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Unformatted text preview: Chapter 6 Process Energy Calculations and Synthesis of Safe and Efficient Energy Flow Sheets CHBE 241 Department of Chemical and Biological Engineering The University of British Columbia 2006 1 Introduction Chemical processes require energy. Energy is needed to drive separation processes like distillation and crystallization. Reactor conversion depends on temperature, which must be controlled by energy supply or withdrawal. Designing chemical processes that work and that are economical requires careful consideration of energy needs. Averaged over all chemical industries, energy costs contribute about 8% to the total expense of product manufacture. Energy costs are very important in determining the profitability of commodity products (e.g., food), but are less important for specialty products (e.g., drugs). For example, with nitrogen-based fertilizers the energy costs are about 70% of the total manufacturing costs. Even for those processes and products where energy costs are not very important, control of energy needs is crucial for the safety and reliability of the process. For example, sterilization of cell culture broths requires heating of the broth to a sufficiently high temperature for a sufficiently long time. 2 Energy Sources Just as the earths resources provide the raw materials needed for making products, the earths resources supply energy. Combustible fuels (mostly fossil fuels) serve as the major source of raw materials for energy. Figure 1: Fossil fuels constitute the largest energy resource, although the contribution from renewable energy resources is growing. (a) Large coal pile stacked at a power plant (b) Offshore oil platform (c) Natural gas storage tanks 3 Energy Sources (2) Solid fuels: Coal, waste paper, bagasse from sugarcaneanything solid that burns. These fuels are often relatively inexpensive, but may require special burners and solids handling facilities, and typically leave an ash residue. Solid fuels were once the dominant fuel source; now in the US they supply about 25% of energy needs. Liquid fuels: In the early 20th century, crude oil (petroleum) became the primary source of liquid fuels. Ethanol and biodiesel, produced from renewable agricultural resources, are becoming increasingly important. Lighter (more volatile) and cleaner liquids are used for transportationgasoline, jet fuel, and diesel. Less volatile (heavier) oils are burned in industrial and home-heating furnaces. About 3540% of US energy needs are supplied by crude oil. Gas Fuels: Natural gas is relatively inexpensive, widely available, and clean burning. It supplies about 25% of the US energy demand. 4 Energy Distribution: Electricity, Heating Fluids, and Cooling Fluids Electricity is not an energy source, but a means of distributing energy. It is clean, generally reliable, and a convenient source of energy for doing work....
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This note was uploaded on 04/05/2010 for the course CHEBE 241 taught by Professor Ali during the Winter '08 term at The University of British Columbia.
- Winter '08