I
HOLGER C. ANDERSEN, WILLIAM J. GREEN, and DUANE R. STEELE Research & Development Division, Engelhard Industries, Inc., Newark, N. J.
Catalytic Treatment of Nitric Acid Plant Tail Gas The nuisance of nitric acid tail gas can be turned to advantage by processing the gas with a fuel over platinum metals catalyst F O R THE PRODUCTION of nitric acid, ammonia-air mixtures are passed over platinum metals catalyst to convert the ammonia to nitric oxide. This nitric oxide is then further oxidized and absorbed in water to produce " 0 3 . Absorption efficiency is usually such that the stack gas contains a n appreciable percentage of nitrogen oxides. For this paper, the tail gas was assumed to have a composition in the range:
NO 0
+ NO1
2
H?O NP
0 2-0 5 5 2 5-57, Saturated at absorber conditions Balance
although small concentrations of other compounds are present in actual tail gases. The higher nitrogen oxides are toxic, and the stack gas therefore presents a n air pollution problem. The problem is esprcially serious in view of the large amounts of gas involved-approximatel) 1,000.@00 SCFH for a 240ton per day nitric acid plant. Llihile the SO2 can be removed by various scrubbing processes such as that recently described by Streight (76), N O is amenable to removal by such means only for certain KO-NO, ratios. Catalytic treatment of the gas stream with a reducing substance can, on the other hand. reduce ,311 the oxygen-containing
compounds in the stream, except H,O. Furthermore, heat evolved may be economically recovered, either in steam boilers, or through improvement in turbine efficiency. Catalytic processing to produce pure nitrogen wi!l be published later. The idea of catalytic treatment of nitric acid plant tail gas is not new. In 1924, for example, Fauser ( 9 ) was granted a patent for a catalytic process. However, most literature references give little specific data on catalytic process conditions required to achieve the several possible objectives. The results of several years' experimental work are summarized, and these results are related to a number of specific tail gas catalytic processes in the table below.
with Conax glands (Conax Corp., Buffalo, N. Y . ) which carried bare wire Chromel-Alumel couples for rapid and complete temperature response. Thermocouple output was generally recorded on a Weston 6-point potentiometric recorder. With the gas flows employed in this series of experiments-100 to 600 SCFH-conditions were sufficiently adiabatic so that observed temperature increments were usually 80% or more of those calculated from heats of reaction. Gases. Nitric oxide, nitrogen dioxide, technical grade methane, ethylene, ethane, propane, propylenr, butane, carbon monoxide, and carbonyl sulfide Ivere obtained from the Matheson Co., Inc. Linde or Air Products nitrogen was used. Hydrogen was supplied by National Cylinder Gas Co. or Air Products. For some experiments, fuel mixtures were made u p by admitting the calculated pressures of desired gases to an evacuated cylinder. Procedure. In most experiments, nitrogen and air were first metered a t the desired flow values; nitric oxide was then added, and the mixture was brought to desired temperature with the preheater. Except for one case, NO2 was not metered as such, but was formed in varying degree from the metered flows of
Experimental
Apparatus. With the exception of a few small-scale experiments made for exploratory purposes, most quantitative experiments were made with an apparatus of semi-pilot plant scale, shown as a flow sheet. The rotameters were calibrated at 120 p.s.i.g., against wet test or dry gas meters. Several different reactors were used, varying from 1 to 2 inches in diameter. These reactors were of Type 304 stainless steel and were fitted
Catalyst Selection Chart
To Produce To Produce Heat and Remove NO and NO, Heat i Power)
Process Conditions
To Remove NO NO,
Usable fuels
Ammonia
Natural gas, hydrogen, carbon monoxide, bleeder gas, hydrocarbon mixtures, coke oven gas, vaporized kerosine
Assumed fuel
Ammonia
Natural gas
2-5
3-5
+
~
0 2
in tail gas,
yo
~
~~
Natural Gas
2-3
3-5 ~~~~
Number of stages
1"
1
I 1
1 .____._____~
1st stage
~
Preferred catalystb Space vel., SCFH/CF Min. inlet temp., O C. Amt. fuel, vol. of tail gas stream Temp. rise, O C., app.
