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3.3 Energy Use and Intensity in the U.S. Petrochemical Industry Average Energy Efficiency Because of the emphasis on ethylene as the most important product from steam cracking in the past, the specific energy consumption per tonne of ethylene (SECe) is a common measure of energy consumption for cracking. Modern plant values for SECe are 14 GJ/tonne of ethylene for ethane cracking (13 MBtu/ton, HHV) and 20-27 GJ/tonne of ethylene for naphtha/gas oil cracking (19-26 MBtu/ton, HHV) (Phylipsen et al., 1998a). The cracker product mix is influenced by feedstocks and processing conditions (or severity which includes parameters such as temperature, pressure and furnace residence time). Using the specific energy consumption per tonne of ethylene (SECe) as a measure of energy efficiency would mean that all energy consumption is allocated to ethylene, and none to the other products. This means that using another feedstock or severity will affect the SECe, even if total energy consumption does not change. In order to be able to compare different processes and feedstocks (with different yields for the various products) another allocation has to be used. In order to exclude effects from changing product yields, energy consumption should be allocated over all products formed in a particular process (on a mass basis). Solomon Associates Ltd., a company performing energy efficiency analyses and comparisons for refineries and petrochemical plants world-wide, has developed an extensive network for this type of analysis. Participating companies provide Solomon with very detailed data on production, throughput, energy consumption, installed technology. In return, Solomon offers companies a comparison of their own plant’s performance with that of all the other participating plants. Plant performance is measured by comparing the plant’s actual energy consumption to a reference level of energy consumption, based on the most efficient technology available, using the plant’s own configuration, including feedstock input, severity, and product mix. Specific energy consumption is defined as the net energy consumption (lower heating value, LHV4) per unit of high value chemicals (including hydrogen, ethylene, propylene, a mixed butenes fraction and a BTX fraction5). The energy efficiency is presented as the ratio of the actual SEC to the reference SEC; the energy efficiency index, in which 100 equals the efficiency of the state-of-the-art reference system. According to Solomon the energy efficiency index (EEI) of the naphtha/gas oil-based crackers in the U.S. is 163 (in 1995). These crackers, located in Texas and Louisiana, account for about 40% of ethylene production (10Mt) (Solomon, 1998). The EEI of all cracking activities in North America (including the U.S., Canada and Mexico) equals 175 (Solomon, 1995). The U.S. accounts for 85% of the capacity in North America (O&GJ, 1998). 4 Lower heating value (LHV) and higher heating value (HHV) are both measures of energy consumption. Higher heating values measure the heat that is freed by the combustion of fuels in case the chemically formed water is condensed. Lower heating value measure the heat of combustion in case the water formed remains gaseous. Although the energy difference between HHV and LHV may not always be technically recoverable, from the perspective of energy efficiency analysis HHV is considered preferable, because it is a better measure of the energy inefficiency of processes. In the US HHV is commonly used in energy reporting, in many other countries LHV is more common. Organisations such as Eurostat, the IEA and the UN also use LHV (Phylipsen et al., 1998). 5 Measured in tonnes of benzene 12 1PDF Image | Energy use and energy intensity
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