CFD Analysis of Gas Distributor in Packed Column--Prediction of Gas Flow and Effect of Tower Interna

Transactions of Tianjin UniversityVoL 10 No 4DeC.2004CFD Analysis of Gas Distributor in Packed ColumnPrediction of gas flow and effect of tower internalsGeometry StructureZHANG LU-hong張呂鴻), ZHoU Hai-ying(周海鷹),LⅫin-gang李鋼),DU Yu-ping(杜玉萍)School of Chemical Engineering and Technology Tianjin University Tianjin 300072, ChinaAbstract Computational fluid dynamics( CFD simulations were carried out on the gas flow patterns of twintangential annular deflector gas distributor in the absence of liquid flow in a packed column( 6. 4 m in diame-ter), and the gas flow field in the column was presented close to reality on the whole. Furthermore after ame.lioration of this gas distributor frame turbulence energy and turbulence energy dissipation rate were both decreased greatly. Simulation results showed that the flow pattern and the distribution of gas flow were strongly af-fected by the column bottom frame the proper column bottom frame could decrease the flow pressure drop greatly. Multifold factors such as the column bottom geometry structure and distributor structure which affects theKeywords packed column gas distributor CFD simulationArticle I:006-4982(2004)4-0270-05As one of internal parts, the feed gas distributor correlations. However, it is necessary to find a more rigoplays an important role in a packed column. The smooth ous approach to the distributor design and operation to exoperation and better quality of products are greatly influ- pand commercial applications which requires more fundaenced by its performance especially in the column with mental understanding of the involved physical processes inlarge diameter short packed bed and low pressure drop. the distributor.Due to the importance of the uniformity of inlet gas distriodynamIcsof the gas flow wasbution, it is essential to design a good feed gas distributor. studied in the absence of liquid flow. In particular specif-However, few theoretical and experimental approaches ic attention was paid to the gas flow and factors whichhave been carried out in this area so far due to the com- could affect flow field in terms of column internal geometrylexity of the fluid flowl 2Jstructure. To address the complex internal geometry of theThe liquid flow structure is improved by a new distrib- column a computational fluid dynamics( CFD )approachutor design which can create flows with good initial uni- was used to analyze the fluid flows. This is particularlyformity. This is achieved by the development of packing for necessary due to the presence of baffles which greatly in-the liquid flow redistribution and internal devices for elimi- fluence gas flow form in the column. The basic CFD modelnation of the wall flow. The gas flow in packed columns for gas-phase flow which will be described in this paperhas been intensively studied in order to improve the distri- will be further developed to analyze the more complex gasbution uniformity or to develop special-designed distribu- liquid( two-phase )flows, as basis for future optimizationof packed column design and operationThe uniformity of gas flow depends on the type of gasIn previous measurements, CFD modeling and CFDdistributors. The initial distribution is particularly impor- analysis of flow patterns about gas distributors have beentant.It is valid especially for packings with high efficiency carried out simply and conventionally. Ref. 7 modeleand low pressure drop. Most modern packings are of this andtype. The investigations have been focused on the rate of flow中國煤化 Dynamics and gas-phaseuniformity created by various types of gas inlets or speciallyTHCNMHGbutor. In this distribudesigned distribution devicesUntil recently the design of gas distributors has beenZHANG Li-hong, bon in 1966 female Dr associate Profmainly based on empirically established procedures andE-mailzhanglvh@eyou.comZHANG Li-hong et al aFD Analysis of Gas Distributor in Packed Columntor the gas flow was treated with a boundary layer approxi- sionand the finite element numerical means which was 1. 3 CFD model implementationnovel because of its new geometry structure and first nuhe simplified outlet in the column represented americal simulationcompromise between model complexity and the ability toresolve flow and pressure fields in the regions far from the1 Geometry and computational approachentry and exit boundaries as the distributor was very corplex in structure. Fig. 2 shows the computational grid1.1 Modeled equipmentTwin-tangential annular flow gas distributor is generalthe uniform gas distribution Isure drop and low entrainment in large-scale columns. Itwould be used as the basis of CFD geometry. This distributor was applied to a packed column with a diameter of 6. 4m. The geometry and calculation domain are displ6400Fig 2 Computational grids1. 4 Boundary conditionsSuppose that the gas flow at the inlet is turbulent flownd its velocity is the average uinlet =Q/Ainlet for example, Q=39.5 m/s, and Ainlet =1. 