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 hydrolysate extracting arrangement configured for extracting hydrolysate in a batch cooking system for producing pulp and to a batch cooking system for producing pulp.

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.

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. When removing hydrolysate from a cooking vessel for extraction of sugar the connection pipes and tanks used in the system for extraction of sugar often have problems with scaling and need to be cleaned. Any cleaning and interruption in production processes are of course negative.

SUMMARY

An object of the present invention is to improve hydrolysate extraction efficiency in a pulp production system. This is achieved by a method for extracting hydrolysate, in a hydrolysate extracting arrangement and 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 comprising 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;

removing hydrolysate from the batch cooking vessel; and

cooling the removed hydrolysate on its way from the batch cooking vessel to a hydrolysate extracting tank to less than 140 °C as soon as possible after removal from the batch cooking vessel, wherein cooling is performed within a time period of 0-2 minutes from when the hydrolysate is leaving the batch cooking vessel.

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 is connected to a hydrolysate outlet of the batch cooking vessel and comprises: a hydrolysate removing conduit comprising a batch cooking vessel connection, which is configured to be connected to the hydrolysate outlet of the batch cooking vessel of the batch cooking system;

a hydrolysate extracting tank connected to the hydrolysate removing conduit and configured to receive hydrolysate removed from the batch cooking vessel through the hydrolysate removing conduit; and

a cooling device configured to cool any hydrolysate removed from the batch cooking vessel to less than 140 °C as soon as possible after removal from the batch cooking vessel, wherein cooling is performed within a time period of 0-2 minutes from when the hydrolysate is leaving the batch cooking vessel, wherein the cooling device (15) is provided in connection with the hydrolysate removing conduit (7) and the removed hydrolysate is cooled on its way from the batch cooking vessel to a hydrolysate extracting tank. According to another aspect of the invention a hydrolysate extracting arrangement configured to extract hydrolysate in a batch cooking system for producing pulp is provided, said hydrolysate extracting arrangement comprising: a hydrolysate removing conduit comprising a batch cooking vessel connection, which is configured to be connected to a hydrolysate outlet of a batch cooking vessel of the batch cooking system;

a hydrolysate extracting tank connected to the hydrolysate removing conduit and arranged for receiving hydrolysate removed from the batch cooking vessel through the hydrolysate removing conduit; and

a cooling device arranged for cooling any hydrolysate removed from the batch cooking vessel to less than 140 °C as soon as possible after removal from the batch cooking vessel, wherein cooling is performed within a time period of 0-2 minutes from when the hydrolysate is leaving the batch cooking vessel, wherein the cooling device (15) is provided in connection with the hydrolysate removing conduit (7) and the removed hydrolysate is cooled on its way from the batch cooking vessel to a hydrolysate extracting tank.

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 after it has been removed from the batch cooking vessel the tendency of the hydrolysate to create scaling 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 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 the method further comprises the step of extracting sugar from the removed hydrolysate.

In one embodiment of the invention the method further comprises the step of performing alkaline cooking in the batch cooking vessel after the removing 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 cooling the removed hydrolysate comprises cooling the hydrolysate to less than 120 °C. In one embodiment of the invention the step of cooling the removed hydrolysate comprises cooling the hydrolysate to less than 100 °C.

In one embodiment of the invention the step of cooling the removed hydrolysate comprises cooling the hydrolysate within a time period of 0-1 minute from when the hydrolysate is leaving the batch cooking vessel.

In one embodiment of the invention the cooling device is a heat exchanger.

In one embodiment of the invention said batch cooking vessel is a pressurized vessel and said batch cooking system comprises further a hydrolysis arrangement connected to the batch cooking vessel and 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 further comprises a liquor adding arrangement connected to the batch cooking vessel and configured for adding alkaline liquor to the batch cooking vessel for performing alkaline cooking in the batch cooking vessel after hydrolysis has been performed in the batch cooking vessel and after hydrolysate has been removed out from the batch cooking vessel.

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

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic illustration of a batch cooking system according to one embodiment of the invention. Figure 2 is a flow chart of a method for extracting hydrolysate according to one embodiment of the invention.

