REACTION WIT
s.
H. SMlLEY
AND
c.
SUPERHEATED STEA
R. SCHMITT
Design ond Developmenf Deparfmenf, Carbide and Corbon Chemicals Co., Oak Ridge, Tenn.
EY
RSIVE studies on the chemistry of fluorine xithin tlie ast decade have advanced the world's most artive element irom the status of a laboratory curioeity to that of commercial availability and utility. However, its extreme reactivity, corrosivity, and high toxicity present' challenging problems in storage aiid handling and, coilcurrent irith increased industrial usage, in disposal. A variety of methods for disposal of fluorine have been investigated (4, 6, 9, 10, 14) 18)-atniaaphcric dilution, reaction vith water or steam, reaction with caustic sods solutions, combustion v i t h hydrocarbon fuels, reaction with carbon or sulfur, absorption by lime or limestone! absorption by inorganic fluorides of lower valence, and reaction Tt-ith hydrogen. Satisfactory operation of a plant providing for the continuous disposal of fluorine a t an average rate of 60 pounds per dag TTas reported by Landau (8). Cndesirable features associat'ed v i t h this met,hod, however, m-ere t,he high equipment, reagent, and operational costs. Other methods for the disposal of relatively large quantities of fluorine under widely variant conditions of flow rate and concentration appear unattractive from the standpoint of additional limitation-Le., general operating unnianagesbility, health hazards, or explosive by-products formation. The disposal of most industrial waste gases is accomplished by discharge into the atmosphere from some type of stack, and theoretical predict,ions (1, 17) for the rate of dispersal of these gases from' a preferred stack height are available. In using these predictions, however, it is important to consider the interrelation of stack height, rate of gaseous emission, prevailing niet,eorological conditions, and topographical feat'ures, and thcir effects on gas concentration at ground level ( I S ) . For a contaminant such as fluorine, lvhich is capable of producing adverse effects on vegetation and acute hygienic problems, topographical restrictions aiid thermal variations usually present an unfavorable sit'uation for the application of bhe atmospheric dilution technique (5,16). As in the case of atmospheric dilution methods, disposal of fluorine with water or steam is of interest because of the potential low cost of such an operation. The reaction of fluorine with water was reported (9, 18) t o proceed in t x o ways under apparently identical conditions, by a smooth uninhibited burning reaction characterized by a purple flame, and by a slower inhibited reaction in whichno burning occurred. Violent explosions took place as the inhibited reaction suddenly shifted to the burning reaction. The reaction of fluorine with low t,emperature steam was also reported to be explosive (16). -4lthough this previous work indicated that t,he reaction between fluorine and water (or steam) was uncontrollable and, consequent'ly, not reliable as a disposal method, it. v,-as believed that if preheated fluorine and superheated steam were used, from a t'hermodynamic standpoint the uninhibited burning rcaction, as shown by the equation +
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norild be favored A pilot plant, unit mas constructed for investigating the reaction betmeen fluoriue and steam a t high tempci alures. Nozzle Injects Preheafed Fluorine and Superheated Steam into Water Coaled Reactor
The pilot plant unit, as s h o ~ in n Figure 1, consisted essentiallgof a steam metering system, steam preheater, fluorine preheater, reactor nozzle, reactor chamber, and water spray tower. Steam Metering System. Steam was reduced from 100 to 10 pounds per square inch gage hy means of a pressure reducing valve. The flow of steam was controlled by a 1/2-inch needlc valve and calibrated by alloxring the Eteam to pass through a spiral tubular condenser mounted in B cold water bath, wherc the amount of steam condensate collected per unit time could b~ determined. AEter t,he desired steam f l o ~rak ~ was obtained for a particular needle valve setting, steam waa introduced int,o the steam preheater by by-passing the condenser coil. Steam Preheater. The steam preheat,cr was of the baffledhead, hairpin-tube type construction, consisting of six 8-foot &inch lengths of 1/2-inch, Schcdule 40 Monel pipe encased by a '/,-inch thick by 71/*-inch dianieter stainless steel shell. The shell was heated electrically by Nichrome wire (10 kw.) embedded in refractory cement and controlled by a 440-volt Variac. The shell of the preheater was insulated by an inner 2-inch layer of Johns-Manville Superex insulation and an outer 1-inch layer of 85% niagneeia insulation, covered with silicate treated canvas. To facilitate future maintenance, the preheater insulation mas fabricated in removable sections. Fluorine Preheater. The fluorine preheater, of similar baffledhead, hairpin-tube type design, consisted of six 9-foot 6-inch lengths of '/*-inch, Sshedule 40 Monel pipe encased by a st'ainless steel shell and heated elect,rically by beaded Nichrome wire of 10 krT. capacity. Reactor Nozzle. The reactor nozzle assembly is shown in detail in Figure 2. Pure fluorine or fluorine-air mixtures were introduced into the inner section of 3/8-inch Monel tubing, and steam was admitt'ed to the outer section of b/s-inch Monel tubing. Concentricity of the nozzle was provided by four Ahort pieces of '/,,-inch Monel rod brazed to the outer 6/&ch tubing a t a distance of 1 inch from the jet end. The nozzle extended TTithin the reactor chamber for a distance of 2 inches. Reactor Chamber. The reactor chamber consisted of a 12foot length of 2-inch, Schedule 40 Xonel pipe inclined 2 inches per linear foot. Three Eample taps, spaced equidistantly along the reactor, leading to 250-ml. filt'er flasks, were provided for withdrawing samples of the fluorine-steam gaseous mixtures. The outlet line from each flask connected to a common vacuum manifold leading to a water aspirator. Reactor chamber temperatures were measured by seven Chr0me1-~4lumel thermocouples connected to a Micromax rc-
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
Voil. 46,No. 2
PILOT PLANTS ~~~~~~
corder. The first thermocouple was located a t t’he fluorinesteam jet inlet, 4 inches from the reactor end, and the remaining six thermocouples w x e fipaced equidistantly along t,he pipe at 2foot intervals. All thermocouples indicated reactor shell temperatures. S’
SWING CHECK VALVE
Figure 1.
