PHOTOCHEMICAL AND THERMAL PRODUCTION OF TITANIUM TRICHLORIDE R O D N E Y 6. B E Y E R ' A N D D A V I D M . M A S O N Department of Chemical Engineering, Stanford University, Stanford, Calif.
The rate of the gas-phase reduction of titanium tetrachloride b y hydrogen to form titanium trichloride and hydrogen chloride was measured in a batch reactor over the temperature range 430" to 500' C. The reaction rate appears to be first order with respect to both titanium tetrachloride and hydrogen giving an over-all empirical second-order rate expression. The temperature dependence of the specific reaction-rate constant indicates an activation energy of 41 kcal. per gram mole. Traces of mercury vapor catalyzed the reaction, The reaction i s also photon-catalyzed, with the rate being increased many orders of magnitude over the thermal rate. The photochemical reaction permits the production of titanium trichloride near room temperature where titanium trichloride formation i s thermodynamically favored; whereas to obtain practical rates in the thermal reaction, temperatures around 800" C. are commercially employed where thermodynamically titanium trichloride formation is much less favored. is of importance as an intermediate in the production of titanium metal (72) and as an olefin polymerization catalyst (2). A convenient method of producing titanium trichloride is by reducing titanium tetrachloride vapor with hydrogen (70) in accordance with: ITANIUM TRICHLORIDE
+ 1/2 HP(9)%. HC1 (g) + Tic13 ($1
TiCL (9)
(1)
I t is of interest to determine the dependence on composition and temperature of the rate of the forward step of this equilibrium reaction. Recent studies on the thermodynamic properties of titanium chlorides (7, 6) have shown that this reaction is exothermic having a standard free energy and enthalpy of reaction a t 298' K. of about - 5 and - 11 kcal. per gram mole, respectively. Thus the equilibrium lies far to the right a t 25" C. although the rate of the thermal reaction is very low. Above 800° C., the rate becomes practically large, but the equilibrium yield of titanium trichloride becomes very low. There is observed in the absorption spectrum of titanium tetrachloride vapor strong absorption in the near ultraviolet region with a peak a t 280 mp ( 9 ) which suggests that the forward step of the above reaction potentially might be photochemically catalyzed near room temperature. Besides the favorable thermodynamic conditions for the formation of titanium trichloride near room temperature, a stable brown allotrope which may have a specificity as a polymerization catalyst is produced below about 250' C.; whereas in the conventional thermal process which is run around 800" C. a violet allotrope, metastable a t room temperature, is always produced. Reduction near room temperature of titanium tetrachloride vapor by atomic hydrogen produced a t low pressures in an electric arc has been observed (8). However, the rate of production of titanium trichloride by the electric arc technique was approximately 1 gram per hour, requiring a large power output of 1 kw.
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Present address, United Technology Corp., Sunnyvale, Calif. I&EC PROCESS D E S I G N A N D DEVELOPMENT
Experimental
Measurement of the rate of the forward step of the above reaction may be made by following in a given reactor the change in pressure with time due to the mole change in gaseous species; or by analyzing chemically the amount of solid titanium trichloride formed with time in several identical reactors. The latter technique was finally employed after it \$as found that in the first method contamination of the system by manometer fluid occurred and the small pressure changes were difficult to measure. The reactors were 2-inch diameter Pyrex (Corning S o . 7740) cylindrical bulbs about 5 inches in length with an 11mm. diameter borosilicate glass tube about 6 inches long attached to one end to allow the reactor to be filled and evacuated. The end of this tube was reduced to a diameter of about 6 mm. to permit both sealing it easily by melting the glass prior to a run and breaking it easily after a run. The volume of each reactor was 0.2 liter. Sealed borosilicate glass vials, 4 mm. in diameter and 1 inch long, containing liquid titanium tetrachloride were added to the reactors. Each was drawn out to a breakable thinwalled tip which could be broken when desired to release the titanium tetrachloride in the reactor by shaking the reactor. Initially, the vials were sealed to a manifold which in turn was attached to a distillation flask containing purified titanium tetrachloride equipped with a break-off seal. After the whole system was evacuated, the break-off seal on the flask as broken with a magnetically driven plunger and titanium tetrachloride was allowed to distill into the vials. After approximately the desired quantity of titanium tetrachloride liquid was collected in the vials as estimated volumetrically, they wcre sealed off and removed from the manifold. At the beginning of a run, a vial was placed in the reactor, the system was heated to approximately 150" C. and evacuated to desorb gases. The desired quantity of hydrogen gas was introduced to the reactor a t room temperature by measuring the final pressure with either a mercury manometer or a stain-
