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1 Inlet receiver
2 Stabilizer tower
3 HP amine tower
4 Mole sieve dryers
5 Turboexpander
6 Demethanizer
7 De-ethanizer
8 Depropanizer
9 Debutanizer
Legend
®
OIL & GAS JOURNAL1455 West Loop South, Suite 400
Houston, TX 77027 www.ogjonline.com
Editorial direction and coordination fromWarren R. True, Chief Technology Editor
Cryogenic Natural Gas Plant
Content by the Wood Group Mustang team: Randy Sharp, Naga Jonnalagedda, Marty Ogg, and David Harsh
Artwork & rendering: Tim Mayo, Industrial 3D – A Check6 company Graphic coordination: Chris Jones, Xenon Group | xenongroupdesign.com
ONSHORE GAS PROCESSINGThis gas processing complex is typical of plants being built to process production
from major gas reserves in various regions of the US, especially shale gas regions. Natural gas liquids (NGLs) produced from such a plant may include ethane, propane, normal and isobutanes, and condensate. The gas remaining after processing (residue gas) moves via a separate pipeline for further processing, as necessary, or to end-use markets.
Inlet receiving 1The gas processing plant’s inlet receiving consists of a slug catcher and an inlet
receiver vessel. The slug catcher employs gravity to separate hydrocarbon liquids that accumulate in the gathering pipeline from the producing field. The receiver vessel receives the vapor and the vapor-liquid overhead from the stabilizer units compression, ensuring that only vapor enters the gas processing unit.
The separated vapor flows to a filter-separator to remove remaining liquids or particulates from the gas stream before flowing to a high-pressure amine contactor to remove such acid-gas contaminants as hydrogen sulfide (H2S), carbon dioxide (CO2), and mercaptans.
Liquid handling, stabilizing 2NGLs from the slug catcher and various other process units flow to the stabilizing
unit where they are blended, filtered, and heated. The heated liquids then enter a stabilizer feed separator, which separates flashed vapor from residual water and NGLs before the NGLs flow to the stabilizer tower for stabilizing. Distillation separates the lighter, more volatile hydrocarbons and returns them to the gas processing system.
The overhead product of the stabilizer tower is a natural gas stream rich in propanes and butanes, which is then compressed and moves to the inlet receiver to enter the gas processing portion of the plant.
The stabilizer tower's bottom product is stabilized condensate or natural gasoline, primarily pentanes and heavier (C5+), and is sold as a product or blended with other products from the gas processing plant.
GAS PROCESSINGAmine treating 3
Following inlet separation and filtering, the mixed gas stream must be treated to remove any sour components before final processing and fractionation. This prevents H2S contamination of the products, and CO2 freezing in downstream cryogenic NGL extraction during ethane (C2) recovery.
Mercaptans are removed to prevent contamination of liquid propane (C3), butane (C4), and liquid gasoline (C5+).
Sour gas enters an amine contactor tower, flowing upward through trays while lean amine flows downward. The lean amine reacts with the sour components in the gas, removing them from the gas stream.
The gas is now “sweet” and the amine liquid is rich. Sweet inlet gas, now saturated with water, leaves the top of the amine contactor. It then passes through the sweet gas scrubber to catch any condensed or carryover amine and water from the contactor. The sweet gas then moves to the inlet coalescing filter upstream of a molecular sieve dehydration unit.
Rich amine leaves the bottom of the amine contactor and enters an amine regenerator. This unit heats the rich amine, boiling it at low pressure to drive off the sour components and regenerates the lean amine for re-use in the amine contactor. The low pressure sulfur-rich vapor is sent to a sulfur unit for conversion to elemental sulfur. Sour overhead vapor can also be disposed of by thermal oxidation.
Mole sieve dehydration 4Because the downstream NGL recovery section operates at cryogenic temperatures
(colder than −150° F. or −100° C.), removing water is necessary to prevent equipment from freezing. Molecular sieve dehydration achieves the proper level of dryness.
After leaving the sweet gas scrubber, treated gas enters a coalescing filter, which removes any residual particles of entrained hydrocarbon liquid, free water, and amine liquid that would contaminate the mole sieve beds.
The gas enters the mercury guard bed, a vertical vessel filled with a sacrificial mercury adsorbent, which removes mercury to prevent damage to aluminum
exchangers further downstream. After the mercury guard bed, the gas enters the mole sieve dryers, which are vertical vessels containing beds of solid granular molecular sieve desiccant and bed support materials.
