Superheated Steam Drying Research Articles

A comparative study on the effect of superheated steam and hot air drying on microstructure of distillers' spent grain pellets using X-ray micro-computed tomography

The spatially correlated microstructure of a material during any processing operation such as drying is very important as it influences the thermo-physical properties of the material. The heterogeneity of the distillers' spent grain (DSG) pellets' microstructure was evaluated using the X-ray micro-computed tomography. The effect of two drying methods, i.e. hot air and superheated steam (SS) drying on the microstructure of this porous material was compared. The DSG pellets were prepared by mixing different amounts of distillers' solubles (10, 30, and 50% w/w) with the coarse grain fraction of the spent grains. The effect of solubles on the microstructure (total porosity, open porosity, closed porosity, and connectivity) and stability (dimensional/volumetric changes and change in density) of the pellet prior to and post drying (hot air and SS drying) was analyzed statistically. The results showed that SS drying caused an overall expansion of the pellet in the range of 90–133%. The increase in the open porosity of the pellet during SS drying enhanced the drying process resulting in a decrease in the drying time by about 81% when compared to hot air drying. Also, by increasing the concentration of solubles above 30% w/w, the dimensional stability of pellets improved by 50% for both drying methods thereby improving the mechanical strength of pellets. These results are crucial in optimizing the composition of pellets from the point of view of the pellet's dimensional stability and drying efficiency.

Modeling of effective moisture diffusivity and activation energy of distillers' spent grain pellets with solubles during superheated steam drying

Drying is an essential unit operation needed for safe storage and handling of the wet Distillers' spent grain (DSG), a major by-product of the ethanol industry. For the simulation and modeling of the drying process, a detailed study on different pre-requisite parameters such as the thermo-physical properties and effective moisture diffusivity of the material to be dried is required. The present study reports the effective moisture diffusivity and activation energy of the DSG pellets during superheated steam (SS) drying. Cylindrical DSG pellets at two moisture mass fractions (25 and 35%) and three mass fractions of distillers' solubles (0, 10, and 30%) were dried at three SS temperatures (120, 135, 150, 165, and 180 °C) and three SS velocities (0.5, 1.0, and 1.5 m/s), respectively. The experiment-based moisture diffusivity of the DSG pellet with and without solubles was determined by using the drying characteristic and the analytical solution of Fick's law of diffusion. The results showed that the effective diffusivity increased with an increase in SS temperature and velocity and its value was in the range of 2.49 × 10−9 to 17.9 × 10−9 m2/s. The dependency of the moisture diffusivity on temperature and moisture mass fraction was established by using Arrhenius equation and Levenberg-Marquardt optimization algorithm. The model coefficient and constants were compared with the calculated values of instantaneous effective moisture diffusivity with the mean relative percentage deviation ≤10%. These findings could serve as a fundamental input for the numerical modeling of SS drying of DSG.

A concept and industrial testing of a superheated steam rotary dryer demonstrator: Cocurrent-triple pass design

ABSTRACTThis study aims to provide a pertinent demonstration of the feasibility of superheated steam drying (SHSD) at industrial demonstrator scale. The dryer is of co-current, direct contact, and triple pass rotary type. The dryer is designed to operate under pressure (up 2 bar abs.) and at high temperatures (up 600°C). Given the operating conditions of pressure and temperature, the control and the instrumentation of the dryer were the subjects of a special study to ensure safe tests and measurements. The demonstrator was manufactured and set-up on an industrial site. Drying experiments on pressed beet pulp are being carried out. At the end of this study, we hope to show first results to demonstrate the drying feasibility and also to provide relevant data concerning the operation of SHSD.

Continuum-scale modeling of superheated steam drying of cellular plant porous media

In this work, a continuous model is developed to describe the dynamics of heat and mass transfer in cellular plant porous media during the superheated steam drying process at atmospheric pressure. This model accounts for the advective liquid and vapor flows in the intercellular void space as well as for the diffusive liquid flow across the solid cell membranes of the porous medium. The numerical results are verified against drying experiments for potato samples, which were carried out by a magnetic suspension balance at three different superheated steam temperatures (160 °C, 180 °C, 200 °C). A comparison between the simulation results and the measured data shows that the drying characteristics of a plant porous medium can fairly be predicted by using the continuous model developed herein. The influence of the cell membrane water conductivity on the spatio-temporal distribution of the moisture content and of the temperature within the porous medium is studied by numerical simulations. It is observed that the water diffusion across the cell membranes controls the dynamics of the heat and mass transfer in the porous medium, and thus the drying kinetics.

