TECHNICAL FIELD

The present invention relates to a method for extracting hydrolysate in a continuous cooking process for producing pulp. It furthermore relates to a prehydrolysis vessel system and to a hydrolysate extracting arrangement in a continuous cooking system for producing pulp.

BACKGROUND

In some pulp production, for example in dissolving pulp production, a lignocellulose raw material is first subject to a hydrolysis before an alkaline cooking is performed. The hydrolysis is normally performed by adding steam or liquid to the lignocellulose raw material. During the hydrolysis the hemicellulose, mainly C5 sugars, are degraded and released from the wood. If the sugars should be extracted a hydrolysate comprising the dissolved sugars needs to be removed from a vessel where the hydrolysis is performed before the pH is changed and an alkaline cooking is performed. In the alkaline cooking process the dissolved sugars after the hydrolysis are broken-down to non-valuable components.

Handling of hydrolysate may be problematic due to its very sticky consistence and tendency to create scaling for example in pipes, screens and on tank surfaces. Hydrolysate can be removed through screens provided in the prehydrolysis vessel in a continuous cooking system. However, due to the sticky consistence of the hydrolysate and its tendency to create scaling there will be problems with scaling in the screens. Hereby the screens need to be cleaned and the production efficiency may be decreased. In the worst case the production even needs to be stopped during cleaning. Any need for cleaning and any interruption in production processes are of course negative.

SUMMARY

An object of the present invention is to improve production efficiency in a continuous pulp production process. A further object of the present invention is to improve hydrolysate extraction efficiency in a continuous pulp production system.

This is achieved by a method for extracting hydrolysate in a continuous cooking system for producing pulp and by a prehydrolysis vessel in a continuous cooking system for producing pulp and by a hydrolysate extracting arrangement in a continuous cooking system for producing pulp according to the independent claims.

According to one aspect of the invention a method for extracting hydrolysate in a continuous cooking process for producing pulp is provided, said method comprising the steps of: cooling a content in a prehydrolysis vessel of a continuous pulp production system such that the content has been cooled down when the content arrives at a hydrolysis outlet of the prehydrolysis vessel on its way from an inlet provided in one end of the prehydrolysis vessel to an outlet provided in an opposite end of the prehydrolysis vessel;

removing a hydrolysate of said content through the hydrolysate outlet of the prehydrolysis vessel, wherein said content in the prehydrolysis vessel is cooled to such a degree that the removed hydrolysate has a temperature between 70 °C and 130 °C.

According to another aspect of the invention a prehydrolysis vessel system in a continuous pulp production system is provided, wherein said prehydrolysis vessel system comprises:

(a) a prehydrolysis vessel comprising:

an inlet provided in one end of the prehydrolysis vessel for receiving a content to be treated in the prehydrolysis vessel;

an outlet provided in an opposite end of the prehydrolysis vessel; and a hydrolysis outlet; and

(b) a hydrolysate extracting arrangement connected to the prehydrolysis vessel, said hydrolysate extracting arrangement comprising:

a hydrolysate removing conduit comprising a prehydrolysis vessel connector which is connected to the hydrolysate outlet of the prehydrolysis vessel; and at least one cooling device which is provided to the prehydrolysis vessel such that it can cool down a content of the prehydrolysis vessel such that the content has been cooled down when the content arrives at the hydrolysis outlet of the prehydrolysis vessel on its way from the inlet of the prehydrolysis vessel to the outlet of the prehydrolysis vessel, wherein said cooling device is configured for cooling down the content in the prehydrolysis vessel to such a degree that a hydrolysate removed through the hydrolysate outlet has a temperature between 70 °C and 130 °C.

