DILUTE PHASE VERTICAL PNEUMATIC CONVEYING OF CORK STOPPERS

Transcription

1 DILUTE PHASE VERTICAL PNEUMATIC CONVEYING OF CORK STOPPERS R. Barbosa a, and C. Pinho b a DEMGI Escola Suerior de Tecnologia de Viseu Camus Politécnico de Reeses , Viseu, Portugal rbarbosa@demgi.estv.iv.t b CEFT-DEMEGI, FEUP Universidade do Porto Rua Dr. Roberto Frias, s/n Engineering Porto, Portugal. ct@e.u.t Corresonding author ABSTRACT The neumatic conveying o cork stoers is used in the cork rocessing industries with equiments designed and built urely on an emirical basis. Exerimental studies to characterize this tye o neumatic transortation have been, so ar, oriented towards the study o horizontal conveying rocesses, either or steady state transortation or or the acceleration zone. However studies were carried out on the determination o the ressure dro on vertical transortation o cork stoers. Here the exerimental aaratus and rocedure are described, and the irst exerimental data that have been obtained are shown. In consequence a simle correlation or the ressure dro in the steady state region o the conveying ie is roosed. The correlation is a unction o dimensionless arameters used to characterize the two hase low under analysis. Three standard stoers sizes and a single ie diameter were used in the exeriments, all carried out at ambient temerature. Keywords: Vertical neumatic conveying, ressure dro, cork stoers. NOMENCLATURE a itting arameter, Eq. (2) b itting arameter, Eq. (2) c itting arameter, Eq. (2) d itting arameter, Eq. (2) d s diameter o the stoer, m d eq equivalent diameter o a stoer, m d article diameter, m D ie diameter, m Fr Froude number, U / g D g acceleration o gravity, m s -2 l length o the stoer, m L ie length, m N number o exeriments ressure, N m -2 Re Reynolds number, ρ U D µ U intersiticial gas velocity, m s -1 Greek symbols θ loading actor t overall riction actor µ luid viscosity, N s m -2 ρ luid density, kg/m 3 ρ gas density, kg/m 3 ψ shericity o the stoer Subscrits t cal t ex total calculated total exerimental INTRODUCTION Pneumatic conveying is used in some stes o the roduction o cork stoers, where article damage through interarticle or article and conining walls imaction has a minor eect uon the quality o the stoers. The develoment o small distance transort systems or stoers has been essentially an emirical rocess and only recently has been subjected o a systematic exerimental aroach. The research work that has been carried out so ar reers only to horizontal neumatic conveying, either in steady state (Neto and Pinho, 1998), or during acceleration conditions (Pinho, 1999; Pinho, 2001). In the horizontal neumatic conveying o cork stoers, ine cork dust is released and acts as a lubricant reducing riction eects, Neto and Pinho (1998). The deosition o this ine dust is still enhanced by electrostatic generation, Smeltzer et al (1982). The conveying air ressure dro is then reduced, with the increase o the solids mass loading. This situation has already been ound on solid-liquid systems; Lee et al (1974) studied olymeric solutions containing susended ibbers and Radin et al (1975) studied the drag reduction in several dilute solidliquid susensions. Szikszay (1988) argued that, as the measured exerimental data reerred to both the solids and the conveying air ressure dro, it was unreasonable to searate them. Weber (1991) also suggested that a single riction actor or the gassolid mixture should be used, instead o two searate riction actors, one or the air and the other or the solids. This question o drag reduction with the 36 Engenharia Térmica (Thermal Engineering), Vol. 5 N o 02 December

2 addition o articles into a low stream has been recently reviewed in the book o Fan and Zhu (1998), who resented a henomenological model to account or the drag reduction. For vertical neumatic conveying the imortant research oints concern the acceleration region, the determination o correlations or the solids riction actor (Rautiainen et al, 1999) and the deinition o low choking conditions. Recent examles o studies on the characterization o the acceleration region are the ublications o Dzido et al (2002) and Namkung and Cho (2002), although the works o Yang and coauthors are still a basic reerence (Yang and Keairns, 1976). The mostly recommended correlations or the calculation o the riction actor are the ublications o Yang, namely Yang (1973), Yang (1974) and Yang (1978). For the deinition o the choking conditions, the work o Yang (1975) is again requently recommended in many recent