Fluorine Disposal System
A cooling coil was tack v-elded around the first 5 feet of rcactor length a t the gas inlet end. The coil, consisting of 3/8-incli copper tubing, was positioned with the first foot of reactor C ~ O S P wrapped with 3/8-inch space from center t o center, the next’ 2 feet of reactor were wra.pped with a 1-inch space from center to center, and the remaining 2 feet were wrapped with the coil spaced 2 inches center to center. Water or steam were provided as coolants. The entire length of the reactor chamber was covered with a 1-inch thickness of Johns-Rlanville Superex pipe insulation, in order to prevent condensation of gases a t the outlet end of the chamber. Spray Disposal Tower. The outlet end of the 2-inch reactor pipe led into the approximate center of a 7-foot length of 4-inch, Schedule 40 blonel pipe. This water spray tower was essentially open t,o the atmosphere a t both ends. The steam-hydrogen fluoride gaseous mixture entering the tower was condensed by water sprayed downward from two overhead nozzles and was prevented from escaping through the bottom of the tower hy a static leg of water. A &inch pipe cap, tack-welded to the top of the 4-inch tower formed an umbrella, which, in the event of sudden upward surge, would deflect any liquid spray downward. S s an additional safety feature, two ’/2-inch swing check valves were installed in the fluorine and steam lines downstream from the respective preheaters to prevent backward flow of inlet gases in the event of pressure build-up or explosions within the reactor. Fluorine Concentration and Temperature and Steam Excess Are Reaction Control Factors
It was found that the reaction bet’ween fluorine and superheated steam, presumably occurring as a flame, is easily initiated and readily controllable, in contrast with previous expericnce reported in thc literature for t’he water or low temperature steam reaction. The quantitative conversion of fluorine to hydrogen fluoride occurs exothermically with no potentially explosive by-product formation-i.c., fluorine oxide, hydrogen peroxide. The efficiency of the react,iori is greatly influenced by the inlet fluorine concentration, amount of steam excess, and inlet gas preheat temperature. With high dilution of the inlet fluorine, as a result of employing low inlet fluorine gas concentrations or large steam excesses, the heat of reaction is apparently sufficiently disfiipated so that the burning temperature necessary to ensure complete reaction is not obtained. Using undiluted fluoFebruary 1954
rine or air-diluted fluorine containing greater than 50 mole % fluorine, a minimum preheat inlet gas temperature of 500’ P. is sufficient to initiate and sustain the reaction with steam quautities ranging from 200 to approximately 1000% in excess of stoichiometric requirements. Best results are obtained by eniploying a 200 to 500% steam excess. Using airdiluted fluorine cont,aining less than 50 mole yo fluorine, a minimum preheat temperature of 750” F. appears necessary for complete reaction. Forty-seven experimental runs of approximately 1-hour duration x-ere made in order to determine the effects of inlet gas temperature, inlet fluorine concentration and flow rate, and steam excess on the completeness of reaction. A secondary objective was to identify any intermediate resct,ion products. A summary of results obtained under some of the experimental conditions employed is given by Table I. Completeness of reaction was indicated nualitativelv“ bv “ the absence or aresence of fluorine NEUTRALIZING PIT odor a t the water spray tower and by the heat of rcaction developed within the reactor chamber. Reactions for which no fluorine odor was detected a t the gas outlet of the water spray tower were classified “good,” those for which a fluorine odor IT ~ L S detected a t the start of a run, but which subsided as the run progressed, were classified “fair,” and those for which a fluoiiiic odor was perceptible throughout the run were classified “bad.’ ’ Since the most sensitive test for elemental fluorine is based on its characteristic odor ( I @ , which makes fluorine readily detectable even in trace quantities in the atmosphere ( l a ) , the fluorine odor test was considered to be the mopt reliable indication of rcaction completeness.
2 ‘ SOH. 40 Y O N L L PIPE YONEL TUBIN6
F O U R &‘YONEL
R O D S IBRAZED TO OUTER TUBE1
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MONEL TUUlNO LO30 WALL)
Figure 2.
rt.cOPPER
2.SOH.
40 MONEL PIPE
TEE
i STEAM
f MONEL
PLATE
Reactor Nozzle Assembly
An attempt was made to melasure quantitatively the completrnew of reaction for nine runs ranging from good to bad reaction efficiencies (as judged by the odor criterion), by determining th
