CRYSTALLIZATION OF SODIUM ORGANIC SALTS PRODUCED IN PARTIAL WET OXIDATION OF BLACK LIQUOR Karhan Özdenkci1,a, Jukka Koskinen1, Kristian Melin1, Kuppa Venkata Sarada1, 1. Aalto University, Research Group of Plant Design, Espoo, Finland a. Corresponding author (
[email protected])
ABSTRACT: Due to environmental issues and sustainable development aspects, it has recently become favorable to integrate biorefineries to process industry to produce biofuel, energy and chemicals. For instance, in pulp industry, black liquor (waste stream) has a significant biorefinery potential. One alternative treatment involves partial wet oxidation of black liquor to produce organic acids or the sodium salts of these acids. The purpose of this study is to investigate salt and acid recovery from partially wet oxidized black liquor. The paper illustrates crystallization of sodium formate and sodium acetate (two of major salts) in aqueous solution with respect to temperature and pH. 1. INTRODUCTION Renewable energy has become an urgent need due to the increasing energy demand and environmental issues of fossil fuels. Moreover, even though biomass is potentially the greatest renewable energy supply, 1st generation bioprocesses consumes food source and requires additional water and land. Therefore, it has become favorable to integrate biorefineries with process industry. Pulp and paper industry has a significant biorefinery potential: black liquor (waste of pulping process) is an important fuel source. However, the commercial treatment includes complete combustion of the organic content: not utilizing the biofuel potential and not feasible for nonwood mills. Moreover, the most investigated alternative (gasification followed by Fischer-Tropsch synthesis) has issues of economic feasibility and synthesis selectivity. An alternative treatment suggested by Mudassar, et al., (2012) involves partial wet oxidation to produce organic acids (mainly acetic, formic, lactic and glycolic acids) and their sodium salts by neutralizing sodium carbonate. Then, the
salts can be separated from other components by crystallization. This paper investigates crystallization of sodium formate and sodium acetate in aqueous solution with respect to temperature and pH. The results are analysed to suggest a recovery process. 2. SALT AND ACID DISSOCIATION The aqueous sodium formate-acetate system has four reactions: dissolution of salts and dissociation of acids. The dissolution of sodium formate:
The dissolution of sodium acetate:
The dissociation of formic acid:
The dissociation of acetic acid:
It should be noted that the dissolution/ dissociation products above assumes thermodynamically ideal mixture. For real solutions, the excess chemical potential should be included as well by activity coefficients. On the other hand, typical activity coefficient models, e.g. Debye-Hückel and Davies models, are valid in low concentrations, thus being inaccurate in the concentration ranges of salt crystallization in this study. The solubility/dissociation products are determined by fitting experimental data to the empirical equation involving the temperature dependence:
Figure 1. Sodium acetate solubility product versus temperature Table 1. The parameters of empirical temperature dependence equations A B C D Ksp1 59.62 -13610 5.35 -0.132 Ksp2 -1.45 -2131 3.8 -0.033 Ka2 -1.504 -2608 1.62 -0.033 The dissolution/dissociation products of sodium acetate and acetic acid were given at various temperatures by Fournier, et al. (1998). The dissolution product of sodium formate is fitted to the solubility data given by Yaws (2012). The data is in molal concentration and it is calculated in molar concentration by assuming 1000 g
of water would have a volume of 1 L. This assumption is expected to have minor impact but more precise calculation can be performed by inserting sodium formate density. Figure 1 shows measured and calculated solubility product of sodium acetate vs. temperature as an example. The fitted parameters are given in Table 1. The temperature dependence of formic acid dissociation constant was determined by Kim et al., 1996 as:
The partially wet oxidized black liquor can be characterized to determine total acetate and total formate content (i.e. the sum of ion and acid). Therefore, the solubility products are modified assuming that acid dissociation reactions are in equilibrium:
and similarly,
where 1 and 2 are the modified solubility products of the salts in terms of total acetate/formate content (i.e., the sum of acidic and ionic forms). 3. SALT AND ACID RECOVERY The feed concentrations are selected based on the partially wet oxidized wheat straw black liquor in Mudassar, et al., 2012. The concentrations are:
The partial wet oxidation downstream is a liquid at high temperature, pressure and pH (e.g. 20 bars, 200 ºC and pH 9). Thus, the stream can be concentrated by water evaporation. The concentrations after water evaporation are:
Then, the species distribution calculated by analytical solution of:
is
Figure 2. Acid recovery after 70 % water evaporation: [Formate] = 1.40 M and [Acetate] = 2.80 M.
where a and b represent crystallized sodium formate and sodium acetate. Figure 2 and Figure 3 show the salt and acid recovery with respect to water evaporation, temperature and pH. 3.1 The effect of water evaporation The feed concentrations are too low to recover salts. Nevertheless, the product stream of partial wet oxidation can be concentrated by water evaporation and the steam can be utilized in pulping process or a CHP plant. Since the downstream of partial wet oxidation is at high temperature and pressure, the prior water evaporation rate can be controlled through a multistage flash operation. 70 % of water evaporation is still insufficient for salt crystallization. Therefore, Figure 2 shows only acid concentrations after 70 % water evaporation. Figure 3 shows the results for 90 % water evaporation. In this case, sodium acetate is recovered depending mainly on pH and slightly on temperature. However, the sodium acetate recovery is less than 65 % and sodium formate cannot be recovered yet.
Figure 3. Acid and salt recovery after 90 % water evaporation: [Formate] = 4.20 M and [Acetate] = 8.40 M.
