Thermal Denitration of Ammonium Nitrate Solution in a Fluidized-Bed

Article pubs.acs.org/IECR

Thermal Denitration of Ammonium Nitrate Solution in a Fluidized-Bed Reactor Sandip Bhowmick,*,† Hanmanth Rao, and D. Sathiyamoorthy‡ †

Chemical Engineering Division and ‡Powder Metallurgy Division, Bhabha Atomic Research Centre, Mumbai-400085, India ABSTRACT: Ammonium diuranate (ADU) filtrate, which contains mainly ammonium nitrate (80−100 g/L), is generated during hydrometallurgical processing of uranium. This filtrate stream poses a disposal problem because of its high nitrate content and residual radioactivity. Fluidized-bed thermal denitration is considered as a suitable chemical-free disposal option for the aqueous waste nitrate stream. Hence, investigations to explore the decomposition of ammonium nitrate in a fluidized bed have been carried out. To enable theoretical analysis and performance evaluation of the process, a mathematical model was developed. The model is based on two-phase theory of a bubbling fluidized bed. Model calculations were used to predict the axial concentration profile of ammonium nitrate in the emulsion and bubble phases and the axial temperature profiles of gas bubbles, emulsion gas, and emulsion particles. The mechanism of decomposition of ammonium nitrate in a fluidized bed was explored, and the conversion of ammonium nitrate was estimated. Model predictions were compared with experimental data available from a benchscale plant. Good agreement was obtained between the model predictions and the experimental measurements. A steady-state parametric study indicated that conversion is enhanced with an increase in bed temperature and feed concentration. It was found that operation at higher feed concentration leads to local hot spots. The required reaction-zone length for complete conversion of ammonium nitrate vapor in the emulsion phase was found to decrease significantly with increased bed temperature. No marked effect of u/umf on conversion was observed. Optimum values of process parameters to maximize the conversion were derived.

1. INTRODUCTION A generic production process for natural uranium conversion normally begins with yellowcake dissolution in nitric acid, followed by a solvent extraction process. The nuclear-pure uranyl nitrate solution, received after solvent extraction, is contacted with ammonia to precipitate uranium as ammonium diuranate. The slurry is then filtered to separate precipitated ammonium diuranate (ADU). ADU filtrate contains ammonium nitrate along with small quantities of radioactive impurities. This waste liquid stream poses a disposal problem because of its high nitrate content and associated residual radioactivity. Fluidized-bed thermal denitration has been considered as an advance technology to treat such waste nitrate streams generated in the nuclear fuel cycle. This chemical-free treatment option is environmentally benign for the disposal of aqueous waste nitrate streams. In the thermal denitration process, the nitrate stream is sprayed into a hot fluidized bed. The bed is maintained at the desired temperature by an induction heating system. In this process, ammonium nitrate decomposes predominantly into nitrogen, oxygen, and water vapor, with the formation of a small amount of NOx. The radioactive impurities in the ADU filtrate are trapped in a small quantity of seed material in the fluidized-bed reactor. Several experimental works, including plant-scale applications, have been reported in the literature on fluidized-bed thermal denitration technology.1−6 Using a synthetic solution of ammonium nitrate as a simulant of ADU filtrate, the thermal denitration of ammonium nitrate solution has been carried out at the Chemical Engineering Division of Bhabha Atomic Research Centre in a bench-scale fluidized-bed reactor of 150-mm diameter. Regarding the thermal denitration of ammonium nitrate solution in a fluidized-bed reactor, no works have been published, and a systematic investigation is still lacking. In this work, a mathematical © 2012 American Chemical Society

model has been developed for the thermal decomposition of ammonium nitrate in a bubbling fluidized bed to study the effects of various operating parameters on conversion. The model was validated by comparing the measured conversions during campaign runs with the model predictions. Optimum values of process parameters to maximize the conversion were derived. Details of the experimental work in the bench-scale plant are not included in this article.

2. MODEL FORMULATION The present model is based on the two-phase theory of fluidization.7 According to the two-phase theory of fluidization, a fluidized bed is assumed to consist of two phases: a bubble phase and an emulsion phase. All of the gas in excess of minimum fluidization flows through the bed as bubbles, whereas the emulsion stays at minimum fluidizing conditions. Gases in all bed phases including the emulsion are assumed to be in plug flow. Exchange of gas occurring between the bubble and emulsion phases is taken into account. Particles in the emulsion phase are fully back-mixed. Mass and energy conservation equations have been derived for the two bed phases. Okasha developed a mathematical model to simulate the combustion of liquid fuel in a bubbling fluidized bed.8 In this model, mass balance differential equations are presented for the considered species in the different bed phases. For the modeling of the fluidized-bed thermal denitration process, some relevant ideas with respect to the mass balance of ammonium nitrate vapor in the bubble and emulsion Received: Revised: Accepted: Published: 8394

