JPH0215791B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an energy-saving and resource-saving reliquefaction method for a methane-based mixed gas, and particularly to a purposeful reliquefaction method for a methane-based mixed gas with a relatively high ethane content. We aim to provide the following.

A typical example of the LNG reliquefaction process is the cascade cycle. An example is shown in FIG. In FIG. 1, evaporated gas from a cargo tank 1 is reliquefied in a refrigeration cycle consisting of a compressor 2, a condenser 3, and an expansion valve 4. (No. 1 cycle) This No.
The refrigerant in the condenser 3 in one cycle is ethane that circulates through the compressor 2', condenser 3', and expansion valve 4' (No. 2 cycle), and the refrigerant in the condenser 3' in the No. 2 cycle is Freon (registered trademark) circulates through the compressor 2'', seawater heat exchanger 5, and expansion valve 4''.
is used. (No. 3 cycle) This process is generally used when the methane concentration is high, and there is still no dedicated reliquefaction process when the methane concentration in the evaporated gas is as low as 60%, such as in ethane tankers. Instead, the LNG reliquefaction process described above is simply used as a substitute. However, in such a process, when the cargo tank is empty or docking, it is easy to maintain the Freon (registered trademark) cycle, which is the No. 3 cycle, but the ethane cycle, which is the No. 2 cycle, cannot be maintained. Because the heat input creates high pressure, it is extremely difficult to maintain this pressure, as all lines in the ethane cycle must be made of pressure-resistant piping of 40 kg/cm ² or more.

Therefore, the present inventor has developed a liquefaction process for a mixed gas with a low methane concentration of about 60% methane and about 40% ethane, such as evaporated gas from an ethane ship, which can save energy and reduce the area occupied by the device. As a result of intensive research, we have found that LNG with a low methane concentration
They focused on the fact that an ethane-rich liquid is produced during the reliquefaction process, and came up with the idea of using this ethane-rich liquid as a refrigerant for a mixed gas with a low methane concentration, and completed the present invention.

That is, the present invention provides a reliquefaction method in which a methane-based mixed gas evaporated from a liquefied methane-based mixed gas storage tank is re-liquefied and recovered into the storage tank, in which the methane-based mixed gas is passed through a compressor and then a circulating ethane condenser. A part of the ethane-rich condensate taken out from the separator is expanded by an expansion valve, and the ethane-rich gas is introduced into the circulating ethane condenser as a refrigerant to condense the methane-based mixed gas, and then, The ethane-rich gas is introduced into the compressor to be integrated with the methane-based mixed gas, and the ethane-rich condensate and methane-rich gas taken out from the separator are recovered into the storage tank via the expansion valve. This is a distinctive reliquefaction method.

A specific example of the present invention will be described below with reference to FIG.

In FIG. 2, the same reference numerals as in FIG. 1 indicate the same equipment as explained in connection with FIG.
(referred to as an expansion valve), 6 is a separator, 12, 15,
14 and 13 correspond to the Freon (registered trademark) cycle, which is the No. 3 cycle in FIG. 1, and are a compressor 12, a seawater heat exchanger 15, an expansion valve 14, and a condenser 13, respectively.

In the present invention, a separator 6 is provided in the reliquefaction cycle A shown in FIG. is provided, and the methane-based mixed gas is cooled in the condenser 3 by the ethane-rich gas expanded in the second expansion valve 4 and turned into a refrigerant.

Therefore, it can be said that the feature of the present invention is that the No. 2 cycle of the conventional reliquefaction process cascade cycle (FIG. 1) is superimposed on the No. 1 cycle. In other words, the process in Figure 2 is
The process shown in Figure 1 is modified, and a separator 6 for gas-liquid separation is provided between the condenser 3 and the expansion valve 4 in the No. 1 cycle in Figure 1, and the expansion valve in the No. 2 cycle in Figure 1 is 4' is connected to the liquid phase of separator 6 (4 in Figure 2), and the No. 2 cycle compressor 2' in Figure 1 is removed and connected to the No. 1 cycle compressor 2 in Figure 1. Furthermore, the condenser 3' of the No. 2 cycle in Fig. 1 is placed between the compressor 2 and the condenser 3 of the No. 1 cycle in Fig. 1 (13 in Fig. 2). ).

The method of the present invention has the above-mentioned configuration, and its operation will be explained using FIG. 2.
Evaporated gas in the cargo tank 1 is compressed by a compressor 2, and a condenser (made by Freon (registered trademark)) 13
and 3 (by circulating ethane rich gas), and is expanded by the first expansion valve 4 via the separator 6, liquefied, and returned to the cargo tank 1. on the other hand,
A part of the liquid in the separator is expanded and gasified by the second expansion valve 4, cools the condenser 3, and is injected into the intermediate stage of the compressor 2 and compressed. Furthermore, condenser 3
In addition, the condenser 13 is cooled by compression, cooling, and expansion cycles 12, 15, and 14 using Freon (registered trademark).

Further, the effects of the present invention are as follows.

The gas and liquid in the separator 6 are methane-rich and ethane-rich, respectively, and a part of this liquid is taken out to form a circulation line B in order to obtain the same effect as the No. 2 cycle (ethane cycle) in FIG. In addition, by arranging the condensers 3 and 13 of the No. 1 and No. 2 cycles in Fig. 1 in series, it became possible to eliminate the ethane compressor 2' in Fig. 1. , the required power can be significantly reduced.

On the other hand, when there is no cargo, the ethane line of the cascade cycle shown in Figure 1 becomes high pressure due to heat input, so the entire line had to be made of pressure-resistant piping of 40 kg/cm ² or more. According to the present invention, since the liquid can be taken out from the ethane rich line together with the cargo, there is no need for pressure-resistant piping that exceeds the operating pressure, resulting in a reduction in product costs.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of a conventional cascade cycle reliquefaction apparatus for methane-based mixed gas, and FIG. 2 is an explanatory diagram of an apparatus for carrying out the method for reliquefying methane-based mixed gas of the present invention.

Claims (1)

[Claims] 1. In a reliquefaction method for reliquefying a methane-based mixed gas evaporated from a liquefied methane-based mixed gas storage tank and recovering it in the storage tank, the methane-based mixed gas is sent to a separator via a compressor and then a circulating ethane condenser, A part of the ethane-rich condensate taken out from the separator is expanded with an expansion valve, and the ethane-rich gas is introduced into the circulating ethane condenser as a refrigerant to condense the methane-based mixed gas, and then,
The ethane-rich gas is introduced into the compressor to be integrated with the methane-based mixed gas, and the ethane-rich condensate and methane-rich gas taken out from the separator are recovered into the storage tank via the expansion valve. Characteristic reliquefaction method.