I
HOLGER C. ANDERSEN and WILLIAM J. GREEN Research and Development Division, Engelhard Industries, Inc., Newark,
N. J.
Removing Carbon Monoxide from Ammonia Synthesis Gas Data are now available for applying the selective CO oxidation process to ammonia synthesis streams at higher pressures
IN
A RECENT PUBLICATION, conditions were shown for converting carbon monoxide in ammonia synthesis gas selectively to carbon dioxide with limited oxygen additions (7). The necessary selectivity was provided by a supported catalyst of the precious metals group, preferably platinum. When catalyst temperature was controlled to 160' C. or lower, CO could be removed from COz-free feeds to less than 10 p.p.m. Feed gases containing u p to 2.5% CO could be purified. From streams containing appreciable COZ, partial CO removals (from 40 to 80 p.p.m.) were achieved. A process was described which provided for bulk CO removal from the COz-rich stream in a first stage, followed by scrubbing and thorough CO removal in a second catalytic stage. Experiments in the cited work were made in the pressure range of 1 to 1.7 atmospheres absolute. In many cases, i t is economically preferable to operate a t somewhat higher pressures, and this article reports results of recent experiments on large laboratory apparatus in the pressure range of 100 to 200 p.s.i.g. Generally, results were in close conformity with those inferred earlier ( 7).
Experimental
A catalytic reactor consisting of 2-inch I.D. stainless steel was employed. Hydrogen, nitrogen, methane, carbon monoxide, and carbon dioxide could be fed downflow into the reactor via calibrated rotameters. I n experiments employing steam, water was fed through a rotameter into a small flash boiler. Cas flows were large enough so that adiabatic conditions were closely approached. Chromel-alumel thermocouples registered temperatures above and in the catalyst bed. The bed temperatures given here refer to the highest temperature measured in the catalyst bed. I n the effluent, carbon monoxide was determined by means of a Liston-Becker nondispersive infrared analyzer Model 15A. The sensitivity could be adjusted from 2 to 10 p.p.m. Oxygen was determined on an Engelhard Minoxo Indicator, with a sensitivity of 1 p.p.m or better.
Results Previous work had demonstrated the necessity of maintaining catalyst temperature below 160' C. in order to remove CO to the 10 p.p.m. level. At low pressures, temperature could be conveniently controlled by incorporating steam with the dry synthesis gas. I n addition, the pilot plant reactor provided for cooling of the bed by coils through which water could be circulated. I n the present series of laboratory experiments a t elevated pressure, substantial concentrations of water vapor could not be included without raising the gas temperature unduly. I n addition, attempts to cool the bed with cooling coils, or by spraying liquid water onto the bed, proved impractical. Cooling by spraying liquid water into gas streams during catalytic processing is successful in large scale reactors ( Z ) , but is difficult to achieve in laboratory apparatus. Complete removals of carbon monoxide could therefore not be obtained from gases containing 1 to 2y& A more fruitful approach appeared to be one of determining the limiting gas compositions for complete selective CO removal in a simple adiabatic reactor. On a commercial scale, the same results could then be expected for a n adiabatiL reactor; or. improved results could be obtained by using reactor designs providing for bed cooling. In streams free of COz, good purifications could be obtained with inlet CO levels below about 0.5% (Table I).
co.
Table I.
However, oxidation of most of the CO could be achieved for much higher inlet CO levels. These findings indicated that relatively high CO levels could be handled in two stages, as in the earlier low pressure work. The following process steps, in addition to those normally provided in a shift conversion sequence, would then be required :
1. Catalytic oxidation stage number one. 2. Catalytic oxidation stage number two. The alternative of employing methanation as the second catalytic stage was discussed previously (7). This alternative applies also in high pressure operation. A very effective catalyst for this purpose is ruthenium ( 3 ) , which in contrast to nickel, is nonpyrophoric and requires no reduction a t start-up or after exposure to air. 3. COz scrubbing downstream of stage number two. Experiments illustrating two-stage removal are shown i n Figure 1. In the first stage, from 1 to 2% CO is reduced to 0.2 to 0.4%; i n the second, 0.3 to 0.5% CO is reduced to 5 to 10 p.p.m. Oxygen removal is also complete provided inlet temperature is maintained above a minimum threshold of about 60' C . A number of other aspects of this type of catalytic processing deserve mention: Carbon dioxide, when present in substantial amount, prevents thorough removal of CO (Table 11). At inlet levels of about 0.1% COZ,however, the effect is very slight. Hence COZ-
Under Essentially Adiabatic Conditions, Removal of C O to Low Levels Could Be Attained When the Inlet C O Was Below about 0.5%
Catalyst, 0.5% Pi on '/s-inch pellets; space velocity, 10,000 standard cubic feet (0' C., 1 aim.) gas per hour per cubic foot catalyst
Inlet CO% (Dry Basis) 0.12 0.3a
Pressure,
Best CO Attainable in Outlet,
P.S.I.G.
P.P.M.
185 130 130 100 100
0 0 0 0 0.5
2
5
0.5 9 1.0 650 1.Ob 500 0.22% CH4 and 0.1% COn in feed.
