Ind. Eng. Chem. Res. 1988,27, 1242-1245
1242
Influence of Sodium Carbonate/Vanadium Concentration Ratios on Vanadate (V) Equilibria and on the Reoxidation of V(1V) in a Hydrogen Sulfide Removal Process Sonet Vermaire* and Rob de Haan Sastech, P.O. Box 1, Sasolburg 9570, RSA
The reoxidation rate of V4+and the reaction order in [V4+]in a H2Sremoval process were determined at different alkalinities. In addition, the V5+species present in the oxidized solutions were determined with 51VNMR. The latter results helped in interpreting the kinetic results. The order in [V4+] was found to be 1 in 0.57 M Na2C03with a rate constant of 2.22 X s-l. At this alkalinity and a t 40 "C, the only V5+species present was the dimeric species [H,V207](4-n)-.As the total alkalinity (expressed as M Na2C03)was lowered to 0.47,0.38, and 0.28, a second concurrent reoxidation reaction occurred which coincided with the appearance of the cyclic tetramer V4012-in the oxidized liquor. Thus, a high carbonate-to-vanadium ratio stabilizes the dimer as opposed to the tetramer which is predominant a t p H 8.5 at low carbonate concentrations. When reduced, the dimer reoxidized faster than the V4+ species from V4O1;-. A first-order dependency of the reoxidation reaction on carbonate concentration was also found. Low concentrations of hydrogen sulfide are commonly removed from low-pressure sour gas streams by liquidphase redox processes using transition metals, e.g., Sulfolin, Stretford, LO-Cat, Shafer, etc. (Weber, 1986; Hammond, 1986; Shafer, 1983; Hardison, 1980). The Sulfolin process currently in use at Sasol2 and 3 in South Africa to clean Rectisol off-gas uses vanadium in an aqueous alkaline solution for the oxidation of hydrogen sulfide to elemental sulfur (Weber, 1983). The reaction sequence (unbalanced) is absorption: H2S + CO2-
-
H2S oxidation:
HS-
-
HS- + 2v5+ regeneration: 2v4+
H2S
2v4++ s i
-+
+ o2
total reaction:
+ 7202
+ HC03-
2v5+
S
(1)
(2)
(3)
HzO
The reduced solution is reoxidized with air in the presence of an organic nitrogen compound, diethanolamine (DEA) to facilitate reoxidation. However, it was recently found that reoxidation in the absence of DEA is as efficient if the alkalinity is increased (de Haan et al., 1986). As the reoxidation step is the rate-limiting step in this process, it is important to establish the most favorable conditions to facilitate the reoxidation of the reduced liquor. The objective of this study was to determine the influence of total alkalinity on the reoxidation rate of V4+and on the equilibria of different V5+ species in the redox system. The technique used for the study of V5+ species was FT-NMR spectroscopy. Although the vanadate equilibria have been studied extensively (O'Donell and Pope, 1976; Howarth and Richards, 1965; Heath and Howarth, 1981; Habayeb and Hileman, 1980; Petterson et al., 1983), they had not yet been studied under conditions of high carbonate concentrations, as exist in a hydrogen sulfide removal process. Experimental Section (a) Kinetic Experiments. Elvan K sodium ammonium vanadate was used as received from the suppliers to pre0888-5885/88/2627-1242$01.50/0
pare solutions in distilled water with 0.04 M V5+and 0.25 M NaSCN, and alkalinities are expressed as 0.095, 0.28, 0.38, 0.47, and 0.57 M Na2C03. The oxidation experiments were performed batchwise. The solutions with V5+were reduced with a CO2/H2Sgas mixture at varying times to achieve different V4+ initial concentrations. After reduction, pure COz was bubbled through the solution for 30 min to ensure that no unreacted HzSremained. Oxidation was carried out with an air/COz mixture because air alone tended to strip COz from the solution and change the pH. The air-tO-CO, ratio was kept constant throughout the experiments and was sufficient to keep the pH constant. The gases were introduced through porous disks and further distributed in the mixture by agitation with a magnetic stirrer. The pH (8.4), dissolved oxygen content (3.5-5.5 mg/L), and temperature (38 "C) were kept constant throughout the experiments. The dissolved oxygen content was measured with a YSI oxygen meter and probe. Samples of 1 mL were withdrawn with syringes and analyzed polarographically for V5+. As all the air was expelled from the syringes, samples were stable for at least 3-4 h. The V4+ concentration was obtained from the difference between the total vanadium concentration and the V5+concentration. (b) Spectroscopy. 