Ind. Eng. Chem. Res. 1997, 36, 2375-2379
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Extraction of Formic Acid from Sodium Formate Liang Hu* and Adeyinka A. Adeyiga Department of Chemical Engineering, Hampton University, Hampton, Virginia 23668
This paper presents the laboratory results of extraction of formic acid from the reaction of sodium formate with carbon dioxide: HCOONa(w) + CO2(g) + H2O(w) + Am(o) a NaHCO3(s,w) + HCOOHAm(o). The reaction was operated at room temperature. Sodium formate (aqueous solution) was reacting with carbon dioxide in the presence of organic tertiary amine. The reaction can be divided into two steps: HCOONa(w) + CO2(g) + H2O(w) a NaHCO3(s,w) + HCOOH(w) (1); HCOOH(w) + Am(o) a HCOOHAm(o) (2). In the first step, sodium bicarbonate and formic acid were formed. Produced formic acid was immediately extracted into an organic phase by tertiary amine as expressed in step two. When proper diluents were used, solid sodium bicarbonate occurred in an aqueous phase. The experimental investigation showed that temperature, pressure, and concentration of sodium formate affected extraction equilibrium. But diluents and their ratio with tertiary amine affected extraction equilibrium significantly. It was found that the solvation of formed complex HCOOHAm(o) in diluents has a great influence on the extraction of formic acid by tertiary amine. The high polarity of the diluent and the low solvophobic property of the diluent with tertiary amine are beneficial to the extraction of formic acid. Introduction Use of a decomposition-extraction method to replace an acidic anion in salt with bicarbonate ion HCO3- has been reported since the early 80’s (Toyo Soda Mfg. Co. Ltd., 1982; Ninane et al., 1984). The outstanding work reported by K. Kosswig and F. V. Praun (1983) can be described by the following reaction:
NaCl(w) + CO2(g) + H2O(w) + Am(o) a NaHCO3(w,s) + AmHCl(o) Aqueous solution of sodium chloride reacted with carbon dioxide in the presence of extractant organic amine, the acidic anion Cl- in NaCl was replaced by HCO3-. Produced HCl was extracted by organic amine into an organic phase. Reported organic amine used was trioctylamine. The purpose of this research was to develop a new route for the manufacture of soda. The key for the success of the process is to select the proper organic phase which included extractant and diluent. Hissamoni et al. (1980) reported that aqueous solution of potassium chloride reacted with carbon dioxide produced potassium bicarbonate and compound AmHCl. The reaction can be expressed as follows:
KCl(w) + CO2(g) + H2O(w) + Am(o) a KHCO3(w,s) + AmHCl(o) This paper presents the laboratory results of extraction of formic acid from the reaction of sodium formate with carbon dioxide:
NaOOCH(w) + CO2(g) + H2O(w) + Am(o) a NaHCO3(w,s) + AmHOOCH(o) Sodium formate (aqueous solution) reacted with carbon dioxide in the presence of organic tertiary amine. Produced formic acid was immediately extracted into * Author to whom all correspondence should be addressed. Phone, 804/727-5530; Fax, 804/727-5189; e-mail, leon@ et.hamptonu.edu. S0888-5885(96)00544-1 CCC: $14.00
an organic phase. Utilized tertiary amine was N235, a mixture of C7-C12 tertiary amine R3N. Experimental Section The experiments of this research consisted of two parts, which used different equipment. In the first part, determination of the influence of diluents on the extraction of formic acid, the experiments were carried out by using a set of separation funnels. The separation funnels were made of glass with the volume of 25 mL. The liquid samples were filled into funnels, which merged into a water bath. The water bath temperature was controlled by a thermostat. The water bath temperature error was less than (0.1 °C. When the liquid sample temperature reached the experimental temperature, the separation funnels were well shaken. They were then settled for phase separation. Both phases after separation were analyzed. The initial concentration of formic acid in the aqueous solution was 0.43 mol/ L. The volume of an organic phase was 10 mL. The phase ratio, organic:water, was 