M. A. Baich
Westinghouse Savannah River Company
Aiken, SC 29808

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Two bench-scale process simulations of the proposed cesium eluate evaporation process of concentrating eluate produced in the Hanford site Waste Treatment Plant were conducted. Evaporation was performed at a temperature of 55 +/- 3° C and a vacuum of 27-28 in Hg. The primary objective of these experiments was to determine the physical properties and the saturation concentration of the eluate evaporator bottoms while producing condensate at ~0.50 molar HNO3.

Chemical composition profiles and physical property data were developed for the evaporator condensate and bottoms for the duration of the process simulation to enable validating the eluate evaporator computer model developed by A. S. Choi using the OLI/ESP software. The specific gravity of the evaporator bottoms ranged from a starting value of 1.25 g/ml to a final saturation value of 1.33 g/ml. The heat capacity of the evaporator bottoms fell by 10% by the end of these experiments (See Table 6). Viscosity measurements of the evaporator product concentrate indicated a Newtonian viscosity twice that of water (2 centipoise).

While not part of the original task request (TSP-W375-99-00008), the average evaporator decontamination factor (DF) was determined in Run 2 to be 5x10^-6.

The data indicates that the number of batches (A batch of feed is defined as the volume of eluate equal to the initial evaporator charge.) of eluate feed to reach saturation is specific for the composition of the eluate used in these tests and in particular the sodium concentration. Sodium nitrate was the only species formed at saturation. Eluate used in Run 2 had approximately twice the sodium concentration as that of Run 1, and therefore saturation was achieved in Run 2 after approximately 17 batches of fed versus greater than 65 batches of eluate feed in Run 1. The 25 ° C saturation concentration determined in Run 2 was; 7.10 molar acid, 2.06 molar sodium, and 9.03 molar nitrate. Condensate generated in Run 1, averaged 0.43 molar in total acid, (target acidity 0.5 molar) while condensate generated in Run 2 averaged 0.55 molar acid. Thus, the acidity in the evaporator bottoms was higher than the target in Run 1 and less than the target in Run 2. This could also affect the observed solubility.

Detailed interpretation of all the data from these process simulations compared to the SRTC Cesium Eluate Evaporation model will be presented by A. S. Choi in a separate report.

The baseline low activity waste (LAW) flowsheet for the River Protection Project (RPP) Waste Treatment Plant (WTP) includes pretreatment of Envelope A, B and C supernate by removing cesium using an ion-exchange column. When the ion-exchange column is spent, the cesium will be eluted with a 0.5 molar nitric acid solution. The cesium ion exchange resin is conditioned for re-use. The cesium eluate solution will then be concentrated in a vacuum evaporator to minimize storage space and recover ~0.5 molar nitric acid for re-use. In order to preclude formation of solids during the storage of evaporator products, BNFL and DOE have respectively set additional criteria for limiting the concentration of the evaporator bottoms to 80% of saturation at 25 ° C.

As part of the BNFL/WSRC Work-for-Others Agreement, SRTC was tasked to perform two small-scale scouting evaporation experiments using eluate simulant of Envelope A and B (Tanks AW-101 and AN-107, respectively) ion exchange columns. The data from bench-scale evaporation tests will be used to validate the cesium eluate evaporator model developed by Alex Choi using the OLI/ESP software.

The principal objectives of these tasks were to determine the following:

• Major precipitating species of the simulant and their solubility limits
  
• Maximum volume of eluate that can be processed for a given evaporator charge
  
• Vapor/liquid equilibrium
  
• Expected evaporator operating parameters (temperature, pressure)
  
• Composition of the evaporator concentrate and condensate
  
• Physical properties of the concentrate (e. g. specific gravity, heat capacity, and viscosity)

The baseline composition of the column eluate from AN-107 (Simulant B), [1] was previously approved by BNFL and was used in Run 1.The baseline composition of the column eluate from AW-101 (Simulant A), [2] was subsequently approved by BNFL and was used in Run 2. The composition of these simulant solutions was based on the composition of the actual cesium eluate solutions derived from small-scale tests with waste samples from tanks 241-AN-107 and 241-AW-101. All chemicals used were of reagent grade and deionized water was used to prepare the solutions. Tables 1, 2 and 3 list the target compositions of each eluate simulant, required chemical addition to achieve the target composition, and the analysis of each of the prepared simulant.

