RU2382023C2 - Method for isomerisation of material containing paraffins with 5 to 7 carbon atoms, and device for realising said method - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to a method for isomerisation of starting material containing paraffins, having 5 to 6 carbon atoms in which at least 15 wt % of the starting material is linear paraffins, to obtain an isomerisation product containing isomerised paraffins, where the method involves: (a) isomerisation of the starting material under isomerisation conditions, including presence of an isomerisation catalyst, reactor temperature from 40°C to 250°C, operating absolute pressure in the reactor from 100 kPa to 10 MPa and volume rate for supply of liquid ranging from 0.2 to 25 l of the isomerised hydrocarbon material in h/l of catalyst volume, so as to obtain an isomerisation product containing linear paraffins, but in concentration less than in the starting material, (b) distillation of at least a portion of the isomerisation product to obtain a low-boiling fraction containing linear paraffins and a high-boiling fraction containing normal hexane, cyclohexane, methylpentanes and methylcycloheptane, (c) bringing at least a portion of the fraction containing normal hexane from step b) into contact with the inlet side of selectively permeable membrane to obtain a retained fraction with low concentration of methylcycloheptane to as to obtain a fraction diffused through the membrane at the outlet side of the membrane, where the said fraction has high concentration of normal hexane and methylpentanes and contains at least 75 wt % normal hexane in the fraction containing normal hexane in contact with the membrane; (d) removal of the retained fraction from step c).

EFFECT: reduced operational costs for the deisohexaniser without adverse effect on octane number of the overhead fraction from the deisohexaniser.

11 cl, 1 dwg

Description

FIELD OF THE INVENTION

The present invention relates to the improvement of isomerization methods for paraffins containing from 5 to 7 carbon atoms, for example, to obtain an isomerization product having an increased octane number (according to the research method) for mixing in compounded gasoline, and especially in such methods using an isohexane removal column (deisohexanizer).

State of the art

Methods for the isomerization of paraffins into 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 4-8 carbon atoms, mainly from 5 to 7 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) (HF im).

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 the context 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, such as those 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 overhead of a deisohexanizer often has an RN (octane by research method) in the range from 86 to 88.

One of the problems associated with the deisohexanizer is that high reflux ratios are required, and thus, a large boiler is required to separate closely boiling compounds having substantially different octane ratios, such as methyl pentanes and dimethylbutanes. In addition, a deisohexanizer is required in order to isolate from a mixture of hydrocarbons, normal hexane, which is used for isomerization to high octane isomers, and compounds containing 7 or more carbon atoms. Often the heat load for a deisohexanizer can be in the range of 15 or more kilocalories (62.8 kJ / mol) per liter of feed.

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 can be used to separate 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. In a computer simulation of membrane separation using MFI zeolite on alumina, Example 1 of this publication shows that a membrane surface area of 5000 square meters is required in order to remove approximately 95% by weight of n-pentane from the overhead of a deisohexanizer distillation column. At a flow rate of 75,000 kg / h of raw material into the membrane separator (containing 20.6% by weight of n-pentane), it turned out that the permeability of n-pentane used in the simulation is of the order of 0.01 gram mol / (m ² • c) at a temperature of about 300 ° C. It is noted that the magnitude of the NF the product after removal of n-pentane is 91.0.

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 Pat. No. 6,818,333 describes thin zeolite membranes which, as indicated, have n-butane permeability of at least 6 • 10 ^-7 mol / (m ² • m • Pa) and relative n-butane / isobutane selectivity of at least 250.

Mostly in refineries where large volumes of hydrocarbon feeds are isomerized, membranes with a very large surface area must be provided to achieve the desired separation of linear paraffins. For example, ZSM-5 / silicalite (MFI) membranes (sieve membranes) supplied by NGK Insulators, Ltd., Japan, which have linear paraffin permeability selectivity compared to branched paraffins, provide a flow in the operating range of about 0.1 from up to 1.0 milligram mol per second per square meter with a pressure drop of approximately 15 to 500 kPa. Thus, the cost of commercializing such a membrane separation system using these membranes, including membranes of the type described in US Pat. No. 6,818,333, makes them uncompetitive compared to adsorption separation or distillation separation systems, such as, for example, a deisohexanizer.

