GYROPROCESS EQUIPMENT, THE PUREO I L
cO\IP\Xk,
M ~ R C UHOOK, S PA.
Vapor-Phase Cracking c. R.
F
WAGNER,
The Pure Oil Company, Chicago, J11.
OR a history of this art reference should be made to articles by Rittman and associates ( 7 ) and by Lomax, Dunstan, and Thole (6'). It is more or less surprising to learn that in the first attempts to crack hydrocarbons hightemperature vapor-phase conditions were employed. Thus Dalton in 1809 worked with electric sparks on hydrocarbon vapors. In 1825 Faraday produced benzene and several of the unsaturated hydrocarbons while using the same procedure. I n 1855 Silliman conducted his classic researches on the production of oil gas from petroleum. I n the 1860's we find many distinguished names connected with research on vapor-phase cracking-Breitenlohner, Berthelot, Thorpe, Young, and others. The commercial development (accelerated by the World War) of vapor-phase cracking was temporarily checked by the work of Burton ( 1 ) and his associates, followed by Cross, Dubbs, and others, in the field of liquid-phase cracking. Yields, quality of distillate (from a treating standpoint), coke formation, and fuel economy were all overwhelmingly in favor of the lower temperature liquid-phase operation. Then came the scarcity of crude supply a t the close of the war, the development of tetraethyllead, and the increased compression ratio of all automobile engines. The overproduction of crude oil in the last decade has not affected the popularity of the high-compression engine, and, as a result, the octane number of motor fuels has steadily risen since 1922 or 1923. Part of this increase has been secured by increasing the volatility of the fuel and part by the addition of tetraethyllead, benzene, or other nonpetroleum products, but most of it has been done by increasing the percentage of cracked gasoline and by raising the temperature of the cracking operations. The first successful vapor-phase cracking installation in the United States (excluding the plants of Rittman, Alexander, and others who were able to operate so long as gasoline was worth 15 cents per gallon or more) was built in 1927 (8). Today a large percentage of the cracking plants in the country are
operating under vapor-phase conditions, utilizing the principles exemplified in that first plant.
TYPESOF VAPOR-PHASE CRACKING There are two general types of vapor-phase cracking operation: (1) high-pressure operation a t temperatures ranging between 950' and 1050' F. and (2) low-pressure operation a t temperatures in excess of 1050" F. The first type is represented by the de Flores process, which operates a t pressures in excess of 100 pounds gage and a t temperatures of about 1035" F. A modified de Flores operation is employed in the Dubbs and Kellogg units now being built, using reaction chambers and operating a t around 970" F. and at pressures in excess of 300 pounds gage. The second of these types is exemplified by the Gyro process, which operates a t a terminal pressure of less than 50 pounds gage and a t temperatures ranging from 1070" to 1150' F. Rittman ( 7 ) and his associates stated the case for vaporphase cracking as follows: "In procuring the desired products of the cracking reaction the physical and chemical properties of an original oil are of secondary importance compared with the influence of temperature, pressure, time, and concentration." It has since been pointed out ( 6 ) that, in the lower pressure ranges a t least, the main effect of pressure is upon the time factor. Geniesse and Reuter ( 2 ) have shown that, for high-temperature cracking at atmospheric pressure, time and temperature are the only factors of importance. It iq their conclusion that the same results can be obtained a t higli temperatures with short reaction times as are produced a t low temperatures and long reaction times. They show that an increase of 31 " F. in reaction temperature approximately halves the time required to produce a given amount of cracking. In a later contribution (3) they show that the reaction velocity constant, IC, may be calculated from the monomolecular equation:
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1 N D L S T R I ,4L .4N D E X G I N E E R I N G C H E h l I S T K Y
February, 1934
a
where a
k = l / t log, a-x n = initial quantity of gas oil charge
-5
=
quantity of recycle stock (after fractionating our gas, gasoline, and tarry polymers produced in the reaction) remaining at time t
Their calculations indicate an energy of activation of the order of 97,120 B. t. u. per pound mole for all petroleum hydrocarbons except ethylene. It is felt that most of the simpler olefins should likewise be excepted because of t>heirtendency to polymerize a t high temperatures, some of them being much more susceptible than ethylene. The polymerization of the simpler olefins explains a part of the difference between the results of the two types of vaporphase cracking processes. This is revealed by an examination of the gases produced as follows: G a s FROM FLORES UNIT
GASFROM GYROPROCESS UNIT
DE
%
% Hydrogen Methane Ethylene Ethane Propylene Propane C
