Intermittent Drying

22

Intermittent Drying Karim Allaf, Sabah Mounir, Mohamed Negm, Tamara Allaf, Hanintsoa Ferrasse, and Arun S. Mujumdar

Contents 22.1 Introduction...................................................................................................................................................................... 491 22.2 Intermittent Drying to Improve Quality........................................................................................................................... 492 22.3 Application to Drying of Fruits, Vegetables, and Cereals................................................................................................ 492 22.4 Application to Rice Drying.............................................................................................................................................. 495 22.5 Classification Based on Physical Parameters................................................................................................................... 495 22.5.1 Infrared/Heat Pump Drying (IR/HPD)................................................................................................................ 495 22.5.2 Rotating Batch Vacuum Dryers............................................................................................................................ 495 22.5.3 Fluidized Bed Dryers............................................................................................................................................ 495 22.5.4 Microwave Drying................................................................................................................................................ 495 22.5.5 Aeration Process................................................................................................................................................... 496 22.5.6 Détente instantanée contrôlée (French for “Instant Controlled Pressure Drop”) Coupled to Intermittent Air Drying............................................................................................................................................................ 496 22.5.6.1 Multiflash Drying MFD......................................................................................................................... 497 22.6 Closing Remarks............................................................................................................................................................... 500 References.................................................................................................................................................................................. 500

22.1 Introduction Various intermittent drying operations have recently increasingly been used at laboratory, pilot, and industrial scales. They all aim to improve product quality and/or process performance by reducing energy consumption. Intermittent drying is a drying operation with several periods of varying external dehydration conditions with different drying rates. In intermittent drying airflow temperature, velocity and/or humidity, heat input, pressure, and/or other external drying parameters (such as microwave or RF) are applied in a discontinuous manner to suit the drying kinetics and quality requirements of the material. The idea is often to introduce tempering periods to even out the internal moisture content and temperature fields. The quality of the final dried product can also be enhanced along with process performance. In each tempering period, moisture gets redistributed inside the material. This produces several desired effects. Quality is enhanced by avoiding or reducing overheating or overdrying of the surface layer. Quality degradation can be due to surface cracking, breakage, or crusting, and under certain conditions even scorching of the material. The drying rate can also increase as the material surface gets rewetted to that higher heat/mass transfer can occur at the exposed surface of the material. In this chapter, selected recent works related to intermittent drying and its impact in terms of kinetics and quality are reviewed. Drying intermittency for batch drying can be implemented through modifications and/or periodic variations, cyclic or arbitrary changes of the main process parameters depending on the needs of the drying material

and type of dryer. Note that in batch drying the process conditions can be varied with time to optimize the external heat/ mass transfer rates and quality parameters. In continuous drying, the variations in process conditions can take place along distance within the dryer. Note further that some continuous dryers are by their inherent nature intermittent but this intermittency is inherent and not forced and it cannot be controlled at will. For example, multicylinder contact dryers for paper provide heat for drying by conduction only while the sheet is in contact with the steam-heated cylinder and not while the sheet is in the open draw between consecutive cylinders. In spouted bed drying, heat transfer to the particles takes place primarily in the spout zone while tempering occurs as the particles descend under gravity in the annular moving bed zone before being re-entrained in the spout. Even the rotary dryer essentially operates in an intermittent mode wherein heat/mass transfer occurs principally as the particles shower down after being lifted to the top during a revolution and the cross flow of hot air flowing axially in the dryer provides convective heat transfer for drying. While the bed of particles is carried by the flights to the top, they undergo a tempering process. In all three examples, the periodicity of heat input is governed by design parameters of the dryer such that one cannot independently fix the periodicity of heat input. Hence, we do not refer to these as intermittent dryers. In intermittent drying, three main parameters are often considered for temporal variation for batch drying and spatial variation in continuous drying: (1) airflow rate, (2) vessel pressure (between two different levels), and (3) heat input by modifying airflow rate and temperature, or microwave power. 491

