CA1160403A

CA1160403A - Method of treating kraft black liquor

Info

Publication number: CA1160403A
Application number: CA000378921A
Authority: CA
Inventors: Richard L. Gay; Samuel J. Yosim; Kenneth M. Barclay
Original assignee: Rockwell International Corp
Current assignee: Boeing North American Inc
Priority date: 1980-07-11
Filing date: 1981-06-03
Publication date: 1984-01-17
Also published as: JPH0253552B2; SE8104134L; JPS5747991A; BR8104337A; FI812191L

Abstract

Abstract A method of treating black liquor solids in which the dried solids are treated in a single reaction zone in a bed of molten salt which comprises a major amount of sodium carbonate and a minor amount of sodium sulfide. A source of oxygen is introduced into the molten salt in a stoichiometric amount less than that required for complete combustion of the combustible constituents of the black liquor solids to provide a combustible off-gas which is withdrawn from the upper portion of the reaction zone and to produce a melt in a lower portion of the zone containing less than 1% of the sulfur constituents as sulfate and less than 1% of the initial combustible carbon content of the black liquor solids.

Description

1 lB0~03 METHOD OF TREATING KRAFT BLACK LIQUOR

Field of the Invention This invention relates tb the treatment of kraft black liquor for the recovery of values therefrom. It particularly relates to the treatment of the dried solids from such liquor in a molten salt to avoid loss of valuable constituents to produce a combustible product gas and a molt~n product suitable for use in the kraft pulping process.
Background Art In the kraft or sulfate pulping process, wood pulp is digested in an aqueous solution generally comprising sodium sulide. After removal of the digested wood pulp, there is let a so-called "black liquor" which contains from about 10-15% organic and inorganic solids. The original treated chemicals (sodium sulfide and sodium hydroxide) as well as their reaction and decomposition products ~sodium carbonate, sodium sulfite, and sodium sulfate) are relatively expensive and must be recovered for reuse in order for the sulfate process to be economical. Thus, in conventional practice, the black liquor is evaporated by passing it through multiple effect evaporators to produce a liquor having a solids concentration of about 50-65% by weight. The concentrated Liquor is then fed to a recovery furnace in which the organic constituents are burned and leave the furnace as gaseous products of combustion while the sodium salts collect in the base of the recovery furnace as a molten ash or smelt.
The recovery furnace generally is operated in a manner such that therelare two zones within the furnace. Combustion air is supplied to the furnace to create an upper oxidizing zone in which the water content of the black liquor is evaporated and the organic materials are decomposed into combustible gases which are also oxidized in that zone.
The oxidized inorganic sodium salts now comprising sodium sulfate and sodium carbonate and some residual carbon content fall to a zone at the bottom of the furnace which is maintained under reducing conditions.

l 1604~3 --2~

In order to assure that a high percentage of the sulfate is converted to sulfide, it has been the practice heratofore to maintain a bed of carbon in the bottom of the ~urnace. A molten smelt of sodium salts collects at the bottom of the furnace. The smelt is withdrawn, dissolved in water, and treated with lime to convert the sodium carbonate to sodium hydroxide, and this regenerated mixture of sodium sulfide and sodium hydroxide is recycled for use in digesting additional wood. To improve the economics of the kraft process, it has been the practice to recover, as much as possible, the heat generated when the organic constituents of the black liquor are burned in the recovery system. Thus, the recovery furnaces generally axe lined with boiler tubes through which water is passed to generate steam which could be used as process steam in the pulping operation.
Obviously, the conventional kraft recovery furnace described above is a relatively complex piece of apparatus which combines oxidation reduction and steam generation in onP unit under high-temperature conditions. The cost of the equipment is, of course, very high. There is also the risk of an explosion within the recovery furnace which would occur if water is accidentally introduced into the pool of smelt at the base of the furnace. Such introduction could occur if a dilute black liquor having a high water content is introduced into the furnace or if the boiler tubes in the wall of the furnace ruptured parmi~ting water to flow into the unit. In view of this vary real possibility of a serious explosion, great care must be taken in the operation of the conventional kraft recovery furnace. Another problem inherent in the operation of the recovery furnace is that, due to incomplete reduction of the sulur values, the smelt withdrawn contains significant portions of sulfate which is recycled and has no significant value in the process. Also, a part of the carbon bed is withdrawn with the smelt and its heating value lost from the system. Clearly, there is an obvious need or an improved method of treating kraft black liquor which 1 160~L0~