1:10. The concentration of formic acid in both phases was titrated by sodium hydroxide. The second part of the experiment which measured the extraction of formic acid from sodium formate used a stirring cell. The structure of the cell is shown in Figure 1. The stirring cell is made of glass with 100mm inner diameter and 130-mm depth. Two agitating blades, one for liquid, one for gas, were driven by a direct current motor. The agitating speed was monitored by a laser meter for rotating rate. The experimental apparatus is shown in Figure 2. Carbon dioxide from cylinder 1 past through buffer bottle 3 and pressure stable tube 4. Gas flow rate was controlled and measured by rotating flow meters 5 and 8. The gas clean system consists of two U-tubes. U-Tube 6 was filled with silicon gel. U-Tube 7 was filled with active carbon. Gas was saturated with moisture in bottle 9, which contained the same solution as that in stirring cell 14, and then absorbed in stirring cell 14. Gas was measured by foam film flow meters 10 and 12 before and after reaction cell 14. The difference between the two flow rates is the gas absorption rate. After measurement, gas was released. The absorption rate of gas, carbon dioxide, at time t was determined by the difference of two flow rates, in © 1997 American Chemical Society
2376 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997
and dissociation of the pair of extracted ions would not occur. When diluent is a solvent with high dielectric constant, the pair of extracted ions which had been extracted by tertiary amine could dissociate in the solvent (diluent):
R3NH+A- a R3NH+ + Awith dissociation constant Kdiss
Kdiss )
Figure 1. Stirring cell.
Figure 2. Experimental apparatus: 1, CO2 cylinder; 2, regulator; 3, buffer bottle; 4, pressure stable tube; 5, 8, rotating flow meters; 6, silicon gel U-tube; 7, active carbon U-tube; 9, saturator; 10, 12, foam film flow meters; 11, motor; 13, liquid feed funnel; 14, stirring cell; 15, thermostat; T1, T2, T3, thermometers.
and out of the stirring cell, with two foam film flow meters. As a result of the measurement, the relationship of absorption rate r and time t would be obtained. By integration of absorption rate with time, r-t, the total amount of carbon dioxide absorbed into the liquid phase can be obtained. By stoichiometry, the amount of formic acid which extracted into the organic phase can be calculated. The equilibrium formic acid concentration in the organic phase was titrated by sodium hydroxide standard solution. The mass balance of formic acid from integration of r-t data and the titration of the organic phase should match well. If the error is higher than 5%, the data are discarded. Results and Discussion 1. Influence of Diluents on Extraction of Formic Acid. The experimental results of the extraction of formic acid by tertiary amine with different diluents are shown in Table 1. As seen, the influence of diluents is significant. Early research showed (Sekinene, T.; et al., translated by Teng, T., 1981) that acid extracted by tertiary amine existed in diluent in the form of ion-ion attraction. The extraction equilibrium can be expressed as follows:
R3N + N+ + A- a R3NH+Awith equilibrium constant Kex
Kex )
[R3NHA] [R3N]RHA
where RHA is acid activity in water. When the diluent is a solvent with low dielectric constant, the association
[R3NH+][A-] [R3NH+A-]
Obviously, solvation of R3NH+A- in diluent is a key for the distribution ratio of acid extraction by tertiary amine. When the diluent is in favor of the solvation of R3NH+A-, the distribution ratio of acid extraction by tertiary amine would be high, otherwise it would be low. So, the dielectric constant or polarity of diluent plays a very important role in the distribution ratio of acid extraction. The higher the dielectric constant, the higher the solvation of R3NH+A-, the higher the extraction distribution ratio. For a tertiary amine with high hydrophobic property, for instance, the carbon number more than 7, tertiary amine would be solvophobic for water. When diluent is highly soluble in water, this would prevent complex