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Sulfur Recovery from Acid Gas Using the Claus Process and High Temperature Air Combustion (Hitac) Technology

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Sulfur Recovery from Acid Gas Using the Claus Process and High Temperature Air Combustion (Hitac) Technology American Journal of Environmental Sciences 4 (5): 502-511, 2008 ISSN 1553-345X © 2008 Science Publications Sulfur Recovery from Acid Gas Using the Claus Process and High Temperature Air Combustion (HiTAC) Technology 1Mohamed Sassi and 2Ashwani K. Gupta 1Department of Chemical and Mechanical Engineering, The Petroleum Institute, P.O. Box 2533, Abu Dhabi, UAE 2Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, USA Abstract: Sulfur-bearing compounds are very detrimental to the environment and to industrial process equipment. They are often obtained or formed as a by-product of separation and thermal processing of fuels containing sulfur, such as coal, crude oil and natural gas. The two sulfur compounds, which need special attention, are: hydrogen sulfide (H2S) and sulfur dioxide (SO2). H2S is a highly corrosive gas with a foul smell. SO2 is a toxic gas responsible for acid rain formation and equipment corrosion. Various methods of reducing pollutants containing sulfur are described in this paper, with a focus on the modified Claus process, enhanced by the use of High Temperature Air Combustion (HiTAC) technology in the Claus furnace. The Claus process has been known and used in the industry for over 100 years. It involves thermal oxidation of hydrogen sulfide and its reaction with sulfur dioxide to form sulfur and water vapor. This process is equilibrium-limited and usually achieves efficiencies in the range of 94-97%, which have been regarded as acceptable in the past years. Nowadays strict air pollution regulations regarding hydrogen sulfide and sulfur dioxide emissions call for nearly 100% efficiency, which can only be achieved with process modifications. High temperature air combustion technology or otherwise called flameless (or colorless) combustion is proposed here for application in Claus furnaces, especially those employing lean acid gas streams, which cannot be burned without the use of auxiliary fuel or oxygen enrichment under standard conditions. With the use of HiTAC it has been shown, however, that fuel-lean, Low Calorific Value (LCV) fuels can be burned with very uniform thermal fields without the need for fuel enrichment or oxygen addition. The uniform temperature distribution favors clean and efficient burning with an additional advantage of significant reduction of NOx, CO and hydrocarbon emission. Key words: Sulfur recovery, acid gas, Claus process, flameless or colorless combustion INTRODUCTION 2H2S(g) + SO2(g) (3/n) Sn(g) + 2H2O(g) Hydrogen sulfide is present in numerous gaseous with waste streams from natural gas plants, oil refineries, −1 H = −108 kJ moL wastewater treatment, among other processes. These r streams usually also contain carbon dioxide, water- vapor, trace quantities of hydrocarbons, sulfur and Therefore, higher conversions for this exothermic, ammonia. Waste gases with ammonia are called sour equilibrium-limited reaction call for low temperatures gases, while those without ammonia are called acid which lead to low reaction rates, imposing the use of a gases. Sulfur must be recovered from these waste catalyst. The catalytic conversion is usually carried out streams before flaring them. in a multi-stage fixed-bed adsorptive reactors process, Sulfur recovery from sour or acid gas typically to counteract the severe equilibrium limitations at high involves application of the famous Claus process[1] conversions. This technology process can possibly using the reaction between hydrogen sulfide and sulfur provide about 96-97% conversion of the influent sulfur dioxide (produced in the Claus process furnace from the in H2S to S. However, higher removal requires critical combustion of H2S with air and/or oxygen) yielding examination of the process and use of near isothermal elemental sulfur and water vapor: reactor since the conversion is critically dependent Corresponding Author: Mohamed Sassi, Department of Chemical and Mechanical Engineering, The Petroleum Institute, P.O. Box 2533, Abu Dhabi, UAE 502 Am. J. Environ. Sci., 4 (5): 502-511, 2008 upon exothermic and endothermic conditions of the as ammonia and BTX (benzene, toluene, xylene) are reactions. Flameless combustion has been shown to supposedly destroyed. The main two reactions in the provide uniform thermal field in the reactor so that the Claus furnace are: reactor temperature is near uniform[2,3,4]. In addition it has been shown to result in compact size of the reactor, H2S + 3/2O2 SO2 + H2O (1) reduce combustion generated pollutants emission up to with [5] −1 50% and increase energy efficiency up to 30% . The Hr = −518 kJ moL application of this technology appears to offer great advantages for the process under consideration. 