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T he B est P ractices In Moisture Control of Sour Natural Gas as ExportGas: Case Study on Line Water Content Analyzer Assessment in a GasPlantGholamreza Bahmannia, SPGC

Copyright 2011, Society of Petroleum Engineers

This paper was prepared for presentation at the SPE Asia Pacific Oil and Gas Conference and Exhibition held in Jakarta, Indonesia, 20–22 September 2011.

This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not beenreviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, itsofficers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission toreproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abst rac t

The dry sour gas is 4th gas plant's main product for transfer and injection into Aghajari Oil Field as EOR projects.Moisture control and monitoring in sour gas is a critical parameter because of severe corrosion problems that are associatedwith sour gas. For assessment of Michell online process moisture analyzers, in comparison to laboratory Michell’s ceramicmoisture sensor and portable monitor (ASTM D-5454) and Chilled mirror device as manual dew point tester (ASTM D-1142),a periodical test has been done. The results showed acceptable accuracy range of online analyzers and the calibration periodconfirmed. In this paper all recorded data has analyzed by statistical methods .

1. Introduction

Based on ISO 14532:2005 (Natural gas – Vocabulary) defines water dew point as “The temperature above which nodetectable condensation of hydrocarbons occurs at a specified pressure”. [11] The application of moisture analysers for themeasurement of natural gas is not simple and straightforward. There are many aspects to be considered that are unique tonatural gas and that can greatly affect the reliability of both the instrument in service and of the measurement data that it

provides.The 4 th gas plant (Phases 6-7-8) as member of the South Pars gas treating complex in south of Iran, north of Persian Gulf is

the largest plant in the world which has designed to treating and producing the dry sour natural gas for transfer and injectioninto Aghajari Oil Field in EOR projects.(Fig.1)

Figure 1: The 4th gas treating plant facilities overall view

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Three 32” sub-sea pipelines transport well stream fluids produced at each wellhead platform, 105 km to the onshore treating plant to produce the following products:

Table 1: The 4th Gas Plant’s main products

Product Production Rate

Dry Sour Gas 93.5 [MMSCMD]

Stabilized Condensate 159,000 [BBL/D]

LPG 4,452 [TON/D]

Rich Ethane Gas 9.5 [MMSCMD]

Moisture control of export gas is a key and critical quality parameter in 4th gas plant and its efficiency is important in orderto meet pipeline needs and to prevent severe corrosion problems that are associated with sour gas if any droplet exist, to ensurethat the gas is safe to transport and has no risks.[6,2]Acuurately monitoring water dew point in sale gas is therefore vital if theintegrity and quality of the gas are to be maintained, and pipeline design conditions complied with. In 4 th gas treating plant,same other typical processing plants, after extraction from the gas field, natural gas is processed to remove water and heavyhydrocarbon components, before transmission through the pipeline network. Figure 2 shows process units of this plant for theremoval of both excess moisture and heavy hydrocarbons. Dehydration process performs by molecular sieve beds andhydrocarbon dew point control process is based on turbo-expander system. The composition of inlet gas shown in Table 2 andit is clearly indicates presence of significant amount of acid gases, more than 4000 ppm H2S and about 2 mol percent carbondioxide content into saturated sour gas of South Pars field.

Figure 2 : Process flow diagram of 4th gas treating plant

The dry sour gas pipeline design specifications and permissible moisture content was defined by gas nationaltransmission management as below that is vital quality control measures:

• Water dew point @44barg : 0 degree C• H/C dew point @55barg : 0 degree C

For control and monitoring of water dew point in 4 th gas plant, three kind of measurement devices are applicable now;Ametek online process moisture analyzers, which are mounted in dehydration unit outlet streams, Michell’s ceramic moisturesensor and portable monitor (ASTM D-5454) and Chilled mirror device as manual dew point tester (ASTM D-1142) which arelaboratory test. The question of paper is how we can sure that on-line analyzer measuring data is reliable through itscalibration periods?

2. Material and Methods

In order to demonstrate the on-line analyzer efficiency in 4 th gas plant dehydration unit, following steps defined andconducted by competent staff.Full composition test of dehydration feed was analyzed by gas chromatograph NGA-GC TypeC, VARIAN CP-3800. This test was done through weekly through six months and used for obtain natural gascharacteristics.As we know phase envelope with hydrate and water dew point curves is an excellent tool to find out whatform/phase water is in at operating conditions, during start-up, during shut-down and during upsets. The water content of a gasdepends on the system temperature, pressure and composition of the water containing gas. In this research, the curves was

plotted by ASPEN-HYSYS software. The 4th gas treating plant enjoys an on-line moisture analyzer on the outlet stream ofeach dehydration unit for indication and control of dew point accurately, as process unit efficiency. In addition in order to

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evaluate this online analyzer, two other quality control devices were chosen as following table characterizations:

