Note: Descriptions are shown in the official language in which they were submitted.
--2--
Field of l:he Inventlon:
The present invention relates to a process -
for recovering and reconstituting nitric acid in a
nitric acid process for the e~traction of alumina
values from clay and more specifically to an irnpro~ed
method for reconstituting NOx gases produced in
such a process.
.
L~ O , ~ h ` ~ C n ~ i 7 : .
In order to provide an economical'y
useful nitric acid extraction process for alumina,
the nitric acid must be recovered from the total
process in sufficient quantity as to provide a high
percentage of acid recirculation. It is known in the
art that substantial nitric acid may be recovered by
direct condel-sation of HN03 in the decomposition
$tep of such process. Such recovery recycles about
67% of the nitric acid used in the process. ~lowever,
substantial amounts o~ the acid exits other stacJes of
the decomposition in the form of NOx gases.
The recovery of nitric acid solutions from
nitrous gases produced by the catalytic combustion of
. ammonia in air is a well-~nown art that is practic~ed
commercially around the world. The basic process
comprises contacting the ammonia oxidation yases at a
pressure of 3 to 6 atmospheres absol~te, or even
higher, in a bubble cap-tray absorption columll con-
taining of the order of a hundred trays in counter-
current relationship with a supply of water introduced
at the top of the column. Variations of the technoloqy
are concerned substantially with design of -the bubble
cap trays, oxidation of the ammonia under pressure, or
oxidation at ;bout atmospheric pressure to reduce
catalyst consumption followed by the compression of
the cool gases, the recovery and re-use of the heat
produced in the ammonia oxidation reaction, and
. . . '
~..
~ 3~
especially recently methods of reducing the approxi
matel~ 1,000 ppm NOx concentration in the tail gas
before releasing this gas to the atmosphere.
rrhe chemistry o~ the conversion of NOx
gases to nitric acid in solution is yenerally
consi.dered as consisting of ~ overall reactions which
ser~e to de~ine the mass balance between the liquid and
vapor streams. Reaction 1: 3No~(g? ~ H20(1)~ a~
2HNO3(aq) ~ NO(g~ which oacurs principally in the
liquid phase and reaction 2: 2NO(g) ~ 02;(g) - 2NQ~(g)
which occurs substantially in the gas phase. The'rate
of reaction 1 is thought to depend primarily upon the
rate of absorption o~ NO2 into the liquid~stream~which~
depends~upon the partial pressure~o~ NO2~i~n ~he gas~ ~
stream~and is thus slowed down by the presence of lar~e
quantities of inert~gase;s such as N2, and~concen-
trations of NO in the gas phase~which t~nd to drive~
reaction 1 in the~reverse~direction~ Once the NO2~has
een absorbed the reacti~ons in the liquid phase appea~r
to proceed at satis~ac~ory velocities. '~he rate o
reaction 2~is proportiol1al ~o the product o$ the s~qu~ar~;
o~ the~pa~rtial pre~ss~ure oE NO and the part;ial pressure~
f 2 and~can~be quite slow in ~the presence o~lar~e
amounts of~inert, gases,s;uch;as N2. In the~ammoni~a
oxidation process f'or makitlg nitric acid the'feed gases
rom the;~oxidizer~comprise on the order of 70 volume
percen~ N2~and the~proportion~of N2 increases as the~
NOx gases~are absorbed from the gaseous stream.
Additiona1~N2;is added~with air~to prov1de~some oxygen
in the tail gas to;drive reaction 2 toward~completiol1.
Thus, altho'ugh nitric acid has been reaovered ~rom
ammonia;oxidation gase6 at about atmospheric~pressure
using 2 or 3 absorption towers in series it has been
found more economi~al to compress the gases~to 3 to 6 ;
atmospheres absolute so as to increase the partial
pressure of the reac~ting gases sufEiciently to pe~rmit
,. ;
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~L5~ 32
-- 4
carrying out the reconstitution in a single tall column.
On the other hand NOX gases produced by the thermal decom-
position of aluminum nitrate material in properly constuc-
ted, indirectly-heated decomposers contain little or no inert
gases. A typical composition of such a gas, before any air in-
leakage, is about 25 volume percent f (NO2 + NO), about 12 1/2
volume percent 2 and about 62 1/2 volume percent water vapor.
