BRPI0608728A2 - process and apparatus for isomerizing a feed load - Google Patents

Abstract

PROCESS AND APPARATUS FOR ISOMERIZING A FEEDING LOAD In an isomerization process where the isomerization effluent (108) is fractionated in a desiso-hexanizer (116) to provide a top fraction (118) containing dimethyl-butanes and a fraction of higher boiling (122) containing normal hexane, the higher boiling fraction is contacted with a selectively permeable membrane (124) to provide a methyl-cyclo-pentane containing trap (128). If desired, the permeate containing normal hexane can be recycled for isomerization. The preferred membranes are sieving membranes having a C6 Permeate Flow Index of at least 0.01 and a C ~ 6 ~ Permeate Flow Ratio of at least 1.25: 1.

Description

"PROCESS AND APPARATUS FOR ISOMERIZATION OF A FOOD LOAD"

BACKGROUND OF THE INVENTION

This invention relates to improved processes for isomerization of 5 and 6 carbon paraffins, for example, to provide isomerate having improved Search Octane Index (RON) for blending in gasoline mixtures, and particularly for processes using a desisoexanizer.

Processes for isomerization of paraffins into higher branched paraffins are widely practiced. Particularly important commercial deisomerization processes are used to increase branching, and thus the octane value of refinery streams containing 4 to 8 paraffins, especially 5 and 6 carbon atoms. The isomerate is typically mixed with a refinery refiner effluent to provide a mixed gasoline mixture having a desired search octane index (RON).

The isomerization process proceeds towards a thermodynamic equilibrium. For this reason, the isomerate will still contain normal paraffins which have low octane indices and therefore impair the isomerate octane index. As long as suitable high octane mixing streams such as alkylate and deforming effluent are available and lower octane index gasolines such as RON 85 and 87 are in demand, the presence of these abnormal paraffins in the isomerate has been tolerated.

Where circumstances require higher RON isomerates, isomerization processes have been modified by separating normal paraffins from the isomerate and recycling them to the deisomerization reactor. Thus, not only are normal paraffins detrimental to the octane index removed from the isomerate but also their return to isomerization reactor increases the portion of the feed converted to highly desired branched paraffins.

Major processes for the separation of normal isomerate paraffins are the use of adsorptive separation as described in US 4,717,784 and 4,804,802, and distillation. The most commonly practiced isomerization processes that recycle normal paraffins use a desanalyst. A desisoexaniser is one or more distillation columns where a top fraction containing branched C6 paraffins such as dimethyl butanes (2,2-dimethyl butane and 2,3-dimethyl butane) and heavier components is obtained as the isomerate product for, for example. for example, mixing for gasolines, and as a side stream containing hexanonormal and similar boiling components such as methyl pentanes (2-methyl pentane and 3-methyl pentane) and methyl cyclopentane is recycled to the deisomerization reactor. The problem with a desisoexanalizer is that the lowest boiling product stream contains n-pentane that has a low RON value.

The use of adsorptive separation instead of a desanalyst allows n-pentane to be removed, thereby providing a high RON engine fuel, often within the range of 91 to 93.

Separation of linear branched paraffins has also been proposed, but a practical commercial application for membranes has yet to be found. US 5,069,794 describes microporous membranes containing crystalline molecular sieve material. In column 8, lines 11 et seq., Potential membrane applications are described including the separation of linear and branched paraffins. See also, US 6,090,289, discloses a layered composite containing molecular sieve that could be used as a membrane. Potential separations in which the membrane may be used which are described starting at column 13, row 6 include the separation of normal paraffin from branched paraffins. US 6,156,950 and 6,338,791 discuss permeation separation techniques which may have application for the separation of normal paraffins from branched paraffins and describe certain separation schemes in connection with isomerization. US2003 / 0196931 describes a two-stage isomerization process for improving hydrocarbon feeds of 4 to 12 carbon atoms.

Recently, Bourney, et al. In WO 2005/049766 describes a process for producing high octane gasoline using a membrane to remove, inter alia, n-pentane from an isomerized stream derived from the top fraction of a desisoexanizer. A side section of the desanalyst is as a cleaning fluid on the permeate side of the membrane. Permeate mixture and cleaning fluid is recycled to the deisomerization reactor. In a computer simulation based on the use of a KlFI over alumina membrane, example 1 of the publication indicates that 5,000 square meters of membrane surface area is required to move 95 mass percent of n-pentane from the top fraction of a desisoexanalizing distillation column. At the feed rate to the permeator (75,000 kg / h having 20.6 mass percent n-pentane), the n-pentane flow used in the simulation appears to be on the order of 0.01 gram-mol / m2, s a300. ° C. The RON of the product with the n-pentane removed is 91.0.

The use by Bourney, et al., Of a desisoexanizer side section as a cleaning fluid for membrane separation in recycling of valuable high octane compounds such as methyl pentanes and methyl cyclopentane back to the isomerization reactor . In Table 1, Bourney, et al. State that the concentration of methylcyclopentane is 9.7 percent by mass. There is no methylcyclopentane in the product stream. In addition, the cleaning stream contains 4.5 mass percent 2,3-dimethyl butane which is also recycled to the isomerization reactor. Target the isomerization reactor will distribute the isomers in the equilibrium direction, they sacrifice by pass the high RON to RON fuel efficiency.

Use of zeolite membranes is suggested as a suitable technique for separating linear molecules. See, for example, paragraphs0008 and 0032. US 6,818,333 describes thinned zeolite membranes that have a n-butane permeability of at least 6 χ 10 mol / m .s.Pa and a selectivity of at least 250 paraisobutane n-butane.

Changes in environmental and fuel efficiency regulations can have a profound effect on isomerate demand for higher octane indices. For example, requirements to reduce benzene content of gasolines would need to increase the octane index and once-though isomerization processes will be required to be retroactively reformed for a process that separates and recycles normal paraffins to the isomerization reactor. Even existing processes that use deexanalysts may be required to provide improved octane index isomerate.

Therefore, simple-to-operate processes are desired to improve the desanalyzer top fraction octane index.

For the purposes of the following discussion of the invention, the following membrane properties are defined.

Microporous

Microporosa and microporosity refer to those having effective diameters of between 0.3 nm and 2 nanometers.

Mesoporous

Mesoporous and mesoporous refer to the pores having diameters between 2 nanometers and 50 nanometers.

Macroporous

Macroporous and macroporosity refer to those having effective diameters greater than 50 nanometers.

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

Molecular Sieves

Molecular sieves are materials having microporosity and may be amorphous, partially amorphous or crystalline and may be serzeolytic, polymeric, metallic, ceramic or carbon.

Screening membrane

Screening membrane is a composite membrane containing a continuous or discontinuous selective separation medium containing molecular sieve barrier. A barrier is the structure that exists to selectively block fluid flow in the membrane. In a continuous de-peeling membrane, the molecular sieve itself forms a continuous layer that is desirably to be defect free. The continuous barrier may contain other materials such as would be the case with mixed matrix membranes. A discontinuous screening membrane is a discontinuous molecular sieve assembly in which spaces, or voids, exist between the particles or regions of molecular sieve. These spaces or voids may contain or be filled with solid material. The sieve particles or regions of the sieve are the barrier. Separation by sieving membranes may be about steric properties of the components to be separated. Other factors may also affect permeation. One is the sortivity or its lack of one-component and molecular sieve material. Another is the interaction of the components to be separated into the molecular sieve microporous structure. For example, for some dezeolite molecular sieves, the presence of a molecule, for example n-hexane, in one pore may prevent more 2-methylpentane from entering that portion than of another n-hexane molecule. As a consequence, zeolites that would not appear to offer such selectivity for the separation of normal and branched paraffins only from a molecular size standpoint may in practice provide greater separation selectivities.