[1] BNFL Task Specification: Eluate Physical Properties and Evaporation Test Specification, BNF-003-98-0176
[2] Use of Eluate Composite Composition for AW-101, SL-644 approved by E-Mail from Mike Johnson

TABLE 1. Simulant Compositions Used in Cesium Eluate Evaporation Bench-Scale Runs.

	AN-107 Eluate Simulant (a)	Run 1			AW-101 Eluate Simulant (b)	Run 2
Ionic Species	Target (mg/L)	Analysis (mg/L)	Ionic Species		Target (mg/L)	Analysis (mg/L)
Al	40	40.5	Al		141	133
Na	1100	1190	Na		2230	2173
K	40	43.3	K		382	401
Cl	267	225	Cl		0	0
NO3	28200	31700	NO3		33000	32790
Cs	53	52	Cs		74	73
H	409	412	H		407.7	440
Ca	0	0	Ca		2.1	2.2
Cr	0	0	Cr		3.48	3.2
Ni	0	0	Ni		5.89	5.5
Zn	0	0	Zn		12	11.1

a) Per Technical Task Plan BNF-003-98-0176, Revision 1
b) Per Eluate Composite Composition for AW-101, SL-644

 

TABLE 2. 40 Liter Simulant Make-up for Run 1.

Compound	MW	Milligrams Reagent per Liter	Gms for 40 Liters of Eluate
AlCl3	133.34	198	7.92(a)
NaCl	58.44	96	3.84(d)
KCl	74.56	76.1	3.04(b
NaNO3	84.99	3927	157.08(g)
CsCl	168.36	67.1	2.68(c)
HNO3 (e)	63.01	25750	14189.8(f)
		SUM---->	14364.36
water		Water By Diff.,-->	25635.7

TABLE 3. 40 Liter Simulant Make-up for Run 2.

Compound	MW	Milligrams Reagent per Liter	Gms for 40 liters of Eluate
Al(NO3)3.9H2O (a)	375.13	1960.5	78.42
NaNO3 (a)	84.99	8244	329.76
KNO3 (a)	101.11	988	39.52
HNO3 (b)	63.01	25690	1580.92
Ca(NO3)2.4H2O(a)	236.15	12.4	0.50
Cr(NO3)3.9H2O (a)	400.15	26.8	1.07
Fe(NO3)3.9H2O (a)	403.95	89	3.56
Ni(NO3)2.6H2O (a)	290.81	29.2	1.17
Zn(NO3)2.6H2O (a)	297.47	54.6	2.18
CsNO3 (a)	194.91	108.5	4.34
		SUM---->	2041.44
		Water By Diff.,-->	37958.56

(a) gms Metal Reagent req’d = (Target mg/L)(MW_reagent g /g-mol)(40L)/ (1000mg/g)(MW_metal g /g-mol)
(b) gms of 16.0M HNO3 req’d = (407.7 mg H/L)(40L)(1000 mL/L)(1.0387 gm/mL)/(1000 mg/g)(16.0 g-mol H/L)

The purpose of Run 1 was to duplicate the Cs eluate evaporation by feeding eluate to a constant volume evaporator. Samples were pulled to determine the specific gravity, chemical composition and heat capacity of both the evaporator pot and overhead condensate. Because the samples were such a significant fraction of the evaporator inventory, the level that the evaporator was controlled at was adjusted after each sample was pulled. Table 4 below summarizes the eluate evaporated, the sample removed, and the number of batches fed to the evaporator.

