US3620969A

US3620969A - Desulfurization by selective adsorption with a crystalline zeolitic molecular sieve

Info

Publication number: US3620969A
Application number: US866484A
Authority: US
Inventors: Philip Hain Turnock; Max Nai Yuen Lee; Krishan Dayal Manchanda
Original assignee: Union Carbide Corp
Current assignee: Katalistiks International Inc; Honeywell UOP LLC
Priority date: 1969-10-15
Filing date: 1969-10-15
Publication date: 1971-11-16
Anticipated expiration: 1988-11-16
Also published as: GB1326630A; DE2050323B2; JPS4918601B1; DE2050323A1; FR2064350B1; FR2064350A1; NL7015074A; CA945538A

Abstract

Sulfur compounds are removed from liquid hydrocarbon streams by selective adsorption on zeolitic molecular sieves and periodic desorption therefrom using a hot purge gas having a high water content. Regeneration of the adsorbent is carried out only to the degree that less than about 5 and more than about 2 weight percent water remains adsorbed on the zeolite.

Description

United States Patent [72] Inventors Philip Ilain Turnock Katonah; Max Nai Yuen Lee, Yorktown Heights; Krlshan Dayal Manchanda, Scarsdale, all of N.Y.

[21] Appl. No. 866,484

[22] Filed Oct. 15, 1969 [45] Patented Nov. 16, 1971 [73] Assignee Union Carbide Corporation New York, N.Y.

[54] DESULFURIZATION BY SELECTIVE ADSORPTION WITH A CRYSTALLINE ZEOLITIC MOLECULAR SIEVE 3 Claims, 4 Drawing Figs.

[52] U.S.Cl 208/245,

[51] Int.Cl ....C10g25/04 [50] Field of Search 208/208,

Pl/RGE GAS SWEETENED PRODUCT Primary Examiner-Delbert E. Gantz Assistant ExaminerG. J. Crasanakis Attorneys-Thomas l. OBrien and Richard G. Miller ABSTRACT: Sulfur compounds are removed from liquid hydrocarbon streams by selective adsorption on zeolitic molecular sieves and periodic desorption therefrom using a hot purge gas having a high water content. Regeneration of the adsorbent is carried out only to the degree that less than about 5 and more than about 2 weight percent water remains adsorbed on the zeolite.

PATENTEUNHV 16 I971 8,620,969

SHEET 2 OF 2 PHIL IP H. TURNOC KRISHAN 0. MANCHANDA MAX IV. V. LEE

BY flmlm 2/ nd/m ATTORNEY DESULFURIZATION BY SELECTIVE ABSORPTION WITH A CRYSTALLINE ZEOLITIC MOLECULAR SIEVE This application relates in general to an improved process for sweetening hydrocarbon liquids and more particularly to a process for removing sulfur compounds from natural gasoline which employs as the selective absorbent for the sulfur-containing impurities a zeolitic molecular sieve dehydrated only to the extent that it contains a residual loading of water of from about 2 to about 5 weight percent.

The removal of sulfur compounds, particularly hydrogen sulfide and the alkyl mercaptans from hydrocarbon streams is desirable for many reasons, depending in part upon the intended use of the final sweetened product. Since a very large percentage of the lighter hydrocarbons are ultimately used as fuel per se, the presence of sulfur compounds is objectionable because of the unpleasant odor imparted and the air pollution resulting from the combustion. When used as fuels for internal-combustion engines, the sulfur compounds are deleterious to the effectiveness of known octane improvers such as tetraethyllead. The heavier hydrocarbons are largely subjected to hydrocarbon conversion processes in which the conversion catalysts are, as a rule, highly susceptible to poisoning by sulfur compounds.

Several methods for sweetening hydrocarbon streams have been proposed and utilized in the past, including both chemical and physical techniques. The chemical processes have involved purely chemical reactions such as scrubbing with monoethanolamine or countercurrent extraction using a hot potassium carbonate solution, and chemisorption methods in which iron oxide sponge perferentially collects the sulfur compound on its surface.

Selective physical absorption of sulfur impurities on crystalline zeolitic molecular sieves is a more recently proposed technique and is now perhaps the most widely used method. Both liquid phase and vapor phase processes have been developed, with liquid phase operation being preferred for use with olefinic streams and low-boiling paraffinic hydrocarbon streams.

