No description available.

As immiscible organic contaminants migrate through the subsurface environment, a significant portion of non-aqueous phase liquids (NAPL's) is trapped by capillary forces and remains in the subsurface as immobile blobs or ganglia. Residual saturations of NAPL on the order of 5-30 percent have been observed in saturated subsurface systems. The NAPL can partition into the aqueous phase and serve as a long-term source of groundwater contamination. NAPL-aqueous mass transfer rates impact the distribution and the rate of movement of the contaminant within the subsurface. The mass transfer coefficient is a function of many variables including aqueous phase velocity and NAPL-aqueous interfacial area. A one-dimensional column apparatus and experimental procedure was developed to study the nature of mass transfer between the aqueous phase (water), and a non-aqueous phase liquid (NAPL) in porous media. The NAPLs used in the experiments were composed of soluble and insoluble (inert) compounds. These experiments were designed to investigate the aqueous mass transfer of benzene, toluene, ethylbenzene, and xylene from and inert compound (soltrol or hexadecane). The results of this research indicate that the rate of NAPL-aqueous interphase mass transfer increases as a function of increasing aqueous phase velocity and percent NAPL saturation. Miller's equation was used to compare the model performance. Comparison of the experimental K values obtained by the literature was performed. The results indicated a satisfactory model performance. / Master of Science

The purpose of this investigation was to study the effect of the direction of flow with varying phase velocities on the overall, mass-transfer coefficient in a horizontal, unpacked, extraction tube, for the system n-butanol-water, and for the system 1,1,2-trichloroethane-sectione-water. / Master of Science

A two-stage (carbon oxidation stage one, ammonia oxidation stage two) biological aerated filter was operated for 10 months on-site at a domestic wastewater treatment plant. Over the study, the system was operated at different hydraulic loading rates that resulted in a range of applied organic and ammonia mass loadings. Performance was monitored regularly for water quality parameters in the effluent and along the length of the reactors. It was found that nitrification performance was significantly influenced by organic loading rates greater than 1.2 kg cBOD5/m³-d. Additional experiments were conducted in which a constant mass of ammonia was applied (Phase 1: 1.40 ± 0.08 kg NH₃-N/m³-d; Phase 2: 1.31 ± 0.02 kg NH₃-N/m³-d) to the N column, the second stage of the system, over a range of hydraulic loading rates (5.1 -15.8 m/h). Phases of testing were defined by the background hydraulic loading rate applied to the system (Phase 1: 8.3 m/h; Phase 2: 7.1 m/h) at which the reactors were allowed to reach a steady effluent quality for at least one week prior to testing. Organic loading was minimized and kept relatively constant throughout the hydraulic loading rate experiments (0.65 ± 0.2 kg cBOD5/m³-d) in order to obtain an evaluation of nitrification capacity with minimal competition from heterotrophic bacteria. Results indicated that nitrification performance improved by 17% as the applied velocity increased over the indicated range. A steady-state biofilm model capable of predicting substrate flux was applied to the data in an attempt to explain the improvement in performance with hydraulic loading rate from a fundamental standpoint. Mass transfer coefficients, KL, were derived from the model for conditions in which the experimentally observed flux correlated with the model predictions. Derived KL values were lower than estimations offered by correlation equations but increased with velocity at a similar rate. The model failed to account for changes that may have occurred in biofilm kinetics and structure throughout the length of the reactor. / Master of Science

No description available.

