US2952630A

US2952630A - Separation of hydrocarbons using zeolitic molecular sieves

Info

Publication number: US2952630A
Application number: US622894A
Authority: US
Inventors: Frank T Eggertsen; John W Gibson
Original assignee: Shell Oil Co
Current assignee: Shell USA Inc
Priority date: 1956-11-19
Filing date: 1956-11-19
Publication date: 1960-09-13
Anticipated expiration: 1977-09-13
Also published as: GB826089A; NL108276C; FR1196019A; DE1067161B

Description

p 1960 F. T. EGGERTSEN ETAL 2,952,630

SEPARATION OF HYDROCARBONS USING ZEOLITIC MOLECULAR SIEVES Filed Nov. 19, 1956 owmu INVENTORI FRANK T. EGGERTSEN JOHN W. GIBSON THEIR ATTORNEY United States Patent 2,952,630 SEPARATION OF HYDRGCARBONS USING ZEO- LITIC MOLECULAR SIEVES Frank T. Eggertsen, Orinda, and John W. Gibson, Oakland, Califi, assignors to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed Nov. 19, 1956, Ser. No. 622,894 6 Claims. (Cl. 208310) This invention relates to a process for the separation of normal hydrocarbons from branched and/or cyclic hydrocarbons. it relates more particularly to an improved vapor-solid contacting process for the fractionation of hydrocarbon mixtures containing a plurality of both normal, i.e. linear, hydrocarbons and non-normal hydrocarbons.

Various methods have been proposed heretofore for the selective removal of more polar hydrocarbons from mixtures containing them and less polar hydrocarbons, by' the use of solid adsorbents which exhibit selective adsorptivity for the more polar substances. Thus, aromatics are selectively adsorbed by silica gel, activated carbon, alumina, and the like and are separable thereby from non-aromatics, both cyclic and acyclic. The socalled Arosorb process thus uses silca gel to recover aromatic hydrocarbons, such as benzene, toluene and the xylenes, from petroleum naphtha streams, such as gasoline boiling range fractions produced by catalytic reforming, as by platforming. Except in special cases, however, equally satisfactory processes for the separation of straight-chain hydrocarbons from non-straight-chain hydrocarbons have not been made available.

Normal parafiins are frequently undesired components in hydrocarbon fuels and lubricants: they depress the octane number of gasoline, raise the pour point of lubricating oil, and raise the freezing point of diesel and jet fuels.

It has been proposed to utilize certain natural and synthetic zeolites having rigid three-dimensional anionic networks and having intracrystalline interstitial channels sufliciently large to accommodate straight-chain hydrocarbons but sufficiently small to exclude branched-chain and/or cyclic hydrocarbons, to separate straight-chain hydrocarbons from other hydrocarbons (Barrer, US. 2,306,610; Black, U.S. 2,522,426). Although normal hydrocarbons are selectively sorbed by such solid substances, no commercial process has been developed to utilize this separation effect. Although the rate of sorption of the normal hydrocarbons is rapid, the regeneration of the sorbent for reuse generally is relatively slow and time-consuming so that the large-scale application of the separation effect is economically infeasible.

It is a principal object of the present invention to provide an improved process for the separation of normal hydrocarbons from hydrocarbon mixtures containing them. It is a more specific object to provide an improved process for the separation of normal hydrocarbons from a hydrocarbon naphtha fraction which contains a plurality of normal hydrocarbons in admixture with non-normal hydrocarbons. These objects will be better understood and others will appear to those skilled in the art from the more detailed description of the invention which will be made with reference in part to the accompanying drawing, wherein:

Figure 1 is a schematic flow sheet of one mode of practicing this invention; and I Figure 2 is a schematic flow sheet of another mode of practicing this invention.

Zeolites having rigid three-dimensional anionic net- 2,952,630 Patented Sept. 13, 1960 works and having intracrystalline interstitial channels whose narrowest cross section has essentially a uniform diameter, e.g. about 4 or 5 Angstrom units, are well known to the art. They are commonly designated molecular sieves. The intracrystalline channels are generally designated pores. Such zeolites are described, for example, in a paper, entitled Zeolites as Absorbents and Molecular Sieves, by R. M. Barter in Annual Reports on the Progress of Chemistry for 1944, vol. 61, pp. 31-46, London (1945). Another molecular sieve is described by Black in U.S. 2,442,191. More recently certain synthetic molecular sieves have become commercially available from Linde Air Products Company. One such molecular sieve is designated MS-4A. It is a zeolite of average composition 0.96:0.04 Na O, 1.00 A1 0 1.92:0.09 SiO plus an amount of water depending on the degree of dehydration; the crystals are cubic, with unit cells measuring, on an edge, approximately 12.26 Angstrom units, and are characterized by an essentially uniform pore diameter of about 4 Angstrom units. Another available sieve is designated MS-SA. This is made from MS-4A by replacement of approximately of the sodium ions with calcium ions by ion exchange. These are also cubic crystals, having the same unit cell dimensions as MS-4A, and are characterized by an essentially uniform pore diameter of about 5 Angstrom units. The crystallites of Lindes sieve materials generally have diameters of from 500 to 5000 Angstrom units.

Zeolites become active for selective sorption by a treatment designed to drive ofi the water originally present in the interstitial spaces. The spaces vacated remain and become available for the sorption of compounds of appropriate maximum critical molecular cross section. The zeolites may be subjected to temperatures of 600 C. and, in some cases up to 800 C. or more without destruction of their crystalline structure. In some cases, repeated contact with steam can be tolerated Without substantially affecting their structure.

