The Impact on Catalyst Performance of Poisons & Fouling Mechanisms Wsv

7/21/2019 The Impact on Catalyst Performance of Poisons & Fouling Mechanisms Wsv

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The Impact on Catalys t Performance dueto poisoning & fouling mechanisms

Gerard B. Hawkins

Catalysts promote chemical reactions or accelerate the rate at which a chemicalreaction approaches equilibrium. The catalyst provides a suitable surface forreactants to adsorb onto and for products to desorb from. The role of the catalystis to lower the activation energy of a reaction by providing a suitable reactionpathway 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 anyreaction is the adsorption of reactants onto the catalyst surface followed by thedesorption of the products from the catalyst surface.

It is not therefore surprising that catalysts are very sensitive to any impurities (orpoisons) that can impact upon and affect that active surface. Poisons, as thename implies, have a deleterious effect upon the catalyst surface, in contrast topromoters that can enhance both catalyst activity and selectivity.

A poison can be simply defined as any substance that changes the chemical orphysical properties of the surface, leading to an adverse effect in terms of activityor life. Poisons impair the catalyst performance by reducing catalyst activity viacompetitive adsorption onto the active sites or by alloy formation with the activesites and the result is to effectively remove these active centres from the desiredreaction scheme. Poisoning by a chemisorptions mechanism is directly due tothe fact that the poison is more strongly absorbed than a reactant. Poisons cansometimes be beneficially introduced into a process stream to modify activity orsurface acidity; selective inhibition of some active sites by partial poisoning toimprove catalyst selectivity is employed in some hydrogenation /dehydrogenation reactions.

Most of the catalytic reaction encountered in hydrocarbon processing areperformed with porous catalysts to provide an adequate surface area for metalsdispersion and subsequent reaction, and are classified as diffusion limited tosome degree. As a consequence of this, only a small amount of a particularcontaminant, even present in trace concentrations, is actually necessary to blockoff 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 therelative rates of different reactions may change to give different catalystselectivity.

We generally distinguish between plant mal-operation, which can cause catalystfouling, and thus catalyst deactivation, and true poisoning. In plant mal-operationthe impurity levels are higher and transient in nature. Typically poisons andfoulants are thought of as separate entities, although both cause catalysts todeactivate. Typical foulants are dust or catalyst particulates, tramp ironcontamination or even coke formation. The first two enter the catalyst bed with

the feedstock whilst coking is often due to unselective reactions within thecatalyst bed causing a dramatic increase in pressure drop.

Poisons deactivate catalysts with lower surface areas more easily than thosemanufactured with high surface areas. This is illustrated in the table below for aneffective contamination of 500 ppmw.

The steam reforming catalyst has the lowest surface area due to the stringentrequirements for high stability and robustness because of the high temperature ofoperation compared to the other refinery duties. The active metal is not so welldispersed 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 activemetal per unit surface area is affected. This is a rather simplistic way of looking ata complex poisoning problem but does highlight the severe effect of even lowlevels of contaminants on a catalyst and consequently, the performance inservice.

Catalyst Type %metaloxide

Surfacearea (m2/g)

Wt. Metaloxide perunit surfacearea

ppmimpuritiesper unitsurface area

Cat. Reforming 1.0 200 0.00005 2.5

Hydrodesulphurization 16 300 0.0005 1.7Catalytic 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 duringthe course of the reaction or by some remedial post reaction treatment – temporary poisons. This implies that the loss of activity – selectivity may besubstantially recovered if the source of poisoning is removed and remedial actiontaken to adequately clean the catalyst surface

In all cases, as the name implies, there is a significant effect on the catalystsperformance both from direct and indirect effects. The extent of any poisoningeffects depends upon the level of poison deposited on the catalyst and thesecondary effects which result from induced changes to the catalyst surface byprocess conditions.

The primary effects of low levels of poisons are often the precursor to significantchanges in catalyst structure and activity.

Poisons may simply lie on the catalyst surface and physically block access to thelarge inner pore structure or be strongly adsorbed onto and react with catalyticsites changing the structure of the catalyst surface. Changes in catalyst surfacechemistry are often reflected in changes to the activity – selectivity of theprocess. In severe cases of poisoning the reaction mechanisms are substantiallymodified, allowing non-selective secondary reactions to predominate with a lossof process economics.

