Browsing by Author "Moodley, Kuveneshan."

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An analysis and control of volatile organic Compound (VOC) emissions from petroleum storage tanks.
(2022) Naidoo, Theasha.; Moodley, Kuveneshan.; Naidoo, Prathieka.
Climate change is a growing phenomenon with its effects becoming more prominent to life on earth. According to the latest report by the Intergovernmental Panel on Climate Change (IPCC), some of the effects of climate change are irreversible. However, the implementation of large-scale reduction strategies on emissions may limit climate change over the long-term and provide short term air quality benefits. Petrochemical industries are a major contributor of Volatile Organic Compound (VOC) emissions as the need for storage facilities are expanding to accommodate for the increase in demand of organic liquids storage capacity. The Durban South Basin is a major industrial hub consisting of South Africa’s largest capacity oil refinery (SAPREF) and Engen refinery, soon-to-be tank terminal, located near a residential area. Therefore, the implementation of emission monitoring and reduction strategies are critical in ensuring climate resilience and the health and well-being of residents living within close proximity to the refineries. While there has been some progress in addressing climate change, emission data indicates that storage tanks contribute 42% of VOC emissions to total emissions from oil refineries. Due to limited studies conducted, there is a gap in the knowledge and understanding of proper monitoring and control practices of VOC emissions from petroleum storage tanks in Durban, South Africa. Therefore, the aim of this study is to provide strategies for implementation, such as simulation modelling using Aspen Plus ® and recommended process conditions, to achieve safe control and handling of emissions and to perform an Environmental Impact Assessment (EIA) for analyses of its potential effect on the environment and health of communities. Estimation of VOC emissions for crude oil and petroleum products (Ultra-Low Sulphur Diesel (ULSD), Unleaded Petrol 95 (ULP 95), Jet Fuel (JET A1) and Marine Gas Oil (MGO) were based on the AP-42 method, Aspen Plus ® simulations, manual flash calculations according to the Rachford-Rice iterative method and empirical correlations (such as the Vasquez-Beggs and Valko-McCain empirical correlation methods). The effects of atmospheric conditions, tank roof type, type of stored organic liquid and varying parameters (such as temperature, pressure and feed flowrate) on the VOC emissions from petroleum storage tanks were assessed to determine the most suitable monitoring method. The potential effect of Nitrogen blanketing (using the API 2000 7th ed. Standards) and Vapour Treatment on the reduction of VOC emissions from petroleum storage tanks were studied to determine its effectiveness as a control method. This study found that the Aspen Plus ® simulation method is an effective tool in monitoring VOC flashing emissions due to its reliability from its repeatability with the estimation crude oil test system in which the Aspen Plus ® and literature VOC measurement was consistent. Its ability to account for variations using the thermodynamic property models (Soave-Redlich-Kwong (SRK) for crude oil and Peng-Robinson (PENG-ROB)) for the product mixtures) further justifies the use of Aspen Plus ® as an effective monitoring method. Manual flash calculations under-estimated emissions across the organic mixture systems due to its less rigorous approach as it uses simplified equations which includes estimates of process conditions whereas Aspen Plus ® is able to account for variations is process conditions. The estimates determined using empirical correlations were mostly invalid due to the limited appliable range. All mixtures indicated a significant reduction in working and breathing losses when stored in an Internal Floating Roof Tank (IFRT) compared to a Floating Roof Tank (FRT). However, MGO was the exception. It was observed that these tanks should operate at 90 % capacity, with turnovers of 0 – 10 per year and a white painted shell, to ensure minimum emissions. Optimal operating tank temperatures should be maintained at 293.15 – 303.15 K and at pressures below 91 kPa. The installation of vapour recovery units is recommended for FRTs, and these measures are 90 % efficient. Due to the high API gravity of the ULP 95 mixture, the ULP 95 mixture should be targeted as a key mixture for control of VOC emissions as it has the potential to emit greater VOC emissions.
Application of the truncated perturbed chain-polar statistical associating fluid theory (tPC-PSAFT) to alcohol/alkane mixtures at high pressures.
(2020) Hussain, Mishqah.; Moodley, Kuveneshan.
