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Ind. Eng. Chem. Fundam. 1982, 27, 369-374
almost perfect agreement with the experimental results. Use of the Stefan-Maxwell interaction coefficients for hydrated species, however, required an approximation of the water flux to be taken into account in order to get good experimental agreement. Further studies with different ionic species and a better attempt to account for the flux of water and the associated pressure gradient are areas clearly needing further study.
f i = mole fraction of species i in resin phase xw = water content of resin (wt water/wt resin) ro = resin particle radius
Nomenclature
Subscripts
Greek Letters p =
resin density
Superscripts
* = tracer ion species a = active ionic sites o = solvent (water) Literature Cited
aij = Stefan-Maxwell interaction coefficient for ionic species Ci = molar concentration of species i di = an effective self-diffusion coefficient for the Nernst-
Boyd, G. E.; Soldano, E. A. J . Am. Chem. SOC.1953, 75, 6091, 6107. Helfferlch, F. “Ion Exchange”; McGraw-Hill: New York, 1962; Chapter 3. Graham, E. E. Ph.D. Thesis, Northwestern University, Evanston, IL, 1970. Lupa, A. J. Ph.D. Thesis, Northwestern University, Evanston, IL, 1967.
Planck equation ) = an effective tracer ion diffusion coefficient for the hernst-Planck equation K = equilibrium distribution ( Z C ~ X N ~ / E N ~ Z C ~ ) Ni = flux of species i referred to fixed coordinates x i = mole fraction of species i in solution phase di
Received for review March 2, 1981 Accepted May 25, 1982
Adsorption Characteristics of Glucose and Fructose in Ion-Exchange Resin Columns Young Sung Ghlm and Ho Nam Chang’ Department of Chemical Engineering, Korea Advanced Institute of Sclence and Technology, P.O. Box 150 Chongyangni, Seoul, Korea
Adsorption characteristics of glucose and fructose were investigated with ion-exchange resin in carbonate form and water as an eluant. Rate and equilibrium constants were evaluated by analyzing the moments of elution curves. To elucidate the effect of eluant flow rate on rate constants, the nonlinear form of the moments was fully examined. Comparing the transport steps of two optical isomers, the contribution of intraparticle diffusion to axial dispersion was confirmed. For glucose, the more adsorbable component, surface diffusion was recognized in the process of intraparticle diffusion. In most flow rate ranges adsorption was a major resistance, particularly for the smallest particle size.
Table I. Effect of Counterions on the Distribution Ratio at 30 “C. (DOWEX 1-X8 Was Used for Anion Exchangers, and DOWEX 50W-X8, for Cationic Exchangers. Both Were of 200-400 Mesh in Dry Mesh Designation)
Introduction During the past decade, analysis of the response to input disturbances has been used to study the dynamic behavior of process equipment particularly for packed bed systems. Among them, Smith and colleagues (Suzuki and Smith, 1975; Ramachandran and Smith, 1978) drew some useful information on the transport phenomena in gas-solid chromatography using the moments derived by Kucera (1965). However, work related to liquid systems is comparatively scarce (Mehta et al., 1973; Ammons et al., 1977; Foo and Rice, 1979). This may be due to the difficulties often associated with experiments or to the lack of information on the systems in comparison with those of gases. In the present work, two solutes, glucose and fructose, are considered for chromatographic separation. Ion-exchange resin in carbonate form is used as a chromatographic medium and water as an eluant. We first compare the several ionic forms of the resin to confirm the involvement of adsorption process. Model equations are established based on the linearity of the equilibrium isotherm. The first two moments are derived from the solution in the Laplace domain, but they are a function of eluant flow rate. Emphasis is placed upon the effect of 0196-4313/82/1021-0369$01.25/0
ionic form
so, * co, 2 -
H,PO,Ca2
+
Sr2+ Zn2+
distribution ratio fructose
glucose
0.45 1.43 0.16 0.30 0.30
0.27 1.00
0.20
0.30
0.08 0.80 0.80
flow rate on rate constants, and the differences between liquid and gas systems are clarified. Adsorption and Partition Chromatography Glucose and fructose are two isomeric sugars. Their structures and chemical properties are so similar that only a few reports dealt with their separation in an industrial scale in spite of its great practical interest (Bieser and Rosset, 1977). From an analytical viewpoint, many experiments have been carried out (Jandera and Churcek, 1974). Some of them used the differences in the strengths of the sugar-ion complexes and some used the differences 0
