Adsorption of Lead Chloride Vapors Chaim Aharoni' and Mark Neuman Faculty of Chemical Engineering, Technion, Israel Institute of Technology,Haifa, Israel
Amos Notea Depafinmnt of Nuclear Engineering, Technion,Israel lnstltute of Technobgy, Haifa, Israel
Vapors of lead chloride flowed through a bed of alumina at temperatures between 570 and 65OoC. The concentration of adsorbed lead was measured directly during the adsorption process by low-energy y-radiation and distribution curves were obtained. The rate of adsorption increases with the flow rate. The amount adsorbed at saturation depends on the internal surface area of the adsorbent as well as on the particle size. It does not depend on the concentration of lead in the gas phase.
Lead salts act as potent poisons for the automotive exhaust catalysts and are dangerous atmospheric pollutants by themselves. A convenient means of protecting the exhaust catalyst is to use a pre-bed in which vapors of lead salts are removed by adsorption. Doubts have been expressed concerning the practicability of using pre-beds with the high-leaded fuels actually in use, 1to 4 ml of TEL per gallon of fuel resulting in 4 to 6 lb of lead exhausted per year for an average car. But there is a significant and unavoidable trend toward the use of low-leaded fuels and for these, adsorption of lead in a pre-bed should be practical and could give adequate protection for 50,000 miles of driving. Alumina, with its unique qualities of large surface area and high heat resistance, seems to be one of the most appropriate adsorbents for the removal of lead salt vapors. The literature mentions pre-beds containing alumina granules, alumina coated steel mesh, and alumina impregnated with phosphates and other additives; the purpose of the impregnation is apparently to convert the lead salts into other less volatile compounds (Behrens, 1966; Brandenburg and Leak, 1966; Henderson et al., 1967; Kenward, 1973). The increasing interest in processes for the adsorption of lead has created an increasing interest in relevant fundamental data, but such data are still scarce. One of the reasons for this scarcity is the difficulty of performing measurements a t the required high temperatures and low concentrations. In the present work we have studied the adsorption of vapor of pure lead chloride on alumina at temperatures between 570 and 650°C. We have used flows of lead chlorides of 0.2 to 2.3 g/hr on an adsorbent bed of about 18 cm3 at pressures well below condensation. We were able to measure the concentration of the lead on the adsorbent without disturbing the adsorption process by using a y-radiation gauge. The concentration profiles were thus straightforwardly determined and not estimated from the breakthrough curves as usually practiced.
Experimental Section The adsorbent mainly studied was the activated alumina gel H151 manufactured by Alcoa. It has a specific surface area of 390 m2/g and its heat resistance is high. In most of the runs the material was crushed and sieved and the fraction 350-1620 p was used. Some runs were performed with other particle sizes, and in one case the material was im-
pregnated with NaOH (by immersion into a 1 N solution, decantation, and drying). Other materials used were SAHT99 alumina with a surface area of 2.6 m2/g, AMC alumina catalyst carrier with a surface area of 0.02 m2/g, both materials manufactured by Carborundum, and Synthad, a carbonaceous calcium phosphate clay with a surface area of 90 m2/g, manufactured by Kerr McGee. Before an adsorption run the bed was heated to 600OC and maintained a t this temperature and under a vacuum of Torr for 3 hr in order to get rid of materials that could be volatilized during the run. A flow of lead chloride vapor was obtained by maintaining a charge of the pure salt a t an adequate temperature between 500 and 600°C at a small distance from the entrance of the adsorbent bed and applying vacuum at the other end of the bed Torr). The bed and the space between the charge and the bed were always hotter than the charge. The temperatures used were above the melting point of lead chloride and the charge had to be melted in situ before the run in order to avoid the large pulse of vapor emitted by the powder due to its large surface area. The liquid melt gave a constant flow of vapor. The flow rate was estimated from the weight of the melt at the beginning and at the end of the run and also from the adsorption curves. The flow rate is plotted against the temperature of the melt and against the corresponding vapor pressure in Figure 1. The melt was analyzed for chloride content before and after the run; no decomposition of the salt was noticed. Apparatus. The charge of lead chloride was placed a t the bottom of a quartz tube, 26 mm diameter, and a bed of adsorbent 7 cm in length was arranged in the same tube above it. Two thermocouple wells ran through the charge and through the bed. The tube was placed in an oven and connected to a vacuum pump. The temperature of the oven was fairly constant in the middle of the oven and it dropped toward its extremities. Advantage was taken of this natural profile to give the lead chloride source and the adsorbent bed different temperatures using the same oven. The lead chloride was placed at a convenient location in the colder area at the bottom of the oven, and the bed was placed in the middle of the oven in the constant temperature portion. Temperature variation was 2 to 5OC along the bed, but mostly less than 2OC. The maximum temperature and the shape of the temperature profile could be modified by adding or removing insulating material and by controlling the energy input. TemInd. Eng. Chern., Process Des. Dev., Vol. 14, No. 4, 1975
417
Vapur F e s s w e of WI. ,mHg c
-
lo'
0
,
012
Lead folk lead nitrate solulions
I
026 cm Pb
I
036
1
Oa
Figure 2. Attenuation curves: curve I, absorption by lead foils or lead nitrate solutions in presence of the other absorbers (oven, quartz tube, adsorbent); curve 11, absorption by lead foils in absence of other absorbers.
