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Feb. 26, 1963 3,078,640 R. M. MILTON SEPARATION OF SULFUR COMPOUNDS FROM VAPOR MIXTURES Filed Dec. 18, 1959 3 Sheets-Sheet l FIG. I. ZEOLITE A ADSORPTION CAPACITY For Various Temperature Ratios (AdsGoZcrAbiena‘vl/mIOs)gd W°SECUIOAL/DMGPH‘°TRUBNED 5 I; 6 3 m / / I / / OJ 0.2 0.3 0.4 0.5 0.6 0.1 0.8 TEM PERATURE RATIO T2/Tl (Tlond T2 in °K) INVENTOR. ROBERT M. MILTON w A TTORNE)’. Feb. 26, 1963 3,078,640 R. M. MILTON SEPARATION OF SULFUR COMPOUNDS FROM VAPOR MIXTURES Filed Dec. 18, 1959 3 Sheets-Sheet 2 \ IN6 _' _ w N (v amcaz pawmov slums ool/awqmspv sums) oasuosov swoauvoouom ozuvamvs %.LH9|3M INVENTOR. ROBERT M. MILTON BY WM ATTORNE)’. Feb. 26, 1963 3,078,640 R. M. MILTON SEPARATION OF SULFUR COMPOUNDS FROM VAPOR MIXTURES Filed Dec. 18, 1959 5 Sheets-Sheet 3 ZEOLITE A ADSORPTION CAPACITY For Various Temperature Ro?os an a LIT 232:M0o6m95;wgq“:532.0863 4s|m m n m m m o2 TE OM. . P 4E 5 EUmmaWTz RonTM 0 . T2G A d BY o5 o. 7 0.8 INVENTOR. ROBERT M. MILTQN . FI63. A TTORNE Y. United States Patent O?ice 8,078,640 Patented Feb. 26, 1963’. 1 2 ecules are excluded. Not all adsorbents behave in the‘ 3,078,640 manner of the molecular sieves. SEPARATION OF SULFUR COMPOUNDS FROM VAPOR MIXTURES Such common adsorb ents are charcoal and silica gel, for example, do not ex-. hibit molecular sieve action. Robert M. Milton, Buffalo, N.Y., assignor to Union ' Zeolite A consists basically of a three-dimensionalv Carbide Corporation, a corporation of New York framework of SiO.; and A104 tetrahedra. The tetrahedra. are cross-linked by the sharing of oxygen atoms so, that Filed Dec. 18, 1959, Ser. No. 860,526 7 Claims. (Cl. 55-73) the ratio of oxygen atoms to the total of the aluminum and , This invention relates to a method for adsorbing ?uids silicon atoms is equal to two or O/(Al-l-Si) =2. More particularly, the invention relates to amethod of adsorbing hydrogen sul?de with adsorbents of the mo lecular sieve type. Still more particularly, the invention relates to a method for preferentially adsorbing hydro example, an alkali or alkaline earth metal ion. balance may be expressed by the formula ' The and separating a mixture of ?uids into its component parts. ' 10 electrovalence of the tetrahedra containing aluminum is, balanced by the inclusion in the crystal of a cation, for. gen sul?de from a vapor mixture containing at least one " 15 member of the group consisting of hydrogen, carbon di oxide and normal saturated aliphatic hydrocarbons con-, This -Al2/ (Ca, Sr, Ba,-Na2, K2) =1 One cation ‘may be‘ exchanged for another‘ by ion"ex-‘" change techniques which are described below...The spaces between the tetrahedra are occupied by water molecules" taining less than nine carbon atoms per molecule. prior to dehydration. This separation is advantageous in, for example, sweet ening the de?ned vapor streamsthereby preventing the. 20 Zeolite‘ A may be activated by heating to effect ‘the. loss of the water‘of hydration.‘ The dehydration results deposition of sulfur compounds which can cause plugging in crystals interlaced with channels of molecular dimen-v and corrosion of transmission pipes, valves, regulators and the like. Also, the sulfur compounds may produce un desirable side reactions with other materials contacting the vapor mixture. > sions that offer very high surface areas for the adsorption of foreign molecules. These interstitial channels will not‘ 25 accept molecules that have a maximum dimension of the minimum projected cross-section in excess of about 5.5v A. Factors in?uencing occlusiongby the activated zeolite.‘ A crystals are the size and polarizing power of ‘the inter stitial cation, the polarizability and polarity of the ‘oc Broadly, the invention-comprises mixing-molecules, in a fluid state, of the materials to be adsorbed or separated with at least partially dehydrated crystalline synthetic metal-aluminum-silicates, which will be described more particularly below, and effecting the adsorption of the 30 c'luded molecules, the dimensions and'shape of the'sorb‘ed the process of the invention is in some respects similar to‘ molecule‘relative to those‘ of the channels, the duration and severity of dehydration and desorption, and the pres naturally occurring zeolites. Accordingly, the term “zeo lite” would appear to be appropriately applied to these will be understood that the ‘refusal characteristics of‘ adsorbate by the silicate. The synthetic silicate used in. ence of foreign molecules in the interstitial channels. ‘Itv zeolite A are quite as important as the adsorptive or posi'-‘ materials. There are, however, signi?cant differences be tive adsorption characteristics. tween the synthetic and natural silicates. To distinguish Although there are a ‘number of cations that may be the synthetic material used in the method of the inven present in zeolite A it is preferred to formulate or.syn-. tion from the natural zeolites and other similar synthetic thesize the sodium form of the crystal since the reactants silicates, the sodium-aluminum-silicate and its derivatives taught hereinafter to be useful in the process of the in 40 are readily available and water soluble. The sodium "in the sodium form of zeolite A may be easily exchanged vention will be designated by the term “zeolite A.” While for other cations as will be shown below. Essentially the structure and preferred method of making zeolite A the preferred process comprises heating a proper mix will be discussed in some detail in this application, addi~ tional information about the material and