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(11) | EP 0 251 533 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Alkaline black liquor treatment |
| (57) The capacity of a chemical recovery plant for alkaline black pulping liquors is increased
by diverting a portion (38) of the black liquors (1) to a wet oxidation step (39).
Organic material is oxidized and water is evaporated. The resulting oxidized black
liquor (42) is combined with unoxidized black liquor (43) to produce a black liquor
mixture (44,50) of reduced volume and a higher ratio of pulping chemicals to organic
matter. The liquor mixture (50) is burned in a recovery furnace (52) at an increased
rate. |
[0001] The invention relates to an improvement in the use of wet oxidation as a means for increasing the solids throughput capacity of an alkaline black liquor recovery boiler which is limited by the black liquor firing rate (BTU/hr), while simultaneously reducing the quantity or sulphur lost in flue gas emissions.
[0002] Alkaline pulping of wood or other cellulosic material results in a black liquor which contains high concentrations of non -cellulosic organic wood components as well as some cellulose and spent pulping chemicals. For example, pulping of wood by the kraft process, i.e. using sodium sulfate as the makeup pulping chemical, results in a black liquor in which the concentration of organic matter is about equal to that of the spent pulping chemicals. Such a black liquor will have a heat of combustion of about 6600 BTU per pound of total solids (3670 Kcal. per Kg.), or 13,200 BTU per pound (7330 Kcal. per Kg.) of organic matter. The recovery of pulping chemicals is widely practiced and usually comprises concentration of the black liquor to 60-65 percent solids by several stages of evaporation, followed by drying and combustion of the concentrated black liquor in a furnace. Makeup chemicals are typically added to the concentrated black liquor prior to combustion, to compensate for losses in the overall pulping process.
[0003] Pulping chemicals are recovered from the bottom of the furnace and steam is generated utilizing the heat of combustion of the organic matter in the black liquor. A typical process for chemical (soda and sulfur) recovery from kraft black liquor consists of the following unit processes:
a. weak black liquor of 12-20 percent solids is concentrated to 40-60 percent solids in a multiple effect evaporator;
b. the black liquor from step (a) is further concentrated in direct or indirect contact evaporators to a concentration which is constrained by either:
aa. - the upper limit of solubility of solids, above which the liquor viscosity is excessive, and the liquor handling system becomes plugged, and/or
bb. - the maximum thermal content (BTU/gal) which can be handled by the furnace;
c. the black liquor may optionally be air-oxidized at atmospheric pressure and somewhat elevated (e.g. 200°F or 93°C) temperatures, thus converting the most easily oxidized reduced sulfur compounds to thiosulfate. This "BLOX" process reduces the quantity of total reduced sulfur emissions from the direct contact evaporators but will not prevent formation of H₂S during pyrolysis in the furnace. This oxidation step is usually provided prior to concentration in the direct contact evaporator (step b, above) and no appreciable oxidation of organic constituents occurs;
d. makeup chemicals are added to the concentrated black liquor; and
e. the concentrated black liquor is dried and burned in a chemical recovery boiler, producing useful steam and a smelt of e.g. sodium sulfide for re-use in the kraft process.
[0004] The recovery furnace is normally the single most expensive item of equipment in a pulp mill. Typically it has been designed with minimal excess capacity or firing rate. The mechanical nature of a recovery furnace resists capacity expansion by modification and "fine tuning" of the operation to a greater extent than other parts of the plant. As a result the recovery furnace at most pulp mills today imposes an upper limitation upon production capacity of the entire mill. It is obvious that any method or apparatus which will inexpensively increase the throughput of the recovery furnace is extremely valuable.
[0005] Any proposed method for increasing the throughput of pulping chemicals must consider the limitations of viscosity and heating value which occur at high solids concentrations. It is often not practical to produce black liquor of greater than 65 percent solids. In fact, when certain materials such as eucalyptus or bamboo are pulped, the resulting black liquor becomes extremely viscous, and concentration by conventional methods to beyond 50 percent solids or even 35 percent solids in some circumstances is impractical.
[0006] Wet oxidation has been proposed in Schoeffel U.S. Patent 2,696,424 as a method for substantially completely destroying organic matter in acidic waste sulfite liquor to produce a calcium sulfate residue which can be roasted to yield useful CaO and SO₂. The step of furnace oxidation is replaced by wet oxidation.
[0007] Wet oxidation is a widely-practiced method of destroying oxidizable organic matter in such materials as sewage sludge, sodium-based black liquor, paper coating waste streams, and toxic wastes. Oxidation in the aqueous phase with oxygen or oxygen containing gas is conducted at temperatures of 250 to 650°F (120 to 340°C), and at pressures necessary to maintain a liquid phase, typically 200 to 4000 psig (1500 to 28,000 KPa). In most cases thermal and/or mechanical energy is recovered from the vapor and/or liquid streams from the wet oxidation process. For example, Pradt U.S. Patent 4,013,560 describes a prior art wet oxidation process with energy recovery.
[0008] Schoeffel U.S. Patent 3,097,988 discloses a process for treating alkaline black liquors by "complete" wet oxidation.
