PRODUCTION OF OLEFINS

Abstract: A process for the catalytic cracking of a olefin-rich feedstock which is selective towards light olefins in the effluent, the process comprising contacting a hydrocarbon feedstock containing one or more olefins, with an MEL-type crystalline silicate catalyst, which has been subjected to a steaming step and has a silicon/aluminium atomic ratio of from 150 to 800, at an inlet temperature of from 500 to 600 DEG C, at an olefin partial pressure of from 0.1 to 2 bars and the feedstock being passed over the catalyst at an LHSV of from 10 to 30h<-1>, to produce an effluent with an olefin content of lower molecular weight than that of the feedstock.

Description: PRODUCTION OF OLEFINS The present invention relates to a process for cracking an olefm-πch hydrocarbon feedstock which is selective towards light olefins m the effluent. In particular, olefmic feedstocks from refineries or petrochemical plants can be converted selectively so as to redistribute the olefin content of the feedstock m the resultant effluent. It is known m the art to use zeolites to convert long chain paraffins into lighter products, for example m the catalytic dewaxmg of petroleum feedstocks. While it is not the objective of dewaxmg, at least parts of the paraffinic hydrocarbons are converted into olefins. It is known m such processes to use crystalline silicates for example of the MFI or MEL type, the three- letter designations "MFI" and "MEL" each representing a particular crystalline silicate structure type as established by the Structure Commission of the International Zeolite Association. Examples of a crystalline silicate of the MFI type are the synthetic zeolite ZSM-5 and silicalite and other MFI type crystalline silicates are known m the art. An example of a crystalline silicate of the MEL type is the synthetic zeolite ZSM-11. GB-A- 1323710 discloses a dewaxmg process for the removal of straight-chain paraffins and slightly branched-chain paraffins, from hydrocarbon feedstocks utilising a crystalline silicate catalyst, m particular ZSM-5. US-A-4247388 also discloses a method of catalytic hydrodewaxmg of petroleum and synthetic hydrocarbon feedstocks using a crystalline silicate of the ZSM-5 type. Similar dewaxmg processes are disclosed m US-A-4284529 and US-A-5614079. The catalysts are crystalline alummo- silicates and the above- identified prior art documents disclose the use of a wide range of Si/Al ratios and differing reaction conditions for the disclosed dewaxmg processes. GB-A-2185753 discloses the dewaxmg of hydrocarbon feedstocks using a silicalite catalyst. US-A-4394251 discloses hydrocarbon conversion with a crystalline silicate particle having an aluminium-containing outer shell. It is also known m the art to effect selective conversion of hydrocarbon feeds containing straight-chain and/or slightly branched-chain hydrocarbons, m particular paraffins, into a lower molecular weight product mixture containing a significant amount of olefins. The conversion is effected by contacting the feed with a crystalline silicate known as silicalite, as disclosed m GB-A-2075045 , US-A-4401555 and US-A-4309276. Silicalite is disclosed m US-A-4061724. Silicalite catalysts exist having varying silicon/aluminium atomic ratios and different crystalline forms. EP-A-0146524 and 0146525 the name of Cosden Technology, Inc. disclose crystalline silicas of the silicalite type having monoclmic symmetry and a process for their preparation. These silicates have a silicon to aluminium atomic ratio of greater than 80. WO-A-97/04871 discloses the treatment of a medium pore zeolite with steam followed by treatment with an acidic solution for improving the butene selectivity of the zeolite catalytic cracking . A paper entitled "De-alummation of HZSM-5 zeolites: Effect of steaming on acidity and aromatization activity", de Lucas et al , Applied Catalysis A: General 154 1997 221-240, published by Elsevier Science B.V. discloses the conversion of acetone/n- butanol mixtures to hydrocarbons over such dealummated zeolites. It is yet further known, for example from US-A-4171257, to dewax petroleum distillates using a crystalline silicate catalyst such as ZSM-5 to produce a light olefin fraction, for example a C 3 to C 4 olefin fraction. Typically, the reactor temperature reaches around 500°C and the reactor employs a low hydrocarbon partial pressure which favours the conversion of the petroleum distillates into propylene. Dewaxmg cracks paraffmic chains leading to a decrease m the viscosity of the feedstock distillates, but also yields a minor production of olefms from the cracked paraffins. EP-A-0305720 discloses the production of gaseous olefms by catalytic conversion of hydrocarbons. EP-B-0347003 discloses a process for the conversion of a hydrocarbonaceous feedstock into light olefms. WO-A-90/11338 discloses a process for the conversion of C 2 - 12 paraffmic hydrocarbons to petrochemical feedstocks, m particular to C 2 to C 4 olef s. US-A-5043522 and EP-A-0395345 disclose the production of olefms from paraffins havmg four or more carbon atoms. EP-A-0511013 discloses the production of olefms from hydrocarbons using a steam activated catalyst containing phosphorous and H-ZSM-5. US-A-4810356 discloses a process for the treatment of gas oils by dewaxmg over a silicalite catalyst. GB-A-2156845 discloses the production of isobutylene from propylene or a mixture of hydrocarbons containing propylene. GB-A-2159833 discloses the production of a isobutylene by the catalytic cracking of light distillates . It is known m the art that for the crystalline silicates exemplified above, long chain olefms tend to crack at a much higher rate than the correspondmg long chain paraffins. It is further known that when crystalline silicates are employed as catalysts for the conversion of paraffins into olefms, such conversion is not stable against time. The conversion rate decreases as the time on stream increases, which is due to formation of coke (carbon) which is deposited on the catalyst. These known processes are employed to crack heavy paraffmic molecules into lighter molecules. However, when it is desired to produce propylene, not only are the yields low but also the stability of the crystalline silicate catalyst is low. For example, m an FCC unit a typical propylene output is 3.5wt%. The propylene output may be increased to up to about 7-8wt% propylene from the FCC unit by introducing the known ZSM-5 catalyst into the FCC unit to "squeeze" out more propylene from the incoming hydrocarbon feedstock bemg cracked. Not only is this increase m yield quite small, but also the ZSM-5 catalyst has low stability in the FCC unit. There is an increasing demand for propylene m particular for the manufacture of polypropylene. The petrochemical industry is presently facing a major squeeze m propylene availability as a result of the growth m propylene derivatives, especially polypropylene. Traditional methods to increase propylene production are not entirely satisfactory. For example, additional naphtha steam cracking units which produce about twice as much ethylene as propylene are an expensive way to yield propylene since the feedstock is valuable and the capital investment is very high. Naphtha is m competition as a feedstock for steam crackers because it is a base for the production of gasoline m the refinery. Propane dehydrogenation gives a high yield of propylene but the feedstock (propane) is only cost effective during limited periods of the year, making the process expensive and limiting the production of propylene. Propylene is obtained from FCC units but at a relatively low yield and increasing the yield has proven to be expensive and limited. Yet another route known as metathesis or disproportionation enables the production