US2736688A - Method for distilling oils - Google Patents

Description

United States Pate 2,736,688 Patented Feb. 28, 1956- ice METHOD FR DISTLLNG LS Wheaton W. Kraft, Scarsdale, N. Y., assigner to 'ljhe Lummus Company, New York, N. Y., a corporation oi Delaware Application February 16, 1953, Serial No. 336,998

2 Claims. (Cl. 1196-73) This invention relates to a method for distilling oil. It is an improvement on my Patent U. S. 2,149,058, and a continuation-in-part of my co-pending application Serial No. 180,418, filed August 19, 1950, and now abandoned.

It is well known that the initial boiling point of any overhead distillate is not subject to control, since it is determined by such light ends as may be present in the charging stock. By ordinary methods, control of the initial boiling point may be effected only by making sure that the light ends are not present in the charging stock, but any such control is not commercially feasible since the preliminary removal of light ends from a large body of charge material would require an excessive quantity of heat and/or stripping steam.

The object of the present invention is to provide a method whereby the initial boiling point of any overhead distillate may be economically controlled. Another object is to provide a method which avoids the diiculties of simultaneously controlling the specified end points of several fractions from a single column. With these ob` jects in view, the present invention comprises the method hereinafter described.

In the accompanying drawing, Fig. l is a diagram of the preferred form of apparatus for practicing the present invention. Fig. 2 shows a modification of a part of the overhead condensing system.

The present invention is herein illustrated and described as embodied in a system for obtaining kerosene of controlled initial boiling point, although it may be applied to heavier or lighter fractions.

The crude oil in line is heated in heater 12 or by other suitable means and passed to the vaporization zone ld of a primary column 16. Column 16 is generally operated at a pressure of from -100 p. s. i. depending on the composition of the crude oil introduced. In handling stocks having relatively high percentage of naphtha, for which this invention is particularly suited, an operating pressure of about 50-100 p. s. i. is preferred. The portion of the oil remaining unvaporized passes through a bottom stripping zone 13 into which steam may be introduced at 2d. Above the point of introduction of the feed, a rectifying section 22 is provided by which the end boiling point of the distillate products are controlled. Vapors including light gasoline and steam are passed through overhead line 24 to -condenser 26, where the vapors are condensed. The condensate is passed to decanter 28 from the lower end of which water is removed through line 30. Light gasoline is withdrawn by line. 34 and passed to a gasoline stabilizer, not shown. Reflux is returned to column 16 through line 32.

The bottoms withdrawn from primary column 16 through line 36 contain some of the components present in the distillate products viz. light gasoline, light naphtha and heavy naphtha. If substantially complete removal of the small quantity of these light components were attempted, an uneconomically large quantity of steam would be required in line 20, or a very considerable heating would be required in the reboiling circuit through reboiler 38, hereinafter referred to. This would require increased relluxing in the rectifying section of the column 16 which would be reflected in high condenser duty and a large diameter tower. According to the present invention, no attempt is made to effect complete separation in the column 16.

It is one of the important features of the present invention to remove substantially all of the naphtha present in the charge in column 16. While the light and heavy naphtha side streams 92 and 94 in column 16 represent only a relatively small proportion of the total feed to column 16, the presence of naphtha in the second column as described in U. S. 2,149,058, adds considerably to the burden on the secondary atmospheric column. The importance of naphtha removal in the primary column with no appreciable increase in the size of the primary column will be apparent from the description which follows.

A portion of the bottoms from the first column 16 is heated in a pipe still reboiler 38 from which it is returned to column 16 for reboiling purposes n place of or in conjunction with the steam introduced at 20. The amount of 4material returned through line 40 is controlled by valve 42.*. Reboiling instead of steam stripping, may be practiced when the overhead gasoline contains material which is not condensable with the available cooling water. in such cases, the column is preferably operated under super-atmospheric pressure, and no steam is used. Whether the distillation is carried out with steam, or by reboiling, or both, no attempt at complete separation in the primary column is made.

