Characteristics of Silver Iodide Ice Nuclei Origination from Anhydrous Ammonia-Silver Iodide Complexes Part I
By R.L. Steele and F.W. Krebs
Technical Paper No. 73 Department of Atmospheric Science Colorado State University Fort Collins, Colorado Characteristics of Silver Iodide Ice Nuclei Originating from Anhydrous Ammonia-Silver Iodide Complexes
R. L. Steele (1) and F. W. Krebs (2)
Part I
Abstract: Introduction
Two methods of generating silver iodide ice nuclei The development of silver iodide nucleating devices have been investigated. Neither employs thermal proc has been underway for a number of years. Most gen essing. A 3000 liter isothermal cloud chamber control erators have used a solution of potassium and silver lable over the range of 0 to -200 :!:. 0.1 C was designed iodide in acetone. These have had varying degrees of and fabricated to study nucleation processes along with success. Other devices using pyrotechnics and electric other support equipment. arcs have been employed. A review of the generator technology indicated a need for the development of simple Anhydrous ammonia as the AgI carrier was employed and reliable generators of medium and high capacity. since solution preparation is simple and direct, high Furthermore, little seemed to be known about the in concentrations (60%/AgI by mass) can be used, and cor fluence of various generator performance parameters on rosion and deposition problems are virtually nonexistent. nucleating effectiveness. The problems encountered in handling the solutions of AgI-KI in acetone indicated the In a simple system in which the complex is sprayed need for another carrier of AgI. into ambient air via a commercial nozzle the results are 12 negative, 10 nuclei per gram at -20C. Another system, The search for the new carrier began in 1962. It using Dautrebande's principle of obligatory filtration, was found that anhydrous ammonia was indeed an excel shows promise. Performance at -13C is 3 x 1014 nuclei lent carrier of AgI. It can be complexed with ammonia per gram. The threshold temperature of both non-ther in concentrations of as high as 60%. Information in the mal systems is about 50 C lower than for other disper literature regarding the complex was scant. Tominaga sion methods. Ultra pure silver iodide is a poorer nu did some preliminary work in 1954. He found that the cleant than the commercial materials. complex when passed through an electric furnace pro duced a microaerosol of Agl. He did not report any in The electron microscope was used to study the ef formation on its effectiveness as an ice nuclei. Bilz and fluents from both generators. The shape and physical Stollenwerk in the 1920's did some work on the physical characteristics of effective particles are quite different properties of the complex. Corrin and his associates from those found in thermal generators. The marked are currently working with the nucleating properties of difference in effectivensss between thermal and non ultra pure silver iodide which has been exposed to anhy thermal systems is attributed to impurities. drous ammonia in its preparation. Corrin's work and the work of others suggested that the NH3-AgI complexes would have good nucleating properties.
The above preliminary work led to research activity aimed at developing a simple, reliable generator using anhydrous ammonia as a silver iodide carrier. In order to carry out this work effectively a program was outlined to determine the dominant variables effecting generator performance.
1. Assoc. Prof. - Mechanical Engineering, Colorado State Univer sity, Fort Collins, Colo. 2. Development Department - E.I. DuPont de Nemours & Co. Inc., Wilmington, Delaware, formerly - Graduate Research Asst. Mech. Eng., C.S.U. Support-Apparatus welded to the shell on 6" centers. The outer shell which is 505 feet diameter is of 10 GA steel. The annular space between shells is filled with polyurethane insulation. Apparatus was needed to carry out the experimental The close tube spacing, heavy inner shell and insulation phases of the research project. The most critical need thickness were required boundary conditions in the heat was a large cloud chamber controllable over a tempera transfer analysis to maintain ~ O. lC variance on the ture range of about 0 to -20C. The design of this facility walls of the chamber. The large tubes are necessary to was undertaken with the following specifications: maintain two phase flow throughout the circuit in order to avoid a hydrostatic temperature gradient. 1. Wall temperature - variance - ~ O.IC over the range of 0 to -20C. Adequate controls were needed to maintain these 2. Automatic control of temperature. wall temperatures. A L & N CAT controller with asso 3. Volume -- 3000 liters. ciated hardware was provided for this purpose. The 4. Adequate multipoint cloud and wall temperature refrigerant temperature is controlled rather than cham measurement. ber air. This results in an air temperature higher than the controller indicates but this bias can easily be ac A schematic diagram of the chamber showing all counted for. associated equipment is depicted in figure (1). The chamber is a closed double shell which is cylindrical in Monitoring of cloud and wall temperatures is man shape. The 4.5 feet diameter by 5 feet high inner shell datory. Two copper resistance thermometers which had is of 1/4 aluminum with I" O. D. tubes at 6 inch spacing been calibrated with a platinum resistance thermometer
LeG[ND
B- H,AT E.,,," ... C7 P"",., v.,,,,
~. S'GIo ----2..-- SOl.EN.O\O-O"E~"'TE.t) VAI-'IE. ----M-- E'LPAIolS\OW 'JA ... "E. -----.I.-- MET'E.ftlPolG OJtl~ICE. ""1"'11 o o"e.ILI'"I.OW Lou .. ~ PU.S'iloIJ~E. C:. "~f. -I __ ".~:". c,,"O C'-"~J R. .. ~1.I4 ...... "'.,. V.. oo/. D.",,,,,, ...,,,, H.I.C.