RU2770964C1 - Method for utilization of stripping gas from a liquefied natural gas (lng) tank - Google Patents

Abstract

FIELD: gas industry.

SUBSTANCE: invention relates to the field of storage of liquefied natural gas (LNG), in particular to ensuring the utilization of stripping gas from an LNG tank, and can be used in the cryogenic gas industry. The method involves storing liquefied natural gas in a tank. When exceeding the nominal pressure, a safety system is triggered that requires filling the gas ejector chamber with high-pressure natural gas in order to create a rarefaction area, which makes it possible to entrain excess stripping gas from the tank and compress the mixture. Next, the gas flow is fed by an ejector into a storage tank, from where it is used for the technological needs of the tank farm or is subject to re-liquefaction.

EFFECT: reduction of cryogenic fuel losses during vapor discharge and ensuring the environmental safety of LNG storage.

1 cl, 2 dwg

Description

The invention relates to the storage of liquefied natural gas (LNG), in particular to ensure the utilization of boil-off gas from an LNG tank, and can be used in the cryogenic gas industry.

There is a known method of maintaining a supercooled state of liquefied natural gas in a tank (RF patent No. 2628337, published on August 16, 2017), which includes the removal of a part of the cryogenic fluid, its cooling with liquid nitrogen and reintroduction back into the liquid zone of the storage tank, which eliminates discharge of gas from the reservoir.

The disadvantage of this method is the need to use a refrigerant, which must be cooled down on the heat exchanger, as well as the use of a pump if it is necessary to circulate liquefied natural gas from the tank and back.

A known method for storing LNG StarLiteLNG (LINDE AG. StarLNG. Advanced standardized small and medium-sized LNG plants. [online] [found 2021-04-25]. Found at https://www.linde-engineering.ru/ru/images/ StarLNG_Russian_tcm480-458277.pdf), which includes a complex of compressors and an expander for re-liquefaction of boil-off gas, which makes it possible to regenerate fuel without emissions to the flare or dispersion candle.

The disadvantage of this method is the capital, operating and energy costs for increasing the pressure of the boil-off gas using a compressor.

There is a known method for storing and shipping liquefied natural gas (RF patent No. 2680914, published on February 28, 2019), which includes pumping LNG after the installation of liquefied natural gas through a cryogenic pipeline, first into a ground cryogenic tank, and then into a gas tanker, while the boil-off gas generated during the filling of the floating storage and the reloading of the LNG into the gas tanker is returned using the boil-off gas receivers through the cryogenic pipeline to the boil-off gas compressor and then to the natural gas liquefaction process.

The disadvantage of this method is the need to use a boil-off gas compressor to remove and compress it, which entails, in addition to capital, energy and operating costs, emissions during compressor operation due to fuel combustion.

A known method of maintaining the low-temperature regime of liquefied gas in the tank (B.S. RACHEVSKY. Liquefied hydrocarbon gases. Moscow, OIL and GAS, 2009, ISBN 5-7246, c. 312-313), which consists in the fact that evaporating as a result of inflow heat from the outside, the gas passes through the heat exchanger and enters the suction of the compressor, after which it is fed into the condenser, where it condenses at a constant pressure, the condensed liquid is additionally supercooled by the oncoming gas flow in the heat exchanger and then throttled in the valve to a pressure corresponding to the storage mode, entering the LNG tank .

The disadvantage of this method is the need to use a compressor to compress the boil-off gas, which leads to capital, operating and energy costs.

A known method for discharging vapors from a liquefied natural gas (LNG) tank (RF patent No. 2677022, publ. tank pressure.

The disadvantage of this method is the need to install a pump in the tank to supply LNG to the ejector, which is accompanied by additional energy and material costs.

The technical result is the exclusion of hydrocarbon losses during the utilization of boil-off gas from a liquefied natural gas (LNG) reservoir.

The technical result is achieved by the fact that the removal of boil-off gas from the tank is carried out by a gas ejector, the working medium of which is a high-pressure natural gas stream, which is reduced to the operating pressure on the pressure regulator after a signal is given to open the valve on the natural gas supply line to the ejector at exceeding the nominal pressure in the tank, and the resulting mixture of gases is sent to a container for storing the mixture, from where it is transported for the technological needs of the tank farm or for re-liquefaction.

The method is illustrated by the following figures:

fig. 1 is an algorithm for implementing a method for utilizing boil-off gas from a liquefied natural gas (LNG) tank;

fig. 2 is a schematic diagram of an ejector system for the utilization of boil-off gas from an LNG tank, where:

1 – LNG pipeline;

2 – LNG tank;

3 - safety valve;

4 – boil-off gas pipeline;

5 - check valve;

6 – high pressure natural gas pipeline;

7 - pressure regulator;

8 - valve;

9 – pipeline of natural gas of working pressure;

10 - gas ejector;

11 - pipeline for a mixture of boil-off and natural gases;

12 - storage tank for a mixture of boil-off and natural gases.

