Evaluation of a vibrating plate extraction column by application of steady state and unsteady state backflow models

[摘要] ENGLISH ABSTRACT:Liquid-liquid extraction is a branch of solvent extraction that employs addition of an immisciblesolvent, as a separating agent, to a liquid feed. Various types of equipment can be used, howeverif the process requires more than three stages, typically an extraction column, operated in acountercurrent manner would be employed. In order to scale-up and design a commercialextraction column, it is necessary to quantify the extraction system hydrodynamics and masstransfer characteristics. The principal objectives of countercurrently operated extraction columnsconcern the mass transfer rate and permissible throughput. The performance of a countercurrentextraction column can be adversely affected by axial mixing, which disturbs countercurrent plugflow.Various methods have been devised whereby it is possible to evaluate the performance of acolumn extraction and scale it up to ensure that the commercial operation achieves the sameseparation achieved on a pilot scale. Classical axial dispersion models allow quantification ofaxial mixing and mass transfer rates. Two Backflow models were derived to describe theperformance of a Vibrating Plate Extraction (VPE) Column, one for steady state and one forunsteady state operation. The steady state model consisted of a series of simultaneous equations,which were solved using the Excel solver function. The unsteady state model consisted of 54ordinary differential equations, which were solved stagewise using a fourth order Runge Kuttaprocedure.The steady state model was based on a dissociation extraction process, whereby meta-cresol (mcresol)was separated from para-hydroxy-benzaldehyde (PHB). The process used the differingde-protonation constants of the two components and the fact that the solubility of the ionicspecies of each was low in the organic solvent. The extraction system was quantified using acombination of acid-base and extraction theory. Experimentally determined concentrationprofiles, measured along the length of the column, were force fitted to the model, therebyallowing determination of the model parameters. The mass transfer coefficients ranged between0.0098 and 0.189 Imin, and it was found that backmixing of the dispersed phase was negligible,while that of the continuous phase was low (varying between 0 and 0.3).The unsteady state model, used to describe the dynamic response of a VPE, was based on asystem whereby tert-butyl hydroquinone (TBHQ) was recovered from a purge stream.Conductivity measurements of the raffinate were used to determine the residence timedistribution in the column, and hence allowed determination of the extent of axial mixing. It waspreferable that the column be operated with minimum settler volumes, otherwise buffering in thesettlers occurred, thereby masking axial mixing effects. This method did not facilitate accuratedetermination of backmixing, at least two other conductivity measurements in the columnneeded to be measured.

[发布机构] Stellenbosch University