Study on the Kinetics of Catalytic Hydrogenation of U(VI) in Nitric Acid

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Study on the Kinetics of Catalytic Hydrogenation of U(VI) in Nitric Acid Solution using a Bubble Reactor Niranjan K. Pandey, Ramakrishna Reddy, Satyabrata Mishra, Remya Murali, and Jyeshtharaj B Joshi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04293 • Publication Date (Web): 23 Feb 2018 Downloaded from http://pubs.acs.org on February 25, 2018

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Graphical abstract

Scanning electron microscopy image of Pt/SiO2 catalyst

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Study on the Kinetics of Catalytic Hydrogenation of U(VI) in Nitric Acid Solution using a Bubble Reactor 1

Niranjan K. Pandey, 1Ramakrishna Reddy, 1Satyabrata Mishra, 1Remya Murali, And 2,3Jyeshtharaj B. Joshi*

1

Reprocessing Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, TN – 60310, India 2

3

Homi Bhabha National Institute, Anushaktinagar, Mumbai - 400 094, India

Department of Chemical Engineering, Institute of Chemical Technology, Matunga, Mumbai 400 019, India

ABSTRACT The kinetics of hydrogenation/reduction of uranium [U(VI)] has been investigated using a platinum catalyst loaded on silica substrates in a bubble column reactor at atmospheric pressure and room temperature. Effects of catalyst loading, hydrogen flowrate and concentrations of nitric acid and hydrazine have been investigated. Several kinetic models are derived based on Langmuir-Hinshelwood mechanism and each of them are evaluated. The experimental results indicate that surface reaction between dissolved hydrogen and adsorbed reactants on the catalyst surface is the rate controlling in the hydrogenation of uranyl ion. In order to test the applicability of the developed rate expression, the mass balance equations for uranium, nitric acid and hydrogen around the bubble reactor have been solved numerically. The comparison of experimental and predicted results showed a reasonably good agreement.

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1. INTRODUCTION In the aqueous reprocessing of spent nuclear fuels by PUREX process1, uranium and plutonium are co-extracted by tributyl phosphate leaving the fission products in the raffinate phase. Further, separation of plutonium and uranium from each other is achieved by selectively reducing the plutonium [Pu(IV)] to poorly extractable [Pu (III)] oxidation state by means of a reducing agent. Among the various reductants, uranous [U(IV)] nitrate is widely adopted as a reducing agent for the U/Pu partitioning (separation) in the contemporary salt free nuclear fuel reprocessing flowsheet. In Indian reprocessing plants, [U(IV)] is produced by electrochemical reduction of U(VI) using Pt/titanium electrodes with hydrazine nitrate as a stabilizing/holding agent for uranous. Hydrazine nitrate acts as a scavenger for the nitrous acid which is produced by autocatalytic decomposition of nitric acid and prevents the re-oxidation of U(IV) to U(VI). The major drawback of the electrochemical method is its limited conversion efficiency (50-60%) which results in increased uranium processing load. Other disadvantages include slow kinetics, frequent recoating of electrode and secondary waste generation during decontamination of electrode as reported by Sahu et al.2 Other methods reported in the published literature are: reduction of U(VI) with hydrogen, formic acid and hydrazine in the presence of platinum catalyst loaded on alumina/silica substrates. Considerable amount of work is available in the published literature dealing withcatalytic reduction of U(VI),2-8however there is a lack of comprehensive information on the kinetic aspects of catalytic reduction of U(VI). The usual data reported so far are conversion obtainable under different process conditions with different catalysts in different solution media. For the design of any of the process equipment, it is important to know the diffusion characteristics, reaction kinetics and the extent of back-mixing in the system. Hence, it was thought important to undertake a systematic study on the kinetics of hydrogenation of U(VI) for the production of U(IV). A careful analysis of the available literature indicates that the systematic effort is needed to understand the roles of all the variables affecting the rate of hydrogenation of U(VI). There is a need to develop a kinetic model in such a way that it will incorporate all the mass transfer and intrinsic kinetics terms. In view of this, in the present work, an attempt has been made to understand the mechanisms of catalytic reduction of U(VI) with hydrogen and hydrazine using 2% Pt catalyst loaded on silica in semi-batch slurry reactor (bubble column reactor).The main objective was to determine (1) mechanism of reaction (2) intrinsic 2 ACS Paragon Plus Environment


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kinetics of hydrogenation of U(VI) using 2% Pt catalyst loaded on silica in semi-batch slurry reactor (3) mathematical model for the performance of semi-batch reactor under isothermal and atmospheric pressure. Such information will provide the starting basis for further development and optimization of U(VI) catalytic reduction reactor for pilot plant or commercial scale operations.