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Although not common it is also possible to vary the air humidity. These operating process variables can be programmed in a fixed or variable frequency mode with fixed or variable amplitude. Since a very large number of combinations and permutations are available for optimization purposes, one often must rely on a liable mathematical model to affix the optimal conditions. A generalized classification scheme of the diverse types of intermittency is presented in Table 22.1. This classification is based mainly on the process parameters and the cycle frequency (i.e., cycle time). Other classification schemes are possible as well especially when different modes of heat input are employed in simultaneous or sequential fashion. Whatever the type of intermittency, intermittent drying involves successive drying/tempering periods. Heat can be provided for drying using diverse heat sources and modes of heat transfer at time-varying rates in batch dryers; it may be lowered, increased, or stopped altogether. The drying period ensures heat/mass interaction between the external surrounding medium and the product. The frequency of this cyclic operation is chosen to obtain the most effective overall drying efficiency rather than rate. In fact, in batch dryers, supplying heat intermittently to heat-sensitive materials can increase the drying rate but can lower overall heat consumption and also yield a better-quality product, for example, grains, fruits, vegetables, etc. The duration of the tempering period when no heat is supplied or it is supplied at lower rate is estimated to allow the internal moisture to migrate to the surface; a more uniform distribution of internal moisture and temperature occurs within the material, and when heat is supplied in the following cycle there is adequate moisture at the drying surface to provide a higher mass transfer potential for drying. Intermittency conditions have time-dependent operating parameters regardless of the type of batch dryer. In batch drying, the frequency of heat input can be controlled in different ways while the drying material resides in the same chamber. However, such temporal variations can be also translated to spatial variations for a continuous dryer. For example, a continuous carpet dryer provides such intermittency by establishing along its length sections of varied airflow rate, temperature, and even relative humidity. Plug flow fluidized beds and vibrated bed dryers often have different velocity and temperature sections along the length of travel of the drying solids to optimize dryer performance. It is also possible to translate microwave (MW) power intensity variations through spatial distribution of MW generators along the length of the dryer. In fact, even the MW power itself can be pulsed. Under the term intermittent drying, we include all types of dryers that use time-varying operating conditions in batch mode and spatial variations in continuous mode.

22.2 I ntermittent Drying to Improve Quality Many well-known continuous drying processes were naturally intermittent processes. Indeed, sun drying (grains, fruits, herbs, etc.) can be studied as natural intermittent drying processes with almost 24 h drying/tempering cycles. Intermittent

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Handbook of Industrial Drying

drying processes have successfully been applied at industrial scales in drying wood, rice, and pasta (to remove cracking risks and reduce broken ratio); banana, guava, and potatoes (to avoid crust formation); and soybean, corn, and wheat (to assure homogeneous drying rate and quality) (Nishiyama et al. 2006; Thomkapanich et al. 2007; Tuyen et al. 2009; Holowaty et al. 2012). Indeed, for some thermal-sensitive biological-origin materials, problems related to germinability, vigor, protein denaturation, and crack formation might result for continuous hot air drying. Intermittent drying should mitigate these problems to a great extent. Generally, intermittency should be considered when the drying kinetics are internally controlled, that is, enhancing external heat/mass transfer rates or even volumetric heating does not increase the drying kinetics substantially since the drying surface can reach equilibrium moisture content and continuing external heating simply heats up the material rather than enhancing evaporation of moisture. Time is needed for the water inside the solid to migrate to the surface and be available for evaporation. This also means that in intermittent drying the overall drying time maybe enhanced but heat is used more effectively without the material reaching damage temperatures or causing physical damage due to cracking (e.g., wood) due to large temperature or moisture gradients in the material being formed. Intermittent drying schemes are considered to produce lower drying severity on surface as well as lower temperatures in the volume. They also have to reduce the necessary drying time in obtaining the same final moisture content. Product tempering and increasing of surface moisture occur when the air temperature has been switched from on to off periods. Moreover, the decreasing of drying front formation was observed under intermittent drying conditions (Pan et al., 1999). Comparing both stationary and intermittent drying, Kowalski and PawŁowski (2011) highlighted results that use of intermittent drying improves the material quality rather than drying under steady fixed process conditions. By finite element modeling, it has been shown that intermittent heating with MW can lower maximal stresses generated in a drying solid (e.g., ceramic, wood, etc.). Thus, the danger of crack formation is reduced.

22.3 Application to Drying of Fruits, Vegetables, and Cereals Conventional processes of sun drying, solar drying, airflow drying, vacuum drying, etc., have the advantage of being easy to use and control and affordable. In the case of sun/ solar drying, the possibility of directly processing fruits and vegetables near the harvesting area means good suitability for many products. In addition, the advantage of using free and renewable energy often reduces significantly the operating energy cost. However, their generally poor sensory quality and nutritional/hygienic value, and possible difficulties in further processing (e.g., grinding, rehydration, etc.) of the finished product are barriers to their extensive adoption.

