US2056725A - Art of cracking - Google Patents

Description

0. G. KAASA ET AL 2,56,7

ART OF CRACKING Filed Aug. 1, 1953 I INVENTORS 0mm 6, fiaaga Thomas B; hlmba/l gm @MWLW ATTORNEYS Patented Oct. 6, 1936 UNITED STATES PATENT OFFICE ART OF CRACKING Maine Application August 1, 1933 Serial No. 683,140

4 Claims. (01. 196-58) In one type of cracking operation, for the production of gasoline from higher boiling oils, a stream of oil is heated to a high cracking temperature, upwards of 900 F. for example, in a 5 heater, the resulting hot oil products are discharged into a separator in which residual tar is separated from a vaporized fraction, and the resulting vaporized fraction is discharged into a fractionating system in which gasoline or a gasoline-containing fraction is separated from one or more higher boiling fractions and a gas mixture incondensible at ordinary atmospheric temperatures and pressures. Up to the point at which further decomposition of the separated liquid tar involves excessive production of such incondensible gases, the recovery of liquid products other than and more valuable than the tar tends to increase as the temperature of the separated residual tar as discharged from the separator increases. From this standpoint, it is desirable to increase the temperature at which the separated residual tar is discharged from the separator, but increase of this temperature entails some difficulties. In particular, increase of this temperature, in the range upwards of 750 F. for example, tends to increase the temperature of the separated vapor mixture to a point such that diiiiculties with respect to coke formation in and corrosion of the separator and contamination of the vaporized fraction with tar constituents are frequently involved. This invention relates to improvements in this type of operation, and more particularly with respect to resolution of the hot oil products discharged from the heater into a residual tar and a vaporized fraction, which assist in eliminating such difficulties in operations in which the temperature of the separated residual tar as discharged from the separator substantially exceeds 750 F.

One form of apparatus embodying, in operation, the improvements of the invention is illustrated diagrammatically and conventionally, in the'accompanying drawing in which Fig. 1 represents, in elevation and partly in section and with parts broken away, this form of apparatus, Fig. 2 represents on an enlarged scale a fragmentary elevation in section of a portion of the separator illustrated in Fig. 1 and Figs. 3 and 4 are details on a. further enlarged scale of the distributing means illustrated in Figs. 1 and 2.

55 heating coil 5 and thence, through connection l0,

into the lower part of the separating tower l, a connection H for discharging tar from the lower part of the separating tower l, a connection 12 for discharging a vaporized fraction from the upper part of the separating tower 1 into the lower part of the fractionating tower 8, and a connection I3 for discharging a vapor mixture escaping uncon-densed from the fractionating tower 8 to a conventional condenser and receiver (not shown). 0

The separating tower 1 is divided, internally, into three zones, an unbafiled zone in the lower part for initial separation of the hot oil products into a vapor mixture and a residual tar, a baffled zone in the intermediate part in which vapors 15 are not compelled to pass through liquid bodies for limited fractionation of the separated vapor mixture with a minimum risk of obstruction through accumulations of coke or cokey material, and a bafiled zone in the upper part in which vapors are compelled to pass through liquid bodies for relatively close fractionation of the vaporized fraction discharged to the separating tower through connection 12. A separating tower of this general type is described, for example,fin Letters Patent Number 1,810,048, granted June 16, 1931 to Sinclair Refining Company on an application of Eugene C. Herthel.

This type of separating tower is particularly advantageous with respect to avoidance of contamination of the vaporized fraction with tar constituents due to the relatively closeiractionation accomplished in the upper part of the tower, but due to the necessity for maintenance of liquid bodies in this part of the tower to secure this fractionation, tends to accentuate difiiculties with respect to coke formation, particularly in the upper part of the tower. For example, as the temperature of vapor mixture entering the upper part of the tower increases with increase of the temperature of the separated residual tar as discharged from the lower part of the tower, the maintenance of these liquid bodies,-particularly those through which the vapor mixture first passes, becomes a matter of increasing diificulty until actual formation of coke in this part of the tower begins. Although this vapor mixture is in approximate equilibrium with the liquid separated in the lower part of the tower, the relations involved appear to be much the same as would be expected were the vapor mixture superheated. This superheat of the vapor mixture also accentuates any corrosion effects. 1' 4 In accordance with this invention, the resolution of the hot-oil products into a residual tar and.

a vaporized fraction is carried out in three steps, first an initial separation of the hot oil products into a vapor mixture and a residual tar in an unbafiled zone, second a cooling of the vapor mixture separated in the first step by direct heat exchange with oil introduced as a liquid in a baffled zone in which vapors are not compelled to pass through liquid bodies and from which condensed vapor and any unvaporized portion of the introduced oil is refluxed only to this unbafiled zone, and third a fractionation of the vapor mixture escaping from the second step in a bafiled zone in which vapors are compelled to pass through liquid bodies maintained therein and from which liquid oil is refluxed only to the second zone, the residual tar being discharged from the first zone and the vaporized fraction from the third zone. The mixture is cooled in the second zone sufficiently to eliminate this superheat thereby facilitating the maintenance of the liquid bodies in the third zone as well as avoiding difiiculties with respect to coke formation and corrosion. Limited fractionation as well as cooling is effected in the second zone and further cooling as well as fractionation is effected in the third zone.

