## 5 Solketal Production
### Processes
Thus, considering a payback period of five years, the cost to produce 3720 t of solketal/year was estimated at US$128.50/ton, a much lower value than what was estimated in other studies and which can be considered unrealistic [10; 25; 120].  
Dmitriev et al., one of the few authors that endorsed the use of traditional homogeneous catalysts, performed experiments with H\({}_{2}\)SO\({}_{4}\) and a series of heterogeneous catalysts and concluded that H\({}_{2}\)SO\({}_{4}\) was the most economically appealing for the industrial production of solketal. The authors proposed a similar flowsheet to Zaharia et al. comprising a CSTR, where the reactants and the catalyst would be directly fed, followed by a vessel to collect the reaction products and then three Rectification Columns with the same separation procedure suggested by Zaharia et al. The neutralization of H\({}_{2}\)SO\({}_{4}\) would be carried by adding NaOH in the stream between the vessel and the first column. The authors highlighted that the most energetically demanding step would be acetone recovery because a large reflux ratio is necessary to assure the water content in the unconverted acetone is low [121].  
After stating the many benefits of using heterogeneous catalysts, as higher productivity, no need to purify the products by neutralization, the consequent absence of wastewater due to not needing this step and the prevented corrosion of the apparatus, the authors counteracted the mentioned benefits. They stated that the low miscibility between acetone and glycerol impairs the reaction; thus, low conversion is attained and there is the need of adding a solvent in the reaction mixture. By using H\({}_{2}\)SO\({}_{4}\), there would be no need to use a solvent to enhance the reactants miscibility, which would diminish the equipment costs (no need for a pre-mixer before the reactor) and operational costs (no need to separate ethanol from the products). Additionally, the glycerol recycled back to the reactor has high impurity content, which would quickly deactivate the heterogeneous catalyst and greatly increase the operational costs. The study affirms the proposed flowsheet is simple, reliable, economically efficient and would reach a solketal yield near 100%. No economic evaluation data were provided [121].  
Da Silva et al. proposed a similar flowsheet (Figure 11) but performed a more meticulous economic analysis. The flowsheet comprises two mixers, one for uniting fresh and recycled glycerol, followed by the second mixer where acetone, glycerol from the first mixer, and the Lewis catalyst (solid-supported SnF\({}_{2}\)) are added. The second mixer's outlet stream is the inlet of the reactor (generic conversion reaction vessel operating at 298 K and 1 bar), which is expected to reach a glycerol conversion of 80%. The condensate stream with the products, unconverted reactants and catalyst follows to a filter to remove the catalyst. The filter's outlet stream is inserted in the first distillation column to separate the remainder of acetone and water, which is the most energetically demanding step. The bottom stream follows to the second distillation column to obtain high purity solketal (99.6%) and the unreacted glycerol that will be recycled back to the system. Acetone is not recycled [122].  
Figure 10: Flowsheet of the industrial solketal production plants proposed by Zaharia et al. (without vessel) and Dmitriev et al. (without mixing equipment) [120; 121].  
One of the most significant economic concerns of an industrial plant is the catalyst cost. SnF\({}_{2}\) is highly recoverable and recyclable; therefore, it was assumed to be replaced twice a year, accounting for only 0.005% of the annual operating costs. The other consumables account for nearly 17.50% of the operating costs. The total cost of producing solketal was US$1229/ton; thus, when compared to the selling price stated by Al-Saadi et al.