5%
500
Temperature o i PbCI,
500 melt
,OC
Figure 1. Dependence of the flow of lead chloride vapor on the temperature and vapor pressure of the molten salt.
peratures were checked periodically a t various heights in the bed and they did not change by more than 2°C during the run (10-20 hr). The radioactivity source that was chosen is a combination of a pure @-emitter,promethium-147, with energies up to 224 keV and a target material, samarium oxide. The resulting radiation contains a peak corresponding to the characteristic X-rays of samarium 40 to 46 keV and a continuous tail resulting from the bremstrahlung emission. The source had an activity of about 5 Ci. The detector was a scintillation spectrometer comprising a 1 X 1 in. NaI crystal activated by thallium and used with suitable electronics: power supply, amplifier, discriminator, tuner, and scaler (manufactured by Ludlum Corp., U.S.A.). The discrimination was held above the noise level a t about 30 keV. As the energy spectrum of the photons emitted from the source is continuous, the spectra observed when no absorber was present differs from the one obtained with absorption between source and detector. The beam passed through the oven, the quartz tube, and the adsorbent bed containing the adsorbate. Count rates of the order of lo4 to lo5 cpm were obtained. The uncertainty, defined as twice the standard deviation, (95.5% confidence level) was of the order of f 2 0 0 to f 7 0 0 cpm, yielding a relative error of about 1%.The background radiation was of the order of 500 cpm and thus could be neglected in most of the measurements. Collimation was by means of a hole, 3 mm wide, bored in the lead shielding around the source. The source and the detector were mounted on the same adjustable height carriage and rigidly maintained in coaxial position on opposite sides of the oven a t the desired height. Results and Discussion Calibration of the Radiation Gauge. Photons of continuous energetic spectrum attenuate on traversing a medium according to a curve which may be described by a sum of exponents. However for a limited absorber-thickness range the calibration curve is describable by a single exponent whose coefficient corresponds to an effective energy. It means that the calibration had to be carried out a t exactly the same geometry as the final measurements and in the presence of the adsorbent, its container, and the oven. The expected lead concentration was estimated and the calibration curves were prepared for this range. The simulation of known amounts of lead concentration on the absorbent was 418
Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 4. 1975
done employing lead foils and lead nitrate solutions. The calibration procedures described in the following were employed a t seven locations vertically along the bed. (1) A varying number of lead foils, each foil 0.15 mm thick, were taped around the tube containing the adsorbent and placed in the oven, and the absorption was measured. The count rate obtained is plotted against the thickness of the layer of lead in Figure 2, curve I. (2) The tube containing the adsorbent and placed in the oven was filled with lead nitrate solutions of varying concentration from 0 to 0.35 g/cm3 and the photon beam absorption was measured. A solution of concentration c occluded in the voids between the adsorbent particles, corresponds to a slab of pure lead of thickness 1 when the same number of atoms of lead are present along the beam path per unit section of the beam, Le., when 1 and c are related by
