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Due to its non-thermal nature, electrohydrodynamic (EHD) drying of foods is considered extremely energy-efficient electrotechnology. Reported experimental results show that energy consumption in the EHD drying depends on multiple factors such as the applied voltage and discharge current, design and geometry of electrodes, environmental conditions, and drying material properties. Unfortunately, the systematic overview of experimental findings related to energy consumption and energy efficiency of EHD drying is still missing. This current review is focused on the energy aspects of EHD drying of foods to provide a roadmap for industrial-scale applications. The challenges associated with the industrial scaling of this novel technology and future directions of research are identified and discussed.

Electrohydrodynamic (EHD) drying is a novel technology, which appears to be beneficial for drying of heat-sensitive biomaterials with high moisture contents. Although numerous publications on EHD drying of fruits, vegetables, and other plant-based foods report high energy efficiency and superior product quality, the technology is still not commercialized. One of the reasons is incomplete information in published studies on the design of EHD dryers, including electrode geometry, dried materials, and experimental conditions, which hinder the comparison of experimental findings. Another reason is the gap in the knowledge about the actual mechanisms behind the EHD drying, including convective moisture removal by airflow and diffusive moisture transport inside the food. In this review, key findings in published reports on plant-based foods and food model systems have been critically analyzed and generalized using the same uniform metrics. The limiting factors, most favorable conditions, and future perspectives of EHD drying for plant-based foods are discussed.

The theoretical background for electrohydrodynamic (EHD) drying, linking mass transfer to electric field strength and charge transfer, has been proposed. The relationship between drying rat...

This paper presents the results of experimental study and mathematical simulation of multi-pin electrohydrodynamic dryer as a function of emitters’ density and a gap between discharge and c...

Due to the inherent hydrodynamics of Impinging stream dryers the heat transfer rates from gas to particle depend on time as well as space. Drying occurs by combined convective and conductive heat transfer. Also, drying with simultaneous grinding can be performed if metallic beads are used as the inert particles. Drying in impinging streams is purely convective. Thus, typically both constant and falling drying rate periods can be observed if the processing materials have both internal and external resistances to mass flow. To prevent falling of the sprayed droplets onto the dryer wall, supplementary airstreams are introduced tangentially through the dryer wall. At a certain limiting ratio of the gas-to-particle terminal velocity, drying with simultaneous disintegration may actually result in drying with simultaneous agglomeration when processing some organic materials such as bone-meat extract.

On behalf of the members of the Faculty of Process and Environmental Engineering, Lodz University of Technology in Poland, Chemical Engineering Committee of the Polish Academy of Sciences and mysel...

A theoretical approach to quantify the electric field intensity, space charge density and mass transport in a multiple pin-plate electrohydrodynamic dryer has been developed. It was found that: (i) the corona onset voltage depends largely on the electrode geometry, decreasing for smaller gaps and larger spacing between pins, (ii) the charge flow rate (convection current) depends on voltage, gap and electrode geometry, and (iii) airflow mass flux generated by multiple-pin electrode is proportional to the voltage and square root of electric current. The theoretical model of electrically-induced airflow predicts velocity profile and occurrence of aerodynamic vortices in a single channel of flowing air. The model has been experimentally validated in drying experiments with wet paper tissue. Drying with 2 × 2 cm electrode showed good correlation between model-predicted air mass flux and water transport from the wet material. Smaller spacing between pins (1 × 1 cm) significantly affected charge distribution, aerodynamics of airflow and mass transfer.

Electrohydrodynamic (EHD) drying is considered as energy efficient non-thermal technology suitable for dewatering of heat-sensitive materials, including food products. A factorial experimental design was used to identify significant factors affecting mass transfer in EHD drying. The experiment revealed significant effects of voltage, distance (gap) between electrodes, configuration of discharge electrode, air cross-flow and the characteristics of material surface on mass transfer. Strong coupling between mass and charge transfer was found for all voltages, gaps and configurations of discharge electrode. The effect of air cross-flow at 1.0 m/s on mass transfer was additive to the effect of ionic wind, consistently increasing total mass transfer by 5.0–5.1 g/h in all experimental conditions. These results led to the conclusion that the effect of EHD is convective in nature, enhancing mass transfer due to ionic wind. The effect of material surface characteristics was attributed to different hydrodynamic conditions of air boundary layer. Industrial relevance This research is focused on better understanding of the factors that play a significant role in EHD drying, and therefore it is important for industry to facilitate practical applications of EHD in bioprocessing and food engineering.