According to another aspect of the invention a hydrolysate extracting arrangement configured for being connected to and extracting hydrolysate from a prehydrolysis vessel in a continuous pulp production system is provided, said hydrolysate extracting arrangement comprising: a hydrolysate removing conduit comprising a prehydrolysis vessel connector which is arranged for being connected to a hydrolysate outlet of the prehydrolysis vessel; and

at least one cooling device configured such that it can be provided to the prehydrolysis vessel and such that it can cool down a content of the prehydrolysis vessel such that the content has been cooled down when the content arrives at a hydrolysis outlet of the prehydrolysis vessel on its way from an inlet provided in one end of the prehydrolysis vessel to an outlet provided in an opposite end of the prehydrolysis vessel, wherein said cooling device is configured to cool down the content in the prehydrolysis vessel to such a degree that a hydrolysate removed through the hydrolysate outlet has a temperature between 70 °C and 130 °C.

Hereby a method and a system for extraction of hydrolysate in a pulp production system is provided which is more effective and reliable than prior art systems. Thanks to the cooling of the hydrolysate before it is removed through a hydrolysate outlet of the prehydrolysis vessel the tendency of the hydrolysate to create scaling, for example in the hydrolysate outlet which can be one or more screens, is much decreased. When cooling the hydrolysate the furfural formation is reduced and also the lignin degradation is reduced. This will reduce the tendency to build scaling. Furthermore the stability of the sugar solution is radically improved due to lowering the sugar degradation rate and a more effective sugar extraction from the hydrolysate can be provided. A more reliable process is provided thanks to less scaling and less need for cleaning of the system and a better sugar yield can be provided thanks to reduced sugar degradation. In one embodiment of the invention said prehydrolysis vessel comprises a cooling section where said cooling is performed, which cooling section is a length section of the prehydrolysis vessel, in which length section the hydrolysis outlet is provided and which length section is less than 1/3 or less than 1/5 of a total length of the prehydrolysis vessel and wherein said cooling section is closer to the outlet than to the inlet of the prehydrolysis vessel.

In one embodiment of the invention said cooling of a content in the prehydrolysis vessel comprises cooling the content to such a degree that the removed hydrolysate has a temperature between 70 °C and 120 °C or between 90 °C and 110 °C. In this embodiment of the invention the cooling device is configured to cool the content to such a degree that the removed hydrolysate has a temperature between 70 °C and 120 °C or between 90 °C and 110° C.

In one embodiment of the invention said cooling comprises adding liquid with a temperature between 20 °C and 100 °C or a temperature between 30 °C and 80 °C or a temperature between 40 °C and 70 °C to the content in the prehydrolysis vessel. In this embodiment the cooling device comprises a liquid adding device which is configured to add a liquid having a temperature between 20 °C and 100 °C or a temperature between 30 °C and 80 °C, or a temperature between 40 °C and 70 °C to the content of the prehydrolysis vessel.

In one embodiment of the invention said cooling comprises circulating a part of the removed hydrolysate through a heat exchanger (41) for cooling it down to a temperature between 20°C and 100 °C or a temperature between 30 °C and 80 °C or a temperature between 40 °C and 70 °C and adding the cooled hydrolysate to the content of the prehydrolysis vessel. In this embodiment the cooling device comprises a hydrolysate circulation arrangement which is configured to circulate a part of the hydrolysate removed through the hydrolysate removing conduit through a heat exchanger for cooling it down to a temperature between 20 °C and 100 °C or a temperature between 30 °C and 80 °C or a temperature between 40 °C and 70 °C and add the cooled hydrolysate to the content of the prehydrolysis vessel.

In one embodiment of the invention said cooled hydrolysate is added into the prehydrolysis vessel through nozzles provided around a periphery of an enclosing wall of the prehydrolysis vessel.

In one embodiment of the invention at least a part of said cooled hydrolysate is added to the content of the prehydrolysis vessel through an outlet of a liquid adding pipe protruding into the prehydrolysis vessel, wherein said outlet is positioned closer to a central axis (A) of the prehydrolysis vessel than to an enclosing wall.

In one embodiment of the invention the step of removing a hydrolysate comprises removing the hydrolysate through at least one screen provided in an enclosing wall of the prehydrolysis vessel, wherein said at least one screen is the hydrolysate outlet.

