US2011030A - Fractional distillation - Google Patents

Description

Aug. 13, 1935. D. J. BERGMAN w 2,011,030

FRACTIONAL DISTILLATION Filed Dec. 26, 1935 F RACTIONATOR CONDENSER RECEIVER' INVENTOR DONALD J. ERGMAN ATTORNEY Patented Aug. 13, 1935 UNITED STATES PATENT OFFICE FRACTIONAL DISTILLATION Application December 26, 1933, Serial No. 703,953

, 7 Claims.

This invention particularly refers to improvements in the fractionation of vapors of a complex nature such as hydrocarbon oil and is particularly well adapted to use in processes for the pyrolytic conversion of hydrocarbon oils.

In processes of the general nature wherein heated vaporous products are subjected to fractionation for the recovery of desirable low-boiling components and separation of the condensate formed during their fractionation into two or more fractions of diiferent'boiling characteristics, the present invention provides an improved method and means for reboiling low-boiling frac-' tions of the condensate in order to free them of entrained gases and low-boiling components within the range of the desired overhead product. This is accomplished in the present invention by indirectly contacting the low-boiling fractions with the hotter, higher boiling fractions of the condensate and, as an additional advantageous feature of the invention, said higher boiling fractions, after being cooled by indirect contact and heat exchange with the low-boiling fractions which they serve to reboil, are commingled with the vaporous products entering the fractionator for the purpose of partially cooling the same, to condense their heavy ends and thereby decrease the load in the first fractionating stages wherein said heavy ends would normally be condensed.

Although it is not intended to limit the invention to use in any particular type or class of distillatory system the features of the invention are particularly well adapted to use in processes for the conversion of hydrocarbon oils wherein the cracked hydrocarbor vapors are subjected to fractionation, whereby their insufliciently converted components, boiling above the range of the desired final light distillate product of the process, are condensed as reflux condensate and separated into two or more selected low-boiling and high-boiling fractions while fractionated vapors of the desired end-boiling point are sub- Jected to condensation and the resulting distillate collected as the final light distillate product of the process.

As applied to the conversion of hydrocarbon oils, one specific embodiment of the invention comprises subjecting a hydrocarbon oil to pyrolytic conversion conditions of elevated temperature and superatmospheric pressure. separating the resulting vaporous and non-vaporous conversion products, subjecting the vaporstoa primary stage offfractionation whereby their highboilingcomponents are condensed as reflux condensate, subjecting vaporous products from said primary fractionating stage to secondary fractionation for the formation of lower boiling reflux condensate, comprising the remaining corn ponents of the vapors boiling above the range of the desired final light distillate product of the process, subjecting fractionated vapors of the desired end-boiling point to condensation, collecting the resulting products, directing a regulated quantity of the reflux condensate from the 10 primary fractionating stage to indirect contact and heat exchange with the reflux condensate from the secondary fractionating stage whereby to reboil the latter and commingling a regulated quantity of the primary reflux condensate, after said heat exchange, with the vaporous conversion products entering the primary fractionating stage for the purpose of partially cooling the same to assist in condensing their high-boiling components.

Many modifications of the specific embodiment of the invention above described, as applied to the conversion of hydrocarbon oils, are within the scope of present invention; for example, I the reflux condensate from either fractionating stage of the process may be returned to further conversion, together with the raw oil charging stock supplied to the system, or the reflux condensate from either or both fractionating stages may be subjected to further conversion under independently controlled conversisn conditions in the same cracking system or may be Withdrawn from the system to storage or to further treatment, as desired.

The accompanying diagrammatic drawing illustrates one specific form ofcracking system in which the process may be practiced.

Referring to the drawing, raw oil charging stock for the process may be supplied through line I and valve 2 to pump 3 by means of which it is fed through line 4 and valve 5 to conversion in heating coil 6. The charging stock may, of course, be preheated in any well known manner, not illustrated, prior to its introduction into the heating coil and, when reflux condensate from either fractionator of the system is returned to the heating coil for further conversion therein, a

regulated portion of the charging stock may be supplied to the fractionator by well known means,

not shown.

