The Influence of Ionic Strength on the Electroassisted Filtration of

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Cite This: Ind. Eng. Chem. Res. 2017, 56, 12789-12798

The Influence of Ionic Strength on the Electroassisted Filtration of Microcrystalline Cellulose Jonas Wetterling,†,‡ Sandra Jonsson,† Tuve Mattsson,†,‡ and Hans Theliander*,†,‡ †

Department of Chemistry and Chemical Engineering, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden Wallenberg Wood Science Center, The Royal Institute of Technology, Chalmers University of Technology, SE-100 44 Stockholm, Sweden

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‡

ABSTRACT: The production of materials such as microfibrillated cellulose and cellulose nanocrystals is gathering significant research interest by combining mechanical strength and toughness with a low density, biodegradability and renewability. However, one of the challenges with production on an industrial scale is to obtain an energy-efficient solid−liquid separation which is difficult because of the high specific filtration resistance of these materials. This study investigates electroassisted filtration as a method to facilitate the dewatering of cellulosic materials and the influence of ionic strength on the electrofiltration behavior. Electroassisted filtration is found to improve the dewatering rate of the studied cellulosic material, and the potential improvement compared to pressure filtration increased with the specific surface area of the solid material. Increasing the ionic strength of the system increased the power demand of the electroassisted filtration, and the major potential for industrial application is thus for systems with a limited ionic strength.

1. INTRODUCTION

In this study the influence of ionic strength on the electroassisted filtration behavior of cellulosic materials is studied using a one-sided dead-end filtration equipment. A mechanically treated microcrystalline cellulose is used as a model material for cellulosic materials with high specific surface areas. The influence of the ionic strength of the suspension on electroosmotic dewatering as well as on the pressure filtration behavior is investigated in order to differentiate between the different contributions to the electrofiltration behavior. The influence of electrolysis reactions is studied by performing experiments on buffered suspensions that neutralize the electrolysis products. The influence of the specific surface area of the cellulosic material on the electrofiltration behavior is also considered.

Production of materials based on cellulosic particles with high specific surface areas, such as microfibrillated cellulose and cellulose nanocrystals, is gathering significant research interest by combining mechanical strength and toughness with a low density, biodegradability, and renewability.1 The potential applications of nanoscale cellulosic particles has a wide range and includes films, hydrogels, foams, aerogels, or as a component in composite materials.2 However, production on a commercial scale requires the development of processes from laboratory scale to industrially viable methods.3 One of the main process challenges with scale up of production is to obtain an energy-efficient solid−liquid separation: conventional mechanical dewatering through filtration can become unfeasible due to the operation time and/or equipment size required for production of materials with high specific surface areas. Electrofiltration is an assisted filtration technique that has been used to facilitate dewatering of materials where the combination of a high specific surface area and the formation of compressible filter cakes impede mechanical dewatering through pressure filtration.4,5 The energy demand of the dewatering operation can therefore be decreased compared to thermal drying.6−8 Electroassisted filtration utilizes the charge of the solid material to induce an electroosmotic flow and has been shown to have the potential to improve dewatering of materials such as wastewater sludge,6,9,10 biopolymers11−13 and hydrogels.14 However, the usage of electroassisted filtration is strongly influenced by the pH and the ionic strength of the suspension.15−17 The potential for usage of electroassisted filtration in the production of cellulosic materials will therefore be highly dependent on the process conditions. © 2017 American Chemical Society

2. BACKGROUND In electroassisted filtration an electric field is used to influence the filtration operation of solid materials with a surface charge. Colloidal particles will move relative to the surrounding fluid by electrophoresis, thereby influencing the filter cake growth.18 The electric field will also result in electroosmotic flow, providing a driving force for separation.19−21 The electroosmotic flow rate during the electrofiltration experiments can be described by the Helmholtz−Smoluchowski equation:22 Q eo = − Received: Revised: Accepted: Published: 12789

Dε0ζ (1 − ϕc)AE μ

(1)

August 29, 2017 October 13, 2017 October 18, 2017 October 18, 2017 DOI: 10.1021/acs.iecr.7b03575 Ind. Eng. Chem. Res. 2017, 56, 12789−12798

