NaHCO3 crystallization using vacuum membrane distillation : effect of organic impurities, optimization of the yield and comparison with conventional crystallization

Details

Supervisors

Luis Alconero, Patricia

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil en chimie et science des matériaux, à finalité spécialisée

Abstract

A potential future post-combustion CO2 capture and utilization application is crystallization. Crystallization is a solid-fluid separation process driven by supersaturation. It induces solid particles/crystals from a liquid solution. This is a quite complex process but well essential for a lot of chemical and engineering applications. Among the main applications of crystallization there are : carbon capture and utilization, waste water treatment, production of high-value added compounds or recovery of drugs and high-purity chemicals. Conventional crystallization processes suffer from some limitations and drawbacks. The main ones are the high energy cost of the process, the difficult control of supersaturation and the non-optimal reproducibility of the crystal characteristics. To face these problems, the use membrane operations with solution crystallization may be a solution. Indeed, there are more and more researches about membrane technologies since the 80’s that show potential to provide a precise regulation of supersaturation, fast crystallization rates, well-controlled nucleation and growth kinetics and allowing of heterogeneous nucleation on their surface. Different types of membrane crystallization exist. In this work one particular type is studied : vacuum membrane distillation-crystallization process (VMD/VMCr). The working principle is the following : the feed solution flows at one side of the membrane and the volatile species evaporate through the hydrophobic porous membrane and are recovered/condensed at the other side of the membrane (permeate). The evaporation is due to the difference in partial pressure between the permeate side which is vacuum and the volatile molecules in the feed solution. The evaporation leads to progressive concentration of the feed solution and will end up with crystallisation. This work focuses on N aHCO3 feed solutions and on the effect of organic impurities (amino acids and enzymes) on the results of the VMD process, on finding some ways to improve the yield of the VMD process and on the comparison of the results obtained with VMD process with the ones obtained with conventional cooling crystallization process. The results are analyzed in terms of : (1) relative supersaturation, (2) transmembrane flux, (3) overall mass transfer coefficient, (4) water recovered, (5) crystals induction time, (6) crystals yield, (7) crystals morphology, (8) crystals PSD and (9) crystals purity. The results show that the higher the concentration of organic impurities added, the higher the relative supersaturation, the more water recovered, the longer the induction time and the less pure the crystals. The PSD curve is a bit affected by the addition of organic impurities but the shape remains similar and the CV increases with the increase of organic impurities concentration. All these effects are more pronounced with Sarcosine than with Arginine and it is not really significant with Enzymes. The morphology of the crystals is also a bit affected with the addition of amino acids but not in the case of enzymes. The transmembrane flux and overall mass transfer coefficient remain constant with the addition of organic impurities. The yield can be easily increased by decreasing the temperature of the solution once it has crystallized or by letting the crystallized solution a certain time at 36 [◦C]. The yield can reach 44 [%] when the temperature is decreased to 5 [◦C] and 12 [%] when the crystallized solution is let 15 [h] at 36 [◦C]. The crystals are bigger when the temperature is decreased to 5 [◦C] and thinner crystals are observed when the solution is let 15 hours after crystallization. The PSD curve of the crystals is quite similar and the CV decreases when the time and the temperature increase. The other parameters remain constant. Finally, cooling conventional crystallization shows less efficient results than the VMD process : higher induction time, lower yield, broader PSD and higher CV. High cooling rate induces less pure crystals that are not spread homogeneously over the whole area.