acid gas water content and physical properties: previously
TRANSCRIPT
Acid Gas Water Content and Physical Properties: Previously Unavailable
Experimental Data for the Design of Cost Effective Acid Gas Disposal
Facilities, an Emission Free Alternative to Sulfur Recovery Plants
By
M.A. Clark, W. Y. Svrcek. W.O. Monnery: University of Calgary
A.K.M. Jamaluddin: Hycal Energy Research Laboratories
E. Wichert: Sogapro Engineering Ltd.
Abstract
The economics of recovering sulfur from sour natural gas have become unfavorable for small
fields. Hydrocarbon producing companies require a cost effective yet environmentally sound
alternative to deal with acid gas. Compressed acid gas re-injection into producing, depleted or
non-producing formations has emerged as a viable alternative to traditional sulfur recovery. Most
injection schemes include dehydration facilities to remove the saturated water from the gas,
preventing corrosion and hydrate formation. An alternative, less costly approach is proposed:
keep the water in the acid gas phase throughout the injection circuit, eliminating the need to
dehydrate.
To design an optimized injection strategy, determination of thermodynamic and physical
properties such as water content, dewpoint, bubble point, hydrate conditions and density of the
acid gas is necessary. A comprehensive joint industry/research project is underway to obtain
data for several different acid gas compositions over a range of operating temperatures and
pressures. An online, repeatable and precise method of water content analysis using gas
chromatography has been designed and tested. Pure CO2 data has been generated from the
equipment and matches accepted values with an average standard deviation of 5 %. To date the
phase behavior of an acid gas mixture containing 89.5% CO2, 9.9% H2S and 0.6% CH. has been
studied and compared to equation of state predictions. Other mixture data is being generated
and should be completed in six months. Until this or other experimental data becomes available,
operating companies will be forced to rely on equation of state predictions and over-design
accordingly .
1/16
Introduction
The unfavorable economics of constructing small Claus sulfur recovery plants, due to their
inherent operating difficulties when the H2S feed concentration is low and the decreased demand
and oversupply of elemental sulfur, necessitate an altemative technology. Air emission standards
and regulations are increasingly stringent, compounding the need for an environmentally-friendly,
cost-effective method to deal with acid gas. Acid gas compression and re-injection into depleted
reservoirs or disposal zones, similar to produced water disposal, is a viable alternative to
traditional sulfur recovery processes with the added advantages of reducing greenhouse gasemissions and providing pressure support for producing reservoirs. 1. 2, 3
Acid gas mixtures, separated from hydrocarbons in a sweetening plant, are compressed through
several stages, dehydrated and pumped into a reservoir. Phase behavior, water content and
physical properties of the acid gas are required for facility design. Phase behavior data is used to
avoid the two-phase region during multi-stage compression to ensure equipment is not damaged
by the appearance of a liquid phase. The injection pressure required is dependent on several
factors including the formation pressure and the density of the acid gas mixture.1.4 The
hydrostatic head of the acid gas column in the well bore aids injection, particularly if the fluid is
injected above its critical point as a dense phase. Water content data determines the need for
and aids in the design of dehydration facilities.
Although experimental data is available for pure hydrogen sulfide and pure carbon dioxide, littlework has been done with acid gas mixtures5, 6, 7, 8, 9. Equation of state predictions can be used for
mixtures, but their applicability for these polar/non-polar mixtures has not been proven and liquid
density calculations are known to be unreliable. An acid gas mixture study is underway to fill the
information gap, providing the required thermodynamic and physical properties over a range of
temperatures and pressures of a series of acid gas mixtures. This pertinent, practical project is a
joint effort between industry and research institutions.
Background
Hydrogen sulphide and carbon dioxide are removed from sour gas by absorption with a
regenerative solvent in an amine plant. The acid gas mixture of H2S, CO2 and a small amount of
light hydrocarbons leaves the sweetening unit saturated with water at the amine still conditions of
low pressure and high temperature, represented by point A in Figure 1. The gas mixture is then
compressed from points A to F in 3 or 4 stages. After each stage, the gas mixture is cooled,
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without entering the two-phase region. Condensed water is removed after each stage. After the
last stage, the mixture travels down a pipeline into the disposal well. Ideally, at the final
compressor discharge pressure the mixture will be supercritical. Further cooling in the pipeline
will increase the density without a phase change, increasing the hydrostatic head of fluid in the
well and reducing the required injection pressure. The operator must ensure that the mixture
does not cool below its water saturation temperature, to avoid corrosion and hydrate plugging of
the pipeline and wellbore.
