US2078899A - Cracking process - Google Patents

Description

Apfil 27, 1937. g, YDN R 2,078,899

i CRACKING PROCESS Filed July 23, 1932 Patented Apr. 27, 1937 UNITED S PATENT orFrcE CRACKING PROCESS Harold Sydnor', Westfield, N. J., assignor to Standard Oil Development Company, a corporation of Delaware Application July 23, 1932, Serial No. 624,256

1 Claim.

. ly in section, of the apparatus used for carrying out the invention.

The efforts of the petroleum refining art have lately been directed tothe production of good knock rating gasoline. This purpose can be achieved in the so-called tube and tank cracking process in which the oil to be crackedds passed through heating coils into a so-called digesting or soaking drum and the cracked prod not is then separated into its constituents such as tar, cycle stock, naphtha and gas. When it is required to produce a good knock rating gasoline in such a process it is necessary to maintain a higher cracking temperature than was formerly usual. ,A serious limitation to such high ternperature tube and tank processes at the present time is the tendency of the release lines and 3 I valves immediately adjacent to the drum tonclog due to the accumulation of coke which terminates the run sooner than would otherwise be necessary. I have found ficulty can be eliminated by transferring the cracked material from the soaking drum into a separator held substantially under the same pressure as the soaking drum and by introducing fresh feed into the separator to cool down its contents and check the further cracking. By thus that this coking difmaintaining full soaker pressure on the products leaving the soaking drum until they are cooled by direct contact with the incoming fresh feed, the coking of the lines, adjacent to the soaking drum becomes eliminated.

Among the several other objects and advantages of the present invention the following are mentioned.

1. -A hi her than the usual preheatof the'oil. entering the heating tubes is obtained since the necessity of recirculating cold tar to the bottom of the separator for cooling becomes eliminated and the excess heat content of the cracked efliuent from the soaking drum is used for preheating the fresh feed. The advantage of carrying out the direct contact of the incoming feed with the cracked products before release of pres;- sure becomes greater ture is increased since the amount of heat to be dissipated betweenpthe soaking drum and the separator ofa unit operating with a low pressure as the coil outlet temperaseparator increases with the cracking temperature. High preheat is essential to the development of a high temperature, high pressure cracking, since it means less heating surface to reach the, cracking temperature at a given feed rate, which in turn means less tubes and less pressure drop through the heating tubes.

2. It is Well known that naphthas produced by high temperature cracking are unstable and contain a high percentage of gum forming constituents which have to beremoved by acid treatment. It has been applicants experience that the stability of the produced naphtha against gum formation increases when the high pressure separator of the present invention is used. Due to increased stability of the gasoline, the treatment of the same with sulfuric acid for the removal of gum forming constituents may be eliminated and the gasoline. finished by simple clay treatment as will be described hereinafter. H

3. I also found that the use of the high pressure separator in a cracking process results in the formation of a gasoline having greater resistance to detonation with increasing engine jacket temperature when used as armotor fuel. The resistance to detonation is a function of the chemical constitution of the gasoline and is expressed in octane numbers as well known in the art. The octane number varies with the conditions under which the engine is run and more specifically with the engine jacket temperature, there being a drop in the octane number as this temperature increases. I found that the drop in the octane number with increasing engine jacket temperature will be less when the gasoline is prepared bya cracking process using a high pressure separator according to the present invention. l. In onemodification of the present process the fresh feed is introduced at the top of the separator while the cracked product is introduced at or near the bottom of the same so that the hot vapors and the cooler liquid pass in countercurrent to each other. It will be seen that a clean and fresh distillate feed stock is thus obtained for the passage through the heating tubes of the furnace and any diificulty due to deposition of coke in the heating tubes on accountof unclean charging stock is eliminated.

v Referring now to the drawing the same will be described in conjunction with the method of op- .erating the process.