September 5, 2011 May 23, 2012 June 1, 2012 June 1, 2012 dx.doi.org/10.1021/ie202018t | Ind. Eng. Chem. Res. 2012, 51, 8394−8403

Industrial & Engineering Chemistry Research

Article

Table 1. Correlations and Equations Used in the Model parameter minimum fluidization velocity at elevated temperature

equation(s)

ref Subramani et al.11

Ar = 1650

Remf Ar =

d p3ρg (ρp − ρg )g μg

2

, Remf =

d pρg umf μg

voidage at minimum fluidizing conditions

εmf = 0.3507Ar 0.0337Remf −0.0704

Subramani et al.11

average bubble size at bed height z

db = 0.21z 0.8(u0 − umf )0.42 exp[− 0.25(u0 − umf )2 − 0.1(u0 − umf )]

Cai et al.12

bubble rise velocity

ub = u0 − umf + 0.711 gdb

fraction of the bed in bubbles

δ=

Davidson and Harrison13 Kunii and Levenspiel14

gas interchange coefficient

u0 − umf ub − umf

Kunii and Levenspiel14

⎛ D0.5g 0.25 ⎞ ⎛u ⎞ ⎟ Kbc = 4.5⎜ mf ⎟ + 5.85⎜ 1.25 ⎝ db ⎠ ⎠ ⎝ db ⎡ Dε (0.711 gd ) ⎤ mf b ⎥ Kce = 6.77⎢ ⎢⎣ ⎥⎦ db 3 1/Kbe = 1/Kbc + 1/Kce

solids circulation flux

Werther15

⎞ ⎛ 1 J = 0.67(1 − εmf )ρp ⎜ − 1⎟(u0 − umf ) ⎠ ⎝ Øb Øb = {1 − 0.3 exp[−8(u0 − umf )]}e−ωz ω = 7.2(u0 − umf ) exp[− 4.1(u0 − umf )]

particle-to-gas heat-transfer coefficient

Nup = 0.0282Rep1.4Prg 0.33 Nup = 1.01Rep0.48Prg 0.33 Nu p =

wall-to-bed heat-transfer coefficient

hpg d p Kg

, Rep =

Chen and Chen16

0.1 ≤ Rep ≤ 50 50 ≤ Rep ≤ 104

d pu0ρg μg

, Prg =

Cpgμg Kg Kunii and Levenspiel7

0 hwb = (1 − δ)/(1/(2Kew /d p + αwc pgρg u0)

+ 1/{1.13[Ke0ρp (1 − εmf )c pp/τ ]0.5 }) heat-transfer coefficient between the bubble and emulsion phases

⎡ ⎛ 2hpg ⎞⎤ 3 1 ⎟⎥ hbe = Cpgρg umf + ubγpCppρp ⎢1 − exp⎜⎜− 3τb ⎢ ⎛ ⎞4 3εmf 4 Cppρp d p ⎟⎠⎥⎦ ⎝ ⎜1 + ⎟ ⎣ ⎝ u b / (εmf u mf )−1 ⎠ 1

Toi et al.17

in at a height 800 mm above the distributor when u/umf ≥ 7. Depending on the temperature, decomposition of ammonium nitrate can follow various modes of reaction, as follows:18−20

phases have been developed from fuel combustion models.8−10 In the thermal denitration process, the feed solution is introduced into the fluidized bed as a spray. It is assumed that liquid droplets in the spray are deposited on the surface of hot emulsion particles and vaporized. Hence, ammonium nitrate vapor is released into the emulsion phase, travels through the emulsion phase, and undergoes decomposition. Transfer of ammonium nitrate vapor from the emulsion to the bubble phase through gas interchange between these phases is considered. Energy balance differential equations have been derived for the bed phases to include the heat of reaction in the fluidized-bed thermal denitration model. All hydrodynamics- and heattransfer-related parameters of a bubbling fluidized bed were estimated using the equations listed in Table 1. An increase in bubble size due to atomizing air and vapor generation was taken into account by considering the superficial gas velocity as the total volume of the fluidizing air, the atomizing air, and the vapor divided by the cross-sectional area of the bed. The bubble size can be determined by the equation given in Table 1. The transition from the bubbling to the slugging regime is taken into account by checking whether the bubble size is greater than 0.7Dt, where Dt is the column diameter. It was estimated that slugging should set

169 ° C

NH4NO3 ⎯⎯⎯⎯⎯⎯→ NH3 + HNO3 ΔG573,1 = 12.94 kJ/mol

(1)

170 − 230 ° C

NH4NO3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ N2O + 2H 2O ΔG573,1 = −288.84 kJ/mol > 230 ° C

NH4NO3 ⎯⎯⎯⎯⎯⎯⎯⎯→

3 1 N2 + NO2 + H 2O 4 2

ΔG573,1 = −379.39 kJ/mol > 260 ° C

(2)

NH4NO3 ⎯⎯⎯⎯⎯⎯⎯⎯→ N2 +

(3)

