Rectification By Dividing The Feed Gas Into Partial Streams

Abstract

Improved rectification of a feed gas is achieved by dividing the feed gas into two or more partial streams that are then introduced at different levels in the rectification zone.

Description

BACKGROUND OF THE INVENTION

This invention relates to a process for improved rectification by dividing the feed gas mixture into partial streams of high-liquid and low-liquid contents at different temperatures. More particularly, this invention relates to such rectification processes where the feed gas mixture is at a higher pressure than that of the rectifying column, and enters the column, after expansion, as a wet vapor.

It is known that rectification is accomplished more easily when the feed gas mixture, before entering the column, is divided into two partial streams of different liquid contents. This is achieved by heating one stream with a source of heat, and cooling the other stream with a source of refrigeration. Thus, the first partial stream is a wet vapor of low-liquid content and the second stream is a wet vapor of high-liquid content. The second partial stream is fed to the column a few trays above the first one.

In such process, free energy is added to the feed gas mixture by means of the aforesaid sources of heat and refrigeration.

SUMMARY OF THE INVENTION

It is the object of this invention to carry out a rectifying process with divided feed gas stream, without the necessity of adding free energy to the feed gas stream.

A process was found for the rectification of a gas mixture, in which the feed gas mixture is divided into two partial streams of different liquid contents. The partial stream with the higher liquid content is fed to the column a few trays above the tray where the partial stream of low-liquid content is introduced. The characteristic feature of this process is that, after its expansion, one partial stream transmits refrigeration to the other partial stream and is then fed into the column, while the other, now cooled, partial stream is also expanded and fed into the column a few trays above the first partial stream.

A slight improvement of this process can be achieved by likewise dividing the second, now precooled, partial stream before expansion into two partial streams. Again, one stream transmits refrigeration after expansion to the other stream, and is then fed into the column a few trays above the tray where the original first partial stream was introduced. The second partial stream, thus further cooled, is then also expanded and fed to the column a few trays still further up.

In the description which follows, a two-component gas mixture is used to explain the process in detail. Of course, the process is not limited to two-component mixtures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a process diagram in which the feed gas is divided into two partial streams;

FIG. 2 shows the relevant McCabe/Thiele diagram; and

FIG. 3 shows a process diagram in which the feed gas is divided into three partial streams.

DESCRIPTION OF PREFERRED EMBODIMENTS

According to this invention, the feed gas mixture 12 is divided into two partial streams as shown in FIG. 1. After expansion through throttle valve 1 and heating in heat exchanger 2, the first partial stream enters rectifying column 4 by way of line 3 as a wet vapor of relatively low liquid content. The second partial stream is cooled in heat exchanger 2, then expanded through throttle valve 5, and introduced into rectifying column 4 by line 6 as a wet vapor of relatively high liquid content, a few trays above the tray where the first partial stream enters the column.

From the relevant McCabe-Thiele diagram, FIG. 2, it is obvious that rectification without dividing the feed gas stream or without additional heat applied at the bottom of the column as well as refrigeration applied at the top of the column, is not possible, since the two outer rectification lines intersect at point A, beyond the equilibrium curve.

By producing cooler and warmer feed gas streams, in accordance with this invention, it is possible without adding any free energy, that the rectification lines intersect at two points below the equilibrium curve. Thus, rectification is achieved. In FIG. 2, Z stands for the composition of the feed gas mixture and F/D is the ratio between liquid volume and vapor volume in a partial stream. It is true that, when entering the column, the two partial wet vapor streams have the same composition and the same total energy (enthalpy) as the undivided feed gas stream. However, the two partial streams have a higher free energy than a feed gas stream that is expanded without having been divided. According to their different liquid/vapor ratios, the two partial streams have different temperatures. The irreversible expansion of the feed gas mixture to column pressure, is performed more reversibly due to the division, so that less energy is lost during the pressure reduction and thus more free energy is available for the rectification process.

FIG. 3 shows the process of this invention with a division of the feed gas stream into three partial streams. The symbols used in FIG. 3 are the same as used in FIG. 1. To begin with, the feed gas stream 12 is divided into two partial streams. The first stream is expanded to column pressure through throttle valve 1, heated in heat exchanger 2, and fed to rectifying column 4 through line 3. The second partial stream is cooled in heat exchanger 2 and then again divided into two partial streams. One of these is expanded through throttle valve 7 to column pressure and thereby cooled, then somewhat heated in heat exchanger 8 and fed to rectifying column 4 through line 9, a few trays above the tray where the first partial stream was introduced. The remaining or third partial stream is further cooled in heat exchanger 8, then expanded through throttle valve 10 and introduced by line 11 into column 4 as the coldest partial stream with the highest liquid content. It is therefore fed to rectifying column 4, several trays higher than the trays where the two other partial streams enter the column.

As an illustrative example of the process of this invention, the rectification of a mixture of 50 mol-percent N.sub.2 and 50 mol-percent CH.sub.4 is carried out to effect the removal of nitrogen from natural gas. The economical double-column process, as known for air separation plants, is used in spite of the high CH.sub.4 content, by dividing the feed gas stream for the lower pressure column. The feed stream for the lower pressure column consists of the liquid crude methane from the sump of the higher pressure column. This liquid crude methane is divided into two partial streams, one of which is passed through a throttle valve to produce a wet vapor stream that is then passed in heat exchange relation with the other partial stream to cool it further. The thus further cooled partial stream is then passed through a throttle valve to produce a wet vapor stream of greater liquid content than that of the first mentioned wet vapor stream. The wet vapor stream of greater liquid content enters the lower pressure column at a tray above the tray where the first mentioned wet vapor stream is introduced.

Without this division of the feed stream to the lower pressure column, the crude methane from the higher pressure column could be separated into pure methane and pure nitrogen only by recycling a fluid through a coil in the top of the lower pressure column to effect cooling and through a coil in the bottom of said column to effect heating.

Claims

What is claimed is:

1. In the process for the rectification of a feed gas mixture at elevated pressure, the improvement which comprises dividing said feed gas without rectification into two partial streams of the same composition, passing one partial stream directly in series through an expansion valve to expand said one partial stream to substantially rectification pressure, then in countercurrent heat exchange relation with all of the other partial stream to effect cooling of said other partial stream and then into a rectification zone operating at substantially a single rectification pressure, at a level overlying the lowermost trays thereof as wet vapor of relatively low liquid content, passing all of the cooled other partial stream through an expansion valve, and introducing all of the expanded other partial stream as wet vapor of relatively high liquid content into said rectification zone at a tray above the tray at which said wet vapor of relatively low liquid content is introduced.

2. In the process for the rectification of a feed gas mixture at elevated pressure, the improvement which comprises dividing said feed gas into at least two partial streams of the same composition, expanding one partial stream through a expansion valve, passing the expanded partial stream in countercurrent heat exchange relation with another partial stream to effect cooling of said other partial stream, then introducing said expanded partial stream as a wet vapor of relatively low liquid content into a rectification zone, dividing the cooled other partial stream to provide a third partial stream, expanding the remainder of said cooled other partial stream through a expansion valve, passing the expanded remainder of said cooled other partial stream in countercurrent heat exchange relation with said third partial stream to effect further cooling of said third partial stream, then introducing the expanded remainder of said cooled other partial stream as a wet vapor of relatively high liquid content into said rectification zone at a tray above the tray at which said wet vapor of relatively low liquid content is introduced, expanding the further cooled third partial stream through a expansion valve, and introducing the expanded third partial stream into said rectification zone at a tray above the tray where the expanded remainder of said cooled other partial stream is introduced.