US2158504A - Process for cracking oils - Google Patents

Description

May 16, 1939- w. K. HoLMl-:s 2,158,504

PROCESS FOR CRACKING OILS Original Filed July 25, 1928 EXPAMS/o/v v (CHAMBER mfg-A( HL nu@ ATTORN EY Patented May 16, 1939 2,l58,5td

UNITED STATES ATEFT FEQ PROCESS FOR CRACKING OILS Will K. Holmes, Tulsa, Okla., assigner to The Texas Company, New York, N. Y., a corporation of Delaware Original application July 25, 1928, Serial No. 295,304. Divided and this application October 28, 1937, Serial No. 171,400

1 Claim. (C1. Nil-48) This application is a division of my co-pending residual material is accomplished in the single application Serial No. 295,304, filed July 25, 1928. stage, thus eliminating the necessity for further This invention has reference to the production distillation of the tar. of gasoline or similar light hydrocarbons through Another feature of the invention is in the prothe cracking or conversion of higher boiling oils vision of a plurality of separate heating units, 5

into lower boiling oils by the application of heat such as coils, in a system of the above general and pressure. lt relates particularly to the discharacter' so that the several oil bodies requiring position of the heavy residual oils which result the application of heat may be subjected to the from ordinary processes for effecting conversion conditions most suited to their requirements. 1o of the character mentioned, Thus, the lighter condensates which are knocked 10 An object of the invention is to provide for back in the fractionation of the vapors generated maximum efficiency in the utilization of the heat in the system may be Subj etod t0 Somewhat dfemployed in the cracking process. In attaining ferent treatment from the fresh charge of oil this end, it is proposed to provide a closed cyclic being constantly introduced into the system. A system which, in the course of operation, will further distinction as to operating conditions may bring about the formation of only two linal prodbe brought about between the two condensates ucts in the nature of a light, marketable gasoline evolved, i. e., that evolved from the fractionation product and a heavy fuel oil. Incidental to the of the vapors from the pressure cracking system accomplishment of the foregoing result, it is conand that evolved from the fractionation of the 2e templated to strip the residual material resulting vapors released in the tar stripping chamber. 20 from the conversion in such a way as to yield a A still further feature of the invention has to maximum .amount of the lighter oils, as well as do with a preferred and adVemtagoOrlS mode 0f stock which may be recycled for further cracking. operation of the system With particular rofererlo By virtue of this phase of the process, there is to the amount of backtrack or condensate rebrought about a greater total yield of the lighter turned from the fractionator associated with the constituents since there is not only effected a more conversion unit to such unit. I-Ieretofore, it has complete separation of the desired lighter ends, been the usual practice to return condensate of but a greater percentage of the intermediate conthis character in an amount less than twice the stituents which may be effectively subjected to ammini? Of fresh Charge ntrO-llCed H1130 JShe SYS- 90 further cracking are recycled While hot. tem and frequently little more than equal to the 30 Brieny, the invention nos in view the employamount of fresh Charse- It has been discovered, ment of a tar or residue distillation chamber however, that the return of o relatively large directly in conjunction with a cracking system of amount of this condensate, say, for examplo, three any suitable form in such a way that the heat or four or more times the amount of fresh charge, which would otherwise be wasted is put to full brings about very advantageous results, particu- 35 advantage. This includes not only the utilization larly in a system embodying the several features of the heat content of the residues themselves, of the present invention. One obvious advantage but also of the heat of the flue gases which are is that the speed 0f flOW Of the liquid, Combined generated in the course of heating the stills erncharge and returned condensate through the bodied in the cracking units. Toward this end it heating unit, such as a coil, is increased to such 40 is proposed to mount an expansion chamber dian extent that the danger of depositing carbon rectly in the furnace surrounding the stills, or and developing hot spots is largely eliminated. otherwise directly in the path of the flue gases At the same time the burden placed upon the from this furnace and to discharge into such heating unit is not excessive due to the fact that chamber the residues taken from the stills. Due the condensate returned to it is already very not 45 to a considerable reduction in pressure on the oil, and nee-ds to receive very little additional heat. which is contemplated in its passage into the With the foregoing and other incidental obchamber, volatilization of a large percentage of jects and advantages in view, the invention will the material will take place immediately and furnow be described in detail by reference to the ther volatile constituents will be .driven off accompanying drawing in which the single figure through the external application of the heat of illustrates diagrammatically one suitable form of the' flue gases or other source.V By the application apparatus for carrying out the invention. of external heat to the residue or tar stripping Referring now to the drawing, there may be chamber, a satisfactory separation between the provided a pump 8 connected to any suitable volatile constituents and the heavier fuel oil or source of supply of any suitable type of cracking 55 stock, such as gas oil, for example, which it is desired to employ in this system. This material may be forced by the pump through a line 9, a heat exchange coil I0 and a line II intoa coil I2 forming one section of a heating coil mounted in a furnace I4. If desired, the heat exchange coil I3 may be by-passed either in part or in whole. The coil I2 may either be a distinct unit, which may be termed an economizer by virtue of the fact that it is merely in the path of the flue gases which are more directly applied to the heating of a lower section I3 of the coil, or, if desired, the two sections may constitute merely separate portions of a single coil. From the outlet end of the economizer section I2, a line I5 is adapted to convey the charge to the upper end of a second heating coil I6 mounted in another furnace I1, which may receive heat from any suitable source.

