JP2003012856A - Liquid-surface detecting method for pyrolysis vessel and method for pyrolysis - Google Patents

Abstract

PROBLEM TO BE SOLVED: To detect a liquid surface of a melt-plastic in a pyrolysis vessel for pyrolizing a plastic, and to prevent a danger by an abnormal arising of the liquid surface. SOLUTION: A pressure in a detecting pipe 4 and a pressure in a pyrolysis vessel 2 are detected by pressure detectors 5 and 6, respectively, wherein the detector 4 having a closed upper end and a opened lower end is extended from an upper part to lower inside of the pyrolysis vessel 2. When the liquid surface of the melt-plastic arises abnormally and gets in the lower end of the detector 4, the elevation of the liquid surface is detected from a pressure difference by the rise of its inner pressure.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detecting the liquid level of molten plastic in a thermal decomposition vessel for thermally decomposing plastic, and a thermal decomposition method for recovering an abnormal rise in the liquid level.

[0002]

2. Description of the Related Art Conventionally, a useful component which can be used as a chemical raw material or a fuel from a generated gas generated by thermally decomposing a large amount of waste plastic (hereinafter simply referred to as plastic) at high temperature in the absence of oxygen. A pyrolysis method for recovering is widely used. Styrene-based plastics such as polystyrene and olefin-based plastics such as polyethylene are suitable for recovery as chemical raw materials or fuels. For example, by thermally decomposing these plastics, they can be recovered as fuels such as heavy oil, light oil and gasoline. In addition, styrene monomer and the like can be recovered by thermal decomposition of styrene-based plastic and the like.

Pyrolysis is carried out by a pyrolysis device. Two types of general pyrolysis devices have been developed: one provided with a tank-type pyrolysis container and one provided with a tubular reactor. The tank-type thermal decomposition container is suitable when a large amount of molten plastic is held in the tank and is pyrolyzed over a certain retention time, and can be applied to either continuous operation or batch operation mode.

In the case of the batch operation system, a certain amount of plastic is supplied into the thermal decomposition vessel and the thermal decomposition reaction is repeated in a sealed state, and the thermal decomposition vessel is cooled each time the processing is completed. Is low. Therefore, the continuous operation method is desirable in order to improve the processing efficiency.

The tubular reactor has an elongated tubular shape, and while the plastic supplied from one end is heated from the surroundings while being melted and pyrolyzed while passing through the inside of the pipe, the produced gas is generated at the other end. It is designed to be discharged from the department. Since the tubular reactor has a small internal volume, the amount of molten plastic retained is small and the reaction time is short, and it is exclusively applied to the continuous operation system.

On the other hand, since the tubular reactor has a short residence time, part of the molten plastic may be discharged from the discharge side without being decomposed. Therefore, the pyrolysis vessel having a larger cross-sectional area than the reactor is discharged to the discharge side. It is desirable to add a (post-reactor). When a pyrolysis container is added, the undecomposed molten plastic can be completely pyrolyzed within the residence time to some extent.

When the plastic is supplied to the thermal decomposition container, there are a method of directly supplying the finely crushed plastic into the thermal decomposition container and a method of supplying the molten plastic to the thermal decomposition container with a supply device, and the latter method. This method is suitable for continuous operation of the pyrolysis vessel. In the case of a pyrolysis vessel added to a tubular reactor, molten plastic from the reactor is supplied together with the produced gas.

In continuous operation using a tank-type pyrolysis vessel, the liquid level of the molten plastic in the pyrolysis vessel fluctuates depending on the difference between the amount of plastic supplied, the amount of produced gas discharged and the amount of decomposition residue. To do. In addition, when operating continuously using a tubular reactor, the liquid level of the molten plastic in the pyrolysis vessel added to the discharge side of the reactor depends on the amount of plastic supplied to the reactor and the thermal decomposition. It varies depending on the difference between the amount of generated gas discharged from the container and the amount of decomposition residue.

