TECHNICAL FIELD

The present invention relates to a method of achieving optimal liquor-to-wood ratios, especially in the pre-impregnation vessel and the top of the digester in association with the continuous production of paper pulp.

STATE OF THE ART AND PROBLEMS

When producing chemical pulp in a continuous cooking process, liquid flows are often utilized for transporting the chips between different process vessels. In order to obtain maximum closure of the process, it is important that these liquid flows, once they have completed their task, are returned and used once again. This problem is most usually solved by means of so-called "circulations" at different points in the process. Thus, as an example, chips are transferred from the bottom of the pre-impregnation vessel to the top of the digester itself with the aid of a circulation which is usually called C5 or the transfer circulation. The function of C5 is as follows: once the chips have migrated down through the pre- impregnation vessel, and their treatment in this vessel has been completed, they are fed out at the bottom of the vessel using a feeding-out device. Recirculated liquor, which assists in conveying out the chips into the conduit, is also supplied to this feeding-out device. The pressure of this liquor is sufficiently large to convey the chips right to the top of the digester. Located at the top of the digester is a top separator, the function of which is to separate the chips from a part of the circulation liquor. An additional part of the liquor accompanies the chips some distance down into the upper part of the digester,

and is then separated off with the aid of a screen section in the digester wall and combined with the liquor which was separated off in the top separator. The transfer circulation is closed by this combined liquor stream being returned to the feeding-out device in the bottom of the pre-impregnation vessel via a heat exchanger for heating of the liquor.

The circulation liquor which migrates down through the upper part of the digester together with the chips assists in bringing about an even and stable downwardly directed movement of the chip column. This is because the chip bed offers a resistance to the flow of liquor, resulting in the formation of a downwardly directed flow resistance force on the bed. Another way of generating an extra force on the chip bed is to permit the level of chips to extend above that of the liquor in the top of the digester. In accordance with Archimedes' principle, the chips which are situated above the surface of the liquor exert a greater gravitational force on the underlying chips than dp the chips which are immersed in the liquor. However, the combined forces also have another effect on the chip bed. The bed is not only pressed downwards, but is also pressed together, so that the space between the chip pieces decreases. This is expressed as the degree of packing increasing in the chip column. Too great a degree of packing has been found to be capable of causing difficulties and is therefore something which is preferably to be avoided. At the screen sections, the liquid flows radially outwards towards the wall of the digester, and a very compressed bed provides a flow resistance which results in a large force on the screen section. In association with this, friction arises against the screen wall which, in the worst case, leads to that the chip column catches, that is the downwardly directed force is not adequate to overcome the friction against the screen wall, with the result that movement of the chip column ceases.

In order to avoid too great a degree of packing at the top of the digester, the level of liquor can be raised, and the liquor-to-wood ratio decreased, in the digester top. If this latter ratio is decreased, there will consequently be less liquid in relation to the quantity of wood. However, a decrease in the liquor-to-wood ratio often gives rise to problems with feeding out chips from the bottom of the pre- impregnation vessel, since a certain quantity of liquid is required for this procedure to function satisfactorily. This results, in turn, in difficulties due to an increased tendency for plugs to occur in the top separator of the digester and in the transfer circulation. Thus, there is no entirely satisfactory method available at present for avoiding excessive degrees of packing at the top of the digester without adversely affecting the transfer of chips from the pre- impregnation vessel to the digester top.

SOLUTION AND ADVANTAGES

Using the method described here, the operational efficiency of the pre-impregnation vessel can be improved at the same time as problems associated with too great a degree of packing at the top of the digester can be avoided.

The concept behind the method which has been devised is to bring about the possibility of decreasing the liquor-to-wood ratio at the top of the digester at the same time as the liquor-to-wood ratio in the pre- impregnation vessel is increased. This is brought about by a pipe being drawn from the pressure side of pump C5, but prior to the heat exchanger, in the return conduit of the transfer circulation. The pipe is connected, downstream of a branching point, to a number of connection pieces immediately below the top separator of the pre-impregnation vessel. The reason for the pipe being connected prior to the heat

exchanger is that the liquor drawn off should not be heated before it is conveyed to the top of the pre- impregnation vessel, in order to avoid starting the cooking at that point. The impregnation temperature should be relatively low, as compared to the digesting temperature.

Hand valves are expediently installed at the connection of the new conduit to the return conduit of the transfer circulation as well as at the connections below the top separator. It can also be appropriate to fit a non-return valve downstream of the hand valve at the transfer circulation, and to locate a drainage valve downstream of the non-return valve. A control loop consisting of a flow meter and control valve is also installed on the conduit.

One contemplated embodiment is to utilize platform connection pieces which are present on the pre-impregnation vessel. In this case, it is appropriate to place pipe bends, which can readily be disassembled, between the hand valves on the conduit in the vicinity, of the vessel and the platform connection pieces, so that the latter can be used for building a platform.