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-recycle" isomerization processes into cyclic processes with the separation and return of normal paraffins to the isomerization reactor. Even in existing cyclic processes in which deisohexanizers are used, there may be a need for an isomerization product with an increased octane number.

In accordance with this, methods are being developed to reduce the operating costs of a deisohexanizer without negatively affecting 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.

but. Microporosity

Micropores and microporosity refer to pores that have an effective diameter of about 0.3 to 2 nanometers.

b. Mesoporosity

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

from. Macroporosity

Macropores and macroporosity refer to pores that have an effective diameter of more than 50 nanometers.

d. Nanoparticles

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

e. molecular sieves

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

f. 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.

g. 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 to 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.

h. 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 a paraffin feed containing 5 to 7 carbon atoms, and these methods use deisohexanizers to isolate and recycle normal paraffins in the isomerization reactor. Advantageously, in these methods, operating costs for the deisohexanizer can be reduced without affecting the octane number of the overhead from the deisohexanizer. In accordance with the methods of the present invention, the deisohexanizer is operated in such a way that a significant portion of the normal hexane contained in the feed entering the deisohexanizer is removed with the bottoms stream and recovered from the bottoms stream by membrane separation.

Normal hexane is the desired linear paraffin for recycling to the isomerization reactor to convert to high-octane dimethylbutanes. Since the methods of the invention extract normal hexane from the bottoms stream, the thermal load on the deisohexanizer can be significantly reduced, often at least about 10 percent. In addition, since the volume of the bottoms stream from the deisohexanizer is typically less than about 20 percent of the volume of the feed stream, a much smaller membrane surface area will be required than for membrane separation of the isomerization product. In preferred aspects of the methods of the invention, the research octane number of the overhead from the deisohexanizer (C ₄₊ RH IM) is at least about 88, preferably at least about 88.5.

Broadly, the methods of the present invention include: (a) isomerizing a feed containing linear paraffins, including linear paraffins having from 5 to 7 carbon atoms, where at least 15 percent of the feed is linear paraffins, under isomerization conditions, including the presence of an isomerization catalyst, to obtain an isomerization product containing linear paraffins, including normal hexane, but in a lower concentration than in the feedstock,

(b) distilling at least a portion, preferably at least about 90 percent by weight and most preferably almost all of the isomerization product in order to obtain a low boiling fraction containing dimethylbutanes, a higher boiling point side stream containing normal hexane, and methyl pentanes, and a bottoms stream containing normal hexane and a heavy residue, where the side stream contains less than about 75, preferably between about 20 and 70 percent by weight of normal hexane distillation in this step b), wherein the bottoms stream contains at least about 25, preferably from about 30 to 80 percent by weight of the normal hexane to be distilled in this step b), and (c) contacting at least a portion preferably at least about 90 percent by weight and most preferably almost all of the bottoms from step b), with the inlet side of the selectively permeable membrane under conditions including a sufficient membrane surface area and a pressure drop of membrane in order to obtain a retained bottoms fraction in which there is a reduced concentration of normal hexane and to obtain on the outlet side of the membrane a diffused fraction having an increased concentration of normal hexane, wherein said diffused fraction contains at least about 50, preferably at least about 75 and most preferably at least about 90 percent by weight of normal hexane contained in the bottoms stream, contact th e membrane.

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

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, this device for isomerizing a feedstock containing paraffins with a carbon number of 5 and 6 to obtain a gasoline fraction includes:

(a) an isomerization reactor (106), in which there are devices for receiving 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 outlet pipe of the low boiling fraction is used to remove the low boiling fraction along line (122), the outlet pipe of the higher boiling fraction to select the side fraction ( 118) and the outlet pipe of the bottom residue (120); and

(c) a membrane separator (124), in which a side nozzle for liquid feed inlet is connected to a bottom residue outlet pipe (120), a liquid outlet pipe on the outlet side of the separator is connected to an outlet pipe of a higher boiling fraction (118) of hexane removal apparatus (116) and a side pipe for the exit of raw materials (126).

