Stream Dryer Research Articles

Mathematical Modeling and Design of Parboiled Paddy-Impinging Stream Dryer Using the CFD-DEM Model.

Impinging stream dryers (ISDs) are effective for removing moisture from particulate materials because of the complex multiphase transport of air particles in ISDs. Nowadays, computational techniques are powerful to simulate multiphase flows, including dilute and dense-phase gas-solid flows and hence, the use of a reliable computational model to simulate the phenomena and design a dryer has recently received more attention. In this study, computational fluid dynamics, combined with the discrete element method (CFD-DEM) and falling drying rate model, were used to predict the multiphase transport phenomena of parboiled paddy in a coaxial ISD. The design of an impinging stream pattern for improving residence time in a drying chamber of ISD was also investigated. The results showed that the CFD-DEM, in combination with the falling drying model, could be well-utilized to predict the particle motion behavior and lead to more physically realistic results. The predicted change of moisture content in parboiled paddy was in good agreement with the experimental data for 17 cycles of drying. Although the prediction of mean residence time was lower than the experimental data, the predicted mean residence time was a similar trend to the experimental data. For ISD design, the simulation revealed that the use of two stages of impinging stream region (two streams collide at the top of the drying chamber at the first stage and then the gas particles flow on the incline floor to collide with the other stream at second stage) in a drying chamber could increase the residence time approximately 75% and reduce drying cycle from 17 to 10 cycles when it was considered at the same final moisture content.

Energy Requisite and Drying Capacity of a Superheated Steam Dryer

This research was carried out to determine the energy requisite in terms of the specific thermal energy consumption, the specific electrical energy consumption and the drying capacity of a superheated steam dryer. The superheated stream dryer consists of a boiler housing, steam transfer pipes made of galvanized steel, super-heater made up of two (2) heater bands of 4000W (used to convert saturated steam to superheated steam), superheated chamber made of galvanized steel, the drying chamber made using stainless steel, a PID temperature controller (for regulating the temperature of superheated steam) as well as a water heater of 1500W to raise the temperature of water to saturation temperature. The result observed revealed that, drying capacity ranged from 0.1 to 0.36 kg/h, specific thermal energy consumption ranged from 9.22 to 19.99 kJ/kg and specific electrical energy consumption ranged from 15.41 to 57.17kWh/kg as influenced by temperatures of 160-180 °C, bed depths of 1 to 5 cm and tempering time of 20 and 40 minutes. Results obtained proves that drying paddy rice in a superheated steam dryer in comparison with hot air dryer has lesser energy consumption and saves operational cost.

Evaluation of physical and cooking quality of parboiled germinated brown rice using a coaxial two-impinging stream dryer

The coaxial two-impinging stream dryer was developed and experimentally used for parboiled germinated paddy drying. The physical quality such as moisture content, color and cracking were investigated. In addition, the cooking quality such as texture properties and rehydration were evaluated as well. The drying parameters were conducted for drying temperatures of 50, 60, 70 and 80°C, drying air velocity at 28 m/s and 44.14 ± 14 %d.b. for initial moisture content. The experimental result found that the final moisture content was reduced to about 38.36, 36.15, 31.80 and 30.70 %d.b. for drying temperatures at 50, 60, 70 and 80°C, respectively. The cracking was little occurred, which was less than 2.73 ± 0.27% for all drying conditions. From the investigation of color changes, it is found that the L-, a-, and b- values were negligible changed in the range between 44.537 ± 0.548 to 46.608 ± 0.743, -3.117 ± 0.460 to -3.182 ± 0.345, and 6.174 ± 0.585 to 6.861 ± 0.406, respectively. The values of total color difference change (E) were approximately 45.079 ± 0.530 to 47.220 ± 0.786. The increase of drying air temperature affected the increase of textural properties. Separately, considering the following maximum values of hardness, gumminess, adhesiveness, cohesiveness and chewiness were about 686.15 ± 4.89, 445.50 ± 54.07, -0.070 ± 0.03, 0.625 ± 0.02, 6090.72 ± 743.0, respectively. The rehydration percentages were ranged from 60.333 ± 6.72% to 65.105 ± 6.24% for drying temperature at 80°C. Keywords: Parboiled rice, impinging stream dryer, nutritional quality, cooking quality, texture property

