Alan R. Greenberg

Real-time measurement of inorganic fouling of RO desalination membranes using ultrasonic time-domain reflectometry

Fouling is a major problem that limits the wider application of membranes for liquid separations. Progress towards improved methods for the control of this generally complex process would be facilitated by the development of additional techniques for the study of fouling under realistic operating conditions. This paper describes a novel approach to the real-time, noninvasive, in situ measurement of membrane fouling using ultrasonic time-domain reflectometry (TDR). Membrane fouling of commercial composite membranes was studied during reverse osmosis desalination of calcium sulfate solutions at three different axial velocities under conditions of constant feed pressure and concentration. The ultrasonic TDR measurements are compared with traditional indicators of fouling including flux decline, changes in permeate concentration, and magnitude of membrane surface coverage. Overall, the results show a good correspondence between the decline in the ultrasonic signal amplitude and the development of a fouling layer. In addition, the data suggest that ultrasonic TDR may be particularly responsive to fouling layer initiation and its subsequent removal during membrane cleaning. While the present results show promise, additional work is required to fully evaluate the capabilities of the technique and its applicability in commercial membrane separation systems.

Investigation of membrane fouling and cleaning using ultrasonic time-domain reflectometry

Abstract Progress in developing a means for control and prevention of fouling has been impeded by the lack of a suitable noninvasive fouling-measurement technique. In addition, fouling remediation strategies have traditionally relied upon the end-of-the-cycle recovery of productivity for estimating the degree of fouling removal. This paper describes the application of ultrasonic time-domain reflectometry (TDR) for real-time measurement of membrane fouling layer growth and its removal. The experimental results obtained using an automated reverse osmosis system under controlled pressure, temperature and CaSO 4 feed-concentration conditions show that ultrasonic signal amplitude measurements provide a sensitivity to the dynamics of fouling-layer growth that is comparable to that obtained from the flux-decline behavior. For experiments conducted at axial velocities of 4.6 and 9.8 cm/s ( Re = 178 and 379, respectively), a sequential two-mode CaSO 4 fouling layer growth was observed; the layer growth occurred as randomly-oriented rosettes initially, followed by the growth of laterally-oriented flat crystals during the later stages. Ultrasonic TDR measurements were capable of distinguishing these two modes of growth. Overall, the implications of the ultrasonic TDR behavior with respect to fouling were confirmed by independent measurement techniques. The ultrasonic technique was also successfully employed for monitoring membrane cleaning at ambient conditions. The end-of-the-cleaning-cycle membrane characterization showed that the ultrasonic measurements correspond well with the permeability recovery and surface analysis.

Dense polymer film and membrane formation via the dry-cast process part I. Model development

Abstract The dry-cast membrane-formation process is a major phase-inversion technique by which asymmetric membranes are manufactured. In this paper a fully predictive model which incorporates coupled heat and mass transfer is developed to describe the evaporation of both solvent and nonsolvent from an initially homogeneous polymer/solvent/nonsolvent system. This unsteady-state, one -dimensional, coupled heat- and mass-transport model allows from local film shrinkage owing to excess volume of mixing effects as well as evaporative solvent and nonsolvent loss. The model can predict composition paths into the ternary phase diagram which determine the onset of phase transition. The ternary phase diagram is predicted using the Flory-Huggins theory allowing for composition-dependent interaction parameters. The model is applied to the well-characterized cellulose acetate/acetone/water-system for which sufficient experimental data are available to permit determination of the friction coefficients in the ternary mass-transport model. The model is solved numerically using a software package based on the method of lines which is capable of handling moving boundary problems. The modeling studies indicate that for a given polymer/solvent/nonsolvent/support system, the most influential parameters are the gas-phase mass transport, initial cast film thickness, and initial composition. Of particular importance, the model can predict the general morphological characteristics associated with the formation of dense polymer films and symmetric as well as asymmetric membranes.

Dense polymer film and membrane formation via the dry-cast process part II. Model validation and morphological studies

Abstract In order to validate the dry-cast model developed by us, a multifaceted experimental approach was undertaken whereby three process variables can be followed independently in real time. Since it is extremely difficult to determine experimentally the concentration and temperature profiles within the cast polymer solution, experiments were designed so as to provide information on the coupled mass- and heat-transfer processes by measuring other process variables. The experimental data-acquisition technique combined gravimetric, inframetric, and light-reflection analyses which provided information on the overall mass change, surface temperature, and the onset and duration of phase separation, respectively. Structural studies were conducted using scanning electron microscopy. These studies revealed that macrovoids or “fingers” can be fomred in dry-cast membranes. A hypothesis for the formation of fingers based on the model predictions and experimental observations is proposed.

MATERIAL AND COMPOSITIONAL PROPERTIES OF SELECTIVELY DEMINERALIZED CORTICAL BONE

Timed immersion in buffered ethylenediamine-tetraacetic acid (EDTA) was used to selectively alter the mineral content at each level in the cortical bone structural hierarchy. The effects on the mechanical behavior were investigated using a combination of experimental techniques which provide collectively a wide range of resolution (5 microns to 3 mm). Optical microscopy and histological analysis demonstrated a heterogeneous structure consisting of a mineralized tissue core surrounded by a layer of demineralized tissue (collagen) whose thickness varied depending on the immersion time. The mechanical behaviors of treated samples with (intact) and without (core) the surrounding demineralized layer were evaluated using three-point flexure. Overall, the intact specimens became significantly less brittle with increased immersion time in buffered-EDTA. For the core specimens, there was a systematic decrease in the elastic flexural properties (E, sigma e, epsilon e). The site-specific properties of the specimens were determined using microhardness testing, scanning acoustic microscopy, and wavelength dispersive analysis. The mineralization and site-specific properties of the mineralized cores were not significantly affected by buffered-EDTA immersion; however, histomorphometric analysis showed a decrease in the mineralized volume fraction via widening of the pre-existing vascular channels. The experimental hierarchy was effective in discerning site-specific property changes and the localized heterogeneities resulting from the buffered-EDTA treatment. Based on the results of this study, buffered-EDTA treatment can be used to facilitate the determination of material and physical properties of intact and demineralized tissues within a single cortical bone sample.

