Aerosol Deposition in Turbulent Pipe Flow Benjamin Y. H. Liu’ and Timothy A. Ilori2 Particle Technology Laboratory, Mechanical Engineering Department, University of Minnesota, Minneapolis, Minn. 55455
A model is proposed for calculating the effective particle diffusivity, t p , in the boundary layer of a turbulent fluid. The model is based on the analogy between the diffusion of particles caused by Brownian motion and that caused by fluid turbulence. The model gives the result t p = t L ~ T , where c is the eddy diffusivity, v is the rootmean-square fluctuating velocity of the fluid in a direction normal to the wall, and T is the particle relaxation time. Results are presented showing that the particle deposition rate based on this effective particle diffusivity and the “diffusion free-flight” model is in general agreement with experimental data without th:! need to use either unreasonably high or arbitrary free-flight velocities as in previous theories in which particle and eddy momentum diffusivities are assumed equal.
+
When a turbulent gas stream containing suspended particles is flowing along a surface or inside a pipe, particles are deposited by the action of fluid turbulence and by other mechanisms which may be operative. Mechanisms such as gravitational settling, Brownian diffusion, image force attraction, and space charge precipitation may act singly or in combination with fluid turbulence to cause particle deposition in turbulent flow. The turbulent deposition of aerosol particles in pipes has received considerable attention in recent years. The phenomenon is of interest in many practical air pollution and aerosol research problems. For instance, in the sampling of atmospheric aerosols for quantitative particle size distribution analysis (Whitby et al., 1972a and 1972b), samples must often be obtained through a long sampling pipe under turbulent flow conditions. Knowledge of the rate of particle deposition, especially the particle size dependence of the deposition rate, is necessary to correct for sampling errors. From a theoretical point of view, turbulent deposition indicates the nature of fluid-particle interaction taking place in a turbulent fluid and gives a measurable quantity by which such interactions can be elucidated. In their pioneering turbulent-deposition studies on vertical pipes, Friedlander and Johnstone (1957) found that the experimental particle deposition rate is considerably higher than can be accounted for by the mechanism of turbulent diffusion alone, if the particle and eddy momentum diffusivities are assumed equal. They found that the particle must be endowed with a finite momentum to account for the apparent ability of the particle to penetrate much of the boundary layer with little or no resistance. In the theory of Friedlander and Johnstone, particles are assumed to be transported by eddy diffusion to within one stopping distance from the wall, at which point particles make a free flight to the wall, where they arrive with zero velocity. The particle stopping distance is calculated by use of an initial free-flight velocity of 0.9 u*, where u* is the friction velocity, or the rms (root-mean-square) raTo whom correspondence should be addressed. Present address, University of Lagos, Lagos, Nigeria.
dial fluctuating velocity of the fluid in the turbulent core. In the subsequent development of this “diffusion freeflight” model by Davies (1966a and 1966b) and Beal (1968), the same assumption was made regarding the equality of particle and eddy diffusivities. However, different initial free-flight velocities were used. Beal assumed that the initial free-flight velocity is equal to one half of the axial fluid velocity at the point where free flight begins, while Davies used the local fluctuating velocity of the fluid as the initial free-flight velocity. The theory of Friedlander and Johnstone and that of Beal give results that are in reasonable agreement with experimental data while Davies’ theory gives deposition rates that are too low, sometimes by more than two orders of magnitude. We are thus faced with the problem of having to use what appear to be unreasonably high, or arbitrary, freeflight velocities to obtain reasonable agreement with experimental data. When the more reasonable free-flight velocity of Davies is used, the results then become too low. This suggests that the assumption that the particle and eddy diffusivities are equal is fundamentally incorrect. The above conclusion has been reached independently by several investigators. Sehmel (1970 and 1971) suggested that the effective particle diffusivity through the boundary layer is higher than the eddy diffusivity and calculated this effective diffusivity from measured deposition rates. Rouhiainen and Stachiewicz (1970) sought to explain the high particle diffusion rate by the shear lift experienced by a particle moving through the shear field in the boundary layer. Similarly, Hutchinson et al. (1971) sought an explanation on the basis of a statistical, random walk model. In spite of the difficulty with the “diffusion free-flight’’ model described above, the model does appear to have considerable merit. The model provides a simple, straightforward descr,iption of the particle deposition process, utilizing fundamental concepts that have proved useful in other applications. Further, according to Fuchs (1964), the particle stopping distance can be regarded as a particle “mean free path.” Thus, in the regime where the particle stopping distance is of the same order of magnitude as the thickness of the boundary layer, the turbulent deposition process is similar to the process of heat, mass, momentum, or electric charge transfer to aerosol particles in the regime of finite Knudsen numbers (Knudsen number = mean free path of the gas molecules/particle radius). And the “diffusion free-flight’’ model is analogous to the “limiting sphere” or “vacuum shell” model that has proved so successful in treating processes involving the transfer of momentum (Fuchs and Stechkina, 1962), mass (Fuchs, 19591, and electric charge (Fuchs, 1963) to aerosol particles in the finite Knudsen number regime. The purpose of this paper is to propose a model for calculating the effective particle diffusivity through the boundary layer and to show that this would give reasonable results on the particle deposition rates.
