Catalyst poisons & fouling mechanisms the impact on catalyst performance

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The Impact on Catalyst Performance due
to poisoning & fouling mechanisms
Gerard B. Hawkins
Catalysts promote chemical reactions or accelerate the rate at which a chemical
reaction approaches equilibrium. The catalyst provides a suitable surface for
reactants to adsorb onto and for products to desorb from. The role of the catalyst
is to lower the activation energy of a reaction by providing a suitable reaction
pathway and is applicable to all types of catalysis, i.e., homogeneous,
heterogeneous and enzymatic catalysis
Heterogeneous catalysis is a surface phenomenon. An essential precursor to any
reaction is the adsorption of reactants onto the catalyst surface followed by the
desorption of the products from the catalyst surface.
It is not therefore surprising that catalysts are very sensitive to any impurities (or
poisons) that can impact upon and affect that active surface. Poisons, as the
name implies, have a deleterious effect upon the catalyst surface, in contrast to
promoters that can enhance both catalyst activity and selectivity.
A poison can be simply defined as any substance that changes the chemical or
physical properties of the surface, leading to an adverse effect in terms of activity
or life. Poisons impair the catalyst performance by reducing catalyst activity via
competitive adsorption onto the active sites or by alloy formation with the active
sites and the result is to effectively remove these active centres from the desired
reaction scheme. Poisoning by a chemisorptions mechanism is directly due to
the fact that the poison is more strongly absorbed than a reactant. Poisons can
sometimes be beneficially introduced into a process stream to modify activity or
surface acidity; selective inhibition of some active sites by partial poisoning to
improve catalyst selectivity is employed in some hydrogenation /
dehydrogenation reactions.
Most of the catalytic reaction encountered in hydrocarbon processing are
performed with porous catalysts to provide an adequate surface area for metals
dispersion and subsequent reaction, and are classified as diffusion limited to
some degree. As a consequence of this, only a small amount of a particular
contaminant, even present in trace concentrations, is actually necessary to block
off access to the huge inner pore structure and thus significantly reduce activity.

Sometimes catalyst activity is decreased without affecting the selectivity when
some of the sites are totally deactivated while others are unaffected.
If, However, some active sites are modified without losing all activity, then the
relative rates of different reactions may change to give different catalyst
selectivity.
We generally distinguish between plant mal-operation, which can cause catalyst
fouling, and thus catalyst deactivation, and true poisoning. In plant mal-operation
the impurity levels are higher and transient in nature. Typically poisons and
foulants are thought of as separate entities, although both cause catalysts to
deactivate. Typical foulants are dust or catalyst particulates, tramp iron
contamination or even coke formation. The first two enter the catalyst bed with
the feedstock whilst coking is often due to unselective reactions within the
catalyst bed causing a dramatic increase in pressure drop.
Poisons deactivate catalysts with lower surface areas more easily than those
manufactured with high surface areas. This is illustrated in the table below for an
effective contamination of 500 ppmw.
The steam reforming catalyst has the lowest surface area due to the stringent
requirements for high stability and robustness because of the high temperature of
operation compared to the other refinery duties. The active metal is not so well
dispersed due to the low surface area carrier and the effect of 500 ppmw impurity
is an order of magnitude higher per unit surface area and therefore more active
metal per unit surface area is affected. This is a rather simplistic way of looking at
a complex poisoning problem but does highlight the severe effect of even low
levels of contaminants on a catalyst and consequently, the performance in
service.
Catalyst Type %metal
oxide
Surface
area (m2/g)
Wt. Metal
oxide per
unit surface
area
ppm
impurities
per unit
surface area
Cat. Reforming 1.0 200 0.00005 2.5
Hydrodesulphurization 16 300 0.0005 1.7
Catalytic Rich Gas 60 150 0.004 3.3
Steam Reforming 15 10 0.015 50
Effective contamination of 500 ppmw

Poisons may be deposited on and remain fixed to the catalyst surface –
permanent poisons- or are deposited on the surface and may be removed during
the course of the reaction or by some remedial post reaction treatment –
temporary poisons. This implies that the loss of activity – selectivity may be
substantially recovered if the source of poisoning is removed and remedial action
taken to adequately clean the catalyst surface
In all cases, as the name implies, there is a significant effect on the catalysts
performance both from direct and indirect effects. The extent of any poisoning
effects depends upon the level of poison deposited on the catalyst and the
secondary effects which result from induced changes to the catalyst surface by
process conditions.
The primary effects of low levels of poisons are often the precursor to significant
changes in catalyst structure and activity.
Poisons may simply lie on the catalyst surface and physically block access to the
large inner pore structure or be strongly adsorbed onto and react with catalytic
sites changing the structure of the catalyst surface. Changes in catalyst surface
chemistry are often reflected in changes to the activity – selectivity of the
process. In severe cases of poisoning the reaction mechanisms are substantially
modified, allowing non-selective secondary reactions to predominate with a loss
of process economics.
To maintain the activity-selectivity and cycle life of the catalytic process, and to
retard unselective side reactions, it is imperative that the catalyst surface is
maintained in a poison free ‘clean’ state. Unfortunately, it is impossible to
completely remove all the potential contaminants and therefore prevent some
degree of poisoning from occurring during normal process operations. Often, the
usual effect of trace contamination is to slowly deactivate a catalyst and permit
an acceptable service life to be achieved but catastrophic failure can occur
depending upon the particular poison and the concentration level.
The main sources of poisons are the feedstock to be processed but additional
sources include steam-raising systems and occasionally the catalyst itself can be
the source. Sulfur from non-optimum desulfurization and inorganic contaminants
(carried over silica sodium, calcium ions.) from a steam drum are fairly common.
Catalyst suppliers also need to ensure that residual sulfate and chloride levels
are kept low during the manufacturing process.