1 O2 + 2H 2O 2

ΔG573,1 = −413.63 kJ/mol

(4)

Gibbs free energy changes of all four reactions were determined at 300 °C and 1 atm using the chemical thermodynamics 8395

dx.doi.org/10.1021/ie202018t | Ind. Eng. Chem. Res. 2012, 51, 8394−8403


Article

place is called the reaction zone. An infinitesimally small representative zone, Δz, of the fluidized bed was considered, and mass and energy balance differential equations were formulated for the bubble and emulsion phases. For the emulsion phase, two energy balance equations were set up, one each for the emulsion gas and particles. Profiles of the axial concentration of ammonium nitrate and temperature were obtained by solving these equations. 2.3. Fluidized-Bed Denitrator Model. The following aspects for modeling the mass balance in the reaction zone were considered: (1) Decomposition of ammonium nitrate takes place in both the bubble and emulsion phases. (2) Ammonium nitrate is exchanged between the bubble and emulsion phases. The mass balance of ammonium nitrate in the emulsion phase was modeled as follows:

software FactSage 6.1. Reaction 4 has the most negative Gibbs free energy change. Hence, according to thermodynamic laws, this is the most probable reaction. Reaction 4 is first-order with an activation energy of E = 102.6 kJ/mol, a frequency factor of K0 = 4.55 × 107/s, and a heat of reaction of Hreaction = 118.86 kJ/mol. 2.1. Assumptions on the Interactions between the Spray and the Fluidized Bed. Okasha and Miccio studied the pneumatic feeding of liquid fuel into a fluidized-bed combustor operating at 850 °C.21 Their model predicted that part of the liquid fuel is released as vapor inside the jet as a result of vaporization of the fuel droplets. This vapor was assumed to contribute to the jet bubbles that periodically detached from the jet. The remaining part of the injected liquid fuel reached the jet boundaries, was deposited on the particles in the emulsion phase, and vaporized. Interaction between the spray and fluidized bed was found to be a combined effect of several microprocesses. Because of the disruptive action of high-velocity atomizing air, the liquid stream disintegrated into liquid drops. The mixture of atomizing air and droplets had a very high velocity at nozzle outlet, so the mixture emerged as a jet from the twin fluid nozzle with enough kinetic energy to even pierce the bed. Particles were sucked into the jet stream, and a small fraction of total droplets was deposited on the entrained particles. Atomizing air detached from the jet as jet bubbles. The operating temperature of the thermal denitration process (∼350 °C) is quite low compared to the fuel combustion process. At this operating temperature, radiative heat transfer from the fluidized bed to the solution droplets can be neglected. All of the heat has to be transferred through the jet surface and by the entrained solid particles. The sublimation point of ammonium nitrate is 210 °C. Vaporization of ammonium nitrate inside the jet is therefore quite unlikely because of the very small residence time of the droplets. Evaporation of water, however, does take place in the jet. Jet bubbles thus do not contain any ammonium nitrate vapor and can be treated the same as distributor bubbles. Most of the droplets follow the jet stream and get deposited on hot particle surface in the emulsion phase. Precipitation of droplets on emulsion particles and variation of drop concentration with distance from the jet outlet have been described in the literature.22,23 Liquid droplets depositing on an emulsion particle are assumed to form a uniform shell. The rates of evaporation of water and ammonium nitrate from particle surface depend on the heat-transfer area and the heat-transfer coefficient between the particle and the liquid film. As drops are deposited over a large surface provided by the particles, the area of heat transfer is large. To the boiling point of the liquid, the particle-to-liquid film heattransfer coefficient is the same as the particle-to-gas heat-transfer coefficient. At the boiling point, the particle-to-liquid film heattransfer coefficient is equal to the film boiling heat-transfer coefficient. Because of the rapid evaporation of the liquid film, the vaporization process was almost completed in the spray zone, and the exchange of only dry particles was assumed to occur between the spray zone and the rest of the fluidized bed. Ammonium nitrate vapor is released into the emulsion phase, and its initial concentration in the emulsion phase is CeAN,0 = mAN/(MANumfAb). The initial concentration of ammonium nitrate vapor in the bubble phase is zero. Because of the continuous wetting and liquid evaporation, the spray zone temperature is lower than the bed operating temperature (by 5− 7 °C). Decomposition of ammonium nitrate in the spray zone is neglected. 2.2. Reaction Zone. The bed available above the droplet deposition zone where ammonium nitrate decomposition takes

(disappearance of ammonium nitrate from the emulsion phase) = (decomposition of ammonium nitrate in the emulsion phase) + (transfer of ammonium nitrate to the bubble phase) −[(1 − δ)AbΔz]ΔCeAN = K 0e−E / RTeg CeAN[(1 − δ)εmf AbΔz] + Kbe(CeAN − C bAN)(δAbΔz)