A furnace I, of any suitable form, supplies the required heat to the oil passing through heatiij coil 6 tobring. it to the. desired conversion temperature, preferably at a substantial super-at mospheric pressure, and the heated products are discharged through line 8 and valve 9 into reaction chamber I0.

Chamber III, in the particular type of cracking system here illustrated, is also preferably maintained at a substantial superatmospheric pressure and although not illustrated in the drawing, it is preferably well insulated, in order to prevent the excessive loss of heat therefrom by radiation, so that conversion of the heated products from heating coil 6 may continue in the reaction chamber. vaporous and non-vaporous conversion products separate in chamber I0, the vapors being withdrawn, in the case illustrated, through line II and valve I2 to fractionation in fractionator I3. Residual liquid conversion products, when such are produced, are withdrawn from the lower portion of chamber III through line I4 and valve I5-to cooling and storage or to any desired further treatment and may, when desired, be subjected to further vaporization at substantially reduced pressure for the recovery of its low-boiling components by well known means not illustrated. The process may also be operated for the production of substantially dry coke in chamber ID, in which case the coke may be allowed to accumulate within the chamber until further operation of the chamber is impractical, following which it may be cleaned and prepared for further operation and, when desired, a plurality of coking chambers similar to chamber Ill, but not illustrated in the drawing, may be employed to provide additional space for the deposition of coke. when a plurality of coking chambers are employed they may be simultaneously operated or, preferably, are alternately operated, cleaned and prepared for further operation so that the duration of the operating cycle of the process will not be limited by the capacity of the coking zone.

Fractionator I3 comprises a zone of primary fractionation, wherein the heavy components oi. the vapors supplied thereto from chamber I0 are condensed as reflux condensate. The reflux condensate produced in fractionator I3 which may, for the sake of convenience, be termed primary or high-boiling reflux condensate, is withdrawn from the lower portion of fractionator I3 through line I6 and valve I1 to pump I8 by means of which a regulated portion thereof, in the case illustrated, isreturned through line I9, line 20, valve 2| and line 4 to further conversion in heating coil 6. Components of the vapors supplied to fractionator I3, boiling below the range of the reflux condensate formed therein, pass as fractionated vapors from the upper portion of fractionator I3 through line 22 and valve 23 to fractionator 24 wherein they are subjected to further fractionation. Components of the fractionated vapors supplied to secondary fractionator 24, boiling above the range of the desired final light distillate product of the process, are condensed as secondary or low-boiling reflux condensate in this zone while fractionated vapors of the desired end-boiling point pass from the upper portion of fractionator 24 through line 25 and valve 26 to be subjected to condensation and cooling in condenser 21. The resulting distillate and gas passes from condenser 21 through line 28 and valve 29 to collection and separation in receiver 3!]. Uncondensable gas may be released from the receiver through line 3| and valve 32 Distillate may be withdrawn from receiver through line 33 and valve 34 to storage or to any desired further treatment. When desired, a regulated 'portion' of the distillate collected in receiver 33 may be recirculated, by well known means not shown in the drawing, to the upper portion of fractionator 24 to serve as refluxing and cooling medium to assist fractionation of the vapors in this zone and maintain the desired vapor outlet temperature from the fractionator, thus controlling the end-boiling point of the final light distillate product of the process.

The condensate formed in fractionator 24, which may be termed secondary or low-boiling reflux condensate will ordinarily contain an appreciable quantity of low-boiling components within the range of the desired light' distillate product of the process which is removed as the overhead stream from fractionator 24. In the present invention, a novel and advantageous means is provided for reboiling the secondary reflux condensate for the purpose of substantially freeing the same of desirable low-boiling components as well as any entrained gases. In the case here illustrated, this is accomplished by withdrawing the secondary reflux condensate' from the lower portion of fractionator 24 through line 35 and valve 36 to a suitable heat exchanger 31, which serves as a reboiler, and the heat for reboiling the secondary reflux condensate is provided by the hotter primary reflux condensate from fractionator I3, a regulated portion or all of which is diverted from line I9 through line 38 and valve 39 to heat exchanger 31, wherein it is passed in, indirect contact with the secondary reflux condensate supplied to this zone. The vaporsand gases evolved by reboiling of the secondary refluxgcondensate are withdrawn from heat exchanger 31 and, in the case here illustrated, are returned through line 40 and valve H to fractionator 24. The reboiled secondary changer 31 through line 42 and valve 43 to storage or to any desired further treatment and, in

the particular case here illustrated, a liquid level ,1

controller 44, of any suitable form, actuates valve 43, insuring the removal of only stabilized liquid through line 42.

