## 6 Conclusions and Future Prospects  
Like almost all other sectors, the glycerol market was affected by the COVID-19 crisis mainly because many biodiesel plants have diminished production. Despite that, it was attested by the Independent Commodity Intelligence Services that glycerol supply is safe, and the shortcut was caused by logistic problems. Considering that relying on such chemical as raw material is advantageous due to its low cost, favorable physicochemical properties, and the environmental benefits of valorizing it. The main issue of the glycerol side-stream produced by the biodiesel industry is its inability to be absorbed by the traditional markets for pharmaceuticals and cosmetic applications. Thankfully, new uses for glycerol were discovered and, along with that, new treatment methods and even processes capable of using crude glycerol. Among the many subproducts of glycerol, solketal stands out due to its versatility and promising improvements in fuels when used as additive. Moreover, solketal does not require high-purity reactant, depending on how it is proposed to be produced.  
New alternative catalysts have been studied to allow the use of crude or less treated glycerol and to diminish the cost of producing solketal. Additionally, new processes have been proposed to overcome the thermodynamic limitation inherent to this ketalization reaction. Nevertheless, the literature on the use of crude glycerol is still scarce, as well as on alternative technologies that can be applied at large scale to keep up with the glycerol production rate. The Process Intensification strategies proposed in the open literature for the production of solketal present great potential to be applied at industrial scale and an immeasurable prospective environmental benefit. Besides enabling the reaction to achieve higher conversion, these strategies can diminish or even eliminate the need for downstream treatment, appointed by some authors as the most energy demanding step (consequently the costiliest) [25; 121; 122]. Therefore, the most promising strategies are the continuous processes that result in purer product streams, as the Simulated Moving Bed Reactor, thoroughly investigated for the production of other chemicals [146; 147; 148; 149; 150; 151], Membrane Reactors [152; 153; 154; 155], and other hybrid reaction-separation technologies.  
Unfortunately, these Process Intensification strategies are developed at a slow pace, and only few patents propose these strategies at an industrial scale. Additionally, there is a gap in the literature on studies that evaluate the environmental impacts of producing solketal, a common point for all the process mentioned in the present review. Considering that this indicator has been gaining strength, it is relevant that the studies perform a Life Cycle Analysis and provide these type of data to evidence if the proposed technologies are competitive from both economic and environmental aspects. This proves the research on solketal still has plenty of room for investigation.  
This work was financially supported by: Base Funding--UIDB/50020/2020 of the Associate Laboratory LSE-LCM--funded by national funds through FCT/MCTES (PIDDAC); Project PTDC/QEQ-ERQ/2698/2014--POCI-01-0145-FEDER-016866--funded by FEDER funds through COMPETE2020--Programa Operacional Competitividade e Internacionalizacao (POCI); Isabella Correa gratefully thanks the FCT--Fundacao para a Ciencia e a Tecnologia for the Doctoral Grant (2020.07258.BD) through NORTE2020--Programa Operacional Regional do Norte from the FSE--Fundo Social Europeu of the UE.  
The authors declare no conflict of interest.