A liquid phase hydrocarbon sweetening process comprises passing a sulfur-containing hydrocarbon fraction such as a stabilized or a full range natural gasoline through a bed of a molecular sieve adsorbent having a pore size large enough to adsorb the sulfur impurities, recovering the nonadsorbed effluent hydrocarbon until a desired degree of loading of the adsorbent with sulfur-containing impurities is obtained, and thereafter purging the adsorbent mass of hydrocarbon and regenerating the adsorbent by desorbing the sulfur-containing compounds therefrom.

The adsorbent regenerating operation is conventionally a thermal swing or combined thermal and pressure swing-type in which the heat input is supplied by a hot gas relatively inert toward the hydrocarbons, the molecular sieve adsorbent and the sulfur-containing adsorbate. Natural gas is ideally suited for use in purging and adsorbent regeneration, provided that it can subsequently be utilized in situ as fuel wherein it constitutes an economic balance against its relatively high cost. Frequently, however, the sweetening operation requires more natural gas for thermal-swing regeneration than can be advantageously consumed as fuel, and in effect, this constitutes an inadequacy of regeneration gas. The result is a serious impediment to successful design and operation of sweetening processes, especially when desulfurization is carried out at a location remote from the refinery, as is frequently the case.

One potential system for generation of an inert atmosphere combines fuel and air in nearly stoichiometric proportions to obtain a combustion product devoid of oxygen. The combustion products, consisting primarily of carbon dioxide and water in nitrogen, amount to about nine times the volume of natural gas consumed. The generated gases ordinarily undergo subsequent treatments to reduce the levels of carbon dioxide and water. Thus, the cost of any such inert gas supply is generally influenced considerably by the required purity and moisture level in the gas. The investment in equipment to supply low-pressure inert gas at a -40 F. dew point can readily amount to twice that required for generation of a comparable gas supply at a F. dew point.

In accordance with the present invention it has been surprisingly found that the presence of relatively large amounts of residual adsorbed water on large pore zeolitic molecular sieve adsorbents does not diminish their selectivity nor unduly diminish their capacity for adsorbing sulfur-containing impurities from liquid hydrocarbon streams. ln marked contrast to this finding, it is well known that in vapor phase-sweetening processes, the presence of even relatively small amounts of water on the molecular sieve adsorbent has such an adverse effect on its selectivity and capacity as to render the operation commercially unfeasible. For example, the dynamic equilibrium capacity (450 p.s.i.g., 86 F.) of fully dehydrated sodium zeolite X (i.e., about 0.1 weight-percent residual H O) for H 8 from a natural gas containing 540 p.p.m. (volume) H S is es sentially the same as when the H 8 concentration in liquid bu tane is about 1,050 p.p.m. (volume). With gas phase adsorption under the same conditions of temperature and pressure, the presence of a residual H O loading on the sodium zeolite X of only 1.8 weight-percent causes a decrease in the equilibriurn adsorption capacity for H 5 of 48 percent. In contrast, sodium zeolite X containing as much as 4.1 weight-percent residual water loading has been found to undergo a dynamic equilibrium capacity decrease for H 8 from a liquid phase system of only 30 percent with comparable H S concentrations. The present discovery thus makes it now possible to carry out a liquid hydrocarbon-sweetening process in which the thermal-swing desorption of the sulfur-containing compounds from the molecular sieve adsorbent is accomplished using a purge gas having a high water content. It is even possible to utilize steam as the purge gas and this is a preferred embodiment of the regeneration procedure of the present invention.

The hydrocarbon stocks suitably treated in accordance with the present invention are not critical with respect to origin, constituent molecular species or relative proportions of the molecular species within the feedstock. Thus, the stocks can be hydrocarbonaceous product resulting from the destructive hydrogenation of coal or they can be obtained from deposits of natural gas or petroleum. Sulfur-containing condensates from natural gas, i.e., the LPG compositions rich in propane and butanes are well suited to the present process as are natural gasolines and relatively light petroleum fractions boiling between about 44 to about F. which are mostly comprised of C to C hydrocarbons. Moreover, liquid or liquefiable olefin or olefin-containing streams, such as those used in alkylation processes, containing propylene, butylene, amylene and the like are also suitably employed.

