Portable apparatus and method for venting a storage vessel

A mobile method and apparatus for venting volatile organic compound vapors from a liquid storage vessel which includes the steps of introducing a purge medium to a liquid storage vessel containing volatile organic compound vapors and establishing an uniform and continuous stratified interface between the purge medium and the volatile organic compound vapors. The introduction of the purge medium is continued causing the continuous stratified interface to move within the vessel purging the virtually undiluted volatile organic compound vapors from the vessel and into a vapor recovery line which delivers the volatile organic compound vapors to a vapor control device. Preferably, the purge medium comprises carbon dioxide or nitrogen.

TECHNICAL FIELD OF THE INVENTION 
The present invention relates to a portable apparatus and its use for 
venting residual vapors from a liquid storage vessel. More particularly, 
the present invention relates to a method and mobile apparatus for venting 
residual vapors from a liquid storage vessel by introducing a gas to the 
storage vessel after removal of all liquids to provide a motive force to 
vent the vapors. 
BACKGROUND OF THE INVENTION 
Description of the Prior Art 
Volatile liquids, such as benzene, petroleum and the like, are often stored 
in tanks at bulk terminals, refineries and end-user facilities, and 
transported in tanks aboard barges or ships, tank trucks and rail cars. 
All such containers shall be referred to herein as liquid storage vessels. 
While resident in these liquid storage vessels, volatilization of the 
liquid occurs leaving residual vapors which must be removed before workmen 
can be permitted to enter the vessel and before certain types of work, 
particularly hot work such as welding, may be done to the vessel. 
Currently, such residual vapors are purged by flooding liquid storage 
vessels with a sufficient volume of water or air to dilute the vapors and 
carry them out of the vessel. The resulting mixture of diluted vapors, in 
many cases, are simply emitted to the atmosphere and surrounding water 
supply where they pollute the environment. Emissions handled in this 
manner lead to severe environmental hazards. For example, the inhalation 
of benzene vapors may cause depression of bone marrow activity, 
convulsions and paralysis. In addition, hydrocarbons are a major 
contributor to the formation of smog which has been proven to increase 
respiratory disorders among the population. 
In addition to these environmental problems, water flushing facilities must 
overcome many economic hurdles. Adequate water for such facilities may be 
expensive due to limited water resources or to restrictions concerning the 
reuse or recycling of the water. If the water must be reused or recycled, 
it must be treated to remove contaminants that might pollute the 
environment or contaminate the next vessel to be flushed. 
The environmental problems associated with air flooding could be eliminated 
by sending the mixture of vented air and vapors to a control device where 
the harmful vapors would be contained rather than emitted to the 
atmosphere. Unfortunately, three or more times the storage vessel volume 
must be cycled through the vessel to ensure that all of the residual 
vapors are purged from the vessel. Clearly, such a solution is expensive 
because of the large amount of air which would have to be heated in a 
combustion device before the volatiles they carry would be destroyed. The 
size of the collection piping and combustion equipment associated with 
such a process, in addition to the amount of fuel required to combust the 
vapors, similarly would be quite large, thereby prohibitively increasing 
the cost of such a process. 
There have been several patents in the prior art which attempted to address 
the problem of removing vapors from storage tanks and collecting the gases 
which are forced out of the storage tank to reuse such gases for 
combustion. 
U.S. Pat. No. 291,085 shows apparatus for removing flammable gases from oil 
tanks which includes devices for causing an induced current of air to pass 
into a storage tank above the surface of the fluid (such as fuel oil) and 
at the same time conduct displaced gases to a point where they may be used 
as fuel or discharged with safety into the atmosphere. The patent which 
issued in 1884 teaches the use of air as a medium for forcing gaseous 
vapors from a storage tank. 
It has been learned over the past hundred years that air is an unsafe 
medium for use in cleansing storage tanks and also can result in corrosion 
of the tank. The device shown by U.S. Pat. No. 291,085 is relatively 
simple and primitive and does not include the safety features or efficient 
means for recapture of vapors for other uses as is claimed by the present 
invention. 