~ t...oI4tl. Oil...,. - CSU ,Q_II·':; Figure 1 Schematic Flow Diagram - Isothermal Cloud Chamber and bridge certified to o. 01C were used to calibrate 24 A continuous water droplet generator was designed thermocouples of which 12 were later imbedded in the and fabricated to supply the chamber with a cloud. The walls at random pOints. The remaining 12 were placed water for the generator is first chilled to about 10 C in a at predetermined points in the chamber proper to meas small tank which is fitted with a refrigeration coil. This ure cloud temperature. The thermocouples were moni is a part of the refrigeration system for the chamber tored with a L & N speedomax recorder which scans 12 proper. The chilled water is then passed through a pOints in approximately 15 seconds to an accuracy of commercial spray nozzle to produce sma 11 droplets about O. 1C. which are thence passed through a Dautrebande obligatory filtration process. The effluent from this, a uniform The inner shell of the chamber is fitted with a velvet cloud made up of particles about 10 JI. in diameter, is liner to minimize frost accumulation and shattering from directed into the cloud chamber on either a time pro the walls and to provide good visual inspection of the grammed or continuous basis via an aspirator supplied nucleation process. A fan is provided to facilitate cool with chamber air. down and warm up. Figure 2 Isothermal Cloud Chamber Performance of the cloud chamber and associated nucleating characteristics of silver iodide microaerosols equipment has been within expectations. Before a cloud which is discussed in a subsequent section of this paper. is introduced, temperatures throughout are within ~ O.lC. When a cloud is introduced, a gradient of 20 C per meter develops due to the initial temperature difference between Preliminary Laboratory Work the cloud droplets and the chamber air. The temperature at a given point however remains to within:!:. O. 1C. A Little information is available on the thermodynamic photograph of the chamber is shown in Fig. 2. properties of silver iodide ammonia complexes. Certain of these were needed in order to carry out the research, A dilution facility was needed to sample the effluent in particular P-T data and Joule-Thompson coefficients. of generators for evaluation in the chamber. Along with Other fluid characteristics were needed. The following this, a good air supply was provided for testing thermal results are reported. generating systems. This is shown schematically in Fig. 3. A 5400 cfm blower is mountediin the enclosure Silver iodide complexes easily with anhydrous am into which the effluent is directed. High capacity gener monia at ambient temperatures up to 60% concentration. ators of all types require pre-dilution to avoid agglom The resulting solution is colorless and has about the eration effects. An air supply is shown on the right same viscosity as anhydrous ammonia. It does not ap which is used for testing thermal generators. This con pear to attack ferrous materials and is incompatible with sists of a two stage axial flow fan fitted with an inlet about the same materials as anhydrous ammonia, e. g. control cone, honeycombs, nozzle and metering orifice. copper bearing alloys. The complex has a solid state which is unstable and white in color. The solid when ex A metering unit was needed in order to account for posed to ambient air decomposes to AgI and ammonia silver iodide consumption and other fuels or carriers vapor in a short time. used in producing nuclei. The unit is shown in Fig. 4. Provision is made for supplying and metering most any The P-T data closely follows that of anhydrous am generator that uses liquid or gaseous fuels and liquid monia as is shown in Fig. 5. The liquid complex is un carriers. stable in a concentration range of 10 to 14%. This is indicated in the figure. Point scatter below 80 psia was The substance under study, i.e. silver iodide micro marked. This confirms the work of Biltz and Stollenwerk aerosol, must be sampled in order to inject it in the mentioned previously. The only apparent explanation of cloud chamber and to process for later study in the this behavior is a time rate of change in the number of laboratory. Samples were needed for chemical analysis. AgI molecules bonded to ammonia molecules or an un This was done with standard millipore filter apparatus. stable state of the complex. Samples for the chamber were taken in a 700 cc syringe and diluted as required. Samples for electron micros The J -'1' data also closely resembles ammonia, as copy were taken with a thermal precipitator using a is shown in Fig. 6 for the range in question. The data standard grid coated with collodion. taken is rough in terms of modern P-T and J -T meas urements. However, it was sufficient to predict the be The above preliminary work permitted the study of havior of the complex in various flow processes. PRE - DILUTION GENERATOR SUPPLY rDILUTION CHAMBER \ GENEr-RA--:T""["'"O_R..!.-_...... SAMPLING LOCATION -- " / /)....~::::---~ -"-...... J , \ ( ,'._, 1...) _ ...... ____ --1 ' ... j I / ~'::::.::..._J"/ cfm CENTRIFICAL FIGURE 3 Ag I GENERATOR DILUTION AND SAMPLING EQUIPMENT ARRANGEMENT L.J. ~ ~~. 17' 1 trGt '-I- ~··V··.-" - -- ~- ~ i (,) ! .. '~'. .:;," • • • • • • .. ~;"~~"'JI~ -~------. ....){+ - -~:'r.~-_ ..... --- Figure 4 220 Universal Metering and Dispensing Unit 0 14,3 % 20,6 % 200 • - / 0 28.6 % '" 36,0 % , 180 0 45.0 % i", IJ/;I ~. I e 160 J~.o '" e N Jo'"JJ\!~I .s:: c! oj e .=() 140 " "- ]1,;1 ~i .0'" ANHYDROUS AMMONIA-~ 7'1 120 w -. JJ#-l a:: ,,~o '" / ::J o tie,",,' / '"w '"a:: 100 - Cl. . 00,1;/4J o .l/ 80 ~ . -0 II'" o /6eVe -/e/ e 60 bl ~!t~? . 40 / ~/l e Figure 5 ~!~e/ P.7 Data for AgI-NH3 Complexes ~-;;~ (Sample 2) 20 e-::: -30 -20 -10 o 10 20 30 40 50 TEMPERATURE, DEGREE C FIGURE 5 P.T DATA FOR Agl-NH 3 COMPLEXES (SAMPLE 2) AgI Microaerosol Production from Complexes without nucleation properties of silver iodide. This is borne out Thermal Processing in Figure 7. It should be emphasized that the effect could be much more pronounced than is indicated. The pure substance could have been contaminated by the contain Two systems were investigated. These were: (a) ex ment vessels and piping even though stainless steel was pansion of the complex at medium pressures (125-200 used throughout and care was taken to be sure the sys psia) at ambient temperature to atmospheric pressure tem was clean. via a commercial hydraulic nozzle. (b) expansion of the complex at medium pressure and ambient tempera It was also found that when commercial AgI was ture to atmospheric pressure using the Dautrebande complexed with research grade ammonia only hex plates principle of obligatory filtration. These will be dis were grown from the embryos. The other complexes cussed separately. grew rods at the higher temperatures, 1. e. ultra pure AgI and research grade NH3 and commercial AgI and The following performance variables were examined agricultural grade NH3. Only hex plates grew at tem for system (a).: peratures of -8 to -9C with research grade NH3 and 1. Effectiveness over the temperature range of 0 commercial AgI while the other complexes grew rods at to -20C. temperatures down to about -llC. This also suggests 2. Concentration over the range of 0.03% to 24%. that the effect of impurities on surface