51VNMR spectra were recorded at 52.6 MHz using a Varian VXR2OO instrument. Spectra were recorded at 23 "C unless otherwise stated. The pH of the samples was adjusted to 8.5 if required by bubbling COz through the sample. Capillary liquid VOC13 was used as the standard. Chemical shifts are reported in ppm with negative values in the low-frequency direction relative to VOC13. Results and Discussion The discussion is comprised of two parts. The first deals with the influence of the carbonate/vanadium ratio (C/V) on the vanadium species present, and the second relates the V5+equilibria to the kinetic results. Although carbonate is mostly present as bicarbonate at pH 8.5, the total alkalinity will be expressed as M NazC03for convenience. (a) 51VNMR Results. Various 51V NMR studies (O'Donell and Pope, 1976; Howarth and Richards, 1965; Habayeb and Hileman, 1980; Petterson et al., 1983) of vanadates in aqueous solutions showed that the [(V03!,]"species is predominant at a pH of ca 8.5 and at a vanadium 0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 1243 Table I. slV NMR Assignments at a DH of ca. 8.5 vanadium (V) species C/V HV02V2074HVzc17~v4013~ (VO&nn537
2.4 7.0 11.8 14.0
562
554 562.8 562.8
566-574
576.4 572 574 574 578.4 578.0 577.4 577.1
569.7 568.7 568.7
564-565
v5015~
V6017~-
584.5 582 586.8 585.7
reference Petterson et al., 1983 O'Donell and PoDe. 1976 Habayeb and Hiieman, 1980 Howarth and Richards, 1965 this work this work this work this work
a n = 3.4.
concentration of M. Most authors identified the species as the cyclic tetramer V4012-, while Habayeb and Hileman concluded that the resonance at 576 ppm is due to the tetrameric and trimeric (V3O9%)species, with a fast rate of interconversion between them. The NMR assignments in this work are listed in Table I, together with data from the literature. We are in general agreement with the assignments made by the various authors whose results also show fair agreement with one another. As the solution used in the commercial H2S scrubbing process contains NaSCN, it first had to be established whether the addition of NaSCN had any effect on the vanadate equilibria. A comparison of 51V NMR spectra of samples with and without NaSCN revealed that no shifts or no new peaks appeared with added NaSCN. The only effect was that the 562 ppm resonance was slightly smaller in the sample without NaSCN. All work reported here was done with 0.25 M NaSCN in the liquor. 51V NMR spectra of the experimental solutions (see Experimental Section (a)) were run at a pH of 8.5. At a carbonate/vanadium ratio (C/V) of 2.4, the major resonance was at 578.4 ppm with small peaks at 554.0, 569.7, and 586.8 ppm. The 578 ppm resonance, which is ascribed to the cyclic tetramer (V4O12-),is also the major resonance in aqueous solutions containing no carbonate; Petterson et al. (1983) found that it comprised ca. 70% of the total vanadium at [VI = 0.04 M, and Habayeb and Hileman (1980) reported that it was 87% at [VI = 0.25 M. As the C/V was increased, the peak at 562 ppm, ascribed to the dimer, increased relative to the 578 ppm resonance (see Figure 1)and the broadened peaks indicate exchange between the cyclic tetramer and the dimer according to V40,24-+ 20H-
-
2HVz0,3-
(4)
Because of the overlap of the peaks, the different species dould not be quantified by integration, but Figure 1 shows clearly the change in area and shape of the peaks with increasing C/V. At a C/V of 14, the dimer has moved closer to the tetramer (564-565 ppm) and the small peak at 569 ppm (HV4013~) has disappeared. The peak at 586 ppm which appears at lower C/V's was assigned by Petterson et al. (1983) to V50155-,but Habayeb and Hileman (1980) advanced convincing arguments that the resonance was due to the hexamer, V601,4-. Results from this work tend to agree with the assignment to the hexamer, because at the disappearance of this peak no monomer is formed which would be expected if the pentamer dissociated: V50155- + OHv4012*- + HV0:(5)
-
The dimer and tetramer could be formed by the dissociation of the hexamer: V60174-+ 20H- + C 0 3 2 V,OlZ4-+ HV20,3- + HC03- (6)
-
1
I
562,7
I
I
1
582,7
PPm Figure 1. 51V NMR spectra of liquors with sodium carbonate to vanadium concentration ratios (C/V) of (a) 14, (b) 12, (c) 7, and (d) 2.4.
