1152
Ind. Eng. Chem. Res. 1988,27, 1152-1157 Power Research Institute: Palo Alto, CA, 1976; EPRC1976, Project S128-1, pp B-9-B-10. Pederson, S. K. “Nuclear Power Plant Design.” In Nuclear Power; Ann Arbor Science: Ann Arbor, 1978; Vol. I., p 416. Penneman, R. A.; Audrieth, L. F. Anal. Chem. 1948, 20, 1058. Powell, E. M. In Kent’s Mechanical Engineer’s Handbook; Salisbury, J. K., Ed.; Wiley: New York, 1970; pp 7-16. Schmidt, E. W. Hydrazine and Its Derivatives; Wiley: New York, 1984; p 827. Watt, G . W.; Chrisp, J. D. Anal. Chem. 1952, 24, 2006.
Commonwealth Edison Company “Byron and Braidwood Twin Unit Stations (CAE/CBE) and (CCE/CDE) Design Parameters”, Chicago, IL, 1974. Ellis, S. R. M.; Jefferys, G. V.; Hill, P. J. Appl. Chem. 1960,10, 347. Gordon, A. J.; Ford, R. A. T h e Chemist’s Companion; Wiley: New York, 1972; pp 486-487. Hawkins, G. A. In Mark’s Standard Handbook for Mechanical Engineers, 8th ed.; Baumeister, T., Ed.; McGraw-Hill: New York, 1978; pp 4-48. Kesler, G . W. In Mark’s Standard Handbook for Mechanical E n gineers, 8th ed.; Baumeister, T., Ed.; McGraw-Hill: New York, 1978; pp 9-30. Key, G. L.; Fink, G. C.; Helyer, M. H. Steam Generator Chemical Cleaning: Demonstration Test No. 2 i n a Pot Boiler; Electric
Received for review June 15, 1987 Revised manuscript received November 17, 1987 Accepted December 7, 1987
Design of Experiment and Parameter Estimation in a Bistable System: Ethylene Oxidation on Platinum Moshe Sheintuch* and Moshe Avichai D e p a r t m e n t of Chemical Engineering, Technion-Israel I n s t i t u t e of Technology, Haifa 32000, Israel
A methodology for designing experiments aimed a t determining the set of limit points in a bistable system is developed and applied, in an automated system, to nonisothermal oxidation of ethylene on platinum. Bifurcation diagrams are automatically traced by fine-scanning domains that were identified t o include limit points in the preceding coarse scan. Model discrimination shows that a Langmuir-Hinshelwood rate expression may account for the observed qualitative features. The parameter estimation procedure is aimed at fitting the limit points by identifying and locating singular points and special features of this set. We derive the defining conditions and search for parameters that satisfy them within the uncertainty of the experimental data. Good description of the bifurcation diagrams and sets is achieved with few parameters. The current strategy for developing a rate expression for a single reaction involves usually three steps: experimentation, model discrimination, and parameter estimation (Froment and Bischoff, 1979; Froment and Hosten, 1984). The same procedure may be applied to systems that exhibit steady-state multiplicity. These steps should be designed then to reveal and predict the set of limit points (bifurcation set), which is the most discriminatory feature of the systems. Typically, one carries out measurements of a state variable (x in Figure la) by varying one or more operating conditions. The state variable is usually the reaction rate, product concentration, or, in nonisothermal studies, the catalyst temperature. Typical operating conditions are concentrations of reactants and the feed or reactor temperature. The experimental work is aimed therefore a t mapping the stable folds of the steady-state surface and the bifurcation set. High-resolution experimentation is necessary, near the limit points, in order to achieve good estimates for their location. High-resolution experimentation will also reduce the uncertainty in the location of the cusp point and other singular points in the plane. A systematic approach for model discrimination relies on the qualitative features of the bifurcation diagrams and set (Harold et al., 1987): number of stable solutions, existence of isolated branches (i.e., extremum points of the bifurcation set), and the slope of the ignition and extinction lines. Once a proper kinetic model has been derived, the third step, parameter estimation, is applied. In a standard computational procedure, one tries to minimize the deviation between model predictions and the data. In a multivalued rate surface, distance is not properly defined in the domain where the model predicts bistability while experimentally the solution is unique (or vice versa), i.e., within the region bounded by experimental and predicted 0888-5885/88/2627-1152$01.50/0