properties is in 3. Type of anhydrous ammonia and silver iodide. deed significant. The difference between research grade 4. Effluent droplet size data. and agricultural grade ammonia is largely water content 5. Effluent solid data (residue fro m evaporated and trace impurities. The commercial AgI was from the droplet). same batch so there was likely no differences here. 6. Threshold temperature. It is now necessary to depart from this discussion The effectiveness of system (a) (Prototype 1) is and examine the droplet and particle characteristics. A shown in Figure 7. The performance of this system is further discussion of nuclei characteristics will follow. negative in that it is too low to be economical. A cost of $10 per 1015 nuclei is predicted. The general shape of Effluent droplet data were taken to relate droplet the curves agree with Fletcher's predictions except for and particle characteristics. The mass mean diameter a shift in the origin. of the droplets issuing from the simple hydraulic nozzle varies from 4.3 to 7.1.u for the 0.3% and 25% concen However, other findings resulted from the effective trations respectively. All droplets measured were less ness study which yield some insight into the nature of the than 20.u in diameter. For the same nozzle conditions, nucleation process. the mass mean diameter of water droplets issuing from the same nozzle is approximately 20,u. It is not sur Effects of ammonia and silver iodide purity were prising that a volatile liquid such as the complex produces observed. The complex prepared with agriculture grade droplets. The expansion of the complex through the noz ammonia and commercial silver iodide was more effec zle is meta stable. At some point down stream of the tive than when research grade ammonia was used with nozzle exit, the two phase flow rapidly changes to the ultra pure silver iodide. The threshold temperature for stable state. The change of state occurs rapidly, causing the latter case was about 10 C lower than for the former. the large droplets a small distance from the nozzle to "explode" into many small droplets. The ultra pure AgI was prepared by M. L. Corrin at the University of Arizona who is engaged in research on When the particles produced from vaporization of the surface properties of AgL His material is prepared the complex at ambient temperatures are examined the by reacting pure silver and iodine directly in a hard following is observed: (a) The number of effective ice vacuum·. Thus this material has no significant impurities. nuclei per droplet is a function of the concentration. At The substance was complexed with research grade am very low concentrations (0.03%) only 0.15 effective monia to determine the change in effectivensss due to the nuclei are formed per drop. This increases with con absence of impurities which are thought to enhance the centration to a value of 15.4 nuclei per drop at 25% con- 10 0 8 0 9 ~?; ~ ,~ ~ ANHYDROUS AMMONIA 0 «P u.. 6 V w w a: (.!) ; ~ ~ 4 0 -- ~aP" w a: ~ 2 0 / -- - « a: , w a.. :::E I~ ~ 0 --1------j~ • 9.3 0/0 0 14.3 0/0 ~o- 0- 20.3 0/0 -20 -- ... - -- ..- . ---- 9 37_0 0/0 -4(' / o 20 40 60 80 100 120 140 160 ISO 200 PRESSURE Ibs/ inch2 FIGURE 6 JOULE - THOMSON EFFECT IN AGI - NH3 COMPLEXES 13 • • 10 . • 1:& - «C> - • :::E « • a: 12 • 0 (.!) 10 0 "- W ..J L:. U => 2 (( (f) 0_03 (f) 10 w • 2 .A 0.30 % w COMMERCIAL Ag I > 3.00 %%} i= • AGRICULTURAL NH3 u • % w 015.00 u.. 10 u.. A 24.00 % w 10 0 3.00 %--- PURE Agi RESEARCH NH3 9 10 -6 -8 -10 -12 -14 -16 -IS -20 -22 TEMPERATURE, °C FIGURE 7 PERFORMANCE CURVES FOR THE PROTOTYPE I GENERATOR centration. This result together with the number of drops as usual. This is borne out by the flatness of the effec per gram of AgI which decreases with concentration tiveness curve at the low temperatures. In fact the explains the peak in the effectiveness of the system as a curve obtained is quite similar to a Fletcher theoretical function of concentration, which was a maximum at a curve for a single particle size if a shift in the origin is concentration of 0.3%. made. Electron micrographs were taken of the AgI from Finally, the poorer performance could be attributed the complex. One of these is shown in Figure 8 and is to the presence of ammonia. This is not indicated (see typical of those taken for concentrations of 3 and 25%. other pertinent remarks). The question of photolytic Note the two distinct forms. The large generally spher activation s h 0 u 1 d also be considered. However, no ical particles are probably hollow. The small particles measurable change in effectiveness was observed when are solid. One explanation of this geometry is as fol samples were taken at night. This is a subject of con lows: The surface of a complex droplet vaporized first tinuing investigation but its effect in this case is not leaving a shell Of AgI or solid complex with a liquid significant. complex center. However, the liquid in the drop is not in stable equilibrium so the pressure and temperature Weare still left with a question regarding the rea rise due to a heat interaction with the surroundings. sons for the behavior of the AgI particles from the non When the pressure is higher than the solid shell will thermal system. support, a hole is blown in the shell and the liquid com plex is expelled. This material then reaches the stable Before pursuing this it is necessary to discuss the state of small solid particles and ammonia vapor. Holes microscopic characteristics of particles which do pro were observed in the shells in some cases which support duce numbers of nuclei which are an order of magnitude this explanation. higher. An electron micrograph of an AgI particle orig inating from the NH3-Ag1 complex which has been ther From the above analysis it would seem that the mally processed is shown in Figure 9. The effectiveness number of nuclei per drop would be essentially constant. of this system is 1. 