Acid Gas Removal and Water Gas Shift Reactor: Progress Report #3 Department of Mechanical and Aerospace Engineering Chemical Engineering Program University of California, San Diego CENG 124B Nicole Law Caitlin Nichols David Tamayo May 8 2008 N.L., D.T did Aspen calculations. C.N was the main writer. D.T. did the process flow diagram. N.L did tables. Abstract The process for producing methanol from coal has been designed and modeled in AspenPLUS up to, and including, the water gas shift reactor. Once the coal is gasified, acid gas is completely removed using a separator and then reacted in the Claus process to produce 0.79 MT/hr of elemental sulfur. The chosen method for acid gas removal is an absorption process and physical wash of Rectisol. The product gas is then reacted in a water gas shift reactor, producing 13.28 MT/hr of hydrogen gas and 91.26 MT/hr of carbon monoxide. This produces the correct molar ratio, 2:1, of hydrogen gas to carbon monoxide gas to produce methanol. Introduction Alternative energy production via coal gasification is a highly efficient, and relatively clean, process. However, as with all systems, the methods involved in waste processing are highly important; safety, environmental concerns, efficiency, and economic considerations are all factors that must be accounted for in the development of this coal gasification system. Instead of releasing potentially harmful syngas components into the atmosphere, it is important to apply operations that will produce components with smaller risk factors safely and efficiently. Coal gasification, in particular, is associated with gases harmful to the environment, such as H2S, CO2, and CO. Various processes may be applied in order to transform these unfavorable components into those that are less hazardous; some components may be processed into those that have beneficial future purposes. Two processes utilized in coal gasification are acid gas removal and the water-gas shift reaction. H2S and CO2 may be treated via an acid gas removal process which is selected based on gas purity, raw gas composition, the selectivity of the process, as well as various other factors. Absorption processes are characterized by washing the synthesis gas with a liquid solvent, and ultimately removing acid components; adsorption systems involve the adsorption of impurities onto a solid carrier bed. An absorption process was more applicable for the removal of H2S and CO2 in the coal gasification process. Several different washing techniques such as chemical, physical, physical-chemical, and oxidative washes may be applied as methods of absorption. The Rectisol process, a type of physical wash, was the method utilized in this project. In particular, this process involves the use of cold methanol as a solvent, and operates within a -30 to -60oC temperature range. Though the energy usage (and therefore operating cost) is high due to necessary refrigeration, this process provides the most substantial H2S and CO2 recovery. Sulfur recovery is an additional process necessary in order to convert the toxic H2S into alternative products, namely elemental sulfur and sulfuric acid. This sulfur recovery is modeled a system known as the Claus process. Although there are several Claus processes available, the Oxygen Claus process will be utilized in this project. The Oxygen Claus process is a specific system chosen mainly for economic reasons, as it is significantly cheaper to oxidize H2S via pure oxygen versus air. A water-gas shift reaction is applied in order to rid the syngas of CO. This reaction produces hydrogen from a synthesis gas. In this process, CO is reacted with steam as the source of hydrogen. This reaction is a sub-process that may ultimately be contributed to the production of methanol. It is ideal to convert the maximum amount of CO as is possible in order to produce large amounts of the desired H2 product. The H2 may be utilized in further processing for methanol production. This system operates with a variety of catalysts within the 200 to 500oC temperature range, and occurs as either high temperature, medium temperature, or low temperature shifts. A low temperature shift is the most