RU2364583C2 - Methods of isomerisation of paraffins having 5 and 6 carbon atoms, with nethylcyclopentane isolation and device for its realisation - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to method of isomerisation of initial raw material, containing paraffins, which have 5 and 6 carbon atoms, in which, at least, 15 wt% of initial raw material represent linear paraffins, in order to obtain izomerization product, which contains isomerised paraffins, including: a) initial raw material isomerisation in isomerisation conditions including reactor temperature from 40° to 250°C and working absolute pressure from 100 kPa to 10 MPa in presence of isomerization catalyst, in order to obtain isomerization product containing linear paraffins but in concentration lower than in initial raw materal; b) distillation of, at least, part of isomerisation product, in order to obtain low-boiling fraction, containing dimethylbutanes and light paraffins, and high-boiling fraction, containing normal hexane, methylpentanes, dimethylbutanes and methylcyclopentane; c) contacting of, at least, part of fraction containing normal hexane from stage b), with inlet side of selectively permeable membrane in conditions including sufficient area of membrane surface and pressure difference on membrane in order to obtain retainable fraction, in which there is higher concentration of methylcyclopentane and dimethylbutanes, and obtain on outlet side of membrane diffused fraction, which has higher concentration of normal hexane and methyl pentanes, said diffused fraction containing, at least, 75 wt % of normal hexane, contained in fraction including normal hexane and contacting with membrane; and d) removal of retained product from stage c). Claimed invention also relates to device for realisation of claimed method.

EFFECT: application of claimed method allows to obtain isomerisation product possessing higher octane number.

15 cl, 1 dwg

Description

FIELD OF THE INVENTION

The present invention relates to improved methods for the isomerization of paraffins having 5 and 6 carbon atoms, for example, to obtain an isomerization product having an increased octane number according to the research method (OR) for mixing in compounded gasoline, and especially in such methods using isohexane removal columns (deisohexanizer).

State of the art

Methods for isomerizing paraffins to substantially more branched paraffins are widely used in practice. Particularly important industrial isomerization methods are used to increase the degree of branching and thus increase the octane number of oil product streams containing paraffins with carbon atoms from 4 to 8, mainly 5 and 6 carbon atoms. Typically, the isomerization product is mixed with the stream leaving the reforming apparatus to obtain a compound gasoline mixture having the desired octane number (by the research method) (RH I.M.).

The isomerization process is limited by thermodynamic equilibrium. Therefore, the isomerization product may still contain normal paraffins, which have a low octane number and thus reduce the octane number of the isomerization product. Provided that appropriate high-octane mixing streams are available, such as alkylation and reforming products, and gasolines with low octane numbers, for example, with high purity units, are needed. 85 and 87, the presence of such normal paraffins in the isomerization product is allowed.

In conditions of the need for isomerization products with increased HF isomerization processes are modified by isolating normal paraffins from the isomerization product and recycling them to the isomerization reactor. Thus, not only normal paraffins, which reduce the octane number, are removed from the isomerization product, but they are also returned to the isomerization reactor, which increases the degree of conversion of the feed to more desirable highly branched paraffins.

The main methods used to isolate normal paraffins from an isomerization product are adsorption separation, which is described in US Pat. Nos. 4,717,784 and 4,804,802, and distillation. In practice, isomerization methods are most often used, in which a deisohexanizer device is used to recirculate normal paraffins. A deisohexanizer is one or more distillation columns in which an overhead containing branched C ₆ paraffins, such as dimethylbutanes (2,2-dimethylbutane and 2,3-dimethylbutane) and lighter components obtained as an isomerization product, for example, for compounded gasolines, and a side stream containing normal hexane and closely boiling components such as methylpentanes (2-methylpentane and 3-methylpentane) and methylcyclopentane are recycled to the isomerization reactor. The problem of the deisohexanizer is that the low boiling point product stream contains n-pentane, which has a low OP value of m.

The use of adsorption separation instead of a deisohexanizer provides the removal of n-pentane, thus producing motor fuel with a high value of the RPh IM, often in the range of 91-93.

In addition, it was proposed the separation of linear and branched paraffins, but the membrane has not yet found industrial application. US Pat. No. 5,069,794 discloses microporous membranes containing a crystalline material, a molecular sieve. Column 8, line 11 and below, describes possible applications for membranes, including the separation of linear and branched paraffins. See also US Pat. No. 6,090,289 for a layered composite material containing a molecular sieve that can be used as a membrane. Among the possible separation processes in which membranes can be used and which are disclosed starting from line 6 in column 13, the separation of normal paraffins and branched paraffins is indicated. US Patent Nos. 6156950 and 6338791 discuss a permeability separation technology that may find application for separating normal paraffins from branched paraffins and describes separation schemes in connection with the isomerization process. US2003 / 0196931 discloses a two-stage isomerization process to improve the quality of a hydrocarbon feed containing 4 to 12 carbon atoms.

Recently, Bourney et al., In WO 2005/049766, described a process for producing high octane gasoline using membranes to remove, among others, n-pentane from an isomerized stream obtained from the overhead of a deisohexanizer. The deisohexanizer side fraction is used as a flushing fluid at the outlet side of the membrane. The mixture of the permeated product and the washing fluid is recycled to the isomerization reactor. Computer simulation of membrane separation using MFI zeolite on alumina in Example 1 of this publication showed that a membrane surface area of 5000 m ^{2 is} required in order to remove approximately 95% by weight of n-pentane from the overhead of the deisohexanizer distillation column. When the flow rate of the feed to the membrane separator was 75,000 kg / h (containing 20.6% by weight of n-pentane), it turned out that the permeability of n-pentane used in the simulation was of the order of 0.01 gram mol / (m ² · c) at a temperature of 300 ° C. It is noted that the magnitude of the NF the product after removal of n-pentane is 91.0.

Using Bourney et al. A side fraction from a deisohexanizer as a flushing fluid for membrane separation results in the recycling of valuable high-octane components, such as methyl pentanes and methyl cyclopentane, back to the isomerization reactor. In table 1, Bourney and others indicate that the concentration of methylcyclopentane is 9.7 wt.%. Methyl cyclopentane is not present in the product stream. In addition, the washing stream contains 4.5 wt.% 2,3-dimethylbutane, which is also recycled to the isomerization reactor. Since the process of isomerization leads to an equilibrium distribution of isomers, these compounds are destroyed in a single pass with the formation of high-octane fuel (HF im).

The use of zeolite membranes is proposed as a suitable process for the isolation of linear molecules. For example, see paragraphs 0008 and 0032. US patent No. 6818333 describes thin zeolite membranes, which, as indicated, have a n-butane permeability of at least 6 · 10 ^-7 mol / (m ² · m · Pa) and a relative selectivity of n-butane / isobutane of at least 250.

Changes in environmental legislation and fuel efficiency can have a profound effect on the demand for a higher octane isomerization product. For example, the need to reduce the benzene content in gasolines will lead to the need to increase the octane number of the isomerization product, and it will be necessary to modernize the "non-recirculation" isomerization processes into processes with the separation and recycling of normal paraffins to the isomerization reactor. Even in existing processes using deisohexanizers, there may be a need for an isomerization product with an increased octane rating.

In accordance with this, economically attractive and easy-to-use methods are being developed to increase the octane number of the overhead of the deisohexanizer.

In connection with the subsequent consideration of the present invention, the following membrane properties are determined.

Microporosity

Micropores and microporosity refer to pores that have an effective diameter of from about 0.3 to 2 nm (nm).

Mesoporosity

Mesopores and mesoporosity refer to pores that have an effective diameter of about 2 to 50 nm.

Macroporosity

Macropores and macroporosity refer to pores that have an effective diameter of greater than 50 nm.

Nanoparticles

Nanoparticles are particles having a basic size of up to about 100 nm.

Molecular sieves

Molecular sieves are microporosity materials, which can be amorphous, partially amorphous or crystalline, and can be zeolite, polymer, metal, ceramic or carbon.

