US2304289A - Conversion of hydrocarbon oils - Google Patents

Description

Dec. 8, 1942. Q Q TONGBERG 2,304,289

CONVERSION OF HYDROCARBON OILS Filed Dec. 2, 1939 2 Sheets-Sheet 1 TTEATEP jg l8 57 /0 Mix/Arc I 77mm JOJ-VGNT ll 1i 21 014 7-0.2: czne ap I Patented Dec. 8, 1942 ii if CGNVERSEON 0F HYDROCARBON OILS Carl 0. Tongberg, Westfleld, N. 3., assignor to Standard (iii Development Company, a corporation of Delaware This invention relates to the cracking of hydrocarbon oils and pertains more particularly to a method of pretreating the oils to be cracked to improve the cracking characteristics thereof.

One of the objects of the present invention is to provide an improved method of treating oils, which are to be subjected to cracking, which would improve the yield and quality of the motor fuel resulting from the cracking operation.

Another related object of the invention is to provide an improved method for selectively removing from the oil to be cracked, coke forming constituents while at the same time retaining in the oil a greater. amount of constituents which can be cracked to advantage into lower boiling motor fuel constituents.

Other more specific objects and advantages of the invention will be apparent from the detailed description hereinafter taken in connection with the accompanying drawings. In the drawings Figure l is a diagrammatic illustration of an apparatus suitable for carrying the invention into efiect and Figure 2 is a graphic chart showing the relative amounts of difierent types of hydrocarbons present in'two typical oils as determined by the specific dispersion of various fractions.

While the invention in its broader phases has a more general application to the cracking and conversion of hydrocarbon oils, it finds particular application to the cracking of oils in the presence of a catalyst wherein the presence of coke-forming constituents rapidly contaminates the catalyst and reduces the activity thereof.

It is more or less commonly known that aromatic hydrocarbons are less suitable for cracking than non-aromatic oils in that there is more degradation of oil into low grade products, such as coke and gas, for a given yield or conversion into gasoline when cracking aromatic oils than when cracking non-aromatic oils, such as paraffins' and naphthenes. In other words the ratio of gasoline to coke or gas for any given conversion, is higher when cracking non-aromatic oils than when cracking aromatic oils. In view of the above, it has been proposed to pretreat the oil to be converted to remove the aromatic constituents before subjecting the oil to cracking. This treatment may be accomplished for example, by the use of selective solvents having a higher solvent power for aromatic constituents than for nonaromatic constituents. This type of treatment is of particular advantage when the oils are to be cracked in the presence of a catalyst since the coke which forms during the cracking operation collects on the surface of the catalyst and rapidly reduces its activity. Any hydrocarbon oil which contains a high percentage of coke-forming constituents, which give a low ratio of gasoline to coke, is therefore particularly objectionable when cracking in the presence of a catalyst although it is objectionable to a lesser extent in thermal cracking since it reduces the yield of gasoline obtained and introduces coking difliculties in the cracking unit.

Heretofore it was generally believed that all aromatic constituents were objectionable and should be removed from the oil before cracking in order to obtain a high ratio of gasoline to low grade products.

It has been found after extensive research that alkylated simple ring aromatics are not objectionable and can be employed to advantage in the feed stock passing through the cracking unit. It has been further found that the coke forming constituents are the condensed ring aromatics such as those of the naphthalene and anthracene type.

In accordance with the present invention the oil to be cracked is treated under conditions which will segregate the condensed ring aromatics but which will retain in the oil to be cracked, at least a major portion of the simple aromatic hydrocarbons. The segregation of the condensed ring aromatics from the remainder of the feed stock is preferably accomplished, according to the present invention, by solvent extraction under controlled conditions.

Any of the numerous known types of solvents having a higher selective solvent power for aromatics than for non-aromatic constituents may be employed in carrying out the present invention. Among such solvents may be mentioned for example, sulfur dioxide, dichlordiethylether, furfural, phenol, chlorinated hydrocarbons such as carbon tetrachloride, nitrated hydrocarbons, oxygenated hydrocarbons such as ethers, alcohols, and the alcohol ethers such as methyl Cellosolve and many others. It is important in the present invention to control the solvent extraction conditions, such as the temperature, the amount of solvent, and other factors, to remove substantially all of the condensed ring aromatics.

One convenient method of insuring removal 01' the condensed ring aromatics is to employ the density of the oil. It may be expressed by the formula g where m is the refractive index at one wave length, m is the refractive index at another wave length and d is the density of the oil. (See for example Science of Petroleum vol. 2 page 1174 published by Oxford University Press of London,

. analytical edition, 1938, page 559.)

