CN112201309A - P-graph-based ethylene full-process superstructure model modeling method - Google Patents
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Abstract
The invention discloses a P-graph-based modeling method for a full-flow superstructure of ethylene, which is used for establishing a model by taking a process of converting straight-chain alkane into olefin as a target object, and comprises four parts of cracking, quenching, compressing and separating, wherein the quenching and compressing processes are simplified into an operation unit type node, ethane circulation existing in the process is subjected to ring opening, and an optimal solution set and a near suboptimal solution set obtained on the basis of a P-graph algorithm are analyzed. The invention provides an ethylene full-process superstructure model based on a P-graph framework, and has the advantage of being capable of obtaining a series of suboptimal solutions. In the modeling process, a method for screening related equipment of a complex industrial process is provided according to the characteristics of 'three-pass-one-reverse' and a problem to be researched.
Description
Technical Field
The invention belongs to the technical field of ethylene cracking, and particularly relates to a P-graph-based modeling method for a full-flow superstructure model of ethylene, which is called PECMA (P-graph based ethylene cracking and analysis method for short).
Background
The ethylene production in China faces the problems of heavier cracking raw materials, higher energy consumption and long-term shortage of the cracking raw materials, and meanwhile, oil refining is in a state of excess capacity for a long time due to the adjustment of the economic structure in China. Therefore, the ethylene production enterprises and the oil refining production enterprises together convert the surplus oil refining capacity into the high-quality ethylene cracking light raw material, and become the first choice of the current Chinese ethylene production enterprises. In the process, the excess refinery gas in the refinery is pretreated and converted into high-quality light raw materials to improve the benefit of an ethylene production device, and meanwhile, the improvement of the yield of high value-added light products and the insufficient treatment capacity of subsequent equipment of a cracking furnace in the ethylene plant form a new contradiction, thereby greatly limiting the further improvement of economic benefit and environmental benefit. Since the analysis resolves this conflict involves individual units including cleavage, separation, etc. Therefore, it is necessary to establish an ethylene full-flow model and search the optimum degree of raw material lightening caused by refining integration under the current equipment condition based on the model.
In addition, the solution of the contradiction relates to the synchronous optimization of the structural working condition and the parameters. The method is characterized in that models of equipment possibly involved in all processes are placed in a network, logistics are represented by edges of the network, and the established model capable of synchronously performing parameter and structure optimization is called a superstructure model and is one of the best methods for synchronously solving structure and parameter optimization. As a superstructure modeling method, a series of algorithms (including a maximum structure generation algorithm (MSG), a solution structure generation algorithm (SSG) and an acceleration branch definition Algorithm (ABB)) in a P-graph method enable a superstructure model to obtain an optimal solution and a large number of near-to-suboptimal solutions through one-time optimization calculation, the model constructed based on the method has better visualization performance, and a high-quality suboptimal solution set is greatly beneficial to guiding optimization decision of a process or analyzing bottleneck equipment. A basic P-graph diagram is shown in figure 1.
In the past, superstructure modeling based on P-graph does not relate to ethylene or similar large-scale industrial devices, and a P-graph method is directly applied to superstructure modeling of a whole process of a large-scale ethylene industrial device, and some technical problems need to be solved, for example:
1) large industrial devices such as ethylene often contain a large amount of equipment, and in order to simplify the complexity of superstructure modeling, it is necessary to study how to screen equipment to be modeled for the problem to be studied and merge other equipment in the process;
2) aiming at the contradiction between the logistics circulation of large industrial devices such as ethylene and the like and the stability of a P-graph algorithm, how to perform ring-opening on the established maximum structure of the superstructure and ensure the minimum influence of the ring-opening on the process is required to be researched.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provide a P-graph-based ethylene full-flow superstructure model modeling method, model complexity is simplified by combining problem screening equipment and associated process equipment in a model, and contradiction between logistics circulation and P-graph algorithm stability is solved by realizing maximum structural ring-opening of the superstructure and ensuring minimum influence of ring-opening on the process.