To maintain the activity-selectivity and cycle life of the catalytic process, and toretard unselective side reactions, it is imperative that the catalyst surface ismaintained in a poison free ‘clean’ state. Unfortunately, it is impossible tocompletely remove all the potential contaminants and therefore prevent somedegree of poisoning from occurring during normal process operations. Often, theusual effect of trace contamination is to slowly deactivate a catalyst and permitan acceptable service life to be achieved but catastrophic failure can occurdepending 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 bethe 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 levelsare 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 downthrough the bed (e.g. chloride, sulfurs.) Those poisons that are volatile will beeasily removed as the temperature increases and hence we do not seesignificant contamination levels on the catalyst surface near the outlet of the bedwhere a reasonable temperature gradient exists from inlet to outlet. An exampleof this would be in a steam reformer duty.

The distribution of the poison within the reactor and across the catalyst particlesis determined by the kinetics of the poisoning reaction and the mobility of thepoison. 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 catalystsemploy copper as an active component. Copper catalysts are very susceptible tochloride attack and readily form copper chlorides, which because of their lowmelting points are sufficiently mobile to migrate from the exterior to the interior ofthe pellet. Chlorides can also react with some metals form volatile metal chloridespecies and increase the rate of sintering of the catalyst.

The inlet section of the catalyst bed may have the highest reaction duty. Smallchanges to the activity of the inlet catalyst are therefore more important than arechanges in activity to catalyst at higher temperatures and will have a major effecton the overall process.

Poisoning, and the effects of poisoning, can be a complicated process tounderstand or model due to the various reaction pathways and mechanismsinvolved. The actual severity of a poisoning episode in commercial plants isdependent upon several inter-related factors. For example, feedstockcomposition, plant throughput, contaminant concentrations, duration of episodeetc..

The effects of permanent poisons cannot be mitigated against, as the poisonscannot be adequately removed. Loss of activity may be reflected in a number ofdifferent ways but often is indicated by increased hydrocarbon slippage,

reduction in cycle length, increased pressure drop across the catalyst, Blockagesof the porosity, removal of active metal sites and agglomeration of the activemetal by poisons often lead to an increase in the relative rate of carbondeposition. This can lead to severe process problems and the need forpremature 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 ppbconcentrations. The primary effects of sulfur poisoning are a significant loss inactivity of the catalyst as the metal is effectively removed as a less active metalsulfide. Removal of the poisoning source of sulfur contamination may permit theloss in activity- selectivity to be recovered, as sulfur is a temporary poison, if thecatalyst remained unchanged during the poisoning episode. This is only correctwhen the secondary effects of sulfur poisoning do not result in a change ofcatalyst structure and the original bulk catalyst state is retained.

Secondary Effects.

Sulfur poisoning of catalysts can have catastrophic consequences. Sulfur isreadily absorbed on to active metal sites resulting in catalyst deactivation andchanges in reaction selectivity. Catalytic processing of ‘heavy are verysusceptible to carbon formation due to an increase in thermally induceddehydrogenation reactions in the sulfur poisoned catalyst areas. Carbon depositsonce formed may be autocatalytic in further promoting dehydrogenation andthermal cracking of the hydrocarbon feedstock. The initially active catalyst israpidly changed to a carbon encapsulated, heat absorbing, inert black body.

Post treatment action to effectively remove the carbon deposition from a catalystnormally 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 theelevated temperature and the steam atmosphere can induce sintering of theactive metal sites. Also, if the coking has occurred within the pore structure thenan aggressive oxidation may damage the catalyst due to the large evolution ofgas as carbon dioxide causing internal stresses within the catalyst pellet. Thebuild up of pressure can be sufficient to disintegrate the catalyst pellet. The lossof active surface area through temperature or hydrothermal ageing has theknock-on effect of reducing catalyst activity.

High temperature stable metal oxide sites may also be formed in someprocesses in areas of deactivated catalyst that are subject to localizedoverheating due to carbon deposition. Under normal operating conditions it maynot be possible to re-reduce the catalyst to the active metal sites and thereforethe 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 involvingboth physical and chemical changes to a catalyst surface. The secondary effectsof poisoning often have as significant effect as the primary causes. Removal ofcatalyst poisons from the feedstock to be utilized is essential and can only beachieved by close attention to desulfurization or other processes from whichpoisons emanate. In addition to increased control of the source of poisons theadoption 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 isprobably several orders of magnitude higher as the poison is deposited on thesurface where the active metal concentration is lower. Analysis of a deactivatedcatalyst needs to be supplemented by additional information, such as surfaceanalysis 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 metalsor metal oxides as the primary active phase. A sulfur compound stronglychemisorbs onto and reacts with the active catalytic sites altering the surfacestructure.