2. EXPERIMENTAL 2.1 Materials and methods Catalyst used in hydrogenation process was platinum (2%) loaded on silica catalyst in the form of small spherical particles which was supplied by M/s. Arora Mathey Ltd. Kolkata. U3O8powder was supplied by NFC Hyderabad, India. The catalyst particles were characterized and the results of characterization are in presented in Table 1 and Fig. 1. To observe the surface morphology of Pt/SiO2 catalyst SEM image was taken on table-top Mini- SEM, South Korean make (Model SNE-3000M) at room temperature. The SEM image of Fig. 1 indicates that the catalyst has a globular shape and possesses high surface area. Specific surface area of Pt/SiO2 was determined by using Surfer Gas Absorption Porosimeter (Thermo Fisher Scientific S. p. A. Milan, Italy) and the particle size distribution was analyzed with the help of a laser light scattering particle size analyzer (Mastersizer, M/s. Malvern Worcestershire, UK). Pt in the form of a catalyst participates in the reaction and provides an alternative reaction pathway to the reaction product. The rate of the reaction is increased as this alternative path has lower activation energy than the reaction path not mediated by the catalyst. Nitric acid used for the dissolution of U3O8 and hydrazine for stabilizing agent for uranous were of analytical grade.

2.2 Experimental setup and procedure The glass reactor (1) (ID= 35 mm, height= 450 mm) used for the catalytic reduction experiment is shown in Fig. 2. At the bottom of the cylindrical reactor, a zero size glass frit (2) was fused for sparging H2 gas into the solution (uranyl nitrate-hydrazine) of the reactor. The sparged gas was used for mass transfer as well as for creating sufficient liquid motion to keep the fine catalyst particles in suspended condition. Through tube (3) connecting to the bottom of cylindrical reactor, H2 or argon at fixed flow rate was passed into the solution to be reduced through the glass frit. The tube had two side valves, one just above the glass frit (4A) 3 ACS Paragon Plus Environment

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and another one at top portion (4B). Sampling and catalyst removal was carried out by opening the valve at bottom portion (5). The gas coming out through solution during experiment, was allowed to pass through water scrubbers (6) and (7) (capacity: 500 mL) before releasing to the atmosphere. At the top of the tube, a stoppered funnel (8) was fused to facilitate the addition of solution to the reactor. A stock solution of uranyl nitrate was prepared by dissolving U3O8 powder in nitric acid of required concentrations. From this solution, about 200 mL of feed solution containing ~ 106 g/L of uranium with required concentration of nitric acid and hydrazine were charged into reaction vessel and to this 2% Pt loaded on silica was added. Prior to reaction, the solution in the reactor was flushed with argon gas for five minutes to remove air and then H2 gas at desired flow rate was passed for catalytic reduction. Sampling was done at regular intervals to estimate the U(IV) generation and variation in acidity as well as hydrazine concentrations. Total uranium and uranous concentrations were determined by modified Davis-Gray method and redox titration with standard potassium dichromate.9 Free acidity and hydrazine concentration were estimated by standard acid-base titration. Several experimental runs, as a function of operating variables affecting the rate of reduction, were conducted and the details of experimental conditions are listed in Table 2. The liquid depth in the reaction vessel with no gas flow was about 300 mm whereas the height of gas-liquid mixture with gas flow was a function of gas flowrate and it varied between 330 and 370 mm. Though the ratio of height of gas-liquid mixture to diameter of the reaction vessel was 3.5 to 37 (maximum), the liquid phase was assumed to be well mixed. All the experiments were conducted at atmospheric pressure and room temperature. Reproducibility of the experimental data measurement was found to be within 6% deviation as indicated by few repeated experiments.