The feed flow to a reactor generally ensures that poisons are deposited on the
inlet catalyst and will be retained at the inlet (e.g., silica) or migrate further down
through the bed (e.g. chloride, sulfurs.) Those poisons that are volatile will be
easily removed as the temperature increases and hence we do not see
significant contamination levels on the catalyst surface near the outlet of the bed
where a reasonable temperature gradient exists from inlet to outlet. An example
of this would be in a steam reformer duty.
The distribution of the poison within the reactor and across the catalyst particles
is determined by the kinetics of the poisoning reaction and the mobility of the
poison. As poison concentrations are usually low and poisoning reactions fast,
most poisoning reactions are strongly diffusion limited with consequent poison
deposition near the outside of the catalyst pellet. Methanol synthesis catalysts
employ copper as an active component. Copper catalysts are very susceptible to
chloride attack and readily form copper chlorides, which because of their low
melting points are sufficiently mobile to migrate from the exterior to the interior of
the pellet. Chlorides can also react with some metals form volatile metal chloride
species and increase the rate of sintering of the catalyst.
The inlet section of the catalyst bed may have the highest reaction duty. Small
changes to the activity of the inlet catalyst are therefore more important than are
changes in activity to catalyst at higher temperatures and will have a major effect
on the overall process.
Poisoning, and the effects of poisoning, can be a complicated process to
understand or model due to the various reaction pathways and mechanisms
involved. The actual severity of a poisoning episode in commercial plants is
dependent upon several inter-related factors. For example, feedstock
composition, plant throughput, contaminant concentrations, duration of episode
etc..
The effects of permanent poisons cannot be mitigated against, as the poisons
cannot be adequately removed. Loss of activity may be reflected in a number of
different ways but often is indicated by increased hydrocarbon slippage,
reduction in cycle length, increased pressure drop across the catalyst, Blockages
of the porosity, removal of active metal sites and agglomeration of the active
metal by poisons often lead to an increase in the relative rate of carbon
deposition. This can lead to severe process problems and the need for
premature catalyst change-out.

A primary contaminant found in all virgin feedstocks is sulfur species. These
compounds are poisons for the majority of catalytic processes, even at low ppb
concentrations. The primary effects of sulfur poisoning are a significant loss in
activity of the catalyst as the metal is effectively removed as a less active metal
sulfide. Removal of the poisoning source of sulfur contamination may permit the
loss in activity- selectivity to be recovered, as sulfur is a temporary poison, if the
catalyst remained unchanged during the poisoning episode. This is only correct
when the secondary effects of sulfur poisoning do not result in a change of
catalyst structure and the original bulk catalyst state is retained.
Secondary Effects.
Sulfur poisoning of catalysts can have catastrophic consequences. Sulfur is
readily absorbed on to active metal sites resulting in catalyst deactivation and
changes in reaction selectivity. Catalytic processing of ‘heavy are very
susceptible to carbon formation due to an increase in thermally induced
dehydrogenation reactions in the sulfur poisoned catalyst areas. Carbon deposits
once formed may be autocatalytic in further promoting dehydrogenation and
thermal cracking of the hydrocarbon feedstock. The initially active catalyst is
rapidly changed to a carbon encapsulated, heat absorbing, inert black body.
Post treatment action to effectively remove the carbon deposition from a catalyst
normally involves some form of oxidation to remove the carbon as carbon
dioxide.
C + O2 - CO2
Steaming at an elevated temperature is generally the method of choice but the
elevated temperature and the steam atmosphere can induce sintering of the
active metal sites. Also, if the coking has occurred within the pore structure then
an aggressive oxidation may damage the catalyst due to the large evolution of
gas as carbon dioxide causing internal stresses within the catalyst pellet. The
build up of pressure can be sufficient to disintegrate the catalyst pellet. The loss
of active surface area through temperature or hydrothermal ageing has the
knock-on effect of reducing catalyst activity.
High temperature stable metal oxide sites may also be formed in some
processes in areas of deactivated catalyst that are subject to localized
overheating due to carbon deposition. Under normal operating conditions it may
not be possible to re-reduce the catalyst to the active metal sites and therefore
the catalyst suffers an irreversible change.

