Patent Application
20090126883
The present invention relates to a Kraft pulping process employing modified cooking technology in conjunction with polysulfide pulping technology in a cooking vessel to obtain higher pulping yields than previously obtained with either modified cooking or polysulfide pulping.
Polysulfide (PS) is a pulping additive which has been used commercially to increase pulping yield. A higher pulping yield improves process economics by decreasing wood consumption and/or increasing pulp throughput. Polysulfide is commercially produced by catalytic oxidation of part of the sulfide ions contained in Kraft pulping alkali solution, often called “white liquor” in the art of Kraft pulping. This oxidation process is currently the most commercially viable technology that converts sulfide in white liquor to polysulfide, giving the resultant liquor an orange color. Polysulfide alkali liquor thus is also called “orange liquor” in the art.
Polysulfide is found to be effective in increasing pulping yield only when it is applied to the beginning of a cook, e.g., to an impregnation stage where the temperature is typically below ˜140° C. (˜284° F.) and a retention time of typically 15-45 minutes. At or above ˜140° C. (˜284° F.), polysulfide starts to decompose rapidly and loses its effectiveness as a pulping yield enhancer. Pulping yield increase from polysulfide pulping is found to increase proportionately with amounts of polysulfide added to the beginning of a cook (up to about 7% polysulfide charged on wood). Thus in polysulfide pulping, all polysulfide liquor (orange liquor) is most preferably added to the beginning of a cook so as to maximize pulping yield increase. This feature works well with conventional Kraft pulping. In conventional Kraft pulping, which had been the only commercial practice until the late 1970s, the total alkali charge required for a cook is added to the beginning of the cook.
In modified Kraft pulping (modified cooking) developed in the late 1970s, the total alkali charge is divided into at least two and often more than two additions. Typically, only about 45-75% of the total alkali is added to the beginning of a modified cook. By splitting the total alkali charge into several additions to different cooking stages, alkali concentration profile in modified cooking is more even throughout the cook than in conventional Kraft cooking. Of particular importance is the concentration of effective alkali (EA) in the early cooking stage, where the cooking temperature goes from an impregnation temperature of typically ≦135° C. (≦275° F.) to full cooking temperature, typically between 150 to 175° C. (302 to 347° F.). When the EA concentration is too high in this early cooking stage, excessive losses occur in pulping yield and pulp strength. Therefore, modified cooking with a more even alkali profile, particularly a lower EA concentration in the early cooking stage, results in significantly higher pulping yield and pulp strength than conventional Kraft pulping, where the total alkali charge is all added to the beginning of a cook and the EA concentration is high at the early stage.
However, when current commercial polysulfide pulping technology is applied to modified cooking, only 45-75% of the total available polysulfide is added to the beginning of a cook, since only 45-75% of the polysulfide-containing alkali liquor is added to the beginning of the cook. As a result, compared to conventional cooking with polysulfide, only a fraction of the total pulping yield increase is realized because the yield increases are proportional to the amount s of polysulfide added to the beginning of a cook as discussed before. This means that in the prior art, current modified cooking cannot take full advantage of polysulfide pulping for maximum yield increases. In other words, the current modified cooking technology is not completely compatible with the current commercial polysulfide pulping technology.
The present invention overcomes the aforementioned incompatibility of modified Kraft pulping with current commercial polysulfide pulping technology. It obtains all benefits of modified cooking as compared to conventional cooking, and the full yield improvement of polysulfide pulping.
The invention comprises a method directed to Kraft pulping employing a modified cooking process in conjunction with polysulfide pulping technology in a cooking vessel to obtain higher pulping yields than is obtained with modified cooking without polysulfide, conventional cooking with polysulfide or polysulfide pulping applied to modified cooking as taught in the prior art. In the present invention, the entire cooking alkali dosage required in the form of polysulfide liquor is added to the beginning of a cook, usually an impregnation stage, as in the case of conventional cooking. At the end of the impregnation stage, when all polysulfide has essentially reacted with lignocellulosic material to increase pulping yield at temperature below ˜135° C. (˜275° F.), at or below which no significant carbohydrate degradation occurs, e.g., near the end of the impregnation stage, part of the cooking liquor (first quantity) high in effective alkali (EA) concentration is removed from the cooking process and replaced with a cooking liquor (second quantity) low in EA concentration and that is removed from another process point, and which may be equal to, greater than, or smaller than the first quantity. The removed first quantity of cooking liquor is then added elsewhere in the pulping process, where the EA concentration is low, for instance near where the second quantity of cooking liquor is removed. By performing this cooking liquor “exchange,” the full yield benefit from polysulfide pulping is realized while at the same time a more uniform EA concentration profile is achieved to obtain the benefits of higher pulp yield and strength from modified cooking.