316 m" then u;inlet =30I&m/s)04002)Outleak dFig 1 Solution domain( unit: mm01.2 CFD model numerical consideration3)Wallcolume method and k-e turbulent model are employed to wall is calculated by the wall function o hes c EThe simulations were done by the CFdFor visent, which used hybrid meshes. In this paper the finite enforced at walls. Each physical value in ze to the1.5 Solutisimulate the flow field of the gas distributorTurbulence model must be considered carefully be-Both the processing of the model topology and gridcause of the complex flow and high gas velocity in the anand the solution of the conservation equations for mass andulus channel. Ideal straight pipe flow which was of fully momentum had been carried out by using the aforemendeveloped turbulence could be applied to the inlet. tioned package Fluent with 519 552 total grids shown inTherefore standard k-e model was used to deal with this Eig.2. Steady computation convergered after 13 h of 1 700re[9] The simplec algorithm 10 JIterationspressure correction. The working fluid was chosen as incompressible and Newtonian. As a convergence criterion, 2中國煤化工scussionthe sum of the absolute mass source residuals over all cellCNMHGwas less than 0. 1% of the total flowrate. The variation of it- 2.1TYHues wasFig 3 shows gas flow velocity vector in the column. Ituals for all the equations were consistently decreasing and can be obviously observed that not all of the gas ascendedapproaching their limits of the accuracy for a single preci- along straight line. Some of the gas came into being re-Transactions of Tianjin University Vol 10 No 4 2004verse flow with the rising gas flow and velocity distribution tributor itself as well. Therefore we are narrowing down onbecame uniform gradually. The zones opposite to inlet had the discussion about how those structures affect gas flowthe reverse flow26.1623.573 Effect of geometry structure20,993. 1 Streamlined disposalFig 5 shows the computational grids of the distributorstructure. To reduce energy loss inlet and annulus chan-dified to a streamline foshown in Fig. 6.1064Simulation conditions were the same as prototype, inclu8.05ding turbulence model boundary condition and so on. Sol-ver converged after 1 500 iterationsFig 3 Gas velocity vector in a tangent plane (x=0)In short the gas flow field was largely influenced bytower bottom frame. The column internals hindrance probably resulted in the gas reverse flowfiles by the column bottom geometry structure and the deFig 4 illuminates the velocity contour of different pr4896044.057263.21849Fig 5 Computational grids of the distributor structure1.54090.70217181416289×102652933.491712.57×1034.330482.25×1035.16926193X106.00803161×1036.84681-768581,28×1038.52436964X102936313642X102-10.20193.21X102C a) Gas velocity contour in a tangent plane(x=0)C a) Prototype4.52×1034.07×1023.62X103.17×10271X102.26X10中國煤化工CNMH1.09×10C b)Gas velocity contour in a columnFig 4 Distributor profile velocity contours( b)Modified patternFig 6 Turbulent dissipation rate E(z=0)272ZHANG Li-hong et al aFD Analysis of Gas Distributor in Packed ColumnThe turbulence degree can be reflected by the turbu- was reduced greatly. It can be concluded that the better-lence kinetic energy h, and turbulent dissipation rate e is a ment of the distributor structure can help reduce energyparameter that characterizes energy loss. Good distributor loss. Fig. 6 illuminates that the gas distribution becamenust have little gas turbulence which results in low pres- more uniform after modifiedsure drop. Tab. I shows comparison between prototype and 3. 2 Effect of column bottom structuremodified pattern k in different zones. It can be observedActually the gas flow in column is quite complexthat the turbulence kinetic energy of modified distributor because there is a complicated flow room. To study thisflow gas flows in three different column bottoms were simTab 1 Comparison of k in different zones of two distributors ulated. Fig. 7 shows three column figures. All the simula-Maximum k of different zones/(tion conditions except the column bottom structures wereDistributorTangent planthe same. The velocity distribution of the tower section a-Inlet sectionbove the gas distributor in three columns was dissimilarPrototypesee Fig. 8). Altogether, a favorable bottom structureModified pattern14.5l1.2could help gas flow upward(a) Taper bottom(b) Flat bollon(c) Are bottomFig 7 Tower bottom configurations5824.283.512902291.070460.320.15-1340.76(a) Taper botton(b) Flat bottom(e)Arc bottomFig 8 Velocity distribution 1.5 m above distributor in cross section direction of column中國煤化工energy and turbulence4 ConcluHCNMHGand the distribution ofThe rule of the liquid flow through a twin-tangential gas flow are strongly affected by the column bottomannular deflector gas distributor in a column was basically frame. Multifold factors such as the column bottom gerevealed by CFD simulations. After amelioration of this ometry structure and distributor structure, which affects273Transactions of Tianjin University Vol 10 No 4 2004the distribution capacity, must be consideredreactors internals using VOF simulations[ J ]. Chemical EngineerinScience200156(11)385-6391Nomenclature[4] Dong Yiren, Ping Huiqiang, Xu Lian. Study on performance of thethree type gas distributors in packed tower[ J ] Petro-Chemical E-k-Turbulence kinetic energy kg"/uipment, 1994 23(4):11-14( in Chinese8--Turbulent dissipation rate ,o;[ 5] Edwards D P, Krishnamurthy K R, Potthoff R W. Development ofAinlet -Inlet area mproved method to quantify maldistribution and its effect on struc-Ptured packing column performance[ J]. Trans IChemE, 1999,7710)b56-662w rate[6] Muir L A, Briens C L. Low pressure drop gas distributors for packedu -Inlet velocity m/sdistillation column: J] Can Chem Eng, 1986 64( 12): 1027-References[7] Zhang Luhong Li Xingang Jiang Bin et al. Numerical simulation of[1] Ali Q H. 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