Sulfur may be the primary initiating poison; carbon and steam the secondary
major poisons.
The effects of poisons are therefore a complex interplay of many factors involving
both physical and chemical changes to a catalyst surface. The secondary effects
of poisoning often have as significant effect as the primary causes. Removal of
catalyst poisons from the feedstock to be utilized is essential and can only be
achieved by close attention to desulfurization or other processes from which
poisons emanate. In addition to increased control of the source of poisons the
adoption of high performance ‘poison traps’ is an alternative avenue to explore.
The extent of poisoning, as determined by most analytical techniques, is reported
as a ‘bulk’ value. The true level of poisoning, as seen by the catalyst phase is
probably several orders of magnitude higher as the poison is deposited on the
surface where the active metal concentration is lower. Analysis of a deactivated
catalyst needs to be supplemented by additional information, such as surface
analysis and element distribution across the pellet.
Permanent Poisons
- Arsenic, lead, mercury, cadmium…
- Silica, Iron Oxide….
Temporary Poisons
- Sulfur, carbon
Typical Poisons in hydrocarbon processing
Sulfur
Sulfur species are poisons for all catalytic processes employing reduced metals
or metal oxides as the primary active phase. A sulfur compound strongly
chemisorbs onto and reacts with the active catalytic sites altering the surface
structure.

The actual chemistry involved can be complex as the metal sulfides are not
always stoichiometric and can be polymorphic in nature. It has been reported that
that the specific interaction depends upon the oxidation state of the sulfur
impurity, i.e, how many electron pairs are available for bonding of s & p- valence
electrons with d-orbitals of the metals and whether they are shielded by other
ligands. The reported order of decreasing toxicity for sulfur poisons is:
H2S< SO2 < S03
Changes in catalyst surface chemistry are reflected in changes to the activity-
selectivity of the process. Sulfur is considered a temporary poison for many
catalysts, such as nickel based steam reforming catalysts. This implies that the
loss in activity-selectivity of the catalyst may be substantially recovered by
removal of the source of contamination and the adsorbed sulfur poisoning the
catalyst surface. The specific surface area and the porosity of the catalyst are
important in determining how long the catalyst will continue to perform in the
presence of sulfur. It is possible to recover most, if not all the activity-selectivity
following a minor (or short duration) sulfur-poisoning excursion. In severe cases
of sulfur poisoning the reaction mechanisms are substantially modified permitting
non-selective secondary reactions to predominate.
Severe sulfur poisoning invariably leads to carbon laydown on the catalyst
surface. To maintain the activity-selectivity balance and cycle length of the
catalytic process, and to retard unselective side reactions, it is essential that the
catalyst surface is maintained in a ‘sulfur free clean’ state.
The temporary effect of sulfur poisoning is used to advantage in the start up of
high activity precious metal catalysts in Catalytic Reformers. The fresh catalyst
has a very high initial activity and start-up conditions at feed-in may lead to
thermal runaways, excessive hydrocracking and dehydrogenation reactions
resulting in carbon deposition and loss of cycle life.
Low levels of sulfur may be introduced to the catalyst during start-up to moderate
the initial high activity by adsorbing on ‘super active’ sites that promote
hydrogenolysis and coking. This pre-treatment with a poison then allows the
feed-in to be controlled without excessive carbon laydown. Under normal run
conditions, with sulfur free feedstock, sulfur adsorbed on the catalyst surface
bleeds off and a stable activity is obtained.

Chlorides
The effect of chloride poisoning is very similar to, but more rapid in action, than
the thermal ageing of the catalysts. The chloride ion is very mobile and can
migrate through the whole process gas system. It can then react with active
metal sites (such as Ni or Cu) to prevent free access to these sites and so alter
the structure of the reduced metal crystallites and so enhance the thermal
sintering process. This type of ‘ageing’ is very destructive and because the
structure has been destroyed, the catalyst cannot be properly regenerated even
if the poison source is removed. The chloride ion is extremely reactive and so the
symptoms of poisoning appear very quickly. Also, chlorides attack many metal
alloy surfaces and induce stress corrosion cracking even at concentrations in the
low ppm chloride range.
The initial chloride contaminant can be present as organic or inorganic forms in
the process feedstock or by inefficient removal in the boiler feed water system
supplied from the demineralization units.
It should also be stressed that many chlorides, particularly organic chlorides,
have relatively high vapor pressures and so organic chlorides, such as solvents
for maintenance work cleaning turbines etc, must also be banned from the sites
to avoid possibilities of chloride ion ingress.
Boiler Feed water impurities
The main impurities associated with a boiler feed water system are silica,
sodium, calcium and other metal ions. Correct control of the demineralization
plant and the steam system is crucial to minimize solids carryover into the
process stream.
The primary cause of industrial catalyst deactivation is due to a physical pore
blocking or pore narrowing effect. The deposited contaminants hinder the
transport of reactants into the active interior pore structure of the catalyst.
Transport of the reactants becomes more difficult through increased diffusion
resistance caused by the plugging in the mouth of the pores.
Silica is often transported in the steam in the form of microencapsulated hydrated
silica droplets, rather than actually dissolved in the steam, and a properly
designed impingement plate or demister pad should reduce any solids carryover
to acceptable limits.