applicable, as very little H2S is present in this inlet gas. Results acid gas air drycoal gases 0 0 0 3.02 0 0 0 1.8 0 88 0 1.74E-11 0 0 0 3.71E-05 0 0 0 6.58 0 0 0 0 0 0 0 185.39 0 0 0 4.36 0 0 0 0.1 1.58 0 0 1.58 0 0 0 9.29E-03 0 0 0 2.97E-03 0 0 0 0 0 0 0 0 0 0 120.83 0 0 0 0 0 1.58 88 120.83 202.8423 Table 1: Mass Flowrates in tonnes/hr inburner oxy products solids steam sulfur1 sulfur2 syngas syngas2 water 0 0 3.03 0 9 0.14 0.42 3.03 1.7 59.45 1.8 0 1.79 0 0 0 0 1.79 1.8 0 15.76 0.37 1.74E-11 0 0 0 0 1.74E-11 0 0 1.49 0 3.71E-05 0 0 0 0 3.71E-05 3.71E-05 0 5.91 0 6.46 0 0 0 0 6.46 13.28 0 0 0 0 0 0 0 0 0 0 0 0 0 185.39 0 0 0 0 185.39 91.26 0 0 0 4.36 0 0 0 0 4.36 152.54 0 0 0 0.1 0 0 0 0 0.1 0 0 0 0 1.58 0 0 1.32 0.79 0 0 0 0 0 9.29E-03 0 0 0 0 9.29E-03 0 0 0 0 2.97E-03 0 0 0 0 2.97E-03 0 0 0 0 0 0 0 0.49 0 0 0 0 0 0 0 0 0 0 0.74 0 0 0 0 0 0 0 0 0 0 0 0 0 15.12 0 15.12 15.12 0 0 0 0 0 0 40.08 0.37 217.8423 15.12 9 1.95 1.95 201.142 260.58 59.45 Water Nitrogen Oxygen Sulfur Hydrogen Carbon Carbon Monoxide Carbon Dioxide Methane Hyrdogen Sulfide Hydrogen Cyanide Ammonia Sulfur Dioxide Sulfur (S8) coal ash total Table 2: Stream properties Stream names analogous to ASPEN streams Stream ACIDGAS AIR (Oxygen) DRYCOAL GASES INBURNER OXY PRODUCTS SOLID STEAM SULFUR1 SULFUR2 SYNGAS SYNGAS2 WATER Temperature (°C) 1452.6 25.0 1452.6 25.0 25.0 1452.6 242.0 1000.0 200.0 1452.6 260.0 300.0 Pressure (MPa) 3.5 0.1 0.1 3.5 0.1 0.1 3.5 3.5 0.1 0.1 3.5 2.0 0.1 WATER (Steam) 300.0 0.1 SYNGAS2 Legend Temperature (°C) Pressure (MPa) 1452.6 3.5 SYNGAS Water-Gas Shift 260.0 2.0 25.0 0.1 DRYCOAL GASES Separation Steam 1000.0 0.1 ACIDGAS Shell Gasifier 25.0 0.1 OXYGEN SOLIDS (Slag) 242.0 3.5 SULFUR2 1452.6 3.5 OXYGEN Acid Gas Removal 200.0 0.1 25.0 0.1 Figure 1: Process flow diagram: Gasifier, acid gas removal, water-gas shift Stream names are analogous to ASPEN streams Table 1 illustrates the stream flowrates acquired from the Aspen simulation. Important flowrates are the outlets streams of CO and H2 in the syngas2 stream. These results are 13.2 MT/hr of hydrogen and 91.26 MT/hr of carbon monoxide. The molar streamflows are 6600 kmol/hr of hydrogen and 3300 kmol/hr of carbon monoxide. The 2:1 molar ratio of hydrogen to carbon monoxide is achieved, but this is not a sufficient amount of carbon monoxide to make the desired 5000 MT/day of methanol. The results will only produce about half that desired amount of methanol. Table 2 illustrates the stream properties and will be vital to the design and selection of materials for the processing plan Discussion Process Selections Acid Gas Removal and Claus Process H2S gas is extremely toxic, and also harmful to the environment; therefore the high purity of H2S immediately following acid gas removal must be managed effectively. Sulfur recovery can be applied via a method known as the Claus process; products that result are either liquid or solid elemental sulfur, or sulfuric acid. A modified Claus process involves a two-stage process. The first stage involves the combustion of one third the total H2S to form SO2 and water; the second is a low-temperature catalytic stage that involves the reaction of SO2 with the remaining H2S to ultimately yield water and elemental sulfur. The reaction is presented as follows: 3 O2 SO2 + H 2O 2 3 2H 2S + SO2 2H 2O + S8 8 ___________________________ 3 3 3H 2S + O2 3H 2O + S8 2 8 H 2S + (1) (2) (3) This modified Claus process serves as a basis for other variants of this method. Specifically, the Oxygen Claus process was chosen due to its economic feasibility. Using oxygen instead of air lowers the cost of concentration within the Rectisol process, as well as the cost of the Claus unit itself. Sizing of the Claus unit is based on the volume of gas input; a lower nitrogen concentration in the feed requires a correspondingly smaller, and therefore cheaper, Claus unit. It is important to note that this process requires the oxidation of all components to molecular nitrogen in order to prevent the plugging of liquid and sulfur lines. Water-Gas Shift A water-gas shift reaction is used in order to produce hydrogen via CO in the syngas. The temperature of operation varies between 200 to 500oC, depending