The partially stripped bottoms from column 16 are then heated in pipe still 39. A single heater and reboiler may be used but it is found desirable to use a separate reboiler system as shown. The heated bottoms are delivered through line 44 to the vaporizing zone 46 of a secondary column d8, which has a bottom stripping zone SQ into which steam may be introduced at 52. Above the feed there are provided fractionating sections, indicated as two sections :'54- and 56 and two side stream drawoi pots 6l? and 62. It will be understood that the number of fractionating and side-stream drawols may be different from the number shown depending on the number of side products to be removed. During operation, gas oil accumulates in section 56 and is withdrawn from drawol 62 by line 55 to stripper 57. In normal operation it is generally desirable to remove separate light and heavy gas oil side streams from the secondary tower. Steam is admitted to stripper 57 through line 59. Lightvvapors and steam return through line 6l to a point in the tower above drawoil 62. Stripped gas oil is removed through line 7d. The kerosene accumulating in section 58 is collected in drawo 6@ from which a suiiicient amount is permitted to overflow to the next lower deck to serve as reflux in the column. Kerosene is withdrawn from drawott 66 through line 63 a portion of the liquid passing to stripper 65. Steam is admitted through line 67 to stripper and light vapors and steam are returned to the tower through line 69. Stripped kerosene is removed through line 72. Reboilers, not shown, are generally provided for gas oil and kerosene strippers 57 and 65 respectively. i

A part of the kerosene is passed through line '74, pump 76, cooler and then recycled to section 58 of tower 48. This circulating retiux system removes heat .from the towerthereby condensing the kerosene cut and providing reflux. The initial boiling point of the kerosene at '72 is controlled by the stripping operation and the end point by the fractionation provided in the sections 54 and 56.

The uncondensed vapors passing out of the top of the column are passed through line Si) to condenser 82 wherein they are condensed. The condensate is passed through line iid to decanter S6 from which the water is removed at 89. The amount of vapors passing through line V will be a small part of the total vapors entering section n 58. The condensate from 86 is returned to the primary column 16 through line $8, preferably at a point at which the composition of the liquid in the column approximates that of the returned material. Valved branch lines 90 permit introduction of the returned material to selected decks. The material returned by SS includes components within the boiling range of the light distillate product which is withdrawn at 34.- and also components within the boiling range of naphtha and kerosene. When this material is fractionated in the section 22, it is so divided that the components belonging in the light distillate are r and become part of the distillate at 3d, the part of the components belonging in the naphtha fractions are vaporized to he later removed from the column at 92 and 94, while the heavier components ow down the column into the residue which is withdrawn from the bottom oi the column.

ln one embodiment of this invention, the amount of overlapping boiling point material in line was raised from 1% to 2% of the total feed to the unit. This is of course a negligible amount and the slight disadvantages resulting from the handling of this additional amount ot recycle are more than offset by the compensating advantages of substantial reduction in tower size.

When operated according to the drawing, light gasoline may be drawn oilc at C14, light naphtha at 22, heavy naphtha at 94, gas oil at 70, residue at 95 and kerosene at 72. The material in line 80 will be, in addition to the 1% of heavy naphtha such as would be present in the original case, only an additional 1% of material having an endpoint greater than that of the heavy naphtha (both percentages being with reference to the feed in line The advantages of recycling this additional 1% are readily apparent and are reflected in both initial expense and operating cost.

It will be seen that the control in the tractionating sections of both columns is not critical since components which are not initially allocated to their unique products find their way back to the first column for further separation.

Withdrawing an uncorrected material as overhead from the secondary column having a boiling point range overlapping the boiling range of the gasoline overhead and naphtha side streams of the first tower provides the desired boiling range of naphtha in the first tower without enlarging the first tower and at considerable savings in the heat requirement of the system.

The return of a small amount of overlapping boiling point material through line S3 provides control of endpoint of the side streams and overhead product in the primary column. Such control is particularly desirable when specific product boiling rance requirements must be satisfied.

The importance of my invention over prior distillation practice for control of initial boiling point of overhead, and in particular over the method disclosed in U. S. 2,149,058, is readily apparent from the following comparative data based on operating conditions and requirements for plants of like capacity and product yield operated under the old method (Unit A) and my improved method (Unit B) respectively.

Table 2 Unit A Unit B Terno. Temp.

Y Kg. Gal./ Igevel, Kg. CaL/Hr. Ieell, Hr.

Heat available:

Secondary Tower 327 121 9,000, 000 343 121 10, 120,000 Benzine p None 168 30 729, 000 Naplltha Product.. 166 30 9, 330, 000 213 30 1,310,000 Kerosene Produet 220 30 1, 504,000 220 30 1, 504, 000 Lt. Gas Oil Product 279 40 2,320,000 279 l0 2,320,000 Hy. Gas Oil Product 310 40 1, 260,000 310 40 1, 200,000 Heat Required:

Kerosene Reboiler. 585,000 585,000 Lt. Gas Oil Reboiler 484, 000 484, 000 Hy. Gas Oil Reboiler 258,000 258, 000 Stabilizer- Reboiler. 1, 692, 000 1, 692, 000 Primary Flash 13, 757, 000 22, 538,000 Primary Rebell/L 4, 334, 000 3, 590, 0 Secondary Flash l 13, 600, 000 2 6, 880, 000

Total 34, 711, 000 36,027, 000

1 Includes naphtha reboiler (305,000 kg. oak/hr.) 2 Includes benzine plus naphtha reboiler (580,000 hg. cal./hr.).

Table 3 Unit A Unit B Heat Pic t-Up Kg. Cal./ Temp., Kg. 09.1./ Temp.,

Hr. C. Hr. C

Stripper Reboiler vs. Sec.

