Environ. Sci. Technol. 1984, 18, 224-230
mination should be done with care. Registry No. NHBr2, 14519-03-0; NHs, 7664-41-7; NH2Br,
Science: Ann Arbor Science: Ann Arbor, 1978; Chapter 14, pp 213-225. (7) Morgenthaler, L. P. “Basic Operational Amplifier Circuits for Analytical Chemical Instrumentation”; McKee-Pedersen (Pacific) Instruments: Danville, CA, 1968; p 63. (8) Braddock, J. N. Dissertation, Universitv of North Carolina. Chapel Hill, NC, 1973, Appendix A. ” (9) American Public Health Association ”Standard Methods for the Examination of Water and Wastewater”, 13th ed.; American Public Health Association: washington, DC, 1971; pp 115-116. (10) Kelley, C. M.; Tartar, €3. W. J. Am. Chem. SOC.1956, 78, 5752. (11) Weil, I.; Morris, J. C. J. Am. Chem. SOC.1949, 71, 3123. (12) Gray, E. T., Jr.; Margerum, D. W.; Huffman, R. P. In “Organometalsand OrganometaJloids: Occurrenceand Fate in the Environment; Brinckman, F. E.; Bellama, J. D.; Eds.; America1 Chemical Society: Washington, DC, 1978; ACS Symp. Ser. No. 82, pp 264-277.
14519-10-9; PO*, 14265-44-2.
Literature Cited Bongers, L. H.; Burton, D. T.; Liden, L. H.; O’Connor, T. P. In “Water Chlorination-Environmental Impact and Health Effects”; Jolley, R. L.; Gorchev, H.; Hamilton, D. H., Eds.; Ann Arbor Science: Ann Arbor, MI, 1977; Vol. 2, Chapter 58, pp 735-752. Farkas-Hmley, H. In “Bromine and Ita Compound”;Jolles, Z. E., Ed.; Academic Press: New York, 1966; Part VI, Chapter 2, pp 554-562. Mills, J. F. In “Chemistry of Wastewater Technology”; Rubin, A. J. Ed.; Ann Arbor Science: Ann Arbor, 1978; Chapter 13, pp 199-212. Johnson, J. D.; Overby, R. J. Sanit. Eng. Diu., Am. SOC. Cir. Eng. 1971,97,617-628; J. Environ. Eng. Diu. (Am.SOC. Civ. Eng.) 1973, 99, 371-373. Galal-Gorchev, H. A.; Morris, J. C. Znorg. Chem. 1965,4, 899-905. Cromer, J. L.; Inman, G. W.; Johnson, J. D. In “Chemistry of Wastewater Technology“; Rubin, A. J., Ed.; Ann Arbor
Received for review August 16,1982. Accepted August 29,1983. This work was supported by the Nuclear Regulatory Commission, Division of Safeguards,Fuel Cycle and Environmental Research, Office of Nuclear Regulatory Research, Contract NRC-04- 77-11.
Reactive Foams for Air Purification Susann M. Brander, Gudrun 1. Johansson,* Bengt 0. Kronberg, and Per J. Stenius The Institute for Surface Chemistry, S-I14 86 Stockholm, Sweden
H The use of water-based foams for removal of gaseous
pollutants from air has been investigated. Hydrogen sulfide, formaldehyde, and acetaldehyde can effectively be absorbed in a foam containing substances that react chemically with the gaseous pollutant. The influence of the foam characteristics on the absorption of gas has been studied for a single continuously flowing liquid film. Formaldehyde, acetaldehyde, and propionaldehyde were used as model compounds. It was found that the chemical structure of the foaming agent did not influence the rate of gas absorption. To achieve efficient absorption of slightly soluble gases, it was necessary to use a high renewal rate of the liquid film. The presence of a liquid-crystalline phase in the film increased the solubility of hydrophobic gas molecules.
