US2534870A

US2534870A - Distillation

Info

Publication number: US2534870A
Application number: US764689A
Authority: US
Inventors: Wheaton W Kraft
Original assignee: Lummus Co
Current assignee: CB&I Technology Inc
Priority date: 1947-07-30
Filing date: 1947-07-30
Publication date: 1950-12-19
Anticipated expiration: 1967-12-19

Description

Dec, 119, 1950 W. W. KRAFT DISTILLATION Filed July 30, 1947 1N VEN TOR.

Wkeaikzz Patented Dec. 19, 1950 DISTILLATION Wheaton W. Kraft, Scarsdale, N. Y., assignor to The Lummus Company, New York, N. Y., a

corporation oi Delaware Application July 30, 1947, Serial No. 764,689

8 Claims. (01. 202-40) This invention relates to improvement in methods of and apparatus for vacuum distillation and more particularly for the vacuum distillation of heavy hydrocarbons oi the type of petroleum oils.

In refining petroleum oils by vacuum distillation it is customary to use a substantial amount of steam to obtain a lower equivalent pressure through the well-known partial pressure effect, the steam usually being removed from the tower at a point above the product overhead. However, as the steam concentrates at the top of the tower, a portion of the useful hydrocarbons accumulates with it and these hydrocarbons are subsequently carried over by the steam to the condenser system. To completely separate the valuable oil from the steam it i necessary to cool the steam-oil mixture in one or more stages until temperatures of about 120 F. or lower are reached. Since the system is operated under a vacuum and since a large amount of vapors must be cooled and condensed, an excessively large surface area is required in the condensers to transfer the heat from the vapors to the liquid cooling medium especially when the temperature of the vapors approaches that of the cooling medium. The large area required in these condensers means, of course, a large pressure drop through the condensers, and consequently, elaborate and expensive Vacuum producing devices are needed. In addition, to minimize pressure drop in the connecting lines, it is necessary to mount these large condensers near the top of the tower, which means costly structural supports for the condensers, or to supply very large vapor lines from the tower to the condensers if they are supported at a lower elevation.

The principal object of my invention is to substantially reduce the loss of hydrocarbons in the steam vapors and also to use liquid-to-liquid coolers in the overhead product and reflux lines which can be maintained in any convenient location and which have a much higher heat transfer rate in comparison with liquid-vapor conclensers and coolers designed for low pressure drop.

A further object of my invention is to provide more effective control of a vacuum distillation step by utilizing a part of a circulating reflux stream in a final contact section to materially aid in the control of the temperature at the top of the distillation column and to reduce the amount of oil vapors passing to the steam condenser.

More specifically, I provide for cooling a side stream at a high temperature level thus utilizing to maximum advantage the temperature difierence between the available cooling medium and the sidestream, returning a part of the cooled sidestream to the column to serve as circulating reflux and subcooling the balance of the stream to. a temperature sufficiently low to provide a final vapor absorbent to separate substantially all of the hydrocarbons from the strip ping steam.

Further objects and advantages of my invention will appear from the following description of a preferred form of embodiment thereof taken in connection with the attached drawing in which Figure l is a schematic flow diagram of a part of a iractionating system, and

Figure 2 is a modification of the system shown in Figure 1.

In principle, my invention involves the combination with a circulating reflux system of a final cooling and absorbing operation whereby all of the hydrocarbons may be separated as product from the steam before the steam is passed from the column. In accordance with this principle, I introduce through line H, located at some intermediate point on column in of Figure l, a hydrocarbon feed such as a partially vaporized reduced crude oil. It is to be understood that one or more sidestreams may be removed from the middle of the column and that steam and vapors from sidestream strippers may be introduced through one or more lines It.

The column is provided with a series of liquidvapor contact stages which may be of any of a number of well-known types. In the accompanying drawing I have shown a series of shower decks below two bubble cap trays, but it will be apparent from the following description that any combination of any type trays might be used. The column I ll of Figure 1, as shown, is provided with a series of shower decks it above the point of feed, the lower-most shower deck Ilia being provided with a draw-off 20 for a liquid which is removed by pump 22 and passed through cooler 2 with a major part of the stream returned through valved line 26 to the uppermost shower tray l8b. The balance of the liquid is passed through line 34 and sub-cooler 36 where the temperature of the liquid is finally adjusted before being returned to the column at the upper tray 30 through the valved line 41.

I find that it is entirely satisfactory to operate the cooler 24 at a temperature such that the liquid removed through line 20 from tray Ila, which may have a temperature of about 500 1'. can be reduced to 200 F. The major proportion of this material i then reintroduced to the uppermost shower tray "b, the exact amount depending upon the specified end product desired. It is thus possible to maintain a temperature approaching 200 F. at the upper part of the column. With the temperature control as outlined above the section of the column l between the drawofi line and the return 26 becomes a condenser wherein most of the heat is removed from the tower and wherein most of the hydrocarbon vapors are condensed to liquid. It may be compared in part to the overhead condensing system in an ordinary column wherein heat is removed from the tower in a reflux condenser. This is, in fact, a circulating reflux system wherein the heat is removed by a liquidto-liquid heat exchange step thereby materially reducing the physical size of the cooler and its associated liquid-liquid heat exchanger.

