Note: Descriptions are shown in the official language in which they were submitted.
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AIR SEPA~TION METHOD AND APPARATUS TO PRODUCE NITROGEN
p~FT A.TFn APPT ~CAT~ONS
lhis is a co~ t;on-in-part of U.S. Patent Application 08/374,060 filed ~anuary
19, 1995, which was a co~ aLion-in-part of 08/329,035 filed October 25, 1994.
RACKGROUNn OF THF INVFNTION
The present invention relates to a method of sepalatillg air by a low te~lpe.al~lre
rectification process employing a ~iicti~ on column to produce a nitrogen product. More
particul~rly, the present invention relates to such a method and a~p~al~s in which a
portion of oxygen enriched liquid produced as a column bottoms in the ~lictill~tion column
S vaponzed and then c~p~ndPd to supply refrigeration and another portion of the oxygen
enriched liquid, after eXp~ncion~ is used to c~dPnce nitrogen vapor in a head c~ndencPr
~tt~rhPd to the distillation column. Even more particularly, the present invention relates
to such a method and app~allls in which the portion of the oxygen enriched liquid is
vaporized by a part of the inr~ming air and under certain conditions an additional
10 condencing stream of lesser oxygen content than air withdrawn from the tlictill~tion
column. The part of inComing air and the additional cond,oncinp strearn are thereby
liquefied and introduced into the column as additional reflux streams to m~int~in the rate
and/or concenL~alion of nitrogen production to prior art levels.
Nitrogen is produced by low telll~.dl~ rectification of the air in an air sep ation
plant. Often such plants employ a single ~ictill~tion column and are known in the art as
Ditrogen generators. After air has been filtered, collll,lesscd and purified, the air is cooled
to a tc~ a~ suitable for its rectification. This te.,l~ldl lre is norrnally at or near the
dew point of the air. Thereafter, the air is introduced into a distillation column having
20 liquid-vapor contarting elements which can be forrned by trays and/or paC~inec~ either
structured or random. In the distillation column an ~cc~n~ing vapor phase of the air is
~159~08
cont~ ted by a descendinp liquid phase. The result of such cont~ct is that the liquid phace
becoll,es cve~nore concen~aled in oxygen to produce a oxygen enriched liquid column
bottoms and the PCc~ ing vapor phase becollles eve~nore concentrated in nitrogen to
produce a nitrogen rich ~-apor tower overhead.
ln order to reflux the column, a head condenser is provided in which the nitrogen
vapor tower overhead is partially condence~ The con~lPnc~e is retumed to the rli~till~tion
column as reflux. Typically, an oxygen cnriched liquid stream composed of the column
bottoms is ~ lo~ed, exr~r~ded to a low ~Il~.a~lre, and then introduced as the coolant
10 for the head condenser. The product is removed from the top region of the column
mostly as a vapor.
ln any type of air separation plant, there is ~lltillual heat leakage into the plant
and enthalpy differences bel..~en the air feed and product streams at the warm end of the
lS plant. Such heat leakage ll~lileS refrigeration to bc supplied to the air separation plant.
If the nitrogen product is to be rn~in1~in~Pd at column pressure, refrigeration is generally
supplied from outside the column envelope. Work expansion obtained from the vaporized
oxygen enriched liquid, all of which is vaporized in the head conflencpr~ or by exp~nding
air from a higher pressure down to column pressure are usual methods of supplying
20 refrigeration. There are also "liquid assist plants" in which liquid nitrogen is added to the
column from an external source in order to supply the requisite refrigeration.
As will be ~ cu~se~l the present invention relates to an air separation technique
in which refrigcration is generated in a manner that rcduces the cnergy PYpen~iture in
2S producing a nitrogen product This is Aec4...plished by more efficiently using energy for
air separation and making cnergy, folll,elly in excess, available for refrigeration.
SUMM~Y OF THF lNVF~llON
The present invention provides a method of separating air to produce a nitrogen
product. In accordance with the method, the air is separated by a low te~ re
- 2~59308
rectification process employing a tlictill~tion column to produce an oxygen rich liquid
column bottoms and a nitrogen rich vapor tower overhead. A head condenser is provided
to condense at least part of the nitrogen rich vapor tower overhead to reflux the
till~tion column.