U.S. Pat. No. 1,918,100 shows a gas-gathering system which is basically a 
closed system in which vapors which collect in a storage tank are pumped 
into a secondary vapor storage tank partially filled with water and from 
the vapor storage tank are recaptured through a compression and condensing 
process to provide dry gas for other uses such as combustion. The patent 
states as its primary objective the provision of a method and apparatus 
for maintaining a hydrocarbon gas at all times within the storage tanks 
above the liquid levels thereof with the specific end in view of 
preventing air or oxygen from entering the tanks and mixing with the gases 
contained therein. 
It should be noted at this point that U.S. Pat. No. 1,918,100 specifically 
teaches away from the method and apparatus of the U.S. Pat. No. 291,085 
patent in that U.S. Pat. No. 291,085 teaches the use of air as a medium 
for moving vapors out of a storage tank, and U.S. Pat. No. 1,918,100 
specifically provides a method to prevent air or oxygen from entering the 
tank and mixing with the gases. 
Although the U.S. Pat. No. 1,918,100 patent is a more modern gas collection 
system apparatus and method, it does not show nor suggest the present 
invention which includes control of the flow of a purge medium to provide 
a laminar flow to create a continuous stratified interface between the 
volatile vapors and the purge medium. Nor does U.S. Pat. No. 1,918,100 
teach or suggest any mechanism for detection of completion of the purging 
operation nor mixing with a high BTU material for later combustion. Nor 
does either prior art patent introduce gas at the bottom of the tank as is 
shown and claimed with respect to one embodiment of the present invention. 
SUMMARY OF THE INVENTION 
The mobile method and apparatus for venting a liquid storage vessel of the 
present invention overcome the above-noted disadvantages and drawbacks 
which are characteristic of the prior art. 
The present invention is directed to a method which comprises the steps of 
introducing a purge medium to a liquid storage vessel containing volatile 
organic or other vapors and establishing a uniform and continuous 
stratified interface between the purge medium and the volatile organic 
compound vapors. The introduction of the purge medium is continued causing 
the continuous stratified interface to move within the vessel purging the 
undiluted volatile organic compound vapors from the vessel. 
In a preferred embodiment, a purge medium, preferably carbon dioxide, is 
introduced to the bottom of a liquid storage vessel containing relatively 
light volatile organic compound vapors establishing a uniform and 
continuous stratified interface between the purge medium and the volatile 
organic compound vapors. The purge medium is introduced at a controlled 
flow rate, a predetermined pressure, and at a temperature that is lower 
than that of the vapors to be displaced causing the continuous stratified 
interface to rise within the vessel purging the undiluted volatile organic 
compound vapors from the top of the vessel. 
In an alternate preferred embodiment, a purge medium, preferably nitrogen, 
is introduced to the top of a liquid storage vessel containing relatively 
heavy volatile organic compound vapors establishing a uniform and 
continuous stratified interface between the purge medium and the volatile 
organic compound vapors. The introduction of the purge medium is at a 
controlled flow rate, and a predetermined temperature and pressure causing 
the continuous stratified interface to descend within the vessel purging 
the undiluted volatile organic compound vapors from the bottom of the 
vessel. 
In a preferred embodiment, the undiluted volatile organic compound vapors 
are purged into a vapor recovery line which delivers the volatile organic 
compound vapors to a vapor control device. 
The present invention also is directed to apparatus for performing the 
above-described methods. A preferred embodiment of the apparatus is 
portable and can easily go from one storage vessel to another storage 
vessel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, and particularly to FIG. 1, a schematic 
representation of a preferred embodiment of a liquid storage vessel 
venting apparatus arranged to perform the method of the present invention 
is generally shown at 8. A liquid storage vessel 10 of typical 
construction has apertures for receiving and dispensing various fluids, 
including volatile organic compounds such as benzene and petroleum 
products which generate vapors 12. The present invention is fully 
operational on all standard types of storage vessels 10. 
According to this embodiment, a gas 14 is introduced at or near the bottom 
of the vessel 10 to provide a motive force to purge the vapors 12 from the 
vessel 10. The gas 14 preferably is heavier than, colder than and inert 
with respect to the vapors 12, and is introduced to the vessel 10 in a 
laminar, or near laminar, flow with little or no turbulence. As the gas 14 
enters the vessel 10, an uniform stratification develops with the vapors 
12 forming a layer above a layer formed by the gas 14. In this manner, 
mixing of the gas 14 with the vapors 12 is avoided and the vapors 12 
remain virtually undiluted. As shown in FIG. 1, an uniform interface 16 
develops between the gas 14 and the vapors 12. As more gas 14 is 
introduced to the vessel 10, the interface 16 approaches the top of the 
vessel 10 driving the undiluted vapors 12 from the vessel 10. Preferably, 
the gas 14 is such that it may be dispersed into the environment without 
contaminating the surrounding area. Preferably, the gas 14 comprises 
carbon dioxide. Those of ordinary skill in the art will recognize that 
other gases (eg. nitrogen) that are heavier or lighter than the vapors 12 
may also be utilized as the gas 14. 
