Establishing a facility to measure packed column hydrodynamics

[摘要] ENGLISH ABSTRACT: Distillation continues to be the most widely used method of separation in the processingindustry, in spite of its inherently low thermodynamic efficiency. Two of the criticaldistillation research needs that arose from the US-Initiative Vision 2020 were to develop abetter understanding of the physical phenomena as well as developing better predictivemodels. Also, characterisation of modern packing materials is required to assist in the CO2capture optimisation.This thesis deals with both these aspects by establishing a facility that can accuratelymeasure the hydraulic capacity of packed columns. This setup eliminates mass transferand 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 packingmaterials 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, andc.) a decrease in Height Equivalent to a Theoretical Plate (HETP). The onset of flooding iswhere the shear forces between the gas and liquid become so large (relative to thegravitational forces) that a net upwards movement of liquid occurs, resulting in liquiddroplets being heavily entrained. This is normally accompanied by a sharp increase inHETP, pressure drop and liquid hold-up.The prediction of these operating limits is of great value but, despite the manycontributions that were made from 1960 to 2010, there is still room for improvement. Theoperating region of particular interest is between the loading and flooding point, especiallyfor fluids with physical properties significantly different from that of water. In the past, thisoperating region was not of great importance, but industries are constantly striving toincrease their production with minimal capital expenditure. Thus, packed columns arebeing pushed to their limits and a good understanding of the phenomena occurring nearthese operational limits is now required.A 400 mm diameter glass packed bed setup (with a bed height of 3000 mm) wasdesigned and constructed to test the effect of the following parameters on packed bedpressure 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 theabove mentioned parameters on entrainment can also be measured. Initially,hydrodynamic tests on random packing materials (1.5 Pall® Rings, 1.5 IMTP®, 1.5Intalox® 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, thatwould be able to predict the pressure drop and the liquid hold-up over most of therandom 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 followingdeficiencies:· Limited to only the pre-loading region.· Tested (and thus applicable) only over a very select group of packing materials withno attempt to generalise.· Lacked the proper validation of significantly variable fluid properties overmultitudes of liquid and gas rates especially, at higher gas and liquid rates.The experimental setup was successfully commissioned, noting the following maximumexperimental errors: Vapour flow factor - 2.6 %; liquid rate - 0.75 %; packed bed pressuredrop - 0.75 %; liquid hold-up - 1.25 % and entrainment - 1.05 %. Significant deviations wereobserved between the experimental hold-up and the hold-up from the predictive model ofBillet (using Pall® Rings). Careful inspection revealed that this predictive model potentiallyuses two definitions for hold-up at flooding, one which has a theoretical basis and theother purely empirical. Upon substituting the theoretical value with the empirical value, asignificant improvement was observed between the measured and predicted results.Deviations were still observed near the flooding point and were attributed to the difficultyof obtaining reliable flooding data. The range of liquid hold-up prediction by Billet was onlyverified up to a liquid rate of 82 m3/(m2·h) and the pressure drop prediction only verifiedup to a liquid rate of 60 m3/(m2·h). This reinforces the need for high liquid, high gas ratedata. Due to the empirical nature of the liquid hold-up at flooding prediction, and sincepressure drop prediction is directly linked to liquid hold-up, another model was used tocompare the experimental pressure drop data.The KG-TOWER® simulator was used to predict IMTP® data and compare it to theexperimentally measured values. It was found that the experimental IMTP® data followedthe 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 measurethe parameters in question.The experimental data from the different random packings were compared to one anotherby using a statistical method to determine the loading point and onset of flooding. Thismethod uses prediction confidence intervals by fitting empirical curves to each operatingregion and was found to be useful in determining these critical points from experimentalhydraulic data (in the absence of HETP data).The only useful comparison was between IMTP® and Intalox® Ultra™ as they both haveroughly the same density, size and void fraction. It was found that, on average, thepressure drop of Intalox® Ultra™ is 20 % lower than that of IMTP® over the entireoperating range. The hydraulic operating range of Intalox® Ultra™ was found to be onaverage 16 % larger than that of IMTP®.It is recommended that further testing should be done to investigate the influence of fluidproperties (specifically liquid viscosity and to a lesser extent surface tension) on thehydraulic capacity of packed columns. Also, high gas and high liquid rate data should begenerated to assist current modelling techniques. Lastly, a comparative characterisationbetween Intalox® Ultra™ and Raschig Super-Rings would serve as a benchmark for fourthgeneration random packings.

[发布机构] Stellenbosch University