Christopher J. Martino and Michael R. Poirier
Westinghouse Savannah River Company
Aiken, SC 29808

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In December of 1999, the 2H evaporator was shut down due to the inability to lift solution from the evaporator pot. Visual inspection of the evaporator pot showed significant buildup of solids on most of the exposed surfaces of the evaporator pot. A sample confirmed the material was sodium aluminosilicate scale mixed with sodium diuranate. Due to the large amount of uranium and the high enrichment, a Potential Inadequacy in the Safety Analysis (PISA) was declared in January 2000.

SRS will clean the evaporator with a nitric acid solution that contains depleted uranium. The depleted uranium that is added to the evaporator will contain tributylphosphate (TBP) and its decomposition products, which will decompose to form 1-butanol during cleaning. High Level Waste Engineering requested SRTC to determine a conservative rate of butanol release from, and a maximum concentration of butanol in, the evaporator pot as a function of evaporator temperature, air sparge rate, and butanol concentration.

SRTC performed the analysis by modeling the air-sparging process as a bubble column. The butanol release rate is calculated by solving three mass balances: a mass balance of butanol in the entire evaporator pot, an overall butanol balance in the bubble column, and a differential mass balance for the bubble column. The authors used conservative and bounding assumptions to perform the analysis.

The results of the analysis are the following:

• At sparge rates of 50 – 150 SCFM, the butanol formed during 2H evaporator cleaning is rapidly removed from the pot.
• The peak liquid butanol concentration in the evaporator pot, which occurs with the highest temperature (100° C), the lowest sparge rate (50 SCFM), and the maximum initial MBP (300 mg/L) concentration, is 14.2 mg/L.
• At 75° C, a sparge rate of 50 SCFM, and an initial MBP concentration of 150 mg/L, the maximum butanol concentration in the evaporator pot is 3.7 mg/L butanol.
• At 90° C, a sparge rate of 50 SCFM, and an initial MBP concentration of 150 mg/L, the maximum butanol concentration in the evaporator pot is 6.9 mg/L butanol.
• At 100° C, a sparge rate of 50 SCFM, and an initial MBP concentration of 150 mg/L, the maximum butanol concentration in the evaporator pot is 10.8 mg/L butanol.
• The maximum liquid butanol concentration in the evaporator pot decreases with increasing sparge rate.

The 242-16H (2H) evaporator is located in the H-Area Tank Farm. The evaporator pot is a 304-L stainless steel vessel 8 feet in diameter and 16.5 feet tall with a normal operating capacity of ~1950 gallons. The design pressure and temperature are 15 psig and 160° C, respectively. Figure 1 shows the evaporator vessel. The conical shape at the bottom of the vessel is designed for efficient removal of concentrated waste.

In December of 1999, the 2H evaporator was shut down due to the inability to lift solution from the evaporator pot. Visual inspection of the evaporator pot showed significant buildup of solids on most of the exposed surfaces of the evaporator pot. A sample of the material at the bottom of the evaporator cone confirmed the material was sodium aluminosilicate scale mixed with sodium diuranate. This material was ~7.4 wt. % uranium with ~3% ²³⁵U enrichment. Due to the large amount of uranium and the high enrichment, a Potential Inadequacy in the Safety Analysis (PISA) was declared in January 2000.

After reviewing options for cleaning the evaporator, SRS chose cleaning with dilute nitric acid. Depleted uranium (~ 300 g/L uranyl nitrate) will be added for isotopic dilution. A nitric acid/depleted uranium solution will be added to the evaporator pot. The evaporator will be heated with steam and agitated with an air lance during dissolution of the scale material. Once cooled, the material will be transferred to a neutralization tank, neutralized with NaOH, and transferred to a waste tank.¹

The history of the depleted uranium, added during evaporator cleaning, involves its mixture with tributyl phosphate (TBP). Over time, the TBP has partially hydrolyzed to form dibutyl phosphate (DBP), monobutyl phosphate (MBP) and butanol according to equation [1].

