Mass transfer in structured packing

Erasmus, Andre Brink (2004-12)

Thesis (PhD) -- University of Stellenbosch, 2004.


ENGLISH ABSTRACT: Structured packing is a popular column internal for both distillation and absorption unit operations. This is due to the excellent mass transfer characteristics and low pressure drop that it offers compared to random packing or trays. The main disadvantage is the lack in reliable models to describe the mass transfer characteristics of this type of packing. The recent development of the non-equilibrium model or rate based modelling approach has also emphasized the need for accurate hydraulic and efficiency models for sheet metal structured packing. The main focus of this study was to develop an accurate model for the mass transfer efficiency of Flexipac 350Y using a number of experimental and modelling techniques. Efficiency is however closely related to hydraulic capacity. Before attempting to measure and model the efficiency of Flexipac 350Y, the ability of existing published models to accurately describe the hydraulic capacity of this packing was tested. Holdup and pressure drop were measured using air/water and air/heavy paraffin as test systems. All experiments were performed on pilot plant scale 200mm ID glass columns. Satisfactory results were obtained with most of the models for determining the loading point and pressure drop for the air/water test system. All of the models tested predicted a conservative dependency of capacity on liquid viscosity for the air/paraffin test system. Efficiency and pressure drop were measured using the chlorobenzene/ethylbenzene test systems under conditions of total reflux in a 200mm ID glass column. Widely differing results were however obtained with the different models for the efficiency of Flexipac 350Y. Experiments were subsequently designed and performed to measure and correlate the vapour phase mass transfer coefficient and the effective surface area of Flexipac 350Y independently. The vapour phase mass transfer coefficient was measured and correlated by subliming naphthalene into air from coatings applied to specially fabricated 350Y gauze structured packing. The use of computational fluid dynamics (CFD) to model the vapour phase mass transfer coefficient is also demonstrated. The effective surface area for vapour phase mass transfer was measured with the chemical technique. The specific absorption rate of CO2 into monoethanolamine (MEA) using n-propanol as solvent was determined in a wetted-wall column and used to determine the effective surface area of Flexipac 350Y on pilot plant scale (200mm ID glass column). The efficiency of Flexipac 350Y could be modelled within an accuracy of 9% when using the correlations developed in this study and ignoringliquid phase resistance to mass transfer for the chlorobenzene/ethylbenzene test system under conditions of total reflux. The capacity and efficiency of the new generation high capacity packing Flexipac 350Y HC was also measured and compared with that of the normal capacity packing Flexipac 350Y. An increase in capacity of 20% was observed for the HC packing for the air/water system and 4% for the air/heavy paraffin system compared with the normal packing. For the binary total reflux distillation the increase in capacity varied between 8% and 15% depending on the column pressure. The gain in capacity was at the expense of a loss in efficiency of around 3% in the preloading region.

AFRIKAANSE OPSOMMING: Gestruktureerde pakking is 'n populêre pakkingsmateriaal en word algemeen gebruik in distillasie en absorpsie kolomme. Dit is hoofsaaklik as gevolg van die goeie massa-oordragseienskappe en lae drukval wat dit bied in vergelyking met 'random' pakking en plate. The hoof nadeel is egter die tekort aan akkurate modelle om die massa-oordrags eienskappe te bepaal. Om modelle te kan gebruik waar die massaoordragstempo direk gebruik word om gepakte hoogte te bepaal, word akkurate kapasiteits- en effektiwiteitsmodelle vir gestruktureerde plaatmetaalpakking benodig. Die hoof doelwit van hierdie studie was om 'n akkurate model te ontwikkel vir die massa-oordragseffektiwiteit van die plaat metaal pakking Flexipac 350Y deur gebruik te maak van verskillende eksperimentele- en modelleringstegnieke. Effektiwiteit is egter direk gekoppel aan hidroliese kapasiteit. Bestaande modelle in die literatuur is eers getoets om te bepaal of hulle die hidroliese kapasitiet van Flexipac 350Y akkuraat kan voorspel. Vir die doel is vloeistofterughou en drukval gemeet deur gebruik te maak van die sisteme lug/water en lug/swaar parafien. Alle eksperimente is in loodsaanlegskaal 200mm ID glaskolomme uitgevoer. Meeste van die modelle was relatief akkuraat in hulle berekening van die ladingspunt en die drukval vir die lug/water toets sisteem, maar was konsertief in voorspellings van die groothede vir die lug/swaar parafien sisteem. Effektiwiteit en drukval was gemeet deur gebruik te maak van die binêre toetssisteem chlorobenseen/etielbenseen onder totale terugvloei kondisies in 'n 200mm ID glaskolom. Daar is 'n groot verskil in die effektiwiteitsvoorspelling deur die verskillende modelle. Vervolgens is eksperimente ontwerp en uitgevoer om die dampfase massaoordragskoeffisiënt en die effektiewe oppervlakarea vir Flexipac 350Y onafhanklik te meet en te korreleer. Die dampfase massaoordragskoeffisient is gemeet en gekorreleer deur naftaleen te sublimeer vanaf spesiaal vervaardigde 350Y gestruktureerde pakking van metaalgaas. Die gebruik van numeriese vloeimeganika (CFD) om die dampfase massaoordragskoeffisient te bereken word gedemonstreer. Die effektiewe oppervlakarea vir dampfase massaoordrag is bepaal deur van 'n chemiese metode gebruik te maak. Die spesifieke absorpsietempo van CO2 in monoetanolamien (MEA) met n-propanol as oplosmiddel is gemeet in a benatte wand kolom en gebruik om die effektiewe oppervlakarea van Flexipac 350Y te bepaal op loodsaanlegskaal (200mm ID). Die effektiwiteit van Flexipac 350Y kon met 'n akkuraatheid van binne 9%gemodelleer word deur vloeistoffaseweerstand te ignoreer en van die korrelasies gebruik te maak wat in hierdie studie ontwikkel is. Die effektiwiteit en kapasiteit van die nuwe generasie hoë kapasiteit pakking Flexipac 350Y HC is ook gemeet en vergelyk met die normale kapasiteit pakking Flexipac 350Y. 'n Verhoging in kapsiteit van 20% is gemeet vir die HC pakking in vergelyking met die normale kapasiteit pakking vir die lug/water sisteem en 'n 4% verhoging in kapasiteit vir die lug/swaar parafien sisteem. Die verhoging in kapasiteit het gevarieër tussen 8% en 14% in die binêre totale terugvloei distillasie toetse en was afhanklik van die kolom druk. Die verhoging in kapasiteit was ten koste van 'n verlaging in effektiwiteit van ongeveer 3% onderkant die ladingspunt.

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