XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
H2S REMOVAL FROM CO2 VENT STREAM OF AMINE PLANTS
Federico Patti and Francisco Sanchez Carelli – FLARGENT
ABSTRACT
In amines plants, venting CO2 into the atmosphere is a problem which demands
hard work during the design stage by making calculations so as to avoid
contamination of CO2 at floor level. But when the gas that is treated in the plant
contains significant amounts of H2S, it is vented to the atmosphere with the CO2.
In this case, it is difficult to avoid presence of H2S at floor level even raising the
height of the vent. Depending on the H2S ppm concentration on the vent, the
effect can be smell or as a simple odor of rotten eggs or as a serious threat to the
health of operators.
On the other hand, Sulfatreat, a non-hazardous, non regenerative, granular
product, is a technology well known on the removal of H2S from natural Gas flows
at low and medium pressure, prior injecting the gas into pipeline.
The purpose of this paper is to present the results of a technology analysis that
ended in the selection of Sulfatreat as the optimum process for the removal of H2S
from CO2 vent stream of amine plants. Since the process is already in operation,
this work not only examines the theoretical aspects that led to that conclusion, but
also is complemented by field measurements taken after the Star-Up.
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XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
CASE STUDY
The Natural Gas Industry has in carbon dioxide (CO2) one of the main obstacles,
since it causes serious problems in all treatment processes stages, transportation
and sales.
Thinking on Natural Gas as a fuel, high level of CO2 increases the inert content
reducing the gas heating value, which results on a lower sells price. Upstream end
users, in transport gas pipelines CO2 can also cause problems, since in
combination with water generates carbonic acid (H2CO3) which at high pressure,
as in any gas pipeline transportation, it is highly corrosive and can reduce the life
of carbon steel pipes. Because of this, it is common for Natural Gas market
regulators to limit the CO2 maximum allowable content so as to be sold in houses
and in industries. For example, in Argentina, the CO2 content in natural gas for
consumption and/or transportation is limited in 2%.
That is why there are different processes for removal of CO2 from Natural Gas in
order to achieve the allowable values by the regulations and laws. One of the most
common processes is the technology of amines, whose optimal range of use is
when the inlet flow contains up to 10% CO2 and is required 2% at output. Amines
are organic chemical compounds obtained from replacing from ammonia by alkyl
radicals one or more hydrogen atoms. In contact with hydrocarbon gas streams
that contain CO2, the amine has the ability to absorb the CO2 removing it away
from the gas. This contact between the gas with CO2 and the amine is produced
on mass transfer equipment, usually known as contactor column.
The amine rich in CO2 must then be regenerated, since the process is a closed
cycle. This means that the CO2 must be removed from the amine so this one can
be re-used. This is accomplished in other mass transfer equipment usually known
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XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
as regeneration column; the CO2 is desorbed from the amine at high temperatures
by means of heating.
The CO2 separated from the amine is usually vented to the atmosphere, for which
during the design of this type of facility is common to take precautions when
defining the vent height above ground level mainly it shall be avoided a high
content of CO2 in the air that plant operators breathe because it reduces the
partial pressure of oxygen in air. Dispersion analyses are reliable tools commonly
used in this stage.
In the graph below (taken from the GPSA manual) shows a diagram of a typical
amine plant process. The CO2 vent flow is referenced as "Acid Gas" at the top
right.
When the gas to be treated contains high levels of H2S (sulfide hydrogen), in
addition to CO2 it is normal that the H2S & the CO2 go together in the
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XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
atmospheric vent stream. The reaction of the amine with the H2S is similar to the
reaction with the CO2, so that in contact with the gas it will remove both
components equally. And in the regeneration will separate both (CO2 & H2S) from
the amine.
When the sulfur content in the inlet gas is low, the presence of H2S in the vent
stream can be smell just like a harmless odor of rotten eggs. But when the sulfur
input reaches higher levels, the level of H2S in the vent reaches values that would
seriously threaten plant operators’ health. Even at low concentrations, H2S acts as
an irritant to the eyes and respiratory tract. Moderate concentrations cause
headaches, dizziness, nausea and vomiting in that order. The greatest danger,
however, is from its acute effects. Massive dosage that is contact with high
concentrations (1000 ppm or more) can cause immediate loss of consciousness
(in 1-2 seconds) which is rapidly followed by respiratory failure and death (2-3
minutes). In Argentina, there is a legal framework take into consideration this and
set a maximum of 7.5 ppmv gaseous emissions from a vent at 30 meters height.
