Pallab Ghosh

NANOFILTRATION AND REVERSE OSMOSIS MEMBRANES: THEORY AND APPLICATION IN SEPARATION OF ELECTROLYTES

Membranes are widely used for separation of ions from a mixture in aqueous solution. Major applications include removal of sulfate from seawater, removal of salt from cheese whey, and separation of sodium chloride from seawater for manufacture of table salt and brine for soda industry. The conventional reverse osmosis membranes are not capable of selectively separating electrolytes from the aqueous mixture. The charged nanofiltration membranes that have been developed over the last decade are now being used for some of these applications. This review is based on the theories of transport of solute and water through these membranes and the mechanism of permselectivity. The application of these theories to estimate the membrane design parameters, i.e., pore size, porosity, charge density and thickness has been discussed. The capabilities of different nanofiltration membranes have been illustrated with examples and compared with reverse osmosis membranes. The information provided here is expected to provide an extensive information on the subject.

Coalescence of Air Bubbles at Air–Water Interface

The rest time of air bubbles at flat air–water and water–organic interfaces is studied in the present work. Effects of cationic and anionic surfactants, alcohol, salts and bubble-size on rest time are investigated. Wide distributions in rest times are observed in all the systems, which establishes the stochastic nature of the process. The stochastic model of Ghosh and Juvekar (2002; Chem Eng Res Des 80: 715–728) is used to fit the bubble rest time distributions. The results show that the magnitude of the rest time is determined by the strength of the interfacial repulsive force and the magnitude of surface diffusivity of the surfactant molecules. Entanglement of surfactants by hydrophobic interaction is believed to be a major factor behind the high rest time in many of the systems studied, apart from the repulsive electrostatic double layer, hydration and steric forces. The nature of the repulsion differs from system to system depending on the type of the adsorbed species. The work provides further support to the viewpoint (Ghosh and Juvekar, 2002) that the hydrodynamic drainage of the thin liquid film trapped between the bubble and the flat interface is complete once the bubble comes to a rest on the interface and the lubrication force plays a negligible role in supporting the weight of the bubble.

Analysis of the Drop Rest Phenomenon

An alternative to the film-thinning model for coalescence of drops at the horizontal liquid–liquid interface has been proposed and elucidated through analysis of drop rest time distribution, both in the presence and absence of added surfactants. The model is based on the assumption that the drop rests at the interface as a result of the repulsive force generated by a high concentration of adsorbate molecules on the opposite faces of the barrier ring. The repulsive force could be DLVO type (double layer force) or non-DLVO type (hydration/steric). High concentration of the adsorbate builds up at the barrier ring during the approach of the drop at the interface, when the resulting shear displaces the adsorbate on the faces of the entrapped film towards the barrier ring. The repulsive force at the barrier ring decays with time, due to back-diffusion of the adsorbate towards the centre of the film. Coalescence occurs when the repulsive force at the barrier ring is too weak to support the weight of the drop. The major cause of the distribution of drop rest time during the experiment involving sequential addition of drops is the drop-to-drop fluctuation in the surface excess of the adsorbate. This fluctuation is caused by the interfacial disturbance resulting from coalescence of the previously added drops and also by non-uniform distribution of the adsorbate at the barrier ring. Based on these ideas, a mathematical model for the rest time distribution has been developed. The model fits well to our own experimental data and those reported in the literature. Validity of the model is established through comparison of the predicted trends of the rest time distribution, with those observed under a variety of experimental conditions. The evidence, which reveals the shortcomings of the currently accepted film-thinning model, has also been presented.

Microbubble-aided water and wastewater purification: a review

Abstract Microbubble-based methods, in recent times, have been widely used for purification of water and wastewater. Microbubbles have several physicochemical properties, which make them eminently suitable for wastewater treatment. In this review, these properties have been analyzed in detail from the perspective of application. Various types of microbubble generators and their operation principles have been discussed. The transport of gas into the aqueous phase has been explained, and the correlations to predict the volumetric mass transfer coefficient have been presented. Many practical applications using ozone, oxygen and air microbubbles, some of which are currently at various stages of commercialization, have been presented. Other important uses of microbubbles for wastewater treatment, namely, removal of fine solid particulate matter and oil, have also been discussed. In addition, directions for future research of microbubble technology and their potential applications have been identified.

Design of heat exchanger networks using randomized algorithm

A randomized algorithm with stream splitting for design of heat exchanger networks is presented in this work. The algorithm has provisions for splitting any one of the process streams. We have studied three benchmark problems taken from literature. The results obtained from these studies clearly indicate the strength of the randomization method in finding a cost-effective network. This random search method can find better networks, which are sometimes unnoticed by other optimization techniques. The simplicity of this method as well as the networks obtained is an additional attractive feature, which should encourage the designers to use it. From the results of this study, randomization is recommended as a reliable check for designing heat exchanger networks.