Sodium can also be carried over and is destructive for catalyst surfaces, as it will
react with deposited silica and other catalytically active components. Deactivation
by silica is therefore a purely physical (and not a chemical) effect of particulates
on the surface blocking access to the active sites. Deactivation in this manner,
accompanied by subsequent carbon deposition, is a consequence of pore
blocking mechanisms.
Heavy Metals
The usual heavy metals that are encountered hydrocarbon streams, though
much more infrequently now are: Arsenic, lead, vanadium, mercury, nickel and
cadmium. These metals may be present in a number of forms, ranging from
metallic, inorganic or organo metallic species and all are permanent poisons for
precious metal and base metal catalysts.
Heavy metals are considered a very severe permanent poison for all catalytic
systems. The heavy metals also tend to be absorbed onto the walls of the
containment vessels from where they may be slowly re-released to further poison
future catalyst charges.
Arsenic and mercury are particularly problematic as they readily alloy with many
active metal species to form an inactive surface alloy. This effectively reduces
the available surface area for subsequent reaction. If arsenic poisoning has
occurred it is essential to thoroughly clean the reactor vessel by a combination of
chemical and mechanical methods. Washing and wire brushing the vessel before
re-charge of a fresh catalyst batch should provide a greater degree of safety
against future contamination.
Most of these heavy metals will be removed in the hydrodesulfurization reactors,
as they tend to build up on the Comox or Nimox Surface.

Industrial experience has shown that specially tailored pore size distributions
within the HDS catalysts lead to increased metals removal and longer HDS cycle
life before the need for a catalyst change-out. As concentrations should be
limited to < 5ppb to protect catalysts.
Vanadium and nickel are known poisons encountered in FCC units. Vanadium is
initially present as organometallic species in the feedstock but this converts to a
vanadium oxide species in the regenerator. The vanadium is then able to migrate
to zeolite crystals and form a low melting eutectic with the silica alumina of the
zeolite. This leads to a permanent destruction of the zeolite crystal structure and
a significant loss in activity.
Nickel is a poison when deposited on a catalyst surface as it can act as a strong
dehydrogenation catalyst, which contributes, to carbon deposition. The effect of
nickel is well documented in FCC units where nickel also increases the unwanted
‘light ends’ gas production. Nickel does not appear to affect the intrinsic activity
but does adversely affect the selectivity of the reaction.
Water can act as a poison for some catalysts, particularly those operating under
reducing conditions. A number of mechanisms are possible for catalyst
deactivation. Hydrothermal ageing is much more rapid than thermal sintering
alone and leads to a permanent loss of activity through crystallite growth and
agglomeration. Water may dissolve some soluble components within the catalyst
system. For example, excess water has a negative impact upon catalytic
reformer catalysts by dissolving chlorides from the catalyst and reducing the acid
site density on the catalyst surface. This affects the ability of the catalyst to
perform skeletal isomerization reactions.
Organo nitrogen components can also be poisons for a variety of catalysts: such
as hydrocracking and catalytic reforming catalysts. These compounds react to
form ammonia which then neutralizes acid functions on the catalyst surface.
Generally, this effect is reversible if the source of organo nitrogen is removed and
the acidity restored. In catalytic reforming, the basic nitrogen compounds adsorb
on the acid sites and reduce isomerization and cracking activity but do not
appear to have little effect on dehydrogenation activity.
Iron oxide is a known poison for many hydrocarbon processing catalysts.
Unfortunately, iron is a known catalyst for many reactions itself and therefore can
promote dehydrogenation reaction pathways, leading to carbon deposition on the
catalyst surface.