TBP ® DBP + BuOH ® MBP + 2BuOH ® 3BuOH + H₃PO₄ [1]

A current analysis of the uranyl nitrate in a tanker car indicates the concentrations of butyl phosphates in the cleaning solution would be 5 mg/L TBP, 112 mg/L DBP, and an unknown concentration of MBP.²

The butanol could create a flammability hazard in the evaporator pot. The air lance will provide a mechanism to remove butanol from the evaporator pot. High Level Waste Engineering requested SRTC to determine the rate of butanol release from the evaporator pot and the maximum liquid butanol concentration as a function of evaporator temperature, air sparge rate, and butanol concentration in the evaporator. This report describes an analysis performed using conservative and bounding assumptions. These assumptions lead to a lower bound on the butanol removal rate and an upper bound on the liquid phase butanol concentration.

Most commercial applications of gas stripping and its analog, gas absorption, are continuous processes with countercurrent gas and liquid flows. Counter-current gas stripping or absorption is usually performed in packed beds, but occasionally bubble columns are used. In a bubble column, gas is sparged into the bottom of a cylinder of either stagnant or down-flowing liquid. In contrast, flow in the evaporator will likely be localized co-current upward flow of gas and liquid in an air plume, and downflow recirculation of liquid in the remaining evaporator area. This scheme is similar to an airlift reactor, with the exception that the riser and downcomer areas of an airlift reactor are always physically bound.

In actuality, the evaporator-sparging process is between the bubble column model and the airlift reactor model. Gas-liquid mass transport in bubble columns has been more widely studied and resulting correlations have less uncertainty than the correlations for airlift reactors. Thus, the authors chose to model the mass transport in the evaporator as a bubble column with an adjustment for the upward liquid flow in the air plume as encountered in an airlift reactor.

Figure 2. Modeling of Evaporator Sparging.

Figure 2 contains a schematic of how the 242-16H evaporator can be modeled as a bubble column during air sparging. Illustration a) represents the full evaporator pot being sparged with the air lance. Air introduction takes place through a single orifice at the end of the lance. Due to this sparging method, the rising air is radially confined to a region near the center of the evaporator cylinder. Illustration b) shows a bubble column with a batch charge of liquid. Air is introduced through the bottom of the column in a manner that assures the even distribution of gas over the column cross section. Illistration c) combines the bubble column concept with the evaporator sparging situation. A portion of the evaporator contains an upflow of liquid and gas fluid that is approximated by a bubble column, while the remainder of the evaporator contains a well-agitated downflow of liquid. Thus, the effective diameter of the bubble column that approximates the air rising in the evaporator is less than the diameter of the entire evaporator pot. The size of the spout of air in the evaporator, and thus the effective bubble column diameter, is an unknown parameter affecting the mass transfer calculations.

In the evaporator pot, the butanol is transferred to the air bubbles and removed from the liquid as the bubbles enter the vapor space. Three mass balances are needed to evaluate this model: a mass balance of butanol in the entire evaporator pot, an overall butanol balance in the bubble column, and a differential mass balance for the bubble column using the log mean concentration gradient as the driving force.

An overall mass balance on the column is described by equation [2]

[2]

where V_L is the liquid volume, C_L is the liquid phase concentration, t is time, r_hydrolysis is the rate of butanol formation, Q_G is the air sparge rate, and C_vHbc is the butanol concentration in the gas bubbles leaving the column. The concentration of butanol in the gas bubbles leaving the column is needed to solve equation [2].

That concentration can be calculated by performing a mass balance on the butanol in the bubble column. That mass balance is described by equation [3]

N_AA_bc = u_GA_bc(C_vHbc - C_vo) = u_LA_bc(C_LHbc – C_Lo) [3]

where N_A is the mass transfer flux, u_G is the gas phase superficial velocity, A_bc is the cross sectional area of the bubble column, C_vHbc is the butanol concentration in the gas exiting the bubble column, C_vo is the butanol concentration in the gas entering the column, u_L is the liquid superficial velocity, C_LHbc is the butanol concentration in the liquid exiting the bubble column, C_Lo is the butanol concentration in the liquid entering the bubble column.