Corrosion and hydrates may occur when the gas is saturated with water. Due to the safetyhazard associated with acid gas equipment failure, most injection schemes currently include
dehydration facilities to ensure the acid gas is undersaturated throughout the system.
Unfortunately. dehydration facilities and stainless steel comprise a major portion of the capitalcost of re-injection facilities. Methanol injection is an option to combat corrosion and hydrate
formation, but can significantly increase operating expenses. When the gas is not in contact with
a water phase and is undersaturated, hydrates cannot form until the temperature drops
sufficiently that the gas can no longer hold all the water in solution and "free" water is available for
the formation of hydrates. If the mixture is below its saturated hydrate formation curve, hydrates
form preferentially to a liquid water phase once free water is available.
Although there is little experimental data on acid gas mixtures, the solubility of water in pure
hydrogen sulfide and pure carbon dioxide lead to some interesting hypotheses. The ability of the
pure compounds to hold water in the vapor phase decreases as the pressure increases up to
about 3000 kPa (400 psi) for H~ and 6000 kPa (900 psi) for CO2. At higher pressures the water
holding capacity of the gases increases, corresponding to a higher water absorption capacity in
the liquid phase or dense phase compared to the vapor phase. In both cases, increasing the
temperature allows more water to be absorbed in the gas phase. Small amounts of methane
substantially reduce the water absorption ability of both components.
If it is assumed that solubility of water in the gas mixtures mimics the trend of the individualcomponents, then a minimum water holding capacity exists at some pressure. 1 Figure 2
demonstrates how this information could eliminate the need for dehydration facilities. Points A
through G in Figures 1 and 2 correspond to the same stages in a re-injection facility. Over each
compression stage, the pressure and temperature increase and after each compression stage the
gas is cooled. Initially the water holding capability of the gas decreases from stage to stage, until
the minimum water holding capacity is reached. If the condensed water is removed at this point,
the gas will be undersaturated with water throughout the rest of compression. Stainless steel will
not be required in the compressors or coolers after point E. If the temperature of the compressed
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gas does not drop to the new water saturation temperature, point G, in the pipeline or wellbore,
dehydration can be eliminated and stainless steel materials and methanol injection will not be
necessary .
!~II)II)G)...
no
Temperature
c c,- 0
>-';:'511),- Q).Q~=a.
OEU)o'aU
CC)IV C-.-C ...Q) =-0C II)0 IV
U<.?~
Q)'C- .-IV U
~ct Pressure
If it is concluded that dehydration cannot be completely eliminated due to a particular set of
conditions, for example in an extremely cold climate, the experimental data will still be beneficial.
The operator will know the inlet water content of point A and the conditions of the lowest water
solubility of the system. The glycol contactor tower, regenerator and circulation system can then
be designed appropriately.
Currently, estimates of the density, water content and phase behavior of acid gas mixtures are
being predicted with equation of state models, as experimental data is unavailable. The
equations can produce considerable error and result in over or underdesigned facilities.
Experimental data results in proper decisions on the equipment and materials required for each
particular set of conditions, which can lead to considerable cost-savings.
Apparatus and Experimental Procedure
The apparatus consists of a temperature controlled air bath, a high-pressure cell with a sight
glass, a positive displacement pump. a Hewlett Packard 6890 gas chromatograph (GC), and an
Anton Parr density meter. The sight cell has an internal volume of approximately 80 cm3 (5 in3)
and a maximum working pressure of 70 MPa at 150°C (10000 psi at 300°F). The 2.5 cm (1 in)
thick sight glass located on the front and back of the cell allows visual observation of the cell
contents throughout experimentation. As shown in Figure 3. the high-pressure sight cell is
mounted in the center of the oven as are the cylinders containing gas mixtures and distilled water.
The density measuring cell is mounted at the bottom of the oven and wired to the density meter
outside the oven.
A resistance detection thermometer is located inside the oven next to the sight cell and is wired to
a digital temperature controller. The temperature controller switches the three oven heaters on or
off depending on the difference between the set and detected temperatures. Two fans in the
oven circulate the air, preventing temperature gradients within the oven. The oven can be heated
up to 150°C (300°F) and the temperature is measured and controlled to within :t 0.1 °C (O.2°F).
The temperature of the sight cell contents is measured to within :t O.1°C with a second resistance
detection thermometer installed in a port on the side of the sight cell.
