A natural gas turbine expander outlet state prediction method based on coupling of structural and thermodynamic parameters

Abstract

The present invention provides a method for predicting the outlet state of a natural gas turbine expander by coupling structure and thermodynamic parameters, and belongs to the technical field of thermal process modeling and calculation of non-variable volume expansion devices. The method calculates the theoretical outlet state under an ideal isentropic expansion process by obtaining the expander structural parameters, operating conditions, and thermophysical property data of a natural gas multi-component mixture, and estimates the total enthalpy loss during the expansion process. The outlet enthalpy value is corrected based on the principle of conservation of energy to obtain the actual outlet thermal state. Furthermore, a constant pressure and constant enthalpy multi-component flash evaporation calculation is performed based on the actual enthalpy value to obtain the outlet temperature, gas phase ratio, liquid phase ratio, and molar component distribution of the gas and liquid phases. The present invention can accurately predict the outlet state parameters of a non-variable volume natural gas expansion device under typical operating conditions, provide theoretical support for the design and optimization of processes such as liquefied natural gas production, pressure energy recovery power generation, and deep cold separation of light hydrocarbons, and improve system efficiency and energy recovery levels.

Description

Technical Field

[0001] The present invention belongs to the technical field of thermodynamic process modeling and calculation of non-variable capacity expansion devices, and relates to a method for predicting the outlet state of a natural gas turbine expander by coupling structure and thermodynamic parameters. Background Art

[0002] Natural gas turbine expanders are core equipment in processes such as liquefied natural gas (LNG) production, natural gas pressure energy recovery for power generation, and cryogenic separation of light hydrocarbons. Accurately predicting their outlet state parameters has a significant impact on process system design optimization, energy recovery efficiency, and operational safety. Overestimating the turbine expander outlet temperature can lead to improper downstream equipment selection, impacting separation efficiency and product quality. Inaccurately predicting the outlet liquid fraction can lead to incorrect capacity configuration of gas-liquid separation equipment, causing process instability and even equipment damage and safety accidents. Therefore, establishing an accurate method for predicting the outlet state of natural gas turbine expanders is crucial for improving process design and operational reliability.

[0003] In actual engineering applications, the complex multiphase flow and heat transfer processes within natural gas turbine expanders make it difficult to directly measure the outlet state parameters. Existing prediction methods mainly include the ideal isentropic model method, the empirical efficiency correction method, CFD numerical simulation method, and artificial neural network method. Although the ideal isentropic model method is computationally simple, it ignores the energy loss during the actual expansion process and has low prediction accuracy. The empirical efficiency correction method uses total efficiency or variable efficiency for correction, but fails to achieve dynamic coupling with specific structural parameters and operating conditions, and its applicability is limited. Although the CFD numerical simulation method has high accuracy, it is computationally intensive and cannot meet the needs of real-time engineering calculations. The artificial neural network method requires a large amount of training data and lacks physical mechanism support, resulting in insufficient generalization ability. Summary of the Invention

[0004] The purpose of the present invention is to provide a natural gas turbine expander outlet state prediction method that couples structure and thermodynamic parameters to solve the technical problem of accurately predicting the natural gas turbine expander outlet state parameters in actual engineering applications while considering the actual thermophysical properties, energy loss mechanism and multi-component phase equilibrium.

[0005] The present invention provides a method for predicting the outlet state of a natural gas turbine expander by coupling structure and thermodynamic parameters, comprising:

[0006] Step 1: Collect the impeller structural parameters of the natural gas turbine expander;

[0007] Step 2: Collect the operating parameters of the natural gas turbine expander;

[0008] Step 3: Obtain the thermophysical properties of the natural gas multi-component mixture through the thermophysical properties database;

[0009] Step 4: Calculate the theoretical outlet state parameters of the ideal isentropic expansion process;

[0010] Step 5: Calculate the total loss enthalpy increment during the expansion process;

[0011] Step 6: According to the law of conservation of energy, consider the effect of energy loss on the outlet enthalpy and calculate the actual outlet enthalpy;

[0012] Step 7: Determine the gas-liquid phase equilibrium state, use the actual outlet enthalpy value to perform constant pressure and constant enthalpy multi-component flash calculation, and output the natural gas turbine expander outlet temperature, gas phase ratio, liquid phase ratio and gas-liquid two-phase composition.

