention Patent No. 202410418755.7. Due to the pore-forming agent added during preparation, the resulting porous layer high-throughput tubular reactor has pores. Of course, micropores can also be processed on the inner tube 230 through post-processing to make it porous.

[0034] The other end of the inner pipe 230 and the water-containing space 250 are connected, respectively, to the outlet of the phenol wastewater storage tank 100 and the inlet of the ground tank 340. In a first specific embodiment, the outlet of the phenol wastewater storage tank 100 is connected to the other end of the inner pipe 230, and the end of the outer pipe 220 corresponding to the sealing end 240 is connected to the inlet of the ground tank 340, that is, the inlet of the ground tank 340 is connected to the water-containing space 250.

[0035] Specifically, phenol wastewater is transported to the inner pipe 230 through the outlet of the phenol wastewater storage tank 100. The phenol wastewater entering the inner pipe 230 (which has pores) will enter the water channel 290. During the process of the phenol wastewater entering the water channel 290 through the inner pipe 230, the phenol wastewater will be catalytically degraded. After catalytic degradation, the phenol wastewater will enter the water holding space 250 through the water channel 290, and then enter the ground tank 340 through the water holding space 250. In other words, the phenol wastewater first enters the inner pipe 230, then enters the water channel 290 through the inner pipe 230, and finally enters the ground tank 340 through the water holding space 250.

[0036] In the second specific implementation, the outlet of the phenol wastewater storage tank 100 is connected to the water-containing space 250, that is, the outlet of the phenol wastewater storage tank 100 is connected to one end of the outer pipe 220 corresponding to the sealing end 240, and the other end of the inner pipe 230 is connected to the inlet of the ground tank 340.

[0037] Specifically, this application can also degrade phenol wastewater through a second specific implementation method. Phenol wastewater is transported to the water-containing space 250 through the outlet of the phenol wastewater storage tank 100, and then enters the water channel 290. The phenol wastewater entering the water channel 290 will enter the inner pipe 230 (which has pores) through the inner pipe 230. During the process of the phenol wastewater entering the inner pipe 230, it will be catalytically degraded. Finally, it will be transported to the ground tank 340 through the other end of the inner pipe 230. That is to say, the phenol wastewater first enters the water channel 290, then enters the inner pipe 230 through the inner pipe 230, and finally enters the ground tank 340 through the inner pipe 230.

[0038] This application sets up an outer pipe 220 and an inner pipe 230. The inner pipe 230 is set inside the outer pipe 220. One end of the inner pipe 230 is a sealed end 240. There is a water-containing space 250 between the sealed end 240 and the corresponding end of the outer pipe 220. Taking the first specific embodiment as an example, phenol wastewater is transported to the inner pipe 230 through the outlet of the phenol wastewater storage tank 100. The phenol wastewater entering the inner pipe 230 will enter the water channel 290 through the inner pipe 230 (which has pores). During the process of the phenol wastewater entering the water channel 290 through the inner pipe 230, since the inner pipe 230 is a ZnO@PDA / POMs nanocomposite material inner pipe, the phenol wastewater will be catalytically degraded and exert its catalytic oxidation property. After catalytic degradation, the phenol wastewater enters the water-containing space 250 through the water channel 290, and then enters the ground tank 340 through the water-containing space 250. This application utilizes an inner tube 230, and the ZnO@PDA / POMs nanocomposite material inner tube has a high degradation rate for phenol wastewater, thus improving the catalytic degradation rate of phenol wastewater. At the same time, the catalytic degradation of phenol wastewater reduces harm to the human body and also reduces pollution to the atmospheric environment. Furthermore, the coal chemical phenol wastewater treatment system disclosed in this application has a simple structure, making phenol wastewater treatment simple.

[0039] In this application, for the sake of brevity, the specific connection method (hereinafter referred to as the connection method in the first specific embodiment is used as an example, and the same applies to the second specific embodiment.

[0040] In a preferred embodiment, a first pressure gauge 270 is provided at the outlet of the phenol wastewater storage tank 100, and a second pressure gauge 280 is provided at the inlet of the ground tank 340. That is, the first pressure gauge 270 is provided between the outlet of the phenol wastewater storage tank 100 and the inner pipe 230, and the second pressure gauge 280 is provided between the outer pipe 220 and the ground tank 340.

[0041] Specifically, phenol wastewater is transported to the inner pipe 230 through the outlet of the phenol wastewater storage tank 100. Because the inner pipe 230 has pores, a certain pressure difference exists when the phenol wastewater passes through the pores between the inner pipe 230 and the outer pipe 220 (water channel 290), preventing complete passage. Therefore, under normal circumstances, a certain pressure difference is required, specifically manifested in a pressure difference between the first pressure gauge 270 and the second pressure gauge 280. When the pressure difference between the first pressure gauge 270 and the second pressure gauge 280 increases (the pressure difference between the first pressure gauge 270 increases and the pressure difference between the second pressure gauge 280 decreases), it indicates that the inner tube 230 is blocked, and the system needs to be stopped for inspection. When the pressure difference between the first pressure gauge 270 and the second pressure gauge 280 is close, it indicates that the inner tube 230 is damaged or the catalyst in the inner tube 230 is depleted, and a new inner tube 230 needs to be replaced. The cause of the fault can be determined by observing the pressure difference between the first pressure gauge 270 and the second pressure gauge 280 to facilitate maintenance.

