PROCESS FOR SEPARATING NORMAL PARAFFINS FROM BRANCHED AND CYCLIC HYDROCARBONS

Abstract: Normal paraffins are separated from mixtures of the same with cyclic hydrocarbons having more than six carbon atoms per molecule and/or branched chain hydrocarbons by contacting the mixture at adsorption conditions with an adsorbent comprising silicalite, whereby the normal paraffin is selectively adsorbed. It may then be recovered from the adsorbent by desorption with a desorbent material. Prefer­ ably a simulated moving bed countercurrent flow adsorption system is used.

Description: This invention relates to a process which utilizes a solid, silicon-containing adsorbent to separate normal paraffins from a feed mixture of normal paraffins with branched-chain and cyclic hydrocarbons. There is an abundance of prior art in the separation field, especially art relating to countercurrent fixed bed type operations, which deal with the separation of normal paraffins from other classes of hydrocarbons using a solid adsorbent. Examples of such are U.S.-A-2,957,927, 3,239,455, 3,405,057, 4,000,059 and 4,006,197. The most common adsorbents used throughout the prior art processes for the above separations are the crystalline aluminosilicates, the best known of which are the zeolites. The crystalline aluminosilicates function as "molecular sieves", that is, they contain pores having cross-sectional diameters which will accept certain molecules in a mixture of molecules of specific size and shape, e.g. normal paraffins, while rejecting others, e.g. branched chain and cyclic hydrocarbons, thereby separating the accepted molecules from the mixture. A new crystalline silica molecular sieve type absorbent material known as "silicalite", having structure containing no tetrahedrally coordinated aluminum has been proposed in U.S.-A-4,061,724. The separation process utilizing silicalite contemplated in US-A-4,061,724 comprises, in general terms, the separation of an organic compound from an aqueous solution. It has now been discovered that silicalite is particularly suitable for separating a normal paraffin from a mixture of the same with branched chain and cyclic hydrocabons. In brief summary the invention provides a process for separating a normal paraffin from a mixture of the same with a cyclic hydrocarbon having more than six carbon atoms per molecule and a branched chain hydrocarbon by contacting the mixture at adsorption conditions with a silicon-containing solid adsorbent to effect the selective adsorption of the normal paraffin by the absorbent, characterised in that the absorbent used is silicalite and subsequently recovering the absorbed normal paraffin by desorption with a desorbent material at desorption conditions, containing no tetrahedrally coordinated aluminum, that the desorbent material comprises a normal paraffin having a boiling point at least 5°C different from the boiling point of the paraffin being separated, and that the feed mixture contains little defins. In a preferred embodiment the present invention provides a process for separating a normal paraffin from a mixture of the same with a cyclic hydrocarbon having more than six carbon atoms per molecule and a branched chain hydrocarbon using a silicon-containing solid adsorbent, which process comprises the steps of: (a) maintaining net fluid flow in a single direction through a column of the adsorbent, which column contains at least three zones having separate operational functions occurring therein and serially interconnected with their terminal zones being connected to provide a cyclic fluid flow path through the column, (b) maintaining an adsorption zone in the column comprising the adsorbent located between a feed input port at its upstream boundary and a raffinate output port at its downstream boundary, (c) maintaining a purification zone in the column immediately upstream from the adsorption zone and comprising the adsorbent located between an extract output port at its upstream boundary and the feed input port at its downstream boundary, (d) maintaining a desorption zone in the column immediately upstream from the purification zone and comprising the adsorbent located between a desorbent input port at its upstream boundary and the extract output port at its downstream boundary, (e) passing the feed mixture into the adsorption zone through the feed input port so as to effect the selective adsorption of the normal paraffin by the adsorbent in the adsorption zone and withdrawing a raffinate stream from the raffinate output port of the adsorption zone, (f) passing a desorbent material into the desorption zone through the desorbent input port so as to effect the displacement of the normal paraffin from the adsorbent in the desorption zone, (g) withdrawing an extract stream comprising the normal paraffin and desorbent material from the extract output port of the desorption zone, (h) withdrawing a raffinate stream comprising the branched chain and/or cyclic hydrocarbon(s) present in