The sight cell volume and pressure are controlled by the addition and withdrawal of mercury
through a port at the bottom of the sight cell. Sample fluids are pumped in and out of a port at the
top of the sight cell. The pump measures volume displacement with a precision of :f: 0.02 cm3(0.001 in1. A dead-weight calibrated digital pressure gauge is connected to the pump outlet.
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The oven is connected to a motor via a steel arm. The motor rotates the arm, rocking the oven
and its contents in a 1800 arc. The mercury in the sight cell agitates the fluids and enhances
mixing when the oven and cell are rocked.
A heated, insulated line runs between the cell and the GC. A metering valve is used to control
the flow and delivery pressure to the GC. A thermocouple downstream of the metering valve
verifies that the temperature of the depressurized gas is sufficiently high to prevent water
condensation.
Figure 3 - Experimental Apparatus for Acid Gas S~dy
6/16
The apparatus is contained in a sour gas laboratory. The lab is continuously flushed with fresh
air pumped in from the ceiling and drawn out of vents located in the bottom four comers of the
room to an incinerator stack. A permanent H~ monitor is located close to the floor, below the
apparatus. If the monitor detects 10 ppm HzS, an alarm sounds outside the sour gas lab and the
incinerator fires up. At 20 ppm the main lights in the lab shut off and emergency lights flash.
Personnel worKing in the lab with supplied air breathing apparatus (SABA) may not hear the
alarm and the flashing lights ensure their evacuation. The lab is equipped with a video camera
for continuous remote monitoring of personnel performing dangerous worK. An -HzS Panic
Button- which summons emergency rescue and medical services is located immediately outside
the sour gas lab.
A gas mixture is synthesized from pure components in the laboratory using partial pressures.
Concentrations are verified using the GC. The system is purged and gas is transferred to the
sight cell. The gas in the cell is saturated with water at the desired temperature and pressure and
allowed to equilibrate while rocking. At equilibrium, usually reached within a few hours, a stable
free water phase should be visible in the cell, ensuring the gas is fully saturated. Equilibrium is
verified by consistent GC measurements. Gas samples are then transferred to the density
measuring cell and the GC for analysis, while maintaining constant cell pressure and
temperature.
The density measuring cell contains a U-shaped tube, electromagnetically excited by the density
meter and vibrating at its natural frequency. The addition of sample fluid to the tube causes a
frequency change. The density is calculated from the change in frequency using two calibration
constants and is read directly from the meter in g/cm3. The density meter has been calibrated at
different temperatures and pressures with methane and pentane. Tests with carbon dioxide have
revealed an accuracy of:t 0.5 % of the density reading compared to accepted literature values.
Water Content Analysis
The accurate measurement of the small amount of water (as little as 100 ppm) that is soluble in
the acid gas phases is crucial. Literature on the subject of water content of acid gases and
analytical methods used to determine moisture content was researched and reviewed. In 1984
and 1987. Kobayashi and SongS.1O measured the water content of carbon dioxide and a carbon
dioxide/methane mixture using a "tedious modified chromatographic technique" developed by
Block and Lifland11 , The technique uses a glycerol packed absorption column to withdraw water
from a measured quantity of gas. The amount of water withdrawn is determined in a calibrated
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split column gas chromatograph system. Kobayashi and Song reported an accuracy of 1: 5-6%
based on calibration results.
In 1985, Huang et al.9 analyzed two mixtures of H~, CO2, CH4 and water by direct
chromatography. The GC was equipped with a therrno-conductivity detector (TCD) and a 3.2 mm
OD by 1 m long Porapak as column and the experiments were run between 30°C and 60°C.
Calibrations were performed with pure carbon dioxide. The reported repeatability of .t; 0.2 mole
percent is reasonable at high temperatures, where the water content is greater than 2 mole
percent, ten times the repeatability. At other conditions, the water content measured less than
0.2 mole %, the repeatability of the analyses. Carroll et. al.7 reported in 1989 that water
measurements in H2S performed by iodometric titration were accurate to within 0.1 mole
percentage for amounts above 1 mole % water, but were inaccurate for measurements below 1
mole % water.
None of these procedures determine water content with the precision, accuracy. repeatability and
simplicity necessary for our project. Consultation and experimentation with Hewlett Packard
resulted in the use of direct chromatography, as in Huang's method, with a TCD. A Plot-Q
capillary column, 30 m long by 0.53 mm diameter with a 40Jlm film thickness of
Divinylbenzene/styrene porous polymer is being used. Hydrogen is used as the carrier gas for
improved resolution.
