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I n d . Eng. Chem. Res. 1991,30, 2592-2595
The Netherlands; Akzo Chemicals Division: Amersfoort, The Netherlands, 1991; paper H-1. Moore, P. K.; Akgerman, A. Comments on Upgrading of Middle Distillate Fractions of Syncrude from Athabasca Oil Sands. Fuel 1985, 64, 721-722. Nooy, F. M.; Lee, S. L.; Yoes, J. R. Applications of Ketjenfine-840. Proceedings of the Ketjen Catalyst Symposium, Scheveningen, The Netherlands; Akzo Chemicals Division: Amersfoort, The Netherlands, 1986; paper H-3. Wilson, M. F.; Kriz, J. F. Upgrading of Middle Distillate Fractions of Syncrudes from Athabasca Oil Sands. Prepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1983, 28, 640-649. Wilson, M. F.; Kriz, J. F. Selected Aspects of Catalytic Refining of Middle Distillates from Athabasca Syncrudes. Prepr.-Am. Chem. Soc., Diu. Pet. Chem. 1984a, 29, 284-291. Wilson, M. F.; Kriz, J. F. Upgrading of Middle Distillate Fraction of Syncrude from Athabasca Oil Sands. Fuel 198413, 63 (21, 190-196.
Wilson, M. F.; Fisher, I. P.; Kriz, J. F. Hydrogenation of Aromatic Compounds in Synthetic Crude Distillates Catalyzed by Sulfided Ni-W/A1203. J . Catal. 1985,95, 155-166. Wilson, M. F.; Fisher, I. P.; Kriz, J. F. Cetane Improvement of Middle Distillate from Athabasca Syncrudes by Catalytic Hydroprocessing. Ind. Eng. Chem. Prod. Res. Deu. 1986, 25 (4), 505-511. J a m a l A. Anabtawi,* Syed A. Ali Petroleum and Gas Technology Division The Research Institute King Fahd University of Petroleum and Minerals Dhahran 31261, Saudi Arabia
Received for review December 3, 1990 Revised manuscript received June 11, 1991 Accepted September 13, 1991
Reaction of Trace Mercury in Natural Gas with Dilute Polysulfide Solutions in a Packed Column The natural gas produced around the world can contain traces of mercury which have to be removed. I t is difficult t o purify gas to desired mercury levels using conventional techniques. By scrubbing with dilute polysulfide solution, the residual mercury in the gas can be removed from about 0.1 to below 0.01 ppb, a reduction of 90%. In the system, the gas is passed through a packed tower wetted with a solution containing 3 ppm of polysulfide salt. Stainless steel packings are effective for this application. In addition to promoting gas-liquid contact, the stainless steel packings adsorb and concentrate polysulfides which react with Hg in the gas to form insoluble HgS, and thus remove Hg from the gas.
Introduction Natural gases found around the world contain trace amounts of mercury (Phannenstiel et al., 1975). The mercury contents were reported to be 200-300,180,50-80, 1-9 and 0.005-0.04 ppb (parts per billion) for gases from Sumatra, Groningen, Algeria, the Middle East, and North America, respectively (Bodle et al., 1980). For gas with a molecular weight of 22.4, the Hg concentration of 1 pg/m3(NTP) equals 1ppb. Thus, for natural gases having a molecular weight of about 22, Hg concentrations in terms of pg/m3(NTP) are close to that in ppb. Mercury can be a major source of concern in liquefied natural gas (LNG) plants (Leeper, 1980), and many potential methods for Hg removal from natural gas have been suggested (Bodle et al., 1980). For example, sulfur-impregnated activated carbon at about 175 OF was found to be better than other non-carbon-based materials (Situmorang and Muchlis, 1986; Biscan et al., 1986). The concentration of mercury in the reactor effluent was found to be temperature dependent and was determined by the thermodynamic equilibrium at the prevailing reaction temperature. At 79.4 "C,the equilibrium concentration of mercury in the effluent was found to be 0.06 ppb. In an effort to reduce the residual mercury concentration to below 0.01 ppb, the present study was undertaken to investigate the reaction of mercury with aqueous solutions of polysulfides at room temperature. Reaction of Hg and S. Sulfur reacts with mercury reversibly in the gas to form solid HgS HgS(s) * 2Hg(g) + S&) Keq F? (PHg)2(PS,) where K is the thermodynamic equilibrium constant (atm3)an! P h and Ps2 are partial pressures of Hg and S2 (atm), respectively. 0888-5885/91/2630-2592$02.50/0
The equilibrium constants, KB9,over the temperature range of 610-650 K have been reported by Kelley (1937). Based on Kelley's data, the plot of In Kegvs 1000/T is a straight line (see Figure 1)and linear extrapolation to a lower temperature could be justified. In addition, this relationship has been verified for a commercial operation at 340 K. Thus, Kw can be represented as follows: In Keq= -44544/T
+ 51.068
