## 5 Solketal Production
### Processes
of US$3000/ton, one can assume implementing a solketal production plant would be highly profitable [122, 123].  
Al-Saadi et al. simulated and compared the performance of three industrial flowsheets: (I) a one-stage solketal process plant operated at 8.5 wt% Sulphonic Acid-Functionalized Copolymer catalyst (ST-DVB-SO\({}_{3}\)H), 323 K, and a glycerol to acetone molar ratio equal 1:6, obtaining 87% solketal yield; (II) a similar plant with a glycerol to acetone molar ratio equal to 1:12, obtaining 98% solketal yield; (III) and a two-stage plant with the first step operating at 8.5 wt% ST-DVB-SO\({}_{3}\)H, 323 K, and a glycerol to acetone molar ratio equal to 1:10, obtaining 84% solketal yield in the first step, followed by removal of formed water, and another reaction under the same conditions to achieve 98% solketal yield. The two-stage plant (Figure 12) was found to be the most economically appealing, because the separation costs were lower than the one-stage processes, resulting, according to the authors, in a Net Present Value (NPV) 45.7% higher when compared with (I) and 0.57% higher when compared with (II); therefore, (III) will be described further [25].  
The flowsheet comprises two mixers, the first to unite recycled and fresh acetone, followed by the second where glycerol is inserted. Then, the stream is heated in a heat exchanger up to the reaction temperature, which occurs in a plug-flow reactor packed with the catalyst. The first reaction's product stream is separated in a distillation column, the acetone recovered as distillate is reinserted in the second reactor and the bottom stream containing glycerol, water, and solketal is fed to a separation unit (not specified), where water is removed. The products stream is cooled in a heat exchanger prior to being fed to the second reactor, where almost all the remainder glycerol is converted. What follows is the second distillation column, where acetone is collected and recycled back to the system and solketal is the bottom product with a content of 98% solketal and 2% glycerol. According to the authors, the separation cost is lower because, in the second reaction, only 0.16% of the products' stream is water. The cost of producing solketal in the described plant, considering a capacity of 100,000 t/years and a 20-year lifetime, was estimated at $2058/ton, well below the selling value reported by the authors US$3000/ton [25].  
Figure 11: Flowsheet of the industrial solketal production plant proposed by da Silva et al. [122].  
An even deeper analysis was proposed by Chol et al. in a study that compared three scenarios comprising the treatment of crude glycerol and its valorization into solketal and glycerol carbonate. The authors concluded that the most economically advantageous flowsheet performs the valorization into both products, with glycerol equally divided for both routes [124].  
The flowsheet (Figure 13) will be briefly described, with more focus on the solketal production steps. Initially, crude glycerol and methanol were fed into a mixer to lower glycerol viscosity so the mixture can undergo a saponification reaction using KOH in the first reactor. The resultant stream was fed to a second reactor to be acidified using HCl before being transferred to a gravity separator for sedimentation, where glycerol (bottom layer) was separated from free fatty acids (light liquids) and vapors. The resultant glycerol was cooled in a heat exchanger prior to solvent extraction using petroleum ether in a liquid-liquid extraction vessel. Petroleum ether was then recovered in a flash separator for reuse and glycerol was neutralized with KOH in another reactor. Then, the resultant glycerol was fed into a membrane separator, where impurities were separated from the glycerol, methanol, and water solution, and in turn, vaporized in a flash separator. Glycerol was cooled and split into two streams. Half of the glycerol was converted into glycerol carbonate by feeding it to a mixer, where it was combined with recovered methanol, then heated and inserted in the reactor with dibutyltin oxide (catalyst) and excess CO\({}_{2}\). The other half of glycerol, i.e., the stream leading to solketal production, was pressurized (12,000 kPa) before being fed into a conversion reactor with acetone and heterogeneous catalyst Amberlyst-15. The reaction was carried at 343 K for 1.5 h. The separation was promoted by a flash separator, where acetone with trace water was vaporized and recovered and the solketal stream was split into pure solketal and a recycle stream that was sent back to the reactor to improve conversion, since glycerol conversion is only 35% but can be enhanced with product recycle, according to the authors [124].  
The economic analysis of implementing this industrial flowsheet is also promising, the cost of treating one ton of crude glycerol would be US$5045, while the revenue cost would be US$8036, resulting in a profit of almost US$3000/ton of glycerol.