Figures 3a and 3b show diagrams of furfural and HMF content in hydrolysate at different temperatures.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 shows schematically a batch cooking system 1 for producing pulp according to one embodiment of the invention. Said batch cooking system 1 comprises a batch cooking vessel 3 and according to the invention a hydrolysate extracting arrangement 5 which is connected to a hydrolysate outlet 11 of the batch cooking vessel 3.

The batch cooking vessel 3 can be a traditional batch cooking vessel used for production of pulp. Only some of the components and features in the batch cooking vessel 3 which are relevant for the invention will be described here. The batch cooking vessel 3 comprises an inlet 4a for receiving a lignocellulose raw material and an outlet 4b for expelling a content of the batch cooking vessel 3 after it has been treated in the vessel. The treatment provided to the lignocellulose raw material in the batch cooking vessel 3 comprises first an acid hydrolysis followed by an alkaline cooking. The batch cooking vessel 3 is a pressurized vessel. The acid hydrolysis of the lignocellulose raw material is performed by the addition of a fluid to the batch cooking vessel 3. The fluid added can be steam and/or liquid added into the batch cooking vessel 3. The steam and/or liquid can be water and can optionally comprise an acid component for improving hydrolysis. In the case of only water the acid comprised in the lignocellulose material is considered being enough for achieving the hydrolysis. In Figure 1 this is illustrated by a hydrolysis arrangement 21 connected to the batch cooking vessel 3 which is configured for adding at least one fluid to the batch cooking vessel for performing the hydrolysis as described above. According to the invention it can be suitable to add a liquid for the hydrolysis, possibly with a previous steam treatment. The liquid can be for example water. According to the invention, which will be described in more detail below, a hydrolysate will be removed from the batch cooking vessel and by adding a liquid for performing the hydrolysis an extraction of hydrolysate can be more effective. If sugar is extracted from the hydrolysate also this sugar extraction can be more effective when a liquid is added for the hydrolysis compared to a hydrolysis accomplished by the addition of only steam.

The batch cooking vessel 3 further comprises a liquor adding arrangement 27 configured for adding alkaline liquor to the batch cooking vessel 3 for performing alkaline cooking in the batch cooking vessel 3. The alkaline cooking is performed after hydrolysis has been performed in the batch cooking vessel. According to the invention the alkaline cooking is also performed after a hydrolysate has been removed out from the batch cooking vessel 3. If for example sugar should be extracted from the hydrolysate, the hydrolysate comprising the dissolved sugars needs to be removed from the batch cooking vessel where the hydrolysis was 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.

According to the invention a hydrolysate should be removed from the batch cooking vessel 3 after the hydrolysis has been performed and before an alkaline cooking procedure is started. The hydrolysate is removed through a hydrolysate outlet 11 provided in the batch cooking vessel 3. The hydrolysate outlet 11 can for example be provided in an upper half of the batch cooking vessel. However it can also be provided in a lower half of the batch cooking vessel. The hydrolysate can either be pushed out from the batch cooking vessel 3 by increasing pressure inside the batch cooking vessel, for example by the addition of another fluid to the batch cooking vessel, for example water or liquor, from for example a bottom part of the batch cooking vessel 3. Another alternative is to pump or drain out the hydrolysate from the batch cooking vessel 3.

The batch cooking system 1 can be a system for producing dissolving pulp. When producing dissolving pulp hemicellulose is removed from the lignocellulose raw material by an acid hydrolysis before lignin is removed in the alkaline cooking procedure.

According to the invention a hydrolysate extracting arrangement 5 configured for extracting hydrolysate in a batch cooking system 1 for producing pulp is provided. Said hydrolysate extracting arrangement 5 is during use connected to a hydrolysate outlet 11 of a batch cooking vessel 3 in the batch cooking system 1 as described above. The hydrolysate extracting arrangement 5 comprises a hydrolysate removing conduit 7 comprising a batch cooking vessel connection 8, which is configured to be connected to the hydrolysate outlet 11 of the batch cooking vessel 3. The hydrolysate extracting arrangement 5 comprises further a hydrolysate extracting tank 13 connected to the hydrolysate removing conduit 7 and arranged for receiving hydrolysate removed from the batch cooking vessel 3 through the hydrolysate removing conduit 7 and a cooling device 15 arranged for cooling any hydrolysate removed from the batch cooking vessel 3. The cooling device 15 can be provided in connection with the hydrolysate removing conduit 7 as shown in Figure 1. Suitably the hydrolysate should be cooled down as soon as possible after removal from the batch cooking vessel 3. For example, the cooling device 15 can be positioned such that cooling of the hydrolysate is performed within a time period of 0-2 minutes from when the hydrolysate is leaving the batch cooking vessel or within a time period of 0-1 minute from when the hydrolysate is leaving the batch cooking vessel.