Constitutive equations, such as equations of state (EoS) characterize mathematical relationships between state functions under set physical conditions and are imperative for the accurate design of chemical processes (Devilliers, 2011; Al-Malah, 2015). Most models, however, fail to accurately predict thermophysical properties of complex mixtures such as those exhibiting molecular association and hydrogen bonding. The Statistical Associating Fluid Theory (SAFT), based on thermodynamic perturbation theory, explicitly accounts for molecular association, hence, providing a more suitable prediction of thermophysical properties (Devilliers, 2011). This work investigates the performance of the truncated Perturbed Chain-Polar Statistical Associating Fluid Theory (tPC-PSAFT) model in accurately accounting for the effect of molecular association on compressed liquid density in liquid alkane-alcohol mixtures at elevated pressures. This was achieved by comparing the density predictions calculated by the tPC-PSAFT model to novel experimental density data. Isothermal measurements were conducted utilizing an Anton Paar DMA HP densimeter with a upplier stated uncertainty ranging between 0.1 and 1 kg.m-3. Measurements were conducted in the temperature and pressure ranges of 313.15 to 353.15 K and 0.1 to 20 MPa, respectively, over the entire composition range. Furthermore, a test system consisting of ethanol (1) + n-heptane (2) was used to confirm the reliability of the experimental setup and procedure. The density data obtained for the test system was compared to literature and demonstrate excellent correlation of the data, with a maximum relative difference of 0.0005, confirming the reliability of the procedure utilized in this study. The density data of six novel binary systems namely, butan-1-ol/butan-2-ol/2-methylpropan-1-ol (1) + n-octane/n-decane (2) are presented in this work. The maximum expanded combined uncertainties for pressure, temperature, composition and density were 0.032 MPa, 0.02 K, 0.0002 mole fraction, and between 1.10 to 1.12 kg.m-3, respectively. Density data obtained experimentally for all six binary systems comply with the general trend regarding temperature and pressure in that the density of the liquid mixtures decreased with an increase in temperature and increase with an increase in pressure. Furthermore, derived thermodynamic properties namely, the excess molar volume, thermal expansivity and isothermal compressibility were computed for each of the binary systems. Large positive deviations from ideality were noted for the excess volumes for all systems. This is attributed to the different shapes and sizes of the molecules as well as the attractive mixture interactions when compared to those of the individual pure components. In addition, the thermal expansivity and isothermal compressibility demonstrate highly non-linear behaviour which is indicative of systems comprising complex mixtures. The experimental data were compared to correlations/predictions resulting from five models namely, the Modified Toscani-Szwarc (MTS) equation of state (EoS), the Benedict-Webb-Rubin-Starling (BWRS) EoS, Peng-Robinson (PR) EoS, Perturbed Chain-Statistical Associating Fluid Theory (PC-SAFT) model and the truncated Perturbed Chain-Polar Statistical Associating Fluid Theory (tPC-PSAFT) model. Both the MTS and BWRS EoS demonstrated excellent correlation of the data for all six of the binary systems attributed to the empirical nature of the model and the significant number of fitting parameters employed. The maximum root mean square deviation (RMSD) was found in the butan-2-ol (1) + n-octane (2) binary system at RMSD = 4.72 x 10-4. In addition, improvements in model performance were noted for the BWRS EoS at higher temperatures and pressures. The PR EoS demonstrated poor correlation of the density data of the mixtures (exceeding RMSD = 0.024), attributed to the poor prediction of the pure component data by the model and the use of a single binary interaction fitting parameter in the cases of the mixtures. Density predictions from the PC-SAFT model demonstrated significant deviation from experimental data (exceeding RMSD = 0.011) in that the PC-SAFT model underpredicts densities for the binary systems. Furthermore, a progressive deterioration in the model’s performance was noted as the respective alcohol concentration increases. Accurate prediction of the density was however noted for the 2-methylpropan-1-ol binary systems in the alcohol dilute region. In addition, some improvement in model performance was observed at higher pressures and temperatures for the butan-2-ol and 2-methylpropan-1-ol binary systems. The tPC-PSAFT model demonstrated improvement in accurately predicting the density data, for all six systems, when compared to those obtained via the PC-SAFT model, with an improvement in excess of 72% in some cases. In addition, the model performs well in the alcohol dilute region and at high pressures and temperatures. However, a progressive deterioration in the model’s performance is noted as the concentration of the alcohol in solution is increased. This was unexpected as both the PC-SAFT and tPC-PSAFT models explicitly account for molecular association and were theorized to perform well in predicting the alcohol mixture behaviour. The model’s poor performance can be attributed to the lack of high precision pure component parameters currently available in the literature that do not effectively characterize the density of the systems under high pressure. All five models exhibit similar trends to that of the experimental data despite their individual merits and shortcomings.
Automation of a static-synthetic apparatus for vapour-liquid equilibrium measurement.
(2012) Moodley, Kuveneshan.; Naidoo, P.; Ramjugernath, Deresh.; Raal, Johan David.
