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

Beet-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.     

Inversion Temperature and Pinch Analysis,Ways to Thermally Optimize Drying Processes

This work studies the compatibility and suitability of a combined inversion temperature and pinch analysis with the process selection for air and superheated steam spray drying of milk solids. The inversion temperature is a good starting point for an energy analysis because it is a simplified rate-based approach to comparing the steam and air drying systems. pinch analysis enables process integration, at least on a heat recovery and heat exchanger network level.

The resulting inversion temperature for the studied system was estimated as 182°C for the dryer inlet temperature. However, mass and energy balances showed that a minimum inlet temperature for spray drying of 184°C was required for the superheated steam dryer in order to ensure that the outlet solids temperature above the dew point temperature.

The inversion temperature is still very relevant in the early stages of a design process because it allows a quick assessment of which drying medium should result in a smaller dryer. It was evident that the steam system is better from an energy perspective because of the recoverable latent heat of the water vapor carried out of the dryer with the recycled steam. The steam system has between 82 and 92% of thermal energy recovery potential as condensable steam, compared with 13–30% energy recovery of the air system. However, other important design and operational factors are not discussed here in detail.

Combining the inversion temperature and pinch analysis suggests that superheated steam drying both gives better energy recovery and is likely to give smaller dryers for all operational conditions.     

OPTIMUM OPERATING CONDITIONS IN DRYING FOODSTUFFS WITH SUPERHEATED STEAM

It is inferred from experimental data that in drying foodstuffs with superheated steam, the initial drying rate has a direct effect on the rate at which the overall drying takes place. That is, the faster the initial drying rate, the shorter the overall drying time. This criterion is very convenient because at the beginning, water moistens the sample external surface so evaporation does not depend on internal sample characteristics, but only on external convective heat and mass transfer rates. Mass and energy balance equations are solved and the result converted into a general initial drying rate equation, in which all dryer characteristics are grouped into one dimensionless parameter. The initial drying rate equation is mathematically maximized and the optimum working conditions determined. The result shows that initial drying rate always increase with increases of either the superheated steam temperature or velocity, but once these two variables are fixed, there exists at least one “optimum” pressure at which the initial drying rate is a maximum. Finally, the initial drying rate and optimum condition equations are applied to three model dryers, a dryer for a flat sheet, a fixed bed dryer and a rotary dryer. In each case, numeric values are computed and plotted as drying rate versus pressure curves, in which the optimum drying rate is also included. Also presented is a chart to compare the optimum pressures as functions of temperature and steam velocity for the three dryers.     

DRYING OF SUGAR BEET FIBER WITH HOT AIR OR SUPERHEATED STEAM

A comparative study of drying thin layers of sugar beet fiber with hot air or with superheated steam in a specially designed pilot dryer is reported. Our present interests are focused on drying rate and on the quality of the dried product. Steam superheated at 130-150°C yields 90 % of dry matter (DM), however steam condensation on the cold product at the beginning of the process may penalize the drying time. The color of the fiber is not modified until reaching 80 % DM. Drying with air heated at temperatures lying in the 40 to 105°C range does not alter the white color up to 90 % DM. The water retention capacity of the original fiber, 14 g water/g DM remains unchanged whatever drying agent is used. The scaling up of the primary hot air drying set up to a ten times larger experimental dryer introduce no modification of drying time, water retention capacity or fiber color. Thick layer drying experiments permit to define parameters of a belt dryer for an industrial fiber processing plant.     

DRYING OF SUGAR BEET FIBER WITH HOT AIR OR SUPERHEATED STEAM

A comparative study of drying thin layers of sugar beet fiber with hot air or with superheated steam in a specially designed pilot dryer is reported. Our present interests are focused on drying rate and on the quality of the dried product. Steam superheated at 130-150°C yields 90 % of dry matter (DM), however steam condensation on the cold product at the beginning of the process may penalize the drying time. The color of the fiber is not modified until reaching 80 % DM. Drying with air heated at temperatures lying in the 40 to 105°C range does not alter the white color up to 90 % DM. The water retention capacity of the original fiber, 14 g water/g DM remains unchanged whatever drying agent is used. The scaling up of the primary hot air drying set up to a ten times larger experimental dryer introduce no modification of drying time, water retention capacity or fiber color. Thick layer drying experiments permit to define parameters of a belt dryer for an industrial fiber processing plant.     

