Gas Bypassing and Circulation of Sand and Char Particles in a Dual System with Fluidized Bed and Pneumatic Transport: Effects of Design and Operation Parameters

Abstract

Dual and multi-fluidized solid circulation systems are widely used in the processes for catalyst regeneration, coal and biomass gasification, cleaning of gases, thermal cracking, drying and incineration of waste. The solids circulation systems can use external or internal circulation. The external circulation is characterized by two beds (riser and downcomer) under different fluidized conditions connected by means of a cyclone and a non-mechanical valve (e.g. fluidized bed siphon, L-valve, etc.). In the case of internal circulation system, one vessel is divided into two or several compartments by internal wall(s). The particles must be fluidized at least in one chamber and generally the fluid dynamic regimes have to be different in the chambers (for example slow fluidized bed and fast fluidized bed). The walls generally must be provided in the lower part by orifices enabling flow of solids between adjacent chambers and by an overflow or impactor for return of particles in the upper part. In a very simplified arrangement the internal circulation system (ICS) can be represented by a cylindrical vessel with a central draft tube. The circulating mass flow of solids (or mass flux) in ICS is dependent on geometrical/design arrangement, height of bed of solids, gas flow rate (aeration) in chambers and on solid particle properties. For practical and safety reasons (for example in combustion/gasification or in a sorption/regeneration process) the mixing (leakage) of one gas with another gas in the adjacent chamber should be often minimized. Unfortunately, total separation of gaseous atmospheres in an ICS is practically excluded. Our study has been concentrated on influence of geometric parameters (height of cylindrical chamber for pneumatic transport (PT), flat perforated sheet and conical gas distributor, different upper part for return of solids, total orifice cross sectional area) and operating parameters (gas flow rate in PT-column, height of fluidized bed) on circulating flow of solids. In a limited extend gas leakage study on a dual fluidized bed (FB)-PT system for circulation of solids has been performed.