SELECTIVE CATALYTIC REMOVAL OF A M M O N I A FROM GASES CONTAINING ACRYLONITRILE Bismuth phosphomolybdate is an effective catalyst for the vapor-phase selective oxidation of ammonia in the effluents of the propylene ammoxidation reaction. Such a n oxidation required very different reaction parameters of temperature, contact time, and gaseous linear velocity from those necessitated for propylene ammoxidation. Up to 90% of the ammonia was oxidized while acrylonitrile was little affected.
ammoxidation of propylene to acrylonitrile, some ammonia remains unreacted under the optimum conditions of the process. During the product recovery, ammonia reacts to form undesirable by-products and resins, diminishing the yield of acrylonitrile. Sulfuric acid has been used to neutralize the ammonia and the separation of pure, fertilizer-grade ammonium sulfate has been claimed (Schoenbeck, 1967), but a simpler and more economical solution of the problem is desirable. Other suggested means of ammonia removal comprise neutralization by volatilizing formic acid into the gaseous
mixture (Hadley, 1967), neutralization by the use of an ammonium sulfate-sulfuric acid mixture and recovering crystallized ammonium sulfate (Borrel and Newman, 1964), and separation by treating with washing liquids that absorb specifically acrylonitrile (Sennewald et al., 1964). In the ammoxidation of propylene with ammonia and air to acrylonitrile over a silica-supported bismuth phosphomolybdate catalyst, ammonia reacted completely under certain conditions, but not under other conditions in a laboratory-scale fixed-bed reactor (Table I). On the
I N THE
Table I. Effect of Gaseous linear Velocity on Catalyst Performance % Conversion (Based on C,+Hs)
Catalyst Volume", M1
Feed N H 3 j C 3 H 6 Molar Ratiob
To A C R N d
To A C N d
R Acrolein in Off-Gas
% N H 3 in
V H S v'
25 5 25 5 25 25
1.1 1.1 2.5 2.5 1.1 2.5
665 665 665 665 133 133
25 54 54' 69' 46 69'
1.1 7.8 2.1 9.4 8.6 5.4
2.4 0 0.7 0 0 0
0 0.30 0 2.45 0.075 0.67
Off-Gas
h-inch pellets of silica-supported bismuth phosphomolybdate ' F o r 25-ml catalyst uolume, reactor is a 14-inch-0.d. U-tube containing catalyst. For 5-ml catalyst uolume, reactor is a g-inch-o.d. straight tube containing same catalyst. Temperature of synthesis in all r u m is 4650 C. For molar ratio 1.1, C3Hs is 7.95% and N H 3 is 8.80%. For ratio 2.5, C3Hs is 5.42% and NH3 is 13.8455, Balance is air in both feeds. V H S V = uolume of total feed ( S T P ) passing through catalyst bed per hour c volume of catalyst bed. d A C R N = acrylonitrile, A C N = acetonitrile. 'Numbers may be somewhat high because of some water condensation in off-line in case of extraN 2 + H 2 0 ) andlor low V H S V . high NH, in feed ( N H , + 0 2 +
Table II. Removal of Ammonia from Acrylonitrile Stream over Bismuth Phosphomolybdate Catalyst
NO.
First Second Reactor Temp, C
11601-17 -21A -23 -43 -47 -49 -50 -53 -54 11444-28
4701465 470/ 515 470 / 540 480/540 4651540 4501540 4501565 4551540 4551540 4501565
Code
NHj 5.40 4.94 4.94 8.31 7.73 7.80 7.80 7.45 10.22
...
Feed". 5; CSHs 5.62 5.77 5.77 7.36 6.92 6.92 6.92 7.38 7.05
...
02
Off-Gas from First Reactor, 7;
First Reactor V H S V', Hr -'
Second Reactor V H S vh, Hr -'
NH,
C3HG
ACRN
ACN
Acrolein
02
coi
331 441 441 217 222 189 189 188 188 189
2340 2900 2810 2810 3355 2810 2730 2810 2810 2350
0.43 0.84 0.84 1.17 1.38 1.34 1.34 1.45 4.64 1.22
0.73 1.47 1.47 0.44 0.41 0.54 0.54 0.45 0.57 0.41
3.02 3.11 3.11 3.33 3.92 4.27 4.27 4.27 3.96 4.46
0.19 0.12 0.12 0.15 0.16 0.22 0.22 0.15 0.15 0.13
0.13 0.11 0.11 0 0 0 0 0 0 0
10.6d 11.4d 11.4d 3.7 3.8 4.9 4.9 5.5 6.0 4.9
0.98 0.42 0.42 3.02 2.88 2.77 2.77 2.48 2.03 2.76
18.6 18.7 18.7 17.6 17.9 17.8 17.8 17.8 17.3
...