A supply of gas 14 is provided in an insulated tank 18. Preferably, the gas 
14 is stored in tank 18 in a chilled liquid state under pressure. A valve 
24 is provided between tank 18 and a fill line 20. Valve 24 may be a 
shut-off valve, a pressure release valve, a pressure control valve, or a 
combination of such valves. 
When valve 24 is opened gas 14, either in its liquid state or as a gas, 
traverses fill line 20 to heat exchanger 26. Heat exchanger 26 will heat 
gas 14 to a desired temperature, preferably between 50.degree. F. and 
110.degree. F. 
Heat exchanger 26 may heat gas 14 directly or indirectly. The preferred 
embodiment of heat exchanger 26 will indirectly heat gas 14 by utilizing a 
gas fired water heater. However, a person skilled in the art will 
recognize that any medium (water, oil, metal heat sink, etc.) may be 
heated by any type of energy source (gas, propane, natural gas, 
diesel,electricity, solar energy, etc.) or mixture of energy sources such 
that when gas 14 passes through heat exchanger 26 gas 14 can absorb the 
heat from the heated medium to increase the temperature of gas 14 to a 
desired temperature. 
Gas 14 passes through a control valve 11 as it leaves heat exchanger 26 and 
continues to travel along fill line 20 towards vessel 10. Control valve 11 
may be a pressure release valve, a shut-off valve, a pressure control 
valve or any combination of such valves. As the pressure of gas 14 is 
reduced the temperature of gas 14 is decreased (due to auto-refrigeration) 
as the gas 14 passes through fill line 20. The temperature and pressure of 
the gas 14 can be monitored along fill line 20 by an optional indicator 
13. Indicator 13 may be one or more temperature and pressure monitoring 
devices. 
In a preferred embodiment using carbon dioxide as gas 14, the heating of 
gas 14 in heat exchanger 26 under storage tank pressure will prevent the 
formation of dry ice when the pressure of gas 14 is further reduced as it 
passes through fill line 20 from the heat exchanger 26 to vessel 10. 
An optional tank entry valve 15 assures that gas 14 is at an appropriate 
pressure when it enters vessel 10. A preferred embodiment of the 
invention, as illustrated in FIG. 1, will reduce the pressure of gas 14 to 
about atmospheric pressure and the temperature of gas 14 to be less than 
or equal to that of vapors 12 before introducing gas 14 into vessel 10 via 
an opening 22 disposed near the bottom of vessel 10. 
The tank 18, fill line 20, heat exchanger 26, indicator 13, valve 24, 
control valve 11 and tank entry valve 15 may be conveniently localized on 
a mobilized gas introductory unit 203 as seen in FIG. 2. Introductory unit 
203 may be a truck as illustrated in FIG. 2 or introductory unit 203 may 
be a trailer, a skid, a boat, or any other mobile vehicle. The preferred 
embodiment illustrated in FIG. 2 contains a large capacity liquid carbon 
dioxide tank 18, an indirect heat exchanger 26, and a compartment 201 for 
storing fill line 20, and assorted valves and indicators and/or monitors. 
FIG. 2 shows a preferred embodiment of heat exchanger 26 that indirectly 
heats gas 14 utilizing a gas fired water heater 9. Gas 14 enters inlet 1 
of gas heating component 3 either as a liquid through line 4 or as vapors 
through line 7. Gas heating component 3 comprises a pipe around which hot 
water is circulated within an enclosure. The gas 14 enters the pipe at 
inlet 1, passes through the pipe (which traverses the gas heating 
component 3 at least one time) and leaves at exit 5. Gas fired water 
heater 9 utilizes natural gas or propane stored in tank 2 to heat the 
water to the appropriate temperature. 
