Purification and compression o f
FLUORINE
J. F. Froning, 31. K. Richards, T. T. Stricklin, and S. G. Turnhull E. I. DU POVT DE VE\IOURS & C O \ I P i \ Y , I\C., W I L \ I I \ ( > T O \ ,
THE coiiipression of fluorine into niche1 arid steel clliri-
DEL.
safet? aspect- of fluorine coiilpre4oii reicilted in t h e follow iiig conclu-ions: (a) Fluorine m a g be iafelj +hippecl I,? coinnioii carrier in riicl\el or steel c>linclers containi n g 5 pouiitls of g a z a t 100 pound* per square inch, (b) operation of app.iratus under fluorine pressure should be carried c u t l)! nieaiis of rcrnote control- e\tetitlinp through a protec t i \ e wall, and (c) i t is in order to proceed rrith field studies t o determine t h e feaiil)ilit> of rientuall! handling fluorine a t 1500-2000 poinids per square inch.
ders a t 100 pounds per square inch has been accomplished b? t h e liquefaction-pressure distillation method. To prebeiit icing conditions during t h e liquefaction, t h e h?drogeii fluoride content of the electrol: tic fluorine was reduced below 0.3 ioliinie per cent. 'Illis w a q accornpli-hed 1): a coni1)inatioii of refrigeration a t -TO3 C. and absorption a t 100' C. with ~ocliiiiiifluoride. The efficiency of renioial of h\clrogeii fluoride from fluorine b? both methods was studied. Extensiie itiiestigatioii of t h e
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S ORDER to provide facilities for transporting fluorine, it Tvas
associated a t temperatures up to 75" C., the datn reported by a number of investigators ( 2 , 4.8, 9) n-ere fiwt utilized to calculate the relation betiveen temperature arid 11reasuie and the equilihrium coiiceiitIatioii in a nonrondensahle gas cuch a< fluorine. Figure 1 alion-5 the effecmt of temperature. Thi. cmrve ivziq duplicated experinientally with hydrogrri fluoiide-iiiti,ogeii mixture. hydrogen fluoride n.ere coolea to -53' C., the volume per cent 111-drogen fluoride n-ould still be 10.6. This value i? calculated assuming that hydi~ogenfluoride is not wsociated but i i monomolecular. Liken-ise the equilibrium concentrations at - 70' and - 78' C . lvoultl he 4 and 27,, respectively. Figure 2 s h o w the calculated effect of pressure on the refrigeration of hydrogen fluoride-fluorine mixtures. I n order t o reduce the concentratioii to the specification limit of 0.57& the pressure irould have to lie 435 pound? per square inch a t -50" C. and 123 pound^ per square inch at -70" c.
necessary t o develop methods for purifying the gas as produced electrolytically, techniques for compressing it, arid tests for selecting cylinders t h a t could be approved by the Interstate Commerce Commission for shipment via common carrier. Hydrogen fluoride n-as the principal impurity t o he removed. The concentration in the gas t o be compressed was limited not oiilp to minimize dilution of the fluorine but, as will he explained, t o facilitate its liquefaction. -1direct method for compresiing fluorine Tvas first to condense it at - 187' C. and then vaporize it in a restricted, calculated volume composed primarily of one or more pressure vessels.