Table 3: Dew point measurement equipmentsEquipment Michell on-

line analyzerMichelllab test

Chilled mirror

Model CERMET II I.S CERMAXI.S

LT/DP-173000/M

Range To -100 C To -100 C To -100 CProb. Al2O3 Capac. Al2O3 MirrorInterval 30 Sec 15 min DailyType Electrical Electrical ManualCalibration 6 months 6 months -

The Cermet II hygrometer is a continuous, on-line instrument for the measurement of absolute moisture content in gas. It isdesigned to operate under a wide range of conditions and provide for the monitoring and/or control of moisture in gases. Theinstrument consists of two component parts: monitor and sensor, each of which is individually calibrated to a single standardallowing combinations of sensor and monitor units to be totally interchangeable. The standard instrument covers the ranges -100° C to +20° C dew point (-148° F to +68° F dew point), g/m3, 0-9999 ppmv as well as 0-1000 lb/mmscf for natural gas.[7]The CERMAX is an advanced portable instrument for the measurement of absolute moisture content of air or other gases. It

is designed to fulfill a wide range of applications by direct read out of moisture content displayed on the LCD display. Theinstrument offers several customer selectable display options based on a calibrated dew point measurement range of -100 to

+20°C.[8] The LT/DP-173000/M is a kind of Chilled mirror devices. The original purpose of the Bureau of Mines chilledmirror device was to measure the water vapor dew point of the stream, so that its water vapor content could be determinedusing a published correlation. The device has since been adapted to hydrocarbon dew point measurement as well, and therecommendations of supplier apply to both applications. Commonly known as the Bureau of Mines Tester, the chilled mirrorhygrometer is a very simple and basic moisture measurement device that has been used for many years as a primary, absolutemoisture measurement tool.[1] In operation, the sample gas stream flows across a temperature controlled polished surface or“mirror”. As the temperature of the mirror is slowly lowered, the water vapor will begin to condense or form dew on themirror. The temperature at which the dew first appears is considered the dew point. Once the temperature Dew point isattained, a simple conversion using Existing charts or tables will provide the actual water Vapor content of the gas.[12]

Figure 4: Chandler dew point Tester,( Chilled mirror)

For logging and analysis of on-line analyzer efficiency, all required data (temperature, pressure, dew point... of gas stream)was collected by process PIMS and laboratory competent personals through 6 months.

3. Results

3.1 Sour wet gas composition analysis

The average feed stream analysis is tabulated in Table 2 and Methan as majority gas component form the major part ofmolecular weight.

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Table 2: Composition of inlet gas to plant

3.2 Phase envelope curves of feed gasFigure 3 demonstrates the phase envelope curves of South Pars Gas field sour gas mixture which enter to process

units.

Figure 3: Phase envelope curves of South Pars Gas field

As shown in this figure, the presence of acid gases shifts all of the curves to the right. In other words, the presence ofacid gases increases the hydrate formation temperature considerably. It also increases the water dew point temperature. Itshould be noted that the water dew point curves have been generated for a fixed amount of water content predicted at specifiedseparator condition.

3.3 Dew point data loggingIn order to compare on line analyzer efficiency, all data was collected through six months, tabulated and plotted as

Fig. 4 during the 6 months calibration periods. Dew point measurement by chilled mirror device in maximum range (-35C)was a confidence base for operators who would see as primary indication of normal dehydration efficiency.

Figure 4 : Trend of three dew point devices

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It is clear that one disadvantage of the Bureau of Mines chilled mirror dew point tester is its subjectivity. Dew pointtemperature measurements depend on the operator’s skill at controlling the refrigerant flow, and on the operator’s ability toidentify and interpret the condensation seen on the mirror. Improved measurement accuracy can be obtained by checking andcalibrating pressure and temperature instruments used with the dew point tester on a regular basis. However, certain sources of

bias uncertainty cannot be eliminated by calibration, and should be considered when assessing measurement accuracy.Table 4shows two remain dew point measurement devices gathered comparable data during 180 days. Because of some processrequirement in maintenance jobs, there were some changes in data which are normal from operator point of view.

Table 4: Six months data collection trend(degree °C)On-Line

AnalyzerPortable Michell

Chilled Mirror

Deviation

-100.1 -98.9 -35 -1.1

-100.1 -100 -35 -0.1

-100.1 -98.7 -35 -1.4

-100.1 -99.2 -35 +1.9

-100.1 -97.7 -35 -2.4

-100 -99.2 -35 -0.8

-97.1 -96.2 -35 -0.9

-96.3 -95 -35 -1.3-63.2 -61.2 -35 -2.0

-54.7 -55 -35 +0.3

-77.7 -78.1 -35 +0.4

-84.2 -85 -35 +0.8

-89.3 -87 -35 -2.3

-93.1 -90.2 -35 -2.9

-99.7 -99 -35 -0.7

-100.0 -98.9 -35 -1.1

Parameter On line analyzer Portable AnalyzerSt. Deviation 14.23141 13.99328

Variance 202.53317 195.81196

Considering the 4th gas plant process condition changes and dewpoint deviations between on line analyzer and portable lab tester, standarddeviation of dew point in on line device and portable one were 14.23 and 13.99. The calculated F-ratio (F=S online

2/S portable2) be obtained as

1.034, which is much greater than critical F-ratio. This high difference shown that those itemst variances have statistically highly significantdifference with 99.6% significance level.