Water content analysis is performed by withdrawing a sample of gas through a metering valve to
the GC sampling loop. The line, valve and loop are heated to prevent condensation. Once the
line and the sampling loop are adequately purged with sample, the GC run is started and the
switching valve sends the carrier gas through the sampling loop. The components are separated
in the column and travel to the TCD. The TCD contains a filament, which is electrically heated to
a constant temperature. The carTier and sample gases flow intermittently over the filament The
difference in power required to maintain the filament temperature with the sample gas compared
to the carTier gas is measured. The resultant peaks, with a height measured in microvolts (~V),
are integrated over time. seconds. to give area counts measured in ~V's.
To convert area counts to mole fractions, a response factor is required for each componentanalyzed. The response factor is determined by running experiments with known concentrations
of each of the components. The percent area count of a component divided by its response
factor equates to the mole percent of that component present in the sample. Determining the
response factors for the components to be tested is commonly referred to as calibrating the GC.
8/16
Between runs the line, valve and loop are purged with heated nitrogen until no water is detected
in the nitrogen stream.
Many tests were run without success until modifications to the gas analysis were perfected. The
GC column was changed from a methyl silicone coated column to the Plot-Q, thus allowing the
runs to proceed at higher temperatures and preventing water freeze out in the column. The heat
tracing and insulation of the line between the sight cell and the GC was increased. Calculations
indicate that for the most severe conditions studied, the gas must be heated to above 200°C to
ensure that water does not condense over the metering valve. The metering valve replaced a
pressure regulator due to carryover between runs, despite lengthy nitrogen purges, attributed to a
large, unswept dead volume in the regulator.
Water Content Experimental Results
While testing the apparatus, the water content of a gas mixture of carbon dioxide and 5.06%
methane with traces of ethane, nitrogen, propane and butane was determined at 6200 kPa (900
psia) and 27°C (80.6°F). Water area counts of 125.0, 119.6, and 126.6 ~V's were obtained over
three successive runs. The measured data has a standard deviation between runs of less than
3% and average water content of 0.135 mole %, proving the repeatability of the equipment at low
water concentrations. The data is plotted vs. published data for a carbon dioxide and 5.31 %
methane gas mixture in figures 4 and 5. Considering the compositional difference between the
two gases, the measured water contents are very dose.
9/16
A series of experiments were run with pure carbon dioxide to verify the accuracy of results over a
range of temperatures and pressures. There is ample experimental data available for pure
10/16
carbon dioxide in the literature. As shown in figures 4 and 5, the application of response factors
to the raw data resulted in a close match to published data. Figure 4 shows the data plotted in 51
units while figure 5 shows the data plotted on a logarithmic scale with field units for ease of
comparison with plots found in the GP5A Engineering Data Book.
The water content of carbon dioxide at 31°C (87.8°F) was measured at three different pressures.
At 2500 kPa (370 psia) and 7200 kPa (1040 psia) the match was within 3% of published data and
the standard deviation between runs at the same conditions was less than 4%. At 5100 kPa (735
psia) some difficulty was encountered obtaining repeatable data and only the last data point of
five experiments is plotted. The match is within 10%. At 50°C, water content was measured at
two different pressures. The match at 10100 kPa (1470 psia) was exact and the repeatability was
within 1%. Difficulties were again encountered at 7600 kPa (1100 psia), the last three of fIVe runs
is plotted, and the match is within 9%.
The poorer matches of 10% and 9% are suspected to be the result of insufficient pressure control
during the experiments, affecting the repeatability and accuracy of the results. Since very good
repeatability is obtained when the experiment is run correctly t any fluctuations in analyzed water
content of greater than 5% alert the operator to inaccurately obtained samples and the
experiments should be repeated. With proper pressure control results with less than 5% error
can be obtained.
Phase Behavior Experimental Results
Several isothemls were obtained to establish the phase envelope of a mixture of 89.5% CO2,
9.9% H2S and 0.6% CH.. The cell temperature was set and allowed several hours to equilibrate.
The cell mercury volume was increased incrementally, resulting in 15-30 psi pressure steps.
Transient and stabilized phase behavior was observed and recorded. The change in mercury
volume was recorded as a function of pressure. At pressures close to the dew and bubble points,
the volume/pressure increment was reduced. By taking a series of data points immediately
above and below the appearance and disappearance of the two-phase region, the dew and
bubble points were established.
