Ind. Eng. Chem. R e s . 1988,27, 1149-1152 (a) the particle size does not modify the hydrocarbon yields obtained in the 0.21-0.84-mm-diameter range of almond shells, (b) the hydrocarbon yields increase with temperature in the 770-905 OC range, and (c) the different chemicals impregnated in the almond shell samples do not increase the hydrocarbon yields. 2. High gas yields can be obtained in a sand fluidized bed reactor with the following operating conditions: sand particles of 0.105-0.210-mm diameter, almond shell particles of 0.297-0.500-mm diameter, inert gas flow rate of 3.6 cm/s through the sand fluidized bed, temperature greater than 850 "C, and volatiles residence time of 1-3 9.
3. The higher yields of hydrocarbons obtained in the fluidized bed reactor correspond to the experiments carried out a t a temperature of 890 "C and at a residence time of volatiles of 2.4 s. Under these conditions, the yields (dry basis) of the different compounds are the following: 8.3% CHI, 4% CzH4,1.5% Hz,45% CO, 0.7% CZH6,0.5% C3H6, 0.02% C3H8, 0.1% CZHZ, 0.2% C4, 28% COZ. 4. From the sets of experiments carried out in the fluidized bed reactor, it has been deduced that the production of the different compounds can be explained taking into account the CO yield, CHI and CO are produced mainly from reactions which are similar. The hydrocarbons of three and four carbon atoms undergo cracking reactions, which lead to the increase of the C2H4 and H2 yields. 5. The yields obtained with the fluidized bed reactor are greater than those from the Pyroprobe 100. Cracking reactions of the volatilized tar and probably a better heat transfer in the fluidized bed reactor would explain the difference indicated above. R e g i s t r y No. NaOH, 1310-73-2; NaCI, 7647-14-5; KC1, 7447-40-7; CdCIz,10108-64-2; CaClz, 10043-52-4; BaClZ,10361-37-2; MnCl,, 7773-01-5; ZnCl,, 7646-85-7; CuCIz, 7447-39-4; NiClZ, 7718-54-9; CoCIz,7646-79-9; CrCl,, 10025-73-7;NiS04, 7786-81-4; Hz, 1333-74-0;CO, 630-08-0; COZ, 124-38-9; CZH6, 74-84-0; C2H4, 74-85-1; C3H8,7498-6; CHI, 7482-8 C,&, 115-07-1;CzH2,74-86-2.
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Literature Cited Antal, M., Jr. "Effects of Reactor Severity on the Gas-Phase Pyrolysis of Cellulose and Kraft Lignin-Derived Volatile Matter". Znd. Eng. Chem. Prod. Res. Dev. 1983,22,366-375. Beaumont, O.,; Schwob, Y. "Influence of Physical and Chemical Parameters on Wood Pyrolysis". Znd. Eng. Chem. Prod. Res. Deu. 1984,23, 637-641. Diebold, J. P. Gasoline from Solid Wastes by a Noncatalytic Thermal Process. American Chemical Society: Washington, D.C., 1980; NO. 130, pp 209-226. Font, R.; Marcilla, A.; Verdfi, E.; Devesa J. "Fluidized-Bed Flash Pyrolysis of Almond Shells. Temperature Influence and Catalysts Screening". Znd. Eng. Chem. Prod. Res. Deu. 1986,25,491-496. Funazukuri, T.; Hudgins, R.; Silveston, P. "Product Distribution