CA1135206A - Control of emissions in fcc regenerator flue gas - Google Patents
Control of emissions in fcc regenerator flue gasInfo
- Publication number
- CA1135206A CA1135206A CA000345482A CA345482A CA1135206A CA 1135206 A CA1135206 A CA 1135206A CA 000345482 A CA000345482 A CA 000345482A CA 345482 A CA345482 A CA 345482A CA 1135206 A CA1135206 A CA 1135206A
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- Prior art keywords
- flue gas
- combustion
- catalyst
- sulfur
- regenerator
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
- Y02A50/00—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE in human health protection, e.g. against extreme weather
- Y02A50/20—Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters
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- Production Of Liquid Hydrocarbon Mixture For Refining Petroleum (AREA)
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Abstract
CONTROL OF EMISSIONS IN FCC REGENERATOR FLUE GAS
ABSTRACT OF THE DISCLOSURE
A system is described for control of sulfur oxides in emissions to the atmosphere from regenerators of cyclic Fluid Catalytic Cracking Units (FCC). By the disclosed system, hot regenerator flue gas is cooled, air (oxygen) is injected into the cooled flue gas unless oxygen is already present, and then the cooled flue gas is contacted with a solid particulate material which has the capability of associating with and binding sulfur oxides in the flue gas to form a stable solid material which is separated from the gases.
ABSTRACT OF THE DISCLOSURE
A system is described for control of sulfur oxides in emissions to the atmosphere from regenerators of cyclic Fluid Catalytic Cracking Units (FCC). By the disclosed system, hot regenerator flue gas is cooled, air (oxygen) is injected into the cooled flue gas unless oxygen is already present, and then the cooled flue gas is contacted with a solid particulate material which has the capability of associating with and binding sulfur oxides in the flue gas to form a stable solid material which is separated from the gases.
Description
~ ~ 3 tj~ ~ ~
l _IELD OF THE INVENTION
The invention is concerned with the operation of catalytic cracking units, especially FCC units, for control of undesirable gases in the flue gas released from the regenerators. More particularly, the invention provides an improvement on known techniques for reducing the content of sulfur oxides in regenerator flue gas by contact with a solid capable of binding sulfur oxides. The invention contemplates selective removal of SO without oxidation of CO in the flue gas by a solid-gas phase contact downstream of regenerator cyclones between cooled flue gas containing a source of oxygen and finely divided particulate solid capable of associating with and binding oxides of sulfur and separating the solid so-contacted from the flue gases before discharging the gases into the atmosphere.
BACKGKOUND OF THE INVENTION
Catalytic cracking of hydrocarbon feedstocks has been character-ized by certain basic steps repeated in cyclic manner. The catalysts are generally highly porous solids characteristically having extensive surface area and possessing acidic sites. During a relatively short period of time, hydrocarbon charge, such as gas-oil, undergoes profound conversions of a complex nature on contact with those surfaces at elevated temperatures upwards of about 850F. and essentially atmospheric pressure.
The temperatures may range up to about 1000F. and the pressure on incoming charge is usually only enough to overcome pressure drop through the reactor and associated product recovery facili-ties, say 30 to 50 psig.
The conversions taking place in the presence of the cracking catalyst include scission of carbon-to-carbon bonds (simple cracking), isomerization, polymerization, dehydrogenation, hydrogen transfer and others leading -to lighter, lower molecular weight compounds as important desired products. In many installations motor gasoline of end point near ~13~
1 about 400F. is a primary product and cracking units are often operated to maximize high quality gasoline within constraints imposed by ability to profitably market the unavoidable by-products such as butane and lighter.
In addition to the gaseous by-products, the reac~ions on cracking catalyst also produce hydrocarbons of very low volatility and very high carbon con~
tent which remain on the active surfaces of the catalyst and mask the active sites, rendering the catalyst inactive. Those deposits of heavy carbon-aceous matter (commonly called "coke") can be removed by burning with air to restore the active surface and thus regererate the activity of the catalyst. Typically the coke contains about 6-7% hydrogen on a weight basis. In commercial plants for practice of catalytic cracking, catalyst inactivated by coke is purged of volatile hydrocarbons, as by steaming, and contacted with air at elevated temperature to burn off the coke.
Combustion of the coke generates carbon dioxide, carbon monoxide and water as combustion products and releases large amounts of heat. To a very considerable extent, the heat so released has been applied to supply the endothermic heat of reaction during the cracking phase of the cycle. In its earliest stages, catalytic cracking was conducted in fixed beds of catalyst provided with heat exchange tubes through which a heat transfer fluid was circulated to abstract heat during regeneration and supply heat during cracking. Continuity of operation was achieved by a complex system of manifolds and valves serving a plurality of reactors such that one is used for cracking while two or more others were purged of volatiles, regenerated~ again purged and ready to assume the cracking function as catalyst in the first reactor became spent.
Further development made available systems in which the catalyst is moved continuously through a reactor, purged, transferred to a regen-erator, again purged and returned to the reactor. These moving catalyst systems are ahle to dispense with the circulating heat transfer medium and ins~ead employ the catalyst itself as a medium for conveying heat from . .............................. . .
5Z~
1 the regenerator to the reactor. The early catalysts such as acid-treated c]ays, and synthetic amorphous silica-alumina composites, resulted in deposition of quantities of coke in excess of the amounts which on complete combustion to carbon dioxide and water will supply the heat of reaction required by the reactor. In some installationsg a portion of the heat was withdrawn by heat exchange coils in the regenerator. That practice was followed in the moving compact bed process known as Thermofor Catalytic Cracking (TCC). Another expedient is ko circulate a portion of catalyst from the regeneratcr through a cooling heat exchanger and back to the regenerator. That practice was found suitable for systems in which a finely divided catalyst is suspended in the hydrocarbon charge in the reactor and in the combustion air in the regenerator. These suspended catalyst systems applied the fluidiæed solids phenomenon and are classed generally as Fluid Catalytic Cracking (FCC).
~- 15 Characteristic of all the systems for many years was a high content of carbon monoxide (CO) in the flue gas from the regenerator, a result of incomplete combustion or partial utilization of ~he fuel value of the coke. C0 in the flue gas is undesirable for other reasons.
That combustible gas can burn in regenerator gas discharge equipment and in flues leading to temperatures which damage those facilities. The loss of potential fuel value has been avoided by providing "C0 boilers"
in which the C0 is burned in contact with steam generation tubes, thus recovering sensible heat from the flue gas as well as fue1 value of the CO .
As designs of moving catalyst systems for charging heavier stocks were developed, the cracker received some hydrocarbons in liquid form, requiring heat input for vaporization of charge, heating the charge to reaction temperature and for endothermic heat of reaction. The "heat balanced" FCC design aids in satisfaction of these requiremen~s. Typically, that design provides a heat sensor in the reacted vapors before removal from 1 the reactor. An automatic control of the valve in the line for return of hot regenerated catalyst from the regenerator to t~e reactor assures return of that amount of hot regenerated catalyst which will n~aintain re-actor top temperature at a desired set point. It will be seen that this control also sets an important reactor parameter, namely ratio of catalyst - to oil (C/0), corresponding to space velocity in fixed bed reactors. It follows that, for a given set of regenerator conditions, C/0 is a depen-dent variable not subject to independent control by the operator.
The advent of zeolite cracking catàlyst as described in patent 3,140,2~9 introduced new considerations in catalytic cracking design and practice. Such catalysts are highly active, inducing more profound con-version of hydrocarbon charge stock than the older catalysts. In addition, they are more selective in that a larger proportion of the conversion pro-ducts are motor gasoline components with lesser proportions of gas and coke.
~ecause of that increased selectivity, the zeolite cracking catalyst rapidly became the catalyst of choice, particularly in areas of high gasoline demand, such as the United States. The more active catalyst has been effectively applied in FCC Units at short catalyst contact times, such as the modern riser reactor units in which hot catalyst is dispersed `~in a column of charge rising through a conduit to an enlarged catalyst disengaging zone. Contact times of 20 seconds or less are common practice in such units. Such short contact times place a premium on high activity of the catalyst. Since activity of the regenerated catalys-t is a function of residual coke remaining on the catalyst after regeneration, it becomes important to reduce residual coke to the lowest level economically attain-able.
The extent of coke burning is a function of time and temperature.
