US2320616A - Stabilization - Google Patents

Description

June 1, 1943. 4 KNIEL ETAL 2,320,616

STABILIZATION Filed May 3, 1959 Lea/a dal fwlgmw@ bym aw laica ma INVENTORS Patented June l, 1943 UNITED STATES PATENT OFFICE STABILIZATION Application May 3, 1939, Serial No. 271,478

2 Claims.

This invention relates to an improved stabilizing and gas recovery system and more particularly to an improvement in the separation and recovery of normally gaseous hydrocarbons containing three to four carbon atoms per molecule.

In the patents to Ostergaard, Nos. 2,134,836 and 2,135,109, issued November l, 1938, a method of cracking petroleum oils has been disclosed, which is particularly characterized in carrying out the cracking process in the presence of normally gaseous hydrocarbons containing three to four carbon atoms per molecule. As described in such patents, the operation is conducted so that the degree of conversion per pass may be increased over that which could be obtained under practical conditions in the conversion of the oil alone. The resulting cracked distillate or pressure distillate is not only greater in quantity than would ordinarily be obtained, but it is of higher antiknock value due in part to the polymerization of the gaseous constituents.

In accordance with our invention we have made an improvement on such general disclosure, our invention being more particularly related to that part of the system in which the gasoline and lighter fractions are partially condensed and stabilized for the separation of the normally gaseous hydrocarbons containing three to four carbon atoms per molecule from those hydrocarbons having more than four carbon atoms per molecule and having boiling points coming within the boiling range of gasoline, which are contained therewith.

It is more particularly the object of our invention to provide an improved method of separating normally gaseous hydrocarbons containing three to four carbon atoms per molecule from gasoline in order to use such gaseous material for enriching a lean oil in a polymerization process.

Another object of the invention is to provide an improved system for the separation of certain gaseous compounds from a hydrocarbon vapor containing gasoline and lighter fractions, whereby the gasoline may be effectively stabilized, and the lighter fractions are available for subsequent absorption by lean oil without having to pass through the stabilizer.

Another object of the invention is to fractionally condense gasoline and lighter fractions to separate the gasoline from the lighter fractions, whereby materially reduced capacity is required for the gasoline stabilizer, and the lighter fractions are immediately available for further use.

Further objects and advantages of our invention will appear from the following disclosure of a preferred form of embodiment thereof, taken in connection with the attached drawing, illustrative of such invention, such drawing being a partial flow sheet showing the manner of operating the process.

As described in the preceding patents, it is found desirable to crack hydrocarbons in the presence of certain gaseous fractions generally known as the C3 and C4 fractions. Under normal operating conditions these are found in the overhead from a fractionating column and are usually so intermixed with gasoline fractions that on condensation the gasolines are overburdened with the C3 and C4 fractions or the C3 and C4 fractions have a substantial percentage of gasoline contained therewith. It has, therefore, been the custom to pass the entire overhead through a stabilizer and vapor feed condenser from which the greater part of the C4 fraction and all of the C3 fraction are rejected as overhead. With such a process, however, a further sacrifice must be made, in that the stabilizer is of unusually large size to accomplish the work required and it .is the principal purpose of our invention to recover the desired fractions with a minimum of equipment and operating difficulties.

In accordance with our invention the tower IIl is indicated as a fractionating tower and normally receives a charge I! from a separating tower, which is either entirely or partly vaporous and which consists primarily of C3 and C4 fractions and gasoline, but also contains fractions in the boiling range of kerosene and gas oil. These latter fractions are, however, condensed in the Atower IIJ and may be recovered either as bottoms I4 or as a side stream I5, to assure the release of only gasoline and the lighter fractions as Ioverhead at I2. Reflux may be introduced at II` The second column generally indicated at I5 is a stabilizer for the purpose of recovering a stable distillate from the gasoline fractions in the overhead I2. Normally this stabilizer column I5 would receive all of the overhead directly, with the result that it is necessarily of a very large size, both in number of decks and .in diameter, to separate the normally gaseous from the normally liquid material.

In accordance with the preferred form of ernbodiment of our invention column I6 is adapted to receive only the fractions which are condensed by the initial condenser I8 and the secondary condenser I9. The condensers I8 and I9 will operate at suitable temperatures, the condenser I8 usually being operated around 160 F. Co-ndensate from condenser I 8 is removed through the line 20, and is introduced into the stabilizer I6 at a desired point. The uncondensed vapors from condenser I8 will be discharged through line 2l and will pass through the secondary condenser I9. Condensate from condenser I9 is removed through line 22 and is introduced at a suitable place above line into the stabilizer IB.

It is well known from physical principles that the vapor in line 2I leaving condenser I8 in equilibrium with the liquid in line 20 is leaner in the less volatile constituents of the overhead feedto said condenser than is the liquid in lineV 20. By the same token the vapor in line 24 is leaner in the less volatile constituents than is'the liquid in line 22 leaving the condenser I v) in equilibrium with said vapor in line 24. The liquid in line 22 is, in turn, leaner in such constituents than is the liquid in line 20. Nor shall this process of Vfractional condensation be necessarily limited to two steps, but any number of steps which may practically be found of benet may be employed, the ultimate aim being to fractionally condense in the condensers I8 and I9, and other condensing units arranged consecutively thereto, all theccnstituents contained in the overhead I2, which it will be desirable to retain in the stable distillate 33, and at the same time to prevent as large a portion of the C4, C3, and lighter fractions, as is possible by these means, from entering the stabilizer I6, but to pass those fractions on to the absorber 26.

