Final Report: Methanol Synthesis | CENG 124B, Study Guides, Projects, Research of Chemistry

Download Final Report: Methanol Synthesis | CENG 124B and more Study Guides, Projects, Research Chemistry in PDF only on Docsity! Final Report : Methanol Synthesis Department of Mechanical and Aerospace Engineering Chemical Engineering Program University of California, San Diego CENG 124B Nicole Law Caitlin Nichols David Tamayo June 9 2008 Everyone contributed to all parts 1 Table of Contents A. Executive Summary B. Overall Project Scope C. Design Basis, Principles and Limitations D. Technology Selection Criteria and Conclusions E. Process Performance Summary F. Project Economics Summary G. Process Description H. Process Flow Diagram I. Environmental and Process Safety Considerations J. Conclusion and Recommendations A. Executive Summary The AIChE 2008 National Design Project Competition involved the production of 5000 metric tonnes (MT) of methanol per day via the gasification of Montana Sub- Bituminous Coal. The Shell Coal Gasification Process (SGCP) was chosen as the preferred gasification process based on its relative efficiency, economic practicality, and adequate environmental concerns. The first step in this system involved the gasification of coal into three main components: carbon monoxide, carbon dioxide, and methane. The following Acid Gas Removal step converted H2S into elemental sulfur. Rectisol was the wash used to separate H2S from and CO2 from valuable syngas streams, and the Claus process converted H2S to elemental sulfur. Ultimately 18 MT/day elemental sulfur were synthesized using these methods, and the water gas shift reactor recovered a 2:1 mol ratio H2 to CO. This is 4,400 MT/day of carbon monoxide and 160 MT/day of hydrogen gas. The final step was methanol production. The system, with the addition of one plug flow reactor, produced 5400 MT/day methanol. B. Overall Project Scope Description The AIChE 2008 National Student Design Competition has proposed a scenario involving the project design of a methanol production facility on the Texas Gulf Coast. Energy needs are at a rise as the world becomes increasingly industrialized. Consequently, the price of crude oil, a major energy source, is in constant escalation. In order to cap this problem, experiments involving alternative energy are currently being explored. Coal gasification to methanol is one very attractive option, as it offers a relatively environmentally safe methanol conversion process. This methanol may be used as a prime energy source such as fuel. The abundance of coal reserves throughout the US will be used as sources for this gasification process. The following tasks were completed: This is the compiled final report, which evaluates numerous aspects of the coal gasification to methanol project. A final flowsheet which integrates the separate processes is included. The commercial technologies chosen will be identified and described. The process will be simulated to obtain mass and energy balances, assisting in the evaluation of capital and operational costs. In the final report, Internal Rate of Return 4 2 32CO H CH OH+ ↔ 2 2 2CO H CO H O+ ↔ + 2 2 3 23CO H CH OH H O+ ↔ + Two-phase reactors are equipped with a porous catalyst in a solid phase and the reaction mixture in a gaseous phase.2 These reactors are placed in a recycling system, as methanol synthesis reactions 6, 7, and 8, are limited by chemical equilibrium. The reaction mixture is cooled after exiting the reactor, and the un-reacted remaining synthesis gas is partially recycled back through the reactor inlet. The kinetics of the reaction system is also important in order to determine methanol conversion; material and heat balances are solved simultaneously via the use of kinetic rate expressions.2 Kinetic data are also indicated, as rate equations are used in the specifications of the plug flow reactor. These rate expressions are listed below. 