would eliminate the possibility of a serious explosion and provide a smelt essentially free o carbon and sulfate.
U.S. Patent No. 1,808,773, directly addressed the problem of incomplete reduction of sulfate in the conventional kraft recovery furnace. Patentee teaches a method of improving the amount of reduction of sulfate to sulfide. In the n~ethod disclosed therein, a high-temperature zone of combustion is maintained in the upper p;art of the furnace by the combustion of black liquor. Intermediate the high-temperature combustion zone and a reducing zone in a lower part of the furnace, there is introduced additional black liquor and salt cake, such as sodium sulfate. Patentee reports that by employing such a method, the percentage of unconverted sulfate could be reduced from 14 to 18% down to as low as 8-12%.
More recently, in U.S. Patent No. 3,322,492, it is proposed to use a series of three fluidized beds for drying, reduction, and for heat recovery. In addition, patentee suggests the use of solid sodium carbonate for reducing sodium sulfate.
In U.S. Patent No. 3,574,051, it is proposed to completely oxidize the black liquor to sodium sulfate.
The sodium sulfate then is reacted with a CO-containing gas at a temperature of from 250-500C to reduce the sulfate to sulfide.
In U.S. Patent No. 3,674,630, it is proposed to use two separate furnaces. In one furnace, 80% of the black liquor is fed under oxidizing conditions. Thus, at least ~0~ of the volatiles of the dried black liquor solids are completely oxidized to CO2 and H2O. In the reduction furnace, molten sodium sulfate-containing smelt is reduced with the remaining 20% of the dried black liquor feed plus auxiliary fuel.
Summary of the Invent on It is an advantage of the present invention that there is provided an efficient single-zone method for the treatment of dried black liquor from a kraft or sulfate pulping process. In accordance with the present invention, 1 lB0403 a kraft black liquor obtained from the solubilization of wood in an aqueous solution of sodium hydroxide and sodium sulfide is filtered to remove the pulp and subsequently dried. The dried black liquor solids, principally comprising sodium carbonate, sodium sulfate, sodium sulfide, some of the pulp, and containing less than about 20 and preferably less than about 5% water, is introduced into a reaction zone containing a bed of molten salt principally comprising a major amount of sodium carbonate and a minor amount of sodium sulfide. A source of molecular oxygen such as air is introduced into a lower portion of the molten salt bed in an amount to provide about 25-45~ of the stoichiometric amount of air required to convert all of the combustible constituents of the black liquor solids to CO2 and steam, excluding, of course, that carbon required to reduce the sulfate values to sulfide.
The pressure within the reaction zone is not critical and may range from one atmosphere up to 100 atmospheres or more. The temperature in the reaction zone is maintained at from 1500 to 2200F. The temperature preferably is maintained by adjusting the stoichiometric amount of air introduced to obtain the desired temperature while still producing a combustible off-gas which is withdrawn from an upper portion of the reaction zone. In the melt, substantially all of the combustible material in the solids are reacted such that there is withdrawn from a lower portion of the melt a stream comprising less than 1~ of the sulfur as sulfate and less than 1% of the initial carbon content of the black liquor solids.
Detailed Description o the Invention The present invention provides a method for the treatment of black liquor solids obtained from the kraft or sulfate pulping process. As used herein, the term "black liquor solids" refers to the material left after the evaporation or removal of water from the black liquor obtained from such pulping processes. Preferably, the black liquor solids contain less than about 20~ water, and it is particularly preferred that they contain less than ` ` 1 ~604~3 about 5% water. It is important that the dried solids have a heating value of at least 5000 Btu's per pound and preferably a heating value in excess of about 6000Btu's per pound. Generally, any of the dried solids will have these required heating values or more unless they have been through a partial pyrolysis or oxidation wherein part of the combustible material was oxidized or the volatile portion thereof driven off. The dried black liquor solids principally comprise sodium carbonate and a minor amount of sulfate in addition to the residual lignin from the pulping operation. The lignin, of course, comprises the principal combustible organic matter in the solids.
It is a particular advantage of the present invention that the dried black liquor solids after treatment will have substantially all the sulfate content reduced to sulide. Generally, in excess of 99~ of the sulfur content is present as sulfide. It is a further advantage of the present invention that substantially all of the combustible organic matter is consumed. Indeed, after treatment, less than 1% of the initial carbon content of the black liquor solids remains. By combustible carbon content, reference is made, of course, to the noncarbonate carbon, i.e., that carbon which will undergo oxidation or combustion~ It is a further advantage of the invention that there is produced a combustible off-gas.
