Water Removal Rate Research Articles

Current distribution in PEM fuel cells. Part 1: Oxygen and fuel flow rate effects

AbstractA segmented electrode/current collector setup was used to examine the effect of oxygen flow rate, anode sparger temperature, and hydrogen starvation on the spatial and temporal distribution of local current densities along a single gas channel in a PEM fuel cell operated on pure hydrogen and oxygen. Uniformity in local current densities at a given voltage was sustained over longer periods of time with higher oxygen flow rates that enhanced liquid water removal. When water removal rate was not sufficient, electrode flooding occurred in segments that were farthest from the gas inlet. Increasing anode sparger temperature resulted in reduction of water removal rate from the cathode by back diffusion leading to performance reduction of downstream segments. When fuel was in excess, the current density distribution was dominated by the hydration pattern of the membrane electrode assembly. When fuel supply was less than the electrochemical reaction demand, segments furthest from the inlet suffered fuel starvation, resulting in loss of performance. The use of one of the electrode segments as a novel in‐line voltage sensor to reflect available reactant concentrations was also demonstrated. Air operation and effect of cell temperature were also examined and the results are presented in the second part of this series. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Effects of different drying rates on the physiological quality of Coffea canephora Pierre seeds

Desiccation tolerance in seeds depends on the species, development stage and drying conditions, especially the water removal rate. Coffea seeds are considered of intermediate performance, because they tolerate relative dehydration compared to orthodox seeds and are sensitive to low temperatures. The objective of this study was to verify the effect of different drying rates on the viability and storability of Coffea canephora seeds. A complete randomized experimental design was used, in a factorial 3 x 5 x 2 design, with three drying rates (fast, intermediate and slow), five final mean water contents after drying (51, 42, 33, 22 and 15 %) and two storage temperatures (10 and 20°C). The germination and seed vigor assessments, using radicle protrusion, cotyledon leaf opening, seedling emergence and emergence speed index, were performed shortly after drying and after two and four months storage. It was observed that with reduction in the water content there was reduction in the germination values and seed vigor, for all the drying rates. The greatest reductions in physiological quality occurred when the seeds were dried quickly and the best results were obtained at the intermediate drying rate. There was an effect of drying rate and storage temperature on the physiological quality of the seeds, and lower germination and vigor values were observed in seeds with lower water content stored at 20°C. C. canephora seeds were tolerant to desiccation down to 15 % water content and can be stored for four months at 10°C. A temperature of 20ºC can be used to store C. canephora seeds, as long as the water content is not reduced to values below 22 % water content.

Synthesis of Ethyl Acetate by Coupling a Heterogeneous Catalytic System with a Pervaporation Unit

The synthesis of ethyl acetate was investigated using a heterogeneous catalyst and a pervaporation unit. Shifting the reaction equilibrium by selective water removal increased the ethyl acetate productivity. Commercial acid resin, Amberlyst 15 (Rohm and Haas), and commercial hydrophilic composite membrane, PERVAP 1000 and 1001 (Le Carbone Lorraine), were used. The effects of catalyst mass, reaction temperature, and initial reactants ratio were studied. The kinetic parameters, such as activation energy and apparent reaction order, were determined. The water production and removal rates obtained in reaction and pervaporation experiments, respectively, were used in a computational code to simulate the behavior of the coupled system. The highest conversion was observed when an initial ethanol/acetic acid molar ratio of 9:1 was used. At this condition, the water production was reduced turning the water removal process more selective, increasing the acetic acid conversion from 51.0% to 63.9%.

