September 2021

Process Controls, Instrumentation and Automation

Monitor ppb monomer impurities and catalyst poisons with process GC-MS

Competition in the ethylene market is fierce and demands for the quality of the ethylene are increasing. High-purity monomers are used to make polyethylene and polypropylene, the polymeric building blocks for two important plastics.

Wasson, J., Russell, T., Wasson-ECE Instrumentation

Competition in the ethylene market is fierce and demands for the quality of the ethylene are increasing. High-purity monomers are used to make polyethylene and polypropylene, the polymeric building blocks for two important plastics. Numerous potential ethylene impurities reduce the quality and value of ethylene by interfering with the subsequent production costs of polyethylene. Non-ethylene hydrocarbons can result in non-ethylene polymers being produced. Contaminants such as oxygenates, sulfurs and metal hydrides like arsine and phosphine have significant impacts on the metallocene catalyst required to produce polyethylene.

For example, the catalyst is poisoned by even trace levels of common byproducts, such as hydrogen sulfide (H2S), acid gases and methanol (CH3OH). This poisoning reduces catalyst efficiency, resulting in lower polymer yields per reaction. Catalyst bed poisoning also necessitates increased maintenance events and associated plant downtime, and, eventually, significant catalyst replacement costs. For these reasons, polyethylene producers demand high-purity ethylene. Careful monitoring of ethylene process streams is required for early detection and remediation of contaminants to meet product specifications.

Analysis

The oxygenates, hydrocarbons, permanent gases and sulfurs potentially found in monomer streams are very different compound classes and require multiple detector types for their complete analysis. Classic process gas chromatographs (PGCs) frequently house a single detector type that necessitates multiple instruments to address the complete analyte list. Laboratory GCs routinely use temperature-programmable convection ovens and capillary chromatography to achieve excellent analyte separation and resultant low sensitivities.

In contrast, most PGCs are limited to isothermal air-bath ovens and use packed columns, resulting in poorer detection limits for analytes of interest. It should be noted that the metal hydrides arsine and phosphine, which are potent catalyst poisons, are not addressable by typical PGCs. Therefore, for the hydrides, or for low-ppb (parts per billion) quantitation of other process stream compounds, offline analysis is required.

Offline analysis poses several challenges for analysis of process streams, the riskiest of which is time. The additional time required to take a manual “grab” sample back to the lab increases the risk that changes in the process might affect product quality before action can be taken. The chemical composition of the sample is also at risk and might not reflect the true composition of the process stream. For example, light components can volatilize out of the sample and be lost in the head space of the sampling container. Reactive components can be lost on the surface of the sampling container. The precision of the PGC measurements is often greater than that of laboratory measurements because there are fewer changes to the sample and increased automation reduces measurement variation.

However, laboratory analysis does offer at least two benefits. Critical processes often have analytical redundancy to safeguard against loss of data in the event of instrument downtime and lab instruments often fulfill this role. Lab GCs have also been relied on to offer advanced methods and additional detector choices, such as mass spectrometers (MSDs), not included in typical PGCs.

MSDs are nearly universal and very sensitive detectors capable of quantifying analytes in the ppb range. Except for hydrogen and helium, an MSD can detect all analytes of interest. MSDs have a broad dynamic range (106) that allows them to be maximally useful in the plant where concentration ranges can vary considerably. The troubleshooting advantages of a mass spectrometer are unparalleled and allow for interrogation of “mystery” peaks, or alternatively, “the usual suspects” that show up from time to time in a process stream. Both types of anomalies are easy addressable by an MSD. Finally, as will be discussed in greater detail here, GC-MSD can offer triple verification of the identity of an analyte. To the best of the authors’ knowledge, no other PGC has offered GC-MS as an option.

The objective in the design of this instrument was to create a PGC capable of lab-quality performance to address the full list of impurities of interest in ethylene and propylene streams. To achieve that goal, temperature-programmed capillary chromatography was combined with GC-MSD, GC-PDHID (pulsed discharge ionization detector) and GC-FID (flame ionization detector). Complete characterization of the C1–C5+ hydrocarbons, permanent gases, common sulfurs and oxygenates, arsine and phosphine in an ethylene stream was achieved in 10 min.