2 3 2 3 2 2 2 2 2 2 3/ 2 1/ 2 0 , 3 1 , 3 1/ 2 1/ 2 ' [ /( )] ' (1 )[ ( / ) ] ps A CO CO H CH OH H p CH OH A CO CO CO CO H H O H H O k K f f f f K r K f K f f K K f − = + + + 2 2 2 2 3 2 2 2 2 2 2 3/ 2 0 , 2 2 , 2 1/ 2 1/ 2 ' ( / )] ' (1 )[ ( / ) ] ps B CO CO H H O CO p CH OH B CO CO CO CO H H O H H O k K f f f f K r K f K f f K K f − = + + + 2 2 3 2 2 3 2 2 2 2 2 2 3/ 2 3/ 2 0 , 3 3 , 3 1/ 2 1/ 2 ' [ /( )] ' (1 )[ ( / ) ] ps C CO CO H CH OH H O H p CH OH C CO CO CO CO H H O H H O k K f f f f f K r K f K f f K K f − = + + + A catalyst is utilized in order to ensure the completion of reactions. Furthermore, relatively high temperatures, roughly 260oC, yield a maximum methanol production. Methanol refining involved a separation step in the resulting stream to produce pure methanol. (7) (8) (9) (10) (11) 5 D. Technology Selection Criteria and Conclusions There are three types of gasifiers available for the coal to methanol process: fluidized bed, fixed bed, and entrained flow. It was concluded that the entrained-flow gasifier was the most efficient for the gasification process with Montana sub-bituminous coal. This decision is based upon specific advantages which including the gasifier’s ability to produce a clean, tar-free gas from virtually any coal. Entrained-flow gasifiers operate at high temperatures, and therefore have the ability to remove a high percentage of ash present in the coal as slag. This is very important regarding the high ash percentage in Montana sub-bituminous coal. Several coal gasification processes are considered next. These processes included: Koppers-Totzek, Shell Coal Gasification Process (SCGP), Noell, Texaco, E Gas, CCP, and Eagle. Ultimately SCGP, a dry coal feed process, is applied based on several characterized advantages including: essentially complete gasification of virtually all solid fuels, production of clean gas without by-products, high throughput, high thermal efficiency and efficient heat recovery, and environmental acceptability.1 The process runs at very high temperatures (from 1400 to 1600oC); these high temperatures allow for easy removal of resulting ash as slag, and also prevent the formation of undesirable pyrolysis by-products. Despite the high operating temperatures, thermal efficiency is still maintained.3 Ultimately a 99% conversion is expected from this process.2 AGR is applied in order to cap dangerous acid gas production. The Rectisol process, a type of physical wash, is the washing method utilized. In particular, this process involves the use of cold methanol as a solvent, and operates within a -30 to -60oC temperature range. Sulfur recovery is modeled as a system known as the Claus process. Although there are several Claus processes available, the Oxygen Claus process is 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.4 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. Final methanol synthesis takes place immediately following the water gas shift reaction. The one plug flow reactor pushes the methanol synthesis to completion, and is specified by kinetic rate data shown as equations 9, 10, and 11. A separator is used after the plug flow reactor to refine the methanol. This piece of equipment ultimately acted to extract pure methanol from the resulting stream; an additional stream leaving the separator was designated as a waste stream. 6 E. Process Performance Summary The completed process produced 5400 MT of methanol/day. This was produced from 5800 MT/day coal fed into a gasifier along with 430 MT/day steam and 4,200 MT/day oxygen gas. The resulting streams from the gasifier were a solid ash waste stream of 730 MT/day and a gas stream of 9,730 MT/day. Hydrogen sulfide is separated from this stream and subsequently enters acid gas treatment where elemental sulfur is produced. The amount of elemental sulfur produced is 18 MT/day. The synthesis gas goes into the water gas shift reactor. A specified amount of water, 2,900 MT/day, is reacted with 9,600 MT/day of the synthesis gas, producing 1,300 MT/day of hydrogen gas and 8,800 MT/day of carbon dioxide gas. This correctly composed gas enters the plug flow reactor, producing 5400 MT/day of methanol. The inlet and outlet streams are specified in the following table. Table 1: Stream Table of Inlet and Outlet Flows The properties of the streams are fairly consistent throughout the process. The temperature raises after the reaction in the plug flow reactor, but pressure does not. Because the reaction is exothermic, the temperature change is realistic. The lack of a pressure change is an incorrect estimation because the reaction creates energy within the system. All the modeled separators gave very desirable results they were modeled very ideally due to unreasonable results when solving using methods such as the Fenske- Underwood-Gilliland equations. kg/hr DRYCOALA STEAM- A AIR-A SOLIDS-A OXY-A WASTE-A ELEM-S-A WATER SYNGAS2 Mass Flow TOTAL kg/hr WATER 18000 835.01 118900.8 3396.678 NITROGEN 3600.833 OXYGEN 176000 741.574 feed streams S 0.074 waste streams H2 26569.529 product streams CO 182525.12 CO2 305083.49 H2S 1579.594 S8 1486.262 METHANOL COAL 241666.666 ASH 30232.5 9 minimal and the chance of solids being present in the reactor would be reduced. Although it was estimated