Introduction The aim of this work has been to investigate if gas pollutants can effectively be removed from air by blowing the air through a foam bed. The removal can be achieved either by simple absorption or by a chemical reaction, in the liquid foam lamella. In order to investigate the effect of foaming agents on the gas pollutant absorption, we have also studied the absorption into a single continuously flowing liquid film. Reactive foams have been used by Viles (1) and Silverman (2, 3) to encapsulate air contaminated with radioactive materials and by Pozin (4) and Helsby and Birt (5) for absorption of COz. Several patents deal with the removal of nitrous oxides (6) or particles (7-10)from air by a foam technique. The main advantage of using a foam as an absorption medium is the large contact area between the gaseous and the liquid phase (1,11). In addition, the rapid destruction and renewal of bubbles in a dynamic foam lead to efficient 224
Environ. Sci. Technol., Vol. 18, No. 4, 1984
mass transfer (12). A low liquid/air ratio can be maintained (typically, 6 X lo4 m3/m3) (13). This implies that a pollutant can be removed from a large gaseous volume to a very small liquid volume. With the proper choice of foam density, foam height, and process equipment the pressure drop in a foam column can be kept quite low, Le., less than 0.5 kPa (11, 13). In a brief survey Jackson (12) has compared the mass transfer operations in a foam with other equipment such as packed towers. For many processes, e.g., absorption of Nz03or conversion of SO2 into H2S04,he found a 10100-fold increase in the absorption rate coefficient. Model equations for the calculations of the mass transfer (11, 14, 15) or foam ratio (16) from measurable physical quantities in dynamic foam columns have been proposed. Weissman (17) found that the mass transfer is decreased by high foam stability and high viscosity of the liquid in the foam. This is probably due to a decreased gas/liquid contact and an increased diffusion barrier. According to Cullen ( l a ) ,impurities in technical surface active agents may cause up to 25% variation in the absorption of gas into a flowing liquid film. Sidorova (19) found that the mass transfer coefficient for desorption of NH3, SOz, COz,and Nz from the liquid in a foam increased with increasing solubility of the gases.
Experimental Conditions and Equipment Chemicals. Table I lists the surfactants used a~ foaming agents. Formaldehyde, HCHO (35%), acetaldehyde, CH3CH0 (for synthesis), and propionaldehyde, CH&HzCHO (purum), were obtained from Merck AG. The hydrogen sulfide, HzS, was supplied by AGA, Stockholm. Other chemicals were of reagent grade. Foam Column. Figure 1is a schematic drawing of the foam column. A foam bed of 500-600 cm3was generated at room temperature (22 f 1OC) by passing a gas flow (VP)
0013-936X/84/0918-0224$01.50/0
0 1984 American Chemical Society
Table I. Foaming Agents type of surfactant sodium decyl sulfate (NaC,,SO,) sodium dodecyl sulfate (NaC,,SO,) sodium tetradecyl sulfate (NaC,,SO,) polyethylene glycol (9-1 0)-p-octylphenol hexadecyltrimethylammonium bromide, CTAB sodium decyl sulfate sodium dodecyl sulfate sodium alkyl sulfate (C12-Cl,) secondary sodium alkanesulfonate (C13-C,8) sodium alkylbenzene sulfonate sodium alkyl ether sulfate ethoxylated primary alcohol ( 3 0 e.0.) alkylphenol-ethylene oxide (14. e.0.) a These cmc values are measured with a Du Nouy Mysels ( 27) standard reference data of cmc values.
trade name
manufactured by Merck AG Merck AG Merck AG
Triton X-100
quality, % 99 (p.a.) 99 99
cmc, ddm, 8.64 2.36 0.65 0.15 0.34 2.0-2.2a 0.42a 0. 5Ba 0.33a 0.32'
technical Fluka AG purum Empicol 0137 Albright & Wilson 90 85 Empicol LZV Albright & Wilson Aarhus Oljefabriker Sulfatol 33 Hoechst AG 60 Hostapur SAS6O Berol Kemi AB 80 Berol 496 Berol Kemi AB 39-41 0.16' Berol475 Berol 081 Berol Kemi AB 100 O.Ola Berol 263 Berol Kemi AB 100 0.03 5a ring balance at 22 ' C . Other cmc values are taken from Mukerjee and
Table 11. Absorption of Form- and Acetaldehyde in Foama concn of concn of removed foaming aldehyde in the gas, pollutant, agent, % vol % foaming agent g/dm3 HCHO~
sodium decyl sulfate sodium decyl sulfate sodium decyl sulfate sodium decyl sulfate sodium decyl sulfate sodium decyl sulfate Sulfatol 33 Sulfatol 33 Empicol LZV Empicol 0137 Berol 496 Berol 081 Berol 263 Berol 475 Hostapur SAS6O
0.060 0.060 0.060 0.060 0.060 0.060 0.030 0.030 0.066 0.256 0.017 0.046 0.013 0.049 0.027
0.02 0.1 0.14 0.33 0.017 0.02 0.1 0.1 0.1
0.1 0.1 0.1
0.1 0.1 0.1
98.4 97.9 96.9 97.5 87.5d 90.2e 97.8 99.2f 96.7 98.3 96.1 96.9 97.1 80.5 80.5
- :
I
L-.,.,.,.....-....I.--~
Flgure 1. Foam column. (1)Foam column (height = 0.58m; diameter = 0.05 m), (2)glass filter (porosity l),(3)sprayer, (4)gas inlet, (5)gas outlet for analysis, (6) vessel for the foam Ilquid, (7) dropping funnel for additives, (8)pump for recirculating liquid, (9)thermostatic water bath, (10)glass wool filters, (11)flowmeter, IIquM, (12)air, (13)gaseous pollutant, (14)flowmeters, gas, and (15)mercury manometer.
