METHOD AND SYSTEM FOR CALCULATING, IN REAL-TIME, THE DURATION OF AUTONOMY OF A NON-REFRIGERATED TANK CONTAINING LNG

Abstract

This invention relates to a method and a system for calculating in real-time the duration of autonomy of a non-refrigerated tank containing natural gas comprising a liquefied natural gas (LNG) layer and a gaseous natural gas (GNG) layer. This invention also relates to a system for calculating, in real time, according to the method of the invention, the duration of autonomy of a non-refrigerated tank, as well as a vehicle comprising an NG tank and a system according to the invention.

Claims

1. A method for calculating in real-time the duration of autonomy of a non-refrigerated tank and defined by a set pressure of the valves p.sub.valve, its shape and its dimensions, as well as its boil off rate, said tank containing natural gas divided into: a layer of natural gas in liquid state (l), defined at a given instant t by its temperature T.sub.liq(t), its composition x.sub.liq(t), and the filling rate of the tank by said natural gas layer; a natural gas layer in gaseous state (g), defined at a given instant t by its temperature T.sub.gas(t) and its composition x.sub.gas(t), and a pressure p(t); said method being characterized in that it consists of an algorithm comprising the following steps: a) at an instant t0, the physical parameters of said natural gas layers are initialized, by measuring using pressure and temperature sensors, the pressure of the gas p(t0), and the temperature of the liquid T.sub.liq(t0); while the respective compositions of the liquid x.sub.liq(t0) and gaseous x.sub.gas(t0) phases are known input data corresponding either to the respective compositions of the liquid and gaseous phases at the time of the loading of the tank, or to average compositions for the type of LNG used; b) for each instant t greater than t0, a predetermined volume of natural gas in the gaseous or liquid state is subtracted, said volume corresponding to the operating state of the tank at this instant t; and a calculation is made, based on the volume of natural gas remaining after subtraction, of the physical parameters p(t), T.sub.gas(t), and T.sub.liq(t), using equations based on the conservation of the mass and of the energy of the liquid and gaseous natural gas contained in the tank; c) as long as the pressure p(t) is less than p.sub.valve, the calculation of the step B is reiterated for the following instant t+?t, with a constant physical time step ?t. d) as soon as during the N iterations of the calculation process of p(t), p(t+?t), . . . , p(t+N*?t), the pressure p(t+N*?t) becomes greater than or equal to p.sub.valve, the calculation is stopped; e) the duration of autonomy sought is equal to the total duration N*?t elapsed by the algorithm at the moment of the stoppage of the calculation.

2. The method according to claim 1, wherein all of the steps a-d are reiterated as soon as time interval ?T has elapsed, in order to recalculate the duration of autonomy at the instant t.sub.0+?T

3. The method according to claim 1, wherein the calculation at the step b of the physical parameters p(t), T.sub.gas(t), and T.sub.liq(t) is carried out according to the steps defined as followed. the temperature of the liquid phase T.sub.liq(t) and of the gaseous phase T.sub.gas(t) are directly determined using the power conversion equation, with as input data the thermal capacities of the natural gas in liquid state and of the natural gas in the gaseous state, the thermal insulation of the tank defined by the manufacturer of the tank and the temperatures at the instant t??t of the liquid LNG and of the gaseous LNG, the mass of liquid evaporated in the gaseous phase is determined by the relationship (5) according to the temperature of the liquid and the pressure determined in the preceding step at the instant t??t:
q.sub.ev=K.Math.(?T.sub.surchauffe).sup.? with: designating a constant relative to the LNG and always being positive, ?T.sub.overheat designating the overheating that is produced during the evaporation phenomenon in the tank of LNG, Q.sub.ev designating the standardized evaporation rate of LNG, and ? designating a coefficient relative to the LNG, with 1?a?2; a coefficient relative to the LNG, with 1?a?2; the pressure p(t) of the gaseous phase is obtained by the Peng-Robinson equation, with as input data the evaporated mass of liquid, the volume of the tank and the temperature of the gas at the instant t.

4. The method according to claim 1, wherein the algorithm is implemented by means of a calculator that calculates the duration of autonomy of the tank, said calculator being connected to a MMI interface that makes it possible to inform an operator as to this duration of autonomy.

