CALCULATION OF MASS TRANSFER IN MULTHWASE FLOW NSF, I/CJCRCCORROSION IN MULTIPHASE SYSTEMS CENTER DEPARTMENT OF CHEMICAL ENGINEERING

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1 Paper 5 Nm CALCULATION OF MASS TRANSFER IN MULTHWASE FLOW L. WANG, M. GOPAL NSF, I/CJCRCCORROSION IN MULTIPHASE SYSTEMS CENTER DEPARTMENT OF CHEMICAL ENGINEERING OHIO UNIVERSITY, ATHENS, OHIO, USA ABSTRACT This paper summarizes the results f mass transfer mechanisms under disturbed liquid-gas flw in 1 cm diameter pipe using electrchemical limiting current density and ptentistatic nise technique. The slutin used is ptassium ferr/ferricyanide disslve in 1.3 N sdium hydrxide system. Mass transfer cefficients in full pipe flw and slug flw are btained. The relatinship between mass transfer cefficient with full pipe flw velcities and with slug flw Frude numbers are studied. The impact f bubbles in slugs n the mass transfer cefficient is revealed, The impact f flw disturbance, including weld beads and pits, are discussed fr bth fhll pipe flw and slug flw. Keywrds: full pipe flw, slug flw, disturbed flw, mass transfer, limiting current methd, plarizatin, crrsin. INTRODUCTION Slug flw exists in the pipelines at high prductin rate f il and gas. High velcity slugs are very turbulent and the crrsin rate is increased greatly in this flw regime. This is due t pulses f gas bubbles entrained in the mixing zne being frced twards the bttm f the pipe. There they impact and can cllapse, causing lcalized crrsin. 998 by NACE Internatinal. Requests fr permissin t publish this manuscript in any frm, in part r in whle must be made in writing t NACE Internatinal, Cnferences Divisin, P.O. Bx 21834, Hustn, Texas The material presented and the views expressed in this paper are slely thse f the authr(s) and are nt necessarily endrsed by the Assciatin. Printed in the U.S.A.

2 Further, severe crrsin is fund in the vicinity f flw disturbances such as weld beads, pits, and pipe cnnectins. Thk is thught t be due t the enhanced turbulence and mass transfer rate arund such bstacles. N infrmatin exists at the present time regarding the mass transfer mechanisms under disturbed flw cnditins. It has been fund that the crrsin rate is strngly related t the mass transfer rate f crrsive ins in the multiphase mixture frm the bulk t the pipe wall. It is suggested that the mass transfer cefficient is the main parameter in the crrsin mdeling (Zhang et al., 1997). Hwever, the mass transfer cetlcients in large diameter pipe flw and in multiphase flw are nt knwn. It wuld be very helpful t measure mass transfer cefficients fr multiphase flw systems f interest in il and gas prductin. Mass transfer measurements using limiting current methd prvide a cnvenient and accurate means t determine the lcal mass transfer cefficient. Small insulated electrdes embedded in flw pipe wall culd prvide lcal mass transfer cefficients. Due t their fast respnse, instantaneus fluctuating values, which directly reveal the lcal turbulence culd be btained by ptentistatic current nise measurement. Reiss (1962) described in detail, the technique f making measurements f mass transfer using a diflhsin cntrlled electrlytic reactin, and the use f this tectilque t measure velcity fluctuatins at the wall. Measurements f mass transfer cefficient and intensity were carried ut in 2.54 cm diameter pipe using the ptassium ferr-ferricyanide electrchemical system. The electrdes were made frm three sizes f nickel wire (1.636 mm,.664 mm,.397 mm in diameter respectively). Shaw andhanratty(1964) measured lcal time average mass transfer cefficients with embedded test electrde. Their experiment differed frm that f Reiss in that the test electrdes, instead f being surrunded by inert surface, are surrunded by active electrde surface. Test electrdes f fur different diameter were used, They fund the very large magnitude f the fluctuatins in the mass transfer rate (mass transfer intensity is as much as.47), the relative sizes f the lngitudinal and circumferential scales, and the lw frequency scales f mass transfer fluctuatins. Sirkar and Hanratty (197) btained the rt-mean-square fluctuating mass transfer cefficient fr a Schmidt number f abut 23 in a 7.62 cm diameter pipe, They used rder-f-magnitude analysis and cncluded that flw fluctuatins in the directin f mean flw have little effect n the mass transfer fluctuatins. The lcal value f mass transfer cefficient they btained is mre accurate cmpared with Shaw & Hanratty. Mizushina (1971) discussed the methd f diffhsin-cntrlled electrchemical reactin and its applicatins in the study f transprt phenmena, including mass transfer measurements, shear stress measurements and fluid velcity measurements. He als prvides a review emphasizing the applicatin f limiting current measurements n micrelectrdes fr lcal and instantaneus shear stress and velcity determinatins, as described by Hanratty (1966). 512