3.2 The effect of pH pH influences the distribution of the dissolved formate and acetate to the ionic form and acidic form. At low pH values, the dissolved acetate and formate content are in acidic form; therefore, ion concentrations are so low that the system does not reach supersaturation with respect to the salts. This fact is reflected to the calculation method as high values of modified dissociation products 1 and thus no solid recovery. Acid 2, concentrations start to decrease after pH 2 and reaches to zero at pH 8. The sodium acetate recovery can be observed after pH 4. The sodium acetate recovery increases with pH and reaches the maximum values at pH 8 while the acid concentrations reaches zero. To sum up, salt recovery requires high pH (6 or above). In contrast, recovering acids in the free form requires low pH (3 for acetic acid and 2 for formic acid, or below). At higher pH (e.g. 4 or 5), the acetate/formate content consists of both ionic and acidic forms; therefore, extracting free form acids can require additional unit operation, such as reactive extraction. 3.3 The effect of temperature As it can be observed from Figure 2 and Figure 3, temperature has minor impact on acid concentration and just noticeable impact on sodium acetate recovery. The trends with respect to pH are unaffected by temperature and the concentration or sodium acetate recovery values are close. The acid concentrations decrease with increasing pH and higher temperature causes slightly sharper decrease. In addition, sodium acetate recovery is higher at 90 ºC. The recovery values are very close to each other in the other presented temperatures. Among those other temperature values, 30 ºC provides slightly higher recovery. This can be expected from Figure 1 as well: the solubility of sodium acetate very slightly
increase with temperature and decreases after around 60 ºC. At 90 ºC, the solubility is already lower than that at 30 ºC. Furthermore, the sodium acetate is recovered in anhydrous form in higher temperatures that 40 ºC but in trihydrate form at low temperatures (Dorn and Steiger, 2007). This fact was not included in this study but it is important to note that it is more favorable to recover the salts in anhydrous form. As a result, 90 ºC is more beneficial temperature: more salt recovery, salt in anhydrous form, and slightly more acid recovery. 4. CONCEPTUAL DESIGN
PROCESS
Based on the results of this study, a preliminary process concept can be suggested for material recovery and energy integration. One option can be a single draft-tube crystallizer unit, which operates at 90 ºC and pH can be adjusted depending on the desired products (acids or salts). On the other hand, under low pH, the vapour phase would contain significant amount of acids besides steam. This would raise a need for additional condensation or distillation unit to separate acids and evaporated water. In addition, the efficiency of steam utilization can be lower in this case. More comprehensive alternative can be as shown in Figure 4, water evaporation followed by crystallization unit at high pH to recover the salt(s) and then another process unit where pH is reduced by adding acid (such as HI) to recover acids. The downstream of partial wet oxidation is at high pH, which is favorable for the recovery process. This stream can be concentrated by water evaporation in a flash (or multistage flash) and can go to the crystallizer. The crystallizer can recover sodium salts and high pH prevents acid in vapour form. Then, acids can be recovered by lowering pH. For the
current, sodium acetate/formate system, the crytallizer can recover sodium acetate at high pH (8-9) and 90 ºC (in anhydrous form). Then, lowering pH in another unit would provide the recovery of formic acid. In addition, the outlet steam of flash and crystallizer can be utilized in the parts of pulping process which require energy or in a CHP plant to produce electricity.
software simulations (such as Aspen Plus) uses very complex models for activity coefficients and physical properties. Therefore, it can be challenge to provide necessary input data for those simulations. This study can be extended by adding other major organic salts (sodium lactate and glycolate) and carbonate system the feed composition, and investigating suitable activity coefficient model. It can be observed that pH has the major influence and temperature has minor effects. In addition, water evaporation is required to enable the salt recovery. Temperature can be selected based on energy integration situation in the whole process. However, different pH values are required for salt recovery and acid recovery. These results lead to a recovery process which involves first separation of salts at high pH and forming acids by lowering pH. In addition, preventing acid in vapour form is important for more efficient utilization of outlet steam. REFERENCES
Figure 4. A preliminary process block diagram 5. CONCLUSION Integrated biorefinery concepts are getting more important to produce biofuels, energy and chemicals. Investigating alternative utilization of black liquor is crucial part of this field due to its lignocellulosic content. Partial wet oxidation is able to produce organic acids or salts. Therefore, this study investigates the crystallization of two of the major salts with respect to pH, temperature and the prior water evaporation rate. This study presents relatively simple model for species distribution and it can be improved easily. In contrast, the
Dorn, J., Steiger, 2007. Measurement and calculation of solubilities in the ternary system NaCH3COO+NaCl+H2O from 278 K to 323 K. J. Chem. Eng. Data, 52(5), 1784 Fournier, P., Oelkers, E. H., Gout, R., Pokrovski, G., 1998. Experimental determination of aqueous sodium-acetate dissociation constants at temperature from 20 to 240°C. Chemical Geology, 151(1-4), 69 Kim, M. H., Kim, C. S., Lee, H. W., Kim, K., 1996. Temperature dependence of dissociation constants for formic acid and 2,6-dinitrophenol in aqueous solutions up to 175°C. Journal of the Chemical Society, Faradat Transactions, 92, 4951 Mudassar, R., Melin, K., Sarada, K., Koskinen, J., Hurme, M., Kallas, J., 2012. Novel treatment method for black liquor and biomass hydrolysate by partial wet oxidation. Proceedings of the 4th International Symposium on Energy from Biomass and Waste, Venice, November 12-15.2012. Yaws, C. L., 2012. Yaws' Handbook of Properties for Aqueous Systems. Knovel