where 0 is the void fraction, L is the length of the path through the bed, Ppb is the density of the lead, and M p b and h!fPbN03 are the molecular weights of lead and lead nitrate. The absorption data recalculated as the thickness of lead according to eq 1are also plotted in Figure 2, curve I. The integral amount of lead uptaken during the adsorption runs served as a further check for the validity of the calibration. The concentration of adsorbed lead was estimated a t various times and various locations on the bed from the radiation measurements and the above-mentioned calibrations. From these measurements an estimation was made of the amount of lead adsorbed at any time on all the bed and the mass flow rate was calculated, neglecting the lead in the gas phase. These values for the flow rate were compared with those obtained directly from the loss of weight of the lead chloride source. Agreement was within experimental error (see Table I). The amount of lead adsorbed during the adsorption runs was expressed as w in g of PbC12/g of adsorbent. An amount w corresponds to a concentration c, referring to the lead nitrate solutions and defined above, when w and c are related by
where p~ is the bulk density of the bed. A correction was applied for the absorption of radiation by water in the case of the solution. Small variations in the absorption of the oven at various bed heights were also taken into account. From Figure 2, curve I, it is seen that log I vs. 1 is linear in the limited 1 range which is of interest in the present work. Thus the relation obeyed here is I = 1, exp(-pO
( 3)
Table I. Adsorption Data Flow rate, g of PbC12/hr
Run"
Bed temp, "C
From wt of sourceb
From adsorption data
w,, g O f PbCl,/ g of adsorbent at saturation
d (w/w,)/dt W/W, = 0.5
at
0.19 f 0.01 0.77 f 0.08 1 612 i 2 0.54 0.56 f 0.03 0.20 f 0.01 1.1 f 0.1 2 571 i 3 0.55 0.60 i 0.03 0.21 0.01 1.0 0.1 3 645 i 2 0.57 0.55 i 0.03 1.35 1.31 0.06 0.19 0.01 2.6 0.2 4 648 i 5 3.3 i 0.3 2.29 2.13 i 0.08 0.20 f 0.01 5 645 i 2 0.73 + 0.07 0.55 0.54 i 0.05 0.19 f 0.02 6 645 i 3 2.2 0.2 0.96 1.06 i 0.09 0.14 i 0.01 7 620 2 0.38 i 0.05 0.20 0.20 i 0.01 0.07 + 0.04 8 645 i 3 Runs: 1 to 5: on H151 alumina, particle size 350 to 1620 p , surface area 222 m2/g. Run 6: adsorbent as above but impregnated with NaOH. Run 7: adsorbent as in runs 1to 5 but particle size 820 to 1620 p. Run 8: On SAHT alumina, particle size 350 to 1620 p , surface area 2.6 m2/g, impregnated with NaOH. Error less than 5 mg/hr.
*
*
*
*
*
,
I
.3.1
% -
-0
5
10
1s
20
25
t , time (hours)
0
6
12
18 t, time (hours)
2L
30
Figure 3. Adsorption against time for various locations in the bed. Run 3, flow rate 0.57 g of PbClzhr, temp, 645 i 2OC. (Figures on the graphs indicate bed length in cm.)
where Io and w are coefficients obtained from the fitting to the experimental data. The values of p obtained correspond to a monoenergetic photon source with energy of about 87 keV. In order to emphasize the need for this careful calibration, the gauge response for the lead foils alone (in absence of the bed material, the glass column, and the oven) is shown in Figure 2, curve 11. The curve is convex toward the 1 axis mainly at low 1 values due to the high attenuation of the low energy photons. In curve I the impact of these photons is not seen due to the additional absorbers existing there. Adsorption Data. The amount adsorbed was plotted against the time a t constant bed lengths, and same of the plots are reproduced here: in Figure 3 those for a particular run (run 3) at various axial locations along the bed and in Figure 4 those for various runs a t a particular location on the bed axis (3 cm). The plots are S-shaped, tend toward a saturation value w,, and have a maximum slope around w/w, = 0.5. Since adsorption below w = 0.03 was not easily determined, the lower part of the curves is missing and the S-shape is not always evident. For measurements at various locations along the bed at the same run, the shape of the adsorption curves is fairly reproducible, but the shapes vary widely for runs at different conditions. The two parameters characterizing the ad-
Figure 4. Adsorption against time at 3 cm in the bed a t various conditions. Numbers on the curves refer to the runs.
