Molten Salt Flow Through Packed Beds Engineering Essay

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This experiment is the study of molten salt flow through graphite pebble channel using packed bed in the molten salt reactor. In this experiment molten salt is used as coolant in the cylindrical type channel because of its high volumetric heat capacity and high temperature. Spherical shaped Graphite pebbles are used as particles in the channel. The high volumetric heat capacity of the molten salt will reduce the pebble size over gas cooled reactors. This reduction in pebble size will increase in power density. The pebble bed advanced high temperature reactor (PBAHTR) is the liquid salt cooled, high temperature reactor designed to use molten salt as its coolant. They are commonly used in the industries involving absorption and adsorption of a solute, distillation, filtration and separation. The operation of the packed bed is flowing one or more liquids through a tower with a fixed bed of particles.

A packed bed is a vertical cylindrical column used in the chemical reactors. The pebble bed reactor concept is based on the reactor core of randomly packed spherical elements containing graphite. The experiment is about the study of pressure drop of one fluid flow (Molten Salt) through the cylindrical column and compares results with the fluent.



Molten salt flow through packed beds introduction:

Packed beds are used to increase the contact between surface area of solid material and a fluid (Molten salt) flowing through a column which in turn increases the transformation between two fluids. When one fluid is used, exchange of ions between the packing material and the fluid will occur. Packed beds have many industrial applications where process involving distillation, extraction, catalysis, adsorption, absorption, and filtration takes place. The industry applications also include exhaust scrubbers which are used to remove pollutants from waste gases before they allowed back into the environment. The set up of the packed bed is a cylindrically-shaped column filled with packing materials. The cylindrical column has the variation in diameter, height and material. And the packing material variation is in shape, roughness, and particle size. The fluid is then pumped through the column. A packed bed that is unrestrained can act as a fluid at the higher fluid flow rates. If the space between the packing increases the particles will oscillate in the column. Under these conditions the packed bed is said to have become fluidization. Fluid flow into the column from bottom to top, passing through the packed materials, there are two pressure nodes above and below the packing that measure the pressure drop across the column. The packing material surface area will increase if the column ensures uniform distribution of the fluid. Fig 1 shows the experimental setup for flow through molten salt. Packed bed is a cylindrical column filled with a packing material. Among different types of packing materials graphite is the most popular.

Fig 1 Experimental set up for flow through packed beds [1]

1.2 Literature Review

The concept of molten salt reactor was introduced by the Oak Ridge national laboratory in 1940s to develop a nuclear engine for a military jet aircraft and support of the aircraft reactor program for the U.S Air Force. After 30 years of research and development followed a design evolution leading to adoption known as single fluid, graphite moderated (MSBR). The design of the reactor will be graphite moderated, thermal neutron spectrum reactor that will generate highly efficient electricity and hydrogen. The area of advance that made single fluid graphite moderated popular was that the idea of employing an under moderated outer zone of the graphite core .Molten Salt reactor is a nuclear fission reactor in which the coolant is molten salt mixtures which can run at high temperature. Molten Fluoride salt coolant as uranium tetra fluoride dissolves the nuclear fuel in many designs .Molten salt reactor contains fluorides of fissile and fertile elements such as UF, PUFand /or ThFare combined with carrier salts to form fluids. Single fluid flow designs have both fissile and fertile combined in one salt.. Whereas the two fluid flow design separate salts for fissile (UF) and fertile (ThF). The advantage of the two fluid flow is that, by the absence of thorium the processing out fission products from the fuel salt is simplified. Later in 1960 it was determined that graphite is the best material for using as particles in the molten salt reactor because it has the capable of long term interaction with the salts. And in the Oak Ridge National Laboratory (ORNL), Molten Salt Reactor experiment was conducted The Molten Salt Reactor Experiment was a test reactor with 7.4MW simulating the neutronic Kernel of an inherently safe epithermal thorium breeder reactor .They used three fuels: Plutonium-239, Uranium-235 and Uranium-233 and the result of breeding thorium is UFThe large breeding blanket of thorium salt was obtained in favor of neutron measurements. The heat coming out of the reactor can be cooled by using the air blowers and radiators.

Recent researches shows interest on the advantages of the high-temperature low pressure primary cooling loop and rely on the ceramic fuel dispersed in the graphite matrix with the molten salt having high temperature and low pressure. These salts have more capacity at taking off the heat from the core, decreasing the need for pumping, piping, and reducing the size of the core. Further molten salt reactors research started with U.S Aircraft reactor experiment (ARE).This experiment was designed as a 2.5 MW nuclear reactor to gain a high power density for use an engine in a nuclear powered bomber. The Aircraft reactor experiment (ARE) was initially developed by the small size that the design provided. While the Molten salt reactor experiment which was conducted in 1960s is the prototype of the thorium fuel cycle breeder reactor nuclear power plant. The advantage of the small core is that it has the fewer materials to absorb neutrons.

The advantages of the molten salt reactor attract to the fourth generation international forum and have drawn attention of many researches again. For the next generation nuclear plant the U.S department of energy is conducting R&D on the Very high temperature reactor (VHTR).

Figure 1 shows the schematic diagram for the flow through packed beds in the cylindrical column with graphite pebbles.

Fig.1. Design to be analyzed

Figure 1 shows the dimensions of the cylindrical column and the dimensions are listed below.

The design is basically a cylinder with a diameter of 0.3 m, and a height of 0.54 m high. The pebble bed will contain graphite pebbles with a diameter of 0.03 m made out of poco-graphite (AXF-5Q); therefore there will be 18 or 19 graphite pebbles per layer. Density of the molten salt is 2020 Kg/m.Viscosity of the fluid is 0.000296 Kg/s-ft. The loop will be operating at a temperature of 700°C, a pressure of 0.2 MPa and a mass flow rate of 4.5 kg/s.. The fluid going through the test section will be Flinak (molten salt).

1.3 Theoretical calculation:

Sabri Ergun conducted many experiments on granular packing in 1950's with the hope of finding the correlation between the pressure drop and the fluid. Ergun concluded that pressure drop over length of the column depends upon the flow rate, size of the column, surface area of the column and shape of the particle; it depends also on the closeness and the orientation of the packing, the viscosity and the density of the fluid also matters. Then Ergun developed an equation 1.1 this equation will work until the fluidization point. If the velocity of the flowing fluid is very high that the tightly packed particles in the column no longer remain stationary, rise along the column and behave similarly to the fluid. Then the Fluidization occurs in the packed beds when the velocity of the flowing fluid is very high Pressure drop through the cylindrical column can be calculated theoretically through the Ergun Equation.

=+1.75 1.1

∆p is Pressure drop, L is the length of packed bed (0.54 m), µ is the Fluid viscosity with 0.000296 Kg/s-ft,G equals to ρV (Mass velocity), V is Superficial velocity (7.8x10), ε is the Inter Particle void fraction(0.49), k is the Conversion factor, (144), g is the gravitational constant(4.17x10),ρ is the Fluid density(2020 Kg/m), D is Effective particle diameter of 0.03 m, Substituting all the calculated values in Eq 1.1. 0.15 MPa pressure drop occurred throughout the test section.