Ethylene (IUPAC name: ethene) is the simplest olefin, or alkene, with the chemical formula C2H4. Olefin fiber is a synthetic fiber made from a polyolefin, such as polypropylene or polyethylene. Ethylene production takes place in an olefin furnace which is also used to produce other hydrocarbons in the olefin family, typically propylene (C3H6) and butadiene (C4H6). It is used in wallpaper, carpeting, ropes, and vehicle interiors. The hydrocarbon feed is pyrolized, or ‘cracked,’ with steam at a high temperature, between 750°C to 900°C (1382°F and 1652°F), and high pressure, between 175 and 240 kPa, in a series of tubular furnaces.
Two main types of feedstock are used:
- Gas feed (ethane, propane)
C2H6 → CH2 = CH2 + H2
C3H8 → CH3CH = CH2 + H2 - Liquid feed (e.g. naphtha)
CnH2n+2 → CH2 = CH2 + C3H6 + C4H8 + CnH2n+
Residence time in the furnace is extremely short, typically 0.08-0.25 seconds.
There are two alternative methods that a typical ethylene plant can use for effluent gas analysis: process gas chromatography (GC) and process mass spectrometry. Mass spectrometry, however, brings two important benefits to the analysis: the reduction of both the footprint and the maintenance required.
The furnace temperature, also known as the cracking severity, determines the type of olefin produced; higher temperatures favor ethylene while lower temperatures favor propylene and butadiene.
The energy transfer through the furnace coils is determined by the burner firing rate and the furnace draft pressure. The plant can therefore operate on a campaign basis, producing various olefins based on market demands, feedstock costs and energy prices.
The furnace effluent product stream is quenched in a heat exchanger. It contains a range of eight to 20 components, including the desired product. A number of additional refining and purification steps then take place but it is the furnace effluent that defines not only the product yield, or slate, but the total plant efficiency and fuel consumption. It essentially drives the overall profitability of the entire complex.
Process Analytical Requirements
Analysis of the furnace effluent provides invaluable information to the plant’s process control system:
- Coking Rate: hydrogen and methane
- Selectivity: ethylene, propylene and butadiene
- Severity: n-pentane, iso-pentane and pentene-1
Traditionally, process GC has been used during this process but long analysis cycle times as well as frequent calibration and maintenance intervals limit the usefulness of this technique. A typical ethylene unit comprises eight to 12 furnaces and requires one GC per furnace just to provide a limited analysis of five components with a cycle time of close to three minutes. Furthermore, additional GC analyzers are required to provide a more complete analysis.
Advantages of Mass Spectrometry
Some process mass spectrometers offer analysis times measured in seconds rather than minutes and the ability to measure inorganic and organic species over a wide dynamic range.
In one example, comparison of furnace effluent analysis by a mass spectrometer compared to a process GC shows analysis of 22 components in 30 seconds, meaning one unit has the capacity to analyze six furnaces in three minutes. The GC is much less efficient and is only capable of analyzing five components in three to six minutes. In addition, the GC does not analyze hydrogen, butadiene or the C5 hydrocarbons, all of which are important to cracker optimization. The mass spectrometer is also more precise; typically around 0.1% relative compared to 0.5% relative for the GC. (See Process Control and Efficiency During Ethylene Production Application Note for more details, including a comparison chart of mean data during furnace effluent gas analysis.)
Olefin manufacturers need fast, reliable online furnace effluent analysis to control cracking severity and to maximize profits. The furnace effluent analysis cycle time needs to be close to three minutes to track process kinetics in this dynamic process. Some mass spectrometers offer this rapid analysis speed along with precision that is on average five times better than process GC.
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