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Kinetics, Catalysis, and Reaction Engineering

Kinetics of Dechlorination of Molten Chloride Salt using Protonated Ultra-stable Y Zeolite Manish Wasnik, Tanner Livingston, Krista Carlson, and Michael Simpson Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02577 • Publication Date (Web): 26 Jul 2019 Downloaded from pubs.acs.org on August 6, 2019

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Industrial & Engineering Chemistry Research

Kinetics of Dechlorination of Molten Chloride Salt using Protonated Ultra-stable Y Zeolite Manish S. Wasnik*, Tanner Livingston, Krista Carlson, and Michael F. Simpson

University of Utah, Department of Metallurgical Engineering

135 South 1460 East, Salt Lake City, Utah 84107 USA

*[email protected]

KEYWORDS Electrorefiner, chloride salt, zeolite, kinetics, ion exchange

ABSTRACT

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Investigation of the kinetics of the ion exchange between protonated ultrastable Y-type (USHY) and surrogate electrorefiner (ER) waste salt was performed to optimize the dechlorination process. The kinetics of the ion exchange reaction was investigated by measuring the amount of unreacted Cl. In theory, the kinetics of the ion exchange reaction in a porous media will be limited by diffusion or reaction controlled mechanisms. Therefore, second order kinetics and diffusion-limited rate models have been derived and compared to experimental data to elucidate the rate-limiting step and develop a predictive model for the apparent rate of reaction. Ion exchange experiments were performed with varying zeolite particle sizes (up to 600 µm) at 625°C. The experiments were performed both with unfluidized and continuously fluidized zeolite particles in a static and a rotating tube furnaces. It was concluded that the process is limited by reaction kinetics inside the zeolite crystals, and a second-order kinetic model best fits the experimental data. Given that the zeolite is not stable at higher temperatures, further increase in the extent of dechlorination for a given batch reaction time requires an increase of the zeolite to salt ratio, which has the undesirable effect of increasing the volume of generated waste per amount of salt.


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1. INTRODUCTION

In support of sustainable nuclear power generation, radioactive chloride salt waste can be generated either from electrorefining spent nuclear fuel or operating a molten salt reactor that utilizes a chloride-based fuel salt.1, 2 Disposal of this salt without suitable long-term containment would present a great risk for release of fission products to the environment and, thus, be considered an impediment to widespread adoption of these technologies. The long-lived radioactive Cl-36 and I-129 isotopes are thermodynamically stable in the salt as anions and represent a long-term disposal challenge due to their high solubility in water. Therefore, a process is needed to immobilize radioactive metals found in chloride salt waste into leach-resistant waste forms. Additionally, there would be benefits to managing radioactive halides including Cl-36 and I-129—either through recycle or isolation into special waste forms. A process demonstrated for accomplishing these goals was recently reported by our group that utilizes H-exchanged ultrastable Y (USHY) zeolite for a high-temperature ion exchange


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reaction with the salt. The ion exchange process was performed with surrogate electrorefiner (ER) salt in a continuously fluidized particle reactor at 625°C for 48 hours to achieve >90% dechlorination using 45-250 µm USHY particulates without zeolite structure degradation as represented stoichiometrically by reaction (1).1 This was a great improvement as compared to the work reported several years ago on the same process by Bagri and Simpson.3 The dechlorination process results in 65% estimated mass or volume reduction in the final waste form relative to the glass bonded sodalite waste forms produced using the ceramic waste form process based on zeolite-4A.1

H(Si𝑂2)2.6(Al𝑂2) +

𝐿𝑖0.58𝐾0.42𝐶𝑙 → 𝐿𝑖0.58𝐾0.42(Si𝑂2)2.6(Al𝑂2) + 𝐻𝐶𝑙(𝑔) (1)

USHY (Faujasite)

ER salt

ion-exchanged zeolite

The high degree of waste dechlorination reported in our previous paper, unfortunately, requires a long residence time in the reactor (24-48 hours). This motivates the study of the kinetics of the ion exchange reaction. Such a kinetic study can potentially elucidate


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the rate-limiting step in the reaction—which could be based on chemical kinetics or diffusion.4 Knowledge of the rate-limiting step is important for determining the optimal conditions to maximize the processing rate.

