(308h) Dynamic Modelling and Control of Membrane Solvent Extraction Unit for Recovery of Rare Earth Elements

Production of rare earth elements from various geothermal and chemical sources has seen a considerable upsurge in the past few years, owing to their applications in multiple industrial sectors, such as catalysis, glass polishing, and ceramics¹. Solvent extraction is a hydrometallurgical unit operation which separates rare earth elements from aqueous acidic solutions with the help of an organic solvent and an extractant. Because the extractant prefers to form a complex with rare earth elements, these elements can be concentrated in the product stream. However, in traditional solvent extraction, the aqueous and organic phases are mixed together in a suspension. This can lead to solvent losses or entrainment of one liquid phase into another, which may lead to poor performance and economic losses. In addition, loading and stripping are done in separate stages, leading to a large physical footprint and long system settling times. Membrane solvent extraction is a novel separation technology in which two aqueous phases flow in channels separated by a membrane that contains the extractant in an organic phase. This system design avoids the issues noted above. However, the mechanism for transport through the membrane is complex, involving both concentration gradients and electric potential difference. This work presents a dynamic model of a membrane solvent extraction unit of a hollow fiber supported liquid membrane (HFSLM) unit².

Models of membrane solvent extraction units for HFSLM have been presented in the literature^3–5. One of the limitations of the existing models that this work seeks to address is the lack of a model for the transport of ionic species through the membrane. The specific configuration modeled in this work is a membrane solvent extraction unit where the extractant is embedded in the microporous fibers of the hollow tube. The feed solution is sent through the center of the tube annulus, and the stripping solution flows around the exterior of the annulus. The dynamic model considers variation of transport variables in both axial and radial directions, leading to a partial differential equation system^4,6. The loading and stripping reactions occur in a reaction zone at the interface of the membrane. Some of the models found in the literature have estimated the membrane flux and the concentration gradient across the membrane by using a material balance on the overall system. In this work, a model for transport of the rare earth elements through the membrane has been developed by considering a combination of the chemical and electrical potentials as the driving force.

One of the control challenges for the membrane solvent extraction units arises from the semi-continuous operation of these units. Thus, the objectives for this control system are the recovery and purity of the rare earth elements. We develop regulatory and supervisory control approaches to solve these control problems to minimize the batch time while maintaining product purity requirements. Disturbance rejection performance is also evaluated from the response of the system to changes in feed solution composition.

Acknowledgments:

This effort was funded by the U.S. Department of Energy’s Process Optimization and Modeling for Minerals Sustainability (PrOMMiS) Initiative, supported by the Office of Fossil Energy and Carbon Management’s Office of Resource Sustainability.

Disclaimer:

This project was funded by the United States Department of Energy, National Energy Technology Laboratory an agency of the United States Government, in part, through a support contract. Neither the United States Government nor any agency thereof, nor any of its employees, nor the support contractor, nor any of their employees, makes any warranty, expressor implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof, or any of their contractors.

References

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