[0009] Pradt U.S. Patent 3,714,911 proposes a wet oxidation process to replace the conventional evaporation and concentration steps.
[0010] Gulley U.S. Patent 4,313,788 discloses a process where the conventional sulfide oxidation step, usually known as BLOX, is conducted at an intermediate point in the multiple effect evaporation system in order to more efficiently recover thermal energy. The process is not concerned with oxidation of organic matter, but with oxidation of reduced sulfur compounds.
[0011] This invention is an improvement in a widely-used process for recovering inorganic pulping chemicals, abbreviated herein as PC, from an alkaline aqueous black liquor which also contains oxidizable organic matter, abbreviated herein as OM. In this prior art process, alkaline black liquor is first concentrated by water evaporation to an intermediate solids concentration, using an evaporator apparatus such as a multiple effect evaporator, long-tube vertical evaporator, thermal compression evaporator and/or Digester Flash Evaporative System (DIFES).
[0012] The evaporated black liquor having an intermediate solids concentration is generally further concentrated, typically by direct contact with hot furnace gases in a plate cascade evaporator or cyclone evaporator. While this further step is generally known as "concentration", performed in a "concentrator", and the prior step is commonly called "evaporation", performed in an "evaporator", both steps are evaporative. In this application, the terms "evaporate" and "evaporation" shall be construed to include evaporation in "evaporators" and/or "concentrators", unless otherwise specifically limited.
[0013] In any case, the black liquor drawn off from the pulping process is concentrated by one or more steps of evaporation.
[0014] At some mills, it is necessary to pass the unconcentrated black liquor through a black liquor air oxidation (BLOX) step to oxidize the reduced forms of sulfur. While the invention presented herein may reduce the quantity of liquor requiring a BLOX step, the need for this step is not generally eliminated thereby.
[0015] The concentrated black liquor is passed to a chemical recovery furnace where it is dried and combusted to produce a smelt of recovered inorganic pulping chemicals (PC). Optionally, makeup chemicals may be added to the concentrated black liquor fed to the furnace.
[0016] The improvement of this invention comprises the steps of:
a. diverting up to about one-half of said alkaline pulping liquor, prior to a final concentration step, to a wet oxidation step;
b. wet oxidizing said diverted portion of the alkaline black liquor in said wet oxidation apparatus at a pressure of 200 - 4000 psig (1500 - 28,000 KPa) and a temperature of 300 - 650°F (150 - 340°C) with oxygen or oxygen-containing gas to destroy at least 5 percent of said oxidizable organic matter, evaporate a portion of the water in said diverted black liquor, and increase the ratio (PC/OM) of inorganic pulping chemicals to oxidizable organic matter contained therein; and
c. combining said wet oxidized portion of the black liquor with the remainder of unoxidized black liquor to form a mixed black liquor having an increased PC/OM ratio, prior to passing said mixed black liquor to said chemical recovery furnace.
[0017] The point at which black liquor is diverted to the wet oxidation step may be prior to, or subsequent to several steps of evaporation and concentration, but is before the final step of concentration.
[0018] The wet oxidized black liquor may be combined with the remainder of unoxidized black liquor at any point in the process prior to combustion in the chemical recovery furnace.
[0019] This invention allows pumping, concentration and combustion of black liquors having increased concentrations of total solids and pulping chemicals, because of the discovery that at a given solids concentration, a mixture of oxidized black liquor and unoxidized black liquor has a lower viscosity than either liquor by itself. Thus the solids throughput of the evaporators and/or furnace may be increased.
[0020] Furthermore, this invention permits ready control over the PC/OM ratio and the final solids concentration in the furnace feed, regardless of continual changes in rate of black liquor production, its total solids concentration and/or its PC/OM ratio.
[0021] The invention will now be described in more detail with reference to various examples thereof and to the accompanying drawings, in which:-
FIG. 1 is a schematic diagram showing the prior art black liquor process as practiced by most alkaline pulp mills.
FIGS. 2 and 3 are schematic diagrams showing alternative prior art processes taught by Pradt U.S. Patent 3,714,911.
FIG. 4 is a graph showing measured viscosities of hardwood kraft black liquor treated according to this invention.
FIG. 5 is a graph similar to FIG. 4, for fir black liquor.
FIG. 6 is a schematic diagram of an embodiment of the invention.
FIG. 7 illustrates the invention wherein wet oxidized black liquor is combined with unoxidized liquor at one or more alternate locations.
FIG. 8 is an embodiment of the invention in which a portion of the black liquor is diverted, after evaporation, to the wet oxidation step.
FIG. 9 is a further embodiment illustrating diversion of portions of the black liquor before and after evaporation to the wet oxidation step.
FIG. 10 is a plot of percentage increase in PC/OM ratio of black liquor entering the furnace, as a function of wet oxidation conditions.
FIG. 11 is a material balance for Example 4 (prior art).
FIG. 12 is a material balance for Example 5, illustrating this invention.
FIG. 13 is a material balance for Example 6, illustrating this invention.