Drying Kinetics and Quality of Shrimp Undergoing Different Two-Stage Drying Processes

An innovative two-stage drying concept is presented in this article. The work considered drying of shrimp using a superheated steam dryer followed by a heat pump (SSD/HPD) or a hot air dryer (SSD/AD) both from drying kinetics and dried product quality points of view. The experiments were performed using the first-stage superheated steam drying temperature of 140°C while the second-stage heat pump drying (or hot air drying) was performed at 50°C. The moisture content of shrimp at the end of the superheated steam drying stage was varied between 30 and 40% (w.b.). The effect of tempering between SSD/HPD was also investigated. Shrinkage, color, rehydration behavior, texture (toughness and hardness), and microstructure of dried shrimp were measured. The results showed that SSD/HPD dried shrimp had much lower degree of shrinkage, higher degree of rehydration, better color, less tough and softer, and more porous than single-stage SSD dried shrimp. It was also found that SSD/AD gave redder shrimp compared to shrimp dried in a single-stage superheated steam dryer. No improvement in terms of shrinkage and rehydration behavior was observed, however.     

Drying Kinetics and Quality of Shrimp Undergoing Different Two-Stage Drying Processes

Abstract

An innovative two-stage drying concept is presented in this article. The work considered drying of shrimp using a superheated steam dryer followed by a heat pump (SSD/HPD) or a hot air dryer (SSD/AD) both from drying kinetics and dried product quality points of view. The experiments were performed using the first-stage superheated steam drying temperature of 140°C while the second-stage heat pump drying (or hot air drying) was performed at 50°C. The moisture content of shrimp at the end of the superheated steam drying stage was varied between 30 and 40% (w.b.). The effect of tempering between SSD/HPD was also investigated. Shrinkage, color, rehydration behavior, texture (toughness and hardness), and microstructure of dried shrimp were measured. The results showed that SSD/HPD dried shrimp had much lower degree of shrinkage, higher degree of rehydration, better color, less tough and softer, and more porous than single-stage SSD dried shrimp. It was also found that SSD/AD gave redder shrimp compared to shrimp dried in a single-stage superheated steam dryer. No improvement in terms of shrinkage and rehydration behavior was observed, however.     

Superheated Steam Drying: An Overview of Pilot and Industrial Dryers with a Focus on Energy Efficiency

According to the principle of sustainability, modern industry should preserve nonrenewable energy sources and develop more efficient processes, especially in terms of energy consumption. The depletion of fossil energy reserves, the environmental impact of greenhouse gases, and the possible threats of environmental taxes are the main reasons to develop new processes in general, and new drying processes in particular, for the existing industries. Using superheated steam as a drying medium instead of hot air can improve the energy efficiency by reusing the energy from exhausted steam and prevent gas emission into the atmosphere by condensation. The present review is focused on both lab-scale pilots—including impingement jet, fluid bed, kiln, fixed bed, and flash drying—described in the literature and existing industrial facilities, with a specific analysis focused on energy efficiency. The usefulness of superheated steam drying pilots for experimental research and for the design of industrial dryers is analyzed. The impact on quality specifications of the dried product for different operating conditions is also presented. Documentation on industrial superheated steam dryers is very rare. Nevertheless, this work presents and analyses the key data available for superheated steam drying of beet, alfalfa, industrial pulp, and paint sludge. Energy recovery and process integration, with a focus on specific technological challenges for industrial dryer implementation, are also presented. This document will result in a discussion of some new ideas for possible R&D in superheated steam drying.     

Industrial Experience in Pressurized Steam Drying of Beet Pulp,Sewage Sludge and Wood Chips.

ABSTRACT

Drying in superheated steam under pressure gives possibilities for pollution free drying, energy recovery. reduced drying time and a very compact drying equipment.

Products are dried i n a pressurized cellular fluid bed by super heated steam blown through the cells. Above the fluid bed dust is separated before the steam is reheated and recirculated to the cells by a fan. The evaporated water leaves the dryer as usable steam at e.g. 3 bar g, and full energy recovery is obtained. Today 14 plants have been builtor are under construction, with capacities from 2 to 40t / h water evaporation.

This technology is suited for drying of beet pulp, residues from starch production of corn or wheat. sludge. spent grain. brown coal. wood chips and bark.

The technology shows great options for integration in steam systems in various industries, whereby drying is possible without using primary energy and pollution is avoided.     

Drying of Fish Press-Cake with Superheated Steam in a Pilot Plant Impingement System

Drying tests for pine sawdust and mackerel press-cake with hot air and with superheated steam were carried out in a pilot impingement cylindrical dryer. Wet particles move axially along the dryer, adjusting the inclination of the equipment, whereas hot gases circulate in cross-flow against the particles, forming a corotational impingement front. Feed rate and residence time of solids were studied experimentally as a function of dryer inclination for hot air and superheated steam as drying media. Drying rates and heat and mass transfer coefficients were found to increase at shorter residence times and higher gas temperatures. Dried mackerel press-cake with superheated steam resulted in a product with high moisture removal and very low losses of the valuable omega-3 fatty acids.     

