The application of different seeding techniques for solution crystallization of ammonium sulphate
Advancing the chemical engineering fundamentals
Crystallization (T2-9)
Keywords: crystallization, seeding, ultrasound, reproducibility, crystal size distribution
The design and operation of solution crystallization processes still pose many challenges despite its wide application in chemical industry. In case of batch-wise operation, the reproducibility between the batches is one of these challenges. Fed-batch experiments show a strong dependency of the final product on the initial conditions of a batch. Primary nucleation results in ill-defined initial conditions and therefore seeding is often applied as an additional process actuator. This contribution will analyze in more detail the influence of seed quality on the reproducibility between batches and the final product quality for an ammonium sulphate water system. In particular, the focus will be on the application of ultrasound to produce small seed crystals with a high surface to volume ratio.
Grinded seeds are favorable because the number and size of the crystals can be determined accurately. The disadvantage of grinded seeds is the occurrence of initial breeding and their lack of stability due to the presence of lattice strain, which causes partial dissolution upon introduction in a crystallizer. An alternative experimental procedure that was developed uses a small draft tube crystallizer in which seeds are produced with primary nucleation. These seeds do not suffer from the lack of stability because they are virtually free of strain. An anti-solvent was used to produce a large amount of nuclei and therefore a large area to volume ratio. The weak point of this procedure is the occurrence of agglomerates which reduces the specific surface area and broadens the CSD.
This drawback can be circumvented by using ultrasound to produce the seed crystals. Ultrasound can produce crystals with a narrow mono-modal distribution with small median size, which is favorable for seeding purposes. Applications of the various procedures are demonstrated in a 75L DT crystallizer.
See the full pdf manuscript of the abstract.
Presented Wednesday 19, 16:20 to 16:40, in session Crystallization (T2-9).
