*3.1.2. Chromatographic column operation*

344 Ion Exchange Technologies

system.

process factors (separation factors).

of the distribution coefficients.

**3.1. Ion exchange column** 

*3.1.1. Batch and column operation* 

**3. Ion exchange separation and analysis** 

are used in large quantities. Distillation is used in industry to separation the isotopes of hydrogen, carbon, boron, nitrogen and oxygen. Chemical exchange is the satisfactory technique for separating isotopes of light elements because of their relatively high simple

Chemical exchange separation of isotopes is based on the equilibrium fractionation of isotopes between two phases, i.e. one of the isotopes is concentrated more abundantly in one of the phases than in the other phase if two chemical species are distributed in each phase. In this method, the enrichment of the isotopes concerned is achieved when the simple process of isotope separation is multiplied, by arranging the countercurrent contacting of two substances. The countercurrent contacting of the two different chemical substances is most easily realized in vapor-liquid or liquid-liquid system. The contacting takes place in conventional systems such as packed columns, mixer-settlers, etc. In this process it is necessary to provide reflux at the product end of a chemical exchange plant; the end which the desired isotope being enriched. In general, chemical refluxing is used to complete the conversion of one chemical species into the other at the end of the multi-stage contactor

Ion exchange isotope separation, which is one of the chemical exchange methods, is based on the chemical equilibrium between a stationary phase and a mobile fluid phase. In the past, many researchers have studied isotope fractionation on the isotopes both for light and heavy elements in this liquid solid system[1-5] and several isotopes especially for nitrogen isotope have been successfully enriched in laboratory scale by using displacement chromatographic technique [6, 7]. In this process a band of adsorbate is eluted through the column by a displacing solution, which the rate of movement of the band is determined by the equilibrium between solution and adsorbent, not of the material of the band but of the displacing adsorbate. Displacement always results in a sharp boundary between the bands of eluted solution and displacing solution. Thus, the adsorption band moves with keeping a constant band length. During the migration the isotopes in the band are rearranged in order

Ion exchanges are generally employed in the two different modes of operations: batch and column operation. The batchwise operation consists of contacting the whole of an electrolyte solution to be treated with a mass of exchanger and then separating the two phases by means of filtration, decantation, etc. It is quite obvious that for those exchange reactions that do not approach completion, a batchwise operation must be repeated many times before complete transformation is realized. The more unfavorable the equilibrium, the larger the

number of stages are necessary for a given degree of exchange or separation.

Chromatography was first applied to colored substances where "bands" of different colors can be seen while they move down the column. The chromatographic column operations are generally conducted in the manners of elution development, displacement development, and frontal analysis [8-10]. In the development techniques, a certain amount of the mixture is introduced at the top of the column and is then "developed" or "eluted" by a suitable agent. In frontal analysis, the mixture is continuously fed to the column. The front boundaries of the various components emerge at different times due to the differences in the migration rates.

### *3.1.3. Preparation of ion exchange column*

In the analytical applications of ion exchange, it is necessary to utilize uniform particle size ion exchangers in order to avoid irregular flow due to the distribution of particle size. For this purpose, the wet resins in the form of H+ were screened prior to the packing. The stainless steel screens of 42, 80, 200 and 400 meshes are used commonly. Then the ion exchanger with water in a beaker was stirred, and poured the slurry of resin into the column.

Under no circumstances must bubble or air be allowed to form in the bed of the ion exchanger beads. In operation, therefore, the bed must never be allowed to drain; the liquid level must not fall below the top of the bed. The water level was kept at 1-2 cm above the top of the bed during the operation.

The fresh ion exchanger provided by the supplier usually contains impurities, such as low molecular weight organic substances, and metallic ions, such as ferric ions. The new ion exchanger bed was, thus, washed in order to remove these impurities and metallic ions by following manner:


### *3.1.4. Apparatus*

In the ion exchange chromatography, high pressure resistant pyrex glass columns with a water jacket were used in the studies of isotope separation. The column of which the inside diameters of 8, 10 and 20 mm were prepared so that the operations are able to be conducted up to the column pressure of 50, 30 and 20 kg/cm2 in each column. To feed solutions, the microprocessor controlled, double plunger type titanium high pressurized pumps were used. The Teflon tubing of 1.0 mm i.d. was employed to connect column and pump. To monitor the column pressure, a pressure gauge with a safety device was placed between the column and the pump. In addition, an air damper was used in order to prevent the piston flow, especially at high flow rate runs, when loading solutions into columns. A schematic diagram of the apparatus used for the ion exchange separation is shown in Figure1. Water jacket was employed to remove the heat of reaction and maintain the column temperature throughout the experiments by circulation the thermostated water.

