FIELD OF THE INVENTION

The present invention is in the field of separating solids from a solution or the like, in particular by using fractional crystallization. The present invention relates to a continuous mode crystallizer, to a method of continuously producing crystals from a solution comprising a plurality of different types of solutes, and to a computer program comprising instructions for operating said crystallizer and carrying out said method.

BACKGROUND OF THE INVENTION

In chemistry, a solution relates to a homogeneous mixture composed of two or more substances. In such a mixture, the so-called solute is a substance dissolved in another substance typically referred to as a solvent. For obtaining such a solution a mixing process may be used, wherein the solutes are dissolved in the solvent; or, the solution may be obtained directly from a source, such as a waste stream, sea-water and the like. In the solution usually the solvent forms the larger (volume, and typically also weight) fraction of the mixture. The amount of solute in a given amount of solution or solvent is referred to as the concentration, which may be expressed in terms of mass/volume (kg/1), in a weight ratio (kg/kg,), or in a molarity, wherein the mass of a certain solute is calculated back to an amount of mole by using the molar weight of said solute (mole/1). The solution typically comprises more than one solute, as (i) typically solutes are present as ions (cations and anions), (ii) more than one type of ions is present inherently. Put different, the solution comprises a plurality of solutes, of which one, or some, typically form a major part. The term "aqueous solution" is used when one of the solvents is water.

In order to separate solutes from the solution, and likewise melt, several chemical -physical processes can be used. One of these processes is crystallization. Typically rather pure solutes can be obtained thereby. Crystallization ion is the process by which a solid forms, where the atoms or molecules are highly organized into a lattice structure known as a crystal. Typically many crystals are formed at the same time. The solid relates to at least one of the solutes or part of the solvent. Some of the ways by which crystals form are precipitating from a solution, freezing, or more rarely deposition directly from a gas. Typical parameters that are controlled may be temperature, pressure, and solvent/fluid evaporation. Crystallization is considered to occur in two major steps. The first is nucleation, the appearance of a crystalline phase from either a supercooled liquid or a supersaturated solvent; the supersaturation provides the driving force for crystallization. The second step is known as crystal growth, which is the increase in the size of particles. In view of crystallization it is noted that minerals, inorganic compounds, and (small) organic molecules typically crystallize easily, and the resulting crystals are generally of good quality, i.e. with high purity.

In the present invention crystallization is mainly used as a chemical solid-liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid crys- tailine phase occurs. In chemical engineering, crystallization occurs in a crystallizer. Crystallization may relate to precipitation, although the result is not amorphous, or poly crystalline, or disordered, as in the case of precipitation, but a crystal.

Most chemical compounds, dissolved in most solvents, show a solubility threshold that increases with temperature, i.e. at a higher temperature more solute can be dissolved in the solvent with respect to a lower temperature. The process of crystallization makes use of this phenomenon, by decreasing the temperature of the solution, thereby lowering the solubility, and forcing crystals to be formed. The temperature difference between the maximum solubility at a certain temperature, and the actual, lower temperature, is often referred to as supersaturation. The present invention relates to crystallization through cooling.

There may be limitations in the use of cooling crystallization. For instance a hydrate instead of an anhydrous crystal may be formed, which may be detrimental, such as in the case of calcium chloride. In the crystallizer a maximum supersaturation will evidently take place in the coldest points. These may be heat exchanger tubes which are sensitive to scaling, and heat exchange may be greatly reduced or discontinued. Due to the decrease in temperature usually an increase of the viscosity of a solution is observed. A too high viscosity may give hydraulic challenges, and the laminar flow thus created may affect the crystallization dynamics. Also crystallization by cooling is not applicable to compounds having a so-called reverse solubility, a solubility that increases with temperature decrease; in such a case crystallization would be obtained by increasing the temperature. An example of such a compound is caesium sulphate.

