Theoretical Models for Membrane Capacitive Deionization for the design of Modular Desalination Processes
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Hellriegel, Ulrich
Critelli, Salvatore
Gabriele, Bartolo
Figoli, Alberto
Hoinkis, Jan
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Dottorato di ricerca in Scienze e ingegneria dell'ambiente delle costruzioni e dell'energia - SIACE
Ciclo XXXIII; Due to climate change, water scarcity will be exacerbated around the globe. To increase
the water availability in regions at risk, water desalination plants can be a solution. Especially
in rural areas, energy e cient technologies are needed so that an operation with
renewable energy as photovoltaic modules can be feasible.
Recent publications showed that the novel technology membrane capacitive deionization
(MCDI) can achieve a lower speci c energy consumption (SEC) than reverse osmosis
(RO), for brackish water desalination with salt concentrations below 2.5 g L-1.
There is still a gap in research between laboratory operation and applied commercial
scaled desalination, regarding experimental but also theoretical model studies. Therefore
the latter is elaborated in the present PhD thesis. Hereby, existing models are
reviewed, adapted and further developed to t to applied MCDI operation for drinking
water production.
Two dimensional nite element methods (FEM) modelling of ion transport, according to
the Gouy-Chapman-Stern theory for electrical double layers (EDL) as well as computational
uid dynamics (CFD) is combined with an adjusted semi-analytical modi ed
Donnan (mD) model, with a constant excess chemical potential att = 2:33 kT, for
the electrosorption of ions into porous active carbon electrodes. It predicts the e uent
salt concentration time-dependently for di erent inputs of applied electrical currents
Icell and voltages as well as inlet concentrations and volume ows. Applied MCDI operation
was optimized for drinking water production with practical experiments, which
support the evaluation of the theoretical ndings. The model ts to experimental data
for Icell = 20 A, however the equations for the voltage over the electrodes need to be
re-assessed so that the model ts for further input parameters.
A CFD model of the water ow through large scaled MCDI modules (> 50 pairs of
electodes) shows the need of constructing spacer thicknesses Sp small enough, to ensure
equal retention times of the water between the electrodes in the module, which is
important for stable diluate concentrations.
Furthermore, an analytical calculation tool is developed, by adjusting the mD model
and introducing an e ective salt adsorption capacity salt; , to predict the maximum
e cient charging time tmax,ch, removal- and recovery rate as well as SEC values for optimized
operation of applied MCDI processes. The model reaches an accuracy of 87% for
the prediction of salt removal, 86% for tmax,ch and 75% for SEC values, compared with
an experimental study and thus can be used to optimize the process design of applied
MCDI desalination plants.Soggetto
Desalination; MCDI; Simulation; Environmental engineering; Calculation tools
Relazione
CHIM/06;