2794
Ind. Eng. Chem. Res. 1993,32, 2794-2199
Chemical and Physical Properties of High-Yield Alkaline Sulfite Green Liquor Nancy J. Sell' and Jack C. Norman Natural and Applied Sciences, University of Wisconsin-Green Bay, Green Bay, Wisconsin 54311
The majority of sodium sulfite pulping liquor recovery systems are based on the reductive burning of the spent liquor, followed by acidification of the resulting smelt solution by C02. This study investigated a number of the physical and chemical properties of the resulting green liquor which might be relevant to the optimum design of this type of sulfite and carbonate recovery system for an alkaline sulfite high-yield process. CO2 gas does generate H2S when bubbled through green liquor; however, a large amount of solid soon is formed. Continuing the flow leads t o increased amounts of HzS, but the ratio of H2Sto C02 remains less than 1.0. Solutions more highly concentrated in Na2S absorb relatively more COz, regardless of the ratios of H2S to C02 in the initial gas stream. The percentage of H2Sreleased increases with increasing Nags concentration. Stripping the green liquor with inert gas, steam, or vacuum does not improve the H2S removal efficiency. The maximum C 0 2 pressure can be generated by decomposing pure 6 M NaHC03. If the starting material is a bicarbonate/carbonate mixture, conversion is incomplete and a portion of the NaHC03 forms a dead load.
Introduction Sodium sulfite pulping systems, for papermaking, are limited by, among other things, an appropriate chemical recovery system. There have been numerous commercial processes which have been developed to recover the inorganic chemicals from these liquors, but the majority of those processes are complicated and potentially lead to unacceptably high reduced sulfur emissions, primarily as H2S (Norman et al., 1990). The development of a more appropriate system has been hindered by the lack of published data regarding the basic chemical/physical properties of the green liquor. This study focused on some of the more relevant properties of one particular type of green liquor, that from an alkaline sulfite high-yield process. The properties reported related to possible process steps utilized by a sodium sulfite/sodium carbonate recovery system. Background. In a pulping process, the pulping liquor, after the digestion and washing steps, is called spent liquor. In a sulfite process it contains the dissolved organics, primarily lignin, which had been removed from the wood and/or other cellulosic raw materials, and various other reaction products. The majority of the dissolved lignin is present as lignosulfonates. In order to separate the inorganic chemicals from the dissolved organic chemicals, in most recovery processes the spent liquors are burned, either reductively or oxidatively, in a recovery furnace. This incineration simultaneously provides the possibility of energy recovery. In systems designed to produce a sulfur product which can be reused in a sulfite system, rather than sold as salt cake (Na2S04)for makeup in a sulfate (kraft) process, the burning is done reductively. The incineration produces a molten smelt, which is then dissolved in water to produce green liquor. It is this green liquor, comprising a mixture of sodium sulfide and sodium carbonate, which is then processed for the recovery of sulfite and carbonate needed for pulping. General Procedure. For this study, the green liquor was simulated by an aqueous solution containing a 4:l mole ratio of Na2S to Na2CO3 mixed so the total Na concentration is 6 mol/L (21% solids). Most of the commercial sulfur recovery systems use carbon dioxide to
acidify the green liquor in order to convert the Na2S to H2S, which is then separated from the liquor by vaporization. The data collected are thus related to the optimization of this process. Those specific properties studied include the following: the vapor pressure of the green liquor as a function of temperature; the conversion of Na2S to H2S as a function of C02 flow; the ratio of H2S to C02 in the expelled gas as a function of C02 pressure in the system and the temperature; the maximum conversion of NazS to H2S in multistage experiments; the relative amounts of H2S and C02 absorbed when the emitted gas is recirculated; the effect of stripping on the H2S emitted; the decomposition of NaHC03 to form C02.