The sulfur-containing impurity present in the hydrocarbon feedstocks comprises at least one but ordinarily a mixture of two or more of hydrogen sulfide, the mercaptans such as ethyl mercaptan, n-propyl mercaptan, isopropyl mercaptan, n-butyl mercaptan, isobutyl mercaptan, t-butyl mercaptan, and the isomeric forms of amyl and hexyl mercaptan, the heterocyclic sulfur compounds such as thiophene and 1,2-dithiol, the aromatic mercaptans exemplified by phenyl mercaptan, organic sulfides and disulfides generally and carbonyl sulfide.

The adsorbent materials employed in the present process are the natural or synthetically prepared crystalline zeolitic aluminosilicates commonly referred to as molecular sieves or zeolitic molecular sieves. Unlike most adsorbents which have a variety of pores of different dimensions, molecular sieve adsorbents are characterized, in part, by having pores of uniform dimension, and thus the particular species selected must have pore diameters large enough to permit passage therethrough of the sulfur compounds present in the hydrocarbon feedstock. Molecular sieves having pores with an apparent minimum dimension of at least 3,8 angstrom units have been found satisfactory when the sulfur compound impurity which is to be adsorbed is hydrogen sulfide. For normal mercaptans having less than seven carbon atoms, the apparent pore size should be at least about 4.6 angstrom units. The sulfur compounds of larger molecular dimensions such as isopropyl mercaptan, isobutyl mercaptan, t-butyl mercaptan, the isomeric form of amyl and hexyl mercaptan, and the heterocyclic sulfur compounds exemplified by thiophene as well as the aromatic mercaptans exemplified by phenyl mercaptan require the use of a zeolitic molecular sieve having apparent pore openings of at least about 6 angstrom units. Since it is uncommon that a sour hydrocarbon feedstock contains no sulfur compounds of large molecular size, it is preferred that the molecular sieve employed have an apparent pore diameter of at least 8 angstroms and more preferably at least 10 angstroms.

The term apparent pore size as used herein may be defined as the maximum critical dimension of the molecular species which is adsorbed by the zeolitic molecular sieve in question under normal conditions. The apparent pore size will always be larger than the effective pore diameter, which may be defined as the free diameter of the appropriate silicate ring in the zeolite structure.

Among the naturally occurring zeolitic molecular sieves suitable for use in the present invention include mordenite and chabazite both having an apparent pore size of about 4 angstrom units, erionite having an apparent pore size of about angstrom units, and faujasite having a pore size of about angstroms. The natural materials are adequately described in the chemical art. The preferred synthetic crystalline zeolitic molecular sieves include zeolites X, Y, L and Q, and large pore mordenites. Zeolite L has an apparent pore size of about 10 angstroms, and is described and claimed in U.S. Pat. No. 3,216,789. Zeolite X has an apparent pore size of about 10 angstroms, and is described and claimed in U.S. Pat. No. 2,882,224, having issued Apr. l4, 1959 to R. M. Milton. Zeolite Y has an apparent pore size of about 10 angstroms, and is described and claimed in U.S. Pat. No. 3,130,007.

Zeolite Q. is described in pending U.S. application Ser. No.

655,318, filed July 24, 1967. Zeolite L is described in U.S. Pat. No. 3,216,789, and the preparation of a large pore synthetic mordenite is disclosed in U.S. Pat. No. 3,436,174, issued Apr. 1, 1969 to L. B. Sand.

The pore size of the zeolitic molecular sieves may be varied by employing different metal cation. For example, sodium zeolite A (U.S. Pat. No. 2,882,243) has an apparent pore size of about 4 angstrom units, whereas calcium zeolite A has an apparent pore size of about 5 angstrom units.

The zeolites occur as agglomerates of fine crystals or are synthesized as fine powders and are preferably tableted or pelletized for large scale adsorption uses. Pelletizing methods are known which are very satisfactory because the sorptive character of the zeolite, both with regard to selectivity and capacity, remains essentially unchanged. Many suitable inert binder materials or compositions are well known in the art including clays, refractory metal oxides and alkali metal silicates, if it is desired to utilize the adsorbents in agglomerated form. In general, the individual molecular sieve crystals are quite small (of the order of 10 microns) and hence in fixed bed operation, at least, it is advantageous to agglomerate the crystals into beads, pellets, extrudate forms, etc., either with or without added binder material.

