Application of Filter Photometers in the Production of Ethylene and Propylene

Transcription

1 Analytical Products Sales Engineering Application of Filter Photometers in the Production of Ethylene and Propylene SC Gary D. Brewer

2 APPLICATION OF FILTER PHOTOMETERS IN THE PRODUCTION OF ETHYLENE AND PROPYLENE Gary D. Brewer Product Manager, Photometers ABB Inc. 843 N. Jefferson St. Lewisburg, WV KEYWORDS Photometer, Infrared Spectroscopy, IR, Near Infrared Spectroscopy, NIR, Ultraviolet Spectroscopy, UV, Ethylene, Propylene ABSTRACT There have been several successful applications of process filter photometers in ethylene plants throughout the world. The process of manufacturing ethylene is extremely fast; therefore, the continuous measurements provided by filter photometers allow for a fast response to process changes for better control and process optimization. The capability of the current generation of filter photometers to use several analytical wavelengths to compensate for spectral interferences have allowed their use in measurements that previously could not be done by photometers. The high reliability and simplicity of filter photometers make them a valuable tool in the process control of an olefins plant. Applications in the IR and NIR spectral regions in both the vapor and liquid phases will be discussed to demonstrate their capabilities and benefits in this manufacturing process. Applications that will be discussed include the measurement of acetylene and ethane at the acetylene converters and the measurement of methyl acetylene and propadiene (MAPD) at the MAPD converters can be measured on a single infrared photometer. Applications at the caustic wash tower and the measurement of carbon dioxide at the furnace decoke will also be discussed. INTRODUCTION Ethylene is one of the highest volume chemicals produced in the world. It is used in the manufacturing of other chemicals such as: polyethylene, ethylene dichloride, vinyl chloride, polyvinyl chloride, ethylene glycol, ethylene oxide, etc Approximately 50% of ethylene produced comes from the cracking of ethane/propane from natural gas and the rest from naphtha and gas oil. The cracking of

3 ethane is an extremely fast process with any individual molecule being in the pyrolysis reactor from 100 milliseconds to a few seconds.(1) Therefore fast measurements are required to control and optimize the process. Filter photometers in the infrared, near infrared, visible, and ultraviolet spectral regions have been used in the hydrocarbon processing industries for many years and have proven to be very robust and easy to maintain. They are normally less expensive than spectrophotometers that are designed for operation in a process environment. A filter photometer uses narrow band pass optical filters for its wavelength selection and thus works with discrete parts of the spectrum.(2) Figure 1 shows a comparison of a full spectrum scan of butane and water along with the filters that would be used to make the measurements on a filter photometer. It demonstrates how photometers work with discrete parts instead of the full spectrum. Process filter photometers are available that measure only single components in liquid or vapor streams or that can measure multiple components in liquid or vapor streams. Some are designed to work with extracting and conditioning the sample and some are designed to utilize fiber optic probes for in-situ measurements. In most designs the reference signal goes through the same optical path as the measure signal and are ratioed. This provides several benefits that make them very stable and reliable in a process analysis: Minimizes drift from source, filter and detector aging Minimizes drift from cell window obstructions Minimizes effect of some particulates in a gas or liquid sample Minimizes effect of some bubbles in a liquid sample % TRANSMISSION FILTER SPECTRA WATER VAPOR SPECTRUM BUTANE SPECTRUM WAVELENGTH (NM) FIGURE 1 NEAR INFRARED SPECTRA OF BUTANE, WATER VAPOR AND FILTERS

4 APPLICATIONS ACETYLENE CONVERTERS Measuring acetylene and sometimes ethane with a fast analysis in the inlet stream to the acetylene converters allows for better control of the converter operation. A typical acetylene converter application is as follows: Measure Components: Acetylene 0-1.5% Acetylene 1% Ethane 0-30% Ethane 25% Methane 0.2% Propane 0.5% Propylene 0.3% Ethylene Balance Figure 2 shows that 3333 wavenumbers (cm -1 ) is a feasible wavelength for measuring acetylene, 2700 cm -1 is a suitable wavelength for measure ethane, and 2500 cm -1 is a feasible reference wavelength. The spectra also indicate that the acetylene measurement will have some interference from both ethane and ethylene. The ethane measurement will not have any significant interference. If ethane is not measured a 2700 cm -1 wavelength filter will be needed to compensate for this interference and a 2075 cm -1 wavelength can be used to compensate for the ethylene interference. Without compensation the acetylene measurement would have about an 8% of full-scale interference from ethane and a 1% of full-scale interference from ethylene. On filter photometers additional wavelengths that pick up the absorbance from an interfering compound and using linear regression to compensate for the interference normally reduces the interference by a factor between 10X to 15X and therefore the acetylene measurement precision would be approximately ±1% of full scale (±0.02% Acetylene). % TRANSMISSION WAVELENGTH (NM) FIGURE 2 INFRARED SPECTRA OF ACEYTLENE, ETHYLENE & ETHANE

