U.S. patent application number 09/867000 was filed with the patent office on 2001-09-27 for thermocouple for use in gasification process.
Invention is credited to Green, Steven R., Powell, David L. JR..
Application Number | 20010024464 09/867000 |
Document ID | / |
Family ID | 22309671 |
Filed Date | 2001-09-27 |
United States Patent
Application |
20010024464 |
Kind Code |
A1 |
Green, Steven R. ; et
al. |
September 27, 2001 |
Thermocouple for use in gasification process
Abstract
An improved apparatus comprising a thermocouple for measuring
the temperature in a gasification process is provided. The
improvement comprises a sapphire envelope for enclosing at least a
portion of the thermocouple. The sapphire envelope may be in the
form of a sapphire sheath fitted over the thermocouple. The
apparatus may also comprise a thermowell, with the sapphire
envelope being provided by the thermowell.
Inventors: |
Green, Steven R.; (El
Dorado, KS) ; Powell, David L. JR.; (Augusta,
KS) |
Correspondence
Address: |
STEPHEN H. CAGLE
HOWREY, SIMON, ARNOLD & WHITE, LLP
750 BERING DRIVE
HOUSTON
TX
77057
US
|
Family ID: |
22309671 |
Appl. No.: |
09/867000 |
Filed: |
May 25, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09867000 |
May 25, 2001 |
|
|
|
09106133 |
Jun 26, 1998 |
|
|
|
Current U.S.
Class: |
374/179 ;
136/230; 136/232; 374/141; 374/E1.014 |
Current CPC
Class: |
G01K 1/10 20130101 |
Class at
Publication: |
374/179 ;
136/232; 136/230; 374/141 |
International
Class: |
G01K 001/08; G01K
001/14; G01K 013/00; H01L 035/02; G01K 007/04 |
Claims
What is claimed is:
1. In an apparatus comprising a thermocouple for measuring the
temperature in a gasification process, the improvement comprising a
sapphire envelope for enclosing at least a portion of said
thermocouple.
2. The apparatus of claim 1, said thermocouple comprising a pair of
wires of dissimilar metal content joined together at one end by a
hot junction and at the other end by a cold junction but otherwise
electrically insulated from each other by an insulating tube,
wherein the sapphire envelope is in the form of a sapphire sheath
having a closed distal end and an open end, said open end having
been fitted over the hot junction and at least a portion of the
insulating tube.
3. The apparatus of claim 1, further comprising a thermowell, said
thermowell surrounding said thermocouple and said thermowell
comprising at least one barrier layer comprised of sapphire.
4. The apparatus of claim 1, further comprising a thermowell, said
thermowell surrounding said thermocouple and said thermowell
comprising an inner protection tube and an outer protection tube,
said inner protection tube comprised of sapphire, wherein said
sapphire envelope comprises said inner protection tube.
5. The apparatus of claim 2, further comprising a thermowell,
wherein the gasification process employs as a reactor a vertical
free-flowing refractory lined cylindrical steel pressure vessel,
and wherein the thermocouple fitted in the sapphire sheath is
installed in the gasification reactor by being passed in succession
straight through a flanged reducer and into the thermowell
connected to the flanged reducer, the thermowell being installed in
the gasification reactor by first passing it through a hole in the
steel wall of the pressure vessel and by then passing it through an
aligned hole in the refractory lining the wall on the inside of the
pressure vessel.
6. The apparatus of claim 2, further comprising a thermowell, said
thermowell surrounding said thermocouple and said thermowell
comprising an inner protection tube and an outer protection
tube.
7. The apparatus of claim 6, wherein the inner protection tube is
comprised of aluminum.
8. The apparatus of claim 6, wherein the inner protection tube is
comprised of sapphire.
9. The apparatus of claim 1, wherein the thermocouple is under the
ambient pressure of the gasification process.
10. The apparatus of claim 1, wherein the temperatures to be
measured range from about 1,700.degree. F. to about 3,000.degree.
F.
11. The apparatus of claim 2, wherein the pair of wires are
comprised of platinum, rhodium, or mixtures thereof.
12. The apparatus of claim 2, wherein the insulating tube is
comprised of alumina.
