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

Hutton, Bridget Margaret (2001-12)

Thesis (MScEng)--University of Stellenbosch, 2001.

Thesis

ENGLISH ABSTRACT: Liquid-liquid extraction is a branch of solvent extraction that employs addition of an immiscible solvent, as a separating agent, to a liquid feed. Various types of equipment can be used, however if the process requires more than three stages, typically an extraction column, operated in a countercurrent manner would be employed. In order to scale-up and design a commercial extraction column, it is necessary to quantify the extraction system hydrodynamics and mass transfer characteristics. The principal objectives of countercurrently operated extraction columns concern the mass transfer rate and permissible throughput. The performance of a countercurrent extraction column can be adversely affected by axial mixing, which disturbs countercurrent plug flow. Various methods have been devised whereby it is possible to evaluate the performance of a column extraction and scale it up to ensure that the commercial operation achieves the same separation achieved on a pilot scale. Classical axial dispersion models allow quantification of axial mixing and mass transfer rates. Two Backflow models were derived to describe the performance of a Vibrating Plate Extraction (VPE) Column, one for steady state and one for unsteady 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 54 ordinary differential equations, which were solved stagewise using a fourth order Runge Kutta procedure. 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 differing de-protonation constants of the two components and the fact that the solubility of the ionic species of each was low in the organic solvent. The extraction system was quantified using a combination of acid-base and extraction theory. Experimentally determined concentration profiles, measured along the length of the column, were force fitted to the model, thereby allowing determination of the model parameters. The mass transfer coefficients ranged between 0.0098 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 a system whereby tert-butyl hydroquinone (TBHQ) was recovered from a purge stream. Conductivity measurements of the raffinate were used to determine the residence time distribution in the column, and hence allowed determination of the extent of axial mixing. It was preferable that the column be operated with minimum settler volumes, otherwise buffering in the settlers occurred, thereby masking axial mixing effects. This method did not facilitate accurate determination of backmixing, at least two other conductivity measurements in the column needed to be measured.

AFRIKAANSE OPSOMMING: Vloeistof-vloeistofekstraksie is 'n vertakking van oplosmiddelekstraksie wat gebruik maak van die toevoeging van 'n onmengbare oplosmiddel as 'n skeidingagent tot die vloeistofvoer. Verskeie tipes apparaat kan gebruik word, maar as die proses meer as drie stadia vereis, sal 'n ekstraksiekolom, in teenstroom bedryf, tipies gebruik word. Om dit moontlik te maak om 'n kommersiele ekstraksiekolom te skalleer en te bedryf, moet die ekstraksiesisteem se hidrodinamika en massa-oordragkarakteristieke gekwantifiseer word. Die hoof doelwitte van ekstraksiekolomme wat teenstroom bedryf word, gaan om die massa-oordrag en toelaatbare deurset. Die skeidingsdoeltreffendheid van 'n teenstroom ekstraksiekolom kan nadelig beinvloed word deur aksiale vermenging, wat teenstroom propvloei versteur. Verskeie metodes is voorgestel wat dit moontlik maak om die doeltreffendheid van 'n ekstraksiekolom te evalueer en te verseker dat dieselfde skeiding verkry word vir 'n kornmersiele aanleg as vir 'n loodsaanleg. Klassieke aksiale dispersiemodelle laat kwantitatiewe berekening van aksiale vermenging en massa-oordragtempos toe. Twee terugvloeimodelle is afgelei om die werksverrigting van 'n Vibrerende Plaat Ekstraksiekolom (VPE) te beskryf. Die gestadidge toestand model bestaan uit 'n stelsel gelyktydige vergelykings wat opgelos is d.m.v. Excel. Die ongestadige toestand model bestaan uit 54 gewone differensiaalvergelykings, wat stapsgewys opgelos is d.m.v. die vierde orde Runge-Kutta metode. Die gestadigde teostand model is gebaseer op 'n dissosiasie ekstraksieproses, waardeur m-kresol geskei is van p-hidroksiebensaldehied (PHB). Die proses maak gebruik van die verskillende protoneringskonstantes van die twee verbindings en die feit dat die oplosbaarheid van beide die ioniese spesies laag is in die organiese oplosmiddel. Die ekstraksiestelsel is gekwantifiseer deur gebruik te maak van 'n kombinasie van suur-basis- en ekstraksieteorie. Die model is gepas op eksperimenteel bepaalde konsentrasieprofiele, gemeet langs die lengte van die kolom. Die massa-oordragkoeffisiente het waardes aangeneem tussen 0.0098 en 0.189 Imin en daar is gevind dat die terugvermenging van die verspreide fase weglaatbaar was, terwyl die van die kontinue fase laag was (tussen 0 en 0.3). Die ongestadige toestand model wat gebruik is om die dinamiese respons van die VPE te beskryf, is gebaseer op 'n stelsel waar tert-butielhidrokinoon (TBHQ) herwin is vanuit 'n bloeistroom. Geleidingsmetings van die raffinaat is gebruik om die residensietydverspreiding in die kolom te bepaaI en het derhalwe toegelaat dat die mate van aksiale vermenging bepaaI kon word. Die kolom moet by voorkeur met minimale skeiervolumes bedryf word, anders is daar 'n buffereffek in die skeiers, wat die aksiale vermenging verskuiI. Hierdie metode Iaat nie die akkurate bepaling van terugvermenging toe nie en minstens twee ander geleidingsmetings in die kolom was benodig.

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