Figure 1. Effect of Temperature on Equilibrium Concentration of Hydrogen Fluoride i n a Noticondensable Gas Figure 2. Calculated Relation betw een t h e Teniperat u r e and Pressure Required to Reduce t h e Yolunie Per Cent of €IF i n a Noncondensable Gas to Various >Ia\im u n i Percentages
The use of compressed fluorine \vas mentioned in the German literature in 1930 and 1940 (1, 1 0 ) . Subsequent information obtained from Germany since the ivar confirms the fact that fluorine had been shipped in cylinders there since that time. Pressures as high as 2200 pounds per square inch irere employed t o olitaiii 1 kg. of gas in a steel cylinder. PURIFICiTIOS
; ipractical purification process was ohtained ivhich permitted compresing fluorine of %yominimum purity. Individual maximum concentrations of 0.5% oxygen, lyoinerts, and 0.005-0.01~c corrosion dust n-ere adhered t o by employing techniques which di,xouraged contaiiiinatioii of the production apparatus n-ith air, nitrogen, or Jvater. The content of hydrogen fluoride was reduced to less than 0.570 in t n o successive steps: first, refrigeration a t -70" C. and atmospheric pressure and, second, chemical absorption with .odium fluoride. The scheme for hydrogen fluoride removal ivas adopted after first coniidering the possibility of carrying out the separation from fluorine entirely by refrigeration. Since hydrogen fluoride is
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Concurrent n-ork on the problem indicated that, a niecliancial compressor suitable for practical continuous operation n.uuld not he readily available to provide sufficiently high pressures for n refrigeration step. Hoivever, it \vas conceirahle that a separation could be effected by condensing the mixture and revaporizing it, in a system of restricted volume-a fractiorinl presure dixtillntion. Practical use of this method was defeated by the fact that the hydrogen fluoride froze a t t,lie relatively high temperature of - 83' C. and interfered n-ith the condensation of fluorine hy solidifying on t h e n-alls of the heat exchanger. At 27, hydrogen fluoride concentration, for example, the efficiency of t h e condenaer decreased too rapidly for convenient use. The most practical conclusion was first t o cool t h e gas mixture t o -70" C. at atmospheric pressure and then reduce the hydro-
INDUSTRIAL AND ENGINEERING CHEMISTRY
2 76
TABLE I. RELATIOSOF T E X P E R A TTO~ R T-APOR E PRESSCRI: OF S O D I U M -4CID FLUORIDE I N PREsESCE O F XITROGES Tpp., C. 2 ju 100" 200 250 2i5 278"
T.apor Pressure of HF over S a F . H F , Nin. Hg
H F in X i t r o g e n , Vol. 1 ;
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t o gas flo~v. More extended performance data have not been obtained. COIIPRESSIO3
Since fen- details concerning the behavior of fluorine under pressure Tvere available in this country except t h e storage results at Columbia 1-niversity ( 7 ) , initial compression experiments and cylinder .-torage tests Tvere carried out in nickel as \vel1 as steel apparatuc. T h e former was knon-n t o lie less reactive t o fluorine 0 Extrapolated vsiues. at atiiio~plieric presiure. -iccordingly, the niaxiniuni n-orking pressure for the initial development program \\-as ret at the limit prescribed for commercially available nickel cylinders-that is, I , ' , 400 pounds per square inch. The results of t h e test v o r k at 400 pounds suggest t h a t a similar investigation a t 1500-2000 pounds may show t h a t fluorine may be shipped at these pressures in steel cylinders. X 2.0S o type of mechanical compre5wr n-ai available which could Z I attain pressures of 400 pounds per square inch n-ithout the use of a lubricant or sealing liquid. Those liquids ivhich suggested themselves either n-ere too reactive t o fluorine or possesbed too 20 40 €0 80 100 120 high a vapor pressure to permit design of a continuously operatFORMATION OF NaF.HF (9.) or without considerable development work. HowFigure 3. Capacity of Fresh ever, a standard single-acting, single-stage air compressor was NaF Pellets for Remoring operated for periods of several hours a t 175 pounds per square €IF from Nitrogen-€IF AIiIture a t 100" C. inch n-liile lubricated