KR100736020B1 - Method of recovery of low grade deuterium oxide having improved purity - Google Patents

Abstract

A method for re-collecting low grade deuterium oxide vapor having improved purity is provided to re-collect the deuterium oxide vapor having improved purity by collecting the low grade deuterium oxide vapor when a density of the deuterium oxide vapor is reduced in an outlet of an absorption tower. A method for re-collecting low grade deuterium oxide vapor having improved purity includes the steps of: absorbing the low grade deuterium oxide vapor to an absorbent of an absorption tower; performing elution to a movable phase by heating the absorbent of the previous step and detaching the deuterium oxide vapor from the absorbent; and classifying a moisture density of the eluted deuterium oxide vapor by the purity and collecting the deuterium oxide vapor when the moisture density of the eluted deuterium oxide vapor at the previous step is reduced from a saturated state.

Description

Method of recovery of low grade deuterium oxide having improved purity

1 is a diagram of H ₂ O, HDO and D ₂ O equilibrium composition according to deuterium purity;

2A is a graph showing elution behavior over time of hard water (H ₂ O) and low grade heavy water (HDO) upon desorption to recover heavy water vapor having a purity of 0.3 mole fraction according to one embodiment of the present invention;

2B is a graph showing elution behavior over time of hard water (H ₂ O) and low grade heavy water (HDO) upon desorption to recover heavy water vapor having a purity of 0.5 mole fraction according to one embodiment of the present invention;

FIG. 2C is a graph showing elution behavior over time of hard water (H ₂ O) and low grade heavy water (HDO) upon desorption to recover heavy water vapor having a purity of 0.7 mole fraction in accordance with one embodiment of the present invention; FIG.

Figure 3a is a graph showing the dew point behavior of the outlet air flow of the adsorption tower over time, when passing the air having a constant relative humidity at any temperature for low grade heavy water vapor saturation according to an embodiment of the present invention; And

Figure 3b is a graph showing the elution behavior with time of desorption hard water (H ₂ O) and heavy water vapor (HDO) in accordance with an embodiment of the present invention.

The present invention relates to a method for recovering low grade heavy water vapor, which is leaked from a heavy water system of a heavy water type nuclear power plant using a reactor grade high purity heavy water (D ₂ O) as a coolant and a moderator, and has a lower purity due to water vapor absorption.

Heavy water is the same as natural water, but its nuclear nature is a mysterious isotope of water that differs greatly from natural water. The heavy water is present in all natural water in an amount of 0.015%, and the specific gravity is 1.1% heavier than natural water. The existence of heavy water first became known to the world after Professor Urei in 1934 won the Nobel Prize for heavy water discovery. Hundreds of fractional distillation of natural water leaves a very small amount of heavy water, which is heavy water. Deuterium constituting the heavy water, like hydrogen, has one electron in the electron orbit of 1 s, and combines with the sp ³ hybrid orbit of oxygen to form a water compound in the form of a stable compound, so that hard water is natural water containing heavy water and hydrogen. Has the same structure. As such, heavy water and hard water having the same structure have different isochemical effects due to the isotope effect due to the atomic mass difference between heavy hydrogen and hydrogen bonded to the oxygen atom, and different chemical or spectroscopic characteristics. Has The heavy water and the hard water have a difference in hydrogen bonding force around 24 kJ / mol and have slight physical properties such as density, melting point, boiling point, triple point, heat of evaporation, heat capacity, and diffusion rate.

Heavy water contributed to mankind because of its neutron response. The neutrons generated during the nuclear fission of uranium react with heavy water and are decelerated into thermal neutrons useful for the chain reaction. If heavy water is used instead of natural water as the coolant of the reactor, the chain reaction proceeds more actively. Because thermal neutrons are easily absorbed and destroyed by natural water, they are hardly absorbed by heavy water. Thus, heavy water can be used to generate nuclear power from unconcentrated natural uranium.

However, since the heavy water has a boiling point of 101.4 ° C. and has substantially the same properties as that of natural water, it is necessary to have a high level of technology for separating and concentrating the reactor to a concentration of 99.8%, which is a nuclear reactor. Currently, heavy water separation uses a chemical exchange method such as hydrogen sulfide method or ammonia method, and heavy water is imported at a high price of about $ 200 / kg due to huge equipment and enormous energy costs.