of propylene from ethylene and butene. Often, combined with a steam cracker, this technology is expensive since it uses ethylene as a feedstock which is at least as valuable as propylene. EP-A-0109059 discloses a process for converting olefms having 4 to 12 carbon atoms into propylene. The olefms are contacted with an alummo-silicate having a crystalline and zeolite structure ( e . g. ZSM-5 or ZSM-11) and having a Sι0 2 /Al 2 0 3 molar ratio equal to or lower than 300. The specification requires high space velocities of greater than 50kg/h per kg of pure zeolite in order to achieve high propylene yield. The specification also states that generally the higher the space velocity the lower the Si0 2 /Al 2 0 3 molar ratio (called the Z ratio) . This specification only exemplifies olefin conversion processes over short periods ( e . g. a few hours) and does not address the problem of ensuring that the catalyst is stable over longer periods (e . g. at least 160 hours or a few days) which are required in commercial production. Moreover, the requirement for high space velocities is undesirable for commercial implementation of the olefin conversion process. Thus there is a need for a high yield propylene production method which can readily be integrated into a refinery or petrochemical plant, taking advantage of feedstocks that are less valuable for the market place (having few alternatives on the market) . On the other hand, crystalline silicates of the MFI type are also well known catalysts for the oligomerisation of olefins. For example, EP-A-0031675 discloses the conversion of olefin- containing mixtures to gasoline over a catalyst such as ZSM-5. As will be apparent to a person skilled in the art, the operating conditions for the oligomerisation reaction differ significantly from those used for cracking. Typically, in the oligomerisation reactor the temperature does not exceed around 400°C and a high pressure favours the oligomerisation reactions. GB-A-2156844 discloses a process for the isomerisation of olefins over silicalite as a catalyst. US-A-4579989 discloses the conversion of olefins to higher molecular weight hydrocarbons over a silicalite catalyst. US-A-4746762 discloses the upgrading of light olefins to produce hydrocarbons rich in C 5 + liquids over a crystalline silicate catalyst. US-A-5004852 discloses a two- stage process for conversion of olefins to high octane gasoline wherein in the first stage olefins are oligomerised to C 5 + olefins . US-A-5171331 discloses a process for the production of gasoline comprising oligomerising a C 2 -C 6 olefin containing feedstock over an intermediate pore size siliceous crystalline molecular sieve catalyst such as silicalite, halogen stabilised silicalite or a zeolite. US-A-4414423 discloses a multistep process for preparing high-boiling hydrocarbons from normally gaseous hydrocarbons, the first step comprising feeding normally gaseous olefins over an intermediate pore size siliceous crystalline molecular sieve catalyst. US-A-4417088 discloses the dimerising and trimerising of high carbon olefins over silicalite. US-A-4417086 discloses an oligomerisation process for olefins over silicalite. GB-A-2106131 and GB-A-2106132 disclose the oligomerisation of olefins over catalysts such as zeolite or silicalite to produce high boiling hydrocarbons. GB- A-2106533 discloses the oligomerisation of gaseous olefins over zeolite or silicalite. WO98/56740 discloses a process for converting a hydrocarbon feedstock to light olefins, using a zeolite catalyst free of added metal oxides with a hydrogenation/dehydrogenation function. The catalyst is a zeolite such as ZSM5 or ZSM11 (or others) and has an Si0 2 /Al 2 0 3 molar ratio of from 2:1 up to 2000:1. It is an object of the present invention to provide a process for using the less valuable olefins present in refinery and petrochemical plants as a feedstock for a process which, in contrast to the prior art processes referred to above, catalytically converts olefins into lighter olefins, and in particular propylene. It is another object of the invention to provide a process for producing propylene having a high propylene yield and purity. It is a further object of the present invention to provide such a process which can produce olefin effluents which are within, at least, a chemical grade quality. It is yet a further object of the present invention to provide a process for producing olefms having a stable olefmic conversion and a stable product distribution over time. It is yet a further object of the present invention to provide a process for converting olefmic feedstocks having a high yield on an olefm basis towards propylene, irrespective of the origin and composition of the olefmic feedstock. The present invention provides a process for the catalytic cracking of an olefm-πch feedstock which is selective towards light olefms m the effluent, the process comprising contacting a hydrocarbon feedstock containing one or more olefms, with an MEL-type crystalline silicate catalyst, which has been subjected to a steaming step and has a silicon/alum ium atomic ratio of from 150 to 800, at an let temperature of from 500 to 600°C, at an olefin partial pressure of from 0.1 to 2 bars and the feedstock being passed over the catalyst at an LHSV of from 10 to 30h λ , to produce an effluent with an olefin content of lower molecular weight than that of the feedstock. The present mvention can thus provide a process wherem olefm- rich hydrocarbon streams (products) from refinery and petrochemical plants are selectively cracked not only into light olefms, but particularly into propylene. The olefm-πch feedstock is passed over an MEL-type crystalline silicate catalyst, with a particular Si/Al atomic ratio and which has been steamed for example at a temperature of at least 300°C for a period of at least 1 hour with a water partial pressure of at least lOkPa. The feedstock may be passed over the catalyst at a temperature ranging between 500 to 600°C, an olefm partial pressure of from 0.1 to 2 bars and an LHSV of from 10 to SOh 1 . This can yield at least 30 to 50% propylene based on the olefm content m the feedstock, with a selectivity to propylene for the C 3 species propylene and propane (i.e. a percentage C 3 / (C 3 +C 3 ) ratio) of at least 92% by weight. In this specification, the term "silicon/alummium atomic ratio" is intended to mean the Si/Al atomic ratio of the overall material, which may be determined by chemical analysis. In particular, for crystalline silicate materials, the stated Si/Al ratios apply not just to the Si/Al framework of the crystalline silicate but rather to the whole material. The feedstock may be fed either undiluted or diluted with an inert gas such as nitrogen. In the latter case, the absolute pressure of the feedstock constitutes the partial pressure of the hydrocarbon feedstock in the inert gas . The various aspects of the present invention will now be described m greater detail however by example only with reference to the accompanying drawings, m which: - Figures la to lc show respectively the variation with time on stream for the olefm yield, the olefmicity and the yield on an olefms basis for components of an effluent which has been catalytically cracked m accordance with a first Example of the present invention; Figures 2a to 2c show respectively the variation with time on stream for the olefm yield, the olefmicity and the yield on an olefms basis for components of an effluent which has been catalytically cracked m accordance with a second Example of the present invention; Figures 3a to 3c show respectively the variation with time on stream for the olefin yield, the olefmicity and the yield on an olefms basis for components of an effluent which has been catalytically cracked m accordance