sorption curves, the saturation value w, and the slope, d(w/w,)/dt, at w/ws = 0.5 are listed in Table I. The deviations between the curves pertaining to the same run at various locations are also noted. The effect of such variables as the vapor flow rate, the temperature, and the nature and geometry of the adsorbing surface can be deduced from the data in Figure 4 and in Table I. In runs 3, 4, and 5 the flow rates are respectively 0.57, 1.35, and 2.29 g of PbClz/hr and the other adsorption conditions are practically identical. The slopes of the resulting adsorption curves increase consistently with the flow rate but the same saturation concentration ws is always reached rapidly when the flow is large and after a longer period of time at smaller flow. The value of w, is also independent of the temperature, at least in the range 571 to 645OC (compare runs 2 and 3). In the mode of operation used in the present work, the adsorbable vapor is not diluted into an inert gas, and the mass flow rate determines the gas phase concentration. The fact that w, is constant at various flow rates means also that it does not depend on the concentration of lead in the gas phase; i.e., adsorption is essentially irreversible and desorption is negligible. The amount adsorbed at saturation depends on the surface area of the alumina. For the alumina H151, with a surface area of 390 m2/g, ws is around 0.2 g of PbCla/g of alumina at various conditions. For the SAHT alumina with 2.6 m2/g, w, drops to 0.07 f 0.04 g of PbCl2Ig of alumina (see run 8) and for the AMC alumina with 0.02 m2/g, adsorption could not be detected. Ind. Eng. Chem., Process Des. Dev., Vol. 14. No. 4, 1975
419
,221
I
I
,
,
,
,
,
,
I
x,cm
Figure 5. Adsorption against distance along the bed at various times. Run 3. Figures on the graphs indicate the time in hours. The points represent the experimental results; the lines represent the plots calculated according to eq 4 with a = 27.4 hr and j3 = 0.10
Figure 7. Adsorption against distance along the bed at various times, Run 5. a = 3.0 hr and fl = 0.16 cm (see caption of Figure 5).
cm.
-
.20
.16-
-
c
s .12am _
. z
3
x,cm
.08-
m
Figure 8. Adsorption against distance along the bed at various times, Run 7. a = 7.5 hr and fl = 0.16 cm (see caption of Figure 5).
3 t 0
1
2
3
4 x . cm
5
6
7
8
Figure 6. Adsorption against distance along the bed at various times, Run 4.a = 8.6 hr and j3 = 0.11 cm (see caption of Figure 5).
Adsorption is also controlled by the particle size of the adsorbent. In run 7 the average particle size is larger than in the other runs; the fraction 820 to 1620 p was used instead of 350 to 1620 p. This does not affect noticeably the slope of the curve at the beginning of the run (compare with run 3 with a flow rate of 0.57 g h r and with run 4 with a flow rate of 1.35) but saturation is reached too early and w, is 0.04 instead of 0.2. With still larger particles, ?$-in. pellets (unground material), adsorption could not be detected a t all. It may be deduced that the saturation w, is not a true equilibrium state; adsorption involves only a part of the internal surface, a layer beneath the external surface whose thickness is determined by diffusion properties. The purpose of experimenting with alumina impregnated with NaOH (run 6) was to find out if adsorption could be improved by decomposing the adsorbed lead chloride 420
Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 4, 1975
into adsorbed lead oxide which is consistently less volatile (PbC12 attains a vapor pressure of 10 mm Hg at 648OC whereas PbO attains the same pressure at 1085OC). The results show that impregnation with alkali does not improve the adsorption, and this is in agreement with the fact mentioned above that adsorption by nonimpregnated alumina is already irreversible. The lowered slope obtained in run 6 is probably due to a decrease of the surface area caused by the impregnation. It may also be noted that high surface area and good thermal stability are not the only qualities required for an adsorbent for lead chloride. The carbonaceous calcium phosphate Synthad which has good thermal stability and a surface area of 90 m2/g did not give any detectable adsorption. Some of the runs are replotted as the amount adsorbed against distance along the bed at various times (Figures 5 to 8). These plots are useful for estimating the bed capacity corresponding to any given maximum concentration of lead in the effluent. It was also found that the distribution of the adsorbate along the bed can be described by an equation similar to the Bohart-Adams equation (Bohart and Adams, 1920) w / w , = (1 - e-')/[1
+
e-'(2