One of the major advantages of the present invention is that it is mobile 
and can be delivered to and used to vent vapors from any type of storage 
tank at any location. 
As shown in FIG. 3, the gas 14 may alternatively be delivered to the vessel 
10 through a fill line 20, which extends through an opening 23, disposed 
on the top of the vessel 10. The distal end of the fill line 20, is 
disposed near the bottom of the vessel 10 to introduce the gas 14 beneath 
the vapors 12. 
Referring to both FIGS. 1 and 3, a vapor recovery line 28 extends from an 
opening 30 disposed in the roof of the vessel 10 to receive the vapors 12 
that are purged from the vessel 10 as it is filled with the gas 14. An 
optional post-vessel control valve 33 may be placed in line 28 as it 
leaves vessel 10. Post-vessel control valve 33 may be a shut-off valve, a 
pressure control valve, a vacuum release valve, a pressure release valve, 
or any combination of such valves. 
A detection/control unit 32 is provided in the recovery line 28 to monitor 
the gas purged from the vessel 10. The detection/control unit 32 may 
contain a gas detector to monitor the presence of vapors 12 purged from 
vessel 10 or to check for the presence of gas 14. Unit 32 may be set to 
detect a predetermined value either for the absence of vapors 12 or the 
presence of gas 14. When that predetermined value is reached, unit 32 may 
signal the closing of valve 24, 11, or 15 through a remote sensing device 
attached to a designated valve. The detection of gas 14 or the absence of 
vapors 12 will signal the completion of the venting process. 
In addition, unit 32 may also be programmed to detect the rate of flow and 
temperature of vapors 12 and/or gas 14. The sensing devices can be 
programmed to signal the damping of valve 24, pressure control valve 11, 
or tank entry valve 15 or to alter the settings on the heat exchanger 26 
to vary the temperature and/or flow rate of gas 14 to maintain the laminar 
flow of gas 14 into vessel 10 and the optimum purging of vapors 12. For 
example, if the vapors 12 entering the vapor recovery line 28 are too hot 
then a signal can be sent to a remote sensing three-way mixing valve in 
heat exchanger 26 to reduce the temperature of the hot water going to the 
indirect heating device or to increase the flow of the cryogen. 
Alternatively, unit 32 may have preset values of various parameters such as 
oxygen concentration, flow rate, temperature, or pressure. Whenever a 
measured parameter falls outside of the preset values for that parameter 
an alarm would sound and the venting operator would know to take the 
appropriate measures to reoptimize the venting system. 
An optional low pressure blower 34 may be provided to receive the vapors 12 
exiting unit 32 and to direct the vapors 12 to a vapor control device 38 
described below. To prevent the vessel 10 from collapsing, care must be 
taken to prevent the blower 34 from creating an excessive vacuum within 
the vessel 10. For example, unit 32 may contain a pressure indicator that 
would signal blower 34 to shut-off, or would sound an alarm, if excessive 
vacuum developed in recovery line 28. 
The vapors 12 may be directed by the blower 34 through a vapor/liquid 
separator 36 which will remove condensed liquid droplets from vapors 12 
flowing through the recovery line 28. After passing through the separator 
36, the vapors 12 may be passed through a detonation arrester 17 and on to 
a vapor control device 38. 
One skilled in the art would recognize that the order of the components 
along the recovery line 28 is not critical. For example, under some 
circumstances one may want the vapors 12 to pass through the vapor/liquid 
separator 36 before it passes through unit 32 or blower 34. Numerous 
permutations of the sequence of vapor handling components exist and may be 
customized for each particular application. 
Vapor control device 38 may be any of a variety of devices. For example, 
the vapor control device 38 may include adsorption chambers of an 
adsorbing substance (such as activated charcoal, resins or catalyst) 
capable of adsorbing gasoline or other vapors from air. Thus, passing 
vapors 12 through adsorption chambers would remove vapors 12 from the air 
intermixed therewith, and thereby permit the release of the cleaned air 
into the atmosphere. 
Vapor control device 38 may also represent a condensation/refrigeration 
unit which would cool the vapors 12 and recover the liquid condensate. A 
preferred embodiment will use the recovered liquids as fuel in a 
combustion engine. Recovered liquids may also be stored under pressure in 
a storage tank for further handling. After removal of the condensed 
volatile organic vapors, the cleaned air may be released into the 
atmosphere. 