bifurcation sets. Parameter estimation procedures have been applied by Hershkowitz and Kenney (1983) to account for ignition points observed in CO oxidation and by Harold and Luss (1987a,b) to predict the bifurcation sets of C2H6and CO oxidation reactions. In the current presentation, we present a systematic procedure for parameter estimation aimed at predicting the limit points of the cusp surface. Fitting their locus in the space of a state variable vs operating conditions (Figure 1)yields a good approximation for the whole surface. The rate vanishes along the concentration axes. At high values of temperature or of a noninhibiting concentration,the rate is limited by mass and/or heat transfer. We fit the projections of limit points into (x&) and (Cb2,Cbl) planes by identifying special features like extremum and cusp points and high-concentration asymptotes. The defining conditions of these points are derived and then applied for parameter estimation. That usually overdefines the problem unless the uncertainty in the location of these points is accounted for. Our search is aimed, therefore, at satisfying all defining conditions within the uncertainty domain of the experimental data. The system employed is nonisothermal ethylene oxidation on a single pellet. The experimentation is performed in a completely automated mode by scanning the (cbl,Cb& operation plane. The methodology of automated tracing of multivalues rate curves is outlined and demonstrated, in another publication (Sheintuch et al., 1988), employing olefins oxidation on an isothermal Pt wire as a model family of reactions. A uniform concentration grid was employed there, and the resulting estimate for the limit point location was not always satisfactory. Reruns, at higher resolution over a narrower domain, had to be manually initiated. The methodology is improved here by incorporating a new feature into the algorithm: after the 0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 1153
w iL? C O M E SCAN
DETERMINE
Figure 1. Cusp surface in the space of state variable (n) vs reactant concentrations and projections of the bifurcation set. Automated experimentation is possible only by varying Cbz. Singular points employed for parameter estimation are denoted.
ignition and extinction points in the coarse scan are identified, a high-resolution scan is automatically initiated in the vicinity of these points. We suggest, therefore, that the computer is better employed for data acquisition rather than for the parameter estimation procedures; that is contrary to the methodology employed in most studies.
Experimental Section Feed composition is achieved by mixing streams of ethylene (from Merck, Munich) with oxygen and nitrogen. A flow controller (Union Carbide, FM 4550) maintained the flow rates at set points that are determined by signah conveyed from the governing (Commodore, PET 9000) microcomputer. The feed is dispersed by a sintered glass and flows through a cylindrical tube. The spherical pellet, 4 mm in diameter with a thin external coating of platinum on alumina (0.3% in average, Engelhard, NY), is placed in a cylindrical microoven. The latter is heated by resistive heating of a 0.5-mm NiCr wire (see Figure 1 in Schmidt and Sheintuch (1986)). The pellet temperature is measured by a (Omega, Type K) thermocouple inserted through a 0.4" hole drilled into the pellet. A new pellet was activated by exposure to air for 2 h at 690 K followed by 8 h at 636 K. Methodology and Experimentation The steady-state manifold in Figure 1 may be constructed from bifurcation diagrams obtained by sweeping c b 2 back and forth (the primary variable) at fixed values of Cbl (secondary variable). For the specific layout in Figure 1,which corresponds to catalyst temperature ( x ) dependence on oxygen (&) and ethylene )I&( concentrations, the identity of the primary and the secondary variables is important. Sweeping Cbl as the primary variable usually reveals only the upper or the lower steady-state branch, since the other branch is isolated. A complete hysteresis is obtained only by sweeping Cb2. Once the user defines the domain of interest (0-5070 oxygen, 0-2% ethylene) and the identities of the primary (oxygen) and secondary variables and certain technical constants, the governing algorithm automatically completes the mapping of the temperature surface by following a sequence of steady states. That sequence may form a uniform grid (Sheintuch et al., 1988) or be designed to optimize the model discrimination or parameter estimation
DIRECTION OR
I
FINE SCANNING REESTABLISH CONDITIONS
Figure 2. Flow chart of the algorithm for experiment design.