In one aspect of the present invention, the low te~pe.dlulc rectification process
includes partially vaporizing an oxygen enriched liquid stream composed of the oxygen
rich liquid column bottoms. The oxygen enriched liquid s~eam is thereaRer separated
into liquid and vapor phases and a liquid phase stream composed of the liquid phase is
10 expanded to create a tc~ a~ difference between the liquid phase stream and the
nitrogen rich vapor tower overhead. The liquid phase stream is introduced into the head
condenser as a coolant stream so thdt heat is transferred from the at least part of the
nitrogen rich vapor to the coolant stredm to thereby cause the condenc~tion of the at least
part of the nitrogen rich vapor tower overhead. A vapor phase stream, composed of the
15 vapor phase is exr~nded with the performance of work to produce a refrigerant stream
utilizèd to at least partially refrigerate the low tem~.dl~re rectification process. A
product stream is extracted from a rem~inin~ part of the nitrogen rich vapor tower
overhead, not utilized in the llictill~tion column as the reflux, to form the nitrogen product.
In another aspect ofthe present invention, the low te~,dtLlre rectification process
includes dividing an oxygen enriched liquid stream composed of the oxygen rich liquid
column bottoms into first and second partial streams. The first partial stream is exr~n~ed
to create a tell~pela~lre difference between the first partial stream and the nitrogen rich
vapor tower overhead. The first partial stream is introduced as a coolant stream into the
head condenser so that heat is ~arLferred from the at least part of ~e nitrogen rich vapor
to the coolant stream thereby c~lcing condenc~tion of the at least part of the nitrogen rich
vapor tower ov~rhead. The æcond partial stream is ~a~l~d and then partially warrned
aflLer having been vaporized. The second par~al stream is eYI-~nded with the ~,~l~lance
of work to produce a refrigerant stream utilized at least to partially refrigerate the low
tcll~clal~lre rèctification process. A product stream is extracted from a rem~ining part of
~lS9308
the nitrogen rich vapor tower overhead not utiliæd in the distillation column as the reflux
to form the nitrogen product.
The present invendon also provides an a~p~alus for S~alil~g air to produce a
S nitrogen product. In accordance with the app~allls~ a filter is provided for filtering the
ur and a CClllpleSSOl iS col~l.P~t ~ to the filter for co~plescing ~e ur. An after-cooler
is provided for removing heat of co~ ,lession from the air and a pre-purification unit is
provided for punfying the air. A rnain heat exchAnee means cools the air to a te~ al~e
suitable for its rectification uld a lictillAtioD column is configured to rectify the air into
10 an oxygen rich liquid column bottoms and a nitrogen rich vapor tower overhead. A head
condenser is connected to the Aictill~tion column to c~ence at least part of the nitrogen
rich vapor tower overhead for reflux to the Aictill~tion column.
In accordance with a further aspect of the present invention, a vaporization means
15 is col-~.ecle~ to the lictillAtion column for partially vaporizing an oxygen enriched liquid
stream composed of the oxygen rich liquid column bottoms and a phase separator is
conn~ cted to the vaporization means for sepa.ating the oxygen enriched liquid strearn into
liquid and vapor phases. The phase sep rator is connP~led to the head co~dencer so that
heat is transferred from the at least part of the nitrogen rich vapor to a coolant stream
20 made up of a liquid phase strearn composed of the liquid phase. The result is to cause
con~nc~tion of the at least part of the nitrogen rich vapor tower overhead and
vaporization of the coolant stream to form a vapo,.2~d coolant stream th~lcf~om. A
~leaàu,c reduction valve is interposed between the phase separator and the head con~ncer
to cxpand the liquid phase stream and thereby create the coolant stream and a t~ lu,e
25 difI~.e.,ce between the coolant stream and the nitrogen rich vapor tower ovahead. The
phase separator is also con~ t~ to the main heat PYrh~nee means so that the vapor
phase stream co..,posed of the vapor phase partially wa~ns. An expansion means is
connected to the main heat exchange means for eyr~n~ine the vapor phase strearn with
the performance of work to produce a refrigerant strearn. The main heat exchange m~nc
30 is in comrnunication with the expansion means so that the refrigerant strearn fully warrns
within the main heat exchange means. A means is provided for extracting a product
- - 2159308
stream composed of a rem~inin~ part of the nitrogen rich vapor tower overhead, not
utilized in the tlictill~tion column as the reflux, to forrn the nitrogen product and the main
heat exchange means is connP~l~d to the product stream cxtracting means so that the
product stream fully warms within the main heat exchange means.
