ROLAND ESQUIVEL
C OAL T O M ET H A NO L D ES I G N R E P O RT METHANOL SYNTHESIS
UNIVERSITY OF C A L I F O R N I A , S A N D I E G O, 9500 GILMAN DR., LA JOLLA, CA 92093
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1. Executive Summary Methanol synthesis was completed with syn gas from the water gas shift reactor. Aspen model simulation Ryield combined with a flash and column distillation produced 1580 Mtonnes of methanol. A column distillation height and diameter were calculated to be 38 feet and 3 feet, respectively. Methanol AA purity will need to be continued in the future to separate the ethanol-methanol binary mixtures to AA specifications. Kinetic rate equations were given in cited articles to be used for LHHW reaction model in Aspen for a Rplug reactor. At this time A Ryield reactor is used to synthesis methanol.
2. Overall Project Scope Description – Coal to Methanol Coal gasification to methanol production is the subject of the 2008 American Institute of Chemical Engineers (AICHe) national student design contest. This report will provide a preliminary engineering design report based on the competition requirements stated by AICHe.
Hydrocarbons derived from petroleum, natural gas or coal is essential to modern life and its quality. The bulk of the world’s hydrocarbons are used as fuels for propulsions, electrical power generation, and heating8. Oil reserves are diminishing and market prices soaring as a primary form of energy in the world. Alternative fuel sources and fuels must be explored. Recent market fluctuation in the price of crude oil and other financial incentives are helping to attract internal investments to exploring alternate energy sources. Methanol derived from the gasification of coal shows many promises in areas of improved environmental carbon capture, production of a variety of synthetic fuels, and in 2008 financial profits are becoming a reality.
A preliminary engineering design report investigating coal processing to methanol production at 5000Mtonne per day capacity is presented. An economic, environmental and process safety evaluation of the production plan is underway. Aspen process modeling simulation is used to verify the chemical engineering feasibility of coal to methanol.
2.1 Overall Report Scope Description – Methanol Synthesis and Refining To continue the Coal to Methanol design report (previous project reports on gasification, water gas shift and acid gas removal are located in the appendices) methanol synthesis, refining and kinetics have been investigated. Aspen simulation modeling is used to estimate the most realistic kinetics of methanol synthesis between the Rstoic, Rplug, Requil, and Rgibbs models. From these simulation and rate law data provide by the AICHe administration a process flow diagram with stream tables is partly developed for the synthesis and refining of methanol. Rigorous kinetic calculations for the following three equations (see section 3 of this report) in a two phase system using the Langmuir-Hinshelwood-
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Hougen-Watson (LHHW) kinetic model have been partly completed. The LHHW parameters are used in the ASPEN simulation RPlug model to specify a plug flow reactor during the synthesis reactions. A Flash and Distillation column were used to refine methanol to the AA requirements. Based on the provided data and Aspen calculations preliminary reactor sizing, column sizing have been completed. Economic analysis for the synthesis and refining phase will be completed in the final design report.
Commercial technologies, processes, and plants currently in use throughout the world are discussed. The technologies critical factors for consideration in a coal to methanol plant are explored. Comparing the simulation results and the research a recommendation of a reliable technology is given.
Environmental and process safety during the synthesis, refining, and by product removal have been researched. Catalyst life is an important factor designing by product removal. Recommendations will be provided based on this research. Alternatives and improvements will be presented.
Based on the Aspen flow diagram, technology research and material balances annual utilities cost and capital investment will be completed in the final report.
3. Design Basis, Principles and Limitations of Methanol Synthesis Methanol is produced by the hydrogenation of carbon oxides over a suitable (copper oxide, zinc oxide, or chromium oxide-based) catalyst1. ( A)
CO + 2 H 2 ↔ CH 3OH
( B)
CO2 + H 2 ↔ CO + H 2O
(C )
CO2 + 3H 2 ↔ CH 3OH + H 2O
The first reaction (A) is the primary methanol synthesis reaction, a small amount of CO2 from the water gas shift reaction in the feed (2–10%) acts as a promoter of this primary reaction and helps maintain catalyst activity. The stoichiometry of both reactions is satisfied when the ratio is 2. Hydrogen builds up in the recycle loop; this leads to an actual R value of the combined synthesis feed (make up plus recycle feed) of 3 to 4. The reactions are exothermic and give a net decrease in molar volume. Therefore, the equilibrium is favored by high pressure and low temperature3. During production, heat is released and has to be removed to keep optimum catalyst life and reaction rate. The produced methanol reacts further to form side products such as dimethyl ether, formaldehyde, or higher alcohols (van Dijk et al. 1995) these by products will be negligible in the preliminary design reported, but noted for awareness. Conventionally, methanol is produced in twophase systems, the reactants and products forming the gas phase and the catalyst forming the solid phase. Gaaf et al, derived the following pseudo rate equation using LHHW reaction models for reactions A, B, C.
