Znd. Eng. Chem. Res. 1995,34, 83-93
83
Sulfidation of Mixed Metal Oxides in a Fluidized-Bed Reactor Sophia C. Christoforou, Evangelos A. Efthimiadis,*and Iacovos A. Vasalos Chemical Process Engineering Research Institute and Department of Chemical Engineering, Aristotelian University of Thessaloniki, P.O. Box 151 7, 54006 University City, Thessaloniki, Greece
Mixed metal oxides were used for the removal of hydrogen sulfide from a hot gas stream. Sorbents were prepared according to the dry and wet impregnation techniques. The desulfurization performance of the metal oxide sorbents was experimentally tested in a fluidized-bed reactor system. Sulfidation experiments performed under reaction conditions similar to those at the exit of a coal gasifier showed that the preparation procedure and technique, the type and the amount of the impregnated metal oxide, the type of the solid carrier, and the size of the solid reactant affect the H2S removal capacity of the sorbents. The pore structure of fresh and sulfided sorbents was analyzed using mercury porosimetry, nitrogen adsorption, and scanning electron microscopy.
Introduction The use of coal for power generation is expected to increase in the coming years because the world reserves of crude oil and natural gas are significantly lower than those of coal. Conventionalpulverized coal power plants produce energy using relatively old technology. The development of new technologies for power generation plants has the following two objectives: to produce energy that costs less than that produced using other competitive methods and to comply with the new environmental standards for the emissions of power plants. New attractive systems for power generation are the integrated gasification combined cycle (IGCC) systems and the gasifier-molten carbonate fuel cells (MCFC) systems. An attractive way to clean a hot gas stream from H2S is a noncatalytic gas-solid reaction. The solid reactant is usually a metal oxide of the form MO,, and the reaction is described by the general equation:
where (s) denotes solid and (g) denotes gas. The solid product of reaction 1is a metal sulfide (MS,). When x = 0, reaction 1 expresses the reaction of a metal with H2S and the gas product of the reaction is H2. Metal oxides have been tested as candidate desulfurization sorbents. Reagent grade metal oxides have been used in order t o estimate the reaction rate constants of the oxides in thermogravimetric analysis reactors (e.g., Westmoreland et al., 19771, and mixed metal oxides, synthesized or commercially available, have been tested mainly in integral reactors. The sorbents are solid mixtures of metal oxides that react with H2S (e.g., zinc ferrite ZnFe204, CuO-FezOs, MnOCuO) or mixtures of an inert oxide with a solid reactant (e.g., zinc titanates ZnzTi04, ZnTiO3, and ZnTi303; FeO-Al203, Fe203-Al203; M110-&03; CaO-MgO). A mixture of two reactive metal oxides acts as follows: the one metal oxide has large desulfurization capacity expressed as kilogram of sulfur adsorbed per kilogram of the metal oxide (e.g., Fen03 and MnO) and the other has high affinity for desulfurization (e.g., ZnO and CuO). The combination of these two chemical compounds is expected to lead to enhanced sorbent performance
* Author t o whom correspondence should be addressed. E-mail:
[email protected]. 0888-5885/95/2634-0083$09.00/0
during sulfidation (Gupta et al., 1992). The addition of an inert solid in a metal oxide sorbent is expected t o stabilize the metal oxide against its reduction to the metal form (Lew et al., 1989). The inert solid can also be used as the porous solid carrier where another metal oxide is impregnated (Wakker et al., 1993). Experimental work has been carried out in order to investigate the effect of the solid reactant, the reactive gas composition, and the reaction conditions on the desulfurization performance of a gas-solid system. Examples of experimental works are those of Woods et al. (1990) and Lew et al. (1992), where sorbents were reacted with an HzS-containing mixture in a thermogravimetric analysis (TGA) system, and those of Tamhankar et al. (1986)and Ayala and Marsh (19911,where the same reaction took place in a packed-bed reactor. In other experimental studies (e.g., Patrick et al., 1993; Lew et al., 1989) the sulfidation kinetics of candidate sorbents were investigated using a TGA reactor and the desulfurization performance of the same samples was tested in a packed-bed reactor. The above experimental studies showed that the sulfidation temperature (at the temperature range of interest: 500-650 "C) and the composition of the reactive gas (concentration of HZ and H2O) weakly influence the desulfurization ability of the sorbents. The size of the solid reactant strongly affects the observed sulfidation rates in a differential reactor (Efthimiadis and Sotirchos, 1992,1993a) and to a lesser extent in a fixed-bed reactor (Efthimiadis and Sotirchos, 1993b). The composition and the initial pore structure of the sorbent are also parameters that may change the desulfurization capacity and the H2S concentration at the exit of a reactor (Tamhankar et al., 1986; Lew et al. 1989; Efthimiadis and Sotirchos, 1993~).The physical properties (density, surface area, pore volume, and pore size distribution) of fresh and reacted sorbents were measured in sulfidation studies (Efthimiadis and Sotirchos, 1993c; Sa et al., 1989; Woods et al., 1991) in order to study the changes in the physical properties of the sorbent during the gas-solid reaction. In all cases sulfidation led to pore volume loss. The same does not apply for the surface area measurements since different studies showed that the reaction may cause an increase, cause a decrease, or have no effect on the surface area of the sorbent. The objective of this study is the evaluation of the desulfurization performance of mixed metal oxides in a fluidized-bed reactor. Commercial sorbents and sorbents prepared from alumina or magnesia substrates 0 1995 American Chemical Society
84 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995
were tested under reaction conditions that are typical for hot coal gas streams. Specifically, experiments were carried out to study the parameters that affect the H2S removal ability of a sorbent, i.e., the technique and the procedure that are followed for the preparation of the sorbent, the percentage and the type of the metal oxide that is impregnated, the porous solid that serves as the substrate where impregnation takes place, and the reaction conditions (H2S concentration and particle size) that are employed during sulfidation. In addition to the sulfidation experiments, the pore structure of unreacted and sulfided sorbents was analyzed. In this way, structural changes due to the preparation procedure of the sorbent and to the chemical reaction were measured.