In one embodiment of the invention the step of removing a hydrolysate comprises displacing the hydrolysate out through the hydrolysate outlet by adding a liquid into the prehydrolysis vessel from a bottom part of the prehydrolysis vessel.

In one embodiment of the invention the method further comprises the step of extracting sugar from the removed hydrolysate.

In one embodiment of the invention the continuous cooking process is a process for producing dissolving pulp.

Further embodiments of the invention are described in dependent claims and in the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures la- Id are schematic illustrations of different embodiments of prehydrolysis vessel systems comprising a hydrolysate extracting arrangement connected to a prehydrolysis vessel in a continuous cooking system according to the invention.

Figure 2 is a flow chart of a method for extracting hydrolysate according to one embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Figures la- Id are schematic illustrations of different embodiments of prehydrolysis vessel systems la, lb, lc, Id comprising a hydrolysate extracting arrangement 3a, 3b, 3c, 3d connected to a prehydrolysis vessel 5a, 5b, 5c, 5d in a continuous cooking system according to the invention. Some of the components are the same in all embodiments and will also be given the same or corresponding reference numbers. Common for all the embodiments is that the hydrolysate extracting arrangement 3a; 3b; 3c; 3d comprises a hydrolysate removing conduit 7 comprising a prehydrolysis vessel connector 8 which is arranged for being connected to a hydrolysate outlet 9 of the prehydrolysis vessel 5a; 5b; 5c; 5d. The hydrolysate outlet 9 can for example be one or more screens provided in an enclosing wall 21 of the prehydrolysis vessel. A reason for removing hydrolysate from the prehydrolysis vessel can be that the hydrolysate can be used in some way, for example for extraction of sugar. Another reason for removing the hydrolysate is that less alkali will be needed in the next step of the pulp production process if at least some of the hydrolysate is removed because the hydrolysate is acidic. Such a prehydrolysis vessel is a pressurized vessel.

The continuous cooking process of the invention can be a process for producing dissolving pulp.

The hydrolysate extracting arrangement 3a; 3b; 3c; 3d comprises furthermore at least one cooling device 11a, 1 lb; 1 lb’; 11c, l ie’, 11c”; l id configured such that it can be provided to the prehydrolysis vessel 5a; 5b; 5c; 5d and such that it can cool down a content of the prehydrolysis vessel such that the content has been cooled down when the content arrives at the hydrolysis outlet 9 of the prehydrolysis vessel on its way from an inlet 13 provided in one end of the prehydrolysis vessel 5a; 5b; 5c; 5d to an outlet 15 provided in an opposite end of the prehydrolysis vessel. A scraper 16 is often provided in the bottom part of the prehydrolysis vessel. The cooling device 11a, l ib; l ib’; 11c, l ie’, 11c”; l id is configured to cool down the content in the prehydrolysis vessel to such a degree that a hydrolysate removed through the hydrolysate outlet 9 has a temperature less than 130 °C. For example, a temperature between 70 °C and 130 °C. In one embodiment of the invention the cooling device 11a, 1 lb; 1 lb’; 11c, l ie’, 11c”; l id is configured for cooling down the content in the prehydrolysis vessel to such a degree that a hydrolysate removed through the hydrolysate outlet 9 has a temperature less than 120 °C or between 70 °C and 120 °C and in another embodiment the cooling device 11a, l ib; l ib’; 11c, l ie’, 11c”; l id is configured for cooling down the content in the prehydrolysis vessel to such a degree that a hydrolysate removed through the hydrolysate outlet 9 has a temperature less than 110 °C or between 90 °C and 110 °C.

The hydrolysate outlet 9 can be provided in the enclosing wall 21 of the prehydrolysis vessel somewhere between the inlet 13 and the outlet 15 of the prehydrolysis vessel and can as mentioned above suitably be one or more screens provided in the enclosing wall. The hydrolysate outlet 9 can be provided closer to the outlet 15 than the inlet 13 of the prehydrolysis vessel.