CH,CHOC 0.05 75.2 sodium decyl sulfate 0.060 74.4 0.2 sodium decyl sulfate 0.060 85.9 0.21 sodium decyl sulfate 0.060 99.6; 0.07 0.060 sodium decyl sulfate 99.8 0.060 sodium decyl sulfate 0.26 99.3f sodium decyl sulfate 0.060 0.36 92.5djf sodium decyl sulfate 0.060 0.32 a V1/W = 0.015 m3/m3,and Vg = 0.6 m3/h. No reactant in solution (if not otherwise indicated). Mean values of two to three determinations. Analysis during first 10 min of experiment. Vg = 1.2 m3/h; V'/Vg = 0.0075 m3/m3. e Vg = 1.2 m3/h;V1/Vg = 0.015 m3/m3. f Na,S,O, added.
through a glass filter, into the absorption liquid. The absorption liquid was an aqueous solution of the foaming agent and the reactant with a total volume of 150-200 cm3. The concentration of the foaming agent was adjusted to give the desired foam height and, hence, was varied as shown in the second column of Table 11. The foam expansion factor (foam volume/liquid volume) was between 4 and 6. The foam level was kept constant by recirculating the drained liquid at constant flow (v')and spraying it on top of the foam bed. Solid reaction products were removed with glass wool filters. The gas flow was a mixture of air and pollutant. Samples for analysis were absorbed in liquid and analyzed. In each experiment, which lasted for 30-60 min, the input flow was analyzed at the beginning and at the end. The output flow was analyzed 2-3 times during the experiment.
4 f
Aldehyde in gas -liquid
-11
Figure 2. Liquid fllm equipment. (1)Gas box (-0.6 dm3), (2) liquid film (-7 cm'), (3)Pt frame, (4)liquid flow, Inlet, (5)sampling of liquid for analysis, (6) gas flow, inlet, and (7) gas flow, outlet.
Apparatus for the Study of a Liquid Film. The absorption of gaseous aldehydes into a single liquid film was studied in the apparatus shown in Figure 2. The solution flows into the gas box and spreads as a liquid film Envlron. Sci. Technol., Vol. 18,No. 4, 1984 225
Table 111. to.,,Values for the Foaming Agents
0
er
6oll f 40 T
1
2
3
4
5
foaming agent
concn, g/dm3
to+
sodium decyl sulfate Sulfatol 33 Empicol LZV Empicol 0137 Berol496 Berol 081 Berol 263 Berol 475 Hostapur SAS6O
0.060 0.030 0.066 0.256 0.017 0.046 0.013 0.049 0.027
90% and requires a molar ratio CuS04/H2Shigher than 2 (Figure 4). It was found that the pressure drop over the foam column (glass filter plus foam bed) was considerably higher in the H2S experiments (>6 kPa) than for the aldehydes (-2 kPa). Figure 5 shows that the pressure drop increases and the removal of hydrogen sulfide decreases rapidly when the liquid/gas flow ratio is reduced. This is explained by the
- 1w
QL sot
Famaklehyde
1.o
- SO
P 3
s
P
0.5
4
Ropbnaldehyde Acetaldehyde
n
8
i T
20
40
60
v
40
80
Absorption tine ( min.)