process. Since our experiment is aimed at improving estimates of the limit points, we employ two approaches to that end (Figure 2). The concentration step size is redetermined at each step from the first and second local derivatives of the bifurcation diagram. The derivatives are obtained by fitting the last three points to a cubic polynomial. That approach is aimed to yield a forward prediction for the location of the infinitely sloped limit point; due to the highly nonlinear shape of the curve, the prediction was usually ineffective and the system usually performed the largest step allowed. The second approach is based on identifying the ignition or extinction points and rerunning the experiment at a higher resolution near those points. Transitions were identified by a relatively sharp increase (or decrease) in temperature. Many points were identified as limit points in the coarse scanning due to a conservative criteria that was employed: a temperature change that is 3 or more times steeper than the previous temperature gradient (Figure 3). In fact at 0.6% CzH4 (near the cusp point), almost all incremental steps in the come run were identified as ignition points. The fine scan revealed that the diagram is continuous. Once the coarse the system returns, in scanning is complete (at 50% 02), two steps, to the conditions prior to the first identified ignition: conditions that are known to yield the lower branch are established (no oxygen) for several minutes before the change to the desired point is achieved. The sampling process is repeated then with smaller steps. If another ignition point is identified in the coarse scan, then the system moves now to fine sample the second domain.
1154 Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 Ts -Tb
lo,
183°C
20
VOI
%02
( O K 1
i
301 d
1
20
35 YO1 %02
' I
CbI(C2H4voI%) 0
$0
20
30
40
50
60
0
10
30
20
40
50
60
cbzlo2,vol%:
Figure 3. Bifurcation diagrams computed and measured by varying oxygen concentration with 0.6 (a) or 1.6 vol % C2H4(b). Lines were computed with eq 4 using parameters shown in Table I.
I 200
J
1
I
I
I
150:
i
i
e
I
05
I
I
,
10 15 cbt ( CzH, VOI
20 '/o)
Figure 4. Comparison of projections of the measured and computed bifurcation set; n denotes the inhibition order of the LangmuirHinshelwood rate expression (eq 1).
A similar approach is applied to determine the extinction points. The good reproducibility of results in the two scans is evident in Figure 3. This approach proved to be very effective, and the bifurcation set is determined by a resolution of 1% oxygen. The projections of the limit points to the (T,-Tb,Cbl) and (Cb2,Cbl) planes are shown in Figure 4.
Model Discrimination We analyze the qualitative features of the bifurcation diagrams and set, following the procedure suggested by Harold et al. (1987): The diagram of T, vs oxygen concentration (&) is inversely unique; i.e., only one Cb2 can sustain a certain surface temperature. Thus, the data may be accounted for by a monotonic dependence of the rate of Cb2. Bifurcation diagrams with varying ethylene concentrations (cb1)may be constructed by taking constant-oxygen cross sections of the T,vs c b 2 experimental diagrams. These diagrams show inverse multiplicity, and the rate expression must exhibit, therefore, a local maximum. The
Figure 5. Bifurcation diagrams with varying Cbl, obtained from cross sections of the cusp surface, showing a continuous upper (a) or lower (b) branch at high or low CbZ, respectively.