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Industrial & Engineering Chemistry Research

products are removed by electrode flushing.8,11,34 The local pH in the filter cake may influence the filtration behavior during electroassisted filtration by changing the particle surface charge. The charge of the particles will in turn influence the electrokinetic effects as well as the specific filtration resistance of the solid material.11,16,17

where Qeo is the electroosmotic flow rate, D is the dielectric constant of the liquid phase, ε0 is the permittivity of vacuum, ζ is the zeta-potential, μ is the fluid viscosity, ϕc is the filter cake solidosity, A is the cross sectional area of the filter cell, and E is the electric field strength. However, for small particles and systems with a low ionic strength the electroosmotic flow rate could be better described by an expression accounting for the surface conductivity of the solid material,23 for example, by Equation 2 that assumes flow in cylindrical capillaries:24 Q eo = −

Dε0ζ (1 − ϕc)AE[1 − G(κa)] μ

3. EXPERIMENTAL SECTION 3.1. Material and Sample Preparation. This study uses a commercially available microcrystalline cellulose (Avicel PH105, FMC Biopolymers) that was mechanically treated in order to act as a model material for cellulosic materials with high specific surface areas. The dewatering experiments were performed at a starting solid content of 5% by volume. The mechanical treatment of the microcrystalline cellulose particles were performed by suspending the particles in deionized water to a solid content of 10% by volume, the suspension was then mechanically treated using an IKA UltraTurrax T50 with a S50N-G45F dispersing element. The dispersing element operated at a rotational speed of 10 000 rpm and was fully submerged in a mixed vessel. The suspension was treated to 269 revolutions/g solid material. Additional experiments were also performed at a higher degree of mechanical treatment of 1186 revolutions/g. After the mechanical treatment the suspension was diluted with deionized water to a solid content of 5% by volume and kept under constant stirring at an ambient temperature of 22 °C for a minimum of 12 h in order to ensure consistent swelling. For suspensions prepared as described above the pH in the suspension was 6.3. Additional filtration experiments were also performed at a pH of 2.9; the pH was modified through the addition of 1 M HCl. The ionic strength of the suspensions were modified through the addition of NaNO3: additions of either 0 g/dm3, 0.125 g/dm3, or 0.5 g/dm3 were used for these experiments. Experiments were also performed for buffered suspensions. The buffered suspension was prepared through the addition of 0.1 g Na2CO3/dm3, resulting in a pH of the suspension of 9.0. 3.2. Material Characterization. The solid density of the microcrystalline cellulose was found to be 1560 kg/m3 using a Micromeritics Accupyc II 1340. The surface charge of the particles was measured by titration with a linear pDADMAC using a Stabino 2.0 particle charge titration analyzer (Particle Metrix GmbH). The pH dependence of the surface charge is shown in Figure 1. The surface charge is negative at neutral and

(2)

where κ−1 is the Debye length, a is the radius of the capillary, and G(κa) is defined by

G (κ a ) =

2I1(κa) κaI0(κa)

(3)

where I0 and I1 are the zeroth-order and first-order modified Bessel functions of the first kind, respectively. For a given system the ionic strength may thus influence the electroosmotic flow rate at a constant electric field strength both through the influence on the zeta potential and through the contribution of surface conduction to the electrical conductivity of the system. The effect of surface conduction is most pronounced when κa < 10.23 For suspensions with a high ionic strength, the electrical conductivity of the system is increased, and a higher current density, and thus a higher power demand, is required in order to maintain an electric field. An increasing current density affects the filtration behavior through an increasing ohmic heating.25,26 Additionally, increasing the ionic strength also promotes electrolysis reactions at the electrodes as the electrolysis products are proportional to the current intensity according to Faraday’s law: np =

1 F

∫ I dt

(4)

where np is the amount of electrolysis products, F is the Faraday constant, I is the electric current, and t is the time. The electrolysis reactions may be influenced by the electrode material as well as by ions in the suspension. The main electrolysis reactions at the anode are4,27 2H 2O → O2,(g) + 4H+ + 4e−

E0 = 1.23 V

M → M n + + ne−

(5) (6)

and the reactions at the cathode: 2H 2O + 2e− → 2OH− + H 2 Mn + + ne− → M

E0 = −0.83 V

(7) (8)

where M is the electrode material, n a stoichiometric coefficient, and E0 is the standard electrode potential of the reaction at 298 K. The anodes used for electrofiltration are often constructed from titanium meshes coated by mixed metal oxides10,28 or noble metals29,30 in order to prevent corrosion that may result in electrocoagulation.31 The ionic content of the suspension will also influence the electrolysis reactions, and at high ionic strength formation of gases from the anions present in the suspension may become significant. The electrolysis reactions at the electrodes can result in a pH profile between the electrodes32,33 unless the electrolysis

