US3287432A - Selective sorption process - Google Patents

Description

NOV- 22, 1966 E. E. sENsEl. 3,287,432

SELECTIVE soRPTIoN PRocEss Original Filed April l1, 1957 peren/age United States Patent O 6 Claims. (cl. 26o-676) This application is a division of my copending patent application Serial No. 652,147 filed April 11, 1957, now abandoned. v

This invention relates to an improved process for separating straight chain hydrocarbon from non-straight chain hydrocarbon in a mixture thereof, and to a manganese-containing zeolite effective in said process.

By straight chain hydrocarbon is meant any aliphatic or acyclic or open chain hydrocarbon which does not possess side chain branching. Representative straight chain hydrocarbons are lthe normal parans and the normal olens, mono or polyolefins, or straight chain acetylenic hydrocarbons. The non-straight chain hydrocarbons comprise the aromatic and naphenic hydrocarbons as well as the isoparans, isoolenic hydrocarbons, and the like. Straight chain hydrocarbon-containing mixtures which are suitably treated in accordance with this invention include mixed butanes, mixtures of normal alkanes and their isomers, and Various petroleum fractions such as naphtha fraction, a gasoline fraction, a diesel oil fraction, a kerosene fraction, a gas oil fraction and the like. Particularly suitable for treatment in accordance with this invention are straight chain hydrocarbon-containing fractions having a boiling point or a boiling range in the range of 40-550" F. and containing a substantial amount of straight chain hydrocarbons, e.g., 2-35% by volume. More particularly, a petroleum fraction suitable for use in practice of this invention could have an initial boiling point in the range of 40-300 F. and an end point in the range of 15G-550 F. A petroleum fraction for use in the practice of this invention must contain both straight chain and non-straight chain hydrocarbons as demonstrated by the following composition:

Hydrocarbon type: Percent by volume Typical refinery stocks or fractions which are applicable to the practice of this invention are a wide boiling straight run naphtha, a light straight run naphtha, a heavy straight run naphtha, a catalytically cracked naphtha, a thermally cracked or thermally reformed naphtha, a catalytically reformed naphtha and the like.

Heretofore, a synthetic sodium calcium alumino-silicate, a dehydrated crystalline zeolite, having a ratio of calcium to sodium (measured as a molecular ratio of calcium oxide to sodium oxide) between about 2:1 and about 4:1 and designated in the trade as Linde 5A molecular sieve, has been proposed for separating certain straight chain hydrocarbons from non-straight chain hydrocarbons in a gasiform mixture thereof. Broadly, the empirical formula for such sodium calcium aluminosilicate, in dehydrated state, can be Written (ca, Napomzoszsio,

This sorbent can be made by exchanging calcium for some of the sodium in the sodium form of the type A Fice zeolite, then removing crystal water. Properties and structure of the type A zeolite are described in the articles of Breek et al. and Reed et al. which appear on pages 5963-5977 of the Journal of the American Chemical Society, No. 23, volume 78. The formula (less crystal water) represented for the sodium form of the type A zeolite in the above-mentioned articles is N312 (A102) 12- (SOa) 12 which is a multiple of six of the empirical mineralogical oxide formula Na2O.Al2O3.2SiO2. For purposes of simplicity I prefer to use the oxide sort of formula for describing the type A zeolite structure, but it will be understood that both vkinds of formulae are interchangeable for purposes of reference herein to zeolites of type A structure, and, where an oxide formula concluding with Al2O3.2SiO2 is used herein, the material being referred to is a type A zeolite.

Capacity and selectivity of said 5A molecular sieve for straight chain hydrocarbons are good, c g., approximately 40-45 cc. of normal butane per gram of this mineral sorbent at temperature of F. and pressure of 760 mm. Hg as against approximately l to 3 cc. of isobutane per gram of the sorbent under the same conditions. At room temperature and approximately atmospheric pressure this mineral sorbent becomes, for all practical purposes, saturated (i.e., it has no more capacity for a gaseous normal paraffin such as normal butane or those of higher molecular weight) after a contact time of about l5 minutes with the straight chain hydrocarbon vapor. Furthermore, it takes almost 5 minutes to reach 90% of saturation of this sorbent with normal butane at room temperature and atmospheric pressure.

