Drying (original) (raw)
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Toward a diffusion theory of drying
A recent theoretical analysis of drying in granular porous media is used as the basis for a diffusion theory of drying. Boundary conditions at the porous media-surrounding gas interface are analyzed and the previously derived transport equations for mass, momentum, and energy are combined in an appropriate manner to yield a tractable theory of drying.
– Drying is a mass transfer process consisting of the removal of water or another solvent by evaporation from a solid, semi-solid or liquid. The objective of this experiment is to determine of the rate of drying as well as to construct a graph of its drying curve under constant drying conditions. Three trays for three trials were prepared with added 250 grams of sand each and mixed with 40 mL of water for each tray. The mixture was evenly distributed into the trays and placed inside the dryer with a constant heating temperature of 110°C and weighed after the predetermined time interval of 5 minutes. In the experiment, the rate of drying was plotted against the free moisture content to obtain a graph of its drying curve and constant rate of drying calculated are 3.344, 3.04 and 2.848 kg/m 2 ·h with critical moisture content, which serves as the indicator of changing rate from constant to falling rate, as 0.01716, 0.04136 and 0.08056 kg H2O/ kg solid respectively for trials 1, 2 and 3.
Application of inverse concepts to drying
Thermal Science, 2005
Original scientific paper UDC: 536.24:66.021.4 BIBLID: 0354-9836, 9 (2005), 2, 31-44 This pa per deals with the ap pli ca tion of in verse ap proaches to es ti ma tion of dry ing body pa ram e ters. Si mul ta neous es ti ma tion of the thermophysical prop er ties of a dry ing body as well as the heat and mass trans fer co ef ficients, by us ing only tem per a ture mea sure ments, is ana lysed. A math e mat ical model of the dry ing pro cess has been de vel oped, where the mois ture con tent and tem per a ture fields in the dry ing body are ex pressed by a sys tem of two cou pled par tial dif fer en tial equa tions. For the es ti ma tion of the unknown pa ram e ters, the tran sient read ings of a sin gle tem per a ture sen sor located in an in fi nite flat plate, ex posed to con vec tive dry ing, have been used. The Levenberg-Marquardt method and a hy brid op ti mi za tion method of minimization of the least-squares norm are used to solve the pres ent pa rame ter es ti ma tion prob lem. An anal y sis of the in flu ence of the dry ing air ve locity, dry ing air tem per a ture, dry ing body di men sion, and dry ing time on the thermophysical prop er ties es ti ma tion, that en ables the de sign of the proper ex per i ments by us ing the so-called D-op ti mum cri te rion was con ducted. In or der to per form this anal y sis, the sen si tiv ity co ef fi cients and the sen si tiv ity ma trix de ter mi nant were cal cu lated for the char ac ter is tic dry ing re gimes and the dry ing body di men sions.
Classification of Drying Models for Porous Solids
Drying Technology, 1993
Models to describe drying pmesses are necessary for engineering design and optimization. Many resenrch studies in lhe past century have had the objective of developing mathematical models to describe drying pmesses in pomus solids. This review identifies key characteristics of drying models including controlling p m m resistances, internal mechanisms of moisture movement, and melhods of model cufficient determination, model solution, and model validation. Similarities and differences between previous work are noted, and strategies for future drying model development are discussad. INTRODUCTION Development of malhemalieal models lo describe drying of porous solids has been a topic of nsearch in many fields for several decades. Models are needed to enable scientific process design and minimilation of energy and capital costs subject to quality constraints. Until general models are developed which ~c e w f u l l y predict drying rates and internal moisture and temperature profiles. the design of drying processes will remain largely an art. A sound understanding of existing drying theory together with new experimental data will enable further advances in the description of the drying phenomenon. Several review articles have recently been presented on the topic of drying. Keey (1980) describes the historical development of drying theory. Van Brake1 (1980) provides a critical review of the topic of mass transfer during convective drying. Chirife (1983). Bruin and Luyben (1980), Fortes and Okos (1980). Holdswarth (1971). Raswn and Haynknwn (1977). and Van Arsdel (1963) review drying theory as applied to food materials. King (1971) Copyright O 1992 by Mprccl Dekkcr. Inc. 2 WAANANEN. LITCHFIELD. AND OKOS reviews freeredrying of fmds, and Rown (1987) and Simpson (1983-84) review the thmry of wood drying. Fulford (1969) provides a comprehensive review of Soviet research in the drying a m. Novak and Caulman (1975) note that soil physicists and engineers have 'generally ignored each other's literature.' This review attempts to encompass and classify efforts made in all arras of drying related research. Key characteristics of drying models a n discussed including controlling process resistances, internal mechanisms of moisture movement, structural and thermodynamic assumptions, and methods of material property measurement, model solution, and model validation. CONTROLLING RESISTANCES DURING DRYING Typically drying is divided into conslant-rate and falling-rate periods. The drying rate in the former is determined by conditions external to the material k i n g dried including temperature, gas velocity. total pressure, and partial vapor pressure. The controlling resislance may be m i s l e d with the transfer of energy to the solid, or the transfer of mass away from the solid. Mass transfer during the constant-ratc period involves diffusion of w s s r vapor from the material surface through a boundary layer into the drying medium. During the falling-rate period, the drying rate ds-es with time, and the rate of internal nus transfer to the material surface typically controls the pmeess. A falling drying rate may be observed when external nus transfer resistance is eontmlling and the surface vapor pressure of the solid is dec-ing m moisture content dmps. Measurement of sample temperature during drying will help to identify whether s process is controlled by energy ar mass transfer. A sample temperature equal to the wet bulb temperature of the surrounding medium is characteristic of energy transfer control. If the sample renches the dry bulb temperature of the drying medium, mass transfer control is suggested. The importance of internal versus external mass transfer resistance can be inferred from drying studies an samples of different sile (i.e.. varying slab thickness (I) or sphere and cylinder radii (r)). The drying time required to reach a given moisture content will be pmponianal lo I or r for external mass transfer control and proportional to I' or ? for control by internal diffusion. Waananen and Okos (1989) presented procedures for analyzing drying dnls from pnsh cylinders of different sires. Vsceareu. el al. (1974) analyzed the effect of nonisothcml drying conditions on the ~u l t s obtained from samples of different thickness. When he31 transfer effects were considered for sugar beet r m l slabs, the thickness dependence of drying data was consistent with internal nus transfer control. Lilchfield and Okos (1992) CLASSIFICATION OF DRYING MODELS 3 identified internaltransfer wntrol for drying of pasts slabs by varying the velocity of the drying medium and obwwing no change in drying rates. Fulrord, O.D. 1969. A ww<)rol mcerd s w i c l m u a x h on h e d r y i q of.olid..
2021
Drying is one of the important operations in the industries like pharmaceutical, fine chemical and food industries. It is always beneficial to dry the product in less time and less area of drying. The drying of the product reduces its weight and thereby decreases transportation cost and also makes it more durable. Time required for drying and space required for drying are important factors in the operation. The quality of product after drying has to be uniform. Natural and forced circulation drying are two commonly used drying methods. Fluidized bed drying has advantage of uniform contact of drying medium with solid. Solar drying is also used in open and closed drying modes. Also microwave assisted drying has been tried by many investigators and consequently by industries in specific applications. The present review summarizes research carried out on drying, its applications and advancements. Keywords— Natural draft, forced draft, drying time, moisture, solar drying.
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