2 x 1014 at -10C or about an order of However, the shell of AgI or AgI-NH3 may change in magnitude higher than the non-thermal system. The physical properties with concentration. The thickness of threshold temperature for thermally processed AgI is the shell formed may vary with concentration as could normal. The particle size varies from 0.02 to .2 }.I. the thermal properties. These changes in turn would The structure of these particles is markedly different influence the number and size of small droplets expelled. from the non-thermal system. Most important is the The net effect could be a change in nuclei per drop at nebulous material surrounding the particle, the nature of different concentrations. which is not known at this time. There is also no ques tion that the impurities are different in quantity and The remaining item for discussion in this section is species due to the different environments. Carbonaceous nucleation characteristics. The effectiveness cur v e material and traces of sulphur oxides present at high shows that the threshold temperature for this system is temperatures are in the environment of the particle in about 50 lower than for nuclei that has been thermally the thermal system and are absent in the non-thermal processed. The electron micrograph (Fig. 8) is needed system. Using these particles for comparative pur to examine this. In general the particles ranged from a poses also tends to bar the influence of photolytic action maximum of 2}.1 to a minimum of about O.Ol}.1. The since the particles were processed through a radiant larger particles represent most of the mass while the flame and high temperatures in one case and high tem smaller ones represent most of the numbers. According peratures only in the other. The effectiveness depended to Fletcher the large particles should be effective down almost entirely on gas temperatures. to about -4C, yet the effectiveness was t00 low to meas ure at -8C. In view of the foregoing, it may be concluded that impurities are necessary to cause silver iodide to act as One could argue that there were too few large par an ice nuclei, especially at warmer cloud temperatures. ticles, yet the small particles were also not as effective Figure 8 - Typical Electron Photo-micrographs of the output of the Prototype I Generator Dautrebande System data from the Dautrebande system is also in agreement with the hypothesis that impurities are essential to the Recall that in this system the complex is expanded nucleating process. The increase in effectiveness com from medium pressure and ambient temperature to at pared to system (a) is due solely to the increase in mospheric pressure using the Dautrebande principal. A particle numbers. schematic diagram of this system is shown in Fig. 10. The liquid complex is first expanded through a commer It should be emphasized that system (b) is still in cial nozzle into the filtering column which is nothing the laboratory stage of development. The capacity of the more than a laboratory "spine type" distillation column. unit described was only 1. 5 mg of effluent AgI per min The large droplets stopped by the spines form layers and ute. Additional development work is needed if the unit act as filters for the other droplets. Thus, the drops is to be used in field. This would include increasing the are forced through successive liquid layers. Vapor is capacity by at least 4 orders of magnitude and changing also released in the process. Only the smallest drops the size distribution from flat to normal for most field emerge since only those carried in a vapor bubble can applications. get through. The length of the column and the liquid flow rate are the dominant variables in effluent droplet size and rate of flow. The effluent from system (b) was significantly smal ler than for system (a). The droplets could not be seen Other pertinent Remarks -- Systems (a) and (b) with an optical microscope (The droplets from system (a) were about 5 J.I ). Electron micrographs of the solid It has been reported that ammonia increases the residue reveal a structure which is quite similar to sys effectiveness of silver iodide. This was confirmed at tem (a), i. e. relatively large hollow spheres and rela -20C. It was found that addition of ammonia in the cloud tively small individual solid particles. However, the chamber after all the ice crystals had fallen out produced size of the large particles are roughly an order of mag about the same number of nuclei as for the original sam nitude smaller than in system (a) or about 0.1-0.2 J.I in pie. However this effect was not observed at tempera diameter. The smaller particles were in the order of tures above -15C. 0.05 J.I. System (a) was not sensitive to the flow rate of sil The effectiveness of system (b) is as shown in ver iodide. This was varied from 2 to 850 grams per Fig. (11). This unit could be economically attractive. hour with nO noticeable change in effectiveness at a given The flatness of the curve can be explained by the essen temperature. tially flat particle size distribution if one ignores the smaller particles. Assuming only the larger particles close to the measured size were effective, 2 x 1014 shells of O. 25J.1 diameter would be formed per gram of AgI. This calculated value is close to the measured effectiveness and agrees in general with the sizes meas Conclusion ured on the electron micrographs. If the smaller par ticles were effective, only a small portion of them were. Two non-thermal silver iodide ice nuclei generators Again we observe the same general effects as in system have been examined. The first of these is not econom (a). The larger particles should have been effective at icalfy feasible. The second, though promising needs the higher temperatures. Fletcher's theory would pre additional development before it is usable in the field. dict that the effectiveness should be about 2 x 1014 at The physical characteristics 0 f the aerosols issuing -8C to -10C instead of at -13C. The effectiveness was from non-thermal systems are different than in thermal xL 2 x 1014 at -10C in the system in which the simplex systems and are not as effective as ice nuclei particles was thermally processed. The smaller particles should from thermal generators. This is probably due to trace have been effective at the lower temperatures. Thus the differences in impurities. PRESSURE I REGULATOR /ROTAMETER FILTERING COLUMN NITROGEN CYLINDER COLUMN PROTECTOR SAMPLE TANK HYDRAULIC NOZZELE FIGURE 10 SCHEMATIC OF SYSTEM 8 16 10 15 10 0 0> <[ :;E <[ 14 a:: <.9 10 iii" ....J U ::::> Z en en 13 w z 10 w > f= u W IJ... IJ... w 12 10 II L __---l'- __...I...- __....L __ ---l. ___ .l....- __....I..- __...... J 10 -6 -8 -10 -12 -14 -16 -18 -20 TEMPERATURE, "C FIGURE I I PERFORMANCE CURVE FOR SYSTEM 8 Acknowledgements 4. Beltz & Stollenwerk: Halogen Silver Ammoniates, Zeitschrift fuen Anonganische Und Allengenmeine The author is indebted to Dr. James P. Lodge of Chemi 114, pp 174-202, 1920. NCAR who gave time and effort to the solutions of the problems encountered in evaluating the electron micro 5. Fletcher, N.H.: Size Effect in Heterogeneous Nucle scopic data. Mr. B. Razakzada did much of the work ation. The Journal of Chemical Physics, Vol. associated with the P-T & J -T data. 29, No.3. The author also wishes to acknowledge the National 6. Fletcher, N. H.: Nucleation by Crystalline Particles. Science Foundation who sponsored the research. (NSF The Journal of Chemical Physics, Vol. 30, pp. Grant No. GP-1663). 