AQ1

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Rice grains, long-fine

Rice (medium-grain) Fresh dill leaves

Elias et al. (2006)

Aquerreta et al. (2007)

Estürka and Soysalb (2010)

Allaf and Allaf (2014)

Zhang and Litchfield (1991) Hemati et al. (1992) Chou and Chua (2001)

Corn Peanuts Maize Wheat Compound fertilizer Corn (thin layer) Corn Potato

Raw Material

Sabbah et al. (1972) Troger and Butler (1980) Harnoy and Radajewski (1982) Giowacka and Malczewski (1986) Hällstrom (1986)

Study

Intermittent drying: drying period: 20 min Intermittent drying: drying period: 20 min and rest periods: 0–210 min Drying air temperature 30°C–35°C Intermittent profiles by raising the inlet air temperature from 30°C to 35°C for a defined periods 15, 30 and 40 min. The intermittency, α is defined as the fraction of time during which the inlet air temperature is raised to the defined cycle time

Dry aeration: tempering periods: 0–4 h Intermittent drying: airflow interrupted for 1 h in a 4 h drying period Intermittent drying: aeration periods: 1–6 min and rest periods: 3–90 min Sinusoidal heating Intermittent drying: drying periods: 2.5–6 s and rest periods: 4.5–6 s

Drying Scheme

Intermittent drying schemes were seen to produce lower surface temperatures and reduce the drying time necessary to obtain the same moisture content. Product tempering and increased surface moisture occurred when the air temperature was switched from the on to the off period The energy demand, for grains and air movement, for the drying in the intermittent dryer, is smaller than 1/7 of that necessary for the heating of the drying air, independently of the operational handling

Energy and Product and Process Quality

Constant air temp. (T = 90 + 5°C): 84 (kWh/ton) Air in growing temp. (T = 70 − 90 − 110 + 5°C) intermittence relation = 1:3 86.9 (kWh/ton) Air in growing temp. (T = 70 − 90 − 110 + 5°C) intermittence relation = 1:1.5 83.5 (kWh/ton) Thin layer convective dryer: Dryer temp. 60°C and tempering at 60°C Quality of tempering reduced the percentage of fissured kernels to 3.35% and enhanced head rice yield (HRY) independently with two drying steps to 96% 1. Continuous microwave + convective air drying at 30°C, 40°C, and 50°C Microwave-convective air drying could be Microwave 597.20 ± 6.89 W with 2. Pulsed 30 s on-30 s off + convective air drying at 30°C, 40°C, and 50°C used to save in drying time and to produce 3. Pulsed 30 s on-60 s off + convective air drying at 30°C, 40°C, and 50°C high quality dried dill leaves with better Airflow speed of 4. Pulsed 30 s on-90 s off + convective air drying at 30°C, 40°C, and 50°C physical (color) and sensory attributes 1.2 ± 0.20 m s−1 Multiflash drying MFD using instant pressure drop DIC technologies: short cycles (about dozens of seconds) between high-pressure (0.1–1 MPa) and primary vacuum (~3 kPa)