Effects of operating parameters of impinging stream dryer on parboiled rice quality and energy consumption

It is very important to know how operating condition of a coaxial impinging stream dryer affects the drying time, parboiled rice quality and energy consumption. The drying temperature, parboiled paddy feed rate, drying air velocity and impinging distance were experimentally investigated. Increases in drying temperature and air velocity and a decrease in paddy feed rate provided higher evaporation rate whilst impinging distance, ranging from 5 to 13 cm, did not affect the evaporation rate. Collision between kernels within the impinging zone caused the hull’s lemma splitting from the kernel and the percentage of the split kernel strongly depended on the air velocity and feed rate. The acceleration in the rate of drying by changing the above operating parameters did not affect the head rice yield. In addition, the collision of kernels also did not influence the head rice yield since the mechanical properties of rice are strengthened during steaming step in the parboiling process. However, the change of head rice quality was only governed by the moisture content after drying. The total energy consumption including electricity and heat was strongly depended on the air velocity and feed rate whilst it was slightly changed with the drying temperature. From the present study, it was recommended that the parboiled paddy should not be dried below 25% d.b. and the highest temperature that could possibly be used was 190 °C and the inlet air velocity should not be below 15 m/s.

Experimental Investigations of a Pilot Scale Coal Impinging Stream Dryer

ABSTRACT A method for drying hard coals using an impinging stream dryer has been proposed in the article. A series of screening tests were conducted, seeking optimum process parameters of an innovative coal impinging stream dryer using flue gas as the drying medium. The article addresses measurement results obtained for the flue gas and feedstock flow rate, feedstock sort, and initial moisture content on the specific energy consumption, volumetric water evaporation rate, and moisture reduction. The maximum volumetric water evaporation rate was found to be around 230 kgwater/m3·h and the maximum volumetric heat transfer coefficient came to ca. 2.5 kW/m3·°C. In terms of specific energy consumption, the lowest specific energy consumption was found to be around 1.79 kWh/kgwater. It was established that the relation between specific energy consumption and a product of the flue gas flow rate and the initial water (in the feedstock) flow rate is a power function. The best performance of the impinging stream dryer was achieved for high-moisture coal and high flue gas flow rates. The impinging stream dryer proved to be suitable for surface moisture removal from particulate materials that contain very high moisture. The initial tests conducted at the pilot plant successfully demonstrated reliable operation and promising results.

Moisture Content and Physical Properties of High-Moisture Agricultural Material Dried by Impinging Stream Dryer

The objective of this research was to study the drying of high-moisture agricultural material using an impinging stream dryer with multi-stage drying concept. Okara was used in this study as the test agricultural material. Experiments were performed in one-stage and two-stage drying. The final moisture content of the okara at several drying conditions was examined. The physical properties of dried okara were analyzed in terms of color and rehydration ability. It was found from the experiment that with the use of multi-stage drying concept the moisture content of okara could be significantly reduced within a very short period of time. In terms of physical properties, it was observed that the dried okara was darker, redder and more yellow than the fresh okara. Drying at higher temperatures led to higher color changes values of the dried okara. The effect of okara feed rate on the redness and yellowness was not significant, except for the lightness; drying at lower okara feed rates led to a darker dried okara. An increase in the drying temperature and a decrease in the okara feed rate led to a higher rehydration ability of the okara.