Equipment In order to conduct the various experiments, it was necessary to have a stainless steel pressure vessel manufactured. We opted to use only one reactor for the majority of the tests; hence the vessel had to be able to withstand the high pressures which might be generated during the high-temperature decomposition of the bicarbonate. The resulting 304L stainless reactor is able to withstand 4134 kPa (600 psi). The vessel is fitted with a number of various-sized input and output ports which can be used for pH and conductivity sensors, pressure relief valves, and sampling cylinders. Because 304L stainless steel is nonmagnetic, the vessel can be stirred by a conventional magnetic stirring bar. Rather than use this vessel for the flow measurements, those determinations were conducted in a simple glass bubbling apparatus. Pressures above 1 atm were not required for these tests, and the much smaller size of the apparatus permitted use of significantly smaller green liquor samples. COz Flow Determinations
Many commercial systems assume that CO2 can be used to convert NazS to Has. The purpose of these initial tests was to confirm that the conversion did occur and then to determine whether the H2S remains in solution after
0 1993 American Chemical Society 0SSS-5S~5/93/2632-2~94$04.00/0
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then increased back to the initial level. Immediately prior to the conclusionof this run, significant precipitation again occurred. Conclusions. The flow of C02 does generate H2S, as expected. This H2S can be detected immediately in the outflowing gas stream. However, the concentrations are all less than 1000 ppm. In all of these preliminary cases, the C02 flow was stopped soon after the precipitation occurred, for the large volume of solid plugged the bubbler.
Equilibrium Conversion of NazS to HzS
Figure 2. Release of hydrogen sulfide as a function of time for bubbling pure carbon dioxide.
formation or whether it immediately is flushed from the system by the flowing C02. Procedure. One hundred milliliter samples of the simulated green liquor solution were inoculated with 1.0 mL of radioactive (35S) Na2S solution. The samples were maintained at 40 "C in a water bath, and the C02 was bubbled through the liquor. The exhaust gas from the bubbling apparatus was further bubbled through two test tubes in series, each containing 1.0 mL of 0.01 M NaOH solution. The NaOH solution, being slightly basic, absorbed the acidic H2S gas. The NaOH solutions were each then mixed with scintillation counter fluid, and the radioactivity levels,which are proportional to the quantity of H2S released, were then determined in a Beckmann LS 1801liquid scintillation counter. A determination of the background counts, using green liquor which was not inoculated with the radiotracer, was also made, and the average value-50 cpm-was subtracted from each of the other determinations. Results. The results of the initial determination using a flow rate of 25 mL of carbon dioxide/min, are in Figure 1. The HzS level first decreased slightly, corresponding to an initial "sweeping out" of the excess in the system, and then increased. However, when the flow rate was increased to 100 mL/min, substantial precipitation occurred during the taking of the second sample. As this occurred, the H2S emissions rose more than 10-fold(Figure 2). To further investigate this phenomenon, the flow rate was decreased to 50 mL/min. The results are in Figure 3. As can be seen, the emission rate first decreased and
Knowledge about the equilibrium conversion of NazS to H2S in green liquor solutions is potentially helpful in the design of a recovery system. Mai and Babb (1955) investigated this type of system in the mid-l950s, but only at concentrations of 1mol/L or less. Extending their data to higher concentrations, using similar experimental techniques, was not feasible due to the detrimental effect of high pH and high concentrations on pH and conductivity sensors. Therefore, a different approach was used. Procedure. A simulated green liquor sample was placed into the pressure vessel, which was then swept out with C02 gas to replace the majority of the air. The reactor was sealed, and enough C02 was added to bring the pressure up to a predetermined value. For the initial tests, the C02 tank was left open, set so it would maintain the desired pressure. For later trials, the system was totally sealed. Gas samples were analyzed by a Carle 5600 gas chromatograph (GC) with a Supelco Puropacs column. Because the GC is sensitive to sample size, the GC readings only provided information regarding the H2S:C02 ratio. Absolute values were obtained by considering the total pressure of the system and the amount of water vapor present at that given temperature. Because of the highly concentrated green liquor, it was not possible to accurately estimate the water vapor pressure by any theoretical means. Therefore, experimental measurements as to the vapor pressure of green liquor solutions were made by conventional techniques using a manometer and vacuum pump. The results are in Figure 4. The results indicate a 2.3 "C boiling point elevation at 40 "C and 4.0 "C elevation at 60 "C. The effects of two variables were considered: the total pressure and the temperature. For several of the runs, liquor samples were also taken. The sulfide remaining in solution was determined by the radiotracer techniques previously described.
2796 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993
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Figure 6. Molar ratio of HaS/C02 aa a function of temperature for C02 pressure maintained at 3.0 atm, with CO2 cylinder open.