on the catalysts used. In this process, CO is reacted with steam as the source of hydrogen. The equation for the reaction is shown below: CO + H 2O CO2 + H 2 o (4) High temperature shifts use iron oxide-based catalysts, and operate between 300 to 500 C. The catalyst in a high temperature shift is capable of sulfur content up to 100 ppmv; however, varying the sulfur content may ultimately cause the catalyst to lose mechanical strength. Low temperature shifts operate in the range of 200 to 270oC. These reactions are highly sulfur-sensitive. A low temperature water-gas shift was ultimately applied as part of this process. The H2S was lowered to less than 0.1 ppmv in the inlet gas. Therefore, it was not necessary to heat the inlet gas to temperatures beyond 270oC; the low temperature shift is able to efficiently complete the process at lower temperature conditions. Aspen Acid Gas Removal / Claus Syngas, which exited the gasifier, entered a separator, where only H2S was removed and treated. This stream of H2S was then treated to produce elemental sulfur. Two RSTOICH reactors were applied in ASPEN to represent the Claus process. In the first reactor (REACTOR1), the acid gas stream was combusted with oxygen to push the reaction 1 to the right. The operating temperature of the first reactor was assumed to be 1000 °C since it is in the range of temperatures where H2S is combusted. The second stoichiometric reactor (REACTOR2) was run at a comparatively lower temperature, 200°C, than the first reactor. This was done to favor reaction 3 and achieve a higher yield of S8. Water-Gas Shift Syngas leaving the separator was treated in a water-gas shift reactor to shift the stoichiometric ratio between hydrogen and carbon monoxide to 2:1 respectively. To simulate a water-gas shift, an RGIBBS reactor was applied. In order to push the watergas shift reaction to the right and produce more hydrogen, an additional steam stream was added to react with the carbon monoxide in the syngas. It was also assumed that a low temperature shift reactor was used since the amount of sulfur remaining in the syngas was minimal. The reactor had an operating temperature of 260°C. The exiting syngas will undergo methanol synthesis. Health and Safety Concerns Safety and environmental considerations are important aspects of plant operation. Acid gas removal and sulfur recovery by the Claus process have various safety and environmental hazards. Dangerous H2S and CO2 components are part of the Rectisol acid gas removal process; H2S is highly toxic, and CO2 is unfavorable to the environment. In addition, sulfur recovery occurs at extremely high temperatures, as one of its steps is a combustion reaction. Conclusion The acid gas removal and water gas shift processes were modeled to produce an estimate of what to expect during these processes. An absorption method using a physical wash of Rectisol was chosen for the acid gas removal process. When this was modeled in AspenPLUS, all the hydrogen sulfide was removed and no carbon dioxide since the process is highly selective of hydrogen sulfide over carbon dioxide. The hydrogen sulfide was reacted and modeled in the Claus process, producing 0.79 MT/hr of solid elemental sulfur. The synthesis gas produced was separated from the acid gas and then reacted in a low temperature water gas shift reactor to produce the desired ratio 2:1 of hydrogen gas to carbon monoxide. This ratio is necessary to produce methanol from these components. The amount of hydrogen and carbon monoxide produced, were 13.2 MT/hr and 91.26 MT/hr which come out to be about 6600 kmol/hr and 3300 kmol/hr, respectively. At this point, only the processes have been modeled and no preliminary costs have been estimated. Therefore, it is difficult to make recommendations based only on the processes. Future work includes redesigning the gasifier and water gas shift to produce the required amount of hydrogen and carbon monoxide to produce 5000 MT/day of methanol. The next step in design also includes an investigation on how to design the methanol synthesis process. References 1. C. Higman and M. Van der Burgt, in Gasification, Elsevier, Boston, 2003, ch. 8, pp. 298-326.