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,

2/16

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

3/16

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.

5/16

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

7/16

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

References

WIChert, E. and T. Royan, .Sulphur Disposal By Acid Gas Injection-, SPE 35585, Gas

Technology Conference, Calgary, May 1996.

2. Keushnig, H., -Hydrogen Sulphide - If You Don't Like It, Put It Back", Journal of Canadian

Petroleum Technology, June 1995, 34,6.18-20.

3. Longworth, H.L., G.C. Dunn and M. Semchuck, .Underground Disposal of Acid Gas in

Alberta, Canada: Regulatory Concerns and Case Histories", SPE 35584, Gas Technology

Conference, Calgary, May 1996.

4. Carroll, J. and D. Lui, "Phase Equilibrium and Physical Property Considerations for Acid Gas

Injection Systems", Canadian Gas Processors Association, Calgary, November 1996.

5. Song, K. and R. Kobayashi, "Water Content of CO2 in Equilibrium with Liquid Water and/or

Hydrates", SPE Formation Evaluation, December 1987, 500-508.

6. Selleck, F.T., L.T. Carmichael and B.H. Sage, .Phase Behavior in the Hydrogen Sulfide-

Water System., Ind. Eng. Chern., September, 1952,44,9,2219-2226

7. Carroll, J.J. and A.E. Mather, .Phase Equilibrium in the System Water-Hydrogen Sulfide:

Experimental Determination of the LLV Locus., Canadian Journal of Chern. Eng., June, 1989,

67,468-470.

8. Ng, Ho, D. Robinson and A. Leu, .Critical Phenomena in a Mixture of Methane, Carbon

Dioxide and Hydrogen Sulfide-, Fluid Phase Equilibria, 1985, 19,273-286.

9. Huang, S., A.-D. Leu, H.-J. Ng and D.B. Robinson, "The Phase Behavior of Two Mixtures of

Methane, Carbon Dioxide, Hydrogen Sulfide, and Water", Fluid Phase Equilibria, 1985, 19,

21-32.

10. Song, K.Y., and R. Kobayashi, -The Water Content of CO2-rich Fluids in Equilibrium with

Liquid Water or Hydrate," Research Report RR-80, GPA, Tulsa, 1984.

11. Bloch, M.G. and P.P. Lifland, "Catalytic Reforming Improved by Moisture Metering", Chern.

Eng. Prog. 69(9), 1973, 49-52