A normal temperature of the content in the prehydrolysis vessel 5a; 5b; 5c; 5d (i.e. before the cooling according to the invention) can for example be around 160-180°C. At that temperature the hydrolysate is very sticky and difficult to handle. For example heavy scaling is easily produced in components such as tank walls, tubes and screens coming into contact with the hydrolysate. Hereby there are often problems related to scaling in equipment used for removing the hydrolysate. The sticky consistence of the hydrolysate and its tendency to provide scaling is probably due to furfural which is produced from the dissolved sugars and also due to lignin. The furfural production is a function of time, i.e. longer exposure time of sugars in such conditions leads to higher furfural concentration in the solution. Lignin content also increases during the hydrolysis time. Scaling consists much of lignin and furfural together with sugars. Surprisingly it has been found that by cooling down the hydrolysate to for example less than around 130 °C or less than 120 °C or less than 110 °C the sticky consistence and tendency to provide scaling is greatly decreased and extraction of hydrolysis from a pulp production process can be much improved. Hereby also extraction of sugars from a hydrolysate can be improved. The positive effect of the cooling, i.e. the reduction of scaling, is surprisingly high also for relatively small temperature reductions. By cooling the content down before a hydrolysate is removed from the prehydrolysis vessel of a continuous pulp production system according to the invention a much more effective extraction of the hydrolysate will be achieved thanks to less problem with scaling. Hereby the system for continuous production of pulp can also be operated in a more effective way when the problem with scaling is reduced.

Furthermore in these types of cooking systems for producing pulp it is traditionally very important to keep up the high temperatures of the material in order to avoid energy loss. Hereby it would not be obvious for a skilled person to adopt this solution to cool down the content in the prehydrolysis vessel before a hydrolysate is removed as elevated temperature is required for the alkaline lignin removal step after the hydrolysis.

Also common for all the embodiments as shown in Figures la- Id is that said at least one cooling device 11a, l ib; l ib’; 11c, l ie’, 11c”; l id is configured such that it can provide a cooling effect to a content in the prehydrolysis vessel 5a; 5b; 5c; 5d in a cooling section 23 of said prehydrolysis vessel when the cooling device is provided to the prehydrolysis vessel. The cooling section 23 is a length section of the prehydrolysis vessel, in which length section the hydrolysis outlet 9 is provided. Said length section is in one embodiment of the invention less than 1/3 of a total length of the prehydrolysis vessel and in one embodiment less than 1/5 of a total length of the prehydrolysis vessel. Said cooling section 23 is positioned closer to the outlet 15 than to the inlet 13 of the prehydrolysis vessel. The cooling of the content in the prehydrolysis vessel 5a; 5b; 5c; 5d is suitably performed in the vicinity of the hydrolysate outlet 9, i.e. above, below or in level with the hydrolysate outlet 9. The cooling section 23 can be defined as a length section of the prehydrolysis vessel which length section comprises a part of the enclosing wall 21 and the space inside the enclosing wall 21 within this length section. The hydrolysis outlet 9 is positioned within this length section. The cooling is performed somewhere within this cooling section 23 and hereby the cooling is performed in vicinity of the hydrolysate outlet 9, i.e. above, below or in level with the hydrolysate outlet 9. The cooling can be performed in a number of different ways and the cooling device can be embodied in a number of different ways which will be further described in relation to Figures la- Id. These different cooling methods can also be combined in different ways which are all covered by this invention.

