Transport and Storage Considerations

To use any agro-industrial residue, the place where it is generated, the place where it will be used, and its stability may be a concern, since agro-industrial residues, besides their organic nature, may have a high microbial load and usually are easily degraded. Transportation costs can be prohibitive to the whole process, due to distance and/or temperature control needs. In general, it is recommended to place the unit where the residue will be used near by the factory where it is generated, to be reused within a short period of time. If not possible, the logistic of the process must be studied in order to preserve the residue characteristics and consider all transportations costs.

Marrison and Larson (1995) developed the standard expression for transport costs as given in (2.1):

where UTC is the unitary transport cost, in $ per ton × km, and distance is in km. Most transportation of residual biomass is done by trucks, for which fixed costs may vary from 3.7 to 5.7 U$/t and variable costs may range from 0.085 to 0.146 U$/t.km for straw and stover or for wood chips, respectively, in Canada, data corrected for 2012 dollars (Searcy et al. 2007). Rail and barrage transportation have fixed costs of

17.1 and 34.1 and variable costs of 0.03 and 0.01, respectively (Montross and Crofcheck 2010). In Brazil, the fixed costs are around 7.5 U$/t and variable costs are around 0.06 U$/t.km for transportation by trucks.

Storage is usually cheap, but it should be as short as possible, because of the risk of degradation and infestation, and care must be taken about fire and for exposed heaps of wetting by rain.

Pretreatment Strategies

Drying and Concentration

Drying is the process of removal of a volatile substance, usually but not exclusively water, from a solid material through the evaporation. Humidity is the amount of water present in a material, and during the material drying, the humidity is reduced to a final acceptable or an equilibrium value. In most cases, drying is a finishing product operation, but for agro-industrial residues, it is necessary to reduce the water activity, in order to prevent deterioration reactions, microbial growth, chemical redox reaction, and the enzymatic reactions (Pessoa and Kilikian 2005).

According to Coulson and Richardson (1991), drying process is essential for (1) reduction of volume and weight of the material, (2) improvement of storage and handling characteristics of the product, and (3) reduction of transportation, packing, and storage costs.

The liquid content of a residue may be reduced by mechanical processes, such as centrifugation or pressing, but the final solid still contains high humidity and water activity (aw). Drying must be more effective and reduce the humidity to suitable values. Hence, thermal methods are more adequate and will be discussed in this chapter.

2.4.1.1 Drying Process

During the drying process, the liquid aggregated on the material is removed by evaporation, passing from the liquid to the gaseous phase. The evaporation happens at a temperature below the liquid boiling temperature at the internal system pressure. Thus, drying is a complex process that involves simultaneously heat and mass transfer, resulting in significant changes in the physical, chemical, and structural properties. Water loss can cause cellular structural stress, microstructure alteration, increasing porosity, and material shrinkage (Laopoolkit and Suwannaporn 2011).

When the material is put in contact with hot air, heat is transferred from the air tothematerialundertheeffectoftemperaturegradientbetweenthem. Simultaneously, the partial pressure difference of water vapor between the air and the material surface determines the mass transfer of water vapor from the material to the air (Perry et al. 1997).

According to Mc Cabe et al (1993) and Coulson and Richardson (1991), the material can be in different forms such as flakes, granules, crystals, powder, plates, or leaves showing different properties. Also, the liquid to be removed can be at the solid surface or at its interior, or partially outside and inside, in two basic forms: (1) free moisture, the water in excess relative to the equilibrium humidity content, and (2) bound moisture, the water retained in such way that it has a vapor pressure below the free water at the same temperature. It can be inside small capillaries, adsorbed on the surface, or inside the material cells. Aspects of the raw material, characteristics, and quality of the final product must be considered during the drying process.

2.4.1.2 Drying Rate

The drying rate or the variation of the humidity of each material along time is an important parameter for the process. The humidity variation depends on the differences of the moisture inside each material (free/bounded). The curve obtained during

Fig. 2.1 Typical drying curve for constant drying conditions, moisture content as a function of time (Adapted from Foust et al. 1960)

monitoring of the variation of the moisture content with time allows the determination of the drying velocity for moisture content, and the curve shape varies with the structure, kind and granulometry of the material, thickness of the material layer, and kind of dryer (Coulson and Richardson 1991).

The main factors to be evaluated to correctly choose the drying method/equipment are directly related to the mass and heat transfer mechanism, to the physical material characteristics, and to the desirable final quality of the product.