1982 American Chemical Society
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Ind. Eng. Chem. Fundam., Vol. 21, No. 4, 1982
in nonionic sorption of individual sugars. These analytical methods are called adsorption and partition chromatography, respectively. Apart from operations for moment analysis, several ionic forms of the resin were tested. Following usual convention the distribution ratio, k ’, is used to measure the distribution of a solute between the mobile and stationary phase (Morris and Morris, 1974). It can be easily obtained from the elution curve by the equation = V*(1 k? (1) where V, and V, indicate the retention and void volumes, respectively. When the elution curve forms the Gaussian, V, equals the volumetric mean to be explained later. As can be seen in Table I, the distribution ratios of glucose are larger than those of fructose for anionic exchangers, while the reverse is true for cationic exchangers. The distribution ratio represents the elution order of the two sugars, and thus for anion exchangers fructose is followed by glucose. Furthermore, most values listed in Table I are smaller than unity, but those of carbonate are greater. This may be due to the combining effect of interaction between solute and counterions in contrast to other ionic forms where only ionic solvation in the resin phase is responsible (Helfferich, 1962). Thus adsorption must be considered in the carbonate resin system for the separation of glucose and fructose. This observation, however, does not agree with that reported by Hough et al. (1960). They have indicated that no carbonate-sugar complexing occurs on the column, apparently owing to the instability of these complexes in solutions that are not strongly alkaline. A main cause of the variation came from the different pH in the resin phase, which was maintained at 12-13 in the present work. Basic Equations Similar to that described earlier (e.g., Schneider and Smith, 1968), transient material balances of a solute are assumed as
v,
dr
1
1 - (Y
(for t > 0, r = 0 )
The column is idealized as a regular array of uniform homogeneous spheres. It is infinitely long and initially clean, and the pulse is injected at the top as a sample solution. We have assumed that the adsorption effect is dominant in the resin of carbonate form with a linear equilibrium isotherm. The solution of eq 2 to 9 in the Laplace domain is
+ cup coth p
Keka Kes + k,
p=RL(l+
+
with the initial and boundary conditions c=q=n=o (for t 5 0, z > c = 6(t) (for t > 0, z = 0 )
aq - 0 _
at the bed exit, z = L , where
Dr
)
and
By applying the final value theorem of Laplace transforms (Aris and Amundson, 1973) the first two moments can be obtained by (15)
Ke2
R(l
+ Ke)2 3
where ml‘ indicates the first noncentral moment and m2 is the second central moment, i.e., mean and variance. However, in liquid chromatography the elution curves based on the eluate volume are more common. Multiplying both sides by superficial volumetric flow rates, eq 15 and eq 16 can be written as MI’ = V,[1 + @(l + K,)] (17)
where M1’ and M 2 are the moments based on the eluate volume. Data at different void volumes and with particle radii will give the moments as a function of these two variables. Equations 17 and 18 can be used to evaluate the rate and equilibrium constants, K,, k,, D,, and D, if kf is known. All constants necessary for the prediction of the elution curve will then be available. Experimental Section With the ion-exchange resin in carbonate form and water as an eluant, chromatographic curves were obtained at 30 “ C and atmospheric pressure. The columns consisted of a glass tube tapered at the bottom sides whose diameters were fixed as 0.7 cm with various lengths of 30,50,70,90, and 110 cm. The resin bed in the column was supported by a thin pad of fine glass wool. Eluant was fed by gravity from a large reservoir installed to maintain a constant head at the top of the bed. For temperature control water from a thermostat was circulated through a jacket surrounding the column. The flow rates of eluant ranged from 0.5 to 1.5 mL/min. For each operation 50 pL of 1 g/L sugar solution was applied to the top of the bed. One-milliliter fractions of the eluate were collected automatically using a fraction collector (Bucher fractomette alpha 200, Bucher). Sugars in the eluate were assayed with a spectrometer (Spectronic 20, Baush & Lomb) according to the method of Dubois et al. (1956).
