Li Jinke Liu Wei Xu Hongbing
(Tianhua Chemical Machinery and Automation Research and Design Institute)
Abstract: The fluid distribution of cracked gas in the inner tube of quenching heat exchanger was simulated by CFD software, and the fluid distribution under three kinds of flow channels was obtained. The simulation results show that the fluid distribution in the inner tube can be effectively improved by increasing the fluid distributor or changing part of the inner tube structure.
Key words: quenching heat exchanger fluid distribution numerical simulation
CLC number: TQ051·5 Document code: A Article ID: 0254-6094(2010)01-0043-05
In the ethylene cracking unit, the most used of the cracking gas quenching heat exchanger is a conventional single inlet double casing quenching heat exchanger, and the heat exchange unit is an upper and lower elliptical header, an inner tube and an outer tube. The inner tube takes the cracking gas, and the high-pressure feed water passes through the lower elliptical header tube and enters the annulus between the outer tube and the inner tube to exchange heat with the cracking gas, and the density difference of the water vapor is increased and derived by the upper elliptical header [1]. The traditional double-casing quenching heat exchanger has the advantages of simple structure, convenient manufacture and wide application. However, due to the fluid distribution problem at the cracking gas inlet, the flow rate of the cracking gas in the inner tube is unevenly distributed, that is, the flow rate of the cracking gas in the inner tube is large. The residence time is short, the coking is less, and the inner pipe cracking gas flow in the surrounding area is small, the residence time is long, the coking is serious, and some inner pipes are even blocked, which reduces the utilization of the heat exchange pipe (inner pipe) The rate causes the cracking gas outlet temperature to rise faster, which limits the operating cycle of the quenching heat exchanger [2]. Therefore, it is necessary to rationally design the cracking gas inlet flow passage so that the cracking gas entering the double casing quenching heat exchanger is distributed as evenly as possible, so that the cracking gas flow rate and the coking degree in each inner tube are substantially the same, and the cracking gas outlet is lowered. The heating rate is increased, thereby effectively extending the operating cycle of the quenching heat exchanger.
There are many factors affecting gas distribution, such as the arrangement of the inner tube, the shape and structure of the flow path at the cracking gas inlet, the resistance drop when the cracked gas passes through the inner tube, and the flow rate of the cracked gas. The author mainly analyzes the distribution of cracked gas in the inner tube from the structural aspect to solve the problem of uneven fluid distribution in the quenching heat exchanger of a device. In order to solve this problem, three options were selected: changing the geometry of the original design inlet flow channel; adding the fluid distributor on the basis of the former solution; changing the inner tube structure of the central part based on the first solution, by means of the calculation fluid The dynamics (CFD) method, by comparing the simulation results of fluid distribution under three schemes, leads to an optimal scheme to guide the design.
Introduction to 1 3 programs
The first scheme modified the original design of the flow path. The geometry of the cracking gas inlet flow passage (Fig. 1) has a great influence on the fluid distribution of the cracked gas. If the expansion angle α is greater than 6°, severe boundary layer detachment may occur and a large vortex zone may appear [2]; Using only one expansion angle (ie, β = α) will inevitably lead to an excessively long H, which not only increases the residence time of the adiabatic segment, but also causes unnecessary waste of resources. In addition, the length h1 of the distribution space also has a large influence on the gas distribution, and h1 is more favorable for gas distribution, but also prolongs the residence time of the cracked gas in the adiabatic section, resulting in an increase in olefin loss. Therefore, it is important to choose the size of α, β, h and h1 reasonably.
The second option is to provide a fluid distributor between the cracking gas inlet of the quenching heat exchanger and the tube sheet. The main function of the fluid distributor is to eliminate the unevenness of the flow in the cross section of the distribution space and to capture and comminute the coke from the cracking furnace tube, because the flow in the inner tube of the cracking gas inlet is large, so the fluid The distributor is arranged at the center of the flow channel to block the partial cracking gas flowing into the central region. In order to prevent the excessive fluid from being blocked, the flow in the central region is too small, and the distributor is opened at the center and both sides of the fluid distributor. A slot for guiding the cracked gas.
