Establishing a facility to measure packed column hydrodynamics

Lamprecht, Sarel Marais (2010-12)

Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2010.

Thesis

ENGLISH ABSTRACT: Distillation continues to be the most widely used method of separation in the processing industry, in spite of its inherently low thermodynamic efficiency. Two of the critical distillation research needs that arose from the US-Initiative Vision 2020 were to develop a better understanding of the physical phenomena as well as developing better predictive models. Also, characterisation of modern packing materials is required to assist in the CO2 capture optimisation. This thesis deals with both these aspects by establishing a facility that can accurately measure the hydraulic capacity of packed columns. This setup eliminates mass transfer and specific attention can be given to the hydrodynamic behaviour of packed columns. Two phenomena that have a large impact on the mass transfer efficiency of packing materials are the loading and flooding point. The loading point is signified by the following: a.) where the packed column hold-up increases, b.) higher increase in pressure drop, and c.) a decrease in Height Equivalent to a Theoretical Plate (HETP). The onset of flooding is where the shear forces between the gas and liquid become so large (relative to the gravitational forces) that a net upwards movement of liquid occurs, resulting in liquid droplets being heavily entrained. This is normally accompanied by a sharp increase in HETP, pressure drop and liquid hold-up. The prediction of these operating limits is of great value but, despite the many contributions that were made from 1960 to 2010, there is still room for improvement. The operating region of particular interest is between the loading and flooding point, especially for fluids with physical properties significantly different from that of water. In the past, this operating region was not of great importance, but industries are constantly striving to increase their production with minimal capital expenditure. Thus, packed columns are being pushed to their limits and a good understanding of the phenomena occurring near these operational limits is now required. A 400 mm diameter glass packed bed setup (with a bed height of 3000 mm) was designed and constructed to test the effect of the following parameters on packed bed pressure drop and liquid hold-up: · Gas and liquid physical properties · Gas and liquid rates · Type of packing (either random or structured) The experimental setup has been designed so that in the future the influences of the above mentioned parameters on entrainment can also be measured. Initially, hydrodynamic tests on random packing materials (1.5” Pall® Rings, 1.5” IMTP®, 1.5” Intalox® Ultra™) were conducted over a liquid range of 6 - 122 m3/(m2·h). Through a thorough literature study it was found that the most likely semi-theoretical model, that would be able to predict the pressure drop and the liquid hold-up over most of the random packing test range, was the model developed by Billet [1991; 1993; 1995; 1999]. The other models found throughout the literature had at least one of the following deficiencies: · Limited to only the pre-loading region. · Tested (and thus applicable) only over a very select group of packing materials with no attempt to generalise. · Lacked the proper validation of significantly variable fluid properties over multitudes of liquid and gas rates especially, at higher gas and liquid rates. The experimental setup was successfully commissioned, noting the following maximum experimental errors: Vapour flow factor - 2.6 %; liquid rate - 0.75 %; packed bed pressure drop - 0.75 %; liquid hold-up - 1.25 % and entrainment - 1.05 %. Significant deviations were observed between the experimental hold-up and the hold-up from the predictive model of Billet (using Pall® Rings). Careful inspection revealed that this predictive model potentially uses two definitions for hold-up at flooding, one which has a theoretical basis and the other purely empirical. Upon substituting the theoretical value with the empirical value, a significant improvement was observed between the measured and predicted results. Deviations were still observed near the flooding point and were attributed to the difficulty of obtaining reliable flooding data. The range of liquid hold-up prediction by Billet was only verified up to a liquid rate of 82 m3/(m2·h) and the pressure drop prediction only verified up to a liquid rate of 60 m3/(m2·h). This reinforces the need for high liquid, high gas rate data. Due to the empirical nature of the liquid hold-up at flooding prediction, and since pressure drop prediction is directly linked to liquid hold-up, another model was used to compare the experimental pressure drop data. The KG-TOWER® simulator was used to predict IMTP® data and compare it to the experimentally measured values. It was found that the experimental IMTP® data followed the same trends as those from KG-TOWER® within the operating limits of the program. Thus, since the experimental data follows similar trends as models found in the literature, as well as falling within their reliable limits, the experimental setup can correctly measure the parameters in question. The experimental data from the different random packings were compared to one another by using a statistical method to determine the loading point and onset of flooding. This method uses prediction confidence intervals by fitting empirical curves to each operating region and was found to be useful in determining these critical points from experimental hydraulic data (in the absence of HETP data).The only useful comparison was between IMTP® and Intalox® Ultra™ as they both have roughly the same density, size and void fraction. It was found that, on average, the pressure drop of Intalox® Ultra™ is 20 % lower than that of IMTP® over the entire operating range. The hydraulic operating range of Intalox® Ultra™ was found to be on average 16 % larger than that of IMTP®. It is recommended that further testing should be done to investigate the influence of fluid properties (specifically liquid viscosity and to a lesser extent surface tension) on the hydraulic capacity of packed columns. Also, high gas and high liquid rate data should be generated to assist current modelling techniques. Lastly, a comparative characterisation between Intalox® Ultra™ and Raschig Super-Rings would serve as a benchmark for fourth generation random packings.