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 thatthat 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- valenceelectrons with d-orbitals of the metals and whether they are shielded by otherligands. 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 manycatalysts, such as nickel based steam reforming catalysts. This implies that the

loss in activity-selectivity of the catalyst may be substantially recovered byremoval of the source of contamination and the adsorbed sulfur poisoning thecatalyst surface. The specific surface area and the porosity of the catalyst areimportant in determining how long the catalyst will continue to perform in thepresence of sulfur. It is possible to recover most, if not all the activity-selectivityfollowing a minor (or short duration) sulfur-poisoning excursion. In severe casesof sulfur poisoning the reaction mechanisms are substantially modified permittingnon-selective secondary reactions to predominate.

Severe sulfur poisoning invariably leads to carbon laydown on the catalystsurface. To maintain the activity-selectivity balance and cycle length of the

catalytic process, and to retard unselective side reactions, it is essential that thecatalyst 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 catalysthas a very high initial activity and start-up conditions at feed-in may lead tothermal runaways, excessive hydrocracking and dehydrogenation reactionsresulting in carbon deposition and loss of cycle life.

Low levels of sulfur may be introduced to the catalyst during start-up to moderatethe initial high activity by adsorbing on ‘super active’ sites that promote

hydrogenolysis and coking. This pre-treatment with a poison then allows thefeed-in to be controlled without excessive carbon laydown. Under normal runconditions, with sulfur free feedstock, sulfur adsorbed on the catalyst surfacebleeds off and a stable activity is obtained.






Chlorides

The effect of chloride poisoning is very similar to, but more rapid in action, thanthe thermal ageing of the catalysts. The chloride ion is very mobile and canmigrate through the whole process gas system. It can then react with activemetal sites (such as Ni or Cu) to prevent free access to these sites and so alterthe structure of the reduced metal crystallites and so enhance the thermalsintering process. This type of ‘ageing’ is very destructive and because thestructure has been destroyed, the catalyst cannot be properly regenerated evenif the poison source is removed. The chloride ion is extremely reactive and so thesymptoms of poisoning appear very quickly. Also, chlorides attack many metal

alloy surfaces and induce stress corrosion cracking even at concentrations in thelow ppm chloride range.

The initial chloride contaminant can be present as organic or inorganic forms inthe process feedstock or by inefficient removal in the boiler feed water systemsupplied 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 solventsfor maintenance work cleaning turbines etc, must also be banned from the sitesto 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 demineralizationplant and the steam system is crucial to minimize solids carryover into theprocess stream.

The primary cause of industrial catalyst deactivation is due to a physical poreblocking or pore narrowing effect. The deposited contaminants hinder thetransport of reactants into the active interior pore structure of the catalyst.

Transport of the reactants becomes more difficult through increased diffusionresistance caused by the plugging in the mouth of the pores.

Silica is often transported in the steam in the form of microencapsulated hydratedsilica droplets, rather than actually dissolved in the steam, and a properlydesigned impingement plate or demister pad should reduce any solids carryoverto 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. Deactivationby silica is therefore a purely physical (and not a chemical) effect of particulateson the surface blocking access to the active sites. Deactivation in this manner,accompanied by subsequent carbon deposition, is a consequence of poreblocking mechanisms.

Heavy Metals

The usual heavy metals that are encountered hydrocarbon streams, thoughmuch more infrequently now are: Arsenic, lead, vanadium, mercury, nickel andcadmium. These metals may be present in a number of forms, ranging from

metallic, inorganic or organo metallic species and all are permanent poisons forprecious metal and base metal catalysts.

Heavy metals are considered a very severe permanent poison for all catalyticsystems. The heavy metals also tend to be absorbed onto the walls of thecontainment vessels from where they may be slowly re-released to further poisonfuture catalyst charges.

Arsenic and mercury are particularly problematic as they readily alloy with manyactive metal species to form an inactive surface alloy. This effectively reducesthe available surface area for subsequent reaction. If arsenic poisoning hasoccurred it is essential to thoroughly clean the reactor vessel by a combination of

chemical and mechanical methods. Washing and wire brushing the vessel beforere-charge of a fresh catalyst batch should provide a greater degree of safetyagainst 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 cyclelife before the need for a catalyst change-out. As concentrations should belimited to < 5ppb to protect catalysts.

