18
Ind. Eng. Chem. Fundam. 1982, 21, 18-22
Adsorption and Oxidative Desorption of Hydrogen Sulfide by Moo,-TiO, Shhnpel Matsuda, Tomdchl Kamo, Jlnlchl Imahashl, and Fumlto Nakajlma Hitachi Research Laboratory of Hitachi Ltd., Hitachi, Ibaraki, Japan 3 19 12
A mixed oxide of Moo3 and TiO, has been found to adsorb hydrogen sulfide and to be regenerated easily by an oxidative desorption process. The adsorbent takes up nearly one mole of H2S per mole of Moo3 according to the followkrg two reactions: H.$ Moo3 = MoO3*SH, and I-& Moo3 = MoO,*S H20. The oxidattve desorption is performed by introducing an oxygen-containing gas through the adsorbent bed. The desorbed gas contains SO, as welt as a small amount of elemental sulfur. A cyclic test was carried out to examine the regenerabli of the adsorbent, and it was found that the adsorption capacity decreased slightly in the first 10 cycles but remained constant in 10-25 cycles.
+
Introduction There have been known several solid materials for the adsorption or absorption of hydrogen sulfide. The adsorbents are classified into two types, i.e., nonregenerable and regenerable (Nonhebel, 1964). Nonregenerable adsorbents (abrbents) commonly known are ZnO, CuO, and Fe2O3, among which ZnO is commercially used to remove small quantities of hydrogen sulfide from a gas stream in the hydrodesulfurization process of light petroleum fractions (Cockerham and Percival, 1966). Zinc oxide is converted to ZnS as a result of hydrogen sulfide absorption. Since the process accompanies a chemical reaction, it is called "reactive adsorption". The regeneration of the adsorbent may not be impossible, but it is generally very difficult. Regenerable adsorbents well known are active carbon (Ramachandran and Smith, 1978) and molecular sieves (Chi and Lee, 1973). They adsorb hydrogen sulfide at relatively low temperatures by a physicochemical process and they are thermally regenerated, desorbing hydrogen sulfide. The adsorption capacity of these adsorbents is usually influenced by the presence of coexisting gases, such as H20and COP The adsorbents are commercially utilized in the natural gas purification and odor control process. It has been discovered that an adsorbent consisting of Moo3 and Ti02 adsorbs hydrogen sulfide at low temperatures (100-300 "C) and is readily regenerated by an oxidative process, desorbing sulfur dioxide. The desorption is performed by introducing an oxygen-containing gas through the adsorbent bed at about the same temperature as the adsorption process. Characteristic properties of the adsorption and desorption as well as the mechanism will be discussed in the present paper. Experimental Section Experimental Apparatus. Figure 1 is a schematic diagram of the experimental apparatus. The reaction tube which contained an adsorption bed of 40-80 mL adsorbent was a Pyrex vessel, about 50 cm in length and 2.7 cm in inner diameter. The reactor was heated externally and the temperature of the bed was measured by a thermocouple gauge. Gas mixtures used in the adsorption and desorption experiments were prepared by mixing 10% S02/N2,10% H#/N,, 0 2 , H2, and Nz gases from cylinders. When steam was contained in the gas mixtures, N2 gas was passed through a steam generator whose temperature was regulated. The gas leaving the reactor was dried by passing
+
+
through concentrated sulfuric acid solution prior to the gas chromatographic analysis. The concentration of H2Sand SO2in a gas entering and leaving the reactor was measured by a gas chromatograph equipped with a 2 m long column packed with Porapak R. Preparation of Adsorbent. The adsorbent consisting of Moo3 and Ti02 (Mo:Ti = 3 7 atomic ratio) was prepared using metatitanic acid and ammonium paramolybdate according to the flow sheet shown in Figure 2. The adsorbent was molded into a columnar shape of 3 mm diameter and 3 mm length or 6 mm diameter and 6 mm length. The adsorbent was crushed and sieved to 10-20 mesh in most experiments. The adsorbent was found to possess pore volume of 0.17 mL/g (measured by a mercury porosimeter) and surface area of 86 m2/g (measured by the BET method). Materials. Gas mixtures containing 10% S02/N2and 10% H#/N2, and O2gas, H2gas, and N2gas were supplied by Nippon Sans0 Co. as pure grade and were used without further purification. The starting materials of the adsorbent, i.e., metatitanic acid and ammonium paramolybdate were supplied by Ishihara Industry Co. and Wako Chemical Co., respectively. X-ray Analysis. The X-ray diffraction analysis was performed to examine