Ind. Eng. Chem. Res. 1991,30, 1105-1110
Of
Const.n! Extent Re8CtlOn Method Fundrmrntsl
Pure Correlrtlons
1105
The constant extent of reaction method should not be overlooked in other modeling studies. It has a unique advantage of measuring the activation energy without assuming an order of reaction. Like the pseudo-fiiborder method, the constant extent of reaction method is a useful bridge between correlations and fundamental studies (Figure 17). Literature Cited
-
Low Modal Sophlrtlc~tlon-Hlah Hlgh
h8CtlOn Complaxlty LOW
Figuru 17. Constant extent of reaction method helps extend range of kinetic analyses.
reaction. The kinetics appear to be controlled by the types of waxy non-n-paraffins. For a given neutral oil operated under commercial conditions, the kinetics can be satisfactorily modeled by a single kinetic lump. For widely varying conditions, the differences between the individual species'that make up the kinetic lump will be important. The activation energy decreases as the molecular weight of the neutral oil increases, presumably due to the inclusion of highly branched species in the kinetically important lump of waxy non-n-paraffins. Empirical correlations could be used to extend the constant extent of reaction method and predict operating temperatures from neutral oil properties, product pour point, and throughput.
Anderson, R. B. Experimental Methods in Catalytic Research; Academic Press: New York, 1968; p 34. Bliznakov, G.; Bakardjiev, I.; Peshev, 0. New Approach In Treating Kinetic Data. J. Catal. 1970, 16,148-156. Donaldson, R.; Pout, C. R. The Application of a Catalytic Dewaxing Process to The Production of Lubricating Oil Base Stock. Presented before the Division of Petroleum Chemistry, American Chemical Society, New York Meeting, August 1972. Graven, R. G.; Green, J. R. Presented at the Congress of the Australia Institute of Petroleum: Sydney, Australia, September 1980. Krishna, R.; Joshi, G. C.; Purohit, K. M.; Agrawal, P. S.; Bhattacharjee, V.; Bhatacharjee, S. Correlation of Pour Point of Gas Oil and Vacuum Gas Oil Fractions With Compositional Parameters. Energy Fuels 1989, 3, 15-20. Rajadhyaksha, R. A.; Doraiswamy, L. K. Falsification of Kinetic Parameters by Transport Limitations and Its Role in Discerning The Controlling Regime. Catal. Reu.-Sci. Eng. 1976, 13, 209-258.
Zakarian, J. A.; Farrell, T. R. Lube Facility Makes High-Quality Lube Oil From Low-Quality Feed. Oil Gas J. 1986, May 19.
Received for review June 19, 1990 Accepted December 11,1990
Kinetics for the Reaction between Chlorine and Basic Hydrogen Peroxide Gabriel Ruiz-Ibanez and Orville C. Sandall* Department of Chemical & Nuclear Engineering, University of California, Santa Barbara, Santa Barbara, California 93106 The objective of this research was to study the kinetics of the liquid-phase reaction between chlorine and basic hydrogen peroxide (BHP). This reaction is being used to produce singlet delta oxygen for the oxygen-iodine laser. The purpose was to determine the relevant kinetic rate constant of what appears t o be the rate-limiting step in the mechanism proposed in the literature. The experimental approach consisted of the measurement of the rate of absorption of Clz by a liquid B H P solution. Gas absorption measurements of ClZin B H P in a laminar liquid jet absorber were carried out over the temperature range 1.8-8.9 "C to determine the rate constant of this reaction. The system was operated in the second-order regime of gas absorption due to the relatively high value of the reaction rate constant and because of the instability of BHP a t high hydroxide concentrations. The results for the second-order rate constant can be correlated by k (L/(g mo1.s)) = 6.477 X 1020 exp(-9872/T (K)). Introduction Production of singlet delta oxygen [02(lAg)] has shown increasing interest due to its use in the chemical oxygeniodine laser (COIL). Singlet delta oxygen is an excited state of molecular oxygen that has approximately 22.5 kcal/mol more energy than ordinary ground-state oxygen (Demyanovich and Lynn, 1987; Held et al., 1978). In the COIL, singlet delta oxygen transfers its energy to atomic iodine which lases as its atoms release energy. The performance of a COIL depends on the ability to produce 02('Ag) generated from its singlet oxygen generator (Takehisa et al., 1987). The chemical reaction between chlorine and basic hydrogen peroxide (BHP) is the most common reaction used
for the production of singlet oxygen. The stoichiometry of this reaction is as follows: 2KOH + H202+ C12 2KC1+ 2H20 + O,('A,) (1)
-
In the design and modeling of singlet oxygen generators it is important to understand the kinetics of this reaction.