Since in the absorption column water vapor is absorbed in the
liquid stream much more rapidly than NO2 the concentrations
of the reacting gases increase during passage through the
absorption column so that the same or even higher rates of
Reactions 1 and 2 may be achieved at near atmospheric pres-
sure as can be achieved with ammonia oxidation gases at
elevated pressures.
Since both Reactions 1 and 2 are highly exothermic
and the easily reversible Reaction 1 can begin converting
HNO3 from the acid solution to NO2 in the yas at temperatures
as low as 150 to 180F, depending upon the concentration of
HNO3 in the liquid and of NO in the gas phase, the removal of
heat from the absorption column is of major importance. It
is known in the art to remove this heat either by placing
water-cooled cooling coils in the liquid layer maintained on
the upper side of the bubble cap trays or to withdraw a por-
tion of the liquid from each of a number of trays in the
column, pass the liquor through individual heat exchangers,
and return it to the column after cooling. Plants handliny
ammonia oxidation gas typically provide sufficient cooling to
the column by one or the other means so that -the strong acid
exiting the column is cooler than about 120F, or even lower
depending upon the strength of the nitric acid that is being
manufactured.
In contrast to the recovery of nitric acid
- - . ?
-- 5 --
from ammonia oxidation gases there has been very little
- need around the world to recover nitric acid from
eoncertrated NOX streams such as that described above
for the decomposition of aluminum nitrate materials~
Aeeording to the invention, a process for the
reconstitution of NQX gases to nitrie aeid eomprises:
a. contacting the NOX gases in a range from
about atmospherie to about 50 inches of water negative
pressure in countereurrent relationship in one or more
paeked eolumns with 50 to 60 pereent nitrie acid
solution cooled suffieiently to provide aeid solution
exiting said one or more eolumns at a temperature below
about 180F to remove as nitrie aeid a major portion of the
originally introdueed NOX values;
b. eompressing the residual gases from step a
to at least about 2 atmosphere absolute;
c. contaeting the compressed gases from step b,
in a pressurized eolumn with from about 50 to about 60
nitrie aeid to remove the residual NOX values from the
gas; and
d. reeovering the nitrie aeid as it accumulates.
For example, the NOX recovery process of the present
invention comprises eontacting NOX gases eontaining at most
relatively small proportions of inert diluant gases such as N2
in eounter-eurrent relation with a cooled 50-60% nitrie acid
solution in~one or more packed eolumns operating at about
or slightly below atmospheric pressure to remove as nitric
acid a major portion, preferably 90% or more, of the originally-
introduced NOX values, eompressing the residual depleted gases
to 2 to 6 or more atmospheres absolute and eontaeting the
eompressed gases in eounter-eurrent relation with the nitrie
acid solution in a paeked tower to strip substantially all
of the remaining nitric acid values from the gas stream.
_ 5a
.
Detailed Description:
According to the present invention, the reconstitution
of such NOX gases to nitric acid is most efficiently
performed as follows:
NOX gases 7 produced by thermal decompositon of
aluminum nitrate materials and consisting essentially of
NOX, 2' and water vapor and possibly small amounts of N2
from air in-leakage, are partially cooled and absorbed
in recircuIating cooled 50 to 60~ nitric acid solution
from a common receiver tank in one or more packed absorption
towers. Nitric acid gas is contacted in a first packed
absorption tower operating around atmospheric pressure to
5-50 inches water column negative pressure in counter-current
relationship to an amount of cooled nitric acid solution
sufficient to
. .
--5--
keep the temperature of the acid solu-tion leaviny the
column at below about 180F, the unabsorbed gases
ex.itiny the tower are co~pressed to at least about ~0
psig and preferably within a range of about ~0 to about
100 psig and contacted in a second, pressurized packed
absorption colulan in counter-current relationship ~ith
a quantity of the cooled 50-60% nitric acid solution
sufficient to maintain the temperature of the liquor
leaving the secorld coluMn below about 150~F, preferably
below about 130F, and residual gases ~rom the second
tower are passed through a small absorber in
countercurrent relationship to a flow of a small amount
o~ water to absorb excess HC1 gases and then passed to
suitable tai]. gas NOX recovery or destruction means
before venting to the atmosphere~
Before contacting the gases in the said
second, pressuri~ed packed absorption colllmn the
NO~ gases are blended with sufficient air to
provide about 2-10 percent or more O~ in the tail
gas. This air may be introduced at any convenient
location. upstream of the second absorption column and
is pre~erably introduced upstream of the first packed
absorption tower.