Rate of coke burning increases with increased temperature. In any given installation, the volume of the regenerator imposes a constrain on time of ; con,act between catalyst and regeneration air. Temperature of regeneration ' ~ -5-~L3L135;Z l~)~
1 is constrained by thermal stability of the catalyst, which suffers unduly rapid loss of activ-ity on exposure to moisture of the regeneration air at temperatures upwards of about 1400F. In addition, the regeneration temperature must be held to a level which will not cause damage to vessel internals. As regeneration gas rises from a dense bed in a regenerator, burning of C0 can take place in a "dilute phase" containing only a small amount o~ catalyst. Because there is very little catalyst to absorb the heat thus released, the temperature of the gas rises rapidly and may reach levels which cause damge to the cyclones which separate entrained catalyst from regenerator fume, plenum chambers and flues for discbarge of the flue gas. This may be combatted by injecting water or steam to these internals.
Better techniques have been recently proposed and adopted in many plants. According to the system of Patent 3,909,392, catalyst from the dense bed of the regenerator is educ-ted through tubes to the disperse phase, thus providing catalyst mass to absorb heat of C0 combustion and return that heat to the dense bed as the catalyst falls back into that bed.
A widely practiced technique causes C0 combustion to take place in the dense bed by use of a catalyst promoted with platinum or the like in very small amounts. See Patent 4,072,600. By transferring the heat of burnin~
CO to the dense bed, these developments make higher regen:eration tempera-tures available to regenerate catalyst to lower residual coke levels, hence higher activity.
Regeneration temperatures above 1250F., preferably arolmd 1300F. and up to about 1375F., become feasible at residual coke levels of less than 0.1% by weight on catalyst. The necessary result of regen-.
eration at these increased temperatures is that the automatic control to maintain preset reactor top temperature will reduce the rate of catalyst flow from regenerator to reactor below the rates for lower regeneration temperature, thus reducing C/0. In addition, catalyst at these high temper-atures will heat a portion of the charge to excessive levels at which ~35;~6 1 thermal cracking occurs with resultan~ production of gas, olefins and coke.
Operators of FCC Units have also been concerned about emissions of sulfur dioxide and sulfur trioxide (SO ) in the regenerator flue gas.
The hydrocarbon feeds processed in commercial FCC units normally contain sulfur. It has been found that about 2-10% or more of the sulfur in a hydrocarbon stream processed in an FCC system is transferred from the hydrocarbon stream to the cracking catalyst, becoming part of the coke formed on the catalyst particles within the FCC cracking or conversion zone. This sulfur is eventually removed from the conversion zone on the coked catalyst which is sent to the FCC regenerator. Accordingly, about 2-10% or more of the feed sulfur is continuously passed from the conversion zone into the catalyst regeneration zone with the coked catalyst in an FCC unit.
In an FCC catalyst regenerator, sulfur contained in the coke is burned, along with the coke carbon, fo~ming primarily gaseous sulfur dioxide and sulfur trioxide. These gaseous sulfur compounds become part of the flue gas produced by coke combustion and are conventionally removed from the regenerator in the flue gas.
It has been shown that SO in the regenerator flue gas can be substantially cut back by including in the circulating catalyst inven-tory an agent capable of reacting with an oxide of sulfur in an oxidizing atmosphere or an environment which is not of substantial reducing nature to form solid compounds capable of reduction in the reducing atmosphere of the FCC reactor to yield H2S. Upon such reduction, the sulfur leaves the reactor as gaseous H2S and organic compounds of sulfur resulting from the cracking reaction~ Since these sulfur compounds are detrimental to the quality of motor gasoline and fuel gas by-products, the catalytic cracker is followed by downstream treating facilities for removal of sul-fur compounds. Thus the gaseous fractions of cracked product may be scrubbed with an amine solution to absorb H2S which is then passed to .
.
~3S;~
l facilities for conversion to elemental sulfur, e~g. a Claus plant. The additional H2S added to the cracker product stream by chemical reduction in the reactor of the solid sulfur compounds formed in the regenerator im-poses little additional burden on the sulfur recovery facilities.
The technology heretofore proposed has involved circulating thè SO binding agent, the latter either being an integral part of the cracking catalyst particles or taking the form of separate particles having essentially the same fluidization properties as the cracking catalyst.
Suitable agents for the purposes have been described in a number of pre-viously published documents. Discussion of a variety of oxides which exhibit the property of combining with SO and thermodynamic analysis of their behavior in this regard are set out by Lowell et al., SELECTIO~ OF
METAL OXIDES FOR ~EMOVING SO FRO~ FLUE GAS, IND. ENG. CHEM. PROCESS DES.
DEVELOP., Vol. 109 ~o. 3 at pages 384-390 (1971).
An early attempt to reduce SO emission from catalytic cracking ~ units, as described in U. S. 3,699,037, involves adding particles of a ;~ Group II metal compound, especially calcium or magnesium oxide, to a cracking unit cycle at a rate at least as great as the stoichiometric rate of sulfur deposition on the cracking catalyst, the additive preferably being injected into the regeneration zone in the form of particles greater than 20 microns. Particle size was chosen to assure a relatively long residence time in the unit. In putting the invention into practice, the Group II metal compound is recycled at least in part between the reactor and the regenerator, the remainder leaving the cycle along with catalyst fines entrained in regenerator flue gas. Subsequently it was proposed to incorporate the alkaline earth metal compound in the cracking catalyst particles by impregna~ion in order to minimize loss of the sulfur acceptor in the regenerator flue gases. See U. S. 3,835,031. This patent apparently recognizes the need for free oxygen for binding SO with a Group II metal x oxide since the equations for the reaction taking place in the regenerator : .
~1315~
1 is summarized as follows:
l _IELD OF THE INVENTION
The invention is concerned with the operation of catalytic cracking units, especially FCC units, for control of undesirable gases in the flue gas released from the regenerators. More particularly, the invention provides an improvement on known techniques for reducing the content of sulfur oxides in regenerator flue gas by contact with a solid capable of binding sulfur oxides. The invention contemplates selective removal of SO without oxidation of CO in the flue gas by a solid-gas phase contact downstream of regenerator cyclones between cooled flue gas containing a source of oxygen and finely divided particulate solid capable of associating with and binding oxides of sulfur and separating the solid so-contacted from the flue gases before discharging the gases into the atmosphere.
BACKGKOUND OF THE INVENTION
Catalytic cracking of hydrocarbon feedstocks has been character-ized by certain basic steps repeated in cyclic manner. The catalysts are generally highly porous solids characteristically having extensive surface area and possessing acidic sites. During a relatively short period of time, hydrocarbon charge, such as gas-oil, undergoes profound conversions of a complex nature on contact with those surfaces at elevated temperatures upwards of about 850F. and essentially atmospheric pressure.
The temperatures may range up to about 1000F. and the pressure on incoming charge is usually only enough to overcome pressure drop through the reactor and associated product recovery facili-ties, say 30 to 50 psig.
The conversions taking place in the presence of the cracking catalyst include scission of carbon-to-carbon bonds (simple cracking), isomerization, polymerization, dehydrogenation, hydrogen transfer and others leading -to lighter, lower molecular weight compounds as important desired products. In many installations motor gasoline of end point near ~13~
1 about 400F. is a primary product and cracking units are often operated to maximize high quality gasoline within constraints imposed by ability to profitably market the unavoidable by-products such as butane and lighter.
In addition to the gaseous by-products, the reac~ions on cracking catalyst also produce hydrocarbons of very low volatility and very high carbon con~
tent which remain on the active surfaces of the catalyst and mask the active sites, rendering the catalyst inactive. Those deposits of heavy carbon-aceous matter (commonly called "coke") can be removed by burning with air to restore the active surface and thus regererate the activity of the catalyst. Typically the coke contains about 6-7% hydrogen on a weight basis. In commercial plants for practice of catalytic cracking, catalyst inactivated by coke is purged of volatile hydrocarbons, as by steaming, and contacted with air at elevated temperature to burn off the coke.
Combustion of the coke generates carbon dioxide, carbon monoxide and water as combustion products and releases large amounts of heat. To a very considerable extent, the heat so released has been applied to supply the endothermic heat of reaction during the cracking phase of the cycle. In its earliest stages, catalytic cracking was conducted in fixed beds of catalyst provided with heat exchange tubes through which a heat transfer fluid was circulated to abstract heat during regeneration and supply heat during cracking. Continuity of operation was achieved by a complex system of manifolds and valves serving a plurality of reactors such that one is used for cracking while two or more others were purged of volatiles, regenerated~ again purged and ready to assume the cracking function as catalyst in the first reactor became spent.
Further development made available systems in which the catalyst is moved continuously through a reactor, purged, transferred to a regen-erator, again purged and returned to the reactor. These moving catalyst systems are ahle to dispense with the circulating heat transfer medium and ins~ead employ the catalyst itself as a medium for conveying heat from . .............................. . .