Activated zeolites are generally soft, friable materials. Although they may be used as sorbents in pure form, if carefully handled, they can be made up in the form of particles held in shape bythe addition of ten to twenty percent of an inert binder material such as clay. They may also be used admixed with other solids which are stable at the conditions to be used in the process and which are, preferably, non-adsorbents, or relatively nonselective adsorbents so as not to interfere in the desired separations.

The basis for the separation of the normal-hydrocarbons from hydrocarbon mixtures containing them and non-straight-chain hydrocarbons which is effected by these natural or synthetic zeolites appears to be that the normal hydrocarbons are capable of passing into and through the interstitial openings (pores) of the sorbent, Whereas the non-straight-chain hydrocarbons have too large a maximum cross-sectional diameter to do so. This is supported by the results and the following approximate largest cross-sectional dimensions perpendicular to the longitudinal axes, sometimes designated critical cross sections, as determined by the use of Fischer-Hirschfelder scale models, reputed to give fair estimates of molecular size, of some representative hydrocarbons: nparafiins, 4.9 Angstroms; monomethylparaifins, 6.3 Angstroms; gem-dimethylpar-aflins, 6.7 Angstroms; ethylparafiins, 7.2 Angstroms; cyclohexane, 6.6 Angstroms; and benzene, 6.9 Angstroms. These differences in critical cross-section permit the separation of normal hydrocarbons from branched and cyclic hydrocarbons by sorption in molecular sieves whose pore diameters are sufficiently large to admit the normal hydrocarbons but not large enough to admit the other types. Thus, chabazite,

- 'It should be understood that pore diameters herein referred to, which are determined by physical measurements such as X-ray methods or by the sorptive characteristics of the zeolites,'rnay not be precisely accurate numbers. Also, it has been found that normal hydrocarbon molecules may enter pores whose smallest diameters are believed to be at least slightly smaller than the apparent maximum critical cross-section diameter of the molecule. For purposes of this invention, reference to zeolites having substantially uniform intracrystalline interstitial channels (or pores) of from about 5 to about 6 Angstrom units diameter includes those above-mentioned zeolites and otherswhich'h'ave the characteristic of selectively sorbing normal hydrocarbons of four or. more carbon atoms per molecule and notsorbing non-normals, by virtue of their crystal structure. These sorbents may also be; referred to as zeolitic molecular sieves of from about 5 to about 6 Angstrom unit maximum pore diameter;

The necessity for substantial uniformity of the maximum pore diameters is demonstrated by the fact that when a silica' gel having the same average pore diameter as the molecular sieve, but a distribution of sizes including substantial proportions both of smaller and larger pores, is used in the place of the molecular sicve,;a suitable separation is not obtained. a

The pore volume in molecular sieves is quite substantial. It may be as much as about 50% of the total volume of each crystal of sorbent. The weight of normal hydrocarbons which can be sorbed is a function of the available pore volume, the temperature, pressure and other j conditions including the concentration of the normal hydrocarbon in the mixture and the rate at which the mixture is passed in contact with the sorbent. It may be as high as 14% by weight of the sorbent. When operating in vapor phase at temperatures above 250 C. and at atmospheric pressure, the dynamic capacity of a molecular sieve such as Linde MS-SA for normal. hydrocarbons may be in the range from 3 to 6% by weight, or higher, based on the weight of sorbent. The capacity is greater for hydrocarbons of higher molecular weight; it increases at higher pressures but decreases. at higher temperatures and at increasing flow velocities through the bed of sorbent.

Heretofore, it has been considered very important to utilize molecular sieve capacity to its fullest extent when separating .no-rmal hydrocarbons from admixture with other hydrocarbon types. This however, required the use of long desorption periods or of severe desorption conditions such as increased temperature and rapid gas flows. Surprisingly, it has now been found that a greatly superior method for the separation of normal hydrocarbons from admixture with other types consists in utilizing only from one tenth to one half of the capacity of molecular sieve sorbent for normal hydrocarbons. V V

In accordance with this invention, a mixture of hydrocarbonsrfrom which normal hydrocarbons of at least four carbon atoms per molecule are to be separated is passed in contact with a mass of molecular sieve sorbent having a pore diameter of from about 5 to about 6 Angstroms in'relatively brief pulses, followed by relatively brief periods ofsweeping the mass of sorbent with an inert sweeping gas; the pulses of feed and the conditions of desorption-are so'correlatedthat'no more than one tenth to one half of sorbed normal hydrocarbons is removed in the sweeping periodand the amount of normal hydrocarbons in each pulse of feed is approximately equal to the actual capacity of the mass of sorbent for normal hydrocarbons at the beginning of the feed pulse, i.e. about one tenth to one half of the total capacity of the sorbent.

Expressed another way, this is a continuous process for the separation of normal hydrocarbons from admixture with non normals in which portions of'feed are intermittently passed in contact with a mass of suitable molecular sieve sorbent which, once the process is in continuous operation, contains from one half to nine tenths of its capacity of normal hydrocarbons at the beginning of each feed pulse and is substantially full of normals at the end of the feed pulse and the beginning of the sweeping gas pulse. 7

When a pulse of feed hydrocarbon mixture is charged to the sorbent bed, the non-normal hydrocarbons in the mixture are not delayed in their passage through the sorbent mass and consequently they emerge from the mass very rapidly, depending only on the rate at which the mixture is charged. The concentration of the nonnormal hydrocarbons in the efiluent vapor mixture is higher than in the feed vapor mixture passed to the sorbent since the normal hydrocarbons are being retained in the sorbent mass. The non-normals are, therefore, present in the effluent vapors'in a relatively high concentration and are readily condensed therefrom by conventional condensers operating at suitable temperatures e.g., between 15 and 40 C. When the predetermined amount of feed has been charged in any one pulse and further addition of feed is discontinued, the concentration of non-normals in the effiuent decreases very rapidly to essentially zero, as soon as all the non-normals have been swept through the contact mass. During the following period, when only sweep gas is charged to the contact mass, norm-a1 hydrocarbons are desorbed irom the contact mass and appear in the efliuent vapors in relatively low concentration compared to the concentration in which the non-normals appeared.

