Note: Descriptions are shown in the official language in which they were submitted.
2 ~ 9 9
AVOIDING GAS PRESSURE BUILD UP IN TRANSPORTING CRUDE OIL
This application is a continuation-in-part of the fol-
lowing United States Patent Applications: Application Serial No.
08/144,815, filed October 29, 1993, entitled "AVOIDING GAS PRES-
SURE BUILD UP IN TRANSPORTING CRUDE OIL", pending; Application
Serial No. 08/069,073, filed May 28, 1993, entitled "DISPOSABLE
OXIDE FOR SCAVENGING HYDROG~N SULFIDE", pending; and allowed Ap-
plication Serial No. 07,879,513, filed May 4, 1992, entitled
"DISPOSABLE OXIDE FOR SCAVENGING HYDROGEN SULFIDE", abandoned;
Application Serial No. 07,613,857, filed December 14, 1990, enti-
tled "DISPOSABLE OXIDE FOR SCAVENGING HYDROGEN SULFIDE", aban-
doned, Application Serial No. 07,498,412, filed March 26, 1990,
entitled "DISPOSABLE OXIDE AND CARRIER FOR FLUID STREAM PURIFICA-
TION", abandoned; and Application Serial No. 07,400,379, filed
August 30, 1989, entitled "DISPOSABLE OXIDE AND CARRIER FOR GAS
STREAM PURIFICATION~, abandoned. ~.
Crude oil as produced from wells may be mixed with
water and contain water vapor and other gases dissolved under
great pressure; these include sulfur-containing gases, such as -
hydrogen sulfide and mercaptans, which are highly objectionable
because of odor, flammability and/or toxicity.
Soon after production, crude oil may be placed into mo- -~
bile tanks for transportation away from the well-site, before the
dissolved gases have evolved and dissipated. Such gases then
evolve from the liquids during transport at an accelerated rate,
because of the vibration and agitation that accompanies such
movement. The evolved gases will then accumulate in the trans-
port tank or be vented to the atmosphere, unless reacted. Allow-
ing such gases to accumulate in the tank until it is opened cre-
ates the danger of build-up of pressurized gas, and in e~treme
case~ could rupture the tank.
Common practice has been to vent these gases from the
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mobile tank into the atmosphere during transport. However, vent-
ing of hydrogen sulfide and mercaptans is of environmental con-
cern, and is forbidden in some areas. Scrubber apparatus con-
taining a bath of liquid, such as arnmonium hydroxide, have been
mounted on tank cars and tank trucks; and gases evolving from the
tank's contents are bubbled up through the bath so that hydrogen
sulfide and mercaptans are reacted. This process has not been
widely accepted due to significant limitations. In order to bub-
ble through the absorbent liquid bath, gas in the head-space of
the tank must build up sufficient pressure to overcome the liquid
head of the bath. To operate successfully, the bath must be deep
enough for substantially complete mass transfer of the hydrogen
sulfide and mercaptans from the gaseous to the liquid phase. The
greater the liquid depth, the greater will be the pressure of the
gas necessarily exerted between the scrubber and the tank head
space, before any gas can pass through the scrubber. Once this
pressure is reached, tank pressure remains at this level until
the tank is opened, thereby venting the accumulated pressurized
gas into the atmosphere.
2~ Other problems associated with this approach are the
requirements that the transporters regularly check and maintain
the strength of liquid in the device and handle and dispose of
the liquid which may be a corrosive absorbent such as ammonia.
Applicants understand that frequency of replacement and problems
of handliny and disposition have resulted in reluctance to use
that type of scrubber.
The present invention provides an apparatus and a proc-
ess for its use that alleviates the problems above set forth.
Gases evolving from crude oil and associated liquids have their
hydrogen sulfide and mercaptan content reacted during transport
-, while tank pressure reduces toward atmospheric as gas evolution
- approaches completion. Further, the present dry cartridge system
.
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makes operation and replacement easy and trouble-free.