The effective mass transfer area (a [subscript e]), liquid film mass transfer coefficient (k [subscript L]), and gas film mass transfer coefficient (k [subscript G]) of eleven structured packings and three random packings were measured consistently in a 0.428 m packed column. Absorption of CO₂ with 0.1 gmol/L NaOH with 3.05 m packing was used to measure a [subscript e], while air stripping of toluene from water with 1.83 m packing was used to measure k [subscript L], and absorption of SO₂ with 0.1 gmol/L NaOH with 0.51 m packing was used to measure k [subscript G]. The experiments were conducted with liquid load changing from 2.5 to 75 m³/(m²*h) and gas flow rate from 0.6 to 2.3 m/s. Packings with surface area from 125 to 500 m²/m³ and corrugation angle from 45 to 70 degree were tested to explore the effect of packing geometries on mass transfer. The effective area increases with packing surface area and liquid flow rate, and is independent of gas velocity. The packing corrugation angle has an insignificant effect on mass transfer area. The ratio of effective area to surface area decreases as surface area increases due to the limit of packing wettability. A correlation has been developed to predict the mass transfer area with an average deviation of 11%. [Mathematical equation]. The liquid film mass transfer coefficient is only a function of liquid velocity with a power of 0.74, while the gas film mass transfer coefficient is only a function of gas velocity with a power of 0.58. Both k [subscript L] and k [subscript G] increase with packing surface area, and decrease with corrugation angle. A new concept, Mixing Point Density, was introduced to account for effect of the packing geometry on k[subscript L] and k [subscript G]. Mixing Point Density represents the frequency at which liquid film is refreshed and gas is mixed. The mixing point density can be calculated by either packing characteristic length or by surface area and corrugation angle: [mathematical equation]. The dimensionless k [subscript L] and k [subscript G] models can then be developed based on the effects of liquid/gas velocity, mixing point density, packing surface area: [mathematical equation] [mathematical equation]. Mi is the dimensionless form of Mixing Point Density (M), which is M divided by a [subscript P]³. Because Mi is only a function of corrugation angle (θ), it is a convenient transformation to represent the effect of θ on mass transfer parameters. An economic analysis of the absorber was conducted for a 250 MW coal-fired power plant. The optimum operating condition is between 50 to 80 % of flooding, and the optimum design is to use packing with 200 to 250 m²/m³ surface area and high corrugation angle (60 to 70 degree). The minimum total cost ranges from $4.04 to $5.83 per tonne CO₂ removed with 8 m PZ.

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.

Thesis (MScEng) -- Stellenbosch University , 2002. / ENGLISH ABSTRACT: The distillation process is the most widely used separating process in the chemical process industry. The optimal design of the distillation units can lead to reduced capital costs and improved energy utilisation. This is particularly the case for structured packed distillation column where design methods tend to over-predict the column efficiency. This work seeks to contribute to the development of a reliable mass transfer model for structured packing. A gas phase mass transfer correlation was developed based on the evaporation of pure components into an air stream. The mass transfer rates were measured in a short triangular wetted wall channel with a corrugated surface wall. The influence that the geometric configuration as well as surface structure has on the mass transfer rates was investigated. The channel geometry and surface wall structure resembled that of the structured packing (Flexipac 350Y). The influence of the channel geometry on the gas mass transfer rates was not significant except at low flow conditions. The liquid phase resistance was investigated for binary mixtures in the triangular wetted wall channel. It was found that the overall mass transfer rate decreased with increasing composition of the volatile component. The enhancement of the gas phase mass transfer rate masked the presence of the liquid resistance. ii The gas phase mass transfer correlations were used in the mass transfer model to predict the separation efficiency of a structured packed distillation column. The structured packing used to validate the mass transfer model was high-capacity Flexipac 350Y and normal Flexipac 350Y. The binary distillation experiments were done at total reflux conditions for the pressure range O.33-1atm. The accuracy of the separation efficiencies of the mass transfer model improved with increasing pressure for both materials. The mass transfer model was able to predict the variation of separation efficiencies with column loadings. / AFRIKAANSE OPSOMMING: Distillasie is die skeidingsmetode wat die meeste in die chemiese industrie gebruik word. Die optimale ontwerp van distillasie eenhede kan lei tot verlagings in kapitaalkostes en 'n verbeterde gebruik van energie. Dit is veral waar in die geval van kolomme met gestruktureerde pakking waar huidige ontwerpsmetodes, die kolomeffektiwiteit nie akkuraat beskryf nie. Hierdie studie het ten doelom 'n bydrae te lewer tot die ontwikkeling van 'n betroubare massa-oordragsmodel vir gestruktureerde pakking. 'n Gasfase massa-oordragskorrelasie is ontwikkel gebaseer op metings van die verdampingstempo van suiwer komponente in 'n lugstroom. Die massaoordragstempos is gemeet in 'n kort driehoekige benatle wand kanaal waarin die wand 'n bepaalde oppervlakprofiel het. Die invloed van beide die kanaalgeometrie en die oppervlakprofiel is ondersoek. Die geometrie en oppervlakprofiel is soorgelyk aan die van Flexipac 350Y gestruktureerde pakking. Die invloed van die kanaalgeometrie op die gasfase massaoordragstempos is slegs beduidend by lae vloeitempos. Die vloeistoffase weerstand vir binêre mengsels is ondersoek in die driehoekige kanaal. Dit is gevind dat die algehele massa-oordragstempo verlaag met 'n verhoging in die konsentrasie van die vlugtige komponent. Die verhoging in die gasfase massa-oordragstempo verberg die invloed van die vloeistoffase weerstand. Die gasfase massa-oordragskorrelasies is in 'n massa-oordragsmodel gebruik om die skeidingseffektiwiteit van gestruktureerde pakking in "n kolom te voorspel. Metings op hoë-kapasiteit Flexipac 350Y en normale Flexipac 350Y is gebruik om die modelvoorspellings te evalueer. Die binêre distillasies is by totale terugvloei gedoen in die drukbereik 0.33 - 1.0 atm. Daar is gevind dat die model meer akkuraat is by hoër drukke. Die massa-oordragsmodel is in staat om die variasie in skeidingseffektiwiteit met veranderende kolomlading te voorspel.

Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2011. / ENGLISH ABSTRACT: Structured packing is often the preferred choice for column internals because of its low pressure drop and high efficiencies compared to that of trays and random packing. However, the mass transfer phenomena in these gas-liquid contacting devices is still not well understood, even though it is widely used in industry. A contributing factor to this is the lack of understanding and availability of experimental data in the open literature. These shortcomings complicate the design of a distillation column and make practical experience essential. There is thus a need for more experimental data, especially for packings where only limited information is available. The focus of this study was to establish a testing facility that can be used to measure the efficiency of structured packing under total reflux, and not to measure vast quantities of experimental data; the latter will be done in future. The facilities available at Stellenbosch University limited the internal diameter of the column to 0.2 m, which is sufficient to test higher surface area structured packings (≥350 m2/m3). The column is used with a thermosyphon reboiler that uses steam as the heating source and is equipped with a total condenser. Two sections are used for the packed bed that allow for a total packed height of 3.78 m (2x1.89 m). The column is set up to operate under total reflux and was designed to operate at pressures ranging from 0.3 to 1 bar abs, vapour flow rates of 0.73 – 3.65 (m/s) (kg/m3)0.5 and liquid flow rates of 5 – 25 m3/(m2.h). It was found that the 2-butanol/iso-butanol and the p-xylene/o-xylene systems are suitable test mixtures for this pilot plant setup. The VLE data from Kutsarov et al. (1993) and Zong et al. (1983) for p-xylene/o-xylene and 2-butanol/iso-butanol are thermodynamic consistent and was validated by VLE experiments done in this study. It was found that the experimentally obtained efficiency (HETP) and pressure drop data for Mellapak 350Y compared well with published results of Spiegel and Meier (1987). With regard to the predictive models, it was found that i) the SRP model predicted the HETP of Mellapak 350Y structured packing accurately in the pre-loading region and slightly over predicted the HETP in the loading region, whereas ii) the Delft model over predicted HETP and iii) the Billet and Schultes model under-predicted HETP under the entire tested range (i.e. over-predict efficiency). With regard to the pressure drop data i) the Billet model iv accurately predicted the pressure drop over the entire tested range, whereas ii) the SRP model accurately predicted the pressure drop in the pre-loading region and slightly over predicted the pressure drop in the loading region and iii) the Delft model over predicted the pressure drop over the entire range and followed an almost parallel trend to the results from the SRP model. It was also found that information in the field of mass transfer in a packed column is far from saturated, and there is a need for more experimental data and better understanding of the mass transfer phenomena in packed columns. / AFRIKAANSE OPSOMMING: Gestruktureerde pakking het ʼn laer drukval en ʼn