To depend not only on dispersion calculations in protecting human health, since
some time companies are discussing possible technologies to remove efficiently
and simply the H2S from CO2 vents in amines plant.
Although there are many technologies used for H2S removal from gas streams,
the basic conditions to be satisfied for the implementation in the application
described are restrictive, limiting the possible options. These conditions are as
follows:
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XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
a) Remove H2S converting it into a non-toxic waste
b) Be compatible with the venting gas composition (approximately 100% CO2
saturated with water)
c) Be applicable to very low pressure streams
d) Efficiently absorb variations and picks in the operation conditions.
The analysis of technology study made in at least two amine plants in Argentina
concluded that Sulfatreat, a non-hazardous, non regenerative, solid granular iron
oxide (FexOy) product is the alternative that meets the requirements described
above with the lowest economic impact.
The process of Sulfatreat consists in the chemical reaction between H2S from the
gas stream with the iron oxide; products obtained are iron sulfide (FeS2) (also
known as pyrite) and water vapor. The reaction is as follows:
Fex Oy
+
SH2
Fe2 S +
H2O
The iron sulfide (Fe2S) which is the reaction product is neither toxic nor corrosive
or flammable. It is a non-reversible reaction and the product is a stable compound
which does not decompose, so that H2S is removed from the gas stream in an
irreversible way. As a consequence of this the product is not regenerative and
must be replaced when spent, usually between one and three years, disposing it
of as a solid spend waste. The spent product is non-hazardous, which can be
disposed of in landfills or on roads.
The model of flow within the Sulfatreat bed is "Plug-Flow". The following graph
illustrates a typical scheme of Sulfatreats reactors. The gas enters through the top
of the vessel, creating a reaction zone that is moving toward the bottom as the
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XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
product
is
being
spent,
and
leaves the reactor at the bottom
with a concentration of H2S which
is close to 0 ppmV during most of
the life of the product. This
feature gives the system the
ability to absorb instantaneous
concentration
and/or
flow
variations without special field
work or extraordinary investment
in the operating cost in terms of cost per kilogram of H2S removed.
The system can be composed of one
or more reactors (vessels) parallel or
lead-lag configurations, with simple
pipes and instrument installation as
illustrated in the graph on the left. The
reactors
do
not
require
special
internals beyond that ones that form
the bed support (The support of the
bed, a couple of meshes and a foam
filter). So this makes a robust system
that
does
not
require
special
maintenance.
The sizing and configuration of the system depend crucially on the amount of H2S
removed per day of operation and is affected mainly by two variables of operation:
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XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
a) Water saturation: The gas must be saturated with water in vapor state
b) Temperature: the kinetics of the reaction improved as the temperature is higher
Therefore; with high inlet temperatures are obtained smaller beds
The Sulfatreat reactors associated equipment is also very simple; its preference
against other alternatives is justified
as it is not so complex like other
H2S removal technologies. The
graph on the right illustrates a flow
diagram of a typical Sulfatreat
installation. Water injection is to
ensure the saturation of water
required and adding a heater rising the temperature of inlet gas to the system is an
alternative usually used.
Sulfatreat has developed different types of products based on the same reaction
and with different characteristics for each type of application. For example, there is
a special product with low pressure drop, others with higher or lower amount of
iron oxide per mass unit, others with incorporated catalysts that accelerate the
reaction rate when is required by the process conditions, etc. So the amount of
product required for the removal of H2S in an application it depends on the design
and the product that is chosen; but its generally between 8.5 and 11 kg SFT / kg
H2S.
This feature makes that Sulfatreat technology has a range of applicability which is
placed between liquids technology (which initial installation cost is zero but
operation costs are significantly higher) and other regenerative technologies
whose initial investment is high in contrast with its low cost per kilogram of H2S
removed.
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XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
The
following
picture
these
illustrates
ranges
applicability,
of
with
the exception that
these
limits
empirical
are
and
depends mainly on
market
This
conditions.
segment
of
applicability
has
meant
the
that
technology is widespread for sweetening natural gas, mainly in battery systems
installed in production prior to compression or entering dew point plants as the
reaction takes place in any pressure range.