13. The apparatus of claim 2, wherein the insulating tube is
comprised of sapphire.
14. In an apparatus for measuring the temperature in a gasification
process, said gasification process employing a reactor comprising a
vertical free-flow refractory lined pressure vessel, said apparatus
comprising a thermowell and one or more thermocouples, said one or
more thermocouples independently comprising a pair of wires of
dissimilar metal content joined together at one end by a hot
junction and at the other end by a cold junction but otherwise
electrically insulated from each other by an insulating tube; the
improvement comprising a sapphire envelope for enclosing at least a
portion of at least one thermocouple.
15. The apparatus of claim 14, wherein the sapphire envelope is in
the form of a sapphire sheath having a closed distal end and an
open end, said open end having been fitted over the hot junction
and at least a portion of the insulating tube of the
thermocouple.
16. The apparatus of claim 14, wherein said thermowell comprises at
least one barrier layer comprised of sapphire, wherein said
sapphire envelope comprises the at least one barrier layer
comprised of sapphire.
17. The apparatus of claim 14, wherein said thermowell comprises an
inner protection tube and an outer protection tube, said inner
protection tube comprised of sapphire, wherein said sapphire
envelope comprises said inner protection tube.
18. The apparatus of claim 14, said thermowell being installed in
the reactor by first passing it through a hole in the steel wall of
the pressure vessel and by then passing it through an aligned hole
in the refractory lining the wall on the inside of the pressure
vessel, the one or more thermocouples being installed in the
reactor by passing the thermocouples in succession straight through
a flanged reducer and into the thermowell connected to the flanged
reducer.
19. The apparatus of claim 15, said thermowell surrounding said
thermocouple and said thermowell comprising an inner protection
tube and an outer protection tube.
20. The apparatus of claim 19, wherein the inner protection tube is
comprised of alumina.
21. The apparatus of claim 19, wherein the inner protective tube is
comprised of sapphire.
22. The apparatus of claim 14, said thermowell having one or more
inner protection tubes, wherein the number of inner protection
tubes is at least equivalent to the number of thermocouples, and
wherein the distal ends of the one or more protection tubes are
positioned at different points along the length of the
thermowell.
23. The apparatus of claim 14, wherein the one or more
thermocouples are under ambient pressure of the gasification
process.
24. The apparatus of claim 14, wherein the temperatures to be
measured range from about 1,300.degree. F. to about 3,000.degree.
F.
25. The apparatus of claim 14, wherein the pair of wires are
comprised of platinum rhodium, or mixtures thereof.
26. The apparatus of claim 14, wherein the insulating tube is
comprised of alumina.
27. The apparatus of claim 14, wherein the insulating tube is
comprised of sapphire.
28. A thermocouple comprising a pair of wires of dissimilar metal
content joined together at one end by a hot junction and at the
other end by a cold junction but otherwise electrically insulated
from each other by an insulating tube, the thermocouple further
comprising a sapphire sheath having a closed distal end and an open
end, said open end having been fitted over the hot junction and at
least a portion of the insulating tube.
Description
FIELD OF THE INVENTION
[0001] This invention relates generally to a thermocouple used in a
gasification process and, more particularly, to the use of sapphire
to extend the useful life of thermocouples used in a gasification
process.
BACKGROUND AND SUMMARY OF THE INVENTION
[0002] In high temperature gasification processes, a hot partial
oxidation gas is produced from hydrocarbonaceous fuels, for example
coal. In these processes, the hydrocarbonaceous fuels are reacted
with a reactive oxygen-containing gas, such as air or oxygen, in a
gasification reactor to obtain the hot partial oxidation gas.
[0003] In a typical gasification process, the hot partial oxidation
gas will substantially comprise H.sub.2, CO, and at least one gas
from the group H.sub.2O, CO.sub.2, H.sub.2S, COS, NH.sub.3,
N.sub.2, Ar, along with particulate carbon, ash, and/or molten slag
typically containing species such as SiO.sub.2, Al.sub.2O.sub.3,
and the oxides and oxysulfides of metals such as Fe and Ca.