Another embodiment of vapor control device 38 is a flare by which vapors 12 
are combusted prior to their emission to the atmosphere. If the 
combustibility of vapors 12 is insufficient for adequate burning, a supply 
of natural gas, propane, butane or other combustible material may be 
provided to increase the BTU level of vapors 12. 
Those of ordinary skill in the art will recognize that a variety of vapor 
control devices, including a combination of vapor control devices, may be 
utilized as the circumstances may dictate. 
The vapor recovery line 28, detection/control unit 32, blower 34, separator 
36, detonation arrester 17, and vapor control device (preferably flare 
438) may be conveniently localized on a mobile vapor recovery unit 401 as 
seen in FIG. 4. Vapor recovery unit 401 may be a truck as illustrated in 
FIG. 4 or a trailer, a skid, a boat, etc. The preferred embodiment 
illustrated in FIG. 4 has a vapor recovery line 28 connected to unit 32. 
Unit 32 is connected by a pipe to detonation arrester 17, blower 34 and 
separator 36. As vapors 12 pass through separator 36, any liquid 
condensate is removed from the vapors 12 and the vapors 12 are then sent 
into a flare 438. Flare 438 may have an optional hinge 440 that allows its 
smoke stack to fold into a horizontal position when not in use or when the 
vapor recovery unit 401 is moving. FIG. 4 also indicates the vapor 
recovery unit 401 with flare 438 upright and ready for operation. 
A person skilled in the art would recognize that the entire venting system 
described herein, except for the storage vessel, may be localized on a 
single mobile unit so that it can be easily transported from site to site. 
Although FIG. 2 and FIG. 4 show the gas introductory unit 203 and the 
vapor recovery unit 401 on two separate trucks, these two units could be 
placed on a single skid or trailer, or each unit could be placed on a 
separate skid or trailer with both skids/trailers carried on one truck, 
barge, train, etc. 
Once the venting process is completed, gas 14 will fill vessel 10. Gas 14 
may be immediately dispersed directly into the atmosphere, or may be 
stored in vessel 10 to be displaced when vessel 10 is refilled with 
another liquid or gas. 
Referring now to FIG. 5, a schematic of another preferred embodiment of the 
present invention is shown and referred to in general by the reference 
numeral 40. A liquid storage vessel 42 of typical construction has 
apertures for receiving and dispensing various fluids, including volatile 
chlorinated hydrocarbon compounds such as perchlorethylene and other 
chlorinated products which generate vapors 44. 
According to this embodiment, a gas 46 is preferably introduced at a 
temperature that is greater than or equal to vapors 44 at or near the top 
of the vessel 42 to provide a motive force to purge the vapors 44 from the 
vessel 42. This embodiment where gas 46 is introduced at the top of vessel 
42, is preferably used when gas 46 is lighter than and inert with respect 
to the vapors 44, or when vapors 44 are at a much colder temperature than 
feasible for the introduction of the purge gas. Gas 46 is introduced to 
the vessel 42 in a laminar, or near laminar, flow with little or no 
turbulence. As the gas 46 enters the vessel 42, an uniform stratification 
develops with the gas 46 forming a layer above a layer comprised of the 
vapors 44. In this manner, mixing of the gas 46 with the vapors 44 is 
avoided and the vapors 44 remain virtually undiluted. As shown in FIG. 5, 
an uniform interface 48 develops between the gas 46 and the vapors 44. As 
more gas is introduced to the vessel 42, the interface 48 approaches the 
bottom of the vessel 42 driving the vapors 44 from the vessel 42. 
Typically, the gas 46 is chosen such that it may be dispersed into the 
environment without contaminating the surrounding area. Preferably, the 
gas 46 comprises nitrogen, but those of ordinary skill in the art will 
recognize that other gases that are lighter than the vapors 44 may also be 
utilized as the gas 46. This embodiment is preferred for cleaning tanks 
containing heavy gases such as chlorinated hydrocarbons. 
A supply of gas 46 is provided in an insulated tank 50. Preferably, the gas 
46 is stored in tank 50 under high pressure. A valve 56 is provided 
between tank 50 and a fill line 52. Valve 56 may be a shut-off valve, a 
pressure control valve, a safety relief valve or a combination of such 
valves. Gas 46 traverses fill line 52 to heat exchanger 58. Heat exchanger 
58 will heat gas 46 to a desired temperature. As the gas 46 is heated the 
gas expands, increasing the pressure of gas 46. 