upper branch is continuous at high oxygen concentrations (Figure 5a), while it is discontinuous at low oxygen concentrations (Figure 5b). The bifurcation set exhibits a positive slope (Figure 4) along the extinction branch, while the ignition branch shows an opposite dependence. The curve should be continuous, except at the cusp point, and thus there exists a limit point where the slope changes sign. This isola point typically does not coincide with the cusp point. The reaction rate with respect to C1 at the isola point is zeroth order. We conclude again that the rate is a nonmonotonic function of C1. The local reaction order is negative at the ignition points and positive at the extinction points. With this information, we suggest the rate expression kClmlC2mz r= (1) (1 + KlCl + K2C2)" which is in agreement with other studies of ethylene oxidation in Pt (Mandler et al., 1983). Since oxygen inhibition is not evident, we assume K2 = 0. The mass and enthalpy balances on the pellet account for film resistances and reaction: kciQv(Cbi- c,)= a$' (2) ha,(T, - T,) = ( - W r + Q (3) with i = 1, 2 for ethylene and oxygen, respectively (a, = 1, a2 = 3). The relation between the two reactants c b 2 C2 = 3kcl/kc2(Cbl- C,) is important only at high ethylene concentrations, near the extinction line; at other conditions, excess oxygen can be assumed. For that reason and since the molecular weights of the reactants are similar, we assume kcl = kc2. The rate of heating by the oven is Q. In the absence of reaction, Q = hav(Tb- Ta),and we use it to define an equivalent bulk temperature (Tb). Incorporating these assumptions, we derive the following steady-state function of the state variable T, or of x : F X - [(I - X)m1(Cb2- ~ C ~ I X ) ~exp(-E,/RT,)]/ ~DU [I -t K1 eXP(-Ez/RT,)Cbi(l - X ) ] " (4) The degree of conversion is x = (T,- Tb)/XCbl and XCbl is the adiabatic temperature rise where X = (-WkCl/h. The latter may be found from the high concentration asymptote of the rate curve(s), yielding 128 "C for 1 vol % ethylene. There are three exponents (ml, m2,and n) and four parameters to be estimated: Da = ko/kclav,K,, El, and E2. We estimate the parameters for an assumed set of integer exponents.
Parameter Estimation Singular points are defined by several conditions, and in many situations these defining conditions can be com-
Ind. Eng. Chem. Res., Vol. 27, No. 7 , 1988 1155 bined into one equation with fewer parameters. Thus, our methodology is aimed at fitting these singularities. The bifurcation set of the steady-state function F(X,Cb;,Cb2,P1...pn) = 0 is defined by F = F, = 0, which describes a line in the (X,Cbl,Cb2) space. We may use the steady-state equation to eliminate a parameter (like Da) that can be expressed explicitly,p1 = pl(X,cbl, ...,pn). The bifurcation set x = X(Cbl,Cb2) can then be fitted to yield p2...pn.The major disadvantage of this approach is the relatively high uncertainty associated with x (or T,) at the limit point. This approach does not assure good fit of the projection of the bifurcation set to the (Cb2,Cbl) plane. Thus, subsequently we fit this projection to the equation derived from F = F, = 0 by eliminating x (or T,). This provides an estimate of pl, and if the agreement is unsatisfactory, the first approach can be reiterated within the uncertainty domain of x . Singular points in the (X,Cbl,Cb2) space, like the cusp point, provide three conditions and enable the successive estimation of three parameters, as the following application demonstrates. The number of parameters in the bifurcation set can also be reduced by using asymptotic solutions, e.g., at low temperatures the ignition points can be described by an lth order ( I = ml - n) reaction. For ml = 1and E2= 0, values that will soon be justified, the bifurcation set, after elimination of Da, is nKICbl El 3m2Cbl~ -- 1 ZACbl X(1 - X ) 1 + KlCbl(1 - x ) RT, cb2 - 3cblx (5) In excess oxygen, c b 2 >> 3Cbl, the last term is negligible, and another parameter (m2or the term DaCb2"'*) is eliminated. The estimation procedure is divided into two steps: the (T,,Cbl) projection should yield the parameters n, K1, and El, while the projection of limit points into (cbp,cbl) yields m2 and Da. We identify now singularities and asymptotes along the experimental bifurcation set and derive their defining condition, to be used in the estimation process (see location in Figure 1). High Concentration Asymptote. At large Cbl, the ignition surface temperatures (T, - Tb) approach a constant value (50 "e,Figure 4). For KlCbl(1 - x ) >> 1, the bifurcation set (at excess oxygen) can be written in the form
+
That is the bifurcation set for an (1 - n) apparent reaction order. The two parameters of the equation can be extracted from the appropriate linear plot (Figure 6). Extremum Point of the (T&bl) Projection. To fiid the minimum of the limit point, let us rewrite the bifurcation set in the form
where c
'
1
p.
