Method for recovering hydrolysate

Abstract

The method is for producing pulp. More particularly, a displacement batch cooking process comprises recovery of a first treatment liquid by draining of the digester, preferably not using a displacement liquid. The method may preferably be used in a prehydrolysis step, wherein the recovery of the by-products is improved. In the recovery step, after the target P-factor in the prehydrolysis stage is reached, the hydrolysate is withdrawn by draining in at least one phase obtaining a strong first liquid and thereafter any residual hydrolysate may be displaced.

Claims

1. A method for recovering a first treatment liquid in a displacement batch pulping process in a digester filled with comminuted cellulosic material and equipped with a bottom, a top and a middle liquid exchange position, said method comprising: filling the digester with a first treatment liquid during or after a steam prehydrolyzina step wherein the process continues by, after filling the digester with the first treatment liquid during or after the prehydrolyzing step, subjecting the first treatment liquid contained in the digester to circulation within the digester by withdrawing the first treatment liquid from the middle liquid exchange position and reintroducing the withdrawn first treatment liquid to a top and a bottom of the digester, passing the first treatment liquid through the comminuted cellulosic material disposed in the digester at least once, and washing out the carbohydrates dissolved in the hydrolysate; in a first draining phase, draining the first treatment liquid from the digester via one of the middle or bottom liquid exchange positions to obtain a strong first liquid; and sending at least a part of the strong first liquid to a dedicated processing position either for use in a different phase of the batch pulping process or for further processing wherein at least a part the strong first liquid is used for recovering by-products wherein the first treatment liquid is used during prehydrolysis of the comminuted cellulosic material to produce an acidic hydrolysate containing the carbohydrates dissolved in the hydrolysate during the prehydrolysis and wherein after the first draining phase at least a part of the strong first liquid is sent to further processing in a sugar recovery process by using the carbohydrates dissolved in the hydrolysate, and wherein the first draining phase is assisted by displacing steam or gases through the digester, the steam or gases being added via the top liquid exchange position and wherein a pressure and a temperature of the strong first liquid is maintained.

2. The method for recovering a first treatment liquid according to claim 1, wherein the first draining phase comprises the step of withdrawing the strong first liquid through the middle liquid exchange position and in final phases displacing a residual strong first liquid kept in the digester below the middle liquid exchange position via a displacement by using water added to the bottom liquid exchange position to displace the residual strong first liquid out through the middle liquid exchange position.

3. The method for recovering a first treatment liquid according to claim 1, wherein the strong first liquid is kept at a prehydrolysis temperature and stored in a dedicated accumulator tank before processing the strong first liquid in a sugar recovery process by using the carbohydrates dissolved in the hydrolysate.

4. A method for recovering a first treatment liquid in a displacement batch pulping process in a digester filled with comminuted cellulosic material and equipped with a bottom, a top and a middle liquid exchange position, said method comprising: filling the digester with a first treatment liquid during or after a steam prehydrolyzing step wherein the process continues by, after filling the digester with the first treatment liquid during or after the prehydrolyzing step, subjecting the first treatment liquid contained in the digester to circulation within the digester by withdrawing the first treatment liquid from the middle liquid exchange position and reintroducing the withdrawn first treatment liquid to a top and a bottom of the digester, passing the first treatment liquid through the comminuted cellulosic material disposed in the digester at least once, and washing out the carbohydrates dissolved in the hydrolysate; in a first draining phase, draining the first treatment liquid from the digester via one of the middle or bottom liquid exchange positions to obtain a strong first liquid; and sending at least a part of the strong first liquid to a dedicated processing position either for use in a different phase of the batch pulping process or for further processing wherein at least a part the strong first liquid is used for recovering by-products; wherein the first treatment liquid is used during prehydrolysis of the comminuted cellulosic material to produce an acidic hydrolysate containing the carbohydrates dissolved in the hydrolysate during the prehydrolysis and wherein after the first draining phase at least a part of the strong first liquid is sent to further processing in a sugar recovery process by using the carbohydrates dissolved in the hydrolysate; and wherein after the first draining phase, the digester is filled with water until the comminuted cellulosic material is submerged in the water, the water is subjected to circulation, in a second draining phase, the water is drained from the digester via one of the middle or bottom liquid exchange positions to obtain a weak first liquid and at least a part of the weak first liquid is sent to a dedicated second accumulator to be used as being part of the first treatment liquid ahead of prehydrolysis in a subsequent batch cycle.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The present invention is described by a sequential process disclosed in FIGS. 2 to 19; wherein