[0013] Furthermore, the impeller structural parameters of the natural gas turbine expander include: impeller inlet diameter , impeller outlet diameter , flow channel length and design speed ;

[0014] The operating parameters of the natural gas turbine expander include: inlet pressure , inlet temperature , outlet pressure , mass flow and the mole fraction of each component of natural gas ;

[0015] The thermophysical parameters of the natural gas multi-component mixture include: inlet enthalpy , entrance entropy value and entrance density .

[0016] Furthermore, the step 4 is specifically as follows:

[0017] Step 4.1: Based on the isentropic process conditions, solve the ideal outlet temperature through numerical iteration method :

[0018]

[0019] Where, at a known outlet pressure and the mole fraction of each component of natural gas Under the condition of ;

[0020] Step 4.2: Calculate the ideal outlet enthalpy:

[0021]

[0022] Where, at a known outlet pressure , mole fraction of each component of natural gas and ideal outlet temperature Under the condition of .

[0023] Furthermore, in step 5, the friction loss, flow separation loss, centrifugal loss and total loss enthalpy increment are calculated according to the following formula:

[0024]

[0025]

[0026]

[0027]

[0028] in, is the friction loss, is the flow separation loss, is the centrifugal loss, is the total loss enthalpy increment; is the friction coefficient, is the flow channel length, is the separation loss coefficient, is the centrifugal loss coefficient, is the hydraulic diameter, is the inlet flow velocity, is the outlet flow rate, is the circumferential velocity at the impeller inlet, is the average circumferential speed of the impeller, is the peripheral speed at the impeller outlet;

[0029]

[0030]

[0031]

[0032]

[0033]

[0034]

[0035]

[0036]

[0037] in, The ideal outlet temperature The corresponding outlet density, A 1 is the impeller inlet area, A 2 is the impeller outlet area.

[0038] Furthermore, the step 6 is specifically as follows:

[0039]

[0040] in, is the actual outlet enthalpy, is the ideal outlet enthalpy, is the total loss enthalpy increase.

[0041] Furthermore, the step 7 is specifically as follows:

[0042] Step 7.1: Given the natural gas turbine expander outlet pressure and the mole fraction of each component of natural gas Under the condition of and dew point enthalpy ;

[0043] Step 7.2: Judgment and the relative size of

[0044] Step 7.3: If , then the multi-component mixture is gaseous, calculated according to the thermophysical properties database Corresponding temperature , directly output the final temperature and the mole fraction of each component of natural gas ;

[0045] Step 7.4: If , the multi-component mixture is in a gas-liquid two-phase state, and iterative calculation is performed. When the gas phase rate and outlet temperature converge, the natural gas turbine expander outlet temperature, gas phase rate, liquid phase rate and gas-liquid two-phase components are output.

[0046] Furthermore, the step 7.4 is specifically as follows:

[0047] Step 7.4.1: Set the initial gas phase rate , convergence criteria , gas phase rate iteration step ;

[0048] Step 7.4.2: Use the value assigned in step 7.4.1 for the gas phase rate and assign the initial value to the outlet temperature. , temperature iteration step ; At a known outlet pressure , outlet temperature Under the thermophysical properties database, the gas-liquid equilibrium constant of each component is obtained. ;

[0049] Step 7.4.3: Calculate the mole fractions of each component in the gas and liquid phases at the current gas phase fraction:

[0050]

[0051]

[0052] in, For the The gas-liquid equilibrium constant of the component, For the gas phase The mole fraction of the component, The first mole fraction of the component;

[0053] Step 7.4.4: Query the thermophysical properties database to obtain the Gas phase fugacity of components Hedi Liquid phase fugacity of components ;

[0054] Step 7.4.5: If all components meet , then the outlet temperature converges and the current outlet temperature is obtained = , the mole fraction of the gas phase component and the mole fraction of the liquid phase component , proceed to step 7.4.6; if the outlet temperature does not converge, return to step 7.4.2 and reassign the initial value of the outlet temperature. , continue iterative calculation until the outlet temperature converges;

[0055] Step 7.4.6: Use the calculated current outlet temperature Query the thermophysical properties database to obtain the enthalpy of each component , and calculate the total gas phase enthalpy and total liquid enthalpy :

[0056]

[0057]

[0058] Calculate the current gas phase rate Total mixing enthalpy under :

[0059]

[0060] Step 7.4.7: If , then convergence; otherwise, return to step 7.4.1 and re-assign the initial value of the gas phase rate, let Repeat steps 7.4.2 to 7.4.7 until convergence, and output the final gas phase rate , liquid phase rate , outlet temperature , gas phase component mole fraction and liquid phase mole fraction .