Pilot intermittent dryer with total static capacity of 360 kg

Flotation fluid bed Heat pump dryer

Bin dryer Fluidized bed fluidized bed

Thin layer

Dryer Type

Table 22.1 Summarization of Some Examples of Intermittent Drying Processes

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The modern drying methods, such as superheated steam, vacuum microwave drying, etc., have significantly improved the drying kinetics and quality. However, the issues of microbiological load, texture control, and powder manufacturing have persisted. Moreover, the high cost of freeze drying and the loss of some vectors of quality such as flavor and color still limit their application to high value-added products. Intermittent drying processes have been used as alternative drying methods especially for biomaterials. Drying kinetics and dried bio-products quality can be enhanced using right process conditions (Chua et al. 2003). Furthermore, the expected positive effect of temperature variation on product quality has also been reported repeatedly by various researchers (Chou et al. 2000; Chou and Chua 2001). Indeed with proper selection of drying air temperature, the amount of temperature-sensitive ascorbic acid in pieces of guava (Psidium guajava) could be up to 20% higher than in pieces dried under isothermal conditions. Uneven hardness and chewing of dried fruits are common product quality issues. The product rejection rate is relatively high due to these problems. Chong and Law (2011) found that the textural attributes of dried fruits can be controlled or standardized by applying milder drying techniques, namely, intermittent hot air and dehumidified air drying. In addition, identification of the critical moisture content contributing to case hardening of dried product should be considered; this depends on the product being dried. A comparison of the textural attributes has been made for the continuous hot air, continuous dehumidified air, and intermittent hot air–dehumidified drying techniques. It was found that for samples subjected to 5 h dehumidified air and 19 h hot air (5C-19H), intermittent drying had the best hardness and chewiness attributes (Chong and Law 2011). As far as product quality is concerned, Pan et al. (1999) and Chou and Chua (2001) have demonstrated clearly the advantages of intermittent drying. In a vibrated bed batch drying of carrot pieces, the retention of β-carotene in the dried product was higher in intermittent drying. In the same way, the net energy consumption was reduced and even the drying time was somewhat shortened (Pan et al. 1999). In other studies, the most significant advantages of applying the tempering stage were linked to the reduction of energy consumption by reducing the drying time because the drying stage alone increased the product temperature instead of removing moisture. Chou and Chua (2001) studied heat pump-assisted drying of Chinese cabbage seeds; they reported that energy consumption is affected by the intermittent drying ratio. The best intermittency ratio was 1/3 in heat pump for drying of Chinese cabbage seeds. The drying conditions were 40°C as drying air temperature and 40% as air relative humidity. Intermittency was achieved with 1200 s cycles including 400 and 800 s for drying and tempering periods, respectively. Hence, energy consumption and time consumed in the intermittent drying were only 52% and 48%, respectively, compared with the continuous operation.

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Tempering results in reducing (or eliminating) the moisture field inside the grain imposed during the previous drying stage. This uniform distribution of moisture inside the product will in turn increase the drying rate in the next drying stage. The starting accessibility δWs takes place once the following drying stage starts. This offers an initial acceleration of drying process (Mounir and Allaf 2008). Thakur and Gupta (2006) announced a considerable amount of energy (21%–44%), which could be saved by providing a rest period of 30–120 min between the two stages of drying. Kowalski and PawŁowski (2011) state that the intermittent drying is recommended for drying materials that have a tendency to crack during drying. Ceramics and wood are very interesting examples. Through changes of drying conditions at the right time, one can avoid fracture and thus offer good quality of dried products. The best quality of the tested samples was achieved using intermittent drying with the variable air humidity. Studies on intermittent drying with time-dependent airflow rate, humidity, and temperature suggest the advantageous combination of these drying techniques to optimize the quality of dried products and energy consumption. This combination would increase the effectiveness of intermittent drying. A relatively large difference was reported between the total energy consumed for drying and the net energy used for removing the moisture from the material. Instead of supplying low-pressure superheated steam drying (LPSSD) under continuous conditions, Thomkapanich et al. (2007) implemented intermittent LPSSD for thermal-sensitive food products (banana slices). Two intermittent modes, namely, intermittent temperature (70°C–90°C) and intermittent pressure LPSSD, as well as the intermittency period (several minutes) were tested. The effects on the drying kinetics and various quality attributes (color, shrinkage, texture, and ascorbic acid retention) were evaluated. In terms of dried chips quality, intermittent temperature drying led to a higher level of ascorbic acid retention, especially for longer tempering (off) periods. On the other hand, it was noted that the product color in the case of intermittent pressure drying was worse than that in the case of continuous drying. Shrinkage of the samples dried by intermittent pressure drying was also more obvious than in the case of continuous drying. In addition, intermittent pressure drying led to greater degradation of ascorbic acid. This drying scheme is therefore not appropriate for heatand, in particular, oxygen-sensitive products, as oxygen (air) is necessarily introduced into the drying chamber during the off period (Chen and Mujumdar 2009). The energy consumption for intermittent LPSSD was also monitored through the net drying time at various intermittent drying conditions and the results compared to those of continuous LPSSD. The effective drying time of intermittent drying was significantly shorter than that of continuous drying, especially with a longer tempering period leading to high energy saving (up to 65%). Note that the drying rates of intermittent pressure LPSSD were higher than those of continuous drying (in the range of 50%–58%).