Novel Drying Process Using Forced Aeration Through a Porous Biomass Matrix

A novel biodrying process was investigated whose goal was to increase the calorific value of pulp and paper wastewater sludge for efficient combustion in woodwaste furnaces. A sludge-woodwaste mixture was dried using forced air, where drying rates were enhanced by the exothermic metabolic activity of aerobic thermophilic microorganisms. Data were collected throughout the mixture to monitor temperature, humidity, pressure, airflow, and total mass so that moisture and carbon losses could be estimated for three sludge/woodwaste mixtures. A 1:0.5 sludge/woodwaste mass ratio mixture resulted in optimal drying rates, providing favorable pneumatic conditions, and resulting in the most effective removal of moisture compared to 1:1 and 1:0.25 dry-mass ratios. The sludge dry solids content rose from 30.5 to 41.6% in 13 days. Matrix temperature rose as high as 65°C, and it was found that temperature correlated positively with the rate of water removal. Pneumatic conditions improved as the material dried, i.e., permeability increased over time. It was found that the rate of carbon loss was linked to internal heat generation, and the total carbon loss was estimated to be between 5 and 18%.

Hygienization of municipal sludge in automatically operated chamber filter presses with thermal vacuum drying

This paper presents the state of the art of thermal vacuum drying in chamber filter presses for unattended automatic operation. The achieved results are exemplified by the treatment of the two stage digested combined primary, chemical and biological sludge created by the VEAS concept for nutrient removal from municipal wastewater at VEAS. The water removal rate in each stage of the drying process is described, with comments on the low energy needs. The advantages of one-sided heating, the capacity and the drying potential are discussed. The hygienization potential of the process is demonstrated by the effect on thermostable coliform bacteria, Escherichia coli, salmonella, the spores of sulfite reducing anaerobic bacteria, f-specific bacteriophages, the seeds of the weed, wild oat, Avena fatua, and the parasite eggs of the potato cyst nematode, Globodera rostochiensis. A more complete paper with the VEAS-concept is found on the VEAS homepage (www.veas.nu).

Influence of structural properties of alum and ferric flocs on sludge dewaterability

Sludges are made of flocs produced during the coagulation stage. Their dewatering not only depends on the equipment used but also on the chemical, structural and physical characteristics of flocs. In this study, the relation between floc structure and sludge dewaterability is investigated beside the influence of the operating conditions during coagulation. Eight synthetic clay flocs families are produced using clay suspensions with four initial solids concentrations ranging from 0.5 to 5 g L −1 and two coagulants (Al 2(SO 4) 3 and FeCl 3). Flocs structure is characterised by their fractal dimension and by the size of the basic units (clusters, aggregates) they are made from. Fractal dimension values about 2.5 are indicative of rather compact structures with slightly looser flocs obtained when using FeCl 3. Except for clusters, larger in ferric flocs, the basic units sizes are similar whatever the coagulant used and they decrease when the solids concentration during coagulation is increased. Sludge dewaterability is estimated from the bound water content and the capillary suction time (CST) giving respectively information on the extent and on the rate of water removal. Ferric flocs contain about 20% less bound water but exhibit higher CST values and therefore higher resistance to water removal than alum ones. Flocs structure and sludge dewaterability are found linked. The sludge bound water content is found to decrease with the floc fractal dimension: less compact flocs contain more water but less bound water. And the capillary suction time is lower when sludges are made of smaller flocs.

Effect of purge pressure on desorbing water removal rate

The need for a high purity gas supply for vacuum systems and related processing equipment has driven the requirements for the design and operation of components in gas delivery subsystems. Water has been widely regarded as a major contaminant species in such subsystems. For designs that follow generally accepted practices for construction, sealing, and layout, the primary cause of water contamination is associated with ambient atmospheric exposure during maintenance or repair. The subsequent removal of this adsorbed water, and in particular, how this process can be accelerated has been the subject of much interest. While the enhancement of molecular desorption through various methods has received considerable attention, the effective removal of water vapor after desorption is also worthy of attention. This is especially true when considering the typical geometry of gas distribution and delivery subsystems. It is shown that at typical purge pressures, a water molecule desorbing from a surface is likely to be adsorbed again. Thus, desorption may be required multiple times for the molecule to be removed from the subsystem. However, if the purge flow is operated at modest vacuum, the desorbing molecule remains in the purge stream considerably longer and travels farther downstream before being readsorbed; thus improving the removal rate. The primary focus of this investigation is to understand the dependence of the rate of water removal on the pressure of an inert gas purge. In most cases, the implementation of lower pressure purge may be accomplished with little or no change to existing equipment. In addition to the obvious savings and convenience resulting from a time reduction, considerably less purge gas is consumed.