1
0°00
2oool 0
1
0
~
~
~I
,
,
,' , , , I ,
, I
~
~
~
~
1000 2000 3000 4000 T:/l xcb- T, t T b )
Figure 6. Fit of the ignition points a t high concentrations by a Ith order (1 - n) reaction (eq 6).
At excess oxygen, B is negligible and the equation is quadratic in Cbl(1 - x ) . The conditions for c b l and T, at the minimum can be derived analytically, and their experimental values may be employed for estimating the two parameters. The cusp point, defined by F = dF/dx = d2F/dx2= 0, lies in the domain of excess oxygen. It is defined by the quadratic equation in a = E1ACbl/RT,2
(
9
2x-1-=0(9) x2(1 - x ) 2 and eq 5. For given coordinates of the cusp point (T,, Cbl, and &), the three parameters El, K1,and Da are estimated successively from eq 9, 5, and 1 (respectively). The isola point, defined by F = dF/dx = dF/dCbl = 0, occurs at x = 1/a 1/Cbl = (n - 1)(1 - x)Kl (10) and Cb2 is derived from eq 5. The separation of isola and cusp points must be very small to escape experimental detection (Figure 4). When these two points coalesce, at a pitchfork singularity, four defining conditions apply (F= dF/dx = d2F/dx2= dF/dCbl = 0) and four parameters may be estimated. To find a singular point it is necessary to vary a third operating condition (e.g., Tb). The proximity of the isola and cusp points suggests that the coordinates at 183' provide a good initial estimate for the parameters. Substituting x = 1 / a in eq 9 yields the relation -T-b - 2nx(1 - X ) 2 - x (11) x2 - 2nx + n A% For a given x , we can find c b 1 and then in successive order E, (from ax = l ) , K1, and Da. With absolute accuracy of the singular points, these conditions overdefine the four-parameters estimation problem. The estimation problem should be restated, therefore, to find a set of parameters that satisfies all the conditions within the uncertainty domain associated with their determination. While the uncertainty of c b 2 at the limit points has been reduced to 1 vol % , the associated uncertainty in the corresponding temperature may be significant due to the infinite slope at the limit point. From the projections of the bifurcation set, we position
l
1156 Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 Table I. Information and Iterations in the Estimation Process info based cusp data iterations on cbl x K1 remarks 3 0.7 5365 1.5 EI/R too large to pitchfork cusp 3 0.7 0.5 7700 0.7 account for ignition PtS 0.75 0.6 2193 2.71 isola at Cbl = 1.15, 0.7 0.55 4000 2.1 adjacent isola (0.7); Figure 4 ignition pta 3 2720 2.67 see line in Figure 6 pitchfork 2 0.72 6300 E 1 / Rtoo large to 0.67 5300 account for ignition cusp 2 0.7 0.55 5400 PtS 0.7 0.59 4200 4.26 adjacent isola, see Figures 3 and 4 0.7
ignition pts 2
0.60
3500 4200
4.26
see line in Figure 6
the cusp point at around cbl = 0.7% f 0.05% with a corresponding temperature gradient of 45-57 "C. The kola point must be very close to the cusp. The list of iterations is tabulated in Table I. Most tempting, of course, is to use the pitchfork coordinates (eq 11)which yields x = 0.52 in a wide range of concentrations around cbl = 0.7. The estimated parameters are very sensitive to the location of the cusp point and the corresponding temperature. Plotting the ignition points in the coordinates of an lth-order reaction (eq 6 with 1 = 1 - n, Figure 6) provides us with the following estimates: At large cbl we find 1 = -2, E1/R = 2800. This slope motivated us in considering n = 3 (with ml = 1). The value of E1/R along the ignition line is somewhat lower than that at the cusp, suggesting that E2 > 0 and is small. Positive E2 is contrary to the interpretation that Kiis the adsorption coefficient and should decline with temperature. That justifies our choice of E2 = 0. These values still lead to a wide separation of the cusp and isola points. Proximity of these two points may be achieved with E1/R = 4000 at the expense of poorer presentation of the ignition branch (Figure 6; move the n = 3 line parallel to itself to intercept at 4000). Better approximation of the ignition line is achieved with n = 2 and E1/R = 4200 (Figure 6). That is within the range of values derived from the cusp point (Table I), and the isola point is within a close