Methanol from Natural Gas by ICI s LP Process (High Efficiency Design) Aspen Model Documentation

Transcription

1 Methanol from Natural Gas by ICI s LP Process (High Efficiency Design) Aspen Model Documentation Index Process Summary About This Process Process Definition Process Conditions Physical Property Models and Data Chemistry/Kinetics Key Parameters Selected Simulation Results: Blocks Streams References PEP Process Module 1 25 Aug 1999

2 Process Summary The Methanol from Natural Gas by ICI s LP (Low Pressure) Process Model illustrates the use of ASPEN PLUS to model a Methanol reforming and synthesis process using ICI s LP process. The design capacity for a typical plant is 825,000 Metric Ton/yr, at 0.90 stream factor. The whole process includes three sections: natural gas reforming section, methanol synthesis section, and distillation section. The final product is Methanol. PEP Process Module 2 25 Aug 1999

3 About This Process There are three types of processes commonly employed for the manufacture of Methanol from Natural Gas. These are the ICI s LP Process, the Lurgi Combined Reforming Process, and the ICI s LCM Process. Table 1 provides basic information on these processes and examples of some of the companies that have commercialized the process technology. All of these processes use Ni based catalyst in Natural Gas reforming reactors and Cu based in the Methanol synthesis reactor. Table 1. Summary of Processes for Methanol Synthesis Process Reformer Reactor Catalyst Company Primary Secondary Temp. Pressure Reforming Synthesis ICI LP Steam saturated None 520 F 1500 psi ICI 574 ICI 100% Natural Gas Properietary flow Cu based Lurgi Steam saturated Primary reforming 500 F 1480 psi Ni based Cu based Tenneco Combined 50% Natural Gas gas, 50% Natural Productivity Chemical, flow Gas flow, and is 1kg / liter Sabah Gas pure Oxygen Catalyst / hr Industries Steam/Carbon = H2 : CO = 2.5: :1 ICI LCM Steam saturated Primary reforming C1100 psi Primary Proprietary BHP 100% Natural Gas gas and pure Proprietary Cu based Petroleum flow Oxygen Ni based Productivity Conoco Steam/Carbon = H2 : CO = Secondary is 1kg / liter 1.42 : :1 Proprietary, Catalyst / hr supported Rh on!"#l The original ICI process rejected substantial quantities of heat energy into cooling water and air. In late 1974, ICI introduced the Reduced Energy Concept. Several changes were made at that time and thereafter: Replacement of LP stream reboilers in the distillation train by ones that were heated by reformer process gas. Inclusion of a boiler feedwater heating system in the reformer gas cooling system and in the methanol synthesis loop, to recover energy which had been discarded in the original 1967 design. Enhanced heat recovery from flue gases by the introduction of an air preheater in the reformer convection section. The Improved Distillation design was added to the design package in Instead of the conventional two-column system, a four-column arrangement was deployed, which produced chemical grade overhead and PEP Process Module 3 25 Aug 1999

4 rejected water in the bottom stream. The first of these columns is operated under pressure. This permits the condensation of overhead vapor at a sufficiently high temperature for use as reboil energy in the next column, which operates at near atmospheric pressure. The pressure column produces slightly impure methanol, rejects water that is virtually methanol free and removes most of the higher alcohols as a sidestream. The atmospheric pressure column does the final refining. The reduction in energy is obtained through the use of lower reflux ratios. The High Efficiency Design (1979) version of the process introduced additional energy saving features: The use of a feedstock saturator. The natural gas is scrubbed countercurrently with hot water. Energy for heating the water is low grade and comes mainly from the methanol synthesis loop. A greater extent of heat recovery from the reformer product, flue gas, and synthesis product streams. PEP Process Module 4 25 Aug 1999

5 Process Definition An ASPEN PLUS model is developed to simulate the ICI s LP process. The whole process includes three sections: natural gas reforming section, methanol synthesis section, and distillation section. 1) The Reforming Section: In the ICI LP process, there is only one reformer. Unlike the Lurgi or LCM process, there is also only steam reforming process in this section. A feedstock water saturator (C101) is employed to furnish 50% of the process reformer steam. The natural gas feed is desulfurized in M101 with active carbon. Preheating of the reformer reactants to 1000F takes place in E101, and the reaction is carried out in the radiant section of reforming furnace F101. The reformed stream, stream 6, comes out of F101, and through a series of heat exchangers (E105, 306, 106, 107, 108, 109, 110), flows into the synthesis section. The reforming gas stream 6 comes out of the reformer at a temperature of 1600 F, and leaves the section at a temperature of 100 F. V102, V103, V104 and V105 separate the water like de-saturators, and discharge the water into the recycle streams 32 and 10. 2) The Synthesis Section: The synthesis section consists of reactor R201 and heat exchangers and flashes. The Syngas stream comes out of the reforming section and is compressed by K201 to 1500 psia. After exchanging heat with the synthesis reactor R201 product in heat exchangers E202, E203, E204 and E205, the syngas reacts in R201 to produce crude methanol, stream 17. V201 and V202 separate uncondensed gases. The impure methanol synthesized, stream 20, flows to the distillation section for purification. 3) The Distillation Section: The distillation section includes four distillation towers, C301, C302, C303 and C304. The overhead vapor from C301 is cooled in E302, and the condensate, mainly methanol with some light ends, is returned as reflux. The uncondensed vapor mainly consisting of dimethyl ether is removed from reflux drum V301 to blend with the synthesis section purge. The bottom product from C301 is fed to C302, the refining column, which is operated at 100 psia at the base. Most of the water is separated in the bottom, stream 26, and the wet methanol is further purified in C303. C304 picks up some methanol left in the bottoms of C303 which is then sent to the final product, stream 31. PEP Process Module 5 25 Aug 1999