Ind. Eng. Chem. Fundam., Vol. 21, No. 4, 1982
Table 11. Bed Characteristics av particle radius, mm 0.0465 0.0805 0.176
void fraction 0.427 0.401 0.410
Glucose
_I -
Fructose
70
Characterization of Resins. The column was packed with DOWEX 1 resin with a cross-linkage of 8% divinylbenzene obtained from Bio-Rad Laboratories, Richmond, CA. The resins of three particle sizes (R = 0.0465, 0.0805, and 0.176 mm) were used, selected from 50-100, 100-200, 200-400 mesh in dry mesh designation which were commercially available. Average particle sizes were determined by screen analysis on wet swollen resins after treating in a usual manner (Cooper, 1977). But the size of 200-400 mesh was measured by a microscope (A0 Series 20, American Optical Co.) with a filar micrometer ocular because the particles were too small for screen analysis. The void fraction of the bed was calculated after applying a non-sorbable sample, e.g., 0.5 N Na2C03solution for C032-. No significant dispersion of the originally applied sample was observed in the effluent, and the results are given in Table 11. The small void fraction was found for the middle-sized particles, which was somewhat unexpected. In general, large particles have been known to pack more densely than small particles. Giddings (1965) surmised that this variation would only be measurable for very small particles with a diameter less than 0.05 mm, but the effect has been observed also for large particle sizes (Schneider and Smith, 1968; Suzuki and Smith, 1971). Direct measurement of the porosity of the wet swollen resin was very difficult because of water within the pores. The porosity was thus estimated as (vol. of wet swollen resin) - (vol. of dry resin) t, = (vol. of wet swollen resin) considering the property of gel type resins (Kun and Kunin, 1967) and the volume of dry resins obtained by Freeman (1966). The porosity of the resin was estimated as 0.555 for all particle sizes. Analysis a n d Discussion Volumetric mean and variance were estimated by assuming the elution curves as Gaussian (Morris and Morris, 1964) rather than by integrating the curve. A short column and high flow rate caused some deviation from the Gaussian due to insufficient retention time. Care was taken to obtain a curve close to the Gaussian by adjusting the retention time. Adsorption Equilibrium. A necessary condition for the chromatographic model employed here is the linearity of the adsorption isotherm. This can be shown in Figure 1, which indicates that the first noncentral moments were independent of the concentration in the injected pulse. Moreover, the linear region was extended over a considerable range taking into account the logarithmic scale of abscissa. This unusal phenomenon could be ascribed to the unstable binding of sugar-ion complex which was confirmed by evaluating the adsorption equilibrium constant. From eq 17 it follows that MI’ - Vo = Vo(1 K,) 4 A plot of (MI’ - Vo)/4 vs. Vo should be a straight line through the origin, and the slope gives K,(Figure 2). The equilibrium constant of glucose was larger than that of
+
371
I
5
10
“5
20
vo
Figure 2. Dependence of reduced first moment on the void volume. Table 111. Rate and Equilibrium Constants of Glucose and Fructose on Ion-Exchange Columns. (Resin Bed, 0.7 cm Diameter (DOWEX 1-X8, CO,z-); Temperature, 30 “C; Sample, 0.05 mg of Each Sugar) glucose fructose K.5 k f , l/min q
D,, cm2/min Ti
0.868 2.670 17.1 4.67 X 0.93
0.360 0.439 22.5 1.75 x 2.49
fructose, as was mentioned earlier, but the values in Table I11 were even smaller than unity and demonstrate that the desorption rate constants were larger than those of the adsorption. Determination of Rate Constants. One interesting phenomenon in moment analyses of fixed bed operations is that the second moment consists of separate and additive terms. If rate constants such as D, and kfare independent of the eluant flow rate, one can evaluate them only by arranging the data in a linear relationship (Schneider and Smith, 1968). However, such an effect cannot be ignored in liquid systems even at a low flow rate. Axial dispersion in packed beds is commonly expressed as a sum of molecular diffusion and eddy diffusion (Suzuki and Smith, 1971)
D D, = - + qRU re
(20)
372
Ind. Eng. Chem. Fundam., Vol. 21, No. 4, 1982
i
1 'i'
'i
2
I
I
20
25
"2s
Figure 3. Dependence of (M2/2V0Q,)on (M{2/LVoQ,): (0) R = R = 0.176 mm. The results 0.0465 mm; (A) R = 0.0805 mm; (0) obtained by nonlinear optimization technique (-1 are compared with those ignoring the effect of flow rate on mass transfer coefficients (- - -).