5Z~
1 the regenerator to the reactor. The early catalysts such as acid-treated c]ays, and synthetic amorphous silica-alumina composites, resulted in deposition of quantities of coke in excess of the amounts which on complete combustion to carbon dioxide and water will supply the heat of reaction required by the reactor. In some installationsg a portion of the heat was withdrawn by heat exchange coils in the regenerator. That practice was followed in the moving compact bed process known as Thermofor Catalytic Cracking (TCC). Another expedient is ko circulate a portion of catalyst from the regeneratcr through a cooling heat exchanger and back to the regenerator. That practice was found suitable for systems in which a finely divided catalyst is suspended in the hydrocarbon charge in the reactor and in the combustion air in the regenerator. These suspended catalyst systems applied the fluidiæed solids phenomenon and are classed generally as Fluid Catalytic Cracking (FCC).
~- 15 Characteristic of all the systems for many years was a high content of carbon monoxide (CO) in the flue gas from the regenerator, a result of incomplete combustion or partial utilization of ~he fuel value of the coke. C0 in the flue gas is undesirable for other reasons.
That combustible gas can burn in regenerator gas discharge equipment and in flues leading to temperatures which damage those facilities. The loss of potential fuel value has been avoided by providing "C0 boilers"
in which the C0 is burned in contact with steam generation tubes, thus recovering sensible heat from the flue gas as well as fue1 value of the CO .
As designs of moving catalyst systems for charging heavier stocks were developed, the cracker received some hydrocarbons in liquid form, requiring heat input for vaporization of charge, heating the charge to reaction temperature and for endothermic heat of reaction. The "heat balanced" FCC design aids in satisfaction of these requiremen~s. Typically, that design provides a heat sensor in the reacted vapors before removal from 1 the reactor. An automatic control of the valve in the line for return of hot regenerated catalyst from the regenerator to t~e reactor assures return of that amount of hot regenerated catalyst which will n~aintain re-actor top temperature at a desired set point. It will be seen that this control also sets an important reactor parameter, namely ratio of catalyst - to oil (C/0), corresponding to space velocity in fixed bed reactors. It follows that, for a given set of regenerator conditions, C/0 is a depen-dent variable not subject to independent control by the operator.
The advent of zeolite cracking catàlyst as described in patent 3,140,2~9 introduced new considerations in catalytic cracking design and practice. Such catalysts are highly active, inducing more profound con-version of hydrocarbon charge stock than the older catalysts. In addition, they are more selective in that a larger proportion of the conversion pro-ducts are motor gasoline components with lesser proportions of gas and coke.
~ecause of that increased selectivity, the zeolite cracking catalyst rapidly became the catalyst of choice, particularly in areas of high gasoline demand, such as the United States. The more active catalyst has been effectively applied in FCC Units at short catalyst contact times, such as the modern riser reactor units in which hot catalyst is dispersed `~in a column of charge rising through a conduit to an enlarged catalyst disengaging zone. Contact times of 20 seconds or less are common practice in such units. Such short contact times place a premium on high activity of the catalyst. Since activity of the regenerated catalys-t is a function of residual coke remaining on the catalyst after regeneration, it becomes important to reduce residual coke to the lowest level economically attain-able.
The extent of coke burning is a function of time and temperature.
Rate of coke burning increases with increased temperature. In any given installation, the volume of the regenerator imposes a constrain on time of ; con,act between catalyst and regeneration air. Temperature of regeneration ' ~ -5-~L3L135;Z l~)~
1 is constrained by thermal stability of the catalyst, which suffers unduly rapid loss of activ-ity on exposure to moisture of the regeneration air at temperatures upwards of about 1400F. In addition, the regeneration temperature must be held to a level which will not cause damage to vessel internals. As regeneration gas rises from a dense bed in a regenerator, burning of C0 can take place in a "dilute phase" containing only a small amount o~ catalyst. Because there is very little catalyst to absorb the heat thus released, the temperature of the gas rises rapidly and may reach levels which cause damge to the cyclones which separate entrained catalyst from regenerator fume, plenum chambers and flues for discbarge of the flue gas. This may be combatted by injecting water or steam to these internals.
Better techniques have been recently proposed and adopted in many plants. According to the system of Patent 3,909,392, catalyst from the dense bed of the regenerator is educ-ted through tubes to the disperse phase, thus providing catalyst mass to absorb heat of C0 combustion and return that heat to the dense bed as the catalyst falls back into that bed.
A widely practiced technique causes C0 combustion to take place in the dense bed by use of a catalyst promoted with platinum or the like in very small amounts. See Patent 4,072,600. By transferring the heat of burnin~
CO to the dense bed, these developments make higher regen:eration tempera-tures available to regenerate catalyst to lower residual coke levels, hence higher activity.
Regeneration temperatures above 1250F., preferably arolmd 1300F. and up to about 1375F., become feasible at residual coke levels of less than 0.1% by weight on catalyst. The necessary result of regen-.
eration at these increased temperatures is that the automatic control to maintain preset reactor top temperature will reduce the rate of catalyst flow from regenerator to reactor below the rates for lower regeneration temperature, thus reducing C/0. In addition, catalyst at these high temper-atures will heat a portion of the charge to excessive levels at which ~35;~6 1 thermal cracking occurs with resultan~ production of gas, olefins and coke.
Operators of FCC Units have also been concerned about emissions of sulfur dioxide and sulfur trioxide (SO ) in the regenerator flue gas.
The hydrocarbon feeds processed in commercial FCC units normally contain sulfur. It has been found that about 2-10% or more of the sulfur in a hydrocarbon stream processed in an FCC system is transferred from the hydrocarbon stream to the cracking catalyst, becoming part of the coke formed on the catalyst particles within the FCC cracking or conversion zone. This sulfur is eventually removed from the conversion zone on the coked catalyst which is sent to the FCC regenerator. Accordingly, about 2-10% or more of the feed sulfur is continuously passed from the conversion zone into the catalyst regeneration zone with the coked catalyst in an FCC unit.
In an FCC catalyst regenerator, sulfur contained in the coke is burned, along with the coke carbon, fo~ming primarily gaseous sulfur dioxide and sulfur trioxide. These gaseous sulfur compounds become part of the flue gas produced by coke combustion and are conventionally removed from the regenerator in the flue gas.
It has been shown that SO in the regenerator flue gas can be substantially cut back by including in the circulating catalyst inven-tory an agent capable of reacting with an oxide of sulfur in an oxidizing atmosphere or an environment which is not of substantial reducing nature to form solid compounds capable of reduction in the reducing atmosphere of the FCC reactor to yield H2S. Upon such reduction, the sulfur leaves the reactor as gaseous H2S and organic compounds of sulfur resulting from the cracking reaction~ Since these sulfur compounds are detrimental to the quality of motor gasoline and fuel gas by-products, the catalytic cracker is followed by downstream treating facilities for removal of sul-fur compounds. Thus the gaseous fractions of cracked product may be scrubbed with an amine solution to absorb H2S which is then passed to .
.
~3S;~
l facilities for conversion to elemental sulfur, e~g. a Claus plant. The additional H2S added to the cracker product stream by chemical reduction in the reactor of the solid sulfur compounds formed in the regenerator im-poses little additional burden on the sulfur recovery facilities.
The technology heretofore proposed has involved circulating thè SO binding agent, the latter either being an integral part of the cracking catalyst particles or taking the form of separate particles having essentially the same fluidization properties as the cracking catalyst.
Suitable agents for the purposes have been described in a number of pre-viously published documents. Discussion of a variety of oxides which exhibit the property of combining with SO and thermodynamic analysis of their behavior in this regard are set out by Lowell et al., SELECTIO~ OF
METAL OXIDES FOR ~EMOVING SO FRO~ FLUE GAS, IND. ENG. CHEM. PROCESS DES.
DEVELOP., Vol. 109 ~o. 3 at pages 384-390 (1971).
An early attempt to reduce SO emission from catalytic cracking ~ units, as described in U. S. 3,699,037, involves adding particles of a ;~ Group II metal compound, especially calcium or magnesium oxide, to a cracking unit cycle at a rate at least as great as the stoichiometric rate of sulfur deposition on the cracking catalyst, the additive preferably being injected into the regeneration zone in the form of particles greater than 20 microns. Particle size was chosen to assure a relatively long residence time in the unit. In putting the invention into practice, the Group II metal compound is recycled at least in part between the reactor and the regenerator, the remainder leaving the cycle along with catalyst fines entrained in regenerator flue gas. Subsequently it was proposed to incorporate the alkaline earth metal compound in the cracking catalyst particles by impregna~ion in order to minimize loss of the sulfur acceptor in the regenerator flue gases. See U. S. 3,835,031. This patent apparently recognizes the need for free oxygen for binding SO with a Group II metal x oxide since the equations for the reaction taking place in the regenerator : .
