US1950058A - Treating hydrocarbon oils - Google Patents

Description

March 6, 1934. c. w, MacKAY 1,950,058

TREATING HYDROCARBON OILS Filed May 19, 1950 '0 '0 l 22 o a N V N t 3 l; K) E no Q q Q N h INVENTOB BY A H/ ATTORNEY Patented Mar. 6, 1934 UNITED STATES PATENT TREATING HYDROCARBON OILS tion of Delaware Application May 19, 1930, Serial No. 453,462

2 Claims.

This invention relates to the cracking of hydrocarbon oils and pertains especially to a process wherein the oil is subjected to cracking in a plurality of stages. The invention contemplates in general a process in which the oil to be converted or resultant residues of conversion are passed through a plurality of stages of progressively reduced pressures.

The invention has in View a process in which the oil is passed through a series of successive cracking and distilling stages in which the pressure is lowered from one stage to another so that, although cracking may be carried on in the several stages, distillation for separating out 1 evolved products may be facilitated by reason of the reductions in pressure. The invention contemplates as a preferred method of operation the introduction of fresh charging stock to an intermediate stage of the system to thus subject :0 the charging stock to cracking at the intermediate pressure and then passing a resultant residue to another cracking stage or stages operating at lower pressures while reflux condensate obtained from the system is directed to a higher pressure cracking stage.

The invention contemplates one method of operation in which the oil is distilled and cracked in a plurality of stages under varying pressure and temperature conditions and the evolved vapors removed from the several vaporizing stages and conducted to a common fractionating zone wherein they are fractionated under a pressure preferably lower than that obtaining in the several vaporizing stages. The invention has in view the fractionation of the vapors derived from several cracking and distilling stages to separate out a light gasoline or naphtha distillate and a cycle stock which is adapted to be conducted to a predetermined cracking and vaporizing stage.

In carrying on the cracking of hydrocarbon oils in a commercial way the usual charging stock is a composite of many hydrocarbons of varying boiling points. The temperature and pressure requirements best adapted for cracking the several component fractions in the charging stock vary and consequently when this composite charge is introduced to a cracking zone it is impossible for the conditions therein to be best suited for all of the several fractions in the stock. This condition may be aggravated in ordinary plant operations by the practice of using as a charging stock a mixture of virgin stock and cycle stock. This cycle stock is obtained as the result of previous cracking operations, being obtained either directly from a cracking unit or as the result of the distillation or stripping of oil or residue Withdrawn from a cracking operation. As stated, it is common practice to utilize as a cracking stock to a cracking unit a mixture of this cycle stock and virgin stock and consequently the pressure and temperature conditions applied to the mixed stock must necessarily be a compromise between those conditions best adapted for the treatment of either stock separately. My invention seeks to provide a complete process and apparatus adapted for treating the oil in a plurality of stages under various temperature and pressure conditions so that the cycle stocks may be thus treated as they are produced. In this way a straight virgin stock may well constitute the entire initial charge to the system.

In accordance with my invention charging stock is introduced to a predetermined cracking and distilling stage and oil or residue from this stage is passed to a succeeding stage operating at lower pressure than the first and this operation is continued through any desired number of stages so that while cracking may be carried on in the several stages a stripping or distilling operation is also promoted by reason of the progressive reductions in pressure. In one method contemplated by the invention the reduction in pressure may be carried on to such an extent that no further cracking of any consequence may take place in the latter stage or stages, only a, stripping or distilling action taking place in these stages. a

In order to more fully explain the invention reference will now be had to the accompanying drawing which is a flow diagram illustrating a specific example of the invention. v