less steel Bourdon gage. The end of the reactor tube was then sealed off with a torch. Just prior to inserting the reactor bulbs into the constant temperature air oven maintained to 1 2 " C. they were shaken to break the tip off the vial allowing the liquid titanium tetrachloride to flash vaporize into the reactor space. At the desired elapse of time, the reactors \\ere removed from the oven, air-cooled for approximately '2 minutes, followed by quenching in cold water. Then the reactor tips were attached by polyethylene tubing to the vacuum system, and the tips were crushed by compressing the polyethylene tubing with pliers. The reactor was heated slightly and volatile species titanium tetrachloride, hydrogen, and hydrogen chloride were evacuated leaving the nonvolatile titanium trichloride solid and broken pieces of the vial in the reactor. The titanium trichloride solid was dissolved by adding dilute sulfuric acid to the reactor. The solution of Tic13 obtained was analyzed by the following standard colorimetric technique ( 7 7 ) . The solution was diluted to 50 ml. in a volumetric flask after the normality was adjusted with 2 ml. of 85 wt.% orthophosphoric acid. and the titanium oxidized with 5 ml. of 3 wt.% hydrogen peroxide. The optical absorbance of the resulting solution containing the yellow peroxytitanate complex was measured with a Beckman Model B spectrophotometer. The concentration of titanium in the unknown was related to optical absorbance by a spectrophotometric calibration curve obtained from solutions of known titanium content. The amount of titanium tetrachloride that had been added to the reactor was determined precisely from the difference between the initial weight of the vial plus titanium tetrachloride and the weight of the clean dry broken pieces of the vial removed from the reactor after the test. Technical grade titanium tetrachloride was used in these experiments. I n order to remove noncondensable impurities it was triply distilled under vacuum to a colorless product. Electrolytic hydrogen, of 99.975% purity and with an oxygen content of less than 1 part in 2 million, was used as obtained. The ultraviolet radiation experiments were conducted by exposing the reactors previously described to either solar radiation or radiation from a carbon arc image-furnace (7). I n the latter device, a pair of ellipsoidal mirrors were used to focus the electric carbon arc image o n the sample. A flux density of about 75 cal. per sq. cm. second over an area of 0.3 sq. cm. was measured with a platinum calorimeter. Although there is no spectral information currently available for this particular source, high intensity carbon arcs of this type emit radiation below 300 mp wave length, where titanium tetrachloride absorbs strongly (4, 9 ) . The Pyrex (Corning No. 7740) glass used in the reactors. however, cuts out a large portion of ultraviolet radiation below this wavelength (5). The photon density was measured at the arc-image zone by exposing an actinometer solution (3)to the arc light in the same manner that the reactors were exposed. Five milliliters of 0.02.l' uranyl oxalate. diluted to 100 ml , were irradiated for 45 seconds and analyzed for the per cent decomposition by titrating with a standardized 0.OlAVK M n 0 4 solution. The average quantum efficiency of uranyl oxalate is about 0 55 between 435 and 254 m p . The light path was over 5 cm. so that the absorbance was assumed to be unity. The average of two tests gave a value of 8 X 10-6 gram moles photons/second. The quantum efficiency (3) is expressed by the ratio of the gram moles of product formed to the gram moles of incident photons.
Kinetics of Thermal Reaction in the Presence of Mercury Vapor. T h e data obtained on the rate of thermal reduction of titanium tetrachloride, by hydrogen in the dark with a trace of mercury a t a partial pressure corresponding to its vapor pressure a t room temperature (0.0025 mm. Hg), are presented in Table I and Figure 1. The mercury vapor was automatically introduced by manometers during pressure measurements. The amounts of titanium trichloride that would be formed a t each temperature if equilibrium were established were calculated from thermodynamic data ( 7 , 6 ) and are included in Table I. The highest amounts of TiC13, measured experimentally, are sufficiently lower than the equilibrium values indicating that the effect on the kinetics measurement of the reverse step of the reaction in Equation 1 is minimal. Also, the amount of titanium trichloride actually formed compared with the initial amount of titanium tetrachloride present shows that the degree of advancement of the reaction is small. Thus a plot of concentration of titanium tetrachloride us. time would tend to be linear. In Figure 1, the data were fit with the best straight line going through the origin, and the average initial reaction rates were determined from the slope of the lines. These values are shown in Table I along with specific rate constant4 based on the following empirical rate expressions:
(3)
(4)
O n the basis of the results obtained a t 450" C. for two different initial concentrations of titanium tetrachloride, the second-
0
53
I30 I30
I50 I I50
250 I 250
200 I 200
REACTION TIME,
300 I 300
350 1
400 400
350
MiN
Figure 1 . Thermal rate of formation of titanium trichloride in presence of mercury vapor Initial
A B C
D E
450 430 450 470 500 VOL. 2
0.81 1.84 1.73 1.86 1.82 NO.
