Venturi Scrubber Performance Model

(9) US. Dept. of Commerce, National Oceanic and AtmosphericAdmin., Environmental Data Div., Asheville, N.C., “Local ClimatologicalData”, Jan., Feb., and Mar. 1974.(10) Samson, P. J., Neighmond, G., Kencha, A. J., J. Air Pollut.Control Assoc., 25,1232-7 (1975).(11) “Recommended Guide for the Prediction of the Dispersion ofAirborne Effluents”, M. E. Smith, Ed., American Society of MechanicalEngineers, 1968.(12) Hanna, S. R., J. Air Pollut. Control Assoc., 21,774-7 (1971).(13) Leahy, D. M., Atmos. Enuiron., 9,817-23 (1975).(14) Lebedeff, S. A., Hameed, S., ibid., pp 333-8 (1975).(15) Munn, R. E., ibid., 7,311-18 (1973).(16) Padmanabhamurty, B., J. Air Pollut. Control Assoc., 25,956-7(1975).(17) Rahn, K., Wesolowski, J. J., John, W., Ralston, H. R., ibid., 21,406-9 (1971).(18) Davies. C. N.. J. Aerosol Sci.. 5.293-300 (1974).(19) Lee, R: E., Jr., Caldwell, J.,’ Akland, G. ‘G., Fankhauser, R.,Atmos. Enuiron., 8,1095-109 (1974).(20) Lyon, J., “Chicago Tribune”, Sect. 1, p 10, Mar. 16,1975.Received for review April 18,1975. Resubmitted June 20,1977. AcceptedOctober 24,1977. Work supported by the U.S. Atomic EnergyCommission under Contract No. AT(11-1)-578.Venturi Scrubber Performance ModelShul-Chow Yung, Seymour Calvert”, and Harry F. BarbarlkaAir Pollution Technology, Inc., 4901 Morena Blvd., Suite 402, San Diego, Calif. 92117Leslle E. SparksIndustrial Environmental Research Laboratory, Utilities and Industrial Power Division, U.S. Environmental Protection Agency,Research Triangle Park, N.C. 2771 1A new model for predicting the particle collection performanceof venturi scrubbers is presented. The model assumesthat particles are collected by atomized liquid only in thethroat section. The particle collection mechanism is inertialimpaction, and the model uses a single drop collection efficiencyequation based on the inertial impaction parameter.The model takes the form of an analytic expression for theparticle penetration based on particle diameter and venturiscrubber parameters. No arbitrary empirical constants areused. A relation for the maximum throat length necessary fora venturi is also presented. The model is compared with experimentaldata from small- and large-scale venturi scrubbers.Venturi scrubbers are a type of gas-atomized spray scrubber.They employ gradually converging, then diverging sections.The venturi throat is the section between the convergingand diverging sections. Liquid usually enters the venturi upstreamof the throat through nozzles. Alternatively, the liquidmay flow along the converging section walls until reaching thethroat. At the throat the liquid is shattered into drops byhigh-velocity gas. Sometimes the liquid is introduced aspreatomized drops at or before the throat through pressurenozzles.Particle collection by liquid drops is the predominant occurrencein the venturi scrubber. Inertial collection, interception,diffusion, electrostatic collection, and gravitationalcollection are several of the mechanisms causing collection bydrops. All investigators have concluded that inertial impactionis the principal mechanism of particle collection in a venturiscrubber for particles larger than 0.5 pm in diameter. Forparticles smaller than 0.1 Mm in diameter, diffusional collectionprevails.By performing a material balance for the dust over a differentialscrubber volume with the assumption of constantliquid holdup, Calvert (I, 2) obtained a differential equationfor the prediction of venturi scrubber performance. Thisequation for change in particle concentration is:--= dc 1.51Up-Ud)(1)C Uddd (QG) ?tdzOther investigators, such as Ekman and Johnstone (3), Morishimaet al. (4), Dropp and Akbrut (5), Boll (6), and Behieand Beeckmans (7), have presented slightly different equationsto describe the particle collection in a venturi scrubber.However, all of their equations can be transformed intoEquation 1.No analytic solution has been given to Equation 1. Yung etal. (8) have performed a numerical solution of this equationand presented graphical results. Calvert ( I) and Calvert et al.