with a third Example of the present invention; Figures 4a to 4c show respectively the variation with time on stream for the olefm yield, the olefmicity and the yield on an olefms basis for components of an effluent which has been catalytically cracked m accordance with a first Comparative Example ; Figures 5a to 5c show respectively the variation with time on stream for the olefm yield, the olefmicity and the yield on an olefms basis for components of an effluent which has been catalytically cracked in accordance with a second Comparative Example; Figures 6a to 6c show respectively the variation with time on stream for the olefm yield, the olefmicity and the yield on an olefms basis for components of an effluent which has been catalytically cracked in accordance with a third Comparative Example; and Figures 7a to 7c show respectively the variation with time on stream for the olefm yield, the olefmicity and the yield on an olefms basis for components of an effluent which has been catalytically cracked m accordance with a fourth Comparative Example . In accordance with the present invention, cracking of olefms is performed m the sense that olefms m a hydrocarbon stream are cracked into lighter olef s and selectively into propylene. The feedstock and effluent preferably have substantially the same olefm content by weight. Typically, the olefm content of the effluent is within ±15wt%, more preferably ±10wt%, of the olefm content of the feedstock. The feedstock may comprise any kind of olefm-containing hydrocarbon stream. The feedstock may typically comprise from 10 to 100wt% olefms and furthermore may be fed undiluted or diluted by a diluent, the diluent optionally including a non-olefimc hydrocarbon. In particular, the olef - contammg feedstock may be a hydrocarbon mixture containing normal and branched olefms m the carbon range C 4 to C 10 , more preferably m the carbon range C 4 to C 6 , optionally in a mixture with normal and branched paraffins and/or aromatics the carbon range C 4 to C 10 . Typically, the olefm-contammg stream has a boiling point of from around -15 to around 180°C. In particularly preferred embodiments of the present invention, the hydrocarbon feedstocks comprise C 4 mixtures from refineries and steam cracking units. Such steam cracking units crack a wide variety of feedstocks, including ethane, propane, butane, naphtha, gas oil, fuel oil, etc. Most particularly, the hydrocarbon feedstock may comprises a C 4 cut from a fluidized-bed catalytic cracking (FCC) unit in a crude oil refinery which is employed for converting heavy oil into gasoline and lighter products. Typically, such a C 4 cut from an FCC unit comprises around 50wt% olefm. Alternatively, the hydrocarbon feedstock may comprise a C 4 cut from a unit within a crude oil refinery for producing methyl tert -butyl ether (MTBE) which is prepared from methanol and isobutene. Again, such a C 4 cut from the MTBE unit typically comprises around 50wt% olefm. These C 4 cuts are fractionated at the outlet of the respective FCC or MTBE unit. The hydrocarbon feedstock may yet further comprise a C 4 cut from a naphtha steam-cracking unit of a petrochemical plant m which naphtha, comprising C 5 to C g species having a boiling point range of from about 15 to 180°C, is steam cracked to produce, inter alia , a C 4 cut. Such a C 4 cut typically comprises, by weight, 40 to 50% 1 , 3-butadιene, around 25% isobutylene, around 15% butene (m the form of but-1-ene and/or but-2-ene) and around 10% n- butane and/or isobutane. The olefm-containing hydrocarbon feedstock may also comprise a C 4 cut from a steam cracking unit after butadiene extraction (raffmate 1) , or after butadiene hydrogenation. The feedstock may yet further alternatively comprise a hydrogenated butadiene-rich C 4 cut, typically containing greater than 50wt% C 4 as an olefm. Alternatively, the hydrocarbon feedstock could comprise a pure olefm feedstock which has been produced m a petrochemical plant . The olef -containing feedstock may yet further alternatively comprise light cracked naphtha (LCN) (otherwise known as light catalytic cracked spirit (LCCS) ) or a C 5 cut from a steam cracker or light cracked naphtha, the light cracked naphtha being fractionated from the effluent of the FCC unit, discussed hereinabove, in a crude oil refinery. Both such feedstocks contain olefms. The olefm-containing feedstock may yet further alternatively comprise a medium cracked naphtha from such an FCC unit or visbroken naphtha obtained from a visbreakmg unit for treating the residue of a vacuum distillation unit in a crude oil refinery. The olefm-containing feedstock may comprise a mixture of one or more of the above-described feedstocks. The use of a C 5 cut as the olefm-containing hydrocarbon feedstock accordance with a preferred process of the invention has particular advantages because of the need to remove C 5 species m any event from gasolines produced by the oil refinery. This is because the presence of C 5 m gasoline increases the ozone potential and thus the photochemical activity of the resulting gasoline. In the case of the use of light cracked naphtha as the olefm-contammg feedstock, the olefm content of the remaining gasoline fraction is reduced, thereby reducing the vapour pressure and also the photochemical activity of the gasoline . When converting light cracked naphtha, C 2 to C 4 olefms may be produced m accordance with the process of the invention. The C 4 fraction is very rich in olefms, especially m isobutene, which is an interesting feed for an MTBE unit. When converting a C 4 cut, C 2 to C 3 olefms are produced on the one hand and C 5 to C 6 olefms containing mainly iso-olefins are produced on the other hand. The remaining C 4 cut is enriched m butanes, especially m isobutane which is an interesting feedstock for an alkylation unit of an oil refinery wherem an alkylate for use m gasoline is produced from a mixture of C 3 and C 5 feedstocks. The C 5 to C 6 cut containing mainly iso-olefins is an interesting feed for the production of tertiary amyl methyl ether (TAME) . Surprisingly, the present inventors have found that accordance with the process of the mvention, olefmic feedstocks can be cracked selectively m the presence of an MEL-type catalyst so as to redistribute the olefmic content of the feedstock m the resultant effluent. The catalyst and process conditions are selected whereby the process has a particular yield on an olefm basis towards a specified olefm m the feedstocks. Typically, the catalyst and process conditions are chosen whereby the process has the same high yield on an olefm basis towards propylene irrespective of the origin of the olefmic feedstocks for example the C 4 cut from the FCC unit, the C 4 cut from the MTBE unit, the light cracked naphtha or the C 5 cut from the light crack naphtha, etc. , This is quite unexpected on the basis of the prior art . The propylene yield on an olefm basis is typically from 30 to 50% based on the olefm content of the feedstock. The yield on an olefm basis of a particular olefm is defined as the weight of that olefm m the effluent divided by the initial total olefm content by weight. For example, for a feedstock with 50wt% olefm, if the effluent contains 20wt% propylene, the propylene yield on an olefm basis is 40%. This may be contrasted with the actual yield for a product which is defined as the weight amount of the product produced divided by the weight amount of the feed. The paraffins and the aromatics contained m the feedstock are only slightly converted m accordance with the preferred aspects of the invention. In accordance with the present mvention, the catalyst for the cracking of the olefms comprises a crystalline silicate of the MEL family which may be a zeolite or any other silicate m that