GB2067173A - Improvements regeneration of absorbents used in desulfurizing fossil fuels - Google Patents

Abstract

Hydrated alkali metal sulfides o used to remove sulfur from fossil fuels form alkali metal polysulfides which are converted to alkali metal sulfides for recycling and elemental sulfur by thermal or thermal reduced pressure hydrolysis of the alkali metal polysulfides. Additional sulphide for recycling can be formed by conversion of polysulphide remaining in the hydrolysis products by treatment with H2S.

Description

SPECIFICATION Improvements in desulfurizing fossil fuels This invention is concerned with a process for preparing alkali metal monosulfides suitable for the desulfurization of fossil fuels.

In my co-pending Application No. 7912188, there is disclosed and claimed a process for desulfurizing fossil fuels, in particular petroleum residues, by contacting petroleum residues with an alkali metal sulfide hydrate or hydroxide at a temperature comprised between 1 200C and 3250C from 3 to 60 minutes and separating said residues now having a reduced sulfur content. The economics of the said desulfurization process would be markedly enhanced by an inexpensive reagent recycling process.

The present invention seeks to provide such a recycling process.

According to the invention there is provided a process of desulfurizing petroleum residues by treating with an alkali metal monosulfide hydrate reagent at a process temperature of between 1200 and 3250C. to form also the corresponding alkali metal polysulfides, and recycling the reagent, the process comprising the steps of: a) separating at said process temperature a mixture of unreacted alkali metal monosuifide hydrate and alkali metal polysulfides from desulfurized petroleum residues; b) hydrolizing in a closed system said polysulfides by adding to said mixture at said temperature at least one mol of water per mol of hydrate and polysulfide thereby forming steam and hydrogen sulfide.

c) removing said steam and said hydrogen sulfide from said system thereby reducing the water content therein to that of the alkali metal monosulfide hydrate and separating the highest sulfur content polysulfide from the said hydrate; and d) separating said hydrate from said polysuifide for recycling to said process.

Alkali metal polysulfides with an intermediate sulfur content between that of the monosulfide and that of the highest sulfur valence (Na2S4 or K2Ss) are unstable in hot water and decomposed to an equilibrium state of the given alkali metal sulfides and K2S5 or Na2S4. Stepwise this decomposition may be represented as follows: 1. 4K2S2 + 8H20 o 3(K2S.2.66H20) + K2S5 potassium monosulfide hydrate 2. 4K2S3 + 5.33H20 e 2(K2S.2.66H20) + 2K2Ss.

3. 4K2S4 + 2.66H20 -ltK2S.2.66H20) + 3K2S5.

The actual water content of the sulfide hydrate formed varies according to the temperature of the heat water solution.

Shown in greater detail, the reaction is illustrated below: 4. 4K2S2 + 8H20 e 4KOH + 4KHS + 4S + 4H20 (at 1001500C).

In turn, the 4H20 and the 4KHS react in a composition-decomposition equilibrium 5. 4KHS + 4H20 ~ 4KOH + 4H2S The soluble atoms of sulfur react with KOH as follows 6. 4S + 6KOH < K2S203 + 2K2S + 3H20 In turn, K2 S2 03, reacts with at least 0.25 volume thereof of the H2S, as follows: 7. K2S2O3+3H2SK2S5+3H2O The purpose of the hot water decomposition of the intermediate polysulfides is to increase the life of the reagent monosulfide hydrate before recycling is required by the formation of high sulfur content polysulfides.

The alkali metal sulfide hydrate in melt condition with no water in excess of its water of hydration does not dissolve either K2S5 or Na2S4 allowing these high sulfur content polysulfides to be separated by a liquid solid operation. Thus a melt of the anhydrous high sulfur content polysulfide is separated from the hydrate melt by a liquid liquid separation.

The separated K2S5 or Na2S4 is decomposed thermally or thermally under reduced pressures.

At one half Torricelli K2S6 at 1 300 yields K2S5 plus sulfur K2S5 at 1900C yields K2S4 + S K2S4 at 2600C yields K2S3 + S K2S3 at 3500 yields a mixture of K2S and K2S2 + S K2S2 at 6500C yields K2S + S The sodium sulfides all display instability at the melting points which are 4450C, for Na2S2, and 2750C for Na2S4.

