Method for recovering concentrated hydrolysate after hydrolysis of cellulose material

Abstract

A method for obtaining a strong hydrolysate from cellulosic material after a hydrolysis in a batch digester is disclosed. According to the invention the cellulosic material is exposed to a 2 stage hydrolysis with a first steam phase hydrolysis followed by a liquid phase hydrolysis, and wherein the steam phase is conducted such that the degree of packing of the cellulosic material results in at least a 20% up to 100% packing increase. The liquid phase hydrolysis includes adding hot and preferably acidified hydrolysis liquid and not establishing a total L/W ratio above 3.5, but sufficient to keep the cellulosic material under the level of the hydrolysis liquid.

Claims

1. A batch process for recovering concentrated hydrolysate after hydrolysis of lignocellulose material in batch digesters comprising the following stages in sequence; a. Subjecting the lignocellulose material to a steam phase hydrolysis wherein (1) the resulting total L/W ratio formed by steam condensate and lignocellulose moisture do not exceed 1.5 if the lignocellulose material only contains natural lignocellulose moisture before the steam phase hydrolysis or (2) the resulting total L/W ratio does not exceed 2.5 if the lignocellulose material has been subjected to washing or any corresponding liquid treatment with subsequent draining before the steam phase hydrolysis, and wherein the lignocellulose material is subjected to a first P-factor exposure during the steam phase hydrolysis resulting in a packing degree increase of at least 20%; b. Subjecting the lignocellulose material to a liquid phase hydrolysis by adding hydrolysis liquid covering the packed lignocellulose material from the steam phase hydrolysis wherein the total L/W ratio formed by a liquid content comprising steam condensate, lignocellulose moisture and added hydrolysis liquid does not exceed a range of from 2.5-3.5, and wherein the lignocellulose material is subjected to a second P-factor exposure during the liquid phase hydrolysis, and c. Recovering a hydrolysate after the liquid phase hydrolysis which hydrolysate corresponds to a volume of from 0.5-2.0 in total L/W ratio and which is diluted only by the added hydrolysis liquid; wherein total P-factor established in the steam phase hydrolysis and the liquid phase hydrolysis lies in the range of from 200-1500, and the first P-factor exposure is 50-95% of the total P-factor and the second P-factor exposure is 5-50% of the total P-factor.

2. The method defined in claim 1, wherein that the liquid phase in the batch digester is subjected to circulation during the liquid phase hydrolysis such that the liquid content is circulated at least 2 times through the digester.

3. The method defined in claim 1, wherein the recovery of the hydrolysate after the liquid phase hydrolysis is obtained by draining free liquid from the batch digester in at least an initial recovery phase.

4. The method defined in claim 1, wherein the recovery of the hydrolysate after the liquid phase hydrolysis is obtained by displacing free liquid from the batch digester using another displacement liquid in at least a final recovery phase.

5. The method defined in claim 1, wherein the total P-factor established in the steam phase hydrolysis and the liquid phase hydrolysis exceeds 400.

6. The method defined in claim 4, wherein the displacement liquid used is a weak hydrolysate displaced and diluted from a previous hydrolysis stage.

7. The method defined in claim 5, wherein the P-factor established in the steam phase hydrolysis exceeds 300.

8. The method defined in claim 1, wherein the packing degree increase after the P-factor exposure during the steam phase hydrolysis exceeds 50%.

9. The method defined in claim 4, wherein the added hydrolysis liquid used at least in part comprises a weak hydrolysate displaced and diluted from a previous hydrolysis stage.

10. The method defined in claim 9, wherein the weak hydrolysate comprises additional acidifier.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a schematic flow chart of the process implemented in a batch digester according to one embodiment of the present invention;

(2) FIG. 2 is a principal layout of the filtrate tank farm used for handling the treatment liquors in a batch digester system according the invention;

(3) FIG. 3a to 3c is showing the increase of packing degree that is developed inside a commercial batch digester during steam hydrolysis;

(4) FIG. 4 is a diagram showing how the packing degree increases during development of the P-factor during steam hydrolysis.

DETAILED DESCRIPTION OF THE INVENTION

(5) The cooking process according to the invention implemented in a batch digester is schematically shown in FIG. 1 as a flow chart

(6) The Fill phase: The displacement batch pulping process according to the invention is started by filling the digester with the lignocellulosic material i.e. with the chips. The chip flow enters into the top of the digester. Low pressure (LP) steam is used to ensure good chip packing over the whole digester cross-section using a swirling steam generator in the inlet. During the chip filling, air is evacuated through suction screens arranged in the inlet. Chip filling is stopped after the digester level switch has operated and the capping valve is then closed. The chips are preheated from ambient temperature to about 60-90 C. during the filling phase.

(7) The Heat phase: Heating of the chips to full hydrolysis temperature is continued by using first further low pressure (LP) steam from the top and bottom of the digester and the heating is finally continued with medium pressure (MP) steam, until the required temperature is reached i.e. 150-170 C. The digester is kept at this temperature and pressure until the prehydrolysis step is completed, i.e. the required P-factor is reached.

