E. B. PECK Ė "OXIDATION OF METHANE."
E. B. Peck. The utilization of light hydrocarbons as chemical raw materials is an important development for this country as it was in Germany. In this country we have large supplies of natural and refinery gases, while the war economy of Germany required the use of every hydrocarbon to best advantage whether pure hydrocarbons or diluted as in coke oven gas or tail gases from syntheses.
This discussion will deal with two similar processes, the first producing synthesis gas from either dilute or concentrated hydrocarbons and the second producing synthesis gas and acetylene from rich hydrocarbon gases.
Synthesis gas is a refined water of specified carbon monoxide (CO) and hydrogen (H2) content as required for synthesis to make ammonia, methanol, higher alcohols or
synthetic oil. Acetylene (C2H2) has a wide use in synthetic chemistry for making synthetic rubber, plastics, and a variety of other products. In Germany it was even hydrogenated to ethylene and then polymerized to aviation lubricating oil.
The basic reaction in each process was partial combustion according to the reaction.
CH4 + Ĺ 02 = CO + 2H2 + 8 Kcal/gm mol.
When this reaction is carried out at high enough temperatures, about one third of the carbon is converted to acetylene.
When ammonia gas is made the combustion can be carried out with air but in other cases pure oxygen (97-98% 02) is used. Pure oxygen was widely used in the German chemical industry and we saw a number of Frankl-Line units that fractionate air economically at low pressures. The principal reason for using oxygen instead of steam for converting hydrocarbons to synthesis gas is that is supplies heat and oxygen can be saved to the extent heat is conserved. For this reason, an essential part of these processes is preheating the gases before combustion.
The safe handling of preheated explosive mixtures of hydrocarbons and oxygen is a development that was back up by extensive laboratory research on explosive limits, flame, velocities, combustion gas analyses and allied problems. This work is covered by a number of German patents.
Both processes are much the same. When only synthesis gas is made, the combustible gas and oxygen are separately preheated to around 400oC by heat exchange with the cracked gases. Then they are mixed, ignited and passed over a nickel catalyst to adjust the gas composition according to the water gas equilibrium. Some water is added with the inlet gas before preheating to supply oxygen that is needed chemically, but does not have to be supplied as pure oxygen for heat.
The mixer and burner are a vertical pipe about 30 inches in diameter, the two parts separated by a flame arrester. The gases enter at the top, the combustibles tangentially at 0.20.3 ats (gage) pressure and the oxygen through a concentric jet at 0.5-0.6 ats pressure. Mixing is supposed to be complete in 1.5 meters of travel down the pipe.
The flame arrester is a plate of tubes 20 millimeters in diameter in such number as to impart a gas velocity greater than the flame velocity. This is an optimum diameter for these tubes. The length is, as I remember it, about 100milimeters. In practice they used two flame arrestors spaced about 50 millimeters apart and with the tubes staggered through the cross section of pipe. This gives further mixing of the gases as well as more safety.
The gases are ignited from pilot flames on the bottom side of the last flame arrestor. They are fed with jets of pure oxygen.
In the combustion section the gases have a velocity less than the flame velocity to insure uniform combustion.
The gases enter the catalyst bed at 650oC, except when the sulphur content of the gas exceeds 15 mgm per cubic meter, when the temperature must be increased. Coke oven gas that is not desulphurized requires 200oC higher temperature. The catalyst is nickel oxide on magnesite and the activity is maintained by adding nickel as nickel nitrate solution to the combustibles before preheating. From the catalyst bed the gases go through two parallel heat exchangers where they preheat the feed gases.
The material balance and utilities for this process when producing 15,000 M3 of synthesis gas per hour from coke over gas are:
|Coke oven gas
|Oxygen (97% 02)
|Steam (low pressure)
|Cooling water 15oC
ANALYSIS OF GASES
|COKE OVEN GAS
When acetylene and synthesis gas are made the temperature is much higher and only pure hydrocarbons are cracked (methane or ethane). The reaction time must be controlled to thousandths of seconds and to this end the combustion gases are quenched with water at a very short distance from the flame arrestor. These gases have no preheat value so the feed gases are preheated to around 650oC in direct fired coils.
The mixer, flame arrestors, and combustion space are the same as in the former process except for quenching the reaction. Water nozzles spray water upward so that the flame length is not more than Ĺ the cross section (700 mm l.d. by 350 mm long). If the reaction space is not longer, then acetylene decomposes to carbon black. If the gas velocity is increased to prevent decomposition of acetylene then the flame becomes unsteady. This is the most critical part of the process.
Some carbon black if formed in any event. The gases are therefore scrubbed in a tower with lumps of coke, sprayed with water. The water washes down the carbon black to a settling tank where it is removed and the clear water recycled.
This washed gas contained 7-5% acetylene, if made from methane and around .9% if made from ethane. It is separated by scrubbing with water at 18 ats (gage) or higher. The Germans have tried other solvents and considered butrolactam promising.
