CFD based parametric analysis of gas flow in a counter-flow wet scrubber system

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1 Internatonal Journal of Envronmental Protecton and Polcy 2013; 1(2): Publshed onlne July 10, 2013 ( do: /.epp CFD based parametrc analyss of as flow n a counter-flow wet scrubber system Bashr Ahmed Danzomo 1, *, Momoh-Jmoh Enyomka Salam 1, Rasuddn Mohd Khan 1, Mohd Iskhandar Bn Mohd Nor 2 1 Department of Mechatroncs Enneern, Internatonal Islamc Unversty, P. O. Box 10, 50728, Kuala Lumpur, Malaysa 2 Chemcal Enneern Department, Unversty of Malaya, 50603, Kuala Lumpur, Malaysa Emal address: bdzomo@mal.com(b. A. Danzomo) To cte ths artcle: Bashr Ahmed Danzomo, Momoh-Jmoh Enyomka Salam, Rasuddn Mohd Khan, Mohd Iskhandar Bn Mohd Nor. CFD Based Parametrc Analyss of Gas Flow n A Counter-Flow Wet Scrubber System. Internatonal Journal of Envronmental Protecton and Polcy. Vol. 1, No. 2, 2013, pp do: /.epp Abstract: Envronmental protecton measures reardn ndustral emssons and thtened reulatons for ar polluton led to the selecton of a counter-flow wet scrubber system based on applcablty and economc consderatons. The flow dynamcs of as transportn partculate matter and aseous contamnants s a key factor whch should be consdered n the scrubber desn. In ths study, as flow feld were smulated usn ANSYS Fluent computatonal fluds dynamc (CFD) software based on the contnuty, momentum and k-ε turbulence model so as to obtan optmum desn of the system, mprove effcency, shorten expermental, perod and avod dead zone. The result shows that the resduals have done a very ood ob of convern at mnmum number of teratons and error of 1E-6. The velocty flow contours and vectors at the nlet, across the scrubbn chamber and the outlet shows a dstrbuted flow and the velocty profles have fully conformed to the recommended profle for turbulent flows n ppes. The total pressure wthn the scrubber cross-secton s constant whle the mnmum and maxmum pressure drops was obtaned to be 0.30pa and 3.03pa whch has conformed to the recommended pressure drop for wet scrubbers. From the results obtaned, t can be deduced that the numercal smulaton usn CFD s an effectve method to study the flow characterstcs of a counter-flow wet scrubber system. Keywords: Computatonal Flud Dynamcs, Counter-Flow Wet Scrubber, Parametrc Analyss, Gas Flow 1. Introducton A counter-flow wet scrubber system s an ar polluton control devce that uses lqud spray to control partcles contaned n a partculate matter (PM) and aseous contamnants that are ben emtted from ndustral producton. The smplest type of the scrubber s the cylndrcal or rectanular system n whch ar and contamnants passes nto a chamber where t contacts a lqud spray produced by spray nozzles located across the flow passae. As ndcated by [1], counter-flow scrubbers have mportant advantae when compared to other ar polluton control devces. They can collect flammable and explosve dusts safely, absorb aseous pollutants, collect msts, cool hot as streams and t has the potental to save 35 to 40 percent over the cost of conventonal ar polluton control equpment for brck klns whle meetn new standards for the control of waste as (dust) emssons from cement fabrcaton, hydroen fluorde, hydroen chlorde (HCl) and partculate matter (PM) [1]. Such technoloy has been successfully used for medcal waste ncneraton and other ndustral applcatons. The unt causes very lttle pressure drop, can handle lare volume of ases and t can be employed for the dual purpose of absorbn aseous pollutants whle removn partculates. Accordn to USEPA and NACAA [2], the flow dynamcs of the as transportn the partculate matter (PM) and aseous