PROCESS FOR ISOMERIZATION OF A HYDROCARBON FEED STREAM

Abstract: This invention relates to a process for the production of an isomerate (e.g. a gasoline blending component) wherein after isomerization, a separatory sieve (preferably ferrierite) selectively adsorbs unreacted normal paraffins and mono-methyl-branched paraffins but not di-methyl-branched paraffins.

Description: The invention is concerned with a process for isomerization of a hydrocarbon feed stream comprising a normal paraffin having at least six carbon atoms per molecule. It is desirable to produce gasoline with a relatively high octane number by methods other than lead addition. Two major techniques are currently available to acquire high octane gasoline pools without lead addition. First, hydrocarbons can be reformed in the presence of a reforming catalyst, such as a platinum-rhenium catalyst, to a high octane gasoline. Second, normal paraffins can be isomerized to branched paraffins possessing high octane qualities. This invention concerns the latter of these two processes. From the standpoint of increasing the octane number, it is desirable that hydrocarbons in gasoline have maximum branching. For example, methyl pentanes have lower octane ratings than do dimethylbutanes. Thus, in an isomerization process, it is beneficial to maximize the content of the dimethylbutanes (di-branched paraffins) in the product at the expense of methylpentanes (mono-methyl-paraffins). In GB-A-1,378,204 a process is described for isomerizing normal paraffins having from 5 to 6 carbon atoms which comprises passing said feedstock mixed with hydrogen through a reactor containing a catalyst composition the molar ratio of hydrogen to hydrocarbon having at least 5 carbon atoms entering said reactor being from 2 to 30, thereafter passing the effluent condensate from the reactor through an adsorption bed containing an adsorbent capable of selectively adsorbing normal paraffins, desorbing normal paraffins from said adsorbent bed with a hydrogen stream and combining effluent mixture of hydrogen and normal paraffins with the hydrocarbon feedstock to the reactor. Adsorbents described to be suitably used are zeolitic molecular sieves having an apparent pore diameter of between 4 and 6 Angstroms. Suitable molecular sieves include zeolite R, zeolite T, chabazite, erionite, small-pore natural mordenite and synthetic mordenite. Particularly suitable is zeolite A. It has now been found that a means to accomplish this goal is to employ a specific separatory tectosilicate sieve having channel dimensions between 5.5 x 5.5 and 4.5 x 4.5 but excluding 4.5 x 4.5 Angstroms, and being sufficient to permit entry of unreacted normal paraffins and mono-methyl branched paraffins but restrictive to prohibit entry of more highly-branched paraffins. This will enable recycle of not only normal paraffins to an isomerization zone, but also mono-methyl-branched hydrocarbons such as methylpentanes. Thus, the product stream of this type of isomerization process contains an increased proportion of dimethylbutanes, the most highly branched (and highest octane) form of C₆ saturates. In refineries which restrict production of gasoline due to octane limitations, the resultant octane enhancement could be used to increase gasoline production. The invention therefore relates to a process for isomerization of a hydrocarbon feed stream comprising a normal paraffin having at least six carbon atoms per molecule which process comprises the following steps: a) passing said hydrocarbon feed stream to an isomerization zone maintained at isomerization conditions and containing an isomerization catalyst to produce an isomerization zone effluent stream comprising di-branched paraffins, mono-methyl-branched paraffins and unreacted normal paraffins; b) passing said isomerization zone effluent stream to a separatory tectosilicate sieve; c) separating with said tectosilicate sieve at separation conditions an isomerate product stream; and d) desorbing paraffins from the separatory sieve and recycling at least a portion thereof to said isomerization zone and/or to admixture with said hydrocarbon feed stream, characterized in that in step (b) the separatory tectosilicate sieve is a select separatory tectosilicate sieve having channel dimensions between 5.5 x 5.5 and 4.5 x 4.5 Angstroms, but excluding 4.5 x 4.5 Angstroms, and being sufficient to permit entry of said