family. An example of an MEL zeolite is ZSM-11 which is known m the art. Other examples are Boralite D and sιlιcalιte-2 as described by the International Zeolite Association (Atlas of zeolite structure types, 1987, Butterworths) . The preferred crystalline silicates have pores or channels defined by ten oxygen rings and a high silicon/alummium atomic ratio . Crystalline silicates are microporous crystalline inorganic polymers based on a framework of X0 4 tetrahedra linked to each other by sharing of oxygen ions, where X may be tπvalent ( e . g. Al,B,...) or tetravalent ( e . g. Ge, Si,...). The crystal structure of a crystalline silicate is defined by the specific order m which a network of tetrahedral units are linked together. The size of the crystalline silicate pore openings is determined by the number of tetrahedral units, or, alternatively, oxygen atoms, required to form the pores and the nature of the cations that are present m the pores. They possess a unique combination of the following properties: high internal surface area; uniform pores with one or more discrete sizes; ion exchangeability; good thermal stability; and ability to adsorb organic compounds. Since the pores of these crystalline silicates are similar m size to many organic molecules of practical interest, they control the ingress and egress of reactants and products, resulting m particular selectivity m catalytic reactions. Crystalline silicates with the MEL structure possess a bidirectional intersecting straight pore system with straight channels along [100] having pore diameters of 0.53-0.54 nm. The crystalline silicate catalyst has structural and chemical properties and is employed under particular reaction conditions whereby the catalytic cracking readily proceeds. Different reaction pathways can occur on the catalyst. Under the process conditions, having an mlet temperature of around 500 to 600°C, preferably from 520 to 600°C, yet more preferably 540 to 580°C, and an olefm partial pressure of from 0.1 to 2 bars, most preferably around atmospheric pressure, the shift of the double bond of an olefm the feedstock is readily achieved, leading to double bond isomerisation. Furthermore, such isomerisation tends to reach a thermodynamic equilibrium. Propylene can be, for example, directly produced by the catalytic cracking of hexene or a heavier olefmic feedstock. Olefmic catalytic cracking may be understood to comprise a process yielding shorter molecules via bond breakage. The catalyst preferably has a high silicon/alummium atomic ratio, greater than about 150, whereby the catalyst has relatively low acidity. Hydrogen transfer reactions are directly related to the strength and density of the acid sites on the catalyst, and such reactions are preferably suppressed so as to avoid the formation of coke during the olefm conversion process, and composition of the olefmic feedstock. Such high ratios reduce the acidity of the catalyst, thereby increasing the stability of the catalyst. Moreover, the use of Si/Al atomic ratios of greater than 150 has been found to increase the propylene selectivity of the catalyst, .e. to reduce the amount of propane produced. This increases the purity of the resultant propylene. If the Si/Al atomic ratio is above 800, the catalyst has been found to have low activity and stability with respect to the production of propylene by the catalytic cracking process. The catalyst for use m the catalytic cracking process of the present invention is manufactured by steaming an as-synthesised or commercially available crystalline silicate of the MEL-type. The MEL crystalline silicate catalyst for use m the mvention most typically comprises a ZSM-11 catalyst which may be synthesised either using diammooctane as the templatmg agent and sodium silicate as the silicon source or tetrabutyl phosphonium bromide as the templatmg agent and a silica sol as the silicon source. Thus the ZSM-11 catalyst may be prepared by mixing sodium silicate with 1,8 diammooctane together with aluminium sulphate to form a hydrogel which is then allowed to crystallise to form the crystalline silicate. The organic template material is then removed by calcining. Alternatively, the ZSM-11 catalyst is produced by reacting tetrabutyl phosphonium bromide and sodium hydroxide together with the silica sol prepared from colloidal silica. Again, a crystallisation is performed to produce the crystalline silicate and then the product is calcined. In order to reduce the sodium content of the crystalline silicate, the crystalline silicate is subjected to an ion exchange with a salt. Thereafter the material is dried. Typically, the crystalline silicate is subjected to ion exchange with ammonium ions, for example by immersing the crystalline silicate m an aqueous solution of NH 4 C1 or NH 4 N0 3 . Such an ion exchange step is desirable if the amount of sodium ions present in the crystalline silicate is so high that a crystalline sodium silicate phase is formed following calcination of the crystalline silicate which would be difficult to remove. In accordance with the present invention, the initial crystalline silicate is modified by a steaming process which, without being bound by theory, is believed to reduce the tetrahedral aluminium m the crystalline silicate framework and to convert the aluminium atoms into octahedral aluminium in the form of amorphous alumina. Although m the steaming step aluminium atoms are chemically removed from the crystalline silicate framework structure to form alumina particles, those particles appear not to migrate and so do not cause partial obstruction of the pores or channels the framework which would otherwise inhibit the olefmic cracking processes of the present mvention. The steaming step has been found to improve significantly the propylene yield, propylene selectivity and catalyst stability m the olefmic catalytic cracking process. The steam treatment is conducted at elevated temperature, preferably m the range of from 425 to 870°C, more preferably m the range of from 540 to 815°C and at atmospheric pressure and at a water partial pressure of from 13 to 200kPa. Preferably, the steam treatment is conducted m an atmosphere comprising from 5 to 100% steam. The steam treatment is preferably carried out for a period of from 1 to 200 hours, more preferably from 20 hours to 100 hours. As stated above, the steam treatment tends to reduce the amount of tetrahedral aluminium m the crystalline silicate framework, by forming alumina. Following the steaming step, the catalyst is thereafter calcined, for example at a temperature of from 400 to 800°C at atmospheric pressure for a period of from 1 to 10 hours. Following the steaming step, the catalyst may be contacted by a complexmg agent for aluminium which may comprise an organic acid m an aqueous solution thereof or a salt of such an organic acid or a mixture of two or more such acids or salts. The complexmg agent may m particular comprise an amme, such as ethyl diamme tetraacetic acid (EDTA) or a salt thereof, m particular the sodium salt thereof. Following the contacting of the crystalline silicate by the complexmg agent, the crystalline silicate may be subjected to a second ion exchange step for reducing the sodium content of the crystalline silicate still further, for example by contacting the catalyst with an ammonium nitrate solution. The steamed crystalline silicate, preferably ZSM-11, catalyst may be mixed with a binder, preferably an inorganic binder, and shaped to a desired shape, e . g. extruded pellets. The binder is selected so as to be resistant to the temperature and other conditions employed the catalyst manufacturing process and m the subsequent catalytic cracking process for the olefms. The binder is an inorganic material selected from clays, silica, metal