1
5-6 1-2 3-4 7-8 9-1 0
JANUARY 1963
79
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I&EC PROCESS DESIGN A N D DEVELOPMENT
order dependence of the rate on titanium tetrachloride as indicated by Equation 2 fits the data best. as Table I shows, although Equation 4 gives nearly as close a correlation. The deviation from the mean for the pair of measurements is * l l % for k z ; h26T for kS; and ~ 1 2 %for k3'. The titanium tetrachloride initial concentration \vas varied by a factor of 2.7, the hydrogen concentration being kept constant. Further substantiation of this empirical second-order behavior is found in the rate data in the absence of mercury vapor. An Arrhenius-type plot of the average second-order reaction rate constants is shown in Figure 2 indicating from the slope a n apparent activation energy of 41 kcal. per mole. The data of Figure 2 may be expressed analytically as: 10glOk2
=
-8.85
x
103
-+ 8.46
(5)
Kinetics of Thermal Reaction in the Absence of M e r c u r y Vapor. In a separate series of tests, in which mercury vapor present in the manometric system was carefully excluded by employing stainless steel Bourdon gages for pressure measurements, the initial rates were lower by a factor of about five than those previously described where traces of mercury vapor were present. These rate data for measurements a t 450" C. are shown in Table I and Figure 3. Again an empirical rate dependence first order with respect to both titanium tetrachloride and hydrogen in accordance with Equation 2 is indicated by the data. T h e average deviation from the mean for these measurements is 9% for k2; 27% for k3; and 34% for k3'. The initial hydrogen concentration was varied by a factor of 3 and the initial titanium tetrachloride concentration by a factor of approximately 1.5. Kinetics of Photochemical Reaction. T h e catalytic effects of both radiation from the sun and from a carbon-arc on the rates of formation of Tic13 are summarized in Table 11. For the purpose of comparing the photochemical rates with the thermal rates values of kz for the thermal reaction were extrapolated to the temperature of the photochemical experiment
employing Equation 5. Actual values of the initial concentrations of titanium tetrachloride and hydrogen together xvith the extrapolated values of k2 were substituted into Equation 2 to calculate the thermal rates a t the temperatures shown in Table
11. The large catalytic effect of photons on the rate of formation of titanium trichloride is indicated by comparing the values of the rates of the photochemical and thermal reactions as shown in Table 11. Enhancement of rates of the order of lo7 to 10'6 are obtained by photochemical means. Since all the radiation experiments were conducted in borosilicate glass reactors. primarily that portion of the spectrum energy at wave-lengths above 300 mp through the visible range was available for the reaction (5). T h e nonmonochromatic quantum efficiencies calculated for the carbon-arc source experiments have a n average value of about 0.4 gram mole TiCl,/gram mole photons. A mechanism wherein the same order of magnitude of quantum yield is observed is :
+ hv +TiCl3 + C1 H? + C1+ HCI + H TIC14 + H +TiC13 + HCl TiCl,
(6a)
fast
(6b)
fast
(6c)
Equations 6a to 6c indicate a theoretical quantum yield of 2 molecules of Tic13 per photon absorbed. The brown allotrope of titanium trichloride occurred in the photochemical studies where the temperature was less than 250" C.; whereas, in the thermal studies which were conducted above 430" C., the violet allotrope was always produced.
Conclusions T h e general kinetic behavior of the formation of titanium trichloride from titanium tetrachloride and hydrogen including the catalytic effect of mercury vapor and light has been observed, and the data presented herein suggest the usefulness
1
.
1 RE*C*lCV
1
1 I=
,
I
c;
Figure 3. Thermal rate of formation of titanium trichloride a t 450" C. in the absence of mercury vapor
(2) Initial
302 T
Figure 2. constant
Temperature dependence of second-order rate Thermal reaction in presence of mercury H Thermal reaction in absence of mercury
Test No 11.12 13; 14 (not shown) 15
Av.
A
0
TIZ4E,
B C
1.14 0 39
0.35 VOL. 2
NO. 1
JANUARY
1963
81
of a photochemical reactor for the production of titanium trichloride. Currently, the photochemical reaction is being studied in a quartz-annular flow reactor with a n axial mercury ultraviolet light source. For future work, the precise kinetic mechanism of the thermal reaction-including the role of mercury vapor-should be carefully investigated ; the photochemical reaction should be systematically studied using refinements over the present work such as employing a monochromatic light source and measuring the effect of composition on the kinetics in an attempt to elucidate the mechanism of this reaction. Acknowledgment
The authors express their gratitude for grants from the C. F. Braun Fund and California Research Corp. and a gift from Paul D . V. Manning which provided laboratory facilities and instrumentation used in this investigation. They also thank Nevin K. Hiester for the use of the arc-image furnace of the Stanford Research Institute in the photochemical studies. Nomenclature
’ K.