## **4. Nitrogen isotope application**

Enriched isotope of 15N has been a very valuable commercial product for which there is presently a growing demand in various scientific research and industry applications. In the agricultural field, 15N has been extensively used to study nitrogen cycling in soil-plant relationships. The fertilizer efficiency, distribution, assimilation and metabolic plants in the plant, symbiotic nitrogen fixation and the behavior of fertilizer in soils have been studied with use of 15N. Adriana et al. evaluated the nitrogen fixing capacity of a range of commercial cultivars of maize (*Zea mays L.*) by the 15N isotope-dilution method. Biological nitrogen fixation expressed as percent nitrogen derived from air ranged from 12 to 33 regardless of nitrogen fertilization [11]. Their results demonstrated that maize cultivars obtain significant nitrogen from biological nitrogen fixation, varying by maize cultivar and nitrogen fertilization level. In order to improve yields of crop production in many areas, one strategy is to choose crops with high nitrogen use efficiency that can produce economic yields under limited water supply. Sarr et al. performed the pearl millet (*Pennisetum glaucum*  L.R.Br.) and cowpea (*Vigna unguiculata* L. Walp.) in sole crops and intercrops systems for the nitrogen use efficiency of applied fertilizers [12]. 15N-labeled urea at rates of 20 kg ha-1(sole and intercrop cowpea) and 41 kg ha-1(sole millet and intercrop millet) was derived from the nitrogen fertilizer and 84.70% from nitrogen mineralized in soil. In addition, many other publications reported the 15N application in agroecological system [13-17]. In the industrial application, growing attention has been recently placed on the use of 15N isotope for the nitride fuels of FBRs (Fast Breeders Reactors) because of their desirable properties of large thermal conductivities and large breeding ratio [18-20].

#### **4.1. Nitrogen isotope separation process**

The separation of the nitrogen isotope has been studied since Urey et al. first succeeded in concentration of the nitrogen isotope of atomic weight 15 by using chemical exchange between ammonia gas and aqueous solutions of ammonium salts [21]. In their work, the Nitrogen Isotope Separation by Ion Exchange Chromatography 347

**Figure 1.** Schematic diagram of the apparatus for ion exchange separation

346 Ion Exchange Technologies

*3.1.4. Apparatus* 

In the ion exchange chromatography, high pressure resistant pyrex glass columns with a water jacket were used in the studies of isotope separation. The column of which the inside diameters of 8, 10 and 20 mm were prepared so that the operations are able to be conducted up to the column pressure of 50, 30 and 20 kg/cm2 in each column. To feed solutions, the microprocessor controlled, double plunger type titanium high pressurized pumps were used. The Teflon tubing of 1.0 mm i.d. was employed to connect column and pump. To monitor the column pressure, a pressure gauge with a safety device was placed between the column and the pump. In addition, an air damper was used in order to prevent the piston flow, especially at high flow rate runs, when loading solutions into columns. A schematic diagram of the apparatus used for the ion exchange separation is shown in Figure1. Water jacket was employed to remove the heat of reaction and maintain the column temperature