51V NMR spectra run at increasing temperatures showed that in a solution with a C/V of 14 the tetramer rapidly converts to the dimer, so that evan at 40 O C very little tetramer is left (Figure 2). The peak at 561 ppm (30 "C) gradually shifts to 553 ppm (80 "C) with increasing temperature. This indicates deprotonation of the dimer to give VZOT4-.At a C/V of 7 (Figure 31, all the species present convert to the dimer, but the dimer is less stabilized than at a C/V of 14, because even at 80 "C some tetramer is left. Once again the 561 ppm resonance shifts downfield with increasing temperature. The results indicate that in a buffered solution of pH 8.5 a high carbonate-to-vanadium ratio stabilizes the resonance at 562 ppm, which is ascribed to the dimer [H,V,0,](4-n)- and facilitates exchange between the dimer and tetramer. The 562 ppm resonance was found to be more stable than the tetramer at increased temperatures and high alkalinity. The ratio of C03/vanadium rather than actual concentrations seems to determine the stabilization of the dimer. A sample was run that was made up with the same [C03=] as the sample with a C/V of 14, but with 10 times the
1244 Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 n
I ‘I
*OoC
I
I
T
1
I
5427 5612,l 5 k I PPm Figure 3. 51VNMR spectra of a liquor with a Cf V of 7 at increasing temperature. I
1
I
TIHEIX1O3S1
542,l 562,l 502,l PPm Figure 2. 51V NMR spectra of a liquor with a C/V of 14 at increasing temperature.
vanadium concentration (0.40 M) giving a C / V of 1.5. In this case, the tetramer at 578 ppm was the major species as opposed to the dimer. Thus, a large excess of carbonate is necessary to stabilize the dimer. (b) Kinetic Results. As the dissolved oxygen content (3.5-5.5 mg/L) and the air/C02 flow were kept constant, the order of the oxidation reaction with respect to [V4+] could be determined. By use of the differential and integration methods as discussed in Benson (1960),the order of the reaction in [V4+] at 0.57 M Na2C03was found to be 1.0. The first-order plot according to the integration method is shown in Figure 4. The firsborder rate constant s-l. was 2.2 x The kinetic equation can then be expressed as a pseudo-first-order rate law of the form -d[V4+]/dt = kl[V4+]
(7)
As the carbon concentration was decreased, the first-order plots deviated from linearity, giving two straight lines (see Figure 4). The reactions at different alkalinities were all repeated at varying initial concentrations ranging from 0.014 to 0.039 M [V4+]. The plots in Figure 4 are thus normalized curves of In [V,4+]/[VO4+]versus time. A t
Ln
I Vt.1 I v:1 ‘
Figure 4. First-order plots for the reoxidation of V4+ at carbonate 0.095, (X) 0.25, ( 0 ) 0.38, (A)0.47, and (+) 0.57 concentrations of (0) M Na2C03.
alkalinities of 0.47, 0.38, and 0.28 M Na2C03, the normalized first-order curves of experiments done at a particular alkalinity, but at different initial V4+concentrations, when plotted together showed that the first part of the plots lies on the same line, whereas the second parts did not. This indicates that, whereas the first part is independent of initial concentration (which is expected for a first-order reaction), the second part shows a dependency on [Vo4+]. The gradients of the first and second lines
Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 1245
i b
2
4
6
8
10
12
14
clv Figure 5. Rate constants as a function of the carbonate/vanadium ratio (C/V).