A Theoretical Model for the Nongray Radiation Drying of Polyvinylalcohol/Water Solutions

A theoretical analysis of heat transfer and moisture variation was performed while a PVA solution was exposed to high-intensity nongray irradiation and/or air flow convection. Effective absorption coefficients were incorporated in the radiative transfer analysis. The influence of various radiation and convection parameters on the transfer of heat and moisture variation in the coated layers on an optically thick substrate was investigated. The effects of radiation and convection parameters on the transfer process were presented in terms of the rate of water content removal, heat transfer, and moisture distribution. Results were compared to those of drying when using convective heat. It is evident that the use of thermal radiation combined with convective heat will help in improving the drying rate. Numerical results show that both the radiative energy absorbed by the solution and the substrate and the distribution of water mass fraction in the solution are closely related to the rate of water removal from the solution during the process.

Diagnostic tool to detect electrode flooding in proton‐exchange‐membrane fuel cells

AbstractAn electrode flooding monitoring device designed for PEM fuel cells with interdigitated flow distributors was proposed and tested. The pressure drop between the inlet and outlet channels can be used as a diagnostic signal to monitor the liquid water content in the porous electrodes, because of the strong dependence of the gas permeability of the porous electrodes on liquid water content. It can be used with no modification to existing fuel‐cell assembly to give real‐time flooding information in the electrode backing layers during operation. The device has been employed to investigate the correlation between the fuel‐cell performance and the liquid water saturation level in the backing layers, the effects of various operating parameters, and the dynamics and hysteresis behavior of liquid water in the backing layers. The results provide, for the first time, direct evidence to show that inadequate water removal causes liquid water build up in the cathode, which in excessive amounts can severely reduce the performance of a PEM fuel cell. It was observed that more than 30 min. were needed for a PEM fuel cell to reach a new steady state when subjected to current‐density changes, and this was attributed to the slow liquid water transport process. The results confirmed that increasing air flow rate or cell temperature increased the liquid water removal rate from the backing layers. The hysteresis behavior of fuel‐cell performance was related to the water imbibition and drainage cycles in the electrode backing layers and was attributed to the difference in the water removal rate by capillary force and the difference in membrane conductivity.

Origin of the Dual Structural Transformation of Trehalose Dihydrate upon Dehydration

Upon dehydration, trehalose dihydrate transforms either toward a glassy amorphous state or toward a polymorphic crystalline phase depending on the dehydration protocol. Although this duality is now admitted, its origin remains an open question. We present here calorimetric and thermogravimetric experiments clearly showing that the main pertinent physical parameter governing this duality is the instantaneous rate of water release and not the thermal treatment used to dehydrate the sample. The latter, by acting on the rate of water loss, has only an indirect influence on the structure of the dehydrated product. A threshold rate of water removal, above which dehydration produces an amorphous state and below which it produces a polymorphic phase, is also defined.

LAB-SCALE IMPULSE DRYING EXPERIMENTS WITH EXTENSION TO HIGHER IMPULSE

Impulse drying uses high temperature and high impulse in the press section of the paper machine. The technique can increase the water removal rate and thereby minimize the size of the dryer section or alternatively increase production capacity. Additionally, the technique improves paper properties compared to conventional wet pressing. This paper contains a discussion of the value associated with extending the press impulse in impulse drying. Experiments were conducted in a lab-scale press simulator. Experiments with 150 g/m2 liner were conducted with four different press impulses, 160, 260, 360 and 450 kPa s. The impulse was varied by changing the dwell time between the pressurization and the depressurization phase. It was shown that increasing the nip length and press load beyond conventional shoe press limits gave noteworthy press dryness and sheet strength advantages. The impulse drying experiments were also compared with single and double-felted wet pressing experiments. Post Masters Student. Professor of Engineering and Assistant Vice President of Research.

