**3.2 Economical drying**

Reducing the carbon footprint is unquestionably a big industrial focus. Making the shift from freeze drying to spray drying may, in fact, represent a significant step toward reduced energy consumption, thereby lowering the overall climate footprint. *Baeghbali et al.* [40] compared the energy consumption of spray drying and freeze drying, showing that the spray dryer required less than 10% of the energy consumed

*Excipient-Free Spray Drying of Bioactive Recombinant Proteins Produced in Plants DOI: http://dx.doi.org/10.5772/intechopen.112944*

**Figure 4.** *Continuous spray drying system with sample bag connection design.*

in the freeze drying process, even though noting that the spray drying process was suboptimal and improvements could be made [40].

The DoE approach, in this study, allowed evaluation of the process based on the estimated energy expenditure. Since drying air flow was kept constant throughout the experiments, the aim would be to minimize the inlet air temperature (energy usage per unit time) while maximizing the feed flow rate (reduced processing time) without affecting the quality attributes negatively. Other researchers have shown a strong correlation between the inlet temperature and the outlet temperature [39, 41, 42], whereas they show lesser [39] or even non-significant influence [42] of the feed flow rate on the temperature relationship despite testing much wider feed flow rate ranges than in the current study (3–20 ml/min and 7.3–17.5 ml/min, respectively). This indicates that to reduce the energy consumption in the process, a design space should be created with low outlet temperature and high feed flow rate. In the current study, the best results were achieved at low outlet temperature, which supports reduced energy consumption, whereas the optimized low feed flow rate increases the energy expenditure. The strongest drive in the model to keep feed flow rate at low levels comes, however, from the sharp drop in powder output at higher feed flow rates, meaning a loss in yield at higher feed flow rates. Therefore, considering that the spray drying operation is the last step in the manufacturing process, a drop in yield results in wasted energy and resources in all manufacturing steps upstream from the spray dryer, hence justifying the use of lower feed flow rate for overall reduction in energy consumption.

To evaluate the energy usage of the process, a compact energy meter, Energy-230 Micro LCD (*Vemer, Italy*), was connected to the spray dryer and the dehumidifier. This equipment is designed to display the consumption of active energy in a single-phase system. The electricity usage was roughly 10 kWh when drying one liter of *h*EGF solution. This is consistent with the results of *Baeghbali et al*. [40], showing that a labscale spray dryer coupled with a dehumidifier can manufacture high-quality, dry *h*EGF powder with only a fraction of the energy required for freeze drying [40].
