ELECTROSTATIC PRECIPITATOR ANALYSIS D .
N . HANSON
AND
C .
R .
W l L K E
Department of Chemical Engineering, University of Califomiu, Berkeley, Calif. 9.2720 Relations are developed to compare space charge with conventional electrostatic precipitators. Space charge precipitators have wall area requirements similar to conventional units, if the drops added to the gas are of the same general sire as the particles to be precipitated. Space charge precipitators are considerably simpler in construction than conventional units and would be highly desirable if large quantities of small drops could be produced. Present designs of atomizing nozzles are not satisfactory for this purpose, but condensation of steam to form droplets appears feasible.
IN the conventional method of electrostatic precipitation of particulate matter from gases, the gas stream is passed through a corona to charge the particles and then through an imposed electric field which causes the particles to drift to a collecting wall or screen. I t is possible to achieve high collection efficiencies, but the equipment is necessarily relatively large and expensive. The presence of extensive high voltage equipment in the collection section imposes complexity of mechanical construction, and the collecting surfaces must be cleaned periodically with hammers or other impact devices. The method of precipitation described is capable of high collection efficiency with equipment that is generally simpler than conventional designs. This type of precipitator operates on the principle of mutual repulsion of charged particles to a grounded wall. One possible apparatus and process are illustrated schematically in Figure 1 for precipitation of solid particles in a three-stage single-tube unit. The gas stream enters the tubular section, where the solid particles are charged by a conventional corona. Charged droplets, normally water, are then injected in the form of a fine spray from a charged nozzle. The charge on the droplets and quantity of spray are set to give the desired electrical fields in the tube. As the charged particles and drops pass through the grounded section of the first stage, a fraction of the water and solids is forced to the grounded wall by the electric fields created by the space charge. Precipitated solid parti-
CGRONA
idARGEG ChARGEO
I_-
GRGUNDEG T J E E WALL
WATER PLUS SOLIDS OUT
Figure 1 . Three-stage single-tube precipitator
cles are entrained in the coalesced water, which runs down the walls and is drained from the precipitator. The concentration of space charge falls in the direction of flow as particles and drops are removed from the gas, and the rate of Precipitation falls off correspondingly. T o counteract this effect, as the stream enters the second stage a charged water spray is again injected to re-establish the desired initial level of space charge. Precipitation then occurs as before. The process is repeated in the third stage. By establishing a sufficiently high initial level of space charge and rebuilding it a t each stage through introduction of water particles, a high degree of precipitation can be accomplished in a few stages with relatively short residence times. Other methods of operation would be equally effective. The other principal method investigated here is to introduce the drops substantially uncharged from external sources, brought in a carrier gas. The drops are then charged by a corona discharge just downstream of inlet position. The theory of precipitation of particles in cylindrical tubes by the action of space charge has been discussed by Faith, Bustany, Hanson, and Wilke (1967), but space charge precipitators have not been compared with conventional electrostatic precipitators. I n the present work, such a comparison has been developed. The equations on which it is based have been derived for one size of particle, but if the space charge of particles is small they apply to systems of any particle-size distribution. Since particle charging is necessary to all precipitators, a suitable particle-charging section must be used upstream of each precipitator, operating through a corona or other means, and the charging section is not included in the comparison. The particles under consideration are assumed to have a uniform charge, and drops added to the system in the space charge precipitators are assumed to be all of the same size and charge, but not necessarily the same size and charge as the particles. Fields due to space charge of the particles present before addition of charged drops are assumed to be negligible compared to fields imposed by electrodes or fields created by the added charged drops. Plug flow of gas is assumed. The devices donsidered are shown in Figure 2 . T o cornpare the different cases, the same cross-sectional area for gas flow is chosen for each. In the tubular cases, the VOL. 8 N O . 3 J U L Y 1 9 6 9
357
- I, C A S E 1.