EMBL-.- 3,019,000 3, 019, 000 343 285 Primary Re (il 4, 334, 000 3, 590,000 Secondary Flash (red) 13, G00, 000 6, 880, 000 Crude vs. Product Streams 4,748,000 15 82 4,748,000 15 v) 82 Crude vs. Secondary Reflux 3,010,000 82 121 5,040,000 82 142 267 127 285 148 Crude vs. Secondary Btms 6,000,000 121 192 5,950,000 142 210 Primary Flash (tired), None 6, 800, 000

Total 34, 711, 000 36, 027,000 Fired Load 17,934,000

Hoot Recovered 16, 777, 000 18,757,000

1 Total Heat Saving (7,800,000 B. T. U,/Hr.).

1n the foregoing data, Table l shows the diterences in overall tower requirements as regards operating conditions and reux loads. Table 2 provides a comparison of major streams available for heat recovery and of heating requirements for all services. While the total heat input is more for Unit B, the greater heat recovery (Table 3) in Unit B, provides a net decrease in fired heat of about 4%, which means considerable savings in fuel requirements.

An alternative condensing scheme is shown in Fig. 2 wherein a condensation system dierent from that of Fig. 1 is shown. Vapors from the tower pass through line 100 to partial condenser 102.. A suitable amount of kerosene is condensed and returned to the tower l through line 1.04. The lighter vapors which are not condensed in condenser 102 are passed through line 103 to condenser and cooler in which they may be condensed before being passed through line 112 to column i6. In this case, the circulating reliuX system 74, 76 and 78 of Fig. l will also be eliminated.

According to my present improvement over my original patent, l have found that it is possible to eliminate decks at the top of tower l0 and to thereby reduce the height of the tower by as much as 15% to 20% without sacrificing any advantages. Furthermore, it is found that by virtue of the decreased vapor load at the top of the second tower 48, the diameter may be substantially reduced by as much as 5% to 10%. The cost of a tower is a direct function of its diameter and height. lt is apparent that since the cost of towers is perhaps the largest single item in the construction of a petroleum reiinery, the savings will be substantial.

In addition, my improved process provides a basically heat economical system which also makes it economical from a cooling water standpoint. The method makes it possible to reduce the size and number of heat exchangers.

The invention having been thus described, what is claimed is:

l. The method of separating a hydrocarbon mixture by fractional distillation which comprises, heating the mixture and passing it to a primary fractionation column to obtain a light distillate and a bottoms under conditions which incompletely remove from the bottoms, materials boiling within the range of the light distillate, withdrawing the' light distillate as an overhead from said column, withdrawing an intermediate side stream of controlled end point from said column, stripping the remainder of the mixture in said column to produce the desired bottoms, withdrawing said bottoms, heating one portion of the withdrawn bottoms in a separate reboiling circuit and reboiling said primary column with the hot bottoms, heating a second portion of said bottoms and passing said second portion to a secondary column, stripping said bottoms in the lower part of said secondary column and withdrawing stripped bottoms as product from said secondary column, substantially reducing the vapor load at the top of said secondary column by reiiuxing said secondary column at the top thereof with a reiiux circuit comprising withdrawing a condensed side cut from the upper part of said column, passing a portion of the withdrawn side cut to a separate stripping operation, cooling the remaining portion of the withdrawn side cut and reintroducing said cooled portion into said tower above the point of withdrawal so as to control the amount of overhead in said column, withdrawing an overhead from said secondary column which includes all of the material boiling within the range of the overhead of said primary column, condensing said overhead, withdrawing a portion of said condensed overhead as product and returning the remaining portion of condensed overhead comprising not more than 2% by weight of the initial feed to the primary column.