Renewed sanitation technology: A highly efficient faecal-sludge gasification–solid oxide fuel cell power plant

Sustainable development goals for 2030 aim at the extensive reduction of the global sanitation breach; this might be achieved by renewed sanitation technologies and while providing sanitation recover valuable products such as energy. Consequently, this work presents a gasification–solid oxide fuel cell (SOFC) power plant that was configured for high-efficiency energy recovery from faecal sludge. The main limitations of faecal sludge gasification are the production of impurities, such as tar, and the high energy requirements for both the endothermic gasification process and removing the high moisture content in the feedstock. However, results from this work indicate that a superheated steam dryer combined with an indirectly heated multistage gasifier and a gas-cleaning unit can overcome the mentioned limitations. The external heat for the gasifier is supplied by the process heat available and a microwave plasma torch, and there is sufficient heat to drive a micro steam turbine. Thermodynamic calculations indicated that the plant could reach a net electrical efficiency of the order of 65%. As a result, a gasification–SOFC power plant is more suitable for energy recovery than any other process such as biochar production by pyrolysis; hence, it might become a technology that is financially feasible and can be used globally for sanitation purposes.

Integrated system of rice production and electricity generation

Research and development of approaches to improve energy efficiency in the rice industry can help stakeholders to make informed decisions. In this study, an enhanced integrated system of both rice production and power generation was proposed. The integrated system mainly consisted of superheated steam drying, husking, polishing, torrefaction, steam gasification, and power generation. In addition, suitable technology options for power generation and rice production processes for increasing the energy efficiency were also investigated. Furthermore, to contribute to minimization of the exergy loss, recovery was performed by combining the concept of heat circulation and process integration. Results show a considerably higher energy efficiency of the proposed integrated system. In a single rice production system, processing of 200 t rice grain d−1 can generate surplus electricity of about 3.4 MW with an electricity production efficiency of about 32%. A high economic benefit could be achieved by synergetic integration in the rice industry.

Superheated steam drying of single wood particles: A characteristic drying curve model deduced from continuum model simulations and assessed by experiments

ABSTRACTComputational fluid dynamics (CFD) and CFD coupled with discrete element method (DEM) are powerful approaches to describe the drying behavior of real drying towers. In these approaches, the heat and mass interaction between fluid phase and the wet solid is required as an essential input. In this work, a new methodology for establishing simple drying model of single wood particles is presented. First, a spatially resolved continuum-scale model that describes the coupled heat and mass transfer within a single porous wood particle during superheated steam drying under atmospheric pressure is developed. The thermophysical properties of wood particle required as input data for the continuum model are determined experimentally. Then sophisticated continuum model is reduced to a simpler lumped model, which is referred to as the characteristic drying curve (CDC) model. The continuum model simulations are performed in a board range of operating drying conditions and the associated results are used to establish correlations for the CDC model parameters. The results of both sophisticated and reduced models are validated against the experimental observations made using a magnetic suspension balance. The sensitivity analysis performed with the continuum model indicates a strong dependency of the critical moisture content on particle size and drying condition. The established CDC model can be implemented in the CFD or CFD-DEM model of superheated steam dryers.

Numerical analysis of single and multiple particles of Belchatow lignite dried in superheated steam

Low production costs have contributed to the important role of lignite in the energy mixes of numerous countries worldwide. High moisture content, though, diminishes the applicability of lignite in power generation. Superheated steam drying is a prospective method of raising the calorific value of this fuel. This study describes the numerical model of superheated steam drying of lignite from the Belchatow mine in Poland in two aspects: single and multi-particle. The experimental investigation preceded the numerical analysis and provided the necessary data for the preparation and verification of the model. Spheres of 2.5 to 30 mm in diameter were exposed to the drying medium at the temperature range of 110 to 170 °C. The drying kinetics were described in the form of moisture content, drying rate and temperature profile curves against time. Basic coal properties, such as density or specific heat, as well as the mechanisms of heat and mass transfer in the particular stages of the process laid the foundations for the model construction. The model illustrated the drying behavior of a single particle in the entire range of steam temperature as well as the sample diameter. Furthermore, the numerical analyses of coal batches containing particles of various sizes were conducted to reflect the operating conditions of the dryer. They were followed by deliberation on the calorific value improvement achieved by drying, in terms of coal ingredients, power plant efficiency and dryer input composition. The initial period of drying was found crucial for upgrading the quality of coal. The accuracy of the model is capable of further improvement regarding the process parameters.