Using the coupling described above, the thrust on the chip column in the pre-impregnation vessel can be increased by increasing the liquor-to- wood ratio at that site, at the same time as the liquor-to-wood ratio at the top of the digester can be lowered. The reason for this is that, by virtue of the new conduit, liquor can be conveyed from the transfer circulation to the top of the pre-impregnation vessel. Here, it provides additional assistance to the downward movement of the chip column and contributes to efficient operation of the feeding-out device at the bottom, despite the fact that lower quantities of liquor are being supplied to the feeding-out device by the "normal" route via the transfer circulation. Due to the efficient operation of the feeding-out device, the supply of extra liquor to the top separator of the

digester, which liquor is normally needed for the operation of the top separator, can be decreased. The decreased flow of liquid to the digester thereby provides a lower liquor-to-wood ratio at the top of the digester and thereby decreases the risk of high degrees of packing occuring at that point.

It is known from SE 334 809 to convey a portion of the liquor in the return conduit to the top of the pre-impregnation vessel. This liquor is, however, drawn off from the return conduit at a point downstream of the heat exchanger, whereby there is a risk that the temperature in the top of the pre- impregnation vessel might become too high. Moreover, in SE 334 809, the positive effect on the liquor-to-wood ratio of the two vessels, when transferring liquor to the top of the pre-impregnation vessel, is not shown. The object of SE 334 809, is rather to be able to operate the digester without the circulation / circulations in the upper part of the digester, something which has proved not to bee able to achieve. Another positive result of the method according to the invention is that there is equalization of the alkali concentration, and an increase in temperature, in the pre-impregnation vessel.

A further advantage is that the feed-in flow of black liquor can be decreased or completely shut off and the flow of flushing liquor to the pre-impregnation vessel can be decreased by at least the same quantity as the flow in the new circulation. Feed-out from the pre-impregnation vessel will in any case proceed in a more stable manner.

In addition to this, it is an advantage that the temperature in the transfer circuit can be raised somewhat, so that the flow of steam for heating the digester top can be decreased.

Another very positive result is that it will be possible to lower the kappa number of the pulp leaving the digester by the order of 1.5 units.

DESCRIPTION OF THE FIGURE

The method which has been devised will be described below on the basis of Figure 1, which shows a part of the flow diagram for the cooking process.

A known embodiment consists of a digester 1 and a pre-impregnation vessel 2. Both chips, down from the vessel, and liquor, in the return conduit 12 from the transfer circulation, are supplied to the feeding- out device 5 at the bottom of the pre-impregnation vessel. The chips are conveyed, together with the liquor, out into an input conduit 9 to the top separator 7 of the digester, where a part of the liquor is separated off and passed to conduit 10. The remainder of the circulation liquor is conveyed some further distance down through the upper part of the digester, together with the chips, and is then separated off through a screen section 8, in order to be combined, in conduit 11, with the liquor in conduit 10. The liquor is then pumped through the C5 pump 4 to a heating device 3 and then back to the feeding-out device 5 of the pre-impregnation vessel.

The basic embodiment is the same for the invention, but that which is novel is, in a preferred embodiment, a conduit 13, of size DN150, for example, which is connected to the return conduit of the transfer circulation on the pressure side of the C5 pump 4 but prior to the heating device 3. Downstream of a branching point, the conduit is connected to a number of connection pieces 18 below the top separator 6 of the pre-impregnation vessel. A hand valve 14 and, downstream of this, a non-return valve 15 and a drainage valve 16, are expediently fitted to the said conduit 13 at its connection to the transfer circulation. Hand valves 17 are also expediently fitted at all the connections to the pre-impregnation vessel 2. A control loop 19, consisting of a flow meter and a control valve, is installed on the novel conduit 13.

One contemplated form of embodiment in association with an existing pre-impregnation vessel is to utilize the platform connection pieces of the vessel for connecting the novel conduit 13. In this case, it is useful to locate pipe bends 20, which can readily be disassembled, adjacent to the vessel in between the connection pieces 18 and the hand valves 17, so that the connection pieces can also be used for constructing a platform. The heating device 3 preferably consists of a heat exchanger.

EXAMPLE

Tests were made at a mill for production of kraft softwood pulp. Through the installation of a conduit in which a constituent stream according to the invention could be conveyed to the top of the pre-impregnation vessel, the liquor-to-wood ratio in the top of the digester could be lowered from 3.8:1 to 3.0:1. The feed-in flow, of black liquor could be completely shut off and the supply of steam to the top of the digester could be lowered by 1.5 - 2 tons / hour, due to a higher temperature in the input conduit. At a flow rate of 100 m 3/h in the new conduit, the flow of flushing liquor to the bottom of the pre-impregnation vessel could be decreased by more than that amount. The operation of the feeding-out device became more stable. Indications also showed a Kappa number which was approximately 1.5 units lower than usually without any decrease in viscosity of the pulp. No negative effects were registred.

The invention is not limited by the embodiment in the Figure, but can be varied within the scope of the subsequent patent claims. Thus, the person skilled in the art will readily perceive, for example, that the size of the conduit and the type of valves can

be selected in different ways, and that the exact location of the connections can be varied.