U.S. patent application number 12/877318 was filed with the patent office on 2010-12-30 for low vapor pressure high purity gas delivery system.
Invention is credited to Thomas John Bergman, JR., Shrikar Chakravarti, Michael Clinton Johnson, Christos Sarigiannidis.
Application Number | 20100326537 12/877318 |
Document ID | / |
Family ID | 39204669 |
Filed Date | 2010-12-30 |
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United States Patent
Application |
20100326537 |
Kind Code |
A1 |
Sarigiannidis; Christos ; et
al. |
December 30, 2010 |
LOW VAPOR PRESSURE HIGH PURITY GAS DELIVERY SYSTEM
Abstract
Systems, apparatuses and methods for vapor phase fluid delivery
to a desired end use are provided, wherein the conditions of the
system are monitored to determine when the water concentration or
supply vessel surface temperature exceeds a specified value or when
the low vapor pressure fluid pressure falls below a specified value
for the purpose of removing a first supply vessel from service by
discontinuing vapor flow from the first supply vessel and
initiating vapor flow from a second supply vessel.
Inventors: |
Sarigiannidis; Christos;
(Williamsville, NY) ; Bergman, JR.; Thomas John;
(Clarence Center, NY) ; Johnson; Michael Clinton;
(Grand Island, NY) ; Chakravarti; Shrikar; (East
Amherst, NY) |
Correspondence
Address: |
PRAXAIR, INC.;LAW DEPARTMENT - M1 557
39 OLD RIDGEBURY ROAD
DANBURY
CT
06810-5113
US
|
Family ID: |
39204669 |
Appl. No.: |
12/877318 |
Filed: |
September 8, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11540220 |
Sep 29, 2006 |
7813627 |
|
|
12877318 |
|
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Current U.S.
Class: |
137/14 |
Current CPC
Class: |
F17C 2225/0123 20130101;
F17C 2250/032 20130101; F17C 2227/0383 20130101; F17C 2221/013
20130101; F17C 2223/043 20130101; F17C 2250/0452 20130101; F17C
2250/043 20130101; F17C 2223/035 20130101; F17C 2225/035 20130101;
F17C 13/026 20130101; F17C 13/025 20130101; F17C 2270/0518
20130101; F17C 2221/01 20130101; F17C 2250/0439 20130101; F17C 5/06
20130101; Y10T 137/0379 20150401; F17C 2223/0153 20130101; F17C
2227/0302 20130101; F17C 2205/0142 20130101; Y10T 137/0396
20150401; F17C 7/04 20130101 |
Class at
Publication: |
137/14 |
International
Class: |
F15D 1/00 20060101
F15D001/00 |
Claims
1. A method for delivering vapor phase fluid under pressure from a
vessel comprising the steps of: providing at least a first and
second vessel having a fluid therein, each vessel having a vessel
wall; providing a heater in communication with each of the first
and second vessel; heating the vessel to achieve a predetermined
pressure within the first and second vessel; providing a controller
in communication with the heater; withdrawing an amount of vapor
phase fluid from the first or second vessel; providing a sensor to
monitor at least one condition in the first and second vessels,
said condition selected from the group consisting of: vapor phase
fluid pressure; vessel wall temperature, vapor phase fluid low
vapor pressure contaminant concentration, and combinations thereof,
to determine the key fluid level in the first and second vessels;
monitoring the condition in the first and second vessels to
determine the key fluid level in the first and second vessels;
providing a controller in communication with the sensor and a valve
having an on/off position, said valve directing flow from the first
or second vessel to an end use, said sensor optionally activating
the valve on/off position.
2. The method of claim 1, further comprising the step of:
activating a first valve in communication with the first vessel to
the off position, said valve diminishing the flow of vapor phase
fluid from the first vessel to an end use when the condition
reaches a predetermined level; and activating a second valve in
communication with the second vessel to the on position, said valve
increasing the flow of vapor phase fluid from the second vessel to
an end use when the condition reaches a predetermined level.