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Intermittent Drying

22.4 Application to Rice Drying Tempering of rice during drying has become a common practice to reduce fissuring. Li et al. (1998) studied the fissuring of rough rice in intermittent drying through experimental and numerical methods. The moisture distribution inside the rice kernel during the tempering stages was obtained by a diffusion model. The moisture gradients were used to analyze the hydro stresses in the rice kernel during intermittent drying. Discontinuing the drying process with tempering can decrease the stresses in the rice kernel. Higher intermittency ratio subsequently results in lower percentage of fissured rice. During the tempering period, the moisture in the kernel was equalized throughout moisture diffusion; it causes reduction of the stresses in the rice kernel. Less fissured rice was found after intermittent drying. Consequently, the quality of the final product is improved (Golmohammadi et al. 2011).

22.5 Classification Based on Physical Parameters Intermittent drying operations can be classified according to the frequencies involved in the drying/tempering cycles (Table 22.1). In the case of heat-sensitive materials, such operations should mainly aim to control and preserve (or increase) the quality of final product. The improvement of end product quality can generally be expected with a slightto-moderate increase in drying time, which would imply even a slight increase in energy consumption. Note also that intermittent drying should be considered only when all of the drying occurs in the falling rate, that is, it is controlled by internal transport of heat and mass.

22.5.1 Infrared/Heat Pump Drying (IR/HPD) An infrared-augmented heat pump dryer (HPD) can be used for faster removal of surface moisture during the initial stages of drying, followed by intermittent drying over the rest of the drying process. Therefore, an IR-assisted HPD offers the advantage of compactness, simplicity, ease of control, and low equipment cost when both surface and internal moisture are present (Mujumdar 2000).

22.5.2 Rotating Batch Vacuum Dryers Intermittent rotation of the rotating batch vacuum dryer can alleviate the stickiness problem when the drying material is at high moisture content (Mujumdar and Sakamon Devahastin 2006). Intermittent application of vibration, fluidization, or spouting can also save energy when the drying rates are slow and there is little need for high external heat and mass transfer rates applied continuously.

22.5.3 Fluidized Bed Dryers Mujumdar and Law (2006) reported that the periodic fluidization of sections of the bed allows the entire bed to

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fluidized in sequence once over a cycle. A pulsating flow can also fluidize a bed of particles. This results in saving drying air and electrical power but leads increased operation time due to intermittent heat input. Mechanical damage to particles can be minimized since continuous particle–particle collisions leading to attrition and dusting are avoided. From experimental studies, Mujumdar and Law (2006) observed a reduction in drying time of up to 40% in vibrated bed drying. Comparing the preservation of β-carotene for continuous and intermittent air drying, Pan et al. (1999) have shown that 87.2% of β-carotene in squash could be preserved in intermittent drying, while only 61.5% was found in conventional continuous drying.

22.5.4 Microwave Drying Zhang and Mujumdar (1992) used a finite element model for intermittent volumetric heating in a thermal drying process. It was thus possible to reduce drying-induced stresses in a grain kernel. During the resting period, moisture content and temperature tend to redistribute more uniformly. In the case of intermittent volumetric heating, the temperatures of the nodes did not increase continuously, and the values of the node temperatures were much lower than those in continuous heating. While the moisture content at different points in the body become more uniform. Since the moisture field of the body is more uniform, it is clear that the drying-induced stresses decrease as well. Other numerical studies were conducted by Gong et al. (1998). Their aim was to understand the effect of intermittent microwave heating on the drying behavior of clay and its internal stress development. Numerical results confirmed that the maximum tensile and compressive stresses within the dried clay could be reduced significantly when microwave heating was employed intermittently. Itaya et al. (2001) demonstrated the beneficial effect of microwave drying of ceramics. With continuous microwave heating or microwave heating with long cycle times, cracks developed in the ceramic samples were tested. With higher frequency of pulsation, crack formation could be avoided or delayed at lower moisture contents although the sample attained higher temperature. They observed that the mechanical crack formation was affected by the intermittency of microwave energy input so care must be taken in selecting the intensity and frequency of heat input. Gunasekaran (1999) has demonstrated the advantages of pulsed microwave vacuum drying of cranberries both in terms of energy efficiency and quality of the dried product in terms of its color (redness) and texture. He found that the longer the power-off time relative to the power-on, the better were the energy efficiency and product quality. He employed MW oven operated at pressures of 5.33 and 10.67 kPa with power-on to total cycle time ranging from 1 to 6. Experimental data show that when the power-on time was shorter, more of the microwave energy was used for evaporating moisture. Furthermore, a longer power-on

