US2852138A - Method of and apparatus for clarifying liquids - Google Patents

Description

Sept. 16, 1958 H. KNIGHT ETAL METHOD OF AND APPARATUS FOR QLARIFYINGLIQUIDS Filed Sept. 4, 1956 INVENTORS CYR/L H. KNIGHT LO C. V4IQCO BY M .X

Aflorney METHOD OF AND APPARATUS FOR CLARIFYING LIQUIDS Cyril H. Knight and Lorne C. Varcoe, Woodbridge,

Ontario, Canada, assignors to Dorr-Oliver Incorporated, Stamford, Conn., a corporation of Delaware Application September 4, 1956, Serial No. 607,789

3 Claims. (Cl. 210-83) This invention relates to clarification, Settling and like tanks.

In many industrial processes involving the use of clarification and settling tanks the need arises for keeping the tank adequately supplied regardless of fluctuations in the supply of the mother liquor thereto. The usual expedient adopted to even out the feed rate is to provide storage or surge tanks by means of which a reserve supply is built up, but such storage or surge tanks introduce certain difficulties, for example, the need arises for maintaining the feed and, because the tanks are quite large and often located at different levels, problems of floor space and plant layout develop. Typically, in one field in which the present invention is especially useful, namely in the recausticizing of kraft green liquor such as is used in sulphate pulp processes in the pulp and paper industry, it has been general practice to provide a green liquor surge or storage tank in the flow line between the smelt dissolvers and the green liquor clarifier.

It is an object of the present invention to provide a structure in which clarification or settling equipment and facility for storage and/or for accommodating surges in supply or demand are embodied in a single unit.

It is, incidentally, a feature of the invention that not only is the normally expected simplification of plant layout and saving of floor space achieved, but also that improved operation can result as will later be understood from the ensuing description.

It is, moreover, an advantage of the present invention that the objects of the invention can be achieved by comparatively simple modification of existing equipment.

In order that it may be clearly understood and more readily carried into effect, the invention will now be described, by way of example, with reference to the accompanying drawing which is a sectional elevation taken through a clarifier according to the present invention.

In the drawing the reference numeral indicates a tank structure having a peripheral wall 11, a bottom' closure 12 and a dished top cover 13. At a point intermediate the top and bottom of the tank there is provided a tray 14 of dished form disposed generally horizontally and having a central opening 15 leading downwardly into a feedwell 16. Mother liquor to be treated is fed into the feedwell 16, preferably tangentially, through the feed supply conduit 17 and flows downwardly into the lower compartment 18 until that lower compartment is full, whereafter the liquor flows upwardly from the feedwell 16 into the upper storage or surge compartment 19. Solids settling out from the liquor in the lower compartment, which can be regarded as the clarifying compartment, collect on the bottom 12 and are raked inwardly toward a central discharge cone 20 by the rotating rake structure 21. Clarified liquor flows out of the main thickening or settling compartment through outlets provided at angularly spaced points immediately subjacent the underside of the tray 14 through suitable connection conduits 22 (only one of which is seen in the drawing) connected with a common manifold 23 from which the clarifier liquor is discharged for subsequent use or treatment.

The raking mechanism 21 is carried by a shaft 24 supported by a suitable bearing structure conveniently mounted on the usual diametrically extending bridge structure 25 which also conveniently mounts a prime mover 26 or equivalent drive gear for rotating the shaft and thereby the rake mechanism 21.

Raking mechanism 27 is preferably also provided in the storage or surge compartment 19 for raking toward the opening 15 and feedwell 16 solids settled out from the liquor in the storage or surge compartment 19, such rake mechanism conveniently being carried for rotation by the shaft 24. The liquid level in the storage or surge compartment 19 can be prevented from exceeding a predetermined level by overflow through the overflow drain conduit 28 which may feed back to the source of mother liquor supply.

A sludge discharge conduit 29 leads out from the cone 20 and branches out into the conduits 30 and 31. The conduit 30 serves for conducting the sludge away for disposal when the valves 32 and 33 are open, whereas, when the valve 33 is closed and the valves 32 and 34 are open, wash liquid can be introduced through the conduit 31 for cleaning purposes.

Throughput of liquor and therefore residence time in the clarifying compartment 18 is normally controlled by suitable valve means. Thus, the feed rate through the conduit 17 can be controlled by the valve 17a, and similar control valves such as 22a can be operated for regulating the clarified liquor discharge to the discharge manifold 23. A discharge control valve 28a can also be provided in association with the upper compartment overflow 28, and in all cases the control means can be actuated manually or provision may be made for automatic control.

A cover drain 35 is preferably provided through which any moisture or overflow collected in the dished cover plate 13 can flow into the upper compartment 19. The cover drain 35 is particularly useful if the overflow 28 is provided with a valve which can be closed, for, in that event the cover drain 35 can serve to vent the upper compartment 19 and by overflow can serve to indicate when the upper compartment 19 is full. In that respect, it may be a desirable manner of operation deliberately to close the overflow outlet valve 28a in order to ensure that the upper storage or surge compartment is full in anticipation of a subsequent interruption of mother liquor supply.

If initial filling with mother liquor is etfectedwith the lower compartment discharge valves 22a closed, air will be entrapped beneath the underside of the tray 14. If that happens, the effect would be that on opening the valves 22 for starting operation, there would be a rush of liquor upwardly in the unit to replace the air being released through the discharge line, with the result that agitation could occur and the unit would not function properly as a clarifier. It is therefore desirable to provide meansfor venting the top of the lower compartment 18 to atmosphere, for example, through the vent pipe 36. Though only one such pipe is shown in the drawing it will in practice he preferred to have three such vent pipes spaced apart.