Materials Commercial and synthesized sorbents were the solid reactants of this study. Metal oxide sorbents were prepared using the dry and wet impregnation techniques. We chose t o use an inert porous solid as the carrier, where the solid reactant-a metal oxide-was impregnated. As a result, the desulfurization capacity of the sorbent was attributed to the sulfidation reaction of the metal oxide that was impregnated on the inert carrier. Alumina and magnesia were candidate solid carriers. Our thermodynamic computations showed that alumina does not react with H2S to produce the corresponding sulfide at the temperature range of 5001000 K, at 1 atm, and for the gas mixture used in our experiments (HzS-Nz mixtures). Similar to our thermodynamic predictions for alumina, only about 1% of magnesia can react with H2S at the reaction conditions of interest. These oxides are commercially available as porous catalysts or as solid substrates where another solid is deposited. There are a variety of porous stones based on alumina and magnesia differing in the composition, the structure of the pore space, and the other physicochemical properties that characterize a porous solid. We chose to use the above two metal oxides as the solid carriers of our synthesized metal oxides because they are inert to an H2S-containinggas mixture and they are commercially available. The solid carriers were provided in cylindrical or spherical pellets. The samples were mechanically crushed to reduce the size of the solid reactant. Sieves were used to separate particles of different sizes. A wide range of particle sizes was employed in the experiments of this study: 90-125, 125-180, and 425-600 pm. Metal Oxide Sorbents. We initially used metal oxide sorbents that were available in our laboratory to test the desulfurization performance of our fluidizedbed facility. A porous zinc oxide sorbent was supplied by Harshaw (sample Zn-0401), and two mixed oxides based on Ca were supplied by Amoco. We will refer to the two calcium-based sorbents as sample AlCa and AlCaC, because they are mixtures of alumina with 46 wt % CaO and 55 wt % CaC03, respectively. Our test experiments showed that when the reactor was loaded with particles of alumina, no H2S was adsorbed at the reaction conditions of this study. Therefore, the commercial metal oxides were mixed with inert material (Amocat, a y-alumina supplied by Amoco) in order t o decrease the required time for complete sulfidation of the solid in the fluidized bed reactor. The particles of sorbent AlCaC were calcined under nitrogen at high temperatures t o decompose the CaC03 to CaO. The solid samples used in the mercury porosimetry analysis of this study were in particle form (125-180 pm); thus, the measurements of the porosimeter for
AICoCJ50 AICaC,&50 A o
OC OC
Zn-&401
Pore Diameter, microns Figure 1. Pore size distributions of commercial metal oxide sorbents.
Table 1. Solid Carriers Used for the Preparation of the Metal Oxide Sorbents type of wt % surface name company alumina alumina area, mZ/g y-alumina 100 282 Catapal B Catapal 95.7 121 Amoco y-alumina Amocat a-alumina 80.3 30 SA 3232 Norton a-alumina 80.3 20 SA3235 Norton a-alumina 98.7 31 puralox Condea
diameters larger than about 10 pm were attributed to the invasion of mercury in the interparticle space and they are not presented. The mercury porosimetry data of the three metal oxides are shown in Figure 1. The pore size distribution of sample AlCaC depends on the calcination temperature (750 or 850 "C) as shown in Figure 1. Significantly higher overall pore volume was measured for the AlCa sample than for the other sorbents, and the most uniform pore size distribution (sharp increase in the cumulative pore volume versus pore diameter curve) was that of the Zn-0401 sample. Alumina Solid Carriers. The alumina carriers that were used in the preparation of the mixed metal oxide sorbents are listed in Table 1. Samples Catapal B, Amocat, and Puralox consist mainly of alumina while the Norton samples are mixtures of alumina and silica (80.3 wt % alumina and 17.9 wt % silica). Traces of other metal oxides (NazO, Fe203, TiO2, CaO, MgO, and K20) also exist in the solid samples. The pore structure of the carriers was measured using mercury porosimetry and nitrogen adsorption. The surface area values shown in Table 1imply that samples made of y-alumina consist of a network of small pores (high surface area). Small pores may be blocked when a metal oxide is impregnated on the precursor solid or when the product of the gas-solid reaction occupies more space than the solid reactant. The mercury intrusion results of Figure 2 describe the pore size distribution of the porous samples. Notice that the overall pore volume of Amocat (y-alumina)is twice that of the a-alumina solids. If we convert the mercury porosimetry results of Figure 2 to porosities, then we estimate 67% porosity for Amocat and 50% porosity for the a-alumina samples. The two samples supplied by Norton have a significant number of large pores ('0.2 pm pore diameter), but they differ in the size of the most probable pore diameter (defined as the diameter where the slope of the pore size distribution curve has its largest value): 0.1 pm for sample SA-3235 and 0.03 pm for sample SA-3232. This remark is in accordance with our BET measurements that sample SA-3232 has higher surface area than sample SA-3235. The solid sample of Condea (Puralox) has small pores ( c 0 . 0 6pm), which are not present in the Norton samples.
Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 85
I
% A 2 2 3 2
C2NO
Amsot SA-4235
cMo
c1-0
PU$OX
Pore Diameter, microns Figure 2. Pore size distributions of solid carriers used in the preparation of the metal oxide sorbents.
I. &O C&O
>" 0.15
0,Ol
I
1
10
1
100
Pore Diameter, microns Figure 4. Mercury porosimetry data for the magnesia samples.
c2-0 CZ.Mg0 WAer Treated M g 2 - Z S Fresh
100
Pore Diameter, microns Figure 3. Effect of calcination temperature on the pore size distribution of magnesia samples.
Magnesia Solid Carriers. Magnesia is commercially supplied in two forms: the ignited and the caustic magnesia. Both types of MgO were produced from magnesite (MgC03) after calcination of the carbonate a t about 1800 "C (ignited magnesia) and 1000 "C (caustic magnesia). The heat treatment of MgC03 at high temperatures causes the sintering of the porous solid. This becomes obvious from Figure 3, where the pore size distribution of the two types of magnesia is presented. Heat treatment at 1800 "C gave rise to almost complete loss of the void space in the interior of the solid. The mercury porosimetry results shown in Figure 3 indicate that the ignited magnesia is not suitable for our desulfurization studies since it is almost a nonporous solid. Four samples of porous magnesia were used as the solid carriers where metal oxides were impregnated. Sample C.MgO was supplied by Magnomin, while samples CLMgO, C2.Mg0, and C3.MgO were supplied by Grecian Magnesite. We compare the pore size distribution of the four magnesia samples in Figure 4. Notice that the solids supplied by Grecian Magnesite have similar pore structure since they are prepared from the purification of the same stone (the compositional analysis of all solids is given in Table 2). Considerably smaller pore volume was measured for the C.Mg0 sample than that of the other solids. All porous solids are classified as materials with medium surface area (Table 4). The smallest surface area was measured for the C3.MgO sample (22.7 m2/g),and the largest for the C1.MgO sample (51.65 m2/g). The surface area measurements and the compositional analysis shown in Table 2 imply that the purification of the stone (i.e., higher percentage of MgO) leads to an increase in the surface area of the porous solid. Sorbent Preparation Procedure. Solid sorbents were prepared using the dry impregnation (incipient wetness) technique as follows: Initially particles of the
Pore Diameter, microns Figure 5. Mercury porosimetry results at different stages of the Mg2-Zn6 sorbent preparation.