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time tended to increase the product temperature and resulted degradation in heat-sensitive materials. Several studies have been carried out to investigate the effect of intermittent microwave drying on shrinkage (Raghavan and Silveira 1999; Raghavan and Venkatachalapathy 1999). Sanga et al. (2001) have reported experimental and numerical analysis of intermittent microwave-convection drying of heat-sensitive materials such as carrot and potato pieces. They measured the drying rate online along with temperature distribution in the drying product using fiber-optic probes. They showed that a diffusion model including temperature and concentration dependent diffusivity as well as allowing for shrinkage in an empirical fashion yielded results, which were in agreement with the measured drying rates. Chen and Wang (2001) made a theoretical analysis of the effect of the intermittent microwave heating patterns on batch fluid drying of porous particles. They considered three MW power patterns, namely, uniform, sinusoidal, and rectangular. Under constant electric field strength conditions, their results indicated that the magnitude and distribution of the moisture, temperature, and pressure within a particle could be substantially affected. More importantly, intermittent heating with a rectangular wave pattern had the most microwave energy consumption but the shortest drying time. For an apple particle size of 5 mm, airflow rate of 2 m/s, and temperature of 60°C, the drying time was, respectively, 1600, 2000, and 2400 s for rectangular, sinusoidal, and constant microwave heating. The values of microwave energy consumed for the three cases were, respectively, 2145, 1980, and 1560 kJ/kg water evaporated. The model could be extended to include kinetics of quality change (e.g., color and vitamins). Also, a more detailed parametric study including different MW energy patterns could be carried out. It should also be possible to dynamically optimize such a process for the best quality at lowest energy consumption or minimum overall cost.

22.5.5 Aeration Process Based on his work on peanuts drying, Farouk (1967) observed that for any particular aeration time shorter cycling periods were more effective than the longer ones. When drying by heating and aeration alone, the higher temperature and shorter aeration periods were more effective in rapid drying. The intermittent heating–drying process was more economical in terms of aeration time than the continuous aeration process for the aerating periods of less than 6 h per cycle. Filho et al. (1982) conducted intermittent drying experiments for soybean comprising a convective heating period followed by an aeration period. In other words, warm and cool air were used for intermittent heating and cooling periods repeatedly until the moisture content of beans reached the desired end value. They observed that soybean intermittent drying took longer than continuous drying while preventing damage compared to the continuous process at the same temperature.

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22.5.6 Détente instantanée contrôlée (French for “Instant Controlled Pressure Drop”) Coupled to Intermittent Air Drying Intermittency is considered as an intensification scenario depending on both product and drying process. Mounir et al. (2012) and Allaf and Allaf (2014) analyzed the different intensification modes of conventional air drying. The first intensification depends on the airflow characteristics, which are the flow rate, temperature, and humidity. The drying rate can effectively increase with airflow rate till a maximum value. The latter depends on water effective diffusivity:

ρ w v W A]r =rsurface < kA  pW ( Ts ) − pW ,air  − Deff A

dρ w  < kA  pW ( Ts ) − pW,air  dr  r =rsurface

− Deff k > kl =

dρ w  dr  r =rsurface

pW ( Ts ) − pW,air

(22.1)

(22.2)

(22.3)

where, ρ w is the apparent water content (kg H2O/kg dry basis or % db) vW is the water velocity within the product (m s−1) A is the exchange surface (m2) k, kl is the coefficient of mass transfer, and its critical value, respectively (m−1 s) pW ( Ts ) and pW ,air are the vapor pressure of water at the surface temperature Ts and the vapor pressure of humidity in surrounding air (Pa) Deff is the effective diffusivity of water in the material (m2 s−1) r is the one-dimension position in the material Once adequate airflow rate, temperature, and moisture are defined to intensify the external exchange processes, drying operation usually becomes completely dependent on internal mass transfer rate. To calculate the effective diffusivity, Deff , one has to combine experimental results with Fick-type model. However, diffusion phenomenon has to be studied out the starting drying time (t ≠ 0). Hence, a starting accessibility is defined as the difference between the initial moisture content Wi and the value Wo calculated by extrapolating the AQ3 diffusion model till t = 0. The starting accessibility δ Ws (kg H2O/kg dry basis or % db)

δ Ws = Wi − Wo

(22.4)

The effective diffusivity Deff becomes smaller with shrinkage, which results in a more compact structure. A very effective intensification method can take place by proper texturing of the material using instantaneously controlled pressure drop, DIC. A second airflow drying stage can then improve both the process performance and final product quality (Albitar et al. 2011).

