Article pubs.acs.org/EF
Sulfur-Leaching Facts from Sulfur-Impregnated Porous Carbons in the Mercury Removal Process K. Suresh Kumar Reddy,* Ahmed Al Shoaibi, and C. Srinivasakannan Chemical Engineering Department, The Petroleum Institute, Post Office Box 2533, Abu Dhabi, United Arab Emirates ABSTRACT: The use of sulfur-impregnated porous carbon is widely adopted industrially for gas-phase mercury removal. It is claimed that it suffers from the disadvantage of sulfur leaching because of condensation of lighter hydrocarbons in the micropores of the porous carbon. To quantify the claim of sulfur leaching because of solubilization, specific experiments to assess the leaching of sulfur from sulfur-impregnated and metal-sulfide-based porous carbons were conducted. The results indicate a large variability in the sulfur-leaching capacity depending upon the source of sulfur-impregnated porous carbon. The work concludes the importance of the sulfur-impregnation protocol to minimize the sulfur leaching.
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INTRODUCTION Emission of mercury is a major environmental concern because it is considered to be one of the most toxic metals found in the environment and, additionally, a hazardous air pollutant (HAP) as per the recommendations of the Clean Air Act (CAA) of 1990. Mercury has been recognized to cause both acute and chronic toxicity on the nervous, renal, and reproductive systems.1 Mercury discharged into the atmosphere persists in the environment and creates a long-term contamination. Furthermore, food-chain transports, bioaccumulation, high toxicity to mammals, and severe health problems because of ingestion of mercury are well-documented, rendering strict control of mercury emission mandatory to very low levels.2 Mercury in natural gas is normally present in a low concentration; however, some sources contain sufficiently high mercury concentrations to cause both safety and health concerns. Table 1 shows the range of mercury levels in wellhead gas in a number of countries.
disposal of mercury-contaminated equipment, and (v) emissions to the environment. Owing to its adverse environmental effects as well as the operational issues of the processing industry, it is mandatory to develop and use an efficient removal system. The elemental mercury removal is quite difficult because of its high volatility and chemical inertness at a low concentration in relation to the total gas stream. Many methods are currently applied in practice, such as reversible and irreversible adsorption and photochemical oxidation. Utilization of sulfur-impregnated carbons was reported widely with substantial industrial adoption for the mercury removal from flue gases as well as in natural gas processing. Studies usually concentrate on the modification of the carbon support, optimization of the sulfur-impregnation procedure, and addition of promoters, such as halides, thiols, etc. Among the sulfur-impregnated material, a UOP adsorbent with the equilibrium adsorption capacity of around 150 μg/g at 30 °C is reported to possess the highest adsorption capacity, which had a surface area of ∼600 m2/g. However, the high surface area carbon generated in our laboratory and impregnated with sulfur was found to have an equilibrium adsorption capacity of ∼160 μg/g at 30 °C,4 slightly but not significantly better. Sulfur-impregnated porous carbon is reported and believed to be effective only in treating dry gas. The microporous structure of common porous carbon tends to become filled with liquid hydrocarbons (C3−C4+) by capillary condensation, at operating conditions close to the dew point of a gas stream. This can lead to a significant reduction of the adsorption capacity and a change of the mass-transfer zone. Additionally, it is reported that it results in sulfur being physically leached by the liquid hydrocarbons, resulting in sulfur loss and sulfur contamination in the downstream processes further to the reduced effectiveness. As a development over the sulfurimpregnated carbons, metal-sulfide-based mercury adsorbents were promoted to be an alternative, offering better activity and
Table 1. Range of Elemental Mercury Levels in Wellhead Natural Gas3 location
elemental mercury concentration in μg/m3 (ppbv)
South America Far East north Africa Groningen (Netherlands) Middle East eastern U.S. pipeline midwest U.S. pipeline North America
69−119 (8−13) 58−193 (6−22) 0.3−130 (0.03−14) 180 (22) 1−9 (0.1−1) 0.019−0.44 (0.002−0.05) 0.001−0.10 (0.0001−0.01) 0.005−0.040 (0.0005−0.004)
Mercury has been recognized to cause both acute and chronic toxicity on the nervous, renal, and reproductive systems.3,4 The presence of mercury is additionally known to contribute to (i) exposure of workers to high levels of mercury during maintenance operations, (ii) corrosion of process equipment because of liquid metal embrittlement or amalgam corrosion, (iii) poisoning of expensive downstream precious metal catalysts, such as platinum or palladium, (iv) difficulty in © 2015 American Chemical Society
Received: May 4, 2015 Revised: May 28, 2015 Published: June 2, 2015 4488
DOI: 10.1021/acs.energyfuels.5b00995 Energy Fuels 2015, 29, 4488−4491
Article
Energy & Fuels
from the activation furnace after cooling to room temperature with a continuous flow of nitrogen.4
overcoming sulfur-leaching problems because of liquid hydrocarbon condensation. However, metal oxides or metal sulfides in an alumina matrix suffer reduced performance as the alumina softens, with a lower crushing strength, and eventually fuse as a solid concrete block inside the adsorption column.12 The effectiveness of sulfur-impregnated carbons is reported to lose its activity because of leaching of condensed hydrocarbon in the micropores; the present work attempts to verify the authenticity of the statement with a suitably designed experiment covering a range of sulfur-loaded porous carbons and metal-sulfide-based porous carbon. Toward which different carbon-based adsorbents along with our in-house development, sulfur-loaded carbon and metalsulfide-based porous carbon were tested with liquid naptha, with the objective to assess the elution of sulfur from the carbon matrix upon exposure to the liquid naptha. Additionally, naptha produced from refineries around the world does possess parts per million levels of mercury, which are being treated for mercury removal to avoid processing and environmental concerns. The typical levels of mercury in various petroleum products are provided in Table 2.5 It should be noted that the
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RESULTS AND DISCUSSION The various commercial and in-house prepared carbons were tested for the equilibrium gas-phase mercury adsorption, and the sulfur utilization efficiency was estimated in the range of 0.5−4%. The efficiency was calculated in comparison to the theoretical sulfur adsorption capacity based on the percentage of sulfur loaded on the porous carbon. The low efficiency indicates the importance of the technology/process conditions to distribute the sulfur in the porous matrix. The low sulfur utilization shows the inability of mercury to bind with sulfur, owing to bulky layers of sulfur embedded on the porous matrix. An increase in efficiency corroborates with the ability to disperse sulfur at the thickness of an atomic layer. The availability of free sulfur, not being bound to the porous carbon, provides enough opportunity to become solubilized with the liquid hydrocarbon. The results of sulfur-leaching experiments were conducted under identical experimental conditions, and the results are shown in Table 3. The results clearly indicate that leaching is
Table 2. Mercury Levels in Refinery Products product
amount (ppb, w/w)
LPG gasoline diesel naphtha petroleum coke