(9) have integrated Equation 1 for constant throat diameterand an approximate drag coefficient relationship and obtainedan explicit equation for predicting particle penetration. Itinvolves an empirical constant that requires experimentalevaluation. This paper presents a modification to Calvert’sequation, which does not contain empirical constants.Performance ModelSince most of the particle collection occurs in the venturithroat, we will apply Equation 1 to the throat section with thefollowing simplifying assumptions:The flow is one-dimensional, incompressible, and isothermal.Particles are collected only by atomized liquid drops. Thedrop diameter is the Sauter mean diameter predicted by theempirical correlation of Nukiyama and Tanasawa (IO). Althoughin actuality a distribution of drop sizes will exist, theuse of a single representative size simplifies calculations andgives reasonable results.Liquid drops are uniformly spread across the duct, andthe drop diameter is invariant with axial distance.There is no wall loss of liquid drops.At any cross section of the scrubber the liquid volumefraction is small.At the throat entrance, liquid drops have zero axial velocity.Drag coefficient is governed by the following relationshipas suggested by Hollands and Goel (11):Co = (constant) NR~-~,~This relationship is adequate for Reynolds numbers rangingfrom 10 to 500. This is the range of Reynolds numbers of interestin venturi scrubbers. ‘‘CD~” is determined from the“standard curve” using a Reynolds number calculated on thebasis of the relative velocity applying at the throat entrance.Particle collection is primarily due to inertial impaction.We will use the simplified equation by Calvert (2) to estimatethe single drop target efficiency456 Environmental Science & Technology 0013-936X/78/0912-0456$01 .OO/O 0 1978 American Chemical Society

‘ = (K,, + 0.7)Kp 2where “Kp” is the inertial impaction parameter(3)(4)Particles are flowing at the same velocity as the gas, i.e.,up = UG.The only force acting on the drops is the drag due to therelative motion of the gas and the drops. Thus, a force balanceon the liquid drops givesEquations 1-5 are the governing equations for venturiscrubber performance. Equations 1 and 5 are nondimensionalizedby using “ UG~” as the characteristic velocityscale, 2 ddp~/3 CD~PG as the characteristic length scale, and“c0” as the characteristic particle concentration. Using thesedefinitions, Equations 1, 3, and 5 becomeThe solution to Equation 8 is:By substituting Equations 7 and 8 into 6, we obtain:__--dc* ~QLPL (1 - u:)’,’C* QGPGCD~ 0.7 2(1 -.;+-) KPO(5)du; (11)0”z0ck-

problem, particle collection data from bench scale venturisare not reliable. Therefore, Equation 12 should be evaluatedwith large industrial scale venturi scrubbers.Brink and Contant (12) and Calvert and Yung (13) reportedperformance test results on large industrial scaleventuri scrubbers. The venturi scrubber tested by Brink andContant was a Pease-Anthony venturi installed in a phosphoricacid plant. Figure 2 shows their curve along with theoreticalpredictions. The particle aerodynamic resistance diameteris used in this and the next figure. Raabe (14) suppliesthe following definition:n 1(14)A particle density of 1.7 g/cm3 (for P205) was assumed in thecalculation.As can be seen, the penetration predicted by Equation 12is lower than the experimental data. However, compared withEquation 1, Equation 12 gives better predictions.Calvert and Yung (13) measured the performance of anAmerican Air Filter Kinepactor 32 venturi scrubber. Figure3 shows the predicted and experimental fractional penetrationcurves. The experimental curves were calculated from simultaneousinlet and outlet cascade impactor data. Theprediction agrees with the measurements.DiscussionEquation 12 was derived from Equation 1 with some simplifyingassumptions. Therefore, Equation 1 should give abetter fit with experimental data than Equation 12. However,when compared with available experimental data, the reverseis true. This probably is caused by a nonuniform liquid dropdistribution as speculated by Calvert (1) and Boll (6).Figure 4 is a graphical presentation of the solution ofEquations 9 and 12. Under a specific operating condition, theperformance of the venturi scrubber depends on the throatlength as revealed by Figure 4. The longer the throat, the moreefficient the scrubber. This dependence is more profound forlarger