~1315~
1 is summarized as follows:
2 1 2 g 4 ~imilar use of reactive alumina either as a discrete fluidizable entities or as a component of catalyst particles is described in patents 4,071,436;
4,115,250 and 4,115,251. Use of oxidants including platinum or chromium as adjuncts to alumina is suggested in these patents. A related use of cerium oxide or an alumina support is described in `U. S. 4,001,375.
In the prior art techniques aforementioned, emphasis was on reversibly reacting sulfur oxides in the flue gas, and doing so while '10 the gases were still in the regenerator. Since the sulfur loade~ parti cles were carried to the reactor to be converted to gaseous hydrogen sulfide under the reducing atmosphere created by the cracking operation, the agents used to bind and then release sulfur were necessarily limited to those in-- herently capable of doing so under the constraints of temperature and time imposed by the operation of the reactor and the regenerator. Such procedures offer promise as means to reduce Sx emissions from refineries but they leave much to be desired.
; With units operating with high sulfur feedstock, relatively large amounts of sulfur acceptor are needed to accomplish reductions in su]fur oxide levels. This will result in appreciable dilution of the active catalyst in the cracking reaction cycle whether the sulfur acceptor is a part of the catalyst particles or is present as discrete entities circulated with catalyst inventory. A basic limitation is that conditions of time and temperature for operating cyclic cracking units, especially heat balanced FCC units, are geared to maximizing production of desired ;~ products and conditions that will favor this result, are by no meansthose that are optimum for reversibly reacting sulfur oxides in the regenerator and carrying the sulfur back to the reactor for conversion ` `
at least in part to hydrogen sulfide. The generally recognized need or ; desirability oE having excess oxygen in the regenerator beyond that needed ~ ~3~;2~3~
to burn coke, in order to convert oxides to sulfur to trioxide which will then react with metal oxide to form a sulfate, imposes severe limitations on those refineries that operate regenerators in conventional m~de, i.e., without full combustion. rmus, introduction of air (oxygen) to promote SO~ association with metal oxide is basically inconsistent with the operation of a regenerator in conventional im~omplete combustion mode.
SUMM~RY OF THE INVENTION
According to the present invention, there is provided in a process for cata.lytic cracking of a sulfur-containing hydrocarkon charge by oontacting said charge at cracking temperature with a circulating inventory of cracking catalyst whereby the catalyst acquires an inactivating carkonaceous deposit c~ntaIningsulfur~ separating vaporous ~roducts of reaotion including hydrogen sulfide from circulating catalyst inventory containing said deposit,regenerating the so separated inventory by contact with air at a temperature to burn said carbonaceous deposit thus generating oxides of carbon and sulfur and regenerating -the catalyst, separating products of combustion from regenerated catalyst and returning regenerated catalyst to renewed CGntact with hydrocarhon charge; the improvement which comprises contact.ing said products of combustion after separation from said regenerated catalyst at a temperature substantially below the temperature of such separation and in the presence of oxygen with a particulate solid capable of assoclating with gaseous oxides of sulfur in an oxidizing atmosphere to thereby induce transfer of said oxides of sulfur from said particulate solid and separating said particulate solid with transferred oxides of sulfur from said products of c~mbustion now containing a reduced content of oxides of sulfur.
Thus, the efficiency of removing Sx from FCC flue gas by solid particulate sulf.ur oxide acceptors such as reactive metal oxides is im-proved in accordance with the invention by oontacting oxygen-conta.ining flue gas with particulate sulfur oxide acceptor at a temperature kelow that prevailing upstream of cyclones in the regenerator. That principle is appl.ied in accordance with this invention by cooling the regenerator flue ~L1352(~6 gases downstream of cyclones, separating regenerated catalyst from ccmbustion gases, injecting air (oxygen) into the cooled flue gas to provide sufficient ~ree oxygen for oxidation of SOx, unless the flue gas already contains such amount of oxygen, and contacting the oooled oxygen-containing flue gases with finely divided suspendable particulate solid sulfur oxide acceptor. Usable sulfur oxide acceptors ~mbrace those materials which will associate with Sux in cooled o~ygen-containing regenerator flue gases to form a solid material that is stable at the temperature of contact. Contact between particulate sulfur acceptor and oooled flue gas is carried out in a manner such that the particulate solid is suspended in the gases and is conveyed therein, preferably under turbulent conditions, to transfer Sx in flue gas to the solid particles.
The particulate sulfur oxide acceptor with bou~nd Sx is then separated from flue gases now having a reduced Sx content. Optionally the separated solid sulfur oxide acceptor with bound Sx is processed external to the regenerator and external to the reactor by thermal or chemi-10a . :
S~6 1 cal means or a combination thereof to restore the solid to a state or condition whereby it is once again capable of being recylced in cooled flue gas in the presence of oxygen to pick up S0 .
In one embodiment, the invention is practiced to reduce S0 emissions from an FCC regenerators operating in conventional mode, i.e., regenerators emit~ing flue gases containing less than 0 5~ 2 and containing at least 500 ppm C0. The flue gases are cooled to a temperature below 1300F., preferably below 1100F., downstream from regenerator cyclones and then air or oxygen is injected external to the regenerator whereby C0 combustion is precluded and interference with the operation of the regenerator is avoided. Practice of the invention permits intro-duction of oxygen to regenerator gases while avoiding afterburning or change-over to complete combustion. Suspendable particles of metal oxide may be injected and suspended in the oxygen-enriched regenerator ; gases external to the regenerator. Contrary to prior art practice, S0 pickup takes place downstream of regenerators and reactors, e.g., in flue gas lines, and the S0 acceptor is not cycled between reactor and regenerator. When acceptor particles of suitable chemical composition are used they may be introduced into the regenerator, but they are not recycled to the reactor as in prior art practice.
In another embodiment, principles of the invention are utilized to reduce S0 in flue gas from regenerators operated in complete C0 combustion mode (i.e., at least 0-5% 2 and less than 500 ppm C0). The metal oxide is contacted with flue gases previously cooled below about 1300F., downstream of the cyclones. Air (oxygen) is injected into the cooled flue gases if necessary. Metal oxide is introduced to the cooled flue gas either e~ternal to the regenerator te.g., in the flue gas line) or into the regenerator at any suitable point.
By the system of the invention, all or essentially all finely d1v~ded particles Oe the S0 accep~ are removed with flue gases and ., ~ ~3~;2~
1 the agent for removing SO is not present in the reactor as a diluent which reduces activity of the catalyst charge in the reactor. The invention may be practiced in a manner such that the agent for removing SO is als~o not present in the regenerator. This also represents a departure and offers advantages over those prior art techniques which re~uire the presence of the agent for binding SO in the regenerator, thus placing limitations on the composition and quantity of such agent that could be used without impairing the operation of the regenerator.
Another major point of distinction is that in the present invention oxygen conducive to conversion of gaseous oxides of sulfur to stable solid sulfate formation is introduced into previously cooled flue gas. This prevents undesired temperature rise in the regenerator with the potential of afterburning or excessive temperature rise. The temperature to which the flue gas can be cooled can be flexibly altered without upsettlng the operation of the regenerator to facilitate utili-zation of any selected Sx acceptor at a temperature thermodynamically most favorable to that acceptor. This flexibility was not possible in ~-the past because of constraints imposed by the need to utilize the sulfur acceptor under conditions dictated by the need to operate units in heat 2~
balance. Improved opportunity for SO association is provided because the solid acceptor can be conslderably finer than would be possible if such material or a major part had to be retained in the regenerator and the reactor. Additionally the invention a~fords favorable contact conditions for transferring Sx to a solid sulfur oxide acceptor by permitting contact of a finely divided solid with the flue gas stream under turbulent conditions.
DESCRIPTION OF T~IE DRAWING
Typical apparatus for practice of the invention is shown in the diagrammatic elevation in the single figure of a typical regenerator ~L3L3S~
1 section of an FCC annexed hereto. As illus~rated in that diagram, conduit 2 is used to transfer hot stripped catalyst from the lower portion of the reactor stripping zone to the regeneration zone shown in the drawing.
The regenerator may be of any desired style, but is prelerably designed and operated to yield regenerated catalyst at minimum residual coke level after regeneration in either a conventional or complete combus~ion modes.
A modern version of regeneration is characterized by "fast fluid"
riser to which catalyst suspended in regeneration gas is supplied from a dense fluidized bed undergoing regeneration at high temperature. The spent catalyst from the reactor and hot catalyst separated at the top of the riser are both introduced in the dense fluidized bed where the "fire" for regenerating spent catalyst is lighted by the hot regenerated catalyst so recycled. See Patents 3,893,812 and 3,926,778.