Table 2. Chemical Composition of the Greek Magnesia Samples suppliers chem composition, %
Magnomin C.MgO 85 2.43 6.21 1.2 0.02 2.3 0.4 0.01
LOI," %
2.44
Grecian Magnesite C1.MgO C2.MgO C3.MgO 96.41 90.03 78.61 1.08 1.7 3.4 0.89 6.26 12.84 0.1 0.16 0.4 0.01 0.13 0.44 0.12 1.0 0.08 0.03 0.14 0.03 0.01 0.09 0.4 1.5
1.63
3.91
" LO1 = loss of ignition. porous solid carrier were impregnated with a quantity of distilled water equal to the pore volume of the solid, the latter determined by mercury porosimeter analysis. The wet mass was then dried a t 120 "C for 2 h and calcined a t 600 "C for 12 h, under no flow conditions. The heat treatment of the fresh solid with water changed the pore size distribution of the porous material as shown in Figures 5 and 6 for two magnesia carriers. The BET surface areas (Table 4) of the untreated C2.MgO and C3.MgO samples were lower and higher than those of the water-treated solids, respectively. This water treatment changed the desulfurization performance, as will be discussed in another section of this work. The water-treated particles were impregnated with metal nitrate or sulfate solutions. The soaked mass was dried a t 120 "C and then calcined at high temperatures. The chemical composition of the calcined solids was determined using the inductively coupled plasmdatomic emission spectroscopy (ICP/AES) technique.
86 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995
Table 3. Metal Oxide Sorbents Prepared at CPERI C3.Mg0 Wqter Treated M g 3 - Z n 6 L f Impreg. Mg3-Zn6Xet Impreg.
solid carrier ~~
0.01
1
10
100
Pore Diameter, microns Figure 6. Mercury porosimetry results a t different stages of the Mg3-Zn6 sorbent preparation.
ZnO-containing sorbents derived from sulfate solutions were prepared using the above procedure at different calcination temperatures (650 and 950 "C) using alumina as the solid carriers. Calcination of the ZnO sorbents at low temperatures may lead to the formation of zinc oxysulfate (ZnO. 2ZnS04) that decomposes to ZnO releasing SO2 during heat treatment under inert atmosphere. The effect of the calcination temperature on the decomposition of metal sulfates was studied by Mu and Perlmutter (1981) and Tagawa (1984) in TGA reactors. Their results showed that calcination temperatures higher than 850 "C are needed to attain complete dissociation of ZnS04 to ZnO. In accordance with their results we measured SO2 when sorbents were initially calcined at 650 "C and then heated at higher temperatures under nitrogen. Following this result, higher calcination temperatures were used to reduce the ZnSO1. However, the X-ray diffraction (XRD)analysis of the produced samples showed the existence of zinc peroxide (ZnOn) in our sorbents calcined at 950 "C. The problem of the remaining ZnSOd or the produced ZnOz in the metal oxide sorbents was eliminated by using metal nitrate solutions (27.5 and 55 wt %) in place of the metal sulfates. Mixed-metal oxides were also prepared using copper, calcium, and manganese nitrates. The calcination of all sorbents took place a t 600 "C for 12 h. Besides dry impregnation, we also used the wet impregnation technique to prepare sorbents made from the same metal oxide and solid substrate as those prepared by the former technique. In this way we examined the effect of the preparation technique on the desulfurization performance of the sorbents. Greek magnesia was the solid carrier and Zn(NO3)z solution (27.5 wt S )the initial solution used for the preparation of a mixed metal oxide sorbent based on ZnO. The porous solid was treated with water according to the procedure described previously, and it was then impregnated with the solution until a suspension was produced. This suspension was heated in a rotary evaporator at 75 "C for 1.5 h, under vacuum. The soaked mass was then dried at 120 "C for 2 h and calcined at 600 "C for 12 h. The mercury porosimetry data of a sorbent prepared by the wet impregnation technique are shown in Figure 6 along with those of a sorbent prepared by the dry impregnation technique. Their pore size distributions are similar with the exception of the mercury intrusion data at high compression pressures. The similarity in the internal pore structure of the two samples is supported by the surface area measurements. The sorbents prepared in our laboratory were analyzed using the XRD and the ICP/MS techniques. A
sorbent name
wt % CaO impregnated in magnesia metal oxide, prep sorbents wt% technique (ICP/AES) (ICP/AES)
~~~
y-alumina Amocat a-alumina SA-3232 SA-3235 Puralox Puralox Puralox magnesia C.MgO C1.MgO C2.MgO C2.MgO C2.MgO C2.MgO C3.MgO C3.MgO C3.MgO
Al-Zn2
dry impreg
ZnO, 12%
Al-Zn3 Al-Zn4 Al-Zn6 Al-Znll Al-Ca12
dry impreg dry impreg dry impreg dry impreg dry impreg
ZnO, 13.06% ZnO, 12% ZnO, 6.2% ZnO, 11.4% CaO, 11.78%
Mg-Zn6 dry impreg Mgl-Zn6 dry impreg Mg2-Zn6 dry impreg Mg2-Cu6 dry impreg Mg2-Mn6 dry impreg Mg2-Ca6 dry impreg Mg3-Zn6 dry impreg Mg3-Zn12 dry impreg Mg3-Zn9- dry impreg cu3 C3.MgO Mg3-Ca6 dry impreg C3.MgO Mg3r-ZnGQ dry impreg C3.MgO Mg3-Zn6W wet impreg
a
2% 1% 1.6% 1.6% 1.6% 1.6% 3% 2.8% 2.5% 2.8% 3% 3%
ZnO, 5.4% ZnO, 5.4% ZnO, 5.1% CuO, 5.1% Mnz03,4% CaO, 6% ZnO, 5.2% ZnO, 9.4% ZnO, 7.3% CuO, 2.5% CaO, 5.74% ZnO, 4.7% ZnO, 5.5%
+
Without carrier pretreatment with HzO.
