SOLVENT CONVERSION OF THE GAS PURIFICATION … · SOLVENT CONVERSION OF THE GAS PURIFICATION SYSTEM...
Transcript of SOLVENT CONVERSION OF THE GAS PURIFICATION … · SOLVENT CONVERSION OF THE GAS PURIFICATION SYSTEM...
SOLVENT CONVERSION OF THE GAS PURIFICATION SYSTEM AT THE BADAK NGL PLANT
RÜDIGER WELKER AND VOLKER SCHUDA BASF
Alemania
Presented at Venezuelan Gas Processors Association (AVPG)
XIV International Gas Convention May 10-12, 2000
Caracas, Venezuela
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SOLVENT CONVERSION OF THE GAS PURIFICATION SYSTEM AT THE BADAK NGL PLANT
ABSTRACT PT Badak operate the world´s largest natural gas liquefaction plant in Kalimantan, Indonesia. The acid
gas removal process was originally employing Monoethanolamine [MEA] which has been encountered
with stress corrosion cracking. In the context of a capacity increase in 1989, MEA was replaced by a
formulated Methyldiethanolamine solution. The hopes placed in this solvent conversion, however,
have proven to be unfounded as the plant still suffered from severe corrosion: failures of major
equipment and the pipework caused numerous extra shutdowns and adversely effected plant safety.
Considering high on-stream times essential to economic production, a second conversion project is
being carried out between 1997 and 1999, comprising the solution swap in a total of eight acid gas
removal units to BASF aMDEA solvent. This paper will provide a detailed consideration of the solvent
conversion. The operational experiences gained with aMDEA and the previous solvent will be
compared, featuring results from solvent analyses, equipment inspection and plant reliability studies.
Plant records prove the complete eradication of earlier corrosion and scaling problems. Solvent
circulation rates have been reduced by 30 %, providing energy savings and operational freedom. The
selection of BASF aMDEA solvent has thus made a substantial contribution to both the reliability and
the economy of the Badak NGL plant.
Rüdiger Welker Volker Schuda
BASF Fax: +49 – 621 – 60 – 41398
Copyright BASF AG Ludwigshafen / Germany
January 2000
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1. INTRODUCTION In 1977 PT. Badak, a subsidiary of the Indonesian states oil & gas company Pertamina, started
production of LNG with two trains, each of 800,000 t/yr capacity. During the period 1982 to 1999, six
further trains were constructed. At the end of 1999, PT. Badak was operating the world’s largest
natural gas liquefaction plant, with an annual capacity of about 21 millions tons of LNG, one million
tons LPG Propane & Butane and one million tons of hydrocarbon condensate; the 9th train is presently
being designed.
2. PROCESS HISTORY OF THE ACID GAS REMOVAL UNIT AT PT. BADAK The PT. Badak LNG facilities utilize an amine based natural gas purification process in order to
achieve the specification of less than 50 ppm CO2 in the treated gas. The acid gas removal unit of all
trains comprises a single stage absorber, a high pressure flash-vessel and a stripper regenerator.
Both columns are equipped with bubble cap trays, the heat exchangers are either shell and tube or
plate and frame types. Fin-fan air coolers are used for most of the cooling duty of the lean amine
solvent.
Fig. 1: Process Flow Diagram of the Acid Gas Removal Unit at PT. Badak
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2.1 First solvent swap from MEA to a formulated MDEA
The original solvent in the trains A – D right from start -up was MonoEthanolAmine (MEA). Despite the
addition of corrosion inhibitors, major equipment was subject to severe corrosion and scale formation,
whichin turn caused foaming and solvent carry-over. Many leaks were caused by stress corrosion
cracking as a result of primary amine attack.
In the context of a capacity increase commencing 1990, a formulated MDEA solvent system replaced
the MEA solution consecutively in Trains A-D. The newer Trains E-G were directly commissioned with
this alternative solvent.
Understandably, the corrosivity of the solvent was investigated extensively. Following the swap,
solvent analyses showed no untoward heavy metal content in the solution for about two months.
Thereafter, a low concentration of heavy metals was observed and subsequently a steep increase of
the iron content in the solution took place.
Fig. 2: Increasing iron content during the operation of the former formulated MDEA-solvent
The iron content reached a level of 700 ppmw, representing an oversaturated solution, followed by a
slight reduction to about 250 ppmw as a result of precipitation of iron carbonate complexes. This
coincided with a considerable accumulation of heat stable salts. At this time, the formulated MDEA
became corrosive and abrasive. The operating conditions approached an equilibrium state of
continuous equipment corrosion on the one hand, and scaling of material surfaces with deposit layers
on the other hand. This required excessive defoamer dosage rates, which in turn enhanced the
formation of scale.
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Fig. 3: Scaling of tubes: The hot end bundles of the exchangers exhibited considerable scaling on
both the shellside (lean amine) but most severe on the tubeside (rich amine) causing
excessive pressure drop across the bundles.
Fig. 4: Example of two tubes of above shown heat exchanger
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All major equipment as well as large sections of the pipework were subject to severe corrosion attack.
