Energy Aspects in Electrohydrodynamic Drying

Drying Technology An International Journal

ISSN: 0737-3937 (Print) 1532-2300 (Online) Journal homepage: http://www.tandfonline.com/loi/ldrt20

Energy Aspects in Electrohydrodynamic Drying T. Kudra & A. Martynenko To cite this article: T. Kudra & A. Martynenko (2015) Energy Aspects in Electrohydrodynamic Drying, Drying Technology, 33:13, 1534-1540, DOI: 10.1080/07373937.2015.1009540 To link to this article: http://dx.doi.org/10.1080/07373937.2015.1009540

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Date: 17 September 2015, At: 09:03

Drying Technology, 33: 1534–1540, 2015 Copyright # 2015 Taylor & Francis Group, LLC ISSN: 0737-3937 print=1532-2300 online DOI: 10.1080/07373937.2015.1009540

Energy Aspects in Electrohydrodynamic Drying T. Kudra and A. Martynenko

Downloaded by [Dalhousie University], [Alex Martynenko] at 09:03 17 September 2015

Department of Engineering, Dalhousie University, Truro, Canada

Acknowledging favorable quality of products obtained through electrohydrodynamic (EHD) drying, there is a lack of knowledge about energy aspects of this promising, yet not commercialized, technology. This article is a critical review of EHD research, which may be crucial for future studies and industrial applications. In particular, effects of electrodes configuration and operating parameters on drying kinetics, energy consumption, and energy efficiency of EHD as compared to conventional drying are examined. Some engineering considerations for using EHD in industrial dryers are also discussed. Keywords Biomaterials; Drying rate; Energy; Ionic wind; Quality

INTRODUCTION Electrohydrodynamic (EHD) drying refers to the removal of water from a drying material placed in the strong electric field due to the so-called ‘‘corona wind.’’ This wind originates from a sharp end of the electrically conducting pin as a result of ions leaving this pin and impinging the surface of the material being dried on their way toward the plate electrode.[1,2] To avoid ambiguities and misinterpretation encountered in published papers related to treatment of biomaterials in a high-voltage electric field, a distinction for EHD should be made at the outset from: i. electroporation (electropermeabilization), where the high-voltage electric field applied, usually in a pulsed mode (typically, exposure time is in the order of microseconds), results in electric discharge. Pulsed electric field (PEF) perforates the hardly permeable skin and disrupts the cellular structure of fruits, berries, or vegetables, facilitating mass transfer for osmotic dehydration[3] or extraction of valuable compounds.[4,5] A comprehensive yet concise overview of PEF applications, including modification of material properties, inactivation of enzymes, and microorganisms is given in the technical literature.[6] ii. altering the properties of fruits, vegetables, and other biomaterials by exposure to a high-voltage electric field Correspondence: T. Kudra, Department of Engineering, Dalhousie University, Truro B2N 5E3, Canada; E-mail: tadeusz. [email protected]

between two parallel plate electrodes to extend the shelf life of perishable foods.[7] iii. pretreatment in electric field prior to osmotic dehydration,[8,9] air drying, [10,11] and vacuum freezedrying.[12] iv. any other technologies, exemplified by roller pressing assisted by electroosmosis,[13] which exploit the characteristics of either high- or low-voltage electric fields.[14,15] Following the aforementioned exceptions, EHD drying= dewatering is further referred only to these applications, where high-voltage electric field generates the corona wind that imposes a specific hydrodynamic effect when impinging the drying material. Although hydrodynamic effects in fluids are well documented, EHD drying has recently gained extensive interest because it is regarded as a non-thermal technology, particularly suitable for heat-sensitive biomaterials. With respect to product quality, the following can be quoted as examples of positive attributes: lower shrinkage,[16,17] higher rehydration ratio,[16] and preserved vitamin content.[18] Moreover, no discernible degradation is generally noted in terms of color,[16–19] though Li et al.[20] reported distinctive browning of okara cake just under the needle electrode. These quality attributes are well enhanced when EHD is combined with vacuum freeze drying.[21] Regarding energy consumption, it is claimed to be much lower in both EHD and combined EHD-hot air drying than in simply hot air drying.[1,2,19,22,23] One of the reasons is the EHD enhancement of drying rate. Numerous researchers indicate significant increase of drying rate, typically in the range from 1.3 to 4.52, depending on the type of biomaterial, initial=final moisture content, and operating conditions.[20,24–26] It results in appreciably shorter drying time by 15–40%.[20,22] Interestingly, the energy consumption of EHD combined with vacuum freeze drying was lower than freeze drying or EHD drying alone.[21] However, even though a prototype has been designed and tested,[27] yet no industrial dryer exploiting the EHD principle was commercially available at the time of writing this article. Apparently, the widely quoted low electric energy consumption based on the only power delivered