Particle Diffusivity Through the Boundary Layer Let us first consider the case of the Brownian d.iffusion Volume 8, Number 4, April
1074
351
of aerosol particles caused by molecular impact. According to Einstein’s equation, D = kTB (1) where D is the Brownian diffusivity of the particles, k is the Boltzmann constant, T is the absolute temperature, and B is the dynamic particle mobility. Further, according to the principle of equipartition of energy, we have ‘/L mu,!! = iiT (2) where m is the mass of an individual particle and u d denotes the rms fluctuating velocity of the particle causing the diffusive transport. Thus, if we consider the diffusive transport of particles along the x-axis, then u d denotes the rms fluctuating velocity of the particle along the same xaxis. Combining these equations, we have
D
= uII2~
(3)
where T is the particle relaxation time given by T
=
mB
(4)
Further, since d
= L ‘ d-
1
(5)
Equation 3 can be written as
D = L‘(/X~ (6) where hd is the particle stopping distance corresponding to the rms fluctuating velocity of the particle associated with its random, fluctuating, Brownian motion. Equation 6 provides a rather general relationship between the particle diffusivity, D, the rms fluctuating velocity, u d and the associated stopping distance, h d . The equation is similar to the kinetic theory equation for the diffusivity of a gas, if h d is interpreted as an equivalent particle mean free path. The boundary layer adjacent to a surface in contact with a turbulent fluid is characterized by the existence of fluctuations caused by the incursion of turbulent eddies from the turbulent core. A comparatively large and rapid turbulent eddy entering the boundary layer from the turbulent core will encounter the slow-moving fluid in the boundary layer and break up into eddies of a smaller size and velocity. If a particle of a finite mass is entrained by such a turbulent eddy and brought into the boundary layer, it would be projected a greater distance and travel farther into the boundary layer than would the associated turbulent eddy. Thus, on the average, particles would be transported with an effective diffusivity which is higher compared to the eddy diffusivity, t , of the turbulent fluid. To calculate the effective particle diffusivity, t p , we assume that the particle is perfectly entrained by the turbulent eddy. This implies that the random fluctuating motion of the particle is identical to that of the turbulent eddy when it enters the boundary layer. Thus, if u is the rms .fluctuating velocity of the fluid in a direction perpendicular to the wall, the particle will also have the same velocity and will be projected a distance equal to h = U T farther than the turbulent eddy when the latter is thoroughly dissipated by turbulent mixing with fluids in the boundary layer. This gives rise to an additional diffusivity, = u x = i”T (7) obtain1.d by analogy from Equations 6 and 3. Thus, the total ef’fectiveparticle diffusivity is given by
The quantity t’ will be referred to as the inertial diffusivity of the particles. As Equation 7 shows, t’ arises from the interaction between the turbulent eddying motion of the fluid as characterized by the velocity, u, and the finite particle inertia as characterized by the particle relaxation time, T. As one would expect, t ’ --* 0 as T 0. Thus, t ’ t when particles are reduced to molecular dimensions. It should be noted that the specific model proposed here requires particles to be perfectly entrained by the turbulent eddies. The diffusivity, e ’ , resulting from particle inertia manifests itself only during the dissipative phase of the eddying motion when the original eddy is broken up into smaller eddies in the boundary layer. If the particles were not perfectly entrained, we would obtain a situation described by Davies as follows: “Larger particles are caused by their inertia to lag behind or to shoot ahead of the local fluid motion (at random). On the average these tendencies cancel and the eddy diffusivity coefficient of the particles is the same as that of the fluid although the movements of the particles are more sluggish.” Undoubtedly, the condition described by Davies would prevail in the turbulent core. As the analyses by Tchen and others (see Soo, 1967) have shown, the finite particle mass can only cause a reduction in the particle diffusivity in comparison with the eddy diffusivity of the fluid in the kind of turbulent motion prevailing in the turbulent core. The effective particle diffusivity given by Equation 8 can be made dimensionless as follows:
