Production of nitrogen

Details Production of nitrogen Production of nitrogen

1998-11-24

2001-07-10

Doerrler, William (Department: 3744)

Refrigeration

Cryogenic treatment of gas or gas mixture

Separation of gas mixture

C062S650000, C062S900000

Reexamination Certificate

active

06257019

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a method of and apparatus for producing nitrogen by the separation of air.
The separation of air by rectification is very well known indeed. Rectification is a method in which mass exchange is effected between a descending stream of liquid and an ascending stream of vapor such that the ascending stream of vapor is enriched in a more volatile component of the mixture to be separated and the descending stream of liquid is enriched in a less volatile component of mixture to be separated.
Conventionally, air is separated in a double rectification column comprising a higher pressure rectification column, a lower pressure rectification column, and a condenser-reboiler, of which the condensing passages communicate with an upper region of the higher pressure rectification column and the reboiling passages communicate with a lower region of the lower pressure rectification column. Nitrogen is thereby separated in the higher pressure rectification column and is condensed in the condenser-reboiler. Part of the resulting condensate is used as reflux in the higher pressure column and another part of the condensate is so used in the lower pressure rectification column. An oxygen-enriched liquid air fraction is taken from the bottom of the higher pressure rectification column and is introduced into an intermediate mass exchange region of the lower pressure rectification column. A nitrogen fraction is obtained at the top of the lower pressure rectification column and an oxygen-enriched fraction at its bottom. A nitrogen product is therefore obtained at the pressure of the lower pressure rectification column. Many industrial processes, for example, the enhanced recovery of oil or gas, require nitrogen to be supplied at an elevated pressure, often well in excess of that at which the higher pressure rectification column operates. In order to reduce the amount of work required to raise the pressure of the nitrogen product from that of the lower pressure rectification column to that demanded by the process to which the nitrogen is to be supplied, it is known to take some of the nitrogen product as vapor from the higher pressure rectification column. A feature of such processes is that for a given size of air separation plant and a given purity and pressure of the nitrogen product, the total power consumption at first falls with increasing nitrogen recovery to a minimum and then rises again. This phenomenon results from two opposing factors. The ideal separation work (and hence power consumption) is at a minimum when the nitrogen recovery is very low and the waste product is still essentially air. It is at a maximum when the waste gas contains no nitrogen. However, the process efficiency (actual work input/ideal work input) is very low when the recovery is very low because the plant is much bigger than it needs to be and losses of work arising from pressure drops and temperature differences are large. Conversely, when the recovery is high, the process efficiency is higher. As the recovery is reduced from 100%, there is a minimum power at an optimum recovery, where the falling separation power is just balanced by the increasing losses of work that are caused by the plant getting larger. The total power consumption of the process also typically includes the power consumed in compressing the nitrogen product. Taking a part of the nitrogen product from the higher pressure column reduces the power consumed in compressing the nitrogen products but reduces the nitrogen recovery.
Other expedients may also decrease the nitrogen recovery. For example, the production of a liquid nitrogen product requires a part of the incoming air to be condensed. This in turn reduces the vapor flow available for condensation in the condenser-reboiler. Again, in order to compensate a larger, less efficient, plant is required.
In practice, known double column air separation plants for generating nitrogen are not necessarily designed either for a minimum power consumption or for maximum nitrogen recovery. Rather, there is generally a preferred operational envelope represented by a particular region of a graph of power consumption plotted against nitrogen recovery, the actual optimum depending on extraneous economic circumstances. It is aim of the present invention to provide methods and apparatuses for producing nitrogen which effectively enable the preferred operational envelope to be shifted in the direction of reduced power consumption without reducing nitrogen recovery, or in the direction of increased nitrogen recovery without increasing power consumption, or in both directions.
SUMMARY OF THE INVENTION
According to the present invention there is provided a method of producing nitrogen, comprising separating nitrogen from air and condensing nitrogen so separated, wherein most or all of the nitrogen is separated by rectification and some of the condensed nitrogen is employed as reflux in the rectification, characterized in that the nitrogen is both separated and condensed at three or more different pressures.
The invention also provides apparatus for producing nitrogen, comprising an arrangement of separation vessels for separating nitrogen from air, some or all of the separation vessels being rectification columns, a plurality of condensers for condensing the nitrogen being arranged to return, in use, at least some of the condensed nitrogen to the arrangement of rectification columns to serve as reflux therein, characterized in that three or more of the nitrogen condensers are arranged to condense nitrogen at different pressures from one another and are in communication with different separation vessels which in turn are operable at different pressures from one another.
By separating and condensing nitrogen at three or more different pressures, the nitrogen condensation load is shared between the condensers, thus enabling relatively efficient operation (e.g. with relatively low power consumption) of the overall air separation process to be maintained under conditions of relatively high nitrogen recovery which would otherwise lead to inefficient operation of a conventional process employing but a single nitrogen condenser, for example, a conventional double rectification column process. In particular the method and apparatus according to the present invention allow the lowest pressure separation to be conducted at a pressure in excess of 3.5 bar absolute while at the same time enabling a nitrogen product to be taken, particularly in vapor state, from the highest pressure separation which is typically conducted at a pressure in excess of 8.5 bar absolute. In a typical example, at constant air compression power, about 80% of the total nitrogen product may be produced at the highest separation pressure at about 86% nitrogen recovery, whereas in a comparable conventional double column process only 60% of the total nitrogen product is produced at the pressure of the higher pressure rectification column. Because a greater proportion of the nitrogen is taken from the higher pressure rectification column, the total power consumption is reduced when producing a nitrogen product at a pressure above that of the higher pressure rectification column. Taking an increased share of the nitrogen product from the higher pressure rectification column is not the only way of realizing a lower power consumption. It is alternatively possible in some examples of the method and apparatus according to the invention to keep this share constant, and reduce the power consumed in compressing the air while essentially maintaining nitrogen recovery. To enable this to be achieved an arrangement of columns comprising not only higher pressure and lower pressure columns, but also an auxiliary rectification column which receives air at a lower pressure then the higher pressure column, is typically used. The method and apparatus according to the invention alternatively make possible at a given nitrogen recovery and power consumption storage of a liquid nitrogen product at a greater rate than in comparable k

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