5 MAPD CONVERTERS Measuring methyl acetylene and propadiene with a fast analysis in the inlet stream to the MAPD converters allows for better control of the converter operation. A typical MAPD converter stream is: Measured Components: Methyl Acetylene 0-2% Methyl Acetylene 1% Propadiene 0-2% Propadiene 1% Ethylene 0.4% Propane 6% Propylene Balance Figure 3 shows that 3333 cm -1 is a feasible wavelength to measure methyl acetylene, 1960 cm -1 is a suitable wavelength to measure propadiene and 2500 cm -1 is an acceptable wavelength to use as a reference. Figure 2 shows that propylene has a small absorbance at the reference wavelength and also at the propadiene measure wavelength and therefore this interference will need to be compensated for. The propylene spectrum in Figure 2 indicates that 1850 cm -1 will be a good wavelength to use for the propylene interference compensation. If the propylene interference were not compensated, then 100% propylene would create a 7% of full-scale error on the propadiene measurement and about a 0.7% of full-scale error on the methyl acetylene measurement. After compensation by using the propylene filter above and linear regression the interference will be reduced to less than ±0.7% of full scale (±0.014% propadiene) on the propadiene channel and to less than ±0.07% of full scale (±0.0014% methyl acetylene) on the methyl acetylene channel. There is benefit to the process control for a fast analysis of methyl acetylene and propadiene on the outlet of the MAPD converters. The required ranges of ppm methyl acetylene and ppm propadiene are too low for most filter photometers to provide a reliable analysis. % TRANSMISSION BLACK: 15% PROPADIENE BLUE: 100% PROPYLENE RED: 7% METHYL ACETYLENE WAVENUMBER (CM -1 ) FIGURE 3 INFRARED SPECTRA OF METHYL ACETYLENE, PROPADIENE, AND PROYLENE

6 ETHYLENE FRACTIONATOR The ethylene fractionator (splitter) is where the ethylene is the purification part of the process where the desired product, ethylene, is removed from the ethane. The filter photometer provides a fast analysis time that allows for optimal control of a fractionator tower. A typical stream of an ethylene fractionator tower is: Measure Components: Ethylene 0-35% Ethylene 20% Propylene 3% Propane 3% Ethane Balance Figure 4 shows that 1905 cm -1 is feasible wavelength to measure ethylene and 1961 cm -1 is suitable for a reference wavelength. The spectra show that the other stream components do not absorb at these wavelengths and therefore interference compensation will not be needed. % TRANSMISSION BLACK: 100% ETHANE RED: 25% ETHYLENE BLUE: 5% PROPYLENE WAVENUMBER (CM -1 ) FIGURE 4 INFRARED SPECTRA OF ETHANE, ETHYLENE, PROPANE, AND PROPYLENE ETHANE FRACTIONATOR In the ethane fractionator tower the ethane is removed from the process stream and is returned to a pyrolysis furnace for conversion into ethylene. The fast measurement of ethane allows for optimum control of the fractionator. A typical stream at the ethane fractionator is:

7 Measured Components: Ethane 0-30% Ethane 10% Methane Trace Ethylene Balance Figure 5 shows that 2775 cm -1 is a suitable wavelength to measure the ethane and 2500 cm -1 will work well as a reference wavelength. Since this is essentially a binary stream there are not any spectral interferences to deal with and is a straightforward measurement. % TRANSMISSION BLACK: 35% ETHANE RED: 100% ETHYLENE WAVENUMBER (CM -1 ) FIGURE 5 INFRARED SPECTRA OF ETHANE AND ETHYLENE CAUSTIC WASH TOWER SOLUTION ANALYSIS The caustic wash tower uses a sodium hydroxide solution to remove the carbon dioxide from the hydrocarbon stream out of the furnaces. The solution must contain excess caustic at all times to prevent the carbon dioxide in the sample from proceeding to other parts of process. A fast and accurate analysis of the sodium hydroxide content of the solution allows the wash tower to be operated at lower excess caustic concentrations and thus a reduction in operation cost and maintenance. Measure Components: Sodium Hydroxide 0-10% Sodium Hydroxide 5% Sodium Carbonate 1% Dissolved Hydrocarbons 0.1% Water Balance Near infrared filter photometers have been used for online caustic measurments in ethylene and other hydrocarbon processing industries for many years.(3) The photometer provides a precise analysis,