In carrying out the invention in the apparatus illustrated, for example: A stream of oil is heated to a temperature of 940-990 F., for example, in the heating coil 5 under a pressure of 400-600 pounds per square inch, for example, as discharged therefrom. This oil may be raw gas oil supplied through connection [4 or the higher boiling fraction or part of the higher boiling fraction discharged from the lower part of fractionating tower 8 supplied through connections [5 and I6 or a mixture of the two. A pressure of 30-60 pounds per square inch, for example, is maintained in the lower part of the separating tower l, the pressure on the hot oil products discharged from the heating coil 5 being reduced as they pass through valve 11. A stream of cooler oil is introduced into the lower part of separating tower 1, together with the hot oil products discharged from the heating coil 5, through connection l8, the rate at which this oil is supplied being regulated, in conjunction with the refluxing maintained in the tower I, to maintain the temperature of the separated residual tar in the lower part of the separating tower l at a, temperature of 770 F. or 780 F. or 790 F., for example. The oil introduced through connection [8 may be raw gas oil or a cooled part of the higher boiling fraction discharged from the lower part of fractionating tower 8 or a reduced crude oil or a similar stock or a mixture of such stocks for example. The separated residual tar, including the tar constituents of the hot oil mixture discharged from the heating coil 5 and any tar constituents of the oil supplied through connection I8, is discharged through connection II. A tar leg I 9, of reduced cross section, is with advantage provided to facilitate, by maintenance of a liquid level within the tar leg, the maintenance of liquid discharge through connection II in conjunction with the maintenance of a minimum volume of separated liquid tar within the separating tower l. The vapor mixture resulting from the initial separation, in the lower part of the separating tower 1, of residual tar from the hot oil products introduced through connection In passes upwardly between the baffles 20 in the intermediate part of that tower. Oil supplied through connection 2| is introduced as a liquid through the spray nozzles 22 at a rate regulated to maintain the temperature of the vapor mixture leaving the intermediate part of the tower at 30 F. or 40 F.

or 50 F., for example lower than that of the vapor mixture entering the intermediate part of the tower. The oil supplied through connection 2| may be raw gas oil or a cooled part of the higher boiling fraction discharged from the lower part of fractionating tower 8 or a fraction of boiling range lower than that of the fraction discharged from the lower part of fractionating tower 8, for example. The oil supplied through connection 2| is with advantage adistilled oil, substantially free from tar constituents, is with advantage a fraction vaporizing substantially completely under the conditions prevailing in the separating tower l, and is with advantage a fraction substantially free from those constituents of the hot oil products introduced into the separating tower through connection [0 of boiling range abutting that of the tar separated therein. In the upper part of separating tower I the partially cooled vapor mixture is subjected to relatively close fractionation. In the apparatus illustrated the bafiles provided in the upper part of the separating tower 1 are of conventional bubble tray type. Liquid bodies through which the vapor mixture passes in this upper part of separating tower l are maintained either by the introduction of oil as a liquid through either or both of connections 23 and 24 or by the return of reflux condensate from the reflux condenser-heat exchanger 25 or by both of these means conjointly. Residual tar is discharged from the separating tower I through connection I I and the vaporized fraction separated therein is discharged through connection I2. The vapor mixture discharged through connection I2 is subjected, in the fractionating tower 8, to conventional fractionation, for example.

The operation of the separating tower 1 may with advantage be conducted generally as described in an application of Eugene C. Herthel and Carlton L. Schmidt, filed April 1, 1933, Serial Number 663,954.

The form of spray nozzle illustrated more in detail in Figs. 2, 3 and 4 has proved advantageous. It consists of a spoon-shaped piece of metal 26 welded to the end of the supply tube. Open end spray nozzles of-this general type tend to eliminate all difiiculty with plugged or coked sprays. As installed in a separating tower such as that illustrated in Figs. 1 and 2, the spray nozzles are with advantage adjusted, by the simple expedient of bending the part 26 up or down with respect to the supply tube, to discharge a spray covering the open end of the bafiie over which the particular spray nozzle is positioned. This can be done directly by using water supplied through connection 2! under normal operating pressure as the test medium.