Another challenge with crystallization is that it is difficult to crystallize a solute in a continuous mode. In particular production rate, scalability, the above scaling in the heat exchanger, may be problematic. Such is especially the case for fraction crystallization. In chemistry, fractional crystallisation use is made of differences in solubility between solutes being present. It fractionates (forms fractions) via differences in crystallization, which differences may be rather absolute, namely crystallization or not. If a mixture of two or more substances in solution are allowed to crystallize, for example by allowing the temperature of the solution to decrease or increase, the precipitate may contain more of the least soluble substance. The proportion of components in the precipitate depends on their solubility quantified by their so- called solubility products. In a case that the respective solubility products at a given temperature are very similar, a cascade process may be needed to effectuate a complete separation. This technique is often used in chemical engineering to obtain very pure substances, or to recover saleable products from waste solutions.

Some prior art may be considered. US 8 475 597 B2 recites a process for crystallizing, by progressively cooling, in multiple stages arranged in series in a crystallization vessel, a descending continuous flow of a saturated sucrose solution at a temperature from about 78° to about 120° C, each stage maintaining the sucrose solution being crystallized at a predeter- mined temperature, until reaching a temperature from about 25 to 40° C., obtaining substantially pure sucrose crystals. A suspension containing sugar seeds is introduced in the crystallizing equipment, in the first stage, jointly with the saturated sucrose solution of 1.05-1.15. In another embodiment of the invention, the saturated solution is fed and its temperature is controlled, already in the first stage of the vessel, to obtain a supersaturation between 1.05 and 1.15, inducing the formation of small crystals used as crystallization seeds. US 3 253 818 A recites an apparatus comprising a screw press with a diminishing screw and interruptions therein to accommodate teeth projecting inwardly from the casing wall which may be used to form solid oxymethylene polymers from a mixture containing at least 50% by weight of trioxane alone or with comonomers. The mass is maintained at a temperature between 0 DEG and 116 DEG C. and discharged as a dry powdered solid after a reaction time of 0.5-2 minutes. The product may be treated with a catalyst deactivator such as water or an aliphatic amine. GB 828 837 A recites a crystallization process wherein a partially crystallized mixture is passed through a filtration and liquid removal zone, a reflux zone and a melting zone, part of the melt being withdrawn and the remainder being refluxed in the reflux zone, the materials in these zones are subjected to a pulsating pressure in the direction of the passage of the crystals, the pressure being applied at a point upstream from the point of liquid removal and downstream from the points at which the mixture is partially crystallized and propelled towards the zones. A feed mixture containing a component separable by crystallization is fed by a pump to a chilling section A, where a crystal mass is obtained together with mother liquor, which is passed to a filter section B. Filtrate passes through a filter screen and is removed by an outlet. A pulsation producing member E, comprising a cylinder and a reciprocating piston, provides a continuous series of pressure waves travelling in the same direction as the crystal mass which advances through a reflux section C towards a melting section D. As the crystal mass approaches the melting section D some of the crystals are melted to provide a reflux which is removed with the mother liquor in the filter section B, and molten product is removed through a line.

The present invention relates to an improved continuous mode crystallizer, a method of continuously producing crystals from a solution comprising a plurality of different types of solutes, and further aspects thereof, which overcomes one or more of the above disadvantages, without jeopardizing functionality and advantages.

SUMMARY OF THE INVENTION

The present invention relates in a first aspect to a continuous mode crystallizer (100) comprising at least one inlet (1) inlet for providing a flow of a, typically aqueous liquid, undersaturated solution with dissolved solutes, in direct downstream fluid contact with the inlet at least one nucleation zone (2) configured to have a nucleation temperature lower than the temperature of the solution at the inlet, comprising at least one nucleation mixer (12) for mixing the solution, and at least one first mixing zone cooler (22) for nucleating initial crystals, in downstream fluid contact with the mixing zone at least one growth zone (3) configured to have a growth temperature providing a supersaturation of the solution, in particular wherein the growth zone is configured to have a temperature different from that of the nucleation zone, more in particular a lower temperature than that of the nucleation zone, comprising at least one crystallization mixer (13) and at least one second growth zone cooler (23) for growing the nucleated crystals, wherein a flow of solution and a flow of crystals is parallel with respect to one and another, and in downstream fluid contact with the growth zone, and at least one separation zone (4) for separating crystals formed from the solution, in particular a settler, and in particular comprising at least one outlet (5) in downward fluid contact with the at least one separation zone (4). Therewith fractional crystallization in a continuous mode is provided. A high production rate is obtained, at relatively low direct costs and at a low investment. The crystallizer is easily scalable. Heat exchanger scaling is mostly avoided. The CAPEX and OPEX of the present crystallizer is considered to be very attractive. Typically a scale size of 100 1/h is considered a lower limit of such a size.

