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Soda Ash Inputs: Trona Ore Cooling Water Steam/Fuel Electricity Air Brine Purification Inputs: Salt Water Make-up Brine Sodium Carbonate Caustic Soda/ Soda Ash Steam/Fuel Electricity Outputs: Soda Ash Carbon Dioxide Condensed Vapors Waste Sludge Waste Water Outputs: Purified Brine Brine Mud/Sludge For every category, energy use for process heat is distributed according to the various fuel types used throughout the industry. Fuel distribution for 1997 was as follows: fuel oil and LPG - 3 percent; natural gas - 77 percent; coal and coke - 10 percent; other - 10 percent (CMA 1998). The “other” category includes any other fuel source (e.g., byproduct fuel gases). Among the three types of chlorine cells, the mercury cell is the most energy-intensive, with electricity requirements of nearly 3600 kWhr per metric ton of chlorine. The membrane cell is the least energy-intensive in terms of both steam and electricity requirements. Steam requirements are less than half those of the diaphragm or mercury cell. Electricity requirements for the membrane cell are in the range of 2800 kWhr per metric ton of chlorine. The diaphragm cell is intermediate between these energy consumption ranges. Overall, the electrical energy requirements for chlorine electrolysis cells are high, accounting for nearly 130 trillion Btu annually. When losses due to transmission and generation of electricity are considered, they reach nearly 400 trillion Btu annually. Thus, efficient operation of the cell is critical to optimized energy use and cost-effective production. Sources of energy losses in chlorine cells include anode or cathode overvoltage, too large a drop across the diaphragm, oxygen evolution on the anode, and failure to recover heat and energy from hydrogen, chlorine, and cell liquor streams. A key consideration in membrane processes is the purity of the brine. Using very pure brine at an optimum flow rate minimizes blockage through the membrane and allows sodium to penetrate freely. Brine purity is also important in mercury cells. Impurities tend to increase hydrogen by-product and reduce the current efficiency. Another issue is brine flowrate. Flowrates that are too high increase cell temperature and electrical conductivity of the medium. Brine rates that are too low create temperatures and high cell voltages that are higher than the most efficient voltage (3.1 to 3.7 volts) (Sittig 1977, Orica 1999). 6.3 Energy Requirements Electricity Is the Largest Energy Source Used for Production of Chlorine and Sodium Hydroxide Data on the process energy used for the co- production of chlorine and sodium hydroxide isshowninTable6-3. Electricityfuelsthe electrolysis process and represents the primary energy source. The amount of electricity required depends on the design of the cell, the design operating current, concentration of electrolytes, temperature, and pressure. The values shown in Table 6-3 represent an average of energy consumption for the various cell types. Energy in the form of fuels or steam is used primarily for evaporation of the sodium hydroxide solution to a useable state. Some fuels are also consumed in the production and purification of brine feedstock before it is sent to the electrolysis cell. 191PDF Image | The Chlor-Alkali Industry
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