US3494101A - Method and apparatus for deaerating liquid suspensions - Google Patents

Description

Feb. 10, 1970 N, A. L. WIKDAHL METHOD AND APPARATUS FOR DEAERATING LIQUID SUSPENSIONS Filed 061;. 25. 1967 FIG.2

FIG.1

INVENTOR- Nfitl-s LII. u.

Gwen-at" 3,494,101 Patented Feb. 10, 1970 3,494,101 METHOD AND APPARATUS FOR DEAERATING LIQUID SUSPENSIONS Nils Anders Lennart Wikdahl, 42 Bravallavagen, Djursholm, Sweden Filed Oct. 25, 1967, Ser. No. 677,940 Claims priority, application Sweden, Dec. 15, 1966,

Int. Cl. B01d 19/00 U.S. Cl. 5551 13 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus for deaerating or degassing liquid suspensions, particularly aqueous suspensions of cellulosic fibers, by forming a flowing film of stock on a surface within a deaerating chamber under suflicient vacuum to cause boiling at the temperature of the stock. Released vapor is condensed by means within the deaerating chamber and released air and the condensed vapor are removed from the chamber through separate outlets. Deaerated stock is discharged through another separate outlet.

This invention relates generally to new and useful improvements in the removal of air or other gases from liquids and particularly seeks to provide a novel method and apparatus for the deaeration of aqueous suspensions of cellulosic fibers such as those commonly employed in the manufacture of paper, paperboard or sheeted pulp.

In such suspensions, generally referred to as stock, gases, mostly air, are present in three different forms, namely (a) as free gases easy to remove, (b) as free or entrained gases diflicult to remove and (c) as gases dissolved in the liquid.

Until recently the pulp and paper industry paid little or no attention to the presence of these gases in stock suspensions, perhaps because the paper machines were relatively narrow and slow speed and good sheet forma tion could be achieved without much difficulty. However, modern paper machines are much wider and faster in operation and the achievement and maintenance of good sheet formation have become a problem.

The influence of even apparently rather small quantities of air in stock suspensions have now been recognized, and although the physics or mechanics of the effect of this air are not fully known, there is a consensus that it is at least very desirable to use deaerated stock on modern high speed machines.

It is thought that the air content of an undeaerated fiber suspension has a more or less harmful effect on the uniform drainage of water by means of dewatering drums and filters, or through the wire at the wet end of a paper or pulp machine and on the actions of other dewatering elements such as the suction boxes, table rolls, hydrofoils, suction couch roll and first press which is generally a suction press. Such harmful effect further occurs in screening operations of various kinds such as vortical cleaning or hydrocyclone separation.

Foaming and the deposition of pitch as well as harmful effects in connection therewith may also be considerably reduced through the use of deaerated stock.

Furthermore, tests have proved that by means of deaeration the dewatering period for stock was substantially reduced, the wet strength of the paper or pulp web was improved and the air permeability of paper off the reel was reduced. For example, a reduction of the air in a pulp suspension from 0.5% to 0.05% effected nearly a fifty percent reduction in dewatering time.

Various methods for the removal of gases from stock have been tried in the past. In one such method the stock is introduced into a chamber under vacuum, the degree of which is not suflicient to cause boiling under the given operating temperature. In this method the liquid portion of the admitted stock is disintegrated or atomized to near monomolecular thickness of the water film on the fibers and the vacuum becomes effective to reduce the air content of the suspension, for example, from about 1.5% to 0.1%. This type of fine atomization is not considered too practical or economical in view of the large volumes of liquid involved in stock suspensions and in view of the large power requirements for operation of the equipment.

However, the disadvantages of earlier methods of deaerating stock suspensions are overcome through the use of the method and apparatus of this invention.

Therefore, an object of this invention is to provide a novel method and apparatus for removing gases from liquids and particularly for deaerating fibrous stock suspensions.

Another object of this invention is to provide a method of the character stated in which an aqueous stock suspension is continuously admitted to a vacuum chamber as a continuous or uninterrupted film flowing along the inner walls thereof, and the vacuum in the chamber is maintained at a degree suflicient to cause a minor portion of the water to be vaporized through boiling and to release air therefrom. The absolute pressure in the degasifying chamber must consequently be somewhat lower than the boiling point pressure corresponding to the temperature of the suspension.