The objectives of the work reported in this paper are to measure the rate of ion exchange between chloride salt and USHY and to identify the rate-limiting process (reaction, intracrystalline diffusion, intraparticle diffusion, or external mass transfer). In theory, the kinetics of the ion-exchange reaction in a porous media will be limited by diffusion or reaction-controlled mechanisms in the zeolite particle or in the crystals. This has been studied by performing ion exchange tests with a varying particle size of both the USHY zeolite and the salt and measuring the unreacted Cl in the salt. The experimental data on the extent of reaction were fitted to both diffusion and reaction controlled derived models


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2. Ion Exchange Rate Models

2.1 Diffusion Limited Ion Exchange Reaction

Typically, the overall rate of reaction in porous materials is controlled by mass transport within the pore network.5 Granulated zeolite particles are comprised of small zeolite crystals bonded together via an inorganic material. Collectively, the zeolite crystals and binder form a mesoporous structure. These mesopores have diameters on the order of 1–10 μm, while the pores in the zeolite crystals have a much smaller diameter of about 3–8 Å depending upon the type of zeolite. The USHY zeolite has a faujasite structure with an approximate pore diameter of 7.4 Å. 6 The overall mechanism of the dechlorination process of the salt via ion exchange can be visualized as shown in Figure 1. The path of chloride salt molecules is presumed to be following the steps shown in the flow diagram of Figure 1 and culminates in the ion exchange reaction represented by equation (1).7


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Figure 1. (Left) Hypothetical dechlorination process model explaining the diffusion of molten salt molecules inside the zeolite particle and diffusion of HCl gas out of the zeolite particle; (Right) flow diagram explaining the various process steps taking place in the ion exchange reaction


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Assuming this path is correct, intracrystalline porosity and particle size are important parameters for determining the rate of diffusion of salt across the particle. The particle diameter can be varied from 3 mm down to the scale of microns by milling/classifying. If relying upon commercial sources for the granulated zeolite, both zeolite crystal size and zeolite particle pore size are considered fixed. This greatly limits the options for optimizing the effective rate of the ion exchange reaction. However, it is still useful to identify the rate-limiting step in the process to know what changes hypothetically would be beneficial. An experimental method that we can use for this system to identify the rate-limiting step is to measure the extent of reaction with all variables fixed except for the size of zeolite particles. In a diffusion-controlled process, the rate of ion exchange is increased by a decrease in particle size.8 This can be quantitatively analyzed via mathematical derivation based on the diffusion equation. Assuming spherical particles and only salt ions diffusion across the particle limiting transport, the model should be based on equation (2).9:


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(

∂𝐶 ∂2𝐶 2∂𝐶 =𝐷 2+ ∂𝑡 𝑟 ∂𝑟 ∂𝑟

)

(2)

Where, C is concentration of the metal ions (mole equivalents/gram of zeolite), D is diffusivity of metal ions within the zeolite particle (cm2/sec), r is radial position in the zeolite particle, assuming spherical shape (cm) and t is time (s). The solution to equation (2) is given by Barrer10 as shown in equation (3). Co is the constant concentration of the metal ions at the surface of the sphere. C1 is the uniform concentration of the metal ions in the sphere initially i.e. C1 =0. R is particle size radius (cm).