Results. The effect of the system pressure at one temperature, 40 "C, is indicated in Figure 5. Note that the optimum ratio of HzS to COz occurs not at the highest pressure, 3.0 atm, but at about 1.5 atm. At the higher pressures the absolute amount of HzS is the greatest, but the COZlevel has increased even more. The H2S:COzratio is a critical factor, for large percentages of COZin the final gas stream will inhibit the subsequent S" recovery. Figure 6 represents the inverse experimental situation. The pressure was maintained at a total 3 atm, while the temperature was varied from 40 to 80 "C. Note that again the greatest ratio did not occur a t the highest temperature. This figure also illustrates another phenomenon; the
initial C02 pressure of 3.0 atm, with C02 cylinder c l d .
HZS generation appears not to be reversible. The measurements were taken in order of increasing temperature, allowing approximately 1 h between measurements for equilibrium to be established. After the highest temperature reading was made, the vessel typically sat overnight at the lowest temperature in order to permit reversal of the reaction. Instead, the ratios appear to be constantly increasing, indicating larger and larger relative amounts of HzS are present. It was postulated that this could be due to any of three phenomena: (1) A very thick crust, presumably NaHCOs crystals, quickly formed on the top of the reaction mixture. Though the mixture was being stirred, the stirring was not sufficiently vigorous to prevent its formation. This crust could potentially prevent equilibrium from being reestablished as the reactor again cooled simply by blocking transport of the HzS back into the bulk liquor. (2) The reaction may be essentially irreversible. Perhaps the actual reaction mechanism is such that the reverse reaction will not equilibrate in normal time periods. (3) The open COZ tank, which maintained the total pressure at 3 atm, may be driving the reaction further and further toward HzS production. As the C02 reacts, two things occur-more COZis supplied, available for reaction, and the solution becomes more acidic. Since H a is less acidic than COZ,the HzS will be preferentially vaporized from the solution. To determine which of these possibilities is most likely, two changes were made in the experimental method. Initially, the liquor was allowed to absorb as much COZas it would, the pressure was set a t approximately 3 atm, and then the tank was closed off from the system. In this way, the quantity of C02 available was fixed. The results of this test are in Figure 7. In this case, the 40 OC readings are essentially reproducible, within experimental error. As can also be seen from this figure, the H2S:COz ratio is optimum at the lower temperatures, 40-50 "C. To determine whether the NaHCOs crust has any effect, the green liquor was diluted in half. The resulting 3 M green liquor was totally soluble, and a crust did not form during the experiment. The results of measurements similar to those above are in Figure 8. As a result of these modifications, it appears that the open COZtank caused the continuous increase in the H2S: COZratio and that the COZ is preferentially absorbed. Thishas considerablesignificance in the potential recycling of the liquor and/or gaseous streams. Conclusions. The relative amounts of HzSevolved in these experiments are 100-fold or more greater than the initial flow experimentsat atmospheric pressure indicated. During the initial experiments, which were halted when-
Ind. Eng. Chem. Res., Vol. 32, No. 11,1993 2797 K-B H
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ever massive precipitation occurred, the concentration of gaseous hydrogen sulfide was always less than 1000 ppm. In these experiments, the gaseous hydrogen sulfide concentrations are >lo6 ppm. However, the H2S:COz ratios are still less than 1.0, and the optimum ratios appear to be at pressures only slightly greater than atmospheric.
HzS and COz Absorption The entire chemical recovery process is feasible only if relatively concentrated gaseous H2S can be formed. Too large a percentage of C02 in the final gaseous stream could inhibit the ultimate recovery of the sulfide. The results of the initial bubbling experiments indicated the product gas contained very large percentages of C02. To decrease the C02:H2S ratio in this gas stream, it was postulated that it might be feasible to recirculate the gas stream to the incoming green liquor, using the recycled gas as a source of C02 for the initial conversion of NazS to NaHS and possibly Has. If the C02 is preferentially absorbed at the pH values occurring within the process, it will be possible to use this technique to concentrate the H2S remaining within the gas