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CN110779275B - Method for improving energy efficiency of natural gas liquefaction device - Google Patents

Method for improving energy efficiency of natural gas liquefaction device Download PDF

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Publication number
CN110779275B
CN110779275B CN201910958025.5A CN201910958025A CN110779275B CN 110779275 B CN110779275 B CN 110779275B CN 201910958025 A CN201910958025 A CN 201910958025A CN 110779275 B CN110779275 B CN 110779275B
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refrigerant
natural gas
component
variable
percentage
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CN110779275A (en
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高买华
王志刚
李朝阳
赵运涛
曹强强
曹华新
董小强
郭琦
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Shaanxi Yanchang Oil And Gas Co ltd
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0244—Operation; Control and regulation; Instrumentation
    • F25J1/0252—Control strategy, e.g. advanced process control or dynamic modeling
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/0002—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the fluid to be liquefied
    • F25J1/0022—Hydrocarbons, e.g. natural gas
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0244—Operation; Control and regulation; Instrumentation
    • F25J1/0245—Different modes, i.e. 'runs', of operation; Process control
    • F25J1/0249—Controlling refrigerant inventory, i.e. composition or quantity
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2210/00—Processes characterised by the type or other details of the feed stream
    • F25J2210/60—Natural gas or synthetic natural gas [SNG]

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Mechanical Engineering (AREA)
  • Thermal Sciences (AREA)
  • General Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • General Chemical & Material Sciences (AREA)
  • Oil, Petroleum & Natural Gas (AREA)
  • Separation By Low-Temperature Treatments (AREA)

Abstract

The invention liquefies 4 components x of nitrogen, methane, ethylene and isopentane in the process refrigerant by mixed refrigeration cycleiEncoding the natural variable xiBecomes the norm variable ziAnd then, according to the percentage combination scheme of each component in the refrigerant, using flow simulation software to obtain the unit refrigeration capacity of the refrigerant under different parameter conditions, establishing a high-order regression equation between the unit refrigeration capacity of the refrigerant and the standard variable, obtaining each regression coefficient of the high-order regression equation by a planning and solving method, and further calculating to obtain the optimal value of each standard variable under the specified unit refrigeration capacity of the refrigerant, so that the percentage of each component in the refrigerant corresponding to the specified unit refrigeration capacity of the refrigerant can be obtained, and the optimal ratio of the refrigerant in the natural gas liquefaction process is obtained, thereby effectively improving the energy utilization rate of the natural gas liquefaction device, reducing the energy consumption of the system and lowering the production cost.

Description

Method for improving energy efficiency of natural gas liquefaction device
Technical Field
The invention belongs to the technical field of natural gas liquefaction, and particularly relates to a method for improving the energy efficiency of a natural gas liquefaction device.
Background
In the face of increasingly severe world energy crisis and environmental pollution problems, existing energy consuming structures dominated by petroleum and coal are greatly threatened and challenged. Natural gas is used as a high-quality clean fuel, the main component is methane, the natural gas has the advantages of high heat value and less pollution, becomes an important component in the current world energy consumption, is combined with petroleum and coal to form three major pillars of energy, plays important roles of adjusting an energy structure, improving the environmental quality, improving the living level, promoting the economy, coordinating and developing the environment and the like, is widely applied to various fields of industrial production, civil gas, urban heating, automobile gas and the like, and has wide application prospect.
Liquefied natural gas is an application form of natural gas, and the typical production process of the liquefied natural gas is that natural gas produced from a gas field is purified by 'three-way dehydration' (namely dehydration, acid gas removal and other impurities removal), and methane is frozen to-162 ℃ under normal pressure by adopting a refrigeration process to become liquid. The liquefied natural gas is only 1/600 the volume of the liquefied natural gas is the volume of the gas, so that the problem of inconvenient transportation of the gas can be effectively solved, and favorable conditions are provided for realizing transnational trade and recycling of small and scattered natural gas resources.
Natural gas liquefaction processes can be classified into three categories, namely cascade refrigeration cycle, mixed refrigeration cycle and expansion refrigeration cycle, according to different refrigeration modes. Compared with other natural gas liquefaction processes, the mixed refrigeration cycle liquefaction process is favored by a plurality of liquefaction plants and is a process adopted by most basic load type natural gas liquefaction devices, but the device energy efficiency is not high, which is the most main challenge faced by the liquefaction technology. Because the energy consumption cost of the natural gas liquefaction device accounts for more than 80% of the total cost, further improving the energy efficiency of the natural gas liquefaction process has very important practical significance for reducing the energy consumption of the device, reducing the production cost and improving the enterprise competitiveness.