-
-
=
E,,+
where
E+
+
(9)
L‘+’T+
is the dimensionless particle diffusivity, t , = t / u is the dimensionless eddy diffusivity, u , = u / u * is the dimensionless fluctuating velocity (rms value) in a direction normal to the surface, and T + is the dimensionless particle relaxation time, defined by tp+
=
tp/u
T+
=
(10)
TU.+’/V
where u is the kinematic viscosity of the fluid. According to Davies (1966a and 1966b), the experimental data of Laufer (1954) on the radial rms fluctuating velocity of a turbulent fluid in pipe flow can be correlated by the equation r+ = ?‘+/i>,+ + 10) (11) Combining Equations 9 and 11,we have fpf
= e+
+
G, y
(12,
JT+
where y A = y u * / u is the dimensionless distance from the wall. Figure 1 shows the value of t p - according to Equation 1 2 using Owen’s (see Davies, 1966a, p 427) correlation for the eddy diffusivity: E + = ( ~ + / 1 0for ) ~ y+ < 5 E+
= O.OlZ(y+
- 1.6)?for 5
~ , ( d c / d y )(141 makes only a minor contribution to the total deposition rate. where r is the distance from the pipe axis, y is the dis-
+
352
Environmental Science & Technology
sivity. With the boundary conditions c = 0 a t y = 0 and c = c,, a t y = R, Equation 20 can be integrated to give
-ol
where we have defined the diffusion velocity of the vapor as U = 4u/2xR?, and the dimensionless vapor diffusion velocity, U,, and the dimensionless vapor diffusivity, t u + , respectively, as U , = U / u , and t u + = t U J u . I t is reasonable to assume that in the turbulent core the particle diffusivity and the vapor diffusivity are equali.e., t p = t u . This implies that the ratio (f/c,) is the same for both the particles and the vapor, since the boundary layer is usually very thin in comparison with the pipe radius, and the differences in the concentration profiles in the boundary layer for particles and for vapor contribute little to the ratio (i./co). Further, under the same assumption that t p = t u in the turbulent core, we can subtract Equation 21 from Equation 19 to obtain -1- -- 1 + ‘ “ / C , , ’ [ ( R , -R+ -E
v+
DIMENSIONLESS WANE
where b is the boundary layer thickness and b- = bc,/v. Further, under usual conditions, R , >> b,, and Equation 22 can be simplified to
FROM WALL, y,
Figure 1. Dimensionless particle diffusivity
tance from the pipe wall, R is the pipe radius, and c is the local particle concentration a t the pointy or r. In the “diffusion free-flight’’ model used here, Equation 14 applies only in the region y > 6, where 6 is the particle stopping distance from the wall where the free flight begins. The flux of particles, @, arriving at the wall by free flight is given by (15) 4 = 2x(R - 6)u,jcg where ~ ‘ 6 and c6 are the values of v and c a t y = 6. Integrating Equation 15 from y = 6 t o y = R, we have
i:
=
1 ‘Sh2ir(R -
> I t , dg
+
c‘+
(16)
Solving c 6 from Equation 15 and substituting it into the above integral. we have
(23) where we have let b , = 90 as the thickness of the boundary layer. Equation 23 indicates that the deposition resistance for particles ( l / V + ) is equal to the deposition resistance for a vapor ( l / U - ) plus the difference in the deposition resistances for particles and vapor. Since there is an extensive body of experimental data on the molecular diffusion rates in turbulent pipe flow, Equation 23 can be used to calculate the particle deposition velocity from the particle and vapor diffusivities in the boundary layer. Details of the calculation procedure are given in the Appendix.
