US2476010A - Rectification - Google Patents

Description

July 12, 1949. J. v. WHITTLESEY 7 RECTIFICATION Filed July 5, 1946 Duscmzo John. V. Uhu'ttleag Saverzbor 5g dbborneg Patented July 12, 1949 amen RECTIFICATION John V. Whittlesey, Roselle, N. J., assignor to Standard Oil Development Company, a corporation of Delaware Application July 3, 1946, Serial No. 681,235

4 Claims. 1

This invention relates to the rectification of water immiscible composite fluids such as hydrocarbon mixtures containing low boiling constituents, and in particular to an improved refluxing system in the fractional distillation of petroleum distillates with or without a chemical treating effect.

In the design of distillation towers for the fractionation of hydrocarbon mixtures, it is frequently desirable to provide refluxing by means of a circulating reflux system located either at the top of the tower or at intermediate points between the feed inlet and the top. In such a system, one or more plates are provided for removal of heat from the upflowing vapors by means of liquid which is drawn off from the bottom reflux plate or an accumulator pan below it, pumped through an exchanger or cooler and returned to one of the plates for advantageous reflux effect. C'onventionally, the liquid circulated by the pump is condensed from the vapors flowing upward in the tower. The present invention relates to an improvement over such processing in the use of water in the pumparound circuit and thus a lowered initial investment and lowered operating costs.

In the prior art, water has been used as a cooling fluid in the rectification of low boiling petroleum distillates. Water and aqueous solutions have also been employed in fractionation systems as a reflux fluid to impart also a treating efiect. In such uses of water, however, the water after a once-through passage through the system has been discarded for one or more reasons of processing. In the processing according to the invention, water is used as the reflux medium in the rectification of low boiling water insoluble composite fluids, particularly petroleum distillates. The water is separated in the tower and then passed through a circulating system for subsequent use as a reflux agent. Also within the scope of the invention is the use of an aqueous solution to impart a chemical treating effect in the use of an aqueous solution in the pumparound circuit system.

In the processing according to the invention, the fractionation tower is similarly constructed as that employed in prior art processing in which cooled portions of the overhead product are used as reflux. The construction of the reflux plates and accumulator pan follows conventional designs. In the use of water as the circulating medium, the temperature of the hydrocarbon vapors is such at the points of use of water as the refluxing medium as to be below the temperature at which substantial evaporation of the water occurs, so that the efiect of water vapor as a distillation aid is negligible. As a result of the cooling action of the water, hydrocarbons and water collect together on the reflux plate and flow to the liquid collecting system. In order that the processing according to the invention proceed satisfactorily, it is desirable that a sufficient period of time be provided in the accumulator pan to allow the separation of the phases of hydrocarbon and water so that only liquid hydrocarbons overflow from the accumulator pan. Usually a duo-gravity level control system is located on the pan so that the water-hydrocarbon interface is at all times below the level of the overflow weir. Usually additional water can be supplied to the pumparound system at some point in the treating system.

As an additional feature of processing according to the invention, an aqueous solution instead of water may be employed in the pumparound reflux system. Thus, a dilute solution of caustic soda may be employed for the removal of impurities. In such a use of an aqueous solution, a desirable treating effect in combination with the advantages of water as circulating reflux medium may be attained.

In order that the invention may be more fully understood, the following description and illustration of a particular embodiment are presented. Figure 1 presents a flow diagram of a pumparound reflux system in connection with the upper portion of a rectification tower.

In the drawing, the upper portion of a fractionatin tower I0 is shown. The Section of the tower it is shown as containing four plates 12, it, it and I8. Plates l2, I4 and I8 carry vaporliquid contacting devices of the type of bubble caps. These vapor contacting devices are, however, not shown for simplicity of illustration of the inventive features. The plate i6 is fitted with a high weir 2i] and with vapor chimneys or other provision for passage of upflowing vapors. One such chimney 23 is shown in the drawing. By reason of the weir 20, the plate l5 functions particularly as an accumulator pan. Fitted also on the plate It is a duo-gravity level controller 22 connected by conventional control devices to valve 212 which is external to the tower E0. The tower it is accommodated with overhead vapor line 25, reflux removal line 28 and reflux return line 3E The reflux line 28 is connected to the circulating pump 32 which pumps the reflux from the accumulator pan l6 through line 34 in which is situated control valve 42 to the cooler 35 and thence through line 30 as reflux to the tower l0. Through line 38 makeup water or aqueous treating solutions are passed into line 28, the rate of addition being regulated by a control valve 40 in line 38.