A by-pass line I8 may be provided for the purpose of permitting all or a portion of the charge to ilow directly from the line Il to the coil I5, if desired, and for the purpose of regulating the flow through the lines II, I5 and I8, a plurality of valves I9 may be suitably placed in the lines II and I8. It is contemplated as desirable to maintain a pressure of between 15GI and 600 pounds, preferably about li0() pounds, on the oil as it leaves the heating coils, although pressures outside of this range may be employed, if desired.

Upon leaving the coil I6, the temperature of the oil should be between 700 and 950 F., preferably about 850 F., and it may then be passed by a transfer line 23 to one or more of a series oi stills or drums 2|.

While two separate heating coils have been disclosed herein and form a part of an ideal embodiment of the invention, this is not essential and one, for example coil I6, may be eliminated altogether, if desired. ff this is done, the charge, after passing through the section I2, will be passed directly to the section I 3 of the coil in the furnace I4 and will emerge under the same conditions of temperature and pressure as explained above. To bring about this elimination of coil I6, it will only be necessary to close a valve 22 in the line l5 and open a valve 23 in a by-pass connection 24 communicating between the section I2 and section I3 of the coil. In this event, the charge upon leaving the lower end of the coil I3, may be passed by a transfer line 25 to one or more of the series of stills 2|.

It is not necessary that more than a single still or drum should be provided but it has been found preferable to employ a plurality of between two and four. Four have been illustrated in the accompanying drawing and these are interconnected by means of a plurality of lines 26 which provide communication between the liquid bodies, as well as the vapor spaces of the several stills. While the stills may, if desired, be merely heatinsulated, all are preferably mounted within a furnace 21, which is heated by any suitable means and which is adapted to supply heat to the oil in the stills through the mid-portion of the walls of the latter. The heat so applied will preferably be only sufficient to maintain the temperature of the oil above the cracking point by way of compensating for the heat of Vaporization of the lighter constituents which pass off in vapor form.

A vapor line 28, connected to the vapor space of one or more of the stills, may serve to conduct the evolved vapors to a dephlegmator or fractionating tower 29, which may suitably be of bubble tower construction. Here a separation between the heavier portions, which will be condensed, and the light portions, which will remain in vapor form, will take place. The lighter vaporous constituents may be carried through a reflux condenser 30 housing the heat exchange coil I0 and through a line 3I to a suitable condenser 32, and into a storage or accumulator drum 33. Any suitable method may be adopted for providing a cooling medium, as by way of reflux, at the top of the tower 29; for example, a portion of the condensate collected in the drum 33 may be carried through a pipe 34 to a pump 35, which may then serve to return it to the top of the tower through a line 36.

From time to time during the operation of the system, or continuously, if desired, the heavy residual oil which tends to collect in the bottom portions of the stills 2I may be drawn oi through tar-drawing lines 31 and discharged into a main 38, which will convey the residues to an expansion or pressure relief valve 39 and into an expansion chamber 40. Due to the heavy, tarry nature of the residue so withdrawn, it will be found preferable to effect the withdrawal intermittently so that a greater oW may be provided at periodic intervals to keep the connections 31 free from obstructing carbon deposits. Where a battery of stills, such as illustrated, ls employed, it will ordinarily be preferable to withdraw the tar more frequently from the last of the series and to withdraw it after only increasingly greater intervals from the remaining stills. Suitable valves will be provided in the drawoff lines 31 for controlling the withdrawal of the residue. In the event that continuous tar drawing is desired, it will be found advantageous to provide a by-pass line 31I around the valve in the line 31 for, say, the last two stills. One or more of these by-pass lines may be kept continuously open to withdraw a portion of the residue as it accumulates. At the same time it will be found desirable to open up the lines 31 at periodic intervals to draw off greater quantities of the residue and keep these lines cleared.