[0061] The method for predicting the outlet state of a natural gas turbine expander by coupling structure and thermodynamic parameters of the present invention has at least the following beneficial effects:

[0062] (1) The present invention proposes a method for calculating the outlet state of a natural gas turbine expander based on the principle of conservation of energy and multi-component phase equilibrium theory, combining the structural parameters of the turbine expander with the thermodynamic parameters. This method can accurately predict key parameters such as the outlet temperature, gas-liquid phase ratio, and isentropic efficiency. The method has a solid theoretical basis, high calculation accuracy, and is easy to apply in engineering.

[0063] (2) The present invention can be used to predict outlet status in natural gas systems with different components, turbine expander equipment of different specifications, and under various operating conditions. There is no need to change the existing calculation framework; only the turbine expander structural parameters, operating parameters, and natural gas composition information need to be input according to the actual situation. Therefore, the present invention has wide applicability in the fields of liquefied natural gas production, natural gas pressure energy recovery for power generation, and cryogenic separation of light hydrocarbons.

[0064] (3) The prediction method established in this invention takes into account the real gas properties, multi-component effects, detailed energy loss mechanisms, and gas-liquid phase equilibrium behavior. Compared with the traditional ideal isentropic model or empirical efficiency correction method, the prediction accuracy is significantly improved, providing a reliable theoretical basis for process design optimization and equipment selection. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figure 1 The present invention is a flow chart of a method for predicting the outlet state of a natural gas turbine expander by coupling structure with thermodynamic parameters. DETAILED DESCRIPTION

[0066] The present invention provides a method for predicting the outlet state of a natural gas turbine expander by coupling structure and thermodynamic parameters. By collecting the structural parameters and operating parameters of the natural gas turbine expander impeller, combining them with a multi-component natural gas thermophysical property database, and adopting a multi-dimensional flow-thermodynamic coupling algorithm, the temperature, phase state and performance parameters at the outlet of the natural gas turbine expander are predicted in real time.

[0067] like Figure 1As shown, a method for predicting the outlet state of a natural gas turbine expander by coupling structure and thermodynamic parameters of the present invention includes:

[0068] Step 1: Collect the impeller structural parameters of the natural gas turbine expander, including: impeller inlet diameter , impeller outlet diameter , flow channel length and design speed In this embodiment, the impeller geometric parameters are shown in Table 1.

[0069] Table 1 Geometric parameters of natural gas turbine expander impeller

[0070]

[0071] Step 2: Collect the operating parameters of the natural gas turbine expander, including: inlet pressure , inlet temperature , outlet pressure , mass flow and the mole fraction of each component of natural gas .

[0072] This embodiment takes a radial turbine expander as an example, and the internal working fluid is a mixture of methane, ethane, propane, n-butane and nitrogen. The specific values ​​are shown in Table 2 and Table 3.

[0073] Table 2 Operating parameters of the natural gas turbine expander in this embodiment

[0074]

[0075] Step 3: Obtain the thermophysical parameters of the natural gas multi-component mixture through the thermophysical property database, including: inlet enthalpy , entrance entropy value and entrance density .

[0076] Table 3 Thermophysical properties of natural gas mixture in this embodiment

[0077]

[0078] Calculation method of thermophysical parameters:

[0079] Inlet enthalpy calculation: ;

[0080] Entropy calculation at the entrance: ;

[0081] Inlet density calculation: .

[0082] Specifically, in the known The inlet enthalpy, inlet entropy, and inlet density were calculated using the CoolProp thermodynamic database under the following conditions: The enthalpy reference state was the standard reference state defined in the database, and all thermodynamic parameters were calculated using the International System of Units (SI) units.

