Chemical Engineering January 2018 - 56

0.1
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
CO2
H2S
Operation condition
Parameter
Wet gas flowrate
Contactor pressure
Value
10 million std. m3/d
80 bara
Lean glycol circulation rate 4 m3/h
flow) or laboratory analysis of glycol
samples, which can indicate high
hydrocarbon content.
30
40
Contactor temperature, °C
FIGURE 5. As with BTEX compounds, the contactor temperature impacts the concentration of acid gases
present in the glycol
materials can result in excessive glycol
losses due to foaming, reduced
efficiency and additional maintenance
problems. Heavy hydrocarbons
in the glycol can cause coking
on the reboiler surface, creating hot
spots on the firebox and plugging
in the regeneration system. Heavy
hydrocarbon presence can also increase
reboiler heat load due to elevated
boiling points, and result in
glycol losses.
Condensation in the contactor
can be prevented by maintaining
the inlet glycol temperature 3 to 6°C
above the feedgas inlet temperature.
If not maintained, condensation of
the hydrocarbon might occur, which
can cause foaming and increase
glycol losses.
Flash vessel considerations. The
glycol flash vessel is used to remove
light hydrocarbons, acid gases and
small amounts of aromatics by rapid
reduction in pressure (flashing). Degassing
in the flash vessel before
the rich glycol enters the lean/rich
exchanger helps to prevent foaming
and fouling in the exchanger and reboiler.
If hydrocarbons such as BTEX
are present along with CO2 and H2S
in the rich glycol, then a preheating
step is more efficient for the degassing
process. The recommended
preheating temperature is about 70
to 75°C, and the recommended operating
pressure of the flash vessel
is around 3 to 5 bara.
Activated-carbon filter. A properly
designed activated-carbon filter can
effectively remove most foamingand
fouling-promoting compounds
in the glycol. The filter should be
installed downstream of the particle
filters in the rich glycol line. Carbon
filters are usually sized for glycol
loading of 2.5 to 5.0 m3/h per m2 of
filter cross-sectional area. If the rich
glycol contains dissolved components,
such as BTEX, H2S, CO2 or
54
heavy hydrocarbons, it is suggested
to install two full-flow activated-carbon
filters in parallel with no bypass
line. If the filters are not designed
for full flowrates, then foaming and
corrosion impurities in the rich glycol
will enter into the lean/rich exchanger,
regenerator system and
circulation pumps. The impurities
will cause exchanger fouling, leading
to poor heat transfer and poor
regenerator performance, which affects
overall glycol purity. If the lean/
rich exchanger is of the plate-andframe
type, then it will cause frequent
maintenance and mechanical
damage. This will also result in failure
of the glycol circulation pump in
the long term.
Canister- or cartridge-type synthetic-carbon
filters are generally
preferred when compared to loose
charcoal beds (installed as fill into
a vessel) because they are easier
to maintain and avoid unnecessary
exposure of workers to BTEX components.
Contaminated spent charcoal
is difficult to dispose of unless it
is contained in a canister.
A synthetic-type carbon filter derived
from petroleum products can
be used effectively for aromatic and
acid-gas removal. The main advantages
of these types of filters are
high surface area and high adsorption
capacity compared with activated
carbon derived from woodbased
charcoal, coconut shell or
bituminous coal.
The replacement of a synthetictype
carbon filter cannot be determined
using the conventional approach
of filter differential-pressure
measurement, because syntheticbased
carbon does not create a
pressure differential after filter exhaust.
The filter replacement interval
should be determined based
on visual examination (color comparison
between inflow and out50
The
ideal solution for the operational
problems described in this
article involves good mechanical
design of the inlet separator, effective
preheating of the rich glycol before
entering the glycol exchanger
and proper selection of a synthetic
carbon filter with two filters in parallel
and full-flowrate capacity with
no bypass line. This will effectively
eliminate most foaming and corrosion
problems by removing the hydrocarbons
and other troublesome
impurities from the glycol, which will
result in minimized operating and
maintenance problems. Reducing
the glycol circulation rate is the most
effective way of decreasing the absorption
of BTEX and acid gases in
circulating glycol.
■
Edited by Mary Page Bailey
References
1. Campbell, J.M., " Gas Conditioning and Process - Vol.
2: The Equipment Modules, " 8th Ed., 2004.
2. Gas Processors Suppliers Association (GPSA) Data Book,
13th Ed., 2012.
3. Stewart, M. and Arnold, K., " Gas Dehydration Field Manual, "
1st Ed., Gulf Professional Publishing, August 2011.
4. Kohl, A.L. and Nielsen, R.B., " Gas Purification, " 5th Ed.,
Elsevier Science, August 1997.
Author
Krishnan Madan Mohan is a senior
process engineer at WorleyParsons
Engineering Oman (P.O.
Box 795 Al-Khuwair,
Oman;
Muscat,
24473394; Email:
Telephone: 968k.mohan@
worleyparsons.com).
He has 15
years of professional experience
in the oil-and-gas industry, including
activities related to feasibility
studies, concept selection, basis of design and frontend
engineering design (FEED) and detail design. His
experience includes debottlenecking, upgrades, design
modifications for glycol-based gas dehydration units
and sour-water stripper units, flare and relief studies,
including detailed blowdown studies, equipment sizing
and thermal design for heat exchangers and air coolers.
He holds a B.Tech. (chemical) degree from the National
Institute of Technology (NIT), Tiruchirappalli, India.
Suman Pachal is a process engineer
with WorleyParsons Engineering
Oman (Same address as
above; Email:
suman.pachal@
worleyparsons.com). He has more
than 10 years of design consultancy
experience in the oil-andgas,
petroleum-refining and petrochemicals
sectors.
Before
joining
WorleyParsons, he worked
as a process engineer for Technip India in New Delhi.
His areas of expertise include line hydraulics, equipment
sizing, relief valve sizing and vent and blowdown
calculations. He holds a B.Tech. (chemical) degree from
the University of Calcutta, Kolkata, India.
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