[0009]

It is desirable that the liquid level of the molten plastic in these pyrolysis vessels is kept constant at a predetermined level. However, as described above, when the amount of plastic supplied to the thermal decomposition vessel does not match the amount of produced gas discharged and the amount of decomposition residue, the liquid level changes. In particular, if the liquid level rises abnormally, there is a risk of molten plastic overflowing from the pyrolysis container, so strict control is required. Therefore, it is desirable to constantly detect the liquid level of the molten plastic in the pyrolysis container and, when an abnormal rise is detected, limit the plastic supply to the pyrolysis container to restore the liquid level to a normal value.

Heretofore, no attempt has been made to detect the liquid level of the molten plastic in the pyrolysis container. The present inventors examined various level meters for detecting the liquid level of molten plastic. However, the detection location is high temperature and filled with explosive gas, so explosion-proof function is required, molten plastic is a highly viscous liquid, and it is necessary to solve problems such as carbon sticking such as caulking. Therefore, it was concluded that general mechanical level meters and optical level meters are difficult to use.

Therefore, an object of the present invention is to solve a conventional problem in detecting the liquid level of molten plastic, and an object thereof is to provide a liquid level detecting method by a new method therefor. Another object of the present invention is to provide a thermal decomposition method adapted to recover the abnormal rise of the liquid level in the thermal decomposition container.

[0012]

As a result of various studies, the present inventors have found that a pressure detection method or a temperature detection method is suitable, and have completed the present invention. That is, the first aspect of the present invention is a method for detecting the liquid level of molten plastic in a pyrolysis container that thermally decomposes plastic by a pressure difference detection method. In this method, the detection tube with the upper end closed and the lower end opened is extended from the upper part of the pyrolysis container to the lower part inside with the lower end first, and the difference between the pressure inside the detection pipe and the space pressure inside the pyrolysis container is increased. The liquid level of the molten plastic is detected from the above. (Claim 1)

A second aspect of the present invention is a method for detecting the liquid level of molten plastic in a thermal decomposition container for thermally decomposing plastic by a temperature difference detection method.
Then, this method is characterized by detecting the presence or absence of the liquid surface of the molten plastic at the predetermined position in the thermal decomposition container from the difference between the space temperature and the temperature of the molten plastic. (Claim 2)

In the liquid level detection method using the temperature difference detection method, the temperature detector provided at a predetermined position in the pyrolysis container or at a predetermined position on the outer surface of the side wall of the pyrolysis container is used to measure the space temperature in the pyrolysis container at that position. And the temperature difference between the molten plastics can be detected. (Claims 3 and 4)

In the liquid level detection method of either the pressure difference detection method or the temperature difference detection method, the heating temperature of the thermal decomposition vessel may be 500 ° C. or higher. (Claim 5)

In a third aspect of the present invention, the liquid level of the molten plastic in the thermal decomposition container detected by the liquid level detection method of either the pressure difference detection method or the temperature difference detection method is abnormally increased. In this case, the thermal decomposition method is characterized in that the supply of plastic into the continuously operating thermal decomposition container is reduced or stopped, and the liquid level of the molten plastic is restored to a normal range (claim 6).

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a process flow diagram of an apparatus for carrying out the molten plastic liquid level detection method for a thermal decomposition container according to the present invention. In the figure, 1 is a thermal decomposition device, 2 is a tank-type thermal decomposition container, 3 is a heating unit for heating the thermal decomposition container 2, 4 is a detection tube, 5 and 6 are pressure detectors, 7 is a control device, and 8 is A supply device for supplying molten plastic to the pyrolysis container 2, 9 is a heating drive unit, 10 is an extrusion unit, 11 is a discharge valve for discharging decomposition residues, and a to e are pipes.

The tank-type pyrolysis container 2 is made of a sealed type using a heat-resistant metal material such as stainless steel. An inlet for introducing the molten plastic from the feeder 8 and an outlet for discharging the produced gas are provided on the upper portion of the thermal decomposition vessel 2, and an outlet for the reaction residue is provided on the bottom of the conical shape.
Further, a scraper (not shown) is provided at the bottom to stir the contents and scrape the caulks generated. The processing capacity of the thermal decomposition vessel 2 is proportional to its internal volume, but with an internal volume of several hundreds of liters, 1
A processing capacity of about 00 Kg / h can be obtained.