Figure 5. Comparison of pressure drop (-) and percent removal of hydrogen sulfides (- -) as a function of absorption time in the foam column. CuSO, was used as a reactant for H2S. Molar ratio CuSO /H2S = 2.5-5.0, concentration H,S = 0.054 vol % and V g = 0.6 m /h. (0, A) V ' / V g = 0.015 m3/m3; (0,A) V ' / V g= 0.0075 m3/m3.
-
where g and 1 denote gas and liquid, respectively. Therefore, at equilibrium
t
increased amount of precipitated CuS in the column which increases the flow resistance in the filter as well as in the foam. We conclude that efficient absorption of gaseous pollutants with reactants that form solid products is also possible, but it is necessary to ensure an efficient removal of the precipitate from the foam column. Experiments with a Single Foam Film. The absorption of gaseous pollutants in the foam column is obviously dependent not only on the properties of the foam lamella and the solubility (absorption capacity) of the impurities in the foam liquid but also on the parameters of the equipment used such as foam height, gas/liquid flow ratio, pressure drop, volume ratio of the foam, etc. In order to study the absorption process independently of these parameters, we also investigated absorption into a single liquid film as a model for the foam lamella in the experimental setup shown in Figure 2. Theoretical Considerations. The cell in Figure 2 is not designed to ensure that equilibrium prevails in all experiments. Rather, it allows one to draw conclusions concerning the rate-determining factors in different cases. To show this, we will derive equations for equilibrium conditions and compare the calculated and experimental results. If the distribution of aldehyde molecules between the gas and the liquid phase reaches equilibrium, Henry's law may be used: P A = kHXA = CiRT
(1)
where P A is the aldehyde partial pressure, X Ais the mole fraction of aldehyde in water, C i is the aldehyde concentration in the gas phase, kH is the Henry law constant, and R and T have their usual meaning. At low concentrations
where Cf4 is-the molar concentration of aldehyde in the liquid and VHDo is the molar volume of water. Inserting eq 2 into eq 1 we obtain (3)
Experimentally, however, the flow rate rather than the concentration of aldehyde is of interest. The aldehyde flow, Q, is related to the gas or liquid flow, W , by Qg
= CRWg
Q' = Caw
(4)
(5)
The total aldehyde flow in the gas box is expressed by Qbt
+ Q1
(6)
K (W / W 9 l+K(W/Wg)
(7)
=
Qg
Combining eq 5 and 6 gives 1
=
tot
Thus, at constant flow rates of gas and liquid the aldehyde uptake in the liquid, Q', should depend linearly on the aldehyde concentration in the gas phase, or Qtot,with a slope given by the constants in eq 7. Equation 7 can also be used to predict the dependence of the aldehyde uptake in the liquid on the flow rate of the liquid film. Figure 6 shows the predicted relative aldehyde uptake in the liquid film, Q1/Qtot,as a function of the liquid flow rate, or K ( W / Wg),keeping the gas flow rate, Wg, constant. In this calculation kH = 7.14 mmHg for formaldehyde (22),kH = 2.3 X lo3 mmHg for acetaldehyde (23),and kH = 5.6 X lo3 mmHg for propionaldehyde (24)were used. The figure shows clearly that, keeping Qtotconstant, there are two extreme cases: (i) K ( w'/ W) > 1 ; Le., kH is small. The aldehyde uptake in the liquid will be almost independent on the liquid flow rate and almost equal to the maximum possible value (Q9. In this case the absorption capacity of aldehyde in the liquid is very large, and the aldehyde content of the gas in equilibrium will be very low. This is the case of formaldehyde as will be discussed below. An intrinsic efficiency, f, of gas uptake in the film may be defined by
where Pkt is the initial partial pressure of the aldehyde and
P O d l tis the experimental partial pressure in the gas outlet. f i and Q'(ca1cd) are the calculated partial pressure of the Environ. Scl. Technoi., Voi. 18, No. 4, 1984
227
12-
10 A
c 8-
a Y
0
F y. 0
64-
.-5
tz
Qtot
- HCHO-flow ( p o l / m h
-4 )
Flgure 7. Uptake of formaldehyde into a flowing liquld film, Q' ( W ' = 2.5 X dm3/min), at varlous concentration of aldehyde In the gas flow, Qtot. The followlng surface active agents and concentrations (g/dm3) were used as film stabilizers: (0)NaC,,SO, (7.5), (0)NaC,oSO4 (5.12), (0)NaCl,S04 (1.8), (0)NaC12S04(1.2), (A)NaC14S04 (0.25), (A)Trlton X-100 (0.05), (V)Emplcol LZV (0.066), ('I Empicol ) LZV (0.264), (half-fllled up trhngle) EmpicolOl37 (1.12), (half-filled down (0) Berol 496 (0.113), (M) Berol 081 triangle) Hostapur SASBO (0.08), (0.046), (left half-filled squares) Berol 475 (0.05), (right half-filled squares) Berol 263 (0.01), (Q) Sulfato133 (0.294), (crossed box) CTAB (0.192), (solid star) CTAB hexanol (32 32). The calculated llnes show the maximal absorption of the aldehydes into the fllm at equilibrium conditions.
