US3515632A - Counter-flow washing method for removing liquid from a product distributed therein - Google Patents

Description

c. A. BERG HOLM 3,515,632 FLOW WASHING METHOD FOR REMOVING LIQUID FROM I Junez; 1970 CQUNTER- -A PRODUCT DISTRIBUTED THERIN Original Filed June 11, 1965 United States Patent 3,515,632 COUNTER-FLOW WASHING METHOD FOR REMOVING LIQUID FROM A PRODUCT DISTRIBUTED THEREIN Carl Arne Bergholm, Sundsvall, Sweden, assignor to Svenska Cellulosa Aktiebolaget, Sundsvall, Sweden Continuation of application Ser. 'No. 463,207, June 11, 1965. This application July 7, 1969, Ser. No. 846,627 Claims priority, application Sweden, June 12, 1964, 7,219/ 64 Int. Cl. D21c 9/02 U.S. Cl. 16260 4 Claims ABSTRACT OF THE DISCLOSURE In washing cellulosic pulp a vertical column of the same is maintained by introducing pulp suspension into the lower end of the column, introducing washing liquid into the upper end of the column, removing suspension of pulp in washing liquid from the upper end of the column and withdrawing separated liquid from the lower end of the column. In withdrawing separated liquid the same is effected at a rate equal to the rate of supply of washing liquid, which rate is such that the downward pressure exerted on the pulp is insufficient to effect substantial concentration of pulp in said suspension. Thereby it is ensured, that the downward pressure exerted in the column does not equal the internal shear resistance of the contents of the column.

This application is a continuation of Ser. No. 463,207 filed June 11, 1965 and now abandoned.

The present invention relates to a countercurrent method of and means for washing and separating a product, especially a fibrous material, from a solution or other liquid, in which said product is suspended or otherwise distributed. The invention has particular reference to the washing of cellulose pulp, obtained by the cooking of vegetable materials with alkaline or acid cooking liquors and/or by mechanical defibration of the vegetable material.

The principal object of the invention is to provide a new method of separating a cellulose fiber pulp, obtained by cooking or by mechanical treatment, from the solution in which the fiber pulp is suspended, and to achieve this while recovering the solution as concentrated as possible with respect to organic and inorganic soluble constituents.

In the sulphate cellulose industry there is used, at present, either periodical diffuser washing or multistage filter washing.

In the former case the wash tanks, the so-called diffusers, are filled at the top by blowing the stock from a cellulose digester, the pulp, then having a fiber concentration of about 15%, being washed by supplying wash liquid from above. The wash liquor first consists of diluted waste liquid from some other diffuser and then of clean water. The wash liquor is drained from the bottom screen of the diffuser, first as a concentrated solution, which is to be evaporated, and then as a solution of a somewhat lower concentration, the latter solution being, as a rule, returned to the cooking section, and, finally, as a solution of a low concentration, the last named solution being used as wash liquor in connection with some other cooking operation. After the washing the ditfuster is emptied of its contents of cellulose.

In multistage filter washing the fiber suspension is diluted with concentrated return waste liquor to a fiber concentration of about 1%, the suspension liquor being 3,515,632 Patented June 2, 1970 then sucked away on a rotary-drum vacuum filter. The fiber material is then again suspended in waste liquor from following filters and so on, in 3-5 stages.

Both these methods of washing pulp suffer from heavy drawbacks. The diffuser washers are very voluminous and require much manual work. Because of unfavorable constrictions in the diffusers the result of the Washing becomes uneven and the average degree of cleanness of the pulp is relatively low. Filter washing plants, as a rule, yield relatively good washing results but the costs of installation and wear are considerable and the problems in connection with the tendency of foaming of the waste liquors are often difiicult to master. Moreover, certain losses of heat cannot be avoided.

In the sulphite cellulose industry there is a similar situation but there the washing is often done within the digester. If there is no recovery of chemicals and no combustion of waste liquor, then the demands on the effectivity of the washing become less than in the sulphate cellulose industry. The introduction of a soluble base and heavier demands on waste water purification, however, tend to make the problems in connection with the washing of the sulfite cellulose just as important as those in the sulphate cellulose industry.

The present invention involves a new way of washing a product, particularly cellulose pulp or other fibrous material, which is suspended or, in some other way, distributed in a liquor and the invention is characterized in that said product together with said liquor is, periodically, pumped into a tank and in that a liquid washing medium is supplied to said tank at an intake, located at a distance from the place where said product is being pumped in and is caused to penetrate said product during the periods when no pumping-in of said product is taking place, the washing medium being supplied at such a slow rate that the pressure drop within said tank is less than the pressure drop required for a substantial compression or displacement of said product within said tank.