More specifically, the invention comprises, in an embodiment, the steps of: (a) adding the total alkali charge in the form of polysulfide liquor to the first stage of a cook operated at between 100-140° C. within about 15-45 minutes; (b) at the end of the first stage, removing from the vessel a first quantity of cooking liquor relatively high in effective alkali (EA) concentration, which is to be added back to the vessel in a later stage; (c) adding to the end of the first stage a second quantity of cooking liquor, which was removed from a later stage of the cook and is relatively low in EA concentration; (d) heating the cook to full cooking temperature; (e) wherein the second quantity cooking liquor is removed about 0-200 minutes after the full cooking temperature is reached; (f) adding the first quantity of cooking liquor to the vessel to a later stage in the cooking process than its point of removal, or to another cooking process; and (g) continuing cooking to completion. The quantities, as well as the removal and addition points or times, of the first and second cooking liquors are controlled to obtain an EA concentration profile that is similar to that of current modified cooking and more uniform than that of conventional Kraft cooking.
The foregoing, as well as other objects and advantages of the invention, will become apparent from the following detailed description when taken in conjunction with the accompanying drawings, wherein like reference characters designate like parts throughout the several views, and wherein:
a & 1b are schematic flow diagrams of a cooking process according to a preferred embodiment of the present invention;
a & 7b show an exemplary installation of the present invention in a battery of batch digesters.
The cooking process of the present invention is indicated generally for a pulping process with one impregnation stage and one concurrent cooking stage at 10 in
Another embodiment of the present invention is depicted in
The terms of downstream and upstream are referenced to the free liquor flow direction inside the cooking vessel in a continuous digester, or to the process time of a batch cooking system with multiple batch digester vessels. By adjusting the quantities of the first and the second of cooking liquor and the process points for their removal and addition, one skilled in the art of Kraft pulping is able to achieve a relatively even EA concentration profile in the subsequent cooking stages (Cook Stages 1 and 2), comparable to that obtained from current modified cooking. Thus, the present invention enables one to achieve the full potential benefits of pulp yield increases from PS pulping, as well as the higher pulp yield and strength from a more even EA concentration profile as obtained in modified cooking, thereby overcoming the incompatibility of prior art modified cooking when using commercially available polysulfide pulping technologies.
Yet another embodiment of the present invention is to (a) add the total required alkali charge in the form of polysulfide cooking liquor (orange liquor) to the very first stage of a cook, usually an impregnation stage, and control the stage conditions, typically around or below 135° C. (275° F.) for 15-45 minutes, such that essentially all polysulfide has reacted with lignocellulosic material and no substantial carbohydrates degradation and polysulfide thermal decomposition occur; and (b) adjust the amounts of the first quantity and the second quantity of liquors to be removed from certain process points and to be added back to the cook at other process points, as well as their relative removal and addition process points, so as to keep the maximal concentration of effective alkali at or below 18 g/L as NaOH (0.45M NaOH or 14 g/L as Na2O) throughout all cooking stages that follow the impregnation stage.
Alternatively, the present invention can be practiced where the maximal effective alkali concentration in all cooking stages that follow the impregnation stage is controlled to be at or below 24 g/L as NaOH (0.6M NaOH or 18.6 g/L as Na2O).
Another way to practice the present invention is to control the maximal alkali concentration at or below 12 g/L as NaOH (0.3M NaOH, or 9.3 g/L as Na2O) in all cooking stages that follow the impregnation stage.
Table 1 summarizes the pulping yields from cooking mixed southern US hardwood furnish to 15 Kappa number at the laboratory. These results are also depicted in
CK-Ref denotes reference cooks of conventional Kraft cooking, which is comprised of: (a) heating up the chips with low-pressure steam at ˜100° C. (˜212° F.) for 10 minutes in a laboratory digester vessel equipped with external circulation and an electric heater; (b) draining off all free steam condensate; (c) adding all cooking alkali liquor (in form of white liquor with a sulfidity of ˜30% on active alkali (AA) basis), corresponding to EA/wood charge of 20.0% as NaOH (15.5% as Na2O) at the beginning of a cook, and bringing the cooking liquor/wood ratio to 3.5 by adding the proper amount of water to the cook; (d) heating up the cook from about 60° C. to 120° C. in 15 minutes; (e) maintaining the cook at 120° C. for 30 minutes to effect an impregnation stage; (f) heating up the cooking to full cooking temperature of about 160° C. (320° F.) in 30 minutes and maintaining the cook at this temperature for 100 minutes to reach a target Kappa number of ˜15; (g) cooling the cook down to below 100° C.; (h) washing the cooked chips with tap water; (i) processing the washed cooked chips into fibers (pulp) by mechanical mixing in a dilute water suspension; and (j) screening the pulp using a laboratory flat screen with 0.25 mm (0.01″) slots before determination of pulping yield, rejects, Kappa number and other pulp properties.