In one modification of this invention, the effluent vapors are passed through a condenser operating at a relatively higher temperature during the period when anon-normals are present in the efiluent, which essentially coincides with the feed pulse period, and the effluent vapors are then switched to a different condenser operating at a lower temperature e.g., between 0 and C. 'to recover the normal hydrocarbons present in the efliuent vapors during that period.

In another mode of practicing this invention, advantage is taken of the diiference in concentration of the nonnormals and the normals in the effluent vapors by passing the total product mixture continuously through a first condenser which is so adjusted in temperature. that substantially all of the hydrocarbons in the efiiuent are condensed when the non-normals are present but'the normals are not condensed when they are present alone in low concentration during the sweeping portion of the cycle, and the gas mixture leaving the first condenser is then passed through a second condenser operating at a substantially lower temperature to condense all me remaining hydrocarbons from the sweep gas. In this mode, the receiver for the first condenser collects essentially all the non-normal hydrocarbons and the receiver for the second condenser collects essentially only the normal hydrocarbons passing through when there are no non-normals present, as well as the small amount of uncondensed nonnormals-which pass through the first condenser in the feed pulse of the cycle.

This invention takes advantage of .the fact that the rate of sorption of normal hydrocarbons on passage of a hydrocarbon mixture containing them into contact with a molecular sieve is rapid and that the normal hydrocarbons are held bythe sieve'quite tenaciously so that o it is not possible to completely desorb normal hydrocarbons from the sieve when applying only heat and vacuum except by using very high temperatures or extremely long desorption periods, it does appear that the normal hydrocarbons are not entirely held in place on the sieve mass but that they gradually diffuse through the sieve in the direction of lower normal paraffin concentration or in the direction of flow of the vapor mixture. It is, therefore, possible to desorb the normal hydrocarbons from a molecular sieve mass by passing a so-called sweep gas through the contact mass. The normal hydrocarbons are then contained in the total vapor mixture leaving the bed. At a given temperature, the amount of time and the amount of sweeping gas required for recovery of a given amount of a sorbed normal hydrocarbon are interchangeable variables. Thus, a very small amount of sweeping gas passed slowly through a bed of normal-rich sorbent permits removal of the same amount of normal hydrocarbons as a large amount .of sweeping gas passed through a bed in a much shorter period of time, conditions otherwise being equal.

It is characteristic of the process of this invention that, since the normal hydrocarbons are not completely removed from the sorbent bed before additional feed is charged, the efliuent from the sorbent bed continuously contains a small concentration of normal hydrocarbons. It is, therefore, not in the nature of the process of this invention to make a complete separation of all normal hydrocarbons from a mixture containing them. It is, however, possible to reduce the concentration of normal hydrocarbons in the non-normal portion of the eflluent to a low value, e.g., below and preferably in the range of from 2 to 5%, by suitably adjusting the length of time of the feed pulse, the flow rate of the feed and sweeping gas during the sweeping period and the temperature and pressure maintained in the sorbent bed as Well as those of the product condensers mentioned above.

In the process of this invention the temperature of the molecular sieve sorbent mass is suitably maintained in the range from 250 to about 600 C. It is better to maintain a temperature of at least 350 and no more than 500 C.; temperatures between 350 and 450 C. are preferred.

Within the above range, the temperature is generally chosen sufiiciently high that the ratio of the absolute temperature in the contacting zone to the absolute boiling temperature of the highest boiling component in the feed mixture at the operating pressure is at least 1.1. It is preferably chosen to be below the temperature at which the least stable hydrocarbon in the feed undergoes appreciable thermal cracking at the process conditions. The thermal stability of the sorbent must not be exceeded.

It is an advantage of this invention that the separation process can be carried out isothermally, i.e., the temperature of the sorbent mass can be maintained essen tially unchanged throughout each cycle. There may be some difference in the temperature during the adsorption and desorption of normals, e.g., up to 100 C. For example, the feed may be relatively cool, thus cooling the sorbent during the charging step, or the heat of sorption of normals from a warm feed may heat the sorbent during the charging step. The temperature of the sorbent may be maintained by indirect heating or cooling means or by control of the feed and stripping gas temperatures.

The pressure is suitably in the range from atmospheric pressure to 1000 lbs/sq. in. gauge, and preferably between 100 and 750 p.s.i.g. The efiect of operating at the higher pressures in this range is to increase the capacity of the bed for sorbing normal hydrocarbons, but at the same time to increase the actual quantity of sweeping gas required (measured at standard conditions) in the desorption step. Pressures above 1000 psig. can, therefore, be employed if the amount of sweeping gas required does not become uneconomically high.

In operating according to this invention the amount of 6 normal hydrocarbons sorbed in the mass of molecular sieve sorbent does not fall below of the capacity of the sorbent, once continuous operation has been established. It is preferable to operate with a sieve mass containing at least about two-thirds of its capacity of normal hydrocarbons at all times and the lowest normal hydrocarbon content of the sieve mass in continuous operation may be as high as nine-tenths of its total dynamic capacity. Total dynamic capacity here refers to the amount of normal hydrocarbon which is retained by the sieve when a hydrocarbon mixture of the composition to be charged is passed into contact with activated molecular sieve sorbent, either fresh or regenerated so as to be completely free of normal hydrocarbons, at the conditions of temperature, pressure and gas flow to be employed in the separation process.