its preparation ture in aqueous solution of the oxides, or of materials‘. whose chemical compositions can be completely. repre may be found in an application ?led December 24, 1953, Serial No. 400,388, now US. Patent 2,882,243. sented as mixtures of the oxides, NaZO, A1203, SiOz and _ It is the principal object of the present invention to provide a process for the selective adsorption of hydrogen ' H2O, suitably at a temperature of about 100° ‘C. for periods of time ranging from 15 minutes to 90 hours or ‘longer. The product which crystallizes from the hot sul?de from ?uids. A further object'of the invention is to provide a method whereby hydrogen sul?de 'may be 50 rnixt-ure is ?ltered off and washed With distilled 'water adsorbed and separated by crystalline synthetic metal until the effluent wash water in equilibrium with the zeo aluminum-silicate from ?uid mixtures with other ,7 mol ' lite has a pH of from about 9-to 12. The material, after ecules. . activation, is ready for use as a molecular sieve. I In the drawings, ' ‘ " ‘ Zeolite A may be distinguished from other zeolites and FIG. 1 is a graph showing the amount of sulfur com 55 silicates on the basis of its X-ray powder diffraction pat tern. The X-ray patterns for several of the ion yex— pounds adsorbed versus the temperature ratio T2/T1 for changed forms of zeolite A are described below. ‘Other various forms of zeolite A; FIG. 2 is a graph showing the amount ofC; through C8 characteristics that are useful in identifying zeolite A are; its composition and density. ‘ > ~ ~ normal saturated aliphatic hydrocarbons adsorbed versus the temperature ratio Tz/Tl for various forms of zeolite 60 The basic formula for all crystalline zeolites where‘ “M” represents a metal and “)1” its valence may be repre-v A; and ~ FIG. 3 is a graph showing the amount of carbon dioxide sented as follows: ‘ ' adsorbed versus the temperature ratio T2/T1 for various forms of zeolite A. ’ I ~ Certain adsorbents, including zeolite A, which selec-. tively adsorb molecules on the basis of the size and shape of the adsorbate molecule are referred to as molecular sieves. These molecular sieves have a sorption area avail M2O2Al2O3:XSlO2:YHzO 65 In general a particular crystalline zeolite will have values The value X for a particular zeolite will vary somewhat since the . for X and Y that fall in a de?nite range. aluminum atoms and the silicon atoms both-occupy esi able on the inside of a large number of uniformly sized pores of molecular dimensions. With such an arrange 70 sentially equivalent positions in the lattice. Minor vari ations in ‘the relative’ numbers of these atoms do not ment molecules of a certain size and shape enter the pores signi?cantly alter the crystal structure or physical prop‘ and are adsorbed while larger or differently shaped mol 3,078,640 3 erties of the zeolite. =For zeolite A, numerous analyses Form of Zeolite A have shown that an average value for X is about 1.85. The X value falls within the range 1.85 $0.5. The value of Y likewise is not necessarily an invariant for all samples of zeolite A particularly among the vari ous ion exchanged forms of zeolite A. This is true be cause various'exchangeable ions are of different size, and, Percent of Exchange Sodium ______________________________ _ . Lithium since there is no major change in the crystal lattice di 100 __ Density, gJce. l. 99i0.1 65 1. 92i0. 1 Potassium . _ _ . . . . . _ _ . . . . _ _ _ _ _ . .. 95 2. 08:1;0. 1 Cesium _ _ _ . _ _ _ . . . . . _ _ _ _ _ . l _ _ _ -- 31 2. 265A). 1 Magnesium- _-_ 75 2. 04=t;0. 1 Calcium ______ _. 93 2. 05:};0. 1 Thallous ............ ._ 80 about 3.36 mensions upon’ ion exchange, more or less space should In making the sodium form of zeolite A, representa be available in the pores of the zeolite A to accommo~ 10 tive reactants are silica gel, silicic acid or sodium silicate date water molecules. For instance, sodium zeolite A as a source of silica. Alumina may be obtained from was partially exchanged with magnesium, and lithium, activated alumina, gamma alumina, alpha alumina, and the pore volume of these 'forms, in the activated alumina trihydrate, or sodium aluminate. Sodium hy condition,‘ measured with the ‘following results: 15 droxide may supply the sodium ion and in addition assist in controlling the pH. Preferably the reactants are water soluble. A solution of the reactants in the proper pro portions is placed in a container, suitably of metal or glass. The container is closed to prevent loss of water Na .......................................... _. 0 5.1 Mg -. -_- . 75 '5. 8 20 and the reactants heated for the required time. A con K‘ i 95 4 venient and preferred procedure for preparing the reactant Ca .......................................... __ ,93 5 mixture‘ is to make an aqueous solution containing the sodium aluminate and hydroxide and add this, preferably The average value for Y thus determined for the'fully with agitation, to an aqueous solution of sodium silicate. hydrated sodium zeolite A was 5.1; and in varying con ditions of hydration, the value of Y can vary ‘from 5.1 25 The system is stirred until homogeneous or until any gel which forms is broken into a nearly homogeneous mix. to essentially zero. The maximum value of Y has been After this mixing, agitation may be stopped as it is un found in 75% exchanged magnesium zeolite A, the fully necessary to'agitate the reacting vmass during the forma hydrated form of which has .a Y value of 5.8. In gen tion and crystallization of the zeolite, however, mixing eral an increase in the degree