RU2844225C1 - Boil-off gas recovery system for liquefied natural gas storage and shipment systems - Google Patents

Abstract

FIELD: cryogenic equipment.

SUBSTANCE: invention relates to cryogenic engineering and can be used in gas industry. Liquefied natural gas storage and shipment boil-off gas recovery system includes an LNG tank equipped with an LNG receiving line and a pump connected by the LNG shipment line to the LNG loading system, which is connected to the tank by a boil-off gas line of the discharge system, and is equipped with a total boil-off gas line. Total boil-off gas line is connected to the lower part of the LNG denitrogenation unit below the reduced LNG input level, but above the steam reflux input level.

EFFECT: utilization of boil-off gas of LNG storage and shipment systems, obtained at liquefaction of natural gas with high content of nitrogen, and system simplification.

1 cl, 1 dwg

Description

The invention relates to the field of cryogenic technology and can be used in the gas industry for the production of liquefied natural gas (LNG).

During large-scale LNG production, cold strip gas flows periodically arise during the filling of transport tanks and are sent to a utilization system that includes either a strip gas circulation compressor or a strip gas recondenser with the return of recondensed strip gas to the LNG tank.

The closest in technical essence to the claimed invention is a system for producing liquefied natural gas equipped with a recondenser [RU 2728305, published 07/29/2020, SPK F25J 1/0022; F25J 1/0025; F25J 1/0052; F25J 1/0072; F25J 1/0204; F25J 1/0212; F25J 1/0244; F25J 1/0245; F25J 1/0249; F25J 1/0251; F25J 1/0258; F25J 1/0261; F25J 1/0262; F25J 1/0272], including a subsystem (system) for utilizing the stripping gas of the storage and unloading systems (Fig. 1-3), consisting of an LNG tank equipped with an LNG receiving line and a pump connected by an LNG unloading line connected by an LNG unloading line to the LNG unloading system, which is connected to the tank by a stripping gas line of the unloading system, and equipped with a total stripping gas line on which a recondenser is located, connected by a recondensate line equipped with a pump, to the LNG tank, wherein the recondenser is connected either to the main refrigeration unit of the LNG liquefier, or to a separate refrigeration machine, or to a buffer refrigerant storage tank, connected to an external source of refrigerant or to the refrigeration unit of the LNG liquefier.

The main disadvantage of this system is the impossibility of using LNG storage and loading systems for utilization of boil-off gas, obtained by liquefying natural gas with a nitrogen content above 2-2.5% vol. due to nitrogen accumulation in the cycle "LNG tank with loading system - recondenser - pump - LNG tank", which leads to an increase in pressure in the LNG tank and the need to bleed excess pressure from the tank into the atmosphere or flare system, which results in LNG losses. In addition, this system is unnecessarily complicated due to the presence of additional equipment - a recondenser with a pump, as well as either a separate refrigeration unit, or additional capacity of the LNG liquefier refrigeration unit, or an external source of refrigerant (liquid nitrogen).

The objective of the invention is to utilize the boil-off gas of LNG storage and loading systems obtained by liquefying natural gas with a high nitrogen content, as well as to simplify the system.

The technical result - utilization of stripping gas of LNG storage and shipment systems obtained during liquefaction of natural gas with a high nitrogen content, and simplification of the system - is achieved by using the existing LNG denitrogenation system for utilizing stripping gas of LNG storage and shipment systems, by connecting the total stripping gas line to the lower part of the LNG denitrogenation unit column below the reduced LNG input level, but above the steam irrigation input level.

The claimed technical result is achieved by the fact that in a system including an LNG tank equipped with an LNG receiving line and a pump connected by an LNG unloading line to an LNG unloading system, which is connected to the tank by a stripping gas line of the unloading system, and equipped with a total stripping gas line, the feature is that the total stripping gas line is connected to the lower part of the LNG denitrogenation unit column below the reduced LNG input level, but above the steam irrigation input level, while a pump is installed on the LNG unloading line.

If the LNG denitrogenation unit is located remotely from the LNG tank, a blower or compressor may be installed on the total stripping gas line. If the LNG tank is located remotely from the LNG unloading system, a blower or compressor may also be installed on the stripping gas line of the unloading system. If necessary, the total stripping gas line is connected to a reducing device located on the nitrogen-containing LNG feed line to the LNG denitrogenation unit column and made in the form of an ejector. If necessary, the LNG denitrogenation unit column is equipped with a liquefied nitrogen-containing fuel gas outlet line.

