WO1985001058A1

WO1985001058A1 - Process and apparatus associated with gas purification

Info

Publication number: WO1985001058A1
Application number: PCT/US1984/001372
Authority: WO
Inventors: Robert C. Butler; John Edward Thelen
Original assignee: Pall Corporation
Priority date: 1983-08-25
Filing date: 1984-08-23
Publication date: 1985-03-14
Also published as: IL72756A0; EP0153406A1; AU3318484A; ES8505829A1; IT1178505B; ZA846511B; PT79128A; ES535401A0; IT8422420A0; PT79128B

Abstract

An apparatus and method for dehydration and sweetening of wet, sour natural gas has no atmospheric discharge of acid gas pollutants. An iron sponge (17) of hydrated iron oxide is placed in a closed purge or regeneration loop of a desiccant gas dryer (12) having a desiccant capable of adsorbing both water vapor and hydrogen sulfide. Preferably the desiccant dryer (12) is a gas-fired external heat reactivated adsorbent dryer (12') and the iron sponge (17) is placed after the cooler (15) and separator (16) in the purge loop. The iron sponge (17) preferably has two beds (41, 42), so that each bed may be intermittently regenerated by taking the bed off stream and introducing oxygen from air and sprinkling water on the bed. The hydrogen sulfide is concentrated into about 10% of the process flow.

Description

Process And Apparatus Associated With Gas Purification

Technical Field

This invention relates generally to the field of natural gas conditioning, and specifically to a system for sweetening and dehydrating natural gas without requiring the atmospheric discharge of sulfur dioxide, commonly known in the art as "acid gas."

Background Art

Natural gas conditioning involves mechanical and chemical steps for removing contaminants from natural gas as the natural gas is transmitted from the wellhead to the consumer. Two common contaminants that must be eliminated from natural gas are water vapor and hydrogen sulfide. The presence of water vapor in natural gas can disrupt service to consumers due to the freezing of gas lines or the formation of hydrates or combinations of water with hydrocarbon gases. The water vapor content typically must be reduced to below 5-7 pounds per million standard cubic feet of gas. Hydrogen sulfide is highly toxic and corrosive and is readily detected by the consumer due to its foul odor like rotten eggs. A proviso of nearly all gas purchase and sales contracts stipulates that no gas is to be sold or otherwise delivered that contains greater than one grain of hydrogen sulfide per 100 cubic feet of gas.

In a natural gas system, gas is gathered from several wellheads and at the terminus of the gathering system the gas is brought up to pipeline pressure and then dehydrated and desulfurized. The removal of sulfur compounds such as hydrogen sulfide is commonly known as "sweetening". The most widely used processes for sweetening and dehydration are liquid chemical solvent processes using glycol for dehydration and amine for sweetening. It is reported that some natural gas conditioning plants use a combination of triethylene glycol (TEG) and diethanolamine (DEA) in solution for simultaneous dehydration and desulfurization. The liquid chemical solvent process operates on the principle that the water vapor or hydrogen sulfide is preferentially dissolved into the chemical solvent. The chemical solvent is regenerated, for example by heating, to liberate the dissolved water vapor or hydrogen sulfide. In addition to alkanolamines for hydrogen sulfide removal, alkaline salt solutions such as hot pot, Catacarb, and Benfield are sometimes used. Additionally, there are physical solvent processes such as Sulfinol, Selexol, and Fluor, and also direct conversion processes such a Stretford, Thylox, Takahax, and Ferrox.

In distinction to the solvent processes and direct conversion processes, there are dry bed processes involving the contact of water vapor or hydrogen sulfide with a solid material. The iron sponge process is one well known dry bed process for removing hydrogen sulfide from natural gas. In the iron sponge process hydrogen sulfide and mercaptans are removed by passing sour gas through vessels loosely packed with iron sponge. The sponge consists of wood shavings impregnated with a hydrated form of iron oxide. The wood shavings serve only as an inexpensive carrier for the active iron oxide powder. Hydrogen sulfide is removed by reacting with the iron oxide to form iron sulfide. Typically the life of the sponge is extended by periodically or continuously regenerating the bed with air. The spent sponge reacts with oxygen to reconvert the iron sulfide to iron oxide and elemental sulfur. The process is most suited to sweetening small volumes of gas with low contents of hydrogen sulfide and carbon dioxide, since the iron sponge process will not remove carbon dioxide, and replacement of the bed is more frequent for higher contents of hydrogen sulfide. It should be noted that the iron sponge process does not remove water vapor.

