(656e) Development of Sorption Enhanced Reactive Processes for the Sustainable Production of Solketal through the Ketalization of Glycerol

Several research groups around the world have recently identified Solketal as one of the most promising solution towards the valorization of glycerol which has become one of the major topics to ensure the sustainability of the entire biodiesel production chain, since this polyol is obtained as a by-product of the Biodiesel production representing approximately 10 wt% of the total mass of diesel produced. Solketal presents interesting properties as diesel additive which would allow its addition to the final biodiesel formulation not only increasing the quality and the amount of diesel produced but also reducing the overall process by-products formation.

For the synthesis of Solketal, glycerol is commonly reacted with acetone in the presence of an acid catalyst, forming water as by-product [1]. Following the general “Green Chemistry” guidelines, the use of heterogeneous catalysts as ion-exchange resins, zeolites and some silica base materials has been proposed [2]; however, most of the research published focused in batch reactor studies for the optimization of the reaction conditions. In another approach, multifunctional reactors have also been proposed for the production of solketal, which can represent a sustainable alternative for the production of this compound, due to their ability to overcome the thermodynamic limitations associated with this chemical reaction by continuously removing one of the products from the reaction media, which will also result in an increase of the process productivity and in a reduction of its capital cost. In fact, Clarkson et al. [3] and Roldan et al. [4] have proposed the use of reactive distillation and membrane reactors for this purpose, respectively. Although the results presented where satisfactory in terms of glycerol conversion and solketal yield (typically above 90%), it is commonly acknowledged that distillation processes present a high energy demand and zeolite membranes lack long term stability. In this context, the present work represents an innovative approach for the production of solketal making use of sorption enhanced reactive processes, which can outcome the previously studied multifunctional reactors performances based on the high purities and productivities attained by adsorptive-reactors in similar systems [5] and by the process energy efficiency and lower operating temperatures.

A previous study concluded that Amberlyst-35 and ethanol were the most suitable stationary and mobile phases for the synthesis of solketal through a sorption enhanced reactive process, respectively, and fundamental adsorption and reaction data were determined through independent set of experiments providing essential knowledge for the definition of some of the most relevant process operating conditions.

The synthesis of solketal was then experimentally performed in a fixed bed adsorptive reactor. The maximum conversion achieved in a transient state by feeding an equimolar mixture of glycerol and acetone diluted in 50% of ethanol (at 313 K and 5 mL/min) was 60%, while the steady state conversion achieved under these conditions was only 44%. Nevertheless, this value is still close to the reaction equilibrium conversion which is approximately 50%. By changing the reactants molar ratio in the feed mixture from 1:1 to 2:1 (with an excess of acetone) the maximum conversion attained reached 80% in a transient state and the equilibrium conversion was achieved at steady state. These results demonstrate the potential of sorption enhanced reactive processes for the synthesis of solketal. Moreover, it was possible to develop a mathematical model that was able to accurately describe the experimental results obtained in the fixed bed adsorptive reactor experiments considering isotherm operation and an axially dispersed plug flow at a constant bed length and porosity. The non-idealities of the mixture were accounted for by determining the activity coefficients through the UNIFAC method and the internal and external mass transfer coefficients between the liquid and the solid phase were estimated and lumped in a global mass transfer coefficient.

Nevertheless, the fixed bed adsorptive reactor productivity is known to be limited requiring, at the same time, the use of significant amounts of solvent for regeneration purposes, and it would not allow the implementation of a continuous production process. Therefore, the production of solketal in a Simulated Moving Bed Reactor (SMBR) was studied, as this chromatographic reactor overcomes all the aforementioned drawbacks. For that purpose, glycerol and acetone were used as reactants and were introduced in the unit through the feed stream while ethanol was used as desorbent. As solketal is the less retained species it will be collected in the raffinate stream while water, the most retained one, will be collected in the extract stream. Consequently, water is selectively removed from the reaction media leading to the displacement of the thermodynamic reaction equilibrium and considerably increasing the reaction conversion. To evaluate the performance of this technology, a comprehensive simulation study was performed, based on the True Moving Bed Reactor model approach reported elsewhere [6], to optimize the design variables associated with the SMBR operation following to main approaches: using a conventional 4 sections SMBR and feeding a 1:1 mixture of glycerol and acetone, and using a non-conventional 5-sections SMBR in which pure glycerol and pure acetone are fed through independent streams at different columns of the unit (typically feeding acetone closer to the extract port and glycerol closer to the raffinate port to promote the counter-current movement of both of these species based on their adsorption selectivity).

The results for the conventional 4-sections SMBR demonstrated that the purity (in an eluent-free basis) attained in the raffinate stream was 90% which is relatively low compared with the values obtained for similar acetalization reactions performed in SMBR (which are typically comprised between 95% and 99% [5]). The low equilibrium conversion together with the low adsorption selectivity between acetone and solketal when using Amberlyst-35 as catalyst/adsorbent and the significant mass transfer resistances associated with this system represent the most likely causes behind the technology limited performance. On the other hand, the 5-section SMBR unit was able to achieve a solketal purity of 95% without compromising the productivity of the process and without leading to any increase in the desorbent consumption. The main factor contributing for this result was the counter-current contact between the two reactants promoted by feeding them independently at different locations within the unit. As the more retained reactant (glycerol) was fed closer to the raffinate port and the less retained one (acetone) was fed closer to the extract port, a considerable increase in the overall reaction rate and reactants conversion was observed throughout the unit. Additionally, by feeding acetone to a column placed at a larger distance from the raffinate collection port, it was possible to enhance the separation between solketal and acetone.

Therefore, it is possible to conclude that sorption enhanced reactive processes represent a potential alternative for the sustainable production of solketal using glycerol as raw material.

References

[1] Bruchmann, B., Haberle, K., Gruner, H. and Hirn, M., Preparation of cyclic acetals or ketals, US5917059 A, 1999.

[2] Trifoi, A.R., Agachi, P.Åž. and Pap, T., Glycerol acetals and ketals as possible diesel additives. A review of their synthesis protocols. Renewable and Sustainable Energy Reviews, 2016. 62: p. 804-814.

[3] Clarkson, J.S., Walker, A.J. and Wood, M.A., Continuous Reactor Technology for Ketal Formation: An Improved Synthesis of Solketal. Organic Process Research & Development, 2001. 5(6): p. 630-635.

[4] Roldán, L., Mallada, R., Fraile, J.M., Mayoral, J.A. and Menéndez, M., Glycerol upgrading by ketalization in a zeolite membrane reactor. Asia-Pacific Journal of Chemical Engineering, 2009. 4(3): p. 279-284.

[5] Rodrigues, A.E., Pereira, C.S.M. and Santos, J.C., Chromatographic Reactors. Chemical Engineering & Technology, 2012. 35(7): p. 1171-1183.

[6] Rodrigues, A.E., Pereira, C.S.M., Minceva, M., Pais, L.S., Ribeiro, A.M., Ribeiro, A., Silva, M., Graça, N. and Santos, J.C., Simulated Moving Bed Technology: Principles, Design and Process Applications. 2015: Elsevier Science.