Unlike axial dispersion in gaseous systems, axial dispersion in liquid systems is directly proportional to U because of small diffusivities. Thus the molecular diffusion term can be dropped D, = qRU (21) The dependence of film diffusion on flow rate can be regarded by the following semiempirical correlation (Foo and Rice, 1975) 2Rkf/D = 2 1.45Re'12Sc'f3 (22)
+
The molecular diffusivity, D,was calculated from the Wilke-Chang equation using the molar volume of the diffusing solute from the additive method by Schroeder (Reid et al., 1977). Glucose and fructose show the same value, 4.36 X cm2/min. In spite of low Reynolds numbers, we cannot neglect the second term in eq 22 because of high Schmidt numbers. Introducing eq 21 and 22 into eq 18 yields the final form of the second central moment. However, the rate constants cannot be determined by simple linear regression because of the nonlinearity of the expression with respect to flow rate. Consequently, the nonlinear optimization method proposed by Rosenbrock (1960) was adopted and the results are given in Table 111. Effect of Flow Rate on Rate Constants. The solid lines in Figure 3 are the result of computer simulations using the constants obtained in this study. They were constructed in the plane of (M2/2VoQs)vs. (M1'2/LVOQs) and would be in linear form if the mass transfer coefficient was independent of flow rate. Considering only the relation in eq 21, eq 18 may be rearranged as (23)
But all the solid lines in Figure 3 are slightly convex upward due to the ordinate intercept b containing kf which is a function of flow rate as given in eq 22. However, Mehta et al. (1973) and Ammons et al. (1977) obtained the rate constants from the dotted lines in Figure 3 assuming that b is independent of flow rate. Significant differences between the solid and dotted lines are observed, which subsequently renders the rate constants obtained by the previous investigators less reliable. In fact, Ammons et al. obtained the internal tortuosity factor of 0.048 which is quite unusual even considering the involvement of surface diffusion.
We also suggest that the same argument be applied to the gas system where external tortuosity factor was less than 1 (Padberg and Smith, 1968; Adrian and Smith, 1970). They thought that the axial dispersion was determined only by molecular diffusion assuming that the flow rate was sufficiently small, but their assumption may not be appropriate in some circumstances. Contribution of Intraparticle Diffusion to Axial Dispersion. In eq 21 we can find that 9 is the reciprocal of the Peclet number based on the particle size. In many works reviewed by Levenspiel and Bischoff (1963), the Peclet number in packed beds filled with liquid is nearly constant for all Reynolds numbers. We had also expected that the two sugars would have the same values, but the results showed that the value of fructose was greater than that of glucose (Table 111). In 1975, Suzuki and Smith reported that the axial dispersion may be influenced by intraparticle diffusion. They suggested that for the region dominated by molecular diffusion, axial dispersion in the gas system increased due to the contribution of intraparticle diffusion. However, in the present liquid system where the eddy diffusion plays an important role, axial dispersion was decreased due to intraparticle diffusion. This may have been caused by dropping the first term in eq 20, but the reason is not clear at this moment. Involvement of Surface Diffusion. In a liquid system, transport in the pore volume is carried out mainly by ordinary molecular diffusion due to the short mean free path with respect to pore diameter. If the porous gel is represented by an assembly of parallel, cylindrical capillaries, the effective diffusivity can be used to define a tortuosity factor 1
D r =-D Ti
