US2217356A - Mineral oil distillation - Google Patents

Description

Oct 8, 1940.- R. v. BECKNELL.

MINERAL OIL DISTLLTION Filed May 3. 1957 3 Sheets-Sheet l lIAII liv Oct. 8, 1940. R. v. BECKNELL MINERAL OIL DISTILLATIQN Filed lay 3, 193" 3 Sheets-Sheet 2 IPO/mld Y Eea/(cned IY Sriovwa .Oct 8, 1940- R. v. BECKNELL MINERAL OIL DISTILLATION a sheets-sheet 5 Filed Hay 3, 1937 n VPUL, Condensae i Patented Oct. 8, 1940 I y t UNITED STATES PATENT OFFICE MINERAL'OIL DIsTILLA'rIoN Ronald V. Becknell, Chicago, Ill., assigner, by

mesne assignments, to High Vacuum Processes, Inc., New York, N. Y., a corporation of Delaware Application May 3, 1937, Serial No. v140,501

2 Claims. (Cl. 196-106) This invention relates to mineral oil distillaparamount. However, there are many practical tion; and it relates more particularly to sysdiiiiculties tending to complicate the problem tems for the manufacture of lubricating oil by of thus cutting down the time factor while at the distillation of suitable mineral oil charging same time enabling production of overhead lubri- 5 stocks under very high vacuum, that is, under4 cating oil distillates of the desired high grade 5 very low absolute pressure. and satisfactory character. Although these An important object of the present invention diiculties are encountered to a greater or less is to provide a high vacuum distillation system extent in the high vacuum distillation of the for the production of lubricating oils as overheavier fractions of any lubricating oil charghead distillates, wherein the time during which ing stock, they are particularly acute in the case 1t] the mineral oil material undergoing distillation of paraflin base stocks typied by Pennsylvania is subjected to heat at the necessary temperacrudes. This is because, in order to vaporize tures shall be minimized, thus minimizing the the heavier lubricating oil fractions, such stocks amount of thermal decomposition or cracking must be heated, even when distilled under the that occurs in the distilling operation. very high vacua mentioned, to final distillation 15 A further object of the invention is to accomtemperatures well above that at which substanplish this objective by providing for such a systial cracking occurs rather rapidly. Accordtem component apparatus units which, in addiingly, while the present invention is of utility tion to cooperating in the general combination in high vacuum distillation of lubricating oil or system to minimize the time factor aforesaid, charging stocks generally, it has particular ap- 20 shall individually possess novel features of conplicability to paraiin base charging stocks for struction and operation which render them of the reasons `iust pointed out. Therefore, in the general utility in oil refining and distillation, explanation of the principles of the invention besides giving them special utility in an appahereinafter to be set forth, reference will be ratus combination or system of the character made more particularly, for the sake of a cony26 above referred to. crete and illustrative example, to a system par- Other objects and advantages of the invention ticularly well adapted for high vacuum distillawill appear more fully as the description protion of a lubricating oil charging stock derived ceeds. from Pennsylvania or similar paraffin base Although the employment of very high vaccrude; but it will. be understood that this is 30 uum, e'. g. absolute pressure of 25 mm. mercury merely typical of the various uses of which the absolute and much lower, enables the manufacsystem and its component units are capable and ture of lubricating oils as overhead distillates is in no way intended to limit the scope of the which can be marketed and used directly aS invention.

nished lubricating oils, either without any fur- A desirable practical embodiment of the in- 85 ther rening acid treatment and ltration or Vention is illustrated in the accompanying drawwith only relatively slight refining, it is neverings, whereintheless aiact that, notwithstanding the much Fig. 1 illustrates more or less diagrammatilower temperatures at which distillation can be cally in the form of a general layout or ow 40 effected under such very low absolute pressures, sheet a typical high vacuum distillation system 40 a certain amount of unavoidable cracking or de- Within the scope of the invention; composition occurs, in taking oi the heavier Fig. 2 is a sectional elevation of a modified lubricating cuts or fractions from the reduced form of fractionating tower or column'which in charging stock if the stock is held for any subsome cases may be used to advantage in place stantial length of time at temperatures substanof the particular form of tower illustrated in 45 tially higher than about 625 F. Since the Fig. 1;

amount of cracking undergone by mineral oil Fig. 3 is a vertical section of the upper part always depends not only upon the temperature of the tower or column on the line 3-3 of Fig. 2; to which it is heated but also upon the time it Figs. 4 and 5 are horizontal sections on the is subjected to that temperature, and since it is lines 4 4 and 5 5, respectively, of Fig. 2; 60 in some cases necessary to go to fairly high Fig. 6 is a sectional detail, on an enlarged temperatures in order to get oi certain desired scale, of the construction of entrainment elimilubricating values of the charging stock, the nator devices of a special typel which may be importance of so conducting the distillation as advantageously employed in the towers shown in to minimize the time factor mentioned becomes Figs', 1 and 2; 55

system therein illustrated is particularly well adapted for high vacuum distillation ofa paraffin base crude charging stock for production of overhead lubricating oil distillates. In this ing two products therein made.

stance, the system is of the double flash type,

comprising two separate flash and fractionating towers I and I I. The ilrst is for the production of average lubricating oil distillates containing component fractions having viscosities upto as high as 150 seconds at 210 F., for example; while the second, which is considerably smaller, is for the production mainly of overhead bright stock having an average viscosity as high as 200 seconds at 210 F. All figures for viscosity given herein are to be understood as referring to viscosity determined by means of the Saybolt universal viscosimeter.

