Direct contact membrane distillation of humic acid solutions

Direct contact membrane distillation process has been conducted for the treatment of humic acid solutions using microporous polytetrafluoroethylene and polyvinylidene fluoride membranes. The membranes were characterized in terms of their non-wettability, pore size and porosity. Water advancing and receding contact angles on the top membrane surfaces were measured. Experiments were also carried out employing pure water as feed at different mean temperatures and the water vapor permeance of each membrane was determined. Different humic acid concentrations in the feed solution, pH values and transmembrane temperature difference were tested. The direct contact membrane distillation technique is more adequate for the treatment of humic acid solutions than the applied pressure-driven separation processes, as lower membrane fouling was detected.     

Heat and mass transfer in membrane distillation

Equations for heat and mass transfer in membrane distillation (MD) have been developed and tested experimentally. The concept of temperature polarisation is introduced and shown to be important in the interpretation of experimental results. Vapour transport through the membranes tested is reasonably described by combined Knudsen and molecular diffusion. The significance of temperature polarisation in the design and operation of large-scale MD modules is discussed, and hollow fibre and tubular systems shown to be potentially the most effective.     

Predicting the effect of membrane spacers on mass transfer

Like many separation processes, ultrafiltration and reverse osmosis are often compromised by concentration polarization. Such polarization can be mitigated by static mixers and other flow barriers placed as spacers next to the membrane surface. These spacers can be shaped like ladders, herringbones, and helices. The effect of these spacers can be successfully predicted without adjustable parameters from extensions of the Lévêque equation. The predictions are in agreement with results of computational fluid mechanics and with electrochemical experiments. They supply a tool for optimizing spacer design.     

Fouling in direct contact membrane distillation process

The formation of deposit on the membrane surface (fouling) is one of the major operating problems of membrane distillation process. The influence of fouling on the performance of this process was investigated during the concentration of wastewater with proteins, bilge water, brines, and the production of demineralized water. The experiments were performed with polypropylene capillary membranes. The morphology and composition of the fouling layer were studied using Fourier transform infrared with diffuse reflectance spectroscopy and scanning electron microscopy coupled with the energy dispersing spectrometry. Fouling with various intensity was observed in most of the studied cases. Permeate flux decline was mainly caused by an increase in the heat resistance of the fouling layer. However in the case of non-porous deposit, a magnitude of the permeate flux was also determined by a resistance of water transport through the deposit layer. It was found the deposits were formed not only on the membrane surface, but also inside the pores. Salt crystallization in the membrane pores besides their wetting, also caused the mechanical damage of the membrane structure. The intensity of the fouling can be limited by the pretreatment of feed and a selection of the operating conditions of membrane distillation.     

Mass transfer in osmotic membrane distillation

Osmotic membrane distillation is a novel athermal membrane process that facilitates the maximum concentration of liquid foods under mild operating conditions. In the present study, the effect of various process parameters such as type, concentration and flow rate of the osmotic agent; type (polypropylene membranes) and pore size (0.05 and 0.2 μm) of the membrane; temperature with respect to transmembrane flux was studied. Experiments were performed with real systems (pineapple/sweet lime juice) in a flat membrane module. Osmotic agents namely sodium chloride and calcium chloride at varying concentrations are employed. For both the osmotic agents, higher transmembrane flux was observed at maximum osmotic agent concentration. In comparison with sodium chloride, higher transmembrane flux was observed in case of calcium chloride. A mass transfer-in-series resistance model has been employed, considering the resistance offered by the membrane as well the boundary layers (feed and brine sides) in case of real systems for the first time. The model could predict the variation of transmembrane flux with respect to different process parameters.     