[0022] This invention is a process for improving the operation of a pulp mill's black liquor chemical recovery plant. The conventional recovery plant to which this invention applies is illustrated in FIG. 1, and treats the black liquor in one or more steps of concentration by evaporation, followed by combustion of the concentrated black liquor in a chemical recovery furnace.
[0023] Weak black liquor 1 from the pulp mill enters evaporator 2 where moisture 3 is evaporated off to concentrate the liquor to an intermediate solids concentration. Evaporator 2 typically comprises several stages and is then known as a multiple-effect evaporator.
[0024] Evaporated black liquor 4 is further concentrated in concentrator 5, and additional moisture 6 is driven off.
[0025] Concentrated black liquor 7 together with makeup pulping chemicals 8 is sprayed into recovery furnace 9 where air 10 is introduced to burn the organic matter in the black liquor. Product smelt 12 of pulping chemicals is recovered; oxidation products and steam are discharged in offgas 11.
[0026] In the past, wet oxidation technology has been applied to black liquor chemical recovery as shown in FIGS. 2 and 3.
[0027] In FIG. 2, weak black liquor 1 is first wet oxidized with air or oxygen 14 in wet oxidizer 13, producing an offgas 15 which contains moisture. The wet oxidized black liquor 16 then passes through evaporator 17 to produce a liquor 19, of intermediate concentration. After further evaporation in concentrator 20, the black liquor 22 and optional makeup chemicals 23 are burned with air 25, in recovery furnace 24. Water vapor is discharged from the system in streams 15, 18, 21 and 26. A smelt 27 of chemicals is recovered.
[0028] In FIG. 3, another prior art process, weak black liquor 1 is wet oxidized in wet oxidizer 28 with air or oxygen 29. The wet oxidation step includes an integral flash evaporation step to remove a large amount of water 30 and concentrate the black liquor prior to optional mixing with makeup chemicals 32 and combustion with air 34 in furnace 33. The chemical smelt 36 and offgases 35 are discharged from the furnace.
[0029] In the process of FIGS. 2 and 3, the wet oxidation step is typically operated to provide a partial oxidation; for example, 30 - 60 percent of the organic matter (OM) is destroyed. For many black liquors, such wet oxidation increases rather than decreases the black liquor viscosity, reducing the maximum solids concentration which can be passed through the concentrator and piping to the furnace. If a nearly complete oxidation is performed, the low heating value of the black liquor will necessitate the addition of considerable auxiliary fuel to the furnace.
[0030] In either case, wet oxidation of the entire volume of black liquor requires a large and relatively expensive system.
[0031] The effect of wet oxidation upon black liquor viscosity is illustrated for hardwood kraft black liquor and fir black liquor in FIGS. 4 and 5, respectively. The degree of oxidation was 37.4 percent and 38.6 percent, respectively. The reduction in viscosity achieved by combining raw black liquor with wet oxidized liquor is also shown in FIGS. 4 and 5. Unexpectedly, and most importantly, the mixed liquor has a lower viscosity than either the raw black liquor or wet oxidized liquor, at the same solids concentration. These viscosity measurements were made with a Brookfield rotating spindle viscometer at 180°F (82°C).
[0032] The present invention is distinguished from prior processes utilizing wet oxidation in that a portion, rather than all, of the black liquor stream is wet oxidized. The wet oxidized portion, generally having an elevated viscosity (at the same total solids concentration) is combined with the unoxidized black liquor, either before or after the unoxidized liquor is concentrated by evaporation. Unexpectedly, the liquor mixture has a viscosity lower than either the unoxidized or wet oxidized black liquor alone, and it is possible to concentrate the liquor to a higher concentration prior to burning in the recovery furnace. Furthermore, the mixed black liquor has a higher PC/OM ratio, increasing the furnace throughput.
[0033] This invention is particularly useful as a modification of an existing chemical recovery plant to increase its throughput and efficiency without a prohibitively large capital expenditure. As will be seen in the ensuing description, the invention not only achieves an increased throughput in the recovery furnace, but may also overcome limitations on throughput presented by the evaporator and/or concentrator.
[0034] The particular embodiments shown in FIGS. 6 through 10 and described herein illustrate several practical applications of the invention, but are not exhaustive thereof.
[0035] Turning now to FIG. 6, an embodiment of this invention, a flow 1 of alkaline pulping weak black liquor is diverted by splitter 37 into a portion 38 which is passed to wet oxidizer 39. The diverted liquor is wet oxidized at a pressure of 200 - 4000 psig (1500 - 28,000 KPa) and at a temperature of 300 - 650°F (150 - 340°C) with oxygen or an oxygen containing gas 40 to destroy all or part of the oxidizable organic matter (OM). A fraction of the water in the liquid is evaporated and is discharged in offgas 41.
[0036] The fraction of weak black liquor diverted to the wet oxidizer comprises up to about one-half, but may be as little as 5 percent of the flow.
[0037] The fraction of OM destroyed in the wet oxidizer may range from about 5 percent to nearly quantitataive. The ratio of inorganic pulping chemicals (PC) to oxidizable matter (OM) is increased as OM is destroyed. For example, PC/OM of the oxidized liquor is doubled when the wet oxidation step is operated at 50% oxidation.