S
In accordance with a still further aspect of the present invention, the head
conden~er is connl~c~ed to the (lictill~tion colllmn so that heat is transferred from the at
least part of the nitrogen rich vapor to a coolant st~earn made up of a firct partial stream
composed of the oxygen rich liquid colurnn bottorns. This causes the contlen~tion of the
10 least part of the nitrogen rich vapor tower overhead and vaporization of the coolant stream
to form a vaporized coolant stream. A pressu,e reduction valve is interposed between the
~ictill~tion column and the head condenser to expand the fir t partia] stream and thereby
create the coolant stream and a te~latule difference between the coolant stream and the
rlitrogen rich vapor tower overhead. A vaporization means is conn~cted to the ~ictill~tion
15 column for vaporizing a second partial strearn ~.ll~osed ofthe oxygen rich liquid column
bottoms. The vaporization means is also com~ ed to the main heat exchange means so
that the second partial strearn partially warms. An expansion meanc is conn~cted to the
main heat exchange means for expanding the second partial stream with the performance
of work to produce a refrigerant strearn. The main heat exchange means is in
20 cornmunication with the exparlsion means so that the refrigerant stTeam fillly warms
within the main heat cxchange means. A means is provided for extracting a product
stream composed of the reTn~ining part of the nitrogen rich vapor tower overhead not
utilized in the dictill~tion colurnn as the reflux to form the nitrogen product. The main
heat ~xch~n~e means is also connecl~ to the product stream extracting means so that the
25 product stream fully warms within the meat exrh~nge means.
The present invention functions by talcing advantage of the larger-than-nPc~s~ ydriving forces that are employed in the ~ tjoD of air to create the nitrogen product.
In the present inven~ion, the oxygen enriched liquid acts as a coolant for con~en~ing
30 reflux to the column and serves to supply at least part of the refrigeration needs of the
plant, independent of aforementioned typical refrigeration processes
~15 9308
Since not all of the oxygen cnriched liquid is being utilized in a reflux
condencAtion role, there is potentially an insufflcient supply of reflux produced by the
head condenser. In order to co~ e for such reduced reflux production, intermediate
reflux can be supplie~, at the vcry least, by liquid air and ~.efc.e..tially by both liquid air
5 and another reflux stream of lesser oxygen content than air. Thus, in yet still another
aspect, thc present invention enco...~ ~s a method in which the oxygen cnriched liquid
stream or a part thereof is partially or wholly vaporized by indirectly ~xch~nging heat
with part of the air to bc separated and preferably, with another vapor stream withdrawn
from the column of lesser oxygen content than air, thereby callcing the part of the air to
10 be separated and if present, the other vapor strearn to liquefy. The part of the air to be
separated and preferably, the other liquefied vapor strearn withdrawn from the colurnn are
then introduced into the dictil~tion column as i~ ,...r~;~te reflux streams to m~int~in
production of the product strearn at a level that would have been obtained had the entire
oxygen rich liquid stream been utilized to condense the at leact part of the nitrogen rich
15 vapor tower o~. lLead. Prior to the partial ~a~.i~alion of the oxygen rich liquid stream
or the complete vapol~ion of part of the oxygen enrich liquid stream, the oxygenenriched liquid is eYp~ e~ to produce a te~lalule difference for the indirect heat
eYch~nge with the part of the air and preferably, if present, the vapor strearn withdrawn
from the column.
nFSCRTPTlON OF THF nRA~lING
While the specification concludes with claims distinctly pointing out the subject
matt that Applicant regards as his invention, it is believed the invention will be better
2~ understood when taken in com1e~lion with the ~ ...p~-ying drawing in which:
Fig. 1 is a sch~m~tic illustration of an air separation plant ope.dtiilg in accordance
with a method and a~p~al~s of the present invention; and
2159308
Fig. 2 is an altemative embodiment of Fig. 1. In order to avoid needless repetition
in the explanation of Fig. 2, the numbering scheme used in Fig. I is carried over to Fig.
2 for co,llponents and streams tht share common function.
nFT,~TT.F.n nF,SCRTPTION
With ,~fe.ence to Fig. 1, a single column nitrogen generator 10 is illustrated. An
~c~ing air strearn 12 is filtered by a filter 14 to remove dust particles and the like. Air
stream 12, after having been filtered7 is coll~ ess~d by a colllplessor 16 and thereafter,
10 the heat of colllpless;on is removed by a conventional after-cooler 18. Water, carbon
dioxide and heavy trace colllponents of the air such as hydrocarbons are removed by a
pre-purification unit 20 conn~cted to aftercooler 18. Pre-purification unit 20 can comprise
several beds of adsorbent o~,~Ling out of phase for regeneration purposes.