[0004] The hot partial oxidation gas in the gasification reactor
will commonly be at a temperature ranging from 1,700.degree. to
3,000.degree. F., and more typically in the range of about
2,000.degree. to 2,800.degree. F., and at a pressure commonly in
the range of about 1 to about 250 atmospheres, and more typically
in the range of about 15 to 150 atmospheres.
[0005] Thermocouples are commonly used for measuring temperature in
these high temperature processes. The thermocouples can be used to
measure the temperature in the gasification reactor. They may also
be used to measure the temperature in downstream process steps in
which the effluent is cooled and particulate and gaseous
contaminants are removed.
[0006] Thermocouples are pairs of wires of dissimilar metals which
are connected at both ends. The content of the wires must be
sufficiently dissimilar to allow for a difference in electrical
potential between them. Except for the ends, the two wires are
electrically insulated from each other. The electrical insulation
is commonly provided by a tube of insulating material having two
non-intersecting holes passing lengthwise through the tube. Typical
insulating materials include high temperature, high purity
ceramics, such as alumina.
[0007] When the two junctions of the wires are at different
temperatures, a difference in electrical potential exists between
them. The difference in electrical potential and therefore the
difference in temperature can be measured by a voltage measuring
instrument placed in the thermocouple circuit or alternatively by a
voltage measuring instrument that is sent signals by a transmitter
placed in the thermocouple circuit.
[0008] The choice of dissimilar metals used for the thermocouple
will vary depending on, among other things, the expected
temperature range to be measured. For instance, one type of
thermocouple commonly employed under the conditions present in a
gasification reactor has one wire that contains platinum and about
30% rhodium and a second wire that contains platinum and about 6%
rhodium. Other pairs of metals are used for different temperature
ranges.
[0009] One problem apparent with the use of thermocouples in the
environment present in a gasification process, particularly the
environment present in the gasification reactor, is the relatively
short lifespan of the thermocouples. The relatively short lifespan
is due in part to the extremely high temperatures and corrosive
atmosphere that prevails during the operation of the gasification
reactor. An unprotected thermocouple left in this environment is
quickly attacked and rendered useless. Such attack can be most
severe when the thermocouple comes into contact with molten slag
present in the reactor.
[0010] To alleviate this problem, thermocouples are commonly
inserted into a refractory thermowell disposed along the outer wall
of a gasification reactor or other exterior process surface. The
refractory thermowells would include barriers of chrome-magnesia,
high chrome, or similar slag resistant materials, and may
incorporate other refractory and non-refractory materials such as
Al.sub.2O.sub.3, MgO, and stainless steel.
[0011] When used in a gasification reactor, the thermowell may be
introduced by passing it through an opening in the outer wall of
the reactor pressure vessel. The thermowell may then pass through a
corresponding opening in a refractory material, or series of
refractory materials, commonly used to line the inner surface of
the reactor pressure vessel. The thermowell may extend into the
open space of the reactor or it may be set back at a slight
distance from the interior of the reactor.
[0012] Unfortunately, positioning the thermocouple inside a
thermowell has not provided a complete solution. Over time, molten
slag will breach the thermowell. The breach is commonly due to the
effects of erosion and corrosion as well as thermal and/or
mechanical stress. However, the breach may also be due, totally or
in part, to an inherent fault in the thermowell. The breach,
typically small initially, allows molten slag to enter the
thermowell where it can come in contact with the thermocouple,
rendering it useless.
[0013] It would therefore be beneficial to have a means to increase
the lifespan of thermocouples used in a gasification process.
[0014] In accordance with one aspect of the present invention, an
improved apparatus comprising a thermocouple for measuring the
temperature in a gasification process. The improvement comprises a
sapphire envelope for enclosing at least a portion of the
thermocouple. The sapphire envelope may be in the form of a
sapphire sheath fitted over the thermocouple. The apparatus may
also comprise a thermowell, with the sapphire envelope being
provided by the thermowell.
[0015] In accordance with another aspect of the invention, an
improved apparatus for measuring the temperature in a gasification
process comprising a thermowell and one or more thermocouples is
provided. The improvement comprises a sapphire envelope for
enclosing at least a portion of at least one thermocouple. The
sapphire envelope may be in the form of a sapphire sheath fitted
over the thermocouple. The thermowell may contain at least one
barrier layer comprised of sapphire, with the sapphire envelope
being equivalent to the barrier layer comprised of sapphire.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 depicts a thermocouple produced in accordance with
one aspect of the invention.