2.3 Reproducibility of experimental data In order to check the reproducibility of experimental data total four experiments were conducted with two different initial conditions. Fig. 3 shows the typical experimental results of two runs with same initial conditions. The standard deviation for the experimental data represented in Fig. 3 was found to be less than 3%. 3. RESULTS AND DISCUSSION 3.1 Experimental results 4 ACS Paragon Plus Environment


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The effect of various parameters such as catalyst loading, nitric acid, hydrazine concentrations and hydrogen flowrate on the hydrogenation/reduction of U(VI) was studied. The effects of individual parameters on hydrogenation of U(VI) are shown in Figs. 4-7. 3.2 Reaction Stoichiometry In the catalytic hydrogenation process, U(VI) is chemically reduced to lower valence state i.e. U(IV) which, results from the following two reactions: i.

Catalytic reduction of U(VI) by hydrogen

ii.

Indirect catalytic reduction of U(VI) by hydrazine

The major fraction of U(VI) reduction is caused by hydrogen. Hydrazine reduces U(VI) much more slowly than does hydrogen. Hence, as a reactant, it is not considered in the development of kinetic rate expression a sour experimental data confirmed that the rate of reduction only with hydrazine is very slow as observed from Fig. 8. Moreover, it is common to add hydrazine as a holding reductant to prevent U(IV) re-oxidation by nitrous acid, which is likely to be produced during the hydrogenation process. Following reactions take place during the hydrogenation process: 7 Pt UO2 ( NO3 ) 2 + 2HNO3 + H 2 → U( NO3 ) 4 + 2H 2 O

Pt A + 2B + C → D + 2E

(1) (1a)

The rate of this reaction increases with an increase in the pressure of hydrogen and with the catalyst loading. Therefore, in industrial conditions, hydrogen is used at fairly high pressure (typically about 40 atm.) and the platinum is deposited on silica grains, in order to meet both high catalytic activities with good hydraulic properties. The rate of the reaction also increases with temperature, but cannot be operated at temperatures significantly higher than room temperature, as at high temperature another reaction occurs; the catalytic reduction of nitric acid by hydrogen, according to: Pt

HNO 3 + H 2 → HNO 2 + H 2 O

(2)

3.3 Kinetic Model of the Reaction Hydrogenation of U(VI) is a heterogeneous catalytic reaction involving gas-liquidsolid phases. Various steps occur in the series when a gas-liquid-solid reaction occurs such as

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diffusion, adsorption, surface reactions and desorption of the products (Mundale et al.10). The following assumptions were made while studying the catalytic hydrogenation of U(VI): 1.

The catalyst activity is assumed to be constant during the experimental runs, i.e., no poisoning or the deactivation of the catalyst occurs.

2.

Desorption of the products offered no resistance11 to mass transfer. Also, desorption rates of the products are assumed to be very fast.

3.

Isothermal condition around and within the catalyst is assumed (i.e. temperature gradient is zero). A Langmuir-Hinshelwood (L-H) type model was proposed to describe the hydrogenation

of U(VI) to U(IV). Several model equations (Table 3) were derived and the model parameters were evaluated, including those with molecular adsorption of all the reactants and dissociative hydrogen adsorption. These model equations are derived on the assumption that one of the three elementary steps of hydrogenation process (adsorption of U(VI) and hydrogen, surface reaction between adsorbed molecules and desorption of products) is the rate-controlling step.

3.4 Solubility of Hydrogen in Uranyl Nitrate Solution The solubility of hydrogen in water at room temperature was calculated using Henry’s Law constant obtained from Perry’s Chemical Engineering Handbook and its value is 7.8x10-4 mol.L-1.atm-1. The solubility of hydrogen gas in mixed electrolyte solution was estimated by the following equation reported by Schumpe.12

log(C G , 0 / C G ) = ∑ (h i + h G )C i

(3)

The values of hi of different cations and anions present in the solution and the value of hG for hydrogen gas at 298.15K have been reported by Weisenberger and Schumpe,13 which were used to calculate the solubility of hydrogen gas in the mixed electrolyte solution used in the present investigation. In the present investigation experiments were conducted at three different acidities keeping uranium concentration more or less constant. The solubility of hydrogen in solution as a function of nitric concentration at room temperature and atmospheric pressure and uranium concentration of about 100 g/L is shown in Fig. 9.