As seen in the annexed drawing, spent catalyst passes by conduit 2 to a lower section of the regenerator, where it is fluidized along with the recycled hot regenerated catalyst from conduit 3 with air introduced by conduit 4. The resulting mixture passes upwards through conduit 4 to an enlarged lower section 5 of the regenerator where it enters a dense fluid-ized bed maintained by the regeneration gas. Catalyst from the dense bed in section 5 is entrained by hot regeneration gas to pass upward through a riser 6 to discharge port 7 into enlarged disengaging zone 8. Disengaging zone 8 may be equipped with a plllrality of cyclone separator combinations comprising first and second cyclonic separation means attached or spaced apart from the riser discharge for separating catalyst particles from the regenerat:Lon gas, as is customary in the art. A single state of separation is shown. Catalyst separated from regeneration gas in cyclone 9 flows downward through dipleg 10 and collects as a fluidized bed in the lower portion of disengaging zone 8 about riser 6. A portion of the hot catalyst collected is recycled by line 3 back to the bed in the lower section 5 for the purpose stated. Another portion for return to the ~ ~3~ 16 l reac-tor section is w~thdrawn by line 11.
Hot regenerator fume essentially free of catalyst exists the cyclonic separation 9 and enters plenum chamber area 12 where the fume can be quenched to the desired temperature by water or steam through the temperature controller sensor located in the flue gas line 13, which con- ;
trols the valve 14 in the water/steam line. In conjunction with this quench system or as an alternate system, steam generator 15 could be used to lower the fume to the desired temperature.
After quenching to below 1300F. in the complete combustion mode or below 1100F. in the conventional mode, air or oxygen is injected : ~
through line 16 into flue gas line 13 controlled at between 0.5 and 3 ~ -mole percent oxygen by oxygen analyzer 17. The S0 acceptor is injected through line 18. Its rate of injection is determined by the S0 analyzer 19 so that the exiting flue gas after treatment for removal of the Sx acceptor particles is essentially free of S0 .
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1 DESC~IPTION OF SPECIFIC EMBODIME~TS
In preferred embodiments, the invention contemplates cooling regenerator flue gas from FCC urits and contacting the cooled flue gas in the presence of free oxygen with a finely divided solid metal compound capable of reacting with or sorbing Sx under conditions to favor transfer of Sx from the flue gas to the finely divided solid metal compound. A
typical metal sulfate formed by the contact exhibits a vapor pressure which determines the minimum level of SO which must exist in the vapor phase for any particular temperature. The higher the temperature, the higher the vapor pressure resulting in a higher concentration of Sx in the vapor phase at the expense of sulfur bound to the solid sulfur acceptor. The amount of sulfur removed as a stable solid oxide of sulfur, e.g., sulfate, by the sulfur acceptor upon separation from the flue gases is constrained by the temperature and contact conditions between the flue gas and solid acceptor.
That constraint is avoided in large part and other advantages obtained by injecting a stream of very finely divided metal compound capable of forming a s~able sulfate in contact with a cooled stream of regenerator flue gas containing air (o~ygen) added downstream from the regenerator cyclones, whereby Sx acceptor and flue gas are contacted at a lower temperature (lower Sx vapor pressure) than that prevailing in the regenerator proper and in the presence of oxygen that could not be in~roduced upstream of a regenerator operating in conventional mode. It ls preferred ~hat the reduced temperature contact of ~solid acceptor with flue gas be on a full-flow basis; tha-t is, the total stream of flue gas be contacted at reduced temperature with the total stream of particulate solid acceptor as shown in the annexed drawing. The invention will be further discussed with respect to that full-flow basis, but it will be apparent to those skilled in the art how ~he invention may be applied to a lesser stream of flue gas by diverting a portion of flue gas directly ~35~
1 to heat recovery and stack.
For the purposes discussed, the invention contemplates conveying regenerator vapors separated from catalyst particles in whole or part, through a heat exchange zone, such as a heat exchanger or steam genera-tor, to reduce the temperature of the vapors below about 1300F., and preferably belo~ 1100F. ~or a conventi~nal regenerator. Preferably the g~s will be cooled at least 100F. below the temperature at the point at which the flue gas and regenerated catalyst were separated itl the disengaging zone.
This value is subject to variation depending principally on the chemical composition of the solid sulfur oxide acceptor. When the regenerator is operated to produce incomplete combustion of the coked catalyst particles, and temperatures are at least about 1100F. and up to about 1250F. at the point of separating flue gas from regenerated catalyst, the flue gas will be cooled to a temperature most preferably below 1100F., for example to a temperature in the range of about 400 to 1050F. On the o-ther hand, when the regenerator is operated to maintain a temperature above 1300F., up to about 1~50F., and regenerating air is used to achieve major conver-sion of CO to CO2, there is no problem of CO combustion in the flue gas lines or other equipment. In this case flue gas can be cooled to tempera-tures as low as about 400F. to below 1300F.~ e.g., to 400F. to 1250F.
It will be apparent that a solid acceptor capable of binding SO in the presence of oxygen will take up those compounds in the flue gas line in an amount to approximate thermodynamic equilibrium with the regenerator flue gas under conditions of contact time and temperature conducive to establishing equilibrium. At the lower temperature of contact with cooled regenerator flue gas, the Sx acceptor will be capable of picking up more Sx than would be possible if the contact were at the normal temperature of flue gas exiting the disengaging zone of the regenerator.
It will also be apparent that it is essential to precool flue ` , .35;~
1 gas contalning CO before introducing air to promote S0 pickup since afterburning would result if the reverse sequence were to be followed.
It is immaterial from a theoretical point of view whether cooling is applied before the solid Sx acc~ptor is introduced into the flue gases.
However, for practical reasons it is preferred that cooling be applied to the flue gas in the absence of solid S0 acceptor. The degree of cooling f~om regenerator temperature will be significant, at least 25F.
and preferably at least 100F. as mentioned above, in order that an acceptance of SO by the solid acceptor shall be increased a significant amount to achieve the advarLtage of increased Sx uptake at reduced temperature.
The solid reactant can be introduced into the regenerator in well-known manner when the regenerator is one that operates in full combustion mode with the proviso that such material is either initially (as charged) in the form of particles 40 microns or finer, preferably 20 microns or finer, or that the material is introduced as particles attri-table in the regenerator to particles 40 microns or finer. When the re-generator is operated under conditions of incomplete combustion and flue gases contain carbon monoxide, the solid sulfur oxide acceptor is preferably introduced in the flue gas line in the form of particles 40 microns or finer when the acceptor includes one or more components capable of promoting or otherwise interfering with the operation of the regenerator.
An example would be a material capable of promoting combustion by catalytic or other effect. When the regenerator is operated under the incomplete combustion mode and the acceptor does not contain such interfering material, it is entirely feasible and within the scope of the invention to inject the acceptor into the regenerator. Examples of acceptors that can be utilized in this matter are those composed of a Group II metal compound, e.g., calcium and/or magensium oxide.
To facilitate separation of the sulfur oxide - loaded acceptor ~3S~
1 particles from sulfur oxide depleted flue gas, it may be advantageous to in~ject into the flue gas acceptor particles that are significantly larger than 40 microns, for example particles in the size range of 1/8" to 60 microns. The upper limit of particle size is dictated by the need to employ particles capable of being suspended in the flue gas and conveyed therein.
This size ~7ill ~herefore vary with the velocity of the gas in the flue gas line. A suitable particle size may be determined applying considerations well within the skill of the art.
A large number of oxides and combinations of oxides for reaction with Sx are described in patents and technical literature such as cited above. These are all potentially capable of being utilized with improved results when principles of this invention are applied. In general, the oxides are those which form sulfates with SO in the presence of free ~ oxygen, the sulfates being stable at the tempera~ure of cooled gases pro-; 15 duced in a regenerator. The solids are "stable" in the sense that they do not melt, sublime or decompose at such temperatures. Among the oxides earlier described for the purpose mentioned and which may be used in practice of the invention are reactive oxides of alumina as described in 0 4,071,436; oxides of Group IIA metals, typified by calcium and magnesium set forth in patent 3,699,037; cerium oxides as described in patent 4,001,375; and the several metal components described in German Offenlegungschrift DT 2657403, including compounds of sodium , scandium, titanium, iron, chromium, molybdenum, magnesium, cobalt, nickel, antimony, copper, zinc, cadmium, rare earth metals and lead. These metals are of varying effectiveness at different temperatures and application of the knowledge and skill of the art to the conditions of a particular situation will indicate the optimum temperature for use.
; The invention expands considerably upon the composition of metal compounds that can be used as a practical manner since there is no need to restrict compound to a metal oxide which will repeatedly bind .