Figure 1. PH dependence of the microcrystalline cellulose particles surface charge as measured by titration with a linear pDADMAC. 12790

DOI: 10.1021/acs.iecr.7b03575 Ind. Eng. Chem. Res. 2017, 56, 12789−12798

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local filter cake solidosity through the attenuation of γ-radiation from a 241Am source (109 Bq) with a NaI(Tl) scintillation detector (Crismatec Scintibloc, ORTEC DigiBASE). The radiation source and the detector is used to measure the attenuation of radiation in the range 33−83 keV at a distance of 6 mm from the top of the filter medium. The local solidosity can then be calculated using the Beer−Lambert law for two phases:39

alkaline conditions, whereas the surface charge is close to zero at a pH of 3. The charge of the particle surface arises from acidic groups introduced during the production of the microcrystalline cellulose particles through acid hydrolysis35 or from charged groups native to the wood material. The pH dependence of the surface charge of the microcrystalline cellulose was independent of the ionic strength of the suspension. The effect of the mechanical treatment on the particle size distribution is shown in Figure 2. A limited decrease in particle

ϕ=−

⎛ nγ ⎞ 1 ⎜⎜ln + μγ ,l dγ ⎟⎟ (μγ ,s − μγ ,l )dγ ⎝ nγ ,0 ⎠

(9)

where ϕ is the local solidosity of the filter cake, dγ is the average path length of the radiation through the filter cake, nγ and nγ,0 are the numbers of observations made during measurement and for the empty filtration cell, respectively, and μγ,s and μγ,l are the attenuation coefficients for the solid material and the fluid phase. The attenuation coefficients were determined to 19.7 m−1 for water and 29.1 m−1 for microcrystalline cellulose using a calibration procedure described elsewhere.40 During electrofiltration experiments the filter cell is equipped with two platinum electrodes connected to a DC power supply (EA-PSI 5200-02 A, Elektro-Automatik). The power supply maintains a constant applied voltage and registers the applied current and the power demand, thereby allowing the electrical resistance of the system to be determined. The cathode is an expanded platinum mesh (Unimesh 300) and is placed beneath the filter medium, whereas the anode is placed inside the filter cell with a constant electrode separation of 25 mm. The anode is a mesh with 10 mm square openings in order to minimize interference with the suspension. The mesh is constructed from platinum wire with a diameter of 0.127 mm. The anode was placed on a supporting rack, which decreased the internal diameter of the filter cell to 50 mm for the 30 mm closest to the filter medium. The filtration experiments were performed using a hydrophilic polyethersulphone filter (Supor) with a nominal pore size of 0.45 μm. An additional Munktell grade 5 filter was placed beneath the filter medium during experiments to provide support. Filtration experiments were performed at an applied filtration pressure of 0.3 MPa and additional electroosmotic dewatering experiments were performed without an applied filtration pressure.

Figure 2. Volume-based particle size distribution of the microcrystalline cellulose particles after different degrees of mechanical treatment.

size can be observed as a result of particle disintegration. However, the mechanical treatment has been shown to have a large influence on the particles’ surface structure; the mechanical treatment increases the roughness of the particle surface and thus the external specific surface area. The effect of the mechanical treatment on the particle surfaces and the corresponding effect on the filtration behavior was extensively characterized in an earlier publication and an increasing mechanical treatment resulted in an increasing specific surface area of the material and a higher specific filtration resistance.36 3.3. Filtration Equipment. Filtration experiments were performed using a bench-scale dead-end filter press designed for measurement of the local filtration properties.37 The equipment was modified for electroassisted filtration experiments and is described in detail in an earlier publication.38 The filter cell is cylindrical with an internal diameter of 60 mm and a height of 175 mm. A constant filtration pressure was applied with a piston capable at delivering a maximum pressure of 6 MPa. The position of the piston was registered with a position sensor (Temposonic EP-V-0200M-D06-1-V0, repeatability 0.2 mm) and the filtrate exiting the filter cell was collected and weighed (Mettler Toledo SB16000, repeatability 0.5 g). The local hydrostatic pressure in the filter cell was measured using four water-filled tubes mounted from the bottom of the filter cell and connected to pressure transducers. The pressure probes have openings with 0.6 mm diameter perpendicular to the direction of flow at different heights from the filter medium (between 2 mm and 8 mm). The temperature inside the filter cell was measured using two PFA coated K-type thermocouples located 5 and 20 mm, respectively, from the filter medium. The section of the filter cell closest to the filter medium consists of a Plexiglas cylinder (115 mm high), allowing visual observation of the filter cake as well as measurement of the