In the processing according to the invention, normal distillation of a composite fluid is carried out in the tower I0. The vapors passing to. the upper part of the tower l and passing through the plates !8, 16, I4, and I2, are brought in contact with water supplied to the plate I2 through line 30. From the plate [2, the mixture of water and condensed fluid passes downward over the plate 14 to the plate l6, while the uncondensed' vapors pass overhead through line 26. On the plate [6, as a result of the high weir 20, di-phase separation occurs between the condensed fluid and water. The condensed fluid, usually a hydrocarbon, overflows the weir 20 onto the plate l8, while the water is removed through line 28, through pump 32 and line 34 through cooler 36, and returned as reflux liquid through line 30. Suitable level control for di-phase separation is maintained on the plate 16 as a result. of the controller 22 and its efi'ect upon the valve 24. Control of the rate of reflux circulation is obtained by means of valve 42. Instead of water employed as reflux, a solution desirable for treating vapors subject to reflux action may be circulated by passing through line 38 the suitable treating solution.

The following will serve as av specific example of processing according to the invention. In a tower used for distillation of naphtha containing hydrocarbon materials in the boiling range from methane to 280 F. end point, it is desired to separate between normal pentane and isohexane taking overhead essentially all of the normal pentane and lower boiling materials. Vapors rising to plate 16 are at a pressure of 107 p. s. i. g. and a temperature of 220 F. Uncondensed vapors leave the tower through line 26 at 106 p. s. i. g. and 215 F. Under these conditions a water phase may be maintained on the plate 16 and the liquid temperature on this plate may be held, for example, at 170 F. The water is removed through line 28 through pump 32 and line, is cooled to 120 F. in cooler 36 and returned as reflux liquid through line 30. If hydrogen sulfide were present in the upflowing vapors a solution of caustic soda, of perhaps Baum strength might be used instead of water for circulation in the refluxsystem thus subjecting the upflowing vapors to treatment for removal of the hydrogen sulfide.

The processing according to the invention is thus suitable for rectification oi fluids which are essentially immiscible with water and aqueous treating solutions. Such processing according to the invention permits smaller reflux equipment due to the high specific heat and thermal conductivity of water. When a treating solution is also employed in addition to the advantage of water as a reflux medium, further added processing advantages are attained.

- What is claimed is:

1. In the fractional distillation of complex hydrocarbon mixtures containing constituents of lower vapor pressure than water in fractionating tower, in which said mixtures are passed upwardly as vapors through said tower in countercurrent relation to a liquid condensate of said vapors flowing downwardly therethrough, the steps which comprise introducing an aqueous material into the upper portion of said tower into direct intimate contact with condensible vapors passing into said portion, condensing said vapors and forming a two phase mixture of condensate and said aqueous material, flowing said mixture downwardly through said upper portion of the tower, in countercurrent contact with upflowing vapors therein into a separation zone in said upper portion, accumulating the mixturein said zone, separating thev condensate from the aqueous material as an upper layer in said zone, continuously overflowing the upper condensate layer from said zone into the lower portion of the tower, into intimate contact with ascending vapors therein, continuously removing. aqueous material from said zone and the tower, cooling; said aqueous material externally of the tower and recirculating the cooled material to the upper-portion of. said tower.

2. The fractional distillation as in. claim 1 wherein the aqueous material is water.

3.. The fractional distillation as in claim 1 wherein the aqueous material is an aqueous solution reactive withv the vapors and condensate thereof toremove impurities in said vapors and condensate.

4.. The fractional distillation as in claim 3 wherein the aqueous material is an aqueous solution of caustic soda.

JOHN V. WHIT'ILESEY.

. REFERENCES CITED The following referenlces' are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,573,025- Averill, Jr. Feb. 16, 1926 1,622,941 Buerger Mar. 29,, 1927 1,683,830 Pollack Oct. 23, 1928 1,756,156 Herthel; et al. Apr. 29, 193.0 2,073,258 Wall-is Mar. 9, 193'? 2,125,325 Youker Aug. 2, 1938 2,134,836 Ostergaard- Nov. 1,. 1938 2 ,394,662 Camp at 'al. Feb. 12, 1946

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