The high temperature and high pressure heavy water circulated in the moderator or coolant heavy water system of heavy water type nuclear power plant inevitably generates a small amount of leakage, and some of it is recovered as a liquid state, but as soon as it leaks, it evaporates to form heavy water vapor. Deuterium can be contaminated with tritium. Therefore, in order to prevent workers from being exposed to heavy water vapor contaminated with tritium and to recover expensive heavy water, the heavy water vapor must be recovered quickly.

The heavy water of the reactor grade purity (99.8% or more) of the expensive reactor, once leaked from the system, becomes a low graded purity with low purity due to inflow of water vapor into the air, but is recovered and driven by hot steam through a series of purification systems. Upgrade from reactor to reactor-grade purity.

The heavy water upgrade is composed of a multi-stage water distillation process. The multi-stage water distillation process is based on the principle of gas-liquid equilibrium in which the heavy water composition of each liquid phase at a certain temperature forms a slightly higher heavy water composition than the gaseous phase. Heavy water is recovered, and most of the heavy water is recovered at the top, and it is a continuous distillation process operated so that only hard water components can flow out into the product. The upgrader is designed to be capable of supplying purity at several water supply points through the tower. A reboiler is provided in the lower part of the upgrader to evaporate the condensed heavy water again, and the upper part is liquefied upstream by the condenser and refluxed downward. Through this process, the concentration of the heavy water concentration in the upper part of the tower is lower than 1% through the whole distillation column and the reactor-grade heavy water purity is formed in the lower part. Manipulate the Reflux Ratio, defined as the condensate flow rate relative to the top product. Therefore, in order to satisfy the predetermined heavy water composition and recovery rate, most of the heavy water vapor boiled up by the reboiler is liquefied by the condenser, so the energy consumption of the steam heat source supplied to the heat exchanger of the reboiler is enormous for continuous operation. Are classified.

Because the recovered low grade heavy water differs in purity at each leak site, it is supplied to the water supply point close to the concentration distributed internally in the subsequent upgrade device by concentration so that it is operated so as not to distort the concentration balance inside the entire upgrader. The principle of the upgrader is to induce the differential phase equilibrium between the gas and liquid in the filling device, which is the contact device of the internal upflow gas and the downflow liquid, and concentrate the heavy water component toward the liquid to recover the reactor grade purity heavy water. The upgrader induces an upflow of the heavy water vapor by means of a reboiler located at the bottom, and maintains an appropriate reflux ratio by the condenser located at the top to recover a predetermined economic heavy water component. Therefore, the higher the purity of the heavy water, the more the water is supplied to the lower part of the upgrader, thereby reducing the burden of re-investment, thereby inducing the economical upgrade of heavy water.

Meanwhile, since the feedwater of the recovered heavy water having a low purity of the heavy water flows into the upper part of the tower, the burden of the recovery unit increases, and the reflux ratio is increased, thereby increasing the energy consumption of the reboiler that needs to send the corresponding upstream. The upgrader is divided into a section with a stripping section and an enrichment section with a water supply point at the bottom. The higher the water supply point, the lower the purity of the water is, and the flow rate in the distillation column increases and reboils. The heat source consumed by is increased. Therefore, in order to maximize the efficiency of the upgrade process, the low grade heavy water recovered from the subsequent water distillation process to the appropriate water supply point by purity should be supplied.

In general, the liquid heavy water leaked into the reactor building, which is blocked from the outside, is guided and recovered by a sump, and the vaporized heavy water vapor is managed to be adsorbed and recovered by a dryer.

However, currently used steam recovery method does not supply the low grade heavy water efficiently to the appropriate water supply point by purity in the subsequent water distillation process because the heavy water vapor adsorbed on the adsorption device is low in purity and difficult to recover selectively depending on the purity. There is no problem.

Therefore, the present inventors are easy to adsorb and desorption of low grade heavy water vapor, and while studying the recovery method of the purified heavy water vapor, the low grade heavy water vapor is adsorbed to the adsorbent and then hard water is heated and desorbed for regeneration and heavy water recovery. Focusing on the different elution rates with, it was confirmed that heavy water vapor could be selectively recovered and completed the present invention.

It is an object of the present invention to provide a method for selectively classifying and recovering the purity of low-grade water vapor whose purity is lowered due to absorption of water vapor due to leakage.