the feed mixture from the desorption zone, and (i) periodically advancing through the column of adsorbent in a downstream direction with respect to fluid flow in the adsorption zone, the feed input port, raffinate output port, desorbent input port and extract output port to effect the shifting of zones through the absorbent and the production of extract and raffinate streams, characterised in that the absorbent used is silicate containing no tetrahedrally coordinated aluminum, that the desorbent material comprises a normal paraffin having a boiling point at least 5°C different, from the boiling point of the paraffin being separated, and that the feed mixture contains little olefins. In the context of the present invention, the feed mixture is a mixture of one or more extract components and one or more raffinate components as fed to the process. An "extract component" is a compound or type of compound that is more selectively adsorbed by the adsorbent while a "raffinate component" is a compound or type of compound that is less selectively adsorbed. In this Invention normal paraffins from the feed stream are extract components while feed stream isoparaffins and cyclic hydrocarbons are raffinate components. The term "desorbent material" means a material capable of desorbing the extract component from the adsorbent. The terms "raffinate stream" and "extract stream" mean streams through which respectively most of the raffinate components and the extract material which has been desorbed by the desorbent material are removed from the adsorbent. The composition of the raffinate and extract stream can vary from essentially 100% desorbent material to essentially 100% raffinate components or extract material as the case may be. Although it is possible by the process of this invention to produce high purity (99+%) normal paraffins at high recoveries (90% or higher), it will be appreciated that an extract component is never completely adsorbed by the adsorbent, nor is a raffinate component completely non-adsorbed by the adsorbent. Therefore, small amountsof a raffinate component can appear in the extract stream and, likewise, small amounts of an extract component can appear in the raffinate stream. The extract and raffinate streams then are further distinguished from each other and from the feed mixture by the ratio of the concentrations of an extract component and a raffinate component appearing in the particular stream. More specifically, the ratio of the concentration of the adsorbed normal paraffins to that of the non-adsorbed components will be lowest in the raffinate stream, next highest in the feed mixture, and the highest in the extract stream. Likewise, the ratio of the concentration of the non-adsorbed components to that of the adsorbed normal paraffins will be highest in the raffinate stream, next highest in the feed mixture, and the lowest in the extract stream. Feed stocks used in the process of this invention include hydrocarbon fractions having a carbon number range of from 2 to 30 carbon atoms per molecule. Typically, the carbon number range of the hydrocarbon fractions will be rather narrow, for example from three to ten carbon numbers. A C₁₀ - C₁₅ kerosine fraction or a C₁₀ - C₂₀ gas oil fraction is a typical feed stream. The feed streams will contain normal paraffins and isoparaffins and cyclohydrocarbons, including aromatics having more than six carbon atoms per molecule, in varying concentrations. However, they should have little olefins. Depending on the type of crude from which the hydrocarbon fraction is derived and the carbon number range of the fraction, the normal paraffin concentration will typically range from 15 to 60 vol.% of the feed and the aromatic concentration from 10 to 30 vol.% of the feed. There may be more unusual feed streams which have aromatic concentrations of only 2 to 4 vol.% of the feed stream. Since the feed aromatics other than benzene, like the isoparaffins, cannot enter the pores of adsorbent used in this process because their cross-sectional diameter is too great, almost all of the aromatics appear in the raffinate stream. The feed aromatics can include monocyclic aromatics such as alkylbenzenes; indanes or alkylindanes; and bicyclic aromatics including naphthalenes, biphenyls, or the acenaphthenes. Desorbent materials used in adsorptive separation processes vary depending upon such factors as the type of operation employed. In connection with the nature and function of desorbents attention is directed to the passage in GB-A-2049667 from page 4 line 35 to page 5 line 6, which is incorporated herein by reference. In the process of this invention, it has been found that desorbent materials comprising paraffins having average boiling points substantially different from, i.e. 5°C or more above or below that of the feed mixture, meet these requirements and are particularly effective. The adsorbent to be used in the process of this invention is silicalite containing no tetrahedrally coordinated