Technical Improvements in the
Strong Nitric Acid Process
Sumitomo's process becomes more economical when it incorporates
a weak acid process for coproduction.
Tetsuya Ohrui, Katsuo Ohkubo, and Osamu Imai
Sumitomo Chemical Co. Ltd.
Tokyo, Japan
Sumitomo Chemical has developed a process to directly
synthesize strong nitric acid from ammonia and air. Using
technology developed over the past 40 years, in 1972 a plant
was constructed to produce simultaneously 770 Mlb (350
metric t) of strong nitric acid and 165 Mlb (75 metric t) of
weak nitric acid per day. Successful implementation of these
processes required several plant modifications to deal with
environmental regulations and some corrosion difficulties;
these will be discussed in the second section of the article.
Our processes for the direct synthesis of strong nitric acid
are: 1) a high pressure oxidation-high pressure absorption
process (also called Mono Pressure Process, MPP) and 2)
low pressure oxidation-high pressure absorption (also called
the Dual Pressure Process, DPP). The MPP becomes more
competitive when it is used for the coproduction of strong
and weak nitric acids.
Mono Pressure Process
Figure 1 shows the flow diagram of the simple production
of strong nitric acid, which was improved based on our
experience in operating the current 935 Mlb (425 metric t)
per day plant.
In the MPP, liquid ammonia is vaporized in the ammonia
evaporator (1) and is mixed with compressed air (4) at a
temperature of 390-480°F (200-250°C). The 10-11%
ammonia contained in the reaction stream is oxidized to NO
in the presence of a platinum-rhodium catalyst in ammonia
burner (7) at a pressure of 128-143 psia (800-1000 kPa) and a
temperature of 1,650-1,740°F. (900-950°C). The oxidation
reaction formula is:
and at the same time, NOx contained in the stream is partly
converted to 40-50-wt% HNO3. To enrich the NOx content,
60-68-wt% weak nitric acid coming from the weak nitric acid
rectifier (17) reacts with NO and is decomposed. The
reaction formula is:
NO + 2HNO3 -> 3NO2 + H2O
The heat of 40-50-wt% recycled nitric acid at a temperature of 210-250°F ( 100-120°C) is recovered in the reboiler of
the weak nitric acid rectifier (17). Excess water contained in
the acid is discharged from the system through the weak
nitric acid rectifier.
Unreacted NO contained in the gaseous stream coming
from the decomposer-condenser (9) is almost completely
oxidized by 65-70-wt% HNO3 to NO2 in the vent absorber
(11). NO2 gas from the vent absorber is sent to the NO2
absorber (14) where it is absorbed in about 85-wt% nitric
acid at 32-68°F (0-20°C) to form red fuming nitric acid, with
1
2
3
4
5
G
7
S
9
10
AMMONIA EVAPORATOR
AMMONIA SUPERHEATER
AIR f ILTEH
AÎH COMPRESSOR
AMMONIA MIXiR
MIXED GAS FILTER
AMMONIA BURNER
WASTE HEAT BOILER
DECOMPOSER -COKDEHSER
CONDENSER-COOLER
11 «HT ABSORBER
12 REACTOR
13 Na04 BLEACHER
14 NOa ABSORBER
15 STRONG NITRIC ACID
RECTIFIER
16 STRONG MTRIC ACID
BLEACHER
17 WEAK NITRIC ACID
RECTIFIER
18 WEJK NITRIC ACtt>
BLEACHER
19
20
21
22
NOx COMPRESSOR
TAIL GAS HEATER
CAÏM.YTIC COMBUSTOR
GAS EXPANDER
23 STEAM TURBINE
STRONG NITRIC ACtO
4NH3 + 502 -> 4NO + 6H2Û
The yield is about 95%. By means of heat recovery in the
waste heat boiler (8), the temperature of the NO-rich reaction
stream goes down to about 390°F (200°C). The stream is
cooled in the decomposer-condenser (9) to remove moisture,
164
mm N.TRIC ACID
Figure 1. Strong nitric acid process: Mono Pressure
Process.