ublications, like or examle Raczek and Palica (1997) and Xu et al (2001). These last authors resent a thorough analysis on the undamentals o susension collasing in vertical conveying. However, in all these more recent ublications, whatever is the subject being treated, acceleration, riction or choking conditions, the order o magnitude o the size o the articles under consideration is quite dierent rom what is ound in the neumatic transortation o cork stoers, hence the justiication or the resent study. On the other end, although drag reduction was detected in horizontal conveying o cork stoers, nothing was known as ar as vertical conveying was concerned, and one o the main aims o this work was to ind out how imortant this could be. For the short distances used in neumatic conveying o cork stoers, the steady state transort conditions are diicultly reached and the acceleration regime becomes dominant. The theoretical determination o the acceleration length is a diicult task and the Rose and Duckworth (1961) equation is still the reerence or this matter. However, it is becoming recognized that the acceleration length does not change very much with the solids loading actor and also that, such equation gives acceleration lengths below those measured or the transort o large articles, Marcus et al (1985). The resent exerimental measurements will allow the determination o the imortance o this henomenon in the vertical conveying, comared to what has been reviously ound under horizontal neumatic conveying conditions, (Neto and Pinho 1998; Pinho, 1999; Pinho 2001). EXPERIMENTS The exerimental system as schematically shown in Fig 1, was comosed by a 6 m long horizontal ortion where the stoers were introduced into the conveying ie, a vertical testing section o 6.8 m length and the recovery circuit comosed by a small horizontal ortion o 1.6 m and a diagonal ortion 5.9 m long. Figure 2 resents a icture o the installation. The tube was a PVC ie with 125 mm nominal diameter ie (121 mm internal diameter, D). Cork stoers, moving in closed circuit, were introduced into the neumatic conveying circuit through the eeding hoer, installed in the horizontal ortion o the duct. This hoer was also receiving the corks ater their tour into the system. Figure 1. Schematic diagram o the exerimental aaratus. Conveying air was introduced into the system through a centriugal an controlled by a ABB requency variator model ACS , with a resolution o 0.1 Hz/50 Hz. Air low was measured through an oriice late low meter, ositioned ustream the eeding hoer, and equied with a dierential ressure transducer with an oerating range o 0 to 1245 Pa. The oriice late low meter was reviously calibrated by means o a Pitot tube, installed beore the eeding hoer. To carry on with the calibration rocess, the conveying ie had to be disconnected rom the entrance o the eeding hoer and through the Pitot tube an average conveying air velocity was obtained with the log-linear method alied to the measurement o the air velocity in six oints along the ie diameter. To reduced the air leakage through the eeding system o the corks, the hoer connection with the conveying ie was closely isolated with aluminium adhesive tae. With the continuous reading o conveying air temerature, corrections in the air low rate were made or the air temerature increase with attrition eects inside the transorting ie, as well as to account or changes in the ambient air temerature. Engenharia Térmica (Thermal Engineering), Vol. 5 N o 02 December

3 Along the vertical testing section, ressure tas were itted hal a meter aart. The irst ressure ta is 0.5 m away rom the bend connecting the horizontal eeding section to the vertical testing section. For measurements o the ressure dro along the conveying ie, dierential ressure transducers in the 0 to 490 Pa range were used. The transducers were rom Series T o Modus Instruments, Inc., and had outut signals o 0-1 V dc. The transducers oututs were checked towards U-tye water manometer readings. A data acquisition system (DAS 1601 lus EXP 16 boards) installed on a ersonal comuter received inormation rom the dierential ressure transducers on use (or the oriice late ressure dro and the conveying ressure dro), as well as temerature readings o the conveying air through a tye T thermocoule. The accuracy o dierential ressure dro measurements considering also the data acquisition rocess was o 9.7 Pa or the Pa ressure transducer, and o 7.9 Pa or the Pa ressure transducer. subsequent ressure tas, it was ossible to get an overall