"Balance nitrogen. V H S V = volume of total feed (STPI passing through catalyst bed per hour t volume of catalyst bed. ' A C R N = acrylonitrile, ACN = acetonitrile. dSupplernental air added to of-gas from first reactor. Amount of oxygen indicated includes ox-ygen from ~
338 Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 2, 1970
basis of these observations, a procedure, based on a tworeactor system for the selective oxidation of ammonia in the presence of acrylonitrile was developed. Experimental
The first reactor of the two-reactor system was a l-inch0.d. stainless steel, fixed-bed U-tube, suspended in a salt bath. I t contained a pelleted ("i6-inch) antimony oxideuranium oxide catalyst made according to the procedure of Callahan and Gertisser (1965). The second reactor was a stainless steel, fixed-bed straight tube heated by an electric tube heater. I t contained a pelleted (%,- or 731inch) silica-supported bismuth phosphomolybdate catalyst. The feed mixture, containing propylene, ammonia, oxygen, and nitrogen, was passed through the first reactor, the off-gas from which was carried through a heated tube into the second reactor. A layer of stainless steel balls as a preheater was placed before the catalyst in the second reactor. T h e recorded temperature of the second reactor was the temperature of the center of the catalyst bed. The performance of the first catalyst alone was determined by keeping the temperature of the second catalyst a t 150" to 200°C. The off-gas from the second reactor was carried through a heated tube and analyzed by means of Perkin-Elmer Fractometers, Model 154. Suitable Porapak (porous crosslinked polymer based on ethylvinylbenzene from Waters Associates, Inc.) columns were used for nitriles, acrolein, and ammonia. Carbon dioxide and propylene were analyzed on a silica gel column, while a molecular sieve column was used for carbon monoxide. Oxygen was determined by a Beckman oxygen analyzer.
acrylonitrile was evident. The fate of hydrogen cyanide supposedly present in the products of the propylene ammoxidation was not determined. Addition of supplemental air to the off-gas from the first reactor was shown to work satisfactorily. This might be advantageous for an extended catalyst life. The temperature rise in the catalyst bed as a result of the oxidation of ammonia and propylene was insignificant as measured because of the quick attainment of equilibrium in the small laboratory reactor. I n a largescale operation, a suitable heat exchange device might be required t o remove heat. When the concentration of ammonia in the second reactor feed was abnormally high, the oxidation of ammonia still approached 90% of the total present. This also resulted in the formation of more acrylonitrile. Conclusions
The process is simple. Most of the ammonia in the effluents of the ammoxidation reaction can be oxidized to nitrogen and water, while acrylonitrile is unaffected. I n a large-scale operation, the heat recovered could be utilized in the form of superheated steam. Acknowledgment
Valuable advice and discussions were provided by R . A. Smiley. S. W. Ward assisted in the experimental work. Literature Cited
Borrel, Marcel, Newman, F. C. ( t o . Societe d'ElectroChimie, d'Electro-Metallurgie et des Acrieries Electriques d'Ugine), French Patent 1,357,087 (April 3, 1964). Callahan, J. L., Gertisser, Berthold (to Standard Oil Co., Ohio), US.Patent 3,198,750 (Aug. 3,1965). Hadley, D. J. (to Distillers Co.), Brit. Patent 1,063, 403 (March 30, 1967). Schoenbeck, R., Hydrocarbon Processing 46, 124 (1967). Sennewald, Kurt, Vogt, Wilhelm, Kandler, Joachim (to Knapsack-Griesheim A.-G.), Can. Patent 683,907 (April 7, 1964). S . C. Malhotra E . I . du Pont de Nemours & Co., Inc. Gibbstoux, N . J . 08027 RECEIVED for review May 21,1969 ACCEPTED September 8,1969
Results
Table I1 shows results from typical runs. The reaction conditions for the first reactor were adjusted to yield desired concentrations of ammonia, propylene, and oxygen in the effluent. I n the second reactor, most of the ammonia present was oxidized by proper adjustment of temperature, contact time, and gaseous linear velocity. Along with ammonia, some propylene was also oxidized and, in certain cases, an additional amount of acrylonitrile was formed in the second reactor. I n two cases, when the second reactor temperature was 565" C, some degradation of
Off-Gas from Second Reactor,
co 0.96 0.48 0.48 0.65 0.70 0.79 0.79 0.83 0.71 0.92
Conv. tc ACRN' N H I 54 54 54 45 57 62 62 58 56
...
0.17 0.10 0.02 0.57 0.42 0.14 0.06 0.22 0.48 0.09
CIH, A C R N 0.48 0.64 0.60 0.10 0.07 0.08 0.08 0.04 0.06 0.02
3.23 3.37 3.35 3.44 3.93 4.29 4.18 4.21 4.07 4.30
5
NH,Con-
ACN
Acrolein
OL
C02
CO
0.23 0.25 0.12 0.12 0.12 0.15 0.12 0.10 0.15 0.09
0.05 0.12 0.24 0 0 0.03 0.03 0 0 0
10.1 9.3 9.0 2.0 1.9 1.8 1.0 2.2 0.7 0.7
1.33 1.22 1.69 4.04 4.14 4.35 5.30 4.36 3.97 5.11
0.98 0.85 0.79 0.82 1.06 1.26 1.12 0.84 1.32
Conuersion to ACRN
sumption in ~ i c o n d Reactor, cc
58 58 58 47 57 62 60 57 58
60 88 98
51 70 90 96 85 90 93
Second Reactor Size , Inch I D 5/16 5/16 51 16 5/16 7/16 5/8 5/8 518 518 5/16
Gas Linear Flou through Second Reactor, Ft See. 0.74' 0.98' 0.98' 0.98 0.60 0.25 0.25 0.25 0.25 0.98
supplemental air. All analyses shoun uith same degree of dilution of feed or product species. VHSV giuen on undiluted basis. 'Linear uelocity of gas through second reactor calculated on undiluted basis.
Ind. Eng. Chem. Process Des. Develop., Vol. 9,No. 2, 1970
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