The primary reflux condensate passed through heat exchanger 31, and somewhat cooled thereby, is directed therefrom through line 45 and as a special feature of the present invention-a regulated portion or all of this material is directed through line 46 and valve 41 into line II, to

commingle with and partially cool the vaporous a reflux condensate is withdrawn from heat exproducts supplied to fractionator I3, thereby assisting in the condensation of the heavy components of the vapors and decreasing the load imposed upon the fractionator in condensing the primary reflux condensate. The remainder, if any, of the primary reflux condensate from heat exchanger 31, not required for cooling the vapors supplied to fractionator I3, is directed, in the case illustrated, through line 48, valve 49, line 20 and line 4 to heating coil 6 for further conversion. It will be understood that, when desired, the cooled primary reflux condensate from-heat exchanger 31, instead of being commingled with the vapors in line II may be directed to fractionator I3, and preferably to the lower portion thereof, in any other well known manner, not

shown.

It will be understood that the invention, as applied to he conversion of hydrocarbon oils, is not limited to the type of cracking system nor to the specific form of apparatus illustrated and described. For example, a single fractionator mary and secondary fractionators illustrated being removed from suitable points in the single fractionator in any well known manner. Also, when desired, reboiling of the secondary reflux condensate may be accomplished within the fractionator instead of in an external heat exchanger such as illustrated, by means of a closed coil or the like within a well or reservoir provided within the fractionator for the accumulation of the secondary reflux condensate, through which coil the primary reflux condensate may be passed. It is also within the scope of the invention, instead of returning primary reflux condensate to the same heating coil to which the charging stock is supplied and withdrawing the secondary reflux condensate from the system, either the primary reflux condensate or the secondary reflux condensate may be supplied to a separate cracking coil of the same system and therein subjected to independently controlled conversion conditions or both the primary and secondary reflux condensate may be supplied to separate cracking coils or the secondary reflux condensate may be subjected to conversion, together with the charging stock, particularly when a relatively low-boiling charging stock is employed, and the primary reflux condensate subjected to milder conversion conditions in the same system.

In a cracking process such as illustrated and above described, the preferred operating condition may be approximately as follows: the heating coil to which the primary reflux condensate and charging stock is supplied may utilize an outlet conversion temperature of from 850 to 950 F., or thereabouts, preferably with a superatmospheric pressure at this point in the system of the order of 100 to 500 pounds, or thereabouts, per square inch. Substantially the same or somewhat reduced pressure may be employed in the reaction chamber relative to that prevailing at the outlet from the heating coil. The primary fractionator may be operated at substantially the same pressure as that employed in the reaction chamber or at a substantially reduced pressure and the pressure employed in the secondary fractionator may be substantially equalized with or reduced relative to the pressure employed in the primary fractionator. Condensing and collecting portions of the system may employ pressures substantially equalized with or somewhat reduced relative to the pressure employed in the secondary fractionaton I I As a specific example ofthe operation of the invention in an apparatus such as illustrated and above described, the charging stock is a 32 A. P. I. gravity mid-continent gas oil which is subjected, together with the primary reflux condensate, to a conversion temperature, measured at the outlet from the heating coil, of approximately 935 F., at a superatmospheric pressure of about 350 pounds per square inch. Substantially the same pressure is employed in thereaction chamber. A reduced pressure of approximately 200 pounds per square inch is employed in the primary fractionator and the pressure in the secondary fractionator and succeeding condensing and collecting portions of the system is further reduced to approximately 75 pounds per square inch. This operation may produce, per barrel of charging stock, about 52% of 400 F. end-point motor fuel of good anti-knock value, about 16% of pressure distillate bottoms, comprising the secondary reflux condensate, which are substantially free of components boiling above 400? F., and about 26% of liquid residue, the

remaining 6%, or thereabouts, being chargeable principally to uncondensable gas and a small amount of coke.