The technical scheme is as follows:
a P-graph-based ethylene full-process superstructure model modeling method comprises the following steps:
1) establishing a model by taking a process of converting straight-chain alkane into olefin as a target object, wherein the process comprises four parts of cracking, quenching, compressing and separating, and the quenching and compressing processes are simplified into an operation unit type node;
2) the ethane circulation existing in the process is subjected to ring-opening, a ring-opening superstructure is constructed, and M is used2And M1Representing ethane, the product of the ethane cyclic cracking process being M3And M4The corresponding yields are then as follows:
in the formula, rnThe yield of recycled ethane for the ratio corresponding to the superstructure edge n is shown in equation (3):
each time the cycle number is ncThen pass through nc＝NcAfter a second cycle, product M3And M4The yields of (a) are expressed by the following equations (4) and (5), respectively:
over a number of iterations, the percentage of remaining ethane is shown in equation (6):
if eta2< 0.0005, the remaining amount of ethane is approximately neglected, r'1-r1Then, the corrected variation value is:
3) analyzing the optimal and near suboptimal solution sets obtained based on the P-graph algorithm:
generating an objective function and a constraint according to the maximum structure and parameters of the PNS problem, and giving an optimal solution and a large number of suboptimal solutions based on an accelerated branch definition algorithm, wherein the established model comprises three types of constraints: (1) upper feed supply and lower product yield (ethylene) constraints; (2) upper and lower limits of the processing capacity of the equipment are restricted; (3) material balance of each node;
calculating an objective function as shown in equation (9) according to the raw material price and the product price:
the optimal parameters under each feasible structure are solved according to the objective function (9), and sequencing is carried out according to the objective function maxprofit so as to give optimal and suboptimal solutions; firstly, an index for measuring the weight of the raw material is calculated based on the optimal solution and the secondary solution, and the weight index adopts the weight ratio of the light raw material as the formula (10)LTo show that:
in the formula, FiIs a raw material RiOf (2) a traffic, coefficienti(l) Is reacted with a starting material RiCoefficient of property when starting material RiWhen the average carbon chain length l of (2) is not more than 6, coefficienti(l) 1, otherwise coefficienti(l)＝0；
The influence of the refinery-ethylene integration process on the environment is characterized by the index SPI as shown in formula (11):
Further, the fuel consumed by the cracking process is synchronously considered due to sustainability indicators
The fuel consumption of the cracked section is estimated based on historical data fit based on the flow rate of the feedstock and the properties of the feedstock.
Further, the catalytically dry gas in the feed needs to be removed from the acid gases that are not involved in the cracking process.
The technical effects obtained by the invention are as follows:
1. an ethylene full-Process superstructure modeling algorithm constructed based on a P-graph (Process graph) algorithm is used for carrying out simulation on an ethylene full-Process under a refining and chemical integration background, equipment required by modeling is analyzed, screened and deleted by applying domain knowledge, the problem of expression of an ethane circulation phenomenon in a superstructure model is solved based on a ring-opening technology, an optimal solution scheme and a near-suboptimal solution scheme are calculated, and the optimal light raw material ratio suitable for a current device is obtained. The optimal solution and a large number of near suboptimal solutions of the process can be obtained based on the P-graph algorithm, and decision and analysis are facilitated.
2. The established superstructure modeling comprises various working conditions of the existing equipment under the condition of load change, and the process can be synchronously subjected to structural optimization and parameter optimization based on the superstructure modeling, so that the search domain of the process optimization problem is enlarged.
3. Based on the limit concept, the ethane circulation process which is easy to generate dead circulation for superstructure calculation is subjected to ring opening by adjusting parameters, and the modeling precision of a superstructure model is ensured, and meanwhile, the complexity of the model and optimization calculation is reduced.
4. The degree of process-refining integration is described by the mass ratio of the light feedstock mass to the total cracking feedstock mass.
Drawings
FIG. 1 is a schematic diagram of a P-graph based modeling method for a full-flow superstructure model of ethylene.
Fig. 2 is a loop-opening flow diagram of the superstructure.
FIG. 3 is a diagram of a P-graph-based maximum structure model for ethylene cracking full-flow modeling.
FIG. 4 is a flow chart of a P-graph based modeling method for a full-flow superstructure model of ethylene.