R3NHA from solvation and finally result in a decrease of the distribution ratio of formic acid extraction. On the basis of the above explanation, for the extraction of formic acid by tertiary amine N235, the chemical bond between anion (HCOO-) and cation (R3NH+) of complex R3NH+OOCH- in the organic phase is an electrostatic interaction. So, the extraction equilibrium highly depends on the dielectric constant and polarity of a diluent. The higher the dielectric constant (high polarity) is, the lower the potential energy of two interacting charges, the more stable the complex R3NH+A- in diluent, and the higher the distribution ratio of extraction. However, tertiary amine N235 is a highly hydrophobic compound. When diluent is partially soluble in water, the original diluent will have different characteristics from the newly formed diluent (original diluent + water), which repulses tertiary amine N235 because of the insolubility of N235 with water. Tertiary amine N235 is solvophobic with newly formed diluent. The solvophobic action of tertiary amine N235 to the newly formed diluent will limit the solvation of complex R3NH+OOCH- and increase the energy potential of the two interacting charges and, furthermore, reduce the distribution ratio of extraction of formic acid by tertiary amine N235. Influence of Polarity, Dielectric Constant, Solvophobicity, and Hydrogen Bond. Table 2 lists the distribu tion ratio of extracting formic acid with N235 and different alcohols as diluent. It can be seen that the distribution ratio increases with increasing carbon chain of the alcohol. To explain this, we used the solvophobic action. As the carbon number of the alcohol increased, the hydrophilicity of the alcohol decreased. Even though the dielectric constant of the alcohol decreased, the solubility of water in the diluent decreased, which reduced the solvophobicity of N235 with diluent and increased the solvation of complex R3NH+OOCH- in diluent. Finally, the distribution ratio of extraction was increased. But a further increase of carbon chain will rarely affect the solubility of water in diluent (alcohol). So, the influence of solvophobicity on extraction equilibrium was very small, while the polarity of diluent
Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2377 Table 1. Distribution Ratio of Extracting Formic Acid with Tertiary Amine N235 + Diluents (25 °C)
no.
N235, vol %
diluents
diluent, vol %
1 2 3 4 5 6 7 8
50 50 50 50 50 50 50 50
9
50
10
50
11
50
12 13 14 15
50 40 65 50
2-ethylhexyl alcohol isopropyl alcohol butanol decanol chloroform methyl isobutyl ketone cyclohexanone nitrobenzene toluene nitrobenzene benzene nitrobenzene benzene nitrobenzene benzene toluene tributyl phosphate tributyl phosphate tributyl phosphate ethanol kerosene
50 50 50 50 50 50 50 15 35 20 30 25 25 10 10 30 50 60 35 20 30
Table 2. Distribution Ratio of Extracting Formic Acid with N235 + Alcohol (25 °C)
diluents
solubility of water in diluent
dielectric const
distribution ratio
isopropyl alcohol butanol 2-ethyl hexyl alcohol decanol
20.5 very little very little
18.3 17.1 8.1-17.1 8.1
2.03 14.4 45.6 39.7
Table 3. Influence of Solvophobic and Polar Action on Distribution Ratio (25 °C)
diluents
solubility of water in diluent
dielectric const
distribution ratio
chloroform methyl isobutyl ketone cyclohexanone
0.061 1.8-2.2 8.0
4.806 13.11 18.3
6.25 7.17 2.51
Table 4. Influence of Hydrogen Bond of Diluent on Distribution Ratio (25 °C)
diluents
solubility of water in diluent
dielectric const
distribution ratio
butanol cyclohexanone
20.5 8.0
17.1 18.3
14.4 2.51