FIG. 1 illustrates apparatus incorporating the present
invention mounted on a conventional tank truck. -
FIG. 2 illustrates a more detailed rear view of the ap-
plication of the present invention to a tank truck as illustrated
in FIG. 1.
FIG. 3 illustrates an exploded view of a hydrogen sul-
fide adsorbing cartridge of the type used in the present inven-
tion. .
Referring to FIGS. 1 and 2, a mobile tank with an at-
tached scrubber for removing hydrogen sulfide and mercaptans from
gases vented from the tank, generally designated 8l includes a
mobile tank 60, having a head space vent 40, means to receive and
to discharge such gases, which means may include a scrubber 9,
and means 39 to conduct such gases from the head space vent 40 to
the scrubber 9, and to vent from the scrubber outlet to the at-
mosphere. Means for mounting on the tank may be any obvious
means; hence none are shown.
The mobile tank 60 usually is fitted with wheels and is -
pulled by a truck, as shown in FIG. 1, or is a railroad tank car.
The tank may be a conventional truck tank of approximately 5000
gallons capacity and having a tank pressure limitation of 15
psig. The scrubber 9 includes a reaction column 10, preferably a
replaceable, disposable cartridge, shown in an exploded view in
FIG. 3, which includes an upright hollow cylinder or tube 12 hav-
ing in inlet end 24 and an outlet end 26 and constructed of a ma-
terial which is resistant to the corrosive effects of hydrogen
sulfide or mercaptans, preferably polyvinyl chloride (PVC).
Referring to FIG. 3, the cartridge 10 has screen sup-
port means 15 at its inlet end 24, and filter means 14 at its
outlet end 26. The screen support means 15 and filter means 14
are particle-retaining, gas porous discs, preferably of foamed
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~ 2120~99
polypropylene, which have passages or pores small enough to re-
tain the material that fills the tube 12 (described below). The
screen support means 15 and filter means 14 are retained in the
cartridge 10 by preferably expanded metal screens 20. The filter
means 14 and screen support means 15 serve to retain in ~he car-
tridge 10 the material that fills the tube, while permitting
vented gas to flow through.
To retain the expanded metal screens 20, we provide, at
each end of the tube 1~, a preferably PVC coupling or sleeve 16,
divided at its mid-height by a narrow shoulder 18, against whose
inner surface fits the screen support means 15 and filter means
14 as shown in FIG. 3. Outwardly of shoulder 18, the sleeve 16
is preferably internally threaded. A screen 20, which may be ex-
panded metal, supports the adjacently-inward screen support means
15 or filter means 14 against substantial deflection and is it-
self retained in the sleeve 16 by a threaded bushing 22.
The tube 12 is filled with reactive iron oxide sup-
ported on inert particulate porous support particles 30. The ~
iron oxide is of the type which reacts with hydrogen sulfide and ~ -
mercaptans and preferably includes a crystalline phase of Fe203
together with an Fe304 moiety. Such iron oxide, supported on
calcined, crushed sized and pre-moistened montmorillonite clay is
obtainable under tradenamet "SulfaTreat~, from Gas Sweetener As-
sociates, Inc., St. Louis, Missouri, U.S.A., and results in reac-
tion products which are safe, stable and easily disposable. The
support particles have been sized to eliminate particles large
enough to permit channeled flow of gas therethrough and to elimi-
nate particles so small as to obstruct gas flow and are prefer-
ably substantially between 4 mesh and 30 mesh in size. - -
The ratio of iron oxides relative to the inert carrier
particles may vary, but is preferably between 9 and 35 lbs. of
iron oxide per 100 lbs. of carrier. The weight of water used to
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pre-moisten the calcined, crushed carrier particles, permeate
their pores and fissures so that the oxide particles will adhere
thereto, may be about the same as the weight of the particles.
The length and diameter of the tube 12 may be varied to
meet conditions encountered in use, such as different capacities
of tanks and different levels of hydrogen sulfide and mercaptans
as well as other dissolved gases in the crude oil and/or associ-
ated liquids being transported.
