(611c) Flow Patterns in High-Density Circulating Fluidized Beds

FLOW PATTERNS IN HIGH-DENSITY
CIRCULATING FLUIDIZED BEDS

T.
Hensler^1,x, K.-E. Wirth^1,*

¹ Institute of Particle
Technology, University of Erlangen-Nuremberg, D-91058 Erlangen, Germany

^x Presenting author:
Timo Hensler, Cauerstr. 4,
D-91058 Erlangen, Germany, e‑mail: timo.hensler@fau.de, phone: +49/9131/85-28291, fax:
+49/9131/85-29402

* Corresponding author: Prof. Dr.‑Ing.
habil. KarlErnst
Wirth, Cauerstr. 4, D-91058 Erlangen, Germany, e‑mail: karl-ernst.wirth@fau.de, phone: +49/9131/85-29403, fax:
+49/9131/85-29402

KEYWORDS:
                        Circulating fluidized bed, high-density riser, multi-phase
flow, fluid‑solid interaction, solids distribution, X-ray computer
tomography, residence time behavior, tracer gas, capacitance probe, state
diagram

TYPE OF PRESENTATION: oral

SCIENTIFIC FIELD:               Particle
Technology Forum

ABSTRACT

The presented work deals with the
investigation of flow patterns in a high-density gas‑solid circulating
fluidized bed riser in pilot plant scale. Circulating fluidized beds provide
excellent conditions for gas-solid reactions due to intense contacting of the
reactants. In order to intensify the interaction of the reacting phases,
investigations have focused on the maximization of the solids holdup in the
reaction zone within the past two decades [ISSANGYA 1999].
Recent results show that solid concentrations up to 30 vol.% [WANG 2014,
HENSLER 2014] can be achieved in the riser section of high-density circulating
fluidized beds. Even though high-density risers offer good prospects for
exceedingly efficient operation of future processes, their implementation and
stable handling of the desired flow condition remains challenging. In order to
gain detailed insight into the flow behavior of the gas and the solid phase within
high-density risers, fluid-dynamic investigations are performed in a pilot plant
scale cold‑flow circulating fluidized bed riser with an inner diameter of
190 mm and a height of 11.3 m. Within the scope of the measurements
the solids holdup within the riser is varied in the range from 3 to 35 vol.%
and superficial gas velocities range from 3.3 to 11.5 m s^-1.
Experimental investigations focus on the characterization of flow patterns in
the riser section with regard to both the solid and the fluid phase. For
determination of the distribution of the solids over the cross section of the
riser, the plant is equipped with a 160 kV X‑ray computer tomography
system. This allows for non-invasive scanning of the riser cross section, by
which the time averaged distribution of particles is derived with high spatial
resolution. Moreover the motion of particle clusters is investigated using
capacitance probes. In this way radial velocity profiles of particle clusters
are derived under various flow conditions. The behavior of the fluid phase is
investigated by the injection of a tracer gas. By tracking the distribution of
the tracer gas in axial and radial direction of the riser, the residence time
behavior of the gas as well as the coefficient of axial dispersion is
determined. Based on the experimental findings the investigated flow conditions
are categorized according to the dense suspension upflow model proposed by
Grace et al. [GRACE 1999] and Wirth's steady state diagram of vertical
multi-phase flow [WIRTH 1991]. Subsequently both models are compared to
identify analogies. The gained insight into the flow structures of high-density
risers provides valuable information for the design of multiphase reactors for
fast catalytic reactions. Moreover, the results allow for drawing conclusions
on the interaction of the fluid and the solids phase in vertical upflow and
thus provide a strong basis for the optimization of multi-phase simulations.

GRACE J. R., ISSANGYA A. S., BAI
D., BI H., (1999), Situating the High-Density Circulating Fluidized Bed, AIChE
Journal, 45, 2108-2116

HENSLER T., ZHANG Y., MLECZKO L.,
ASSMANN J., BELLINGHAUSEN R., SCHWIEGER W., WIRTH K.-E., (2014), High-Density
Risers as Reactor System for Benzene Synthesis by Non-oxidative
Dehydroaromatization of Methane, Proceedings of the 11th International
Conference on Fluidized Bed Technology, Chemical Industry Press, 155-160

ISSANGYA A. S., BAI D., BI H. T.,
LIM K. S., ZHU J., GRACE J. R., (1999), Suspension Densities in a High-density
Circulating Fluidized Bed Riser, Chem. Eng. Sci., 54, 5451-5460

WANG C., ZHU J., Li C., BARGHI
S., (2014), Detailed Measurements of Particle Velocity and Solids Flux in a
High Density Circulating Fluidized Bed Riser, Chem. Eng. Sci., 114, 9-20

WIRTH K.-E., (1991), Fluid
Mechanics of Circulating Fluidized Beds, Chem. Eng. Technol., 14, 29-38