(8) St Hyd phase: According to the invention the prehydrolysis step is carried out in a steam phase, where the acids of the cellulose are hydrolyzed by the steam and acidic conditions are created in the digester. The end-pH of the steam prehydrolysis phase varies depending on the cellulose or wood species and the prehydrolysis conditions itself. The pH is typically measured in the condensate formed and varies from 2.5 to 4.0. The steam hydrolysis phase continues until a predetermined packing degree has been obtained, wherein the cellulose material is subjected to a first P-factor exposure during the steam phase hydrolysis resulting in a packing degree increase of at least 20%, and preferably after a P-factor exceeding 100 and more preferably over 400.

(9) W hyd phase: Once the steam phase hydrolysis has ended and the packing degree has increased is the batch digester filled, preferably as fast as possible, with a small predetermined amount of liquid (A1) that will form a suspension of the digester content. The hemicellulose rich condensate from the steam phase hydrolysis will be readily suspended in this liquid. As shown in the flow chart may the suspension be subjected to circulation during this phase solving the hemicellulose condensate evenly in the entire liquid volume. The liquid added is preferably already heated to the full hydrolysis temperature and may contain additional acidifiers. The hydrolysis then continuous in a second water hydrolysis phase further dissolving hemicellulose into this liquid from the cellulose material.

(10) Hyd Ext phase: After the total P-factor is reached, i.e. after the steam hydrolysis and the water hydrolysis, the extraction step is started by a first draining phase obtaining an undiluted strong hydrolysate (E1), followed by introducing hot washing liquid (A2) to the digester displacing the residual strong hydrolysate from the cellulose material.

(11) The first volume of displaced strong hydrolysate is essentially undiluted and is extracted to a dedicated strong hydrolysate tank in flow E1, and may be sent directly to further processes such as C5-sugar processes.

(12) Hyd Wash phase: When the concentration of the displaced strong hydrolysate is dropping, or immediately before it starts to drop, the flow E1 to the strong hydrolysate tank is blocked and finally displaced liquids routed in line E2 are collected in a weak hydrolysate tank, wherein residual hemicellulose is caught in the liquid. As this liquid is almost at hydrolysis temperature it is as shown used as the liquid added, via line A1, as the liquid for forming the water hydrolysis phase.

(13) Neutr phase: The temperature of the hot washing liquid is between 100-174 C., preferably between 140-160 C. and it is pumped into the digester from the bottom thereof. According to one embodiment hot water from Hot W/HOT WATER accumulator as shown in FIG. 2 is used as the washing liquid.

(14) According to another embodiment sodium hydroxide may be added to the hot water prior introducing it into the digester, if there is a need to increase the pH of the chips during the recovery step to enhance the stopping of the prehydrolysis.

(15) As shown in the flow chart may also the neutralization phase include addition of white liquor, either cold or as shown here as heated white liquor in the B1 flow.

(16) BL Imp phase: The following kraft cooking process starts with addition of hot black liquor in flow C1 and additional white liquor in flow B2, while displacing used neutralization liquor in flow F1 from digester.

(17) Heat phase: After impregnation is the digester content exposed to circulation while adding medium pressure steam MP ST, heating the content to full cooking temperature.

(18) Cook phase: After heating to full cooking temperature the circulation continues during the cooking stage.

(19) Displ and Discharge phases: After cooking the final black liquor is displaced in flow G1 by adding displacement liquid in form of wash liquid in flow D1 in a first displacement phase, and continues with displacement of residual black liquor in flow G2 by adding more wash liquid in flow D2 in a second phase. Once ended the produced pulp is suitable for dissolving pulp production, Diss Pulp, is pumped out from the batch digester.

(20) In FIG. 2 is shown a principal layout of the filtrate tank farm used for handling the treatment liquors in a batch digester system according the invention and described above.

(21) WASH LIQUID TANK: Starting from the left hand side the tank farm includes a wash liquid tank, Wash Liq, receiving wash liquid that may be filtrate from brown stock washing stages after cooking or any alkaline filtrate from bleaching stages following brown stock washing. The temperature of the wash liquid is conventionally at least 70-80 C. and the wash liquid tank may be an atmospheric tank.

(22) HOT WHITE LIQUOR TANK: White liquor, conventionally holding a temperature about 70-90 C. from the recovery process, is fed to a hot white liquor tank, Hot WL, via an indirect heat exchanger where the white liquor is heated by the residual heat in the spent cooking liquors that is to be sent to evaporation stages in the recovery process. The heated hot white liquor is sent to both the neutralization phase as well as the black liquor impregnation stage ahead of the kraft cooking stage.

(23) FINAL BLACK LIQUOR TANK: Final black liquor obtained from both the final stages of neutralization and after cooking is sent to a final black liquor tank, Hot BL 2, and as shown is the residual heat value in these liquors used in 2 indirect heat exchanger heating the white liquor, WL, as well as the warm water, WW, sent to hot water tank Hot W. As this tank receives liquors of different pH levels is the tank normally under circulation to even out these differences and avoid settling in the tank.