[0042] In a preferred embodiment, the device includes at least two sets of the catalytic reaction apparatus 210, which are arranged in parallel. Each set of the catalytic reaction apparatus 210 includes at least two catalytic reaction apparatuses 210, and the at least two catalytic reaction apparatuses 210 are arranged in series.

[0043] Specifically, two sets of catalytic reaction devices 210 are set up, one for use and one for standby. One is in normal use and the other is on standby. When the inner tube 230 in the catalytic reaction device 210 in use is exhausted, the other one is replenished in time and replaced with the standby one. Each set of catalytic reaction devices 210 includes at least two catalytic reaction devices 210, which are connected in series. After phenol wastewater is transported to the catalytic reaction device 210 through the phenol wastewater storage tank 100, the inner tube 230 of the catalytic reaction device 210 is provided with pores. Phenol wastewater passes through the pores between the inner tube 230 and the outer tube 220, and the phenol wastewater is degraded and catalyzed during the passage. By using two catalytic reaction devices 210 connected in series, when the catalytic reaction device 210 is not effective in degrading phenol wastewater, the other catalytic reaction device 210 can be used to degrade the phenol wastewater again, thereby increasing the catalytic degradation rate of phenol wastewater. At the same time, the catalytic degradation of phenol wastewater reduces harm to the human body and reduces pollution to the atmospheric environment.

[0044] In a preferred embodiment, a liquid level sensor 110 is provided on the phenol wastewater storage tank 100.

[0045] Specifically, a liquid level sensor 110 is installed on the phenol wastewater storage tank 100 to detect the liquid level of phenol wastewater in the phenol wastewater storage tank 100. When the phenol wastewater is detected to be too low, it is replenished to the phenol wastewater storage tank 100 in a timely manner. When the phenol wastewater is detected to be too high, the addition of phenol wastewater to the phenol wastewater storage tank 100 is stopped.

[0046] In a preferred embodiment, a metering pump 120 is connected to the inlet of the phenol wastewater storage tank 100. Specifically, by setting the metering pump 120, phenol wastewater is delivered from the outlet of the phenol wastewater storage tank 100 and pumped into the inner pipe 230 to provide pressure.

[0047] In a preferred embodiment, the coal chemical phenol-containing wastewater treatment system disclosed in this application further includes a gas-liquid separator 300 and a waste gas treatment buffer tank 330. The other end of the inner pipe 230 and the water-containing space 250, connected to the inlet of the ground tank 340, are connected to the inlet of the gas-liquid separator 300. The gas outlet of the gas-liquid separator 300 is connected to the waste gas treatment buffer tank 330, and the liquid outlet of the gas-liquid separator 300 is connected to the inlet of the ground tank 340. Taking a first specific embodiment as an example, the end of the outer pipe 220 corresponding to the sealing end 240 is connected to the inlet of the gas-liquid separator 300, the gas outlet of the gas-liquid separator 300 is connected to the waste gas treatment buffer tank 330, and the liquid outlet of the gas-liquid separator 300 is connected to the ground tank 340.

[0048] Specifically, the water-containing space 250 is connected to the inlet of the gas-liquid separator 300, the gas outlet of the gas-liquid separator 300 is connected to the waste gas treatment buffer tank 330, and the liquid outlet of the gas-liquid separator 300 is connected to the inlet of the ground tank 340. When phenol wastewater permeates through the pores between the inner pipe 230 and the outer pipe 220, it undergoes catalytic degradation, producing a gas-liquid mixture. Since this mixture is inconvenient to handle within the water-containing space 250, it is transported through the water-containing space 250 to the gas-liquid separator 300. The gas-liquid separator 300 separates the mixture, and the separated waste liquid is transported through the liquid outlet of the gas-liquid separator 300 to the ground tank 340 for storage. The separated waste gas is transported through the gas outlet of the gas-liquid separator 300 to the waste gas treatment buffer tank 330.

[0049] In a preferred embodiment, the gas-liquid separator 300 includes a tank 310 and an agitator. The agitator is disposed in the tank 310, and the agitating part of the agitator is disposed inside the tank 310. The tank 310 is divided into an upper side and a lower side according to the liquid level inside the tank 310. The inlet and gas outlet of the gas-liquid separator 300 are disposed on the upper side of the tank 310 and are arranged opposite to each other. The liquid outlet of the gas-liquid separator 300 is disposed on the lower side of the tank 310.