1476-1482, June 1959. 7. Fletcher, N.H.: Entropy Effect in Ice Crystal Nu cleation. The Journal of Chemical Physics, Vol. References 30, pp. 1476-1482, June, 1959. 1. Corrin, M. L., Edwards, H. W., and Nelson, J.A.: 8. Rowland, Layton and Smith: Photolytic Activation of The Surface Chemistry of Condensation Nuclei: II. Silver Iodide in the Nucleation of Ice. Journal of The Preparation of Silver Iodide Free of Hygro Atmospheric Sciences, Vol. 2. scopic Impurities and Its Interaction with Water Vapor. Journal of the Atmospheric Sciences, Vol. 9. Robertson, Charles E.: Analytical Methods for De 21, pp. 565-567, September 1964. termining Silver Iodide Smoke Characteristics (Paper presented at National Meeting of American 2. Corrin, Storm, N. S.: The Surface Chemistry of Meteorological SOCiety on Cloud Physics and Se Condensation Nuclei 1. The Sintering of Silver vere Storms, October 1965). Iodide. Journal of Physical Chemistry, Vol. 67, pp. 1509-1511, July 1963. 10. Tominaga, H. and Kinomaki, S.: Note on a Method of Spraying AgI-in-Liq. NH3 Solution for the Pur 3. Dautrebande, L. : Microaerosols - Physiology, Phar pose of Artificial Seeding. Tohoku University, macology, Therapeutics: New York. Academic Science Report, 5th Series Geophysics, Vol. 6, Press, 1962. pp. 39-42, August, 1954. Characteristics of Silver Iodide Ice Nuclei Originating from Anhydrous Ammonia - Silver Iodide Complexes Part II - Thermal Systems by R. L. Steel and F. W. Sciacca Technical Paper No. 78 Department of Atmospheric Science Colorado State University Fort Collins, Colorado CHARACTERISTICS OF SILVER IODIDE ICE NUCLEI ORIGINATING FROM ANHYDROUS AMMONIA - SILVER IODIDE COMPLEXES PART II - THERMAL SYSTEMS by R. L. Steele (1) and F. W. Sciacca (2) ----Abstract The characteristics of Silver Iodide Ice Nuclei originating from a complex of silver iodide and ammonia without thermal processing is dis cuss 1. Assoc. Prof. Mechanical Engineering C"lorado State University, Fort Collins, Colo. 2. General Electric Co., San] ose, Calif., formerly Graduate Research Ass't. Mechanical Engineering CSU Introduction The non-thermal dispersion of AgI from a complex AgI and NHS by non·~thermal means is described in another paper (1). The effectiveness of this system is too low to be economic. This finding led to the develop ment of a thermal system. It was believed that if the complex of AgI-NHS were burned, the effectiveness would be improved. This opinion was based on the success of other thermal systems (2) (S) (4) (5). Such sys tems are thought to be more effective due primarily to the chemical env:~ronmental effects of the combustion process. In order to carry out the combustion of ammonia in air it is neces sary to burn it in a supporting flame. Various combustionprocesseswere investigated. It was found that a system using No.1 Fuel Oil as the sup porting fuel showed promise. It was further concluded that the use of preheated air enhanced the combustion of the combination fuel, i. e., No.1 Fuel Oil and a complex of NHS-AgI. The advantages of preheat are well documented in the literature for many diverse combustion processes. An analytical and experimental feasibility study demonstrated the need for preheat to improve generator effectiveness. Combustion System lower nozzle; the complex is admitted in the upper noz The experimental work referred to in the intro zle. Cooling NH3 is admitted in the chamber surround duction permitted design of the prototype pre h eat ing the nozzles to prevent vaporization of the complex in generator. The system is depicted in Fig. (1). The the line. A swirl plate and a eonvergent cone encompass combustion air passes through the outer annulus to the the nozzle section so that the high temperature air flow exit end of the generator, turns and passes back to the will enter the combustion seetion as a highly turbulent inlet end. The air is preheated by the heat interaction vortex to promote rapid mixing of fuel and air. This with the hot gases passing through the central core or rapidly vaporizes the fuel so that it will ignite and burn combustion section. Liquid fuel is admitted through the in an intense short flame. Fuel flows of 15 gal/hr have Fuel supply line NH3 - Agi Supply line Cooling NH3 line Air swirler and cone Intermediate r;::::...L..:=t----NH -Agi nozzle shell cap 3 Outlet L....!:::;:::::;:::;::::;..,--Fuel spray nozzle ~-r~------~ Inner s/Iell Intermediate shell Outer s/;'el/ FIG. PREHEAT GENERATOR been processed by the unit with complete combustion. Flame temperatures of 30000 F have been measured. A photograph of the preheat unit in operation is shown as Fig. (2). Other photos of the generator are shown in Fig. (3). Combustion Variables Effecting Nuclei Effectiveness The dominant variables which effect the nucleating characteristics of silver iodide particles which have passed through a combustion process are for the most part unknown, (4) (6). The more important independent variables (and there may be others) may be listed as follows: 1. Geometrical Configuration of Combustion Chamber 2. Temperature (Reaction Zone) 3. Supporting Fuel 4. Atomization 5. Quench rate 6. Stoichiometric ratio Another related variable is the effect of tempera ture alone, i. e. particle characteristics resulting when the silver iodide is exposed to a high temperature, non reacting gas stream. Items one and two in the list are essentially fixed by a given prototype. The literature search coupled with the experimental feasibiUty study led to the design described in the previous section. The maximum tem perature achievable is also lagely fixed by the geomet rical configuration as well as the he a t interaction Fig. 2 ') p- - .