Detailed Description
In order to make those skilled in the art better understand the technical solution of the present invention, the P-graph-based ethylene full-flow superstructure modeling method provided by the present invention is described in detail below with reference to the following examples. The following examples are intended to illustrate the invention only and are not intended to limit the scope of the invention.
The invention aims at carrying out modeling research on a specific ethylene plant under the integrated background of oil refining and ethylene. In the actual research process, an optimization model is established by taking economic benefit as a target, material flow and equipment working conditions of an ethylene process as decision variables and actual conditions of equipment and materials as constraints, and an optimal scheme is comprehensively determined based on economic indexes and carbon emission corresponding to suboptimal solutions so as to guide actual production. In different schemes, the optimization of the analysis process by taking the ratio of the light raw material to the total raw material (light hydrocarbon rate) as an independent variable and taking economic and environmental benefits as a dependent variable is used for exploring the optimal degree of refining integration for a specific device.
Equipment for screening and deleting models required to be modeled by using process knowledge
The goal of building an ethylene full-process superstructure model is to optimize process paths and operating parameters, and for an ethylene production process comprising a large number of devices, the devices that affect the above goal should be focused, and it is not necessary to connect all the devices into the network of the superstructure, so at the very beginning of the modeling process, the devices that are closely related to the analysis index should be screened out for modeling. On the other hand, a plurality of series or parallel devices should be simplified into one operation unit as much as possible to control the complexity of the P-graph modeling and improve the modeling efficiency.
The set analysis index mainly focuses on the influence of the economic benefit and the process of cracking on the environment, while the economic benefit index mainly focuses on the increment generated in the process of converting low-value hydrocarbon (mainly straight-chain alkane) into high-value-added olefin, so that a model of equipment (cracking furnace group) related to chemical reaction needs to be established.
The main flow of the cracking process comprises four parts of cracking, quenching, compressing and separating. The rapid cooling and compression processes are mainly physical processes, relate to energy consumption and phase change and are the key points for researching energy efficiency, but in the process of researching economy and environment, a large number of equipment models related to the two parts can be simplified into an O-type node (using OMC)iRepresentation). Under normal production conditions (i.e. conditions involved in the plant), the separation part has little influence on the economic benefits and environmental indicators (emissions) of the whole process, but because the upgrading of the co-produced raw material is not matched with the processing capacity of the existing separation part, and the mismatching can greatly limit the upgrading degree of the feasible cracked raw material, the separation part is also one of the key points of modeling. In addition, since the research involves environmental influences, such as carbon dioxide emissions, fuel systems not included in the main process should also be added to the model, taking into account in the modeling and optimization process simultaneously.
Second, the ethane circulation existing in the process is subjected to ring-opening based on the limit thought
Because in the actual cracking process, ethane generated by cracking heavy raw materials needs to be recycled as cracking raw materials, and the process containing recycling does not meet the non-recycling process axiom of 5 axioms in P-graph. For this purpose, a superstructure of the solution ring needs to be constructed. For complex P-graph networks, the ring is often very difficult to separate, and deleting an edge may mean modifying parameters of a large number of edges and nodes, but the P-graph structure of other products generated by ethane circulation in the ethylene cracking process is relatively simple, and only the P-graph structure is usedOnly the weight of the edge needs to be modified. As shown in FIG. 2, M2And M1All represent ethane, O1Represents the corresponding operation unit of the cracking process (actually, the whole cracking process, which is simplified to be one operation unit for convenience). Assuming that the product of the ethane recycle cracking process is M3And M4Their yields can be expressed by equation (1) and equation (2), respectively:
the yield of recycled ethane is shown in equation (3):
each time the cycle number is ncThen pass through nc-NcAfter a second cycle, product M3And M4The yield of (c) can be expressed by the following formula (4) and formula (5), respectively:
the percentage of remaining ethane is shown in equation (6):
if eta2< 0.0005, the amount of ethane remaining can be approximately neglected, usually r'1＝r1Then, the corrected variation value is:
thirdly, analyzing the optimal and near suboptimal solution sets obtained based on the P-graph algorithm
And automatically generating an objective function and a constraint according to the maximum structure and the parameters of the PNS problem, and giving an optimal solution and a large number of suboptimal solutions based on an accelerated branch-and-bound (ABB) algorithm. The established model contains three types of constraints: (1) the upper feed supply and lower product yield (ethylene) constraints, which are determined by the feed capacity of the refinery light feedstock; (2) the upper and lower limits of the processing capacity of the equipment are usually determined by design values or data of actual operation of the plant; (3) material balance of each node.