A plausible mechanism for the reaction, as proposed by Hurst and Goldberg (1980) and Storch et al. (1983),is given as follows. First, BHP is prepared in typical concentrations by mixing 6 M KOH and 90% (wt) hydrogen peroxide; the solution reaches equilibrium according to H202 + OH- H2O + HOO(2)
-
Typically an excess of hydrogen peroxide is used and the
0888-588519112630-1105$02.50/0 0 1991 American Chemical Society
1106 Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991 C
concentration of hydroxyl ion is almost zero according to the equilibrium constant for the reaction given by Balej and Spalek (1979). Then chlorine gas reacts with BHP according to C12 + HOO- A C1HOOClc100-
kl
+ HOOCl
H+ + C100-
2 02 + c1-
1
(3) (4)
(5)
-
where the H+ ion is then neutralized by H+ + HOO- Hz02
(6) And f d y , the hydrogen peroxide formed here reacts with more KOH according to eq 2. The reaction in eq 4 is expected to be very fast compared to the others since it involves only a proton transfer. C100- reacts spontaneously in the gas phase, and it is expected to decompose fast in the liquid phase although it can be slightly stabilized by polar solvents. It appears that the rate-limiting step for the overall reaction is eq 3. Thus, the rate of reaction of chlorine per unit volume of liquid in aqueous alkaline peroxide is given by kl[C12]. [HOO-1, where Itl is the second-order rate constant for the reaction. The value of this rate constant has been estimated, based on similar reactions, to be in the order of lo7m3/(kg mol& For example, Sandall et al. (1981) used the system of chlorine and concentrated aqueous sodium hydroxide, for which the second-order rate constant was found to be 3 X lo7 m3/(kg mobs) a t 0 OC. Held et al. (1978) studied the reaction between hydrogen peroxide and hypochlorous acid in basic and acid media. For the preferred basic path they found the rate constant to be 4.4 X lo7mS/(kgmobs) at 25 OC. The purpose of this research is to study the kinetics of the liquid-phase reaction between chlorine gas and BHP aqueous solutions for the generation of singlet delta oxygen. This includes the measurement of the important reaction rate constant It, and its correlation with temperature using the Arrhenius expression. The BHP concentration of interest in this work is an aqueous solution made by mixing 90% (wt) hydrogen peroxide and 6 M potassium hydroxide to yield a solution of 4 M in HOO-. Such concentration is similar to that used in a singlet delta oxygen generator. The experimental approach taken in this project involves in general the measurement of the rate of absorption, under specific conditions, of a gas into a liquid solution using a laminar liquid jet absorber. Kumar and McCluskey (1987) measured the gas absorption rate of chlorine by BHP liquid solutions in an unstirred batch reactor under instantaneous regime conditions, and their procedure for preparation and analysis of solutions, as well as safety indications, is followed here. Experimental Apparatus The apparatus used to measure the kinetic rete constant kl is the laminar liquid jet absorber shown in Figure 1. This apparatus was modified from the original design as described by Al-Ghawas et al. (1989), and a detailed description of the changes is given by Ruiz-Ibanez (1990). This absorber is a gas-liquid contacting device with very small contact time between the gas and the liquid in the range of 10-3-10-2 s. It consists of an absorption chamber, H, constructed from a 0.31-m-long, 0.076-m-inside-diameter Pyrex glass cylinder and is enclosed by a constanttemperature jacket, I, constructed from a 0.31-m-long,
Figure 1. Laminar liquid jet absorber.