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with corona wind needs to be carefully re-examined. Thus, this study aims to analyze certain energy-related issues in EHD drying, which could be useful for industrial scaling of this innovative technology towards proces and equipment design. ENERGY-RELATED ISSUES IN EHD DRYING With due respect to authors of fundamental research, the EHD drying of commodity materials such as wheat, apples, and carrots is rather an academic curiosity than a technically and economically justifiable large-scale processing. EHD application could be justified only for low-temperature drying of valuable medicinal plants, probiotics, and nutraceuticals. The key criterion in industrial viability of EHD drying appears to be the energy efficiency leading to process economics. Therefore, this section presents some key issues related to energy consumption and energy efficiency of EHD drying. Principle of EHD Drying The principle of EHD drying is based on the use of the ‘‘corona’’ wind, also called the ‘‘ionic’’ or ‘‘electric’’ wind, where electric discharge from a high-voltage electrode creates the jet of high-energy ions and gas molecules targeting the dried material. For this reason, the electrode generating this specific gas jet is considered as the ionic wind (EHD) generator. The corona discharge appears when a high voltage is applied between two electrodes with substantially different radii of curvature, such as a flat surface and needle (sharp pin), or a flat surface and thin wire, giving point-to-plate and wire-to-plate configurations, respectively. For example, the corona wind emitted from the needle forms the gas jet on its way to the plate electrode. Observation of the jet in dark environment indicated its conical form with the cone angle of 2a (Fig. 1). According to the widely quoted Warburg empirical law[28] (also confirmed by analytical solutions[29]), the maximum semi-vertical

FIG. 1. Schematics of the EHD system in needle-to-plate configuration: 1–high voltage power supply, 2–needle electrode, 3–ions, 4–corona wind cone, 5–drying material, 6–plate electrode.

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cone angle of the corona wind in point-to-plate discharge in air is equal to 60 . This rule has been proven for direct current (DC) of positive polarity in point-to-plate and wire-to-plate configurations. It is reasonable to assume validity of the same jet geometry for other modes of electric current supply; however, this assumption should carefully be tested for various alternating current (AC) wave forms. So far, no consensus exists on the performance of alternating current (AC), which was found to be more efficient than DC for water evaporation[30] or less efficient for drying of carrots[26] and paper.[31] Typically, the point-to-plate electrode produces a highly non-uniform electric field, so the strongest evaporation takes place just below the point electrode and diminishes toward the periphery. In the case of evaporation from liquid solutions and suspensions, the vortex-like turbulent motion was observed in water[32] and aqueous suspension of whey protein,[33] which disappeared when the suspension started to solidify. Thus, such vortex motion is unlikely to exist in drying of solid materials, such as carrots and potatoes. It should be noted that the jet of corona wind impinging the dried material rebounds from the material surface, which affects the neighboring jets emitted from a multineedle or multi-wire electrode. However, this boundary phenomenon was not considered in previous configurations of multi-needle electrodes. Electrode Configuration Although the plate electrode may be either solid or perforated, the emitting electrode is usually a vertical singleand multi-needle type[16–20,25,26,31–37,39,40] or the wire-type placed in parallel to the plate electrode.[21–24,27,29,30,38,41–43] The single- and multiple-wire electrodes were studied because they better fit the foreseen industrial units such as band dryers.[27] As indicated by Lai and Wong,[36] the performance of the wire electrode is better than the needle type when the applied voltage is lower than 15 kV. Over this critical voltage, the needle electrode performs better. This phenomenon can be attributed to the corona wind geometry,[36] which for the wire electrode resembles a slot-type jet (Fig. 2a), whereas the needle electrode produces a conical-type jet with circular area on the material surface (Fig. 2b). Figure 2b presents the theoretically optimal staggered arrangement of the multiple needle electrode, which would provide close to uniform exposure of a material under drying. Rationally, the best configuration of the multi-needle electrode is with the needles spaced to produce impinging corona jets with circular areas almost touching each other. The distance between the needles and their configuration should be determined experimentally because of possible interference of neighboring jets and the effect of jets rebounding from the surface of dried material. The gap between the electrode and material is another important design parameter, which has to be properly chosen to


