CATALYTIC COMBUSTION OF AEROSOLS JACK G. C H R I S T I A N A N D J .
ENOCH J O H N S O N
L . S. Naual Research Laboratory, Washington 25, D . C.
The presence of fluids of relatively high vapor pressure aboard nuclear submarines presents the possibility that these substances may find their way into the submarine atmosphere as aerosols. Since part of the air of the submarine i s passed continuously through a catalytic burner as part of the air revitalization system, it was desirable to learn the fate of aerosols in the burner. The catalyst used is an unsupported mixture of copper and manganese oxides. A laboratory-scale burner which duplicates shipboard burners as to catalyst, temperature, and residence time showed that aerosols of dioctyl phthalate, a hydrocarbon-base lubricating oil, and a commercial triaryl phosphate ester mixture were converted to substantially the theoretical amount of COz at 300" C. Thus, moderate to high concentrations of the aerosols studied are effectively destroyed by the catalyst under conditions of operation used on nuclear submarines.
T HAS BEEN SHOWN
in a laboratory study that Hopcalite,
I a n unsupported catalyst consisting of a mixture of copper and manganese oxides, is a good catalyst for the combustion of hydrocarbons of several structural types as well as for the combustion of a number of oxygenated organic compounds (2). lYith the exception of methane, all the compounds studied were converted essentially quantitatively to CO2 and water a t 300" to 400" C. Methane was oxidized to the extent of only 30% even a t 400" C. The presence of liquid substances of comparatively low vapor pressure aboard nuclear submarines provides the possibility that such substances may find their way into the submarine atmosphere in the form of aerosols. Aerosols of hydraulic fluids or lubricating oils could result from leaks in high pressure systems or from oils dripping onto a hot surface. Since the air in the submarine is passed continuously through a catalytic burner containing Hopcalite (5). it was desirable to knoiv whether the Hopcalite would effectively catslyze the combustion of such aerosols. This question became of even greater moment with the consideration of triaryl phosphate esters for use aboard nuclear submarines as hydraulic fluids and compressor lubricants ( 3 ) . No reference to studies of the catalytic combustion of aerosols was found in the recent literature. These esters have a n important advantage over some other substances in that they are remarkably resistant to ignition while effective as lubricants. However, these phosphoruscontaining esters are known to produce toxic reactions upon ingestion by man (7), and it was important to learn whether they are destroyed or detoxified by oxidation upon passage through the combustor.
Aerosols were prepared using air from which CO:! was removed, passed through the l1/* X 12 inch preheater section, and then through a ll/s X 5 inch bed of catalyst a t a space velocity of 21,000 hours-'. The completeness of combustion was determined primarily by measurement of the COZ produced by means of a continuously recording Liston-Becker Model 15A nondispersive infrared analyzer equipped with a n 8.25-inch analyzer tube. The system was calibrated by using breathing air of known CO2 content and tank nitrogen for zero adjustment. Standard dynamically produced mixtures of decane in air were used to check the analytical procedures and the combustion effectiveness of the catalyst. The reproducibility of the over-all method a t the lowest concentrations of lubricating oil (75 p g . per liter) was +2%. Aerosols were produced by means of a n aerosol generator patterned after the one described by Thompson (6) and shown in Figure 2. In this generator, the aerosol is produced by a shearing action of the air o n the fluid contained in can B. The aerosol then passes to can C, where the larger particles are separated out, and thence to the jet impactor D where particles larger than a certain designated size are removed by impaction on the plate. The size range of particles removed is determined by the air flow. the slit width, and by the distance from the slit to the impactor plate. To produce the aerosols
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INFRA RED ANALYZER
Experimental
A laboratory unit (Figure 1) was built to duplicate the conditions of operation of shipboard catalytic combustion units insofar as flow rate, residence time, and temperature are concerned ( 2 ) . Hopcalite (Mine Safety A4ppliancesCorp.), the catalyst used throughout this work, conformed to the military specification (7) which provides that the catalyst consist of mixed copper and manganese oxides. Spectrographic analysis a t this laboratory did not show the presence of more than traces of elements other than copper and manganese. /Vet chemical analysis revealed 78.3y0MnOz, 13.17, CuO, and loss on ignition 7.9%, this loss presumably being water or chemisorbed gases.
KOH SOLUTION
Figure 1 .