oxides such as Zr0 2 and/or metals, or gels including mixtures of silica and metal oxides. The binder is preferably alumma-free . If the binder which is used m conjunction with the crystalline silicate is itself catalytically active, this may alter the conversion and/or the selectivity of the catalyst. Inactive materials for the binder may suitably serve as diluents to control the amount of conversion so that products can be obtained economically and orderly without employing other means for controlling the reaction rate. It is desirable to provide a catalyst having a good crush strength. This is because m commercial use, it is desirable to prevent the catalyst from breaking down into powder-like materials. Such clay or oxide binders have been employed normally only for the purpose of improving the crush strength of the catalyst. A particularly preferred binder for the catalyst of the present invention comprises silica. The relative proportions of the finely divided crystalline silicate material and the inorganic oxide matrix of the binder can vary widely. Typically, the binder content ranges from 5 to 95% by weight, more typically from 20 to 50% by weight, based on the weight of the composite catalyst . Such a mixture of crystalline silicate and an inorganic oxide binder is referred to as a formulated crystalline silicate. In mixing the catalyst with a binder, the catalyst may be formulated into pellets, extruded into other shapes, or formed into a spray-dried powder. Typically, the binder and the crystalline silicate catalyst are mixed together by an extrusion process. In such a process, the binder, for example silica, m the form of a gel is mixed with the crystalline silicate catalyst material and the resultant mixture is extruded into the desired shape, for example pellets. Thereafter, the formulated crystalline silicate is calcined m air or an inert gas, typically at a temperature of from 200 to 900°C for a period of from 1 to 48 hours. The binder preferably does not contain any aluminium compounds, such as alumina. This is because as mentioned above the preferred catalyst for use m the mvention has a selected silicon/alummium ratio of the crystalline silicate. The presence of alumina the binder yields other excess alumina if the binding step is performed prior to the aluminium extraction step. If the aluminium-containing binder is mixed with the crystalline silicate catalyst following aluminium extraction, this re-alummates the catalyst. The presence of aluminium m the binder would tend to reduce the olefm selectivity of the catalyst, and to reduce the stability of the catalyst over time. In addition, the mixing of the catalyst with the binder may be carried out either before or after the steaming step. The various preferred catalysts of the present invention have been found to exhibit high stability, m particular being capable of giving a stable propylene yield over several days, e . g. up to ten days. This enables the olefm cracking process to be performed continuously m two parallel "swing" reactors wherem when one reactor is operating, the other reactor is undergoing catalyst regeneration. The catalyst of the present invention also can be regenerated several times. The catalyst is also flexible m that it can be employed to crack a variety of feedstocks, either pure or mixtures, coming from different sources in the oil refinery or petrochemical plant and having different compositions. In the process for catalytic cracking of olefms m accordance with the invention, the present inventors have discovered that when dienes are present the olefm-containing feedstock, this can provoke a faster deactivation of the catalyst. This can greatly decrease the yield on an olefm basis of the catalyst to produce the desired olefm, for example propylene, with increasing time on stream. The present inventors have discovered that when dienes are present in the feedstock which is catalytically cracked, this can yield a gum derived from the diene being formed on the catalyst which in turn decreases the catalyst activity. It is desired m accordance with the process of the invention for the catalyst to have a stable activity over time, typically for at least ten days. In accordance with this aspect of the invention, prior to the catalytic cracking of the olef s, if the olefm-contammg feedstock contains dienes, the feedstock is subjected to a selective hydrogenation process m order to remove the dienes. The hydrogenation process requires to be controlled in order to avoid the saturation of the mono-olefms. The hydrogenation process preferably comprises nickel-based or palladium-based catalysts or other catalysts which are typically used for first stage pyrolysis gasoline (Pygas) hydrogenation. When such nickel -based catalysts are used with a C 4 cut, a significant conversion of the mono-olefms into paraffins by hydrogenation cannot be avoided. Accordingly, such palladium-based catalysts, which are more selective to diene hydrogenation, are more suitable for use with the C 4 cut . A particularly preferred catalyst is a palladium-based catalyst, supported on, for example, alumina and containing 0.2-0.8wt% palladium based on the weight of the catalyst. The hydrogenation process is preferably carried out at an absolute pressure of from 5 to 50 bar, more preferably from 10 to 30 bar and at an mlet temperature of from 40 to 200°C. Typically, the hydrogen/diene weight ratio is at least 1, more preferably from 1 to 5 , most preferably around 3. Preferably, the liquid hourly space velocity (LHSV) is at least 2h 1 , more preferably from 2 to 5h x . The dienes m the feedstock are preferably removed so as to provide a maximum diene content m the feedstock of around 0.1% by weight, preferably around 0.05% by weight, more preferably around 0.03% by weight. In the catalytic cracking process, the process conditions are selected m order to provide high selectivity towards propylene, a stable olefm conversion over time, and a stable olefmic product distribution m the effluent. Such objectives are favoured by the use of a low acid density m the catalyst (i.e. a high Si/Al atomic ratio of greater than 150) m conjunction with a low pressure, a high let temperature and a short contact time, all of which process parameters are interrelated and provide an overall cumulative effect { e. g. a higher pressure may be offset or compensated by a yet higher let temperature) . The process conditions are selected to disfavour hydrogen transfer reactions leading to the formation of paraffins, aromatics and coke precursors. The process operating conditions thus employ a high space velocity, a low pressure and a high reaction temperature. Preferably, the LHSV ranges from 10 to 3011 1 . The olefm partial pressure preferably ranges from 0.1 to 2 bars, more preferably from 0.5 to 1.5 bars. A particularly preferred olefin partial pressure is atmospheric pressure (i.e. 1 bar) . The hydrocarbon feedstocks are preferably fed at a total mlet pressure sufficient to convey the feedstocks through the reactor. The hydrocarbon feedstocks may be fed undiluted or diluted in an inert gas, e . g. nitrogen. Preferably, the total absolute pressure in the reactor ranges from 0.5 to 10 bars. The present inventors have found that the use of a low olefin partial pressure, for example atmospheric pressure, tends to lower the incidence of hydrogen transfer reactions in the cracking process, which in turn reduces the potential for coke formation which tends to reduce catalyst stability. The cracking of the olefins is preferably performed at an inlet temperature of the feedstock of from 500 to 600°C, more preferably from 520 to 600°C, yet more preferably from 540 to 580°C, typically around 560°C to 570°C. The catalytic cracking process can be performed