The energy utilization rate of the natural gas liquefaction device is the ratio of the energy effectively utilized by the liquefied natural gas to the actually consumed energy, and the expression is as follows:
Figure BDA0002228013500000021
in the formula, psi is the energy efficiency of the natural gas liquefaction plant,%; qLNGThe total cooling capacity absorbed by the liquefied natural gas is kW.h/h; zeta is the cold energy conversion efficiency,%; qRThe total cooling capacity provided by the refrigerant is kW.h/h.
The energy utilization rate of the natural gas liquefaction device comprehensively reflects the energy consumption level and the utilization effect of the natural gas liquefaction device, and the energy utilization rate is a comprehensive index for measuring the effective utilization degree of energy. When the cold energy provided by the refrigerant is far less than the cold energy required by the liquefaction of the natural gas, the liquefaction rate of the natural gas is insufficient; when the cold energy provided by the refrigerant is far more than the cold energy required by the liquefaction of the natural gas, the cold energy carried by the refrigerant is surplus.
However, it should be noted that, in the actual production process, when 4 components of nitrogen, methane, ethylene and isopentane are selected as the mixed refrigeration cycle refrigerant, because there are often certain fluctuations in the gas quality of the raw natural gas, the processing capacity of the device, etc., it is difficult to ensure that the cold absorbed by the liquefied natural gas and the cold provided by the refrigerant form an ideal match and optimization, so it is often caused that field technicians cannot respond to the change of the operation conditions in time by accurately and reliably adjusting the refrigerant composition, thereby resulting in a low energy efficiency of the natural gas liquefaction device.
Disclosure of Invention
The invention aims to provide a method for improving the energy efficiency of a natural gas liquefaction device so as to overcome the technical defects.
Therefore, the invention adopts the following technical scheme:
a method of increasing the energy efficiency of a natural gas liquefaction plant, comprising the steps of:
s1 determining total cold absorbed by liquefied natural gas
Determining the total cold quantity required to be absorbed by the liquefied natural gas according to the theoretical natural gas processing capacity of the natural gas liquefaction device and the cold quantity required to be absorbed by the liquefied unit natural gas corresponding to the theoretical natural gas processing capacity;
s2 constraint condition for setting refrigerant ratio
The method comprises the following constraint conditions: with xiRepresenting the percentage of each component in the refrigerant, the proportion of each component must be non-negative and their sum must be 1, where i ═ 1, 2, 3, or 4;
constraint two: the percentage of a certain component in the refrigerant is not lower than the lower limit line ai；
S3 pairs of natural variables xiPerforming normalization processing
For natural variable xiEncoding the natural variable xiBecomes the norm variable ziSo that x isiIs converted into z of more than or equal to 0iLess than or equal to 1, and further determining the component number of the refrigerant;
s4 determining the refrigerating capacity of the refrigerant unit under different parameter conditions
Determining the percentage combination scheme of each component in the refrigerant according to the component number in the refrigerant, and obtaining the unit cold energy of the refrigerant under different parameter conditions through a refrigeration cycle liquefaction process flow simulation experiment;
s5 determining regression coefficients
According to the experimental result of the percentage combination scheme of each component in the refrigerant, the unit cold capacity and the standard variable z of the refrigerant under different parameter conditions are establishediThe high-order regression equation is solved, and each regression coefficient is further solved to obtain an accurate expression of the high-order regression equation;
s6 calculating the optimum value of each specification variable corresponding to the specified refrigerating capacity per unit
Obtaining specified refrigerating capacity per unit according to the refrigerating capacity conversion efficiency, the refrigerating capacity circulation quantity and the total refrigerating capacity required to be absorbed by the liquefied natural gas, and calculating the optimal value of each standard variable corresponding to the specified refrigerating capacity per unit by a planning and solving method based on the high-order regression equation obtained in the step S5;
s7 determining the optimal ratio of refrigerant
And (4) converting the optimal values of the standard variables in the step (S6) into the optimal values of the corresponding natural variables to obtain the optimal ratio of the refrigerant, wherein the energy utilization rate of the natural gas liquefaction device is maximized under the optimal ratio of the refrigerant.