stream. Procedure. To determine the advantages, if any, of recirculating the resulting H2S/C02 gas to increase the percentage of H2S, various mixtures of these two gases were equilibrated with a series of simulated green liquors. Equilibrium occurred very rapidly, within minutes. In every case, 1.0 mL of green liquor was placed in a 110-mL bottle with a septum cover. A 10-mL sample of the gas mixture was then injected into the bottle with a syringe. The bottles were maintained a t 35 "C in an incubator. Two separate cylinders of gas were used to prepare the gas mixtures, one containing H2S and the other containing C02. The flow rates of the gases from these cylinders were measured independently and adjusted to produce three different mixtures, corresponding to nominal C02: H2S mole ratios of 2575, 5050, and 7525. The separate C02 and H2S gas streams were then mixed by using a Y -connector. Four simulated green liquors were prepared, using various ratios of Na2CO3 and NazS, under the constraint that the total Na concentration remain at 6 M. The resulting solutions were thus composed of (a) 2.4 M NazS + 0.6 M Na2C03, (b) 1.8 M Na2S + 1.2 M Na2C03, (c) 1.2 M NazS + 1.8 M Na2C03, and (d) 0.6 M Na2S + 2.4 M Na2C03. In this way, it was possible to simulate the reacting of the gaseous stream at various stages within the process, to permit optimization of the H2S concentrating
H 2 S / C 0 2 EQUILIBRIUM ABSORPTION STUDIES
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process. Sulfide was analyzed by a standard iodine method (American Public Health Association, 1985). Results. The results of these experiments are in Figures 9 and 10. Each point is the average of duplicate samples. The standard deviations are also indicated in Figure 10. The 0.0 M Na2S value in Figure 10 corresponds to the initial gas concentration prior to injection into the bottle containing the green liquor. Note that the solutions more highly concentrated in Na2S (2.4 M) absorb relatively more CO2, regardless of the ratios of H2S to CO2 in the initial gas stream. For all gas streams, the percentage of H2S released increases with increasing Na2S concentration. Kubelka (1968) indicated, under other process conditions, that C02 and H2S have different absorption kinetics in aqueous solution and that advantage can be taken of this behavior to obtain H2S releases may times greater than would be calculated from the equilibrium partial pressures of H2S and C02 above the solution. This appears to have been verified also under these process conditions, especially in solutions with high NazS levels.
HzS Stripping The H2S formation process inevitably leaves some H2S dissolved in the remaining green liquor. The amount of dissolved H2S must be minimized for efficient operation of the recovery process. It was thought that a stripping process might serve to remove more of this dissolved gas. Procedure. In order to test the stripping efficiencies, spent green liquor samples remaining from the H2S
2798 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993
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Figure 11. Percent (by mass) of sulfide remaining in green liquor after various stripping methods.
Figure 12. Partial pressure of C02 as a function of temperature for solutions and slurries of NaHC03.
pressure formation studies were used. In every case, approximately 40-mL samples were taken. The S2-content was determined by radiotracer techniques. Five different "stripping" procedures were investigated: (1)Flash the sample to 1atm. A liquor subsample was taken and analyzed immediately after the sample was removed from the pressure vessel. (2) Permit the flashed sample to equilibrate at room temperature and pressure. (3) Apply a moderate vacuum (-0.5 atm). (4) Boil the sample. This is equivalent to bubbling saturated steam through the green liquor. (5) Bubble N2 gas through the sample. For methods 2-5, the treatment was applied for approximately 10 min. Results. The results obtained for the various pressure treatments (1-3 atm) of the green liquor were averaged for each of the stripping methods and are given in Figure 11. Note that there is only a maximum of 5 % difference in the percentage of S2-removed (93% retained compared to 98%) regardless of the stripping method used. Not only do the treatments not differ appreciably in their results, but simply flashing the sample to 1 atm, testing the sample immediately after removal from the pressure vessel, was as effective as any. Conclusions. Stripping the green liquor did not improve the H2S removal efficiency. The major factor which determines the solubility is the pH, and as long as the stripping process does not alter that, the Na2S/NaHS/ H2S remaining in solution appears to be fixed.