The third scheme is to change the size of the lower end of the inner tube at the central portion, and reduce the flow rate of the cracked gas in the inner tube at the central portion by reducing the cross-sectional area of ​​the inner tube. This kind of scheme does not need to add other distribution parts, does not need to consume external energy, and has a simple structure, but a weld seam is added to the heat exchange tube.
2 Numerical simulation
The simulation process includes five steps: geometric modeling, meshing, pre-processing, iterative calculation, and post-processing.
2.1 Geometric Modeling
The object of this simulation is the inlet flow path and inner tube of a type of cracking furnace quenching heat exchanger. Firstly, the modeling software is used to establish the flow path model at the inlet. Since the structure of the second scheme equipped with the fluid dispenser is more complicated than the first and third schemes, the simulation process is described by taking the second scheme as an example.
Figures 2 to 4 show the flow path model after the distributor is added, the geometry of the fluid distributor, and the lower structure of the inner tube at the center when the third option is used.
2.2 Dividing the grid
The model was meshed using ICEM 9.0 software (Figure 5), using grid types including tetrahedral, pentahedral, and hexahedral meshes. If all hexahedral meshes are used, the direction of the mesh with the streamline can be basically the same, the orthogonality of the mesh is good, and the quality and calculation accuracy can be guaranteed, but the number of tubes in the quenching heat exchanger is Dozens of them, and the geometry of the runner inlet is relatively simple, only a circular shape. If all the hexahedral meshes are used, the mesh at the inlet is also more, which greatly increases the calculation time. To this end, a transition section is provided between the flow channel inlet and the inner tube flow channel, the transition segment mesh is a tetrahedral mesh, and the tetrahedral mesh is connected with the hexahedral mesh by a pentahedral mesh. In this way, most of the grids in the calculation domain are still hexahedral grids, and the grids in the inner tubes are denser, and the grid at the inlets of the runners is sparse, which saves calculation time and ensures calculation accuracy.
As can be seen from Fig. 5, the upper mesh of the flow channel is denser, the number of meshes is larger, and the lower mesh is less. The two-part mesh is connected by the intermediate tetrahedron and the pentahedral mesh, preferably Controls the number of overall meshes. As can be seen from Figures 6 and 7, the position of the fluid distributor in the flow path is directly above the flow path inlet, and part of the fluid blocked by the distributor will flow around the central area to the peripheral inner tube, still having a part of the fluid The flow through the middle of the distributor directly into the inner tube, so that the flow in each inner tube is substantially balanced.
2.3 Pre-processing (boundary condition setting and model selection)
The simulation uses CFX 9. 0 commercial software. The boundary conditions of the calculation domain mainly include inlet, outlet and wall: the inlet adopts the mass flow inlet; the outlet adopts the pressure outlet, and the static pressure of the inlet is determined according to the calculation result of the internal flow field; The wall is made of a non-slip solid wall [3].
The calculation uses the standard k-ε turbulence model [4]. Since the focus of this simulation is to observe the distribution of the fluid, it has a great relationship with the geometry of the flow channel and is less affected by the temperature, so this simulation ignores the influence of temperature.
2.4 Iterative calculation
The basic idea of ​​the numerical calculation method is to discretize the continuous problem into a discontinuous problem and then solve it. CFX uses a finite element method based on finite element method to ensure the numerical accuracy of the finite element method is absorbed on the basis of the conservation characteristics of the finite volume method. CFX is the first commercial software to develop and use fully implicit multi-grid coupled solution technology. This solution technique avoids the iterative process of traditional algorithms requiring “hypothetical pressure terms—solving—correcting pressure terms†while solving Momentum equations and continuous equations, coupled with its multi-grid technology, CFX's computational speed and stability are much higher than traditional methods.