The cooling device 15 can be arranged to cool the hydrolysate to less than 140 °C or less than 120°C or less than 100 °C. A common temperature of the content in the batch cooking vessel 3 can for example be around 170°C. In one embodiment of the invention the cooling device 15 is arranged to cool the hydrolysate to a temperature between 70 and 140 °C and in one embodiment of the invention the cooling device 15 is arranged to cool the hydrolysate to a temperature between 80 and 120°C or between 90 and 100 °C. By cooling the hydrolysate down a much more effective extraction of the hydrolysate will be achieved. The hydrolysate is normally very sticky and difficult to handle. For example heavy scaling is produced in tanks, tubes, circulation lines, screens etc. in equipment used for the handling of 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 140 °C or less than 120 °C or less than 100 °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.

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 hydrolysis.

The cooling device 15 can be a heat exchanger. Another alternative could be to add cold fluid to the hydrolysate when removed from the batch cooking vessel 3. However, this will dilute the hydrolysate which may not be advantageous in all applications.

Figure 2 is a flow chart of the method according to one embodiment of the invention. The method is a method for extracting hydrolysate in a batch cooking process for producing pulp and comprises the steps:

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. This step can comprise steam hydrolysis and/or liquid hydrolysis as described above. Suitably a liquid is added to the batch cooking vessel 3 for the hydrolysis in order to improve hydrolysis extraction.

S3: Removing hydrolysate from the batch cooking vessel 3.

S4: Cooling the removed hydrolysate. This step can comprise cooling the hydrolysate to less than 140 °C or less than 120 °C or less than 100 °C as described above. In one embodiment of the invention this step can comprise cooling the hydrolysate to a temperature between 70 and 140 °C or to a temperature between 80 and 120 °C or to a temperature between 90 and 100 °C. Furthermore the cooling is suitably performed as soon as possible after it has been removed from the batch cooking vessel 3 in order to avoid scaling in the equipment. The hydrolysate is cooled on its way from the batch cooking vessel 3 to a hydrolysate extracting tank 13. The step of cooling the removed hydrolysate can comprise cooling the hydrolysate within a time period of 0-2 minutes from when the hydrolysate is leaving the batch cooking vessel or within a time period of 0-1 minute from when the hydrolysate is leaving the batch cooking vessel. The step of cooling the removed hydrolysate can comprise cooling the hydrolysate by a heat exchanger 15.

According to some embodiments of the invention the method further comprises the step S5: Extracting sugar from the removed hydrolysate. This step is performed after the step S4 of cooling the removed hydrolysate. An extraction yield of sugar from the hydrolysate is much improved when the hydrolysate has been cooled down according to the invention as described above. The stability of the sugar solution is radically improved due to lowering the sugar degradation rate when the hydrolysate is cooled down and a more effective sugar extraction can be provided. In some embodiments of the invention also lignin can be extracted from the hydrolysate. According to some embodiments of the invention the method further comprises the step S6: Performing alkaline cooking in the batch cooking vessel 3 after the removing of the hydrolysate. The step S6 can be performed at any time after the step S3 as described above.

Figure 3a is a diagram of furfural content in hydrolysate at different temperatures after 0, 2, 4 and 6 hours. It shows clearly that the amount of furfural starts to increase at temperatures above 140 °C. Below 140 °C the amount of furfural is very limited.

Figure 3b is a diagram of HMF (hydroxymethylfurfural) content in hydrolysate at different temperatures. Also, HMF content increases at temperatures above 140 °C but is very limited at lower temperatures. Furfural and HMF will together with lignin form sticky scaling in the equipment and hereby scaling can be greatly reduced when the hydrolysate is cooled down to a temperature below 140 °C as soon as possible after removal from the batch cooking vessel according to the invention.