Also, finely divided iron oxide is quite reactive with other contaminant
components, such as Na or Si and accumulate on the catalyst surface and
eventually form new low temperature phases. These can grow and cause
masking of active sites and pore obstruction and plugging which reduce catalyst
activity.
Carbon monoxide (and other electron pair donors such as NH3, PH3, H2S, and
H2O…) is a severe poison for many catalysts due to the reactivity associated
with an unshared pair of electrons. The CO molecule is readily able to coordinate
with metals to form a chemisorbed species that inhibits the surface mobility of
adsorbed species. CO may also form volatile metal carbonyls with some metals
at low temperatures. CO is sometimes used to achieve improved selectivity in
hydrogenation reactions by utilizing a partial poisoning effect with palladium
catalysts.
Poison Catalysts Process
HCl Cu Methanol Synthesis
H2S Ni Steam Reforming
NH3, Organic bases Pt/ chlorided Al2O3 Cat. Reforming
V,Ni Zeolites FCC
metals CoMo NiMo HDS
Hg Pd Hydrogenation
Typical poisons of industrial catalysts
Foulants
A major problem encountered in many plants is an increase in pressure drop
with time on stream. An increase in pressure drop can cause unnecessary
and premature catalyst change out even though useful catalytic life may still
be available. The increase in pressure drop is often caused by reducing the
void fraction of the bed by physical plugging of the catalyst bed, usually in the
inlet section of reactors. The origin of the bed plugging may be from several
different sources

- Heavy metals contained within the feedstock to be processed (e.g., nickel,
vanadium in FCC feedstocks)
- Tramp iron contamination, formed as corrosion products within the pipe
work
- Scale and carbon deposits from heaters and exchangers
- Polymerization caused by reactive molecules within the feedstock
- Particulates contained within the feedstock or caused by upstream attrition
of catalysts.
The contaminants may vary in size from sub micron to several hundred
microns and be deposited in the interstitial voids between the catalyst
extrudates or spheres where they restrict flow, cause channeling or they may
reduce catalyst activity by deposition and encapsulation of the catalyst
surface or by a pore mouth narrowing mechanism leading to eventual total
pore plugging.
Deposition on catalyst surface
Several catalyst companies introduced graded bed technology to act as a
particulate trap and to protect the catalyst bed from solids accumulation and
plugging. Also, improvements in the catalyst support technology with tailored
pore size distributions has allowed for greater metals tolerance in both FCC
and HDS catalysis.
Several catalyst companies have introduced, high voidage, macro porous
ceramics to act as effective filters for many entrained solids.

The porous ceramics can be produced with varying micro-meritic properties
and physical attributes; a wide range of porosities and pore sizes depending
upon the particular application, density variation and structural integrity can
be exploited. The cellular, open pore structure is very effective at reducing
pressure drop build-up and acting as a guard bed for the catalyst.
The foamed structure with a specifically tailored pore size distribution has
almost 100% inter pore connectability, which provides a high tortuosity for the
gas or liquid. This tortuosity increases the chances for collisions between the
entrained solids and cell walls and therefore increased dis-engagement for
solids within the pore structure.
Magnified image of macroporous ceramic

The following notes are a broad description of the nature of carbon ‘coke’
deposits on high severity process catalysts such as Steam Reforming and
HDS catalysts. The influence of catalyst and feedstock on the formation of
carbon is described together with an overview of the ‘wave front’ technique for
the removal of carbon using steam-oxygen (air) procedures.
THE NATURE OF CARBON DEPOSITS FORMED ON CATALYSTS
Carbon deposits formed on hydro processing or steam-reforming catalysts
may be grouped into three broad types:
A Fine, loose, ‘sooty’ type carbons.
B Hard, ‘pitch-like’ polymeric carbons.
C Hard, graphitic type carbons.
Whilst the name ‘carbon’ is loosely applied to these deposits the term is
strictly incorrect as all types of carbon found on catalysts contain low levels of
chemically bonded hydrogen to the skeletal carbon structures.
The carbon deposits are often, in reality, ultra high molecular weight
hydrocarbons, which are formed via thermal-catalytic dehydrogenation (loss
of hydrogen) of the feedstock components. Dehydrogenation reactions
produce highly unsaturated – double bonded – hydrocarbon monomers that
readily polymerize and further dehydrogenate to produce carbon rich
deposits. Often a part of the ‘carbon’ deposit can be solubilized in solvents,
indicating a hydrocarbon nature.
The above types of carbon are often formed as amorphous or filamentous
deposits on the catalyst surface. The filamentous type carbon deposits
usually indicating operations at very low steam to carbon ratios with feeds
containing high molecular weight hydrocarbons or from CO disproportionate
through the Boudouard reaction, via the reaction mechanism
2CO C + CO2.
The nature of the polymerized hydrocarbon ‘carbon’ deposit is very
dependent upon the process operating conditions and the feedstock
composition.