S T A G E D S P A C E CHARGE PRECIPITATOR
Irj-c---
a Stage 1
Stage n
CASE 2
C O N T I N U O U S SPACE C H A R G E P R E C I P I T A T O R
CASE 3
ROD A N 0 T U B E P R E C I P I T A T O R
If the contribution of charged particles to the fields existing in the precipitator is negligible compared to the contribution of drops, Faith et al. have also shown that the precipitation of drops in the stage can be directly calculated by
Hence
CASE 4
P L A T E TYPE PRECIPITATOR
// CASE 5
I n a stage the radial field is proportional to the radius and is a maximum a t the wall a t any cross section in the tube. Since the drop concentration is highest at the inlet of each stage, the limiting field occurs a t the wall at that point. This field is given by Gauss’ law:
CONTINUOUS SPACE CHARGE PLATE PRECIPITATOR
Q J EdS = ; EwrDdL =
4
Figure 2. Precipitator models
cross-sectional area is directly 7 D 2 / 4 , and in the plate cases the product of X and Y is taken to be aD2/4 also. The amount of steel surface in the precipitator walls is then directly given by the wall perimeter around this cross section multiplied by the required axial length of the passage. The wall perimeter is identical in the first three cases, if the central pipe or rod is neglected. The last two cases can be made directly comparable to the first three by choosing X and Y such that the amount of wall perimeter enclosing cross section XY is also nD. Since all the devices would probably be built of multiple units with a similar surrounding shell, the shell area is not considered. The maximum field allowed in each case is the breakdown field of the gas, EBD.Attainment of this field a t some point in each of the apparatuses establishes the limit for the capacity of that case, and determines the residence time needed. Each unit is thus being operated a t the upper limit of its effectiveness. Case 1. Staged Space Charge Precipitator
Consider a precipitator consisting of n stages, of equal length and operated identically. The residence time per stage is then t/n, where t is the total residence time. If the over-all fraction of unprecipitated particles is U p , the fraction of particles entering each stage which are unprecipitated a t the exit of that stage is
Similarly, the fraction of drops entering each stage which are unprecipitated a t the exit of the stage is (Ud)stage and from the work of Faith et al.,
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I & E C PROCESS D E S I G N A N D D E V E L O P M E N T
Substitution of 2 into 1 yields
r (3) and rearrangement gives
(4) Case 2. Continuous Space Charge Precipitator
As an alternate process, charged drops can be introduced approximately continuously (Case 2, Figure 2 ) . A central charged pipe with many nozzles provides the equivalent of a very large number of precipitator stages, allowing introduction of drops a t the same rate as they are precipitated and thus maintaining the space charge a t essentially the same level throughout the length of the precipitator. I n Case 1, where the stages are definite and discrete, the drops and particles enter each stage uniformly distributed over the inlet cross section, and because of the nature of the precipitating fields they remain uniformly distributed across any downstream cross section. In Case 2, since the drops are introduced near the center of the tube, a uniform radial concentration of the drops or particles does not necessarily exist. However, turbulent diffusion tends to remove any concentration gradient, and as one limit it can be assumed that the eddy diffusivity is sufficiently high that a radially uniform concentration of both drops and particles exists. The other limit results from the assumption of zero diffusivity. The correct answer should lie between the limits.
With Turbulent Diffusion. Only the solid particles need be considered. At any cross section the flux of particles a t the wall is given by
By inspection,
N, = X,c,Eu. As in Case 1, the field a t the wall is again the highest, and can be set a t .EHIl and maintained a t that value by the addition of charged drops. Writing the equation of continuity for the particles,
and substituting in 7,
E =[
Wqd 2EO
1' ' ,
independent of r
Since E is independent of r, E = E w , and thus
or
Equation 4 approaches Equation 5 as n approaches regardless of the relative mobility of particles and drops. Without Turbulent Diffusion. I n Case 1 and in the preceding analysis of this case, the drop concentration was uniform a t any cross section and the field due to the drops was thus proportional to the radius. I n the absence of turbulent diffusion, the drop concentration and the field as functions of radius are unknown and must be determined. The field due to the central charged pipe is neglected, since the ratio of diameters is normally great and the voltage used to produce the sprays is not excessively high. The field, which is the same a t all cross sections, may be obtained from Gauss' law. At any radius in the cross section
I n practice the flow of liquid drops is made such that E = EeD, thus defining the drop concentration a t the wall and the amount of water necessary. Since the field is known, the rate of precipitation of solid particles can be established. Consider a particle a t radius r,, a t time 0. The velocity of this particle is
m,
dr dt
U. = hpEnD= -
Calculating the time for this particle to reach the wall,
If 0: is neglected in comparison with D 2 , the fraction of particles unprecipitated in time t is given by
4 Substituting in 10,
Q
2 ~ r d L E= -
t=-
to
2xrdLE = to
I rcdqddL 2 ~ r d r (6)
An additional expression for the field may be obtained from the continuity equation for the drops. T h e flux a t any radius is given by
Nd = X ~ E C ~
(11)
Case 3. Rod and Tube Precipitator
or
E = "re, b i ' cdrdr
D (1- uy) 2XpEn~
I n this case the space charge fields due to the solid particles are neglected in comparison with the fields produced by the central electrode. With this assumption the equations can easily be obtained by the above methods. I n all equations Df is neglected in comparison with 0'. However, this only simplifies the final equations rather than changing form. Breakdown field is stipulated t o occur a t the surface of the central electrode. With Turbulent Diffusion.