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S0 in flue gas and release bound SO as H2S in the reactor since the Sx acceptor is not cycled to the reac~or in practice of the invention and it need not be associated with catalyst inventory in the regenerator. Thus inexpensive materials such as Group II metal or other Group II metal compounds such as hydroxides and carbonates can now be employed in effective manner. Oxides which would have an adverse effect on the cracking operatlon such as sodium aluminates or caustic treated bauxite ore are usable.
When practicing the invention to reduce Sx emissions in a unit in which regeneration is operated under conventional mode and the flue gas containing C0 is sent to a C0 boiler, it will be necessary to avoid use of an acceptor such as alumina or rare earth metals that will decompose to release S0 in the boiler. For this application, compounds of Group II metals, especially calcium oxide, magnesium oxide, or mixtures thereof, are recommended.
In representative practice of the present invention in a conventional FCC unit operating at a 1200F. dense phase regenerator temperature and a 1250F. flue gas temperature with the two stage regenerator cyclones discharging at 105 fps into a plenum chamber, water would be injected into the plenum chamber to cool the flue gas below 1100F. to eliminate the possibility of burning carbon monoxide when the air is injected into the flue gas line. The flue gas line superficial velocity of about 100 fps insures good mixing of the air as it is injected into the entrance of the flue gas line just downstream of the regenerator plenum chamber to control the oxygen level at 2 volume percent. At the first 90 elbow in the flue gas line the Sx absorber, for example minus 20 micron gamma alumina, is introduced to react with the Sx to form a stable sulfate. This material is entrained with the flue gas through the Elue gas slide valve, pressure reducing chamber, the C0 boiler and finally into the flue gas electrostatic precipitator where it is recovered ~.35~
1 with the catalyst fines.
It is to be understood that the foregoing description is merely illustrative of preferred embodiments of the invention of which many variations may be made by those skilled in the art within the scope of the following claims without departing from the spirit thereof.
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4,115,250 and 4,115,251. Use of oxidants including platinum or chromium as adjuncts to alumina is suggested in these patents. A related use of cerium oxide or an alumina support is described in `U. S. 4,001,375.
In the prior art techniques aforementioned, emphasis was on reversibly reacting sulfur oxides in the flue gas, and doing so while '10 the gases were still in the regenerator. Since the sulfur loade~ parti cles were carried to the reactor to be converted to gaseous hydrogen sulfide under the reducing atmosphere created by the cracking operation, the agents used to bind and then release sulfur were necessarily limited to those in-- herently capable of doing so under the constraints of temperature and time imposed by the operation of the reactor and the regenerator. Such procedures offer promise as means to reduce Sx emissions from refineries but they leave much to be desired.
; With units operating with high sulfur feedstock, relatively large amounts of sulfur acceptor are needed to accomplish reductions in su]fur oxide levels. This will result in appreciable dilution of the active catalyst in the cracking reaction cycle whether the sulfur acceptor is a part of the catalyst particles or is present as discrete entities circulated with catalyst inventory. A basic limitation is that conditions of time and temperature for operating cyclic cracking units, especially heat balanced FCC units, are geared to maximizing production of desired ;~ products and conditions that will favor this result, are by no meansthose that are optimum for reversibly reacting sulfur oxides in the regenerator and carrying the sulfur back to the reactor for conversion ` `
at least in part to hydrogen sulfide. The generally recognized need or ; desirability oE having excess oxygen in the regenerator beyond that needed ~ ~3~;2~3~
to burn coke, in order to convert oxides to sulfur to trioxide which will then react with metal oxide to form a sulfate, imposes severe limitations on those refineries that operate regenerators in conventional m~de, i.e., without full combustion. rmus, introduction of air (oxygen) to promote SO~ association with metal oxide is basically inconsistent with the operation of a regenerator in conventional im~omplete combustion mode.
SUMM~RY OF THE INVENTION
According to the present invention, there is provided in a process for cata.lytic cracking of a sulfur-containing hydrocarkon charge by oontacting said charge at cracking temperature with a circulating inventory of cracking catalyst whereby the catalyst acquires an inactivating carkonaceous deposit c~ntaIningsulfur~ separating vaporous ~roducts of reaotion including hydrogen sulfide from circulating catalyst inventory containing said deposit,regenerating the so separated inventory by contact with air at a temperature to burn said carbonaceous deposit thus generating oxides of carbon and sulfur and regenerating -the catalyst, separating products of combustion from regenerated catalyst and returning regenerated catalyst to renewed CGntact with hydrocarhon charge; the improvement which comprises contact.ing said products of combustion after separation from said regenerated catalyst at a temperature substantially below the temperature of such separation and in the presence of oxygen with a particulate solid capable of assoclating with gaseous oxides of sulfur in an oxidizing atmosphere to thereby induce transfer of said oxides of sulfur from said particulate solid and separating said particulate solid with transferred oxides of sulfur from said products of c~mbustion now containing a reduced content of oxides of sulfur.
Thus, the efficiency of removing Sx from FCC flue gas by solid particulate sulf.ur oxide acceptors such as reactive metal oxides is im-proved in accordance with the invention by oontacting oxygen-conta.ining flue gas with particulate sulfur oxide acceptor at a temperature kelow that prevailing upstream of cyclones in the regenerator. That principle is appl.ied in accordance with this invention by cooling the regenerator flue ~L1352(~6 gases downstream of cyclones, separating regenerated catalyst from ccmbustion gases, injecting air (oxygen) into the cooled flue gas to provide sufficient ~ree oxygen for oxidation of SOx, unless the flue gas already contains such amount of oxygen, and contacting the oooled oxygen-containing flue gases with finely divided suspendable particulate solid sulfur oxide acceptor. Usable sulfur oxide acceptors ~mbrace those materials which will associate with Sux in cooled o~ygen-containing regenerator flue gases to form a solid material that is stable at the temperature of contact. Contact between particulate sulfur acceptor and oooled flue gas is carried out in a manner such that the particulate solid is suspended in the gases and is conveyed therein, preferably under turbulent conditions, to transfer Sx in flue gas to the solid particles.
The particulate sulfur oxide acceptor with bou~nd Sx is then separated from flue gases now having a reduced Sx content. Optionally the separated solid sulfur oxide acceptor with bound Sx is processed external to the regenerator and external to the reactor by thermal or chemi-10a . :
S~6 1 cal means or a combination thereof to restore the solid to a state or condition whereby it is once again capable of being recylced in cooled flue gas in the presence of oxygen to pick up S0 .
In one embodiment, the invention is practiced to reduce S0 emissions from an FCC regenerators operating in conventional mode, i.e., regenerators emit~ing flue gases containing less than 0 5~ 2 and containing at least 500 ppm C0. The flue gases are cooled to a temperature below 1300F., preferably below 1100F., downstream from regenerator cyclones and then air or oxygen is injected external to the regenerator whereby C0 combustion is precluded and interference with the operation of the regenerator is avoided. Practice of the invention permits intro-duction of oxygen to regenerator gases while avoiding afterburning or change-over to complete combustion. Suspendable particles of metal oxide may be injected and suspended in the oxygen-enriched regenerator ; gases external to the regenerator. Contrary to prior art practice, S0 pickup takes place downstream of regenerators and reactors, e.g., in flue gas lines, and the S0 acceptor is not cycled between reactor and regenerator. When acceptor particles of suitable chemical composition are used they may be introduced into the regenerator, but they are not recycled to the reactor as in prior art practice.
In another embodiment, principles of the invention are utilized to reduce S0 in flue gas from regenerators operated in complete C0 combustion mode (i.e., at least 0-5% 2 and less than 500 ppm C0). The metal oxide is contacted with flue gases previously cooled below about 1300F., downstream of the cyclones. Air (oxygen) is injected into the cooled flue gases if necessary. Metal oxide is introduced to the cooled flue gas either e~ternal to the regenerator te.g., in the flue gas line) or into the regenerator at any suitable point.
By the system of the invention, all or essentially all finely d1v~ded particles Oe the S0 accep~ are removed with flue gases and ., ~ ~3~;2~
1 the agent for removing SO is not present in the reactor as a diluent which reduces activity of the catalyst charge in the reactor. The invention may be practiced in a manner such that the agent for removing SO is als~o not present in the regenerator. This also represents a departure and offers advantages over those prior art techniques which re~uire the presence of the agent for binding SO in the regenerator, thus placing limitations on the composition and quantity of such agent that could be used without impairing the operation of the regenerator.