In a second aspect the present invention relates to a method of continuously producing crystals from a solution comprising a plurality of different types of solutes, that is a multicomponent comprising solution comprising (a) providing at least one continuous mode crystallizer according to the invention, (b) entering the solution in the crystallizer, (c) mixing the solution and lowering the temperature in the nucleation zone to form initial crystals, (d) further growing the initial crystals in the at least one growth zone, and (e) separating the formed crystals formed from the crystallizer.

In a third aspect the present invention relates to a computer program comprising instructions for operating the crystallizer (100) according to one of the invention, the instructions causing the crystallizer (1) to carry out the following steps: (b) entering the solution in the crystallizer, (c) mixing the solution and lowering the temperature in the nucleation zone to form initial crystals, (d) further growing the initial crystals in the at least one growth zone, and (e) separating the formed crystals formed from the crystallizer.

Thereby the present invention provides a solution to one or more of the above mentioned problems. Advantages of the present invention are detailed throughout the description.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates in a first aspect to a continuous mode crystallizer.

In an exemplary embodiment of the present continuous mode crystallizer the nucleation zone (2) comprises at least one nucleation zone screw (32) for gradually flowing the solution through the nucleation zone, and/or wherein the at least one growth zone comprises at least one growth zone screw (33) for gradually flowing the solution through the growth zone, and/or wherein the at least one separation zone comprises at least one separation zone screw (34) for gradually flowing the solution through the separation zone, in particular wherein each screw individually comprises at least one baffle. A screw is considered to be a mechanical apparatus that converts rotational motion to linear motion. In an exemplary embodiment of the present continuous mode crystallizer the nucleation zone screw and the growth zone screw and optionally the separation zone screw are one and the same screw, in particular wherein the one and the same screw comprises a screw thread void section in between the nucleation zone screw and the growth zone screw, typically substantially in the middle of the two aforementioned, more in particular wherein the screw thread void section comprising no threads.

In an exemplary embodiment the present continuous mode crystallizer further comprises at least one screw actuator for rotating the at least one nucleation zone screw and the at least one growth zone screw and the at least one separation zone screw each individually or combined, in particular at least one actuator adapted to rotate the screw or screws at a rotation speed of 1-240 rpm, in particular 10-60 rpm.

In an exemplary embodiment of the present continuous mode crystallizer each mixer (12,13) individually is adapted to provide a turbulent flow in the respective zone, in particular a turbulent flow from an inlet of the nucleation zone to an outlet of the nucleation zone, such as by rotating with a rotation speed of 60-1500 rpm, in particular 120-1200 rpm.

In an exemplary embodiment the present continuous mode crystallizer comprises at least one shaft, at least one first screw shaft for rotating the respective screw, and at least one second mixer shaft for rotating the respective mixer, in particular wherein the a least one first screw shaft is an eccentric shaft and wherein the at least one second mixer shaft is a central shaft.

In an exemplary embodiment of the present continuous mode crystallizer each zone (2,3) comprises 1-5 mixers (12,13) respectively, and 0-4 voids (41,42), in particular wherein each mixer (12,13) independently may be selected from impellers, such as a Rushton impeller, in particular comprising 2-20 blades, such as 4-12 blades.

In an exemplary embodiment of the present continuous mode crystallizer each mixer (12,13) individually is provided in a screw thread void (41,42) between respective screw threads.

In an exemplary embodiment of the present continuous mode crystallizer each zone (2,3,4) comprises at least one cooler inlet (52,53,54) and at least one cooler outlet (62,63,64), adapted to provide a counter-current cooling flow, in particular wherein flows of coolant in adjacent cooling zones are separated from one and another.