The total flow of drops is then
N d 2 ~ r d L= XdEcd2rrdL and since this is constant and applies a t the wall,
Without Turbulent Diffusion.
or
E= Equating 6 and 7 ,
DEw(cd)w arcd
Equation 12, although of a different form, corresponds to the equation previously derived by Deutsch (19221, in which he expressed the product of mobility and field a t the wall as the drift velocity, w . The term DID,, which does not appear in Deutsch's equation, results from the character of the field as a function of r: VOL. 8 NO. 3 J U L Y 1 9 6 9
359
V
E=-
r In"
Comparison of Wall Area Requirements
D DO
from which
If the total volumetric gas flow, F , is divided among m tubes (or XY cross-sectional units in the case of plate precipitators), the velocity of gas flow in the precipitator is given by u=-
F/m aD2/4
Case 4. Plate-Type Precipitator
and the precipitator length is then
The cross section considered is XY = aD2/4, in order to be common to the tubular cases, and the total steel perimeter is taken to be x D , in order to have the same amount of steel wall per unit axial length of this cross section as in the tubular cases. Since both sides of the plate are active in precipitation, only one wall need be assigned to the cross section and Y thus equals x D . Then, since
L = ut Since all the expressions found for residence time are of the form
length is given by
K D 2
XY=-
4
and Y = x D , it follows that X = 0 1 4 . If space charge of the solid particles is neglected, the field is constant a t all points in the cross section and may be taken as ERU.The methods used above then lead to the following results: With Turbulent Diffusion.
Without Turbulent Diffusion.
The question could be raised as to whether the choice of X made here influences the amount of wall required. The answer is that since Y is inversely proportional to X (Equation 14) and t is directly proportional to X (Equation 15 or 16), product t Y , which determines the total amount of wall, is independent of the choice of
X. Case 5. Continuous Space Charge Precipitator
As shown in Figure 2, a continuous space charge precipitator can be arranged in the form of a plate-type precipitator. The tubes producing the spray are assumed to be placed a t frequent enough intervals to achieve a reasonably uniform input of drops. I n the analysis of the case it was assumed that the same amount of metal was used in the tubes as in one of the walls. As in Case 4, it was found that X = Dl4 and Y = xD. Similar analysis to that of the preceding cases yields the following results: With Turbulent Diffusion.
The wall area required is then
A w = maDL or, per unit gas flow,
Equation 20 shows that the amount of wall area required for treatment of a given amount of gas flow is independent of the tube size chosen. Of the terms which enter into the equation, mobility is determined by particle size and charging mechanism and is unaffected by choice of precipitator type; similarly, the breakdown field is independent of precipitator type. I n comparing the wall area requirements of the various types of precipitators, a direct comparison can be obtained solely from the values of f( U p ) . The expressions for f ( U,) for the various cases are listed in Table I. The difficult question of which expression is more nearly correct for the last four cases arises, but meaningful comparisons can still be made without exact knowledge of the extent of turbulent leveling of particle concentration gradients. The wall area requirements of Case 3 are markedly higher than those of Cases 2, 4, or 5 . This difference results from the low fields found a t the wall of the rod Table I. Expressions for f ( Up) Case 1
3
Without Turbulent Diffusion.