where the diffusivities used here are based on the bulk fluid. In spite of the same pore structure and the almost same molecular structure, the two sugars show different internal tortuosity factors with different intraparticle diffusivities. However, the low value of tortuosity factor for glucose suggests the involvement of surface diffusion. There have been a few studies in the literature on the surface diffusion in liquid-filled pores. Komiyama and Smith (1974), following Satterfield et al. (1973), have obtained the surface diffusivity vs. adsorption equilibrium constant. They stated that the surface diffusion was influenced negatively by the adsorption capacity in general, but in the region of low adsorption capacity where the adsorption isotherms were represented by a linear form, surface diffusion increased with the adsorption capacity. In view of the latter finding, our data in Table I11 are readily explained since in the present system the adsorption isotherms of glucose and fructose are linear and the equilibrium constant of glucose is larger than that of fructose. Relative Importance of Rate Processes. In a fixed bed operation the performance of the resin column relies on the four rate processes: axial dispersion, film diffusion, intraparticle diffusion, and adsorption. We can evaluate the importance of these processes by separating the resistance of each rate process. Using the additivity of terms in the second moment, eq 18 may be arranged as
where A's mean the resistances corresponding to the in-
Ind. Eng. Chem. Fundam., Vol. 21,
No. 4,
1982 373
Us (cmlrnin)
0
50
IO
‘0
I
Y
I
IO
I
I
I
I
MO
15 0
100
I
I
I
I
I
I
20
30
40
50
60
250 I
I
I
I
I
I
70
80
90
I
Q, (mllmin)
Figure 4. Effect of flow rate on relative contribution of each rate process to the second moment. The curves are simulated for glucose with resins, R = 0.176 mm: (0)adsorption resistance; (0) film resistance; (A)intraparticle diffusion resistance; ( 0 )axial dispersion resistance.
Table IV. Relative Importance (%) of Rate Processes at the Flow Rate of 5.0 mL/min or 13.0 cm/min glucose fructose R, mm adsorption resistance film
resistance intraparticle diffusion resistance axial dispersion resistance
Eluate volume (ml)
100
0.0465 0.176 88.4 47.8
Figure 5. Comparison of experimental data points (0)with the predicted elution curve (-) and the Gaussian curve (- - -) for frucbe. Resin bed, 0.7 X 67 cm; flow rate, 0.55 mL/min or 1.43 cm/min. Statistically, the elution curve is the s u m of independent individual distributions from various successive or simultaneous causes. It always approaches Gaussian distribution as the number of individual steps is increased. In other words, the retention time in the bed is important to characterize the shape of the peak. Short columns of high flow rate often produce skewed elution curves accompanying long tails. The extent to which a curve departs from the Gaussian may be measured by the skew and excess using the third and the fourth moments (Grubner, 1968).
0.0465 87.7
0.176 48.8
2.7
11.9
1.4
5.9
3.3
26.5
4.6
35.3
a = slope in eq 23
11.0
c = concentration of the solute in the interparticle space, g/L E = c in the Laplace domain
Nomenclature b = ordinate intercept in eq 23
6.2
12.7
5.6
volved arguments. Since the axial dispersion and film diffusion vary with flow rate, each resistance in eq 25 will also be influenced by flow rate. Figure 4 illustrates their dependences for glucose with largest particle size (R = 0.176 mm). We should notice the large variations in the low flow rate range, which might lead to an erroneous result, but the curves are gradually changing even in the higher flow rate range and asymptotically approach the shape in plug flow. As flow rate increases, resistances of axial dispersion and f i b diffusion which are dependent upon flow rate are diminished, while those of intraparticle diffusion and adsorption grow. In most regions adsorption resistance is rate-controlling, which is accentuated for small particles. Relative importances for small and larger particles are given in Table IV a t the volumetric flow rate of 5.0 mL/min. I t is interesting that glucose and fructose have almost the same constitutions in spite of variations in rate constants. Owing to the comparatively small particle size, intraparticle diffusion resistance does not play a leading part, but for larger particles intraparticle diffusion still holds its importance as can be seen in Table IV. Prediction of Elution Curves. The rate constants discussed up to now can be used to predict the elution curve by transforming eq 10 to 14 into the real time domain. Among many approximation techniques, we inverted the equations numerically following the simple curve fitting procedure suggested by Dang and Gibilaro (1974). Figure 5 shows a typical predicted curve together with the experimental data points and the Gaussian curve having the same mean and variance. The agreement between the first two is fairly good, but the curves do not deviate greatly from the Gaussian either.