In the apparatus thus illustrated a system involving three distilling stages is shown but it is to be understood that the invention contemplates any number of cracking and distilling stages that it may be desired to employ. Each of the several stages shown in the drawing includes a. heating coil and a pair of stills. The several stages as viewed in the drawing from left to right are intended to operate at progressively reduced pressures. The first stage includes a. heating coil 10A and stills 11A and 12--A, the second stage includes a heating coil 10--B and stills 1l--B and l2B and the third stage includes a heating coil 10C and stills 11-0 and 12C. In the arrangement shown the heating coil of each stage is connected to one of the stills as 11--A, 11B and 11--C and the stills of each pair are interconnected by vapor equalizing lines 13 and liquid flow lines 14. Although a f LTD single still or reaction chamber may be employed in each stage, it is advantageous to use a plurality of stills, preferably two, introducing the oil from the heating coil into one of the stills as 11A, 11B and 11C and withdrawing oil largely from the second stills 12A, l2-B, and 12-C.

The coils 10A, 10B, and 10C are suitably disposed in furnaces so that they may be heated to the tem eratures desired. A convenlent method of applying heat to the coils is to install all of these coils in a single furnace applying the highest temperaturegases to the coil l0-A and progressively less highly heated gases to the coils 10-43, and 10C. The stills 11-A, 12-A, l1B, 12-B, etc. may be located outside this furnace and suitably insulated to retard loss of heat or, if desired, they may be mounted in suitable furnaces or heating chambers.

The oil to be treated may be drawn from a suitable source, not shown, by a pump 15 and directed through a charging line 16 to an intermediate stage of the system, such as the coil 10-B. The fresh charging stock may, before being introduced to the coil 10-B, be preheated as by heat exchange with vapors or residual products obtained in the system and may furthermore be stripped by distillation of lower boiling constituents, such as components of the gasoline range, before being introduced to the coil 10-B. The oil is heated to a cracking temperature under superatmospheric pressure in the coil 10B and the cracking reaction may be initiated or carried on to such extent as may be desired in the coil after which it is discharged into the still 11-13. The oil undergoing conversion passes to the second still 12-B and evolved vapors leave the stills through a vapor line '17 having a pressure reducing valve 18. Liquid oil is withdrawn through a transfer line 19 having a pressure reducing valve 20 and is directed to the heating coil 10C wherein the oil is heated under a pressure lower than that obtaining in the still l2B to the temperature desired which may be a cracking temperature or may be merely a temperature sufficient for vaporiaation without material cracking. The oil is passed from the coil 10-C to the stills 11-C and 12-0 and the evolved vapors are removed through a vapor line 21 having a pressure reducing valve 22, while liquid oil or residue may be withdrawn through lines 23 and 24 to a tar header or residue line 25.

The vapor lines 21 and 17, as well as a vapor line 26 havin a pressure reducing valve 27 and leading from the still 12A, are allin communication with a vapor header or manifold 28 which leads to a dephlegrnatcr or fractionating tower 29. Thus the vapors evolved in the several stages of distillation which may be operating under different pressures are taken oil? through the lines 26, 17, and 21, respectively, and by reason of the pressure reducing valves 27, 18 and 22 are discharged at a lower pressure into the tower 29. The line 28 is shown provided with a valve 30 for further controlling the reduction of pressure and'delivery of vapors to the tower 29. The pressure in the tower 29 will not exceed the pressure obtaining in he lowest pressure stage distilling zone (the stills 11C and. 12C) and may, if desired, be lower than that obtaining in any of the distilling stages. The tower 29 is of any suitable type, such as a bubble tower or packed tower, to effect the desired fractionation. A vapor fraction is taken off to a condenser coil 31 and the final gasoline or naphtha distillate collected in a receiver 32. The reflux condensate is removed from the tower 29 through a line 33 to a hot oil pump 34 which forces the condensate through a line 35 and thence into the heating coil 10A.

