Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2001-02-01
2001-12-18
Doerrler, William C. (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
Reexamination Certificate
active
06330812
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to the cryogenic separation of air by distillation for the production of primarily gaseous nitrogen.
BACKGROUND ART
Nitrogen is among the most heavily produced and used chemicals. It finds application in the petroleum, glass, foods, electronics, pharmaceutical, and metals industries. Cryogenic separation of air is a principal means of producing nitrogen.
Cryogenic air separation plants, chiefly for the production of gaseous nitrogen, exist in a number of configurations. These, in turn, group around single distillation column and double distillation column designs. There are many variations of these designs in each category. In most cases the objective is to produce nitrogen at the lowest energy consumption for any given delivery pressure; but aspects such as capital cost and particular features of convenience are equally important.
A simple single-column system has a relatively low nitrogen recovery, the balance of the air being discharged as an impure product containing a substantial amount of nitrogen. Means have been suggested in more complex designs for increasing the nitrogen recovery in such systems and reducing the amount of energy required per unit of product nitrogen.
Two-column systems have inherently greater nitrogen recoveries than simple single-column systems. Nevertheless, simple two-column systems do not necessarily have lower unit energy requirements than improved single column systems. Well-designed systems of either configuration compete for lowest unit energy consumption. The elements of energy consumption, capital cost, and particular convenient features remain important considerations.
OBJECT OF THE INVENTION
An object of the invention is to provide a process for a two-column cryogenic distillation of air which achieves high nitrogen recovery, low unit energy consumption, and, though nitrogen is produced by each distillation column operating at different pressures, the product gaseous nitrogen is delivered at a single pressure, a desirable and convenient feature, while maintaining high nitrogen recovery and low unit energy consumption.
SUMMARY OF THE INVENTION
Double distillation column systems which are designed to produce principally nitrogen have the following requirements:
1. The condenser condensing nitrogen overheads from the high pressure column must boil a stream which boils at a temperature lower than said nitrogen condensing temperature.
2. A vapor stream resulting from the aforementioned boiled stream which enters the low pressure column for further separation must be at or above the operating pressure of the low pressure column.
3. The pressure of the low pressure column must be high enough such that at least a portion of the nitrogen overheads from the low pressure column can be condensed in a condenser against a boiling stream which boils at a colder temperature than the condensing nitrogen overheads. This boiling stream can be the bottoms liquid product from the low pressure column which is reduced in pressure upon entry into the condenser.
It can be seen then that such a system described above becomes easier to effect as the pressure difference between the high pressure column and the reduced pressure derived from the bottoms product from the low pressure column becomes greater. This pressure difference, or some function of this pressure difference, when coupled with the quantity of nitrogen actually recovered, has a direct impact on the requisite energy to produce a nitrogen product. A greater pressure difference indicates a higher energy consumption.
Another feature desirable but not essential to such processes is the recovery of all or most of the nitrogen at the pressure of the high pressure column, where part of the reflux made in the low pressure column condenser is pressurized and returned as additional reflux to the high pressure column.
The current invention improves on this process by conducting the condensation of vapors at the pressure of the high pressure column, all of which may be the overhead vapor from the high pressure column, in at least two stages of coolant vaporization in series. The composition of the boiling stream becomes richer in oxygen as the extent of vaporization increases. At essentially a constant temperature of vaporization, the first stage of vaporization occurs at a higher pressure and the second stage at a lower pressure. The vapor from the first stage is both richer in nitrogen and higher in pressure than the vapor from the second stage, and constitutes a feed to the low pressure column. Therefore, the pressure of the low pressure column is maximized—a desirable effect for a given high pressure column pressure, and oxygen is preferentially rejected from the column system from the second stage condenser. Because the composition of the liquid bottoms from the low pressure column are related to the composition of the vapor feed to the bottom of the low pressure column, these bottoms are richer in nitrogen and vaporize at a colder temperature when transferred to the low pressure column condenser and reduced in pressure, allowing the low pressure and high pressure columns to operate at pressures closer together. The low pressure column condenser typically operates just above atmospheric pressure. The effects of reducing head pressure and rejecting an oxygen-rich mixture from the second or last stage of the high pressure column condenser lead to lower compression energy and higher nitrogen recovery, which minimize unit energy expenditure for the nitrogen produced.
REFERENCES:
patent: 4337070 (1982-06-01), Rohde
patent: 4717410 (1988-01-01), Grenier
patent: 5098457 (1992-03-01), Cheung et al.
patent: 5341646 (1994-08-01), Agrawal et al.
patent: 5761927 (1998-06-01), Agrawal et al.
patent: 5768914 (1998-06-01), Xu et al.
patent: 5916261 (1999-06-01), Bonaquist et al.
patent: 5956972 (1999-09-01), Naumovitz
patent: 6196022 (2001-03-01), Horst et al.
LandOfFree
Method and apparatus for producing nitrogen from air by... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for producing nitrogen from air by..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for producing nitrogen from air by... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2582507