The method by which the sour hydrocarbon stream and the molecular sieve adsorbent are brought into contact in order to selectively adsorb the sulfur compounds therefrom is not a critical part of this invention. For example, the operation can involve one or more fixed adsorbent beds, moving beds, slurry beds or combinations thereof and the hydrocarbon charge material can flow in direct, countercurrent or cocurrent contact with the molecular sieve. The use of fixed adsorbent beds is most frequently used in hydrocarbon stream sweetening, and incorporates the following processing steps in conventional sequence: (a) adsorption of undesirable sulfur impurities from the liquid hydrocarbon stream; (b) displacement of liquid from the bed voids; (c) volatilization of occluded liquid; (d) regeneration of the molecular sieve; (e) cooling of the adsorbent; and-(f) filling the bed for further feed purification.

With specific reference to a typical method of operation in P10. 1, two beds, 10 and 11 of crystalline zeolitic molecular sieve material are provided and piped in parallel flow relation so that when one bed is on the adsorption stroke, the other bed is being regenerated by purging and cooldown. In this manner, a continuous supply of sulfur compound-depleted hydrocarbon liquid is available for consumption. If a continuous supply is not required, it may be preferable to employ a single bed of zeolitic molecular sieve adsorbent, and provide a product liquid supply during the intermittent periods when such bed is on adsorption stroke.

The sour sulfur compound-containing liquid hydrocarbon feed stream is introduced through conduit 12, preferably at ambient temperature although there is no sharply defined critical region in this respect. Choice of the optimum temperature depends on an economic balance between savings in zeolitic molecular sieve material by virtue of higher adsorptive capacities at lower temperatures, and the cost of heat exchangers to obtain the lower temperature. Viscosity may also be a limitation on heavy naphtha streams. With regard to feed pres sure, the only limitation in this respect is that the pressure be sufficiently high to keep the feed in the liquid phase throughout the adsorber bed to avoid internal flashing with consequent poor contact with the molecular sieve and attrition of particles.

It has been found that the adsorption step may be efficiently performed with feed liquid superficial linear velocities of 0.1 to 20 feet per minute, and preferably between 1 and 10 feet per minute. The reasons for these criticalities are as follows: at low superficial linear velocities, a thin film of liquid exists on the exterior surface of each zeolitic molecular sieve particle, primarily due to the viscosity of the hydrocarbon feed liquid. The sulfur compound must pass through this film for flow through the pores and into the inner cage of the crystal structure for adsorption thereby, and such passage is resisted by the film so as to decrease the adsorption rate. As the feed liquid velocity increases the thickness of the liquid film decreases, thereby reducing the external film resistance and increasing the rate of adsorption. Finally, as the superficial linear velocity is further increased, the liquid film is substantially eliminated and the efficiency of the adsorption step becomes primarily dependent on contract time between the sulfur compound containing feed stream and the zeolitic molecular sieve. That is, sufficient time must be provided for the sulfur compound to transfer from the feed stream to the molecular sieve, and higher feed liquid velocities will of course reduce such contact time. Still another characteristic of higher superficial linear velocities is increased pumping costs. It has been found that a superficial linear velocity of below 0.1 feet per minute produces an excessively high external film resistance, while a velocity of over 20 feet per minute does not permit sufficient contact time for high adsorptive efficiency. Within this broad range, the adverse effect of the external liquid film is essentially eliminated at a velocity above 1 foot per minute. Also, the required bed length becomes unduly long when the superficial linear velocity is above 10 feet per minute, due to the reduced contact time and lower adsorptive efficiency.

The process will efficiently handle feed streams containing minute traces of sulfur on the order of 5X10""" weight-per cent up to those containing 2 weight-percent sulfur compounds. This process is particularly advantageous in the sulfur trace concentrations because of the relatively high sulfur loadings attainable on crystalline zeolitic molecular sieves.

The upper limit of 2 weight-percent sulfur compound con centration in the feed liquid is based on the fact that liquid phase adsorption becomes impractical when the sulfur concentration exceeds this level since the duration of the adsorp tion step becomes relatively short as compared with the necessary duration of the desorption and cooldown steps. Stated in another way, a prohibitively large adsorption bed would be required to obtain an adsorption step of specified duration if the sulfur concentration exceeds 2 weight-percent.

