PROCESS TO PRODUCE POLYPROPYLENE FROM
C3 OLEFINS PRODUCED SELECTIVELY IN A 1 FLUID CATALYTIC DISINTEGRATION PROCESS
Field of the Invention The present invention relates to a process for producing polypropylene from C3 olefins produced selectively from a stream of catalytically disintegrated or thermally disintegrated naphtha. Background of the Invention The need for low emission fuels has created an increased demand for light olefins in alkylation, oligomerization, MTBE, and ETBE synthesis processes. In addition, a low cost supply of light olefins, particularly propylene, continues in demand to serve as a feedstock for the production of polyolefins, particularly polypropylene. The fixed bed processes for dehydrogenation of light paraffins have recently attracted renewed interest to increase olefin production. However, these types of processes typically require relatively large capital investments and high operating costs. It is therefore advantageous to increase the production of olefins using processes that require relatively small capital investment. Would
it is particularly advantageous to increase the production of olefins in catalytic disintegration (cracking) processes such that olefins could be further processed into polymers such as polypropylene. A problem inherent in producing olefin products using FCC units is that the process depends on a specific catalyst balance to maximize the production of light olefins while also achieving high conversion of the feed components of 650 ° F + (~ 340 ° C + ). Furthermore, even if a specific catalyst balance can be maintained to maximize the overall olefin production, the selectivity of olefins is generally low due to undesirable side reactions, such as extensive decay reactions, isomerization, aromatization and hydrogen transfer. Light saturated gases produced from undesirable side reactions result in increased costs to recover desirable light olefins. Therefore, it is desirable to maximize the production of olefins in a process that allows a greater degree of control over selectivity to C3 and C4 olefins. SUMMARY OF THE INVENTION An embodiment of the present invention is a process for producing polypropylene comprising the steps of
(a) feeding a stream of naphtha comprising less than about 40% by weight of paraffins and between about 15 and about 70% by weight of olefins to a process unit which
it comprises a reaction zone, a separation zone, a catalyst regeneration zone, and a fractionation zone; (b) contacting the naphtha stream with a fluidized bed of catalyst in the reaction zone to form a disintegrated product, the catalyst comprising a zeolite having an average pore diameter of less than about 0.7 nm and where the reaction zone it is operated at a temperature of around 500 to 650 ° C, a hydrocarbon partial pressure of 10 to 40 psia (about 70-280 kPa), a hydrocarbon residence time of 1 to 10 seconds, and a weight ratio from catalyst to feed of about 4 to about 10, thereby producing a reaction product where no more than about 20% by weight of paraffins are converted to olefins and where the propylene comprises at least about 90 mol% of the total C3 products; and (c) passing the catalyst through said separation zone; (d) passing the separated catalyst from the separation zone to the catalyst regeneration zone where the catalyst is regenerated in the presence of an oxygen-containing gas; (e) recycling the regenerated catalyst to the reaction zone; (f) fractionating the disintegrated product to produce a C3 fraction, a C4 fraction rich in olefins, and optionally a C5 fraction rich in olefins; (g) passing at least a portion of the C4 fraction to the reaction zone or the separation zone, or both; and, (h) separating the propylene from the C3 fraction and polymerizing the propylene to form polypropylene.