It is still another advantage of the invention that complex cooling equipment is not required for the removal of heat from the molten salt, since the amount of heat generated by the oxidation reaction of the molten salt is controlled to be essentially only that amount of heat which is required to maintain the salt in its molten condition. The combustible off-gas produced is then combusted in a secondary combustion zone or boiler separate from the molten salt reaction zone. Thus, a given size of molten salt furnace will permit a much higher throughput of black liquor solids where only partial combustion is carried out therein for production of a combustible gas and reduction of the sulfur content to sulfide than where complete oxidation is required.
Further, the separate secondary combustor or boiler for final combustion is much less expensive than would be a larger molten salt furnace which would be required for complete combustion. In addition, the separation of the boiler from the reduction furnace eliminates the possibility of a smelt-water explosion due to a ruptured boiler tube.
Preferably, the dried black liquor solids are treated in a suitable molten salt furnace which typically is refractory lined. Examples of suitable apparatus for use in the practice of the present invention are found, among other places, in U.S. Patent Nos. 3,845,190 and 3,916,617, which are assigned to the assignee of the present invention. Initially, there is provided in the reaction zone defined by the reactor a body of molten salt comprising a major amount of sodium carbonate and a minor amount of sodium sulfide. Preferably, the molten salt at least initially comprises about 70 to 95 wt % sodium carbonate and from about 5 to 30 wt ~ sodium sulfide. The actual steady state composition will depend on operating conditions, amount of sulfur values lost and the type of wood which was digested.
The initial body of molten salt provides a compatible salt medium at practical operating temperatures for the black liquor solids and acts as a dispersing medium for the organic constituents which are to be combusted and for the primary air used for such combustion. The molten salt also acts as a heat sink for the high heat transfer rates for absorbing and distributing the heat of combustion, as a heat source for the distillation of the volatile components of the black liquor solids and as an absorbent for the ash constituents of the black liquor solids.
The temperature in the reaction zone is not particularly critical provided it is sufficiently high to maintain tha salt in a molten form and not so high as to volatilize any significant portion of the organic salt constituents. Generally, the selected temperature will be within the range of from about 1500 to 2200F with a l 16~403 temperature of from 1700 to 1850F being particularly preferred. Pressure is not critical in accordance with the present invention. Thus, the reaction zone may be maintained at any pressure from atmospheric up to lO0 atmosphexes or higher. Generally, for economic reasons, it is preferred that the pressure be within the range of from about atmospheric up to about 20 atmospheres.
A substantially atmospheric operating pressure is particularly preferred since this eliminates the necessity of pressurized lock hoppers for feeding the dried black liquor solids into the reaction zone. However, in some instances, to increase the throughput for a given reactor si~e and provide a pressurized off-gas, it may be desirable to maintain the reaction zone at a higher pressure of, for lS example, 5 to 20 atmospheres.
~ir is introduced into a lower portion of the bed of molten salt in less than the stoichiometric amount required for complete combustion of the combustible constituents of the dried black liquor solids. In order to obtain a product gas having the desired low Btu ~from about 100 to 200 Btu's per standard cubic foot), only from about 25 to 50~ of the stoichiometric amount of air required for complete combustion is used, preferably from about 30 to 45~. When the dried black liquor solids have a heating value of from ~5 about 5000 to 7000 Btu's per pound of solids, particularly good results are obtained when the air is introduced in amounts to provide about 35% of the stoichiometric amount of oxygen required for complete combustion. It will be obvious, of course, that in lieu of air in some instances, it may be desirable to use pure oxygen, oxygen-enriched air or an oxygen and steam mixture to obtain a combustible product gas having a higher heating value per cubic foot than with air only. However, the use of oxygen ordinarily is economically undesirable for the production of a combustible product gas since this would ordinarily require an oxygen plant. Accordingly, the present invention will be particularly described using air as the source of oxygen.