Ce Permeate Flow Index

The permeability of a sieve membrane, that is, the taxane through which a component passes through a given membrane thickness, often varies with changes in conditions such as temperature, absolute, and differential pressure. Thus, for example, a different permeation rate can be determined where the absolute pressure on the permeate side is 1,000 kPa instead of that where the pressure is 5,000 kPa, all other parameters, including pressure differential, are constant. Consequently, a Ce Permeate Flow Index is used here to describe sieving membranes. The Permeate Ce Flow Index for a given membrane is determined by measuring the rate (gram-mol per second) at which a substantially pure normal hexane (hexane preferably at least 95 weight percent) permeates the membrane at approximately 150 ° C at one side pressure. withholding pressure of 1,000 kPa absolute and a permeate side pressure of 100 kPa absolute. The Ce Permeate Flow Index reflects the permeation rate per square meter of retained side surface area but is not normalized to membrane thickness. As a consequence, the PermeatoCe Flow rate for a given membrane will be in the hexanonormal gram-mol units permeating per second per square meter of permeate side membrane surface area.

C6 Permeate Flow Ratio

The Permeate Flow Rate Cs for a given sieving membrane is the ratio of the Permeate Flow Index Ce (n-hexane) to a Permeate Flow Index Ce in which the Permeate Flow Index C6 is determined in the same manner as that of Permeate Flow Index Cemas using substantially pure dimethylbutanes (independent of the distribution between 2,2-dimethylbutane and 2,3-dimethylbutane) (preferably at least 95 weight percent dimethylbutanes).

SUMMARY OF THE INVENTION

By this invention improvements are made in deisomerization processes to improve the octane index of paraffin feed fillers comprising 5 and 6 carbon atoms where the processes use deoxanalizers to provide a lower boiling dimethylbutane containing fraction and a higher boiling normal hexane containing fraction. . In accordance with this invention, membranes are used for recovery of higher octane index components such as methylcyclopentane edimethyl butane fraction containing higher boiling normal hexane. These higher octane index components may be used for blending. with motor fuels. As only a portion of the isomerization effluent is subjected to membrane treatment, the required membrane surface area can be reduced, thereby increasing the economic viability of membrane use.

Preferably, at least a portion of the normal hexane-containing permeaton of the membrane separation is recycled to isomerization. Since the normal hexane concentration in the permeate is higher than that in the higher boiling fraction, the recycle volume is reduced compared to recycle of the same amount of hexanonormal but without the benefit of membrane separation. The ability of the processes of the invention to reduce the recycle volume for the deisomerization reactor can provide several advantages. For example, the reduced recycle volume allows an increase in feed to the deisomerization reactor for a given conversion, thereby increasing the isomerization reactor capacity. Also, by reducing the amount of higher octane components such as methylcyclopentane and dimethylbutanes that would otherwise be recycled to the isomerization reactor, the nature of the isomerization reaction balance will allow more effective amount of higher octane to be produced. per feed-load unit.

The broad aspects of the processes of this invention include:

The. isomerize a feedstock comprising paraffins having 5 and 6 carbon atoms in which at least 15 weight percent of the feedload is linear paraffin under deisomerization conditions including the presence of isomerization catalyst to provide an isomerization effluent containing linear paraffins but at a lower concentration than than that in the feed load.

B. distilling at least a portion, preferably at least 90 mass percent of and most preferably essentially all, isomerization effluent to provide a lower boiling fraction containing dimethyl butanes and lighter paraffins and a normal hexane-containing fraction containing normal hexane, methylpentanes, dimethyl butanes and ethyl cyclopentane,

ç. contacting at least a portion of, preferably at least 90 mass percent of, and most preferably essentially the whole, normal hexane-containing fraction from step b with a retaining side of a selectively permeable membrane under conditions including sufficient membrane surface area and pressure differential across the membrane. to provide a retentate moiety having an increased concentration of methylcyclopentane and dimethylbutanes, and to provide through the permeate side membrane, a permeate fraction having an increased concentration of normal hexane and methylpentanes, said permeate fraction. containing at least 75, preferably at least 90, mass percent of normal hexane contained in the membrane-contacted hexanonormal containing fraction, ed. remove from step c the retention fraction.

Preferably at least a portion of, most preferably at least 90 percent by weight of, and most preferably substantially all, the permeate fraction of step c is recycled to stepa.

Preferably at least 25, more preferably at least 30, by weight percent of the methyl pentanes contained in the current normal hexane containing membrane is contained in the permeate fraction. In many cases, the concentration of normal hexane to total opermate will be less than 90 mass percent, e.g. from 25 to 90, e.g. 40 to 80 mass percent. In some aspects of the processes of the invention, the retained fraction removed is greater than 10, e.g. 15 to 50, by weight percent of the normal hexane-containing fraction contacting the membrane. Thus, the volume of the recycle for deisomerization step a is smaller than in an identical process except that the fraction containing normal hexane is not subjected to membrane separation.

The retentate fraction from step d contains significant amounts of methylcyclopentane and thus has an octanoattractive index. Often at least 50, preferably at least 80, by weight percent of methylcyclopentane in the normal hexane containing fraction containing the membrane is retained in the retention fraction.

Separation of monomethyl pentanes from dimethyl butanes is difficult due to the proximity of the boiling points and thus not only a deso-examiner with extensive number of distillation trays is used, often within the range of 80, but also a large reflux ratio. for power, for example, 2: 1 to 3: 1. As a result, operation of the desanalyst requires substantial re-evaporator heat. Through the processes of this invention, a significant portion of the dimethyl butanes contained in the normal hexane containing fraction remains in the melted fraction and thus is recovered with methylcyclopentane for use in engine fuel mixture. Preferably distillation step b is operated in detail that the normal hexane containing fraction contains dimethyl butanes, for example at least 2, and sometimes from 5 to 30, by weight percent of the dimethyl butanes contained in the isomerization effluent of step a.

Preferably at least 30, more preferably at least 70, weight percent of the dimethyl butanes contained in the hexanonormal containing moiety are retained in the retentate moiety. Thus, the retained removed may be used as engine fuel or added in a mixture to provide engine fuel. As a result, for an existing deexamplifier, the reflux ratio can be reduced, sometimes by 10 to 50 percent, resulting in energy savings without undue octane loss of the product.

Preferably the membrane is a sieve membrane having a C6 Permeate Flow Index of at least 0.01, more preferably at least 0.02, and the C6 Permeate Flow Ratio of at least 1.25: 1, most preferably at least minus 1.3: 1, and often from 1.35: 1 to 5: 1 or 6: 1.

The invention also relates to apparatus suitable for carrying out the processes of this invention. In its broadest aspects, the apparatus of this invention is an apparatus for isomerization of a feedstock comprising paraffins having between 5 and 6 carbon atoms to provide a gasoline fraction comprising:

The. an isomerization reactor (106) being adapted to receive a load on one input and having one output,

B. a de-analyzer (116) having an inlet in fluid communication with the isomerization reactor outlet (106), a lower boiling fraction output adapted to remove a lower boiling fraction via line (118), an outlet to provide a fraction of lateral cut and a higher boiling fraction output; and

ç. a membrane separator (124) having a fluid communicating feed inlet with the outlet to provide a side section fraction of the de-scrubber (116), a fluid communicating feed outlet with line (118) of the boiling defrost output (116), and a permeate output in fluid communication with the isomerization reactor inlet (106).

DESCRIPTION OF THE FIGURE

Figure 1 is a schematic representation of the processes according to this invention using a stabilizing column prior to a desanalyst.

