carbonaceous adsorbents - ali...
TRANSCRIPT
Carbonaceous
Adsorbents
Ali Ahmadpour
Chemical Eng. Dept.
Ferdowsi University of Mashhad
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Contents
Industrial adsorbents
Activated carbonsTypes
Preparation methods
Structures
Applications
Carbon molecular sieves Preparation methods
Structures
Applications
CMS membranesApplications
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Examples of commercial
gas-adsorption separations
Separation AdsorbentI. Gas bulk separations
N-paraffins/iso-paraffins, aromatics
N2/O2
O2/ N2
CO, CH4, CO2, N2, Ar, NH3/H2
Hydrocarbons/vent streams
H2O/ethanol
Chromatographic analytical separations
II. Gas purification
H2O/olefin-containing gas, NG, SG gas, air
CO2/C2H4, natural gas, etc.
Hydrocarbons, solvents/vent streams
Sulfur compounds/NG, hydrogen, LPG, etc.
SO2/vent streams
Indoor air pollutants-VOCs
Tank-vent emissions/air or nitrogen
Zeolite
Zeolite
CMS
Zeolite, Activated carbon
Activated carbon
Zeolite
Inorganic and polymeric agents
Silica, Alumina, Zeolite
Zeolite
Activated carbon, Silicalite
Zeolite
Zeolite
Activated carbon, Silicalite
Activated carbon, Silicalite
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INDUSTRIAL ADSORBENTS
FOR GAS SEPARATIONS
Activated carbons
Carbon molecular sieves (CMS)
Molecular sieve membranes
Zeolites
Activated aluminas
Silica gels
Molecular gates
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Some types of generic sorbents which have dominated the
commercial use of adsorption are: activated carbon, zeolites,
silica gel, activated alumina, and clays.
Worldwide sales of these sorbents in 2001 were:
Activated carbon $1,500 million (demand 1.2 million tons in 2010)
Zeolites $1,070 million
Silica gel $71 million
Activated alumina $63 million
Clays $16 million
Commercial sorbents
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Activated carbons
Activated carbon is a predominantly amorphous solid that
has an extraordinarily large internal surface area and pore
volume.
Its history can be traced to 350 BC when it was used by
Egyptian for the reduction of copper, zinc, and tin ores.
In modern day, it is widely used and is produced as a
powder, granule, or preformed shapes (pellet, extrudate,
and block).
They have unique properties and low cost compared with
other adsorbents.
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Cont.
ACs are extremely versatile adsorbents of major industrial
significance and are used in a wide range of applications for
the removal of species by adsorption from the liquid (e.g. Hg)
or gas phase (e.g. sulphur), in order to effect purification or
the recovery of chemicals.
They are also used as catalysts or catalyst supports.
Activated carbons are prepared from a variety of raw
materials such as: coals, polymers, nutshells, …
The surface area, dimensions, and distribution of the pores in
ACs depend on the precursor and on the conditions of the
carbonization and activation.
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Activated carbon is a "universal adsorption medium". The
lack of chemical uniformity of the surface and the wide
variation of pore size of AC make adsorption of a variety of
molecules possible.
This variety is linked to some serious disadvantages.
Therefore, activated charcoal is more often tailored to
specific requirements, which improves performance even if
at the expense of wide application.
Cont.
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Types of Activated Carbons
Coal-Derived Carbon
For reasons of economy, the most favored startingmaterial for making AC and CMS is coal. This type is usedfor removing color bodies and impurities from a variety ofliquids and for solvent recovery.
Polymer- Derived Carbon
CMS and ACs can also be made from various polymerssuch as polyvinylidene chloride, polyacrylonitrile, phenolformaldehyde, etc.
Carbon Derived from Other Materials
ACs with very high surface areas (2800 m2/g), exceptionaladsorptive capacities and unique structural features can beobtained by chemical activation of coal or petroleum cokewith KOH. They are used for removing color bodies fromwater and hydrocarbons.
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Activated carbon preparation
CrushingNut
Coal
or
ZnCl2
CO2
Steam
KOHActivation
Chemical
Physical
How much? How fast?
Characterization
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Industrial manufacturing
(physical activation)
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AC structure
The structure of AC is best described as a twisted
network of defective carbon layer planes, cross-linked by
aliphatic bridging groups.