Screen membranes

Sieve membranes are a composite membrane containing a continuous or discontinuous selective separation medium that includes a molecular sieve barrier. This barrier is a structure that exists in order to selectively block the fluid flow in the membrane. In a continuous sieve membrane, the molecular sieve itself forms a continuous layer, which they strive to make free from defects. The continuous barrier may contain other materials, such as may be present in mixed matrix membranes. An interrupted sieve membrane is an interrupted set of molecular sieve barriers in which there are spaces or voids between particles or regions of a molecular sieve. These spaces or voids may contain or fill with other solid material. Particles or regions of a molecular sieve are barriers. Separation by sieve membranes may be based on the spatial properties of the components to be separated. Other factors may also affect permeability. One such factor is the sorption ability or its absence due to the component and material of the molecular sieve. Another factor is the interaction of the components to be separated in the microporous structure of the molecular sieve. For example, for some zeolite molecular sieves, the presence of molecules, say n-hexane, in the pore can further impede the entry of 2-methylpentane into this pore compared to another n-hexane molecule. Therefore, zeolites, which only from the point of view of molecular size, presumably, will not have high selectivity in the separation of normal and branched paraffins, in practice can provide a higher separation selectivity.

Permeability Flow Index C ₆

The permeability of the sieve membrane, that is, the speed at which a given component passes through a membrane of a given thickness, often changes with varying conditions such as temperature and pressure, absolute and differential. So, for example, various values of permeability can be found when the absolute pressure at the inlet side is 1000 kPa, compared with the absolute pressure of 5000 kPa, and all other parameters, including differential pressure, are constant. Therefore, in the present invention, the C ₆ permeability flow index is used to describe screen membranes. This permeability flow index for a given membrane is determined by measuring the rate (gram mol per second) at which substantially pure normal hexane (preferably at least about 95% by weight of normal hexane) penetrates the membrane at approximately 150 ° C and absolute the pressure on the inlet side of the membrane is about 1000 kPa; and the pressure on the outlet side of the membrane is about 100 kPa. The C ₆ permeability flow index corresponds to the permeability rate per square meter of surface area from the input side, but is not normalized to the membrane thickness. Therefore, the C ₆ permeability flow index for a given membrane has a dimension of gram-mole of normal hexane passing per second per square meter of membrane surface area from the input side.

The ratio of the flow of permeability With ₆

The ratio of C ₆ permeability fluxes for a given sieve membrane is the ratio of C ₆ permeability flux index (n-hexane) to i-C ₆ permeability flux index (isohexane), where i-C ₆ permeability flux index is determined in the same way as in the case C ₆ , but using substantially pure dimethylbutanes (regardless of the distribution between 2,2-dimethylbutane and 2,3-dimethylbutane) (preferably at least about 95% by weight of dimethylbutanes).

SUMMARY OF THE INVENTION

The present invention has made improvements to isomerization processes to increase the octane number of paraffin feedstocks containing 5 and 6 carbon atoms, and these methods use deisohexanizers to produce a low boiling fraction containing dimethyl butanes and a high boiling fraction containing normal hexane. According to the present invention, membranes are used to recover high octane components such as methylcyclopentane and dimethylbutanes from the high boiling fraction containing normal hexane. These high octane components can be used to mix with motor fuel. Since only part of the isomerization product undergoes membrane separation, the required membrane surface area can be reduced, and thus the economic attractiveness of the use of membranes is increased.

Preferably, at least a portion of the normal hexane-containing substance diffused during the membrane separation process is recycled for isomerization. Since the concentration of normal hexane in the diffused substance is higher than the concentration in the high-boiling fraction, the volume of the recycle stream decreases compared to the volume of recirculation of the same amount of normal hexane, but which has not passed membrane separation. The ability to reduce the volume of stream recycled to the isomerization reactor may provide certain advantages to the methods of the invention. For example, a decrease in the volume of the recycle stream allows an increase in the supply of raw materials to the isomerization reactor at a given degree of conversion and thus increase the productivity of the isomerization reactor. In addition, by reducing the amount of high-octane components, such as methylcyclopentane and dimethylbutanes, which were otherwise recycled to the isomerization reactor, the equilibrium nature of the isomerization process will provide a product with an increased octane number obtained per unit amount of raw material.

Broadly, the methods of the present invention include

a) isomerization of a feedstock containing paraffins having 5 and 6 carbon atoms, where at least 15% by weight of the feedstock are linear paraffins, under isomerization conditions, including the presence of an isomerization catalyst, to obtain an isomerization product containing linear paraffins , but in a lower concentration than in the feedstock,

b) distilling at least part, preferably at least 90% by weight and most preferably almost all of the isomerization product, in order to obtain a low-boiling fraction containing dimethylbutanes and lower paraffins, and a fraction containing normal hexane, methylpentanes, dimethylbutanes and methylcyclopentane,

(c) contacting at least a portion, preferably at least 90% by weight and most preferably almost the entire fraction containing normal hexane, from step b), with the inlet side of the selectively permeable membrane under conditions including a sufficient area the surface of the membrane and the pressure drop across the membrane in order to obtain a retained fraction in which there is an increased concentration of methylcyclopentane and dimethylbutanes, and to obtain on the outlet side of the membrane a diffused fraction having a higher hydrochloric concentration of normal hexane and methylpentane, The diffused wherein said fraction contains at least 75%, preferably at least 90% by weight of normal hexane contained in the fraction comprising normal hexane contacted with the membrane, and

d) removing the retained product from step c).

Preferably, at least a portion, more preferably at least 90% by weight, and most preferably almost all of the diffused fraction from step c) is recycled to step a).

Preferably, at least 25%, more preferably at least 30% by weight of methyl pentanes contained in the stream containing normal hexane and in contact with the membrane is contained in the diffused substance. In many cases, the concentration of normal hexane in the whole diffused substance will be approximately less than 90 wt.%, For example, from about 25 to 90, for example from 40 to 80 wt.%. In some aspects of the methods of the invention, the recovered retained product contains more than about 10%, for example, 15 to 50% by weight of a fraction containing normal hexane and in contact with the membrane. Thus, the volume of the recycle stream at the isomerization stage a) becomes less than in a similar process in which the fraction containing normal hexane is not subjected to membrane separation.

The retained product from step d) contains a significant amount of methylcyclopentane and, thus, has a good octane number. Often at least 50%, preferably at least about 80% by weight of methylcyclopentane in the fraction containing normal hexane and in contact with the membrane remain in the retained product.

The separation of monomethylpentanes and dimethylbutanes is difficult due to the proximity of their boiling points and, therefore, it is necessary not only to use a deisohexanizer with a very large number of distillation plates, often in the range of 80 plates, but also a large ratio of reflux to raw materials, for example from 2: 1 to 3: 1, therefore, the deisohexanizer requires a significant amount of heat in the boiler to operate. By using the methods of the invention, a significant portion of the dimethylbutanes contained in the fraction containing normal hexane remains in the retained product and is thus recovered with methylcyclopentane for use in compounded motor fuels. Preferably, the distillation step b) is operated in such a way that the fraction containing normal hexane includes dimethylbutanes, for example at least 2, and sometimes 5 to 30% by weight of dimethylbutanes contained in the isomerization product leaving step a). Preferably, at least 30%, more preferably at least 70% by weight of dimethylbutanes contained in the fraction containing normal hexane is stored in the retained product. Thus, the recovered retained product can be used as motor fuel or is a component for producing compounded motor fuel. According to the invention, the reflux ratio can be reduced in the existing deisohexanizer, sometimes by 10-50%, which leads to energy savings, without the undesirable loss of the octane number of the product.

Preferably, the membrane is a sieve membrane having a C ₆ permeability flux index of at least about 0.01, more preferably at least about 0.02, and a C ₆ permeability flux ratio of at least about 1, 25: 1, more preferably at least about 1.3: 1, and often from 1.35: 1 to 5: 1 or 6: 1.

In addition, the invention relates to a device suitable for implementing the methods of this invention. In a broad aspect, the device according to the invention is a device for isomerizing a feedstock containing paraffins with carbon numbers 5 and 6 to obtain a gasoline fraction, and includes:

a) isomerization reactor (106), in which there are devices for receiving the feedstock at the inlet and outlet pipe,

b) a hexane removal apparatus (116), in which the liquid fluid inlet is connected to the outlet pipe of the isomerization reactor (106), the low-boiling fraction outlet pipe is used to remove the low-boiling fraction along line (118), the outlet pipe to select the side fraction and the high-boiling outlet pipe fractions; and

c) a membrane separator (124), in which a side nozzle for entering the liquid raw material is connected to an outlet nozzle that selects the side fraction of the hexane removal apparatus (116), a side nozzle for the exit of liquid raw material is connected to the pipeline (118) for the higher boiling fraction of the removal apparatus hexane (116), and the output of the liquid diffused product is connected to the inlet of the isomerization reactor (106).

