Adsorptive separations on nonporous materials in
petrochemical industry
Drs, Prakash Kumar, Jagannath.Das and Vijayalakshmi R.P.
RTG Centre, Reliance Industries Limited, Vadodara`
Petrochemical light gases like CO2, CH4, O2, Ar N2, C3H8, C2H4 and
C2H6 are vented out during manufacture of polymers due to build up
of inert gases like methane, N2 and ethane etc. as these gases do not
participate in the reaction. The presence of these inerts decreases
the overall yield of the catalyst in the reactor. Hence, the industry is
interested in the development of low cost gas separation process for
the removal of these inerts and recovery of ethylene and propylene
monomers which can be put back into the system. Gas separation
technology is closely inter-related with improved molecular sieving
materials. Other separations of interest in petrochemical industry are
separation of aromatics hydrocarbons like xylene isomers which are
conventionally separated using zeolite with simulated moving bed.
Nanoporous materials such as zeolite, carbon molecular sieves
(CMS) and
metal organic framework ( MOF) offers
potential for
various above mentioned separations in petrochemical industry.
CMS because of controlled microporosity are widely used in the
separation of binary gas mixture by techniques such as Pressure
Swing Adsorption (PSA) which take advantage of the difference in
rate of adsorption of the gases to be separated. The sorption kinetics
is controlled by the diffusion of gases through the nano-porous
structure. The rate of diffusion of molecules with close molecular size
can differ thus making it possible for separation. CMS show higher
hydrophobicity as compared with zeolite molecular sieves, a higher
resistance in acid and basic media and their pores are slit shaped
which renders them shape selective towards planar molecule. The
kinetic diameter of the light gases are given in the Table 1.
Table1: Kinetic diameter of light gases
Molecule
Kinetic dia, Ao
N2
3.64
O2
3.46
Ar
3.7
CH4
3.8
CO2
3.3
C2H4
4.2
C2H6
4.4
Sorption isotherms of gases which are present in typical polyolefin
plant off-gases are presented in the present work on different
nanoporous
adsorbents
like
carbon
molecular
sieve,
zeolite
molecular sieve.
In gas separation, molecules smaller than the size of the micropore
constriction rapidly diffuse into micropores whereas access for larger
molecules is denied due to constriction.
A small change in the
effective size of the constriction can considerably affect the diffusion
rate of an adsorbing molecule. Sorption kinetics of these molecules
into CMS was measured using the McBain gravimetric set-up at 1
atmosphere. The results are given in the Figure 1.
Adsorption (g/100g)
Fig.1: Sorption kinetics of gases on
CMS
7
6
5
4
3
2
1
0
CO2
N2
O2
Ar
C2H4
CH4
0
5
10
15
20
25
30
Time / min
Sorption kinetics in figure 1 shows high kinetic selectivity of carbon
dioxide over other gases which makes suitability of carbon molecular
sieve for the adsorptive removal of carbon dioxide from polyolefin offgases using pressure swing adsorption process.
The equilibrium capacity for these light gases are measured using a
volumetric set-up at 25oC and 40C on CMS and zeolite molecular
sieve of X type. The results are given in Figure 2 and 3 respectively.
Fig.2: Adsorption isotherm of gases
on CMS at 25 C
Adsorption (g/100g)
6
N2
O2
Ar
CO2
C2H4
CH4
5
4
3
2
1
0
0
200
400
600
800
1000
P, mmHg
Figure 3: Equilibrium adsorption isotherms on zeolite X of different
gases present in polyolefin off gases.
Adsorption isotherm on 13X at 40oC
5
Propylene
Nitrogen
Hydrogen
Propane
CH4
Ethane
Ethylene
4
m.mol/g
3
2
1
0
0
2000
4000
6000
Pressure mmHg
8000
10000
The adsorption isotherms in above figures shows that equilibrium and
kinetic separation of above gas mixtures can be achieved using
nano-porous adsorbents like carbon and zeolite molecular sieves.
These adsorbents offers suitability for use in separation of gases
using pressures swing adsorption process.
Other latest novel materials which offers good potential in aromatics
separations are metal-organic framework (MOF) having crystalline,
and uniform porous structure, and are quite stable thermally like
zeolites. Various catalytic reactions as well as feasibility of separation
of individual components of C8 aromatics (viz, EB, m-X, and p-X)
have been reported in literature. Very high surface area and
extremely high porosity, gives the advantage of using these materials
as a nano-reaction vessel, and many unusual properties are
observed
in
application
as
catalyst/adsorbent.
This
requires
exploration from both fundamental as well as development point of
view.
In figures given below nonporous framework structure of MOF is
shown suitable for diffusion of gases and aromatics molecules.
PG NMR studies as shown in figure below confirms MOF have higher
diffusivities for ethane and benzene over zeolite X.
In
figure
4
adsorptive
breakthroughs
separations are given on MOF materials.
and
chromatographic
Metal-Organic Frame-Work: Applications
Adsorption and Separation:
• Selective Adsorption & Separation of EB and Xylene isomers
• Separations of linear and non-linear hydrocarbons
• Gas Storage (Hydrogen, applicable to fuel cells etc)
Excellent Separation of EB and Xylenes as reported in Literature
As discussed above nonporous materials like carbon molecular
sieves, zeolite X and latest material like MOF offers huge
opportunities for adsorptive separations in petrochemical industry.