crystallographic forms of molybdenum and titanium oxides in the adsorbent after several kinds of experiments. The Cu Ka line was employed using a Rigaku-Denki Model D3F X-ray diffractometer. Experimental Procedure. At the center of the reactor 40 mL of 10-20 mesh size adsorbent was placed and heated to 150 "C. For the measurement of breakthrough curve an N2gas containing 1% H2Swas passed through the bed at a flow rate of 80 NL/h (normal liter per hour), producing a gas space velocity of 2000 (vol/vol)/h. The analysis of exit gas was performed every 5-10 minutes and the H2S adsorption capacity was calculated from the breakthrough curve. After the adsorption experiment the reactor was completely purged with N2 gas for about 20 min. For the regeneration of the adsorbent, N2 gas containing 3% O2 was introduced into the reactor at a rate of 80 NL/h, until SO2which was the major product in the oxidative desorption became negligible. In an adsorption-deaorption cyclic test the purging procedure described above was also undertaken after the oxidative desorption. A few experiments were conducted to examine thermal desorption products. In this case the adsorption bed was heated from 150 to 450 "C for 1.5 h, while N2 gas or N2
0196-43 i 3 l a 2 l i o 2 i - o o 1 8 ~ o i . 2 5 l o 0 1982 American Chemical Society
Ind. Eng. Chem. Fundam., Vol. 21, No. 1. 1982 19
Figure 3. Adsorption bed after 30 min of H a adsorption. Upper mne with black color is uaed for the adsorption.
Figure 1. %hematic diagram of erperimental apparatus.
New 10min 2Omin 30min -
-water MI x
Ing-hneadinE
fl7OC-Ion 1
T a b l e t t Ing
13mme-m"
I
14SOC-2h I
-Q
60min 120min180min
0
16"
, 'L I
Figure 2. Preparation of Md8-TiOl adsorbent.
Table I. Standard Experimental Conditions gas mixture
space velocity temperature adsorbent
adsorption H,S 1% N, balance
desorption (a) oxidative 0, 3% N, balance (b) thermal H,O 0 , 30% N, balance 2000 vol/(vol h ) 150 'C Moo,-TiO, (Mo/Ti = 3/7 atom) 10-20 mesh, 40 mL
gas containing 30% H20was passed through the bed. The standard experimental conditione are sUmmaRzed ' inTable I. Experimental Results a n d Discussion Adsorption of A2S by Mo03-Ti02. The Mo0,-Ti02 adsorbent has a slightly bluish oolor before H a adsorption. Since the color changes to black-blue at the H2S adsorp tion, the adsorption zone in the bed is easily observed and photographed in the experiment. Figure 3 is a photograph of the adsorption bed after 30 min of adsorption. The boundary between the reacted and unreacted zones is clearly seen. The adsorption zone shifted downward as the adsorption progressed. Using a large size adsorbent with columnar dimensions of 6 mm diameter and 6 mm length, the progress of the H a adsorption in one adsorbent particle was investigated. F i 4 is a photograph showing a crowseetional view of the adsorbent treated by gas for a different duration. It can be seen that the layer of the reacted zone becomes thicker as the adsorption time elapses. The thickness was about 0.6 mm a t 30 min. Adsorption and Oxidative Desorption. A typical breakthrough curve for the H2S adsorption and the SO,
Figure 4. Crwwect~ 'onal view of an adsorbent particle treated by H a for different duration. -mal layer with black mlor is reacted Zone. Adsorbent: 6 m m diameter, 6 mm length; gas mixture: 1% Ha in N1. 0.4g(13 adsorption capacity regenera tion H,O formatio n = a
mmol) (> 85 mmol) 7.0 g of S/100 g of ad. (S/Mo = 0.72 atomic ratio) S desorbed/S adsorbed > 0.89
1
1 5
adsorption 0.90 g oxidative desorption ( 5 0 mmol) 0.50 g (30 mmol)
Only semiquantitative,
measured by the methylene blue method. The decline of the H2S removal after the breakthrough is quite slow, which is a typical characteristic of an adsorption accompanying a chemical reaction. The amount of H a adsorbed during the 150 min was calculated to be 3.22 g or 95 mmol. The adsorption capacity was calculated to be 7.0 g of sulfur per 100 g of adsorbent, or the S to Mo atomic ratio is 0.72, since the weight of the adsorbent used was 44 g, or the Mo content in the adsorbent was 130 mmol. The S to Mo atomic ratio seems to approach unity if the duration of the adsorption is long enough. As shown in Figure 5b, the SO2 concentration in the desorbed gas was about 2% for about 60 min, when N2 gas containing 3% O2 was used in the oxidative desorption. It was observed that the temperature of the bed was raised from 150 to 230 O C due to the high exothermicity of the oxidative desorption reaction. It was estimated that the temperature rise could be 250 "C, if the