Bulk and measured temperatures in direct contact membrane distillation

The aim of this work is the development of a transport model for a direct contact membrane distillation process in laminar flow that allows knowing the velocity and temperature profiles within the flow channels as a function of externally measured temperatures just at the entrances and exits of the flow channels in the membrane module. The second aim is to apply this model to a conventional membrane module, and so calculate the difference between the bulk temperatures and the externally measured ones. For the system studied here, moderately important differences between both temperatures have been obtained when working at low flow rates and high temperatures. It can be concluded from the trends observed in this study that an estimation of this temperature difference has to be made before considering the bulk temperature as equal to the externally measured temperature, above all, in those systems where the thermal boundary layers represent an important portion of the flow channels height, and important temperature drops exist through them.     

Theoretical modeling and experimental analysis of direct contact membrane distillation

A two-dimensional mathematical model was theoretically developed to predict the temperature polarization profile of direct contact membrane distillation (DCMD) processes. A concurrent flat-plate device was designed to verify the theoretical prediction of pure water productivity on saline water desalination. The numerical results from the temperature polarization profile were obtained using the finite difference technique to reduce the two-dimensional partial differential equations into an ordinary differential equations system. The resultant simultaneous linear equations system was solved with the fourth-order Runge-Kutta method. The results show theoretical prediction agreement with the measured values from the experimental runs. A combination of the Knudsen flow and Poiseuille flow models in the present mathematical formulation for membrane coefficient estimation was used to establish theoretical agreement. The influence of the inlet saline water temperature and volumetric flow rate on the pure water productivity as well as the hydraulic dissipated energy are also delineated.     

Treatment of metal-plating waste water by modified direct contact membrane distillation

In this study, the treatability of metal-plating waste water by modified direct contact membrane distillation (DCMD) at different temperature differences (ΔT = 30°C, 40°C, 50°C, and 55°C was investigated. Two different hydrophobic membranes made of poly(tetrafluoroethylene) (PTFE) and poly(vinylidene fluoride) (PVDF) having different pore sizes (0.22 μm and 0.45 μm) were used. The results indicated that conductivity, COD, sulphate, copper, and nickel could be successfully removed by modified DCMD. The rejection efficiencies for conductivity, COD, and sulphate were 99%, 86%, and 99%, respectively. Copper rejection was effective with both membranes while nickel concentration was below the limit of detection in the effluent. It was found that the pollutant rejection efficiency was affected by the raw water characteristics, membrane properties, and influent heating temperatures. In addition to the water quality parameters, the flux was measured to evaluate membrane performance. A high flux was obtained at 65°C (ΔT = 55°C) with 0.45 μm pore size PTFE membrane (24.1 L m^?2 h^?1) and with PVDF membrane (17.1 L m^?2 h^?1). The flux was mainly affected by temperature and membrane properties. As a result, modified DCMD and all the membranes used in this study were effective for the treatment of metal-plating waste water.     

Flow rate influence on direct contact membrane distillation experiments: Different empirical correlations for Nusselt number

Direct contact membrane distillation (DCMD) experiments using distilled water are reported. Influence on the process of feed and permeate flow rates through the cell has been investigated in a wide flow range, from 2 to 8 l/min. Two main effects have been studied, its effect on the heat transfer coefficient and on the effective membrane thickness. An empiric dependence of the membrane thickness with linear velocity through the cell has been included in the equation for mass flux through the membrane obtained from the “Dusty-Gas” model with satisfactory results.     

Concentrating the extract of traditional Chinese medicine by direct contact membrane distillation

This study applies direct contact membrane distillation (DCMD) to concentrating the extract of traditional Chinese medicine (TCM). The trans-membrane flux under various operation conditions was measured in real-time during concentration process. By decoupling the factors affecting the trans-membrane flux decline, it was found that the observed flux decline throughout the process could be attributed to the membrane fouling, the reduction of water vapor pressure and the increase of transport resistance at feed side. Analysis of the combined factors was given to show in detail the mechanism of flux decline. Factors that may affect the flux level, such as feed velocity, feed temperature and pretreatment were experimentally examined. Gas bubbling or sparging was introduced into DCMD system for reducing membrane fouling, and it was found that both gas–liquid two-phase flow at the feed side and gas back-washing within membrane module are effective ways to control membrane fouling.     

