Liquid purification or separation – Processes – Separating
Patent
1991-12-05
1993-07-13
Dawson, Robert A.
Liquid purification or separation
Processes
Separating
210780, 210784, 2103602, 210402, 162 60, B01D 3315
Patent
active
052270750
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process and apparatus in which a bed of material is formed by continuous filtration of a liquid suspension of a fibrous or finely-divided material on a continuously progressing filter surface, and the bed of material is conveyed forward in order to carry out liquid displacements in such a manner that at least a portion of the liquid content of the bed is displaced with another liquid by means of a liquid flow which is perpendicular to the forward-feeding direction of the above-mentioned material.
The filtration and liquid displacement operations of suspensions of material are of great importance, and they are used commonly in different branches of the process industry and especially in the pulp production processes in the wood processing industry, in which they are used:
in the washing of pulp, in which the fiber material is separated from the chemicals used in the digestion process and from the dissolved substances
in the bleaching of pulp in the manner of pulp washing, but also in order to introduce bleaching chemicals into the fiber material in the so-called dynamic (displacement) bleaching
for the preparation of cooking chemicals, for example in the causticization process in the sulfate pulp industry when the white liquor is being separated from the lime mud.
Initially, batch processes were used in the above operations. For example, the washing of pulp was carried out in a so-called diffuser, i.e. a screen-bottomed tank which was filled with pulp from the digester, by rinsing or pushing the pulp into it. Thereafter the so-called basic liquor of the pulp, which contains the chemicals used in the digestion and the dissolved substances, was displaced from the pulp through the screen bottom of the diffuser by adding a washing liquid into the upper section of the diffuser. When the pulp had in this manner been washed sufficiently clean, it was rinsed out from the diffuser, which could thereafter again receive a new batch of pulp for washing.
This type of simple batch washing of pulp was made more effective by using a so-called multistage process. In it the different steps were carried out in principle in the above manner, except that in the first step no washing water was used but a washing liquor diluted with washing water, the washing liquor having been recovered from the second washing step of the previous pulp batch, and that pure washing water was not used until the last step.
In such a batch diffuser wash the displacement of liquid in the fiber bed is effected by a vertical liquid flow, and in this case by a downward flow, taking advantage of the hydraulic pressure difference across the fiber bed, caused by gravity. Since the fiber bed is stationary, neither of the horizontal dimensions is utilized operationally in the displacement; they are used only to provide the washing apparatus physical dimensions (size) and thus to accomplish the desired pulp washing capacity.
In the batch filtering or liquid displacement it is, of course, also possible to use an arbitrarily chosen flow direction for the liquid and, for example, by means of suitable pumping arrangements and devices, to produce the necessary pressure difference in this selected direction across the fiber bed, which direction may be, for example, parallel to a radius which is arbitrarily oriented relative to gravity. It is, however, typical that in a batch washing process only one dimensional direction is used operationally, i.e. for liquid displacement in the pulp bed, whereas the two other dimensional directions, of which one may be tangential, only provide size for the apparatus.
Along with technical development, continuous methods have been adopted for the said operations, methods which have for a long time already been economically so competitive that they are now in an entirely predominant position in the pulp production industry. In a continuous method used for the said filtration and liquid displacement operations, two dimensional directions need to be used operationally in the operation itself: one as i
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Dawson Robert A.
Reifsnyder David
Salomans Oy Hackman Process Ltd.
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