unreacted normal paraffins and said mono-methyl-branched paraffins but restrictive to prohibit entry of more highly-branched paraffins, and that in step (c) said more highly-branched paraffins are separated as an isomerate product stream from said unreacted normal paraffins and from said mono-methyl-branched chain paraffins; and that in step (d) unreacted normal paraffins and mono-methyl-branched paraffins are desorbed from said separatory sieve and at least a portion of said unreacted normal paraffin and mono-methyl-branched paraffin stream is recycled to said isomerization zone and/or to admixture with said hydrocarbon feed stream. It should be noted that the zeolitic molecular sieves employed in known isomerization processes are capable of selectively absorbing normal paraffins employing the molecular size and configuration of the normal paraffin molecules as a selection criteria. Particularly suitable zeolites of this type are Zeolite A and calcium exchanged Zeolite 5A. Other naturally occurring zeolite molecular sieves include chabazite and erionite. The isomerate produced with such processes will have a certain quantity of mono-methyl-branched hydrocarbons derived from the isomerization of normal paraffins in an upstream isomerization zone. In contrast, in the process according to the present invention both normal paraffins and monomethyl-branched chain hydrocarbons are recycled to increase and optimize the quantity of di-branched paraffins in the isomerate; mono-methyl-branched paraffins such as methylpentanes, are absorbed in the tectosilicate sieve and, after desorption, along with normal paraffins, are recycled to the feedstream and/or to the isomerization zone, wherein normal paraffins and mono-methyl-branched hydrocarbons are isomerized to multi-methyl-branched paraffins such as dimethylbutane. The present process increases the degree of branching existent within the isomerate, which increases the octane number. A preferred embodiment of the invention resides in a process for the preparation of an isomerate gasoline stream which comprises: isomerizing normal hexane, and a hereinafter defined recycle stream, in the presence of an isomerization catalyst, to form an isomerization zone effluent stream comprising normal hexane, methylpentanes, and dimethyl butanes, passing said isomerization stream to a select tectosilicate having channel dimensions between 5.5 x 5.5 and 4.5 x 4.5 Angstroms, but excluding 4.5 x 4.5 Angstroms, and being sufficient to permit entry of said normal hexane, and methylpentanes but restrictive to prohibit adsorption of said dimethylbutanes, recovering an isomerate gasoline stream comprising dimethylbutanes and a recycle stream comprising normal hexane, and methylpentanes and recycling at least a portion of said recycle stream to said isomerization zone. The contemplated hydrocarbon feedstream to the process according to the invention is comprised mainly of isomeric forms of saturated hydrocarbons having six or more carbon atoms per molecule. These can comprise C₆ normal paraffins, normal paraffins having more than six carbon atoms per molecule (C₇+) or mixtures thereof. Such feedstocks are usually derived from refinery operations and can contain quantities of C₅-, C₇+ and cyclic paraffins. Olefinic and aromatic hydrocarbons may also be present. The preferred feedstock will contain more than 25 mol% normal hexane. The paraffinic feed material is passed through an isomerization reactor having an isomerization catalyst therein. The isomerization catalyst preferably comprises an aluminosilicate having Group VIII metal deposited therewith. Exemplary of such a catalyst is mordenite with platinum having a range of .005 wt% to 10.0 wt% with a preferred range being from .2 to .4 wt%. Other zeolite molecular sieves are also viable which have a silicate to alumina molar ratio of greater than 3 and less than 60 and preferably less than 15. The zeolitic molecular sieves may have many polyvalent metal cations exchanged with the sieve, such as those of the alkali metals or alkaline earth metals. The catalytic metals associated with the isomerization function are preferably noble metals from Group VIII of the Periodic Table of Elements. These can be exemplified by such metals as platinum, palladium, osmium, ruthenium. The isomerization catalyst can be present per se or it may be mixed with a binder material. The isomerization