The effect of emissivity on heat transfer

Emissivity has been found to play a key role in heat transfer even at relatively low temperatures. Two general equations have been derived which allow one to estimate the emissivity of different materials for this system.     

Shell-side mass transfer of hollow fibre modules in osmotic distillation process

The effect of packing density of hollow fibre modules on mass transfer in the shell side of osmotic distillation process was studied. The osmotic distillation experiments were carried out with several modules of the packing densities ranging from 30.6 to 61.2%. It was found that the Reynolds number was a function of packing density and packing density affected mass transfer performance. The shell-side mass transfer coefficient increased with the brine velocity. The membrane permeability can be predicted from the experimental flux at the maximum brine velocity. The mass transfer correlation was proposed in order to determine the shell-side mass transfer coefficient in the randomly packed modules for osmotic distillation process. The empirical correlation proposed was fitted to the experimental results and it was found that the mass transfer coefficients calculated from the proposed correlation were in good agreement with those from the experimental data. Comparison of the results obtained from the proposed correlation with other correlations in the literature was discussed.     

Experimental study of desalination using direct contact membrane distillation: a new approach to flux enhancement

New membrane distillation configurations and a new membrane module were investigated to improve water desalination. The performances of three hydrophobic microporous membranes were evaluated under vacuum enhanced direct contact membrane distillation (DCMD) with a turbulent flow regime and with a feed water temperature of only 40 °C. The new configurations provide reduced temperature polarization effects due to better mixing and increased mass transport of water due to higher permeability through the membrane and due to a total pressure gradient across the membrane. Comparison with previously reported results in the literature reveals that mass transport of water vapors is substantially improved with the new approach. The performance of the new configuration was investigated with both NaCl and synthetic sea salt feed solutions. Salt rejection was greater than 99.9% in almost all cases. Salt concentrations in the feed stream had only a minor effect on water flux. The economic aspects of the enhanced DCMD process are briefly discussed and comparisons are made with the reverse osmosis (RO) process for desalination.     

An effect of sprinkling density on a value of active surface of irregular heat and mass transfer packing

The results of analysis of an influence of a sprinkling density in the packed columns were reported on a value of an active surface of irregular ring nozzles such as Raschig and Pall rings. Sizes of Raschig rings were 15, 25 and 50 mm; Pall rings, 50 mm. Values of fluid load was 10, 20, 30, 40 and 50 m³ m⁻² h⁻¹). The results of the analysis were presented in graph form.     

Parametric analysis of direct contact sensible heat transfer in spray columns

The present investigation deals with a theoretical analysis of direct contact sensible heat transfer between two immiscible liquids in a countercurrent spray column. The non-dimensional parameters governing the heat transfer process are identified and a study of the effects of the variation of these parameters is made. The predicted column heights and the temperature profiles of both the phases along the column compare favourably with the available experimental data.     

Effect of anion-exchange membrane surface properties on mechanisms of overlimiting mass transfer

Four effects providing overlimiting current transfer in ion-exchange membrane systems are examined. Two of them are related to water splitting: the appearance of additional current carriers (H+ and OH- ions) and exaltation effect. Two others are due to coupled convection partially destroying the diffusion boundary layer: gravitational convection and electroconvection. Three anion-exchange membranes, which differ in surface morphology and the nature of ion-exchange sites within a surface layer, are examined. The ion transfer across these membranes in NaCl solutions is studied by voltammetry, chronopotentiometry, and pH-metry. By excluding the effects of water splitting and gravitational convection, it is shown that the main mechanism of overlimiting mass transfer in narrow membrane cells at low salt concentrations is electroconvection. The reasons explaining why water splitting suppresses electroconvection are discussed. The scenario of development of potential oscillations with growing current and time is compared with that described theoretically by Rubinstein and Zaltzman.     

Investigation of MHD effect on nanofluid heat transfer in microchannels

Journal of Thermal Analysis and Calorimetry - An incompressible preconditioned lattice Boltzmann method (IPLBM) is proposed to investigate the fluid flow and heat transfer characteristics of...