SS2100 - Dry Cracked Gas (Dryer Outlet) Analyzer Solution

Application Note

Cracked Gas Dryer
Molecular Sieves and Activated Alumina are both used in Cracked Gas drying. As the inlet gas is saturated, water concentration can be estimated from the process temperature and pressure, and the cycle time of the dryers is controlled from their known capacity. However, dryers are subject to fouling with polymers and heavy hydrocarbons and channeling through the dryer bed can shorten the effective dryer cycle time.

Critical Control of Moisture
To maintain the efficiency of the process and to ensure the purity of the final Ethylene product, water measurements at the outlet of the dryer are essential.

Traditional Measurement Solutions
Vibrating crystal, AlO3 capacitance probes and electrolytic moisture analyzers have been the traditional methods for monitoring the levels of water in Polymer Grade Ethylene. These methods suffer from slow wet-up and dry-down times, slow drift in calibration and sensitivity to contamination Due to the very short residence times in Ethylene feed storage vessels and the desired to measure Ethylene in flowing pipelines, the delay in measurements can easily lead to excursions in concentrations before it is detected. Due to their slow drift in calibration, capacitance probes have very high maintenance costs due to frequent replacement of the probe.

SpectraSensors' Solution
The SpectraSensors SS2100 is the ideal solution for moisture in Dry Cracked Gas. The use of Tunable Diode Laser technology means that analysis results can be updated every second. Furthermore, the high resolution that is inherent to the laser eliminates errors due to interferences that have hampered other spectrometric approaches.

Also, the non-contact nature of the measurement with no moving parts in the analyzer means that the analyzer is simple to operate and little routine maintenance is needed. Only SpectraSensors applies Differential Spectroscopy where the sample is dried using a metal getter technology and the background spectrum of the sample is collected.

The dryer is then bypassed and the wet sample spectrum is collected The dry spectrum is subtracted from the wet spectrum to get a sensitive measurement free of background gas interferences, and thus changing background gas compositions are compensated. The dryer has a typical lifetime greater than 18 months, and is the only consumable part of the analyzer. See Fig. 3 for typical calibration performance.

Typical Stream Composition for Dry Cracked Gas:

Component	Unit	Typical Concentration	Min. for Application	Max. for Application
Water	ppmv	< 5	0.05	10
Hydrogen Sulfide (H2S)	ppmv	< 2	0	10
Carbon Dioxide (CO2)	ppmv	<5	0	10
Hydrogen (H2)	Mole%	25	10	30
Methane (CH4)	Mole%	20	10	35
Ethane (C2H6)	Mole%	15	5	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	1.0
Butenes	Mole%	0.3	0	1.0
1,3-Butadiene	Mole%	0.5	0	2.0
C5+	Mole%	0.1	0	2.0
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	H2O in Dry Cracked Gas
Typical Measurement Ranges	0 – 10ppm* (other ranges available by request)
Typical Precision	±2% of Full Scale*
Measurement Response Time	1 to ~60 sec
Principle of Measurement	Differential Tunable Diode Laser Absorption Spectroscopy (H2O Scrubber included)
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	A certified blend of H2O in Nitrogen balance is diluted with dried sample in the sample conditioning system under flow control
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 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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