It is known that separation of components by crystallization can be effected by introducing supersaturation of the crystallizing product component in a liquid or gaseous solvent in a crystallizer. Supersaturation is induced by cooling the liquid or by evaporation of the solvent or by applying heat transfer through internals in the crystallizer.

Patent application WO 95/01345 (KEMIRA OY) discloses a process for the preparation of very pure, crystalline melamine. This process comprises a step wherein a gas mixture of melamine and ammonia is fed into a crystallizer. The crystallization of melamine is effected by introducing a cooling liquid-water or liquid ammonia-into the crystallizer. The cooling liquid, when vaporizing, will bind the heat released in the cooling and crystallization of melamine. In such a crystallization process as well as in other cooling crystallization processes, the highest supersaturation occurs at the coldest places in the crystallizer, i. e. against the walls and internals of the crystallizer, resulting in crystal growth on these surfaces. Such crystal growth complicates the crystallizer operation and product handling. Hence, crystal growth against walls and internals is a major concern in crystallizer design.

It is therefore an object of the present invention to provide a crystallization process wherein crystal growth against the walls and internals of the crystallizer is avoided, and wherein the product quality and handling is improved and crystallizer operation is facilitated. A further object of the present invention is to provide a device for carrying out the process of the invention.

Thus, in a first aspect of the invention there is provided a process for the separation of a component from a gaseous medium by crystallization comprising injecting a gaseous medium containing the component to be separated into a cooling liquid in a crystallizer to form free traveling vapor bubbles which upon cooling induce super- saturation of the crystallizing component with subsequent crystallization of the

component at the phase interface of the free traveling vapor bubbles in the cooling liquid.

In a preferred embodiment the gaseous medium comprises besides the component to be crystallized also vapor of a condensable solvent. The component to be crystallized can be supplied as a diluted component in said gaseous medium wherein the condensable solvent is the main component.

Preferably the component to be crystallized comprises a compound which is not totally soluble in the cooling liquid. Typically the compound is essentially insoluble or sparingly soluble in the cooling liquid. Preferred compounds are melamine and compound having a similar crystallizing behaviour to melamine.

Preferably the cooling liquid comprises the condensable solvent of the gaseous medium in liquid state.

Preferably the condensable solvent and the cooling liquid comprise a compound which does not totally dissolve the crystallizing component. Examples of such compounds are ammonia and water.

The temperature of the cooling liquid is lower than the boiling point of the liquid at the system pressure.

The process according to this invention can be applied to components which can be supplied as a diluted component in gaseous medium wherein the condensable solvent is the main component. Preferably the cooling liquid comprises the condensable solvent of the gaseous medium in liquid state. Furthermore the diluted component in gaseous medium can be solidified in the presence of cooling liquid.

The compounds to be crystallized include melamine, paraffin, cresol, xylene, dichlorobenzene or other substituted benzenes or similar compounds when for example water or ammonia is used as cooling liquid.

Other suitable compounds to be crystallized according to the present invention include following organic compounds: 2-aminophenol, isophthalic acid, anthracene, naphthalene, anthranilic acid, 2- naphthol, anthraquinone, phthalic anhydride, benzanthrone, phthalimide, benzoic acid, pyrogallol, 1,4-benzoquinone, salicylic acid, camphor, terephthalic acid, cyanuric chloride and thymol; and following elemental and inorganic substances:

aluminium chloride, iron (III) chloride, arsenic, magnesium, arsenic (III) oxide, molybdenum trioxide, calcium, sulfur, chromium (III) chloride, titanium tetra- chloride, hafnium tetrachloride, uranium hexafluoride, iodine and zirconium tetra- chloride.

The process of the invention can be applied to the crystallizing stage of a melamine production process, for example the process described in WO 95/01345. In that case the component to be crystallized comprises melamine and the condensable solvent and the cooling liquid comprise ammonia.

Preferably the bubbles are formed by injecting the gaseous medium through a nozzle submerged in the cooling liquid.

The gaseous medium and the cooling liquid can be fed counter-currently into the crystallizer.

The process of the present invention is especially useful in the separation of at least two components from a gaseous medium.

In a second aspect of the invention there is provided a device for carrying out the process of the invention. The device of the invention comprises a vessel having a first inlet means for the gaseous medium, a second inlet means for the cooling liquid and an outlet means for the crystallized component, said first inlet means for the gaseous medium comprising a nozzle positioned inside the vessel, said nozzle comprising a housing and having a nozzle inlet, a nozzle outlet and an internal baffle guiding the gaseous medium supplied to the nozzle and forming a labyrinth path connecting the nozzle inlet and nozzle outlet.