in Pyrolysis of Ceilulose in a Microfluidized Bed". J . Anal. Appl. Pyrolysis 1986a, 9, 139-158. Funazukuri, T.; Hudgins, R.; Silveston, P. "Correlation of Volatile Produds from Fast Cellulose Pyrolysis". Ind. Eng. Chem. Process Des. Dev. 1986b, 25, 172-181. Hajaligol, M. R.; Howard, J. B.; Longwell, J. P.; Peters, W. A. "Product Composition and Kinetics for Rapid Pyrolysis of Cellulose". Znd. Eng. Chem. Process Des. Dev. 1982,21,457-465. Nunn, T.; Howard, J. B.; Longwell, J. P.; Peters, W. "Product Composition and Kinetics in the Rapid Pyrolysis of Sweet Gum Hardwood". Ind. Eng. Chem. Process Des. Dev. 1985a, 24, 836-844. Nunn, T.; Howard, J. B; Longwell, J. P.; Peters, W. "Product Composition and Kinetics in the Rapid Pyrolysis of Milled Wood Lignin". Znd. Eng. Chem. Process Des. Dev. 1985b, 24,844-852. Rolin, A,; Richard, C.; Masson, D.; Deglise, X. "Catalytic Conversion of Biomass by Fast Pyrolysis". J. Anal. Appl. Pyrolysis 1983,5, 151-166. Ruiz, F.; Prats, D.; Marcilla, A,; "Activated Carbon from Almond Shells. Chemical Activation. 1. Activating Reagent Selection and Variables Influence. Znd. Eng. Chem. Prod. Res. Dev. 1984a, 23, 266-269. Ruiz, F.; Prats, D.; Marcilla, A. "Activated Carbon from Almond Shells. Chemical Activation. 2. ZnCll Activation Temperature Influence. Znd. Eng. Chem. Prod. Res. Dev. 1984b, 23,269-271. Shafizadeh, F. "Introduction to Pyrolysis of Biomass". J . Anal. Appl. Pyrolysis 1982, 3, 283-305.
Received for review June 11, 1987 Revised manuscript received February 2 , 1988 Accepted February 12, 1988
The Fate of Hydrazine in Pure, Deoxygenated, Aqueous Solutions at Elevated Temperatures and Pressures? George R. B r u b a k e r * and Michael M. Geoffrey Department of Chemistry, Illinois Institute of Technology, Chicago, Illinois 60616
The fate of aqueous hydrazine at temperatures between 373 and 573 K (212-572 OF) a t saturation pressures has been determined. Particular emphasis was placed on high-purity water and careful exclusion of oxygen. At least 97% of the initial hydrazine charge was recovered after up to 3 h in a passivated 316 stainless steel reactor. Statistical analyses indicate that hydrazine recovery is independent of mass fraction of hydrazine in the vapor phase as well as temperature, pressure, and time. The use of dilute aqueous solutions of hydrazine as an oxygen scavenger and passivating agent in boiler feedwater and boiler water treatment for commercial steam generators is widely practiced. Of interest is the extent of hydrazine decomposition at the temperatures and pressures of operation and in a controlled environment. This work was presented in part at the Hydrazine Centennial Conference, Division of Inorganic Chemistry, 193rd National Meeting of t h e American Chemical Society, April 1987 (INOR 292). This work is also contained in part in the M.S. Thesis of M. M. Geoffrey.