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(eq3.1)
(eq.3.2)
ri ' =
KineticFactor [ DrivingForce]
' rCH 3OH , A 3
[ Adsorption]
f k ps' , A 3 K co f co f H32/ 2 − 1/CH2 3OH° ( f H 2 K p1 ) = K (1 + K co fco + K co 2 f co 2 ) f H1/22 + H1/202 f KH 2
H 20
It is of the form stated in equation 3.1a kinetic factor, driving force and adsorption expressions. The kinetic Factor follows the Arrhenius rate law
(eq3.0)
k = A exp[− Ea /( RT )]
Limits on the model are low pressures, kinetic factors > 0, Ea > 0. The equilibrium stated in the AICHe is valid for a temperature range of 373K – 676K for the equilibrium relationship in equation 3.4.
(eq3.4)
1 P y 1 11284 ln K p = ln CH 3OH = ln CH 3OH = −32.918 + 2 2 2 2 PCO PH 2 yCO yH 2 P T (K ) psia
Aspen simulation software has two models for handling kinetics. Following LHHW parameters given by Gaaf et al, the RPlug model and be used to simulate the three reactions A, B, C.
Other design bases are separation principles heuristics for binary mixture separation. After the methanol synthesis, whether a Rplug or Ryield reactor was used by product gases need to be separated from methanol, ethanol, water liquid mixture. Ethanol is added in as a mass balance of 1:100 of methanol produced and produced by the following reaction.
(eq3.3)
2CO + 4 H 2 ↔ C2 H 5OH + H 2O
A flash or knock out drum removes CO2 and CO excess and recycles back to WGS. Further separation of alcohols from water and lastly methanol from ethanol are based on the relative volatilities of the binary components. No azeotropes exist within this system allowing for theoretically perfect separation. Column sizes in diameter and height may limit production of 5000mtone/day of methanol. Calculations are currently used with 3000Mtonnes/day of coal feed and once the final yield is know this will need to be scaled up. At scale up reactor size and column size will be limited by structural limits.
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3.2 Technology Selection Criteria and Considerations Design considerations during the methanol synthesis and refining processes are syngas cooling, acid gas removal, in particular carbon dioxide. The catalyst deactivates primarily because of loss of active copper due to physical blockage of the active sites by large by-product molecules; poisoning by halogens or sulfur in the synthesis gas, which irreversibly form inactive copper salts; and sintering of the copper crystallites into larger crystals, which then have a lower surface area-to-volume ratio1. Fixed Bed technology is a adiabatic quenched low pressure plug flow reactor. The reactor is catalyzed with a Cu/Zn/Al mesh interface. The surface area is several hundred square feet per kilogram catalyst. There are inherent inefficiencies from the cooling of the syngas feed to the heating of the reaction core. The raise and fall of temperature structurally weakens materials of construction. However, this technology is known for the reliability. Isothermal steam raising reactors were developed by Lurgi. They are made up of tubes packed with catalyst and cooling water into steam on the shell. This allows for near isothermal conditions. Conversion is limited by equilibrium. Synetix, formerly ICI, systems produced nearly 61% of the worlds methanol production with adiabatic quenched plug flow reactors.
3.3 Process Performance Summary & Process Descriptions Three Aspen simulations were completed. First a test of Rgibbs, RYield, Rstoic, and Rplug with all reactor specification the same was used to verify the different conversion rates. Rplug specifications were not complete and require more time to complete hand calculation requirements before moving forward with the Rplug model. Ryield model produced the highest conversion of methanol at 500 K and 100 atm. Ryield model was then added to the entire process flow diagram. Lean syngas was feed to the reactor at 400F and 1 bar. 2035 mtonne/day of methanol was produced cleared out of the reactor as product. The product gas contained 1138 mtonnes of CO2, 47 mtonnes of CO and 160 mtonnes of water. 92mtonnes of methanol exited the reactor with excess unreacted by products and syngas. This exit stream could be used as a recycle stream for added efficiencies. 1150 mtonne, 342 mtonnes, and 3.5 mtonnes of CO2, CO and H20 exited the reactor as a split fraction. The crude syngas was then flashed to release acid gas from the liquid product. 454mtonnes of methanol escaped with the CO and CO2 sour gas leaving methanol product of 1580 mtonnes and 148 mtonnes of water. To separate the water methanol binary mixture (ethanol is added after as a mass balance) a Radfrac column distillation was used. The Radfrac was able to achieve the 99.85% (dry basis purity) however, an ethanol – methanol binary mixture will need to be separated to AA specifications. The column used 21 stages, murphee efficiency of 0.8, feed inlet at 16th stage based on Mcabe Thiel step off method. VLE data was provided by Aspen databank. A reflux ratio of 2 and bubble cap spacing with 2 foot spacing. Aspen calculated a 3 foot diameter column. Column height was determined to be 38 feet. 4. Project Economic Summary To be completed on final design report.