The heating part 3 is a high temperature heating gas for the thermal decomposition container 2
Is heated from the surroundings, the heating gas generated in a combustion device (not shown) is supplied via the pipe c, and the heating gas cooled by heat exchange is discharged from the pipe d. The heating unit 3 may be an electric heating type or a steam heating type.

As the feeding device 8, the same one as an extruder generally used as a plastic injection molding machine can be used. Hopper (not shown) provided in the feeding device 8
Is supplied with finely crushed plastic, and the plastic is introduced into the heating drive unit 9, where it is stirred by the drive means and is heated by the heater at 150 ° C. to 250 ° C.
It is heated and melted at ℃. Next, the molten plastic is extruded into the pipe a from the extruding section 10 having a rotating screw,
It is supplied to the pyrolysis container 2.

In this embodiment, the liquid level of the molten plastic in the thermal decomposition container 2 is detected by the pressure difference between the two pressure detectors 5, 6. One of the pressure detectors 5 detects the internal pressure of the detection tube 4, and the other pressure detector 6 detects the space pressure above the pyrolysis container 2, which sends an electric signal according to the detected pressure to the control device 7. Output. In this embodiment, 2
Although two pressure detectors 5 and 6 are used, a differential pressure gauge can also be used.

The detection tube 4 made of a heat resistant material such as a metal tube is closed at the upper end and opened at the lower end, and extends vertically from above the pyrolysis container 2 to the inside. The lower end surface of the detection tube 4 is set slightly below the lower limit of the liquid level range to be measured (for example, the detection level as shown). Although the function of the detection tube 4 is that it has a diameter of about 10 mm or more, it is preferable that it is as large as possible from the viewpoint of preventing blockage due to carbides and the like.

Next, a method of pyrolyzing with the thermal decomposition apparatus of FIG. 1 and a method of detecting the liquid level of the molten plastic in the thermal decomposition container 2 will be described. First, the air in the thermal decomposition vessel 2 is replaced with an inert gas such as nitrogen. For this purpose, the vacuum pump (not shown) connected to the pipe b discharges the air in the thermal decomposition container 2 and supplies the inert gas to the thermal decomposition container 2 from the inert gas supply device. By supplying the inert gas for a certain period of time, the inside of the thermal decomposition container 2 becomes a state in which oxygen does not exist.

Along with the above operation, heating gas from a combustion device (not shown) is supplied to the heating section 3 through the pipe c to raise the pyrolysis container 2 to the pyrolysis temperature. For example, the thermal decomposition temperature of polystyrene is about 350 ° C to 450 ° C, and the thermal decomposition temperature of polyethylene is about 400 ° C to 500 ° C. Further, the finely crushed plastic is heated and melted by the supply device 8. These operations complete the preparation for continuous pyrolysis.

Next, the pyrolysis container 2 maintained at the pyrolysis temperature
The molten plastic is supplied from the supply device 8. The molten plastic supplied to the pyrolysis container 2 is heated there and pyrolyzed. The generated gas generated is sent from a pipe b to a condenser (not shown), where it is cooled and condensed. Condensed components are collected in the oil recovery tank, and non-condensed gas components are recovered in the gas tank. The condensed component can be further distilled by a distillation apparatus and recovered as, for example, purified styrene monomer.

When the thermal decomposition is continued, decomposition residues and caulks are gradually accumulated in the thermal decomposition container 2. The decomposition residue is deposited on the bottom of the thermal decomposition vessel 2, and the coking is scraped off by a scraper and similarly deposited on the bottom. Therefore, these deposits are periodically discharged from the pipe e by opening the discharge valve 11.

The liquid level of the molten plastic in the thermal decomposition container 2 varies depending on the difference between the amount of plastic and the amount of produced gas to be discharged, the amount of decomposition residue, etc. as described above. When the liquid level of the molten plastic is located below the lower end of the detection pipe 4, the outputs of the pressure detector 5 for detecting the internal pressure of the detection pipe 4 and the pressure detector 6 for detecting the space pressure in the thermal decomposition vessel 2 are Will be equal.