The measurement of vapour-liquid equilibrium data is extremely important as such data are crucial for the accurate design, simulation and optimization of the majority of separation processes, including distillation, extraction and absorption. This study involved the measurement of vapour-liquid equilibrium data, using a modified version of the static total pressure apparatus designed within the Thermodynamics Research Unit by J.D. Raal and commissioned by Motchelaho, (Motchelaho, 2006 and Raal et al., 2011). This apparatus provides a very simple and accurate means of obtaining P-x data using only isothermal total pressure and overall composition (z) measurements. Phase sampling is not required. Phase equilibrium measurement procedures using this type of apparatus are often tedious, protracted and repetitive. It is therefore useful and realizable in the rapidly advancing digital age, to incorporate computer-aided operation, to decrease the man hours required to perform such measurements. The central objective of this work was to develop and implement a control scheme, to fully automate the original static total pressure apparatus of Raal et al. (2011). The scheme incorporates several pressure feedback closed loops, to execute process step re-initialization, valve positioning and motion control in a stepwise fashion. High resolution stepper motors were used to engage the dispensers, as they provided a very accurate method of regulating the introduction of precise desired volumes of components into the cell. Once executed, the control scheme requires approximately two days to produce a single forty data points (P-x) isotherm, and minimizes human intervention to two to three hours. In addition to automation, the apparatus was modified to perform moderate pressure measurements up to 1.5 MPa. Vapour-liquid equilibrium test measurements were performed using both the manual and automated operating modes to validate the operability and reproducibility of the apparatus. The test systems measured include the water (1) + propan-1-ol (2) system at 313.15 K and the n-hexane (1) + butan- 2-ol system at 329.15 K. Phase equilibrium data of binary systems, containing the solvent morpholine-4-carbaldehyde (NFM) was then measured. The availability of vapour-liquid equilibrium data for binary systems containing NFM is limited in the literature. The new systems measured include: n-hexane (1) + NFM (2) at 343.15, 363.15 and 393.15 K, as well as n-heptane (1) + NFM (2) at 343.15, 363.15 and 393.15 K. The modified apparatus is quite efficient as combinations of the slightly volatile NFM with highly volatile alkane constituents were easily and accurately measured. The apparatus also allows for accurate vapour-liquid equilibrium measurements in the dilute composition regions. A standard uncertainty in the equilibrium pressure reading, within the 0 to 100 kPa range was calculated to be 0.106 kPa, and 1.06 kPa for the 100 to 1000 kPa pressure range. A standard uncertainty in the equilibrium temperature of 0.05 K was calculated. The isothermal data obtained were modelled using the combined (-) method described by Barker (1953). This involved the calculation of binary interaction parameters, by fitting the data to various thermodynamic models. The virial equation of state with the Hayden-O’Connell (1975) and modified Tsonopoulos (Long et al., 2004) second virial coefficient correlations were used in this work to account for vapour phase non-ideality. The Wilson (1964), NRTL (Renon and Prausnitz, 1968), Tsuboka-Katayama-Wilson (1975) and modified Universal Quasi-Chemical (Anderson and Prausnitz, 1978) activity coefficient models were used to account for the liquid phase non-ideality. A stability analysis was carried out on all the new systems measured to ensure that two-liquid phase formation did not occur in the measured temperature range. A model-free method based on the numerical integration of the coexistence equation was also used to determine the vapour phase compositions and activity coefficients from the measured P-z data. These results compare well with the results obtained by the model-dependent method. The infinite dilution activity coefficients for the systems under consideration were determined by the method of Maher and Smith (1979b), and by suitable extrapolation methods. Excess enthalpy and excess entropy data were calculated for the systems measured, using the Gibbs-Helmholtz equation in conjunction with the fundamental excess property relation.
Carbon dioxide encapsulation in methane hydrates.
(2022) Ndlovu, Phakamile.; Naidoo, Paramespri.; Babaee, Saeideh.; Moodley, Kuveneshan.