The amount of feed charged to the sorbent mass in each pulse during continuous operation is determined by the working capacity of the sorbent mass for normal hydrocarbons at the time the feed pulse is started. The length of the feed pulse may be varied by varying the rate at which the feed is added. Feed pulses as short as from one to a few seconds have been found satisfactory in small scale experiments, since selective sorption of normal hydrocarbons is practically instantaneous at the conditions of this process. It is convenient to control the feed pulse such that the feed is added at a LHSV of up to 20 v./v./hr. (measured during the feed pulse). Any lower rate is operative, e.g., down to l v./v./hr. or less, but rates from 5 to 20 v./v./ hr. are preferred. Much higher rates may be used; limits are set mainly by apparatus limitations. The length of the desorption period is determined according to the extent of desorption desired; it is affected by factors which have already been discussed, e.g., the temperature, sweep gas rate and type of hydrocarbon to be desorbed. It is convenient to fix the desorption period at a value such that the feed rate for the overall process is at a LHSV between 1 and 4 v./v./hr. or higher, preferably between 2 and 4 v./v./l1r. The time during which normal hydrocarbon is desorbed is generally at least as long as the time during which feed is charged and may be up to ten times as long or longer. The time required in the desorption step can be shortened by increasing the flow rate of sweep gas through the sorbent mass during the desorption step. 7

In some cases it may be desirable to make no attempt to remove the lowest molecular weight normal hydrocarbons from a feed stock. In that case the process is modified in such a manner that-the sorbent capacity is considered and used only for the higher normal hydrocarbons which are to be separated, while the lower normal hydrocarbons are permitted to run through the bed at all times and be recovered with the non-normal hydrocarbons.

Figure 1 illustrates a single mode of practicing the present invention. The molecular sieve sorbent, such as Chabazite or Linde molecular sieve type 5A which has been activated by driving off most of the water of hydration, is placed in a fixed bed A. Efiluent from the bed passes to product recovery zone B or C. Zones B and C may each contain a condenser and product receiver or other suitable recovery means, such as a hydrocarbon absorber or adsorber. In starting up the process, sweep gas is passed through line 11 controlled by valve 12. The gas passes into sorbent bed A which is maintained at a desired predetermined temperature by the gas or by other means, not shown. Gas leaves bed A through line 15 and passes through line 16 controlled by valve 17 into product recovery zone B and out through line 18. Valve 20 in line 19, leading to zone C, is kept closed at this time. Hydrocarbon feed mixture, preferably vaporized and preheated, is then added to line 11 through line 13 controlled by valve 14. Flow of feed mixture through bed A results in holdup of normal hydrocarbons from the feed mixture in the molecular concentration than were the non-normals.

sieve .bed A while the non-normal hydrocarbons pass through and are removed from the inert gas and retained in zone B... The sweep gas continues to leave zone B through line 18. Flow .offeed is continued until the molecular sieve mass is substantially filled with normal hydrocarbons. This may be determined by calculation from knowledge of the capacity of the molecular sieve at the conditions of the process and of the content of ,normalfhydrocarbons in. the feed or it may be: determined by a continuous or intermittent analysis of the effluent passing through line for normal hydrocarbon content. When the sieve mass is substantially filledv with normals, the flow of feed is discontinued by closing-valve 14 and-shortly thereafter, when the remaining non-normal hydrocarbons have been flushed out of the bed, valve 17 leading to zone B is closed and valve 20 in line 19, leading to zone C, is opened. Conditions in zone C are. adapted to recover the normal paraflins now being swept out of the mass of molecular sieve and present in the effluent therefrom in much lower At the same time, if desired, desorption of normal hydrocarbons may be accelerated by substantially increasing the rate of sweep gas flow by manipulatingvalve '12 in line ll, or by raisingthe temperature of bed A; the latter is generally an. uneconomic procedure. Normal hydrocarbons are collected in zone C during-this period while essentially hydrocarbon-free sweep gas passes out through line 21. After from one-tenth to one-half of the normal hydrocarbon capacity of the molecular bed has been de-' sorbed, a new pulse of feed is charged by opening valve 14 in line 13; if the sweep gas rate was increased during the desorption step it is again lowered to the desired value for the charging pulse by closing down on Valve 12. Valve 17 is opened, valve 20 is closed, and nonnormal hydrocarbons are again recovered in zone B. The feed pulse is continued until'a sufiicient amount of normal hydrocarbons has been charged to substantially fill the capacity of the molecular sieve mass for normals, as determined by calculation from knowledge of the capacity of'the sieve, the feed rate and the concentration of normals in the feed, or by analysis of the efiiuent in line 15.

As has been previously pointed out, some normal hydrocarbons appear continuously in line- 15 during continuous operation of the process, but the concentration is relatively low as long as the bed has capacity for sorbing additional normal hydrocarbons. If the flow of feed is continued after the bed capacity is filled up at the conditions of operation, the normals will pass through essentially at the rate at which they are fed. When the saturation point is passed, the concentration ofnormals in the vapors in line 15' will show a sudden increase from a relatively low continuous rate and will quickly approach the concentration in which the normals are present in the feed.

'In the continuous operation, the feed pulse with recoveryof the efiluent in receiver C and the sweeping step with recovery of the effluent in receiver D are cyclically repeated. The'cyclic operation may continue for many days without requiring further treatment of the sorbent bed, provided the conditions of temperature, pressure, rate and so forth are controlled within the ranges given in this specification and the feed contains no impurities which tend 'to poison the sorbent bed. The product receivers in zones B and C are periodically emptied or may be continuously emptied through valved lines 22 and 23respectively.