that nitrogen does not react in the gasifier, it actually reacts in trace amounts to form HCN and NH3. Acid Removal: The two syngas streams (GASES-A, GASES-B), which exited their respective gasifiers, each entered a separator, where only H2S was removed and treated. These streams of H2S were then treated to produce elemental sulfur. Two RSTOICH reactors for each stream were applied in ASPEN to represent the Claus process. In the first reactor (REACT1A, REACT1B), 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 (REACT2A, REACT2B) was run at a comparatively lower temperature, 200°C, than the first reactor. This was done to favor reaction (4) and achieve a higher yield of elemental sulfur, S8. The two exit streams from these Claus processes each entered their own separator, where S8 was completely separated from the other remaining components. 1.5MT of elemental sulfur is produced per hour, 12,500 MT annually. Water Gas Shift: Syngas leaving the separators was mixed prior to being 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 water-gas shift reaction to the right in reaction (5) 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. Methanol Synthesis and Refining: Methanol synthesis took place immediately following the water gas shift reaction. This process occurred as a two-phase system, and was based on a series of three reactions. A plug flow reactors was specified in ASPENPlus in order to push the methanol synthesis to completion. Kinetic data ultimately played an important role in the specifications of each chemical reactor. Reactions (6), (7), (8) occurred at differing reaction rates. Kinetic rate expressions, (9), (10), (11), were specified in ASPENPlus. All rate constants within these reactions were accounted for by using associated kinetic parameters and chemical equilibrium constants; specifications within ASPENPlus also accounted for the orders of the fugacity coefficients via adsorption properties. Streams were initially led through catalyst beds, with a bed voidage of .38 and catalyst loading of 20kg; however, failure to produce any methanol led to the omission of any catalyst being input into the reactor. The reactions were run at 260oC, as this temperature maximized methanol production. Furthermore, it was also assumed that ethanol production was not taken into account. A separator was used after the plug flow reactor. This component ultimately acted to extract pure methanol from the resulting stream; an additional stream leaving the separator was designated as a waste stream. 2230 MT/day was the resulting methanol production amount for this particular reactor design. This number falls short of the ideal methanol production of 5000 MT/day methanol as a result of the water gas shift reaction. A scale-up will be performed in order to attempt the targeted production rate of methanol. 1 0 H . P ro ce ss F lo w D ia g ra m 11 I. Safety and Environmental Considerations Safety: Safety considerations are an important aspect of plant operation. In particular, gasification plants are complex, and able to produce a high-pressure toxic gas that may be inflammable or even explosive in the presence of oxygen and an ignition source. There are several safety precautions necessary for the gasification process, as gasification plants are capable of producing high-pressure toxic gases that are inflammable or explosive in the presence of oxygen or ignition sources. First of all, the dangers posed by the primary ignition step are extensive, as the burners operate at pressures ranging from 20-70 bar. The increased pressure causes the risk of combustion to escalate; thus, a mixture of a combustible gas and oxygen should never be present in the reactor. Following shutdown, the gasifier is coated with nitrogen in order to avoid corrosion. If repairs must be made within the gasifier, the immediate environment must be rid of all gases other than air by drawing a good vacuum and breaking it with air, sometimes several times. Spontaneous combustion may be caused by the fuel present, as well as FeS that may have formed as a product of corrosion. Special handling of catalysts is also important; the consumed catalyst must also be discarded. There are several toxic materials that may be formed as a result of the gasification process, such as H2S, COS, NH3, HCN, and CO2. Nitrogen poses a threat due to its lack of smell, and exposure can quickly lead to unconsciousness. In order to avoid this danger, good ventilation of the plant is very important. The presence of CO2 may also pose as a risk, as it is heavier than air, and may have a