This is Part Two of a three-part blog series.
Results
Summarized 2F spectra for the data collected with the AMETEK 5100HD analyzer are shown in Figures 1a and 1b. Each of the spectra in the figures covers the average of 100 separate scans and illustrate the changes in the spectra corresponding to different concentrations of water vapor in ethylene (1a) and propylene (1b) backgrounds.
Figure 1a. 2F spectra of moisture in ethylene
Figure 1b. 2F spectra of moisture in propylene
In this data, the peak amplitude and area of the 2F moisture signal were proportional to the concentration of moisture in the cell, with the sharpest peaks occurring as moisture concentrations were increased. The data in Figure 1a also contain the background spectrum of the sample matrix (i.e., ethylene). This background spectrum was significant in comparison with the absorbance of moisture at low concentration levels, requiring creation of a calibration model. An Inverse Least Squares (ILS) regression was developed to accurately measure the moisture concentrations in the presence of the ethylene sample matrix. The data in Figure 1b demonstrate that 2F spectrum of propylene is very close to spectrum of zero gas nitrogen, because it is a heavier molecular weight substance and has no resolved rotational structure in the spectral vicinity of interest. The regression vector that was calculated as a result of the ILS regression is also shown on Figure 1b. Note that the spectral profile of the calculated regression vectors are analyte specific and correspond to the spectral area where moisture spectra are changing with concentration.
The data shown in Figure 2a and Figure 2b are the responses of the instrument to a series of moisture challenges over the moisture concentration range of 0-50 ppmv in ethylene and in propylene, correspondingly. The duration for each challenge was approximately 20 minutes. The background gas baseline was also tested before validation measurements for each of the background gas ranges. Dried ethylene and propylene were used as the zero gases. The response time (T90) was measured to be 2 minutes for moisture in ethylene and 3 minutes for moisture in propylene and was limited by the propagation of the gas in the sampling system with a flow rate of 2L/min for ethylene stream and 0.5 L/min for propylene stream. Propylene stream through the sample cell was kept at 0.5L/min to avoid turbulence developing for high molecular weight substances flow in the sample cell.
Figure 2a. Moisture readings in ethylene
Figure 2b. Moisture reading in propylene
The results of the validation test are summarized in the Table 1. The accuracy for moisture measurements in ethylene was evaluated as 1 ppmv with repeatability of 0.2 ppmv. The accuracy for moisture measurements in propylene was evaluated as 0.25 ppmv with repeatability of 0.2 ppmv. Repeatability as a degree of agreement between replicate measurements of the same quantity was expressed in terms of standard deviation of the measurements. The value of the accuracy was calculated as the maximum positive and negative deviation of the readings from set value observed during validation test of the analyzer.
No significant trends or correlations with the environmental temperature or sample pressure were observed in the data.
|
Background Gas
|
Ethylene
|
Propylene
|
|
Moisture, ppmv
|
Average (ppmv)
|
STD
(ppmv)
|
Error
(ppmv)
|
Average (ppmv)
|
STD
(ppmv)
|
Error
(ppmv)
|
|
0
|
0.15
|
0.12
|
0.15
|
0.25
|
0.04
|
0.25
|
|
5
|
5.13
|
0.1
|
0.13
|
5.19
|
0.08
|
0.19
|
|
15
|
14.85
|
0.23
|
0.15
|
14.88
|
0.04
|
0.12
|
|
25
|
24.77
|
0.14
|
0.23
|
24.97
|
0.08
|
0.03
|
|
35
|
34.6
|
0.16
|
0.4
|
34.91
|
0.12
|
0.09
|
|
50
|
49.3
|
0.1
|
0.7
|
49.91
|
0.15
|
0.09
|
Table 1. Moisture analyzer performance in ethylene and proplyene
Conclusion
New gas analyzers were built for online measurements of moisture in ethylene and in propylene. Corresponding applications include measurements of moisture in ethylene and propylene in both production processes and product purity at quality control points. The moisture measurements in ethylene gas yielded an accuracy of +/-1 ppmv and repeatability of 0.2 ppmv in the concentration range of 0 to 50 ppmv. The moisture measurements in propylene gas yielded an accuracy of +/-0.25 ppmv and repeatability of 0.2 ppmv in the concentration range of 0 to 50 ppmv.
To learn more about AMETEKs TDLAS solutions for this application, click here.
Read Part One of this series – The Importance of Measuring Water in Ethylene and Propylene Production
Read Part Two of this series - Instrument and Testing Methodology for Measuring Water in Ethylene and Proplyene Production
Acknowledgements
The authors wish to thank David Elro (AMETEK Process Instruments, Pittsburgh, Pennsylvania) for collaboration in calibration of analyzers for measurements of moisture in ethylene and in propylene.