Mass transfer in structured packing

[摘要] ENGLISH ABSTRACT: Structured packing is a popular column internal for both distillation and absorptionunit operations. This is due to the excellent mass transfer characteristics and lowpressure drop that it offers compared to random packing or trays. The maindisadvantage is the lack in reliable models to describe the mass transfercharacteristics of this type of packing. The recent development of the non-equilibriummodel or rate based modelling approach has also emphasized the need for accuratehydraulic and efficiency models for sheet metal structured packing.The main focus of this study was to develop an accurate model for the mass transferefficiency of Flexipac 350Y using a number of experimental and modellingtechniques. Efficiency is however closely related to hydraulic capacity. Beforeattempting to measure and model the efficiency of Flexipac 350Y, the ability ofexisting published models to accurately describe the hydraulic capacity of thispacking was tested. Holdup and pressure drop were measured using air/water andair/heavy paraffin as test systems. All experiments were performed on pilot plantscale 200mm ID glass columns. Satisfactory results were obtained with most of themodels for determining the loading point and pressure drop for the air/water testsystem. All of the models tested predicted a conservative dependency of capacity onliquid viscosity for the air/paraffin test system. Efficiency and pressure drop weremeasured using the chlorobenzene/ethylbenzene test systems under conditions oftotal reflux in a 200mm ID glass column. Widely differing results were howeverobtained with the different models for the efficiency of Flexipac 350Y. Experimentswere subsequently designed and performed to measure and correlate the vapourphase mass transfer coefficient and the effective surface area of Flexipac 350Yindependently. The vapour phase mass transfer coefficient was measured andcorrelated by subliming naphthalene into air from coatings applied to speciallyfabricated 350Y gauze structured packing. The use of computational fluid dynamics(CFD) to model the vapour phase mass transfer coefficient is also demonstrated. Theeffective surface area for vapour phase mass transfer was measured with thechemical technique. The specific absorption rate of CO2 into monoethanolamine(MEA) using n-propanol as solvent was determined in a wetted-wall column and usedto determine the effective surface area of Flexipac 350Y on pilot plant scale (200mmID glass column). The efficiency of Flexipac 350Y could be modelled within anaccuracy of 9% when using the correlations developed in this study and ignoringliquid phase resistance to mass transfer for the chlorobenzene/ethylbenzene testsystem under conditions of total reflux.The capacity and efficiency of the new generation high capacity packing Flexipac350Y HC was also measured and compared with that of the normal capacity packingFlexipac 350Y. An increase in capacity of 20% was observed for the HC packing forthe air/water system and 4% for the air/heavy paraffin system compared with thenormal packing. For the binary total reflux distillation the increase in capacity variedbetween 8% and 15% depending on the column pressure. The gain in capacity wasat the expense of a loss in efficiency of around 3% in the preloading region.

[发布机构] Stellenbosch University