The nitric acid solution is maintained at
about the acid concentration required for ex'craction,
i.e~ within the range of about S0 to 60~ acid, usually
about 54 to 58%, and is supplied from one or ~nore
surge tanks, as may be desired, through heat exchangers
to the individual packed columns at rates to each
columll such that the acid solution leaving the colurnn
is less than a~out 180F in temperature, prererably
less tharl about 150-Fo Liquid draining from the to~;1ers
is collected in the surge tank for recirculation and
the excess is drawn off as product acid for use, for
instance, for the di~estion of alumina frorn calcined
clay for the manufacture of aluminum nitrate in a
,_ ~ . . ~
,~3
w7~
process for the recovery of metallurgical grade alumina
from clay. More specifically, accordin$ to a pre~erred
method, concentrated NX gases ~rom aluminum nitrate
decomposers are mixed with hot make-up NOx qases from
an NH3 oxidizer, pass through a waste heat boiler,
wherein re-usable heat is extracted, and then blended
with vent gases, and with air introduced through a flow
control.valve that is responsive to an o~ygen meter.
The mixed gases pass in to an open spray tos~er where
they are counter-currently contacted with acid to
remove a portion of the contained heat, water vapor,
and N02 and then pass sequentially through one or more
packed towers, in which they are counter-currently
- contacted with acid which has been cooled in a heat
exchanger, and ti-en compressed in a compressor to 2 to
6 or more atmospher~s absolute pressure be~ore passing
through another packed tower in counter-current contact
with cooled acid. The vapors are then passed throuclll a
bubble cap tower where they are contacted counter-
currently with water to absorb I~Cl values that may be
present and the remaining gases pass throuc3h an
absorber for stripping out any residual NOx before
. exhausting to the a~mosphere. Nitric acid solutio.
draining from all of the towers is colleeted il~ a
tank, in which the acid concentration is control}.ed to
~elow 60~ or preferably below about 58~, by means of
the addit.i.on of relatively strong acid from the
aoresaid acid and heat recovery operations.
The reaction towers are packed absorption
columns wherein the packing may be any desired
cornmercially available packing material which
preferahly has a large void volume per unit of surface
area such as is true of Raschig rings. The large
void volurhe of, for instance, Raschig rings minimizes;
the velocity o flow of the gas through the packing,
thereby providing gas residence time or the relatively
,.
.
~ 2,32
slow Reaction 2 to proceed~ This reaction tlme would
have to be provided by increasing the height of the
tower if packin~ with a lower vold volume were
provided. S~ch rings also simultaneously provide;a
large gas-liquid contact area which in well known
manner facilitates the absorption of N02 into the
liquid and desorption of the reaction product NO from
the li~uid.
Void volumes for a number of packing
material~ available arè in "Chemical Engineers
Handbook" Fi~th Edition edited by Perry and Chilton -
McGraw-Hill Publishing Company, New York,~N.Y.;~Section
18: Gas-Liqu~id Contacts. This section also discusses
the relative efficiencies~ o~ various packing materials
for a~sorption of gases in~to liquids and pres~ent~s
methods of estima~ing absorption rates, h~eat-trans~er
rates, pressure drops, etc. Data also may be obtained
from packing manu~acturers and ~rom other ~ell known
pu~lish~ing sour~ces. ~
~ s mentioned hereinabove, plate-type
a~sorption columns g~enerally;are used for absorp~io~n of
NOx from gas:es~produced by~oxidation o NH~3~wit~h~air ~o
obtain the~maximum~possible void ~olume, and ga~s
residence~time wh~erein Reaction 2 mcty proceed
substanti~ally to~comple~t~ion. For the concentrated
gases addressed;herein, however, an even more important
requirement~is the~abstraction of sensible~heat~ from~
the gas~phase produ~ed therein by the exothermic
Reaction 2,~ whereby the temperature rise of~the~gas ;~
phase, with~the attendant rap~id decrease in the rate of
Reaction 2, is mini~,lzed. Packed to~ers are much more
efficient for the removal or this sensible heat than
tray-type towers, and~in addition eva~oration of water
and acid from the myriads of small droplets dispersed
in the gas phase further assists in minimizing the gas
temperature rise thereby permittiny use of much smaller
,
,. :
, .. . . :
~ 4~
eq~ipment than would be possible with tray-towers.