360
I & E C PROCESS D E S I G N A N D DEVELOPMENT
With Turbulent Diffusion
Without Turbulent Diffusion
and tube precipitator in contrast to the wall fields of the other cases. The disadvantage could be alleviated by use of a larger central rod, but a t best Case 3 could only approach the results of the other cases, and under normal construction would require more area than the others. Comparison of Cases 2 and 5 shows that the two types of continuous space charge precipitators have identical wall area requirements. The precipitator of Case 5 would be more difficult to construct than that of Case 2 and the certainty of maintaining wetted walls in Case 2 appears higher than in Case 5 , leading to the conclusion that Case 2 is more favorable. Comparison of Case 2 with Case 4 shows that if turbulence is effective in removing concentration gradients, the wall area requirements are the same. If turbulence is not effective, Case 2 requires approximately twice the area of Case 4, since in most applications U p is small compared to unity. Results of experiments reported by White (1963) generally confirm the logarithmic character of f ( U p ) although , experiments with relatively large particles have shown definite particle trajectories, indicating that turbulent mixing of such particles does not occur to any great extent. On the basis of the evidence for the precipitation of small particles, those 10 microns or less in diameter, which are of the most importance in precipitator application, turbulent diffusion appears to be effective, and the wall area requirements of Cases 2 and 4 can be considered to be the same. However, Case 2 possesses the distinct advantage over Case 4 of trapping the particles a t the wall in the precipitated liquid phase and preventing re-entrainment. While the walls in Case 4 could presumably be irrigated, assurance of wetting a t all points on the flat plates would be difficult. Comparison of Cases 1 and 2, the two principal variations of space charge precipitator, shows that Case 2 is clearly advantageous if Case 1 is considered to be constructed simply as shown in Figure 2. Values of f ( U p ) are given in Table I1 for both cases, using various values of U pand n, and assuming that Ad = A,. Turbulent diffusion is also assumed in Case 2 . As the degree of precipitation is increased, the advantage of Case 2 mounts. At a precipitation of 99.9‘4, Case 1, even with five stages, still requires twice as much wall area. The importance of the relative mobility of drops and particles in staged precipitators is also apparent from consideration of f ( U p )for Case 1. As the ratio of mobilities, Ad/&,, approaches zero, f ( U p ) for Case 1 also approaches f ( U p )for Case 2, regardless of the number of stages, just as f ( U J for Case 1 approaches f ( U p ) for Case 2 as the number of stages approaches infinity, regardless of the relative mobility of drops and particles.
Thus as the mobility ratio is improved or the number of stages is increased, Case 1 can be made to approach Case 2 . However, Case 1 can be constructed in a way which materially lowers the cost of the wall area required for it and makes the two cases competitive. Power Requirements
All of the precipitators are presumed to have an upstream section in which the particles are charged. The power requirements of these charging sections are thus common to all cases and need not be considered. The additional power requirements of the conventional cases, in which drops are not added, would consist solely of the power necessary to move the particles to the collecting surface and would be small, unless a corona were operated throughout the length of the precipitator, as is often done with rod and tube precipitators of the type in Case 3. Under these circumstances the additional power would be high. The additional power requirements due to introduction of charged drops in the space charge precipitators are not apparent. Cases 1 and 2 are compared. Case 1. I n each stage the concentration of charged drops is brought up to the level ( c d ) , , by introduction from a charged spray. This level is set by the stipulation of breakdown field, E R D ,a t the wall, and it can easily be shown that
The current carried by the drops in the first stage is then I 1
=
(Cd)aqdF
If the fraction of original drops unprecipitated in each stage is
in later stages The total current for n stages is or
Case 2. Consider the case with turbulent diffusion, since it is of greater interest. Charged drops are sprayed in a t the center of the tube a t the same rate as they are precipitated. Under these conditions the concentration a t the wall (and a t all other points if turbulent diffusion is effective) is fixed by the existence of breakdown field a t the wall, and is
Table II. Values of f( U p ) for Space Charge Precipitators (Assuming hd = h,) Case
1
2
n=l n=2 n=3 n=4 n=5
Up= 0.1
Up = 0.01
Up = 0.001
9 4.3 3.5 3.1 2.9
99 18 10.9 8.6 7.6
999 61 21 18.5 14.9
2.3
4.6
6.9
The current carried by the drops a t the wall is then
I
= qd(Nd)w
Aw
(24)
Inserting the drop flux a t the wall and adding the drops which leave in the exit gas,
VOL. 8 NO. 3 JULY 1969
361