While the reduction of temperature to about 200 F. is highly efllcient from the standpoint of operation of the heat exchanger 24 under available cooling medium conditions, the vapors leaving the deck MD at such temperature will include a certain percentage of valuable hydrocarbons which must be recovered from the steam. In order to remove this material, I find it desirable to provide one or more bubble cap trays 30 in the topmost part of the column, such bubie trays serving as a multiple absorption section for hydrocarbons in the vapors passing to the outlet 32. The absorption liquid is supplied to these bubble cap decks by line 34 and is the balance of the stream removed at 20, such stream thence passing through the sub-cooler 36 and being fed to the topmost tray 30 through the valved line 41.

It is thus possible by cooling the small amount of liquid passing through the cooler 36 to a temperature of approximately 100 F. to reduce the temperature at the top of the column to 120 F. or less, maintaining a topmost temperature sufficient to condense and absorb all of the hydrocarbons in the vapors. The vapor at 32 is thus only steam with not more than a trace of hydrocarbons and can be condensed in a barometric type condenser or a surface condenser 38 as shown with condensate removed through line 38 in the well-known manner. Vacuum jet 40 and barometric condenser 42 may also be arranged as usual.

As so far described, the column I0 is in baiance, that is, all of the liquid returned to the top tray 30 is w thdrawn as product and is sufficient in quantity to de-superheat the steam and condense and absorb the residual hydrocarbon vapors entering tray 30a. Preferably, however, an excess amount of the liquid must be recirculated for control purposes. For this reason, the product line 44 is provided with the temperature controlled valve 45, the setting of which is determined by the temperature at some suitabe point in the column such as that shown in Figure 1 below the shower tray I811. As the temperature increases at this point, valve 45 will deliver a lesser volume of product and the excess will accumulate in the trap section of the plate 3"a. The accumulated liquid will overflow into the extended portion 33 of the downflow pipe 3| thereby providing an increased amount of cooling liquid on the trays I8. In this way, the temperature and thus the composition of the product liquid may be more easily controlled. It will be apparent, however, that substantially the same eil'ect may be obtained by substituting for the temperature controlled valve 46, a flow control valve to give a constant rate of output. The difference between the two control methods described is that the former operates to control the end boiling point of the overhead product while the latter provides that such product-i5 a constant percentage of the feed to the column.

In Figure 2, I have shown a modification of the basic system outlined above and as indicated in Figure 1. In Figure 2, those parts having the same function as those in Figure 1 have the same number. In this system the product liquid is withdrawn from tray 30a and passed to the accumulator 46 by way of line 44. From the accumulator 46 it may be then passed by pump 48 through line 50 to the product draw-oil. line 35. Operated in this manner, the system is in balance as mentioned above whereby the product liquid withdrawn is sufficient to absorb the hydrocarbon vapors from the vapor mixture in the last two trays 30. If an excess amount of coolant is required to maintain the temperature at the top of the column, it may be returned directly to the top of the column through the valved line 35a. In this way a closed cycle is provided for the excess through line 44, accumulator 46, line 50 and valved line 35a. Instead of returning a portion of the product through line 35a an excess amount of the liquid may be pumped through the cooler 24 and then passed to cooler 36 to be eventually returned to the trays l8 through valved line 31. Lines 31 and 35a may be used separately or together to provide a liner control of the temperature at the top of the column and composition of the product withdrawn.

Lines

31, 35 and 3511 provide a simple means of assuring an adequate flow of liquid back to the column to maintain any predetermined temperature in the top of the column while at the same ,time providing a simplev means of drawing off a product stream.

It will thus appear that I have accomplished several important features. Cooling is done entirely by relatively small liquid-liquid coolers which can be maintained at any convenient elevation and no vapor-liquid condensers are necessary except for the steam which, because of the absorption step, contains a much smaller amount of hydrocarbon vapors than would be the case if the usual type of condensing system were employed. The major cooling load is accomplished in the shower tray section and the balance is completed in a bubble tray absorption step so that the hydrocarbon vapor load on the overhead condensers is eliminated. Any recycle required for the absorption step is independently subcooled in a liquid-liquid cooler and inasmuch as the amount subcooled is small the heat exchange surface required is correspondingly small.