The sulfur-containing liquid hydrocarbon feed stream is directed from conduit 12 to communicating conduit 13 joining at its opposite end with the inlet and upper end of first zeolitic molecular sieve bed 10. Conduit 13 also contains

flow control valves

14 and 15 arranged in a series relationship.

Discharge conduit 16 communicates with the lower end of bed 10, and contains flow control valves 17 and 18 arranged in series relationship, and valve 17 is at least partially closed during the initial part of the adsorption stroke, so that the bed may be filled with liquid hydrocarbon feed. Control by means of

valves

14 and 15 can be exercised over the rate at which the liquid is introduced during the filling step to avoid channeling of the liquid downwardly through the bed as this may result in premature vaporization in the lower regions of the bed with resultant pressure buildup and violent movement of the bed. Flow distributing devices as are well understood by those skilled in the art, may be employed to prevent such channel- When the zeolitic molecular sieve bed 10 is filled with liquid, the withdrawal of purified liquid hydrocarbon product from the lower end of the bed is begun through conduit 16 and control valves 17 and 18 therein. The sweetened liquid hydrocarbon product stream is discharged from the system through communicating conduit 19. Simultaneously, the sour liquid hydrocarbon feed stream is introduced through valves 14 and at a rate sufficient to maintain bed 10 completely filled with liquid. As the adsorption step or stroke is continued, the sulfur compounds are selectively adsorbed by the molecular sieve in a downwardly advancing zone, in which zone the previously adsorbed hydrocarbon is being displaced by the sulfur compounds. The liquid removal step is downward to afford superior drainage, and thus improve the efficiency of the adsorption step.

The adsorption step may be continued until the appearance of sulfur compounds in the product indicates that the capacity of the molecular sieve has been attained. At this point, however, the free spaces in the bed not occupied by molecular sieve material are filled with sour liquid hydrocarbon which must either be sent to a fresh molecular sieve bed or discharged. Preferably, valve 15 through which the sour feed enters bed 10 is closed when the latter has only sufficient remaining adsorptive capacity to remove the sulfur compound contained in the liquid remaining in the free spaces or interstices of the bed.

At this point, valves 17 and 18 are closed and the sour liquid hydrocarbon feed stream is diverted from conduit 12 through communicating conduit 20 to second zeolitic molecular sieve bed 11 which has previously been desorbed and recooled. The depressurization or draining of first bed 10 should be carried on gradually to prevent excessive flashing, movement of the pellets and attrition. A 15-minute period for the blowdown step has been found satisfactory. 1n the desorption step, a hot purge gas is supplied to conduit 21 at a temperature preferably between 450 and 700 F., the purge gas being nonoxidizing with respect to either sulfur or the hydrocarbons present, and containing moisture to a dew point level of +10 to 160 F. Conventional purge gasses such as methane, hydrogen, nitrogen and carbon dioxide in combination with adequate water vapor can be employed. Economic benefit is achieved by utilizing as the purge gases the oxygen-free combustion product gases of air and methane which formerly were not used, due to their high moisture content, without a drying operation. A typical embodiment of the present process utilizing this type of purge gas is incorporated into the flow diagram of FIG. 1. Into combustion chamber 38 there is charged stoichiometric proportions of natural gas and air. Upon burning, the resultant composition is very high in moisture content, but is essentially free of oxygen and contains CO, and nitrogen as the other principal ingredients. Since merely cooling the combustion products will result in the condensing out ofa substantial quantity of water, the gas mixture from combustion chamber 38 are passed through heat exchanger 42 and into condenser 44. Liquid water is withdrawn from condenser 44 which is operated at a temperature sufficiently low that the purge gas stream emerging therefrom contains moisture at a dew point level commensurate with the temperature and pressure conditions employed in the adsorbent bed during the regeneration. The range of dew point levels for 15-100 p.s.i.a. regeneration gas is described by the following equations: 1.b. Maximum allowable dew point temperature, F.

0.4(Regeneration Temperature, F. )-l20 2. Normal operating limit on dew point temperature, "F.

0.4(Regeneration Temperature, F. )140 3. a. Minimum suggested dew point temperature, "F.