conditions present in the isomerization zone are those selected to maximize the conversion of normal paraffins and mono-methyl-paraffins to di-methyl-branched paraffins. This type of isomerization is favoured in the vapour phase with a fixed bed of isomerization catalyst. Typical operating temperatures include from 200 to 400 °C with pressures of 10 to 40 bar. The isomerization process, which is limited in octane upgrading by thermodynamic equilibria, is frequently measured at 10 points. Even at these select conditions, the effluent from the isomerization reactor will still contain substantial amounts (e.g. 20 to 30 wt%) of normal paraffins and mono-methyl-branched paraffins which are unreacted or partially reacted due to the afore-mentioned equilibrium. After isomerization, it is preferred that the total isomerization zone effluent be transmitted to a separatory adsorption zone. This separatory zone will preferably comprise 3 to 8 adsorbent beds which can be modified to operate in an adsorption/desorption mode. A calcium-5A sieve as applied in known isomerization/separation processes is capable of adsorbing virtually no methylpentane nor di-methylbutane while a sodium ZSM-5 molecular sieve will adsorb both methylpentane and dimethylbutane in substantial quantities; hence, neither of said molecular sieves is applicable in the process according to the invention. The molecular sieve of this invention is capable of adsorbing not only normal hexane, but methylpentanes as well. The most preferred molecular sieve comprises ferrierite, which exhibits greatly increased adsorption capacity toward methylpentane relative to a calcium 5A molecular sieve. In EP-B1-0,012,473 a process for the preparation of ferrierite is described. It has been discovered that both the sodium and hydrogen form of ferrierite are viable although it is preferred that the ferrierite be utilized which has cations exchanged therein selected from the group consisting of the alkali metal, alkaline earth metal or transition metal cations. The tectosilicates of this invention include ferrierite and other analogous shape-selective materials with channel dimensions intermediate those of the calcium-5A and ZSM-5 molecular sieve which selectively adsorbs methylpentanes while dimethylbutanes are excluded. Other preferred tectosilicate sieves would be aluminophosphates, silicoaluminophosphates and borosilicates. It is also feasible that the instant tectosilicate could be a larger pore zeolite that has been ion exchanged with relatively large cations so as to diminish the effective channel size of the tectosilicate to within the afore-mentioned range of dimensions. As exemplary of the type of tectosilicates which are viable for this technique, the following list is given showing the channel size dimensions of various zeolites including those preferred for use in known isomerization processes such as chabazite, erionite, calcium-5A. Zeolites that are too small in pore size do not discriminate between mono-methyl-branched C₆ (i.e. methylpentanes) and di-methyl-branched C₆ (i.e. dimethylbutanes). In fact, they exclude both. Zeolites that are listed as too large do not discriminate between mono-methyl-branched and di-methyl-branched C₆. In fact, they adsorb both. Thus, only zeolites between, and not including, the sizes of the sieves of calcium-5A and ZSM-5 discriminate to accomodate mono-methyl-branched adsorption while excluding di-methyl-branched C₆ paraffins. While ferrierite is the best example of such a sieve, this invention should not be limited to ferrierite per se as the only species which is operable for this process. The adsorption/desorption conditions typically utilized within the multiple tectosilicate sieves comprise a temperature of from 75 °C to 400 °C and a pressure of from 2 bar to 42 bar. Specific desorbents utilized in order to extract the desired trapped normal paraffin and mono-methyl-paraffins will preferably be hydrogen, which can be transferred with the normal paraffin and mono-methyl-paraffin recycle stream to the isomerization zone. The invention is illustrated by way of a schematic flow scheme of the present process. In the Figure a hydrocarbon feed comprising C₆ paraffins and relatively small amounts of C₁-C₅ paraffins, C₇+ paraffins and aromatic compounds are passed through conduit 100 to isomerization zone 103 for the select isomerization of feed material to di-branched