The method is carried out as follows. The LNG tank 2 (Fig. 1, 2) is filled with cryo-liquid through the LNG pipeline 1, where it is stored at an overpressure, the level of which is controlled and, in case of exceeding the nominal value, the safety system is triggered. After that, the valve 8 of the operating pressure 9 installed on the natural gas pipeline is opened, where natural gas flows from the high pressure natural gas pipeline 6, reducing to the operating pressure on the pressure regulator 7 and filling the chamber of the gas ejector 10. Then the safety valve 3 is activated to remove the excess of the resulting boil-off gas and the working fluid of the gas ejector 10, which is natural gas of operating pressure, entrains this flow through the boil-off gas pipeline 4 due to the rarefaction area created in the mixing chamber of the gas ejector 10, while the check valve 5 prevents the flow with a higher pressure from flowing back into the tank. In the gas ejector 10, the mixture of boil-off and natural gases is compressed and it is supplied when the valve 8 is opened on the pipeline of the mixture of boil-off and natural gases 11 into the storage tank for the mixture of boil-off and natural gases 12, from where the mixture is discharged through the pipeline of the mixture of boil-off and natural gases 11 upon opening valves 8 for the technological needs of the tank farm or for re-liquefaction.

произошло повышение температуры с

до

, что привело к повышению давления насыщенных паров в резервуаре с рабочего

до максимально допустимого

- давление срабатывания предохранительного клапана и сбросу избытка отпарного газа. The method is illustrated by the following example. In an LNG tank with a volume

there was an increase in temperature from

before

, which led to an increase in the pressure of saturated vapors in the tank from the working

up to the maximum allowable

- the pressure of the safety valve and the discharge of excess boil-off gas.

- температура потока;

- плотность потока;

- абсолютное давление потока, равное максимально допустимому давлению с учетом предотвращения эффекта запирания в газовом эжекторе;

- показатель адиабаты;

- газовая постоянная для метана.The flow of natural gas coming from the high pressure natural gas pipeline to the gas ejector has the following characteristics:

- flow temperature;

- flux density;

- absolute pressure of the flow, equal to the maximum allowable pressure, taking into account the prevention of the effect of locking in the gas ejector;

- adiabatic index;

is the gas constant for methane.

- секундный массовый расход потока, равный расходу из предохранительного клапана для сброса избытка отпарного газа;

- температура потока;

- абсолютное давление потока, равное давлению поддерживаемому в линии подвода отпарного газа в эжектор;

- диаметр проходного сечения патрубка эжектора;

- площадь проходного сечения патрубка эжектора.The flow of boil-off gas from the LNG tank to the gas ejector has the following characteristics:

- second mass flow rate, equal to the flow rate from the safety valve to dump excess boil-off gas;

- flow temperature;

- absolute flow pressure, equal to the pressure maintained in the boil-off gas supply line to the ejector;

- diameter of the passage section of the ejector pipe;

- area of the passage section of the ejector pipe.

An indicator of the efficiency of the ejector is the absolute pressure of the mixture at the outlet of the gas ejector (1) and the efficiency (3).

, (1)

, (one)

- коэффициент отношения площадей проходных сечений потоков отпарного газа к природному газу (2). Параметр определен с целью сокращения металлоемкости устройства. where

- coefficient of the ratio of the areas of flow sections of boil-off gas flows to natural gas (2). The parameter is defined in order to reduce the metal consumption of the device.

. (2)

. (2)

- площадь и диаметр потока природного газа;

- площадь и диаметр проходного сечения потока смеси на выходе из газового эжектора. Then

- area and diameter of natural gas flow;

- area and diameter of the flow section of the mixture flow at the outlet of the gas ejector.

.Therefore, the absolute pressure of the mixture at the outlet of the gas ejector (1) is

.

, (3)

, (3)

- массовый расход потока природного газа (4):where

- mass flow rate of natural gas (4):

, (4)

, (4)

- критическая скорость истечения потока природного газа из сопла эжектора, соответствующая скорости звука (5):where

is the critical flow rate of natural gas flow from the ejector nozzle, corresponding to the speed of sound (5):

, (5)

, (5)

- коэффициент отношения давлений. where

- pressure ratio coefficient.

, который представляет собой внутренний массовый расход устройства как отношение расходов природного и отпарного газов (6):The ejector is characterized by the ejection coefficient

, which represents the internal mass flow rate of the device as the ratio of the flow rates of natural and boil-off gases (6):

. (6)

. (6)

=186К, где (7):The process of mixing gases in the ejector is close to adiabatic, so the stagnation temperature of the mixture at the outlet of the mixing chamber of the ejector

=186K, where (7):

. (7)

. (7)

Then, the efficiency of the gas ejector (3) is

The advantage of the method is that the boil-off gas utilization system using an ejector makes it possible to reduce the loss of cryogenic fuel and environmental pollution by extracting and compressing excess boil-off gas without using energy costs from the outside, while ensuring the safety of cryogenic storage of LNG, as well as the efficiency of the ejector 16%, and the absolute pressure of the mixture flow at the outlet of the gas ejector is 1.13 MPa, thereby replacing the first stage compressor.

Claims (1)

A method for utilizing boil-off gas from a liquefied natural gas (LNG) tank, including filling the tank with cryo-liquid, storage at excess pressure, and removal of the resulting vapors, characterized in that the boil-off gas is removed from the tank by a gas ejector, the working fluid of which is a high-pressure natural gas flow. pressure, which is reduced to the operating pressure on the pressure regulator after a signal is given to open the valve on the line for supplying natural gas to the ejector when the pressure in the tank exceeds the nominal value, and the resulting gas mixture is sent to a container for storing the mixture, from where it is transported for the technological needs of the tank farm or for re-liquefaction.

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