Another object of this invention is to provide a method of the character stated in which the deaerated stock is discharged from the vacuum chamber and the removed air and vapor are extracted through a condenser associated with the vacuum chamber.

Another object of this invention is to provide apparatus of the character stated which includes a vertically disposed cylindrical vacuum chamber, means for injecting a continuous stream of stock onto the inner walls of the vacuum chamber as an essentially downwardly flowing continuous or uninterrupted film to permit the water of the stock to boil and partly vaporize to effect the removal of air therefrom, and a vertically disposed contact condenser positioned within the vacuum chamber for condensing and ejecting the removed vapor and gases.

Another object of this invention is to provide apparatus of the character stated in which the stock is supplied through an axially disposed pipe having its upper end terminating adjacent the top of the vacuum chamber to define in combination therewith a circumferential slot that effects uniform distribution of the stock onto the full periphery of the inner wall of the vacuum chamber.

Another object of this invention is to provide apparatus of the character stated in which the central portion of the top of the vacuum chamber is provided with an adjustable diaphragm in juxtaposition with the upper end of the stock supply pipe to regulate the area of the circumferential slot and thus regulate the flow rate of stock admitted to the vacuum chamber.

Another object of this invention is to provide apparatus of the character stated in which the jet condenser is constructed concentrically with the stock supply pipe.

A further object of this invention is to provide a modification of the apparatus in which the top of the vacuum chamber is solid and in which the upper end of the stock supply pipe is provided internally with an inflatable venturi-like sleeve having a variable orifice.

With these and other objects, the nature of which will be apparent, the invention will be more fully understood by reference to the drawings, the accompanying detailed description and the appended claims.

In the drawings:

FIG, 1 is a vertical section schematically showing one form of apparatus constructed in accordance with this invention which may be used in practicing the method of this invention; and

FIG. 2 is a view similar to FIG. 1 but showing a modification of the devices for controlling the flow and distribution of the incoming stock.

Referring to the drawings in detail, the invention as illustrated is embodied in a deaerator including an outer vertical shell generally indicated 5 having a relatively large diameter cylindrical upper portion 6 provided with an integral domed top 7 having a central aperture 8 closed by a removable cover 9. The upper shell portion 6 terminates at its lower end in a frusto-conical section 10 which in turn terminates in a smaller diameter cylindrical lower portion 11 having a frusto-conical bottom 12 and a flanged outlet 13.

The lower end of the outer shell 5 extends downwardly through a mounting plate 14 and is rigidly aflixed thereto as by gusset plates 15.

A stock supply pipe 16 extends axially upwardly from the shell bottom 12 and is provided at its upper end with a flared outlet 17 in proximity to the aperture 8 in the shell top 7.

The aperture 8 is covered by a flexible diaphragm 18, which together with the flared outlet 17 of the stock supply pipe 16 defines a circumferential slot 19 for com plete radial distribution of undeaerated stock onto the inner surface of the top 7 from which it flows downwardly as a continuous or uninterrupted film over the entire surfaces of the shell walls. By film it is intended to mean that the flowing stock would have a nominal thickness of about mm., for example, but this thickness could vary rather widely on either side of that figure without appreciably affecting the efliciency of deaeration.

The top cover 9 and the diaphragm 18 together define a chamber 20 that may be filled from a supply pipe 21 by water or other fluid under controllable pressure to vary the degree of flexing of the diaphragm and thus controllably vary the area of the circumferential slot 19 and the flow of stock therethrough.

A condenser is constructed concentrically around the supply pipe 16 and includes an inner tube or shell 22 having a top aperture 23 and a closed bottom 24 sealed around the supply pipe and provided with a flanged outlet 25 extending outwardly through the bottom 12 of the shell 5. The upper end of the tube 22 terminates adjacent the flared outlet 17 of the supply pipe 16. An outer condenser tube or shell 26, having a funnel-shaped open top end 27, in general horizontal alignment with the bottom of the main upper shell portion 6, is concentrically disposed around the inner shell 22 and has a closed bottom 28 sealed around the bottom 24 of the shell 22 and is provided with a flanged outlet 29. A deep skirt 30 is concentric with the upper end of the inner shell 22 and extends downwardly from the flared outlet 17 of the supply pipe 16 into association with the funnel-shaped upper end 27 of the shell 26.