20-30-wt% N2Û4. The N2Û4 is separated from the acid by
air in the N2Û4 bleacher (13). Then, together with air, it is
sent to the reactor (12), where it reacts with water contained
in 70-75-wt% nitric acid from the vent absorber (11) at 100140°F (40-60°C).
The reaction formula is:
AMMONIA EVAPORATOR
AMMONIA SUPERHEATER
AIR FILTER
AIR BLOWER
AIR HEATER
AMMONIA MIXER
MIXED GAS FILTER
AMMONIA BURNER
WASTE HEAT BOILER
NO.1 COOLER-CONDENSER
11
12
13
14
15
16
17
18
N0.2 COOLER-CONDENSER
NO.1 GAS COMPRESSOR
OXIDATION TOWER
TAIL GAS HEATER
NQ.3 COOLER-CONDENSER
N0.2 GAS COMPRESSOR
N0.4 COOLER-CONDENSER
VENT ABSORBER
19 REACTOa
20 Na04 STRIPPER
21 MEDIUM KtTRIC ACtO
BLEACHER
22 NOa ADSORBER
23 STRONG NITRIC ACID
RECTIFIER
24 STRONG NITRIC ACID
BLEACHER
25 SECONDARY AIR FILTER
26 SECONDARY AIR
COMPRESSOR
27 NO, ABATER
28 GAS EXPANDER
29 STEAM TURBINE
2N2Û4 + 2H2O + Ü2 -> 4HNO3
As a result, the acid concentration reaches about 85 wt%.
Since the produced acid contains 20 to 30-wt% N2Û4, it goes
to the N2Û4 bleacher (13). Colorless, about 85-wt%, nitric
acid is concentrated in the strong nitric acid rectifier ( 15) to
over 98 wt%. Then, it is bleached again into the final strong
nitric acid product.
On the other hand, the exhaust gas from the NO2 absorber
(14), containing about 1,000 ppm nitrogen oxides, is sent to
the catalyst combustor (21 ) for removing nitrogen oxides by
reduction with fuel. The exhaust gas from catalytic combustor (21 ) containing less than 50 ppm NOx is used for driving a
gas expander (22) to recover its energy. After further heat
recovery, the exhaust is vented to the atmosphere at 210 to
300°F(100-150°C).
The nitrogen content in the waste water is as low as 0.21b/
Mlb HNO3 (0.2 kg/metric t HNO3) as N, and therefore, even
when required, a simple after-treatment of the waste water is
sufficient.
In Figure 1, the dotted line indicates the flow of the weak
nitric acid product. Weak and strong nitric acids can be coproduced simultaneously in this process, in case the coproduction ratio of weak acid to strong acid is less than 0.5.
The lower the feed rate of 60-68-wt% nitric acid from the
weak nitric acid rectifier (17) to the decomposer-condenser
(9), the higher the coproduction ratio of weak acid to strong
acid. By decreasing the bottom temperature of the
decomposer-condenser, the coproduction ratio can be increased. Theoretically speaking, weak and strong nitric acids
can be simultaneously manufactured at any coproduction
ratio. According to our plant operation records, the coproduction ratio varied from 0.2 to 0.6.
An economic assessment of the MPP is shown in Table 1.
If the plant capacity is relatively small, the mechanical efficiency of steam turbines drops to increase utility consumption. It is desirable to coproduce weak nitric acid in view of
improved steam consumption.
Dual Pressure Process
In the Dual Pressure Process (DPP) (Figure 2), ammonia
gas is oxidized at the atmospheric pressure in the ammonia
burner (8). The NO-rich reaction stream is fed to the cooler
condensers (10) and (11) for removal of moisture. Since the
concentration of nitric acid generated there is as low as
l-3-wt%, the acid is discharged directly for water removal.