Preferably the outer walls of the housing are made of heat insulating material to avoid heat losses to the environment.

Preferably the internal baffle is made of a heat conducting material and extends to the tip of the nozzle. This baffle material conducts heat from the hot gas entering through the nozzle inlet to the nozzle tip.

The nozzle can additionally comprise a piston movable backward-and-forward within the housing.

One of the advantages of the present invention is that there is no crystal growth against walls and internals of the crystallizer. The present invention makes crystallization occur at the phase interface of free traveling vapor bubbles in a

subcooled solvent. The crystals originating at the interface are trapped and collected in the liquid phase immediately. Crystal growth is stopped in the subcooled liquid phase, resulting in small particle sizes typically within the range of 0.011000, um.

According to the present invention it is possible to control the particle size of the crystals of the crystallizing compound by changing the temperature of the cooling liquid and/or the size of the submerged nozzle. For example by lowering the temperature of the cooling liquid the grain size will decrease and by decreasing the nozzle size the grain size will decrease.

An other advantage is that the relatively high heat transfer rates at the liquid-gas interface assures rapid cooling of the gaseous feed to the boiling point and transfer of sublimation heat and high supersaturation levels. This will result in high nucleation rates and therefore small crystals are formed.

The invention is hereafter described in more detail with reference to the enclosed drawings in which Fig. 1 shows schematically a preferred crystallizing device of the present invention, and Fig. 2 is a partial cross-sectional view of a preferred nozzle of a crystallizing device of the present invention.

Referring to Fig. 1 the device of the invention comprises a closed vertical cylindrical vessel 1 having an inlet opening 2 for cooling liquid and an outlet 3 for slurry discharge containing the crystallized product compound (e. g. melamine) and cooling liquid (e. g. ammonia). The vessel is filled with the cooling liquid. The liquid is supplied to the vessel at a temperature well below the boiling point of the liquid phase at the system pressure and at a flow rate sufficiently high to remove the heat transferred from the gas phase, throughout the vessel volume.

The heated gas mixture containing the compound to be crystallized (e. g. melamine) and vapour of a condensable solvent (e. g. ammonia) as main component is injected into the vessle through a labyrinth nozzle 4 submerged in the liquid. Vapor bubbles will form at the nozzle tip, cool down and finally condense as they rise through the liquid. Cooling of the vapor phase in the bubbles and additionally condensation of the solvent induces supersaturation and as a consequence crystallization of the product compound at the bubble surface.

The sublimation enthalpy of the product compound, the condensation heat of the gaseous solvent and the latent heat of the gaseous feed mixture are transferred to the liquid which fills the vessel. The system pressure and solvent compound are chosen to effect a saturated vapor pressure of the product compound lower than the initial partial pressure in the gas phase at the conditions of the bubble surface. The conditions at the bubble surface are determined by the feed composition, the boiling point of the liquid and the system pressure.

The crystals formed at the phase interface are trapped and collected in the liquid phase immediately. These crystals are essentially insoluble in the cooling liquid but depending on the operating conditions scarce partial dissolution may occur.

A preferred labyrinth nozzle design is depicted in Fig. 2. The nozzle 4 comprises a housing 5 and is provided with a nozzle inlet 6 and a nozzle outlet 7. Furthermore the nozzle contains an internal cylindrical fin 8 made of a heat conducting material. The purpose of this labyrinth nozzle design is to conduct as much heat from the hot gas mixture to the nozzle tip 9 as possible, to avoid crystallization against the walls of the nozzle caused by cooling of the injected gas mixture. The outer walls 10,11 of the housing 5 are made of heat insulating material to avoid heat losses to the environment.

The back end of the housing 5 contains a piston valve 12 that enables quick and easy closing of the nozzle by pushing the piston against the back end of the internal fin 8.

The following examples further illustrate the present invention, but of course, should not be construed as in any way limiting its scope.

Examples Example 1 Several crystallization experiments within the temperature and pressure range given in Table 1 were performed according to the description of the invention using the crystallizing device shown in Figure 1. The test component to be crystallized was p- dichlorobenzene (1,4-dichlorobenzene) purity 99.9 wt-% and the condensable solvent as well as the cooling liquid was water.

The test component was crystallized in this submerged crystallization vessel (height 840 mm, diameter 100 mm) operating at a temperature of Tv °C and a pressure of

Pv bar. The heated gas mixture containing p-dichlorobenzene and steam as main component was injected into the vessel through a nozzle submerged in the sub- cooled liquid. The gaseous medium and the cooling liquid were fed counter- currently into the vessel. Liquid level inside the vessel was controlled leaving free gas space in the top section of the crystallizer. Four transparent monitoring windows were attached to the vessel flanges in various elevations for visual inspection of the operation and to observe the fouling and plugging tendency of the crystallization apparatus.