[0017] FIG. 2 depicts a segmentary view in cross-section of a
portion of gasification reactor wall in which a thermocouple and
thermowell are installed in accordance with one aspect of the
invention.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0018] Gaseous mixtures substantially comprising H.sub.2, CO, and
at least one gas from the group H.sub.2O, CO.sub.2, H.sub.2S, COS,
NH.sub.3, N.sub.2, Ar, along with particulate carbon, ash and/or
molten slag typically containing species such as SiO.sub.2,
Al.sub.2O.sub.3, and the oxides and oxysulfides of metals such as
Fe and Ca are commonly produced by well known partial oxidation
processes in the reaction zone of a free-flow, down-flowing
vertical refractory lined steel pressure vessel. An example of such
a process and pressure vessel are shown and described in coassigned
U.S. Pat. No. 2,818,326 hereby incorporated by reference. In such a
process, the partial oxidation gas will typically be subjected to
cooling and additional purification steps in which particulate
contaminants, gaseous contaminants, and water vapor are
removed.
[0019] The partial oxidation gas produced from such a process will,
depending on chemical composition and intended end use, commonly be
referred to as synthesis gas, fuel gas, or reducing gas. The
generic partial oxidation gas will be referred to herein as
encompassing all of these potentialities.
[0020] The feed used to produce the partial oxidation gas comprises
hydrocarbonaceous fuels. The term "hydrocarbonaceous" as used
herein to describe various suitable feedstocks is intended to
include gaseous, liquid, and solid hydrocarbons, carbonaceous
materials, and mixtures thereof. In fact, substantially any
combustible carbon-containing organic material, or slurries
thereof, may be included within the definition of the term
"hydrocarbonaceous". For example, there are (1) pumpable slurries
of solid carbonaceous fuels, such as particulate carbon dispersed
in a vaporizable liquid carrier, such as water, liquid hydrocarbon
fuel, and mixtures thereof; and (2) gas-liquid-solid dispersions,
such as atomized liquid hydrocarbon fuel and particulate carbon
dispersed in a temperature moderating gas.
[0021] The term "liquid hydrocarbon," as used herein to describe
suitable liquid feedstocks, is intended to include various
materials, such as liquefied petroleum gas, petroleum distillates
and residua, gasoline, naphtha, kerosene, crude petroleum, asphalt,
gas oil, residual oil, tar-sand oil and shale oil, coal derived
oil, aromatic hydrocarbons (such as benzene, toluene, xylene
fractions), coal tar, cycle gas oil from fluid-catalytic-cracking
operations, furfural extract of coker gas oil, and mixtures
thereof.
[0022] "Gaseous hydrocarbons," as used herein to describe suitable
gaseous feedstocks, include methane, ethane, propane, butane,
pentane, natural gas, coke-oven gas, refinery gas, acetylene tail
gas, ethylene off-gas, and mixtures thereof.
[0023] "Solid hydrocarbon fuels," as used herein to describe
suitable solid feedstocks, include, coal in the form of anthracite,
bituminous, subbituminous; lignite; coke; residue derived from coal
liquefaction; peat; oil shale; tar sands; petroleum coke; pitch;
particulate carbon (soot or ash); solid carbon-containing waste
materials, such as sewage; and mixtures thereof.
[0024] Solid, gaseous, and liquid feeds may be mixed and used
simultaneously; and these may include paraffinic, olefinic,
acetylenic, naphthenic, and aromatic compounds in any proportion.
Also included within the definition of the term "hydrocarbonaceous"
are oxygenated hydrocarbonaceous organic materials including
carbohydrates, cellulosic materials, aldehydes, organic acids,
alcohols, ketones, oxygenated fuel oil, waste liquids and
by-products from chemical processes containing oxygenated
hydrocarbonaceous organic materials, and mixtures thereof.