3 Selman and Tbias (1978) cmpiled ver a hundred mass transprt crrelatins pertaining t different transprt, flw, and agitatin cnfiguratins. The thery and practice f limiting-current technique fr the measurement f mass-transprt cefficients were described. Landau (198 1) used limiting current technique t determine the mass transprt rates f laminar and turbulent flws. He als discussed the cnsideratin f current distributin, selectin and placement f the electrdes, mdes f current applicatin and the electrchemical system selected. Campbell and Hanratty (1983) studied the structure f the velcity field clse t a wall by measuring the transverse cmpnent f the fluctuating velcity gradient simultaneusly at multiple lcatins n the wail. Velcity fluctuatins f all frequencies appear t have the same transverse scale. Measurement were held in 8 inches pipe system. It is fund that frequency spectra and intensities are universal prperties and nt strngly rdtected by the particular design f the experimental flw system. Mass transfer cefficient is calculated frm the average limiting current using the fllwing equatin: K= I~/(n FACJ (1) where K = mass transfer cefficient 1~= limiting current n = number f mles reacted F= Faraday s cnstant A = surface area f the electrde cb = bulk cncentratin f the ptassium ferrcyanide Mass transfer deviatin is calculated frm the ptential static nise curve using the fllwing equatin: DevK = [.Z(k - Avg~2/N]l 2 (2) where Dev~ = mass transfer deviatin k = instantaneus mass transfer cefficient Avg~ = mean value f the instantaneus mass transfer cefficients N= ttal number f the instantaneus mass transfer cefficients EXPERIMENTAL SETUP The verall layut f the system is shwn in Figure 1. The flw lp is a 1 m lng 1 cm in diameter Plexiglass pipe. A 1 m3 stainless steel tank is filled with 1,3 N sdium hydrxide and.1 M ptassium ferricyanidelferrcyanide. Nitrgen is stred in a pressured gas tank and is added int the system thrugh a pressurized regulatr and a needle valve. 513

4 The test sectin is a 1 cm diameter Plexiglas pipe with electrdes and disturbance blck. The rientatin f the electrdes in the directin f flw are as fllws: the reference electrde, disturbance blck, wrking electrde and cunter electrde. The verall layut f it is shwn in Figure 2. The disturbance blck simulates the disturbance inside a pipe line, such as weld bead and pits. The weld bead is a small hump. The pit is a small cylinder hle. The cunter electrde is a ring electrde munted flush with the pipe wall. The wrking and reference electrdes are hastally pins inserted in the Plexiglass blck, laying ut equal-distantly in a line at the bttm f the pipe. The distance between tw cnsecutive electrdes is 4.5 mm and the surface area f it is 7.85x 1-7 square meter. The dimensins f the weld beads and the pits are described in Table 1 and 2. The data were taken by Gamry sflware CMS 1 installed in a Pentium cmputer. Tw types f results were cllected. The first was the DC plarizatin curve, frm which the andic limiting current was determined. The ther was electrchemical current nise curve that was measured by setting the ptential value at which the limiting current was reached. Table 1, The Dimensin f the Weld Bead Test Sectin Test Sectin 1 mm Weld Bead 2 mm Weld Bead Height (mm) 1 2 Table 2. The Dimensin f the Pit Test Sectin Test Sectin Pit #1 Pit #2 Pit #3 Pit #4 Pit #5 Pit #6 Diameter (mm) Depth (mm) EXPERIMENTAL TEST MATRIX Full pipe flw and slug flw experiments were cnducted fr the ferrlferncyanide system. In rder t study the effect f disturbance resurce, nine test sectins were studied at three different velcities. Table 3 shwed the Experiments carried ut. Table 3. Experimental Test Matrix Flw Velcity (fr fill pipe flw, m/s).5, 1, 2 Frude Number (fr slug flw) 4, 6, 9 Test Sectin 2 Weld Bead, 6 Pit, 1 Smth Sectin Electrde 1,2, 3,4, 5 514

