Process for the production of carbon molecular sieves

Details Process for the production of carbon molecular sieves Process for the production of carbon molecular sieves

1994-03-21

1996-03-12

Cross, E. Rollins

Catalyst, solid sorbent, or support therefor: product or process

Solid sorbent

Free carbon containing

502429, 502430, 502431, 502433, 502436, 502437, 423445R, 95902, B01J 2020

Patent

active

054985897

DESCRIPTION:

BRIEF SUMMARY
SPECIFICATION

1. Field of the Invention
The invention relates to a process for producing carbon molecular sieves.
2. Background of the Invention
From German Patent 36 18 426 and German Patent 21 19 829 mentioned therein it is known to produce carbon molecular sieves for the separation of gases with small molecular size, particularly O.sub.2 and N2, in the following manner: finely ground hard coal is oxidized with air in a fluidized bed. After adding coal-tar pitch as a binder and water, the coal is molded and carbonized at temperatures up to 900.degree. C., subsequently activated with steam at temperatures of 800.degree.-900.degree. C. and the preliminary product of the low-level activation is treated with carbon splitting hydrocarbons at 750.degree.-850.degree. C.
The carbon molecular sieves produced this way are used for obtaining nitrogen from air in pressure swing processes. The operating costs of pressure swing processes consist primarily of compression costs for the required air compression. The so-called specific air consumption, i.e. the ratio of the used amounts of air to the produced amount of nitrogen (m.sup.3 air/m.sup.3 nitrogen) should be as low as possible. This specific air consumption is directly related to the nitrogen adsorptivity of the carbon molecular sieve, which results from the difference between the diffusion rates of nitrogen and oxygen in the carbon molecular sieve.
In order to establish the characteristics of carbon molecular sieves, up to now test methods have been used which take into account the differences in the diffusion rates of the gases to be separated.
In German Patent 36 18 426 and German Patent 21 19 829 a quality test is disclosed, wherein a certain volume of carbon molecular sieve (e.g. 200 ml) which has been previously evacuated, is traversed by air for instance for 1 minute and subsequently evacuated for 1 minute. Subsequently the absorbed amount of gas is measured and the maximum oxygen concentration as well as the average oxygen concentration in the evacuated gas is determined (CMS-"quality test").
However, the best way to prove the quality of a carbon molecular sieve is to perform practice-related pressure swing tests in a pressure swing installation. Thereby it is important that the amount of air required for obtaining 1 m.sup.3 of nitrogen be as low as possible. At the same time the tendency is to keep the nitrogen amount produced per cubic meter of CMS volume as high as possible, in order to be able to keep the volume of the adsorber as small as possible. Consequently the air
itrogen ratio L/N (m.sup.3 (i.N) air/m.sup.3 (i.N>) N.sub.2) as well as the nitrogen production ratio (m.sup.3 (i.N) N.sub.2 /m.sup.3 CMS.times.h) are the quality features for a carbon molecular sieve for the separation of nitrogen.


OBJECT OF THE INVENTION

It is the object of the invention to produce a carbon molecular sieve by means of which in the pressure swing process particularly low air
itrogen ratios and particularly high specific yields of nitrogen can be obtained.


SUMMARY OF THE INVENTION

Through tests it has been found that carbon molecular sieves having these properties can be produced in that 500.degree. and 850.degree. C. with a dwelling time of 55 to 65 minutes, in order to obtain a bulk density of 530-560 g/l and 900.degree. C. for 165 to 195 minutes, until through the steam gasification and sintering a bulk density between 590 and 650 g/l is reached.
It has been found that when a coal finely ground to a grain size of 95%<20.mu.m is used in the production of carbon molecular sieves, low rates of air consumption and high nitrogen production can be achieved.
For the quality of the carbon molecular sieve it is also very important to follow the bulk density requirements during the carbonization and activation. Thereby the dwelling time of the carbonization product in the rotary kiln is set so that the activated product will absorb five to six times the volume of air in a quality test and that during the desorption of this air a maximum oxygen concentration in

REFERENCES:
patent: 3951856 (1976-04-01), Repik et al.
patent: 3979330 (1976-09-01), Munzer et al.
patent: 4102812 (1978-07-01), Robinson et al.
patent: 4880765 (1989-11-01), Knoblauch et al.
patent: 4921831 (1990-05-01), Nakai et al.
patent: 5248651 (1993-09-01), Henning et al.

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