in a fixed bed reactor, a moving bed reactor or a fluidized bed reactor. A typical fluid bed reactor is one of the FCC type used for fluidized-bed catalytic cracking in the oil refinery. A typical moving bed reactor is of the continuous catalytic reforming type. As described above, the process may be performed continuously using a pair of parallel "swing" reactors. Since the catalyst exhibits high stability to olefinic conversion for an extended period, typically at least around ten days, the frequency of regeneration of the catalyst is low. More particularly, the catalyst may accordingly have a lifetime which exceeds one year. After the catalytic cracking process, the reactor effluent is sent to a fractionator and the desired olefins are separated from the effluent . When the catalytic cracking process is employed to produce propylene, the C 3 cut, containing at least 92% propylene, is fractionated and thereafter purified in order to remove all the contaminants such as sulphur species, arsine, etc.. The heavier olefins of greater than C 3 can be recycled. For example, olefin-rich streams from refinery or petrochemical plants are cracked into light olefins, in particular propylene. The light fractions of the effluent, namely the C 2 and C 3 cuts, can contain more than 92% olefins. Such cuts are sufficiently pure to constitute chemical grade olefin feedstocks. The present inventors have found that the propylene yield on an olefin basis in such a process can range from 30 to 50% based on the olefinic content of the feedstock which contains one or more olefins of C 4 or greater. In the process, the effluent has a different olefin distribution as compared to that of the feedstock, but substantially the same total olefin content. The various aspects of the present invention are illustrated below with reference to the following non-limiting Examples. Exampl e 1 In this Example a ZSM-11 zeolite having an MEL structure was synthesised using diaminooctane as a templating agent and sodium silicate as the silicon source. The catalyst is prepared by mixing the following three solutions A, B and C where solution A comprised 550ml of sodium silicate (27wt%) ; solution B comprised 17.87g of 1,8 diaminooctane in 522ml of distilled water; and solution C comprised 2.82g of Al 2 (S0 4 ) 3 .18H 2 0 and 550ml of distilled water. Solutions A and B were mixed in a 2 litre autoclave and a hydrogel was obtained by slowly adding solution C. The initial pH value of 11.95 was adjusted to a pH of 11 by adding 35ml of sulphuric acid (97wt%) . After stirring for 1 hour the crystallisation reaction was performed at 150°C for a period of 2 days in a 2 litre volume stainless steel autoclave, with stirring at a speed of 150 rpm. The resultant product was washed with 10 litres of distilled water, then dried at 110°C for 16 hours, and finally calcined at a temperature of 600°C for a period of 10 hours in order to remove the organic template material . A scanning electron micrograph of the resultant catalyst showed that the ZSM-11 crystals prepared from the sodium silicate were short and ovate-like with dimensions of around 5 microns by 5 to 6 microns. The powder also contained small particles, which could be attributed to crystalline silica, which was identified by an x-ray diffraction spectrum of the material. The x-ray diffraction spectrum exhibits peaks present in MEL-type crystalline silicates with peaks corresponding to small contaminations of dense silicon oxide faces such as quartz and cristobalite . The resultant crystalline silicate was then subjected to ionic exchange by reacting the crystalline silicate in three successive ion exchange reactions for respective periods of 5, 18 and 5 hours by heating the crystalline silicate under reflux with stirring together with a 0.5M ammonium nitrate solution (8.4ml/g zeolite) . This ion exchange reduced the sodium content of the crystalline silicate. The material was then dried at a temperature of 110°C for a period of 16 hours. Thereafter the zeolite was subjected to a steaming step in which the zeolite was loaded into a tubular reactor and flushed with nitrogen. The temperature was increased up to 550°C. At 350°C a stream of a steam and nitrogen atmosphere containing 72 vol% of steam was passed over the catalyst . The steam treatment was continued for a period of 48 hours. Following the steaming step, the zeolite was heated under reflux for a period of 18 hours with a 0.06M ethyl diamine tetraacetic acid (EDTA) salt solution of EDTA-Na 2 (4.2ml/g zeolite) and then washed with distilled water. Such an extraction was employed with the aim of removing extractable aluminium from the catalyst but in fact such an extraction did not occur because of the unavailability of aluminium in the catalyst for such an extraction treatment. This is because the steaming step although removing Al from the silicate framework does not produce such extractable Al . Finally, the catalyst was ion-exchanged with a 0.12M NH 4 Cl solution (4.2ml/g zeolite) under reflux for a period of 18 hours. The zeolite was then washed with water to remove excess chlorine. The ZSM-11 catalyst thus obtained was dried at 110°C and then calcined for a period of 3 hours at a temperature of 400°C. The catalyst thus obtained was then employed an olefmic catalytic cracking process. The catalyst was reduced to grains of 35-45 mesh and 10ml of the grains were loaded into a tubular reactor having an internal diameter of 10mm and a length of 300mm. On either side of the catalyst m the reactor, the reactor cavity was filled w th inert granulates of silicon carbide of 2mm particle size. A thermocouple well was placed mside the reactor to measure the temperature profile m the catalyst bed. The reactor was heated up at a rate of 50°C/hour under nitrogen to a temperature of 560°C. Then a hydrocarbon feed, comprising an LCCS feed having the composition of Feed No. 1 specified m Table 1, was fed through the reactor tube with the aid of a pump at a liquid hourly space velocity (LHSV) of lOh 1 . The mlet temperature was adjusted to around 560°C and the outlet pressure was set at 1 bara . The effluent of the reactor was analysed on-line with chromatographic equipment. The results were analysed to calculate the variation of the yield m weight percent of various hydrocarbon species against time on stream, the olefmicity per carbon number versus time on stream, and the normalised yield on an olefm basis versus time on stream. The results are shown m Table 2 and m Figures la to lc. It may be seen from Table 2 and Figures la to lc that the initial propylene yield is around 16.5% and after a time on stream of over 17 hours is around 14.5%. No propylene was present the original feedstock. After a time on stream of 17 hours, the propylene purity, as represented by the percentage C 3 / (C 3 +C 3 ) , was around 87%, indicating a high selectivity of the catalyst for the production of propylene as compared to propane. The yield on an olefm basis for the production of propylene was initially around 35% and after 17 hours on stream was above 30%. Thus the use of the catalyst m accordance with the invention enables selective catalytic cracking of olefms to produce propylene with a high yield and selectivity towards propylene, and with good stability of the catalyst. Example 2 In this Example, a ZSM-11 catalyst was synthesised using tetrabutyl phosphonium bromide as the templatmg agent and a silica sol available m commerce as colloidal silica under the trade designation Ludox HS-40 from the company E.I du Pont de Nemours & Co . , Inc. as the silicon source. Such a silica source contains only a small amount of aluminium. In this Example, a ZSM-11 zeolite with an Si/Al ratio of 160 was prepared by mixing the following two solutions A and B, solution