RU2127855C1 - Method of liquefaction of natural gas - Google Patents

Abstract

FIELD: gas industry. SUBSTANCE: high-pressure natural gas is divided into two flows, one of which (main) is directed to a preliminary heat exchanger, and the other - to a cooled vortex tube. After the preliminary heat exchanger the main flow is fed to a recuperative heat exchanger, and then, to a throttle valve. The formed liquid phase is gathered in a hot well, and the gas phase after passage of the recuperative heat exchanger is connected to the cold flow from the vortex tube and supplied to the preliminary heat exchanger. The use of the cooled vortex tube increases the coefficient of liquefaction of natural gas by 1.2 to 1.8 times as compared with the throttling cycle. EFFECT: facilitated procedure. 2 cl, 2 dwg

Description

 The present invention relates to cryogenic technology, and in particular to a method for liquefying natural gas.

 To produce liquefied natural gas (LNG), throttling fluidization cycles with various methods for pre-cooling natural gas (GH) are widely used.

 The maximum liquefaction coefficient is achieved in cascade refrigeration schemes where individual hydrocarbons or mixtures thereof are used as external refrigerant for cooling a direct gas stream. Due to the use of sophisticated, expensive and energy-intensive equipment, such liquefaction methods prove to be economically viable only when organizing large-scale production of liquefied natural gas, measured in millions of tons / year. The same drawback (the need to use sophisticated expensive equipment) is inherent in small and medium-capacity plants, where technological schemes using internal circulation refrigeration circuits are used, which are based on the principle of isentropic expansion of a part of the flow of liquefied gas in expander units (Heilandt cycle and its variants).

 With regard to facilities that are already reducing pre-compressed natural gas supplied through main gas pipelines - gas reduction stations (GDS) and gas reduction stations (GRP) or its distribution - gas-filling compressor stations (CNG filling stations), the simplest liquefaction process can be applied - the classic throttle cycle . The liquefaction in it is based solely on the regenerative utilization of a non-condensable part of the liquefied stream by a direct flow of high pressure cold gas [1] (prototype). Technologically, it consists in cooling the gas in a recuperative heat exchanger, throttling and separating the resulting vapor-liquid mixture in a condensate collector with the outlet of vapors to the recuperative heat exchanger, and liquid to the consumer.

 The method has several advantages (low cost, ease of implementation, reliability), but is characterized by a small value of the liquefaction coefficient. Due to this reason, an increase in the liquefaction coefficient is usually achieved by introducing additional sources of cooling capacity into the cycle.

 We propose a method for liquefying GHG in the throttle cycle using a refrigeration circuit, which is based on the principle of energy separation of gas in a cooled single or multi-stage vortex tube (OVT).

A specific feature of the OVT design is that its hot end is provided with an external circuit (jacket) into which cooling gas or liquid is supplied. As a result, all (gas μ = 1) entering the OBT nozzle inlet leaves it cooled by 20-35 ^o [2, 3].

OVT does not allow to obtain large cooling effects (ΔT _x ), but differs in increased cooling capacity (μ • ΔT _x ). The high temperature of the peripheral layers of the vortex flow in the OVT (ΔT _g = 100-120 ^o ) when installed at a high temperature level makes it easy to remove heat from them into the environment, which provides a noticeable effect of cooling the entire gas stream even when gas or liquid is supplied to the cooling jacket with the temperature of the cooling medium. The combination of these properties allows the highest possible efficiency to use OVT in the regenerative throttle cycle of GHG liquefaction.

 A schematic diagram of the proposed method is shown in FIG. 1.

 High-pressure natural gas (point 0 in the diagram), being divided into two streams, enters the preliminary heat exchanger (main stream) and OBT, respectively.

 From the OBT, the cooled low-pressure gas (point 8) is introduced into the return flow of the non-condensing GHG in the cycle in front of the T-I heat exchanger (point 6).

 The high-pressure gas (main stream), having passed the preliminary heat exchanger (point 1) and the main heat exchanger (point 2), is throttled (point 3) and enters the condensate collector, where it is separated into liquefied natural gas (point 4) and saturated steam (point 5) .

 The non-condensing cold GHG, leaving the unit, is preheated to ambient temperature, transferring its cold to a direct flow of high-pressure gas (point 6 after the T-II heat exchanger and point 7 after the T-I heat exchanger), and then it is introduced into the jacket of the cooled vortex tube ( OBT).

 To confirm the possibility of carrying out the invention below is its calculated justification.