DRYING CHARACTERISTICS OF OSMOTICALLY DEHYDRATED FRUITS COATED WITH SEMIPERMEABLE EDIBLE FILMS

This paper deals with key issues of the coating technology applied to plant materials prior to osmotic dehydration and convective drying.Coatings created with solutions of starch products and pectin were examined.It was shown that coating of foods to be dehydrated influences mass transfer in a plant tissue during osmotic pre-treatment as well as during convective drying. Each coating should be individually examined. The properties of coatings depend on their composition and the method used for the fabrication of a coating.Coating of foods with an artificial barrier on the surface may efficiently hinder the penetration of solute inside the food, not affecting much the rate of water removal. The coated plant materials had a greater water loss/solids gain ratio then the uncoated ones. Osmotic dehydration seems to be one of possible pre-treatment methods before drying. The conditions of convective drying are changed upon osmotic dehydration of plant materials. It was shown that changes in drying rate depend on the kind of coating substance. Bigger differences were observed at higher water content and these differences narrowed as water content in dried material decreased. The positive effect of coatings on the physical properties of dried fruits was found.

Experiments in a pilot-scale impulse unit – heat transfer and water removal rates

This is the first paper out of two dealing with heat transfer, water removal and energy efficiencies in an impulse unit in the pilot-scale research paper machine EuroFEX, located at the Swedish Pulp and Paper Research Institute (STFI) in Stockholm, Sweden. The paper produced was made of kraft pulp and with basis weights of 60-120 g/m(2). The roll surface temperature ranged from ambient temperature to 350 degreesC and the press loads were between 400-800 kN/m. The paper was produced at machines speeds of 300-800 m/min. An energy balance was conducted for the impulse unit to determine the different heat flows. It was determined that about 60% of the electrical power input was transferred to the paper web. The major heat loss was caused by wafer cooling of the induction heater. It was concluded that the heat transfer to the paper web was dependent on the roll surface temperature, the machine speed and the ingoing moisture ratio. The heat transfer to the paper web was not affected by the press load the basis weight or the roll cover. The water removal rate depended mainly on the roll surface temperature, the press load and the ingoing moisture ratio. (Less)

EFFECT OF DRYING AIR TEMPERATURE AND GRAIN TEMPERATURE OF DIFFERENT TYPES OF DRYER AND OPERATION ON RICE QUALITY

Quality of aromatic rice (KDML-105) and non-aromatic rice (Suphanburi I) was studied using different methods and scales of drying. At the lab scale, rough rice at high moisture (21±1% w.b.) was subjected to shade drying, sun drying, oven drying at 45 °C and 60 °C and fluidized bed drying (for different time periods) at 120 °C, with tempering for 2hrs. On a commercial scale, single stage drying (sun drying, column drying) and multi stage drying in a fluidized bed dryer coupled with column and shade drying were investigated. Head rice yield, color, hardness, stickiness and hardness ratio, water absorption, and structure of milled rice were examined for each method of drying. The results revealed that water absorption and ratio of stickiness and hardness decreased but the b-value and hardness increased with increase in dry grain temperature, longer exposure/drying time, longer tempering time, and rapid rate of water removal. Head rice yield of samples dried at high temperature (>60 °C) followed by tempering and of those dried at low temperature (<45 °C) in case of lab scale drying was increased and no significant difference (p<0.05) was seen relative control samples. For single stage drying, at low temperature, and multi stage drying in the fluidized bed drier (interment tempering), at high temperature, head rice yield was decreased and exhibited significant difference (p<0.05) relative to control sample. The yield however increased for a sample subjected to a 12hr tempering period and then shade drying stage subsequent to drying in a fluidized bed drier. Drying of rough rice at low temperature (<45 °C) did not affect cooking and eating qualities of rice which were affected by drying at high temperature (>60 °C). Cooking and eating qualities however changed at both single and multi stage drying on a commercial scale.

