Hydrocarbon Processing - September 2021 - 83

Process Controls, Instrumentation and Automation
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 (H2
-
S, 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
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).
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 sampleinjection
valves. Upon actuation, the injected
sample was delivered, via a temperature-controlled
bridge, to the analyzer
inside the shelter (FIG. 4).
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
186
, UOP 1021-195
, UOP 1022.
,
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 H2
S and
COS. The MDL for COS was 10 ppb,
while that for H2
S 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.
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
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. 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.
Hydrocarbon Processing | SEPTEMBER 2021 83
Hydrocarbon Processing - September 2021

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