oxidative desorption was carried out adiabatically. The oxygen concentration in the desorbed gas was also monitored with a Beckman Oxygen Analyser (Model 777) and found to be negligible as long as SO2was formed and to rise to 3% at 90 min. During the oxidative desorption, deposition of elemental sulfur was observed in the downstream portion of the adsorption bed. After the experiment, the deposited sulfur was scraped out from the reactor and was found to weigh not less than 0.4 g. The total amount of sulfur desorbed was more than 85 "01, Le., 72 mmol as SO2and more than 13 mmol as elemental sulfur. The regeneration factor which is defined as the ratio of S desorbed to S adsorbed was more than 0.89. The analysis of the experimental data shown in Figure 5 is summarized in Table 11. It is mentioned here that the formation of H20 which was measured by the weight increase in the H2S04trap (see Figure 1) was observed during the adsorption as well as in the oxidative desorption process. The amount of H 2 0 was 50 and 30 mmol in the adsorption and oxidative desorption, respectively. The total amount of HzO formed (80 mmol) was slightly smaller than that of H2S adsorbed (95 mmol) due to incompleteness of the HzS04trap. Cyclic Test. An adsorption-desorption cyclic test was performed to examine the regenerability of adsorbent using an adsorbent of 3 mm diameter and 3 mm length. A gas mixture containing 1% H2S,30% H20, and Nz balance was used in the adsorption and 3% O2 balance N2 in the oxidative desorption (regeneration). The adsorption was carried out until the HzS removal became less than 80%, and the desorption until SO2concentration in the desorbed gas became less than 0.02%. The reactor was purged for 10 min by N2 after adsorption and desorption. The test result for 25 cycles is shown in Figure 6 in which the vertical axis represents the amount of HzS adsorbed at the
20 C y c l e numner
10
25
15
30
Figure 6. Change of adsorptioncapacity in cyclic test. Adsorbent: 3 mm diameter, 3 mm length; gas mixtures: 1% H2S and 30% H20 in N2 (adsorption), 3% O2 in N2 (desorption);gas space velocity: lo00 (vol/vol)/h; temperature: 150 O C . a 3 0 0
-4.p
------
A d s o r p t on
Purging
Q
T me
b
D e s o r p t i o n b y H20
\
c4
il
Figure 7. Adsorption of H2S followed by desorption by H20. Gas mixtures: 1% H2Sin N2 (adsorption),100% N2 (purging),and 30% HzO in Nz (desorption);gas space velocity: 2000 (vol/vol)/h.
H a removal higher than 98%. The HzS uptake decreased slightly in the first 10 cycles but stayed considerably constant in the 10-25 cycles. Thus it has been shown that the adsorbent consisting of MooBand TiOz is able to adsorb HzS and to be regenerated by the oxidative desorption process. Effect of Water Vapor. It was found that the H2S adsorption capacity was lowered by the presence of H 2 0 in the gas mixture; for example, the presence of 30% HzO decreased the capacity by a factor of 2. The desorption of HzS by water vapor was examined; an example is shown in Figure 7. The adsorption was carried out for 3.5 h, where the H2S breakthrough had already occurred. After the reactor was purged for 30 min, N2gas containing 30% H 2 0 was introduced into the adsorption bed. As shown in Figure 6, the effluent gas contained high concentrations of H2S,more than 1.2% at maximum. The amount of H2S desorbed by H20 was 0.31 g, which corresponded to about one-tenth of the adsorbed HzS (3.56 g). Thus it is shown that some fraction of the adsorbed HzS is easily replaced by H20. Another experimental result showing the effect of water vapor is given in Figure 8, in which the H2S adsorption in presence of HzO was followed in the absence of HzO. The adsorption was carried out for 90 min using a gas mixture containing 1% HzS and 30% H20. The H2S removal rose from 30% to 90%, when the H20 supply was stopped at 90 min. The adsorbent picked up an additional 0.86 g of S before the H2S removal returned to 30% in the absence of H20, while it picked up 1.44 g of S in the presence of HzO. From these observations it is clear that H2S and H 2 0 adsorb on the adsorbent competitively, or HzS adsorbed is replaced by H 2 0 and vice versa. Thermal Desorption. When the adsorption bed was heated from 150 to 450 OC after the H2Sadsorption, it was
Ind. Eng. Chem. Fundam., Vol. 2 1, No. I , 1982
21
a , New
80
c b , A f t e r HqS a d w r o t i o n
I
20
-
-
H 2 0 30%-
I
c H 2 0 absent
I
I
__c
-
I
2
6
deg 1
Figure 10. X-ray diffraction pattern of adsorbent in various stages. a
After
T ~ m e mlr rb; D e s o r p t i o n b y
H,O+
N?'