0.3(Regeneration Temperature, F.)-140 The above equations establish the following limits for dew point temperatures of the purge gas at normal regeneration temperatures ranging from 500700 F.:

a. to leave about 2 weight-percent H O on the molecular sieve.

b. to leave about 5 weightpercent H O on the molecular sieve.

The purge gas is then reheated in heat exchanger 42 and thereafter through conduit 21 and control valve 22 therein to branch conduit 23 containing valve 24. Conduit 23 joins inlet conduit 13 between

valves

14 and 15, and the hot purge gas is introduced therethrough to the upper end of first zeolitic molecular sieve bed 10 for downward flow and removal of the adsorbed sulfur compound. The cooled and sulfur compound laden purge gas is discharged from the lower end of first bed 10 through conduit 16 containing valve 17, and directed through branch conduit 25 containing valve 26 therein to discharge conduit 27 for use as desired. Since the purge gas has been selected with respect to water content in accordance with the foregoing equations, cessation of the desorption step at a time when a desired amount of the sulfur-containing compound has been removed from the bed will result in a regenerated adsorbent bed having about 2 to about 5 weightpercent adsorbed water.

In a preferred embodiment, the liquid hydrocarbon held in the interstices of the first zeolitic molecular sieve bed 10 is drained therefrom at the end of the adsorption step and before the previously described desorption step is initiated. This permits recovery of the interstitial liquid and improves the efficiency of the cycle. Such drainage may be effected in any of several wellknown ways following the closing of liquid hydrocarbon feed inlet valve 14. At this point a cool gas purge at temperature below about 250 F. can be used before the desorption step, thereby effecting additional removal of the hydrocarbon holdup.

At the end of the previously described desorption step, the reactivated first bed 10 is recooled by a controlled introduction of sour liquid feed through conduit 13 into the upper end of bed 10 for downward flow therethrough. The precooling step is conducted downwardly from the inlet to the discharge end to prevent excessive temperature rises and flashing since the downwardly advancing liquid front recools the rising convective currents of generated vapor. To achieve this flow, valves 14, l5, l7 and 26 are opened, and valve 24 is closed. The amount of vaporized feed can be condensed and returned to the feed, or if very small in quantity, can be passed to waste or flared. lt has been found that about 20-35 gallons of coolant are required per pounds of molecular sieve to be cooled. The coolant is preferably fed at a rate of l-4 gallons per minute per square foot of bed cross section, the maximum rate being 8 gallons per square foot per minute. Cooling is continued until the bed is full of liquid and essentially at the temperature of the adsorption stroke. Valve 26 is then closed and valve 18 opened, and the first bed 10 is placed back on the adsorption stroke.

It should be noted that the second bed ll of zeoliticmolecular sieve material is operated in a manner analogous to that of first bed 10 so that during the adsorption step, sour feed is introduced through conduit 12 to communicating conduit having flow control valves 31 and 32 arranged in a series relationship at the upper end of bed 11. The sweetened hydrocarbon liquid is withdrawn from the lower end of second bed 11 through conduit 33 having

flow control valves

34 and 35 therein arranged in series. During the desorption stroke, the hot purge gas is introduced through valve 36 in branch conduit 23 communicating with conduit 20, the sulfur compound-laden purge gas discharged from the lower end of second bed 11 is removed from the system through valve 37 in conduit 25.

In accordance with the present invention, it. is not necessary that a purge gas of limited water content be employed such as in the foregoing description. Since we have now found that molecular sieve adsorbents containing as much as 5 weightpercent residual adsorbed water are feasible for use in liquid hydrocarbon sweetening processes, and moreover since dehydration of a molecular sieve adsorbent to a water content of from 2 to 5 weight-percent is easily and economically accomplished, we have further found that a steam purge can be used in a preferred embodiment of the present invention provided only that an adsorbent drying step is included in the regeneration procedure.

The temperature and pressure of a steam purge steam are not narrowly critical factors, provided of course, that the usual precautions are taken to prevent zeolite agglomerate shattering and zeolite hydrolysis at elevated temperatures. Agglomerate shattering results from severe nonuniform thermal stress induced when hot activated agglomerates are contacted with liquid water to produce high-adsorption exotherms. Hydrolytic stability varies with the silica/alumina ratio of the zeolites and with crystal lattice type, and is at least in part concerned with metal cation replacement or removal from the zeolite. Thus, the steam purge can vary from very wet steam to superheated steam to maintaining a purge temperature of about 212 up to about 400 F.