[0025] In the reaction zone of a gasification reactor, the
hydrocarbonaceous fuel is contacted with a free-oxygen containing
gas, optionally in the presence of a temperature moderator. The
reaction time will typically be in the range of about 1 to 10
seconds, and preferably about 2 to 6 seconds. In the reaction zone,
the contents will commonly reach temperatures in the range of about
1,700.degree. to 3,000.degree. F., and more typically in the range
of about 2,000.degree. to 2,800.degree. F. Pressure will typically
be in the range of about 1 to about 250 atmospheres, and more
typically in the range of about 15 to about 150 atmospheres. As the
partial oxidation gas proceeds downstream, the temperature of the
flow will be reduced as the gas is subjected to various cooling,
washing, and other steps.
[0026] In accordance with the present invention, temperature may be
measured at various locations within the gasification process by
thermocouples having employed therewith a sapphire envelope. The
use of a sapphire envelope in accordance with the various
embodiments of the invention, amongst other advantages, increases
the useful life of the thermocouple over conventional
thermocouples. In its various embodiments, the sapphire envelope
will enclose at least a portion of a thermocouple with which it is
employed. The use of the sapphire envelope is particularly
advantageous when it is used in conjunction with thermocouples
positioned so as to measure the temperature in the gasification
reactor, as the detrimental effects of high temperatures, molten
slag, and corrosives are most prevalent in the reactor.
[0027] In one embodiment of the present invention, the sapphire
envelope is manifested in the form of a sapphire sheath 24 that
fits over at least a portion of a thermocouple. In this embodiment,
illustrated in FIG. 1, a thermocouple 10 is provided. The
thermocouple 10 is comprised of a pair of wires 12 and 14. The
wires have dissimilar metal content such that a difference in
electrical potential can develop between them when the thermocouple
is exposed to a heat source. The wires, for example, may both
contain platinum and rhodium as their primary substituents with the
amounts of platinum and rhodium being different in the two wires.
Preferably one of the wires has about 30% rhodium while the other
wire has about 6% rhodium. For both wires, the remainder is
primarily platinum.
[0028] The wires are joined to each other at a hot junction 16 and
cold junction 18. The terms "hot" and "cold" are used because when
employed to measure the temperature of a gasification reactor the
hot junction 16 is positioned closer to the heat source. The
difference between the electrical potential of the two wires, being
representative of the temperature at the hot end, is measured. It
is not critical how the difference in potential is measured. In
fact, various means are known to those of ordinary skill in the art
for measuring the difference in electrical potential. Any of these
methods can be used in the present invention. For example, a
voltage meter can be placed in the thermocouple circuit.
Alternatively, and preferably, the cold junction 18 is provided at
a temperature transmitter. The signal generated by the temperature
transmitter can then be relayed to a control room or other location
by signal transfer means 20.
[0029] Except for the hot and cold junctions, the two wires 12 and
14 are otherwise electrically insulated from each other. While it
is not critical how insulated, in this embodiment, the electrical
insulation 22 is provided by a high temperature, high purity
ceramic tube. Such a ceramic tube can be made of, for example,
alumina.
[0030] If the thermocouple disclosed to this point was utilized
alone or in combination with a typical thermowell in order to
measure the temperature of a gasification reactor, the thermocouple
would, as disclosed, succumb to the slag and other detrimental
materials present in the reactor relatively quickly. It is for this
reason that in the present embodiment a sapphire sheath 24 is
provided to fit over at least a portion of the thermocouple. The
sapphire sheath 24 is substantially resistant to attack from the
slag and other products of the gasification process. The completed
thermocouple, comprising the improved sapphire sheath 24, can thus
be viewed as having a distal end 26 adjacent to the hot junction
16.
[0031] It is necessary that the sapphire sheath 24 enclose at least
the hot junction 16. Preferably, and as subsequently detailed, the
sapphire sheath 24 will be of sufficient length such that before
the molten slag reaches the top of the sapphire sheath 24 the
molten slag will cool and reach a state of nominal or zero flow or
a breach will form at some other point on the sapphire sheath.