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SEM images of ACs
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Pore volume distribution of ACs
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Pore size distribution of ACs
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Adsorption isotherms of
different adsorbents
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AC market
Demand for AC in the US is forecast to increase nearly 5%
per year to 1.2 million tons in 2010.
Since AC is used primarily as an adsorbent to remove
organic compounds and pollutants from liquid and gas
streams, the market is heavily impacted by the
implementation of various environmental regulations.
These laws, as well as ACs low cost and excellent
performance, drive demand in many large volume markets,
such as municipal and industrial water treatment, industrial
air purification, automotive emission canisters and solvent
recovery.
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Some applications &
advantages
Removal of organic compounds from gas or liquid streams up to 1000 mg/L (cost effective), feasible up to 5000 mg/L
GAC is used for toxic organic compounds from ground water and industrial waste streams
PAC is used for biological treatment
Higher BOD and COD removal than biological treatment
More insensitive to changing influent composition than biological treatment
Enhanced removal of toxic compounds and priority pollutants
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Activated Carbon Systems
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Breakthrough curve in
Adsorption columns
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AC contactor
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Activated Carbon Evaluation
Process
It is important to establish a well defined Test
Protocol so that the operating parameters are the
same for each activated carbon being evaluated.
Fixed Parameters:
Quality of feedstock
Temperature
Contact time
Activated carbon dose rate
Particle size of activated carbon
pH of color measurement
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Activated Carbon Selection and
Optimization
Granular activated carbon vs. powdered activated carbon
Dose rate
Contact time
Particle size of activated carbon
Temperature vs. viscosity
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Carbon molecular sieves
Carbon molecular sieves (CMS) represent one member of
the family of activated carbons.
CMS can be made from activated carbons by a post
treatment which narrows the pore size distribution to
produce a material with a bimodal pore distribution
having a predominance of pores less that 6Å.
Key to the performance of these materials is specific size
selectivity.
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Cont.
The main distinction between CMS and AC is that
activated carbons separate molecules through differences
in their adsorption equilibrium constants.
In contrast, an essential feature of the CMSs is that they
provide molecular separations based on rate of adsorption
rather than on the differences in adsorption capacity. This
behavior is clearly evident in pressure swing adsorbers
where gas dynamics dominate.
The separation of nitrogen from air by PSA is the single
most important application of CMS.
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Kinetic separation of CMS
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Preparation method
General methods of manufacturing CMS materials
include carbonizing coal or nut hulls in an inert
atmosphere to produce chars.
These chars may then be activated in a reactive
atmosphere (e.g.. air or steam) to develop the necessary
porosity.
Non-O2 selective materials are made selective to O2 by
depositing a gate-keeping layer on the pore
entrances. This layer is often formed by pyrolysis of
carbonaceous materials such as pitch, benzene, or furfuryl
alcohol.
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Major approaches for CMS
preparation
Carbon
precursors
Porous carbonPyrolysis
Activation
CMS with
lower adsorption
capacity
CMS with
higher adsorption
capacity
Modification
CVD
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CMSs prepared by different
methods
(A) Formed by coke deposition ; (B) Formed by steam activation
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CMS structure
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Applications
Pressure swing adsorption is a promising process for air
separation, and the adsorbent is crucial to successful
implementation of the technology.
At present, carbon molecular sieves from coal and plants
are used as the most common adsorbent in PSA.
The major problem with currently available CMS is small
effective pore volume (~0.3mL/g) while the ability to
control the size of the pore openings in a CMS remains a
major challenge in preparing CMS.
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Cont.
A variety of separation processes are carried out in
industry by PSA on granular beds formed by CMS such
as:
O2/N2
CO2/N2
CO2/CH4
C2H4/C2H6
H2/CH4
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Pressure swing adsorption
PSA systems are used widely in the refining,
petrochemical and air separation industries.
Sample applications include H2 purification, reactant
recovery from vent streams and air separation to
produce enriched oxygen for combustion or nitrogen for
inerting atmospheres.
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Cont.
The PSA process operates on the principle that an
adsorbent’s capacity for an adsorbed component increases
with increasing partial pressure. This principle is applied
in cyclical manner where process steps typically include
high-pressure adsorption, stepwise depressurization, and
purge (generally with a light component and
repressurization).