CARBON FORMATION
Carbon deposits form by thermal and catalytic dehydrogenation of
hydrocarbons on the catalyst surface or in the gas phase. The carbon
forming mechanism is highly complex and encompasses both thermal
cracking – breakage of C-C bonds and loss of hydrogen from one fragment;
and catalytic dehydrogenation – loss of hydrogen without C-C bond scission
reaction routes.
The mechanisms are favored by high temperature (550-650o
C), low hydrogen
partial pressures, acidic catalysts and high molecular weight or unsaturated
feed components.
FEEDSTOCK COMPOSITION EFFECTS
The composition of the feedstock used has a significant effect on the nature
of the carbon deposit and therefore on the ease of carbon removal.
TYPE A CARBONS
Loose, sooty type A carbon deposits (whisker carbon) are generally found
when processing low molecular weight feedstocks – such as a good quality
Natural Gas.
Except under exceptional circumstances pure methane will not form
dehydrogenated species, which give rise to unsaturated monomers. Methane
will not form a double bond on the single carbon atom, and hence
dehydrogenation of methane leads to the deposition of single atom pure
carbon species, which are readily removed in the steam reforming reaction
under normal process conditions.
If not removed by steam, the carbon forms loose agglomerates and is easily
removed by oxidation in air. Carbon from methane does not normally lead to
resinous-polymeric deposits. The methane decomposition reaction can be
depicted by:

Whisker carbon
CH4 C + 2H2
TYPE B CARBONS
This type of carbon deposit, which can be broadly described as a heavy
resinous-polymeric like deposit, is the more common form of carbon found on
the majority of catalytic processes such as high severity Catalytic Reforming,
steam reforming, HDS and Cat Cracking. The deposits are formed from the
dehydrogenation of alkanes, naphthenes and aromatic feed components from
ethane upwards.
Dehydrogenation reactions on the catalyst surface give rise to unsaturated –
double bond – compounds, which may further polymerize to higher molecular
weight compounds.
These in turn dehydrogenate and polymerize further to the ultimate carbon
deposit, a resinous, carbon rich and hydrogen deficient coke. Cracking of
the C-C bonds of the skeleton without cracking off the Hydrogen leads to the
formation of highly reactive methane fragment species. These species are
formed rapidly without the deposition of polymeric coke.
The level of potential polymer precursors in the feed and the process
conditions dictate the rate of carbon builds up.

dehydrogenation
H H H H
C = C—C—C—H + H2 Polymerization Coke
H H H H H H H
H--C—C—C—C--H
H H H H
Butane
If this type of carbon deposit is allowed to build up and is subjected to high
temperatures over long periods it will further dehydrogenate, cyclize and form
condensed ring, graphitic type C carbon.
CnH2n+2 CnH2n +n H2 CnHn + H2 Aromatics + H2 Coke
TYPE C CARBONS
The hard, graphitic, unreactive type of carbon deposit generally derives from
processing feedstocks, which contain significant levels of single and
condensed aromatic ring hydrocarbons – Benzene and above.
The aromatic type molecule is hydrogen deficient, by nature of the aromatic
double bond structure, and is capable of further dehydrogenation and linking
of aromatic rings to form multi-ring, graphitic form, hydrogen deficient carbon
polymers.
The graphitic forms of condensed ring aromatics are normally associated with
high heat duty, hydrogen deficient processes. The deposits build up slowly
with time on stream and are exceedingly difficult to remove even with full air
oxidation techniques.
Desired
pathway C+C+C+C
Undesired
pathway

EFFECT OF CARBON FORMATION
Carbon or ‘coke’ formation is one of the primary deactivating side reactions in
catalytic hydrocarbon processing reactions in refinery and petrochemical
plants. Carbon may be formed due to poor catalyst activity or due to
uncontrolled side reactions occurring on the catalyst surface. The poor activity
or selectivity may be induced by poisoning, e.g. Sulfur, chlorides or entrained
boiler feed-water deposits etc. Carbon is considered a temporary poison for
many process catalysts as the poisoning effect is reversible.
Electron micrograph of carbon deposit on a nickel catalyst surface
Carbon deposition may lead to serious catalyst deactivation and possible
physical damage or degradation. If operating conditions stray into a region
where there is a net buildup of carbon on the catalyst, this may result in:
Loss of process economics due to less effective conversion of hydrocarbon
feedstock as the catalyst progressively deactivates due to the carbon blinding
the catalyst surface. This surface masking with carbon blocks facile access to
the large inner pore structure and the active nickel component thus
significantly reducing activity.
Pressure drop increase as carbon (derived from thermal cracking reactions)
progressively fills the interstitial void space between the catalyst particles.
Even a small buildup of carbon may lead to dramatic increases in pressure
drop due to restricted flows and possible channelling.