Specifically, the total refrigeration capacity absorbed by the liquefied natural gas in S1 is calculated according to the following formula (i):
Figure BDA0002228013500000041
in the formula, QLNG,OPTThe total cooling capacity absorbed by the liquefied natural gas is kW.h/h;
Figure BDA0002228013500000045
the cooling capacity required to be absorbed by liquefied unit natural gas, kW.h/m3Natural gas;
qLNGis naturalTheoretical working Capacity of gas, m3Natural gas/h.
Specifically, the components of the natural gas liquefaction process refrigerant in the first constraint condition of S2 include nitrogen, methane, ethylene and isopentane, and the percentage of each component is x1、x2、x3And x4X, theniShould satisfy
Figure BDA0002228013500000042
Meanwhile, 4 components of the natural gas liquefaction process refrigerant meet the second constraint condition: x is the number ofi≥ai (Ⅲ)
I.e. x1≥a1、x2≥a2、x3≥a3、x4≥a4，a1+a2+a3+a4＜x1+x2+.x3+x4＝1
Thus, x1＝[1-(a1+a2+a3+a4)]z1+a1；
x2＝[1-(a1+a2+a3+a4)]z2+a2；
x3＝[1-(a1+a2+a3+a4)]z3+a3；
x4＝[1-(a1+a2+a3+a4)]z4+a4；
Wherein a isiIs the lower limit of a certain component in the refrigerant.
Specifically, the natural variable x is set in S3iBecomes the norm variable ziUsing the following formula (iv) for conversion:
Figure BDA0002228013500000043
namely, it is
Figure BDA0002228013500000044
In the formula, xiIs the percentage of each component in the refrigerant;
aiis ziZero level of (d);
ziis a natural variable xiThe corresponding specification variables.
Further, the method for determining the unit refrigeration capacity of the refrigerant under different parameter conditions in the S4 is obtained by adopting ChemCAD, PRO/II, Aspen Plus, Aspen HYSY or Pro Max software to perform full-flow simulation on the refrigeration cycle liquefaction process.
Specifically, the higher-order regression equation in S5 is:
Figure BDA0002228013500000051
wherein,
Figure BDA0002228013500000052
in the formula, i, j and k are certain components in the refrigerant;
biis a monobasic component action parameter;
bjiis a binary component interaction parameter;
bkjiis a ternary component interaction parameter;
b1234is a quaternary component interaction parameter;
χi、χj、χkthe experimental result of the percentage combination scheme of the unary component;
χkj、χki、χjithe interactive experiment result of the percentage combination scheme of the binary components;
χkjithe interactive experiment result of the percentage combination scheme of the ternary components is shown;
χ1234is a four-elementThe percentage of the components is combined with the interactive experimental result of the scheme.
Specifically, the refrigerant unit capacity χ specified in S6OPTCalculated according to the following formula (VIII)
Figure BDA0002228013500000061
In the formula,
Figure BDA0002228013500000062
is the specific unit cooling capacity of the refrigerant, kW.h/kg refrigerant;
QLNG,OPTthe total cooling capacity required to be absorbed by liquefied natural gas is kW.h/h;
zeta is the cold energy conversion efficiency,%;
qRkg refrigerant/h as the refrigerant circulation amount.
Specifically, the conversion of the optimal value of each specification variable in S7 into the optimal value of the corresponding natural variable is performed according to the following formula (ix):
Figure BDA0002228013500000063
the invention has the beneficial effects that:
the invention liquefies 4 components x of nitrogen, methane, ethylene and isopentane in the process refrigerant by mixed refrigeration cycleiEncoding the natural variable xiBecomes the norm variable ziThen, according to the percentage combination scheme of each component in the refrigerant, the unit cold quantity of the refrigerant under different parameter conditions is obtained by using the process simulation software, and the unit cold quantity of the refrigerant and the standard variable z are establishediThe high-order regression equation is obtained through a planning solving method, and the specified refrigerating capacity per unit is further calculated and obtained
Figure BDA0002228013500000064
Each normative variable z under the conditioniSo as to obtain the refrigerant corresponding to the unit cold quantity of the specified refrigerantThe optimal percentage of each component is the optimal proportion of the natural gas liquefaction process refrigerant. The device field verification result shows that the method for improving the energy utilization rate of the natural gas liquefaction device can accurately match and optimize the cold quantity required by the liquefied natural gas by adjusting the composition of the refrigerant, effectively improve the energy utilization rate of the natural gas liquefaction device, reduce the energy consumption of a system and reduce the production cost, has wide application range and can be used for various refrigeration cycle flows.