Numerous modifications of the system shown in Figure 1 will occur to those skilled in this art. For example, instead of providing separate product recovery means in zones B and C a'single means may be placed in line 15; zones B and C, in that case, merely provide product accumulators. The recovery means in line 15 may consist ofja single condenser operating at a temperature suf- 8 ficiently low to permit adequate condensation. of normal hydrocarbons during the desorption step, or of a single condenser operating at diiferent temperatures for the recovery of non-normals and of normals,.or it may comprise two condensers operating at difierent temperatures, connected in such manner that the non-normals are recovered at. a relatively higher temperature than the normals.

A preferred modification is illustrated schematically in Figure 2. The apparatus shown in Figure 2 comprises a feed tank E; a sorbentbed F located in a heatable vesselG; a first product condenser H; a product analyzer I adapted to determine the concentration of normal hydrocarbons in a vapor mixture; A means I for recovering. the remaining hydrocarbons from a vapor stream, which may be a condenser or an absorber; productaccumulators K and L, and product receivers M and N. Since the process diagram is schematic in nature, much necessary associated equipment which can be readily supplied by those skilled in. the art is not shown to avoid unnecessary complexity.

In starting up the process, a flow of inertgas is commenced through line 211'- controlled by'valve 212. This gas passes through bed P, which is brought to a desired temperature by passing heating medium through the jacket, the heating medium entering through line 214 and passing out through line 215. Other means of heating the bed F may be employed, e.g., liquid coils imbedded in the sorbent bed or electrical or other means. The gas may be used as a direct heating medium. The gas passes out of the bed through line 215, through condenser H, line 218, accumulator K, line 219, condenser or'absorber J, line 220, accumulator L and finally through line 221. It may be taken out of thesystem through line 222, controlled by valve 224, or it may be returned to line 211, for further use, by means of line 225 containing compressor 0. Once the flow of gas is established at a desired rate, feed hydrocarbon mixture is passed from tank E through line 226 containing vaporizer 228 and into line 211. As the resulting mixture passes through the bed, normal hydrocarbons are retained in bed F and nonnormal hydrocarbons are swept out through line 216, condensed in condenser H and recovered in accumulator K. The condenser temperature is maintained by controlling the rate of flow of cooling medium which enters the condenser through line 229 and leaves it through line 231. The flow controlling means is valve 230, which may be manually adjusted or may be regulated from analyzer I once continuous operation is established. Analyzer I may be, for example, a differential refractrometer which continually determines the difierence in refractive index between amixture having the composition corresponding to original feed minus the amount of normal hydrocarbons to be removed therefrom and the actual liquid condensate which has been collected in at least one complete cycle of operation in vessel K; the condensate is sampled through'line 232. The differential reading is used to control valve 230, e.g., by controlling the air-pressure supplied to the valve to open it further and thus lower the temperature of the condenser. for additional hydrocarbon recovery or to open it and raise the condenser temperature, thus keeping the concentration of normals in the recovered liquid in a predetermined range.

The flow of feed is continued until the capacity of sorbent bed F for normal hydrocarbons is substantially used up. This may be calculated from a knowledge of bed capacity at the conditions employed, the concentration of normal hydrocarbons inthe feed and feed rate. It may also be determined by analyzing the vapor mixture in line 218, e.g., by passing a continuous sample of the mixture to an analyzer similar to analyzer I.

When the sorbent bed F has been filled essentially to capacity with normal hydrocarbon, flow of feed mixture is discontinued by closing 'valve 227. in. line 226 and the 9 flow of gas is continued. The result is that the condensation of non-normal paraflins is quickly completed; the efliuent vapors from bed F thereafter pass through condenser H and accumulator K without change and the normal hydrocarbon content in the vapors is substantially completely recovered by further condensation at a lower temperature or by absorption in a suitable absorbent liquid in recovery vessel J. The resulting normal hydrocarbon liquid is retained in accumulator L. The sweep gas substantially free of hydrocarbons passes through line 221. Preferably, the sweep gas is returned for further use through line 225. After a predetermined relatively short period of desorption in which the amount of normal hydrocarbon removed from bed F is equal to one-tenth to one-half of the bed capacity, the flow of feed to bed F is re-established by opening valve 227. This automatically results in non-normal hydrocarbons appearing in the effluent line 216 in substantial concentration and again being condensed in condenser H and recovered in accumulator K.

It is advantageous to utilize at least two separation columns in parallel, manifolded so that difierent steps of the process can be carried out in different units simultaneously and processing streams of hydrocarbons and of sweep gas move substantially continuously through the lines to and from the manifolds.

The sieve material may be provided in a suitably sized vessel, generally an upright cylindrical vessel with The material a length from 2 to 10 times the diameter. is suitably supported by a screen grid in the bottom and, if desired, a plurality of suitable screen grids are prov'ided at spaced intervals throughout the column to sup- 7 port the sieve material.

By supporting the sieve material on a plurality of 7 screen grids, a small zone of free space can be left between the top of sieve material disposed on one grid and the underside of the next grid thereabove. In that case, advantage can be taken of reduced pressure drop resulting from passing the larger volume of eluting gas a through the sieve mass under conditions less likely to cause packing of the zeolite mass and tending to expand the volume of the mass.

The bed of zeolite material may be disposed horizontally instead of upright and it may be provided as a fixed, stationary mass, or it may be adapted to be moved as a mass, as in an elevating system or as annular segmental packing of a rotatable vertical or horizontal contactor providing solid particulate contacting material in the annular space between concentric rotatable cylindrical screens or perforate partitions, suitably provided with means for delivering and removing fluid streams to and from outer and inner surfaces of the mass and periodically to change the nature of the fluid delivered to and removed from any particular segment and to reverse the flow of fluid therethrough.

