EXTRACTING ARRANGEMENT

TECHNICAL FIELD

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

BACKGROUND

In some pulp production, for example in dissolving pulp production, lignocellulose raw material is first subject to a hydrolysis for removing unwanted hemicelluloses and then an alkaline cooking is performed for removing lignin. The hydrolysis in a batch cooking system is normally performed by adding steam 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 and taken care of 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 sugars are broken-down to non-valuable components.

During production of dissolving pulp from lignocellulosic material it is generally desirable to remove hemicellulose in order to obtain a pulp with a high content of alpha-cellulose. This is also done for various other kinds of specialty chemical pulp.

The removal is performed by a prehydrolysis step where a treatment liquor such as steam or water is introduced, and hemicellulose is hydrolysed from the wood chips during acid conditions. Afterwards, the slurry thus created is neutralized by an alkaline neutralizing liquor to produce a neutralized hydrolysate. Hemicellulose is broken down by the contact with alkali into smaller components and is removed from the slurry in subsequent washing stages during the pulp production.

In some applications, it is now desirable to preserve the hemicellulose in hydrolysate and remove it from the slurry for further use in other fields of industry such as the food industry, livestock industry, or biofuels. However, only a small part of available hemicellulose is removed using prior art methods. It is also difficult to keep the hemicellulose separate from the alkali added to the hydrolysate and used for neutralization, and to remove and preserve hemicellulose without lowering the efficiency of the pulp process.

Since the economic value of the finished pulp is at the moment significantly higher than that of the removed hemicellulose, any commercially viable process for extracting and removing hemicellulose cannot be allowed to impair the pulping process.

There is therefore a need for a method for extracting hemicellulose from lignocellulose material that takes the above considerations and drawbacks into account and that enables removal of hemicellulose in a more efficient way while at the same time having a minimal impact on the process for producing pulp.

Steam hydrolysis only in batch cooking will give very low sugar yield to hydrolysate as the steam condensate volume is too low to access most of the dissolved sugars, which can be still inside the chips after the hydrolysis. Water hydrolysis of liquid phase hydrolysis is suitable if higher and controllable sugar recoveries are sought for. In such process the chips are heated up with steam and after reaching the target hydrolysis temperature, or at a specific P-factor, preheated liquid is added to the digesters and the dissolved sugars will move to the liquid phase to be more easily removed from the digester and the recovery of the sugars is improved compared to e.g. steam hydrolysis. However, the addition of large volumes of water may result in low dry solids in the hydrolysate resulting in additional evaporation need in hydrolysate evaporation and also in higher water flow to kraft liquor evaporation.

SUMMARY

An object of the present invention is to provide an effective method for extracting hydrolysate in a pulp production system where the amount of added fresh water can be limited. This is achieved by a method for extracting hydrolysate and by a batch cooking system according to the independent claims.

According to one aspect of the invention a method for extracting hydrolysate in a batch cooking process for producing pulp is provided. Said method comprises the steps of: providing lignocellulose raw material to a batch cooking vessel; performing acid hydrolysis of the lignocellulose raw material in the batch cooking vessel;

displacing a first part of a hydrolysate from the batch cooking vessel by adding to the batch cooking vessel at least one first displacement liquid comprising recycled hydrolysate from a hydrolysate recirculation tank, wherein said recycled hydrolysate has been transferred to the hydrolysate recirculation tank from the batch cooking vessel during a previous batch cooking process;

receiving the first part of the hydrolysate in at least one hydrolysate extraction tank;

displacing a second part of the hydrolysate from the batch cooking vessel by adding to the batch cooking vessel at least one second displacement liquid comprising alkaline neutralization liquor, such as for example white liquor or black liquor; and

receiving the second part of the hydrolysate in at least one hydrolysate recirculation tank.