(2) FIG. 1 shows how a conventional liquid displacement front is developed through the batch digester;

(3) FIG. 2 shows the first chip filling phase with low pressure steam distribution and heating;

(4) FIG. 3 shows an optional first heating phase of the filled digester using low pressure steam;

(5) FIG. 4 shows an optional subsequent second heating phase using medium pressure steam;

(6) FIG. 5 shows an optional prehydrolysis stage in steam phase;

(7) FIG. 6 shows the start of the hot water or hydrolysate filling of the digester;

(8) FIG. 7 shows the circulation phase of the hot water or hydrolysate filled digester;

(9) FIG. 8 shows the first phase of the inventive draining of the digester via mid screen withdrawal;

(10) FIG. 9 shows the final displacement of residual hydrolysate below mid screen using displacement with hot water;

(11) FIG. 10 shows an optional 2.sup.nd hot water filling;

(12) FIG. 11 shows an optional circulation following the 2.sup.nd hot water filling;

(13) FIG. 12 shows an optional mid screen draining of the circulated 2.sup.nd hot water;

(14) FIG. 13 shows the next phase with displacement of the residual 2.sup.nd hot water volume with neutralizing hot white liquor;

(15) FIG. 14 shows how the 2.sup.nd hot water volume has been displaced with hot white liquor;

(16) FIG. 15 shows how the hot white liquor pad is further filling the digester;

(17) FIG. 16 shows the next phase starting with hot liquor fill, displacing the hot white liquor pad upwards through the digester volume,

(18) FIG. 17 shows how hot liquor fill is displacing the hot white liquor pad upwards while the residual hot water pad is displaced to HBL 2;

(19) FIG. 18 shows how hot liquor fill is displacing the residual hot white liquor pad to HBL 2;

(20) FIG. 19 shows the digester has been filled completely with hot liquor ahead of the subsequent kraft cook.

BACKGROUND PRIOR ART AND DEFINITIONS USED IN THIS DESCRIPTION

(21) In FIG. 1 is shown a conventional displacement cycle in a batch digester, where a 1.sup.st liquor is displaced via a top screen TS by adding a 2.sup.nd displacement liquor via an inlet in the bottom.

(22) The batch digester includes a bottom, a top and a middle liquid exchange position.

(23) The bottom liquid exchange position includes at least an inlet for adding different liquors, and conventionally there may be dedicated inlets for each type of liquors or steam to be introduced.

(24) The mid liquid exchange position includes at least a mid screen MS which is the essential withdrawal position used when withdrawing and circulating the cooking liquor, but the screen may also be used for adding treatment liquors or steam. Using the mid screen also as a distributor in special phases helps to keep the screen open. A pump is located in the withdrawal line from the mid screen, and if filled grey as in FIG. 1, the pump is shut off.

(25) In commercial batch digesters of today, the volume of the digester is about 300 m.sup.3, which of course may vary dependent on intended capacity of the digester. In such a digester the mid screen is typically located in lower of the digester, with a volume of 100 m.sup.3 below mid screen and thus 200 m.sup.3 above mid screen.

(26) The top liquid exchange position includes at least one feed screen FS in chip inlet and a larger top screen TS. The feed screen is a steam injector of a well known design that adds a swirling motion to the inflow of comminuted cellulose material during filling such that an even upper surface of comminuted cellulose material is obtained. The top screen is a screen used to withdraw gases but also withdrawing displaced liquors.

(27) The three figures show from left to right show how the displacement front is developed inside the batch digester during the displacement process. In the first phase, left figure, is hot liquor added, and here filling the entire bottom cone part, as displacement liquor to bottom of digester will displace the 1.sup.st liquor out through the top.

(28) In the second phase, middle figure, is the displacement front moving upwards as more hot liquor is added and as indicated is the displacement front no longer a perfect horizontal displacement front, but instead are streaks of hot liquor penetrating the chip volumes faster than in other areas. This applies especially to the chip volumes close to digester wall.