[0083] Step 4: Calculate the theoretical outlet state parameters of the ideal isentropic expansion process, specifically:

[0084] Step 4.1: Based on the isentropic process conditions, solve the ideal outlet temperature through numerical iteration method :

[0085]

[0086] Where, at a known outlet pressure and the mole fraction of each component of natural gas Under the condition of .

[0087] Step 4.2: Calculate the ideal outlet enthalpy:

[0088]

[0089] Where, at a known outlet pressure , mole fraction of each component of natural gas and ideal outlet temperature Under the condition of .

[0090] In specific implementation, the desired outlet temperature is solved iteratively using the thermodynamic database. =154.1K; ideal outlet enthalpy =-390.6kJ / kg.

[0091] Step 5: Calculate the total enthalpy loss during the expansion process.

[0092] In this embodiment, friction loss, flow separation loss, centrifugal loss and total loss enthalpy increment are calculated according to the following formula:

[0093]

[0094]

[0095]

[0096]

[0097] in, is the friction loss, is the flow separation loss, is the centrifugal loss, is the total loss enthalpy increment; is the friction coefficient, is the flow channel length, is the separation loss coefficient, is the centrifugal loss coefficient, is the hydraulic diameter, is the inlet flow velocity, is the outlet flow rate, is the circumferential velocity at the impeller inlet, is the average circumferential speed of the impeller, is the peripheral speed at the impeller outlet;

[0098]

[0099]

[0100]

[0101]

[0102]

[0103]

[0104]

[0105]

[0106] in, The ideal outlet temperature The corresponding outlet density, A 1 is the impeller inlet area, A 2 is the impeller outlet area.

[0107] In this embodiment, the friction loss , the flow separation loss is , the centrifugal loss is Total loss enthalpy increment .

[0108] Step 6: According to the law of conservation of energy, considering the effect of energy loss on the outlet enthalpy, calculate the actual outlet enthalpy, specifically:

[0109]

[0110] in, is the actual outlet enthalpy, is the ideal outlet enthalpy, is the total loss enthalpy increase.

[0111] Step 7: Determine the gas-liquid phase equilibrium state, use the actual outlet enthalpy value to perform constant pressure and constant enthalpy multi-component flash calculation, and output the natural gas turbine expander outlet temperature, gas phase ratio, liquid phase ratio, and gas-liquid two-phase composition, specifically:

[0112] Step 7.1: Given the natural gas turbine expander outlet pressure and the mole fraction of each component of natural gas Under the condition of and dew point enthalpy ;

[0113] Step 7.2: Judgment and the relative size of

[0114] Step 7.3: If , then the multi-component mixture is gaseous, calculated according to the thermophysical properties database Corresponding temperature , directly output the final temperature and the mole fraction of each component of natural gas ;

[0115] Step 7.4: If , the multi-component mixture is in a gas-liquid two-phase state, and iterative calculation is performed. When the gas phase rate and outlet temperature converge, the natural gas turbine expander outlet temperature, gas phase rate, liquid phase rate and gas-liquid two-phase components are output.

[0116] In this embodiment, , so iterative calculation is performed, including:

[0117] Step 7.4.1: Set the initial gas phase rate , convergence criteria , gas phase rate iteration step ;

[0118] Step 7.4.2: Use the value assigned in step 7.4.1 for the gas phase rate and assign the initial value to the outlet temperature. , temperature iteration step ; At a known outlet pressure , outlet temperature Under the thermophysical properties database, the gas-liquid equilibrium constant of each component is obtained. ;

[0119] Step 7.4.3: Calculate the mole fractions of each component in the gas and liquid phases at the current gas phase fraction:

[0120]

[0121]

[0122] in, For the The gas-liquid equilibrium constant of the component, For the gas phase The mole fraction of the component, The first mole fraction of the component;

[0123] Step 7.4.4: Query the thermophysical properties database to obtain the Gas phase fugacity of components Hedi Liquid phase fugacity of components ;