We claim:

1. In the resolution of the total hot oil products discharged from a heater in which a stream of oil is heated to a high cracking temperature into a residual tar and a composite vaporized fraction including all of said hot oil products lower boiling than said residual tar, the improvement which comprises effecting an initial separation of the hot oil products into a vapor mixture and a residual tar in an unbafiled zone, cooling the separated vapor mixture by direct heat exchange with a cooling oil introduced as a liquid in a zone provided with open end baffies and in which zone vapors are not compelled to pass through liquid bodies and from which zone liquid is refluxed only to the said unbafiied zone, said cooling oil being introduced in the form of jets arranged to discharge sprays over the open end of said baffles, thereafter further cooling and fractionating the remaining vapor mixture in a baffled zone in which vapors are compelled to pass through liquid bodies maintained therein and from which liquid is refluxed only to the first-mentioned bafiled zone, including in the cooling oil introduced as a liquid in the first-mentioned bafiled zone a liquid cooling oil in addition to liquid refluxed thereto from the second-mentioned baffled zone, and discharging the residual tar at a temperature substantially exceeding 750 F. and the said composite vaporized fraction from the said unbaffied zone and from the second-mentioned baffled zone respectively.

2. In the resolution of the total hot products discharged from a heater in which a stream of oil is heated to a high cracking temperature into a residual tar and a composite vaporized fraction including all of said hot oil products lower boiling.

than said residual tar, the improvement which comprises effecting an initial separation of the hot oil products into a vapor mixture and a residual tar in an unbailled zone, cooling the separated vapor mixture by direct heat exchange with a cooling oil, vaporizing substantially completely under the prevailing conditions, introduced as a liquid in a baffled zone in which vapors are not compelled to pass through liquid bodies and from which zone liquid is refluxed only to the said unbaffled zone, thereafter further cooling and fractionating the remaining vapor mixture in a baffled zone in which vapors are compelled to pass through liquid bodies maintained therein and from which liquid oil is refluxed only to the first-mentioned baflled zone, including in the said cooling oil introduced as a liquid into the first-mentioned bafiled zone liquid oil in addition to the liquid refluxed from the second-mentioned baffled zone, and discharging the residual tar at a temperature substantially exceeding 750 F. and the said composite vaporized fraction from the said unbafiled zone and from the second-mentioned baflied zone respectively.

3. In the resolution of the total hot products discharged from a heater in which a stream of oil is heated to a high cracking temperature into a residual tar and a composite vaporized fraction including all of said hot oil products lower boiling than said residual tar, the improvement which comprises effecting an initial separation of the hot oil products into a vapor mixture and a residual tar in an unbafiled zone, cooling the separated vapor mixture by direct heat exchange with a cooling oil, vaporizing substantially completely under the prevailing conditions and substantially free from those constituents of the hot oil products of boiling range abutting that of the residual tar separated in the said unbailled zone, introduced as a liquid in a baflled zone in which vapors are not compelled to pass through liquid bodies and from which zone liquid is refluxed only to the said unbaflled zone, thereafter further cooling and fractionating the remaining vapor mixture in a baffied zone in which vapors are compelled to pass through liquid bodies maintained therein and from which liquid oil is refluxed only to the first-mentioned bafiled zone, including in the said cooling oil introduced as a liquid into the first-mentioned baffled zone liquid oil in addition to the liquid refluxed from the second-mentioned baffled zone, and discharging the residual tar at a temperature substantially exceeding 750 F. and the said composite vaporized fraction from the said unbaffled zone and from the second-mentioned bafiled zone respectively.

4. In the resolution of the total hot oil products discharged from a heater in which a stream of oil is heated to a high cracking temperature into a residual tar and a composite vaporized fraction including all of said hot oil products lower boiling than said residual tar, the improvement which comprises effecting an initial separation of the hot oil products into a vapor mixture and a residual tar in an unbafiled zone, cooling the separated vapor mixture by direct heat exchange with a cooling oil, vaporizing substantially completely under the prevailing conditions, introduced as a liquid at a plurality of points spaced along the path of vapor flow in a baflled zone in which vapors are not compelled to pass through liquid bodies and from which liquid oil is refluxed only to the said unbafiled zone, thereafter further cooling and fractionating the remaining vapor mixture in a baflled zone in which vapors are compelled to pass through liquid bodies maintained therein and from which liquid oil is refluxed only to the first-mentioned baflled zone, including in the said cooling oil introduced as a liquid into the first-mentioned baiiled zone liquid oil in addition to the liquid refluxed from the second-mentioned baified zone, and discharging the residual tar at a temperature substantially exceeding 750 F. and the composite vaporized fraction from the said unbaffled zone and from the second-mentioned zone respectively.

ORIN G. KAASA. THOMAS B. KIMBALL.

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