US2315122A - Distillation process - Google Patents

Description

17 Water March so, 1943. w. w. KRAFT 2,315,122

'msnm-JuxowrnocEss Filed Feb. 28, 1958 v I INVENTOR W/zeaton 'BY V A ORNE Y Patented Mar. 30, 1943 DISTILLATION PROCESS Whcaton W. Kraft, New York, N. Y., assignor to The Lummus Company, New York, N. Y., a corporation of Delaware Application February 28, 1938, Serial No. 193,168

1 Claim.

This invention relates to improvements in petroleum refining processes and more particularly to an improved arrangement for the condensing and cooling of the lowest boiling range product topped crude bottoms product removed at It. In this case; however, the'topping unit H3 is preferably operated underpressure which may be in the neighborhood of fifty pounds per square inch.

from a topping unit. 5 The overhead vapors are discharged into the heat It is well known that in the operation of topexchanger I5 which is water cooled by Water ping units, at substantially atmospheric pressure, entering at l 6 and discharging at I]. the amount of uncondensed gaseous material The heat exchanger [5 is in the nature of a reaches extremely high proportions especially partial condenser and cooler, but has such heat when operating on light or wild crude oils. l removal capacity that adequate reflux liquid may These amounts may range as high as and are be obtained. The heat transfer surface is much frequently in the range of 3-5%, depending upon smaller than usual in this apparatus for even the availability of cooling water at a suitable though it handles the entire overhead, 'the mean temperature and other operating conditions. temperature 'difierence between the relatively With a unit having a capacity for 25,000 barrels hot vapors and the cooling water is quite large Der day of crude oil, it will be appreciated that due to the pressure operation and the fact that the amount of such uncondensed material is very t efi s I101; Cooled to end Product p substantial and, even more objectionable, it reature. quires such a large extent of apparatus to handle he condensed l q d and Uncondensed VdDOIS it, that it becomes a major problem in petroleum from the first heat exchanger or partial conrefining. denser I 5 are then collected in the receiver I8,

The principal object of the present invention is the liquid level of which is maintained at a deto provide an improved method for reducing to sired level by the control l9.' The reflux liquid is negligible proportions, the uncondensed gaseous removed at 20 and p p into the reflllX line 2! material loss from a topping operation and s0 to the top of the fractionating column It. 'Atemabsorbing the gas that it can be readily handled pera ure' n rol i p vi e at 22 to r l the in combination with the liquid whereby extensive amount of reflux in accordance with the Vapor equipment is not required. temperature discharging overhead at i2. The Another object of the invention is to provide reflux, which in usual practice, is several times an improved process for condensing and sub-coolthe volume of final product discharged at 32, is ing the overhead from a topping unit so that the not as cool as is customary but as the reflux is apparatus may be of low cost and will require purely a heat removal process, the higher rel'elatively smaller amounts of Cooling water at turn temperature is not objectionable for more available temperatures with a maximum efliciency eflux m y be etu de much higher maxiof cooling. mum temperature of the overhead, more than A more specific object of the invention i t balances this higher minimum temperature with operate a topping unit under such pressure, and the resulting higher mean temperature difference. to condense only so much of the vapors, and to e u oo de ed se fro e ce ver l8 sub-cool only the end product, that higher heat so ove head at 4 unde p s e control 25,.With removal efliciency is obtained by a higher mean 40 e Vapor e 24 jo d With p t line S0 temperature difference. and the uncondensed h t th vap r nd iqu pr t e com ined gas is reduced to a negligible quantity, nd i n and introduced into the final condenser or subhandled as absorbed in the liquid end product. Cooler nd o r 6 hr h line 2 Th fin Further objects and advantages of t invoncooler 26 is also water cooled With the inlet at tion will appear from the following description 23 and the discharge t of a preferred form of embodiment thereof taken Pressure t l 25 is OOIlVenieIltlY inserted in in connection with the attached drawing which line 24 t ta the pre u o ac o at illustrates diagrammatically t t part of a pgcolumn l0 and partial condenser l5 substantially troleum refining plant which specifically perconstant and equal. Although condenser 26 is tains to the condensation and sub-cooling of the a d at a p e Which y be the Same lightest fraction from a t p i unit lower than the pressure on the rest of the system, The toppin u it to in accordance t my nit is desirable to utilize a pressure control as at vention, is of any suitable type with the lightest 5 to p v t flu a n t p ure n t fraction discharged as Overhead Vapor at '2, the discharge end of the system from affecting the heated crude oil being introduced at l3, and operation in the fractionating column and the partial condenser. It will be understood, of course, that this pressure control device may be so placed as to maintain both of the condensing stages and the fractionating column under the same pressure.