A In this instance, it will be assumed that it is desired to make, in each tower, two side stream fractions of lubricating oil distillations containing wax, which are to be sufficiently fractionated to give the lower (higher viscosity) side stream producta high flash point. In tower I0, therefore, the fractionating section, consisting most desirably of a single fractionating or. bubble tray or plate I 2, is located between the points at which these two cuts (e.v g., light distillate and heavy distillate) are removed from that tower, in order to give the desired fractionation between the Bubble tray I3 is similarly arranged in tower II for the same purpose. The two cuts made in the second tower may be an overhead bright stock cut and a relatively small cut of considerably lower average viscosity representing in part the overlap between vapor and liquid residue in the flashing eiected in the rst tower.

In the present example, it is further assumed that the charge to the distillation system is a residual product resulting from topping crude in a preceding atmospheric pressure topping operation in which the light ends, including gasoline, naphtha, kerosene and most of the gas oil have already been removed, but substantially without removal of lubricating oil compounds of the crude. In a typical instance, the topped crude thus charged into the system may represent 40 per cent of the original crude. Generally, the crude will have been treated with caustic soda before the topping operation, as by adding, say, 0.2 pint of 40 Baum caustic soda solution per barrel of original crude. Since the water from this addition, as well as that originally contained in the crude oil, will have been practically all removed in the'atmospheric topping unit, the

' topped crude charged into the high vacuum vdistillation unit or system shown in Fig. 1 will be practically dry but will of course contain soda. Where the crude employed is one containing no light en ds, such as gasoline and kerosene, such `crude may be charged directly into the system and the caustic soda introduced into the crude stream after it has been preheated to a. temperature of approximately 300 F., the crude being then discharged into an atmospheric dehydrating tank or tower (not shown) where the water naturally contained in the crude, as well asv the water from the soda solution, is evaporated before entering the first high vacuum fractional/lng tower ofsthe system.

Assuming the charging stock to be a topped'- Pennsylvania crude, it may enter the system from the topping unit through line I4 at around 200 F., being pumped by the crude charging pump I5 through heat exchange in the two fractionating towers, including pressable wax distillate condenser I6 and heavy distillate condenser I1 in tower I0, and condenser I8 in tower I I, thereby being pre-heated to a temperature of, say, about 485 F. The pre-heated charging stock then goes to the. pipe still heater indicated generally at I9, passing first through convection tube bank 20, where it is heated to approximately 595 (e. g.) and then through the radiant tube bank 2 I, where it is heated to a temperature at the heater outlet of, say, 745 F., at substan tially which temperature it is discharged at 22 into the vaporizing section of tower I0, entering the tower in this instance through three inlet nozzles 23, as more clearly shown in Fig. 5. These nozzles are provided with restricted discharge openings whereby the hot charging stock is given a very high velocity flow (e. g., 50 to 100 feet per second) and is directed tangentially against a plurality of evaporating members 24 (three in this instance) which, as here shown, take the form of hollow inverted frusto-conical shells. The hot oil thus projected against these vaporizing members near the upper edges of their inner faces is caused to circulate rapidly over those faces and spread out in a thin film. The slight downward tapering of the shells causes the downward pull of gravity on the oil to be balanced to a certain extent by the centrifugal force due to the rapid whirl given the oil, thus tending to prolong the time required for it to flow from the top of each evaporating member to the bottom. In this operation, a considerable part of the oil undergoes evaporation, the unvaporized liquid residue dropping from the lower edges of members 24 to a series of evaporating pans or trays 25 of any suitable design, shown here as of the disc and doughnut type, and thence to the bottom of the tower. The unvaporized liquid residue, amounting to, say, 25 per cent of the charging stock, leaves the bottom of the tower immediately through line 26 and is discharged into accumulator tank 21, the upper part of which is connected by vent line 28 to tower II, as shown.

The combination of evaporating members 24, 25, arranged asfshown in the vaporizing section of tower I0 provides a very large evaporating surface favoring extremely rapid evolution of oil vapors under the low absolute pressure prevailing in this part of the tower, which may advantageously be in the neighborhood of mm. of mercury. Also, by immediately leading unvaporized liquid residue from the bottom of the tower (typically at around 6259 F.), no pool of hot liquid residue accumulates and lies in the bottom of the fractionating tower to increase the time factor of this residue in the tower and thus give rise to some cracking in said tower with resultant production of cracked vaporsr passing upwardly through the tower to contaminate overhead lubricating cuts obtained from this tower. Instead, by thus passing the unvaporized liquid residue immediately into the accumulator tank 21, any decomposition products formed by holding the hot liquid residue prior to its further treatment in the next ilash stage and volatile at the temperatures involved are given off in said accumulator tank and therefore pass over through vent line 23 into tower il where they will not be detrimental.

