*3.1.2 Combined conduction-convection heating rotary dryer*

For drying the high-moisture paddy in the field conditions, a combined conduction and convection heating rotary dryer for 0.5 t/h capacity was designed and developed

#### **Figure 2.**

*A schematic drawing of combined conduction and convection-type rotary dryer.*

by Likitrattanaporn et al. [32] by using liquefied petroleum gas (LPG) as the heat source. Moreover, a trial rotary dryer proposed with a system of concurrent flow comprising two main parts, a discharge cover, and a double cylinder is illustrated in **Figure 2**. The forward motion of the paddy happens by rotary motion of the cylinder and inclination angle. The air is inflated within the cylinder by a suction fan positioned on the discharge cover top. A 1-hp motor with a 1–60 reduction gear was utilized for driving the rotary dryer. LPG bulb on the entry edge heats the air. The heated air stirs to another end by the suction fan. Through the forward movement, the paddy contacts the external surface for the interior cylinder. Where, the conduction heating occurs followed by a cascading activity over the interior of the outer cylinder, which causes a convection heating. After this, paddy is placed into the emptying cover and got out of the dryer. When the suction fan absorbs the humid air, comparatively less humidity is taken away through the last or third pass at 100°C and 110°C, meaning 1.5% and 1.7%, respectively. At 120°C, the humidity content of 2.1% could be extracted. Clearly, this is because there was less free water available at the third pass of drying [33].

#### **3.2 Fluidized-bed dryers**

Fluidized-bed dryers (FBDs) are used extensively for the drying of wet particulate and granular materials with sizes ranging from 50 μm to 5 mm that can be fluidized, and even slurries, pastes, and suspensions that can be fluidized in beds of inert solids [34]. Here, the product is held aloft in a high-velocity hot air stream, thus promoting good mixing and heat transfer for uniform and rapid drying (**Figure 3**). FBDs are commonly used in processing many products such as chemicals, carbohydrates, foodstuff, biomaterials, beverage products, ceramics, medicines in agglomerated or powder form, healthcare stuffs, fertilizers, agrochemicals and pesticides, detergents, pigments, and surface-active agents, tannins, polymer and resins, as well as products for combustion, calcination, incineration, waste management, and environmental protection processes. Fluidized-bed operation offers imperative benefits such as good materials mixing, high amounts of heat and mass transfer, as well as easy material moving [34].

The air passages across a cribriform tray from below the foodstuff in addition to check it. The fragments are consumed in one end and from above. This support pushing along pieces formerly in a dryer, which they leave at the other termination. This is fortified by the lower-humidity fragments having lesser density and mass. This *Drying Technology Evolution and Global Concerns Related to Food Security… DOI: http://dx.doi.org/10.5772/intechopen.109196*

system has a suitable thermal efficiency and reduces individual fragments over warming [35].

The traditional fluidized bed is made by passaging a gas stream from bottom of bed for special materials. With little gas speeds, the bed of units is packed, which rests on a gas distributor plate. The passages of fluidizing gas is done over wholesaler, which regularly spread crossways bed [35]. The pressure droplet through the expansion of bed as the velocity of fluidizing gas is improved. At a particular gas speed, a fluidized bed is formed when the whole bed weight is completely supported by gas stream. This state is identified as minimum fluidization,and the corresponding gas velocity is named minimum fluidization velocity. Pressure drop through the bed remains near the same as pressure drop at minimum fluidization even if the gas velocity is increased further. **Figure 4** demonstrates numerous particulate bed regimes from filled to bubbly bed when the gas velocity is boosted. The graphs express the bed pressure drops and bed void age under many regimes [35].

By superficial gas velocities greater than the lowest fluidization velocity, typically from 2 to 4 umf, the fluidized bed is controlled. The minimum fluidization velocity could be assessed experimentally and using numerous correlations [36]. It should be noted that these correlations have limitations such as particle size, column dimensions, and operating parameters. Thus, they are valid in a certain range of criteria and operating conditions.

#### *3.2.1 Fluidized-bed dryer's benefits and limitations*

Commonly fluidized-bed drying advantages included a removal of moisture high rate, easy material transport inside the dryer, high thermal efficiency, control easing, as well as low cost of maintenance. The fluidized-bed dryer's limitations comprise high electrical power consumption, high pressure drop, poor quality of some particulate stuffs, unsimilarity product quality for certain categories of such dryers, pipes and vessels erosion, fine particles entrainment, and particle pulverization or attrition, as well as fine particle agglomeration, etc. For detailed discussion, see [37].