idea o the evolution o the ressure dro, as a unction o article entrance eects in the vertical ortion o the ie and also o the article acceleration. Aterwards, by knowing the length o the steady state conveying region, ressure dro data obtained in this ortion o the vertical ie were treated in order to obtain an emirical correlation. To calculate the mass low rate o transorted cork stoers, a basket was used as collection device or conveyed articles. The batch o collected stoers during a inite time interval was weighted and the solids mass low rate could then be calculated. The measurement o the voidage raction during neumatic conveying was carried out through the knowledge o the amount o cork stoers that in a given instant remain inside the ie. Having the exerimental installation working in steady state conditions, the eeding hoer was suddenly stoed and all the cork stoers being conveyed were immediately collected and weighted. This was, or the stoage instant, the amount o articles remaining inside the conveying system. Comarisons between the overall volume o the corks and the inside volume o conveying ie, gave the voidage ractions under normal conveying conditions. Table 1 resents the hysical characteristics o the tested cork stoers, size, shericity and density. Table 1. Characteristics o cork stoers. Size (l d s ) (mm mm) deq (m) ψ ρ (kg/m 3 ) RESULTS AND DISCUSSION Figure 2. Exerimental installation. In the irst lan it can be seen the oriice late air low meter and the corks eeding hoer. For each tested situation characterized by a deinite air low rate and solids loading actor θ (ratio between the solids mass low rate and the conveying air mass low rate), ressure dro measurements were made throughout the straight vertical transorting ie, using all the available ressure tas. Through the adequate grouing o the ressure dro measurements, like or examle the sequential combination between the irst and The irst aroach towards the exeriments was to analyze the imortance o the acceleration region on the evolution o the ressure dro. As can be seen rom Fig. 3, where some tyical results are resented or the dierential ressure dro between the irst ressure ta and the subsequent ones, ater a sudden initial rise in the ressure dro due to the entrance eects, the acceleration o the articles justiies the ollowing strong reduction in the measured values. Only ater steady state transortation conditions are reached, the evolution o the ressure dro ollows a smooth trend. This steady state situation arrives at around 3.5 to 4 meters beyond the entrance o the stoers. This order o magnitude o the entrance/acceleration length is similar to what was ound or the horizontal conveying o cork stoers (Pinho, 1999; Pinho 2001). On the other end, 38 Engenharia Térmica (Thermal Engineering), Vol. 5 N o 02 December

4 ressure dro data obtained with ressure tas close to the end o the vertical conveying ie were not lotted as the low deceleration induces the increase o the ressure dro, and again unsteady conveying conditions are obtained. From this igure it is evident that, to get reliable ressure dro data or steady state neumatic conveying, values beyond 4 m should be used or the determination o emirical correlations. vertical neumatic conveying such abnormal situation does not haen, as the exeriments have shown, Fig. 4. Total, gas lus solids, ressure dro is higher than the ressure dro corresonding to the sole low o air, as the exerimental oints in Fig. 4 stand, comaratively to the corresonding values obtained through the Blasius equation. 100 /L [Pa/m] x22,θ= x22,θ= x22,θ= x22,θ= x22,θ= x22,θ= x24,θ= x24,θ= x24,θ= x24,θ= L [mm] /L [Pa/m] x24, θ= 0.226, Fr = Blasius' equation L [mm] Figure 4. The overall, air lus solids, ressure dro and the corresonding values calculated through the Blasius equation. Figure 3. Evolution o overall ressure dro along the conveying ie. The lot o ressure dro er unit length as a unction o several arameters, to characterize the oeration o a neumatic conveying system, is a tyical rocedure ound in the technical literature (Marcus et al, 1990; Rhodes, 1998); in the situation o Fig. 3 it allows a quick deinition o the acceleration region. In classical models or neumatic conveying, Barth (1960a; 1960b), Yang (1973; 1974; 1978) and Yang and Keairns (1973; 1976), the steady state overall ressure dro is considered to be the addition o two individual contributions, the gas and the solids ressure dro. More recently, Rhodes (1998) continues to adot, as a general aroach, the addition o