The reflux condensate is heated to a cracking temperature in the coil 10A, preferably a higher cracking temperature than that obtaining in any of the other cracking stages, while under a higher pressure than that of any of the other stages. The cracking or decomposition is initiated in the coil 10-A or carried on to the extent desired and the oil is then discharged into the still 1l-A from which the oil undergoing cracking passes to the still 12--A. The evolved vapors pass out through the line 26 and pressure reducing valve 27 to the vapor header 28 by which they are directed into the tower 29, as has heretofore been explained. Liquid oil is drawn off through a transfer line 36, having a pressure reducing valve 37, and passed to the coil 10-13 to be commingled with the fresh charging stock entering through the line 16.

The stills 11A and 12A are shown equipped with lines 38 and 39, respectively, extending to a line 40 for the purpose of withdrawing liquid from the lower end of the stills. Thus when liquid is being drawn off from the still 12A through the line 36 and passed regularly to the coil 10-B, it is sometimes desirable to take shots or draw comparatively small quantities of liquid '10-." through either or both of the lines 38 or 39 in order to remove from the stills heavier constituents including coke that may accumulate in the lower portion of these stills. In an alternative m thod of operation no liquid is withdrawn 111 through the line 36 and all of the liquid is removed from the stills through either or both of the lines 38 and 39. The material thus directed into the line 40 may be passed into the still 11-B through the line 41 or it may be passed into the still 1lC through the line 42. The stills 11B and 12B are shown provided with draw-off lines 43 and. 44 to either withdraw comparatively small quantities of liquid, while regularly passing the bulk of the liquid through the transfer line 19 1 530 to the coil Ill-A, or to regularly withdraw all of the liquid from the stills 11--B and 12B so as to pass it through the lines 40 and 4-2 to the still 11-0.

It is thus seen that in the practice of my invention the oil being converted is subjected to cracking conditions in a plurality of stages under varying pressure and temperature conditions, a residue obtained in one cracking and distilling stage being passed to successive stages operating at successively reduced pressures. The fresh charging stock may be introduced to an intermediate stage of the system while the reflux condensate obtained is directed to a stage or stages operating at higher pressures and preferably also 1135 under higher temperatures than that of the other stages. In one modification of the invention the vapors that are collected in the line 28 from all of the distilling stages may be passed successively through a plurality of fractionating towers or g; fractionating sections of a tower in such a way as to obtain a plurality of reflux condensate fractions, in lieu of the single fraction that may be withdrawn through the line 33 shown in the drawing, and these several fractions may be Ill-1t passed to separate cracking distilling stages, the lower boiling point fractions being passed to the higher pressure and temperature stage and the remaining fractions to cracking stages operating at lower pressures and temperatures.

In one modification of the invention the fresh charging stock, instead of being passed to an intermediate stage of the system, may be directed to a higher pressure stage and thus the charging line 16 is shown with a branch line so as to pass the charging stock to the coil 10-A. When this method of operation is used it is generally preferable to introduce the reflux condensate being pumped through the line 35 to an intermediate portion of the coil 10A and a branch line 46 is shown for thus admitting the condensate to the coil l0A. While this modified method may be employed at times, such for example as when it is desired to charge a comparatively light charging stock to the system or when it is desired to charge a highly refractory cycle stock that may be on hand, it is ordinarily preferable to introduce the fresh charging stock to an intermediate stage of the system, such as the coil 10B and stills l1-B and 12-B.

In a preferred manner of practicing the invention a virgin stock, that is, a stock which has not been subjected to a previous cracking operation, is charged by the pump 15 to an intermediate cracking stage represented by the coil 10-13. While the practice of the invention does not preclude the employment of a cycle stock, or a mixture of cycle and virgin stocks, as the fresh charging stock to the system, ordinarily if my invention be applied to all of the cracking operations of a refinery there need be no occasion for the accumulation of any cycle stock in tankage, since any cycle stock obtained as a result of distillation in any stage of the system may be directly used up in the process, as has been explained.