In the normal hydrocarbon removal step of this invention helium, hydrogen, nitrogen and methane can be sucessfully used as sweep gases. Other inert gases, i.e., gases which do not react with either the sorbent, the vessel or the. reactants, are also suitable. For example,

argon, flue gas (preferably scrubbed to remove reactive impurities), propane and other gases and vapors can be used as sweep gas.

It is not essential to use a sweep gas while the hydrocarbon feed is being charged. The feed may be added to the heated sorbent as a liquid, to be quickly vaporized by contact withsorbent, or it may be added as a vapor. 7

If desired, sweep gas flow may be maintained through the sorbent mass at all times and the feed added as liquid or as vapor to the sweep gas. It will generally be preferred to maintain a relatively low rate of inert gas flow or no gas flow at all during the time when feed is added,

and a substantial gas flow during the time when sorbed normal hydrocarbons are desorbed. The first sweep gas added after feed is discontinued serves to flush the 7 10 remaining non-sorbed hydrocarbons out of the sorbent mass. The rate of gas flow during this flushing step may diifer from that used during the sweeping step.

The process of the present invention is useful in the separation of normal hydrocarbons from mixtures containing normals having a carbon number of at least 4 and generally at least 5. It is particularly suitable for mixtures containing normals from n-pentane or n-hexane through n-decane or n-dodecane and corresponding olefins, but may be used with mixtures containing normals up to n-eicosane.

The invention will be further illustrated by means of the following examples, in which Example I illustrates runs carried out in accordance with the mode described by means of Figure 2, and Example II a run carried out in accordance with the mode described by means of Figure 1.

The feed used throughout these runs was a dehexanized C -C Platformate fraction. This fraction had a refractive index (R. I.) of 1.4538, and contained about 41% wt. saturated compounds, 59% wt. aromatics and 0% wt. olefins. The content of normal parafiins was about 12% wt.

EXAMPLE I Sorbent bed'F consisted of ca. parts by weight of Linde molecular sieve sorbent type 5A, in the form of irregularly shaped particles of average 1-2 mm. diameter, containing about 80% wt. of the actual zeolite and 20% of a clay binder. The bed was placed in an externally heated vertical cylindrical vessel of about 20:1 height to diameter ratio.

Throughout the run, the bed was maintained at a temperature of about 450 C. A flow of dry nitrogen through the bed was maintained throughout the run at a rate of about volumes (at STP) per bulk volume of sorbent per hour. The nitrogen was passed in at the top of the sorbent bed.

Condenser H was maintained at about 18 C. by

' means of cooling water, and condenser J at about -l80 C. by means of a refrigerant.

The feed was added to the flowing nitrogen at the rate of 0.72 liquid volume per bulk volume of sorbent per hour, in portions of 20 parts by weight, each, during a period of 6.7 minutes. After each feed pulse, flow of nitrogen without feed addition was continued for 20 minutes. The non-normal product was separately recovered from receiver K after each feed pulse; it was measured and analyzed for normal paraflins by means of refractive index. The hydrocarbons not recovered in receiver K were recovered in receiver L. This product was measured only at the end of the run, i.e. after several cycles had been completed, and its volume and refractive index then determined. a

The results of the first run, in which the feed was charged in pulses of 20 parts by weight, are set out in Table I.

Table 1 Product 1st Condenser (H) Feed- Analysis Cycle Weight Nor- (parts) Weight Percent mal Par- (parts) of Feed afiins,

(wt.) percent Product 2d Condenser (J) It became'quicklyapparent; merelyfrom an inspection of the refractive index of the product from receiver K, that normalparaflins were being removed only to an undesirably small extent. This was an indication that the feed pulses were too large, exceeding the capacity of the bed for normal parafiins.

-Insanother run in which the same conditions were maintained except that'the amount of feed charged in each pulse was only about 10.0 parts by weight, it was found that the capacity of the bed was not exceeded. The results of this run are set out in TableII. This run was continued f or15 cycles, and was then voluntarily termi- 12 volumes per bulk volume of sorbentper hour, in portions ofca. parts by weight, each. Feed addition in each cycle lasted ca. seconds, flushing of non-normals ca. 15 seconds and desorption of normals ca. seconds. Be cause of the rapid feed rate, the sorbent bed gradually cooled from an inlet temperature of ca. 525 C. to one of ca. 300 C, at the end of 10 cycles. Flow was discontinued for 5 minutes after each set of ten cycles'to permit the bed to be heated to its original temperature. The

10 average bed temperaturewas about 450 C. Recovery means B represents a condenser and reactor operating'at 18 C.,'followed by a set of cold traps at --180 C. Almost 98% of the normal-lean product was recovered nated.

Table 11 Product 1st Condenser (H) Feed-- Analysis Cycle Weight (parts) Weight Percent (parts) of Feed Normal Satu- Aroma- Olefins, (wt.) Paraflins, rates,'pertics, perpercent percent cent wt. cent wt wt.

10.0 8. 3 83 3. 7 10.0 8. 7 r 87 3. 5 10.0 8. 7 87 3. 5 10.1 9.3 V 92 4.8 9. 9 8.8 89 5.0 10.0 8. 4 84 4.9 9.9 9.0 91 5.2 14. 7 13. 3 91 4. 9 9. 9 8.7 88 5. 7 10.0 8. 5 4.4 9. 9 8. 1 82 4. 6 l0. 5 8. 8 84 4. 8 10.0 8.6 86 3.7 10.0 s. e as 4.4 10. 4 8. 1 78 4. 1

Product 2d Condenser (3') EXAMPLE ,II

Sorbent bed A was of the same size and shape as bed F of Example I. The run consisted of 5 0 cycles. In each cycle, dry nitrogen flow was maintained, but the rate was varied from 270 volumes (at STP) per bulk volume in the condenser-receiver.

bed of silica gel.