The oil vapors rising from the evaporatlng section 2li-25 of tower I0 and having a temperature of, say, about 645 F., pass upward through troughs 30, suitably supported at their ends by' the tower wall and separated by spaces or vapor slots 3l. These slots are surmounted by inverted semi-cylindrical trough-like caps 32. As indicated by the arrows in Fig. 6, vapors passing upward at high velocity (e. g. 40 feet per second) through the slots or spaces 3i are compelled by caps 32 to reverse their direction and to pass downwardly between each cap and trough, which are so positioned relatively as to constrict the vapor passage somewhat at this point and thereby to increase the vapor ow velocity (e. g. to 60 feet per second). After leaving this constricted passage at such higher velocity, the vapors are forced to turn through a full 180 angle and then pass on up through the tower, while the entrained liquid particles are thrown directly downward and caught in the bottom oi' the troughs. InA order to prevent or minimize splashing and re-entralnment oi' the trapped liquid, a body 33 of suitable trapping material that is foraminous, reticulate, or otherwise of such character as to permit ready passage of liquid therethro-ugh, is provided in the lower part of each trough 30. For this purpose, a plurality of layers of screen wireor the like serve admirably, the assemblage being most desirably supported in the trough at some little distance above the bottom, as shown, to provide a shielded space for reception of the separated liquid. Liquid collecting in these protected spaces thus provided in the troughs 30 passes through outlets 34 into a common drain 35, by which the liquid is returned to some lower point in the tower (e. g., to the bottom).

This type of construction for the entrainment separators involves but a very low pressure drop, on the order of only 0.5 mm. mercury per separator device under conditions normally prevailing in the system, making a total drop of only 1.5 mm. for the entire entrainment separating section here illustrated.

In the form of the invention illustrated in Fig. l, the vapors leaving the entrainment separating section pass next through the fractionating section provided by bubble tray i2, already referred to hereinabove, wherea heavy waxbearing lubricating distillate cut is fractionated from the lighter vapors. Reiiux for this iractionating tray is provided by the fractionating condenser i? previously referred to, which is located above the tray and cooled by incoming charging stock on its way to the pipe still heater, thus constituting a part of the heat exchange whereby said charging stock is preheated.

in order to enable this fractionating of the heavy distillate cut to be accomplished with the minimum pressure drop, and in order to provide high ratio of slot area to tower area, the bubble tray and bubble caps 'of the fractionating device l2, which may be of the general type more fully illustrated in Figs. 2 and 3, may advantageously be of the special construction shown in greater detail in Fig. 8. In a typical instance, the bubble tray may be designed to carry an effective liquid head of approximately 1 inch, causing a pressure drop oi approximately 1.4 mm. Hg. The vapor velocity for this pressure drop can be as high as feet per second, depending on the density of the oil vapor; and the liquid head of 1 inch assumed, diminishes or increases accordingly as this vapor velocity is reduced or increased.

In the construction shown in Fig. 8, the bubble tray comprises a series of terminally supported troughs 3l in spaced parallel arrangement, with which cooperate caps 38 in the form of inverted troughs respectively located above the spaces cr slots 39 between troughs 3l. In this construction, the customary teeth 40 provided at the lower edges oi the cap troughs 38, instead of extending clear down to the desired nominal liquid level Lz'on the inside of the caps from the norma1 level L1 on the outside of the caps, as is the usual practice, extend only half of this distance (which distance is here assumed to be 1 inch), as clearly shown in Fig. 8. In the ordinary type cap, the free unit slot area is, only half the total area below the top of the slots, while in this design the free slot area. is I5 per cent of the total area below the top of the slots, which increases the slot area per unit length or per cap by approximately 50 per cent.

The overflow from the bubble tray passes through down pipes 40a into a catch basin 4I located inside the tower (Fig. 1), which is` equipped with an outside' oat type cage liquid level controller 42, the heavy distillate fraction or cut collected in this catch basin and having -a temperature of, say, 540 F., being pumped away by pump 43 through cooling means 44 to storage. 'Ihis heavy lubricating distillate fraction, in a typical instance, may have an average viscosity of 80 seconds at 210 F. and may comprise 25 per cent of the charging stock.

The uncondensed vapors leaving the bubble tray fractionator I2 pass upwardly through the heavy distillate fractional condenser I1 which, likewise, is designed for very low pressure drop (e. g., approximately l mm. mercury). A sultable arrangement of the tubes H* of this condenser is shown in Fig. '7, where the tube bank is fteen rows of tubes high and the tubes are arranged on a triangular pitch. 'I'here are no horizontal baiies of any kind in this condenser and at all times the liquid condensate is falling downward in a direction opposite to the rising vapor. in this way, a certain amount of reboiling of the condensate takes place in the condenser itseli, which is desirable for present purposes, particularly because it prevents the condensate` being cooled considerably below its boiling point and concomitant undesirable con- Y densation, as well as absorption and retention therein, of lighter hydracarbons. 'Ihe charge oil used to cool this condenser Il enters the top layer of tubes of the unit and thence through the succeeding layers of tubes in series down c between the cooling medium and vapor as is practical without using a condenser unit of excessive size.- The purpose of this is to prevent cooling the condensate on each tube much below character to the liquid catch plates 8I'82 shown on a somewhat larger scale in Fig. 2; thence through the wax distillate condenser I6 at the top of the tower. As here shown, this condenser is a separate unit located on the exterior of the tower, but it may belocated inside the tower if desired. Its construction and operation are virtually the same as those of the heavy distillate condenser Il. The wax distillate collecting in catch basin 46, having a temperature of, say, 420 for example, may be withdrawn by pump 41 equipped with liquid level controller 4l, and pumped through cooler 48 to storage. This cut which has a viscosity of 115 seconds at 100 F., for example, may constitute approximately 44 per cent of the charging stock.