The drawing shows a schematic representation of the methods according to the invention using a stabilization column to a deisohexanizer.

DETAILED DESCRIPTION OF THE INVENTION

Isomerization

Any suitable feedstock containing paraffins may be used in the methods of the present invention. Raw naphtha is most often used as a feedstock in isomerization processes. Raw naphtha includes paraffinic, naphthenic and aromatic hydrocarbons and may contain small amounts of olefins boiling in the range of the gasoline fraction. Raw materials that can be used include straight run naphtha, petroleum gas, synthetic naphtha, thermal cracking gasoline, catalytic cracking gasoline, partial reforming naphtha or raffinates after aromatic hydrocarbon extraction. Essentially, this feedstock is included in the full range of the gasoline fraction or in the range from 0 to 230 ° C. Typically, the feedstock is light naphtha having a boiling point of about 10 to 65 ° C and a boiling point of about 75 to 110 ° C; preferably the boiling point lies lower than about 95 ° C.

Typically, naphtha as a feedstock contains small amounts of sulfur-containing compounds in a concentration of less than 10 parts per million (parts by weight per million) per element. Preferably, the naphtha feed is obtained from contaminated feed using conventional pre-treatment processes, such as hydrogenation treatment, hydrotreating, or hydrodesulfurization, so that contaminants such as sulfur, nitrogen, and oxygen compounds, respectively, in H ₂ S, NH ₃ and H ₂ O that can be isolated from hydrocarbons by fractionation. Preferably, a catalyst known in the art and containing an inorganic oxide support and metals that are selected from groups VIB (IUPAC 6) and VIII (IUPAC 9-10) of the Periodic Table of Elements can be used for such a conversion. Water can play the role of a promoter of catalytic activity, acting as a base, and sulfur temporarily deactivates the catalyst due to platinum poisoning. Typically, as a result of the above hydrotreating of the feedstock, the content of water-forming oxygen-containing compounds and deactivating sulfur-containing compounds is reduced to an acceptable level and generally there is no need for other means of removing sulfur and water from hydrocarbon streams, such as adsorption separation systems. The scope of the present invention includes an optional pretreatment step for the feed.

The main components of the preferred feedstock are cyclic and acyclic paraffins containing from 4 to 8 carbon atoms in the molecule (C ₄ to C ₈ ), mainly C ₅ -C ₆ , and in addition, small amounts of aromatic and olefinic hydrocarbons may be present . Typically, the concentration of C ₇ and heavier components is approximately less than 20 percent of the weight of the feedstock, and the concentration of C ₄ and lighter components is less than 20%, preferably less than 10% of the weight of the feedstock. The mass ratio of the components With ₅ to With ₆ in the preferred raw material is from 1:10 to 1: 1.

Although there are no specific restrictions on the total cyclic hydrocarbon content of the feed, typically the feed contains approximately between 2 and 40% by weight of cyclic naphthenic and aromatic hydrocarbons. Although the content of aromatic hydrocarbons in the raw naphtha usually does not exceed the content of alkanes and cycloalkane hydrocarbons, it can be from 2 to 20% and more typically from 5 to 10% of the total mass. Benzene usually constitutes the bulk of the aromatic hydrocarbons in the preferred feedstock, optionally together with less toluene and higher boiling aromatic hydrocarbons in the above boiling range.

Typically, linear paraffins are present in an amount of at least 15%, often from 40%, preferably at least 50%, to almost 100% of linear paraffins based on the weight of the feed used in the methods of the invention. For the initial naphtha, the amount of linear paraffins is usually at least 50%, for example from 50 to 90 wt.%. The mass ratio of non-linear paraffins to linear paraffins in the feedstock is often less than 1: 1, for example from 0.1: 1 to 0.95: 1. Non-linear paraffins include branched acyclic paraffins and substituted or unsubstituted cycloparaffins. In addition, other components, such as aromatic and olefinic compounds, as described above, may be present in the feed.

The feed is passed into one or more isomerization zones. According to the concept of the present invention, in the case of recycling normal hexane, the feedstock and the recycle stream are usually mixed before entering the isomerization zone, but they can be introduced separately, if desired. In any case, the total flow of raw materials into the isomerization zone is here called the raw material for isomerization. Recycling can be carried out in one or more threads. As discussed below, the recycle stream contains linear paraffins. The concentration of linear paraffins in the isomerization feed will depend not only on the concentration of linear paraffins in the feed, but also on their concentration in the recycle stream and on the ratio of the amounts of recycle stream to the feed, which can vary over a wide range. Often the raw material for isomerization has a linear paraffin concentration of at least about 30%, for example between 35 and 90, preferably from about 40 to 70% by weight, the molar ratio of non-linear paraffins to linear paraffins is between 0.2: 1 and 1.5: 1 and sometimes between 0.4: 1 and 1.2: 1.

In the isomerization zone, the isomerization feed is subjected to isomerization conditions, including in the presence of an isomerization catalyst, preferably in the presence of a limited but defined amount of hydrogen, as described in US Pat. Nos. 4,804,803 and 5,326,296, which are both incorporated by reference. Typically, paraffin isomerization is considered a first order reversible reaction. Thus, the flow from the isomerization reactor will have a higher concentration of non-linear paraffins and a lower concentration of linear paraffins in comparison with the feed for isomerization. In preferred embodiments of the present invention, the isomerization conditions are sufficient to isomerize at least 20%, preferably between 30 and 60% by weight of normal paraffins in the isomerization feed. Typically, under isomerization conditions, a level of at least 70%, preferably at least 75%, for example, from 75 to 97% of the equilibrium composition of C ₆ paraffins present in the isomerization feed is achieved. In many cases, in a stream from an isomerization reactor, the weight ratio of non-linear paraffins to linear paraffins is at least 2: 1, preferably between 2.5 and 4: 1.

The isomerization catalyst is not essential in a broad aspect of this invention, and any suitable isomerization catalyst may find use. Suitable isomerization catalysts include acid catalysts using, to maintain the desired acidity, chlorinated and sulfated catalysts. The isomerization catalyst may be amorphous, for example based on amorphous alumina, or zeolite. Typically, the zeolite catalyst may still contain amorphous binder material. The catalyst may include sulfated zirconia and platinum, as described in US Pat. No. 5,036,035 and European Patent Application 0666109 A1, or a platinum group metal on chlorinated alumina, as described in US Pat. Nos. 5705730 and 6214764. Another suitable catalyst is described in US patent No. 5922639. US Pat. No. 6,818,589 discloses a catalyst comprising a tungstate support on a Group IVB metal oxide or hydroxide (IUPAC 4), preferably zirconium oxide or hydroxide, in which at least the first component is a lanthanide element and / or yttrium component and, at least the second component is a platinum group metal. These documents are included in the present invention, since they disclose catalytic compositions, operating modes, and isomerization techniques.

Contacting within the isomerization zones can be carried out using a fixed bed catalyst system, a moving bed system, a fluidized bed system, or in a batch mode. A fixed bed system is preferred. The reagents can be contacted with particles in the catalyst bed in an upward, downward, or radial direction of flow. Upon contact with the catalyst particles, the reactants can be in the liquid phase, the mixed vapor-liquid phase, or in the vapor phase, with excellent results being obtained mainly when using the present invention in the liquid phase mode. The isomerization zone may be in one reactor or in two or more separate reactors, between which there is a suitable device to maintain the desired isomerization temperature at the entrance to each zone. The preferred scheme includes two or more series reactors, which provides an improved isomerization mode by controlling the temperature in individual reactors and the possibility of partial replacement of the catalyst without stopping the process.