contamnants nto the scrubber system s a key factor whch should be consdered n desnn the system for as and partcle separatons. If the dra force of the as s not suffcent enouh to transport the partcles wthn the nlet and outlet ducts and nto the scrubbn chamber, the partcles may depost alon the duct ways and the chamber and the as may develop another swrln acton wthn the chamber. Parametrc CFD analyss has shown to play a crucal role n the enneern desn of flud-dynamc devces. Wthout t, enneern methods can be costly and tme consumn. An understandn of the staes nvolved n parametrc CFD analyss ves user a better understandn of how that devce behaves over ts entre operatn envelope and quckly

2 Internatonal Journal of Envronmental Protecton and Polcy 2013; 1(2): produces better desns. Hudson [3], Kennedy et al. [4] and Mnhua et al. [5] ndcated that, computatonal flud dynamcs (CFD) approach has promsn applcatons n both dry and wet counter-flow scrubber systems for ndustral ar polluton applcatons. It has the beneft of developn a vrtual prototype whch can evaluate and compare desn alternatves wthout materal, constructon and testn cost, the advantae of enabln and mprovn enneern desn wth tht marns, smulatn complex flud flow phenomena at a reduced cost and demonstratn proect feasblty pror to constructon as n [6,7,8,9]. The man obectve of ths study s to perform a CFD based parametrc numercal analyss of the as flow dynamcs of a counter-flow wet scrubber system so as to explore the velocty and pressure feld profles and optmze the as-pm flow n the scrubber system so as to avod dead zone wthn the systems cross secton. 2. Materals and Method 2.1. The Case Study Specfcatons The studed wet scrubber has been desned usn hydraulc smltude method by consdern data obtaned from cement ndustry as shown n Table 1. Schematc daram of the system s descrbed n Fure 1. Table 1. Summary of Kln Shop Exhaust PM-Laden Gas Data Operatn Parameter Operatn Data Volume Flow Rate 104,885 m 3 /hr Mass Flow 119,084 k/hr Gas Temperature C Dust Burden (Inlet) 62,500 µ/m 3 Dust Burden (Outlet) 25,000 µ/m 3 Source: Ashaka Cement Company [10] Fure 1. Geometrc Sketch of the Scrubber Consdern the fact that, the scrubber as outflow s our subect of concern n ths study; a curved transton usn hood desn approach was provded so as to create shape and ood flow characterstcs of the as out of the scrubber wth mnmum frcton loss. However, ths transton was not provded at the slurry ext, because ts flow characterstcs have not been consdered n the study Mathematcal Modeln The approprate overnn equatons for the as flow are the ncompressble Nerver Stokes Equaton (NSE) for a snle phase flow obtaned from the conservaton of mass and momentum descrbed by Catn, et al. [7]. Contnuty equaton Momentum equaton ( U U ) = µ eff ( U ) = 0 P Where, and are the drecton vectors for the three coordnates (x, y, and z) whle U and U are the three velocty components (u, v, w) of the as flow, µ eff s the effectve dynamc vscosty of as, s the as densty, P s the as pressure and s the as volume force n -drecton respectvely. To avod the unnecessary consumpton of the computer tme for the soluton of the full-scale, turbulence model for predctn the effects of turbulence n the as phase should be consdered. Averan s often used to smplfy the soluton of the overnn equatons of turbulence, but models are needed to represent scales of the flow that are not resolved. One of the most effectve vscosty models for the smulatons of the turbulent flow s the Harlow-Nakayama k-ε model of the turbulent flow descrbed n (3) and (4). The model provdes the tme averaed values of veloctes and pressure of the as or ar throuhout the system. k equaton ( ku x µ t µ t k µ δ k ) U k J = ε (1) (2) (3)