Enriched isotope of 15N has been a very valuable commercial product for which there is presently a growing demand in various scientific research and industry applications. In the agricultural field, 15N has been extensively used to study nitrogen cycling in soil-plant relationships. The fertilizer efficiency, distribution, assimilation and metabolic plants in the plant, symbiotic nitrogen fixation and the behavior of fertilizer in soils have been studied with use of 15N. Adriana et al. evaluated the nitrogen fixing capacity of a range of commercial cultivars of maize (*Zea mays L.*) by the 15N isotope-dilution method. Biological nitrogen fixation expressed as percent nitrogen derived from air ranged from 12 to 33 regardless of nitrogen fertilization [11]. Their results demonstrated that maize cultivars obtain significant nitrogen from biological nitrogen fixation, varying by maize cultivar and nitrogen fertilization level. In order to improve yields of crop production in many areas, one strategy is to choose crops with high nitrogen use efficiency that can produce economic yields under limited water supply. Sarr et al. performed the pearl millet (*Pennisetum glaucum*  L.R.Br.) and cowpea (*Vigna unguiculata* L. Walp.) in sole crops and intercrops systems for the nitrogen use efficiency of applied fertilizers [12]. 15N-labeled urea at rates of 20 kg ha-1(sole and intercrop cowpea) and 41 kg ha-1(sole millet and intercrop millet) was derived from the nitrogen fertilizer and 84.70% from nitrogen mineralized in soil. In addition, many other publications reported the 15N application in agroecological system [13-17]. In the industrial application, growing attention has been recently placed on the use of 15N isotope for the nitride fuels of FBRs (Fast Breeders Reactors) because of their desirable properties of large

The separation of the nitrogen isotope has been studied since Urey et al. first succeeded in concentration of the nitrogen isotope of atomic weight 15 by using chemical exchange between ammonia gas and aqueous solutions of ammonium salts [21]. In their work, the

throughout the experiments by circulation the thermostated water.

thermal conductivities and large breeding ratio [18-20].

**4.1. Nitrogen isotope separation process** 

**4. Nitrogen isotope application** 

considerable quantities of material containing up to 75% atom of 15N was obtained by exchange between ammonia and aqueous ammonium nitrate solutions. Later, in 1955, Spinder and Taylor [22] developed the chemical exchange method involving the exchange between nitric acid and nitric oxide as Equation 1:

$$^{15}\text{NO} + H^{14}\text{NO}\_3 = ^{14}\text{NO} + H^{15}\text{NO}\_3 \tag{1}$$

The fractionation factor of the exchange reaction obtained at 1.0 M nitric acid 1.062 is extremely favorable compared with the separation factors of most other possible separation methods of nitrogen isotopes by chemical system. In this process, the 15N concentrates in the nitric acid and thus 15N enriches in the lower part of the exchange column. In Japan, 15N has been commercially produced using this technique, which is called NITROX process. In Romania, Axente et al. have studied this process for thirty years using a laboratory-scale experimental plant [23]. Some other isotope separation methods such as distillation and thermal diffusion have also proved to be satisfactory for 15N separation [24-27].

Ion exchange process on nitrogen isotope separation has been studied since Spedding et al. first succeeded in the enrichment of 15N by cation exchange chromatography [6]. In their experiment, the separation factor of the isotopic exchange between the two phases of the dilute ammonium hydroxide solution and the cation exchange resin (Dowex 50-x12) in ammonium form was measured as 1.0257±0.0002. A few years later, the studies on the nitrogen isotope separation by ion exchange were reported by two Japanese research groups Ishimori[28] and Kakihana [29]. Ishimori investigated the influence of operating temperature and concentration of ammonium solution on the isotope effects in a batch equilibrium system. Kakihana measured the separation factors of nitrogen isotopes for NH3 and NH4Cl in acetone water and ethanol water mixture systems. Afterwards, in 1980s, Park and Michaels studied the separation process developed by Spedding et al. under various operating conditions using columns packed with sulfonated polystyrene-divinylbenzene copolymers [30]. In the recent years, Krugrov et al. studied NH3-NH4+ system using SMB (simulated moving bed) process for various flow rates under total reflux [31] and the theoretical analysis of separating nitrogen isotopes by ion exchange was reported by Aoki et al [32].