Pressure drops increase from catalyst breakage due to carbon formation
(derived from polymerization reactions or from the Boudouard reaction) within
the catalyst particle pore structure.
These processes may occur gradually if operating conditions have only just
moved into net carbon formation or may occur almost instantaneously if an
incident leads, for example, to a slug of liquid hydrocarbon hitting the catalyst.
Catalysts are often supported on ceramic carriers such as alumina, calcium or
magnesium aluminates. These refractory carriers are insulators and are
prone to damage by both thermal shocks and induced thermal stresses.
All refractory materials suffer from micro-cracks formed during the initial
manufacturing process. This makes the carriers susceptible to induced
thermal stresses that can cause eventual failure though a classic brittle
fracture mechanism.
Steaming or steam/ air decokes therefore need to be carefully controlled to
avoid excessive temperature gradients across the catalyst pellets that may
propagate crack formation and eventually catastrophic catalyst failure. Once
cracking is initiated, under heating or cooling transients, the cracking is
extended under repetitive thermal cycling. This means it is imperative to
operate the catalyst under steady state conditions, and to avoid carbon
deposition or poisoning, if a long service life is to be achieved.
‘COMMERCIAL’ CARBON DEPOSITS
In reality the nature of the carbon deposits on a commercial catalyst cannot
be broken down into easily defined groups and the actual deposits are
complex mixtures of all types of carbon polymer. It is however important to
understand how the carbon deposits will react to oxidative removal and the
possible upset conditions that may occur before embarking on carbon
removal.
Loss of control over the oxidative carbon burn or insufficient carbon removal
will lead to catalyst and possible hardware damage or loss of throughput due
to poorly regenerated catalyst.

Dependent on the nature of the process, the feedstock and the catalyst, the
deposited carbon will range from the sooty, easily burnt, to the graphitic,
unreactive carbon. Those processes operating with light feeds at low severity
– i.e. naphtha HDS – will have a significant amount of light, easily burnt off
carbon, at the end of the cycle life.
A steam reformer processing naphtha or natural gas with heavy end
components, which has suffered a low steam to carbon operation over a
prolonged period, or a unit with aromatic or heavy aliphatic feed
contaminants; will produce a significant amount of graphitic and resinous
carbon.
The naphtha hydrotreater will be able to be regenerated easily at low
temperature with low oxygen levels whereas the steam reformer will require
higher temperatures and oxygen levels to remove the carbon.
The requirements of temperature and oxygen cannot be prejudged prior to
regeneration (as normally samples of the carbon are not available) and
therefore each regeneration carried out is unique and liable to upset unless
adequately monitored and controlled.
Reactor size and catalyst volume are important considerations – it is easier to
control the burning of carbon coated catalyst in a 100mm diameter steam
reformer tube than in a 2 m diameter x 4 m carbon coated HDS catalyst
loaded reactor.
CARBON BURNING IN AIR
Examination of the ignition temperatures of varying molecular weights of
carbon polymers ‘carbons’ in pure air shows an almost linear graph of
temperature with carbon number. That is the light carbons burn first and
graphite last. Broadly a light carbon deposit will start to burn in air from 300o
C
and the graphitic type carbon from 580o
C to as high as 680o
C in air.
The oxidative burning of carbon is highly exothermic, with potential
temperatures in excess of 1000o
C produced at the point of carbon burn.
When a mixture of light carbon ‘soot’ and graphite is heated in air to a
temperature at which the ‘soot’ begins to burn, but below that temperature at
which the graphite normally burns, it is found that both ‘soot’ and graphite
burn. The exotherm from soot burning is sufficient to raise the temperature to
above that required to pyrolyze the graphite and both forms of carbon are
removed by the runaway exotherm.

Removing the potential for graphite burning is only achieved by removing the
source of oxygen. Reduction of the temperature, whilst retaining the oxygen
level will not be sufficient to stop gross carbon burning. This summarizes the
basis of the regeneration procedure commonly used in a wide variety of
catalytic hydrocarbon processing reactors employing the wave front
technique.
Regeneration of carbonized catalysts by heating the catalyst in an inert
atmosphere, up to or above the temperature at which the light carbon burns,
and then introducing an initial pulse of low level of oxygen is an established
commercial procedure. The procedure allows an oxygen and carbon burn
wave front to move, in a controlled manner, through the catalyst bed. If the
oxygen level is low enough on the first burn, the exotherm – which is a
function of the rate of carbon burn – will not be sufficient to substantially
increase the surface temperature and therefore uncontrolled burning does not
take place. The technique allows the carbon to be removed by burning off
layers of carbon in steadily increasing levels of oxygen.
CARBON REMOVAL BY STEAMING
Steaming is a recognized procedure for decoking catalysts. The method is
simple and does not have the potential risks associated with oxygen decoking
– i.e. over burn and runaway exotherms.
Steaming however is only suitable for the light/medium type of carbon
deposits, which are readily gasified on the catalyst surface by low
temperature steam in conjunction with the catalyst nickel component. With
the heavier and graphitic types of carbon the rate of removal by steam is too
slow to make this method attractive.
The catalyst surface composition plays a more significant part in catalyzing
carbon removal by steam than it does when using the oxygen removal
procedure. Additions of alkali, such as potash, to the catalyst composition
enhance the rate of carbon removal, as does a high intrinsic metal dispersion.
In those cases where the carbon deposition is judged to be ‘light’ i.e. of recent
origin and from a clean feed, steaming is an adequate and effective method
to remove carbon. In those cases where the deposits are ‘hard’ – old and
from a feedstock containing high molecular weight or aromatic components,
steaming will not be an adequate means of removing the carbon case.