Vanadium and nickel are known poisons encountered in FCC units. Vanadium isinitially present as organometallic species in the feedstock but this converts to avanadium oxide species in the regenerator. The vanadium is then able to migrateto zeolite crystals and form a low melting eutectic with the silica alumina of thezeolite. This leads to a permanent destruction of the zeolite crystal structure anda significant loss in activity.

Nickel is a poison when deposited on a catalyst surface as it can act as a strongdehydrogenation catalyst, which contributes, to carbon deposition. The effect ofnickel is well documented in FCC units where nickel also increases the unwanted‘light ends’ gas production. Nickel does not appear to affect the intrinsic activitybut does adversely affect the selectivity of the reaction.

Water can act as a poison for some catalysts, particularly those operating underreducing conditions. A number of mechanisms are possible for catalystdeactivation. Hydrothermal ageing is much more rapid than thermal sinteringalone and leads to a permanent loss of activity through crystallite growth andagglomeration. Water may dissolve some soluble components within the catalyst

system. For example, excess water has a negative impact upon catalyticreformer catalysts by dissolving chlorides from the catalyst and reducing the acidsite density on the catalyst surface. This affects the ability of the catalyst toperform skeletal isomerization reactions.

Organo nitrogen components can also be poisons for a variety of catalysts: suchas hydrocracking and catalytic reforming catalysts. These compounds react toform ammonia which then neutralizes acid functions on the catalyst surface.Generally, this effect is reversible if the source of organo nitrogen is removed andthe acidity restored. In catalytic reforming, the basic nitrogen compounds adsorbon 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 canpromote dehydrogenation reaction pathways, leading to carbon deposition on thecatalyst surface.






Also, finely divided iron oxide is quite reactive with other contaminant

components, such as Na or Si and accumulate on the catalyst surface andeventually form new low temperature phases. These can grow and causemasking of active sites and pore obstruction and plugging which reduce catalystactivity.

Carbon monoxide (and other electron pair donors such as NH3, PH3, H2S, andH2O…) is a severe poison for many catalysts due to the reactivity associatedwith an unshared pair of electrons. The CO molecule is readily able to coordinatewith metals to form a chemisorbed species that inhibits the surface mobility ofadsorbed species. CO may also form volatile metal carbonyls with some metalsat low temperatures. CO is sometimes used to achieve improved selectivity in

hydrogenation reactions by utilizing a partial poisoning effect with palladiumcatalysts.

Poison Catalysts Process

HCl Cu Methanol SynthesisH2S 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 dropwith time on stream. An increase in pressure drop can cause unnecessaryand premature catalyst change out even though useful catalytic life may still

be available. The increase in pressure drop is often caused by reducing thevoid fraction of the bed by physical plugging of the catalyst bed, usually in theinlet section of reactors. The origin of the bed plugging may be from severaldifferent 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 hundredmicrons and be deposited in the interstitial voids between the catalystextrudates or spheres where they restrict flow, cause channeling or they may

reduce catalyst activity by deposition and encapsulation of the catalystsurface or by a pore mouth narrowing mechanism leading to eventual totalpore plugging.

Deposition on catalyst surface

Several catalyst companies introduced graded bed technology to act as aparticulate trap and to protect the catalyst bed from solids accumulation andplugging. Also, improvements in the catalyst support technology with tailoredpore size distributions has allowed for greater metals tolerance in both FCCand HDS catalysis.

Several catalyst companies have introduced, high voidage, macro porousceramics to act as effective filters for many entrained solids.






The porous ceramics can be produced with varying micro-meritic propertiesand physical attributes; a wide range of porosities and pore sizes dependingupon the particular application, density variation and structural integrity canbe exploited. The cellular, open pore structure is very effective at reducingpressure drop build-up and acting as a guard bed for the catalyst.