hoër effektiwiteit in vergelyking met willekeurige pakkings en plate, en is daarom dikwels die voorkeur keuse vir pakkings materiaal in ʼn distilleer kolom. Die massa-oordrags verskynsels in hierdie gas-vloeistof kontaktors word egter nog nie goed verstaan nie, ten spyte van die grootskaalse aanwending in die nywerheid. ʼn Bydraende faktor is die tekort aan eksperimentele data in die ope literatuur. Die tekortkomings bemoeilik die ontwerp van distilleerkolomme en maak praktiese ervaring ʼn noodsaaklike vereiste. Daar is dus ʼn behoefte aan meer eksperimentele data veral vir pakkings waar daar min of geen data beskikbaar is nie. Die fokus van die studie was om ʼn toetsfasiliteit op te rig wat gebruik kan word om die effektiwiteit van gestruktureerde pakking onder totale terugvloei kondisies te bepaal, en dus nie om ‘n groot hoeveelheid data te genereer nie; laasgenoemde sal wel deel uitmaak van toekomstige studies. Die fasiliteite beskikbaar by die Universiteit van Stellenbosch het die binne diameter van die kolom beperk tot 0.2 m. Die diameter is voldoende om gestruktureerde pakkings met ʼn hoë oppervlakarea te toets byvoorbeeld pakkings met areas 350 m2/m3 en hoër. Die kolom gebruik ‘n verdamper (met stoom as energie bron) om die vloeistof te verdamp en ‘n totale kondensator (verkoel met verkoelingswater) om die damp te laat kondenseer. Twee seksies van 1.89 m elk word gebruik vir die gepakte bed en die kolom het dus ‘n totale pakkingshoogte van 3.78 m. Die kolom is opgestel vir totale terugvloei en is ontwerp om bedryf te word by drukke tussen 0.3 en 1 bar abs, damp snelhede van 0.73 tot 3.65 (m/s) (kg/m3)0.5 en vloeistof vloeitempo’s tussen 5 en 25 m3/(m2.h). 2-butanol/iso-butanol en p-xylene/o-xylene is gevind om geskik te wees as mengsels vir die toetsopstelling. Die damp-vloeistof fase-ewewig data van Kutsarov et al. (1993) en Zong et al. (1983) vir p-xylene/o-xylene and 2-butanol/iso-butanol is termodinamies konsistent en is gevalideer deur damp-vloeistof fase ewewig toetse in die studie. Daar is gevind dat die eksperimenteel bepaalde effektiwiteit en drukval data vir Mellapak 350Y pakking goed vergelyk met gepubliseerde data van Spiegel and Meier (1987). Die eksperimenteel bepaalde effektiwiteit data is met waardes van beskikbare modelle model vergelyk en daar is gevind dat: i) die SRP voorspel die effektiwiteit van Mellapak 350Y pakking akkuraat in die ‘pre-loading’ gebied maar toon afwykings van die eksperimentele data in die ‘loading’ gebied, ii) die Delft model voorspel ‘n hoër hoogte ekwivalent aan ‘n teoreties plaat (HETP) oor die hele gebied terwyl iii) die Billet en Schultes model weer ‘n laer HETP voorspel oor die hele gebied. Met betrekking toe die drukval data i) voorspel die Billet model die drukval akkuraat oor die hele gebied, ii) die SRP model voorspel die drukval korrek in die ‘pre-loading’ gebied maar begin afwyk van die eksperimentele data in die ‘loading’ gebied en iii) die Delft model voorspel groter waardes vir drukval oor die hele gebied en volg amper ʼn parallelle tendens met die SRP model. In die studie is daar gevind dat daarin die gebied van massa-oordrag nog ʼn tekort is aan eksperimentele data en daar baie navorsings geleenthede is.

No description available.