MC-Ref denotes reference cooks carried out with a modified cooking process, comprising essentially the same steps as outlined above for the CK-Ref cooks, expect for step (c), adding only 65% of the total alkali charge at the beginning of a cook, and step (f), adding the second EA addition equal to 20% of the total alkali charge to the cook by a metering device before heating up the cook to 157° C. (˜315° F.) in 30 minutes, maintaining the temperature for 45 minutes before adding the third EA addition equal to 15% of the total alkali charge, and continuing the cook at this full cooking temperature for another 150 minutes to reach a target Kappa number of ˜15.
CK-PS and MC-PS represent polysulfide (PS) cooks performed using the aforementioned CK-Ref and MC-Ref procedures, respectively, and instead of white liquor using PS liquor, produced by catalytic oxidation of white liquor, containing an amount of total polysulfide equivalent to 0.7% charge on wood and with a sulfidity of ˜14% on AA. In addition, a charge of anthraquinone (AQ) equal to 0.05% on wood was added to these PS cooks with the first EA charge at the beginning of a cook.
The MC-EPS cooks were done using the present invention, and were performed in the following steps: (a) heating up the chips with low-pressure steam at ˜100° C. (˜212° F.) for 10 minutes in a laboratory digester vessel equipped with external circulation and an electric heater; (b) draining off all free steam condensate; (c) adding 0.05% AQ and the total required alkali charge in the form of PS liquor (containing an equivalent of 0.7% PS on wood with a sulfidity of 14% on AA basis), corresponding to EA/wood charge of 20.0% as NaOH (15.5% as Na2O) at the beginning of a cook, and bringing the cooking liquor/wood ratio to 3.5 by adding proper amount of water to the cook; (d) heating up the cook from about 60° C. to 120° C. in 15 minutes; (e) maintaining the cook at 120° C. for 30 minutes to effect an impregnation stage; (f) collecting a first quantity of cooking liquor relatively high in EA concentration, in an amount equivalent to about 1.2 times the total wood charge by weight through a cooling device from the digester vessel for use in the next MC-EPS cook; (g) adding to the digester vessel via a metering device a second quantity of cooking liquor relatively low in EA concentration collected from a previous MC-EPS cook; (h) heating up the cook to full cooking temperature of about 157° C. (315° F.) in 30 minutes and maintaining the cook at this temperature for 45 minutes; (i) collecting a second quantity of cooking liquor in an amount equivalent to about 1.2 times the total wood charge by weight through a cooling device from the digester vessel and storing this second quantity of cooking liquor relatively low in EA concentration for use in the next MC-EPS cook; (j) adding to the digester vessel via a metering device the first quantity of cooking liquor collected from a previous MC-EPS cook, and maintaining the full cooking temperature during this liquor exchange; (k) continuing the cook at this full cooking temperature for another 150 minutes to reach a target Kappa number of ˜15; (1) cooling the cook down to below 100° C.; (m) washing the cooked chips with tap water; (n) processing the washed cooked chips into fibers (pulp) by mechanical mixing in a dilute water suspension; and (o) screening the pulp using a laboratory flat screen with 0.25 mm (0.01″) slots before determination of pulping yield, rejects, Kappa number and other tests.
The results show that modified cooking of southern US mixed hardwood to 15 Kappa number (MC-Ref) resulted in a pulp yield increase of about 0.9% on wood over conventional reference cooks (CK-Ref). Charging the total required alkali charge in the form of PS liquor containing about 0.7% PS and 0.05% AQ, both on OD wood basis, to the beginning of a conventional Kraft cook (CK-PS) increased the pulp yield by about 2.1% over conventional reference cooks, and about 1.2% points over the MC-Ref cook. As expected based on teaching from the prior art, when 65% of the total PS liquor was added to the beginning and the balance of the PS liquor to the subsequent cooking stages of a modified cook (MC-PS), the total pulp yield increase was only 1.4% on wood over that of the MC-Ref (2.1% over CK-Ref), which is significantly lower than the expected sum of (0.9%+2.1%)=3.0% yield increases from both modified cooking and PS addition. When applying the present invention, i.e., the enhanced PS process with modified cooking (MC-EPS), the total pulp yield increase was found to be 3.3% on wood, which is approximately the sum of the 0.9% increase from modified cooking over conventional Kraft cooking and the 2.1% expected from PS pulping.
