JPS5823432B2 - Safety device in pyrolysis furnace for ethylene production - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a safety device in a pyrolysis furnace for producing ethylene.

In ethylene plants that produce useful petrochemical raw materials such as ethylene by thermally decomposing ethane, carbon dioxide, naphtha, kerosene, light oil, heavy light oil, etc., the above-mentioned hydrocarbons are A gas mixture with water vapor is decomposed through an externally heated reaction tube.

A pyrolysis furnace has a large number of reaction tubes arranged in a row, and a burner is used to heat the reaction tubes.

The number of burners varies depending on the furnace model, but even in the case of a new type of furnace with a small number of burners, there are, for example, several dozen per furnace, and in an existing furnace of another type, there are over a hundred, so if there are 12 burners, there are Even if the total number is small, it will cover about several blind areas, and if it is large, it will reach more than ten blind areas.

For safety reasons, cracking furnaces must be shut down in the event of an emergency such as a large-scale earthquake, typhoon, snowfall, equipment failure, or outage of electricity, pressurized air, water, etc.

Conventionally, the only way to stop the operation was to manually close the valves of the burner and feed pipe for raw material (hereinafter referred to as naphtha), but as there are a large number of burners as mentioned above, these valves must be closed at once. It takes time to operate one by one,
It also requires effort.

For example, in the case of an existing furnace with a large number of burners, it is necessary to extinguish approximately 800 burners to reduce the heat load to the point where the fluid outlet temperature of the furnace can be lowered to 700°C and the naphtha can be shut off. There are approximately 1,600 burners that need to be extinguished, so it takes at least 4 workers two hours to shut off the naphtha from the time a problem occurs to the entire house. It will take 4 hours to complete.

Furthermore, the only way to dispose of the naphtha that has passed through the furnace before reaching this naphtha shutoff is to release it outside and burn it (flare out), which causes pollution and wastes 30% of the raw material.
A 10,000-ton ethylene plant releases 180 tons per hour, resulting in waste.

In order to avoid this, it may be possible to close the main valves of the fuel and naphtha and cut off their supply all at once, but if the supply of fuel and naphtha is abruptly stopped, normally 1200
As the temperature of the furnace decreases rapidly by approximately °C, thermal stress (temperature shock) is applied to the furnace and the reaction tube through which the naphtha passes due to the thermal contraction, causing cracks and cracks to occur in the furnace wall and reaction tube. There is a risk that the joints of the reaction tubes may become misaligned, causing damage to the furnace.

The present invention solves the above problem, and the whole picture is DDC (
It is controlled by a direct, digital, controller), and in an emergency, by pressing a button, the entire burner and raw material supply are controlled to a substantially constant temperature at the exit of the furnace, and both are linked to automatically and quickly shut down.

An embodiment of the present invention will be described in detail below with reference to the drawings.

Fig. 1 shows an outline of an ethylene production plant to which the present invention is applied, where 1, 1... is a furnace, 2, 2...
...Burners installed in each house, 3, 3...
Reaction tubes for passing naphtha, which is a raw material, are installed in each house 1, 1 (4 each), 4, 4...
・is the dilution steam pipe connected to it, and these burners 2
゜2... are reaction tubes, 3, 3... are steam pipes, 4, 4... are control valves, 5, 5......
- Connected to the fuel tank 8, naphtha tank 9, and dilution steam source 10 via 6, 6...7, 7...

Temperature detectors 11, 11, . . . made of thermoelectric materials are provided at the outlet of each reaction tube 3, 3, . . ., respectively, and heat exchangers 12, 12, . The outlet of this bus 13 is connected to a purification system 15 via a normally open valve 14, and a normally closed valve 1 that opens in an emergency.
6 to a ground flare 17 or an elevated flare.

The output signals of each of the above detectors 11, 11... are AD
The input is inputted to a direct digital controller 21 via a converter 20, and this controller 21 also controls a control valve 23 for ignition of the burners 2, 2, etc. via an ignition/extinguishing operation panel 22. The control valves 5, 5...6, 6...7°7...
. . . Controls the valves 14 and 16, and performs control as shown in FIGS. 2 and 3 based on the signal from the emergency push button 25.

Next, the operation of the above device will be explained with reference to FIGS. 2 and 3.

In the furnace 1, the raw material naphtha undergoes an endothermic reaction and is thermally decomposed by the combustion heat of the fuel.

Therefore, in order to maintain a constant furnace outlet temperature, the amount of fuel and the amount of naphtha must be proportionally reduced in a constant relationship.

In this control, in the present invention, the naphtha flow rate is decreased at a constant rate, the furnace outlet temperature is controlled to be constant according to the amount of fuel, and the burner is extinguished in accordance with the opening degree of the control valve 5. is reduced at a constant rate, the burner is extinguished at a constant rate, and the furnace outlet temperature is sometimes controlled at a constant level by the amount of naphtha.