The foamed structure with a specifically tailored pore size distribution hasalmost 100% inter pore connectability, which provides a high tortuosity for thegas or liquid. This tortuosity increases the chances for collisions between theentrained solids and cell walls and therefore increased dis-engagement forsolids 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 andHDS catalysts. The influence of catalyst and feedstock on the formation ofcarbon is described together with an overview of the ‘wave front’ technique forthe 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 catalystsmay 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 isstrictly incorrect as all types of carbon found on catalysts contain low levels ofchemically 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 (lossof hydrogen) of the feedstock components. Dehydrogenation reactionsproduce highly unsaturated – double bonded – hydrocarbon monomers thatreadily polymerize and further dehydrogenate to produce carbon richdeposits. 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 filamentousdeposits on the catalyst surface. The filamentous type carbon depositsusually indicating operations at very low steam to carbon ratios with feedscontaining high molecular weight hydrocarbons or from CO disproportionatethrough the Boudouard reaction, via the reaction mechanism

2CO C + CO2.

The nature of the polymerized hydrocarbon ‘carbon’ deposit is verydependent upon the process operating conditions and the feedstockcomposition.






CARBON FORMATIONCarbon deposits form by thermal and catalytic dehydrogenation ofhydrocarbons on the catalyst surface or in the gas phase. The carbonforming mechanism is highly complex and encompasses both thermalcracking – breakage of C-C bonds and loss of hydrogen from one fragment;and catalytic dehydrogenation – loss of hydrogen without C-C bond scissionreaction routes.

The mechanisms are favored by high temperature (550-650oC), low hydrogenpartial pressures, acidic catalysts and high molecular weight or unsaturatedfeed components.

FEEDSTOCK COMPOSITION EFFECTS

The composition of the feedstock used has a significant effect on the natureof the carbon deposit and therefore on the ease of carbon removal.

TYPE A CARBONS

Loose, sooty type A carbon deposits (whisker carbon) are generally foundwhen processing low molecular weight feedstocks – such as a good qualityNatural Gas.

Except under exceptional circumstances pure methane will not formdehydrogenated species, which give rise to unsaturated monomers. Methanewill not form a double bond on the single carbon atom, and hencedehydrogenation of methane leads to the deposition of single atom purecarbon species, which are readily removed in the steam reforming reactionunder normal process conditions.

If not removed by steam, the carbon forms loose agglomerates and is easilyremoved by oxidation in air. Carbon from methane does not normally lead toresinous-polymeric deposits. The methane decomposition reaction can bedepicted by:






Whisker carbon

CH4 C + 2H2

TYPE B CARBONS

This type of carbon deposit, which can be broadly described as a heavyresinous-polymeric like deposit, is the more common form of carbon found onthe majority of catalytic processes such as high severity Catalytic Reforming,steam reforming, HDS and Cat Cracking. The deposits are formed from thedehydrogenation of alkanes, naphthenes and aromatic feed components fromethane upwards.

Dehydrogenation reactions on the catalyst surface give rise to unsaturated – double bond – compounds, which may further polymerize to higher molecularweight compounds.

These in turn dehydrogenate and polymerize further to the ultimate carbondeposit, a resinous, carbon rich and hydrogen deficient coke. Cracking ofthe C-C bonds of the skeleton without cracking off the Hydrogen leads to theformation of highly reactive methane fragment species. These species areformed rapidly without the deposition of polymeric coke.

The level of potential polymer precursors in the feed and the processconditions 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 hightemperatures over long periods it will further dehydrogenate, cyclize and formcondensed 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 fromprocessing feedstocks, which contain significant levels of single andcondensed aromatic ring hydrocarbons – Benzene and above.

The aromatic type molecule is hydrogen deficient, by nature of the aromaticdouble bond structure, and is capable of further dehydrogenation and linkingof aromatic rings to form multi-ring, graphitic form, hydrogen deficient carbonpolymers.

The graphitic forms of condensed ring aromatics are normally associated withhigh heat duty, hydrogen deficient processes. The deposits build up slowlywith time on stream and are exceedingly difficult to remove even with full airoxidation 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 incatalytic hydrocarbon processing reactions in refinery and petrochemicalplants. Carbon may be formed due to poor catalyst activity or due touncontrolled side reactions occurring on the catalyst surface. The poor activityor selectivity may be induced by poisoning, e.g. Sulfur, chlorides or entrainedboiler feed-water deposits etc. Carbon is considered a temporary poison formany 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 possiblephysical damage or degradation. If operating conditions stray into a regionwhere there is a net buildup of carbon on the catalyst, this may result in:

Loss of process economics due to less effective conversion of hydrocarbonfeedstock as the catalyst progressively deactivates due to the carbon blindingthe catalyst surface. This surface masking with carbon blocks facile access tothe large inner pore structure and the active nickel component thussignificantly 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 pressuredrop 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) withinthe catalyst particle pore structure.