DETAILED DESCRIPTION OF THE INVENTION

Isomerization

Any suitable paraffin-containing feedstock may be used in the processes of this invention. Naphtha feed loads are the most frequently used as feed loads in isomerization processes. Naphtha feedstock comprises paraffins, naphthenes, and aromatics, and may comprise small amounts of olefins boiling within the gasoline range. Feed charges that may be used include straight-run naphtha, natural gasolin, synthetic naphtha, thermal gasoline, catalyzed-down gasoline, partially reformed naphtha or aromatic extraction raffinates. The feed load is essentially encompassed by the range of a full range naphtha, or within the range of 0 ° C to 230 ° C. Normally the feedstock is light naphtha having an initial boiling point of 10 ° C to 65 ° C and a final boiling point of 75 ° C to 110 ° C; preferably, the final boiling point is less than 95 ° C.

Naphtha feedloads generally contain small amounts of sulfur compounds totaling less than 10 parts per million parts by weight (mppm) on an elemental basis. Preferably the naphtha feedstock has been prepared from a feedstock contaminated by the step. conventional pretreatment such as hydrotreating, hydrorefining or hydrodesulfurization to convert such contaminants as sulfur, nitrogenous and oxygenated compounds into H2S5 NH3 and H2O5 respectively, which may be prepared from fractional hydrocarbons. This conversion will preferably employ a catalyst known in the art comprising an inorganic oxide support and metals selected from Groups VIB (IUPAC 6) and VIII (IUPAC 9-10) of the Periodic Table. Water may act to attenuate the acidity of the catalyst by acting as a base, and temporarily over-catalyze the catalyst by platinum poisoning. Feed-load hydrotreating as described herein typically reduces water-generating oxygenates and sulfur-deactivating compounds to desirable levels, and other media. Sorbent systems for the removal of sulfur and water from hydrocarbon streams are generally not required. It is within the scope of the present invention that this optional pretreatment step be included in the present process combination.

Preferred main components of the feedstock are cyclic and acyclic paraffins having from 4 to 8 carbon atoms per molecule (C4 to Cg), especially C5 and C6, and smaller amounts of olefinic and aromatic hydrocarbons may also be present.

Typically, the concentration of C7 and heavier components is less than 20 percent by weight of the feedstock, and the concentration of C4 and lighter components is less than 20, preferably less than 10, by weight percent of the feedload. The mass ratio of C5 to C6 components in the preferred feedstock is 1: 10a to 1: 1. Although there are no specific limits on the total feed content of cyclic hydrocarbons, the feedstock generally contains between 2 and 40 weight percent. cyclic comprising naphthenes and aromatics. The aromatics contained in the naphtha feedstock, although generally totaling less than cycloalkane alkanes, may comprise from 2 to 20 mass percent and more usually from 5 to 10 mass percent of the total. Benzene normally comprises the main aromatic constituents of the preferred feedstock, optionally together with smaller amounts of high boiling aromatic toluene and within the boiling ranges described above.

In general, linear paraffins make up at least 15, often 40, preferably at least 50, mass percent essentially the total feedstock used in the inventive processes. For naphtha feed loads, linear paraffins are typically present in amounts of at least 50, e.g. 50 to 90, by weight percent. The mass ratio of nonlinear paraparaffins to linear feeds is often less than 1: 1, for example 0.1: 1 to 0.95: 1. Nonlinear paraffins include cyclic branched paraffins and substituted or unsubstituted cyclo paraffins. Other components such as aromatic and olefinic compounds may also be present in the feed fillers as described above.

The feed load is passed in one or more deisomerization zones. In aspects of this invention where normal hexane is recycled, the feed charge and the recycle are usually mixed before entering the isomerization zone, but if desired may be separately introduced. In any case, the total feed to the deisomerization zone is referred to herein as the isomerization feed. Recycling may be provided in one or more streams. As discussed later, the recycle contains linear paraffins. The concentration of lineal paraffins in the isomerization feed will not only depend on the concentration of linear paraffins in the feedstock but also on the recycle concentration and the relative amount of feedstock recycle, which may fall within a wide range. Often, the isomerization feed has a linear paraffin concentration of at least 30, for example between 35 and 90, preferably 40 to 70, by weight, and a molar ratio of non-linear paraparaffin paraffins of 0.2 : 1 to 1.5: 1, and sometimes between 0.4: 1 to 1.2: 1.

In the isomerization zone the isomerization feed is subjected to the isomerization conditions including the presence of isomerization catalyst preferably in the presence of a limited but hydrogen-containing amount as described in US 4,804,803 and 5,326,296, both of which are incorporated herein by reference. Paraffin isomerization is generally considered a reversible first order reaction. Thus, the isomerization reaction effluent will contain a higher concentration of nonlinear paraffins and a lower concentration of linear paraffins than the isomerization feed. In preferred embodiments of this invention, isomerization conditions are sufficient for isomerization of at least 20, preferably from 30 to 60, by weight percent of normal paraffins in the isomerization feed. In general, isomerization conditions reach at least 70, preferably at least 75, for example, 75 to 97 percent of the equilibrium for C paraffins present in the deisomerization feed. In many cases, the isomerization reaction effluent has a ratio of nonlinear paraffins to linear paraffins of at least 2: 1, preferably from 2.5 to 4: 1.

The isomerization catalyst is not critical to the broad aspects of the process of this invention, and any suitable isomerization catalyst may find application. Suitable isomerization catalysts include acid catalysts using chloride to maintain the desired acidity and sulfated catalysts. The isomerization catalyst may be amorphous, for example based on amorphous alumina, or zeolitic. A zeolitic catalyst would normally still contain an amorphous binder. The catalyst may comprise a sulphated zirconia and platinum as described in US 5,036,035 and European Application 0.666.109 Al or a chlorinated superalumin platinum group metal as described in US 5,705,730 and 6,214,764. Suitable catalyst is described in US 5,922,639. US 6,818,589 describes a catalyst comprising a tungstate support of a Group VIB metal oxide or hydroxide (IUPAC 4), preferably zirconium hydroxide or oxide, at least one first component which is an elementolanthanide and / or yttrium component, and at least one second components being a metal component of the platinum group. These documents are incorporated herein because of their teaching on catalyst compositions, isomerization operating conditions and techniques.

Counting on the isomerization zones can be effected by using the catalyst in a fixed bed system, a moving bed system, a fluidized bed system, or in a batch type operation. A fixed bed system is preferred. The reagents may be contacted with upstream, downstream, or radial flow catalytic particle oil. The reactants may be in the liquid phase, a mixed liquid-vapor phase, or a vapor phase when contacted with catalytic particles, with excellent results being obtained by applying the present invention in a primarily liquid phase operation. Isomerization azone may be in a single reactor or in two or more separate reactors with suitable means to ensure that the desired deisomerization temperature is maintained at the inlet for each zone. Two or more sequential reactors are preferred to allow improved isomerization through intermediate reactor temperature control and partial catalyst replacement without a process stop. Isomerization conditions in the isomerization zone include reactor temperatures typically ranging from 40 ° C to 250 ° C. . Lower reaction temperatures are generally preferred for the purpose of favoring equilibrium mixtures having the highest concentration of high branched isoalkanes high in octane and to minimize feed cracking for lighter hydrocarbons. Temperatures within the range of 100 ° C to 200 ° C are preferred in the present invention. Reactor operating pressures generally range from 100 kPa to 10 absolute MPa, preferably between 0.5 and 4 absolute MPa. Net hourly space velocities range from 0.2 to 25 isomerizable hydrocarbon feed volumes per hour per catalyst volume, with a range of 0.5 to 15 h -1 being preferred.

