Vacuum Stripping and Gas Stripping

PROCESS & COMPRESSION SYSTEMS
Vacuum Stripping and Gas Stripping
WATER
Vacuum stripping and gas stripping towers for dependable water deaeration
Total capability
As part of the original Serck Baker waterflood
technology portfolio, Cameron’s HiperVac
and HiperStrip deaerators are the product for
more than 30 years operating experience on
over 100 installations worldwide. Supplied as
either stand-alone units or as part of a full
waterflood system, the vacuum stripping and
gas stripping deaerators provide reliable and
consistent water deaeration.
Gas vacuum deaeration
Vacuum deaeration is the most common
deaeration technology. Packed towers are
continuously evacuated by specially selected
vacuum systems, reducing the oxygen partial
pressure to create a driving force for mass
transfer from the liquid to the gas phase.
The design of the Gas Vacuum Deaerator is
an economic balance between tower
diameter, number of stages, packed bed
heights and vacuum pump size. Computeraided design procedures help select the
optimum configuration by taking into account
salinity and temperature, air leakage under
vacuum conditions, liquid distribution and
500,000 bpd vacuum stripping module mounted for offshore India
the foaming tendency of the water. Materials
of construction are selected to give excellent
corrosion resistance (in a service where
oxygen levels are high enough for corrosion
to occur but too low to replace the passive
film on 316 stainless steel).
World Wide Experience
Cameron Dearator systems have been applied to a variety of water sources with a wide range of
feed water temperatures:
Location
Temperature
Water Source
Brunei
20º F
River and produced water mixed
Arabian Gulf
18º F
Seawater
Indonesia
18º F
River delta
USA
15º F
Produced water
Alaska
4º F
Ice melt/seawater
UK
4º F
Estuary
Data Sheet | TC9814-061
Tower internals
In general, packed beds are preferred to
tray towers because material selection is
more reliable, tower fabrication and lining is
easier, installation of internals is simpler and
susceptibility to misalignment is lower.
How it works
Water enters at the top of the deaeration
tower. It is distributed evenly across the vessel
cross-section. The water trickles down over a
bed of polypropylene mass transfer packing,
where it is broken up into thin films, thereby
forming a large interfacial area between the
water and the surrounding vapor phase. A
vacuum system extracts all gases from the
vapor phase, thereby lowering the partial
pressure oxygen in the vapor phase to near
zero. This creates a driving force for oxygen
molecules dissolved in the water to diffuse to
the liquid surface and on into the vapor phase,
thereby reducing the concentration of oxygen
in the water.
Two-stage efficiency
The first stage of the tower performs bulk
removal of oxygen at a relatively high
pressure, whilst in the second stage, final
removal is achieved at a lower absolute
pressure (higher vacuum). For successful
removal of oxygen, it is necessary to achieve
the optimum balance between a number of
variables including tower diameter, packing
shape, size and quantity, vacuum capacity,
internals efficiency, etc.
Deaerator internals
Top distribution system
The top distributor may be selected as a header and lateral system, or as
a tray distributor depending on the application. A header and drilled
lateral system is typically used to evenly distribute water over the surface
of the packing. If even distribution is not achieved, the mass transfer
packing will not perform properly. The top distributor must work at all
flow rates.
Mass transfer packing
The mass transfer packing must create a very large liquid/vapor contact
area and must have a high number of drip points per unit volume, while
allowing large flows of water to pass through. The optimum packing size
for most applications is 2”, though this may vary.
Second stage packing
Two stages of mass transfer packing are typically used. At any given
temperature, it is necessary to achieve a certain pressure to achieve the
desired oxygen residual. This is typically 25 mm Hg absolute for warm
water applications. Theoretically, it is possible to use a single stage tower
and extract the entire vapor at this low pressure, but the necessity of
using an ejector to achieve the low pressure required would mean having
to pump large volumes of motive air. It is more efficient to extract the
40,000 bpd top distribution tray
PROCESS & COMPRESSION SYSTEMS
bulk of oxygen at higher pressure in the first
stage without an ejector, leaving a relatively
light duty for the lower pressure stage.
Interstage seal
This is the most important component in any
vacuum deaeration system. The interstage seal
is required to maintain a pressure differential
between the stages of the tower. This seal
is typically of the order of 25 mm Hg or 14
inches of water.
Some manufacturers use crude orifice
plate devices or downcomers, which while
performing well at design conditions, may
not hold up sufficient liquid level at low flows
to maintain the necessary seal. Slugging and
bypassing of vapor can result.
Cameron’s system uses an upcomer/seal pot
arrangement, which guarantees a minimum
water hold up, and therefore pressure seal
at all flow rates, even when there is no flow.
Following the seal section, the water falls onto
a corrugated redistribution plate designed to
optimize distribution and turndown.
2,300,000 bpd 32 Horizontal Hiperfilters SFX for Prudhoe Bay, Alaska
Vacuum system
Sump section
The vacuum system must extract two vapor
After passing through the second stage packing, the water passes to a
streams continuously. The first stage stream
sump section to allow for level control, residence time, oxygen scavenger
will be at about 40 to 50mm Hg abs while
reaction time, extraction pump NPSH and injection pump rundown.
the second stage will be 20 to 25 mm Hg abs
(lower in cold water). There are two types of
vacuum pumps available:
Single-stage – 100 mm Hg abs minimum
suction pressure
Two-stage – 40 mm Hg abs minimum suction
pressure
If a single stage pump is used, it is necessary
to raise both extract streams to at least 100
mm Hg abs. If a two-stage pump is used, it
can be connected directly to the first stage
and only the second stage extract need be
raised in pressure. In both cases, atmospheric
air ejectors are used to achieve the necessary
vapor compression.
Skid mounted vacuum pump package
PROCESS & COMPRESSION SYSTEMS
Gas stripping deaerators
Where natural gas of suitable quantity
and quality is available, gas stripping
via the gas stripping deaerators may
be an attractive alternative to vacuum
deaeration.
A stream of natural gas is introduced
in counter-current flow to the water.
The oxygen-free gas creates a driving
force for mass transfer. Gas stripping
systems may employ either packed
bed or tray towers.
The design of packed towers takes
into account backmixing of the gas
phase as a result of the high liquid to
gas ratios employed for deaeration.
52,500 bpd HiperVac tower for West Africa FPS
Cutaway of vacuum stripping deaeration tower
5 of 8 towers supplied for Arco’s 2 million bpd project in Prudhoe Bay, Alaska
L O C AT I O N S
United States of America
11210 Equity Dr., Suite 100
Houston, TX 77041
USA
TEL +713.849.7500
United Kingdom
Cameron House
61-73 Staines Road West
Sunbury-on-Thames
Middlesex, TW16 7AH
England, UK
TEL +44.1932.732000
O T H E R L O C AT I O N S
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2 Gul Circle (Gate 2)
Jurong, Singapore 629560
TEL +65.6861.3355
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Japan
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Calgary
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Colombia
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© 2011 Cameron | 12/11 TC9814-061
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