The removing of a hydrolysate from the prehydrolysis vessel 5a; 5b; 5c; 5d according to the invention can comprise displacing the hydrolysate out through the hydrolysate outlet 9 by adding a liquid into the prehydrolysis vessel from a bottom part 25 of the prehydrolysis vessel. Therefore said prehydrolysis vessel 5a; 5b; 5c; 5d can comprise a liquid adding device 27 which is configured for adding liquid to the bottom part 25 of the prehydrolysis vessel for displacing cooled hydrolysate out through the hydrolysate outlet 9. The liquid that is added can be for example cooking liquor, such as white or black liquor and it can be added from nozzles in the enclosing wall 21 in the bottom part 25 and/or from a central tube 27’ provided into the vessel through a bottom 26 of the vessel as shown in Figures la- Id. Such a liquid adding device 27 is normally provided in this type of prehydrolysis vessel for allowing transfer of the material out through the outlet 15 of the prehydrolysis vessel. The central tube 27’ of the liquid adding device 27 is provided according to the invention for providing the further effect of displacing the cooled hydrolysate out through the hydrolysate outlet 9.

The hydrolysate extracting arrangement 3a; 3b; 3c; 3d can in some embodiments further comprise a sugar extracting arrangement 31 (only shown in Figure la, but it can be provided in all embodiments of the invention). The sugar extracting arrangement is connected to the hydrolysate removing conduit 7 and is arranged for extracting sugar from the hydrolysate. Sugar extraction will be more effective compared to in prior art because of the lower temperature of the hydrolysate. Sugar degradation reaction is reduced in the hydrolysate thanks to the lower temperature of the hydrolysate and a total sugar yield can hereby be improved.

Figure la shows one embodiment of a prehydrolysis vessel system la according to the invention. A hydrolysate extracting arrangement 3a which is connected to the prehydrolysis vessel 5a comprises in this embodiment a hydrolysate removing conduit 7 and a cooling device 11a. The cooling device 11a is in this embodiment a liquid adding device which is configured for adding a liquid having a temperature between 20 °C and 100 °C or a temperature between 30 °C and 80 °C or a temperature between 40°C and 70 °C to the content of the prehydrolysis vessel 5a. The liquid can be for example water or washing liquid or another liquid and should be provided to the prehydrolysis vessel 5a inside the cooling section 23. The liquid can suitably have a pH around 7 or can be slightly acid or slightly alkaline. In this embodiment a liquid adding pipe 33a is provided to the prehydrolysis vessel 5a. Said liquid adding pipe 33a is protruding into the prehydrolysis vessel 5a and has an outlet 34a positioned within the cooling section 23 and suitably closer to a central axis A of the prehydrolysis vessel than an enclosing wall 21 of the prehydrolysis vessel, i.e. centered within the prehydrolysis vessel 5a in vicinity of the hydrolysate outlet 9, i.e. above, below or in level with the hydrolysate outlet 9. The cooling device 11a of the hydrolysate extracting arrangement 3a can therefore be connected to the liquid adding pipe 33 for providing a liquid into the prehydrolysis vessel for cooling the content of the prehydrolysis vessel. Hereby the content inside the prehydrolysis vessel 5a can be cooled down in vicinity of the hydrolysate outlet 9 (i.e. the content is cooled down above, below or in level with the hydrolysate outlet 9) and a hydrolysate removed through the hydrolysate outlet 9 can have a lower temperature than without this cooling. The temperature of the removed hydrolysate can be measured, and a temperature of the liquid added through the liquid adding pipe 33 can be controlled such that a suitable temperature of the removed hydrolysate is achieved, such as for example less than 130 °C or less than 120 °C or less than 110 °C as discussed above. Figure lb shows another embodiment of a prehydrolysis vessel system lb according to the invention. A hydrolysate extracting arrangement 3b which is connected to a prehydrolysis vessel 5b comprises in this embodiment a hydrolysate removing conduit 7 and a cooling device 1 lb, 1 lb’. The cooling device comprises in this embodiment two parts. A first part 1 lb can be connected to a liquid adding pipe 33b provided to the prehydrolysis vessel 5b. Said liquid adding pipe 33b is protruding into the prehydrolysis vessel 5b and has an outlet 34b positioned within the cooling section 23 and suitably closer to a central axis A of the prehydrolysis vessel than an enclosing wall 21 of the prehydrolysis vessel, i.e. centered within the prehydrolysis vessel 5b in vicinity of the hydrolysate outlet 9, i.e. above, below or in level with the hydrolysate outlet 9. The second part l ib’ of the cooling device is in this embodiment configured for adding a cooling liquid into the prehydrolysis vessel 5b through nozzles 37 provided around a periphery of an enclosing wall 21 of the prehydrolysis vessel 5b. Said nozzles 37 are provided within the cooling section 23 of the prehydrolysis vessel 5b, i.e. in vicinity of the hydrolysate outlet 9, e.g. above, below or in level with the hydrolysate outlet 9. Hereby a cooling liquid can be both added into the prehydrolysis vessel 5b from the enclosing wall 21 and be added to the content in the prehydrolysis vessel from a central place within the prehydrolysis vessel in vicinity of the hydrolysate outlet 9, i.e. above, below or in level with the hydrolysate outlet 9. Hereby the hydrolysate can be effectively displaced and cooled down. Said nozzles 37 can be provided such that they direct a cooling liquid into the prehydrolysis vessel. Nozzles 37’ can however also be provided within an outer compartment 38 which is provided outside the prehydrolysis vessel such that it surrounds the screen 9 (the prehydrolysis outlet) provided in the enclosing wall 21 of the prehydrolysis vessel. Hereby cooling liquid can be added to the hydrolysate when it has been removed out from the prehydrolysis vessel through the screen 9 and when the hydrolysate is provided in said outer compartment 38.