This method, which thus may be defined as being semicontinuous, at the liquor outlet of the wash tank yields as wash solution having a concentration almost as high as the concentration of the cooking liquor of the stock discharged into the wash tank. At the same time, in connection with the method according to the invention, there is delivered a uniform and thoroughly washed pulp at that end of the wash tank Where the washing medium is introduced.

The invention is based on the observation that cellous pulp of normal concentrations, when being piped, moves through the pipe like a plug, a certain minimum pressure drop per linear meter of the pipe being required for overcoming the friction of the pulp against the pipe wall. Consequently, also in case of extremely low rates of flow there is required a finite gradient of pressure.

Now, if wash liquid is supplied at a sufiiicently slow rate it will flow through the network of fibers without displacing the latter in relation to the wall. Consequently, there will be no compression of the network of fibers against the bottom screen of the wash tank as in conventional periodical diffusers. Due to this fact the cellulose material, while the wash liquor is passing therethrough, will stay loose all the time and be homogeneously distributed within the wash tank contrary to what occurs in conventional diffusers, in which the pulp is compressed and in which, in connection within the diffusion under a relatively high wash liquid pressure, inhomogeneities arise within the pulp, so that it is easier for the wash liquor to pass through certain parts of the pulp and more difiicult for it to pass through others, which will lead to an uneven result of washing that cannot, in many cases, be accepted.

In connection with the periodical pumping-in of fresh, unwashed stock into the other end of the wash tank, in accordance with the present invention, the pressure drop becomes greater than the friction of the plug of pulp against the interior wall of the tank, by which the network of fibers in the liquor will move together in a direction opposite to the direction of flow of the wash liquor. The two movements, washing and, respectively, the introduction of a fresh, unwashed plug of pulp into the wash tank are, therefore, carried out at different periods of time, the latter movement being, normally, only a fraction of the washing time. It may be mentioned that the rate of flow of the wash liquor may be kept higher the higher the friction of the plug of pulp against the interior wall of the wash tank. The upper limit, as regards the rate of movement of the wash liquor through the plug of pulp, i.e. the pressure drop above the same during the washing, is determined by the interior shear resistance of the fiber network of the plug of pulp. In case this limit is exceeded the plug of pulp will be compressed against the bottom screen of the wash tank, the favorable washing conditions in accordance with the invention being then lost. A certain degree of compression of the network of fibers may, however, be tolerated in certain cases, depending on the concentration of the plug of pulp in respect of cellulose fibers. Due to the fact that the friction of the plug of pulp against the interior wall of the wash tank is a condition for the maintenance of a certain pressure drop above the plug of pulp during the washing operation, then the rule is that the production of washed pulp per unit of cross sectional area of the wash tank becomes larger the smaller the size of said cross sectional area, because the relation between the size of the area of friction and said cross sectional area increases with decreasing cross sectional area, calculated in respect of one and the same length of the tank in the direction of flow of the wash liquor. Another way of putting it would be to say that the rate of supply of the wash liquor may be kept higher the larger is the relation between the area of the wall of the tubular Wash tank and the cross sectional area thereof. When uniform tubes are concerned the yield of washed pulp per unit of the cross sectional area becomes larger the narrower are the tubes.

A more detailed description of the invention will be given below, reference being had therein to the accompanying drawing, showing diagrammatically one embodiment of an apparatus for carrying out the method according to the invention. The apparatus is of the so-called diffuser type and comprises the vertical tube 1, which at its bottom is provided with a supply pipe 2 extending from a container or some other source (not shown) of unwashed pulp, coming from a digester (not shown), and a bottom screen 3 and also a vertically adjustable overflow 5. Near the upper end 7 of the tube there is provided water sprinkling means 4 for supplying wash water to the upper end of the tube.

