US2347527A - Cracking of hydrocarbons - Google Patents

Description

Patented Apr. 25, 1944 2,347,527 CRAOKINGOF HYDROCABBONS Byron M. Vanderbilt, Oranford, N. 1., assignor to Standard Oil Development Company, a corporation of Delaware No Drawing. Application December 27, 1940,

Serial No. 372,007

3 Claims. (01. 260-680) The present invention relates to improvements in the art of treating hydrocarbons, and, more 'particularly, it relates to pretreatment of the carbons in ferrous and other metallic equipment more coke or carbon is formed and deposited on the inner surfaces of the equipment than when the equipment is constructed from glass or silica. It has been shown, for example, that when 2- butene was cracked to form butadlene, using KAZS alloy tubes to form the cracking coil, the deposition of carbon was copious and soon plugged the reactor so that it was necessary to discontinue the operation in order to remove the coke or carbon. When this equipment. was replaced by quartz equipment, the deposition of carbon on the equipment was greatly reduced. My explanation as to why the deposition of carbon is excessive on certain ferrous alloys such as KAZS is that it contains metals which oxidize, and the oxides of these metals act as active catalysts in the decomposition of hydrocarbons which they contact at crackin conditions.

I propose to pretreat metallic equipment, such a cracking coils', with an acid having an ionization constant greater than whereupon the equipment is rendered substantially passive toward causing the formation of excessive quantities of coke when subsequently employed in the cracking operation. I prefer to use an aqueous solution of such an acid, but gaseous acidic substances, such as hydrogen chloride, hydrogen iodide, hydrogen bromide, acetic acid, nitric acid, oxides of nitrogen, sulfur dioxide, or sulfur trioxide either in pure form or mixed with inert gases such as nitrogen or admixed with steam or water vapor, may also be employed. The treatment is advantageously carried out at ordinary temperatures, but if the acid is applied in gaseous form a sufliciently high temperature must be employed to keep it vaporized, or at temperatures up to 875 F.

It is also advantageous to combine the action of an acid and a reducing agent for rendering ferrous and ferrous alloy tubes less catalytic toward hydrocarbons. This may be accomplished,

' for example, by first carrying out the acid treatment followed by treatment with hydrogen at elevated temperatures. Or, the order may be reversed, or, if desired, they may be combined by using a mixture of hydrogen chloride and hydrogen at elevated temperatures, or an aqueous acid solution containing an active metal such as zinc or tin may be employed at ordinar or slightly elevated temperatures.

I have found that metallic tubes to he used in the cracking process should be treated with the acids or their anhydrides for a period of from one minute to several hours, depending upon the type and concentration of the agent used. When the treatment is discontinued, the hydrocarbon oil or vapors undergoing cracking may be then passed through the pretreated tubes or other equipment.

The following are concrete examples illustrating my invention, but they are set forth with the understanding that they do not constitute a restriction or limitation of m invention:

Example 1 A series of test runs were carried out by a standardized procedure and in special equipment designed to give a semi-quantitative measure of carbon deposition during the cracking of butene-2 to butadiene in the presence of a. metallic surface. In these tests the cracking zone consisted of the annular space between an inner KA2S steel tube (18 inches long by H inch outside diameter, and closed at both ends) and an outer quartz tube (36 inches long by inch inside diameter) heated in an electrical furnace with a 13 inch heating zone. This type of cracking zone was used in these tests so that comparative results could be obtained quickly and so that the metal surface (the outside surface of the inner closed metal tube) of the cracking zone could beeasil inspected and cleaned between tests. The quartz tube was also cleaned between each test. After the clean reaction zone was given a chosen pretreatment, it was flushed with nitrogen gas and the cracking reaction test run was carried out by feeding 2-butene at a constant flow rate of 25 cc./sec. and at a furnace temperature Of 867 0., giving a gas temperature of about 830 C. and a contact time (time of residence in the cracking zone) of 0.015 second. The flow of 2-butene was usually maintained for about 19 minutes, except in the cases where the reaction zone became plugged with carbon in a much shorter time. Since the annular reaction zone was only approximately /;4 inch in radius, carbon deposition was immediately detectable by an increase in resistance to flow of the butene feed, as measured by a mercury manometer at the entry end of the furnace. The normal operatins pressure with a similar all-quarts reaction zone of the same size was about 25 mm. of mercury. With the abovedescribed metal-quarts reaction zone, lack of any pretreatment of the reaction zone resulted in a rapid build-up of the line pressure duringthe cracking reaction. When the pressure reached 80 mm. of hydrogen, the annular zone was considered to be plugged.