aluminum. Silicalite is a hydrophobic crystalline silica molecular sieve. Due to its aluminum free structure, silicalite does not show ion-exchange behaviour, and is hydrophobic and organophilic. Silicalite thus comprises a molecular sieve but not a zeolite. Silicalite is uniquely suitable for the separation process of this invention for the presumed reason that its pores are of a size and shape (about 6 angstrom units in diameter) that enable the silicalite to function as a molecular sieve, i.e. accept the molecules of normal paraffins into its channels or internal structure, while rejecting the molecules of hydrocarbons of other structural classes. A more detailed discussion of silicalite may be found in the article "Silicalite, A New Hydrophobic Crystalline Silica Molecular Sieve"; Nature , Vol. 271, 9 February 1978, incorporated herein by reference. The adsorbent may be employed in the form of a dense compact fixed bed which is alternately contacted with the feed mixture and desorbent materials. In the simplest embodiment of the invention the adsorbent is employed in the form of a single static bed in which case the process is only semi-continuous. In another embodiment a set of two or more static beds may be employed in fixed bed contacting with appropriate valving so that the feed mixture is passed through one or more adsorbent beds while the desorbent materials can be passed through one or more of the other beds in the set. The flow of feed mixture and adsorbent materials may be either up or down through the desorbent. Any of the conventional apparatus employed in static bed fluid-solid contacting may be used. The particles of silicalite adsorbent will preferably have a particle size range of 16-60 mesh (Standard U.S. Mesh) or 250 to 1200 microns. Countercurrent moving bed or simulated moving bed countercurrent flow systems have a much greater separation efficiency than fixed adsorbent bed systems and are therefore preferred. In the moving bed or simulated moving bed processes the adsorption and desorption operations are continuously taking place, which allows both continuous production of an extract and a raffinate stream and the continual use of feed and desorbent streams. One preferred embodiment of this process utilizes what is known in the art as the simulated moving bed countercurrent flow system. The operating principles and sequence of such flow system are described in the passage at page 8 line 53 - page 10 line 2 of GB-A-2049667, which passage is incorporated herein by reference. It is contemplated that at least a portion of the extract stream will pass into a separation means wherein at least a portion of the desorbent material can be separated to produce an extract product containing a reduced concentration of desorbent material. Preferably, but not necessary to the operation of the process, at least a portion of the raffinate stream will also be passed to a separation means wherein at least a portion of the desorbent material can be separated to produce a desorbent stream which can be reused in the process and a raffinate product containing a reduced concentration of desorbent material. The separation means will typically be a fractionation column, the design and operation of which is well known to the separation art. Reference can be made to U.S.-A-2,985,589, and to a paper entitled "Continuous Adsorptive Processing - - A New Separation Technique" by D.B. Broughton presented at the 34th Annual Meeting of the Society of Chemical Engineers at Tokyo, Japan on April 2, 1969, both incorporated herein by reference, for further explanation of the simulated moving bed countercurrent process flow scheme. Although both liquid and vapor phase operations can be used in many adsorptive separation processes, liquid-phase operation is preferred for this invention because of the lower temperature requirements and because of the higher yields of extract product that can be obtained with liquid-phase operation over those obtained with vapor-phase operation. Adsorption conditions typically include a temperature range of from 40°C to 250°C and a pressure sufficient to maintain liquid-phase. Desorption conditions typically include the same range of temperatures and pressures as used for adsorption. The size of the units which can be utilized in the process of this invention can vary anywhere from those of pilot-plant scale (see for example U.S.-A-3,706,812) to those of commercial scale and can range in flow rates from as little as a few cc an hour up to many thousands of gallons per hour. Commercially available adsorbents heretofore used in the separation of the present invention have some entrance diameters less than 6 angstroms, examples of which are chabazite, Type A (both sodium and calcium forms), faujasite, mordenite, etc. A serious problem with these adsorbents is the low exchange rate for displacement of feed straight chain hydrocarbons with desorbent molecules and thus the long and inefficient cycle times required to effect desorption. Surprisingly, silicalite, which has some entrance diameters of 6 angstroms, does not exhibit such low exchange rate. The following Examples are presented in further illustration but there is no intention to limit the invention to the operating conditions and materials disclosed in them. EXAMPLE 1 (Comparative) Commercially available, clay-bindered (about 20% by weight), 5A type, Ca exchanged molecular sieves containing 2 wt.