ELECTROOSMOTIC DEWATERING (EOD) OF CLAYS AND SUSPENSIONS: COMPONENTS OF VOLTAGE IN AN ELECTROOSMOTIC CELL

In the electroosmotic dewatering (EOD) of clays, suspensions and fines, a continuous or interrupted DC voltage, V, is applied between the anode and the cathode. In the previous literature on this subject the components of V have not been completely clarified.

Following a brief outline of the EOD that emphasizes the central role of the electrochemical double layer, we provide an analysis of the components of V, Three case are distinguished

(1)Threshold voltage V-n, for initiating the EOD and its connection to the so-called open-circuit voltage;

(2)EOD at very low voltages viz. around IV;

(3)EOD at very high rates for prolonged times where electrodes such as Al can form barrier oxides and some “caking” is also observed near the a node, This attempt to define the components of V under different conditions of EOD is aimed to identify parameters and processes that arc involved in the effect of V on EOD; RT for example, it is generally not recognized that —pH can form a significant part of V F under some conditions of EOD     

SOME OBSERVATIONS ON ELECTROOSMOTIC DEWATERING (EOD) UNDER A.C. ELECTRIC FIELD CONDITIONS

ABSTRACT

The electroosmotic dewatering (EOD) reported by Yoshida et al. (3) under low-frequency A.C. conditions is shown to be equivalent to EOD carried out under D.C. conditions with periodic polarity reversal. The interpretation of their results is carried out here in terms of electrochemical considerations (e.g., pH gradients, zeta potentials) with lesser role assigned to factors considered paramount by Yoshida, e.g., heating effect in the dry region near the anode.

More importantly, it is pointed out that the practical advantages of EOD under A.C. conditions claimed by Yoshida et al. are based on an invalid comparison of their results with EOD under continuous D.C. conditions; a comparison under equivalent conditions would be between low-frequency A.C. conditions and those under D.C. field but with periodic polarity reversal — the enhanced dewatering claimed under “A.C. conditions” by Yoshida would disappear when EOD under D.C. conditions with periodic interruption and/or polarity reversal is considered.     

ELECTROCHEMICAL TREATMENT OF TUMORS (ECT): ELECTROOSMOTIC DEWATERING (EOD) AS THE PRIMARY MECHANISM

ABSTRACT

A recent series of experimental papers on the electrochemical treatment (ECT) of tumors provide empirical data and observations amenable to a theoretical analysis of this phenomenon from an electrochemical point of view. An attempt is made here to develop the idea that ECT is a case of electroosmotic dewatering (EOD) of the tumor tissue with the consequent changes in pH, with the concomitant role of reactions at the electrodes. Some speculative suggestions are made to enhance the efficacy of the ECT process; in particular, a multi-electrode system is proposed to achieve complete necrosis of a large tumor     

A New Theoretical Approach To Electroosmotic Dewatering (Eod) Based On Non-Equilibrium Thermodynamics☆

ABSTRACT

A novel theoretical approach to electroosmotic dewatering (EOD), with or without a pressure gradient, of clays, sludges and other colloidal suspensions is proposed. The treatment is based on nan-equilibrium thermodynamics as developed in the work of Overbeek, De Groot and others. The interpretation of electrokinetic phenomena in terms of the cancepts of irreversible thermodynamics when combined with Onsager's relations, it has been shown by Overbeek, provides a complete framework for understanding all electrokinetic phenomena.

We have applied this approach here to the electroosmotic dewatering. both in the presence and absence of applied hydrostatic pressure.

The approach provides much clarification on the nature and significance of currents and fluxes observed during EOD: these are composed of three components, during combined pressure electroosmotic dewatering: (i) electrochemicavelectrical current; (ii) hydrodynamic flux: (iii) electroosmotic current.

We have also shown the manner in which the proposed new approach to EOD based on irreversible thermodynamics can be connected to the conventional approach based on the Helmholzu-Smoluchowski equation.     

A review of “ MODERN COATING AND DRYING TECHNOLOGY” Edward Cohen and Edgar Gutoff VCH Publishers,Inc New York NY 1992, 310 p

ABSTRACT

Recently some workers in the field of electrostatics have proposed a “new” method of electroosmotic dewatering (EOD) in which a third electrode (called the “gate” electrode) is placed between the anode and the cathode to enhance the EOD. These authors present a conceptual analysis in terms of the notions of a field-effect transistor (FET). We show here that the proposed method is simply a variation of the multistage EOD already practiced by the workers in this field. It is further demonstrated that an analysis based on the FET model is not applicable to the phenomena involved in the proposed EOD method.     