Basics of GC-MS

An in-depth review of GC-MS is beyond the scope of this paper and a basic understanding will be adequate for the purpose of this discussion.

GCs—using capillary columns carefully chosen for the chemical properties of their surface coating—spatially resolve analytes within a sample by virtue of the variable interaction of those analytes with the column coating. As the separated compounds elute from the column, they are directed to a detector for quantitation.

As the name suggests, the mass analyzer of a quadrupole mass spectrometer uses the mass of a molecule for identification and the abundance of that mass for quantitation. There are several important steps of this process, outlined in FIG. 1.1

FIG. 1. Quadrupole mass analyzer.<sup>1</sup>
FIG. 1. Quadrupole mass analyzer.1

Compounds within the GC column eluates are delivered to the inlet of the MSD where they are exposed to electrons released by a heated filament. This interaction does two things. First, the electrons bombard the molecule and fragment it in a structure-dependent fashion—the unique chemical structure of the molecule dictates the way the molecule will break apart under electron bombardment. This fragmentation pattern, as well as the relative abundance of the different fragments, is unique to the molecule. The impact of the electrons also strips an electron from the intact molecule and its fragments confer a charge on them.

The charged intact molecules (called molecular or parent ions) and fragments (called fragment or daughter ions) are accelerated and guided through the instrument by a tunable, DC and RF electric field in the quadrupole mass analyzer.2 This field functions as a filter by allowing only selected ions, which resonate with the field, to make it to the detector while non-resonant ions are deflected away. It is the mass-to-charge ratio (m/z) that is selected but, because the charge is 1, the ion mass is readily determined. The selected ions then strike an electron multiplier, which greatly amplifies the signal and sends it to the data system for quantification.

The observed m/z, the mass spectra, for methanol and diethyl sulfide are shown in FIG. 2. These unique compounds have distinct spectra with characteristic peaks. Methanol has a parent m/z at 32 and a dominant fragment m/z at 31. Diethyl sulfide has a parent m/z at 90 and a dominant fragment m/z at 75. The fragmentation patterns, as well as the ratios of the abundances of the fragments, are constant and defining for these molecules. This allows for unambiguous identification of methanol and diethyl sulfide.

FIG. 2. Example of mass spectra: methanal (top) and diethyl sulfide (bottom) have unique mass spectra. Source: NIST Chemistry WebBook, SRD 69.
FIG. 2. Example of mass spectra: methanal (top) and diethyl sulfide (bottom) have unique mass spectra. Source: NIST Chemistry WebBook, SRD 69.

The importance of this fact is highlighted in FIG. 3. Methanol and diethyl sulfide often co-elute and are observed as a composite peak, seen at 8.84 min in this example (FIG. 3A). With an MSD, this composite peak can be separated by examining extracted ion chromatograms (EICs), which are individual chromatograms for a desired m/z (FIG. 3B, diethyl sulfide and 3C, methanol). As expected, the retention time of the individually extracted and resolved compounds matches that of the composite peak. EICs allow for quantitation of each individual contributor to the composite peak and clearly demonstrates one of the greatest advantages of the MSD.

FIG. 3. Extracted ion chromatograms resolve co-eluting compounds: (A) chromatogram showing co-elution of diethyl sulfide and methanol at 8.84 min; (B) EIC of diethyl sulfide; and (C) EIC of methanol.
FIG. 3. Extracted ion chromatograms resolve co-eluting compounds: (A) chromatogram showing co-elution of diethyl sulfide and methanol at 8.84 min; (B) EIC of diethyl sulfide; and (C) EIC of methanol.

MSDs also allow for the identification of unknown peaks that might appear in a chromatogram. The analyzer can be configured to identify the mass (m/z) of the peak, which can then be found in a mass spec library. Alternatively, if the presence of a specific compound is suspected, the indicative masses (m/z) for this analyte can be quickly added to the analyte list to determine if the compound is present. Subsequent analysis of a prepared sample can be confirmatory and quantitative.