(29) In a later phase, right hand figure, is shown how these streaks of hot liquor reach the top screen while still large volumes of the chip content lower down in digester has not been displaced at all. This effect is resulting in a dilution of the displaced 1.sup.st liquor sooner than expected compared to if the displacement front had a perfect horizontal front without these streaks.

(30) This example of prior art show the conventional displacement of a 1.sup.st liquid with a different 2.sup.nd liquid, and the 2.sup.nd liquid here is hot liquor.

(31) Only one digester is shown but typically are a number of digesters used operated in sequence and thus in different phases of the cook. If for example 5 digesters are operated the first digester is started and then the remaining digesters are started at some time interval which time interval may correspond to of the total cooking cycle time for one digester. Cooked pulp may then be blown to a blow tank at regular intervals, and the process liquids stored in accumulators and atmospheric tanks may be used in another digester minimizing inactive dwell time for the liquids used. The piping system is simplified showing only one liquid addition point for WL, Wash filtrate, LP.sub. and MP-steam but in a real system are individual piping connected to the the digester.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

(32) The present invention may be applied to any batch cooking phase where the 1.sup.st liquid needs to be kept in an undiluted form at largest possible volumes. Hence, the 1.sup.st liquid may be a hydrolysate or any other process liquid that catch dissolved compounds from the comminuted cellulosic material treated in the batch digester. However, the first liquid may also be warm or hot liquor from a preheating sequence ahead of any type of cook. The first liquid may also be different impregnation liquors or liquids containing polysulfide, anthraquinone, CCE filtrate or similar.

(33) In following figures are shown a sequential prehydrolysis process ahead of a kraft cook, where the invention is applied in the phase of recovering the hydrolysate after prehydrolysis. For further processing of the hydrolysate it is essential that the hydrolysate has as high carbohydrate content as possible, catch as much as possible of the dissolved carbohydrates, while avoiding dilution of the hydrolysate with water or chemicals that may hamper the recovery process. High water content reduces the heat economy of the subsequent recovery process.

(34) A Prehydrolisis Kraft Process Batch Sequence

(35) In FIG. 2 is shown the very first phase of batch cooking, where comminuted cellulose material is fed into digester during low pressure steam addition trough top and bottom liquid exchange positions, and venting gases trough middle point liquid exchange position. The steam added to top is injected using a conventional swirl inducing inlet that helps spreading the comminuted cellulose material in an even layer inside digester.

(36) In FIG. 3 is shown an optional extension of the low pressure steam heating in a filled digester heating the comminuted cellulose material towards the condensation point of the low pressure steam, typically at some 130-140 C. at the most.

(37) In FIG. 4 is shown an optional final steam heating phase where the filled digester is heated towards full prehydrolysis temperature using medium pressure steam.

(38) In FIG. 5 is shown an optional steam phase prehydrolysis, where the temperature is maintained for a sufficient time to subject the material in the digester to full hydrolysis, i.e. reaching the necessary P-factor.

(39) In FIG. 6 is shown filling of the digester with hot water and/or hydrolysate, which could be done directly after FIG. 2, 3 or 4 or alternatively after a steam phase prehydrolysis shown in FIG. 5. Thus, water or hydrolysate is introduced to bottom liquid exchange position until the liquid level cover the comminuted cellulose material inside the digester.

(40) In FIG. 7 is shown a subsequent circulation phase in the digester where the treatment liquid is withdrawn from middle liquid exchange position and reintroduced to both top and bottom liquid exchange position. If this is a wash-out phase after steam phase hydrolysis (FIG. 5), the circulation ratio of the treatment liquid may be well over 1.5 times the total volume of the treatment liquid, possibly up to 5-10 times, especially if the prehydrolysis is done in steam phase. If this is the establishment of a water filled prehydrolysis, the digester will be kept at this temperature and pressure until the prehydrolysis step is completed, i.e. the required P-factor has been reached. P-factor is a defined factor to control the prehydrolysis stage, taking the temperature and time into account (as e.g. H-factor); Herbert Sixta, Handbook of Pulp, Volume 1, Wiley-VCH Verlag, 2006, pages 343-345.