Drying of Fish Press-Cake with Superheated Steam in a Pilot Plant Impingement System

Drying tests for pine sawdust and mackerel press-cake with hot air and with superheated steam were carried out in a pilot impingement cylindrical dryer. Wet particles move axially along the dryer, adjusting the inclination of the equipment, whereas hot gases circulate in cross-flow against the particles, forming a corotational impingement front. Feed rate and residence time of solids were studied experimentally as a function of dryer inclination for hot air and superheated steam as drying media. Drying rates and heat and mass transfer coefficients were found to increase at shorter residence times and higher gas temperatures. Dried mackerel press-cake with superheated steam resulted in a product with high moisture removal and very low losses of the valuable omega-3 fatty acids.     

DRYING PAPER IN SUPERHEATED STEAM

The industrial potential for drying paper in superheated steam has been investigated in a series of studies designed to treat paper properties - drying process relations as well as process engineering aspects. Properties measurements show that for paper made from mechanical pulps, drying in superheated steam produces a better bonded sheet which is thereby stronger and has a lower scattering coefficient. Surface properties of such steam dried paper are improved, including reduced tinting. Desorption equilibrium shows that completely dry paper can be obtained at very low superheats. Impingement drying rates can be about twice as high for drying in superheated steam as in air. A hybrid cycle was developed in which drying is partially by superheated steam impingement drying and partially by conventional cylinder drying. Preliminary design and techno-economic evaluation indicates that three factors, paper properties, dryer size and energy efficiency, in various combinations specific to each case, may provide industrial potential for this new technology for drying paper.     

DRYING PAPER IN SUPERHEATED STEAM

ABSTRACT

The industrial potential for drying paper in superheated steam has been investigated in a series of studies designed to treat paper properties - drying process relations as well as process engineering aspects. Properties measurements show that for paper made from mechanical pulps, drying in superheated steam produces a better bonded sheet which is thereby stronger and has a lower scattering coefficient. Surface properties of such steam dried paper are improved, including reduced tinting. Desorption equilibrium shows that completely dry paper can be obtained at very low superheats. Impingement drying rates can be about twice as high for drying in superheated steam as in air. A hybrid cycle was developed in which drying is partially by superheated steam impingement drying and partially by conventional cylinder drying. Preliminary design and techno-economic evaluation indicates that three factors, paper properties, dryer size and energy efficiency, in various combinations specific to each case, may provide industrial potential for this new technology for drying paper.     

造纸污泥干化热力计算及方案选择

文章利用空心浆叶干燥机作为干化造纸污泥的设备,对采用过热蒸气和饱和蒸气作为热源的两个方案进行了热力计算。比较结果表明:把污泥的含水率从80%干化到40%,利用0.4 MPa,200℃过热蒸汽为热介质来干燥污泥时,耗气量是1712 kg/h;利用0.4 MPa,150℃的饱和蒸气作为热源,耗气量是1800 kg/h。     

COMPARISON OF SUPERHEATED STEAM AND AIR OPERATED SPRAY DRYERS USING COMPUTATIONAL FLUID DYNAMICS.

In this paper a numerical simulation of a spray dryer using the computational fluid dynamics (CFD) code Fluent is described. This simulation is based on a discrete droplet model and solve the partial differential equations of momentum, heat and mass conservation for both gas and dispersed phase.

The model is used to simulate the behaviour of a pilot scale spray dryer operated with two drying media : superheated steam and air Considering that there is no risk of powder ignition in superheated steam, we choosed a rather high inlet temperature (973 K). For the simulation, drop size spectrum is represented by 6 discrete droplets diameters, fitting to an experimental droplets size distribution and all droplets are injected at the same velocity, equal to the calculated velocity of the liquid sheet at the nozzle orifice.

It is showed that the model can evaluate the most important features of a spray dryer : temperature distribution inside the chamber, velocity of gas, droplets trajectories as well as deposits on the walls. The model predicts a fast down flowing core jet surrounded by a large recirculation zone. Using superheated steam or air as a drying medium shows only slight differences in flow patterns. Except for the recirculation which is tighter in steam.

The general behaviour of droplets in air or steam are quite the same : smallest droplets are entrained by the central core and largest ones are taken into the recirculation zone. In superheated steam, the droplets penetrate to a greater extent in the recirculation zone. Also, they evaporate faster. The contours of gas temperature reflect these differences as these two aspects are strongly coupled. In both air and steam there is a “cool” zone which is narrower in steam than in air. Finally, the panicle deposit problem seems to be more pronounced in air than in steam.

Adding to the inherent interest in using superheated steam as a drying medium, the model predicts attractive behaviour for spray drying with superheated steam. In particular. under the conditions tested with the model, a higher volumetric drying rate is obtained in superheated steam.     