A comprising 9g of sodium hydroxide, 41.25g of tetrabutyl -phosphonium bromide and 2.74g of Al 2 (S0 4 ) 3 .18A 2 0 and solution B comprising 247.5g of colloidal silica available m commerce under the trade designation Ludox HS-40. Solution A was poured into a two litre autoclave and a hydrogel was obtained by slowly adding solution B and 82.5g of distilled water. The pH of the solution was 12.78. After stirring for a period of 1 hour, the crystallisation reaction was performed at 155°C for a period of 60 hours a stainless steel autoclave, with the reaction mixture bemg stirred at a speed of 150 rpm. The product was washed with 10 litres of distilled water, dried at 110°C for 16 hours and then calcined at a temperature of 600°C for a period of 10 hours. Thereafter the resultant crystalline silicate was subjected to ion exchange by means of a 0.5M ammonium nitrate solution under reflux and stirring three successive exchange operations having respective periods of 5, 18 and 5 hours. The ion exchange was employed to reduce the sodium content of the crystalline silicate obtained. The ion-exchanged crystallmed silicate was then dried at a temperature of 110°C for a period of 16 hours. The catalyst was then subjected to the same steaming, extraction and second ion-exchange reactions as for the crystalline silicate of Example 1. The crystalline silicate thereby obtained was then subjected to the same catalytic cracking process as for Example 1 using a LCCS feedstock having the composition of Feed No. 2 specified m Table 1. The feedstock had been hydrogenated over a hydrogenation catalyst prior to the catalytic cracking process thereby to reduce the diene content of the feed to a value of 0.05wt%. The results are shown m Table 2 and Figures 2a to 2c. The initial propylene yield was 17wt% and the propylene selectivity is the (i.e. percentage C 3 / (C 3 +C 3 ) ratio) of the effluent was high, being 92% at the start of the run to reach a value of 94% after a time on stream of 30 hours. The catalyst had a good stability and the yield on an olefin basis was above 30% after a time on stream of 160 hours. Example 3 In this Example, the same steps were employed to produce the steamed zeolite as for Example 2 but the zeolite following steaming was not subjected to a subsequent extraction and second ion exchange. After the steaming step, the catalyst was dried at a temperature of 110°C for a period of 16 hours. The zeolite crystals thereby obtained were, prior to ion exchange, homogeneous m shape and exhibited elongated ovate-like shapes with dimensions of 1.5-1.7 by 2.2-3.2 microns. The resultant catalyst was subjected to the same catalytic cracking process as for Example 2 employing the same hydrogenated LCCS feedstock as for Example 2. The results are shown m Table 2 and m Figures 3a to 3c. It may be seen that the initial propylene yield was above 17wt% and decreased to reach a value of 14.6wt% after 100 hours on stream. The selectivity for propylene m the C 3 cut was initially around 90% and reached around 93% after 30 hours on stream. It may also be seen from a comparison to Examples 2 and 3 that the use of an extraction step with EDTA does not increase the Si/Al atomic ratio of the catalyst. This suggests that any attempt to extract aluminium from the zeolite is not the critical step to obtain a zeolite exhibiting a good activity and selectivity for propylene. Comparative Example 1 In this Comparative Example, the catalyst comprised the same catalyst as that of Example 1 but with the catalyst having bemg produced m the absence of the steaming step and subsequent extraction and second ion exchange steps. The catalyst was employed under the same process conditions for the catalytic cracking of the same LCCS feedstock as for Example 1 and the results are shown m Table 2 and m Figures 4a to 4c. It may be seen that as compared to Example 1, the use of an unsteamed catalyst accordance with the Comparative Example had a lower initial yield and selectivity and a lower final yield and selectivity for the production of propylene than for Example 1. The initial yield of propylene was around 14wt% and reached 13wt% after 18 hours. The propane production was reasonably high therefore as compared to Example 1. It may thus be concluded from a comparison of Example 1 and Comparative Example 1 that the use of steaming m accordance with the invention greatly increases the selectivity of the catalyst for the production of propylene . Comparative Example 2 In this Comparative Example 2, a crystalline silicate ZSM-11 catalyst was prepared using a process similar to that employed m Examples 2 and 3, except that the catalyst was not subjected to a steaming step. Thus a ZSM-11 catalyst having an Si/Al atomic ratio of 286 was prepared by mixing two solutions A and B, solution A comprising 9g of sodium hydroxide, 41.25g of tetrabutyl -phosphonium bromide and 1.37g of Al 2 (S0 4 ) 3 .18H 2 0 and solution B comprising 247.5g of colloidal silica m the form of Ludox HS-40. Solution A was mixed m a 2 litre autoclave and thereafter a hydrogel was obtained by slowly adding solution B and 82.5g of distilled water. The pH of the solution was 12.8. After stirring for a period of 1 hour, the crystallisation reaction was performed at a temperature of 155°C for a period of 60 hours m a stainless steel autoclave under stirring at a speed of 150rpm. The product was washed with 10 litres of distilled water, dried at 110°C for a period of 16 hours and calcined at a temperature of 600°C for a period of 10 hours m order to remove the organic template material . The resultant crystalline silicate was subject to ion exchange by means of a 0.5M ammonium nitrate solution (8.4ml/g zeolite) under reflux and stirring m three successive reactions for respective periods of 5, 18 and 5 hours m order to reduce the sodium content of the catalyst . The material was then dried at a temperature of 110°C for 16 hours. The resultant catalyst was then employed m a catalytic cracking process as for Example 1, the LCCS feedstock having the same composition as for that of Example 1, i.e. Feed No. 1. The results are shown m Table 2 and Figures 5a to 5b. The zeolite employed m Comparative Example 2 had not be subjected to a steaming process but had a higher amount of acid sites than for the earlier Examples and Comparative Examples. The propylene production was initially around 17wt%, with a decrease activity of around 5wt% after 100 hours on stream. The olefmicity was initially low at less than 86% and after 100 hours on stream was less than 95%. The low olefmicity is believed to be the result of still a too high acid site density. Such a high acid density enhances hydrogen transfer reactions, thereby producing paraffins. Although the results for Comparative Example 2 show a reasonably good propylene yield and selectivity at the indicated time on stream m Table 2, nevertheless if additionally a steaming process had been employed at such acid density as compared to those employed for Examples 2 and 3, then the propylene yield and selectivity would have been yet higher. It was noted that for Comparative Example 2 the final propylene yield and selectivity are both lower than that for Example 2 and Example 3. Comparative Example 3 In this Comparative Example, an unmodified ZSM-11 catalyst having a very high Si/Al atomic ratio of 843 was prepared as described below. A solution A comprising 9g of sodium hydroxide and 41.25g of tetrabutyl -phosphonium bromide was mixed m a 2 litre autoclave and then a hydrogel was obtained by slowly adding a solution B comprising 247.5g of colloidal silica under the trade designation Ludox HS-40 and 84.5g of distilled water. The pH of the solution was 12.82. After stirring for a period of 1 hour, the crystallisation reaction was