0888-5885/8812627-1 149$01.50/0
The concentration of dissolved O2 found in the steam turbine loop of a commercial electric generating station is a function of the efficiency of mechanical deaeration, percent makeup or blowdown used, and the efficiency of chemical O2 removal. Correspondingly, the amount of residual hydrazine found in the steam turbine loop is a function of these same parameters. Residual dissolved O2 in the feedwater of the steam turbine loop may be reduced to less than 0.007 ppm by the use of countercurrent mechanical deaerators (Kesler, 1978). Blowdown or makeup in these systems varies greatly. Schmidt reports 60% makeup for the Olin Chemicals 0 1988 American Chemical Society
1150 Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988
owned Doe Run plant and 7540% makeup for O h ’ s Lake Charles plant (Schmidt, 1984). The Babcock & Wilcox (1987) “Babcock-205 NSS” design employs a “OnceThrough Steam Generator (OTSG)”, which is effectively a 100% makeup system. Typical pressurized water reactor designs specify essentially zero makeup (Pederson, 1978). Baker and Marcy (1956) reported average hydrazine concentrations of 0.009 ppm in the saturated steam of a 1350 psi (9.18 MPa) boiler at 772 K and 0.004 ppm in the condensate for a feed concentration of 0.03 ppm. Baker and Marcy (1956) also reported hydrazine concentrations in the condensate of a 115 psi (0.78 MPa) boiler at 447 K which averaged 0.87 ppm, for feed concentrations averaging 12.6 ppm. Schmidt (1984) reports data, from Olin Chemicals’ Doe Run and Lake Charles Plants, which indicates r e s i d d hydrazine concentrations ranging from 0.05 to 0.3 ppm. Work performed by Ellis and co-workers showed that the rate of hydrazine decomposition, as a function of temperature and the relative concentrations of hydrazine and oxygen, is slow and complex up to temperatures of 80 OC (353 K) (Ellis et al., 1960). In contrast, Baker and Marcy (1956) concluded that at 772 K (9.18 MPa saturation pressure) hydrazine reacts rapidly with dissolved 02. Design data for Commonwealth Edison’s (1974) Byron and Braidwood PWR facilities show a secondary loop steam flow rate of 3.78 X lo6 lb/h (1.72 X lo6 kg/h) at a pressure of 990 psia (6.73 MPa), a temperature of 543.3 O F (557 K), and a maximum moisture content of 0.25 wt %, for a feedwater temperature of 440 O F (500 K). Calculations on the energy flux from these data indicate an approximate secondary loop residence time of 10 s at 100% efficiency. The area of the secondary loop is calculated from enthalpy data (Hawkins, 1978). The mean secondary loop volume is calculated from heat capacity data (Powell, 1970). The mean residence time is calculated based on the mean specific volume of the steam. This report presents the results of experiments performed using deoxygenated, ca. 8.5 % aqueous solutions of hydrazine, in pure water and at temperatures ranging from 473 to 573 K at saturation pressures. The times at the specified temperature, for each experiment performed, ranged from 1 to 3 h or 360-1080 times the estimated residence time in the boiler turbine circuit. Experimental Section
Water employed in these experiments was doubly deionized by passing previously deionized water through a Barnsted Ultrapure DI cartridge. Resistivites were maintained between 1 and 2 MDcm. AU glassware employed in these experiments was cleaned with a commercial detergent (typical household white detergent), rinsed with doubly deionized water, soaked in 2% aqueous NaJEDTA), and rinsed with 10 rinse volumes of doubly deionized water. Glassware requiring cleaning with dichromate cleaning solution was soaked in 2% aqueous N%(EDTA) for a minimum of 2 h between rinses. Glassware, when not in use, was stored in 2% aqueous Na,(EDTA) and sealed against the atmosphere. Residual hydrazine was removed from analytical glassware by washing with household chlorine bleach solution followed by dilute aqueous ammonia before repeating the normal cleaning procedure. All glassware used in these experiments was Pyrex. Solutions of ca. 8.5% aqueous hydrazine were prepared using purified grade 85% hydrazine hydrate solution (Fisher Scientific Co., catalog number H 318-100) and doubly deionized water. Solutions were sparged with