Δz umf

Δz umf

⎛ε ⎞ dCeAN δKbe = −⎜ mf ⎟K 0e−E / RTeg CeAN + dz umf (1 − δ) ⎝ umf ⎠ × (C bAN − CeAN)

(5)

The mass balance of ammonium nitrate in the bubble phase was modeled as follows: (disappearance of ammonium nitrate from the bubble phase) = (decomposition of ammonium nitrate in the bubble phase) + (transfer of ammonium nitrate to the emulsion phase)

−(δAbΔz)ΔC bAN = K 0e−E / RTbC bAN(δAbΔz) × (δAbΔz)

Δz ub

Δz + Kbe(C bAN − CeAN) ub

⎛1⎞ dC bAN K = −⎜ ⎟K 0e−E / RTbC bAN + be (CeAN − C bAN) dz u ub ⎝ b⎠ (6)

Considerations for the energy balance over a small representative section of reaction zone are schematically shown in Figure 1. They include (1) heat generation due to the exothermic decomposition of ammonium nitrate, (2) heat transfer between the bubble and emulsion phases, (3) heat transfer between the emulsion gas and emulsion particles, 8396



Article

mbg Cpg ΔTb = K 0e−E / RTbC bANHreactionVbg − hbeS(Tb − Tep)

Δz ub

Δz ub

dTb ⎡ 1 = ⎢ K 0e−E / RTbC bANHreactionVbg dz ⎣ ub ⎤ 1 + hbeS(Tep − Tb)⎥ (mbg Cpg) ub ⎦

(10)

3. MODEL RESULTS The differential equations for the mass and energy balances were solved numerically using a fourth-order Runge−Kutta method. Model inputs are presented in Table 2. Table 2. Data Used for Numerical Calculations Figure 1. Block diagram of various modes of energy transfer in the reaction zone.

(4) heat transfer between the wall and the emulsion particles, and (5) heat transfer due to solids circulation. The energy balance for the emulsion gas was modeled as follows: (energy accumulation in the emulsion gas) = (energy generated in the reaction) + (energy transferred to the emulsion particles) meg C pg ΔTeg = K 0e−E / RTeg CeANHreactionVeg − hpg a*(Teg − Tep)

Δz umf

parameter

value(s)

bed temperature (°C) bed material mean sand size (μm) sand density (kg/m3) fluidized-bed height (mm) fluidizing air flow feed flow rate (L/h) ALR ammonium nitrate concentration in the feed solution (g/L)

300−380 silica sand 300 2600 600−800 2−5umf 6 1.2 75−200

3.1. Profiles of the Axial Concentration of Ammonium Nitrate and Temperature. Variations of the axial concentrations of ammonium nitrate in the different phases were estimated under specific conditions: u/umf = 3, Tbed = 623 K (350 °C), feed rate = 6 L/h, ALR = 1.2 (air-to-liquid ratio in an external mixtype nozzle, mass flow rate of air/mass flow rate of liquid), and NH4NO3 concentration in the feed = 150 g/L. Model calculations considering specific conditions are presented in Figure 2. As liquid

Δz umf

⎡ 1 ⎤ 1 =⎢ K 0e−E / RTeg CeANHreactionVeg + hpg a*(Tep − Teg)⎥ dz umf ⎣ umf ⎦

dTeg

(meg C pg) (7)

The energy balance for the emulsion particles was modeled as follows: (energy accumulation in the emulsion particles) = (transfer of energy from the emulsion gas) + (transfer of energy from the gas bubbles) + (transfer of energy from the wall)

Ṁ pCppΔTep = hpg a*(Teg − Tep) + hbeS(Tb − Tep) + hwb(1 − δ)(πDt Δz)(Tw − Tep)

Ṁ p = JAb

(8) Figure 2. Model predictions of the axial concentration profile of ammonium nitrate in the bubble and emulsion phases.

(9)

The energy balance for the bubble gas was modeled as follows: droplets in the spray are assumed to be deposited on the hot surface of emulsion particles and vaporized, ammonium nitrate vapor is released into the emulsion phase. Hence, the initial concentration of ammonium nitrate vapor in the emulsion phase of the reaction zone can be high. The gas velocity through the

(energy accumulation in the bubble gas) = (energy generated in the reaction) − (energy transferred to the emulsion particles) 8397