3. The method of claim 1, wherein the vapor phase fluid is a
non-air based gas selected from the group consisting of: ammonia,
boron trichloride, carbon dioxide, chlorine, dichlorosilane,
halocarbons, hydrogen bromide, hydrogen chloride, hydrogen
fluoride, methylsilane, nitrous oxide, nitrogen trifluoride,
trichlorosilane and mixtures thereof.
4. The method of claim 1, wherein the low vapor pressure
contaminant is water.
5. The method of claim 1, wherein the first and second vessels are
made from a material selected from the group consisting of: 304
stainless steel, 316 stainless steel, Hasteloy, carbon steel and
mixtures thereof.
6. The method of claim 1, wherein the first and second vessels are
selected from the group consisting of: ISO container vessels, ton
container vessels and drum container vessels.
7. The method of claim 1, wherein the heater is an electrical
resistance heater selected from the group consisting of: silicon
blanket heaters, band heaters, heating bars, heating tape and
combinations thereof.
8. The method of claim 1, wherein the vapor phase fluid is selected
from the group consisting of: high purity vapor phase fluid,
ultra-high purity vapor phase fluid, and combinations thereof.
9. The method of claim 1, further comprising the steps of:
substantially controlling the amount of heat delivered to the first
or second vessel; and maintaining a substantially constant pressure
within the first or second vessel.
10. The method of claim 2, wherein the end use is the manufacture
of a device, said device selected from the group consisting of: a
semiconductor, a liquid crystal display, a light emitting diode and
a solar cell.
11. A device made according to the method of claim 2, selected from
the group consisting of: a semiconductor, a liquid crystal display
and a light emitting diode.
Description
RELATED APPLICATIONS
[0001] The present application is a division of U.S. patent
application Ser. No. 11/540,220, filed Sep. 29, 2006, which is
incorporated by reference herein in its entirety.
FIELD OF THE INVENTION
[0002] The present invention relates generally to the efficient
delivery of low vapor pressure high purity gases from delivery
vessels. More particularly, the present invention relates to
methods and apparatuses for the efficient delivery of low vapor
pressure high purity gases from a plurality of heated supply
vessels.
BACKGROUND OF THE INVENTION
[0003] Non-air gases (i.e. gases that are not derived from air) are
commonly used in the manufacture of products such as
semiconductors, LCDs, LEDs and solar cells. For example, nitrogen
trifluoride is used as a chamber cleaning gas, while silane and
ammonia can be used for deposition of silicon and silicon nitride
respectively during chemical vapor deposition (CVD) processes.
[0004] Semiconductor, LCD, LED and solar cell manufacturers often
require a supply of non-air gas in the vapor phase, at high or
ultra-high purity at a high flow rate with the capability of
supplying the gas in the vapor phase in a discontinuous flow
pattern. The presence of low-volatility contaminants in these gases
(i.e. contaminants that are less volatile than the non-air gas) is
particularly undesirable, since they can deposit on the product
substrate and deteriorate, or otherwise adversely affect product
performance. For example, water, is a common low volatility ammonia
contaminant that can deposit on LED sapphire substrates, resulting
in reduced LED brightness and yield loss. For such applications,
vapor phase moisture levels in ammonia that exceed 1 ppb can be
detrimental to the processes, and the products produced
thereby.
[0005] New semiconductor products have large throughput and
consequently require large quantities of non-air gases.
Additionally, due to the batch nature of semiconductor process tool
operation, the use pattern of non-air gases is often preferably
discontinuous.
[0006] Many non-air gases are transported and stored as liquids or
vapor/liquid mixtures. Such gases are known as low vapor pressure
gases and include, for example, ammonia, hydrogen chloride, carbon
dioxide and dichlorosilane. Low vapor pressure gases typically have
a vapor pressure less than about 1500 psig at a temperature of
about 70.degree. F. According to known methods, because low vapor
pressure gases are supplied as liquids or vapor/liquid mixtures, a
device for heating/boiling these gases is required so that vapor
phase product can be supplied to the desired end use, such as, for
example, the semiconductor, LED, LCD or solar cell manufacturing
process. This boiling is commonly achieved by applying heat to the
supply vessel outer wall, as described, for example, in U.S. Pat.