the same as in Equation 23 for Case 1 except for the difference in the last term. The last term in Equation 23 approaches the last term in Equation 25 as n approaches m , but the term in Equation 25 is always higher than that in Equation 23, showing that the current requirements of Case 2 are higher than those of Case 1. The current requirements of either type of space charge precipitator are inversely proportional to the tube diameter chosen, and the use of larger tubes is indicated in order to minimize the amount of power required. The current requirements are also increased as the relative mobility of drops to particles increases and the use of drops of low mobility is indicated. Water Requirements
The water flow necessary for drop production can be obtained easily from the power requirements. If W is the volumetric flow rate of water and Dd is the drop diameter,
However, it is more convenient to express the charge on the drops by means of the relation between mobility and charge, and for this purpose Stokes’ law is assumed with the Cunningham correction for small drops included. Thus
where the Cunningham correction is taken as
(Perry, 1963, p. 1019). .i is the mean free path of molecules in the gas and is assumed here to be 0.1 micron. Use of Equation 26 leads directly to equations for the water requirements: Case 1.
Case 2 (Turbulent).
With a normal charging device the mobility of a drop might be expected to be roughly proportional to or independent of the diameter, depending on drop size. Thus, as a guide, the water requirements would be approximately proportional to the drop diameter or the square of the drop diameter. I t is obviously important to use small drops. Requirements for Typical Industrial Gas Flow
T o examine the magnitude of equipment necessary and the general feasibility of space charge precipitators, an example of each type was calculated. Typical charged spherical particles were assumed t o have the mobilities given by Perry (1963). Thus particles of 1-micron diameter were taken to have a mobility of 1.12 x 10 ’ sq. meter:volt.sec. Such particles had a lower mobility than particles of any other diameter and 362
I & E C PROCESS D E S I G N A N D DEVELOPMENT
Table 111. Continuous Space Charge Precipitator Design
Specifications Gas flow = 100,000 cu. ft./min. 9gC.Crecovery of 1-micron particles Drop size = 5 microns Drop mobility = 5.09 x 10 ’ sq. meter/volt.sec. Gas velocity = 10 ft.isec. Tube diameter = 2 ft.
Requirements Wall area = 10,400 sq. ft. Number of tubes = 53 Tube length = 31.1 ft. Precipitator diameter = 17.2 ft. Power for sprays = 1.20 kw. Water flow in drops = 299 gal./min.
hence the design of a precipitator to effect a high recovery of 1-micron particles provides a severe test of the effectiveness of space charge precipitators. The desired recovery was set a t 99%. Breakdown field was assumed to be 20,000 volts per cm. Gas velocity in the precipitator was set a t 10 feet per second or lower and a unit was designed to process 100,000 cu. feet per minute of gas. Table I11 shows a typical design utilizing a continuous space charge precipitator as analyzed in Case 2 with turbulent diffusion. Many choices of tube diameter are possible; a tube diameter of 2 feet was selected for the design. Drop diameter from the sprays was taken to be 5 microns, and for calculation of the power requirements it was assumed that the sprays were charged to 10,000 volts. Precipitator cross-sectional area was taken as 1.4 times the tube cross-sectional area to allow for reasonable spacing of the tubes. The resulting design shows that a continuous space charge precipitator containing 53 tubes, each 2 feet in diameter and 31 feet long, is required. A conventional precipitator would require as much area or more, depending on the type used. The additional power requirements of‘ the space charge precipitator are seen to be small, approximately 1 kw., and the operating costs due to these power requirements would be negligible. However, the water requirements for the sprays are excessively high. Even with 5-micron drops, a flow rate of 299 gallons per minute, or 2490 pounds per minute, is needed. T o produce drops in this quantity by atomization with air is clearly unfeasible; if 1 pound of air per pound of water were used, a compressed air flow roughly equal to one third the flow of gas being treated would be required. Atomization by impact nozzle or by the sudden expansion of superheated water as reported by Brown and York (1962) might offer a possible method of producing the drops, but the ability of such devices to produce drops of the order of a few microns in diameter is questionable. Vonnegut and Neubauer (1952) produced drops of approximately 1-micron diameter from a highly charged liquid point. However, the rate of drop production was small, and the feasibility of successfully operating a large number of liquid points is highly doubtful. There appears to be no existing type of charged nozzle which will conveniently produce along the center line of a tube drops of the size and quantity required for a continuous space charge precipitator. The drops could be produced externally in an uncharged state, introduced along the center line, and charged as they leave the central
Table IV. Staged Space Charge Precipitator Design
4
Specifications Gas flow = 100,000 cu. ft./min. 99% recovery of 1-micron particles Drop size = 0.1 micron Drop mobility = 2.75 X lO-'sq. meter/volt.sec. Tube diameter = 1 ft. Number of stages = 4 Requirements for precipitator Total precipitator volume = 14,700 cu. ft. If diameter = 20 ft. Stage length = 11.7 ft. Gas velocity = 5.3 ft./sec. Water flow in drops = 0.217 gal./min. Requirements for drop production Carrier air flow (NTP) = 2000 cu. ft.imin. Temperature of carrier air = 40" F. Steam required = 7.6 lb../min.