Thus, the upper end of the shell 22 and the skirt 30 together define a generally cylindrical space 31; the stock supply pipe 16 and the shell 22 together define a generally cylindrical space 32; and the lower end of the shell 22 and the shell 26 together define a generally cylindrical space 33.

Coolant liquid for the condenser is introduced into the upper part of the space 31 through a supply pipe 34 that enters the unit adjacent the bottom thereof and extends upwardly through the space 32 and terminates at its upper end in an annular shower head 35.

In FIG, 2 of the drawings there is illustrated a modification of the above described apparatus in which an outer shell generally indicated 36 includes a relatively large diameter upper cylindrical portion 37 corresponding to the portion 6 of the shell 5, closed at its top by an unbroken domed end 38.

An axial stock supply pipe 39, corresponding to the supply pipe 16, is provided at its upper end with a relatively large diameter closure disc 40 having a central aperture 41. A double-walled venturi nozzle 42 formed from an elastomeric material such as rubber, is retained within the upper end of the stock supply pipe 39 and is provided with an annular cavity 43 adapted to receive water or other fluid under controllable pressure from a supply pipe 44 to controllably vary the configuratlon and orifice diameter of the venturi. Stock flowing through the venturi of the nozzle 42 and the disc aperture 41 as substantially a free jet will deviate in essentially radial directions upon the shell top 38 in such a way that the et will break up radially to form a continuous or unlnterrupted flowing film on the upper wall surfaces of the shell portion 37.

In use, the deaerating unit receives heated stock from a pulp mill, bleach plant, or other source at the temperature naturally reached as the result of prior process operations. Such a temperature is generally well below that of boiling, although it could for example lie within the range of -140 F. However, regardless of whether or not the stock temperature lies within that range, the deaerating portion of the unit defined primarily by the shell portions 6 and 10 is maintained under a vacuum such that the absolute pressure therein is kept somewhat lower than the boiling point pressure corresponding to the actual temperature of the admitted stock.

Thus, when undeaerated stock is injected either through the slot 19 of FIG. 1 or the venturi nozzle 42 of FIG. 2, it will flow as a continuous or uninterrupted film along the walls of the outer shell and due to the vacuum condition therein, the liquid will tend to boil and a small portion, less than 1% and preferably less than 0.1% will be vaporized. It is this boiling and vaporization that effect deaeration of the stock. The heat required for vaporization is taken from the heat content of the stock.

The outlet 25 is connected to a Vacuum pump (not shown) to create the desired vacuum in the deaerating chamber through the spaces 32 and 31. Cooling liquid is sprayed onto the inner surface of the skirt 30 and falls together with condensate absorbed thereby from the lower end thereof as a curtain into the space 33 from which it is discharged through outlet 29.

The released air and vapor from the deaerated stock flowing along the surface of the shell portion 6 are drawn through the curtain of liquid falling from the skirt 30 and thence into the spaces 31 and 32. Any foam emanating from the stock suspension is broken by the liquid curtain and is discharged through the outlet 29 along with the used cooling liquid and the major portion of the condensate. Although the major portion of the vapor is condensed in the space 31, the rest thereof is carried over together with the released air into the space 32 from which they are removed through the vacuum connection 25.

The efficiency of deaeration may be increased by causing the released air and vapor to flow in countercurrent with respect to the downfiowing film of stock in the shell 6 as the result of increasing the diameter of the funnelshaped top 27 of the shell 26 and also increasing its height so that its upper end terminates slightly below the top of the cylindrical shell portion 6 or 37.

If a longer retention time is desired for the flowing film on the vertical surface of the shell portion 6, suitable deflectors (not shown) may be employed to provide a tangential component of travel and thus increase the length of travel.

The deaerated stock is withdrawn from the unit through the outlet 13. Outlets 13, 25 and 29 constitute barometric legs.

It is of course to be understood that variations in arrangements and proportions of parts may be made within the scope of the appended claims.

