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Downstream Processing Plant and Equipment
Published in Juan A. Asenjo, Separation Processes in Biotechnology, 2020
Once experimental vacuum drying rate data have been determined, scale-up is relatively easy (Spotts and Waltrich, 1977). The rate of vacuum drying is dependent on the vacuum, the wall temperature and its surface area, and the effective heat transfer coefficient between the wall and the inside of the moist solid. The rate of moisture removal during the drying process is not constant but increases in rate during the early phase, may remain constant during removal of the bulk of the internal moisture, and will then fall toward the end of the cycle. The dryer volume is sized according to the overall rate, whereas the vacuum equipment is sized to maintain the desired pressure at the maximum evaporation rate. Some typical data for rotary and conical vacuum dryers are given in Table 29. These values may be used for preliminary assessment of dryer sizing prior to experimentation which yields the average and peak water removal rates per kilogram of product charged under the expected operating conditions. The full-scale dryer volume may be estimated by linear scale-up of the dryer wetted area to meet the actual evaporation rate, and then for the geometry of choice, identifying the nearest standard dryer with a working volume wetted area equal or greater than the calculated value.
Diffusivity in Drying of Porous Media
Published in Peng Xu, Agus P. Sasmito, Arun S. Mujumdar, Heat and Mass Transfer in Drying of Porous Media, 2020
Chien Hwa Chong, Chung Lim Law, Adam Figiel, Tommy Asni
Another type of drying that is also commonly used for drying woods is vacuum drying. Vacuum drying offers various advantages like high drying rate at lower temperature (lower boiling point), shorter drying times, color preservation and better control of volatile organic compound emissions. However, it is usually combined with other drying technology or pretreatment technology. Steam, ultrasound and microwave mode of inputs are applied in vacuum drying of wood. The microwave conditioning produces better product quality compared to steam conditioning (He and Wang 2015). This is because during the microwave heating process, the heat absorbed drives internal water to the surface of the wood structure. He et al. (2017) also reported that microwave pretreatment enhanced permeability by effectively decreasing the moisture content; drying rate also accelerated by 171% compared to the untreated sample. Other methods like ultrasound-assisted vacuum drying show a faster drying and higher effective water diffusivity than the normal hot-air drying when removing free water.
Drying
Published in C. Anandharamakrishnan, S. Padma Ishwarya, Essentials and Applications of Food Engineering, 2019
C. Anandharamakrishnan, S. Padma Ishwarya
For almost all food products, drying involves the removal of liquid water from the material via the gaseous phase. Drying techniques can be categorized based on the methods used for heat addition and water vapor removal (Ishwarya et al., 2015). In hot-air (HA) drying, moisture removal is facilitated by direct contact between the product and hot air at atmospheric pressure (Geankoplis, 2006). With respect to the phase diagram, hot air drying occurs in zone AB (Figure 10.1) in which the liquid water is converted to the gas phase. On the other hand, freeze-drying takes place in the zone below the triple point D of the phase diagram. In the freeze-drying process, water is removed from the frozen product via sublimation during which the solid water (ice) (zone C) is directly converted to the gaseous phase (zone B) without passing through the liquid phase (zone A). The pressure at the triple point of the phase diagram, 4.58 mm Hg refers to the water vapor pressure and not the total pressure of the system. Thus, from the phase diagram of water, it is evident that sublimation can occur at atmospheric pressure as long as the water vapor pressure is below 4.58 mm Hg. Therefore, the requirement of vacuum is not a prerequisite for the freeze-drying process. In contrast, vacuum drying takes place under reduced pressure involving the gas and liquid phases.
Valorization of fruits, vegetables, and their by-products: Drying and bio-drying
Published in Drying Technology, 2022
Choon Hui Tan, Ching Lik Hii, Chaleeda Borompichaichartkul, Puttapong Phumsombat, Ianne Kong, Liew Phing Pui
Vacuum drying is commonly used for perishable and heat sensitive raw materials, which are widely used in the pharmaceutical and chemical industries and food products.[66] The physical conditions used to add heat and remove water vapor can describe vacuum drying processes like pressure and temperature.[67] Compared to traditional atmospheric drying, vacuum drying has significant advantages, such as shorter drying time, lower drying temperature range, and an oxygenless drying environment.[68] Furthermore, vacuum drying consumes less energy due to the ambient pressure, which creates total pressure differences within the product, which is the primary driving force for moisture transfer and aids in the preservation of color, flavor, and other functional contents in the food products.[69] This is why vacuum drying is quick and efficient. The greater the pressure difference, the faster the drying rate. These characteristics may assist in enhancing the quality and nutritional values of dried products. Table 6 shows the application of the vacuum drying method on different food products.
Implication of ultrasonic power and frequency for the ultrasonic vacuum drying of honey
Published in Drying Technology, 2021
Mengmeng Jiang, Xiting Bai, Jun Sun, Wenxue Zhu
On the other hand, vacuum drying has the advantage of potentially shorter drying time, using low temperature and less energy, and reducing the oxidation of food compounds during drying.[10] Meanwhile, ultrasonic assisted dehydration can facilitate the removal of water due to the ultrasonic waves that can create microscopic channels. Furthermore, the ultrasound produces cavitation phenomena and sponge effect that can aid in the removal of the tightly bound water presented in the material.[11] The combination of vacuum and ultrasound as a new drying technique was initially reported by Başlar et al.[12] The drying studies on salmon and trout fillets, beef, chicken, and green beans have been reported in the literature.[13,14] However, there are no studies on the effect of ultrasonic power and frequency on the honey drying with the combination of ultrasound and vacuum. Compared with other drying methods, it combines the advantages of both vacuum drying and ultrasonic drying with higher drying rates, lower temperatures and better energy efficiency.[15,16]
Superheated steam drying and its applicability for various types of the dryer: The state of art
Published in Drying Technology, 2020
Sanjay Kumar Patel, Mukund Haribhau Bade
Based on the pressure of the drying medium, SSD is categorized into three groups viz.: (i) vacuum pressure (Below atmospheric pressure), (ii) atmospheric pressure, and (iii) above atmospheric pressure. Vacuum drying is mainly preferred for heat-sensitive materials such as fruits, vegetables, herbs, spices, bio-products, and woods. In this drying system, the temperature requirement is significantly less (lower than 100 °C), so by maintaining pressure below the atmospheric pressure, superheated steam is generated at a lower temperature. The initial exhaustive analysis covering both the drying kinetics and quality of the dried product (carrots) under vacuum pressure has been conducted by Devahastin et al.[8] For the next category, the products such as paper, tissues, ceramics, coal, industrial effluents, and detergents are dried under atmospheric or near atmospheric pressure conditions using superheated steam. This type of system is most commonly used compared to drying at the other two categories based on pressure, i.e., higher or lower than the atmospheric pressure. The last group used for the drying of beet pulp, sludge, wood chips, and so forth with dryers as flash and conveyor, typically works at above atmospheric pressure. The primary function of this type of system is to evaporate surface moisture instantly. Generally, it is noticed that higher pressure than atmospheric has no significant effect on drying kinetics, as discussed in more detail later.