Nos. 6,025,576 or 6,614,412. In such systems, vapor phase low vapor
pressure gas is withdrawn from the supply vessel. Sufficient heat
is applied to boil liquid phase low vapor pressure gas at the rate
that vapor phase low vapor pressure gas is withdrawn from the
supply vessel, thereby theoretically maintaining supply vessel
pressure.
[0007] U.S. Pat. No. 6,025,576 describes a configuration whereby
vapor phase, low vapor pressure gas is withdrawn from a heated
transport vessel that uses heaters that are only in tensioned,
non-permanent contact with transport vessel. The contaminants that
have a lower volatility than the low vapor pressure gas
preferentially remain in the liquid, producing low contaminant
level vapor. Vapor is drawn from the vessel until liquefied gas
occupies only about 10% volume of the cylinder, which brings the
contact area of the liquefied gas to below the heater level.
[0008] U.S. Pat. No. 6,614,009 discloses a system configuration
whereby vapor phase, low vapor pressure gas is withdrawn from a
large heated transport vessel (e.g. isotainer) that includes
permanently positioned heaters. These heaters are preferably
located so as to minimize direct heating above the lowest expected
liquid level to maximize purity. However, the '009 patent does not
describe a means to maximize low vapor pressure gas utilization by
maintaining a supply vessel in service until the moisture level
exceeds some value.
[0009] U.S. Pat. No. 6,581,412 describes a system whereby vapor
phase, low vapor pressure gas is withdrawn from a heated transport
vessel that employs heaters which are in contact with the transport
vessel. This patent describes a method for controlling the
temperature of a liquefied compressed gas in a supply vessel
comprising: positioning a temperature measuring means onto the wall
of the compressed gas supply vessel, monitoring the temperature of
the supply vessel and controlling heater means to heat the
liquefied gas in the supply vessel. However, the '412 patent does
not describe a means to identify the appropriate time to remove a
supply vessel from service.
[0010] U.S. Pat. No. 6,363,728 describes a means for controlling
heat input to a low vapor pressure gas contained in a heated
transport vessel. The system comprises a heat exchanger disposed on
a delivery vessel to provide or remove energy from a liquefied gas,
pressure controller for monitoring pressure and a means for
adjusting the energy delivered to the vessel contents. However, the
'728 patent does not describe a means to identify the appropriate
time to remove a supply vessel from service.
[0011] A typical, known means of addressing present operational
challenges in the industry is to remove the supply vessel from
service when the mass of low vapor pressure gas remaining in the
supply vessel falls to a pre-set value (typically from about 10% to
about 20% of the initial mass). However, this approach fails to
recognize that the key liquid level (that is, the liquid level at
which a vessel should be removed from service) will be different
depending on the key parameter that is selected (vessel pressure,
wall temperature or water level).
[0012] A significant problem exists in the field, as no useful
means exists for determining efficiently when a low vapor pressure
gas supply vessel should be removed from service. Presently known
systems risk removing a supply vessel from service too early or too
late. As a result, if the supply vessel is removed from service too
early, low vapor pressure gas will be wasted. If the supply vessel
is removed from service too late, several deleterious effects can
occur. For example, the contaminant level can build beyond
tolerable limits, resulting in adverse effects in the end use, such
as, for example, semiconductor, LED, LCD or solar cell
manufacturing processes. Such potential adverse effects include,
for example, yield loss.
SUMMARY OF THE INVENTION
[0013] According to one embodiment, the present invention is
directed to a method and apparatus for vapor phase fluid delivery
to a desired end use, wherein the conditions of the system are
monitored to determine when the water concentration or supply
vessel surface temperature exceeds a specified value or when the
low vapor pressure fluid pressure falls below a specified value for
the purpose of removing a first supply vessel from service by
discontinuing vapor flow from the first supply vessel and
initiating vapor flow from a second supply vessel. Preferably, the
liquid level at which this occurs is located near the plane
determined by the upper edges of the heaters.