The swing between the high adsorption pressure and
regeneration at low pressure is completed in rapid cycles,
generally on the order of a few minutes, to minimize the
adsorbent inventory.
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PSA highlights
1960: Drying of air and other gases.
1961: n-Paraffin removal
1966: H2 purification
1970: O2 production from air by zeolite
1976: N2 production from air by CMS
1977: N2 production from air by zeolite
1977: Poly-bed for large scale H2 production
1980: Single-bed for O2 production (medical use)
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PSA systems
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PSA for N2/O2 separation using CMS
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PSA process for N2 production
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PSA for hydrogen recovery
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Hydrogen purification
System for capacities
from 100 to 10,000
Nm3/h and purities of up
to 99.999+% by volume.
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Custom designed adsorption
and purification equipment
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CHEMICAL PLANTS
Applications for drying and purification or processing ofchemical products are numerous. The following are just afew applications:
XYLENE, BENZENE, BUTADIENE, PROPYLENE,ETHYLENE, ACETONE, ETHANOL/METHANOL,REFRIGERANTS, ACETYLENE, TOLUENE,METHYLENE CHLORIDE, H2, AMMONIA
25% of the nitrogen used in the U.S. is used by thechemical processing industry. This nitrogen, in most cases,is dried by adsorption. Without this dry nitrogen blanket,there would be corrosion of pipelines and vessels,poisoning and consumption of valuable catalyst andinhibition of reactions.
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CMS membranes
Carbon molecular sieve membranes with exceptional gas
separation properties have been manufactured.
The great advantage of these materials is that they can
work in a continuous way through a simple and energy-
saving process.
Carbon molecular sieve membranes have shown promise
for the separation of carbon dioxide from methane.
A potential application of these membranes is the
removal of carbon dioxide from poor quality landfill gas
in order to make it suitable for use as an engine fuel or to
upgrade it to pipeline quality natural gas.
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Cont.
These membranes are of particular interest because they
offer increased permeance, selectivity, and stability over
their predecessors. The improved stability over other
types of membranes is obtained due to the inert materials
used in production.
Carbon membranes with molecular sieve properties can
be obtained by deposition of carbon molecular sieves on
a macroporous material (Carbon Composite Membranes).
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Cont..
One of the most important problems in the manufacture of
Carbon Composite Membrane is the difference in shrinkage
between the support and the CMS during carbonization.
This leads to cracking and deformation of the membrane
which modifies or even suppresses its performance.
By using supports and molecular sieves of a similar
chemical nature, the effect of the shrinkage would be
reduced or eliminated.
Macroporous carbon materials, such as chars and ACs with
a well developed macro/mesoporosity, are interesting
materials to be used as supports of the carbon molecular
sieve films.
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Applications
The most important applications and area of research formolecular sieve membranes are:
Development of cost-effective oxygen-separationmembranes. These can provide substantial cost reductionfor oxygen separation compared to conventionalcryogenic methods.
Improved H2 recovery and CO2 removal. Currently,developing high-temperature ceramic membranes for H2
recovery from gas streams, as well as low-temperature approaches to H2 recovery and CO2
removal are under investigation.
Other novel approaches for O2 and H2 separation areinvestigating under different operating conditions.
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H2 Separation
H2 separation in gasification-based systems can be a main
source of low cost H2 for use in refineries, as fuel for fuel
cells, and for H2 product gas.
Various ceramic membranes, including both high and low-
temperature membranes, are being tested for hydrogen
separation.
The technology is based on ion-conducting ceramic
membranes (ICCM) for the selective transport of hydrogen.
The membranes is composed of composites of a proton
conducting ceramic and a second metallic phase to promote
electrical conductivity.
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Cont.
Due to the extremely high flux characteristics of
composite membranes they have excellent gas separation
properties.
One of the prominent examples is the SSFTM membrane
developed by Air Products and Chemicals, which has
been commercialized and is able to produce highly
enriched hydrogen streams at high pressure.
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CO2 separation
Separation of CO2 is a key technology in the reduction of
greenhouse gases emissions to the environment.
Development of low-cost, advanced CO2 separation
technologies are being investigated, including production
of CO2 hydrates, and dry scrubbing processes with
regenerable sorbents.