individual ressure dro contributions. In these aroaches, it is imlicitly assumed that the overall ressure dro or a neumatic conveying situation should be higher than any individual contribution. However, this is not always the real situation and the alication o any o such models, or others ollowing the same hilosohy, would mean that some individual contributions should be negative, leading to a hysically inaccurate condition. For horizontal neumatic conveying o cork stoers such is the situation, the overall, gas lus solid, ressure dro, is smaller than the ressure dro corresonding to the exclusive low o air (Neto and Pinho, 1998; Pinho, 1999; Pinho, 2001). For the However, as a irst aroach to the treatment o the exerimental results, the adoted methodology was again to consider a unique overall riction actor or the gas-solid mixture, as suggested by Weber (1991). This is temting, because the exerimental determination o neumatic conveying ressure dro is eectively the measurement o a combined (gas lus solid) ressure dro, and consequently the total ressure will then be given by, 2 L U t = t ρ (1) D 2 where the global riction actor t, is written as a simle unction o dimensionless arameters, Szikszay (1988), d e b c d ρ t = a θ Fr D ρ (2) Knowing exerimental data o t L, or dierent tested situations covering several stoers diameters d and densities ρ, loading actors θ and air interstitial velocities U, t is calculated by means o Eq. (1) and correlated with the dimensionless numbers reerred in Eq. (2), θ, Fr, d /D and ρ /ρ, through itting arameters a, b, c, d and e, obtained through the alication o a Engenharia Térmica (Thermal Engineering), Vol. 5 N o 02 December

5 commercial sotware or nonlinear regression analysis NLREG. The obtained values or those arameters guarantee the best it to the set o rovided exerimental data obtained in the exeriments this ormula is o 17.4 % whereas the mean deviation, calculated through 2 tcal t ex Mean Deviation % = N 100 tex (4) tex Correlation 38x22 38x24 45x as deined by Wen and Chen (1982), is 9.2 %. Exerimental and calculated t are comared in Fig. 5. I an alternative and more elegant correlation is adoted through the use o rounded values or the itting arameters a, b, c, d and e, the result is 4/ 5 4/ 5 3 1/ 5 4/ 5 d eq ρ t = 2 10 θ Fr D ρ (5) tcal Figure 5. Comarison between exerimental and calculated values or the overall riction actor. For the determination o steady state ressure dro in horizontal neumatic conveying, as reerred in Tab. 1, three stoers sizes were used, 38 22, 38 24, (mm mm). Tested loading actors covered the range rom 0.10 to 0.35, the air low Reynolds number ( Re = ρ U D µ ) went rom to , while the Froude number ( Fr = U / g D ) ranged rom 15.3 to 30.7, all values tyical or cork stoers conveying systems. In the deinition o the Reynolds number, µ is the dynamic viscosity o the conveying air. Small values or the loading actor are common in the neumatic conveying o stoers, because o the combined inluence o having to transort large size articles with the use o centriugal ans having lat working curves romoting low blockage situations, Wirth and Molerus (1986). This aects minimum conveying conditions and imoses restrictions on the maximum ossible loading actor. At the same time, the extreme ragility o stoers imoses some limitations on the maximum recommended transort velocity, reducing even urther the range o working conditions. The obtained correlation was then, d eq ρ t = θ Fr D ρ (3) The maximum absolute deviation between exerimental t ex and calculated data tcal with In these circumstances the absolute deviation between exerimental tex and calculated data tcal with Eq. (5) is o 17.5 %, while the mean deviation calculated again through Eq. (4) will be 9.4 %. These data were obtained through a set o about 40 exerimental runs, or each run twenty results or twelve ressure dierentials were registered and each registered value corresonded to an average o a 1000 readings. CONCLUSIONS From the reliminary results o a study on vertical neumatic conveying o cork stoers, it was realized that contrary to what was ound or horizontal conveying, in the vertical transortation regime the total ressure dro is higher than the ressure dro corresonding to the single low o air. The order o magnitude o the acceleration length is identical to that o horizontal conveying, although a deeer analysis o the exerimental results is still under way. Finally, as a reliminary aroach, a simle emirical correlation or the overall (gas lus solid) ressure dro in the steady state conveying regime was roosed. By sacriicing a little o the accuracy o the original correlation, a more elegant one can be obtained through the use o rounded itting arameters. REFERENCES Barth, W., 1960a, Physical and Economical Asects o Transortation o Solids in Fluids and Gases, (In German), CIT 32, No. 2, Engenharia Térmica (Thermal Engineering), Vol. 5 N o 02 December

