Optimization of WF6 desorption process on NaF nano adsorbent

dissertation

to obtain a master's degree

in the field of nano chemical engineering

abstract

In the uranium enrichment unit by centrifuge, UF6 gas enters the cold trap after passing through the centrifuge and performing the enrichment process It is solidified and collected at low temperature.

One ??of the absorbents used to absorb uranium hexafluoride is sodium fluoride. The equilibrium curve of UF6 absorption by sodium fluoride adsorbent is one of the important information for the design of absorption towers. The advantage of sodium fluoride compared to other UF6 absorbers is the possibility of desorption, regeneration of the absorber and reuse of UF6. In this thesis, surface adsorption of UF6 on sodium fluoride at pressures of 10, 20, 30, and 40 millibars is carried out and based on the Langmuir isotherm diagram, the amount of saturation adsorption at ambient temperature is obtained, then desorption of UF6 is carried out at different temperatures and pressures based on the needs of the industry, and the effective factors in desorption are investigated and the best process conditions are presented. In the following, the rate of loss of absorption capacity after successive absorption and desorption under the same process conditions has been investigated and reported. Qualitative analysis of UF6 before and after desorption has been done and reported. Studies have been conducted and qualitative analyzes have been reported on how to obtain sodium fluoride and its cavity structure.

Key words: surface adsorption, surface desorption, isotherm, sodium fluoride, uranium hexafluoride

1-1-                                                                                                                                                                             or separate different components from a mixture by appropriate methods. The type of method chosen for separation depends on various factors such as the nature of the substance and mixture, size, concentration, phase of the substance and so on. It depends. In general, separation based on the chemical driving force is divided into five general categories, and each mixture can be separated using one of these methods or a combination of them.

The general methods of separation are:

1- Separation by creating a new phase (distillation)

2- Separation by adding a new phase (use from the solvent)

3- Separation by creating a barrier (using a membrane)

4- Separation using solid particles (surface absorption)

5- Separation by magnetic or electric field

1-1-1-               Surface absorption

Surface adsorption is a type of separation process in which some components of a fluid phase are transferred to the adsorbent surface. In general, in solid materials, the structure of the surface is different from the structure of the solid mass, so that the surface is not completely saturated in terms of energy, and when the solid is exposed to a gas, the gas molecules are attached to the centers on the surface and are absorbed. This phenomenon is called gas absorption by the solid [1]. Normally, small solid particles are kept stationary in a bed and the gas continuously passes through that bed. Finally, the solid is almost saturated and separation is no longer possible. At this time, the flow is transferred to the second bed until the adsorbent is saturated, replaced or regenerated.

Surface adsorption is proposed as the most important process in separation for low concentrations. In surface absorption, unlike the absorption process in which the separation occurs in the fluid mass, the separation takes place on the solid surface.

An important point that should be considered in surface absorption is that this method of separation is used as an efficient method when separation is considered at low concentrations. Since the particles used in surface absorption become saturated after a while and lose their initial absorption ability, this method is not very useful in high concentrations because the adsorbents are quickly saturated and lose their ability.. Most of the adsorbents are very porous materials and surface adsorption mainly takes place on the walls of the cavities or specific locations inside the particle. Since the holes are generally very small, the internal surface area is several times larger than the external surface area and reaches approximately between 500 and 1000 square meters per gram. Differences in molecular mass, shape, or polarity make some molecules more tightly held on the surface. It is also possible that the pores are too small to accept the larger molecules, causing the materials to separate. In many cases, the adsorbed component is held tightly enough and its complete separation from the fluid is possible with very little absorption of other components. In this case, the absorbed material can be processed in a concentrated or almost pure form by regenerating the adsorbent.

Surface absorption applications in the vapor phase include the recycling of organic solvents used in paints, printing inks and solutions used for cast iron.

Surface absorption on carbon is used to separate pollutants such as CO2, N2O and other odorous compounds from the air to As in most new cars, charcoal cans are used to prevent gasoline from entering the air inside the car. Drying of gases is often done by surface absorption of water on silica gel, alumina or other inorganic porous solids. Zeolites or molecular sieves are natural or artificial alumina and silicates that are effective in producing low dew point gases. Surface adsorption on molecular sieves is also used to separate oxygen and nitrogen, to prepare pure hydrogen in order to separate normal paraffins from branched paraffins and aromatic compounds.

The direction of mass transfer in this process is from the liquid phase to the solid phase and the opposite is known as the surface deposition process. In order to recover some valuable materials (such as precious metals) from absorbent surfaces, the disposal process with suitable efficiency is widely used.

Gas flow speed and time required for the desired cycle determine the size of absorbent bed. By using longer beds, the absorption cycle can be extended to several days, but the increase in pressure drop and the higher initial investment of the absorption column will make it uneconomical.

Separation processes through surface absorption are almost similar, in this way, the mixture to be separated is placed in contact with an insoluble phase (adsorbent) and an inhomogeneous distribution of the main components between the adsorbed phase on the solid surface and the fluid mass occurs, and the separation is carried out. There are two types of separation in surface absorption: 1 physical surface absorption or van der Waals absorption 2 chemical surface absorption 1-1-1-1 Physical surface absorption or van der Waals absorption This type of absorption is related to the reversible process that is the result of absorption through intermolecular forces between solid and Surface adsorbed materials are obtained. For example, when the intermolecular forces between a solid and a gas are greater than the intermolecular forces of the gas alone, even if the gas pressure is lower than the vapor pressure at the temperature of the solid, the gas molecules are adsorbed on the surface of the solid. This absorption is usually accompanied by heat and is slightly larger than the latent heat of vaporization. The adsorbed material is not replaced in the solid crystal structure and does not dissolve in it, but remains completely on the solid surface.

Most of the time, in the equilibrium state, the partial pressure of the adsorbed material is equal to the pressure of the gas phase in contact, and by lowering the gas pressure or increasing the temperature, the adsorbed gas is easily removed. Physical absorption is not specific and like condensation generally happens with any solid gas system and does not depend on the type of absorbent or absorbent, provided that the combination of temperature and pressure is suitable. Reversible (physical) surface absorption is not limited to gases but has also been observed in liquids.

Abstract

In the uranium enrichment plant by gas centrifuges, the gaseous UF6 is enriched by passing through a cylindrical centrifuge and then is desublimated and stored by a cold trap in low temperatures.

Sodium fluoride (NaF) is an applicable adsorbent of UF6. The equilibrium diagram of UF6 adsorption by sodium fluoride is very important for designing adsorption towers of UF6.