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numerous experiments in a wide range of air velocities from 0.1 to 5 m=s revealed a generally negative impact of air flow on the efficiency of EHD drying because of suppression of ionic wind.[23,24,36,38,43] The exception was the drying of kiwi fruits with a 17-needle electrode where the effect of cross-flow air velocity was found insignificant.[37] The effect of cross-flow on EHD performance was thoroughly examined by Lai and Sharma[34] and Dalvand et al.[37] To quantify interaction between convective airflow and ionic wind, the dimensionless EHD number as the ratio between ionic wind velocity ue and cross-flow air velocity u has been introduced[42]:

FIG. 2. Projected area of corona wind impinging the surface of drying material: (a) single wire electrode, (b) multi-needle electrode in staggered arrangement.

optimize the both electric field strength and material surface area covered by the jet of electric wind. Not only electric field strength, but also electric charge density has to be controlled in the gap. Air Temperature Despite the fact that heat generally enhances drying, it was found that the efficiency of EHD drying drops with increasing air temperature.[39] The experiments on combined EHD=hot air drying showed EHD enhancement of drying rate as 1.9, 1.6, and 1.5 for temperatures 20, 35, and 50 C, respectively.[39] Decrease in EHD efficiency of drying with temperature might be related to the non-Fickian mechanism of drying with negligible effect of thermodiffusion. These experiments clearly demonstrated advantages of EHD application for low-temperature drying. In experiments with partially wetted glass beads,[43] it was found that EHD drying is mostly a surface phenomenon. This is an obvious obstacle for EHD in the final stages of drying to remove moisture trapped deep inside the material. To overcome this obstacle, it has been proposed to create the vertical thermal gradient by auxiliary heating from below.[35] A preliminary study has shown that low-power heating at 118.9 W=m2 can effectively accelerate the EHD drying due to enhanced thermodiffusion from the sample core to its surface.[35] Hence, with the backup of auxiliary heating, the EHD phenomenon could be used for industrial drying at low air temperature. However, possible accumulation of moisture in the gap between electrodes might require additional air convective flow. Convective Cross-Flow Cross-flow convection was initially applied in EHD drying with the intention of enhancing drying rates. However,

NEHD ¼

ue u

ð1Þ

The EHD number reflects interaction of two orthogonal forces: electric force of ionic wind Fe and inertial force of airflow F. It was found that ionic wind velocity is directly proportional to the electric field strength and can be calculated from the following relationship[32]:

ue ¼ E

rffiffiffiffi e0 q

ð2Þ

The silent assumption incorporated in this formula is that air density q is constant, independent of water vapor density and electric charge density, but this requires further experimental verification. The effect of EHD was significant only at low air velocity (NEHD > 1), when ionic wind was not suppressed by cross-flow convection.[23,26,36,37,42] The velocity of ionic wind is usually in the order of several meters per second, which definitely entails the aerodynamic effect on a drying material, disturbing the boundary layer at the material’s surface. Under experimental conditions in the study by Pogorzelski et al.,[26] the ionic wind generated by the needle electrode at 5.27 kV=cm was 1.45 m=s, which is much higher than the cross-flow air velocity of 0.1 m=s. These authors confirmed the findings by others that ionic wind is the major driving force in the first drying period. The considerable reduction of the EHD effect by convective airflow (NEHD < 1) could be explained by domination of the inertial force over electric force, thus resulting in classical convective air drying. In these experiments, the drying rate was independent of electric force, increasing with air velocity due to the direct effect on the boundary layer mass-transfer resistance (boundary condition of the third kind). Multifactorial experimentation proved negligible effect of low air velocity in the range 0–0.4 m=s on EHD efficiency and significant effect of air velocity on energy consumption.[37] It could be explained through substantial energy consumption by convective blower, which is