of ion exchange ‘of the magnesium form of zeolite A results in an increase in the 30 during formation and crystallization has not been found to be detrimental. The initial mixing of ingredients is con Y fvalu'e; Larger values, up to 6, may be obtained with Ion Exehanged Form of Zeolite A more fully ion exchanged materials. Percent Na Ions Replaced ‘ Value of Y veniently done at room temperature but this is not essen ' In ‘zeolite A synthesized according to the preferred pro tial. ' In the synthesis of zeolite A, it has been found that the cedure, the ratio Na2O/Al2O3 should equal one. But if all of the excess alkali present in the mother liquor is 35 composition of the reacting mixture is critical. The not washed out of the precipitated product, analysis ‘may crystallizing temperature and the length of time the show a ratio greater than one, and if the washing is car crystallizing temperature is maintained are important variables in determining the yield of crystalline material. ried too far, some sodium may be ion exchanged by hydrogen, and the ratio will drop below one.’ Thus, a typical analysis for a thoroughly washed sodium zeolite Under some conditions, for example too low a tempera 40 ture for too short a time, no crystalline materials are pro duced. Extreme conditions may also result in the pro A ‘ is 0.99Na2O:1.0Al2O3:1.85SiO2:5.1H2O. The ratio duction of materials other than zeolite A. Nam/A120, has varied as much as 23%. The composi ' The sodium form of zeolite A has been produced at tion for zeolite A lies in the range of ‘ MO El a 45 100‘? C., ‘essentially free from contaminating materials, from reacting mixtures whose compositions, expressed as mixtures of the oxides, fall within either of the follow ing ranges. where “M” represents a metal and “n” its valence. ' Thus the formula for zeolite A may be 'written as follows: 50 ‘“ 1.0 =1: 0.2M 2 O:Al;0;:1.85 i 0.5Si0z2YH2O F In this formula f‘M” represents a metal, “n” its valence, Range 1 Sim/A1203 __________________________________ __ NaiO/SiO; __________________________________ __ HgO/NauO . . ._. .____ Range 2 0.5-1. 3 1. 0-3. 0 1. 3-2. 5 0. 8-3. 0 35-200 35-200 and ‘.‘Y” may be any value up to 6 depending on the ' When zeolite A has been prepared, mixed with other chemical analysis. When other materials as well as water tivity, characteristic of zeolite A are present in the prop identity of the metal and the degree of dehydration of 55 materials, the X-ray pattern of the mixture can be repro duced by a simple proportional addition of the X-ray pat the crystals. terns of the individual pure components. The pores of zeolite A are normally ?lled with water Other properties, for instance molecular sieve selec and in this case, the above formula represents their are in the pores of zeolite A, chemical analysis will show 60 erties of the mixture to the extent that zeolite A is part of the mixture. a lower value of Y and the presence of other adsorbates. The adsorbents contemplated herein include not only The presence in the pores of non-volatile materials, the sodium form of zeolite A as synthesized above from such as sodium chloride and sodium hydroxide, which a sodium-aluminum-silicate-water system with sodium as are not removable under normal conditions of activation at temperatures of vfrom 100° C. to 650° _C. does not 65 the exchangeable cation but also crystalline materials ob tained from such a zeolite by partial or complete replace signi?cantly alter the crystal lattice or structure of zeolite ment of the sodium ion with other cations. The sodium A although it will of necessity alter the chemical compo cations can be replaced at least in part, by other ions. These replacing ions can be classi?ed in the following zeolite A were determined by the ?otation of the crystals 70 groups: metal ions in group I of the periodic table such as potassium and silver, and group II metal ions such as on liquids of appropriate densities. The technique and calcium and strontium with the exception of barium. liquids used are discussed in an article entitled “Density Other cationic metal zeolites are too complex in their of Liquid Mixture” appearing in Acta Crystallographica, preparation for use in the present invention. 1951, vol. 4, page 565. The densities ofseveral such crystals are as follows: 75. The spatial arrangement of the aluminum, silicon, and sition. The apparent density of fully hydrated samples of 5 3,078,640 oxygen atoms which make up the basic crystal lattice of the zeolite remains essentially unchanged by partial or complete substitution of the sodium ion by other cations. The X-ray patterns of the ion exchanged forms of the zeolite A show the ‘same principal lines at essentially the same positions, but there are some differences in the rela_ gt. tive intensities of the X-ray lines, due to the ion exchange. Ion exchange of the sodium form of zeolite A (which for convenience may be represented as NazA) or other 6 large to enter the pore system of the zeolite freely, but are not large enough to be excluded entirely, there is a finite rate of adsorption and the amount adsorbed will vary with time. In general, the recorded data represents the adsorption occurring within the ?rst one or two hours, and for some borderline molecules, further adsorptionl may be expected during periods of ten to ?fteen hours. Washing techniques, different heat treatments and the crystal size of the sodium zeolite A powder can cause forms of zeolite A may be accomplished by conventional 10 very'appreciable differences in adsorption rates for the‘ ion