[0124] Step 7.4.5: If all components meet , then the outlet temperature converges and the current outlet temperature is obtained = , the mole fraction of the gas phase component and the mole fraction of the liquid phase component , proceed to step 7.4.6; if the outlet temperature does not converge, return to step 7.4.2 and reassign the initial value of the outlet temperature. , continue iterative calculation until the outlet temperature converges;

[0125] Step 7.4.6: Use the calculated current outlet temperature Query the thermophysical properties database to obtain the enthalpy of each component , and calculate the total gas phase enthalpy and total liquid enthalpy :

[0126]

[0127]

[0128] Calculate the current gas phase rate Total mixing enthalpy under :

[0129]

[0130] Step 7.4.7: If , then convergence; otherwise, return to step 7.4.1 and re-assign the initial value of the gas phase rate, let Repeat steps 7.4.2 to 7.4.7 until convergence, and output the final gas phase rate , liquid phase rate , outlet temperature , gas phase component mole fraction and liquid phase mole fraction .

[0131] In this embodiment, the gas phase rate =96.9%, liquid phase rate( ) =3.1%, outlet temperature =161.0K. Impeller isentropic efficiency: = 89.9%

[0132] Through the structural and thermodynamic parameter coupling prediction method described in the above technical solutions and embodiments of the present invention, the outlet state of a radial natural gas turbine expander under cryogenic conditions can be accurately predicted, with an isentropic efficiency of 89.9% and a liquid phase fraction of 3.1%, which are highly consistent with similar radial turbine expanders in the industry (88%-92%).

[0133] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A method for predicting the outlet state of a natural gas turbine expander by coupling structure and thermodynamic parameters, characterized in that: include: Step 1: Collect the impeller structural parameters of the natural gas turbine expander, including: impeller inlet diameter D1, impeller outlet diameter D2, flow channel length L and design speed ω; Step 2: Collect the operating parameters of the natural gas turbine expander, including: inlet pressure P1, inlet temperature T1, outlet pressure P2, mass flow rate m and mole fraction z of each natural gas component i ; Step 3: Obtain the thermophysical properties of the natural gas multi-component mixture through the thermophysical property database, including: inlet enthalpy H1, inlet entropy S1 and inlet density ρ1; Step 4: Calculate the theoretical outlet state parameters of the ideal isentropic expansion process; given the outlet pressure P2 and the mole fraction z of each component of natural gas i Under the condition of 2,ideal ; Given the outlet pressure P2 and the mole fraction of each component of natural gas z i And the ideal outlet temperature T 2,ideal Under the condition of 2,ideal ; Step 5: Calculate the total enthalpy loss increment during the expansion process based on the impeller structural parameters, the operating parameters of the natural gas turbine expander, and the thermophysical parameters; Step 6: According to the law of conservation of energy, consider the effect of energy loss on the outlet enthalpy and calculate the actual outlet enthalpy; Step 7: Determine the gas-liquid phase equilibrium state, use the actual outlet enthalpy value to perform constant pressure and constant enthalpy multi-component flash calculation, and output the natural gas turbine expander outlet temperature, gas phase ratio, liquid phase ratio, and gas-liquid two-phase composition; The step 6 is specifically as follows: H 2,act =H 2,ideal +ΔH loss,tot Among them, H 2,act is the actual outlet enthalpy, H 2,ideal is the ideal outlet enthalpy, ΔH loss,tot is the total loss enthalpy increment; The step 7 is specifically as follows: Step 7.1: Given the natural gas turbine expander outlet pressure P2 and the mole fractions of the natural gas components z i Under the condition of dew and dew point enthalpy H dew ; Step 7.2: Determine H 2,act and H dew the relative size of Step 7.3: If H 2,act >H dew , then the multi-component mixture is gaseous, and H is calculated according to the thermophysical properties database. 2,act The corresponding temperature T 2,act , directly output the final temperature T 2,act and the mole fraction z of each component of natural gas i ; Step 7.4: If H 2,act ≤H dew , the multi-component mixture is in a gas-liquid two-phase state, and iterative calculation is performed. When the gas phase rate and outlet temperature converge, the natural gas turbine expander outlet temperature, gas phase rate, liquid phase rate and gas-liquid two-phase components are output.