In the embodiment as shown in Figure lb the hydrolysate extracting arrangement 3b comprises further a hydrolysate circulation arrangement 39. The hydrolysate circulation arrangement 39 is configured for circulating a part of the hydrolysate removed through the hydrolysate removing conduit 7 through a heat exchanger 41 for cooling it down to a temperature between 20 °C and 100 °C or a temperature between 30 °C and 80 °C or a temperature between 40 °C and 70 °C and adding the cooled hydrolysate to the content of the prehydrolysis vessel 5b through the first and/or the second part l ib, l ib’ of the cooling device. In another embodiment the cooling device may comprise only one of the first and second parts l ib, l ib’. The hydrolysate circulation arrangement 39 hereby connects the hydrolysate removing conduit 7 with the first and/or second parts l ib, l ib’ of the cooling device through a heat exchanger 41 and transfers a part of the removed hydrolysate from the hydrolysate removing conduit 7, through the heat exchanger 41 and to the cooling device l ib, l ib’ for adding it to the cooling section 23 of the prehydrolysis vessel 5b when the hydrolysate extracting arrangement 3b is connected to the prehydrolysis vessel 5b. The cooling provided by the heat exchanger 41 to the circulated hydrolysate can be controlled in dependence of a measured temperature of the removed hydrolysate. As discussed above a temperature of the removed hydrolysate should be controlled to be less than 130 °C or less than 120 °C or less than 110 °C.

Recirculation of the hydrolysate may be advantageous because no additional liquid from outside the system need to be added into the prehydrolysis vessel for providing the cooling and hereby unnecessary dilution of the hydrolysate is avoided. However in some processes it may be suitable to add some extra liquid as shown in the embodiment in Figure la. For example hydrolysis extraction yield may be improved by the addition of some extra liquid. It may be suitable to have a system where both these alternatives can be utilized in combination or independently. One such system is shown in Figure lc.

