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Background The production of industrial grade silicon (IGS) is vital to businesses such as the metals industry, semiconductor grade silicon manufacturing, silicones production, silicon nitrate production, and many others. In 1995, Western world demand of silicon was 790,000 metric tons. Western demand is projected to grow at an average rate of 5% into the twenty-first century. By the year 2000, this demand is expected to reach almost 1,000,000 metric tons . The goal of this project is to design a grass roots facility that will safely and efficiently produce 25,000 tons per year of IGS from carbon and silicon dioxide. Process Description A BFD of the overall process is shown in Figure 1. Unit 100 – Reaction Section A PFD for Unit 100 is shown in Figure 2. Coke particles, Stream 1, and
pulverized SiO2, Stream 2, are combined and pneumatically conveyed into a refractory lined fluidized bed, H-101, where they are preheated to 1500°C. The heat is supplied to this process by the combustion of natural gas and air in the bottom portion of the vessel. At this temperature, no reaction occurs between the SiO2 and the carbon. The off gas from H-101 is then cooled in a thermal regenerator and sent to Unit 200. The heated solids, Stream 6, are then transported to a graphite lined, electrically heated reduction furnace, R-101, where the reaction takes place and molten silicon is formed. The molten silicon is removed in Stream 14, and the unreacted SiO2 and carbon, Stream 13, are sent
The off gases from Unit 100. The following reactions lead to the production of molten silicon : 2SiO2 (l ) + 4C ( s ) ! SiO( g ) + SiC + 3CO ( g ) .8 MPa to provide the necessary pressure to produce the high-pressure steam. Stream 12. are fed to a steam boiler. Boiler feed water. Unit 200-. The waste solids.2 to a waste treatment unit. is pumped up to 40. Stream 18 is then sent to a turbine. Stream 18. Within the fluidized bed any sulfur present as sulfur dioxide in the off gas is absorbed and reacted with limestone to form calcium sulfate. approximately 2045°C. which contains limestone particles. E-201. the electrodes are gradually used up and must be continually fed (replaced) to the reactor. which uses three graphite electrodes to reach the necessary temperature. Necessary Information and Simulation Hints The reduction furnace used in this process is an electric smelting furnace. T-201. Because carbon is consumed in the reaction. Stream 9. Once through the boiler.Co-generation Loop A PFD for Unit 200 is shown in Figure 3. The off gas from R-101. A conversion of about 90% is reached. the cooled off gases from Unit 100. for the reaction to occur. Stream 22. Stream 10 is then compressed and mixed with Stream 8 and sent to Unit 200 as Stream 12. R-101. are then sent to a waste treatment facility and the exiting gas is vented to the atmosphere. are sent to a fluidized bed. Stream 16. Stream 15. where they supply the energy to produce highpressure steam. where electricity is generated and subsequently consumed in the process. is then cooled in a thermal regenerator.
oxygen.3 SiO ( g ) + SiC ( s ) ! 2 Si(l ) + CO ( g ) SiO2 (l ) + S ( s ) ! SO2 ( g ) + Si (l ) In order to meet environmental regulations provided by the Clean Air Act. These regenerators take the form of packed beds of ceramic material. For these services it is recommended that fixed bed heat regenerators be used. Hot gas will flow through one bed while the other bed is cooled (by passing air or steam through it. Due to the high temperature involved. Operation of these units is cyclic with two units operating in parallel. the SO2 must be removed. The limestone is fed in at the top of the fluidizing gas. The removal of SO2 by limestone may be inefficient at the temperature of Stream 15. Placing the fluid bed upstream of E-201 or other methods of SO2 removal such as caustic scrubbing should be considered. Additional heat recovery from the air and steam will allow additional utility savings. The use of limestone in a fluidized bed significantly reduces the SO2 present in the gas stream. Equipment Summary . nitrogen. The requirement is that the off gases from this unit must contain no more than 0. With a limestone to sulfur ratio of 2. The reaction that occurs takes place between the limestone (calcium oxide) and the sulfur dioxide to yield calcium sulfate.) At the end of the cycle the flows are reversed with the hot gas now flowing through the cooled bed and air or steam being heated in the bed of hot solids.5 to 1. it has been reported that sulfur dioxide is undetectable in the exiting gas stream . water vapor.5 ppm SO2. the recovery of heat from Streams 7 and 9 cannot be done in conventional heat exchangers (recuperators). The fluidizing gas consists of carbon dioxide. and sulfur dioxide.