CARBON BURN CONTROL METHODS
Regeneration of a carbon coated HDS catalyst in a reactor of dimensions 2m
dia x 4m ht is extremely hazardous. Due to the bulk compacted bed of
catalyst a runaway exotherm is unavoidable without the correct controls on
the regeneration. Without adequate oxygen control, temperatures of 1200o
C
have been observed and the catalyst fuses into inert lumps. HDS catalyst
regenerations are however carried out on a routine basis, both in situ and ex
situ, without catalyst damage. Oxygen analyzers and temperature indicators
at the inlet, in the catalyst bed and exit the reactor are used to indicate the
inlet oxygen and the temperature of the burn wave.
In the case of the multi-tube steam reformer, temperature indication of the
burn is not a viable control option. This is due to the mode of process heating
and the amount of carbon burnt per tube per wave would not be sufficient to
increase the temperature by a significant value. If the tubes glow during the
burn, the regeneration has been uncontrolled and catalyst damage will result.
There is only one control procedure to be recommended for regenerating a
catalyst – monitor the exit carbon dioxide with a carbon dioxide meter (must
be infra red) coupled to a chart recorder. The chart recorder will give an
immediate trend line when the carbon commences burning. The carbon
dioxide meter should read 0-5% to obtain the required accuracy.
Drager tubes and Orsat are not recommended, as they are neither accurate
enough nor capable of continuously monitoring the exit gas streams. Oxygen
inlet analysis cannot be used when the diluent is steam and therefore the rate
of oxygen (air) addition must be controlled by means of a readily accessible
and controllable air inlet valve. Inlet oxygen analyzers should be used when
the diluent is inert gas.
CATALYST – REACTION WITH STEAM
Typically steam is used as the inert diluent during the regeneration of steam
reforming catalysts.
Using the regeneration procedures outlined below, steam will not have a
detrimental effect on the catalyst, as the temperatures will be maintained
within the operating envelope of the catalyst in steam. However, if the
catalyst is steamed at temperatures significantly higher than normal operating
temperatures the structure of the catalyst may be changed in such a way that
will reduce the reforming activity (metal agglomeration and sintering). High
temperature steaming increases the time required to fully re-reduce the
catalyst once returned to an on- stream condition.

Similarly, high temperatures in an oxidizing atmosphere will reduce the
activity by sintering the active nickel phase and produce a less easily
reducible catalyst.
High temperatures in steam and oxygen are to be avoided at all times.
Temperatures within the normal process-operating envelope will not
deactivate the catalyst: Low steam flow-high heat fluxes will rapidly deactivate
the catalyst.
Steam contact with the catalyst surface at the point of carbon burn will not
deactivate the catalyst if the burn exotherm is correctly controlled.
MAXIMUM OXYGEN CONCENTRATION
There is not a maximum oxygen level above which the catalyst cannot be
exposed – only a maximum temperature. The catalyst should not be exposed
to temperatures higher than 650o
C in a pure air stream for extended periods.
‘With lower oxygen concentrations the temperature can be allowed to go up to
700o
C in 5% oxygen for extended periods.
TEMPERATURE OF THE CATALYST SURFACE DURING CARBON
BURNS
With the correct control of the regeneration conditions, and an even carbon
burn, the surface of the catalyst will not substantially rise above that of the
process gas or steam flow.
In cases where the carbon burn front is allowed to burn in a high of an
uncontrolled oxygen level the surface temperature of the catalyst will rise
substantially, resulting in sintering and agglomeration of the active metal
phase. The immediate catalyst surface and pore internals will be subjected to
localized over-heating but this increase will not normally be noted in the exit
gas temperature and the overheating will result in catalyst breakdown and
subsequent high pressure drop.

CONDITIONS TO BURN OFF CARBON COATED CATALYST
The oxidative (air) burning of carbon is the recommended method to use for
the removal of carbon deposits. However the method should only be used if
the correct analysis equipment is available and correct control of the air
volumes can be maintained.

Information contained in this publication or as otherwise supplied to Users is believed to be
accurate and correct at time of going to press, and is given in good faith, but it is for the User to
satisfy itself of the suitability of the Product for its own particular purpose. GBH Enterprises,
C
2
PT, Catalyst Process Technology gives no warranty as the fitness of the Product for any
particular purpose and any implied warranty or condition (statutory or otherwise) is excluded
except to the extent that exclusion is prevented by law. GBH Enterprises, C
2
PT accepts no
liability for loss or damaged resulting from reliance on this information. Freedom, under Patent,
Copyright and Designs cannot be assumed.

Catalyst poisons & fouling mechanisms the impact on catalyst performance

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