According to another aspect of the invention a batch cooking system for producing pulp is provided. Said batch cooking system comprises a batch cooking vessel and a hydrolysate extracting arrangement, which hydrolysate extracting arrangement is configured to extract hydrolysate in the batch cooking system, whereby the hydrolysate extracting arrangement comprises: at least one hydrolysate extraction tank connected to a hydrolysate outlet of the batch cooking vessel;

at least one hydrolysate recirculation tank connected to a hydrolysate outlet of the batch cooking vessel, wherein said hydrolysate recirculation tank also is connected to an inlet of the batch cooking vessel such that hydrolysate removed from the batch cooking vessel can be recirculated back to the batch cooking vessel for displacing hydrolysate in a next following batch cooking process.

According to another aspect of the invention a hydrolysate extracting arrangement is provided which is configured to be connected to a batch cooking vessel in a batch cooking system for producing pulp, said hydrolysate extracting arrangement being configured to extract hydrolysate in the batch cooking system, whereby the hydrolysate extracting arrangement comprises: at least one hydrolysate extraction tank configured to be connected to a hydrolysate outlet of the batch cooking vessel;

at least one hydrolysate recirculation tank configured to be connected to a hydrolysate outlet of the batch cooking vessel, wherein said hydrolysate recirculation tank also is configured to be connected to an inlet of the batch cooking vessel such that hydrolysate removed from the batch cooking vessel can be recirculated back to the batch cooking vessel for displacing hydrolysate in a next following batch cooking process.

Hereby, by separating the first part of the hydrolysate for further use, only the purest sugar solution is extracted. A second part of the hydrolysate is instead used for displacing hydrolysate in a next batch cooking process. Hereby a more efficient hemicellulose extraction can be achieved both because less water needs to be evaporated and because there is a decreased risk of inclusion of alkaline in the extracted hydrolysate. The second part of the extracted hydrolysate may possibly contain small amounts of impurities, e.g. alkali and Na- ions and by not using this second part for extraction of hemicellulose but instead use the second part for displacing hydrolysate in a next batch the problem of possible impurities, e.g. alkali and Na-ions in the extracted hydrolysate is much decreased or even eliminated. Furthermore, by recirculating parts of the hydrolysate the volume of fresh water needed is limited which is suitable for the pulp processing, e.g. less evaporation needs, and for water saving reasons.

In one embodiment of the invention the method comprises the further step: recirculating said second part of the hydrolysate from the hydrolysate recirculation tank to the batch cooking vessel for displacing a first part of a hydrolysate from the batch cooking vessel in a next batch cooking process performed in the batch cooking vessel.

In one embodiment of the invention the method further comprises the step of extracting sugar from the hydrolysate in the hydrolysate extraction tank.

In one embodiment of the invention the method further comprises the step of performing alkaline cooking in the batch cooking vessel after the displacing of the first and second parts of the hydrolysate. In one embodiment of the invention the batch cooking process is a process for producing dissolving pulp.

In one embodiment of the invention the step of performing acid hydrolysis of the lignocellulose raw material in the batch cooking vessel comprises steam hydrolysis and/or liquid hydrolysis.

In one embodiment of the invention the step of receiving the second part of the hydrolysate in at least one hydrolysate recirculation tank comprises receiving the second part of the hydrolysate in at least two hydrolysate recirculation tanks alternately such that one of the hydrolysate recirculation tanks can be cleaned while the other is used.

In one embodiment of the invention the hydrolysate extracting arrangement further comprises: a first hydrolysate removing conduit which is connecting a hydrolysate outlet of the batch cooking vessel and the hydrolysate extraction tank and which comprises a first valve;

a second hydrolysate removing conduit which is connecting a hydrolysate outlet of the batch cooking vessel and an inlet of the at least one hydrolysate recirculation tank and which comprises a second valve;

at least one recirculation conduit which is connecting an outlet of the at least one hydrolysate recirculation tank with an inlet of the batch cooking vessel; at least one recirculation pump connected to the recirculation conduit; and a control system connected to the first and second valves, wherein said control system is configured to control the first and second valves such that a first part of the hydrolysate which is displaced from the batch cooking vessel is transferred to the hydrolysate extraction tank and a second part of the hydrolysate which is displaced from the batch cooking vessel is transferred to the at least one hydrolysate recirculation tank.