The foregoing is a summary of the present invention, and in order to provide a clear understanding of the technical means of the present invention and to be implemented in accordance with the present specification, preferred embodiments of the present invention are described in detail below.
Detailed Description
Example 1:
the embodiment provides a method for improving the energy efficiency of a natural gas liquefaction device, which comprises the following steps:
s1 determining total cold absorbed by liquefied natural gas
Determining the total cold quantity required to be absorbed by the liquefied natural gas according to the theoretical natural gas processing capacity of the natural gas liquefaction device and the cold quantity required to be absorbed by the liquefied unit natural gas corresponding to the theoretical natural gas processing capacity;
s2 constraint condition for setting refrigerant ratio
The method comprises the following constraint conditions: with xiRepresenting the percentage of each component in the refrigerant, the proportion of each component must be non-negative and their sum must be 1, where i ═ 1, 2, 3, or 4;
constraint two: the percentage of a certain component in the refrigerant is not lower than the lower limit line ai；
S3 pairs of natural variables xiPerforming normalization processing
For natural variable xiEncoding the natural variable xiBecomes the norm variable ziSo that x isiIs converted into z of more than or equal to 0iLess than or equal to 1, and further determining the component number of the refrigerant;
s4 determining the refrigerating capacity of the refrigerant unit under different parameter conditions
Determining the percentage combination scheme of each component in the refrigerant according to the component number in the refrigerant, and obtaining the unit cold energy of the refrigerant under different parameter conditions through a refrigeration cycle liquefaction process flow simulation experiment;
s5 determining regression coefficients
According to the experimental result of the percentage combination scheme of each component in the refrigerant, the unit cold capacity and the standard variable z of the refrigerant under different parameter conditions are establishediThe high-order regression equation is solved, and each regression coefficient is further solved to obtain an accurate expression of the high-order regression equation;
s6 calculating the optimum value of each specification variable corresponding to the specified refrigerating capacity per unit
Obtaining specified refrigerating capacity per unit according to the refrigerating capacity conversion efficiency, the refrigerating capacity circulation quantity and the total refrigerating capacity required to be absorbed by the liquefied natural gas, and calculating the optimal value of each standard variable corresponding to the specified refrigerating capacity per unit by a planning and solving method based on the high-order regression equation obtained in the step S5;
s7 determining the optimal ratio of refrigerant
And (4) converting the optimal values of the standard variables in the step (S6) into the optimal values of the corresponding natural variables to obtain the optimal ratio of the refrigerant, wherein the energy utilization rate of the natural gas liquefaction device is maximized under the optimal ratio of the refrigerant.
The invention liquefies the component x of the process refrigerant by the mixed refrigeration cycleiEncoding the natural variable xiBecomes the norm variable ziThen, according to the percentage combination scheme of each component in the refrigerant, the unit refrigeration capacity of the refrigerant under different parameter conditions is obtained by using the process simulation software, and the unit refrigeration capacity of the refrigerant and the standard variable z under different parameter conditions are establishediThe high-order regression equation is obtained through a planning solving method, and each normative variable z under the specified unit cold capacity condition of the refrigerant is further calculatediThe optimal value of the natural gas liquefaction process can be obtained, so that the percentage of each component in the refrigerant corresponding to the unit cold quantity of the specified refrigerant can be obtained, and the optimal ratio of the natural gas liquefaction process refrigerant can be obtained, thereby obviously improving the ratio of the components in the refrigerantThe energy utilization rate of the natural gas liquefaction device is high, the energy consumption of the system is reduced, and the production cost is reduced; the method has wide application range and can be used for various refrigeration cycle processes.
