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Treatment and Handling of Separated Fluids (Oil, gas and water)

Vertical Separator
 After the gross separation of the produced fluid stream has taken place, further treating of the individual gas, oil, and water phases often must be carried out to meet the specifications of pipeline sales contracts. For example:
• natural gas cannot contain excessive amounts of CO2 or H2S;
• the gas must be delivered to a pipeline at a specified pressure;
• the gas must not contain water vapor,which can condense and form hydrates or can cause corrosion and added pressure losses (typical contracts call for less than 7 lb/MMscf or 112 kg/million m3);
• crude oil may not contain excessive amounts of sediment or emulsified water (BS&W) or salt dissolved in water (typically less than 1% BS&W and less than 10 lb salt per 1000 bbl or 28 kg salt per 1000 m3);
• produced water must be cleansed of dispersed oil before disposal
Compressors may be required to raise the pressure of the gas streams to pipeline pressure. Several stages of compression can be utilized to compress the individual low pressure and intermediate pressure gas streams. The two basic types of compressors used in production facilities are reciprocating and centrifugal compressors. Reciprocating compressors use a piston and cylinder arrangement with inlet (suction) and outlet (discharge) valves synchronized with the piston movement. Centrifugal compressors use finned wheels that rotate at high speed and cause an increase in gas velocity, which is then converted to increased pressure. Centrifugal compressors are normally operated by gas-fired turbines. Compressor installations must be designed with a certain degree of flexibility to handle the changes in gas volumes and pressures that occur over the life of a field. A type of separator known as a scrubber, or suction drum is typically found just upstream of a compressor, to prevent liquids from entering the compressor cylinders.

We can expect to find gas-sweetening equipment for the removal of hydrogen sulfide, if this contaminant is present. Such equipment normally relies on a chemical reaction between the "sour" gas and another compound (for example, iron oxide in the "iron sponge" method, or an amine solution) to selectively remove the corrosive and toxic compound.
Dehydration of the gas stream is typically accomplished in most oil processing facilities by using a liquid desiccant such as triethylene glycol to absorb water vapor as the gas stream is bubbled through the chemical solution. The "wet" glycol can then be heated to drive off the accumulated water and reused in the closed system. Other approaches to gas dehydration include:
• contacting the gas with a solid desiccant such as silica gel, or a molecular sieve. These units achieve very low moisture content levels but are relatively expensive and more common in gas processing plants than production facilities;
• cooling the gas by refrigeration or by expansion of the gas stream in a low temperature extraction (LTX) unit to condense the water. This can cause solid hydrocarbon-water compounds (hydrates) to form. These hydrates must be melted to keep them from plugging the system.

Looking at the separated oil stream we find our major problem to be the removal of emulsified water from the oil. Oil-treating vessels typically rely on settling-time, chemicals, heat, electricity, or a combination of these methods to break the film surrounding each water droplet and cause the droplets to coalesce and form larger drops, thus allowing the density difference to separate the two phases. Small amounts of emulsified water that contain large amounts of salt will require removal, as salt can foul refinery equipment. This may be accomplished by mixing the treated oil with fresh water and re-treating until the dissolved salt content of the entrained water is sufficiently low, a type of process that is common at the inlet to refineries and in certain parts of the Middle East.

In areas where large volumes of crude oil must be stored and transported, crude stabilization is sometimes used to adjust the vapor pressure of the oil and prevent loss of the more volatile components during transport. In a stabilizing process, fractionating columns may be used to reclaim condensate from separated gas. This condensate is then returned to the crude oil to increase the volume of liquid sold, and its API gravity (and therefore price).

Both the gas and oil streams must be metered before sale. Orifice meters, which measure pressure drop across an orifice and relate the drop to gas flow rate, are the most common type of gas measurement device in the oil field. Oil volumes are typically measured by gauging levels in storage tanks by positive displacement meters that segment the oil flow and keep an account of the discrete volumes, or by turbine meters. Lease automatic custody transfer (LACT) units are sometimes used to automatically measure oil volumes, monitor oil quality, and record a legal transfer of custody from seller to buyer.

The produced water stream that flows from three-phase separators and/or the oil treating vessels must undergo further treatment to remove droplets of oil that have escaped the earlier processes. Skim tanks allow the water to remain in one place long enough for coalescence of the oil droplets and gravity separation to occur. The inlet on such a vessel is designed to spread out the oily water and force the oil droplets to rise through the water in the vessel, where they coalesce and are skimmed off the surface. In plate coalescers, closely spaced parallel plates allow the oil droplets to rise a relatively short distance to a plate surface, where coalescence and capture occur. Precipitators, or filters, which employ a bed of coalescing medium such as wood shavings or hay, have been common in the past but are difficult to maintain. "SP" packs cause coalescence of the oil droplets in the water by forcing the water through a serpentine path so as to create turbulence, but without shearing the oil droplets and making them smaller. Gas flotation units utilize dissolved or dispersed gas in the water to contact the oil drop-lets and bring them to the surface in a froth. After the water has been treated to specification by using one or several of these methods, it may be metered and then disposed of overboard at offshore locations, injected into disposal or injection wells as water or steam, or even used to irrigate fields in some areas.

Treatment and Handling of Separated Fluids (Oil, gas and water) Reviewed by Industri Migas on 11:46 PM Rating: 5

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