Newly designed superheated steam dryer bearing heat recovery unit: Analysis of energy efficiency and kinetics of Kelp drying

ABSTRACTIn this work, a newly designed superheated steam dryer (SSD) bearing exhaust heat recovery unit (RD) was fabricated. The dryer was designed for the superheated steam (SS) to be generated by foodstuff being dried, and the latent heat of the exhaust SS was recovered by microtube radiators and exploited to heat the RD unit. The system was then applied in Kelp drying, in which the drying kinetics was analyzed and fitted with mathematical models. The energy efficiency by the system was further evaluated and compared with hot air (HA)-drying. The results showed that the Kelp drying process in SSD could be described by two stages: heat-upstage (stage I) and superheated steam stage (stage II). While in stage I, different heat-up times of 10, 15, and 20 min were required to generate superheated steam at temperatures of 110–150°C, in stage II, the moisture content in Kelp was decreased to approximately 50 ± 5% (wet basis) within 60, 50, and 30 min at 110, 130, and 150°C, respectively. Moreover, the Midilli and Kucuk model best described Kelp drying curve in both stages, whereas the logarithmic model best fitted with that in stage II. Finally, the energy efficiency for SSD-RD was in the range of 1.127–1.425 kWh/kgwater compared with 2.406–2.508 kWh/kgwater for HA operating under the same conditions, demonstrating that the SSD-RD was able to reduce the energy input by at least 46.14%.

Investigation of drying characteristics in superheated steam drying of UF-impregnated Chinese fir

Urea formaldehyde (UF) resin-impregnated Chinese fir (Cunninghamia lanceolata (Lamb.)Hook.) was dried at different temperatures in an atmospheric pressure superheated steam dryer. Drying characteristics, moisture content, drying rate, temperature profile, drying defects, and color change were investigated. The moisture content was reduced from 66.21 to 11.79% within 30 h without causing severe drying defects; in contrast, the conventional hot air process required 7–8 days. After 25 h of drying, the temperatures at both the center and the surface of wood remained stable. After 34.5 h, the surface temperature gradually approached the steam temperature. The color of the superheated steam dried Chinese fir appeared slightly more intense yellow and red than the control. Investigation of the UF-impregnated Chinese fir wood by scanning electron microscopy (SEM) revealed that the majority of the lumens and voids, including the microvoids in wood structure, was filled with urea formaldehyde resin.

Superheated Steam Drying of Cashew Kernels with Testa

Thermal processing of cashew kernels is determined by removal of moisture, and therefore, an indication of the product quality and the entire process economics. This paper reported an improved design of superheated steam drying system for cashew kernels with the objective of determining the drying characteristics and the effect of superheated steam on product quality. The range of drying steam temperature at atmospheric pressure was 120- 140ºC with velocity varied from 1-3 m/s. The total colour difference and browning index were used to examine the change in colour of dried kernels. The results showed that higher temperature and velocity of steam promoted higher drying rate and lower final moisture content of the product. The high temperature of steam was found to be the major cause of colour degradation of dried kernel besides steam velocity.

Quality Evaluation of Onion Slices Dried Using Low Pressure Superheated Steam and Vacuum Drying

The effect of combined low pressure superheated steam drying (LPSSD) followed by vacuum drying (VD) on quality of onion slices at different temperature (60-80 o C~10 kPa) was determined. In the first stage, the moisture content of onion slices was reduced to 50 % (w.b.) from 88-90 % (w.b.) using LPSSD; and then it was further reduced up to 7-8 % (w.b.) by VD. It was found that among the various drying temperatures studied, retention of colour, rehydration ratio, thio-sulphinate content and antioxidant capacity of dehydrated onion was better at 60 o C, followed by 70 o C, whereas total phenol contents were higher at 80 o C. The results of control dried sample and sample dried at 60 o C had significant differences in time taken for drying, thio-sulfinate content, total phenol content and antioxidant capacity, whereas insignificant effects of drying temperature was observed on rehydration and water activity of dried onion slices.