high-purity-grade argon, sealed, and refrigerated between uses. All reactions were run in a 316 stainless steel reactor having a contained volume of 85 mL. The reactor was passivated using a 1% aqueous hydrazine solution and an air sparge for 2 h at 453 K (Key et al., 1976). The reactor design is presented schematically in Figure 1 and the components are listed in Table I of the supplementary material. Deoxygenated, ca. 8.5 % aqueous hydrazine was transferred under argon, at a pressure slightly in excess of atII passivated reactor. mospheric pressure, into ~evacuated The reactor was then placed into a tube oven and brought to the test temperature. Temperature control was maintained at *4 K via an iron/constantan thermocouple in contact with the outer surface of the reactor and an Omega Model 4001JC temperature controller. The test temperature was maintained for a set time, a t the end of which the heated solution was thermally quenched employing dry ice and 2-propanol and under an argon atmosphere. In the first set of experiments, aqueous hydrazine solutions were stored in a volumetric flask and were transferred through the air, into a graduated separatory funnel. This separatory funnel facilitated sparging of the solutions with argon and transfer of the solutions into the reactor. Quantitative analysis of reactant and product hydrazine concentrations was performed by the Jamieson iodometric titration as described by Penneman and Audrieth (1948). The experimental setup for charging the reactor is shown schematically in Figure 2 and the components are listed in Table I1 of the supplementary material. As our familiarity with the experiment increased, we made improvements in the method. In all subsequent experiments, the aqueous hydrazine solutions were stored in a specially designed vessel which facilitated sparging, storage, and transfer. The design of this vessel eliminated the exposure of the aqueous hydrazine solutions to the air once they were sparged. Quantitative analysis of reactant and product hydrazine concentrations was performed spectrophotometrically using p(dimethy1amino)benzaldehyde as the indicator (Watt and Chrisp, 1952). The experimental setup for charging the reactor is given in Figure 3 and the components are listed in Table I11 of the supplementary material. The setup for quenching the reactions is shown in Figure 4 and the components are listed in Table IV of the supplementary material. Results
Our working parameters and statistically significant results are summarized in Table I. Data for percent hydrazine unreacted are reported as the average of no less than three analytical determinations plus/minus the error at the 95% confidence level. The first data were collected a t 473 and 523 K, and hydrazine was quantified by iodometric titration. A preliminary review of these data indicated that hydrazine, under the conditions of these experiments, was unreactive. This result obviated the requirement of a rigorous statistical experimental design; since hydrazine was found to be unreactive, we expected that verification of this result at our 573 K temperature limit together with reasonable replicates at lower temperatures should produce statistically significant results. In order to establish statistical significance, the Q test (Gordon and Ford, 1972) was chosen for the first pass sorting of data and the F test (Chao, 1969) was then applied to establish the equality of the variances involved. In applying the F test, several questions should be asked: are the results independent of the experiment, analytical method, and temperature?
Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 1151 Table I. kscovery of ca. 8.5% Aqueous Hydrazine reaction time, h
vol reacted, mL
1 2 3 4
1 1 2 3
25 25 25 25
5 6 7 8 9
3 3 3 3 3 3 3 3
65 30 65 65 65 23 55 55
sample
10 11
12
saturation pressure, MPa temp, K Analysis of Jamieson Titration 473 1.55 523 3.98 523 3.98 523 3.98
1.5 4.4 4.4 4.4
Analysis by Spectrophotometric Determination 373 0.10 0.016 1.55 1.3 473 523 3.98 7.7 523 3.98 7.7 523 3.98 7.7 573 8.59 12 573 8.59 0.076 8.59 0.076 573
Table 11. Babcock & Wilcox Babcock-205 NSS Recommended OTSG Feedwater Specifications max total solids, ppb max cation conductivity, Q-’ cm-l max dissolved oxygen, ppb max total silica, ppb max total iron, ppb max total copper, ppb pH at 77 OF (adjusted with ammonia)
mass % vapor phase
50 0.5 7 20 10 2 9.3-9.5