where T is temperature, K. In order to reduce the concentration of Hg in the treated gas to below 0.01 ppb, the reaction temperature must be decreased to lower the equilibrium constant. However, at low temperatures, the reaction rate becomes low. One approach to increase the rate of reaction is to create a large surface area of sulfur for reaction by dispersing it on a high surface area carrier such as activated carbon (Biscan et al., 1986). However, when it is operated at low temperatures, the mass-transfer zone in the adsorption bed increases, which not only necessitates a large bed but also diminishes the adsorption capacity of the adsorbent. Another approach is to dissolve sulfur in water and use the sulfurcontaining water to remove the mercury from the gas by scrubbing. The solubility of sulfur in pure water at 20 OC is low, at 1.9 X mol of SB/kg,or 4.86 ppb (Boulegue, 1978), but it can be increased by increasing the pH of the water to >7. In alkaline water, the solubility of sulfur increases by the formation of polysulfides, Sz2-,and much of the sulfur in the solution exists as polysulfides. To increase the pH of the solution, NaOH, KOH, and amines such as monoethanol amine, diethanol amine or ammonia can be used. Polysulfide ions react with mercury to give HgS, according to
0 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No. 12, 1991 2593
I
POLYSULFIDE
SOLUTION
RESERVOIR
UT
x 103, * K
RECYCLE PUMP
Figure 1. Effect of temperature on equilibrium constant.
Polysulfides have been found to be effective in removing elemental mercury from wastewaters (Findlay and McLean, 1981). At the optimum pH of 9-11, the Hg in the wastewaters from a mercury-cell chlor-alkali plant was reduced from 570 to 1-4 ppb by adding 50 ppm of NaHS and 50 ppm of Na2S,. Reaction of Trace Mercury in Gas with Polysulfides. In the gas-treating scheme, the mercury-containing gas is contacted with a sulfur/polysulfide aqueous solution. The mercury is removed from the gas by reacting with the sulfur/polysulfide to form insoluble HgS in the solution. Preliminary experiments indicated that, in addition to the concentration of polysulfides, the rate-limiting steps are the adequate contact between the gas and the liquid and the rate of mass transfer of mercury from the gas into the liquid phase. In order to accelerate these two rate-controlling steps simultaneously, the gas is contacted with the liquid by allowing it to flow over high surface area packings which are wetted with a thin film of the solution. High surface area packings, such as stainless steel Propack Cannon packings, ensure a good contact between the gas and the liquid. The thin liquid film shortens the diffusion path of mercury in the liquid and minimizes the masstransfer limitation. The stainless steel packing surface also serves to adsorb and enrich the concentration of polysulfides and accelerate the reaction.
Experimental Section Hg-Containing Gas. Nitrogen gas was purified with activated carbon. The purified gas was passed through a mercury generator to obtain the mercury-containing feed gas for the test. The mercury generator was a porous capsule containing pure mercury. The mercury content in the gas was controlled within the range of 0.05-1 ppb by changing the configuration of the mercury generator. Scrubbing Solution. The aqueous solution of various polysulfides was prepared by diluting a stock solution with water. The stock solution of polysulfides was prepared by reacting NazS with sulfur and water. Equipment Setup. A schematic of the flow system is shown in Figure 2. The absorption columns were made of two glass tubes of 2.5-cm i.d. and 15-cm length. The columns were packed with 15 cm3of packings. The column packings were divided into two beds in an attempt to improve liquid distribution. Two types of packings were investigated: 0.16-in. stainless steel Propack Cannon packing and rings of 1/4-in. Tygon tubing. Test Procedure. The absorption solution was pumped to the top of the column, flowed down through the packing, and finally was collected at the bottom for recycling. Feed gas containing mercury was introduced at the bottom of the column, passed through the wet column and a Teflon
Figure 2. Experimental setups. Table I. Hg Removal in Batch Systems (Scrubbing Solution, 400 cm3; Temperature, 50 "C) run no. 1 2 3 4 5 6 7 8
sulfur," gas rate, no. of stage PPm L/h 100 100
100 100 50 50 100 100
11.46 11.46 11.46 11.46 11.28 11.28 5.00 5.00
1 1 2 2 1
1 1 2
Hg
Hg,ng/m3
removal,
feed product
% 51 41 91 89 41 55 90 90
0.76 0.78 0.78 0.73 0.78 0.85 0.05 0.05
0.38 0.46 0.07 0.08 0.46 0.38 0.005 0.005
Solution concentration of sulfur from polysulfides.
demister, and finally exited at the top and was analyzed for mercury. All runs were conducted at 25 O C and 1atm. The mercury in the gas reacted with the S,2- in the solution to form HgS which deposited on the wet packings and was eventually washed off by the solution. Pressure drop through the column was measured with a water manometer. The feed and exit gases were analyzed for mercury using a Jerome Mercury Vapor Analyzer (Model 301).