@+ PACKING
Figure 3. Modified stages for staged space charge precipitators
pipe. B u t such means of drop production and introduction would be more conveniently used in a staged precipitator and should be examined in conjunction with a staged device. A staged precipitator built like the continuous precipitator just considered would require more tube length, and hence if built as a series of tube bundles would be more expensive to construct than the continuous device. However, modifications to the stages could result in considerable simplification and allow low cost construction (Figure 3). At the entrance to each stage a short tube bundle is used for the introduction and charging of drops. The drops could be introduced ahead of the tube bundle in an uncharged state and charged by coronas operating inside each tube, or they could be introduced through the manifold that carries the corona wires. The diameter of tubes chosen for the shortened tube bundle will fix the allowable concentration of charged drops, as it does for a stage of long tubes. Downstream of the short tube bundle the only requirement is that a sufficiency of grounded wall be present that breakdown field not be exceeded a t any point. This can be accomplished by filling the space with a loosemesh packing or any other highly-open packing material. The total length of stage is exactly the same as would be required if the tubes extended for the whole length of the stage, since as Faith et al. showed, the fraction precipitated is independent of the passage diameter, but is determined only by residence time. The short tube bundle itself need only be long 'enough to accomplish the drop charging, and a t gas velocities typical of precipitators will be only a few inches in length. Staged space charge precipitators then are best designed by calculating the residence time and precipitator volume required by means of Equation 4. Table IV shows the design of a typical staged space charge precipitator to accomplish the same precipitation as that for which the continuous unit of Table I11 was designed. I n this case
the drops were taken to be 0.1 micron, the tube diameter in the short tube bundles was set a t 1 foot, and the number of stages was chosen to be 4. The required staged precipitator was found t o have a total volume of 14,700 cu. feet. Selecting an arbitrary diameter of 20 feet, each stage would be 12 feet long. The power requirements were not calculated, since Equation 23 calculates only the power dissipated by the charge on the drops and does not include the inefficiency of the coronas; however, the total electrical power is expected to be small. The 0.1-micron drops were assumed to be made in equipment external to the precipitator by mixing chilled air and steam. If air a t 40°F. and steam a t atmospheric pressure are mixed in proportions shown in Table IV, the resulting supersaturation ratio is sufficient to produce either nucleation on ions or spontaneous nucleation and condensation. The use of chilled air with a suitable concentration of ions would effectively control the number of drops made and hence their ultimate size. The design of Table IV represents one possible combination of equipment size and operating requirements. All of its features appear reasonable; however, operating and capital costs should be optimized to determine the best tube size and stage number. Nomenclature
A = area, sq. meters c = concentration of drops or particles, number/ cu. meter Dd = diameter of drops, meters D = precipitator tube diameter, meters Do = central electrode diameter, meters E = electrical field, voltsimeter EBD= limiting acceptable, or breakdown, field, volts/ meter f = function F = gas flow rate, cu. meters/second I = electrical current, amperes Cunningham correction to Stokes' law drag precipitator length, meters m = number of tubes in precipitator N = flux of drops or particles, numberisq. meter/ sec. n = number of stages in staged space charge precipitator VOL. 8 N O . 3 JULY 1 9 6 9
363
r
S t
UP
U
V W
W
X Y
drop or particle charge, coulombs charge enclosed within surface area S, coulombs radial length, meters surface area, sq. meters time, seconds over-all fraction of original particles unprecipitated in a precipitator fraction of entering drops of particles unprecipitated in a precipitator stage gas velocity, metersisec. potential applied to central electrode, volts drift velocity water flow rate for drop production, cu. meters/ sec. width of cross section in plate-type precipitators, meters height of cross section in plate-type precipitators, meters
GREEKLETTERS e,
= permittivity of free space, 8.85 x lo-’’ sq.