These processes may occur gradually if operating conditions have only justmoved into net carbon formation or may occur almost instantaneously if anincident leads, for example, to a slug of liquid hydrocarbon hitting the catalyst.

Catalysts are often supported on ceramic carriers such as alumina, calcium ormagnesium 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 initialmanufacturing process. This makes the carriers susceptible to inducedthermal stresses that can cause eventual failure though a classic brittlefracture mechanism.

Steaming or steam/ air decokes therefore need to be carefully controlled toavoid excessive temperature gradients across the catalyst pellets that maypropagate crack formation and eventually catastrophic catalyst failure. Oncecracking is initiated, under heating or cooling transients, the cracking isextended under repetitive thermal cycling. This means it is imperative to

operate the catalyst under steady state conditions, and to avoid carbondeposition 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 cannotbe broken down into easily defined groups and the actual deposits arecomplex mixtures of all types of carbon polymer. It is however important tounderstand how the carbon deposits will react to oxidative removal and the

possible upset conditions that may occur before embarking on carbonremoval.

Loss of control over the oxidative carbon burn or insufficient carbon removalwill lead to catalyst and possible hardware damage or loss of throughput dueto poorly regenerated catalyst.






Dependent on the nature of the process, the feedstock and the catalyst, thedeposited 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 offcarbon, at the end of the cycle life.

A steam reformer processing naphtha or natural gas with heavy endcomponents, which has suffered a low steam to carbon operation over aprolonged period, or a unit with aromatic or heavy aliphatic feedcontaminants; will produce a significant amount of graphitic and resinouscarbon.

The naphtha hydrotreater will be able to be regenerated easily at lowtemperature with low oxygen levels whereas the steam reformer will requirehigher temperatures and oxygen levels to remove the carbon.

The requirements of temperature and oxygen cannot be prejudged prior toregeneration (as normally samples of the carbon are not available) andtherefore each regeneration carried out is unique and liable to upset unlessadequately monitored and controlled.

Reactor size and catalyst volume are important considerations – it is easier tocontrol the burning of carbon coated catalyst in a 100mm diameter steamreformer tube than in a 2 m diameter x 4 m carbon coated HDS catalystloaded reactor.

CARBON BURNING IN AIR

Examination of the ignition temperatures of varying molecular weights ofcarbon polymers ‘carbons’ in pure air shows an almost linear graph oftemperature with carbon number. That is the light carbons burn first andgraphite last. Broadly a light carbon deposit will start to burn in air from 300oCand the graphitic type carbon from 580oC to as high as 680oC in air.

The oxidative burning of carbon is highly exothermic, with potentialtemperatures in excess of 1000oC produced at the point of carbon burn.

When a mixture of light carbon ‘soot’ and graphite is heated in air to atemperature at which the ‘soot’ begins to burn, but below that temperature atwhich the graphite normally burns, it is found that both ‘soot’ and graphiteburn. The exotherm from soot burning is sufficient to raise the temperature toabove that required to pyrolyze the graphite and both forms of carbon areremoved by the runaway exotherm.






Removing the potential for graphite burning is only achieved by removing thesource of oxygen. Reduction of the temperature, whilst retaining the oxygenlevel will not be sufficient to stop gross carbon burning. This summarizes thebasis of the regeneration procedure commonly used in a wide variety ofcatalytic hydrocarbon processing reactors employing the wave fronttechnique.

Regeneration of carbonized catalysts by heating the catalyst in an inertatmosphere, 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 establishedcommercial procedure. The procedure allows an oxygen and carbon burnwave 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 afunction of the rate of carbon burn – will not be sufficient to substantiallyincrease the surface temperature and therefore uncontrolled burning does nottake place. The technique allows the carbon to be removed by burning offlayers 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 carbondeposits, which are readily gasified on the catalyst surface by lowtemperature steam in conjunction with the catalyst nickel component. Withthe heavier and graphitic types of carbon the rate of removal by steam is tooslow to make this method attractive.