Electrohydrodynamic (EHD) drying is considered as energy-efficient nonthermal technology suitable for dewatering of heat-sensitive materials. This technology relies on the ionic discharge from the vertical pin or fine horizontal wire, impinging wet material deposited on the plate electrode of opposite polarity. The critical issue for the scaling of EHD dryer is the geometry of a multipin/wire discharge and collecting electrodes, in particular the spacing between pins/wires and the gap between discharge electrode and material surface. This paper presents the results of experimental study and mathematical simulation of multipin discharge current to maximize total charge and mass transfer at the material surface. A mathematical model for discharge current based on Poisson’s and Warburg fundamental equations was developed and validated in experiments with multipin electrodes of different spacing (1, 2, 3, 4, and 6 cm) and gaps from 2 to 4 cm. It was demonstrated that linear relationship between total ...

Background Electrohydrodynamic (EHD) drying is non-thermal and energy efficient technology, which appears to be a viable alternative to thermal drying. However, transport phenomena in EHD are not well understood, which limits practical applications of EHD for drying of biomaterials. Scope and approach This paper provides a comprehensive review of the fundamentals of electrically-induced physical processes relevant to the theory of EHD drying with particular emphasis on yet existing gaps in the knowledge. Key findings and conclusions Essential phenomena pertinent to high-voltage electric field and electric wind were examined in terms of heat and mass transfer at the gas-solid interface. The gaps in the knowledge caused by simplifying assumptions in theoretical and experimental studies, as well as omitting some crucial electrically-induced transport phenomena were identified. These include: (i) quantifying energy balance in EHD drying, (ii) uncovering electrically-induced mechanisms of mass transfer in solid, gas and gas-solid boundary layer; (iii) identifying interactions and possibly synergetic effects between electric charge and heat/mass transfer processes; (iv) determining whether average electric wind velocity adequately represents spatial distribution of wind velocity in non-uniform electric field, (v) explaining of some experimental findings, such as temperature drop at the liquid-gas interface, distortion of the free water surface and tangential component of electric wind at the material plane.

Combustion of fuels provides more than 90% of the worlds׳ primary energy demand at the costs of negative environmental impacts. In the recent years, substantial research has been carried out to explore clean and efficient combustion technologies to meet the ever-increasing demand for sustainable energy production using both fossil and renewable fuels at stringent environmental standards. In relation to advanced combustion technologies such as moderate or intense low-oxygen dilution (MILD), pulse combustion is one of the most promising techniques to burn diversified fuels ranging from high-grade natural gas and propane to low-grade fuels such as biogas, syngas or biodiesel high combustion efficiencies and minimal pollutant emissions (e.g., SO x , NO x ). In spite of growing interest in pulse combustion technology, its state-of-the art is largely missing in the literature. The aim of this paper is to provide a comprehensive yet concise review of pulse combustion regarding the phenomenon and background of such combustion, modeling, experimental findings and industrial applications. First, the basics of pulse combustion are presented which include pulse combustor classification and types, operation principles, and advantages and disadvantages. In view of sustainability, pulse combustion characteristics with different fuel (e.g., biogas) are fully elaborated. Then, pulse combustion models described in topical literature are classified into different types and thoroughly examined. Next, experimental and numerical studies performed by numerous researchers are summarized, and the applications of pulse combustion in different fields, particularly in drying are presented, and its great potential to promote the utilization of renewable fuels is analyzed. Finally, the barriers, challenges, and R&D for pulse combustion are identified and a constructive discussion on these issues is provided. Key publications for the readers seeking broader and in-depth information are also given.