[0050] Specifically, the agitation device includes a propeller and a motor. The motor is located on the outer side of the tank 310 and connected to the propeller. The propeller is located inside the tank 310. After the gas-liquid separator 300 separates the gas-liquid mixture, since the waste liquid still contains a small amount of waste gas, the agitation device stirs the gas-liquid mixture in the gas-liquid separator 300, causing the waste gas to overflow. The waste gas is then transported to the waste gas treatment buffer tank 330 through the gas outlet.

[0051] In a preferred embodiment, the bottom of the tank 310 is inverted conical, and a drain outlet 320 is provided at the bottom of the tank 310. Specifically, some impurities still exist in the treated phenol wastewater, which will gradually settle at the bottom of the tank 310 and be discharged through the drain outlet 320.

[0052] In a preferred embodiment, the inner tube 230 is detachably disposed within the outer tube 220. Specifically, the inner tube 230 is disposed within the outer tube 220, which includes a tube body and an annular sealing plate 350 disposed at one end of the tube body. The inner tube 230 can be inserted into the tube body, and the annular sealing plate 350 is sealed to the end of the inner tube 230 with sealant 260 (and also sealed to the end of the outer tube 220 with sealant 260). When the inner tube 230 is worn out and needs to be replaced, the sealant 260 is first removed, then the worn-out inner tube 230 is pulled out from the outer tube 220, a new inner tube 230 is placed back into the tube body, and a layer of sealant 260 is applied to the tube wall to seal the outer tube 220 and the inner tube 230. By making the inner tube 230 detachable, the cost of the equipment is saved.

[0053] Obviously, the above embodiments of this application are merely examples for clear illustration and are not intended to limit the implementation of this application. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all possible implementations here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A treatment system for phenol-containing wastewater from coal chemical industry, characterized in that: Includes phenol wastewater storage tanks, catalytic reaction units, and underground tanks, among which: The catalytic reaction device includes an outer tube and an inner tube. The inner tube is disposed inside the outer tube, and there is a preset distance between the inner wall of the outer tube and the outer wall of the inner tube to form a water channel. One end of the inner tube is a sealed end, and there is a water-containing space between the sealed end and the corresponding end of the outer tube. The water-containing space is connected to the water channel. The other end of the inner tube is sealed to the end of the outer tube away from the sealed end. The inner tube is a ZnO@PDA / POMs nanocomposite material inner tube. The other end of the inner tube and the water-containing space are connected, one to the outlet of the phenol wastewater storage tank and the other to the inlet of the ground tank.

2. The treatment system for phenol-containing wastewater from coal chemical industry as described in claim 1, characterized in that: The outlet of the phenol wastewater storage tank is equipped with a first pressure gauge, and the inlet of the ground tank is equipped with a second pressure gauge.

3. The treatment system for phenol-containing wastewater from coal chemical industry as described in claim 1, characterized in that: It includes at least two sets of the catalytic reaction devices, and the at least two sets of the catalytic reaction devices are arranged in parallel.

4. The treatment system for phenol-containing wastewater from coal chemical industry as described in claim 3, characterized in that: Each group of catalytic reaction devices includes at least two catalytic reaction devices, and the at least two catalytic reaction devices are arranged in series.

5. The treatment system for phenol-containing wastewater from coal chemical industry as described in claim 1, characterized in that: The phenol wastewater storage tank is equipped with a liquid level sensor.

6. The treatment system for phenol-containing wastewater from coal chemical industry as described in claim 1, characterized in that: A metering pump is installed at the outlet of the phenol wastewater storage tank.

7. The treatment system for phenol-containing wastewater from coal chemical industry as described in claim 1, characterized in that: It also includes a gas-liquid separator and a waste gas treatment buffer tank. The other end of the inner tube and the water-containing space, one of which is connected to the inlet of the ground tank, is connected to the inlet of the gas-liquid separator. The gas outlet of the gas-liquid separator is connected to the waste gas treatment buffer tank, and the liquid outlet of the gas-liquid separator is connected to the inlet of the ground tank.

8. The treatment system for phenol-containing wastewater from coal chemical industry as described in claim 7, characterized in that: The gas-liquid separator includes a tank and an agitator. The agitator is disposed in the tank, and the agitating part of the agitator is disposed in the tank. The tank is divided into an upper side and a lower side according to the liquid level in the tank. The inlet and gas outlet of the gas-liquid separator are disposed on the upper side of the tank and are arranged opposite to each other. The liquid outlet of the gas-liquid separator is disposed on the lower side of the tank.

9. The treatment system for phenol-containing wastewater from coal chemical industry as described in claim 8, characterized in that: The bottom of the tank is inverted cone shape and is equipped with a drain outlet.

10. The treatment system for phenol-containing wastewater from coal chemical industry as described in claim 1, characterized in that: The inner tube is detachably installed inside the outer tube.

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