Results Figure 2 shows the result of calculation based on Equa-
(19) where V - = V / c , is the dimensionless deposition velocity, R- = Ru,/v is the dimensionless pipe radius, 6, = 6u,/v is the dimensionless distance from the wall where the free flight begins. Equation 19 cannot be evaluated directly because the integral is improper a t the tube axis (y- = R - ) . However, this difficulty can be avoided by considering the similar case of the radial diffusive transport of a vapor to a perfectly absorbing pipe under the same turbulent flow conditions in a pipe of the same radius, R. We have, similar to Equation 14, $ L = 2x(R - y ) t , ( d c / d y ) (20) where & is the molar vapor flux to the wall per unit length of the pipe and t D is the corresponding vapor diffu-
, , , , , / , ,l
l
where c o is the particle concentration at the tube axis It is convenient to define a mean particle deposition velocity, 1’ = @ / 2 x R c (18) based on the average particle concentration, f , in the pipe. Using Equation 17 and expressing all quantities in a dimensionless form, we have
, , , ,,,,,,
, , ,
,
, , / ,
, , ,/,
l
IEDLANDER-JOHNSTONE
J/
II I
I .I
’
, , , , , ,,,,,
0.6
, , ,
0 I
,,,,I
, , , I ,,
I IO 100 MMENSIONLESS RELAXATION TIME, T+
“‘‘‘‘‘l
1000
Figure 2. Theoretical and experimental particle deposition velocities Volume 8 , Number 4 , April 1974
353
I
E
,
.
,,,,, , , ,,,,,,, ,
, ,,,,,, ,
,
,,,,,,,,
Re = 5,000 0
30
Re: 50.000
INVESTIGATOR (1
I
121 ILORI
v
142
I31 FARMER e t 0 1
I
O
I
a
151 POSTMA B SCHWENDIMAN 4
0
( 6 1 WELLS
a relationship first obtained by Davies (1966a). In addition, in the Stokes law regime, T = 2a2p,/9Up, ( A-5) where p p and pf are the particle and fluid densities, respectively. Thus. in dimensionless form, Equation A-5 becomes
PP 15
SEHMEL
141 ALEXANDER
a
+
L’;,+ = 6+/(6+ 10) (A-3) Equations A-2 and A-3 can be used to solve for the two unknowns, v6+ and 6-, with the result
3
-
.
e
n a
0
, , ‘1”’4
,,,,,
, , I
a
COLDREN
CHAMBERLAIN
100 IPOO DIMENSIONLESS RELAXTION TIME, T+
I 18 0
l0,OOO
100,000
Figure 3. Theoretical, experimental particle deposition velocities
tion 23 for the dimensionless particle velocity, V - ; V, is shown to be primarily a function of the dimensionless particle relaxation time, T & , with the Reynolds number having only a moderate effect at high deposition rates. The higher deposition velocities of the lower density particles result from the larger “interception” effect, since for particles of the same relaxation time the lower densities give a larger particle radius. The use of b - = 90 as the boundary layer thickness in Equation 23 is somewhat arbitrary. but the final results are not affected to a great extent by the actual boundary layer thickness used. For instance, changing b - from 90 to 30 gives results that are indistinguishable from those shown in Figure 2 for 6 + < 30 and for i up to about 50. Also shown in Figure 2 are the experimental data of Friedlander and Johnstone (1957). The data of Sehmel (1968). Ilori (1971). Farmer et al. (1970). Alexander and Coldren (1951), Postma and Schwendiman (1966). and Wells and Chamberlain (1967) are compared in Figure 3 with the theoretical predictions. The data of Montgomery and Corn (1970) have been presented elsewhere in a similar form, and they are not included in this comparison. Although there is general agreement between the available experimental data and the theory of Friedlander and Johnstone and that proposed in this paper. the data are too widely scattered to make possible finer distinctions between theories. It should be noted that the theory of Davies appears to be too low, in comparison with both the experimental data and the other theories. These comparisons show that more accurate experimental data are needed to determine whether the theoretical model presented here is adequate to describe the turbulent deposition process in the regime where the particle stopping distance is of the same order of magnitude as the thickness of the boundary layer.
T+ = ( ? ‘J)
(4-6)
( p , / p )a+?
With Equations A-4 and A-6, the dimensionless free-flight velocity, ~ ’ g + , can be calculated as a function of i L .And
MMENSlONLESS RELAXATION TIME, T,
Figure 4. Relationships between T, and
6-
APPENDIX Free-Flight Distance and Velocit) The free flight of particles to the wall begins at > = 6 with an initial velocity L ~ where , 6 = L‘,T a ( A-1) The particle radius. a, is included in the equation to account for the “interception effect.” In dimensionless form, Equation A-1 becomes 6+ = L‘,+T+ a+ (A-2) where 6- = 6 u * / v and a _ = a L * / v . Using Davies’ correlation for the rms fluctuating velocity, Equation 11. we have
+
+
354
Environmental Science & Technology
DIMENSIONLESS RELAXATION TIME, T+
Figure 5. Relationships between
i-
and v..
by means of Equation A-2, 6, can be determined. The results are shown in Figures 4 and 5.