Desiccant dryers and sweeteners for natural gas treatment have been marketed for many years and are in wide use throughout the world. The usual type is composed of multiple desiccant bends, one of which is processing (drying and sweetening, or drying) gas while the others are being regenerated. During regeneration purge gas is passed in counterflow to the direction in which the gas was processed. A single processing cycle for the dryer and sweetener, then, is comprised of an adsorption interval or half cycle and a regeneration interval or half cycle for each bed. The spent bed is typically regenerated by passing heated purge gas through the chamber in order to cause desorption of the adsorbed hyrdogen sulfide or water vapor. The purge gas is typically disposed of in an incinerator which oxidizes the hydrogen sulfide to sulfur dioxide acid gas which is discharged to the atmosphere.

It should be noted that the chemical processes for sweetening natural gas also result in a hydrogen sulfide or acid gas stream which must be disposed of. If more than approximately 5-10 tons per day of sulfur are produced.' by the desulfurization plant, the plant is typically provided with a "Claus" reactor for sulfur recovery. With the addition of catalytic conversion steps, the efficiency of sulfur recovery for the Claus method approaches 99%. The unrecovered sulfur is discharged to the atmosphere as sulfur dioxide acid gas via a tailgas incinerator.

Presently "acid rain" is becoming a severe environmental problem and the acid rain is caused, to some extent, by the discharge of sulfur dioxide acid gas by the natural gas industry.

Disclosure Of Invention

The primary object of the invention is to provide a method and apparatus for drying and sweetening natural gas that does not require the atmospheric emission of sulfur dioxide acid gas.

Another object of the invention is to provide apparatus for drying and sweetening natural gas having a reduced capital cost and a reduced operating cost compared to current technology using amine for hydrogen sulfide removal and glycol for water vapor removal.

Yet another object of the invention is to provide a drying and sweetening plant for natural gas which is economical for small as well as large scale requirements.

In accordance with the invention, an iron sponge of hydrated iron oxide is placed in a closed purge or regeneration loop of a gas fractionator or desiccant dryer having a desiccant capable of adsorbing hydrogen sulfide. Preferably the desiccant dryer is a gas-fired external heat reactivated adsorbent dryer and the iron sponge is placed downstream of the cooler and separator in the purge loop. The iron sponge preferably has two beds, so that each bed may be intermittently regenerated by taking the bed offstream and introducing a specific flow of oxygen and sprinkling a specific flow of water on the bed. It should be noted that since the purge flow is typically less than 10% of the process flow, the iron sponge becomes very economical. Moreover, the moderate temperature and high humidity of the purge flow into the iron sponge prevents the formation of hydrates in the sponge and also prevents the iron oxide in the sponge from dehydrating. The iron sponge need not be completely effective in removing the hydrogen sulfide, since the outlet stream from the iron sponge is fed back into the inlet of the desiccant dryer. The desiccant dryer and iron sponge components are suitable for small as well as large scale requirements, and the system operates efficiently regardless of scale.

Brief Description Of Drawings

Other objects and advantages of the invention will become apparent upon reading the attached detailed description and upon reference to the drawings in which:

FIGURE 1 is a functional block diagram of the gas sweetener and dryer according to the invention;

FIG. 2 is a block diagram of the preferred embodiment of the gas sweetener and dehydrator; and

_.

FIG. 3 is a detailed schematic diagram of the preferred embodiment shown in FIG. 2.

While the invention has been described in connection with the preferred embodiment, it will be understood that there is no intention to limit the invention to the particular embodiment shown but it is intended, on the contrary, to cover the various alternative and equivalent forms of the invention included within the spirit and scope of the appended claims. The process parameters given below are exemplary, and any specific embodiment of the invention can accommodate a wide range of process parameters.