” Kirk-Othmer Encyclopedia of Chemical Technology. Combustion in the Circulating Fluid Bed: An Alternative Approach in Energy Supply and Environmental Protection. G.. U. pp. 1997. 21.. H.. & A. Serbent. “Silicon. 17B. 2. Peterson. 1110-1111. April 9-11. Dept. I. 4th ed. and H. vol. “Thermodynamics of the Si-C-O System for the Production of Silicon Carbide and Metallic Silicon. Daradimos. 3. 1980. V. Claveau. . Malinsky. 1986). N. Of Energy. Nagamuri. Schmidt.4 C-101 E-101A/B E-102A/B H-101 R-101 E-201 P-201A/B R-201 T-201 References Compressor Thermal Regenerator Thermal Regenerator Fluidized Bed Preheater Reduction Furnace Steam Boiler Boiler Feed Water Pump Fluidized Bed Scrubber Turbine 1. (Sept.” Metallurgical Transactions.S. 503-514.
0 ----- 9 2045 1 9745 ------9745 -- 10 480 1 9745 ------9745 -- 11 12 600 600.5 Stream Tables for IGS Production Stream Temp.0 1.0 ------9745 -----428 6109 10764 772 428 6109 1019 772 .4 9745 18073. (atm) Total Flow (kg/h) Component Flows (kg/h) Silicon Dioxide Silicon Carbon Natural Gas Oxygen Nitrogen Carbon Dioxide Water 1 25 1 3071 --3071 ------ 2 25 1 7382 7382 -------- 3 25 1 10453 7382 -3071 ------ 4 25.7 ---363.0 ----428 6109 1019 772 8 600. (°C) Press. (°C) Press.5 1.7 ----- 5 25 2 7965 ----1856 6109 --- 6 1500 1 10453 7382 -3071 ------- Stream Temp.5 8328.0 363.0 1. (atm) Total Flow (kg/h) Component Flows (kg/h) Silicon Dioxide Silicon Carbon Natural Gas Oxygen Nitrogen Carbon Dioxide Water 7 1500.0 1.0 2.5 8328.
(°C) Press.0 85. (atm) Total Flow (kg/h) Component Flows (kg/h) Silicon Dioxide Silicon Carbon Natural Gas Oxygen Nitrogen Carbon Dioxide Water 13 2045 1 1605 738 -867 ------ 14 15 16 17 18 2045 260.3 --------- .6 Stream Temp. (°C) Press.8 3100 18073.7 1 1.0 14967.0 -3100 ----------428 6109 10764 772 ------14967 -------14967 -------14967 Stream Temp.0 85.0 14967. (atm) Total Flow (kg/h) Component Flows (kg/h) Silicon Dioxide Silicon Carbon Natural Gas Oxygen Nitrogen Carbon Dioxide Water 19 100 1 14967 -------14967 21 260 1 18073 ----428 6109 10764 772 22 260.4 40.0 184.5 5.0 194.8 40.0 14967.0 1.
FEEDFORWARD control is installed between the two utilities. Sand to Coke (1:2) FT X RSP TT FT TC FT FC 3. natural gas and air in order to maximize energy consumption .FC 1. FEEDBACK control is installed on line 6 back to the natural gas utility stream to ensure maximum control of utilities supplied to the system and highest cost save FT FF 4. RATIO control is installed to maintain the relationship between the two feed streams on unit 100. CASCADE control is installed between the desired Molten SIlicon product and Coke feed in order to maximize supply for the product AC AT 2.
Limestone Make-up and gaseous feed (from unit 100 in 2.PC PT FT RSP FC X AC 7. RATIO control is installed with a valve as an actuator to maintain the relationship between two inlet streams on unit 200. budget pending.5:1 direct scale) AT 5. FEEDBACK control on the steam to atms (steam trap) could be potentially installed as a back up. FEEDBACK control is combined with the feedforward loop to ensure maximum control with simultaneous inﬂuence from the exiting high pressure steam . Ratio would not be ideal because purities are inconsistent (feed gas inlet composition is variable) FT FF X 8. This could serve as a ﬁnal check for FDA approval and connect to the Limestone to Make-up which would indirectly mitigate unwanted steam product in the atms stream FT 6. FEEDFORWARD control is installed on unit 200 from the feed gas to the boiler feed water.
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