It has been proposed, for example, to separate normal butane from isobutane as one typical operation with said 5A molecular sieve, and to separate higher normal parafiins land olefins from non-straight chain hydrocarbons in other operations by the process which comprises contacting the mixture of non-straight and straight chain hydrocarbons in vapor phase with said 5A molecular sieve, thereby selectively sorbing some of the straight chain hydrocarbon; withdrawing the resulting hydrocarbon mixture depleted of straight chain hydrocarbons; and desorbing sorbed straight chain hydrocarbon from the laden mineral sorbent to dit it for reuse. Alternating from sorption to desorption and vice-versa can be done very simply and rapidly by an essentially isothermal pressure swing technique. This technique is more fully described hereinafter.

Efficiency of a plant for separation of straight from non-straight hydrocarbon using the contacting process outlined above is a function of the mineral sorbents selectivity for, and sorbing and desorbing rates for the straight chain hydrocarbons in process. This becomes particularly evident in the instance of a fixed bed contacting plant wherein a substantial shortening of the sorbing phase of the operating cycle coupled with only a small reduction in capacity of the mineral sorbent for straight chain hydrocarbons will permit a greater number of operating cycles a day and, consequently, will increase the production significantly.

I have now found, in a process for separating straight chain from non-straight chain hydrocarbons in a mixture thereof, that use of a synthetic crystalline zeolite of type A structure having 0.25 to 0.95 of its exchangeable cation content as divalent manganese under certain controlled operating conditions hereinafter described can shorten significantly the sorbing time without a proportional sacrifice in sorbing capacity or any significant loss in selectivity. Viewed from one aspect, my invention permits the processing of a larger quantity of particular hydrocarbon 3 mixture to desired specifications with a given Weight of sorbent than has been possible heretofore. Thus, by contacting the hydrocarbon mixture with the aforementioned manganese-containing type A zeolite under vapor phase sorbing conditions, I iind that I can obtain 80-95% of saturation capacity of said zeolite for a straight chain hydrocarbon or hydrocarbons present in a very short time, generally in substantially less than 4 minutes, e.g., 1-3 minutes, and usually in a time as short as l-2 minutes, or even less.

As this sorption of the straight chain hydrocarbon is occurring, there is withdrawn from sorbing contact a hydrocarbon mixture containing a reduced amount of straight chain hydrocarbon. When 80-90% saturation of my zeolite with sorbed hydrocarbons has been eiected in Vthe aforesaid short time, the feed mixture of hydrocarbons is shut off, and the laden zcolite subjected to desorbing conditions whereby previously sorbed straight chain hydrocarbon is driven off and the zeolite made readily for another cycle.

Not only can sorbing be made materially faster using the above processing technique and said manganese-con taining type A zeolite (as compared to using the conventional sodium calcium alumino-silicate 5A molecular sieve), but also desorbing appears to proceed at a correspondingly faster rate under comparable desorbing conditions. For greatest processing throughput with a given amount of mineral sorbent, desorbing is advantageously discontinued when the zeolite contains straight chain hydrocarbon material from previous sorbing operations amounting to roughly l-20% of saturation capacity. Thus, in preferred operation both sorbing and desorbing is only 80-90% complete, but done very rapidly with short contact time.

While the hydrocarbon contacting operations with my sorbents are conducted preferably as a cyclic process with a fixed bed of sorbent particles, it is possible also to use moving or uidized bed contact. This is particularly true when the particles of sorbent are stabilized by methods described in the following U.S. patent applications, all of which are assigned to The Texas Company: Riordan et al., Serial N o. 544,244, tiled on November 1, 1955; Hess et al., Serial No. 544,185, tiled on November 1, 1955; and Ray, Serial No. 599,231, filed on July 20, 1956, now U.S. Patent No. 2,947,709.