v i t h a fluorocarbon. This oil blackened quickly in use and occasionally fired. Either a lubricant must be avoided or the compression must be carried out in a sufficient nuniher of stages to reduce the temperature of compression t o the gen fluoride content from 4 to 0.57, or leas by use oi'a c1ieriiic:il absorber. Sodium fluoride has been used for hydrogen fluoi,itle Imint where the lubricant will not be attacked. absorption since the days of Xoissan ( 6 ) . Before a plant ~ ) K J T-Se of the liquefaction-pressure distillation principle obviated the need for moving parts. Liquid nitrogen (boiling point, cedure xvas developed, t h e vapor pressure characteristics of such :I tem Tvere studied by determining the vapor pressure of aodiui,i - 1\16' C.) TVBS a readily available coolant for condensing acid fluoride in equilibrium with nitrogen. Table I summarize. fluorine, n-liicli boils a t -187" C. and possesses a vapor pres.urr of 280 mm. mercury a t -196" C. (S), n-hich corresponds this data. Tlie information indicated that hydrogen fluoride tu a v:i~uuniof 19 inches. Tlie apparatus (Figure 4) consisted of could be abaorbed from fluorine b y sodium fluoride a t 100" C. niitl that, therefore, t h e 0.570 limit could easily be met. Pilot plant tlirottling valve A , vacuum gage, pressure ihutoff valve B , nickel vessel sei,ving as combined condenser, receiver, and pressure performance shon.ed this t o be true. Temperatures belox 100" C. still, lirel-ure gage, safety disk, piping and valves for handling were aroided because there 1i-a; a tendency for semifluid polyacitl cj-liiiders, and refrigeration facilities. Fluorine was bled in \vliicli plugged the reaction bed. through .I at a constant rate of 1.5 pounds per hour. The e- on a small scale dereloped the curve slion-11in initial vacuiini \vas 1-1 inches and decreased to 0 as the receiver in fluoride pellets ahsorbed one mole of hydiofilled, almost completely, with ahout 7 ponnd. of liquid fluorine. ractically 1007, efficiency before t h e hydrogen .it this point valve B as shut, the cylinder feed valves were fluoride vapor pre>.ure in the exit gas began to rise. aliened. t h r cooling bath ?\-as lon-ered (by remote control), and In the pilot plant a toner containing a bed of sodium a c i t l vaporization was perniitted t o occur into the cylinder: one a t a up1)lied in l/s-inch size 12)- The Harshan- Clieiiiitime, pacli having a capacity of four pounds of fluorine. The ca1 Company), 3 inches in diameter and 4 feet high, m a fir.? cylinders cwnstituted swept n-ith iiitrogen 087, of the volume at 275-300' C. t o reunder pres-ure. LIQUID niore all but ahout Since t h e critical NITRO G E N i 0.027, of the !iyirotemperature of fluog e n f l u o r i d e . The rine is approximately resulting sodium -130," C., i t n-ould fluoride ivas used t o be impractical t o inprocess 1.5 pounds crease the efficiency per hour of fluorine of the liquefaction by containing 4 volume carrying it out under 70hydrogen fluoride. pressure. For t h i s I.C.C.3A 2000 STL. Such a t o y e r was Fame reason it would CYLINDER operated through five be desirable t o in60-hour cycles, alcrease t h e TV o r k i n g ternately absorbing pressure of shipping and regenerating apcylinders, since the proximately l2y0 by rffert on t h e weight weight of hydrogen that can be carried is ELEVATO? FOR f 1u or i d e , w i t h ou t almost directly proCOOLIP!G BATH portional. sufficient deterioraAverage refrigerant tion of its pellets t o Figure 4. Schematic Diagram of Liquefaction-Pressure Distillation consumption was 7 increase r e s i s t a n c e Unit for Packaging Fluorine 0.001 0 1s 11.5 55.6 93.0 100.0
0.01 1.4 87 422 706 760
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Initial reaction
Damage to the brick and valve after 4 pounds of fluorine has escaped
Glow ahout 30 seconds later as a n adjacent brick reacts with gas escaping through t h e partially deatro5ed valve
Close-up of damage to t h e valve
Photographs Taken during Deliberate Ignition (by Contaminating with Grease) of a Small Bar-Stock Steel Needle Talle Throttling Fluorine a t 100 Pounds per Square Inch
pounds of liquid nitrogen per pound of fluorine.