In one embodiment of the invention the control system is also connected to the at least one recirculation pump and wherein the control system is configured to control the recirculation pump such that a content in the hydrolysate recirculation tank is transferred to the batch cooking vessel for displacing the first part of the hydrolysate from the batch cooking vessel. In one embodiment of the invention said batch cooking system further comprises an alkaline neutralization liquor tank connected to an inlet of the batch cooking vessel via an alkaline neutralization liquor conduit and an alkaline neutralization liquor pump provided in connection with said alkaline neutralization liquor conduit, wherein the control system further is connected to said alkaline neutralization liquor pump and is configured for controlling said alkaline neutralization liquor pump to add alkaline neutralization liquor from the alkaline neutralization liquor tank into the batch cooking vessel for displacing at least a part of the second part of the hydrolysate.

In one embodiment of the invention said batch cooking vessel is a pressurized vessel and said batch cooking system further comprises a hydrolysis arrangement connected to the batch cooking vessel, which hydrolysis arrangement is configured for adding a fluid to the batch cooking vessel for performing a hydrolysis of a lignocellulose raw material provided in the batch cooking vessel.

In one embodiment of the invention the hydrolysis arrangement is configured for adding steam and/or liquid to the batch cooking vessel for performing the hydrolysis.

In one embodiment of the invention the batch cooking system is a system for producing dissolving pulp.

In one embodiment of the invention the hydrolysate extracting arrangement comprises at least two hydrolysate recirculation tanks such that they can be used alternately.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows schematically a batch cooking system according to one embodiment of the invention.

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

DETAILED DESCRIPTION OF EMBODIMENTS

The method according to the invention is suitable for use with any process where lignocellulosic material undergoes hydrolysis as part of a pulping process. The term lignocellulosic material is used herein to mean materials containing lignin, cellulose and hemicellulose. One example of such materials is wood, others include other agricultural or forestry wastes. The lignocellulosic material is commonly divided into small pieces, chips or fragments before the pulping process is initiated.

The lignocellulosic material is supplied to a batch cooking vessel 3 in a batch cooking system 1. One embodiment of a batch cooking system 1 is schematically shown in Figure 1. A treatment liquor is added to the batch cooking vessel 3 so that hydrolysis of the lignocellulosic material can be performed. A hydrolysis arrangement 21 is shown schematically in Figure 1, which hydrolysis arrangement 21 is configured to add treatment liquor for performing hydrolysis. The treatment liquor may be a gas or a liquid that serves to create acidic conditions in the batch cooking vessel 3. In some embodiments, the gas or liquid itself has a low pH, but in some embodiments the gas or liquid is water or condensate in the form of steam or fluid and the acidic conditions are created by the lignocellulosic material itself releasing acidic substances when in contact with the treatment liquor at elevated temperatures. This is well known within the art and will not be described in detail herein.

Suitably, steam and/or water is supplied as treatment liquor for the hydrolysis. Suitably, a combination is used where steam is first supplied and water added later. The steam will extract hemicellulose and other substances from the lignocellulosic material in a more efficient way than a liquid. Furthermore, this makes the process more economical due to the possibility of heating the water in a separate vessel before supplying it to the batch cooking vessel 3 hereby preventing a lowering of the temperature inside the batch cooking vessel 3.

During hydrolysis, hemicellulose is hydrolysed from the lignocellulosic material and forms part of a slurry that comprises the lignocellulosic material and treatment liquor. In order to remove a hydrolysate from the batch cooking vessel 3 a displacement liquor is supplied to the batch cooking vessel 3. The hydrolysate comprises hemicellulose and also other substances that are in liquid form or that are dissolved in liquid of the slurry. Larger particles such as the wood matrix of the hydrolysed lignocellulosic material will be retained inside the treatment vessel.

