# Tannin Extraction from Chestnut Wood Waste: From Lab Scale to Semi-Industrial Plant
## 3 Results and Discussions
### Semi-Industrial-Scale Subcritical Water Extraction (Si-SWE)  
The extraction of CW was also investigated for a suitable scale-up in a semi-industrial plant, able to work in subcritical water. In detail, 13.5 kg of CWC were loaded into the reactor baskets, using an S/L ratio of 1 to 10. The extraction in these conditions was carried out starting from a larger-sized matrix with respect to the one exploited at a small scale. Figure 8: Tannin percentage of diafiltration retentates; labels and sphere radius indicate the polyphen percentage. Sum of tannins and monomeric polyphenols is normalized to 100. This expedient was necessary to meet industrial requirements, as production wastes are usually about 3 cm-sized. The extraction under these conditions was carried out starting from a larger-sized matrix than the one used in the small scale. This was necessary for meeting the requirements of the industry, since the production waste is usually about 3 cm in size. In the development phase of the process, they were used as such to avoid another grinding step, which would excessively burden the total energy consumption and treatment time. It should be noted that conventional laboratory-scale reactors could not handle matrices of this size without comminution. Nevertheless, the use of a 2-3 cm chip size was possible due to the efficient mass transfer provided by the extractor through the forced recirculation of subcritical water through the material confined in perforated baskets. The screening of SI-SWE of CWC took the lead from the conditions applied in the first set of lab-scale extractions. Hence, a first treatment was carried out at 120 \({}^{\circ}\)C for 5 min and the resulting extract was sampled for subsequent analysis. The system was discharged and then re-filled with the same amount of water, bringing the temperature up to 150 \({}^{\circ}\)C for 15 min. The results expressed in yield and TPC are shown in Table 8.  
The yields obtained are significantly lower than the same conditions carried out on a laboratory scale (2.89% versus 12.15% and 3.47% versus 5.48%). This difference is most likely due to the different size of the wood chips affecting the mass transfer. This could be compensated by longer extraction times, which are, however, still below the times usually used in industry for conventional extraction. Concerning the TPC selectivity, this parameter is comparable for the extracts achieved at 120 \({}^{\circ}\)C (646.36 mgGAE/gext lab scale versus 516.33 mgGAE/gext pilot scale) while for the second step, the pilot scale had a much higher result (242.77 mgGAE/gext versus 666.94 mgGAE/gext). This phenomenon can be explained by the slower extraction kinetics, which left more residual metabolites in the post-extraction matrix at 120 \({}^{\circ}\)C, then recovered by the treatment at 150 \({}^{\circ}\)C. The global process yield is 6.26%, with a corresponding polyphenol recovery of 37 gGAE for each kilogram of processed matrix. A second set of extraction was lately performed in the semi-industrial plant, considering some protocol modifications such as a longer time and smaller particle size (_approx._ 1 cm), to increase the process yield and obtain a more exhaustive extraction of the matrix. The experimental protocol involved three different steps: the first one carried out at 100 \({}^{\circ}\)C for 60 min, the second at 120 \({}^{\circ}\)C for 30 min, and the last one, 150 \({}^{\circ}\)C for 30 min (as shown in Table 9). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Temperature (\({}^{\circ}\)C)** & **Time** & **Yield** & **TPC Selectivity** & **TPC Yield** \\  & **(min)** & **(\%)** & **(mgGAE/gext)** & **(mgGAE/gDM)** \\ \hline
120 & 5 & 2.89 & 516.33 \(\pm\) 9.65 & 14.92 \(\pm\) 0.28 \\
150 & 15 & 3.47 & 666.94 \(\pm\) 18.12 & 23.12 \(\pm\) 0.63 \\ \hline Global Process & & 6.26 & - & 37.36 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Extraction yield and TPC analyses for double sequential extraction.