Another major point of distinction is that in the present invention oxygen conducive to conversion of gaseous oxides of sulfur to stable solid sulfate formation is introduced into previously cooled flue gas. This prevents undesired temperature rise in the regenerator with the potential of afterburning or excessive temperature rise. The temperature to which the flue gas can be cooled can be flexibly altered without upsettlng the operation of the regenerator to facilitate utili-zation of any selected Sx acceptor at a temperature thermodynamically most favorable to that acceptor. This flexibility was not possible in ~-the past because of constraints imposed by the need to utilize the sulfur acceptor under conditions dictated by the need to operate units in heat 2~
balance. Improved opportunity for SO association is provided because the solid acceptor can be conslderably finer than would be possible if such material or a major part had to be retained in the regenerator and the reactor. Additionally the invention a~fords favorable contact conditions for transferring Sx to a solid sulfur oxide acceptor by permitting contact of a finely divided solid with the flue gas stream under turbulent conditions.
DESCRIPTION OF T~IE DRAWING
Typical apparatus for practice of the invention is shown in the diagrammatic elevation in the single figure of a typical regenerator ~L3L3S~
1 section of an FCC annexed hereto. As illus~rated in that diagram, conduit 2 is used to transfer hot stripped catalyst from the lower portion of the reactor stripping zone to the regeneration zone shown in the drawing.
The regenerator may be of any desired style, but is prelerably designed and operated to yield regenerated catalyst at minimum residual coke level after regeneration in either a conventional or complete combus~ion modes.
A modern version of regeneration is characterized by "fast fluid"
riser to which catalyst suspended in regeneration gas is supplied from a dense fluidized bed undergoing regeneration at high temperature. The spent catalyst from the reactor and hot catalyst separated at the top of the riser are both introduced in the dense fluidized bed where the "fire" for regenerating spent catalyst is lighted by the hot regenerated catalyst so recycled. See Patents 3,893,812 and 3,926,778.
As seen in the annexed drawing, spent catalyst passes by conduit 2 to a lower section of the regenerator, where it is fluidized along with the recycled hot regenerated catalyst from conduit 3 with air introduced by conduit 4. The resulting mixture passes upwards through conduit 4 to an enlarged lower section 5 of the regenerator where it enters a dense fluid-ized bed maintained by the regeneration gas. Catalyst from the dense bed in section 5 is entrained by hot regeneration gas to pass upward through a riser 6 to discharge port 7 into enlarged disengaging zone 8. Disengaging zone 8 may be equipped with a plllrality of cyclone separator combinations comprising first and second cyclonic separation means attached or spaced apart from the riser discharge for separating catalyst particles from the regenerat:Lon gas, as is customary in the art. A single state of separation is shown. Catalyst separated from regeneration gas in cyclone 9 flows downward through dipleg 10 and collects as a fluidized bed in the lower portion of disengaging zone 8 about riser 6. A portion of the hot catalyst collected is recycled by line 3 back to the bed in the lower section 5 for the purpose stated. Another portion for return to the ~ ~3~ 16 l reac-tor section is w~thdrawn by line 11.
Hot regenerator fume essentially free of catalyst exists the cyclonic separation 9 and enters plenum chamber area 12 where the fume can be quenched to the desired temperature by water or steam through the temperature controller sensor located in the flue gas line 13, which con- ;
trols the valve 14 in the water/steam line. In conjunction with this quench system or as an alternate system, steam generator 15 could be used to lower the fume to the desired temperature.
After quenching to below 1300F. in the complete combustion mode or below 1100F. in the conventional mode, air or oxygen is injected : ~
through line 16 into flue gas line 13 controlled at between 0.5 and 3 ~ -mole percent oxygen by oxygen analyzer 17. The S0 acceptor is injected through line 18. Its rate of injection is determined by the S0 analyzer 19 so that the exiting flue gas after treatment for removal of the Sx acceptor particles is essentially free of S0 .
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1 DESC~IPTION OF SPECIFIC EMBODIME~TS
In preferred embodiments, the invention contemplates cooling regenerator flue gas from FCC urits and contacting the cooled flue gas in the presence of free oxygen with a finely divided solid metal compound capable of reacting with or sorbing Sx under conditions to favor transfer of Sx from the flue gas to the finely divided solid metal compound. A
typical metal sulfate formed by the contact exhibits a vapor pressure which determines the minimum level of SO which must exist in the vapor phase for any particular temperature. The higher the temperature, the higher the vapor pressure resulting in a higher concentration of Sx in the vapor phase at the expense of sulfur bound to the solid sulfur acceptor. The amount of sulfur removed as a stable solid oxide of sulfur, e.g., sulfate, by the sulfur acceptor upon separation from the flue gases is constrained by the temperature and contact conditions between the flue gas and solid acceptor.
That constraint is avoided in large part and other advantages obtained by injecting a stream of very finely divided metal compound capable of forming a s~able sulfate in contact with a cooled stream of regenerator flue gas containing air (o~ygen) added downstream from the regenerator cyclones, whereby Sx acceptor and flue gas are contacted at a lower temperature (lower Sx vapor pressure) than that prevailing in the regenerator proper and in the presence of oxygen that could not be in~roduced upstream of a regenerator operating in conventional mode. It ls preferred ~hat the reduced temperature contact of ~solid acceptor with flue gas be on a full-flow basis; tha-t is, the total stream of flue gas be contacted at reduced temperature with the total stream of particulate solid acceptor as shown in the annexed drawing. The invention will be further discussed with respect to that full-flow basis, but it will be apparent to those skilled in the art how ~he invention may be applied to a lesser stream of flue gas by diverting a portion of flue gas directly ~35~
1 to heat recovery and stack.
For the purposes discussed, the invention contemplates conveying regenerator vapors separated from catalyst particles in whole or part, through a heat exchange zone, such as a heat exchanger or steam genera-tor, to reduce the temperature of the vapors below about 1300F., and preferably belo~ 1100F. ~or a conventi~nal regenerator. Preferably the g~s will be cooled at least 100F. below the temperature at the point at which the flue gas and regenerated catalyst were separated itl the disengaging zone.
This value is subject to variation depending principally on the chemical composition of the solid sulfur oxide acceptor. When the regenerator is operated to produce incomplete combustion of the coked catalyst particles, and temperatures are at least about 1100F. and up to about 1250F. at the point of separating flue gas from regenerated catalyst, the flue gas will be cooled to a temperature most preferably below 1100F., for example to a temperature in the range of about 400 to 1050F. On the o-ther hand, when the regenerator is operated to maintain a temperature above 1300F., up to about 1~50F., and regenerating air is used to achieve major conver-sion of CO to CO2, there is no problem of CO combustion in the flue gas lines or other equipment. In this case flue gas can be cooled to tempera-tures as low as about 400F. to below 1300F.~ e.g., to 400F. to 1250F.
It will be apparent that a solid acceptor capable of binding SO in the presence of oxygen will take up those compounds in the flue gas line in an amount to approximate thermodynamic equilibrium with the regenerator flue gas under conditions of contact time and temperature conducive to establishing equilibrium. At the lower temperature of contact with cooled regenerator flue gas, the Sx acceptor will be capable of picking up more Sx than would be possible if the contact were at the normal temperature of flue gas exiting the disengaging zone of the regenerator.
It will also be apparent that it is essential to precool flue ` , .35;~
1 gas contalning CO before introducing air to promote S0 pickup since afterburning would result if the reverse sequence were to be followed.
It is immaterial from a theoretical point of view whether cooling is applied before the solid Sx acc~ptor is introduced into the flue gases.
However, for practical reasons it is preferred that cooling be applied to the flue gas in the absence of solid S0 acceptor. The degree of cooling f~om regenerator temperature will be significant, at least 25F.
and preferably at least 100F. as mentioned above, in order that an acceptance of SO by the solid acceptor shall be increased a significant amount to achieve the advarLtage of increased Sx uptake at reduced temperature.
The solid reactant can be introduced into the regenerator in well-known manner when the regenerator is one that operates in full combustion mode with the proviso that such material is either initially (as charged) in the form of particles 40 microns or finer, preferably 20 microns or finer, or that the material is introduced as particles attri-table in the regenerator to particles 40 microns or finer. When the re-generator is operated under conditions of incomplete combustion and flue gases contain carbon monoxide, the solid sulfur oxide acceptor is preferably introduced in the flue gas line in the form of particles 40 microns or finer when the acceptor includes one or more components capable of promoting or otherwise interfering with the operation of the regenerator.
An example would be a material capable of promoting combustion by catalytic or other effect. When the regenerator is operated under the incomplete combustion mode and the acceptor does not contain such interfering material, it is entirely feasible and within the scope of the invention to inject the acceptor into the regenerator. Examples of acceptors that can be utilized in this matter are those composed of a Group II metal compound, e.g., calcium and/or magensium oxide.