Article

emulsion is equal to the minimum fluidization velocity. The residence time of the emulsion gas in the bed is relatively higher than that of the gas bubbles. Therefore, decomposition of ammonium nitrate takes place predominantly in the emulsion phase. In principle, the initial concentration of ammonium nitrate in the bubble phase is zero. Because the rate of mass exchange between the emulsion and bubble phases is relatively low, a small fraction of ammonium nitrate vapor could be transferred to the bubble phase from the emulsion phase. The residence time of gas bubbles in the bed is very short as compared to the residence time of the emulsion gas. These factors lead to negligible decomposition of ammonium nitrate in the bubble phase. Ammonium nitrate that enters into the bubble phase escapes from the bed unconverted. Figure 3 presents the axial temperature profiles of

particles along the axial direction. The fraction of ammonium nitrate decomposed in the bubble phase is negligibly small. Consequently, no increase in gas-bubble temperature is observed. 3.2. Ammonium Nitrate Conversion. Steady-state parametric studies were conducted to assess the effects of operating variables on the conversion of ammonium nitrate. One operating parameter was varied while others were kept constant at specific conditions. Effect of Bed Temperature. The influence of bed temperature on conversion along the bed height is shown in Figure 4. It is

Figure 4. Influence of bed temperature on ammonium nitrate conversion.

clear that the conversion increases with increasing bed operating temperature. The decomposition rate constant is a strong function of temperature and increases exponentially with temperature. The magnitudes of the rate constant at 623 and 663 K are 0.114 and 0.375 s−1, respectively. A 40 K increase in bed operating temperature causes a factor of 3.3 increase in reaction rate constant. Hence, the results appear to be quite reasonable. Effect of Bed Height. In Figure 5, the ammonium nitrate vapor concentration in the emulsion phase is plotted against the

Figure 3. Model predictions of the axial temperature profiles of bubble gas, emulsion gas, and emulsion particles.

the emulsion gas, gas bubbles, and emulsion particles. An overall energy balance for the spray zone was carried out to estimate the temperature in the spray zone, which is lower than the operating temperature in the bed (623 K). The spray-zone temperature is a function of the liquid feed rate, the quality of the dispersion, and the interaction of the dispersion with the fluidized bed. In the beginning of the reaction zone, the temperature of the emulsion gas and emulsion particles is equal to the spray-zone temperature. The ammonium nitrate decomposition reaction is exothermic. In the front end of the reaction zone, the rate of decomposition of ammonium nitrate is high, and the rate of heat generation is also high. An initial sharp increase in the emulsiongas temperature indicates this fact. At the upper region of the reaction zone, the rate of reaction is low because of the low concentration of ammonium nitrate in the emulsion phase. The emulsion-gas temperature passes through a maximum and then decreases and approaches the particle temperature. In the upper region of the reaction zone, the heat-transfer rate from the emulsion gas to the particles is higher than the heat-generation rate. Hence, the temperature gradient between the emulsion gas and particles is reduced. On the other hand, considering the temperature profiles of the emulsion particles and gas bubbles, it is noted that there is almost no increase in temperature along the bed height. The parameter ρCp of the solid particles is much larger than ρCp of the gas. Therefore, when reaction heat is transferred to the emulsion particles from the emulsion gas, the increase in particle temperature is negligible. Also, effective particle mixing maintains a uniform temperature among the emulsion

Figure 5. Axial concentration profile of ammonium nitrate vapor in the emulsion phase for different bed temperatures.

dimensionless bed height for different bed temperatures. When the bed operating temperature is 623 K (350 °C), a significant 8398



Article

portion of the unconverted ammonium nitrate within the emulsion phase reaches the bed surface. Hence, a low conversion is obtained. Ammonium nitrate in the emulsion phase is almost completely decomposed when the bed temperature is at or above 643 K (370 °C). Complete conversion is not obtained because of the escape of a small amount of ammonium nitrate through the bubble phase. It is noted that, at a bed operating temperature of 370 °C, nearly a unit bed height (∼0.9Dt) is required for almost complete decomposition of ammonium nitrate vapor in the emulsion phase. The required bed height above the spray zone depends on various factors, including nozzle location, nozzle orientation, feed rate, and bed operating temperature. Model calculations were done for a constant nozzle location (500 mm above the distributor), nozzle orientation (inclined downward as per the experimental setup), and feed rate (6 L/h). The bed height required for good conversion was found to be completely dependent on these parameters. Therefore, an optimum bed height can be determined only if these parameters are fixed. Effect of Operating Velocity Ratio (u/umf). The effects of u/umf on conversion for different bed temperatures are presented in Figure 6. No marked effect of u/umf on conversion can be seen.

Figure 7. Axial temperature profile of the emulsion gas for different values of u/umf (Tbed = 598 K).

Figure 8. Axial temperature profile of the emulsion gas for different values of u/umf (Tbed = 623 K). Figure 6. Effect of fluidization velocity on conversion for different bed temperatures.