In the specific operation above described, the yield of motor fuel is increased by some 2%, or more, over that obtainable in a process not utilizing the features of the present invention, due to reboiling of the pressure distillate bottoms and in addition a smaller primary fractionator may be employed than that which would be required in a similar process not utilizing the feature of cooling the vapors supplied to this zone in the manner provided by the present invention.

I claim as my invention:

1. In a process for the fractional distillation of materials of relatively wide boiling range wherein heated vapors are subjected in a primary fractionating zone to fractionation for the formation of condensate of a relatively high-boiling nature and the resulting fractionated vapors subjected in a secondary fractionating zone to further fractionation for the formation of a relatively low-boiling condensate and the recovery of lower boiling fractionated vapors of the desired characteristics, the improvement which comprises subjecting said relatively low-boiling condensate formed in said secondary zone to reboiling by indirect contact and heat exchange with a regulated quantity of the higher-boiling condensate from said primary zone thereby cooling the latter, and returning such cooled condensate after said heat exchange, to said primary zone.

2. In a process for the conversion of hydrocarbon oils wherein an oil is subjected to conversion conditions of elevated temperature and superatmospheric pressure, the resulting vaporous and non-vaporous conversion products separated, the vapors subjected to fractionation in successive primary and secondary fractionating zones, whereby their components boiling above the range of the desired final light distillate product of the process are condensed as reflux condensate and separated into relatively low-boiling and highboiling fractions, fractionated vapors of the desired end-boiling point subjected to condensation and the resulting products collected, the improvement which comprises reboiling said low-boiling fractions of the reflux condensate formed in said secondary zone by indirect contact and heat exchange with a regulated quantity of the highboiling fractions of the reflux condensate from said primary zone thereby cooling the latter, and returning such high-boiling fractions of the reflux condensate, cooled by said heat exchange, to said primary zone. I

3. A process of the character deflned in claim 2 wherein high-boiling fractions of the reflux condensate are returned to further conversion in the same system.

4. YA conversion process which comprises cracking hydrocarbon oil, fractionating the resultant vapors in a primary fractionating zone thereby forming relatively heavy reflux condensate, further fractionating the vapors in a secondary fractionating zone to form a lighter reflux condensate, withdrawing heavy reflux condensate from said primary zone and passing the same in indirect heat exchange relation with said lighter condensate to reboil the latter and thereby cooling the withdrawn heavy condensate, then returning such cooled heavy condensate to said primary zone, and continuously supplying heavy reflux condensate from the primary zone to the cracking step.

5. A conversion process which comprises cracking hydrocarbon oil, fractionating the resultant vapors in a primary fractionating zone thereby forming relatively heavy reflux condensate, further fractionating the vapors in a secondary fractionating zone to form a lighter reflux condensate, withdrawing heavy reflux condensate from said primary zone and passing the same in indirect heat exchange relation with said lighter condensate to reboil the latter and thereby cooling the withdrawn heavy condensate, then returning such cooled heavy condensate to said primary zone by commingling the same with the vapors entering this zone thereby to decrease the load in the latter, and continuously supplying heavy reflux condensate from the primary zone to the cracking step.

6. A conversion process which comprisw cracking hydrocarbon oil, fractionating the resultant vapors in a primary fractionating zone thereby forming relatively heavy reflux condensate, further iractionating the vapors in a secondary fractionating zone to form a lighter reflux condensate, continuously circulating heavy reflux condensate in a closed ring from said primary zone through a cooling zone and then back to the .primary zone, passing the thus circulated heavy condensate, during its flow through said cooling zone, in indirect heat exchange relation with said lighter condensate to reboil the latter, and continuously supplying heavy reflux condensate from the primary zone to the cracking step.

'I. The process as defined in claim 6 further characterized in that the heavy condensate circulated in said ring is returned to the primary iractionating zone by being commingled with the vapors entering this zone whereby to decrease the load of the primary zone.

DONALD J. BERGMAN.

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