Refrigeration – Processes – Fractionally solidifying a constituent and separating the same
Reexamination Certificate
2000-06-19
2002-10-22
Doerrler, William C. (Department: 3744)
Refrigeration
Processes
Fractionally solidifying a constituent and separating the same
Reexamination Certificate
active
06467305
ABSTRACT:
BACKGROUND OF THE INVENTION
The application relates to an installation for the crystallisation of a liquid from a solution, a mixture of liquids or a suspension, comprising:
a nucleation zone for forming seed crystals, with a feed line for supplying mother liquor to the nucleation zone,
a ripening zone in fluid communication with the nucleation zone for making the seed crystals grow and
a separator connected to a discharge from the ripening zone for separating crystals and mother liquor.
DESCRIPTION OF THE RELATED ART
An installation of this type for freeze-concentrating aqueous substances such as fruit juice, beer, tea or coffee is disclosed in U.S. patent application No. 4,459,144 in the name of the Applicant. With this installation the aqueous liquid is fed to a surface-scraped heat exchanger. The surface-scraped heat exchanger forms a nucleation zone in which seed crystals are formed which are between 5 and 10 micrometers in size. The seed crystals are transferred from the surface-scraped heat exchanger to a growth zone in a separate adiabatically stirred ripening vessel where the average crystal size increases to approximately 300 micrometers. The ice containing slurry from the ripening vessel is fed to a separator for separating the crystals and the solution, which separator in this case consists of a washing column. Via a filter in the ripening vessel in the first stage the solution is recirculated through the surface-scraped heat exchanger associated with the ripening vessel and a portion is fed to the second stage of the freeze-concentrator, which again has a surface-scraped heat exchanger as nucleation zone and a ripening vessel as growth zone. The ice containing slurry formed in the ripening vessel of the second stage is fed to the ripening vessel of the first stage, the ice containing slurry moving in counter-current to the solution. The final concentration of the concentrated product discharged from the third stage can be approximately 50% (m/m) dissolved solids.
In the nucleation zone the seed crystals are produced by extracting heat from the liquid within a short time. During this operation some of the liquid is converted to crystals. For this purpose the residence time of the liquid in the nucleation zone must be short and high temperature differences between the scraped surface and the liquid must be employed. Thus, the liquid content of the nucleation zone must be low. As a consequence of the low surface temperature, crystals are produced on the surface of the nucleation zone, such that these crystals have to be scraped off at a high frequency.
In the known surface-scraped heat exchangers the scrapers are mounted on a rotary shaft. The shaft itself is constructed as an impeller along the heat-exchange surface, with a narrow gap between the surface and the rotor, such that the liquid volume is as small as possible and a short residence time is obtained. For heat-exchange surface areas of up to about 1 m
2
, heat exchangers of this type are available commercially. For larger installations, however, 20 heat exchangers, for example, each with a surface area of 12 m
2
, can be needed. Because of the low tolerances in respect of the gap for the rotary scraper and the high degree of dimensional and shape accuracy, as well as the high demands in respect of running and wear characteristics of the scrapers, surface-scraped heat exchangers are very expensive. Moreover, because of the special construction process and the machining operations required the ability to obtain them is uncertain.
The use of surface-scraped heat exchangers on a larger scale is also problematical. For a ripening vessel with a capacity of 36 m
3
it is possible to use a heat-exchange surface area of 24 m
2
in the nucleation zone. With a circulation stream through the nucleation zone and the ripening zone of 72 m
3
/hour (that is to say the content of the zones is passed twice per hour over the heat-exchange surface), 36 tonnes of filter cake must be re-suspended per hour if a filter is used in the ripening zone. A residence time of 10 seconds in the heat exchanger means that the heat exchanger has a capacity of 200 litres. This means that a gap width of at most 8 mm can be used for a total surface area of 24 m
2
. If the stream impinges on the heat-exchange surface in the widthwise direction, all ice formed must be discharged without problems over the longitudinal direction through the 8 mm gap, in which the scrapers are also accommodated. This constitutes a major problem when scaling up.
SUMMARY OF THE INVENTION
Therefore, one aim of the present invention is to provide a crystallisation installation for separating off and purifying liquids from a solution, a mixture of liquids or a suspension, such as, for example, effluent, with which installation the nucleation zone is of relatively simple construction, is relatively inexpensive and can be scaled up in a simple manner.
To this end the installation according to the present invention is characterised in that the nucleation zone comprises a vacuum crystalline. It has been found that seed crystals for feeding to the ripening zone can be formed in a simple manner by means of a vacuum crystalline. Using a vacuum crystalline known per se seed crystals in the desired size range of between 3 and 10 micrometers can be obtained with residence times and heat extraction per unit time which correspond to those in a scraped heat exchanger. Moreover, if the mother liquor contains undissolved particles, such as, for example, precipitated salts, it is possible to obtain relatively high percentage ice contents with the vacuum crystalline, which is not achievable with a scraped heat exchanger because of the small gap dimensions.
It is pointed out that a vacuum crystalline is known per se from, for example, U.S. Pat. No. 1,865,614. Such a vacuum crystalline can be used in the installation according to the present invention. The nucleation zone and ripening zone can each be located in a separate vessel. The separator preferably comprises a washing column, such as, for example, is described in NL-A 7 106 457.
The vacuum crystalline can consist of a single vessel that can be connected selectively to two ice condensers. The first ice condenser is able to remove the water vapour from the nucleation vessel by freezing said vapour in contact with a cooling coil. After a predetermined period the ice formed on the cooling element will be removed with the aid of steam, the first condenser of the nucleation vessel being shut off, whilst the second ice condenser is connected to the nucleation vessel. However, it is also possible to use two nucleation vessels, each nucleation vessel provided with its own, preferably integral, ice condenser.
In one embodiment the nucleation zone is provided with a line for feeding a heating medium to the nucleation zone in order to prevent ice crystals forming on the surface of the nucleation zone. The heating medium can remove heat from the feed or can be made up of the feed itself by, for example, feeding a line system for the feed over the wall of the nucleation vessel before supplying the feed to the feed vessel.
REFERENCES:
patent: 2516832 (1950-07-01), Rosenbloom
patent: 4459144 (1984-07-01), Van Pelt et al.
patent: 6247321 (2001-06-01), Roodenrijs
Jansen Halbe Anne
Roodenrijs Jacobus Petrus
Doerrler William C.
Drake Malik N.
Niro Process Technology B.V.
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