10 to 20 times higher than energy consumption of the equivalent EHD wind generator.[44] Moreover, the experimental data on EHD=hot air drying indicate that the overall energy consumption could be by 1000 times higher than energy consumption of the EHD generator.[22] It follows that the direct effect of convective cross-flow is not beneficial for industrial EHD drying for at least two reasons: (i) reducing the effect of the vertical ionic wind due to perpendicular air flow and (ii) increasing the overall energy consumption. Indirect effect of convective cross-flow on the EHD efficiency due to reduced humidity should be further investigated. However, this effect is significant only for the first period of drying. An alternative solution to control humidity is an air dehumidifier, sufficient to maintain equilibrium vapor pressure. Effect of Humidity and Pressure In spite of the fact that humidity and air pressure play a significant role in the process of drying, the effect of these conditions on the efficiency of EHD drying has never been studied. Humidity measurements were used in experiments of Lai and coworkers[34,36,42,43] to calculate the Sherwood number, representing conditions of mass transfer. Bai et al.[21] presented results of vacuum freeze drying, which revealed better performance of EHD drying at ambient temperature 18 C and relative humidity of 45% versus vacuum freeze drying (conditions were not specified, however). Unfortunately, the presented experimental design did not allow separate evaluation of temperature and low-pressure effects. This gap in the knowledge requires careful research of these factors, which are critical for the industrial scaling of the EHD technology. Drying Kinetics The kinetics of EHD drying were found to be similar to the kinetics of convective drying, which means that depending on the processed material and moisture content, both constant and falling rate periods are recognized. In some cases, the short increasing rate period was also noted.[20,22] Figure 3 presents drying curves for two kinds of food products with definitely constant drying rate at the beginning of drying.[26] One of the measures to quantify the efficiency of EHD drying is the ratio of the mass flow rate of the EHD-dried sample over the control one dried under ambient conditions without an electric field.[45] Based on this definition, the EHD enhancement has been expressed through the drying rate ratio for the materials dried in the first drying period.[26] Hence, for experiments presented in Fig. 3, the enhancement ratio was calculated as 2.01 for miscanthus and 4.52 for carrot.[26] Alternatively, drying enhancement could be expressed through the ionic wind velocity [Eq. (2)], which is proportional to easily measurable applied voltage.[45] For solid biomaterials with complex diffusivity,

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FIG. 3. Drying curves of carrot and miscanthus at DC positive polarity; seven needles, air velocity 0.1 m=s (excerpt from Pogorzelski et al.[26]).

such as biscuits, with the assumption of coupled heat=mass transfer and negligible boundary effects, it was proposed to characterize drying enhancement with a ratio of heat transfer coefficients.[45] It is important to notice that EHD enhancement of drying rate is not constant over the entire drying process. EHD enhancement in the falling rate period is reported to be in the range from 0.97 to 3.2 for a variety of biomaterials.[20,23–25,36] However, much higher values of 4.52 for carrot[26] and 6.8 for paper towel[45] have been obtained for the constant rate period, which points to the prevailing hydrodynamic effect of the corona wind on the evaporation of surface water. In the case of open surface water, the evaporation was enhanced by 7.3 at cross-air velocity of 0.125 m=s.[38] Our own attempts to calculate the EHD enhancement for the materials dried in both the constant and falling rate periods are underway. Energy Consumption Electric energy consumption is widely acclaimed to be much lower in EHD drying than in convective drying. For example, Yang et al.[46] reported energy savings due to EHD drying at 50 C in the range from 50 to 85% as compared to oven drying and fluid bed drying at 150 C. This large-scale EHD-based dryer with 2 m2 surface area and heating power of 1.5 kW provided 0.4 kW of corona power at 40 kV of operating voltage. Unfortunately, no specific information was given on EHD design details and experiments with the oven and fluid bed dryers. In general, energy consumption depends on many parameters, such as applied voltage, positive or negative polarity, electrode configuration, and the electrode-tomaterial spacing. The complex dependence of energy consumption on numerous parameters can be exemplified through nonlinear decrease of energy consumption with