2H2S + SO2 3/2S2 + 2H2O (2) The UAE, which pumps about 2.4 million bpd of with −1 crude oil, is also home of the world’s fifth biggest gas Hr = +47 kJ moL reserves at about 200 trillion cubic feet. Abu Dhabi Gas Industries (GASCO), an operating company of the Abu This last endothermic reaction is responsible for up Dhabi National Oil Company (ADNOC), is leading a to 67% conversion of the sulfur at about 12000C. drive to boost gas production in the UAE from five to Moreover, many side reactions take place in the seven billion cubic feet per day. This calls for sulfur furnace, which reduce sulfur recovery and/or produce recovery capacity of over 3000 metric tons per day with unwanted components that end up as ambient pollutant the associated SOx and NOx emissions. Therefore, the emissions. Therefore, it would be useful to combine the adoption and further development of flameless endothermic and exothermic process using an combustion technology for sulfur recovery among other isothermal reactor offered by the flameless (or commercial and industrial heating processes is expected colorless) oxidation combustion. to be very crucial and beneficial, both economically and A vast majority (about 92%) of the 8 million metric environmentally. tons of sulfur produced in the United States in 2005 was The conventional Sulfur recovery process is based recovered from industrial by-products using the Claus upon the withdrawal of sulfur by in-situ condensation process[8]. However, the traditional Claus process does within the reactor. The selective removal of water face limitations and various process improvements have should, however, be a far more effective technique as been investigated in order to satisfy the increasingly its effect on the equilibrium composition in the mass stringent emission regulations and the need to process action equation is much greater. The in-situ gas streams and fuels with higher sulfur content. New combination of the heterogeneously catalyzed Claus technologies have to be developed in order to achieve reaction and an adsorptive water separation seems 100% removal of sulfur compounds from industrial flue especially promising, as both reaction and adsorption gases. The Claus process and its various derivatives and exhibit similar kinetics and pressure can be adapted to improvements are described here for treatment of H2S the needs of the adsorptive separation. Such an containing streams. The use of HiTAC technology as a adsorptive reactor will lead to almost complete reliable and cost-effective alternative for improvement conversion as long as the adsorption capacity is not of lean acid gas treatment in the Claus process is exhausted. There are numerous possibilities for proposed and described. Finally, the flameless or implementing these two functionalities, ranging from colorless combustion is proposed and described for fixed-beds with homogeneous catalyst/adsorbent processing acid-rich gas. mixtures to spatially structured distributions or even fluidized beds. The Traditional Sulfur Recovery Process: The three For the sulfur recovery process most of the main steps of sulfur recovery from sour gas are previous studies have concentrated on the Claus described below. catalytic conversion reactors and the Tail Gas [6] Treatment Unit TGTU . However, some previous Amine Extraction: Gas containing H2S is passed studies have identified the Claus furnace as one of the through an absorber containing an amine solution most important yet least understood parts of the (Monoethanolamine (MEA), Diethanolamine (DEA), modified Claus process[7]. The furnace is where the Methyldiethanolamine (MDEA), Diisopropylamine combustion reaction and the initial sulfur conversion (DIPA), or Diglycolamine (DGA)), where the hydrogen (through an endothermic gaseous reaction) take place sulfide is absorbed along with carbon dioxide. A typical and also where the SO2 required by the downstream amine gas treating process, shown in Fig. 1, includes an catalytic stages is produced and the contaminants (such absorber unit and a regenerator unit as well as 503 Am. J. Environ. Sci., 4 (5): 502-511, 2008 lean acid gases are involved (low calorific value) then auxiliary fuel, oxygen enrichment or a by-pass stream has to be used. The H2S-content and the concentration of other combustible components (hydrocarbons or ammonia) determine the location where the feed gas is burned. Claus gases (acid gas) with no further combustible contents apart from H2S are burned in lances surrounding a central muffle. Gases containing ammonia, such as the gas from the refinery's Sour Water Stripper (SWS) or hydrocarbons are converted in the burner muffle. Claus Catalytic Stage: The remaining H2S, from the Claus furnace, is reacted with the SO2 at lower temperatures (about 470-620 K) over an alumina- or titanium dioxide-based catalyst to make more sulfur: Fig. 1: Flow diagram of a typical amine treating process [9] 2H S + SO 3/8S + 2H O (3) used in industrial plants 2 2 8 2 −1 accessory equipment[9]. In the absorber, the down- Hr = −108 kJ moL flowing amine solution absorbs H2S and CO2 (referred to as acid gases) from the up-flowing sour gas to On average, about 70% of H2S and SO2 will react produce a sweetened gas stream (i.e., an H2S-free gas) via reaction (3). Note that in the catalytic stage mostly as a product and an amine solution rich in the absorbed S8 is produced, which is an exothermic reaction acid gases.
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