The corrosion problems led to several unplanned shutdowns, and the need to repair or replace the
corroded equipment. This happened not only in the old trains A to D, but also in the new ones E, F
and G, which were never operated with the MEA solvent and which utilize much more stainless steel
equipment.
Fig. 5: Window patching in order to repair the flash drum
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Fig. 6: Leaking heat exchanger tube sheets after operating the former solvent for one year
The tubes of the lean solution air cooler also exhibited severe thinning and a great number leaked.
This proves the corrosive/abrasive nature of the former solvent as CO2 pitting corrosion attack can be
ruled out for a fully regenerated lean solution.
PT. Badak pursued a cost intensive reengineering option to overcome the corrosion problem, and
initiated a “material upgrade” program. Sections of the rich and lean solution piping were replaced by
stainless steel in order to cope with the erosion corrosion attack, in particular of elbows, flanges,
reducers/expanders. Heat exchanger bundles were upgraded from S.S. series 304 to series 316. The
regenerator columns of trains F, G and H were completely constructed from stainless steel. Plant
reengineering, however, was not successful, as Fig. 7 depicts the increase of the Chromium ion
content indicating corrosion of stainless steel material.
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Fig. 7: Increasing chromium -ion content in the former formulated solvent
The hopes placed in this first solvent change to a formulated MDEA proved to be unfounded, as the
unit still suffered from leaks due to corrosion and severe scaling, causing production losses and high
maintenance costs. Every remedy that PT. Badak tried to improve the plant operation was merely
“curing the symptoms”. Consequently a new approach to tackle the problems was sought.
3. SOLVENT SWAP In 1995 BASF proposed to PT. Badak the solvent conversion in the existing units to aMDEA. Two
years later, after a profound evaluation, PT. Badak decided to carry out a field test in one train. The
decision was mainly based on the records of over 100 aMDEA reference plants, many of which were
converted to solve operational problems with the former solution, and not a single aMDEA plant ever
being converted to another solvent.
The older Train C was chosen for the trial: A straight solvent swap without any plant modifications was
carried out in October 1997. In a rapid turnaround, the previous solvent was drained off, the unit
cleaned, filled with aMDEA-solution and re-started.
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3.1 Cleaning Procedure
The corroded equipment surfaces were washed off with a water jet. Corrosion cavities were then
hollow-ground and coated with an epoxy protection layer. The trays and the column base of the
absorber and stripper were covered with metal dust and residues and had to be cleaned with a
vacuum cleaner.
The heat exchanger tube bundles were removed and cleaned with a high pressure water jet. The
tubing at the hot end in the solvent heat exchangers was mostly replaced as the firm deposit layer
inside the tubes could not be removed. The outer surface of the tubes was covered with a greyish-
brown layer (mainly FeCO3) after cleaning, which could partly be removed.
The system was then flushed by circulating caustic solution, especially for the removal of grease,
corrosion and decomposition products, rust or other residues to minimize the impact of the operational
history on the aMDEA performance.
Afterwards, the whole unit was filled with condensate, heated up to about 90 °C and circulated for 6
hours. This procedure was repeated once more, with samples being taken to check for the absence of
residual suspensed particles in filtration tests. The condensate analysis after 6 hours circulation
showed no foam activity, no suspended particles and the samples had a clear colouration.
The aMDEA-premix was introduced into the system and diluted with water to an amine concentration
of 40 wt%. The solvent circulation was started and afterwards the feed gas throughput stepwise
increased until 110% of the former capacity was reached within 3 days. The CO2-slip was less than
10 ppmv at a 30% lower solvent circulation rate. That means an easy access to a capacity increase of
the existing unit or, in turn for a new plant, a more compact design with reduced CAPEX & OPEX.
4. BENEFITS OF THE SOLVENT SWAP TO aMDEA The swap to BASF aMDEA solvent in the world’s largest LNG plant may be the fastest solvent change
which has ever been carried out. Within two years the former solvent was exchanged in all 7 existing
trains while the 8th train was commissioned with aMDEA in October 1999.
PT. Badak carried out the conversion project completely and rapidly, because the improvements were
noticeable, even before a visual equipment inspection proved the eradication of corrosion and scaling.
The experience of about 2 years of operating aMDEA at PT. Badak can be summarized as follows:
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Benefits of the Solvent Swap
- no corrosion damage; iron content in the solution less than 10 ppmw; corrosion
rate measured from coupons lower than 0.1 mm per year.
- plant reliability increased by more than 10%
- solvent make-up 47 % lower
- steam consumption 20 % lower
- solvent circulation rate 30 % lower
Fig. 8: Benefits of the solvent swap
5. CONCLUSION PT. Badak declared the conversion to BASF’s aMDEA
® solvent a full success, as the plant reliability
was considerably increased while the maintenance costs decreased drastically. Earlier corrosion,
scaling and associated operational problems encountered with the former solvent were completely
eradicated which also contributes to plant safety.
This example shows the immense potential available in existing facilities to improve their profitability
and competitiveness, and the easy access of its utilization - i.e. just a straight solvent swap.