PROCESS FOR THE SEPARATION AND ISOMERISATION OF N-PARAFFINS IN THE PRESENCE OF A ZEOLITE MEMBRANE

Field of the Invention

The present invention provides a process for the separation of different n-paraffins from each other. Background of the Invention 5 n-Paraffins are commonly used as feedstock for chemical processes, for instance for the production of chemicals such as benzene or toluene. In addition, n-paraffins are used as feedstock for the preparation of isoparaffins, which are suitable as blending component in

10 gasoline. In a crude oil refinery, such isoparaffins are generally produced by the isomerisation or hydro- isomerisation of n-paraffins present in straight run naphtha or in the effluent of hydrocarbon cracking processes like hydrocracking, fluid catalytic cracking or

15 thermal cracking.

Isomerisation processes are generally operated at elevated temperature. The optimal isomerisation temperature, i.e. the temperature at which the highest n-paraffin to isoparaffin conversion is obtained, depends

20 on the carbon number of the n-paraffin. Generally, the optimal (hydro-) isomerisation temperature decreases with increasing carbon number of the n-paraffin.

An undesired side effect of the isomerisation process is the cracking of n-paraffins to lower hydrocarbons,

25 e.g. hydrocarbons with at most 4 carbon atoms, thereby lowering the isoparaffin yield. The tendency of n-paraffins to crack increases with increasing carbon number of the n-paraffin and with increasing isomerisation temperature.

Generally, in isomerisation processes, as for instance described in US 4717784, a mixture of n-paraffins with different carbon numbers is isomerised. The isomerisation temperature is based on a trade-off between conversion and cracking of the individual n-paraffins in the mixture. As a consequence, the conversion is low, generally as low as 25%, and substantial amounts of unconverted n-paraffins remain in the product stream. The unconverted n-paraffins are typically separated from the product stream and recycled to the isomerisation process.

Due to the tendency of C7+ n-paraffins, i.e. n-paraffins with more than 8 carbon atoms, to crack at temperatures that are needed to isomerise C5/C5 n-paraffins, the amount of C7+ n-paraffins in the isomerisation feedstock should be kept to a minimum. Generally, no more than 2 wt% of C7+ n-paraffins should be allowed. In order to achieve this, the C7+ n-paraffins need to be separated from the feed by distillation. To allow each n-paraffin to be isomerised at the optimum isomerisation temperature, it would be advantageous if a mixture of n-paraffins could be separated into different n-paraffin streams, each with a narrower carbon number distribution than the feedstock. Summary of the Invention

It has now been found that it is possible to separate a mixture of n-paraffins into different n-paraffin streams with each a narrower n-paraffin carbon number distribution than the feedstock by using a membrane process.

Accordingly, the present invention provides a process for the separation of different n-paraffins from each other, wherein a hydrocarbonaceous feedstock comprising

different n-paraffins with a carbon number in the range of from 5 to 10 and having a n-paraffin carbon number distribution is contacted with an inorganic porous membrane with an average pore diameter in the range of from 3.5 x 10 ^" 1O to 10 x 10 ^" 1O m at a trans-membrane pressure in the range of from 1 to 50 bar, to obtain at least two product streams each comprising at least 50 wt% of n-paraffins with a carbon number in the range of from 5 to 10, wherein each of the at least two product streams has a n-paraffin carbon number distribution that is narrower than the n-paraffin carbon number distribution of the feedstock and the average carbon numbers of the n-paraffins in each of the at least two streams differ from each other . An advantage of the process according to the invention is that in a single process step different separate n-paraffin product streams are obtained from the feedstock. These product streams each have a different average carbon number and a narrow carbon number distribution. Such product streams can subsequently each be isomerised at its optimal isomerisation temperature in order to obtain a high yield of isoparaffin and to reduce n-paraffin cracking. Such product streams, i.e. product streams having a different average carbon number and a narrow carbon number distribution, can be obtained by distillation. However, this would require a complex distillation process comprising several separate distillation stages. Detailed description of the invention The process according to the present invention is a process for the membrane separation of different n-paraffins with a carbon number in the range of from 5 to 10 into at least two different n-paraffin product

streams. A feedstock comprising different n-paraffins with a carbon number in the range of from 5 to 10 and having a n-paraffin carbon number distribution is contacted with an inorganic porous membrane with an average pore diameter in the range of from 3.5 x 10 ^" 1O to

10 x 10 ^" 1O m at a trans-membrane pressure in the range of from 1 to 50 bar. In this process at least two product streams are obtained. The product streams have a n-paraffin carbon number distribution that is narrower than the n-paraffin carbon number distribution of the feedstock. Furthermore, the product streams differ from each other in average n-paraffin carbon number. As the feedstock is contacted with the membrane, n-paraffins in the feedstock will permeate through the membrane as a consequence of the trans-membrane pressure. The preference of permeation through the membrane depends on the carbon number of the n-paraffin. A n-paraffin with a higher carbon number will preferentially permeate over a n-paraffin with a lower carbon number, i.e. C _] _Q preferentially permeates over C9 to C5, C9 preferentially permeates over Cg to C5, Cg preferentially permeates over

C7 to C5, C7 preferentially permeates over Cg to C5 and Cg preferentially permeates over C5.