Diffusion Velocity of Vapor and Vapor Diffusiuity i n the Boundary Layer The diffusion velocity, U,, of a vapor in turbulent pipe flow can be obtained from the existing correlations for heat and mass transfer in turbulent pipe flow. For instance, according to Diesseler (see Eckert and Gross, 1963), the Stanton number, St, for the diffusive transport of a vapor in turbulent pipe flow can be correlated with the Reynolds number, Re, and the Schmidt number, Sc = D , J u , by the equation 0.0396 Re-’ St = __ (A-7) 1 1.7Re-’ YSc - 1)Sc-l ’ where D , is the molecular diffusivity of the vapor. The Stanton number is defined as St = U / o , where ti is the mean flow velocity in the pipe. Thus,
+
U , = U/u,, dimensionless diffusion velocity of vapor V = deposition velocity of particles, cm/sec V- = V/u,, dimensionless deposition velocity of particles u = local root-mean-square (rms) fluctuating velocity of
fluid, cm/sec = rms fluctuating velocity of particle due to Brownian motion, cm/sec u, = friction velocity, cm/sec u + = u/u,, dimensionless rms fluctuating velocity of fluid u g = rms fluctuating velocity of the fluid a t the pointy = 6 where free flight begins u6+ = u g / u * , dimensionless rms fluctuating velocity of fluid at y - = 6, y = distance from wall, cm y , = yu,/u, dimensionless distance from wall ud
Greek Letters
6 = distance from wall where free flight begins, cm = u / c * = (Sthi, (-4-8) 6, = 6u,/u, dimensionless distance from wall where free flight begins where ti, = ii/u,. For the case Sc = 1, Equations A-7 and t = eddy diffusivity, cm2/sec A-8 give U + = 0.0396 Re-! 4 ~ + (-4-9) c ’ = inertial diffusivity of particle, cm2/sec c p , t L = particle and vapor diffusivity, cm2 and the corresponding dimensionless vapor diffusivity in c+,tp-,tL= t / u , t p / u and t L / v = dimensionless values of the boundary layer is given by t , en, and e ( , t + = l + t+ (A-10) p f , p P = fluid and particle densities, g/cm3 Equations A-9 and A-10 have been used in Equation 23 to X = stopping distance of particle corresponding to an inieveluate the dimensionless particle deposition velocity. tial velocity of u h d = stopping distance of particle corresponding to an Concentration Ratio, (E/c,) initial velocity of u d u = kinematic viscosity of fluid, cm2/sec The ratio of the average particle or vapor concentration T = particle relaxation time, sec in the pipe to the particle or vapor concentration a t the pipe axis, (?/c,), can be calculated by use of the fact that T, = T U * ~ / U ,dimensionless particle relaxation time in fully developed turbulent flow, E/co = u / u o where a is 4,4, = particle and vapor flux to pipe wall per unit the mean flow velocity in the pipe and uo is the velocity length of pipe, number/cm, or mol/cm a t the pipe axis. Based on the velocity profile measurement of Nikuradse (see Schlichting, 1960), the ratio ii/uo Literature Cited varies from 0.791 for Re = 4000 to 0.865 for Re = 3 x 106. An average value of 0.82 has been used in Equation 23 for Alexander, L. G., Coldren, C. L., “Droplet Transfer from Suspending Air to Duct Walls,” Ind. Eng. Chem., 43, 1325 (1951). C/c, for the calculation of the particle velocities. Beal, S. K., “Transport of Particles in Turbulent Flow to Chan-
u+
Acknowledgment The assistance of Jugal Agarwal is hereby acknowledged.
