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Hematite Precipitation
Objective: To Estimate Parameters for a Gas-Liquid-Solid Multiphase
Batch Reactor
This REX example involves a three-phase reactor. Here, oxygen is fed to oxidize Fe ions in an
aqueous solution, thus creating hematite which is precipitated out. This process promotes the
removal of ferrous ions from liquid solutions. The model is adapted from the work of Ruiz et al. [1].
You may download the REX file to view the model. This example shows how easy it is to set up
multiphase fed-batch reactors where the pressure is controlled by manipulating the gas flow entering
or leaving the reactor.
Features Illustrated
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Three phase (Gas-Liquid-Solid) model
Fed-Batch reactor with Pressure Control
Reaction Model
All reactions take place in the liquid phase. First, there is the oxidation of ferrous ions to ferric:
2Fe2+ + 2H+ + ½ O2 → 2Fe3+ + H2O
This is followed by the hydrolysis of ferric ions:
2Fe3+ + 3H2O → Fe2O3 + 6H+
Oxygen enters the reactor from a gas feed and dissolved oxygen oxidises the ferrous ions. Due to
poor solubility of Fe2O3 , it mostly precipitates.
Setting up the REX Project
A multiphase Batch with Gas, liquid and solid phases are defined in Reactor node, with reaction in
the liquid phase. The compound distribution is then specified as shown below. In this example, all
species are available in liquid phase. Oxygen is also present in Gas phase, while Fe2O3 is present in
both the liquid and solid phase:
For the O2 gas-liquid equilibrium, Henry constant values must be defined in the Phase Distribution
node. You may use a polynomial or exponential correlation. For simplicity, we use a constant value in
this example. Likewise, the solid-liquid equilibrium for hematite must be specified in the Solubility tab
of the Phase Distribution node.
The oxygen feed is set by a pressure controller. As oxygen is consumed, the gas inflow replenishes it
while keeping the reactor pressure constant. Thus, inflows are enabled in Reactor node, and the
inflow composition is defined in Flow node as shown below.
Experimental Data
The experimental design includes pressure and temperature variation. Conditions include pressure
setpoints at 15, 45 and 90 psi and temperature settings at 160, 180 and 190 C. Measurements of
Fe2+ and Fe2O3 are provided as a function of time. For other species, the initial content is known,
however they are not measured with time. A snapshot of the Total moles data for the set P15psiT180C is shown below:
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In this example, the compounds measurements are available as total moles only. Compounds
measurements in the phases are not available. Only the phase volumes are available as shown
below:
Parameter Estimation and Results
Pre-exponentials and Activation Energies are estimated for the reactions by reconciling the measured
Fe2+ and Fe2O3 total moles:
Pressure is weighted to keep it close to its set-point by adjusting the inflow of fresh O2 . All variables
selected are weighted using the automatic weight generator for Uniform Percentage Error, while
ignoring the zeros in the data.
After executing the run, we highlight the most significant results. For Fe2O3 , the predictions are quite
good:
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There is no data for Fe2O3 in the third and fourth sets, thus they appear as zero. These values do not
affect the parameter estimation, as the zero measurements were specified to be ignored. In the chart
above, we can select the Ignore Zero Measurements checkbox to hide the zero data points.
We now examine the Fe2O3 in the liquid and solid phases:
We observe that liquid Fe2O3 concentration initially increases until the solution is saturated and then
becomes constant.
As for the Solid phase, it stays at zero until the solution is saturated. Once the solubility limit is
reached, precipitation starts and the solid phase keeps increasing.
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The predictions for Fe2+ are shown below.
If you wish to see all the compound profiles for a single set, you may execute the Single Set action in
the Model Data Comparison node as shown below for the first set:
References
1. Ruiz, M.C., Castillo, D., Padilla, R., 2007, Precipitación de Hematita en Medio Sulfato, IV
International Copper Hydrometallurgy Workshop, May 16-18, Viña del Mar, Chile.
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