In an exemplary embodiment the present continuous mode crystallizer comprises an inner tube (6) and an outer tube (7) incorporating said inner tube (6), in particular with a thermal insulation material (8) between said inner tube (6) and said outer tube (7).

In an exemplary embodiment of the present continuous mode crystallizer the at least one second growth zone cooler is adapted to cool the solution to a temperature below the freezing point thereof, such as below 0°C.

In an exemplary embodiment of the present continuous mode crystallizer comprises at least one controller for controlling the temperature and/or temperature gradient, such as in the at least one mixing zone, and/or in the at least one growth zone.

In an exemplary embodiment the present continuous mode crystallizer comprises at least one controller for controlling the flow rate and/or flow rate gradient, such as in the at least one mixing zone, and/or in the at least one growth zone, and/or in the at least one separation zone.

In an exemplary embodiment of the present continuous mode crystallizer the at least one growth zone comprises at least one cylinder adapted to flow the solution through.

In an exemplary embodiment of the present continuous mode crystallizer the inlet, the mixing zone, the growth zone, the separator, and the outlet are provided within one crystallizer, in particular within one reactor.

In an exemplary embodiment of the present continuous mode crystallizer the crystallizer comprises at least one optically transparent section providing a visual view of the solution, in particular wherein a transparent section extends substantially over the full length of the screws or combined screw. It is noted that this does not imply the screw section is fully transparent.

In an exemplary embodiment of the present continuous mode crystallizer the crystallize has a volume of 1 O' ³ - 10 m ³ .

In an exemplary embodiment the present continuous mode crystallizer comprises at least one flow sensor, wherein the at least one flow sensor is adapted to control a flow of the solution by partly or fully opening or closing at least one valve.

In an exemplary embodiment the present continuous mode crystallizer comprises at least one pump for maintaining a flow of the solution.

In an exemplary embodiment the present continuous mode crystallizer comprises at least one temperature sensor, typically at least one sensor per zone, wherein the at least one temperature sensor is adapted to control the temperature of at least part of the solution in the crystallizer by activating or de-activating a respective cooler of heater, in particular by controlling said respective heater or cooler.

In an exemplary embodiment the present continuous mode crystallizer comprises at least one filter for removing pollutants.

In an exemplary embodiment the present continuous mode crystallizer comprises at least one crystal size detector, such as a light-scattering detector, in particular wherein the crystal size detector is adapted to control at least one of the at least one valve, of the at least one cooler, of the at least one heater, and of the at least one pump.

In an exemplary embodiment of the present continuous mode crystallizer comprises at least one weight detector for determining a weight of separated crystals, in particular wherein the at least one weight detector is adapted to control at least one of the at least one valve, of the at least one cooler, of the at least one heater, and of the at least one pump.

In an exemplary embodiment of the present method a supersaturation for a first type of crystal is provided, in particular A 1-10 K, more in particular A2-5 K and/or (dl) wherein a supersaturation of a first type of crystal is maintained during 1-120 minutes, in particular 10-60 minutes, such as 20-40 minutes.

In an exemplary embodiment of the present method step (dl) is repeated 1-10 times for a subsequent type of crystal, such as in 2-10 continuous crystallizers.

In an exemplary embodiment of the present method the solute comprises >0.1 mole solutes/1, which is a value being typically too high for performing osmosis, in particular >0.2 mole/1, more in particular > 0.5 mole/1, such as >1 mole/1.

In an exemplary embodiment of the present method in the at least one mixing zone crystals are kept in suspension.

In an exemplary embodiment of the present method steps (a)-(e) are performed in a continuous mode.

In an exemplary embodiment of the present method the solution is selected from a chemical solution, such as from a water treated solution, a brine solution, a sea water solution, from food solutions, from mining solutions, from agricultural solutions, from petrochemical solutions, from pharmaceutical solutions, etc.

In an exemplary embodiment of the present method the solution is cooled in the nucleation zone to a eutectic point thereof.

In an exemplary embodiment of the present method the solution comprises a solvent selected from water, an alkanol or alcohol, in particular a Ci-Ce alkanol or alcohol, such as a C2-C4 alkanol or alcohol, or a combination thereof.

