Process for the Separation of C.sub.5+ Hydrocarbons From a Gaseous Stream

Various other objects, features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood when considered in connection with the accompanying drawings, in which like reference characters designate the same of similar parts throughout the several views, and wherein:

FIG. 1 is a schematic flowsheet of a preferred comprehensive embodiment of the invention; and

FIG. 2 is a graph illustrating the effect of the temperature on the quantity of the C.sub.5+ hydrocarbons remaining in the vapor phase during condensation of the feed gas mixture, as well as the quantity of LPG in the condensate.

In the installation shown schematically in FIG. 1, the gaseous mixture to be separated, e.g., a refinery gas or natural gas, is introduced via conduit 1 under elevated pressure, for example between 15 and 25 bar, and at approximately ambient temperature into a heat exchanger 2 wherein it is cooled against process streams to be heated as well as against a cooling cycle 3 to a sufficient extent to condense most of the C.sub.5+ hydrocarbons as well as a portion of the C.sub.3 and C.sub.4 hydrocarbons in equilibrium therewith. In the separator 4, the thus-formed condensate separated from the gaseous fraction is withdrawn via conduit 5, expanded to a lower pressure in valve 6, and reheated in heat exchanger 2 before being introduced, after partial vaporization, in a rectifying column 7. Via conduit 8 a product stream is obtained in the bottom of the rectifying column 7 which consist essentially of C.sub.5+ hydrocarbons as well as minor amounts of C.sub.3 and C.sub.4 hydrocarbons. This product stream is discharged, after being cooled in heat exchanger 9, via conduit 10 as the product and can be fed, for example, to a debutanizer that is generally present in a relatively large plant; in the debutanizer, the product stream can be further fractionated without much additional expenditure into C.sub.3 /C.sub.4 fraction and a C.sub.5+ fraction. A portion of the liquid withdrawn from the bottom of the rectifying column 7 is introduced via conduit 11 into a heat exchanger 12, heated therein, and recycled, as reboiler fluid into the lower zone of the rectifying column 7.

Rectifying column 7 generally operates at a pressure below the critical pressure of the liquid product stream, preferably at a pressure of 20-60% of the feed gas pressure. Typical operating pressures for column are about 5-15 bar. The operating temperatures of column 7 are generally about -20.degree. to +50.degree. C. at the head and about +50.degree. to 120.degree. C. at the bottom. The temperature difference between the head and the bottom of column 7 is about 20-80K.

At the head of the rectifying column 7, via conduit 13, a light overhead product is obtained no longer containing any C.sub.5+ hydrocarbons. This product is cooled in heat exchanger 2 and thereafter is passed via conduit 14 into a further heat exchanger 15 wherein most of the head product is condensed. The condensate is passed via conduit 16 into a phase separator 17 wherein the thus-formed liquid is separated and withdrawn via conduit 18. After increasing the pressure to the pressure of the crude gas by means of pump 19, the liquid is conducted through heat exchanger 15 and thereafter introduced to the head of a recontacting column 20. The residual gas stream obtained in separator 17 is withdrawn via conduit 21 and fed into a residual gas conduit which will be described below.

The recontacting column 20 is arranged above the separator 4 and separated from the latter by a flue plate 22. Through the flue plate, the gaseous fraction is passed from the separator 4 directly into the lower zone of the recontacting column where it is then passed upwardly through a mass transfer zone 23 containing packing or several plates, and into contact with the condensate fed via conduit 18. During this step, higher boiling components still present in the gas are scrubbed out whereas a portion of the scrubbing liquid is vaporized and remains in the gaseous fraction. In the lower zone of the recontacting column, a liquid is obtained loaded with higher boiling components; this liquid is withdrawn via conduit 24 and introduced as reflux liquid to the head of the rectifying column 7.

The gaseous fraction exiting from the recontacting column 20 is withdrawn via conduit 25 and is partially recondensed in heat exchanger 15 against process streams to be heated as well as against external refrigeration provided in a cooling cycle 26, in order to separate out C.sub.3 and C.sub.4 hydrocarbons. The thus cooled mixture is passed into a phase separator 27. Condensate is discharged therefrom via conduit 28, and after being reheated in heat exchanger 15 is passed into a further rectifying column 29 where it is separated into a bottoms LPG fraction discharged via conduit 30 and an overhead product containing lower-boiling components, said product being withdrawn via conduit 31. A portion of the bottoms LPG is branched off via conduit 32, heated in heat exchanger 33 and returned as reboiler fluid into the lower zone of column 29.

