2.0 Natural gas processing
• natural gas is gaseous form of petroleum
mostly methane (C1), some ethane(C2), propane (C3), butanes (C4),
pentanes (C5), hexanes (C6) and C7+
1.6 trillion m3 (56.5 trillion ft3)
Reserves at Year End:
Natural Gas:
Production:
Natural Gas:
484 million m3/d (17.1 billion ft3/d)
Natural Gas – Nymex Henry
$8,300 US$/MJ (8.77 US$/mmbtu)
Hub
Prices:
Exports:
Natural Gas:
Share of Primary Energy
Natural Gas:
Consumption (2000):
Crude Oil:
289 million m3/d (10.2 billion ft3/d)
30%
38%
Coal:
11%
Electricity – Hydro:
10%
Electricity – Nuclear:
3%
Other:
10%
C1 , C2, H2O
C1 , C2, H2S, CO2 etc..
Dehydration/
Compression
Acid Gas
Removal
H2S, CO2
Inlet
Separators
Sulphur
Recovery
HC, SO2, CO2
S
condensate (C2-C5+)
Condensate
Stabilization
C5+
C3 , C4
Propane/Butane
Processing
e.g. deep cut,
turboexpansion
Simplified PFD for Sour Gas Processing Plant
Point of processing is to meet pipeline/storage/use specifications
Pipeline Specification (Typical)
Oxygen 10 ppm
Nitrogen 3 %
CO2 2-3% pipeline to 100 ppm for LPG plant feed
H2S low as 4 ppm (0.25grains/100 scf) for pipeline higher for fuel gas
CS2, COS, RSH 20 grains/100 scf
Natural Gas Liquid (NGL) Specifications:
H2S, Sulfurs
Pass Copper Strip, ASTM D-2420
CO2
varies – 0.35 LVP of Ethane
content 1000 ppm or
less, depends on application
Acid Gas
Injection
Dehydration
acid gas
Dewpoint
Control and
Compression
Gas
Sweetening
gas
from
wells
Inlet
Separation
C2-C5+
light gases
natural gas
to market
Condensate
Stabilization
C5+
Simplified PFD for Sable Island
C1, C2
some C3-C4
+H2O
C1, C2
some C3-C4
dehy
Amine
plant
C1, C2
some C3-C4
+contaminants
comp
SO2, CO2, CO etc..
acid gas
Claus Plant
Inlet Sep
C5+,C3-C4
S
C3-C4
Condensate
stabilization/
fractionation
C5+
Sour Gas Plant in AB
2.1 Auxiliary Equipment
a) fired equipment
- heat exchangers throughout plant, furnaces used in utility and SRU
2 types (figure 8-2)
i. direct fired
- combustion gases heat process stream which is contained in pipes
ii. fire tube
- combustion gases are surrounded by a liquid that either is used as a heat transfer
medium or is the process stream itself
direct fired
firetube
application
characteristics
regeneration gas heaters
amine reboilers
lower space
forced/natural combust
line heaters
C3+ vaporizers
gly/am reboilers
low P steam gen
more equip/controls
higher ηthermal
low heat duty
skid mount
forced/natural combust
less hot spot
b) HE
- discussed in section 1.3
c) cooling towers
- detail in section 1.3
- purpose cool process water by ambient air  achieved by maximize
evaporation of H2O in droplets exposed to maximum air flow over longest
time (picture)
- mech draft – fans move air and natural draft – use density
d) pumps/turbines
- mostly centrifugal type due to lower cost, smaller space, and low
maintenance
e) compressors/expanders
- compressors used inlet and sales gas to boost pressure
+ displacement
dynamic
thermal
f) refrigeration
- used in:
 NGL/LPG recovery
 HC dewpoint control
 reflux condensation for light HC fractions
 LNG plants
- refrigerant type selected by T requirements, availability, economics,
previous experience
e.g. natural gas plant may use C2 and C3 while due availability and
economics olefin plant may use ethylene and propylene
i. mech refrigeration
- most common
- simple cycle of expansion, evaporation, compression, condensation
ii. Absorption Refrigeration
- if low cost of n.gas, low level heat source, and electricity rates
from GPSA Handbooks
2.2 Inlet Separators
•
a)
discussed fractionators in general, separator is like one stage of a
fractionator where adjust P of incoming gas to separate v and l
4 major sections
A. primary section – sep main portion of free l by abrupt change in
momentum or direction (nozzle)
B. secondary or gravity sectn – use gravity to enhance sep of entrained
droplets
• gas moves at low velocity w/ little turbulence
C. coalescing sectn – coalescer (wire, mesh, vane elements, cyclonic
passage) or mist extractor
• removes droplets can’t be sep by gravity by impingement on surface
• limits l carryover into gas (<0.013 mL/m3)
D. sump/l collection – recover l from ii and iii – provides surge V for
degassing a slug catching
b) orientation
•
vertical – high v:l ratio or total gas V low
•
horizontal – used large V total fluids and large amounts of dissolved gas in l
•
spherical – occasionally used where high P and compact size needed, l volumes
are small
•
new are small valve types on platforms
from GPSA Handbooks
2.5 Fractionation
•separate gas mixtures into individual products
•in next section discuss bulk separation of NGLs from gas
which differs from this discussion
•absorption -type units also used  use trays/packing
a)types of fractionators at gas plants
•demethanizer – product bottom is C2+, OH is C1
•deethanizer - product bottom is C3+, OH is C1/C2
commercial C3, C3/C4 (LPG), C4, C4/gasoline, natural
gasoline
e.g. at gas plant in AB deethan run depending price butane
•depropanizer
•debutanizer
from GPSA Handbooks
from GPSA Handbooks
b) Product specs
• material balance around column is 1st step in design calcs  need to
assume product stream compositions
• defined in terms of
 % recovery of component in OH or bottom OR
 composition of component in either product OR
 specify physical properties (Pvap) in either product
c) design
• in fractionation there usually 2 components which are key in separation
 lightest component in bottom (LK)
 heaviest component in OH (HK)
• these components are adjacent to each other in volatility
• in hand calcs make the assumption all components heavier than than
heaviest in OH are in bottoms