The differential mass balance for the bubble column is described by equation [4]

N_AdA_bc = K_Ga(C_v^* - C_v)A_bcdz = K_GaDC_vA_bcdz [4]

where K_G is the overall gas phase mass transfer coefficient, a is the interfacial area, z is the vertical distance, C_v is the vapor phase butanol concentration, and C_v^* is the vapor phase butanol concentration that is in equilibrium with the bulk liquid butanol concentration (C_L).³

Following an analogy with heat transfer, the derivative of DC with respect to mass transfer rate can be expressed in terms of the overall change in DC and the total mass transfer rate in the column as described by equation [5].³

[5]

Substituting equation [4] into equation [5] produces equation [6]

[6]

Rearranging equation [6], changing the concentration to vapor concentration, and integrating over the column height produces equation [7]

[7]

where H_bc is the height of the bubble column. Integrating equation [7] produces equation [8].

[8]

Since the resistance to mass transfer in the liquid phase is much greater than the resistance to mass transfer in the vapor phase, the overall gas phase mass transfer coefficient, K_G, can be replaced by the liquid phase mass transfer coefficient, k_L, divided by the Henry’s Law constant.³ Solving equation [8] for the mass flux and replacing the overall gas phase mass transfer coefficient with the liquid phase mass transfer coefficient divided by the Henry’s Law constant (H) produces equation [9].

[9]

To solve equation [9], one needs the liquid phase mass transfer coefficient, the interfacial area, the height of the bubble column, the Henry’s Law constant and the concentration gradient at the inlet and outlet of the column.

Equations [10] – [12] define the concentration gradients used in this analysis.

DC_v = C_v^* - C_v [10]

DC_vHbc = C_vHbc^* - C_vHbc = HC_LHbc - C_vHbc [11]

DC_vo = C_vo^* - C_vo = C_vo^* = HC_Lo [12]

Combining equations [3] and [9] produces equation [13]

[13]

where u_b is the bubble rise velocity. The bubble rise velocity is used in equation [13] rather than the gas superficial velocity, because it is faster, it reduces the contact time, and is more conservative. Substituting equations [11] and [12] into equation [13] produces the following expression

[14]

Rearranging equation [3] gives the following expression for the outlet liquid phase butanol concentration

C_LHbc = C_Lo – u_GC_vHbc/u_L [15]

Substituting equation [15] into equation [14] and rearranging produces equation [16]

[16]

In equation [16], the term H_bc/u_b is the contact time (t_c) of the gas bubbles with the liquid. Equation [16] can be simplified to form equation [17]

[17]

Taking the exponential of both sides of equation [17] produces

[18]

Solving equation [18] for C_vf produces equation [19].

[19]

A stripping efficiency can be defined by equation [20]

[20]

and equation [19] can be rewritten as

C_vHbc = F H C_Lo [21]

Substituting equation [19] into the overall mass balance, equation [2], gives the following expression for the transport of butanol from the liquid to the vapor in the evaporator.

[22]

To solve equation [22], one needs the following information: the air sparge rate (Q_G), the evaporator liquid volume (V_L), the Henry’s Law constant (H), the initial liquid phase butanol concentration (C_Lo), the gas bubble superficial velocity (u_G), the liquid phase superficial velocity in the bubble column (u_L), the bubble rise velocity (u_b), the liquid phase mass transfer coefficient (k_L), the interfacial area (a), and the height of the bubble column (H_bc).

Since the liquid in the evaporator pot is not stagnant, the actual contact time between the gas and liquid is less than in a standard bubble column. The gas bubbles will entrain liquid and cause it to flow upward in the center of the evaporator pot. Equation [23] provides a means for calculating the superficial liquid velocity.⁴

[23]

In equation [23], A_e is the cross sectional area of the evaporator pot and A_bc is the cross sectional area of the bubble column. Since the evaporator pot does not have a draft tube, the actual liquid velocity should be less than predicted by equation [23], which is conservative.

To be conservative, the bubble velocity was calculated as the sum of the bubble velocity and the liquid velocity in the bubble column.

u_b = u_G/e_G + u_L/(1-e_G) [24]

To solve equation [22], one needs to know the mass transfer coefficient and the interfacial area. The mass transfer coefficient was obtained from equation [25]

[25]

where D_i is the molecular diffusivity, d_vs is the Sauter mean bubble diameter, n_L is the kinematic viscosity, g is gravitational acceleration, r_L is the liquid density, s is surface tension.⁵

The Sauter mean bubble diameter is determined from equation [26]

[26]

where D_bc is the diameter of the bubble column and u_G is the interstitial gas velocity.⁵ In equation [26], when the bubble column diameter is larger than 0.3 m, a value of 0.3 m is used for D_bc.