In the process according to the invention at least two product streams each comprising at least 50 wt% of n- paraffins with a carbon number in the range of from 5 to 10 are obtained. The at least two product streams may be permeate streams or the retentate stream and at least one permeate stream. Preferably, a retentate stream and at least two permeate streams are obtained, wherein the permeate streams are the product streams . The permeate streams then each comprise at least 50 wt% of n-

paraffins, each have a n-paraffin carbon number distribution that is narrower than the n-paraffin carbon number distribution of the feedstock and they differ from each other in average n-paraffin carbon number. If the product steams are permeate streams, then the at least two permeate streams are preferably obtained by continuously supplying the feedstock to the membrane in a flow direction parallel to the membrane surface. As the feedstock flows along the membrane surface it will first become depleted in the preferentially permeating n- paraffins. As a consequence both the composition of the feedstock and the composition of the permeate will change along the length of the membrane: the average carbon number of the n-paraffins in the permeate will decrease in the downstream direction of the membrane. Thus, different permeates can be recovered from different sections of the membrane. For instance, a first permeate stream may be obtained from a first section and a second permeate stream may be obtained from a second section, wherein the first section is located upstream to the second section. The average n-paraffin carbon number of the first permeate stream is then higher than the n- paraffin number of the second permeate stream. Reference herein to downstream or upstream is with regard to the flow direction of the feedstock.

Alternatively, at least two permeate streams each comprising at least 50 wt% n-paraffins can be obtained by supplying an amount of feedstock in a batchwise manner to a dead-end membrane separation unit. Initially, the permeate will predominantly comprise the preferentially permeating higher n-paraffins. In time, the feedstock will become depleted in the higher n-paraffins and n- paraffins with a lower carbon number will permeate

through he membrane. As a consequence, the composition of the permeate will change. By recovering a first permeate stream during a first time interval followed by recovering a second permeate stream in a second later time interval, two permeate streams are obtained, each having at least 50 wt% of C5-C10 n-paraffins and each with a n-paraffin carbon number distribution that is narrower than that of the feedstock and each with a different average carbon number of the n-paraffins. It will be appreciated that it is also possible that the at least two product streams include the retentate stream and a permeate stream. This requires the feedstock to comprise far more than 50 wt% of n-paraffins with a carbon number in the range of from 5 to 10, typically more than 70 wt%, since the n-paraffins preferably permeate through the membrane and a retentate stream still comprising more than 50 wt% of n-paraffins with a carbon number in the range of from 5 to 10 is to be obtained . The feedstock may be any hydrocarbonaceous feedstock comprising n-paraffins. The feedstock may also comprise other hydrocarbonaceous components like isoparaffins, olefins, naphthenes and aromatics .

Preferably, the feedstock comprises at least 50 wt% of hydrocarbons with a carbon number in the range of from 5 to 18, based on the total weight of the feedstock, more preferably at least 75 wt%, more preferably at least 90 wt%.

Preferably, the feedstock comprises in the range of from 5 to 99 wt% of n-paraffins with a carbon number in the range of from 5 to 10, based on the total weight of hydrocarbons in the feedstock, more preferably in the

range of from 20 to 99 wt%, even more preferably of from 50 to 99 wt%.

Preferably, in the range of from 50 to 99 wt% of the n-paraffins with a carbon number in the range of from 5 to 10 in the feedstock have a carbon number in the range of from 5 to 7, more preferably of from 70 to 99 wt%, even more preferably of from 80 to 99 wt%.

Examples of suitable feedstocks are straight-run naphtha, naphtha obtained by a cracking process like hydrocracking, fluid catalytic cracking, thermal cracking, delayed coking, visbreaking and/or flexicoking and synthetic naphtha. Reference herein to synthetic naphtha is to naphtha obtained from a Fischer-Tropsch process. Straight-run naphtha and naphtha obtained from cracking processes typically comprise in the range of from 10 to 70 wt% n-paraffins with a carbon number in the range of from 5 to 10. Synthetic naphtha will generally comprise n-paraffins with a carbon number in the range of from 5 to 10 in excess of 70 wt%. Further examples of feedstocks are the effluent of a catalytic reformer or fuels .

The feedstock may comprise sulphurous or nitrogenous compounds. It may therefore be advantageous to desulphurise and/or denitrogenate the feed prior to supplying it to the membrane. However, it will be appreciated that the sulphurous or nitrogenous hydrocarbonaceous components will typically not permeate through the membrane and will consequently not be present in the permeate . The feedstock is contacted with the membrane at a trans-membrane pressure in the range of from 1 to 50 bar, preferably of from 5 to 30 bar. Reference herein to trans-membrane pressure is to the pressure difference

between the retentate side of the membrane and the permeate side of the membrane. At the permeate side of the membrane, a vacuum or sweep gas may be applied to maintain the trans-membrane pressure. Examples of suitable sweep gases are helium, nitrogen, propane, n-butane, hydrogen or a mixture thereof. Hydrogen or hydrogen-comprising mixtures are particularly preferred if the permeate is subsequently hydro-isomerised . The feedstock is typically contacted with the membrane at a temperature in the range of from 50 to 200 ⁰ C, preferably of from 70 to 120 ⁰ C.

