JPS586116B2 - Processing method for displacement purge gas from a low-temperature liquefied gas carrier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides for the purpose of inspecting, repairing, cleaning, etc. a combustible low-temperature liquefied gas storage tank installed in a combustible low-temperature liquefied gas carrier such as a low-temperature LPG carrier, for example, to prevent residual gas from remaining in the tank. This invention relates to a method for condensing and recovering residual liquefied gases when replacing combustible liquefied gases such as propane and butane with inert gases such as nitrogen.

When inspecting, repairing, or cleaning the liquefied gas storage tank of a flammable low-temperature liquefied gas carrier, flammable gases such as propane and butane remaining in the tank are usually disposed of by replacing them with inert gas such as nitrogen. be done.

The gas discharged at this time (hereinafter referred to as purge exhaust gas) initially contains almost no inert gas, but then contains 5 to 15% inert gas for about 80 minutes of the total replacement work time, and then rapidly increases. The replacement work is completed when it is confirmed that the liquefied gas concentration has decreased and is within a safe concentration range.

Conventionally, this purge exhaust gas has been treated by burning it in a flare stack, or by condensing and recovering it using a compressor, condenser, refrigerator, etc. for residual gas recovery.

However, burning the purge exhaust gas in a flare stack is problematic in that it causes various problems such as wasted resources and air pollution due to the combustion of a large amount of liquefied gas.

For example, in large ships such as ocean-going LPG carriers, a large amount of LPG of 150 to 200 tons remains in the tank, and burning such a large amount of LPG is extremely contrary to resource conservation.

In addition, the method of recovering residual gas using a compressor, etc. requires large capacity compressors, condensers, refrigerators, etc., and it is not economically viable because it is used infrequently. .

On the other hand, a direct compression type evaporated gas reliquefaction device conventionally installed on a flammable low-temperature liquefied gas carrier is composed of a compressor 2, a condenser 3, and an expansion valve 4, as shown in FIG.
The evaporated gas generated from the flammable low-temperature liquefied gas storage tank 1 is pressurized by the compressor 2 and sent to the condenser 3, where it is condensed by exchanging heat with seawater, and then passes through the expansion valve 4 to the flammable low-temperature liquefied gas. It is returned to the gas storage tank 1.

For example, in a large ship such as an ocean-going LPG carrier, four to five series of conventional reliquefaction equipment with a capacity of 1200 to 1350 m 3 /H are installed.

It is conceivable to introduce the aforementioned purge exhaust gas into a conventional reliquefaction device and condense and recover the liquefied gas in the same gas.
This equipment condenses and re-liquefies liquefied gas that does not contain non-condensable gases such as nitrogen, and when re-liquefying gas that contains non-condensable gases such as nitrogen such as purge exhaust gas, the condensation temperature is 3.
Since the temperature is approximately 5 to 40°C, a large amount of liquefied gas is mixed into the non-condensable gas and discharged, resulting in a liquefaction rate of 60 to 40%, resulting in extremely poor efficiency.

The present invention aims to improve the above-mentioned problems by efficiently recovering liquefied gas in purge exhaust gas by using a reliquefaction device conventionally installed on a combustible low-temperature liquefied gas carrier. As mentioned above, the liquefied gas in the non-condensable gas, which could not be condensed using the conventional reliquefaction equipment alone, can be refrigerated using at least one of the several series of conventional reliquefaction equipment. Cold heat obtained by operating a refrigeration cycle (as an example, approximately -40°C in the case of propane)
This provides a method for condensation recovery in an additional low temperature condenser.

An embodiment of the present invention will be described below based on FIG. 2.

In this example, the inert gas is nitrogen, the residual gas in the tank is propane, and there are two reliquefaction lines, but the present invention is not limited to this example.

In addition, in the figure, in order to clarify the gist of the invention, parts that are not important to the explanation are omitted.

In the figure, reference numeral 1 denotes a tank for storing flammable low-temperature liquefied gas, and a pipe 7 for introducing nitrogen gas and a pipe 8 for removing residual gas communicate with this tank 1.

Further, pipes 9 and 13 are connected to this pipe 8, a compressor 2a is connected to the pipe 9, a compressor 2b is connected to the pipe 13 via a valve 12, and the outlets of these compressors 2a and 2b are connected. A condenser 3a, 3b is connected to each by a tube 10.14.

In addition, each of the condensers 3a . The gas phase part of 3b is the pipe 17,1
8.19, the tube 20 passes through the tube side of the low temperature condenser 5.
This separator 6 and the liquid phase portion of the condenser 3a are connected to a pipe 23 via pipes 21, 22, and 11.
The liquid phase part of the condenser 3b is connected to the pipe 2 through the valve 15.
2 and the condenser 5 via the expansion valve 4.
is connected to the pipe 13 via a valve 16 through the shell side of the pipe.

Now, by introducing nitrogen gas into the liquefied gas storage tank 1 through the pipe 7, the propane gas remaining in the tank 1 is discharged through the pipe 8.

In this case, in order to maintain a high concentration of propane gas in the purge exhaust gas, it is desirable to introduce nitrogen gas from the upper part of the tank and to discharge residual gas from the lower part of the tank, as shown in the figure.

While the propane gas concentration in the exhaust gas is initially approximately 100%, the device operates in the same manner as a conventional reliquefaction device with valves 12 and 15 open and valve 16 closed.

That is, the exhaust gas passes through pipe 9, valve 12, and pipe 13, is pressurized by compressors 2a, 2b, and is introduced into condensers 3a, 3b through pipes 10.14.