played the important role. This is why 2-ethylhexal alcohol has a higher distribution ratio than decanol. As above, Table 3 showed that the distribution ratio of extraction of formic acid was not increased with the increase of the polarity of the diluent because of the influence of solvophobicity. Besides polarity and solvophobicity, some other interaction between solute and solvent affected the solubilization of complex R3NH+OOCH-, such as hydrogen bond. Table 4 showed that butanol had a higher distribution ratio than cyclohexanone, even though butanol had a higher water solubility than cyclohexanone and they had similar dielectric constants. The reason is that butanol formed a hydrogen bond with complex R3NH+OOCH-, which was helpful for the solvation of complex R3NH+OOCH- in diluent. In general, the higher the polarity, the lower the solubility of water in diluent, and some kinds of interactions between solute and diluent which increase the solvation of complex R3NH+OOCH- increase the distribution ratio of extraction of formic acid. Ratio of Extractant with Diluent. Table 5 showed that the influence of different ratios of extractant N235 with
concn of formic acid in aq phase, mol/L
concn of formic acid in org phase, mol/L
distribution ratio
0.00743 0.03885 0.01916 0.00941 0.0261 0.0247 0.0339 0.0283
0.3388 0.07887 0.2759 0.3736 0.1631 0.1771 0.0851 0.1409
45.6 2.03 14.4 39.7 6.25 7.17 2.51 4.98
0.0186
0.2381
12.3
0.0176
0.2482
14.1
0.0306
0.1181
3.86
0.02321 0.01487 0.01632 0.03550
0.02351 0.1544 0.1446 0.1123
9.27 10.4 8.6 3.16
Table 5. Influence of Different Ratios of Extractant N235 with Diluent on Distribution Ratio (25 °C) concn concn of formic of formic 2-ethylhexyl acid in acid in amine, alcohol, kerosene, aq phase, org phase, distribution vol % vol % vol % mol/L mol/L ratio organic phase composition
50 40 30 20 50 50 50 100 80 70 60
50 50 50 50 40 30 20 20 30 40
10 20 30 10 20 30
0.00733 0.00875 0.0120 0.0181 0.0100 0.0139 0.0193 0.0323 0.0166 0.0113 0.00855
0.3399 0.3257 0.2932 0.2322 0.3132 0.2742 0.2202 0.08966 0.2463 0.3002 0.3277
46.4 37.2 24.4 12.8 31.3 19.7 11.4 2.78 14.8 26.6 38.3
diluent on the distribution ratio of formic acid extraction from aqueous solution. From Table 5, it can be seen that the distribution ratio of extraction of formic acid is highest at the extractant:diluent volume ratio of 1:1. The results can be explained by the concentration of R3N and the solvation of complex R3NH+A-. High R3N concentration is in favor of the extraction of formic acid. However, high R3N concentration limited the solvation of complex R3NH+A-. As a results, a maximum distribution ratio will appear. 2. Extraction of Formic Acid from Sodium Formate. The extraction of formic acid from the reaction of sodium formate with carbon dioxide can be described as following:
HCOONa(w) + CO2(g) + H2O(w) + Am(o) a NaHCO3(s,w) + HCOOHAm(o) Obviously, when the organic phase has been determined, the equilibrium will be affected by the following factors: temperature, pressure, and concentration of sodium formate. Temperature. The influence of temperature on the equilibrium of extraction of formic acid from sodium formate is shown in Figure 3. It can be seen that the amount of formic acid extracted from sodium formate at 15 °C was 2.5 times more than that at 40 °C. This mean low temperature is in favor of extraction. Furthermore, low temperature reduced the solubility of NaHCO3 and made more NaHCO3 crystal. But low temperature also limited the solubility of sodium for
2378 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997
Figure 3. Influence of temperature on the equilibrium of extraction. PCO2 ) 1 atm; phase ratio (o/w) ) 8; [NaOOCH]ini ) 7.35 mol/L. Organic phase composition: tertiary amine N235, 50% (vol); 2-ethylhexyl alcohol, 50% (vol).
Figure 6. Relationship of equilibrium constant and temperature.
Concentration of Sodium Formate. Figure 5 showed the relationship of the concentration of OOCH- in the aqueous phase and in the organic phase. The results showed that the higher the OOCH- concentration in the aqueous phase, the higher the OOCH- concentration in the organic phase. The more OOCH- was extracted into the organic phase, the more sodium bicarbonate was produced. So, high concentration of sodium formate will be beneficial for the extraction of formic acid. Extraction Process Enthalpy. From Gibbs-Helmholtz equation
| ) -S
(1)
| ) -∆S
(2)
∂G° ∂T
p
we have Figure 4. Influence of partial pressure of carbon dioxide on the equilibrium of extraction. T ) 16 °C; phase ratio (o/w) ) 1; [NaOOCH]ini ) 7.35 mol/L. Organic phase composition: tertiary amine N235, 50% (vol); 2-ethylhexyl alcohol, 50% (vol).
