Chemical apparatus and process disinfecting – deodorizing – preser – Physical type apparatus – Crystallizer
Reexamination Certificate
1999-04-06
2001-06-05
Utech, Benjamin L. (Department: 1765)
Chemical apparatus and process disinfecting, deodorizing, preser
Physical type apparatus
Crystallizer
C422S254000
Reexamination Certificate
active
06241954
ABSTRACT:
The present invention relates to a method for the crystallisation of a liquid from a mother liquor comprising a solution, a suspension or a mixture of liquids, comprising the following steps:
feeding the mother liquor to a zone to form a crystal slurry
discharge of the crystal slurry from the zone via a crystal slurry discharge line and
feeding said slurry to a separator.
The invention also relates to an installation for the crystallisation of a liquid from a solution, a suspension or a mixture of liquids, comprising:
a crystalliser connected to a feed line for feeding the solution, the suspension or the mixture, comprising a vessel having a cooling element for the purpose of forming crystals in the vessel,
a filter located in the vessel, which filter is connected to a discharge line for liquid filtered from the vessel, and
a crystal slurry discharge line, connected to the vessel, for discharging a suspension of crystals from the vessel.
Netherlands ter inzage legging (laid open application) no. 8104086, in the name of the Applicant, discloses a recrystallisation vessel in which a scraped-surface heat exchanger is accommodated inside the vessel. The scraped-surface heat exchanger constitutes a separate nucleation zone for the crystals, with an average retention time of the mother liquor of between approximately 10 and 100 seconds. Following nucleation, the ice crystals are fed from the heat exchanger to the maturing vessel, where the average retention time is of the order of several hours. One or more combined screening/washing devices, such as a wash column, are mounted on the vessel to separate the crystals formed form the mother liquor. A stirrer and a filter, set up in a fixed position, are also fitted in the vessel. The mother liquor is withdrawn from the vessel through the filter and recycled via a pump to the feed for the heat exchanger. The depleted mother liquor which, via the filter, has been removed from the vessel can also be withdrawn from the circuit through a discharge line. A disadvantage of such an installation is that, as a consequence of the separate nucleation and growth of the crystals and the resultant differences in retention time in the respective separate nucleation and maturing zones, the filter has to cope with large volume flows and is a critical element in this set-up.
To achieve efficient separation of the crystals from the mother liquor using a wash column, the mother liquor in the recrystallisation installation must not be too viscous and the difference between the melting point of the crystallised substance and the equilibrium temperature of the mother liquor must not be too great. Both increase as the content of the substance to be recovered decreases. If there is too great a difference in melting point, the separation of the crystals from the concentrated mother liquor by means of packed-bed wash columns can no longer take place efficiently. Furthermore, the crystals formed must be as spherical as possible with a suitable size of about 300 &mgr;m and the size distribution must be uniform. If the pure substance to be recovered is water, a very good result is obtained with separate nucleation and growth, as is described, for example, in Netherlands Patent Application no. 8104086 in the name of the Applicant, the disclosure of which is hereby incorporated herein by reference thereto.
However, such a system is less suitable for the majority of other products coming under consideration, because, as a result of their shape, the crystals formed in the nucleation zone do not melt sufficiently rapidly in the maturing vessel. If these crystals do not melt in the maturing vessel, the average crystal size becomes much smaller, which has a severe restrictive effect on the capacity of the crystal mother liquor separator, such as a wash column.
In practice, crystallisation installations in which a distinction is no longer made between nucleation and growth zones is used for these other products. In these installations it is not the mother liquor but the crystal slurry that is passed over a scraped heat-exchanging surface. In some embodiments said surface is incorporated in the vessel and in other embodiments the crystal slurry is fed over one or more external scraped-surface heat exchangers.
Both embodiments have the disadvantage that it is not possible to adjust the crystal content in the vessel independently of the freezing point line in the phase diagram for the product. With installations of this type one is therefore forced to operate with a crystal content far removed from that desirable from the standpoint of process technology. Filter installations located outside the vessel are indeed used, but this has the major disadvantage that some of the crystals break, which again results in a small average crystal size.
One aim of the present invention is to provide a method and an installation for separating solutions, mixtures of liquids or suspensions with crystallisation of the solvent, with which better control of the crystal content in the crystalliser is possible and an appreciable improvement in the capacity can be achieved.
To this end the method according to the present invention is characterised in that the zone is a zone of combined nucleation and growth, mother liquor being withdrawn via a discharge line from the zone via a filter located in the zone.
The installation according to the present invention is characterised in that the cooling element is equipped to cool the entire contents of the vessel, the vessel forming a single chamber for both nucleation and growth of the crystals.
The combination of the filter device in the vessel as known to the Applicant from the separate nucleation and growth systems with the combined nucleation as described above leads to unexpectedly good results. The possibility of operating crystallisation installations at crystal contents optimum from the process technology standpoint appreciably increases the capacity of existing installations and reduces the investment costs for new installations.
Because nucleation and growth according to the present invention take place in a single vessel, it is no longer necessary to recycle large volume flows through the vessel via the filter as according to the prior art. With the installation according to the present invention, a relatively small quantity of depleted mother liquor can be withdrawn from the vessel via the discharge line, as a result of which the crystal content in the combined nucleation and growth zone can be controlled.
No more than 30%, preferably no more than 10%, of the volume of the vessel is discharged per hour via the discharge line from the installation according to the present invention, which discharge line preferably is not directed back to the crystalliser vessel. In contrast to the recirculation stream from the known installation, where, for example, at least twice the capacity of the maturing vessel is recycled per hour through the nucleation zone (and thus also through the filter), the quantity of mother liquor discharged via the filter in the installation according to the invention is relatively small. Consequently, damage to the crystals on the filter is minimised.
Preferably, the crystal slurry discharge line from the installation according to the present invention is connected to a mixing vessel that is also connected to a feed line for supplying the mother liquor from a reservoir to the mixing vessel. In the mixing vessel the crystals are mixed with the mother liquor before being separated off. By this means the viscosity of the crystal slurry is reduced and the freezing point raised, so that the capacity of the wash column increases, Because the crystal slurry in the mixing vessel is mixed with fresh mother liquor the solution separated off from the wash column is still relatively rich in substance to be separated off and this liquor is therefore not discharged but is recycled to the crystalliser via the feed line.
In a further embodiment of an installation according to the present invention, the crystalliser has a stirrer with the filter
Jansen Halbe Anne
Jansen Johannes Josephus Andreas Gerardus
Chen Kin-Chan
Handal & Morofsky
Niro Process Technology B.V.
Utech Benjamin L.
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