PRESSURIZED ELECTROOSMOTIC DEWATERING IN A FILTER CYCLE*

This paper describes the role of pressurized electroosmotic dewatering (EOD) in a filter cycle. A laboratory-scale filter-press was used to filter a highly conductive silica suspension under constant pressure, followed by washing and mechanical precompression of the filter cake and finally by pressurized EOD at constant electric current. The influence of filter cycle parameters (filtration and washing pressure and duration, mechanical pressure, and electric field intensity) on the final cake dryness and energy consumption was studied. Electrodes of different materials, forms, and surfaces were used. The optimal conditions of each filter cycle operation were found to minimize the energy consumption during EOD and maximize the cake dryness. With mechanical pressure and electric field intensity increasing, the total energy consumption increased, but the specific energy consumption (per kg of expressed water) decreased, and the final filter cake was dryer. The pressurized EOD also used less energy than thermal drying.

     

Electrochemical Aspects of Electroosmotic Dewatering of Clay Suspensions

ABSTRACT

An attempt is made to elucidate the electrochemical aspects of the electroosmotic dewatering (EOD) of clays as reported in some recent work, especially that on interrupted DC power electroosmotic dewatering published by Rabie, Mujumdar and Weber (2). These authors showed that the dewatering by EOD stops after the DC power has been on for several minutes or hours; on interruption of their power and on short-circuiting of the electrodes, conditions can be created again for some further dewatering by DC power EOD. This discovery, of Rabie et al. is interpreted as a fuel cell effect and it is shown that it affords clues to several other electrochemical strategies for the possible enhancement of the efficiency of the EOD by DC power.

Further, the open circuit potentials observed by Rabie et al. (2) on the interruption of DC power are given an electrochemical interpretation which leads to quantitative estimates in agreement with the experimental values.     

ELECTROOSMOTIC DEWATERING BY A “NEW” METHOD USING A “GATE” ELECTRODE: FIELD EFFECT TRANSISTOR (FET) MODEL OR SIMPLY A MULTISTAGE DEWATERING?

Recently some workers in the field of electrostatics have proposed a “new” method of electroosmotic dewatering (EOD) in which a third electrode (called the “gate” electrode) is placed between the anode and the cathode to enhance the EOD. These authors present a conceptual analysis in terms of the notions of a field-effect transistor (FET). We show here that the proposed method is simply a variation of the multistage EOD already practiced by the workers in this field. It is further demonstrated that an analysis based on the FET model is not applicable to the phenomena involved in the proposed EOD method.     

Electro-osmotic dewatering of soaked hemp stems

ABSTRACT

Hemp stems were immersed in water as a pretreatment to enhance the production of hemp fiber. To reduce the high moisture content, the soaked hemp stems were dewatered using a bench-type electro-osmotic roller press. Variables like applied voltage (12, 24, and 36?V), roller pressure (1000, 2000, and 3000?kPa), and duration of soaking of hemp stems (12, 24, and 36?h) were subjected to investigation and the percentage of total water expelled due to electro-osmotic dewatering (EOD) at various levels of experiments was recorded. Hemp stems soaked for 24?h treated at a roller pressure of 2,000?kPa at an applied voltage of 36?V showed the maximum water removal after EOD process. The water removal was found to be increasing with increase in applied voltage and roller pressure. Soaking time up to 32?h leads to an increase in water removal and then it started decreasing. The probable reason for that was the penetration of surface water into micropores and its adhesion to the lignocellulosic bonds. Electro-osmotic permeability of hemp stems at various levels of voltages, roller pressures, and soaking times was studied and the result proved that electro-osmotic permeability was inversely proportional to applied voltage and it was independent of the applied pressure.     

PRESSURIZED ELECTROOSMOTIC DEWATERING IN A FILTER CYCLE

This paper describes the role of pressurized electroosmotic dewatering (EOD) in a filter cycle. A laboratory-scale filter-press was used to filter a highly conductive silica suspension under constant pressure, followed by washing and mechanical precompression of the filter cake and finally by pressurized EOD at constant electric current. The influence of filter cycle parameters (filtration and washing pressure and duration, mechanical pressure, and electric field intensity) on the final cake dryness and energy consumption was studied. Electrodes of different materials, forms, and surfaces were used.