OPTIMAL OPERATION OF A CONCURRENT-FLOW CORN DRYER WITH A DRYING HEAT PUMP USING SUPERHEATED STEAM

A numerical model of a concurrent-flow dryer of corn using superheated steam as drying medium is solved applying a shooting technique, so as to satisfy boundary conditions imposed by the optimal design of a drying heat pump. The drying heat pump is based on the theory of minimum energy cycles. The solution of the model proves the applicability of the heat pump to a concurrent-flow dryer, achieving a Specific Energy Consumption as low as 1080 kJ/kg     

COMPARISON OF SUPERHEATED STEAM AND AIR OPERATED SPRAY DRYERS USING COMPUTATIONAL FLUID DYNAMICS.

ABSTRACT

In this paper a numerical simulation of a spray dryer using the computational fluid dynamics (CFD) code Fluent is described. This simulation is based on a discrete droplet model and solve the partial differential equations of momentum, heat and mass conservation for both gas and dispersed phase.

The model is used to simulate the behaviour of a pilot scale spray dryer operated with two drying media : superheated steam and air Considering that there is no risk of powder ignition in superheated steam, we choosed a rather high inlet temperature (973 K). For the simulation, drop size spectrum is represented by 6 discrete droplets diameters, fitting to an experimental droplets size distribution and all droplets are injected at the same velocity, equal to the calculated velocity of the liquid sheet at the nozzle orifice.

It is showed that the model can evaluate the most important features of a spray dryer : temperature distribution inside the chamber, velocity of gas, droplets trajectories as well as deposits on the walls. The model predicts a fast down flowing core jet surrounded by a large recirculation zone. Using superheated steam or air as a drying medium shows only slight differences in flow patterns. Except for the recirculation which is tighter in steam.

The general behaviour of droplets in air or steam are quite the same : smallest droplets are entrained by the central core and largest ones are taken into the recirculation zone. In superheated steam, the droplets penetrate to a greater extent in the recirculation zone. Also, they evaporate faster. The contours of gas temperature reflect these differences as these two aspects are strongly coupled. In both air and steam there is a “cool” zone which is narrower in steam than in air. Finally, the panicle deposit problem seems to be more pronounced in air than in steam.

Adding to the inherent interest in using superheated steam as a drying medium, the model predicts attractive behaviour for spray drying with superheated steam. In particular. under the conditions tested with the model, a higher volumetric drying rate is obtained in superheated steam.     

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

In 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/kg_water compared with 2.406–2.508?kWh/kg_water for HA operating under the same conditions, demonstrating that the SSD-RD was able to reduce the energy input by at least 46.14%.     

SUPERHEATED STEAM IMPINGEMENT DRYING OF TORTILLA CHIPS

ABSTRACT

Low-fat snack products are the driving forces for the drying of tortilla chips before frying. Super-heated steam impingement drying of foods has the advantage of improved energy efficiency and product quality. The temperature profile, drying curves, and the physical properties (shrinkage, crispiness, starch gelatinization and microstructure) of tortilla chips dried at different superheated steam temperatures and heat transfer coefficients were measured. Results indicated that the steam temperature had a greater effect on the drying curve than the heat transfer coefficient within the range of study. The microstructure of the samples after steam drying showed that higher steam temperature resulted in more pores and coarser appearance. The modulus of deformation and the shrinkage of tortilla chips correlated with moisture content. A higher steam temperature caused less shrinkage and a higher modulus of deformation. The pasting properties showed that samples dried under a higher steam temperature and a higher heat transfer coefficient gelatinized less during drying and had a higher ability to absorb water. Comparison of the superheated steam drying and air drying revealed that at elevated temperatures the superheated steam provided higher drying rates. Furthermore, there was a less starch gelatinization associated with air drying compared to superheated steam drying.     

STEAM DRYING OF WOOD CHIPS IN PNEUMATIC CONVEYING DRYERS

A model for a pneumatic conveying dryer is presented. Although the main emphasis is put on superheated steam drying of wood chips, it can be used for other porous materials as well

The model includes a comprehensive two-dimensional model for the drying of single wood chips which accounts for the main physical mechanisms occurring in wood during drying. The external drying conditions in a pneumatic conveying dryer were calculated by applying the mass, heat and momentum equations for each incremental step in dryer length. A plug flow assumption was made for the dryer model and the single particle and dryer models were solved in an iterative manner. The non-spherical nature of wood chips were accounted for by measuring the drag and heat transfer coefficients

Model calculations illustrate the complex interactions between steam, particles and walls which occur in a flash dryer. The drying rate varies in a very complex manner through the dryer. The internal resistance to mass transfer becomes very important in The drying of less permeable wood species such as spruce. Two effects were observed as the particle size was increased: firstly the heat transfer rate decreased, and secondly the residence time increased. To some extent, these effects compensate for each other, however, the net result is that larger chips have a higher final moisture content.