The sweetening process using a steam purge step is in all respects the same as exemplified hereinbefore except that at the termination of the desorption of the sulfur compounds, the adsorbent will contain substantially more than the permissible 5 weight-percent adsorbed water, and an adsorbent drying step must therefore be included. For this purpose an inert gas stream such as nitrogen, hydrogen, natural gas, and the like, which has a water dew point not greater than a value determined by the expression 0.4T-140 in which T is the temperature of the purge gas stream in degrees Fahrenheit, can be brought into contact with the adsorbent crystals in an amount, for a time period, and at a temperature such that dehydration of the zeolite adsorbent to a water content .of from 2 to 5 weight-percent is achieved. v

In FIGS. 2, 3 and 4 of the drawings are three typical drying systems which can be used following a steam purge desorption. Other systems or modiffrions of these three exemplifications will be obvious to those skilled in the art.

In accordance with the schematic diagram of FIG. 2, the inert dry purge gas such as methane or nitrogen is introduced into a loop at line 50 and is impelled by blower Sll through heater 52 from which it emerges at a temperature of about 550 The hot gas thereafter enters cocurrently the molecular sieve bed 53 which contains the wet steam desorbed zeolite adsorbent. After the drying gas carrying water desorbed from the zeolite emerges from the effluent end of the bed it is passed through a water-cooled condenser 54, operated at a temperature sufficiently low to condense out water and produce a gas stream having a dew point temperature of less than F. which is thereafter recyc ed along with any makeup gas which may be required.

The system of FIG. 2 is modified in FIG. 3 by the addition of a bed 56 containing any desired desiccant through which the drying gas emerging from condenser 54 is passed prior to being recycled. This modification of the system of FIG. 2 permits less rigorous cooling in the condenser 54 with the consequent increase in the dew point of the gas stream emerging therefrom. The desiccant bed 54 can contain an activated molecular sieve adsorbent, or silica gel, calcium chloride or any other available desiccant material. After emerging from the desiccant bed 56 the drying gas is thereafter cycled through blower 51, heater 52 and molecular sieve bed 53 which is being dried. The modification of FIG. 3 substantially decreases the drying cycle time because of the more rapid transfer of moisture from the molecular sieve in bed 53 to the drier gas stream passing therethrough.

The drying scheme of FIG. 4 utilizes butane or other readily condensable regeneration media which is impelled by pump 57 into vaporizer superheater 58 before entering cocurrently wet molecular sieve bed 53. The water-laden drying gas emerging from bed 53 is cooled to condense the butane in condenser 54 and thereafter the liquid butane is separated from the water it has carried from the bed 53 and recycled.

From the foregoing, other alternate procedures and modifications of the various steps of the process of this invention will be obvious to those skilled in the art. Such obvious variations are considered to be within the proper scope of this invention.

What is claimed is:

1. Process which comprises contacting in the liquid phase a sour hydrocarbon feedstock containing sulfur compounds in an amount of not greater than 2 weight percent with a crystalline zeolitic molecular sieve to adsorb selectively and isolate said sulfur compounds from said hydrocarbon feedstock, thereafter desorbing sulfur compounds by contacting the molecular sieve with a purge gas stream containing sufficient water vapor to load the molecular sieve with from 2 to 5 weight-percent water at equilibrium, and thereafter contacting said water-containing molecular sieve with sour hydrocarbon feedstock in the liquid phase.

2. Process according to claim 1 wherein the temperature of the purge gas is between about 450 and about 700 F., the

pressure of the purge gas in contact with the zeolitic molecular sieve adsorbent is from about 15 to about p.s.i.a., and the dew point of the purge gas stream is between 10 and 160 F.

3. Process according to claim 2 wherein the dew point of the purge gas stream is between 60 F. and F.

Claims

2. Process according to claim 1 wherein the temperature of the purge gas is between about 450* and about 700* F., the pressure of the purge gas in contact with the zeolitic molecular sieve adsorbent is from about 15 to about 100 p.s.i.a., and the dew point of the purge gas stream is between 10* and 160* F.
3. Process according to claim 2 wherein the dew point of the purge gas stream is between 60* F. and 140* F.