An objective function maxprofit shown in formula (9) is automatically calculated according to the provided raw material price and product price:
and solving the optimal parameters under each feasible structure according to the objective function, and sequencing according to the objective function maxprofit to give optimal and suboptimal solutions. Firstly, an index for measuring the light weight of the raw material is calculated based on the optimal solution and the secondary solution, and the light weight index adopts the light raw material proportion rale as the formula (10)nTo show that:
in the formula, FiIs a raw material RiOf (2) a traffic, coefficienti(l) Is reacted with a starting material RiIs of a natureCoefficient of relevance when starting material RiWhen the average carbon chain length l of (2) is not more than 6, coefficienti(l) -1, otherwise coefficienti(l)-0。
The influence of the refinery-ethylene integration process on the environment is characterized by the index SPI as shown in formula (11):
And then, carrying out multi-index analysis on the optimal and suboptimal solution sets obtained by modeling, and analyzing the limit of light weight and the influence of the light weight on the economy and emission of the whole ethylene flow by combining the visual hypergraph chart corresponding to each solution so as to provide an operation guide with operability.
Example 1
To verify the effectiveness of the process, there were 11 cracking furnaces (N) in a given refinery-ethylene co-production ethylene plant before co-productionF11), one of which is ready for use. The ethylene plant uses naphtha (NAP, R)1)，Hydrogenated tail oil (HVGO, R)2) Liquefied petroleum gas (LPG, R)3) Recycle of ethane (R)4) And C5 (R)5) As cracking feedstock, hydrogen (H2, P) is produced1) Methane (CH4, P)2) Ethane (C2H6, P)3) Ethylene (C2H4, P)4) Propylene (C3H6, P)5)，C4(P6) And pyrolysis gasoline and diesel (C5+, (P)7) Etc.).
The distribution of the raw materials in the whole plant before co-production is shown in Table 1, in which case the weight reduction of the raw materials is 4.16% and the SPI is 3.405X 10 by applying the formulas 9, 10 and 114year·km2The economic benefit is 51582 $/h.
TABLE 1 Pre-Cogeneration Condition
According to the calibration test of the whole ethylene flow, the working conditions of different separation systems are changed when the load is greatly changed as shown in table 2. The calibration test is carried out on the working state of the whole process under the condition of load change.
TABLE 2 variation of operating conditions of the separation system with load
In the refining and chemical integration process, light raw materials are increased. The supply of light feedstocks to refiners upstream of the ethylene production facility is shown in table 3. This supply is the maximum degree of process lightening.
TABLE 3 feedstocks for refinery-ethylene integrated post-ethylene plants
Based on the data and the equipment screening method, the main equipment for modeling is determined to be a cracking furnace group and analysis equipment, the compression part and the quenching part are combined for modeling, and material balance modeling needs to be carried out on the refinery gas pretreatment and fuel pipe network system. And then, performing ring-opening calculation based on the formulas 1 to 8 to establish a superstructure model. Some of the parameters used for the superstructure model optimization are shown in table 4.
TABLE 4 parameters associated with index calculation
The superstructure created is shown in fig. 3. The calculation of multiple indexes is performed on the obtained optimal solution and the near suboptimal solution based on the formulas 9 to 11, and the results are as follows:
the maximum profit appears as three peaks in all sub-optimal solutions. The highest peak in economic indicator corresponds to a maximum profit of $ 126677/hour, an increase of 21.79% compared to the performance before refinery-ethylene integration ($ 100722/hour). The SPI value for this solution was 47.77(km2 years), although better than that before co-production (48.02km2 years), but it was not the optimal solution under current environmental policy tightening. The second highest peak in the economic indicator corresponds to an SPI of 26.07km2 x years, which represents an economic benefit of $ 122336/hour, and the economic benefit of this solution is slightly reduced (3.43%) but the environmental protection effect is greatly improved (SPI is reduced by 45.43%) compared to the most profitable solution. In conclusion, the optimized proposal that the light hydrocarbon ratio is 11.91 percent, the economic benefit is improved by 21.47 percent, and the negative impact (SPI) on the environment is reduced by 40.27 percent is recommended.