In another embodiment of the present invention the catalyst is a catalyst of type ZSM-5. In one embodiment of the present invention a C5 fraction rich in olefins is also recycled. In another embodiment of the present invention, the feedstock contains from about 5 to 35% by weight of paraffins, and from about 20 to 70% by weight of olefins. In another embodiment of the present invention the reaction zone is operated at a temperature of about 525 to about 600 ° C. Detailed Description of the Invention Feeds that are suitable for producing relatively high C2, C3, and C4 olefin productions are ebullient currents in the. Naphtha range and containing less than about 40% by weight, preferably from about 5 to about 35% by weight, more preferably from about 10 to about 30% by weight, and most preferably from about 10%. to 25% by weight of paraffins, and from about 15% by weight, preferably from about 20% by weight to about 70% by weight of olefins. Food can also contain naphtho and aromatics. Naphtha boiling range streams are typically those having a boiling range of from about 65 to about 430 ° F (from about 18 to about 225 ° C), preferably from about 65 to
-S- around 300 ° F (from around 18 to around 150 ° C). The naphtha feed can be a thermally disintegrated or catalytically disintegrated naphtha. Naphtha streams can be derived from the fluid catalytic disintegration (FCC) of gas oils and residues, or they can be derived from delayed or fluid coking of residues. Preferably, the naphtha streams used in the practice of the present invention derive from the fluid catalytic disintegration of gas oils and residues. FCC naphthas are typically rich in olefins and / or diolefins and relatively light in paraffins. It is within the scope of the present invention to feed or co-fuse other olefinic streams that are not catalytically or thermally disintegrated naphthas, such as a refined MTBE, into said reaction zone with the primary feed. It is believed that this will increase the conversion of propylene. The process of the present invention is carried out in a process unit comprising a reaction zone, a separation zone, a regeneration-catalyst zone, and a fractionation zone. The naphtha feed is fed into the reaction zone where it makes contact with a hot, regenerated catalyst source. The hot catalyst vaporizes and disintegrates the feed at a temperature of about 500 to 650 ° C, preferably about 525 to 600 ° C. The disintegration reaction deposits coke in the
catalyst, thereby deactivating the catalyst. The disintegrated products are separated from the coked catalyst and sent to a fractionator. The coked catalyst is passed through the separation zone where a separation medium, such as steam, separates the volatiles from the catalyst particles. The separation can be carried out under low stringency conditions to retain a larger fraction of the hydrocarbons adsorbed for energy balance. The separated catalyst is then passed to the regeneration zone where it is regenerated by means of burning the coke in the catalyst in the presence of a gas containing oxygen, preferably air. Decooking restores catalyst activity and sianeously heats the enter catalyst 650 and 750 CC. The hot regenerated catalyst is then recycled to the reaction zone to react with fresh naphtha feed. Chimney gas formed by burning coke in the regenerator can be treated for particle removal and for carbon monoxide conversion. The products disintegrated from the reaction zone are sent to a fractionation zone where several products are recovered, particularly a C3 fraction, a C4 fraction, and optionally a Cs fraction. The C4 fraction and the C5 fraction will typically be rich in olefins. At least a portion of one or both of these fractions can be recycled to the reactor. They can be recycled to either the main reactor section, or a riser section, or a separation section. HE
prefers that they are recycled to the top of the separation section, or the separation zone. Recycling at least a portion of one or both of these fractions will convert at least a portion of these olefins to propylene. Although attempts have been made to increase conversions to light olefins in the FCC process unit itself, the present invention uses its own distinctive process unit, as previously described, and receives naphtha from a suitable source in the refinery. The reaction zone is operated at process conditions that will maximize the selectivity of olefins (particularly propylene) C2 to C4 with relatively high conversion of C5 + olefins. Suitable catalysts used in the present invention contain a crystalline zeolite having an average pore diameter of less than about 0.7 nanometers (nm), said crystalline zeolite comprising from about 10 to about 50% by weight of the total fluidized catalyst composition . It is preferred that the crystalline zeolite be selected from the family of medium pore size crystalline aluminosilicates (< 0.7 nm), otherwise known as zeolites. Of particular interest are medium pore zeolites with a molar ratio of silica to alumina of less than about 75: 1, preferably less than about 50: 1, and more preferably even less than about 40: 1, although some embodiments can have silica to alumina ratios greater than 40: 1. The diameter of pores, also known