Example 2:
the embodiment provides a method for improving the energy efficiency of a natural gas liquefaction device, which comprises the following steps:
s1 determining total cold absorbed by liquefied natural gas
Appointing natural gas processing energy q which should be reached by a natural gas liquefaction device according to production tasks issued by enterprisesLNGAnd the corresponding cold energy required to be absorbed by the liquefied unit natural gas
Figure BDA0002228013500000081
To determine the total amount of cold Q that the liquefied natural gas needs to absorbLNG，OPT(ii) a In particular, the total refrigeration Q that the liquefied natural gas needs to absorbLNG，OPTCalculated according to the following formula (I)
Figure BDA0002228013500000082
In the formula, QLNG,OPTThe total cooling capacity absorbed by the liquefied natural gas is kW.h/h;
Figure BDA0002228013500000083
the cooling capacity required to be absorbed by liquefied unit natural gas, kW.h/m3Natural gas;
qLNGis the theoretical processing capacity of natural gas, m3Natural gas/h;
it should be noted that, for different refrigeration cycle liquefaction processes, the refrigeration amounts absorbed by the natural gas units need to be different, for example, a cascade refrigeration cycle liquefaction process, a single-stage mixed refrigerant refrigeration cycle liquefaction process, a propane pre-cooling single-stage mixed refrigerant refrigeration cycle liquefaction process, a multi-stage mixed refrigerant refrigeration cycle liquefaction process, a single-stage expansion refrigeration cycle liquefaction process, a propane pre-cooling single-stage expansion refrigeration cycle liquefaction process, a two-stage expansion refrigeration cycle liquefaction processThe cold energy required to be absorbed by the liquefied unit natural gas is 0.32 kW.h/m respectively3Natural gas, 0.40 kW.h/m3Natural gas, 0.37 kW.h/m3Natural gas, 0.34 kW.h/m3Natural gas, 0.64 kW.h/m3Natural gas, 0.54 kW.h/m3Natural gas and 0.54 kW.h/m3Natural gas;
s2 constraint condition for setting refrigerant ratio
The method comprises the following constraint conditions:
the components of the natural gas liquefaction process refrigerant comprise nitrogen, methane, ethylene and isopentane, and the percentage of each component is x1、x2、x3And x4X, theniShould satisfy
Figure BDA0002228013500000091
Meanwhile, 4 components of the natural gas liquefaction process refrigerant meet the second constraint condition: x is the number ofi≥ai (Ⅲ)
I.e. x1≥a1、x2≥a2、x3≥a3、x4≥a4，a1+a2+a3+a4＜x1+x2+.x3+x4＝1；
Thus, x1＝[1-(a1+a2+a3+a4)]z1+a1；
x2＝[1-(a1+a2+a3+a4)]z2+a2；
x3＝[1-(a1+a2+a3+a4)]z3+a3；
x4＝[1-(a1+a2+a3+a4)]z4+a4；
Wherein a isiIs the lower limit line of a certain component in the refrigerant;
s3 pairs of natural variables xiPerforming normalization processing
For natural transformationQuantity xiEncoding the natural variable xiBecomes the norm variable ziSo that x isiIs converted into z of more than or equal to 0iLess than or equal to 1, and further determining the component number of the refrigerant;
specifically, the natural variable xiBecomes the norm variable ziUsing the following formula (iv) for conversion:
Figure BDA0002228013500000101
namely, it is
Figure BDA0002228013500000102
In the formula, xiIs the percentage of each component in the refrigerant;
aiis ziZero level of (d);
ziis a natural variable xiA corresponding specification variable;
it is to be noted that if each component xiIs 0, then xi＝ziAt this time, the natural variable xiAnd a specification variable ziAre equal in value;
s4 determining the refrigerating capacity of the refrigerant unit under different parameter conditions
Determining the percentage combination scheme of each component in the refrigerant according to the component number in the refrigerant, and performing full-flow simulation on the refrigeration cycle liquefaction process by adopting professional software such as ChemCAD, PRO/II, Aspen Plus, Aspen HYSY or Pro Max to obtain the unit refrigeration capacity χ of the refrigerant under different parameters, namely
Figure BDA0002228013500000103
See table 1 below for details:
Figure BDA0002228013500000104
Figure BDA0002228013500000111
s5 determining regression coefficients
According to the experimental result of the percentage combination scheme of each component in the refrigerant, the unit cold quantity and the standard variable z of the refrigerant under different cold parameters are establishediThe high-order regression equation is solved, and each regression coefficient is further solved to obtain an accurate expression of the high-order regression equation; specifically, the high-order regression equation is:
Figure BDA0002228013500000112
wherein,
Figure BDA0002228013500000113
in the formula, i, j and k are certain components in the refrigerant;
biis a monobasic component action parameter;
bjiis a binary component interaction parameter;
bkjiis a ternary component interaction parameter;
b1234is a quaternary component interaction parameter;
χi、χj、χkthe experimental result of the percentage combination scheme of the unary component;
χkj、χki、χjithe interactive experiment result of the percentage combination scheme of the binary components;
χkjithe interactive experiment result of the percentage combination scheme of the ternary components is shown;
χ1234the results of the interactive experiment of the percentage combination scheme of the quaternary components;
s6 calculating the optimum value of each specification variable corresponding to the specified refrigerating capacity per unit
According to the cold energy conversion efficiency zeta and the refrigerant circulation quantity qRAnd the total cooling capacity Q required to be absorbed by the liquefied natural gasLNG,OPTObtain the unit cold quantity of the specified refrigerant
Figure BDA0002228013500000121
In particular, a specified refrigerating capacity per unit
Figure BDA0002228013500000122
Calculated according to the following formula (VIII):
Figure BDA0002228013500000123
in the formula,
Figure BDA0002228013500000124
is the specific unit cooling capacity of the refrigerant, kW.h/kg refrigerant;
QLNG,OPTthe total cooling capacity required to be absorbed by liquefied natural gas is kW.h/h;
zeta is the cold energy conversion efficiency,%;
qRkg refrigerant/h as the refrigerant circulation amount.