623 K compared to 598 K. A lower fluidization velocity reduces the heat-transfer coefficient between the particles and the gas. The reaction heat released directly into the emulsion gas is dissipated to the emulsion particles at a lower rate at lower fluidization velocity. Hence, the increase in the emulsion-gas temperature is higher at lower u/umf. Additionally, this rise in the emulsion-gas temperature enhances the rate of reaction. Because of this phenomenon, slightly higher conversion is obtained at lower u/umf. u/umf is known to have a significant effect on heat transfer. It should be noted that the effect of u/umf on the spray−fluidized bed interaction was not considered in the above analysis. The effect of u/umf is expected to be significant in the spray deposition zone, where the heat transfer from the particles to the liquid film is important. However, there is an upper limit on the operating value of u/umf to avoid slugging flow. Effect of Feed Concentration. Figure 9 depicts the effect of the concentration of ammonium nitrate in the feed solution on the conversion for different bed temperatures. It is noted that conversion is greater at higher feed concentrations. Axial temperature profiles of the emulsion gas for different feed concentrations were estimated considering a constant bed temperature of 643 K. These theoretical results are shown

In the present study, model calculations were carried out only for the reaction zone assuming that ammonium nitrate vapor enters mainly into the reaction zone and that its temperature is the same as the emulsion-gas temperature. It has already been established that a high fluidizing air velocity leads to larger gas bubbles. The gas interchange coefficient between the emulsion and bubble phases and the residence time of gas bubbles inside the bed for larger bubbles are small. Therefore, the possibility of ammonium nitrate vapor being transferred from the emulsion phase to the bubble phase and unconverted ammonium nitrate vapor escaping through the bubble phase is reduced. Hence, conversion increases very slowly with fluidizing air velocity at a bed temperature of 598 K (325 °C). As the gas exchange between the emulsion and bubble phases is a very slow process, no pronounced effect is observed. The reverse trend when bed temperature is at or above 623 K (350 °C) can be explained by referring to Figures 7 and 8. Comparison of the two figures indicates that a significant increase in the emulsion-gas temperature occurs at a bed temperature of 623 K. This difference is quite realistic because the reaction rate is significantly higher at 8399



Article

Figure 9. Influence of feed concentration on conversion for different bed temperatures.

in Figure 10, where it can be observed that the increase in feed concentration also contributes to a local rise of

Figure 11. Schematic sketch of the fluidized-bed denitrator. Figure 10. Axial temperature profile of the emulsion gas for different feed concentrations.

Limited data are available on the experimental conditions and corresponding measured conversions (see Table 3). Using the experimental conditions as inputs to the model, the ammonium nitrate conversion was estimated. The model results are presented along with the experimental data for comparison. Also, comparisons between the experimental data and model predictions for the bed temperature and bed height as process variables are presented in Figures 12 and 13, respectively. It can be observed that the model-predicted percentage conversion was always slightly lower than the value determined experimentally. In the experimental setup, the freeboard of the denitrator and the off-gas lines to the heat exchanger were heat traced to avoid deposition of unconverted ammonium nitrate. Decomposition of ammonium nitrate in the freeboard and off-gas lines, which is not considered in the model, might be the reason for obtaining slightly higher conversions experimentally. As the concentration of ammonium nitrate vapor in the off-gas is very small, the rate of decomposition of ammonium nitrate is very low. Hence, the difference between the model calculations and experimental results is quite low. Rigorous validation of the model would require axial concentration and temperature data along the bed height, which were not measured in the experiments. Such experiments are recommended for further extensive validation of the model.

emulsion-gas temperature. Thus, the feed concentration has to be appropriately selected to obtain maximum conversion while restricting the rise of emulsion-gas temperature to within acceptable limits. 3.3. Comparison with Experimental Data. A series of experiments was carried out to study the effects of various process parameters on the conversion of ammonium nitrate. To investigate the influence of a particular process parameter, this parameter was varied over a range keeping other process parameters constant. In the fluidized-bed denitrator, the feed nozzle was located 500 mm above distributor, inclined downward (Figure 11). In this configuration, it was estimated that the deposition of the droplets was almost completed below the location of the spray nozzle. The rate of heat transfer from the particles to the deposited droplets was so high that water evaporation and volatilization of ammonium nitrate were completed in the spray zone. For such a nozzle arrangement, the bed available above the spray nozzle can be considered as the reaction zone. With reasonable approximation, it was assumed that the reaction-zone length was equal to the height of the fluidized bed (in millimeters) − 500 mm. 8400



Article

Table 3. Comparison between Experimental Data and Model Predictionsa run 1

2

300 40 35.32

800 300 2 325 46 41.70

feed concentration (g/L) fluidized-bed height (mm) reaction-zone length (mm)

4

5

6

700 200 1.3

600 100 0.67

75 800 300 2

35 31.80

55 50.10

150

h/Dt bed temperature (°C) conversion of ammonium nitrate (%)

3

experimental measurement model prediction

350 60 55.33

50 47.76

For all runs, the following conditions were used: u/umf = 7, feed flow rate = 6 L/h, air-to-liquid ratio (mass flow rate of air/mass flow rate of solution) = 1.2, active nozzle orientation downward (α = −45°) at an elevation of 500 mm (from the distributor), average size of bed particles = 300 μm. a

(2)

(3) (4)

Figure 12. Comparison between experimental data and model predictions for the bed operating temperature.

