Liquid purification or separation – Filter – Movable medium
Patent
1990-01-18
1992-03-03
Sever, Frank
Liquid purification or separation
Filter
Movable medium
210404, B01D 3306
Patent
active
050930010
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for recovering crystals from a slurry containing crystals of, for example terephthalic acid.
BACKGROUND ART
In the production of terephthalic acid by a liquid phase oxidation of paradialkylbenzene, terephthalic acid produced will deposit within the mother liquor as crystals forming a slurry containing crystals of terephthalic acid. By separating the crystals out of this slurry, a crude product of terephthalic acid is obtained. After dissolution of the resulting crude crystals and subjecting to purification by treatments by oxidation, reduction and so on, terephthalic acid is caused to deposit as crystals to form a slurry containing the crystals. By separating the crystals from this slurry, purified terephthalic acid is obtained.
In either of the above-mentioned cases, the slurry has heretofore been subjected to procedures of separating the crystals by filtration or by centrifugation, washing the so separated crystals by re-slurrying in a wash liquor, separating again the crystals from this re-slurry and, if necessary, repeating these operations. Such a conventional technique reveals disadvantages that the procedures are complicated and that a larger apparatus has to be installed therefor.
As an alternative for this technique, a method had been proposed in which a sucking filtration, filter cake washing and a further sucking filtration are successively effected within a rotary sucking filter (designated hereinafter as RSF. In this technique, a sequence of operations of a sucking filtration, a filter cake washing, a further sucking filtration and removal of the filtered cake is realized in succession while rotating a cylindrical filter medium (filter cloth), wherein an ordinary pressure prevails outside the filter medium and the filtration pressure is attained by sucking out of the internal of the filter medium. However, this technique has a disadvantage that the stuffing or blockage of the filter medium proceeds quite steeply and the operation of apparatus is forced to be interrupted.
The object of the present invention is to obviate the disadvantage mentioned above and to propose a novel method for recovering crystals from a slurry under the use of RSF, which enables to recover the crystals by effecting a sequence of operations of filtration, washing and further filtration without suffering from stuffing of the filter medium.
DISCLOSURE OF THE INVENTION
The present invention proposes a method for recovering crystals from a slurry by supplying the slurry to a rotary sucking filter while rotating a cylindrical filter medium thereof to effect a sequence of operations of sucking filtration, washing and further sucking filtration, said method comprising conducting a sucking filtration while pressurizing the slurry side so as to maintain such a temperature and pressure that the filtrate of the rotary sucking filter will not reach a condition of supersaturation.
Investigating the cause for the stuffing of filter medium upon recovery of crystals using an RSF, it has now been discovered that this is due to deposition, together with an increase in the viscosity of the slurry, of the solute from the mother liquor onto the surfaces of the filter medium resulting from supersaturation of the mother liquor reached by a decrease in the temperature of the slurry caused by the sucking of the filter drum. Therefore, the present invention incorporates a sucking filtration under a pressurized condition, in order to avoid the supersaturation of the mother liquor as mentioned above.
Thus, according to the present invention, the sucking filtration in the RSF is carried out while pressurizing the slurry side on the rotating filter medium so as to maintain such a temperature and pressure that the filtrate of the rotary sucking filter will not reach a condition of supersaturation. In this manner, a premature stuffing of the filter medium due to the deposition of crystals from the mother liquor together with an increase in the viscosity of the slurry is
REFERENCES:
patent: 4263148 (1981-04-01), Symens et al.
Mitsui Petrochemical Industries Ltd.
Sever Frank
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