SS2100 - CO2 Caustic Tower Feed

Application Note

 Caustic Wash Tower
Hydrogen Sulfide is required in Ethylene cracking furnaces to passivate the coil metallurgy, and also to limit the formation of CO2 and CO. Any CO2 entering the furnaces with the feedstock, CO2 formed by the water shift reaction and CO2 formed by decoking must be removed in the Caustic Wash Tower. CO2 and H2S are removed in the Caustic Wash Tower (Fig. 1) which is typically located after the third stage of compression on the cracked gas. The acid gases are absorbed in a dilute caustic solution.

 Critical Control of Carbon Dioxide
As Carbon Dioxide and H2S concentrations can change depending on the feedstock and operating conditions of the process, the loading on the Caustic Wash Tower is highly variable. Measurement of CO2 at the Caustic Wash Tower Inlet is critical to the unit caustic control.

 Traditional Measurement Solutions
On-line gas chromatographs have been the traditional method for monitoring the levels of CO2 in cracked gas streams. Unfortunately, even with the latest in chromatography techniques, the analysis can take 3-6 minutes between measurement updates. Due to rapid changes in the acid gas concentration, this delay may not be acceptable. Also, gas chromatographs consume carrier and flame fuel cylinder gases, as well as requiring hydrocarbon-free air for flame ionization detectors, so the consumable costs of GC’s are high. Spectroscopic methods using broadband light sources and narrow band pass filters are prone to interferences from changing background concentrations.

 SpectraSensors’ Solution
The SpectraSensors SS2100 offers a new solution to this control measurement. The use of Tunable Diode Laser technology means that analysis results can be updated every second if desired. Furthermore, the high resolution that is inherent to TDL technology eliminates errors due to interferences that have hampered other spectrometric approaches. There are no consumables required for CO2 analysis at normal concentrations, so cost of ownership is low.

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Data Sheet

Performance
Target Components	CO2 in Caustic Wash Tower Feed
Typical Measurement Ranges	0 – 500 ppm*
Typical Precision	±2% of Full Scale
Measurement Response Time	1 to ~60 sec*
Principle of Measurement	Tunable Diode Laser Absorption Spectroscopy Non-differential
Environmental Temperature Range	-20º to 50ºC (-4º to 122ºF) -10º to 60ºC (14º to 140ºF) optional
Sample Inlet Pressure	70 kPag (10 psig) typical 210 kPag (30 psig) maximum
Cell Temperature	Maintain at 50ºC +/- 0.2ºC
Maximum Cell Pressure	70 kPag (10 PSIg)
Sample Flow Rate	3 - 4 L/min (6.4 to 8.5 scfh)*
Validation	Certified blend of CO2 in Nitrogen balance
Electrical
Power	100-240 VAC, 50-60 Hz standard
Max Current	Controller: 1 A @ 120 VAC
Controller to Cell Cable Length	1m standard (3m, 5m & 10m available optionally)
Communication	Current Loop Output 4-20 mA Isolated, 1200 ohms @ 24 VDC max load Serial: ASCII Text RS232C standard Modbus RS232C
Digital Outputs	4 ea. 12 VDC for valve operations Scrubber (if required), Process/Val, Val 1, Val 2 5 SPDT (Form C) Dry Contact Alarms Common Fault, Val 1 Active, Val 2 Active, Val Fail, one user-assignable to any alarm
LCD Display	Concentration, Cell Pressure and Temperature, Diagnostic Data
Physical
Controller Enclosure	NEMA 4X - 304 Stainless Steel standard
Controller Dimensions	343mm H x 305mm W x 165mm H (13.5" H x 12" W x 6-7/16" D)*
Weight Approximately	13.1 Kg (28.6 lbs)*
Sample Cell Dimensions	8m Herriot Cell 559 mm H × 127 mm W (22” H × 5” W)
Sample Cell Construction	316L Series Polished Stainless Steel standard SilcoNert® coated
Number of Sample Cells	1 (single channel SS2100) or 2 (dual channel SS2100)
Area Classification	CSA Certified for Class I, Division 2, Groups ABCD Temp Code T3C  ll 2G Ex d llB+H2 T5; Tamb : -20 ÷ +60 °C
Dimensions with Sample System	1678 mm H x 613 mm W x 427 mm D (66” H x 24-1/8” W x 16-13/16” D)
Weight with Sample System (approx.)	68 kg (150 lbs.)
* Application specific; consult factory

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Drawings

Typical Accessories

Photos

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