Results and Discussion Mercury Removal in Batch Systems. To assess the kinetics of Hg removal by scrubbing, preliminary studies were conducted in a batch system. The gas-scrubbing bottle fitted with medium-pore glass frit was filled with 400 cm3 of polysulfide solution containing 50 or 100 ppm of sulfur. The Hg-containing gas was passed through either one or two bottles to stimulate one- and two-stage operation, and the effluent gas was analyzed for Hg contents. The results in Table I show that the reaction is mass transfer limited for high Hg content feeds and becomes thermodynamic limited when the Hg contents of the feeds are low. In the system configuration, the Hg removal from high Hg feed is about 40-50% (runs 1and 2) and about 90% (runs 3 and 4) for one- and two-stage operations, respectively. The amount of Hg removed in each stage, whether in a one-stage or two-stage operation, was about the same. These results indicate that the contact between the gas/Hg and the solution/sulfides is the critical parameter. This conclusion is supported by the fact that the Hg removal was not affected by chwges in sulfide concentrations from 100 to 50 ppm (cf. runs 1 and 2, 5 and 6). For feed with 0.W ppb of Hg, the Hg content of the effluents remained 0.005 ppb for one- and two-stage operation (runs 7 and 8), indicating that the concentration of Hg in the effluent has reached its equilibrium value after
2594 Ind. Eng. Chem. Res., Vol. 30, No. 12, 1991
2
d
0 01
8
0 005
PRODUCT
x
0,001 0
4
12
8
16
20
24 28
32
36
40
44
48
52
60
56
72 76
64 66
ON S T R E A H T l M L , cavs
Figure 3. Hg concentration in feed and product and Hg removal vs on-stream time.
6C e
E"
50 1 0 0.5
5.10 0.01
; 0.01 0.005 0,001
0
I
,
4
6
', !2
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,
I
,
,
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24
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, 36
, 40
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,
,
,
,
55
60
64
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I
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T l r E , DAYS
Figure 4. Hg concentration in product and Hg removal vs onstream time.
the first stage. In fact, the calculated equilibrium Hg concentration at 50 O C was 0.007 ppb, which is in good agreement with the 0.005 ppb found for runs 7 and 8. Mercury Removal in a Column with Stainless Steel Packing. Water containing 3 ppm of polysulfide circulated through a stainless steel packing column is effective in removing mercury from gases to a low level at room temperature. In the 2.5-cm-id. column packed with 30 cm of stainless steel Propack Cannon packing, the mercury concentration in the gas was reduced from 0.08 to 0.003 ppb (Figure 3). This corresponds to a mercury removal efficiency of 96%. There was no breakthrough for 72 days, and the mercury concentrations of the effluent remained constant at about 0.003 ppb throughout the entire testing period. Judging from the consistent results from the repeated tests throughout the run, the data are considered reliable. Effect of Packing on Mercury Removal. Unexpectedly, the stainless steel packing alone, wetted with water, was found to be effective for removing mercury from gases. In a control run, the stainless steel packings were operated with water alone, without the addition of polysulfide. This control column removed mercury from 0.3 to 0.008 ppb for 30 days (Figure 4). This corresponds to 98% mercury removal. However, its mercury adsorption capacity is limited. Mercury broke through after 30 days. The mercury concentration in the effluent gas increased and the mercury removal efficiency dropped (Figure 4). It is important to note that the drop in mercury removal efficiency was gradual, and it took 16 days to drop from 96 to 50% (Figure 4). Apparently, reaction rate of mercury with the stainless steel packing is not very high, so that the distribution of mercury in the packed column does not form a sharp moving band. To positively demonstrate that the wet stainless steel packings themselves are effective for removing mercury, another control experiment was carried out. In this test, a column was packed with Tygon (poly(viny1chloride)) rings in lieu of the stainless steel packings. In this column,