где

массовая доля холодного потока, выходящего из ОВТ (в данном случае μ = 1,0)

средняя массовая теплоемкость холодного потока, выходящего из ОВТ при изменении параметров между точками 8 и 7 в теплообменнике (Т-I).The basic system of equations of the energy and heat balance of the operation of the constituent elements of the regenerative cycle, according to the accepted designations of nodal points (see Fig. 1), has the form:
(G ₁ + G ₂ ) = K ₀ i '+ (G ₁ + G ₂ - K ₀ ) i ₇ + A • G ₂ (1)
G ₁ • i ₃ = K ₀ • i '+ (G ₁ - K ₀ ) i''(2)
G ₁ (i ₁ -i ₃ ) = (G ₁ -K ₀ ) (i ₆ -i '') (3)
G ₁ (i ₆ -i ₁ ) =) G ₁ + G ₂ -K ₀ ) (i ₇ -i ₆ ) (4)
Legend:
i ₀ is the enthalpy of the input stream into the preliminary heat exchanger (T-I) and into the vortex tube (OVT);
i ₁ - the enthalpy of the output stream from the preliminary heat exchanger (T-I);
i ₃ - the enthalpy of the output stream after throttling on the throttle washer (Др);
i ₆ - enthalpy of the reverse flow of non-condensed steam at the outlet of the main heat exchanger (T-II);
i ₇ is the enthalpy of the total return flow from the OBT and (T-II) at the outlet of the preliminary heat exchanger (T-I);
i '= i ₄ - enthalpy of liquid on the condensation line;
i '' = i ₅ - vapor enthalpy on the condensation line, dimension: [i] = KJ / kg;
G ₁ , dimensionless specific is the flow rate of the main (going to decrease) gas flow;
G ₂ , dimensionless specific - gas flow through OBT;
Note: G ₁ + G ₂ = 1;
K ₀ - GHG liquefaction coefficient in the installation;
A - kJ / kg, specific cooling capacity of OBT, calculated by the equation:

Where

mass fraction of the cold stream leaving the OBT (in this case, μ = 1.0)

average mass heat capacity of the cold stream leaving the OVT when changing the parameters between points 8 and 7 in the heat exchanger (T-I).

температурная эффективность ОВТ,
где ΔT_s= (T₀-T₈) - эффект охлаждения [2, 3] в ОВТ;

эффект охлаждения в идеальном детандере;

степень расширения газа в охлаждаемой вихревой трубе.

temperature efficiency of OVT,
where ΔT _s = (T ₀ -T ₈ ) is the cooling effect [2, 3] in OVT;

cooling effect in an ideal expander;

the degree of expansion of the gas in the cooled vortex tube.

 Solving the system of the above equations (1) ... (4), it is possible to calculate the value of the liquefaction coefficient of natural gas in the installation.

можно оценить интегральную эффективность принятого технического решения.Comparing it with the theoretically achievable value of K _dr for an ideal throttle cycle:

you can evaluate the integrated effectiveness of the technical solution.

от входного давления природного газа для температуры T = 300K приведена на графике (фиг. 2).Parameter Dependence

from the inlet pressure of natural gas for a temperature T = 300K is shown in the graph (Fig. 2).

In this case, two options for the expansion of gas in a cooled vortex tube are considered:
- single-stage (2.0 <P <6.0 MPa),
- two-stage (6.0 <P <20 MPa).

 From the graph it follows that in the input pressure range from 2 to 6 MPa (a single-stage cooled vortex tube) and from 6 to 20 MPa (a two-stage cooled vortex tube), the proposed liquefaction method provides an increase in the real liquefaction coefficient of natural gas against an ideal throttle cycle of at least 1 , 2-1.8 times.

 The expansion of high-pressure gas is carried out stepwise in two or more cooled pipes, the gas inlet of which is the exit from the previous vortex tube.

Literature:
1. Ivantsov O.M., Dvoiris A.D. Low temperature gas pipelines. M., Nedra. 1980, p. 207 - 209.

 2. Merkulov A.P. Vortex effect and its application in technology. M., Mechanical Engineering, 1969, p. 65 to 69.

 3. Dyskin L. M. Vortex thermostats and air dryers. N. Novgorod. NNSU, 1991. 5 - 16.

Claims (2)

 1. The method of liquefying natural gas, which consists in cooling the gas in a recuperative heat exchanger with natural gas not condensed in the cycle, throttling it and separating the resulting vapor-liquid mixture in the condensate collector, characterized in that the initial high-pressure gas stream is divided into two parts, one of which (the main ) first fed into the preliminary, and then into the main heat exchanger, and the other into the vortex tube, which is formed from Lednov cold gas is fed to the pre-heat exchanger for further cooling the main part of the natural gas stream fluidisable.  2. The method according to claim 1, characterized in that the expansion of the high pressure gas is carried out stepwise in two or more cooled pipes, the gas inlet of which is the outlet from the previous vortex tube.

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