tendency to leak through open valves. Finally, environments with high concentrations of oxygen have the ability to cause more vigorous form of combustion; materials such as metals that normally oxidize at slow rates may behave as fuels for fire in such conditions. Materials must be selected in order to minimize the ignitability probability. Furthermore, system geometry must be constructed so that velocities of oxygen lines are low. Oxygen compressors and fireproof walls may be incorporated in a plant in order to ensure the safety of personnel, Environmental Issues: Several considerations must be taken into account when operating a gasification plant. From an environmental standpoint, it is imperative that a gasification process is performed under acceptable pollution regulation guidelines for the given country in which the plant operates. Various waste effluents, as well as greenhouse gases, threaten natural surroundings, and must therefore be kept to a minimum. 1. Gaseous Effluents Gaseous effluents are certainly contributors to environmental deterioration. Those that are emitted via the use of fossil fuels are oxides of sulfur, nitrogen, and particulate matter, as well as components like carbon monoxide which are not fully combusted; oxides of sulfur, nitrogen, and particulate matter are all subsequently removed in the intermediate fuel gas. Sulfur compounds are of primary concern due to the role they play in acid rain. Modern techniques can remove sulfur readily by 98-99%. Particularly for methanol production, sulfur levels can be reduced below the 10 ppbv mark in the flue gas alone. 14 There are several toxic materials that may be formed as a result of the gasification process, such as H2S, CO, and CO2. The presence of CO2 may also pose as a risk, as it is heavier than air, and may have a tendency to leak through open valves. CO is a colorless, odorless, and tasteless gas, and is highly toxic. The SCGP/Prenlfo process aims at economic efficiency, whilst taking into consideration environmental and safety conditions.1 The main products of concern regarding coal gasification are CO, H2, and CO2; H2S, although not produced as abundantly, also poses a safety and environmental threat. The threat of greenhouse gases to the environment is a major factor that must be taken into consideration. CO2 emission is a major factor in coal gasification. The potential damage of CO2 emissions has ultimately become a long-term issue. The correlation between CO2 concentrations and the rise of global temperatures is indisputable. The component H2S, is harmful to the environment as well; it is a toxic, flammable gas.4 Excess water from the gasification section is usually stripped to remove components like H2S. Finally, environments with high concentrations of oxygen have the ability to cause more vigorous form of combustion; materials such as metals that normally oxidize at slow rates may behave as fuels for fire in such conditions. Materials must be selected in order to minimize the ignitability probability. Furthermore, system geometry must be constructed so that velocities of oxygen lines are low. Oxygen compressors and fireproof walls may be incorporated in a plant in order to ensure the safety of personnel. J. Conclusion and Reccommendation: The process designed and modeled in this report yields 5400 MT/day of methanol. The initial capital costs is $750 million dollars and one can break even after 3 years of operation. After 20 years of operation, approximately $3.3 billion is made. These values are very approximate, but give highly desirable results. Many of the results from the Aspen simulations were not entirely realistic, so it is recommended that this design should be reconsidered and redesigned. Additional components such as pumps, cooling systems, and additional reactors need to be added. All such components will add to the capital costs and decrease the cash flow, lowering the overall internal rate of return. 15 Works Cited: 1. http://en.wikipedia.org/wiki/Claus_process 2. Graaf, G.H., and A.A.C.M. Beenackers, Comparison of two-phase methanol synthesis processes, Chemical Engineering and Processing 35, (1996) 413-427. <http://dx.doi.org/10.1016/S0255-2071(96)04147-5> 3. E. Vogt, M Van Der Burgt, J Chesters, and J Hoogendoorn, in Development of the Shell-Koppers Coal Gasification Process, Royal Society of London, London, 1981, Vol. 300, No. 1453, pp. 111-120. 4. C. Higman and M. Van der Burgt, in Gasification, Elsevier, Boston, 2003, ch. 8, pp. 298-326. 5. http://www.fischertropsch.org/DOE/DOE_reports/91034/91034_25/91034_25_ appdx_e.pdf