the mercury broke through quickly. After 3 days of operation, the column had a 17% mercury removal efficiency. It is well-known that Tygon is inert to mercury. We believe that the inertness of the Tygon surface to mercury leads to poor mercury removal efficiency. These results underscore the contribution of adsorption of the stainless steel packing in overall mercury removal. The total surface area and the contact efficiency of the packings are important in mercury removal. When the mercury removal efficiency with water in the column containing stainless steel packing reached 50%, 3 ppm of polysulfide was added to the circulating water. Instantaneously, the mercury removal efficiency returned to 95% (Figure 4). This result demonstrates that the packing is effectively distributing the polysulfide over itself to provide the required stage for reaction and for promoting gasliquid contact. For comparison, when 3 ppm of polysulfide was added to the column packed with Tygon rings with a low surface area, the mercury removal efficiency increased from 17 to 50430%. Further increases in polysulfide concentration to 10 and 20 ppm did not improve the mercury removal efficiency. These results show that because of the low surface area in the Tygon column, the mercury removal efficiency is limited by gas-liquid contact. The effects of packings to adsorb polysulfide on mercury removal were investigated by bubbling the gas through vigorously stirred polysulfide solutions in the absence of packings. The concentration of polysulfide required for 95% removal of mercury was greater than 200 ppm. Since the same high mercury removal was achieved in the stainless steel packing using 3 ppm of polysulfides, it was concluded that the effective concentration of active sulfur on the surface of the packings is higher than that of the bulk liquid phase leading to higher mercury removal efficiency. These results suggest that the stainless steel packings contribute to overall mercury removal in, a t least, three ways: (1)they adsorb and react with Hg itself; (2) they promote gas-liquid contact; (3) they adsorb and concentrate polysulfides on the surface to facilitate reaction between Hg and polysulfides. Effect of Polysulfide Concentration, In the stainless steel packed column, the concentration of polysulfide required for adequate mercury removal is low. The packing adsorbs and concentrates polysulfide on the surface to facilitate the solution for scrubbing. Figures 3 and 4 show that 3 ppm is sufficient to achieve over 95% mercury removal. In the experiment, column 1 (Figure 4) was removing 50% of mercury with pure water at 43 days of on-stream time. As 3 ppm of polysulfide was added, the mercury removal efficiency jumped to over 95%. The real minimum polysulfide concentration required was never determined. The concentration of polysulfide required for adequate mercury removal depends on the nature and contacting efficiency of the packings. In contrast to the Tygon rings, the stainless steel packing is particularly effective because it can adsorb and retain active sulfur species to achieve the effect of enhancing the sulfur concentration on the surface for reaction with mercury. Even though the sulfur concentration was very low at 3 ppm, it is 2000 times more than the stoichiometric requirement of the reaction with the mercury in the gas. This calculation is based on the assumption the gas to scrubbing solution weight ratio is 15. Residual Activity of Packings and Resiliency of the System. The stainless steel packings apparently adsorb active sulfur and remain reactive for mercury removal for
Ind. Eng. Chem. Res., Vol. 30, No. 12, 1991 2595 Table 11. Effect of Gas Velocity on Hg Removal (Polysulfide Concentration, 3 ppm; Temperature, 25 "C; Pressure, 1 atm) diameter, cm packing height, cm gas flow, cms/min superficial gas velocity, cm/s Hg removal, %
2.5 30 400
1.4 97
2.5 30 670
2.3 95
0.7 30 300 13 92
an extended period of time. Thus, the polysulfide addition to the system can be intermittent. In column 1 (Figure 4), 3 ppm of polysulfide was introduced into the system on the 44th day of operation for 3 days. As mentioned above, the mercury removal jumped from 50 to 95% (Figure 4). The 3 ppm of polysulfide solution was then drained, and the column was rinsed and refilled with water. Without polysulfide in the circulating water, the mercury removal remained at -95% for an extended period of time, showing the residual sulfur activity of the packing. Effect of Superifical Gas Velocity. When stainless steel Propack Cannon packing was used, over 90% mercury removal could be achieved at a superficial gas velocity of up to 13 cm/s in a 30-cm-high bed. The superficial gas velocity in the 2.5cm columns ranged between 1.4 and 2.3 cm/s. In order to test it at a higher gas velocity, a smaller 0.7-cm-diameter column was used a t a flow rate of 300 cm3/min, which corresponded to a superficial gas velocity of 13 cm/s. As shown in Table 11, increasing the superficial gas velocity by an order of magnitude only decreased the removal efficiency from 97 to 92%. For a constant superficial gas velocity, the mercury removal efficiency should increase with an increase in the height of packings due to increased gas to liquid contact time. Effect of Superficial Liquid Velocity. For mercury