In accordance with the invention the above described apparatus is used in the following manner. From the digester there is supplied, for a period of, for example, 30 seconds, through the supply pipe 2 cellulose pulp having a suitable fiber concentration, for example 4%, at such a high pressure that the plug 6 of pulp will be pressed upwards within the tube 1, so that pulp will be displaced and flow over the rim 7. The amount of pumped-in pulp is determined by the pump pressure, the time of pumping and the diameter of the supply pipe 2, and the whole operation is adjusted in such a manner that the plug 6 of pulp will move, for example, one-tenth of the entire length of the tube 1 during each pumping-in period. The pumping-in having become finished the plug 6 of pulp is standing still and such an amount of wash water from the sprinkler 4 is supplied to the top 7 of the tube that it will not, in any mentienable degree, flow over the rim 7 but will pass through the plug 6 of pulp. In connection herewith there will occur a pressure drop above the plug 6 of pulp, said pressure drop depending upon the level to which the overflow 5 has been adjusted. In each separate case the overflow 5 is set to the lowest level possible without the fiber material of the plug 6 of pulp packing on the bottom screen 3 to any mentionable degree. A certain degree of packing, corresponding to an increase by, at most, half the original concentration and preferably by at most 25%, for example 540%, thereof calculated with respect to the whole tube, may, however, be allowed. It is also possible to say that the liquor pressure above the plug 6 of pulp is adjusted in such a manner that the gradient of pressure within the plug of pulp comes close to but will not at all, or not in any mentionable degree, exceed a figure corresponding to the shear resistance of the fiber network of the plug of pulp at the beginning of the displacement of the liquor. As is known, said shear resistance increases with increasing fiber concentration and may thus increase in connection with the compression of the plug of pulp. If the gradient of pressure is somewhat greater at the beginning of the wash than the interior shear resistance of the plug of pulp some compression may, therefore, occur but it will cease as soon as the pulp concentration has reached a balanced figure.

While Wash liquor is being supplied from the sprinkler 4 said clean wash liquor will displace the waste liquor contained in the plug 6 of pulp. Then a slowly proceeding diffusion wash of cellulose pulp is obtained at the upper end of the tube 1. When the washing has been proceeding for such a long time that a length of the plug of pulp, corresponding to a pumped-in length of new pulp, has become washed clean at the upper part of the tube, new pulp is pumped into the tube 1 through the supply pipe 2, the washed pulp Welling over at the top 7 of the tube .1 and is taken care of in a manner not shown. In the diffusion wash taking place at the upper portion of the tube 1 the clean wash liquor displaces the waste liquor Without diluting the latter in any mentionable degree. The waste liquor obtained from the overflow 5 will then be of about the same concentration as the pure waste liquor, which is, for easily understood reasons, an essential advantage. Consequently, in connection with the method according to the invention, there are no dilute waste liquors requiring a special treatment, contrary to what is the case in diffuser washing plants hithereto m use.

When the apparatus according to the invention has been put into steady operation, there will be a pulsatory flow of clean pulp from the upper end of the tube 1 and a likewise pulsatory flow of concentrated waste liquor from the overflow 5.

Experiments have shown that the diffuser according to the invention can be used for washing pulp of various fiber concentrations and yet yield a good production capacity. In accordance with the invention the degree of concentration may be between 1 and 25%, preferably between about 2 and about 15%, and it is preferred to operate all the time at one and the same concentration in each separate case. With regard to the feeding and the uniformity of the washing concentrations within the interval of 3-8%, for example, a concentration of about 4%, is to be preferred. Within the range of temperature from 50-100 C. the production capacity is approximately proportional to the temperature, which should be at least 50 C. and preferably at least C. Contrary to the conditions prevailing in connection with filter washing plants there are no drawbacks connected with the use of high temperatures in the method according to the invention. If desired, the diifuser may even be operated at temperatures above C. at pressures above atmospheric pressure.

The supply of wash liquor should be shut off during the periods when unwashed pulp is pumped into the dif-- fuser, since no washing takes place during said periods. On the other hand a continuous supply of wash water would do no other harm than dilute the washed pulp product to some degree.

According to the invention it is very convenient to de sign the wash tanks, the so-called diffuser elements, with a square cross sectional area and to assemble them to form blocks, comprising a number of, for example, 20 x '20 elements of equal height, to which the supply of pulp may take place simultaneously for one row of elements and then immediately be shifted to another row. The size of each element should be such that the relation between the bottom or cross-sectional area and the area of one meter of height should amount to 0.5 at most and'preferably to about 0.1. If this condition is satisfied in respect of elements having non-treated interior surfaces, experiments show that the production capacity in washing sulphate pulp will be of the order 100 kg. per hour and square meter of bottom area or tons of pulp per hour when using a battery of x 20 elements each having a square cross sectional area, the side of the square being 0.5 m. The wash becomes very effective, corresponding to losses of sulphate amounting to 10 kg. per ton of pulp with a factor of dilution of the order 95%. A suitable height of the elements is 2-4 m.