Gas 46 passes through a control valve 51 as it leaves heat exchanger 58 and 
continues to travel along fill line 52 towards vessel 42. Control valve 51 
may be a shut-off valve, a safety relief valve (to release excess 
pressure), a pressure control valve, or a combination of such valves. The 
temperature and pressure of the gas 46 can be monitored along fill line 62 
by an optional indicator 53. Indicator 53 may be one or more temperature 
and pressure monitoring devices. 
An optional tank entry valve 55 assures that gas 46 is at an appropriate 
pressure and flow rate when it enters vessel 42. A preferred embodiment of 
the invention, as illustrated in FIG. 5, will introduce gas 46 at or near 
atmospheric pressure and at a temperature equal to or more than that of 
vapors 44. Gas 46 is introduced into vessel 42 via an opening 54 disposed 
near the top of vessel 42. The gas is introduced at a desired temperature, 
pressure and flow rate. 
Since heat exchanger 58, indicator 53, valve 56, control valve 51, and tank 
entry valve 55 are the same as the corresponding devices illustrated in 
FIG. 1 and described in connection thereto, these devices will not be 
described in detail here. Furthermore, the tank 50, fill line 52, heat 
exchanger 58, indicator 53, valve 56, control valve 51 and tank entry 
valve 55 may be conveniently localized on a mobilized gas introductory 
unit 203 as seen in FIG. 2. Introductory unit 203 may be a truck as 
illustrated in FIG. 2 or introductory unit 203 may be a trailer, a skid, a 
boat, etc. 
Since introductory unit 203 will have corresponding devices when 
incorporating the embodiment illustrated in FIG. 5 to those discussed 
above where unit 203 incorporated the embodiment illustrated in FIG. 1, 
these devices and unit 203 will not be described in detail here. 
A vapor recovery line 60 extends from an opening 62 disposed near the 
bottom of the vessel 42 to receive the vapors 44 that are purged from the 
vessel 42 as it is filled with the gas 46. Alternatively, and as shown in 
FIG. 6, a vapor recovery line 60, extends into the vessel 42 through an 
opening 63 disposed on the top of the vessel 42. The proximal end of the 
recovery line 60, is disposed near the bottom of the vessel 42 to receive 
the vapors 44 as the vessel 42 fills with the gas 46. A post-vessel 
control valve 65 may be located along recovery line 60 close to opening 
63. Post-vessel control valve 65 may be a shut-off valve, a pressure 
release valve, a pressure control valve, a vacuum release valve, or any 
combination of such valves. 
Referring to both FIGS. 5 and 6, a detection/control unit 64 may be 
provided in the recovery line 60 to monitor the gas purged from the vessel 
42 or to check for the presence of the gas 46. When the gas 46 is detected 
in a predetermined quantity by the gas detector 64, the venting process 
has been completed. 
A low pressure blower 66, a vapor/liquid separator 68, a detonation 
arrester 67, and a vapor control device 70 may be provided to receive the 
vapors 44 exiting the detection unit 64 to process them for further 
handling. Since these devices are the same as the corresponding devices 
described in connection with the previous embodiment, they will not be 
described here in detail. 
The present invention will be further illustrated by the following specific 
examples, it being understood that while these examples may describe in 
detail some of the preferred features of the invention, they are merely 
provided for the purpose of illustration and are not intended to limit the 
broader aspects of the present invention. 
EXAMPLE 1 
A test of the present invention was conducted to vent the gaseous contents 
of a liquid storage vessel carried aboard a barge. For the purpose of this 
test, volatile organic compound vapors were removed from the tank so that 
it contained only air. 
Liquid carbon dioxide stored in an insulated tank truck at about 0.degree. 
F. and about 300 psia was released into a fill line, and then to a heat 
exchanger where the liquid was warmed to approximately 80.degree. F. and 
vaporized. The gas was released to a pressure reducing valve which reduced 
the pressure of the gas from 300 psia to about 10 psia causing the carbon 
dioxide gas to auto-refrigerate to about 30.degree. F. In this way the 
pressure of the carbon dioxide within the fill line was dropped to 
atmospheric pressure without forming solids. 
