This work compares lime, frequency and state-space analyses of pressure mea
surements from fluidized beds. The experiments were carried out in a circul
ating fluidized bed, operated under ambient conditions and under different
fluidization regimes. Interpretation of results in time domain, such as sta
ndard deviation of the pressure fluctuations, may lead to erroneous conclus
ions about the flow regime. The results from the frequency domain (power sp
ectral and state-space analyses (correlation dimension, D-ML, and Kolmogoro
v entropy, K-ML, together with a non-linearity test) of the pressure fluctu
ations are generally in agreement and can be used complementary to each oth
er. The power spectra can be divided into three regions, a region correspon
ding to the macro-structure (due to the bubble flow) and, at higher frequen
cies, two regions representing finer structures that are not predominantly
governed by the macro structure of the flow. In all fluidization regimes, t
he measured pressure fluctuations exhibited an intermittent structure, whic
h is not revealed by power spectral analysis of the original signals. Fluct
uations with pronounced peaks in the power spectrum and in the auto-correla
tion function, corresponding to passage of single bubbles through the bed,
are non-linear with a low dimension (D-ML < 5.5). For D-ML < 5.5, the Kolmo
gorov entropy is proportional to the amount of energy in the spectral range
of the intermittent structures observed, whereas for D-ML > 5.5 both K-ML
(bits/cycle) and D-ML are insensitive to changes in the distribution of ene
rgy in power spectra. Thus, the state-space analysis reflects that non-line
arity is mostly found in the macro-structure of the flow. Fluidized bed tim
e series treated in this work are available at http://www.entek.chalmers.se
/ similar to fijo (C) 2000 Elsevier Science Ltd. All rights reserved.