Brief Description of the Drawing

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 gasoline, synthetic naphtha, thermal cracking gasoline, catalytic cracking gasoline, partial reforming naphtha or raffinates after aromatic hydrocarbon extraction. Essentially, this feedstock enters 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 approximately 10 ° to 65 ° C and a boiling point of approximately 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 in H ₂ S, NH ₃ and H ₂ O, respectively which can be isolated from hydrocarbons by fractionation. It is preferable for such a conversion to use 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. 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 and feedstocks, the content of water-forming oxygen-containing compounds and deactivating sulfur-containing compounds is reduced to an acceptable level, and in general 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 7 carbon atoms in the molecule (C ₄ to C ₇ ), 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 by weight of the feedstock. Although there are no specific restrictions on the total cyclic hydrocarbon content of the feed, typically the feed contains approximately 2 to 40 percent by weight of cyclic naphthenic and aromatic hydrocarbons. Although the content of aromatic hydrocarbons in raw naphtha usually does not exceed the content of alkanes and cycloalkane hydrocarbons, it can be from 2 to 20 percent and more typically from 5 to 10 percent 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, the feedstock used in the method of the present invention includes at least about 15%, often from about 40, preferably at least about 50 percent, to about 100% linear paraffins. In the initial naphtha, linear paraffins are usually present in an amount of at least about 50%, for example, from 50 to 90 percent by weight. The mass ratio of non-linear paraffins to linear paraffins in the feedstock is often less than 1: 1, for example, from about 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 feedstock is passed into one or more isomerization zones. In accordance with 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 may be carried out in one or more flows. 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 about 35 and 90, preferably from about 40 to 70 percent by weight, the molar ratio of non-linear paraffins to linear paraffins is between about 0.2: 1 and 1.5: 1, and sometimes approximately 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 about 20%, preferably between 30 and 60 percent by weight of normal paraffins in the isomerization feed. Usually, under isomerization conditions, a level of at least about 70%, preferably at least about 75%, for example, from 75 to 97 percent 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 about 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 patent. US 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 the vapor phase, and excellent results are 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.

Isomerization conditions in the isomerization zone include the temperature of the reactor, which usually varies from about 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 from about 100 ° to about 200 ° C. is preferred. The operating absolute pressure in the reactor typically ranges from about 100 kPa to 10 MPa, preferably between about 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 from about 0.5 to 15 hours ^-1 being preferred.

Hydrogen is mixed or left with the isomerization feedstock in the isomerization zone in order to provide a molar ratio of hydrogen to hydrocarbon feedstock of from about 0.01 to 20, preferably from about 0.05 to 5. Hydrogen can be supplied completely from an external source or replenished by hydrogen recirculated 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.

Deisohexanizer and membrane separation

The stream leaving the isomerization reactor 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 about 50 and 500 kPa, and the temperature of the bottom residue is between about 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 stream to the deisohexanizer 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 percent by weight of dimethylbutane; from 10 to 40 percent by weight of normal pentane; and from 20 to 60 percent 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 percent by weight.

The design of the deisohexanizer provides a boiling fraction that contains methylpentanes, methylcyclopentane, a portion of normal hexane and cyclohexane, and a bottoms stream that contains normal hexane, cyclohexane and C ₇ + hydrocarbons. At least a portion of the higher boiling fraction is recycled. As noted earlier, distillation can be operated in a more economical mode by using lower reflux ratios and thus more normal hexane gets into the bottoms stream. The reflux value used 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 about 1.5: 1 to 2.5: 1 based on the weight ratio phlegm to raw materials.

At least a portion, preferably at least about 50%, and more preferably at least about 80 percent to almost 100% of the mass of the bottom stream of the deisohexanizer is fed to the inlet side of the selective membrane in order to obtain a retained product that has a reduced concentration of normal hexane, and to obtain on the other side of the membrane a diffused fraction containing normal hexane. Preferably, the diffused fraction contains at least about 75 percent by weight of normal hexane in contact with the membrane. The diffused fraction often contains at least about 50%, and sometimes between about 60 and 95 percent by weight of normal hexane.

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 about 0.01 and a C ₆ relative permeability flux of at least about 1.25: 1. Sieve membranes are described in more detail below.

In membrane separation, the pressure drop often varies from about 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 on the temperature of the overhead of the deisohexanizer. 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 overhead of the deisohexanizer. Often the temperature is in the range of about 25 ° C 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 about 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.