thermal desorption
MOO,
A A
II
A Ti02
A Graph1 t e
-? 20
30
40
50
b # A f t e r H? r e d u c t i o n
I
'
60
1
Figure 9. Thermal desorption from Mo03-TiOz adsorbed H a . Bed temperature is denoted by dashed line. Adsorbent was treated by Nz gas containing 1% HzS for 2.5 h at a space velocity of 2000 (vol/vol)/h prior to the desorption.
Figure 11. X-ray diffraction pattern of adsorbent after thermal desorption and Hz reduction.
observed that the effluent gas contained H2S and SOz. A typical thermal desorption in the absence and presence of H 2 0 is shown in Figure 9a and b, respectively. The desorption of H2S occurred at the start of the heating, while SO2was observed above 200 "C. The amount of H2S and SO2 in the desorbed gas in Figure 9a was 2 and 26 mmol, respectively, while the H2Suptake by the adsorbent was 77 mmol. Since a fraction of adsorbed H2S was desorbed by presence of H20, a larger amount of H2S was desorbed when desorption gas contained H 2 0 as shown in Figure 9b. The amount of H2S and SO2 in the desorbed gas in Figure 9b was 20 and 27 mmol, respectively. In addition to H2S and SO2 small amounts of elemental sulfur were formed in thermal desorption. It should be emphasized that SO2 was formed despite the fact that no oxygen was supplied in the gas. The SO2, therefore, must be formed by the oxidation of adsorbed H2S by oxygen atoms in the adsorbent. Consequently, the adsorbent is reduced in the thermal desorption process. Crystallographic Analysis of Adsorbent. The change of crystallographic form of MOO, and Ti02 was followed by the powder X-ray diffraction method. The X-ray diffraction patterns of the new adsorbent, after the H2S adsorption, and after the oxidative desorption are shown in Figure loa, b, and c, respectively. In the new
adsorbent there appear peaks due to Moo3, TiOz, and graphite, which was used as a lubricating material in tabletting process. As shown in Figure lob, Moo2 as well as MOO, exists after the H2S adsorption; i.e., Moo3 is partly reduced during the adsorption. This observation is in agreement with the observation that H20 was formed during the adsorption. The diffraction pattern of the adsorbent after the oxidative desorption is the same as that of the new adsorbent. Peaks due to anatase Ti02 are unchanged throughout the cycle. Figure l l a shows the X-ray diffraction pattern of the adsorbent after the thermal desorption. It is seen that MOO, is not present at all and Moo2 gives sharp peaks. In order to investigate a reduced form of MOO,, the adsorbent was treated by hydrogen at 350 "C for 2 h; the diffraction pattern is shown in Figure llb. Molybdenum trioxide was completely reduced to MOO, by hydrogen. The relative intensity of Moo2 to Ti02 was higher after the H2 reduction than that after the thermal desorption. It may be said that MOO, is reduced to Moo2and probably to lower oxidation states after the thermal desorption. The adsorbent which had undergone thermal desorption liberated SO2when an oxgyen-containing gas was introduced. Consequently, there is a possibility that molybdenum had an intermediate form of oxide and sulfide or a mixture of
22
Ind. Eng. Chem. Fundam., Vd. 21, No. 1, 1982