3 18 Bashr Ahmed Danzomo et al.: CFD Based Parametrc Analyss Of Gas Flow In A Counter-Flow Wet Scrubber System ε equaton ( εu ) x c1 ε µ t k = µ t µ δ ε J ε c 2 2 ε k (4) profle by usn effcent meshn parameters such as; the rowth rate and mesh refnement as shown n Fure 3. A rowth rate of 1.4 and a refnement of 3 were used across the scrubber nlets, outlets and the wall parts so as to provde a smooth flow transton across the system. Where k s the turbulent knetc enery, ε s the turbulent knetc enery dsspaton rate; µ t s vscosty coeffcent of turbulent flow whle δ k, δ ε, c 1, c 2 are constants; c 1 = 1.44, c 2 = 1.92, δ k = 1.0, δ ε = 1.3 [7]. The standard k-ε model was based on the hypothess of the sotropc eddy-vscosty, whch was modeled throuh the flow felds of the turbulent knetc enery and the specfc dsspaton rate. Accordn to [11], the model leads to stable calculatons that converes easly and allows reasonable predctons for many flows. Also, [7] ndcated that k-ε s the most commonly used turbulence model whch has many advantaes; ts concept s smple and t has been mplemented n many commercal CFD codes. The model has demonstrated the capablty to smulate many ndustral processes effectvely as n [6, 12, 13, and 14] Numercal Computaton Methods A dmenson of the scrubber n Fure 1 has been used to enerate a 3D eometry of the scrubber system usn CAD. The eometry was then mported nto ANSYS Fluent [15] for processn. As shown n Fure 2, the ppe eometry for the scrubbn lqud spray and the slurry outlet duct were suppressed from the man eometry so as to analyse only the as flows across the scrubbn chamber. Fure 2. Scrubber Geometres n ANSYS Fluent Workbench Usn the ANSYS Fluent [15] workbench and mesh wndows the volume for the flow dynamcs was flled and the boundary condton defned. The calculaton doman of the scrubber eometry was appled to produce ood mesh Fure 3. Boundary Condton and Mesh Profle of the Scrubber Geometres Ths yelded a statstcs of 91,051 nodes and 459,870 elements and an aspect rato of and skewness of However, these values are wthn the requred rane of mesh skewness 1.0 and aspect rato 10 respectvely. For the boundary condton, velocty nlet has been selected at the as nlet whle pressure outlet was chosen for the as outlet. Values for the nlet veloctes were chosen from the rane of USEPA and NACAA [16], Nala et al. [17] and Garba [18] recommended PM transport veloctes; 0.32, 0.79 and 1.27m/s. The smulaton starts by dentfyn and applyn condtons at the doman boundares and then solvn the overnn equatons on the computatonal mesh teratvely as a steady-state. Snce ths s a pressure based soluton method, the PISO (Pressure Implct wth Splttn of Operators) alorthm recommended by ANSYS Fluent [19] for solvn steady flow calculatons (especally when the soluton nvolves turbulence model) has been used. Also, [19] ndcated that enerally for tranular and tetrahedral meshes, a more accurate result s obtaned by usn the second-order dscretzaton. There are sx dfferental equatons to be solved n 3D whch resulted to sx resduals for converence; contnuty, x-velocty, y-velocty, z-velocty and k-ε turbulent equatons respectvely. The CFD soluton were calculated usn effcent resdual of 1E-6 startn from 50 teratons whch were ncreased to hher value dependn on the delay for the solutons to convere. These resduals represent a knd of averae error n the soluton. An absolute ntalzaton was set to compute from the as nlet. Consdern the fact that, the flow feld n the wet scrubber s complcated. The as flow CFD smulaton of the scrubber system s based on the follown assumptons:

