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
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.
SVOBODA Karel;
KALISZ Sylwester;
MARTINEC Jiri;
POHORELY Michael;
2008-09-15
CHISA
JRC47305
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