[0014] In a further embodiment, the present invention is directed
to a method for delivering vapor phase fluid under pressure from a
vessel by providing at least a first and second vessel, each vessel
having a vessel wall, providing an amount of vapor phase fluid from
the first or second vessel and providing at least one heater in
communication with the first vessel wall and at least one heater in
communication with the second vessel wall. Each vessel is heated
before being brought on line to achieve a predetermined pressure
within the first and second vessel as needed. At least one heat
controller is provided in communication with the heaters for
controlling the amount of heat delivered to the first and second
vessel walls and the liquid phase fluid contained within the first
and second vessels. A device to monitor a condition selected from
the group consisting of vapor phase fluid pressure, vessel wall
temperature and vapor phase fluid water concentration in the first
and second vessels is provided for monitoring the condition
selected from the group consisting of vapor phase fluid pressure,
vessel wall temperature and vapor phase fluid water concentration
in the first and second vessels to determine the key fluid level in
the first and second vessel. A second controller is provided in
communication with the device and at least one valve having an
on/off position. The valve directs flow from the vessel to an end
use, with the second controller activating the valve on/off
position and activating the valve to an off position when the key
fluid level reaches a predetermined level in a vessel, and opens a
valve to direct vapor phase fluid from a second vessel to the end
use.
[0015] In a still further embodiment, the present invention is
directed to an apparatus and system for efficiently delivering a
vapor phase fluid to an end use. The apparatus comprises at least a
first and second vessel, each vessel having a vessel wall, and each
vessel containing an amount of vapor phase fluid. A heater is
placed in communication with the first and second vessel. A heat
controller is in communication with the heater, with the heater
controller controlling the amount of heat delivered to the first
and second vessel and the liquid phase fluid contained within the
first and second vessels. A device to monitor a condition selected
from the group consisting of vapor phase fluid pressure, vessel
wall temperature and vapor phase fluid water concentration in the
first and second vessels is placed in communication with the vapor
phase fluid. A second controller is placed in communication with
the device and with a valve having an on/off position. The valve
directs flow from the vessel to an end use, with the second
controller activating the valve on/off position to an off position
when the key fluid level reaches a predetermined level, and opens a
valve to direct vapor phase fluid from a second vessel to the end
use.
DETAILED DESCRIPTION OF THE DRAWINGS
[0016] Other objects, features, embodiments and advantages will
occur to those skilled in the art from the following description of
preferred embodiments and the accompanying drawings, in which:
[0017] FIGS. 1a and 1b are cross-sectional diagrams of conventional
supply vessel systems with heating features positioned adjacent to
the outer vessel wall.
[0018] FIG. 2 is a graph charting vapor pressure as a function of
liquid level in the vessel relative to heating units.
[0019] FIG. 3 is a graph charting vessel wall temperature as a
function of liquid level relative to heating units.
[0020] FIG. 4 is a graph charting vapor phase water concentration
as a function of liquid level relative to heaters.
[0021] FIG. 5 is a schematic diagram of a conventional low vapor
pressure fluid supply system.
[0022] FIGS. 6-8 are schematic diagrams of preferred embodiments of
the low vapor pressure fluid supply systems of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0023] Known techniques in the field of low vapor pressure high
purity gas delivery systems fail to recognize that the key liquid
level will vary depending on whether pressure degradation, vessel
wall temperature increase or water level increase is most
important. In the example cited in U.S. Pat. No. 6,025,576,
allowing the liquid level to fall below the heater would cause the
pressure to degrade and the water level to increase before the
vessel is removed from service. The '576 patent also fails to
recognize that the key liquid level will vary depending on
equipment and operational parameters, such as heater configuration
and vapor draw rate.