- I)]
(4)
where T = t l a and X = xlb, a and b are constants empirical-
ly determined for each adsorption run. In order to estimate these parameters advantage was taken from the fact that a t w / w s = 0.5 eq 4 reduces to ex =
e‘
-
1
(5 )
and [ ~ ( w / z u J / ~ A ] , = -0.25
In the present experiments, the parameters do not have the significance implied by the Bohart-Adams model since the assumptions on which it is based are not entirely applicable. The adsorption runs were performed in absence of a carrier gas and the relevant mass balance equation is
a2c
( 6)
D--7rax
p was estimated from the slopes of the plots of w against x at various times averaged for a given run, and a! was estimated by introducing the value of p in eq 5 and solving. The plots of w against x are recalculated according to eq 4 and compared with the experimental data (Figures 5 to 8). The Bohart-Adams equation, like other equations found in the literature, refers to an experimental procedure in which the adsorbable vapor is diluted in a stream of inert gas flowing at constant rate through the bed, and the mass balance is expressed by
where U L is the constant linear velocity of the inert gas and C is the concentration of the adsorbate in the gas phase. The assumptions concerning the dependence of the rate of adsorption on the concentration of the adsorbate at the two phases is expressed by
where k is a constant. Combination of eq 7 and 8 and integration gives an equation similar to eq 4 but in which the above-mentioned parameters a! and /3 have a well-defined physical significance a! = ts/NT and fl = LINT where NT is the number of transfer units, t , is the saturation time, and L is the length of the bed.
ac
v L - ax
+
= 0 PBr
where D is a diffusion coefficient and U L is a function of C. The computation of a distribution based on eq 9 is beyond the scope of the present work, but the results indicate that it can be approximated by eq 4. The rate equation on which the Bohart-Adams model is based (eq 8) is probably applicable to the present experiments. It was shown above that adsorption does reach a saturation value w s and that the rate of adsorption increases with the mass flow rate which determines the gas phase concentration. Acknowledgments The authors thank Alcoa, Carborundum, and Kerr McGee companies for kindly supplying samples of adsorbents. Literature Cited Behrens, M. D., (to Texaco Inc.). US. Patent 3.247,665 (Apr 26, 1966). Bohart, G., Adams, E.. J. Am. Chem. Soc.. 42, 523 (1920). Brandenburg. J. T., Leak, R. J., (to Texaco Inc.), US. Patent 3,227,659 (Jan 4, 1966). Henderson, D. S..et al. (to W. R. Grace & Co.), U.S. Patent 3,295,919 (Jan 3, 1967). Kenward, M., New Scientist,58, No. 845,344 (1973).
Receiued for reoiew January 22, 1975 Accepted April 8, 1975
Mass Transfer for Two-Phase Cocurrent Downflow in a Packed Bed Nicholas D. Sylvester. and Punya Pltayaguisarn Department of Chemical Engineering, University of Tuisa. Tulsa, Oklahoma 74 104
Gas-liquid and liquid-solid mass transfer coefficients were measured for cocurrent, two-phase downflow in a 6-in. i.d. column packed with 1/8 X in. cylindrical pellets. The liquid flux was varied between 2620 and 14520 Ib/hr ft2 and the gas flux between 167 and 687 Ib/hr ft2 covering the gas continuous, transition, and pulsing flow regimes. The mass transfer coefficients were correlated empirically and the experimental data compared favorably with those of others.
Introduction
A trickle-flow system commonly consists of a fixed bed of packing with liquid flowing downward through the bed and gas or vapor flowing cocurrent to the liquid. Flow patterns under trickle-flow conditions were first studied by Larkins and White (1961) and were later clearly categorized (Weekman and Myers, 1964) as the gas continuous, transition, and pulsing regimes. However, Sat0 et al. (1973) have recently reclassified trickle-flow into four patterns. The first three patterns are the same as those defined by
Weekman and Myers and the fourth pattern was denoted the dispersed bubble flow regime. Trickle-bed reactors are used extensively in the petroleum industry on a very large scale for such processing as hydrodesulfurization of heavy oil fractions, hydrocracking of high boiling point stocks, and hydrotreating and refining of lubricating oils and waxes. Recently, Schuit and Gates (1973) reviewed the chemistry and engineering of catalytic hydrodesulfurization. In the engineering section some of the problems associated with separation processes, mass transfer effects, fluid flow and mixing effects, catalyst Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 4, 1975 421