(5) (6)

(7)

Figure 13. Comparison between experimental data and model predictions for the bed height.

■

the temperature distributions in the bubble and emulsion phases. The model predictions exhibit good agreement with the experimental results. It was verified with the model that both higher bed temperature and higher feed concentration improve the decomposition of ammonium nitrate. No significant effect of operating velocity ratio (u/umf) on the conversion of ammonium nitrate was observed. The required reaction-zone length for complete conversion of ammonium nitrate vapor in the emulsion phase is an inverse function of bed temperature. The model test results suggest that a small fraction of ammonium nitrate vapor is transferred by gas interchange to the bubble phase from the emulsion phase and then escapes from the bed unconverted. Hence, complete decomposition of ammonium nitrate was found not to be possible. Higher feed concentration contributes to an appreciable local rise in the temperature gradient between the emulsion gas and particles. The model predicts that about 85−90% conversion of ammonium nitrate can be achieved by adjusting the experimental parameters. The recommended operating conditions based on the model results are as follows: Tbed = 633−643 K (360−370 °C), u/umf = 3−4, feed concentration = 175−200 g/L. However, these recommendations need to be confirmed by further experimental work. Development of a comprehensive model for the spray deposition zone based on momentum and energy exchange between the gas−liquid spray and the bubbling fluidized bed and integration of this model with the reaction-zone model developed in this work will improve the predictive capability of the overall performance of fluidized-bed denitrators.

AUTHOR INFORMATION

Corresponding Author

4. CONCLUSIONS A mathematical model that simulates the decomposition of aqueous solutions of ammonium nitrate in bubbling fluidized beds has been developed. The model is based on the two-phase theory of fluidization. From the theoretical and experimental investigations reported herein, the following conclusions were drawn: (1) The proposed model is able to predict the axial variation of the vapor-phase concentration of ammonium nitrate and

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ 8401

NOMENCLATURE Ab = cross-sectional area of fluidized-bed denitrator, m2 ALR = air-to-liquid ratio Ar = Archimedes number = [dp3ρg(ρp − ρg)g]/μg2 dx.doi.org/10.1021/ie202018t | Ind. Eng. Chem. Res. 2012, 51, 8394−8403


Article

a* = contact area between the emulsion gas and the emulsion particles, m2 CbAN = concentration of ammonium nitrate in the bubble phase, mol/m3 CeAN = concentration of ammonium nitrate in the emulsion phase, mol/m3 Cpg = specific heat of the gas, J/(kg K) Cpp = specific heat of the particles, J/(kg K) D = molecular diffusion coefficient of the gas, m2/s Dt = column diameter, m db = bubble diameter, m dp = particle diameter, m E = activation energy, J/mol G = Gibbs free energy, kJ/mol g = acceleration of gravity, m/s2 Hreaction = heat of reaction, J/mol of ammonium nitrate h = height of the reaction zone, m hbe = bubble−emulsion heat-transfer coefficient, W/(m2 K) hpg = heat-transfer coefficient between the particles and the gas, W/(m2 K) hwb = heat-transfer coefficient between the wall and the bed, W/(m2 K) J = solids circulation flux in the fluidized bed, kg/(m2 s) K0 = frequency factor, s−1 Kbc = coefficient of gas interchange between the bubbles and the cloud, s−1 Kbe = coefficient of gas interchange between the bubbles and the emulsion, s−1 Kce = coefficient of gas interchange between the cloud and the emulsion, s−1 K0e = effective thermal conductivity of a fixed bed with a stagnant gas, W/(m K) K0ew = effective thermal conductivity of a thin layer of bed near the wall, W/(m K) kg = thermal conductivity of the gas, W/(m K) ks = thermal conductivity of the particles, W/(m K) MAN = molecular weight of ammonium nitrate, mol/g mAN = mass flow rate of ammonium nitrate into the fluidized bed, g/s mbg = mass of the bubble gas, kg meg = mass of the emulsion gas, kg Ṁ p = solids circulation rate, kg/s Nup = Nusselt number for gas−particle heat transfer = hpgdp/kg Prg = Prandtl number = Cpgμg/kg Remf = particle Reynolds number at minimum fluidizing conditions = dpumfρg/μg Rep = particle Reynolds number = dpuoρg/μg S = contact area between bubble and emulsion, m2 Tb = temperature of bubble gas, K Teg = temperature of the emulsion gas, K Tep = temperature of the emulsion particles, K Tw = wall temperature, K u0 = superficial gas velocity, m/s ub = velocity of a bubble rising through a bed, m/s umf = superficial gas velocity at minimum fluidizing conditions, m/s Vbg = volume of the bubble gas, m3 Veg = volume of the emulsion gas, m3 z = height of the fluidized bed, m