The feed stock consisting of fresh feed distillate and cycle stock is pumped from accumulator I through line 2 and preheater 3 to the heatingcoil 4 arranged in furnace 5. The oil is heated in the coil to a cracking temperature and then discharged into the digester or soaking drum 6 in which the desired degree of cracking takes place. The hot cracked effluent leaving the drum by line 1 is then transferred into the high pressure separator 8 in which it is cooled by partially preheated fresh feed in order to check the further progress of the cracking and to simultaneously vaporize the vaporizable constituents of the latter. Fresh feed from tank 9 is pumped through heat exchange coil I0, heat exchanger H and line l2 to the top plate of the separator 8. Valve I3 and line 14 allow the passage of a small part of the preheated fresh feed to the bottom of the soaking drum mainly for the purpose of preventing the coking of line 1. Release valve l5 allows the maintenance of a high pressure on coil 4, drum 6 and separator 8. It will be understood that there is considerable pressure drop in the elongated coil 4 due to the passage of the oil therethrough at a high velocity but the pressure in drum 6 and separator 8 will be substantially the same since the pressure drop in the short line I is comparatively small. The preheated fresh feed is preferably introduced at the top plate of separator 8 which is provided with discs and doughnuts or equivalent separating and fractionating means. The introduced fresh feed cools thehot cracked effluent and checks thereby the further cracking, at the same time it is vaporized so that it leaves the separator in admixture with the cracked vapors through vapor line 16.

The liquid level control I1 connected with the release valve l8 and the bottoms line H! allows the maintenance of a certain low liquid level in the separator and the removal of the unvaporized bottoms therefrom. The bottoms consists of tarry matter from the cracking and from the fresh feed in case an unclean charging stock is used as fresh feed. The vapors leaving through line [6 will be substantially free of any heavy tarry matter and are subjected to partial condensation and fractionation in heat exchangers 3, l I and fractionating tower 20. The latter is provided with fractionating means such as bell cap plates and a heat exchange coil It at the top of the tower. The liquid which condenses in the tower 20 is collected in pan 2| from which it flows through line 22 into the accumulator l. Similarly the condensed oil from heat exchangers 3 and II flows through 23 into the accumulator I to provide a clean charging stock for the cracking coil. There is connected with the lower end of the fractionating tower 20 a low pressure separator 24 into which the bottoms leaving the high pressure separator 8 through line l9 are discharged. Here the lower boiling constituents contained in the high pressure separator bottoms evaporate due to the release of the pressure and pass through center opening of pan 2 I into the fractionating tower. Unvaporized tar is removed through line 25 and tar cooler 26 to a storage tank (not shown). The uncondensed oil vapors leave the top of the fractionating tower 20 through vapor line 21, are condensed in final condenser 28 and are obtained as naphtha after the separation and removal of the permanent gases through line 29. Due to the stability of the naphtha obtained in my process the sulfuric acid treatment may be eliminated and the stock may be finished by a clay treatment in a clay treating apparatus diagrammatically shown at 30 whereby a treated naphtha is ob-' tained.

The operating conditions may be varied within a considerable range. The pressure maintained on the soaking drum and the high pressure separator is preferably higher than about lbs. per square inch and is usually around 1,000 lbs. per square inch although pressures higher than that may also be employed. The temperature to which the oil is heated in coil 4 may also vary within a broad range. It is usually higher than 800 F. and is preferably between 900 and l000 F.

The following example will illustrate my invention.

A 31 A. P. I. West Texas gas oil was cracked according to the present invention, using a mixed feed rate to the cracking coil of 25,000 gallons per hour of which 50% was fresh feed. The conditions of the cracking were the following:

F. Coil outlet temperature 950 Soaking drum temperature 900 High pressure separator temperature 825 Coil inlet temperature 775 The pressures were the following:

Lbs. per sq. inch Hot feed pump 1,000 Soaker 750 High pressure separator 7 50 Bubble tower 100 Low pressure separator 100 The following yields were obtained:

Percent Distillate of 400 F. end point by volume" 54.6 Fuel oil of 9 A. P. I. gravity by volume 31.6

Gas by weight 15.4

ing zone to effect the required amount of crack- 1 ing, passing substantially all of the cracked effluent to an enlarged high pressure separating zone, maintaining substantially the same high pressure in the digesting and separating zones,

introducing fresh feed oil below cracking tem- 5' perature to the process, passing a part thereof into said digesting zone in proximity to the point of withdrawal of the heated oil therefrom, passing another part of said fresh feed oil into the separating zone, contacting said last mentioned part of said fresh feed oil with cracked efiluent from the digesting zone to effect vaporization of the fresh oil at cracking temperature therein, removing oil vapor from the separating zone and fractionating the same under lower pressure than that in the separating zone to obtain the said relatively clean distillate charging stock and naphtha, returning clean distillate charging stock thus obtained to said heating zone as charge oil therefor, separately removing unvaporized bottoms from the separating zone and eliminating said bottoms from the system without subjecting them to thermal conversion.

HAROLD SYDNOR.

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