All components of the installation can be implemented in any manner known from the prior art.

Utilization of the stripping gas from LNG storage and loading systems obtained during liquefaction of natural gas with a high nitrogen content and simplification of the utilization system are achieved by using the LNG denitrogenation unit available at the LNG liquefaction plant by connecting the total stripping gas line to the lower part of the LNG denitrogenation unit column below the reduced LNG input level but above the steam reflux input level. In this case, the methane contained in the stripping gas is condensed and returned to the LNG tank, and the heat supplied with the stripping gas flow helps to reduce the steam reflux feed to the column.

The proposed system for utilization of boil-off gas from liquefied natural gas storage and loading systems is shown in the drawing and includes LNG tank 1, pumps 2 and 3, and LNG denitrogenation unit column 4. If necessary, the system includes gas blowers/compressors 5 and an ejector 6.

During operation of the proposed system for utilizing strip gas from liquefied natural gas storage and unloading systems, denitrogenated LNG is withdrawn from the bottom of column 4 and divided into two parts. The first part 7 is fed by pump 2 to tank 1, from which LNG is fed by pump 3 to the unloading system (not shown) via line 8. The second part of denitrogenated LNG 9 is heated in heater 10 and returned to the bottom of column 4 as steam reflux. Strip gas from the unloading system is fed to tank 1 via line 11, and the total strip gas is sent to column 4 via line 12 above the level of introduction of steam reflux 9, but below the level of introduction of nitrogen-containing LNG 13. Column 4 is also equipped with lines for supplying acute reflux 14 and for removing gaseous nitrogen 15.

If necessary, the stripping gas of the unloading system and/or the total stripping gas can be supplied using gas blowers/compressors 5. If necessary, the total stripping gas is supplied directly to the nitrogen-containing LNG stream using ejector 6. If necessary, liquid nitrogen-containing fuel gas with the standard calorific value is removed from column 4 via line 16.

The efficiency of the proposed system is confirmed by an example.

At a peak flow rate of 8.4 thousand ^nm3 /h of total stripping gas 12 with a pressure of 0.04 MPa, containing, % vol.: methane 79.9; nitrogen 20.0; hydrocarbons C2 ₊ the rest, at minus 159.8 °C is sent to column 4 above the level of introduction of steam reflux 9, but below the level of introduction of nitrogen-containing LNG 13. 37.0 t/h of steam reflux 9 with a temperature of minus 147.0 °C are also fed to column 4, as well as 276.0 t/h of nitrogen-containing LNG 13 with a nitrogen content of 50.0% vol. and a temperature of minus 185.3 °C and 18.5 t/h of acute reflux 14 with a temperature of minus 192.1 °C are fed. From the top of column 4, 154.0 thousand ^nm3 /h of gaseous nitrogen 15 with a methane content of 1.0% by volume are withdrawn, 141.9 t/h of denitrogenated LNG are withdrawn from the bottom of column 4, which is then divided into two parts, 104.8 t/h of the first part 7 are fed to tank 1, and the second part 9 is heated in heat exchanger 10 and returned to column 4 as steam reflux, and 4.9 t/h of liquid nitrogen-containing fuel gas with a standard calorific value of 16 are withdrawn from the middle part of the column below the level of input of stream 13.

From tank 1, 104.4 t/h of denitrogenated LNG 8 with a nitrogen content of 1.0% is fed into the unloading system by pump 3, and 8.0 thousand ^nm3 /h of stripping gas 11 are sent to tank 1 during LNG unloading into the tanker, and the total stripping gas 12 is removed from tank 1.

The prototype system is more complex due to the presence of additional equipment - a recondenser with a pump, as well as either a separate refrigeration unit, or additional capacity of the LNG liquefier refrigeration unit, or an external source of refrigerant (liquid nitrogen), and under the conditions of the example is inoperative due to the gradual accumulation of nitrogen in the system.

Thus, the proposed system is simpler, allows for the utilization of stripping gas from LNG storage and loading systems obtained by liquefying natural gas with a high nitrogen content, and can be used in industry.

Claims (1)

A system for utilizing strip gas from liquefied natural gas storage and unloading systems, comprising an LNG tank equipped with an LNG receiving line and a pump connected by an LNG unloading line to an LNG unloading system, which is connected to the tank by a strip gas line of the unloading system, and equipped with a total strip gas line, characterized in that the total strip gas line is connected to the lower part of the LNG denitrogenation unit column below the reduced LNG input level, but above the steam reflux input level.