3.1 Drying brewer’s spent grain in a pan
Figure 4 shows that, under the conditions evaluated, the increase in drying air temperature and the decrease in the initial biomass load favored drying, reducing the time required to reach dynamic equilibrium. For example, by keeping the initial load at 1.5 kg and increasing the drying air temperature from 67.7°C to 101.2°C, a decrease was observed in the total drying time from 420 min to 240 min. By keeping the drying air temperature at 67.7°C and decreasing the initial load from 1.5 kg to 1.0 kg, a decrease was observed in the total drying time from 420 min to 330 min. The results indicated the potential use of rotary dryers in the processing of malt bagasse, reducing its initial moisture from approximately 80% (wb) to a moisture of approximately 8.5% (wb) in dynamic equilibrium in the four conditions evaluated. The product obtained is suitable for thermal conversion through combustion, gasification or pyrolysis, since the biomass must have a moisture content between 5 and 25% (wb) [6].
3.2 Oven drying rate
Figure 5 shows the drying rate as a function of malt bagasse moisture for the three different temperatures evaluated. Under the conditions evaluated, two distinct periods were identified, a stabilization period in which an increase in the drying rate was observed until the biomass reached a moisture ratio of approximately 0.9, followed by a period with a decreasing drying rate that extended until the end of the operation. The absence of a well-defined drying period at a constant rate is a strong indication that there was no excess of free water on the surface of the BSG and that the diffusion of water molecules from the inside of the BSG particles to its surface controlled drying mechanisms. It was also observed that the increase in drying rate was favored by the increase in air temperature. In Figs. 4 and 5, the results of the drying rates are shown, which indicate that the total processing time and the drying rate are highly dependent on the temperature of the drying air and that the process can be controlled through this variable.
3.3 Adhesion forces in brewer’s spent grain
Figure 6 shows that there was a reduction in the adhesion forces between the brewer’s spent grain and the copper surface by removing water from the particles.
It was observed that the decrease in the moisture ratio from 1.0 to 0.25 was sufficient to reduce the adhesion force per surface area from approximately 1.4 to 0.14 N·cm− 2. When the moisture ratio was less than 0.05, the adhesion forces were considered zero since no adhesion of the BSG particles to the copper surface was observed.
Other authors have observed a decrease in the adhesion force as water is removed from the materials since, in conditions of high humidity, capillary forces prevail over other mechanisms of surface interaction, such as van der Waals forces and electrostatic forces [18]. The presence of water between the surfaces of the adhesive and the adherent would be responsible for the formation of liquid bridges through a process known as "wetting", and the water may come from steam condensation on the surface, the presence of excess water, or migration inside the solid particles [10]. Moreover, expressive adhesion forces between the spent grain and the surface showed that high sugar content led to a stronger interaction with metal surfaces and a tendency for agglomeration [12]. Deng et al. (2020) evaluated the influence of humidity on the forces that cause urban sewage to adhere, identifying a relationship between humidity and the shear and tensile stresses involved in the adhesion of the material and between humidity and the forces of compression involved in the cohesion of the material [19]. Kobayashi et al. (2021) used resins as accelerators in the drying of urban sewage and observed that, regardless of the presence of resin, moisture directly influenced the contact angle [20].
The influence of the moisture content of the spent grain on the adherence seems to be consistent with the work of Stroem et al. (2009) who evaluated the agglomeration profile of the biomass in a rotary drum dryer. According to their work, brewer’s spent grain tends to adhere to the dryer walls, especially close to the solids feed, where there was a higher moisture content and the adherence mechanisms were associated to the water condensation from the superheated steam [5]. However, in the present work, the atmosphere had controlled humidity during the adhesion tests and, consequently, vapor condensation would not have a significant influence on the adhesion mechanisms. It is possible, therefore, that the moisture inside the biomass particles controlled the interaction between the phases.
3.4 Biomass movement
As shown in Fig. 7, the average motion of solids inside the rotating dryer was strongly dependent on the moisture content of the biomass. The results indicate that with high moisture content, at the beginning of drying, the solids had low fluidity, but as the moisture ratio reached 0.6, fluidity increased. A decrease in the movement of solids was observed when the water was removed, from a moisture ratio of 0.6 to dynamic equilibrium. The differences in motion of solids inside the rotating dryer could be visually noticed through its opening. Under conditions of high moisture content, the biomass formed a single agglomerate of solids with low mobility. By removing water from the biomass, some particles were detached by the mechanical action of the strips and started moving with higher mobility compared to the agglomerated particles. As the moisture ratio was close to 0.6, the agglomerated biomass was completely detached, and the solids reached their maximum motion. To this point, the solid-wall interaction played a key role in the solid motion as they were adhered to the surface, lifted to a specific height, and dropped, finally returning to the bottom bed through an "avalanche" movement. As the water evaporated, the adhesion between the solids and the walls of the dryer decreased, and consequently, they were not lifted by the rotation movement but were mixed by the strips.