The potassium sulfides are all stable at their melting points at atmospheric pressure (except K2S6) At 760 mm of mercury K2S5 yields K2S4 + sulfur at 3000 C.

K2S4 yields K2S3 + S at 4600 C.

K2S4 yields K2S3 + S at 6200 C.

K2S3 yields K2S2 + S at 7800C.

An alternative is to dissolve the polysulfides in aqueous solution and pass H2S into the solution thereby forming the hydrosulfide of the alkali metal. The alkali metal hydrosulfide neither dissolves nor reacts with sulfur in alkaline aqueous solutions, the sulfur being precipitated.

Following the separation of this precipitated sulfur the solution may be heated to both reduce the water content and to drive off H2S. When one half of the H2S of the hydro-sulfide present has been driven off, the alkali metal sulfide is reformed and with the proper amount of water present (depending upon the temperature and the alkali metal) the particular hydrate is formed. The H2S driven off is used to precipitate sulfur from the next batch.

Due to the diminished degree of hydrolysis encountered, as the sulfur content of polysulfides is increased, the thermal or thermal reduced pressure decomposition of high sulfur content alkali metal polysulfides may precede the formation of alkali metal hydrosulfide by treatment of alkali metal hydroxide with hydrogen sulfide, as shown in equation 4 above.

In the hot water hydrolysis of the intermediate sulfur content alkali metal polysulfide, the alkali metal hydrosulfide and the alkali metal hydroxide are formed along with atomatic sulfur. The atomic sulfur combines with the alkali metal hydroxide to form the alkali metal thiosulfate (equation 6). A small quantity of the atomic sulfur combined with any non-hydrolized alkali metal polysulfide to form a still higher sulfur content alkali metal polysulfide. The higher the sulfur content the less hydrolysis is exhibited. The higher the temperature of the hydrated melt the greater is the degree of hydrolysis of all the alkali metal sulfides and polysulfides.

As shown by equation 5 the alkali metal hydrosulfide is unstable in hot water and decomposes to additional alkali metal hydroxide and hydrogen sulfide in a reaction which is reversible in a closed system. The addition of hydrogen sulfide under pressure will largely convert the hydrolysis product (the alkali metal hydroxide) to the alkali metal hydrosulfide. The hydrosulfide will expel the atomic sulfur as elemental sulfur which then is removed from the system.

The hydrogen sulfide which comes from the decomposition of the hydrolysis product, (the alkali metal hydrosulfide) will react with the thiosulfate, in equation 7 to form the tetrasulfide of sodium or the pentasulfide of potassium. Only sulfur ion as the alkali metal sulfide itself with all the atomic sulfur is coordinated into sodium tetrasulfide or the potassium penta-sulfide.

The aqueous tension of the remaining alkali metal monosulfide melt is less than that of sodium tetrasulfide or potassium pentasulfide. These polysulfides then exist as anhydrous melts above the metling point or as anhydrous solids below the melting point. These anhydrous solids or liquids are practically insoluble in the slightly hydrolized melts of the alkali metal monosulfide hydrates. Separation thus takes place. The fossil fuel forms the top layer, the mono sulfide forms the next layer, the bottom layer consists of the potassium pentasulfide or the sodium tetra sulfide. The bottom layer is then removed and treated. The middle layer is an excellent reagent for desulfuring additional fossil fuels. The process, beginning with a mixture of alkali metal sulfides derived from the desulfurization of the fossil fuels and the separation of the fossil fuels from the melt is as follows: 1.Pressure condensation of steam to produce hydrolysis. This hydrolysis decomposes intermediate sulfur content polysulfides and establishes an equilibrium between the alkali metal monosulfide and the sodium tetrasulfide or potassium pentasulfide. The addition of one mole of water is adequate for producing hydrolysis. This step is carried out at a selected process temperature of between 1200 to 3250C.

2. The reduction of the water content of the system.

The gases produced in step 1 which are steam and H2S at a temperature above the critical pressure of water at the selected process temperature, are withdrawn from the system. The water is condensed (a) by cooling in a gas stream or (b) by compression of the gases to still'higher pressure while cooling to keep the gases from heating during the condensation. The condensed water is separated and the remaining gases are returned to the system from which they came.