particles, i.e., larger “Kpo”. Thus, in designing a venturiscrubber, we should use a long throat. However, we should notuse a longer throat then necessary because the pressure dropwill increase with increasing throat length. The final choice1.0 cI I I I I 1 1 1 1 I I I 1 lllq1 \0L, DINCNSIONLESS THROAT LChGTHFigure 4. Theoretical particle penetration and drop velocity as a functionof dimensionless throat lengthof throat length should be a compromise of the two. A closeinspection of Figure 4 reveals that a dimensionless throatlength (L) of 2-3 would be sufficient. Further increase in dimensionlessthroat length will increase pressure drop withlittle gain in scrubber performance.NomenclatureB = dimensionless parameter characterizing liquid-to-gasmass flow rate ratioCD = drag coefficient, dimensionlessc = particle mass concentration, g/cm3c* = c/co, dimensionlessd = diameter, cm or pmd,, = particle aerodynamic resistance diameter, pm(Equation 14)Kp = inertial parameter, dimensionlessL = 3 cDofJC&/2 ddpL, dimensionless throat lengthlt = venturi throat length, cmNR~ = Reynolds number, dimensionlessPt = particle penetration for particles with diameter d,,fraction or percentQ = volumetric flow rate, cm3/su = linear velocity, cm/su* = u/uc0, dimensionless linear velocityz = length parameter defined by Equation 10z = axial coordinate, cmz* = 3 CDoPCZ/:! ddpL, dimensionless coordinateGreek Lettersp = density,g/cm3p = viscosity, poiseq = single drop collection efficiency, fractionSubscriptsd = liquiddropG = gasL = liquid1 = at throat exit or distance “lt” from throat entrancep = particleo = at throat entranceI I0 .01 I 1 1 1 1 1 10.1 1 10PARTICLE AERODYNAMIC RESISTANCE DIAMETER,pmFigure 3. Comparison of Calvert and Yung’s ( 73) data with theory forventuri scrubbersLiterature Cited(1) Calvert, S., “Source Control by Liquid Scrubbers”, in “Air Pollution”,Arthur Stern, Ed., 2nd ed., Chap. 46, Academic Press, NewYork, N.Y., 1968.(2) Calvert, S., AIChE J., 16,392-6 (1970).(3) Ekman, F. O., Johnstone, H. F., Znd. Eng. Chem., 43, 1358-63(1951).(4) Morishima, N., Yoshida, T., Kosoka, Y., Nonaka, Y., FuntaiKogaku Kenkyu Kaishi, 9,357-62 (1972).458 Environmental Science & Technology

(5) Dropp, L. T., Akbrut, A. J., Tepoloenergetika, 7,63-8 (1972).(6) Boll, R. H., Ind. Chem. Fundam., 12,40-50 (1973).(7) Behie, S. W., Beeckmans, J. M., Can. J. Chem. Eng., 51, 430(1973).(8) Yung, S., Calvert, S., Barbarika, H. F., “Venturi Scrubber PerformanceModel”, EPA 600/2-77-172,1977.(9) Calvert, S., Lundgren, D., Mementa, D. S., J. Air Pollut. ControlAssoc., 22,529-32 (1972).(10) Nukiyama, S., Tanasawa, Y., in “Perry’s Chem. Eng. Handbook”,Perry and Chilton, Eds., 5th ed., McGraw-Hill, New York,N.Y., 1973.(11) Hollands, K.G.T., Goel, K. C., Ind. Eng. Chem. Fundam., 14,16-22 (1975).(12) Brink, J. A., Contant, C. G., Ind. Eng. Chem., 50,1157 (1958).(13) Calvert, S., Yung, S., “Evaluation of Venturi Scrubber”, FinalRep. to EPA, Contract 68-02-1328, Task 6, Feb. 1975.(14) Raabe, 0. G., J. Air Pollut. Control Assoc., 26,856-60 (1976).Receiued for review June 30,1977. Accepted October 31,1977. Studysupported by EPA under Contract No. 68-02-1328, Task No. 13.NOTESAmbient Air Hydrocarbon Concentrations in FloridaWilliam A. Lonneman*, Robert L. Seila, and Joseph J. BufaliniEnvironmental Sciences Research Laboratory, Research Triangle Park, N.C. 2771 1In May of 1976 a three-day study was undertaken in St.Petersburgrnampa, the Everglades, and Miami areas to assessthe importance of naturally emitted hydrocarbons to ozoneproduction. Some samples were collected within inches ofcitrus trees in an effort to observe the occurrence of naturalterpene hydrocarbon emissions. Other samples were collectedin remote areas to determine hydrocarbon composition andto investigate natural hydrocarbon contribution to the overallhydrocarbon burden. No significant natural hydrocarboncontribution was observed in any of the collected samples. TheClo terpene hydrocarbons were below our limit of detectabilityof 1 ppb carbon. The meterological conditions during thisstudy were not conducive for the buildup of high hydrocarbonconcentration, and low ozone levels were observed. The resultssuggest that natural hydrocarbon emissions are not as importantas