Further, based on the high-order regression equation obtained in step S5, the refrigeration capacity of the specified refrigerant unit is calculated by a planning solution method
Figure BDA0002228013500000125
The corresponding optimal value of each specification variable;
s7 determining the optimal ratio of refrigerant
The optimal ratio of the refrigerant obtained in the step S6 is obtained, under the optimal ratio of the refrigerant, the cold energy absorbed by the liquefied natural gas and the cold energy provided by the refrigerant can form ideal matching and optimization, and the energy utilization rate of the natural gas liquefaction device reaches the maximum.
It should be noted that, the following conversion formula is adopted to convert the optimal value of each normative variable into the optimal value of the corresponding natural variable:
Figure BDA0002228013500000126
the invention relates to a4 components x of nitrogen, methane, ethylene and isopentane in the mixed refrigeration cycle liquefaction process refrigerantiEncoding the natural variable xiBecomes the norm variable ziThen, according to the percentage combination scheme of each component in the refrigerant, the unit refrigeration capacity of the refrigerant under different parameters is obtained by using process simulation software
Figure BDA0002228013500000131
Establishing unit cold quantity of starting refrigerant
Figure BDA0002228013500000132
And a specification variable ziThe high-order regression equation is obtained through a planning solving method, and the specified refrigerating capacity per unit is further calculated and obtained
Figure BDA0002228013500000133
Each normative variable z under the conditioniThe optimal value of the natural gas liquefaction process refrigerant is obtained, so that the percentage of each component in the refrigerant corresponding to the unit cold quantity of the specified refrigerant can be obtained, and the optimal ratio of the natural gas liquefaction process refrigerant is obtained, so that the energy efficiency of the natural gas liquefaction device is effectively improved.
Example 3:
the embodiment provides a method for improving energy efficiency of a natural gas liquefaction device, which specifically comprises the following steps:
firstly, a natural gas liquefying device in Yanan area adopts a mixed refrigeration cycle liquefying process (the amount of cold absorbed by natural gas in a liquefying unit is 0.3400 kW.h/m)3Natural gas) if the natural gas processing capacity of the plant is 24680m3And (2) natural gas/h, the total cold absorbed by the suitable liquefied natural gas is as follows:
Figure BDA0002228013500000134
and secondly, setting constraint conditions of refrigerant ratio. With x1、x2、x3、x4Respectively represent nitrogen in the refrigerantThe percentage of each of the 4 components methane, ethylene and isopentane requires that the proportions of each component must be non-negative and that their sum must be 1, i.e. that
Figure BDA0002228013500000135
Meanwhile, x is in addition to the above-mentioned constraints1、x2、x3And x4Are not limited by other constraints, i.e. a1＝a2＝a3＝a4＝0。
Third step, for x1、x2、x3And x4Encoding the natural variable x1、x2、x3、x4Becomes the norm variable z1、z2、z3、z4(ii) a In view of a1＝a2＝a3＝a40, thus x1＝z1，x2＝z2，x3＝z3，x4＝z4。
Fourthly, determining a corresponding basic data combination scheme according to the component number in the refrigerant, and obtaining the unit cold capacity χ of the refrigerant under different parameter conditions through a process simulation experiment, which is detailed in the following table 2.