These results agree with those obtained by Ma et al. (2019), who used a three-dimensional discrete element model to study the surface interactions between individual particles and their surroundings through capillary bridges. The moisture content of the solids had an influence on the repose angle, varying from 13.76° for dry materials to 18.94° for wet materials, showing that dry solids have higher flowability. Furthermore, in the rotating drum, the dry particles had a lower surface angle and rolled down the slope in an unsteady flow, while the wet particles showed a higher surface angle due to the capillary effect. The particles fell as a coherent whole with uniform velocity in an "avalanche" motion [21].
Thus, it can be understood that the variation in solids’ movement, shown in Fig. 7, is caused by the formation of agglomerates and by the interaction between the BSG particles and the internal surface of the dryer. In addition, it has been shown that the decrease in biomass humidity makes the movement of particles closer to what is considered ideal for rotating equipment. This behavior results in a homogeneous mixture and free movement of the particles.
3.5 Thermal behavior of malt bagasse drying
Figure 8 shows the increase in the temperatures of the internal surface of the pan and the biomass during drying. It can be observed that the temperature profile was similar in all conditions evaluated, even with the constant increase of Tw and Tsolids. At the beginning of the process, Tw and Tsolids presented similar values; later, Tw presented higher values, indicating more intense heating on the pan surface; and finally, Tsolids presented higher values at the end of the process.
It can be observed that the increase in the maximum temperature reached by the internal surface of the pan and by the malt bagasse, as expected, was mainly associated with the air temperature at the blower outlet. The biomass was heated to a maximum temperature of approximately 46.5°C with the drying air at 67.7°C, while for the drying air at 101.2°C the maximum temperature reached by the biomass was considerably higher, approximately 65°C. The same was observed for the temperature of the internal surface of the pan, with maximums of approximately 41°C and 59°C for the drying air at 67.7°C and 101.2°C, respectively. The increase in the total mass of biomass did not change the temperatures in the dryer, influencing only the total processing time.
Figure 9 presents the results obtained from the dimensionless temperature θ analysis through the drying. It can be noticed that the drying could be divided into three different regions. It can be observed that in Region 1, with a moisture ratio greater than 0.6, the dimensionless moisture remains practically constant, with values close to 1.0 due to the proximity between the temperatures of the biomass and the internal surface of the pan, indicating the thermal equilibrium between the phases. In Region 2, with a humidity ratio between 0.6 and 0.25, a constant reduction of the dimensionless humidity to values close to 0.85 is observed, indicating a more intense heating of the internal surface of the pan in relation to the BSG. Finally, in Region 3, with a humidity ratio lower than 0.25, a constant increase in θ could be observed with the decrease in humidity to values close to 1.1. This indicates that in this region, unlike Region 2, the BSG temperature increased more rapidly relative to the inner surface of the pan. The variation of θ as a function of humidity is a strong indication that the heat transfer between the BSG particles and the dryer wall was more efficient in conditions of high humidity. These results match those observed in earlier studies, as adhesion and cohesion in wet conditions usually affect the interface area [12] and there are differences between heat transfer coefficients in free-flowing and sticky materials [22].
Drying in a rotary dryer has allowed an approach to biomass drying, especially the BSG, in which it was possible to corroborate that this type of reactors can improve drying efficiencies. A drying ratio of 0.6 has proven to be the best to achieve fluidization in the rotary dryer. A single pan proves to be very efficient in the drying and fluid dynamic behavior of the BSG, as evidenced in Figs. 4, 5, and 6. This information is relevant in drying processes, especially for biomass from the brewing industry, because it can predict drying behavior in this type of reactor. It is then necessary to continue testing with more developed equipment and larger dimensions to corroborate the good behavior shown in this study. The pretreatment of this type of residual biomass turns out to be quite interesting because barley, which is the main raw material used in the beer industry, and its residues have a relatively small average particle size (2 mm). This size is normally the same, with even slightly larger sizes up to 3 mm (Fig. 1), but it is always the same because, due to the beer process, they must be small particles, which means that the drying pretreatment does not need crushing or grinding.
Finally, dry BSG waste is interesting for its use in thermal processes due to its calorific value of 21 MJ kg− 1, which could be used in the same industry to obtain energy and use it within industrial plants, reducing some costs [16, 23]. Instead, its use in pyrolysis or gasification turns out to be quite interesting, due to the possible obtaining of by-products such as H2, which can be used as raw material in refinery processes and as biofuel. Likewise, some authors have verified that BSG residues have high contents of phenolic waste that can be used in the cosmetic industry. Becoming a low-cost raw material for this industry and a raw material of sustainable origin for polyphenols as well as other bioactive molecules [24].