4. RESULTS AND DISCUSSION Electroosmotic dewatering of microcrystalline cellulose without an applied filtration pressure is studied in section 4.1 in order to evaluate the influence of the ionic strength of the suspension. The effect of the ionic strength on the pressure filtration behavior is then discussed in section 4.2 before the combined effect of ionic strength on electroassisted filtration behavior is considered in section 4.3. 4.1. Electroosmotic Dewatering. 4.1.1. Influence of Ionic Strength. The filtrate flow rate for electroosmotic dewatering without an applied filtration pressure can be seen in Figure 3: suspensions with two different levels of ionic strength and two levels of constant applied voltage are shown. The dewatering rate was found to increase with the applied electric field strength for both the investigated suspension conditions. During the electroosmotic dewatering operation solid material is accumulated between the filter medium and the anode, resulting in a thickening of the suspension. As the dewatering operation progresses a compressible cake is formed 12791


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given in Figure 3. The electroosmotic flow rate is found to be largely unaffected by the specific surface area, indicating that the specific surface area of the microcrystalline cellulose particles has not increased the surface conductivity of the material in a way that would decrease the electroosmotic flow rate. 4.1.2. Electrolysis Reactions and Buffered Suspensions. In Figure 3 the dewatering rate can be seen to decrease during the electroosmotic dewatering operation. This effect was most pronounced for systems at high ionic strength and at high applied electric field strengths. The dewatering rate for electroosmotic dewatering at an applied voltage of 60 V/cm is shown in Figure 5: suspensions at two different levels of ionic Figure 3. Electroosmotic filtrate flow rate for microcrystalline cellulose suspensions (treated with 269 rev/g) at different ionic strengths through the addition of NaNO3.

with an increasing solid content in the direction of the filter medium.38 For suspensions with an addition of 0.125 g NaNO3/dm3 the dewatering rate in Figure 3 was found to be higher during the early stage of the dewatering operation compared to experiments performed at lower ionic strengths. This is contrary to the behavior expected from the Helmholtz−Smoluchowski equation, see eq 1: according to the equation an increase to the ionic strength is expected to decrease the absolute value of the zeta-potential and thereby decrease the electroosmotic flow rate. The observed behavior is in agreement with previous studies using bentonite and sodium kaolinite: the rate of electroosmotic dewatering was shown to increase as the ionic strength increased for salt concentrations of 10−3 to 10−2 M (corresponding to the added salt levels used in this study).15,41 A plausible explanation for the observed increase in flow rate is that surface conduction contributes to decrease the electroosmotic flow rate at the low ionic strengths. The electroosmotic flow rate could thus be better described by an expression that accounts for the surface conduction, for example, eq 2.23 The electroosmotic filtrate flow rate of a microcrystalline cellulose with a higher specific surface area, a result of a higher degree of mechanical treatment, is given in Figure 4 for two levels of ionic strength. The observed behavior is similar to the microcrystalline cellulose with a lower specific surface area

Figure 5. Electroosmotic filtrate flow rate for microcrystalline cellulose suspensions (treated with 269 rev/g) with different additions of NaNO3 or buffered with addition of Na2CO3. An electric field of 60 V/ cm was applied.

strength through the addition of NaNO3 is shown as well as a suspension to which Na2CO3 has been added in order to provide buffering ability. The dewatering rate of the buffered suspension is maintained further during the electroosmotic dewatering operation than for the unbuffered suspensions. This behavior indicates that the observed change to the filtrate flow rate for suspensions without buffer is a result of electrolysis reactions at the anode. The acidic electrolysis products at the anode lowers the pH in the filter cell, thereby decreasing the charge of the microcrystalline cellulose particles and, consequently, the electroosmotic flow rate. The electrical resistance during electroosmotic dewatering of systems with different ionic strengths is shown in Figure 6; the electrical resistance is shown to decrease with an increasing ionic strength. For systems at a low ionic strength, that is, without the addition of NaNO3, the electrical resistance decrease during the dewatering operation as a result of accumulation of ionic electrolysis products. This effect is less pronounced for suspensions with addition of NaNO3 due to the higher ionic strength of the system and the electrical resistance is instead increasing somewhat during the dewatering operation as the solid content between the electrodes increase. At the end of the electroosmotic dewatering operations in Figure 6 the electrical resistance of the unbuffered suspensions are shown to increase sharply, an effect that occurs as the filtrate flow rate decreases. This behavior may be a result of the decreasing pH in the filter cell from the electrolysis reactions at the anode. As the charge of the particles surfaces in the filter cell decreases, giving a lower electroosmotic flow rate. The behavior is similar to the increasing electrical resistance that can