The following e~amples are intended to better
describe and more clearly point out the advantages and
preferred manipulative steps of eacll of the steps of
the process of the instar,t invention.
The following modified technique might be
applied in the processes of the following examples
as :EQ1.10WS:
Concentrated NOx gases from ANW decomposers
are mixed ~ith hot make-up NOx gases from an NH3-
oxidizer, pass thro~gh a waste heat boiler, wherein
re-usable heat is extracted~ and then blended with vent
gases and air~ The mixed gases pass in to an open
spray tower where they are counter-currently contacted
witl acid to remove a portion of the contained heat,
water vapor, and N02 and then pass sequentially through
two packed towers, in which they are counter-currently
contacted with acid which has been cooled, and then
compressed in a ccmpLessor to 2 to 6 or more
atmospheres absolute pressure before passing through a
third packed tower in counter~current contact with
cooled acidO The gases are then passed through a
bubble cap tower where they are contacted counter-
currently with water to absorb HCl values that may be
present and the rernaining gases pass through a NOx
absorber or stripping out any residual NOx before
exhausting to the atmosphere. Nitric acid solution
draining from all four of the towers is collected in a
tank in which the acid concentration is controlled to
below ~0%, or preferabIy below about 58%, by means of
the addition of relatively strong acid
Example 1
Feed gas having an estimated rate (in pound-
moIs per hour) and temperature shown in Column 2 of
Ta~le 1 is mixed with makeup gas from an atmospheric
.
l~4,)~2
pressure Nll3-oxidation ~nit at a rate shown in
Column 3, Table 1 and passed through a waste-heat
boiler in which heat is extracted and is blende(1 with
air from Column ~, Table 1 to produce an assumed
column feed gas as given in Column 5~ Table 1. To
better illustrate the simplicity of the absorption
process it is assumed that the gas is cooled only to
about 250F and that no water is condensed in the
heat exchanger, although such is not common practice
in the nitric-~acid-from-ammonia industry. It is also
thought that, in many instancesj heat exchangers or
cooling ancl dewatering the gas may be more ex~ensive
than liquid-acid-to-water heat exchangers for removing
the same quantity of heat.
';ince the column feed gases will reconstitute
to about 64% nitric acid, an unnecessarily-high concen-
tration that would increase unduly the difficult~ o~
absorption and reconstitutionJ about 191 tons per hour
of about 54~ nitric acid solution are ed to the
product holding tank. Such addition alone maintains
the concentration in the product tank at about 5~9gO~ a
satisfactorily low value. In addition about ~.4 tons
of dilution water are added to reduce the mean concen-
tration in the tank to about 55~ nitric acid.
Example-2
-
The column feed ~as from Example 1 is
introduced to a first packed tower, which comprises 160
square feet of internal cross section area f about 14.3
feet inside diarneter, and is packed with 2~ nch metal
Raschig rings. The to~er is fed with about 4565 GPM of
acid solution which is cooled to about 100F and which
is distributed over the packing and drains throuc~h the
packing in counter-cu rent relation to the rising feed
gas. The liquid absorbs nitric acid and water (and a
litt]e NO2) from the feed ga5 and d~ains from the
z
colwlln at a rate o about ~810 GPM at a temperature of
about 175F. At the gas and liquid ratex existing at
the base of the packing the pres~ure drop is about 1~5
inches water column per foot of pack.ing, just below
flooding conditions, but the absorption rate of NO2,
and particularly E~2Or is so rapid that the ~as volume
decreases to about l/2 of the initial volume within
about a ~oot of effective packing height, whereby the
pressure drop is reduced to well below that required
~or ~looding.