The higher mean temperature difference in the partial condenser I 5 requires a much smaller amount of heat exchange surface than the subcooler 26 per unitof heat removed; This is due to the fact that the cooling water entering the sub-cooler 26 is not very much below the necessary temperature of the sub-cooled end product. By virtue of not sub-cooling reflux material, it can be seen that a very great decrease in total heat exchange surface results. This approximately balances the difference in volumes of materials handled.

The resulting end product, which may be a light gasoline is discharged at 32, and is foundto con,-

tain less than a negligible quantity of uncondensed gas, and this may be directly run to a suit-' able stabilizing unit. No separation of gas from liquid is required, and the end product'has such a relatively small proportion of gas, it may be handled as a liquid without difiiculty. No excessively large equipment isrequired under the cir cumstances. V

As an'example of operating conditions under which this process operates, I have used pressures within the fractionating column II] in the neighborhood of fifty pounds per square inch with equivalent temperatures of approximately 290" F. This compares with normal atmospheric pressure operating temperatures of 175 F. The reflux then returns to the fractionating column at about 140 to 150 F. instead of approximately 105 F.

With cooling water at a minimum temperature of 95 F., it will be seen that, in the partial cooler I5, a very large amount of heat may be removed with a relatively high mean temperature differ ence which would not exist if allofthe overhead had to be cooled to 105 F.; cooling the end product onlyto this'lowtemperature thus makes a direct saving in heat exchange surface.

reducing the uncondensed gas from 5% to 0.5% or less, approximately ninety per cent of the gas handling equipment can be eliminated. This saves on gas separators, gas conduits, gas holders, and gas compressors, all of which are extremely costly. A typical multiple pass shell and tube heat exchanger for the final cooler 26 is most satisfactory for the required absorption of the liquid and vapor during the sub-cooling.

While I have shown a preferred form of embodiment of my invetnion, I am aware that modifications may be made thereto and I, therefore, desire abroad interpretation of my invention The pressure operation also permits direct handling of the end product as a liquid and by within the scope and spirit of the disclosure herein, and of the claim appended hereinafter.

What I claim is: The method of condensing hydrocarbon vapors and gases obtained as an overhead from the fractionation of hydrocarbons under superatmospheric pressure in which the hydrocarbons undergoing fractionation are crude petroleum, and the fractionation treatment is a topping operation, which comprises cooling the vapors and gases to produce a partial condensation thereof, separating the resulting condensate from the uncondensed vapors and gases, returning a sufllcient portion of said condensate as reflux for said fractionation, combining the uncondensed vapors and gases with the remainder of said condensate, cooling the mixture to a temperature lower than that of the partial condensation to effect a complete condensation of the vapors and an absorption of the gases in the condensate so that the final product comprises a liquid containing less than of uncondensed material, and controlling the pressure at which the uncondensed vapors and gases are separated from the partial condensate in the partial condensation stage independently of the pressure on the final condensation and cooling stage, the pressure on the partial condensation stage being substantially equal to the pressure of fractionation and higher than the pressure on the final condensation and cooling stage. A

. WHEATON W. KRAFT.

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