Coal mining and petroleum refining processes face extreme pressure under climate change and global warming threats. Hence alternative sustainable and renewable energy sources must be made available for the rising energy demands. Natural gas found in permafrost and seabed areas in the form of gas hydrates possess vast amounts of low-carbon methane gas, which can replace fossil-based energy sources. The capture and storage of carbon dioxide gas in natural gas hydrate beds with the release of methane gas is a sustainable route under intense research. This study investigates the methane-carbon dioxide (CH4-CO2) replacement reaction mechanisms and the improvement of the process using different techniques, namely, additives, secondary gas, and thermal stimulation. Firstly, the gas hydrate dissociation measurements for the former gases utilized in the study were performed. This was followed by kinetic measurements with nanoparticles (aluminum oxide, copper oxide, and graphene nanoplatelets) and chemical additives (zinc oxide powder, graphite powder, and magnesium nitrate hexahydrate crystals) in the presence of sodium dodecyl sulfate (SDS) to affect kinetic or thermodynamic improvement in hydrate formation. The kinetic parameters investigated were induction time, hydrate storage capacity, water consumed in hydrate formation, fugacity of the gaseous phase, and the ratio of gas consumed to moles of water. Graphene nanoplatelets were selected for replacement reaction based on promising results obtained from the kinetic studies. The CH4-CO2 replacement process was performed in a 52 cm3 equilibrium cell using deionized water and nanoparticles. Also, a new experimental setup with a 300 cm3 reaction vessel was designed and assembled for CH4-CO2 replacement in the presence of synthetic silica sand. The results from kinetic studies showed an improvement in the hydrate formation kinetics due to the presence of nanoparticles. The CO2 hydrate formation kinetics obtained a maximum storage capacity of 51 (v/v), with 1.2 wt.% graphene nanoplatelets which also produced a maximum water conversion of 25%. When nanoparticles were added, the induction time for CO2 hydrate in deionized water was reduced from 9 minutes to less than one minute. Graphite powder with a concentration of 1.2 wt.% had the highest rate of gas uptake of 0.0024 (mol of gas/ mol of water. min). In CH4 kinetics, the induction time was reduced from 18 minutes with deionized water to less than one minute due to addition of nanoparticles. A maximum storage capacity of 28.5 (v/v), water-to-hydrate conversion of 13.09%, rate of gas uptake of 0.0089 (mol of gas/ mol of water. min), and gas consumption of 0.0238 moles were obtained with 0.1 wt.% CuO + 0.05 wt.% SDS. Also, CH4-CO2 replacement measurements showed that an 80 mol% N2/20 mol% CO2 gas mixture yielded a CH4 replacement efficiency of 17.04% at a temperature of 274.77 K and pressure of 5.34 MPa. The highest amount of CO2 sequestrated was 57.03%, and 28.77% was the highest CH4 replacement efficiency. These results were obtained using pressurized CO2 with application of thermal stimulation at a temperature of 275.90 K and pressure of 5.66 MPa. In the replacement reaction with silica sand, the maximum amount of CH4 replaced was 37.49% with the pressurized CO2 at a pressure of 7.01 MPa and temperature of 276.43 K. Applying thermal stimulation and adding secondary gas (N2) improved CO2 sequestration from 51.73% to 76.63%. These outcomes are vital in applying hydrates in gas storage and CO2 sequestration.
Design and optimization of a separation process for butanediol dehydration for use as a biofuel.
(2020) Mavalal, Shivan.; Moodley, Kuveneshan.
Ongoing research in incorporating renewable biofuels into the transport sector are fuels that can be used interchangeably with petroleum derived fuels. These fuels are termed “drop-in” fuels and can be used in the pure state or as a blending component. Diols such as butane-1,4-diol and butane-2,3-diol have been identified as appropriate drop-in fuels in various transport applications as they can improve octane numbers and heating values of the fuel blend. The butanediols are generally produced by the energy intensive process of chlorohydrination of butene with a subsequent hydrolysis step or hydrogenation and hydrolysis on the industrial scale. A potentially lower energy-impact process for the production of these diols is the biochemical process route which involves the fermentation of biomass (a renewable feed) by certain classes of bacteria. A low concentration aqueous mixture of the butanediols is produced, that must be dehydrated before use. Conventional distillation can be used for the dehydration and subsequent purification step, but the process is energy intensive as high-pressure steam must often be used as the heating medium, due to low concentrations of the butanediols and their high boiling points relative to water. Hence, there is merit in exploring lower-energy alternate separation schemes. The most promising options presented in the literature are hybrid techniques involving solvent extraction using butan-1-ol and recovery by distillation to first remove excess water and subsequently concentrate the butanediol product composition. However, those processes were designed based on model parameters extrapolated mostly from liquid-liquid equilibrium data only, and a limited set of vapour-liquid equilibrium (VLE) data. This yielded broadly qualitative designs in the literature. To improve this, in this work, novel isothermal VLE