[0024] Co-pending and commonly assigned U.S. patent application
Ser. No. 11/476,042, filed Jun. 28, 2006 describes, according to
certain embodiments, a means for attaching heaters to the lower
portion of a supply vessel containing low vapor pressure gas. This
application states that known low vapor pressure gas supply systems
can produce "hot spots" and vigorous low vapor pressure gas
boiling, which can result in the delivery of contaminants to the
customer. This application further describes the accumulation of
moisture due to simple vapor/liquid equilibrium, and that, because
of this equilibrium based moisture accumulation, a percentage of
the low vapor pressure gas must be discarded (typically 10%-20%).
The contents of this co-pending and commonly-assigned U.S. patent
application are incorporated by reference in its entirety herein,
as if made a part of the present application.
[0025] As a result, in known systems, the supply vessel is likely
to be removed from service too early (i.e. prior to the on set of
the challenges listed above) or too late (after the supply vessel
wall temperature, water level or have exceeded acceptable limits).
If the supply vessel is removed from service too early, some of the
low vapor pressure gas that could be utilized will be wasted. If
the supply vessel is removed from service too late, one of the key
parameters could exceed acceptable limits. For example, the water
level could become too high, which would have an adverse effect on
the semiconductor, LED, LCD or solar cell manufacturer process,
resulting in poor product quality or product loss. Allowing the
water level to exceed acceptable limits could also increase the
cost of ammonia purification downstream of the supply vessel at
those sites where ammonia purification systems are utilized.
[0026] According to one embodiment of the present invention, the
systems and apparatuses of the present invention recognize and use
these variations to maximize low vapor pressure product utilization
without negatively impacting the semiconductor, LCD, LED or solar
cell manufacturing process.
[0027] It is difficult for conventional low vapor pressure gas
supply systems to consistently meet semiconductor, LED, LCD and
solar cell manufacturer requirements. For example, heat transfer
becomes ineffective when a significant portion of the heat is
applied to that portion of the supply vessel wall that is not in
contact with liquid phase low vapor pressure gas. Experiments were
conducted to determine the ability to transfer heat to liquid phase
ammonia as the liquid level falls, causing the portion of the
supply vessel wall that is in contact with liquid phase ammonia to
decrease. While ammonia was selected for illustrative purposes, the
methods and apparatuses of the present invention also lend
significant advantage to the processing of gases including, but not
limited to boron trichloride, carbon dioxide, chlorine,
dichlorosilane, halocarbons, hydrogen bromide, hydrogen chloride,
hydrogen fluoride, methylsilane, nitrous oxide, nitrogen
trifluoride, trichlorosilane, and mixtures thereof. As depicted in
FIG. 1, vapor phase ammonia was withdrawn from a supply vessel at a
constant rate via conduits 4 and 13. To replenish the withdrawn
vapor and maintain supply vessel pressure, heat was applied to the
outside, bottom surface of the supply vessel using surface mounted
heaters 3 and 12. The ability to transfer heat to the liquid phase
ammonia was determined by monitoring the vessel pressure using
pressure measuring devices 6 and 15. If heat transfer is
ineffective, the supply vessel pressure will fall.
[0028] FIG. 2 shows the pressure measured as a function of liquid
level (x-axis positive values indicate that the liquid level is
above the heater and vice versa). Note that when the liquid level
is above the heater, the supply vessel pressure is generally
sustained (heat transfer is effective). When the liquid level
approaches the heater, the supply vessel pressure is not sustained
(heat transfer is ineffective). Therefore, at some liquid level
referred to as "key pressure liquid level", the supply vessel
pressure will no longer be sustainable. This key pressure liquid
level will vary from system to system and will depend on a number
of variables, such as vapor draw rate, heater configuration, heater
temperature and contact intimacy between the heater and supply
vessel wall. The key pressure liquid level is likely to be lower
than the point at which the liquid level is equal to the heater
level, although as shown in FIG. 2, it may also be located above
the heater level.