[0038] For many applications the optimal design is achieved when a relatively small fraction, 5 to 35 percent, of the black liquor is wet oxidized to destroy at least 30 percent of the oxidizable organic matter.
[0039] Wet oxidized black liquor 42 is combined with the remainder 43 of unoxidized black liquor to produce a mixture 44 having increased PC/OM and decreased viscosity relative to unoxidized black liquor 1. The mixture 44 is evaporated to an intermediate solids concentration in evaporator 45, and the resulting black liquor 47 is concentrated in final concentrator 48. Moisture 46, 49 is driven off by thermal energy input, usually steam, not shown.
[0040] Makeup chemicals 51 are optionally added to the concentrated black liquor 50 and the mixture passed to the chemical recovery furnace 52. Oxidizable organic matter is burned with air 53, producing offgas 54, and a smelt 55 of pulping chemicals is recovered.
[0041] Turning now to another embodiment, shown in FIG. 7, a portion 57 of weak black liquor 1 is diverted by splitter 56 to wet oxidizer 58. The liquor is wet oxidized with air or oxygen containing gas 59 to produce a wet oxidized liquor 61.
[0042] The wet oxidation conditions may be controlled to evaporate a considerable portion of water, which leaves the system in stream 60. Wet oxidized black liquor 61 may then be combined with unoxidized black liquor at a location preceding recovery furnace 76. For example, wet oxidized liquor 61 may be combined with remaining unoxidized black liquor:
a. before the evaporator 66, thus mixing wet oxidized black liquor 63 with remaining unoxidized black liquor 64 to form mixture 65; this embodiment is identical to that of FIG. 6;
b. following the evaporator 66 thus mixing wet oxidized black liquor 69 with evaporated unoxidized black liquor 68 to form mixture 70; and/or
c. following the concentrator 71, thus mixing wet oxidized black liquor 74 with concentrated unoxidized black liquor 73 to form mixture 75.
[0043] In most cases, all of the wet oxidized liquor 61 will be combined with unoxidized liquor at a single point.
[0044] Optionally, the flow of wet oxidized liquor 61 may be split by splitter 62 into two or three streams 63, 69, 74 to achieve optimal operation of the evaporator, concentrator and furnace.
[0045] If, for instance, the evaporator 66 is more severely limited in flow than concentrator 71, all or part of the wet oxidized liquor may be combined as streams 69 and/or 74 rather than as stream 63. If it is added as a single stream, of course, splitter 62 is not needed or used.
[0046] Wet oxidized black liquor is generally added as stream 74 following the concentrator only when the wet oxidizer 58 is operated at high oxidation conditions of about 80 percent or greater, and a large portion of water is removed in the wet oxidation step. In this case, the solids content of liquor stream 74 is roughly the same as that of concentrated black liquor 73, and autogenous combustion can be achieved in the furnace 76. A combustible material will support autogenous combustion when the released thermal energy meets or exceeds the total thermal energy required by the furnace to operate at the desired conditions of temperature, throughput rate and degree of combustion.
[0047] Adding all or a portion of wet oxidized black liquor as stream 69 to evaporated liquor 68 has special advantages. Since the maximum solids concentration which can be achieved in concentrator 71 is usually limited by black liquor viscosity, adding wet oxidized liquor prior to the concentrator reduces viscosity and permits a higher solids concentration to be achieved.
[0048] FIG. 8 shows a further embodiment illustrating the invention. The weak black liquor 1 is first passed through evaporator 80, to evaporate a large portion of water as vapor 81. Black liquor 82 of intermediate concentration is divided into two fractions by splitter 83. Diverted portion 84 is wet oxidized in wet oxidizer 85 by air or oxygen containing gas 86 resulting in an offgas 87, containing water vapor. Wet oxidized black liquor 88 is combined with the remainder 89 of unoxidized black liquor from evaporator 80. The mixed stream 90, having an increased PC/OM ratio and reduced viscosity, is passed to concentrator 91 where further water vapor 92 is driven off, and the concentrated liquor 98 is burned in recovery furnace 99 with air 100 to burn the remaining organic matter. Offgas 101 and a smelt 102 of recoverable chemicals are produced.
[0049] A particular advantage to this embodiment is that the fraction of black liquor to be wet oxidized is a much smaller volume of a more concentrated stream. The reduced size of the wet oxidation system results in appreciable cost savings over that of the embodiments in FIGS. 6 and 7.
[0050] In FIG. 8, the wet oxidized black liquor 94, may optionally be diverted to a splitter 95 which divides the stream into two fractions. A first fraction 96 is combined with the remainder of evaporated unoxidized black liquor 89, forming mixture 90. A second fraction 97 of wet oxidized black liquor is combined with concentrated mixed black liquor 93 prior to burning in furnace 99.
[0051] In the additional embodiment shown in FIG. 9, black liquor to be wet oxidized is diverted both before and after the evaporation step, in order to optimally control the concentration of the wet oxidizer feed.
