Report (29/11/2007)
0730-0915
The process area comprises the following main units:
gas receiving and metering
-acid gas removal
-dehydration
-mercury removal
-heavy hydrocarbons separation
-fractional/stabilization
-liquefaction
The processes in MLNG and MLNG Dua are essentially similar. Natural gas from the wells offshore, after flowing through the slug catcher and the gas metering station, shall be treated in these process units to remove CO2,H2S, water, mercury and heavy hydrocarbons before being fed into the main cryogenic for liquefaction.
Before 0900
Slug Catcher (U100)
The slug catcher unit is used to separate the bulk of hydrocarbon liquid and any free water from the natural gas.
0920-1020
Gas Metering Station
The Gas Metering Station is designed to maintain a constant plant inlet pressure, remove any entrained and condensing liquids and accurately measure gas supplies to the processing modules. The unit also provides an emergency fuel gas supply to U4400 and U5400.
The facility comprises 4 identical runs
a) pressure/flow control station
b) a gas/liquid separator with SMSM internals
c) metering run
1020- 11.20
Sufinol Unit U1100
The purpose of the unit is to reduce the acid gas content of the feed gas stream to avoid freezing and corrosion in the liquefaction section and ensure that the LNG is within specification.
The design of the acid gas removal unit is based on the Shell Sulfinol process which is an absorption –regeneration process for the removal of CO2,H2S and other organic sulphur compounds in the NG stream.
A mixed solvent consisting of the following components is used:
50% wt DIPA (Di-isopropanol amine)-chemical absorbent
25% wt Sulfolane (Tetrahydrothiopene dioxide)-physical absorbent and antifoam agent
25% wt water-viscosity control
Process
a) Absorption of the acidic and sulphur compounds from the feed gas by both chemical and physical absorption. Take place in the Absorber column (C1101/2) at the feed gas pressure.
Absorption is a process whereby a substance penetrates into the body of another. Favorable conditions for good absorption
-high pressure
-low temperature
-good contact between gas and liquid
-sufficient solvent or high capacity
b) Regeneration of the Sulfinol solution which takes place in two stages:
-Flashing to remove co-absorbed and entrained hydrocarbons in V1101/2
-Stripping to remove absorbed gases from the solvent at reduced pressure and elevated temperature in C1103.
Desorption is the process of removing absorbed materials from the absorbent. Condition apply: low pressure and high temperature
Treated gas
Absorbed Materials
Absorber
Fat sulfanol
Lean sulfanol
Stripper
The absorber-stripper goes together. The absorber recovers components and the stripper removes them so that the sulfinol can return to the absorber in proper condition.
Dehydration Unit U1300
The function of the dehydration unit is to reduce the moisture content of the natural gas stream preventing ice formation and thus blockage in downstream equipment at sub-zero temperatures.
The natural gas leaving the sulfinol unit is approximately 80% saturated with water and the moisture content is reduced to less than 1 ppm on leaving the driers.
Removing of water from the gas is done in two stages:-
a) Cooling the gas against high pressure propane in E1401 A & B
b) Drying by absorption with regenerable molecular sieves
Mercury Removal Unit C-1351
The purpose of the unit is to remove mercury present in the feed gas to the liquefaction unit to protect the aluminium Main Cryogenic Heat Exchanger against corrosion. Mercury has been found to corrode aluminium rapidly even in trace quantity.
This unit is located downstream of the Dehydration Unit U1300 as the absorbent performs better under dry conditions.
Liquefaction Unit U1400
In the liquefaction unit, treated natural gas is passed through the Scrub Column C1401, where heavier hydrocarbons are removed together with some lighter hydrocarbons; subsequently it is in stages cooled, liquefied and subcooled through the main cryogenic heat exchanger (E1405), HP and LP Flash Drums (V1403 and V1404). The LNG is rundown using pump P1402 to the storage tanks (T-3101/2/3/4/5) where it is stored at atmospheric pressure and the temperature of about -161°C.
The liquefaction is performed with the help of three cooling circuits
a) cooling water
b) propane circuit
c) multi component refrigerant (MCR) circuit
MCR takes heat out of NG at the lowest temperature. Propane takes heat out of MCR and NG at intermediate temperature. The cooling water takes the heat out of propane and MCR at ambient temperature.
Flash gas generated in HP and LP Flash Drums removes heat from LNG in V1403 and V1404 and light MCR in E1406 and E1407.
Refrigeration power is provided through the operation of compressors driven by steam turbines.
The LNG leaving U1400 at a T = -161°C and has the following product specification
C1 86.5 mol% min
N2 1 mol% max
C4 1.8 mol% max
C5+ 0.09 mol% max
LNG has the following characteristics in storage and transport:
-By condensing, the volume of the gas is reduced by the factor of approximately 600.
-LNG is heavier than air at -136°C and lower temperature.
-By subcooling the LNG to a temperature of -161°C, the liquid can be stored at atmospheric pressure so that there is no need for HP tanks for storage and shipping.
p/s: my first day at work, a lil bit interesting-plus got this briefing from the HSE department. i.e-use the special mask while the alarm detecting a H2S leaking. more than 1000ppm of H2S in the air can lead to death (immediate fatal).
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