Nomenclature a = particle radius, cm a + = a u,/u, dimensionless particle radius B = dynamic mobility of particle, cm/sec-dyne b = boundary layer thickness, cm b , = bu*/u, dimensionless boundary layer thickness c, ?, co = local, average, and centerline concentration of particles or vapor, number/cc, or mol/cc D,D, = Brownian diffusivity of particles and vapor, cm2/sec k = Boltzmann constant m = mass of an individual particle, gram R = pipe radius, cm R- = Ru*/u, dimensionless pipe radius Re = u ( P R ) / v ,Reynolds number r = distance from pipe axis, cm Sc = D L / v ,Schmidt number St = U/ti, Stanton number U = diffusionvelocity of vapor, cm/sec u,ti,u0 = local, average, and centerline velocity of fluid in pipe, cm/sec ti+ = t i / u * , dimensionless mean flow velocity of fluid in Pipe
nel or Pipe Walls,” WAPD-TM-765, Bettis Atomic Power Laboratory, Westinghouse Electric Co., Pittsburgh, P a . , 1968. Davies, C . S . , “Deposition from Moving Aerosols,” in Aerosol Science, C. N . Davies. Ed.. Chap. 7, pp 393-446, Academic Press, New York, N.Y. (1966a). Davies. C. N . , “Deposition of Aerosols from Turbulent Flow Through Pipes,” Proc. Roy. SOC.(London). A, 289, 235-46 (1966b). Eckert, E. R. G., Gross, J . F.. “Introduction to Heat and Mass Transfer,” p 140, McGraw-Hill, New York, N.Y., 1963. Farmer: R., Griffith, P., Rohsenow, W. M . , “Liquid Droplet Deposition in Two Phase Flow,” ASME Paper S o . 70-HT-1 (1970). Friedlander, S. K., Johnstone, H . F., “Deposition of Suspended Particles from Turbulent Gas Streams,” Ind. Eng. Chem., 49, 1151-6 (1957). Fuchs, N. A., “Evaporation and Droplet Growth in Gaseous Media.” Pergamon Press. New York. N.Y.. 1959. Fuchs, 3. A,: “Mechanics of Aerosols.” ’ pp 182-3, Pergamon Press, New York, K.Y., 1964. Fuchs, S . A , , “On the Stationary Charge Distribution on Aerosol Particles in a Bipolar Ionic Atmosphere,” Geofis. Pura. A .P.P ~ . , 56,185 (1963). Fuchs, N. A.. Stechkina. I. B., “Resistance of a Gaseous Media to the Motion of Spherical Particles of a Size Comparable to the Mean Free Path of the Gas Molecules,” Trans. Farad. SOC.,58, 1949 (1962). Hutchinson, P., Hewitt, G. F., Dukler, A. E . , “Deposition of Liquid or Solid Dispersions from Turbulent Gas Streams: A Stochastic Model, Chem. Eng. Sci. (Great Britain), 26, 419-39. 1971. Ilori, T . A., “Turbulent Deposition of Aerosol Particles Inside Pipes.” PhD Thesis, Mech. Eng. Dept., U. of Minnesota, Minneapolis, Minn.. 1971. Laufer, J.. “The Structure of Turbulence in Fully Developed Pipe Flow,” NACA R e p t . 1174 (1954). Volume 8, Number 4, April 1974
355
Montgomery, T. L., Corn, M., “Aerosol Deposition in a Pipe with Turbulent Air Flow,”J. Aerosol Sci., 1,185 (1970). Postma, A. K., Schwendiman, L. C., “Studies in Micro-metrics: Particle Deposition in Conduits as a Source of Error in Aerosol Sampling,” HW-65308, Hanford Laboratories, Richland, Wash., 1966. Rouhiainen, P. O., Stachiewicz, J . W., “On the Deposition of Small Particles from Turbulent Streams.” J . Heat Trans.. 92. 169-77 (1970) Schlichting, H., “Boundary Layer Theory,” p 506, McGraw-Hill, New York, N.Y., 1960. Sehmel, G. A , , “Aerosol Deposition from Turbulent Airstreams in Vertical Conduits,” BNWL-578, Pacific Northwest Laboratory, Richland, Wash., 1968. Sehmel, G. A,, “Particle Deposition from Turbulent Air Flow,” J . Geophys. Res., 75,1766-81 (1970).
Sehmel, G. A., “Particle Diffusivitie: and Deposition Velocities Smooth Surface. J. Coll. Interface SCZ., 37. over a Horizontal ~. 891-906 (1971). Soo,S. L., “Fluid Dynamics of Multiphase Systems,” Blaisdell, Waltham, Mass., 1967. Wells, A . C., Chamberlain, A. C., “Transport of Small Particles to Vertical Surfaces,”Brit. J . Appl. Phys., 18,1793 (1967). Whitby, K. T., Liu, B. Y. H., Husar, R. B., Barsic, N. J., “The Minnesota Aerosol-Analyzing System Used in the Los Angeles Smog Project,”J. Coll. Interface Sci., 39, 139-64 (1972a). Whitby, K. T., Husar, R. B., Liu, B. Y. H., “The Aerosol Size Distribution of Los Angeles Smog,” ibid., pp 177-204 (1972b). ~~~
Received for review March 5, 1973. Accepted November 17, 1973. Based in part on PhD thesis of Ilori, 1971. Work supported under Contract AT (11-1)-1248,Atomic Energy Commission.