Effects of convective cross-flow of air in electrohydrodynamic (EHD) drying on drying rate of 5 mm slices of champignons have been investigated. Electro-convection issued from discharge electrode (42 needles arranged into 6×7 rows with 2×2 cm spacing, 18 kV DC voltage and 3.5 cm gap) provided average ionic wind velocity of 1.0 m/s flowing perpendicularly to the surface of champignons slices, while forced air stream at atmospheric pressure 1000 kPa, superficial velocity 1.0 m/s, temperature 22-24°C, and relative humidity 25-40%, was blown parallel to the surface of champignons slices. To study interactions between forced air cross-flow and electro-convection, the experimental protocol was designed, exploring three cases in various combinations: (1) Sole EHD, (2) air cross-flow, and (3) EHD with simultaneous air cross-flow. The case # 3 was found to be the most efficient, resulting in 10.2 g/h of water evaporation whereas drying rate was 6.6 g/h (# 1) and 3.6 g/h for (# 2). Such numbers imply that these effects are additive. In some combinations the effect of air cross-flow was the same (3.6 g/g), but electro-convection was significantly suppressed to 3.2 g/h likely because air stream removed surface water, which reduced charge transfer and electro-diffusion.In trials with different initial moisture content it was found that drying kinetics followed exponential decay in the wide range of initial moisture contents from 4.9 to 12.0 g/g (db). Drying rate due to forced air convection was found to be independent of moisture content, whereas drying rate due to electro-convection significantly depended on the moisture content. For example, the EHD drying rate of fresh-cut champignons slices with initial moisture content 10.74 g/g was 0.237 g/h, while the slices after two days in the cooler (initial moisture content dropped to 4.92 g/g) it was 0.418 g/h. Also, it was found that electro-convective drying could not remove all residual water. At the end of drying the equilibrium moisture content attained 0.2 - 0.3 g/g (aw~0.3).It appears that performance of EHD drying depends also on the product porosity as water can exist as free in open pores or be trapped in closed pores. In some experiments we observed rotation of champignon slices in the plane perpendicular to ionic wind. It happened at the end of drying when slices were light enough to be lifted by electrostatic force and dragged by the vortex. This phenomenon could be attributed either to the effect of DC electric field on polarized water molecules trapped in closed pores, or it could be electrostatic effect of ionic wind on charged porous body. Also, the hypothesis that EHD has both linear and rotational (vortex) components require further investigation.

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.

Following background to the phenomenon of electrohydrodynamics with concise review of basic features like shorter drying time, lower energy consumption and better product quality, the selected key factors affecting EHD drying are examined. These include the geometry of discharge electrodes, effects of air humidity on drying rate, depression of material temperature, and cooling effect of ionic wind. Examples are given for: (i) prototype EHD dryers of multi-belt types, and (ii) pilot-scale multi-belt EHD dryer in vertical arrangement that can be aggregated into one unit of higher capacity, and vertical cylindrical EHD dryer with vibrated shelves. Keywords: ionic wind; corona discharge; drying; energy; quality

Implementation of a lignin-based biorefinery into one of the existing kraft pulp mills calls for increased consumption of resources such as steam (by up to 21.5%), water (by up to 3%), carbon dioxide (by up to 16.2%), and sulphuric acid (by up to 11.3%). To compensate for these extra demands on resources, an advanced process integration method was used to identify steam, water, and chemicals savings options and resource recovery opportunities within the kraft process. Given the importance of the lignin-based biorefinery, an economic viability assessment was carried out toward four scenarios, namely: a reference case relating to a stand-alone kraft pulp mill without a pulp production increase but with/without advanced process integration (scenarios #1 and #2) as well as to an integrated biorefinery with a pulp production increase by 5, 10 and 15% (scenarios #3 and #4).

Aside from special dewatering techniques such as mechanical pressing, centrifuge, permeation, osmosis, or sorption processes, conventional drying of wet materials by hot air, either used alone or i...

Following background information on energy efficiency measures, this paper presents the methodology and calculation results for energy performance of several industrial dryers quantified in terms of the specific energy consumption and compared to the results obtained from the Baker and McKenzie's adiabatic dryer model for convective dryers. Examples of performance assessment are given for an indirectly heated spouted bed dryer with inert particles and a spray dryer with integrated fluidized bed. Because the energy performance determination is based on temperature and humidity of the ambient and exhaust air, the calculation method is also given for gas-fired direct dryers represented by a natural-gas heated pneumatic dryer for which combustion air and generated water vapor have to be accounted.

Background information on process intensification including the rationale, basic methods in process engineering, and interactions between material streams and processing methods is given. The general rules are then examined with respect to heat and mass transfer intensification, and pertinent to drying techniques and technologies (e.g., microwave drying, pulse combustion drying, intermittent drying, drying by alternation pressure, etc.) are highlighted. Milk processing for powdery product and bakery production is given as an example of process intensification with respect to water evaporation.