Hydrogen is mixed with or remains with the isomerization feed into the isomerization zone to provide a hydrogen to docosahexaenoic acid ratio of 0.01 to 20, preferably 0.05 to 5. Hydrogen may be supplied entirely externally to the process or supplemented by hydrogen recycled for feed after separation of effluent from isomerization reaction. Light hydrocarbons Small inert quantities such as nitrogen and argon may be present in hydrogen. Water should be removed from hydrogen supplied externally to the process, preferably by an adsorption system as known in the art. In a preferred embodiment the molar hydrogen to hydrocarbon ratio in the reactor effluent is equal to or less than 0.05, generally avoiding the need for reactor effluent hydrogen recycling for feed.

Especially where a chlorinated catalyst is used for isomerization, the isomerization reaction effluent is contacted with an absorbent to remove any chloride components as described in US 5,705,730.

The effluent from the isomerization reaction is subjected to one or more separation operations to provide an increased octane index product fraction and, optionally, to remove other components such as hydrogen, lower alkanes, and especially with respect to chlorinated catalysts, halogen compounds.

In a commonly practiced isomerization process, isomerization is conducted in a liquid phase and the effluent from the deisomerization reaction is passed into a product separator in which a hydrogen and lower alkane-containing gaseous top fraction is obtained. At least a portion of this hydrogen may be recycled to the isomerization reactor to provide at least a portion of hydrogen desired for isomerization. Bottom liquid fractions are passed through a distillation (desisoexaniser) assembly to provide a lower boiling fraction containing dimethyl butanes and a higher boiling fraction containing normal hexane. More often, the desanalyst is adapted to provide the normal hexane-containing stream and provides a background stream comprising normal heptane. The desanalyst may be a stuffed or tray column and typically operates with a top pressure of between 50 and 500 kPa (gauge) and a deep temperature of between 75 ° C and 170 ° C.

The composition of the lowest boiling fraction of this analyzer will depend on the operation and design of the assembly and any separation processes to which the isomerization effluent has been subjected. For example, if the stream for the desanalyst contains light such as C1 to C4 compounds, the desanalyst may be adapted to provide a top fraction containing these light, and a side-melt fraction containing C5 compounds and C6 branched compounds, especially dimethyl butanes. Typically the lowest boiling fraction contains 20 to 60 weight percent dimethyl butanes; 10 to 40 percent by weight of normal pentane and 20 to 60 percent by weight of isopentane ebutane. Depending on the operation of the desanalyst, the lower boiling fraction may also contain significantly, for example, at least 10 percent by weight of methyl pentanes. The desanalexiser may also be adapted to provide a C5 rich current in addition to the lower boiling stream.

The highest boiling fraction containing hexane also typically contains methyl pentanes and methyl cyclopentane. As stated at the outset, the processes of this invention allow the desisoexaniser to be operated more economically resulting in a higher concentration of dimethyl butanes in the normal hexane containing fraction. Often the fraction containing normal hexane will contain 2 to 10 mass percent dimethyl butanes; 5 to 50 mass percent of normal hexane; 20 to 60 percent by weight of methyl pentanes, and 5 to 25 percent by weight of methyl cyclopentane.

Typically, the desexanalizer will be designed to provide a side stream containing methyl pentanes, methyl cyclopentane, hexanonormal, dimethyl butanes and cyclohexane, and a bottom stream containing cyclohexane and C7 + hydrocarbons. If the normal hexane-containing fraction had been the bottom fraction of the deexaminer, that fraction could also contain heavier hydrocarbons.

As stated above, the ability to recover dimethyl butanes from the highest boiling fraction containing normal hexane allows distillation to be conducted with a lower reflux ratio. The ratio of flow used will depend on the nature of the column feed as well as the column design and thus may vary over a wide range, for example, from 1.5: 1 to 2.5: 1 on a mass reflux basis for feed.

At least a portion, preferably at least 50, and more preferably at least 80, mass percent of substantially all of the normal hexane-containing fraction of the desoexanizer is contacted with the retention side of a selective membrane to provide a retention fraction of the reaction effluent. isomerization having a reduced concentration of total linear paraffins, and to provide across the membrane on a permeate side a fraction of permeate having an increased concentration of total linear paraffins. The permeate fraction contains at least 75 mass percent of normal hexane, preferably at least 75 mass percent of normal hexane, in contact with the membrane. The retentate preferably contains at least 50, more preferably at least 80 to substantially all of the omethyl cyclopentane contained in the membrane contacting fraction. In preferred aspects of the invention, the membrane allows at least 25, for example, 30 to 90, mass percent of the methyl pentanes contained in the normal hexane-containing fraction to contact the membrane to permeate.

A pressure drop is maintained across the membrane in order to effect the separation described at appropriate permeation rates. Membrane may be of any suitable type including diffusion and sizing, and may be constructed of inorganic, organic or composite materials. For diffusion membranes, the driving force is the differential partial pressures between the retained and permeate sides. In separation membranes, absolute pressure drop becomes a significant component of the driving force independent of concentrations or partial pressures. Preferred membranes are sieving membranes having a Ce Permeate Flow Index of at least 0.01 and a C6 Permeate Flow Ratio of at least 1.25: 1. Separation membranes are discussed in more detail below.

In membrane separations, the pressure drop is often within the range of 0.1 to 10, preferably 0.2 to 2, MPa. In practice, normal hexane containing fractions will be contacted with the retained side of the membranes without further compression to minimize capital and operating costs. The temperature for membrane separation will depend in part on the nature of the membrane and the temperature of the fraction. Thus, for polymer-containing membranes, temperatures should be sufficiently low that membrane resistance is not unduly affected. In most cases, the temperature for the separation is the temperature of the desanalyst fraction. Often the temperature is within the range of 25 ° C to 150 ° C. Thus, the conditions of membrane separation may provide a liquid or gaseous or mixed phase over membrane retention olate. Regardless of the fluid phase over the holding oil, the permeate may be a gas. If the fluid on the membrane retaining side is in the liquid phase, the permeate may be in the liquid, gaseous or mixed phase.

Any suitable selectively permeable membrane may be used in the apparatus and processes of this invention. Preferred membranes are sieving membranes. The membranes used in the processes of this invention are characterized by having a high flux, that is, having a C6 Permeate Flow Index of at least 0.01. The membranes may be in any suitable shape such as hollow fibers, plates, and the like which may be mounted in a separate separator unit such as beam or plate hollow fibers or spiral flame membranes. The physical design of the membranes should allow, when mounted on the separator unit, sufficient pressure drop across the membrane to provide desirable flow. For hollow fiber membranes, the high pressure side (retained side) is usually on the outer side of the hollow fiber. The permeate flux may be concurrent, countercurrent or transverse current with respect to fluid flow over the membrane holding side. Sufficient membrane surface area is provided under steady state conditions of at least 75, preferably at least 80, and with most preferably at least 90, by weight percent of the normal hexane contained in the desanalyst fraction is contained in the permeate. Normal hexane concentration will depend on membrane selectivity. Although the membrane may be highly selective and provides a permeate containing 99 mass percent or more of hexanonormal, advantageous embodiments of this invention may be achieved with lower purity permeates. The normal hexane concentration for total opermate in these embodiments will be less than 90 mass percent, e.g. 25 to 90, e.g. 40 to 80 mass percent. The remaining effluent will typically be methylpentanes and some methylcyclopentane and dimethylbutanes passing through the membrane.

Preferred high flow permeation membranes allow a portion of branched paraffins to permeate. Relative permeation rates will depend on the molecular configuration of the paraffins. Methyl pentanes will pass more readily through the membrane than dimethyl butanes and cyclopentane.

Preferably at least a portion of the permeate is recycled to the isomerization step. The permeate will contain linear paraffins which will be subjected to isomerization conditions during the deisomerization step.