AFRIKAANSE OPSOMMING: Distillasie is vandag nog die skeidingsproses wat die meeste gebruik word in the prosesnywerhede ten spyte van ‘n lae termodinamiese effektiwiteit. Twee van die kritieke distillasie navorsing behoeftes wat vanuit die US-Initiative Vision 2020 ontstaan het, was om die fisiese verskynsels beter te verstaan, asook om beter voorspellende modelle te ontwikkel. Die karakterisering van moderne pakking materiale is ook nodig vir die optimering van die verwydering van CO2 uit uitlaatstrome. Hierdie tesis spreek beide van hierdie faktore aan deur ‘n fasiliteit op te rig wat die hidrouliese kapasiteit van gepakte kolomme akkuraat kan meet. Hierdie opstelling elimineer massa-oordrag en dus kan spesifieke aandag gegee word aan die hidrodinamiese gedrag van gepakte kolomme. Twee verskynsels wat ‘n groot impak het op die massaoordrag effektiwiteit van pakkingsmateriale is die ladingspunt en die vloedpunt. Die ladingspunt word deur die volgende gekenmerk: a.) waar die vloeistof inhoud in die gepakte bed toeneem, b.) ‘n toename in drukval en c.) ‘n afname in die hoogte ekwivalent aan ‘n teoretiese plaat (HETP). Die vloed gebied word gekenmerk waar die skuifkragte tussen die vloeistof en gas so groot raak (relatief tot die gravitasionele kragte), dat daar ‘n netto opwaartse beweging van vloeistof druppels in die kolom is. Hierdie gaan normaalweg gepaard met ‘n skerp toename in HETP, drukval en vloeistof inhoud. Die voorspelling van hierdie bedryfslimiete is baie waardevol, maar ten spyte van die bydrae wat tussen 1960 en 2010 gemaak was, is daar nog steeds ruimte vir verbetering. Die spesifieke bedryfsgebied van belang is die gebied tussen die ladingspunt en die vloedpunt en spesifiek vir sisteme waar die fisiese eienskappe van die vloeistowwe drasties verskil van die van water. In die verlede was hierdie gebied van minder belang gewees, maar maatskappye probeer deesdae hul produksie opstoot met minimale kapitale uitleg. Dus is ‘n goeie kennis van massa-oordrag verskynsels naby aan die bedryfslimiete van kardinale belang. ‘n 400 mm Diameter gepakte kolom (met ‘n bed hoogte van 3000 mm en bestaande uit glas) opstelling is ontwerp en gebou om die effek van die volgende parameters te toets op gepakte bed drukval en vloeistof inhoud: · Gas en vloeistof fisiese eienskappe · Gas vloeistof vloeitempos · Tipe pakking (beide ongeordend en gestruktureerd) Die eksperimentele opstelling is ontwerp om die bogenoemde eienskappe op vloeistofmeesleuring te meet vir toekomstige navorsing. Hidrodinamiese toetse op ongeordende pakkingsmateriale (1.5” Pall® Ringe, 1.5” IMTP®, 1.5” Intalox® Ultra™) is uitgevoer vir vloeistof vloeitempos tussen 6 en 122 m3/(m2·h). Vanuit ‘n deeglike literatuurstudie is daar gevind dat die mees toepaslike semi-teoretiese model, wat die drukval sowel as die vloeistof inhoud kan voorspel oor al die bedryfsgebiede, is die model wat deur Billet [1991; 1993; 1995; 1999] ontwikkel is. Die ander modelle in die literatuur het ten minste een van die volgende tekortkominge gehad: · Is slegs van toepassing in die voor-ladings gebied. · Is slegs van toepassing vir ‘n paar pakkingsmateriale en geen poging is aangewend om dit te veralgemeen