Air stream 12 having thus been filtered and purified is then introduced into a main
heat eyrh~nger 22. The air to be separated enters main heat exçh~n~er 22 and is then
fully cooled to a temp~ suitable for its rectification. In this regard, the term "fully
cooled" as used herein and in the claims means cooled to a te.llpe,alule at which the
rectification is conduct~ The tcrm "fully warmed" as used herein and in the claims
means warrned to a telllpe~atulc of the warm end of main heat exchanger 22. The term
"partially warrned" meanc warmed to a temperature above the rectification te.ll~,al lre
but below the telllpelal~e of the warrn end of main heat exchanger 22.
After having been fully cooled within main heat exch~nPer 22, air stream 12 is
then divided into first and second subsidiary streams 24 and 26 ~s~cti~ely. A junction,
fo~med by T-sections of pipe, headers and the like, is co.-..ccted to main heat eYrh~nger
for this ~ ose. First subsidiary stream 24 co~slituhs a major portion of the air to be
separated and is introduced into a single ~ tillAtion column 30 which is provided by
liquid-vapor co~t~rting elements 32, 34 and 36 which can be trays and/or structured
30 p~r~in~, random p~c~ing and etc. Distillation column 30 rectifies the incoming air into
an oxygen rich liquid column bottoms that collects within bottom region 38 of distillation
- ~159308
column 30 and a rlitrogen rich vapor tower overhead which collects in a top region 40 of
distillation column 30. A head condenser 42 is connect~Pd to tlictill~tion column 30 to
condPnce at least part of the nitrogen rich vapor tower overhead collected in top region
40 of ~ictill~tion column 30. To this cnd, part of a nitrogen vapor stream 44 is cxtracted
5 from top region 40 of listill~tion column 30 and is introduced into head cor ~enCpr 42.
Nitrogen vapor stream 44 is in part cond~pnce~ by a coolant s$ream 46, which in turn
~a~o~ s to produce a va~ol;~d coolant strearn 47. After c~n~Pn~tion~ nitrogen vapor
stream 44 is ~ ed as a reflux stream 48 and to top region 40 of ~ictill~tion column 30.
An oxygen enriched liquid stream 50, composed ofthe oxygen rich liquid column
bonoms, is extracted from bottom region 38 of iictill~tion column 30. Oxygen enriched
liquid stream 50 can then be preferably subcooled within a subcooler unit 52 to minimi7P
vapor formation upon subsequent valve expansion. Thereafter, oxygen cnriched liquid
stream 50 is partially va~olized within a vaporizer 54 after having passed through a
15 ples~ reduction valve 55 (described in more detail hereinafter) and then introduced into
a phase se~a~or 56 to separate oxygen enriched liquid s$ream 50 into liquid and vapor
phases.
A liquid phase stream 58 colllposed of the liquid phase is extracted from phase
20 separator 56 and is then passed through a pres~ule reduction valve 60 to sufficiently lower
the t~,ll~,al~lre of liquid phase stream 58 that it can serve as the coolant for head
condenser 42. Thus, liquid phase stream 58 after passage through plC5~Llle reduction
valve 60 is converted into coolant stream 46 which has been dicc~lcsed hereinabove.
Phace separator 56 is also conn~le~d to the main heat exch~nger 22 so that a vapor
phase stream 62, cu~ os~ of the vapor phase, partially wa~ns within main heat
e~k~ .~e~ 22. Vapor phase stream 62 af~er having been partially warmed is exr~nde~ in
a ~ der 64 or other cxpansion m~chin~ connPc1ed to main he~at exch~n~er 22.
The expansion ûf vapor phase strearn 62 produces a refrigerant strearn 66.
2159308
In the illustrated embodi~nent, re~igerant strearn 66 also partially warms within
subcooler unit 52 as does vaporized coolant strearn 47 and a product stream 68. As
illustrated, vaporized coolant strearn 47, aRer subcooler unit 52, fillly wanns within main
beat exr-h~nPer 22 to form a waste nitrogen stream labelled WN~. Part of warrn vaporized
5 coolant stream 47 can be fed to pre-purification unit 20 for bed 1~ gene.aLion purposes.