The stabilizerv I, of course, is operated under the usual conditions of heating as by a reboiler circuit, including the bottoms discharge 33; reboiler 3'I and vapor return 32, with stable distillate removed' at 33. The stabilizer overhead, within which are included the balance of the Cs and C4 fractions which cannot be in the distillate 33, is removed at 34, condensed at 35, drawn off through line 31, collected in receiver 38, and with that part of the condensate acting as rei'iux being recycled through pump 39, and line 4U to the top of the tower. Any part of thel condensate in excess of that being recycled as reflux may be drawn off at 42. The balance of the condensate enters the bottom of vabsorber 26.

It will be found that some of the overhead 34 from the stabilizer` is C3 and C4 fractions; which will appearl either in the uncondensed portion iny line 43 or from the liquid overflow 44 from the receiver 38. The uncondensed overhead from the stabilizer removed by line 43 can be con-V veniently joined with the uncondensed C3 and C4 fraction removed from the secondary condenser I9 through line 24 with all of the gaseous material introduced into the absorber through line 46.

In accordance with the disclosure in the Ostergaard patents heretofore-mentioned, it is found desirable to scrubthe C3 and C4 fractions with the lean oil introduced at 23 to obtain a rich oil removed at 48 from the bottom of. the absorber. Gases lighter than C3 will be removed as overhead at 45.

By condensing the fractionating tower overhead vapors in two or more steps and removingr the condensate from each step separately, far better operating conditions are obtained. The final vapor is lean enough in gasoline fractions to be by-passed directly to the absorber tower gas inlet, which materially relieves the load on the upper fractionating section ofthe stabilizer tower. Furthermore, the condensates removed from the several condensing steps are of different composition and are available at different temperatures and may, therefore, be introduced into the stabilizer tower fractionating sections at different points with resulting beneficial effect on the fractionating conditions.

In a representative case the use of a two-step condensation of the hydrocarbon vapors, as compared to a single step condensation to the same terminal temperature, gave only one-half the gasoline fractions in the uncondensed vapor. The net overhead from the stabilizer was reduced by This results in a stabilizer of smaller diameter and having less fractionating decks.

VAs an example, under normal circumstances a stabilizer 'I' in diameter having twenty fractionating deckscould be replaced with a stabilizer 5--6 indiameterY having but twelve decks. This materially overbalances the possible additional cost of heat exchangers and renders the entire system more flexible and better contlolled.

While we have shown a preferred form of embodiment of our invention, we are aware' that other modifications may be made thereto and we;

therefore, desire a broad interpretation of the inventionY as described herein and as claimed hereinafter.

We claim: 1. In a process wherein a vaporous hydrocarbon mixture consisting of normally liquid con-- stituents having 5 or more carbon atoms per molecule and falling within the boiling` range of gasoline and of normally gaseous constituents having 4- or less carbon atoms per molecule isv subjected to fractionation to separate the gasoline constituents therefrom, the steps which comprise subjecting said vaporous hydrocarbon mixture to a plurality of partial condensation stages maintained at progressively lower temperatures to produce a plurality of liquid fractions of progressively lower boiling range and containing substantially all the normally liquid constituents having 5 or more carbon atoms per molecule and to produce a nal gas fraction containing predominantly only normally gaseous constituents having 4 or less carbon atoms per molecule, separatelyv removing said liquid fractions from said partial condensation stages, removing the nal gas fraction from the last of said partial condensation stages, directly introducing said separate liquid fractions into diierent portions of a stabilization zone in such a manner that the higher temperature fractions will have no deleterious` effect on the lower temperature fractions to `eliminate from said separate fractions the greater portion of any normally gaseous constituents of 4 or less carbon atoms per moleculel unavoidably contained therein, removing from said stabilization zone a stabilized liquid gasoline fraction comprising, predominantly constituents having 5 or more carbon atoms per molecule. and maintaining the pressure on said partial condensation stages and on said stabilizingv zone substantially the same.

2. In a process wherein a hydrocarbon stock is subjected to cracking under conditions of ele-v Avated temperature and pressure in admixture with. normally gaseous hydrocarbons predominantly containing from 3 to 4 carbon atoms per molecule and wherein the cracked products are separated into a liquid portion and a Vapor portion and wherein the vapor portion is fractionated to separate therefrom the constituentsV heavier than gasoline, the improvement Whichcomprises subjecting the remainder of Vsaid vapor portion to a plurality of partial condensation stages maintained at progressively lower temperatures to produce a plurality of liquid fractions of progressively lower boiling range and containing substantially all the normally liquid hydrocarbons having 5 or more carbon atoms per molecule and to produce a final gas fraction containing predominantly only normally gaseous hydrocarbons having 4 or less carbon atoms per molecule, separately removing said liquid fractions from said partial condensation stages, removing the ilnal gas fraction from' the last of said partial condensation stages, directly introducing said separate liquid fractions into diierent portions of a stabilization zone in such a man-ner that the higher temperature fractions will have no deleterious effect on the lower temperature fractions to eliminate from said separate fractions the greater portion of any normally gaseous hydrocarbons of 4 or less carbon atoms per molecule unavoidably contained therein, re-

moving from said stabilization zone a stabilized liquid gasoli-ne fraction comprising predominantly hydrocarbons having 5 or more carbon atoms per molecule, combining said final gas fraction and said normally gaseous hydrocarbons eliminated in said stabilization zone, introducing said combined gases into an absorption zone, introducing into said absorption Zone a hydrocarbon liquid to absorb substantially all the hydrocarlO bons having 3 or more carbon atoms per mole- AUGUST HENRY SCHUTTE. LUDWIG KNIEL.

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