Because of this, the DPP does not need a weak nitric acid
rectifier and decomposer-condenser. Therefore, steam
consumption in this process is considerably better than in the
Mono Pressure Process.
Compared with the Mono Pressure Process, the DPP also
STRONG NITRIC ACID
ACID CONDENSATES
Figure 2. Strong nitric acid process: Dual Pressure
Process.
is characterized by the high pressure N2Û4 stripper (20), and
atmospheric pressure medium nitric acid bleacher (21), used
to easily separate N2Û4 from the red fuming nitric acid. NOx
contained in the exhaust gas from the NO2 absorber (22) is
removed by selective catalytic reduction with ammonia.
Using this method, NOx content can be abated to less than
100 ppm. In this case, the nonselective reduction with fuel
can be alternatively used.
Table 1 shows the economic assessment data on the DPP.
This process is superior in steam consumption to the MPP,
which is preferable in terms of environmental protection.
Local conditions and environmental regulations will dictate
which process should be employed. It is less economical to
coporduce weak nitric acid in the DPP.
Coproduction increases economy
Since a relatively high coproduction ratio of weak acid to
strong acid can eliminate the need for removing excess water
from the reaction system, the weak nitric acid rectifier and
decomposer-condenser are not necessary in the coproduction process. A higher coproduction ratio than 1.5 is satisfactory for generating 60-wt% and 98-wt% nitric acid.
The process flow differs somewhat, depending upon the
coproduction ratio. Figure 3 shows the process flow for a
coproduction ratio of 2-3. A strong acid process can be easily
added to the existing conventional weak acid plant by
AMMONIA SUPERHEATER
ABU FILTER
AIR COMPRESSOR
AMMONIA MIXER
MIXED GAS FILTER
MMQNiA BURNER
WASTE HEAT BOtLER
10 ABSORBER
11
12
13
14
19
NOa ABSORBER
REACTOR
MaO« BLEACHER
STRONG WTRtC AfitD RECTIFIER
STRONG NITRIC ACID BLEACHER
16 VENT SCRUBBER
17 TAIL GAS HEATER
18 CATALYTIC CQMBUSTOR
19 GAS EXPANDER
20 STEAM TURBINE
WEAK NITRIC ACID
-«-!
j
-«»CESS WATER
STRONG NITRIC ACID
Figure 3. Coproduction process: physical absorption.
165
Table 1. Economie assessment data on Sumitomo strong nitric acid process
Coproduction of Strong and
Weak Nitric Acids
Strong Nitric Acid (98% HNO3)
Process
Mono Pressure
Process
Dual Pressure
Process
Mono Pressure Process
Dual Pressure
Process**
Oxidation pressure (psia)
133 - 147
15 -30
133 - 147
133 - 147
Absorption pressure (psia)
133 - 147
133 - 147
133 - 147
133 - 147
Plant capacity (Mlb/d)
(as 100% HNO3)
770
265
Installed cost (ISBL)*
in billions of yen
2.8
1.85
Ammonia (Ib)
288
288
284
288
288
Catalyst (10-6lb)
250
250
45
250
250
0
0
0
0
200
270
0
0
0
0
0.05
0.1
220
260
210
220
190
210
Electricity (kWh)
25
70
150
25
9
7
Fuel (Ib)
38
38
0
38
38
265
1.8
98% HNO3: 770
70% HNCh:
J 165
98% HNO3: 330
60% HNOJ3: 660
98% HNO3: 330
60% HNOJ3: 660
2.45
SlOmillion ***
3.2
Consumption per Mlb HNO3
Process water (Mlb)
Steam (630 Ib/in2,
752 F) (Mlb)
Boiler feed water (Mlb)
Cooling Water (Mlb)
NOx abatement system
NOx in tail gas (ppm)
0.1
-470
0.52
Nonselective catalytic
reduction
< 200
Selective
< 200
< 200
< 200
< 0.2
0.6- 1.5
< 0.2
0
N in waste water
(lb/Mlb-HNO3)
Nonselective
*Excluding piling, license fee, engineering fee and catalysts
**Integration of 330 Mlb/d SNA to the existing 990 Mlb/d WNA plant
***Additional installed cost (excluding electricity and instrument) and additional consumptions
Applicable metric conversions:
l psi = 6.895 kPa
l Ib = 0.4536 kg
l Ib/mlb = l kg/ton
l mlb/ = 0.4536 ton
166
1 ,390
0
branching the NO-rich reaction stram from the cooler-condenser (9) to link the two. Since a coproduction ratio higher
than 4 increases the absorption temperature of NO2 gas with
about 85-wt% nitric acid at the NO2 absorber, the process
removes the need for refrigeration facilities, further
improving its economic advantage. The most noticeable
feature of our technology is that, by minor modification of
the exising weak nitric acid plant (e.g., modifying the
absorber) it becomes possible to also produce strong nitric
acid easily, as indicated in Figure 3. With this technology, it
is possible to manufacture strong acid directly without a
previous concentration stage of weak acid with sulfuric acid
or magnesium nitrate.