as the effective pore diameter, it is measured using standard adsorption techniques and hydrocarbon compounds of known minimum kinetic diameters. See Breck, Zeoli te Molecular Sieves, 1974 and Anderson et. al., J. Catalysis 58, 114 (1979), both of which are incorporated herein by reference. The medium pore size zeolites that can be used in the practice of the present invention are described in "Atlas of Zeolite Structure Types", eds WH Meier and DH Olson, Butterworth-Heineman, Third Edition, 1992, which is incorporated in the present by reference. Medium pore size zeolites generally have a pore size of about 5 to about 7A and include for example, structure type zeolites MFI, MFS, MEL, MTW, EUO, MTT, HEU, FER, and TON ( IUPAC Zeolite Nomenclature Commission). Non-limiting examples of such medium pore size zeolites include ZSM-5, ZSM-12, ZSM-22, ZSM-23, ZSM-34, ZSM-35, ZSM-38, ZSM-48, ZSM-50, silicalite, and silicalite 2. The most preferred is ZSM-5, which is described in US Pat. Nos. 3,702,886 and 3,770,614. The ZSM-11 is described in the patent US 3,709,979; the ZSM-12 in the patent US 3,832,449; ZSM-21 and ZSM-38 in US Patent 3,948,758; ZSM-23 in US Patent 4,076,842; and ZSM-35 in US Patent 4,016,245. All of the above patents are incorporated herein by reference. Other suitable medium pore size zeolites include silicon and aluminum phosphates (SAPO), such as SAPO-4 and SAPO-11 which is described
in U.S. Patent 4,440,871; chromosilicates; gallium silicates; iron silicates; aluminum phosphates (ALPO), such as ALPO-11 described in US patent 4,310,440; titanium aluminosilicates (TASO), such as TASO-45 described in EP-A 229,295; boro silicates, described in US Pat. No. 4,254,297; titanium aluminophosphates (TAPO), such as TAPO-11 described in US Patent 4,500,651; and iron aluminosilicates. Medium pore size zeolites may include "crystal blends" which are believed to be the result of faults occurring within the crystal or crystalline area during the synthesis of zeolites. Examples of crystalline mixtures of ZSM-5 and ZSM-11 are disclosed in US Patent 4,229,424, which is incorporated herein by reference. The crystalline mixtures are in themselves medium pore size zeolites and should not be confused with physical mixtures of zeolites in which crystals other than crystallites of different zeolites are physically present in the same catalyst compound or hydrothermal reaction mixtures. The catalysts of the present invention can be held together with a component of inorganic oxide matrix material. The inorganic oxide matrix component binds the catalyst components together such that the catalyst product is sufficiently hard to survive collisions between particles and with the reactor wall. The inorganic oxide matrix can be made from a sol of
inorganic oxide that dries jto "agglutinate" the catalyst components together. Preferably, the inorganic oxide matrix is not catalytically active and will comprise silicon and aluminum oxides. It is also preferred that the separated alumina phases are incorporated into the inorganic oxide matrix. Species of oxyhydroxides of aluminum-g-alumina, boehmite, diaspore, and transition aluminas such as α-alumina, b-alumina, g-alumina, d-alumina, e-alumina, k-alumina, and r-alumina can be used . Preferably, the alumina species are an aluminum trihydroxide such as gibbsite, bayerite, nordstrandi-ta, or doyelite. The matrix material may also contain phosphorus or aluminum phosphate. Preferred process conditions include temperatures of from about 500 to about 650 ° C, preferably from about 500 to 600 ° C; partial pressures of hydrocarbons of about 10 to 40 psia (about 70-about 280 kPa), preferably about 20 to 35 psia (about 140-about 245 kPa); and a catalyst to naphtha (w / w) ratio of about 3 to 12, preferably about 4- to 10, where the catalyst weight is the total weight of the catalyst compound. Water vapor can be introduced concurrently with the naphtha stream into the reaction zone, with water vapor comprising up to about 50% by weight of the naphtha feed. Preferably, the residence time of naphtha in the zone of