5 RESULTS AND DISCUSSION Full Pipe F1OWResults The results f smth test sectin, pit #1 and 2 mm weld bead sectin are shwn in Figure 3. When the flw velcity is.5 mls, the average mass transfer cefficients in these three test sectins are.83 x1-4 m/s,,96x 1-4 rds and 1,x 1-4rds respectively, As the flw velcity increases t 2. m/s, the mass transfer cefficients increase t 1.72 x1-4m/s, 2.43 x1-4 rds and 3.12 x1-4mls respectively. It is seen that the mass transfer cefficients increase with increase f full pipe flw velcity in disturbed and undisturbed flw. At the same i-lw velcity, such as Imls, the mass transfer cefficients f the Pit #1 sectin and 2 mm weld bead are 1.51 x1-4 m/s and 1.79x 1-4rids, bth being higher than the mass transfer cefficient f smth sectin which is 1.17X 1 4mls. Thk is due t the enhanced turbulence causing by the disturbance inside the pipe. As the flw velcity increases, the mass transfer cefficients in disturbed flw increase faster than thse in undisturbed flw. In Figure 4, the mass transfer cefficients fr 1 mm weld bead and 2 mm weld bead test sectin are cmpared. Fr 1 mm weld bead, the mass transfer cefficient at the first electrde, which is the nearest ne t the weld bead, is 2.59x 14 mls, It increases t 3.68x 1 4mfs at the secnd electrde. Then it decreases t 3.15 x1-4 m/s at the thkd electrde and slightly increase t 3.31x 1_ nds at the fitlh electrde. Fr 2 mm weld bead, the mass transfer cefficient at the first electrde is 1.99x 14 m/s, lwer than 1 mm weld bead. It increases t 3.44 x 14 m/s and 3.49 x1-4 nds at the secnd and third electrde, slightly higher than 1 mm weld bead at third electrde. Then it decreases t 2.86x 1-4nds at the fifth electrde. This can be explained by the flw structures affected by the weld bead. The weld bead acts as a bump causing mre t the dwnstream and frms a relatively quiescent area next t the weld bead. In this quiescent area, the flw velcity is relatively lw and s n the mass transfer cefficient. The higher the weld bead, the lnger it takes fr the flw t redevelp. The maximum mass transfer cefficient appears at the pint where the turbulent eddy hits the bttm f the pipe after flwing ver the weld bead. Fr 1 mm weld bead, this ccurs near the secnd electrde, whereas fr 2 mm weld bead, thk ccurs dwnstream the third electrde, where they reach the highest mass transfer cefficient. Mass transfer cefficients in Pit #1 and Pit #6 are cmpared in Figure 5 Fr Pit #1, it is seen that the mass transfer cefficient at the first electrde, which is lcated at the bttm f the pit, is O.5x 1-4 rids, much lwer than the values f ther electrdes. Pit #1 is the smallest pit in ur experiments, and the turbulent flw can nt reach the bttm f the pit. Therefre, the liquid inside the pit is stagnant, causing the mass transfer cefficient t decrease t extremely lw value. The K value greatly increases t 1.33 x 1-4mls at the secnd electrde and then slightly increases t 1,47x 1-4mls at the fifth ne, Fr Pit #6, the mass transfer cefficient at the first electrde is.89x 1-4mls, much higher than Pit #1. This is due t the large size f Pit #6, because flw is mre turbulent in the large pit than in the small ne. The mass transfer ceillcient increases t 1.82x 1-4mls at the secnd electrde and slightly decreases t 1.76x 1-4m/s at the fifth ne. These results shw that the larger pit causes mre turbulent dwn stream, than the smaller ne and results in higher mass transfer cefficient. 515