At a level in the column corresponding to
about 12-l/2 feet of fully-effective packing over 90%
of the ~eed NOX values have been absorbed and the gas
flow rate and composition is that given in Column 2,
. Table 2u Bo~h reactions ~l) and ~2) are continuing but
at reduced ratesO
~ t a column height equal to about 2~ feet
of fully-effective packing the gas rate and compositi.on
is that given in Column 3r Table 2.
~ample 3
_
The gas from the 12-l/2 ~t. effective ~acking
height of Example 2 is compressed to 3.0 atmospheres
absolute and fed into a packed absorption column. The
packed column has.an inside cross-sectional area of
S3.6 square ~eet, inside diameter about 8.26 feet r and
is fed with about 1200 ~PM of 55% acid cooled to about
114F which absorbs acid, heat and water vaFor from the
gas during counter-current contact in the packing so
that the liquid dralning from.the column comprises
about 1213 GPM at a temperature of about 130~. The
column is packed with 2-inch Raschig rings.
Because of the high partial pressures of
bo~h NO and 2 in the compressed gas, NO is oxidized to
~ y Reac~ion 2 at a rate o nearly S pound moles NO
per hour per cubic foot of void volumel with an
~12-
attendant high rate of heat release to the ~as~ Since
the rate of Reaction 2, which is very important in this
column, is inhibited by high gas temperatures it is
desirable to use a liq-lid seal of cool acid in the
coMpressor, and to make ti~e compressor to column
connection relatively short so that oxidation occurs
mainly within the coluMn where the gas is cooled by
contact with the liquid and liquid sprayn
The quantities of N0x remaining, including
minor amounts of N203 and N204, and HN03 vapors, at
column heights equivalent to the listed packing height
in feet of fully effective packing are given in Table
3.
If the system feed gas contains HCl in excess
of the small amounts soluble in 55% nitric acid, the
off-gas from the selected effective packing height o
the column is passed throucJh a ~ist eliminator to
another column wherein it is contacted in counter-
current relation with 200 to 300 pounds per hour o
water, as needed to keep the concentration o~ the wea]c
acid exiting below about 25 wt.% total acid, whereby
the HCl and HN03 vapors are absorbed along with rninor
amounts of the contained N0x values. The absorption,
as HN03, of the N0x values is minimiæed by limiting
the number of trays and the gas residence time to the
minimum values required for proper design for absorblng
the very-easily-absorbed HCl. It is known to treat the
HCl-contair,ing liquid with 020ne to convert the HCl to
C12 gas, which is removed and absorbed in caustic
liquor, and return the HCl-free acid to the process.
The tail gas exits the columll thro~gh a mist
eliminator, passing a ('2 concentration sensor, to an
N0x stripping unit which is preferably a proprietary
Pura Siv M unit manufactured for sale to the industry
by Union Carbide Corporation, New York, New York, that
is known to strip the N0x concentration to S0 ppm or
..... , . . . :
1 3w
less and permit recycling of the recovered N0x to the
reconstitution system (~SP 3,473,893, 11ardison).
It is a]so known to catalytically reduce the
N0x to N2 with ammonia or methane or to absorb the N0x
in nitric acid solutions from which all residual N0x
has been stripped by treatment with hot air in a
stripping column. Acid offered for commercial sale
usually has been bleached, that is, stripped in SUC11 -
manner .
The stripped tail gas is then exhausted to
the atmosphere through, if desiredr power recovery
means well known to the industr~0
Example 4
. .
The gas frvm the 24 foot effective packing
height of Example 2, listed in CO1Umn 3 0~ Table 2, is
compressed to 6.0 atmospheres absolute pressure and
contacted in a 12.5 square foot inside area, 4 foot
insi~e diameter, column packed with 2 inch Raschic3
rings and supplied as in Example 3 with about 280 GPM
of 55~ acid solution at about 10~~ which, ar~ter
absorbing heat, nitric acid and water vapor, exits the
column at about 130Fo The N0x and HN03 contents o~
the residual gas at various effective packing heights
are listed in Table 4. The gas exiting the column
through the mist eliminator is treated as described in
Example 3.