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3.5 Semi-batch Reactor (bubble reactor) Model In order to verify the applicability of the kinetic model, reaction is carried out in bubble reactor to which hydrogen is added continuously in such a way that pressure is maintained constant (~1 atm.) and experimental data on the liquid phase concentration of uranium (i.e. U(VI)) and nitric acid as a function of time were obtained. The gas and the liquid phase in the reactor are assumed to completely back-mixed. The mass balances for uranium, nitric acid and hydrogen in the liquid is written as

m dC A = − cat rA dt VL

(4)

m dC B = −2 cat rA dt VL

(5)

(

dC C m k a V = − cat rA + L GL R C*C − C C dt VL VL

)

(6)

The initial conditions for the Eqs. (4-6)] can be written as at t = 0; CA = CA0; CB = CB0 and CC = 0 Eqs. (4 - 6) imply that the mass-transfer resistances around the catalyst particles and inside the particles are ignored. As, Choudhary et al have reported that mass transfer resistances in and around the porous catalyst can be eliminated by using small catalyst particle size (≤45 µm).14 In the present investigation average particle size was < 45 µm. The system of ordinary differential equations [Eqs. (4-6)] were solved numerically by using Runge-Kutta method to obtain concentrations of uranium, nitric acid and dissolved hydrogen as a function of time. For this purpose, intrinsic rate parameter, equilibrium constants and volumetric gas-liquid mass transfer coefficient ( k L a GL ) were determined from superimposing the experimental time vs concentration data on to the diagrams of numerical solution of model Eqs. (4-6) using a nonlinear regression technique. The best values were obtained by minimizing the following objective function: f = ∑ ([C A ]exp − [C A ]calc + [C B ] exp − [C B ]calc ) p

2

(7)

1


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where, ‘p’ is the number of observations. Calculations were performed using MATLAB, which has built-in algorithm to perform a nonlinear least-squares data fit. The estimated parameters for all the models are summarised in Table-4. It is observed that only the parameters of model I, III, IV, X and XI (Table 4) have positive values but the model III was found to give the best fit for the experimental data. Logically speaking if we visualise all the possible kinetic models screened out from eleven models based on different controlling mechanisms (described in Table 3 of the manuscript) one can make out that Model-XI is the most appropriate model describing the physical/chemical phenomena occurring in the system. But, the limitation of this model is in fitting the experimental data where we have observed that model Eqs. (4 -6) do not converge at all the initial guess values of rate parameters while solving them numerically. Most of the time it provided complex values of some of the rate parameters. Whereas, such limitations are not there with the Model-III and also, it was found to give the best fit of the experimental data even though it may not describe fully the physical/chemical phenomena occurring in the system. Therefore, we are inclined to select Model-III. The detailed explanation about the selection of appropriate kinetic model among the rival models has been provided in the supporting information of this manuscript. The estimated values of volumetric gas-liquid mass transfer coefficient for the hydrogen superficial velocities of 0.14, 0.35, 0.49 and 0.64 cm/s were found to be 0.07, 0.098, 0.22 and 0.25 (1/s), respectively and it was almost found to be independent of nitric acid concentration in the range studied in the present investigation. The comparisons between experimental and estimated results (from the model) for some typical cases are presented in Figs. (10 and 11), which show a reasonably good agreement.

As it is observed from these figures, the process is partially influenced by gas-

liquid mass transfer resistance, because with an increase in the hydrogen flowrate the extent of conversion increases. Based on the observations presented above, it is appropriate to comment that surface reaction between dissolved hydrogen and adsorbed reactants A and B (uranium and nitric acid respectively) on the surface of the catalyst explains the catalytic hydrogenation of U(VI) (i.e. Model III). The derivation of kinetic rate expression represented by Model III has been described in supporting information. The hydrogen flow rates studied in the present investigation are: 5, 12, 17 and 22 lph in order to analyse the effects of it on the initial rate. The objective was to find out flowrate range where diffusional resistance (gas-liquid) is minimum (i.e., where initial rate is almost 8 ACS Paragon Plus Environment