yo
Pi
Pd
30,000 180 0.3
20,000-40,000 440-500 0.5-1.4
20-40
130 per
Pd
yo O 2
removed a
~
1 1
~
60,000 460 1.1-1.7
130 per % 0 removed
I f additional temperature rise i s desired, a second stage operating with All catalysts are commercially available from Engelhard Industries, Inc.
Hz can
2
~
2nd stage
Pd
Pd
20,000-40,000 440-500 0.8-1.3
60,000
j
E L 4
130 per % 0 2
'
130 per % 0 2
removed
removed
be employed
VOL. 53, NO. 3
MARCH 1961
199
(t?C t-
PREHEATER
NITRIC OXIDE
' -f CATALYST
c
T1
L
MANOMETER
1 -
FUEL G A S
T,
1
I
t
1
REACTOR
WWNSTREAM SAMPLE
do
,
2 ,
1 , 2, 3, 4. First, second, third, and fourth ignition determinations
Tail gas treatment was studied in apparatus providing gas flows up to 600
SCFH S O and air. \\'-hen catalyst temperature and flo\vs were steady, fuel )\-as introduced. If the catalyst temperature was below the ignition point, fuel was turned off and the catalyst temperature raised to a higher level by increasing the preheater fuel or kvattage: and fuel again admitted Ichen a new steady temperature level had been attained. Under conditions of ignition the catalyst temperature rose rapidly to a new steady value. At this time, gas samples \cere taken for analysis, and a complete record of flows, temperatures, and pressure drop was made. When the saturator was used, water vapor flow was calculated from the drop of liquid level in the saturator. Sitrogen oxides were determined by several methods. .4t low levels, combined NO and NO, was usually measured by shaking a known volume of the gas with air, a small partial pressure of butadiene, and sodium hydroxide solution; the resulting sodium nitrite was then estimated colorimetrically with a-napht hylamine-sulfanilic acid (Griess reagent). The method is an adaptation of that reported by Liebhafsky and Winslow ( 7 2 ) and Fulwiler (70). The authors' experience confirmed the statements by Liebhafsky that the method converts each mole of N O to 1 mole of nitrite, whereas each mole of NOnis converted to '/2 mole nitrite. Results are therefore indicative of N O Analytical.
+ '1. NO?.
LVith the 5-liter sample bulb
used in this Lrork, sensitivity is judged to he approximately 3 p.p.m. .4 few determinations were made by oxidizing NO
with acid permanganate, according to Guyer and Weber ( 7 7 ) , and then determining nitrite with Saltzmann reagent ( 7 5 ) . This method, which registers NO S O ? , is inherently more sensitive than the air oxidation method, but is subject to greater uncertainties in completeness of oxidation and N O ? absorption in the colorimetric reagent. Nitrogen dioxide, NO,, \vas determined as a separate species by its light absorption in a 48-inch long glass tube; light transmission was registered by a b'eston photocell No. 594, and the unit was calibrated with known mixtures of N O 2 in nitrogen. Sensitivity was approximately 100 p.p.m. .Ammonia \vas determined, in essentially acid-free catalyst effluents, by passage of the gas through a knoivn volume of standard sulfuric acid containing methyl orange, and observing the gas volume registered on a wet test meter when indicator color changed. Oxygen was measured by Orsat analysis at high levels, and by Super Sensitive Indicator (Engelhard Industries, Kewark, N. J . ) at the parts-per-million level (5). Carbon monoxide and carbon dioxide were determined either by Orsat analysis or by infrared absorption using a PerkinElmer Model 21 spectrophotometer.