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decreasing voltage and electrodes gap, thus increasing the electric field strength up to the threshold value of 5.2 kV=cm.[37] Also, the total moisture removed in a given time with three wires was found higher than with one wire, which translates into higher energy efficiency.[47] In the review by Singh et al.,[2] it was pointed out that energy consumption depends on the electrode configuration and water availability. The lowest values of energy consumption in the range from 14.7–2727 kJ=kg referred to evaporation from the open water surface or free water saturating the bed of 3- and 6-mm glass beads whereas the higher values were pertinent for food products.[2,36,42] Although numerous publications refer to energy advantages of EHD drying, the scale-up route toward the industrial apparatus requires considering all aspects of energy consumption. Namely, it should be noted that the energy consumption in published papers is based on the ‘‘net’’ energy calculated from the applied voltage and current passing from the corona electrode to plate electrode[19,24,26,31,39,40,48] and is rarely determined from the consumed electric power.[41,44] However, typical EHD apparatus is composed of various electricity-based units (Fig. 4), each of them being characterized by certain energy efficiency. Thus, in any comparison, the factual electric energy delivered to the drying system should be considered. This does not mean that the energy consumed by the EHD drying will exceed the one for air drying, but the savings effect might not be so appealing. For instance, Zander[49] indicated that the major fraction of energy was consumed by peripheral equipment, so the ratio of the net power to the supplied power varied from 0.01 to 0.17, depending on the apparatus and experimental conditions. Our own studies were performed with a multiple-needle electrode formed from 60 steel nails with 60 conical tip arranged in six rows and 10 columns with 10-mm square spacing, connected to the DC voltage source (SPELLMAN, Model RHR20P10=FG=RC, USA).[44] The experiments indicated from 85 to 99% energy lost in the low-to-high voltage convertor. Clearly, determination of the factual energy use for EHD drying is needed to get the real picture of energy consumption for the comparison of various competitive drying technologies.

FIG. 4. Typical components of EHD setup in DC configuration: 1–voltage controller=stabilizer, 2–step-up transformer, 3–rectifier, 4–polarity switch, 5–safety resistor, 6–capacitor, 7–applicator. In AC configuration, the applicator (7) is directly connected to the step-up transformer (2); measuring instruments and air supply system are not shown.

Energy Efficiency The energy performance of a dryer and drying process is characterized by various indices such as volumetric evaporation rate, surface heat losses, steam consumption, unit heat consumption, and energy (thermal) efficiency.[50] The ratio of energy used for moisture evaporation to the total energy supplied to the dryer is the most frequently quoted as an efficiency in technical literature. However, the superior index to compare efficiency of drying operation is specific energy consumption, defined as the amount of energy required to evaporate unit mass of water and therefore given in kJ=kg.[50] This index, further termed as energy efficiency, can be determined from two measurable variables, namely the electric (thermal) power (kW) and the drying rate (kg=s). The energy efficiency over the entire period of EHD drying varies broadly in the range from 90 to 720 kJ=kg,[26] which corresponds well with 100–800 kJ=kg for singleand multiple-needle electrode and 200–5000 kJ=kg for wire electrode.[2] As a reference, the energy efficiency of the convective belt dryer, which so far is the only configuration of the foreseen industrial EHD dryer,[27] attains 3800–3950 kJ=kg.[51,52] It should be noted that the energy performance in kg=kJ, quoted in some papers,[34,35,42,43] is a reciprocal to energy efficiency, but mutual conversion is quite straightforward. From the analysis of literature data, it follows that the energy efficiency of EHD drying varies in time. Thus, the concept of instantaneous drying indices[50] could be used to determine the temporal characteristic of the energy performance to be further exploited for optimization of EHD drying. In such a case, the real-time quantification of energy performance requires simultaneous measurements of two variables, namely the power consumption and the evaporation rate. Instrumentation for real-time measurements on the lab scale is available,[53] but the procedure of experimental verification is needed for further scaling-up to the industrial prototype. CONCLUSIONS 1. EHD proved to be a promising tool for acceleration of low-temperature drying of high value materials such as medicinal plants, probiotics, nutraceuticals, and other heat-sensitive biomaterials. Yet, detailed analysis of quality=energy issues should be performed. 2. The kinetics of EHD drying are similar to conventional drying, which means that depending on the processed material, both constant and falling rate periods are recognized. In general, the drying rate in EHD processing is higher, typically from 1.3 to 4.52, and thus drying time is shortened by 15 to 40%, depending on the material, moisture content, and operating conditions. When combined with convective drying, the



accelerating effect of EHD diminishes with air temperature and cross-flow air velocity. 3. Energy consumption in EHD drying is discernibly lower than that in convective drying, yet not as small as that calculated from applied voltage and current of EHD generator. 4. Because of various hypotheses regarding the mechanism of water removal due to direct hydrodynamic impact of electric wind and possibly turbulent vortex phenomenon, as well as other non-thermal effects, such as lowering of entropy, this technology requires further studies toward optimization and careful engineering design for industrial applications. NOMENCLATURE E electric field strength, V=m u superficial velocity, m=s dimensionless number, NEHD db dry basis Indices e

electric

Greek letters a semi-vertical cone angle, o Eo permittivity of free space, F=m q density, kg=m3

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