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independent of flowrate). Figure 12 shows the plot of initial rate vs. hydrogen flow rate from which it is observed that initial hydrogenation rate of uranium is almost independent of hydrogen flowrate above 17 lph. From Figures 4 to 7 that initial part of concentration curves (up to about 60% conversion) the overall rate is practically constant (zero order with respect to U(VI) concentration) and becomes practically first order beyond 80% conversion. Similar observations have been reported by Toppinen et al15 for the hydrogenation of benzene, toluene, ethylbenzene and cumene. It may be pointed out that the Model III (Table 3) accommodates these observations to a good extent. Figure 13 shows the calculated concentration profile of dissolved hydrogen by solving Eq. (6) in the reactor during the hydrogenation process at three different flow rates namely 5, 17 and 22 lph. It is noticed that at hydrogen flow rate of 17 lph (0.49 cm/s) dissolved hydrogen concentration oscillates in the range 5.7x10-4–7.5x10-4 kmol.m-3during the hydrogenation process. The following observations can be made from this figure: (1) As the hydrogen flow rate is increased from 6 to 22 lph (superficial velocity 0.14 to 0.64 cm/s) the dissolved hydrogen concentration (CC) increases because of an increase in mass transfer coefficient. (2) Since the overall rate remains practically constant (Figs. 4-7), the value of CC also remains constant. However, beyond 80% conversion, the rate decreases with time and hence the value of CC also increases with time. The present work is concerned with the experimental work carried out at room temperature and atmospheric pressure. In the future, research will be extended to different size of catalyst particles, high pressures and temperatures.

4. CONCLUSIONS The kinetics of hydrogenation/reduction of uranium [U(VI)] has been investigated using a platinum catalyst loaded on silica substrates in a bubble reactor at atmospheric pressure and room temperature. The effects of operating parameters such as catalyst loading, hydrogen flowrate and concentrations of nitric and hydrazine on reduction rate of U(VI) have been analysed. Rate equations based on Langmuir-Hinshelwood mechanism has been derived and the kinetic parameters have been estimated using experimental concentration vs. time data. The surface reaction between dissolved hydrogen and adsorbed reactants A and B (uranium and nitric acid respectively) on the surface of the catalyst was found to be controlling the rate of catalytic hydrogenation of U(VI). In order to test the applicability of 9 ACS Paragon Plus Environment

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the developed rate expression, the mass balance equations for uranium, nitric acid and hydrogen around the bubble column reactor were solved numerically. The comparison between experimental and predicted results showed a reasonably good agreement. Kinetic models presented here can be used for design and scale-up of hydrogenation reactor for the production of uranous [U(IV)] from U(VI).

5. AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Tel.: +91-22-25597625

6. NOMENCLATURE

a GL

= gas-liquid interfacial area, (m-1)

CA

= concentration of U(VI), (kmol.m-3)

CB

= concentration of nitric acid, (kmol.m-3)

CC

= concentration of hydrogen in liquid phase, (kmol.m-3)

CA0 CB0 CC0

= initial concentrations reactants A, B and C, (kmol.m-3)

C*C

= equilibrium concentration of hydrogen in liquid phase (kmol.m-3)

CG,0

= solubility of gas in water, (kmol.m-3)

Ci

= concentration of cations and anions, (kmol.m-3)

hi

=ion-specific salting parameter, (m3.kmol-1)

hG

= gas-specific parameter, (m3.kmol-1)

k

= rate constant, (m3.kg-1.s-1) or (kmol.kg-1.s-1)

KA, KB, KC

= equilibrium constants, (m3.kmol-1)

kL

= gas-liquid mass transfer coefficient, (m/s)

mcat

= mass of catalyst, (kg)

rA

= rate of reaction component A, (kmol.kg-1.s-1)

t

= time (s)



VL

= volume of liquid in the reactor (m3)

VR

= volume of reactor (m3)

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7. SUPPORTING INFORMATION The supporting information includes: (1) the detailed explanation about the selection of appropriate kinetic model among the rival models and (2) the derivation of kinetic rate expression represented by Model III.