removal, water only serves to wet the packings and distribute the polysulfide for reaction with mercury. As a result, the scrubbing solution to gas ratio required is much lower than normally circulated in scrubbers for gas treating. In the laboratory, the minimum water circulation rate was limited by the lower limit of the pump available. In an attempt to determine the minimum liquid velocity and/or the solution/gas ratio required for efficient mercury removal, the pump was operated at 1-2 cm3/h for a superficial velocity of 0.2-0.4 cm/h. Since this rate was below its specified capacity, the pump did not operate well at such low rates and several times the columns ran dry for periods of hours. However, the mercury removal efficiency was not affected significantly. It is tempting to conclude that the liquid velocity or liquid/gas ratio is not a critical factor, as long as the packing is kept wet with the polysulfide solution or coated with active polysulfide. Apparently, the stainless steel packings adsorbed the polysulfides. The adsorbed polysulfides served as a reservoir and continued to react with Hg after the circulation of polysulfide solution was stopped. Fate of HgS Product. Mercury reacts with polysulfides to form HgS of low solubility which is deposited on the stainless steel packings. Since HgS is a black precipitate, the stainless steel packings gradually turned dark during the test. Because of its cavities and irregular surface, the stainless steel packings can expect to hold significant amounts of HgS. At a typical gas residence time of 10 s, the capacities of the packing are estimated to be
550 and 5.5 years for treating gases containing 1and 100 ppb Hg, respectively. The testing period was too short to quantify the holding capacity of the stainless steel packings for HgS, however. As the HgS deposits on the packings increases, the HgS deposits could be dislodged and end up in the wastewater. However, no HgS was found in the wastewater in this study because the test was too short. HgS in wastewater could be recovered by filtration or centrifuging.
Conclusions By scrubbing with dilute polysulfide solution, over 90% of the residual mercury in the gas was removed to a level below 0.01 ppb. The gas was contacted with stainless steel packings wetted with a solution containing about 3 ppm of polysulfides. The mercury in the gas reacts with the polysulfides to form insoluble mercuric sulfide and is removed from the gas. In the laboratory tests, the system was shown to be effective and to have a large capacity for mercury removal. The stainless steel packings promote gas-liquid contact and adsorb mercury and polysulfide to facilitate the reaction between mercury and polysulfide to form insoluble mercuric sulfide. The polysulfide can be added to the system continuously or intermittently. For over 90% mercury removal, the gas superficial velocity can be as high as 13 cm/s. The liquid velocity is not critical because the packings serve as a reservoir for polysulfides. Acknowledgment I acknowledge the invaluable contributions by R. K. DeMura, who faced the tedious work with enthusiasm, and Drs. C. A. Audeh, N. Y. Chen, and F. P. Ragonese for their valuable discussions. Registry No. Hg,7439-97-6.
Literature Cited Biscan, D. A.; Gebhard, R. S.; Matviya, T. M. Impact of Process Conditions on Mercury Removal from Natural Gas Using Activated Carbon. Eighth International LNG Congress, Los Angeles, June 15-19, 1986; paper 1 11-5. Bodle, W. W.; Attari, A.; Serauskas, R. Considerations for Mercury in LNG Operations. Papers-International Conference on Liouified Natural Gas (ICLNBT). V 6the (1) 1980: Session 11, _Paper i; ISSN 0197-2782. Boulegue, J. Solubility of Elemental Sulfur in Water at 298K. PhosDhorus Sulfur 1978.5 (1). 127-128. Findlay, D. M.; McLean, R: A.' N. Removal of Elemental Mercury from Wastewaters Using Polysulfides. Environ. Sci. Technol. 1981,15,(ll),1298-1390. Kelley, K. K. Contribution to the Data on Theoretical Metallurgy-The Thermodynamic Properties of Sulfur and Ita Inorganic Compounds. Bur. Mines Bull. 1937,No. 406, 52. LeeDer, J. E. Mercurv-LNG's Problem. Hydrocarbon Process. f980,59,(ll),237-240. Phannenstiel. L. L.: McKinelv. C.: Sorensen. J. C. Mercurv in Natural Gas. 'Progress in Rejrigeration Science and Teihnology, Proceedings of the International Congress of Refrigeration, 14th, Moscow, 1975. Situmorang, M. S. M.; Muchlis, M. Mercury Problems in the Arun LNG Plant. 8th Int. Gas Union-Int. Inst. Refrig.-Inst. Gas Technol, Jt. Int. LNG Congr., Los Angeles, June 15-19, 1986; paper 1 11-6.
Tsoung Y.Yan Mobil Research and Development Corporation P.O. Box 1025, Princeton, New Jersey 08543-1025 Received for review April 24, 1991 Revised manuscript received August 20, 1991 Accepted September 5, 1991