[0029] The key liquid level will also vary from system to system
based on, for example, vapor draw rate, heater configuration,
heater temperature and contact intimacy between the heater and
supply vessel wall. For example, at low vapor draw rates, the key
pressure liquid level will be lower than at high vapor draw rates,
since the heater area required to maintain supply vessel pressure
is lower at low vapor draw rates.
[0030] The supply vessel wall temperature may increase beyond
design limits locally when a significant portion of the heat is
applied to that portion of the supply vessel wall that is not in
contact with liquid phase low vapor pressure gas. Experiments were
conducted to determine the effect of liquid level on supply vessel
wall temperature. The results are shown in FIG. 3 (x-axis positive
values indicate that the liquid level is above heater and vice
versa). It was determined that when the liquid level drops below
the key temperature liquid level, the supply vessel wall
temperature begins to increase in that portion of the supply vessel
wall that is not in contact with liquid phase low vapor pressure
gas. Supply vessels are designed to operate near ambient
temperature and typically have a very low maximum acceptable
operating temperature. A typical maximum acceptable operating
temperature is about 125.degree. F. Operating at temperatures in
excess of the maximum acceptable operating temperature is a safety
issue and could result in vessel failure. As shown in FIG. 3, this
temperature limitation is approached as the liquid level falls
below the key temperature liquid level. The key temperature liquid
level (-0.7 inches, liquid level below the heater) is different
than the key pressure liquid level (0.35 inches, liquid level above
the heater).
[0031] The low-volatility contaminant level in the vapor phase
substantially exceeds equilibrium levels when a significant portion
of the heat is applied to that portion of the supply vessel wall
that is not in contact with liquid phase low vapor pressure gas.
Because they do not evaporate readily, low-volatility contaminants
preferentially remain in the liquid phase as vapor phase low vapor
pressure gas is withdrawn from the supply vessel. As a result, as
explained above, the low-volatility contaminant concentration in
both the vapor and liquid phases increases with time.
[0032] The low-volatility contaminant level resulting from this
phenomenon is referred to as the equilibrium contaminant level.
Experiments were conducted to determine the low-volatility
contaminant level observed in vapor ammonia drawn from the supply
vessel as liquid level falls, causing the portion of the supply
vessel that is in contact with liquid phase ammonia to decrease. In
these experiments, the low-volatility contaminant was water. The
results are shown in FIG. 4. Note that the water concentration
observed as the liquid level decreases reflects the projected
equilibrium concentration until the key water liquid level is
reached. At that key water liquid level, the water concentration
substantially exceeds predicted equilibrium values. For these
experiments, the key water liquid level occurs when the liquid
level falls about to a level substantially equivalent to the heater
level.
[0033] As stated above, previously known systems fail to recognize
that the key liquid level will vary depending on whether pressure
degradation, vessel wall temperature increase or water level
increase is most important. Allowing the liquid level to fall below
the heater would cause the pressure to degrade and the water level
to increase before the vessel is removed from service. Previous
systems also fail to recognize that the key liquid level will vary
depending on equipment and operational parameters, such as heater
configuration and vapor draw rate. According to one preferred
embodiment, the present invention recognizes and uses these
variations to maximize low vapor pressure product utilization
without negatively impacting the semiconductor, LCD, LED or solar
cell manufacturing process.
[0034] Further, presently known methods and systems do not describe
a means to maximize low vapor pressure gas utilization by
maintaining a supply vessel in service until the moisture level,
wall temperature or pressure exceed some value, and further fail to
provide a means to identify the appropriate time to remove a supply
vessel from service.
[0035] When the water concentration or supply vessel surface
temperature exceeds a specified value or when the low vapor
pressure fluid pressure falls below a specified value, the supply
vessel is removed from service by discontinuing vapor flow from the
first supply vessel and initiating vapor flow from a second supply
vessel. The liquid level at which this occurs is located near the
plane determined by the upper edges of the heaters.
[0036] According to one embodiment, the present invention provides
a means to maximize low vapor pressure gas utilization without
supply vessel pressure degradation, supply vessel overheating or
high water level product delivery to the semiconductor, LCD, LED or
solar cell manufacturer. Supply vessel overheating is an issue with
respect to safe operation. Pressure degradation and high moisture
level are an issue with respect to semiconductor, LCD, LED or solar
cell yield.