This separation of acetylene was carried out as follows: the gases were scrubbed in a packed tower at 18 ats. And the solvent stripped in three stages. First the pressure was released to 3 ats and this gas which contained only 38% acetylene was recycled. Then the solvent was reduced to 1.1 ats where 35% of the acetylene rich gas was stripped out and the remaining acetylene taken off with vacuum; about 40 mm of mercury pressure absolute. This gas contained about 70% acetylene, the remainder being largely carbon dioxide, which could be scrubbed out by conventional methods. The Germans did not find this necessary for their subsequent processing.
The scrubbed synthesis gas contained 0.2% acetylene and about 7% methane and to remove these, the gas was treated again with oxygen, as in the first process to make only synthesis gas.
The material balance and utilities for this process when producing one ton of acetylene per hour and 10,000 cubic meters of synthesis gas, are as follows:
ANALYSIS (N2 FREE BASIS)
|Ca 5% N2 IN GAS.
|For 2nd combustion
|First cracked gas
|Final Synthesis gas
OTHER CONSUMPTIONS PER HOUR
|M3 @ 15oC
|The Plant cost in Reich marks is summarized as follows:
|Acetylene generators (5 units)
|Acetylene scrubbing plant
|Secondary burning plant
In conclusion it should be pointed out that the Koppers people have a competitive process for converting hydrocarbons to synthesis as with steam instead of oxygen using a blow and run process. Similarly, there are processes for converting hydrocarbons to acetylene by other heating methods (1) are the arc process and (2) by blow and run processes.
Finally, solid fuels are converted to synthesis gas either by blow and run processes or by partial combustion with oxygen. The Lurgi people made synthesis gas from lignite and oxygen under pressure of 20-28 ats. (gage.)
I. H. Jones. I would like to ask if it was Dr. Peckís original unabridged report which went to T.A.C. for reproduction?
E. B. Peck. No, itís the abbreviated one. I havenít sent the original report since we handed it over in our own handwriting.
I. H. Jones. We would like to get hold of that original somehow. It seems to me it would be more valuable to industry.
W. C. Schroeder. Did you hand the original over to C.I.O.S., or who did it go to?
E. B. Peck. In fairness to John Paul Jones, I think that Holroyd came in, grabbed it, and said this is part of the Ludwigshafen report, weíll take it up to Billingham, and thatís where it got abstracted.
J. P. Jones. I think that is correct. I know that I personally did no changing whatsoever, except to dot a few "iís" or cross a "t" here or there in any reports. I doubt if Dr. Peckís original handwritten report was typewritten. I donít know, we were extremely short of adequate stenographic help. If it was incorporated as part of the Ludwigshafen report, it did go to Dr. Holroyd, what Dr. Holroyd did with it I had no knowledge, and had no control over it. But I did understand that with some of the reports that Peck and Evans wrote, he had some questions. What he did as the result of his questions, I do not know.
W. C. Schroeder. I think weíd better write to Holroyd, apparently heís the man who has the original material, and see if we can locate it.
C. S. Snodgrass. I would suggest, Mr. Chairman, that instead of writing to Dr. Holroyd direct, you address an official letter to the Ministry of Fuel and Power, requesting this and other original manuscript which should properly form part of the records and which you havenít been able to locate, indicating where you think they may be.
W. C. Schroeder. Thatís a very excellent suggestion, in view of the fact that our liaison is supposed to be with the Ministry.
C. S. Snodgrass. I think you should place the responsibility squarely on them, and indicate that these manuscripts are the property of the U.S. group.
A. R. Powell. I have one question, of Dr. Peck. In that last process you mentioned, the carbon, calcium and oxygen were used to make carbide. Didnít you intent to say "coal, lime"?
E. B. Peck. Yes, coal, lime, and oxygen. I think I would say in fairness to the British on this report, they probably took the view that our calculations were not essential data, and, therefore, could be deleted because anybody else could do them. In fact, there is a note down here to that effect. There are no costs in this process; the writer states that fuller details and utilities, consumption, labor requirements, etc., no, thatís not the one. The above data should be sufficient for the purpose to be costed under British and American conditions. Holroyd was going to do the same under British conditions, and they were both going in the report; and then I think they felt that the report got so bulky those were things any competent engineer could do, and they might just as well be deleted.
W. C. Schroeder. I still think it is of great interest to this group, if the work has been done it should be available to everybody.
V. Haensel. I would suggest that instead of writing that you send a wire to the British Military of Fuel and Power, and get rapid action. If it goes by mail, we wonít see it for another six months.
NOTE: The information referred to in this discussion was requested informally by W.F. Faragher, but has not been received. Mr. Faragher (who is now abroad) will again make an effort to secure this information at his first opportunity to visit London. Mr. L.P. Evans has stated on November 21, 1946 that the essential information is in Bureau of Mines Information Circular 7375. L. L. N.
W. C. Schroeder. Mr. Fraser has spoken to us earlier on "Refineries near Hamburg." He will now say words regarding the preparation of coal-oil fuel mixtures in the form of "Schaumkohle", of foam coal. Mr. Fraser.