Method for calculating in real time the methane number MN in the liquid phase of a liquefied natural gas

Abstract

A method for calculating in real time the methane number of a liquefied natural gas contained in a tank, in particular in an on-board tank.

Claims

1. Method providing the actual value of the methane number MN of a liquefied natural gas for triggering corrective action to the incorrect combustion of hydrocarbons in an engine, or to a system that regulates the engine functioning, of a transporter, the transporter including a transporter tank, the method implemented by computer to calculate in real time the methane number MN of the liquefied natural gas contained in the transporter tank, the transporter tank containing natural gas for use in combustion, the transporter tank including a tank liquid holding space for holding the natural gas in a liquid phase and an expansion space for containing the natural gas in a gaseous phase, the natural gas being distributed into: a layer of natural gas in the liquid state defined at a given instant t by the temperature T(t) thereof and the density p(t) thereof, said layer of natural gas in the liquid state held in the tank liquid holding space and being in balance with a layer of natural gas in the gaseous state (g) contained in the tank expansion space, the layer of natural gas in the gaseous state defined at a given instant t by the pressure P(t) thereof; said method, at a given instant t, including the following steps of: A. positioning at least one temperature sensor in the tank expansion space, positioning a plurality of additional temperature sensors in the tank liquid holding space, positioning a density sensor in the tank liquid holding space, and positioning a pressure sensor in the tank expansion space; B. determining, by measuring, the temperature To(t) and the density po(t) of the layer of natural gas in the liquid state with the plurality of additional temperature sensors and the density sensor in the tank liquid holding space, and the pressure Po(t) of the layer of natural gas in the gaseous state (g) with the pressure sensor in the tank expansion space; C. approaching the calculation of the composition of liquefied natural gas contained in the tank, by a stressed minimum calculation algorithm, said algorithm comprising the following sub-steps: c1) determining, by calculating stressed minimum, a first composition of density po(t) and to pressure Po(t), or a first envelope of compositions having the same density po(t) to pressure Po(t); c2) determining, by calculating stressed minimum, a second envelope of compositions having the same density po(t) to temperature T.sub.0(t) and to pressure Po(t), and of which the temperature balance T.sub.eq(t) to pressure Po(t) is equal to T.sub.0(t); c3) determining a non-singular point of the second envelope giving a composition approaching the real composition of LNG; and c4) calculating the methane number MN from said approaching composition; and providing the engine, or the system that regulates the engine functioning, the correct value of the methane number MN of the natural gas entering the engine, so that the engine, or the system that regulates the engine functioning, triggers the corrective action regarding the engine functioning in general, especially the combustion of the natural gas; and, and triggering the corrective action when the calculated methane number MN indicates the incorrect combustion of hydrocarbons.

2. Method according to claim 1, wherein step c1) for determining the first envelope of compositions is carried out as follows, by seeking to resolve the following equations: $\min \rho \left(x,P•\right)-{\rho }_0$ $s.t.\{\begin{array}{c}\rho \left(x,P•\right)-{\rho }_0\ge 0\\ .Math.x_i=1\\ x_i^l\le x_i\le x_i^u\end{array}$ with: p meaning the density calculated from x and Po, p0 meaning the density measured, Po meaning the pressure measured, x meaning the composition vector, composed of x.sub.i, x.sub.i meaning the molar fraction of the component i, the exponents 1 and u respectively making reference to the lower and upper limit of this molar fraction.

3. Method according to claim 1, wherein step c1) for determining the first envelope of compositions is carried out as follows, by seeking to resolve the following equations: $\min \left({\left(\rho \left(x,P•\right)-{\rho }_0\right)}^2\right)$ $s.t.\{\begin{array}{c}.Math.x_i=1\\ x_i^l\le x_i\le x_i^u\end{array}$ with: p meaning the density calculated from x and Po, p0 meaning the density measured, Po meaning the pressure measured, x meaning the composition vector, composed of xi, x.sub.i meaning the molar fraction of the component i, the exponents 1 and u respectively making reference to the lower and upper limit of this molar fraction.

4. Method according to claim 1, wherein step c2) for determining the second envelope of compositions is carried out as follows by seeking to resolve the following equations: $\min T_{\mathrm{eq}}\left(x,P•\right)-T_0$ $s.t.\{\begin{array}{c}{T}_{\mathrm{eq}}\left(x,P•\right)-T_0\ge 0\\ \rho \left(x,P•\right)-{\rho }_0=0\\ .Math.x_i=1\\ x_i^l\le x_i\le x_i^u\end{array}$ with: T.sub.0 meaning the temperature measured, T.sub.eq meaning the temperature balance of LNG at pressure Po, p meaning the density calculated from x and Po, p0 meaning the density measured, Po meaning the pressure measured, x meaning the composition vector, comprised of x.sub.i, x.sub.i meaning the molar fraction of the component i, the exponents 1 and u respectively making reference to the lower and upper limit of this molar fraction.