The uncondensed gas oil vapors leaving the wax or light distillate condenser I6 at, say, about 355 F. in a typical instance, pass to gas oil condensing means 49, consisting most desirably of duplicate vertical type condensers mounted at the side of the tower as indicated at 49 in Fig. 2, the vapors flowing in a single pass downwardly parallel to the tubes 50 so that the vapor and condensate are cooled to as low a temperature as possible and as quickly as possible. The liquid gas oil condensate, which in a typical instance has a.v viscosity rof around 52 seconds at 100 F. and amounts to perhaps 6 per cent of the charge, falls into a compartment 5I at the bottom of each condenser and is pumped away to storage through line 5ta. These condensers are watercooled.

'Ihe non-condensable gas leaving the gas oil condenser-or condensers is withdrawn through large-diameter vapor line 52 by suitable vacuumproducing means, such as the 3-stage ejectors 53 which may be located on the framework and platform 54 at the top of the tower, in the manner indicated in Fig. 2. One of the chief advantages in locating the gas oil condensers on the side of the tower, as indicated at 49 (Fig. 2), is to leave the spaceat the top of the tower free for mounting the ejectors. By this method, it is possible to have the entire systemsupported by the tower itself, thus eliminating all separate structural steel supports for the different units. This is` a substantial advantage, because towers or columns operated at high temperature and having a considerable height expand as much as two vinches or more in being heated up from the cold,

rendering necessarythe installation of large expansion bends o r spring mountings where the condensers and ejector-s are mounted on a structural framework separate from the tower.

The ejectors maintain an absolute pressure of approximately 4 mm. Hg at the outlet of the gas oil condensers; and with all units designed for very low pressure drops, as described, an absolute pressure at the point of flash (22) as low as 10 mm. Hg is easily maintained under these assumed conditions.

The side stream wax-containing distillates produced by the operation described hereinabove are sufficiently closely fractionated to provide products of adequately high flash points and, suitable for subsequent de-waxing by available solvent de-waxing processes, such as the benzol-acetonetoluol and benzol-methyl-ethyl-ketone processes.

prevailing at this point in the tower.

the heavy distillate and the lighter distillate and also to practically reverse the volume pro-portions of these cuts, so that the heavy distillate cut shall amount to approximately, 43 per cent of the charge oil and the'light distillate approximately 26 per cent. The latter will then be a pressable wax distillate, properly speaking; while the heavy distillate, whose wax content will be largely amorphous, will nevertheless include relatively light fractions or ends carrying pressable wax. Removal of these lighter ends from the heavy distillate can be effected in various ways as, for example, in a high vacuum re-fractionating tower (not shown) provided with a sufiicient number of bubble trays to accomplish the desired closer fractionation. In order to accomplish this fractionation the heavy distillate cut is heated in the Ire-fractionating tower to a sufllciently high temperature to re-vaporize all of the pressable wax distillate, the necessary heat being provided by means of a separate tubular heater. The refractionating tower may, for example, be constructed with four bubble trays above the point of the charge inlet and two bubble trays below the charge inlet. The tower should also be equipped with a re-boiler below the series of bubble trays; and it should also be provided with a reflux condenser for supplying reflux to the bubble trays. The overhead pressable wax distillate vapor leaving this re-fractionating tower may be returned to tower I at a point above the wax distillate drawoil (4B) and below the fractionating pans 45. This auxiliary refractionating tower would operate, under these circumstances, at an absolute pressure approximating 11 mm. Hg at the point of vaporization below the bubble traysystem; and since only the lighter ends of the heavy distillate have to be re-vaporized in order to substantially free the heavy distillate of its pressable wax distillate component, the outlet temperature of the separate tubular heater employed with this auxiliary re-fractionating tower is not high enough to cause troublesome decomposition.

The liquid residue from tower I0 collecting in accumulator tank 21 is pumped therefrom by pump 54a through the radiant tube bank 55 of pipe still heater i9, this bank being separate from vthe first mentioned radiant tube bank 2l and located in a separate firing chamber 56, which has its own firing means l controllable independently of firing means 58 of the other heater chamber; but the firing gases from both heater chambers exit through the common flue 59 in which the aforementioned convection tube bank is located. In radiant tube bank 55, the residue pumped from accumulator tank 2l is very rapidly heated to a temperature of approximately 825 F. at which it is discharged into tower Il at 60, where the second ash vaporization occurs at the very low absolute pressure (e. g. 6-8 mm. Hg) Because of the particular design and arrangement of the pipe still heater i9, it is possible to heat the aforesaid residue from 670 F., the approximate temperature at which it ordinarily enters tube `bank 55, to 825 F., in a relatively very short period of time, only about 'l0 seconds in a typical instance, considerably lessthan half the time which would be required if the heater were designed in the conventional manner. In other words, the time factor involved in this final heating of the charging stock residue is reduced to approximately 40 per cent of what it would be with the ordinary type of pipe still heater.

