Layer-based and suspension-based melt crystallization (MC) processes, having a high energy saving potential compared to separation techniques like distillation, were compared in the Joule 1 project.
Layer processes are proven technology, but their relatively low separation efficiency leads to a strong reduction of the energy efficiency. The energy saving and separation potentials of a single step suspension process are better. However, the complete separation of the pure crystals from the impure mother liquor is essential for the utilization of these potentials.Therefore the ongoing Joule 2 project focuses on the application of wash columns as solid-liquid separation devices coupled to a suspension crystallizer.
For a number of systems, like phenol and p-xylene, the technical feasibility of such a combination has been proven.
Pilot plant experiments in the Joule 2 project will be started soon. However, for a number of industrially important bulk chemicals the size (distribution) and/or the shape of the crystals which are formed in a suspension crystallizer to the column is yet nog good enough to apply wash columns directly. Examples of such chemicals are e.g. caprolactam and bisphenol A.
Cooling belts and wash columns are technically proven unit operations. In the proposed research project these technologies will be combined for the first time for the development of a widely applicable MC process. It is expected that the impurities from a feed melt are concentrated in grain boundaries in the granules during solidification on a cooling belt. The granules have a relatively large specific surface area compared to a solid layer and comparable to that of crystals in suspension-based MC processes. Due to this increase in surface area it is expected that the removal of impurities from the granule by means of mechanisms like diffusion washing and sweating, which have been shown to be effective in layer MC processes, will go faster and/or better for granules than for layers.
Furthermore, the narrow size distribution
of granules and their almost spherical shape seem to be perfect for wash columns.
It is expected that the properties of the granules can be controlled to a large extent by the granulation process and will be less strongly related to the properties of the compound than the properties of crystals in a suspension crystallization.
The fact that the size of the granules will be larger than that of crystals formed in a suspension crystallizer will make the processing of melts with a high viscosity in a wash column relatively easy for the new technology. For these reasons, granulation of organic melts followed by purification of granules using wash columns seems to be a very promising and widely applicable technology.
The final aim of the project is the development and testing of a pilot in which the combination of granule formation and wash columns technology is applied to industrially important chemicals.
KEYWORDS (max 10) melt crystallization, separation, granulation, wash columns, purification, organic bulk chemicals
Funding SchemeCSC - Cost-sharing contracts