∂∆G° ∂T
p
Because ∆G° ) ∆H° - T∆S° and ∆G° ) -RT ln K
| ) -RT ln K - RT ∂ ln∂TK|
∂∆G° ∂T
-∆S° )
p
p
∆H° ∆G° - ∆H° ) -R ln K T T
(3) (4)
Substituted eqs 3 and 4 into eq 2, we have
| ) RT∆H°× T
∂ ln K ∂T
Figure 5. Influence of the initial concentration of NaOOCH on the equilibrium of extraction. T ) 25 °C; PCO2 ) 1 atm; phase ratio (o/w) ) 8. Organic phase composition: tertiary amine N235, 50% (vol); 2-ethylhexyl alcohol, 50% (vol).
mate which will reduce the amount of extraction of formic acid. Pressure. The influence of partial pressure of carbon dioxide on the equilibrium of extraction of formic acid is shown in Figure 4. It can be seen that higher partial pressure of carbon dioxide will increase the extraction of formic acid from its salt solution. The partial pressure of carbon dioxide increased from 1 atm to 5 atm; the concentration of formic acid extracted into the organic phase increased about 1.7 times. Figure 4 shows the relationship between the pressure of carbon dioxide and the concentration of formic acid in organic phase. When the partial pressure of carbon dioxide is higher than 16 atm, the increase of formic acid concen tration in the organic phase with partial pressure of carbon dioxide becomes slow. The increase of the partial pressure of carbon dioxide will be beneficial to the extraction of formic acid. But high pressure will cost more energy and equipment investment.
p
(5)
Equation 5 expresses the relationship between the equilibrium constant and temperature. If the extraction enthalpy remains constant in the range of temperature measurement, the relationship of ln K and T will be linear. The slope of the line will be ∆H°/RT/T. If ∂ ln K/∂T|p < 0, i.e., ∆H° < 0, the extraction process is exothermic. If ∂ ln K/∂T|p > 0, the extraction process is endothermic. The extraction process enthalpy ∆H° ) slope × R × T × T. Figure 6 shows the relationship of ln K and T. From that, we obtained ∆H° ) -14.30 kcal/ mol. The extraction process is exothermic. Conclusion (1) It is possible to extract formic acid by using tertiary amine N235 with proper diluents from the reaction
NaOOCH(w) + CO2(g) + H2O(w) + Am(o) a NaHCO3(w,s) + AmHOOCH(o) (2) The extraction of formic acid with tertiary amine N235 is affected significantly by the diluent. In general, high polarity and dielectric constant, low solvophobicity of N235 with diluent, and formation of a special interac-
Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2379
tion such as hydrogen bond is in favor of extraction of formic acid. (3) The influence on the extraction equilibrium of the temperature, partial pressure of carbon dioxide, and initial concentration of sodium formate has been measured. The experimental results showed that the low temperature, high partial pressure of carbon dioxide and high initial concentration of sodium formate are in favor of the extraction of formic acid from the reaction:
NaOOCH(w) + CO2(g) + H2O(w) + Am(o) a NaHCO3(w,s) + AmHOOCH(o) But low temperature will reduce the solubility of sodium formate. (4) The extraction process is exothermic. Notation (w) ) water phase (o) ) organic phase (s) ) solid phase (g) ) gas phase [ ] ) concentration R ) activity r ) reaction rate t ) time G ) Gibbs energy
H ) enthalpy K ) equilibrium constant P ) pressure R ) gas constant S ) entropy T ) temperature V ) volume
Literature Cited Hissamori, et al. Technol. Rep. Kansai Univ. 1980, 21, 75. Kosswig, K.; Praun, F. V. Sodium carbonate and hydrogen chloride from sodium chloride and carbonic acidsa new process. Chem. Econ. Eng. Rev. 1983, 15, 6, 30. Ninane, L.; Breton, C.; Guerdon, C. Device and method for producing an organic solution of a water-insoluble organic base. Ger. offen. 3,415,303, 1984. Taziya, S.; Yogo, H. Solvent extraction chemistry, Teng, T., translator; Atomic Energy Publishing House: Beijing, China, 1981. Toyo Soda Mfg. Co. Ltd. Sodium bicarbonate. Jpn. Kokai Tokkyo Koho 57-7826, 1982.
Received for review September 3, 1996 Revised manuscript received February 7, 1997 Accepted February 10, 1997X IE9605443
X Abstract published in Advance ACS Abstracts, April 1, 1997.