The interfacial area is determined from equation [27]

a = 6e_G/d_vs [27]

where e_G is the gas phase holdup.⁶

The gas phase holdup is determined from either equation [28] or [29].^5,6

[28]

[29]

Equations [25] – [29] are solved to determine the mass transfer coefficient and the interfacial area. Both equations [28] and [29] are solved for gas phase holdup, and the lower value calculated is used in equation [27], to be conservative. Once the mass transfer coefficient and interfacial area are known, equation [22] can be solved to determine the amount of butanol removed from the evaporator pot.

Table 1 shows the input parameters used in the calculation. The diffusivity of butanol in water at 25° C was found in the technical literature.⁷ The diffusivity was calculated at other temperatures with the Wilke-Chang equation.⁸ The effect of nitric acid on diffusivity is not known. Data on the diffusivity of KCl in electrolytes shows a change in diffusivity of less than 20% as salt concentration increased from 0 – 2 N.⁸ Therefore, the diffusivity values were multiplied by 0.8 for conservatism.

The viscosity of the solution was measured by SRTC at 25° C.⁹ The nitric acid solution will have a lower viscosity than the neutralized solution. Sensitivity analyses performed showed minimum effect of viscosity on the butanol removal rate. A higher viscosity increases the resistance to mass transfer, and is conservative. The viscosity at other temperatures was calculated with equation [30]

m^-0.2661 = m_K^-0.2661 + (T – T_K)/233 [30]

where T is temperature and m_K is the known viscosity at a known temperature T_K.⁸

SRTC calculated the density of this solution at 25° C.¹ The density was calculated at other temperatures by assuming the temperature correction was the same as for 1.5 M nitric acid.¹⁰

The height of the bubble column was assumed to be 100 inches. The actual height will be larger, because the cone height (~ 5 ft) is not included. A larger height would increase the contact time and improve butanol removal. The impact of bubble column diameter was investigated. Smaller bubble column diameters decrease butanol removal. A bubble column diameter of 0.4 m was selected as the minimum reasonable for the 2H evaporator for sparge rates of ³ 50 SCFM. The Henry’s Law constant was extrapolated from butanol-in-water data at 25 ° C by using vapor pressure data for butanol and assuming no change in water solubility.¹¹ The Henry’s Law constant in water is less than the Henry’s Law constant in salt solution.¹² One would expect a similar result for nitric acid. Therefore, using Henry’s Law constants for water would be conservative. The surface tension of water was used.¹³ This value shows very little sensitivity to temperature or the addition of nitric acid.¹³

Figure 3 presents the production rates of butanol for the cases investigated in the subsequent graphs. These production rates are based on the rate approximations developed in other reports² and correspond to a sparge rate of zero. At any set of stable conditions, the rate of butanol formation is greatest as the cleaning solution first reaches the desired temperature. Because it is exclusively a product of the hydrolysis reactions, the amount of butanol in the evaporator pot does not decrease with time.

Figure 3. Formation of 1-butanol in the evaporator pot
from the hydrolysis of buytylphosphates, without air sparging.

Figures 4 through 7 are conservative predictions of the liquid-phase concentration profiles of butanol in the evaporator during cleaning. Each figure contains four sparge rates (0, 50, 100, and 150 SCFM) for a single temperature (75, 90, or 100 ° C) and initial MBP concentration (150 or 300 mg/L). Butanol removal is a function of sparge rate, with greater sparge rates leading to greater degrees of butanol removal. While butanol removal rates are higher at higher temperatures, the increase in hydrolysis rates at higher temperatures causes the amount of butanol remaining in the pot to increase with temperature. Comparing Figures 6 and 7, a higher initial concentration of MBP leads to a higher peak butanol concentration.

