FINAL TECHNICAL REPORT
September 1, 1996, through August 31, 1997
Project Title: PREPARATION OF CARBON MOLECULAR SIEVES FOR OXYGEN
SEPARATION FROM AIR
ICCI Project Number: 96-1/4.2A-3
Principal Investigator: Mark P. Cal, ISGS
Other Investigators: Kenneth J. Slota, UIUC; Anthony A. Lizzio, ISGS
Mark J. Rood, UIUC
Project Manager: Daniel D. Banerjee, ICCI
ABSTRACT
Carbon molecular sieves (CMS) have become an increasingly important class
of adsorbents for use in gas separation and recovery processes. The overall
objective of this project was to produce Illinois coal-based CMS suitable
for producing an enriched oxygen stream of >85% from an air feedstock.
Theoretically, there are two ways to recover enriched oxygen, either as the
high pressure product of adsorption or the low pressure product of desorption
in a pressure swing adsorption (PSA) process. The high pressure product involves
altering the surface chemistry of the char making the CMS selectively adsorb
nitrogen. This is a novel approach that attempts to take advantage of nitrogen's
higher quadrupole moment. The low pressure product of desorption involves
the traditional CMS, which selectively adsorbs oxygen. Using carbon deposition
(CD), generating a narrow pore size distribution (PSD) in the 3-4 range will
be attempted. This range is based on the kinetic diameters of 3.46 for
O2 and 3.68 for N2. Char preparation techniques included
pyrolysis, activation, and CD with benzene, or surface treatment with nitric
acid, calcium, or ammonia.
Our initial CMS research used IBC-102 coal pellets, since commercial CMS
is pelletized for use in PSA processes. However, to increase experimental
productivity, it was decided to switch to granular coal as the raw starting
material. The best CMS pellet produced thus far had an
O2/N2 selectivity of 3.36 with an O2 capacity
of 6.88 cc/g. However, selectivity was well below the goal of 20. One granular
CMS sample produced an O2/N2 selectivity of 29.5. During
the five minute adsorption experiment, this same CMS demonstrated a selectivity
as high as 58; however, capacity was extremely small. The best surface treated
sample attained an O2/N2 selectivity of 0.91 and an
oxygen purity of up to 36%. However, these were well below our goals of a
selectivity of 0.2 and >85% O2 purity.
Future research recommendations include varying percent burn-off during
activation, further study of the two-step hydrocarbon deposition process,
and further surface treatment with calcium, chlorine, and possibly hydrogen
to make a more N2 selective CMS.
EXECUTIVE SUMMARY
Background
Gas separations are a major production cost in the chemical industry today.
Production of industrial gases by pressure swing adsorption (PSA), is expected
to grow much faster than by the conventional method, cryogenic distillation.
Commercial interest in carbon molecular sieves (CMS) is growing and these
materials are currently being used to replace less efficient adsorbents (e.g.,
zeolites) in industrial gas separation processes.
Carbon molecular sieves are microporous materials having pore size dimensions
similar to the critical dimensions of the gas molecules to be separated.
CMS separate gases based on kinetic adsorption. The pore surface area of
CMS is usually less, and the pore size distribution narrower than that of
typical activated carbons. Commercial CMS, used in nitrogen recovery,
preferentially adsorb oxygen from air. The challenge is to produce a CMS
which produces >85% oxygen purity. Most CMS made thus far at ISGS can
produce an enriched nitrogen stream (90-95%) from an air feedstock, while
a different CMS was able to produce an oxygen enrichment of approximately
40%.
Goals and Objectives
By either producing CMS that preferentially adsorbs nitrogen or CMS with
high oxygen adsorption capacity, we believe that it is possible to produce
a gas separation system that could produce up to 90% oxygen. We outlined
an experimental approach to develop these CMS.
The proposed research consisted of 5 tasks.
Task 1. Chars were prepared from IBC-102 coal under specified heat treatment and activation conditions. The pore structure and surface chemistry of the chars were tailored for oxygen separation using chemical activation, carbon deposition, and surface treatment.
Task 2. The kinetics of O2 and N2 adsorption and desorption
on chars were measured at 25%C.
Task 3. The physical and chemical properties of the CMS products were evaluated
to gain additional insight into the fundamentals of preparation and properties
of CMS.
Task 4. A process flowsheet for oxygen recovery with Illinois coal-based
CMS was developed.
Task 5. Technical and management reports were prepared and submitted to the ICCI.