The cold water feed, at a temperature between was supplied to the vessel at a flow rate sufficiently high to remove the heat transferred from the vapour feed. The observed crystal formation was extremely rapid showing that nucleation rate is fast enough to generate small crystals, essentially of size 30-50 pm with a narrow size distribution.

No crystal growth on the walls and internals was observed inside the crystallizer.

Also dust formation was not observed above the liquid layer.

The formed crystals were easily trapped and collected to the liquid phase. This product slurry containing crystals and cooling liquid was continuously taken out from the bottom of the crystallizer. The flow rate of the cold water feed was sufficiently high to carry all crystals to the product slurry.

After separation from the cooling liquid a sample was taken from the crystals for analysis. The purity of the p-dichlorobenzene was determined before and after crystallization by using gas chromatography. The operating conditions and results are given in Table 1.

This example proves that the crystallization process and the device for carrying out the process according to the present invention succeeds in avoiding crystal growth against the walls and internals and that the product quality is good.

Example 2 Several crystallization experiments were performed as described in example 1. The test component to be crystallized was fully refined paraffin wax with melting point at 54-56 °C and purity 99.8 wt-%, and water was used as the condensable solvent as well as the cooling liquid.

The heated gas mixture containing paraffin and steam as main component was injected into the vessel through a nozzle submerged in the liquid. The gaseous medium and the cooling liquid were fed counter-currently into the vessel.

The cold water feed, at a temperature of was supplied to the vessel ata flow rate sufficiently high to remove the heat transferred from the vapor feed. The observed crystal formation was extremely rapid showing that nucleation rate is fast enough to generate small crystals.

No crystal growth on the walls and internals was observed below the cooling liquid surface of the crystallizer. Also dust formation was not observed above the liquid layer. During the experiments with paraffin a floating crystal layer was observed above the liquid surface due to the density difference dH20 > dparaff, n, resulting in some crystals temporarily to attach to the vessel wall. However, this was not a hindrance to the operation of the crystallizer. Because the sufficient cooling water feed created a downward flow pattern in the bottom section the settling behavior of the crystals was suitable for continuous operation, removing all crystals formed in the crystallizer.

The formed crystals were easily trapped and collected to the liquid phase. This product slurry containing crystals and water was continuously taken out from the bottom of the crystallizer.

The purity of paraffin was determined before and after crystallization by using gas chromatography. The operating conditions and results are given in Table 1.

The crystallization process and the device construction for carrying out the process according to the present invention is equally suitable for materials of varying properties as is shown by this example.

Example 3 Similar experiments to the ones described in examples 1 and 2 were performed using melamine (purity > 99.0 wt-%, containing melem 700 ppm, ammeline 300 ppm and ureidomelamine 1500 ppm) as the test component to be crystallized and ammonia, as the condensable solvent as well as the cooling liquid.

The heated gas mixture containing melamine and ammonia vapor as main component was injected into the vessel through a nozzle submerged in the liquid.

The gaseous medium and the cooling liquid were fed counter-currently into the vessel.

The cold ammonia feed, at a temperature of 50-55 °C, was supplied to the vessel at a flow rate sufficiently high to remove the heat transferred from the vapor feed, The observed crystal formation was extremely rapid showing that nucleation rate is fast enough to generate very fine crystals.

No crystal growth on the walls and internals was observed below the cooling liquid surface of the crystallizer. No dust formation which could follow the gas flow going out from crystallizer was observed above the liquid layer as it often happens in conventional spray crystallizers. No blocking or operational problems were observed in the subsequent gas treatment unit operations.

The formed melamine crystals were easily trapped and collected to the liquid phase.

The liquid level was maintained by the liquid ammonia feed and by the removal of the product slurry containing melamine crystals and liquid ammonia from the bottom of the crystallizer.

The purity of melamine was determined before and after crystallization by using high performance liquid chromatography (HPLC). The operating conditions and results are given in Table 1.

This example shows that a crystallization process according to the present invention is suitable for varying temperature and pressure conditions in addition to those benefits described in examples 1 and 2.

Table 1 Example Test com-Cooling Tv Pv Crystal Purity in feed Purity of ponent liquid °C Bar growth wt-% crystals a ainst walls wt-% 1 p-dichloro-Water 18-30 1.0-1.3 None 99.9 99.9 benzene i t I I I 2 Paraffin Water 16-22 1.0-3.3 None below the 99.8 99.8 liquid surface 3 Melamine Ammonia 55-59 31-3 2 None 99.0 99.9