Abstract—Fluidization at vacuum pressure has been a topic that is of growing research interest. Several industrial applications (such as drying, extractive metallurgy, and chemical vapor deposition (CVD)) can potentially take advantage of vacuum pressure fluidization. Particularly, the fine chemical industry requires processing under safe conditions for thermolabile substances, and reduced pressure fluidized beds offer an alternative. Fluidized beds under vacuum conditions provide optimal conditions for treatment of granular materials where the reduced gas pressure maintains an operational environment outside of flammability conditions. The fluidization at low-pressure is markedly different from the usual gas flow patterns of atmospheric fluidization. The different flow regimes can be characterized by the dimensionless Knudsen number. Nevertheless, hydrodynamics of bubbling vacuum fluidized beds has not been investigated to author’s best knowledge. In this work, the two-fluid numerical method was used to determine the impact of reduced pressure on the fundamental properties of a fluidized bed. The slip flow model implemented by Ansys Fluent User Defined Functions (UDF) was used to determine the interphase momentum exchange coefficient. A wide range of operating pressures was investigated (1.01, 0.5, 0.25, 0.1 and 0.03 Bar). The gas was supplied by a uniform inlet at 1.5U mf and 2U mf . The predicted minimum fluidization velocity (U mf ) shows excellent agreement with the experimental data. The results show that the operating pressure has a notable impact on the bed properties and its hydrodynamics. Furthermore, it also shows that the existing Gorosko correlation that predicts bed expansion is not applicable under reduced pressure conditions. Keywords—Computational fluid dynamics, fluidized bed, gas- solid flow, vacuum pressure, slip flow, minimum fluidization velocity. I. INTRODUCTION N the fine chemical industry, the manufactured particulates usually require drying. In some cases, these products include organic solvents that are flammable and pose a risk of explosion with typical fluidized beds. Therefore, the low- oxygen environment of vacuum fluidized beds allows for drying outside of the flammability conditions. Additionally, thermolabile substance can also be dried with vacuum fluidized beds with low risk of material degradation [1]-[4]. Nevertheless, the poor fluidization quality at reduced pressure has been a major detractor [2], [4]. This is also supported by limited experimental and numerical data of such operation. Lanka Dinushke Weerasiri and Subrat Das are with the Deakin University, Geelong, Australia, School of Engineering (e-mail: dinushke.werrasiri@deakin.edu.au, subrat.das@deakin.edu.au). Daniel Fabijanic is with the Deakin University, Geelong, Australia, Institute of Frontier Materials (e-mail: daniel.fabijanic@deakin.edu.au). William Yang is with the CSIRO Process Science and Engineering, Clayton, Victoria, Australia (e-mail: william.yang@csiro.au). Kawamura and Suezawa [5] were the first to study reduced operating pressure in the range of 0.133 to 13.33 kPa for beds with Group B powders. They found similar fluidization characteristics to atmospheric conditions. The fluidization at low-pressure is markedly different to the usual gas flow patterns of atmospheric fluidization. The gas flow is no longer in the laminar flow regime due to the increase in mean free path of gas molecules. The gas can be molecular state, viscous or intermediate state. The different flow regimes can be characterized by the Knudsen number (Kn) (1). It is a ratio of mean free path of molecules () to the characteristic length (diameter of the solid particle). The mean free path can be calculated from (2) where,  is the Boltzmann constant,  is the temperature,  is the diameter of gas molecule and  is the gas pressure [1], [2].  ൌ ఒ ஽ (1) ൌ ௄ √ଶ గక మ ௉ (2) The gas is described to be in a rarefied or molecular state (also known as Knudsen flow) when a gas flow has a Kn >> 1. The gas particles’ collisions are mostly with the container wall rather than with each other. The viscosity of the gas is negligible at this state. Slip flow regime is reached when the mean free path of molecules is comparable to the characteristic length (Kn ≈ 1). The gas flow in this state is characterized by molecular phenomenon and viscosity. In laminar flow (Kn << 1), the characteristic dimension is larger than the mean free path. The gas is governed by the viscosity and the Hagen-Poiseuille law applies. Generally the pressure drop in a particular bed is expressed by the well-known semi-empirical correlation by Ergun [6]. Ergun [6] reported that the loss of pressure in a fluidized bed was due to both viscous (left term) and kinetic energy losses (right term). ∆௉ ௟ ൌ 150 ௨ ೘೑ ሺଵఌ ೒ ሻ మ ఓ ೒ ఌ ೒ య థ మ ௗ మ ൅ 1.75 ఘ ೞ ሺଵఌ ೒ ሻ௨ ೘೑ మ థௗ ఌ ೒ య (3) Kusakabe et al. [7] studied fluidization of fine particles at reduced operating pressure. They combined the expressions for throughput of gas in viscous (4) and intermediate (5) flow as put forth by Dushman et al. [8] to propose an expression for minimum fluidization velocity ( ௠௙ ) (6). ൌ గ஽ ర ଵଶఓ ௉ௗ௉ ௗ௫ (4) Lanka Dinushke Weerasiri, Subrat Das, Daniel Fabijanic, William Yang Numerical Study of Bubbling Fluidized Beds Operating at Sub-atmospheric Conditions I World Academy of Science, Engineering and Technology International Journal of Mechanical and Mechatronics Engineering Vol:13, No:10, 2019 656 International Scholarly and Scientific Research & Innovation 13(10) 2019 ISNI:0000000091950263 Open Science Index, Mechanical and Mechatronics Engineering Vol:13, No:10, 2019 waset.org/Publication/10010817