US2064847A - Conversion of hydrocarbons - Google Patents

Description

Dec. 22,1936. J. c. MORRELL 2,064,847

CONVERSION OF HYDROCARBONS Filed Dec. 21, 1955 Condenser 23 fiZ/8 7Z for: (@0510? Cflorrell,

Patented Dec; 22, 1936 "FES PAT orric CONVERSION OF I i ROCARBONS Application liecembcr 21, 1933, Serial No. 703,381

. 4 Claims.

This invention relates particularly to the conversion of hydrocarbons, either liquid or gaseous, which are of a petroleum origin, though it may also be applied to the treatment of indi- 5 vidual hydrocarbons or blended mixtures.

' In a more specific sense the invention is concerned with the conversion of hydrocarbons by a process of heat treatment adapted to produce therefrom substantial yields of gasoline boiling range fractions which have unusually high antiknock characteristics in comparison with similar fractions produced" by ordinary pyrolytic processes.

The art of cracking heavy liquid hydrocarbons 15 to produce practical yields of good quality gasoline has undergone a rapid and extensive development in the past twenty years and commercial processes in operation at the present time are somewhat similar in their general characteristics. The 20 art of producing liquid hydrocarbons by the pyrolysis and polymerization of gaseous hydrocarbons is not so extensively developed though there are a few commercial installations which are claimed to be successful from an economical standpoint. From a practical standpoint, particularly when dealing with hydrocarbon mixtures of the complicated nature of liquid petroleum fractions and with hydrocarbon gas mixtures of varying 30 composition, much stress is laid upon the empirical determination of the proper inter-relation of such factors as temperature, pressure and time in the case of different hydrocarbon mixtures employed as starting material. Chemically, two general types of reactions are recognized as occurring either consecutively or simultaneously during pyrolytic operations. In the simplest aspect, the first type of reaction involves dehydrogenation and dealkylation of either straight chain or cyclic hydrocarbon compounds, with resulting production of hydrogen and unsaturated hydrocarbons of different types. The second type of reaction is consequent upon the first and evidently consists in the recombination of unsaturated residues or radicals to form new compounds of less unsaturation and higher molecular weight. The process of the present invention is a contribution to the cracking art in respect to the more effective utilization and control of'these types of reactions.

In one specific embodiment the present inven-' tion comprises subjecting hydrocarbons to pyrolysis in a primary stage under conditions of relatively high temperature, low pressure and short time to limit the conversion reactions to the formation of large percentages of unsaturated hy-' dro'carbons, subjecting the products of said primary conversion stage to further conversion under lower temperatures and high superatmospheric pressures to produce liquids of gasoline boiling 5 range, fractionating the products to eliminate heavy residues unsuitable for further heat treatment and produce intermediate insufficiently converted boiling range fractions suitable for further heat treatment, fixed gases which may be 10 withdrawn from the process or recycled, and gasoline boiling range fractions. The process is capable of considerable modification in the details of operation and may be conducted-in plants whose component parts bear different relationships in the matter of capacity. Furthermore, operations upon gaseous hydrocarbons may be somewhat different than those conducted upon liquid fractions so that a complete description of various alternative types of operation would consume a great deal of space. However, the following description of an operation involving the conversion of relatively heavy hydrocarbon gas mixtures such as, for example, those obtainable from the stabilizers of gasoline plants will be in point and to assist in this desoription the attached diagrammatic drawing has been provided which shows, in side elevation by the use of conventional figures not drawn to scale, the essential parts of a plant in which the process may be conducted.

Referring to the drawing, hydrocarbon gas of the character mentioned above may be introduced through a line I containing a control valve 2 to a tubular heating element 3 disposed to re- 5 ceive heat from a furnace 4. Since subatmospheric pressuresare commonly employed in the first conversion stage, partly to assist in controlling the time factor and otherwise influencing the nature of the reactions, no pumping equipment is, 40 as a rule, necessary at this point; the gases being supplied to the heating coil at atmospheric pressure.

The design of coil 3 may vary considerably. In some cases, where very low time factors are found to be most effective, it may consist of a single length of refractory tubing of relatively small cross section and in other cases it may consist of a bundle of parallel tubes or channels to increase the capacity and reduce the pressure drop without increasing the time factor. The details of construction of this primary heater can usually be worked out after a few preliminary experiments inlaboratory or semi-plant apparatus have indicated the optimum conditions of operation.

The materials entering into the construction of heating element 3 may include not only metals and alloys but also refractories such as fire clay, sillimanite, carborundum, etcetera, if found desirable or expedient.

As a rule the temperature range employed in the primary conversion stage is from 900 to 1500 F. and the pressure is below atmospheric, frequently as low as that corresponding to an absolute pressure of 50 mm. of mercury. The time under which hydrocarbons are exposed to this range of conditions is practically always less than 0.1 second and may be of the order of 0.01 second in some cases. The actual temperature, pressure and time employed will depend upon the character of the charging stock and the overall results desired.

In the plant shown in the drawing the heated and primarily converted products pass through line 5, containing control valve 6, and enter coil 1 in a heat exchanger or cooler 31, which serves the dual purpose of quickly reducing the temperature of the conversion products to a point where decomposition reactions are substantially arrested and further lowers the temperature so that the compression temperature attained after the pressure is increased for the second stage conversion and polymerization reactions is not too high for best results. In the drawing shown 31 appears as a heat exchanger in which the recycled gases from the process are flowing through a separate but closely adjacent coil 36 in a direction generally countercurrent to the flow of the hot conversion products. The description of this feature of cooling by heat exchange will constitute the subject matter of a subsequent paragraph.

