Written by Mr. Pascal FEUCHT (FEUCHT SA) in collaboration with Mr. RIGAL (Director of COGNIS, Boussens)
COGNIS, a Company belonging to the American financial group Goldman Sachs and the British financial group Permira, manufactures various sterols extracted from the processing of vegetable oils at the Boussens site south of Toulouse. The sterols are used in pharmaceutics for the treatment of cholesterol and cardiovascular diseases, in the food and agriculture industries as nutritive compounds, and in the cosmetic industry.
The increase in the demand for sterol and sterol derivatives led to the creation of a new manufacturing unit. After a phase of traditional synthesis, this sterol is isolated in the form of a melted product which must be processed by cooling.
The choice of a drum flaker, which has been in use since the launching of the product, was a matter for consideration within the context of this new facility. This proven technology, however, has the disadvantage of producing dust during the breaking up of the solid film into flakes. These dust particles cling partially to the scales, but also cover the mechanical parts of the flaker. Therefore, a careful cleaning of the machine between the various production campaigns is required.
The flakes produced and put in big bags also tended to stick to each other.
Among the different possible solid forms, the pelletizing process has been considered, since COGNIS already possesses a cooling band. A pellet-making machine with superposed disks, occupying less space on the floor, was another solution.
But finally, the prilling technology was adopted.
The granules obtained contain no dust particles and exhibit a very good flow behavior, therefore making their dosage easier for the user of the product.
This solid form also presents the advantage of melting more easily than the pellets, which are more massive and contain some dust particles.
The prilling technology was validated following the production of a batch at the industrial site of GMF-Gouda, which also permitted sampling by the final clients.
Two prilling technologies were in competition: the traditional prilling technology and the cryogenic prilling technology.
COGNIS has chosen the latter, which uses liquid nitrogen at - 196°C to congeal the droplets produced.
This cryogenic prilling process was developed and patented by the Dutch Company GMF-Gouda, represented in France by FEUCHT SA.
The melted product is introduced into one or several prilling heads through a pump or from a pressurized melting machine. Each head is composed of a lower plate with a great number of holes, and an upper vibrating metal membrane.
The stimulation of this membrane, at a frequency defined according to the viscosity of the product, creates the separation of the liquid jets of the product in a succession of perfectly calibrated drops. These drops go through a column in which liquid nitrogen is injected at - 196°C, and this causes an immediate solidification of the external wall of the droplets. The inner solidification is obtained in the fluidized bed located under the cryogenic columns. The annular fluidized bed carries the granules that need to be solidified in a toroidal movement. This assembly (Fig. 1) functions in closed loop with nitrogen. There are no mechanical moving parts.
Fig. 1 The Closed-Loop-System produces compact prills
A cyclone, with an automatic cleaning filter as well as a cooler on the gaseous nitrogen loop completes the installation. The granules are drawn out of the fluidized bed by gravity and fall into an alveolar chamber which feeds the processing line.
When the system was first put into service, it was necessary primarily to regulate the flow and the pressure of liquid and gaseous nitrogen according to the flow of the melted product, as well as the different temperature limits for the control of the procedure.
Concerning the application described above, the rate of dust particles produced in the system and recovered in the cyclone is below 1%. These particles are recycled in a melting machine.
The consumption of liquid nitrogen is about 30 to 35% of the flow of melted product supplied.
This cryogenic prilling technology is innovative, compact, and it allows production levels reaching 10 T/hour. It is an alternative to the traditional technologies inasmuch as the products absorb the extra cost represented by the liquid nitrogen.