Lastly, GC-MSD can offer triple confirmation of an analyte by agreement of the expected retention time, correctness of the spectrum of the molecular and fragment ions, and the coincidence of those ions in the extracted ion chromatograms at the expected retention time.

Case study: Instrument design

The process GC monomer analyzera (FIG. 4) used in this study was designed and built by the authors’ company. It was designed for one of the largest petrochemical companies in North America to speciate C1–C4 hydrocarbons and numerous common catalyst poisons (H2S, COS, methyl mercaptan, ethyl mercaptan, methanol, arsine and phosphine) in an ethylene matrix.

Temperature-programmed capillary chromatography was required to resolve and quantify such diverse analytes. Numerous capillary columns were housed in two patented, programmable, micro-convection ovens (MCOs) that wrap around the analytical columns to control temperatures precisely and rapidly. Packed columns were also used and housed in an isothermal oven. Electronic pressure and flow programming controlled the performance and resolving power of the analytical columns.

Column eluates were directed to three multiplexed detector trains (MSD, PDHID and FID). Combining the resolving power of capillary chromatography with the selective and discriminatory power of the MSD made GC-MS the best choice to address the oxygenates, sulfurs and metal hydrides. A proprietary mass spectrometerb was used, and a proprietary PDHIDc was selected for the permanent gas analysis to achieve the low-ppb sensitivities requested.3 The common combination of a methanizer and an FID causes tailing of the CO and CO2 peaks, which decreases the sensitivity for these analytes. PDHID does not suffer from this limitation. An FID is the best detector for hydrocarbon analysis and a proprietary FIDd was chosen for this application. The most common hydrocarbon impurities in monomer streams are hydrocarbons lighter than the monomer of interest. In the case of ethylene, methane and ethane are the most frequently observed contaminants. Heavier hydrocarbons were also potentially present and were characterized, as well.

This instrument used a single chromatographic method to control sample acquisition and simultaneous analysis by the three detector trains. Unambiguous analyte detection and quantification were completed in < 10 min.

A custom sample conditioning system was designed to be mounted outside of the shelter and to accept eight sample streams: two ethylene streams and six calibration streams. Stream-selection valves delivered the chosen sample to sample-injection valves. Upon actuation, the injected sample was delivered, via a temperature-controlled bridge, to the analyzer inside the shelter (FIG. 4).

FIG. 4. Process GC monomer analyzer and sample system: (A) process gas chromatograph within the analyzer shelter; and (B) sample conditioning system mounted outside of the shelter, directly opposite the PGC. Samples are ported from the sample system to the analyzer through sample lines housed within a temperature-controlled heated bridge (black flexible tubing).
FIG. 4. Process GC monomer analyzer and sample system: (A) process gas chromatograph within the analyzer shelter; and (B) sample conditioning system mounted outside of the shelter, directly opposite the PGC. Samples are ported from the sample system to the analyzer through sample lines housed within a temperature-controlled heated bridge (black flexible tubing).

Once inside the analyzer, column-selection valves directed the sample to the appropriate analytical column for chromatographic separation and elution to the appropriate detector for identification and quantification. This design, with the sample system outside of the shelter and the analyzer inside the shelter, allowed for the bulk of the hazardous sample to be kept away from potential ignition sources inside the shelter. Only very small aliquots of sample gas (0.1 ml–1 ml each) were delivered to the Class 1, Division 2 hazardous area rated analyzer. Where appropriate, inert tubing and hardware were used to prevent analyte loss within the instrument.

Case study: Results

A 50-m capillary column in MCO2 resolved methanol, methyl mercaptan and ethyl mercaptan from a 10-ppm calibration blend (FIG. 5). The reference methods used were UOP 1015-174, UOP 1021-195, UOP 1022-186, UOP 1023-187 and UOP 1024-188. Calculated minimal detectable limits (MDLs) were methanol (25 ppb), methyl mercaptan (10 ppb) and ethyl mercaptan (15 ppb). Simultaneously, another capillary column in MCO2 resolved H2S and COS. The MDL for COS was 10 ppb, while that for H2S was 40 ppb. Arsine and phosphine (10 ppm each, as shown in FIG. 5) were also quantified with MDLs of 15 ppb and 30 ppb, respectively. Note that the retention times for many of these analytes are very similar; it would be difficult to resolve and quantitate them with another type of detector. However, the MSD’s ability to generate an extracted ion chromatogram allowed for complete resolution and ppb level quantitation of these important compounds. The results of the process GC-MS system showed outstanding performance consistent with laboratory GC-MS systems.