0.165-m-inside-diameterPyrex glass cylinder. Both cylinders are held between two stainless steel flanges and the ends are sealed with Teflon gaskets. Ethylene glycol to the jacket is supplied from the recycled to a constanttemperature circulating bath. The temperature of the supply bottle, D, is maintained at about 5 "C below the temperature of the experiment by cooling with a solution of acetone and dry ice. The liquid feed is sent to the apparatus by pressurizing the supply bottle and passes through a coil in the constanttemperature jacket. It is then fed to a l-cm-inside-diameter delivery glass tube. The delivery glass tube can slide in the vertical direction and can be locked in position by a swage-lock nut with Teflon ferrules. The gas supply is fed through a coil in the constant-temperature jacket. It is then introduced into the absorption chamber at the base and is exhausted at the top of the chamber. Chlorine was put in contact with a concentrated solution of KOH to be neutralized before it was sent to the atmosphere according to the recommendations of Kumar and McCluskey (1987). The jet nozzle assembly is fitted onto the end of the glass delivery tube by three O-rings. The jet nozzle is a square-edged orifice, 5.1 X lo4 m in diameter, drilled in m thick stainless steel. This nozzle design was a8X that recommended by Raimondi and Toor (1959) for absorption rate results closest to the theoretical values for rodlike flow and no interfacial resistance. The receiver is 0.01-m-inside-diameter glass tube fitted into a funnelshaped base and is capped by a Teflon plug in which a 0.001 m hole is drilled. A hole in the base allows draining of any liquid overflow. The jet is centered by manipulating the mount of the delivery tube relative to the top flange. The jet is considered centered when all of its contents empty into the receiver with no overflow. A leveling device consisting of a valve at the exit line is used to precisely adjust the liquid level in the receiver tube. If the level is low, gas entrainment occurs; if it is high, then the liquid overflows. The liquid level has to be readjusted after any changes in the liquid flow rate. The length of the jet is measured by a cathetometer with an error less than f 5 X m. The temperature of the system is monitored by thermometers in the constant-temperature jacket, in the jet chamber, and in the liquid delivery tube. The tempera-
Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991 1107 tures are controlled to within k0.30 "C. After the jet is flowing satisfactorily, the gas is turned on for enough time for the jet chamber and all the tubings to fill with the gas. Then the rate of gas absorption is found by measuring the concentration of chloride ion in the liquid BHP solution using a selective chloride ion electrode. The analysis procedure described by Kumar and McCluskey (1987)was followed. BHP solutions were prepared by mixing the right amounts of 6 M potassium hydroxide aqueous solution and 90% (wt) hydrogen peroxide to yield a solution of 4 M concentration of HOO-. The reactants used were KOH pellets for Fisher Scientific with a composition of approximately 85% KOH, 14% HzO, and 1% K2C03, and H202aqueous solution provided by the U.S.Air Force Weapons Laboratory with a purity of about 90%. The temperature was kept at approximately -20 "C during the mixing and closely monitored to avoid an uncontrolled decomposition of the solution. Also, the KOH aqueous solution was prepared using distilled and deionized water and it was treated with Decolorizing Carbon to adsorb any impurities that may enhance the decomposition of the BHP solution. Safety equipment was used at all times to avoid contact with the concentrated solutions. In most cases the BHP solutions were used immediately or within a few hours after preparation to avoid significant changes in its concentration due to decomposition.