Figure 4. Electroosmotic filtrate flow rate for microcrystalline cellulose suspensions at the higher degree of mechanical treatment (1186 rev/ g) and at different additions of NaNO3. 12792


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instead largely unchanged throughout the dewatering operation. The electrolysis reactions at the electrode thus have a large influence on the operation by affecting not only the rate of the electroosmotic dewatering but also the specific energy demand of the operation. 4.2. Pressure Filtration. The ionic strength will not only influence the electrokinetic effects during electroassisted filtration, it will also influence the pressure filtration behavior. The filtrate flow rate during pressure filtration experiments without an applied electric field is shown in Figure 8 for

Figure 6. Electrical resistance of the system during electroosmotic dewatering of microcrystalline cellulose suspensions (treated with 269 rev/g) at an applied electric field of 60 V/cm. Suspensions at different ionic strengths as well as a buffered suspension with the addition of Na2CO3 is included in the figure.

be observed for the desiccation of filter cakes34 and electrofiltration at high dry contents of the filter cake.42 For the buffered suspensions in Figure 6 the electrical resistance of the system is increasing during the early stage of the electroosmotic dewatering as acidic electrolysis products forming at the anode are neutralized by the buffer. The ionic strength of the buffered suspension therefore decreases during the electroosmotic dewatering operation, resulting in an increased electrical resistance. After an initial increase the electrical resistance is stabilized when an ionic strength profile has been established between the electrodes as a result of the reaction between electrolysis products and the buffer. 4.1.3. Specific Energy Demand. The specific energy demand during electroosmotic dewatering is given in Figure 7 for

Figure 8. Filtrate flow rate during pressure filtration of microcrystalline cellulose suspensions (treated with 269 rev/g) without an applied electric field. Suspensions with different ionic strengths from the addition of NaNO3 are included; filtration pressure of 0.3 MPa and a suspension pH of 6.3.

microcrystalline cellulose suspensions with different additions of NaNO3; increasing the ionic strength of the suspension resulted in an increased filtrate flow rate. By lowering the repulsive electrostatic interactions between particles in the system attractive interactions between the particle surfaces are promoted. The aggregating behavior has a large effect on the filtration resistance of the microcrystalline cellulose as the external specific surface area of the particles that are subjected to drag throughout the filter cake is decreased.43 This effect can be obtained either by increasing the ionic strength of the suspension or by decreasing the charge of the particle surfaces by changing the pH of the suspension. The inverse filtrate flow rate for pressure filtration experiments at different ionic strengths are given in Figure 9. The average specific filtration resistances at the different ionic strengths of the suspension were evaluated using the filtration equation given in eq 10. However, the somewhat convex shape of the inverse filtrate flow rate indicates that sedimentation may contribute to cake growth and the calculated values for the average specific filtration resistance and the filter medium resistance should therefore only be considered as rough estimates.

Figure 7. Specific energy demand during electroosmotic dewatering of microcrystalline cellulose suspensions (treated with 269 rev./g) at an applied electric field of 60 V/cm. Suspensions at different ionic strengths as well as a buffered suspension with addition of Na2CO3 is included in the figure.

μ(αavgcV + R mA) dt = dV A2 ΔP

suspensions at two levels of ionic strength as well as for a buffered suspension. Increasing the ionic strength of the system can be seen to increase the specific energy demand for the electroosmotic dewatering. The specific energy demand for water removal is also shown to increase during the operation for unbuffered suspensions, an effect that is more pronounced at higher ionic strengths. For buffered suspensions with the addition of 0.1 g Na2CO3/dm3 the specific energy demand is

(10)

where t is the filtration time, V is the filtrate volume, μ is the fluid viscosity, αavgc is the average filtration resistance per filtrate volume, Rm is the flow resistance of the filter medium, A is the filtration area, and ΔP is the applied filtration pressure. The average filtration resistance and the flow resistance of the filter medium at a filtration pressure of 0.3 MPa are calculated and given in Table 1. The average filtration resistance of the 12793