Screening membranes

Preferred sieving membranes may be of various types, molecular sieve membranes, contendoporous ceramic membranes, metal membranes, polymeric membranes or decarbon membranes, or composite membranes having a high porous polymeric, ceramic, molecular sieve support with a barrier or thin layer (molecular sieve), for example zeolitic, polymeric, metal, ceramic or carbon, having microporosity.

The membranes may be continuous or discontinuous. A discontinuous membrane comprises a microporous barrier assembly of small particle size while a continuous membrane comprises a continuous microporous barrier layer. The membranes may be formed of a single material or may be composite containing the microporous support and barrier and optionally another structure. When preparing a continuous, thin barrier, as the thickness of the depen- ding layer decreases, the difficulty of obtaining a free defect layer increases. Since the processes of this invention do not require high selectivity, the membranes may contain minor defects. Continuous membranes are typically prepared by depositing or growing on a meso / macroporous structure of a continuous thin layer of barreiramicroporous. Discontinuous microporous barrier assemblies of an anonymous size allow very small permeation thicknesses to be obtained but with potential for deviation. Discontinuous membranes use a meso / macroporous structure with which the microporous barrier is associated.

Examples of zeolite barrier include small pore size molecular sieves such as SAPO-34, DDR, AlPO-14, AlPO-17, AlPO-18, A1PO-34, SSZ-62, Zeolite 3A, Zeolite 4A, Zeolite 5A, Zeolite KFT., H-2K-5, LTA, UZM-9, UZM-13, ERS-12, CDS-1, Phyllipsite, MCM-65, and MCM-47; medium pore molecular sieves such as comosilicalite, SAPO-31, MFI, BEA, and MEL; grandetal 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. Numerous types of molecular screens are available in colloidal form (nanometric department size) such as A, X, L, OFF, Mg, and SAPO-34. Zeolites may or may not be metal exchangers. Other types of screening materials include carbon sieves; polymers such as PIMs (intrinsic microporosity polymers) as described by McKeown, et al., Chem. Commun., 2780 (2002); McKeown5 et al. Chem. Eur. J., 11: 2610 (2005); Budd, et al., J. Matater. Chem., 13: 2721 (2003); Budd, et al., Adv. Mater., 16: 456 (2004) eBudd, et al., Chem Commun., 230 (2004); polymers in which porosity is induced by pore-forming agents such as poly (alkylene oxide), poly (vinyl pyrrolidone); cyclic organic hosts such as cyclohextrins, calixeenes, crown ethers, and spheres; microporous organometallic structures such as MOF-5 (or IRMOF-1); glass, ceramic and metal forms into which microporosity has been introduced.

In composite membranes, a meso / macroporous structure is used. The meso / macroporous structure serves one or more functions depending on the membrane type. It may be the support for the membrane composite, it may be an integral part of the microporous barrier formation, it may be the structure on which or in which the microporous barrier is located. The meso / macroporous structure may be continuous or discontinuous, and the meso / macroporosity may therefore be channels through the meso / macroporous structure material or be formed between the particles forming the meso / macroporous structure. Examples of the latter are AccuSep ™ inorganic filtration membranes available from Pall Corp. having a zirconia layer on a porous metal support in which zirconia is in the form of spherical crystals.

The meso / macroporous structure preferably defines channels or pores within the range of 2 to 500 nanometers, preferably 10 to 250 nanometers, more preferably between 20 and 200 nanometers in diameter, and has a high flow. In more preferred embodiments, the C6 Permeate Flow Index of the meso / macroporous structure is at least 1, and more preferably at least 10, and sometimes at least 1,000. Meso / macroporous structure may be isotropic or anisotropic. Bone / macropores may be relatively rectilinear or tortuous.

The meso / macroporous structure may be composed of inorganic, organic or organic and mixed inorganic material. The selection of material will depend on the conditions of the separation as well as the type of macroporous structure formed. The meso / macroporous structure material may be the same or different from the molecular sieve material. Examples of porous structure decompositions include metal, alumina such as alpha-alumina, gamma-alumina and transition aluminas, molecular sieve, ceramics, glass, polymer, and carbon. In preferred embodiments, substrate defects are repaired prior to the provision of the barrier or barrier precursor. In another embodiment, the substrate may be treated with a desyllic sun to partially occlude pores and facilitate barrier deposition or barrier precursor. The silica particles will still provide sufficient space between the interstices to allow high flow rates. Other techniques are to coat the support with silicone rubber or another polymer that allows high flow but occludes defects in the support or barrier.

If the meso / macroporous structure does not serve in this way, the membrane may contain a porous support for the macroporous structure. Porous support is typically selected based on its intended strength, tolerance to porosity and separation conditions.

AccuS ep ™ inorganic filtration membranes available from Fall Corp. and similar types of meso / macroporous structures are particularly advantageous because the meso / macroporous structure can be fined in this way by preventing the growth of undue sieve-molecular thickness. In addition, zirconia is relatively inert to zeolite formed precursor solutions and synthesis and calcination conditions, making it a preferred meso / macroporous structure for these depeneering membrane types.

High flow is achieved by at least one of the following techniques: first, using a larger pore than required for normal alkane passage; and second, using an extremely thin pore-containing layer. Where high flow is achieved using microporosomal, less selective microporous barriers, adequate separation can be achieved. The pores for these membrane types often have an average diameter greater than 0.50 nm (average length and width), for example 0.50 to 0.70 or 0.8 nm. Preferably, the structures have an aspect ratio (length to width) of less than 1.25: 1, for example 1.2: 1 to 1: 1. For sieviramolecular containing membranes, exemplary structures are USY, ZSM-12, SSZ-35, SSZ-44, VPI-8, and Cancrinite. In some cases, a permeating molecule in a microporopole may aid in increasing selectivity. For example, a normal hydrocarbon in one pore may decrease the rate at which a hydrocarbon-amplified can a branched hydrocarbon may enter the pore compared to another normal hydrocarbon.

High flow can also be achieved by using very thin barreiramicroporous on a continuous or discontinuous muitofine membrane. The microporous barrier may, if desired, be selected from screening structures having microporous which are substantially impermeable to the desired group to be retained on the retaining side. In general, the pores for these membrane types have a pore diameter of up to 0.55 nm, for example 0.45 to 0.54 nm. The pore aspect ratio of these membranes can vary widely, and is usually within the range of 1.5: 1 to 1: 1. For membranes containing molecular sieve, exemplary structures are ZSM-5, silicalite, ALPO-11, A1PO-31, ferrierite, ZSM-11, ZSM-57, ZSM-23, MCM-22, NU-87, UZM-9, and CaA

Membranes comprising a micropore porous discontinuous assembly are characterized in that the barrier has a size greater than 100 nanometers, and the microporous barrier is associated with a meso / macroporous structure defining flow-defective pores, in which the barrier is positioned to prevent flow. fluid through the pores of the meso / macroporous structure. A sieviramolecular barrier is "associated" with a meso / macroporous structure when it is positioned over or in the structure whether or not it is attached to the structure. As a consequence, nanometer-sized islands or particles of sieve size are used as barriers to the membranes. The discontinuous barreiramicroporous is positioned to prevent fluid flow through the fluid flow channels defined by the meso / macroporous structure. The barrier may be at least partially occluding the opening of a fluid flow channel of the meso / macroporous structure and / or within the fluid flow channel. Due to the small size of the microporous barrier discontinuous mounting particles or forming islands, some separation selectivity is achievable despite the discontinuity.

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 consideration when selecting the components for sieving membranes. With more spherical sieve particles, such as silicalite, it is preferred to select a meso / macroporous structure having portions that are close to the effective mesmodi diameter. In this way; Molecular sieve particles, positioned within or partially within, the pores of the structure / macroporous, will provide minimal void space by deviation. Platelet and molecular sieve particles are more flexible than regular because they may overlap with little or no void space. In some cases a combination of molecular sieve configurations may be desirable. For example, a spherical molecular sieve may be inserted into the spores of a meso / macroporous structure with smaller, more plaque-like molecular sieve particles being subsequently introduced. Complementary functions are those that the sphere serves as a support for plate-like particles and plate-like particles overlap to reduce deviation. Although molecular sieves are possibly different compositions, and thus have different configuration and microporosity size, the benefit is enhanced separation without undue loss of permeability.