The carbon dioxide gas was then introduced to the bottom of the vessel 
through an 8 inch line at a flow rate of approximately 7,000 cfh. It was 
admitted in a nonturbulent, metered flow to create an even and uniform 
stratification between the air present in the vessel and the incoming 
carbon dioxide gas. An air-carbon dioxide gas interface formed in the 
vessel and continually rose toward the roof of the vessel as more carbon 
dioxide gas was admitted. As the interface rose, the air in the vessel was 
forced toward and through an opening in the roof of the vessel without 
experiencing any significant mixing of the air and carbon dioxide gas, 
leaving the vessel completely void of air and full of carbon dioxide gas. 
EXAMPLE 2 
All liquids are removed from a liquid storage vessel carried aboard a 
barge. The liquid storage vessel contains volatile organic compound 
vapors. 
Liquid carbon dioxide stored in an insulated tank truck at about 0.degree. 
F. and about 300 psia was released into a fill line, and then to a heat 
exchange where the liquid was warmed to approximately 80.degree. F. and 
vaporized. The gas was released to a pressure reducing valve which reduced 
the pressure of the gas from 300 psia to about 10 psia causing the carbon 
dioxide gas to auto-refrigerate to about 30.degree. F. In this way the 
pressure of the carbon dioxide within the fill line was dropped to 
atmospheric pressure without forming solids. 
The carbon dioxide gas is then introduced to the bottom of the vessel 
through an 8 inch line at a flow rate of approximately 7,000 cfh. It is 
admitted in a nonturbulent, metered flow to create an even and uniform 
stratification between the volatile organic compound vapors present in the 
vessel and the incoming carbon dioxide gas. A volatile organic compound 
vapor-carbon dioxide gas interface forms in the vessel and continually 
rises toward the roof of the vessel as more carbon dioxide gas is 
admitted. As the interface rises, the air in the vessel is forced toward 
and through an opening in the roof of the vessel without experiencing any 
significant mixing of the volatile organic compound vapors and carbon 
dioxide gas, leaving the vessel completely void of volatile organic 
compound vapors and full of carbon dioxide gas. 
The purged volatile organic compound vapors, virtually undiluted by 
employing the present invention, are forced into a recovery line connected 
to the vessel. The carbon dioxide gas is introduced into the vessel until 
a detector in the recovery line detects the presence of the carbon dioxide 
gas. At this point, due to the uniform stratification maintained within 
the vessel, all of the residual vapors originally in the vessel are purged 
from the vessel, and the vessel is filled solely with carbon dioxide gas. 
A blower attached to the recovery line directs the vapors through a 
vapor/liquid separator to remove any condensed liquids within the vapor 
stream. The vapors are then directed to a combustion device where, if 
needed, the vapors are mixed with natural gas and combusted prior to their 
emission to the atmosphere. 
EXAMPLE 3 
All volatile liquids are removed from a liquid storage vessel. 
A truck carrying a large insulated refrigerated tank filled with liquid 
carbon dioxide, a heat exchanger, and a fill line with various meters and 
gauges is driven close to the liquid storage tank. A flexible connecting 
hose is withdrawn from a storage area on the truck and is used to connect 
the fill line to the storage vessel. 
Another truck containing a vapor recovery hose, a detection/control unit, a 
blower, a detonation arrester, and a vapor control device is also driven 
close to the storage vessel. The vapor recovery hose is withdrawn from its 
storage compartment and connected to the outlet of the storage vessel. 
The heat exchanger, on the first truck, has a gas fired circulating water 
heater that is turned on to allow the water in the circulating water 
heater to reach the appropriate temperature. Then a valve, between the 
carbon dioxide refrigerated tank and the heat transfer device, is opened 
and liquid carbon dioxide that has been stored in the refrigerated tank at 
about 0.degree. F. and about 300 psia is released into a fill line. The 
carbon dioxide is then passed through the fill line into a heat exchanger 
where the temperature of the carbon dioxide gas is raised to about 
80.degree. F. 
The carbon dioxide gas continues to pass through the fill line through 
several pressure control valves to continually reduce the pressure of the 
carbon dioxide gas. The pressure of the carbon dioxide as it enters the 
storage vessel is preferably from about 5 psia to about 1 psia. As the 
pressure of the carbon dioxide drops the temperature of the carbon dioxide 
also drops. The temperature of the carbon dioxide is kept as low as 
possible, generally from about -20.degree. F. to about 50.degree. F. 