Preferably, at least a portion of the diffused substance containing normal hexane is recycled to 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. The membranes can be formed from the same 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. The discontinuous aggregates of microporous nanometer-sized barriers make it possible to achieve 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, AlPO-14, AlPO-17, AlPO-18, AlPO-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, Philipsit, MCM-65 and MCM-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 (nanometer sized 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 (polymers with intrinsic microporosity), 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 cyclodextrins, 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 may be a carrier for the membrane composite material, may be an integral part of the forming microporous barrier, and may 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 about 20 and 200 nanometers and having high permeability. In more preferred embodiments, the C ₆ permeability flow index for the meso / macroporous structure is at least about 1, and most preferably at least about 10, and sometimes at least about 1000. The meso / macroporous structure may 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 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 such a function, then the membrane may contain a porous carrier for the meso / macroporous structure. Typically, the porous carrier is selected taking into account strength, endurance under the conditions of the proposed 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 zeolite-forming precursor solutions, 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 of greater than about 5.0 Å (average length and width), for example, from about 5.0 to 7.0 or 8 Å. Preferably, these structures have a length to width ratio of less than about 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 penetrating 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, both in a continuous and intermittent membrane. 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 about 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 about 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 array of microporous barriers are characterized in that the barrier has a main size of less than about 100 nanometers, 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, nanometer-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, have a more spherical shape, it is preferable to choose a meso / macroporous structure 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 be a minimum volume of voids for the bypass flow. Particles of a molecular sieve with a lamellar 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 molecular sieve particles can be drawn into the interior of 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 the 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 will 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 nanometers. 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 a 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 dialkylalkoxysilane. 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, approximately between 0.1 and 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 nanometer-sized molecular sieve solution or suspension. The concentration of the molecular sieve in the suspension should be low enough 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 the 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 of between about 2 and 30%, for example, from 5 to 20 percent 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 about 30 ° C and 150 ° C, for 1-50 hours. A vacuum may be used to facilitate drying. When zeolites are used as a molecular sieve, calcination, for example, at a temperature of between about 450 ° C 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, reduction in 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 helps to get 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 about 5 and 20 nanometers, 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 the zeolite layers can be grown on and between silicon dioxide 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 of forming a barrier layer is to use a liquid precursor of a zeolite molecular sieve 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; AlPO; SAPO and borosilicates. The precursor compound includes the elements mentioned above, 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 of about 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 about 20: 1, and for aluminophosphate molecular sieves, the molar ratio of water to aluminum should be at least about 20: 1.

Crystallization conditions often range from about 80 ° C to 250 ° C with an absolute pressure in the range from 100 to 1000, often from 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 about 50 hours, for example from 10 to 40 hours. Preferably, the crystallization time is sufficient to form crystals, but less than the time required to form a molecular sieve layer with a thickness of about 200 nanometers, for example, from about 5 to 50 nanometers. 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 approximately 350 ° to 550 ° C to remove any organic matter.

Mostly, for some zeolite molecular sieve materials, obtaining particles smaller than 100 nanometers is difficult. Moreover, even when using seed crystals, the particle size may be larger than desired. Another embodiment in the preparation of an intermittent barrier membrane is the synthesis of zeolite in the open space between particles (substrates) having a main size of less than about 100 nanometers, respectively, the main size of the microporous barrier can be less than about 100 nanometers. The substrate particles play the role of the nucleation site of zeolite crystals and, thus, they are selected from materials that are capable of nucleation during zeolite growth. Examples of such materials are silica, mainly silica particles having a main size between about 5 and 50 nanometers, and other zeolites having a main size of less than about 100 nanometers. 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 guarantee 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 structure of the carbon carrier 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 about 20 to 50 nanometers.

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 an ultrafiltration membrane (effective pore diameter from 1 to 100 nanometers) or a microfiltration membrane (effective pore diameter from 100 to 10,000 nanometers), 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 precipitated material is needed to obtain a local reduction in pores or holes throughout the carrier to a size that provides the desired sieve effect, without overly reducing the diameter of the remaining porous structure of the carrier. Examples of vapor deposited materials include silanes, paraxylene, 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 about 1 and 80 nm, preferably between about 2 and 50 nanometers.