[0052] A portion 104 of weak black liquor 1 is diverted by splitter 103 to wet oxidizer 105. The remaining black liquor 106 is evaporated in evaporator 107, discharging water 108 and black liquor 109 at an intermediate concentration. A portion 111 of black liquor 109 is diverted by splitter 110 to wet oxidizer 105 which wet oxidizes the mixture of black liquor streams 104 and 111 with air or oxygen containing gas 112. Water vapor is discharged in offgas 113.
[0053] In this embodiment, black liquor is diverted to the wet oxidizer 105 at two locations. Each diverted liquor has a different solids concentration, and a calculation of the total fraction of black liquor which undergoes wet oxidation is based upon the weight of inorganic pulping chemicals in each diverted stream.
[0054] In FIG. 9, wet oxidized black liquor 115 is combined with the remaining intermediate concentration black liquor 114 to form mixed black liquor 116. Liquor 116 is further concentrated by evaporation in concentrator 117. Water vapor 118 is discharged and concentrated black liquor 119 is dried and burned in furnace 120 with air 121 to produce offgas 122 and smelt 123 of recovered pulping chemicals.
[0055] Where evaporator 107 is a multiple effect evaporator, black liquor may be diverted from an intermediate stage or stages to provide an optimum solids concentration to the wet oxidizer 105.
[0056] Where black liquor is diverted to the wet oxidizer in several streams of different solids concentration, the flow rate of each diverted stream may be continuously controlled to provide proper operation of the wet oxidizer. For example, where black liquor strength (solids concentration) and/or rate may vary with time, it may be desirable to control each diverted flow rate to achieve wet oxidation of a constant volumetric rate of black liquor containing a constant fraction of the total solids.
[0057] The splitter shown in the FIGS. 6-9, 11 and 12 is any device which will divert a portion of the black liquor flow to the wet oxidizer, and may include a flow control valve or valves, or a positive displacement pump.
[0058] While energy input to the evaporation devices and energy recovery from the furnace are integrally related to successful operation of the chemical recovery system, they are well known and are not described at any length in this application or shown in the drawings.
[0059] The effect of wet oxidizing a portion of the black liquor upon the PC/OM ratio of black liquor entering the furnace is illustrated in FIG. 10. With a furnace limited by heat release, the PC/OM ratio must be increased in order to increase throughput. Suppose it is desired to increase throughput by 15 percent. Many combinations of (a) percent diversion to the wet oxidizer and (b) percent oxidation will achieve the desired result. From FIG. 10, three such combinations are:
1. 65.5% oxidation of 20% of the black liquor;
2. 37.5% oxidation of 35% of the black liquor; and
3. 27.5% oxidation of 50% of the black liquor.
[0060] While the portion of black liquor which is wet oxidized may range from 5 to about 50 percent, in most cases the optimal diverted portion is 5 to about 35 percent. Of course, quantitative wet oxidation of 50 percent of the black liquor will increase the PC/OM ratio of furnace feed by a factor of 2.0. For most applications, the optimal degree of oxidation may lie in the 30 - 70 percent range, but in particular cases the degree of oxidation (percentage OM reduction) in the wet oxidation step may be as low as 5 percent or as high as essentially 100 percent.
[0061] When the degree of oxidation exceeds about 15 percent, the wet oxidizer may be operated at pressures and temperatures to result in evaporation of a large quantity of water. In some cases, the wet oxidized black liquor may be sufficiently concentrated to have a heating value equal to or greater than the furnace requirement for autogenous combustion.
[0062] The energy released in the wet oxidation step may be recovered and used to heat and evaporate water from the black liquor, for example, in the evaporator and/or concentrator. Excess steam may be used in other operations of the mill.
[0063] The invention may be used with any alkaline pulping liquor, and is particularly applicable to kraft cellulose pulping liquors. While batch-wise operation of wet oxidation systems may be cost effective for certain small pulp plants, continuous operation is the preferred mode of operation.
[0064] Selection of the wet oxidation operating conditions will take into account both capital and operating costs to arrive at the most cost-effective design of the wet oxidizer.
[0065] In each of the embodiments it is understood that make-up chemicals may typically be added to the concentrated black liquor prior to drying and combustion in furnace.
[0066] As seen in the foregoing descriptions, numerous possible flowsheets exist for using this invention. The flowsheet applied to a particular existing mill will depend on many factors, including:
a. maximum existing capacity of each of the evaporator, concentrator and furnace;
b. black liquor characteristics;
c. cost of thermal energy;
d. capital cost and operating cost of the wet oxidized unit as a function of its capacity;
e. desired chemical recovery capacity; and
f. the desired sophistication of control over the process.
EXAMPLE 1
[0067] Actual test data and illustrations of the invention based on the data are presented. Viscosities are measured with a Brookfield rotating spindle viscometer. Viscosity values in centipoise (cp) obtained at 100 rpm and 180°F (82°C) were used for comparing the various samples. Although each pulp mill may use its own particular type of viscometer and test conditions, the results are comparable to the data presented herein. For example, some mills use viscosity measurements obtained at 200°F (93°C) where the viscosity is considerably lower than at 180°F (82°C).
[0068] For the sake of simplicity, the examples assume that all of the material measured as "ash" represents pulping chemicals (PC). While not exactly true, the difference is insignificant for the purpose of the examples.