---... ~ Fig. 3 barriers and the fuel used. The maximum temperature data leads to the conclusion that the silver iodide passes is variable to an extent by a change in the stoichiometric through the reaction zone as the compound AgI and that ratio but it is a dependent variable in this sense. The it is therefore in a meta stablE: state in the reaction zone. chosen prototype provided a means for investigating the The above discussion raises the related question - effect of the remaining variables. if the AgI passes through the reaction zone as AgI how Fuel - A detailed study 0 f fuels to support the are small particles formed? This will be discussed in combustion of the ammonia was not undertaken. A given the section on microscopic aspects. fuel effects all of the variables listed. The preliminary work was done with #1 fuel oil. This material when 14 used with ammonia produced good results in the cloud 10 0> chamber. It was chosen due to this and its low cost, f= • (J 13 combustion chamber. The correct atomizing nozzle w 10 LL LL must be chosen. This is not a problem though choice of W such variables as spray angle and type, (hollow or solid cr cone) are best determined by experiment. Complete ~cr w ( Nuclei tested (It - I OOG ) vaporization of the fuel was assumed to be a necessary z W •• condition for the production of effective nuclei. Fuel <.? which does not vaporize will not undergo a chemical re 0.6 0.7 action of significance in the flame zone. This results in a lower flame temperature which reduces the evapora FIG. 4 VARIATION OF GENERADR EFFECTIVENESS WITH tion of the ammonia spray. Poor spray characteristics STOICHIOMETRIC RATIO of the ammonia reduces the number of resulting AgI particles. The nozzle type chosen provided good atom Stoichiometric Ratio ization of fuel and ammonia. The flame was smokeless The effect of stoichiometric* ratio on generator at normal stoichiometric ratios with no carbon monoxide effectiveness was evaluated at 8% complex concentration detectable at ratios greater than 1. 1. and at a temperature of -10C. This temperature was Quench Rate - The rate at which a gas containing chosen since nucleation parameters are more sensitive silver iodide is quenched strongly effects the nuclei ef than at -20C. The effectiveness was measured in the fectiveness. This was established by Fuquay (5) and isothermal cloud chamber designed for evaluating ice others. A satisfactory explanation of this necessary nucleating devices (1). A plo t of effectiveness vs. stoi condition is needed. The history of the interaction of the chiometric ratio is shown in Fig. 4. Agl and the burning fuel must be known. At about 1460 C Certain hypotheses can be drawn regarding the AgI d.. transforms to AgI which melts at 5580 C and FJ effect of air-fuel ratio. ThiE, ratio can affect the envi boils at 15060 C. The flame temperature is conSiderably ronment of the silver iodide in at least two ways. First, higher than the melting point temperature of (3 AgI. The the temperature will vary to a degree. Second, the following questions arise: (a) Does the AgI dissociate in chemical constituents of the environment will vary. At the flame to Ag and 12? (b) Is there sufficient time for stoichiometric fuel ratios of less than 1. 0 a reducing this reaction to occur? (c) Does a passive resistance atmosphere prevails. The constituents of this atmos prevent this? If item (a) does not occur and if there is phere could conceivably inte:~act (but not necessarily insufficient time and a passive resistance, the AgI can react) with the silver iodide particles in a way which pass through the reaction zone as AgI in a meta stable would influence their nucleation characteristics. From state. This argument supports the need for a rapid the data presented above, this interaction appears to be quench rate. detrimental. Alternately, 01' possibly in addition, the Experimental evidence supports the meta stable temperature of the products issuing from the generator state hypothesiS. The effluent from the thermal system is lower in the range of .6 to 0.8 than it is in the range was sampled for analysis by means of a high tempera 0.8 to 1. O. This very likely influences the character ture water cooled gas probe. The sampled gas was thus istics of the particles produced. very rapidly cooled to ambient temperature in an attempt Optical pyrometer meLsurements indicated that to "freeze" the chemical composition prevailing in the the flame temperature was highest near stoichiometric gas stream. The particulate matter was collected on a millipore filter and chemically analyzed. No detectable amount of silver as metallic silver or oxides of silver . were found. This was also the case for iodine. This *The ratio of actual air required to the theoretical amount used for combustion of the fuel oil and ammonia. PRE- DILUTION GENERATOR SUPPLY rDILUTION CHAMBER :\BLOWER SAMPLING LOCATION -- .... , / --;',)...... ,---+------,, ~ \ ( n r-t------..... \,...) I I ~~':a-'::~ 5400 cfm CENTRIFICAL FIGURE 5 Ag I GENERATOR DILUTION AND SAMPLING EQUIPMENT ARRANGEMENT conditions, being approximately 2400oF. Also, at these The curve shows a sharp increase in effectiveness with operating conditions the most complete combustion was increases in temperature in the 10000 F range with a obtained. Fewer possible contaminants were present. definite flattening trend as the temperature approaches In light of the above discuBsion, the following statements 3000 F. It is probable that the effectiveness would de are purported: crease jf even higher temperatures were used. Other 1. High temperatures (at least up to 24000 F) are investigators have found this same trend for other silver conducive to the formation of large numbers of iodide generating systems. For one particular system active silver iod.ide nuclei. employing acetone and potassium iodide as the silver 2. A reducing atmosphere (of the nature produced iodide, D. M. Fuquay found that optimum performance by incomplete combustion) is somewhat detri was obtained using temperatures of about 2200oF. The mental to the formation of active ice nuclei. 3. The environment produced by the use of air 14 fuel ratios above 1. 15 times stoichiometric 10 sharply reduces generator performance. Analysis and discussion of the above items are a 0 subject of continuing investigation. The mechanism of c. • C •. • - • • • • fIII# .. •• • •• • • • ., • ,. • •• .. •• • ., . • • .. .. • • • ... •. •• • • .. , .. • • . .. • o' .'6 • ,. 10000A Figure 7 - Typical Electron Photo-micrograph.' of the output of a Thermal Generator 1------. -.• • 1"',;'.'·. ~.'.Ii ...•• ,..".' ~' •• \. • •• • ••• #I • • ~.. .•• .':. I • I: ••..• · •.. a.,' .1; •.•..... : ., " ,," ...... , .. . 