where E is the enhancement factor for absorption with chemical reaction. Finally, for the case of instantaneous reaction the rate of gas absorption is calculated as
Procedure
from which the contact time required can be calculated as
Since chlorine is only consumed in the system by reaction 3, the reaction rate constant k, can be determined by the experimental measurement of the rate of absorption of chlorine in a BHP solution. The proper relationship between this rate constant and the gas absorption rate mubt be used depending on the gas absorption regime. If the jet can be regarded as a cylindrical rod in uniform motion, the time of exposure, t , to the gas of each element of its surface is the length, h , of the jet divided by its velocity, 4 L / r d 2 (where L is the volumetric flow rate of liquid and d the diameter of the jet). Then t = *d2h/4L (7) Thus, if one measures the rate of absorption of gas into the jet, q, one can calculate the amount Q absorbed by unit area of surface during a time of contact t q = rdhQ/t (8) or Q = qd/4L The contact time can be varied by changing h and L , and thus Q can be determined as a function of contact time. According to the penetration theory (Danckwerts, 1970) for the case when the dissolved gas reacts with the liquid under fast first-order or pseudo-first-orderreaction kinetics the rate of absorption is calculated as q = TA*h(DAkI)'/' (10) where DAis the diffusivity of the gas in the liquid solution, A* is the concentration of the gas in the gas-liquid interface and kI is the first- or pseudo-first-order rate constant. When the dissolved gas undergoes a reaction with the liquid and the reaction rate depends on both the concentration of dissolved gas and the concentration of liquid (which cannot be regarded as uniform), the kinetics are described by a second-order regime. For this case the rate of gas absorption is calculated as q = ~EA*(DAL~)'/~ (11)
q = 4EiA*(o~Lh)'/~ (12) where Ei is the enhancement factor for the instantaneous case and it can be approximated as Ei = 1 B0/zA* (13)
+
where Bo is the concentration of reactant in the bulk of the liquid solution and z is the number of moles of reactant that react per mole of gas. The first-order regime of gas absorption is always preferred for kinetic studies due to the relatively simple relationship between gas absorption rate and kinetic rate constant. However, the following condition has to be satisfied to operate in this regime:
MI2< Ei/2
(14)
where M is the dimensionless rate constant given by M = (T/4)k11Bot (15) Thus, by substituting eqs 13 and 15 into eq 14 1 Bo (TkIIB"t/4)1/2< 2 +4A*
(16)
If the value of kn can be taken as approximately lo7 m3/(kg mobs) and for a common BHP solution Bo a 4 kg mol/m3 and A* 0.04 kg mol/m3, then the contact time between gas and liquid in the absorber, t , should be less than 2 X s in order to satisfy eq 14. The liquid laminar jet is one of the gas absorption devices that provides the shortest contact times. The minimum contact time in this device is the order of lo9 s, which makes it impossible to achieve conditions to operate in the first-order regime, provided that the value of 10' m3/ (kg molos) for the rate constant is correct. There are two common practices to relax the condition given by eq 14. The first is to increase the concentration of reactant in the liquid solution, BO. Unfortunately, in this case this concentration could not be increased significantly due to the increased instability of the solution. The second is the dilution of the gas to reduce its concentration as the gas-liquid interface, A*. However, this option introduces the problem of gas-side mass-transfer resistance which is very complicated to deal with in the analysis of the experimental data. It seems that the only alternative to determine the rate constant k, is to operate in the second-order regime of gas absorption. Gas absorption accompanied by chemical reaction in the second-order regime is not described explicitly by any analytical equation. However, a good approximation given by Brian et al. (1961) can be used to determine the second-order rate constant k,. Their equation is
1108 Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991
centration of reactant is depleted completely, instantaneous regime conditions). For the conditions of our experiments, Bo is more likely to be closer to the instantaneous regime conditionsthan to the pseudo-first conditions according to the contact time estimates given in eq 17. The experimental values of solubility and diffusivity of chlorine in 35% hydrogen peroxide are used in both of these cases due to the uncertainty of the estimates, and since the values of these properties are expected to be similar in both solutions. These values have been estimated in a previous work (Ruiz-Ibanez et al., 1991). In H
(&)
g mol
D (cm2/s) = -1.904
01
10
1
100
(WLR Figure 2. Enhancement factor as a function of the dimensionless
rate constant and the instantaneous enhancement factor for the case of second-order reaction regime.
Equation 18 is shown plotted in Figure 2. This result was found to represent very well the numerical results of Brian et al. The procedure to determine the second-order rate constant can be outlined as follows: 1. Under the instantaneous regime conditions (large enough contact time), measure the gas absorption rate of chlorine in BHP solution and determine the instantaneous enhancement factor, Ei, from eq 12.
2. Under second-order regime conditions (short contact time) measure the gas absorption rate of chlorine in BHP solution and determine the enhancement factor, E, from eq 11.