When the liquid level of the molten plastic rises above the lower surface (detection level) of the detection tube 4, the molten plastic enters the detection tube 4 closed at the upper end and compresses the gas. Then, the internal pressure of the detection tube 4 rises in proportion to the head difference from the rise of the liquid level of the thermal decomposition container 2, and the pressure detector 5 outputs an electric signal corresponding to the pressure rise to the control device 7. The pressure rise is about 0.1 kPa when the liquid level rises 10 mm from the tip of the detection tube 4.

When an excessive amount of molten plastic infiltrates into the detection tube 4, the generated gas or vapor intrudes into the detection tube 4 at any time to automatically replace and expel the molten plastic. The internal pressure of is always settled to the pressure corresponding to the head difference according to the liquid surface. On the other hand, the electric signal of the pressure detector 6 for detecting the space pressure of the thermal decomposition container 2 is output to the control device 7 as the internal pressure itself of the thermal decomposition container 2 regardless of the liquid surface of the molten plastic.

The control device 7 is composed of, for example, a microcomputer device, and performs calculations and control operations in accordance with a control program stored in advance in its storage section.
The controller 7 calculates the difference (pressure difference) between the pressure detection values from the pressure detectors 5 and 6, and when it exceeds a preset value, the liquid level of the molten plastic rises above the allowable level. Then, the supply reduction command or the operation stop command is issued to the supply device 8 to reduce or stop the supply of the molten plastic to the thermal decomposition container 2.

When the supply of the molten plastic to the thermal decomposition container 2 is reduced or stopped, the liquid level of the molten plastic in the thermal decomposition container 2 gradually decreases. When the liquid level decreases, the internal pressure of the detection container 4 also decreases accordingly. However, when the control device 7 determines that the liquid level has decreased to the normal range,
The supply device 8 is returned to the normal supply state. In this way, the control device 7 constantly monitors the abnormal state of the liquid surface of the molten plastic in the thermal decomposition container 2 and performs control to restore it to the normal state.

FIG. 2 is another process flow diagram of an apparatus for carrying out the molten plastic liquid level detection method for a thermal decomposition container according to the present invention. This embodiment is characterized in that the liquid level detection of the molten plastic in the thermal decomposition container 2 (detection of the liquid level at the detection level) is performed by a temperature detection method.
Therefore, parts other than the temperature detector 12 shown in FIG. 2 are the same as those in FIG. 1, and those parts are denoted by the same reference numerals as those in FIG.

The temperature detector 12 has a temperature detecting section such as a thermocouple or a resistance temperature detector, and a transmitting section for amplifying the temperature detecting section and transmitting it to the control device 7. The temperature detecting section is connected to the transmitting section by a conductor. Is housed at the tip of a heat-resistant protective tube 13 made of ceramic or metal. The tip portion of the protective tube 13, that is, the temperature detection position is set to the upper limit position (for example, the detection level shown in the figure) that allows the liquid surface of the molten plastic.

When the liquid level of the molten plastic rises to the detection level, the tip of the protective tube 13 located at the space position is immersed in the molten plastic. Then, the detection value of the temperature detection unit housed in the portion rapidly changes from the temperature of the space in the thermal decomposition container 2 to the temperature of the molten plastic.

As described above, the thermal decomposition temperature of general polystyrene is about 350 ° C to 450 ° C, and the thermal decomposition temperature of polyethylene is about 400 ° C to 500 ° C. According to the experiment, when polystyrene was continuously pyrolyzed using the apparatus of FIG. 2, the temperature of the molten plastic was 370 ± 20 ° C.
The temperature of the heating gas of the heating unit 3 is 500
The temperature of the space inside the thermal decomposition vessel 2 was about 450 ° C. due to the effect of radiant heat from the side wall. Therefore, the temperature difference between the space and the molten plastic is practically 60-9.
The temperature is about 0 ° C., and if there is such a large difference, the presence or absence of molten plastic at the detection level can be reliably detected.

The above-mentioned temperature change is transmitted from the transmitter to the control device 7, whereby the control device 7 reduces the supply amount of the molten plastic supplied from the supply device 8 to the thermal decomposition container 2 as in the case of FIG. Stop. As a result of the control, when the liquid level of the molten plastic recovers below the detection level, the temperature detector 12 detects it and transmits it to the control device 7, and the control device 7 transfers from the supply device 8 to the thermal decomposition vessel 2. Increase or restart supply of molten plastic.