AEROSOL GENERATOR
FILTER
WET TEST METER
Schematic diagram of apparatus VOL. 2
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for this study, air was passed through the generator a t 1 cu. ft. per minute using a slit width of 1 mm. and impactor plate distance of 3 mm. Since the production of an aerosol depends on a shearing action and, hence, on the viscosity of the fluid, it was necessary to control the temperature of the fluid. T o this end, cans B and C were placed in a water bath whose temperature could be controlled to within 1 0 . 2 ' C. After leaving the impaction container, the aerosol was passed directly into the bottom of the furnace without dilution, and a portion of the stream was diverted for analysis (Figure 1). The aerosol was analyzed gravimetrically by passing a known volume of the stream through a glass fiber filter and weighing the material collected. The glass fiber filters used were the binderless Type A from the Gelman Instrument Co. The aerosol content of the air stream was determined by collection of the aerosol both before and after passage through the furnace. A second filter placed in series showed no weight increase. The mass median diameter and number diameter of the aerosols were measured by a light-scattering method described by Knudsen and White ( 4 ) . The low vapor pressure fluids studied were dioctyl phthalate (DOP), Navy Symbol 2190 lubricating oil (a), and a commercial triaryl phosphate fluid (TAP). D O P was studied because it is a chemical entity and thus of unambiguous composition, permitting ready calculation of the theoretically equivalent COS. Also, this compound is considered a standard in work with aerosols since it produces good stable aerosols whose characteristics are well known. The 2190 lubricating oil was chosen for study since it is in wide use in the Navy as a lubricant and hydraulic fluid. The TAP: a commercial mixture of cresylic acid phosphate esters: is an example of a class of fluids being used by the Navy as hydraulic fluids and compressor lubricants in nuclear submarines, as well as elsewhere. T h e particular mixture of phosphate esters selected for a given task is determined by the viscosity characteristics desired. The sample used in this study had a viscosity of 45.2 cs. at 100" F. The TAP fluid was analyzed for carbon and hydrogen content to permit calculation of the theoretical amount of CO:! to be expected upon complete oxidation. The phosphorus content of the TAP was also determined, since it was desired to learn the fate of this element upon passage of the TAP through the catalytic reactor. -4nalysis showed 69.847, C, 6.35% H, and 7.85y6P. Phosphorus analyses were carried out both on the reactor cffiuent air collected in water bubblers and aqueous extracts of the catalyst which had been exposed to the T.4P aerosol. All phosphorus analyses on the effluent air and catalyst extracts were carried out using a modification of a method described by Carpenter and associates ( I ) . The analyses were conducted by photometric mezsurenient of the absorption of the phosphomolybdate blue complex formed from the solution being analyzed using light of 575 mp in a Lumetron Model 402-E photometer. The limit of the method is about 0.1 mg. total phosphorus, whether collected as inorganic phosphorus or TAP aerosol.
Results and Discussion
Various concentrations of aerosols of 21 90 lubricating oil. DOP. and TAP were converted to the theoretical amount of COZ over Hopcalite catalyst a t 300' C., as shown in Table I. \Ve have no direct evidence as to whether the aerosol survives the passage through the preheater as a distinct physical state. The total flow through the combustor was approximately 1 cu. ft. per minute in all cases. The desired concentrations of T.4P aerosol were achieved by regulation of the temperzture 236
l&EC
PRODUCT RESEARCH A N D
DEVELOPMEN1
Figure 2. A. 8. C.
Aerosol generator
Air inlet tube Fluid reservoir Settling chamber
D.
E.
Jet impactor Aerosol reservoir
of the aerosol generator to the appropriate value. The relation between the aerosol generator temperature and the concentration of aerosol produced is given in Table 11. Table I11 shoivs dimensional properties of the aerosols produced, as determined by the light-scattering method previously mentioned (4). These physical properties describe stable aerosols well suited for this work in that they will not tend toward too ready impingement. A glass fiber filter placed at the furnace exit during combustion shoxved that negligible amounts of TAP aerosol passed through the furnace in the concentration range of 18 to 60 pg. per liter.
Combustion of Aerosols Over Hopcalite at 300' C. ;Ierosoi Eitent of Substonce Concn., ConrmPrsion t o St i t d i d ~ 5jLitar . CO?. 50 100 2190 lube oil 100-1 50 40-100 100 DOP 20-60 a p 1 - r ~ 11 ~ .5 Th P
Table 1.
Table 11.
Concentration of TAP Aerosol Produced at Several Temperatures Genrrtitor 1 r o ~ o lCuncn . Lemp, * C g Llt I 30 20 35 25 4(1 43 60 45
Table 111.
Particle Size of Aerosols at 25" C. .\lass .Wedtan Diametrr, p
1)OP ThP
1.3 1 '
.Viim b e l Dianirter,
0 82 1 0
p
As part of a study of the combustion of phosphatic aerosols, i t was of interest to learn whether phosphorus or any of its
compounds passes through the combustor under the same conditions of operation as normally used on shipboard. Such materials as the various oxides of phosphorus, for instance, Lvould likely have a deleterious effect upon the crew and equipment. O n the other hand, should the phosphorus be retained on the catalyst in some form, a possible consequence might be a loss in catalytic activity. T o resolve this question, two extended runs were conducted during which T A P aerosols were passed through the combustor a t various concentrations. T h e first series of extended runs totaled 40 hours. During this first extended series, the average concentration of T A P was 30 pg. per liter, and the average air flow rate was 0.9 cu. ft. per minute. A total iveight of 1.8 grams of T A P was passed into the catalyst bed, \vhich weighed 80 grams. During part of the time, a portion of the furnace effluent air was bubbled through water. No phosphorus in any form was detected in this water. At the end of the 40-hour period the Hopcalite catalyst was removed in layers. The bottom layer of the catalyst-Le., that in first contact with the T A P aerosol- was coated with a thin layer of gra) material, the next to bottom layer was less gray, and so on until the sixth or topmost layer appeared the same as unused Hopcalite. Portions of each of these six segments of the 5-inch catalyst bed were extracted with hot 107, NaOH and the phosphorus content measured by a determination of the absorption a t 575 mp of the phosphomolybdate blue complex formed upon reduction by hydrazine. These analyses are summarized in Table IV.