p
= gas viscosity, newtons.sec./sq. meter
SUBSCRIPTS d = drop o = drop or particle concentration at inlet t o precipitator p = particle W = a t wall literature Cited
Brown, R., York, J. L., A .I.Ch.E. J . 8 , 149 (1962). Deutsch, W., Ann. Physik 68, 335 (1962). Faith, L. E., Bustany, S. N., Hanson, D. N., Wilke, C. R., Ind. Eng. Chem. Fundamentals 6, 519 (1967). Perry, J. H., Ed., “Chemical Engineer’s Handbook,” 3rd ed., McGraw-Hill, New York, 1963. Vonnegut, B., Neubauer, R. L., J . Colloid Sci. 7, 61622 (1952). White, H. J., “Industrial Electrostatic Precipitation,” pp. 181-94, Addison-Wesley, Reading, Mass., 1963.
coulomb/newton ‘sq. meter A = drop or particle mobility, sq. meters/volt sec. A = mean free path of gas, meters
RECEIVED for review February 9, 1968 ACCEPTED March 2, 1969
HYDROGENOLYSIS OF ETHANE ANDOF PROPANE OVER A COMMERCIAL RUTHENIUM CATALYST Kinetic Study D .
G . TAJBL’
Institute of Gas Technology, Chicago, Ill. 60616 The kinetics of ethane and propane hydrogenolysis were studied over a commercial 0.5% ruthenium on ?-alumina catalyst. The ethane hydrogenolysis rate was first-order-dependent on ethane partial pressure and -2-order-dependent on hydrogen partial pressure over a temperature range of 160’ to 220’C. a t 1- to 2-atm. total pressure. The propane hydrogenolysis rate was first-order-dependent on propane partial pressure and -3/2-order-dependent on hydrogen partial pressure over a temperature range of 140’ to 170’C. a t 1-atm. total pressure. The apparent activation energies were 4 2 kcal. for ethane hydrogenolysis and 35.8 kcal. for propane hydrogenolysis obtained in a well-mixed gas-solid reactor operating over a wide range of conversions. Possible interpretations of the kinetic results are given.
RECENTLY, two papers
were published on the use of ruthenium as catalyst for the synthesis of liquid and gaseous fuels from carbon monoxide and hydrogen (Fischer-Tropsch synthesis). Karn et al. (1965) produced liquid and gaseous hydrocarbons a t moderate pressures and temperatures over a dilute (0.5 weight %) ruthenium on alumina catalyst. Tajbl et al. (1967) were interested in producing light gaseous paraffins during the conversion of carbon monoxide to methane over a similar catalyst at higher pressures. If not suppressed, hydrocarbon hydrogenolysis reactions, which are catalyzed by metals of the I Present address, Mobil Research and Development Corp., Paulsboro, K.J. 08066 364
I & E C PROCESS D E S I G N A N D D E V E L O P M E N T
Group VI11 transition series, can lower the yield of the valuable hydrocarbon product. No data were available at the time of these publications that could be used to assess the effect of hydrogenolysis reactions on the yield of hydrocarbon products in such processes over ruthenium. This lack of data led to the present research into the reaction kinetics of ethane and propane hydrogenolysis over a commercial ruthenium catalyst that was very similar to the catalysts used by Karn et al. (1965) and Tajbl et al. (1967). The most widely studied hydrogenolysis reaction is that of ethane catalyzed by metals of the Group VI11 transition series. Taylor and coworkers studied ethane hydrogenolysis