performed at a temperature of 155°C for a period of 60 hours in a stainless steel autoclave under stirring at a speed of 150 rpm. The product was washed by 10 litres of distilled water, dried at 110°C for 16 hours and then calcined at a temperature of 600°C for 10 hours. Thereafter the crystalline silicate obtained was subjected to ion exchange by means of a 0.5M ammonium nitrate solution (8.4ml/g zeolite) under reflux and stirring m three successive ion exchange operations for respective periods of 5, 18 and 5 hours m order to reduce the sodium content of the crystalline silicate. The synthesised product was then dried at a temperature of 110°C for a period of 16 hours. The thus-obtained catalyst was employed in a catalytic cracking process similar to that employed in Example 1. The feedstock comprised an LCCS having the composition of Feed No. 3 specified in Table 1 and the results are shown in Table 2 and in Figures 6a to 6c. At the start of the catalytic cracking process, the propylene production was around 15wt% and decreased rapidly to reach 9wt% after 23 hours. The low activation rate was believed to be resultant from the low amount of acid sites found in the zeolite. The propylene yield was low as compared to the Examples of the invention. Comparative Example 4 In this Comparative Example, the ZSM-11 catalyst comprised the same catalyst as that of Example 2 but which had not been subjected to steaming and extraction steps. The catalyst of Comparative Example 4 was subjected to ion exchange with the ammonium nitrate solution as for the catalyst of Example 2, and then dried at 110°C for a period of 16 hours. The as prepared catalyst was then employed in a catalytic cracking process similar to that of Example 1 using the same hydrogenated feed as for Example 2. The results are shown in Table 2 and in Figures 7a to 7c. It may be seen that in contrast to the results for Example 2, the results for Comparative Example 4 show a lower initial propylene yield and selectivity and a lower final yield and selectivity after the indicated time on stream. Thus when comparing the results of Comparative Example 4 and Example 2, it may be seen that the steaming step for the production of the catalyst in accordance with the invention increased the propylene yield and selectivity. TABLE 1 Feed No . Feed No . Feed No . Breakdown Summary of Total Cl PI 0.00 0.00 0.00 C2 P2 0.00 0.00 0.00 02 0.00 0.00 0.00 D2 0.00 0.00 0.00 C3 P3 0.00 0.01 0.00 03 0.00 0.23 0.00 D3 0.00 0.00 0.00 C4 iP4 0.21 0.09 0.25 nP4 0.56 0.27 0.64 i04 0.00 0.20 0.00 n04 2.99 1.29 3.46 D4 0.00 0.00 0.00 C5 iP5 20.04 13.57 20.67 nP5 2.96 2.18 3.04 cP5 0.39 0.39 0.39 i05 12.19 9.93 12.18 n05 9.73 8.40 9.72 c05 0.78 0.72 0.75 D5 0.33 0.00 0.31 C6 iP6 13.68 17.61 14.03 nP6 1.76 1.55 1.78 cP6 3.02 3.67 3.15 i06 5.82 6.38 6.75 nO6 9.33 4.68 5.35 c06 0.00 3.40 0.19 D6 0.11 0.05 0.00 A6 2.03 1.95 1.91 C7 iP7 0.62 7.50 5.50 nP7 1.03 0.54 0.26 cP7 3.02 3.34 2.41 n07 5.93 0.00 2.31 i07 0.25 3.90 0.65 c07 0.00 1.61 0.84 D7 0.00 0.00 0.00 A7 2.61 2.98 2.20 C8 iP8 0.00 1.41 0.83 nP8 0.20 0.00 0.00 cP8 0.07 0.39 0.24 i08 0.00 0.00 0.00 n08 0.00 0.00 0.00 c08 0.00 0.00 0.00 A8 0.35 1.77 0.19 Total 100.00 100.00 100.00 Paraffins (P) 47.05 52.53 53.19 Olefins (0) 47.52 40.73 42.20 Dienes (D) 0.44 0.05 0.31 Aromatics (A) 5.00 6.70 4.30 Total 100.00 100.00 100.00 TABLE 2 Ex: ion exchanged in three stages with a 0.5M ammonium nitrate solution under reflux for 5, 18 and 5 hours respectively. E : extracted with a 0.06 N EDTA-Na, solution under reflux for 18 hours. S : steamed at 550°C under a stream of steam-nitrogen for 48 hours.

Claims: CLAIMS : 1. A process for the catalytic cracking of an olefm-πch feedstock which is selective towards light olefms m the effluent, the process comprising contacting a hydrocarbon feedstock containing one or more olefms, with an MEL crystalline silicate catalyst, which has been subjected to a steaming step and has a silicon/alummium atomic ratio of from 150 to 800, at an mlet temperature of from 500 to 600°C, at an olefm partial pressure of from 0.1 to 2 bars and the feedstock being passed over the catalyst at an LHSV of from 10 to SOh 1 , to produce an effluent with an olefm content of lower molecular weight than that of the feedstock. 2. A process accordmg to claim 1, where the catalyst has been steamed at a temperature of at least 300°C for a period of at least 1 hour at a water partial pressure of at least lOkPa. 3. A process according to claim 1, wherem the catalyst has been steamed at a temperature of from 425 to 870°C at a water partial pressure of from 13 to 100 kPa for a period of from 1 to 200 hours. 4. A process according to any foregoing claim, wherein the catalyst comprises a ZSM-11 catalyst. 5. A process according to any foregoing claim, wherem the feedstock comprises C 4 to C 10 hydrocarbons. 6. A process according to any foregoing claim, wherem the effluent includes C 3 compounds. 7. A process according to claim 6, where propylene comprises at least 92% of the C 3 compounds present in the effluent . 8. A process according to claim 6 or claim 7, wherein the catalytic cracking has a propylene yield on an olefinic basis of from 30 to 50% based on the olefin content of the feedstock. 9. A process according to any foregoing claim, wherem the olefm contents by weight of the feedstock and of the effluent are withm ±15% of each other. 10. A process according to any foregoing claim, wherein the mlet temperature is from 540 to 580°C. AMENDED CLAIMS [received by the International Bureau on 27 November 2000 (27 1 1 00) , original claims 6 and 7 amended, other claims unchanged (2 pages)] CLAIMS : 1. A process for the catalytic cracking of an olef -rich feedstock which is selective towards light olefins in the effluent, the process comprising contacting a hydrocarbon feedstock containing one or more olefins, with an MEL crystalline silicate catalyst, which has been subjected to a steaming step and has a silicon/alum ium atomic ratio of from 150 to 800, at an mlet temperature of from 500 to 600°C, at an olefin partial pressure of from 0.1 to 2 bars and the feedstock being passed over the catalyst at an LHSV of from 10 to 301 1 , to produce an effluent with an olefm content of lower molecular weight than that of the feedstock. 2. A process according to claim 1, wherem the catalyst has been steamed at a temperature of at least 300°C for a period of at least 1 hour at a water partial pressure of at least lOkPa. 3. A process according to claim 1, wherem the catalyst has been steamed at a temperature of from 425 to 870°C at a water partial pressure of from 13 to 100 kPa for a period of from 1 to 200 hours. 4. A process according to any foregoing claim, wherein the catalyst comprises a ZSM-11 catalyst. 5. A process according to any foregoing claim, wherein the feedstock comprises C 4 to C ι0 hydrocarbons. 6. A process according to any foregoing claim, wherein the effluent includes C 3 compounds. 7. A process according to claim 6, wherein propylene comprises at least 92% of the C 3 compounds present in the effluent . 8. A process according to claim 6 or claim 7, wherein the catalytic cracking has a propylene yield on an olefinic basis of from 30 to 50% based on the olefin content of the feedstock. 9. A process according to any foregoing claim, wherein the olefin contents by weight of the feedstock and of the effluent are within +15% of each other. 10. A process according to any foregoing claim, wherein the inlet temperature is from 540 to 580°C.

Inventors: DATH JEAN-PIERRE, VERMEIREN WALTER, DATH, JEAN-PIERRE, VERMEIREN, WALTER
Applicants: DATH JEAN PIERRE, FINA RESEARCH, VERMEIREN WALTER, DATH, JEAN-PIERRE, FINA RESEARCH S.A., VERMEIREN, WALTER
Application year: 2000
Main classification: C10G11
IPC codes: B01J29, C10G11