Refrigeration – Separator for solidified constituent of liquid mixture
Reexamination Certificate
1998-12-17
2001-06-19
Capossela, Ronald (Department: 3744)
Refrigeration
Separator for solidified constituent of liquid mixture
Reexamination Certificate
active
06247321
ABSTRACT:
The invention relates to an apparatus for crystallizing a liquid from a solution, a mixture of liquids or a suspension, comprising:
a nucleation zone with a cooling unit for forming crystals,
a growth zone which is situated below the nucleation zone and is connected, via a delivery line, to the nucleation zone for feeding a mixture of liquid and crystals to the growth zone, and
a recycling path between the growth zone and the nucleation zone for mass transfer from the growth zone to the nucleation zone, the growth zone being situated in a tank, which extends along a vertical axis and a topmost end of which comprises the nucleation zone and which, at a bottommost end, is provided with an intake connected to the delivery line, the recycling path being formed by a growth zone section situated between the intake and the nucleation zone.
Such a crystallization apparatus is disclosed by W. L. McCabe, J. C. Smith: “Unit Operations of Chemical Engineering”, McGraw-Hill, New York XP002074939, pp. 777-778. This publication demonstrates a vacuum crystallizer including a conical crystallizer vessel having a nucleation zone at its top end and having a constricted and extended bottom end as the growth zone. Positioned in the nucleation zone is a draft tube, in which crystal slurry is drawn in by suction from a propeller. The bottom rim of the vacuum tank extends into the nucleation zone and forms an annular chamber which effects separation between seed crystals and larger crystals. The seed crystals are removed via a bypass and introduced at the bottom end of the growth zone. A crystal slurry discharge line is situated in the centre of the growth zone.
The known apparatus has the drawback that the crystal slurry is pumped over with a high velocity from the nucleation zone to the growth zone with the aid of the propeller or screw, so that the degree of separation between nucleation and growth is low. Furthermore, the known apparatus is operated batchwise rather than continuously. Moreover, with the known apparatus there is the risk that in the event of certain substances becoming concentrated it is possible for sediment to precipitate, which may lead to blockage of the feed of the seed crystals.
American patent application number 4,459,144, in the name of the Applicant, discloses a freeze concentration apparatus comprising three identical stages connected countercurrently. This involves a water-containing substance such as fruit juice, beer, tea or coffee being fed in, in each stage, to a surface-scraped heat exchanger. The surface-scraped heat exchangers form nucleation zones in which seed crystals are formed having a size of between 5 and 10 microns. From the surface-scraped heat exchangers the seed crystals are transferred to a growth zone in a separate adiabatic stirred recrystallizer tank, where the average crystal size increases to about 300 microns. The ice slurry from the recrystallizer tank of the first stage is fed to a separation device for separating the crystals and the solution, which in this case is formed by a wash column. Via a filter in the recrystallizer tank of the first stage, some of the solution is recirculated via the surface-scraped heat exchanger associated with the recrystallizer tank, and some is fed to the second stage of the freezeconcentration apparatus which in turn comprises a surface-scraped heat exchanger as the nucleation zone and a recrystallizer tank as the growth zone. The ice slurry formed in the recrystallizer tank of the second stage is fed to the recrystallizer tank of the first stage, the ice slurry moving countercurrently to the solution. The final concentration of the concentrated product discharged from the third stage can be around 50%.
A further multistage countercurrent freezeconcentration apparatus, where the crystal slurry is transferred via a centrifuge from the recrystallizer tank of the second stage to the recrystallizer tank of a first stage, is disclosed by the Netherlands patent application number 8902621.
In the case of cryoconcentration of foodstuffs using the known multistage countercurrent installations, the concentration factor that can be achieved is determined by the viscosity of the concentrate. However, when industrial waste water like that, for example, released in the production of styrene monomer, propylene oxide or other types of effluent is concentrated, the concentration factor that can be achieved is limited by the dissolved substances in the feed becoming supersaturated. As a result of the concentration of dissolved substances in the separation section, such as the wash column for example, being kept close to the feed concentration, for example owing to the crystal slurry being mixed with the feed, no problems will occur at that point. Downstream of the filters in the recrystallizer, or ripening tanks, however, the velocity is so low that, as concentrations increase, certain substances will precipitate there. This sediment may give rise to blockages. The known freezeconcentration apparatuses are therefore less suitable for a number of substances such as, in particular, industrial waste water or effluent.
It is an object of the present invention to provide an apparatus by means of which aqueous solutions, suspensions or mixtures of substances from which one component readily precipitates as a sediment, can be concentrated in an efficient manner. It is a further object of the present invention to reduce the number of apparatus required for freezeconcentration and the number of rotating parts and to bring about a general simplification of the process conditions, which leads to a relatively inexpensive method and apparatus.
To this end, the apparatus according to the present invention is characterized in that in or near the nucleation zone and/or the top end of the growth zone a liquid feed line is connected into the tank and in that in or near the nucleation zone and/or the top end of the growth zone a crystal slurry discharge line is connected to the tank for discharging crystal slurry from the tank.
During operation of the apparatus according to the invention, the vertically aligned tank containing the growth zone is filled with crystal slurry as far as the nucleation zone. Via the feed line, crystals formed in the nucleation zone are fed, together with the solution, to the bottom end of the growth zone. This causes the ice slurry in the growth zone to be pushed upwards very slowly, at such a low velocity that heavier undissolved fractions can settle on the bottom of the tank and the lighter ice can float up. The sediment can be discharged from the growth zone at the bottom end of the tank. In the apparatus according to the present invention, no filter is required between the growth zone and the nucleation zone as in the case of the freezeconcentration apparatuses according to the prior art. This allows reliable processing of suspensions to take place, without sedimentation of solids resulting in the apparatus becoming blocked.
As a result of the liquid being fed in via the liquid feed line, the level in the ripening tank is maintained, while the discharge of the crystals via the crystal slurry discharge line takes place at such a flow rate, that a constant level of the ice slurry is maintained. As a result, a stirrer, if present, will keep turning continuously in the ice slurry. Owing to the crystal slurry being discharged at the top end of the growth zone, preferably exclusively at the top end of the growth zone, all the nuclei formed are able to be effectively removed by ripening and cannot reach the nucleation zone for a second time.
In one embodiment of the crystallization apparatus according to the present invention, the nucleation zone communicates with the growth zone and is provided with evaporation means for evaporating liquid in the nucleation zone. The term “communicates” refers here to the crystal slurry at the top end of the growth zone being able to move into the nucleation zone without passing an intermediate filter, intermediate pump, system of lines or other component. In the nucleation zone cooling
Capossela Ronald
Handal & Morofsky
Niro Process Technology B.V.
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