Drying of soy sauce residue in superheated steam at atmospheric pressure

ABSTRACTSoy sauce residue needs drying to avoid fermentation and oxidation during storage and transportation, and its reutilization as a useful resource is expected. Superheated steam drying was applied to investigate the effects of drying conditions on the drying characteristics and the content changes of salt and protein. The results showed that the inversion temperature was about 130°C, beyond which superheated steam drying was faster than hot air-drying. The drying time approaching equilibrium moisture content was reduced with elevated drying temperature as well as higher steam mass flow rate in the present experimental conditions. The effect of bed thickness on drying time was not obvious when drying temperature increased. Interestingly, the salt content of soy sauce residue could be decreased by 34.8% due to condensate water in the initial drying period (wetting), while protein content had no significant loss (p < 0.05) after superheated steam drying.

Discrete pore network modeling of superheated steam drying

ABSTRACTA nonisothermal two-dimensional pore network model is developed to describe the superheated steam drying of a capillary porous medium. The complex void space is approximated by a network of spherical pores interconnected by cylindrical throats. In this model, the condensation of water vapor at the network surface as well as the network drying are taken into account. During the network drying period, the liquid transport is driven by capillary action, whereas vapor transport occurs because of convection. The condensation of water vapor within the pores is modeled based on newly formulated liquid invasion rules. The simulation results, presented as temperature and moisture content profiles over time, indicate qualitative agreement with available experimental observations. The inclusion of the liquid invasion rules is shown to accommodate more of the condensed water mass compared to earlier models, in which condensation is only partly treated. Due to the viscous vapor flow, the vapor overpressure within the network, which is the driving force of vapor transport, is reproduced in these simulations. The influence of vapor overpressure on the disintegration of the liquid phase is also discussed.

Three-dimensional CFD modelling of superheated steam drying of a single distillers’ spent grain pellet

The classic method of modelling drying processes using the transfer coefficients can be replaced by a method of solving Reynolds-Averaged Navier-Stokes equations applicable to the drying medium. This specifically applies to superheated steam (SS) as the drying medium, where the mass transfer coefficient cannot be defined by heat transfer analogy. In this study, the heat and mass transfer phenomena of SS drying of distillers’ spent grain (DSG) pellets were numerically studied using a commercial Computational Fluid Dynamics (CFD) package by combining the drying models related to the moist cylinder with the model describing the external flow of SS. A three-dimensional (3D) model of the DSG pellet and the drying chamber was created. The governing differential equations for mass and energy balance of the pellet and steam-flow around it were solved using the finite volume method and SIMPLEC algorithm within the CFD package (ANSYS CFX). The validation of the numerical model with experimental observations showed a good agreement with a mean relative percentage error less than or equal to 10%. The obtained mathematical model could serve as a basic tool for optimization and design of large-scale SS dryers for DSG.

Life cycle assessment of superheated steam drying technology as a novel cow manure management method

Common methods of managing dairy manure are directly applying it to the farm field as a fertilizer. For direct application without any type of treatment, the majority of nutrients in the manure run off to the local river and lake during precipitation periods. The algae bloom is one of the environmental outcomes due to eutrophication of the lakes, which may jeopardize the quality of drinking water. In this study, superheated steam drying (SSD) technology is investigated as an alternative manure management method. Rapidly dried cow manure can be used as alternative fuel. Evaluations of energy payback time (EPBT) and life cycle assessment (LCA) of the SSD technology are presented in the SSD scenario and the results are compared with those of the direct field application (FA) of fresh manure and anaerobic digestion (AD). The heat required for the generation of superheated steam in the SSD scenario is provided from combustion of the dry manure to reduce energy costs. The results for the SSD process show 95% and 70% lower eutrophication and global warming potential in comparison to the FA scenario. Acidification potential for SSD turned out to be 35% higher than FA. The comparison of SSD with AD for their EPBT and normalized impacts indicated that the proposed SSD scenario has higher environmental sustainability than AD (70% lower impact), and is likely an economically better choice compared to conventional AD method (87% lower EPBT) for the future investment.