Main heat eYr-h~nper 22 is in co~ul~ication with turboeYr~n-ler 64 so that refrigerant
stream 66 cventually fillly warms within main heat cyr~n-er 22 and is discha~ged as a
waste st~eam, desigr ~tcd as WN2. A product strcam 68 is formed which is con~posed of
the nitrogen vapor tower overhead collected in top region 40 of ~ictill3tion column 30.
10 Product stream 68 co~liLuLes a reTn~ining portion of the nitrogen vapor tower overhead
that is not used in fo~ming reflux to ~lictill~tion column 30. After partial warming in
subcooling unit 52, product strearn 68 fuIly warms within main heat eY~h~nger 22 and is
discharged as a product stream, labelled PN. Against the partial wam~ing of the foregoing
mentioned steams, as mentioned previously, oxygen enriched liquid strearn 50 subcools.
In single col-~mn nitrogen generator 10 oxygen enriched liquid strearn 50 is
partially vaporized in vaporizer 54 and thus, only part of oxygen enriched liquid stream
50 is used as coolant for head conden~r 42. As a result, there is less reflux contributed
through condenc~tion of tower overhead in single colu nn nitrogen generator 10 than in
20 a nitrogen generator of the prior art. If no other reflux were added (an operation
col-t~.,.}-lated by the subject invention) a nitrogen generator ofthe present invention would
have a lower production rate and/or produce nitrogen at a lower purity than a prior art
desigIL However, the present invention also c~ nt~nplates an operationa] embodiment in
which a c~,.,~ ,c,.l;on for such reduced reflux is effected by the provision of i,.~ .",~Ai~t~
25 reflux s~ns introduced into lower portions of lictill~tion column 30 where additional
liquid reflux is particularly nPedetl
The int~rmPdi~te reflux allows single column nitrogeD ge,le,8lor 10 to have the
sarne production rate of product and purity as could be cxpected in a similar prior art
30 plant design. To this end, second subsidiary stream 26 is liquefied within ~a?~l,ze. 54.
In order for there to be a t~ difference between oxygen emiched liquid st~earn
?159308
50, af'~er having been subcooled, and second subsidiary stream 26, pressure reduction
valve 55 is provided to reduce the plC~ and thereby the t~ pc,dtllre of oxygen
enriched liquid stream 50. This reduction in ple~ e of oxygen enriched liquid stream
50 is below the ~ ; of ~ tion column 30 and yet results in a sufficient ~Ics~l.e5 for oxygen cnriched liquid stream 50 that vapor stream 62, derived ther~olll, can serve
in a refrigeration role. At lower d~ tion column pless~lies, for ~ nre below 8
bar(a), additional reflux to ~i~tit~ on column 30 is produced by liquefaction of a vapor
stream 72 c~l,acled from lictill~tion column 30 at about the same point as second stream
26, after liquefaction, is intrc~uced into ~i~till~tion column 30. Vapor stream 72 is then
10 liquefied within vaporizer 54 and introduced as additional reflux above the point of
introduction of the liquefied second subsidiary stream 26. As is evident, I,~s~l,e
reduction valve 55 also serves to provide a ten~pe.atule difference between oxygen
enriched liquid stream 50 and vapor stream 72.
A possible variation to a~lus 10 involves operation of r~ tjon colllmn 30
at high p.essu~ In such case, an expansion m~rhine might also be ~t ~rh~i to coolant
stream 46. This would in~ l~ase total plant refrigeration and therefore the amount of liquid
produced. Additionally, such t~ A~.~nder could also be used to drive a recycle
COnl~ 5SOI to recycle part of the oxygen enriched liquid contained within coolant stream
20 46 back into distillation column 30 to also increase production. As can also be
appreciated, partial vaporization of oxygen enriched liquid stream 50 is not restricted to
the illustrated embodiment in which partial vaporization is effected through liquefaction
of a portion ofthe il~collling air. For i~ c~ in a proper low ples~ule colurnn
application, a stream from the col-~mll, not having the cxact composition of liquid air,
25 could be used in place of liquefied air.