Figure 2.1 shows a typical drying curve. AB segment represents the unsteadystate period. BC segment is the constant-rate drying part, where the entire exposed material surface is saturated with water and drying proceeds from a free liquid surface without the influence of the solid. CD segment is the period with a decreasing drying rate, where there is a lack of free liquid at the material surface as the liquid movement to the surface is slower than the movement of mass from the surface to the drying air. At point D and at lower moisture values, there is no significant amount of liquid at the material surface. The wet part of the surface dries by convective transfer of heat and mass to the drying gas stream, and vapor from inside of the material diffuses to the dry places of the surface and then into the gas stream. This mechanism is slower than the convective transfer from the saturated surface. All evaporation occurs from the interior of the solid, and the material moisture contents continue to fall until the equilibrium moisture content (EMC) is reached (XE) and the drying process stops.

Figure 2.2 shows simulated drying curves for a 1,000 kg of Miscanthus grass. The initial biomass humidity is 30 %, reasonable for grasses and straw partially airdried in the field. The drying temperature and the initial air moisture determine the drying rate and the equilibrium moisture content.

Fig. 2.2 Simulated drying curves for Miscanthus at several temperatures. Inlet air is at 20 °C and 50 % relative humidity. Data based on the equilibrium curve of Fig. 2.3 and literature thermodynamic data for water

Materials with free water, such as fruit residues, may have an initial lower drying rate than a constant drying rate period and finally an exponential decay of the drying rate as shown in Fig. 2.1; materials without free water (as is the case) show only the falling rate period.

The EMC of a material depends on the ambient temperature and relative humidity, being inversely proportional to the temperature and directly proportional to the relative humidity (RH). To know the behavior of a certain material is useful to determine the suitable humidity for storage, considering the usual weather.

Figure 2.3 shows EMC isotherms for selected biomasses. While Jatropha seeds, for example, show relatively stable moisture content for a wide range of RHs, the EMC of a grass such as Miscanthus grows from 10 to 20 % with RH going from 40 to 80 %. The RH in the isotherm may be used to predict the aw of the solid, which should be below 0.8 to prevent the growth of most fungi.

2.4.1.3 Equipment

The classification of the drying equipment depends on the heat transfer method and the properties and characteristics of the material to be dried. In general, drying equipment can be divided into two groups: (1) direct dryers where the material is put in direct contact with the drying gas heat and (2) indirect dryers where the heat is supplied through other way, such as radiation, conduction, high-frequency electric field, and microwave (Mc Cabe et al. 1993; Perry et al. 1997). Both types can be used for residue drying.

Fig. 2.3 Equilibrium moisture curves for 5 biomasses. Regression models using the Oswin modified equation and data from Arabhosseini et al. (2010), Kallemullah and Kailappan (2004), Kartikaa et al. (2012), Fioretin et al. (2010), Achargee et al. (2011)

There is a large variety of equipment and drying processes. The first criterion to be analyzed is the volume and characteristic of the material to be dried. Later, the heating method, material feeding mode, and cost must be evaluated (Al-Kassir et al. 2005).

For industrial scale, tray dryers, conveyors and tunnel dryers, rotatory dryers, and fluidized and fixed bed dryers can be used. According to Perry et al. (1997), the main dryers are:

Conveyors and tunnel dryers: The material is transported through a drying tunnel, where hot air circulates, transversal, countercurrent, or parallel to the material. The advantage of this equipment is the operational flexibility, and it can be used to dry materials of various sizes and forms, as the hot air velocity can be adjusted and the residence time does not depend on the particle characteristics, although it will determine the material final humidity.

Rotatory dryers: Consists of a big slightly inclined cylinder, rotating around an axis, with internal paddles that enhance the thermal exchange between the hot air and the material. Beyond the cylinder movement and the gravity action, the material is constantly rebutted favoring the drying and driven to the dryer discharge while the volatile mass is transported by the gas flow. Normally, dryers operate in countercurrent, where the hot air enters opposite to the material discharge, enhancing the process thermal yield. In this case, parameters such as density, form, particle size, and the equipment inclination significatively influence the biomass velocity along the dryer and the retention time.

Tray dryers: It consists of a thermal isolated camera, with heating systems and forced ventilation through trays placed in shelves. The air movement allows heat conservation and improves drying efficiency. It is the simplest equipment, used mainly for discontinuous operation and in low scale.

 
< Prev   CONTENTS   Next >