The overhead product withdrawn via conduit 31 from column 29 is cooled sufficiently in heat exchanger 15 to be partially condensed and is then introduced into a further recontacting column 34. The condensed proportion of the overhead product here scrubs the gaseous fraction withdrawn from the separator 27 via conduit 35. During this step, C.sub.3 and C.sub.4 hydrocarbons that have remained in this fraction are scrubbed out essentially at the pressure of rectifying column 29. The resultant liquid fraction is withdrawn via conduit 36 and conveyed by means of pump 37 as reflux liquid to the head of the rectifying column 29.

The gaseous fraction leaving the recontacting column 34 passes via conduit 38 into a further heat exchanger 39 and is cooled therein so that there are formed, by partial condensation and separation of the condensate in a separator 40, a gaseous fraction containing essentially hydrogen and a liquid fraction containing higher-boiling components. The hydrogen fraction is withdrawn via conduit 41, partially heated in heat exchanger 39, and then engine-expanded in two stages in expansion turbines 42, 43. The thus-expanded hydrogen is conducted via conduit 44 to the cold end of heat exchanger 39 and is heated in heat exchange against the gaseous stream introduced via conduit 38. Subsequently, the hydrogen is passed via conduit 45 into a heat exchanger 46 arranged within separator 27 and here cooling the gaseous fraction further, thereby to separate a portion of the C.sub.3 and C.sub.4 hydrocarbons that have remained in the gas. Thereafter, the hydrogen is further heated in heat exchangers 15 and 2, and is discharged from the installation as a product stream via conduit 47.

The liquid fraction obtained in separator 40 is withdrawn via conduit 48, expanded in valve 49, and thereafter heated in heat exchanger 39. Also this fraction is subsequently further heated in phase separator 27 against the gaseous fraction present therein; for this purpose, heat exchanger 50 is provided. Then, after being mixed with the residual gas conducted via conduit 21 from separator 17, the resultant mixture is heated in heat exchangers 15 and 2 and is finally discharged as a residual gas fraction via conduit 51. The compositions, flow rates, temperatures and pressures of many of the streams for the Example illustrated in FIG. 1 are provided in Tables I and II.

In FIG. 2, the compositions of the fractions obtained in separator 4 are plotted as a function of the temperature. In order to attain a sufficiently high separation of C.sub.5+ hydrocarbons which will permit the elimination of a subsequent treatment of the LPG product obtained in column 29, it is generally necessary to provide that the gaseous fraction to be further fractionated, i.e., the non-condensed gas in separator 4, contain a residual content of C.sub.5+ hydrocarbons of less than 3%, for example 2%. It can be seen from FIG. 2 that this can be accomplished by partial condensation only if the gas, prior to phase separation, is cooled to very low temperatures. However, this entails the disadvantageous effect that at that point a very large proportion of the C.sub.3 /C.sub.4 components is concomitantly condensed as well. Consequently, only a relatively small amount of LPG product of adequate purity can be obtained from the rectifying column 29. For example, with a typical partial condensation temperature of +3.degree. C., an unduly high C.sub.5+ hydrocarbon content is obtained in the gaseous phase, being about 8 mol %. Furthermore, about 32 mol % of LPG is condensed in the liquid phase and lost for the subsequent separating stage. Due to the high LPG proportion of this fraction, further processing in conventional rectification columns frequently available in refineries is realizable only at a substantial expense. In comparison, with the use of the process of this invention, with the same gas mixture in conduit 25, the result is a gas composition with merely about 2.3 mol % of C.sub.5+ hydrocarbons, while simultaneously the C.sub.5+ fraction obtained in conduit 10 contains about 22% of the LPG present in crude gas so that about 78% of the LPG present in the crude gas can be discharged via conduit 30 as a product stream having the desired purity. The figures given above are valid for a slightly changed feed gas composition. For the feed gas listed in Table I, stream (1) the C.sub.5+ fraction obtained in conduit 10 would contain even less LPG. In this case stream 10 contains 18.7% of the LPG present in the crude gas.

The preceding example can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding example.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.