### **4.2. Nitrogen isotopic analysis with mass spectrometry**

If an aqueous solution of ammonium hydroxide is placed in contact with a cation exchanger, the following exchange reaction occurs:

$$\mathrm{^{14}NH\_4^+ + ^{15}NH\_4OH} = \mathrm{^{15}NH\_4^+ + ^{14}NH\_4OH} \tag{2}$$

the result of which, under equilibrium conditions, is that the heavy nitrogen isotope, by means of the ammonium ion, is distributed more abundantly in the solid phase, which is the cation exchanger. In order to multiply the elementary effect for separating 15N, the following chromatographic separation process developed by Spedding was employed [6]: the ion exchanger bed was first converted to the hydrogen form by passing 2.0 M hydrochloric acid through the column until the bed was saturated. The bed was then rinsed with distilled water and an ammonium hydroxide solution of certain concentration was added. At the lower end of an ammonium band, the following reaction takes place:

$$H-R + NH\_4OH \to NH\_4-R + H\_2O\tag{3}$$

Where -R denotes the fixed anion of cation exchanger. Since the equilibrium constant of this exchange reaction is on the order of 109, the reaction is far to the right, resulting in an extremely sharp boundary. The ammonium ion comes to chemical equilibrium within the system very rapidly and isotopic equilibrium tends to be approached as the solution passes over the cation exchanger. In the frontal analysis, the effluent is collected in fractions at the bottom of the column when the ammonia solution of which the front part of the ammonium band is emerged. Prior to the collection, certain volumes of hydrochloric acid are initially added in the sampling tubes to prevent the ammonia from degassing. Since the 15N isotope is depleted in the front of the band, 14N isotope is enriched in the effluent.

348 Ion Exchange Technologies

al [32].

been commercially produced using this technique, which is called NITROX process. In Romania, Axente et al. have studied this process for thirty years using a laboratory-scale experimental plant [23]. Some other isotope separation methods such as distillation and

Ion exchange process on nitrogen isotope separation has been studied since Spedding et al. first succeeded in the enrichment of 15N by cation exchange chromatography [6]. In their experiment, the separation factor of the isotopic exchange between the two phases of the dilute ammonium hydroxide solution and the cation exchange resin (Dowex 50-x12) in ammonium form was measured as 1.0257±0.0002. A few years later, the studies on the nitrogen isotope separation by ion exchange were reported by two Japanese research groups Ishimori[28] and Kakihana [29]. Ishimori investigated the influence of operating temperature and concentration of ammonium solution on the isotope effects in a batch equilibrium system. Kakihana measured the separation factors of nitrogen isotopes for NH3 and NH4Cl in acetone water and ethanol water mixture systems. Afterwards, in 1980s, Park and Michaels studied the separation process developed by Spedding et al. under various operating conditions using columns packed with sulfonated polystyrene-divinylbenzene copolymers [30]. In the recent years, Krugrov et al. studied NH3-NH4+ system using SMB (simulated moving bed) process for various flow rates under total reflux [31] and the theoretical analysis of separating nitrogen isotopes by ion exchange was reported by Aoki et

If an aqueous solution of ammonium hydroxide is placed in contact with a cation exchanger,

<sup>14</sup> <sup>15</sup> <sup>15</sup> <sup>14</sup> *NH NH OH NH NH OH* 44 44

the result of which, under equilibrium conditions, is that the heavy nitrogen isotope, by means of the ammonium ion, is distributed more abundantly in the solid phase, which is the cation exchanger. In order to multiply the elementary effect for separating 15N, the following chromatographic separation process developed by Spedding was employed [6]: the ion exchanger bed was first converted to the hydrogen form by passing 2.0 M hydrochloric acid through the column until the bed was saturated. The bed was then rinsed with distilled water and an ammonium hydroxide solution of certain concentration was added. At the

Where -R denotes the fixed anion of cation exchanger. Since the equilibrium constant of this exchange reaction is on the order of 109, the reaction is far to the right, resulting in an extremely sharp boundary. The ammonium ion comes to chemical equilibrium within the system very rapidly and isotopic equilibrium tends to be approached as the solution passes

(2)

*H R NH OH NH R H O* 4 42 (3)

thermal diffusion have also proved to be satisfactory for 15N separation [24-27].

