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Freeze Drying
Published in Arun S. Mujumdar, Handbook of Industrial Drying, 2020
Athanasios I. Liapis, Roberto Bruttini
In real freeze drying processes, the secondary drying stage is considered to start when all the ice has been removed by sublimation (end of primary drying stage). It is then considered that during the secondary drying stage most of the water that did not freeze (bound water) is removed. The bound moisture is present due to mechanisms of (a) physical adsorption, (b) chemical adsorption, and (c) water of crystallization. While the amount of bound water is about 10-35% of the total moisture content (65–90% of the total moisture could be free water that was frozen and then removed by sublimation during the primary drying stage), its effect on the drying rate and overall drying time is very significant. The time that it takes to remove the sorbed water could be as long or longer than the time that is required for the removal of the free water.
Deterioration of materials
Published in A. M. Sowden, The Maintenance of Brick and Stone Masonry Structures, 2020
The natural durability of a stone thus depends on a number of factors. One factor is the ability of the stone to resist leaching which will increase its porosity and reduce its cohesiveness. Resistance to leaching depends upon the relationship between total porosity and crystallite size; leaching is more severe for microporous stones which tend to hold the moisture and thus absorb atmospheric pollutant gases over a longer period. Salts generated by the action of rainfall that has absorbed pollutant gases tend to crystallize in the evaporation zone close to the stone surface. Crystal growth invariably occurs as a result of the absorption of water of crystallization. The degree of expansion depends on the nature of the salt involved which in turn depends on the stone and the pollutant gases, whilst resistance to crystallization stress induced by crystal growth depends on the porous properties of the stone; a macroporous stone is more resistant to crystallization stress than a microporous stone, apparently because the latter tends to hold water for a protracted period. This relationship also holds for crystallization stress resulting from freezing, so that the pore-size distribution within a stone, i.e. the proportions of microporosity and macroporosity or small and large pores, will generally define durability in the most reliable way within a particular group of stones of similar structure, although resistance to crystallization also depends on the cohesiveness of the stone so that stones of very low total porosity tend to be more durable in any case.
Industrial Biotechnology
Published in Firdos Alam Khan, Biotechnology Fundamentals, 2020
Practically, all of the cobamides formed in the fermentation are retained in the cells, and the first step is the separation of the cells from the fermentation medium. Large high-speed centrifuges are used to concentrate the bacteria to a cream, while filters are used to remove Streptomycetes cells. The vitamin B12 activity is released from the cells by acid, heating, cyanide, or other treatments. Addition of cyanide solutions decomposes the coenzyme form of the vitamin and results in the formation of the cyanocobalamin. The cyanocobalamin is adsorbed on ion exchange resin IRC-50 or charcoal and is eluted. It is then purified further by partition between phenolic solvents and water. The vitamin is finally crystallized from aqueous-acetone solutions. The crystalline product often contains some water of crystallization.
Investigating Compatible Drying Technique for Safe Utilization of Thar Coal, Pakistan
Published in International Journal of Coal Preparation and Utilization, 2022
Syed Nasir Mehdi, Zahid Mahmood Khan, Hafiz Umar Farid, Sadiq Hussain
To understand the characterization and behavior of coal, firstly we need to understand the physical properties. The properties of coal have significant impact on various processes. The microstructure and porosity are closely interrelated to complex heterogeneous reactivity of coal. Each type of coal has a certain set of physical parameters which includes moisture, density, specific gravity, and surface Area. Moisture of coal is the percentage of water trapped in coal structure except water of crystallization of mineral matter. Pore characteristics generally depend upon the grade of coal. Porosity is the measure of percentage volume of pores to the total volume of the substance, such substance is known as porous substance. Porous nature of substance affects density, specific gravity, and also physical properties. Based on dimension and shape, pores are classified into micro-pores (<2 nm), meso-pores (2–50 nm), and macro-pores (>50 nm). The sample size was 50 mm for the stress strain test. Coal has different specific gravity depending upon the type and nature of the coal.
Development of self-polarized PVDF films on carbon fabrics for sensing applications
Published in The Journal of The Textile Institute, 2022
Rukhsar Ali Khan, Munir Ashraf, Amjed Javid, Kashif Iqbal, Abher Rasheed, Nadeem Nasir
A microscopic view of the films is shown in Figure 9. Higher porosity has been observed in films prepared by solidification through deionized water. As there is no salt present in water so a rapid movement of DMF from films solution to water occurs that results in pores formation. Drying method also plays an important role to generate porosity in films. Drying in oven leads to rapid drying of films that increases the number of pores in the films. Many studies show that PVDF molecules could make intermolecular forces with several molecules having high polar groups that help to induce β-phase within the polymer (Chen et al., 2012). PVDF molecules interact with water of crystallization of hydrated salt and form β-phase in the films. When the carbon fabric having PVDF films was dipped into aqueous solution of hydrated salt the promotion of β-phase and solidification of PVDF solution into the films occur simultaneously. The interaction of water molecules with PVDF polymer has been utilized to achieve films with ordered molecular structure. The molecular chains orient themselves at air–water interface in such a way that hydrophobic part orients upward whereas hydrophilic part moves downward due to strong interaction between water and films. The OH group of water molecules could form hydrogen bond with C–F group of PVDF when carbon fabric was submerged into aqueous solution of hydrated salt. It makes the PVDF polymer in oriented form to promote β-phase.
Preparation and evaluation of microcapsules containing Rimulus Cinnamon and Angelica Sinenis essential oils
Published in Journal of Dispersion Science and Technology, 2022
Wensheng Lai, Yanling Liu, Yanhui Kuang, Sisi Zhang, Chuanping Zhang, Chuyuan Li, Bohong Guo
Scanning electron microscope (SEM) was used to evaluate the morphological structure of EOs microcapsules prepared by spray drying and freeze drying under the optimal preparation conditions (Figure 4). Clearly, the microcapsule powder prepared by freeze-drying has irregular shape and hollow surface (Figure 4A, B). The main reason is that the emulsification system was destroyed by the water of crystallization during the emulsion precooling. In subsequent vacuum freeze drying, the sublimation of ice crystals caused the formation of powder porous surface with different shapes.[38] The freeze-dried microcapsules are cake-shaped and need to be ground to form a powder product, but the grinding will also cause irregular appearance of the microcapsules.[39]