Use of ultrasonic TDR for real-time noninvasive measurement of compressive strain during membrane compaction

A major limitation in previous compaction studies has been the inability to obtain direct simultaneous measurements of permeate flux and membrane thickness changes in real-time. In this paper we describe the development and application of ultrasonic time-domain reflectometry (TDR) for quantifying membrane compressive strain. The non-invasive nature of the technique allows standard performance data including permeate flux to be simultaneously measured under realistic conditions. Representative data for commercial reverse osmosis membranes are presented to demonstrate the potential benefits of this more complete approach to compaction studies. Results are also described for ultrasonic TDR experiments using cellulose acetate membranes which show the effect on membrane compressive strain behavior of changes in overall porosity and upstream pressure. Additional results show the variation in compressive strain during both the creep and recovery phases of a single pressurization cycle as well as over multiple cycles. Given these capabilities, the use of ultrasonic TDR should enable improved experimental and modeling analyses regarding the effects of operational and structural parameters on membrane performance.

A new technique for the simultaneous, real-time measurement of membrane compaction and performance during exposure to high-pressure gas

Abstract Membrane compaction accounts for a major source of the flux decline that occurs during gas separation using polymeric membranes. Previous membrane compaction studies have been limited by the inability to simultaneously measure membrane performance. In this study, we report the development of a technique based on ultrasonic time-domain reflectometry (TDR) that enables the simultaneous, real-time, noninvasive measurement of membrane compaction and performance during gas separation. In order to demonstrate the utility of the technique, representative data are presented for membrane compaction and pressure-normalized flux of a commercial asymmetric cellulose acetate (CA) gas separation membrane. Data obtained during the recovery cycle, i.e., after the pressure difference across the membrane is removed, are also described. These preliminary results indicate that the ultrasonic TDR technique can be successfully applied to quantify the effect of high-pressure gases on polymeric films and asymmetric membranes.

Flow-visualization during macrovoid pore formation in dry-cast cellulose acetate membranes

Video-microscopy flow-visualization (VMFV) is adapted to study the development of macrovoid (MV) pores in the dry-casting of cellulose acetate (CA)/acetone/water solutions. Particle tracer velocities provide the first direct evidence for the presence of solutocapillary-driven convection that can enhance mass-transfer to a MV. Three phases of MV development are observed: fast initial growth, slow growth, and collapse. During the latter, MVs were observed on occasion to initiate far from the demixing front. These studies have led to a significantly modified hypothesis for MV development. Extremely rapid initial MV growth is thought to occur owing to coalescence of dispersed phase microdroplets. To ensure net mass-transfer to a growing MV, it is postulated that a homogeneous supersaturated solution layer must exist between the demixed fluid layer and the homogeneous stable solution layer. Fast growth also involves convective mass-transfer to the MV whose surface is initially entirely immersed in this homogeneous supersaturated solution layer. Slow growth involves net transport that results from both convective mass-transfer to the MV across the portion of its surface in contact with the homogeneous supersaturated solution layer, and convective mass-transfer from the portion of its surface that extends into the homogeneous stable solution layer. Active collapse is thought to occur owing to skin formation at the MV surface. Passive collapse occurs when the convective mass-transfer from the MV in the homogeneous stable solution layer exceeds that entering the MV in the homogeneous supersaturated solution layer.

The thermal properties of beeswaxes: unexpected findings.

SUMMARY Standard melting point analyses only partially describe the thermal properties of eusocial beeswaxes. Differential scanning calorimetry (DSC) revealed that thermal phase changes in wax are initiated at substantially lower temperatures than visually observed melting points. Instead of a sharp, single endothermic peak at the published melting point of 64°C, DSC analysis of Apis mellifera Linnaeus wax yielded a broad melting curve that showed the initiation of melting at approximately 40°C. Although Apis beeswax retained a solid appearance at these temperatures, heat absorption and initiation of melting could affect the structural characteristics of the wax. Additionally, a more complete characterization of the thermal properties indicated that the onset of melting, melting range and heat of fusion of beeswaxes varied significantly among tribes of social bees (Bombini, Meliponini, Apini). Compared with other waxes examined, the relatively malleable wax of bumblebees (Bombini) had the lowest onset of melting and lowest heat of fusion but an intermediate melting temperature range. Stingless bee (Meliponini) wax was intermediate between bumblebee and honeybee wax (Apini) in heat of fusion, but had the highest onset of melting and the narrowest melting temperature range. The broad melting temperature range and high heat of fusion in the Apini may be associated with the use of wax comb as a free-hanging structural material, while the Bombini and Meliponini support their wax structures with exogenous materials.

Tensile behaviour of grass

The tensile behaviour of ryegrass (Lolium perenne L.), which was grown, harvested and tested under controlled conditions, is described. Whereas some of the grass leaf specimens behaved in a predominantly brittle manner, others evinced a semi ductile mode such that a proportional limit could be identified. Results indicated that the tensile properties depended upon specimen location and the tensile test strain rate. The data showed that as strain rate was increased, the stiffness, toughness and strength increased, while ductility decreased. Comparison of test results as a function of water content did not reveal statistically significant differences in any of the mechanical parameters. Analysis of the leaf structure suggests that the epidermal cells play a major role as a load-bearing component.