Table IV. Analysis of Catalyst Sections Exposed to 1.8 Grams of TAP Aerosol during 40 Hours' Operation First extended series
Sampls hTo.
4
5 6
Distance from Bottom of Bed, In. 0 .0-0.5
0.5-1.0 1 .o-2.0 2.0-3.0 3.0-3.8 3.8-5.0
p, %
0.37 0.25 0.20 0.21 0.19 0.08
Photomicrographs (about 50X) of sections of granules of catalyst having the greatest phosphorus content show that the surface coating is quite thin. X-ray diffraction studies revealed very little of the exact chemical nature of the coating. T h e indication is that the crystallites present are quite small, of the order of 100 A. The total phosphorus lodged on the catalyst was calculated from the phosphorus concentration and weight of each of the several layers. Such a calculation yielded a value of about 170 mg., rvhich is in reasonably good agreement \vith the theo-
retical amount of 140 mg. expected from the 1.8 grams of TAP passed into the catalyst. Despite the apparent quantitative retention of phosphatic material on the catalyst, no diminution of the catalytic activity for the combustion of hydrocarbons was observed. .4t the beginning of the first extended series of runs with TAP, COS was obtained in greater than theoretical amounts. Toward the end of the series, the COZ measured was nearly the theoretical amount. Simultaneous measurements of the TAP aerosol concentration showed the stream passing vertically into the furnace to be the same as the stream moving horizontally to the analytical filter (Figure 1). Thus, any anomalous results cannot be laid to anisotropy. Since Hopcalite is known to hold a certain amount of adsorbed COZ a t a given temperature, it may be that the lodging of some form of phosphorus o n the catalyst results in the displacement of Con. During the second extended series of runs, TAP aerosols of even higher concentrations, 60 fig. per liter and greater, were introduced into the furnace. Under these conditions, some of the T A P emerged from the furnace exit as liquid TAP. The Hopcalite catalyst thus exposed to such large concentrations of TAP aerosol was then examined for its activity toward combustion of n-decane and 1,2,4-trimethylbenzene (TMB). The initial efficiencies for the combustion of these compounds a t 325' C. were 62 and 78% for n-decane and TMB, respectively, compared with values of 98 to l O O ~ , for their combustion over fresh Hopcalite (2). During subsequent runs with n-decane totaling several hours' duration, the combustion efficiency of the used catalyst rose to 80%. Thus, Hopcalite catalyst whose catalytic activity for hydrocarbons had been diminished by high TAP aerosol concentrations seemed to undergo a t least a partial regeneration upon continued use for hydrocarbon combustion. The used catalyst was examined for the presence of orgznic phosphorus by Soxhlet extraction with isopropyl alcohol and for inorganic phosphorus by extraction with 10% aqueous NaOH. Only inorganic phosphorus was found. No phosphorus was removed from the catalyst by Soxhlet extraction with water alone. literature Cited
(1) Carpenter, H. M., Jenden, D. J., Shulman, N., Tureman J. R., A M A Arch. Ind. Health 20, No. 3, 234 (1959). (2) Johnson, J. E., Christian, J. G., Carhart, H. W., 2nd. Eng. Chem. 53, 900 (1961). (3) King. H. F., Coil, J. A , , ASTM Spec. Trch. Publ. No. 267,
59-70 (1960). (4) Knudsen, 'H. [V., \\bite, L.. Naval Research Lab. Rept. P-2642, Sept. 14, 1945. f5) Ramskill. E. .A,. S . A . E . Tranr. 70. 350 (1962). (6) Thompson, J. K., Re;. ,VRL Pro&, 14117,1~ly 1956. (7) L . S Dept. of Navy, Bur. Ships Code 2368, Mil. Spec. Mil-C21665 (SHIPS), Dec. 12, 1958, amended May 11, 1959. (8) Z b t d , MIL-L-17331 (SHIPS),Sept. 30, 1952. RECEIVED for review September 26, 1962 ACCEPTEDMay 28, 1963 Division of Prtroleum Chemistry, 142nd Merting. ACS, ..\tlantic City, S . J.. September 1962.
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