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8. REFERENCES (1) Lanham,W. B.; Runion, T. C. PUREX process for plutonium and uranium recovery; USAEC, ORNL-479, 1949. (2) Sahu, A.; Vincent, T.; Shah, J.G.; Wattal, P.K. Catalytic reduction of U (VI) to U (IV) using hydrogen. J. Radioanal. Nucl. Chem. 2014,300, 163. (3) Rainey, R.H. Hydrogen reduction of Pu(IV) to Pu(III). Nuclear Applications.1965, 1. 310. (4) Schafer, A.C. Reprocessing of the Elk River Reactor Fuel in the. ITREC Plant, Reactor Fuel-Proc. Technol. 1969, 12, (3), 259. (5) Swanson, J.L. Platinum catalyzed hydrazine reduction of U (VI). Technical report, BNWL-1584, UC-4, 1971. (6) Abdounnabi, H.M.; Ananiev, A.V. Preparation of concentrated uranium (IV) nitrate solutions for the application in the nuclear fuel cycle, Proceedings of the International Conference on Fast Reactors and Related Fuel Cycles, Vol. IV, Kioto, Japan, 1991. (7) Tison, E.; Bretault, Ph. COGEMA Experience in uranous nitrate production, Proceedings of the WM 06 Conference, Tucson, AZ, La Hauge, France, 2006. (8) Rao, K.; Sreenivasa, Shyamlal R.; Narayan, C.V.; Jambunathan, U.; Ramanujam, A.; Kansra, V.P. Uranous nitrate production for PUREX process applications using PtO2 catalyst and hydrazine nitrate as reductant, BARC, Report, BARC/2003/E/009, 2009. (9) Chitnis, R.T.; Kulkarni, R.T.; Rege, S.G.; Mukherjee, A. Volumetric method for the determination of uranium in the active process solutions, J. Radioanal. Chem. 1978, 45(2), 331. (10) Mundale, V.D.; Kalam, A.; Joglekar, H.S.; Joshi, J.B. Spent Regeneration of Activated Carbon by Wet Air Oxidation. Can. J. Chem. Eng.1991, 69, 1149. (11) Doraiswamy, L.K.; Sharma, M.M. Heterogeneous reactions analysis, examples, and reactor design of fluid–fluid–solid reactions, Vol. 2, Wiley, New York,1984. (12) Schumpe A., The estimation of gas solubilities in salt solutions, Chem. Eng. Sci. 1993, 48, 153. (13) Weisenberger, S. and Schumpe, A. Estimation of gas solubilities in salt solutions at temperatures from 273 K to 363 K. AIChE J. 1996, 42, 298. (14) Choudhary, V. R.; Sane, M. K.; Tambe, S. S. Kinetics of Hydrogenation of o-Nitro phenol to o-Aminophenol on Pd/C Catalysts in a Stirred Three Phase Slurry Reactor. Ind. Eng. Chem. Res.1998, 37, 3879.



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(15) Toppinen, S., Rantakyl, T.K., Salmi, T., Aittamaa, J. Kinetics of the Liquid-Phase Hydrogenation of Benzene and Some Monosubstituted Alkylbenzenes over a Nickel Catalyst, Ind. Eng. Chem. Res. 1996, 35, 1824-1833. (16) Kittrell, J.R. Mathematical Modeling of Chemical Reactions, Adv. Chem. Eng. 1970, 8, 97-183. (17) Choudhary, V.R. and Doraiswamy, L.K. A Kinetic Model for the Isomerization of nButene to Isobutene. Ind. & Eng. Chem. Process Des. Dev., 1975, 14(3), 227-235.


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TABLES

Table 1. Characteristics of platinum catalyst particles Catalyst

: 2 wt% Pt/silica

Particle size (d32) (µm)

: 36.6

BET surface area (m2/g)

: 258.98

3

Pore volume (cm /g)

: 1.19

Table 2. Details of experimental conditions Variable

Effect of catalyst loading Effect of nitric acid concentration Effect of hydrazine concentration Effect of hydrogen flow rate

Catalyst loading (C:U) (g/g) 1:75 1:100 1:150 1:200 1:150

1:150

1:150

Conditions Uranium H2 flowrate (g/L) (M) (lph)

HNO3 (M)

N 2 H 5+ (M)

~ 1.5

~0.5

17

~ 112

0.47

7.1E-4:0.47:1.5

~1.0 ~ 1.5 ~2.0

~ 0.5

17

~ 106

0.45

6.8E-4:0.45:1 7.1E-4:0.45:1.5 7.3E-4:0.45:2

~ 1.5

~0.25 ~ 0.5 ~ 0.75

17

~ 106

0.45

7.1E-4:0.45:1.5

~0.5

5 12 17 22

~ 106

0.45

7.1E-4:0.45:1.5

~ 1.5

CC0:CA0:CB0



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Table 3. Kinetic models for the different controlling mechanism Model No

Controlling Mechanism

Rate Model

Dissolved hydrogen directly reacts with adsorbed reactant A and B on catalyst surface kC A I Adsorption of A controlling rA = (1 + K B C B ) II

III

IV

Adsorption of B controlling

rA =

rA =

Surface reaction controlling

(1 + K A C A )

kK A K 2B C A C 2B C C

(1 + K A C A + K B C B )3

All the reactants molecularly adsorbed on the catalyst surface and reacts kC A rA = Adsorption of A controlling (1 + K C + K C ) B