New York and Toronto: also Ward and Kurtz, Journal of Industrial and Engineering Chemistry, All of the condensed ring aromatics normally present in hydrocarbon oils have a value as determined by the equation above 200, when determined according to a method used by Ward and Kurtz whereas the simple ring aromatics have values below 189 and above 100 and the naphthenes and paraflins have values below 100. By controlling conditions under which the solvent extraction is carried out, so that all of constituents in the extracted oil have a specific dispersion below 200, it is possible to remove all of the condensed ring aromatics which are the objectionable cokeforming constituents, while at the same time re= taining in the oil the simple uncondensed aromatics many of which can be cracked to advantase.

In practical operations it is not possible to.

obtain a sharp division 'between the condensed and uncondensed ring aromatics. As a result even when carefully controlling the solvent extraction the extract may contain some uncondensed ring aromatics. However, by controlling the degree of extraction, so that substantially all constituents in the extracted oil have specific dispersions below 200, substantially all of the condensed ring coke forming constituents can be removed while retaining within the oil a considerable amount of single ring aromatics.

The solvent extraction may be carried out in any suitable equipment designed for the purpose.

The oil is passed through a line H! to a mixing chamber i I wherein it is intimately admixed with a solvent having a relatively high solvent power for aromatic constituents. The solvent may be introduced into the mixing tank through line I2. For illustrative purpose methyl Cellosolve will be employed as the solvent. The relative proportions of solvent and oil will depend upon the selectivity of solvent, nature of the oil, temperature and other factors.

The solvent and oil after being thoroughly mixed in chamber H, are passed through line l3 to a settling tank I wherein the mixture separates into a rafiinate phase having a relatively higher concentration of non-aromatic hydrocarbons than the original oil treated and an extract ph'ase comprising the solvent and an oilfraction rich in aromatics. The rafilnate phase is passed through line to a solvent distilling chamber I6, wherein it is heated in any suitable manner such as by heating coil II to vaporize any solvent contained therein. The vaporized solvent passes overhead through line l8 to a condenser I9 wherein it. is condensed. The condensed solvent may be returned to the mixing tank ll through line which merges with line l2.

The oil after being freed of solvent in distilling 7 chamber I6 is removed therefrom through line 2| and is passed to'a cracking unit (not shown).

The extract phase separated in the bottom of the settling tank I4 is withdrawn through hne in the same manner as in separator 22. Oil

separated from the extract in the second reflux separator 21 may be passed through line 28 to the first reflux separator 23 and combined with oil separated therein for return through line 24 to the mixing tank ll.

Extract from the second reflux separator 21 may be passed through line 2! to a third stage reflux separator 30 wherein it is subjected to further separation in the sme manner as in the reflux separator 23, such as, by the introduction or water or by decrease in temperature or both. Water may be introduced into the second and third reflux separators through lines 3| and 32. Oil separated in the third reflux separator may be passed to the second stage reflux separator through line 33 and therein admixed with oil and extract contained therein. By passing the oil phase in countercurrent to the. extract phase through the successive reflux separators, as before described, a more effective separation can be obtained. Extract separated in the third stage reflux separator 3|! is withdrawn therefrom through line 34 and may be passed to a solvent still 35 which may be heated in anysuitable manner such as by heating coil 36 to vaporize solvent from the extract. The solvent so vaporized may be passed through line 31 and combined with solvent recovered from the solvent still I6. The extract may be removed from the solvent still 35 through line 38 and rejected from the system. This extract will contain the con densed ring aromatics.

While three reflux separators have been shown, it will be understood that more or less may be provided as conditions may dictate. The use of a plurality of such reflux separators results in more eflective separation of the condensed ring aromatics from oil.

As hereinbefore described the volume of arcmatic extracts removed from the oil can be readily controlled by regulating the amount of water introduced into the reflux separator or by varying the temperature in the separators or both.

In accordance with the present invention, the amount of extract removed is preferably controlled so that all constituents having a specific dispersion above about 200 are removed. In some cases it may be desirable to remove all constituents having a speciflc'dispersion above since constituents having specific dispersions above 150 are less suitable for cracking than others. If the specific dispersion drops below these values, it means that simple uncondensed ring aromatics having good cracking characteristics are being rejected in the extract with the consequent unnecessary sacrifice in volume of feed passing through the cracking unit. If the specific dispersions of any portion of the rafllnate exceeds these values it means that coke forming constituents are being retained therein.