When the liquid water content is reduced to that of the hydrated alkali metal mono-sulfide hydrate melt, the K2S5 or NA2S4 separates in liquid or solid form depending upon the temperature.

The separated mono-sulfide melts are then used for desulfuring additional fossil fuels.

The separated potassium pentasulfide or the sodium tetrasulfide can be decomposed thermally or thermally with reduced pressures to lower sulfur content sulfides and elemental sulfur. Sodium sulfides decompose to the monosulfides beginning at 4450C. The greater the temperature, the more rapid its decomposition. Reduced pressure can be used to decompose it more rapidly at lower temperatures.

Potassium pentasulfide is decomposed to potassium trisulfides and elemental sulfur 4200C to 4600C. At those same temperatures, when the pressures are reduced under 10 mm of mercury or less, elemental sulfur is distilled and a resulting mixture of potassium sulfide and disulfide is obtained in proportions of 2 potassium sulfide to 1 potassium disulfide. When this mixture is converted to hydrated potassium sulfide reagent a slight excess of water will convert the potassium disulfide to 3/4 potassium sulfide hydrate and 1/4 part potassium pentasulfide and this mixture is used as a reagent.

The separated potassium pentasulfide may also be made into a hot water solution and within a closed system sufficient hydrogen sulfide is added under pressure to convert the compound to essentially potassium hydrosulfide and free sulfur. The sulfur is then separated and the excess hydrogen sulfide is allowed to escape until a melt of the potassium- sulfide hydrate, which exists at the selected process temperature (any temperature from boiling point to critical temperature of water) is formed. The same amount of boiled off hydrogen sulfide (exactly 1/2) is used over and over again to throw out sulfur.

The amount of water added to the system should be that needed to form that potassium sulfide hydrate which exists at the selected process temperature.

Outlined in greater detail the process of the invention includes the following steps.

1. The fossil fuel is separated from the spent alkali metal sulfide polysulfide reagent mixture at the process temperature of 1200--3250C.

2. A stoichiometric quantity (at least 1 mol) of water per mol of potassium sulfide and polysulfide is added to the reagent mix in a closed system.

3. The H2S and H20 generated are allowed to escape into a compressor at a pressure slightly above the critical pressure of water at the selected temperature of the system and the water is condensed. The uncondensed volatiles are then returned to the process system.

4. When the amount of liquid water of hydration is the same as that of the alkali metal hydrate melt, hydrated potassium pentasulfide or sodium penta sulfide precipitate.

5. The precipitated polysulfides are then removed from the bottom of the process system.

6. The potassium pentasulfide or sodium tetrasulfide are heated to 4200--4600C under atmospheric conditions under which conditions the elemental sulfur is immiscible.

7. The immiscible sulfur is then separated from the polysulfide liquid by a liquid-liquid separation.

8. Reduced pressures of no more than 10 mm of mercury are used to distill the rest of the removable sulfur.

9. Water is added to the mixture formed from step 8 and a slight excess of water above that of the theoretical hydrate which exists at the system is used.

10. The mixture of step 9 is next hydrated to the actual water content of the hydrate.

In practice up to 10% of the K2S5 has been left, with the hydrated melt reagent. The sulfur removed is 97 1/2% per mol of sulfur in excess of that present as sulfide ions.

The invention is further illustrated in non-limiting fashion by the following example: EXAMPLE Following the removal of organic and elemental sulfur from fossil fuels by combination of these sulfur forms with alkali metal sulfide hydrates, preferabiy potassium sulfide hydrate, to form alkali metal polysulfide, the alkali metal polysulfide is decomposed to alkali metal sulfide hydrate and elemental sulfur.

1. 500 ml of the mixed potassium sulfide hydrate melt and potassium polysulfide was withdrawn from the process system. The mixture was at 2300 C.

2. 1 50 grams of cold H20 was added to the mixture in a closed air-tight vessel with agitation.

3. The temperature was allowed to drop to 1 500C. the gases of the system were permitted to exit the system through a tube. The gases were compressed to above the critical pressure of H20 at 1 500C and some water was condensed. The gases were returned to the mixture.