Table 1 shows the economic data on our coproduction
process. The minimum economic capacity for the simple
production of strong nitric acid is about 110 Mlb/d (50 metric
t/d). In case of simultaneous production, the minimum
capacity for strong acid goes down to 22 Mlb/d (10 metric
t/d).
Special stainless stee! "SN-1"
The development of the strong nitric acid process has kept
pace with the development of construction materials.
To select the most suitable construction material for the
strong nitric acid plant, we conducted corrosion tests on a
number of materials. However, the initial corrosion test
periods were too short, causing us to overlook the acceleration of corrosion over a period of time, a fatal defect in stainless steel. For example, in the case of SUS 304L, after 4,000
hr fieldtesting at 85-wt% and 75°C, corrosion accelerated to
3-4 times the initial rate. Though not as dramatic as this example , similar phenomena appeared in other parts of the plant.
To overcome the problem various steps were taken. Aside
from reducing carbon content of the materials, we developed
methods of predicting long-term corrosion resistance with
short-term evaluation of artificially corroded material
samples.
Construction materials such as aluminium, titanium, high
silicon cast iron and glass-lining have been used for the
strong nitric acid process. However, none of them could
satisfactorily protect a plant from corrosion in the long run.
Even conventional stainless steel is seriously corroded by
strong nitric acid at a high temperature. This phenomenon is
called "transpassive".
As a result of intensive research and development conducted jointly with Nippon Stainless Steel Co., Ltd., a
special stainless steel has been developed. Named "SN-1,"
the material has excellent corrosion resistance against any
concentration of nitric acid over long periods of commercial
plant operation.
The chemical composition of SN-1 is: 0.03-wt% C;
4.00-wt% Si; 17.50-wt% Cr; 14.00-wt% Ni; 1.00-wt% Mn;
P and S at less than 0.03 wt%; plus some other additives.
Its characteristic features are summarized here:
(1) It contains 4-wt% Si to improve corrosion resistance
against strong nitric acid.
(2) Content of C is curbed to a relatively low level in order
to prevent the intergranular corrosion caused by me sensitization of the material.
(3) Another element is added for stabilizing the C component and making the microstructure of the material more
fine.
Si plays the most important role in improving resistance
against strong nitric acid. Si content is controlled within a
narrow range around 4 wt%, taking into account the relationships among corrosion resistance, hot workability, fabricability, weldability, etc. The contents of Cr and Ni are set at
17.5 wt% and 14 wt%, respectively, to keep the austenitic
microstructure and to maintain sufficient corrosion resistance and the mechanical properties of the material when Si
content is 4 wt%.
Accordingly, SN-1 can be machined and fabricated in the
same manner as common stainless steel. Further, SN-I reqires no annealing, which is necessary for other stainless
steels having chemical compositions similar to SN-1.