6 Barth, W., 1960b, Technical Flow Problems in Conveying Dust and Gas Mixtures, (In German), Mitt. VGB., No. 79, Dzido, G., Palica, M. and Raczek, J., 2002, Investigations o the acceleration region in the vertical neumatic conveying, Powder Technologt, Vol. 127, Fan, L. S., and Zhu, C., 1998, Princiles o Gas-Solid Flows, Cambridge University Press, Cambridge Series in Chemical Engineering, Cambridge. Lee, W. K., Vaseleski, R. C. and Metzner, A. B., 1974, Turbulent Drag Reduction in Polymeric Solutions Containing Susended Fibbers, A.I.Ch.E. J., Vol. 20, No..1, Marcus, R. D., Hilbert Jr., J. D, and Klinzing, G. E., 1985, Flow Through Bends and Acceleration Zones in Pneumatic Conveying Systems, Bulk Solids Handling, Vol. 5, No. 4, Marcus, R. D., Leung, L. S, Klinzing, G. E. and Rizk, F., 1990, Pneumatic Conveying o Solids, Chaman and Hall, London. Namkung, W. and Cho, M., 2002, Pressure Dro in a Vertical Pneumatic Conveying o Iron Ore, Ind. Eng. Chem. Res.,Vol. 41, Neto, P. and Pinho, C., 1998, Dilute Phase Horizontal Pneumatic Conveying o Cork Stoers: A Preliminary Exerimental Study, Proceedings o the Symosium on Thermal and Fluids Engineering, CSME Forum SCGM, 19 th -22 nd May, Toronto, Vol. 1, Pinho, C., 1999, Acceleration o Cork Stoers in Pneumatic Conveying, 15 th Brazilian Congress o Mechanical Engineering, 22 nd -26 th November, Águas de Lindóia, São Paulo, SP, Brazil. Pinho, C., 2001, Horizontal Pneumatic Conveying o Cork Stoers, Proceedings o Cobem 2001, Fluid Mechanics, Vol. 8, 21-29, 26 th to 30 th November 2001, Uberlândia, MG, Brazil. Raczek, J. and Palica, M., 1997, Selection o the gas working velocity in vertical neumatic conveying systems or binary mixtures, Chemical Engineering and Processing, No. 36, Radin, I., Zakin, J.L. and Patterson, G.K., 1975, Drag Reduction in Solid-Fluid Systems, A.I.Ch.E. J., Vol. 21, No..2, Rautiainen, A., Stewart, G., Poikolainen, V. and Sarkomaa, P., 1999, An exerimental study o vertical neumatic conveying, Powder Technology, Vol. 104, Rhodes, M., 1998, Introduction to Particle Technology, John Wiley and Sons, Chichester. Rose, H.E. and Duckworth, R. A., 1961, Transort o Solids Particles in Liquids and Gases, The Engineer, No. 14, 21 st and 28 th March. Smeltzer, E. E., Weaver, M. L. and Klinzing, G. E., 1982, Pressure Dro Losses due to Electrostatic Generation in Pneumatic Transort, Ind. Eng. Chem. Process Des. Dev., Vol. 21, No. 3, Szikszay, G., 1988, Friction Factor or Dilute Phase Pneumatic Conveying, Bulk Solids Handling, Vol. 8, No. 4, Weber, M., 1991, Friction o the Air and the Air/Solid Mixture in Pneumatic Conveying, Bulk Solids Handling, Vol. 11, No. 1, Wen, C.Y. and Chen, L.H., 1982, Fluidized Bed Freeboard Phenomena: Entrainment and Elutriation, AIChE J., vol. 28, No. 1, Wirth, K.-E. and Molerus, O., 1986, Solids and Gas-Solid Flows, Encycloedia o Fluids Mechanics, Vol. 4, Ch. 15, N. P. Cheremisinos Editor, Gul Publishing Comany. Xu, G., Nomura, K., Gao, S. and Kato, K, 2001, More Fundamentals o Dilute Susension Collase and Choking or Vertical Conveying Systems, AIChE J., Vol. 47, No. 10, Yang, W. and Keairns, D.L., 1973, Estimating the Solid Particle Velocity in Horizontal Pneumatic Conveying Lines, The Canadian Journal o Chem. Eng., Vol. 51, Yang, W., 1973, Estimating the Solid Particle Velocity in Vertical Pneumatic Conveying Lines, Ind. Eng. Chem. Fundam., Vol. 12, No. 3, Yang, W., 1974, A Correlation or Solid Friction Factor in Vertical and Horizontal Pneumatic Conveying, AIChE J., Vol. 20, No. 3, Yang, W., 1975, A Mathematical Deinition o Choking Phenomenon and a Mathematical Model or Predicting Choking Velocity and Choking Voidage, AIChE J., Vol. 21, No. 5, Yang, W. and Keairns, D.L., 1976, Estimating the Acceleration Pressure Dro and Particle Acceleration Length in Vertical and Horizontal Pneumatic Transort Lines, Pneumotransort 3. Third International Conerence on the Pneumatic Transort o Solids in Pies, 7 th - 9 th Aril,. D D7-98. Yang, W., 1978, A Correlation or Solid Friction Factor in Vertical Pneumatic Conveying Lines, AIChE J., Vol. 24, No. 3, Engenharia Térmica (Thermal Engineering), Vol. 5 N o 02 December