Best Mode For Carrying Out The Invention

Turning now to the drawings, there is shown in FIG. 1 a functional block diagram of a gas sweetener and dehydrator 10 according to the invention. Wet, sour natural gas is received on an inlet 11 which is fed, for example, by a pumping station at the terminus of a gathering system from wellheads. A pumping station (not shown) generates an inlet stream at a pressure of 240 P.S.I.G. and a temperature of about 80° Fahrenheit. The inlet stream is saturated with water vapor and has a hydrogen sulfide content of about 30 grains per 100 cubic feet. The wet, sour gas stream from the inlet 11 is received by a gas fractionator or, more specifically, a dryer and sweetener 12 which has an outlet 13 emitting a dry, sweet gas stream, and a purge outlet 14 exhausting a stream of wet, sour purge gas. The dry, sweet gas stream, for example, has a -20° Fahrenheit dewpoint, contains less than 0.3 grains of hydrogen sulfide per 100 cubic feet of gas, and is at a pressure of 230 P.S.I.G. The purge is fed to a cooler 15 which lowers the temperature of the purge stream so that the water vapor condenses to water droplets and in the process liberates heat of condensation which is received by the cooler. The water vapor droplets are collected by a separator 16 and drained from the purge stream.

An iron sponge 17 removes hydrogen sulfide from the stream of sour purge gas. The iron sponge 17 is placed downstream of the separator 16 in the purge loop, it being presumed that the purge loop starts from the purge outlet 14 of the fractionator 12. The iron sponge 17 is comprised of at least one bed of iron sponge material consisting of wood shavings holding an active material of hydrated iron oxide. The hydrated iron oxide chemically reacts with the hydrogen sulfide so that the hydrogen sulfide and iron oxide become converted to iron sulfide and water. Moreover, as will be explained in detail below, the spent iron sponge material consisting of iron sulfide is preferably intermittently regenerated by introducing oxygen and sprinkling water on the beds in^" a controlled fashion so that the iron sulfide is reconverted to hydrated iron oxide and elemental sulfur is liberated. The regeneration reaction is highly exothermic and the heat of reaction is removed from the sponge by the controlled sprinkling of water so that the reaction temperature is moderated.

The purge gas flowing from the iron sponge 17 is fed back to an inlet 18 of the fractionator 12. The purge loop, in other words, is "closed" as that term is used in describing adsorbent fractionators or desiccant dryers. In the art, the purge loop is "closed" when all of the purge gas from the purge outlet 14 is fed back to at least one input 18 of the fractionator 12. The purge gas stream, however, may not necessarily be closed in the sense of the same purge gas being recirculated. For the purpose of this patent application, a purge loop will be considered to be closed when the purge flow from a fractionator 12 is fed back to at least one input 18 of the fractionator regardless of whether the same or identical purge gas is recirculated through the purge loop, so that the purge loop is "closed" from the atmosphere. The significance of the term "closed" in the desiccant dryer art becomes clear when it is understood that for most desiccant dryer applications the purge outlet 14 is merely exhausted to the atmosphere, rather than being recirculated.

The gas sweetener and dehydrator 10 shown in FIG. 1 is a synergistic combination of the fractionator 12 and iron sponge 17. .The dryer and sweetener 12 cannot by itself be operated with a closed purge loop and remove hydrogen sulfide, since eventually the hydrogen sulfide would saturate the active material in the fractionator. The iron sponge 17, on the other hand, cannot remove water vapor from the wet, sour gas inlet. Of greater importance, however, is the fact that the combination of the fractionator 12 and the iron sponge 17 permits more economical and reliable operation of the iron sponge 17 than if the iron sponge 17 were used alone or in combination with a conventional dehydration plant such as a glycol dehydrator. The required diameter of the iron sponge vessel 17 is primarily dependent upon the rate of flow of gas through the iron sponge. If the iron sponge 17 were used in combination with a glycol dehydrator, the iron sponge 17 would have to carry the full volume of the wet, sour gas inlet stream or the dry gas outlet stream. But when used in combination with the fractionator 12, the iron sponge 17 need only carry the purge flow. For the process parameters given above, the purge flow is less than 8% of the inlet flow. Thus, the diameter and the resulting minimum wall thickness of the iron sponge vessel 17 is correspondingly reduced. Preferably, the iron sponge 17 can be sized somewhat larger than the minimum required size such that a lower pressure drop across the sponge will result from the reduced purge flow rate, saving energy in circulating the purge flow. The purge flow into the iron sponge is also near saturation and thus at about the ideal 90% relative humidity for maintaining the hydration of iron oxide in the sponge. ' If an iron sponcje were used in series with a glycol dehydrator, the glycol dehydrator would have to be placed after the sponge since the sponge does not remove water vapor. But then the system would be inefficient for drying and sweetening gas which was not saturated with water vapor, since water vapor would have to be added to the inlet stream to the iron sponge to prevent dehydration. In the present invention the inlet stream to the iron sponge is saturated not because water vapor was added, but because the water vapor from the gas inlet stream 11 is concentrated by the fractionator 12.