The propane liquid condensed through heat exchange with seawater in the condensers 3a and 3b is sent to a land-based propane tank or the like through a pipe 11, a valve 15, and pipes 22 and 23.

When the nitrogen concentration in the exhaust gas increases over time, valves 12 and 15 are closed and valve 16 is opened to operate the compressor 2b series in a refrigeration cycle using propane as a refrigerant, which is a non-condensable gas. Propane gas that evaporates along with nitrogen is condensed and recovered in a low-temperature condenser 5.

That is, exhaust gas from the storage tank 1 passes through pipes 8 and 9, is pressurized by the compressor 2a, passes through the pipe 10, and is transferred to the condenser 3a.
However, if the exhaust gas contains nitrogen, a portion of the propane gas in the gas is condensed by exchanging heat with seawater in the condenser 3a, and then passes through the pipes 11 and 23 to the However, the remaining uncondensed water is introduced into the tube side of the low-temperature condenser 5 through pipes 17 and 19 along with nitrogen gas.

A refrigerant propane, which will be described later, is introduced into the shell side of the low-temperature condenser 5, and the propane gas in the nitrogen gas has a temperature of -35°C.
After being cooled and condensed to a certain extent, it is introduced into the gas-liquid separator 6 through the pipe 20 together with the nitrogen gas, which still contains a small amount of uncondensed propane gas.

The propane liquid separated by the gas-liquid separator 6 is passed through pipes 21, 22.
The nitrogen gas is sent to land through a pipe 23 together with the propane liquid condensed in the condenser 3a, and the nitrogen gas is sent to a flare stack or the like through a pipe 24 because it contains a small amount of propane.

If you have time to carry out the replacement work, you can reuse this small amount of nitrogen gas containing propane as a purge gas.
It is possible to improve the propane recovery rate and save nitrogen gas.

On the other hand, the propane gas pressurized by the compressor 2b is introduced into the condenser 3b through the pipe 14, where it exchanges heat with seawater and condenses.

The propane liquid exiting the condenser 3b is expanded through the expansion valve 4 and introduced into the low-temperature condenser 5, where it exchanges heat with the uncondensed nitrogen and propane mixed gas introduced through the aforementioned pipes 17 and 19. The gas is vaporized and sucked into the compressor 2b through the valve 16 and pipe 13, and the pressure is increased again to repeat the same operation.

Further, if nitrogen gas is mixed into the series of this compressor 2b, it is discharged from the condenser 3b through the pipe 18 and treated in the same manner as the uncondensed gas discharged from the aforementioned condenser 3a.

As in the above example, when the cargo of a flammable low-temperature liquefied gas carrier is propane and the replacement inert gas is nitrogen, the recovery rate of residual propane in the liquefied gas storage tank is lower than that of the conventional reliquefaction equipment alone. However, according to the method of the present invention, the recovery rate becomes about 90, and just by adding a low-temperature condenser, the recovery rate can be greatly improved.

In addition, as mentioned above, large ships such as ocean-going LPG carriers are equipped with 4 to 5 reliquefaction devices with a capacity of 1200 to 1350 m3/H.
It is installed in series, and the inflow of nitrogen gas for replacement is 3000 ~
At 4000 m3/H, it is possible to process with a total of 4 lines, 3 lines for purge exhaust gas and 1 line for refrigeration cycle.

As described above, according to the method of the present invention, by adding a low-temperature condenser to the evaporative gas reliquefaction device conventionally installed on flammable low-temperature liquefied gas carriers, the device can be used effectively and This allows for efficient recovery of residual gas in the ship's liquefied gas storage tanks, which had been burned and disposed of, thereby preventing resource waste and air pollution.

[Brief explanation of the drawing]

FIG. 1 is a system diagram of a conventional direct compression type reliquefaction apparatus, and FIG. 2 is a system diagram showing an embodiment of the method of the present invention. 1... Flammable low temperature liquefied gas storage tank, 2a, 2b.
... Compressor, 3a, 3b... Condenser, 4... Expansion valve, 5... Low temperature condenser, 6... Gas-liquid separator.

Claims (1)

[Claims] 1 Equipped with multiple series of direct compression type evaporative gas reliquefaction equipment consisting of a flammable low-temperature liquefied gas storage tank and a compressor, condenser, and expansion valve for reliquefying the evaporative gas in this tank and returning it to the tank. On low-temperature liquefied gas carriers, for purposes such as inspection, repair, and cleaning of flammable low-temperature liquefied gas storage tanks, the flammable liquefiable gases remaining in the tanks, such as propane and butane, are replaced with inert gases such as nitrogen. In case,
In order to separate and recover the flammable gas in the displacement purge gas formed by mixing these flammable gases and inert gases,
At least one train of the evaporated gas reliquefaction equipment installed on the carrier ship is operated in a refrigeration cycle using the liquefied gas, which is the original cargo of the ship, as a refrigerant, and this refrigerant and the above-mentioned replacement barge gas are used to reliquefy the evaporated gas in the remaining trains. After the combustible and liquefiable gas contained in the displacement purge gas is condensed and separated through the device, the uncondensed gas rich in inert gas discharged is exchanged with the uncondensed gas rich in inert gas in a low-temperature condenser to form the uncondensed gas rich in inert gas. A method for processing displacement purge gas from a low-temperature liquefied gas carrier, characterized by further condensing and separating the combustible and liquefiable gas, and recovering the separated combustible liquefied gas in a tank different from the storage tank.