Prediction of Vapor‐Liquid Equilibria Using Peng‐Robinson and Soave‐Redlich‐Kwong Equations of State

Knowledge of vapor-liquid equilibria is essential for chemical process design. Development of algebraically simple cubic equations of state, particularly Peng-Robinson and Soave-Redlich-Kwong equations of state has encouraged vapor-liquid equilibria calculation using the equation of state approach. To extend applicability to complex mixtures, several modifications to these equations of state were proposed. Also, many new mixing rules have been developed besides the already existing ones that incorporate excess free energy models. Some of these EoS/GE models are found to be applicable to complex systems in which the components can be highly polar, differ vastly in size or may be forming association. In spite of a large number of publications on this field, this subject is still open for further research. In this article, an overview on the work done using the most successful of the cubic equations of state, i.e., Peng-Robinson and Soave-Redlich-Kwong equations of state is presented. In the later part of this article, VLE predictions (using these two equations of state) of some interesting systems like water/hydrocarbon mixtures, polymer solutions, electrolyte solutions and refrigerant mixtures are discussed. The objective is to present the available information for ready reference that will prove to be useful from a designers point of view. It is seen from the existing literature that with a proper choice of equation of state and mixing rule based on the available knowledge on the properties of the system of interest, VLE of highly complex systems can be predicted with accuracy. Several comparative studies that are cited here (and discussed) will help the designer to pick-up the right combination for the system under investigation.

COALESCENCE OF AIR BUBBLES IN SURFACTANT SOLUTIONS: ROLE OF SALTS CONTAINING MONO-, DI-, AND TRIVALENT IONS

This work presents a study on adsorption of ionic surfactants at the air-water interface in the presence of a 1:1, 2:1, or 3:1 salt. Coalescence of air bubbles in the aqueous solutions of these surfactants and salts was studied by measuring the bubble rest time. Cetyl trimethyl ammonium bromide (CTAB) was used as the cationic surfactant and sodium dodecyl sulfate (SDS) was used as the anionic surfactant. NaCl, MgCl2, and AlCl3 were used as the salts. It was observed that surface tension decreased and rest time increased with the addition of salt. In some of the systems, rest time decreased when the amount of salt was increased beyond a certain concentration. The amount of salt required to reduce the surface tension decreased in the order: NaCl > MgCl2 > AlCl3. A similar sequence was observed for the bubble rest times also. A stochastic model was used to fit the bubble rest time distributions.

Heat exchanger network synthesis: the possibility of randomization

Abstract This paper presents a new approach to heat exchanger network (HEN) design making extensive use of randomization techniques. It is exceedingly simple to implement and gives new insight into the hardness and the cost landscape underlying a given problem. At the same time, the results from our algorithm may be used as good initial solutions required by most non-linear optimization problem formulations of HEN design. Practical networks involve trade-off between a number of factors, all of which are difficult to incorporate in a single design methodology. Our approach is blind to any design heuristic and generates a sufficiently large number of networks that can be further evaluated to pick up the most suitable network depending on specific design requirements. However, the current version of the algorithm is limited to HEN synthesis problems that can be solved without stream splitting. We have experimented with the three standard literature problems and obtained results that compare well with the previously published results, which justify further research in this direction.

Prediction of vapor-liquid equilibria of binary systems using PRSV equation of state and Wong-Sandier mixing rules

Abstract The Peng-Robinson equation of state modified by Stryjek and Vera (1986) has been used with Wong-Sandier mixing rules (1992) to predict vapor-liquid equilibria of forty-three binary mixtures involving organic alcohols, esters, ketones, amines, etc. The Wong-Sandier mixing rules are theoretically correct in the sense that they reproduce quadratic composition dependence of the second virial coefficient. However, recent studies indicate that the basic premises of these rules break down for highly asymmetric systems and at high pressures. The systems studied have low to moderate asymmetry. It has been observed that the non random two liquid (NRTL) model provides a good representation of G E models and the results obtained from optimization compare well with those reported in DECHEMA data series in spite of the asymmetry inherent to these systems. The results obtained from this study can be used directly to predict vapor-liquid equilibria involving such systems.

Oxidation of As(III) to As(V) using ozone microbubbles

The use of ozone in the treatment of water and wastewater is rapidly increasing due to its high oxidizing power. Arsenic is one the most toxic elements found in water. As(III) and As(V) are the major sources of arsenic poisoning. It is known that As(V) can be more easily removed from water by adsorptive methods than As(III). In this work, oxidation of more toxic As(III) to less toxic As(V) was studied in a pilot-plant by using ozone microbubbles. The microbubbles were effective in dissolving ozone in water. The oxidation was fast over a wide range of pH (e.g., 4-9). The role of hydroxyl radical in the oxidation of As(III) under acidic conditions was investigated by using 2-propanol as the hydroxyl radical scavenger. Under acidic conditions, the addition of 2-propanol slowed down the oxidation, which proves that hydroxyl radicals were involved in the oxidation process. The effect of carbonate ions on the rate of oxidation was investigated. It was found that the generation of carbonate ion radical from the carbonate ion accelerated the oxidation of As(III). The kinetics of oxidation of As(III) by ozone was studied.