E=
9
4A* (DALh)'/'
(20)
3. With E and Ei measured, calculate the value of the dimensionless rate constant, M, by using eq 18. For the values of E and Ei in our case, it can be shown that the denominator in eq 18 is very close to unity. Thus, M can be solved as
M=
P ( E i - 1) Ei - E
(21)
4. The kinetic rate constant can then be determined by using eq 15 as (22)
Knowledge of the solubility, A*, and diffusivity, DA, of chlorine are required in order to determine k l in this manner as shown in eqs 19 and 20. For the first case, the instantaneous regime conditions, these properties are needed for a solution of aqueous hydrogen peroxide, since the concentration of reactant (HOO-) is zero in the plane beneath the surface. For the second case, the second-order regime conditions, these properties are required for an aqueous BHP solution of concentration between Bo (if the concentration of reactant is not depleted near the interface, pseudo-first-order conditions) and zero (when the con-
X
4149.01 = 16.9428 - T (K)
+ (8.18 X 10-e)T (K)
(23)
(24)
In order for the reaction between chlorine and BHP solution to be in the second-order regime and as far as possible from the instantaneous regime, the concentration of reactant, B O , needs to be high, yielding a high value of the instantaneous enhancement factor, Ei,according to eq 13. This can be observed from Figure 2.
Results Experimental runs with very concentrated BHP solutions (>4 M) were attempted, but in none of these was it possible to obtain measurements. The instability of the solutions plus the fact that the system is highly reactive did not permit the experimental setup to reach steady conditions. At these higher BHP concentrations the main difficulty encountered was foaming caused by the rapid decomposition of Hz02. For the results presented here, a BHP solution of concentration equal to 4 M in the HOOion was prepared according to the procedure described previously. The rate of gas absorption, q, was calculated from the chloride concentration, [Cl-1, as q = L[C1-]/2
(25)
Measurement of Ei.The laminar liquid jet absorber was adjusted to yield a high contact time by using low liquid flow rates, L, and large heights, h, in order to carry out gas absorption in the instantaneous regime. The height of the liquid jet was set at the highest possible value without the appearance of turbulence at the end of the liquid jet. The gas absorption rate, q, for chlorine was measured, and the instantaneous enhancement factor, Ei, was calculated by using eq 19. The contact time was kept to 9.43 X s. The results for in a range of 7.72 X a height of the liquid jet of h = 0.0277 m are shown in Figure 3. The enhancement factor was measured at the same conditions with slightly lower contact time, and no appreciable change was noticed in its value (within experimental error) indicating that indeed the regime was instantaneous. The results are correlated by Ei = 40.128 + 2.8957T ("C)
(26)
which is the equation for the line in Figure 3 and predicts the experimental results with a mean deviation of 4.05%. Measurement of E. For the second-order regime experiments the laminar liquid jet was adjusted to yield considerably lower contact times than that in the instantaneous regime. With a height of the jet of h = 0.0123 m the contact time was kept in a range of 2.6 X to 2.9 X s. Again, the gas absorption rate, q, of chlorine was measured and the enhancement factor, E , was calculated according to eq 20. The results are shown in Figure 4.
Ind. Eng. Chem. Res., Vol. 30, No. 6,1991 1109
0
70
60
w-
50
40-
\ 30
0 L
8
4
4 -5
20-10
5
0
10
J 3.54 3.56 3.58 3.60 3.62 3.64
15
'Oi.52
T ("(3
in (K)x 10'
Figure 3. Instantaneousenhancement factor as a function of temperature for chlorine in basic hydrogen peroxide solution (4 M).
w
35
-
30
-
25
-
3.66
Figure
5. Rate constant for the reaction between chlorine and peroxyl ion as a function of temperature.
values obtained for the second-order rate constant can be correlated as a function of temperature by the Arrhenius equation as In kl = 47.92 - 9872.0/T (K) (28) or k , (L/(g mol-s)) = 6.4773 X lozoexp (-9872.0/2' (K)) (29) which gives a value of 1.3 X lo5 m3/(kg mo1.s) at 0 "C compared to the value of 3 X 10' m3/(kg mol-s) found by Sandal1 et al. (1981) for the reaction between chlorine and concentrated aqueous sodium hydroxide at the same temperature. Equation 29 predicts the experimental results with an average mean deviation of 13.9% and is valid for the temperature range between 0 and 10 "C.
Acknowledgment 20
0
2
6
4
8
10
T ("C)
Figure 4. Enhancement factor as a function of temperature for chlorine in basic hydrogen peroxide Solution (4 M).(Contact time in the range 2.6
X
to 2.9
X
lo-* 8.)