FIG. 3 is a further process flow diagram of an apparatus for carrying out the molten plastic liquid level detection method for a thermal decomposition container according to the present invention. Similar to the example of FIG. 2, this embodiment is also characterized in that the liquid level of the molten plastic in the pyrolysis container 2 is detected (the presence or absence of the liquid level at the detection level is detected) by the temperature detection method. However, in this example, the temperature detector 1
2 is different from the example of FIG. 2 in that the temperature detection unit 2 is provided in contact with a predetermined position (detection level) on the outer surface of the side wall of the pyrolysis container 2.
Since the other parts are the same as those in FIG. 2, the same parts are designated by the same reference numerals as those in FIG. 2, and the duplicated description will be omitted.

The same temperature detector 12 as that shown in FIG. 2 is used, and the temperature detector is housed at the tip of the protective tube 13. The protection tube 13 penetrates the heating section 3 from the outside of the apparatus, and is attached so that its tip portion contacts the outer surface of the side wall of the pyrolysis container 2. Then, the portion of the protective tube 13 in contact with the heating gas of the heating unit 3 is covered with a heat insulating material so that the temperature detected by the temperature detection unit is not affected by the heating gas.

As a result, the temperature detector can detect the temperature of the space in the thermal decomposition container 2 or the temperature of the molten plastic through the side wall. For example, when the side wall is heated with a heating gas of 500 ° C., the surface temperature of the side wall is about 480 ° C. when the liquid level of the molten plastic has not reached the detection level (the temperature of the space inside the thermal decomposition vessel 2 is about 450 ° C.). When the liquid level reaches 380
Degree (The temperature of the space inside the thermal decomposition vessel 2 is about 370 ° C)
become.

By thus detecting the temperature difference on the outer surface of the side wall, it is possible to detect the presence or absence of the liquid surface of the molten plastic at the detection level, and the controller 7 receives the detection signal as in the example of FIG. The supply device 8 is controlled so as to recover the liquid surface.

Depending on the type of plastic to be treated or the purpose of treatment, the temperature of the heating gas is 600 to 800.
It may be thermally decomposed at a high temperature of about ℃. For example, when recovering styrene monomer from polystyrene, it is desirable to set the thermal decomposition temperature to about 700 ° C. When the thermal decomposition is performed at such a high temperature, the temperature difference due to the presence or absence of the liquid surface also increases in proportion thereto, so that the presence or absence of the liquid surface by the temperature detection method can be detected with higher reliability.

Further, it is also effective to measure the temperature of the heating gas or to provide temperature detectors 12 at a plurality of locations in the thermal decomposition vessel 2 and compare the results of the temperature measurements. The comparison calculation and the like are performed by a program stored in the control device 7 in advance, and optimal control can be performed. Further, when pyrolyzing at a high temperature, the reaction rate becomes high, and the residence time of the plastic in the pyrolysis container 2 becomes short, so
The internal volume of the pyrolysis container 2 is also designed to be small accordingly. In such a case, the margin of the thermal decomposition container 2 with respect to the excessive supply amount of plastic is small, so that the risk of molten plastic overflowing from the thermal decomposition container 2 increases. Therefore, the rapid transmission of the detected value by the electric signal and the automatic control of the plastic supply by the control device 7 are effective.

Further, the detected value of the plastic supply amount to the thermal decomposition container 2 (for example, the detected value of the rotating screw speed of the extrusion section 10 in the supply device 8) is fed back to the control device 7 to perform PID (proportional integral derivative) control or the like. You can also do Although the embodiment of FIGS. 1 to 3 is a case where the method of the present invention is applied to the tank-type thermal decomposition container 2, the same applies to the thermal decomposition container added to the discharge side of the tubular reactor. Can be applied to.