Formation of Bis(chloromethy1) Ether from Formaldehyde and Hydrogen Chloride Lawrence S. Frankel, Keith S. McCallum,l and Ledelle Collier Rohm and Haas Co., 5000 Richmond St , Philadelphia, Pa. 19137 ~
_ _ _ _ _ ~
____
The formation of bis(chloromethy1) ether from formaldehyde and hydrogen chloride in moist air was investigated by combining measured amounts of monomeric formaldehyde and hydrogen chloride with air in glass vessels or in Saran bags. The resulting amount of bis(ch1oromethyl) ether was determined by high resolution mass spectrometry and verified in some instances by gas chromatography/mass spectrometry or dual column gas chromatography. Very low yields of bis(chloromethy1) ether are produced by the reaction. and normally this finding would be of little interest, but for the recently reported carcinogenicity of bis(chloromethy1) ether. During the course of exploratory studies in our laboratories, bis(chloromethy1) ether (BCME) was observed to form under surprising conditions. This prompted a series of experiments to determine if BCME might be formed by the adventitious combination of formaldehyde and a source of hydrogen chloride. A preliminary announcement of this finding was made last year (Chem. & Eng. News, 1973). Initially, it appeared that threshold limiting value (TLV) concentrations of formaldehyde and hydrogen chloride yielded approximately 1 ppb bis(chloromethy1) ether. Subsequent work has indeed confirmed that bis(ch1oromethyl) ether is formed a t concentrations of HC1 and formaldehyde higher than the TLV levels, although yields are lower than originally thought. These facts are being reported now so that potentially hazardous operations may be recognized in situations where significant quantities of formaldehyde could react with chloride ion in the presence of moist air t o produce bis(chloromethy1) ether, a reported carcinogen (Laskin et al.. 1971); (Thiess et al., 1973). Apparatus and Procedure Gas phase reactions were carried out either in glass flasks or Saran bags. To whom correspondence should be addressed. 356
Environmental Science & Technology
Reaction Flasks. Three-neck, round-bottom flasks were provided with a septum and glass stopcocks. T o prepare a reproducible surface, the flasks were rinsed with water, then baked a t 200°C for at least a n hour prior to use. Gases were mixed by agitating several 4-mm glass beads with a Teflon-coated magnetic stirrer. The flask was partially evacuated. Aliquots of gaseous formaldehyde and hydrogen chloride were introduced through the septum. One of the stopcocks was momentarily opened to allow the contents of khe flask t o return t o atmospheric pressure. At the end of the reaction, the gases were flushed out through a n adsorber for analysis. Saran Bags. Saran bags (Anspec Co., P.O. Box 44, Ann Arbor, Mich. 48109) were equipped with a glass “T,” one arm of which was fitted with a septum. The bags were twice filled with air and emptied prior to use (Boettner and Dallos, 1965). The reactants were slowly introduced through the septum while air was being pumped into the bag. After filling, the bag was kneaded t o thoroughly mix the reactants. Experiments were planned so that bags were reused a t higher concentrations of reactants in subsequent experiments. Humidity Control. Reaction vessels were generally filled with air from a room held constantly a t 26°C and 40% RH. Some experiments were carried out under lower humidity conditions. The 100% relative humidity atmosphere was attained by drawing the air through water in tandem fritted glass disc scrubbers. A dry atmosphere was obtained by drawing room air through a n 0.5-in. diameter cartridge charged with 42 grams of Linde 4x Molecular S‘ ieves. Calibration and Gas Standards. The primary standard of calibration was prepared by adding a measured volume of BCME into a known volume of ethylene dichloride. Concentrations between 1/4-lY~ were convenient. These solutions are stable indefinitely and obviate the necessity of frequently handling liquid BCME. Gas standards were most conveniently and reproducibly prepared by pumping a measured volume of gas over a thermostated FEP Teflon (Du Pont) tube containing liquid BCME (O’Keeffe and Artman, 1966). The rate of dif-