Table 4. Calculated Run 1 Batch Addition Summary

Eluate fed	Pot Volume	Samples	Batch
(ml)	(ml)	(ml)	#
0	820	20	0.0
4000	800	20	5.0
8000	780	20	10.3
12000	760	20	15.8
16000	740	20	21.6
20000	720	20	27.8
24000	700	20	34.3
28000	680	20	41.2
32000	660	20	48.5
36000	640	20	56.3
40000	620	20	64.5

Figure 1 shows a drawing of the evaporator configuration for Run 1. System vacuum was used to pull feed into the evaporator. A needle valve (V-4) was used to control this flow to control evaporator level. Samples of the evaporator bottoms were taken after every 4 liters of condensate generated. Analyses of the evaporator bottoms and the corresponding condensate analysis can be used to estimate vapor-liquid equilibrium data. The target evaporator level was reduced by the sample volume, as samples of the evaporator were pulled. The initial evaporator charge of 800 ml was a 50-50 volume mixture of concentrated nitric acid and water to give an initial acidity of 8.02 molar. An air bleed to the vacuum pump controlled system pressure at 27-28 mm Hg, which resulted in an operating (boiling) temperature of 55 +/- 3 °C. The normal operating temperature for the proposed WTP cesium eluate evaporator is 50 to 60^oC (System Description for Cesium Nitric Acid Recovery: PT-320, SD-W375PT-PR00004, revision 2, February 16, 2000). Little or no variation in these conditions was observed during the entire evaporation. The condenser was operated at 5 °C. While the condenser for the proposed cesium eluate evaporator system does not operate at this temperature, this experiment was conducted at a reduced temperature to ensure complete condensation of evaporator overheads. Condensate was periodically removed by stopping vacuum and emptying the collection flask. Specific gravity of the evaporator contents was determined by pulling a sample into the hydrometer sample tube, reading the hydrometer, and returning the sample to the evaporator.

In Run 2, the evaporator configuration was modified to allow pumping the eluate into the evaporator and pumping condensate out (see Figure 2). This change was made to allow ease of operation. Peristaltic pumps were used for this purpose. These flows were balanced to match volume of eluate fed with condensate generated to maintain constant volume operation. Evaporation was performed at a temperature of 55 +/- 3° C and a vacuum of 27-28 in Hg. Specific gravity of the evaporator bottoms was determined by the same method as in Run 1. No samples of the evaporator bottoms were taken during operation so that no salts would be lost to samples. Extra eluate was added periodically to dilute the evaporator bottoms when condensate acidity exceeded 0.5 molar. Table 8 summarizes the acidity of each carboy of condensate collected.

Due to the higher sodium concentration of the eluate used in Run 2, only half of the projected eluate was evaporated before solids formed. In an attempt to determine the saturation point at 25°C, several adjustments of the acidity in the evaporator bottoms were made. Nitric acid solutions of various strengths and volume were used for this purpose.

FIGURE 2. Drawing of Eluate Evaporator Setup for Run 2

Table 5 shows the analytical analysis and specific gravity of the samples taken from the evaporator. No solids were formed after all 40 L of the eluate was fed. Only after additional concentration was performed, did solids appear when left overnight. Sample "Conc-11" was taken after all of the eluate feed was fed to the evaporator and the evaporator bottoms were further concentrated from 620 to 480 ml. XRD analyses of the solids formed indicate that only sodium nitrate had precipitated. Figure 3 shows the XRD scan. A substantial, but un-quantified amount of solids were formed. An ion balance for each of the samples demonstrated closure within 10%.

TABLE 5. Run 1 Concentrate Analysis and Specific Gravity.

Sample			H+	Al	Cs	K	Na		NO3	Cl		SpG
			Molar	mg/L	mg/L	mg/L	mg/L		mg/L	mg/L
Conc-1			6.98	215	278	210	5210		478000	801		1.250
Conc-2			6.79	394	666	412	9966		490000	503		1.290
Conc-3			7.00	626	ND	ND	ND		513000	320		1.290
Conc-4			6.30	790	1070	784	24700		517000	246		1.285
Conc-5			6.44	1020	1550	963	28000		514000	238		1.300
Conc-6			6.43	1220	1780	1360	38700		567000	336		1.300
Conc-7			6.42	1440	1990	1560	48600		566000	329		1.315
Conc-8			5.92	1710	2300	1690	47800		550000	1014		1.315
Conc-9			5.22	1870	2470	1860	50400		546000	1066		1.320
Conc-10			4.98	1960	2480	2300	56800		528000	425		1.324
Conc-11 (supernate)a			7.90	2720	3720	3100		29000	550000	433	1.330

a. Composition of aqueous fraction of the evaporator bottoms after 40L of eluate had been fed
to the evaporator and the volume of the evaporator bottoms concentrated from 620 mL to 480 mL

Table 6 presents the measured heat capacity as a function of temperature for each of the concentrate samples. The heat capacity of the evaporator bottoms fell from that of water; to about 90% of the heat capacity of water at the end of concentration.