The feedstock may be in liquid and/or gaseous form when contacted with the membrane. Preferably, the process conditions and/or feedstock are chosen such that the feedstock is liquid when contacting the membrane.

In order to prevent the permeation of undesired components when the feedstock becomes depleted in C5 to

C]_o n-paraffins, it is advantageous that the feedstock comprises n-butane and/or propane. This can be achieved by co-supplying naphtha and a small amount of n-butane and/or propane to the process.

The membrane used in the process according to the invention is an inorganic porous membrane. Reference herein to a membrane is to a selective barrier. The membrane may consist of a single membrane layer or may be a composite of more than one membrane layers or of a porous support layer and one or more membrane layers.

Preferably, the average pore diameter of the membrane is in the range of from 4 x 10 ^" 1O to 9 x 10 ^" 1O m, more preferably of from 4 x 10 ^~10 to 6 x 10 ^~10 m. The pores may have any form, for instance round or slit-shaped. Reference herein to average pore diameter of the membrane is to the average pore diameter of the membrane layer

determining the separation properties. It will therefore be appreciated that the ranges for the average pore diameter, as mentioned hereinabove, do not apply to the porous support layer. The membrane preferably has a porosity in the range of from 10 to 90%, preferably of from 15 to 60%.

The membrane layer (s) may be composed of any inert inorganic material having a high affinity to paraffins and a pore diameter in the range of from 3.5 x 10 ^" 1O to 10 x 10 ^~ 10 m. The inert inorganic material should not comprise acid sites. Examples of such materials are refractory oxides or zeolites, preferably zeolites, more preferably silicalite or Ca5A.

Preferably, the membrane has a porous support layer. The porous support layer preferably is a refractory oxide support layer or a porous metal support layer, more preferably alumina, titania, zirconia or porous stainless steel. Such porous support layers are typically used to provide mechanical stability. A particularly suitable membrane comprises silicalite supported on a porous stainless steel support layer. The silicalite may be deposited on the support by means known in the art, e.g. in-situ crystallisation from a gel. This way a continuous defect free layer of b-orientated silicalite crystals with a thickness in the range of from 2 to 50 μm, preferably of from 2 to 10 μm, is obtained without inter-crystalline voids. Such a layer has an average pore diameter in the range of from 5.1 to

5.6 x 10 ^"10 m. The membrane may be used in any configuration known in the art, for example hollow fibre, tubular or as flat sheet .

Optionally, the membrane may comprise one or more functional components, e.g. catalysts, preferably in the form of catalyst-comprising layers.

Preferably, the process further comprises an isomerisation step, more preferably a hydro-isomerisation step, to allow for further conversion of the product streams. Preferably, at least one of the product streams is hydro-isomerised by contacting the product stream with a hydro-isomerisation catalyst in the presence of hydrogen at a temperature in the range of from 50 to

300 ⁰ C. Preferably, the at least one product stream is contacted with the hydro-isomerisation catalyst at a pressure in the range of from 10 to 50 bar.

Preferably, the permeate side of the membrane is in fluid communication with the hydro-isomerisation catalyst. More preferably, the membrane comprises the hydro-isomerisation catalyst. For instance, the permeate side of the membrane may be impregnated or coated with the hydro-isomerisation catalyst. It will be appreciated that such a configuration will have practical significance if the hydro-isomerisation temperature does not exceed the temperature range given for contacting the feedstock with the membrane. If the hydro-isomerisation catalyst is in fluid communication with the permeate side of the membrane, the n-paraffins can be converted shortly after permeating through the membrane. Consequently, the concentration of n-paraffin at the permeate side of the membrane is kept low and therefore the driving force for permeation, i.e. the concentration gradient over the membrane, remains high.

The catalyst may be any hydro-isomerisation catalyst known in the art. Preferably, the catalyst comprises a refractory oxide or zeolite with disposed thereon a

catalytic metal, preferably a group VIII noble metal, more preferably Pt. Suitable catalysts are for example Pt on sulphonated zirconia, Pt on mordenite and chlorinated Pt on alumina. The catalyst may comprise a binder. Suitable binders are well known in the art and include refractory oxides, such as silica, alumina, titania, zirconia and mixtures thereof .

Preferably, at least two product streams are hydro- isomerised. The product streams are contacted with the hydro-isomerisation catalyst and each product stream is contacted with the hydro-isomerisation catalyst at a different temperature. As explained hereinabove, the isomerisation temperature decreases with increasing carbon number of the n-paraffins in the hydro- isomerisation feed. It will be appreciated that in order to obtain high conversion and high yield it is preferred that each product stream is contacted with the catalyst at a different temperature. The hydro-isomerisation reaction is thermodynamically limited, resulting in an isomerisation effluent comprising an equilibrium reaction product wherein some of the n-paraffins remain. The isomerisation effluent may subsequently be recycled to the membrane in order to obtain unreacted n-paraffins .