Figure lc shows another embodiment of a prehydrolysis vessel system lc according to the invention. A hydrolysate extracting arrangement 3c which is connected to a prehydrolysis vessel 5c comprises in this embodiment a hydrolysate removing conduit 7 and a cooling device 11c, l ie’, 11c”. The cooling device comprises in this embodiment three parts, whereof a first part 11c and a second part l ie’ are identical to the first and second parts l ib, 1 lb’ as described above in relation to Figure lb and will not be described in detail again. The first and second part 11c, l ie’ are hereby connected to a hydrolysate circulation arrangement 39 and to a liquid adding pipe 33b and to nozzles 37 as described above. The third part 11c” is identical to the cooling device 11a as described in relation to Figure la and will not be described in detail. The third part 11c” of the cooling device is connected to a liquid adding pipe 33a and can add a cooling liquid to the content of the prehydrolysis vessel 5c. Hereby, in this embodiment these three alternative ways of cooling the content in the prehydrolysis vessel 5c can be combined for an optimal cooling effect. Alternatively an operator of the system can choose one or more of the separate parts 11c, l ie’, 11c” of the cooling device for cooling the content. The temperature of the added liquid (either circulated hydrolysate or other liquid added through the third part 11c”) can be controlled such that the removed hydrolysate has a suitable temperature such that scaling is minimized.

Figure Id shows another embodiment of a prehydrolysis vessel system Id according to the invention. A hydrolysate extracting arrangement 3d which is connected to a prehydrolysis vessel 5d comprises in this embodiment a hydrolysate removing conduit 7 and a cooling device l id. It comprises furthermore a hydrolysate circulation arrangement 39 as described above in relation to Figure lb. The hydrolysate circulation arrangement 39 is configured for circulating a part of the hydrolysate removed through the hydrolysate removing conduit 7 through a heat exchanger 41 for cooling it down to a temperature between 20 °C and 100 °C or a temperature between 30 °C and 80 °C or a temperature between 40 °C and 70 °C and adding the cooled hydrolysate to the content of the prehydrolysis vessel 5d through the cooling device l id. The cooling device l id transfers the cooled circulated hydrolysate to a number of nozzles 37. The nozzles 37 are in this embodiment distributed not only around the periphery of the enclosing wall 21 but also distributed along the length of the prehydrolysis vessel 5b within the cooling section 23. In this embodiment the prehydrolysis vessel 5d comprises a number of screens 51 which are provided in sections and nozzles 37 are provided in between said screens 51. Hereby the cooled hydrolysate can be more effectively distributed over the screens and prevent scaling in the screens. Of course the prehydrolysis vessel system Id according to this embodiment can also be provided with another cooling device 11a which is adding another cooling liquid as described in relation to Figure la.

Figure 2 is a flow chart of the method for extracting hydrolysate in a continuous cooking process for producing pulp according to one embodiment of the invention. The steps are described in order below:

SI : Cooling a content in a prehydrolysis vessel of a continuous pulp production system such that the content has been cooled down when the content arrives at a hydrolysis outlet of the prehydrolysis vessel on its way from an inlet provided in one end of the prehydrolysis vessel to an outlet provided in an opposite end of the prehydrolysis vessel. Said cooling is suitably performed in a cooling section of said prehydrolysis vessel, which cooling section is a length section of the prehydrolysis vessel, in which length section the hydrolysis outlet is provided Said length section is in one embodiment of the invention less than 1/3 of a total length of the prehydrolysis vessel and in one embodiment less than 1/5 of a total length of the prehydrolysis vessel. Said cooling section is closer to the outlet than to the inlet of the prehydrolysis vessel. The cooling can be provided by adding liquid with a temperature between 20 °C and 100 °C or a temperature between 30 °C and 80 °C, or a temperature between 40 °C and 70 °C to the content in the prehydrolysis vessel. The liquid can be for example water or washing liquid or circulated hydrolysate as described above. S2: Removing a hydrolysate of said content through the hydrolysate outlet of the prehydrolysis vessel.

Said content in the prehydrolysis vessel is cooled to such a degree that the removed hydrolysate has a temperature less than 130 °C or less than 120 °C or less than 110 °C as described above. This can also be described as in interval between 70 °C and 130 °C or 120 °C or 110 °C.

The method can optionally also comprise a further step:

S3 : Extracting sugar from the removed hydrolysate. Thanks to the cooling of the content in the prehydrolysis vessel the sugar degradation is reduced, and the sugar extraction yield can be increased.