+
Results Catalysts. T ~ v oimportant criteria of catalyst usefulness are fuel-oxygen ignition temperature and catalyst life. Supported rhodium or palladium catalysts ignite methane in tail gas a t the
Hydrogen Reduces NO2 to NO in the Presence of Excess 0 D. reactor. Tail gas, 0.370 NO, 37; 0 2 , balance Nn.
100 ml. catalyst in 1.76-inch I.
velocity:
Pressure,
H?.
0.5 Pd
100 4h 4h 100
0.5 Ru
100
0.32 1.21 1.63 0.6 1.28 0.15 0.6
Catalyst, % 0 . 5 Pt o n 1 , ' B - h . cylinders
1 2 0 , 0 0 0 SCFHICF
T ~ I ~ I PC. ,. P.S.I.G. Added, % Inlet Bed 119 178 214 174 181 250 252
140 276 352 221 282 261 273
In
% KO? Out
0.19 0.34 0.32 0.13 0.12 0.14 0.13
0.035 0.13 0.04 0.04 0.016 0.02 0.01
Determined by shaking v-ith H202solution and titrating H S O I formed. NO, metered into feed gas.
200
1000
Figure 1 . Oxidizing treatment usually increases ignition temperature requirement, but effect is partially reversible
VSNT
I
d
Table I.
I
251400 500 600 700 eo0 LIR F V R N A C I N G T E M P E R I T U R E ( F O R i 6 H O U R S l *C.
--
lo\\est temperatures (7). For use of other hydrocarbons, such as for example propane or ethylene, the preferred catalyst is platinum (6). The values of these ignition temperatures vary with operating conditions. To obtain accelerated aging data, the ignition temperature of CHr-tail gas mixture on several catalysts, before and after furnacing a t several elevated temperatures was determined. O n fresh catalysts. ignition occurred a t 310' to 490' C. (Figure 1). After the catalysts \\ere aged, the ignition temperatures rose appreciably. Each subsequent ignition then occurred a t a lower temperature (points 1 , 2 , 3. and 4 in that order), hchich was attributed to reducing action of the gas stream on the catalyst partially oxidized in the furnacing treatment. This pattern of behavior was characteristic of all the catalysts employed. Although the palladium catalyst \ \ a s slightly superior to the rhodium, both in the initial and aged states, palladium and rhodium are so similar in activity
INDUSTRIAL A N D ENGINEERING CHEMISTRY
% KO
2
Space
+ NO?"
In
Out
0.32
0.22
0.27
0.24
0.32
0.28
A
B
R \ I
P
- _ THEORETICAL _ _ _ _ _ _ _ _( A- )-
W
4 W LL u
cl Q W
3 4
a W
I -W i
REDUCING
0 32
y. y.
SPACE VELOCITY o 28.400 S C F H K F X 56,800
O24t
113,600
Figure 2. Nitrogen oxides are removed thoroughly when fuel is in excess over the oxygen content Catalyst, 0.3% Rh on '/,-inch spherical support Amount, 100 ml. Pressure, 100 p.s.i.g. Reactor, 1.76 inch I.D. Inlet temp, 432 to Inlet gas, 0.3% NO metered, 3% 0 2 449O c. metered, bal. N2 metered
Theoretical fuel requirement shown as region bounded by limits A-B calculated by assumption of either 1.
Y02, rather than 2.