Employing the above quipment and procedure, a series of runs was made on the cracking of 2-butene in the reaction zone followingvarious pretreating methods for making the ma steel tube less catalytic toward carbon deposition, the rate of pressure increase while maintaining a constant butene flow rate being taken as a measure of the rate of carbon deposition. The following table gives the pretreatments used and the results obtained:

1 such, as for example, 2%-nitric acid, followed by washing with water. Thereafter methane or hydrogen at atmospheric or higher pressure should be kept in the cracking tubes while bringing them to cracking temperature. when necessary to shut down the cracking coil, hydrogen or methane should be kept in the tubes while cooling and air should be excluded from con- Eilect on meeting of fl-butene Test Method oipretreating the KAQS steel tube m 3 Nature of carbon deposit I A Mechanical cleringwith brush or suitable m copious and onl coherent. B Nflspamedovercleanedtubeatrateoflommiorl hrs.atlll'0- C Hes overcleanedtubeatrateoflOeeJeeciorl hrs.at000- Hard layer, butscaledoif y D Cleaned tube treated with 25% NaOH at 95 0. for lihr Copier: layer, scalled oi! s Cleaned tube treated with fused NaOH at sooc. for 1% hrs. and Veryeooty. r Carbo alfifm t'fi m fifi h s u t v mu and h n on a m rose on was a eryeop yer muc 700 -740 0. for-M hr. y soot carried out of acne by gases. 0 Excel of arbon deposit removed mechanically and then sir pessed 1% min Do.

reaction zone at MP-811' C. for )4 hr.

H Cl e a ned tube treated with acetic acid by weight for 2 hrs. at room Only 5 innipressure in- Thin uniform coating. 1 Cleaned tube treated with flnormal HNOs for 2 min. at room temper- 7 mm. premise in Small and coherent layer.

aims and washed with water. ii no further lncrease in I Carbon burned out with air at 630M560 C. for 45 min. followed by mm. pressure increase in Quite hard layer, peeled 0B treatment with S0; at 679-800 0. for 4s min. 10 min. K Mechanically cleaned tube treated with S0, at 640-8l0 C. for 40 3 mm. pressure increase in Very small layer, peeled oif min. followed by He at Sir-870 O. for 10 min. 0 min. 0811!.

Example 2 An all-metal cracking zone consisting of the annular space between two K4128 steel tubes, one (18 inches long by 11/32 inch outside diameter and closed at both ends) fitting inside the other (36 inches long by 95 inch inside diameter), was pretreated with 0.2 normal BNO: for 20 minutes at room temperature, washed with water, dried, and treated with hydrogen at NW-845 C. for minutes. 2-butene was then fed at a rate of 25 cc./second to the cracking zone which was maintained at a furnace temperature of about 860 0., giving a gas cracking temperature of about 820' G. and a contact time of 0.015 second. A run of 22 minutes duration was made with a line pressure increase of only 9 mm. of hydrogen during this period. At the completion of the run, a very small coherent layer of carbon was found in the heated portion of the reaction zone. In this experiment a 57.1% conversion of the fi-butene was obtained with a 30.3% yield of butadiene based on butene converted. This compares very well with the 30.5% yield which is obtained at the same conversion in all-quartz equipment.

Although the above tests were carried outwith KAZS steel tubes, while other steel alloys .such as HGT-3 (25% Cr, 20% Ni, 0.15% C, and the balance Fe.) or Ascoloy (2'6%-30% Cr, 1% Mn, 0.6% Ni, and the balance Fe) etc. may be more applicable for the construction of reaction zones for high temperature 'cracking, the results obtained will likewise apply to the other steel alloys since iron is believed to be the chief oftact with the interior of the cracking tubes imtil they are cold. Decoking of the tubes should be by mechanical means, avoiding burning out with air. Hydrogen is preferably passed through the cracking tubes at the operating temperature for one-half to one hour after each shutdown prior to the introduction of the feed stock. If, at any time, the carbon deposit appears to be excessive,

the tubes may be cooled, the excess carbon removed mechanically, and the tubes pretreated by one of the methods described above.

It will be distinctly understood that the precise details hereinbefore enumerated are purely illustrative of my invention and many modifications of it may be made without departing from the spirit thereof, which invention is based on the proposition that alloys which are used in the construction of cracking coil tubes may. and

chromium, and the like, to catalyze the hydrocarbon decomposition and to thereby cause excessive coke deposition can be greatly minimized by treating the insides of the tubes with an acid having an ionization constant greater than 10-, employing also, if desired, or necessary, a treatment with hydrogen as hereinbefore set forth.

What I claim is: 1. In a process for eifecting pyrolysis of gaseous hydrocarbons in metallic apparatus, the effectiveness of which becomes reduced during the efl'ectiveness of such apparatus becomes thus reduced, removing carbonaceous deposits from the metal surfaces, thereafter treating said metal surfaces with an acidic substance having an ionization constant greater than 10-, thereafter treating said metal surfaces with hydrogen, and reemploying the apparatus for-the pyrolysis of said gases.

2. In a process for effecting pyrolysis of hydrocarbons in metallic apparatus, the effectiveness of which becomes reduced during the course of the operation, the. method of treating said metal surfaces to reduce coke deposition thereon which comprises treating said metal surfaces with an acidic substance having an ionization constant greater than 10*, thereafter washing said metal surfaces to'remove the acidic substances retained thereon, and subsequently treating the metal surfaces with hydrogen prior to contacting the same with said gaseous hydrocarbons to be pyrolyzed.

3. In a process for effecting pyrolysis of gaseous hydrocarbons wherein said gaseous hydrocarbons are contacted with metal surfaces at relatively high temperatures and wherein carbonaceous deposits tend to form on said metal surfaces under operating temperature conditions, the method of reducing the amount of carbonaceous deposits formed on said surfaces which comprises initially contacting said surfaces with an acidic substance having an ionization constant greater than 10' and continuously maintaining an atmosphere of hydrogen in contact with said metal surfaces.

BYRON M. VANDERBILT.