The difference between this enhancement factor and the instantaneous enhancement factor measured at the same temperature and HOO- concentration was used as a proof of the second-order regime conditions. The line in Figure 4 is the equation that correlates E as a function of temperature and is given by E = 22.1073 + 1.94422' ("C) (27) which predicts the experimental results with a mean deviation of 3.89%. However, the enhancement factor is also a function of contact time. Calculation of k The dimensionless rate constant, M, can be calculated from the values of the instantaneous enhancement factor, Ei, and the enhancement factor for the second-order regime, E, according to eq 21. And finally, the second-order kinetic rate constant is calculated by using eq 22. The results are shown in Figure 5. The
This work was sponsored by the U.S.Air Force through Research Contract No. F29601-88-K-0005.
Nomenclature A* = concentration of gas A at the gas-liquid interface, g mol/L Bo = concentration of reactant in the liquid solution, g mol/L d = diameter of the liquid jet, cm DA = diffusion coefficient of gas A into liquid solution, cm2/s E = enhancement factor for absorption with chemical reaction Ei = instantaneous enhancement factor h = length of the liquid jet, cm kl = reaction rate constant for eq 3, L/(g mol-s) k2 = reaction rate constant for eq 4, l / s k3 = reaction rate constant for eq 5, l / s kI = first-order reaction rate constant, l / s kII = second-order reaction rate constant, L/(g m o l d L = volumetric flow rate, cm3/s M = dimensionless rate constant q = total rate of gas absorption, g mol/s Q = total amount of gas absorbed per unit area in time t , g mol/cm2 t = time, s z = stoichiometric coefficient, moles of reactant that react per mole of gas
Znd. Eng. Chem. Res. 1991,30, 1110-1116
1110
drogen Peroxide Reaction. Ind. Eng. Chem. Res. 1987, 26,
Literature Cited Al-Ghawas, H. A.; Hagewiesche, D. P.; Ruiz-Ibanez, G.; Sandall, 0. C. Physicochemical properties important for carbon dioxide absorption in aqueous methyldiethanolamine. J. Chem. Eng. Data 1989, 34, 385.
Balej, J.; Spalek, 0. Calculation of equilibrium composition in more concentrated systems H202-KOH (or NaOH)-H20. Collect. Czech. Chem. Commun. 1979,44, 488-494. Brian, P. L. T.; Hurley, J. F.; Hasseltine, E. H. Penetration theory for gas absorption accompanied by a second order chemical reaction. AIChE J. 1961, 7, 226-231. Danckwerts, P. V. Gas-Liquid Reactions; McGraw-Hill: New York, NY, 1970; pp 18-20, Demyanovich, R. J.; Lynn, S. Process design and evaluation of a continuous chemical plant for the singlet oxygen-iodine laser. Fusion Technol. 1987, 12,488-501. Held, A. M.; Halko, D. J.; Hurst, J. K. Mechanisms of chlorine oxidation of hydrogen peroxide. J. Am. Chem. SOC.1978, 100, 5732-5740.
Hurst, J. K.; Goldberg, I. B. Discussions at O2('A,)/I2 Laser Contractors Meeting. Air Force Weapons Laboratory: Kirtland Air Force, NM, March 1980. Kumar, A.; McCluskey, R. J. Study of the Chlorine-Alkaline Hy-
1323-1329.
Raimondi, P.; Torr, H. L. Interfacial Resistance in Gas Absorption. AZChE J. 1959,5, 86-92. Ruiz-Ibanez, G. Kinetic Study for the Reaction between Chlorine and Basic Hydrogen Peroxide. Ph.D. Dissertation, Department of Chemical & Nuclear Engineering, University of California, Santa Barbara, 1990. Ruiz-Ibanez, G.; Bidarian, A.; Davis, R. A.; Sandall, 0. C. Solubility and Diffusivity of Oxygen and Chlorine in Basic Hydrogen Peroxide Solutions. Submitted for publication in J. Chem. Eng. Data 1991.