Table 4. Surface Area Measurements sample
state of sample
surface area, "Vg
Al-Zn3 Al-Zn3 Al-Zn6 Al-Znl 1 Al-Ca2 AI-Ca3 C.MgO Mg-Zn6 Mg-Zn6 C1.MgO Mgl-Zn6 Mgl-Zn6 C2.MgO C2.MgO Mg2-Zn6 Mg2-Zn6 C3.MgO C3.MgO Mg3-Zn6 Mg3-Zn6W Mg3-Zn6 Mg3-Zn12 Mg3-Zn12 Mg3r-Zn6 Mg3r-Zn6 Mg3-Ca6
unreacted (dry impreg) sulfided unreacted (dry impreg) unreacted (dry impreg) calcined at 750 "C calcined at 850 "C fresh unreacted (dry impreg) sulfided fresh unreacted (dry impreg) sulfided fresh water-treated unreacted (dry impreg) sulfided fresh water-treated unreacted (dry impreg) unreacted (wet impreg) sulfided unreacted (dry impreg) su 1fided unreacted (dry impreg) unreacted sulfided (dry impreg)
93.93 94.60 28.36 27.18 111.40 87.90 42.50 20.89 21.70 51.65 28.31 29.90 34.70 29.00 27.40 29.49 22.70 37.50 20.56 20.90 19.55 17.13 19.37 19.60 19.50 11.00
list of the sorbents prepared in this study is given in Table 3, where the sorbents were classified according t o the precursor solid. In the same table the preparation technique and the weight percent of the impregnated metal oxide are also presented. Notice that CaO exists in all sorbents derived from the magnesia substrate since this metal oxide exists in all the magnesia-based solid carriers. About 5 wt % metal oxide was impregnated on most of the sorbents, while the percentage of CaO in the precursor MgO-containing solids was lower than 3 wt %.
Experimental Procedure and Conditions The desulfurization performance of various metal oxides was investigated in a bench-scale unit, schematically shown in Figure 7. The main part of the reaction unit was a quartz fluidized-bed reactor of 3.5 cm o.d.,
Ind. Eng. Chem. Res., Vol. 34, No. 1,1995 87
I
Air
T /N21,N2 H2
Vent
rw A
I
Analyzer
Filter .-L
?i! 7
Cyclone
1% 1o x c-
Water Bath Figure 7. Schematic of the fluidized-bed reactor and the gas analysis system.
equipped with a fritted, plate-gas distributor. The reactor was vertically mounted in an electric, three-zone furnace. A constant temperature profile was maintained throughout the experiment using three temperature controllers connected to the three zones with thermocouples. The reaction temperature was measured by a K-type thermocouple. In order to protect this thermocouple from corrosive gases, it was placed in a 1/4 in. 0.d. quartz tube. The tip of the thermocouple lay about 2 cm above the gas distributor. Experiments were performed at the temperature range of 600-900 "C and a t ambient pressure. The solid reactant was particles of mixed metal oxides. The composition and flow rate of the feed gas were controlled by a system of odoff valves and mass flow controllers. The reactive gas in the sulfidation experiments was an HzS-containing mixture. Hydrogen sulfide was supplied from a mixture of 1%H2S in N2. Other gases, notably H2, CO, and CO2, could also be added in the feed gas. The flow rate of gases was monitored by rotameters and the flow rate was also periodically measured during the experiments using a water meter. The feed rate of the reactive gases could vary between 500 and 2000 cm3/min. The unreacted H2S a t the exit of the reactor was measured as follows: The gas stream after the fluidizedbed reactor was fed in a cylindrical stainless steel reactor of 1.58 cm o.d., where the unreacted H2S was oxidized with air to S02. The configuration of this reactor was similar t o that of the sulfidation reactor; i.e., it was vertically mounted inside an one-zone furnace. The reaction was performed a t 700 "C and a t ambient pressure. The SO2-containinggas mixture was fed to a pulse fluorescent analyzer to measure the SO2 concentration. We examined the possibility of incomplete conversion of the H2S to SO2 as follows: The exit stream from the oxidation reactor passed through an H2S scrubber that was loaded with a CdS04 solution. Following that, the solution was titrated. When the appropriate conditions were employed, all H2S was converted to S02. Thus, the hydrogen sulfide concen-
tration a t the exit of the fluidized-bed reactor was easily determined from the SO2 concentration measured in the analyzer. Even if the reactive gas did not contain H20, water is present a t the exit of the reactor system because it is produced during the sulfidation and the oxidation reactions. However, water may cause problems for the SO2 analyzer and it was, thus, removed from the exit stream with the use of drierite. Standard gas mixtures were used t o ensure that the signal of the SO2 analyzer was linear with respect to the SO2 concentration. The lower detectable SO2 concentration of the analyzer was 1 ppm. Sorbents were prepared from the calcination of CaC03 under nitrogen. The extent of the CaC03 decomposition was monitored using a nondispersive infrared analyzer. The experimental data were collected and stored with an analog/digital system connected to the analyzers. Typical Reaction Conditions. The standard reaction conditions that were used to test the desulfurization capacity of sorbents in the bubbling fluidized bed (BFB) were the following: reactive gas flow rate (25 "C, 1atm) 1000 cm3/min;superficial velocity 2 cm/s; reaction temperature 600 "C; inlet H2S concentration 2000 ppm; reactor loading 28 g; particle size 125-180 pm. The reaction conditions for the oxidation of the unreacted H2S to SO2 were the following: reaction temperature (25 "C, 1 atm) 700 "C; flow rate of air 1000 cm3/min. The above reaction conditions were employed in all the sulfidation runs unless it is mentioned differently. Results and Discussion
The desulfurization performance of a metal oxide depends on the nature and the physicochemical properties of the sorbent and on the composition of the reactive gas. Our first set of experiments was carried out in order to test the repeatability of our experimental results under identical reaction conditions. In Figure 8 we compare experimental data for sorbents AlCa, AlZnll, and Mg3-Zn6 obtained in two different runs under the standard reaction conditions (particle size 125-180
88 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995
6
1
6001
E ' a
AlZg-2 AIZg-3 AlZg-4 AlZt-1 1
a
800 C
0 ._ U
600
P
+C Q)
U C
0
0 v,
N
I
'0
0.1
0.2
0,3
0.4
0,5
0,6
Normalized Time, t/tmin
Normalized Time, t/tmin Figure 8. Reproducibility of the sulfidation experiments.