proximity. Both sets of parameters (n = 2 or 3) yield similar approximation of the (T,,Cbl) projection (Figure 4). The deviation from experimental results is within the uncertainty associated with T,. We need to estimate now the remaining parameters and find if a better discrimination may be achieved from the (Cb&,l) bifurcation sets. We estimated Da by fitting the extinction line at the vicinity of the cusp point. The second-order inhibition model ( n = 2) yields good agreement with the experimental observations and is certainly superior to n = 3 (Figure 4) in describing the extinction line. To describe the ignition line, let us derive analytically the bifurcation set at large Cbl; we already showed that along the ignition line T, approaches a constant and thus x = (T, - Tb)/Cbldeclines like 1/Cb From the rate equation, we find that c b 2 varies like Cbln-'lm2. With m 2= 1,the ignition line is parabolic at n = 3 or linear at n = 2. The experimental curve suggests n - 1< m2.We employed m2 = n - 1to account for the bifurcation sets shown in Figure 4. The ignition line is better described by n = 3. Employing m2= n = 2 may improve the fit of the ignition line at the expense of the extinction one. The parameters estimated from the bifurcation set yield, as expected, good approximation of the whole bifurcation
40
t
i
'0
10
20
30
0
20 C~*LO,VOl.~/. 10
30 0
I IO
I 20
30
)
Figure 7. Bifurcation diagrams at various temperatures and 1.2 (upper row) or 2 vol % C2H, (lower row).
10
c b l ( C2H4 VOI
20
'10 )
Figure 8. Bifurcation sets at 239 (a) and 208 "C (b).
diagram: The lines in Figure 3 were computed with n = 2, E1/R = 4200. A deviation is evident only near the cusp point where the computed curve approaches the high concentration temperature more moderately than the experimental data.
Concluding Remarks The algorithm employed was found to be effective and time saving in automated tracing of the bifurcation diagrams and set of bistable systems. In the case of tristability, the intermediate steady state may be completely or partially nested within the main hysteresis loop. The data at 2% C2H4and 183 or 208 "C suggest the existence of such an intermediate branch (Figure 7 , data points along this branch are shown). To establish that conclusively, it would have been necessary to fine scan the intermediate branch with decreasing Cb2. The algorithm, however, is not designed to establish intermediate branches. The second hysteresis loop is evident at 208 "C and 1.2% ethylene (Figure 7 ) when it lies outside the large loop. Tristability may exist at 183 "C only at large concentrations (Cb2 > 2%). It is clearly evident at 208 OC, and the bifurcation set is made of two overlapping cusps (Figure 8). We could not trace the second extinction line (broken line) except in the vicinity of the cusp point. Tristability is not evident at 239 "C (Figure 8). The proposed model (eq 4) may account for three stable steady states (Tsotsis et al., 1982) for certain parameters. That implies that the cusp condition (eq 9), which is quartic in x , should acquire three solutions. The parameter estimation procedure becomes more discriminatory as it in-
Znd. Eng. Chem. Res. 1988,27, 1157-1162
volves now more defining conditions. Since the evidence for tristability is not conclusive at 208 "C,we do not pursue this avenue. Acknowledgment Work was supported by the Israel Academy of Sciences and Humanities. Nomenclature a = stoichiometric coefficient a , = surface-to-volume ratio ci,Cbi = surface and bulk concentration, vol % Da = Damkohler number E i / R = activation energy, K F = steady-state function h = heat-transfer coefficient K i= adsorption equilibrium coefficient k , ko = reaction rate constant and preexponential factor, respectively k , = mass-transfer coefficient 1 = apparent reaction order m = order of reaction n = inhibition order Q = heat input by oven r = reaction rate T,, Tb, T , = ambient, apparent bulk, and solid temperature, respectively x = degree of conversion
1157
Greek Symbols dimensionless exothermicity, C b l E I X / R T ~ (-AH) = heat of reaction X = adiabatic temperature rise per 1 vol % Cbl
a=
Subscript i = species i Registry No. Pt, 7440-06-4; C2H4, 74-85-1.