The optimal conditions of each filter cycle operation were found to minimize the energy consumption during EOD and maximize the cake dryness. With mechanical pressure and electric field intensity increasing, the total energy consumption increased, but the specific energy consumption (per kg of expressed water) decreased, and the final filter cake was dryer. The pressurized EOD also used less energy than thermal drying.     

PRESSURIZED ELECTROOSMOTIC DEWATERING IN A FILTER CYCLE *

This paper describes the role of pressurized electroosmotic dewatering (EOD) in a filter cycle. A laboratory-scale filter-press was used to filter a highly conductive silica suspension under constant pressure, followed by washing and mechanical precompression of the filter cake and finally by pressurized EOD at constant electric current. The influence of filter cycle parameters (filtration and washing pressure and duration, mechanical pressure, and electric field intensity) on the final cake dryness and energy consumption was studied. Electrodes of different materials, forms, and surfaces were used. The optimal conditions of each filter cycle operation were found to minimize the energy consumption during EOD and maximize the cake dryness. With mechanical pressure and electric field intensity increasing, the total energy consumption increased, but the specific energy consumption (per kg of expressed water) decreased, and the final filter cake was dryer. The pressurized EOD also used less energy than thermal drying.     

THE USE OF LOCAL THERMODYNAMIC PATH FOR QUALITY PROBLEMS DURING CONVECTIVE DRYING

ABSTRACT

The pharmaceutical drug under investigation in this paper has two hydration states with several polymorphic forms. The two hydration states are tetrahydrate and monohydrate. Two polymorphs of monohydrate are in competition. Process must lead to the metastable therapeutic form (TF) although the non therapeutic form (NTF) is the stable one. The sensitive stage is drying

From desorption isotherms coupled with infrared spectroscopy, an equilibrium diagram of the different forms versus the external conditions (temperature and relative humidity) is performed. A macroscopic study achieved on tangential convective drying shows that the equilibrium diagram does not insure to obtain the metastable form after drying. It is deduced from an experimental analysis of internal temperature and water activity that the final form depends on the local thermodynamic path. Indeed, if NTF favourable conditions occur within the product during the process, this irreversible form remains although the final form predicted by the diagram is TF

It is concluded that NTF is promoted by a slowing down of transfers and an enhanced water vapour removing (greater exchange surface).     

STEAM DRYLNG OF WOOD FUELS

ABSTRACT

The objective of this work is to investigate some of the important aspects in the design of a steam band dryer for wood fuels. For this purpose the drying of the material in a bench-scale fixed bed dryer has been studied.

Drying times and thermal efficiencies for experiments conducted under different conditions are compared. The investigated materials are soft-and hardwood chips and softwood bark.

The thermal efficiency, the part of the sensible heat which is used during one passage of the steam through the bed, increases with increasing mass load (mass of dry matter per unit area) and with decreasing steam mass flux. At a mass load of 30 kg/m the thermal efficiency is about 0.85 even at steam mass fluxes as high as 0.6 kg/m² s (1.2 m/s). The thermal efficiency proves to be almost independent of pressure and temperature of the steam.

Due to the very inhomogeneous materials the steam mass flow distribution was uneven. This causes a decreased thermal efficiency. When bark     

EFFECT OF CREEP AND MECHANO-SORPTIVE EFFECT ON STRESS DEVELOPMENT DURING DRYING

ABSTRACT

Constitutive equations to quantify wood deformation under combined mechanical loading and moisture content change (1] were coupled with the moisture distribution developed during drying to predict stress and strain in 50 by 190-mm Douglas-fir heartwood lumber.

Two combinations of temperature and relative humidity were used to dry the wood. The overall board shrinkage and the immediately released and set strains were measured as a function of time. Those strains were compared with analytic results, which showed good agreement.

The roles that four strain components played in the development of stress-both at board surface and center were compared for different drying conditions. The significance of creep and mechano-sorptive strain in relieving the stress was demonstrated by varying the model parameters.     

SEPARATION OF IONIC SPECIES UNDER SUPERCRITICAL WATER CONDITIONS

ABSTRACT

The unique characteristics of supercritical water (SCW) offer potentially attractive processing options that can be explored for reaction and separation purposes. While supercritical water oxidation (SCWO) can achieve high organic conversion efficiencies, low and relative solubilities of inorganic species in SCW may be further utilized for in situ separation of potential by-products from the SCWO process effluent.