The use of a hydrocyclone is proposed to concentrate the liquid feed such as slurry or diluted suspension prior to thermal drying. The hydrocyclone of special design, called a CANMET hydrocyclone, allows partial separation of three-phase systems such as fine solids suspended in an oil–water emulsion. Thus, when combined with a dryer, preconcentration of the solid–liquid suspension in a hydrocyclone makes it possible to reduce thermal load to the dryer and therefore to lower energy consumption for water evaporation, which translates into reduced emissions of greenhouse gases. This article presents the design characteristics and operational features of the CANMET hydrocyclone and evaluates drying energy savings in a hybrid system built on this hydrocyclone and a convective dryer.

An algorithm to examine the energy performance of convective dryers was developed and transformed into an Excel-based calculation tool. Provided with the input data for a given industrial dryer, this tool allows the energy use to be quantified in terms of the specific energy consumption and energy efficiency. The energy use can then be compared with the corresponding values for an ideal adiabatic dryer to identify the potential for energy savings. The algorithm accounts for direct and indirect dryers as the single- and multistage units operated in closed or open cycles. In addition, the maximum energy efficiency can be determined for nonhygroscopic materials through the sorption isotherms. The tool permits the identification of the major sources of dryer inefficiency and allows energy savings to be calculated for several low- and medium-cost measures such as dryer insulation, partial recycling of exhaust air, feed preheating, and others. The tool is built as a modular system comprising the following main ...

Using available correlations for heat transfer, a comparative analysis of drying rates in CO2 and in air was performed for several basic types of dryers. Higher heat transfer rates were found for dryers with active hydrodynamics, which translates into shorter drying time for materials dried in the first drying period. These results were validated by experiments on drying wheat kernels fluidized by air and by CO2. Shorter drying times by about 20% were confirmed for CO2, which offers energy savings of about 3% of the heat input to the dryer. Additional energy savings of 4% of the heat load can be expected for drying at temperatures below 100°C because of the lower wet-bulb temperature for CO2 than that for air. The potential for CO2 abatement was evaluated based on a case study for drying of distillers' spent grain.

Blueberries and cranberries were dried using a hybrid technology consisting of pretreatment, osmotic dehydration, and convective drying. The performance of this hybrid technology was evaluated in terms of quality attributes such as color, taste, rehydration ratio, water activity, as well as sugar and anthocyanins content. Of thirteen different pretreatment methods tested for cranberries, cutting the berries in halves was the simplest and the best one. No pretreatment was needed for blueberries. Osmotic dehydration was performed in batch and continuous modes using sugar solution of 67.5–70°Brix. Convective drying was then carried out in a fluidized bed, a pulsed fluidized bed, or a vibrated fluidized bed. Quality profiles for both berries were determined after each step of such a hybrid technology and compared with quality of berries dried in the vibrated fluidized bed and freeze dryer. Recommendations for full-scale processing are given.

The energy performance of a dryer and a drying process is characterized by various indices such as volumetric evaporation rate, steam consumption, unit heat consumption and energy (thermal) efficiency. Of all indices, energy efficiency is the most frequently quoted, in technical specifications. A thorough analysis of available information, including the Handbook of Industrial Drying, points to the inconsistency of terminology, definition and data interpretation. Thus, reported data on energy efficiency vary significantly and frequently contradict both drying theory and industrial practice. To establish a common platform to deal with energy issues, this article provides a concise overview of the most common definitions of energy efficiency, along with a critical review of the published data. A need for energy audit and benchmarking is pointed out. To eliminate shortcomings of the energy efficiency as a lumped parameter, and to allow analysis of energy consumption over time (batch drying) or distan...

A device has been developed, to characterize the stickiness of materials prone to shearing that is based on torque measurement during intermittent stirring. Experiments were carried out, using materials of various origin, and over a range of moisture contents that span the pasty to free-flowing powder consistencies. To reflect the complex nature of stickiness, often noted by researchers during the drying of slurries and pastes, the concept is proposed, of a stickiness region, adjacent to the stickiness curve. Referring to the two-dimensional visualization of a drying process with evolving moisture content and material temperature, possible trajectories are suggested, to deal with the stickiness problem. #The copyright is held by © Her Majesty the Queen in Right of Canada, as represented by the Minister of Natural Resources, 2003.

An original method was proposed for the determination of the mean residence time in a continuous dryer, based on the step-change in the solids feed rate. The method has been validated through experiments performed in a pilot-scale rotary dryer. The effect of the solids flow rate, gas flow rate, dryer rotation speed, and dryer slope was quantified. Several design correlations to predict the residence time in rotary dryers were critically evaluated, and a new, more accurate correlation was derived. #The copyright is held by © Her Majesty The Queen in Right of Canada, represented by The Minister of Natural Resources, 2003.