In order to achieve the above object, the present invention provides a method for selectively classifying and recovering low-grade heavy water vapor of which purity is low due to absorption of water vapor due to leakage.

Hereinafter, the present invention will be described in detail.

The heavy steam recovery method according to the present invention,

Adsorbing the low grade heavy water vapor to the adsorbent of the adsorption tower (step 1);

Heating the adsorbent of step 1 to desorb heavy water vapor from the adsorbent to elute the mobile phase (step 2); And

At the time when the water concentration of the heavy water vapor eluted to the mobile phase by step 2 is reduced in saturation state, the concentration is collected by purity (step 3)

It is configured to include.

Step 1 is a step of adsorbing the low grade heavy water vapor to the adsorbent packed in the adsorption tower. Examples of the adsorbent packed in the adsorption tower include silica gel, activated alumina, zeolite molecular sieve and the like. They can be used selectively depending on the operating temperature and water vapor pressure.

As the water adsorbent packed in the adsorption tower according to the present invention, porous zeolite molecular sieves may be preferably used among the adsorbents. Preferably, Na-A type, Na-X type, and Ca- with a part of sodium substituted with calcium Type A etc. can be used.

Since the zeolite exhibits a high water content and excellent thermal stability, the zeolite desorbs all moisture which is physically adsorbed at high temperature, and thus can be used for regeneration. Therefore, the zeolite molecular sieve is advantageous when the adsorption temperature is high and the water vapor pressure is low.

In addition, adsorption capacity and adsorption rate should be considered for heavy water vapor recovery or triple steam decontamination application. At this time, the adsorption capacity decreases with increasing temperature. Among the adsorbents, silica gel and activated alumina are highly dependent on temperature, but since zeolite molecular sieves are relatively less affected by temperature, they are stable even at high temperatures up to 500 ° C. and low humidity of 30% or less. Therefore, as the adsorbent according to the present invention, it is preferable to use a zeolite molecular sieve which has an excellent adsorption capacity and relatively little dependency on temperature and water vapor pressure. Such zeolite molecular sieves may be natural zeolite molecular sieves as well as synthetic zeolite molecular sieves.

Further, the water adsorption rate is determined by the residual water content of the adsorbent, the zeolite molecular sieve has a higher residual water content than other water adsorbents and decreases even in a low humidity range of 30% or less in terms of adsorption capacity of water. In addition, the zeolite molecular sieve has a property that the regeneration is kept constant. That is, the moisture is not gradually desorbed, but when the temperature of the adsorption layer reaches the critical point, there is an advantage that the recovery of moisture is easy due to the characteristic of sudden desorption. The regeneration of the adsorbent includes heat-swing procedures and vacuum-swing procedures, and the heat regeneration temperature is, for example, 200-350 ° C. for the synthetic zeolite Na-A. The adsorption capacity of such a water adsorbent is slightly reduced by the generation of surface hydroxyl groups and bound water as the number of regeneration increases.

The zeolite molecular sieve can be used as a hydrate of aluminosilicates of alkali and alkaline earth metals, synthesized or commercially available by conventional methods. For example, one embodiment of the present invention used a synthetic zeolite manufactured industrially in Lindesa, USA. It is a sodium salt with a crystal structure similar to that of natural zeolites. The chemical formula is generally represented by Na _m (Al ₂ O ₃ ) _m (SiO ₂ ) _{n x} H ₂ O (m≤n) and silicon oxide (SiO _4). ) Tetrahedron and aluminum oxide (AlO ₄ ) tetrahedron are crystals that form a three-dimensional network, and the meshes form a cavity, and sodium ions are present therein. The cavities loosen the bonds of each atom in crystal structure, so that the particulate matter can be adsorbed. This space can be filled with up to about 18% moisture, and even if all the physically adsorbed moisture is desorbed at high temperatures, the skeleton remains intact, allowing regeneration. The cavity of the synthetic zeolite has a uniform diameter, and thin pores are connected vertically and horizontally, and according to the size of the pores, 4A (Na-A type) and 5A (calcium salt of zeolite, Ca -A type) and 13X (Na-X type). Since the synthetic zeolite contains a large amount of thin holes having a uniform diameter, it has a distinct adsorption force and has a function of sieving molecules, and thus may be used as various chemically active catalysts, catalyst carriers, and hygroscopic agents.