FIG. 5. Sulfurs, oxygenates and hydrides by MSD. Extracted ion chromatogram results for 1.) methanol with an m/z at 31; 2.) methyl mercaptan with an m/z at 47; 3.) ethyl mercaptan with an m/z at 62; 4.) carbonyl sulfide with an m/z at 60; 5.) hydrogen sulfide with an m/z at 34; 6) arsine with an m/z at 76; and 7.) phosphine with an m/z at 34.
FIG. 5. Sulfurs, oxygenates and hydrides by MSD. Extracted ion chromatogram results for 1.) methanol with an m/z at 31; 2.) methyl mercaptan with an m/z at 47; 3.) ethyl mercaptan with an m/z at 62; 4.) carbonyl sulfide with an m/z at 60; 5.) hydrogen sulfide with an m/z at 34; 6) arsine with an m/z at 76; and 7.) phosphine with an m/z at 34.

At the same time, the PDHID train quantified CO, CO2, hydrogen (H2), O2/argon (Ar) and nitrogen (N2) from a 10-ppm calibration blend. The reference method used was ASTM D8098-179. Two column sets in the isothermal oven, each with one packed and one capillary column, were used to separate the analytes (10 ppm each, as shown in FIG. 6). The chromatogram shows two sections with a rising baseline at 3.7 min resulting from the switch between the column sets. The first column set resolved and quantified H2, O2/Ar composite, N2 and CO. The second column then set quantified CO2. The observed baseline shift is a consequence of switching column sets during the run. The MDLs for these analytes were H2 (100 ppb), O2/Ar and N2 (500 ppb), CO (20 ppb) and CO2 (30 ppb). Methane was observed on each column set but analyzed by the FID (shown below).

FIG. 6. Permanent gases by PDHID. Chromatogram of PDHID demonstrating 1.) hydrogen, 2.) oxygen/argon composite, 3.) nitrogen, 4.) methane*, 5.) carbon monoxide, 6.) hydrogen/air composite*, 7.) methane* and 8.) carbon dioxide. *Not quantified.
FIG. 6. Permanent gases by PDHID. Chromatogram of PDHID demonstrating 1.) hydrogen, 2.) oxygen/argon composite, 3.) nitrogen, 4.) methane*, 5.) carbon monoxide, 6.) hydrogen/air composite*, 7.) methane* and 8.) carbon dioxide. *Not quantified.

While the MSD and PDHID trains were analyzing their respective compounds, the FID speciated and quantified hydrocarbons. The reference method used was ASTM D6159-1710. Here, an ethylene stream was interrogated for hydrocarbons (200-ppm refinery gas mix), which were resolved on a 50-m column in MCO1 and speciated by a flame ionization detector (FID, shown in FIG. 7). C1–C4 hydrocarbons were speciated while heavier species were backflushed as C5+. The MDL was determined to be 2 ppm for these components.

FIG. 7. Hydrocarbons by FID. Chromatogram showing results for 1.) C<sub>5</sub>+ backflush, 2.) methane, 3.) ethane, 4.) ethylene*, 5.) propane, 6.) propylene, 7.) acetylene, 8.) propadiene, 9.) isobutane and 10.) n-butane. *Matrix.
FIG. 7. Hydrocarbons by FID. Chromatogram showing results for 1.) C5+ backflush, 2.) methane, 3.) ethane, 4.) ethylene*, 5.) propane, 6.) propylene, 7.) acetylene, 8.) propadiene, 9.) isobutane and 10.) n-butane. *Matrix.