paraffins. An off gas stream may be removed in conduit 105 with an isomerization zone effluent stream in conduit 107 containing normal paraffins, mono-methyl-branched paraffins and more highly-branched paraffins. This effluent stream is passed to a multiple separatory zone 109 which contains particular tectosilicates useful in this invention having channel dimensions between 4.5 × 4.5 and 5.5 × 5.5 A. These tectosilicates may be prepared by any known ferrierite synthesis procedure. this separatory tectosilicate sieve is selective for adsorbing or trapping normal and mono-methyl-branched paraffins. Thus, the effluent in conduit 111 from separation zone 109 contains a substantial concentration of di-methyl-branched paraffins. As a result of this different separatory function versus known integrated isomerization-/-separation processes, recycle stream 113 will be very different. In fact, the difference is derivative of the presence of mono-methyl-branched paraffins in addition to the normal paraffins, all of which are recycled to feed paraffins 100 via conduit 113 and/or a portion of the same may be passed directly to the isomerization zone via conduit 115. A slip stream 17 may also be present to remove unwanted impurities or to extract normal paraffins and mono-methyl-branched paraffins which are not recyclable to exhaustion. The invention is further illustrated by way of the following Examples. EXAMPLE 1 In this example, sorption capacities of sodium ferrierite, calcium-5A and sodium ZSM-5 sieves were determined in regard to normal hexane, 3-methylpentane and 2,3-dimethylbutane. The particular denoted zeolite was placed on a pan in a Cahn balance, sample chamber was evacuated, and heated to 550 °C for one hour. The particular zeolite was thus dried and following drying was cooled to 105 °C. At this time, hydrocarbon vapours were admitted to the evacuated chamber to a level of 0.13 bar. Weight changes due to the adsorption of hydrocarbon into the zeolite were recorded. An exposure time of three hours was allowed for the branched hydrocarbons to approach equilibrium weight, whereas an exposure time of only one half hour was required for normal hexane. The results of this adsorption are shown in Table 2. Each combination of zeolite plus solvent was subjected to at least three separate determinations. Listed in the Table are results of individual determinations, as well as mean and standard deviation values for each set of determinations. The sorption capacities are reported as weight gain in the sieve relative to the dry weight of the pure zeolite. As shown in Table 2, the calcium-5A zeolite adsorbed very little branched hydrocarbon and thus is a preferred adsorbent for the recycle of pure normal paraffins to exhaustion as exemplified in prior art processes. The ratio of 3-methylpentane/normal hexane sorption capacities is 0.018. In contrast, sodium ferrierite adsorbed little dimethylbutane but adsorbed a substantial amount of 3-methylpentane. The ratio of the 3-methylpentane/normal hexane sorption capacity is about 20 times greater for the sodium ferrierite than for calcium-5A zeolite. The sodium ZSM-5 sieve adsorbed virtually identical amounts of the mono- and di-branched isomers. Thus, the afore-described sodium ferrierite has the capability to effect a separation between 3-methylpentane and 2,3-dimethylbutane. EXAMPLE 2 The sorption capacities of a hydrogen ferrierite towards the same three hydrocarbons were determined and are presented in Table 3. These data clearly show the comparison of the hydrogen ferrierite versus the calcium-5A zeolite. The ratio of 3-methylpentane/normal hexane sorption for the hydrogen ferrierite is about 25 times greater than that for the calcium-5A sieve. A comparison of the sodium ferrierite and hydrogen ferrierite, i.e. see Tables 2 and 3, underscores the discovery that appropriately sized tectosilicates can be tailored to modify the relative sieving capabilities. EXAMPLE 3 A sample of ammonium ferrierite was tableted to particles having a size of 0.35-1.41 mm and placed in a glass tube. The glass tube was placed in a tube furnace under a flow of nitrogen and heated to 500 °C for 2 hours to expel ammonia and thus produce the hydrogen form of ferrierite. Under the same nitrogen flow, the molecular sieve was cooled to room temperature while the nitrogen flow was diverted through a gas saturation tower containing a mixture of