50 The results of this run are set out in Table III.

Recovery means C represents a set of cold traps operating at -180 C. followed by a Non-normal product was-removed after each set of ten cycles; it was measured and analyzed for normal parafiins content by means of refractive index. The product in cold traps C was measured and analyzed at the end of the Table III Product, Condenser B Feed Cycle Charged, 7 Normal (pts. Percent Paraflius Satu- Aroma- Olefins, wt. (pts) wt of Content, rates, pertics, perpercent 7 feed percent cent v. cent v. v.

110..... 78. 6 78. 6 11-20-..- a 100 81. 6 81. 6 3. 7 21-30--.. B 100 82. 0 82.0 3. 7 31-40 a 100 81. 7 81. 7 3. 7 4150 H 100 82.3 82. 3 3. 7

Con-

denser Tot 406.2 From Cold Trap-.- 9. 3

Total. V 493. 6 415. 5 84. 6 3. 7

. Product Cold Traps C l-50. 493.6 69.8 14.2 s 50 72 r '28 0 We claim as our invention:

1. A process for the separation of normal hydrocarbons from a mixture of a plurality of normal hydrocarbons of at least four carbon atoms per molecule with non-normal hydrocarbons of similar boiling range which comprises: (1) passing a vapor stream of the hydrocarbon mixture into a particulate fixed mass of a solid zeolitic material having a rigid three-dimensional anionic network and having substantially uniform intracrystalline interstitial channels of from about to about 6 Angstrom units diameter while maintaining a contacting temperature of from 250 C. to 600 C. to selectively retard the passage of at least the higher molecular weight normal hydrocarbons through the mass while the remaining hydrocarbons pass through the mass and are recovered as a non-normalhydrocarbon-enriched product; (2) discontinuing passage of the mixture to the mass before normal hydrocarbons of at least the highest carbon number appear in substantial proportion in the effluent from the mass; (3) passing a gaseous eluting material through the zeolite mass to elute from about one-tenth to about one-half of the retained hydrocarbons from the whole of the zeolitic mass as a normal hydrocarbon enriched stream; and (4) repeating the cycle of steps 1, 2 and 3.

2. A process in accordance with claim 1, wherein the hydrocarbon mixture is a gasoline boiling range reformate hydrocarbon mixture.

3. A process in accordance with claim 1, wherein the process is operated at a pressure of from about one to about 1000 p.s.i.g.

4. A continuous process for the separation of normal hydrocarbons from a feed mixture of a plurality of normal hydrocarbons of at least four carbon atoms per molecule with non-normal hydrocarbons of similar boiling range which comprises: (1) establishing a continuous flow of an inert gas through (a) a particulate mass of a solid zeolitic material having a rigid three-dimensional anionic network and having substantially uniform intracrystalline interstitial channels of from about 5 to about 6 Angstrom units diameter, while maintaining a contact temperature of from 250 to 600 C., (b) a first hydrocarbon recovery means comprising a condenser, and (c) a second hydrocarbon recovery means; (2) periodically adding to said gas, upstream from said mass, a portion of said feed mixture to produce a vapor mixture, which passes into said mass; (3) continuing said addition for a period no longer than that required for those normal paraflins desired to be separated to appear in the vapor mixture at the outlet and of said mass in substantial proportion; (4) recovering hydrocarbons, substantially reduced in content of those normal hydrocarbons desired to be separated, in said condenser by maintaining it at a temperature sufiicient to condense hydrocarbons from the vapor stream during the period When feed is charged to the mass, and (5) recovering mainly normal hydrocarbons desired to be separated in said second recovery means, during the period when feed is not being charged to the mass.

5. A continuous process for the separation of normal hydrocarbons from a feed mixture of a plurality of normal hydrocarbons of at least four carbon atoms per molecule with non-normal hydrocarbons of similar boiling range which comprises: (1) establishing a continuous flow of an inert gas through (a) a particulate mass of a solid zeolitic material having a rigid three-dimensional anionic network and having substantially uniform intracrystalline interstitial channels of from about 5 to about 6 Angstrom units diameter, while maintaining a contact temperature of from 250 to 600 C., (b) a first condenser maintained at a first temperature, and (c) a second condenser maintained at a second temperature; (2) periodically adding to said gas, lip-stream from said mass, a portion of said feed mixture to produce a vapor mixture; (3) continuing said addition for a period no longer than that required for those normal paraffins desired to be separately recovered to appear in the vapor mixture at the outlet end of said mass in substantial proportion; (4) recovering hydrocarbons, substantially reduced in content of normal hydrocarbons desired to be separated, in said first condenser by maintaining it at a temperature sufficient to condense hydrocarbons from the vapor stream during the period when feed is charged to the mass, and (5) recovering mainly normal hydrocarbons desired to be separated in said second condenser by maintaining it at a lower temperature suflicient to condense hydrocarbons from the vapor stream during the period when feed is not being charged to the mass.