The full-process ethylene superstructure model based on the P-graph framework has the advantage of being capable of obtaining a series of suboptimal solutions. In the modeling process, a method for screening related equipment of a complex industrial process is provided according to the characteristics of 'three-pass-one-reverse' and a problem to be researched. Based on the limit principle, a ring-opening strategy of a specific situation in the logistics circulation of the complex industrial process is given. Practical cases verify the effectiveness of the modeling method, and reveal the advantages and limitations of the light ethylene cracking raw material under the integrated oil refining-ethylene background: for the plants involved in the case, the light feedstocks are temporarily not completely converted into feedstocks for ethylene cracking, limited by the processing capacity of the separation apparatus. Under the optimal condition, the light degree (light raw material ratio) of the raw materials can reach 11.91 percent, the economic efficiency of the ethylene process can be improved by 21.47 percent, and the emission can be reduced by 40.27 percent.
The present invention is not limited to the above-described examples, and various changes can be made without departing from the spirit and scope of the present invention within the knowledge of those skilled in the art.
Claims (3)
1. A P-graph-based ethylene full-process superstructure model modeling method is characterized by comprising the following steps:
1) establishing a model by taking a process of converting straight-chain alkane into olefin as a target object, wherein the process comprises four parts of cracking, quenching, compressing and separating, and the quenching and compressing processes are simplified into an operation unit type node;
2) the ethane circulation existing in the process is subjected to ring-opening, a ring-opening superstructure is constructed, and M is used2And M1Representing ethane, the product of the ethane cyclic cracking process being M3And M4The corresponding yields are then as follows:
in the formula, rnThe yield of recycled ethane for the ratio corresponding to the superstructure edge n is shown in equation (3):
each time of the loop is numbered asncThen pass through nc＝NCAfter a second cycle, product M3And M4The yields of (a) are expressed by the following equations (4) and (5), respectively:
over a number of iterations, the percentage of remaining ethane is shown in equation (6):
if eta2< 0.0005, the remaining amount of ethane is approximately neglected, r'1＝T1Then, the corrected variation value is:
3) analyzing the optimal and near suboptimal solution sets obtained based on the P-graph algorithm:
generating an objective function and a constraint according to the maximum structure and parameters of the PNS problem, and giving an optimal solution and a large number of suboptimal solutions based on an accelerated branch definition algorithm, wherein the established model comprises three types of constraints: (1) upper feed supply and lower product yield (ethylene) constraints; (2) upper and lower limits of the processing capacity of the equipment are restricted; (3) material balance of each node;
calculating an objective function as shown in equation (9) according to the raw material price and the product price:
the optimal parameters under each feasible structure are solved according to the objective function (9), and sequencing is carried out according to the objective function maxprofit so as to give optimal and suboptimal solutions; firstly, an index for measuring the weight of the raw material is calculated based on the optimal solution and the secondary solution, and the weight index adopts the weight ratio of the light raw material as the formula (10)LTo show that:
in the formula, FiIs a raw material RiOf (2) a traffic, coefficienti(l) Is reacted with a starting material RiCoefficient of property when starting material RiWhen the average carbon chain length l of (2) is not more than 6, coefficienti(l) 1, otherwise coefficienti(l)＝0；
The influence of the refinery-ethylene integration process on the environment is characterized by the index SPI as shown in formula (11):
2. The P-graph based ethylene of claim 1The modeling method of the full-flow superstructure model is characterized in that the fuel consumed by the cracking process is synchronously considered due to the sustainability index
The fuel consumption of the cracked section is estimated based on historical data fit based on the flow rate of the feedstock and the properties of the feedstock.
3. The modeling method for the full-flow superstructure model of ethylene based on P-graph according to claim 1, characterized in that the catalytic dry gas in the feed needs to remove the acid gas not participating in the cracking process.
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