Figure 9. Sulfidation of ZnO-containing sorbents prepared by
pm, reaction temperature 600 "C, reactive gas flow 1000 cm3/min, H2S concentration 2000 ppm). The experimental curves of the duplicate experiments fall on those of the initial runs in all cases. We present the experimental data as the evolution of the H2S concentration with the normalized time &in (breakthrough curve), where tmi, is the minimum required time for the complete sulfidation of the metal oxide. When the exit stream has almost zero H2S concentration, the dimensionless time that corresponds to the breakthrough point (sharp increase of the reactive gas concentration in the reactor) is a measure of the sorbent utilization, i.e., the average conversion of the solid reactant. Inert Material. In the sulfidation runs with commercial sorbents (samples Zn-0401, AlCa, AlCaC), we loaded the reactor with about 2 g of the solid reactant particles and 28 g of inert particles (y-alumina, Amocat), so that t~~ was of the order of a few hours. The addition of the inert solid in the reactor loading could affect the desulfurization capacity of the sorbent. Therefore, we compared the breakthrough curves of an experiment performed when the reactor was loaded with a mixture of sample AlCa (2 g) and alumina (28 g) with that of an experiment performed when the reactor was loaded with sample AlCa. Our experimental data showed that the presence of inert solid in the reactor loading improves the H2S adsorption ability of the sorbent when the breakthrough curves are presented as a function of the normalized time. The dilution of the metal oxide with inert solid increases the contact time between the solid reactant and the reactive gas and gives rise to more efficient desulfurization. Following that, in all the experiments with the commercial sorbents we loaded the reactor with about 2 g of the sorbent and 28 g of inert solid (alumina). Preparation Procedure and Technique. Gupta et al. (1992) prepared a zinc ferrite sorbent by impregnating zinc and iron nitrate solutions on a-alumina. Their experiments in a TGA reactor showed that the sorbent capacity was influenced by the calcination temperature. However, poor capacity was, in general, measured for the sorbent when it was tested in a benchscale reactor. The wet impregnation technique was also employed by Wakker et al. (1993) to prepare manganese- and iron-containing sorbents. Wakker et al. attributed the deactivation of the fresh sorbents to the thermal sintering of the porous solid (the solid carrier was y-alumina). In this study sorbents were prepared using a-alumina, a porous material of stable pore structure at reaction conditions typical for sulfidation. Moreover, relatively small amounts of the reactant metal oxide were impregnated on the solid carriers. A sulfate or nitrate solution of the metal can be used to impregnate the metal oxide on the solid substrate.
11).
sulfate (AlZn-2, AIZn-3, and AlZn-4) or nitrate solutions (AlZn-
E 3001 a a 250 -
r
0 '3 200
?
2
Mg3zZn6 Mg3rA-Zn6
-
150-
e,
2
1ooc
.5
Normalized Time, t/tmin Figure 10. Effect of heat treatment with water at 600 "C on the sulfidation results.
Initially, three samples of ZnO sorbents (samples AlZn2, AlZn-3, and AlZn4) were prepared using zinc sulfate and three types of alumina (Amocat, 4-3232, and Al3235, respectively). These materials showed poor desulfurization capacity as shown in Figure 9. The ability of the sorbents to remove the hydrogen sulfide was significantly improved when a metal nitrate was used in the impregnation procedure (sample AlZn-11 in Figure 9). The problem associated with the use of metal sulfates in the preparation technique was discussed previously. It was, thus, decided t o use metal nitrate solutions for the preparation of the sorbents. In the description of the preparation procedure we mentioned that the solid substrate was heat treated with water and then calcined at 600 "C. We followed this procedure in the preparation of the metal oxide sorbents because we believe that the presence of water at high temperatures (vapor water is produced during sulfidation) may affect the interior structure of the porous solid. We tested the effects of the water treatment on the desulfurization performance of sorbents reacting sorbent Mg3-Zn6 and sorbent Mg3r-Zn6 under identical reaction conditions with an HzS-containing mixture. The former sample was prepared according to the procedure described earlier, while the latter one was prepared without any heat treatment with water. The breakthrough curves of the two experiments (see Figure 10) show that sample Mg3-Zn6 has a higher breakthrough point than sample Mg3r-Zn6. The experimental data of the untreated sample presents a sharp increase in the H2S concentration a t normalized times 0.15 and 0.35, while in the time interval between the above two times, the H2S concentration remained stable at 50 ppm. This behavior verifies our initial assumption that the water-treated sorbent has a stable pore structure and, thus, adsorbs larger amounts of H2S before the breakthrough point. These results led us t o
Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 89
Ea
I
1.000,
" t Rnn
I
0,z
l i m e , min dry (Mg3-Zn6)and wet (Mg3-Zn6W)impregnation techniques. I
zool 100
0,l
0.2
0,3
0,6
0,8
1
Normalized Time, t/tmin
Figure 11. Breakthrough curves of sorbents prepared using the
c
0,4
0,4
0,5
0,6
Normalized Time, t/tmin Figure 12. Experimental results using sorbents impregnated with 5.2 wt % (Mg3-Zn6)and 9.4 wt % (Mg3-Zn12)ZnO.
use the heat treatment with water in all sorbents before the impregnation with a metal oxide. Samples Mg3-Zn6 and Mg3-Zn6W were prepared using the dry and wet impregnation techniques, respectively. The experimental data of the runs with the above two sorbents under identical reaction conditions are shown in Figure 11. The difference between the sulfidation data of these two experiments is small (the breakthrough point of the sorbent prepared with the wet impregnation technique was somehow lower than that of the other sorbent of Figure 111, implying that the impregnation technique does not affect the desulfurization performance of the sorbents significantly. Metal Oxide Content in the Sorbent. A small percentage (about 5%)of metal oxide was impregnated on the sorbents so that the pore structure of the sorbent remained unchanged when the metal oxide was added to the solid carrier. In Figure 12 we compare the experimental data of two sulfidation runs, where sorbents were impregnated with different amounts of ZnO. The sorbent with the larger percentage of ZnO has a lower breakthrough point than the other sorbent. When 9.4%ZnO was impregnated on the solid, the concentration at the exit of the reactor was about 25 ppm after about 75 min and a second breakthrough point was noticed at about 225 min. A possible explanation for this behavior is that a larger percentage of the metal oxide in the sorbent gives rise to a thicker layer of the reactant oxide in the porous solid carrier. As a result, diffusion of the reactive gas in this product layer may decrease the overall sulfidation rate. Besides that, when a relatively large amount of metal oxide is impregnated on the porous substrate, pore blockage may take place. Particle Size. Intraparticle diffusional limitations may control the observed rate of a gas-solid reaction. When this is the case, reduction of the particle size is expected to improve the desulfurization capacity of the
Figure 13. Effect of particle size on the experimental results of sorbent AlCa.