6. A cyclic process for the separation of a normal hydrocarbon from non-normal hydrocarbons, said normal hydrocarbons having from about five to about .ten carbon atoms per molecule, which comprises (1) passing a vapor stream of a mixture of said hydrocarbons into a particulate fixed mass of a solid zeolitic material having a rigid three-dimensional anionic network and having substantially uniform intracrystalline interstitial channels of from about 5 to about 6 Angstrom units diameter while maintaining a contacting temperature of from 250 C. to 600 C. to selectively retard the passage of the normal hydrocarbon through the mass while the non-normal hydrocarbons pass through the mass and are recovered as a non-normal-hydrocarbon-enriched product; (2) discontinuing passage of the mixture to the mass before the normal hydrocarbon appears in substantial proportion in the effluent from the mass; (3) passing a gaseous elutiug material through the zeolite mass to elute from about one-tenth to about one-half of the retained hydrocarbon from the whole of the zeolitic mass as a normal hydrocarbon enriched stream; and (4) repeating the cycle of steps 1, 2 and 3.

References Cited in the file of this patent UNITED STATES PATENTS 2,306,610 Barrer Dec. 29, 1942 2,522,426 Black Sept. 12, 1950 2,586,889 Vesterdal et al Feb. 26, 1952 2,628,933 Eagle et al. Feb. 17, 1953 2,644,018 Harper June 30, 1953 2,651,603 Martin et al. Sept. 8, 1953 2,818,137 Richmond et al. Dec. 31, 1957 2,818,455 Ballard et al. Dec. '31, 1957 2,859,173 Hess et al. Nov. 4, 1958 2,859,256 Hess et al. Nov. 4, 1958 OTHER REFERENCES Article by Barrer, Quarterly Reviews of the Chemical Society (London), vol. 111, 1949, page 300.

Chemical Engineering News, Nov. 29, 1954 (vol. 32), p. 4786, (article, Selective Adsorption With Zeolites).

Claims

1. A PROCESS FOR THE SEPARATION OF NORMAL HYDROCARBONS FROM A MIXTURE OF A PLURALITY OF NORMAL HYDROCARBONS OF AT LEAST FOUR CARBON ATOMS PER MOLECULE WITH NON-NORMAL HYDROCARBONS OF SIMILAR BOILING RANGE WHICH COMPRISES: (1) PASSING A VAPOR STREAM OF THE HYDROCARBON MIXTURE INTO A PARTICULTE FIXED MASS OF A SOLID ZEOLITIC MATERIAL HAVING A RIGID THREE-DIMENSIONAL ANIONIC NETWORK AND HAVING SUBSTANTIALLY UNIFORM INTRACRYSTALLINE INTERSTITIAL CHANNELS OF FROM ABOUT 5 TO ABOUT 6 ANGSTROM UNITS DIAMETER WHILE MAINTAINING A CONTACTING TEMPERATURE OF FROM 250*C. TO 600*C. TO SELECTIVELY RETARD THE PASSAGE OF AT LEAST THE HIGHER MOLECULAR WEIGHT NORMAL HYDROCARBONS THROUGH THE MASS WHILE THE REMAINING HYDROCARBONS PASS THROUGH THE MASS AND ARE RECOVERED AS A NON-NORMALHYCARBON-ENRICHED PRODUCT, (2) DISCONTINUING PASSAGE OF THE MIXTURE TO THE MASS BEFORE NORMAL HYDROCARBONS OF AT LEAST THE HIGHEST CARBON NUMBER APPEAR IN SUBSTANTIAL PROPORTION IN THE EFFUENT FROM THE MASS, (3) PASSING A GASEOUS ELUTING MATERIAL THROUGH THE ZEOLITE MASS TO ELUTE FROM ABOUT ONE-TENTH TO ABOUT ONE-HALF OF THE RETAINED HYDROCARBONS FROM THE WHOLE OF THE ZEOLITIC MASS AS A NORMAL HYDROCARBON ENRICHED STREAM, AND (4) REPEATING THE CYCLE OF STEPS 1, 2 AND 3.

6. A CYCLE PROCESS FOR THE SEPARATION OF A NORMAL HYFROCARBON FROM NON-NORMAL HYDROCARBONS, SAID NORMAL HYDROCARBONS HAVING FROM ABOUT FIVE TO ABOUT TEN CARBON ATOMS PER MOLECULE, WHICH COMPRISES (1) PASSING A VAPOR STREAM OF A MIXTURE OF SAID HYDROCARBONS INTO A PARTICULATE FIXED MASS OF A SOLID ZEOLITIC MATERIAL HAVING A RIGID THREE-DIMENSIONAL ANIONIC NETWORK AND HAVING SUBSTANTIALLY UNIFORM INTRACRYSTALLINE INTERSTITIAL CHANNELS OF FROM ABOUT 5 TO ABOUT 6 ANGSTROM UNITS DIAMETER WHILE MAINTAINING A CONTACTING TEMPERATURE OF FROM 250*C. TO 600*C. TO SELECTIVELY RETARD THE PASSAGE OF THE NORMAL HYDROCARBON THROUGH THE MASS WHILE THE NON-NORMAL HYDROCARBONS PASS THROUGH THE MASS AND ARE RECOVERED AS A NON-NORMAL-HYDROCARBON-ENRICHED PRODUCT, (2) DISCONTINUING PASSAGE OF THE MIXTURE TO THE MASS BEFORE THE NORMAL HYDROCARBON APPEARS IN SUBSTANTIAL PROPORTION IN THE EFFLUENT FROM THE MASS, (3) PASSING A GASEOUS ELUTING MATERIAL THROUGH THE ZEOLITE MASS TO ELUTE FROM ABOUT ONE-TENTH TO ABOUT ONE-HALF OF THE RETAINED HYDROCARBON FROM THE WHOLE OF THE ZEOLITIC MASS AS A NORMAL HYDROCARBON ENRICHED STREAM, AND (4) RETAINED HYDROCARBON FROM THE WHOLE OF THE ZEOLITIC MASS AS A NORMAL HYDROCARBON ENRICHED STREAM, AND (4) REPEATING THE CYCLE OF STEPS 1, 2 AND 3.