experimental data were measured for the binary systems of water/butan-1-ol in combination with the butanediol component species; butane-1,4-diol and butane-2,3-diol, utilizing a dynamic-analytical apparatus at sub-atmospheric conditions. For the binary systems of water (1) + butane-1,4-diol (2)/butane-2,3-diol (2), measurements were performed at temperatures ranging from 353 – 373 K. For the binary system of butan-1-ol (1) + butane-1,4-diol (2)/butane-2,3-diol (2), measurements were performed at temperatures ranging from 353 – 388 K. Temperature ranges were selected to maintain conditions up to atmospheric pressure which are commonly used in industry for these applications. For both sets of binary measurements, the P-T-x-y data was modelled using the γ-Φ approach. To account for the liquid-phase non-ideality, the Non-Random Two-Liquid and Universal Quasi-Chemical activity coefficient models were used while the Hayden and O’Connell correlation in the virial equation of state was used to account for the non-ideality in the vapour-phase. For all binary systems considered in this study, the experimental P-T-x-y data was concluded to be of good quality as thermodynamic consistency tests such as the area test and point test ii were passed with tolerances of below 10 % and 0.01, respectively, and the root mean square deviations in pressure and the absolute average deviation values in the vapour-phase mole fraction was found to be within the experimental uncertainty in these measurements. The binary parameters regressed from the experimental VLE data were used to improve the simulated separation design to purify butane-1,4-diol and butane-2,3-diol from the aqueous mixtures that result from the biological process pathways proposed in the literature. This was executed by exploring the design potential of a hybrid extraction-assisted distillation separation process in comparison to conventional distillation. Separation techniques such as conventional distillation, heterogeneous azeotropic distillation and liquid-liquid extraction are utilized in the novel proposed separation process. To achieve the dehydration of the butanediol constituents, butan-1-ol was used as the solvent in the liquid-liquid extraction step. The design of the separation process was performed using Aspen Plus® and optimized using standard procedures to reduce duties and costs. The simulation was used to investigate the technical and economic feasibility of the process with further optimization of the design by considering heat-integration. Conventional distillation was found to be the most economically feasible process alternative for the butane-1,4-diol purification, with an estimated total annual cost in the range of $4,532,846.67 and $4,635,070.52 for a payback period of 3 years, while extraction assisted distillation with heat integration was found to be the economically viable option for butane-2,3-diol purification with total annual costs in the range of $2,997,204.58 and $3,988,868.70 for a payback period of 3 years.
Liquid-liquid extraction of neodymium.
(2022) Bayeni, Thulani Tholithemba.; Naidoo, Paramespri.; Moodley, Kuveneshan.; Williams-Wynn, Mark Duncan.
Neodymium is classified as a rare earth element (REE). These elements possess a unique set of optical, electrochemical and magnetic properties that allow for their use in electronics manufacturing, medicine, catalysis and clean technologies. The global neodymium supply from primary source mining is isolated to a few countries, therefore developing technologies to recover neodymium and other rare earth elements from electronic waste is an emerging research area with economic incentive. The readiness of these technologies for industrial implementation is dependent on data for the extraction of neodymium from aqueous acidic solution into an organic phase for recovery. The available literature on these processes is limited. To address the gaps in the available literature, in this study, the distribution coefficient of neodymium in liquid-liquid equilibrium systems was measured across a range of nitric acid concentrations (0.1 – 2.9 M). The distribution coefficient is a measure of the affinity of a solute for the organic solvent to the aqueous phase. The extractant solutions used were composed of various concentrations of phosphorous acid diluted with n-dodecane. The tested extractant solutions are 0.1, 0.5 and 1 M of di(-2-ethylhexyl)phosphoric acid in n-dodecane. To investigate possible enhancements to the performance of the extractant, trace amounts of the ionic liquids (ILs) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (at concentrations of 0.019 and 0.19 M) and tributylmethylphosphonium methyl sulfate (at concentrations of 0.01, 0.1 and 0.25 M) were added to the organic extractant (0.5 M HDEHP in n-dodecane). The distribution coefficient data obtained for an extractant concentration of 0.5 M HDEHP was also used to determine its performance in a liquid-liquid extraction column by way of elementary mass balance calculations. The experiments performed in this study were undertaken using a bank of 6 stirred equilibrium cells immersed in a water bath maintained at a temperature of 298.15 K. Each vessel was filled with 5 ml of the aqueous and the organic solutions and mixed vigorously for 12 hours before being allowed to gravimetrically settle for 8 hours. Samples of the aqueous phase were withdrawn from the vessels, diluted using de-ionised water and analysed by way of inductively coupled plasma optical emission spectroscopy (ICP- OES). The equilibrium