SS2100 - H2S 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, so H2S or other sulfur compounds are added to the feedstock. 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.

For optimal efficiency, the caustic strength and low rate through the tower must be matched to the inlet CO2 and H2S concentrations, so measurement of the H2S is critical.

Traditional Measurement Solutions
Lead Acetate Tape devices have been used for on-line H2S analysis, but the on-going cost and maintenance effort of replacing the tapes is high. Moreover, many regard the Lead Acetate tape to be a hazardous material with associated handling and disposal problems. On-line gas chromatographs are another method widely used for monitoring the levels of H2S 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. Gas chromatographs consume carrier and flame fuel cylinder gases, as well as requiring sulfur-free air for flame photometric 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.

Only SpectraSensors employs Differential Spectroscopy to cope with the changes in background spectrum of the sample caused by changing operating conditions and feedstocks. The sample gas is passed through a copper nanoparticle scrubber to remove H2S, and the spectrum of the H2S-free sample is measured. The scrubber is then bypassed to measure the raw sample with H2S, and the H2S is measured by spectral subtraction. As the H2S-free background spectrum can be applied repeatedly to the sample, the scrubber is only used as needed, controlled by logic in the electronic controller. The scrubber lifetime in normal service is a minimum of 18 months, so consumable costs and maintenance are low.

Component	Unit	Typical Concentration	Min. for Application	Max. for Application
Hydrogen Sulfide (H2S)	ppmv	500	0	1000
Carbon Dioxide (CO2)	ppmv	200	10	500
Hydrogen (H2)	Mole%	25	15	30
Methane (CH4)	Mole%	20	10	30
Ethane (C2H6)	Mole%	15	10	30
Ethylene (C2H4)	Mole%	25	20	40
Acetylene (C2H2)	Mole%	0.3	0	0.5
Propylene (C2H6)	Mole%	7.5	0	15
Propane (C3H8)	Mole%	7.5	0	15
Methyl Acetylene (Propyne C3H4)	Mole%	0.03	0	0.1
Propadiene (C3H4)	Mole%	0.02	0	0.1
Carbon Monoxide (CO)	Mole%	0.05	0	0.1
Butanes	Mole%	0.05	0	0.1
Butenes	Mole%	0.3	0	0.5
1,3-Butadiene	Mole%	0.5	0	1
C5+	Mole%	0.1	0	0.5
Total	Mole%	100

 

The background stream composition must be specified for proper calibration and measurement performance. Specify the Normal composition, along with the minimum and maximum expected values for each component, especially water, the measured component. Other stream compositions may be allowable with approval from SpectraSensors. 

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Data Sheet

Performance
Target Components	H2S 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 H2S 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	28m Herriot Cell 559 mm H × 127 mm W (22” H × 5” W)
Sample Cell Construction	316L Series Polished Stainless Steel standard
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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