The copyright is held by © Her Majesty the Queen in Right of Canada, as represented by the Minister of Natural Resources, 2002.

This paper provides a comprehensive overview of pulse combustion, as applied to industrial drying. A concise description is given, of the pulse combustion process and its basic features. Pulse combustion experimental data on drying kinetics for solid materials are compared to those of continuous combustion, and interpreted in terms of water bonding and internal/external drying conditions. Two drying mechanisms are postulated, based on data from the literature and from the authors' experiments in the drying of solids and dispersed liquids. Experimental (LDA analysis) and theoretical (CFD modeling) approaches are presented, to describe the flow field generated by the pulse combustor in the drying chamber. Environmental aspects (acoustic noise and pollutant emissions) are discussed. Finally, the issues of user-acceptance and reliability of pulse combustion drying are raised, in regards to noise level and combustor-dryer interaction.

Drying of raw hog manure, a highly diluted suspension of organic and inorganic matter was studied experimentally in the multistage screw-in-trough dryer. Laboratory tests included material characteristics (rheological properties, thermodynamic equilibrium, critical settling point), boiling pattern, and kinetics of convective drying at temperatures from 90 to 120°C and air velocity from 0.5 to 1.5 m/s. Three phases of manure drying were identified, namely boiling and release of volatile compounds, evaporation from viscous liquid, followed by drying of a soft paste, and finish drying of semi-dry granular product. Drying kinetics were quantified in terms of reduced moisture content and a generalized drying curve using the characteristic drying time concept. Field tests were carried out in the pilot multistage screw-in-trough dryer to validate design calculations. ©Her Majesty the Queen in Right of Canada, represented by the Minister of Natural Resources, 2002.

Hydrodynamics and drying kinetics for the pulp and paper primary sludge dried in a pulsed fluid bed dryer with relocated air stream are presented. Batch experiments have indicated that drying of disintegrated sludge to the required 12% moisture content takes place during the first drying period at practically constant material temperature close to the wet bulb temperature with respect to the inlet air conditions. Equations were developed for pressure drop, minimum pulsed-fluidization velocity, dynamic bed height, and volumetric mass transfer coefficient. Continuous experiments under drying conditions determined from the average residence time concept have confirmed that transportation of disintegrated sludge along the dryer follows the plug flow model. ©Her Majesty the Queen in Right of Canada, represented by the Minister of Natural Resources, 2002.

This study reports on finish drying of osmotically pretreated (dehydrated and sugar-infused) cranberries. The halved fresh berries pretreated in a standard osmotic solution (67.5 Brix at 50°C for 5 h) were then freeze-dried, vacuum-dried and air-dried in various dryers (cabinet-air-through, fluid bed, pulsed fluid bed, and vibrated fluid bed dryers) in order to identify the best drying technology. Energy consumption and product quality were chosen as the comparison criteria. The best product quality, quantified by the anthocyanins content, rehydration ratio, color and taste, was noted for freeze-dried berries. As all other drying methods gave similar, albeit slightly lower quality products than freeze-drying, the selection of a drying method could be based on the unit heat consumption. The vibrated fluid bed and the pulsed fluid bed should be favored because of the highest energy efficiency. Even though sugar infused into cranberries during osmotic pretreatment reduces drying rates during the sec...

A great progress has been done in computer software and hardware, which obviously broaden the application of computers to drying technologies. This paper provides the state-of-the art in computer-aided tools for drying environment. The following applications of computers are identified and discussed in depth: mathematical modeling and simulation for process calculation and equipment design (microscale approach– at the level of a single drop or particle, and macroscale approach– at the level of a dryer or drying system); process control; modeling tools; software packages; artificial intelligence systems; databases and information transfer.

Drying of recycled polypropylene powder was studied experimentally in a laboratory pulsed fluid bed dryer (PFB) with relocated air stream. It was proved that fluidization of fine particles having a large specific surface area and a broad size distribution is technically feasible when using the composite supporting grid. Drying and hydrodynamic characteristics for a pulsed fluid bed of fine particles are found to be similar to the ones for coarse particles. Equations for minimum pulsed fluidization velocity, pressure drop, and surface and volumetric heat transfer coefficients are given.