Figure BDA0002228013500000141
Fifthly, according to the experimental result of the percentage combination scheme of each component in the refrigerant, the unit cold capacity x and the standard variable z of the refrigerant under different parameter conditions1、z2、z3、z4And further solving each regression coefficient of the high-order regression equation, wherein the high-order regression equation comprises the following specific steps:
χ＝b1z1+b2z2+b3z3+b4z4+
b12z1z2+b13z1z3+b14z1z4+b23z2z3+b24z2z4+b34z3z4+
b123z1z2z3+b124z1z2z4+b134z1z3z4+b234z2z3z4+
b1234z1z2z3z4
the experimental result of each number is substituted into the equation to obtain
Figure BDA0002228013500000151
At this time, the process of the present invention,
χ＝0.1928z1+0.2214z2+0.3083z3+0.4246z4+
0.0124z1z2+0.0882z1z3+0.0680z1z4-0.0358z2z3-0.0776z2z4-0.4150z3z4-
1.0836z1z2z3-2.1360z1z2z4+1.1472z1z3z4+1.5204z2z3z4+
33.2640z1z2z3z4
sixthly, passing the unit cold capacity χ of the refrigerant and the standard variable z1、z2、z3、z4The higher order regression equations in between and the associated constraints can be predicted as follows:
when ζ is 27%, qRWhen the ratio is 68500kg of refrigerant/h, Q is causedLNG,OPT8391.2000 kW.h/h, so
Figure BDA0002228013500000161
Calculating the unit cold capacity χ of the suitable refrigerant by a planning and solving methodOPTCorresponding specification variables z1、z2、z3And z4。
0.4537＝0.1928z1+0.2214z2+0.3083z3+0.4246z4+
0.0124z1z2+0.0882z1z3+0.0680z1z4-0.0358z2z3-0.0776z2z4-0.4150z3z4-
1.0836z1z2z3-2.1360z1z2z4+1.1472z1z3z4+1.5204z2z3z4+
33.2640z1z2z3z4
Therefore, the temperature of the molten metal is controlled,
Figure BDA0002228013500000162
seventhly, using proper cold agent unit cold capacity chiOPTEach normative variable z under the condition1、z2、z3、z4Conversion to natural variable x1、x2、x3、x4Thus, an appropriate refrigerant ratio can be obtained.
Because of x1＝z1，x2＝z2，x3＝z3，x4＝z4Therefore, it is
Figure BDA0002228013500000163
Therefore, when the percentage of nitrogen in the refrigerant is 20.17% (V/V), the percentage of methane is 19.17% (V/V), the percentage of ethylene is 28.09% (V/V), and the percentage of isopentane is 32.57% (V/V), the cold energy absorbed by the liquefied natural gas can be ideally matched with the cold energy provided by the refrigerant, so that the energy efficiency of the natural gas liquefaction device is remarkably improved.
The field verification result shows that the energy efficiency of the natural gas liquefaction device before optimization is only 89.65%, and the energy efficiency of the natural gas liquefaction device after optimization is as high as 97.78%, which is improved by about 8.13% compared with that before optimization, so that the purposes of energy saving, consumption reduction, quality improvement and efficiency improvement are effectively achieved, field technicians are ensured to accurately and reliably adjust the refrigerant composition to timely respond to the change of the natural gas quality of the raw material, the processing capacity of the device is improved, the energy consumption of the system is reduced, and the production cost is reduced.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.

Claims (6)

1. A method of increasing the energy efficiency of a natural gas liquefaction plant, comprising the steps of:
s1 determining total cold absorbed by liquefied natural gas
Determining the total cold quantity required to be absorbed by the liquefied natural gas according to the theoretical natural gas processing capacity of the natural gas liquefaction device and the cold quantity required to be absorbed by the liquefied unit natural gas corresponding to the theoretical natural gas processing capacity;
s2 constraint condition for setting refrigerant ratio
The method comprises the following constraint conditions: with xiRepresenting the percentage of each component in the refrigerant, the proportion of each component must be non-negative and their sum must be 1, where i ═ 1, 2, 3, or 4;
constraint two: the percentage of a certain component in the refrigerant is not lower than the lower limit line ai；
S3 pairs of natural variables xiPerforming normalization processing
For natural variable xiEncoding the natural variable xiBecomes the norm variable ziSpecifically, the following formula (IV) is used for conversion so that xiIs converted into z of more than or equal to 0iLess than or equal to 1, and further determining the component number of the refrigerant;
Figure FDA0002822347530000011
namely, it is
Figure FDA0002822347530000012
In the formula, xiIs the percentage of each component in the refrigerant;
aiis ziZero level of (d);
ziis a natural variable xiA corresponding specification variable;
s4 determining the refrigerating capacity of the refrigerant unit under different parameter conditions
Determining the percentage combination scheme of each component in the refrigerant according to the component number in the refrigerant, and obtaining the unit cold energy of the refrigerant under different parameter conditions through refrigeration cycle liquefaction process flow simulation software;