Figures 4 through 7 illustrate that when the evaporator is sparged at any constant rate, the balance of the hydrolysis rate and the air stripping rate cause the butanol to reach a maximum concentration and a slow decline. Table 2 contains the maximum butanol concentration for each of the stripping cases and the corresponding cleaning time at which this maximum occurs. The highest temperature (100° C), lowest sparge rate (50 SCFM), and highest initial MBP concentration (300 mg/L) leads to the highest peak in butanol concentration (14.2 mg/L). Table 3 contains a summary of the butanol concentration profile for this, the most conservative stripping case.

Figure 4. Predictions, at several sparge rates, of 1-butanol concentration in the
evaporator pot as a function of time. T = 75 ° C, initial MBP concentration is 150 mg/L.

Figure 5. Predictions, at several sparge rates, of 1-butanol concentration in the
evaporator pot as a function of time. T = 90 ° C, initial MBP concentration is 150 mg/L.

Figure 6. Predictions, at several sparge rates, of 1-butanol concentration in the
evaporator pot as a function of time. T = 100 ° C, initial MBP concentration is 150 mg/L.

Figure 7. Predictions, at several sparge rates, of 1-butanol concentration in the
evaporator pot as a function of time. T = 100 ° C, initial MBP concentration is 300 mg/L.

Table2. Prediction of the maximum butanol concentration in the evaporator pot and
the time into the cleaning process corresponding to the maximum butanol concentration.

Table3. Butanol concentration in the evaporator pot at the worst
stripping case (T=100° C, Q_G=50 SCFM, and initial MBP concentration of 300 mg/L).

As defined by equation [20], the approach of the actual vapor-phase concentration leaving the evaporator to the vapor-phase concentration corresponding to an equilibrium with the bulk liquid-phase is the stripping efficiency, F . Table 4 contains F values for the conditions of interest, as well as several intermediate values used in the calculation. Note that F is a function of the Henry’s Law constant, and the values for F presented here are calculated using the Henry’s Law constants of butanol in water. While the lower Henry’s Law constants for butanol in water, as compared with salt solutions, leads to lower stripping rate, these lower constants lead to higher stripping efficiencies.¹⁴

Table 4. Paremeters calculated for D_bc = 0.4 m.

Figure 8. Dependence of the stripping efficiency on the effective bubble column diameter.

The effective bubble column diameter, D_bc, used in these calculations has a large effect on the stripping efficiency. Figure 8 presents the relationship between the stripping efficiency and the bubble column diameter for the moderate temperature (90° C) and sparge rate (100 SCFM). At this sparge rate, the stripping efficiency sharply decreases below diameters of about 0.4 m. This decrease is due to an unrealistic increase in the bubble velocity. Such an increase would correspond to slug flow, which would not occur in this situation where the air plume is not horizontally bound. As long as D_bc << D_e, any slug flow would expand horizontally to restore churn-turbulent flow, thus increasing the effective bubble column diameter, contact time, and stripping efficiency.⁶

The authors have not been able to find, in the related literature, a clear predictive relationship for calculating the effective bubble column diameter representative of our system. Thus, stripping efficiencies are calculated assuming a relatively small bubble column diameter, D_bc = 0.4 m. This effective diameter leads to contact times of 0.24 to 0.38 s, which are lower than expected values and conservative. Yet, the high stripping efficiencies obtained result in butanol stripping rates that are nearly the butanol-in-water equilibrium rates.

The conclusions of the analysis are the following:

• At sparge rates of 50 – 150 SCFM, the butanol formed during 2H evaporator cleaning is rapidly removed from the pot.
• The peak liquid butanol concentration in the evaporator pot, which occurs with the highest temperature (100° C), the lowest sparge rate (50 SCFM), and the maximum initial MBP (300 mg/L) concentration, is 14.2 mg/L.
• At 75° C, a sparge rate of 50 SCFM, and an initial MBP concentration of 150 mg/L, the maximum butanol concentration in the evaporator pot is 3.7 mg/L butanol.
• At 90° C, a sparge rate of 50 SCFM, and an initial MBP concentration of 150 mg/L, the maximum butanol concentration in the evaporator pot is 6.9 mg/L butanol.
• At 100° C, a sparge rate of 50 SCFM, and an initial MBP concentration of 150 mg/L, the maximum butanol concentration in the evaporator pot is 10.8 mg/L butanol.
• The maximum liquid butanol concentration in the evaporator pot decreases with increasing sparge rate.

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