% water (pore entrance diameters of 5 angstroms ) were obtained. A portion of these were evaluated for normal paraffin exchange in a dynamic test apparatus in the following manner. A first mixture of 16% n-tetradecane in isooctane was introduced into one end of a fixed bed, thereby contacting a 40 cc bed of these fresh molecular sieves at 2070 kPa gauge, 232°C and 3 liquid hourly space velocity (LHSV). When the molecular sieve cavities were full of n-tetradecane as evidenced by a GLC analysis of the effluent from the other end of the fixed bed, a desorbent second mixture containing 16% chemically pure n-decane in chemically pure isooctane was introduced 'into one end of a fixed bed at the above conditions to effect the displacement of n-tetradecane within the sieve cavities by n-decane. This was continued until the effluent contained no n-tetradecane by GLC analysis. The first mixture was thereupon reintroduced into said one end again until the effluent contained no n-decane. The steepness of the concentration gradient for the appearance of n-tetradecane in the effluent was observed and taken as a measure of the rate of sorption of n-tetradecane. Likewise, the steepness of the concentration gradient from the appearance of n-decane in the effluent was observed and taken as a measure of the rate of sorption of n-decane. The volume of the first mixture required to change the concentration of C₁₄ in the effluent from 10% to 90% of total normal paraffin contained in the effluent can be taken as a measure of the rate of sorption of n-C₁₄. The volume of the second mixture required to change the concentration of C₁₀ in the effluent from 10% to 90% of total normal paraffin contained in the effluent can be taken as a measure of the rate of sorption of n-C₁₀. In either case, the higher the volume the slower the rate of sorption. The volume required for the 10% to 90% change is hereinafter referred to as the "breakthrough slope". For the above commercial adsorbent the breakthrough slope of n-C₁₀ was 60.0 ml, while that of n-C₁₄ was 12.8 ml . EXAMPLE II The test as set forth in Example I was repeated except that the adsorbent comprised silicalite particles. The breakthrough slope observed for n-C₁₀ was 19.3 ml and that for n-C₁₄ was 12.6 ml. The above data illustrates that the use of silicalite enables a much more rapid rate of sorption of n-C₁₀ as compared to the use of the commercial 5A adsorbent. Silicalite thus does not show a preference for one normal paraffin over another as exhibited by the 5A adsorbent, i.e. the n-paraffins are adsorbed and desorbed as a class. Furthermore, unlike the commercial adsorbent which requires a relatively high water content (2 wt.%) to function, the silicalite adsorption system needs no water. Water contamination of the product is then eliminated by use of silicalite.

Claims: 1. A process for separating a normal paraffin from a feed mixture of the same with a branched chain paraffinic hydrocarbon and a cyclic hydrocarbon having more than six carbon atoms per molecule, by contacting the mixture at adsorption conditions with a silicon-containing solid adsorbent to effect the selective adsorption of the normal paraffin by the adsorbent, and subsequently recovering the adsorbed normal paraffin by desorption with a desorbent material at desorption conditions, characterised in that the absorbent used is containing no tetrahedrally silicate coordinated aluminum, that the desorbent material comprises a normal paraffin having a boiling point at least 5°C different from the boiling point of the paraffin being separated, and that the feed mixture contains little olefins. 2. A process as claimed in claim 1, characterised in that the adsorption and desorption are carried out at a temperature of from 40 to 250°C and at a pressure sufficient to maintain liquid phase. 3. A process as claimed in claim 1 or 2, characterised in that the normal paraffin has from 2 to 30 carbon atoms per molecule. 4. A process as claimed in claim 3, characterised in that the feed mixture has a carbon number range of from 3 to 10. 5. A process as claimed in claim 1 or 2, characterised in that the feed mixture is a C₁₀-C₁₅ kerosine fraction or a C₁₀ -C₂₀ gas oil fraction, containing 15 to 60 vol % of normal paraffins and 10 to 30 vol % of aromatics. 6. A process as claimed in any of claims 1 to 5, characterised in that no water is added to the adsorbent.

Inventors: KULPRATHIPANJA, SANTI, NEUZIL, RICHARD WILLIAM
Applicants: UOP INC.
Application year: 1982
Main classification: C07C9
IPC codes: C10G25, C07C7