The preferred manganese-containing sorbents of my invention are most conveniently prepared by exchanging manganese for sodium in the hydrated sodium form of the type A zeolite, Na2O.Al2O3.2SiO2.4.5HZO, for example, by agitating such hydrated parent zeolite for 1/2 to 12 hours in a 0.1 to N aqueous manganese salt solution, discarding the salt solution, and repeating the treatment with fresh solution until the necessary proportion of the sodium originally present in the structure has been replaced by manganese. Operating at room temperature and pressure tive changes of aqueous 1 N manganese chloride are usually adequate to obtain sufficient manganese substitution for purposes of practicing my invention. After calcining or otherwise ridding the resultant sorbent of Water, it is receptive to straight chain hydrocarbons.

Alternatively, a hydrated sodium-calcium form of the type A zeolite, e.g., the Linde 5A molecular sieve, or a hydrated sodium-lithium or hydrated sodium-potassium form of the type A zeolite, can be treated with a manganese salt solution in a similar manner to produce a similarly useful type A zeolite having 0.25-0.95 of its exchangeable cation content of divalent manganese. The fraction of exchangeable cation content referred to herein is computed as the ratio of the number of equivalents of divalent manganese to the sum of the equivalent of all the exchangeable metals, eg., Mn++, Na+, Li+, K+, Ca++ etc., in the resulting type A structure.

Among the manganese salts useful in the ion exchanging are manganese nitrate, chloride, bromide, iodide, fluosilicate, formate, and sulfate. The parent sodium form of the type A zeolite can be made by the processes shown in the following U.S. patent applications, both of which are assigned to The Texas Company: Sensel, Serial No. 617,734, now U.S. Patent N o. 2,841,471 and Estes, Serial No. 6l7,735, now U.S. Patent No. 2,847,280, both filed on October 23, 1956.

FIGURE 11 of the drawing shows curves plotted from experimental results finding the percentage of saturation (ultimate capacity) obtained with n-butane at room temrperature (75 F.) and atmospheric pressure for various contact times lof .the nJbutane with two selective mineral sonbents, one (Ca2, Na)O.Al2O3.2SiO2, i.e., the Linde 5A molecular sieve, wherein the ratio of Ca to Nag Was about 3:1, 'and the other a typical sodium-manganese type A zeolite of this invention wherein 39% of the exchangeable cation content in the structure -Was divalent manganese. Inspection of the ligure shows that the conventional zeolite attained only about 73% of saturation with the normal hydrocarbon in 2 minutes, whereas the manganese-containing zeolite of my invention attained about 88% |of sa-turation in 2 minutes. Ultimate capacity of the two scribe-nts for n-butane under the test conditions Was practically the same. N-butane siorbing rate characteristics for manganese-containing type A zeolites having lbroadly 0.25 to 0.95 of their exchangeable cation content as divalent manganese and corresponding to the 'above test zeolite are about the same. However, for economy and efficiency of preparation, those having 0.35-07 of the exchangeable cation content as divalent manganese are preferred.

FIGURE 2 shows curves plotted from experimental results finding straight and non-straight chain :hydrocarbon capacity and selectivity characteristics at about room temperature (75 F.) and atmospheric pressure for type A zeolites wherein the content of divalent lmanganese in the zeolite was varied over a wide range. As these manganesecontainin-g type A zeolites are made most conveniently and preferably ilzvy exchanging a portion lof Na+ ions for Mn++ ions in the sodium [60mn of the type A zeolite, the test zeoli-tes were made that way and the x axis indicates the percentage of sodium replaced by manganese in the type A structure. The capacities of the manganese-containing ty-pe A zeolites for the non-straight chain hydrocarbon, isofbutane, and the straight chain hydrocarbons, normal butane and ethane, are indicated on the y axis.

While the foregoing experimental work was done mainly wit-h butane and isoibutane, it will be understood that these two hydrocarbons are representa-tive of the two broad classes of hydrocarbons forpurposes of this invention, namely straight-chain and non-straight chain hydrocanbons, and that hydrocarbons `of higher molecular weight, e.'g., up to about 550 F. nunmal boiling point, can fbe treated similarly except with the reservation that temperature and/or pressure conditions must tbe such 'that the hydrocarbons are in vapor phase for sonption.