sorbent. The sulfidation experiments in a TGA reactor using limestone-derivedcalcines and commercial porous zinc oxides of Efthimiadis and Sotirchos (1992, 1993a) showed that a t high temperatures ('500 "C) the removal of H2S from hot coal streams is strongly affected by the size of the solid reactant (particles in the range 50-350 pm). In this study sulfidation experiments were carried out using particles of three sizes: 90-125, 125-180, and 180-225 pm of sample AlCa. Figure 13 shows experimental results for sorbent AlCa, Le., a mixture of CaO and A l 2 0 3 , mainly, reacted with HzS at the standard reaction conditions. It is evident from this figure that the optimum size, for the applied experimental conditions, is from 125 to 180 pm. These experimental data apparently contradict the above remarks about the relation of the sorbent particle size and the reaction rate. It is believed that when the size of the reactant particles is small (90-125 pm) lower desulfurization capacity is measured than when it is large (125-180 pm), because particles of the former size tend to agglomerate during the sulfidation reaction. A similar behavior was noticed in other experiments of this study (for instance the experiments using the commercial grade ZnO of Harshaw). Our explanation was verified by the optical examination of sulfided samples; i.e., agglomeration of the 90-125 pm particles was observed after the completion of the sulfidation experiments. The internal pore structure of a solid may change during the size reduction procedure as a result of the disappearance of those large pores that feed the internal small pores. If the fragmentation procedure has the above result on the porous solid, then the pore structure of the sample depends on the particle size of the examined material. We compared the pore size distribution of samples of different particle sizes using mercury porosimetry, and we found small differences between these data, implying that the internal pore structure of our samples does not depend on the crushing procedure. We chose to use particles of 125-180 pm in our sulfidation experiments since particles of this size exhibit good desulfurization performance without the problem of the agglomeration. HzS Concentration. In previous sulfidation studies researchers examined the dependence of the H2S concentration on the desulfurization performance of metal oxide sorbents. Westmoreland et al. (1977) measured the initial reaction rates of MnO, ZnO, CaO, and VzO3 in a TGA reactor using different initial H2S concentrations. Their results showed a linear dependence between the rates of reaction and the reactive gas concentration; therefore, a first order reaction was estimated for the sulfidation of the above oxides. Efthimiadis and
90 Ind. Eng. Chem. Res., Vol. 34,No. 1, 1995
Time, min
Time, min
E
5001
0
Normalized time, t/tmin
0.2
0.3
0.4
0.5
(
Normalized Time, t/tmin
Figure 14. Breakthrough curves of magnesia solid carriers presented as a function of the reaction (a) and the normalized (b) time.
Figure 15. Breakthrough curves of sorbents impregnated with ZnO presented as a function of the reaction (a) and the normalized (b) time.
Sotirchos (1993a,c)used TGA experiments to prove that the sulfidation of ZnO is of first order with respect to the H2S concentration. They used the evolution of the ZnO conversion with reaction time at 500 and 600 "C to demonstrate that the sulfidation data obtained using different H2S concentrations are bundled in a single curve when the conversion is plotted as a function of a time that accounts for the amount of H2S that passes through the reactor. We used low H2S concentration in our sulfidation runs to simulate the H2S concentration at the exit of a coal gasifier. When we performed experiments with initial H2S concentration of 1000 and 2000 ppm-the other reaction conditions were standard-we measured higher H2S concentration at a given reaction time for higher initial H2S concentration. Solid Carrier. The compositional analysis of the four magnesia carriers (shown in Table 2) showed the existence of CaO in all solids. We tested the ability of the sorbent to adsorb H2S before the impregnation with a metal oxide experimentally. The breakthrough curves of the four samples are presented in Figure 14 as a function of the reaction and the normalized time. The latter is defined as the ratio of the real time and the minimum time for complete sulfidation of the CaO that exists in the porous solid. Notice that when the experimental data are plotted as a function of the normalized time, all solids have a similar breakthrough point of about 0.2. About 5 wt % ZnO was impregnated on four types of magnesia-based substrates. The breakthrough curves of the sulfidations runs using the MgO-ZnO sorbents reacted under standard reaction conditions are shown in Figure 15. Notice that when the data are presented as a function of the normalized time, the experimental curves of the four runs fall into each other. The normalized factor (t-) was defined as the time required for complete sulfidation of the ZnO and the CaO that exist in each sorbent. The improvement in the desulfu-
rization capacity of a magnesia sorbent impregnated with a metal oxide becomes obvious when the breakthrough points of Figure 15a are compared to those of Figure 14a. Our sulfidation data showed that the desulfurization capacity of sorbents made from different magnesia substrates did not depend on the type of the solid carrier. On the other hand, if we compare the desulfurization ability of zinc oxide impregnated on alumina (Puralox) and magnesia ( c w e s of samples AI-Znll and Mg3-Zn6 in Figure 81, we conclude that sorbents prepared from the magnesia substrate clearly adsorb larger amounts of H2S than those from the alumina substrate, implying the importance of the precursor porous stone to the desulfurization capacity of the sorbent. The mercury porosimetry data (Figures 2 and 4) and the breakthrough points of ZnO impregnated on different substrates indicate that solids with small pores and small overall pore volume have lower desulfurization capacity than solids with larger pore sizes and higher pore volume. Since the surface area of the reacted solids did not differ significantly (Table 41, our experimental data indicate that the surface area should not be considered as the physical property of a solid that determines its desulfurization capacity. Metal Oxide. Different metal oxides were impregnated on the C2.MgO solid carrier. We tested some of the metal oxide sorbents that can thermodynamically adsorb H2S under the typical reaction conditions of this study: ZnO, CuO, Mnz03, and CaO. The experimental data of Figure 16 show that all sorbents except Mg2Ca6 have the same breakthrough point. We also reacted a sorbent prepared from the impregnation of ZnO and CuO (7.3 wt % ZnO, 2.5 wt % CaO, and 2.5 wt % CuO) with the HzS-containing reactive gas, and we compared the experimental data of this run with those of another run where the Mg3-Zn12 sorbent (9.4 wt % ZnO and 2.8 wt % CaO) was used. The addition of CuO to the Mg0-Zn0-Ca0 solid mixture did not affect the
Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 91 I
4001 L
E 3501 756 850 8-
300
-5 200 8
150 C 0 100
0
v, 50 0
Normalized Time, t/tmin Figure 16. Breakthrough curves of a magnesia carrier (CP.Mg0) impregnated with different metal oxides. r
hl
0,2 0,3 0,4 0,5 0,6 0,7 Normalized Time, t/tmin Figure 18. Effect of calcination temperature of a Al203-CaC03
I
'0
0,l
mixture on the breakthrough curve of the sorbents derived from the decomposition of CaC03.