s5 determining regression coefficients
According to the experimental result of the percentage combination scheme of each component in the refrigerant, the unit cold capacity and the standard variable z of the refrigerant under different parameter conditions are establishediAnd further solving each regression coefficient of the high-order regression equation to obtain an accurate expression of the high-order regression equation, wherein the high-order regression equation is specifically as follows:
Figure FDA0002822347530000021
wherein,
Figure FDA0002822347530000022
in the formula, i, j and k are certain components in the refrigerant;
biis a monobasic component action parameter;
bjiis a binary component interaction parameter;
bkjiis a ternary component interaction parameter;
b1234is a quaternary component interaction parameter;
χi、χj、χkthe experimental result of the percentage combination scheme of the unary component;
χkj、χki、χjithe interactive experiment result of the percentage combination scheme of the binary components;
χkjithe interactive experiment result of the percentage combination scheme of the ternary components is shown;
χ1234the results of the interactive experiment of the percentage combination scheme of the quaternary components;
s6 calculating the optimum value of each specification variable corresponding to the specified refrigerating capacity per unit
Obtaining specified refrigerating capacity per unit according to the refrigerating capacity conversion efficiency, the refrigerating capacity circulation quantity and the total refrigerating capacity required to be absorbed by the liquefied natural gas, and calculating the optimal value of each standard variable corresponding to the specified refrigerating capacity per unit by a planning and solving method based on the high-order regression equation obtained in the step S5;
s7 determining the optimal ratio of refrigerant
And (4) converting the optimal values of the standard variables in the step (S6) into the optimal values of the corresponding natural variables to obtain the optimal ratio of the refrigerant, wherein the energy utilization rate of the natural gas liquefaction device is maximized under the optimal ratio of the refrigerant.
2. The method for improving the energy efficiency of a natural gas liquefaction plant according to claim 1, wherein the total refrigeration capacity absorbed by the liquefied natural gas in the step S1 is calculated according to the following formula (i):
Figure FDA0002822347530000031
in the formula, QLNG,OPTThe total cooling capacity absorbed by the liquefied natural gas is kW.h/h;
Figure FDA0002822347530000032
the cooling capacity required to be absorbed by liquefied unit natural gas, kW.h/m3Natural gas;
qLNGis the theoretical processing capacity of natural gas, m3Natural gas/h.
3. The method of claim 1, wherein the components of the natural gas liquefaction process refrigerant under the first constraint of S2 comprise nitrogen, methane, ethylene and isopentane, and the percentage of each component is x1、x2、x3And x4X, theniShould satisfy
Figure FDA0002822347530000041
Meanwhile, 4 components of the natural gas liquefaction process refrigerant meet the second constraint condition: x is the number ofi≥ai(Ⅲ)
I.e. x1≥a1、x2≥a2、x3≥a3、x4≥a4，a1+a2+a3+a4＜x1+x2+.x3+x4＝1
Thus, x1＝[1-(a1+a2+a3+a4)]z1+a1；
x2＝[1-(a1+a2+a3+a4)]z2+a2；
x3＝[1-(a1+a2+a3+a4)]z3+a3；
x4＝[1-(a1+a2+a3+a4)]z4+a4；
Wherein a isiIs the lower limit of a certain component in the refrigerant.
4. The method for improving the energy efficiency of the natural gas liquefaction plant according to claim 1, wherein the method for determining the refrigeration unit capacity under different parameter conditions in S4 is obtained by performing full-process simulation on a refrigeration cycle liquefaction process by adopting ChemCAD, PRO/II, Aspen Plus, Aspen HYSY or Pro Max software.
5. The method for improving the energy efficiency of a natural gas liquefaction plant according to claim 1, wherein the refrigeration unit specified in S6 is cold
Figure FDA0002822347530000042
Calculated according to the following formula (VIII)
Figure FDA0002822347530000043
In the formula,
Figure FDA0002822347530000044
is the specific unit cooling capacity of the refrigerant, kW.h/kg refrigerant;
QLNG,OPTthe total cooling capacity required to be absorbed by liquefied natural gas is kW.h/h;
zeta is the cold energy conversion efficiency,%;
qRkg refrigerant/h as the refrigerant circulation amount.
6. The method for improving energy efficiency of a natural gas liquefaction plant according to claim 1, wherein the transformation of the optimal values of the specification variables to the optimal values of the corresponding natural variables in S7 is performed according to the following formula (ix):
Figure FDA0002822347530000051
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