**Figure 4.** *Various regimes of a bed of particles at different gas velocities.*

Similar to drying, the fluidized bed has located a varied range of industrial purposes in many industries for dedusting, granulation, mixing, coating, agglomeration, incineration, chemical reactions, gasification, combustion as well as cooling, etc. Many procedures could be combined to fluidized bed drying in the same processor for achieving more than two procedures in one unit. Practices that could be favorably integrated to fluidized-bed drying are explained briefly in the following. The mixing influence in a fluidized bed is commonly suitable for particles size within 50 and 2000 mm. In the case of fine particles, which are less than 50 mm, or for the case of particles that are hard to fluidize when moistened, vibration is generally applied to enhance the mixing effect and fluidization characteristic [35–37]. For large particles, the supplement of internals or the usage of the spouting mode can assist to develop the operation. Good particle mixing is essential for fluidized-bed drying. So, the awareness of particle characteristics and properties is required to ensure good fluidized-bed dryer performance. Moreover, the particles bed can be fluidized by an energetic flow or by bed fluidizing sections regularly such as the entire bed is fluidized in order once over a cycle. Evidently, this process results in saving drying air and therefore electrical power, but it leads to an elongated operating time due to the intermittent heat input mode. In addition, intermittent fluidization can decrease the mechanical damage problem to particles because of continual vigorous particles clash and corrosion-induced dusting [35–37].

Spray drying, coating, agglomeration, and granulation share similar basic operative standard. The fine spray of the solution paste slurry suspension is sprayed in a fluidized bed of inert particles or the drying substance oneself, which is already overloaded in the room drying. Solid particle development and formation take place in the room as evaporation and drying enthuse moisture [35]. In granulation, the growth of solid particles is acheived by succeeding wetting and liquid feed coating onto the solid particles and coated layer solidification by hot drying air. In the coating, a layer of the *Drying Technology Evolution and Global Concerns Related to Food Security… DOI: http://dx.doi.org/10.5772/intechopen.109196*

priceless active agent could be coated on a less expensive substrate or adding a surface agent on solid particles, which is necessary for downstream processing. Through spraying a proper binder onto the solid particles bed, granulated or agglomerated solid big-size particles are formed [35–37]. During most situations, sole spray drying is not power efficient to remove the whole wet content in solids. It is because a significant amount of heat and time are necessary for the elimination of internal water, which is stuck in the solids internally. The drying system of the fluidized bed could be combined as the second phase of drying for removing interior moisture. This could be followed by the third phase of fluidized bed cooling for preventing condensation troubles during the packaging in various applications [35–37].

#### **3.3 Drum dryers**

The drum dryer is normally used to dry slurries, pastes, concentrated solutions, or viscous on rotating steam-heated drums [38, 39]. This is because of the moisture boiling off and flashing or of irreversible thermochemical transformations of their content, which take place on their first interaction with the hot drum surface [40–42]. The viscous slurry or paste is automatically spread by spreading action of both counter rotating drums into a fine sheet, which follows to warmer drum within single-drum dryers or splitting sheets on both hot cylinders in double-drum dryers. The adhering fine paste sheet is then speedily dried conductively by excessive heat flux of steam condensation in the drum. In the case of humid slurries, which generate wet slabs, the wet thin slab drying could be improved by blowing heated dry air on the sheet outward. The fine slab has heat-sensible supplies, which could be dried also at a minor temperature in the vacuum [43].

It can be used in the irreversible thermochemical transformations during the slurry's first contact with the hot drum. This is to impart the certain required qualities together of the dried product [44]. In the case of starch slurries, to produce pregelatinized starch, the starch can be gelatinized before the sheet is dried. When the thin sheet is exposed to the high heat flux and high temperature for a short period, the pored structure is imparted to the dried slab, because of the speedy formation of vapor bubbles in the slab during drying. The pored goods are premium in immediate food constructions because they are more readily wetted and can be simply rehydrated. For this, a drum dryer is broadly utilized over the universe in pregelatinized starch fabrication for immediate food construction.

#### *3.3.1 Types of drum dryers*

The drum dryer was first patented for use in the manufacture of pregelatinized starch in Germany [45]. Number and configuration of drums, heating systems, and product removal have been considered in many experiments. The drum diameter varies from 0.45 to 1.5 m, and its length varies from 1 to 3 m. The drum wall thickness is between 2 and 4 cm. The drum dryer is classified according to the steam-heated drums number and configuration as well as the atmosphere pressure around the drying sheet.

#### *3.3.1.1 Atmospheric double-drum dryer*

This dryer type has a higher rate of production, it can handle a wide range of goods as well as it is more efficient [38–40, 46]. Across the pendulum nozzle or a header, there are several nozzles, the paste or slurry is nourished onto the tweak of two

**Figure 5.** *Double-drum dryer with nip feed.*

**Figure 6.**

*Twin-drum dryer with applicator roller feeds.*

steam-heated drums counter-rotating toward each other, making a boiling pool at the tweak as illustrated in **Figure 5**. The feedstuff could be nourished in the drum tweaks and applicator cylinder as shown in **Figure 6**. Starch slurries turn into gelatine in boiling pool to form pastes, which be extra sticky. The counter-rotation of the drums spread the slurry or paste into two soft slabs on both drums that subsequently dry conductively.