6 Process Conditions The process conditions are as listed in Table 2. Table 2. Component Methane CO2 CO Hydrogen Nitrogen Oxygen Ethane Propane Methanol Dimethyl Ether N-Butanol Acetone Water Process Conditions Description Raw material Inert gas and Inert gas Reforming component Raw material Raw material Product Reforming component Operating Conditions Reformer Inlet Pressure: Temperature: Synthesis Reactor Pressure: Temperature: 295 psia 1460 F 1500 psia 520 F All reactors use ASPEN PLUS REQUIL and RSTOIC reactors. PEP Process Module 6 25 Aug 1999

7 Physical Property Methods and Data The NRTL-RK thermodynamic method is used with Henry s Law. All the major binary interaction parameters are in the Aspen data bank. PEP Process Module 7 25 Aug 1999

8 Chemistry/Kinetics Primary Reformer: F101 Reaction Conversion/Extent CH4 + H2O = CO2 + 4 H lbmol/hr CH4 + H2O = CO + 3 H lbmol/hr C2H6 + 2 H2O =2 CO + 5 H2 100% Conversion of Ethane C3H8 + 3 H2O =3 CO + 7 H2 100% Conversion of Propane Aspen s RSTOIC model is used to model this reactor. Combustion Reactions in Reformer Furnace: F101S Reaction CH4 + 2O2=CO2+2H2O 2C2H6+7O2=4CO2+2H2O 2CO+O2=2CO2 2H2+O2=2H2O 2CH3OH+3O2=2CO2+4H2O C3H8+5O2=3CO2+4H2O 2CH3OCH3+6O2=4CO2+6H2O H2S+1.5O2=SO2+H2O Conversion/Base Component 0.995/CH /C2H6 1/CO 0.998/H /CH3OH /C3H8 1/CH3OCH3 1/H2S Aspen s RSTOIC model is used to model this reactor. Synthesis Reformer: R201 Reaction Reaction s Extent CO + 1 H2O = CO2 + 1 H F Temperature Approach CO2 + 3 H2 = CH3OH + 1 H2O 27 F Temperature Approach 2CH3OH =DM-Ether + H2O lbmol/hr 4CO + 8 H2 = 1 Butanol + 3 H2O lbmol/hr 3CO + 5 H2 = 1 Acetone + 2 H2O lbmol/hr Aspen s REQUIL model is used to model this reactor. Molar Extent is used to control the low yield of by-products. PEP Process Module 8 25 Aug 1999

9 References Scott, R.H. (to ICI), Process for Purifying Methanol by Distillation US 4,013,521 (March 22, 1977) Pinto, A. (to ICI), Methanol Distillation Process US 4,210,495 (July 1, 1980) Dunster, M., et al., Reduced Natural Gas Consumption for Methanol Production, Inst. Chem. Eng. Symp. Ser. 44,5 (1976), Pinto, A. et al., Optimizing the ICI Low-Pressure Methanol Process Chem. Eng., 84, 14, July 1977, Pinto, A. et al., Impact High Fuel Cost on Plant Design Chem. Eng., 84, 14, July 1977, Masson, J.R. Energy Saving in LP Methanol Industries,, March 3-8, Pinto, A. (to ICI), Steam Hydrocarbon Process US 4,072,625 (Feb 7, 1978) Camps, J.A., et al. Synthetic Gas Production for Methanol : Current and Future Trends, Am. Chem. Soc., Symp. Ser., 116 (1979, publ. 1980), Rowell, G. M. (to Humphreys & Glasgow), Synthesis Gas Production, British 2,066,841 (July 15, 1981) Supp, E. Improved Methanol Process, Hydrocarbon Processing, 60, 3 (March 1981) Methanol (ICI Low Pressure Process, Hydrocarbon Processing 60, 11 (November 1981), Strelzoff, S., Methanol: Its Technology and Economics, paper in Methanol Technology and Economics, Chemical Engineering Progress Symp. Ser., 66, 98 (1970), Dybkjaer, Ib., Topsoe Methanol Technology, Chem. Econ. Eng. Review, 13, 6 (149), (June 1981), Brennan, J. R., Recover Power with Hydraulic Motors, Hydrocarbon Processing, 60, 7 (July 1981) Reports Satish Nirula, Methanol, Process Economics Program Report No. 43 B (August 1982). Report by: Sherif Aly August 25, 1999 PEP Process Module Aug 1999