Mg2-246 Fresh Mg2-ZnQ Sulfided
c 0 ._ u u S Q,
1.500 1.000 500
t
A
A
V
Time, min
Figure 17. Typical calcination evolution curves of carbonates at different temperatures.
shape of the breakthrough curve, and similar results were obtained for the two sulfidation runs. The experimental data of Figure 16 clearly show that the type of metal oxide that is impregnated on the solid carrier has little effect on the desulfurization performance of the sorbent. We attribute this behavior to the control of the intraparticle diffusional limitations on the observed reaction rate. These limitations are expected to be similar for the sorbents of Figure 16 since the same porous solid was used as the solid substrate. It is interesting that when the impregnated metal oxide is CaO the utilization of the sorbent is lower than that of the other samples of Figure 16. The measurement of the physical properties of samples Mg2-Ca6 and Mg3-Ca6 showed that their surface area is relatively low (11 m2/g for Mg3-Ca6), the overall pore volume is almost equal to that of the precursor solid, and the pore size distribution is narrower than that of the carrier. It is, however, difficult to attribute the lower desulfurization capacity of the CaO sorbents to changes in their internal pore structure due to the impregnation of CaO. The analysis of these sorbents with the XRD technique showed the existence of calcium silicates in these samples. Therefore, a possible explanation for the HzS removal capacity of Mg2-Ca6 in Figure 16 is that either calcium silicates do not react with H2S at 600 "C or the reaction rate is significantly smaller than that of CaO. Calcination Temperature of CaC03-Al203. The carbonate-containing metal oxide (sample AlCaC) was calcined a t high temperatures to release C02 and form CaO. The calcination reaction of CaC03-Alz03 took place in the fluidized bed reactor under NZflow a t 750 "C (sorbent AlCa-2) and a t 850 "C (sorbent AlCa-3). The effluent COZversus time plot during the calcination is shown in Figure 17. The release of C02 was faster when the calcination took place a t 850 "C than that a t 750 "C. The decomposition of CaC03 a t two temperatures
Pore Diameter, microns Figure 19. Comparison of mercury intrusion data of the unreacted and sulfided Mg2-Zn6 samples.
gave sorbents differing in their pore structure, but with the same chemical composition. The ICP/AES analysis of the two sorbents showed that the CaO content is about 4156, in accordance to the CaC03 content. Calcination of the porous solids at 850 "C led to lower pore volume and surface area and caused a shift of the most probable diameter toward larger pore sizes. The porosimetric data of the AlCa-2 and AlCa-3 samples are shown in Figure 1 and the surface areas are shown in Table 4. Sorbents prepared at different calcination temperatures were reacted with HzS to examine their ability t o remove HzS. The experimental curves of Figure 18 show that the AlCa-3 sorbent has better H2S adsorption ability than AlCa-2. It can be, easily, concluded that the calcination temperature determined the structure of the interior pore space of the calcined sample and severely affected the sulfidation rate of the sorbents. A possible explanation for this behavior is that the sample calcined at 750 "C (with surface area and porosity higher than those of the sample calcined at 850 "C) is characterized by a large number of small pores that may become plugged during sulfidation causing the formation of inaccessible pore space. When this is the case, the absence of small pores in the sample calcined a t the higher temperature can improve the desulfurization capacity of the sorbent with respect to that of the sorbent calcined a t the lower temperature.
Characterization of the Sulfided Sorbents The solid product of the sulfidation reaction occupies more space than the initial oxide (for instance, the ratio between the molar volume of ZnS and ZnO is 1.6). Therefore, the pore structure of the sulfided solids may be different than that of the unreacted sample. A
92 Ind. Eng. Chem. Res., Vol. 34,No. 1, 1995
Figure 20. SEM micrographs of sample C3.MgO fresh ( a )and impregnated with CaO (b).
detailed investigation of the structural changes during sulfidation was performed in the work of Efthimiadis and Sotirchos (19934. They measured the evolution of the pore size distribution with the extent of the sulfidation reaction, and their results showed that the desulfurization performance of a porous solid depends on the initial pore size distribution and on the interaction of pores of different size in the porous solid. Structural property changes during the sulfidationregeneration reactions of zinc ferrite were also measured by Woods et al. (1990). In both studies lower overall pore volume and average pore diameter were measured after sulfidation. Changes in the internal pore structure of a sorbent (surface area and pore volume loss) may decrease its H2S sorption ability. This means that a sorbent of stable pore structure during sulfidation is expected to maintain its desulfurization capacity during multiple sulfidation-regeneration cycles.
We analyzed the pore structure of our sorbents reacted with H2S using mercury porosimetry and nitrogen adsorption. Typical results are shown in Figure 19 for a zinc oxide sorbent impregnated on a magnesia substrate. Negligible differences were measured between the interior void space of the solid before and after the first sulfidation reaction implying that structural changes due to the reaction do not practically affect its pore structure. Notice that the structural changes that are due to a gas-solid reaction with solid product formation are restricted by the presence of inert solids in the sorbents. Similar results were obtained for the other samples of this study. The examination under a scanning electron microscope (SEM) of a porous sorbent helps to develop a better picture of its pore structure. SEM micrographs of Al203-Fe203 mixtures, unreacted, reduced, and sulfided, were examined by Patrick et al. (1993). The
Ind. Eng. Chem. Res., Vol. 34,No. 1, 1995 93
structural changes of their samples (initially smooth pore surfaces were converted to textured, faceted crystals, and vice versa) depended on the involved solid reactants. On the other hand, Lew et al. (1992) used SEM micrographs to estimate the microstructure of zinc titanates, and they postulated that the changes in the sulfidation rate with the extent of the sorbent conversion are the result of nonuniform grain size distributions. Their micrographs showed the coexistence of small spherical grains and larger platelike grains; therefore, the initial high rate was attributed to the reaction of the former grains and the lower rate a t high conversions to the latter grains. In Figure 20 we present SEM micrographs of sample C3.MgO (particle size 125-180pm) before and after the impregnation with CaO. Notice that the initially relatively smooth surface of the solid substrate takes a grainy texture after the impregnation procedure. A similar microstructure was observed in the heat-treated sample, and the development of the grainy texture was attributed to the escape of the vapor water from the interior of the porous solid. Finally, SEM micrographs showed that the sulfided metal oxides have a smoother internal surface than the unreacted samples.