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, for example with 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 it is not required that the membranes have a high C ₆ permeability flux ratio, 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 unable to diffuse into the micropores of the barrier, and on the other hand there is a crosslinking agent. Major defects and, to some extent, minor defects are filled with these large, reactive molecules and fixed by crosslinking. Then, the unreacted portion of the large molecules can be removed, as well 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 are nanometer in 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 about 450 ° C and 650 ° C are used for a period of approximately 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 approximately less than 200 nm, preferably approximately less than 100 nanometers. Agglomeration can be carried out in the presence or absence of a pressure drop across the membrane. Preferably, the pressure differential 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 about 1 nanometer, 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 nanometers, 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 of about 1: 2 to 100: 1, preferably between about 3: 1 and 30: 1.

Mortar and particles can be mixed, for example, in a slurry and then placed in conjunction with a microporous structure, or can be provided after 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 about 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 about 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. A dilute aqueous solution of silicon tetraalkoxide is typically used, for example, containing between about 0.5 and 25 percent silicon tetraalkoxide by weight to provide a 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 nanometers, 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 about 100 nanometers are likely to have a specific configuration due to their crystal structure. Some zeolites tend to have a plate-like 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 may be polymer, including oligomeric; carbon; and inorganic, such as fuming silica, zeolite, alumina and the like.

The sieve membranes used in the methods of the present invention are characterized in that they have high permeability, that is, have a C ₆ permeability flow index of at least about 0.01. The type of sieve membranes can be different, for example, molecular sieves, pore-containing ceramics, metals, polymer or carbon membranes, or composite membranes having a highly porous polymer, metal, molecular sieve, ceramic or carbon carrier with a thin sieve layer, for example, a molecular sieve or carbon. High permeability is achieved due to at least one of the following techniques: firstly, the use of larger pores than is necessary for the passage of normal alkanes; and secondly, the use of a very thin layer containing pores. In the first step, it should be recognized that the selectivity of the membrane with larger pores may decrease. However, in the methods of the present invention, selectivity is not a very important parameter.

Not limited to theory, it is believed that the preferred class of membranes are those in which the sieve pores are large enough so that branched hexanes can pass through the pores, but their resistance to movement will be greater than for normal hexane. Often the pores for membranes of this type have an average diameter greater than about 5.0 Å (average length and width), for example, from about 5.0 to 7.0 Å. Preferably, the structures have a length to width ratio of approximately less than 1.25: 1, for example, from 1.21: 1 to 1: 1. For membranes containing a molecular sieve, examples of structures are USY, ZSM-12, SSZ-35, SSZ-44, VPI-8 zeolites and cancrinite.

Another class of preferred membranes are membranes with increased selectivity for the separation of normal hexane from branched hexanes, where the sieve structure impedes the passage of branched hexanes through a suitably formed porous structure. Typically, pores for membranes of this type have an average diameter of approximately not more than 5.5 Å, for example, from about 4.5 to 5.4 Å. The ratio of length to pore width in these membranes can vary within wide limits and is 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. In the latter type of membranes, the thickness of the sieve layer is an important parameter for achieving high permeability. However, when the thickness of the sieve layer decreases, the difficulty of obtaining a defect-free layer increases. Since the methods of the present invention do not require high selectivity, the membranes may contain small defects, that is, those that have a relatively small effective diameter. Larger defects are less satisfactory and relatively rare within their presence in order to maintain the desired C ₆ permeability flow ratio. For example, with a membrane having a ZSM-5 barrier layer, a C ₆ permeability flux ratio of 1.5 can only be obtained if one third of the fluid passes through the barrier layer. Membranes containing ZSM-5 and silicalite are preferred because of their ability to selectively pass methyl pentanes over dimethyl butanes.

As an example, one technique for producing a composite membrane is the formation of sieve structures within a porous substrate. The porous substrate may be any suitable inorganic material that has adequate strength to withstand pressure and temperature differences during use. Examples of porous substrate compositions include metal, alumina such as alpha alumina, gamma alumina and transition alumina, molecular sieve, ceramic, glass, polymers and carbon. The porous substrate is highly porous and preferably has a C ₆ permeability flow index of at least about 1, preferably at least about 10. Often, the porous substrate can have a pore or hole size in the range of 2 to 100, preferably approximately from 20 to 50 to nm. These pores or holes can be almost straight or winding and can be defined as passing through a solid or through the space of voids between the particles of the substrate. In preferred embodiments, the defects in the substrate heal before the deposition of the barrier layer or the precursor of the barrier layer. In another embodiment, the substrate may be treated with silicasole to partially clog the pores and facilitate deposition of the barrier layer or barrier layer 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.