A) Remaining petroleum residues withdrawn along with the alkali metal sulfide-polysulfide separated when the liquid H20 content of the process-system equaled that of the hydrated melt.

B) The K2S5 was precipitated as an insoluble solid when its water content reached that of a K2S hydrate, for that temperature and pressure.

The liquid K2S hydrate was drawn off through a separating funnel, leaving lighter petroleum residues and solid K2S5.

The solid K2S5 was separated from petroleum residue by a liquid solid separation.

The separated K2S5 was divided in half relying on visual approximation. The total K2S5 had a weight of 55 grams.

One half the K2S5 was heated to 4600C under 1 mm of mercury pressure. Sulfur was distilled leaving a mixture of K2S-K2S2 after 37 minutes distillation time. A slight excess of water was added and 3/4 of the K2S2 was converted into K2S hydrate and 1/4 of the K2S2 converted into K2S5. The H20 content was reduced as before. The K2S hydrate was used to desulfurize additional fossil fuel while the K2S5 was added to the other half of the K2S5 being recycled.

14.25 grams of sulfur was recovered from the original half of the 55 grams of K2S5.

The K2S5 was made into a 70% aqueous solution (70% K2S5 and 30% H2O) at 900 C. H2S was passed into the system contained within a closed system. Approximately 50% of the polysulfide sulfur was removed as elemental sulfur when the hydrolysis product contained insufficient KOH to form K2S.

The sulfur was removed.The H20 content was reduced as before and a mixture of K2S-K2S hydrates resulted and 7.3 grams of elemental sulfur were recovered.

Claims (10)

1. A process of desulfurizing petroleum residues, by treating with an alkali metal monosulfide hydrate reagent at a process temperature of between 1200 and 3250C to form also the corresponding alkali metal polysulfides, and recycling the reagent, the process comprising the steps of: a) separating at said process temperature a mixture of unreacted alkali metal monosulfide hydrate and alkali metal polysulfides from desulfurized petroleum residues; b) hydrolizing in a closed system said polysulfides by adding to said mixture at said temperature at least one mol of water per mol of hydrate and polysulfide thereby forming steam and hydrogen sulfide; c) removing said steam and said hydrogen sulfide from said system thereby reducing the water content therein to that of the alkali metal monosulfide hydrate and separating the highest sulfur content polysulfide from the said hydrate; and d) separating said hydrate from said polysulfide for recycling to said process.

2. A process according to Claim 1, comprising the further steps of separating said polysulfide from any petroleum residue; heating a portion of said polysulfide to distil off sulfur thereby leaving additional monosulfide and disulfide; adding an excess of water; and dehydrating the resulting mixture to form additional reagent alkali metal monosulfide and additional alkali metal polysulfide.

3. A process according to Claim 1, wherein polysulfide is treated with hydrogen sulfide in a closed system with water, the sulfur formed is removed, and the water content of the system is reduced to that of the alkali metal hydrosulfide hydrate.

4. A process according to Claim 3, wherein excess hydrogen sulfide is allowed to exit from the system until said water content of said system is reduced.

5. A process according to Claim 3 or Claim 4, wherein the water which is added is that needed to form that monosulfide hydrate which exists at the selected process temperature.

6. A process according to any preceding Claim, wherein said alkali metal is potassium and said reduction in water content in step c) provides K2S.2H2O at 1 500C or above and K2S. 5H2O below 1500C.

7. A process according to any one of Claims 1 to 5, wherein said metal is sodium or potassium.

8. A process according to any preceding Claim, wherein said polysulfide is heated to 4600C to distil off sulfur and leave behind sulfides of lower sulfur content including the monosulfide of said alkali metal.

9. A process according to any preceding Claim, wherein said alkali metal monosulfide hydrate reagent is formed by the following reactions: 4K2S2 + 8H20 e 3 (K2S.5H20 or 2H2O) + K2S5 4K2S3 + 5.33 H20 o 2 (K2S.2.66H20) + 2K2S5 4K2S4 + 2.66H2O e (K2S.2.66H2O) + 3K2S5

10. A process of desulfurizing petroleum residues, substantially as hereinbefore described with reference to the Example.