[0032] In the present embodiment, the sapphire sheath 24 is
substantially tubular having an enclosed end, being equivalent to
the distal end 26 of the thermocouple, and an open end 28, the
opening at the open end 28 being capable of receiving and fitting
over the existing thermocouple comprised of the two wires 12 and 14
and the electrical insulation 22 surrounding and insulating the
wires. In a variation of the embodiment, an enlarged plug 30 of
sapphire is provided at the enclosed end of the sapphire sheath 24.
The enlarged plug 30 increases the time it takes for the slag to
penetrate the sapphire sheath 24. The presence of enlarged plug 30,
in its simplest form, may be due to the fact that the sheath may be
inherently thicker at the enclosed end than on its sides.
[0033] In the present embodiment, the sapphire sheath 24 fits over
and covers only a portion of the existing thermocouple. The open
end 28 advantageously should fit tightly over electrical insulation
22. Platinum foil wrapped around the electrical insulation 22 or
wrapped around the inner surface of the sheath 24 can be
advantageously used to provide a good fit for the sapphire sheath
24. In other embodiments, the sapphire sheath 24 may extend over
and cover a larger portion, if not substantially all of the
existing thermocouple. In still other embodiments, sapphire may be
used to both electrically insulate the two wires as well as sheath
the wires. In such an embodiment, the sapphire sheath 24 and the
electrical insulation 22 would both be comprised of sapphire.
[0034] In other embodiments of the invention, any one of the
disclosed thermocouples having a sapphire sheath 24 is
advantageously combined with a thermowell. The combined apparatus
is advantageously used to measure the temperature in a gasification
process, particularly in a gasification reactor. Any thermowell
commonly used or subsequently developed by one of ordinary skill in
the art can be employed. Such thermowells would include barriers of
chrome-magnesia, high chrome, or similar slag resistant materials,
and may incorporate other refractory and non-refractory materials
such as Al.sub.2O.sub.3, MgO, and stainless steel.
[0035] In a preferred thermowell, illustrated in combination with a
thermocouple of the present invention in FIG. 2, the thermowell is
comprised of an inner protection tube 62 and an outer protection
tube 64. The inner protection tube 62 can be formed from a high
density low porosity refractory, such as alumina or magnesia. A
castable refractory material, typically a high density low porosity
refractory, is then poured around the inner protection tube 62 and
allowed to set so as to form the outer protection tube 64 around
all but the opening of the inner protection tube 62. Preferably,
this castable high density low porosity refractory material is
comprised of chromium oxide or chromia-magnesia.
[0036] In this embodiment the thermocouple 10 is inserted into the
thermowell, distal end 26 first. The thermocouple 10 is passed
through a flanged reducer 76 and into the thermowell in contact
with and mated to the flanged reducer 76. The distal end 26 of the
thermocouple 10 is positioned adjacent to the tip 66 of the
thermowell. A gap of about 0.125 to about 0.25 inches is preferably
maintained between the inside surface of tip 66 of the thermowell
and the distal end 26 of the thermocouple.
[0037] The upstream ends of the wires 12 and 14 of the thermocouple
10 extend past the back end of the electrical insulation 22, and/or
the sapphire sheath 24 if the sheath is coterminous with the
electrical insulation 22. The wires pass through a pressure sealing
fitting 70. The pressure sealing fitting 70 contacts a bushing 72
which fits into a removable flange 74. The flange 74 mates with
flange reducer 76 that is mated to the outer steel wall 40 of the
pressure vessel gasification reactor.
[0038] The thermocouple 10 and thermowell assembly is held in place
by bolting or clamping together flange 74 to flange reducer 76 and
similarly bolting or clamping together flange reducer 76 to the
outer steel wall 40 of the pressure vessel gasification reactor.
The use of two separate connections provides for increased
efficiency in that a thermocouple 10 can be replaced without
removing the thermowell. Instead of mating flanges, threaded caps
and nozzles or other connection means can be used.
[0039] The thermowell, with or without attached thermocouple 10, is
passed in succession straight through a hole in the steel wall 40
of the pressure vessel gasification reactor and then through an
aligned hole in the refractory 42 lining the wall on the inside of
the pressure vessel. The tip 66 of the thermowell is preferably
positioned so as to be retracted from about 0.25 to about 0.75
inches, preferably 0.5 inches, from the face of the refractory 42
lining the inside steel wall of the pressure vessel reactor. In
this manner, the rate of erosion is reduced as opposed to when the
tip 66 of the thermowell is positioned even with the face of the
refractory 42 or beyond the face of the refractory 42.