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electrofiltration behavior is highly dependent on the applied voltage, and the filtrate flow rate is found to be largely increased compared to pressure filtration without an electric field as shown in Figure 8. The electroassisted filtration in Figure 10 could be used to obtain a higher filtrate flow rate at a filtration pressure of 0.3 MPa than could be obtained by modification of the particle surface charge by decreasing the pH of the suspension. The potential improvement of the filtrate flow rate through electroassisted filtration is thus larger than what could be obtained by modification of the suspension conditions. In Figure 10 the filtrate flow rate at an applied voltage of 10 V/cm is found to decrease during the filtration operation to a somewhat higher degree than the experiments performed at higher applied voltages. This behavior is similar to pressure filtration experiments without an applied electric field where the flow rate decreases as a result of filter cake formation. The influence of the electric field on filter cake formation can be observed in Figure 11 using measurements of the local

Figure 9. Inverse filtrate flow rate during pressure filtration of microcrystalline cellulose suspensions (treated with 269 rev/g) without an applied electric field. Suspensions with different ionic strengths from addition of NaNO3 are included; filtration pressure of 0.3 MPa and a suspension pH of 6.3.

Table 1. Average Specific Filtration Resistance and Filter Medium Resistance for Pressure Filtration Experiments at an Applied Filtration Pressure of 0.3 MPa (g NaNO3/dm3)

αavgc [m−2]

Rm [m−1]

ΔP [MPa]

0 0.125 0.500

3.0 × 10 1.0 × 1015 1.5 × 1014

8.1 × 1012 2.7 × 1012 1.1 × 1012

0.32 0.32 0.30

15

microcrystalline cellulose decreases with an increasing ionic strength of the suspension. The calculated filter medium resistances of the forming filter cake in Table 1 are higher than the flow resistance of the filter medium determined through permeability experiments with water without the presence of microcrystalline cellulose particles, 1.4·1010 m−1, thereby indicating that the Rm determined from eq 10 not only describes the filter medium resistance but also includes the resistance of the material deposited on the material during the early stage of the filtration operation. The rate of the filtration operation is however mainly controlled by the flow resistance of the forming filter cake and not by the filter medium. 4.3. Electroassisted Filtration. The filtrate flow rate for electroassisted filtration of microcrystalline suspensions without the addition of NaNO3 is shown in Figure 10. The

Figure 11. Local solidosity in the filter cell measured 6 mm from the top of the filter medium during electroassisted filtration of microcrystalline cellulose suspensions (treated with 269 rev/g) without the addition of NaNO3 at a filtration pressure of 0.3 MPa.

solidosity in the filter cell at a fixed distance of 6 mm from the filter medium. For experiments with an applied voltage of 10 V/cm the measured filter cake solidosity increased during the operation as a result of filter cake formation. At higher applied voltages the increase to the measured solidosity is more limited and corresponds to a thickening of the suspension in the filter cell rather than filter cake formation. The effect of the electric field has been described by an electrofiltration model in a previous publication and was described as a combination of electroosmotic dewatering and the influence of the electrophoretic force on the filter cake growth.38 4.3.1. Influence of Specific Surface Area. The electrofiltration behavior of microcrystalline cellulose suspensions subjected to different degrees of mechanical treatment is shown in Figure 12. Increasing the mechanical treatment resulted in a slower pressure filtration operation of the material when no electric field was applied as the specific surface area of the material was increased. However, when an electric field was applied the rate of electrofiltration was found to be largely unaffected by an increased mechanical treatment. The filtrate flow rate for microcrystalline cellulose at the higher degree of mechanical treatment is given in Figure 13. The filtrate flow rate is found to be very similar for electroosmotic dewatering without an applied filtration pressure and for electrofiltration at an applied filtration pressure of 0.3

Figure 10. Filtrate flow rate during electrofiltration of microcrystalline cellulose suspensions (treated with 269 rev/g) without the addition of NaNO3 at a filtration pressure of 0.3 MPa. 12794


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Figure 12. Obtained filtrate during electrofiltration of microcrystalline cellulose suspensions subjected to the different degrees of mechanical treatment. Experiments were performed at a filtration pressure of 0.3 MPa and without the addition of NaNO3.

Figure 14. Filtrate flow rate during electrofiltration of microcrystalline cellulose suspensions (treated with 269 rev/g) with an addition of 0.125 g NaNO3/dm3 and a filtration pressure of 0.3 MPa. Pressure filtration at pH 2.9 without an applied electric field is included for reference.