and sealed, and the temperature was slowly increased to 140or 180"C. Gas chromatograph samples of the resulting vapor were taken and the total pressure recorded at every 20 "C. In contrast to the previous pressure vessel studies, in this case the water vapor content was not known and could not be measured by conventional vapor pressure techniques. In addition, it would have been very difficult to determine the water content with the GC, for the H2O peak occurs far from the C02 peak. Therefore, the vessel was sealed without flushing the air space, and the 1atm of air present served as the GC calibration reference. To determine the minimum remaining NaHC03, the 1 M bicarbonate solution was analyzed. After the temperature trials were completed, the vessel temperature was lowered to 140 "C, approximately the temperature needed to produce 3 atm of CO2 when this solution was used. The CO2 was slowly bled from the system and the remaining solution allowed to equilibrate at 1atm. In one case, the solution was allowed to evaporate to dryness. In the second, the sample remained in solution. After equilibration, the temperature was lowered to room temperature to permit opening of the vessel. The spent solution was analyzed for both carbonate and bicarbonate by a Winkler titration. Results. The vapor pressure of C02 as a function of temperature for all three solutions/slurries is in Figure 12. The pure bicarbonate 6 M slurry decomposed to give the largest quantities of COz. The carbonate/bicarbonate mix, which contains 3 mol of bicarbonate (thus was comparable to 50 7% conversion of Na2C03 to NaHCOa), was barely able to supply 3 atm of C02 at temperatures in the range of 160-180 "C (320-356 OF). The 1M bicarbonate solution was not able to provide 3 atm of C02 at the temperatures we considered, as can be noted. To determine the maximum decomposition of the NaHC03, two separate experiments were conducted as noted above. In both cases a 1M NaHC03 solution was heated to 140 "C under pressure. After equilibrium was established, the vessel was opened and the gas allowed to escape slowly, while the temperature was maintained at 140 "C. In the first case, the solution was allowed to evaporate to dryness. The resulting powder was found to contain only 1 % bicarbonate. In the second case, the sample was not permitted to evaporate to dryness. The resulting solution was analyzed similarly and corresponded to -68% conversion of bicarbonate to carbonate. This remaining bicarbonate would unavoidably form a "dead load" chemical within the recovery process.
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Bicarbonate Decomposition For the processes to be economical, it is necessary to recycle the C02. During the H2S formation stage, the C02 reacts with the NazC03to form NaHC03. This latter must be decomposed to regenerate the COa. One of the simplest ways to regenerate C02 is to thermally decompose the bicarbonate. These tests were designed to determine the following: (1) whether 3 atm of C02 could be formed at reasonable temperatures and pressures, (2) the minimum amount of NaHC03 necessary in solution to produce that 3 atm, and (3) the amount of NaHC03 remaining in solution that cannot be decomposed at the operating temperatures. Procedures. Three different mixtures of NaHC03 were used for these tests: (1)a 1M pure NaHC03 solution, (2) a 6 M pure NaHC03 slurry, and (3) a slurry 6 M in Na, which contained 3 mol of NaHC03 and 1.5 mol of Na2C03. In all cases the samples were placed in the pressure vessel
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Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 2799 C02 + H2S
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In a "one-pass" scheme, large quantities of HzS will be generated only in conjunction with the use of even larger volumes of COz gas. However, the relative amounts of the two gases can be optimized if the COdHzS mixture is recirculated. Because of the preferential COZremoval by highly concentrated green liquors, it is preferable the C o d H2S gaseous mixture be recirculated to the first carbonation stage. After the HzSis collected, it can be burned to form SOz. The SO2 can then be mixed with the NaOH and NaC03 from the recaustization step to form white liquor (pulping liquor) for reuse in the pulping operation.
Acknowledgment We sincerely thank the Wisconsin Department of Development,Technology DevelopmentFund, for partial support for this study.
Literature Cited
PROPOSED FLOW DIAGRAM Figure 13. Proposed process flow diagram for recoveryof chemicals from green liquor.
Conclusions Figure 13 is a proposed process diagram for the recovery of sulfide chemicals. At these high concentrations, it is impractical to do the carbonation in one stage due to the extensive bicarbonate precipitation which inevitably occurs as the pH is lowered sufficiently to generate the HzS. The first stage should correspond to the initial precipitation and very little HzS production; the second stage will also generate some precipitate, but substantial amounts of gaseous HzS.
American Public Health Association. Titrimetric (12) Method. In Standard Methods for the Examination of Water and Wastewater, 16th ed.; AmericanPublic Healthhociation: Washington, DC, 1985; pp 476-477. K u b e h , V. Absorptionand Desorption of Gawa During Green Liquor Carbonation. Proceedings of the International Symposium on Recovery of Pulping Chemicals; May 13-17, Helsinki; 1968; pp 269-279. Mai, K. L.; Babb, A. L. Vapor-Liquid Equilibria by Radioactive Tracer Techniques. Ind. Eng. Chem. 1955,49 (9),1749-1757. Norman, J. C.; Sell, N. J.; Ciriacks, J. A. Avoiding TRS Emissions from Sodium SulfitePulping Recovery Procesees. Tappi J. 1990, 73 (6),229-231. Received for review December 1, 1992 Revised manuscript received June 28,1993 Accepted July 19, 1993. Abstract published in Advance ACS Abstracts, October 1, 1993. @