normal hexane, 3-methylpentane and 2,3-dimethylbutane. The molecular sieves were exposed to hydrocarbon-containing streams of nitrogen for 1 hour. Samples of the hydrocarbon reservoir were taken at the beginning and at the end of the gas saturation period. The purpose of sampling both at the start and at the end of the experiment was to verify that the ratio of hydrocarbons remained essentially constant throughout the experiment. A portion of the hydrocarbon-containing vapour stream was diverted directly through a cold finger that was immersed in a dry ice/acetone bath to collect a sample of the actual hydrocarbon vapours. Following exposure to these vapours, the hydrocarbon-saturated ferrierite sample was removed from the glass tube and placed on a vacuum line. The sample was evacuated to below 0.001 bar through a cold finger immersed in liquid nitrogen. The sample was heated to 40 °C and the sorbed hydrocarbon was quantitatively removed from the zeolite. It was experimentally determined that this first trapping was quantitative. The results of the adsorption were analyzed by gas chromatography and are shown in Table 4. As shown in Table 4, very little dimethylbutane entered the hydrogen ferierite. Because the adsorption capacity of normal hexane is greater than that of 3-methylpenthane and because the rate of the adsorption is faster for the unbranched molecules, they hydrocarbon recovered from the zeolite pores was enriched in normal hexane. A substantial amount of methylpentane was adsorbed along with the normal hexane.

Claims: A process for isomerization of a hydrocarbon feed stream comprising a normal paraffin having at least six carbon atoms per molecule which process comprises the following steps: a) passing said hydrocarbon feed stream to an isomerization zone maintained at isomerization conditions and containing an isomerization catalyst to produce an isomerization zone effluent stream comprising di-branched paraffins, mono-methyl-branched paraffins and unreacted normal paraffins; b) passing said isomerization zone effluent stream to a separatory tectosilicate sieve; c) separating with said tectosilicate sieve at separation conditions an isomerate product stream; and d) desorbing paraffins from the separatory sieve and recycling at least a portion thereof to said isomerization zone and/or to admixture with said hydrocarbon feed stream, characterized in that in step (b) the separatory tectosilicate sieve is a select separatory tectosilicate sieve having channel dimensions between 5.5 x 5.5 and 4.5 x 4.5 Angstroms, but excluding 4.5 x 4.5 Angstroms, and being sufficient to permit entry of said unreacted normal paraffins and said mono-methyl-branched paraffins but restrictive to prohibit entry of more highly-branched paraffins, and that in step (c) said more highly-branched paraffins are separated as an isomerate product stream from said unreacted normal paraffins and from said mono-methyl-branched chain paraffins; and that in step (d) unreacted normal paraffins and mono-methyl-branched paraffins are desorbed from said separatory sieve and at least a portion of said unreacted normal paraffin and mono-methyl-branched paraffin stream is recycled to said isomerization zone and/or to admixture with said hydrocarbon feed stream. The process of claim 1 wherein said isomerization zone is maintained at a temperature of 200 °C to 400 °C and a pressure from 10 bar to 40 bar. The process of claim 1 wherein said isomerization catalyst comprises an aluminosilicate having a Group VIII metal deposited therewith. The process of claim 1 wherein said tectosilicate sieve comprises ferrierite. The process of claim 4 wherein said ferrierite has cations exchanged therein selected from the group consisting of the alkali metal, alkali earth metal and transition metal cations. The process of claim 1 wherein said tectosilicate sieve comprises an aluminophosphate. The process of claim 1 wherein said tectosilicate sieve comprises a borosilicate. The process of claim 1 wherein said tectosilicate sieve comprises silicoaluminophosphate. The process of claim 1 wherein said separation conditions include a temperature from 75 °C to 400 °C and a pressure of from 2 bar to 42 bar.

Inventors: EVANS, WAYNE EARL, STEM, STEPHEN CHARLES
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1987
Main classification: C07C5
IPC codes: C07C7, B01J29, C07C9, C07C67, C07B61, C07C5, C07C1