exchange methods. A preferred continuous method borderline molecules. is to pack zeolite A into a series of vertical columns with The calcium and magnesium exchanged zeolite A have suitable supports at the bottom; successively pass through molecular sieve adsorptive properties characteristic of the beds a Water solution of a soluble salt of the cation to be introduced into the zeolite; and change the ?ow from the ?rst bed to the second bed as the zeolite'in the ?rst bed becomes ion exchanged to the desiredsextent. materials with larger pores than exist in sodium zeolitev A. These two forms of divalent ion exchanged zeolite A behave quite similarly and adsorb all molecules ad sorbed by sodium zeolite A plus' some larger molecules. , To obtain hydrogen exchange, a water solution of an There are numerous other ion exchanged forms of acid such as hydrochloric acid is effective as the exchang zeolite A such as lithium, ammonium, silver, zinc, nickel, ing solution. For sodium exchange, a water solution of 20 hydrogen and strontium. In general, the divalent ion sodium chloride is suitable. Other convenient reagents exchanged materials such as zinc, nickel and strontium are: for potassium exchange, a water solution of potas zeolite A have a sieving action similar to that of calcium sium chloride or dilute potassium hydroxide (pH not over and magnesium zeolite A, and the monovalent ion ex-_ about 12); for lithium, magnesium, calcium, ammonium, changed materials such as lithium and hydrogen zeolite nickel, or strontium exchange, water solutions of the 25 A behave similarly to sodium zeolite A, although some chlorides of these elements; for zinc exchange, a water di?erences exist. ‘A solution of zinc nitrate; and for silver exchange, a silver Another unique property of zeolite A is its strong nitrate solution. While it is more convenient to use Water preference for polar and polarizable molecules, provid-f soluble compounds of the exchange cations, other solu ing of course that these molecules are of av size and tions containing the desired cations or hydrated cations 30 shape permitting them to enter the pore system of the may be used. zeolites. This is in contrast to charcoal and silica gel Among the ways of identifying zeolite A and distin which show a main preference based on thevolatility guishing it from other zeolites and other crystalline sub of the adsorbate. A selectivity for polar and polarizable stances, the X-ray powder diffraction pattern has been molecules is not new among adsorbents. Silica gel ex: found to be a useful tool. The X-ray powder diffraction hibits some preference for such molecules, but the extent lines of zeolite A are set forth in the previously referenced of this selectivity is so much greater with zeolite A that US. Patent 2,882,243 to R. M; Milton, incorporated here separation processes based upon this selectivity become in ‘by reference. _The zeolites contemplated herein exhibit adsorptive Zeolite A shows a selectivity for adsorbates,.provided properties that are unique among known adsorbents. The that‘ they are small enough to enter thev porous network common adsorbents, like charcoal and silica gel, show of the zeolites, based on the boiling points of the, ad adsorption selectivities based primarily on the boiling sorbates, as well as on their polarity and polarizability. point or critical temperature of the adsorbate. Activated For instance, hydrogen which has a low boiling point is zeolite A on the other hand exhibits a selectivity 2based on feasible. . _ ,_ . the size and shape of the adsorbate molecule. Among 45 not strongly adsorbed at room temperature. A further important characteristic of zeolitev A is its those adsorbate molecules whose size and shape are such property of adsorbing large amounts of adsorbates' at as to perrmit adsorption by zeolite A,'a very strong pref: low adsorbate pressures, partial pressures or concentra erence is exhibited toward those that are polar and polar~ tions. This property makes zeolite A uniquely useful in izable. Another property of zeolite A that contributes the more complete removal of adsorbable impurities from to its unique position among adsorbents is that of adsorb ing large quantities of adsorbate either at very low pres sures, at very low partial pressures, or at very low con centrations. One or a combination of one or more of gas and liquid mixtures. It gives them a relatively high adsorption capacity even when the material being ad; sorbed from a mixture is present in very low» concentra-f tions ,and permits the'e?icient recovery of minor‘c'om-i thesethree adsorption characteristics or others can make zeolite A useful for numerous gas or liquid separation 55 ponents of mixtures; This characteristic is all the more important since adsorption processes are most frequently processes where adsorbents are not now employed. The used when the desired component is present in low “con; use of zeolite A permits more e'l?cient and more economi centrations or low partial pressures. High adsorptions oal operation of numerous processes now employing other at low pressures or concentrations on zeolite A are illus~ adsorbents. Common adsorbents like silica gel and charcoal do 60 trated in the following table, along with some compara'é tive data for silica gel and charcoal. not exhibit any appreciable molecular sieve action where as the various forms of zeolite A do. This is shown in the tables following in the speci?cation, for typical sam pics of the adsorbents. In these tables the term “Weight percent adsorbed” refers to the