The precision of the micro-convection oven temperature and electronic pressure controls were also examined in a separate engineering study (FIG. 8). Retention time reproducibility is a measure of how tightly controlled the temperature, pressure and backpressure in a system are. To examine the retention time reproducibility, data taken over two days from eight injections of a hydrocarbon mixture were obtained. The relative standard deviation (%RSD) of the results was calculated and found to be excellent for all eight compounds in the blend. Peak area reproducibility is measure of a system’s ability to repeatably deliver the same amount of a sample to the analytical column. It is a function of split inlet performance and mass flow control. To evaluate these parameters, 5% methane in argon was injected 12 times. Again, %RSD results obtained rival those obtained on laboratory GCs and are unparalleled in online PGCs.

FIG. 8. A screenshot of micro-convection oven performance.
FIG. 8. A screenshot of micro-convection oven performance.

Takeaway

The goal of this study was to design a single PGC capable of ppb detection of analytes of importance for ethylene and propylene producers and their customers. A novel approach using process GC-MS was developed to take full advantage of the sensitivity and specificity of MSDs. Tightly controlled and temperature-programmable ovens allowed for the use of fused silica capillary columns for the greatest resolving power. Parallel chromatography with the MSD, PDHID and FID detector trains allowed for analysis of an extensive range of possible monomer contaminants at ppb levels not previously attainable with process GCs. Together, the data presented here clearly demonstrate that, when using the same technologies available to laboratory GCs, lab-quality performance can be achieved from a process instrument. HP

NOTES

Wasson-ECE Instrumentation’s Eclipse Neutrin

Agilent Technologies’ 5977B Mass Spectrometer

Valco PDHID

Wasson-ECE FID

References

  1.  Figure modified from the original, online: https://commons.wikimedia.org/wiki/File:Quadrupole_ion_trajectory.svg
  2.  Silverstein, R. M., F. X. Webster and D. J. Kiemle, Spectrometric identification of organic compounds, 7th Ed., Wiley, 2005.
  3.  Wentworth, W. E., S. V. Vasnin, S. D. Stearns and C. J. Meyer, “Pulsed discharge helium ionization detector,” Chromatographia, Vol. 34, September/October 1992.
  4.  UOP Standard 1015-17, “Determination of trace oxygenates in polymer grade ethylene and propylene by gas chromatography mass spectrometry,” Honeywell UOP, Des Plaines, Illinois, 2017, online: https://www.astm.org/Standards/UOP1015.htm
  5.  UOP Standard 1021-19, “Determination of trace methyl mercaptan, ethyl mercaptan, and isopropyl disulfide in polymer grade ethylene and propylene by gas chromatography mass spectrometry,” Honeywell UOP, Des Plaines, Illinois, 2019, online: https://www.astm.org/Standards/UOP1021.htm
  6.  UOP Standard 1022-18, “Determination of trace carbonyl sulfide and hydrogen sulfide in polymer grade ethylene and propylene by gas chromatography mass spectrometry,” Honeywell UOP, Des Plaines, Illinois, 2018, online: https://www.astm.org/Standards/UOP1022.htm
  7.  UOP Standard 1023-18, “Determination of trace arsine and phosphine in polymer grade ethylene and propylene by gas chromatography mass spectrometry,” Honeywell UOP, Des Plaines, Illinois, 2018, online: https://www.astm.org/Standards/UOP1023.htm
  8.  UOP Standard 1024-18, “Determination of trace permanent gases in polymer grade ethylene and propylene by gas chromatography-pulsed discharge helium ionization detector,” Honeywell UOP, Des Plaines, Illinois, 2018, online: https://www.astm.org/Standards/UOP1024.htm
  9.  ASTM Standard D8098-17, “Standard test method for permanent gases in C2 and C3 hydrocarbon products by gas chromatography and pulse discharge helium ionization detector,” ASTM International, West Conshohocken, Pennsylvania, 2017, online: https://www.astm.org/Standards/D8098.htm
  10.  ASTM Standard D6159-17, “Standard test method for determination of hydrocarbon impurities in ethylene by gas chromatography,” ASTM International, West Conshohocken, Pennsylvania, 2017, online: https://www.astm.org/Standards/D6159.htm

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