The isomerization conditions in the isomerization zone include the temperature of the reactor, which usually varies from 40 to 250 ° C. Generally, lower process temperatures are preferred, which are favorable for obtaining equilibrium mixtures having the highest concentration of high octane highly branched isoalkanes, and to minimize cracking of the feed to form lighter hydrocarbons. In the present invention, a temperature range of 100 to 200 ° C. is preferred. The operating absolute pressure in the reactor typically ranges from 100 kPa to 10 MPa, preferably between 0.5 and 4 MPa (absolute pressure). The volumetric flow rate of the liquid varies from about 0.2 to 25 liters of isomerizable hydrocarbon feed per hour per liter of catalyst volume, with a range of 0.5 to 15 h ^-1 being preferred.

Hydrogen is mixed or remains with the isomerization feed in the isomerization zone in order to provide a molar ratio of hydrogen to hydrocarbon feed from 0.01 to 20, preferably from 0.05 to 5. Hydrogen can be supplied completely from an external source or replenished with hydrogen, recycle in the feed, after separation from the stream leaving the isomerization reactor. Light hydrocarbons and small amounts of inert gases such as nitrogen and argon may be present in hydrogen. Water must be removed from hydrogen coming from an external source, preferably using an adsorption system that is known in the art. In preferred embodiments, the molar ratio of hydrogen to hydrocarbons in the stream leaving the reactor is 0.05 or less, so there is usually no need to recycle hydrogen from the stream leaving the reactor to the feed.

Especially in the case of using a chlorinated catalyst for isomerization, the stream leaving the isomerization reactor is contacted with the sorbent in order to remove any chloride components, as described in US patent No. 5705730.

Distillation and membrane separation

The product of the isomerization process is subjected to a separation procedure one or more times in order to obtain a higher octane product fraction and, optionally, to remove other components, such as hydrogen, lower alkanes and mainly halide compounds in connection with chlorinated catalysts.

In the usual practice of the isomerization process, the isomerization reaction proceeds in the liquid phase, and the isomerization stream leaving the reactor enters the product separator, which forms a gaseous overhead containing hydrogen and lower alkanes. At least a portion of this hydrogen can be recycled to the isomerization reactor to provide at least a portion of the hydrogen desired for isomerization. The liquid bottoms are fed to a distillation unit (deisohexanizer) to obtain a low boiling fraction, dimethyl butanes and a higher boiling fraction containing normal hexane. Most often, a deisohexanizer operates in a mode in which a stream containing normal hexane is formed as a side stream and a bottoms stream containing normal heptane is obtained. The deisohexanizer may be a column with a nozzle or plates, and in normal operation, the overpressure at the top of the column is between 50 and 500 kPa, and the temperature of the bottom residue is between 75 and 170 ° C.

The composition of the low boiling fraction leaving the deisohexanizer will depend on the operating mode and design of the distillation unit and on any separation processes to which the stream from the isomerization reactor is subjected. For example, if the deisohexanizer stream contains light hydrocarbons, such as C ₁ -C ₄ , the deisohexanizer operating mode may provide an overhead containing these light hydrocarbons and a side overhead containing C ₅ compounds and branched C ₆ compounds, mainly dimethyl butanes. Typically, the low boiling fraction contains from 20 to 60% by weight of dimethylbutanes; from 10 to 40% by weight of normal pentane; and from 20 to 60% by weight of isopentane and butane. Depending on the operating mode of the deisohexanizer, the low boiling fraction may also contain a significant amount of methyl pentanes, for example at least about 10% by weight. In addition, the deisohexanizer can operate in a C ₅ rich stream mode, in addition to the low boiling fraction.

The boiling fraction containing normal hexane also contains methyl pentanes and methyl cyclopentane. As noted above, the methods according to the invention provide a more economical mode of operation of the deisohexanizer, which leads to an increased concentration of dimethylbutanes in the fraction containing normal hexane. Often a fraction containing normal hexane may contain from 2 to 10 wt.% Dimethylbutanes; from 5 to 50 wt.% normal hexane; from 20 to 60 wt.% methylpentanes, and from 5 to 25 wt.% methylcyclopentane. Typically, a deisohexanizer will be designed to produce a side stream that contains methyl pentanes, methyl cyclopentane, normal hexane, dimethyl butanes and cyclohexane, and a bottoms stream that contains cyclohexane and C ₇₊ hydrocarbons. If the fraction containing normal hexane is a bottoms stream of a deisohexanizer, this fraction also contains such higher hydrocarbons.

As noted above, the possibility of extracting dimethylbutanes from the higher boiling fraction containing normal hexane allows distillation at a lower reflux ratio. The used reflux value will depend on the nature of the feed entering the column, as well as on the construction of the column, and thus can vary over a wide range, for example, from 1.5: 1 to 2.5: 1, based on the ratio of the reflux mass to the feed.

At least a portion, preferably at least 50%, and more preferably at least 80% to almost the entire mass of the fraction containing normal hexane, from the deisohexanizer is contacted with the inlet side of the selective membrane in order to obtain a retained fraction of the product isomerization, which has a reduced total concentration of linear paraffins, and get on the other side of the membrane diffused fraction, which has an increased total concentration of linear paraffins. Preferably, the diffused fraction contains at least 75% by weight. normal hexane, preferably at least 75% wt. normal hexane, in the fraction in contact with the membrane. The retained product preferably contains at least 50%, more preferably at least 80 to almost 100% methylcyclopentane contained in the fraction in contact with the membrane. In preferred aspects of the present invention, the membrane provides at least 25%, for example from 30 to 90% by weight of methyl pentanes contained in the fraction containing normal hexane and in contact with the permeable membrane.

A pressure differential is maintained on the membrane in order to achieve the desired separation with an acceptable permeability rate. Membranes of any suitable type may be used, including diffusion and sieve, and they may be composed of inorganic, organic, or composite materials. For diffusion membranes, the driving force is the difference in partial pressures of the product at the inlet and outlet sides. In sieve membranes, the absolute pressure drop is a significant part of the driving force, regardless of partial pressures or concentrations. Preferred membranes are sieve membranes having a C ₆ permeability flux index of at least 0.01, and a C ₆ relative permeability flux of at least 1.25: 1. Sieve membranes are described in more detail below.

In membrane separation, the pressure drop often varies from 0.1 to 10, preferably from 0.2 to 2 MPa. In practice, the overhead of the deisohexanizer will be in contact with the inlet side of the membrane without additional compression to minimize capital and operating costs. The temperature of the membrane separation will depend in part on the nature of the membrane and the temperature of the fraction. So, for membranes containing polymers, the temperature should be low enough so that the strength of the membrane does not deteriorate excessively. In most cases, the separation temperature coincides with the temperature of the deisohexanizer fraction. Often the temperature is in the range of 25 to 150 ° C. Thus, in the conditions of membrane separation, it is possible to obtain a liquid or gaseous or mixed phase at the inlet side of the membrane. Regardless of the phase state of the fluid at the inlet side, the diffused substance may be gaseous. If the fluid on the inlet side of the membrane is in the liquid phase, the diffused substance may be in the liquid, gaseous, or mixed phase.

Any suitable selectively permeable membrane may be used in the apparatus and method of the present invention. Sieve membranes are preferred. The membranes used in the method according to the invention are characterized in that they have high permeability, that is, have a C ₆ permeability flow index of at least 0.01. These membranes can be of any suitable shape, such as hollow fibers, sheets and the like, which can be mounted in the separator unit in the form of a bundle of hollow fibers or a flat plate or sheet membranes twisted into a spiral. After mounting in the separator unit, the physical design of the membranes must provide a sufficient pressure drop across the membrane to obtain the desired permeability. For hollow fiber membranes, typically the high pressure region (inlet side) is outside the hollow fibers. The flow of diffused substance can be directed in parallel, countercurrent or cross-flow relative to the fluid flow on the inlet side of the membrane.

Sufficient membrane surface area and pressure drop across the membrane are provided in order to obtain, under stationary conditions, a diffusible fraction having at least 75%, preferably at least 80% and more preferably at least 90% by weight of normal hexane contained in a fraction from a deisohexanizer. The concentration of normal hexane will depend on the selectivity of the membrane. Although the membrane can be highly selective and can provide a diffused substance containing 99 wt.% Or more of normal hexane, advantageous embodiments of this invention can be carried out with lower purity of the diffused substance. In these embodiments, the concentration of normal hexane in the whole diffused substance will be less than 90 wt.%, For example from 25 to 90, for example from 40 to 80 wt.%. The remainder of the stream is usually methylpentanes and some methylcyclopentane and dimethylbutanes that pass through the membrane.