anthropogenic sources of hydrocarbon in the formationof ambient ozone.It has been suggested that certain areas of Florida have highambient ozone levels largely because of the high hydrocarbonemissions from citrus-type trees and perhaps through thedecay of organic mat.ter in the large number of swamps in thearea. Previous studies (1-3) have shown that several terpene-typehydrocarbons are contained in the volatile fractionof orange oil. These hydrocarbons include limonene, myrcene,alpha -pinene, and many others. One can therefore speculatethat citrus trees may liberate many of these hydrocarbons intothe environs during the growing process. Being olefinic innature, these hydrocarbons are expected to have very highreactivity, i.e., in the presence of sunlight and oxides of nitrogen,the hydrocarbons rapidly oxidize and ozone is produced(4). Therefore, many of the manifestations usuallyobserved when auto exhaust is irradiated with sunlight canalso be observed by the irradiation of naturally producedhydrocarbons in the presence of NO,.This three-day study was undertaken as a part of theFlorida Ozone Transport Study (5), the purpose of which wasto investigate occurrence, causes, and transport of high ambientozone in Florida. If we consider the short duration of thiseffort, the results are taken to have a suggestive rather thanconclusive nature.ExperimentalBag samples were collected in 2-mil Tedlar bags by meansof a battery-operated Thomas pump. The internal parts of thepump consisted only of Teflon and stainless steel components,thus minimizing pump contamination. Earlier laboratorystudies have shown that the pump is inert in both emissionsand adsorption of hydrocarbons up to Clo. A 10-L volume ofsample air was collected usually over a 2-5-min time period.The Tedlar bags were always covered with aluminized ScotchPak. This precaution was taken since the hydrocarbons wouldphotooxidize in the bags when exposed to sunlight. The bagsamples were shipped by commercial air freight to the ResearchTriangle Park, N.C., facility for detailed gas chromatographichydrocarbon analyses. Detailed descriptions of thegas chromatograph (6) and the cryogenic concentration apparatus(7) systems are published elsewhere. Usually, a0.5-1-L volume of sample is sufficient to obtain subpartper-billioncarbon sensitivity for most components.A total of 24 grab bag samples was collected over a three-daysampling program of May 13,14, and 18. Eight samples werecollected at ground level sites, while 16 samples were collectedfrom an aircraft at 2000 ft during the ozone sampling program.Not all of the GC data from this study are presented in thisreport since for the most part the compositional details of thesamples were quite similar.Results and DiscussionThe detailed hydrocarbon results of some selected samplesare shown in Table I. The data are separated into groundsamples (G), aircraft samples (A), and one downtown Miamiground-level sample (D). The geographical locations at whichthe bag samples were collected are shown in Figure 1.Samples G-2 and G-3 were collected in an orange grove nearDunedin, Fla. These samples were collected close to the citrustrees where ambient air dilution processes are minimal, andnatural hydrocarbon concentrations are expected to be thegreatest. Sample G-3 was collected at a distance of approximately10 ft from the citrus trees, while sample G-2 was collectedwithin inches of the fruit trees. No significant changeswere observed in the CI-Clo hydrocarbon distribution. In fact,the terpene hydrocarbons previously observed in orange oil(1-3) were not observed in either of the collected bag samples,even though these compounds would appear as separate peakson our gas chromatographic column and our sensitivity is lessthan 1.0 ppb carbon (0.1 ppb compound). Since we did notobserve any of the Cl0 terpenes in our bags, our concern wasthat these hydrocarbons could possibly be adsorbed on thepump or Tedlar bag surfaces. We excluded these possibilitieswhen later laboratory studies performed at low terpene concentrations(5-25 ppb carbon) showed no pump losses andexcellent bag storage characteristics.This article not subject to U.S. Copyright. Published 1978 American Chemical Society Volume 12, Number 4, April 1978 459