50
100
dot-CH$l-$CHO-flow
150 ( pmollmin. )
Figure 9. Uptake of propionaldehyde into a flowing liquid fllm, Q' (W' = 2.5 x dms/min), at various concentrations of aldehyde in the gas flow, Qtot. See Figure 7 for symbols and details.
+
+
l2
2-
-1 wL-
Lwid flow in the film ( 103d&mkr.)
Flgure 10. Aldehyde u take in the liquid film, Q', as a functlon of the flow rate of the fllm, W W g = 10 dm3/mln. (U) HCHO, (0)CH,CHO, and (0)CH3CH,CH0.
P.
100
200
do'- CH$HO-flow
300
( pmol/min. )
Figure 8. Uptake of acetaldehyde into a flowing liquid film, 0' ( W ' = 2.5 X dm3/mln), at various concentrations of aldehyde in the gas flow, Qtot. See Figure 7 for symbol and details.
aldehyde in the outlet and the calculated liquid uptake of the aldehyde assuming equilibium conditions. Influence of Type of Surfactant in the Film. Figures 7-9 show the uptake of form-, acet-, and propionaldehyde into the liquid films formed by 12 different surfactants with varying concentrations. No reactant was added to the film liquid. In all experiments the flow rate dm3/min. Neither of the liquid film, W', was 2.5 X 228
Environ. Sci. Technol., Vol. 18, No. 4, 1984
the chemical composition nor the concentration of surfactant has any significant effect on the rate of absorption as long as the surfactant is dissolved in homogeneous solution below the critical micelle concentration, cmc. (The values of cmc are given in Table I. The results for CTAB-hexanol are discussed below). Since there is no variation in the aldehyde uptake on changing surfactant, we conclude that the variations in efficiency of absorptidn indicated in Table I1 must be due to variation in the properties of the foam bed. It is also obvious that the rate of transport across the surfactant film cannot be a ratedetermining step in the absorption process. Using eq 7, we can calculate the aldehyde uptake in the film at equilibrium as a function of Qtot. The predicted uptake is shown in Figures 7-9. The predicted slopes are respectively 5, 0.1, and 0.04 for form-, acet-, and propionaldehyde. The experimental slopes are 0.18,0.05, and 0.04, respectively. The predicted faster uptake of the formaldehyde is confirmed experimentally. The intrinsic efficiencies according to eq 8 are 0.04 for formaldehyde, 0.5 for acetaldehyde, and 1 for propionaldehyde. Hence, in the case of formaldehyde the system is far from equilibrium, and the experimental line in Figure 7 probably describes the maximum uptake made possible by the design of the gas box.
-
Hexanol Surfactant molecules 1
0.1
0.2
0.3
00
0.4
Concentration of aldehyde in the gas flow ( Volume% ) Figure 11. Theoretlcal possible absorption at equilibrium condltlons
(calcd)compared with experimental results for absorption of form- and acetaldehyde in the foam column. Vg = 0.6 m3/min, and V ' / V g = 0.015 m3/m3.
Influence of the Liquid Flow Rate in the Film. Figure 10 shows clear differences between formaldehyde and the two other compounds. The uptake of formaldehyde is independent of the liquid flow rate, corresponding to K(W/Wg) >> 1 (Figure 6). (With Wg = 10 dm3/min, K(W/We) = 36 dm3/min and w' = 2.5 X for formaldehyde.) Thus, the uptake in the film confirms that conditions for formaldehyde are far from equilibrium. The uptake of acet- and propionaldehyde, on the other hand, increases linearly with increasing flow rate. This corresponds to K ( W ' / W )
