Ind. Eng. Chem. Res. 1994,33,2989-2995
2989
Oxidative Absorption of Hydrogen Sulfide by a Solution of Ferric Nitrilotriacetic Acid Complex in a Cocurrent Down Flow Column Packed with SMV-4 Static Mixers? Johannes F. Demmink, Harm J. Wubs,and Antonie A C. M. Beenackers. Department of Chemical Engineering, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
The reactive absorption of hydrogen sulfide into a solution of the ferric chelate of nitrilotriacetic acid (NTA) was studied a t 13 "C in a cocurrent down flow column packed with stainless steel Sulzer SMV-4 static mixers. The concentration of ferric chelate varied from 200 to 30 mourn3; the pH ranged from 8.3 to 6.7. Volumetric liquid-phase mass transfer coefficients for H2S ( k ~ u ) could be measured and were correlated by l z ~ a= 0.0572E~O.~~ for 0.060 IULI0.156 [ d s l , 0.59 I UG I 2.95 [ d s l , and 1.4 x lo2 IE L I 3 x lo3 [N/m2sl, in which E L is the liquid energy dissipation factor and ULand UGare the superficial liquid and gas velocity, respectively. The k ~ values u observed with H2S under reactive conditions appeared to be approximately a factor of 4 higher than those observed for oxygen absorption in the same liquid over the same packing. The cause of the extra volumetric mass transfer obtained with H2S under similar conditions is not well understood yet. The reaction kinetics of hydrogen sulfide with the ferric chelate of NTA was found to be first order in both ferric NTA and H2S for 0.40 < PA < 0.8 Wa, 0 < CF~(III) < 200 mourn3, and 6.7 < pH < 8.2.
Introduction Many commercial processes are available for the removal of hydrogen sulfide from gaseous streams. Most of these processes use gas-liquid contactors in which the hydrogen sulfide is contacted with a liquid to give either another dissolved sulfide-containing component (e.g., amine or hydroxide based processes) or elemental sulfur as a precipitate. The most important representatives of the latter type are the so-called iron chelate based processes. The absorption of hydrogen sulfide in reactive ferric chelate solutions is usually represented by
-
H2S(g)
H,S(aq)
H,S(aq)
+ 2Fe3+chelate"- Si + 2Hf + 2Fe2+chelate"- (1)
In this equation n denotes the charge of the chelant anion. The product, ferrous chelate, can be regenerated into the active ferric form by oxidation of the solution with air or oxygen:
-
02(g)
+
02(aq)
+
02(aq) 4Fe2+chelate"2H20 4Fe3+chelate"-
-
+ 40H-
(2)
This way, the iron chelate can be regarded as a pseudocatalyst in the reaction of hydrogen sulfide with oxygen (Buenger e t al., 1987). The sulfur produced is easily recoverable from the slurry. Another advantage of iron chelate based processes is that they essentially operate a t ambient conditions and are selective to H2S relative t o C02. For an overview of typical operation
* Author to whom correspondence
should be addressed. Presented a t the Symposium on Catalytic Reaction Engineering for Environmentally Benign Processes a t the San Diego ACS Meeting, March 13-18, 1994. t
0888-588519412633-2989$04.5010
Table 1. Typical Operation Conditions of Iron Chelate Based Processes temperature 20-60 "C 6-9 PH pressure < 100 bar typical chelants EDTA, HEDTA, NTA iron chelate concentration 10-1000 m 0 h 3 chelant-iron ratio 1.1-2.0
conditions of iron chelate based processes, as can be obtained from patent and literature data, see Table 1. Experimental data of the oxidative absorption of hydrogen sulfide in industrial or in mini-plants under typical industrial conditions are very scarce in open literature. Neumann and Lynn (1984) studied the oxidative absorption of both hydrogen sulfide and oxygen by iron-NTA solutions in a turbulent wettedwall column, operated at 60-65 "C,with superficial gas velocities (UG) ranging from 0.7 to 1.7 m/s for a constant superficial liquid velocity UL= 0.28 m/s. The inlet H2S pressure, p i , in the gas phase varied from 4 to 9.1 kPa, the total pressure being 101 @a. The concentration of iron-NTA was 100 mol/m3 with pH ranging from 3.9 to 4.1. Notably, these pH values are much lower than those usually applied in industry (see Table 1). According to Lynn and Dubs (1981), the reaction of hydrogen sulfide with ferric NTA is still rapid at these low pH values, whereas the overoxidation of sulfur to sulfur oxides (especially thiosulfate) is virtually eliminated. All research on mass transfer in columns with structured packings is of relatively recent date (Bravo e t al., 1985). Most of these studies have primarily focused on counterflow (see, for example, Billet and Mackowiak, 1988; Bravo e t al., 1985; Fair and Bravo, 1990; Huber and Meier, 1975), especially for application in distillation (Grangriwala, 1987). In acid gas removal systems the use of structured packings is in its infancy. Structured packings applied so far are of a sheet-metal type (Grangriwala, 19871, whereas those usually applied in (vacuum) distillation are from gauze sheet. The application of static mixers in reactive gas absorption has not been reported in open literature yet.
0 1994 American Chemical Society
2990 Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 Gas Clean-up I
A
Absorber
Settler
Regenerator
I
I
Figure 1. Mini-plant. G1, gas-phase sample point at the absorber inlet; G2, gas-phase sample point at the reactor outlet; L1, liquidphase sample point at the reactor sample point at the outlet.