From a process standpoint, there is substantial cooling surface for the desired temperature reduction of the vapors without excessive pressure loss. In addition, the steam is de-superheated before leaving the column and thus, the vacuum apparatus is less expensive for the duty then customarily required. The system has substantial flexibility and may be more easily maintained than is usual for such installations. Also, since the condensation accomplished in trays l6 greatly reduces the volume of vapors passing upwardly through the column in, the final release of steam vapors from tray 30 takes place at greatly reduced velocity with negligible entrainment of hydrocarbon liquids to the steam condenser. It is to be understood that although I have described tower III as a reduced crude tower or column, it could also be used as a rerun tower on a heavy hydrocarbon distillate, and for numerous similar uses.

While I have shown and described a preferred form of embodiment of my invention, I am aware that modifications may be made thereto and I, therefore, desire a broad interpretation of my invention within the scope and spirit of the description herein and of the claims appended hereinafter.

I claim:

1. A method of removing heat from the vapors in a steam stripped distillation column operated under a vacuum which comprises withdrawing a portion of the liquid from the column}, cooling said liquid and returning a major part of the cooled liquid to the column in direct contact with the vapors in said column whereby the heavier components are substantially condensed from said vapors, subcooling the remainder of said cooled liquid, passing the subcooled liquid to an absorption zone in direct contact with the residual vapors whereby substantially all of the lighter components are absorbed, withdrawing a liquid overhead product from said absorptio zone and thereafter removing substantially product-free steam from said column.

2. The method as claimed in claim 1 whereby a part of said overhead product is also subcooled and thereafter recycled to said absorption zone.

3. The method as claimed in claim 1 whereby a portion of said overhead product is in direct contact with the vapors in said column.

4. The method of removing heat from the vapors in a steam stripped, vacuum distillation column having a reflux zone at the top of the column and an absorption zone above the reflux zone which comprises withdrawing a portion of the liquid from the reflux zone, cooling said liquid, returning a major portion of the cooled liquid to said reflux zone in direct contact with the vapors in said zone, subcooling the remainder of said cooled liquid, returning the subcooled 5. In a vacuum distillation column employing steam stripping, a series of liquid-vapor contacting decks, means to cool a portion of the liquid collected on said decks, means to return said cooled liquid to the column in direct contact with vapors rising from said decks, an absorption section above the contacting decks and in vapor com munication therewith, means to subcool a portion of said cooled liquid, means to pass said subcooled liquid to said absorption section, means to withdraw a liquid product from said column, and vacuum producing means connected to the top of the column whereby substantially productfree steam may be removed from said column.

6. A vacuum distillation column as described in claim 5, including means to subcool a portion of said liquid product and means to recycle said subcooled product to said absorption zone.

7. A vacuum distillation column as described in claim 5 and including means to pass a portion 8. In a vacuum distillation column employing.

steam stripping, a series of liquid-vapor contacting decks, an external liquid cooler, means to pass a portion of the liquid collected on said decks to said cooler, means to return a portion of the cooled liquid to the column in direct contact with the vapors leaving said decks, a subcooler, means to pass the remainder of the cooled liquid to said subcooler, an absorption section above the contacting decks and in vapor communication there= with, means to pass the subcooled liquid to said absorption section whereby a liquid product is formed, means to return a portion of the liquid product to said column in direct contact with said vapors, means to withdraw the remainder of said product from said column, means to return a portion of said product through said subcooler, to said column. and means whereby substantially product-free steam may be removed from said column.

WHEATON W. KRAFT.

REFERENCES map The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,456,419 Black May 22. 1928 1,851,550 Tuttle Mar. 29, 1932 1,912,136 Haslam May 30, 1933 1,991,792 Coubrough Feb. 19. 1935 2,125,325 Youker Aug. 2, 1938 2,224,986 Potts Dec. 17, 1940 2,377,736 White June 5, 1945

Claims

1. A METHOD OF REMOVING HEAT FROM THE VAPORS IN A STEAM STRIPPED DISTILLATION COLUMN OPERATED UNDER A VACUM WHICH COMPRISES WITHDRAWING A PORTION OF THE LIQUID FROM THE COLUMN, COOLING SIAD LIQUID AND RETURNING A MAJOR PART OF THE COOLED LIQUID TO THE COLUMN IN DIRECT CONTACT WITH THE VAPORS IN SAID COLUMN WHEREBY THE HEAVIER COMPONENTS ARE SUBSTANTIALLY CONDENSED FROM SAID VAPORS, SUBCOOLING THE REMAINDER OF SAID COOLED VAPORS, SUBCOOLING THE REMAINDER OF SAID COOLED LIQUID, PASSING THE SUBCOOLED, LIQUID TO AN ABSORPTION ZONE IN DIRECT CONTACT WITH THE RESIDUAL VAPORS WHEREBY SUBSTANTIALLY ALL OF THE LIGHTER COMPONENTS ARE ABSORBED, WITHDRAWING A LIQUID OVERHEAD PRODUCT FORM SAID ABSORPTION ZONE AND THEREAFTER REMOVING SUBSTANTIALLY PRODUCT-FREE STEAM FROM SAID COLUMN.