5. Method according to claim 1, wherein step c2) for determining of the second envelope of compositions and carried out as follows by seeking to resolve the following equations: $\min \left({\left(T_{\mathrm{eq}}\left(x,P•\right)-T_0\right)}^2\right)$ $s.t.\{\begin{array}{c}\rho \left(x,P•\right)-{\rho }_0=0\\ .Math.x_i=1\\ x_i^l\le x_i\le x_i^u\end{array}$ with: T.sub.0 meaning the temperature measured, T.sub.eq meaning the temperature balance of LNG at pressure P.sub.0, p meaning the density calculated from x and Po, p0 meaning the density measured, Po meaning the pressure measured, x meaning the composition vector, comprised of x.sub.i, x.sub.i meaning the molar fraction of the component i, the exponents 1 and u respectively making reference to the lower and upper limit of this molar fraction.

6. Method according to any one of the claim 1, wherein the non-singular point of the second envelope is the barycenter of the second envelope or the point of the second envelope which is the closest to the barycenter of the second envelope.

7. Method according to claim 1, wherein the non-singular point of the second envelope is calculated as the re-standardized average of the standardized compositions of the second envelope.

Description

DETAILED DESCRIPTION

(1) More specifically, FIG. 1 shows a horizontal tank 1 containing natural gas being distributed into: a layer of natural gas 11 in the liquid state 1 defined at a given instant t by the temperature T(t) thereof and the density p(t) thereof, this layer of natural gas 11 being in balance with a layer of natural gas 12 in the gaseous state g, defined at a given instant t by the pressure P(t).

(2) The tank 1 comprises: a plurality of temperature sensors 21, 22, 23, 24 to determine the temperature T.sub.0(t) of the LNG, these sensors are arranged on a support 25, one of them 21 being located in the gaseous phase (or expansion space) of the LNG, and three of them being located in the liquid phase (the temperature of the gas is not considered in the calculation), a density sensor 26 to determine the density p.sub.0(t) of LNG in the liquid state (one single sensor is sufficient as the LNG in the liquid state has a homogenous density), and a pressure sensor 31 to determine the pressure Po(t) of the expansion space.

(3) Once measured at each instant, the temperature values T.sub.0(t), of density p.sub.0(t) and the pressure Po(t), these are injected in a calculator implementing the method according to the invention to define a composition approaching LNG. The results of these simulations are presented in example 1 below.

(4) The calculation method according to the invention illustrated in more detail in the example below.

EXAMPLE

(5) The example below is made with a limited number of points (envelope of 6 compositions) for educational purposes in order to facilitate the reading thereof and the understanding of the argument by a person skilled in the art. They can easily understand, by simply deducting that by increasing the number of measuring points, a result of the number is arrived at, of which the precision will increase with the number of measuring points.

(6) The inlet parameters of the method according to the invention are as follows: T.sub.0(t)=−160.76° C.; p.sub.0(t)=448.11 g/cm.sup.3; Po(t)=1190 mbar.

(7) From these values, an LNG composition is thus determined, having the density p.sub.0(t) measured at the pressure measured Po(t) (step b1). This first composition is given in Table 1 below.

(8) TABLE-US-00001 TABLE 1 first composition LNG components % mol (first composition) CH.sub.4 91.09 C.sub.2H.sub.6 7.14 C.sub.3H.sub.8 1.54 iC.sub.4 0.117 nC.sub.4 0.01 iC.sub.5 0 nC.sub.5 0 C.sub.6 0 N.sub.2 0

(9) For this first composition, the temperature balance has been calculated by a conventional phase envelope calculation method (for example, the Rachford-Rice method).sup.[2], which is −158.36° C. The density p(t) is recalculated for verification: p(t)=448.11 g/cm.sup.3 for T.sub.0(t)=−160.76° C.