[0037] FIG. 5 depicts a conventional low vapor pressure fluid
supply configuration. In general, the system intent is to deliver
liquid or two-phase low vapor pressure fluid contained in a supply
vessel to a semiconductor, LED, LCD or solar cell manufacturing
facility and to convert it into vapor phase low vapor pressure
fluid. Supply vessels 20 and 30 containing, for example, vapor and
liquid phase ammonia are installed in parallel so that as one
vessel is consumed, the other can be brought into service without
disrupting supply to the semiconductor, LED, LCD or solar cell
manufacturer. Vapor phase ammonia is withdrawn from whichever
vessel is in service via conduit 21 or 31. It is then transferred
to a gas panel 40, which regulates the ammonia pressure and
temperature prior to delivery to a semiconductor, LED, LCD or solar
cell manufacturing facility via conduit 41.
[0038] As vapor phase ammonia is withdrawn from supply vessel 20 or
30, the supply vessel pressure is maintained using one or more
heater systems 22 and 32 and a closed loop heater control means.
Typically, a pressure transducer 23 or 33 monitors the supply
vessel pressure and sends a signal to a programmable logic
controller 24 or 34, where the signal is compared to a set point
value. Based on the difference between these values, the energy
delivered to supply vessel 20 or 30 from heater system 22 or 32 is
adjusted. This facilitates vaporization of ammonia to sustain the
required supply vessel pressure.
[0039] Although a number of heater types may be employed, a common
heater type is a silicone rubber blanket heater. This silicone
rubber blanket heater may be affixed to the vessel in a variety of
ways. A typical silicon rubber heater is that available from Watlow
Electric Manufacturing Company (St. Louis, Mo.). The heater
preferably is installed so that its heat is evenly distributed to
the bottom of the vessel and such that it does not rise to too high
a level on the vessel. According to one embodiment of the present
invention, a method for discontinuing flow from the vessel is used.
If the heater rises to too high a level on the vessel, a
significant portion of the ammonia will be wasted. The heater
typically covers from about 5% to about 50% of the vessel
circumference, preferably from about 10% to about 40% of the vessel
circumference and most preferably from about 20% to about 35% of
the vessel circumference. The silicone rubber heater typically
operates at a temperature ranging from about 100 to about
500.degree. F., preferably from about 120 to about 300.degree. F.
and most preferably from about 130 to about 200.degree. F. Such a
heating configuration is preferably used with a number of supply
vessel types. For example, a horizontally mounted Y-cylinder, which
initially contains approximately 500 lbs of ammonia, could be
used.
[0040] Ammonia is withdrawn from supply vessel 20 or 30 until the
mass remaining drops to from about 10% to about 30% of the original
level. When this level is reached, the supply vessel is removed
from service and the remaining liquid, which is referred to as the
heel, is discarded. The heel is enriched in contaminants that have
a lower vapor pressure than ammonia, such as water.
[0041] Preferred embodiments of the present invention are depicted
in FIGS. 6, 7 and 8. As described previously, according to
embodiments of the present invention, the present systems and
apparatuses determine the point at which a supply vessel 20 or 30
should be removed from service. More specifically, FIG. 6 depicts a
means for determining the point at which the supply vessel 20 or 30
should be removed from service based on pressure. The pressure at
the outlet of each supply vessel 20 and 30 is monitored using
pressure transducer 23 and 33, respectively. This pressure is
maintained, typically within the range of from about 50 to about
250 psig, preferably within the range of from about 100 to about
200 psig and most preferably in the range of from about 120 to
about 180 psig. When the liquid content of supply vessel 20 or 30
drops to a level at which the desired pressure cannot be sustained
and falls below some predetermined value, a controller 64 will
cause vapor flow from the supply vessel that is in use to cease by
closing either valve 25 or valve 35, depending on which supply
vessel is in service. The switch-over pressure typically occurs
when the pressure decreases by an amount of from about 1 to about
100 psi, preferably when the pressure decreases by an amount of
from about 5 to about 50 psi and more preferably when the pressure
decreases by an amount of from about 5 to about 20 psi. Flow is
then initiated from the supply vessel that was not in service by
opening valve 25 or 35.