The fractionator 12 also concentrates the hydrogen sulfide in the purge. In contrast to an iron sponge in series with the process flow, the iron sponge 17 need only reduce the concentration of hydrogen sulfide in the purge flow to approximately the concentration of hydrogen sulfide in the inlet flow, since the fractionator 12, rather than the iron sponge 17, assures a low level of hydrogen sulfide at the outlet so long as the fractionator 12 is not saturated with hydrogen sulfide.

Further advantages of the present invention are evident from the block diagram of the preferred embodiment 10' for natural gas drying and sweetening shown in FIG. 2. Elements that are the same in the figures are identified by the same reference numerals. It will become evident that the functional block diagram of FIG. 1 is a generic description with respect to the species 10' shown in FIG. 2. In FIG. 1, the gas fractionator 12 is a functional block which could encompass a number of embodiments. The fractionator 12, for example, could be any kind of gas fractionator having a closed purge loop, for example, an internal heat reactivated, external heat reactivated, or pressure swing reactivated desiccant dryer. In practice, due to the low cost and availability of natural gas at the site of a natural gas dehydrator and sweetener, it is preferable that the desiccant dryer and sweetener 12 be an external heat reactivated split-stream adsorbent dryer 12'. Such a heat reactivated desiccant dryer and sweetener 12' has two inputs, a mainstream input 21 and a purge stream input 22. The heat reactivated dryer or fractionator 12' shown uses a natural gas powered heater 23 to heat a portion of the wet, sour gas stream from the inlet 11 before it is applied to the purge inlet 22 of the dryer 12*. Moreover, the motive force for circulating the purge gas is provided by a throttling valve 18' which generates a pressure drop in the main flow stream equal to that which is encountered in the purge loop. In some instances, however, it might be advantageous to employ the pumping station compressor for circulating the purge merely by feeding the purge from the iron sponge 17 to the inlet or lower pressure side of the pumping station compressor.

Although the pressure drop across the throttling value 18' and the flow of gas through the throttling valve represents an energy loss or entropy increase, use of a throttling valve is preferable to the use of a separate pump or compressure due to the fact that the large compressor in the natural gas pumping station (not shown)* providing the wet, sour.inlet gas has efficiencies due to the economy of scale. The throttling valve 18' is adjusted to set the desired purge flow rate. It should also be noted that the heat reactivated system for the dryer 12' also ensures that the purge flow to the iron sponge 17 is at a moderate temperature so that hydrate -formation in the iron sponge 17 is prevented.

Turning now to FIG. 3 there is shown a detailed schematic drawing of the preferred embodiment 10' shown in FIG. 2. The wet, sour gas at the inlet 11 is first passed through a coalescing prefilter 26 to remove any condensation or impurities from the pumping station (not shown). The function indicated by the throttling valve 18' in FIG. 2 is performed by a "Pall Flo" (registered trademark) differential flow control valve 18'' manufactured by Pall Corporation, Glen Cove, New York 11542. The "Pall Flo" differential flow control valve 18'' has a separate purge input 27 which is used by the valve 18'' as a control input. The "Pall Flo" differential flow control valve automatically adjusts its throttling action to keep the purge flow in the control input 27 at a predetermined value, substantially independent of variations in pressure and flow at the inlet 11. A flow sensor 28 monitors the purge flow and activates an indicator 29 so that an operator can manually preadjust, the control valve 18''. It should be noted that if an ordinary throttling valve were used, a servo mechanism responsive to the indicator 29 could provide continuous adjustment 31 to the throttling valve 18' ' to keep the purge flow constant. Thus the feedback 31 from the flow sensor indicator 28 to the throttling valve 18'' could be either manual or automatic.