percentage increase in Tempera- Pressure Weight; Percent‘ Adsorbed ', Adsorbate ture(° O.) (mm.Hg) “ , NazA CaA MgA the weight of the adsorbent. The adsorbents were ac tivated by heating them at a reduced pressure to remove adsorbed materials. Throughout the speci?cation the activation temperature for zeolite A was 350” C. and the pressure at which it was heated as less than about 0.1 millimeter of mercury absolute unless otherwise speci ?ed. Throughout the speci?cation, unless otherwise in dicated, the pressure given for each adsorption is the pressure of the adsorbate at the adsorption conditions. O02 ______ -- g H28 ______ __ 009 ...... -_ 25 1.0 25 25 80 750 5.0 25 0.5 8.5 12.7 6.6 25 11 10.4 21.0 15.2 25 198 723.7 20.3 25.9 0 50 17 - 5.0 5.3 15.0 10.5 18.9; 24.4 15.0 22.2 Charcoal " ' * Silica Gel. . .......... . Any cationic ........ form -_ of000zeolite 21.8 A having a pore size of ‘In borderline cases where adsorbate molecules are too 75 at least 4 Angstroms is suitable for practicing ‘the present 3,078,640 invention.‘ Smaller pore sized forms, such as potassium zeolite A do not admit hydrogen sul?de and the mer captans. The adsorption capacity of adsorbents usually decreases with increasing temperature, and while the adsorption 8 That is, the T; values for these examples were obtained from the preceding portion of the speci?cation and the T2 values were read from the vapor pressure tables in “Industrial and Engineering Chemistry,” vol. 39, page 517, April 194-7. TABLE C capacity of an adsorbent at a given temperature may be su?icient, the capacity may be wholly unsatisfactory at a higher temperature. With zeolite A a relatively high capacity may be retained at higher temperatures. Ion Form Zeolite A Wt. Temperature, Pressure, Percent ° K. mm. Hg 1128 Adsorbed Zeolite A may be activated by heating it in either air, 10 TI/Tl T] a vacuum, or other appropriate gas to temperatures of as high as 600° C. The conditions used for desorption of an adsorbate from zeolite A vary with the adsorbate, 0.5 11 198 0. 5 11 O. 5 11 198 5. 0 12 5.0 12 50 12 50 150 237 2. 7 1. 6 0. 15 5. 0 2. 7 12 50 by either raising the temperature or reducing the pressure, partial pressure or concentration of the adsorbate in contact with the adsorbent or a combination of these steps is usually employed. Another method is to dis place the adsorbate by adsorption of another more strongly held adsorbate. The present process for separating hydrogen sul?de from certain vapor mixtures depends upon two related properties 'of zeolite A with respect to the adsorbed phase. The ?rst property is the selectivity of the in~ ternal surfaces of the crystal towards these strongly polar compounds as compared to normal saturated aliphatic hydrocarbons, hydrogen and carbon dioxide. As previ ously discussed and illustrated by the tables, zeolite A is capable of adsorbing all of these compounds based on 8. 5 16. 4 23. 7 l2. 7 21. 0 6. 6 15. 2 25. 9 11.1 14. 3 5. 8 7. 9 12. 0 2. 2 5. 0 7. 1 9. l 8.1 0.8 0. 5 10. 0 4. 7 8.1 11. 0 298 298 298 298 298 298 298 298 298 298 348 348 348 423 423 423 423 298 423 423 298 348 348 348 T2 133 157 191 133 157 133 157 191 150 158 150 158 174 158 174 191 196 144 136 127 150 144 158 174 0. 45 O. 53 0. 64 0. 45 U. 53 0. 45 0. 53 0. 64 0. 49 0. 52 0.43 0. 46 0.50 0.37 0.41 0. 45 0. 46 0. 47 O. 32 0. 30 0. 40 0. 41 0. 46 0. 50 a consideration of the zeolite A pore size and critical An inspection of Table C will reveal that it includes molecular dimensions of the compounds. For example, 30 adsorbate temperatures from 25° C. to 150° C. and ad the pores of zeolite A are su?iciently large and in fact sorbate pressures 0.5 mm. Hg to 237 mm. Hg. It was do receive methane, ethane, propane, hydrogen and car unexpectedly discovered that hydrogen sul?de, methyl bon dioxide molecules. mercaptan and ethyl mercaptan, all being freely adsorbed Based on these considerations, one skilled in the art would logically conclude that zeolite A would not possess 35 on zeolite A exhibit the same temperature ratio Tz/Tl any particular selectivity for hydrogen sul?de in prefer ence to the other constituents of the present vapor mix ture. Contrary to these expectations, it has been dis relationship to weight percent of sulfur compound ad sorbed. That is, for a given T2/T1 value, the weight per cent adsorbed will be the same for all of the previously de?ned sulfur compounds. The present invention utilizes lectivity for the enumerated sulfur compounds to the sub 40 this relationship in combination with the previously dis cussed polar compound selectivity property of certain stantial exclusion of normal saturated aliphatic hydrocar crystalline zeolitic molecular sieves to provide a novel bons, hydrogen and carbon dioxide. One reason for this covered that zeolite A possesses an extremely strong se selectivity is the highly polar nature of hydrogen sul?de separation process. The present invention combines the previously dis as compared with the other possible constituents of the cussed properties of zeolite A in such a manner that a 45 vapor mixture. novel process is provided for separating hydrogen sul The second interrelated property of zeolite A which ?de ?rom a vapor mixture containing at least one mem is utilized by a preferred form of the present invention ber selected from the group consisting of hydrogen, car is the relationship of the boiling point or vapor tension bon dioxide and normal saturated aliphatic hydrocarbons characteristics of an individual ?uid or clearly related type of ?uid to the capacity of the crystalline