In preferred high permeability sieve membranes, part of branched paraffins is allowed to penetrate. The relative permeability rate will depend on the configuration of the paraffin molecules. Methylpentanes will more easily pass through the membrane than dimethylbutanes and cyclopentane.

Preferably, at least a portion of the diffused substance is recycled to the isomerization step. This diffused substance will contain linear paraffins that can undergo isomerization under the conditions of the isomerization step.

Screen membranes

Preferred sieve membranes can be of various types, for example molecular sieves, ceramics containing pores, metal, polymer or carbon membranes, or composite membranes having a highly porous polymer, metal, molecular sieve, ceramic or carbon carrier with a thin sieve layer or barrier (molecular sieve ), for example zeolite, polymer, metal, ceramic or carbon, having microporosity.

Membranes can be continuous or intermittent. The discontinuous membrane includes a combination of microporous barriers with small particles, while the continuous membrane contains a continuous layer of microporous barriers. Membranes can be formed from one material or they can be composite containing a microporous barrier and a carrier and, optionally, other structures. Upon receipt of a thin continuous barrier layer with a decrease in the thickness of the sieve layer, the difficulties of obtaining a defect-free layer increase. Since high selectivity is not required in the methods of the present invention, membranes may contain few defects. Typically, continuous membranes are obtained by precipitation or growth on the meso / macroporous structure of a continuous thin layer of microporous barrier. Discontinuous aggregates of microporous nanometer-sized barriers allow for a very small permeability thickness, but with a possible workaround. Discontinuous membranes use a meso / macroporous structure to which a microporous barrier is bound.

Examples of zeolite barriers include small pore molecular sieves such as SAPO-34, DDR, AlRO-14, AlRO-17, AlRO-18, AlRO-34, SSZ-62, SSZ-13, zeolite 3A, zeolite 4A, zeolite 5A zeolite KFI, H-ZK-5, LTA, UZM-9, UZM-13, ERS-12, CDS-1, Philipsite, MSM-65 and MSM-47; medium pore molecular sieves such as silicalites, SAPO-31, MFI, BEA and MEL; large-pore molecular sieves such as FAU, OFF, NaX, NaY, CaY, 13X and zeolite L; and mesoporous molecular sieves such as MCM-41 and SBA-15. A number of types of molecular sieves are available in colloidal form (nanoscale particles) such as A, X, L, OFF, MFI, and SAPO-34. These zeolites may have exchange metals or they are absent.

Other types of sieve materials include carbon sieves; polymers such as PSM (self-microporous polymers), such as those described by McKeown, et al., Chem. Commun., 2780 (2002); McKeown, et al. Chem. Eur. J., 11: 2610 (2005); Budd, et al., J. Mater, Chem., 13: 2721 (2003); Budd, et al., Adv. Mater., 16: 456 (2004) and Budd, et al., Chem Commun., 230 (2004); polymers in which porosity is caused by pore-forming agents such as poly (alkylene oxide), polyvinylpyrrolidone; cyclic organic host crystals such as cyclodeketrins, calixarenes, crown ethers, and spherands; microporous organometallic scaffolds such as MOF-5 (or IRMOF-1); glass, ceramic and metal forms into which i microporosity is introduced.

Composite membranes use a meso / macroporous structure. This meso / macroporous structure performs one or more functions, depending on the type of membrane. It can be a carrier for the membrane composite material, it can be an integral part of the forming microporous barrier, m can be a structure on which or in which the microporous barrier is located. The meso / macroporous structure may be continuous or discontinuous and thus the meso / macroporosity may be channels within the material of the meso / macroporous structure or may be formed between particles that form the meso / macroporous structure. Examples of the latter type are AccuSep ™ inorganic filtration membranes supplied by Pall Corp. and having a zirconia layer on a porous metal support, the zirconia being in the form of spherical crystals.

Preferably, the meso / macroporous structure defines channels or pores having a diameter in the range from 2 to 500, preferably from 10 to 250, more preferably between 20 and 200 nm, and having high permeability. In more preferred embodiments, the C ₆ permeability flow index for the meso / macroporous structure is at least 1, and most preferably at least 10, and sometimes at least 1000. The meso / macroporous structure can be isotropic or anisotropic. Meso / macropores can be relatively straight or winding.

The meso / macroporous structure may be composed of inorganic, organic or mixed inorganic and organic materials. The choice of material will depend on the separation conditions, as well as on the type of meso / macroporous structure formed. The material of the meso / macroporous structure may be the same as or different from the molecular sieve material. Examples of porous structure compositions include metals, aluminas such as alpha alumina, gamma alumina and transition alumina, molecular sieves, ceramics, glass, polymers and carbon. In preferred embodiments, the defects in the substrate “heal” to form a barrier or barrier precursor. In another embodiment, the substrate may be treated with a silica sol to partially clog pores and facilitate deposition of the barrier or barrier precursor. Between the particles of silicon dioxide there is still enough space in order to provide a high permeability rate. Another technique is coating the carrier with silicone rubber or another polymer that provides high permeability, but clogs defects in the carrier or in the barrier.

If the meso / macroporous structure does not perform this function, the membrane may contain a porous carrier for the meso / macroporous structure. Typically, a porous carrier is selected taking into account strength, endurance under the conditions of intended separation and porosity.

AccuSep ™ inorganic filtration membranes supplied by Pall Corp. and similar types of meso / macroporous structures are particularly attractive since the meso / macroporous structure can be thin and thereby eliminate the excessive thickness of the grown molecular sieve. In addition, zirconia is relatively inert in solutions of zeolite-forming precursors, and the synthesis and calcination conditions make zirconia a preferred meso / macroporous structure for this type of sieve membranes.

High permeability is achieved, at least due to one of the following methods: in the first, larger pores are used than are required for the passage of normal alkanes; and the second uses a very thin layer containing pores. When high permeability is achieved by using larger, less selective micropores in the microporous barrier, appropriate separation can be realized. Often the pores in membranes of this type have an average diameter greater than 5.0 Å (average length and width), for example from 5.0 to 7.0 or 8 Å. Preferably, these structures have a length to width ratio of less than 1.25: 1, for example from 1.2: 1 to 1: 1. For membranes containing a molecular sieve, examples of structures are USY, ZSM-12, SSZ-35, SSZ-44, VPI-8, and cancrinite. In some cases, molecules that penetrate micropores can enhance selectivity. For example, normal hydrocarbons in pores can reduce the rate at which branched hydrocarbon molecules can enter the pore compared to other normal hydrocarbon molecules.

In addition, high permeability can be achieved through the use of a very thin microporous barrier in both continuous and discontinuous membranes. Optionally, the microporous barrier can be selected from sieve structures having micropores that are substantially impermeable to molecules with functional groups, which it is desirable to keep on the input side. In general, pores in membranes of this type have an average diameter of up to about 5.5 Å, for example from 4.5 to 5.4 Å. The ratio of pore length to width in these membranes can vary over a wide range, usually in the range from 1.5: 1 to 1: 1. For membranes containing a molecular sieve, examples of structures are ZSM-5, silicalite, ALPO-11, ALPO-31, ferrierite, ZSM-11, ZSM-57, ZSM-23, MCM-22, NU-87, UZM-9 and CaA.

Membranes containing an intermittent collection of microporous barriers are characterized in that the barrier has a main size of less than 100 nm, and this microporous barrier is associated with a meso / macroporous structure that defines the fluid flow in pores in which the barrier is positioned so as to impede fluid flow through the pores of the meso / macroporous structure. The molecular sieve barrier is “bound” to the meso / macroporous structure when it is located on the structure (or inside), regardless of the connection with the structure. Therefore, nm-sized particles or molecular sieve islands are used as barriers to membranes. Intermittent microporous barriers are positioned so as to impede fluid flow through fluid flow channels defined by the meso / macroporous structure. This barrier may at least partially plug the opening of the fluid flow passage in the meso / macroporous structure and / or within the fluid flow passage. Due to the small size of the particles or islands forming an intermittent collection of microporous barriers, a certain separation selectivity can be achieved despite the discontinuity of the membrane.