4 ",' • ~ . '. " .. to .. , '.' " •..... ,• ~ I. IJ.l .1 Figure 8 - Typical Electron Photo-micrographs of the putput of the Prototype I Generator • ~...... ,... .. Figure 9 - Electron Micrographs of Typical Agglomerates Fig. 10 Electron Micrographs of AgI Particles from Ice Crystals ., Electron micrographs of AgI particles from ice Holes have been observed in some of the larger shells crystals nucleated at -12C were collected and are shown which supports this argument. in Fig. (10). Note the various shapes and sizes. This The above argument may be applied to the thermal is in disagr"eement with the theory of Fletcher (2) (7), as system. This same process can occur but more rapidly regards the size-temperature relationship. According since evaporation rate from the droplet is much higher to the theory, the particles should be larger than 7200 A in the thermal system due to the higher temperature. to be effective. There is also disagreement in geomet This would mean that the sheil would form sooner on the rical considerations. However, the thermal system drop or when it was larger, since the initial evaporation performance agrees closely with Fletcher's theoretical rate of NH3 from the drop if: quite likely much greater maximums except for a shift in the origin. than the NH3 diffusion rate in the droplet. (The initial No other quantitative and theoretical quantitative complex droplet size is approximately 20~. These are analysis is available to the writer's knowledge. The smaller then the fuel oil droplets due primarily to the work of Corrin (8) (9) strongly suggests a different Joule-Thomson expansion through the nozzle.) mechanism which is based on the mechanism mentioned If we conclude that the shell forms sooner, it is in a previous section. This is also implied in the work seen that the AgI would be '9ssentially prevented from of Rowland, Layton and Smith (10) where it was found decomposing due to the cooling effect of the evaporation that AgI must be photolytically activated to be effective. process and the protection of the shell. Now suppose This observation also suggests that photolytic change of that the shell fails as in the Don-thermal system and ex a particle changes the chemical potential of certain pels the liquid in the interior. These droplets are much sites on the particle surface. smaller than the parent shell (see Fig. 8) and are much The remaining item to be discussed in this sec larger in number. The droplets from the parent shell tion is the hypothesis regarding meta stability of the AgI would in turn form their own shells and rupture again as in the reaction zone. The question of the mechanism above and so on. These processes all require time. small particle production was raised in the section on In the meantime, the L.quid phase is increasing in Quench Rate but was not answered. This same question concentration rapidly. At some point the complex will was implied in the section of effect of temperature alone. be in the solid phase. This state has been observed (1). It is believed that the answer to this question lies This is unstable and decomposes to NH3 and AgI. Thus in the electron micrograph of the aerosol from the non even after all the ammonia has been driven from the thermal system. The large spheres are really hollow liquid droplet, it must also be driven from the solid shells. See discussion in Steele & Krebs (1). The shell phases before silver iodide as such is exposed to the is formed by initial rapid evaporation of the carrier flame. As long as ammonia 1s present, the temperature from the surface of the droplet resulting in a solid shell of the droplet will remain low due to the evaporation of with a liquid interior. Interaction with the environment the ammonia. It is therefore proposed that particles of causes the pressure of the liquid in the interior to rise. the size range encountered in Fig. 7 are formed in the Interaction between the shell and the interior has the manner described above due to insufficient time for result of dissolving the shell due to the escaping tend evaporation. They would thus escape dissociation and ency of the ammonia relative to the AgI in the shell. would pass through the reaction zone as AgI and not as These interactions cause failure of some point in the Ag and 12. shell at which point the liquid in the interior is expelled. Droplet evaporation rate is mentioned in the pre The decrease in effectiveness with temperature results ceeding paragraph. This rate should be estimated since from fewer particles being formed. The evaporation if it is too high, the above explanation will not stand rate decreases with temperature so the concentration of scrutiny. The instanteous rate is given by the equation: Agl at the surface also increases at a lower rate. The shell then forms at a later time in the reaction due to the lower evaporation rate. As the temperature is low dr ~YC fl- - hy (To - To) ered, the shell formation-fracture cycle does not occur c/.t as many times. The result is fewer, but larger par Po - ~~a- +/ hf~ ticles, hence the lower mass efficiency. At the low temperatures, the situation compares tothe non-thermal Where: system. dr de = Time rate of change of radius Other arguments against dissociation to any de .,P = Droplet density gree can also be given. These are developed as follows: = Droplet radius The concentration of the Agl in the gas stream is about df 2 dt:; = Time rate of change of droplet 3.5 x 10- mols AgI per mol of gas. Now suppose that temperature the Agl dissociates into iodine gas and solid silver. The hy = He~~t transfer coefficient concentration of the iodine will also be 3.5 x 10-2 molsl To = Temperature of surroundings mol gas if all of the AgI dissociates. If the total pres 7D = Temperature of drop sure of the stream is one atmosphere, the partial pres Fo = Pressure of surroundings sure of the 12 will also be 3.5 x 10-2 ATM. By contrast, ~ = Surface tension the concentrations of water vapor, carbon dioxide and hi' = HeEt of vaporization nitrogen are orders of magnitude higher. The collision Examination of the equation permits insight into frequency of the 12 with Ag is thus much lower than with the time required to rerr,ove the ammonia from the the other constituents present. complex droplet. Suppose that we consider the time re Now consider the silver. If the temperature of the quired to remove all the r.mmonia from a droplet with gas stream is 23000 F, the silver will not vaporize since an initial concentration of 3% AgI by mass. The initial the boiling point is about 35400 F. Thus the silver will droplets are about 20,u in diameter. The size when all be in the liquid phase in the gas stream. of the NH3 is evaporated is about 1,u. For Simplicity The recombination probability now becomes even and to be conservative in the analysis, the effects of smaller. The iodine is in the gas phase. A single col shell formation may