**4.2. Nitrogen isotopic analysis with mass spectrometry** 

lower end of an ammonium band, the following reaction takes place:

the following exchange reaction occurs:

In the displacement band chromatographic process, an ammonia solution is fed into the column until an ammonium adsorption band of the desired length is formed. The concentration of the fed ammonia solution was adjusted approximately at the concentration of the eluent in each operation. A caustic solution (NaOH or LiOH) was then fed into the column to elute the ammonium band in the reverse breakthrough manner. At the top of the ammonium band, the following reaction takes place:

$$\text{NaNH}\_4-\text{R} + \text{NaOH} \text{(LiOH)} \rightarrow \text{Na(Li)}-\text{R} + \text{NH}\_4\text{OH} \tag{4}$$

which, having an equilibrium constant of about 105, in turn guarantees a very sharp boundary at the rear end of the band. The ammonium ions are continuously released from the cation exchanger and converted to ammonium hydroxide by an equivalent of sodium (or lithium) ions which will be deposited at the rear edge of the band. The ammonium hydroxide solution moves down along the band until it reaches the front edge of the band. The diagram of the displacement band chromatographic process for separating nitrogen isotopes with the use of a cation exchanger is shown in Figure 2. The countercurrent movement of NH3-NH4+ ions in two phases develops a longitudinal isotopic distribution profile within the band. The 15N isotope is enriched in the rear end of the band. The effluent of the rear part of the band was collected in fractions at the bottom of the column.

The volume of the fraction was determined by gravimetry, measuring the weight of the sample solution. The concentrations of ammonium and sodium (lithium) ions in the fractions were determined by an ion chromatography analyzer. The collected samples in the form of ammonium chloride were converted to nitrogen gas by adding a solution of potassium hypobromide(KBrO). The reaction involved is as follows:

$$2\text{NH}\_4\text{Cl} + 3\text{KBrO} + 2\text{NaOH} = \text{N}\_2 + 3\text{KBr} + 2\text{NaCl} + 5\text{H}\_2\text{O} \tag{5}$$

The solution of potassium hypobromide was prepared by following process:


The KBrO solution was stored in a refrigerator since potassium hypobromide is easily decomposed to potassium bromide and oxygen in light or by elevation of temperature. The Rittenberg was employed in order to prepare nitrogen gas samples for isotopic analysis and the apparatus of the nitrogen conversion system is shown in Figure 3. The preparation procedure of nitrogen gas samples is described below:

**Figure 2.** The diagram of the displacement band chromatographic process for nitrogen isotope separation


6. Freeze the solutions again and open the stopcock B so that the gas generated during the melting is evacuated.

350 Ion Exchange Technologies

Rittenberg glass tube A.

solutions to be melted.

vacuum system during evacuation.

**Figure 2.** The diagram of the displacement band chromatographic process for nitrogen isotope separation

1. Put a certain volume of ammonium chloride sample (0.1-1.0 cm3) in one side and a stoichiometrically excess amount of potassium hypobromide in the other side of a

2. A cold trap D is cooled with liquid nitrogen to prevent contamination of water in the

4. Open the glass stopcocks B and C which are used to control the vacuum manipulations to pump out the air by an oil rotary pump. After the evacuation, the stopcock B is closed. 5. The liquid nitrogen vessel is then removed from the Rittenberg tube to admit the

3. Freeze the solutions in the Rittenberg galss tube with liquid nitrogen.


**Figure 3.** Schematic apparatus of the nitrogen gas conversion system

### **5. The effect of migration distance to the nitrogen isotope separation**

The sulfuric strong acid cation exchange resin SQS-6(high porous type, cross linking 8%, particle size 60-90 um in the form of NH4+, provided by Asahi Chemical Industry Co., Japan) was packed uniformly in a pressurized glass column with a water jacket. The microscope view of H+ type SQS-6 resin was shown in Figure 4. The ion exchange resin in the column preliminarily conditioned to the H+ form with 2.0 M HCl solution, then pure water was feed into the top of the column to remove the free H+ in the resins. 0.2M NH4OH solution was feeded into the column to form 1.0 m ammonium adsorption band and the band was eluted by the displacing solution of 0.2M NaOH in the reverse break-through manner. The flow rate and band velocity were controlled by feeding pump and the column pressure was monitored by high pressure pump. The temperature of the column was kept constant throughout the experiments by passing the thermostated water through the water jacket. The effluent, which emerged from the column, was collected using a fraction collector. In order to prevent the NH4OH samples from de-gassing, certain volumes of excess HCl solutions were added in the collection tubes prior to the samplings. Considering the long migration distance for the purpose of large scale production, it is necessary to regeneration column after the ammonium adsorption band pass through certain column. When the rear boundary of ammonium adsorption band enter into the next column, the first column was separated from the connection system and eluted by 2.0M HCl for regeneration, the rinse volume was at least five times larger than that of the resin's volume. The concentrations of NH4+ in the fractions were determined by ion chromatography analyzer and the mass peaks of 14N15N and 14N14N of the samples were measured with the mass spectrometer and the isotopic abundance of 15N was calculated from the ratio of the peak height.