Figure 13. Filtrate flow rate for microcrystalline cellulose subjected to the higher degree of mechanical treatment (1186 rev/g) with different applied electric fields, applied filtration pressures, and pH of the suspension.

Figure 15. Filtrate flow rate during electrofiltration of microcrystalline cellulose suspensions (treated with 269 rev./g) with an addition of 0.5 g NaNO3/dm3 and a filtration pressure of 0.3 MPa. Pressure filtration at pH 2.9 without an applied electric field is included for reference.

In both Figure 14 and Figure 15 the filtrate flow rate obtained during electrofiltration is lower than, or similar to, the filtrate flow rate that can be obtained for pressure filtration of suspensions with a pH of 2.9. Changes to the pH in the filter cell as a result of the electrolysis reactions at the anode may thus have a significant influence on the filtration behavior at these suspension conditions. In fact, if the electric field is removed during the latter stage of the filtration operation at an addition of 0.5 g of NaNO3/dm3, see Figure 16, no change to the filtration behavior was observed. This indicates that electrophoresis and electroosmosis had a minor effect on the system at this stage. The pH profile during the electrofiltration operation was measured through dissection of filter cakes collected at various stages of the filtration operation. The pH in the filter cakes decreased in the direction toward the anode and became increasingly acidic during the filtration operation. At the stage of the filtration operation at which the electric field was removed in Figure 16 a pH about 2−3 was observed throughout the filter cake, resulting in a particle surface charge close to zero. This effect may be avoided by removing the electrolysis products from the system, for example, by using flushed electrodes.8,34 4.3.3. Buffered Suspensions. The filtrate flow rate during electrofiltration of buffered suspensions that neutralize the

MPa. This indicates that the filtration pressure has a very limited contribution to the filtrate flow rate due to the high specific filtration resistance of the solid material. The filtrate flow rate is also found to be higher than the flow rate for pressure filtration that can be obtained by modifying the aggregation of the microcrystalline cellulose through the pH or the ionic strength of the suspension. This behavior implies that the potential improvement to the dewatering rate by the application of an electric field increases with an increasing specific surface area of the solid material. 4.3.2. Influence of Ionic Strength. The filtrate flow rate during electrofiltration of microcrystalline cellulose suspensions is shown in Figure 14 and Figure 15 for additions of 0.125 g of NaNO3/dm3 and 0.5 g of NaNO3/dm3 respectively. The filtrate flow rate can decrease during the electroassisted filtration experiments for both suspension conditions, indicating an increasing filtration resistance as a result of cake formation. When Figure 14 and Figure 15 are compared, the influence of the applied electric field strength on the filtrate flow rate is found to decrease as the ionic strength of the systems is increased. This effect is expected as the filtration resistance of the microcrystalline cellulose is lower at high ionic strengths, thereby increasing the contribution of pressure filtration to the overall filtrate flow rate. 12795


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Figure 16. The filtrate flow rate for electrofiltration of a microcrystalline cellulose suspension (treated with 269 rev/g) with an addition of 0.5 g of NaNO3/dm3 at a filtration pressure of 0.3 MPa. An electric field of 30 V/cm was applied initially and removed during the filtration operation.

Figure 18. Temperature measured 5 mm from the filter medium during electrofiltration of microcrystalline cellulose suspensions (treated with 269 rev/g) suspension with addition of 0.1 g Na2CO3/dm3 and a filtration pressure of 0.3 MPa.

acidic electrolysis products are given in Figure 17. The filtrate flow rate decreased during the early stages of the filtration

Figure 19. Specific energy demand during electrofiltration of microcrystalline cellulose suspensions (treated with 269 rev/g) with the addition of 0.125 g of NaNO3/dm3. The filtration pressure applied was 0.3 MPa.

Figure 17. Filtrate flow rate during electrofiltration of buffered microcrystalline cellulose suspensions (treated with 269 rev/g) suspension with addition of 0.1 g Na2CO3/dm3 and a filtration pressure of 0.3 MPa.

