US3308056A - Use of nitrogen oxides to inhibit coke formation in hydrocarbon thermal cracking processes - Google Patents

Description

March 7, 1967 J. A. SCHULTZ USE OF NITROGEN OXIDES TO INHIBIT COKE FORMATION IN HYDROCARBON THERMAL CRACKING PROCESSES Filed May 18, 1966 Ni/r'ogen Hydro carbon AH'r and 01/049; 680 0/900 Prehea/er flea/ea 90666 Hea/ Cracker C racked gases 60, 602, N2, r

(Inca/7049 7360 0566 L/gu/fi/eo cracked gases INVENTOR. John 4 Salad/ BY HTTORNEYS United States Patent 3,308,056 USE OF NITROGEN OXIDES TO INHIBIT COKE FORMATION IN HYDROCARBON THERMAL CRACKING PROCESSES John A. Schultz, Bay City, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Filed May 18, 1966, Ser. No. 557,335 3 Claims. (Cl. 208128) This is a continuation-in-part of my copending application Serial No. 291,499, filed June 28, 1963, now abandoned.

This invention relates to an improvement in conventional thermal hydrocarbon cracking techniques and, more particularly, to the addition of an oxide of nitrogen to the hydrocarbon feed to minimize carbon formation in the cracking process.

Coke formation as a result of the thermal cracking of hydrocarbons is undesirable and has long been a problem in that the cokeformed tends to plug the reactor tubes. Up until now, this necessitated the stopping of the cracking process periodically and removing the carbon by oxidizing it to carbon monoxide and carbon dioxide with air and steam.

Thus, it is a principal object of the present invention to provide a method for minimizing coke formation in thermal cracking processes thereby minimizing reactor plugging and down time for decoking and thus increasing the efficiency of the cracking operation.

A further object of the present invention is to provide a method for minimizing coke formation in thermal cracking processes, said method producing gaseous products which are readily separable from aromatic hydrocarbon products.

These and other objects and advantages will become more apparent after reading the detailed description of the method of this invention disclosed hereinafter, reference being made to the accompanying drawing showing a flow diagram of the method of the invention.

In accordance with the method of the present invention, a nitrogen oxide product is added to the input hydrocarbon feed to a thermal hydrocarbon cracking process and both said nitrogen oxide product and said feed are submitted to cracking. The nitrogen oxide product oxidizes the carbon or carbon-forming species produced by cracking of the hydrocarbon feed thereby forming byproducts in the cracking procedure. The so-produced byproducts are gaseous products such as carbon monoxide, carbon dioxide, nitrogen and sometimes, primary amines. These gaseous products are easily separated from the desirable products and removed therefrom.

This improvement in cracking, that is, the addition of a nitrogen oxide product to the hydrocarbon feed, may be used in connection with any conventional thermal cracking operation in which hydrocarbon is subjected to pyrolysis. In the process of this invention, as in the standard thermal cracking process, it is necessary that substantially no air or oxygen be present in the reaction zone during the cracking process.

In general, any of the oxides of nitrogen may be used in the method of this invention including nitrous oxide (N 0), nitric oxide (NO), nitrogen dioxide (N0 nitrogen trioxide (N0 dinitrogen tetroxide (N 0 dinitrogen trioxide (N 0 and nitrogen pent-oxide (N 0 3,308,056 Patented Mar. 7, 1967 ICC The range of mole ratios of the nitrogen oxide product to the hydrocarbon feed needed for eflicient operation of the method of this invention is determined by the nature of the hydrocarbon feed, the nature of the nitrogen oxide product and the amount of coke formed during the cracking process in question which must be burned out in order to continue the operation wherein no nitrogen oxide product is added to the feed stream. The quantity of coke burned out may be determined by analysis of the quantity of air and/or steam needed in accomplishing the burn-out. For example, where propylene is to be cracked, a range of from 0.1 to 10 mole percent of a nitric oxide may be used, with a preferred range being from 0.5 to 3 mole percent.

The method of this invention may be carried out at any temperature at which thermal cracking is accomplished, such as from about 600 to 1050 C. In cracking propylene, for example, a temperature in the range of from 650 to 950 degrees centigrade is preferred.

This invention may be carried out at atmospheric or at elevated pressures as understood in the cracking art.

The addition of a nitrogen oxide into the hydrocarbon feed stream has no material effect on the distribution of the cracked products. The product distribution, after the hydrocarbons are cracked either with or without using a nitrogen oxide product, are approximately the same.

The accompanying figure is illustrative of a method of carrying out this invention as described in the following example.

Example In order to demonstrate the effectiveness of the addition of a nitrogen oxide to the feed stream in a hydrocarbon cracking process, the following experiment was conducted.

Cracking apparatus was assembled as illustrated in the accompanying figure. The cracker 14, made of 1 inch S8446 pipe, was maintained at 830 C. and the preheater 10 at approximately 400 C. Propylene (98.6 percent pure) was passed through the unit at 200 liters per hour. Nitric oxide (99 percent pure) was introduced through a Rotameter at 4.4 liters per hour. The cracking process was carried out for 3 hours. The liquid products were collected in the receiver while the gaseous ones were analyzed and their volume measured with a wettest meter.

The propylene feed stream was closed after the three hours and a 50-50 air-argon mixture was then passed through the reactor at such a rate that the heat of combustion of the carbon raised the tube temperature to 1000 C. Burning of the carbon continued while the temperature was maintained at 1000 C. by increasing the amount of air into the reactor and at the same time, proportionately decreasing the amount of argon entering the reactor. Burn-out was complete at 6 minutes whence the temperature of 1000" C. could no longer be maintained notwithstanding the quantity of air allowed to enter the reactor.

For comparison, the same cracking procedure was performed, but Without the addition of nitric oxide to the prolylene feed stream. After three hours, the propylene feed was terminated and the same burn-out procedure aforementioned was performed. Burn-out time required to relieve the reactor tubes of carbon clogged therein was 1 /2 hours.

Thus, it is seen that the addition of nitric oxide greatly decreased the amount of carbon formed as a solid coke during the cracking process.

An analysis and comparison of the product distribution resulting from both of the aforementioned runs revealed no appreciable difference except for the presence of more oxide and nitrogen compounds in the run wherein nitric oxide was added to the feed stream.

Various modifications can be made in the method of the present invention Without departing from the spirit or scope thereof, for it is understood that I limit myself only as defined in the appended claims.

I claim:

1. In a process for the thermal cracking of hydrocarbons, the improvement which comprises admixing from 0.1 to 10 mole percent of an oxide of nitrogen to the hydrocarbon feed prior to subjecting such feed to thermal cracking temperatures.

2. The process of claim 1 wherein the oxide of nitrogen is added to the hydrocarbon feed in an amount of from 5 0.5 to 3 mole percent.

3. The process of claim 1 wherein the oxide of nitrogen is nitric oxide.

References Cited by the Examiner UNITED STATES PATENTS 2,660,032 11/1953 R osenthal 208129 2,178,365 4/1965 Miale 208114 DELBERT E. GANTZ, Primary Examiner.

H. LEVINE, Assistant Examiner.

Claims (1)

1. IN A PROCESS FOR THE THERMAL CRACKING OF HYDROCARBONS, THE IMPROVEMENT WHICH COMPRISES ADMIXING FROM 0.1 TO 10 MOLE PERCENT OF AN OXIDE OF NITROGEN TO THE HYDROCARBON FEED PRIOR TO SUBJECTING SUCH FEED TO THERMAL CRACKING TEMPERATURES.

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