Contact-sorption drying

An interesting option for contact-sorption drying is the use of a sorbent that can be incorporated into the drying material as an integral and essential part of the final product. 

E.G. Tutova and D.S. Slizhuk, Development of a technology for contact-sorption drying of lactic acid and nitrogen-fixing bacteria, in Drying of Capillary-Porous Materials, ITMO Belorussian Academy of Sciences, Minsk, Belarus, pp. 11-18 (1990) (in Russian). 

A very general scheme of contact-sorption drying comprises mixing a solid sorbent with the material being dried followed by separation of these two media once the desired mass transfer has taken place. The solid sorbent is then regenerated and returned to the process. Clearly, the technical justification for the contact-sorption drying depends on whether the sorbent can be easily regenerated and recycled. 

A typical fermentation culture is an aqueous mixture of microorganisms or biopolymers, unreacted residues of nutrients, byproducts, process-controlling additives, and other components, with solid content amounting to only several percent. Thus, the solid sorbent used in contact-sorption drying performs two basic functions ... 

The mechanism of contact-sorption drying is very complex because moisture transfer takes place in heterogeneous and multicomponent systems and is accompanied by thermal effects. The idealized scheme of contact-sorption drying shown in Figure 12.1 reflects phenomena taking place in a dynamic system (e.g., mixing or fluidization), where interaction between the sorbent-material, material-material, and sorbent-sorbent is likely to occur. 

Figure 12.1 General mechanism of contact-sorption drying in a dynamic particulate system.

Analyzing the above mathematical model for the dynamics of the contact-sorption drying, one can conclude that ... 

It is clear that the efficiency of contact-sorption drying attains its maximum at the moment of contact with solid sorbent and drops with time reaching q = 1 when the sorption capacity approaches zero. Because the sorption capacity is maximum at the beginning of the process when the sorbent is dry, and reduces dramatically when the sorbent is close to saturation, it is reasonable to intermpt the contact of the material with the sorbent after a certain time. Based on numerous experiments the following relationship was proposed for determination of the optimum contact time (Tutova, 1988 Tutova and Kuts, 1987) ... 

Once the contact time is chosen, the partially saturated sorbent may be removed and replaced with the new charge of a dry sorbent renewal of the sorbent can then increase the rate of contact-sorption drying even by severalfold (Tutova and Kuts, 1987). Having the contact time established, the rate of sorbent renewal can be expressed by the number of renewals per batch (run) and determined from the ratio of the total amount of moisture to be removed to the amount of moisture absorbed during a contact time ... 

In practice, renewal of the partially wet sorbent can be accomplished either by complete separation of the sorbent from the material being dried, by a counterflow of sorbent-material layers, or by continuous replacement of a fraction of the sorbent as it is in contact-sorption drying in a fluidized bed. These methods are briefly described in Section 12.4 of this chapter. Details of various technologies can be found elsewhere (Kudra and Strumillo, 1998 Tutova and Kuts, 1987 Tutova, 1988). 

Another approach to mathematical modeling of contact-sorption drying that was used to simulate drying of com by mixing with zeolite particles (Alighani, 1990) is based on the following simplified system of Luikov s differential equations for heat and mass transfer (Luikov, 1966 Luikov and Mikhailov, 1961) ... 

Figure 12.3 Nomenclature for the mathematical model of contact-sorption drying.

When selecting a sorbent for contact-sorption drying, the following aspects should be taken into account ... 

Sorption characteristics, which comprise sorption kinetics, sorption isotherms, and sorption capacity, determine the efficiency of contact-sorption drying and selection of the drying method and dryer design. 

Figure 12.14 presents a schematic of the spray-drying system for con-tact-sorption drying of bacterial preparations, while the design of a three-stream nozzle used to disperse solid sorbent within the spray of these preparations is shown in Figure 12.15. Performance data of this nozzle are given in Table 12.2, and characteristics of contact-sorption drying for selected bacterial cultures are specified in Table 12.3.

Table 12.3 Contact-Sorption Drying of Bacterial Preparations on Wheat Bran...

The principle of contact-sorption drying on inert sorbents can also be applied under conditions of reduced pressure. Since the evaporation rate in freeze-... 

Figure 12.19 Basic structures for contact-sorption drying with heat-transferring elements in vertical (a) and horizontal (b) configurations 1—sorbent 2—material 3—solid (metal) structure. (From Tutova et al., 1985.)...

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