The reduced crude stock entering tower H at the approximate maximum temperature (i. e. heater outlet temperature) of 825 as aforesaid, is discharged into arr evaporating section which is desirably similar to that of the fractionating tower l0, employing vaporizing surfaces 6h62, of the same type as 26 and 25 employed in tower l0. The unvaporlzed liquid residue, or that portion of it representing the normal make of ux oil residue, leaving the base of the tower at about 760 in a typical instance, is passed by pump 63 through cooler 66 to storage, this ilux oil residue usually amounting to around 10 per cent of the original charge oil. Instead of passing this residue through the cooler, it ,may be utilized in a heat exchange for recovery of much of its contained heat units. Most desirably, a portion of the total ilnal residue leaving the tower Il is dlverted through line by pump 66 and pumped back to accumulator tank 2l Where it mixes with and substantially raises the temperature of the residue from tower l0 and passes therewith through tube bank 55 of the flnal heater. The volume of flux oil residue thus constantly being re-clrculated, after the system is operating non mally, may desirably be about one-half that oi the residue from tower E@ with which it is mixed and whose temperature it thus quickly and easily raises before entry into final heater 56. This re-circulation accomplishes the further very desirable result of enabling a constant through-put to be maintained through the nal heater and oi providing a suiiciently high rate of flow to enable heeping the tube size in this iinal heater above a minimum A of. say, 1.5 inches internal diameter. At diameters less than this, considerable trouble is likely to be experienced from cols- 4ing and from deposition of soda contained in the charging stock. The pumping rate of the re-circulating pump 66 is controlled by an automatic liquid level control tl located in the accumulator tank, so that a constant liquid level will be maintained in this tank. Without such re-crculation or re-cycling, it is difficult to maintain a constant rate or" flow through the iinal heater because, although liquid residue normally ows from the fractionating tower lil at a substantially constant rate, some :ductuation may occur occasionally due to slight changes in operating temperatures and pressures in tower ES or rate of charge thereto. Consequently, in the absence of said re-cycling, and ii the operation 'of the iinal heater charging pump 5de were controlled by the liquid level in accumulator 2l, any change in rate of flow of residue from tower l0, unless irrimediately noticed by the plant operator, could cause a sudden stoppage of said nal heater charging pump, with resultant damage to the nal heater tubes. By providing the aforesaid recirculation or recycling of residue from. tower li, this diiculty is overcome and a method ormaintaining a constant through-put through the nal heater is provided.

The vapors ashed on" in the vaporizing section of tower ll have a temperature of around '770 (e. g.) They pass up through entrainment separators S6, bubble tray i3, fractional condenser ld and fractionating pans 69, all of 'which are designed for very low pressure drops, being most desirably similar in construction and general function to entrainment separators 2S, bubble tray I 2, fractional condenser I1 and fractionating pans 45, employed in tower I0.

Heavy bright stock, containing wax, is removed as a side stream distillate just below bubble tray i3 at a temperature approximating 670 (e. g.), being pumped by pump l0 through cooler 'Il to `storage, the operation of pump 1,0 being automatically controlled as indicated at 12. In a typical instance... this bright stock amounts to around 10 to l1 percent of the charge and has an average viscosity of 160 seconds at 210 F. 4It has a very high ash point, 600 F. open cup test and even higher in some cases. This is a very unusual product to be obtained by overhead distillation from a charging stock of the Pennsylvenia. type. It can be nished and made color stable by treatment with a comparatively small amount of clay. It is feasible, with this apparatus system, to produce from Pennsylvania crude, as an overhead distillate, heavy bright stocks having average viscosities as high as i90 to 200 (or more) seconds at 210 F. and flash points around 635 open cup. Such products have not been known in the art heretofore. Their boiling points are as much as 100 F. higher than bright stocks obtained as overhead distillates from non-wax-bearing crudes by high vacuum distillation.

In the particular arrangement here shown, a smaller percentage (e. g., between 2 and 3A per cent) of extra-heavy distillate, also containing wax, may be removed as a side stream above the heavy bright stool; cut by pump ll. This product, fractionated out of the vapors leaving condenser le (at about 610 FJ, by the action oi fractionating pans 6d and fractional condenser 69e, is taken from the column at a point loelov.7 condenser #d at a temperature approximately 580 F. and is pumped through cooler 'it to storage, the operation of the pump being automatically controlled in the usual way as indicated at l5. This product may have, typically, a viscosity oi about 90 seconds 'at 25.6 F. and an. open cup ash point of approximately 550. This product can be blended baclr with heavy bright stock or nished separately. It is to be noted that this product, representing in part a slight overlap between the ashed vapors and liquid residue in tower l d, is produced in relatively very small percentage as compared to the percentage that would normally be produced when operating at higher absolute pressures (including atn mospheric). where such overlap would be very much higher.

