(186c) Membrane-Assisted Crystallization of L-Ascorbic Acid
AIChE Annual Meeting
Monday, November 4, 2013 - 3:59pm to 4:21pm
The design and operation of solution crystallization processes still pose many challenges despite its wide application in chemical industry. The performance of the crystallization process and final product are closely related to specific properties of the crystals such as purity, crystal size distribution (CSD), and shape. Many interacting physical phenomena occur in solution crystallization processes, involving for example primary nucleation, growth and attrition of crystals. Supersaturation is a key driving force for these physical phenomena. Therefore, precise control over the generation of supersaturation is of crucial importance. In addition, improved yields and reduced energy consumption for the generation of supersaturation are of interest to improve sustainability of crystallization processes. The latter is especially relevant for evaporative crystallizers.
Membranes offer an interesting opportunity to generate supersaturation as alternative to conventional evaporative crystallization.1,2 They can be used to increase the surface area available for evaporation per unit crystallizer volume and there is no restriction related to entrainment. Furthermore, in the design phase local gradients in supersaturation can be anticipated as a designer can choose the locations where solvent removal takes place. Also, membranes can be used for thermal labile compounds as they can be applied at low temperatures and outside boiling conditions. Finally, membranes offer an opportunity to reduce energy consumption, which is particularly true for application of reverse osmosis. However, before industry can fully embrace membrane-assisted crystallization processes, several challenges have to be met. First, the risk of membrane fouling due to crystallization on the membrane surface has to be mitigated such that stable operation for sustained periods of time can be achieved. Secondly, validated dynamic models for systems that integrate membranes and crystallization are required for selected case studies such that optimal designs and operational policies can be compared to conventional processes.
The objective of this study is to compare the performance of membrane-assisted crystallization of two model systems that both require a different membrane technology. A mini pilot-plant setup has been constructed featuring a separate crystallization vessel and a membrane module. Both sections of the pilot plant can be operated at different conditions, which allows for contacting the membrane surface with an undersaturated solution to prevent membrane fouling while operating the crystallization section at supersaturated conditions. The first model system is an adipic-acid water system for which membrane-assisted crystallization with reverse-osmosis membranes (MaC-RO) is feasible.3,4 The second system studied is crystallization of L-ascorbic acid from water. The high osmotic pressure of this system prohibits application of MaC-RO; hence the experimental setup was modified to apply membrane distillation (MaC-MD). The measured flux over the membrane for MaC-MD was an order of magnitude lower compared to the measured flux for MaC-RO. Therefore, a much larger surface was installed for MaC-MD (1.4 m2 for MaC-MD compared to 0.024 m2 for MaC-RO). The experimental data has been used to model both the tested MaC-RO and MaC-MD processes including crystallization kinetics. Subsequently, the model has been used to reveal optimal operating conditions to obtain a desired supersaturation profile in the crystallizer while minimizing energy consumption. A comparison of the results for both case studies illustrates typical strengths and weaknesses of MaC-RO compared to MaC-MD and provides guidelines for the design and operation of future membrane-assisted crystallization processes.
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This work was supported by the European Commissions Framework 7 program through the OPTICO consortium