V


rA =

VI

Adsorption of C controlling

rA =

VII

kC B


rA =

B

C

C

kC B

(1 + K A C A + K C C C ) kC C

(1 + K A C A + K B C B ) kK A K B C A C 2B C C

(1 + K A C A + K B C B + K C C C )4

Reaction between atomically adsorbed hydrogen and adsorbed molecules of A and B kC A rA = 1 VIII Adsorption of A controlling (1 + K C + (K C ) 2 ) B

rA = IX

X

XI



rA =

C

C

kC B 1 + K C + (K C ) 12  A A C C  

rA =

Adsorption of C controlling

B

kC C

(1 + K A C A + K B C B )2 kK A K 2B K C C A C 2B C C

1 + K C + K C + (K C ) 12  A A B B C C  

5


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Table 4. Rate parameters and equilibrium constants for L-H models Model No

KA (m /kmol) -2.69 7.51 ± 0.36 -0.015 -147.74 0.060 -0.028 0.142 0.015 3

I II III IV V VI VII VIII IX X XI

KB (m /kmol) 0.1184 ± 0.5 0.29 9.7 99.97 0.106 0.038 0.065 0.058 3

KC (m /kmol) 500 -1820 -1840 1870 1790 1830 3

k (m /kg.s) 2e-4 -5.18e-6 5.64 ± 1.43 1.04 -5.27e-6 10.7 -1.85e-7 -2.71e-8 -3.89e-8 2.33e-3 6.51e-5 3



Page 18 of 24

FIGURES

Figure 1 Scanning electron microscopy image of Pt/SiO2 catalyst

Figure 2 Experimental setup made of glass


Figure 3 Concentration vs. time plot representing the reproducibility of experimental data

1 m m

0.8 [U(VI)]/[U(VI)]0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[U(VI)]/[U(VI)]0

Page 19 of 24

:m

cat

:m

cat

U(VI) U(VI)

= 1:75 = 1:100

mcat: mU(VI) = 1:150 mcat:mU(VI) = 1:200

0.6

0.4

0.2

0 0

1000

2000

3000 4000 5000 6000 7000 Time (s) Figure 4 Variation of U(VI) concentration with time as a function of catalyst loading



1 0.83 M HNO3 1.5 M HNO3

[U(VI)]/[U(VI)]t, = 0

0.8

2.1 M HNO3

0.6

0.4

0.2

0 0

2000

4000

6000 Time (s)

8000

10000

12000

Figure 5. Variation of U(VI) concentration with time as a function of initial nitric acid concentration

1 [N2H+5]0 = 0.25M

0.9

[N2H+5]0 = 0.5M

0.8 [U(VI)]/[U(VI)]0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 24

[N2H+5]0 = 0.75M

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

2000

4000

6000 8000 Time (s)

10000

12000

14000

Figure 6. Variation of U(VI) concentration with time as a function of hydrazine concentration 19 ACS Paragon Plus Environment

Figure 7 Variation of U(VI) concentration with time as a function of hydrogen flow rate

1 0.9 0.8 [U(VI)]/[U(VI)]t = 0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[U(VI)]/[U(VI)] 0

Page 21 of 24

0.7 0.6 0.5 0.4 0.3 0.2

Without hydrogen With hydrogen

0.1 0

2000

4000

6000 Time (s)

8000

10000

12000

Figure8. Variation of uranium concentration with and without hydrogen during hydrogenation process 20 ACS Paragon Plus Environment


Fig.9 Solubility of H2 as a function of nitric acid concentration at a uranium concentration of about 100 g/L

[U(VI)]/[U(VI)] 0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 24

Figure10. Comparisons between experimental and estimated concentration profiles of U(VI) as a function of hydrogen flow rate


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[HNO3]/[HNO 3]0

Page 23 of 24

Figure11. Comparisons between experimental and predicted concentration profiles of HNO3 as a function of hydrogen flowrate

Figure 12 Initial hydrogenation ratevs. hydrogen flow rate 22 ACS Paragon Plus Environment

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 24

C*C (kmol.m -3)


Figure 13 Variation of dissolved hydrogen concentration during hydrogenation


Study on the Kinetics of Catalytic Hydrogenation of U(VI) in Nitric Acid

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