(821e) CFD Analysis of Slag Flow and Residence Time in a Packed Bed Reactor

Phosphorus is an essential and
non-replaceable element for all life forms. While the phosphate rock still
remains the main source for the worldwide production of phosphorus today [1],
this practice is not sustainable due to rather fast depletion of this
non-renewable resource [1, 2]. The ecological cost of phosphate rock mining is
also tremendous, ranging from eutrophication [3] to emissions of toxic and
radioactive pollutants [4, 5]. Sustainable recycling of this crucial food
resource will help combating global poverty and providing better conditions for
human life. The main source for recycling of phosphorus is the sewage sludge
and particularly sewage sludge ashes after mono-incineration of the sludge [6].
The European Union is committed to promote research in this direction and the
study presented here is part of the ?RecoPhos? project which has received
funding from the European Union Seventh Framework Programme (FP7/2007-2013)
under grant agreement no. 282856.

The basic idea in RecoPhos
project is thermal reduction of the phosphates in sewage sludge ash to
phosphorus gas in an inductively heated bed of Carbon. In this paper, CFD
analysis and simulation of the fluid flow through the packed bed in the so
called ?InduCarb? reactor is presented. The focus is on the residence time of
the molten ash in the reactor under different operating conditions. Such a
study is crucially important for better understanding of the fluid dynamics in
such a complex system and to provide insight for proper design and optimization
of the reactor. As such, the current study disregards the chemical reactions
taking place and only focuses on the multiphase flow of the non-reacting flow
and how the flow is affected by changes in ash properties, packed bed
characteristics or operating parameters.

The CFD study is performed
employing the latest version of the commercial software Ansys Fluent. The
multiphase model used is the Eulerian granular which is the most complete approach
among Ansys Fluent multiphase models [7]. The flow behavior in the reactor also
depends on whether the reactor is at transient or steady state. The residence
time of the molten ash during start-up i.e. transient state is different than
the residence time during steady state. Both states are considered in this
study. During steady state, the residence time distribution of the molten ash
in the reactor is studied. This is done by pulse injection of a tracer at the
inlet and monitoring its concentration at the outlet at fixed time intervals.
The resulting exit concentration curve is widely used among chemical engineers
to characterize the reactor performance [8]. CFD visualization opportunities e.g.
by visualizing localized concentration of the tracer inside the reactor will
give even more insight for understanding the behavior of such a complex system.
Fig. 1 shows the molten ash flow through the packed bed by color coding the
volume fraction of the ash in relation to the total volume of all phases
including gas, liquid and granular.

Fig. 1. Visualization of the
molten ash flow through the packed bed reactor by color coding contours of ash
volume fraction in the reactor.

The CFD results show
interesting correlations between operating parameters and flow behavior of the molten
ash in the reactor. In particular, the ash feed rate, viscosity and particles
size have shown to be highly influential on the molten ash residence time.
Porosity of the packed bed also affects the distribution of the residence time.
The ash feed rate and the outlet diameter should be designed in such a way that
flooding of the reactor by the molten ash is avoided. If the molten ash
viscosity is too high or the particles in the packed bed are too small, liquid
hold-up in the reactor can become a problem too.

References

1.    Cordell, D., Drangert, J.-O., White,
S. (2009). The story of phosphorus: Global food security and food for thought.
Global Environmental Change 19: 292-305.

2.    Steen, I. (1998). Phosphorus
availability in the 21^st Century: management of a non-renewable
resource. Phosphorus and Potassium 217: 25-31.

3.    Wissa, A.E.Z. (2003). Phosphogypsum
Disposal and the Environment. Ardaman & Associates Inc., Florida. Available:
http://www.fipr.state.fl.us/pondwatercd/phosphogypsum_disposal.htm

4.    Kratz, S., Schnug, E. (2006). Rock
Phosphates and P Fertilizers as Sources of U                            
Contamination in Agricultural Soils. Institute of Plant Nutrition and Soil
Science, Federal Agricultural research Center, Germany, pp: 57-67, in Uranium
in the Environment: Mining Impact and Consequences, Springer Berlin Heidelberg.

5.    Saueia, C. H., Mazzilli, B.P., Favaro,
D.I.T. (2005). Natural radioactivity in phosphate rock, phosphogypsum and
phosphate fertilizers in Brazil. Journal of Radioanalytical and Nuclear
Chemistry, 264: 445-448.

6.   
Donatello,
S., Tong, D.,Cheeseman, C.R. (2010). Production of technical grade phosphoric
acid from incinerator sewage sludge ash (ISSA). Waste Management 30: 1634?1642.

7.    ANSYS FLUENT Theory Guide
(2011). ANSYS Inc., Release 14.0, Canonsburg.

8.    Levenspiel, O. (1972).
Chemical reaction engineering. (2^nd ed.) John Wiley & Sons,
Toronto, Canada.