Hydrocarbon Processing - September 2021 - 81

Process Controls,
Instrumentation
and Automation
J. WASSON and T. RUSSELL, Wasson-ECE
Instrumentation, Fort Collins, Colorado
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. 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 (H2
acid gases and methanol (CH3
S),
OH). 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
Hydrocarbon Processing | SEPTEMBER 2021 81
Hydrocarbon Processing - September 2021

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