[0042] FIG. 7 depicts a further embodiment of the present invention
whereby a means for determining the point at which the supply
vessel 20 or 30 should be removed from service based on supply
vessel wall temperature. The vessel wall temperature is monitored
using temperature elements 74, 76 respectively. This temperature is
typically within the range of from about 0.degree. to about
125.degree. F., preferably within the range of from about
30.degree. to about 125.degree. F. and most preferably within the
range of from about 60.degree. to about 125.degree. F. When the
liquid contents of the supply vessel drop to a level at which the
surface temperature approaches the set point range, typically from
about 70.degree. to about 125.degree. F., preferably within the
range of from about 100.degree. to about 125.degree. F. and most
preferably within the range of from about 115.degree. to about
125.degree. F., a controller 78 will cause vapor flow from the
supply vessel that is in use to cease by closing either valve 25 or
valve 35, depending on which supply vessel is in service. Flow is
then initiated from the supply vessel that was not in service, by
opening valve 25 or 35.
[0043] FIG. 8 depicts a means for determining the point at which
the supply vessel 20 or 30 should be removed from service based on
water concentration. The water concentration at the outlet of each
supply vessel 20 and 30 is monitored using moisture analyzer 80.
The water concentration is typically within the range of from about
0.001 to about 10 ppm, preferably within the range of from about
0.01 to about 5 ppm and most preferably within the range of from
about 0.1 to about 2 ppm. When the liquid contents of the supply
vessel 20 or 30 drops to a level at which the water concentration
increases beyond the level predicted by vapor/liquid equilibrium, a
controller 90 will cause vapor flow from the supply vessel that is
in use to cease by closing either valve 25 or valve 35, depending
on which supply vessel is in service. Flow is then initiated from
the supply vessel that was not in service by opening valve 25 or
35.
[0044] The proposed control mechanisms can be applied to any size
vessel, such as a T-cylinder, a Y-cylinder (ton container) or an
ISO container, tube trailer or tanker that contains any desired
liquid or two phase low vapor pressure gas, such as, for example,
ammonia, thereby producing a vapor phase low vapor pressure gas
stream. For example, ton containers are typically horizontally
oriented and made from 4130X alloy steel and can contain, for
example, 510 pounds of ammonia when filled to capacity. The vessels
may be pre-filled and self-contained, or may be fillable from a
source as would be readily understood by one skilled in the field
of gas delivery systems.
[0045] A number of heater types may be used for delivering heat to
the larger vessel. The most common are electrical resistance
heaters, including blanket heaters, heating bars, cables and coils,
band heaters, and heating wires. Heaters are preferably installed
at the lower portion of the vessel and a heater controller
preferably regulates the amount of heat delivered to the low vapor
pressure gas maintaining the vapor output. Other potentially useful
heater types include, for example, bath heaters, inductive heaters,
heat exchangers that contain a heat transfer medium (such as, for
example, silicone oil), etc.
[0046] Vapor low vapor pressure non-air gas leaving the second
vessel may be further purified by, for example, adsorption,
filtration or distillation means to further improve purity. It is
further contemplated that the gas stream could be sent to a mist
eliminator to remove any liquid phase low vapor pressure gas
droplets that carry over from the supply vessel due to vigorous
boiling. These droplets would be collected by a mist eliminator,
and could be returned to the supply vessel by suitable delivery
means, such as, for example, by gravity.
[0047] While the invention has been described in detail with
reference to specific embodiments thereof, it will be apparent to
one skilled in the field that various changes, modifications and
substitutions can be made, and equivalents employed without
departing from, and are intended to be included within, the scope
of the claims.
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