TABLE 6. Heat Capacity(a) Data for Run 1 Concentrate Samples.

	Cp (a)	Cp (a)	Cp (a)	Cp (a)	Cp(a)	Cp(a)
Temp (oC)	37	47	57	67	77	87
Conc-1	0.80	0.82	0.84	0.85	0.87	0.88
Conc-2	0.78	0.81	0.83	0.85	0.87	0.88
Conc-3	0.79	0.81	0.83	0.84	0.85	0.86
Conc-4	0.80	0.82	0.84	0.85	0.86	0.87
Conc-5	0.66	0.68	0.68	0.70	0.71	0.72
Conc-6	0.76	0.79	0.80	0.82	0.83	0.84
Conc-7	0.78	0.81	0.82	0.83	0.84	0.85
Conc-8	0.78	0.81	0.81	0.82	0.84	0.85
Conc-9	0.73	0.76	0.77	0.78	0.79	0.80
Conc-10	0.74	0.76	0.78	0.79	0.80	0.81
Conc-11 (b)	0.54	0.56	0.57	0.58	0.59	0.60
Water	0.80	0.84	0.86	0.87	0.88	0.89
Conc-10 %Cp of water	92.5	90.5	90.7	90.8	90.9	91.0

(a) Cp, heat capacity, (cal/gm-^oK)
(b) Heat capacity of aqueous fraction of the evaporator bottoms after 40L of eluate had been fed
to the evaporator and the volume of the evaporator bottoms concentrated from 620 mL to 480 mL

Table 7 shows the analysis of the condensate samples taken during Run 1. Analysis indicates that both nitric acid and hydrochloric acid are distilled from the evaporator when chloride ion is present. Evaporator sample analyses are also presented for estimation of vapor-liquid equilibrium relationship.

TABLE 7. Analysis of Condensate and Concentrate Samples from Run 1.

Sample	Cond	Cond H+	Cond NO3	Cond Cl	Sample	Evap H+	Evap NO3	Evap Na
	(gm)	Molar	mg/L	mg/L		Molar	mg/L	mg/L
Cond-1	4060	0.51	30100	145	Conc-1	6.98	478000	5210
Cond-2	8020	0.40	22400	252	Conc-2	6.79	490000	9970
Cond-3	12000	0.40	23700	221	Conc-3	7.00	513000	ND
Cond-4	16100	0.42	26000	192	Conc-4	6.30	517000	24700
Cond-5	20100	0.40	24200	303	Conc-5	6.44	514000	28000
Cond-6	24200	0.42	25800	254	Conc-6	6.43	567000	38700
Cond-7	28100	0.44	26600	205	Conc-7	6.42	566000	48600
Cond-8	32100	0.46	27200	245	Conc-8	5.92	550000	47800
Cond-9	36100	0.42	24500	234	Conc-9	5.22	546000	50400
Cond-10	38500	0.41	20600	272	Conc-10	4.98	528000	56800

Run 2

Due to the higher sodium concentration of the eluate feed, (~2X Run 1) the formation of solid sodium nitrate was noted after 20.4 liters (25.5 batches) of eluate was fed. Once solids were observed, several additions of nitric acid solution were made in an attempt to adjust evaporator concentration for the excess eluate fed and determine the saturation concentration at 25 °C. Acid additions included:

1^st Addition 150 ml of 9.2 molar nitric acid
2^nd Addition 119 ml of 7.5 molar nitric acid
3^rd Addition 72 ml of 11.4 molar nitric acid
4^th Addition 90 ml of 7.9 molar nitric acid
5^th Addition 72 ml of 7.8 molar nitric acid after the removal of 300 ml of evaporator bottoms.

Table 8 presents the mass and acid analysis of the 5 carboys of condensate collected during the second run. Since no chloride was present in the eluate feed for run 2, no species other than nitric acid are distilled from the evaporator and nitrate molarity was assumed to equal the acid molarity.

TABLE 8. Analysis of Condensate Carboys from Run 2.