Residual uncondensed vapors passing condenser @Se at about 406 are conducted to iinai condenser T16 where a so-called header distillate or stink oil, usually amounting to around 2 per cent or less of the charge, is condensed. It consists almost entirely of partially decomposed products resulting from the very high temperature (825) of the nal heater discharge. It is pumped away by pump ide.

.Condenser 'i6 is connected by large-diameter vapor conduit means (like 52) to the vacuum producing means, a 3-stage ejector system in this instance.

Although the partial condenser 6tlg' may be cooled by charge oil like the other partial condensers shown in the illustrated system, it is here shown as provided with an oil trim cooling system, consisting of accumulator tank ll, pump ld and cooler ld. ln this system, a fairly light gas oil is suitable to use as the cooling medium to be circulated. Water cooling of the partial condenser 58* would give rise to dilculties due to vaporization in the tubes of the cooling unit, with deposition 'of silt and excessive tube corrosion.

All the condensers in the system described are' trainment-separating section and fractionating section of each ofthe towers I0 and I I, the resistance to ow of vapors at high velocity upwardly through these towers is reduced to a practical minimum, thus also minimizing absolute pressure drop or difference between point of flashing and the tower outlet through which the final uncondensable vapors and fixed gases are withdrawn by the vacuum-producing equipment. In the typical operation assumed in the foregoing description, this pressure dierence amounts to only 6 mm. Hg in tower I0, and 4 mm. in tower Il. Such drop need not exceed 10 mm. as a maximum in systems embodying the ,principles of the invention. The vacuum-producing equipment employed should in all cases be of such capacity that, in the normal operation of the system, said equipment shall be capable of withdrawing uncondensable vapors and gases from the vapor outlets of the towers with sufllcient rapidity to maintain at said outlet an absolute pressure not substantially exceeding 6... mm. Hg, and most desirably not exceeding 4 mm.

Referring again to the pipe still heater I9, the use of a heater of this particular type, which embodies novel features of construction and arrangement, contributes substantially to the achievement of one of the important objects of the invention, namely, reducing -the time to which the charging stock has tobe subjected to the relatively high flnal heating temperature, besides effecting important economy in fuel. In the construction illustrated, that part of the unit containing the convection tube bank and radiant tube bank 2| and fired by burner means 58 may be of conventional construction. The furnace gases from burner means 58, after being cooled to approximately 1200 F. by radiation to the radiant tube bank 2| pass down through the convection bank 20, being thereby cooled to about 550 F. ently red by burner means 51, comprises only a radiant bank of tubes 55. The furnace gases in this final heater are also cooledto approximately 1200 by radiation to tubes 55, and then pass through openings in the dividing wall between the two furnaces and join the flue gases of the first furnace, passing down through the common flue containing the aforesaid convection tube bank 20. In this way, notwithstanding the fact that the charge oil entering the final heater tubes 55 from accumulator 21 is at a temperature of 670, thus rendering it impractical to provide suflicient heat transfer surface in a convection tube bank to cool the combustion gases to a temperature' lower than approximately 740, the heater construction and arrangement of the present invention, whereby said charge oil from accuf mulator 21 is heated to the high final temperature of 825 by passage solely through the radiant tube bank 55, makes it possible to cool the gases from the nal heater 55 down to the same economical low temperature, approximately 550, as the gases The final heater 55, however, independfrom the rst heater are cooled. This not only results in a fuel saving of approximately 10 per cent in the ilnal heater, but it has the further advantage that all of the heat transfer to the charge oil going to the final tower Il is radiant heat, thus permitting the use of a minimum tube surface area. By reducing this surface area, the volume of oil contained in tubes 55 is correspondingly reduced, resulting in a tower time factor and thus reducing the amount of cracking that occurs. In other words, in the pipe still heater here illustrated, the charge oil going from accumulator 21 to tower Il remains in the nal heater for a period of time on the order of only about 70 seconds, in a typical instance; whereas if a heater of the conventional type were employed, the oil would have to remain in the heater tubes on the order of 175 seconds in order to achieve the same increase in temperature. Otherwise stated, the present arrangement enables a reduction of the time factor, in this stage of heating to the final flashing temperature, to approximately 40 per cent of what would otherwise be required. The importance of this in any process where the charge oil must be heated to a temperature at which cracking occurswith appreciable rapidity is obvious, and this importance is particularly pronounced where, as here, the nal temperature of the 'charge ismuch above that at which it is generally recognized cracking of a relatively heavy lubricating charging stock begins to be appreciable.