In 2007 100 % of Argentinean Sulfatreat applications were operating according to
previous description, with a total processing flow in 16 different applications of
approximately 4,000,000 Sm3/d.
However, since 2005, with the Oil & Gas market having a growing tendency on
operational safety and focused attention on plants environmental impact,
industries began to analyze applications for removal of H2S from CO2 amine vent.
An Amine plant located inside an ethane, propane and butane Fractionation
Complex which is owned by an Argentinean company is the first of these
applications. It started years ago after a feasibility study and technology selection;
then was done the integration analysis to the existing amine plant ending with the
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XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
construction of the plant in the year 2007 and started operation in December of
that same year.
The proper settlement of design conditions for H2S removal plant was crucial to
the viability of the Project, mainly because of the wide dispersion on flows and
concentrations of the stream. These two variables display a high variability
between maximum and minimum values, which was a big challenge in defining the
design parameters to take. Originally the design flow for H2S removal system was
determined from 15.000 Sm3/d minimum up to 271.000 Sm3/d maximum, while
the H2S concentration was define from a minimum of 6 ppmv up to a maximum of
90 ppmv. However, when analyzing and reorder historical data of venting flow was
realized that there is a roughly linear function between the values of the flow and
the values of H2S concentration: if flow increases then the H2S concentration
decreases and vice versa. On the other hand pressure and temperature values
are quite stable because are controlled upstream by operating conditions of the
regeneration column of the amine plant.
At this point the technology advantage on the possibility of absorbing high level
flows and/or concentration without affecting the quality of the treated gas during
almost all of beds life time takes an important part. From an extensive analysis of
historical data using this feature, the final design conditions adopted were:
PARÁMETER
Value
Gas Flow (Sm3/día)
230.000
Minimum Inlet Pressure (Kg/cm2g)
0,63
Minimum Inlet Temperature (ºC)
42
Maximum
inlete
H2S
concentration
(ppmV)
33,5
% CO2
98
Water conten (% saturation)
Saturated
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XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
For these conditions, the required installation for the H2S removal plant is: Two
reactors with the following characteristics:
PARAMETER
Value
Number of Reactors
2
Internal Diameter (in)
160
S/S height (ft)
33
Sulfatreat Bed per vessel (lbs)
232.000
Approximate performance (Days)
1000
As an example, if the design conditions were specified in 271.000 Sm3/d @ 90
ppmv as how was initially thought, the minimum installation requirements would
have been the double of reactors with a resulting impact on the initial investment.
The following pictures, taken during Star-Up, show the two Sulfatreat reactors and
confirm the simplicity of the required installation for these type of applications.
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XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
The Star-Up of the application was performed in late December 2007. During the
first year operation the results were consistent with those predicted during
engineering stage of the Project. H2S levels were maintained without interruption
under the detectable level of the measurement instrument (under 0.2 ppmv)
During the second year operation a problem related to excessive Pressure Drop
appeared. Those problems were attributed to excessive water condensation
through the bed; forcing to insulate the vessels in order to minimize this effect.
Also channelizing appeared in the bed as a secondary effect from the
condensation that’s why the gas by-passed part of the bed generating the
mentioned pressure drop increase.
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XIX International Gas Convention AVPG 2010, May 24th - 26th Caracas, Venezuela
To correct this effect, during a new loading of the product, extreme precautions
were taken in order to generate homogeneous and evenly loading of the product
into the reactors (Following a rigorous loading procedure). This, in conjunction with
the insulation, corrected the effect of channelizing.
CONCLUSION
Sulfatreat, usually associated with sweetening in primary treatment facilities of gas
streams in the upstream segment, is a technology that can be effectively used to
remove H2S from CO2 vent stream coming from amine plant.
The plant application analyzed in this paper it’s a clear evidence of that
conclusion. The operating parameters obtained after starting operation confirm the
calculations made during conceptualization and basic engineering stages.
SulfaTreat has efficiently removed H2S coming from the regenerating column,
handling high levels of flow and concentration, reducing significantly the risks for
operators and residents of neighboring cities to the possible exposure to high
levels of Sulphidric Acid (H2S) in the air.
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