Example 5
Examples 2-4 show t~e use of a single
atmospheric-pressure absorption column ahea~r of the
high-pressure column. r~his example shows the use of a
short, large-diameter atmospheric-pressure column for
removi~g the bulk o the water from the gases followed
by absorption in a taller, smaller-diameter atmospheric
pressure column to complete absorption o~ 90% or more
... .. . .
of the NOX in the column feed gas (Table 1, Column 5).
The column feed gas (Table 1, Column 5) is absorbed
in a first column of 160 square feet cross-section area packed
with 6-1/2 feet effective depth of 2 inch Raschig rings sup-
plied with about 4585 GPM of acid liquor cooled to about 100F,
yielding a liquid effluent of about 4810 GPM at about 162F.
Residual gas exiting at the top of this column is fed to an
atmospheric pressure column which comprises a 7 ft. inside
diameter column packed with 2-inch Raschig rings supplied
through a sprayer with about 845 GPM of acid cooled to about
100F.
At a column height corresponding to about 15 feet
of effective packing height the gas composition and heat con-
tent are essentially the same as those obtained in Example 2
for 12-1/2 feet of effective packing (Table 2, Column 2).
Upon comparison of the total volumes of effective packing it
is seen that the 2-column combination saves about 400 cubic
feet of effective packing volume over the single, large dia-
meter column of Example 2.
Similarly it is found that at an effective packing
height of about 47 feet in the 7-foot diameter column the gas
conditions essentially match those of Table 2, Column 3 for
the 24-foot packing height of Example 2. The savings for this
variation is about 980 cu. ft. of effective packing volume.
,
- 1 5-
.
TABLE 1
~ATE .- E~OUND MOLS/EIOUR
COMPONE~T FEED GAS MAKE UP BLEED AIR COLUi~ Ei'EhD
NO 817:.3 60.7 - ~ ~ 169~.,S~
-~ N(~2 817. 3 : - _ 1525. 8
:~ 2 81~7~. 3~ ~ 40, 9 67. 8 5~71~. 8
H~O ~ 86. 6 ~10~. 6 2. 7~ 41~89. 9
~:~ N2 : 0 ~ 452. 9 256. 6 7~09. 5
Temperature:, F ~ :: 600: ~ 1 boo 77 ~; 2~s0 : ~
P r e s s u r e, : ~
Atm. Abs. ~ :O.95 1 1 : ~0.948
T~BL~ 2
~: : :FLOW -: POUNr~ MO~SfHOUR
GAS ~~ 12~ 2 FT. ~ rr~
:: COMPON13NT :: PA:CKING :: P~ G: : ~: :
NO ~ 2 7 . 5 ~ 4 3
N02:: ; ~ 27. 3: H 10.~
~ N203 ~ : n.21 ~ o.os ~ ~ ~
N~204 ~ 0. 24 ~ 0.~1~7
2 ~ 6 6 . 0 ~ 9 9
~ 29~o ~ ~ ~ 21
: HN03 ~ 3.:1 ~ 20 3
N2 ~ ~ ~709, 5, ; 70~9~. 5
.
'
,;
.:
.
. .,
, : ~-
.:: :
c
.
- 16 -
TABLE 3
RATE - POUND MOLS/HOUR IN GAS
EFFECTIVE LOSS, %
PACKING CONC.OF NOX FEED '$
Ft. NX HNO3NOX, PPM(COLUMN 5, TABLE 1)
19.5 6.12 0.83 7700 0.41
23 5.28 0.83 6700 0.36
33 4.03 0.82 5100 0.29
02 Concentration 8.4 to 8.3 Volume %.
TABLE 4
FLOW - POUND MOLS/HOUR
EFFECTIVE
PACKING LOSSI %
HEIGHT C'ONC. OF NOX FEED
Ft. NX HNO3NOX~ PPM(COLUMN 5, TABLE 1)
9 5.53 0.31 7050 0.34
3.29 0.29 ~200 0.21
2.47 0.29 3200 0.16
3~ 1.7~ 0.28 2250 0.12
-$~