From the foregoing, it will be seen that the invention provides a high vacuum distillation system of which the various component parts are all designed and particularly adapted to cooperate in reducing the period of time during which a charging stock must be subjected to the relatively high temperatures requisite for effecting adequate vaporization and recovery, as overhead distillates, of the lubricating values contained therein. Operation of the system is characterized by extremely rapid vaporization through systematic lming over vaporizing surfaces of large area, as well as high velocity flow of the resultant vapors through entrainment-separating and fractionating means of a type interposing minimum resistance to vapor flow with consequent minimum drop in vapor pressure, while at the same time adequate fractionation for production of the desired products as side stream cuts is accomplished. In addition, large reduction is accomplished in the time factor involved in heating to the final flashing temperature at which charging stock by a procedure involving the employment of flash vaporizing temperatures', especially in the nal flashing stage, so high as to be generally regarded as excessive and dangerous, is suiliciently obvious without further comment.

The distillation system as thus far described is especially well adapted, as already stated, for processing a paraffin base charging stock, such as a topped Pennsylvania crude, where it is desired to make two side stream fractions of wax distillate which are separated by more or less close fractionation as may be desired. In operating on a non-wax-bearing crude, on the other hand, it may often be desired to e'ect in the first ash-vaporizing stage (i. e., in the rst tower of the system) only a fractionation between the gas oil cut and the side stream lube distlllates.v Under these circumstances, certain important practical advantages are attained by using the modified tower construction illustrated in Fig. 2 in place of tower I0 in the system above described and illustrated in Fig. 1. The final tower of that system, tower Il, may also be replaced, if desired, by the modified type shown in Fig. 2.

Referring to Fig. 2, it will be seen that this modified type of tower is provided in its lower part with a vaporizing section consisting of coned evaporating members 24a and evaporating trays or pans 25FL similar ltothe corresponding vaporizing surfaces 24, 25 of tower I0, nozzles 23 being arranged to direct high velocity jets of heated charge oil against the coned members 26B to nlm the charge thereover for a very rapid vaporization at the low absolute pressure (e. g., 10 mm. Hg) prevailing in this section of the tower. Unvaporized liquid residue exits through outlet 26a, while dashed oir vapors travel upwardly through entrainment separator devices 2te', similar in construction toseparators 29 of tower i0. Leaving the last of this series of three entrainment separators 29a, the vapors, instead oi passing next through a bubble tray section as in tower i0, are subjected to fractional condensation by passage through the intertubular spaces of fractional condenser 80 to condense out a heavy lubricating distillate which collects on liquid catch plates or trays Bl, 82 and is withdrawn as a side stream at 83 by an automatically controlled pump (not shown) in the usual manner. The vapors in passing from the entrainment separating section through the liquid catch plates di, t2, undergo a certain amount of fractionation owing to the contact of upwardly iiowing vapors with liquid overowing the edges of the upper set of catch plates Bi. The edges of these latter may be V-notched or serrated so that there may be a uniform distribution of liquid streams falling from these upper catch plates to the lower catch plates 82. The cooling medium employed in the tubes of the heavy distillate condenser d@ is charge oil that is to be preheated, and this condenser is operated at a sufficiently high temperature to fractionally condense a substantial part oi' the higher boiling vapors without danger of condensing any gas oil at this point in the tower, as will be further explained presently.

Vapors leaving the condenser 8d pass next through a fractionating section consisting in this instance of a single bubble tray indicated generally at 83a, the detailed construction of which may be the same as that of bubble trays E2 and i3 hereinabove described.l The reflux for this fractionating section is provided by light distillate condenser 8d in which charge oil to ce preheated is also employed as the cooling medium. The liquid fraction collecting on the bubble tray liows therefrom through down pipes idg into the compartments 85 on either side of the condenser Se. Since the Vapor below these compartments is somewhat hotter than the liquid, a certain amount of re-boiling takes place in these compartments, resulting in closer fractionation of this cut which is drawn ofi as a side stream at 80 by an automatically controlled pump (not shown).

Uncondensed vapors leaving the light distillate condenser Bil' pass thence into the water cooled gas oil condensers 49B, the resultant gas oil condensate collecting in l and being pumped away to storage, while residual uncondensable vapors and xed gas are drawn through large diameter lines 52 by the 3-stage ejector equipment 53; all as hereinabove described in connection with the system shown in Fig. 1.

In processing a non-wax-bearing crude, as has been assumed in describing the modified tower construction of Figs. 2 and 3, where it would not be essential to make more than one lube distillate side stream cut and a gas oil overhead cut, it would be possible to operate with only one fractional condenser, such as the light distillate condenser 84. But in that case. all of the reux heat from the tower would have to be removed by this one fractional condenser at a point where the vapor temperature is comparatively low, and it would be impossible, as a practical matter, to pre-heat the charge oil (used as cooling medium) to a temperature above approximately 350 F. in this one fractional condenser td. In thus preheating the charge to 350", an insufcient amount of heat would be removed and it would be necessary to use some other cooling medium in addition. However, if another condenser is placed below the fractionating section 83a, as here shown, where the vapor temperature is considerably higher than it is above the fractionating section, the charge may be preheated to approximately 100 higher temperature by being utilized as the cooling medium in this second condenser, and all the necessary heat may be removed by the charge.