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5. PFD with Stream Tables synthesis and refining
Methanol Synthesis - Report 4 Stream Table Methanol Syn Feed 3000 tonne/day Coal From To Substream: ALL Mass Flow Mass Enthalpy MASSFLOW METH H2 CO2 CO H2O ETHAN-01 N2 TEMP PRES Substream: MIXED Phase:
NONMETH
1 METHFEED
METHSPLT METHSPLT METHSYN LB/HR BTU/HR
3779.84 145572.6
TONNE/DAY TONNE/DAY TONNE/DAY TONNE/DAY TONNE/DAY TONNE/DAY TONNE/DAY F PSI
2 GAS2 METHSYN
456233.7 145632.5 0 -484763500
0 0 0 0 0 0.01 40.23 400 14.7 Vapor
Synthesis Gases
0 267.71 2286.72 2248.76 163.45 0 0 400 14.7 Missing
3 METHCRUD METHSYN B4
2035.34 0.01 1138.62 47.35 159.95 0 0 211.73 2175.57 Liquid
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4 GAS B4
5 METH1 B4 B1
Distillation
6 METHETHA 7 WATER B1
310601.2 151481.4 159119.8 -1093442000 -527160900 -564954100
91.65 0.07 1148.1 342.05 3.51 0 0 211.73 2175.57 Vapor
Flash Gases
454.71 0.01 1135.15 47.35 11.84 0 0 77 5.8 Vapor
96.72 159023.1 -392188.3 -549940100
1580.63 0 3.47 0 148.11 0 0 77 5.8 Liquid
B1
0 0 1.05 0 0 0 0 -147.56 14.5 Liquid
1580.63 0 2.42 0 148.11 1.58 0 145.55 14.5 Mixed
7. Major Equipment with Sizing for Cost Estimation Methanol synthesis will require a plug flow reactor similar to the ICI adiabatic quench reactor. Sizing and number of reactors required to meet capacity will be completed on final report. Distillation column will be needed for methanol refining to meet AA specifications. Current distillation column can produce 1500 mtonnes of methanol with a feed stock of 3000mtonne of coal. Scale up processes will be limited by column structural height. Prelimary size has been calculated to be: • 3 foot diameter • 38 foot height 8. Environmental and Process Safety Considerations Excessive CO2 emissions are a major concern for coal gasification. There overwhelming evidence of rise in temperature correlates with CO2 atmospheric concentrations increasing, however the mechanisms are not fully understood. Global average temperatures and CO2 concentrations (as determined from Antarctic Ice cores) are some examples identifying the environmental effects of CO2 emissions. The power sector in the united states is a significant contributor (37% of total US CO2 emissions) to pollution. One solution to alleviate the levels of pollution comes from the old coal fired plant operating at 32% efficiency. If the same plants used the same fuel with 43% efficiency increase an improvement on CO2 emissions would drop per produced Kilowatt-hour by 25%. The barriers to reinvest in old coal production plants are not technological but of a financial nature. Lack of capital and incentives for replacement projects has delayed much need infrastructure improvements1. Operator safety to avoid CO2 asphyxiation, due to CO2 heavier than air is a concern. Preventing leaks in equipment and piping should be priority. In addition to CO2 safety, methanol synthesis from CO has its own set of safety concerns. For one the higher heat of reaction of CO has over CO2 heat removal becomes a requirement to prevent heat exhaustion on the materials of construction. Prevent impurity build up due to side reactions increase with Increasing Temperature.
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Reference: 1) C. Higman and M. Van der Burgt, Gasification, Elesevier, Amsterdam, 2003. 2) G.H. Gaaf, “Comparison of two-phase and three phase methanol synthesis processes,” Chemical Engineering and Processing 35 (1996) 413-427. 3) G.H. Gaaf, “Kinetics of Low-Pressure Methanol Synthesis,” Chemical Engineering Science vol 43 (1988) pp 3185-3195 4) R. Probstein, in Synthetic Fuels, McGraw-Hill, New York, 1976, pp. 31-140. 5) H. Fogler, Elements of Chemical Reaction Engineering, 4th ed, Prentice Hall, 2005, pp 1021 – 1026 6) G.A. Olah, A. Goeppert, G.K.S. Prakash, Beyond Oil and Gas: The Methanol Economy. Wiley-VCH, Weinheim, 2006.
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