SN-1: strong and corrosion resistant
In contrast with ordinary stainless steel, corrosion of SN-1
does not accelerate over time.
Figure 4 shows an iso-corrosion rate curve of SN-1. The
corrosion rate of SN-1 is below 3.9 x 10"3 in/y (0.1 mm/y)
against nitric acid at any concentration, when the storing
temperature is 140°F (60°C) or less. Further, against 88-95wt% nitric acid, SN-1 still shows excellent corrosion resistance, up to the boiling point.
120
90
100
Figure 4. Iso-corrosion chart of SN-1 vs. nitric acid.
SN-1 and SUS 304L are sensitized materials. Corrosion rate is the larger value in the lab test data for
the liquid phase and the gas phase.
167
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Table 2 shows the performance of SN-1 in commercial
operation. No abnormality has been recorded to date. For
example, in the case of a 1.76 MMlb (800 metric t) storage
tank of 98-wt% nitric acid made of SN-1 alone, very good
performance was observed.
We already had two 1.10 MMlb (500 metric t) aluminium
product storage tanks. Aluminium is not suitable for larger
tanks ( 1.76 MMlb or 800 metric t) because of relatively poor
mechanical strength and the difficulty of making thick plate.
SN-1 is about six times as strong as aluminium, and even thin
plates guarantee enough strength. In addition, unlike aluminium, SN-1 is free from corrosion, even if 98-wt% nitric acid
is diluted while stored. Therefore, SN-1 was used for the new
1.76 MMlb (800 metric t) tank, though the construction cost
was slightly higher.
Field corrosion tests for 98-wt% nitric acid to compare the
corrosion rate of SN-1 with that of aluminium. As a result,
the corrosion rate of SN-1 was found smaller than that of
aluminium:
Corrosion rate
in liquid phase
In our field test, as the ratio of the oxygen concentration
necessary for complete combustion of fuel to oxygen concentration in exhaust gas goes up, fuel stoichio goes up, NOx
content in the gaseous stream from the catalytic combustor
drops. But, as the contents of unburnt hydrocarbons (i.e.,
paraffins such as methane and ethane) increase, so does
carbon monoxide generated by incomplete combustion. The
increase in content of CH4, €2^6 and CO are in proportion
to the fuel stoichio.
In normal operation, the fuel stoichio is usualy set around
110% as the fuel composition fluctuates. NOX content in the
exit gas is about 40-50 ppm and HCN or NH3 formed is less
than Ippm.
Assuming that the local emission control regulations on
hydrocarbons will become more strict, we tested the
hydrocarbon abater after the catalytic combustor. In this
case, the same catalyst as in the catalytic combustor was used
and space velocity was set at 200,000 h' '. From test results,
CoiTosion rate
in gas phase
Test period
SN-1
(material welded)
0.00097 in/y
(0.0247 mm/y)
0.00098 in/y
(0.0249 mm/y)
14,616 h
Al
(material base)
0.00166 in/y
(0.0422 mm/y)
0.00166 in/y
(0.0422 mm/y)
10,608 h
100
We can say that the current stable operation of our strong
nitric acid commercial plant is in large part due to the
successful development of SN-1.
NOx abatement methods
OOOh-i,480°F
8 90
o
ë «o
While NOx content in the gas from the top of the NO2
absorber is 1,000-2,000 ppm in conventional nitric acid
plants, it reaches only 700-1,000 ppm in our 935 Mlb/d (425
metric t/d) nitric acid plant.