As recorded in Table 1, dew points were observed at 7°C/4100 kPa, 9°C/4342 kPa, 15°C/5045
kPa, 26°C/6355 kPa, 34°C/7479 kPa and 37°C/7955 kPa. Bubble points were observed at
7°C/4528 kPa, 9°C/4755 kPa, 16°C/5510 kPa, 27°C/6900 kPa and 34°C/7844 kPa. Volume of
mercury in the cell is plotted in Figure 6 vs. pressure for each isotherm. As seen in figure 6, two
11/16
The slope changes correspond to the dewdistinct slope changes occurred for each isotherm.
and bubble points, verifying the visual observations.
Figure 6 - Pressure vs Volume Data
9.9% H2S, 89.5% CO2, 0.6% C1
1400
1200.-CI
'iDo-e~f)f)e
Q.
1000
800
800
400
Cell Volume (cc)
12/16
Above 37.5°C (99.5°F), a stable two-phase region was not observed. Some droplets, and
elongated bubbles appeared during a volume/pressure change and while the system wasstabilizing, but upon reaching equilibrium, the system was single phase at all pressures.
At 37.5°C (99.5°F) and 8253 kPa (1197 psia) the critical point was observed. At all other
temperatures the contents of the cell were clear and colorless in the vapor, liquid and two-phase
regions. In the critical region, a small change in pressure (3-5 psi) resulted in the entire cell
contents becoming a murky, grey cloud and then stabilizing out into a variety of shades of yellow.
Above 8303 kPa (1205 psia) the contents were single phase, clear and colorless. At about 8274
kPa (1200 psia), the see-through single phase took on a slightly yellow tint. At the critical point of
8253 kPa (1197 psia), two phases appeared with an indistinct thick yellow interface, a darker
yellow color at the bottom of the cell and a lighter yellow color on top. At 8212 kPa (1191 psia)
the bottom half of the cell was a distinct dark orange liquid and the top half a colorless vapor. At
8198 kPa (1189 psia) the liquid phase faded to yellow and below 7957 kPa (1154 psia) the cell
contents were again a colorless single phase.
In Figure 7, the calculated equation of state phase envelope is plotted along with the
experimental data. The widths of the two phase envelopes are similar, but the calculated
envelope falls below and to the left of the experimental data. The calculated critical point occurs
at 34.9°C and 7633 kPa (94.8°F, 1107 psia), 2.6°C and 620 kPa below the experimentally
determined critical point of 37.5°C and 8253 kPa (99.5°F, 1197 psia). The deviations between
actual and calculated phase behavior emphasize the importance of obtaining an experimental
data set for acid gas mixtures. The equation of state was regressed to fit the phase behaviour
data obtained experimentally. The regressed curves and critical point match the measured data
within the experimental error. The modified equation of state allows some extrapolation to
different conditions, but experimental verification will be necessary until more data becomes
available and a general regression is completed.
13/16
Figure 7 - Pressure vs Temperature Phase EnvelopeExperimental vs Equation of State
{!~I
.. 5.0 10.0 15.0 200
Temperatu,. (C)
ao 30.0 35.0 40.0
The two-phase region can be avoided during compression by cooling the gas to a minimum of
37.5°C between compression stages. The fluid is in the supercritical, dense phase above 8253
kPa and above 37.5°C.
Conclusions & Future Plans
Acid gas re-injection may be the optimum solution for producing small sour gas fields. As the
issue of greenhouse gases heats up, re-injection will become viable for even moderate to large
operations. The operating company must avoid the two-phase region during compression.
Water condensation and hydrate formation in the post-compression equipment must be
prevented to ensure safe, cost-effective operation. Experimental data on the water content,
density. and phase behavior of acid gas mixtures is therefore necessary.
A comprehensive joint interest project is underway to obtain data for several different acid gas
compositions over a range of operating temperatures and pressures. An online, repeatable and
precise method of water content analysis using gas chromatography has been designed and
tested. The reproduction of carbon dioxide data has proven the reliability of the method.
14/16
Experimental phase behavior of a single mixture has shown that actual behavior does deviate
from equation of state predictions. Additional mixture data is being generated and should be
completed in six months. Until this or other experimental data becomes available, operating
companies will be forced to rely on equation of state predictions and over-design accordingly.
Acknowledgements
The authors would like to acknowledge the support of staff at both Hycal Energy Research
Laboratories Ltd. and the University of Calgary.
15/16
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