decreased with decreasing alkalinity. A graphical representation of the gradients of the first lines (k,)versus alkalinity (Figure 5) shows a linear relationship, indicating a first-order dependence of the rate on alkalinity. The plot does not pass through the origin, implying the presence of a second concurrent reaction as is also suggested by Figure 4. To establish whether the deviations in Figure 4 did not indicate a change in order, the van't Hoff differential method (Laidler, 1950) was employed. The plots of (d[V4+]/dt)versus log [V4+],where d[V4+] -dt
[V4+]tz- [V4+]tl t2tl
did not yield straight lines as was the case with 0.57 M Na2C03. These curved lines also indicate that complex reactions, e.g., concurrent reactions, are taking place at alkalinities between 0.47 and 0.28 M Na2C03. At a carbonate concentration of 0.095 M, the first-order plot is a single straight line once more with a very low gradient. The kinetic results may be explained by the results of the 51VNMR investigation. As was seen in Figure 2, the dimer (HV20T3-)is almost exclusively present in a liquor with a C/V of 14 at 40 "C. This would reduce to a single species which was found to reoxidize with a first-order rate dependency on V4+concentration and a rate constant of 2.22 x 10-3 s-1. As the carbonate concentration is lowered, progressively more of the tetramer is present. This coincides with the digression from linearity in the first-order plot. The tetrameric species seems to reduce to a different V4+species which reoxidizes at a slower rate. A t a C/V of 2.4, only the tetramer is present (viz. Figure 1) and the reduced species reoxidizes extremely slowly. Because of the very slow reoxidation at this alkalinity, the order of this reaction is not known, because the data fits a half-, first-, and second-order plot. But as there is an apparent dependence on initial concentration of the second part of the mixed complex reaction plots, it appears that the reoxidation of the tetramer is not first order. With the present data, this order cannot be determined. Tentatively the role of carbonate is proposed to be as follows: It is known that an equilibrium exists between the dimer and tetramer at pH 8 which lies toward the tetramer in aqueous solutions with no or little carbonate. In solutions containing higher concentrations of carbonate, the equi-
U
110 I
Figure 6. Proposed reaction equations for the reduction and oxidation of the dimer (stabilized by carbonate) and the unstabilized tetramer.
librium shown in Figure 6 (eq 8) is proposed where the equilibrium at pH 8.5 in an excess of carbonate is driven to species 1. Thus, species 1reduces according to eq 9 (Figure 6), and the V4+species is stablized by the carbonate. The stabilized reduced species is then easily reoxidized. The cyclic tetramer reduces according to eq 10 (Figure 6) and reoxidizes slowly. Acknowledgment We thank Dr. M. B. Wiege and M. Roux for technical assistance in recording the 51V NMR spectra. We also acknowledge Sasol Limited for permission to publish this work. Registry No. V4+, 22541-76-0; H2S,7783-06-4; sodium ammonium vanadate, 39455-80-6.
Literature Cited Benson, S. W. The Foundations of Chemical Kinetics; McGraw-Hill: New York, 1960. De Haan, R.; Dressler, F. H.; Dry, M. E.; Hesse, H. J. F. A. SA Patent Appl. 8613264, 1986. Habayeb, M. A.; Hileman, 0. E., Jr. Can. J. Chem. 1980, 58, 2255-2261. Hammond, C. A. Enuiron. Prog. 1986, 5(1), 1-4. Hardison, L. G. US.Patent 4 238 462, 1980. Heath, E.; Howarth, 0. W. J. Chem. SOC.,Dalton Trans. 1981, 1105-1110. Howarth, 0. W.; Richards, R. E. J. Chem. SOC.1965, 864-870. Laidler, K. J. Chemical Kinetics; McGraw-Hill: New York, 1950. O'Donell, S. E.; Pope, M. T. J. Chem. SOC.,Dalton Trans. 1976, 2290-2297. Petterson, L.; Hedman, B.; Andersson, I.; Ingri, I. Chem. Scr. 1983, 22, 254-264. Shafer, R. E. U.S. Patent, 4400361, 1983. Weber, G. SA Patent 8310444, 1983. Weber, G. Erdoel Kohle 1986, 371-374.
Received for review October 22, 1987 Accepted February 22, 1988