2. The method for predicting the outlet state of a natural gas turbine expander by coupling structure and thermodynamic parameters according to claim 1, characterized in that: The step 4 is specifically as follows: Step 4.1: Based on the isentropic process conditions, solve the ideal outlet temperature T by numerical iteration method 2,ideal : S(P2,T 2,ideal ,z i )=S1 Among them, when the outlet pressure P2 and the mole fraction of each component of natural gas z are known i Under the condition of 2,ideal ; Step 4.2: Calculate the ideal outlet enthalpy: H 2,ideal =H(P2,T 2,ideal ,z i ) Among them, when the outlet pressure P2 and the mole fraction of each component of natural gas z are known i And the ideal outlet temperature T 2,ideal Under the condition of 2,ideal .

3. The method for predicting the outlet state of a natural gas turbine expander by coupling structure and thermodynamic parameters according to claim 1, characterized in that: In step 5, the friction loss, flow separation loss, centrifugal loss and total loss enthalpy increment are calculated according to the following formula: ΔH cf =ξ cf U m ·(U2-U1) ΔH loss,tot =ΔH f +ΔH sep +ΔH cf Where ΔH f is the friction loss, ΔH sep is the flow separation loss, ΔH cf is the centrifugal loss, ΔH loss,tot is the total loss enthalpy increment; λ is the friction coefficient, L is the flow channel length, ξ sep is the separation loss coefficient, ξ cf is the centrifugal loss coefficient, D h is the hydraulic diameter, v1 is the inlet velocity, v2 is the outlet velocity, U1 is the circumferential velocity at the impeller inlet, U m is the average circumferential speed of the impeller, and U2 is the circumferential speed at the impeller outlet; A1=π(D1 / 2) 2 A2=π(D2 / 2) 2 Where ρ2 is the ideal outlet temperature T 2,ideal The corresponding outlet density is A1, which is the impeller inlet area, and A2, which is the impeller outlet area.

4. The method for predicting the outlet state of a natural gas turbine expander by coupling structure and thermodynamic parameters according to claim 1, characterized in that: The step 7.4 is specifically as follows: Step 7.4.1: Set the initial value of the gas phase rate β j =0.99, convergence criterion∈=10 -3 , gas phase rate iteration step Δβ=0.01; Step 7.4.2: Use the gas phase rate assigned in step 7.4.1, and assign the outlet temperature an initial value of T 2,k =T dew , temperature iteration step ΔT=0.01; when the outlet pressure P2 and outlet temperature T are known 2,0 Next, query the thermophysical properties database to obtain the gas-liquid equilibrium constant K of each component. i ; Step 7.4.3: Calculate the mole fractions of each component in the gas and liquid phases at the current gas phase fraction: Among them, K i is the gas-liquid equilibrium constant of component i, y i is the mole fraction of the i-th component in the gas phase, x i is the mole fraction of the i-th component in the liquid phase; Step 7.4.4: Query the thermophysical properties database to obtain the gas phase fugacity of component i and the liquid phase fugacity of component i Step 7.4.5: If all components meet The outlet temperature converges and the current outlet temperature T is obtained. 2,act =T 2,k , the mole fraction of the gas phase component y i and the mole fraction x of the liquid phase component i , proceed to step 7.4.6; if the outlet temperature does not converge, return to step 7.4.2, reassign the initial value of the outlet temperature, and let T 2,k+1 =T 2,k -ΔT, continue iterative calculation until the outlet temperature converges; Step 7.4.6: Use the calculated current outlet temperature T 2,act Query the thermophysical properties database to obtain the enthalpy H of each component i , and calculate the total gas phase enthalpy H g (P2,T 2,act ) and the total liquid enthalpy H l (P2,T 2,act ): Calculate the current gas phase rate β j The total mixing enthalpy H calc : H calc =β j H g (P2,T 2,act )+(1-β j )H l (P2,T 2,act ) Step 7.4.7: If |H calc -H 2,act |<∈, then convergence; otherwise, return to step 7.4.1, reassign the initial value of the gas phase rate, and let β j+1 =β j -Δβ, repeat steps 7.4.2-7.4.7 until convergence, and output the final gas phase rate β, liquid phase rate 1-β, and outlet temperature T 2,act , gas phase component mole fraction y i and the liquid phase mole fraction x i .

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