be neglected (this produces a delay lision of an 12 molecule with a silver particle may pro in the evaporation). In determining ~ one can neglect duce a molecule of AgI. Furthermore, the reaction will several of the terms for this particular case. ~ and probably stop when a monolayer of Agl forms on the sil 2I are very small compared to the other terms. The ver particle. All of this must occur in a short time as t;;rm containing % is also small for an evaporating the material passes through the combustion chamber. If liquid. Furthermore neglecting this term has the con there is dissociation, there is insufficient time for the servative effect of increasing the evaporation rate. reaction: Insertion of the dominant variables in the equation Ag + 1/212 ¢Agl gives an evaporation rate of about 40,u/sec. The drop to reach equilibrium. lets are in the high temperature zone about 0.5 seconds. This explains why, as mentioned previously, de This rate of evaporation :.8 therefore compatible with tectable amounts of Ag, AgO or 12 were not found in the the hypothesis that all of the ammonia does not leave the exit gas stream. One can counter that a mono-layer of drop before it leaves the system, in which case the AgI AgI on a particle may be enough for the particle to act does not break down. as a nucleus. However, if this were the case, there The non-dissociation hypothesis should also be would have been insufficient Agl in the particle to grow discussed from the tempErature only effects (Fig. 6). the silver whiskers mentioned previously. Effectiveness of the Thermal System The effect of temperature alone on the formation The effectiveness of the thermal system is shown of nuclei was also investigated with the finding that the in Fig. 11 for a complex concentration of 3% and an out nuclei effectiveness increases exponentially as the tem put rate of 133 gms/hr. Not shown is the 500 gm/hr perature is increased to about 30000 F. curve which corresponds to 8% concentration. The out It is hypothosized that the AgI does not decompose put is somewhat lower, e. g. 8 x 1013 nuclei per gram when it is thermally processed using NH3 as the carrier. at -10C. The performance of the thermal ammonia This is confirmed by chemical analysis. The thermo system is quite attractive, however, the system is too dynamiCS of a complex droplet passing through a flame complex in its present state to employ in the field. The do not favor dissociation of the AgI as long as NH3 is IPA system shows as much promise (11) and is much present. simpler. This i s des ignated IPA after the carrier, The effectiveness of the thermal system using the isopropylamine, (CH3) 2CH NH2. This complexes read NH3-AgI complex is higher than acetone-KI-AgI systems ily with AgI and is similar to ammonia in many respects. over a wide temperature range, but the system is too It has the added advantage of having a much higher boil complex for field use. A new system employing IPA as ing point (310 C @ 76mm) than ammonia and is a good carrier is developed to the point that it may be used in fuel. These features eliminate the complexities of the the field. ammonia system. Conclusion ACKNOWLEDGEMENTS The effect of some dominant variables 0 nice The writer wishes to acknowledge: Prof. Maxwell nuclei effectiveness in systems employing thermal proc Parshall of CSU who developed the procedures for sam essinghave been presented. When a complex of ammonia pling and chemically analyzing the effluent, Prof. L. O. and silver iodide are burned with No. 1 fuel oil the ef Grant of CSU and Dr. M. L. Corrin of the Univ. of Ari fectiveness is very sensitive to stoichiometric ratios zona who acted as consultants during the course of the above 1. 15. The optimum ratio is about 0.95. Lower work, Dr. James P. Lodge of NCAR who aided in the stoichiometric ratios lower the effectiveness. An ex analysis of the electron mk:rographs. The research planation of these effects is the subject of continuing was sponsored by the National Science Foundation (NSF investigation. Grant No. GP-1663). 16 10 I ...... / // V / FLETCHERS V HOT NH 3 SYSTEM - 3 % - THEORETICAL-CURVE // /' / .~~ // V- 15 // J ~ . ....- v----- 10 / G / ....- f<"-y .~ --- - // / ./ V7 0' V • <[ // ~IPA-3% I- // -A ~ CJ) "- V~ W I /A ~ ...J / U /- ~IPA _-~-o/~-- :::> Z 14 ,/) rp V CJ) 10 CJ) ~------/ - _.... ~~- w --/f---: z / --- w > J/ / /; , ~ U / W lJ.. / lJ.. / 4V w ~csu MODIFIED SKYFIRE - 3 0/0 13 It 10 / . I I I I -- i I II • NH3 - Agi IPA - Agi - 3 0/0 0 ..- IPA -Agi - 6 /0 J - -- I 12 10 I -6 -€I -10 -12 -14 -16 -18 -20 TEMPERATURE, OF FIG_ 11 COMPARISON OF IPA GENERATOR EFFECTIVENESS References 1. Steele, R. L. and Krebs, F. W.: Characteristics of 7. Fletcher, N. H.: Size ECect in Heterogeneous Nu Silver Iodide Ice Nuclei Originating from Anhydrous cleation. The Journal of Chemical Physics, Vol. Ammonia-Silver Iodide Complexes. Colorado State 29, No.3. University, Mech. Engr. Tech. Paper No. 66-1, Atmospheric Science Tech. Paper No. 73. 8. Corrin, M. L., Edwards, H. W., and Nelson, J.A.: The Surface Chemistry of Coneensation Nuclei: II. 2. Fletcher, N. H.: The Optimum Performance of The Preparation wf Silver Iodide Free of Hygro Silver Iodide Generators. Journal of Meteorology, scopic Impurities and Its Interaction with Water pp. 385-387, August 1959. Vapor. Journal of the Atmospheric Sciences, Vol. 21, pp. 565-567, September 1964. 3. Vonnegut, B.: Early Work on Silver Iodide Smokes for Cloud Seeding. Final Report of the Advisory 9. , Storm, N. S.: The Committee on Weather Control, Vol. II, pp. 283- Surface Chemistry of Condensation Nuclei 1. The 285, Washington: Government Printing Office, 1958. Sintering of Silver Iodide. Journal of Physical Chemistry, Vol. 67, pp. 1509-1511, July 1963. 4. Vonnegut, B.: Techniques for Generating Silver Iodide Smoke. Journal of Colloid Science, pp. 37- 10. Rowland, Layton and Smith: Photolytic Activation 48, February 1950. of Silver Iodide in the Nueleation of Ice. Journal of Atmospheric Sciences, Vol. 2. 5. Fuquay, D. M. and Wells, H. J.: The Project Sky fire Cloud Seeding Generator. Intermountain Forest 11. Steele, R. L.: Preliminary Investigation of a New and Range Experiment Station U. S. D. A., Research Generating System for SLIver Iodide, Proceedings, Paper No. 48, 1957. Western Snow Conference, Seattle, Washington, 1966. 6. Robertson, Charles E.: Analytical Methods for Determining Silver Iodide Smoke Characteristics (Paper presented at National Meeting of American Meteorological Society on Cloud Physics and Severe Storms, October 1965).