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O2 separation
Development of cost-effective oxygen-separation
membranes is a major program.
Since the air separation unit are typically 12 to 15 percent
of the capital cost of a plant, there is a substantial
opportunity to improve the overall costs and efficiency of
the plant with improved air separation technologies.
Nanoporous carbons (NPC) can be fabricated in the form
of supported thin films with minimal defects and
remarkably high size selectivity of oxygen over nitrogen.
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Cont.
Spray coating of porous stainless steel disks with a
solution of polyfurfuryl alcohol (PFA) in acetone are used
to synthesize nanoporous carbon membranes in a
reproducible manner.
The resulting membranes are tested with binary
oxygen/nitrogen mixtures and had an oxygen selectivity
of up to 4.
Two other applications include a Portable Oxygen System
for Respiratory Care and a system to enhance the power
of a Fuel Cell.
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N2 separation
Nitrogen membranes are today's most efficient, highest
purity membrane on the market today.
These high performance air separation membranes are
found in industries including oil and gas, food storage,
chemical, marine, metallurgical, electronics, and many
more.
Molecular gates
Engelhard Corporation has developed andpatented a new family of molecular sieves basedon the technology of titanium silicates.
These materials have unique surface properties,as well as the unique ability to adjust pore sizeopenings.
Conventional molecular sieves (zeolites) havefixed pore openings. Molecules are adsorbedthrough surface attraction. The adsorption forcesdue to polarity and surface attraction allow theadsorption of more polar or higher molecularweight compounds from gas stream mixtures.
Cont.
Titanium silicates have a unique property beyond
that of conventional molecular sieves.
Conventional sieves have fixed pore openings
dependent on the cations exchanged into the
structure, while the titanium silicate structure can
be contracted in a controlled manner.
The net effect is that the pore size of the
Molecular Gate adsorbent can be controlled and
adjusted within 0.1 Å and tailored to produce a
pore size targeted at size selective separations.
Cont..
The potential technical applications are numerousand extremely diverse. Conceptual separationsinclude:
H2S enrichment (CO2 at 3.3Å from H2S at 3.6Å)
Ammonia removal from sour water stripperoverheads (NH3 at 2.6Å and H2O at 2.7Å from H2Sat 3.6Å)
Nitrogen removal from methane (N2 at 3.6Å frommethane at 3.8Å).
Nitrogen removal from NG
The Gas Technology Institute estimates that 11%of production and 16% of natural gas reserves inthe US are contaminated with nitrogen.
Nitrogen levels above 4% often must be removedbefore the gas is permitted into the pipelinesystem.
Current nitrogen removal technologies (includingcryogenic, adsorption, membrane and liquidsolvent systems) are either expensive in capitaland operating costs or only economical at largeflow rates.
Cont.
The new adsorbents will enable flexible,simplified, environmentally friendly separations.
The new technology could yield an additional$1.4 billion of marketable natural gas per year,with potential applications in other fields, such asremoval of sulfur dioxide from stack gases andalcohol dehydration.
Oxygen-enriched airstreams from this newmethod could also improve the economics oftransportable oxygen for medical needs andprovide for cleaner-burning diesel engines.
Molecular gate
MOLECULAR SIEVE Framework of titanium (blue), silicon
(green), and oxygen (red) atoms contracts on heating--at room
temperature (left), d=4.27Å; at 250 °C (right), d=3.94Å.
Molecular gate preparation
The materials are prepared by dehydrating atitanosilicate known as ETS-4 (EngelhardTitanoSilicate-4) discovered in Engelhard'slaboratories and patented in 1990.
ETS-4 normally collapses to an amorphousmaterial on heating. However, by exchangingsodium cations for strontium cations and thencarefully heating the material to control thedehydration, it is able to produce stable materialswith reduced pore sizes.
Cont.
ETS-4 undergoes a systematic, uniform, andcontrollable pore contraction with structuraldehydration to form a new series of materials, calledtitanosilicates, or CTS. The range of effective porecontraction is governed by cationic content.
The initial targets is the separation of nitrogen frommethane in natural gas.
It is possible to reduce initial N2 content of naturalgas at wellhead pressures from 18% to less than 5%,with a methane recovery of at least 90%.
System for the purification of natural gas is currently
under commercialization by Engelhard.
Cont..