zeolite to 50 containing less than nine carbon atoms per molecule. In its broadest form, the process consists of contacting the adsorb the ?uid at a given temperature and pressure. vapor mixture with a bed of at least partially dehydrated More speci?cally, it has been discovered that a rela zeolite A adsorbent material having a pore size of at tionship exists between the amount of ?uid adsorbed and least about 4 Angstroms, thereby adsorbing the hydrogen the temperature ratio T2/ T1 where T1 is the temperature in degrees Kelvin during adsorption, assuming that the 55 sul?de. The 1hydrogen sul?de-depleted vapor mixture is then discharged from the crystalline zeolite A bed. temperature of the ?uid and the adsorbent are in equi It is to be understood that the expression “pore size," librium. T2 is the temperature in degrees Kelvin at as used herein refers to the apparent pore size, as dis which the vapor pressure of the ?uid is equal to the par~ tinguished from the effective pore diameter. The appar tial pressure or vapor tension of the ?uid in equilibrium with the zeolite adsorbent. Stated in another way, T2 is 60 ent pore size may be de?ned as the maximum critical dimension of the molecular species which is adsorbed the temperature at which the vapor pressure of the ad by the zeolitic molecular sieve in question under normal sorbate is equal to the partial pressure of the adsorbate conditions. Maximum critical dimension may be de during adsorption. T2 is actually the dew point of the ?ned as the diameter of the smallest cylinder which will adsorbate at the adsorption conditions. This relationship is clearly shown in FIG. 1 which 65 accommodate a model of the molecule constructed us ing the best available values of bond distances, bond an is a plot of the weight percent of sulfur compounds ad gles, and Van der Waal radii. Effective pore diameter sorbed versus the temperature ratio T2/ T1 for both mono is de?ned as the free diameter of the appropriate sili valent and divalent cationic forms of zeolite A having cate ring in the zeolite structure. The apparent pore a pore size of at least about 4 Angstroms. The tested adsorbate in all cases was hydrogen sul?de, but it has 70 size for a given zeolitic molecular sieve will always be larger than the eifective pore diameter. been found that methyl and ethyl mercaptan behave simi The previously described contact between zeolite A larly. The following table is a summary of the‘ data adsorbent material and the vapor mixture is preferably from which FIG. 1 was prepared, the data for the ?rst e?ected under conditions such that the temperature ratio eight examples having been assembled from tests de scribed in more detail in other parts of the speci?cation. 75 T2/ T1 with respect to the inlet end of the bed and with 3,078,640 respect to at least (ne of the enumerated sulfur com pounds of the vapor mixture is between 0.31 and 1.0, where T1 is the adsorption temperature and is less than 873° K., and T2 is the temperature at which the one sulfur compound has a vapor pressure equal to its par tial pressure in the vapor mixture. The lower limit of 0.31 for the temperature ratio T2/ T1 is ?xed by the dis covery that below this value there is a smaller percentage change in adsorption capacity per unit change in the 10" a vapor pressure equal to the partial pressure of the compound over the zeolite A bed at the end of the re generation. It will be understood by those skilled in the art that at least two adsorbent beds may be provided, with one bed on adsorption stroke and the» other bed on regeneration stroke. The respective flows are then pe-' riodically switched when the ?rst ‘bed becomes loaded with the adsorbate, so that the latter is placed on regen eration stroke and the second bed is placed on-stream. temperature ratio. In contrast, above 0.31 there is a 10 The continuous process is most e?iciently performed if T1, the adsorption temperature, is less than 616° K. change in the temperature ratio. Stated in another way, but higher than 283° K., for previously stated reasons. if it is desired to obtain a certain incremental adsorbate Also, for maximum e?‘iciency during the adsorption loading at a speci?ed adsorption temperature with a stroke, T2 the dew point ofthe hydrogen sul?de of the given feed stream, it would be necessary to ‘increase the 15 vapor mixture, is below 499'0 K. During the regenera pressure of operation by a greater percent if the tem tion stroke, T1 is preferably above 283° K. and below larger percentage change in adsorption capacity per unit perature ratio is below 0.31 than if it is maintained 616° K., also for the previously discussed reasons. above this value in accordance with the invention. Also, It will be understood by those skilled in the art that the temperature ratio of 0.31 corresponds to a bed load the temperature ratio may be adjusted by well-known ing of about 0.5 weight percent adsorbate and if the tem 20 methods as for example heating the bed by direct or perature ratio were reduced below this value, a larger indirect heat transfer, employing a purge gas, or by draw adsorption bed would be required with its attendant ing a vacuum .on the bed during the regeneration stroke. higher investment and operating expenses. Also, during the adsorption stroke, the ratio may be ad The upper limit of 1.0 for the temperature ratio should justed for favorable operation by varying eitheror both not be exceeded, because if the adsorption temperature 25 the temperature and the pressure. is equal to or less than the dew point, condensation of