(41) In FIG. 8 is shown the inventive recovery of the first treatment liquid, which after prehydrolysis is the hydrolysate. Recovery starts with draining the hydrolysate from at least the middle liquid exchange position, and sending the hydrolysate to a strong first liquid accumulator. As shown, the draining may be assisted by adding a hot displacement gas through the digester, such as LP steam. A compressor may assist this displacement gas but the important thing is that no larger volumes of displacement liquids are used. As the comminuted cellulose material are at full prehydrolysis temperature essentially no volumes of condensate is formed using low pressure steam, which may dilute the hydrolysate.

(42) Alternatively could the draining be done without steam addition, and in such case is the pressure reduced while lowering the liquid level, and the liquor will flash off primarily steam in the void above liquor level.

(43) The recovered strong first liquid is thereafter sent to processing in a sugar recovery process using the carbohydrates dissolved in the hydrolysate.

(44) As shown in FIG. 9 is the draining continued until the hydrolysate level is approaching the middle liquid exchange position and the residual volume of hydrolysate kept below the middle liquid exchange position is displaced by adding hot water to the bottom liquid exchange position. The addition of hot water may continue until the hydrolysate is displaced and withdrawn from mid screen to such an extent that the hydrolysate starts to become diluted, which may be detected by a pH or conductivity sensor in withdrawal outlet. As it is only the lower part of the digester that is subjected a displacement, the order of dilution of the hydrolysate is reduced considerably compared with a displacement of the entire batch volume.

(45) Optional 2.sup.nd Wash

(46) After withdrawal of the hydrolysate in FIG. 9, could a second optional wash continue, which purpose is to catch the part of hydrolysate that is wetting the surface of the comminuted material in the digester, as well as enable a diffusion of a part of the hydrolysate that is bound in the comminuted material. As shown in FIG. 10 could then the supply of hot water continue until the volume of comminuted material is submerged in hot water.

(47) In FIG. 11 is then shown a circulation phase where the hot water is withdrawn from mid screen and recirculated to top and bottom. Such an intensified liquid circulation may leach out more of the hydrolysate bound in the comminuted material.

(48) The optional second wash is then as shown in FIG. 12 ended by draining the weaker hydrolysate to a dedicated weak hydrolysate accumulator.

(49) The weak hydrolysate may preferably be used for filling the digester ahead of a liquid filled prehydrolysis, or as wash liquid after a steam phase hydrolysis.

(50) This optional wash requires an extra dedicated accumulator tank for storing the weak hydrolysate, and is only considered in mills where the yield of carbohydrates is optimized at expense of investment costs for the extra accumulator.

(51) Neutralizing Phase

(52) In FIG. 13 is shown the start of the neutralizing phase, where hot white liquor HWL is added to digester in order to swing the pH of the content from acidic conditions to alkaline. The hot white liquor (black in figure) is only added in an amount partially filling the digester, forming a hot white liquor pad. As shown in FIG. 13 is the residual weak hydrolysate withdrawn in mid screen and may be directed to the weak hydrolysate accumulator, until the white liquor pad reaches the mid screen as seen in FIG. 14. In FIG. 15 is shown how the white liquor pad further displace a residual volume of weak hydrolysate upwardly.

(53) Directly after this volume of hot white liquor is added hot liquor fill as shown in FIG. 16. The pad with hot white liquor (black in figure) is pushed upwards swinging the pH to alkaline conditions ahead of the hot liquor filling, the latter having the substantial charge of alkali for the subsequent kraft cook.

(54) At the end of the neutralizing phase is the residual wash water displaced to HBL accumulator, as shown in FIG. 17, and the consumed hot white liquor is also displaced to the HBL accumulator, as shown in FIG. 18.

(55) Start of Kraft Cook

(56) In FIG. 19 is shown how the entire digester finally is filled with Hot Liquor, i.e. the right proportions of black and white liquor necessary for the kraft cook. And the kraft cook continues during circulation similar to that shown in FIG. 7 or 11 but is not disclosed in detail here.

(57) While the present invention has been described in accordance with preferred compositions and embodiments, it is to be understood that certain substitutions and alterations may be made thereto without departing from the spirit and scope of the following claims.