A convenient and rapid 'Way to chan-ge from sor-hing conditions to describing conditions in the practice of my process is rto operate essentially isothermally at a temperature ffnom about 50 to about `800" F. and to sonb under a pressure of Ito 2000 p.s.i.1g., then to desonb at lower pressure in the range from 0 to 100 p.s.i.|g. or even subatmospheric pressure. Such operation is described in U.S. Patent No. 2,859,256, of Hess et al., also assigned to The Texas Company.

The pnocess vorf -my invention can also be operated wherein temperature of sorbin-g contact is 'between 50 and 500 F. and is raised for desorption. Alternatively, desorption can Ibe done at a temperature substantially' above, and at a pressure substantially lbelow fthe s'oribing temperature and pressure to drive off soi-bed straight chain hydrocarbons. Describing can be done advantageously by using a swbatmospheric pressure, eig., 10 to 25 inches of Hg absolute, and/ or a sweep of low molecular weight gas, eig., hydrogen, nitrogen, isopentane, or methane to help drive oi described straight chain hydrocarbon vapors from the mineral sorbent.

The following examples show how type A zeolites 'having manganese as a portion of Itheir exchangeable cation content have been prepared and how such zeolites can 'be used in a renery, but should not ibe construed as limiting the invention.

A type A zeolite in which 37% of the exchangeable ca-tion content was manganese was made as follows: 50 ,grams of pelleted and dehydrated sodium form tof the type A zeolite, marketed as Linde 4A molecular sieve, was allowed to soak tor 38 'hours at 200 F. in 60 cc. of 3 N aqueous MnCl2 solution. The exchanged zeolite was washed with water and dehydrated at about 575 F. for two hours to prepare it for sorption of straight chain hydrocarbons.

A manganese-calcium-sodium form of type A zeolite was made as follows: '30 grams of granular dehydrated calcium-sodium form of t-ype A zeolite was allowed to soak in 72 cc. of 4.5 N aqueous MnCl2 solution at 200 F. for 184 hours. The ygranules were washed thoroughly 'with Wa-ter and dehydrated .to remove zeolitic water. Chemical analysis of the product zeolite indicated a for-.mula (dehydratedv state) as follows: (0.5Mn, 0.35Ca, 0.15Na2)O.A12O3.2SiO2. Capacity of the product at 75 F. and atmospheric pressure, found by test, was 45 cc. of ethane per gram, 39 cc. of n-butane per gram, and 4 cc. of isobutane per gram.

One-half of the product produced in the immediately previous ion exchanging operation was treated for a second .time with 72 cc. of 4.5 N aqueous `MnClZ solution at 205 F. for 83 hours. The granules were washed thoroughly with water, and dried to remove water. Chemical a-nalysis indicated the following formula (dehydrated state) (0.75Mn, 0.14C-a, 0.11Na2)O.A12O3.2SiO2. Capacity of the product at 75 F. temperature and atmospheric pressure, found by test, was 44 cc. of ethane per igram, 41 cc. of normalbutane per tgrarn, a-nd 4 cc. of isohutane per gram. The three foregoing products were capable of attaining from 80-90% saturation with normal butane at room temperature (75 F.) and atmospheric pressure in `substantially less than 4 minutes.

A typical hydrocarbon separation contemplated with my manganese-containing formof type A zeolitie is the removal of straight chain hydrocarbons :from stabilized, catalytically reformed motor naphtha, such naphtha having characteristics, for example, of API gravity, 48.8; refractive index (20 C./4 C.), about 1.444; ASTM distillation IBP., 126 F. and EP., 377 F.; and ASTM research clear octane rating, 87.1.