With reference to Fig. 2, an altemative embodiment of single column nitrogen
generator 10 is illustrated. In nitrogen gencrator 10, oxygen rich strcam 50 after being
subcooled within su~cooler unit 52 is divided into first and second partial strea-m--s 50a
30 and 50b. First partial stream 50a is e~r~n-led in first pl~a~ule reduction valve 60 to form
coolant stream 46. Second partial strearn 50b afier having been e~cr~nded by pressure
2159308
reduction valve 55 is then fuily vaporized within vaporizer 54. The fiully va~l~ed
stream, design~ted by reference number 63, is then partially warrned within rnain heat
exch~nger 22 and exp~nded within turboexpander 64.
S EXAMPLE 1
The following is a charted, c~lull~t~d cxample of a possible operation of singlecolumn nitrogen gc.le.alor 10 (illustrated in Fig. 1) in accoldance with the present
invention. In such example it is ~csllrn~d that colurnn 30 uses low pl~s~ drop
10 s~ructured pflc~ing and has about 100 theoretical stages. Second partial stre~n 26 after
having been liquefied is added to the tlictill~tion column at about six theoretical stages
` from the bottom. Stream 72 is withdrawn from the dictill~tion column at a point of about
six theoretical stages from the bonom and returned after co~dl ncing to a point about
sixteen theoretical stages from the bottom of ~lictill~tioD colurnn 30.
2159308
CHART FOR EXAMPLE 1
Stream Flow Condition Tc~ ule P~esau~e~ 2 Content
No. Nm3/hr Liquid K bar (a) % or parts
Vapor per rnillion
12 13,400 V 305 1.01 20.96
24 11,739 V 93.0 3.17 20.96
26 1,454 V 93.0 3.17 20.96
47 3,901 V 87.6 1.21 53.32
S0 6,670 L 92.6 3.17 41.92
58 3,901 L 90.1 2.27 53.32
62 2,768 V 90.1 2.27 25.85
66 2,768 V 83.0 1.09 25.85
68 6,670 V 88.3 3.10 2 ppm
72 1,454 V 90.6 3.17 9.33
It is to be noted that in such example the letter "L" indicates a liquid condition
and the letter "V" indicates a vapor condition. Top product punty, nitrogen recovery as
fraction of the air feed, and addition and withdrawal points are sensitive to the physical
20 ~ro~, Lies of the data base employed in effec~ ;ng the c~lc~ tions. Losses inherent in
operation of the pre-purification unit have been included in stream 12. As would be
appreciated by those skilled in the art, subcooler 52 would be at a low elevation with
respect to the sump of ~ till~t;on column 30.
25Thus, in a prior art design making a gaseous nitrogen product, identical in quantity,
fractional recovery from air, purity and pressure, where a turboexpander makes
12
- ~159308
refrigeration by exr~nding air into the dictill~tion column, s~eam 12 would normally be
col~plcssed to about 3.94 bar(a). In the present invention, air co~ ,l.ssion will only be
to about 3.45 bar(a).
EXA~LE 2
Thc following is a cha~ed, calculated cxarnple of a possible operation of singlecolurnn nitrogen generator 10 (illus~ated in Fig. 2) in accordance with the present
invention. In such example it is ~sllme~ that column 30 uses low ~le;.~u~e drop
10 s~uctured pa~l~ing and has about 100 theoretical stages. Second partial strearn 26 after
having been liquefied is added to the ~ictill~tion column at about six theoretical stages
from the bottom. Stream 72 is withdrawn from the ~ictill~tion column at a point of about
six theoretical stages from the bottom and retumed af~er con.1~n~ine to a point about
sixteen theoretical stages from the bottom of tli~till~tion column 30.
2159308
CHART FOR EXAMPLE 2
Stre~n FlowCondition Te.. ~pc.atu~e P~saul~, 2 Content
No. Nm3~rLiquid K bar (a) % orparts
Vapor per million
12 13,400 V 305 1.01 20.96
24 11,259 V 92.6 3.10 20.96
26 2,080 V 92.6 3.10 20.96
47 3,889 V 86.3 1.21 41.92
6,669 L 92.4 3.10 41.92
50a 3,889 L 87.0 3.03 41.92
63 2,780 V 92.1 2.18 41.92
66 2,780 V 85.2 1.06 41.92
68 6,670 V 88.0 3.03 4.8 ppm
72 1,008 V 90.3 3.09 9.06
While the present invention has been tescribed with .cife.e,~cc to a ~lcf~.led
embodiment, it will be understood by those skilled in the art that numerous changes,
20 additions, and omissions may be made without departing from he spirit and scope of the
present invention.