T
= absolute temperature,
C
= concentration (gram moles/liter)
e
specific reaction rate constant for (second order) expression in Equation 2 specific reaction rate constant for (third order) expression n Equation 3 specific reaction rate constant for (third order) expression in Equation 4 literature Cited
(1) Altman, D., Farber, M., Mason, D. M., J . Chem. Phys. 25, 531 (1956). (2) Bolton, F. H., IND.EKG.CHEM.53, 79 (1961). (3) Bowen, E. G., “The Chemical Aspects of Light,” 2nd ed., p. 193, Clarendon Press, Oxford, 1946. (4) Ellis, C., Wells, A. A., “The Chemical Action of Ultraviolet Rays,” p. 52, Reinhold, New York, 1941. (5) Zbzd., p. 181. (6) Glassner, A,, “The Thermochemical Properties of Oxides, Fluorides, and Chlorides to 2500 O K.,” Argonne National Laboratory, Rept. No. 5750, 1959. (7) Hiester, N. K., De Le Rue, R. E., Am. Rocket SOC.[Paper] 30, 928 (1960). (8) Ingraham, T. R., Downes, K. LV., Marier, P., Can. J . Chem. 35, 850 (1957). (9) Mason, D. M., Vango, S. P., J . Phys. Chem. 60, 622 (1956). (10) Schumb, W. C., Sundstrom, R. F., J . Am. Chem. SOC.5 5 , 596 (1933). (11).S ~ e l l ,F. D., Snell, C. T., “Colorimetric Methods of Analysis, Vol. IIA, 3d ed., pp. 325-6, Van Nostrand, New York, 1959. (12) Stahler, A., Bachran, R., Berzchte 44, 2906 (1911).
RECEIVED for review April 17, 1962 ACCEPTEDAugust 13, 1962
= time
(g) = gas phase (s) = solid phase
S):mposium on Industrial Photochemistry, 141st Meeting. ACS. LZashington, D. C., March 1962.
SALINE WATER CONVERSION BY PHENOL ADDITION ALLEN
F. R E I D A N D A L B E R T H. H A L F F
University of Dallas, Albert H. Halff Associates, and Ha@ and Reid, Dallas 79, Tex.
The two-phase system from addition of phenol to a saline solution i s utilized to desalinize one phase b y countercurrent flow of the phases. The phenol i s separated primarily b y cooling, the remaining phenol being removed b y liquid-liquid extraction with benzene. The desalinization i s carried out at pH -5 and at room temperature or below, reducing scaling and corrosion. Estimated costs are 59 to 77 cents per 1000 gallons in a 10,000,000-gallon-per-day plant. The investigation has been carried to the pilot plant stage.
THF chemical potential of a
solute is generally changed upon the addition of another solute to the solution. This phenomenon may be utilized to decrease the salt concentration in brackish or sea water. The process studied is based on the facts that when sufficient phenol is added to water two liquid phases are formed, one with a high and one with a low concentration of phenol; and salts dissolved in such a twophase system a t equilibrium have a higher concentration per unit volume of water in the low-phenol solution than in the high-phenol solution. The two phases may then be directed countercurrent to each other by conventional liquid-liquid extraction techniques to produce one solution with a depleted salt concentration and one with an increased salt concentration. A liquid-liquid extraction process for saline water demineralization has been studied in some detail by Hood and Davison 82
I & E C PROCESS DESIGN A N D DEVELOPMENT
( 7 , 3, 4)! who chose solvents which could be primarily separated from the salt-depleted water by heating. They ultimately decided on secondary and tertiary amines as solvents of choice in a process operating with water solutions a t p H > 10 and temperatures up to 80’ C. The phenol process, in contrast, depends on separation of the phenol by cooling and operates with the water solutions at p H 5 and temperatures of 0’ to 25’ C.
-
Step A. Passing a high-phenol salt solution down countercurrent to a n ascending low-phenol salt solution in a packed tower, which is continuously removing salt from the descending solution. Step B. Collecting the low-phenol, high-salt solution from the top of the tower, adding phenol to produce two phases, and refluxing the high-phenol phase down the tower. The low-phenol solution remaining is stripped of its phenol (Steps E and F) and the resulting high-salt solution is produced as waste.