(10) Then, from this first composition calculated in step b1, an envelope of compositions is determined, of which the temperature balance T.sub.eq(t) at pressure Po(t) is equal to the temperature measured T.sub.0(t), that is −160.76° C. (step b2). This envelope comprises six compositions C1 to C6, which are detailed in Table 2 below:

(11) TABLE-US-00002 TABLE 2 envelope of compositions for which T.sub.eq(t) = T.sub.0(t) LNG % mol % mol % mol % mol % mol % mol composition (C1) (C2) (C3) (C4) (C5) (C6) CH.sub.4 91.89 90.07 92.56 91.8 92.57 92.14 C.sub.2H.sub.6 5.97 8.52 5 6.16 5 5.7 C.sub.3H.sub.8 1.45 0.267 1.22 1.35 1.22 1.15 iC.sub.4 0 0 0.27 0.002 0.27 1.15 nC.sub.4 0 0 0.27 0.002 0.27 0 iC.sub.5 0 0 0 0 0 0 nC.sub.5 0 0 0 0 0 0 C.sub.6 0 0 0 0 0 0 N.sub.2 0.679 0.721 0.654 0.682 0.654 0.67

(12) For this envelope of compositions C1 to C6, the temperature balance has been calculated (also by a conventional phase envelope calculation method such as the Rachford-Rice method).sup.[2]): T.sub.eq(t)=−160.76° C., that is the temperature T.sub.0 measured, which shows that the calculation is carried out correctly.

(13) From this envelope of compositions C1 to C6, a non-singular point is determined from the second envelope giving a composition approaching the real composition of LNG. The methane number MN is then calculated (step b4) from the approaching composition obtained in step b3.

(14) In a first case, the barycenter giving a composition approaching the real composition of LNG as taken as a non-singular point (step b3), given in table 3 below:

(15) TABLE-US-00003 TABLE 3 composition of the barycenter LNG components % mol CH.sub.4 91.8383333 C.sub.2H.sub.6 6.05833333 C.sub.3H.sub.8 1.1095 iC.sub.4 0.282 nC.sub.4 0.09033333 iC.sub.5 0 nC.sub.5 0 C.sub.6 0 N.sub.2 0.67666667

(16) The methane number MN is then calculated (step b4) from the approaching composition obtained in step b3. This calculation is simple and uses standard calculation methods of the LNG industry (CARB, GRI H/C, linear coefficient GRI, and MON/KIWA, MWM, AVL), which are empirical methods well-known to a person skilled in the art based on different experiences and engine types. The MWM and AVL methods are the most commonly used public methods in the industry.

(17) The results of these calculations are indicated in Table 4 below for each of the different methods for calculating the methane number, from the composition of Table 3.

(18) TABLE-US-00004 TABLE 4 methane number MN MN (CARB method) 86.9427269 MN (GRI H/C method) 79.9123525 MN (linear coefficient GRI method) 81.1445809 MON/KIWA method 78.9080193 MWM method 78 AVL method 78.7

(19) It is noted, that the different methods give different results. This is normal as there is no agreement today in the industry on the method to be used to calculate the methane number. Different methods exist, and each company has a predilection method. Other calculation methods, less expansive, exist and can all also be used from the composition determined above.

(20) In a second case, from the envelope of compositions C1 to C6 obtained above, the re-standardized average of the standardized compositions of the second envelope is taken as a non-singular point, of which the composition is given in Table 5 below:

(21) TABLE-US-00005 TABLE 5 second composition LNG components % mol CH.sub.4 91.7873393 C.sub.2H.sub.6 6.05721857 C.sub.3H.sub.8 1.10826957 iC.sub.4 0.28047889 nC.sub.4 0.09035225 iC.sub.5 0 nC.sub.5 0 C.sub.6 0 N.sub.2 0.67634147

(22) The methane number MN is then calculated (step b4) from this approaching composition obtained in step b3 (given in Table 5) by using the standard calculation methods of the LNG industry described above (MWM, AVL). The results of the methane number calculations are indicated in Table 6 below for each of the different methods used, from the composition of Table 5.

(23) TABLE-US-00006 TABLE 6 methane number MN MN (CARB method) 86.9479438 MN (GRI H/C method) 79.9169944 MN (linear coefficient GRI method) 81.0463815 MON/KIWA method 78.9166216 MWM method 77 AVL method 78.7

(24) It is observed that the values calculated in Table 6 are close to those of Table 4.

(25) All the differences between Table 4 and Table 6 are less than 0.2%, except for that for the MWM method, of 1.3%. The MWM method is known in the industry to not be a stable method and give difference of absolute 1 for very close compositions, therefore this result could be expected.

LIST OF REFERENCES

(26) [1] http://www.giignl.org/publications [2] Procedure for use of electronic digital computers in calculating flash vaporization hydrocarbon equilibrium. RACHFORD Jr, H. H., & RICE, J. D. s.l.: Journal of Petroleum Technology, 1952, Vol. 4(10), 19-3.