l 16040~

The residence time require~ to reduce ~he sul~ate content of the black liquor solids to sul~ide and achieve substantially complete gasification of the combustible constitutents ln a conventional kraft recovery furna~e generally is in the range of from about 1.0 to 10 seconds.
However, it is a particular advantage of the present invention when using the claimed molten salt bath that the reactions are extremely rapid and generally are complete in less than about l/2 second. Thus, the preferred residence time for practicing the present invention is from about 0.1 to 1 second. The treated solids now comprising carbonate and sulfide and containing less than 1% of the sulfur content as sulfate and less than 1% of the initial combustible organic content are withdra~m rom a lower portion of the molten salt bath while combustible product gas is withdrawn from an upper portion of the reaction zone.
The following example will more clearly illustrate the practice of the present invention but should not be interpreted to unduly limit its generally broad scope.
EXA~PLE
A representative sample of two dried black liquor solids was obtained. Sample A was from the pulping of hardwood, and Sample B was obtained from the pulping of softwood. An analysis of the samples is set forth in Table l.

DRIED BLAC~ LIQUOR SOLIDS ANALYSES
.. . . _ . . . .
Analysis, wt % SamPle A Sample B
.. .. ~ ......... ... . _ _ .
Total Carbon 33.2 33.1 Organic Carbon * 12.2 Hydrogen 3-5 0 7 Total Sulfur 2.0 3.6 Oxygen 40.8 39.3 Na2C3 18.5 44.6 Na2SO4 5.2 4.1 *Not Analyzed ~-1 ~60403 _g_ Tests were carried out in a bench-scale molten sal~ reducer which comprised a 6-inch internal diameter by 36-inch high alumina crucible held in a Type 321 stainless s~eel retainer vessel. The stainless steel vessel, in turn, was contained in an 8-inch ID, four~heating-zone electric furnace. The dried black liquor solids were fed into the reducer using a variable-speed screw feeder. The rate of solids feed for each experiment was determined by running the screw feeder at a constant speed and averaging the weight of solids fed during a discrete time period (usually 10 to 30 minutes).
Prior to starting the test, 3.08 pound-s of sodium carbonate and 1.32 pounds of sodium sulfide were introduced into the reactor and the temperature increased to provide a body of molten salt therein. The test conditions and results are set forth in Table 2.

REDUCTION AND GASIFICATION OF SAMPLE A
DRIED BLACK LIQUOR SOLIDS

Pressure, atm Temperature, F 1700 Superficial Gas Velocity, ft/sec 1.38 Total Test Time, min 14.0 Solids Feed Rate, lb/hr 5.95 Air Feed Rate, scfm @ 70 F 2.i7 Fuel/Air Ratio, % stoichiometric air 40 Off-Gas Composition*
N2' % 49.9 C2' % 4.2 CO, % 29.5

2' % 11.8 CH4~ % 2.1 C2H4, % 2.0 H2S, % 0.53 Ar(+O2), % 0.58 Higher Heating Value, Btu/scf190 *Not normalized to 100%.

From~the table it is seen that using approximately 40% stoichiometric air, a product gas having a higher heating value of 190 Btu's per standard cubic foot, was obtained. Due to the small sample available, the test was limited to a 14-minute run time, and the melt was not analyzed because insufficient material was added during the test to measurably change the salt composition.
Following the test of Sample A, Sample B was tested.
The initial salt in the reducer for this test comprised the salt remaining from the test of Sample A pIus an additional 7.2 pounds of sodium carbonate. The sodium carbonate was added to assure an initial bed depth of at least 6 inches.
The initial salt bed was analyzed and found to comprise 0.04 wt % organic carbon, 5.22 wt ~ total sulfur, and 0.006 wt % sulfate ~as sulfur). This corresponded to a 99.9% reduced sulfur and a melt composition of 12.7 wt ~
sodium sulfide and 87.3 wt % sodium carbonate. The test of Sample B was run in two parts. During Part ~, a solids feed rate of 8.3 pounds per hour and air feed rate of 2.17 standard cubic feet per minute was used. This corresponds to a fuel-air ratio of approximately 30~ of stoichiometric air.
During Part II, a solids feed rate of 6.5 pounds per hour and an air feed rate of 2.17 standard cubic feet were used. This corresponds to a fuel-air ratio of about 36~ of stoichiometric air~ The test conditions and product gas composition are set forth in Table 3.
From the table, it is seen that in Part I, a product gas was produced having a higher heating value of 192 Btu's per standard cubic oot. The heating value is lower for Part II because the stoichiometry was less fuel-rich than Part I. Nonetheless, this test clearly demonstrates production of a combustible off-gas from dried black liquor solids in accordance with the present invention.
Following the test, the melt was analyzed,and the results are set forth in Table 4. From that table it is seen that substantially all of the sulfate was reduced to sulfide, and less than one percent of the carbon content of the dried solids remained.