The invention is further detailed by the accompanying examples, which are exemplary and explanatory of nature and are not limiting the scope of the invention. To the person skilled in the art it may be clear that many variants, being obvious or not, may be conceivable falling within the scope of protection, defined by the present claims.

SUMMARY OF THE FIGURES

Figs. 1-3 show an example of the present crystallizer.

DETAILED DESCRIPTION OF THE FIGURES

Figs. 1-3 show an example of the present crystallizer.

In the figures:

100 crystallizer

1 inlet

2 nucleation zone

3 growth zone

4 separation zone

5 outlet

12 mixer nucleation zone

13 mixer growth zone

22 cooler nucleation zone

23 cooler growth zone 24 cooler separation zone

32 screw nucleation zone

33 screw growth zone

34 screw separation zone

42 screw thread void nucleation zone

43 screw thread void growth zone

44 screw thread void separation zone

52 cooler inlet nucleation zone

53 cooler inlet growth zone

54 cooler inlet separation zone

62 cooler outlet nucleation zone

63 cooler outlet growth zone

64 cooler outlet separation zone

72 baffle nucleation zone

73 baffle growth zone

74 baffle separation zone

81 central axis

82 eccentric connector

91 inner wall

92 outer wall

Fig. l shows a top view of the present crystallizer, wherein the solution flows from bottom to top. The screw 22-24 provides sections in between screw blades, and slow flow of the solution. The screw segments are connected by eccentric connector 82. The mixers are driven by central axis 81. Coolers 22-24 are provided. Cooling inlets 52-54 and cooling outlets 62-64 are provided. A baffle 72-74 cleans the inner wall and provided segmentation. Voids 42-44 are provided, typically comprising space for a mixer.

Fig. 2 shows a top view, in line with fig. 1. An inlet 1 and outlets 5 are shown. A nucleation zone 2, a growth zone 3, and a separation zone 3 are visible. Therein mixers 12-13 are shown. Screw sections 32-34 for the respective zones are shown.

Fig. 3 shows a cross-section A-A. Therein a cooler inlet for the growth zone 53, and a cooler outlet for said zone 63, are shown. Baffles 73 (4) are shown, as well as eccentric connector 82. Also central axis 81 for driving the mixer 13 is shown. Mixer 13 is provided with 12 blades. Inner wall 91 and outer wall 92 are also visible.

EXAMPLES/EXPERIMENTS

The invention although described in detailed explanatory context may be best understood in conjunction with the accompanying examples.

As an example of the fractional operation of the crystallizer a Na2SO4-NaCl-H2O mixed solution is considered. Considered that the solution is saturated at 10 degrees Celsius with re- spect to Na2SO4, Na2SO4-10H2O crystals will form in the crystallizer nucleating at a temperature below 10 degrees Celsius. In the growing area the solution temperature is below the eutectic temperature for the initial composition, producing only Na2SO4-10H2O crystals, whereas the rest of the Na2SO4 and NaCl remains in solution. After this point in time, the apparatus is operated under eutectic conditions, where ice and Na2SO4 - 10 H2O are produced by at a temperature above -21 degrees Celsius. In this manner the maximum extraction of Na2SO4is produced from the solution, without crystal formation of NaCl . The ice and Na2SO4-10H2O are separated in the separation zone by gravitational separation and ice is removed from top outlet whilst salt from the bottom outlet. Both of them are carried by the movement of the screw rotation. The remaining NaCl is crystallized in a subsequent crystallizer using the filtrated solution from Ice and Na2SO4-10H2O.

In the case of CaSCU -Na2SO4-H2O solution, CaSCU and/or Na2SO4 are removed in pure form, depending of their relative concentrations, up to a point where a mixture is produced. These limit operating point is in eutectic (ice+salt) or in just cooling crystallization of one of the salts. Due to the fractional-batch crystallization happening in the moving volume of fluid through the reactor, this mixing product gap is extended in one reactor to produce a solution with minimal percentage of the other present salt, for further treatment in an optional consecutive reactor.