Microwave Processing of Maple Sap to Maple Syrup and Maple Syrup Products

A study of the physical process ofconcentration of maple sap to maple syrup and preparation of maple syrup products by microwave heating is described. Duty cycles of 60, 75 and 100% were used for the microwave application. During the process, some of the drying kinetics are discussed, including the reduction of moisture content with time, the progress of the process in terms of increasing sugar concentration and the power absorbed. Obviously, the rate of water removal was higher in case of the higher duty cycles. The total time required for finishing the syrup was also dependent on the initial mass of the load and the initial sugar content. ‘The products obtained were compared with commercial graded products for the quality and met the highest standards prescribed by the industry. The absence of heat damage or browning of the product was identified as a distinct advantage that could be derived from microwave processing of maple syrup.

Growth Performance Of Hybrid Bass And Hybrid Tilapia In Conventional And Active Suspension Intensive Ponds

Recently developed, active suspension intensive ponds are based on the idea that fish ponds (aer- ated and mixed as required for the well-being of the fish) can also serve as water purification units. The present paper compares water quality and fish growth in conventional intensive ponds (daily water exchange 500%), with active suspension intensive ponds (daily exchange 8%). The fish test- ed were hybrid tilapia (Oreochromis niloticus x O. aureus), already known to perform well in active suspension units, and hybrid bass (Morone saxatilis x M. chrysops), which is commercially cultured in conventional intensive ponds. Water quality in the two types of intensive ponds differed as a result of the “internal water purifi- cation” vs “external water purification” approach. Bacterial development was greater in the active suspension ponds. Several parameters were affected by the different rate of water exchange, including temperature and removal of ammonium. The latter was lower in active suspension ponds due to reduced washout of particles and their associated nitrifying bacteria. Nitrification was greater in tilapia active suspension ponds because of grazing by this fish. Active suspension units operated with less than 2% of the water used in the conventional intensive ponds. Hybrid bass and hybrid tilapia performed similarly well in both types of pond, indicating the economic advantage of culturing them in the water-saving active suspension system. Tilapia graze on suspended parti- cles, leading to additional savings in feed costs. The good performance of hybrid bass in active suspension ponds is herein reported for the first time. * Corresponding author. Email: anamilst@netvision.net.il 2 Email: agyoram@techunix.technion.ac.il

Desiccation survival of entomopathogenic nematodes of the genusHeterorhabditis

The present study aims at determining the desiccation tolerance of entomopathogenic nematodes from the genusHeterorhabditis recently isolated in Israel. We first verified the most suitable desiccation conditions that lead to induction of the anhydrobiotic state usingHeterorhabditis bacteriophora HP88. After direct exposure of infective juveniles (IJs) to 97% and 93% r.h. for 96 h, the survival rate was > 70%. By contrast, exposing HP88 IJs to 88% and 85% r.h. resulted in poor survival ( 93% > 88% > 85% r.h.) at a 24-h or 72-h interval resulted in enhanced survival (30% survival) of IJs at the final r.h. level compared with IJs which were either directly exposed to 85% r.h. (0% survival) or were preconditioned at the higher r.h. levels prior to exposure to 85% r.h. (15% survival).H. bacteriophora HP88 IJs were able to survive for at least 18 days after preconditioning. At 97% r.h. nematode viability remained stable at 70–85% and at 93% r.h. survival ranged between 37% and 60%. The data indicate that survival is influenced by rate of water removal from the nematode’s body and a minimal relative humidity level (>93%). Substantial differences in survival ability were observed among IJs of 12 new heterorhabditid populations, isolated from different climatic regions in Israel, which were preconditioned at 97% r.h. for 72 h following by an additional 72 h at 93% r.h. Maximum survival was recorded with HIS-19 (64%), moderate (40–55%) survival was observed with seven isolates, includingH. bacteriophora HP88, and five isolates displayed poor (<25%) desiccation tolerance.