Sandall, 0. C.; Goldberg, I. B.; Hurlock, S. C.; Laeger, H. 0.;Wagner, R. I. Solubility and rate of hydrolysis of chlorine in aqueous sodium hydroxide at 273 K. AIChE J. 1981,27,856-859. Storch, D. M.; Dymek, C. J.; Davis, L. P. MNDO Study of the Mechanism of 02('$)Formation by Reaction of C12 with Basic HzOz. J. Am. Chem. SOC.1983, 105, 1765-1769. Takehisa, K.; Shimizu, N.; Uchiyama, T. Singlet oxygen generator using a porous pipe. J. Appl. Phys. 1987,61, 68-73.
Received for review May 29, 1990 Accepted January 8, 1991
Development of a Fluidized Bed Catalyst for the Oxidation of n-Butane to Maleic Anhydride Gerhard Emigt a n d Friedrich-G. Martin*?$ Zmtitut frir Chemische Technik, Universitat Karlsruhe, Kaiserstrasse 12, D-7500 Karlsruhe, Federal Republic of Germany, and Znstitut f u r Technische Chemie, Universitdt Braunschweig, Hans-Sommer-Strasse 10, 0-3300 Braunschweig, Federal Republic of Germany
A series of fluidized bed catalysts for the preparation of maleic anhydride (MA) from n-butane was prepared by encapsulation, embedding, or impregnation of the active and selective catalytic material. The catalysts were characterized by their fluidization and their catalytic properties. A fluidized bed catalyst embedded in an amorphous matrix of zirconium hydrogen phosphate was found to be optimal concerning both its selectivity and its attrition resistance. This catalyst was the result of
a systematic catalyst development, the starting point of which was an investigation of pure crystalline phases. The optimization of the performance of fixed bed catalysts by statistical experimental design followed. Finally, the above-mentioned different methods of adjusting the material to a fluidized bed process were studied. 1. Introduction The development of catalysts for the selective oxidation of a paraffin n-butane to maleic anhydride (MA)-a chemical of high interest-was a major breakthrough in heterogeneous catalysis. This process replaced the up to then predominant process of oxidation of benzene. This change was due to the availability of n-butane and the 50% lower cost of the raw material (Emig and Martin, 1987). The fixed bed technology predominates in the production of MA (Irving-Monshawand Klein, 1989; Wellauer, 1985). The world production of MA was about 530000 tons/year in 1985/86 (Gerry and Tsuchiya, 1986; Chemical Prices, 1976-1986). Since then the production has increased steadily by abcut 5% /year (Irving-Monshaw and Klein, 1989). The consumption of MA was 370 X lo6 lb/year in 1989 in the US.and is estimated to be 538 X 106lb/year by 1995 (Irving-Monshawand Klein, 1989). In 1989 the total production of MA was by the fixed bed technology. A major aspect that contributed to the predominance of the fixed bed technology was the shift from benzene to n-butane as a raw material. Nearly the same
Universitht Karlsruhe.
* Universitat Braunschweig.
eauiDment could be used (Chinchen et al. 1987). The only disadvantage was the lower selectivity of this procesi compared with the benzenebased process. The exhaustion of the available production capacities of MA spurred the idea to use other technologies with a higher process efficiency and selectivity. Such improved technologies comprised the separation and purification (Budi et al., 1982) of the product and use of the fluidized bed technology (Contractor and Sleight, 1987; Emig and Martin, 1987; Griesbaum and Swodenk, 1984). Ten years ago, Laguerie (1973a,b) and later Liebenau (1979) investigated the efficiency of a fluidized bed process for the oxidation of n-butane and ruled it out. Laguerie did so because he observed that, in comparison with the same catalyst used in a fixed bed, the fluidized bed gave lower yields of MA. Liebenau most probably got bad selectivities for his fluidized bed catalyst because of the iron oxide he added to increase the attrition resistance. In the early 1980'9, several companies and engineering enterprises, including Badger, Sohio, and UCB, developed fluidized bed processes (A Butane-Based Maleic Anhydride.... 1979). but none of those were imdemented on an industrial scale. Yet all new plants were' fixed bed units. More recently, most research in this promising area has been done by companies and engineering laboratories that
0888-5885/91/2630-1110$02.50/00 1991 American Chemical Society