2. The method of separating a hydrocarbon mixture by fractional distillation which comprises, heating the mixture and passing it to a primary fractionation column operated at a pressure of less than p. s. i. g., withdrawing a light gasoline as overhead from said column having a controlled boiling point, withdrawing a light naphtha and a heavy naphtha as side cuts from said primary column, said naphthas having controlled boiling points, stripping said mixture in a lower part of said primary column, withdrawing from said column a stripped bottoms containing components of said overhead and said side cuts, heating a portion of said bottoms and returning said heated bottoms portion to the lower part of said primary column to reboil the same, heating a second portion of said bottoms to a temperature substantially above the temperature to which said first bottoms portion was heated, and passing the second portion of said heated bottoms to a second column operated at substantially atmospheric pressure, withdrawing a stripped residue as bottoms from said second column, withdrawing a gas-oil from an intermediate part of said second column, substantially reducing the vapor load in the upper part of said second column and controlling the amount of overhead withdrawn therefrom by withdrawing kerosene from a drawof deck of a condensing section at the top of said second column, passing a portion of the condensed withdrawn kerosene to an independent stripping column and withdrawing a stripped kerosene product from said stripping column, cooling the remaining portion of said withdrawn kerosene and returning it as circulating reflux to the top deck of said condensing section, withdrawing from said second column an overhead containing a portion of the kerosene and all of the heavy naphtha and lighter components contained in the feed to said second column as well as the light gasoline present inthe feed to said second column, condensing said overhead and controlling the boiling points of said gasoline overhead and naphtha side cuts in said primary column by returning condensed overhead from the second column to an intermediate part of said primary column, said returned condensate amounting to not more than 2% by weight of the initial feed to the primary column.

References Cited inthe file of this patent UNITED STATES PATENTS 1,997,675 Bahlke et al Apr. 16, i935 2,149,058 Kraft Feb. 28, 1939 2,224,986 Potts et al. Dec. 27, 1940

Claims (1)

1. THE METHOD OF SEPARATING A HYDROCARBON MIXTURE BY FRACTIONAL DISTILLATION WHICH COMPRISES, HEATING THE MIXTURE AND PASSING IT TO A PRIMARY FRACTIONATION COLUMN TO OBTAIN A LIGHT DISTILLATE AND A BOTTOMS UNDER CONDITIONS WHICH INCOMPLETELY REMOVE FROM THE BOTTOMS, MATERIALS BOILING WITHIN THE RANGE OF THE LIGHT DISTILLATE, WITHDRAWING THE LIGHT DISTILLATE AS AN OVERHEAD FROM SAID COLUMN, WITHDRAWING AN INTERMEDIATE SIDE STREAM OF CONTROLLED END POINT FROM SAID COLUMN, STRIPPING THE REMAINDER OF THE MIXTURE IN SAID COLUMN TO PRODUCE THE DESIRED BOTTOMS, WITHDRAWING SAID BOTTOMS, HEATING ONE PORTION OF THE WITHDRAWN BOTTOMS IN A SEPARATE REBOILING CIRCUIT AND REBOILING SAID PRIMARY COLUMN WITH THE HOT BOTTOMS, HEATING A SECOND PORTION OF SAID BOTTOMS AND PASSING SAID SECOND PORTION TO A SECONDARY COLUMN, STRIPPING SAID BOTTOMS IN THE LOWER PART OF SAID SECONDARY COLUMN AND WITHDRAWING STRIPPED BOTTOMS AS PRODUCT FROM SAID SECONDARY COLUMN, SUBSTANTIALLY REDUCING THE VAPOR LOAD AT THE TOP OF SAID SECONDARY COLUMN BY REFLUXING SAID SECONDARY COLUMN AT THE TOP THEREOF WITH A REFLUX CIRCUIT COMPRISING WITHDRAWING A CONDENSED SIDE CUT FROM THE UPPER PART OF SAID COLUMN, PASSING A PORTION OF THE WITHDRAWN SIDE CUT TO A SEPARATE STRIPPING OPERATION, COOLING THE REMAINING PORTION OF THE WITHDRAWN SIDE CUT AND REINTRODUCING SAID COOLED PORTION INTO SAID TOWER ABOVE THE POINT OF WITHDRAWAL SO AS TO CONTROL THE AMOUNT OF OVERHEAD IN SAID COLUMN WITHDRAWING AN OVERHEAD FROM SAID SECONDARY COLUMN WHICH INCLUDES ALL OF THE MATERIAL BOILING WITHIN THE RANGE OF THE OVERHEAD OF SAID PRIMARY COLUMN, CONDENSING SAID OVERHEAD, WITHDRAWING A PORTION OF SAID CONDENSED OVERHEAD AS PRODUCT AND RETURNING THE REMAINING PORTION OF CONDENSED OVERHEAD COMPRISING NOT MORE THAN 2% BY WEIGHT OF THE INITIAL FEED TO THE PRIMARY COLUMN.

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