D = binary molecular diffusivity, cm2/min D,= effective intraparticle diffusivity, cm2/min D, = axial dispersion coefficient, cm2/min k’ = distribution ratio given by eq 1 k, = adsorption rate constant, l/min K, = adsorption equilibrium constant kf = mass transfer coefficient, cm/min L = bed length, cm M/, Mi = ith noncentral and central moments based on eluate volume mi’,m,= ith noncentral and central moments based on time n = concentration of the solute adsorbed on the pore surface, g/L q = concentration of the solute in the pore space, g/L R = radius of the particle, cm r = radial coordinate in the particle, cm Re = Reynolds number Q, = superficial volumetric flow rate, mL/min s = variable of Laplace transformation of t Sc = Schmidt number t = time, min U , U, = interstitial and superficial velocities, cm/min V, = bed void volume, mL Vr = retention volume, mL z = axial coordinate in the bed, cm A = resistance of a rate process corresponding to the argument involved in eq 25 e = void fraction of the bed e, = porosity of the particle 7) = coefficient for axial dispersion given by eq 20 ri,re = tortuosity factors for intraparticle diffusion and for axial dispersion 4 = expression defined by eq 14 Literature Cited Adrlan, J. C.; Smith, J. M. J . Catal. 1970, 16, 57. Ammons, R. D.; Dougharty, N. A,; Smith, J. M. Ind. Eng. Chem. Fundam. 1977, 16, 203. Ark. R.;Amundson, N. R. “Mathematical Methods in Chemical Engineering”, Vol. 2; PrenticaHail: Engelwood Cliffs, NJ, 1973; pp 171-172.
374
Ind. Eng. Chem. Fundam. 1902, 21, 374-370
Bieser, H. J.; de Rosset. A. J. Staerke 1977, 2 9 , 392. Cooper, T. G. "The Tools of Biochemistry"; Wiley: New York, 1977; Chapter 4. Dang, N. D. P.; Gibilaro, L. G. Chem. Eng. J . 1974, 8 , 157. Dubois, M.; Gllles, K. A.: Hamilton, J. K.; Rebers, P. A.; Smith, F. Anal Chem. 1956, 2 8 , 350. Foo, S.C.; Rice, R. G. AIChEJ. 1975, 21, 1149. Foo, S. C.; Rice, R. G. Ind. Eng. Chem. Fundam. 1979, 18, 68. Freeman, D. H. In "Ion Exchange". Vol. I J. A. Marinsky. Ed.; Dekker: New York, 1966; Chapter 5. Giiings. J. C. "Dynamics of Chromatography"; Dekker: New York, 1965; p 199. Grubner, 0. Adv. Chromatogr. 1966, 6, 173. Helfferlch, F. "Ion Exchange"; McGraw-Hill: New York, 1962; p 128. Hough, J. E.; Priddle, J. E.; Theobald, R. S.Chem. Ind. (London) 1960, 900. Jandera, P.; Churacek, J. J . Chromatogr. 1974, 98, 55 Komiyama, H.; Smith, J. M. AIChE J . 1974, 2 0 , 1110 Kucera, E . J . Chromatogr. 1965, 1 , 237. Kun. K. A.; Kunin. R. J . Polym. Sci. Part C 1967, 16, 1457.
Levenspiel, 0.; Bischoff, K. B. Adv. Chem. Eng. 1963, 4 , 95. Mehta, R . V.; Merson, R. L.; McCoy, B. J. AIChE J. 1973, 19, 1068. Morris, C. J. 0. R.; Morris, P. "Separation Methods in Biochemistry"; Pitman: London, 1964; pp 50-60, 63-64. Padberg. G.; Smith, J. M. J. Catal. 1966, 12. 172. Ramachandran, P. A.; Smith, J. M. Ind. Eng. Chem. Fundam. 1978, 17, 148. Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. "The Properties of Gases and Liquids", 3rd ed.; McGraw-Hill: New York, 1977; pp 567-8. Rosenbrock, H. H. Comput. J. 1960, 3, 175. Satterfield. C. N.; Colton, C. K.; Pitcher, W. H., Jr. AIChE J, 1973, 19, 628. Schneider, P.; Smith, J. M. AIChE J. 1966, 1 4 , 762. Suzuki, M.; Smith, J. M. Adv. Chromatogr. 1975, 13, 213. Suzuki, M.; Smith, J. M. J . Catal. 1971, 2 1 , 336.