nie. · Is nie geldig waar die vloeistof eienskappe drasties verskil van ‘n lug/water sisteem nie, sowel as by hoë gas en vloeistof vloeitempos. Die eksperimentele opstelling is suksesvol in werking gestel met die volgende waargenome eksperimentele foute: Gas vloei faktor – 2.6 %; vloeistof vloeitempo – 0.75 %; gepakte bed drukval – 0.75 %; vloeistof inhoud – 1.25 %; vloeistof-meesleuring tempo – 1.05 %. Noemenswaardige verskille is waargeneem tussen die eksperimentele en teoretiese vloeistof inhoud (deur Pall® Ringe te gebruik). Na gelang van noukeurige inspeksie, is daar gevind dat die Billet-model twee moontlike definisies voorstel vir die voorspelling van vloeistofinhoud by die vloedpunt. Een van hierdie is teoreties van aard en die ander een suiwer empiries. ‘n Vervanging van die teoretiese waardes met die empiriese waardes het gelei tot ‘n merkwaardige verbetering tussen die eksperimentele en teoretiese voorspellings. Daar was nog steeds verskille naby aan die vloedpunt, maar dit kon toegeskryf word aan die feit dat min betroubare data naby aan die vloedpunt beskikbaar is. Die voorspelling van vloeistof inhoud deur Billet is slegs gekontroleer tot ‘n vloeistof vloeitempo van 82 m3/(m2·h) en die drukval slegs tot ‘n vloeistof vloeitempo van 60 m3/(m2·h). Die bogenoemde bewys dus die tekort aan hoë gas- en hoë vloeistofvloeitempo data. Die voorspellende model se drukval is gekoppel aan die vloeistof inhoud, en dus is ‘n ander model gebruik om die eksperimentele drukval data teen te vergelyk. Die KG-TOWER® simulasie program is gebruik om die IMTP® drukval te voorspel en dit het goed vergelyk met die eksperimentele data. Dus, aangesien die eksperimentele data dieselfde tendens toon as dié van die modelle in die literatuur en aangesien dit binne die modelle se foutbande val, kan die eksperimentele opstelling die verlangde parameters akkuraat meet. Die eksperimentele data van al drie pakkingsmateriale is teenoor mekaar vergelyk deur gebruik te maak van ‘n statistiese metode wat die ladings- en vloedpunt bepaal. Hierdie metode maak gebruik van voorspellings vertroue intervalle deur empiriese kurwes op die eksperimentele data in elke bedryfsgebied te pas. Hierdie metode is ontwikkel om toepaslike te wees in die afwesigheid van HETP data. Die enigste nuttige vergelyking is tussen IMTP® en Intalox® Ultra™ omdat albei dieselfde pakkingsdigtheid, grootte en pakkings oop ruimte het. Daar is gevind dat die drukval van Intalox® Ultra™ ‘n gemiddeld van 20 % laer is as dié van IMTP® oor die hele bedryfsgebied. Die hidrouliese bedryfsgebied van Intalox® Ultra™ is 16 % groter as dié van IMTP®. Daar word voorgestel dat bykomende toetswerk gedoen moet word om die invloed van vloeistof eienskappe (spesifiek vloeistof viskositeit en vloeistof oppervlak spanning) op die hidrouliese kapasiteit van gepakte kolomme te ondersoek. Bykomende toestwerk by hoë gas- en hoë vloeistofvloeitempo word benodig om die bestaande modelle aan te vul. Laastens, sal ‘n vergelykende studie tussen Intalox® Ultra™ en Raschig Super-Rings die grondslag lewer vir die karakterisering van vierde generasie ongeordende pakkingsmateriale.

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