++ + ++ ++ + +
CHa 202 COn 2H20 CH4 4 N 0 = COz 2H2O 2N2, or 202 COz 2H20 CH4 5CH4 8NO 2 H 2 0 = 5COn+ 8NHa
+
NITRIC ACID TAIL GAS
VOL. 53, NO. 3 e
MARCH 1961
201
that they are interchangeable in most applications. Since palladium is considerably lower in cost, it was chosen for most experiments involving methane and other fuels. General Features of Catalytic Treatment. When fuel in increasing amounts is added to nitric acid tail gas, and the mixture passed over a catalyst above the ignition temperature, the temperature rises as shown in Figure 2. When the fuel exceeds the oxygen stoichiometrically. the nitrogen oxides are reduced to low concentrations. O n the reducing side, some ammonia formation occurs over Pd, Pt, or R h when the fuel contains hydrogen. Others have reported ammonia formation over copper (73). The fuel-oxygen reaction CHa
+ 2 0 2 = CO, + 2 H 2 0
These findings, which were confirmed repeatedly in numerous experiments, do not support the contention of others ( 8 ) that nitrogen oxides elimination can be achieved with use of fuel less than stoichiometrically equivalent to the total oxygen content. However, apparent purification can be achieved with fuel deficiency when NO? is converted to the colorless NO. When 1% CH4 was reacted with a tail gas containing 47c 0 2 , 1100 p.p.m. NO?, and approximately 1900 p.p.m. NO, the effluent contained only 260 p.p.m. KO,, and 2300 p.p.m, NO. The limit of visibility of NOS has been variously estimated a t 50 p.p,m. (76) to 300 p.p.m. ( 8 ) ; the authors' experience accords with the latter value. Similar apparent purifications can be obtained with hydrogen (Table I). High temperatures are registered in the bed when fuel is equal to or greater than that required by oxygen (Figure 2). O n the oxidizing side, the temperature drops off sharply, indicating incomplete reaction. This was attributed to a n oxidation of the metal to a less active form. The oxidized form which is shown by x-ray diffraction is different in color from the reduced form! and the color changes are reversible. Operations on the oxidizing side, which are commer-
(1)
usually proceeds more rapidly on the catalyst than the fuel-nitrogen oxides reactions:
+490
+ 2H2O + 2Nz (21 5CHa + 8x0 + 2 H 2 0 = 5C02 + 8NH3 (3) C H a + 2 N 0 2 = CO, + N, + 2 H 2 0 (4) 7CH4 4- 8 N O z = 7COr + 8NH3 + 2H2O ( 5 ) CHa
CO2
~~
Table 111.
Carbon Monoxide Is Formed with Carbon-Containing Fuels, Especially when Large Excess Is Used
Presbure. 100 p.s.i.g. Space velocity, 30.000 S C F H / C F . 100 nil. catalyst in 1.76 inches I.D. reactor. Inlet gas composition: 02.KO, and C H I as ahown plus 0.77, H20: bal.. iV? Catalyst
0.57' Pd on 1 r - I ~ ~ c h Spherical Support
Low hIethane In out 417 751 3.0 0
Condition Temp., C. 02,Orsat, %
+ '12
NO
NO?, % CHI, rotam.,
0.3
5%
1.7
CH1, infrared CO, infrared,
72 CO?,Orsat or infrared, 70 Hz, Orsat,
Table IV.
0.0015
...
... ...
0.12
... ...
1.5 0.2
0.10
-
Low l I e t h a n e
High Methane In 363 3.0
out 652 0
0.0042
2.8
1.7 1.66
0.35 1.6 0.5
0.0009
4.7 4.4
...
...
In out 401 700 3.00 0 0.3
0.3
,.*
0.57, Pt on 1'8-1nrh Spherical Support
...
High 1Iethane
In out 389 636 3.00 0.1 0.3
0.0002
0.04
4.7 4.41
2.08
...
0.13
...
0.86
... ...
1.3 0.1
...
...
1.5 0.6
Two-Stage Nitric Acid Tail Gas Process Is Used when Oxygen Level in Tail Gas Is High
Catalyst,, 50% of 0.5% Pt; 50% of 0.5% P d , both on ','s-inch cylinaars. Two-stage reactor, 1.05 inches I.D. T o p and lower beds. 100-ml. catalyst (7 in. deep). Tail gas, 0.3% NO, 4.0 0 2 , 1.2 Hz0, bal. N2. Fuel, 26.9% CHd. 28.7 CzH6. 3.0 C ~ H J21.4 . C:Hs, 16.0 CaHs, 4.0 C