2.5 Post-processing (calculation results)
2.5.1 Calculation result of scheme 1 (changing the flow channel geometry)
Since the cracking gas enters the quenching heat exchanger and has a high flow rate itself, the flow rate in the inner tube facing the inlet region, that is, the central portion is large, and the flow rate in the outer inner tube is small. It can also be seen by observing Fig. 8 that the streamline is densely concentrated at the center of the quenching heat exchanger.
In order to better observe the distribution of the fluid, the mass flow rate of the cracked gas in each inner tube was extracted from the calculation results, as shown in Fig. 9.
The circle with the "+" sign indicates the inner tube with the largest mass flow, and the circle with the "-" sign indicates the inner tube with the smallest mass flow, so that the uniformity of the fluid distribution in the inner tube can be obtained (the flow rate is the smallest). The value/maximum is 87.32%.
2.5.2 Calculation result of scheme 2 (increased fluid distributor)
Figure 10 shows the flow line in the flow path after the fluid distributor is added. As can be seen from Figure 8, when the fluid flows through the distributor, the cracking gas entering from the inlet of the flow channel is split, and the flow line near the central area is now flown. There is a decrease, and the fluid flow line in the peripheral area is increased, effectively reducing the flow rate in the central area.
The mass flow rate of the cracked gas in each inner tube was also extracted from the calculation results for comparison of fluid distribution, as shown in Fig. 11.
At this time, the uniformity of the fluid distribution in the inner tube (flow rate minimum/maximum value) is 92.25%, and it can be seen that the mass flow rate in the tube in the central region is controlled, and the fluid distribution is more uniform.
2.5.3 Calculation result of scheme 3 (changing the inner tube structure of the central area)
Figure 12 is a flow chart showing the flow path in the inner tube after changing the inner tube structure. Compared with Fig. 8, there is not much difference in the distribution of the streamline, but the reduction of the diameter of the tube at the lower end of the tube in the central region can reduce the flow rate of the cracked gas through the inner tube, thereby making the distribution more uniform.
Figure 13 shows the specific distribution of the fluid. Although the inner tube with the largest flow rate is still in the central area, the value of the fluid is significantly reduced. The uniformity of the fluid distribution in the inner tube (flow rate/maximum value) is 92.24%, so that the overall distribution of the fluid is obtained. Significant improvement.
3 Conclusion
With the help of CFX 9. 0 fluid analysis software, the author simulated the fluid distribution of the cracked gas in the quenching heat exchanger based on numerical calculation. From the results of the simulation, the uniformity of the fluid distribution is only 87.32% when the inlet flow channel geometry and the structural size are changed; the value can be raised to 92. 25% after the fluid distributor is increased; The uniformity of the lower structure is 92.24%. It can be seen that the fluid distribution in the quench heat exchanger can be made more uniform by increasing the fluid distributor or changing the lower structure of the tube in the central region.
Due to the difference in the number of tubes, the number of inner tubes, and the geometry and size of the inlet runners in the quenching heat exchanger, when the fluid distributor is used to improve the distribution, the structural size of the distributor should be changed according to the flow passage. When improving the fluid distribution by changing the structure of the lower part of the tube in the central region, it is also necessary to determine the optimal size of the inner tube lower structure, the number of inner tubes in the central area, and the range. In summary, CFD software must be used for analysis in different situations to determine the optimal fluid distribution improvement.
references
1. Wang Songhan. Technology and operation of ethylene plant. Beijing: China Petrochemical Press, 2009
2. Gu Datian, Fang Zifeng. The whole book of chemical equipment design - waste heat boiler. Beijing: Chemical Industry Press, 2002
3. Han Zhanzhong, Wang Jing, Lan Xiaoping. Examples and Applications of FLUENT Fluid Engineering Simulation Calculation. Beijing: Beijing Institute of Technology Press, 2004
4. Wang Fujun. Computational Fluid Dynamics Analysis——CFD Software Principles and Applications. Beijing: Tsinghua University Press, 2004
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