Double-cyclone in fluidized bed drying is an important equipment which reflects the conditions of drying in HDPE slurry process. Cyclone is an important unite of fluidized bed drying in order to move the solid particles outward to its wall. Therefore, flow pattern created in fluidized bed will affect industrial cyclones installed in dryer for dust removing. Pressure drop of the cyclones is an effective parameter represents the drying behavior. Substantially, geometry of cyclone, inlet flow rate of gas, density and particle size distribution (PSD) can affect the pressure drop value. Fluidized bed hydrodynamic regime is very complex and must be understood to improve fluidized bed operations through theoretical, industrial and CFD study of double-cyclone. Pressure drop is introduced as parameter related to the cyclone efficiency can be calculated with ANSYS Fluent software in the Eulerian-Lagrangian framework with RNG k-ɛ turbulence model used as a mathematical method. Proper pressure drop concluded from industrial experiments and CFD calculation shows good fluidization of HDPE particles in the bed of nitrogen and powder to reach the best fluidized bed situation and suitable quality of HDPE powdery product. Keywords:CFD; HDPE Particles; Double-Cyclone; Pressure Drop. Article History:Received: 28 July 2017Accepted: 01 January 2018 1-IntroductionHDPE Particles of fluid bed drying in the gas entering double-cyclone are subjected to centrifugal forces which move them radially outwards, against the inward flow of gas and towards the inside surface of the cyclone on which the solids separate [1]. The performance of cyclone is in a relationship with its static pressure drop between input and output [2]. The factors affecting the rate of entrainment of solids from a fluidized bed dryer named particle size distribution (PSD), terminal velocity, superficial gas velocity, particle density, gas properties and gas flow regime. Therefore, it is necessary to understand the gas-particle flow and separation characteristics of the cyclone.With computational fluid dynamics (CFD techniques), it is now possible to sufficiently calculate the pressure drop created in cyclone. Fluid flows have been mathematically described by a set of nonlinear and partial differential equations named the continuity and Navier-Stokes equation [3]. ANSYS Fluent solves conservation equations for mass and momentum and additional transport equations are also solved when the flow is turbulent. Different CFD calculation have been successfully applied by employing the related mathematic model to determine the features of gas-solid flow field for cyclones [4,5]. Turbulence models such RNG-based k-ɛ model which was derived using a statistical technique called renormalization group theory is one of the proper models in this field. In Fluent, the Lagrangian discrete phase based partly on the physical properties of dust particles and partly on the mathematical modeling with reasonable assumptions made to describe the particles transport in a fluid medium [6]....