To facilitate separation of the sulfur oxide - loaded acceptor ~3S~
1 particles from sulfur oxide depleted flue gas, it may be advantageous to in~ject into the flue gas acceptor particles that are significantly larger than 40 microns, for example particles in the size range of 1/8" to 60 microns. The upper limit of particle size is dictated by the need to employ particles capable of being suspended in the flue gas and conveyed therein.
This size ~7ill ~herefore vary with the velocity of the gas in the flue gas line. A suitable particle size may be determined applying considerations well within the skill of the art.
A large number of oxides and combinations of oxides for reaction with Sx are described in patents and technical literature such as cited above. These are all potentially capable of being utilized with improved results when principles of this invention are applied. In general, the oxides are those which form sulfates with SO in the presence of free ~ oxygen, the sulfates being stable at the tempera~ure of cooled gases pro-; 15 duced in a regenerator. The solids are "stable" in the sense that they do not melt, sublime or decompose at such temperatures. Among the oxides earlier described for the purpose mentioned and which may be used in practice of the invention are reactive oxides of alumina as described in 0 4,071,436; oxides of Group IIA metals, typified by calcium and magnesium set forth in patent 3,699,037; cerium oxides as described in patent 4,001,375; and the several metal components described in German Offenlegungschrift DT 2657403, including compounds of sodium , scandium, titanium, iron, chromium, molybdenum, magnesium, cobalt, nickel, antimony, copper, zinc, cadmium, rare earth metals and lead. These metals are of varying effectiveness at different temperatures and application of the knowledge and skill of the art to the conditions of a particular situation will indicate the optimum temperature for use.
; The invention expands considerably upon the composition of metal compounds that can be used as a practical manner since there is no need to restrict compound to a metal oxide which will repeatedly bind .
~13~
S0 in flue gas and release bound SO as H2S in the reactor since the Sx acceptor is not cycled to the reac~or in practice of the invention and it need not be associated with catalyst inventory in the regenerator. Thus inexpensive materials such as Group II metal or other Group II metal compounds such as hydroxides and carbonates can now be employed in effective manner. Oxides which would have an adverse effect on the cracking operatlon such as sodium aluminates or caustic treated bauxite ore are usable.
When practicing the invention to reduce Sx emissions in a unit in which regeneration is operated under conventional mode and the flue gas containing C0 is sent to a C0 boiler, it will be necessary to avoid use of an acceptor such as alumina or rare earth metals that will decompose to release S0 in the boiler. For this application, compounds of Group II metals, especially calcium oxide, magnesium oxide, or mixtures thereof, are recommended.
In representative practice of the present invention in a conventional FCC unit operating at a 1200F. dense phase regenerator temperature and a 1250F. flue gas temperature with the two stage regenerator cyclones discharging at 105 fps into a plenum chamber, water would be injected into the plenum chamber to cool the flue gas below 1100F. to eliminate the possibility of burning carbon monoxide when the air is injected into the flue gas line. The flue gas line superficial velocity of about 100 fps insures good mixing of the air as it is injected into the entrance of the flue gas line just downstream of the regenerator plenum chamber to control the oxygen level at 2 volume percent. At the first 90 elbow in the flue gas line the Sx absorber, for example minus 20 micron gamma alumina, is introduced to react with the Sx to form a stable sulfate. This material is entrained with the flue gas through the Elue gas slide valve, pressure reducing chamber, the C0 boiler and finally into the flue gas electrostatic precipitator where it is recovered ~.35~
1 with the catalyst fines.
It is to be understood that the foregoing description is merely illustrative of preferred embodiments of the invention of which many variations may be made by those skilled in the art within the scope of the following claims without departing from the spirit thereof.
:~ ' ., .
,: .
-:
Claims (21)
1. In a process for catalytic cracking of a sulfur-containing hydrocarbon charge by contacting said charge at cracking temperature with a circulating inventory of cracking catalyst whereby the catalyst acquires an inactivating carbonaceous deposit containing sulfur, separating vaporous products of reaction including hydrogen sulfide from circulating catalyst inventory containing said deposit, regenerating the so separated inventory by contact with air at a temperature to burn said carbonaceous deposit thus generating oxides of carbon and sulfur and regenerating the catalyst, separating products of combustion from regenerated catalyst and returning regenerated catalyst to renewed contact with hydrocarbon charge;
the improvement which comprises contacting said products of combustion after separation from said regenerated catalyst at a temperature substantially below the temperature of such separation and in the presence of oxygen with a particulate solid capable of associating with gaseous oxides of sulfur in an oxidizing atmosphere to thereby induce transfer of said oxides of sulfur from said particulate solid and separating said particulate solid with transferred oxides of sulfur from said products of combustion now containing a reduced content of oxides of sulfur.
the improvement which comprises contacting said products of combustion after separation from said regenerated catalyst at a temperature substantially below the temperature of such separation and in the presence of oxygen with a particulate solid capable of associating with gaseous oxides of sulfur in an oxidizing atmosphere to thereby induce transfer of said oxides of sulfur from said particulate solid and separating said particulate solid with transferred oxides of sulfur from said products of combustion now containing a reduced content of oxides of sulfur.
2. A fluid catalytic cracking process according to claim 1 wherein said contact with hydrocarbon charge and said contact with air are conducted by suspending the catalyst inventory in charge and in air, respectively.
3. A process according to claim 2 wherein said particulate solid is finer than 40 microns or is attritable to particles finer than 40 microns when contacted with said products of combustion.
4. A process according to claim 2 wherein said regeneration is carried out under conditions of incomplete combustion whereby said products of combustion include appreciable carbon monoxide.
5. A process according to claim 2 in which said flue gas is cooled to a temperature in the range of about 400 to 100°F.
6. A process according to claim 2 wherein said particulate solid is injected in a flue gas line conveying cooled products of combustion and is suspended therein and the suspension is subjected to turbulent flow.
7. A process according to claim 2 wherein said particulate solid is a metal oxide finer than 40 microns.
8. A process in accordance with claim 2 wherein said flue gas is cooled by injection of water or steam.
9. A process in accordance with claim 2 wherein said flue gas is cooled in a heat exchanger.
10. A fluid catalytic cracking process according to claim 4 wherein said separated products of combustion from the regenerator are discharged in a flue gas line wherein they are sequentially cooled to a temperature below 1300°F., oxygen is injected without burning carbon monoxide, and particulate solid is injected for reactive contact with oxides of sulfur therein in a manner such as to be suspended in and conveyed in the flue gas line by said products of combustion.
11. A process according to claim 4 in which said flue gas is cooled to a temperature below about 1100°F.
12. A process according to claim 4 wherein said particulate solid comprises an oxide of an alkaline earth metal.
13. A process according to claim 4 wherein said particulate solid suspended in said flue gas is charged to a carbon monoxide boiler and said particulate solid is an oxide of a metal which forms a sulfate which does not decompose in the carbon monoxide boiler.
14. A fluid cracking process according to claim 4 wherein said regenerator is operated under conditions such that said products of combustion include more than 500 ppm CO, and said products of combustion are cooled to a temperature below 1100°F. in a flue gas line wherein air is subsequently injected, while in said flue gas line particulate solid is injected, and then suspended and conveyed under turbulent conditions in the flue gas thus cooled, said particulate solid being an alkaline earth oxide in the form of particles finer than 40 microns.
15. A process according to claim 14 wherein said suspension of particulate solid in flue gas is charged to a CO boiler before separation of said particulate solid from products of combustion.
16. A process in accordance with claim 2 wherein said regeneration is carried out under conditions of complete combustion whereby said products of combustion contain free oxygen.
17. A process in accordance with claim 16 wherein said products of combustion are cooled to a temperature below 1300°F., and additional oxygen is introduced external to the regeneration zone.
18. A process in accordance with claim 16 wherein said particulate solid is a metal oxide introduced downstream of regenerator cyclones.
19. A process in accordance with claim 16 wherein said particulate solid is introduced during regeneration and is in the form of particles 40 microns or finer or particles attritable to particles 40 microns or finer during regeneration.
20. A process in accordance with claim 10 wherein said particu-late solid is introduced into the regenerator.
21. A process in accordance with claim 16 wherein said particu-late solid injected in said flue gas line is composed of particles pre-dominantly larger than 40 microns.
30720.01 -23-
30720.01 -23-
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US2601179A | 1979-04-02 | 1979-04-02 | |
| US26,011 | 1979-04-02 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1135206A true CA1135206A (en) | 1982-11-09 |
Family
ID=21829336
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000345482A Expired CA1135206A (en) | 1979-04-02 | 1980-02-13 | Control of emissions in fcc regenerator flue gas |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1135206A (en) |
-
1980
- 1980-02-13 CA CA000345482A patent/CA1135206A/en not_active Expired
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