6 Figure 6 shws the mass transfer deviatins f 2 mm weld bead at different fill pipe flw velcity. At velcity f 2. m/s, the mass transfer deviatin fr the first electrde is 1.24 x1-5rts. It increases t 1.7 1x 1-5m/s at the secnd electrde and decreases t 1,3x 1-5rds at the fifth electrde. It has the similar trend as mass transfer cefficient in Figure 4. The value f mass transfer deviatin is abut 3Y-4% f the vahre f mass transfer cefficient. The ther test sectins have the similar resuks. Slug Flw Results Figures 7 t 11 reveal very unique instantaneus fluctuatins in slug flw. These are plts f electrchemical nise. In Figure 7, smth sectin, at Frude number 6, it is seen that at time instants f.3,.75 & 1. s, the instantaneus fluctuatin is 2.4x 1-3mls, 1 times higher than the average value. In Figure 8, smth sectin, Frude number 9, there are peaks f 2 different magnitudes. At.65 s, the peaks are 1 times higher than the mean. At.8, 1.2 & 1.5 s, several huge peaks appear, which are 1 times higher than the mean. These are related t the pulses f gas bubbles reaching the electrde surface. When the slug appraches clse t the electrde, the bubbles are frced dwn twards the surface f the pipe wall, and d impact, causing mass transfer cefficient t increase such values. At these higher Frude numbers, the impact f bubbles n the pipe surface causes the mass transfer cefficient t increase dramatically by abut 1 times. These unique mechanisms can explain the ccurrence f severe lcalized crrsin in slug!dw. Figures 9-11 are the results f Pit #1. Figure 9 is shwn fr Frude number 4. The results are similar t thse shwn befre. Fr example, at time instants f.4,.7,.9 & 1.75 s, peaks abut 1 times the average ccur, which indicate that the instantaneus mass transfer cefficient at this time is abut 2.4x 1-3m/s. Figure 1 shws that fr Frude number 6, in additin several peaks which are 1 times greater than the average, tw peaks 1 times the average appear and the value is 2.OX1 2m/s. In Figure 11, at Frude number 9, mst peaks are indicate that the instantaneus mass transfer cefficients at this time are abut 2.3x 1-2m/s, 9-1 times higher than the average. The results f weld beads are similar t the results f pits. Fr smth sectin, n peak appears at Frude number 4, while at Frude number 6, peaks are nly 1 times the average, and at Frude number 9, a few peaks are 1 times high. Fr disturbed sectins, peaks f 1 times the average appear at Frude number 4, at Frude number 9, mst peaks are 1 times high. It shws that disturbed flw is mre turbulent than undisturbed flw. CONCLUSION The limiting current density technique has been demnstrated in multiphase flws fr measuring mass transfer mechanisms using ptassium ferr/ferricyanide disslved in 1.3N sdium hydrxide. S/6

7 In fill pipe flw, mass transfer cefficient increases with fill pipe velcity. Fr the same flw velcity, mass transfer cefficient in disturbed flw is higher than that in undisturbed flw, Fr weld bead, the minimum mass transfer cefficient is reached at the first electrde, which is next t the weld bead, due t decrease in turbulence, The maximum mass transfer rate lcates at the end f this quiescent zne, between the secnd and the third electrde. The higher the weld bead dimensins, the lnger the quiescent zne, Fr pit, the minimum mass transfer cefficient appears at the bttm f the pit. Other dwnstream lcatins experience high values f mass transfer rate. Larger pit creates mre turbulent than smaller nes. In slug flw, average mass transfer cefficients increase with Frude number. At a Frude number f 6, instantaneus increases in mass transfer cefficient, abut 1 times the average value, are seen. These are due t the turbulence f gas bubbles in the mixing zne. At a higher Frude number f 9, there are still peaks 1 times the average, but nw peaks 1 times the average are seen. These are caused by the actual impact f pulses f bubbles n the electrde surface. It is fund that instantaneus averages in slug flw can be 1-2 rders f magnitude higher in the mixing zne cmpared t stratified r fill pipe flw, resulting in highly increased crrsin rate. Fr the same Frude number, disturbed flw is mre turbulent than undisturbed flw, while the size f disturbance des nt cause much effect n it. REFERENCE Campbell, J. A. and Hanratty, T. J. (1983). Mechanism f Turbulent Mass Transfer at a Slid Bundary, A,I,Ch.E. Jurnal, 29,221 Landau, U. (1981). Determinatin f Laminar and Turbulent Mass Transprt Rates in Flw Cells by the Limiting Current Technique, Ph.D. Thesis, Case Western Reserve University, Cleveland Mkzusbina, T., Irvine, T. F., and Hartnett, J. P. (1971), Academic Press 7 Reiss, L. P. (1962). Investigatin f Turbulence Near a Pipe Wall Using a Diffhsin Cntrlled Electrlytic Reactin n a Circular, Ph.D. Thesis, University f Illinis, Urbana Selman, J. R. and Tbias, C. W., (1978) Limiting-Current Mass-Transfer Measurements, Advances in Chemical Engineering, Vl. 11, N. 3, pp. 271 Shaw, P. V. and Hanratty T. J. (1964), Fluctuatins in the Lcal Rate f Turbulent Mass Transfer t a Pipe Wall, A.I.Ch.E, Jurnal, 1, 475 Sirkar, K. K. and Hanratty, T. J. (197). Relatin f turbulent mass transfer t a wall at high Schmidt numbers t the velcity field, J. Fluid Mech., 44, 589 Zhang, R., Gpal, M. and Jepsn, W. P, (1997). Develpment f a Mechanistic Mdel fr Predicting Crrsin Rate in Multiphase Oil/Water/Gas Flws, CORROSION/97, Paper N. 61 SQ17

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