𝐶 ― 𝐶1

𝐶𝑜 ― 𝐶1 = 1 +

2𝑅 𝜋𝑟

∞

( ― 1)𝑛 𝑛

∑𝑛 = 1

sin(

𝑛𝜋𝑟 ―𝐷𝑛2 𝜋2𝑡/𝑅2 𝑅 )𝑒𝑥𝑝

(3)

Equation (3) can be used to derive equation (4), which gives the total uptake of mole equivalents of the metal ions from the salt diffusing in the zeolite as a function of time. Since mole equivalents of metal ions are equivalent to moles of chlorides ions in the


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salt, the uptake of metal ions is the negative of the release of chloride ions—which can be measured. Equation (4) also represents the amount of Cl- ions diffusing into the zeolite but immediately leaving the zeolite particle in the form of HCl gas (refer reaction (1)). 𝐶 (𝑡) represents the concentration of metal ions in mol equivalents/g of the zeolite at time t

, 𝐶(∞) represents the concentration of metal ions in mol equivalents/g of the zeolite at equilibrium, D is diffusion coefficient (cm2/sec).

𝐶(𝑡) 𝐶(∞)

=1―

6

1 ∞ ∑𝑛 = 1 2 2 𝜋 𝑛

𝑒

―

𝐷𝑛2 𝜋2𝑡 𝑅2

(4)

𝐶(𝑡)

Therefore the value 1-𝐶(∞) would represent the fraction of metal ions or Cl ions that did not take part in the ion exchange reaction and remained in the zeolite in their chloride 𝐶(𝑡)

forms post ion exchange. The unreacted fraction of Cl i.e. 1-𝐶(∞) can also be measured experimentally.


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2.2 Chemical Reaction Limited Ion Exchange Reaction

In the case of a chemical kinetics controlled ion exchange process, the rate of the reaction is independent of the particle size of the zeolite. The theory of chemical-kinetics controlled reaction assumes that the intrinsic kinetics of the ion exchange reaction is slow compared to any diffusion process, i.e., the chemical exchange is the rate determining step in the process. To understand the chemical kinetics of the dechlorination process, the ion exchange reaction as shown in equation (1) can be written in an ionic form as equation (5):

(5)

𝐻𝑧+ + 𝑀 + 𝐶𝑙 ― →𝑀𝑧+ + 𝐻𝐶𝑙(𝑔)

The subscripts z denote the zeolite phase. One mole of H+ ions from the zeolite reacts with one mole equivalent of salt to generate one mole of HCl gas with simultaneous ionexchange of one mole equivalent of cations (M+) of the salt into the zeolite. Since the decrease in the concentration of H+ ion in the zeolite is equal to the decrease in


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concentration of Cl- in the salt which is equal to increase in the concentration of gaseous HCl at any time t, the rate of reaction can be expressed as given in equation (6). [𝐻𝑧+ ] is concentration of H+ in the zeolite (mol/g zeolite) at time t, [𝐶𝑙𝑧― ] is concentration of Cl- in the unreacted Cl- ions in the zeolite (mol/g zeolite) at time t,

[𝐻𝐶𝑙(𝑔)] is concentration of 𝐻𝐶𝑙(𝑔) (mol/g zeolite) at time t.

𝑅𝑎𝑡𝑒 = ―

𝑑[𝐻𝑧+ ] 𝑑𝑡

=―

𝑑[𝐶𝑙𝑧― ] 𝑑𝑡

=+

𝑑[𝐻𝐶𝑙(𝑔)] 𝑑𝑡

(6)

Assuming a second-order kinetic model, the second order rate law of the reaction is proportional to the product of the concentrations of reacting species. The time rate of change of [𝐶𝑙𝑧― ] can be expressed in the differential equation (7). k (g/mol-sec) is the second order reaction rate constant. Table 1 shows the derived rate equations for three different initial conditions of zeolite and salt reaction. The detailed derivation of rate equations for each initial condition is given in the supplementary section of this paper.