This paper describes a novel method for separating ionic species under SCW conditions. The concept is based on relative solubilities of different ionic species in SCW. Laboratory-scale demonstration tests were conducted with a Nylon monomer manufacturing process wastewater containing sodium hydroxide, sodium borate, carboxylic acids, and water. The process achieved (1) effective destruction (>99%) of organic components in the wastewater; (2) selective precipitation of sodium (>99.5%) as carbonates produced from oxidation of the organic components; and (3) efficient recovery of boron (>90%) as boric acid in the reactor effluent. The sodium removal efficiency is governed by the solubilities of sodium carbonates in SCW and, therefore, can be directly improved by increasing process temperature. As a result of the temperature increase, both organic destruction and boron recovery efficiencies may be enhanced.

This method of selective separation of ionic species in SCW has potential for a wide range of processing applications.     

ANALYSIS AND MODELING OF BROWNING OF THE GRANNY SMITH APPLE DURING DRYING

ABSTRACT

After proposing simple empirical models for drying kinetics and for sorption isotherms, the authors show that browning during drying of Granny-Smith apple samples can be calculated from models developped in static conditions. The time at which the browning begins (t_B) is similar to the induction time deduced from the static conditions ; it corresponds to a very low sample water content (around 1% dry basis) and to a sample temperature equal to the heating temperature. The “thermic past” (drying method, drying rate, heating temperature before t_B) has no influence on the browning rate, only the heating temperature after t_B is important     

THERMODYNAMIC STUDY OF THE EXTRACTION OF INDIUM(III) AND CADMIUM(II) BY CYANEX 301 FROM CONCENTRATED HCl MEDIA

ABSTRACT

The distribution equilibria of In(III) and Cd(II) between aqueous HCl solutions and kerosene containing Cyanex 301 (bis(2,4,4-trimethylpentyl) dithiophosphinic acid denoted hereafter HL) and 10 % (V/V) of n-decanol are investigated. In(III) and Cd(II) are extracted as InL₃ and CdL₂, respectively and a physico-chemical model is proposed for modelling the distribution data. In the final part of the paper, this, model is used to discuss the optimal conditions for separating indium and cadmium, previously co-extracted with Cyanex 301, by stripping in HCl media.     

ELECTROOSMOTIC DEWATERING PROCESS AND DESIGN EQUATIONS

This paper deals with electroosmotic dewatering that can be very effective for sludge which is difficult to dewater by conventional mechanical techniques. First, the dewatering mechanism of sludge due to electroosmosis is analyzed theoretically based on an electroosmotic flow through a compressible packed bed of particles, and then its processes under the conditions of both constant electric current and constant voltage are discussed both theoretically and experimentally. Further theoretical expressions for the dewatering rate and the electric power consumption are proposed using a simple model in which the sludge bed consists of two parts: a dewatering bed and a dewatered bed. The equations obtained theoretically are demonstrated to be applicable to the design of electroosmotic dewatering equipment.     

ELECTROOSMOTIC DEWATERING PROCESS AND DESIGN EQUATIONS

This paper deals with electroosmotic dewatering that can be very effective for sludge which is difficult to dewater by conventional mechanical techniques. First, the dewatering mechanism of sludge due to electroosmosis is analyzed theoretically based on an electroosmotic flow through a compressible packed bed of particles, and then its processes under the conditions of both constant electric current and constant voltage are discussed both theoretically and experimentally. Further theoretical expressions for the dewatering rate and the electric power consumption are proposed using a simple model in which the sludge bed consists of two parts: a dewatering bed and a dewatered bed. The equations obtained theoretically are demonstrated to be applicable to the design of electroosmotic dewatering equipment.     

COLLAPSE OF STRUCTURE DURING DRYING OF CELERY

ABSTRACT

Collapse of structure of foodstuffs during air drying affects quality. In many materials the soluble components, mainly sugars, are an important part of the tissue in which case collapse may be related to their glass transition temperature (T_g). It has been speculated that collapse occurs at a temperature (T_c) related to, but greater than, T_g. Plant tissues with high moisture contents, such as celery, have low T_gS. Therefore considerable collapse is expected at drying temperatures.

The aim of this study was to determine how air drying temperature affected the quantity characteristics of the tissue. Celery, air dried at temperatures between 5 and 80°C, was examined for volumetric shrinkage, rehydration characteristics and porosity changes. significant shrinkage occurred at all drying conditions. At low water content collapse was limited, probably due to a higher collapse temperature. porosity development was insignificant during drying until the sample was very dry. Lower air-drying temperatures gave a product with improved quality characteristics.