After displacement of the hydrolysate, an alkaline liquid such as white liquor is supplied to increase the pH value and create alkaline conditions in the treatment vessel. Treatment of the lignocellulosic material is then continued in order to produce pulp as is well known within the art. According to the invention a batch cooking system 1 for producing pulp is provided. Said batch cooking system 1 comprises a batch cooking vessel 3 and a hydrolysate extracting arrangement 5. Said hydrolysate extracting arrangement 5 is configured to extract hydrolysate in the batch cooking system 1 and the hydrolysate extracting arrangement 5 comprises at least one hydrolysate extraction tank 7 which is connected to a hydrolysate outlet 11 of the batch cooking vessel 3. According to the invention the hydrolysate extracting arrangement further comprises at least one hydrolysate recirculation tank 9 which also is connected to a hydrolysate outlet 11 of the batch cooking vessel 3. In this embodiment the hydrolysate extraction tank 7 and the hydrolysate recirculation tank 9 are connected to the same hydrolysate outlet 11 of the batch cooking vessel. In another embodiment they could however be connected to different hydrolysate outlets. The hydrolysate recirculation tank 9 is also connected to an inlet 13 of the batch cooking vessel 3 such that hydrolysate removed from the batch cooking vessel 3 can be recirculated back to the batch cooking vessel 3 for displacing hydrolysate in a next following batch cooking process. According to the invention a method for extracting hydrolysate in a batch cooking process for producing pulp is also provided. A flow chart of the method is shown in Figure 2 and the steps of the method are described in order below:

SI : Providing lignocellulose raw material to a batch cooking vessel 3.

S2: Performing acid hydrolysis of the lignocellulose raw material in the batch cooking vessel 3. The step of performing acid hydrolysis of the lignocellulose raw material in the batch cooking vessel 3 can comprise steam hydrolysis and/or liquid hydrolysis as described above.

S3: Displacing a first part of a hydrolysate from the batch cooking vessel 3 by adding to the batch cooking vessel 3 at least one first displacement liquid. Said first displacement liquid comprises recycled hydrolysate from a hydrolysate recirculation tank 9. Said recycled hydrolysate has been transferred to the hydrolysate recirculation tank 9 from the batch cooking vessel 3 during a previous batch cooking process. Another first displacement liquid can additionally be used for displacing the first part of the hydrolysate from the batch cooking vessel 3. This displacement liquid could be for example water.

S4a: Receiving the first part of the hydrolysate in at least one hydrolysate extraction tank 7. S5a: Displacing a second part of the hydrolysate from the batch cooking vessel 3 by adding to the batch cooking vessel 3 at least one second displacement liquid comprising alkaline neutralization liquor, such as for example black liquor or white liquor. Another second displacement liquid can additionally be used for displacing the second part of the hydrolysate from the batch cooking vessel 3. This displacement liquid could be for example water.

S6: Receiving the second part of the hydrolysate in a hydrolysate recirculation tank 9. There could as well be provided more than one hydrolysate recirculation tanks 9 which can be used alternately such that one of the hydrolysate extraction tanks can be cleaned while another is used.

S7: Recirculating said second part of the hydrolysate from the hydrolysate recirculation tank 9 to the batch cooking vessel 3 for displacing a first part of a hydrolysate from the batch cooking vessel 3 in a next batch cooking process performed in the batch cooking vessel 3.

The method can furthermore comprise the step:

S4b: Extracting sugar from the hydrolysate in the hydrolysate extraction tank 7. This step can be performed at any time after step S4a independently of the other steps S5-S7, i.e. after the first part of the hydrolysate has been received in the hydrolysate extraction tank 7. The further process performed in the batch cooking vessel 3 after the second part of the hydrolysate has been displaced, i.e. after step S5a, comprises the steps:

S5b: Performing alkaline cooking of the material left in the batch cooking vessel 3 after the displacing of the first and second parts of the hydrolysate.