Since itis not desired to separate the two lubricating oil distillate cuts from each other by close fractionation when processing a non-waX-bearing crude as in the particular case here assumed (in practice they will ordinarily be pumped together), the average lubricating oil distillate produced will have a relatively broad boiling range, making it a simple matter to fractionally condense, by means of the condenser @d below the fractionating section, a large proportion of the higher boiling vapors while avoiding condensation of gas oil at this point. Another advantage of this arrangement and mode oi operation is that the volume of vapor passing through the bubble tray fractionating section is considerably reduced and the diameter of the tower can be reduced at this point, as indicated at dll in Figs. 2 and 3, thus effecting substantial economy in cost of construction. By thus arranging the second fractional condenser below the iractionating section of the tower, the capacity of a given tower is greatly increased over that of one of otherwise similar construction and arrangement, this increase amounting to as much as 50 per cent in a typical instance. r

Where the system of the invention is employed in processing a non-wax-bearing charging stock, it is usually not necessary to ernploy temperatures in either ashing stage nearly so high as when processing a wax-bearing stock. Nevertheless, even in such lower-temperEl ature operations, employment of the system herein disclosed aords marked advantages in the way of economy and increased yield of desired products of high cpiality.A

Although of general utility, the system herein disclosed is especially well adapted to be employed as a primary high vacuum distillation unit for production of high grade lubricating oil distillates which are to be re-distilled in a rcrun unit (not shown) generally in the presence of a neutralizing agent (e; g. caustic soda), for direct production of finished lubricating oils as overhead distillates that are sweet and stable without having to be acid treated or ltered.

in mixture with the rest of the charge accumu.

lated in tank 21, through the final heater and is re-i'lashed in tower Il. Since the greater part of this residue from the re-run unit is as heavy as the bright stock overhead distillate produced in tower ll, the yield of this bright stock product is thus increased accordingly. This increase may amount to as much as 2.5 per cent of the crude, depending upon the type of charging stock. Moreover, this continuous re-cycling of the re-run residue through the final flashing stage of the primary system has the further advantage that it obviates the necessity for accumulating the re-run residue over a period of time until enough is available to re-flash it in v large, bulk through a primary distillation system such 'as that herein disclosed. Such practice involves loss of time; and it also introduces difficulties from an operating standpoint, particularly because of the character of the residue where soda. is used in the re-run operation. In that case, the re-run residue, besides containing the unused excess of soda, also contains the non-volatile components of the products of reaction of soda with unstabilizing and odor-producing impurities present in the charging stock supplied to the re-run units; all of which makes the processing of such a re-run residue, by itself, a somewhat troublesome problem. This trouble is avoided by constantly feeding it back in small amount into the primary system, as indicated at 88, so that it is extensively diluted, constituting only about 1 per cent (e. g.) of the charge.

Although, in the foregoing detailed descrip.

tion of desirable practical embodiments of the invention, reference has been made at various points to specific dimensions, operating temperatures, pressures, pressure drops, etc., this is `to be understood as only for the purpose of illustrating typical good practice within the scope of the invention and as aifording concrete illustrative examples as an aid to a better understanding of its underlying principles, and that the invention is not restricted to such details. Moreover, various features of apparatus construction and arrangement herein disclosed are believed to be novel per se, as well as in combination with other units of a multi-hash system, whether or not employed in a system operating under vacuum of any degree. The invention is therefore not to be understood as limited in all respects to vacuum distillation systems.

Certain subject-matter disclosed in this application is also disclosed in the copending application of John E. Schulze and Ronald V. Becknell, Serial No. 140,500, filed concurrently herewith.

What is claimed is:

1; lA high vacuum distillation system for the manufacture of mineral lubricating oils as overhead distillates which comprises, in combination, a flash and fractionating tower provided with vaporizing surfaces, and also having an entrainment-separating section and a fractionating section which includes a bubble tray, said entrainment-separating and fractionating sections being so constructed and arranged that the resistance they offer to vapor now in the normal operation of the tower results in a pressure drop therethrough not exceeding 10 mm. Hg, means for delivering heated charging stock to said vaporizing surfaces, means for withdrawing fractionated products from said tower in side stream, an accumulator outside said tower, means for continuously transferring from the base of said tower to said accumulator unvaporized liquid oil residue as rapidly as it separates in said tower, a second flash and fractionating tower having entrainment-separating and "fractionating sections, means for withdrawing liquid oil residue from said accumulator for further heating said residue and for delivering it to said second tower, means for withdrawing fractionated products from said second tower in side stream, and vacuum-producing means connected to thevapor outlet of the fractionating section of each tower and capable of maintaining at such outlet an absolute pressure as low as 4 mm. Hg.

y 2. A fractionating tower for high vacuum distillation of mineral oil comprising, in combination, a vaporizing section; an entrainment-separating section comprising cooperating trough and cap members arranged to abruptly reverse the flow direction of vapors ascending from said vaporizing surfaces, thereby to throw entrained liquid into said trough members, and means for draining separated liquid from said troughs; and a fractionating section, including bubble tray means, locat'edfv above; said entrainment-separating section, said tower having a vapor outlet above said fractionating section, and said entrainment-separating and fractionating sections being so constructed and arranged that the pressure drop between said outlet and said vapor-l izing section does not exceed 10 mm. Hg.

RONALD V. BECmqE'LL.

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