The naph-tha in vapor form is passed through a first lbed of my manganese-containing type A zeolite in -pelleted form at temperature of about 750 F., and under pressure of about 350 p.s..g. for 2 minutes. The zeolite has 35- 70% of its exchangeable cation content manganese with balance of sodium. Saturation of the pellets with straight chain hydrocarbon components is .tiren about 85% complete. The unsorbed naphtha vapors emerge from the bed low in the straight chain hydrocarbon content which detracts from the octane rating of the feed stock. At this point the naphtha feed is shunte-d t-o another similar sorbent bed. A stream of recycle hydrogen from catalytic reforming of the feed napfhtha is passed through the rst bed, briefly under elevated pressure to purge the vessel of unsorbed material. Then -the pressure on the tirst lbed is reduced to O p.s.i.|g. and the hydrogen is continued for about 2 minutes to -desorb all but approximately l5-20% of the straight chain hydrocarbons in the laden sorbent pellets. The effluent vapors from the desorbing operation are used as part of the recycle feed to the naphtha reforming process.

I claim:

1. Process for `the separation of a straight chain hydrocarbon from a n-on-straight chain Ihydrocarbon in admixture therewith which comprises contacting said admix- .ture in vapor phase with a dehydrated crystalline Zeolite of type A structure having 0.25 to 0:95 of its exchange- `able cation content as divalent manganese to adsonb said straight chain hydrocarbon therefrom, discontinuing said contact when about Ito albout 95% of the saturation capacity of said crystalline zeolites or said straight chain hydrocanbon is reached, desorbing said yadsorbed straight chain hydrocarbon, discontinuing `said desonbing when about 80-90% of the adsorbed straight chain hydrocarbon has been described and repeating said contacting and desorbing steps.

2. Process as claimed in claim 1 wherein contacting is carried out to adsorib from about 8O to about `90% of the saturation capacity of said zeolite for said straight chain hydrocarbon.

3. Process as claimed in claim 1 wherein desorbing is carried out to desonb from about 80 Ito about `85% orf the adsorbed straight chain hydrocarbon from said zeolite.

4. Process as claimed in claim 1 wherein the mixture is in contact with the selective adsorbent for a period not in excess of about 5 minutes.

5. Process as claimed in claim 1 wherein contacting is carried ou-t at a temperature between about 50 and 800 F., and a pressure between about 100 and 2000 p.s.i.fg.

6. Process as claimed in claim 5 wherein contacting and desorbing are carried out under substantially isothermal conditions and the describing pressure is less than the contacting pressure.

References Cited by the Examiner UNITED STATES PATENTS 2,859,256 11/1958 Hess et al. 260-676 2,882,243y 4/1959 |Milton 26o-676 2,988,577 6/1961 sensel 26o-67s 2,988,502 6/1961 Richards er a1. 26o- 676 ALPHONSO D. SULLIVAN, Primary Examiner.

PAUL M. COUG'HLAN, Examiner. D. S. ABRAMS, Assistant Examiner,

Claims (1)

1. PROCESS FOR THE SEPARATION OF A STRAIGHT CHAIN HYDROCARBON FROM A NON-STRAIGHT CHAIN HYDROCARBON IN ADMIXTURE THEREWITH WHICH COMPRISES CONTACTING SAID ADMIXTURE IN VAPOR PHASE WITH A DEHYDRATED CRYSTALLINE ZEOLITE OF TYPE A STRUCTURE HAVING 0.25 TO 0.95 OF ITS EXCHANGEABLE CATION CONTENT AS DIVALENT MANGANESE TO ABSORB SAID STRAIGHT CHAIN HYDROCARBON THEREFROM, DISCONTINUING SAID CONTACT WHEN ABOUT 80 TO ABOUT 95% OF THE SATURATION CAPACITY OF SAID CRYSTALLINE ZEOLITES FOR SAID STRAIGHT CHAIN HYDROCARBON IS REACHED, DESORBING SAID ABSORBED STRAIGHT CHAIN HYDROCARBON, DISCONTINUING SAID DESORBING WHEN ABOUT 80-90% OF THE ADSORBED STRAIGHT CHAIN HYDROCARBON HAS BEEN DESORBED AND REPEATING SAID CONTACTING AND DESORBING STEPS.

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