acid concentration of these samples was measured using acid-base titrations with 0.1 M sodium hydroxide solution. In this work the distribution coefficient data of 10 unique systems are presented, 2 test systems to validate the experimental method and 8 unique configurations of nitric acid concentration and extractant composition. The analysis of the distribution coefficient of neodymium showed that neodymium has an inversely proportional relationship to the aqueous [H+] concentration, established by the nitric acid concentration. For the HDEHP in n-dodecane extractant, the maximum distribution coefficient calculated was 274.26 at a nitric acid concentration of 0.2701 M with the 1.0 M HDEHP in n-dodecane. In the ionic liquid doped systems the maximum calculated distribution coefficients were 158.70 at a nitric acid [H+] concentration of 0.1161 M 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide doped extractant and 23.454 at an aqueous acid concentration of 0.0974 M when neodymium was extracted with the 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide doped extractant. The degree of extraction achievable by the addition of ILs is either decreased or increased, based on the concentration and species of ionic liquid. It was found that when ILs are used to enhance phosphorus acid extractant solutions, phase separation within the extractant occurs readily, decreasing the precision of measurements by more than 10%. The calculations for a liquid-liquid extraction column were performed based on a solution of neodymium and iron in nitric acid media extracted using 0.5 M HDEHP in n-dodecane. The results showed that neodymium with a solvent free purity of 96.75% by mass could be obtained using a column in which the extractant to volumetric feed flow ratio is 3.2. It is recommended that further distribution coefficient studies be undertaken to provide insight into the distribution behaviour of multiple ion-containing systems when extracted with IL containing synergistic extractant solutions.
Phase equilibria studies on chemical mixtures encountered in the natural gas industry.
(2022) Zvawanda, Paul.; Naidoo, Paramespri.; Nelson, Wayne Michael.; Moodley, Kuveneshan.
Natural gas processing involves removing impurities from the gas streams. These impurities include carbon dioxide, nitrogen, hydrogen sulphide, water vapour, mercury, and others. These impurities must be eliminated from the gas streams, often using solvents, to meet sales specifications, enhance calorific value, lessen corrosion and blockages in pipelines due to hydrate formation and to allow for cryogenic gas processing. Solvents such as methanol and the lower molecular weight glycols have the most suitable characteristics to be employed as hydrate inhibitors, whilst 2,2′-[Ethane-1,2-diylbis(oxy)] di(ethan-1-ol) (triethylene glycol (TEG)) is mostly used in dehydration plants. In this study, phase equilibria data for mixtures of six chemical species commonly encountered in the processing of natural gas were studied. Phase equilibrium measurements were performed using a combined static (synthetic or analytic) apparatus. The apparatus comprises a horizontal cylindrical sapphire tube fitted with a movable piston that can be used to adjust the cell volume, thereby fixing/controlling the pressure in the process. A mobile Rapid Online Sampler Injector (ROLSI™) was fitted to the equilibrium cell for sampling both the vapour and the liquid phases. Vapour- liquid equilibrium (TPxy) data were measured and modelled for the following test systems, carbon dioxide + n-hexane and carbon dioxide + n-decane over a temperature range of 313.15 to 319.23 K. Bubble point (TPx) data were measured and modelled for the following test systems: carbon dioxide + methanol; carbon dioxide + TEG; methane + methanol; methane + TEG; carbon dioxide + aqueous TEG systems over a temperature range of 298.10 to 323.15 K. Generally, good agreement was observed between the reported literature data and the experimental data measured in this work, thus validating the experimental techniques used. New TPx data were measured and modelled for seven novel systems of this study, namely: methane + propane + methanol; methane + propane + TEG; methane + methanol + TEG; carbon dioxide + methanol + TEG; methane + propane + methanol + TEG; methane + propane + methanol + water + TEG; methane + propane + carbon dioxide + methanol + water + TEG over a temperature range of 283.15 to 323.15 K and in selected composition regions. The composition ranges and conditions are typical of those found in gas pipelines and gas dehydration units. The experimental data were modelled in Aspen Plus V11-12 using appropriate thermodynamic models, i.e., Peng Robinson (PR), Soave-Redlich-Kwong (SRK), Peng Robinson Wong Sandler (PRWS), Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT), and the Cubic Plus Association (CPA) models. The maximum absolute average relative deviation (AARD) in pressure on all the modelled data were 4.89%, 8.67%, 7.39%, 9.63% and 19.7% for the PRWS, SRK, CPA, PR, and PCSAFT models, respectively, indicating that the PRWS model best described most of the systems. The measured data contributes to the information required for the process design, control and monitoring of methanol and/or TEG in gas conditioning systems. Furthermore, the data helps refine thermodynamic models that can predict phase behaviour in multicomponent systems in applications mentioned earlier, including gas hydrate inhibition, subsea gas processing, carbon capture, and storage.