Where zeolitic molecular sieves are used, obtaining small particles is important for high flow in a discontinuous barreiramicroporous. For many zeolites, seed particles are available which are smaller than 100 nanometers in larger dimension.

Molecular sieves are mostly made using organic molds that have to be removed to provide access to the cages. Typically this removal is by calcination. As discussed below, calcination can be performed when mold-containing molecular sieves are positioned in a macropore such that improper agglomeration is avoided simply by limiting the number of nearby particles.

Another technique for preventing agglomeration of zeolite particles during calcination is to silane the surface of the zeolite, for example, with an amino alkyl trialkoxy silane, amino alkyl dialkoxy silane, or amino alkyl dialkyl alkoxy silane. The amount of silanation required will depend on the size of the dazeolite and its composition as well as the conditions to be used for calcination. In general, between 0.1 and 10 millimoles of silane are used per gamma zeolite.

There are various techniques for providing the sieve-particle particles on or in the meso / macroporous support in a manner that meso or macropore occludes in the support. The specific technique to be used will depend upon the size and configuration of the molecular sieve particles, the size and configuration of the meso / macropores in the macroporous structure, and the desired positioning of the molecular sieve within or on the meso / microporous structure.

Especially where molecular sieve is positioned on the surface of a meso / macroporous structure to occlude at least a portion of the pore opening, the meso / macroporous structure may be provided with a size or molecular sieve solution or suspension. The concentration of molecular sieve in the suspension should be sufficiently low that under drying, the resulting sieve layer will not be unduly thickened. Advantageously at least one gentle pressure drop is maintained through the meso / macroporous structure during the coating such that a driving force will exist to insert molecular sieve into any pores in the meso / macroporous structure that have not been occluded. Usually the suspension will be an aqueous suspension, although suspensions in alcohols and other relatively inert liquids may advantageously be used at a concentration of between 2 and 30, for example 5 and 20, by weight. Where a pressure differential is used, the pressure differential is generally within the range of 10 kPa to 200 kPa. One or more molecular sieve coatings may be used, preferably with drying between coatings. Drying is at a high temperature, for example between 30 ° C and 150 ° C, for 1 to 50 hours. Vacuum can be used to aid drying. Where zeolites are used as molecular sieve, calcination, for example, at a temperature of between 450 ° C and 600 ° C may, in some cases, assist in the attachment of the sieviramolecular to the meso / macroporous structure. Calcination can also serve to agglomerate the molecular sieve particles and thus reduce voids and void size. Of course, calcination is not essential for the broad aspects of this invention and is only required where, for example, mold microporous residues. Where the discontinuous nanometer-sized molecular sieve assembly is located outside the pores of the structure / macroporous, it may be desirable to bond at least a portion of the particles on the surface of the structure. This can be accomplished in numerous ways. For example, the surface of the structure may be functionalized with hydroxyl groups or other groups that would be reactive with a zeolitic sieve. For polymer molecular sieves, the surface may be functionalized with groups which react, such as addition or condensation, with functional groups or the polymer. These techniques are well known in the art for other applications.

Similar preparation techniques may be used where it is desired to incorporate at least a portion of the sieviramolecular particles into the pores of the meso / macroporous structure. The sieve particles should be of an appropriate size to fit into the macros / macropores. A pressure differential may be used to insert barrier particles into the pores or ultrasound may be used to assist in the addition of barrier particles within the pores of the support / macroporous. The depth of the porous molecular sieve particles of the meso / macroporous structure should not be so large as to unduly reduce the permeability. Often, any surface deposition of molecular sieve is removed, for example by washing.

If desired, zeolitic molecular sieves may be grown in situ in the pores of the meso / macroporous structure to provide a discontinuous membrane. The synthesis may provide discrete islands or particles among another structure such as the meso / macroporous structure or other particles.

An example of using other particles to prepare discontinuous zeolitic molecular sieve membranes involves provision of silica, which may have a particle size between 5 nanometers and 20 nanometers, in or on the meso / macroporous structure. Silica, due to active hydrolysis on the surface, serves as a nucleation site for a zeolite-forming precursor solution, and zeolite layers can be grown on and between the silica particles.

Materials other than silica particles may be used as nucleation sites including other molecular sieves and de-zeolite crystals. The surface of the meso / macroporous structure may be functionalized to provide a selective location for zeolite growth. Some zeolites have self-nucleating properties and thus can be used in the absence of nucleation sites.

Examples of these zeolites are FAU and MFI. In such cases, it may be desired to maintain the precursor solution under conditions of zeolite formation for sufficient time so that zeolite growth begins prior to contacting the precursor solution with the meso / macroporous structure.

For example, one method of forming a barrier layer is to position zeolitic molecular sieve precursor liquid over a meso / microporous structure. The precursor is allowed to crystallize under hydrothermal crystallization conditions, after which the membrane is washed and heated to remove residual organic material. Molecular sieve material primarily resides in and occludes the pores of the porous substrate.

The molecular sieve may be any suitable combination of elements to provide the desired porous structure. Aluminum, silicon, boron, gallium, tin, titanium, germanium, phosphorus and oxygen have all been used as building blocks for molecular sieves such as silica-alumina molecular sieves. including zeolites; silicalite; A1PO; FROG; and borosilicates. The precursor includes the above mentioned elements, usually as oxides or phosphates, together with water and an organic structuring agent which is usually a polar organic compound such as tetrapropyl ammonium hydroxide. Other adjuvants may also be used such as amines, ethers and alcohols. The polar organic decomposed mass ratio for building block material is generally within the range of 0.1 to 0.5 and will depend on the specific building blocks used. In order to prepare thin layers of molecular sieves on the membranes, it is generally preferred that the precursor solution is rich in water. For example, for silica-alumina molecular sieves, the molar ratio of water to silica should be at least 20: 1, and for molecular aluminophosphate sieves, the mol ratio should be at least 20 moles of water per mol of aluminum.

Crystallization conditions are often within the range of 80 ° C to 250 ° C at pressures within the range of 100 kPa to 1,000 kPa, often from 200 kPa to 500 kPa absolute. The time for crystallization is limited so that it does not form an unduly thick molecular depleting layer. In general, the crystallization time is less than 50 hours, for example 10 to 40 hours. Preferably the time is sufficient to form crystals but less than that required to form a 200 nanometer molecular sieve, e.g. 5 to 50 nanometers. Acristalization can be done in an autoclave. In some cases, microwave heating will crystallize over a shorter period of time. The membrane is then washed with water and then calcined at 350 ° C to 550 ° C to remove any organic materials.

Especially with some molecular sieve materials, the preparation of particles smaller than 100 nanometers is problematic. In addition, even with the use of seed crystals, the particle size may be larger than desired. Another embodiment of the preparation of a discontinuous barrier membrane is the synthesis of zeolite in open regions between particles (substrate particles) having a size smaller than 100 nanometers. Consequently, the largest dimension of the microporous barrier may be smaller than 100 nanometers. The substrate particles serve as a nucleation site for zeolite formation and thus are selected from materials having the nucleation capacity of zeolite growth. Examples of such materials are silica, especially silica having a size greater than 5 nanometers to 50 nanometers and other zeolites having dimensions smaller than 100 nanometers. The use of vaporized silica as the substrate particle is particularly useful for preparing an AlPO microporous barrier.