dewatering experiments without an applied filtration pressure. The optimal electric field strength for an electrofiltration operation will thus be determined by a trade-off between the specific energy demand and the dewatering rate. The specific energy demand for electrofiltration experiments at a constant applied voltage is shown in Figure 20 for systems with different ionic strengths. Increasing the ionic strength of the suspension increased the electrical conductivity of the system. An increased current density is thus required to maintain the electric field, thereby increasing the ohmic heating and the formation of electrolysis products. The energy demand for water removal through electrofiltration thus increased with the ionic strength for the microcrystalline cellulose system. The temperature in the filter cell during electrofiltration experiments at different ionic strengths is shown in Figure 21; the temperature was measured 5 mm from the top of the filter medium. For systems with addition of NaNO3 a significant temperature rise occurred, thereby influencing the filtrate flow rate through the viscosity of the fluid. The specific energy demand of the dewatering of the microcrystalline cellulose used in this study is however increased by an increasing ionic strength of the system.

operation, a result of filter cake formation, before stabilizing. At the higher applied electric field strengths the filtrate flow rate can be seen to increase somewhat at the stabilized level, an effect that may be attributed to an increasing temperature due to ohmic heating, see Figure 18. The filtrate flow rates at the stabilized level in Figure 17 are proportional to the applied voltage during the experiment; the flow rate could therefore be described as controlled by electroosmotic flow according to the Helmholtz−Smoluchowski equation, see eq 1. The filtrate flow rate of the buffered suspension does thus not exhibit the same behavior as suspensions with the addition of NaNO3, as compared with Figure 14 and Figure 15. The difference in behavior indicates that the influence of the electric field during electrofiltration of suspensions with the addition of NaNO3 was decreased as a result of the electrolysis reactions. 4.3.4. Specific Energy Demand. The specific energy demand for electrofiltration experiments performed at different electric field strengths are shown in Figure 19. The energy demand increased with the electric field strength as the current density increased; the same behavior was observed for electroosmotic 12796



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AUTHOR INFORMATION

Corresponding Author

*Tel.: +46 31 772 29 92. Fax: +46 31 772 29 95. E-mail: [email protected]. ORCID

Hans Theliander: 0000-0002-2120-6513 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This study was performed within the framework of the Wallenberg Wood Science Center, and the financial support of the Knut and Alice Wallenberg Foundation is gratefully acknowledged.

Figure 20. Specific energy demand during electrofiltration of microcrystalline cellulose suspensions (treated with 269 rev/g) at different ionic strengths achieved by the addition of NaNO3. The filtration pressure applied was 0.3 MPa, and the electric field was 30 V/ cm.

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Figure 21. Temperature measured 5 mm from the filter medium during electrofiltration of microcrystalline cellulose suspensions (treated with 269 rev/g). Experiments were performed at different additions of NaNO3 and at an applied electric field of 30 V/cm and a filtration pressure of 0.3 MPa.

NOMENCLATURE A = filtration area [m2] a = radius of capillary [m] c = mass of solids per unit filtrate volume [kg/m3] D = dielectric constant of liquid phase [-] dγ = average path length of radiation [m] E = electric field strength [V/m] E0 = standard electrode potential [V] F = Faraday constant [C/mol] I = electric current [A] np = electrolysis product [mol] nγ = number of γ-detector observations [-] nγ,0 = number of γ-detector observations for the empty filter cell [-] Qeo = filtrate flow rate from electroosmosis [m3/s] Rm = flow resistance of the filter medium [m−1] t = time [s] V = filtrate volume [m3]

Greek Letters

5. CONCLUSIONS Electroassisted filtration can be used to improve the dewatering rate of cellulosic materials with high specific surface areas. The potential improvement compared to pressure filtration increases with the specific surface area of the solid material. Increasing the ionic strength of the system increases the power demand of the electroassisted filtration to a large extent. For this reason the major potential for industrial application of electroassisted filtration is for systems with a limited ionic strength. A high ionic strength may however have a beneficial effect on the pressure filtration behavior by affecting the specific filtration resistance. Increasing the ionic strength of the system increases the formation of electrolysis products. Unless the electrolysis products are removed during the filtration operation the resulting effect on the pH in the filter cake may influence the electrofiltration behavior through the effect on the particles surface charge. For the microcrystalline cellulose used in this study the formation of acidic electrolysis products was found to have a detrimental effect on the electroassisted filtration operation.

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αavg = average specific filtration resistance [m/kg] ΔP = pressure drop over the filter cake and filter medium [Pa] ε0 = permittivity of vacuum [F/m] ζ = zeta-potential [V] κ = reciprocal Debye length [1/m] μ = viscosity of fluid [Pa·s] μγ,l = attenuation coefficient of the solid phase [m−1] μγ,s = attenuation coefficient of the liquid phase [m−1] ϕ = local solidosity [-] ϕc = filter cake solidosity [-]

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