Testa of significance were first applied to data sets comprised of different experiments performed at the same temperature. Tests for significance among temperatures were also performed, as appropriate. Q tests indicated retention of all data. Application of the F test resulted in an F(exp), for comparison of the data among temperatures, equal to 2.16 with an F(o.os,lla of 4.03 (Chemical Rubber Company, 1962). Pooling of data produced an average hydrazine recovery of 97% with a standard deviation (spooled) of 2.5. As a comparison to actual process conditions, Table I1 presents typical- water quality specifications from design data for the Babcock & Wilcox (1987) Babcock-205 NSS OTSG system. Since the resistivity of water used in our experiments was maintained between 1 and 2 Ma, the total concentration of ions in the system would typically be less than equiv/L. Calculation of ion concentration in equivalents per liter employed a degree of ionization of unity, a limiting cation conductance for 0.5Pb equal to 61 ohm-l cm-l, and a limiting anion conductance for C1 equal to 65 ohm-l cm-’ at a temperature of 18 OC. The pH of the 8.5% aqueous hydrazine solutions ranged from 11.65 to 12.05. Henry’s law calculation a t 25 “C using a 0.21 volume fraction of oxygen in dry air yields an equilibrium dissolved oxygen concentration in pure water of 2.62 X lo4 M. When the initial hydrazine concentrations used in these experiments are employed, the expected residual hydrazine assuming complete reaction of dissolved oxygen ranges from 99.986% to 99.984%. With respect to dissolved oxygen in the volumes of solutions we actually used, the effect of sparging is undetectable. With respect to the head space available in the reador, the effect of omission of an inert atmosphere would also be undetectable. On the basis of the volumes of solution reacted, a 0.21 volume fraction of oxygen in dry air, and complete reaction of all oxygen present, including dissolved oxygen, the residual hydrazine expected ranges from 99.843% to 98.723%. Our pooled data yield 97% ( f 2 . 5 ) hydrazine recovered. Our data also include mass percent of hydrazine in the vapor phase. Regression analysis of mass percent of hy-
pH of reacted solns
% hydrazine unreacted
98 83 92 89
2
*1
11.20 11. 60 11.85
*1 *9
99 f 3 99 5 97 3 96 f 3 99 4 99 f 3 95 3 92 3
* * * * *
drazine in the vapor phase versus percent hydrazine recovered for the pooled data set yielded a correlation coefficient of 0.469 based on eight data points. Discussion
Aqueous solutions of hydrazine in a pure, deoxygenated system are stable at temperatures of up to 573 K at saturation pressures for up to 3 h. Since our data show limited reactivity of aqueous hydrazine solutions at high temperature, extrapolation of these data through 373 K is reasonable. Significance testing of the data indicates that the stability of aqueous hydrazine is independent of temperature up to 573 K, time up to 3 h, and volume. The extent of decomposition of hydrazine, under the conditions of these experiments, is independent of the mass fraction of hydrazine in the vapor phase. This indicates that, in the absence of oxygen and in a passivated reactor, hydrazine vapor is stable at temperatures up to 573 K at saturation pressures. For pure water containing 0.007 ppm of dissolved O2and employing a 1:l stoichiometry for the reaction of hydrazine and oxygen, 0.007 ppm of hydrazine would be required to completely eliminate the dissolved oxygen. Therefore, hydrazine residuals in excess of that reported from field studies would be expected. The results of our experiments indicate that in a steam turbine loop there exists a continuous source of dissolved oxygen or, in the absence of dissolved oxygen, catalysts are present which provide for the decomposition of hydrazine. We are now exploring the extent of hydrazine decomposition in a deoxygenated system, but in the presence of transition-metal ions typical of the materials of construction of a boiler-turbine cycle. Acknowledgment
This work was supported by the IIT School of Advanced Studies and by the IIT Department of Chemistry. Registry No. HzNNHz, 302-01-2. Supplementary Material Available: Schematic drawings and tables containing associated components lists for those experimental setups presented in the text (9 pages). Ordering information is given on any current masthead page. Literature Cited Babcock & Wilcox, Co. “Babcock-205NSS Design Summarv”, 1987 pp 5-7; Augusta, GA. Baker. M. D.: Marcv. V. M. A S M E Trans. 1956. 78. 299. Chao, L. L. Statistiis: Methods and Analyses; McGraw Hill Book Company: New York, 1969; pp 294-300. Chemical Rubber Company Handbook of Mathematical Tables; Chemical Rubber Company: Cleveland, OH, 1962; p 273. I
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