Conclusions Our sulfidation experiments in a fluidized bed reactor system showed that the preparation procedure affected the desulfurization performance of a sorbent as follows: Heat treatment of the solid carrier with water a t 600 "C improved its capacity to remove H2S before the breakthrough point. Sorbents prepared from sulfate solutions exhibited significantly smaller desulhrization capacity than those prepared from nitrate solutions. Sorbents prepared using the dry and wet impregnation techniques gave similar experimental data. Sorbents derived from different solid carriers exhibited different desulfurization capacity when they were impregnated with the same metal oxide. This behavior was attributed to differences in the initial pore structure of the porous samples. The same breakthrough point was measured when ZnO, CuO, or Mnz03 was impregnated on the same solid carrier, while a smaller breakthrough point was measured when CaO was impregnated on the same substrate. Sulfidation experiments performed using particles of different sizes (90-225 pm) showed that particles of 125-180 pm had the largest desulfurization capacity. Sorbents were prepared by the calcination of a CaC03-Al203 mixture a t 750 and 850 "C. Calcination at the higher temperature gave rise to a sorbent of lower porosity and surface area, but of larger desulfurization ability. The mercury porosimetry and nitrogen adsorption measurements showed that the structure of the internal void space of the sorbents remained almost the same afier sulfidation.
C. We acknowledge all the researchers of CPERI for our useful discussions and the assistance of Dr. Maria Goula for the preparation of some sorbents derived from magnesia. Literature Cited Ayala, R. E.; Marsh, D. W. Characterization and Long-Range Reactivity of Zinc Ferrite in High-Temperature Desulfurization Processes. Znd. Eng. Chem. Res. 1991,30,55-60. Efthimiadis, E. A.; Sotirchos, S. V. Sulfidation of Limestone Derived Calcines. Z n d . Eng. Chem. Res. 1992,31,2311-2321. Efthimiadis, E. A.; Sotirchos, S. V. Reactivity Evolution during Sulfidation of Porous Zinc Oxide. Chem. Eng. Sci. 1993a,48 (51, 829-843. Efthimiadis, E. A.; Sotirchos, S. V. Experimental Validation of a Mathematical Model for Fixed-Bed Reactors. MChE J . 19931.3, 39 (l),99-110. Efthimiadis, E. A.; Sotirchos, S. V. Effects of Pore Structure on the Performance of Coal Gas Desulfurization Sorbents. Chem. Eng. Sci. 1993c,48 (ll),1971-1984. Gupta, R.; Gangwal, S. IC;Jain, S. C. Development of Zinc Ferrite for Desulfurization of Hot Coal Gas in a Fluid-Bed Reactor. Energy Fuels 1992,6, 21-27. Lew, S.;Jothimurugesan, K.; Flytzani-Stephanopoulos,M. HighTemperature H2S Removal from Fuel Gases by Regenerable Zinc Oxide-Titanium Sorbents. Ind. Eng. Chem. Res. 1989,28, 535-541. Lew, S.;Sarofim, A. F.; Flytzani-Stephanopoulos,M. Sulfidation of Zinc Titanate and Zinc Oxide Solids. Ind. Eng. Chem. Res. 1992,31,1890-1899. Mu, J.; Perlmutter, D. D. Thermal Decomposition of Inorganic Sulfate and Their Hydrates. Znd. Eng. Chem. Process Des. Dev. 1981,20,640-646. Patrick, V.; Gavalas, G. R.; Flytzani-Skphanopoulos, M.; Jothimurugesan, K. High-Temperature Sulfidation-Regeneration of Cuo-Al203 Sorbents. Znd. Eng. Chem. Res. 1989,28,931-940. Patrick, V.;Gavalas, G. R.; Sharma, P. K. Reduction, Sulfidation, and Regeneration of Mixed Iron-Aluminum Oxide Sorbents. Z n d . Eng. Chem. Res. 1993,32,519-532. Sa, L. N.;Focht, G. D.; Ranade, P. V.; Harrison, D. P. High Temperature Desulfurization Using Zinc Ferrite: Solid Structural Property Changes, Chem. Eng. Sci. 1989,44,215-224. Tagawa, H. Thermal Decomposition Temperatures of Metal Sulfates. Thermochim. Acta 1984,80,23-33. Tamhankar, S.S.;Bagajewicz, M.; Gavalas, G. R.; Sharma, P. K.; Flytzani-Stephanopoulos,M. Mixed-Oxide Sorbents for HighTemperature Removal of Hydrogen Sulfide. Znd. Eng. Chem. Process Des. Dev. 1986,25,429-437. Wakker, P. J.;Gerritsen, A. W.; Moulijn, J. A. High Temperature H2S and COS Removal with MnO and FeO on y - A l 2 0 3 Acceptors. Ind. Eng. Chem. Res. 1993,32,139-149. Westmoreland, P. R.; Gibson, J. B.; Harrison, D. P. Comparative Kinetics of High-Temperature Reaction Between H2S and Selected Metal Oxide. Environ. Sci. Technol. 1977,11(5),488491. Woods, M. C.; Gangwal, S. K.; Jothimurugesan, K.; Harrison, D. P. Reaction between H2S and Zinc Oxide-Titanium Oxide Sorbents. 1. Single-Pellet Kinetic Studies. Znd. Eng. Chem.Res. 1990,29,1160-1167. Woods, M. C.; Gangwal, S. K.; Jothimurugesan, K.; Harrison, D. P. Kinetics of the Reactions of a Zinc Ferrite Sorbent in HighTemperature Coal Gas Desulfurization. Znd. Eng. Chem. Res. 1991,30,100-107.
Received for review April 11, 1994 Revised manuscript received August 28, 1994 Accepted September 8,1994@ IE940244Q
Acknowledgment This work was funded by the Commission of the European Community, under Contract No. JOUF-0051-
Abstract published in Advance ACS Abstracts, November 1, 1994. @