Preferably, the carbon dioxide is introduced into the bottom storage 
vessel at about atmospheric pressure and at about 0.degree. F. or at less 
than ambient temperature. 
The carbon dioxide is introduced into the vessel at a carefully controlled 
flow rate of 3,000 cfh or greater. The carbon dioxide is admitted in such 
a manner as to establish and maintain essentially laminar flow through the 
vessel. A uniform interface between the organic vapors present in the 
vessel and the carbon dioxide is created and rises toward the roof of the 
vessel as more carbon dioxide is admitted. As the interface rises, the 
vapors are forced toward and through an opening in the roof of the vessel 
without experiencing any significant turbulence and mixing of the vapors 
and the carbon dioxide. 
The purged vapors, virtually undiluted by employing the present invention, 
are pushed into a vapor recovery line. A detector in the recovery line 
signals a valve on the fill line to shut-off the influx of carbon dioxide 
once carbon dioxide is detected in the vapor recovery line. At this point, 
due to the laminar flow of carbon dioxide through the vessel, all of the 
residual vapors originally in the vessel are purged from the vessel, and 
the vessel is filled with carbon dioxide gas. 
A blower attached to the recovery line directs the purged vapors through a 
liquid/vapor separator to remove any droplets of condensed liquid from the 
vapor stream. The vapors are then directed to a refrigeration/condensation 
unit, where the vapors are condensed, collected and recycled to supplement 
the energy source used to fire the heat transfer device. 
It is thus seen that the method and apparatus of the present invention 
provides several advantages. In general, the present invention reduces the 
amount of purge medium necessary to vent residual vapors from all types of 
liquid storage vessels to nearly a single vessel volume. The present 
invention is mobile and can be used when the storage vessel access is 
limited. The present invention will work well with a fill line hose 
connected to the inlet of the vessel and a vapor recovery line connected 
to the vessel outlet. The system is easy to set up and since the system is 
portable, when the vessel has been vented, the system can be moved to the 
next vessel. 
In addition, since the present invention eliminates the need for flushing 
the storage vessel with water or air, it is an environmentally safe and 
efficient way to vent residual vapors. The essentially laminar flow of the 
gas through the vessel increases the efficiency of the vessel purging. In 
fact, the present invention can remove 95-99% of the vapors in a tank or 
barge with only 1.1 to 1.5 vessel volumes of inert gas. 
Since the vapors are essentially undiluted as they leave the vessel, the 
efficiency of selected vapor control methods is vastly increased. For 
example, the combustibility of the vapors might be sufficiently high for 
burning, or at the very least they can be combusted with a minimal 
addition of fuel. Similarly, undiluted vapors allow for the efficient use 
of refrigeration to condense the vapors to a liquid state for collection 
and reuse. Similarly, activated carbon and resins can more effectively 
process concentrated vapors. 
The present invention can also be used to purge residual vapors that are 
either heavier or lighter than the purge medium. 
It is understood that variations of the foregoing can be made within the 
scope of the present invention. For example, numerous purging mediums can 
be used to provide the motive force to vent the vessel 10 or 42. Further, 
the present invention can be used to vent more than just volatile organic 
compound vapors from liquid storage vessels. It is applicable for venting 
any type of vapor from any type of enclosure. 
Further, a variety of detection units may be employed to monitor the 
progress of the venting process. For example, a gas detector can be used 
to signal the presence of purging gas or a flow meter may be used to 
determine the quantity of purging gas that has entered the vessel. A 
single turnover of the vessel volume plus 10% extra gas will virtually 
vent the vapors from the vessel. Since the volume flow rate of the gas and 
the volume of the vessel are known, the flow meter can also be eliminated, 
by calculating and using the time needed to introduce enough gas to equal 
1.1 times the volume of the vessel. In addition, the blower can be 
eliminated if the structural design of the vessel is sufficiently high to 
allow for pushing the vented vapors through the collection piping. 
A latitude of modification, change and substitution is intended in the 
foregoing disclosure and in some instances some features of the invention 
will be employed without a corresponding use of other features. 
Accordingly, it is appropriate that the appended claims be construed 
broadly and in a manner consistent with the scope of the invention.