S5c: Discharging the treated material, i.e. the pulp which can be dissolving pulp. Then the process is repeated from step SI, i.e. new raw material is provided into the batch cooking vessel 3, i.e. a new batch of raw material can be treated in the batch cooking vessel 3.

In some embodiments of the invention the batch cooking process is a process for producing dissolving pulp.

The batch cooking system 1 will now be described in more detail with reference to Figure 1. The hydrolysate extracting arrangement 5 comprises further according to some embodiments of the invention a first hydrolysate removing conduit 15. Said first hydrolysate removing conduit 15 connects a hydrolysate outlet 11 of the batch cooking vessel 3 and the hydrolysate extraction tank 7. The first hydrolysate removing conduit 15 can be one single conduit which is directly connecting the hydrolysate extraction tank 7 and the batch cooking vessel 3 but it can also comprise branched conduits and a conduit part which can be common with a second hydrolysate removing conduit 17 which will be described below. The first hydrolysate removing conduit 15 comprises a first valve 16a.

The hydrolysate extracting arrangement 5 may further comprise a second hydrolysate removing conduit 17 which is connecting a hydrolysate outlet 11 of the batch cooking vessel 3 and an inlet 19a of the at least one hydrolysate recirculation tank 9. The hydrolysate outlet 11 can be the same outlet as the first hydrolysate removing conduit 15 is connected to as shown in Figure 1 but it can also be separate hydrolysate outlets. As described above the first and second hydrolysate removing conduits may comprise a common conduit part which is branched to the separation conduits leading to the hydrolysate extraction tank 7 and the hydrolysate recirculation tank 9 respectively. The second hydrolysate removing conduit 17 comprises a second valve 16b.

The hydrolysate extracting arrangement 5 further comprises at least one recirculation conduit 23 which is connecting an outlet 19b of the at least one hydrolysate recirculation tank 9 with an inlet 13 of the batch cooking vessel 3. A recirculation pump 25 can be connected to the recirculation conduit 23. Hereby hydrolysate received in the hydrolysate recirculation tank 9 can be recirculated back to the batch cooking vessel 3.

The hydrolysate extracting arrangement 5 may further comprise a control system 27 connected to the first and second valves 16a, 16b. The connection may be by wire or wireless. Said control system 27 is configured to control the first and second valves 16a, 16b such that a first part of the hydrolysate which is displaced from the batch cooking vessel 3 is transferred to the hydrolysate extraction tank 7 and a second part of the hydrolysate which is displaced from the batch cooking vessel is transferred to the at least one hydrolysate recirculation tank 9.

The control system 27 may also be connected to the at least one recirculation pump 25. The control system 27 can then be configured to control the recirculation pump 25 such that a content in the hydrolysate recirculation tank 9 is transferred to the batch cooking vessel 3 for displacing the first part of the hydrolysate from the batch cooking vessel 3.

According to some embodiments of the invention the batch cooking system further comprises an alkaline neutralization liquor tank 31 connected to an inlet 14 of the batch cooking vessel 3 via an alkaline neutralization liquor conduit 33 and an alkaline neutralization liquor pump 35 provided in connection with said alkaline neutralization liquor conduit 33. The control system 27 can also be connected to said alkaline neutralization liquor pump 35 and the control system can be configured for controlling said alkaline neutralization liquor pump 35 to add alkaline neutralization liquor from the alkaline neutralization liquor tank 31 into the batch cooking vessel 3 for displacing at least a part of the second part of the hydrolysate.

As discussed above, also other displacement liquors in addition to the first and second displacement liquors can be used for displacing the first and second parts of the hydrolysate. For example water and condensates can be used for the displacing.

The batch cooking vessel 3 is a pressurized vessel and said batch cooking system 1 comprises further a hydrolysis arrangement 21 connected to the batch cooking vessel 3. The hydrolysis arrangement 21 is configured for adding a fluid to the batch cooking vessel 3 for performing a hydrolysis of a lignocellulose raw material provided in the batch cooking vessel 3. The hydrolysis arrangement 2 lean be configured for adding steam and/or liquid to the batch cooking vessel 3 for performing the hydrolysis.