[0040] The thermocouple 10 and thermowell assembly positioned in a
gasification reactor exhibits increased resistance to slag. In the
gasification reactor, molten slag 50 deposits out on the inside
walls of the refractory 42 lining the inside steel wall of the
pressure vessel reactor. The molten slag 50 will migrate toward the
thermowell. As disclosed, over time the effects of erosion and
corrosion as well as thermal and/or mechanical stress may cause a
small breach in the tip 66 of the thermowell. When this occurs, the
molten slag 50 will, moving toward cool spots, migrate through the
breach and enter the inner protection tube 62, thereby coming in
contact with the sapphire sheathed thermocouple. Advantageously,
with the sapphire sheath 24, the wires 12 and 14 and hot junction
16 are shielded from the molten slag 50 and its destructive
effects. The molten slag 50 will continue to migrate up the
interior of the inner protection tube 62 until it cools to the
point at which it achieves a state of zero or nominal flow. Because
of this, the sapphire sheath 24 should be of sufficient length such
that before the slag can reach the open end 28 of the sapphire
sheath 24, one of two things will occur: the molten slag 50 will
achieve thermal equilibrium, cool, and achieve a state of nominal
or zero flow; or a breach will form at some other point on the
sapphire sheath. This second possibility might occur first when the
effects of erosion and corrosion as well as thermal and/or
mechanical stress cause the entire tip 66 of the thermowell to be
removed. When this occurs, the sapphire sheath 24 becomes onset by
the full effects of erosion and corrosion in the gasification
reactor. A breach ultimately forms in the sapphire sheath 24. With
the wires 12 and 14 and the hot junction 16 unprotected, the
thermocouple 10 fails. The selection of an appropriate length for
the sapphire sheath 24 is within the skill of one of ordinary skill
in the art having knowledge of the characteristics of their
specific process, including temperature and gas composition, and
having the benefit of this disclosure.
[0041] In other embodiments, one or more, and preferably three,
thermocouples are inserted into a thermowell having at least a
corresponding number of inner protection tubes 62. In such a
preferred embodiment, the distal ends of the one or more
thermocouples are advantageously positioned at different points
along the length of the thermowell. This arrangement provides for
increased times between thermocouple and thermowell replacement.
For example, in an embodiment in which a total of three
thermocouples are used, slag ultimately penetrating the thermowell
will reach the thermocouple positioned closest to the tip 66 first.
This thermocouple will subsequently fail. It then takes an
additional amount of time for the slag to reach and cause the
failures of the second and third thermocouples. Thus, the process
can be run longer without need for shut down. While the accuracy
provided by the second and third thermocouples is not as good as
the first thermocouple, the difference does not pose a problem for
process control as the readings for the second and third
thermocouples may be corrected based on data gathered prior to the
failure of the first thermocouple.
[0042] In other embodiments, the sapphire envelope can be provided
by utilizing a thermowell fabricated wholly or in part from
sapphire. Such a thermowell could have sapphire, preferably in the
form of sapphire fiber, intermixed throughout the thermowell. Such
a thermowell could also have at least one substantially continuous
barrier layer comprised of sapphire. These thermowells could be
used with a thermocouple that did not have a separate sapphire
sheath. Alternatively, these thermowells could be employed with
sapphire sheathed thermocouples. In one illustrative embodiment of
a thermowell having at least one substantially continuous barrier
layer comprised of sapphire, an inner protection tube 62 of the
thermowell could be formed of sapphire.
[0043] In other embodiments, a thermocouple having a sapphire
sheath could be used without a thermowell to measure the
temperature in a gasification process. However, this alternative is
not preferred where the thermocouple would be exposed to molten
slag. While a thermocouple sheathed with sapphire will withstand
the full effects of erosion and corrosion in the gasification
reactor for a longer time than a thermocouple not having a sapphire
shield, the use of a thermocouple having a sapphire shield in
conjunction with a thermowell dramatically increases the lifespan
of the thermocouple so used.
* * * * *