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𝑑[𝐶𝑙𝑧― ] 𝑑𝑡

(7)

= ― 𝑘[𝐻𝑧+ ][𝐶𝑙𝑧― ]

Table 1. Rate equations for three different initial conditions of the ion exchange reaction between the zeolite and salt

Cases

Initial Conditions

Rate law

Rate equation*

Case 1

At time t=0,

𝑑[𝐶𝑙𝑧― ]

[𝐻𝑧+ ]0 [𝐻𝑧+ ] 𝑙𝑛 ― = 𝑙𝑛 ― + [𝐶𝑙𝑧 ] [𝐶𝑙𝑧 ]0 𝑘([𝐻𝑧+ ]0 ― [𝐶𝑙𝑧― ]0)𝑡

𝑑𝑡

= ― 𝑘[𝐻𝑧+ ]

[𝐶𝑙𝑧― ] (7) [𝐶𝑙𝑧― ]0 ≠ [𝐻𝑧+ ]0

Initial concentration

(16)

of

the H+ ions in the zeolite and Cl- ions in the zeolite are not equal

Case 2

At time t=0,

―

[

𝑑[𝐶𝑙𝑧― ]

𝑑𝑡 ― 2 𝐶𝑙𝑧

]

1

=𝑘

1

[𝐶𝑙𝑧― ]𝑡 ― [𝐶𝑙𝑧― ]0 = 𝑘𝑡

(17)

[𝐶𝑙𝑧― ]0 = [𝐻𝑧+ ]0

(20)


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Initial concentration of 𝐻𝑧+ and 𝐶𝑙𝑧― are equal

Case 3

At time t=0,

Initial

[

concentration of one of the reactants is in

]

𝐶𝑙𝑧― 0

≫[

𝑑[𝐻𝑧+ ] 𝑑𝑡

𝐻𝑧+ 0,

]

= ―𝑘[𝐻𝑧+ ]

[𝐶𝑙𝑧― ]0

(21)

𝑙𝑛[𝐻𝑧+ ]𝑡 = 𝑙𝑛[𝐻𝑧+ ]0 ―𝑘′𝑡 (24)

Then

[𝐶𝑙𝑧― ]𝑡 = [𝐶𝑙𝑧― ]0

excess [𝐻𝑧+ ]0 [𝐻𝑧+ ] *Case 1:A plot of 𝑙𝑛[𝐶𝑙 ― ] vs 𝑡 is a straight line with an intercept 𝑙𝑛[𝐶𝑙 ― ] and slope 𝑘( 𝑧 𝑧 0

[𝐻𝑧+ ]0 ― [𝐶𝑙𝑠― ]0) *Case 2: If this model applies to the system, a plot of 1/[𝐶𝑙𝑧― ]𝑡 vs. t in equation (15) would be a straight line with slope of 𝑘 and an intercept 1/[𝐶𝑙𝑧― ]0.

*Case3: If this model applies to the system, a plot of 𝑙𝑛[𝐻𝑧+ ]𝑡 vs. t in equation (19) would be a straight line with slope of 𝑘′ and an intercept 𝑙𝑛[𝐻𝑧+ ]0.

3. EXPERIMENTAL

3.1 Materials


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Binderless USHY powder zeolite (CBV-400, Zeolyst Inc.) with Si/Al equal to 2.6 and pelletized (~3 mm pellet size) USHY zeolite with a pseudoboehmite binder and Si/Al equal to 2.6 (Riogen Inc.) were used in all the ion exchange tests. The stoichiometric formula of the zeolite was assumed to be H(SiO2)2.6(AlO2) with a molar mass of 216.2 g/mol. The salt used for the study was a surrogate ER anhydrous chloride salt mixture with composition: 4.03 wt% Li, 18.4 wt% K, 4.82 wt% Na, 2.37 wt% Cs, 3.12 wt% Nd, 3.41 wt% Ce, 1.22 wt% Sr, 0.31 wt% Y. Both zeolite and salt were handled in the argon glove boxes (Inert Technology) maintained at

Kinetics of Dechlorination of Molten Chloride Salt ... - ACS Publications

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