Received f o r review March 2, 1981 Revised manuscript received J u n e 10, 1982 Accepted J u n e 23, 1982
Kinetics of Hydrazine Decomposition on Iridium Surfaces Owen I . Smlth' and Wayne C. Solomon' Chemical, Nuclear, and Thermal Engineering Department, University o f California, Los Angeles, California 90024
The global kinetics of hydrazine decomposition on iridium surfaces were determined under conditions such that mass transport resistances from the bulk gas to the catalyst surface could be accounted for quantitatively. The performance of both pure iridium and alumina-supported iridium catalysts was examined over the temperature range of 350-700 K at 131 Pa pressure. The reaction order with respect to hydrazine was found to be between 1 and 1.5 for each surface. The decomposition rate was weakly dependent on temperature, except in the range between 450 and 500 K. The strong temperature dependence observed in this regime is attributed to a change in the nature of the catalyst, probably due to desorption of a strongly bound species (NH,, i = 2,3) from a significant fraction of the surface. The implications of this type of kinetic behavior for catalyst degradation in hydrazine monopropellant thrusters are discussed.
Introduction The catalytic decomposition of hydrazine on iridium surfaces has been of interest in recent years since this process is extensively used in monopropellant thrusters intended for long duration applications. For example, these devices are often used for communications satellite attitude control, where a premium is placed upon reliable operation in a space environment over a period of years. The process of interest is the decomposition of liquid hydrazine into gaseous nitrogen, hydrogen, and ammonia, usually over a bed of alumina-supported iridium catalyst. Failure of such devices has been attributed to the attrition of catalyst particles and subsequent formation of bed voids, resulting in sudden, destructive excursions in chamber pressure (spikes) and eventual loss of activity (washout). Such degradation has been shown to be closely connected with the operation of the thruster, and particularly with the number of starts from ambient temperature. Many mechanisms for the initiation and propagation of catalyst attrition have been proposed but, due to the difficulty encountered in isolation of individual mechanisms in a realistic environment, there are few data available that can be interpreted unambiguously. Proposed attrition mechanisms may be conveniently separated into two categories: those which are only casually related to the heterogeneous hydrazine decomposition rate and those for which a direct relationship exists. Examples of proposed mechanisms which fall into the first category B e l l Aerospace C o m p a n y ,
Buffalo, NY 0196-4313/82/1021-0374$01.25/0
include the mechanical crushing of catalyst particles resulting from the difference in thermal expansion coefficients between the engine and catalyst materials, gas-dynamic erosion caused by high-temperature reaction products, and the removal of iridium (the active agent) by chemical reaction. Mechanisms which fall into the second category include attrition due to thermal shock to individual catalyst particles, and large pressure gradients generated within the catalyst particle resulting from pore blockage by liquid hydrazine (Kesten, 1972). For reactions which exhibit a sudden, large increase in reaction rate with increasing temperature, the second category of attrition mechanisms assume increased importance relative to those of the first. Such kinetic behavior could result from the irreversible adsorption of reactant or product species on a significant fraction of the catalytic surface. This phenomenon has been observed for the decomposition of hydrazine on polycrystalline tungsten and molybdenum film, where part of the surface (the 100 and 111planes) is "poisoned" by irreversible dissociative adsorption of hydrazine for temperatures less than 1000 K (Cosser and Tompkins, 1971; Contaminard and Tompkins, 1971). The relationship observed between performance degradation and number of cold starts to which the thruster is subjected suggests that the predominant catalyst attrition mechanism is probably of the second category. Experimental Procedure The heterogeneous decomposition kinetics of hydrazine were investigated in two phases. In the first phase, 0.025 6 1982 American
Chemical Society