The cooled but still vaporous products from' heat exchanger 31 pass through line 8, containing control valve 9, to a pump or compressor 10 which is designed to efiectively maintain the required low pressure on its suction side and develop the required high pressure on the discharge side,

the latter pressure being as high as 1,000 pounds per square inch if desired, and, although not illustrated, two or more stages of compression may be employed when high discharge pressure is employed. The products discharged from pump I0 pass through line H, containing control valve l2, and enter a reaction chamber l3 provided to allow time for the secondary conversion or polymerizing reactions. Since the production of a certain amount of tarry residual material is usually unavoidable at this point, chamber I3 is provided with a liquid draw line l4, containing control valve l5, for the removal of such tarry or asphalt residual products which are unsuitable for further heat treatment on account of their high carbon content and coke-forming tendencies.

The general character of the reactions which are preferably allowed to take place in chamber l3 correspond to the second type of chemical re- 1 action previously designated. The character of the products ordinarily obtained indicates that a large proportion of the gasoline boiling range liquids recovered as a final product of the process consist of dimers and trimers of such oleflns as propylene, butylenes, and amylenes, while the presence of mixed condensation products such as are formed by the union of ethylene with propylene, etcetera, is also indicated. Such compounds valve ll, to enter a fractionator l8 of suitable design and capacity for producing an overhead vapor mixture comprising fixed gases and gasoline boiling range hydrocarbons and a refluxof intermediate character which can be employed flow through run down line 24, containing con trol valve 25, to receiver 26 which has a gas release line 21, containing control valve 28, for withdrawing fixed gases and a liquid draw line 29, containing control valve 30, for the removal of the gasoline product to storage.

- In the operation being described a portion of the fixed gases of the process are recycled for further conversion by way of line 3 l control valve 32, pump 33, line 34, line 34' and control valve 35' to pass through coil 36 of the heat exchanger and thence through line 38, containing control valve 39 to primary feed line I. By manipulating valves 35 and 35' any required portion of the recirculated gases may be passed through the heat exchanger, the remainder passing directly to line I and thence to the primary heating stage.

The heat exchanger shown in the drawing is ofa type which has proved its value in practice, since it eliminates a number of possible points where leaks might occur in the ordinary vessel type of heat exchanger where one flowing medium surrounds a bundle of small thin walled tubes. The invention is not, however, limited to the employment of this particular type of heat exchanger and as indicated it may not be necessary at all times to resort' to any considerable cooling before the second stage of the process is reached.

While the above description is chiefly concerned with the cracking of a low boiling hydrocarbon mixture, theprocess is applicable as well to the cracking of higher boiling liquid, hydrocarbon fractions such as gas oil distillates which can be readily vaporized prior to or during the initial cracking step. In such cases a preliminary heater is usually indicated so that the oils may be vaporized and brought to a temperature somewhat lower than that required in the primary decomposition step, the primary cracker A plant similar in essential detail to that shown in the drawing may be operated upon a charging stock consisting essentially of butane and isobutane obtained from the stabilizer of a natural gasoline plant so that the hydrocarbons are substantially all of a paraflinic character. The temperature used in the primary cracking step may be of the order of from 1200 to 1300 F., the pres-.-

sure in the neighborhood of 50 mm. of mercury absolute and the time of exposure to these conditions maybe approximately 0.01 second. In the second stage of the process the pressure may be raised to 700 pounds per square inch, the temperature reduced to approximately 1000 F. and the time factor increased to approximately one minute.

The foregoing procedure may yield about 10 gallons of gasoline boiling range distillate per 1,000 cubic feet of charging stock measured as a gas under standard conditions. The octane number of the product having a 400 F. end point may be 95 to 100 determined by the motor method in blends with a low'octane number parafiinic gasoline.

Example 2 When charging the vapors of agas oil distillate of approximately A. P. I. gravity to the process and operating under substantially the same conditions as those given in the first example, it may be possible to produce a yield of to of 400 end point gasoline having the same octane number as that of ,the product produced from the hutane mixture given in the first example. When operating upon such a distillate the vapors at atmosphericpressure may be preheated to some temperature between 800 and 900 F. before entering the primary decomposing stage. If a portion of the charging oil remains liquid under these conditions, the liquid may be separated before admixture of the heated charging stock to the primary cracking coil.

The character of the invention and the results obtainable by its use have been shown in the foregoing specification and appended data respectively, but neither specification nor examples are to be considered as imposing undue limitations upon its generally broad scope.

I claim as my invention:

1. A process for the treatment of gaseous hydrocarbons to convert the same into liquid hydrocarbons, which comprises subjecting said gaseous hydrocarbons to elevated temperatures in a first stage to form unsaturated hydrocarbons and thereafter subjecting the products of this treatment to lower temperatures and higher pressures in a second stage to produce liquid hydrocarbons from the unsaturated hydrocarbons formed in the first stage of the process, separating the liquid hydrocarbons thus formed from residual gases, passing at least a portion of the residual gases in indirect heat exchange with the heated products in transit from the first to the second stage and then supplying the same to said first stage.

2. A process of the character described in claim 1, wherein the temperature of treatment in the first stage is from 900 to 1500 F.

3. A process'of the character described in claim 1, wherein the temperature of treatment in the first stage is from 900 to 1500 F'., and the pressure in the neighborhood of 50 millimeters of mercury.

4. A process of the character described in claim 1 wherein the temperature of treatment in the first stage is from 900 to 1500 F. and the pressure is below atmospheric.

J ACQUE C. MORRELL.

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