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Intermittent Drying

After having optimized the external airflow conditions and before texturing by DIC the partially dried biological material, air drying can be operated in intermittent drying mode. Each tempering period (index k) involves stopping the airflow. Thus, the starting accessibility δ Ws,k can be considered as the contribution of each tempering period to the subsequent drying period. The specific performance of intermittency on drying is seen from the sum of the different starting accessibilities δ Ws,k to be compared to the total moisture removed from the sample: Intermittent drying performance ϵ:

εintermittent

  =

∑

 δ Ws,k   (Wi − We ) n

k =1

(22.5)

where We is the final equilibrium moisture content (kg H2O/kg dry basis or % db). 22.5.6.1 Multiflash Drying MFD Internal water transfer has as driving force the gradient of water content between the core and the surface. However, in the case of vapor transfer, the driving force is the gradient of vapor pressure, which depends only on local temperature and local water activity aw. The values of vapor pressure pW are as higher at higher temperature. Since the temperature is higher at the product surface, this phenomenon defines a paradoxical situation; vapor has to be transferred toward the material core. This is completely opposite to what is required for a drying operation (Al Haddad 2007, 2008).

Paradoxical Drying Step in Coupled Heat and Vapor Transfer During final part of dehydration, it can be assumed that the water transfer as a liquid within the solid matrix is negligible. The main transfer phenomenon is vapor diffusion. However, in similar cases, it is assumed that water evaporation necessarily implies amount of heat able to transform the liquid phase into vapor:

∂ ψ  ∇ −λeff ∇T +  pw M w Lw )  = 0 ( ∂t  RT 

(

)

(22.6)

The vapor transfer is governed by a Fick-type law; the formulations of Allaf (1982) can be related to the vapor pressure gradient through an effective diffusivity D eff:

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 p /T ( pw /T ) vW − vd ) = − Deff ∇  w ( ρs  ρs

  

(22.7)

where ρs is the apparent density of dry material (kg m−3) T is the temperature (K) ψ is the porosity ratio pw is the vapor pressure of water in the porous material (Pa) M w is the molar mass of water (kg mol−1) Lw is the latent heat of water vaporization (J kg−1) λeff is the effective conductivity of the porous wet material (J m−1 K−1) vd is the absolute porous solid velocity (m s−1) vw is the absolute velocity of vapor within the porous solid (m s−1) Deff is the effective diffusivity of vapor within the porous solid (m2 s−1) The temperature distribution is assumed to be steady and the transferred heat is used mainly to evaporate water (Allaf 2009). By neglecting possible shrinkage phenomena, one can assume that ρs = constant and vs = 0; then Equations 22.6 and 22.7 may be transformed into one-dimensional formulation (r) as follows:

( pW /T )  ∂( pW /T )  vW = − Deff   ρs ∂r  

(22.8)

∂ 2T ∂ p  + ψ MLW  W  = 0 ∂r 2 ∂t  RT 

(22.9)

−λ

To achieve such a diffusion operation, the vapor should normally flow from the center toward the surface:

vW > 0 ⇒

∂ ( pW /T ) 0 ∂r

(22.11)

This phenomenon is a paradoxical situation, inducing a two-way motion of vapor. Some part of vapor is transported toward the surrounding medium and another part is transferred toward the material core. The latter is in fact opposite to what is required for a drying operation (Al Haddad 2007, 2008). Then, in the final period of standard hot air drying, the operation is achieved following front progression kinetics.