The external heat reactivated adsorbent fractionator or desiccant dryer 12' is shown having two chambers, a left chamber 32 undergoing adsorption or drying and sweetening and a right chamber 33 undergoing a heated purge. The wet, sour gas inlet stream is directed from the dryer from the dryer input 21 to either the left 32 or right 33 chamber by a dryer chamber inlet switching valve 34 and the dry, sweet gas from the selected drying chamber is released from the selected chamber to the outlet 13 by an outlet switching valve 35. Corresponding purge switching valves 36 and 37 are provided to selectively switch the purge flow between adsorbent chambers 32 and 33. It should be noted that the pair of valves 34, 36 form in effect a single four-way valve, and similarly the pair of valves 35, 37 form another four-way valve. But separate two-way or three-way valves are preferable because of the rather large flows involved and because the purge flow is less than 10% of the throughput. For example, for a throughput of 3.5 M.S.C.F. per day (2,430 S.C.F.M.), the purge flow would be about 328 S.C.F.M. The valves are shown activated by selectively applying pressure at the pilots A and B. Specifically, the left chamber 32 is selected for sorption and the right chamber 33 is selected for purge or regeneration when the pilot A is pressurized. When pilot B is pressurized, the left chamber 32 is selected for purge or regeneration and the right chamber 33 is selected for sorption. The other dual- piloted valves shown operate in a similar fashion.

Preferably a Zeolite molecular sieve type 4A is used for the adsorbent or desiccant, and to process a 3.5 M.S.C.F. per day flow rate, each chamber would be about 15 feet in height and two feet in diameter.

Preferably the adsorption fractionator,or ^' desiccant dryer 12' employs convection cooling at the end of the heated purge of either of the adsorbent chambers 32, 33. It should be noted that the sorption gas flow is always counter to the heated purge so that the lower or exit ends of the desiccant chambers 32, 33 are always maintained in a dry state and substantially free of hydrogen sulfide. At the end of the heated purge portion of the drying cycle, the adsorbent retains the purge heat so that its adsorbent properties would be impaired during an initial period of sorption in the next half cycle unless the adsorbent is cooled before sorption. Thus, the adsorbent should be cooled at the end portion of the purge half cycle, before the flow to the chambers 32, 33 is switched by the valves 34, 35, 36, and 37. During cooling the purge flow is temporarily reversed so that the adsorbent at the bottom of the purging chamber 32 or 33 (33 as shown) will be maintained in a dry state, substantially free of hydrogen sulfide. To temporarily reverse the purge flow, a pair of heat/cool switching valves are provided to function as a four-way flow reversing valve. The switching valves 38, 39 are activated by pilots C and D. It should be noted that the pilots A, B, C, and D may be provided by conventional solenoid valves switching control gas obtained from the dry, sweet gas outlet 13 in response to an electromechanical or electronic controller of the kind commonly used for external heat reactivated split-stream adsorbent dryers employing convection cooling.

The iron sponge generally designated 17 employs two chambers 41, 42 of iron sponge material consisting, for example, of wood chips or shavings

-*. coated with hydrated iron oxide. The iron sponge material is a staple item of commerce sold, for example, by Connelly-G.P.M. , Inc., 200 South Second Street, Elizabeth, New Jersey 07206. For removing

_, QMPI hydrogen sulfide from a 328 S.C.F.M. purge flow, each iron sponge chamber 41, 42 would be sized at about 18 feet in height and 5-1/2 feet in diameter. This should be compared to a size of 15 feet in heigth and two feet in diameter for the adsorbent chambers 32, 33 described above for a 3.5 M.S.C.F. per day throughput. In the preferred embodiment, two beds 41, 42 are used rather than a single bed so that each bed may be intermittently taken off the purge loop while the other continues removing the hyrdogen sulfide from the purge stream. Valves 43 and 44 are provided to be closed for removing the left sponge chamber 41 from the purge flow and similarly valves 45 and 46 are provided to be closed for removing the right sponge chamber 42 from the purge flow. These valves 43-46 may be manually operated or automatic. Since the regeneration of the chambers 41, 42 need only be done on a weekly basis, manual valves are preferred for the system sized according to the present parameters. When either chamber is taken off the purge flow, a valve 47 or 48 may be opened to permit the entry of air into the respective chamber 41, 42 to introduce oxygen to the iron sponge material for regeneration or revivification. The circulation of air into the respective chamber 41, 42 is regulated by a small rotary vane compressor or pump 49. It is contemplated that a flow rate of less than 10 S.C.F.M. should be needed. Preferably the air introduced to the regenerating bed 41, 42 is exhausted to the atmosphere through an open exhaust valve 51, 52 respectively. The regeneration process for the iron sponge* is an exothermic chemical ^" reaction and therefore a pump 53 provides a regulated water spray for cooling the regenerating iron sponge bed. A flow rate of about 1-3 gallons per minute is contemplated for the specific system described. Since the water spray is continuously fed simulataneously with the circulation of air, there is no need to monitor the reaction temperature or regulate the flow of water in response to the reaction temperature. Valves 54, 55, respectively, are provided to be opened to permit the flow of cooling water to the respective bed 41, 42 being regenerated. The cooling water is exhausted along with the air through the drain valves 51, 52 and the cooling water drains into a holding tank or reservoir 56. Flow restrictors 57, 58 in series with the respective drain valves 51, 52 prevent excessive discharge of natural gas when the drain valves are first opened or if the valves 43, 45 are improperly left open during regeneration.

