Ethylene is one of the most important feedstocks for the petrochemical industry. Since it is used in the manufacture of a wide range of compounds, Ethylene has very stringent purity specifications. One of the more important steps in purifying the ethylene is removing the acetylene. There are 4 common methods of Acetylene removal in Ethylene process that have evolved over the years:
- Solvent Extraction using Dimethyl Formamide (DMF)
- Cracked Gas Train or Raw Gas Catalytic Hydrogenation
- Back End Catalytic Hydrogenation
- Front End Catalytic Hydrogenation
Application Note
The Acetylene Converter Unit consists of a series of reactors (Fig. 1) or a single vessel with multiple beds of catalyst (Fig. 2) which reacts the acetylene with hydrogen to form ethylene. The first reactor or bed will reduce the amount of acetylene from the percent levels down to the high ppm ranges. The second reactor or bed will then reduce the acetylene level even further to the low ppm and even ppb levels. Acetylene converters can be located at different points in the process. “Front End” Converters are upstream of the separation section, while “Back End” Converters may be located downstream of the De-Propanizer or De-Ethanizer. Measurements may also be made at the inlet of the Acetylene Converter Unit for feed-forward control strategies. Each stream type presents different analytical challenges for measuring Acetylene.
Critical Control of Acetylene
It is critical that the acetylene be brought down to the proper levels as it exits the final reactor; otherwise, any acetylene present will follow the ethylene all the way to the final product stream. But if the unit operates too aggressively, it could hydrogenate some of the ethylene back into ethane. So the proper operation of the unit requires the fast and precise monitoring of acetylene in between the two reactors (mid bed) and as it exits the final reactor.
Traditional Measurement Solutions
On-line gas chromatographs have been the traditional method for monitoring the levels of acetylene in the process streams. Unfortunately, even with the latest in chromatography techniques, the analysis can take 3-6 minutes between measurement updates. Due to the very short residence times in the reactors, this delay in measurements can easily lead to excursions in concentrations before it is detected.
SpectraSensors’ Solution
The SpectraSensors SS2100 is the ideal solution for this challenging application. The use of Tunable Diode Laser technology means analysis results can be updated every second. Furthermore, the high resolution 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 the analyzer is simple to operate and has no routine maintenance needed.
|
Component |
Unit |
Typical Concentration |
Min. for Application |
Max. for Application |
|
Acetylene (C2H2) |
ppmv |
1500 - 5000 |
0 |
5000 |
|
Ethylene (C2H4) |
Mole% |
65 |
60 |
90 |
|
Ethane (C2H6) |
Mole% |
33 |
0 |
40 |
|
Hydrogen (H2) |
ppmv |
0 |
0
|
1000
|
|
Carbon Monoxide (CO) |
ppmv |
0.5 |
0
|
1000 |
|
Carbon Dioxide (CO2) |
ppmv |
< 1.0 |
0 |
1000 |
|
Methane (CH4) |
ppmv |
50 - 100 |
0 |
1000 |
|
Propylene (C2H6) |
ppmv |
3000 |
0 |
5000 |
|
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.
Data Sheet
Performance |
|
|
Target Components |
Acetylene in Back End Acetylene Converter Midbed |
|
Typical Measurement Ranges |
0 – 3000 ppm |
|
Typical Precision |
±100ppm* |
|
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 |
1 - 2 L/min (2.1 to 4.2 scfh)* |
| Recommended Validation | Certified blend of C2H2 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 |
0.8m cell for ranges >100 ppm
438 mm H × 108 mm W (17-1/4” H × 4-1/4” W)* 28m cell for ranges <100 ppm 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
|
| 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
|
|
Typical Accessories
|
SS2100 ARM9 Electronics
|
|
|
80000-02199 |
Temperature Control Board |
|
23000-00030 |
4-20 mA Current Loop Board
|
|
80000-02203 |
Power Supply Assembly, 120/240 VAC 50/60 Hz
|
|
80000-02300 |
Power Supply Assembly, 24 VDC
|
| 01902-17106 | External Serial Output Cable |
| 01902-30011 | Keypad Assembly |
| 24601-00002 |
Display Assembly
|
| 02203-13100 |
Daughter Board
|
| 45000-02002 |
Relay, DC12V SPDT 3 A/120 VAC
|
| Herriott Cell (8m & 28m) |
|
| 02101-17102 |
Pressure Transducer Assembly
|
| 02101-17103 |
Thermistor Assembly
|
| 02199-00006 |
Spares Kit (o-rings, fuses), AC, Viton
|
| 02199-00007 |
Cleaning Kit (domestic only)
|
| Cell (0.8m) | |
| 01902-17102 |
Pressure Transducer Assembly
|
| 02199-00001 |
Spares/Cleaning Kit (o-rings, fuses), AC, Viton (domestic only)
|
| 02199-00005 |
Spares Kit (o-rings, fuses), AC, Viton
|