One way to form a barrier layer is to place a liquid molecular sieve precursor on a porous substrate. The precursor crystallizes under hydrothermal conditions, after which the porous substrate is washed and heated to remove residual organic material. The molecular sieve material is located 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; AlPO; SAPO and borosilicates. The precursor compound includes the elements mentioned above, 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 of about 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 about 40: 1, and for aluminophosphate molecular sieves, the molar ratio of water to aluminum should be at least about 40: 1.

Crystallization conditions often range from about 80 ° C to 250 ° C with an absolute pressure in the range from 100 to 1000, often from 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 about 50 hours, for example, from 10 to 40 hours. Preferably, the crystallization time is sufficient to form crystals, but less than the time required to form a molecular sieve layer with a thickness of about 200 nm, for example, from about 5 to 50 nanometers. 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 approximately 350 ° to 550 ° C to remove any organic matter.

Another method for producing membranes suitable for use in accordance with the method of this invention involves the deposition of a thin layer of molecular sieve on a porous carrier, such as a polymeric carrier or inorganic carrier, as described above. In preferred embodiments of these membranes, the porous substrate is highly porous and preferably has a C ₆ permeability flow index of at least about 1, preferably at least about 10. Often, the porous substrate can have a pore or hole size in the range of 2 to 200 nm, preferably from about 20 to 100 nanometers. The structure of the polymer carrier may be isotropic, but anisotropic structure is preferred. Pores or openings can be substantially straight or winding and can be defined as passing through a solid or through the space of voids between particles of a substrate. Typical polymeric carriers include polyimides, polyacrylonitrile, polycarbonates, poly (ether) ketones, poly (ether) sulfones and polysulfones.

The precipitated molecular sieve usually has a relatively small particle size, for example, from about 20 to 50 nanometers in the main direction. The application of the molecular sieve to the carrier can be carried out in any conventional manner. For example, the molecular sieve may be in an aqueous suspension and applied to the membrane in the form of a thin coating, for example, a suspension containing from about 5 to 50 percent molecular sieve by weight, and the thickness of the coating before drying is approximately less than 200 nm, preferably between 50 and 100 nanometers.

The deposition process, if desired, may include maintaining on one side of the porous support a reduced pressure to facilitate the placement of molecular sieve particles in the pores of the support. If the molecular sieve is loosely attached to the carrier, for example, is placed in the pores, and the coating composition may contain one or more components that act as a binder material, provided that it does not clog the porous structure of the molecular sieve. Auxiliary agents include one or more polyamides, polyvinyl alcohols, polyvinyl acetate, silicone rubbers and polyacrylates.

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

Another technique that provides high permeability of the composite membrane is the vapor deposition of a thin layer of a substance on the surface of a highly porous carrier, which may be polymer or inorganic, of the type described above. The purpose of the deposited material is to provide a local reduction in the size of pores or holes throughout the carrier to a value that will provide the desired separation without unduly reducing the diameter of the remaining porous structure in the carrier. Examples of vapor deposited materials include silanes, para-xylene, alkylene imines and alkylene oxides.

One technique for depositing molecular sieves on a porous support is to obtain a relatively uniform, diluted suspension of the molecular sieve in a viscous liquid or in a solid polymer, so that after removing the liquid or polymer, for example by calcination, a thin, very homogeneous molecular sieve coating. As an example, a suspension of a molecular sieve (preferably from about 1 to 10 percent by weight) in a hydrocarbon, which is usually solid at room temperature, such as dodecane, is prepared and applied as a coating on the outside of a hollow tubular porous carrier. The temperature of the suspension is such that the viscosity is suitable for maintaining a uniform suspension, but still provides the desired thin coating. The coating thickness is usually from about 5 to 30 microns. A small pressure drop is maintained on the tube wall (approximately 5 to 30 kPa), so that most of the coating is more likely to be involved in all major defects in the carrier than in nanopores. Then the carrier is dried and calcined to remove hydrocarbon