When producing diffuser elements in accordance with the invention, arranged in batteries, it may be suitable in certain cases to make the vertical walls of one element form vertical walls also of juxtaposed elements of the same battery.

The production per unit of the cross sectional area of the diffuser elements may be increased by providing the elements with internal friction surfaces, for example, by providing bars or plates extending perpendicularly to the longitudinal axes of the elements and arranged at such short distances from each other that the pulp will, on the whole, be of the same concentration throughout the whole tube. The production capacity may also be increased by making the element walls rough, for example, by means of sandblasting or by welding on substantially horizontal bars. A 4% pulp shows, against smooth surfaces, a frictional force of about 8 kiloponds per square meter and, against a sandblasted surface, of about 16 kiloponds per square meter. The shear resistance in the network of fibers of the plug of pulp is about 22 kiloponds and the plug of pulp cannot be loaded beyond that figure. At a fiber concentration of 8% the corresponding frictional values are about kiloponds per square meter for smooth surfaces and about 60 kiloponds per square meter for sandblasted surfaces, the shear resistance of the network of fibers being about 110 kiloponds per square meter. Particularly, the last mentioned example shows that it should be possible to utilize the shear resistance by adding to a friction increasing treatment of the inner walls of the elements also extra friction surfaces within the diffuser elements.

EXAMPLE A diffuser element having a square cross sectional area and a side width of 0.3 meter was tested for washing sulphate pulp having a Roe number of about 5. The height of the diffuser element was 2 meters. The ratio between the cross sectional area of the element and the area of its friction surface (=the surface of the inner walls) per meter of length of the element amounted to 0.075. The walls were sandblasted inside. The bottom consisted of perforated sheet metal each hole having a diameter of 3 millimeters, and the pitch amounted to 10 millimeters. At the center of the bottom there opened a pipe the diameter of which was 50 millimeters, said pipe being provided with a nozzle having a diameter of millimeters. Every fifth minute a pulp suspension containing 3.32% of fibers was, for 20 seconds, pumped in through the pipe, which resulted in an average flow of 4 liters per minute. At the top of the diffuser element warm water, having a temperature of 60 C., was uniformly distributed over the top surface of the element by means of a vat having a perforated bottom. The solution from the bottom of the element was discharged through an overflow, which opened 4 decimeters below the top of the element. With this feeding system the apparatus yielded 8 kg. of absolutely dry pulp per hour. The concentration of the leaving pulp amounted to 3.94.1% of fiber. The salt content of the washed pulp corresponded to a loss of sodium sulphate varying between 2 and 10 kg. per ton of pulp. The leaving liquor was, practically, just as concentrated as when mixed with fibers.

The invention is not confined to the embodiment as described and shown, nor to the stated figures of the applied parameters, because various deviations therefrom may be applied within the scope of the invention.

I claim: 1. A method of counter-currently washing and separating a fibrous material from a liquid in which the material is suspended, which comprises establishing and maintaining a vertical column of a suspension of said material in said liquid in the form of a plug in which the fibres form a stable network;

intermittently displacing said plug in upward direction by intermittently introducing new unwashed suspension at the lower end of the column;

intermittently introducing washing medium at the upper end of said column at such periods of time in which suspension is not introduced at the lower end of the column; causing the washing liquid to flow through the network of the plug, thus displacing the liquid from the plug;

continuing to introduce the washing medium until the liquid has been displaced from the plug in a length corresponding to the pump-in length of the plug during one period, thus obtaining an intermittent flow of washed fibrous material suspended in the washing medium at the upper end of the column and withdrawing separated liquid from said lower end of said column at such a rate that the downward pressure exerted on said column is less than sufficient to produce a substantial increase of the concentration of said solid in said suspension;

said downward pressure being less than sufficient to produce a substantial increase of the compression or downward displacement thereof, said washing liquid being supplied at substantially the same rate as is the rate of Withdrawal of separated liquid.

2. A method as defined in claim 1 in which the downward pressure exerted on said column is less than the internal shear resistance thereof.

3. A method as defined in claim 1 in which the concentration of solid in said suspension is maintained substantially uniform from the top to the bottom of said column.

4. A method as defined in claim 1 in which the solid is cellulose pulp and the washing liquid is supplied to said column at a temperature of at least 50 C.

HOWARD R. CAINE, Primary Examiner US. Cl. X.R.

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