■

γp = solid volume fraction in the bubble phase, 1% δ = fraction of the bed in the bubble phase εmf = void fraction at minimum fluidizing conditions μg = viscosity of the gas, kg/(m s) ρg = gas density, kg/m3 ρp = particle density, kg/m3 τ = average residence time of the particles in the vicinity of the wall, s τb = residence time of particles in the bubble phase, s

REFERENCES

(1) Otero, A. R.; Rof, J. S.; Lago, E. C. Fluidized bed calcinations of uranyl nitrate solutions. In Proceedings of the International Symposium on Fluidization; Netherlands University Press: Amsterdam, 1967; p 769. (2) Legler, B. M. Fluidized bed processing in the nuclear fuel cycle. Chem. Eng. Prog. Symp. 1970, 66 (105), 167. (3) Bjorklund, W. J.; Offutt, G. F. Fluidized bed denitration of uranyl nitrate. AIChE Symp. 1973, 69 (128), 123. (4) Fane, A. G.; Charlton, B. G.; Alfredson, P. G. Thermal Denitration of Uranyl Nitrate in a Fluidized Bed Reactor; Technical Report AAEC/ E284; Australian Atomic Energy Commission: Lucas Heights, Australia, 1974. (5) Dotson, J. M.; Peters, T. E. Conversion process for waste nitrogencontaining compounds. U.S. Patent 3,862,296, 1975. (6) Mason, J. B.; McKibbin, J.; Ryan, K.; Schmoker, D. Steam reforming technology for denitration and immobilization of DOE tank wastes. Presented at the Waste Management ’03 Conference, Tucson, AZ, Feb 23−27, 2003. (7) Kunii, D.; Levenspiel, O. Fluidization Engineering; ButterworthHeinemann: Boston, 1991. (8) Okasha, F. Modeling of liquid fuel combustion in fluidized bed. Fuel 2007, 86, 2241−2253. (9) Pre, P.; Hemati, M.; Marchandt, B. Study on natural gas combustion in fluidized beds: Modelling and experimental validation. Chem. Eng. Sci. 1998, 53, 2871−2883. (10) Miccio, M.; Miccio, F.; Faravelli, T.; Frassoldati, A.; Ranzi, E. Modeling homogeneous combustion in bubbling beds burning liquid fuels. J. Energy Resour. Technol. 2007, 129, 33−41. (11) Subramani, H. J.; Balaiyya, M. B. M.; Miranda, L. R. Minimum fluidization velocity at elevated temperature for Geldart’s group-B powders. Exp. Therm. Fluid Sci. 2007, 32, 166−173. (12) Cai, P.; Schiavetti, M.; De Michele, G.; Grazzini, G.; Miccio, M. Quantitative estimation of bubble size in PFBC. Powder Technol. 1994, 80, 99−109. (13) Davidson, J. F.; Harrison, D. Fluidized Particles; Cambridge University Press: Cambridge, U.K., 1963. (14) Kunii, D.; Levenspiel, O. Bubbling bed modelModel for flow of gas through a fluidized bed. Ind. Eng. Chem. Fundam, 1968, 7, 446−452. (15) Werther, J. Convective solids transport in a large diameter gas fluidized beds. Powder Technol. 1976, 15, 155−167. (16) Chen, J. C.; Chen, K. L. Analysis of simultaneous radiative and conductive heat transfer in fluidized beds. Chem. Eng. Commun. 1981, 9, 255−271. (17) Toei, R.; Matsuno, R.; Hotta, H.; Oichi, M.; Fujine, Y. The capacitance effect on the transfer of gas or heat between a bubble and the continuous phase in gas−solid fluidized bed. J. Chem. Engg. Jpn. 1972, 5, 273. (18) Shah, M. S.; Oza, T. M. The decomposition of ammonium nitrate. J. Chem. Soc. 1932, 21, 725. (19) Oommen, C.; Jain, S. R. Ammonium nitrate: A promising rocket propellant oxidizer. J. Hazard. Mater. 1999, A57, 253−281. (20) Gunawan, R.; Zhang, D. Thermal stability and kinetics of decomposition of ammonium nitrate in the presence of pyrite. J. Hazard. Mater. 2009, 165, 751−758. (21) Okasha, F.; Miccio, M. Modeling of wet jet in fluidized bed. Chem. Eng. Sci. 2006, 61, 3079−3090.

Greek Symbols

αw = constant representing the mixing of gas in the vicinity of wall 8402



Article

(22) Heinrich, S.; Mörl, L. Fluidized bed spray granulationA new model for the description of particle wetting and of temperature and concentration distribution. Chem. Eng. Process. 1999, 38, 635−663. (23) Heinrich, S.; Blumschein, J.; Henneberg, M.; Ihlowa, M.; Peglow, M.; Mörl, L. Study of dynamic multi-dimensional temperature and concentration distributions in liquid-sprayed fluidized beds. Chem. Eng. Sci. 2003, 58, 5135−5160.

8403


Thermal Denitration of Ammonium Nitrate Solution in a Fluidized-Bed

Recommend Documents