Direct condrnsation of the gas into the shipping cylinder would have more than tripled this figure. S.AFET\-
.4 fluorine compresiion plant must be designed and operated \vith careful attention to detail, for accidental leaks constitute :i serious hazard. Severe burns would rewlt if a stream of undiluted fluorine n-ere impinged upon the body. Piping and tubing of steel, copper, hr , hlonel, and nickel have been used succesqfully. Corrosion i q not a serious problem if moisture (inclutling t h a t due to air) and I enctive contaminants, such as grease and pipe dope, are kept out of the system. If they are not, not only does t h e system become fouled and plugied, h u t the quantity and disposition of the contaminant may he such t h a t reaction ivith fluorine causes hot spots a t which t h e kindling
temperature of the metal is reached. When this occur$, all available fluorine reacts rapidly with the metal, causing it to melt and burst under pressure. A metal-fluorine flame is produced, and pressure ejects molten metal and reartion products t o a distance of several feet, the whole effect being one of great danger t o personnel. T h e follon-ing safety rule was established for general application t o both users and fillers of cylinders: So apparatus under fluorine pressure may be approached unshielded by a n adequate protective barricade, except a closed cylinder or storage tank of a tested and approved type. There are two reasons for this rule: ( a ) Adequate protective clothing has not been developed against a sudden accidental leak of fluorine from a pressure source; ( b ) more experience should be accumulated before adequate safety against a possible metal-fluorine flame can be guaranteed by simply taking precautions rather than by provid-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
TYPE 440 VALVE FROM TYPE 550 KEROTEST VALVE
c
/16” DEEP RECESS FOR GASKET
Figure 5. Gaslteted Seal between I.C.C. 3BN400 Nickel Cylinder and Modified Chlorine Institute Brass Valve
ing a protective wall. It is true t h a t a study has been made oi bnditions that could cause an accident and that routine operation of the cylinder-filling plant used in this xork has been safely carried out, but i t is intended to develop much more varied operating experience before abandoning the use of remote controls extending through a protective wall. Qualitative field tests showed that, of the metals useful for piping, those possessing the loner corrosion rates a t atmospheric pressure also have the higher kindling temperatures. Sickel, brass, steel, and lead are decreasingly resistant in that order, lead being unsafe for even gasket use. Threaded pipe connections are practical a t atmospheric pressure, provided the minimum amount of dope necessary for lubrication is applied. The least reactive pipe dope ahich has been applied is a paste of powdered fluorspar in a fluorocarbon. For pressure service, screwed joints are back-xelded as a matter of routine, although brass-to-steel threads in good mechanical condition have never required this. It is supposed t h a t the relative resiliencies of these two metals cause the threads to seal without aid from a dope. Annealed copper is recommended for gasketed joints; the seat ing surfaces should be free of nicks and carefully aligned. Valves for pressure service were operated by extension handles. Desirable mechanical features of valves included rugged needle valve design, combined n-ith the use of dissimilar metali for seat and stem-for example, mild and hardened steel; positive turning action when seating; and true alignment of seat and stem since the Teflon (tetrafluoroethylene) packing employed was not sufficiently resilient’ to permit self-alignment of a poorly constructed valve. These features permitted valves to operate many months without significant leakage. They also rule out packless valves, which do not seat with a positive turning action. The valve packing consisted of machined annular rings of Teflon. To ensure added resistance, the bottom ring was made of Teflon compounded with 30y0 calcium fluoride. This packing gave perfect service for indefinite periods unless contaminated with some reactive subshnces or unless permitted to leak (because of a loose packing nut or a scored stem). A slow leak along the rings, easily detected with potassium iodide-starch paper, was shown to cause firing and destruction of the bonnet of a Monel
