FLUIDIZATION

Lester AlfonsoSophia AlfonsoJeanette GrandeIana OngDanagene San Gabriel

ChE 155 Unit Operations Laboratory I

Date performed: July 17, 2013Date Submitted: July 24, 2013

ABSTRACTFluidization is a method by which solids are made to adapt the behavior of fluids to minimize difficulty in handling, notably in industrial processes. Fluidizing was performed in the experiment to replicate the process on a smaller level and obtain data on the principles regarding fluidization. This was done using plastic beads as the solids to be fluidized and tap water as the liquid to be passed through the bed. Increasing the flow rate of the water entering the fluidizing column and noting the corresponding heights to which the solid particles reached allowed for the calculation and correlation of velocity and pressure drop, both of which were plotted against bed height and compared with theoretical values. The minimum fluidizing velocity was also computed for and compared with the theoretical values.The data showed a linear and proportional relationship between velocity and bed height, as well as a constant pressure drop at the bed heights increasing past the corresponding minimum fluidizing velocity.

I. INTRODUCTIONWhen a stream of gas or liquid flowing upwards is introduced into a packed bed of solid particles such that the particles are suspended but not swept out of the vessel, fluidization occurs. Fluidization is the process of giving a bed of solid particles the properties of a dense fluid via expansion and suspension in a medium. Fluidizing occurs as the pressure drop equals the weight of the bed over a cross-sectional area (Green and Perry, 2008). The solid particles then exhibit fluid-like properties and behavior, following the volume of the vessel, with retention of horizontal top level as well as zero angle of repose.The first step in the fluidizing process is the fixed bed stage and concerns the minimum fluidizing velocity, the velocity at which bed fluidization is initiated. While the minimum fluidizing velocity remains greater than the flow rate of the fluidizing medium, the solid particles support each others weight and stay in direct contact.The fluidized bed stage, the second step, is reached when the flow rate of the fluidizing medium is equal to or greater than the minimum fluidizing velocity, but less than the particle terminal velocity. In this stage, separation of solids and consequently bed expansion ensues.Lastly, the mobilized bed stage is attained when the fluid flow rate reaches or surpasses the particle terminal velocity. The bed is expanded further and the particles are pushed out of the container.

Fluidization is categorized into two types. Aggregative fluidization occurs when the particles used are of large size, flow rate is high, and the difference between the fluid and solid densities is great. The fluid passes through the bed in large bubbles and fluidization under these circumstances is uneven.

On the other hand, particulate fluidization follows when the solid and fluid densities are nearly equal and the particles are relatively small. With low fluid flow velocity, fluidization is even and each particle moves through fairly uniform mean free paths.The ratio between kinetic and gravitational energy termed as the Froude number (NFR) and depicted in the equation that follows is used to determine the type of fluidization that will prevail.(Eqn. 3.1)where = Froude Number = Reynolds Number = solid particle density = fluid density = bed height = particle diameter

When the product X computed from these factors is found to be greater than 100, the fluidization is aggregate fluidization. If less than 100, it is taken to be particulate fluidization.Bed porosity is defined as the void fraction of a bed of particles, taken to be the void volume divided by the total bed volume. The porosity during fluidization at any given height can be computed with the following equation.(Eqn. 3.2)where LA and LB are bed heights at A and B, respectively

The pressure drop is then determined using the calculated porosity.(Eqn. 3.3)The Reynolds number is instrumental in the equations used to determine the velocity at which the solid particles begin to be swept out of the vessel, termed the terminal velocity .At < 1: (Eqn. 3.4)At 1 < < 500:(Eqn. 3.5)At 500 < < 2 x 105:(Eqn. 3.6)McCabe (2001) instructs that the bed be fluidized vigorously and the flow rate gradually increased until bed expansion occurs to measure the minimum fluidizing velocity VOM. For large particles, VOM is given as a modification of the Ergun equation: (Eqn. 3.7)When the particles are small and NRe