Within the chamber 40 the pressure will preferably be maintained as near atmospheric as possible in order to produce a maximum flashing effect, but it will be obvious that this pressure will fluctuate to a certain extent due to the intermittent introduction of at least a part of the residue into it. The chamber 40 may be provided With a series of bafile plates AUI and suitable scrapers 402 and, as shown, is preferably mounted directly in the furnace 21, although it may be mounted in any other suitable way to receive the heat of the ue gases generated within the furnace 21 or to receive heat from other sources. It will be apparent that as a result of the reduction in pressure on the residue, and due to the additional heat applied to the chamber 40, a very large percentage of the oil will be flashed, or otherwise converted, into va.- por, which may be carried through a vapor line 4I to any suitable form of dephlegmator or fractionating column 42. This may suitably be in the nature of a rock tower, or preferably a bubble tower. Those portions of the vapors which remain as suc-h in their passage through the tower will be carried through a line 43 to a condenser 44 and may be discharged into an accumulator drum 45. A part of this condensate may be returned by means of a pump 46 and a line 41 to the top of the column 42 as a means of cooling the latter. The products collected in the drum may, under certain conditions of operation, be in the nature of a marketable grade of gasoline, as when the necessary cooling regulation is provided for the tower 42. However, it will be found preferable to operate under conditions not calculated to produce a substantially finished gasoline at this point. Furthermore, it will be found that this product will at best have a greater tendency to go off color, due to polymerization of some of the constituents, than is the case with the producte collected in the drum 33. For this reason it may be found preferable to pass the remainder of the condensate collected in the drum 45 directly into the first mentioned fractionating tower 29, where it will provide some of the necessary cooling and reduce the requirement of reflux from the drum 33. At the same time the condensate so subjected to further fractionation under pressure in the tower 29 will undergo an action of polymerization which will tend to render the product color-stable. It will, in fact, pass in large part with the lighter vaports from the line 28 into the drum 33 as a good quality naphtha. To dispose in this way of the part of the condensate not returned to the column 42, a pipe connection la may be provided from the pump 46, or a separate pump may be employed if desired. It will be clear that it will be necessary to boost the pressure of the condensate by means of the pump 46 from substantially atmospheric to that maintained within the tower 29 and for this reason a different pump may be found preferable. If this mode ,of operation is adopted, the tower 42 may, if desired, be so regulated that a heavier product will be carried over, as for example a fraction including some of the kerosene constituents, a1- though, preferably, the end-point will be kept within the gasoline range. The point of entry of the condensate into the tower 29 will, for best results, depend upon the nature of the material. If its end-point is near that of the final product, it may be introduced through the line 4l!) while if its end-point is higher, it may be introduced through the branch line 41|.

From the base of the fractionating tower 29, the condensate which is knocked back and which may be termed back-trap is withdrawn by means of a pump 58 and may be passed through a line 19 to the heating coil in the furnace I4. Various optional methods of handling the backtrap through the heating coil are afforded by the present disclosure. The particular course through which it is desirable to force this condensate will depend largely upon the method of handling the fresh charge of oil which is selected out of the several ways hereinbefore indicated. If the fresh charge is to be passed .directly to the coil I5 through the by-pass line I8, previously described, then the back-trap may advantageously be passed through a line to the upper end of the coil I2. At the same time the valves 22 and 23 should be so adjusted that the re-heated back-trap will be passed through the coil I3 and be delivered into the transfer line 25 at a somewhat higher temperature than that of the fresh charge of oil leaving the coil I6. This is done for the reason that the back-trapped material is somewhat more difficult to crack than the fresh charge and should preferably be raised to a higher temperature.

Now, if the system is to be so operated that the fresh charge is rst passed through the economizer section I2 within the furnace I4, before being passed to the coil I6, it will be advantageous to pass the back-trap directly to the inlet of the coil I3. To bring about this mode of operation, the line 59 may be provided with a valve 5I which, when closed, will tend to direct the condensate into a line 52 leading directly to the inlet of the coil I3. Since the condensate, before entering the coil, is already at a temperature which may well be in the neighborhood of 700 F., the temperature at the outlet of the coil I3 will still be quite high and probably in excess of that of the fresh charge leaving the coil I6.

Should the system be so operated as to entirely eliminate the coil I6, as has been explained, in which case the fresh charge will be passed through both of the coils I2 and I3, it will be desirable to introduce the back-trap at a lower point in the coil I3 where its temperature will correspond more nearly to that of the fresh charge. To enable the passage of the oil in this way, the line 52 may be provided with a valve 53 which, when closed, together with the valve 5 I, will direct the condensate into a line 54 leading to a suitable point near the bottom of the coil I3. A valve 55 should be provided in this line so as to close it off in the event that one of the first two mentioned modes of operation is adopted.