Carboy	Mass (gm)	H+ (molar)	Batch #
1	4042	0.57	5
2	4036	0.55	10
3	4032	0.50	15
3.45 (a)	1816	0.53	17.3
4	4040	0.56	20
5	4036	0.56	25

(a) 25° C Saturation point calculated from acid additions 3 and 4.

Table 9 presents the calculated cesium DF based upon the concentration of cesium in the eluate and the measured cesium concentration of the condensate.

TABLE 9. Calculated Cesium Decontamination Factor (DF) for Run 2.

Eluate	Cesium (a)	Pot	Pot	Condensate	*DF (b)
Fed (ml)	Fed (mg)	Volume (L)	Cs (mg/L)	Cs (ppb)
4080	301.92	0.8	377.4	0.87	2.3E-06
8160	603.84	0.75	805.1	1.3	1.6E-06
12240	905.76	0.7	1293.9	2.3	1.8E-06
16320	1207.68	0.65	1857.9	36	1.9E-05
20400	1509.6	0.6	2516.0	1.6	6.4E-07

Average DF = 5x10^-6

(a) Based on 73 mg/mL cesium in eluate*
(b) DF = [Cs in Condensate (ppb)]/[(1000)(Cs in pot, mg/mL)]

Table 10 below presents the heat capacity of two of the samples taken from the evaporator. The 300 ml sample was taken before the final acid adjustments were made. The final product sample was taken after the final acid adjustment.

Table 10 Final Product Heat Capacity for Run 2

	300 ml Sample	Final Product Sample	Water	Final Product Sample
T Deg ° C	Cp (cal/gm-° K)	Cp (cal/gm-° K)	Cp (cal/gm-° K)	%Cp of Water
17	0.45	0.70
25	0.57	0.63
27	0.57	0.66
37	0.59	0.69	0.80	86.5
47	0.61	0.72	0.84	85.7
57	0.63	0.74	0.86	86.0
67	0.65	0.76	0.87	87.4
77	0.66	0.78	0.88	88.6
87	0.68	0.81	0.89	90.8

The final evaporator product had a saturation temperature of 18.0 ° C. Table 11 shows the analysis of this material. Analysis of this product ion balances within 10 %.

Table 11 Analysis of Final Evaporator Product and Predicted 25 ° C Saturation

Ionic Species	Analysis	Re-Analysis	25 ° C Saturation (molar)
Al	2400 mg/L	2330 mg/L
Na	45,700 ppm	42400 ppm	2.06
K	7,480 ppm	5570 ppm
NO3	671000 mg/L	571000 mg/L	9.03
Cs	1,430 ppm	1270 ppm
H	7200(b) mg/L	7040 mg/L	7.1
Ca	36(a) mg/L	1331 mg/L
Cr	50(a) mg/L	58 mg/L
Ni	100(a) mg/L	98 mg/L
Zn	210(a) mg/L	193 mg/L

1Extra Calcium is at impurity level of sodium nitrate reagent.

(a) Calculated based on ratio of (Metal ion)/Al in eluate feed times 2400 mg/L.
(b) Calculated by material balance after last acid addition.

Sodium nitrate was the only insoluble species observed to form during the evaporation process. The amount of eluate that can be evaporated before solids form was dependent upon the sodium concentration in the eluate. Acidity of the evaporator bottoms may also affect the amount of eluate which can be fed, but this affect can not be determined from the limited experiments performed here. Should this affect be determined to be significant, operation of a plant unit will require strict control of the condensate acidity not obtained in this investigation. Increased water flush of the cesium columns could further reduces the sodium content of the eluate and allow a greater number of batches to be evaporated before saturation is obtained.

Both nitric acid and hydrochloric acid are distilled from the evaporator when chloride ion is present. This could lead to a continuing increase in chloride concentration of the eluate when this distillate is reused. Higher and higher concentrations of hydrochloric acid in the eluate could then lead to variations in the operation of the evaporator and the physical properties of the evaporator bottoms.

Use OLI/ESP software to predict evaporator ion concentrations at various levels of evaporation. Prepare solutions at these ion concentrations and experimentally determine the physical properties of the solutions made. Compare model predictions with experimental data.