It will also be understood that the present invention is not confined to the particular method of extraction with solvents described and illustrated above but has application to other conventional methods of separation. As a guide to a better understanding of the invention the following examples may be helpful, it being understood that the values and conditions given in the examples are illustrative rather than limited.

7 Two types of oils were selected for determining cracking characteristics. One oil was a cycle gas oil obtained from a catalytic cracking process operating on a 33.4 A. P. I. gravity East Texas gas oil and employing activated clay as a catalyst suspended in finely-divided form in the oil to be cracked. The catalytic cracking conditions, such as temperature, time of contact and catalyst to oil ratio were controlled to obtain about 40% conversion into gasoline. The second 011 was a cycle stock from a thermalcracking unit processing East Texas gas oil at 1000pounds pressure and 880 F. Inspections of the oils are given below:

Each of the above oils was subjected to solvent extraction using methyl Cellosolve as a solvent in a batch treater of the type described in the body of the specification under controlled conditions and the specific dispersions of spot samples corresponding to certain volumes of extract were determined as previously described. The results of these determinations are graphically represented in Figure 2, wherein the specific dispersion is plotted against the per cent extracted.

From these curves it will be noted that the cycle stock from the catalytic cracking process contains between and of condensed ring aromatics having a specific dispersion about 200. The thermal cycle gas oil, on the other hand, contained between and of condensed ring aromatics having a specific dispersion above 200.

Various rafiinate and extractfractions of these two types of oil were subjected to catalytic cracking to determine their cracking-characteristics under the following conditions; The oil to be cracked in the form of superheated vapors was passed through a cracking zone containing activated clay known as "Super Filtrol maintained at 850 F. The oil was fed at a rate of 0.6 volume of liquid-feed per volume of catalyst per hour and the length of the cracking cycle was two hours. The results obtained are tabulated below:

Comparing first the cracking characteristics of the various rafilnate fractions of the catalytically cracked cycle oil with respect to the vuriextracted oil it will be seen from the table that the removal of a 20% extract fraction results in a material improvement both in yield of gasoline and weight ratio of gasoline to coke but that the improv ement is not as striking as that obtained by the removal of an additional 10% of extract. Re-' ferring to Figure 2, it will be noted that the raffinate still contains some condensed ring aromatics having a specific dispersion above 200.

Comparing the 80% rafllnate with the 70% rafllnate, it will be seen that the removal of an additional 10% of aromatic constituents from the rafiinate results in 9.5% increase in gasoline yield, with an actual decrease in amount of coke formed. As a result the ratio of gasoline to coke is increased from 10.7 to 18.8. Referring to Figure 2, it will be noted that the-70% raifinate (30% extract) is substantially free of condensed ring aromatics but still contains substantial amounts of single ring aromatics since this fraction has a specific dispersion of 150.

In the particular stock treated according to this example, it would be necessary to remove about 50% of the oil as extract to insure complete removal of aromatics. However, from the table it will be observed that the removal of an additional 20% of extract does not materially improve the cracking characteristics of the resulting ramnate.

Referring now to the thermally cracked cycle gas .oil, it will be noted that the 70% rafiinate fraction gives only 37.5% conversion as 4 compared to 49% of a similar rafiinate from a catalytically cracked gas oil. From Figure 2, it will be seen that the 70% ramnate fraction from the thermal cycle oil still contains condensed ring aromatics since the 30% extract fraction contains constituents having a specific dispersion above 200. On the other hand, if the 50% raffinate is used as cracking stock, the yield jumps from 37.5 to 49.7 or an increase of 12% with an actual decrease in amount of coke formed. As a result the gasoline to coke ratio increased from v parent from the curve that it is not necessary to remove 50% as extract in order to bring the specific dispersion below 200.

a It will be understood that the curve in Figure 2 holds true only for the particular type of stocks treated and that the relative amounts of condensed and single ring aromatics present will depend upon the source of the oil treated and in case of cycle oils, the nature of the cracking to which it has been subjected and other factors.

Having described the embodiment of the invention and given a specific example thereof, it will be understood that it embraces such other variations and modifications as come within the spirit and scope thereof.

What is desired to be protected by Letters Patent is:

1. In a process for cracking hydrocarbon oils, the improvement which comprises treating the oil to be cracked with a. selective solvent capable of removing aromatic constituents, controlling the degree of treatment to separate a solvent extract phase consisting principally of condensed ring aromatics having a specific dispersion above and a raffinate phase substantially free of condensed ring aromatics but containing substanstituents having a specific dispersion between 100 and 190.

3. The process defined by claim 1, wherein the raflinate phase is cracked in the presence of an 5 active cracking catalyst.

CARL O. TQNGBERG.

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