Multistage Impinging Stream Drying for Okara

Abstract. In this study, a multistage drying concept (one-stage and two-stage drying) was applied in an attempt to enhance the performance and energy efficiency of an impinging stream dryer for okara, which is a soy residue obtained from soymilk production. The volumetric water evaporation rate, the volumetric heat transfer coefficient and the specific energy consumption of the drying system at various drying conditions were evaluated. For one-stage drying, the volumetric water evaporation rate increased with an increase in the okara feed rate and the drying temperature. The effect of the drying temperature was insignificant at the minimum okara feed rate, however. For two-stage drying, the drying temperature insignificantly affected the volumetric water evaporation rate. However, an increase in the okara feed rate led to an increase in the volumetric water evaporation rate. The effects of the drying temperature and the okara feed rate on the volumetric heat transfer coefficient were similar to those of the volumetric water evaporation rate, for both one-stage and two-stage drying. The maximum volumetric water evaporation rate and the maximum volumetric heat transfer coefficient of 1784 kgwater m-3 h-1 and 3385 W m-3 K-1, respectively, were noted during one-stage drying at the drying temperature of 150°C and the okara feed rate of 45 kgdrysolid h-1. Regarding the energy efficiency of the drying process expressed in terms of the specific energy consumption (SEC), an increase in the drying temperature and the okara feed rate led to lower values of the SEC for one-stage drying. The effect of the okara feed rate on the SEC for two-stage drying was similar to that for one-stage drying. However, the opposite trend was observed for the effect of the drying temperature. When a multistage drying concept was applied to the impinging stream dryer, the percentage of moisture content reduction of the okara were substantially decreased. The results obtained from this study implied that an impinging stream dryer with a multistage drying concept may be used to dry okara or other high-moisture particulate materials. Keywords: Impinging stream dryer, Multistage drying, Okara, Specific energy consumption, Volumetric heat transfer coefficient, Volumetric water evaporation rate.

Simulation of flow and drying characteristics of high-moisture particles in an impinging stream dryer via CFD-DEM

ABSTRACTImpinging stream dryer (ISD) is an alternative for drying high-moisture particulate materials. Due to the complex multiphase transport phenomena that take place within an ISD, use of a reliable computational model instead of a tedious experimental route to aid the design of the dryer is desirable. In the present study, computational fluid dynamics were used in combination with the discrete element method (CFD-DEM) to predict, for the first time, the multiphase transport phenomena within a coaxial ISD; results from a model that does not consider particle-particle interactions (CFD) were also obtained and compared with those from the CFD-DEM model. In all cases, high-moisture particles having negligible internal transport resistance were assumed. Both models were used to simulate the gas-particle motion behavior, particle mean moisture content, particle mean residence time, and particle residence time distribution. The simulated results from both models were compared with the experimental data whenever possible. The results showed that the CFD-DEM model could be utilized to predict the particle motion behavior and led to more physically realistic results than the CFD model. The CFD-DEM model also gave predictions that were in better agreement with the experimental mean particle residence time and moisture content data.

Mathematical Model Based on DSMC Method for Particulate Drying in a Coaxial Impinging Stream Dryer

An impinging stream dryer (ISD) belongs to a unique class of dryers that has proved to be an excellent alternative to flash dryers for removing surface moisture of particulate materials due to the collision of streams and particles in the dryer. However, the performance analysis of such devices, from a viewpoint of mathematical modeling, has not been investigated extensively. In this study, a mathematical model based on the direct simulation Monte Carlo (DSMC) method is proposed to describe the drying process of particulate materials in a coaxial ISD. The collisions between particles and the heat exchange between impacting particles are included in the present mathematical model. The predicted results were in good agreement with the experimental data, which indicates the validity of the present model. The drying process and the effects of various parameters, including the feeding mode and impinging distance, on the drying performance of the dryer were then numerically investigated and discussed.