Typically, the size and configuration of the molecular sieve particles and the size and configuration of the meso / macropores in the meso / macroporous structure will be taken into account when selecting components for sieve membranes. When the particles of the molecular sieve, such as silicalites, are more spherical in shape, a meso / macroporous structure is preferably chosen in which there are pores close to the same effective diameter. Thus, in the particles of the molecular sieve, if they are at least partially located in the pores of the meso / macroporous structure, there will remain a minimum volume of voids for the bypass flow. Particles of a molecular sieve with a plate and irregular shape have more elasticity, since they can overlap, leaving little empty space (or it is absent). In some cases, a combination of molecular sieve configurations may be desirable. For example, spherical particles of a molecular sieve can be involved inside the pores of the meso / macroporous structure, followed by the introduction of smaller lamellar particles of the molecular sieve. Additional functions are that the sphere serves as a carrier for lamellar particles, and the lamellar particles overlap in order to reduce bypass flow. Although molecular sieves are likely to have a different composition and thus have a different size and configuration of micropores, the advantage is increased separation, without excessive loss of permeability.

When using zeolite molecular sieves, it is important to obtain fine particles in order to achieve high permeability in the intermittent microporous barrier. For many zeolites, seed crystals are available whose main size is less than 100 nm. Most molecular sieves are obtained using organic templates that must be removed to allow access to the framework. Typically, this removal is carried out by calcination. As discussed below, calcination can be performed when the molecular sieve containing the template is located in the macropore so as to prevent excessive agglomeration, simply by limiting the number of neighboring particles. Another technique to prevent agglomeration of zeolite particles during calcination is to silylate the surface of the zeolite, for example aminoalkyltrialkoxysilane, aminoalkylalkyl dialkoxysilane or aminoalkyl dialkyl alkoxysilane. The degree of silylation required will depend on the size and composition of the zeolite particles, as well as on the conditions that will be used in the calcination. Typically, 0.1 to 10 millimoles of silane per gram of zeolite are used.

There are various techniques for introducing molecular sieve particles onto (or into) a meso / macroporous carrier such that at least partially clog the meso or macropores in the carrier. The choice of a specific technique for use will depend on the size and configuration of the molecular sieve particles, the size and configuration of the meso / macropores in the meso / macroporous structure, and the desired placement of the molecular sieve inside (or on) the meso / microporous structure.

Mainly, when the molecular sieve is located on the surface of the meso / macroporous structure in order to clog at least part of the pore mouths, the meso / macroporous structure can be wetted with a solution or suspension of a molecular size nanometer. The concentration of the molecular sieve in the suspension must be sufficiently low so that the layer of molecular sieve formed after drying is not excessively thick. Advantageously, at least a small pressure drop over the meso / macroporous structure is maintained during the coating in order to provide a driving force involving molecular sieve particles in any pores in the meso / macroporous structure that are not clogged. Typically, the suspension will be an aqueous suspension, although mainly suspensions in alcohols and other relatively inert liquids can be used at a concentration between 2 and 30%, for example from 5 to 20% by weight. When using a differential pressure, the differential pressure is usually in the range of 10 to 200 kPa. One or more molecular sieve coatings may be used, with preferred drying between coatings. Drying is usually carried out at elevated temperatures, for example between 30 and 150 ° C, for 1 to 50 hours. A vacuum may be used to facilitate drying. When zeolites are used as a molecular sieve, calcination, for example, at temperatures between 450 and 600 ° C, in some cases, may contribute to the attachment of the molecular sieve to the meso / macroporous structure. Calcination can also contribute to the agglomeration of molecular sieve particles and thus reduce the number and size of voids. Of course, calcination is not an essential operation for the broad concept of the present invention and is required, for example, only when the template remains in micropores.

When a discontinuous collection of nanometer-sized molecular sieve particles is located outside the pores of the meso / macroporous structure, it may be desirable to link at least a portion of the particles to the surface of the structure. This can be done in various ways. For example, the surface of structures can be functionalized with hydroxyl groups or other functional groups that can react with a zeolite molecular sieve. The surface of the polymer molecular sieves can be functionalized with functional groups that interact, for example, by addition or condensation reactions with functional groups in the polymer. These techniques are well known in the art for other applications.

Similar production techniques can be used when it is desirable to introduce at least a portion of the molecular sieve particles into the pores of the meso / macroporous structure. These molecular sieve particles must be of appropriate size in order to enter the meso / macropores. In order to engage the barrier particles in the pores, a pressure drop or ultrasound can be used, which facilitates the entry of the barrier particles into the pores of the meso / macroporous carrier. The penetration depth of the molecular sieve particles into the pores of the meso / macroporous structure should not be too large to excessively reduce the permeability. Often any deposition on the surface of the molecular sieve is removed, for example, by washing.

Optionally, zeolite molecular sieves can be grown in situ (in situ) in the pores of the meso / macroporous structure to produce discontinuous membranes. In the synthesis, isolated particles or islands between other structures, such as a meso / macroporous structure or other particles, can be obtained.

An example of other particles for producing discontinuous membranes of zeolite molecular sieves includes the use of silicon dioxide, particles of which may have a size between 5 and 20 nm, inside the meso / macroporous structure (or on the structure). Due to the presence of active hydroxyl groups on the surface, silicon dioxide plays the role of the formation site of zeolite nuclei from the precursor solution, and zeolite layers can be grown on and between silica particles.

As the nucleation site, materials other than silica particles can be used, including other molecular sieves or seed crystals of the same zeolite. The surface of the meso / macroporous structure can be functionalized, which provides selective growth centers of the zeolite. Some zeolites have the property of initiating nucleation, and thus they can be used in the absence of a nucleation site. Examples of such zeolites are FAU and MFI. In such cases, it may be desirable to withstand the precursor solution under conditions of zeolite formation for a time that is sufficient for the zeolite to begin to grow before the precursor solution contacts the meso / macroporous structure.

For example, one way to form a barrier layer is to place a liquid zeolite molecular sieve precursor on a meso / microporous structure. This precursor has the ability to crystallize under hydrothermal crystallization conditions; after that, the membrane is washed and heated to remove residual organic material. The molecular sieve material remains mainly inside and clogs the pores of the porous substrate.

The molecular sieve may be any suitable combination of elements that provides the desired porous structure. Aluminum, silicon, boron, gallium, tin, titanium, germanium, phosphorus and oxygen can be used as building blocks of molecular sieves, such as aluminosilicate molecular sieves, including zeolites; silicalites; AlRO; SAPO and boron silicates. The precursor compound includes the above elements, usually in the form of oxides or phosphates, together with water and an organic structuring agent, which is usually a polar organic compound, such as tetrapropylammonium hydroxide. In addition, other auxiliary agents, such as amines, ethers and alcohols, may be used. The mass ratio of the polar organic compound to the materials of the building block is usually in the range from 0.1 to 0.5 and will depend on the particular building block used. In order to obtain thin layers of a molecular sieve in the membranes, it is usually preferred that the precursor solution contains a lot of water. For example, aluminosilicate molecular sieves, the molar ratio of water to silicon dioxide should be at least 20: 1, and for aluminophosphate molecular sieves, the molar ratio of water to aluminum should be at least 20: 1.

Crystallization conditions are often in the range of 80 to 250 ° C. with an absolute pressure in the range of 100 to 1000, often 200 to 500 kPa. The crystallization time is limited so that an excessively thick molecular sieve layer does not form. Typically, the crystallization time is less than 50 hours, for example from 10 to 40 hours. Preferably, the crystallization time is sufficient for the formation of crystals, but less than the time required for the formation of a molecular sieve layer with a thickness of about 200 nm, for example from 5 to 50 nm. Crystallization can be carried out in an autoclave. In some cases, when heated by microwave radiation, crystallization will take place in a shorter time. After that, the membrane is washed with water and then calcined at a temperature of 350 to 550 ° C to remove any organic matter.

Mostly, for some zeolite molecular sieve materials, obtaining particles smaller than 100 nm is difficult. Moreover, even when using seed crystals, the particle size may be larger than desired. Another embodiment of the preparation of an intermittent barrier membrane is the synthesis of zeolite in the open space between particles (substrates) having a principal size of less than about 100 nm. Accordingly, the main size of the microporous barrier may be less than 100 nm. The substrate particles play the role of the nucleation site of zeolite crystals and are thus selected from materials with the ability to nucleate during zeolite growth. Examples of such materials are silica, mainly silica particles having a main size between 5 and 50 nm, and other zeolites having a main size less than 100 nm. The use of fuming silica as support particles is particularly effective in the preparation of the microporous AlPO barrier.