(24) PRIMARY BLACK LIQUOR TANK: The first volume of the spent cooking liquor in flow G1, holding full cooking temperature, is sent to a primary black liquor tank, Hot BL 1, and as shown is this black liquor used in flow C1 to establish the black liquor impregnation stage following neutralization.

(25) WEAK HYDROLYSATE TANK. The residual hemicellulose suspended in the liquid displaced after hydrolysis is sent in flow E2 to a weak hydrolysate tank, Hot Hyd weak, and is used as the suspension liquid when forming the water hydrolysis stage. The residual hemicellulose is thus not wasted and instead brought back to the system where the liquid is used to suspend more hemicellulose from the steam hydrolysis phase.

(26) STRONG HYDROLYSATE TANK: The strongest hydrolysate recovered by draining after the water hydrolysis phase, i.e. flow E1, is sent to a strong hydrolysate tank. Hot Hyd strong. This high concentration liquor may be sent directly to further processing and recovery of commercial products such as C5-sugar production. Normally this tank is also under circulation to avoid settling in the tank. In some systems this tank may also be subjected to cooling in order to avoid the hemicellulose to be further degraded.

(27) HOT WASH WATER TANK: Hot wash water is used to wash out and displace the hydrolysate in the acidic phases as alkaline content is to be avoided here. The wash water is sent to this tank, Hot W, via heaters, and may be put under a heating circulation in the tank.

(28) FIG. 3a to 3c show the increase of packing degree that is developed inside a commercial batch digester during steam hydrolysis. In the first FIG. 3c is the batch digester filled to the top with chips (until the level sensor indicates full), and thereafter is the inlet valve closed and the chips is heated with pressurized steam reaching a hydrolysis temperature of about 170-180 C. After a while is the signal from the level sensor lost, but as the digester is heated and under pressure could no more chips be supplied, as such late furnish would be subjected to other process conditions than rest of the content. What has been realized in this context is that the content is subjected to extensive compaction and at end of the prehydrolysis is the level of content reduced to about half the volume of the digester which is shown in FIG. 3c.

(29) How much the content is compressed has been studied in a small laboratory digester where a press piston could be applied on the content of chips during steam phase prehydrolysis conditions. In FIG. 4 is shown a test where the press piston applies a force of about 14 kPa on the content. This order of force should be compared with a force of about 70 kPa that is fully developed in the bottom of a commercial batch digester with a height of about 20 meter due to the weight of the content. This means that in a commercial batch is a linear force applied on the content from top to bottom ranging from 0 to 70 kPa, i.e. with an average force of about 35 kPa. Hence, applying a moderate force of 14 kPa should mimic the possible compression in average in the entire digester by margin. When the force of 14 kPa is applied initially is an incremental increase of packing seen at about 1.1 in packing degree and this increases slightly to about 1.3 in packing degree after some 70 minutes, which corresponds to a neglect able single digit P-factor. However, when the P-factor increases to about 200 is the packing degree increasing rapidly to about 1.7 and continuous to increase to a packing degree approaching 2.0 at a P-factor of about 700 and after 200-250 minutes. Already at a P-factor of about 400 after some 150 minutes is a packing degree of about 1.9 obtained. The test show that the volume of content has reduced its volume by about half if a P-factor of about 700 is reached during the hydrolysis.

(30) The reason for this high order of compaction during hydrolysis may likely be found in the softening temperature of lignin and possibly hemicellulose in the wood matrix. Prior studies (Goring, Pulp & Paper Mag. Can. 64:T-517, 1963) of thermoplasticity of dry wood components has shown that lignin and hemicellulose has softening temperatures around 127-235 C. and 167-217 C. respectively, while cellulose require a temperature of 231-235 C. for thermal softening. A typical steam phase hydrolysis at about 170-180 C. may thus activate lignin and possibly hemicellulose softening. This may explain why typical black liquor impregnation only has revealed single digit compaction of the cellulose material as the black liquor impregnation typically is conducted at some 110-130 C.

(31) The invention may apply to any kind of cellulose material such as hardwood, softwood and annual plants, including bagasse, bamboo and straw. The invention is preferably applied when the cellulose material is in form of well screened chips, where the total void volume between chips may be as high as of the total volume, but also pin-chips, chopped straw and saw dust with lower order of total void volume.

(32) As noted before could the original cellulose material contain up to 15% of hemicellulose (Eucalyptus Nitens) and liquid draining and displacement techniques may recover of this content. In some processes is the hemicellulose extraction given priority and the pulp after hydrolysis may be exposed to extreme mechanical pressing and washing in order to extract more hemicellulose. But this will be at the expense of losses in pulp strength, and where the residual alpha cellulose instead is used for ethanol production or other uses than paper pulp production. The invention may be used for ethanol production mills or, as shown in FIG. 1, in a paper pulp production mill.