4 Internatonal Journal of Envronmental Protecton and Polcy 2013; 1(2): The as phase flow was only consdered n the scrubber and no lqud phase s taken nto account, 2. The flow s stable and sothermal, so heat exchane amon the phase s not consdered, 3. The as s rearded as ncompressble, 4. The PM transport velocty recommended by USEPA and NACAA [16] was consdered for the smulaton. Graphc dsplays of the velocty felds whch ncludes velocty contours descrbn the as flows from the nlet, across the scrubbn camber to the outlet for the three nlet veloctes consdered n the study s shown n Fures 7, 8 and 9 (a and b)respectvely. 3. Results and Dscusson From the smulaton expermentatons, the resduals does a very ood ob of convern at approxmately 590, 500, and 550 teratons at nlet veloctes of 0.32, 0.79 and 1.27m/s as shown n Fures 4, 5 and 6. Fure 7a. Y-Coordnate Contour Dsplay of the Gas Flow across the Scrubber at 0.32m/s Fure 4. Resduals for the Contnuty, k-epslon and X, Y, Z Veloctes at 0.32m/s Fure 7b. X-Coordnate Contour Dsplay of the Gas Flow across the Scrubber at 0.32m/s Fure 5. Resduals for the Contnuty, k-epslon and X, Y, Z Veloctes at 0.79m/s Fure 8a. Y-Coordnate Contour Dsplay of the Gas Flow across the Scrubber at 0.79m/s Fure 6. Resduals for the Contnuty, k-epslon and X. Y, Z at 1.27m/s

5 20 Bashr Ahmed Danzomo et al.: CFD Based Parametrc Analyss Of Gas Flow In A Counter-Flow Wet Scrubber System Fure 10. Velocty Vector of the Gas Flow wthn the Geometry Fure 8b. X-Coordnate Contour Dsplay of the Gas Flow across the Scrubber at 0.79m/s For the other veloctes; 0.79 and 1.27m/s, the as s partally dstrbuted across the centre and fully dstrbuted across the sde walls. Fures 10 and 11 shows a stream lne flow alon the scrubber nlet and the outlet ducts at 0.32m/s whch have conformed to the Bernoull s theorem Fure 9a. Y-Coordnate Contour Dsplay of the Gas Flow across the Scrubber at 1.27m/s Fure 11. Velocty Vector of the Gas Flow at Inlet 0.32m/s Fure 9b. X-Coordnate Contour Dsplay of the Gas Flow across the Scrubber at 0.79m/s The velocty contours n X and Y coordnates above descrbn the as flow across the scrubber shows a more dstrbuted flow across the spray chamber for a 0.32m/s velocty whch are further supported by the velocty vector dsplays across the chamber shown n Fures 10, 11 and 12. Fure 12. Velocty Vector of the Gas Flow towards the Ext at 0.32m/s Not all flud partcles travel at the same velocty wthn a cylndrcal ppe. The shape of the velocty curve whch s the velocty profle across any ven secton of the ppe depends upon whether the flow s lamnar or turbulent as shown n Fure 13 [20, 21].

6 Internatonal Journal of Envronmental Protecton and Polcy 2013; 1(2): Fure 13. Velocty Profle across a Cylndrcal Ppe for Lamnar and Turbulent Flows As shown n fure 13, f the flow n a ppe s lamnar, the velocty dstrbuton at a cross secton wll be parabolc n shape wth the maxmum velocty at the centre ben about twce the averae velocty n the ppe. In turbulent flow, a farly flat velocty dstrbuton exsts across the secton of ppe, wth the result that the entre flud flows at a ven snle value. The velocty of the flud n contact wth the ppe wall s essentally zero and ncreases the further away from the wall. To valdate the as flow nto the scrubber system, velocty profles for the three veloctes alon the nlet duct was plotted as shown n Fures 14, 15 and 16. across a wet scrubber system s an mportant parameter n evaluatn the performance and operatons of the system especally for PM removal. The pressure drop s a measurement of the resstance to flow as the as passes from one pont to another pont and s smply the arthmetc dfference between the statc pressure at the scrubber