8 since it can make the measurement without the results being affected by the build up of sodium carbonate and other salts in the solution. The photometer does not have any consumables like those needed for online titrators (an analytical method that is sometimes used for this analysis). This liquid sample contains gases that are not dissolved in the sample and these bubbles will create a light scattering issue if they are allowed to reach the analyzer. It is necessary to degas the sample in the sample handling system before the sample enters the analyzer. Figure 6 contains spectra of sodium hydroxide and sodium carbonate in water. It shows that 2200 nanometers (nm) is a wavelength that could be used to measure sodium hydroxide. Sodium carbonate will interfere at this wavelength and will need to be compensated nm is a good wavelength for a reference. The spectral region around 1800 nm can be used to compensate for the sodium carbonate interference since its absorbance is approximately the same at both wavelengths. The sodium hydroxide absorbance at 1800 will reduce the sensitivity by about 30% but there is still plenty of absorbance to provide stable readings. Testing has indicated that after compensation a change from 0 to 10% sodium carbonate will have less than a ±0.5% of full-scale affect on the sodium hydroxide measurement.(4) ABSORBANCE BLACK: 100% WATER RED: 15% NaOH IN WATER BLUE: 5% Na 2 CO 3 IN WATER WAVELENGTH (NANOMETERS) FIGURE 6 NEAR INFRARED SPECTRA OF WATER, SODIUM HYDROXIDE AND SODIUM CARBONATE FURNACE DECOKE During the cracking operation of the feedstock in the pyrolysis reactors to make ethylene carbon builds up on the reactor tubes. This carbon build up reduces the heat transfer, which in turn reduces the cracking efficiency. Approximately every three weeks it is necessary to remove this carbonaceous material to regain cracking efficiency. This is done by injecting steam and air into the reactors, which in combination with heat converts the carbon to carbon dioxide. By monitoring the carbon dioxide concentration during this process the decoking can be optimized and the time can be minimized which

9 reduces the associated costs of this operation. This is an easy analysis from a spectroscopic view but heavy particulate loading along with water vapor concentrations between 35 to90% make this a very difficult stream to draw a representative sample from. A sampling approach that has worked on this application is using a reflux sampler to remove the particulate and remove the free water vapor before sending the sample to the analyzer. Figure 7 is a schematic of a typical reflux sampler. This sampler cools the sample to condense the water and the liquid water is then used to remove the particulate back to the process line before they can be transported to the downstream sample handling system and analyzer. Filter photometers with external heated sample cells are a benefit to this measurement since it allows operating the reflux sampler in a manner to remove the particulates and not completely remove the water and thus minimizes the absorption of the carbon dioxide into the water. A typical decoke stream is shown below: Measured Component: Carbon Dioxide 0-15% Carbon Dioxide 10% Carbon Monoxide 5% Water 8% Oxygen 17% Nitrogen Balance Figure 8 shows that the carbon dioxide can be measured without spectral interferences from the other major stream components. It indicates that 2364 cm -1 is a suitable wavelength for the carbon dioxide measurement and that 2500 cm -1 is good reference wavelength. 1 Column Vortec Cooler Vortec Cooler Exhaust Power Supply Purge Gauge Temp erature Probe Assembly Samp le Stream Shutoff Valve Prop ortional Valve Purge Regulator Temp erature C ontroller Power In Shutoff Valve Solenoid Eductor Eductor Air Regulator FIGURE 7: SCHEMATIC OF A REFLUX SAMPLER

10 % TRANSMISSION BLACK: 1% CARBON DIOXIDE RED: 5% CARBON MONOXIDE BLUE: 2% WATER VAPOR WAVENUMBER (CM -1 ) FIGURE 8 INFRARED SPECTRA OF CARBON DIOXIDE, CARBON MONOXIDE, AND WATER VAPOR CONCLUSION There are several measurements that can be made with filter photometers that can be used to optimize plant operation in the manufacturing of ethylene and propylene. The continuous measurement of acetylene and ethane at the converters is feasible on a multiple component filter photometer. Both methyl acetylene and propadiene can be measured at the MAPD converters and the spectral interference from propylene can be compensated to improve the precision of the measurements. Ethylene and ethane can be measured to improve operation of the fractionation towers. A filter photometer configured for operation in the near infrared region can be used to accurately measure the caustic concentration in the scrubber solution. When coupled with a sample handling system to remove the heavy particulate loading, a filter photometer can be used to measure the carbon dioxide during the decoking operation of the pyrolysis furnaces. There are other areas within the process that can benefit from using filter photometers for the necessary measurements. REFERENCES 1. Burdick, Donald and Leffler, William, Olefin Plants, Ethylene, and Propylene, Petrochemicals in Nontechnical Language, Pennwell Publishing Co., Tulsa, OK, 1990, Goldman, Don, New Developments for Chemical Analysis Using Process Photometry,141, Proceedings for ISA Analysis Division 2001, Houston, TX, 2001.

11 3. Baughman, Dr. Ernie and Watson Jr., Dr. Edgar, On-line Analysis of Caustic Streams by Near-Infrared Spectroscopy, Spectroscopy, Vol. 2, No. 1, January 1986, Cardis, Thomas and Brewer, Gary, On-line Photometer for Caustic Monitoring, 151, Proceedings for ISA Analysis Division 2001, Houston, TX, 2001.