Rapid Drying of Parboiled Paddy Using Hot Air Impinging Stream Dryer

A method for rapid drying of parboiled paddy via the use of an impinging stream dryer was proposed and assessed. The effects of the drying air temperature, number of drying cycles, as well as time of tempering between each drying cycle on the moisture reduction, head rice yield, and whiteness index of the dried parboiled paddy were studied. The drying experiments were carried out at drying air temperatures of 130, 150, and 170°C; inlet air velocity of 20 m/s; impinging distance of 5 cm; and paddy feed rate of 40 kgdry_paddy/h. Parboiled paddy was dried for up to seven cycles. Between each drying cycle the parboiled paddy was tempered for a period of either 0 (no tempering), 15, 30, 60, or 120 min. After impinging stream drying, paddy was ventilated by ambient air flow until its moisture content reached 16% (db). Moisture reduction of the paddy was noted to depend on both the impinging stream drying temperature and tempering time. Drying at a high temperature along with tempering for a suitable period of time could maintain the head rice yield of the paddy at a level similar to that of the reference parboiled paddy. To avoid discoloration and low head rice yield, parboiled paddy should not be dried at a temperature higher than 150°C and should be tempered for at least 30 min.

Numerical Simulation of Multiphase Transport Phenomena During Impinging Stream Drying of a Particulate Material

The objective of the present study was to employ an existing three-dimensional computational fluid dynamic model to investigate the multiphase transport phenomena in a coaxial impinging stream dryer. The continuous-phase equations were solved in the Eulerian frame, while the discrete-phase equations were solved in the Lagrangian frame. Two-way coupling between the continuous and dispersed phases was taken into account in the model. Standard k-ϵ and realizable k-ϵ turbulence models were compared in order to decide which model could better represent the turbulent behavior of the flow in the ISD, while the stochastic approach was used to treat the effect of turbulence on the particle motion. Simulated mean particle moisture content and air humidity ratio at the dryer outlet as well as the particle mean residence time were compared with the experimental data. The results indicated that the model could predict the experimental results within ±10%, with the realizable turbulence model performing better than the standard model. The effects of various parameters including the inlet air velocity, inlet air temperature, material feed flow rate, and impinging distance on the transport and performance behavior of the dryer were then numerically investigated and discussed.

Comparative evaluation of performance and energy consumption of hot air and superheated steam impinging stream dryers for high-moisture particulate materials

Abstract Impinging stream dryer (ISD) is a novel alternative to flash dryer for drying high-moisture particulate materials. A number of studies have been conducted on various aspects of an ISD; however, in almost all of the previous studies hot air was used as the drying medium. Since it is well recognized that use of superheated steam as the drying medium could lead to much reduced net energy consumption of the drying process, it was the aim of the present study to investigate the use of superheated steam as the drying medium in an ISD. Okara (soy residue) was used as a test high-moisture particulate material. The performance of the drying system, in terms of the volumetric heat transfer coefficient and volumetric water evaporation rate, as affected by the superheated steam temperature, steam velocity, material feed flow rate and the dryer geometric parameter viz. impinging distance was assessed. In addition, the ability to reuse the exhausted steam as well as the specific energy consumption of the system were evaluated. Comparison was made with the hot air drying results. The maximum volumetric water evaporation rate was found to be around 807 kgwater/m3h, while the maximum volumetric heat transfer coefficient was around 7950 W/m3K at the steam recycle ratios of 46–63%. The lowest total specific energy consumption of the system was around 3.1 MJ/kgwater at an inlet steam temperature of 190 °C, inlet steam velocity of 20 m/s, material feed flow rate of 20 kgdry solid/h, impinging distance of 5 cm and steam recycle ratio of 63%. Compared with the hot air drying results at the same corresponding conditions, savings of the total specific energy consumption in the range of 9–46% were noted.

