Ind. Eng. Chem. Res. 1992, 31, 1574-1577
1574
RESEARCH NOTES Thermal Decomposition of Dilute Aqueous Formic Acid Solutions The aqueous-phase oxidation of formic acid and formate has been studied in a batch autoclave reador at 260 "C and 2 MPa of 02.The formate is converted to bicarbonate whereas formic acid, besides oxidation, decomposes by a t least two different routes, namely a dehydration or a decarboxylation. In particular the second one is dependent on the reactor vessel used. It is shown to be catalyzed by a mixture of oxides of stainless steel components. The presence of CH&OOH or CH3CH0 promotes the decomposition of HCOOH by way of both decarboxylation and oxidation. In any case formic acid is a relatively short-lived intermediate in the wet-oxidation process.
Introduction The recent years have seen a growing interest in wet oxidation as a method for decomposing organic pollutants. The main reaction products are C02and H20,but a certain amount of organics (measured as COD, chemical oxygen demand) remains in the form of refractory organic compounds among which formic acid is sometimes mentioned on equal terms with acetic acid (Baillod and Faith, 1983; Randall and Knopp, 1980). This contradicts what is stated elsewhere about the stability of formic acid (Barham and Clark, 1951; Mars et al., 1963; Kisfaludi et al., 1988). The last-mentionedresults relate, however, to experiments with gaseous reactants. We have therefore found it worthwhile to try to elucidate the decomposition of dilute aqueous solutions of formic acid under varying conditions. Experimental Section Figure 1 shows the apparatus used. It has been described previously (Smensen et al., 1990), but the main features are repeated here. In a cylindrical vessel an impeller mounted on the lid pumps the liquid through a pipe loop. The autoclave is mounted on a rack, which makes it possible to adjust the temperature by raising or lowering it in a heating bath. After the prescribed reaction time has elapsed the autoclave is moved to a water bath for cooling. Due to the excellent heat-transfer conditions the heating and cooling periods are short as shown in Figure 2. A gas valve is provided, but for liquid sampling the lid must be removed. Two specimens of this autoclave have been manufactured from different materials: One (A) was made of Uddeholm acid-resistant steel UHB 24 (17.2% Cr, 11.1% Ni, 2.7% Mo) which is similar to stainless steel 316. It was somewhat corroded from earlier use. The other (B) was made of Sandvik Sanicro 28 (27% Cr, 31% Ni, 3.5% Mo, 1% Cu) and was quite new at the beginning of these experiments. When closed, the free volume is 1920 mL in autoclave A and 1890 mL in autoclave B. The reactor was charged with 1000 mL of deionized water containing the organic reactants in solution. Formic acid (98-100%), acetic acid (96%), and sodium formate, all of Merck analytical grade, and acetaldehyde, Fluka purum, were used. The pertinent gas-phase O2or N2was supplied from ordinary cylinders. After each experiment the gas phase was analyzed by gas chromatography-mass spectrometry (GC-MS). The residual acids were determined on a Dionex 4000 ion chromatograph. Oxidative Decomposition. The decomposition of
Table I. Conditions under Oxidative Decomposition reaction exPt autoclave weight of organic reactants time, min 1 A 6.2 g of sodium formate 10, 30, 60 2 A 6.1 g of formic acid 10, 20, 30 3 B 6.1 g of formic acid 10, 20, 30 B 6.1 g of formic acid-6.3 g of acetic 10, 20, 30 4 acid 5 B 4.88 g of formic acid-2.35 g of 10, 20, 30 acetaldehyde Table 11. Formate Remaining after Wet Oxidation time at 260 "C,min 0 10 30 60 HCOO, mg/L 4200 2800 1540 480
formic acidlformate performed by the reaction with molecular O2 is described by the following reactions: HCOOH + 0.502 CO2 + HzO (1)
-
+
HCOO- + 0.502
HCOC
(2)
Two experiments were performed to compare the formic acid and formate. Since there were indications that the course of the reaction differed in the two autoclaves, another experiment was performed with formic acid to examine the catalytic effect of the reactor walls. Finally, two experiments were carried out to investigate the possible influence of other components present as found by earlier workers (Friedhofen et al., 1980). Acetic acid and acetaldehyde, which frequently occur in practice, were chosen for this purpose. In all experiments the autoclave was operated at 260 "C. The gaseous reactant charged was 1.9 MPa of O2 added to the initial 0.1 MPa of atmospheric air. The other conditions are given in Table I. After the reaction times indicated, the reactor was cooled to 30 "C, at which temperature gas and liquid samples were taken. If the experiment needed to be continued, the operation was repeated for yet another period, each time with a new gas charge of 1.9 MPa of 02.The reaction times cited represent the period at which 260 "C was maintained, neglecting the heating and cooling periods (Figure 2). This is justified by the rapid fall in reaction velocity with temperature. In experiment 1 no gaseous component evolved. The pH rose from 7.2 to 8.1 owing to the transformation of formate to bicarbonate (equation 2). The remaining formate is shown in Table 11. Experiments 2 and 3 illustrate the effect of the reaction vessel. A considerable amount of gas is produced as seen
OSS8-5885/92/2631-1574$03.0~/00 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 6,1992 1576 Table 111. Analysis Results from Experiments 2 and 3, Table I gas composition, % reaction time, min N2 0 2 Ar H2 Experiment 2 10 3.8 86.7 0.1 0.82 20 4.1 92.8 0.1 0.17 30 3.5 95.7 0.09 co.01
co
CO2
HCOOH left in liquid, mg/L
0.09 0.03 co.01
8.4 2.8 0.7
995 43 7
Experiment 3 0.88 0.37 0.48
0.1 0.1 co.01
3.0 5.0 2.0
4200 870 330
10 20 30
3.0 5.5 2.8
92.8 87.8 94.5
0.19 0.17 0.18
Table IV. Analysis Results from Experiment 4 reaction time, min 10 20 30
Nz 5.4
gas composition, % Ar H2 0.2 2.0
0 2
88.2
co
acids left in liquid, mg/L HCOOH CH&OOH 3200 6275 150 5800 C25 5600
COZ 3.9
0.3
sample containers leaky
0
20
40
60
80
(M) Figure 3. Test of pseudo-first-orderreaction. (0) Formate, autoformic acid, autoclave B. clave A; (v)formic acid, autoclave A; (0)
Table V. Gas Composition (014) after Experiment 5 reaction N, 0, Ar H, co time,min 1.0 84.4 0.20 1.0 10 3.8 93.1 0.19 0.14 20 3.2 0.1 30 3.2 95.6 0.19 CO.01 CO.01 Table VI. HCOOH, CH&HO, and CH&OOH Concentrations (mg/L) after Experiment 5 reaction time, min HCOOH CHICOOH 10 520 1300 20 11 1540 30 11 1470
A
Y- 200 w U
3
2 a 0"
3
100
I-
0
2
4
6
8
1
0
1
2
1
4
TIME (rnin)
Figure 2. Heating and cooling profile.
from Table 111. As an example, we will draw up the mass balance for experiment 2. Initially, the 0.92-L gas volume at 20 OC equals 0.0383 mol of atmospheric air with 78.3% N2. Since the nitrogen is untouched by the reaction, it can be calculated from the analysis after 10 min that the autoclave contains 0.788 mol of gas. The percentages of COz
cot 9.5 3.4 1.0
CH&HO 25 c2