**Figure 4.** Microscope view of H+ type SQS-6 resin

The separation of nitrogen isotopes by means of ion exchange resin is based on the isotopic fraction between ammonia in aqueous solution and ammonium ion in the ion-exchange resin as shown in the Equation 6. The single stage separation factor or the separation coefficient is defined by:

$$\alpha = 1 + \varepsilon = \boxed{15N} \left[ \boxed{14N} \right] / \left[ \boxed{15N} \right] \overline{\boxed{14N}} \tag{6}$$

Where denotes the concentration of isotopes in the aqueous phase and the concentration of the isotopes in the resin phase.

In order to evaluate the chromatographic efficiency of nitrogen isotope enrichment, HETP (height equivalent to a theoretical plate) is introduced. HETP is usually calculated using the slope of the isotopic distribution curve in the steady-state of isotope separation after migration and is calculated by the following Equation 7:

$$H = \frac{\mathcal{E}}{k} + \frac{1}{k^2 L} \tag{7}$$

Where H is the HETP and L is the migration length. The slope k is experimentally determined by using the following Equation 8:

352 Ion Exchange Technologies

migration distance for the purpose of large scale production, it is necessary to regeneration column after the ammonium adsorption band pass through certain column. When the rear boundary of ammonium adsorption band enter into the next column, the first column was separated from the connection system and eluted by 2.0M HCl for regeneration, the rinse volume was at least five times larger than that of the resin's volume. The concentrations of NH4+ in the fractions were determined by ion chromatography analyzer and the mass peaks of 14N15N and 14N14N of the samples were measured with the mass spectrometer and the

The separation of nitrogen isotopes by means of ion exchange resin is based on the isotopic fraction between ammonia in aqueous solution and ammonium ion in the ion-exchange resin as shown in the Equation 6. The single stage separation factor or the separation

denotes the concentration of isotopes in the aqueous phase and

In order to evaluate the chromatographic efficiency of nitrogen isotope enrichment, HETP (height equivalent to a theoretical plate) is introduced. HETP is usually calculated using the slope of the isotopic distribution curve in the steady-state of isotope separation after

> <sup>1</sup> *<sup>H</sup> k k L*

2

1 15 14 / 15 14 *NN NN* (6)

(7)

the

isotopic abundance of 15N was calculated from the ratio of the peak height.

**Figure 4.** Microscope view of H+ type SQS-6 resin

migration and is calculated by the following Equation 7:

concentration of the isotopes in the resin phase.

 

coefficient is defined by:

Where

$$Ln(r - r\_o) = k(L - x) \tag{8}$$

Where r is the isotopic ratio of nitrogen sample and ro is the isotope ratio of feeding material. The term (L-x) is the distance between inner band location x and the rear or front boundary of which the migration length is L. Equation 8 indicates that plotting ln(r-ro) against (L-x) for the experimental data produces a linear line with the slope of k.

Nitrogen isotope separations with different migration distance were performed and the experimental conditions were mentioned in Table 1.


**Table 1.** Experimental conditions of chromatographic nitrogen isotope separation with different migration distance

The chromatographic elution curve for Run 4 as an example is given in Figure 5. It is seen that ideal displacement chromatograms are obtained and a sharp band boundary between the ammonium band and sodium band is maintained during the long migration at the current experimental conditions. The maintenance of the sharp band boundary is most important to obtain highly enriched isotope at the boundary region. Light isotope of 14N was enriched at the front boundary and the heavy isotope 15N was enriched at the rear boundary.