Zeolite growth on the substrate particle may occur before or after the substrate particle is used in membrane composite formation.

Advantageously, growth of zeolite on substrate particles occurs while inserting the synthetic liquor through the composite.

This technique ensures that growth occurs not as a layer on top of the particles, but in the interstices between the particles. Depression fall increases as zeolite growth occurs, and depression fall can be used as an indicator of when proper zeolite formation has occurred.

Polymeric molecular sieves can be synthesized in the meso / macroporous structure. One method for synthesizing a small polymeric sieve-molecular sieve is to functionalize the nanoparticles and / or meso / macroporous structure with a group that can react with a oligomer such as by a condensation or addition reaction.

For example, the functional groups may provide a hydroxyl, amino, anhydride, dianhydride, aldehyde, amic acid, carboxyl, starch, nitrile, or olefinic group for condensation or addition reaction with an oligomer reactive group. Suitable oligomers may have molecular weights of 30,000 to 500,000 or greater and may be reactive polysulfone oligomers; poly (styrene) including styrene-containing copolymers, cellulosic polymers and copolymers; polyamides; polyimides; polyethers, polyurethanes; polyesters; acrylic and methacrylic polymers and copolymers, polysulfides, polyolefins, especially vinyl polymers and copolymers; poly (benzimidazole); polyphosphazines; poly (hydrazides), polycarbodiides, and the like.

In situ synthesis of the molecular sieve, whether organic or inorganic, may be under suitable conditions. A preferred technique involves conducting synthesis while introducing the reagent solution, for example, the precursor solution or the oligomer solution through the meso / macroporous structure. This technique provides the benefit of directing the reagent solution to unclogged voids as well as limiting the extent of molecular sieve growth so that no fresh reagents will be able to enter the reaction site once the molecular sieve has occluded the meso or macropore.

The molecular sieve on the polymeric support membranes or on the polymeric supports themselves may also be pyrolyzed in a vacuum oven to produce a carbon membrane. For such molecular sieve containing membranes, the carbon support pore structure is preferably of sufficient diameter to minimize Resistance to fluid flow with molecular sieve structure performing separation. The temperature of the pyrolysis will depend on the nature of the polymer support and will be below a temperature at which the porosity is unduly reduced. Examples of polymeric supports include polyimides, polyacrylonitrile, polycarbonates, polyetherketones, polyethersulfonase polysulfones, and prior to pyrolysis, the supports have pores or apertures within the range of 2 nanometers to 100 nanometers, preferably 20 nanometers to 50 nanometers.

Continuous membranes may be prepared by any suitable technique. Typically, the thickness of the microporous barrier will be related to the duration or growth of the microporous barrier over the meso / macroporous structure. The microporous barrier can be formed by pore size reduction of an ultrafiltration membrane (effective pore diameters from 1 nanometer to 100 nanometers) or a demicrofiltration membrane (effective pore diameters from 100 nanometers to 10,000 nanometers) by, for example, coating organic or inorganic channel inwardly or preferably at least partially near the channel opening. The deposited material serves to provide a localized reduction of the pores or openings through the support to a size that allows the desired screening without unduly reducing the diameter of the remaining pore structure of the support. Examples of vapor depositable materials include silanes, paraxylene, alkylene imines, and alkylene oxides. Another technique for reducing pore size is to deposit a layer of coke over the meso / macroporous structure. For example, charred gas such as methane, ethane, ethylene or acetylene may be contacted with the structure at a sufficiently high temperature to cause coking. Preferred porous supports are ultrafiltration membranes having pore sizes of between 1 nanometer and 80 nanometers, preferably between 2 nanometers and 50 nanometers.

For continuous, zeolitic membranes, a fabrication technique involves contacting the surface of the meso / macroporous molecular sieve precursor structure and growing the molecular sieve long enough to achieve the desired film thickness. The procedures described above can be used to synthesize the molecular sieve. In some cases, it may be desirable to occlude, for example, with a wax, osmotic / macropore from the support to prevent improper growth of zeolitana in those pores. The wax can be subsequently removed.

Several techniques are available to increase the selectivity of high flow membranes. There are numerous techniques for healing defects in continuous or discontinuous membranes. Since membranes do not need to exhibit high permeate flow ratios to be useful in many applications, any technique that increases flow resistance through defects will serve to improve membrane performance. For example, a silica sun overlay coating may be used to occlude interstitial openings between molecular sieve crystals or larger pores remaining in the support regardless of how the membrane is prepared.

Another technique for occluding large pores is to provide on one side of the barrier layer a large reactive molecule that is not able to permeate the barrier micropores and on the other side a cross-linking agent. Major defects, and to some extent minor defects, become filled with the large reactive molecule and are repaired by crosslinking. The unreacted large molecule component can then be removed as well as the unreacted crosslinking agent. The large molecule may be an oligomer or a large molecule.

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

A generic technique for increasing the selectivity of a screening membrane is to agglomerate adjacent particles of the molecular sieve to reduce or substantially eliminate voids between the particles and between the particles and the pore structure walls in the macroporous structure. Because the particles are of an anomalous size and the number of adjacent particles may be relatively small, agglomeration may occur while the permeate flow rates are still maintained. For polymeric molecular sieves that are thermothermic, the action may occur by heating to a temperature at which agglomeration occurs but not so high that it loses either its microporous structure or its ability to provide the desired meso- or macropore occlusion of the meso / macroporous structure. It can also be performed by calcination of zeolitic molecular sieves. Calcination tends to agglomerate small zeolite particles, especially particles that are not either silanated or untreated to reduce agglomeration tendency. The temperature and duration of calcination will depend on the nature of the zeolitic sieve. Normally temperatures between 45 ° C and 65 ° C are employed for a period of between 2 hours and 20 hours.

The agglomeration technique can be used with respect to molecular sieve particles that are on the structure / macroporous surface as well as those within the structure pores. More preferably, agglomeration is used when the molecular screening particles are located within meso- or macropores of the meso / macroporous structure such that the largest size of the agglomerate is less than 200 nanometers, preferably smaller than 100 nanometers. Agglomeration may be effected with or without a pressure differential across the membrane. Preferably a pressure differential is used to assist in reducing voids through which fluid can deviate from the molecular sieve.

Another generic technique where the batch discontinuous assembly defines voids is at least partially occluding at least a portion of the voids by a solid material within them. Preferably the solid material is a polymeric or inorganic material. The solid material may simply reside in the void or may adhere to or be attached to the sieviramolecular or meso / macroporous structure. The solid material may be a particle or an oligomer which may be preformed and then introduced into the voids or may be formed in situ.

In one aspect, the solid material provides a "mortar" with the microporous barrier particles. Mortar is typically a suitable polymeric material that can withstand the conditions of separation. Representative polymers include polysulfones; poly (styrene) including styrene-containing copolymers; cellulose polymers and copolymers; polyamides; polyethers; polyurethanes; polyesters; acrylic and methacrylic ecopolymers; polysulfides, especially vinyl polymers and polymers; polyalkyls; poly (benzimidazole); polyphosphazines; poly (hydrazides); polycarbodiides, and the like. Preferred polymers are those having porosity such as MIPs (see WO 2005/012397) and polymers in which porosity has been induced by pore-forming agents. These polymers have pores which may be 0.3 nanometer or larger, preferably at least 1 nanometer larger in size, and as a consequence allow fluid flow to and from the deburring particles.

All particles need not be enclosed in the mortar. Often the average thickness of the mortar layer is less than 100 nanometers, and is preferably no larger than the largest particle size. If too much mortar is used, a mixed membrane structure may result, and improperly penalized flow. As a result, the mass ratio of barrier particles to mortar is often within the range of from 1: 2 to 100: 1, preferably from 3: 1 to 30: 1.