Although the temperatures and pressures employed in the practice of my invention may be varied within comparatively wide limits dependent upon the number of cracking stages employed and other variable factors the following figures may be given as a typical example of the inven tion as carried on in three cracking stages with the fresh charging stock. being directed to an intermediate stage represented by the coil 10B and stills 1lB and l2B and cycle condensate being passed to a higher temperature-pressure stage represented by the coil l0A and stills 11-A and l2-A. In the higher pressure stage the pressure may, for example, be around 600- 800 pounds at temperatures of 875-1000 F. The pressure in the intermediate stage represented by the coil 10B and stills ll-B and 12B may, for instance, be around 300-4e0 pounds at temperatures of about SOT-850 F. while the lowest pressure stage, represented by the coil l0--C and stills l1-C and 12-C may, for example, be operated under 100-200 pounds pressure at temperatures of 700-750 F. Under these conditions the tower 29 may be operated under pressures approaching those obtaining in the stills 11-0 or 12C or they may be reduced to such an extent as may be desired.

It will thus be seen that my invention provides a process in which residues developed as a result of cracking and distillation are passed through cracking stages at successively lower pressures so that while the continued cracking of the residual oil is facilitated, as the oil passes through the successive stages of the system, the separating out or removal of the gasoline constituents, as well as the removal of cycle stock adapted for further cracking treatment at higher temperatures and pressures, is facilitated by reason of the successive drops in pressure.

In one method of operation contemplated by the invention the lower pressure distilling stage may be operated essentially as a stripping or auto-tar-distilling operation to recover gasoline and cycle condensate from the cracked residues derived from the cracking stages. Thus for example the apparatus shown in the drawing may be operated with the coil 19A and stills l1-A and 12A under a relatively high pressure, such for example as 600-800 pounds and the stills llB and 12-B operated at a lower pressure, such for example as 200-d00 pounds pressure and the stills 11-C and 12-0 operated at approximately atmospheric pressure. in this m thod of operation it is ordinarily unnecessary to employ the coil 10C, and the tar or residue that is withdrawn from the stills li--B and 12-3 through the lines 43 and 4A and also any tar or residue that may be withdrawn from the stills llA and 12-A through the lines 38 and 39 be passed through the header 40 and line 42 nto the still 11-0. The contained heat of the hot residue from the cracking stills is utilized to effect distillation in the stil s llC and l2-C, which distillation may be aid" employment of a subatmospherio pressure. The distillation is carried on to the extent desired to distill on" the gasoline constituents and cycle stock and produce a residue of the characteristics desired which may for example be a liquid residuum adapted for fuel oil or the distillation may be carried. on to such an extent as to produce a liquid pitch which is withdrawn through either or both of the lines 23 and 24 or, if desired, the 'stillation may be carried on o the extent of coking.

In another method of practicing the invention the temperatures employed may be approximately the same in all of the several cracking stages while the pressures are progressively redu -ed.

Obviously many modifications and variations of the invention, as hereinbefore set forth, may be made without departing from the spirit and scope thereof, and therefore, only such limitations should be imposed as are indicated in the appended claims.

I claim:

that comprises subjecting fresh charging stock :1, if desired, by steam or the 1,2;

to cracking temperature under superatmospheric 1;

pressure to effect cracking and vaporization in an intermediate pressure zone, subjecting condensate oil to cracking temperature under superatmospheric pressure to effect cracking and vaporization in a higher pressure zone, passing residue developed in the higher pressure cone to the intermediate zone for distillation therein, passing residue developed in the int pressure zone comprising residual con from both the cracked fresh charging stock and condensate to a lower pressure zone and suog'ecting the residue to vaporization therein, passing the vapors evolved in several zones to a dephlegmating zone subjecting them to fractionation therein to form a vapor fraction and a reflux condensate, conducting said reflux condensate to said higher pressure zone and condensing said vapor fraction.

2. A process in accordance with claim 1 wherein such conditions are maintained in said lower pressure zone that the residue introduced thereinto for vaporization is reduced to coke.

CHARLES W. MACKAY.

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