Conditions for Best Removal Inlet Bed L O 0 2 temp., temp., @;as CO OC. 'C.
b
1.2 0.8 1.0 1.2 2
78 59 61 129 180
117 99 153 297 349
7800 SCFH/CF.
VOL. 53, NO. 8
AUGUST 1961
645
1.6
I
STAGE I
0
17% CO,
24000
I
STAGE Il
070 co*
T n 160OC.
I0,OOO
z
5,000
1000
I
I
I
I
.04 I3 O
I
.02 1
r*
0
, --
$
I
2
0
I 0:4
velocity,
10,000 SCFH/CF;
were observed, for 71 C. inlet, 150" bed, 130 p.s.i.g., and 10,000 standard cubic feet per hour per cubic foot (SCFH/CF) over platinum catalyst:
Catalyst, 0.5% Pt on 1/8-inch support; space pressure, 1 8 5 velocity, 10,000 SCFH/CF; p.s.i.g.; feed gas, 74% H1, 24% Nt, 0.1 2% CO. and C o t , 0 2 as shown
139 139 140 157 156 154
111 110 119 111 111 111
Table 111.
Outlet,
Inlet % 0 2
0.17 0.17 0.17 0.28 0.28 0.28
COZ 0 1.0 2.0 0 1.0 2.0
P.P.11. CO 6-7 18 36 8-12 25 52
COSp.p.m. in
900
CO p.p.m.in CO p.p.m. out OZp.p.m. in
3000 8 4500 5 4600 4600
0 2
p.p.m. out
CH, p.p.m. in CHI p.p.m. out
Steam incorporated in the process stream appears to offer no advantage in the second "clean-up" stage. The beneficial effects of dilution in decreasing temperature rise are offset by the higher inlet temperatures required to prevent water condensation on the catalyst.
Process Conditions for Selective Oxidation of Carbon Monoxide in Ammonia Synthesis G a s to Carbon Dioxide
Catalyst, Engelhard supported platinum; space velocity, 10,000 standard cubic feet gas and vapor per hour (NTP) per cubic foot catalyst; pressure range, 100 to 200 p.s.i.g.; reactor type, adiabatic (higb.er inlet C O levels may b e to'erable in internally cooled reactors)
Additions Hz0 0 2
Applicable
Inlet Outlet Temp.. CO %CO %Con P.S.I.G. OC. P P.31. 0.0-0.5 0-0.1 0 1 100-200 60-120 < 10 0.5-1-5 0-17 1 1-1.5 160 1000-2000 100-130" 1.5-2 2000-4000 0-17 1 1-1.5 100-130" 160 Higher pressures can be employed, but required inlet temperatures and CO limits have n o t been determined. All compositions on dry basis. Feed
646
gas
co
Pressure,
INDUSTRIAL AND ENGINEERING CHEMISTRY
removal
Inlet gas, 1.5% CO, 17% COZ, and bal. 3Hz-1 Nz; pressure, 130 p.s.i.g.; space velocity, 10,000 SCFH/CF (total gas and vapor); catalyst, 450 ml. 0.370 Pt on '/s-inch support, in 2 inch dia. X 8I/,-inch deep bed
Inlet temperature, O c . Catalyst temperature, Outlet 02, p.p.m.
Table II. High COz Concentrations in Feed Gas Cause Deterioration in CO Remova I
Temp., "C. In Bed
catalyst,
CO
H20 =
scrubbing after the first stage need not be perfectly quantitative. Methane in the feed appears to have little or no effect on the selective process. I n one experiment, the following values
110
% O X Y G E N ADDED
Figure 2. In the first stage, sieam improves and inhibits methanation
Pressure, 130 p.r.i.g.1 space 0.5% Pt on l/&-inch support
I
Ob
Dry 80
"C.
262-396 0-1
1
gas
165 250-3 19 0-1
In the first stage, steam is advantageous, resulting in lower CO values and preventing methane formation (Figure 2). Some methanation occurs in dry streams a t temperatures above about 300° C. T h e experiment of Figure 2 was made with a catalyst even more selective than that of Figure 1. Space velocities of 7500, 10,000, and 15,000 SCFH/CF were studied with a feed containing 0.30/, C O , 0.1% COz, and 0.22y0 CHa a t 130 p.s.i.g. At the highest space velocity the 0 2 to C O ratio had to be increased to 1.5 to produce low C O (8 p.p.m.) in the effluent. Residual oxygens of 1 to 2 p.p.m. were found a t all space velocities.
Process Conditions Recommended process conditions for employment of the selective catalytic method are summarized i n Table 111.
literature Cited (1) Brown, M. L., Green, A. W., Cohn, G., Andersen, H. C., IND.END.CHmi. 52, 841-4 (1960). (2) FIAT Final Report 1107, "The Manufacture of Ethylene by Reduction of Acetylene," April 22, 1947. (3) Rosenblatt, E. F. (to Baker & Co., Inc.), U. S. Patent 2,747,970 (May 29. 1956). RECEIVED for review November 29, 1960 ACCEPTED April 5 , 1 9 6 1