5. A system for calculating in real time, according to the method of claim 3, the duration of autonomy of a non-refrigerated tank and defined by a set pressure of the valves p.sub.valve, its shape and its dimensions, as well as its boil off rate, said system comprising: a tank containing liquefied natural gas divided into: a layer of natural gas in liquid state, defined at a given instant t by its temperature T.sub.liq(t), its composition x.sub.liq(t), and the filling rate of the tank by said natural gas layer in the liquid state; a natural gas layer in gaseous state, defined at a given instant t by its temperature T.sub.gas(t) and its composition x.sub.gas(t) and a pressure p(t); pressure and temperature sensors, said system being characterized in that it is an onboard system further comprising: an onboard calculator (5) connected to said pressure (3) and temperature (4) sensors, said calculator being designed to execute the algorithm of the method, wherein the algorithm is implemented by means of a calculator that calculates the duration of autonomy of the tank, said calculator being connected to a MMI interface that makes it possible to inform an operator as to this duration of autonomy, the MMI interface (6), of the onboard dashboard type of a vehicle, interacting specifically with said onboard calculator (5), to report to an operator (7) the duration of autonomy calculated by means of a calculator connected to the MMI interface that makes it possible to inform the operator as to this duration of autonomy.

6. A vehicle comprising an NG tank and a system such as defined according to claim 4.

Description

[0126] Other advantages and particularities of this invention shall result from the following description, provided as a non-limiting example and made in reference to the annexed figures:

[0127] FIG. 1 shows a block diagram of a tank 1 of NG according to the invention;

[0128] FIG. 2 shows a block diagram of the system according to the invention,

[0129] FIG. 3 shows a block diagram of the method according to the invention,

[0130] FIGS. 4 to 8 are screen captures of dashboards of vehicles each transporting an unrefrigerated tank of N.

[0131] FIG. 1 diagrammatically shown a tank 1 of LNG, which is modelled by a two-layer system with two homogenous layers of NG, a liquid layer 1 (LNG) and a gaseous g layer (GNG).

[0132] FIG. 2 is a block diagram of the system according to the invention, comprising: [0133] a tank 1 containing liquefied natural gas being divided into [0134] a layer of natural gas in liquid state 1 (T.sub.liq(t), x.sub.liq(t), and filling rate z of the tank 1 by the layer of natural gas in the liquid state); [0135] a layer of natural gas g in the gaseous state g (T.sub.gas(t), x.sub.gas(t) and p(t); [0136] pressure 3 and temperature 4 sensors, [0137] a calculator 5 connected to said pressure 3 and temperature 4 sensors, the calculator being able to execute the algorithm of the method such as defined according to claim 4, [0138] a MMI interface 6 interacting with the calculator, to report to a given operator 7 the duration of autonomy calculated according to the method of claim 4.

[0139] FIG. 3 is a block diagram of the method according to the invention, showing the various steps of the method as described hereinabove.

[0140] FIGS. 4 to 8 are screen captures of dashboards of vehicles each transporting a non-refrigerated tank of LNG.

[0141] In particular, FIG. 4 is a screen capture of a dashboard showing the input data specific to the tank (dimensions, boil off rate, maximum allowable pressure). This data is common to all of the examples described hereinafter.

[0142] FIG. 5 is a screen capture of a dashboard showing, for a first example of calculation according to the method of calculation according to the invention, the input data specific to an LNG (composition, temperature, pressure and filling rate z. In this example, the LNG is slightly overheated: temperature of ?160? C. although the equilibrium temperature for this LNG is ?162.31? C.

[0143] FIG. 6 is a screen capture of a dashboard showing, for a second calculation example according to the method of calculation according to the invention, the input data specific to an LNG (composition, temperature, pressure and filling rate z. In this example, the LNG is slightly sub-cooled: temperature of ?157? C. while the equilibrium temperature for, this LNG is ?154.17? C.

[0144] FIGS. 7 and 8 are screen captures giving, respectively for each one of the first (data of FIGS. 4 and 5) and second examples (data of FIGS. 4 and 6), the calculated duration of autonomy of the non-refrigerated tank transported by the vehicle.

LIST OF REFERENCES

[0145] [1] Peng, D. Y. (1976). A New Two-Constant Equation of State. Industrial and Engineering Chemistry: Fundamentals, 15: 59-64. [0146] [2] H. T Hashemi, H. W. (1971). CUT LNG STORAGE COSTS. Hydrocarbon Processing, 117-120.