Phase equilibrium studies and the blending/separation potential of alcohol, ketone and alkane mixtures for process design applications.
(2021) Chetty, Thavashni.; Moodley, Kuveneshan.; Naidoo, Prathieka.
Abstract available in PDF.
Separation of binary homogeneous azeotropic mixtures using pervaporation.
(2021) Mari, Ronel.; Moodley, Kuveneshan.; Naidoo, Paramespri.
The separation of mixtures containing homogeneous azeotropes is often complex and requires the use of enhanced distillation techniques. This leads to a significant increase in capital and operating costs. The use of membrane separation techniques to separate azeotropic mixtures is favoured over extractive distillation, azeotropic distillation and absorption as this is an effective low energy and low-cost alternative. Pervaporation is a membrane-based separation technique often used in industry to dehydrate alcohol-water azeotropes, to remove water from organic solvents or to remove organics from water. The process requires a liquid feed at a pressure high enough to maintain its phase while being depleted of components contained within the feed to form a liquid retentate. A membrane is typically selective for one component with finite permeability for the remaining components in the feed. A vapour phase must be maintained on the permeate side of the membrane by applying a vacuum downstream thereby creating a pressure gradient. A pervaporation unit generally consists of a series of membrane cells grouped together in modules, and interstage heat is applied to the feed of subsequent modules. This investigation focused on the dehydration of alcohols (ethanol, propan-1-ol and propan-2-ol) using a poly(vinyl alcohol) based membrane. An experimental study on ethanol-water under various operating conditions was performed. The effect of permeate pressure (2‒5 kPa), feed temperature (338.15‒348.15 K) and feed water concentration (1‒5 wt.%) are reported in terms of flux and permeate quality. Results confirmed that pervaporation is a suitable method to break an azeotrope. Due to technical issues encountered with the equipment, the experimental determination of pervaporation performance was not pursued further. This prompted an extensive simulation study whereby semi-empirical models were developed for the alcohol-water systems using Aspen Custom Modeler® before exporting to Aspen Plus® for simulation and optimization. Dehydration of an industrial grade propan-2-ol aqueous solution (85 wt.% propan-2-ol) using pervaporation was then rigorously simulated as the final objective, as this is not explored in detail in the literature. Various interstage heat temperatures (363.15, 368.15, 373.15 K) and module arrangements (3, 5 and 8 cells per module) were considered to produce the required retentate stream of less than 2 wt.% water. A total of nine design cases were developed to meet the industry purification requirements (>98 wt.% propan-2-ol in retentate). An economic evaluation (inclusive of operating, investment, and maintenance cost) of the separation was performed. It was confirmed that a membrane setup of 3 modules with 3 cells per module including interstage heating to 373.15 K presented the lowest. total cost of 174.27 $/t. This arrangement provided the most feasible configuration for propan-2-ol dehydration using a PVA-based membrane and when compared to azeotropic distillation from literature, it was found that a saving of 34% could be achieved using pervaporation, assuming a pre-concentrator cost of 1/3 of the total process costs from the literature studies. The comparative economic analysis performed across various processes was based on the total cost per ton of propan-2-ol product, which served as a standardized cost. Two procedural assumptions were applied; an operational time of 300 days per year and 24 hours a day for an industrial plant, and a production rate of 257.69 kg.h-1 propan-2-ol, as per the optimal design case.
A universal segment approach for the prediction of the activity coefficient.
(2016) Moodley, Kuveneshan.; Ramjugernath, Deresh.