Mathematical Modelling of Profiled Haemodialysis: A Simplified Approach

For many renal patients with severe loss of kidney function dialysis treatment is the only means of preventing excessive fluid gain and the accumulation of toxic chemicals in the blood. Typically, haemodialysis patients will dialyse three times a week, with each session lasting 4‐6 hours. During each session, 2‐3 litres of fluid is removed along with catabolic end‐products, and osmotically active solutes. In a significant number of patients, the rapid removal of water and osmotically active sodium chloride can lead to hypotension or overhydration and swelling of brain cells. Profiled haemodialysis, in which the rate of water removal and/or the dialysis machine sodium concentration are varied according to a predetermined profile, can help to prevent wide fluctuations in plasma osmolality, which cause these complications. The profiles are determined on a trial and error basis, and differ from patient to patient. Here we describe a mathematical model for a typical profiled haemodialysis session in which the variables of interest are sodium mass and body fluid volumes. The model is of minimal complexity and so could provide simple guidelines for choosing suitable profiles for individual patients. The model is tested for a series of dialysate sodium profiles to demonstrate the potential benefits of sodium profiling. Next, using the simplicity of the model, we show how to calculate the dialysate sodium profile to model a dialysis session that achieves specified targets of sodium mass removal and weight loss, while keeping the risk of intradia‐lytic complications to a minimum. Finally, we investigate which of the model profiled dialysis sessions that meet a range of sodium and fluid removal targets also predict extracellular sodium concentrations and extracellular volumes that lie within “safe” limits. Our model suggests that improvements in volume control via sodium profiling need to be set against potential problems in maintaining blood concentrations and body fluid compartment volumes within “safe” limits.

Phase Behavior of a PVAL-Based Polymer Proton Conductor

Solid protonic conductor gels were synthesized using poly (vinyl alcohol) (PVAL), hypophosphorous acid (H3PO2) and water as prime chemicals. The samples were characterized by means of impedance spectroscopy, fuel cell measurements, differential scanning calorimetry (DSC), thermogravimetry (TG) and X-ray diffraction. The electrical conductivity of the samples at room temperature showed a sensitive variation between 10—6 and 10—1 S/cm as the acid concentration was increased. Using the raw membranes as electrolytic separator in a fuel cell, voltages up to 726 mV were obtained. DSC thermograms showed a well-resolved step anomaly associated to a glass transition for samples with the highest acid concentrations: at about —130 °C for the first set of samples and at about —120 °C for the other sets, which indicates the amorphous character of the samples. TG traces confirmed that the membranes with higher acid concentrations have higher water contents and that the maximum rate of water removal is at about 50 °C for all samples. X-ray spectra for the raw samples at room temperature show a large peak at about 2θ = 20°, which is smaller for the higher acid content samples and increases when the samples are annealed at 70 °C, indicating that the amorphousness of PVAL complexes increases with the H3PO2 content and drops with the water removal. The results, then, indicated the presence of a separate acid/water phase in the raw samples and an increasing polymer chain intervention in the ionic mobility as the samples are thermally treated.

Thermocapillary effect on drying of a polymer solution under non-uniform radiant heating

This theoretical investigation analyzes the two-dimensional heat and mass transfer characteristics arising from the non-uniform radiant incidence in polymer solutions whose motion is governed by combined thermocapillary/buoyancy laminar flow. Also considered herein is the evaporation of solvent at solution surface. The effects of various radiation input, external convective parameters, and important physical properties of solutions on the drying characteristics are investigated and results are presented in terms of flow, thermal, and concentration fields, and rate of water removal. In addition, the influence of thermocapillary effect on drying features is also examined. Numerical results indicate that absorbed energy and distribution of the solvent concentration dominate the drying rate. At larger surface tension Reynolds number, more mass concentration can be transferred by thermocapillary flow from the interior region to the surface, which then increases drying rate. Furthermore, removing the solid wall effect reduces the drying time required for a solution film with two sides of symmetric surfaces, which is irradiated by an array of infrared radiant heaters.