nlet and outlet (measured at rht anles to the flow). The value of the pressure drop takes nto account the amount of enery loos durn wet scrubbn process. Accordn to [22], the system pressure s computed usn the aue pressure (pressure measured n a system) and the atmospherc pressure ven as; P = P P (5) system aue The ANSYS Fluent [15] uses pressure aue to measure the statc pressure whch s also the scrubber systems pressure at the nlet and outlet as shown n Fures 17a, 17b and 17c for the three veloctes. As shown above, for the 0.32m/s velocty the statc aue pressure was obtaned to be 0.309pa and at the nlet and the outlet of the system. Also, for the 0.79m/s, the statc aue pressure was obtaned to be 1.32pa and pa at the nlet and outlet. However, a statc aue pressure of 3.16pa and 0.131pa at the nlet and outlet for the 1.27m/s was obtaned. The pressure drop, P for the three flows veloctes was computed usn the relaton atm P = P system P (6) ( 1) system (2) Comparson between the dynamc smltude pressure obtaned from the recommended pressure drop for spray tower scrubber and the computed pressure drop from the CFD analyss shown n Table 2 ndcated a closer areement between the two. Table 2. Comparson between the Recommended and Computed Pressure Drop Fure 14. Velocty Profle for the Inlet Gas at 0.32m/s Gas Velocty (m/s) Recommended Pressure Drop (pa) Computed Pressure Drop usn CFD (pa) Fure 15. Velocty Profle for the Inlet Gas at 0.79m/s Snce turbulent model was consdered n the CFD smulaton, from the above fures t can be seen that the velocty profles has fully conformed to the recommended profle for turbulent flows n ppes and cylnders. Accordn to USEPA and NACAA [16], pressure drop Fure 16a. Contour Dsplay of Statc Pressure at 0.32m/s

7 22 Bashr Ahmed Danzomo et al.: CFD Based Parametrc Analyss Of Gas Flow In A Counter-Flow Wet Scrubber System Fure 16b. Contour Dsplay of Statc Pressure at 0.79m/s Fure 17a. Contour Dsplays of Total Pressure at 0.32m/s Fure 17b. Contour Dsplays of Total Pressure at 0.79m/s Fure 16c. Contour Dsplay of Statc Pressure at 1.27m/s The study of pressure across any flud system arse from the Bernoull s equaton whch s snfcant n the study of an ncompressble flud flow that s fundamental to an understandn of the desn and operatons of most flud based devces. Smplfed form of Bernoull s equaton s descrbed by the word equaton as; Statc Dynamc = Constant (7) Equaton (7) above ndcated that the sum of flow statc and dynamc pressure s constant alon a streamlne of flud flow. Accordn to [20], the sum of these two pressures s termed as the total pressure. Fures 18a, 18b and 18c shows total pressure contours for the three veloctes. For 0.32m/s velocty whch s the velocty at whch the scrubber performed optmally n the smulaton analyss, the total pressure s almost constant across the spray chamber. But the pressure s hher across the nlet and slhtly lower alon the ext. For 0.79 and 1.27m/s veloctes, the total pressure s slhtly hher and approxmately constant wthn the chamber. Ths ndcated that the proposed scrubber has conformed to the Bernoull s equaton especally at velocty of 0.32m/s. Total Pressure at 0.32m/s 1.27m/s Fure 17c. Contour Dsplays of Total Pressure at 1.27m/s 4. Concluson ANSYS Fluent software has been used to nvestate the as dstrbuton wthn a counter-flow wet scrubber system based on arflow velocty and pressure felds. The velocty flow contours and vectors at the nlet, across the scrubbn chamber and the outlet (especally at 0.32m/s) shows a dstrbuted flow and the velocty profles have fully conformed to the recommended profle for turbulent flows n cylndrcal ppes. The total pressure wthn the scrubber cross-secton s constant whch follows the Bernoull s prncple. The mnmum pressure drop, P mn for the scrubber system was obtaned to be 0.30pa and the maxmum, P max was 3.03pa whch has conformed to the recommended pressure drop for wet scrubbers. From the par-