This study comprised an investigation into solid-liquid equilibrium prediction, measurement and modelling for active pharmaceutical ingredients, and solvents, employed in the pharmaceutical industry. Available experimental data, new experimental data, and novel measuring techniques, as well as existing predictive thermodynamic activity coefficient model revisions, were investigated. Thereafter, and more centrally, a novel model for the prediction of activity coefficients, at solid-liquid equilibrium, which incorporates global optimization strategies in its training, is presented. The model draws from the segment interaction (via segment surface area), approach in solidliquid equilibrium modelling for molecules, and extends this concept to interactions between functional groups. Ultimately, a group-interaction predictive method is proposed that is based on the popular UNIFAC-type method (Fredenslund et al. 1975). The model is termed the Universal Segment Activity Coefficient (UNISAC) model. A detailed literature review was conducted, with respect to the application of the popular predictive models to solid-liquid phase equilibrium (SLE) problems, involving structurally complex solutes, using experimental data available in the literature (Moodley et al., 2016 (a)). This was undertaken to identify any practical and theoretical limitations in the available models. Activity coefficient predictions by the NRTL-SAC ((Chen and Song 2004), Chen and Crafts, 2006), UNIFAC (Fredenslund et al., 1975), modified UNIFAC (Dortmund) (Weidlich and Gmehling, 1987), COSMO-RS (OL) (Grensemann and Gmehling, 2005), and COSMOSAC (Lin and Sandler, 2002), were carried out, based on available group constants and sigma profiles, in order to evaluate the predictive capabilities of these models. The quality of the models is assessed, based on the percentage deviation between experimental data and model predictions. The NRTL-SAC model is found to provide the best replication of solubility rank, for the cases tested. It, however, was not as widely applicable as the majority of the other models tested, due to the lack of available model parameters in the literature. These results correspond to a comprehensive comparison conducted by Diedrichs and Gmehling (2011). After identifying the limitations of the existing predictive methods, the UNISAC model is proposed (Moodley et al, 2015 (b)). The predictive model was initially applied to solid-liquid systems containing a set of 18 structurally diverse, complex pharmaceuticals, in a variety of solvents, and compared to popular qualitative solubility prediction methods, such as NRTLSAC and the UNIFAC based methods. Furthermore, the Akaike Information Criterion (AIC) (Akaike, 1974) and Focused Information Criterion (FIC) (Claeskens and Hjort, 2003) were used to establish the relative quality of the solubility predictions. The AIC scores recommend the UNISAC model for over 90% of the test cases, while the FIC scores recommend UNISAC in over 75% of the test cases. The sensitivity of the UNISAC model parameters was highlighted during the initial testing phase, which indicated the need to employ a more rigorous method of determining parameters of the model, by optimization to the global minimum. It was decided that the Krill Herd algorithm optimization technique (Gandomi and Alavi, 2012), be employed to accomplish this. To verify the suitability of this decision, the algorithm was applied to phase stability (PS) and phase equilibrium calculations in non-reactive (PE) and reactive (rPE) systems, where global minimization of the total Gibbs energy is necessary. The results were compared to other methods from the literature (Moodley et al., 2015 (c)). The Krill Herd algorithm was found to reliably determine the desired global optima in PS, PE and rPE problems. The algorithm outperformed or matched all other methods considered for comparison, including swarm intelligence and genetic algorithms, with an average success rate of 89.5 %, and with an average number of function evaluations of 1406. The UNISAC model was then reviewed, and extended, to incorporate the significantly more detailed group fragmentation scheme of Moller et al. (2008), to improve the range of application of the model. New UNISAC segment group area parameters that were obtained by data fitting, using the Krill Herd Algorithm as an optimization tool, were calculated. This Extended UNISAC model was then used to predict SLE compositions, or temperatures, of a large volume of experimental binary and ternary system data, available in the literature, (over 4000 data points), and was compared to predictions by the UNIFAC-based and COSMO-based models (Moodley et al., 2016 (d)). The AIC scores suggest that the Extended UNISAC model is superior to the original UNIFAC, modified UNIFAC (Dortmund) (2013), COSMO-RS(OL), and COSMO-SAC models, with relative AIC scores of 1.95, 4.17, 2.17 and 2.09. In terms of percentage deviations alone between experimental and predicted values, the modified UNIFAC (Dortmund) model, and original UNIFAC models, proved superior at 21.03% and 29.03% respectively; however, the Extended UNISAC model was a close third at 32.99%. As a conservative measure to ensure that inter-correlation of the training set did not occur, previously unmeasured data was desired as a test set, to verify the ability of the Extended UNISAC model to estimate data outside of the training set. To accomplish this, SLE measurements were conducted for the systems diosgenin/ estriol/ prednisolone/ hydrocortisone/ betulin and estrone. These measurements were undertaken in over 10 diverse organic solvents, and water, at atmospheric pressure, within the temperature range 293.2-328.2 K, by employing combined digital thermal analysis and thermal gravimetric analysis, to determine compositions at saturation (Moodley et al., 2016 (e), Moodley et al., 2016 (f), Moodley et al., 2016 (g)). This previously unmeasured test set data was compared to predictions by the Extended UNISAC, UNIFAC-based and COSMO-based methods. It was found that the Extended UNISAC model can qualitatively predict the solubility in the systems measured (where applicable), comparably to the other popular methods tested. The desirable advantage is that the number of model parameters required to describe mixture activities is far lower than for the group contribution and COSMO-based methods. Future developments of the Extended UNISAC model were then considered, which included the preliminary testing of alternate combinatorial expressions, to better account for size-shape effects on the activity coefficient. The limitations of the Extended UNISAC model are also discussed.