The former control is shown in FIG. 2, and the latter control is shown in FIG. 3.

FIG. 2 shows a block diagram of the control circuit in the controller 21 in the case of fuel control. In an emergency, when the push button 25 is pressed, the naphtha amount constant reduction circuit 30 operates, and this output signal is sent to the DA converter 24. is sent to the control valves 6, 6, . . . , and the control valves 6, 6, . . . begin to close.

As a result, the reaction tubes 3, 3... of the furnaces 1, 1...
The amount of naphtha flowing in ... decreases at a constant rate, and the amount of heat absorbed decreases, so the temperature of the furnace 1 increases.

This temperature is detected by the detector 11, its output signal is compared in the controller 21 with the output signal of the outlet temperature setting circuit 31 in the comparison circuit 32, and the output signal of the difference is sent to the operation circuit 34 of the control valve 5. , the control valves 5, 5 are activated by the output signal.
.
Erase.

Therefore, the amounts of naphtha and fuel are reduced while controlling the outlet temperature to be constant, and the operation of the furnaces 1, 1, . . . can be stopped.

FIG. 3 shows a block diagram of the control circuit in the controller 21 in the case of naphtha control. When the push button 25 is depressed in an emergency, the fuel amount reduction circuit 40 of the controller 21 is activated.
The output signal is sent to the control valve 5.5, which closes at a constant speed and also extinguishes the burner 2 at a constant speed in accordance with the flow rate.

As a result, the amount of heat generated in the furnace 1 decreases, so the temperature decreases, the detector 11 detects the latter, and its output signal is compared with the output signal of the outlet temperature setting circuit 41 in the controller 21 in the comparison circuit 42. , the difference signal is input to the naphtha amount operation circuit 43.

The output signal of the fuel amount reduction circuit 40 is inputted to this circuit 43 via the feedforward correction circuit 44, thereby calculating the amount of decrease in the amount of naphtha. ..., and the control valve 6.
...1 closes and reduces the amount of naphtha.

Therefore, the amount of endothermic reactions in the furnace 1 is reduced, so
The amount of fuel and naphtha is reduced while controlling the outlet temperature to be constant, and the operation of the furnaces 1, 1, . . . can be stopped.

(The operation referred to here means that the decomposition reaction is stopped due to the stoppage of raw materials such as naphtha.) As described above, according to the present invention, since the operation can be stopped while controlling the outlet temperature of the furnace at a constant level, the furnace wall and reaction tube The furnace operation can be stopped quickly and automatically in a very short time of about 10 minutes because the valves installed in the fuel and raw material supply paths of each house can be controlled by the controller. It is safe and can significantly reduce flare pollution, which has become a big problem in recent years (to 20 tons for 10 minutes), and is therefore economical.

In terms of pollution, in the case of an emergency as mentioned above, the only way to dispose of the naphtha passing through the furnace is to release it outside and burn it.
If this is sent to a normal smoke height of about 120 m and ignited, the amount of gas will be 70 tons per hour, and the length of the flame will be 50 to 70 m.
It makes a noise louder than a horn, and is bright enough to read a newspaper from 400 meters away, even in the middle of the night.

Therefore, it disturbs the sleep of nearby residents and causes anxiety, and also generates nitrogen oxide, etc., which causes pollution.

In order to solve this problem, the controller 21 sends a signal to the valves 14 and 16 by pressing the imaging unit 25 in an emergency, and the valve 1
4 is closed and valve 16 is opened.

Therefore, each house 1... coil pipe 3, 3...
The decomposed naphtha, ie, ethylene gas, etc. produced by the reactor are not sent to the refining system 15, but are sent to the ground flare 17, where they are combusted.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram showing an overview of an embodiment of the present invention, Fig. 2 is a block diagram of the emergency fuel control circuit of the controller, Fig. 3 is a block diagram of the raw material control circuit, and Fig. 4. is a flowchart of the direct digital controller. 1...Furnace, 2...Burner, 3...
...Pipe, 5,6...Control valve, 11...
・Temperature detector, 21... Direct digital controller.

Claims (1)

[Claims] 1. A pyrolysis furnace for ethylene production, a burner installed in this furnace, a reaction tube through which raw materials pass, a temperature detector installed at the outlet of this tube, and the amount of fuel and raw materials supplied to these burners and chain pipes. A safety device in a pyrolysis furnace for ethylene production consists of a control valve that adjusts the flow rate, a flow rate detector, and a direct digital controller that controls these control valves in response to signals from the detector. pyrolysis for ethylene production, characterized in that the outlet temperature of the reaction tube is kept approximately constant by means for reducing the amount of raw material, and means for reducing the amount of fuel supplied in proportion to the decrease in the raw material, and then the furnace is stopped. Safety devices in furnaces.