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2.2 Natural Gas Sweetening Process Waste
Natural gas production has increased to meet the rising demand. Meanwhile, raw natural gas consists of carbon dioxide CO
2
and hydrogen sulfide H
2
S that is known to be pollutants in significant level. These gases must be removed before
piping or shipping because it causes corrosion, reduces the heating value and thus decreases the sales value of the gas Arnold and Stewart, 1989; Kohl, and Nielsen,
1997. Natural gas sweetening processes based on amine absorption have become common in practical application Arnold and Stewart, 1989. This amines solution
combined with anti-corrosive agents are used to absorb acid gases. During shutdown process, the amine waste is generated. Such amine wastes require appropriate
treatment before disposal.
2.2.1 Source of Natural Gas Sweetening Process Wastewater
During gas sweetening process, non reclaimable contaminants tend to accumulate in the system and cause the reduction of efficiency and operational
problems Arnold and Stewart, 1989; Kohl, and Nielsen, 1997. The problem may be partially overcome by a number of strategies: a purging a part of the solution and
replacing it with fresh absorbent; b replace the entire volume of contaminated solution; c inject caustic solution to free amine bond up as heat stable salts and more
CO
2
induced degradation product; and d reclaim the entire solution.
Wastewater from the sweetening process units is exposed to the environment during process operation and turn-around. Periodically turn-around process is
performed to maintain satisfactory process performance. This step produces large quantity of amine waste. The general sources of amine wastewater during process
operation are Arnold and Stewart, 1989:
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a. The reclaimer: The normal generation temperature in the stripping tower
will not regenerate heat-stable salt or compounds such as azodazole-2. Therefore, a reclaimer is usually included to remove these contaminants.
A side stream of from amine circulation is drawn from the bottom of the stripping column. This stream is than heated to boil the water and amine
overhead while the heat-stable salts and azodazole-2 are retained in the reclaimer. This reclaimer is periodically shut down and collected
contaminants are cleaned and removed from the system. The amine bound to contaminant is introduced to the wastewater stream.
b. Foaming problem in absorption tower: Amine systems foam rather easily,
resulting in excessive amine carried over from the absorber. Foaming can be caused by a number of foreign materials such as condensed
hydrocarbon, degradation product, valve grease, etc. c.
Degraded amine: Since the sweetening process is operated in a close loop system, the used amines will be degraded during the process. The
degradation products are removed through reclaimer. Degraded amine is remediated by injection of fresh amine to stripping column.
d. Production of heat stable salt and other solids: Some solid contaminants
may present in the system. These solid contaminants can be produced from heat stable salt or solid. The cake that remains in the filter has to be
backwashed to maintain the operation pressure of the filter. The used water for backwash of the filter becomes wastewater. The amine bonded
to the cake will also go to wastewater. e.
Contamination of hydrocarbon: The liquid hydrocarbon comes from the bottom of absorption tower and inlet separator. At low pressure some
hydrocarbons condense and form liquid. This hydrocarbon mixed with the water in absorption tower gets introduced to the wastewater stream.
f. Several others source of amine wastewater are from: water used to washed
the vessel and others plant equipments i.e. heat exchanger, pumps, and etc; valve leakage and operational upset.
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On the whole, the wastewater from sweetening process units may be combination of raw amine-solution, amine degradation product, thermally stable salts,
heavy hydrocarbon and particulates.
2.2.2 Characteristics of Natural Gas Sweetening Process waste
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