Towards Predictive Simulation of Single Feed Semibatch Reaction Crystallization
Advancing the chemical engineering fundamentals
Crystallization (T2-9)
Keywords: reaction crystallisation, mixing model, semi-batch
Åke C. Rasmuson and Marie Ståhl
Department of Chemical Engineering and Technology,
Royal Institute of Technology, 100 44 Stockholm, Sweden.
Telephone: +46-8-7908227. Fax: +46-8-105228. E-mail: rasmuson@ket.kth.se
Abstract
Reaction crystallization or precipitation is used in the production of both inorganic and organic compounds, for example organic fine chemicals and photographic materials. A common procedure is to feed one reactant into an agitated solution of the other reactant in a semi-batch (or fed-batch) process. In reaction crystallization processes, the solubility of the formed compound is normally low or very low, and a region with high supersaturation and subsequent rapid nucleation and growth will form around the feed point. Nucleation starts before the solution has been completely homogenized, and the crystallization proceeds in a partially segregated solution. The mixing conditions in the crystallizer will have a significant influence on the final product characteristics.
The semi-batch reaction crystallization of benzoic acid have been investigated in several experimental studies of our research group (Åslund and Rasmuson, 1992, Torbacke and Rasmuson, 2001, Torbacke and Rasmuson, 2004), and the kinetics and aging of benzoic acid has also been studied, based on experiments and population balance modeling (Ståhl et al., 2001, Ståhl et al., 2004). The semi-batch experiments showed that mesomixing has a decisive influence on the experimental results, and that the product weight mean size is influenced by the total feed time and by the feed pipe diameter.
In the present work a population balance model is developed of single feed semi-batch reaction crystallization of benzoic acid. The model accounts for chemical reaction, mixing, nucleation, crystal growth and growth rate dispersion, and the aim is to correctly describe the influence on the product weight mean size of process conditions like the reactant concentrations, agitation rate, feed point location, feed pipe diameter, total feeding time and crystallizer volume. Two mechanistic mixing models are evaluated against previous experimental results. When the mixing is described by the engulfment model and growth rate dispersion is accounted for, the model quite well captures the influence of the processing conditions. It is also found that the kinetics of nucleation and crystal growth have a great impact on the results of the simulations, influencing the product weight mean size as well as the response to changes in the processing conditions.
Literature Cited
Åslund, B., Rasmuson, Å. C., (1992). Semibatch reaction crystallization of benzoic acid. AIChE Journal, 38, 328-342.
Ståhl, M., Åslund, B. L., Rasmuson, Å. C., (2001). Reaction crystallization kinetics of benzoic acid. AIChE Journal, 47, 1544-1560.
Ståhl, M., Åslund, B. L., Rasmuson, Å. C., (2004). Aging of reaction-crystallized benzoic acid. Industrial Engineering Chemistry Research, 43, 6694-6702.
Torbacke, M., Rasmuson, Å. C., (2001). Influence of different scales of mixing in reaction crystallization. Chemical Engineering Science, 56, 2459-2473.
Torbacke, M., Rasmuson, Å. C., (2004). Mesomixing in semi-batch reaction crystallization and influence of reactor size. AIChE Journal, 50, 3107-3119.
Presented Wednesday 19, 15:00 to 15:20, in session Crystallization (T2-9).