The batch cooking system 1 can be a system for producing dissolving pulp.

In some embodiments of the invention the hydrolysate extracting arrangement 5 comprises at least two hydrolysate recirculation tanks 9 such that they can be used alternately as described above.

Furthermore according to the invention a hydrolysate extracting arrangement 5 is provided which is configured to be connected to a batch cooking vessel 3 in a batch cooking system 1 for producing pulp as described above. Said hydrolysate extracting arrangement 5 is configured to extract hydrolysate in the batch cooking system 1. The hydrolysate extracting arrangement 5 comprises at least one hydrolysate extraction tank 7 configured to be connected to a hydrolysate outlet 11 of the batch cooking vessel 3 and at least one hydrolysate recirculation tank 9 configured to be connected to a hydrolysate outlet 11 of the batch cooking vessel 3, wherein said hydrolysate recirculation tank 9 also is configured to be connected to an inlet 13 of the batch cooking vessel 3 such that hydrolysate removed from the batch cooking vessel 3 can be recirculated back to the batch cooking vessel 3 for displacing hydrolysate in a next following batch cooking process.

In the embodiment as shown in Figure 1 the hydrolysate extracting arrangement 5 further comprises a first hydrolysate removing conduit 15 which is connected to the hydrolysate extraction tank 7 and which comprises a hydrolysate outlet connector 18, which is configured to be connected to a hydrolysate outlet 11 of the batch cooking vessel 3. Said first hydrolysate removing conduit 15 comprises furthermore a first valve 16a. The hydrolysate extracting arrangement 5 further comprises a second hydrolysate removing conduit 17 which is connected to an inlet 19a of the at least one hydrolysate recirculation tank 9 and which comprises a hydrolysate outlet connector 18, which is configured to be connected to a hydrolysate outlet 11 of the batch cooking vessel 3. The hydrolysate outlet connector 18 of the first and second hydrolysate removing conduits 15, 17 are in this embodiment one and the same connector which is connected to one and the same hydrolysate outlet 11 of the batch cooking vessel. The first and second hydrolysate removing conduits 15, 17 have a common part and are then branched towards the hydrolysate extraction tank 7 and the hydrolysate recirculation tank 9 respectively. The second hydrolysate removing conduit 17 further comprises a second valve 16b. The hydrolysate extracting arrangement 5 further comprises at least one recirculation conduit 23 which is connected to an outlet 19b of the at least one hydrolysate recirculation tank 9 and which comprises a batch cooking vessel inlet connector 20, which is configured to be connected to an inlet 13 of the batch cooking vessel 3. The hydrolysate extracting arrangement 5 further comprises at least one recirculation pump 25 connected to the recirculation conduit and a control system 27 connected to the first and second valves 16a, 16b, wherein said control system 27 is configured to control the first and second valves 16a, 16b such that a first part of the hydrolysate which is displaced from the batch cooking vessel is transferred to the hydrolysate extraction tank 7 and a second part of the hydrolysate which is displaced from the batch cooking vessel is transferred to the at least one hydrolysate recirculation tank 9 when the hydrolysate extracting arrangement 5 is connected to a batch cooking vessel 3 in a batch cooking system 1.

In some embodiments the control system 27 is also connected to the at least one recirculation pump 25 and the control system 27 is configured to control the recirculation pump 25 such that a content in the hydrolysate recirculation tank 9 is transferred to the batch cooking vessel 3 for displacing the first part of the hydrolysate from the batch cooking vessel 3 when the hydrolysate extracting arrangement 5 is connected to a batch cooking vessel 3 in a batch cooking system 1.

In some embodiments of the invention the hydrolysate extracting arrangement 5 comprises at least two hydrolysate recirculation tanks 9 such that they can be used alternately.