[0069] The raw black liquor resulting from kraft pulping of hardwood had the following analysis:
Total Solids, percent 42.0
Organic Matter, percent 20.5
Inorganic Pulping Chemicals, percent 21.5
Chemical Oxygen Demand (COD), percent 45.7
Heat of Combustion, BTU per lb. solids 5911
(Kcal. per kg solids) 3285
PC/OM Ratio 1.05
pH 13.1
Sp. Gr. 1.266
Viscosity, Brookfield, cp at 180°F(82°C),
100 rpm 28
[0070] A first portion of the raw black liquor was evaporated to a concentration suitable for combustion in a recovery boiler. The analysis of the concentrated liquor was:
Total Solids, percent 69.6
Organic Matter, percent 33.8
Inorganic Pulping Chemicals, percent 35.8
Chemical Oxygen Demand, percent 70.0
Heat of Combustion, BTU per lb. solids 6236
(Kcal. per kg. solids) 3465
PC/OM Ratio 1.06
pH 12.4
Sp. Gr. 1.520
Viscosity, Brookfield, cp at 180°F(82°C),
100 rpm 1400
[0071] A second portion of the raw black liquor was subjected to wet oxidation treatment at 400°F (204°C) using oxygen as the oxidant. A total of 37.4 percent of the oxidizable organic matter was destroyed, as determined by the COD reduction. The analysis of wet oxidized black liquor was as follows:
Total Solids, percent 35.7
Organic Matter, percent 13.9
Inorganic Pulping Chemicals, percent 21.8
Chemical Oxygen Demand, percent 27.9
Heat of Combustion, BTU per lb. solids 4061
(Kcal. per kg. solids) 2257
PC/OM Ratio 1.57
pH 9.1
Sp. Gr. 1.264
Viscosity, Brookfield, cp at 180°F(82°C),
100 rpm 28
[0072] A portion of the wet oxidized liquor was concentrated by evaporation to a total solids concentration of 64.6 percent. The analysis of this concentrated wet oxidized liquor was:
Total solids, percent 64.6
Organic Matter, percent 25.7
Inorganic Pulping Chemicals, percent 38.9
Chemical Oxygen Demand, percent 53.3
Heat of Combustion, BTU per lb. solids 3765
(Kcal. per kg. solids) 2092
PC/OM Ratio 1.51
pH 9.3
Sp. Gr. 1.485
Viscosity, Brookfield, cp at 180°F(82°C),
100 rpm 1500
[0073] A further portion of wet oxidized liquor was combined with raw black liquor in equal volumetric proportions and concentrated by evaporation to 69.0 percent total solids. The analysis of this mixture was as follows:
Total Solids, percent 69.0
Organic Matter, percent 31.3
Inorganic Pulping Chemicals, percent 37.7
Chemical Oxygen Demand, percent 56.8
Heat of Combustion, BTU per lb. solids 4846
(Kcal. per kg. solids) 2693
PC/OM Ratio 1.20
pH 11.5
Sp. Gr. 1.550
Viscosity, Brookfield, cp at 180°F(82°C),
100 rpm 1000
EXAMPLE 2
[0075] Assume for example, a chemical recovery system which is limited to operating with a maximum viscosity of 1400 cp in the concentrated black liquor going to the furnace. For the sake of simplicity, the addition of makeup chemicals will be ignored.
[0076] The data of Example 1 will be applied to three modes of pre-furnace operation:
a. simple evaporation and concentration (no wet oxidation;
b. wet oxidation of the total stream of black liquor followed by evaporation and/or concentration; and
c. wet oxidizing one-half of the raw black liquor, combining the oxidized and raw black liquors and evaporating and/or concentrating the mixture.
[0077] By extrapolating the data to achieve a limiting viscosity of 1400 cp under each mode, the furnace feed materials (after concentration) will have the following calculated compositions:
[0078] The advantages of the invention are clearly seen. Wet oxidation of the whole black liquor stream will allow black liquor of 37.9 percent PC to be combusted. This increases the furnace capacity by [(37.9 - 35.8)/35.8] × 100 = 5.9 percent. A relatively large wet oxidation unit will be required to handle the entire stream.
[0079] There are several disadvantages to wet oxidizing the entire black liquor stream. First, the wet oxidation system is relatively large, requiring considerable capital expenditure. Second, wet oxidized black liquor must be fed to the furnace at a higher moisture content (because of increased viscosity). The increased ratio of moisture to OM will often result in inefficient furnace operation.
[0080] It is noted that the viscosity effect of wet oxidation reverses when complete oxidation of the OM is approached. The viscosity of such highly oxidized black liquor may be equal to or lower than the original black liquor.
[0081] If only one-half of the black liquor stream is wet oxidized, and the resulting oxidized black liquor is mixed with raw black liquor, concentrated and combusted, the furnace capacity is increased by a further 3.6 percent, providing a 9.5 percent total increase in furnace capacity.