In a similar way the condensate which collects at the base of the tower 42 and which may be termed cycle stock, is adapted to be withdrawn by means of a pump 56. Several optional courses, may likewise be followed in the disposition of the cycle stock. It may, if desired, be passed through a linev 5l' to a point of juncture with the line 49 so as to merge the two condensates for passage in any one of the three ways indicated with respect to the back-trap. However, a different course may be found desirable, particularly where the fresh charge is being passed first through the coil I2 and then the coil I5. The cycle stock will be of a somewhat lower temperature than the back-trap whereas it will be of a considerably higher temperature than the fresh charge.` .Its

temperature may correspond morenearly to that of the charge after having passed through the coil I2 and for this reason it may be advantageously merged with the charge before the latter enters the coil I6, or, if desired, at an intermediate point in this coil. To effect this disposition of the cycle stock, the line 51 may be provided with a Valve 58 which, when closed, will direct the condensate into a line 59 joining the line I5 or entering the coil I6 at any suitable point. A valve 50 in the line 59 should lbe provided, so that when closed it will enable the passage of the oil in the first way indicated. Obviously, if desired, both of the valves 58 and 60 might be left partially open to any extent desired to split the cycle stock between the two courses available to it.

It has previously been suggested, as a feature of the invention, that the ratio of the backtrapped material to the fresh charge should preferably be quite large, for example in the neighborhood of three or four to one. Thus, where a charging rate of, say, 100 barrels per hour of fresh charge is maintained, the backtrap may to advantage be returned to the extent of 300 or 400 barrels per hour, or even more. It is to be understood, however, that this is optional and that a smaller amount of back-trap may be developed, if desired. At the same time the cycle stock, or condensate from the chamber 42, may well be developed in quantities of 100 to 300 barrels, or more, per hour, although operations tending to produce somewhat less cycle stock than the quantity of charge used are deemed preferable. Where the back-trap and cycle stock 'assume the relatively high values mentioned, and the two heating coils are in operation, it will be found advantageous to pass the cycle stock through the line 59 directly to the coil I6, while 'the fresh charge is passed iirst through the coil I2, and then through the coil I6 and the back-trap is passed only through the coil I3. Operation of the unit in this way will bring about relatively high speeds of flow of the material through the hotter portions of both the coils I3 and I6 so that the oil may be raised to a relatively high temperature without danger of excess deposition of carbon or the burning out of the Vtubes of the coils.

From the base of vthe expansion chamber 40 a residue drawoff line 6I may be provided and may serve to pass the heavy unvaporized portions of the tar to any suitable storage. This residue will be in the nature of a heavy fuel oil fraction, which is unsuitable for further cracking but is of good quality for fuel purposes. It will be recognized that in the preferred mode of operating the system, only two 'products will be formed, these being the gasoline fraction collected at 33 and the fuel oil discharged through the line 6I. In addition, there will, of course, be formed a small amount of coke and a small amount of fixed gases but these may be ignored.

While one suitable form of apparatus for carrying out the invention has been disclosed herein, and certain definite operating conditions have been suggested, it should be understood that these are given merely for the purpose of illustration and that many variations may be made within the scope and spirit of the invention. It is desired to be limited .in the application of the invention only by the scope of the rclaim which follows.

I claim:

In a process for the conversion of higher boiling hydrocarbon oils into lower boiling ones, the process that comprises passing a condensate oil formed as hereinafter specified through a heating coil positioned in a furnace, applying furnace gases to said heating coil to subject the oil stream therein to cracking temperature and effect conversion into lower boiling hydrocarbons, directing the resultant cracked products from said heating coil int-o an enlarged cracking still maintained at cracking temperature under superatmospheric pressure and wherein separation of 'vapors from residue takes place, passing the separated vapors to a fractionating zone wherein the vapors are fractionated to form a distillate comprising gasoline and a higher boiling reflux condensate, introducing said reflux condensate to said heating coil as the condensa'te oil subjected to cracking therein, withdrawing residue from the cracking still and reducing the pressure on the residue so withdrawn to effect vaporization, subjecting the evolved vapors to condensation to form a ashed condensate, passing fresh charging stock through an economizer heating coil positioned in an economizer section of aforesaid furnace and applying furnace gases to the economizer heating coil which have been iirst applied in heating the first-mentioned heating coil, combining said flashed condensate with the heated charging stock from the economizer heating coil and passing the mixture through a separate heating coil positioned in a separate heating zone, heating the mixture therein to a cracking temperature to effect conversion into lower boiling hydrocarbons and passing the resultant cracked products from the latter heating coil into the cracking still.

WILL K. HOLMES.

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