It should be noted that there are, however, a few alternative regeneration schemes that have certain disadvantages. Oxygen or air could be continuously injected into the iron sponge 17, but the oxygen would contaminate the gas stream and the molecular sieve adsorbent in the desiccant chambers 32, 33. The beds 41, 42 could be cooled by the recirculation of natural gas during regeneration of the beds 41, 42 instead of using a water spray, but the recirculation rate would have to be at least 500 S.C.F.M. to hold the bed temperatures down to reasonable levels, and also the mechanical equipment for gas recirculation and cooling would be moe costly than the equipment such .as the pump 52, drain valves 56, 57 and holding tank 58.

Since it is contemplated that the regeneration occurs at a low pressure and exhaust is released to the atmosphere, check valves 61, 62, 63 and 64 are provided to prevent excessive release of natural gas due to operator error in controlling the valves 43, 44, 45, 46 for removing the sponge vessels 41, 42 from the purge stream. Also, a repressurizing valve 65 is provided to permit gradual repressurization of the sponge vessel 41, 42 just having been regenerated.

To prevent fluidization of the iron sponge beds 41, 42 the purge flow is directed downwardly through the beds. A coalescing filter 66 is also placed downstream of the iron sponge 17 in order to prevent any particles or chips from the iron sponge bed from becoming introduced into the recirculated purge flow. The coalescing filter 66 also removes any condensation passing from the iron sponge 17.

In view of the above, a gas sweetener and dryer has been disclosed that has no atmospheric emission of sulphur dioxide acid gas. For the preferred embodiment 10' in FIG. 3, the dryer and sweetener has a reduced capital cost and reduced operating costs compared to current technology using a combined amine and glycol plant. The energy requirements for daily operation in particular are estimated to be about 25% of the requirements for an amine sweetener and glycol dehydrator. Moreover, the drying and sweetening apparatus of the present invention is economical for small as well as large scale requirements, since desiccant dryers are now manufactured in a variety of sizes. Although the invention has been described in terms of processing natural gas, it should also be advantageous for processing synthetic gas such as coal gas or "producer" gas whenever the synthetic gas has an objectionable content of sulfur comppund^'s.

Claims

C AIMS

1. A natural gas dryer and sweetener comprising, in combination, means for receiving an inlet stream of wet, sour natural gas containing hydrogen sulfide and generating an outlet stream of dry, sweet natural gas and a stream of wet, sour purge gas, means for removing heat of condensation and water vapor from the stream of wet purge gas, and means for removing hydrogen sulfide from the stream of sour purge gas, wherein the stream of purge gas having water vapor and hydrogen sulfide removed is fed back to the means for receiving the inlet stream of wet, sour natural gas.

2. The combination as claimed in claim 1, wherein the means for receiving and generating comprises a desiccant gas dryer.

3. The combination as claimed in claim 1, wherein the means for removing hydrogen sulfide comprises an iron sponge.

4. The combination as claimed in claim 3, wherein the means for receiving and generating comprises a desiccant dryer.

5. The combination as claimed in claim 1, wherein the means for removing heat and water vapor comprises a cooler and separator.