reaction is less than about 10 seconds, for example about 1 to 10 seconds. The reaction conditions will be such that at least about 60% by weight of the C5 + olefins in the naphtha stream are converted to C4- products and less than about 25% by weight, preferably less than about 20% by weight. of the paraffins are converted to C4- products, and that the propylene comprises at least about 90 mol%, preferably more than about 95 mol% of the total reaction products with the propylene weight ratio / total C2 products greater that around 3.5. Preferably, the ethylene comprises at least about 90 mol% of the C2 products, with the propylene: ethylene weight ratio being greater than about 4, and the "full range" C5 + naphtha product is improved in octanes both motor and research related to the supply of naphtha. It is within the scope of this invention to pre-coke the catalysts before introducing the feed to further improve the selectivity to propylene. It is also within the scope of this invention to feed an effective amount of single ring aromatics to the reaction zone to improve the selectivity of propylene against ethylene. The aromatics may be from an external source such as a reforming unit or may consist of the heavy naphtha recycle product of the present process. The following examples are presented for purposes
illustrative only and should not be taken as limiting the present invention in any way. Examples 1-13 The following examples illustrate the criticality of the process operating conditions for maintaining purity of chemical grade propylene with samples of disintegrated catalytic naphtha on ZCAT-40 (a catalyst containing ZSM-5) that has been vaporized with water at 1500 ° F (-815 ° C) for 16 hours to simulate a commercial equilibrium The comparison of Examples 1 and 2 shows that increasing the catalyst / hydrocarbon ratio improves the conversion of propylene, but sacrifices the propylene purity. The comparison of Examples 3 and 4 and 5 and 6 shows that reducing the partial pressure of hydrocarbons significantly improves the purity of the propylene without compromising the propylene conversion The comparison of Examples 7 and 8 and 9 and 10 shows that increasing the temperature improves both the propylene conversion and the purity The Comparison of Examples 11 and 12 shows that decreasing the residence time of the catalyst improves the conversion and purity of propylene. Example 13 shows an example where both high propylene conversion and purity are obtained at a reactor temperature and catalyst / hydrocarbon ratio which can be achieved using a conventional FCC reactor / regenerator design for the second stage.
"# Table 1
Table 1 (continued)
= CGL? + CtHd + C-jH,
The above examples [1,2,7 and 8) show that C3"/ C2 => 4 and C3VC2 ~> 3.5 can be achieved by selection of suitable reactor conditions, Examples 14-17 The disintegration of olefins and paraffins contained in Naphtha streams (ie, FCC naphtha, co-chelator naphtha) on small or medium pore zeolites such as ZSM-5 can produce significant amounts of ethylene and propylene.The selectivity to ethylene or propylene and the selectivity of propylene to propane varies as a function of the catalyst and the operating conditions It has been found that the conversion of propylene can be increased by co-feeding water vapor together with catalytic naphtha to the reactor.The catalyst can be ZSM-5 or other zeolites of Small or medium pore Table 2 below illustrates the increase in propylene conversion when 5% by weight of water vapor is co-fed with a catalytic naphtha containing 38.8% by weight of olefins. Propylene conversion increased, the propylene purity decreased. Thus, other operating conditions may need to be adjusted to maintain the desired propylene selectivity.
Table 2
Table 2 (continued)
Examples 18-21 ZCAT-40 was used to disintegrate catalytic disintegration naphtha as described in the previous examples. The coked catalyst was then used to disintegrate a C4 stream composed of 6% by weight of n-butane, 9% by weight of i-butane, 47% by weight of 1-butene, and 38% by weight of i-butene in a reactor at the temperatures and velocity spaces indicated in the following table. As can be seen from the results in the following table, a significant fraction of the feed stream was converted to propylene.
table 3
WHSV, hr-1 35 18 12 6 Temperature, ° C 575 575 575 575 Conversion of butylene,% weight Product Conversions,% weight Ethylene 2.4 4.7 5.9 8.8 Propylene 20.5 27.1 28.8 27.4 Binders 39.7 29.0 25.5 19.2
• Light Saturated L-C, 18.2 19.2 19.8 22.0
10 Products C + 19.3 20.0 20.0 22.6
The resulting light olefins of the preferred process can be used as feeds for processes such as oligomerization, polymerization, co-polymerization, ter-polyerylation, and related processes (hereinafter referred to as "polymerization") to form macromolecules, such as light olefins. they can be polymerized both alone and in combination with other species, according to the polymerization methods known in the art In some cases it may be desirable to separate, concentrate, purify, improve, or otherwise
• process light olefins prior to polymerization. Propylene and ethylene are preferred polymerization feeds. Polypropylene and polyethylene are preferred polymerization products made therefrom.