Concentration of Aqueous Solutions by Freeze Crystallization in Biorefinery Applications

Abstract

Freeze crystallization (FC) is a melt crystallization method, defined as water removal based on ice formation and its separation from aqueous solutions at or slightly below the freezing point of the solution. In this work,freeze crystallization (suspension freeze (SFC) and layer freeze crystallization (LFC)) was applied as a concentration method for different aqueous organic solutions obtained in the emerging fields of biorefinery applications, where a concentration step is typically required in downstream processing.The studied aqueous solutions were 1,5-diazabicyclo[4.3.0]non-5-eniumacetate ([DBNH][OAc]) ionic liquid solution, aqueous pyrolysis oil (PO) extract, acetone-1-butanol-ethanol (ABE) fermentation broths, and synthetic sucrose and ABE solutions. SFC and LFC were applied to concentrate solutions with freezing points in the range from -0.56°C to -7.51°C.The ice nucleation and growth kinetics were influenced to varying degrees by the solution ideality, undercooling, and initial water concentration in feed solution. The ice yields of various solutions obtained by SFC for undercooling degrees in the range of 0.39-1.87 °C were as follows: 19.8-35.6 % for [DBNH][OAc] solutions (60 min freezing),11.6–68% for PO extract (60 min freezing), and 54.6-86.7% for ABE fermentation broths (80 min freezing).The concentration of impurities in washed ice samples was below the detection limit hence the distribution coefficients were effectively zero for PO extract and ABE solutions, and up to 0.26 for [DBNH][OAc] solutions. Hence, SFC was found to be a very efficient concentration method that removes water in the form of very pure ice with high yields.The ice yields for LFC were lower than for SFC because of the smaller cooling area. Thus, the obtained ice yields for 60 min of freezing at an undercooling range of 0.83-2.28 °C were:1.3-5.8% for aqueous sucrose solution and 3.6-7.15% for [DBNH][OAc] solutions with 60 min of freezing. The heat transfer during cooling of aqueous solutions (10 wt.%, 23 wt.% and 30 wt.% sucrose) prior to ice layer formation was investigated by CFD simulations. The solution properties affected the cooling in such a way that the less concentrated the solution was, the faster the undercooling temperature (supersaturation) was reached, and consequently the convective heat transfer coefficient values were higher. SFC enabled the separation of 1-butanol from synthetic ABE solutions, with average cumulative yields of separated 1-butanol were in range 9.6-59.5% and with purities in the range of 84.8-94.9%. Cooling crystallization from solutions was applied as a levoglucosan recovery method from PO extract. The average recovery levoglucosan rate achieved was 24%. In conclusion, FC is an efficient method for concentrating various biorefinery related solutions. Moreover, it enables separation of sparingly miscible 1-butanol from ABE solutions. Furthermore, cooling crystallization from solutions can be considered as a recovery method for valuable biochemicals, such as levoglucosan, from aqueous PO extracts.