6. The combination as claimed in claim 1, further comprising means for circulating the gas through the purge loop including a throttle valve in the inlet stream, and wherein the means for receiving and generating has a separate^* purge stream input, the purge steam input being fed from the higher pressure side of the throttle valve and the stream of purge .

gas having water vapor and hydrogen sulfide removed being fed to the lower pressure side of the throttle valve, the pressure drop across the throttle valve thereby providing a motive force for circulating the gas through the purge loop.

7. The combination as claimed in claim 6, wherein the means for receiving and generating comprises a desiccant dryer.

8. The combination as claimed in claim 7, wherein the means for removing hydrogen sulfide comprises an iron sponge.

9. The combination as claimed in claim 7, further comprising a heater receiving the inlet stream from the higher pressure side of the throttle valve and passing heated natural gas to the purge stream input of the means for receiving and generating.

10. A gas dryer and sweetener comprising, in combination, an adsorbent dryer receiving an inlet stream of wet, sour gas and generating an outlet stream of dry, sweet gas and having a closed regeneration loop, and an iron sponge placed in the regeneration loop.

11. The combination as claimed in claim 10, wherein the adsorbent dryer uses a molecular sieve adsorbent.

12. The combination as claimed .in claim 10, wherein the adsorbent dryer is an external heat reactivated split-stream adsorbent dryer.

13. The combination as claimed in claim 12, wherein the adsorbent dryer is convection cooled at the completion of heat reactivation cycles.

14. The combination as claimed in claim 10, wherein the closed regeneration loop includes a cooler and separator receiving the purge from regeneration of the adsorbent, and the iron sponge is placed downstream of the cooler and separator.

15. The combination as claimed in claim 14, further comprising a coalescing filter placed in the closed regeneration loop downstream of the iron sponge.

16. The combination as claimed in claim 10, wherein the iron sponge comprises at least two beds of iron sponge material and also comprises means for selectively removing each bed at intermittent intervals and applying oxygen to the removed bed to regenerate the iron sponge material in the removed bed.

17. The combination as claimed in claim 16, further comprising means for applying a predetermined flow rate of air and a predetermined flow rate of water spray to the removed bed to maintain the exothermic regeneration reaction below a predetermined temperature.

18. A method for removing hydrogen sulfide from an input stream^' of gas with substantially no atmospheric discharge of sulfur dioxide acid gas, said method comprising the steps of fractionating the input stream into an output stream of sweet gas and a stream of sour purge gas, removing hydrogen sulfide from the purge gas by contact of the purge gas with iron oxide to form iron sulfide, and feeding the purge gas back into the input stream of gas.

19. The method as claimed in claimed 18, wherein the step of fractionating comprises passing the input stream through one of two adsorbent beds to generate the output; stream of sweet gas whi!J.e purge gas is passed through the second adsorbent bed to regenerate the second bed and thereby generate the stream of purge gas, and periodically switching the -2Q-

respective flow of sorbing and purge gas between the respective chambers when the sorbing adsorbent is expected to have adsorbed a design quantity of hydrogen sulfide, the flow of purge gas through each chamber during a half cycle of regeneration being counter to the flow through the same chamber during the other half cycle of sorption.

20. The method as claimed in claim 18, further comprising the step of periodically regenerating the iron sulfide by the application of a predetermined flow rate of oxygen and a predetermined flow rate of water spray to the iron sulfide to thereby generate iron oxide and elemental sulfur, the water spray absorbing the exothermic heat of reaction.

21. A regeneration method for an iron sponge gas sweetener having at least two pressure vessels for sweetening a process gas stream, each pressure vessel containing iron sponge material, an inlet gas valve for permitting entry of pressurized sour gas, an outlet gas valve for permitting the discharge of pressurized sweetened gas, an inlet water valve for receiving water spray, and a drain valve for exhausting excess water spray, the regeneration method comprising the steps of: alternately removing at least one of the pressure vessels from the process gas stream while at least one other pressure vessel remains on stream by closing the inlet and outlet gas valves, reducing the gas pressure in the removed pressure vessel to substantially atmospheric pressure, opening the inlet water spray valve and feeding a continuous predetermined flow rate of water spray into the removed pressure vessel, and opening the drain valve to release excess water spray, and simultaneously feeding a continuous predetermined flow rate of air into the removed pressure vessel as the water spray is fed, thereby maintaining the exothermic regeneration reaction of the iron sponge below a predetermined temperature without the need to monitor the reaction temperature.