Vol. 39, No. 3
valve. The hypothesis may be advanced t h a t a blanketing layer of gaseous reaction products-for example, carbon tetrafluorideordinarily exerts a retarding influence which is removed by t h e sxeeping action of the fluorine over the surface a t a leak. If conditions are such that exothermic heat can accumulate, the Teflon and the surrounding metal will reach their ignition temperatures. Barricades of steel and brick viere employed. Steel plate inch thick was demonstrated to be satisfactory against an impinging fluorine stream and also not to be attacked by a steel-fluorine flame, if i t were fa,r enough away so t h a t the heat could be dissipated with sufficient rapidity to keep the steel from reaching its ignition temperature. Brick construction is recommended if pressure lines must be closer than 6 inches to the n-all or if reservoirs larger than 5 pounds or pressures greater than 400 pounds per square inch are contemplated without further field tefts. Brick is readily attacked by fluorine if ignited, but the penetration is sloiv. Fluorspar-loaded concrete proved much more resistant than hrick. A r h m 6 follon,s of the cylinder testing program carried out:
1. Five-pound nickel cylinders and 4-pound steel cylinders loaded a t 400 pounds per square inch were subjected to both impact and penetration with 30-caliber bullets. S o explosions occurred. The hole in the nickel was clean, whereas the hole in the steel was enlarged somewhat when the edges were ignited by the escaping gas. 2. B small loaded steel cylinder was heated to destruction in a flame. Failure occurred iyithout explosion a t the point where loss of strength and excessive pressure should have caused i t . 3. Maintenance of a full steel cylinder a t 100” C. for ten days caused no change in pressure and no intergranular penetration of the metal. 4. KO leaks occurred with a standard-taper pipe thread joint hetween brass valves and steel cylinders that have been under constant pressure for over two years. However, apparently because nickel has less resilience than steel, a gasket-sealed joint had to be deviLed for nickel cylinders (Figure 5 ) . As a result of theae observations application was made to Interstate Commerce Commission, Bureau of Explosives, permission to ship fluorine by common carrier a t 400 pounds square inch in steel or nickel cylinders. With approval routine operation was carried out in a pilot plant.
the for per
(5)
ACKNOWLEDGMENT
The authors wish to acknowledge the invaluable contributions of J. A. Eckstein, D. A. Bender, G. F. Feldmann, R. C. McHarness, and A. F. Benning to the work outlined in this paper. This work was carried out under the auspices of the Corps of Engineers t-nited States ilrmy, Manhattan Project. LITERATURE CITED
(1) Bockernueller, W., Angew. Chem., 53,419 (1940).
(2) Claussen, W. H . . and Hildebrand, J. H., J . Am. Chem. SOC.,56, 1820 (1934). (3) Cady, G. H., and Hildebrand, J. H., Ibid., 52,3839 (1930). (4) Fredenhagen, K., 2.anorg. allyem. Chem., 210, 210 (1933). ( 5 ) Interstate Commerce Commission, General Rules and Regulations, Miscellaneous Amendments to Parts 71-85, Docket N o . 3666 (Jan. 23, 1946). (6) Moissan, H., ;‘Das Fluor und seine S’erbindungen”, Berlin, Verlag von 11.Kroyn, 1900. (7) Priest, H . F.. and Grosse, A. V.. IKD. ESG.CHEX., 39,279 (1947). (8) Simons, J. H., J . Am. C‘hem. Soc., 46, 2182 (19241. (9) Thorpe, T. E., and Hambly, F. J., J . Chem. Soc., 55, 163 (1889). (10) Tartenberg, H. Y.von, 2. a n o r y . allgem. Chem., 242, 406 (1939). PRESESTED before t h e Sj-niposium on Fluorine Chemistry as IjaUer 39. Division of Industrial a n d Engineering Chemistry, 110th Meeting of t h e M ERIC AS CHEMICAL SOCIETY.Chicago, 111. T h e work described i n t h i s pap& is coi-ered also i n a comprehensive report of work with fluorine a n d fluorinated compounds undertaken i n conneetion with the X a n h a t t a n Project. This report is soon t o be published as Volume I of Division VI1 of t h e M a n h a t t a n Project Technical Series.