These beds remove virtually all the water remaining in the inlet gas to an outlet water content of less than 0.02 ppm (wt). The mole sieve dryers operate in rotation, sequentially absorbing water or being thermally regenerated with hot gas.
The dehydrated gas leaves the bottom of mole sieve dryers and is then filtered to remove dust that could damage or foul downstream equipment in the NGL extraction unit. The filters are rated a nominal 0.5µ. After filtering, about 10% of the total gas flows to the regeneration compressors, which is then heated and used to regenerate the molecular sieve beds to a dry state. The remaining dry gas stream flows to the inlet of the NGL extraction unit.
Cryogenic NGL extraction 5 6The purpose of cryogenic extraction is to cool, condense, and absorb into a liquid
all hydrocarbon components in the plant’s inlet gas heavier than methane, so that these components can be separated and sold as individual products. Performing this separation greatly increases the value of the products, compared with leaving them in the gas stream.
There are many variations of cryogenic gas separation processes, but all function by progressively cooling the gas by applying cooling by refrigeration, separation, and mechanical gas expansion. Propane and propylene are commonly used refrigerants.
Mechanical expansion can be provided by a high pressure-drop control valve or a turboexpander. 5 The rotating turboexpander
is often preferred because it provides improved gas cooling, product recovery efficiency, and recompression via an attached compressor.
Following expansion, the cold gas and refrigerated liquids enter the demethanizer tower. 6 Cold methane-rich residue gas leaves the overhead while ethane and heavier hydrocarbons leave the tower bottom as a liquid. The cold residue gas is cross exchanged in the brazed aluminum feed exchangers to cool incoming gas and then recompressed before entering the residue-gas pipeline for sale or further processing.
NGL fractionationThe liquid stream from the bottom of the demethanizer tower contains
ethane, propane, butanes, and pentane and heavier hydrocarbons. These are separated by sequential fractionation.
Ethane is recovered in the overhead of the de-ethanizer column 7 as a vapor product then typically compressed and sold into a pipeline. If ethane recovery is not required or desired, the demethanizer can be designed to “reject” ethane, leaving it in the residue gas.
The deethanizer bottom liquids contain propane and heavier hydrocarbons. The propane is recovered in the depropanizer 8 as an overhead liquid and sold as an LPG product. Mixed butanes are recovered in the debutanizer tower 9 overhead as liquid and sold as a product or further processed to split the normal butane and isobutane.
The debutanizer bottom liquid is a natural gasoline product similar to the stabilizer bottom product and can often be blended and sold.
Slug catcher
Stabilizer
Stabilizerreboiler
To condensate storage
Inletseparator
Inlet filter separator
Sourgas
Mercuryremoval
Molecularsievedehydration
Aminetreating
Inlet pipeline Inlet gasSweet gas
Water
Hot regenerationgas
Residue-gas compressorafter-cooler
Residuegas compressor
Cryogenicexchangers,separators
Turboexpander
Demethanizer
Surge drum
Residue gas to sales pipeline
Depropanizeroverhead condenser
De-ethanizeroverhead
condenserDe-ethanizerrefluxaccumulator
De-ethanizerreboiler
Depropanizerreboiler
Ethane compression
DepropanizerDe-ethanizer
Debutanizer overheadcondenser
Debutanizerreboiler
Propane storage
Butane storageMixed butane to sales
Stabilized condensate to salesCondensate storage tanks
Condensate cooler
Propane to sales
Ethane product to sales pipeline
Debutanizer
Depropanizer refluxaccumulator
Debutanizer refluxaccumulator
Cyrogenic gas processing
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Typical brazed aluminum exchange
Exchanger separation vessel
Brazed aluminum multipass exchanger
5 Turboexpander module
Low-pressure residue gas
Turboexpander, compressor unit
High-pressure process gas
Compressor residue gas
Expander discharge
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GAS PROCESSING SOLUTIONS EPFC Capabilities for the Entire Natural Gas Value Chain
• GAS CONDITIONING AND TREATING• MODULAR GAS PROCESSING PLANTS• FRACTIONATION
• DEEP ETHANE EXTRACTION• LPG RECOVERY• LIQUID PRODUCT TREATING
Call: + 1 713 375 8885 | Visit: www.CBI.com/gas-processing
cbi_ogj_cryogenic_pstr_mar_2015.indd 1 3/11/2015 1:36:53 PM
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