Similar results were found in laboratory pulping of southern pine, as summarized in Table 2 and depicted in
Modified cooking (MC-Ref) to about 30 Kappa number was found to increase pulping yield by ˜0.5% on wood over conventional Kraft reference (CK-Ref) cooks. Adding 0.05% AQ and 0.7% PS to CK cooks increased the pulp yield by about 1.7% on wood. As expected based on teaching from the prior art, performing PS pulping with MC cooking without the use of the present invention, i.e., splitting the total alkali charge into multiple additions and only adding about 65% of total alkali charge to the beginning of a cook, the total pulp yield increase was only ˜1.5% over CK-Ref and 1.0% over MC-Ref, significantly lower than the expected sum of ˜2.2% (˜0.5% from modified cooking and 1.7% from PS addition). When applying the present invention using the enhanced PS process concept, the total pulp yield increase in the MC-EPS cooks was ˜2.3% over that of CK-Ref and ˜1.8% over that of MC-Ref cooks.
In another laboratory pulping study using a different southern pine furnish, but without adding AQ to any cooks, the results also clearly show the significant advantage of the present invention. The cooking procedures were the same as those described in Example 1 for each type of cook.
As can be seen in Table 3 and
The above three examples clearly demonstrate the advantages of the present invention over the prior art in the use of polysulfide pulping with modified cooking processes.
Amounts of the first and the second quantities of cooking liquor removed from certain process points and added back to other process points should be adjusted to achieve the most preferred EA concentration profile in all cooking stages that follow the impregnation stage. Consideration should also be given to the liquor removal and addition locations with regard to hydraulic balance of the digester, as well as to the ease of chip column movement for improved digester operational stability.
By practicing the present invention, the EA concentration profile in PS pulping with modified cooking in a continuous digester is more even than that in a conventional Kraft cook, retaining all essential benefits from modified cooking. At the same time, since all PS is put to use at the beginning of the cook, maximum pulp yield increase from PS pulping is realized.
As discussed before, amounts of the first and the second quantities of cooking liquor removed from certain process points and added back to other process points should be adjusted to achieve the most preferred EA concentration profile in all cooking stages that follow the impregnation stage. Consideration should also be given to the liquor removal and addition locations with regard to hydraulic balance of the digester, as well as to the ease of chip column movement for improved digester operational stability.
a & 7b illustrate the application of the present invention in a battery of batch digesters 410, 420, 430 and 440 capable of running modified batch cooking. For each digester the 100% required alkali dosage in the form of polysulfide (orange) liquor is added to the beginning of a cook, either together with wood chips or after all required wood chips have been added. Each batch digester, e.g., digester #1, is equipped with a cooking circulation loop 411, consisting of a set of drainer (extraction screen) 412, a circulation pump 413 and a heater 414. The first quantity of cooking liquor 44 high in effective alkali is removed from digester vessel #1 that is just at the end of the impregnation stage, and added to another digester (vessel #4), which completed the impregnation stage and has undergone substantial cooking, e.g., at least 30 minutes at cooking temperature and after the second quantity of cooking liquor low in effective alkali was removed from this vessel. The second quantity of cooking liquor 46 low in effective alkali concentration, removed from digester #3 is added to digester vessel #2 after the first quantity of cooking was removed.
Alternatively, the first quantity and second quantity of removed liquor may be stored in separate liquor tanks before being pumped into another digester at a different cooking stage to achieve the preferred alkali concentration profile.
As can be seen, according to the invention a cooking liquor of relatively high effective alkali concentration is “exchanged” with a cooking liquor of relatively low effective alkali concentration, wherein the cooking liquors of relatively high and low concentrations, respectively, are extracted from the cooking process at different process points or times and reinserted or recycled into the cooking process at other points or times.
While particular embodiments of the invention have been illustrated and described in detail herein, it should be understood that various changes and modifications may be made in the invention without departing from the spirit and intent of the invention as defined by the appended claims.