REDUCTION AND GASIFICATION OF SAMPLE B
DRIED BLACK LIQUOR SOLIDS
Part I Part II
_.~ . _ . _ _ . . . _. . _ . _ . . . ___ _ _ Pressure, atm Temperature, F 1680 1710 Superficial Gas Velocity, ft/sec 1.65 1.26 Total Tes~ Time, min 32 14 Solids Feed Rate, lb/hr 8.3 6.5 Air Feed Rate, scfm @ 70F 2.172.17 Fuel/Air Ratio, ~ stoichiometric air 30 36 Off-Gas Composition*
N2. % 41.8 54-7 C2' ~ 3.9 4.8 CO, % 29.926.7 H2' ~ 11.412.2 CH4~ % 1.8 1.5 C2H4, % 2.252.0 H2S, % 0.300.28 Ar(+O2), % 0.580.57 H20, %** 1.6 1.6 Higher Heating Value, Btu/scf 192 176 *Not normalized to 100%.
**Based on water condensed from the gas sampling line.

MELT ANALYSES FROM S~LE B TEST
Initial Final :
Salt Bed Salt Bed Bed Height, inch 6.5 8.0 Bed Mass, pound* 12.1 15.4 Organic Carbon, wt ~0.04 0.03 Hydrogen, wt ~ ~0.01 <0.01 Total Sulfur, wt %5.22 5.59 Sulfate (as sulfur), wt ~ 0.005 0.006 Reduced Sulfur, %**99.9 99.9 *Based on bed height.
**Based on sulfate analysis.

l 1~04~3 While this invention has been described with reerence to what are now the preferred embodiments, it is obvious that many modifications and changes therein can be made by those skilled in the art without departing from the spirit and sense of this invention. Thus, while the examples illustrating this invention have been described with respect to speci~ic black liquor solids, temperature and other reaction conditions, the invention may be otherwise practiced, as will be readily apparent to those skilled,in this art. Accordingly, the invention should not be limited by the specific embodiments thereof, rather its scope should be determined in accordance with the following claims.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of treating dried black liquor solids comprising: introducing the dried solids into a reaction zone containing a body of molten salt comprising a major amount of an alkali metal carbonate and a minor amount of an alkali metal sulfide; introducing a source of oxygen into said body of molten salt, said oxygen being introduced in less than the stoichiometric amount required to oxidize all of the combustible constituents of the black liquor solids to produce in from about 0.1 to 1.0 second a combustible off-gas, having a heating value of from about 100 to 200 Btu's per cubic foot, which is withdrawn from an upper portion of said zone; and a melt containing less than 1% of the sulfur as sulfate and less than 1% combustible carbon, said melt being withdrawn from a lower portion of said zone.

2. The method of claim 1 wherein said reaction zone is maintained at a temperature of from about 1700° to 1850°F.

3. The method of claim 1 wherein said reaction zone is maintained at a pressure of from about 5 to 20 atmospheres.

4. The method of claim 1 wherein said reaction zone is maintained at about atmospheric pressure.

5. The method of claim 1 wherein said dried black liquor solids have a heating value from about 5,000 to 7,000 Btu's per pound of solids.

6. A method of treating black liquor solids which comprises:
a) providing a reaction zone containing a molten salt consisting essentially of a major portion of sodium carbonate and a minor portion of from about 1 to about 25 wt % sodium sulfide;
b) maintaining said molten salt at a temperature of from about 1500° to 2200°F.;

c) maintaining said reaction zone under a pressure of from about l to 100 atmospheres;
d) concurrently introducing into said molten salt, dried black liquor solids having a heating value of at least 5000 Btu's per pound of solids and air, the air being introduced into a lower portion of the reaction zone in an amount to provide from about 25 to 50% of the amount of oxygen stoichiometrically required for complete oxidation of all the combustible constituents of the black liquor solids; and e) controlling the conditions set forth in the preceding paragraphs to promote in from about 0.1 to 1.0 second production of a combustible off-gas having a heating value within the range of from about 100-200 Btu's per standard cubic foot and withdrawing a stream of molten salt containing less than 1% of the sulfur as sulfate and less than 1% of the initial carbon content of the black liquor solids.