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The granupure process. New technology for energy efficient separation of melts by combination of granulation and wash column purification

Deliverables

The investigation of the size and shape of the pastilles in respect of their purification potential leads to optimised production parameters. The results will be used to adjust the process parameters of the granulation unit in such a way that the pastilles produced give an optimal purification behaviour in the subsequent wash column process. This knowledge will be published to third parties by seminars and congresses. During pastille production, it was found that the purification potential of pastilles produced by a drop forming process from an impure melt can be influenced by the parameter settings. Different pastille properties are required according to the purification process namely sweating, rinsing, diffusion-washing, and crystallization. These can be either the pastilles' internal crystalline structure or their geometrical properties which in turn can both be influenced by the temperature of the pastillation surface and the initial impurity concentration of the melt.
The size and shape as well as the purification potential of the caprolactam pastilles led to some design improvements of the Rotoformer. With these improvements the granulated material can be produced nearly dust-free, regular in shape and size and therefore easy to handle for further (purification) steps. As the result, SPS is now able to offer a perfect fitted equipment for producing optimised pastilles for further purification steps. SPS modified the well known Rotoformer equipment so that caprolactam micropastilles with small tolerances with respect to size and shape can be produced. SPS can offer this modified equipment to the chemical industry with a process guaranty.
The general objective of the project was to develop a new, energy efficient and widely applicable methodology, the so-called GRANUPURE process, for the separation of organic bulk chemicals. The cooling belt technology for solidifying a melt into granules was combined with wash columns in which granule purification and solid-liquid separation are carried out. The cooling belt can be coupled directly to a (gravity) wash column or to a so-called pre-purification unit, like a stirred tank reactor. The energy consumption for the Granupure process is significantly lower than that of a conventional separation technique applied in industry for the production of organic bulk chemicals (especially distillation). Furthermore, the Granupure process combines the advantages of the well-known layer-based and suspension-based melt crystallisation techniques. Due to this, a noticeable saving of energy compared to the existing melt crystallisation processes should be possible for equal or even improved product quality. The Granupure fulfils two important demands, namely the (social and technical) demand for reducing the consumption of energy by the chemical industry and the production of high purity products required by industry. Two well-known and technically proven unit operations, namely cooling belts for granulation and wash columns for purification, are combined for the first time in a separation process. Due to the modular design, the scaling-up strategy for both unit operations is rather straight-forward. In principle, the Granupure process will be applicable to the typical production rates required for organic bulk chemicals (around 100 ktons/year). Lower production rates, which are typical for e.g. fine chemicals and pharmaceuticals, are also covered by the Granupure process. The final aim of the project was to deliver the proof of principle of the Granupure process using an integrated pilot plant which was tested for caprolactam, an industrially important chemical. The maximum production capacity of the integrated pilot plant was about 50 kg/hour. Operation of the integrated pilot plant for caprolactam was difficult but feasible. The measured purification for the option cooling-belt-stirred tank reactor-hydraulic wash column was very good (removal model impurity > 99%), thus delivering the proof of principle for the Granupure process. The logical next stage in the development trajectory is a demonstration of the integrated technology on a scale that is relevant for the potential users of the technology. Typically, such a demonstration should be carried out at a production rate of about 1 ton per hour. The aim is to demonstrate the scale-up potential of the technology and to obtain a convincing reference for the implementation of the Granupure technology (at larger scale). An interested end-user in the chemical industry is required for execution of a demonstration project. Financial support might be required or at least helpful. The result is an innovative separation process and a combination of equipment.
An alternative energy-saving technology based on combination of sweating and vacuum distillation in a single apparatus with their simultaneous occurrence is developed. The combined process is carried out at a pressure close to the triple point pressure of the major component. In this case a sweating liquid changes to the vapour state and the vapour comes out of the pastille and is withdrawn into a condenser. The method of distillation sweating was employed for the pastille purification of caprolactam from the impurities of water and cyclo-hexanone as well as naphthalene from biphenyl and conversely. The results obtained indicate a high efficiency of the process. In contrast to distillation, in the combined process both more volatile impurities (with respect to major component) and less volatile ones are removed. The purification of both naphthalene pastilles containing 4 wt% biphenyl and biphenyl pastilles with 4 wt% naphthalene (but with a lower yield) was successful. However, non-volatile impurities cannot be removed in this way. The technology being developed in the project which combines the production of granular materials with their purification meets the requirements of low-capacity chemical and pharmaceutical productions of relatively expensive products.
A new technology to purify organic solid compounds by melt crystallization has been developed. Solid particles up to few millimetres in size are most suitable for the treatment. The technique can be easily implemented as a section of a chemical production plant. Both the cost of investment for the construction, and the energy consumption for the operation, are low in comparison to traditional technologies. It is based on the dry sweating unit operation, that is partial melting of the solid induced by the contact with a hot gas stream, and is typically applied to improve the purity grade from 95% to 99.5%. The molten fraction is lower than 15% and the impurities can be concentrated in the residue molten stream up to one order of magnitude. The energy consumption is 83 W per kg of treated solid.

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