them. It may be added here that the adsorbent reduced by hyclrogen possesses the ability to adsorb H a at the high temperatures, 200-450 "C. Mechanism of HzS Adsorption and Desorption. It has been found experimentally that (1)Moo3 and Mooz are present after the H a adsorption, (2) HzO is formed during the adsorption as well as during the oxidative desorption, and (3)H a and SOzare desorbed by the thermal desorption. From these observations the following two reactions are postulated for the adsorption Moo3 + HzS = MoO3.SH2 (1) Moo3 + HzS = MoOz.S + HzO (2) In reaction 2, MoOZ*Srepresents that the S atom is chemically adsorbed to Mooz and does not mean that elemental sulfur is deposited on the adsorbent. It was observed that the surface of the adsorbent became yellow when elemental sulfur was deposited on the adsorbent as a result of the low temperature Claw reaction 2HzS + SO2 = 3S(1) + 2H20 (3) As ahown in Figurea 3 and 4, the adsorbent had a dark blue color after the adsorption. The oxidative desorption could be represented by reactions 4 and 5 corresponding to the adsorption reactions 1 and 2, respectively MoO3-SH2+ 3/zOz = Moo3 + SOz + HzO
(4)
MoO2.S + 3/z02 = Moo3 + SOz
(5) These reactions explain the experimental observation that the SOz concentration in the desorbed gas was 2% when the gas contained 3% Oz in the oxidative desorption. Summation of reactions 1 and 4 or 2 and 5 gives the oxidation of HzS to SOz
HzS + 3/20z = SOz
+ HzO
(6)
The adsorption and oxidative desorption of HzS by the present method is, therefore, considered the two step oxidation of H a by Mo03-TiOs. The heats of reaction of 1 and 2 are roughly estimated to be -11 and -17 kcal/mol, respectively, considering the following reactions with known heat of reaction (Weast, 1972) Moo3 HzO = H2MoO4 AHo = -21.0 kcal/mol (7) Moo3 + HZS
MoO3.SH2 + 2Mo03 = 3MoO2
+ SOz + HzO AHo = 0 kcal/mol (11)
MoO2.S + 2Mo03 = 3MoOZ+ SOz AHo =6 kcal/mol (12) Formation of small amounta of elemental sulfur was also observed during the thermal desorption. MoOz-S = Mooz +
'/a8
AHo = 10 kcal/mol
(13)
The desorption of H a by presence of HzO is given by the following replacement reaction MoO3-SH2+ HzO = Mo03.0H2 + HzS (14) Reaction 13 is not completely reversible, because the HzS uptake in the presence of HzO is less than that in the absence of HzO followed by the treatment by an HzO containing gas. Application of Mo03-Ti02 Adsorbent. The Mo03-TiOz adsorbent can be applied to many chemical processes which need removal of HzS from a gas stream. Although the cost of the present adsorbent is considerably higher than that of a conventional adsorbent such as ZnO and FezO3, the operating cost will be much lower since the adsorbent can be used repeatedly. One of the most suitable applications of the adsorbent is to the clean-up of tail gas from a Claus sulfur recovery plant, since the desorbed gas which contains high concentrations of SOz can be recycled to the inlet of the Claus reactor. Thus in this case the function of the adsorbent is to remove H a from a gas containing low concentrations of H a and to generate a gas containing high concentrations of SOz. The application of the MoO3-TiOZ adsorbent to the Claus tail gas cleanup will be discussed in detail in a future publication.
Literature Cited CM,C. W.; Lee, H. AICXESynp. Ser. 1873, 69.95. Codtwt", R. G.; Perckal, G. I d . Eng. chem.ProaeSsaeS. Dev. 1966.
(8)
5, 253. "bd.G.. Ed. "Gas RriRcatkn Recesses"; George Newnes W e d : London. Igs4; pp 276. 321. RamachanGan,P. A.; SmHh, J. M. I d . €hg. a". Ff". 1978, 17, 17. Weast, R. C., Ed. "m of chemistry and physics"; CRC Press: c l e v s land, 1972 D D81.
AHo = -12.3 kcal/mol (9)
Received for review October 5, 1979 Accepted August 12,1981
+
MOSz + '/8S8 = MOSS
Then, the heats of reaction of (4) and (5) are calculated to be -113 and -107 kcal/mol, respectively. Reactions occurring in the thermal desorption are given by MoO3.SH2 = Moo3 + HzS AHo = 11 kcal/mol (10)
AHo = -9.0 kcal/mol
MOO^ + HzO + '/&