It is claimed:

1. In a method for degasifying aqueous suspensions of cellulosic fibers the steps of, continuously supplying a gascontaining suspension to a degasifying chamber, distributing a flow of said suspension as a flowing continuous film over an internal surface of said chamber while maintaining within said chamber a vacuum sufiicient to cause boiling of said suspension at its existing temperature whereby to vaporize a portion of the liquid of said suspension and to release gases therefrom, first condensing the vapor then removing from said chamber condensed vapor and released gases through separate outlets, and removing the degasified suspension from said chamber through a separate outlet.

2. The method of claim 1 in which said suspension is supplied to a centric portion of the degasifying chamber and coaxially therewith.

3. The method of claim 2 in which said suspension is supplied to the degasifying chamber in the form of a free jet radially deviating upon the internal surface of said chamber.

4. The method of claim 1 in which the vaporized portion of said suspension, and gases released therefrom are, through a curtain of coolant liquid and condensate, passed to a condensing zone and moved counter-currently therethrough in direct contact with said coolant liquid.

5. The method of claim 2 in which said suspension is given a tangential velocity component.

6. Apparatus for degasifying a liquid suspension of cellulosic fibers including a closed degasifying chamber having a vertical cylindrical upper portion provided with a domed top, means for axially supplying and radially distributing a flow of the liquid suspension from the central portion of said top outwardly to a vertical surface of said cylindrical upper portion whereby to form a continuous downwardly flowing film of said suspension, means for maintaining said degasifying chamber under a vacuum sufficient to cause said liquid suspension to boil at its existing temperature whereby to release gases and vapor therefrom, means within the degasifying chamber for condensing said vapor, means for removing said gases, means for removing vapor condensate, and means for separately discharging the degasified liquid suspension.

7. The apparatus of claim 6 in which said liquid suspension supplying means includes an axial supply pipe having its upper end terminating adjacent the top of said chamber whereby to define in conjunction with said top a circumferential slot for effecting radial distribution of said suspension.

8. The apparatus of claim 7 wherein said upper end of said axial supply pipe is outwardly flared, and additionally including means operably associated with said top for controllably varying the area of said circumferential slot.

9. The apparatus of claim 7 in which, in said axial supply pipe, at its upper end, an internally positioned inflatable orifice-defining sleeve is afiixed and means for controllably varying the inflation of said sleeve are provided whereby to vary the area of the orifice defined thereby.

10. The apparatus of claim 7 in which said condensing means is constructed concentrically around said axial supply pipe and includes a closed-bottom open-top inner shell connected to a vacuum source, a deep skirt depending from the upper end of said supply pipe and enclosing the upper end of said inner shell, means for supplying coolant liquid to the inside of said skirt from the upper end thereof, an open-top outer shell surrounding the lower portion of said inner shell and terminating at its upper end in a portion of larger diameter encompassing the lower end of said skirt, and means for discharging condensate and used coolant liquid from said outer shell.

11. The apparatus of claim 9 in which said condensing means is constructed concentrically around said axial supply pipe and includes a closed-bottom open-top inner shell connected to a vacuum source, a deep skirt depending from the upper end of said supply pipe and enclosing the upper end of said inner shell, means for supplying coolant liquid to the inside of said skirt from the upper end thereof, an open top outer shell surrounding the lower portion of said inner shell and terminating at its upper end in a portion of larger diameter encompassing the lower end of said skirt, and means for discharging condensate and used coolant liquid from said outer shell.

12. The apparatus of claim 6 wherein the means for removing said gases, the means for removing the vapor condensate and the means for discharging degasified liquid suspension are connected to separate discharge conduits, the height of said means over the discharge ends of said conduits being such that said means are at barometric height above the discharge ends of the conduits.

13. The apparatus of claim 12 wherein the means for supplying suspension are connected to a feed conduit, the

intake of said feed conduit being also so located that said means are at barometric height above the intake.

References Cited UNITED STATES PATENTS 945,640 1/1910 Thelen et al 202-187 2,428,045 9/1947 Sharp et al 194 X 2,751,031 6/1956 Smith et al. 55-51 X 2,876,860 3/1959 Clark et a1 55-177 3,010,692 11/1961 Jentoft 239-534 3,214,903 11/1965 Cochran 239455 X 3.355,364 1967 Hammond 202236 X REUBEN FRIEDMAN, Primary Examiner R. W. BURKS, Assistant Examiner US. Cl. X.R. 5555, 193

Images