Literature Cited Froment, G. F.; Bischoff, K. B. Chemical Reactor Analysis and Design; Wiley: New York, 1979; Chapter 2. Froment, G. F.; Hosten, L. In Catalysis Science and Technology; Anderson, J. R., Boudart, M., Eds.; Wiley: New York, 1984;Vol. 2,p 97. Harold, N. P.; Luss, D. Ind. Eng. Chem. Res. 1987a,26,2092;1987b, 26,2099. Harold, N. P.;Sheintuch, M.; Luss, D. Ind. Eng. Chem. Res. 1987, 26,783. Hershkowitz, M.; Kenney, C. N. Can. J. Chem. Eng. 1983,61,194. Mandler, J.; Lavie, R.; Sheintuch, M. Chem. Eng. Sci. 1983,38,979. Schmidt, J.; Sheintuch, M. Chem. Eng. Commun. 1986,46, 289. Sheintuch, M.; Schmidt, J.; Rosenberg, S. submitted for publication in Ind. Eng. Chem. Res. 1988. Tsotsis, T. T.; Haderi, A. E.; Schmitz, R. A. Chem. Eng. Sci. 1982, 37, 1235.
Received for review July 1, 1987 Revised manuscript received January 1, 1988 Accepted January 21, 1988
The Effect of Copper and Iron Complexation on Removal of Cyanide by Ozone Mirat D. Gurol* Department of Civil Engineering and Environmental Studies Institute, Drexel University, Philadelphia, Pennsylvania 19104
Timothy
E.Holden
Versar, Inc., 6850 Versar Center, Springfield, Virginia 22151
Oxidation of cyanide by ozone in basic aqueous solution was studied in the absence and presence of copper and iron. Free cyanide was oxidized via a fast reaction under mass-transfer-limited conditions. Copper catalyzed the oxidation of cyanide further by entering into an oxidation-reduction reaction. Oxidation of cyanide consumed equal moles of ozone and produced equal moles of cyanate. Upon prolonged ozonation, cyanate was converted to carbon dioxide. Complexation of cyanide with iron hindered the oxidation reaction. Under the experimental conditions, oxidation of each mole of iron-complexed cyanide to carbon dioxide required the consumption of more than 30 mol of ozone. Cyanate was not detected during ozonation of iron-cyanide complex. Complete destruction of free cyanide ion in aqueous solution has been reported unanimously by numerous researchers (Gurol and Bremen, 1985; Rowley and Otto, 1980; Zeelvalkink et al., 1979; Matsuda et al., 1975a,b; Balyanskii et al., 1972; Sondak and Dodge, 1961a,b; Khandelwal et al., 1959; Selm, 1959; Walker and Zabban, 1953). However, there is disagreement as to the effects of metal complexation of cyanide on the reaction rate. For example, copper as Cu(I1) was found to catalyze the oxidation of cyanide (Matsuda et al., 1975a,b; Khandelwal et al., 1959). It was believed that Cu++formed an oxide with ozone; subsequently, Cu++was regenerated by a reaction of the oxide with cyanide (Khandelwal et al., 1959). Matsuda et al. (1975a,b) observed a maximum in the reaction rate which corresponded to a Cu(I1) concentration 0888-5885/88/2627-1157$01.50/0
of 3 mol/L. Khandelwal et al. (1959) however, reported that increasing the Cu(II) concentration did not markedly increase the reaction rate. Sondak and Dodge (1961a,b) studied the reaction in the presence of copper, iron, manganese, and vanadium and claimed no noticable effect of any of these metals on the reaction rate. On the other hand, iron complexes of cyanide in plating wastes were found resistant to ozonation by Mauk et al. (1976) and Streebin et al. (1981), who recommended coupling UV radiation with ozonation for complete destruction of iron-cyanide complexes. This study was undertaken to understand the effects of complexation of cyanide with copper and iron on the kinetics and the mechanism of cyanide oxidation by ozone in aqueous solution. It was hoped that the findings would 0 1988 American Chemical Society