Evaluation of performance and energy consumption of an impinging stream dryer for paddy

Impinging stream dryer has proven to be an excellent alternative means for removing surface moisture of particulate materials. In this study, a coaxial two-impinging stream dryer prototype for paddy, whose surface moisture needs to be removed prior to subsequent processing, was developed and tested. The effects of various operating and geometric parameters, i.e., inlet air temperature, impinging distance, particle flow rate and particle feeding characteristics (single-point feeding vs. double-point feeding), on the overall performance (in terms of the volumetric water evaporation rate and volumetric heat transfer coefficient) and energy consumption of the dryer were then studied. It was found that the developed impinging stream dryer could reduce the moisture content of paddy by 3.4–7.7% (d.b.) within a very short period of time. The maximum value of the volumetric water evaporation rate was found to be about 198 kg water /m 3 h, while the maximum value of the volumetric heat transfer coefficient was about 7013 W/m 3 K. The mean residence time of the particles (paddy) in the system was in the range of 1.81–2.42 s, leading to average drying rate in the range of 1.52–3.83 (% d.b.) s −1 , which is about 250 and 40 times higher than spouted-bed and fluidized-bed dryers, respectively. The lowest total specific energy consumption of the process was 5.1 MJ/kg water when using double-point particle feeding at an inlet air temperature of 110 °C, an impinging distance of 5 cm and particle flow rate of 150 kg dry solid /h.

MATHEMATICAL MODELING AND SIMULATION OF DRYING OF A SINGLE WET PARTICLE IN A COAXIAL IMPINGING STREAM DRYER

In this study two simple mathematical models are proposed to describe the drying process of wet particles in a coaxial impinging stream dryer. The models represent the particles in two conditions, surface and internal moisture, which are solved numerically. The effect of various parameters including particle diameter, air velocity, and air temperature are investigated on the final moisture content of the product. In addition, drying intensity of the coaxial impinging stream dryer is compared with the other common dryers. This comparison leads to the conclusion of the high efficiency of impinging stream dryers at higher air velocity and smaller particles. Comparing the results of the models with available experimental data shows that the numerical predications are in good agreement with the experimental ones.

Performance and Energy Consumption of an Impinging Stream Dryer for High-Moisture Particulate Materials

Impinging stream dryers (ISDs) are a novel alternative to flash dryers for high-moisture particulate materials. However, studies on hydrodynamics and heat/mass transfer as well as drying characteristics of an ISD are still limited or incomplete. The effects of various geometric and operating parameters of an ISD on its performance, in particular, are not conclusive. In addition, ISD was tested mainly with model materials that do not represent the real challenge of higher-moisture materials, which tend to be sticky in nature. In this study, modifications were made to an existing coaxial two-impinging stream dryer of Sathapornprasath et al.[ 1 ] to make it more suitable for high-moisture particulate materials. This newly modified dryer was then evaluated for its performance in terms of the volumetric water evaporation rate and volumetric heat transfer coefficient. Soy residue (okara), which is a by-product of a soymilk production process, was used as a test material. Particle mean residence time was also evaluated and the results were used to support and explain the performance data of the dryer. Finally, the specific energy consumption of the dryer was assessed at various geometric and operating conditions. The maximum volumetric water evaporation rate was found to be around 520 kgwater/m3h and the maximum volumetric heat transfer coefficient was around 4,500 W/m3K for the mean particle residence time of approximately 0.97–1.74 s. In terms of the specific energy consumption the lowest specific energy consumption was found to be around 5.6 MJ/kgwater.

Application of Markov-Chain Analysis and Stirred Tanks in Series Model in Mathematical Modeling of Impinging Streams Dryers

In spite of the fact that the principles of impinging stream reactors have been developed for more than half a century, the performance analysis of such devices, from the viewpoint of the mathematical modeling, has not been investigated extensively. In this study two mathematical models were proposed to describe particulate matter drying in tangential impinging stream dryers. The models were developed based on Markov-Chain analysis and the theory stirred tanks in series. The required parameters for each model were determined by using RTD data obtained in a tangential impinging stream dryer and then the governing equations were solved numerically. Comparing the models with available experimental data shows that the stirred tanks in series model successfully explains the drying behavior in impinging stream dryers. Nevertheless, the developed results of the model that is based on Markov-Chain analysis, are not in exact agreement with corresponding experimental data. Also, the effects of some operating parameters on performance of such dryers were investigated. The results indicate that the moisture removal efficiency of dryer increases when the solid to gas ratio and initial moisture content of particles decreases and the temperature of carrier gas increases.