Drying Kinetics and Quality Attributes of White Radish in Low Pressure Superheated Steam

Abstract In this paper, white radish discs were dried in superheated steam at absolute pressure 95 mbar at four levels of drying temperature from 75 °C to 90 °C. The drying kinetics and various quality attributes of white radish were investigated, compared with the results by vacuum drying. By kinetic modeling of drying processes with Fick’s second law, low pressure superheated steam drying (LPSSD) had slightly lower value of effective moisture diffusivity but higher activation energy than vacuum drying. In comparison with vacuum drying, the rehydration capability of dried samples by LPSSD was better due to porous microstructure. Only 25 % or more the total amount of Vitamin C was preserved after drying to the ultimate moisture content, but it was found that some amount of Vitamin C was taken with the exhaust steam and preserved in the condensate.

Comparative study of moisture absorption and dimensional stability of Chinese cedar wood with conventional drying and superheated steam drying

ABSTRACTThe effects of drying methods on equilibrated moisture content (MC) and swelling efficiency of Chinese cedar (Cryptomeria fortunei) wood were studied in this paper. Drying experiments were conducted with conventional (CON) drying and superheated steam (SHS) drying under atmospheric pressure. Specimens were equilibrated at two environment conditions to measure moisture and dimensional changes, and then the moisture excluding efficiency (MEE) and antiswelling efficiency (ASE) were determined. Results showed that the equilibrated MC of artificial-dried wood was lower than control samples (air drying), and the equilibrated MC of wood with SHS drying was lower than that with CON drying, which indicated that MEE was enhanced in SHS drying process. Similar results were found in swelling efficiency and ASE of artificial-dried wood and the control. The mechanism was studied by dynamic mechanical analysis (DMA) and X-ray diffraction analysis (XRD). The DMA results showed that both of relative storage modulus and relative loss modulus were the highest for SHS-dried wood and the lowest for the control samples. As for the crystalline structure assessed by changes of XRD, the results showed that the cellulose crystallinity and crystallite size of Chinese cedar wood with SHS drying were the highest, and control specimens were the lowest. All the analyses showed that Chinese cedar wood with low hygroscopic and high dimensional stability could be gotten through SHS drying process.

Reducing energy requirement for drying of beet-pulp: Simulation of energy integration between superheated steam and air drying systems

ABSTRACTBeet-pulp dehydration in the sugar industry is a highly energy intensive unit operation. Producing 1 kg of dried beet-pulp requires ∼2–3 kg of water to be removed. The cost saving is a real challenge in sugar factory as the current dryers are underperforming (around 3 MJ/kg of water evaporated) because the heat recovery is limited and only a small proportion of dryer exhaust can be reused. The aim of this study was to investigate by simulation the energy efficiency of an independent multistage drying method that combines superheated steam drying (SHSD) and hot air drying (AD). Two case studies awarding the two types of energy coupling (AD then SHSD or SHSD then AD) are presented and compared. An approach of optimization is developed from energy balance. A number of operating parameters of the two drying configurations are investigated using sensitivity analysis. It proves that both cases allow an energy economy around 40% compared to the conventional dryer. Nevertheless, the SHSD-AD plant may present more benefits for a better quality product.

An experimental verification of numerical model on superheated steam drying of Belchatow lignite

Due to low production costs, lignite is an important component of energy mixes of countries in its possession. However, high moisture content undermines its applicability as fuel for power generation. Drying in superheated steam is a prospective method of upgrading quality of lignite. The study aimed to validate the drying model of lignite from Belchatow mine in Poland. The experimental investigation on superheated steam drying of lignite was previously conducted. Spheres of 10 mm in diameter were exposed to the drying medium at the temperature range of 110-170oC. The drying behaviour was described in the form of moisture content, drying rate and temperature profile curves against time. With the application of basic coal properties (e.g. density, water percentage, specific heat) as well as the mechanisms of heat and mass transfer in subsequent stages of the process, the numerical model of drying was constructed. It was tentatively verified with reference to experimental results both in terms of drying parameters and temperature. The model illustrated drying behaviour in the entire range of conditions. Nevertheless, further development of numerical model is desirable regarding accuracy of the process parameters.