Mortar and particles may be mixed, for example, in a sludge, and then positioned in association with the structuramicroporous, or may be provided after deposition of the particles. Opolymer may be formed in situ in the region containing the deburring particles. Barrier particles may be inert to polymerization and may have active sites to anchor a polymer. For example, the particle may be functionalized with a reactive group which may bind to the polymer or monomer undergoing polymerization, for example by means of a condensation or addition mechanism as discussed above.

One concern is that the mortar occludes the micropore of the molecular sieve. With highly porous polymer such as MIPs, the effect of any occlusion can be attenuated. Often the amount of polymer used for the mortar and its molecular weight and its configuration are that insufficient polymer is present for encapsulation of all molecular sieve particles. Frequently, the mass ratio of polymer to molecular sieve is between 0.01: 1 and 0.3: 1. The average molecular weight of the polymer is sometimes within the range of 20,000 to 500,000, preferably between 30,000 and 300,000.

Mortar may be different from polymer. For example, where the molecular sieve is a zeolite, a silicon tetraalkoxide may react with the zeolite and may by hydrolysis form a silica mass or structure between the molecular sieve particles. Usually a dilute aqueous solution of silicon tetra alkoxide is used, for example, containing between 0.5 and 25 mass percent of silicon tetra alkoxide, to ensure distribution. Functionalization of zeolite with tetraalkyl desoxide alkoxide is also useful for cross-linking the site with organic polymer, especially those containing functional groups such as hydroxyl, amino, anhydride, dianhydride, aldehyde or amic acid groups that can form covalent bonds with organosilicon alkoxide. Also, the same or different zeolite may be grown between zeolite particles and zeolite particles and the meso / macroporous structure using the techniques described above.

Yet another approach to reducing drift is to use two or more sized particles to form the barrier containing layer. If, for example, microporous barrier particles are generally spherical with a nominal larger size of 60 nanometers, the regions between the particles may be sized to allow deviation. Incorporating configurationally compatible departments in these regions can prevent fluid flow and thus result in a larger portion of the fluid being directed to the barrier particles for selective separation. The configuration of the barrier particles will depend on the type of trapping particle used. A microporous zeolitic molecular sieve particle having a size smaller than 100 nanometers will likely have a definite configuration due to its crystal structure. Somezeolites tend to have a platelet-like configuration while others, such as A1PO-14, have a stick-like structure.

Similarly, polymeric, ceramic, glass and carbon molecular sieve particles may have configurations that are not readily modified. As a result, the configuration of open regions between particles may vary.

Sometimes configurationally compatible particles are selected to achieve at least partial occlusion of the region. Thus, for spherical barrier particles, stick-shaped particles or much smaller configurably compatible particles may be desired. The configurationally compatible particles may be of any composition given the size and operating conditions. The particles may be polymeric, including oligomeric; of carbon; and inorganic such as vaporized silica, zeolite, alumina, and the like.

Detailed Description of the Drawing

Referring to Figure 1, a feed load containing linear paraffin is supplied to an isomerization unit via line 102. Hydrogen is provided via line 104. The combined current passed in isomerization reactor 106. Isomerization reactor effluent106 is directed via line 108 to stabilizer column 110. In stabilizer column 110, lighter ones are removed as a top fraction via line112. Lightweight can be used for any suitable purpose including fuel value. The bottom fractions of the stabilizer column 110 are passed through line 114 to the desanalyst 116. A separate fraction is provided via line 118 of the desanalyst 116. A bottom chain of the desanalyst 116 is removed via the line 120. A hexane-containing strand of the desanalyst. 116 is passed via line 122 to the retained side of membrane separator 124. A stream having a lower concentration of linear paraffins is removed from separator 124 via line 128. This stream will contain an increased concentration of methylcyclopentane. As shown, line 128 directs the retention fraction for the combination with the top fraction at line 118. The permeate fraction is recycled via line 126 to the isomerization reactor 106.

Claims (15)

A process for isomerizing a feedstock comprising paraffins having between 5 and 6 carbon atoms in which at least 15 mass percent of the feedstock is linear paraffin to provide an isomerate comprising paraffins, characterized by the following: a. isomerize the feedstock under deisomerization conditions including the presence of isomerization catalyst to provide an isomerization effluent containing linear paraffins but at a lower concentration than that in the feedstock, b. distilling at least a portion of the isomerization effluent to provide a lower boiling fraction containing dimethyl butane and lighter paraffins and a fraction containing higher normal hexane containing normal hexane, methylpentanes, dimethyl butane, emethyl cyclopentane, ç. contacting at least a portion of the hexanonormal containing fraction from step b with a retained side of a selectively permeable membrane under conditions including sufficient membrane surface area and pressure differential across the membrane to provide a retained fraction of the side stream having increased methyl concentration. cyclopentane and dimethyl butanes, and to provide through the membrane on a permeate side, a permeate fraction having an increased concentration of normal hexane and methyl pentanes, said permeate fraction containing at least 75 weight percent normal hexane contained in normal hexane-containing fraction contacted with membrane, ed. remove from step c the retention fraction. Process according to Claim 1, characterized in that at least a portion of the permeate fraction from step c is passed to step a. Process according to Claim 2, characterized in that the normal hexane-containing stream from step b is a lateral stream, and the distillation of step b provides bottom fractions containing C7 hydrocarbon. Process according to Claim 3, characterized in that the concentration of normal hexane in the permeate fraction is less than 90 percent by mass. The process according to claim 4, characterized in that at least 30 mass percent of normal hexane-containing nephration-containing methyl pentanes contacting the retaining side of the depen- ding membrane passes to the permeate side of the membrane, and at least 70 percent in mass of methyl pentanes in the normal hexane-containing fraction contacting the retaining side of the sieving membrane passes to the membrane permeate side, and at least 70 percent by weight of dimethyl butanes in the normal hexane-containing fraction contacting the retained side of the sieving membrane is retained on the retained side of the membrane. Process according to Claim 5, characterized in that the normal hexane-containing fraction comprises methylcyclopentane, and methylcyclopentane is contained in the retentate fraction. Process according to Claim 5, characterized in that the normal hexane-containing fraction contains 5 to 30 percent by weight of dimethyl butanes in the isomerization effluent. Process according to claim 1, characterized in that the membrane is a sieving membrane having a Ce Permeate Flow Index of at least 0.01 and a Permeate Ce Flow Ratio of at least 1.25: 1. Process according to Claim 8, characterized in that the sieving membrane has a pore diameter of 0.50 to 0.70 nm. Process according to Claim 8, characterized in that the sieving membrane has an average pore diameter of 0.45 to 0.54 nm. Process according to Claim 8, characterized in that the sieving membrane comprises ZSM-5 or silicalite. Process according to Claim 1, characterized in that the normal hexane-containing fraction contains at least 2 percent by weight of dimethyl butanes contained in the isomerization effluent. Process according to claim 1, characterized in that the lower boiling fraction contains isopentane. Process according to claim 12, characterized in that the lower boiling fraction is a top fraction. 15. Apparatus for the isomerization of a feedstock comprising paraffins having between 5 and 6 carbon atoms to provide a fraction of petrol, characterized in that it comprises: a. an isomerization reactor (106) being adapted to receive supply load at an input and having an output, b. a de-analyzer (116) having an inlet in fluid communication with the isomerization reactor outlet (106), a lower boiling fraction output adapted to remove a lower boiling fraction via line (118), an outlet to provide a fraction of lateral cut and a higher boiling fraction output; ec. a membrane separator (124) having a fluid communicating feed inlet with the outlet to provide a side section fraction of the de-scrubber (116), a fluid communicating feed outlet with line (118) of the boiling defrost output (116), and a permeate output in fluid communication with the isomerization reactor inlet (106).