Application of Markov Chain Analysis and Tanks-in-Series Model in Mathematical Modeling of Impinging Stream Dryers

In spite of the fact that the principles of impinging stream reactors have been developed for more than half a century, the performance analysis of such devices, from a viewpoint of the mathematical modeling, has not been investigated extensively. In this study two models are proposed to describe the drying performance of particulate materials in two-impinging stream dryers. The models are developed based on the Markov chain analysis and the tanks-in-series model. The required parameters for each model are determined by using RTD data obtained in a two-impinging stream dryer and the governing equations are solved numerically. Comparison of the results of the models with available experimental data shows that the stirred tanks-in-series model successfully explains the drying behavior in impinging stream dryers. Nevertheless, the results of the model that is developed based on the Markov chain analysis are not in exact agreement with corresponding experimental data because of the extremely short residence time of the particles inside the dryer. Also, the effects of some operating parameters on the performance of such dryers are investigated. The results indicate that the drying efficiency of the dryer increases when solid-to-gas flow rate ratio, initial moisture content, and diameter of the particles decrease and when the temperature of the carrier gas increases.

Drying of aqueous lactose solutions in a single stream dryer

Spray-dried lactose is a useful ingredient in many food and drug preparations. In the literature, many articles have attempted to mathematically characterize the drying kinetics and the product quality during spray drying of various dairy products using a number of approaches. As far as lactose is concerned, little mathematical analysis is available regarding its detailed drying kinetics. Recently, the reaction engineering approach (REA) was found to be useful to formulate a drying kinetics model for drying of aqueous lactose droplets. The main objective of this work is to determine the sensitivity of the REA in order to assess its performance over different drying and feed conditions, which may be relevant to an industrial scenario. A single stream drying (S 2D) approach was used for modeling the air-drying of aqueous lactose droplets, as this technique is simple to be used and reduces the computation time. Some laboratory results on the physical characteristics of the air-dried lactose particles are also reported using the S 2D approach.

Performance Evaluation of an Impinging Stream Dryer for Particulate Materials

Impinging stream dryers (ISD) belong to a unique class of dryers that has proved to be an excellent alternative to flash dryers for removing surface moisture of particulate materials due to collision of streams and particles in the dryer. However, very limited works are so far available on the fundamental study of the effects of various operating parameters on the performance of an ISD. The objective of this study was thus to investigate the effects of various operating parameters, i.e., inlet air temperature, inlet air velocity, feed rate of drying particles, and spacing between the two opposed inlets, on the overall performance of an ISD in terms of its volumetric water evaporation rates and volumetric heat transfer coefficients. In this study, it was assumed that the model material (resin) dried only in the unhindered rate period where mainly surface moisture was being removed. Resin had initial moisture content of about 81–85% (d.b.). The maximum volumetric water evaporation rate was found to be around 110 kgwater/m3h while the volumetric heat transfer coefficient was about 880 W/m3K for the average particle residence time of approximately 2 s. Empirical correlations were also developed to predict the experimental data and the maximum error of prediction was found to be about 10%.

Fundamental Research on Drying Process in Institute of Engineering Thermophysics, CAS

Some results of our fundamental research on drying processes are summed up in this article. It consists of three parts: (1) Multistage fluidized bed drying, including particle flowing characteristics, heat and mass transfer between particles and drying medium, drying characteristics of drying materials; (2) Impinging stream drying, the flowing and drying characteristics of a vertical impinging stream dryer, one-stage and multistage semi-circular impinging stream dryer and combined vertical and semi-circular impinging stream dryer are discussed; (3) The effects of rapid transient heat and mass transfer on drying processes, such as time and space scales for nonFourier or nonFickian and influence of extreme heat and mass transfer are also discussed.