[0082] The above calculations assume that the volumetric rate of black liquor to the furnace is the same in all cases. In actual practice, the overall evaporative capacity is increased by the evaporation accomplished in the wet oxidation system. Furthermore, the approximately 20 percent reduction in heating value (BTU per lb. solids) resulting from wet oxidation of part of the black liquor allows an increased processing rate of black liquor in the furnace, without exceeding its maximum allowable firing rate. In practice, the overall increase in capacity, based on recovered chemicals, may be as much as 18 percent at these particular conditions of wet oxidation.
EXAMPLE 3
[0083] A raw black liquor resulting from kraft pulping of fir had the following analysis:
Total Solids, percent 46.3
Organic Matter, percent 23.2
Inorganic Pulping Chemicals, percent 23.1
Chemical Oxygen Demand (COD), percent 49.4
Heat of Combustion, BTU per lb. solids 6030
(Kcal. per kg. solids) 3351
PC/OM Ratio 1.00
pH 13.0
Sp. Gr. 1.307
Viscosity, Brookfield, cp at 180°F(82°C),
100 rpm 40
[0084] A first portion of the raw black liquor was concentrated by evaporation to a concentration suitable for combustion in a recovery boiler. The analysis of the concentrated liquor was:
Total solids, percent 68.8
Organic Matter, percent 34.1
Inorganic Pulping Chemicals, percent 34.7
Chemical Oxygen Demand, percent 72.0
Heat of Combustion, BTU per lb. solids 6273
(Kcal. per kg. solids) 3486
PC/OM Ratio 1.02
pH 11.9
Sp. Gr. 1.890
Viscosity, Brookfield, cp at 180°F(82°C),
100 rpm 2000
[0085] A second portion of the raw black liquor was subjected to wet oxidation treatment at 400°F (204°C), using pure oxygen as the oxidant. A total of 38.6 percent of the oxidizable organic matter was destroyed, as determined by the reduction of COD. The liquor analysis follows:
Total Solids, percent 42.5
Organic Matter, percent 18.2
Inorganic Pulping Chemicals, percent 24.3
Chemical Oxygen Demand, percent 31.6
Heat of Combustion, BTU per lb. solids 4026
(Kcal. per kg. solids) 2237
PC/OM Ratio 1.34
pH 9.0
Sp. Gr. 1.170
Viscosity, Brookfield, cp at 180°F (82°C),
100 rpm 70
[0086] A portion of the wet oxidized liquor was concentrated by evaporation to a total solids concentration of 61.8 percent. Its analysis was:
Total Solids, percent 61.8
Organic Matter, percent 26.7
Inorganic Pulping Chemicals, percent 35.1
Chemical Oxygen Demand, percent 47.1
Heat of Combustion, BTU per lb. solids 4126
(Kcal. per kg. solids) 2293
PC/OM Ratio 1.31
pH 9.3
Sp. Gr. 1.655
Viscosity, Brookfield, cp at 180°F (82°C)
100 rpm 24,000
[0087] A further portion of wet oxidized liquor was combined with raw black liquor in equal volumetric proportions and concentrated by evaporation to 72.2 percent total solids. The concentrated liquor analysis was:
Total Solids, percent 72.2
Organic Matter, percent 34.4
Inorganic Pulping Chemicals, percent 37.8
Chemical Oxygen Demand, percent 60.7
Heat of Combustion, BTU per lb. solids 5247
(Kcal. per kg. solids) 2916
PC/OM Ratio 1.10
pH 11.1
Sp. Gr. 1.550
Viscosity, Brookfield, cp at 180°F (82°C),
100 rpm 1650
[0089] The results with fir black liquor were thus similar to those of hardwood black liquor, and show a decided advantage in wet oxidizing only a portion of the black liquor, and then combining the un-oxidized and oxidized streams.
EXAMPLE 4
[0090] FIGS. 11, 12 and 13 compare material balances for several embodiments of this invention applied to a chemical recovery plant treating 500,000 Kg/day of black liquor containing 80,000 Kg. solids.
[0091] FIG. 11 shows the existing recovery plant, having the flowsheet and indicia of FIG. 1. It is assumed that the limiting factors in these examples are the evaporative capacities of the evaporator and concentrator.
EXAMPLE 5
[0092] In FIG. 12, the invention is adapted to the process of Example 4, using the flowsheet of FIG. 8. The wet oxidizer treats 50 percent of the black liquor from evaporator 80, oxidizing 25 percent of the organic matter. The wet oxidizer is operated to evaporate 73.6 percent of the moisture in the diverted portion of black liquor. The net increase in PC capacity of the recovery plant is 10.0 percent.