The growth of zeolite on a substrate particle can occur before or after using the substrate particles in the formation of the composite membrane.

Advantageously, zeolite growth on a support particle occurs when a synthesis fluid is drawn through the composition. This technique allows us to ensure that growth does not occur in the form of a layer on top of the particle, but in the gaps between the particles. As the zeolite grows, the pressure drop increases, and this pressure drop can be used as an indicator of the achievement of the corresponding degree of zeolite formation.

Polymer molecular sieves can be synthesized in a meso / macroporous structure. One way to synthesize small polymer molecular sieves is to functionalize the nanoparticles and / or the meso / macroporous structure with groups that can interact with the oligomer, such as through condensation or addition reactions. For example, the functional groups may contain hydroxyl, amino, anhydride, dianhydride, aldehyde, amino acid, carboxyl, amide, nitrile or olefin group for the reaction of addition or condensation with the reactive group of the oligomer. Suitable oligomers can have a molecular weight of from 30,000 to 500,000 or more and can be reactive polysulfone oligomers; poly (styrenes), including styrene-containing copolymers; cellulose polymers and copolymers; polyamides; polyimides; poly (ethers); polyurethanes; poly (esters); acrylic and methacrylic polymers and copolymers; polysulfides, polyolefins, mainly vinyl polymers and copolymers; polyallyls; poly (benzimidazole); polyphosphazines; polyhydrazides; polycarbodiids, and the like. The in situ synthesis of both inorganic and organic molecular sieves can be carried out under suitable conditions. A preferred technique involves performing synthesis by passing a reagent solution, for example a precursor solution or an oligomer solution, through a meso / macroporous structure. This technique provides an advantage, since the reagent solution is sent to voids that have not been clogged, and also limits the growth rate of the molecular sieve, since fresh portions of the reagent will not be able to enter the reaction zone when the molecular sieve clogs the meso- or macropore.

In addition, the molecular sieve on the membranes of the polymer carrier or the polymer carriers themselves can be pyrolyzed in a vacuum oven to obtain a carbon membrane. For such membranes containing molecular sieves, the pores in the carbon support structure preferably have a sufficient diameter to minimize the flow resistance of the molecular sieve separation fluids. The pyrolysis temperature will depend on the nature of the polymer carrier and will be lower than the temperature at which the porosity decreases excessively. Examples of polymeric carriers include polyimides, polyacrylonitrile, polycarbonates, poly (simple) ether ketones, poly (simple) ether sulfones and polysulfones, and prior to pyrolysis, the carriers have pores or openings ranging in size from 2 to 100 nm, preferably from 20 to 50 nm.

Continuous membranes can be obtained using any suitable technique. Typically, the thickness of the microporous barrier will be related to the duration of deposition or growth of the microporous barrier on the meso / macroporous structure. A microporous barrier can be formed by reducing the pore size of the ultrafiltration membrane (effective pore diameter from 1 to 100 nm) or the microfiltration membrane (effective pore diameter from 100 to 10,000 nm), for example, by using an organic or inorganic coating on the channel or on the inner surface, or preferably at least partially near the channel opening. The deposited material is needed to obtain a local reduction in pores or holes in the entire support to a size that provides the desired sieve effect, without overly reducing the diameter of the remaining porous support structure. Examples of vapor deposited materials include silanes, para-xylene, alkylene imines and alkylene oxides. Another technique for reducing pore size is to deposit a coke layer on a meso / macroporous structure. For example, a carbonizable gas, such as methane, ethane, ethylene or acetylene, can contact the structure at a temperature high enough to cause coking. Preferred porous supports are ultrafiltration membranes having a pore size between 1 and 80 nm, preferably between 2 and 50 nm.

To obtain zeolite continuous membranes, one of the methods involves contacting the surface of the meso / macroporous structure with a molecular sieve precursor and growing the molecular sieve for a time that is sufficient to achieve a given film thickness. The techniques described above can be used to synthesize a molecular sieve. In some cases, it may be desirable to clog the meso / macropores of the support with, for example, wax to prevent excessive zeolite growth in these pores. Then the wax can be removed.

To increase the selectivity of membranes with high permeability, there are various techniques. Numerous techniques are available for healing defects in continuous or discontinuous membranes. When membranes are not required to have a high permeability flow ratio

With ₆ , which is desirable in many applications, any techniques that increase the resistance to flow through defects will help improve membrane performance. For example, coating a silica sol can be used to plug intermediate holes between molecular sieve crystals or remaining large pores in a carrier, regardless of how the membrane is prepared.

Another technique for clogging large pores is that on the one side of the barrier layer are large reactive molecules that are not able to diffuse into the micropores of the barrier, and on the other hand there is a crosslinking agent. The main defects and to some extent secondary defects are filled with these large reactive molecules and fixed by crosslinking. Then, the unreacted portion of the large molecules can be removed in the same way as the unreacted crosslinking agent. These large molecules can be oligomeric or large molecules.

For discontinuous membranes, a solid can be provided in at least a portion of the voids between the particles or islands of the microporous barrier and between the microporous barrier and the meso / microporous structure.

One common technique to enhance the selectivity of sieve membranes is to agglomerate adjacent molecular sieve particles to reduce or substantially eliminate voids between particles and between particles and walls of a porous structure in a meso / macroporous structure. Since the particles have a nanometer size and the number of adjacent particles can be relatively small, agglomeration can occur, however, while maintaining the desired flow rate of the diffusing substance. For polymer molecular sieves that are thermoplastic, agglomeration can occur by heating to a temperature at which agglomeration occurs, but not so high as to lose the microporous structure of the sieve or its ability to provide the desired occlusion of the meso- or macropores of the meso / macroporous structure. In addition, agglomeration can be performed by calcining zeolite molecular sieves. Calcination causes agglomeration of small zeolite particles, mainly particles that have not been silylated or otherwise processed, which reduces the tendency to agglomerate. The temperature and duration of calcination will depend on the nature of the zeolite molecular sieve. Typically temperatures between 450 and 650 ° C are used for a period between 2 and 20 hours.

The agglomeration technique can be used with respect to molecular sieve particles that are on the surface of the meso / macroporous structure, as well as particles within the pores of the structure. Most preferably, agglomeration is used when the molecular sieve particles are inside the meso- or macropores of the meso / macroporous structure so that the main size of the agglomerated particles is less than 200 nm, preferably less than 100 nm. Agglomeration can be carried out in the presence or absence of a pressure drop across the membrane. Preferably, the differential pressure is used to reduce voids through which the fluid flow can bypass the molecular sieve.

Another common technique in which voids are defined as an intermittent set of barriers is that at least partially clog at least a portion of the voids with a solid. Preferably, this solid is a polymeric or inorganic material. The solid may simply be in the voids or it may be attached or bonded to a molecular sieve or meso / macroporous structure. The solid can be dispersed or oligomeric, and can be preformed and then introduced into voids or it can be formed in place.

In one design, a solid provides a “mortar” for microporous barrier particles. Typically, this mortar is a suitable polymeric material that can withstand separation conditions. Typical polymers include polysulfones; poly (styrenes), including styrene-containing copolymers; cellulose polymers and copolymers; polyamides; polyimides; poly (simple) ethers; polyurethanes; poly (ester) esters; acrylic and methacrylic polymers and copolymers; polysulfides, polyolefins, mainly vinyl polymers and copolymers; polyallyls; poly (benzimidazole); polyphosphazines; polyhydrazides; polycarbodiids and the like. Preferred polymers are those in which porosity is present, such as PSM (see WO 2005/012397) and polymers in which porosity is created using pore-forming agents. These polymers have pores that can have a major size of 0.3 nm or more, preferably at least 1 nm, and therefore fluid can pass to and from the barrier particle.