[0044]

As described above, according to the first aspect of the present invention, the detection tube whose upper end is closed and whose lower end is opened is extended from the upper side of the pyrolysis container to the lower inside thereof with the lower end thereof first, and the pressure inside the detection tube is increased. And the liquid level of the molten plastic is detected from the difference in space pressure in the pyrolysis container. According to the liquid level detection method based on the pressure difference detection method, the liquid level of the molten plastic can be detected reliably and highly reliably in a severe environment of high temperature only by detecting the pressure difference.

The second invention is characterized in that the presence or absence of the liquid level of the molten plastic at the position is detected from the difference between the space temperature and the temperature of the molten plastic at a predetermined position in the pyrolysis container. According to the liquid level detection method based on this temperature difference detection method, the liquid level of the molten plastic can be detected reliably and with high reliability in a severe environment of high temperature simply by detecting the temperature difference.

In the above liquid level detecting method, the temperature of the space inside the pyrolysis container and the temperature of the molten plastic at that position are detected by a temperature detector provided at a predetermined position in the pyrolysis container or at a predetermined position on the outer surface of the side wall of the pyrolysis container. Can be detected. In this way, the liquid level of the molten plastic can be detected more reliably with a simple structure.

In a third aspect of the present invention, when the liquid level of the molten plastic in the thermal decomposition container detected by any of the above liquid level detection methods rises abnormally, It is a thermal decomposition method characterized in that the supply of plastic is reduced or stopped and the liquid level of the molten plastic is restored to a normal range. According to this thermal decomposition method, it is possible to reliably detect an abnormal rise in the molten plastic in the thermal decomposition container 2 and quickly recover it.

[Brief description of drawings]

FIG. 1 is a process flow diagram of an apparatus for carrying out a molten plastic liquid level detection method for a thermal decomposition container according to the present invention.

FIG. 2 is another process flow diagram of an apparatus for carrying out the molten plastic liquid level detection method for a thermal decomposition container according to the present invention.

FIG. 3 is still another process flow chart of the apparatus for carrying out the molten plastic liquid level detection method for the thermal decomposition container according to the present invention.

[Explanation of symbols]

1 Pyrolysis device 2 Pyrolysis container 3 heating section 4 detection tubes 5 Pressure detector 6 Pressure detector 7 Control device 8 feeder 9 Heating drive 10 Extruder 11 discharge valve 12 Temperature detector 13 Protection tube a to e piping

Claims (6)

[Claims] 1. A pyrolysis container 2 for pyrolyzing plastic.
A method for detecting the liquid level of molten plastic in a container, wherein a detection tube 4 having an upper end closed and a lower end opened is extended from the upper side of the thermal decomposition container 2 to the lower inside thereof, with the lower end thereof first. A liquid level detection method for a thermal decomposition container, characterized in that the liquid level of the molten plastic is detected from the difference between the pressure inside 4 and the space pressure inside the thermal decomposition container 2. 2. A pyrolysis container 2 for pyrolyzing plastic.
A method for detecting the liquid level of the molten plastic in the interior of the pyrolysis container 2, wherein the presence or absence of the liquid level of the molten plastic at that position is detected from the difference between the space temperature at a predetermined position in the thermal decomposition container 2 and the temperature of the molten plastic. A method for detecting a liquid level in a pyrolysis container, which is characterized. 3. The temperature detector 12 provided at a predetermined position in the thermal decomposition vessel 2 detects the difference between the space temperature and the temperature of the molten plastic at that position.
The method for detecting a liquid level in a pyrolysis container according to. 4. A temperature detector 12 provided in contact with a predetermined position on the outer surface of the side wall of the pyrolysis container 2 detects the difference between the space temperature in the pyrolysis container 2 and the temperature of the molten plastic at that position. The method for detecting a liquid level in a pyrolysis container according to claim 2. 5. The method for detecting the liquid level of a thermal decomposition container according to claim 1, wherein the heating temperature of the thermal decomposition container 2 is 500 ° C. or higher. 6. Continuous operation when the liquid level of the molten plastic in the thermal decomposition vessel 2 detected by the liquid level detection method according to any one of claims 1 to 5 rises abnormally. A thermal decomposition method, characterized in that the supply of plastic to the thermal decomposition container 2 is reduced or stopped, and the liquid level of the molten plastic is restored to a normal range.