The two parameters of separation coefficient and HETP can be calculated from both front and rear boundary regions and the values were listed in Table 2.The observed at front and rear boundary should be coincident each other. As shown in Table 2, the observed separation coefficients at front and rear boundary are in good agreement in each Run. The separation coefficient is an equilibrium factor and should be constant, being independent of the migration distance. It is confirmed from the results listed in Table 2 in the cases other than Run1. The small values of separation coefficient of Run1 are probably due to the feed material of (NH4)2SO4 solution and one in NH4SO3-R in the ion exchange resin are resemble each other. In such case, isotope effects do not occur between the two chemical species. Due to this fact, the effective length of migration is reduced by approximately 20%. This effect is reflected in the decrease in the separation coefficient in Run1.

It is also confirmed that the values of obtained from Run 4 are slightly reduced. Probably this is due to the remixing in the middle of the adsorption band. Figure 5 showed the chromatographic profile of Run 4. From the figure it can be seen that the isotopic plateau region is not seen, the enriched zone and the depleted zone are directly contacted because of the long migration. It should be noted that the values of obtained from the experiments using natural nitrogen (Run 3) are practically the same as those observed by using enriched nitrogen (Run 5);

**Figure 5.** Chromatographic profiles of ammonium ion concentration and isotopic enrichment of 30 m migration distances.


Run 5 used 80% 15N as feed solution, of which ro =4.

**Table 2.** Experimental results of chromatographic nitrogen isotope separation with different migration distance

The maximum enrichment values are defined as the local enrichment factor of 15N at the boundary as follows:

354 Ion Exchange Technologies

nitrogen (Run 5);

migration distances.

distance

chromatographic profile of Run 4. From the figure it can be seen that the isotopic plateau region is not seen, the enriched zone and the depleted zone are directly contacted because of the long migration. It should be noted that the values of obtained from the experiments using natural nitrogen (Run 3) are practically the same as those observed by using enriched

**Figure 5.** Chromatographic profiles of ammonium ion concentration and isotopic enrichment of 30 m

Run Max. enrichment(%) βrear = rmax/ro Separation coefficient HETP (mm)

1 99.956 2.29 6.4 0.016 0.018 0.37 0.21 2 99.987 3.91 11.1 0.025 0.025 0.74 0.19 3 99.994 9.54 28.7 0.024 0.024 1.22 0.16 4 99.990 21.55 78.9 0.021 0.019 2.6 0.16 5a 99.143 99.678 77.4 0.023 0.022 0.39 1.58

**Table 2.** Experimental results of chromatographic nitrogen isotope separation with different migration

Run 5 used 80% 15N as feed solution, of which ro =4.

Front 14N Rear 15N Front Rear Front Rear

$$
\beta\_{\text{rear}} = r\_{\text{max}} \mid r\_o \tag{9}
$$

Where r is the isotopic abundance ratio of 15N against 14N. The values of calculated are listed in Table 2 and plotted as a function of migration distance in Figure 6. The slope of plots is unity, which means that the maximum enrichment values are proportional to the migration distance up to 30 m. This fact suggests that the enrichment proceeds, forming an ideal shape of exponential enrichment curve at the band boundary region. In addition, it is quite interesting thatvalue of Run 4 using natural nitrogen is in very good agreement with that of Run 5 where 80% enriched nitrogen was used as feed material. This information is important and useful for designing the enrichment plant based on the present method of ion exchange chromatography.

**Figure 6.** Correlation between migration distance and the maximum enrichment of 15N

In order to evaluate the efficiency of nitrogen enrichment in chromatographic migration, HETP is introduced and the calculated values of HETP are listed in Table 2. HETP values are small enough in the rear band region of Runs 1-4 where the enrichment of 15N is steadily proceeding. Similar phenomenon is observed in the front 14N enrichment zone of Run 5. In general, the HETP values are small when the enrichment process does not reach the high level. On the other hand, saturation of isotope enrichment gradually takes place and slope of the enrichment curve sharply decreases when high enrichment is attained. This is the case for the enrichment of 14N in Runs 1-4 and the enrichment of 15N in Run 5.

The results of Run 1-4 clearly indicate that the practical limit of 14N enrichment by the ion exchange packed column is 99.99%. The reason for this limit has not yet been elucidated, but it is estimated that this limit of 99.99% may be applicable for the enrichment of 15N as well. So far, the target of 15N enrichment for nitride fuels is 99.9%. The present work realized the enrichment of 99.678% 15N in Run 5. The results suggest that the 99.9% 15N is attainable by ion exchange enrichment.