The student-t statistics for comparison of two equipment dew point measuremnet is calculated as -98.3. This value is also much greater thancritical student-t value, indicating that the average have highly differences with 99.7% significance level.Hence, the data and also F-ratio andstudent-t statistics show that considerable close data has been obtained. In other word, the dew point accuracy for on line analyzerconfirmed as below by pracical measurements through six month monitoring :

±2.0° C between –60° C & -100° C±1.0° C between +20° C & -60° C

This results changed as below for dew point measurment through one year re-calibration period:

±3.2° C between –60° C & -100° C±2.3° C between +20° C & -60° C

The first results are very close to on line dew point analyzer’s manufacturer reported performance for one year re-calibration period. TheChilled mirror device was applied as measurement basis up to -75°C in some cases.

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Figure 5 : Phase envelope curves of dry sour gas

For adapting of some of analysis based on composition of dry sour gas, the new phase envelope curve was plotted after GC test. Figure 5shows that expected water dew point will occur after dehydration due to its physical properties.

4. Conclusion

The application of a moisture analyser for sour gas measurement requires a sensing technology that can performsatisfactorily in practice.[4,13] This paper demonstrates that on line dew point analyzer through six months re-calibration

period will act in the lowest error range in comparison with other calibrated and trusted devices.In other hand Cermet II as online hygrometer has acceptable accuracy in practice. Statistical information indicates that the vast majority of failures arecaused either by incorrect sampling methods, sampling positions or inadequate protection against harmful substances. Thefrequency of re-calibrations required in order to maintain the performance of the Cermet II hygrometer is primarily dependenton the composition of the gas to which the sensor is exposed: i.e. content of liquid and particulate contaminants, corrosiveelements, etc. In 4 th gas treating plant application, half annual re-calibration ensures that the stated accuracy of the Cermet IIhygrometer is maintained. It seems that the new TDLAS(tuneable diode laser absorption spectroscopy) type hygrometer as anew dew point measurement device enables natural gas processing and transportation facilities to monitor moisture content inreal time with high precision and reliability It is the first technique to monitor the gas itself, rather than monitoring a reactionor a change on the surface of a sensor . It combines a high degree of accuracy with an extremely good speed of response.

5. References

1. Deaton, W.M., Frost E.M., Jr., “Bureau of Mines Apparatus for Determining the Dew Point of Gases Under Pressure”. Bureau of Mines Report ofInvestigation 3399, May 1938.

2. Bukacek, R.F., “Equilibrium Moisture Content of Natural Gases”, Research Bulletin 8. Institute of Gas Technology, Chicago, USA, 1955.

3. ASTM designation D 1142-95, “Standard Test Method for Water Vapor Content of Gaseous Fuels by Measurement of Dew-Point Temperature”.American Society for Testing and Materials, Philadelphia, USA, 1995.4. Oellrich, L.R., Althaus, K., “Relationship Between Water Content and Water Dew Point Keeping in Consideration the Gas Composition in theField of Natural Gas”. GERG Technical Monograph TM 14. Fortschritt-Berichte VDI, Nr 679, 2002.5. Robinson, J.N., Wiekert E., Moore R.G., Heidemann R.A., “Charts help estimate H 2O content of sour gases”. The Oil and Gas Journal (USA), February 6,1978, pages 77-78.

6. NACE Standard MR0175-97, “Standard Material Requirements: Sulfide Stress Cracking Resistant Metallic Materials for Oilfield Equipment”. NationalAssociation for Corrosion Engineers, Houston, USA, 1997.7. CERMET II Hygrometer Installation, Operation and Maintenance Manual,2009,pages 4-68. CERMAX Portable Hygrometer Installation, Operation and Maintenance Manual,2009,pages 4-69. , Darin L. George, Reducing Measurement Uncertainty In Process Gas Quality Measurements, San Antonio, Texas, USA, 200610. Torbjorn V. Lokken, Water Content of High Pressure Natural Gas; Data, ,Prediction and experience from Field, Statoil Hydro Research Center, 2007,

Page 3-511. Andy Benton, Dew Point Measurements in Natural Gas, Natal Brazil, 2010, page 3-412. Chandler Process Instruments General Catalogue, 2010, Page 113. John J. Carroll, The Water Content of Acid Gas and Sour Gas From 100°F to 220°F and Pressures to 10000 PSIA,CANADA,2003,page 2

6. Acknowledments

At first I would like to dedicate my special thanks to all my colleagues in 4 th gas treating plant in SPGC who support me for data gatheringand analysis, especially Mr. F.Bahmani head of process engineering and Mr. A.Sabet our laboratory and quality control head.I would like to

dedicate my special thanks for SPGC and NIGC’s R&D supports and contributions as well.

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