The low grade heavy water vapor is adsorbed by diffusing to the pores through the porous space formed in the inside of the adsorbate having a large specific surface area such as the molecular sieves such as zeolite, and the adsorption low grade heavy water vapor is physically saturated to the liquid state so that the adsorption is It ends. In order to have the adsorption activity again, when the adsorbent is heated to a high temperature, it is detached to the outside of the pores due to the elevated vapor pressure of the heavy water filled in the pores and physically easily desorbed.

Next, step 2 is a step of heating the adsorbent of step 1 to desorb heavy water vapor from the adsorbent to elute the mobile phase.

When the adsorbent saturated and absorbed by the low grade heavy water vapor in step 1 is heated to a high temperature, the vapor pressure of the heavy water filled in the pores rises and is easily detached from the pores. At this time, the heating temperature for desorption of the heavy water vapor is preferably 170 ~ 350 ℃, the mobile phase using a dry air or inert gas such as helium, neon, argon containing a small amount of hydrogen for heavy water vapor for hard water It can be eluted by inducing delayed dropping.

At the time of desorption, the low grade heavy water vapor adsorbed through the micropore passage of the molecular sieve diffuses and elutes to the external mobile phase.

Next, step 3 is a step of collecting the concentrations by purity at the time when the water concentration of the heavy water vapor eluted to the mobile phase by step 2 decreases in a saturated state.

Most of the reactor-grade high-purity heavy water molecules take the molecular form of D ₂ O, and as they are diluted by hard water, deuterated water molecules are present in the form of HDO, so that they are very diluted in the lattice of H ₂ O molecules. In the case of heavy water, three types of hydrogen isotope oxides, ie, H ₂ O, HDO, and D ₂ O, water molecules, etc., have a constant equilibrium concentration depending on the diluted purity. These oxides exhibit the same type of molecular bond vibration. The three hydrogen isotope liquid oxides are always in equilibrium at a constant temperature, and these oxides are equilibrated in the composition as shown in FIG. 1 by a rapid liquid isotope exchange reaction as in Scheme 1. The reaction equilibrium constant is known to be about 3.26 at 25 ° C.

H ₂ O + D ₂ O ⇔ 2HDO, K _{25 ° C} = 3.26

Self-diffusion Coefficient of the hydrogen isotope oxides is H ₂ O: 2.272 × 10 ^-9 m ² / s, D ₂ O: 2.109 × 10 ^-9 m ² / s at 25 ℃, vapor pressure H ₂ O: 3.165 kPa, pure D ₂ O: 2.734 kPa at 25 ° C. Comparing the self-diffusion coefficient and the vapor pressure of the hard water steam and heavy water steam is lower than the hard water of the low grade heavy water vapor, the diffusion through the pores during heat desorption is delayed. By using the relatively slow elution behavior of the heavy water vapor, when the desired concentration of the heavy water vapor decreases as a recovery point, the collected water can be recovered by initially collecting the heavy water higher than the low graded concentration.

The recovery method according to the present invention uses any of the _{two components} selected from the group consisting of hard water (H ₂ O), mixed heavy water (HDO), triple water (T ₂ O) and mixed triple water (HTO, DTO) The hydrogen isotope oxide of can be recovered separately.

Hereinafter, with reference to the accompanying drawings will be described in more detail the present invention.

1 shows H ₂ O, HDO and D ₂ O equilibrium composition diagrams according to deuterium purity. The heavy water (D ₂ O) is diluted with hard water (H ₂ O) and is present in the form of HDO by liquid isotope exchange.

Figure 2a to 2c is a graph showing the elution behavior according to the purity of heavy water at the outlet of the adsorption column during the heat desorption of the adsorbent in order to recover the heavy water vapor with improved purity by the method of the present invention. Over time, heavy water (HDO), which has become lower in purity, has an increased purity (mole fraction) up to a certain point due to a slower elution behavior than hard water (H ₂ O), where the concentration (mole fraction) of the adsorption tower outlet water (HDO) is increased. If the recovery time is selected by dividing the time point decreases to the downward curve, it is possible to recover the HDO of improved purity than the conventional purity.