Given the total amount of gases expected to be evolved
during transport, for pressure less than 100 psig., the inner di-
ameter of the tube 12 rnay be designed to give a predictable back
pressure by using the empirically-determined formula:
Pt ~ Pa = 0.007(v)(h) = 0.007 (4Vh)/~ D2
where: Pt = Tank pressure, psig.
Pa = Atmospheric pressure, 0 psig.
v = Gas velocity, ft./min.
h = Height of bed, ft.
D = Cylinder inner diameter, ft.
V = Gas flow rate, ft.3min.
The height of the cylinder 12, and thus the volume of
the bed of supported iron oxide particles 30, is then determined
by the diameter of the cylinder, the loading of iron oxide on the
carrier particles, the content of hydrogen sulfide and mercaptans
in the gas, and the reactive capacity of said iron oxide. Thus,
if one knows how much gas will be evolved and their hydrogen sul-
fide and mercaptan content, a cartridge may be designed to react
all evolved hydrogen sulfide and mercaptans while never exceeding
a selected maximum back pressure. Since the cartridge 10 con-
tains no free liquid in the bed 30, it will present no minimum
back pressure to gas flowing through it and pressure drop will
approach zero as gas flow rate approaches zero.
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Preliminary tests indicate that a cartridge 10 of 6"
inner diameter and of 48" height may be sufficient for reacting
gas evolved from several tank-loads of 6% hydroyen sulfide crude,
in a 5000 gal. tank truck.
The application of the present invention to a tank
truck is illustrated in FIG. 2. Gases pass from the tank 60 via
its head space vent 40 into a means to conduct the gas 39 which
includes a generally "Un-shaped pipe segment 41 at the lowest
part of which is a drain 42 to remove liquid as often as may be
necessary to avoid free passage of gases from the tank head
space. The other end of the "U"-shaped segment leads into the
lower port 46 of a four-port connector, or cross 44, preferably
of a 4" sch 80 PVC piping system. A second port 47 accommodates
a pressure relief valve 48 which permits high-pressure gas flow
to bypass the cartridge 10. A vacuum breaker 50 is mounted in
the opposite side port 49; it opens to permit air to enter the
system rapidly anytime a vacuum is placed on the tank. The upper
port 52 receives the cartridge 10 with conventional piping compo- ~-
nents. The upper end of the cartridge is connected to a vent 54
through which the scrubbed gases are released to the atmosphere.
Use of the present invention involves filling said mo-
bile tank 60 with the liquid to be transported and joltingly
transporting the liquid from the loading site to the unloading
point; thus continuing such jolting transport as the consequent
evolution and release of gases proceeds at a progressively less-
ening rate. Hence, their scrubbing and discharge proceeds at a
progressively lessened scrubber-induced back pressure. On com-
pletion of transport, the unscrubbed gases in the tank head
space, including unreacted hydrogen sulfide and mercaptans, are
released to the atmosphere. Since the scrubber of the present
invention maintains no minimum back pressure, the gases remaining
in the head space will necessarily be at a lower pressure (and,
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thus in lessened amount) than if a scrubber which maintains some
minimum back pressure had been used.
An advantage of the process using the present invention
is that the acceleration of dissolved gas evolution caused by
transport, when combined with venting through a scrubber which
maintains no minimum back pressure, turns the former problem of
releasing harmful gases to the atmosphere into the advantage of
quickly removing such gases from the liquids being transported
and reacting the gases into safe, stable products during trans-
port.
The term ~mobile tanks" includes tank cars and tank
trucks.
The term, "crude oil and associated liquids", includes
any mixture of crude oil, other hydrocarbon liguids and/or water
which is produced during oil or gas drilling operations and which
may occur either before or after a separation operation performed
after production.
As various modifications may be made in the procedures
herein described and illustrated without departing from the scope
of the invention, it is intended that all matter contained in the
foregoing description or shown in the accompanying drawings shall
~e taken a8 illustrat1ve rather than limiting.
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