8 Internatonal Journal of Envronmental Protecton and Polcy 2013; 1(2): ametrc analyss conducted, t can be concluded that the numercal smulaton usn Ansys Fluent CFD s an effectve method of studyn the flow characterstcs of a counter-flow wet scrubber system. References [1] Frank, R. S. and Nancy, W. E Envronmental Enneers Mathematcs Handbook, CRC Press, Flord, USA, pp [2] Unted States Envronmental Protecton Aency (USEPA) and Natonal Assocaton of Clean Ar Aences (NACAA) Control of Partculate Matter Emssons. APTI 413 Module 8 Student Manual, Onlne: ( anual/sm_ch8.pdf). [3] Hudson Product Corporaton Ar Cooled Heat Exchaner Modeln wth CFD. [4] Kennedy, C., Dwakar, P., Leonad, J.R. and Rosendall, B Computaton Based Enneern of Multphase Process Usn CFD, Bechtel Technoloy Journal, vol. 3, No. 1. [5] Mnhua, B., Qufan W. and Yu, Z Numercal Smulatons of Dust Removal Devce for Qucklme Slackn, Envronmental and Chemcal Enneern Collee, Yanshan Unversty. [6] Gonva C., Prker, S., Tukovoc, Z., Felmayr, C. and Burler, T Smulaton of Off-Gas Scrubbn by a Comned Euleran-Laranan Model, 7 th Internatonal Conference on CFD n the Mnerals and Processn Industres, Melbourne, Australa. [7] Catn L., Shanhon L., Guanmn, Z., Fe W., Dayon W., Honlan, G. And We G Arflow Smulaton of an Umbrella Plate Scrubber, IEEE Conference Publcatons, The 2 nd Conference on Bonformatcs and Bomedcal Enneern, (ICBBE-2008), Shanha, Chna. [8] Shan-hon L., Ca-tn L, Guan-mn Z., S-mn L., Fe W. and Da-yon W CFD Smulaton on Performance of New type Umbrella Plate Scrubber, Transactons of Nonferrous Metals Socety of Chna, vol. 18, pp [9] Dudek, S.A., Roers, J. A. and Gohara, W. E Computatonal Flud Dynamcs (CFD) Model for Predctn a Two-Phase Flow n Flue Gas Desulfurzaton Wet Scrubber, EPRI-DOE-EPA Combned Utlty n Ar Polluton Control Symposum, Atlanta, Geora, USA. [10] Ashaka Cement plc., Gombe State, Nera [11] Karthk, T. S. D Turbulence Models and ther Applcatons, Department of Mechancal Enneern, Indan Insttute of Technoloy, Madras, Inda. [12] Hendaw, M., Molle B. And Folton C Measurement Accuracy Analyss of Sprnkler Irraton Ranfall n Relaton to Collector Shape, Journal of Irraton and Dranae Enneern, vol. 131, no. 5, pp [13] Lee, C. W., Palma, P. C. and Smmons, K Comparson of Computatonal Imae Velocmetry Data for the Arflow n an Aero-enne Bearn Chamber, Journal of Enneern for Gas Turbnes and Power, vol. 127, no. 4, pp [14] Xan, R. B. And Lee, K. W Numercal Study of Flow Feld n Cyclones of Dfferent Heht, Journal of Chemcal Enneern Process, vol. 44, no. 8, pp ANSYS Fluent 13.0 Inc., USA. [15] Unted States Envronmental Protecton Aency (USEPA) and the Natonal Assocaton of Clean Ar Aences (NACAA) APTI: Control of Partculate Matter Emssons, 5th Edton, Chapter 10 Student Manual. Onlne: ( t/413%20student%20manual/sm_ch%2010.pdf). [16] Nala, G. M. Sulaman, A. I. and San, M. U Ar Polluton Control n Cement Factory Usn Horzontal Type Wet Scrubber, Contnental Journal of Appled Scences, vol. 3, No. 1. [17] Garba, M. N Gas Partcle Separaton Usn Wet Scrubber Method, M.En Thess, Department of Mechancal Enneern, Bayero Unversty Kano, Nera. [18] ANSYS Fluent 12.0 Inc., USA. [19] Yunus A. C. and John M. C Flud Mechancs: Fundamentals and Applcatons, Internatonal Edton, McGraw Hll Publcaton, pp [20] Munson, B. R., Youn, D. F., Oksh, T. H. and Huebsch Fundamentals of Flud Mechancs, Sxth Edton, John Wley and Sons, pp [21] Enreern ToolBox, Onlne: (