It is not necessary that all particles be enclosed in a mortar. Often the average thickness of the mortar layer is less than 100 nm, and preferably, approximately does not exceed the main particle size. If too much mortar is used, a mixed membrane structure may result, and permeability will be excessively limited. Therefore, the mass ratio of barrier particles to mortar is often in the range from 1: 2 to 100: 1, preferably between 3: 1 and 30: 1.

Mortar and particles can be mixed, for example, in a suspension and then placed in conjunction with a microporous structure, or can be provided after the particles are deposited. The polymer may be formed in place, in an area containing barrier particles. These barrier particles may be inert with respect to polymerization or may have active sites for polymer attachment. For example, the particle can be functionalized with a reactive group that can bind to the polymer or to the monomer undergoing polymerization, for example, as a result of the condensation or addition processes discussed above.

It matters that the mortar clogs the micropores of the molecular sieve. In the case of highly porous polymers such as PSM, any effect of occlusion can be attenuated. Often the amount of polymer used as a mortar, its molecular weight and configuration are such that not enough polymer is available to encapsulate all particles of the molecular sieve. Typically, the weight ratio of polymer to molecular sieve is between 0.01: 1 and 0.3: 1. The average molecular weight of the polymer is sometimes in the range of 20,000 to 500,000, preferably between 30,000 and 300,000.

Mortar may differ from polymer. For example, when the molecular sieve is a zeolite, silicon tetraalkoxide can interact with the zeolite and form a silica skeleton or mass between the particles of the molecular sieve by hydrolysis. Usually a dilute aqueous solution of silicon tetraalkoxide is used, for example, containing between 0.5 and 25% silicon tetraalkoxide by weight to ensure distribution. The functionalization of zeolite with silicon tetraalkoxide is also effective as a center for crosslinking with an organic polymer, mainly with those polymers that contain functional groups, such as hydroxyl, amino, anhydride, dianhydride, aldehyde or amino acid groups, which can form covalent bonds with organosilicon alkoxide. In addition, particles of the same or different zeolite can be grown between zeolite particles and a meso / macroporous structure using the techniques described above.

Another approach to reducing bypass flow is to use particles of two or more sizes to form a layer containing the barrier. If, for example, particles of a microporous barrier are usually spherical with a nominal basic size of about 60 nm, then the areas between the particles can be significant to provide a bypass flow. The inclusion of configurationally compatible particles in these regions can further restrict the flow of fluid, and thus, as a result, most of the fluid is directed toward the barrier particles for selective separation. The configuration of the barrier particles will depend on the type of barrier particles used. Microporous particles of a zeolite molecular sieve having a basic size of less than 100 nm are likely to have a specific configuration due to their crystal structure. Some zeolites tend to have a plate type configuration, while others, such as AlPO-14, have a rod-like structure. Similarly, particles of polymer, ceramic, glass and carbon molecular sieves can have a configuration that is difficult to change. Therefore, the configuration of the open regions between particles can vary widely.

Sometimes, configurationally compatible particles are selected so as to obtain at least partial clogging of the area. Thus, for spherical barrier particles, rod-shaped or much smaller configurationally compatible particles may be desirable. Configurationally compatible particles can have any suitable composition for a given size and operating conditions. These particles can be polymer, including oligomeric, carbon and inorganic, such as fuming silica, zeolite, alumina and the like.

Detailed description of the drawing

The drawing schematically shows that the feedstock containing linear paraffins enters the isomerization unit through line 102. Hydrogen is supplied through line 104. The combined stream enters the isomerization reactor 106. The stream exiting from the isomerization reactor 106 is sent through a pipe 108 to the stabilization column 110. In this stabilization column 110, light fractions are removed via line 112 as an overhead. These light fractions can be used for any suitable purpose, including as valuable fuel. The bottom residue from the stabilization column 110 via line 114 enters the deisohexanizer 116. The overhead from the deisohexanizer 116 is discharged through line 118. The bottom stream from the deisohexanizer 116 is removed via line 120. The side stream from the deisohexanizer 116, which contains normal hexane, is passed through line 122 to the line upstream side of the membrane separator 124. A stream having a reduced concentration of linear paraffins is removed from the separator 124 via line 128. This stream contains an increased concentration of methylcyclopentane. As you can see, the retained product is sent through line 128 and mixed with the overhead in line 118. The fraction of the diffused substance is recycled through line 126 to the isomerization reactor 106.

Claims (15)

1. The method of isomerization of a feedstock containing paraffins having 5 and 6 carbon atoms, in which at least 15 wt.% Of the feedstock are linear paraffins to obtain an isomerization product containing isomerized paraffins, which includes:
a) isomerization of the feedstock under isomerization conditions, including a reactor temperature of 40 to 250 ° C and a working absolute pressure of 100 kPa to 10 MPa in the presence of an isomerization catalyst, to obtain an isomerization product containing linear paraffins, but at a concentration lower than in the original raw materials
b) distilling at least a portion of the isomerization product in order to obtain a low boiling fraction containing dimethyl butanes and light paraffins, and a high boiling fraction containing normal hexane, methyl pentanes, dimethyl butanes and methyl cyclopentane,
(c) contacting at least a portion of the fraction containing normal hexane from step b) with the inlet side of the selectively permeable membrane under conditions including sufficient membrane surface area and pressure drop across the membrane to obtain a retained fraction, in which there is an increased concentration of methylcyclopentane and dimethylbutanes, and to obtain on the output side of the membrane a diffused fraction having an increased concentration of normal hexane and methylpentanes, wherein said diffund th e fraction comprises at least 75 wt.% of normal hexane contained in the fraction comprising normal hexane contacted with the membrane, and d) removing retentate product of step c). 2. The method according to claim 1, characterized in that at least a portion of the diffused fraction from step c) goes to step a). 3. The method according to claim 2, characterized in that the stream from step b) containing normal hexane is a side stream, and during distillation in step b), a bottoms stream containing C ₇ hydrocarbons is obtained. 4. The method according to claim 3, characterized in that the concentration of normal hexane in the fraction of the diffused substance is less than 90 wt.%. 5. The method according to claim 4, characterized in that at least 30 wt.% Methyl pentanes in the fraction containing normal hexane and in contact with the inlet side of the sieve membrane, penetrate the output side of the membrane, and at least 70 wt. % dimethylbutanes in the fraction containing normal hexane and in contact with the inlet side of the sieve membrane are held on the inlet side of the membrane. 6. The method according to claim 5, characterized in that the fraction containing normal hexane includes methylcyclopentane, and methylcyclopentane is contained in the retained product. 7. The method according to claim 5, characterized in that the fraction containing normal hexane contains from 5 to 30 wt.% Dimethylbutanes in the stream from the isomerization stage. 8. The method according to claim 1, characterized in that the membrane is a sieve membrane having a C ₆ permeability flux index of at least 0.01 and a C ₆ permeability flux ratio of at least 1.25: 1. 9. The method according to claim 8, characterized in that the sieve membrane has an average pore diameter of from 5.0 to 7.0 Å. 10. The method according to claim 8, characterized in that the sieve membrane has an average pore diameter of from 4.5 to 5.4 Å. 11. The method according to claim 8, characterized in that the sieve membrane includes zeolite ZSM-5 or silicalite. 12. The method according to claim 1, characterized in that the fraction containing normal hexane contains at least 2 wt.% By weight of dimethylbutanes contained in the stream from the isomerization step. 13. The method according to claim 1, characterized in that the low-boiling fraction contains isopentane. 14. The method according to p. 12, characterized in that the low-boiling fraction is the overhead. 15. A device for the isomerization of a feedstock containing paraffins having 5 and 6 carbon atoms to obtain a gasoline fraction, which includes:
(a) an isomerization reactor having devices for receiving feedstock at the inlet and an outlet pipe,
(b) a hexane removal apparatus that has a liquid fluid inlet connected to an outlet pipe of the isomerization reactor, an outlet pipe of a low boiling fraction is used to remove the low boiling fraction along a line, an outlet pipe for selecting a side fraction and an outlet pipe of a high boiling fraction; and
(c) a membrane separator in which a side pipe for entering the liquid feed is connected to an outlet pipe that allows the side fraction of the hexane removal apparatus to be selected, a side pipe for output of the liquid feed is connected to a pipe for the higher boiling fraction of the hexane removal apparatus, and the outlet of the diffused liquid is connected with the entrance to the isomerization reactor.

Images