It is also desirable that a sample valve should be provided on the lower compartment discharge or through the tank Wall just below the tray 14, so that samples can be drawn off to determine the clarity of the liquor being discharged. Such a sample valve is indicated by way of example at 37. If desired sample valves can be provided at several elevations on the unit.

In normal clarifying operation the supply of mother Patented Sept. 16, 1958.

liquor through the conduit 17 proceeds until the bottom compartment is full and the top compartment is approximately half full. If, owing to some oversight in operation, the level in the upper. compartment should rise, due to; thehydraulic influence ofiincreased feed, to the extent of filling the upper compartment,'the excess would merely discharge by overflow through. the overflow discharge conduit 28. Whenfilling has been completed the discharge of clarified overflow from the clarifying compartment 18 is adjusted in relation to the normal feed ratev of mother liquor entering through the conduit 17 so that a predetermined level will be maintained in the surge .or storage upper compartment 19. If the rate of feedof mother liquor through the conduit 17 increases for any reason the overflow discharge-conduit 28 accommodates any necessary overflow, the overflow generally being cycled back to the supply source. On the other hand, if the mother liquor feed rate is interrupted the supply to the clarifier compartment 18 is maintained through gravity flow through the feedwell 16.

It is a noteworthy feature of the structure according to the present invention that not'only are storage and surge accommodation facilities combined with a clarifier in a single structure, but, in addition, an operational improvement results. Thus, the preliminary settling of solids which inevitably occurs in the surge or storage upper compartment 19 results in considerable clarification of the liquor therein and the there settled solids or pulp raked into the feedwell 16 pass in isolation through the upper increasingly clarified layersin the clarifying compartment 18 where they merge with the settled solids at the bottom of the lower compartment with minimum contamination of the clarified liquor in the lower compartment. In that respect, it will be noted that the-feedwell 16 is of sufficient depth to reach down to the lower part of the lower compartment 18. The eflect is cumulative because the settled-solids descendingfrom the upper compartment, by contact with downwardly settling solids which have-already reached the lower part ofthe clarifying compartment, tend toamalgamate, as a result of which the overall settling. tendency'is increased. Likewise, the liquorwhich feeds into theclarifying lower compartment from the upper compartment being already partially clarified results in an improved overall clarification.

Another significant factor is that an existing-installa tion can be adapted to the teachings of the present invention with a minimum of structural modification by the simple expedient of installing the tray and feedwell' structure 14 and 16 with the feed conduit leading into the feedwell, and if necessary or desirable, extending the tank walls upwardly. The upper raking structure 27 can be carried by the same driven shaft 24 which carries the lower rake mechanism 21 which is anyway customarily present.

Although the embodiment of the invention-described is especially applicable to the clarification of green liquor in the pulp. and paper industry, the scope of the invention is limited neither to that embodiment nor that environment. The invention is, indeed applicable in any circumstances in which it may be desirable to provide for continuity of supply despite fluctuations in availability at the source. As an indication of a somewhat different environment one can envisage utility of the invention in 4. other processes such, for example, as in its application flotation systems where it is likewise desirable to have facility for storage or to accommodate surges.

We claim:

1. A clarifier comprising a tank having a bottom and a confining marginal wall upstanding therefrom, an annular tray extending inwardly from the marginal wall at a height intermediate the top and bottom thereof to define 'below said tray a clarification compartment in communication through a substantially central opening defined by said tray with a superjacent tray compartment, sediment raking means at the bottom of said clarification compartment operable to rake materials settled therein toward a substantially central sediment discharge outlet, clarified liquor overflow discharge means leading out from the clarification compartment subjacent the periphery of said annular tray, a feedwall leading from the central opening defined by said annular tray into the lower part of the clarification compartment, feed conduit means extending through the marginal wall of the tank and said clarification compartment for discharging mother liquor to be clarified directly into said feedwell and therefrom downwardly until the clarification compartment is full and thereafter upwardly into the subjacent tray compartment, overflow discharge means leading out from said tray compartment at a predetermined elevation above said tray and thereby normally operable to determine a maximum liquid level in said tray compartment, and valve means associated with said clarified liquor overflow means enabling control of the clarified liquor discharge rate independently of the rate of feed supplied through said feed conduit.

2. Apparatus according to claim 1, having the raking mechanismin the clarification compartment carried by a rotatably driven shaft extending through said tray compartment, said 'feedwell and into the lower part of said clarification compartment, and further including raking mechanism in said tray compartment also drivingly carried by said rotatably driven shaft.

3. The improved method of operating a clarifier in which solids are separated from liquids by sedimentation and clarified liquid is discharged from an upper portion of said clarifier; said improved method comprising the steps of substantially fixing the rate of clarified liquid discharge in:proportion to a pro-selected rate of clarifier feed; and maintaining said fixed clarified liquor discharge rate to be substantially constant during. periods of feed rates varying from said preselected rate .by compensating for increased feed rates by forcing excess feed upwardly under the hydraulic influence of such increased feed into a functionally remote superjacent storage zone maintained in hydraulic communication with said clarifier, and automatically compensating for deficiencies in feed resulting from decreased feed rates by gravity flowing material downwardly from said storage zone into said clarifier at a rate proportioned to the decrease in feed rate.

References Cited in the file of this patent UNITED STATES PATENTS 845,520 Callow Feb. 26, 1907 1,578,625 Ankeny Mar. 30, 1926 2,460,834 Logue Feb. 8, 1949

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