Air separation by pressure swing adsorption with a high capacity carbon molecular sieve

• US 5,240,474 A
• Filed: 01/30/1992
• Issued: 08/31/1993
• Est. Priority Date: 01/23/1991
• Status: Expired due to Fees

• Current Assignee: Air Products and Chemicals Incorporated (Linde Plc)
• Original Assignee: Air Products and Chemicals Incorporated (Linde Plc)
• Inventors: Srinivasan, Rajagopalan, Schork, Joan M., Auvil, Steven R.
• Primary Examiner(s): SPITZER, ROBERT H
• Application Number:
  US07/828,264
• Time in Patent Office: 579 Days
• Field of Search: 55/25, 55/26, 55/68, 55/74, 55/75, 55/389, 502/420, 502/432, 502/433, 502/436, 502/437
• US Class Current: 95/96
• CPC Class Codes: B01D 2253/102 Carbon
• CPC Class Codes: B01D 2253/302 Dimensions
• CPC Class Codes: B01D 2253/308 Pore size
• CPC Class Codes: B01D 2256/10 Nitrogen
• CPC Class Codes: B01D 2257/104 Oxygen
• CPC Class Codes: B01D 2259/402 using two beds
• CPC Class Codes: B01D 53/02 by adsorption, e.g. preparative gas chromatography solid sorbent compositions B01J20/00, preparation of inorganic compounds or elements C01
• CPC Class Codes: B01D 53/047 Pressure swing adsorption
• CPC Class Codes: B01J 20/20 comprising free carbon; comprising carbon obtained by carbonising processes
• CPC Class Codes: C01B 21/0461 Carbon based materials
• CPC Class Codes: C01B 32/306 with molecular sieve properties
• CPC Class Codes: Y10S 95/903 Carbon

Abstract

Nitrogen is produced by separating air in a pressure swing adsorption process using a high capacity kinetically O₂ selective material as the adsorbent. An example of such an adsorbent is a carbon molecular sieve made from coconut shell char. Such a coconut shell char having a high oxygen volumetric capacity is provided by crushing and sizing coconut shells to form granules which are then heated in flowing inert gas at a temperature ramp rate of about 2 to 12° C. per minute to a peak temperature of 775 to 900° C. which is then held so that the total heating time is up to 8 hours and thereafter the granular char is cooled in an inert gas atmosphere. The granular char thus produced is kinetically selective for O₂ over N₂ without further modification to narrow the openings of its micropores and has an oxygen volumetric capacity in excess of 8.0 cc/cc at 1 atm. and ambient temperature. Further modification of this char is provided by contacting it with an oxidizing atmosphere of carbon dioxide or a mixture of inert gas and carbon dioxide, H₂ O or O₂ at temperatures ranging from 650° to 900° C. until the gasified char has been altered so that its volumetric oxygen capacity is greater than 9.0 cc/cc at 1 atm. and ambient temperature.

Claims

1. In a process for separating air for nitrogen recovery by pressure swing adsorption, the improvement for achieving enhanced nitrogen recovery which comprises using as an adsorbent in said process a carbon molecular sieve having an oxygen volumetric capacity of at least 9.0 cc/cc at 20°

C. and 1 atmosphere which sieve is made from a granular coconut shell char formed by;

(a) crushing and sizing raw coconut shells to form shell granules having a size suitable for use in an adsorbent bed for pressure swing adsorption,

(b) heating said granules in a flowing stream of inert gas at an average temperature ramp rate of about 2°

C. to 12°

C. per minute to a peak temperature of 775°

to 900°

C,

(c) holding said peak temperature for a period of time so that the total heating and holding steps total not less than 1 hour nor more than 8 hours to produce granular char,

(d) cooling said granular char in an inert gas atmosphere, and

(e) recovering said product of step (d).

2. The process of claim 1 wherein said carbon molecular sieve has an oxygen volumetric capacity of at least 10.0 cc/cc at 20°

C. and 1 atmosphere.

3. The process of claim 2 wherein said carbon molecular sieve has an oxygen uptake rate at least 15 percent slower and a kinetic selectivity (O₂ /N₂) at least 15 percent lower than a carbon molecular which has an oxygen volumetric capacity of 8.5 cc/cc or less but performs about the same in terms of productivity and recovery under equivalent process conditions with optimized cycle times in air separation by pressure swing adsorption.