The many advantages of the invention are illustrated the sulfur compound will occur, thereby essentially elim inating the sieving action of the zeolite A adsorbent. by the following examples. quent loss of adsorption capacity and reduction in pore feed mixture comprising 0.035 mole fraction HZS, 0.035 Example I The broad upper limit of 873° K. for T1 is due to the 30 It is desired to remove hydrogen sul?de from a meth fact that above this temperature, the crystal structure ane-ethane vapor stream provided at 600 p.s.i.a., the of zeolite A will be disrupted or damaged with conse size, thereby fundamentally changing its adsorptive char mole fraction ethane and the remainder methane. The mixture is to be passed through a bed of sodium zeolite acteristics. The present adsorption process is most ef ?ciently performed if T1, the adsorption temperature, is 35 A at a temperature of 218° C. (425° F.). The hydrogen sul?de-loaded‘ zeolite A bed is to be regenerated at the less than 616° K. but higher than 283° K. This is for same temperature under a vacuum pressure su?icient to the reason that above such range, the hydrocarbon con obtain a loading of 1.5 weight percent adsorbate at the stituents of the vapor feed stream in contact with zeolite end of the regeneration stroke. ' A will tend to isomerize, crack, aromatize and polymer The potential capacity of the bed to adsorb hydrogen ize, all of which will clog the pores and cause loss of 40 sul?de at the bed inlet section may be determined as capacity of zeolite A molecular sieve. Also, to employ follows: Since the partial pressure of hydrogen sul?de at adsorption temperatures below about 283° K., a refrig the inlet end will be 21 p.s.i.a., T2 will be 221° K., as erating system would be required which increases the extrapolated from the previously referenced vapor pres complexity and expense of operation. Also, for maxi sure table. Accordingly Tz/Tl will be 221/4911 or 0.45. mum e?‘iciency, T2 the dew point of one of the sulfur This temperature ratio will provide a loading of 7.0 compounds of the ?uid mixture is preferably below 499° weight percent hydrogen sul?de on the zeolite A ad K. corresponding to the critical temperature of ethyl sorbent as determined by a reading of the FIG. 1 graph. mercaptan. This is to more effectively utilize the adsorp The potential capacity of the adsorbent bed inlet end for tive capacity of zeolite A. The present invention also contemplates a process for 50 methane and ethane may be ‘determined in a similar man ner. That is, T2 for ethane is 193° K., so that T2/T1 continuously separating hydrogen sul?de from a vapor will be 0.39. Refer-ring now to FIG. 2, which is a'plot mixture containing at least one member of the group of the weight percent of normal saturated aliphatic hy drocarbons adsorbed versus the temperature ratio T2/T1, containing less than nine carbon atoms per molecule, hydrogen and carbon dioxide. 'Ihis continuous process 55 this corresponds to a potential loading of only about 0.6 weight percent ethane. For methane, T2 is 186° K., so includes two steps, an adsorption stroke and a regenera that T2/T1 will be 0.38. Referring again to FIG. 2, tion stroke. The adsorption stroke is the same as the consisting of normal ‘saturated aliphatic hydrocarbons previously described adsorption where the temperature the potential loading is only about 0.5 weight percent ratio T2/T1 is between 0.31 and 1.0, and the broad range for T1 is less than 873° K. In the regeneration stroke, at least part of the adsorbed hydrogen sul?de is removed by subjecting the zeolite A adsorbent to conditions such that the temperature ratio T2/ T1 at the end of the regen methane. The adsorption stroke can be terminated when the amount of hydrogen sul?de reaches the maximum tolerable concentration. _ ' - On regeneration, the end loading is to be 1.5% ad sorbate, which corresponds to a T2/T1 value of 0.35. If a pressure swing cycle is to be employed, T1 is ?xed eration stroke with respect to at least one of the ad sorbed sulfur compounds, is less than the temperature 65 at 491° K. so that the dew point T2 must change and be equal to 174° K. or —99° C. This corresponds to a hy ratio at the end of the adsorption stroke. Also, the dif drogen sul?de vapor pressure of 50 mm. Hg, which should ference in total adsorbate loading between the ends of be the desorption pressure of the cycle. Thus, regenera the adsorption and regeneration stroke is at least 0.1 tion is accomplished by maintaining a constant adsorption weight percent for increased e?iciency of the overall con tinuous process. A lower differential adsorbate loading 70 bed temperature but drawing a vacuum on the system. would entail prohibitively large adsorber units. During the regeneration stroke, T1 is the regeneration tempera Example II It is desired to remove hydrogen sul?de from a ture and is less than 873° K. for the broad range, and methane-hydrogen stream provided at 200 p.s.i.a., the T2 is the temperature at which the hydrogen sul?de has 75 feed mixture comprising 0.00005 mole fraction HZS, 0.12 3,078,640 11' 12 2. A process for separating hydrogen sul?de from a vapor mixture containing hydrogen sul?de and at least one member of the group consisting of hydrogen, carbon dioxide and normal saturated aliphatic hydrocarbons con taining less than nine carbon atoms per molecule which comprises contacting said vapor mixture with a bed of at least partially dehydrated crystalline zeolite A adsorbent mole vfraction methane, 0.0005 mole fraction propane, the remainder being hydrogen. This