Sulzer SMV static mixers have found a great variety of applications such as in the dispersion of gases in liquids, usually in cocurrent flow, either upflow or horizontal flow, or in gas purification either in cocurrent down flow or in countercurrent flow. These mixers are applied in bubble columns, both for extraction and reaction. The dispersions of gases in liquids are homogeneous, and radial concentration and temperature gradients are reduced. The flow of gasfliquid dispersions is essentially plug flow (Sulzer technical sheet, see also Grosz-Roll et al., 1982). The packing may be particularly attractive for application with fouling solutions because its surface is smooth and therefore less susceptible to fouling than other structured packings such as Sulzer BX and CY. Sulzer SMV is a candidate for application in the SulFerox process of Shell (Fong et al., 1988a,b). It is the aim of this work to study the oxidative absorption of hydrogen sulfide by ferric NTA solutions in a mini-plant under appropriate industrial conditions. We focus our attention on hydrodynamical parameters km and kw and reaction kinetics.
Experimental Section For the experimental mini-plant used see Figure 1. All vessels were of glass. PVC was used for piping and stainless steel for other devices such as pumps, flow meters, and air distributors. The system was operated in cocurrent down flow mode with a solution containing the active component ferric NTA. This liquid is extremely corrosive toward any copper-containing alloys. The liquid was prepared by adding ferrous sulfate to a solution of NTA trisodium salt (B.A.S.F. Trilon A) and sorbitol. Sorbitol was added to prevent precipitation of ferric hydroxide at pH levels greater than 8. The ironNTA-sorbitol ratio was 1:2:0.5. A 4 N sulfuric acid solution was used to adjust the pH to approximately 8.5. The above ferrous chelate solution was aerated to yield the desired ferric chelate solution. The resulting slight increase of pH, see eq 2, was corrected by adding extra 4 N sulfuric acid. The liquid was pumped (Lowara SGM-7) from the regenerator (volume, 60 dm3; diameter, 0.25 m) to the absorber where the liquid was contacted with a mixture of nitrogen and hydrogen sulfide. Before and after gas-
Table 2. Characteristics of Sulzer SMV-4Static Mixers Mat&
stainlesssteel
Element diameter
0.025 m
Element height
0.025 m
spaific ana
760 rnz1rn3bed
Bed porosity
0.81
Crimp height h
5.9 mm
Corrugation side S
6.8 mm
Corrugation bauB
6.6 mm
Equivalent diam*cr a
4.8 mm
Hydraulic diameter
1.4 mm
Critical nuface tension of
0.075 N/m
J Triangular flow channel cross section.
stainless steel
+
+
a = Bh [1/(B 25) 1/25] (Bravo et al., 1985). Hydraulic diameter is defined as 0.5 equivalent diameter (Bravo et al., 1985). Canbeny and Varma, 1986.
Table 3. Packed Bed Characteristics and Experimental Operation Conditions column diameter 0.025 m packing height 0.20 m 8 (+1as liquid-gas distributor) number of elements gas rates 1.02-3.7 kg/(m2 s) liquid rates 88-263 kg/(m2 s) H2S content at inlet 4000-8000 ppmv H2S content at outlet 230-2400 ppmv Table 4. Gas and Liquid Properties gasa density 1.25 kg/m8 (0 "C) viscosity 1.75 x 10-5 Pa s surface tension H2S solubility DBJvBDA a
liquid 1146 kg/m8 1.65 x Pa s 62.3 x N/m 929 (Pa m3)/mol 0.32
Perry and Chilton, 1983.
liquid contacting, gas samples were taken for calculating the absorption efficiency of the column. The HzS content of the gas was analyzed with an HzS gas analyzer Model 825R of Tracor Atlas. Repeated calibration never showed deviations over 2%. Liquid samples were taken a t the outlet of the absorber for analyzing the iron(II1) content (see Wubs and Beenackers, 1994). The liquid was then transported to the settler (volume, 65 dm3; diameter, 0.30 m), where the sulfur partly settled, partly remained suspended, and partly floated on the gas-liquid surface. From this vessel, the liquid was periodically pumped to the regenerator. Depending on the desired type of experiment, the liquid was either fully regenerated or not regenerated at all. All experiments were carried out at 13 "C with pH ranging from 8.2 to 6.7. Gas and liquid flow rates were controlled with gas flow controllers (Sierra Side Trak 111)in order to obtain a well-defined gas mixture. For the main characteristics of the Sulzer SMV-4 static mixers and the operation conditions applied, see Tables 2 and 3, respectively. Gas and liquid properties are given in Table 4.
Theoretical Model The absorption rate of HzS per unit gas-liquid interface followed from
Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 2991
' L 0.8
1
pH,/po, (-)
I"[$)
0'35 0.30 0.25
i
4 1
0.20
Oe6
0.4
0.2 0.05
0.00 0
0
5
10
time
15
t ~1
I -
0
20
40
20
60
Figure 2. Absorption of hydrogen sulfide in SulFerox ferric NTA solution, measured in a stirred cell. First-order dependency on hydrogen sulfide pressure (Wubs, 1994)T = 22 "C,CB = 30 moV m3, Ha x 10.
K.
These two equations are coupled via the H2S solubility:
(5)
From measurements of the H2S absorption rate in a commercial SulFerox ferric NTA solution in a stirred cell reactor, the kinetics with respect to H2S appeared to be first order, see Figure 2 (Wubs, 19941,which is in line with the results obtained with ferric EDTA and ferric HEDTA chelates (Wubs and Beenackers, 1994). Therefore, Ha can be defined as
H a = l/kI,d(DAkl,pc$)
Fe3+Ln-(OH-)
100
80
0
I
120
-
+ H+
Kb
Fe3+Ln-(OH-)
(4)
If the Hatta number Ha
