Liquid/Liquid Extraction


SEPERATION


Liquid/liquid extraction is an important separation mechanism in the nuclear fuel reprocessing cycle and in other industrial plant. This mechanism depends upon utilising the different relative solubility of dissolved species between two liquid phases in order to concentrate preferentially one or more species.

This application demonstrates how FACSIMILE has been applied to a particular separation involving a number of dissolved species in order to simulate both steady state and dynamic behaviour.

PROCESS UNITS


A process unit consists of any device involved in a solvent extraction operation entailing two counter current liquid streams. The two streams, in this example, are respectively an aqueous stream and an organic solvent stream with mass transfer occurring between the streams and possibly chemical reactions occurring in the aqueous stream. There may be a variety of input points with feeds entering at the top, bottom and intermediate positions along the unit.

The minimum requirement is that three chemical components exist in both the aqueous and solvent phases. For the case of extra chemical components in the aqueous phase a minimum of two is required. The chemical components involved are uranium, plutonium, nitric acid and possibly a reducing agent (hydroxylamine or hydrazine or uranium in a particular valence state). The uranium and plutonium components in different valence states are considered as different components.
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Figure 1: Predicted uranium concentration versus measured concentration in an aqueous/organic solvent extraction unit.

Figure 1 shows a graph of the predicted concentration of Uranium versus the measured concentration in an aqueous/organic solvent extraction unit. This graph is just one of many that has been used to validate the model. As a result the model has been and continues to be a valuable tool in the design and optimisation of this type of separation in the nuclear reprocessing industry.

This application demonstrates the usefulness of FACSIMILE for modelling industrial separations involving equilibrium and reaction kinetics for determining mass transfer, in this instance, between two liquid phases. However the model is much more than a steady state simulator. The inclusion of dynamic simulation and transport mechanisms demonstrates the versatility and capability of FACSIMILE.