Since it is not required that the membranes have a high C ₆ permeability flux ratio, which is desirable in many applications, any techniques that increase the resistance to flow through defects will contribute to improving 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 unable to diffuse into the micropores of the barrier, and on the other hand there is a crosslinking agent. Major defects and, to some extent, minor defects are filled with these large, reactive molecules and fixed by crosslinking. Then, the unreacted portion of the large molecules can be removed, as well as the unreacted crosslinking agent. These large molecules can be oligomeric or large molecules.

Detailed description of the drawing

The drawing 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 routed through a pipe 108 to a stabilization column 110 In this stabilization column 110, light fractions are removed via line 112 as 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 side stream from the deisohexanizer is recycled through line 118 to the isomerization reactor 106. The bottom stream from the deisohexanizer 116 is removed through line 120. The overhead from the deisohexanizer 116 is discharged through line 12. line 120 enters the inlet side of the membrane separator 124. A stream having a reduced concentration of normal hexane is removed from the separator 124 through line 126, and the diffused fraction, which is It contains normal hexane and is recycled through line 128 to the isomerization reactor 106.

Claims (11)

1. The method of isomerization of a feedstock containing paraffins having from 5 to 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 the presence of an isomerization catalyst, a reactor temperature of 40 to 250 ° C., an operating absolute pressure in the reactor of 100 kPa to 10 MPa, and a volumetric flow rate of 0.2 to 25 L of isomerizable hydrocarbon feed in h / l of catalyst volume to obtain an isomerization product containing linear paraffins, but at a lower concentration than in the feedstock,
(b) distilling at least a portion of the isomerization product to obtain a low boiling fraction containing linear paraffins and a higher boiling fraction containing normal hexane, cyclohexane, methyl pentanes 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 in order to obtain a retained fraction that has a reduced concentration of methylcyclopentane to obtain a fraction diffused through it on the outlet side of the membrane, having an increased concentration of normal hexane and methyl pentanes, wherein said diffused fraction contains at least 75 wt.% normal hexane contained in the fraction containing n normal hexane in contact with the membrane;
(d) removing the retained fraction from step c). 2. The method according to claim 1, characterized in that at least a portion of the diffused fraction from step c) is fed to step a). 3. The method according to claim 2, characterized in that the stream containing normal hexane from step b) is a side stream and the distillation from step b) provides a bottoms stream containing C ₇ ⁺ hydrocarbons. 4. The method according to claim 2, characterized in that the concentration of normal hexane in the diffused fraction is approximately 90 wt.%. 5. 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 about 0.01 and a C ₆ permeability flux ratio of at least about 1.25: 1. 6. The method according to claim 5, characterized in that the sieve membrane has an average pore diameter of from about 5.0 to 7.0 Å. 7. The method according to claim 6, characterized in that the sieve membrane has an average pore diameter of approximately from 4.5 to 5.4 Å. 8. The method according to claim 5, characterized in that the sieve membrane contains a molecular sieve with a structure that is a zeolite ZSM-5 or silicalite. 9. The method according to claim 1, characterized in that the low-boiling fraction contains isopentane. 10. The method according to claim 9, characterized in that the low boiling fraction is a fraction of the overhead. 11. A device for a method of isomerizing a feedstock as claimed in claims 1 to 10, containing paraffins with a number of carbon atoms of 5 and 6, to obtain a gasoline fraction that includes:
a) an isomerization reactor in which there are nozzles for receiving the feedstock at the inlet and an outlet nozzle,
b) a hexane removal column, in which the liquid fluid inlet is connected to the outlet pipe of the isomerization reactor, the low-boiling fraction outlet pipe is used to remove the low-boiling fraction in line, the boiling fraction outlet pipe to select a side fraction and the outlet pipe for bottoms; and
(c) a membrane separator in which a side nozzle for entering the liquid feed is connected to an outlet nozzle of bottoms, a nozzle for leaving the fluid on the outlet side of the separator is connected to an outlet nozzle of a higher boiling fraction of the hexane removal apparatus, and a side nozzle for leaving the feed.