Among various methods for NOx abatement, the
following two are particulary effective:
1 ) nonselective catalytic reduction — with fuel (generally,
hydrocarbon) and 2) selective catalytic reduction — with
ammonia. In Sumitomo's nonselective catalytic reduction
process, off-gas from the ethylene plant is used as fuel. A
platinum catalyst of honeycomb type is used as the catalytic
combustor. Operating conditions are:
Space velocity:
60,OOph-1,660°F
75,000h-l
Catalyst layer
inlet temperature:
750 - 840°F
(400 - 450°C)
Catalyst layer
outlet temperature:
1290 - 1380°F
(700 - 750°C)
<
O
S
ui
70
K
m
60
50
0.5
0.7
0.9
1.1
1.3
INLET NH3/NOX MOLE RATIO (-)
Pilot test conditions SV=60,000 (ft3/hr)/(m ft3 cat) and 100,000h"
P =100 psia
T = 600°F and 480°F
inlet NOX= 600 ppm
O2 = 2.6%
Treated gas = Outlet gas from absorber
NH3 breakthrough (ppm)
NH3 leak ratio =
inlet NOy
(Ppm)
Figure 5. NOx removal efficiency and NHs leak ratio
vs. inlet NH3/NOX mole ratio.
169
we can say that emission of hydrocarbon is easily controlled
at less than 300 ppm, as CH^.
Selective catalytic reduction is also used at our plant. The
research and development of catalysts for NOX removal with
ammonia was started in 1972. We have operated quite a few
denitration facilities such as boiler and heating furnaces
including seven units with a larger capacity than 3 MMft3/h
(85000 m3/h).
Figure 5 shows the results of a pilot test in which the
catalyst we developed was used for the outlet gas from an
NO2 absorber. It is clear that the activity of the catalyst
slightly drops when the reaction temperature goes below
480°F (250°C), and it gives high denitration efficiency within
the temperature range of 480°F (250°C) to 750°V (400°C).
Since the ammonia content in the outlet gas after denitration
is as low as 10 ppm or less, there is no secondary pollution
problem. It is confirmed that the catalyst life is more than two
years.
Recommendations
Generally speaking, the optimal process for manufacturing strong nitric acid differs depending upon local conditions of the material, utility costs, and environmental
regulations. The strong nitric acid process discussed here
becomes more economical when it incorporates the weak
acid process, and by settling the corrosion problem, our
strong acid process can be easily added to the existing weak
acid process for economical coproduction. It is also possible
to employ various methods for NOX removal in our
Sumitomo process. For the production of strong nitric acid, it
is of vital importance to select the process best suited for the
local conditions, while considering the feasibility of the
coproduction of weak nitric acid.
#
DISCUSSION
hans-JOACHIM VOELKER, UHDE, Germany: What
is the normal pressure used in Sumitomo's mono
pressure process?
HIROMU KORi, Sumitomo Chemical: The pressure
is about 8-9 kg/cm2g.
VOELKER: And in the dual pressure process, what
is the pressure in ammonia burner?
KORE: The pressure is about 2 kg/cm2 g for the dual
pressure process.
VOELKER: Is the NO2 absorber (number 14 in Figure 1) a special contraption, or is it a normal sieve
tray apparatus as used in conventional process?
KORI: The NOa absorber is a rather conventional
type. But this facility has special construction materials. Some parts are constructed with SN-1.
VOELKER: Is this the only difference of material?
KORI: That is the main difference.
VOELKER: The next question is about the reactor,
number 12 in the Figure 1. Is this reactor liketheone
used for the direct process?
KORI: Yes, I think so, but the reactor also has special
construction materials.
170
VOELKER: I have some doubts concerning the
availability of SN-1, due to high silicon content.
Does availability compare with that of a normal 304
materials?
KORI: We can handle SN-1 just like 304 and 304L.
VOELKER: When you carried out your corrosion
tests with SN-1 and with 304 L, did you continually
renew your nitric acids, during the test?
KORE: We used two methods. One was in the actual
plant, and the other, laboratory tests. We find no difference between the two test results. We have now
developed a special test method to shorten the corrosion test.
VOELKER: In the laboratory test for SN-1 and 304L,
did you renew nitric acid several times?
KORI: Yes.
ELLIS: Du Pont: Did you test the susceptibility to
stress corrosion cracking of this material while you
were at it?
KORI: Yes, and we have not observed any stress corrosion cracking.