Hydrocarbon Engineering - August 2019

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August 2019 HYDROCARBON
ENGINEERING 60
significant amount of C1 – C4 mercaptans.
Other components detected included
heavier hydrocarbons including benzene and
toluene as well as minor amounts of dimethyl
ether and acetaldehyde. The sample as
analysed also included some of the feed gas
components, mostly methane, ethane and
propane.
The heavier hydrocarbons and
mercaptans at the low process temperature
likely contributed to MCHE fouling. The
presence of heavier hydrocarbons also
suggested that the upstream HRU was not
operating effectively. In fact, analysis of this
small sample of liquids collected during
testing was the strongest evidence of the
root-cause of MCHE fouling and led to
activities directed at verifying HRU
operation.
Mitigation strategies and
process improvements
The MCHE contamination causing fouling and
freezing issues was identified as heavy
hydrocarbons and mercaptans. The
mercaptans were determined to be
originating from the molecular sieve beds.
Small concentrations of mercaptans in the
feed gas to the beds likely built up slowly
over time until the sieves were saturated and
began to release contaminants during
regeneration. Due to the closed system
design, mercaptans could only be released in
the treated gas over time. The heavy
hydrocarbons were present because of
inefficiencies at the upstream HRU. This
problem was identified only because of the
contamination found during gas testing at the
MCHE inlet. Operations were adjusted to
prevent reduced efficiency at the HRU after
an upset and mitigate MCHE fouling.
After addressing the root-causes for
fouling at the MCHE, solutions were
established considering all the data and
observations. Changes to the HRU operation
solved the fouling at the MCHE. Central to
the success of this onsite work was the
rigorous sampling and testing of
contaminants coupled with process data
analysis and investigation.
Overall, it is important to understand that
the contamination testing and control
strategies for any process unit is a critical step
for ensuring a stable and reliable plant
performance. The majority of plants that do
not consider this important step carefully are
often challenged with high operating costs,
low equipment reliability, unscheduled
shutdowns, and other adverse situations with
direct impacts on plant economics.
Table 3. MCHE inlet contamination analysis results (liquid phase)
Component (liquid phase) Concentration Units
Helium <0.0010 % mole
Nitrogen 0.1221 % mole
Carbon dioxide <0.0050 % mole
Oxygen <0.0050 % mole
Methane 38.6956 % mole
Ethane 17.1773 % mole
Propane 25.9707 % mole
Iso-butane 3.4366 % mole
n-butane 6.6348 % mole
Iso-pentane 1.8470 % mole
n-pentane 1.1454 % mole
Total hexanes 12579 ppm (v/v)
Total heptanes 10611 ppm (v/v)
Total octanes 15461 ppm (v/v)
Total nonanes 11054 ppm (v/v)
Total decanes <1 ppm (v/v)
Benzene 173 ppm (v/v)
Toluene 1016 ppm (v/v)
1,3-dimethylbenzene (m-Xylene) <1 ppm (v/v)
1,4-dimethylbenzene (p-Xylene) <1 ppm (v/v)
1,2-dimethylbenzene (o-Xylene) <1 ppm (v/v)
Table 3. MCHE inlet contamination analysis results (gas phase)
Component (gas phase) Concentration Units
Hydrogen sulfide <0.1000 mg/m3
Carbonyl sulfide <0.1000 mg/m3
Methyl mercaptan 16.1400 mg/m3
Ethyl mercaptan 315.4200 mg/m3
Tert-butyl mercaptan 445.6300 mg/m3
n-propyl mercaptan 788.7800 mg/m3
Isopropyl mercaptan 119.0900 mg/m3
n-butyl mercaptan 96.3700 mg/m3
Methanol <0.5 mg/kg
Dimethyl ether 27.8000 mg/kg
n-propyl alcohol and isopropanol <0.5 mg/kg
Acetone <0.5 mg/kg
Acetaldehyde 2.8 mg/kg
Isobutylaldehyde <0.5 mg/kg
Butylaldehyde <0.5 mg/kg
Helium <0.0010 % mole
Nitrogen 0.2432 % mole
Carbon dioxide <0.0050 % mole
Oxygen 0.0070 % mole
Methane 70.1234 % mole
Ethane 15.5826 % mole
Propane 11.5456 % mole
Iso-butane 0.8563 % mole
n-butane 1.2976 % mole