Figure 2a is a graph showing the elution behavior with time of desorption (H ₂ O) and low grade heavy water (HDO) when desorption to recover the heavy water vapor having a purity of 0.3 mole fraction. As shown in Fig . 2A , the time point at which the concentration (mole fraction) of HDO decreases in the downward curve is selected as the recovery point.

Figure 2b is a graph showing the elution behavior of the hard water (H ₂ O) and low grade heavy water (HDO) during desorption to separate and recover the heavy water vapor having a purity of 0.5 mole fraction. As shown in Fig . 2B , the time point at which the concentration (mole fraction) of HDO decreases in the downward curve is selected as the recovery point.

Figure 2c is a graph showing the elution behavior over time of hard water (H ₂ O) and low grade heavy water (HDO) during desorption to separate and recover the heavy water vapor having a purity of 0.7 mole fraction. As shown in Fig . 2C , the time point at which the concentration (mole fraction) of HDO decreases in the downward curve is selected as the recovery point.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, it is a matter of course that the scope of the present invention is not limited or limited only to the following examples.

< Example  1> heavy steam Elution behavior  Measure

An adsorption tower packed with Linde's synthetic zeolite (1/16 "pellet-shaped molecure 13X) with a diameter of 1 cm in a stainless steel tube at a height of 10 cm was vented to dry air at a flow rate of 0.2 L / min in an atmosphere of 280 ° C. Impurities were completely desorbed and activated until the dew point was below -35 ° C. An air having a relative humidity of 71% at 25 ° C. from a humidifier consisting of the dry air and a 20% by weight low grade heavy water flow regulator and an evaporator. Was passed at a rate of 1 l / min to confirm the dew point transition of the outlet air stream ( FIG. 3A ) to saturate the packed tower with low grade heavy water vapor.

In order to investigate the elution behavior of heavy water vapor during desorption, the quadrupole mass is determined by elution behavior of H ₂ O and HDO vapor in the outlet air stream when the adsorption tower is desorbed with dry air at a flow rate of 0.2 ℓ / min in an atmosphere of 280 ° C. Measured by an analyzer, the results are shown in Figure 3b .

As shown in FIG . 3B , as the desorption time passes, the molar fraction of hard water (H ₂ O) vapor decreases, while the mole fraction of heavy water (HDO) vapor increases to some point. From this it can be confirmed that the elution behavior of the heavy steam is slower than the hard water steam, and thus, when collected at a time when the mole fraction of the heavy steam decreases, the purified heavy steam can be recovered.

As detailed above, the low grade heavy water component (HDO) is slowly eluted when high temperature air passes through the low grade water vapor adsorbed on the zeolite adsorption tower, when the heavy water vapor is desorbed from the adsorbent pores by the vapor pressure corresponding to the temperature. Therefore, by selecting and collecting the point at which the concentration of heavy water vapor at the outlet of the adsorption column decreases, the heavy water of high purity is recovered and fed to the enrichment section of the subsequent heavy water upgrader, so that the reactor class heavy water can be recovered by more economical operation.

Claims (8)

Adsorbing the low grade heavy water vapor to the adsorbent of the adsorption tower (step 1); Heating the adsorbent of step 1 to desorb heavy water vapor from the adsorbent to elute the mobile phase (step 2); And At the time when the water concentration of the heavy water vapor eluted to the mobile phase by step 2 is reduced in saturation state, the concentration is collected by purity (step 3) Method for recovering low grade heavy water vapor improved purity is configured to include. The method of claim 1, wherein the adsorbent is a porous zeolite molecular sieve. 3. The method of claim 2, wherein the porous zeolite molecular sieve is Na-A type, Na-X type or Ca-A type. The method of claim 1, wherein the heating temperature of the step 2 is 170 ~ 350 ℃ enhanced low grade heavy water vapor recovery method. The method of claim 1, wherein the mobile phase of step 2 is a dry air or an inert gas containing a small amount of hydrogen to induce delayed dropping of heavy water vapor to hard water. . The method of claim 5, wherein the inert gas is any one selected from the group consisting of helium, neon, and argon gas. A method for recovering low grade heavy water vapor having improved purity, characterized by separating and recovering hydrogen isotope oxides of two components using the method according to any one of claims 1 to 6. The method of claim 7 wherein the hydrogen isotope oxide of the binary component is selected from the group consisting of H ₂ O, HDO, T ₂ O, HTO, DTO.

Images