The catalyst surface composition plays a more significant part in catalyzingcarbon removal by steam than it does when using the oxygen removalprocedure. Additions of alkali, such as potash, to the catalyst compositionenhance 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 recentorigin and from a clean feed, steaming is an adequate and effective methodto remove carbon. In those cases where the deposits are ‘hard’ – old andfrom 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 2mdia x 4m ht is extremely hazardous. Due to the bulk compacted bed ofcatalyst a runaway exotherm is unavoidable without the correct controls onthe regeneration. Without adequate oxygen control, temperatures of 1200oChave been observed and the catalyst fuses into inert lumps. HDS catalystregenerations are however carried out on a routine basis, both in situ and exsitu, without catalyst damage. Oxygen analyzers and temperature indicatorsat the inlet, in the catalyst bed and exit the reactor are used to indicate theinlet oxygen and the temperature of the burn wave.

In the case of the multi-tube steam reformer, temperature indication of theburn 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 toincrease the temperature by a significant value. If the tubes glow during theburn, the regeneration has been uncontrolled and catalyst damage will result.

There is only one control procedure to be recommended for regenerating acatalyst – monitor the exit carbon dioxide with a carbon dioxide meter (mustbe infra red) coupled to a chart recorder. The chart recorder will give animmediate trend line when the carbon commences burning. The carbondioxide meter should read 0-5% to obtain the required accuracy.

Drager tubes and Orsat are not recommended, as they are neither accurateenough nor capable of continuously monitoring the exit gas streams. Oxygen

inlet analysis cannot be used when the diluent is steam and therefore the rateof oxygen (air) addition must be controlled by means of a readily accessibleand controllable air inlet valve. Inlet oxygen analyzers should be used whenthe diluent is inert gas.

CATALYST – REACTION WITH STEAM

Typically steam is used as the inert diluent during the regeneration of steamreforming catalysts.

Using the regeneration procedures outlined below, steam will not have adetrimental effect on the catalyst, as the temperatures will be maintainedwithin the operating envelope of the catalyst in steam. However, if thecatalyst is steamed at temperatures significantly higher than normal operatingtemperatures the structure of the catalyst may be changed in such a way thatwill reduce the reforming activity (metal agglomeration and sintering). Hightemperature steaming increases the time required to fully re-reduce thecatalyst once returned to an on- stream condition.






Similarly, high temperatures in an oxidizing atmosphere will reduce theactivity by sintering the active nickel phase and produce a less easilyreducible catalyst.

High temperatures in steam and oxygen are to be avoided at all times.Temperatures within the normal process-operating envelope will notdeactivate the catalyst: Low steam flow-high heat fluxes will rapidly deactivatethe catalyst.

Steam contact with the catalyst surface at the point of carbon burn will notdeactivate the catalyst if the burn exotherm is correctly controlled.

MAXIMUM OXYGEN CONCENTRATION

There is not a maximum oxygen level above which the catalyst cannot beexposed – only a maximum temperature. The catalyst should not be exposedto temperatures higher than 650

oC in a pure air stream for extended periods.

‘With lower oxygen concentrations the temperature can be allowed to go up to700oC 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 carbonburn, the surface of the catalyst will not substantially rise above that of theprocess gas or steam flow.

In cases where the carbon burn front is allowed to burn in a high of anuncontrolled oxygen level the surface temperature of the catalyst will risesubstantially, resulting in sintering and agglomeration of the active metalphase. The immediate catalyst surface and pore internals will be subjected tolocalized over-heating but this increase will not normally be noted in the exit

gas temperature and the overheating will result in catalyst breakdown andsubsequent high pressure drop.






CONDITIONS TO BURN OFF CARBON COATED CATALYST

The oxidative (air) burning of carbon is the recommended method to use forthe removal of carbon deposits. However the method should only be used ifthe correct analysis equipment is available and correct control of the airvolumes can be maintained.

Gerard B. HawkinsManaging Director GBH Enterprises Ltd.Catalysts & Process Technology Consultancy

Sales and ServiceP.O. Box 805763Chicago, Ill60680-4119Tel. +1-312-235-2610Tel. +52 (1) 55 1648 7960Fax. +52 55 2614 8349e-mail: [email protected]

Information contained in this publication or as otherwise supplied to Users is believed to beaccurate 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,C2PT, Catalyst Process Technology gives no warranty as the f itness of the Product for any

particular purpose and any implied warranty or condition (statutory or otherwise) is excludedexcept to the extent that exclusion is prevented by law. GBH Enterpri ses, C

2PT accepts no

liability for loss or damaged (other than that arising from death or personal injury caused by GBHEnterprises, C

2PT’s negligence or by a defective product, if proved), resulting from reliance on

this information. Freedom, under Patent, Copyright and Designs cannot be assumed.

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