EXAMPLE 6
[0093] In FIG. 13, the invention is adapted to the process of Example 4, using the flowsheet of FIG. 7. The wet oxidizer treats 15 percent of the weak black liquor, evaporating about one-half of its water and oxidizing 50 percent of the organic matter. The PC capacity of the recovery plant is increased by 5 percent.
a. diverting up to about one-half of said alkaline black liquor, prior to the final concentration step, to a wet oxidation step;
b. wet oxidizing said diverted portion of the alkaline black liquor at a pressure of 200-4000 psig (1500-28,000 KPa) and a temperature of 300-650°F (150-340°C) with oxygen or oxygen-containing gas to destroy at least 5 percent of said oxidizable organic matter and evaporate a fraction of the water in said diverted portion, and increase the ratio (PC/OM) of inorganic pulping chemicals to oxidizable organic matter contained therein; and
c. combining said wet oxidized diverted portion of the black liquor with the remainder of unoxidized black liquor to form a mixed black liquor having an increased PC/OM ratio relative to unoxidized black liquor, prior to passing said mixed black liquor to said chemical recovery furnace.
a. diverting up to about one-half of said alkaline black liquor to a wet oxidation step;
b. wet oxidizing said diverted portion at a pressure of 200-4000 psig (1500-28,000 KPa) and a temperature of 300-650°F (150-340°C) with oxygen or oxygen-containing gas to destroy at least 5 percent of said oxidizable organic matter and evaporate a portion of the water in said diverted portion of black liquor, and increase the PC/OM ratio therein;
c. combining said wet oxidized portion of the black liquor with the remainder of unoxidized black liquor to form a mixed black liquor having an increased PC/OM ratio relative to unoxidized black liquor;
d. passing said mixed black liquor to one or more steps of water evaporation to concentrate said mixed liquor; and
e. passing concentrated black liquor to a chemical recovery furnace where said concentrated mixed liquor is dried and combusted to produce a smelt of recovered inorganic pulping chemicals.
a. diverting up to about one-half of said alkaline black liquor to a wet oxidation step;
b. wet oxidizing said diverted portion at a pressure of 200-4000 psig (1500-28,000 KPa) and a temperature of 300-650°F (150-340°C) with oxygen or oxygen-containing gas to destroy at least 5 percent of said oxidizable organic matter and evaporate a portion of the water in said diverted portion of black liquor, and increase PC/OM ratio therein;
c. passing the remainder of unoxidized black liquor to one or more steps of water evaporation to concentrate said remainder;
d. combining said wet oxidized portion of the black liquor with the concentrated remainder of unoxidized black liquor to form a mixed black liquor having an increased PC/OM ratio relative to said unoxidized black liquor; and
e. passing concentrated mixed black liquor to a chemical recovery furnace where said concentrated mixed liquor is dried and combusted to produce a smelt of recovered inorganic pulping chemicals.
a. concentrating said alkaline black liquor by one or more steps of water evaporation;
b. diverting up to about one-half of said concentrated alkaline black liquor to a wet oxidation step;
c. wet oxidizing said diverted portion at a pressure of 200-4000 psig (1500-28,000 KPa) and a temperature of 300-650°F (150-340°C) with oxygen or oxygen-containing gas to destroy at least 5 percent of said oxidizable organic matter and evaporate a portion of the water in said diverted portion of concentrated black liquor, and increase the PC/OM ratio therein;
d. combining said wet oxidized portion of the concentrated black liquor with the remainder of unoxidized concentrated black liquor to form a mixed concentrated black liquor having an increased PC/OM ratio relative to said unoxidized black liquor; and
e. passing mixed concentrated black liquor to a chemical recovery furnace where said concentrated mixed liquor is dried and combusted to produce a smelt of recovered inorganic pulping chemicals.
a. diverting up to about one-half of said alkaline black liquor to a wet oxidation step;
b. wet oxidizing said diverted portion at a pressure of 200-4000 psig (1500-28,000 KPa) and a temperature of 300-650°F (150-340°C) with oxygen or oxygen-containing gas to destroy at least 5 percent of said oxidizable organic matter and evaporate a portion of the water in said diverted portion of black liquor, and increase the PC/OM ratio therein;
c. combining a first fraction of said wet oxidized black liquor with the remainder of unoxidized black liquor to form a mixed black liquor having an increased PC/OM ratio relative to said unoxidized black liquor.
d. passing said mixed black liquor to one or more steps of evaporation to concentrate said mixed liquor;
e. combining a second fraction of said wet oxidized black liquor with said concentrated mixed black liquor from evaporation step (d) to produce a furnace feed material; and
f. passing furnace feed material to a chemical recovery furnace where said material is dried and combusted to produce a smelt of recovered inorganic pulping chemicals.
a. diverting a portion of said alkaline black liquor from each of two or more stages of evaporation, prior to the final concentration step, to a wet oxidation step in a wet oxidation appartus, whereby the total of diverted portions comprises up to about one half of said black liquor, based on weight of inorganic pulping chemicals contained therein;
b. combining and wet oxidizing said diverted portions of the alkaline black liquor at a pressure of 200-4000 psig (1500-28,000 KPa) and a temperature of 300-650°F (150-340°C) with oxygen or oxygen-containing gas to destroy at least 5 percent of said oxidizable organic matter and evaporate a portion of the water in said diverted portions, and increase the ratio of inorganic pulping chemicals to oxidizable organic matter (PC/OM) contained therein; and
c. combining said wet oxidized portion of the black liquor with the remainder of unoxidized black liquor to form a mixed black liquor having an increased PC/OM ratio relative to unoxidized black liquor, prior to passing said mixed black liquor to said chemical recovery furnace.