mixture is to be passed through a bed of sodium zeolite A at 25° C. The hydrogen sul?de~loaded zeolite A bed is to be regener ated at elevated temperature using the feed mixture as a purge gas to obtain a loading of 0.5 weight percent at the end of the stroke. material, said material being capable of adsorbing mole The potential capacity of the bed to adsorb hydrogen clues that have a maximum dimension of the minimum sul?de at the bed inlet section may be determined as follows: Since the partial pressure of hydrogen sul?de 10 projected cross-section up to about 5.5 Angstroms, and thereafter discharging the hydrogen-sul?de-depleted va~ at the inlet end will be 0.01 p.s.i.a., T2 will be 134° K., por stream from said bed. as determined from the previously referenced vapor pres 3. A process ‘for separating hydrogen sul?de from a sure table. Accordingly, T2/T1 will be 134/298 or 0.45. vapor mixture containing hydrogen sul?de and at least This temperature ratio will provide a loading of 7.0 weight percent hydrogen sul?de on the zeolite A ad 15 one member of the group consisting of hydrogen, carbon dioxide, methane and ethane and at least one member sorbent as determined by a reading of the FIG. 1 graph. of the group consisting of normal saturated aliphatic The potential capacity of the adsorbent bed for methane, hydrocarbons containing more than two but less than propane and hydrogen may be determined in a similar nine carbon atoms per molecules and molecules with a manner. That is, T2 for propane is 152° K. so that T2/T1 will be 0.51. Referring again to FIG. 2, this corresponds 20 maximum dimension of the minimum projected cross section larger than about 5.5 Angstroms which comprises to a potential loading of about 4.0 weight percent propane. contacting said vapor mixture with a bed of at (least par However, as previously discussed, zeolite A will adsorb tially dehydrated crystalline zeolite A adsorbent material hydrogen sul?de to the substantial exclusion of propane ‘having a pore size of at least about 4 Angstroms, and due to the former’s greater polarity. For methane, T2 is 116° K. so that T2/T1 will be 0.39. Referring to FIG. 25 thereafter discharging the hydrogen sul?de-depleted va por stream from the bed. 2, the potential loading is only about 0.6 weight percent 4. A process for separating hydrogen sul?de from a methane. vapor mixture containing hydrogen sul?de and molecules With regard to hydrogen, it has been determined ex with a maximum dimension of the minimum projected perimentally that when T2/ T1 is 0.07, the hydrogen load ing on zeolite A is zero. Thus, in the present example, 30 cross-section larger than about 5.5 Angstroms and at least one member of the group consisting of hydrogen, T2=32° K. and T2/ T1 is 0.11 so that the hydrogen load carbon dioxide and normal saturated aliphatic hydro carbons containing less than‘ nine carbon atoms per mole cule which comprises contacting said vapor mixture with percent adsor-hate, which corresponds to a T2/T1 value of a bed of at least partially dehydrated crystalline zeolite A CO Or 0.31. If a temperature swing-purge cycle is to be em adsorbent material, said material being capable of adsorb ployed, T2 is 134'’ K. as previously determined and T, ing molecules that have a maximum dimension of the must be 433° K. or 160° C. Thus, regeneration is minimum projected cross-section up to about 5.5 Ang achieved by heating either the adsorbent and/or the stroms and thereafter discharging the hydrogen-sul?de inlet gas mixture su?iciently to provide a regeneration temperature of 160° C. while using the inlet gas for 40 depleted vapor stream from said bed. ing is negligible. On regeneration, the end loading is to be 0.5 weight 5. A process as described in claim 1 wherein the vapor purging. mixture contains hydrogen sul?de and methane. Although the preferred embodiments have been de-. scribed in detail, it is contemplated that modi?cations 6. A process as described in claim 1 wherein the vapor mixture contains hydrogen sul?de and ethane. of the process may be made and that some features may 7. A process is described in claim 2 wherein the vapor mixture contains hydrogen sul?de and normal saturated be employed without others, all within the spirit and scope of the invention as set forth herein. aliphatic hydrocarbons containing less than nine carbon When the inlet vapor mixture contains C02, the poten tial capacity of the zeolite A adsorbent for this constituent is similarly determined by reference to the vapor pres atoms per molecule. References Cited in the ?le of this patent sure tables ‘and FIG. 3. This is a continuation-in-part application of copending application Serial No. 400,385, ?led December 24, 1953, UNITED STATES PATENTS in the name of R. M. Milton, now abandoned. What is claimed is: m U! 1. A process for separating hydrogen sul?de from a vapor mixture containing hydrogen sul?de and at least one member of the group consisting of hydrogen, carbon hydrated crystalline zeolite A adsorbent material having a pore size of at least about 4 Angstroms, and thereafter discharging the hydrogen sul?de-depleted vapor stream from the bed. Christensen et al. _____ _._ Dec. 31, 1957 OTHER REFERENCES “Molecular-Sieve Action of Solids” by R. M. Barrer, Quarterly Reviews, London Chemical Society, vol. III, 1949, pages 293 to 320. dioxide, methane and ethane which comprises contacting said v-apor mixture with a bed of at least partially de 2,818,449 60 “Examine These Ways To Use Selective Adsorption,” Petroleum Re?ner, vol. 36, No. 7, July 1957, pages l36~ 140.