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MFD treatment vessel

Specific adiabatic compressor

V2

V1

Vacuum tank

Vacuum pump

AQ5 Figure 22.1 Schematic view of the multiflash drying MFD equipment (V1 and V2 are instant electrovalves). drop rate ( ∆Td /∆t ) = (T − − T + ) /∆t ; (4) other reactor parameters of interest like volume ratio of vacuum tank to processing chamber, intrinsic density or filling ratio, quantity and apparent volume of the product to be processed, etc. The different steps of MFD cycle are presented in Figures 22.2 and 22.3. There is an initial air-compression step. It is a heating step with a positive temperature difference (gap):

A third intensification is usually needed to overcome this paradoxical situation. It is to replace Fick-type vapor diffusion process by a total pressure gradient (TPG) (Darcy-type process), which can be obtained through microwave MW heating, superheated steam drying SHSD or multiflash drying MFD. MFD treatment consists of subjecting biological products to successive, pre- programmed short period cycles of high− pressure P +/low-pressure P . In the cases of plant-based products, the high temperature T + was studied between 20°C and 60°C depending on P + (between 0.1 and 1 MPa). Lowest gas pressure P − tested was close 3 kPa. Each cycle starts by opening the first electrovalve V1 (Figure 22.1). After a treatment time t + , an instantaneous pressure drop is obtained by abruptly opening the electrovalve V2 between the treatment chamber and the vacuum tank whose initial pressure is close to 3 kPa. The abrupt pressure drop (Al Haddad et al. 2008) leads to partial vaporization of water. The system is maintained during t − time at P −. The process is controlled by many operating parameters, such as (1) intrinsic parameters like shape and size of the raw material, initial water content, specific heat, thermal conductivity, effective diffusivity, etc.; (2) operating process parameters like P + , T + , t + , P − , T − , and t − ; (3) kinetic parameters like pressure drop rate ( ∆Pd / ∆t ) = ( P− − P + ) /∆t and temperature

In some experimental conditions carried out on collagen and on yeast, the positive temperature gap was of 10°C–15°C. The initial temperature level Te usually is the boiling point at the lowest pressure P −. After high-pressure/high-temperature processing time t +, pressure instantaneously drops, reaching the minimal temperature level T − . A very short time later, temperature increases and normally reaches the equilibrium level of Te. Vacuum level is maintained for t −. Then, another MFD cycle starts. At each cycle, a final equilibrium temperature depending on the final pressure (usually Te = 27°C at 3.5 kPa) is reached. This operation involves a certain amount of product moisture content, thus modifying the internal water content (dry basis) of ∆Wk:

Multi flash drying MFD

0.6 Static absolute air pressure (MPa)

∆T + = T + − Te

P+

Pressure drop

0.5

t+

0.4 0.3

Cycle 1

Cycle 2

Cycle 3

0.2 0.1 0

t–

P– = 3600 Pa 0

20

40

60

80

100

120

140

160

Time (s)

Figure 22.2 Pressure–time variation strategy multiflash drying MFD treatment.

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499

Intermittent Drying

Multi flash drying MFD

45 40

T+

Temperature (°C)

35

T+

T+

30

Te

25 20 15 10

T–

5 0

T– Cycle 1

0

20

T–

Cycle 2 40

60

80

Cycle 3 100

120

140

160

Time (s)

Figure 22.3 Temperature variation with time under multiflash drying MFD treatment.

Evolution of moisture content dry basis by autovaporization

∆Wk = ( Wk − Wk− 1 ) =

( cp,d + Wk− 1cp,w ) L

(T − T ) e

+

(22.12)

were Wk and Wk−1 are the moisture contents dry basis for MFD cycles k and k − 1, respectively L is the latent evaporation heat of water cp,d and cp,w are the specific heat of dry product and water, respectively Since Te is the starting temperature of the heating stage and the final equilibrium temperature of the decompression stage, it is easy to compare the amount of heat absorbed by the material and the quantity of water removed by autovaporization (Equation 22.12). This allows to identification of an interesting thermal efficiency, η: Thermal efficiency, η:

η=

Qk ≈1 md ( Wk− 1 − Wk ) L

(22.13)

The total input energy input depends only on the mechanical work of the compressor and vacuum pump. • Instantaneous autovaporization Studies conducted by Allaf (2009) and Allaf and Allaf (2014) show the major importance of the rate of DIC pressure drop. Similarly, their studies on multiflash drying MFD showed that the drying rate was substantially faster when achieved by instantaneous pressure drop (between 20 and 200 ms) compared with 2 s decompressions. Indeed, measurements were carried out under the same conditions and it is noteworthy that T −